Polyurethane (PU) synthetic leather (hereinafter referred to as PU leather) is the most ideal material for replacing natural leather products in the world. It is mainly used in footwear, clothing, bags, balls, furniture, automobiles, flexible containers, pipes, and transportation. Belts, canvas and other fields. The production of PU leather began in Germany, the United States, and Japan in the early 1950s. Because it has advantages that other synthetic leathers can't match, such as easy molecular structure design according to needs, good flexibility at low temperatures, etc., it has developed rapidly. Increasingly, the variety of colors and colors is constantly updated. In 1979, China had the first pU leather production facility at the Guangzhou Synthetic Leather Factory. Then Yantai Taicheng Leather Factory introduced a pU leather production line with an annual output of 3 million square meters from Japan. The trial production was completed in 1984 and passed the national acceptance. At present, there are more than 40 pU leather production lines in China, with an annual output of 30 million square meters and a wet method of 24.4 million square meters.
Leather
pU is a thermoplastic elastomer composed of hard segments and soft segments. As the proportion of hard segments increases, its strength, elongation at break and hardness increase. However, if the hard segment is too much, it will lose its elasticity and make the tanning difficult. Generally, the hard segment accounts for 25% to 50% of the total weight of the polymer. Industrially, it is usually obtained by reacting a hydroxyl group-containing polyester or polyether with an isocyanate. Polyester polyols are generally used in the production, among which polyesters made from adipic acid and ethylene glycol are the most widely used, and isocyanates are aromatic and aliphatic, and alicyclic, and the latter synthesized PU is not used for a long time. It turns yellow, but it is expensive and toxic. MDI and TDI in aromatics are currently generally used. Since the spatial orientation of MDI is more regular than TDI, the PU synthesized by MDI has better crystallinity, heat resistance, elasticity and yellowing, and the solid MDI is non-toxic, the freezing point is 41C, and it is fusible. Process production. So although TDI is cheap, most use MDI.
PU leather has the advantages of oil resistance, wear resistance, cold resistance, buckling resistance and good mechanical strength. However, there are also disadvantages such as poor hydrolysis stability, discoloration of light, resistance to mold, heat resistance, and poor moisture permeability.
Due to the lack of pU leather, pU leather sometimes cracks and even peels off piece by piece. Obviously, if the shortcomings of PU leather can be overcome, the scope of application will be further expanded. Japan has established a reference standard for pU leather for automobiles and furniture. The requirements for hydrolytic stability are in the environment of 70C and 95% relative humidity, and the strength is maintained above 60% after 10 weeks; the requirement for light resistance It is the temperature of the blackboard thermometer after 400 hours of exposure to the air fastness tester; the PU leather for the car is 83 ° C, the furniture is 63 ° C, the strength is still more than 60%; the heat resistance requirements (only For PU leather for automobiles, it is kept for 400h in 120C aging oven, and the strength is still more than 60%; the PU leather for furniture is based on 10 years of service life. Since the pU coating is located on the surface of the pU leather, the aging problem of the PU leather actually becomes an aging problem of the pU coating.
Main factors affecting the aging of PU leather
In nature, heat, air, sunlight, and moisture are common, which are factors that affect the aging of PU leather.
Thermal influence
The stability of a polymer usually depends on its chemical structure and the dissociation energy of the bond. PU is not a homopolymer, it is formed by a polyisocyanate, a polyol and an amine chain extender, etc., so increasing the ambient temperature is actually equivalent to providing the dissociation energy of the bond. Due to the action of heat, the following reactions occur:
R-NH-CO-O-R'→ R-NH-R'+CO2↑
As a result of thermal degradation, the mechanical properties of the product are degraded. Under the action of heat, the radical chain reaction is also initiated by oxygen in the air. This reaction starts at about 80 ° C and accelerates the reaction above 100 ° C. Compared to polyether PUs, polyester PUs are quite stable to thermal oxidative cracking.
