Stainless Steel
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Polyamides and Polyimides
Whilst by far the bulk of polyamide materials are used in the form of fibres, they
have also become of some importance as speciality thermoplastics of particular
use in engineering applications. The fibre-forming polyamides and their
immediate chemical derivatives and copolymers are often referred to as nylons.
There are also available polyamides of more complex composition which are not
fibre-forming and are structurally quite different. These are not normally
considered as nylons (see Section 18.10).
The early development of the nylons is largely due to the work of W. H. Carothers and his colleagues, who first synthesised nylon 66 in 1935 after extensive and classical researches into condensation polymerisation. Commercial production of this polymer for subsequent conversion into fibres was commenced by the Du Pont Company in December 1939. The first nylon mouldings were produced in 1941 but the polymer did not become well known in this form until about 1950.
In an attempt to circumvent the Du Pont patents, German chemists investigated a wide range of synthetic fibre-forming polymers in the late 1930s. This work resulted in the successful introduction of nylon 6 (and incidentally in the evolution of the polyurethanes) and today nylons 66 and 6 account for nearly all of the polyamides produced for fibre applications. Mention may, however, be made of nylons 7 (Enanth) and 9 (Pelargone) which have been investigated as fibres in the Soviet Union. Very many other aliphatic polyamides have been prepared in the laboratory and a few have become of specialised interest as plastics materials including nylons 11, 12, 46, 69, 612, 66/610 and 66/610/6. For a variety of reasons the aromatic polyamides were slow in their development. A glassy aromatic polymer, poly(trimethylhexamethy1ene terephthalamide) became available in the early 1960s as Trogamid T, and this was followed by a series of other glassy aromatic polyamides in the 1970s and 1980s. During the same period several aromatic polyamides of more regular structure than the glassy polymers became important as fibres because of their exceptional strength in some cases or because of their fire-retarding properties in others. In the latter part of the period polyamides became available which might be classified as thermoplastic rubbers.
There thus exists a very wide range of materials-fibires, crystalline plastics, amorphous plastics, adhesives and rubbers-which are classified as polyamides. They have the common feature that the amide (-CONH-) group occurs repeatedly in the polymer. Such an amide group can increase resistance to swelling and dissolution in hydrocarbons, increase interchain attraction and hence stiffness and heat deformation resistance, reduce electrical insulation resistance, particularly at high frequencies, and increase water absorption. However, as with other condensation polymers which are classified by the group formed during the condensation reaction, the amide groups form only a small proportion of the molecule, and other chemical groups may also have an important influence on the properties.
Of the many possible methods for preparing linear polyamides five are of commercial importance:
An example of the first route is given in the preparation of nylon 66, which is made by reaction of hexamethylenediamine with adipic acid. The first ‘6’ indicates the number of carbon atoms in the diamine and the second the number of carbon atoms in the acid. Thus, as a further example, nylon 6.10 is made by reacting hexamethylenediamine with sebacic acid (HOOC*(CH,), mCOOH). (In this context the numbers 10,ll and 12 are considered as single numbers: the need to use two digits results simply from the limitations of the decimal system.) Where the material is denoted by a single number, viz nylon 6 and nylon 11, preparation from either an w-amino acid or a lactam is indicated. The polymer nylon 66/6.10 (60:40) indicates a copolymer using 60 parts of nylon 6.6 salt with 40 parts of nylon 6.10 salt.
Closely related to the polyamides are the polyimides and derivatives such as polyamide-imides and polyether-imides. These are discussed in Sections 18.13 and 18.14.
Comparison of nylons 6 and 66 in glass-filled compositions The presence of glass-fibre fillers can to some extent mask the differences between nylons 6 and 66. For example, an advantage of unfilled nylon 66 in injection moulding is that the high T, leads to a high solidification temperature and shorter cycle times. However, in glass-filled grades the more rapid cooling and crystallisation can lead to a poorer surface finish than obtained with corresponding nylon 6 compounds. It is also considered that abrasive wear on screws is greater with nylon 6.
Water absorption decreases with increasing glass-fibre content at about the same rate with both nylons 66 and 6, and since nylon 6 has an intrinsically higher water absorption than nylon 66 the glass-filled grades also have higher levels at similar glass-fibre loadings.
Mechanical properties of freshly injected compositions are similar for the two nylons but, after conditioning, differences arise largely due to the plasticising effect of the moisture present. Thus for tensile and flexural yield stress, tensile strength and modulus of elasticity, nylon 66 gives slightly higher figures. Yield elongation and elongation at break are greater with nylon 6. Izod impact strengths are similar, with nylon 6 giving marginally higher values. The above comments refer to comparisons between the two compositions at the same glass-fibre level. If, however, comparison is made between a nylon 66 composition with a glass content of x% and a nylon 6 compound with a glass content of (x + 5)%, then the differences in mechanical properties become very small. At the same time the nylon 6 material will have slightly easier processing characteristics and surface quality.
Whilst nylon 66 has the higher T,, the long-term heat resistance of typical copper-stabilised nylon 6 is somewhat superior in such properties as impact strength and bending strength compared to nylon 66. However, it is frequently the case that nylon 66 has better resistance to chemicals at elevated temperatures.
PROCESSING OF THE NYLONS
In the processing of nylons consideration should be given to the following points: (1) The tendency of the material to absorb water.
(2) The high melting point of the homopolymers.
(3) The low melt viscosity of the homopolymers.
(4) The tendency of the material to oxidise at high temperatures where oxygen (5) The crystallinity of the solid polymer and hence the extensive shrinkage The above features are particularly marked with nylons 46, 6,66 and 610 and less marked with nylons 11 and 12. Providing they are dry the copolymers may be processed in much the same way as conventional thermoplastics.
In the injection moulding of nylon 66, for example, it is necessary that the granules be dry. The polymer is normally supplied in sealed containers but should be used within an hour of opening. If reworked polymer is being used, or granules have become otherwise damp, the polymer should be dried in an oven at about 70-90°C. Too high a temperature will oxidise the surface of the granules and result in inferior mouldings.
Injection moulding cylinders should be free from dead spots and a temperature gradient along the cylinder is desirable.
Because of the low melt viscosity of the polymer at processing temperatures it will ‘drool’ through normal injection nozzles even when the plunger is retracted. Several types of nozzle have been specially designed for use with nylon and all function by sealing the end of the nozzle, either by allowing a pip of polymer to harden, by the use of spring-loaded valve, by the use of sliding side-closure nozzles or by the use of hydraulic nozzle valve activated at the appropriate stages of the moulding cycle. Variations of this last approach have become popular since they are both positive in action and simple to operate. In designs in which solidified polymer is formed at the nozzle it is necessary to make provision for a cold-slug well in the mould, a feature frequently not possible with single-cavity tools. Where spring-loaded closing devices are used the spring should be kept as cool as possible if rapid thermal fatigue is to be avoided.
Because of the crystallisation that occurs on cooling from the melt the polymers show a higher moulding shrinkage than that generally observed with amorphous polymers. With average moulding conditions this is about 0.01 8 cm/ cm with nylon 66 but by increasing the injection pressure and the injection time the shrinkage may be halved. This is because a high initial mould cavity pressure is developed and a large part of the crystallisation process will be complete before the cavity pressure has dropped to zero. The shrinkage will also be affected by the melt temperature, the mould temperature, the injection speed and the design of the mould as well as by the type of nylon used. The nylons, nylon 66 in particular, may also exhibit a certain amount of aftershrinkage. Further dimensional changes may occur as a result of moulding stresses being relieved by the plasticising effect of absorbed water. It is consequently often useful to anneal mouldings in a non-oxidising oil for about 20 minutes at a temperature 20°C higher than the maximum service temperature. Where this is not known a temperature of 170°C is suitable for nylon 66, with somewhat lower temperatures for the other nylons.
When dimensional accuracy is required in a specific application the effect of water absorption should also be considered. Manufacturers commonly supply data on their products showing how the dimensions change with the ambient humidity.
The particular features of the nylons should also be taken into account in extrusion. Dry granules must be used unless a devolatilising extruder is employed. Because of the sharp melting point it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. The polymerisation casting process mentioned in Section 18.3.2 has been adapted to reaction injection moulding (RIM), a process originally developed for polyurethanes. In this process the reacting ingredients are mixed together by impingement of jets of the materials in a small mixing chamber adjacent to the mould cavity into which the reacting material is then injected. Because of the low injection pressures much lower locking forces are possible than in conventional injection moulding, making the process attractive for large area mouldings. The first polyamides specifically developed for RIM were introduced by Monsanto in 1981 as Nyrim. They are block copolymers of a polyether (such as a poly(ethy1ene glycol), poly(propy1ene glycol) or polybutadiene containing hydroxyl groups) with caprolactam. The reaction components comprise the polyether, caprolactam, adipyl-bis-caprolactam as chain propagator and a caprolactam-magnesium bromide complex as catalyst. The latter has to be protected against moisture, carbon dioxide and oxygen, and thus requires special care in handling. Other polyamide-RIM systems have been developed by Upjohn and Allied Fibers and Plastics.
Unlike polyurethane-RIM processes, nylon-RIM reactions are endothermic and require temperatures of 130-140°C. In contrast to the polyurethane-RIM systems, this enables thick wall parts to be made. Cycle times of 2-3 minutes are comparable to those for polyurethane-RIM. In the development stage, current work is concerned with reducing moulding times and optimising moulding conditions.
