Heat and Chemical Resistance

Fluoroelastomers. The fluoroelastomers were introduced to the mbber industry in the late 1950s by the DuPont Company. They were made by modification of Teflon polymers and designed to have exceUent heat and chemical resistance, but remain elastomeric in nature. They were very expensive and have found use in limited appHcations. However, with the increasing demand in the automotive and industrial market for improved reHabUity and longer Hfe, the elastomeric fluoroelastomers have made significant inroads into these appHcations (see Elastomers, synthetic-fluorocarbon ELASTOTffiRS). 

This copolymer has proved particularly suitable for wire and cable insulation, with many grades being rated at 155°C for 20 000 h continuous exposure. It is extensively used in electrical systems for aircraft, underground railways, computers, telecommunications installations and heating circuits. Because of its toughness combined with its heat and chemical resistance it also finds use for lining pumps and valves and other equipment for the chemical industry and for laboratory ware. 

In 1989 Du Pont introduced Teflon AF, said to be a copolymer of tetrafluoroethylene and trifluoromethyldifluorodioxol. This amorphous fluoro-polymer has a similar heat and chemical resistance to PTFE but possesses several notable properties, including ... 

Asbestos may be used for improved heat and chemical resistance and silica, mica and china clay for low water absorption grades. Iron-free mica powder is particularly useful where the best possible electrical insulation characteristics are required but because of the poor adhesion of resin to the mica it is usually used in conjunction with a fibrous material such as asbestos. Organic fillers are commonly used in a weight ratio of 1 1 with the resin and mineral fillers in the ratio 1.5 1. 

Laminates have been prepared for the manufacture of chemical plant. They have better heat and chemical resistance than the polyester- epoxide- phenolic- or aminoplastic-based laminates but because of the low viscosity of the resins were not easy to handle. Because they were also somewhat brittle, furan-based laminates have been limited in their applications. 

For very difficult to adhere bag substrates, such as poly-coated bags, or where extreme chemical resistance is needed, polyamide hot melts are used. These adhesives are very lightly formulated if at all. They provide soft adhesives with outstanding heat and chemical resistance, but at roughly double the price. 

Fluoroplastic FPs have superior heat and chemical resistance, excellent electrical properties, but only moderate strength. Variations include PTFE, FEP, PFA, CTFE, ECTFE, ETFE, and PVDF. Used for bearings, valves, pumps handling concentrated corrosive chemicals, skillet linings, and as a film over textile webs for inflatables such as pneumatic sheds. Excellent human-tissue compatibility allows its use for medical implants. 

Polystyrene One of the high volume plastics, is relatively low in cost, easy to process, has sparkling clarity, and low water absorption. But basic form (crystal PS) is brittle, with low heat and chemical resistance, poor weather resistance. High impact polystyrene is made with butadiene modifiers provides significant improvements in impact strength and elongation over crystal polystyrene, accompanied by a loss of transparency and little other property improvement. PS is used in many different formulations. 

Fluorocarbons Very high heat and chemical resistance, nonburning, lowest coefficient of friction, high dimensional stability Injection molding Encapsulation Continuous pultrusion... 

The choice of acrylic ester, and hence its polarity, determines the low temperature flexibility, and the heat and chemical resistance of the polymer both alkyl and alkoxy acrylic esters are used as monomers. Within the alkyl acrylic esters, ethyl acrylate has the highest polarity and hence gives... 

High heat resistance and oil resistance. Self extinguishing. Relatively poor cold performance. High cost. Slightly inferior heat and chemical resistance to the fluorocarbon terpolymers. 

Benzenetetracarboxylic dianhydride (pyromellitic dianhydride) is a typical bifunctional acid anhydride, and it is a useful raw material for preparing many useful chemicals. Polyimides and polyimidazopyrrolons prepared from this dianhydride have excellent heat and chemical resistance, as well as excellent mechanical and electrical properties. Pyromellitic dianhydride is produced by the oxidation of 1,2,4,5-tetraalkylbenzenes such as 1,2,4,5-tetramethylbenzene (commonly known as durene) and 4,6-diisopro-pyl-l,3-dimethylbenzene. Durene, in particular, is a fundamental raw material for the production of the dianhydride 1-8). 

SAN (70% styrene, 30% acrylonitrile) has better heat and chemical resistance and is stiffer than PS. The optical clarity is not as good. SAN is used in a variety of houseware applications, particularly those things that will come in contact with food (chemical attack) and those that will end up in a dishwasher (heat attack). Coffee pots and throwaway tableware are good examples. 

