Characteristics and Recycling of Plastic PET

When recycling polyethylene terephthalate, there are usually three ways to distinguish:

Chemical recycling is recycled into the initial raw material, this process leads to the complete destruction of the polymer structure and is converted into pure terephthalic acid (PTA) or dimethyl terephthalate (DMT) and ethylene glycol (EG), or It is transformed into intermediates in the process, such as bis (2-hydroxyethyl) terephthalate.

By mechanical recycling, the original properties of the polymer are maintained or reconstructed.

In the chemical recycling of the transesterification reaction process, other diols/polyols or glycerin are added to prepare polyol polyester, which can be used in other fields, such as polyurethane production or polyurethane foam production.

As long as there is a high-capacity recycling line of more than 50,000 tons per year, the chemical recycling of PET polyester will be cost-effective. Such production lines can only be seen at the production sites of very large polyester manufacturers, if they exist. There have been many attempts to build such a chemical recycling plant, but none of them have achieved great success. So far, even the promising Japanese chemical recycling industry has not made an industrial breakthrough. There are two main reasons for this situation: First, it is difficult to continuously purchase large quantities of waste plastic bottles at a single location, and second, the purchase price of waste plastic bottles has been rising and fluctuating. For example, between 2000 and 2008, the price of packaging bottles rose from 50 Euros/ton to more than 500 Euros/ton in 2008.

At present, PET polyester is recycled or recycled directly in various forms in a polymer state. These processes are typical models of small and medium-sized factories. The cost-effectiveness of factories with a production capacity of 5000-20,000 tons/year can be realized. In this case, almost all types of recyclable materials today may be fed back into the material cycle. These different recycling processes will be discussed in detail below.

In addition to the chemical contaminants and degradation products produced during the first processing and use, mechanical impurities are the main impurity part that causes quality degradation on the recycling line. Recycled materials are increasingly being introduced into manufacturing processes, although these processes are initially designed for new materials only. Therefore, efficient sorting, separation and cleaning processes have become essential for the recovery of high-quality polyester.

Speaking of the polyester recycling industry, our main concern is the recycling of PET polyester plastic bottles, which can be applied to various liquid packaging, such as water, carbonated soft drinks, juices, beer, soy sauce, detergents, household chemicals, etc. Because of the shape and similarity, the bottles are easily identifiable and can be separated from the waste plastic stream through an automatic or manual sorting process. The established polyester recycling industry is mainly composed of three parts:

Collection and waste separation of PET polyester plastic bottles: waste logistics

Production of clean flakes: flake production

Conversion of PET flakes into final products: flake processing

The first part of the intermediate product is PET bottle waste with more than 90% polyester content. The most common form of transaction is bales, but bricks and even loose, pre-cut bottles are also common in the market. In the second part, the recycled bottles are converted into clean PET PET flakes. This step may be more or less complicated, depending on the final quality of the sheet required. In the third step, PET polyester bottle flakes are processed into any kind of products, such as films, bottles, fibers, filaments, ties or intermediates, such as pellets for further processing and engineering plastics.

In addition to this external (post-consumer) form of polyester bottle recycling, there are many internal (pre-consumer) recycling processes. Waste polymer materials will not leave the production site and enter the free market, but will be reused in the same production loop. In this way, fiber waste is directly reused to produce fibers, pre-finished waste is directly reused to produce pre-finished products, and film waste is directly reused to produce films.

10.1 PET polyester plastic bottle recycling

Purification and decontamination

All successful recycling concepts conceal the efficiency of purification and decontamination, which need to be carried out at the right time during the processing and be able to reach the necessary or desired level.

Generally speaking, the following principles are universal: In the process, the earlier and more thoroughly the foreign matter is removed, the more effective the process will be.

PET polyester requires a plasticizing temperature of up to 280°C (536°F), which is why almost all common organic impurities such as PVC, PLA, polyolefin, chemical wood pulp and paper fibers, polyvinyl acetate, melt adhesives, Colorants, sugars, and protein residues are all turned into colored degradation products, which may release additional reactive degradation products. This significantly increases the number of defects in the polymer chain. Impurities have a wide particle size distribution, and large particles (visible to the naked eye and can be filtered) with a size of 60–1000 µm have less impact because their total surface area is relatively small, and therefore the degradation rate is slower. The influence of tiny particles is relatively large (because there are many), and they increase the frequency of defects in the polymer.

In many recycling processes, the motto "Out of sight, out of mind" is very important. Therefore, in this case, in addition to efficient sorting, the removal of visible impurity particles through the melt filtration process also has a special effect.

