Chemical resistance also crosslinking

Thermosetting polymers based on epoxy resins often display superior tensile strength and chemical resistance when compared with their thermoplastic counterparts. Such attributes make epoxy polymers ideal matrices for adhesives and composites. However the applicability of epoxy polymers as matrices for adhesives and composites are often limited by an inherent weakness - low flaw tolerance. Ironically, the same crosslinked chemical structure that imparts high strength and superior chemical resistance also promotes brittle behavior. 

Addition copolymers of cycloolefin compounds with a polar substituents in the side chain exhibit excellent heat resistance and transparency. They are also capable of crosslinking to improve the adhesion properties, the dimensional stability and the chemical resistance (35). 

Primary and secondary aliphatic amines react relatively rapidly with epoxy groups at room or lower temperature to form three-dimensional crosslinked structures. The resulting cured epoxies have relatively high moisture resistance and good chemical resistance, particularly to solvents. They also have moderate heat resistance with a heat distortion temperature in the range of 70 to 110°C. Thus, short-term exposures of cured adhesive joints at temperatures up to 100°C can generally be tolerated. 

These aliphatic amines can also be cured at elevated temperatures to provide a more densely crosslinked structure with better mechanical properties, elevated-temperature performance, and chemical resistance. Table 5.3 illustrates the effect of curing temperature on the bond strength of DGEB A epoxy with two different aliphatic amines. 

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. 

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). 

An explanation is apparent by examining the cure curve. Because the diluent reduces the crosslinking density, it allows the epoxy to react more completely. The greater extent of reaction tends to compensate for the otherwise possibly deleterious effect of the diluent on the properties of the cured system. The use of diluent also provides the practical advantage of lowered viscosity in a formulated system. Since excessive use of diluents can lead to poor chemical resistance and reduced mechanical strength, recommended amounts should not be exceeded. 

Lastly, the polyimide has some inherent photosensitivity. The use of a photosensitive polymer requires fewer processing steps, which also implies an increased yield for the circuit. Ciba-Geigy announced the formulation of inherently photosensitive polyimides in 1985.(5) The use of an inherently photosensitive polyimide is attractive because no sensitizers are added to contaminate the final resin, no functionality is added which degrades the thermal and chemical resistance, and fewer volatiles are present to contaminate the resin or equipment than in some other approaches to photosensitive polyimides. Recent work has shown that these polyimides crosslink on irradiation through hydrogen abstraction by triplet benzophenone and subsequent coupling of the resultant radicals. ... 

Within the different types of epoxies, are found epoxy diacrylates or vinyl ester resins, used to produce specific corrosion and chemical resistant composite systems. Vinyl ester resins are produced by either reacting epoxy resins of glycidyl derivatives with methacrylic acid, or from BPA and glycidyl methacrylates, where an active monomer (usually styrene) as crosslinker, hardener (usually organic peroxides), accelerators (cobalt) are added to the system. In the thermoset epoxy systems, there are also the mould releasers , which can be either internal such as, lecithin, or stearates of zinc and calcium, certain organic phosphates that are mixed in the resin, or, external - such as, fluorocarbons, silicone oil, and certain waxes, that are directly laid on the mould. 

The chemical resistance of PEEK is shown in Table 6.3. PEEK exhibits a remarkable chemical resistance, comparative to fluoropolymers. PEEK is approved by the FDA. PEEK undergoes crosslinking by irradiation in vacuum under stress. The tensile properties of PEEK sheets after UV radiation show a tendency to embrittlement. This is caused not only by crosslinking but also by the orientation of molecular chains resulting from the temperature rise of the specimens. Furthermore, the tensile stress applied during exposure accelerates molecular scission and disturbs the crosslinking. ... 

Series of monodisperse latexes with small, uniform, and well-controlled particle diameters are useful as size reference materials for calibrating particle sizing equipment. Usually, the particles are comprised of polystyrene, and may also be crosslinked with divinylbenzene to improve chemical resistance. 

PP can be rotational moulded as an alternative to PA, polyester and crosslinked PE for such rotational mouldings as automotive parts, toys, food handling systems and chemical tanks [30]. It also provides higher heat/chemical resistance and stififiiess than normally achieved by rotational mouldable LLDPE. 

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