In nature, heat, air, sunlight, and moisture are ubiquitous which are all factors that affect the aging of PU leather.
1. The effect of heat
The stability of a polymer usually depends on its chemical structure and bond dissociation energy. PU is not a homopolymer, it is formed by polyisocyanates, polyols and amine chain extenders, so increasing the ambient temperature is actually equivalent to providing bond dissociation energy.Due to the effect of heat, a thermal degradation reaction will occur, resulting in a decrease in the mechanical properties of the product. Under the action of heat, the oxygen in the air will trigger a chain reaction of free radicals. This reaction starts at about 80°C and accelerates the reaction above 100°C. Compared with polyether PU, polyester PU is quite stable to thermal oxidative cracking.
2. The effect of light
The PU prepared with aromatic isocyanate causes ultraviolet light degradation under illumination and the product changes from colorless to yellow. This is due to the formation of quinones under light, and the cross-linking reaction causes light aging and embrittlement, while the color of PU prepared with aliphatic and alicyclic isocyanates is basically stable.
3. The influence of moisture in the air
Water has two effects on PU. The first is plasticization, that is, water molecules penetrate into the PU macromolecular network and form hydrogen bonds with polar groups in it which weaken the interaction between adjacent molecules, thereby making the mechanics Performance drops. This process is reversible. After the water is discharged, it can still repair the mechanical properties of PU. The second is hydrolytic degradation which leads to a significant and permanent decrease 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 polyether PU under moisture and the leached acid stays in the material, further accelerating the catalytic hydrolysis (autocatalytic hydrolysis), while the polyether PU has both hydrolysis reaction and oxidation under moisture. reaction. For PU, acid, alkali and mold can accelerate the hydrolysis reaction.
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