Increasing strength and stiffness

Although thermoplastics comprise un-linked chain molecules, they can nevertheless have a branched structure. In contrast to elastomers and duromers, these branches do not cause cross-linking between the chains, but they can inhibit a geometrically dense packing necessary to form crystalline regions. This is sketched in figure 8.25. 

The most important methods to increase the strength or stiffness of a polymer are a direct consequence from what we saw in the previous section. They are  

One method of improving the strength or stiffness is particularly important in polymers Combining them with other materials to form composites. This is the subject of chapter 9. 

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Air and Oil Filters. Liquid resole resins are used to coat and penetrate the cellulose fibers of filters and separators in order to increase strength and stiffness and protect against attack by the environment. The type of phenoHc to be used depends on both the final property requirements and the papermaking process. 

Fillers offer a variety of benefits increased strength and stiffness, reduced cost, shrinkage reduction, exothermic heat reduction, thermal expansion coefficient reduction, improved heat resistance, slightly improved heat conductivity, improved surface appearance, reduced porosity, improved wet strength, reduced crazing, improved fabrication mobility, increased viscosity, improved abrasion resistance, and/or impact strength. Fillers also can have disadvantages. They may limit the method of fabrication, inhibit cure of certain resins, and shorten pot life of the resin. 

With regard to costs, honeycomb core is currently around 40 to 50/lb, making this a cost-effective application even at current LCP prices. LCP films could gain a place in honeycomb through increased strength and stiffness, reduced weight, and equivalent or lower cost. 

Recent advances further enhance their commercial potential in metal matrix composites such as aluminum, nickel, and copper ceramic matrix composites, such as alumina, zirconia and silicon nitride and glass ceramic matrix composites such as lithium aluminosilicate. Silicon carbide whiskers increase strength, reduce crack propagation, and add structural reliability in ceramic matrix composites. Structural applications include cutting tool inserts, wear parts, and heat engine parts. They increase strength and stiffness of a metal, and support the design of metal matrix composites with thinner cross sections than those of the metal parts they replace, but with equal properties in applications such as turbine blades, boilers and reactors. 

The principal material used for RIM is polyurethane, however nylon, polyethylene, epoxy, and polyester can also be used. Chopped glass fibers and inorganic fillers have been added to these materials to increase strength and stiffness. The process then becomes known as reinforced reaction injection molding (RRIM). 

Polymer C (with randomly mixed MDI/DBDI hard phase), however, showed advantages of increased strength and stiffness, plus increased strain recovery compared to polymer A, thus offering improvements in all these properties [61,254]. 

As the temperature is lowered, the mechanical and elastic properties of PS and PU foams are affected. In general, the properties reflect increased strength and stiffness as the temperature decreases. Figures 8 through 13 contain experimentally observed low temperature mechanical and elastic characteristics for these foam systems. 

Improved composite properties. These can include increased strength and stiffness, better impact strength, increased abrasion resistance, reduced adsorption and less swelling in the presence of water and other fluids. 

Under irradiation up to medium doses mainly crosslinking occurs in the amorphous, inter-lamellar phases, resulting in increased strength and stiffness and reduction in strain at break. 

Reinforcement, e.g., glass fibre Increased strength and stiffness... 

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