The strain energy density function (W-function) is one of the basic function describing mechanical properties of rubber. And it is important to determine the functional from of the W-function for the study of rubber elasticity from industrial application point of view such as stress analysis of rubber structural component. Two functional forms of the W-function for carbon filled systems are presented based on biaxial extension experiments. The first is the simple form, as follows:
W(I₁, I₂)=C₅(I₁-3)+C₂(I₂-3)+C₃/N+1(I₁-3)^N+1
where C₂, C₃, C₅ and N are constants and these are determined by pure shear deformation (strip-biaxial deformation) experiment. The more precise form of the W-function as the second approximation is as follows:
W(I₁, I₂)=C₅(I₁-3)+C₂(I₂-3)+C₃/N+1(I₁-3)ⁿ⁺¹(I₂-3)^-m
The constants for this equation, except n and m, are the same constants as those for the simpler from and n or m can be determined by not only the strip-biaxial deformation experiment but also by general-biaxial deformation experiments and the other constant is derived by the relation N=n-m. The third method is the method to estimate the constants for the second equation with uniaxial extension and strip-biaxial experiments. The accuracy of these methods for determining the W-function is high enough for the industrial applications.

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We carried out a three-dimensionally stressed fatigue test of a carbon black reinforced NR, with structural and physical stability variable depending on the mixing process. We investigated in detail the influence of the change in structure and phusical property due to fatigue, formation of anisotropic layer structures and fatigue fracture patterns.
The results are as follows: the changes in structure and phusical property of the NR due to fatigue are not so great as those of systems in which is formed a stable structure with progress of mixing and in consequence the formation of anisotropic layer structures from layer orientation of carbon black and orientation of rubber molecular chains becomes less. In addition, fatigue fracture pattern changes from a lamellar flaking fracture parallet to the circumference, to a fracture perpendicular to the circumference.

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Effects of water on properties of novel polyurethane elastomers based on poly (β-methyl-δ-valerolactone) glycol (PMVL) and poly (3-methylpentamethylene adipate) glycols (PMPA) were compared with those of general purpose polyurethane elastomers based on poly (caprolactone) glycol (PCL) and poly (butylene adipate) glycol (PBA). The polyurethanes were prepared from polyols (PMVL, PMPA, PCL, and PBA), 2, 4-TDI, and 1, 4-butanediol. Results of the measurements of water uptake and pulsed NMR indicated that the water absorbed in the novel polyurethanes was bound more strongly than in general purpose polyurethanes and that the amount of water taken up by the novel polyurethanes was smaller than that by general polyurethanes. Results of stress-strain relationships and dynamic viscoelastic properties suggested that water acted as plasticizer and broke microphase separation. PMVL- and PMPA-based polyurethanes kept better mechanical properties than PCL- and PBA-based polyurethanes after immersion in hot water.

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