Protection principle of rubber protective wax

1. Protection principle of rubber protective wax

As the name suggests, rubber protective wax is a special wax specially prepared and processed to protect rubber products from light, oxygen and ozone. Its main protection principle is to add a certain amount of rubber protective wax to make it dissolve in the rubber compound and reach a certain degree of saturation. When the rubber products are affected by the external temperature, the rubber protective wax in the rubber products will also be affected by the external temperature and migrate to the surface of the rubber products (the migration process is from low temperature to high temperature, from low carbon to high carbon), forming a wax film on the surface of the rubber products with smooth surface, uniform thickness, good tightness, tight structure, strong toughness, elasticity and strong adhesion, which is not easy to fall off. Because the formation of wax film on the surface of rubber products effectively curbs the erosion and aging of light, oxygen and ozone on the surface of rubber products, prevents the surface of rubber products from cracking, and thus extends its service life. At the same time, it must be pointed out that in order to have this ability, proper chemical antioxidant should be added in the rubber compound preparation, so as to play a common role and make it play a good additive role, namely, the synergistic effect of anti-aging system. Rubber protective wax has its special physical resistance and containment ability to light, oxygen and ozone. Its applicable temperature range is - 5 ℃ -+55 ℃, and it can play a good protective effect in this temperature range.

2. Structure of rubber protective wax

The composition and structure of rubber protective wax is quite different from that of ordinary paraffin wax. The internal structure of rubber protective wax is determined according to the temperature range of rubber products and the ability to resist ozone erosion. According to the requirements and adaptability of anti-aging of rubber compound, it measures and screens the internal structure of various petroleum waxes and microcrystalline waxes, and is processed and combined to form a special rubber protective wax with different group composition, different melting point and different fractions, but with a certain carbon number distribution structure and content. Generally speaking, this is the internal quality of rubber protective wax. Its use characteristics can be determined according to the temperature range of ozone erosion on rubber and the operating conditions of use. At present, the rubber protective wax at home and abroad is basically divided into four types, namely, medium-temperature protective wax, medium-high temperature protective wax, high-temperature protective wax and all-weather protective wax. Enterprises can make appropriate selection according to the characteristics of rubber products and different rubber requirements (see the carbon distribution diagram of four types of rubber protective wax).

3. Difference between rubber protective wax and common paraffin wax

At present, some enterprises use ordinary paraffin instead of rubber protective wax in order to reduce costs and pursue profits. In the short term, they can obtain certain benefits, but in the long term, it is not worth the loss. In addition to the poor adaptability mentioned above, the most important reason for using ordinary paraffin is that the internal quality of ordinary paraffin is unstable and the carbon number distribution and content are incomplete due to the different components of crude oil used by various oil refining enterprises. Therefore, when using ordinary paraffin as protective wax, spray, whitening, rainbow and other phenomena often appear on the surface of rubber. The rubber surface is rough and matt, which seriously affects the appearance of rubber products, makes the products lack the ability of anti-aging, affects the user's recognition of the products, and ultimately affects the sales of the products. Therefore, we recommend not to use ordinary paraffin wax as rubber protective wax, especially for tire sidewall compound and rubber products under high temperature conditions. The rubber protective wax suitable for the use of rubber compound should be correctly selected to improve product quality, create famous brands and increase benefits.

4. Selection of rubber protective wax

The rational selection of rubber protective wax is an important and non-negligible link in the preparation of rubber products. The correctness, practicability and economy of selection must be considered during selection. The following points must be paid attention to: ① grade of rubber products ② temperature range of rubber products ③ working conditions of rubber products ④ service life of rubber products, etc. Rational selection of rubber protective wax required for rubber products depends on the internal composition of the selected protective wax, that is, its carbon number distribution structure. During use, attention must be paid to the reasonable preparation of a certain amount of rubber protective wax to ensure that the rubber products have a sufficient amount of rubber protective wax to migrate to the surface during the whole process of use, so as to achieve the anti-aging effect in the whole process. At present, the preparation of protective wax for domestic rubber products is generally 1-4 parts of the total rubber. The user is hereby reminded to pay attention to the quality of rubber softener when selecting all-weather rubber protective wax. For example, the wax content in rubber softener is on the high side (the reference data sp is on the high side). In addition to the front peak light component wax of all-weather rubber protective wax, spray will often appear when combined, so pay attention when selecting. Rubber protective wax is mainly divided into ordinary paraffin wax and microcrystalline wax.

Paraffin wax is obtained from vacuum distillate oil by solvent dewaxing and deoiling, refining, molding and packaging. Paraffin wax is divided into soap wax, crude wax, semi-refined wax and fully refined wax. Each type is divided into multiple brands according to the melting point. The commonly used brands are 54 # and 58 # paraffin.

