The change in Mooney viscosity (ML1+4) with aging was followed for silica filled compounds containing various silanes and polar additives. Several mechanisms for the aging stability are postulated and evaluated through experimentation. The type of silane or polar additive used can cause the ML1+4 to increase or even decrease during aging. When bis(triethoxy silanes) are used in silica filled rubbers, the ML1+4 growth during aging is caused by hydrolysis. Silica-silica bridging was found to be responsible for the ML1+4 growth in rubber compounds containing a more thermally stable polysulfide or a sulfur-free bis(triethoxy silane). When the bis(triethoxy silane) is bis(3-triethoxysilylpropyl) tetrasulfide (TESPT), a fraction of TESPT is attached to the unsaturated rubber to give polymer-silica attachments. These attachments further enhance the hydrolytic ML1+4 increase during aging. Chemical coating of the silica with a monofunctional silane or a physical coating with a trialkyl amine compound effectively stops the ML1+4 increase upon aging. The prevention of ML1+4 growth is so efficient that a reduction in the ML1+4 can be realized by absorption of ambient moisture. The extent of ML1+4 reduction caused by moisture depends on the degree of hydrophobation of the coated silicas. Hydrolytic stability was also studied with an amine or a sugar fatty acid ester that formed either strong or weak polar associations to the silica.

ABSTRACT:The preparation and characterization of a class of alkoxy-modified silsesquioxane (AMS) compounds containing less than 5 wt % of latent alcohol is described. The AMS derived from octyl triethoxysilane (OTES) behaves as a shielding agent in silica-filled rubbers while significantly reducing the volatile organic compounds that are released during the manufacture of rubber articles. Analysis of AMS by high pressure liquid chromatography and 29 Si NMR shows that it has a plethora of structures present that can best be described as a highly condensed opened oligomeric, branched arrangement of silsesquioxanes units. This structure is moisture stable, hydrocarbon soluble, and reactive with silica at compounding temperatures. The AMS has been prepared by acid-or base-catalyzed hydrolysis and condensation in alcohol solution to form a product that often separates from the lower density reaction mixture. The kinetics of the AMS formation was examined and showed two separate rate processes occurring. The co-AMS prepared from OTES and 3-mercaptopropyl trimethoxysilane yielded an effective shielding agent and coupling agent when used in silica-filled vulcanizates.

A technique is described allowing a relatively simple determination of χ as a function of vr from swelling in heptane. A good measure of the true νe of the cured elastomer at all values of vr was demonstrated by substituting this relationship for χ in the Flory-Rehner (F-R) equation. The relationship was established over a wide range of vr values by using samples that had the νe of the cured elastomer determined by tensile retraction (TR). Applying this function to samples treated using the thiol probe method of Campbell gave an improved measure of the types of crosslinks present in sulfur-cured stocks. An identical equation describing χ as a function of vr in heptane was obtained with NR, EPDM and SBR containing up to a 0.31 volume fraction of carbon black (CB) and other fillers. The presence of up to 10 % of clay, talc, silica, resins or metal oxides in the CB had no noticeable effect on the relationship measured. However, when the filler contained about 50% silica a distinctly different slope in the relationship was found. The percent S1, S2 and Sx distribution measured was contrasted between measurements made by 13C NMR, swelling with χ = constant or χ as a function of vr.

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The identification of accelerators in rubber vulcanizates has traditionally been a difficult and tedious task. The mass spectrometric method described in this paper offers a simple and expedient approach to the identification of 2-mercaptobenzothiazole sulfenamides. Neither complicated sample preparation nor solvent extraction of the vulcanizates is required as in the more common methods employed. Only about one-half gram of vulcanizate is needed for the analysis. The sample tube is attached to the mass spectrometer, heated for 15–20 min at 150–200° C and then the mass spectrum of the volatiles is recorded. The characteristic amine fragment ions are utilized for identification of the amine portion while the presence of benzothiazole is indicated by a peak at m/e=135. Spectra obtained from analysis of seven vulcanizates containing different accelerators are presented and the results discussed. The identification of the accelerator in a competitor's tread is also discussed.