Chain-extraction chemical recovery is the introduction of some chemical groups in the recycled PET molecular chain to extend the molecular chain and improve the performance of recycled recycled materials.
The residual moisture in PET pellets and their contaminants (PVC, adhesives, etc.) will cause pyrolysis and hydrolysis of recycled PET, which will cause some problems related to the reduction of the relative molecular mass, which will lead to its intrinsic viscosity. decline. The reduction of these two parameters will affect the mechanical properties of the reclaimed material, and the use of the chain extender to extend the chain can compensate for the reduction of the molecular chain length. These chain extenders are polyfunctional compounds that are thermally stable and can work with PET under conventional processing conditions. The rapid reaction of the terminal hydroxyl group and carboxyl group o' this process is generally irreversible. Commonly used chain extenders are diisocyanate, bisoxazoline, di-anhydride, diepoxide, carbodiimide, and the like. The main chain chemical structure of the recycled material obtained by this method has other characteristic groups such as carbamate besides the main PET.
Diisocyanate
N. Torres et al. added the chain extender hexamethylene diisocyanate (HMDl) during processing. New PET pellets or recycled PET bottle tablets containing 0.9% HMDI were injection molded into standard test bars. After the chain extender modified to recover the relative molecular mass of Mw. 51,300 g/mol, intrinsic viscosity (η) of 0,84 dL/g, higher than new material PET (Mw is 40,500 g/mol, [η] is 0.72 dL/g), mechanical properties are also improved . From the industrial point of view, chemical modification of recycled PET after recycling is a good method. The obtained and recycled materials have properties similar to those of the new PET.
Bisoxazoline
George P. Karayannidis and EleniA. Psalida first prepared the 2,2-(1,4-phenylene)bis(2-oxazoline) (PBO) using a two-step method and used it as a chain extender to modify and recover high molecular weight PET. The beverage bottle oPBO has a very high activity for macromolecules containing terminal carboxyl groups but no hydroxyl end groups, whereas PET has these two groups. For better results, phthalic anhydride was initially added to the sample prior to the addition of PBO to react with the hydroxyl end of the initial polymer, successfully adding carboxyl-terminated carboxyl groups. The chain extension modification of the initial PET sample proved that PBO is a very effective chain extender. The initial intrinsic viscosity of recycled PET was 0.78 dL/g; the [η] of PET obtained without the chain extender during recovery was 0.69 乩/g, and the molecular weight was Mn=19,800; the chain extension was added. The PET (η) prepared at a reaction time of 5 min had a value of 0.85 dL/g, M = 25,600, and did not produce unwanted by-products.
Homopolytetracarboxylic dianhydride
Firas Awaja and Fugen Daverl used the chain extender homopolytetramine dianhydride (PMDA) in a commercial reactive extrusion processing system for the recovery of PET flakes, and studied the relationship between PMDA concentration and extruder residence time for recovered PET. Effect of Performance When the mass fraction of oPMDA is 0.3%, the extruded PET has a higher (η) and lower carboxyl content than the ordinary extruded PET, compared to other contents, PMDA is 0. At .2%, its [η] is higher and the carboxyl content is lower. At a residence time of 45 s and a PMDA of 0.15%, the [η] of PET is 0.73 dL/g, which is obtained by ordinary extrusion. PET (ηh) was only 0.67dL/g. (η) increased linearly with residence time when the residence time was higher than 45 s. In the studied PMDA content range, the elastic modulus of the recycled PET after reactive extrusion was higher than that of the recycled PET sample. High; elastic modulus drops when the residence time is high
Tetrafunctional epoxy additive
SoniaJapon et al. use a tetrafunctional epoxy additive to induce recycled PET to produce randomly-branched molecules, increasing the extensional viscosity of recycled PET.
Second, physical recycling
Physical recycling of PET bottles (also called mechanical recycling) includes the following three methods: simple recycling, dissolution/reprecipitation, and melt extrusion. In general, the chemical composition of the material during physical recovery should not change. The physical recycling method has low cost, relatively low investment and small impact on the environment, but the recycled material will still contain a certain amount of impurities, and the recycling process will degrade the product's sexual energy.
Simple recovery method
The simple recycling method refers to a method of obtaining a regenerated waste PET bottle sheet of a desired size by sorting, washing, and crushing. The PET flakes thus recovered and recycled can be used for chemical recovery or shaped articles and can even be reused for the production of PET beverage bottles.
