Light and white soles are prone to yellowing in the process of use and storage, thus affecting the appearance and quality of the whole shoe, resulting in various economic disputes and irreparable losses. Recently, during the quality inspection of footwear products, the author found that the yellowing resistance of some light and white soles made of some materials was poor in the experiment process, and encountered the phenomenon of yellowing in the use and storage of the shoes made of light and white soles produced by many enterprises; and even found that Hg / t3689-2001 test method for yellowing resistance of footwear for light and white soles made by some enterprises When the test is carried out to determine the yellowing resistance of the material under the similar sunlight and ultraviolet radiation, the degree of color change on the surface of the sample is not obvious, but the finished shoes made of this sole are prone to yellowing and other phenomena in the use and storage process. According to this, the author wrote this article through analysis and summary.
1、 The cause and influencing factors of yellowing of light and white sole
Light, White PU, thermoplastic elastomer SBS (TPR), PVC and other sole are prone to yellow under the long-term irradiation of natural sunlight and ultraviolet or under the action of heat, oxygen, stress, micro moisture, impurities, improper process, etc. EVA light and white sole with good stability sometimes turn yellow during use and storage. The main reasons are as follows Factor:
（1） The effect of polymer structure on its properties
Polymer structure has a great influence on yellowing resistance of light and white sole. There is bond energy between polymer macromolecular chain bonds. When the energy provided is greater than bond energy, the molecular chain is easy to produce active center, which will make the polymer degrade gradually in the process of use and storage. The bond energy is related to the polymer structure. The alkenes with tertiary carbon atom structure in the main chain have poor stability and are easy to produce active centers, which lead to degradation and poor yellowing resistance. When the main chain contains - C - C = C - structure, the single bond in the double bond position also has relative instability. The bond energy of - c-c-bond on the main chain is also affected by the substituent energy and atom on the side chain. The polarity and distribution regular substituent can increase the strength of the primary key - c-c-bond and improve the yellowing resistance, while the irregular substituent is the opposite. For example, the asymmetric chlorine atom in the main chain of PVC sole easily reacts with the adjacent hydrogen atom to produce dehydrochlorination reaction, which makes the polymer degrade and lead to the yellow change of the sole; and the macromolecular chain contains: On the other hand, these structures are sensitive to polar substances such as water, acid, alkali and amine, and also easily lead to yellowing of the sole. For example, due to the presence of groups in the macromolecular chains of light and White PU sole, the sole is prone to yellowing during use and storage.
（2） The influence of light
The effect of light can make the light and white sole yellow. Light itself is a kind of energy, but the energy of light varies with different wavelengths. Usually, the maximum wavelength of damage to the sole is ultraviolet light below 400nm. Although ordinary sunlight can activate oxidation, its effect is very weak. The destructive effect of light is mainly caused by the absorption of light energy by rubber and plastic materials. When the material absorbs light energy, the molecular chain on the absorption site will produce the cracking of carbon carbon bond or hydrocarbon bond. Diene rubber and plastic materials are very sensitive to light, which is easy to happen, and make the material yellow. For example, when Hg / t3689-2001 method is used to test the yellowing resistance of light and white soles such as PVC, PU, SBS, etc. under the condition of approximate sunlight and ultraviolet radiation, it can be observed that the surface color of the sample is yellowing, which is mainly caused by the destructive effect of light on the material. As far as pure vinyl chloride is concerned, it is stable to light and does not absorb 300-400nm ultraviolet light. However, vinyl chloride thermal degradation produces a small amount of double bond or carboxyl group, which can absorb ultraviolet light and cause photochemical reaction and discoloration of materials. PU sole contains Group is easy to produce photooxidation reaction, which leads to yellowing of sole; thermoplastic elastomer SBS sole, because of polybutadiene unsaturated double bond in molecular structure, will produce obvious photooxidation aging effect in the oxygen atmosphere under strong light irradiation, and the material will soon form hydroperoxides, so the light resistance of TPR sole is poor and prone to yellowing.
（3） Effect of heat and oxygen
Oxygen can react with rubber and plastic materials to make the materials oxidized, and heat can accelerate the oxidation process of materials. Therefore, thermal oxidation is also the basic cause of aging and yellowing of rubber and plastic materials. In the shoe making process, the main process is to make the rubber and plastic materials produce thermal decomposition, generate the activation center, in the presence of oxygen in the air, oxygen makes the active center generate extremely unstable peroxide structure. The activation energy ed of peroxide structure is relatively low, (the activation energy of thermal degradation of polystyrene is 22.6 kcal / g, and the activation energy of formation of peroxide structure is reduced to 10 kcal / g). It is easy to form free radicals, and then produce chain degradation reaction in the process of use and storage, resulting in discoloration of light and white shoe materials. In addition to the activation of oxidation, the thermal effect can also lead to the fracture of - c-c-bond and double bond.
