1、 Definition and explanation of the phenomenon of glue spray painting

During the process of parking on the surface of mixed rubber or rubber products, there may be different color changes (such as redness, blurriness, discoloration, etc.), which is called "spray painting". Its essence is that molecules containing chromophores inside the adhesive migrate to the surface and adhere to it.

Different material molecules contain specific functional groups, such as carbon carbon double bonds, carbonyl groups, aldehyde groups, carboxyl groups, azo groups, nitroso groups, - C=S, etc. These functional groups are capable of absorbing light of specific wavelengths while reflecting light of other wavelengths, resulting in different colors when exposed to sunlight. These functional groups that can cause color changes are called "chromophores". When substances containing chromophores migrate to the surface of the adhesive, it will cause the phenomenon of spray painting.

These molecules with chromophores mainly come from small molecule volatiles in rubber raw materials, reaction intermediates, or small molecule volatiles in release agents. These substances have good compatibility with the rubber material, and during the processing, they evaporate and migrate to the surface of the rubber material, becoming miscible with or adsorbed by the rubber surface.

2、 The source of chromophores

Raw rubber system

The residual unpolymerized monomers (such as acrylonitrile, styrene, ENB) in raw rubber are one of the sources of chromophores.

Reinforcement filling system (main factors)

During the combustion and cracking process of carbon black, functional groups such as carbon carbon double bonds, carbonyl groups, aldehyde groups, carboxyl groups, azo groups, - C=S, as well as impurities such as sulfur and tar, will remain on the surface. These substances are prone to migrate to the surface of the product, causing bluish or colorful phenomena. Excessive use of carbon black, low oil absorption value, and low structural degree are more likely to lead to spray painting. Generally speaking, carbon black with small particle size and high structure has strong adsorption capacity, more residual oily substances, and is more prone to discoloration. For example, N220 and N330 carbon black have a high carboxyl content on their surface and are prone to bluing; However, larger particle size carbon black such as N550, N660, and N774 have lower coloring strength and are more prone to redness.

Rubber oil (main factor)

If the purification of rubber oil is insufficient, the content of polycyclic aromatic hydrocarbons is high, or the content of substances containing nitrogen, sulfur and other elements is high and the flash point is low, it may lead to spray painting phenomenon. In addition, excessive addition of recycled rubber oil and poor compatibility may also cause spray painting.

Other additives

Antioxidants, accelerators, activators, release agents, etc. containing carboxyl, heterocyclic, azo, azo and other chromophore groups, or under high-temperature oxidation conditions, may also cause spray painting phenomenon.

3、 The discoloration problem of non black rubber products

For non black rubber materials or products in contact with non black rubber materials, discoloration is particularly important. The following experimental schemes may help improve the color resistance of the adhesive, but it should be noted that these general schemes may not be applicable to all situations. Any measures to improve color fastness may have an impact on other properties, and the specific effect needs to be evaluated based on the actual situation.

Oil selection

Non aromatic oils should be preferred because aromatic oils are more prone to discoloration than paraffin and cyclohexane oils.

Sulfurization method

Peroxide vulcanized rubber materials usually have better color resistance than sulfur vulcanized rubber materials.

Selection of antioxidants

Phenolic antioxidants have better resistance to discoloration than amine antioxidants. Among phenolic antioxidants, bisphenol antioxidants exhibit the best resistance to discoloration. Research has found that bis (tetrahydrobenzaldehyde) pentaerythritol formaldehyde can endow CR, IIR, CIR, or BIIR adhesives with excellent ozone resistance and color change resistance. In addition, compounds such as tetrahydro-1,3,5-triazolethione tri-n-butyl ester, 3,5-di-tert-butyl-4-hydroxyphenylcyanoacetate, and 2,4,6-trimethylphenylenediamine melamine have also been shown to have good ozone resistance and color change resistance.

Use saturated rubber

In order to ensure that rubber products maintain good smoothness throughout their entire use, the rubber compound should contain sufficient saturated rubber components. Even without the use of antioxidants, these saturated rubbers can provide good ozone resistance (because antioxidants sometimes migrate to the surface, affecting the smoothness). Common saturated rubbers include EPDM, BIR, CIR, and brominated isobutylene styrene rubber (BIMS).

Use shading agents

By combining UV resistant and color changing additives, the color changing components can be masked (minimizing the use of color changing components).

4、 The impact and solutions of spray painting phenomenon

The phenomenon of spray painting (such as bluish or iridescent) usually does not affect the performance of the product, but for products that require high color and light appearance requirements (such as rubber shoes, toys, and other daily necessities), this issue may lead to returns or rejection. The main reasons for rubber products turning blue include the following:

Softener: The oil has already been oxidized before adding to the rubber compound, or adding too much softening oil to the large molecule aromatic hydrocarbon rubber can easily cause the vulcanized rubber to turn blue.

Carbon black: When producing carbon black, the large aromatic hydrocarbons are not fully burned, and adding rubber can easily cause the product to turn blue.

Antioxidants: Excessive use of darker colored antioxidants, or the use of additives such as PEG, SA, 4010NA, may also lead to discoloration issues. Gumaron resin has a darker color and is also prone to color issues.

Sulfurization temperature: When adding additives that are prone to bluing, a high mixing temperature may cause the vulcanized product to turn blue.

Uneven mixing: Uneven mixing and dispersion of rubber can easily lead to excessive concentration of certain additives, resulting in spray painting phenomenon.

Incomplete solubility of paraffin oil in carbon black: The larger the amount of carbon black used, the smaller the oil absorption value and structural degree, and the more likely it is to cause discoloration. For example, N550 and N774 are more prone to reddening, while N330 is more prone to blueing.

Insufficient sulfurization or insufficient sulfurizing agent: may lead to frost spraying, which in turn can cause discoloration.

Solution

Replace carbon black: If there are experimental conditions, you can try different combinations of carbon black; If there are no experimental conditions, the brand of carbon black can be changed. A simple testing method is to put 30 grams of carbon black into a beaker, immerse it in water, boil it, and observe whether there are oil droplets on the surface of the carbon black after cooling.

Adjust the amount of oil: Reduce the amount of oil appropriately, and try to choose high flash point, high viscosity oil and softener. Naphthalene oil performs better than paraffin oil in inhibiting discoloration. In EPDM, the use of heavy substance anti-aging agents such as 4010 can be avoided.

Replace the filler: If there is discoloration in the filler, adjust the filler ratio and try to use materials such as nano aluminosilicate and Qiang Wei powder. Adding 10 to 20 parts can help solve the discoloration problem.

Adjust the vulcanization system: prevent the occurrence of frost spraying.

Improve processing technology: enhance dispersibility.

Post processing: For products that have already been produced, they can be processed by boiling or baking (but this method may have some impact on the product's lifespan).

(From "1500 Practical Methods for Improving the Performance of Rubber Compounds")