Light effect
PU prepared with aromatic isocyanate causes ultraviolet light degradation under illumination, and the product changes from colorless to yellow, which is due to the formation of mites under illumination:
The photoaging embrittlement occurs due to the crosslinking reaction, while the PU prepared with the aliphatic or alicyclic isocyanate is substantially stable in color.
And the influence of moisture in the air
Water plays two roles for PU. The first is plasticization, that is, water molecules infiltrate into the PU macromolecular network, forming hydrogen bonds with the polar groups, weakening the interaction between adjacent molecules, thus making the mechanics Performance degradation. This process is reversible, and after the water is discharged, it can still repair the mechanical properties of PU. The second is hydrolytic degradation, resulting in significant and permanent reduction in mechanical properties.
Polyether PU is more stable to water and moisture than polyester PU, but the moisture resistance of both is worse than water resistance. This may be due to the degradation of the polyether PU under moisture, the leaching of the acid remaining in the material, further accelerating the catalytic hydrolysis (autocatalytic hydrolysis), while the polyether PU is both hydrolyzed and oxidized under moisture. reaction.
For PU, acid, alkali, and mold can accelerate the hydrolysis reaction.
Method for improving PU aging resistance
In order to improve the aging resistance of PU, the currently used methods include adding additives and changing the structure and synthesis method of the monomer.
Additive method
The additives include heat stabilizers, light stabilizers, hydrolysis stabilizers and other additives, and the addition amount is generally less than 2%, and the effect is remarkable.
Heat stabilizers
Phenol and amine anti-aging agents are often added to the thermal stability of PU. 4. Phenols often use hindered phenols, polyfunctional derivatives of phenols and sulfonic acid derivatives containing phenols. 264, 1010, 330, 2246, 3114 can be used in hindered phenol, 264 is commonly used in industry, and 2246 and 3114 are better. The polyfunctional derivative of phenol can be a derivative of an N,N-dialkylhydrazide group. In addition, if 2246 is used in combination with hydroquinone or catechol, it has a synergistic effect and can prevent PU from being colored during the manufacturing process.
The most widely used amine anti-aging agents are p-phenylenediamine derivatives such as anti-aging agents H, DNP, 4010Na, etc. In addition, diphenylamine and mannarylnaphthylamine dimers can also be used to increase the heat of PU. stability.
Light stabilizer
Mainly added benzophenone and benzotriazole light stabilizers, among which the commonly used and significant effects are UV-9, UV-531, UV-24, UV-P, UV-327, etc. If the ultraviolet light (wavelength 290-400 nm) absorbent is properly matched with the heat stabilizer, a good effect can be obtained, and synergistic anti-aging effect is obtained. For PU, the light stabilizer is combined with the heat stabilizer, and the effects are remarkable as UV-P and 2246, UV-531 and 264. Recently appeared triazine derivative compounds. It is both a heat stabilizer and a light stabilizer.
Hydrolysis stabilizer
In order to improve the hydrolytic stability of PU, polycarbodiimide hydrolysis stabilizer is commonly used. The polycarbodiimide has a sterically hindered aromatic structure, reacts with a terminal carboxyl group to form an unstable intermediate, and then rearranges into a stable and neutral N-acyl urea, thereby improving the hydrolysis resistance of the PU.
Changing monomer structure and synthesis method
Adding additives to prevent aging is effective, but when the additive is gradually depleted, it loses its anti-aging effect. For example, the hydrolysis stabilizer polycarbodiimide can increase the hydrolysis resistance of PU by 4 times, but it will be gradually depleted, and there will be no water-repellent solution after depletion. The fundamental approach is to change the monomer structure and synthesis method. To solve the yellowing problem, you can use aliphatic or alicyclic isocyanates. Since the ester group is hydrophilic, reducing the amount of the ester group can improve the hydrolysis resistance of the PU. The aging resistance of different types of PUs is summarized in the table. It can be seen that PU synthesized from aliphatic or alicyclic isocyanate and polycarbonate diol has the best aging resistance.
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