The structure of polyurethane varies widely, and its performance can be adjusted in a wide range, and there are many kinds of products. PU products are divided into two categories: foamed products and non-foamed products. Foam products include soft, hard and semi-hard foams; Non-foam products include coatings, adhesives, synthetic leather, elastomers and elastic fibers (spandex).
Due to its excellent performance and wide application, PU has developed rapidly. However, factories that produce polyurethane foam at the same time will produce a large number of scraps, mold overflow and waste products every year, as well as wastes from various application fields of polyurethane, such as old polyurethane foam and elastomer in scrapped cars, which also need to be treated. At present, the recycling methods of polyurethane are mainly divided into three categories: physical recycling, chemical recycling and energy recycling.
I. Physical recovery method
Physical recovery is a method of directly utilizing waste materials without destroying the chemical structure of polymer itself and changing its basic composition.
① Burying method
Burying is the most primitive method of garbage disposal. It adopts the method of burying, which makes the garbage decompose in the soil for a period of time at a certain temperature and humidity, and gradually become harmless substances, while the polyurethane waste is difficult to decompose by burying. With the decrease of available landfill space and the need of resource recovery, landfill is no longer applicable.
② crushing method
Polyurethane waste and old waste are cut or crushed and screened before application to obtain small pieces or fine powder with required particle size. Generally speaking, rigid polyurethane foam is easy to break, so its breaking technology is relatively mature, and most of them have been put into commercialization, such as precision cutting technology, Flachmatritsen extrusion and other technologies. It can be crushed into particles with the particle size less than 1 mm, and the fine fragments or powder of this waste polyurethane after crushing are mostly used as fillers to be mixed into raw materials for recycling. Dow Chemical Company claims that the cost of reusing waste polyurethane as filler to produce RIM products is lower than that of using new raw materials. In Japan, waste rigid polyurethane foam has been used as lightweight aggregate of mortar.
③ Bonding, processing and molding.
This method is the most common method in recycling waste polyurethane. The key points are as follows: firstly, the waste rigid polyurethane foam is crushed into fine sheets, then the polyurethane adhesive is spread, and then high-temperature gas such as steam is directly introduced to melt or dissolve the polyurethane adhesive, then the powdered waste polyurethane is bonded, and then the foam is solidified into a certain shape under pressure.
④ Extrusion molding
Another method of bonding processing is extrusion molding. Extrusion molding is to transform molecular chain into medium-long chain and PU material into soft plastic material through thermodynamics. This material is suitable for plastic parts with high strength and hardness, but not high requirements for elongation at break. For soft microcellular PU foam waste, it can be crushed into powder, mixed with thermoplastic polyurethane, granulated in an extrusion molding machine, and made into soles and other products by injection molding. Bayer in Germany has done research in this respect.
⑤ Others
Cut the corner waste produced in production into small pieces, which can be directly used as packaging buffer filler or padding. Polyurethane foam can also be used as artificial soil and natural soil cover. Adding water and chemical fertilizer to open-cell flexible polyurethane foam can cultivate many kinds of plants, which grow rapidly without pests and weeds.
Second, the chemical recovery method
Chemical recovery method refers to the degradation of polyurethane into reusable liquid oligomers or even small molecular organic compounds in the presence of chemical reagents, catalysts, heat and air, thus realizing the recycling of raw materials. Its advantage is that the insoluble thermosetting polyurethane waste can be recovered.
The general technological process of recycling waste polyurethane by chemical method is as follows: sorting waste PU materials, washing, pulverizing into particles, putting them into a reaction kettle, adding a degrading agent at about 200℃, vacuum distillation for separation and purification, inspection and warehousing.
① alcoholysis
At present, alcoholysis is the most widely studied and applied method, and its main purpose is to recover reusable polyols to synthesize polyurethane materials. Generally, low molecular alcohol is used as the degradation agent, and under the action of some catalyst, at 150~ 250℃? Polyurethane can be degraded into oligomer in the temperature range of 0℃ and normal pressure, and the degradation products obtained by this method can be used directly. As for the alcoholysis mechanism, most people think that the main reaction in the alcoholysis process is that under the action of alcohol and catalyst, carbamate groups in polyurethane are broken and replaced by short alcohol chains, releasing long-chain polyols and aromatic compounds:
r 1NH coor 2+hor 3 oh r 1nhcoor 3 oh+R2OH
Because many groups participate in the reaction during the degradation process, many side reactions will occur. The main side reaction is that under the action of alcoholysis agent, urea group breaks to form amine and polyol;
r 1NH CONH R2+hor 3 oh 1NH coor 3 oh+r2nh 2
② ammonolysis
Polyurethane foam is easy to decompose in amine, and its reaction is very similar to transesterification. From polyurethane or polyurethane-urea, compounds with relatively low molecular weight containing hydroxyl and amino groups and unsubstituted urea are generated. The reaction is characterized by low temperature and can be carried out at 150℃. Under appropriate conditions, the produced polyol can be separated from amine. During the period of 1997, Russian Anon used hexamethylenediamine as ammonolysis agent to study cross-linked polyurethane rubber, and the obtained ammonolysis product was used as catalyst for semi-rigid polyurethane foam.