The relative molecular weight of paraffin wax is lower than that of microcrystalline wax, which is generally between 300 and 500. The carbon atom number is 20 to 35, the relative density is 0.86 to 0.94, and the melting point is 30 to 70 ℃. The paraffin wax is in large crystalline form. The main components of commercial paraffin are normal alkanes, in addition to isoalkanes, monocyclic alkanes, bicyclic alkanes, monocyclic aromatics and bicyclic aromatics.       

Microcrystalline wax is a refined synthetic wax with microcrystalline properties. It has the characteristics of good luster, high melting point and light color. Its structure is compact, firm and smooth, and it can melt with various natural waxes, improve the melting point of its low wax and improve the performance of crude wax. The microcrystalline wax itself is as white as jade and polished by friction, which has a very beneficial effect on the production of light-colored products.

Microcrystalline wax is refined from vacuum residue oil. Its crystal is fine, its relative molecular weight is 500~700, carbon atom number is 35~55, and its drop point is 60~90 ℃. The molecular structure of microcrystalline wax is more complex than that of paraffin wax. The mass fraction of normal alkanes and aromatic hydrocarbons is small, while the mass fraction of isoalkanes and long-chain cycloalkanes is large.       

Microcrystalline wax has good oil absorption performance, can form stable and uniform paste with a variety of solvents and waxes, and has emulsifying properties. It can be used as processing aids such as shoe polish, car wax, polishing wax, floor wax, polishing wax, Chinese medicine pills, protective agents, candles, wax toys, dental materials and cosmetics. In addition, the use of this raw material in the waxed paper formula can make the handwriting clear and significantly improve the abrasion resistance.

The chemical property of microcrystalline wax is relatively active. It reacts violently with fuming sulfuric acid and generates foam and heat, while the paraffin wax does not react and its chemical property is not active.

5. Main factors affecting the protective performance of rubber protective wax

The main influencing factors include the carbon atom number distribution of the protective wax, the proportion of normal isomers, the use temperature, the degree of crosslinking, the compounding agent, the type of rubber compound, the load, the filler, the softener, etc. The carbon atom number distribution and the ratio of normal isomers are the key factors, which have a decisive impact on the protective performance of rubber protective wax.

Carbon number distribution of protective wax:

Alkanes with low carbon atoms have small relative molecular weight, low melting point, low branching degree, and are easy to migrate from rubber to rubber surface. However, when the temperature rises, alkanes with lower carbon atoms will be dissolved in rubber, so the supersaturation at high temperature will reduce the migration rate, even the migration rate is zero.

Alkanes with high carbon atoms have large molecular weight, high melting point and high branching degree, so their migration resistance is large and their migration speed is small. When the number of carbon atoms is higher, the migration speed is slower, and the resulting wax film is very thin and cannot form a protective layer, which has little protective effect on rubber.

Chemical composition of protective wax:

Rubber protective wax is mainly composed of normal alkanes (straight-chain alkanes), isomeric alkanes (branched alkanes) and cyclic hydrocarbons (with long-chain cycloalkanes and aromatics). When the relative molecular weight is the same, the migration speed of normal alkanes is faster than that of isoalkanes. The more branches of isoalkanes are, the slower the migration speed is. Cyclic hydrocarbons with long side chains migrate more slowly.

The n-alkane crystals are large and loose, and ozone in the atmosphere is easy to penetrate the wax film and attack the rubber surface. When a certain amount of isoparaffins are added to normal alkanes, because the regularity of crystalline molecules is destroyed, a fine, amorphous and dense wax film can be obtained, thus enhancing its ability to resist ozone erosion.

Service temperature:

The use temperature of rubber is - 5~55 ℃, and ozone has aging effect on rubber. When the use temperature is lower than - 5 ℃, because of the scarcity of activated molecules, the two cannot react chemically, that is, the rubber will not age due to the effect of ozone; When the operating temperature is above 55 ℃, ozone will decompose to produce harmless oxygen, so rubber will not age due to ozone.

At low temperature, the mobility of each component of the protective wax decreases, and the sprayed wax film is alkane with low carbon atom number, and the crystalline wax film is porous and loose. Because of the slow migration speed and the slow and thin formation of wax film, ozone began to erode the surface of rubber before the formation of wax film, so it is difficult for rubber protection wax to resist ozone at low temperature.

At higher temperatures, because hydrocarbons with low carbon atoms are dissolved in the rubber, the wax film is formed slowly and thin, so it is also difficult to form an appropriate protective film. The relationship between specific temperature and the number of carbon atoms of alkanes with the highest migration rate is as follows:

Therefore, the composition of carbon atom number of rubber protective wax should be controlled properly, otherwise the protective wax cannot be guaranteed to play its due protective role at any temperature point within the range of 0~55 ℃.