Chemical recycling of waste polymer materials

Chemical cycle is one of the important methods of polymer material cycle, which refers to the degradation reaction of polymers under the action of heat and chemical reagents to form low-molecular weight products, which can be further utilized, such as monomer repolymerization, oil products can be further processed. At present, the main methods of chemical cycle are chemical degradation. Chemical degradation can be divided into depolymerization, pyrolysis, hydrogenation and gasification.

The present situation of chemical cycle development of polymer materials

1.       Step - by - step polymer material

The progressive polymer materials mainly include polyester and polyurethane, which are represented by polyethylene terephthalate. Mainly used for film, fiber and fabric, beverage bottle, etc. Waste materials can react with polyols in the presence of catalysts and the products can be condensed with unsaturated polyacids to make unsaturated polyresin. Different esters can be obtained by alcoholysis with different alcohols, either as monomers or as plasticizers. PET can be hydrolyzed under acidic or alkaline conditions. It can be hydrolyzed under normal pressure in strong acid (such as sulfuric acid and nitric acid) medium. The hydrolysis rate is fast. If it is hydrolyzed in an aqueous solution of alkaline (such as NaOH), 3-sh should be reacted at 21 2500C and 1.4-2.0mpa. After the reaction, TPAO weak alkali (such as hydrogen and oxygen) can be precipitated, and can also be used to hydrolyze PET waste to obtain monomer. Saponification reaction under atmospheric pressure has been applied in silver and TPAO polyurethane is from recycled PET film condensation polymer materials, can be hydrolyzed into the pluralistic alcohol and amine, with a special extruder hydrolysis, dibasic acid can be obtained by the purification of the product and diamine, diamine and reaction with phosgene, preparation of diisocyanate, used for foam production. But the cost of this process is large and the recovery benefit is not high. PU alcoholysis is a widely used method at present. PU waste materials can be alcoholyzed to obtain polyol mixture, which cannot be separated effectively at present, but this product can be used as a component in the manufacture of foam plastics and elastomers.

2.       Additive polymer material

Polystyrene (ps) in addition to used as paint, binder, but also used to crack styrene ps under the action of heat can be cracked into styrene, the yield of 65%, above. Japanese scientists have cracked PS in solvent method, decomposed it at 400 to 500 ℃ for 1 20min, and the condensate obtained can be distilled into styrene with a purity of 96%.High purity styrene can be obtained by fractionation using metal oxide as catalyst in melting state (>350 ℃).In addition, lead alloy is used as heating medium to crack ps. Under the action of appropriate catalyst or irradiation, polyolefin can undergo chemical reactions to form materials with good performance, such as polyethylene (PE), which can be crosslinked with crosslinking agents (such as peroxides) to produce PE materials with good performance. Waste polyolefin can be chlorinated, and the chlorinated polyolefin can be used in binders and coatings. In addition, polymer polymerization to make oil is a common cycle method.

3.       Hybrid polymers and composites

A common method of making use of a variety of polymer mixtures is cracking them to make oil. The mixture is cracked at high temperatures to produce gas and oil, which can be used as fuel or refined directly at the refinery. Chemical plants have high requirements on the organochlorine content of oil, generally no more than 10 * 10-6. However, the organochlorine content of oil obtained from cracking of plastic mixture can reach (50 to 200) * 10-6, so it is very important to dehalogen before cracking or during cracking. In addition, waste plastics often contain heavy metal element compounds, cracking oil refining to consider the poisoning of the catalyst. Most of the resins of composite materials are thermosetting resins such as unsaturated polyvinegar resin, epoxy resin and phenolic resin, etc. The waste materials of composite materials are not only used as powder filler, combustion to take heat and chemical auxiliary fuel, but also used for cracking and recycling oil products and raw materials. For example, after the decomposition of glass fiber reinforced plastics at 380 ℃ at atmospheric pressure of 50 ℃, the further decomposition at 450 ℃ and 550 ℃ can obtain oil products. Due to the large amount of residue, a special decomposition furnace needs to be designed to complete the pyrolysis process, and the research work is still under way. Another example is the pyrolysis of phenolic resin in the experimental vulcanization bed (722 ℃), the products include aliphatic hydrocarbon (mass fraction is 5 · 24%), phenol (8, 25%), carbon black (42).2%), gas (24, 3%), etc.

