Rubber thermal diffusivity

Thermal diffusivity is of little interest in many thermal insulation applications, for example civil engineering, where approximately steady state conditions normally exist. However, in rubber processing when temperatures are changing rapidly it is of more value than conductivity. 

The typical polymer or rubber sample would be classified as optically transparent or opaque and thermally thick except possibly for the strongest bands. In this case the signal intensity would be proportional to the product of the optical absorption coefficient (P) and the thermal diffusion length and show a - 3/2 dependence on the modulation frequency (to). The angular modulation frequency is a product of the interferometer mirror velocity and the wavenumber ... 

A discussion of surface analysis requires a review of the depth being sampled during PA-FTIR spectroscopy. The depth being sampled during PA-FTIR analyses of rubbers is the thermal diffusion depth (Dt). This is a function of the thermal diffusivity of the sample, the wavenumber, and the mirror velocity. 

For illustrative purposes a thermal diffusivity of 1.3 x 10 3 cm2/s is often used as being typical of rubber and polymers. Some values from the literature for various materials are given in Table 2.1. Using the value of 1.3 x 10"3 it can be calculated that a depth of 3 to 11 Jim is being sampled at 2000 cm"1 as indicated in Table 2.2. This is an order of magnitude greater than that sampled by ATR techniques. 

The two slabs of materials, steel mold and rubber, were assumed to be in perfect contact at the interface. He desired to employ the same modulus and the same incremental time At for both materials, so the thickness of the slices had to be different for the mold and the rubber. The heat balance at the interface led to the conclusion that the ratio of the thicknesses of the slices had to be taken as equal to the square root of the thermal diffusivities amo, j and... 

The heat flux through the thicknesses of the rubber and of the mold is expressed by the same Equation 2.8, with their respective values for the thermal diffusivity and... 

Erensdorff H.K. 1974. Thermal diffusivity measurements on elastomers. Rubber Chem. Technol. 47 849-57. 

Hands D. and F. Horsfall. 1977. The thermal diffusivity and conductivity of natural rubber compounds. Rubber Chem. Technol. 50 253-65. 

For the rubber in sheet 1 with the values of the thermal diffusivity a and of the thermal capacity Q... 

Th is chapter deals with the measurement of thermal conductivity, thermal diffusivity. and specific heat. Other properties that are sometimes included under the umbrella term "thermal properties" are dealt with in other parts of this volume. In most cases it does not matter whether the sample is a rubber or a plastic, the experimental techniques arc the... 

Figure 18.29 Thermal diffusivity versus filler volume fraction for Ti02- and nanosilica-filled natural rubber (NR) composites.

Song and co-workers [14] point out that in MTA three images based on topography, thermal conductivity and thermal diffusivity can be obtained simultaneously. They used MTA to study the phase separation process in a 50 50 (by weight) PS-polyvinyl methyl ether (PVME) blend and natural rubber-nitrile rubber blends. 

Molecular orientation can have a significant effect on thermal conductivity, and presumably also on thermal diffusivity. Generally, thermal conductivity values can be expected to increase along the direction of orientation or machine direction and decrease in the cross directions. For example, rubber vulcanizates show increases of up to 50% in the direction of stretch (64), and a glassy pol3rmer such as poly(methyl methacrylate) shows increases up to 20% (37). 

Oxidative ageing of rubbers is limited by the rate of diffusion of oxygen into the rubber product and is usually confined to the outer 3 mm. Antioxidants are used to protect rubbers from the effects of thermal oxidation and the vast majority of compounds will contain one or more. Peroxide vulcanisates are usually protected with dihydroquinolines. Other antioxidants react adversely with the peroxide inhibiting the crosslinking reaction. 

It is clear, that MEK is a "good" solvent for both the elastomers and the epoxy resin. Note that at 10% rubber, the MEK absorption nearly doubles. This implies that a much higher concentration of MEK is present in the rubber phase than in the epoxy phase. This is possible because the MEK diffuses more rapidly into the rubbery CTBN phase owing to its greater segmental thermal motion. 

Functional properties and stability of rubbery materials Chapters 1, 3, 4, 7, 12 and 13, give examples of applications of spectroscopic techniques for the characterisation of thermal stability and degradation, kinetics of thermal decomposition, ageing, oxidation and weathering, self-diffusion of small molecules in rubbery materials, adhesion of rubbers to metals, fluid adsorption and swelling. 

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