APPLICATIONS
Consumption of polyamide plastics in the late 1990s has been estimated at about 1 300 000 t.p.a. Western Europe and the United States each have about 42% of this market and Japan about 16%. This is probably about 20% of the total production of polyamide materials and virtually all of the rest of polyamide production goes into fibres where the market is shared very roughly equally between nylons 6 and 66. The large-scale production of these materials enables them to be available at a substantially lower price than the other nylons, which do not have the benefit of the economies of scale. Hence the other nylons are usually restricted to applications where nylons 6 and 66 are unsuitable. It has been estimated that in the 1990s in Western Europe, plastics usage was split as follows: nylon 6 48%: nylon 66 40% nylons 11 and 12 10%: the rest 2%. For historical reasons, usage of the various types varies from country to country. For example, in the United States and the United Kingdom, nylon 66 was the first to be developed and remains well entrenched, whilst for similar reasons nylon 6 is more dominant in Germany. The substantial market penetration of nylons 11 and 12 in France also reflects long-standing French commercial activity with these types. It remains to be seen what effect the recent introduction of nylon 46 will have on the balance of usage.
It has been reported that out of a total 1997 Western European consumption of 532000 tonnes, 387000 tonnes, i.e. almost 75% of the total, was injection moulded. Film use accounted for 70 000 tonnes, monofilaments 19 000 tonnes, extruded sheet, rod and tube 24 0o0 tonnes, wire and cable 10 000 tonnes and other processes such as powder coatings, hot melts, blow moulding and monomer casting about 22 000 tonnes. Insofar as data can be compared, patterns in the USA are similar. Over two-thirds of injection moulding usage is in transport and electrical/electronic applications. The polyamides still have the biggest tonnage of the so-called engineering plastics although this could well be overtaken by the polycarbonates in the near future (see Chapter 20). The two classes of materials do, however, have quite dissimilar properties and are seldom directly competitive.
The nylons have found steadily increasing application as plastics materials for speciality purposes where their toughness, rigidity, abrasion resistance, good hydrocarbon resistance and reasonable heat resistance are important. Because of their high cost they have not become general purpose materials such as polyethylene and polystyrene, which are about a third of the price of the nylons. The largest applications of the homopolymers (nylon 6, 66, 610, 11 and 12) have been in mechanical engineering. Well-known applications include gears, cams, bearings, bushes and valve seats. In addition to the advantageous properties cited above, nylon moving parts may be frequently operated without lubrication, are silent running and may often be moulded in one piece when previously a metal part required assembling of several parts, or alternatively, extensive machining with consequent waste of material. It may be noted that in this area the newly introduced nylon 46 has become of interest in auto automatic gears, gearboxes, engine differentials and the clutch area because of its exceptional ability (for a polyamide) to withstand severe mechanical and thermal loading.
In recent years the nylons have met increased competition from acetal resins (Chapter 19), the latter being superior in fatigue endurance, creep resistance and water resistance. Under average conditions of humidity the nylons are superior in impact toughness and abrasion resistance. When a nylon is considered appropriate it is necessary to consider the relative importance of mechanical properties, water resistance and ease of processing. For the best mechanical properties nylon 66 would be considered but this material is probably the most difficult to process and has a high water absorption value. Nylon 6 is easier to process but has slightly inferior mechanical properties and an even higher water absorption. Nylons 11 and 12 have the lowest water absorption, and are easy to process, but there is some loss in mechanical properties. Sterilisable mouldings have found application in medicine and pharmacy. Because of their durability, nylon hair combs have found wide acceptance in spite of their higher cost.
Nylon film has been used increasingly for packaging applications for foodstuffs and pharmaceutical products. The value of nylon in this application is due to low odour transmission and to some extent in the ability to boil-in-the-bag. Film of high brilliance and clarity, particularly from nylon 11, is available for point-of-sale displays.
Although the nylons are not generally considered as outstanding electrical insulators, their toughness and, to some extent, their temperature resistance, have led to applications in coil formers and terminal blocks. Indeed, the new nylon 46 materials would appear to be of particular interest here. Acetal resins, polysulphones, modified PPO and polycarbonates, however, present a challenge to applications in this sphere. Nylon monofilaments have found application in brush tufting, wigs, surgical sutures, sports equipment, braiding and outdoor upholstery. Nylons 610 and 11 have found extensive application in these fields because of their flexibility but nylon 66 is also used for brush tufting less than 0.0035in. in diameter. Nylon 66/610 copolymer is used in the manufacture of a monofilament for angling purposes.
Extruded applications of nylon, other than film and monofilament, are less commonly encountered because of the low melt viscosity of the polymers. Uses include cable sheathing which requires resistance to abrasion and/or chemical attack, flexible tubing for conveying petrol and other liquids, piping for chemical plant, rods for subsequent machining, as the tension member of composite belts for high-duty mechanical drive and for bottles requiring resistance to hydrocarbons. Nylons 11 and 12 are frequently preferred because of their ease of processing but the high molecular weight 6,66 and 610 polymers find occasional use. The 661610 and 66161016 polymers have also been used in the past where tough leather-like extrudates have been required. Their cost, and perhaps also their obscurity, have resulted in their current use being minimal. Nylon 11 is also used in powder form in spraying and fluidised bed dipping to produce chemical-resistant coatings. Although more expensive than the polyolefin and PVC powders, it is of interest because of its hardness, abrasion resistance and petrol resistance.
As previously mentioned, mouldings have been produced by the polymerisation casting of caprolactam. The ability to produce large objects in this way enables one to envisage new horizons for the use of plastics engineering and other applications. Such polymerisation-cast polymers also possess certain advantageous properties. The polymers tend to have a somewhat higher molecular weight and also a 45-50% crystallinity, again higher than for melt processed materials. This leads to a higher tensile strength, hardness, modulus and resistance to creep. The comparatively stress-free mouldings also have a reasonably consistent morphological structure. A disadvantage is the shrinkage of 4-4.5% which occurs during polymerisation.
Amongst the products made by polymerisation casting are propellers for small marine craft, conveyor buckets used in the mining industry, liners for coal washing equipment and main drive gears for use in the textile and papermaking industries. There is persisting interest in nylon-RIM materials as alternatives to polyurethane-RIM. Advantages of the nylon materials are the better shelf life and lower viscosity of the reaction components, ability to mould thick-walled articles, absence of a need for mould lubrication and the ability to avoid using isocyanates with their associated hazards. The main disadvantages of nylon-RIM are the need to have heated storage tanks and elevated temperature reactions, difficulties in catalyst handling and the high water absorption of the product. Possible markets include exterior car body components and appliance and business machine components.
The glass-reinforced nylon plastics are now of substantial importance and take about 30-40% of the UK market. The rigidity, creep resistance, low coefficient of friction and high heat deflection temperature have enabled these materials to replace metals in many applications. Furthermore their good low-frequency electrical insulation properties and non-magnetic characteristics may also be utilised. For these reasons glass-fibre-filled grades are widely used in housings and casings, in domestic appliances, and in car components, including radiator parts. They are also extensively used in the telecommunications field for relay coil formers and tag blocks. Glass-bead-filled nylons have been used in bobbins. Carbon-fibre-reinforced nylon 6 and nylon 6/12 mixtures have been offered commercially and found use in aerospace and tennis racket applications. More recently interest has been shown in the appearance of exceptionally tough nylon plastics.
Initial materials of this super-tough type were blends of nylon 66 with an ionomer resin (see Chapter 11). More recent materials are understood to be blends of nylon 66 with a modified ethylene-propylene-diene terpolymer rubber (EPDM rubber-also see Chapter 11). One such modification involves treatment of the rubber with maleic anhydride, this reacting by a Diels-Alder or other reaction with the double bond in the rubber due to the diene component. A twophase structure is formed in which the rubber exists not simply as particles embedded in the polyamide matrix but in the form of a reticular structure. Glass-fibre-filled grades of these toughened polymers are also available but these do not show the same improvement in toughness over normal glass-fibrefilled nylons.
Since large tonnage production is desirable in order to minimise the cost of a polyamide and since the consumption of nylons as plastics materials remains rather small, it is important that any new materials introduced should also have a large outlet as a fibre. There are a number of polyamides in addition to those already mentioned that could well be very useful plastics materials but which would be uneconomical for all but a few applications if they were dependent on a limited outlet in the sphere of plastics. Both nylon 7 and nylon 9 are such examples but their availability as plastics is likely to occur only if they become established fibre-forming polymers. This in turn will depend on the economics of the telomerisation process and the ability to find outlets for the telomers produced other than those required for making the polyamides.
18.10 POLYAMIDES OF ENHANCED SOLUBILITY
Polyamides such as nylon 6, nylon 66, nylon 610, nylon 11 and nylon 12 exhibit properties which are largely due to their high molecular order and the high degree of interchain attraction which is a result of their ability to undergo hydrogen bonding. It is, however, possible to produce polymers of radically different properties by the following modifications of the molecular structure.
(1) Replacement of some or all of the -CONH- hydrogens by alkyl or alkoxy-alkyl groups to reduce hydrogen bonding which results in softer, lower melting point and even rubber polymers (N-substitution).
(2) Use of acids or amines containing large bulky side groups which prevent close packing of the molecules.
(3) Use of trifunctional acids or amines to give branched structures.
(4) Copolymerisation to give irregular structures.