Chemical cycle process and equipment

1.       Reaction Still

Reaction kettle is a common chemical equipment for chemical circulation, and its matching equipment is condenser, storage tank, distillation tower, etc. The products of raw materials (such as polyolefin) degraded in the reaction kettle can be monomer, chemical raw materials, etc., such as PET pyrolysis and then polymerization into PET, or unsaturated polyvinegar resin can be produced. According to the demand, the reactor can be designed into a tank reactor to facilitate heating and slag removal. The reactor can also be designed as a tubular reactor, which can increase the cracking temperature, shorten the time, and continuously crack, suitable for the cracking of ps, PMMA and other polymers.

2.       Fluidized bed reactor

Fluidized bed reactor is a kind of bed reactor. W. kanaminsky et al., Germany, used propane combustion to heat carrier gas or water vapor, and used heater to heat sand and carrier gas to 500 ° c at 00 ° c. The carrier gas should be enough to promote sand fluidization in the reactor. The polymer is squeezed into the fluidized bed by the extruder, and the polymer material is cracked in the fluidized bed. The resulting gas and carrier gas are separated by condensation and separation, and the pyrolysis products are obtained. Fluidized bed cracking apparatus has the advantages of fast heating, high efficiency, even cracking temperature and closed system. Cracking reaction is best carried out under inert carrier gas. If air is used as carrier gas, the product is easy to be oxidized and the thermal energy of the obtained oil is 10700 lower.

3.       Extrusion cracking equipment

Extrusion cracking equipment is composed of two extruders in series. The first extruder has a vent hole. The waste polymer materials were cracked at low temperature on the first extruder. The main purpose was to remove HCI from the waste materials. The intermediate products of cracking are then entered into the second extruder for high-temperature cracking, which turns the polymer into a low-molecular compound or oil or gas. After separation, HCG cracking reaction can be carried out continuously, which can be decomposed mechanically and thermally. In addition, new devices are being researched and developed.

Development and application of degradable polymer plastics

Development and application of biodegradable polymer plastics, biodegradable plastics is a kind of new type with the function of degradation of polymer materials, in use process, it has to do with the same kind of common plastic with the corresponding health and relevant application performance, and after its complete function, the material can rapidly degraded in the natural environment conditions become easy to be given environment fragments or crushed, and with the passage of time further degradation become eventually oxidation products (CO2 and water), return to nature. 

Based on the environmental pollution caused by plastic waste, as well as the demand of environmental protection and human needs, it is urgent to study degradable polymer materials. In a specific time and under certain environmental conditions, the chemical structure of biodegradable plastics will change. According to the reasons for the changes in its chemical structure, biodegradable plastics can be divided into two categories: biodegradable plastics and photodegradable plastics.

1.     Degradation mechanism of degradable plastics

Generally speaking, degradable plastic refers to a kind of plastic that can be decomposed into small molecules through the action of microorganisms in soil or solar radiation.It must meet the requirements of the use of products and easy to process on the basis of the basis of biodegradable properties. The nature of the action of sunlight on polymer materials is the comprehensive effect of ultraviolet light in sunlight and oxygen in air, so it is also called photooxidation degradation. Take polyolefin as an example to explain the mechanism of photooxidation degradation. In essence, photooxidation causes chain breaking or crosslinking of polymers, and some oxygen-containing functional groups, such as carboxylic acids, peroxides, ketones and alcohols, are formed in this process. Catalyst residues in polymers and initiation of peroxide and carboxyl groups introduced during processing are the main sources of degradation.

Under the action of microorganisms (mainly fungi, bacteria or algae, etc.), polymers can be eroded or metabolized to cause changes in their chemical structure and decrease in molecular weight. The mechanism of action can be mainly divided into two situations :

(1) biophysical action. That is, after the erosion of plastic products by microorganisms, biological cells growth, promote the decomposition of polymers, ionization or proton, this physical action on the polymer caused mechanical damage, the high molecular weight of the polymer into oligomer fragments, so as to achieve the purpose of physical degradation.

