Crystallinity chemical resistance

Although the main uses for benzoic acid are as a chemical raw material, it also has numerous direct uses. Benzoic acid is used in substantial quantities to improve the properties of various alkyd resin coating formulations, where it tends to improve gloss, adhesion, hardness, and chemical resistance. Benzoic acid terminates chain propagation in alkyd resins (qv) and promotes crystallinity in the final product. 

The PEEK resia is gray, crystalline, and has excellent chemical resistance T is ca 185°C, and it melts at 288°C. The unfilled resia has an HPT of 165°C, which can be iacreased to near its melting poiat by incorporating glass filler. The resia is thermally stable, and maintains ductiUty for over one week after being heated to 320°C it can be kept for years at 200°C. Hydrolytic stabiUty is excellent. The resia is flame retardant, has low smoke emission, and can be processed at 340—400°C. Crystallinity is a function of mold temperature and can reach 30—35% at mold temperatures of 160°C. Recycled material can be safely processed. Properties are given ia Table 16. 

The properties of PBT and PC resins and of a blend of these two resins are given in Table 18. The chemical resistance of crystalline PBT is reduced, but that of amorphous PC is increased. Hydrolytic stabiUty is good throughout. Impact performance is lower than that of the components. It can be improved by modifiers. A commercial example of this type of resin blend is the General Electric Xenoy resin which is used in automotive bumpers. 

A 50 50 mol/mol copolymer of hexafluoroisobutylene (CH2 = C(CF3)2) and vinylidene fluoride was made available by Allied Chemical in the mid-1970s as CM-1 Fluoropolymer. The polymer has the same crystalline melting point as PTFE (327°C) but a mueh lower density (1.88g/cm ). It has excellent chemical resistance, electrical insulation properties and non-stiek characteristics and, unlike PTFE, may be injeetion moulded (at 380°C). It is less tough than PTFE. 

Meta-phenylenediamine, a crystalline solid with a melting point of about 60°C, gives cured resins with a heat distortion temperature of 150°C and very good chemical resistance. It has a pot life of six hours for a 200 g batch at room temperature whilst complete cures require cure times of four to six hours at 150°C. About 14 pts phr are used with the liquid epoxies. The main disadvantages are the need to heat the components in order to mix them, the irritating nature of the amine and persistent yellow staining that can occur on skin and clothing. The hardener finds use in the manufacture of chemical-resistant laminates. 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

As regards the general behaviour of polymers, it is widely recognised that crystalline plastics offer better environmental resistance than amorphous plastics. This is as a direct result of the different structural morphology of these two classes of material (see Appendix A). Therefore engineering plastics which are also crystalline e.g. Nylon 66 are at an immediate advantage because they can offer an attractive combination of load-bearing capability and an inherent chemical resistance. In this respect the arrival of crystalline plastics such as PEEK and polyphenylene sulfide (PPS) has set new standards in environmental resistance, albeit at a price. At room temperature there is no known solvent for PPS, and PEEK is only attacked by 98% sulphuric acid. 

Chemical resistance Amorphous polymers Crystalline polymers... 

Crystallinity Molecular weight Flammability Spatial configuration Chemical resistance... 

Polyester, thermoplastic TP polyesters have different grades. Polybutylene tereph-thalate (PBT) a crystalline polymer and an excellent engineering material. It has marginal chemical resistance but resists moisture, creep, fire, fats, and oils. Molded items are hard, bright colored, and retain their impact strength at temperatures as low as — 40°F (-40°C). Uses include auto louvers, under-the-hood electricals, and mechanical parts. 

Crystallinity increases the impermeability and, consequently, chemical resistance. Generally, HDPE is slightly more resistant than LDPE. 

Crystallinity increases impermeability and, consequently, chemical resistance. 

The growth of polyesters is hindered by their sensitivity to hot water above 60°C, fire behaviour except for FR grades, mould shrinkage for crystalline products, opacity (except amorphous PET), need of UV protection, limited chemical resistance, higher cost than commodities... 

PP is the lowest density plastic. It has fair-to-good impact strength and excellent colorability. It s translucent in thick sections, and it also has good chemical resistance. The properties can vary widely with different degrees of crystallinity. PP has good resistance to heat and low water absorption. That makes it a suitable material for many medical instruments that need. sterilization by steaming. 

Poly(ether ether ketone) (PEEK) is an aromatic, high performance, semicrystalline polymer with extremely good thermal stability, chemical resistance, and electrical and mechanical properties. This polymer shows little solubility in organic solvents due to the crystallinity. One of the first ways to characterize PEEK was by sulfonating the polymer. By adding sulfonic acid groups to the backbone, the crystallinity decreased and solubility increased.Commercially available Victrex appears to be one of the more interesting poly(arylene ether) s used for postmodification. 

Tg = 70-85 °C, higher than polyolefins because polar C—Cl bond gives dipole-dipole intermolecular attractions Low crystallinity Good impact strength Good chemical resistance Resistant to insects and fungi Non-flammable... 

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. 

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