(5) Reduction in molecular weight.
The techniques of N-alkylation may be effected by the use of N-alkylated or N,”-dialkylated diamines, or by the use of an w-N-alkylaminocarboxylic acid of type R,NHRCOOH. The polymers thus have repeating units of the general form.
Polyphthalamide plastics
As with the aliphatic polyamides such as nylons 6 and 66, the polyphthalamides were developed as plastics materials only after their sucessful use in the field of fibres. Such materials were introduced in 1991 by Amoco under the trade name of Amodel.
As might be expected of a crystalline aromatic polar polymer, the material has a high T, of 310°C and a high Tg of 127"C, the ratio of the two having a value close to the 2/3 commonly found with crystalline polymers (see Section 4.4). Also, as to be expected, the material exhibits high strength and rigidity and good chemical resistance, particularly to hydrocarbons. A typical glass-reinforced grade has a continuous use temperature of 18O"C, similar to that of polysulphone and only exceeded by a small number of polymers (see Table 9.1). Commercial polymers are generally modified by glass- or mineral-fibre reinforcement. Standard grades have a UL94 Flammability Rating of HB but the use of flame retardants allows grades to be produced with a V-0 rating at 0.8 mm thickness. Also of note are such good electrical properties as a high Comparative Tracking Index of 550 V and an ASTM D495 Arc resistance of about 140 s. The manufacturers stress ease of processing as a particular feature of the material. Recommended melt temperatures are in the range 320-340°C and mould temperatures are 135-165°C. Mould shrinkage of glass-filled grades is usually of the order of 0.2-0.4% in the flow direction and up to twice this value in the transverse direction. The materials are notable for their ability to withstand vapour phase and infrared soldering processes.
POLYIMIDES
The polyimides have the characteristic functional group below and are thus closely related to the polyamides. However, the branched nature of the
functional group facilitates the production of polymers with a backbone that consists predominantly of ring structures and hence high softening points. Furthermore, many of the structures exhibit a high level of thermal stability so that the polymers have become of some importance in applications involving service at higher temperatures than had been hitherto achieved with plastics materials. The first commercial materials were introduced by Du Pont in the early 1960s when they marketed a range of products obtained by condensing pyromellitic dianhydride with aromatic amines, particularly di-(4-aminophenyl) ether. These included a coating resin (Pyre ML) film (originally H-film, later named Kapton) and in machinable block form (Vespel). In spite of their high price these materials have found established uses because of their good performance at high temperature. Unfortunately, by their very nature, these polymers cannot be moulded by conventional thermoplastics techniques and this led in the early 1970s to the availability of modified polyimides such as the polyamide imides typified by Torlon (Amoco Chemicals), the polyester imides (e.g. Icdal Ti40 by Dynamit Nobel) and the polybismaleinimides such as Kine1 (Rhone-Poulenc).
By the mid-1970s there were over 20 suppliers in the United States and Western Europe alone although some companies have now withdrawn from the market. In this section discussion will be confined to the ‘true’ polyimides whilst the modified materials will be considered in Section 18.14. The general method of preparation for the original polyimides is shown in
The pyromellitic dianhydride is itself obtained by vapour phase oxidation of durene ( 1,2,4,5tetramethylbenzene),u sing a supported vanadium oxide catalyst. A number of amines have been investigated and it has been found that certain aromatic amines give polymers with a high degree of oxidative and thermal stability. Such amines include m-phenylenediamine, benzidine and di-(4-aminophenyl) ether, the last of these being employed in the manufacture of Kapton (Du Pont). The structure of this material is shown.
In addition to the intramolecular condensation leading to the linear polymer some intermolecular reaction may also occur which leads to cross-linking and hence greatly restricts mouldability. In order to prevent premature gelation the reaction mixture should be anhydrous, free from pyromellitic acid and reacted at temperatures not exceeding 50°C.
Films may be made by casting (I) and heating to produce the polyimide (11). Tough thin film may be obtained by heating for 1-2 hours at 150°C but thicker products tend to become brittle. A substantial improvement can be obtained in some cases if a further baking of solvent-free polymer is carried out at 300°C for a few minutes.
A measure of the heat resistance can be obtained by the weight loss at various temperatures. Tabie 18.12 gives details of the weight loss of three polypyromellitimides after various heating times at 325°C. The first commercial applications of polypyromellitimides were as wire enamels, as insulating varnishes and for coating glass-cloth (Pyre.ML, Du Pont). In film form (Kapton) many of the outstanding properties of the polymer may be more fully utilised. These include excellent electrical properties, solvent resistance, flame resistance, outstanding abrasion resistance and exceptional heat resistance. After 1000 hours exposure to air at 300°C the polymer retained 90% of its tensile strength.
The polymers also have excellent resistance to oxidative degradation, most chemicals other than strong bases and high-energy radiation. Exposure for 1500 hours to a radiation of about 10 rads at 175°C led to embrittlement but the sample retained form stability.
Some typical properties of a fabricated solid grade (Vespel-Du Pont) are given in Table 18.13 together with some data on a graphite-loaded variety and a commercial polyamide-imide (Torlon 2000-Amoco). The limited tractability of the polymer makes processing in conventional plastics form very difficult. Nevertheless the materials have been used in the manufacture of seals, gaskets and piston rings (Vespel-Du Pont) and also as the binder resin for diamond grinding wheels.
Laminates produced by impregnation of glass and carbon fibre with polyimide resins followed by subsequent pressing have found important uses in the aircraft industry, particularly in connection with supersonic airliners. Such laminates can be used continuously at temperatures up to 250°C and intermittently to 400°C.
MODIFIED POLYIMIDES
The successful introduction of the polyimides stimulated attempts to produce somewhat more tractable materials without too serious a loss of heat resistance. This led to the availability of a polyamide-imides, polyester-imides and the polybismaleinimides, and in 1982 the polyether-imides. If trimellitic anhydride is used instead of pyromellitic dianhydride in the reaction illustrated in Figure 18.35 then a polyamide-imide is formed (Figure 18.37). The Torlon materials produced by Amoco Chemicals are of this type.
Both the polyimide and polyamide-imide reactions described above require starting materials of high purity and the use of capped amines (in fact diisocyanates or diurethanes) has been suggested (Figure 18.38). It is understood that one of these reactions has been used by Rhone-Poulenc to produce their Kennel fibres. Closely related is the Upjohn process involving the selfcondensation of the isocyanate of trimellitic acid, although in this case the product is a true polyimide rather than a polyamide-imide (Figure 18.39). Whereas the polyimides are modified polyimides described above are produced by condensation reactions the polybismaleinimides may be produced by rearrangement polymerisation. This avoids the production of volatile low molecular mass by-products.
The key starting materials in this case are the bismaleimides, which are synthesised by the reaction of maleic anhydride with diamines (Figure 18.40).
A variety of bifunctional compounds react with the bismaleimides to form polymers by rearrangement reactions. These include amines, sulphides and aldoximes (Figure 18.41). If the bismaleimide-amine reaction is carried out with a deficiency of amine the polymer will have terminal double bonds which allows a cure site to give a thermosetting polymer via a double bond polymerisation mechanism. This approach was developed by Ciba-Geigy with their product P13N (Figure 18.42).
The polybismaleinimides, typified by the Rhone-Poulenc material Kinel, may be processed like conventional thermosetting plastics. The original polymers have double bonds at the ends of the chains and polymerisation occurs through them during the moulding process to bring about cross-linking, in this case without the formation of any volatile by-products. The properties of the cured polymers are broadly similar to those of the polyimides and polyamide-imides, Moulding temperatures vary from type to type but are usually in the range 200-260°C followed by post-curing for about 8 h at 250°C. Unfilled polybismaleinimides are used for making laminates, impregnating glass and carbon fibre fabrics, for making printed circuit boards and for filament winding. Grades are also available filled with a diversity of materials such as glass fibre, asbestos, carbon fibre, molybdenum sulphide, graphite and PTFE. They find use in aircraft and spacecraft construction, and in rocket and weapons technology. Specific uses include brake equipment, rings, gear wheels, friction bearings and cam discs.
The polyester-imides form yet another class of modified polyimide. These are typified by the structure shown in Figure 18.43. Polyimides and related materials have also been used in a number of specialist applications. Polyimide foams (Skybond by Monsanto) have been used for the sound deadening of jet engines. Polyimide fibres have been produced by Rhone- Poulenc (Kennel) and by Upjohn.
Polyamide-imides The polyamide-imide Torlon was marketed in the early 1970s as a compression moulding material and from the mid- 1970s an injection moulding grade has been available. In solution form in N-methyl-pyrrolidone it has been used as a wire enamel, as a decorative finish for kitchen equipment and implements and as an adhesive and laminating resin in spacecraft. The compression moulding grade, Torlon 2000, can accept high proportions of filler without serious detriment to many properties.
Polymers of this type have exceptional good values of strength, stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a tensile load of 70 MPa the creep modulus drops only from 4200 to 3000 MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500 and 2500 MPa respectively. If the test temperature is raised to 150°C the creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa in 100 h.
Three months immersion in water leads to a 5% w/w absorption of water which at this level leads to a reduction in the heat distortion temperature (ISO) of 100 Celsius degrees.