（4） Influence of other factors
The reasons for yellowing of light and white soles are also related to the additives added to the materials, the existing moisture, impurities and processing technology. Some substances (such as initiator, catalyst, acid, alkali, etc.) added in the polymerization process of materials can not be removed cleanly, or materials absorb water in the transportation and storage process, mixed with various chemical or mechanical impurities will reduce the stability of the polymer. The materials that are easy to decompose and leave the base can make the light and white sole produce chain degradation phenomenon, while the sole containing polar substances such as acid, alkali and water will produce random degradation reaction; some auxiliaries will produce migration in the sole to make the sole yellow, and the use of polluting auxiliaries is also easy to make the material yellow; the impurities in the sole will produce strong catalysis, water and trace Metal elements can promote the process of photooxidation, especially metal ions can make EVA light and white soles with better weather resistance yellow in use and storage, which is also the reason why EVA soles have better performance in yellow resistance test, but yellow in use and storage, which can be wiped off with HF; shoe material forming and processing technology can also have certain effect on the Yellow resistance of the soles Sound, when the shoe material is processed into the sole, the macromolecules will break and form free radicals due to the action of shearing and other forces, and then degrade in the use and storage process, and make the material yellow, which is also a main factor that causes the yellow change of EVA light and white sole with good stability; in the process of shoe forming, the light and white sole will also be improper with adhesive and treatment water It may cause pollution due to contact or yellowing due to chemical reaction between a certain substance in leather and a substance in shoes (this phenomenon is obvious on the surface, but it is complicated to analyze the causes). In a word, the reasons that affect the yellowing of light and white soles are quite complex. It needs specific analysis of specific problems. Any situation may not be a single factor, which requires us to analyze and summarize in our work, and find out the methods to avoid the yellowing of soles and improve the yellowing resistance of soles.
2、 Measures to improve yellowing resistance of light and white sole materials
The yellowing of light and white sole materials in use and storage will not only affect the appearance and quality of the whole shoe, but also cause irreparable economic losses. Therefore, we should take precautions to improve the yellowing resistance of light and white sole materials according to the possible causes of yellowing. For this purpose, the following measures can be taken according to the specific situation:
（1） According to the characteristics of the polymer, antioxidant, stabilizer and other additives are considered in the formula to improve the Yellow resistance of light and white sole. Antioxidants and stabilizers can maintain the stability of polymer structure. Antioxidants have the ability to interact with oxygen to form stable substances, which greatly slows down the thermal oxygen effect. Stabilizers have the function of terminating or changing the chain reaction by interacting with free radicals. In fact, they are the receptors of free radicals, which can capture free radicals and eliminate the factors that cause degradation yellow. For example, HCI will precipitate from PVC shoe material, which will cause autocatalytic effect, and the performance of UV resistance and light is relatively poor. Lead salt, metal soap and other stabilizers and uv-71, uv-73, uv-12 and other anti UV agents should be added to fix the structure. For the sole of EVA and other rubber and plastic materials with good stability, stabilizers (such as three salt slurry) should be added to make its storage better.
In general, the stability of polymer processing increases with the increase of stabilizer or antioxidant. For the SBS sole with poor performance of heat-resistant oxygen aging, photo oxygen aging and ozone aging under stress, different additives can be used according to different aging reasons of the polymer itself. For example, for the thermal oxygen aging of the sole, diaryl secondary amine, p-phenylenediamine and other amines and hindered phenols represented by 2,6-di-tert-butyl-4-methylphenol can be added according to the thermal oxygen aging mechanism Compounds are used to improve the properties of the sole, so as to improve the yellowing resistance of the sole. However, the practice results show that amines pollution is serious, which will affect the yellowing resistance of the sole, so it should be used less, but the combination of phenols and peroxides decomposition agent has the best effect (such as 1010 quaternary amyl and DLTDP can achieve high stability); and in order to improve the light and oxygen aging resistance of SBS sole Yes, diphenylketones, benzotriazoles and substituted acrylonitrile can be used. General photooxidation or thermal oxidation preventer can also effectively prevent ozone aging and achieve good yellowing resistance. At the same time, according to the properties of diene containing unsaturated double bond, the UV aging resistance of the compound can also be improved by means of hydrogenation.
（2） Strictly control the technical indicators of raw materials and use qualified raw materials. The quality of polymer is largely affected by the process of synthesis. For example, when the macromolecule structure contains double bond or branched chain, the molecular weight dispersion is large, the raw material is impure or mixed with various chemical or mechanical impurities such as initiator, catalyst, acid, alkali or metal powder due to the poor purification in the later stage, the stability and processability of polymer will deteriorate. Some impurities can play a catalytic role in degradation, so how to strictly control the technical indicators of raw materials, using qualified raw materials is particularly important.
（3） Polymer materials should be dried strictly before being processed into sole. In particular, polymers containing polyester polyether, polyamide and other groups are easy to absorb water from the air during storage and degrade materials during processing, thus forming a life center