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Physical recovery methods of waste polyurethane
Time:2021/3/4 9:16:32    

Physical recycling, that is direct recycling.It is directly recycled using physical methods without destroying the chemical rstructure of the polymer itself or changing its composition.The recycling methods of waste PU materials include thermo-compression molding, bonding pressure molding, extrusion molding and use as fillers, etc.,

1. Compression molding

The compression molding method is to cut the PU waste into 0.5-3mm pellets under normal pressure, preheat it at 140-200℃ for 2-12 mins and then heat it at high temperature (185195℃), high pressure (3080MPa) and high shear under the action of shear force for 1 to 3 minutes,so that the urethane chain (-NHCOOR) and the urea chain (-NHCONHR) between PU molecules may undergo a chemical reaction to form new chemical bonds,or can be bonded by coordination bond or hydrogen bond to combine PU particles and press them into finished or semi-finished products.

  The tensile strength, modulus of elasticity and elongation at break of recycled products obtained by hot pressing of waste PU have a large decrease, while the decrease in hardness and tear resistance is small, the surface finish of the product is poor as well. Therefore, it is only suitable for areas that do not require high elongation at break and surface properties, such as wheel covers, spare wheel covers, fenders, fender linings, small tool boxes and other passenger car parts.Generally only good dimensional stability, heat resistance and aging resistance are required.

  There is also a thermo-mechanical degradation and kneading technology in the hot press molding method to recycle waste PU, that is,mixed with certain thermoplastic polymer materials (resin) and finally hot-pressed into products after the action of heat and mechanical shearing force. The main point of this technology is that the recycled waste PU is heated to 150°C in a kneader to transform it into a softened plastic state.Due to the high frictional heat generated by kneading, the PU decomposes when the temperature reaches 200and then it is cooled to room temperature, crushed into powder in a pulverizer, then mixed with polyisocyanide (PI) powder and pressed at 150, 20MPa to get finished product.This technology uses thermo-mechanical degradation to make the polymer structure highly three-dimensionally branched with many functional groups, so it is easy to cross-link with high-concentration PI to obtain high-hardness products.Its performance is similar to that of hard rubber which can be used to make thick-walled or thin-walled products such as shells, tool boxes, packaging products and chassis.

2. Adhesive pressure molding

  This is the most common method in the recycling of waste PU.The main point is: first pulverize waste PU into flakes, sprinkle PU adhesive, etc. and then directly pass in high temperature gas such as water vapor to melt or dissolve the PU adhesive to bond the powdered waste PU, then add Press and solidify into a certain shape of foam.

  Adhesive pressure molding method is effective for the recycling of various waste PU, but it has the longest history of recycling waste soft PU foam. This method has also been used for the recycling of waste PU such as semi-rigid foam, rigid foam and reactive polyurethane (RIM) recently.The biggest drawback of this method is that the performance of recycled foam products is reduced.It is only suitable for low-grade parts such as furniture and automobile linings.Its application area is narrow, process is cumbersome, the labor needed is large and the economic value is not high.In addition, this technology has also begun to be used in the regeneration of waste polyolefin plastics.

3. Extrusion

After the waste is mixed with three types of thermoplastic elastomers, EPDM (ethylene propylene rubber), NBR (nitrile rubber) and SBS (styrene-butadiene-styrene block copolymer), granulate through an extruder and then process by injection molding, extrusion molding or calender molding to obtain finished products.

4. Used as filler

  It is to pulverize various PU wastes into particles with a particle size of about 3mm after screening and cleaning and then grind them into powders with a diameter of 180-300μm which are used as fillers and mixed with fresh PU raw materials to make finished products.According to the report by the US-China Chemical Company, waste PU as a filler is mainly used to produce RIM (reaction injection molding) products, energy absorbing foam and sound insulation foam. The literature reports that if the obtained waste PU powder is added to the raw material for the production of the original part and the same part is produced again,the amount can reach 20% since the powder has the same structure as the raw material, while the mechanical properties of the final product are not significantly weakened.In Japan, waste rigid PU foam has been used as a lightweight material for mortar.In addition, the waste PU elastomer can be used as an elastic layer or surface material for track and field stadiums and multi-purpose venues after being crushed.

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