Torlon-type polymers are unaffected by aliphatic, aromatic, chlorinated and fluorinated hydrocarbons, dilute acids, aldehydes, ketones, ethers and esters. Resistance to alkalis is poor. They have excellent resistance to radiation. If a total of lo3 Mrad is absorbed at a radiation dosage of 1 Mradh the tensile strength decreases by only 5%. For compression moulding the moulding compound is preheated at 280°C before moulding at 330-340°C at moulding pressures of 30 MPa (4350 lbf/in2). The mould is cooled to 260°C before removal. For injection moulding melt temperatures are about 355"C, whilst mould temperatures are about 230°C. In order to achieve high-quality mouldings prolonged annealing cycles are recommended. For example, for a 12 mm thick article the annealing cycle is: 36 h at 150"C, 36 h at 177"C, 36 h at 204"C, 36 h at 232°C and finally 48 h at 260"C, a total time of 192 h. For a 6mm section the total recommended time is 120 h and for a 3 mm section, 48 h.
Uses of the polyamide-imides include pumps, valves, gear wheels, accessories for refrigeration plant and electronic components. Interesting materials may be made by blending the polymer with graphite and F'TFE. This reduces the coefficient of friction from the already low figure of 0.2 (to steel) to as little as 0.02-0.08.
Polyamide-imides may also be produced by reacting a diacid chloride with an excess of diamine to produce a low molecular mass polyamide with amine end groups. This may then be chain extended by reaction with pyromellitic dianhydride to produce imide linkages. Alternatively the dianhydride, diamine and diacid chloride may be reacted all together.
Polyetherimides In 1982 General Electric introduced Ultem, a polyetherimide with the following structure:
The presence of the either linkages is sufficient to allow the material to be melt processed, whilst the polymer retains many of the desirable characteristics of polyimides. As a consequence the material has gained rapid acceptance as a hightemperature engineering thermoplastics material competitive with the polysulphones, poly(pheny1ene sulphides) and polyketones. They exhibit the following key characteristics:
(1) Very high tensile strength without the use of reinforcement.
(2) A glass transition temperature of 215"C, a deflection temperature of 200°C
(3) A high UL Temperature Index of 170°C (for mechanical with impact).
(4) Flame resistance (LO1 of 47 and UL94 V-0 rating at 0.41 mm thickness).
(5) Very low smoke emission, superior even to polyethersulphone.
(6) Excellent hydrolytic stability (a weakness of many polyimides).
and a Vicat softening point of 219°C.
Some typical properties of polyetherimides are given Table 18.15. Although the polymer has a regular structure, it is amorphous, the natural polymer being transparent and orange in colour. The polyetherimides are competitive not only with other high-performance polymers such as the polysulphones and polyketones but also with polyphenylene sulphides, polyarylates, polyamide-imides and the polycarbonates. Because of its high stability, the processing 'window' (range of processing conditions) is wider than for many other thermoplastics. The main points to bear in mind are:
The need to use dry granules.
The need to use high melt temperatures (340-425°C).
The low moulding shrinkage of 0.005-0.007 cm/cm (typical of an amorphous material).
(4) The high melt strength, facilitating thermoforming and blow moulding techniques.
The markets for pol yetherimides arise to an extent from stricter regulations concerning flammability and smoke evolution coupled with such features as high strength, toughness and heat resistance. Application areas include car under-thebonnet uses, microwave equipment, printed circuit boards and aerospace (including carbon-fibre-reinforced laminated materials). The polymer is also of interest in flim, fibre and wire insulation form. General Electric now also offer polyetherimide-polycarbonate blends. Although these materials are not transparent and have a lower specification than the basic polyetherimide, they are less expensive and find use in microwave oven trays and automotive reflectors. Also of interest is the polysiloxane-polyetherimide copolymer marketed as Ultem Siltem STM1500, which is considered further in Chapter 29.
ELASTOMERIC POLYAMIDES
Although some of the polyamides described in Section 18.10 are somewhat rubbery, they have never achieved importance as rubbers. On the other hand, the past decade and a half has seen interest aroused in thermoplastic elastomers of the polyamide type which may be considered as polyamide analogues of the somewhat older and more fully established thermoplastic polyester rubbers. Most of the commercial polymers consist of polyether blocks separated by polyamide blocks. The polyether blocks may be based on polyethylene glycol, polypropylene glycol or, more commonly, polytetramethylene ether glycol. The polyamides are usually based on nylon 11 but may be based on nylons 6 or 66 even a copolymer
In 1978 Hiils (Mumcu et ~ 1 . ’ d~e)sc ribed the properties of a block copolymer prepared by condensation of polytetramethylene ether glycol with laurin lactam and decane- 1,lO-dicarboxylic acid. The materials were introduced as XR3808 and X4006. The polyamide XR3808 is reported to have a specific gravity of 1.02, a yield stress of 24MPa, a modulus of elasticity of 300MPa and an elongation of break of 360%. The Swiss company Emser Werke also introduced similar materials. The currently available grade is Grilamid ELY 60 (formely ELY 1256). Somewhat related are the Monsanto Nyrim materials processed by reaction injection moulding techniques (see Section 18.8).
A wide range of polyether-polyamide block copolymers were first offered by Atochem in 1981 under the trade name Pebax. These are made by first producing a low molecular weight polyamide using an excess of dicarboxylic acid at a temperature above 230°C and under a pressure of up to 25 bar. This is then combined with a polyether by reaction at 230-280°C under vacuum (0.1-1OTorr) in the presence of a suitable catalyst such as Ti(OR)4. Products varying widely in their properties can be produced by variation of (1) The nature of the polyamide block.
(2) The nature of the polyether block.
(3) The lengths of the two blocks.
(4) The relative amounts of the two blocks present.
Variation in the polyamide block nature and length is a prime influence causing variations in T,, specific gravity and chemical resistance. Variation in the polyether block is the prime influence causing variations in Tg , hydrophilic properties and antistatic properties.
Further variation in properties is obtained by incorporating such additives as antistatic agents, ultraviolet stabilisers and antioxidants. As a result of this flexibility in formulation, the range of physical properties possible is somewhat greater than normally achieved with thermoplastic polyesters (see Section 25.9) or thermoplastic polyurethane rubbers (see Section 27.4.4). For example, hardness can range from Shore A60 (a fairly soft rubber) to Shore D63, which is commonly rated as a moderately hard plastics material. Typical properties of five basic materials in the Pebax range and one hydrophilic grade material (Grade 4011) are given in Table 18.16 in order to illustrate the range of properties available.
Due to the polyether blocks, these polymers retain their flexibility down to about -40°C and only Grade 6333 breaks in an Izod test at this temperature (using specimens of thickness 3.2 mm). The materials generally show excellent resistance to crack growth from a notch during flexure; some grades are reported generally more transparent than hard ones as a result of the lower amount of crystalline polyamide block material.
These polymers may be extruded and injection moulded on standard equipment used for thermoplastics. Typical melt temperatures range from about 230°C for the harder grades down to about 200°C for the softer polymers. Mould temperatures are about 25-30°C.
The thermoplastic elastomer polyamides have found use in conveyor and drive belts, ski and soccer shoe soles, computer keyboard pads, silent gears in audio and video recorders and cameras, and thin film for medical applications. A further range of segmented block copolymers have been developed by Dow through reaction of aromatic di-isocyanates with dicarboxylic acids together with a dicarboxy-terminated poly01 using a reaction of type (5) given in Section 18.1 (Nelb et al., 1987). The isocyanate (usually MDI-see Section 27.2) reacts with the dicarboxylic acid (typically adipic or azelaic acid) to give an aromatic polyamide block with a high T, in the range 230-270°C. This is combined to the poly01 (either polyester or polyether-see Chapter 27) via the carboxy terminal groups of the latter.
In a typical process, reaction is carried out at elevated temperatures in a polar solvent. The general polymer reaction scheme is as follows:
Because of the aromatic nature of the polyamide block, the overall polymers can have higher softening points than obtained with other thermoplastic elastomers. For example, some grades will retain a tensile strength of about 15 MPa at 150°C (i.e about half that of the room temperature strength). The polymers also show good heat aging properties, with, for example, tensile strength increasing after 5 days of exposure at 150"C, due to an annealing process. Where polyester polyols are used there is a good strength retention after exposure to a temperature of 175°C. Although these materials are available in a range of levels of hardness (Shore 88A to 70D), this is in a somewhat harder range of rubbers than the Pebax-type materials, and they are similar to the polyester thermoplastic rubbers discussed in Chapter 25.9.
Other companies interested in thermoplastic polyamide rubbers have been Dow (following on work by Upjohn) and Akzo, whose initial development grades have been trade marked Arnetal.
Applications of the elastomeric polyamides include keyboard pads, sports footwear, loudspeaker gaskets and, in the case of filled grades, watch straps.
POLYESTERAMIDES
Tn Chapter 25 it will be shown that polyesters, condensation polymers containing the repeat -COO- group, may be produced by reactions analogous to the methods used to produce polyamides as summarised in the first section of this chapter. It is also quite feasible, by using appropriate starting materials to make polyesteramides. These materials, which are effectively copolymers, do not have the regularity of the common polyamides and to date have not become of great significance but two types will be mentioned here in passing. In the 1940s IC1 introduced a material marketed as Vulcaprene made by condensing ethylene glycol, adipic acid and ethanolamine to a molecular weight of about 5000 and then chain extending this with a diisocyanate. This rubbery material found some use as a leathercloth and is dealt with further in Chapter 25.
