Hardness and Modulus

All the materials increased in hardness or were essentially unchanged. Generally, the increase was no more than 10° with some compounds showing negligible change but nitrile R increased by up to 26°. Hardness measurements were mostly consistent with relatively modest scatter, although W, for example, had several aberrations in the first 10 years. 

The figures for stress at 100% elongation were generally compatible with the hardness results in that there was a rise for most materials, typically in the 50 to 100% region. The nitriles P and R (hot dry) and some of the SBR exposures suffered higher rises whereas the values for acrylate S and polysulphide W showed little change and for silicone X decreased. 

The results for stress at 300% elongation were less consistent, mostly where the data was incomplete. Where 300% elongation was retained for 40 years, the stress generally increased and compared reasonably with the 100% figures, although the changes were less. 

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Fig. 6.6 (a) A comparison of mechanical and thermal properties of PMMA hybrids with SWNT, expanded graphite (EGr) and functionalized graphene sheets (FGS). (b) Percentage synergy in hardness and modulus of the hybrids with binary nanocarbon fillers (from [44]). 

Table 5.2 lists polymers and their tendency toward crystallinity. Yield stress and strength, and hardness increase with an increase in crystallinity as does elastic modulus and stiffness. Physical factors that increase crystallinity, such as slower cooling and annealing, also tend to increase the stiffness, hardness, and modulus of a polymeric material. Thus polymers with at least some degree of crystallinity are denser, stiffer, and stronger than amorphous polymers. However, the amorphous region contributes to the toughness and flexibility of polymers. 

The physical properties of natural rubber and synthetic rubber compounds are affected greatly by the type and amount of fillers used. Carbon black is the most commonly used filler. Increasing amounts of carbon black increases the hardness and modulus of the vulcanizates. Resilience and resistance to impinging type abrasion decrease along with elongation. Tensile strength and tear strength... 

The optimum cure of such a stock is quoted as 45 mts at 138°C. The above details on optimum cure are related only to natural rubber and do not apply to SBR in which case hardness and modulus continue to increase beyond the optimum cure point. However this gives an insight into the cure state after all of the vulcanizate in general. Tear resistance and cut growth and permanent set, however, have been used to determine the optimum cure of SBR. 

Fig. 9.8. Relationship between scratching hardness and modulus of elasticity. (After Decneut, 1967)...

FIG. 25.8 Correlation between indentation hardness and modulus (see Tables 13.12 and 25.4 for reference numbers). 

The main problem is related to exposure of these materials to high temperatures in the presence of oxygen. These conditions favor the oxidative degradation of the elastomer, which leads to opposing changes in its structure. On the one hand, chain scission can occur, and on the other hand, cross-links can be generated. If chain scission dominates, the hardness and modulus of the elastomer will decrease. If, however, cross-links are generated, both the hardness and the modulus increase and the strain at fracture will decrease. Other mechanical properties are altered by an increase in... 

CVD is known to polymerize an order of magnitude more easily (Ref. 143) than the analogous bis-allyl carbonate marketed under the designation CR-39 which is the leading material for casting prescription eyewear since decades. The crosslinked homopolymer of CVD exhibits similar properties but with the advantages of much better scratch resistance and higher hardness and modulus. 

Mahoney E, Holt A, Swain M, Kilpatrick N The hardness and modulus of elasticity of primary molar teeth an ultra-micro-indentation study. J Dent 2000 28 589-594. 

High tensile strength, modulus Low hardness and modulus... 

Desoto Corp. [124] patented a composition of an epoxy resin, specific polyol, and onium salt The composition, upon photocuring, gives a balance of hardness and modulus. Increased epoxy resin curing speeds by addition of a sensitizer to a sulfonium salt containing system was disclosed anonymously in 1986 [125]. Also in 1986, a photocurable composition having excellent thermal, electrical, and weathering properties, which consists of an epoxy resin and onium salt, was described by Daicel Chemical Industries [126],... 

Plasticizers have a marked effect on T. One would normally expect that the plasticized polymer would nave a lower strength, hardness, and modulus than the unplasticized material. Actually incorporation of small amounts of a plasticizer such as di(2-ethylhexyl) phthalate (OOP) causes an increase in these values. At the 10-15% level of plasticizer. X-ray diffraction studies and IR analyses show an increase in the amount of crystallinity, probably due to an ordering of polymer chains. This result is the so-called "anti-plasticizer effect" and is dependent on the type of plasticizer. As the amount of plasticizer is increased, however, these properties (tensile strength and modulus) do decrease as shown in Figure 17. The effect on T as a result of increasing levels of DOP plasticizer is shown in Frgure 18. 

PCO/PE Block copolymers with Ethylene or Propylene and norborene blocks low melt viscosity thus good processability, high elongation at break, impact strength, toughness, hardness, and modulus Epple and Brekner, 1994... 

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