Reasons and Solutions for Rubber Frosting
In the rubber processing process, the form of frost spraying on the rubber surface sometimes appears in one form, but sometimes it appears in two or three forms simultaneously.
（1） Spray or seepage of the compounding agent
1. Cause of occurrence
Sulfur is the most commonly used vulcanizing agent for rubber. It is also a highly sprayable compounding agent, and the solubility of sulfur in different rubbers varies. Temperature has a significant impact on the solubility of sulfur. Sulfur is more easily soluble in NR, CR, SBR, and BR, but less soluble in CIIR, IIR, and EPDM. Sulfur has different dissolution states in NR. When the sulfur dosage is less than the saturation solubility, sulfur is in a stable dissolution state. When the sulfur dosage is greater than the saturation and supersaturation solubility, sulfur is in a metastable and unstable dissolution state, respectively. At this time, a decrease in temperature will cause sulfur crystals to precipitate and migrate to the rubber surface, resulting in sulfur spray phenomenon. In addition, the solubility of sulfur in rubber is closely related to its crystalline form. Ordinary sulfur exhibits a circular structure at room temperature and is a rhombic crystal α Sulfur, soluble in rubber, α After heating the sulfur crystal to 96 ℃ and then cooling it, it becomes β Sulfur crystals, β The solubility of sulfur crystals in rubber is relatively low, β Sulfur crystals become liquid when heated to 120 ℃ μ Sulfur, μ When sulfur is heated to boiling and rapidly cooled, it becomes elastic sulfur, which is insoluble sulfur. Insoluble sulfur has the characteristics of being insoluble, non crystalline, and non migratory, and adding it to rubber will not cause ejection.
2. Solution measures
Adjust the amount of sulfur in the adhesive to ensure that it is in a stable dissolved state under the storage and use conditions of the adhesive or product. Under the conditions of ensuring product quality and processing technology, a raw rubber system with high sulfur solubility is used. Under the conditions of production cost and processing technology, using insoluble sulfur to completely or partially replace soluble sulfur can lead to the transformation of insoluble sulfur into soluble sulfur if the processing temperature is too high.
（2） Promoters and antioxidants
1. Cause of occurrence
Due to significant differences in polarity and structure between promoter and antioxidant molecules and rubber molecules, their compatibility with rubber is poor. When their dosage exceeds solubility or temperature changes significantly! It is easy to produce ejection phenomenon. Promoters have higher solubility in rubber with high polarity and low saturation, such as SNR and CR. The solubility is relatively low in rubber with low polarity and high saturation, such as IIR and EPDM. When using only one accelerator and antioxidant, to achieve the expected effect, the dosage of both the accelerator and antioxidant is large and prone to spray phenomenon.
2. Solution measures
Use two or more types of accelerators and antioxidants together! Keep the dosage of each promoter and antioxidant within its solubility range. On the premise of meeting the physical properties and processing requirements of the rubber material, promoters and antioxidants with high solubility are used as the main promoters and antioxidants respectively! Use promoters and antioxidants with low solubility as auxiliary promoters and auxiliary antioxidants. At the same time, attention should be paid to using a combination of synergistic promoters and antioxidants. If terminated chain reaction type antioxidants are used in combination with destructive peroxide type antioxidants. Thus effectively reducing the dosage of accelerators and antioxidants. When the accelerator is combined with sulfur, if the accelerator is combined with sulfur, it will strengthen the phenomenon of frost spraying. Other accelerators should be used instead.
（3） Inorganic fillers
1. Cause of occurrence
Inorganic fillers mainly refer to white carbon black, calcium carbonate, and magnesium carbonate. Its morphology and physical properties are completely different from rubber, and its compatibility with rubber is poor. When the amount of inorganic filler is large, under the elastic stress inside the rubber material, the inorganic filler will detach from the surface of the rubber. Especially when the cracked surface sprays out. In addition, rubber products that lack sulfur and are stored in moderate humid and hot environments are also prone to filler ejection.