Dissolution/Reprecipitation
The dissolution/reprecipitation method is a method of recovering a PET bottle by a process such as solvent dissolution and precipitation.
J. G. Poulakis et al. studied the use of a dissolution/reprecipitation process to recover PET bottles. In a 2L vessel with a stirrer, reflux condenser, and thermometer, 0.5 L and 1.5 L PET bottles were minced and dissolved in a concentrated solution of N-methyl-2-pyrrolidone (NMP) at 165 °C. A solution with an upper concentration limit of 0.2 kg/L was formed and stirred for 90 minutes to dissolve the polymer. After addition of n-octane, the polymer precipitated from the solution as fine particles and was filtered and washed with n-hexane. Polymer and solvent recovery is very good. This dissolution/reprecipitation process is performed once or twice without affecting the relative molecular mass properties of the final material. After removing other soluble or insoluble substances, the recovered material is completely comparable to the new material in many different applications. This process division is very often suitable for separating PET from other polymer wastes.
Melt extrusion
In the melt extrusion method, the classified PET bottles are first crushed into pieces, washed with water, decontaminated, dried, and then subjected to processes such as extrusion (usually a twin screw extruder), melt filtration, and granulation to obtain recycled PET pellets. Methods. In addition to pure melt extrusion, melt extrusion is often performed by adding blended modifying components to improve the properties of the regrind.
Blend PP-g-MAH modification
Abdulrasoul Oromiehie and AlirezaMamizadeh used single-screw, co-rotating twin-screw and counter-rotating twin-screw extruders to study the recovery of PET bottles and their performance improvement. The results show that the intrinsic viscosity of the recycled PET mixture decreases as heat history and recycled PET content increase. This shows that as the content of recycled PET increases in the mixture, the thermal history and shear degradation of the recycled PET reduce the relative molecular mass and the melt viscosity decreases.
In order to improve the performance of recycled PET, Abdul-rasoul Or_omiehie and AlirezaMamizadeh added different amounts of maleic anhydride grafted polypropylene (PP-g-MAH) to the new PET/recovered PET (65/25, mass ratio). The results showed that the intrinsic viscosity and relative molecular mass of the system increased with the increase of the amount of PP-g-MAH (from 5% to 10%). This is because the PP-g-MAH functional group in the molten state reacts with the PET end group or ester group to form a block or graft copolymer at the interface, which improves the adhesion and mechanical properties.
Blending SEBS-g-MAH toughening
Zhong Yuzhen et al. used MAH grafted styrene-ethylene/butylene-styrene triblock copolymer (SEBS-g-MAH) to toughen and recover PET material. The results show that the addition of 30% SEBS-g-MAH improves the notched impact strength of recycled PET by more than 10 times. SEM shows that SEBS-g-MAIl is uniformly dispersed in the recovered PET, but the size is somewhat large, ranging from 0.89 txm to 0.981 xm. To obtain a super-tough mixture, a finer dispersion is required.
Solid phase extrusion modification
This is an improved melt extrusion method. Tang Qiqi et al. studied the crystallization performance and degradation degree of solid phase extrusion PET. The material used was an irregularly-recovered PET bottle tablet having a diameter of 3 to 5 mm, and [η] was 0.825 dL/g. The recycled PET bottle tablets were extruded in a twin-screw:extruder. The solid-phase extrusion method uses a processing temperature of 100°C and a spindle speed of 50 r/min. DSC, research shows that solid-phase extrusion recovery. PET has a higher crystallization rate, and its crystallinity is 10% higher than melt-extrusion-recovery PET, and its grain size distribution is narrower. Melt flow rate and intrinsic viscosity tests have found that solid phase extrusion causes low degradation of recycled PET. Therefore, relative to the commonly used melt processing methods, the solid phase extrusion method can effectively reduce the degradation of the PET during processing, and the material crystallization is more complete and favorable: to improve its mechanical properties
Conclusion
Traditionally, waste plastics are all discarded, landfilled, or burned, regardless of their type, performance, or use, causing serious environmental pollution in society. Due to the requirements of environmental protection, the increasing public awareness of environmental protection, the energy crisis, the urgent need for resource utilization, and the reduction of land resources, with the increasing use of PET bottles, the recycling of PET bottles is getting more and more. Concern, its recycling has become a problem that must be resolved. Because there are many methods for recycling PET bottles, this paper only categorizes the main methods. Many methods have entered the commercialization process, their recycling application technology is also very mature, and the variety of products obtained.
Source: China Packaging News