(2) biochemical action -- direct action of enzymes. This situation is caused by the erosion of enzymes secreted by fungi or bacteria, which leads to the splitting or oxidative disintegration of plastics, and causes the splitting or oxidative degradation of insoluble polymers into water-soluble fragments, generating new small molecular compounds (CH4, CO2 and H2O) until the final decomposition.

There are generally two hypotheses about the mechanism of biodegradation of polymer materials that lead to biodegradation. The other is an invasive cut from the end of the chain. Therefore, the structural properties of materials, such as composition, main and side chain structure, size of end groups, and presence or absence of spatial steric resistance, are the key factors affecting their degradation performance. Among them, the main chain properties have a greater impact. If the main chain of the polymer contains bonds that are easily hydrolyzed, it will be easily biodegraded. Secondly, if the backbone is flexible, the degradation rate will be relatively fast, whereas if the backbone is rigid and orderly, the degradation rate will be slow. 

The biodegradability of polymer materials is reduced by branching and crosslinking. For example, the introduction of hydrophobic groups at the end of polylactic acid (PLA) molecular chain can reduce the erosion rate at the initial stage of degradation. This is because in the original degradation process, PLA's erosion mainly depends on the structure of molecular chain end, and the addition of hydrophobic groups leads to the decline of its erosion rate. In addition, some researchers have studied the chemical structure of polymers and the relative molecular weight of materials that play an important role in their degradation.

2.    Development of biodegradable plastics

The development direction of biodegradable plastics in the future can be as follows:

(1) biodegradable plastics were prepared by studying the biodegradation mechanism of degradable polymers, and the block copolymerization of biodegradable plastics with existing ordinary polymers, microbial polymers and natural polymers was studied and developed.

(2) to search for microorganisms that can produce polymer plastics, explore new polymers, analyze their synthesis mechanism in detail, improve their productivity through existing methods and genetic engineering methods, and study efficient methods of cultivating microorganisms.

(3) pay attention to the control of degradation rate, develop efficient degradation promoters and stabilizers to improve the biodegradation performance of degradable plastics, reduce their cost, and expand the market application.

(4) research and establish a unified definition of degradable plastics, enrich and improve the evaluation method of biodegradation, and further understand the degradation mechanism.

Measurement for shear viscosity spectrum of polymer melts by using screw extruder capillary

Most polymer materials are processed in the melt state, which involves melt flow and deformation, which not only affects the processing process itself, but also affects the final performance of the product. Therefore, the study of rheological properties of polymer materials has been a hot topic. Accurate measurement of rheological parameters is the basis of in-depth study of rheological properties.

Shear viscosity is an important parameter to characterize the rheological behavior. The so-called shear viscosity of polymer melt is the ratio of shear stress and shear rate that melt is subjected to in the process of flow. Polymer melt causes pseudoplastic fluid, and its flow behavior has the characteristics of shear thinning. It is usually necessary to use the relationship curve between shear viscosity and shear rate, namely shear viscosity spectrum, to fully reflect the processing characteristics of polymer melt.

The basic method of measuring melt viscosity is to try to make the melt flow through a long and thin capillary tube, such as a round capillary tube. The shear stress can be calculated by measuring the pressure drop at both ends of the melt as it flows through the capillary tube. The shear rate can be calculated by measuring the flux of melt per unit time. Thus the melt viscosity can be obtained.

The conventional way to get the melt out of the capillary tube is to use piston propulsion. The advantage of this method is that it USES fewer test materials and can obtain higher shear stress. The high pressure capillary rheometer is based on this principle [4].However, the disadvantage of this test method is that the material cannot be tested under the actual processing conditions, and it is difficult to obtain the rheological properties of the polymer melt when it is processed. Especially in the study of blending modification of several polymer materials, the polymer melt needs the strong shearing action of screw to achieve the purpose of blending. High pressure capillary rheometer is not suitable for testing such materials.

The screw extrusion capillary rheological test device can solve the above problems. The device USES the propulsive force of the screw to make the polymer melt flow through the capillary tube. Therefore, the shear viscosity of polymer melt can be measured under conditions closer to real processing. This method is particularly suitable for the measurement of the rheological properties of thermoplastic materials and their mixtures. Because the measurement simulates the real experimental environment, the obtained test parameters can more accurately describe the behavior of materials in actual processing.

