Elastomers measurements

Figure 4. Stress-strain curves for the TIP A polyurethane elastomer measured at the indicated temperatures at an extension rate of 0.94 min 1. Arrows indicate...

Figure 4. Temperature dependence of log( ) and tan (S) for nematic elastomers measured at 30 Hz (reproduced with permission from [8]).

Silica filled PMPS elastomer measured by swelling... 

Shore hardness. Durometer hardness is a property that, as apphed to elastomers, measures resistance to indentation. Shore A scale is used for soft elastomers, with shore D scale for harder materials. 

T3q>e of information to be recorded depends on the materials fiom which the test specimens are made. For metals, calculated corrosion rate and type/depth of localized corrosion must be documented. For polymers and elastomers, measured property changes, such as thickness or hardness, are documented, in addition to appearance change. The information for polymers and elastomers is usually recorded immediately after the coupons are removed and after a designated interval, to provide time for them to stabilize. In all cases, performance data may be linked to a photographic record of the coupons. 

Table 5.89 Resistance of polyamide 11 to oils compared to thermoplastic, polyester-based elastomers measured according to ASTM no. 2 and no. 3, 70 hours storage at 150 °C, tensile test [921]...

FRE French, R.N. and Koplos, G.J., Activity coefficients of solvents in elastomers measured with a quartz crystal microbalance,F/ttir// Aase qr //., 158-160, 879, 1999. 

One can take advantage of the large chemical shift dispersion of to resolve nuclei in different chemical environments in elastomers. Measurements of the relaxation rates at each resolvable carbon are also possible because the low abundance of C precludes averaging of relaxation rates by spin diffusion. In this manner structural information can be obtained and the solid state NMR spectra give quantitative indications of the type and amount of the chemical crosslinked structures. It has been possible to relate the reinforcement mechanism of carbon black to the relative amount of chemical versus physical crosslinks (entanglements) using NMR and equilibrium swelling measurements. 

Acid-Base Behavior. The relative acidity-basicity of the filler, generally determined by measuring the pH value of a slurry of a specific mass of filler in 100 mL of deionized water, can influence the behavior of a filler in some systems. For example, the curing behavior of some elastomers is sensitive to the pH value of carbon black. 

International Rubber Hardness. The International mbber hardness test (ASTM D1415) (2) for elastomers is similar to the Rockwell test ia that the measured property is the difference ia penetration of a standard steel ball between minor and major loads. The viscoelastic properties of elastomers require that a load appHcation time, usually 30 seconds, be a part of the test procedure. The hardness number is read directly on a scale of 0 to 100 upon return to the minor load. International mbber hardness numbers are often considered equivalent to Durometer hardness numbers but differences ia iadenters, loads, and test time preclude such a relationship. 

Quahty control testing of siUcones utilizes a combination of physical and chemical measurements to ensure satisfactory product performance and processibihty. Eor example, in addition to the usual physical properties of cured elastomers, the plasticity of heat-cured mbber and the extmsion rate of TVR elastomers under standard conditions are important to the customer. Where the siUcone appHcation involves surface activity, a use test is frequently the only rehable indicator of performance. Eor example, the performance of an antifoaming agent can be tested by measuring the foam reduction when the sihcone emulsion is added to an agitated standard detergent solution. The product data sheets and technical bulletins from commercial siUcone producers can be consulted for more information. 

As shown, ethylene—acrylic elastomers will function for greater than 24 months at 121°C, or 6 weeks at 170°C continuous service. Exposures up to 190—200°C can be tolerated, although service life at these temperatures are measured ia days rather than weeks. 

Properties such as low permanent set, low creep and low hysteresis are really measures of the efficiency of the heat fugitive network system. This is a complex function of the morphology. As a very general statement, the problem would seem to be less important with the harder grades of thermoplastic elastomer. 

As mentioned earlier, the contact-mechanics-based experimental studies of interfacial adhesion primarily include (1) direct measurements of surface and interfacial energies of polymers and self-assembled monolayers (2) quantitative studies on the role of interfacial coupling agents in the adhesion of elastomers (3) adhesion of microparticles on surfaces and (4) adhesion of viscoelastic polymer particles. In these studies, a variety of experimental tools have been employed by different researchers. Each one of these tools offers certain advantages over the others. These experimental studies are reviewed in Section 4. 

JKR-type mea.surements on soft elastomers. In their original paper, Johnson and coworkers [6] reported on adhesion measurements between soft materials. 

JKR type mea.surement.s on monolayers depo.sited on. soft elastomers. The recent interest in the JKR experiments has been stimulated by the work of Chaudhury and coworkers [47-50J. In a 1991 paper, Chaudhury and White-sides [47] reported their extensive studies on the measurement of interfacial work of adhesion and surface energies of elastomeric solids. The motivation for this work was to study the physico-organic chemistry of solid surfaces and interfaces. 

Fig. 17. Adhesion energy G measured as a function of the surface density of the interfacial chains. It may noted that the strength measured in a peel test (a) is about 5 times larger than that measured using the JKR method (b). Further, a maximum exists in the value of G as function of the surface chain density. This is because of swelling effects at larger values of surface chain density. The open symbols represent the data for elastomer molecular weight Mo = 24,000 and the closed symbols represent the data for Mo = 10,000.

As reviewed so far, the contact-mechanics-based techniques (JKR and SFA methods) have been effective in the understanding molecular level mechanisms related to the adhesion of elastomers and in measuring the surface and interfacial energies of polymers and self-assembled monolayers. The current work in this area is aimed at understanding contact induced interfacial rearrangements and the role of specific interactions. The recent progress of these studies is discussed in this section. 

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