In the rubber processing industry, the problem of mold scaling is a common phenomenon. In the process of vulcanization, a layer of sediment is formed on the wall of the mold and accumulates gradually in the subsequent production cycle. Previous literature has discussed the effects of various factors that cause mold scaling. It is now found that zinc sulfide is the most annoying reaction by-product of scaling in vulcanization process for various sulfides (and Zinc Oxide) contained in polymer rubber mixture. No semi - permanent demoulding agent or permanent (metal) cover can avoid this deposition. The conclusion is that the scaling is initially caused by zinc sulfide (inorganic sediments) attached to the mold and formed a gray sedimentary layer. As a function of temperature, the composition of low molecular weight in the mixture is attached to the microcrystal of zinc sulfide and causes deposition (organic deposition) in the second stage. The oxidation products are formed in a certain period of time, and the deposition of carbon is caused.

By understanding the causes of mold scaling and the internal mechanism of the formation of zinc sulfide microcrystals on metal surface, it is possible to produce a suitable way for the current processing process, so as to reduce the formation of zinc sulfide and prevent mold scaling. It is possible to reduce this phenomenon by the study of the cause of scaling.

There are two possible solutions to prevent or reduce the formation of dirt: change the composition of the mixture or improve the surface of the mold.

Change the composition of the mixture to reduce the scale of the mold

The mold scale caused by Zinc Oxide or vulcanization must be reduced or eliminated. Most of the sediments are associated with high content of sulphides and Zinc Oxide, which are often used in tire rubber products. According to the volume, the tire is the largest of the global rubber products (up to 75%). As a result, most of the experiments are made with a mixture of NR/BR compounds and SBR compounds used in tire production. The influence of zinc sulfide, the short-term vulcanization experiment and the composition of the compound are investigated by changing the composition of the mixture to reduce the scale of the mold.

The determination of zinc sulfide

This study begins with a review of the formation of zinc sulfide, which is the source of the initial fouling. The vulcanization experiment shows that zinc sulfide is formed on the surface of the metal. By amplifying the sediment of the inner plug-in by magnifying the microscope 1000 times, we can identify the visible initial microcrystal and then analyze it with RMA method (Rontgenmicroanalysis), as shown in Figure 1. The RMA element analysis detected the presence of zinc and sulfur. According to the ratio of sulfur and zinc detected, it is concluded that microcrystals are mainly composed of dissolvable zinc sulfide (Figure 2). In order to determine the existence of zinc sulfide, a physical analysis method (AP-TPR) is used to analyze the content of H2S in the vulcanized mixture (indirect method). A molding vulcanization test was used to determine the formation process of zinc sulfide in the presence of iron. The experiment was carried out in a closed pipe at 200 and no oxygen conditions. The test tube contained thirty isoalkane, Zinc Oxide, sulfur and high surface area element iron. In this experiment, zinc sulfide was also detected with the aid of RMA. As expected, the results of zinc sulfide formation were found in all two experiments. However, there is no evidence that zinc sulfide is formed on the interface between mixture and mold, or ZnS is formed as a by-product of zinc and sulfur reaction during sulfuration.

In order to determine the content of zinc sulfide in rubber mixture, another method was applied. The molded rubber was grinded at low temperature and made into small particles, then extracted with acetone, and treated with a mixture of hydrochloric acid and acetic acid, and the metal sulfide was decomposed. The produced hydrogen sulfide was absorbed by the cadmium acetate buffer solution, and the cadmium sulfide was determined by the iodic method. In addition, the extracted rubber is hydrolyzed in the microwave oven in sulfuric acid and nitric acid. The hydrolysate was scanned by ICP-ES.

It is concluded from these results that zinc sulfide is formed as a reaction product of Zinc Oxide and sulfur. In vulcanization production, this product is useful and useful for the formation of zinc sulfide microcrystals between the rubber products and the mold surface.

The most acceptable assumption is that zinc sulfide is formed as a reaction product of Zinc Oxide and sulfur. This common chemical reaction is described in various rubber manuals. A simplified reaction mechanism is:

2RH+Sx+ZnO+ (catalyst) R-S (x-1) -R+ZnS+H2O

Most of the 100 parts of the tire mixture contain 5 copies of Zinc Oxide and about 2 portions of sulfur. For a tire mixture, it can be calculated: a formula based on 100 parts of rubber (about 175 total), containing 2.8% (weight) Zinc Oxide and 1.1% (weight) sulfur. It is calculated from the reaction formula that about 0.6 grams of zinc sulfide is generated for every 1 grams of Zinc Oxide. It is clear that a considerable amount of zinc sulfide can be produced. In fact, only zinc sulfide, which exists in the upper layer of the tire, is a microcrystal of zinc sulfide (probably caused by a metal surface). About 500 times of die pressure can be performed before the mold must be cleaned.