Mechanical property retention, additive

Polyimides (PI) were among the eadiest candidates in the field of thermally stable polymers. In addition to high temperature property retention, these materials also exhibit chemical resistance and relative ease of synthesis and use. This has led to numerous innovations in the chemistry of synthesis and cure mechanisms, stmcture variations, and ultimately products and appHcations. Polyimides (qv) are available as films, fibers, enamels or varnishes, adhesives, matrix resins for composites, and mol ding powders. They are used in numerous commercial and military aircraft as stmctural composites, eg, over a ton of polyimide film is presently used on the NASA shuttle orbiter. Work continues on these materials, including the more recent electronic apphcations. 

Previous BCB/BMI resin studies have demonstrated that the addition of 40X to 60X BCB to a BMI results in a resin with thermal stability dramatically improved over that of the BMI itself The 1 1 siolar ratio BCB/BMI system used in this study has exhibited the retention of thermal mechanical properties at elevated tesiperatures as well. 

Finally, the use of low-molecular-weight species to improve flow properties called plasticizers normally reduces stiffness, hardness, and brittleness. Plasticization is usually restricted to amorphous polymers or polymers with a low degree of crystallinity because of the limited compatibility of plasticizers with highly crystalline polymers. Other additives, such as antioxidants, do not affect the mechanical properties significantly by themselves, but can substantially improve property retention over long periods of time, particularly where the polymer is subject to environmental degradation. 

In addition to characterizing the many properties introduced by the choice of monomers and the polymerization process itself, considerable further characterization is required to quantitatively describe the properties imparted by spinning and subsequent downstream processing. These important properties relate to the crystalline order and microsiruciure of the fibers, and die resultant performance characteristics, such as crimp retention, abrasion resistance, mechanical properties, etc. 

Melt in a plasticator must be freed of gaseous components that include moisture and air from the atmosphere and from plastics, plasticizers, and/or other additives as well as entrapped air and other gases released by certain plastics. Gas components such as moisture retention in and on plastics have always been a potential problem for all processors. All kinds of problems develop on products (splay, poor mechanical properties, dimensions, etc.). This situation is particularly important when processing hygroscopic plastics (Chapter 1). One major approach to this plastic degrading situation is by using plasticators that have vents in their barrels to release these contaminants. 

The thermally stable plastics are correspondingly classified in two classes heat stable and high-temperature-stable plastics. The emphasis in heat stable plastics is on the resistance to mechanical deformation at higher temperatures. Such plastics can be applied at temperatures up to 250-300°C, whereas conventional plastics can only be used up to about 100°C. Many engineering plastics belong to the heat stable plastics (see also Section 36.4). Thermal dimensional stabilities of at least 180°C, tensile strengths of at least 45 MPa and flexural moduli of at least 2 200 MPa at this temperature with retention of at least 50% of the mechanical property values at 115° C in air for at least 11.5 years (100 000 h) are required of these polymers. In addition, the polymers should be resistant to as many chemicals as possible at temperatures of 80°C and higher. 

Ticona and Xydar from BP-Amoco. The key advantages of Vectra are its fast cycling and ease of processing. Xydar offers improved retention of mechanical properties at high temperatures. Both Vectra and Xydar are very resistant to chemical and solvent attack. In addition, LCPs are little affected by radiation [6]. 

Addition of a small amount of PSF to a variety of resins improves hardness, the notched Izod impact strength, plateabflity, hydrolytic stability, and shape retention at high temperatures. Many PSF blends of or with engineering resins have been developed, viz., with PA, PEST, PC, PPE, or POM. They have high HDT, heat resistance, strength, stiffness, mechanical properties, and ESCR. Polysulfone blends have been foamed using water and either N2 or CO2 (Bland and Conte... 

APP and APP-based systems are very effident halogen-free flame retardants mainly used in polyolefins (PE, PP), epoxies, polyurethanes, unsaturated polyesters, phenolic resins, and others. APP is a nontoxic, environment friendly material and it does not generate additional quantities of smoke due to intumescence. Compared to other halogen-free systems, APP requires lower loadings. In thermoplastic formulations, APP exhibits good processability, retention of good mechanical properties. 

Polymethylpentene has a high crystalline melting point of 240°C, coupled with useful mechanical properties at 204°C and retention of form stability to near the melt point. However, the polymer is brittle (fiber or rubber additives are usually advised for improved toughness), ages poorly (the use of antioxidants is recommended), has high gas permeability, and is relatively expensive. Polymethylpentene s chemical resistance is very good and typical of the polyolefins. Its transparency is close to the theoretical optimum for thermoplastics. Polymethylpentene also has excellent electrical properties with power factor, dielectic constant, and volume resistivity on the same order as PTFE fluorocarbon. 

In addition to offering resistance to degradation at high temperatures, polysulfones maintain their mechanical properties at high temperatures without reinforcement. The effect of temperature on PSF tensile stress-strain behavior is shown in Fig. 13.2. It can be seen that the retention of useful properties extends to approximately 150°C for PSF. This useful temperature range approaches 180°C for PES and PPSF... 

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