Spraying frost on the soles will cause the following adverse effects:
1. Affect the appearance of the product
2. Reduce the viscosity of the compound.
3. Reduce the bonding effect between rubber compound and skeleton material.
4. Pollute the materials in contact
5. Reduce the physical and mechanical properties and service properties of the products.
6. Waste materials and increase the cost of products
The following will focus on an in-depth analysis of the problem of frosting on the outsole. Rubber can be divided into unvulcanized rubber (hereinafter referred to as rubber compound) and vulcanized rubber (hereinafter referred to as products). Rubber frosting includes frosting on the surface of rubber and frosting on the surface of products. Frosting (bloom) is the phenomenon that liquid or solid mixtures migrate from the inside of rubber to the surface of rubber. It can be seen that the internal compounding agent of the rubber is precipitated and frost spraying is formed. Generally speaking, the forms of rubber frosting can be divided into three types. That is, Powder spraying, Wax spraying , fuel injection (also known as exudation).
Powder spraying is a powder compounding agent such as curing agent, accelerator, active agent, antioxidant, filler and so on, which precipitates on the rubber surface and forms a layer of powder.
Wax spraying is that waxes such as paraffin wax and ground wax precipitate on the rubber surface to form a wax film.
Fuel injection is a liquid compounding agent such as softener, tackifier, lubricant, plasticizer and so on, which precipitates on the rubber surface and forms a layer of oil. In practice, the form of frosting on rubber surface sometimes appears in one form, sometimes in two or three forms at the same time.
The reasons for spraying frost on the outsole of vulcanized shoes:
(1). Improper formulation design
Saturated ejection: common in sulfur, accelerators, active agents, antioxidants.
Migration ejection: common in processing aids, migratory antioxidants. Antistatic agent.
Generation ejection: common in sulfur vulcanization system with accelerators and reaction products.
Reaction retention: excessive low molecular weight substances are common in organic peroxide vulcanization systems.
Stress ejection: common in inorganic fillers, such as calcium carbonate.
The solubility of various auxiliaries in rubber is different, the smaller the solubility of auxiliaries in rubber, the easier it is to spray frost caused by excessive coordination (that is, the content of auxiliaries in rubber exceeds its solubility in rubber). Excessive frosting often causes other components to spray together (this phenomenon is called passive frosting), although these passive frost sprays are far from being saturated in rubber.
(2) improper process operation.
Uneven mixing results in poor dispersion and local over-saturation.
The mixing temperature is too high, so that the compounding agent is locally excessive.
The weighing is not accurate (more, less, missing, misweighed).
When the vulcanization temperature is too high, the degradation of polymer results in frosting.
The vulcanization temperature is too low, resulting in undersulfur frosting due to incomplete reaction.
The curing time is not enough, resulting in insufficient sulfur spray frosting.
Improper operation of spray release agent or mold washing water, resulting in whitening on the rubber surface.
(3) fluctuation of raw material quality
Due to different materials, different preparation methods, different processes and different batches of raw materials, there are great differences in the synthesis of raw rubber, such as polymerization temperature, catalyst, synthetic monomer, etc., resulting in different solubility. Purity, moisture, ash, pH value and physical properties changed.
(4) poor storage conditions.
Temperature: the solubility of compounding agents in rubber generally rises and falls with the rise and fall of temperature. Time (pressure\ humidity): the pressure of rubber storage, the humidity of the surrounding air and time also have an effect on the solubility of the compounding agent, but generally have little effect. However, if the pressure is high, the compounding agent in the pressurized part of the rubber will form a crystal nucleus, precipitate from the rubber surface, and form frosting; if the humidity of the air is too high, the effect of the highly polar compounding agent in the rubber on the raw rubber (non-polar) is weakened, and the solubility of the compounding agent decreases, which leads to frosting. The longer the storage time is, the more obvious the frosting on the rubber surface is. Because the temperature and humidity of the air in the storage environment vary with the seasons and vary greatly, it is easy to cause the solubility of the compounding agent to change, which leads to frosting.
(5) Rubber aging
The aging of rubber mostly leads to the destruction of the complete and balanced network structure of vulcanizate, which also destroys the chemical or physical combination of various compounding agents with raw rubber molecules and compounding agents in the rubber system. the solubility of the compounding agent in the rubber system was reduced. As a result, those locally supersaturated ingredients will precipitate from the rubber to form frosting.
(6) temperature change
The solubility of auxiliaries in rubber varies with temperature. In general, the solubility increases when the temperature is high, and decreases when the temperature decreases. Because rubber products are usually used at room temperature, once the external temperature is lower than room temperature, some auxiliaries in the formula precipitate close to their solubility, resulting in frosting. For example, rubber overshoes produced in summer are qualified when they are inspected at the factory, but they are found to spray frost in winter.
(7) lack of sulfur
The solubility of auxiliaries in rubber is affected by curing conditions. Taking NR as an example, under the condition of positive vulcanization, the cross-linking density is the highest, the free sulfur decreases, the probability of spraying sulfur decreases, and the chance of other auxiliaries shuttling through the three-dimensional network is also reduced, so the probability of frosting is reduced; on the contrary, under the condition of under-sulfur, the cross-linking density of the network is relatively small, and the rate of frosting increases accordingly.
(8) uneven force
When rubber is subjected to external forces, it often leads to stress concentration and surface fracture, which accelerates the precipitation of the original supersaturated auxiliary particles, forms frosting on the crack surface, and extends to the periphery.
(9) uneven mixing
Uneven mixing leads to uneven dispersion of compounding agents in rubber, and local frosting occurs when the compounding auxiliaries exceed their solubility.