2. Solution measures
Using surface treatment agents such as coupling agents to modify inorganic fillers, the filler particles chemically bind with rubber molecules and form a network structure to reduce the migration of the filler. Adjust the formula and processing technology of the rubber material to ensure the degree of vulcanization of the product. Improve the aging resistance of products and prevent surface cracking. Sealed storage of adhesive materials and products to avoid long-term exposure to humid and hot environments.
（4） Oils
1. Cause of occurrence
Oil leakage is the instantaneous local separation of rubber molecular chains during movement, and the result of oil molecules drilling out from between rubber molecules under a certain kinetic energy. The probability of oil molecules drilling out is closely related to the mobility, link length, oil dosage, viscosity, compatibility between molecular structure and rubber, and temperature of rubber molecular chains. Generally speaking, oils with appropriate dosage, high viscosity, complex molecular structure, and good compatibility with rubber are less likely to leak out.
2. Solution measures
Try to choose oils with good compatibility with raw rubber, such as aromatic oil and coal tar. On the premise of meeting the requirements of rubber process and physical properties of the product, the amount of oil used should be minimized as much as possible, and polymer plasticizers such as liquid NBR and polyester plasticizers should be used to replace plasticizer oil.
（5） And spray frost with glue
1. Cause of occurrence
Due to significant differences in molecular structure, relative molecular weight and distribution, crystallization ability, surface tension, thermodynamic compatibility, and high viscosity of different rubbers, the Brownian motion of molecules is difficult. Therefore, it is difficult to achieve a homogeneous distribution state with a mixed adhesive system, and it is generally in a non-uniform distribution state. The solubility of various compounding agents varies greatly in different rubbers. At vulcanization temperature, the compounding agent diffuses and migrates to the rubber phase with high solubility in the mixed system, resulting in different content of each compounding agent in different rubber phases. And there may be both oversulfur phase and undersulfur phase simultaneously. At the end of vulcanization, the solubility of the mixture decreases with the decrease of pressure and temperature. The coordination agent in the rubber phase containing more unreacted coordination agents will diffuse and migrate again, and when it migrates to the surface of the product, it forms a spray.
2. Solution measures
Choose rubber with good compatibility and use it together. Adopting a reasonable mixing process and strict operation to ensure uniform dispersion of the compounding agent and prevent local agglomeration. Select appropriate co vulcanizing agents to reduce the difference in crosslinking density of each rubber phase and reduce the migration and diffusion of the blending agent. The dosage of the mixing agent should be appropriate to avoid rapid cooling of the product after vulcanization.
（6） Frosting caused by high-temperature vulcanization
1. Cause of occurrence
In the production of rubber products, high temperature rapid vulcanization process is often used to improve production efficiency. However, during high-temperature vulcanization, the oxidation reaction of rubber molecules (causing the main chain of rubber molecules to break and transform) is intense. This can result in a low cross-linking density and uneven structure of the rubber material, resulting in weak physical and chemical bonding between rubber molecules and coordination agent molecules. The mixing agent is prone to migration in rubber, resulting in frost spraying.
2. Solution measures
According to the characteristics of raw rubber and blending agents, use suitable high-temperature vulcanization temperature and time. The vulcanization temperatures of NR, oil-filled SBR, CR, SBR, and NBR should not exceed 240, 250, 260, 300, and 300 ℃, respectively.
Select effective and semi effective vulcanization systems for the vulcanization system (mainly forming single sulfur bonds and double sulfur bonds with good heat resistance); The main accelerator is sulfosamide or thiazole type accelerator, and the auxiliary accelerator is guanidine or thiuram type accelerator; Using sulfur donors (such as vulcanizing agent DTDM) to partially replace sulfur. Use additives such as pine tar, asphalt, rosin, and liquid coumarone resin that have a certain dissolution and wetting effect on the powder.
epilogue
Choosing suitable materials, designing reasonable formulas, and adopting suitable mixing and vulcanization processes are effective means to prevent or reduce the spraying of admixtures and improve the quality of rubber products. In addition, anti frost agents can be used to quickly solve and improve problems.
Frosting phenomenon during rubber processing:
