Cause analysis of rubber frosting

1. Overview of frost spraying
Rubber can be divided into unvulcanized rubber (hereinafter referred to as rubber) and vulcanized rubber (hereinafter referred to as products). Rubber spray includes spray on the surface of rubber and spray on the surface of products. Bloom is the phenomenon that liquid or solid compounding agents migrate from the inside of rubber to the surface of rubber [1]. It can be seen that the internal compounding agent of rubber precipitates, forming spray frost. The forms of rubber spray can be summarized into three types. Namely, powder spray, wax spray and oil spray (also known as exudation).
Powder spraying refers to the precipitation of powdery compounding agents such as vulcanizing agent, accelerator, activator, antioxidant and filler on the rubber surface to form a layer of powder.
Wax spraying refers to the precipitation of paraffin, ground wax and other waxes on the rubber surface to form a wax film.
Oil injection is the precipitation of liquid compounding agents such as softener, tackifier, lubricant and plasticizer on the rubber surface to form a layer of oil. In practice, the form of spraying frost on the rubber surface sometimes appears in one form, sometimes in two or three forms at the same time.
(1) Improper formulation design:
Saturated spray: common in sulfur, accelerator, activator, antioxidant
Migration spray: common in processing aids, migration antioxidant Antistatic agent
Formation and ejection: common in sulfur vulcanization system, accelerator is used together with reaction product
Reaction retention: common in organic peroxide vulcanization system with excessive low molecular weight substances
Stress ejection: common in inorganic fillers: such as calcium carbonate
(2) Improper process operation
Uneven mixing leads to poor dispersion and partial saturation
The mixing temperature is too high, which makes the mixture locally excessive
Inaccurate weighing (over weighing, under weighing, missing weighing, wrong weighing)
The vulcanization temperature is too high, and the polymer degrades, causing frost spray
The sulfuration temperature is too low, resulting in incomplete reaction and under sulfuration spray
Insufficient vulcanization time leads to insufficient sulfur spray
The sprayed release agent or mold washing water is improperly operated, resulting in the whitening of the rubber surface
(3) Raw material quality fluctuation
Due to different raw materials, production methods, processes and batches, there are great differences in raw rubber synthesis processes: differences in polymerization temperature, catalysts, synthetic monomers, etc., resulting in different solubility Changes in purity, moisture, ash, pH value, physical properties, etc
(4) Poor storage conditions
Temperature: the solubility of compounding agent in rubber generally rises and falls with the rise and fall of temperature
Time (pressure and humidity): the pressure of rubber during storage, the humidity of surrounding air and time also have an impact on the solubility of the mixture, which is generally not significant [6]. However, if the pressure is high, the compounding agent in the rubber at the pressure part will form crystal nuclei, which will separate out of the rubber surface and form spray frost; If the humidity of the air 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 seasons, and the difference is large, it is very easy to cause the solubility of the mixture to change, resulting in frost spraying.
(5) Rubber aging
Rubber aging 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, raw rubber molecules and compounding agents in the rubber system, and reduces the solubility of compounding agents in the rubber system. Therefore, those admixtures that are partially supersaturated will separate out from the rubber and form frost spray.