Zinc oxide in reclaimed rubber products can be used not only as a vulcanizing activator but also as a reinforcing agent, colorant, and thermal conductor. Reclaimed rubber itself contains some zinc oxide residues in the original rubber formula. Improper control of zinc oxide dosage or incorrect addition can lead to quality issues such as insufficient vulcanization, whitening of the product, accelerated thermal oxidation aging, or decline in physical and mechanical properties. So, what issues should be noted when using zinc oxide in reclaimed rubber products?
1. Pay attention to the activity and dispersibility of zinc oxide
When using zinc oxide as a vulcanizing activator in reclaimed rubber products, attention must be paid to the activity and dispersibility of zinc oxide. Common zinc oxides on the market include indirect zinc oxide, direct zinc oxide, active zinc oxide, and nano zinc oxide. In the production of reclaimed rubber products, priority is given to activity and dispersibility, rather than pure purity. Due to its large specific surface area and high activity, active zinc oxide in reclaimed rubber formulations can be used 20%-30% less than ordinary zinc oxide, with better dispersion and less likely to agglomerate.
2. Strictly control the amount of zinc oxide added to reclaimed rubber products
Zinc peroxide is added during the production of waste tires and waste latex products, and a considerable proportion remains in the reclaimed rubber after desulfurization; Therefore, when designing reclaimed rubber product formulations, this residual content needs to be E5LYY1219 included. Based on 100 parts reclaimed rubber, add 2-3 parts more zinc oxide to tire reclaimed rubber formulations, 3-4 parts zinc oxide to latex reclaimed rubber formulas, 4-5 parts zinc oxide to butyl reclaimed rubber formulas, and 3-5 parts zinc oxide to the EPDM reclaimed rubber formula.
Excessive zinc oxide usage not only increases the cost of recycled rubber products, but unreacted zinc oxide forms excess zinc soap with stearic acid, which migrates to the surface, leading to issues such as frosting, whitening, and reduced adhesion in reclaimed rubber products. When using active zinc oxide or nano zinc oxide in the formulation of reclaimed rubber products, the amount can be reduced to 40%-70% of that of ordinary zinc oxide, with attention to precipitation and scorching.
3. Pay attention to the ratio of zinc oxide to stearic acid in the reclaimed rubber formula
In the production of reclaimed rubber products, zinc oxide reacts with stearic acid during mixing to form zinc stearate, which can significantly improve the solubility of zinc oxide in the compound and form more active complexes with accelerators, truly exerting vulcanization and activation. Typically, the dosage of stearic acid is 60%-75% of the zinc oxide dosage. Too little stearic acid can lead to incomplete zinc oxide activation and reduced vulcanization efficiency of reclaimed rubber; Excessive stearic acid can affect the tensile stress of the vulcanized reclaimed rubber and may even cause frosting.
4. Pay attention to the timing of zinc oxide addition during reclaimed rubber mixing
The feeding sequence directly affects the quality of the reclaimed rubber mixed. Zinc oxide particles tend to carry negative charges on the surface and exhibit electrostatic repulsion with the reclaimed rubber substrate, resulting in poor dispersibility in the compound. It is recommended to add zinc oxide after thorough plasticizing of the reclaimed rubber and before adding a large amount of filler. This ensures that the zinc oxide is fully encapsulated by the rubber and avoids excessive temperature rise caused by excessive shearing. When mixing reclaimed rubber, adding zinc oxide at the initial stage significantly shortens the compound's scorching time and increases the risk of early vulcanization; Adding zinc oxide in the later stage of mixing causes the compound to form aggregates due to its high viscosity and small free volume, resulting in localized loss of activity.
The correct feeding sequence for reclaimed rubber mixing is as follows: plastic rubber → stearic acid→ zinc oxide→ small materials→ fillers→ softeners→ sulfur. Zinc oxide should avoid being added in batches with acidic additives.
When using zinc oxide in reclaimed rubber products, it is essential to select the appropriate zinc oxide based on actual needs and reasonably determine the amount of zinc oxide added. We will continue to share key points in the formulation design and production process of reclaimed rubber products in the future.
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