Vulcanized rubber powder is made from waste rubber processed mechanically into powder of different particle sizes. On a microscopic level, it consists of fragments with an interlinked network structure, contains a small amount of unsaturated double bonds, and has re-crosslinking capability. It can be reintroduced into tire rubber compounds to reduce raw material costs. When using vulcanized rubber powder in tire rubber compounds, it’s necessary to strictly control the amount added and promptly adjust the original tire formulation to ensure the finished tire meets performance standards.
1. How vulcanized rubber powder is used in tire rubber compounds
There are two ways to use vulcanized rubber powder in tire compounds: partial blending or using 100% vulcanized rubber powder directly.
(1) Adding rubber powder to tire compounds in a certain proportion
In conventional tire production, rubber powder is usually added in a certain proportion. It cannot replace the original rubber entirely in any part, whether the tread or sidewall. For example, adding 5-15 parts of 60-120 mesh rubber powder to truck tire tread compounds can take advantage of the interfacial properties of the rubber-powder blend, improving fatigue resistance, wear resistance, and low heat buildup, thereby extending tire life.
(2) Using 100% vulcanized rubber powder to produce non-motor vehicle tires or inner-tube vehicle tires’ reinforcement strips
Traditional non-motor vehicle tires typically use natural rubber, but due to scarce resources and high prices, the final product costs are high. Making tires with recycled rubber requires complex processes like waste rubber crushing, desulfurization, and refining, which makes production costly. By crushing waste tires into 30-mesh rubber powder and vulcanizing directly with sulfur, non-motor vehicle tires can be produced. Some tire manufacturers also use 100% vulcanized rubber powder to make the reinforcement strips of inner-tube vehicle tires.
2. Key points of adjusting tire formulations when using vulcanized rubber powder
Tires for different applications have different requirements for the physical properties and vulcanization kinetics of compounds. When adding vulcanized rubber powder, the formulation needs to be appropriately adjusted.
(1) Adjusting physical properties of tire compounds with rubber powder
The core of adjusting physical properties is determining the right proportion of added rubber powder to ensure key performance indicators are met. For example, in tread compounds, wear resistance is critical. The blend containing rubber powder must have wear resistance not lower than the original compound. Other properties like tensile strength and stress at set elongation may decrease slightly but should not drop too much.
(2) Adjusting vulcanization kinetics of tire compounds with rubber powder
The key to adjusting the vulcanization system is to make sure the scorch time and optimal cure time of the mixed compound meet the original standard, otherwise, the degree of curing will be affected. Adjusting the amount of sulfur can correct stress at set elongation, but changing sulfur content also affects cure time, so adjustments need to be done simultaneously.
When blended compounds with rubber powder are vulcanized, sulfur and accelerators in the base rubber migrate directionally to the powder surface due to concentration differences. The more powder added, the more serious the migration, leading to uneven and lower crosslink density at the interface. High-modulus rubber powder increases set stress and reduces elongation at break. If the rubber powder is already vulcanized, it’s difficult to mix thoroughly with the base rubber, reducing mechanical properties.
When adjusting formulations after adding rubber powder, three key points must be kept in mind: the powder proportion should be based on tire type and specific part (no more than 15 parts for truck tread compounds); adjust the vulcanization system according to the powder amount to restore scorch and cure time to baseline; and pay attention to uneven interface crosslink density caused by sulfur and accelerator migration to the powder surface. When using vulcanized rubber powder to cut costs, coordinating adjustments of physical properties and the vulcanization system ensures tire compound performance remains stable while saving costs.
Exclusive original article [commercial authorization] reprint, excerpt and excerpt in any form are prohibited without written authorization. Focus on Hongyun rubber: learn the process formula and raw material technology of producing rubber products from recycled rubber to help you reduce costs and increase profits