Chemical/environmental resistance

For applications with low glass content, the choice of resin is more likely to depend upon cost (polyester being the cheapest), processability or chemical/environmental resistance (Chapter 9). 

Although thermal performance is a principal property of thermal insulation (13—15), suitabiHty for temperature and environmental conditions compressive, flexure, shear, and tensile strengths resistance to moisture absorption dimensional stabiHty shock and vibration resistance chemical, environmental, and erosion resistance space limitations fire resistance health effects availabiHty and ease of appHcation and economics are also considerations. 

During World War II, several new synthetic elastomers were produced and new types of adhesives (mainly styrene-butadiene and acrylonitrile copolymers) were manufactured to produce adequate performance in joints produced with new difficult-to-bond substrates. Furthermore, formulations to work under extreme environmental conditions (high temperature, resistance to chemicals, improved resistance to ageing) were obtained using polychloroprene (Neoprene) adhesives. Most of those adhesives need vulcanization to perform properly. 

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. 

Thermoplastics which are used for corrosion protection can be applied in coatings as thin as 0.025 mm by solution techniques and in excess of 5 mm by extrusion or plastisol dipping. They are used where environmental resistance, chemical resistance, abrasion resistance, sound deadening or cushioning are required. They are used in those market areas that necessitate metallic mechanical strength plus thermoplastic corrosion resistance. 

These coatings provide the most effective fire-resistant system available but originally were deficient in paint color properties. Since, historically, the intumescence producing chemicals were quite water-soluble, coatings based thereon did not meet the shipping can stability, ease of application, environmental resistance, or aesthetic appeal required of a good protective coating. 

Fibers (qv) have been defined by the Textile Institute as units of matter characterized by flexibility, fineness, and a high ratio of length to thickness (3). For use in textile applications, fibers should have adequate temperature stability, strength, and extensibility. Other important qualities include cohesiveness or spinability and uniformity. There are also several secondary characteristics that improve customer satisfaction and therefore may be desirable. These include cross-sectional shape, specific gravity or density, moisture regain, resiliency, luster, elastic recovery, and resistance to chemicals, environmental conditions, and biological organisms. 

Given the existence of interphases and the multiplicity of components and reactions that interact to form it, a predictive model for a priori prediction of composition, size, structure or behavior is not possible at this time except for the simplest of systems. An in-situ probe that can interogate the interphase and provide spatial chemical and morphological information does not exist. Interfacial static mechanical properties, fracture properties and environmental resistance have been shown to be grealy affected by the interphase. Careful analytical interfacial investigations will be required to quantify the interphase structure. With the proper amount of information, progress may be made to advance the ability to design composite materials in which the interphase can be considered as a material variable so that the proper relationship between composite components will be modified to include the interphase as well as the fiber and matrix (Fig. 26). 

Poly (methyl methacrylate) is characterized by crystal-clear hght transparency, unexcelled weatherability, and good chemical resistance and electrical and thermal properties. It has a useful combination of stiffness, density, and moderate toughness. PMMA has a moderate Tg of 105°C, a heat deflection temperature in the range of 74 to 100°C, and a service temperature of about 93°C. However, on pyrolysis, it is almost completely depolymerized to its monomer. The outstanding optical properties of PMMA combined with its excellent environmental resistance recommend it for applications requiring light transmission and outdoor exposure. Poly (methyl methacrylate) is used for specialized apphcations such as hard contact lenses. The hydroxyethyl ester of methaciyhc acid is the monomer of choice for the manufacture of soft contact lenses. Typical applications of poly(methyl methacrylate are shown in Table 15.6. 

Table 2. Impeller for Chemical Handling Pump. Design Criteria Strength and Stiffness, Toughness, Short-Term and Long-Term Heat Resistance and Environmental Resistance.

There are two major allyl plastics, diallyl phthalate (DAP) and diallyl isophthalate (DAIP). Both of these are widely used in fiber RP forms. The allyl plastics are usually compression or transfer molded performing well in automated equipment (Chapter 5). They retain their physical and electrical characteristics under prolonged exposure to severe environmental conditions. They have high heat and moisture resistance, excellent electrical performance, good chemical resistance, dimensional stability, and low creep. These plastics are used where they provide environmental resistances. 

The current concentration on the use of synthetic instead of natural fibrous materials stems from the desire to standardize the physical and mechanical characteristics of materials and to adapt them to the demands of the technical designers and users of membrane constructions. Above all, characteristics such as mechanical firmness, flexibility, workability, durability, resistance to UV radiation and chemicals, dirt resistance and cleaning behaviour, flammability, moisture stability and environmental compatibility determine the costs and therefore the choice of materials. 

Many practical benefits can be obtained by blending polymers. Blending allows for the beneficial properties of two polymers to be combined in one material while shielding their mutual drawbacks. Deviations in the mle of mixing can lead to properties of the blend over and above those of its components. Thus, processibility, chemical and environmental resistance, adhesion, and mechanical properties of polymer blends are superior to those of their homopolymers. 

F flow property, MR mold release, impact property, HDT" heat distortion temperature, S stiffness, MRT moisture resistance, DS dimensional stability, M moldability, LW low warpage, TS thermal stability, LWA low water absorption, lubricity, BM blow moldability, CR " chemical resistance, ER environmental resistance, and SC stress cracking. 

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