Scaling of rubber mould is a common phenomenon in rubber processing industry. During the vulcanization process, a layer of sediment formed on the die wall and accumulated gradually in the subsequent production cycle. Previous literatures have discussed the effects of various factors causing scaling of rubber moulds. It is now found that zinc sulfide is the most annoying by-product of scale formation during vulcanization for various sulfides (and zinc oxide) contained in the polymer rubber mixture. No semi-permanent release agent or permanent (metal) coating can prevent this deposition. The conclusion is that the scaling is initially caused by zinc sulfide (inorganic sediment) attached to the die and forms a gray deposit layer. As a function of temperature, the low molecular weight components of the mixture adhere to the microcrystals of zinc sulfide and cause the second stage of deposition (organic deposition). In a certain period of time, oxidation products are formed and carbon deposits are induced.
By understanding the causes of scaling in rubber molds and the internal mechanism of zinc sulfide microcrystal formation on the metal surface of the molds, it is possible to produce a suitable method for the current processing process to reduce the formation of zinc sulfide and prevent scaling in rubber molds. It is possible to reduce this phenomenon by studying the causes of scaling.
There are two possible solutions to prevent or reduce the formation of dirt: changing the composition of the mixture or improving the surface of the mould.
Change mixture composition to reduce scale
Scale caused by zinc oxide or sulphidation must be reduced or eliminated. Most sediments are associated with high sulphide and zinc oxide content, which are commonly used in tire rubber products. By volume, tyres are the largest type of rubber products in the world (up to 75%). Therefore, most of the experiments were carried out with the mixture of nr/br compounds and SBR compounds commonly used in tire production. The effects of zinc sulfide, short-term vulcanization experiments and chemical composition on scale reduction by changing the composition of the mixture were investigated.
Scaling mechanism of mould
It is concluded that the in-situ formation of zinc sulfide in the mixture is the reaction product in the process of sulfurization; the nano-sized zinc sulfide crystals diffuse to the oxidation surface of hematite iron mould to form a layer of znsfe type; or in the same process of sulfurization, or in the follow-up. During the sulfidation process, additional nano-sized zinc sulfide crystals were deposited, which further grew to form micron-sized zinc sulfide crystals.
mixture
Reducing the level of zinc oxide in rubber mixtures can reduce the scaling of rubber moulds; using nano-sized zinc oxide can eliminate or reduce the scaling of rubber moulds, however, in order to achieve excellent mechanical and dynamic performance, more research must be done; other metal oxide catalysts can not solve the scaling problem of rubber moulds; yes, nano-sized zinc oxide can eliminate or reduce the scaling of rubber moulds. Alternative catalysts, such as amines or mfas, can also reduce mold scaling. However, those catalysts/accelerators have an unpleasant smell and some are toxic. In addition, another zinc catalyst (zbec) must be used in order to obtain a considerable sulfidation rate.
coating
There is no electrochemical reaction involved in the solution of mould scaling problem; metal coating is not helpful to reduce mould scaling, and a closed barrier must be formed on the surface of the mould; plastic coating, such as PTFE type, can solve the scaling problem. However, these coatings can not be used in tire moulds due to manual laying problems and spatial instability; magnetite (fe3o4) coatings are potential candidates. The results should be very promising, but in order to verify the ability of magnetite (fe3o4) coating to prevent scaling, experiments must be carried out on tire moulds.