Some 50 years later, in the 1990s Bayer produced their BAK polyesteramides by co-reacting either hexamethylene diamine or &-caprolactam with adipic acid and butane glycol. These materials do have sufficient regularity to be crystallisable and are of interest as biodegradable plastics and are discussed.
The early development of the nylons is largely due to the work of W. H. Carothers and his colleagues, who first synthesised nylon 66 in 1935 after extensive and classical researches into condensation polymerisation. Commercial production of this polymer for subsequent conversion into fibres was commenced by the Du Pont Company in December 1939. The first nylon mouldings were produced in 1941 but the polymer did not become well known in this form until about 1950.
In an attempt to circumvent the Du Pont patents, German chemists investigated a wide range of synthetic fibre-forming polymers in the late 1930s. This work resulted in the successful introduction of nylon 6 (and incidentally in the evolution of the polyurethanes) and today nylons 66 and 6 account for nearly all of the polyamides produced for fibre applications. Mention may, however, be made of nylons 7 (Enanth) and 9 (Pelargone) which have been investigated as fibres in the Soviet Union. Very many other aliphatic polyamides have been prepared in the laboratory and a few have become of specialised interest as plastics materials including nylons 11, 12, 46, 69, 612, 66/610 and 66/610/6. For a variety of reasons the aromatic polyamides were slow in their development. A glassy aromatic polymer, poly(trimethylhexamethy1ene terephthalamide) became available in the early 1960s as Trogamid T, and this was followed by a series of other glassy aromatic polyamides in the 1970s and 1980s. During the same period several aromatic polyamides of more regular structure than the glassy polymers became important as fibres because of their exceptional strength in some cases or because of their fire-retarding properties in others. In the latter part of the period polyamides became available which might be classified as thermoplastic rubbers.
There thus exists a very wide range of materials-fibires, crystalline plastics, amorphous plastics, adhesives and rubbers-which are classified as polyamides. They have the common feature that the amide (-CONH-) group occurs repeatedly in the polymer. Such an amide group can increase resistance to swelling and dissolution in hydrocarbons, increase interchain attraction and hence stiffness and heat deformation resistance, reduce electrical insulation resistance, particularly at high frequencies, and increase water absorption. However, as with other condensation polymers which are classified by the group formed during the condensation reaction, the amide groups form only a small proportion of the molecule, and other chemical groups may also have an important influence on the properties.
Of the many possible methods for preparing linear polyamides five are of commercial importance:
An example of the first route is given in the preparation of nylon 66, which is made by reaction of hexamethylenediamine with adipic acid. The first ‘6’ indicates the number of carbon atoms in the diamine and the second the number of carbon atoms in the acid. Thus, as a further example, nylon 6.10 is made by reacting hexamethylenediamine with sebacic acid (HOOC*(CH,), mCOOH). (In this context the numbers 10,ll and 12 are considered as single numbers: the need to use two digits results simply from the limitations of the decimal system.) Where the material is denoted by a single number, viz nylon 6 and nylon 11, preparation from either an w-amino acid or a lactam is indicated. The polymer nylon 66/6.10 (60:40) indicates a copolymer using 60 parts of nylon 6.6 salt with 40 parts of nylon 6.10 salt.
Closely related to the polyamides are the polyimides and derivatives such as polyamide-imides and polyether-imides. These are discussed in Sections 18.13 and 18.14.
Comparison of nylons 6 and 66 in glass-filled compositions The presence of glass-fibre fillers can to some extent mask the differences between nylons 6 and 66. For example, an advantage of unfilled nylon 66 in injection moulding is that the high T, leads to a high solidification temperature and shorter cycle times. However, in glass-filled grades the more rapid cooling and crystallisation can lead to a poorer surface finish than obtained with corresponding nylon 6 compounds. It is also considered that abrasive wear on screws is greater with nylon 6.
Water absorption decreases with increasing glass-fibre content at about the same rate with both nylons 66 and 6, and since nylon 6 has an intrinsically higher water absorption than nylon 66 the glass-filled grades also have higher levels at similar glass-fibre loadings.
Mechanical properties of freshly injected compositions are similar for the two nylons but, after conditioning, differences arise largely due to the plasticising effect of the moisture present. Thus for tensile and flexural yield stress, tensile strength and modulus of elasticity, nylon 66 gives slightly higher figures. Yield elongation and elongation at break are greater with nylon 6. Izod impact strengths are similar, with nylon 6 giving marginally higher values. The above comments refer to comparisons between the two compositions at the same glass-fibre level. If, however, comparison is made between a nylon 66 composition with a glass content of x% and a nylon 6 compound with a glass content of (x + 5)%, then the differences in mechanical properties become very small. At the same time the nylon 6 material will have slightly easier processing characteristics and surface quality.
Whilst nylon 66 has the higher T,, the long-term heat resistance of typical copper-stabilised nylon 6 is somewhat superior in such properties as impact strength and bending strength compared to nylon 66. However, it is frequently the case that nylon 66 has better resistance to chemicals at elevated temperatures.
PROCESSING OF THE NYLONS
In the processing of nylons consideration should be given to the following points: (1) The tendency of the material to absorb water.
(2) The high melting point of the homopolymers.
(3) The low melt viscosity of the homopolymers.
(4) The tendency of the material to oxidise at high temperatures where oxygen (5) The crystallinity of the solid polymer and hence the extensive shrinkage The above features are particularly marked with nylons 46, 6,66 and 610 and less marked with nylons 11 and 12. Providing they are dry the copolymers may be processed in much the same way as conventional thermoplastics.
In the injection moulding of nylon 66, for example, it is necessary that the granules be dry. The polymer is normally supplied in sealed containers but should be used within an hour of opening. If reworked polymer is being used, or granules have become otherwise damp, the polymer should be dried in an oven at about 70-90°C. Too high a temperature will oxidise the surface of the granules and result in inferior mouldings.
Injection moulding cylinders should be free from dead spots and a temperature gradient along the cylinder is desirable.
Because of the low melt viscosity of the polymer at processing temperatures it will ‘drool’ through normal injection nozzles even when the plunger is retracted. Several types of nozzle have been specially designed for use with nylon and all function by sealing the end of the nozzle, either by allowing a pip of polymer to harden, by the use of spring-loaded valve, by the use of sliding side-closure nozzles or by the use of hydraulic nozzle valve activated at the appropriate stages of the moulding cycle. Variations of this last approach have become popular since they are both positive in action and simple to operate. In designs in which solidified polymer is formed at the nozzle it is necessary to make provision for a cold-slug well in the mould, a feature frequently not possible with single-cavity tools. Where spring-loaded closing devices are used the spring should be kept as cool as possible if rapid thermal fatigue is to be avoided.
Because of the crystallisation that occurs on cooling from the melt the polymers show a higher moulding shrinkage than that generally observed with amorphous polymers. With average moulding conditions this is about 0.01 8 cm/ cm with nylon 66 but by increasing the injection pressure and the injection time the shrinkage may be halved. This is because a high initial mould cavity pressure is developed and a large part of the crystallisation process will be complete before the cavity pressure has dropped to zero. The shrinkage will also be affected by the melt temperature, the mould temperature, the injection speed and the design of the mould as well as by the type of nylon used. The nylons, nylon 66 in particular, may also exhibit a certain amount of aftershrinkage. Further dimensional changes may occur as a result of moulding stresses being relieved by the plasticising effect of absorbed water. It is consequently often useful to anneal mouldings in a non-oxidising oil for about 20 minutes at a temperature 20°C higher than the maximum service temperature. Where this is not known a temperature of 170°C is suitable for nylon 66, with somewhat lower temperatures for the other nylons.
When dimensional accuracy is required in a specific application the effect of water absorption should also be considered. Manufacturers commonly supply data on their products showing how the dimensions change with the ambient humidity.
The particular features of the nylons should also be taken into account in extrusion. Dry granules must be used unless a devolatilising extruder is employed. Because of the sharp melting point it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. The polymerisation casting process mentioned in Section 18.3.2 has been adapted to reaction injection moulding (RIM), a process originally developed for polyurethanes. In this process the reacting ingredients are mixed together by impingement of jets of the materials in a small mixing chamber adjacent to the mould cavity into which the reacting material is then injected. Because of the low injection pressures much lower locking forces are possible than in conventional injection moulding, making the process attractive for large area mouldings. The first polyamides specifically developed for RIM were introduced by Monsanto in 1981 as Nyrim. They are block copolymers of a polyether (such as a poly(ethy1ene glycol), poly(propy1ene glycol) or polybutadiene containing hydroxyl groups) with caprolactam. The reaction components comprise the polyether, caprolactam, adipyl-bis-caprolactam as chain propagator and a caprolactam-magnesium bromide complex as catalyst. The latter has to be protected against moisture, carbon dioxide and oxygen, and thus requires special care in handling. Other polyamide-RIM systems have been developed by Upjohn and Allied Fibers and Plastics.
Unlike polyurethane-RIM processes, nylon-RIM reactions are endothermic and require temperatures of 130-140°C. In contrast to the polyurethane-RIM systems, this enables thick wall parts to be made. Cycle times of 2-3 minutes are comparable to those for polyurethane-RIM. In the development stage, current work is concerned with reducing moulding times and optimising moulding conditions.
APPLICATIONS
Consumption of polyamide plastics in the late 1990s has been estimated at about 1 300 000 t.p.a. Western Europe and the United States each have about 42% of this market and Japan about 16%. This is probably about 20% of the total production of polyamide materials and virtually all of the rest of polyamide production goes into fibres where the market is shared very roughly equally between nylons 6 and 66. The large-scale production of these materials enables them to be available at a substantially lower price than the other nylons, which do not have the benefit of the economies of scale. Hence the other nylons are usually restricted to applications where nylons 6 and 66 are unsuitable. It has been estimated that in the 1990s in Western Europe, plastics usage was split as follows: nylon 6 48%: nylon 66 40% nylons 11 and 12 10%: the rest 2%. For historical reasons, usage of the various types varies from country to country. For example, in the United States and the United Kingdom, nylon 66 was the first to be developed and remains well entrenched, whilst for similar reasons nylon 6 is more dominant in Germany. The substantial market penetration of nylons 11 and 12 in France also reflects long-standing French commercial activity with these types. It remains to be seen what effect the recent introduction of nylon 46 will have on the balance of usage.
It has been reported that out of a total 1997 Western European consumption of 532000 tonnes, 387000 tonnes, i.e. almost 75% of the total, was injection moulded. Film use accounted for 70 000 tonnes, monofilaments 19 000 tonnes, extruded sheet, rod and tube 24 0o0 tonnes, wire and cable 10 000 tonnes and other processes such as powder coatings, hot melts, blow moulding and monomer casting about 22 000 tonnes. Insofar as data can be compared, patterns in the USA are similar. Over two-thirds of injection moulding usage is in transport and electrical/electronic applications. The polyamides still have the biggest tonnage of the so-called engineering plastics although this could well be overtaken by the polycarbonates in the near future (see Chapter 20). The two classes of materials do, however, have quite dissimilar properties and are seldom directly competitive.
The nylons have found steadily increasing application as plastics materials for speciality purposes where their toughness, rigidity, abrasion resistance, good hydrocarbon resistance and reasonable heat resistance are important. Because of their high cost they have not become general purpose materials such as polyethylene and polystyrene, which are about a third of the price of the nylons. The largest applications of the homopolymers (nylon 6, 66, 610, 11 and 12) have been in mechanical engineering. Well-known applications include gears, cams, bearings, bushes and valve seats. In addition to the advantageous properties cited above, nylon moving parts may be frequently operated without lubrication, are silent running and may often be moulded in one piece when previously a metal part required assembling of several parts, or alternatively, extensive machining with consequent waste of material. It may be noted that in this area the newly introduced nylon 46 has become of interest in auto automatic gears, gearboxes, engine differentials and the clutch area because of its exceptional ability (for a polyamide) to withstand severe mechanical and thermal loading.
In recent years the nylons have met increased competition from acetal resins (Chapter 19), the latter being superior in fatigue endurance, creep resistance and water resistance. Under average conditions of humidity the nylons are superior in impact toughness and abrasion resistance. When a nylon is considered appropriate it is necessary to consider the relative importance of mechanical properties, water resistance and ease of processing. For the best mechanical properties nylon 66 would be considered but this material is probably the most difficult to process and has a high water absorption value. Nylon 6 is easier to process but has slightly inferior mechanical properties and an even higher water absorption. Nylons 11 and 12 have the lowest water absorption, and are easy to process, but there is some loss in mechanical properties. Sterilisable mouldings have found application in medicine and pharmacy. Because of their durability, nylon hair combs have found wide acceptance in spite of their higher cost.
Nylon film has been used increasingly for packaging applications for foodstuffs and pharmaceutical products. The value of nylon in this application is due to low odour transmission and to some extent in the ability to boil-in-the-bag. Film of high brilliance and clarity, particularly from nylon 11, is available for point-of-sale displays.
Although the nylons are not generally considered as outstanding electrical insulators, their toughness and, to some extent, their temperature resistance, have led to applications in coil formers and terminal blocks. Indeed, the new nylon 46 materials would appear to be of particular interest here. Acetal resins, polysulphones, modified PPO and polycarbonates, however, present a challenge to applications in this sphere. Nylon monofilaments have found application in brush tufting, wigs, surgical sutures, sports equipment, braiding and outdoor upholstery. Nylons 610 and 11 have found extensive application in these fields because of their flexibility but nylon 66 is also used for brush tufting less than 0.0035in. in diameter. Nylon 66/610 copolymer is used in the manufacture of a monofilament for angling purposes.
Extruded applications of nylon, other than film and monofilament, are less commonly encountered because of the low melt viscosity of the polymers. Uses include cable sheathing which requires resistance to abrasion and/or chemical attack, flexible tubing for conveying petrol and other liquids, piping for chemical plant, rods for subsequent machining, as the tension member of composite belts for high-duty mechanical drive and for bottles requiring resistance to hydrocarbons. Nylons 11 and 12 are frequently preferred because of their ease of processing but the high molecular weight 6,66 and 610 polymers find occasional use. The 661610 and 66161016 polymers have also been used in the past where tough leather-like extrudates have been required. Their cost, and perhaps also their obscurity, have resulted in their current use being minimal. Nylon 11 is also used in powder form in spraying and fluidised bed dipping to produce chemical-resistant coatings. Although more expensive than the polyolefin and PVC powders, it is of interest because of its hardness, abrasion resistance and petrol resistance.
As previously mentioned, mouldings have been produced by the polymerisation casting of caprolactam. The ability to produce large objects in this way enables one to envisage new horizons for the use of plastics engineering and other applications. Such polymerisation-cast polymers also possess certain advantageous properties. The polymers tend to have a somewhat higher molecular weight and also a 45-50% crystallinity, again higher than for melt processed materials. This leads to a higher tensile strength, hardness, modulus and resistance to creep. The comparatively stress-free mouldings also have a reasonably consistent morphological structure. A disadvantage is the shrinkage of 4-4.5% which occurs during polymerisation.
Amongst the products made by polymerisation casting are propellers for small marine craft, conveyor buckets used in the mining industry, liners for coal washing equipment and main drive gears for use in the textile and papermaking industries. There is persisting interest in nylon-RIM materials as alternatives to polyurethane-RIM. Advantages of the nylon materials are the better shelf life and lower viscosity of the reaction components, ability to mould thick-walled articles, absence of a need for mould lubrication and the ability to avoid using isocyanates with their associated hazards. The main disadvantages of nylon-RIM are the need to have heated storage tanks and elevated temperature reactions, difficulties in catalyst handling and the high water absorption of the product. Possible markets include exterior car body components and appliance and business machine components.
The glass-reinforced nylon plastics are now of substantial importance and take about 30-40% of the UK market. The rigidity, creep resistance, low coefficient of friction and high heat deflection temperature have enabled these materials to replace metals in many applications. Furthermore their good low-frequency electrical insulation properties and non-magnetic characteristics may also be utilised. For these reasons glass-fibre-filled grades are widely used in housings and casings, in domestic appliances, and in car components, including radiator parts. They are also extensively used in the telecommunications field for relay coil formers and tag blocks. Glass-bead-filled nylons have been used in bobbins. Carbon-fibre-reinforced nylon 6 and nylon 6/12 mixtures have been offered commercially and found use in aerospace and tennis racket applications. More recently interest has been shown in the appearance of exceptionally tough nylon plastics.
Initial materials of this super-tough type were blends of nylon 66 with an ionomer resin (see Chapter 11). More recent materials are understood to be blends of nylon 66 with a modified ethylene-propylene-diene terpolymer rubber (EPDM rubber-also see Chapter 11). One such modification involves treatment of the rubber with maleic anhydride, this reacting by a Diels-Alder or other reaction with the double bond in the rubber due to the diene component. A twophase structure is formed in which the rubber exists not simply as particles embedded in the polyamide matrix but in the form of a reticular structure. Glass-fibre-filled grades of these toughened polymers are also available but these do not show the same improvement in toughness over normal glass-fibrefilled nylons.
Since large tonnage production is desirable in order to minimise the cost of a polyamide and since the consumption of nylons as plastics materials remains rather small, it is important that any new materials introduced should also have a large outlet as a fibre. There are a number of polyamides in addition to those already mentioned that could well be very useful plastics materials but which would be uneconomical for all but a few applications if they were dependent on a limited outlet in the sphere of plastics. Both nylon 7 and nylon 9 are such examples but their availability as plastics is likely to occur only if they become established fibre-forming polymers. This in turn will depend on the economics of the telomerisation process and the ability to find outlets for the telomers produced other than those required for making the polyamides.
18.10 POLYAMIDES OF ENHANCED SOLUBILITY
Polyamides such as nylon 6, nylon 66, nylon 610, nylon 11 and nylon 12 exhibit properties which are largely due to their high molecular order and the high degree of interchain attraction which is a result of their ability to undergo hydrogen bonding. It is, however, possible to produce polymers of radically different properties by the following modifications of the molecular structure.
(1) Replacement of some or all of the -CONH- hydrogens by alkyl or alkoxy-alkyl groups to reduce hydrogen bonding which results in softer, lower melting point and even rubber polymers (N-substitution).
(2) Use of acids or amines containing large bulky side groups which prevent close packing of the molecules.
(3) Use of trifunctional acids or amines to give branched structures.
(4) Copolymerisation to give irregular structures.
(5) Reduction in molecular weight.
The techniques of N-alkylation may be effected by the use of N-alkylated or N,”-dialkylated diamines, or by the use of an w-N-alkylaminocarboxylic acid of type R,NHRCOOH. The polymers thus have repeating units of the general form.
