Solution to scaling problem of rubber mold

In rubber processing industry, mold scaling is a common phenomenon. During the curing process, a layer of sediment formed on the mold wall and gradually accumulated in the subsequent production cycle. Previous literature has discussed the influence of various factors that cause mold scaling. It is now found that for various sulfides (and zinc oxide) contained in the polymer rubber mixture, zinc sulfide is the most annoying reaction by-product causing scaling during the curing process. No semi-permanent release agent or permanent (metal) coating can avoid this kind of deposition. The conclusion is that scaling is initially caused by zinc sulfide (inorganic deposit) attached to the mold and forms a gray deposit layer. As a function of temperature, low molecular weight components in the mixture attach to the microcrystals of zinc sulfide and cause the second stage of deposition (organic deposition). Oxidation products are formed within a certain period of time and cause carbon deposition.
By understanding the causes of mold scaling and the internal mechanism of the formation of zinc sulfide microcrystals on the metal surface of the mold, it is possible to produce a method suitable for the current processing process to reduce the formation of zinc sulfide and prevent mold scaling. It is very 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 mold.
Change the composition of the mixture to reduce mold scale
The mold scale caused by zinc oxide or vulcanization must be reduced or eliminated. Most of the deposits are related to high content of sulfide and zinc oxide, which are usually used in tire rubber products. By volume, tire is the largest rubber product in the world (up to 75%). Therefore, most of the experiments are carried out with the mixture of NR/BR compound and SBR compound commonly used in tire production. In the aspect of reducing mold scale by changing the composition of the mixture, the influence of zinc sulfide, short-term vulcanization experiment and the influence of compound composition were investigated.
◆ Determination of zinc sulfide
This study begins with the investigation of the formation of zinc sulfide, which is the source of the original dirt. The vulcanization experiment showed that zinc sulfide was formed on the metal surface. Detect the deposit of the insert with a 1000-fold magnification microscope to determine the visible initial microcrystalline, and then analyze it with the RMA method (Rontgen microanalysis), 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 insoluble zinc sulfide (Fig. 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 mixture after vulcanization (indirect method). A molding vulcanization experiment is used to determine the formation process of zinc sulfide in the presence of iron. The experiment is carried out in a closed pipeline at 200 ℃ and without oxygen. The test tube contains isotriacontane, zinc oxide, sulfur and high surface area element iron. In this experiment, zinc sulfide was also detected by RMA. As expected, both experiments showed the formation of zinc sulfide. However, there is no evidence that zinc sulfide is formed at the interface between the mixture and the mold, or that ZnS is formed as a by-product of the reaction of zinc and sulfur during the curing process.
In order to determine the content of zinc sulfide in the rubber mixture, another method was applied. Molded rubber is ground at low temperature and made into small particles, which are extracted with acetone and treated with a mixture of hydrochloric acid and acetic acid. The metal sulfide is decomposed. The generated hydrogen sulfide is absorbed by cadmium acetate buffer solution, and the formed cadmium sulfide is determined by iodometry. In addition, the extracted rubber is hydrolyzed in sulfuric acid and nitric acid in a microwave oven. The hydrolysate was scanned by ICP-ES.
From these results, it can be concluded that zinc sulfide is formed as a reaction product of zinc oxide and sulfur. In vulcanization production, this reaction product is available and useful for the formation of zinc sulfide microcrystals between rubber products and mold surfaces.
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 tyre mixtures contain 5 parts zinc oxide and about 2 parts sulfur per 100 parts. For a tire mixture, it can be calculated that a formula based on 100 parts of rubber (about 175 parts in total) contains 2.8% (weight) zinc oxide and 1.1% (weight) sulfur. From the reaction formula, it can be calculated that about 0.6 grams of zinc sulfide will be generated for each gram of zinc oxide. Obviously, a considerable amount of zinc sulfide can be produced. In fact, only zinc sulfide in the upper layer of the tire is microcrystalline zinc sulfide (possibly caused by the metal surface). Before the mold must be cleaned, about 500 times of molding can be performed.
◆ Experiment
It has been known that the insert (small metal sheet) used as the surface of the mold can, in principle, easily analyze whether it contains zinc sulfide microcrystals through the RMA method, but this is a very expensive test. Therefore, a simple test method was developed to determine the initial (visible) microcrystals on the insert. With the aid of an optical microscope with a magnification of 500, single microcrystals of 0.5 to 1 micron can be seen. In order to produce microcrystals, vulcanization experiments were carried out with different mixtures, different temperatures and different times. Two mixtures were selected and shown in Table 1, including the mixture based on s-SBR tread and the intermediate mixture based on NR/BR mixture. Both mixtures are used as the base compound for various vulcanization experiments.
In order to carry out the vulcanization experiment, a simple press mold is manufactured, which is suitable for up to 8 inserts. This press mold is manufactured for a short-term experiment of up to 20 cycles. Visually inspect the insert after every 5 consecutive cycles. In this way, various parameters, such as mixture parameters of different additives, or parameters related to inserts, such as metal selection, roughness, or coating, can be detected. Preliminary experiments were carried out on two basic tire mixtures. The tire mixture was vulcanized at 160 ℃ for 20 minutes and at 200 ℃ for 2 minutes (the data was calculated from the rheometer curve). The results showed that there was no difference in the number of zinc sulfide microcrystals found by visual inspection. In order to compress the time, all further experiments were carried out at 200 ℃. After 20 vulcanization experiments in a short time, further experiments are carried out with the help of injection molding, up to 500 vulcanizations.
◆ Influence of mixture composition
▲ Selection of zinc
Experiments were carried out on the composition of the mixture. As has been shown, zinc sulfide is formed as a reaction product of zinc oxide (or zinc-containing component) and sulfur. It is not easy to remove sulfur or zinc sulfide from the formula. Because these two components are necessary in rubber formula. Natural sulfur can improve mechanical strength and bonding, while zinc oxide can activate the curing system.
The rheometer experiment shows that without changing the maximum torque of the rheometer, the level of zinc oxide can be reduced from 5 parts to 3 parts (every 100 parts of rubber), which can be almost reduced by a factor of 2 times. However, even with the decrease of this level, the deposit of zinc sulfide still has no obvious change (Table 2). Similarly, there is no obvious difference between the use of zinc oxide (RS) and the replacement of zinc oxide with a smaller particle size.
However, using 0.25 parts of nano-zinc oxide instead of zinc oxide with a particle size of 40 nm (the maximum value of the same rheometer), the level of zinc oxide can be reduced by 20 times, and the difference in sediment is obvious. In the short-term vulcanization experiment with nano-zinc oxide, the number of times the die was used up to 20 cycles, and there was no zinc sulfide deposition.