Low modulus of elasticity

In structural applications for plastics, which generally include those in which the product has to resist substantial static and/or dynamic loads, it may appear that one of the problem design areas for many plastics is their low modulus of elasticity. The moduli of unfilled plastics are usually under 1 x 106 psi (6.9 x 103 MPa) as compared to materials such as metals and ceramics where the range is usually 10 to 40 x 106 psi (6.9 to 28 x 104 MPa). However with reinforced plastics (RPs) the high moduli of metals are reached and even surpassed as summarized in Fig. 2-6. 

Carswell and Nason assigned five classifications to polymers (Figure 14.8). It must be remembered that the ultimate strength of each of these is the total area under the curve before breaking. The soft and weak class, such as PIB, is characterized by a low modulus of elasticity, low yield (stress) point, and moderate time-dependent elongation. The Poisson ratio, i.e., ratio of contraction to elongation, for soft and weak polymers is about 0.5, which is similar to that found for many liquids. 

Fig. 4.5.11. Stress curves—elastic recovery for glazes with a high modulus of elasticity (1) and with a low modulus of elasticity (2).

The best known polymeric materials are natural rubber and synthetic rubber. These materials have low modulus of elasticity. The flexibility of these materials enables their application in tubing, belting and automotive tires as encountered in everyday usage. Resistance to chemicals, abrasive attack and insulating property can be advantageous in corrosion control applications. 

An alternative to the acrylamide-based composite hydrogel dressing is an acrylamide grafted to a polyurethane film to give a transparent, flexible gel with an equilibrium water content of approximately 50 /o. The hydrated gel has a low modulus of elasticity and a high water permeability that facilitates the adherence of the gel to the wound surface. The permeability... 

In recent years, the concept of light construction has led to the development of hybrid structural parts. Using parts made of pure plastic runs into repeated difficulties because of the material s low modulus of elasticity, since the only way to compensate for this drawback is to use much thicker structures. Combinations of sheet metal materials with plastics facilitate new solutions that do justice to the concept of lightweight construction and provide new properties at the same time (sound insulation, thermal conductivity, dynamic stability, corrosion protection). The precondition for successful development of such hybrid structural parts is a reliable bond between the two materials. In certain cases, this must be realized by adhesives that are flexible with regard to the a problem (different heat expansion coefficients of the materials), but still provide high strength levels. 

The outstanding property of rubber in general is resilience, or low modulus of elasticity. The flexibility and physical properties of rubber account for its application in general engineering and automobile industries whereas its chemical, wear and abrasion resistance as a sacrificial material, plus its insulating properties are utilised in many corrosion and erosion applications in process industries. Rubber lined mild steel, pipes and tankages have been standard materials of construction for hydrochloric acid service for many years. 

In another example, ] a problem with poor physical properties of molded fluoroplastics was overcome. Transfer molded material was found to have low modulus of elasticity above 150°C and was prone to irreversible cold flow. The solution involved embedding a metal or plastic insert as a core material in the mold. The choice of the core material depended on the end use performance requirements. 

Purified elastin has a pale yellow color and a bluish fluorescence in uv light. Resists acid and alkaline hydrolysis. Practically insol in a wide range of hydrogen-bond-breaking solvents at temps up to 100° and swells, but does uot dissolve, in phenolic solvents. It appears practically impossible to bring elastin into oln except by hydrolytic reagents capable of rupturing peptide bonds. Elastin is one of only a few polymeric substances which, in the presence of water, exist in a form with rubber-Like extensibility and low modulus of elasticity. Enzymes which dissolve fibers of the insol protein... 

Silicone-based chemistries match well with applications that require a low modulus of elasticity or high-temperature resistance. They also correspond with applications that require a combination of bonding and sealing. Curing mechanisms can include heat, moisture, or solvent evaporation. Room temperature vulcanization (RTV) materials are designed to polymerize as a solvent evaporates. 

Adhesives formulated from flexible, elastomeric resins are now commercially available and effectively used to dissipate stresses. They are low-stress or no-stress adhesives that have a low modulus of elasticity. Several guidelines that can be followed to reduce stresses are the following ... 

VLSIC in TQFP/thin Low modulus of elasticity blends Epoxy, cyanate-ester blends... 

PTFE is made from tetrafluoroethylene under pressure with a peroxide catalyst in the presence of excess water for removal of heat. The polymer is highly crystalline (over 94% crystallinity) with an average molecular weight of 0.5-5 xlO g/mol. This polymer has a very high density (2.15-2.2 g/cm ), low modulus of elasticity (0.5 GPa) and tensile strength (14 MPa). It also has a very low surface tension (18.5 erg/cm ) and friction coefficient (0.1). 

The foamed plastic has such a low modulus of elasticity and such low strength that it contributes little to either the stiffness or the bending strength of the T beam. It must, however, be stiff enough to prevent buckling or wrinkling of the mat or the roving. 

A principal structural deficiency of glass fabric (E-glass) RPs was their low modulus of elasticity. Development of new, higher modulus fibers. 

The high tensile strength, the low modulus of elasticity, and the hardness of titanium alloys... 

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