Polyphthalamide plastics
As with the aliphatic polyamides such as nylons 6 and 66, the polyphthalamides were developed as plastics materials only after their sucessful use in the field of fibres. Such materials were introduced in 1991 by Amoco under the trade name of Amodel.
As might be expected of a crystalline aromatic polar polymer, the material has a high T, of 310°C and a high Tg of 127"C, the ratio of the two having a value close to the 2/3 commonly found with crystalline polymers (see Section 4.4). Also, as to be expected, the material exhibits high strength and rigidity and good chemical resistance, particularly to hydrocarbons. A typical glass-reinforced grade has a continuous use temperature of 18O"C, similar to that of polysulphone and only exceeded by a small number of polymers (see Table 9.1). Commercial polymers are generally modified by glass- or mineral-fibre reinforcement. Standard grades have a UL94 Flammability Rating of HB but the use of flame retardants allows grades to be produced with a V-0 rating at 0.8 mm thickness. Also of note are such good electrical properties as a high Comparative Tracking Index of 550 V and an ASTM D495 Arc resistance of about 140 s. The manufacturers stress ease of processing as a particular feature of the material. Recommended melt temperatures are in the range 320-340°C and mould temperatures are 135-165°C. Mould shrinkage of glass-filled grades is usually of the order of 0.2-0.4% in the flow direction and up to twice this value in the transverse direction. The materials are notable for their ability to withstand vapour phase and infrared soldering processes.
POLYIMIDES
The polyimides have the characteristic functional group below and are thus closely related to the polyamides. However, the branched nature of the
functional group facilitates the production of polymers with a backbone that consists predominantly of ring structures and hence high softening points. Furthermore, many of the structures exhibit a high level of thermal stability so that the polymers have become of some importance in applications involving service at higher temperatures than had been hitherto achieved with plastics materials. The first commercial materials were introduced by Du Pont in the early 1960s when they marketed a range of products obtained by condensing pyromellitic dianhydride with aromatic amines, particularly di-(4-aminophenyl) ether. These included a coating resin (Pyre ML) film (originally H-film, later named Kapton) and in machinable block form (Vespel). In spite of their high price these materials have found established uses because of their good performance at high temperature. Unfortunately, by their very nature, these polymers cannot be moulded by conventional thermoplastics techniques and this led in the early 1970s to the availability of modified polyimides such as the polyamide imides typified by Torlon (Amoco Chemicals), the polyester imides (e.g. Icdal Ti40 by Dynamit Nobel) and the polybismaleinimides such as Kine1 (Rhone-Poulenc).
By the mid-1970s there were over 20 suppliers in the United States and Western Europe alone although some companies have now withdrawn from the market. In this section discussion will be confined to the ‘true’ polyimides whilst the modified materials will be considered in Section 18.14. The general method of preparation for the original polyimides is shown in
The pyromellitic dianhydride is itself obtained by vapour phase oxidation of durene ( 1,2,4,5tetramethylbenzene),u sing a supported vanadium oxide catalyst. A number of amines have been investigated and it has been found that certain aromatic amines give polymers with a high degree of oxidative and thermal stability. Such amines include m-phenylenediamine, benzidine and di-(4-aminophenyl) ether, the last of these being employed in the manufacture of Kapton (Du Pont). The structure of this material is shown.
In addition to the intramolecular condensation leading to the linear polymer some intermolecular reaction may also occur which leads to cross-linking and hence greatly restricts mouldability. In order to prevent premature gelation the reaction mixture should be anhydrous, free from pyromellitic acid and reacted at temperatures not exceeding 50°C.
Films may be made by casting (I) and heating to produce the polyimide (11). Tough thin film may be obtained by heating for 1-2 hours at 150°C but thicker products tend to become brittle. A substantial improvement can be obtained in some cases if a further baking of solvent-free polymer is carried out at 300°C for a few minutes.
A measure of the heat resistance can be obtained by the weight loss at various temperatures. Tabie 18.12 gives details of the weight loss of three polypyromellitimides after various heating times at 325°C. The first commercial applications of polypyromellitimides were as wire enamels, as insulating varnishes and for coating glass-cloth (Pyre.ML, Du Pont). In film form (Kapton) many of the outstanding properties of the polymer may be more fully utilised. These include excellent electrical properties, solvent resistance, flame resistance, outstanding abrasion resistance and exceptional heat resistance. After 1000 hours exposure to air at 300°C the polymer retained 90% of its tensile strength.
The polymers also have excellent resistance to oxidative degradation, most chemicals other than strong bases and high-energy radiation. Exposure for 1500 hours to a radiation of about 10 rads at 175°C led to embrittlement but the sample retained form stability.
Some typical properties of a fabricated solid grade (Vespel-Du Pont) are given in Table 18.13 together with some data on a graphite-loaded variety and a commercial polyamide-imide (Torlon 2000-Amoco). The limited tractability of the polymer makes processing in conventional plastics form very difficult. Nevertheless the materials have been used in the manufacture of seals, gaskets and piston rings (Vespel-Du Pont) and also as the binder resin for diamond grinding wheels.
Laminates produced by impregnation of glass and carbon fibre with polyimide resins followed by subsequent pressing have found important uses in the aircraft industry, particularly in connection with supersonic airliners. Such laminates can be used continuously at temperatures up to 250°C and intermittently to 400°C.
MODIFIED POLYIMIDES
The successful introduction of the polyimides stimulated attempts to produce somewhat more tractable materials without too serious a loss of heat resistance. This led to the availability of a polyamide-imides, polyester-imides and the polybismaleinimides, and in 1982 the polyether-imides. If trimellitic anhydride is used instead of pyromellitic dianhydride in the reaction illustrated in Figure 18.35 then a polyamide-imide is formed (Figure 18.37). The Torlon materials produced by Amoco Chemicals are of this type.
Both the polyimide and polyamide-imide reactions described above require starting materials of high purity and the use of capped amines (in fact diisocyanates or diurethanes) has been suggested (Figure 18.38). It is understood that one of these reactions has been used by Rhone-Poulenc to produce their Kennel fibres. Closely related is the Upjohn process involving the selfcondensation of the isocyanate of trimellitic acid, although in this case the product is a true polyimide rather than a polyamide-imide (Figure 18.39). Whereas the polyimides are modified polyimides described above are produced by condensation reactions the polybismaleinimides may be produced by rearrangement polymerisation. This avoids the production of volatile low molecular mass by-products.
The key starting materials in this case are the bismaleimides, which are synthesised by the reaction of maleic anhydride with diamines (Figure 18.40).
A variety of bifunctional compounds react with the bismaleimides to form polymers by rearrangement reactions. These include amines, sulphides and aldoximes (Figure 18.41). If the bismaleimide-amine reaction is carried out with a deficiency of amine the polymer will have terminal double bonds which allows a cure site to give a thermosetting polymer via a double bond polymerisation mechanism. This approach was developed by Ciba-Geigy with their product P13N (Figure 18.42).
The polybismaleinimides, typified by the Rhone-Poulenc material Kinel, may be processed like conventional thermosetting plastics. The original polymers have double bonds at the ends of the chains and polymerisation occurs through them during the moulding process to bring about cross-linking, in this case without the formation of any volatile by-products. The properties of the cured polymers are broadly similar to those of the polyimides and polyamide-imides, Moulding temperatures vary from type to type but are usually in the range 200-260°C followed by post-curing for about 8 h at 250°C. Unfilled polybismaleinimides are used for making laminates, impregnating glass and carbon fibre fabrics, for making printed circuit boards and for filament winding. Grades are also available filled with a diversity of materials such as glass fibre, asbestos, carbon fibre, molybdenum sulphide, graphite and PTFE. They find use in aircraft and spacecraft construction, and in rocket and weapons technology. Specific uses include brake equipment, rings, gear wheels, friction bearings and cam discs.
The polyester-imides form yet another class of modified polyimide. These are typified by the structure shown in Figure 18.43. Polyimides and related materials have also been used in a number of specialist applications. Polyimide foams (Skybond by Monsanto) have been used for the sound deadening of jet engines. Polyimide fibres have been produced by Rhone- Poulenc (Kennel) and by Upjohn.
Polyamide-imides The polyamide-imide Torlon was marketed in the early 1970s as a compression moulding material and from the mid- 1970s an injection moulding grade has been available. In solution form in N-methyl-pyrrolidone it has been used as a wire enamel, as a decorative finish for kitchen equipment and implements and as an adhesive and laminating resin in spacecraft. The compression moulding grade, Torlon 2000, can accept high proportions of filler without serious detriment to many properties.
Polymers of this type have exceptional good values of strength, stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a tensile load of 70 MPa the creep modulus drops only from 4200 to 3000 MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500 and 2500 MPa respectively. If the test temperature is raised to 150°C the creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa in 100 h.
Three months immersion in water leads to a 5% w/w absorption of water which at this level leads to a reduction in the heat distortion temperature (ISO) of 100 Celsius degrees.
