Solution to scaling problems in rubber molds

In the rubber processing industry, mold scaling is a common phenomenon. A deposit is formed on the mold wall during the vulcanization process and gradually accumulates in subsequent production cycles. The previous literature discusses the effects of various factors that cause mold fouling. It has now been found that zinc sulfide is the most annoying reaction by-product of fouling during vulcanization for the various sulfides (and zinc oxide) contained in the polymer rubber mixture. There is no semi-permanent release agent or permanent (metal) coating to avoid this deposition. The conclusion is that the scale is initially caused by zinc sulfide (inorganic deposits) attached to the mold and forms a gray deposit. As a function of temperature, the low molecular weight components of the mixture adhere to the microcrystals of zinc sulfide and cause a second stage of deposition (organic deposition). An oxidation product is formed for a certain period of time and causes deposition of carbon.
By understanding the cause of mold fouling and the inherent mechanism of zinc sulfide microcrystal formation on the metal surface of the mold, it is possible to create a solution to the current processing to reduce the formation of zinc sulfide and thereby prevent mold fouling. It is quite possible to reduce this phenomenon by studying the cause 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
Mold scale due to zinc oxide or vulcanization must be reduced or eliminated. Most of the deposits are associated with high levels of sulfides and zinc oxide, which are often used in tire rubber products. Tires are the largest of the world's rubber products by volume (up to 75%). Therefore, most of the experiments were carried out using a mixture of NR/BR compounds and SBR compounds commonly used in tire production. In terms of reducing mold scale by changing the composition of the mixture, the effects of zinc sulfide, short-term vulcanization experiments, and the effects of compound components were examined.
◆ Determination of zinc sulfide
This study began by examining the formation of zinc sulfide, which is the source of the initial fouling. Vulcanization experiments have shown that zinc sulfide is formed on the metal surface. The deposits of the insert were examined by a microscope magnified 1000 times to determine the initial microcrystals visible and then analyzed by the RMA method (Rontgen microanalysis) as shown in FIG. RMA elemental analysis detected the presence of zinc and sulfur. Based on the detected ratio of sulfur to zinc, it is concluded that the crystallites are mainly composed of insoluble zinc sulfide (Fig. 2). To determine the presence of zinc sulfide, a physical analytical method (AP-TPR) was used to analyze the H2S content of the vulcanized mixture (indirect method). A molding vulcanization test was conducted to determine the formation of zinc sulfide in the presence of iron. The test was carried out in a closed pipe at 200 ° C and anaerobic conditions. The test tube contained iso-triacontane, zinc oxide, Sulfur and high surface area elemental iron. In this experiment, zinc sulfide was also detected by means of RMA. As expected, both experiments yielded the results of zinc sulfide formation. However, there is no evidence that zinc sulfide is formed at the interface of the mixture and the mold, or that ZnS is formed as a by-product of the reaction of zinc and sulfur during the vulcanization process.
In order to determine the content of zinc sulfide in the rubber mixture, another method was applied. The molded rubber is ground at a low temperature to form small particles, which are then extracted with acetone and treated with a mixture of hydrochloric acid and acetic acid, and the metal sulfide is decomposed. The generated hydrogen sulfide was absorbed by a cadmium acetate buffer solution, and the formed cadmium sulfide was measured by an iodometric method. Further, the rubber to be extracted is hydrolyzed in a microwave oven in sulfuric acid and nitric acid. The hydrolyzate was subjected to elemental scanning using 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 present and useful for the formation of zinc sulfide microcrystals between the rubber product 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 tire mixes contain 5 parts zinc oxide and about 2 parts sulfur per 100 parts. For a tire mixture, one can be calculated: a formulation based on 100 parts of rubber (a total of about 175 parts) containing 2.8% by weight of zinc oxide and 1.1% by weight of sulfur. From the reaction formula, it is calculated that about 0.6 g of zinc sulfide is generated per 1 g of zinc oxide. Obviously, a considerable amount of zinc sulfide can be produced. In fact, only the zinc sulfide present in the upper layer of the tire is the crystallite of zinc sulfide (possibly caused by the metal surface). Molding can be performed approximately 500 times before the mold must be cleaned.
◆Experiment
It is known that the insert (small metal sheet) used as the surface of the mold can in principle be easily analyzed by the RMA method for containing zinc sulfide microcrystals, but this is a rather expensive test. Therefore, a simple test method was developed to determine the initial (visible) crystallites on the insert. With a 500-magnification optical microscope, a single crystallite of 0.5 to 1 micron can be seen. In order to produce crystallites, vulcanization experiments were carried out on different mixtures, at different temperatures and times. Two mixtures were selected and shown in Table 1, including an intermediate mix of s-SBR based tire face mix and NR/BR based mixture. Both mixes were used as masterbatches for various vulcanization experiments.
For the vulcanization experiment, a simple stamper was fabricated for up to 8 inserts, which were fabricated for short-term experiments of up to 20 cycles. The insert was visually inspected after every 5 consecutive cycles. Thus, various parameters, such as mixture parameters for different additives, or parameters regarding the insert - such as metal selection, roughness, or plating, can be detected. Preliminary experiments were carried out on two basic tire mixes, the tire mix was vulcanized at 160 ° C for 20 minutes and at 200 ° C for 2 minutes (data calculated from the rheometer curve). The results showed that there was no difference in the number of zinc sulfide microcrystals found by visual inspection. For the compression time, all further experiments were carried out at 200 °C. After 20 cycles of vulcanization experiments in a short period of time, further experiments were carried out by injection molding, up to 500 vulcanizations.
◆The effect of the composition of the mixture
▲Zinc choice
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 a zinc-containing component) and sulfur. It is not easy to remove sulfur or zinc sulfide from the formulation. Because both components are required in the rubber formulation. Natural sulfur can increase mechanical strength and bonding, while zinc oxide activates the vulcanization system.
Rheometer experiments show that the level of zinc oxide can be reduced from 5 parts to 3 parts (per 100 parts of rubber) without changing the maximum torque of the rheometer, and can be reduced by almost a factor of two. However, even with this decrease in level, there was no significant change in the deposit of zinc sulfide (Table 2). Similarly, substitution with a smaller particle size zinc oxide has no significant difference in deposits compared to the use of zinc oxide (RS).
However, by replacing 0.25 parts of nano zinc oxide with zinc oxide having a 40 nm particle size (the same rheometer maximum), the zinc oxide level can be reduced by a factor of 20, and the difference in deposits is significant. With the short-term vulcanization experiment of nano zinc oxide, the number of compression moldings was up to 20 cycles, and no zinc sulfide deposition was shown.