Why do rubber products turn white?

In life, many people must have seen that rubber products will turn white after being used for a period of time. For example, paint tends to turn white when the humidity is high.

Spray frost

Rubber can be divided into unvulcanized rubber (hereinafter referred to as rubber) and vulcanized rubber (hereinafter referred to as product). Rubber bloom includes bloom on the surface of the rubber and bloom on the surface of the product. Bloom is the phenomenon of liquid or solid compound moving from the interior of the rubber to the surface of the rubber [1]. It can be seen that the internal compound of the rubber separates, forming blooming. To sum up, the methods of rubber spray frost can be roughly divided into three types. That is, powder spraying, wax spraying, and oil spraying (also called exudation).

Powder spraying is when vulcanizing agents, accelerators, active agents, antioxidants, fillers and other powdery ingredients are separated on the surface of the rubber to form a layer of powder.

Spray wax is when waxy substances such as white wax and ozokerite are separated on the surface of the rubber to form a wax film.

Oil injection is when liquid ingredients such as softeners, tackifiers, lubricants, and plasticizers are separated on the surface of the rubber to form a layer of oil. In practice, the frost on the rubber surface sometimes appears in one way, and sometimes it appears in two or three ways at the same time.

(1) Improper formula planning:

Full spray: common in sulfur, accelerators, active agents, antioxidants. Relocation spray: common in processing aids, removable antioxidants. Antistatic agent generation and ejection: common in accelerators and reaction products in sulfur vulcanization systems. Reaction retention: common in organic peroxide vulcanization systems. Excessive stress eruption of low molecular substances: common in inorganic fillers: such as calcium carbonate.

(2) Improper technical operation:

Uneven mixing results in poor dispersion, and some parts exceed the fullness level.

The rubber mixing temperature is too high, causing excessive amounts of compounding agents.

Inaccurate weighing (overweighing, underweighing, missing weighing, wrong weighing)

The vulcanization temperature is too high and the polymer degrades to cause blooming.

The vulcanization temperature is too low, causing under-sulfur bloom caused by incomplete reaction. The vulcanization time is insufficient, causing under-sulfur bloom. The release agent or mold washing water sprayed is improperly used, causing the rubber surface to appear white.

(3) The quality of raw materials fluctuates due to different raw materials from different places of origin, different production methods, different technologies, and different batches. The raw materials are very different. The raw rubber synthesis technology: differences in polymerization temperature, catalysts, monomers, etc., lead to differences in solubility. Changes in moisture, ash, pH value, physical properties, etc.

(4) Poor storage conditions and temperature: The solubility of the compound in the rubber usually rises and falls with the rise and fall of temperature (pressure humidity): the pressure of the rubber during storage, the humidity of the surrounding air and time also have an impact on the solubility of the compound, and usually have little effect. However, if the pressure is high, the compounding agent in the rubber at the pressurized part will form crystal nuclei, which will separate from the rubber surface and cause blooming; 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) will be weakened, and the solubility of the compounding agent will be reduced, and then Resulting in blooming; the longer the storage time, the more obvious the blooming on the rubber surface. Because the temperature and humidity of the air in the storage environment vary with the seasons, and the differences are large, it is easy to cause changes in the solubility of the compound, thus leading to blooming.

(5) Rubber aging

Rubber aging mostly causes damage to the complete and balanced network structure of the vulcanized rubber, and then damages the chemical or physical connection between various compounding agents and raw rubber molecules and compounding agents in the rubber system, reducing the solubility of the compounding agents in the rubber system. Therefore, those parts of the compound that are in an over-saturated state will separate from the rubber, causing blooming.