Polyurethane foam is easily decomposed into hydroxyl and amino compounds in amino-containing compounds.
③ Alcoholamine method
At 80- 190℃, polyurethane can be degraded into oligomer by using alkanolamines such as monoethanolamine, diethanolamine and dimethyl ethanolamine, and catalysts such as NaOH, Al (OH)3 and sodium methoxide can promote the degradation reaction speed of polyurethane. The main reactions of ethanolamine method are carbamate cleavage and urea cleavage.
④ Alkali degradation method
Alkaline degradation method is to degrade rigid polyurethane foam into oligomer at about 160 ~ 200℃ with MOH(M is one or more of Li, K, Na and Ca) as degradation agent. When nonpolar solvent ester or halogenated hydrocarbon and water are added to the degradation product, the degradation product is divided into two layers, the upper layer is distilled to obtain polyol, which can be directly used to produce polyurethane foam again, and the lower layer is concentrated, crystallized, recrystallized or vacuum distilled to add phosgene to generate isocyanate. The disadvantage is that the reaction is carried out under the condition of high temperature and strong alkali, which requires high equipment, high production cost and difficult industrialization.
⑤ Hydrolysis method
In 1970s, it was found that polyurethane flexible foam was degraded into diamine and polyether polyol by hot water steam under certain pressure. However, the hydrolysis of polyurethane is different from the hydrolysis of polyester, and it is not the reverse reaction of polymerization. In addition to diamines and polyols, CO2 is sometimes released from the hydrolysate. During the hydrolysis reaction, the reaction can be accelerated by increasing the temperature and pressure or in the presence of solvent. After separation and purification, polyurethane can be synthesized from polyol, and diamine can also be converted into isocyanate. Because hydrolysis is carried out at high temperature and high pressure, it requires high conditions and equipment, and the purification technology of hydrolysate is very difficult, so this method has not been widely used.
⑥ Hydrogen degradation method
Hydrogen degradation method is theoretically suitable for the recovery and utilization of all organic compounds. After the waste is crushed, it is put into a hydrogenation reactor to react at 40MPa and 500℃, and the degradation products similar to refined oil products can be obtained. However, due to economic factors, hydrogenolysis is only applicable when there are a large number of PU wastes to be treated.
⑦ Thermal degradation method
Thermal degradation method generally destroys the structure of waste materials in inert gas or oxidizing atmosphere at a high temperature of 250 ~ 1200℃ to obtain a mixture of gas and liquid fractions. At present, this method is mainly suitable for recycling the mixture of waste plastics and waste rubber tires, and the recycling of polyurethane waste is still in the early stage of development.
⑧ Phosphate ester method
Phosphate ester method is a new theory to degrade polyurethane. Under the action of dimethyl phosphate, diethyl phosphate and tris (1- methyl -2- chloroethyl) phosphate, polyurethane will be degraded. The product obtained by degrading polyurethane with phosphate ester contains phosphorus, which can be used as a non-reactive additive to improve the flame retardant performance, and can also be used to synthesize flame retardant polyurethane after being treated with hydroxyl-containing compounds, amines or metal salts.
Third, the energy recovery method
When polyurethane is burned, its calorific value is about 7000kcal/ kg, and the heat it can provide is equivalent to the energy provided by coal with the same weight. When the physical recovery and chemical recovery of waste polyurethane are influenced by technical and economic factors and have no practical significance, the waste can be crushed into particles and used as fuel to replace coal, oil and natural gas to recover energy for baking cement or generating electricity. Due to the high recovery cost of polyurethane materials treated by chemical methods, almost all polyurethane foam recovery methods in Japan adopt incineration treatment.
The United States Polyurethane Industry Alliance (API) conducted a series of experiments, pointing out that adding waste polyurethane elastomer and other components of solid plastic waste (up to 20%) to municipal solid waste can obviously improve its fuel calorific value. Although the volume of polyurethane after sintering will be reduced to 1% of the initial volume, which will reduce the volume of polyurethane waste, it will bring secondary pollution. In the process of energy recovery, a large number of gases such as NOX, HCl and trace CHCl3 will be produced, which is very harmful to the environment. Therefore, if energy method is needed to recover polyurethane, the emission of reaction products must be strictly controlled.