Hardness is one of the most fundamental and important performance indicators of rubber materials. It not only relates to the tactile feel, support, and durability of the product, but also directly affects downstream customers' perception of product quality. In the actual production process, rubber products with excessive hardness, large fluctuations, and unstable batches are high-frequency causes of customer complaints and internal and external quality problems. Hardness control may seem simple, but it is actually the result of precise collaboration across the entire chain of formula, process, and management system. This article combines practical experience to summarize the five most critical devil details in rubber hardness control, from formula design to vulcanization molding, with a deep analysis of the entire process, providing reference and implementation for quality, quality inspection, and production departments.
Detail 1: The precise design of the filling system in the basic formula is mainly controlled by the type of base rubber and the filling system, among which the filling plays a key role in "skeleton support".
1. Carbon black selection and particle size control
The smaller the particle size of carbon black, the larger the specific surface area, the stronger the reinforcement effect, and the more significant the improvement in hardness. The general rule is as follows: N220 carbon black → with a large increase in hardness (suitable for high hardness products) N330 carbon black → with excellent comprehensive performance and moderate hardness N550 and N660 carbon black → mainly used for soft rubber, improving fluidity, and relatively low hardness
2. Function of white carbon black (silicate filler)
White carbon black can also increase hardness in rubber, but its effects become more complex due to its hygroscopicity and dispersibility. The white carbon black system needs to be used in conjunction with coupling agents (such as Si69), otherwise not only will the hardness be unstable, but it will also cause fluctuations in the vulcanization curve.
3. Strict control over the total amount of fillers
The contribution coefficient of hardness varies among different fillers. Generally speaking, for every 10 phr increase in carbon black, the rubber hardness roughly increases by 3-5 Shore A degrees. However, it should be noted that excessive filler not only increases hardness, but also sacrifices elasticity and flexural flexibility, which needs to be balanced. Practical case reminder: A factory once caused a hardness exceeding the standard by more than 3 degrees due to fine-tuning of carbon black batches (with slightly different particle sizes), leading to large-scale rework. It is reminded to establish dual control standards for particle size distribution and DBP oil absorption value when purchasing raw materials.
Detail 2: Management of subtle changes during the plasticization and mixing stages
The mixing processing technology, especially the plastic dissolution control, is the hidden key that determines the fluctuation of the hardness base. 1. Excessive or insufficient plastic deformation
If the plasticization (natural rubber) is excessive, the molecular chains will break too much, the rubber network will become loose, and the hardness will decrease after vulcanization. On the contrary, insufficient plasticity, poor processing performance of rubber materials, and uneven dispersion of fillers also lead to unstable hardness. 2. Reasonable application of quick refining agents and operating oils
The ratio of the amount of quick refining agent (such as Pepperizer) added to the operating oil (such as AR oil) will significantly affect the initial viscosity and final hardness. An increase in oil content leads to a decrease in plasticity and a decrease in product hardness, but excessive use can cause a decrease in rebound. 3. Mixing temperature and time management
In the mixing stage, it is recommended to control the temperature: the temperature of the first stage should not exceed 135 ℃, and the temperature of the second stage should be about 90-100 ℃. Time control: avoid premature crosslinking (Scorch) caused by long-term mixing at high temperatures, which may affect the final hardness. Attention should be paid to the temperature rise curve of each batch of mixed rubber, and the Max Temp and Energy values of each batch of rubber can be used as important process data for hardness prediction.
Detail 3: Precise balance between vulcanization system and crosslinking density. The vulcanization system is the last key barrier for rubber hardness molding. High cross-linking density, tight rubber molecular network chains, and increased hardness; On the contrary, the hardness decreases. 1. Accurate setting of sulfurizing agent type and dosage. Sulfur system: For every 0.1phr increase in sulfur content, the hardness increases by approximately 1-2 degrees. Peroxide systems (such as DCP systems): Different types of cross-linking bonds (C-C bonds) contribute more to hardness and are suitable for products with high temperature resistance and high hardness requirements. The type and proportion of accelerators (such as CBS, MBTS) in the additive system directly affect the vulcanization rate and crosslinking density. The content of active agents (such as ZnO/Tearic Acid) should be strictly balanced, as excessive levels can lead to early sulfurization and affect hardness consistency. 3. The analysis of Moving Die Rheometer (MDR) data shows that changes in ML and MH values directly reflect the workability and hardness tendency of the rubber compound after vulcanization. T90 (90% vulcanization time) should be strictly standardized, and different types of T90 exceeding the tolerance by ± 2 minutes may cause hardness fluctuations of 2-4 degrees. Case tip: When improving a high hardness nitrile rubber compound, by adjusting the CBS/MBTS ratio (from 1.2/0.3 to 1.0/0.5), the hardness of the product was successfully controlled from 82 ± 3 degrees to within 82 ± 1.5 degrees.