Nylon-4,6 was developed by DSM Engineering Plastics in 1990 and sold under the trade name Stanyl giving a nylon that has a higher heat and chemical resistance for the automotive industry and in electrical applications. It has a of 295°C and can be made more crystalline than nylon-6,6. A number of other nylons, such as the aromatic nylons and aramids, are strong and can operate at high temperatures, and they have good flame-resistant properties. 

Ethylene-Vinyl Acetate. Two ethylene-vinyl acetate copolymers were evaluated (U.S. Industrial Chemicals NE 630X-1 and U.C.C. DQD 1868). Similar behavior was shown by both samples—i,e,y upon exposure to increasing dosages the tensile strength showed a maximum between 10 and 20 megarads with little effect upon elongation. Heat and chemical resistance appeared to be below that of irradiated polyethylene. 

ABS is a feasible choice for blending with PTT, because of their potential combination of good impact strength, modulus, heat and chemical resistance, and abrasion resistance (31). 

The large part of the epoxy resin backbone contains aromatic rings, which provide a high degree of heat and chemical resistance. 

Crosslink density Increases rigidity Increases heat and chemical resistance Tensile strength increases and peel and impact decrease with increasing crosslink density... 

In a heat-cured epoxy resin system, the hydroxyls generally react with either epoxy or acid groups. Hence, heat-cured diepoxies can be regarded as being potentially tetrafunc-tional, rather than difunctional. Conventional diepoxy resins, therefore, yield much more highly crosslinked structures with higher heat and chemical resistance after heat cure than after room temperature cure. 

Epoxy novolac High heat and chemical resistance ET ... 

Epoxy novolac acrylates These are specialty products. They are mainly used in die electrical and electronics industries because of their excellent heat and chemical resistance. However, they provide rigid coatings with relatively high viscosity and high costs. 

Aromatic amines Moderate heat and chemical resistance Solid at room temperature Rigid Long elevated-temperature cures Composites Electrical encapsulation Adhesives... 

With liquid DGEBA epoxy resins, DETA is normally used at the stoichiometric concentration of 10 to 11 parts per hundred (pph), and TETA is used at a concentration of 14 pph. However, both curing agents can be used at mix ratios as low as 70 to 75 percent of stoichiometry for greater toughness and increased pot life at the sacrifice of heat and chemical resistance. The effect of the mix ratio of DETA and TETA on the heat deflection temperature of castings is shown in Fig. 5.3. 

DEAPA cured epoxies have a less densely crosslinked structure than do DETA or TETA cured epoxies. This results in lower heat and chemical resistance and less hardness however it also improves the toughness and peel strength. The other physical properties are very similar to those of DETA or TETA cured epoxies. 

Pyromellitic dianhydride (PMDA) is a solid having a melting point of 286°C. It contains two anhydride groups symmetrically attached to a benzene ring. Because of the compactness of the molecule, PMDA achieves very high crosslink densities and, therefore, high heat and chemical resistance. PMDA cured epoxy adhesives have a heat distortion temperature on the order of 280 to 290°C. 

Latent imidazole catalysts have also been developed to provide cure rates considerably faster than those of dicyandiamide cured epoxy resins.18 They also exhibit excellent adhesive characteristics and heat and chemical resistance. 

Diluents will also affect the performance properties of the adhesive. Diluents generally lower the degree of crosslinking and degrade the physical properties of the cured epoxy. This reduction in crosslink density increases the resiliency of the adhesive, but it also reduces tensile strength as well as heat and chemical resistance. These effects are more pronounced at elevated temperatures than at room temperature. The degree of these effects will depend on whether the diluent has epoxy functionality (reactive diluents) or whether the diluent is incapable of reacting with the epoxy system (nonreactive diluents). 

Monofunctional epoxy diluents are used primarily with DGEBA epoxy blends. The most common monofunctional diluents are butyl glycidyl ether and phenyl glycidyl ether. The effect of butyl glycidyl ether and other reactive diluents on the viscosity of epoxy resin is shown in Fig. 6.3. Because the monofunctional diluents reduce crosslink density, they are used at relatively low levels to avoid degrading heat and chemical resistance or other properties of the adhesive. 

Epoxy-nitrile Nitrile-epoxy adhesives are composed of solid epoxy resin modified with carboxyl-terminated butadiene nitrile (CTBN) copolymer. The CBTN is introduced into die epoxy resin at elevated temperatures. The modification provides toughness and high peel strength without sacrificing heat and chemical resistance. The film adhesives are widely used in the aerospace industry in the construction of jetliners. 

However, the single-phase epoxy nitrile adhesive achieves its high peel strength by bulk elongation. Other properties, such as heat and chemical resistance, are generally degraded as they are when other flexibilizers or plasticizers are added to the epoxy. The two-phase version of this adhesive hybrid solves many of these problems. 

---