Torlon-type polymers are unaffected by aliphatic, aromatic, chlorinated and fluorinated hydrocarbons, dilute acids, aldehydes, ketones, ethers and esters. Resistance to alkalis is poor. They have excellent resistance to radiation. If a total of lo3 Mrad is absorbed at a radiation dosage of 1 Mradh the tensile strength decreases by only 5%. For compression moulding the moulding compound is preheated at 280°C before moulding at 330-340°C at moulding pressures of 30 MPa (4350 lbf/in2). The mould is cooled to 260°C before removal. For injection moulding melt temperatures are about 355"C, whilst mould temperatures are about 230°C. In order to achieve high-quality mouldings prolonged annealing cycles are recommended. For example, for a 12 mm thick article the annealing cycle is: 36 h at 150"C, 36 h at 177"C, 36 h at 204"C, 36 h at 232°C and finally 48 h at 260"C, a total time of 192 h. For a 6mm section the total recommended time is 120 h and for a 3 mm section, 48 h.
Uses of the polyamide-imides include pumps, valves, gear wheels, accessories for refrigeration plant and electronic components. Interesting materials may be made by blending the polymer with graphite and F'TFE. This reduces the coefficient of friction from the already low figure of 0.2 (to steel) to as little as 0.02-0.08.
Polyamide-imides may also be produced by reacting a diacid chloride with an excess of diamine to produce a low molecular mass polyamide with amine end groups. This may then be chain extended by reaction with pyromellitic dianhydride to produce imide linkages. Alternatively the dianhydride, diamine and diacid chloride may be reacted all together.
Polyetherimides In 1982 General Electric introduced Ultem, a polyetherimide with the following structure:
The presence of the either linkages is sufficient to allow the material to be melt processed, whilst the polymer retains many of the desirable characteristics of polyimides. As a consequence the material has gained rapid acceptance as a hightemperature engineering thermoplastics material competitive with the polysulphones, poly(pheny1ene sulphides) and polyketones. They exhibit the following key characteristics:
(1) Very high tensile strength without the use of reinforcement.
(2) A glass transition temperature of 215"C, a deflection temperature of 200°C
(3) A high UL Temperature Index of 170°C (for mechanical with impact).
(4) Flame resistance (LO1 of 47 and UL94 V-0 rating at 0.41 mm thickness).
(5) Very low smoke emission, superior even to polyethersulphone.
(6) Excellent hydrolytic stability (a weakness of many polyimides).
and a Vicat softening point of 219°C.
Some typical properties of polyetherimides are given Table 18.15. Although the polymer has a regular structure, it is amorphous, the natural polymer being transparent and orange in colour. The polyetherimides are competitive not only with other high-performance polymers such as the polysulphones and polyketones but also with polyphenylene sulphides, polyarylates, polyamide-imides and the polycarbonates. Because of its high stability, the processing 'window' (range of processing conditions) is wider than for many other thermoplastics. The main points to bear in mind are:
The need to use dry granules.
The need to use high melt temperatures (340-425°C).
The low moulding shrinkage of 0.005-0.007 cm/cm (typical of an amorphous material).
(4) The high melt strength, facilitating thermoforming and blow moulding techniques.
The markets for pol yetherimides arise to an extent from stricter regulations concerning flammability and smoke evolution coupled with such features as high strength, toughness and heat resistance. Application areas include car under-thebonnet uses, microwave equipment, printed circuit boards and aerospace (including carbon-fibre-reinforced laminated materials). The polymer is also of interest in flim, fibre and wire insulation form. General Electric now also offer polyetherimide-polycarbonate blends. Although these materials are not transparent and have a lower specification than the basic polyetherimide, they are less expensive and find use in microwave oven trays and automotive reflectors. Also of interest is the polysiloxane-polyetherimide copolymer marketed as Ultem Siltem STM1500, which is considered further in Chapter 29.
ELASTOMERIC POLYAMIDES
Although some of the polyamides described in Section 18.10 are somewhat rubbery, they have never achieved importance as rubbers. On the other hand, the past decade and a half has seen interest aroused in thermoplastic elastomers of the polyamide type which may be considered as polyamide analogues of the somewhat older and more fully established thermoplastic polyester rubbers. Most of the commercial polymers consist of polyether blocks separated by polyamide blocks. The polyether blocks may be based on polyethylene glycol, polypropylene glycol or, more commonly, polytetramethylene ether glycol. The polyamides are usually based on nylon 11 but may be based on nylons 6 or 66 even a copolymer
In 1978 Hiils (Mumcu et ~ 1 . ’ d~e)sc ribed the properties of a block copolymer prepared by condensation of polytetramethylene ether glycol with laurin lactam and decane- 1,lO-dicarboxylic acid. The materials were introduced as XR3808 and X4006. The polyamide XR3808 is reported to have a specific gravity of 1.02, a yield stress of 24MPa, a modulus of elasticity of 300MPa and an elongation of break of 360%. The Swiss company Emser Werke also introduced similar materials. The currently available grade is Grilamid ELY 60 (formely ELY 1256). Somewhat related are the Monsanto Nyrim materials processed by reaction injection moulding techniques (see Section 18.8).
A wide range of polyether-polyamide block copolymers were first offered by Atochem in 1981 under the trade name Pebax. These are made by first producing a low molecular weight polyamide using an excess of dicarboxylic acid at a temperature above 230°C and under a pressure of up to 25 bar. This is then combined with a polyether by reaction at 230-280°C under vacuum (0.1-1OTorr) in the presence of a suitable catalyst such as Ti(OR)4. Products varying widely in their properties can be produced by variation of (1) The nature of the polyamide block.
(2) The nature of the polyether block.
(3) The lengths of the two blocks.
(4) The relative amounts of the two blocks present.
Variation in the polyamide block nature and length is a prime influence causing variations in T,, specific gravity and chemical resistance. Variation in the polyether block is the prime influence causing variations in Tg , hydrophilic properties and antistatic properties.
Further variation in properties is obtained by incorporating such additives as antistatic agents, ultraviolet stabilisers and antioxidants. As a result of this flexibility in formulation, the range of physical properties possible is somewhat greater than normally achieved with thermoplastic polyesters (see Section 25.9) or thermoplastic polyurethane rubbers (see Section 27.4.4). For example, hardness can range from Shore A60 (a fairly soft rubber) to Shore D63, which is commonly rated as a moderately hard plastics material. Typical properties of five basic materials in the Pebax range and one hydrophilic grade material (Grade 4011) are given in Table 18.16 in order to illustrate the range of properties available.
Due to the polyether blocks, these polymers retain their flexibility down to about -40°C and only Grade 6333 breaks in an Izod test at this temperature (using specimens of thickness 3.2 mm). The materials generally show excellent resistance to crack growth from a notch during flexure; some grades are reported generally more transparent than hard ones as a result of the lower amount of crystalline polyamide block material.
These polymers may be extruded and injection moulded on standard equipment used for thermoplastics. Typical melt temperatures range from about 230°C for the harder grades down to about 200°C for the softer polymers. Mould temperatures are about 25-30°C.
The thermoplastic elastomer polyamides have found use in conveyor and drive belts, ski and soccer shoe soles, computer keyboard pads, silent gears in audio and video recorders and cameras, and thin film for medical applications. A further range of segmented block copolymers have been developed by Dow through reaction of aromatic di-isocyanates with dicarboxylic acids together with a dicarboxy-terminated poly01 using a reaction of type (5) given in Section 18.1 (Nelb et al., 1987). The isocyanate (usually MDI-see Section 27.2) reacts with the dicarboxylic acid (typically adipic or azelaic acid) to give an aromatic polyamide block with a high T, in the range 230-270°C. This is combined to the poly01 (either polyester or polyether-see Chapter 27) via the carboxy terminal groups of the latter.
In a typical process, reaction is carried out at elevated temperatures in a polar solvent. The general polymer reaction scheme is as follows:
Because of the aromatic nature of the polyamide block, the overall polymers can have higher softening points than obtained with other thermoplastic elastomers. For example, some grades will retain a tensile strength of about 15 MPa at 150°C (i.e about half that of the room temperature strength). The polymers also show good heat aging properties, with, for example, tensile strength increasing after 5 days of exposure at 150"C, due to an annealing process. Where polyester polyols are used there is a good strength retention after exposure to a temperature of 175°C. Although these materials are available in a range of levels of hardness (Shore 88A to 70D), this is in a somewhat harder range of rubbers than the Pebax-type materials, and they are similar to the polyester thermoplastic rubbers discussed in Chapter 25.9.
Other companies interested in thermoplastic polyamide rubbers have been Dow (following on work by Upjohn) and Akzo, whose initial development grades have been trade marked Arnetal.
Applications of the elastomeric polyamides include keyboard pads, sports footwear, loudspeaker gaskets and, in the case of filled grades, watch straps.
POLYESTERAMIDES
Tn Chapter 25 it will be shown that polyesters, condensation polymers containing the repeat -COO- group, may be produced by reactions analogous to the methods used to produce polyamides as summarised in the first section of this chapter. It is also quite feasible, by using appropriate starting materials to make polyesteramides. These materials, which are effectively copolymers, do not have the regularity of the common polyamides and to date have not become of great significance but two types will be mentioned here in passing. In the 1940s IC1 introduced a material marketed as Vulcaprene made by condensing ethylene glycol, adipic acid and ethanolamine to a molecular weight of about 5000 and then chain extending this with a diisocyanate. This rubbery material found some use as a leathercloth and is dealt with further in Chapter 25.
Some 50 years later, in the 1990s Bayer produced their BAK polyesteramides by co-reacting either hexamethylene diamine or &-caprolactam with adipic acid and butane glycol. These materials do have sufficient regularity to be crystallisable and are of interest as biodegradable plastics and are discussed.
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