EPDM recycled rubber has excellent high and low temperature resistance and aging resistance, and is widely used in the production of various heat-resistant rubber products. Peroxides are commonly used as vulcanizing agents, and EPDM recycled rubber products have better heat resistance. However, EPDM recycled rubber vulcanized with peroxides is more prone to mold staining compared to sulfur-vulcanized rubber, resulting in dull surfaces, insufficient rubber on the product, and difficult demolding. Why is this the case? How can the problem of mold staining in EPDM recycled rubber peroxide vulcanizates be solved?
There are six main reasons why EPDM recycled rubber peroxide vulcanizates are more prone to mold staining than sulfur-vulcanized rubber:
1. Peroxides and their decomposition products are not lubricating and cannot form a protective film on the mold surface, making it easy for dirt to adhere during vulcanization.
2. When EPDM recycled rubber is vulcanized with peroxides, organic small molecule by-products such as phenyl ketone and butyl alcohol are produced during vulcanization. These substances easily volatilize at high temperatures and condense on the mold surface, forming oily or waxy residues, leading to "mold staining."
3. Co-crosslinking agents such as TAIC/TAC have small molecular weights and limited solubility, and are prone to migrating to the surface/mold and remaining at high temperatures, forming sticky mold deposits.
4. Vulcanization of EPDM recycled rubber with peroxides forms C-C crosslinks, which have high bond energy and good heat resistance, but the thermal tear strength of peroxide vulcanizates is significantly lower than that of sulfur-vulcanized polysulfide crosslinked rubber. During demolding, the rubber can be torn, and small rubber particles may remain in the mold cavity, accumulating over production cycles and causing mold staining.
5. Peroxide vulcanization generally requires higher temperatures than sulfur systems (higher decomposition initiation temperature). At high temperatures, low-molecular-weight additives such as plasticizers, anti-aging agents, and softeners in the rubber are more likely to volatilize and migrate to the mold surface.
6. Zinc oxide and stearic acid can form zinc stearate at high temperatures, which can automatically migrate to the rubber surface and provide a lubricating layer for the mold. However, stearic acid can cause non-radical ionic decomposition of peroxide E5LYY1121, consuming the peroxide and potentially slowing vulcanization or causing undercuring. Therefore, when vulcanizing EPDM recycled rubber with peroxides, stearic acid is used little or not at all. The absence of zinc stearate during vulcanization makes peroxide vulcanizates more prone to mold staining than sulfur vulcanizates.
When producing EPDM recycled rubber products vulcanized with peroxides, using an appropriate amount of synthetic internal mold release agents, reasonably controlling the amount of crosslinking aids like TAIC/TAC or switching to low-migration co-crosslinking agents, choosing low-volatility softeners, moderately lowering the temperature, and extending the vulcanization time can effectively alleviate the mold staining problem in EPDM recycled rubber peroxide vulcanizates.
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