Effect of Tackifier Addition on Cushion Compound Formulation for Tire Retreading Application

Tire retreading is a prospective industry. Old tires are repaired and retreaded with suitable tread compounds to fulfill the requirement as the new ones. One of the important components in tire retreading process is cushion compound. Cushion compound consists of unsaturated rubber, in this case natural rubber Hevea brasiliensis was used, less phr of filler compared to the retread compound, and additives such as peptizer, tackifier, processing oil, antioxidant, activator, accelerator and curatives. Tackifier is an important component in cushion compound since its role to make a bonding between different layer, the initial tire after buffing and new retread layer. Tackifier should has good resistance, good compatibility and does not affect the rheological and dynamical properties of bonded rubber. The general tackifier that used in industries are hexamethyl tetramine as methylene donor and resorcinol as methylene acceptor. There is certain reaction between those two additives that determine how good the performance of cushion compound and its effect to retreading process. To obtain optimum reaction, comparison between resorcinol and hexamethyl tetramine were varied as 1:1 (FRR1), 1:2 (FRR2) and 1:3 (FRR3). Hardness test, compression test, rebound resilience, tensile and tear strength, and FTIR were done to observe the optimum variation for retread application. Compared to the control with no tackifier at all, FRR2 showed the optimum result with 21.75 MPa (min. 19 Mpa) and 454,54% elongation at break (min. 450%). The most interesting result was observation by using FTIR, it was detected that the crosslink density was significantly higher than other formulation. It is a new breakthrough which is minimum tackifier with certain treatment could give better performance.

Mooney Viscosity Stability and Polymer Filler Interactions in Silica Filled Rubbers

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.

Sulfur Vulcanization of Simple Model Olefins, Part II: Characterization and Reactivity of Intermediate Vulcanization Products of 2,3-Dimethyl-2-Butene

In a study of the mechanism of the sulfur vulcanization of unsaturated rubber, 2,3-dimethyl-2-butene (C6H12) was used as a simple, low-molecular model alkene. Only equivalent allylic positions are present in this alkene. Treating C6H12 with a mixture of ZnO, S8 and the accelerator tetramethylthiuramdisulflde at 140°C for 20 minutes yields a mixture of addition products (C6H11—Sn—C6H11) and also intermediate products (C6H11—Sn—S(S)CN(CH3)2). The formation of C6H11—Sn—C6H11 from these intermediate products only proceeds in the presence of the zinc dimethyldithiocarbamate complex and free alkene.

Mechanisms of Antiozonant Protection: Antiozonant-Rubber Reactions during Ozone Exposure

Rubber (IR or BR) vulcanizatcs containing DOPPD antiozonant have been aged under ozone, and the soluble constituents have been removed by acetone extraction. Small amounts of nitrogen from the antiozonant remain in the rubber after extraction; this nitrogen is presumed to be chemically attached to the polymer network. Some of this “unextractable nitrogen” may be due to reaction of ozonized rubber with antiozonant, but other explanations are possible. The amount of unextractable nitrogen in the vulcanizate before ozone aging is about the same, within experimental error, as the amount remaining after static ozone aging. Model compound experiments (with cis-9-tricosene or squalene) show that chemical reactions can take place between the ozonized olefins (or unsaturated rubber) and DOPPD. The chemical structure for a model reaction product has been determined. Despite these findings, it seems unlikely that these types of reactions will play a significant role in the overall protection of rubber vulcanizates from ozone attack. That is, these products can form only after the rubber starts to become ozonized, and it has been shown that ozone attack on the rubber does not occur until after the antiozonant is nearly completely consumed. The results of this work are consistent with previous studies in our laboratory that indicate a combined “scavenger-protective film” mechanism is principally responsible for antiozonant protection. There is no definitive evidence to date to show that ozonized rubber reacts with antiozonant during ozone aging of rubber vulcanizates.

Infrared Spectra of Reaction Products of Natural Rubber with Alkylphenol Formaldehyde Resins

Infrared spectroscopic studies were carried out with reaction products of natural rubber with 101K tertiary butylphenol-formaldehyde resin without activators and in the presence of stannous chloride dihydrate and zinc oxide. No reduction was observed in unsaturated rubber content during reaction with resin. Reaction of resin with rubber probably takes place at the α-carbon atom and not at the double bonds. A reduction in the proportion of resin phenol hydroxyl is basically due to its participation in recombination reactions of radicals formed during breakdown of ether bridges of resin. At 150° C activators accelerate decomposition of resin ether bridges, but increase the stability of phenol hydroxyl. In the case of zinc oxide, phenol hydroxyl is sufficiently resistant even at 180° C.

The Mechanism of Accelerator Action

In the year 1839, Goodyear discovered that, on treatment with sulfur at elevated temperatures, natural rubber loses its plasticity and becomes elastic. Around the time of World War I, it was discovered that the reaction of the sulfur with the unsaturated rubber hydrocarbons could be promoted by the action of so-called vulcanization accelerators. The reaction then takes place not only faster and at lower temperatures but, in addition, one also needs less sulfur and the vulcanizates acquire considerably better technical properties. Accordingly a tremendous number of compounds have been examined as accelerators of vulcanization and much effort has been applied to develop accelerators with the most satisfactory properties. Today, accelerators are known which permit vulcanization at room temperature. In the past, the mechanism of accelerator action has been studied mostly with the methods of organic and physical chemistry. In the following, an attempt will be made to consider the problem from the viewpoint of the inorganic chemist. One can attempt to interpret the catalysis from two points of view. Either the reactivity of the unsaturated natural rubber hydrocarbon is enhanced, or the sulfur will be converted into a more reactive form. Twiss concluded from his investigations that the latter possibility is the more likely. Subsequently, this assumption has been supported by further observations. In particular, the discovery of Peachey pointed in this direction, since natural rubber is vulcanizable at room temperature if it is treated successively with hydrogen sulfide and sulfur dioxide gases. The sulfur which is formed by this reaction is in the nascent state and is very reactive.

Isoprene and Rubber. Part 30. Hydromethylrubber

When rubber is reduced at 270° under high pressure, a hemi-colloidal hydrorubber is obtained, and it was proved by Geiger and Huber that the product has a higher molecular weight and is less cyclicized if a good catalyst is used in large quantity (for example, active nickel produced by the method of Kelber), while according to the original experiments of Fritschi, who carried out the hydrogenation in the presence of very little platinum, a more degraded and somewhat cyclicized hydrorubber is obtained. The saturated hydrorubber is much more stable than the unsaturated rubber since the loosening action of the double bonds is lacking. A hydrorubber of the average molecular weight of 10,000 is still relatively stable at 270°, while a hemi-colloidal rubber with this molecular weight will he cracked to still smaller fragments at this temperature, and these fragments are then changed by cyclicization. This behavior can be clearly seen in methylrubber. The following reduction proves that it is even more easily decomposed than rubber itself. With nickel as catalyst, Geiger obtained from methylrubber by reduction at 270° and 100 atmospheres a hemi-colloidal hydromethylrubber which had an average molecular weight of 1600 and therefore had a degree of polymerization of about 20. If rubber is reduced under the same conditions a higher molecular product is obtained with an average molecular weight of 3000 to 10,000. Judged by reduction experiments, the chain of butadiene rubber is still more stable, since the hydrobutadiene rubber prepared under the same conditions had the highest average molecular weight. The cleavage of the chains, as in the following formula, is therefore favored by the methyl groups: