Analysis of the main causes of rubber whitening (frosting)
1. Frosting overview
Rubber is divided into unvulcanized rubber (hereinafter referred to as rubber compound) and vulcanized rubber (hereinafter referred to as product). Rubber frosting includes rubber surface frosting and product surface frosting. Bloom is the phenomenon that liquid or solid additives migrate from the rubber interior to the rubber surface [1]. It can be seen that the internal compounding agent of the rubber is precipitated, forming the spray frost. To sum up, there are three types of rubber spray. Namely, powder spraying, wax spraying and oil spraying (also known as exudation).
Powder spraying refers to the precipitation of vulcanizing agent, accelerator, active agent, antioxidant, filler and other powder coordination agents on the rubber surface to form a layer of powder.
Wax spraying refers to the precipitation of wax like substances such as paraffin wax and paraffin wax on the rubber surface to form a wax film.
Oil injection is a layer of oil formed by the release of softener, tackifier, lubricant, plasticizer and other liquid additives on the rubber surface. In practice, rubber surface frosting sometimes occurs in one form, sometimes in two or three forms at the same time.
(1) Improper formulation design:
Saturated ejection: common in sulfur, accelerator, active agent and antioxidant
Migration spray: common in processing aids, migration antioxidant Antistatic agent
Generation and ejection: it is commonly used as accelerator and reaction product in sulfur curing system
Reaction retention: common in excess of low molecular substances in organic peroxide curing system
Stress ejection: common in inorganic fillers, such as calcium carbonate
(2) Improper process operation
Poor dispersion caused by uneven mixing, and local saturation is exceeded
The mixing temperature is too high, resulting in local excessive mixing agent
Inaccurate weighing (excessive weighing, insufficient weighing, missing weighing, wrong weighing)
High curing temperature, polymer degradation causes frost spraying
Under sulfur spray caused by incomplete reaction due to too low vulcanization temperature
Insufficient curing time, resulting in sulfur deficiency and frost spraying
Improper operation of the sprayed mold release agent or mold washing water causes the rubber surface to turn white
(3) Raw material quality fluctuation
Due to different raw materials from different producing areas, different preparation methods, different processes, and different batches, the raw rubber synthesis process: polymerization temperature, catalyst, synthesis monomer, etc. are different, resulting in different solubility Changes in purity, moisture, ash, pH, physical properties, etc
(4) Poor storage conditions
Temperature: the solubility of the compounding agent in rubber generally rises and falls with the rise and fall of temperature
Time (pressure  humidity): the pressure under which the rubber is stored, the humidity of the surrounding air and time also have an impact on the solubility of the compound, which generally has little impact [6]. However, if the pressure is high, the compounding agent in the rubber at the pressure part will form a crystal nucleus, which will separate from the rubber surface and form a spray; If the air humidity is too high, the effect of the polar compounding agent in the rubber on the raw rubber (non-polar) will be weakened, and the solubility of the compounding agent will be reduced, resulting in frost spraying; The longer the storage time is, the more obvious the frost spraying on the rubber surface is. Because the temperature and humidity of the air in the storage environment vary with the season, and the difference is large, it is very easy to cause the solubility of the mixture to change, which leads to frost spraying.
(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 between various compounding agents and raw rubber molecules in the rubber system, and reduces the solubility of compounding agents in the rubber system. Therefore, those ingredients that are partially supersaturated will be separated from the rubber, forming a spray
Frost spraying refers to the phenomenon that sulfur, SA, wax, accelerant, antioxidant, filler and other additives pull the surface from the inside of the rubber and precipitate. After the oxidation reaction of the rubber product surface without frost is caused by sunlight, the surface has obvious changes [such as bronze color], which is called rainbow frost spraying.
In rubber technology, it has been pointed out that frost spraying is mainly caused by the phenomenon that the dosage of the compounding agent exceeds the solubility in the rubber or vulcanizate and moves from metastable state to steady state. Frosting will damage the performance of the rubber compound. If the self adhesion decreases, the appearance of the vulcanizate will be poor and become unqualified products. In particular, the anti-aging agent against ozone aging is expected to restrain the surface. However, there is no need to spray frost. Waxes need to prevent the surface from being sprayed with frost to form a protective film for physical protection. In addition, for products whose surface requires matting, it is necessary to control some compounding agents to spray frost moderately, and use the frost to destroy the surface oxide film to achieve matting effect.

For silicone rubber, please check the details:
https://www.siliconerubbers.net/product-general-grade-htv-silicone-rubber.html