In the rubber processing industries, the rubber mold scaling is a common phenomenon. The deposit will be formed on the mould wall surface in the curing process, and accumulates gradually in the later productive cycle. The earlier document have elaborated the influent on the kinds of factors of the mould scaling . Now, it have been discovered that as to the kinds of sulfide (and zinc oxide) in the polymer rubber mixture, the zinc sulfide is the most annoying response by-product to cause the scaling in the curing process. Not one-and-a-half kinds of permanent release agents or permanent (metal) coating can avoid such kind of deposition. The conclusion is that the scaling is at first caused by zinc sulfide (inorganic deposit) which stick on the mould, and then forms a gray deposit. As the temperature function, the low molecular ingredient in the mixture which adheres to the zinc sulfide minicrystal causes the second stage deposition (organic deposits). It forms the oxidation product within certain amount of time and causes the carbon deposition.
By understanding the formed intrinsic mechanism of zinc sulfide minicrystal on the metal surface and the causation of the rubber mould scaling. It may get a suitable method to the present processing treating processes and to reduce the formation of the zinc sulfide, thus to prevent the rubber mold scaling. Based on the scaling origin research, it is very possible to reduce such kind of phenomenon.
There are two possible solution to prevents or reduce the scaling formation: to change the ingredient of the mixture or to improve the mold's surface.
Changing the ingredient of the mixture to reduce reduction mold scaling
The mold scaling which caused by the zinc oxide or the sulfur chemical combination must be reduced or eliminated. Most of the deposit have the relation with the high content sulfide and zinc oxide, but these ingredients usually need to apply in the tire rubber product. According to the volumeter, tire is the biggest rubber product (achieves 75%) in the world. Therefore, the majority experiments are done with the blend of nr/br compound and sbr compound which are commonly used in the tire production. In the respect of reducing the mould scaling by changing the ingredient of the mixture ,some influence have been inspected ,such as the zinc sulfide influence, the short-term curing experiment and the compound ingredient influence.
The mechanism of mold scaling
the following conclusion can be gotten from the zinc sulfide crystal formed on the iron mold surface : the original formation of the zinc sulfide in the mixture is the resultant of the curing process; The zinc sulfide crystal with nanometer size proliferate to the oxidized surface of the hematite profiled iron pattern and form znsfe coating; Either in the identical curing process or in the following curing process, this crystal seed body grows further after the extra nanometer size zinc sulfide crystal deposition and form the micron size zinc sulfide crystal.
Reducing the level of the zinc oxide in the rubber mixture can decrease the rubber mold scaling; using the nanometer size zinc oxide may eliminate or reduce the mold scaling, however, in order to achieve the most superior machinery and power performance, more research must be conducted; Other metal oxide catalyst cannot solve the mold scaling problem; the following catalyst are in choice such as amine or mfas, they can also reduce the mold scaling, however, these catalyst/accelerator has the ill-smelling flavor and some of them are virulent. In addition, in order to obtain the suitable curing speed, other zinc catalyst (zbec) must also be used .
The solution to the mould scaling does not involve the electrochemistry response; The metal plating does no help to the mold scaling reduction , the mold surface must form a seal barrier; The plastic coating, like ptfe, can solve the scaling. But, due to the handwork laying and instability spatial, these coating cannot apply to the tire mold; The magnetite (fe3o4) coating is possible candidate. the outcome should be promising, but in order to confirm the magnetite (fe3o4) coating ability on preventing the scaling, experiment must be carried on on the tire mold.
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