Shear viscosity spectra of polymer melts can be measured using specialized test instruments, such as high pressure capillary rheometers, or combined revolutions. However, these devices are expensive and limited in practical use, especially in the application of large-scale industrial production. In fact, it is not necessary to rely on the special test instrument, as long as the shear viscosity test principle, you can use a simple small single-screw extruder and capillary mold, constitute a low-cost shear viscosity spectrum test device. Combined with computer data processing, the shear viscosity spectrum of polymer melt can be easily and quickly obtained. This method is especially suitable for small and medium-sized enterprises to carry out product development and raw material inspection.

Polymer plastic properties

1.Definition

The main components of polymer plastics are mainly polymer compounds, whose molecular weight is about 1 000, which is only for the physical concept. At present, polymer plastics are widely used, but the daily use of plastic is synthesized by polymer compounds, usually called high or macromolecule, is synthesized by monomer raw materials or raw materials into integrated materials, and through the later development, and fusion resin, color, stabilizer and other additives. The shape of polymer plastic can be changed freely and can be designed according to the user's requirements.

2.Characteristics

Polymer plastics not only have their own unique, that is, arbitrary changes in the shape, but also have viscoelasticity. Polymer plastic, if subjected to external forces, its body will produce high elastic deformation, viscous flow, polymer plastic deformation is mainly related to time. In addition, polymer plastic also has high specific strength, low strength and other characteristics, the strength of polymer plastic is generally lower, but its density is lower, so the strength becomes higher. Polymer has many characteristics, such as high stability, high wear resistance, expansion, high insulation, so it is widely used in various fields, by many people's favor.

The types of polymer plastics are also diverse. At present, the classification of polymer plastics in China mainly includes seven categories: rubber, polymer coatings, polymer materials, matrix composites, plastics, fibers and plastics.

a.   Rubber.

The price of rubber molecular chain is small, flexible, this is from the point of view of the field of physics. Rubber with the size of the external force will change its shape, with instability, if the external force removed, the original state of rubber can be quickly restored.

b.  Polymer adhesive.

The adhesive materials of polymer adhesive are mainly natural compounds. In practice, it is usually divided into two kinds: natural, synthetic adhesive. The most widely used is synthetic adhesive.

c.   polymer coating.

The main components of polymer coatings are mainly polymers, which are made in a relatively simple way, mainly adding solvents or additives in its production process. Polymer coating is divided into 3 kinds commonly, namely grease, synthetic resin and natural resin, often use in daily life.

d.  polymer materials.

The commonly used high molecular weight materials mainly include high molecular enzyme and high molecular transparent materials. Polymer materials have certain functions: energy, material, information conversion, transmission and magnetism.

e.   fiber.

Fiber is common in daily life, usually divided into two kinds: natural fiber and chemical fiber. Fiber has the characteristics of small deformation force, small secondary valence force and high modulus in physical concept.

f.    Plastics.

Plastic is a kind of material that USES more at present, also be the commonest material. The main components of plastics are usually synthetic resins, natural polymers, and the integration of other additives, such as plasticizers, fillers and so on. Plastic is generally based on synthetic resin, so it can be divided into thermosetting plastic, thermoplastic.

3. Physical properties

In general, if the polymer plastic placed in isothermal conditions, easy to produce the product. In the actual processing, the crystallization temperature is not completely consistent. If the crystallization temperature difference is large, it will affect the crystallization process. At the same time, high stress is often formed in the process of spinning forming, film stretching forming and extrusion forming, which leads to the tendency of accelerating the product. In addition, plastics apply techniques in shear stress. And under the influence of tensile stress, the melt will form longer fibrous crystals. The higher the stress and strain rates are, the more elongated chains and the higher the melting point.

4. Chemical properties

Generally speaking, if the polymer plastics are placed in high stress and high temperature conditions, the molecular structure will have some changes. In this reaction, polymer plastics usually release a large amount of degraded substances, thus releasing more harmful substances. To this end, in the processing should be clear raw material indicators, choose high-quality raw materials. At the same time, a small amount of antioxidant and stabilizer can be added in the formula to improve the anti-degradation performance of the polymer.