1. Dihydric alcohol alcoholysis
The alcoholysis method has been the most studied in all the researches on the recycling of polyurethane waste by chemical methods. The technology is relatively mature and has formed a certain industrial scale. Using alcohol compounds as decomposing agents, under heating, the polyurethane waste is decomposed into polyether polyols, that is, alcoholysis. Polyurethane waste materials use ethylene glycol diols as alcoholysis agents, and react and degrade into low molecular oligomeric polyols under the protection of medium temperature or medium depression/catalyst and inert gas. The degradation products are stable and the composition is relatively simple. Easy to separate and purify. Glycol alcoholysis polyurethane mainly occurs in two types of bond scission, namely c-N bond scission and C-() bond scission, resulting in polyols or polyols and amine-terminated polymers. For rigid polyurethane foam, it is more suitable to be processed by alcoholysis process, which is characterized by mild alcoholysis conditions, lower reaction speed than hydrolysis and pyrolysis, and allows waste materials to contain other impurities, such as polyurethane or polyamide fibers, Polycarbonate and polymethanol, etc.
The alcoholysis reaction is related to the catalyst used. The catalysts used in the alcoholysis reaction include basic catalysts such as dibutyl tin dilaurate, tetrabutyl titanium, triethylene diamine, sodium hydroxide, potassium acetate, etc., which have high catalytic efficiency and are beneficial to the dissociation of urethane bonds to generate amines and carbon dioxide . The rate of alcoholysis is related to the chemical composition of waste materials, catalysts, reaction temperature, reaction time, type and amount of alcoholysis agent. Under the same conditions, the amount of catalyst used is much faster for alcoholysis. The amount of PMI foam alcoholysis agent is faster than that of alcoholysis, but when the ratio of the amount of alcoholysis agent to waste reaches 1:1, the reaction speed of alcoholysis agent will not increase much. As the amount of alcoholysis agent increases, the average molecular weight of the alcoholysis product decreases.
2. Alkali degradation method
The alkaline degradation method uses MOH (M is one or a mixture of Li, K, Na, and ca) as the degradation agent, and degrades the rigid polyurethane foam into oligomers at about 160-200°C. When non-polar solvents (esters or halogenated hydrocarbons) and water are added to the degradation products of PMI foam, the degradation products are divided into two layers. The upper layer is distilled to obtain polyols, which can be directly used to produce polyurethane foam again, and the lower layer is concentrated, Crystallized, recrystallized or vacuum distilled diamines can be added with phosgene to generate isocyanates. The disadvantage is that because the reaction is carried out under high temperature and strong alkali conditions, it requires high equipment, high production costs, and difficult industrialization.