Workers sort the incoming plastics that are mixed with non-recyclable waste.

Bundles of crushed blue PET plastic bottles.

Bundles of crushed PET plastic bottles are classified by color: green, transparent and blue.

Generally speaking, there are many ways to make PET flakes from collected bottles, because different waste streams are different in composition and quality. From a technical point of view, there is not only one way. At the same time, there are many engineering companies that provide wafer production plants and components, and it is difficult to make a design for this or that plant. However, some processes also share most of these principles. According to the composition and impurity level of the incoming material, the following basic process steps are common. [31]

Unpacking

Sort and select according to different colors, remove foreign objects, including flakes, paper, glass, sand, soil, stones and metals, and remove foreign polymers, especially polyvinyl chloride.

Pre-cleaning without cutting

Dry cutting or combined with prewash

Remove stone, glass and metal

Air screening removes flakes, paper and labels

Grinding, drying and/or humidifying

Remove low-density polymers (e.g. cups) by density difference

Hot wash

Caustic cleaning, surface corrosion, maintaining intrinsic viscosity and decontamination

rinsing

Rinse with water

dry

Air screening flakes

Automatic sheet sorting

Water cycle and water treatment technology

Flake quality control

Impurities and material defects

Taking into account the extension of the service life, the increase in the frequency of terminal applications and the repeated recycling, the number of impurities and material defects accumulated in the polymer materials will continue to increase (both in the process of processing or in use). As far as the recycling of PET polyester plastic bottles is concerned, the above-mentioned defects can be divided into the following categories:

Reactive OH- or COOH- end groups are transformed into inert or non-reactive end groups, for example, terephthalic acid is dehydrated or decarboxylated to form vinyl ester end groups, OH- or COOH- end groups are degraded with monofunctional Products such as single carboxylic acids or alcohols react. As a result, the reactivity of the recondensation or re-solid phase polycondensation is reduced, and the molecular weight distribution becomes wider.

Thermal oxidative degradation causes more end groups to convert to COOH groups. As a result, the reaction performance is reduced, and the acid autocatalytic decomposition reaction caused by the heat treatment process in a humid environment increases.

The number of multifunctional macromolecules has increased. Accumulation of gel and branched long chain defects.

The number, concentration and types of heterogeneous polymers and organic and inorganic impurities have increased. Under the new thermal stress, organic foreign matter will react through decomposition. This allows further degradation products (base material and colored material) to be released.

In air (oxygen) and humid environments, hydroxyl and peroxide groups will be generated on the surface of polyester products. Ultraviolet rays accelerate this process. In the further treatment process, hydroperoxide will generate oxygen free radicals, which are the source of oxidative degradation. The destructive effect of hydroperoxide will occur before the first heat treatment or during the plasticizing process, and appropriate additives such as antioxidants will promote this process.

Taking into account the above chemical defects and impurities, in each cycle, the following polymer characteristics need to be continuously modified. These characteristics can be detected by chemical and physical experimental analysis.

especially:

Increase of COOH end groups

Increase of heavy color number

Increased ambiguity (transparent product)

Increase in oligomer content

Filterability reduction

Increased content of by-products such as acetaldehyde and formaldehyde

Increase in extractable foreign pollutants

Light color reduction

Decrease of intrinsic viscosity or dynamic viscosity

The crystallization temperature decreases and the crystallization speed increases

Decrease in mechanical properties such as tensile strength, elongation at break or modulus of elasticity

Broadened molecular weight distribution

PET PET bottle recycling also has an industry standard process provided by various engineering companies. [32]

10.2 Examples of polyester recycling processing

The polyester recycling process is almost as diverse as the manufacturing process based on primary pellets or melt. Depending on the purity of the recycled materials, polyester can be mixed with virgin polymers or used in most polyester manufacturing processes using 100% recycled polymers as raw materials. But there are some exceptions, such as low-thickness biaxial polyester film, special-purpose optical film, yarn spun at a speed greater than 6000 m/min, microfilament and microfiber can only be produced from virgin polyester .

Simple regranulation process of bottle flakes

This process requires drying, crystallization, plasticization, filtration, and granulation to convert waste bottles into flakes. The product is an amorphous reconstituted granule with an intrinsic viscosity in the range of 0.55-0.7 dℓ/g, which depends on the completion of the pre-drying process of the PET polyester flakes.

The characteristics of the product are: low content of acetaldehyde and oligomers in the particles; reduced viscosity, the particles are amorphous, and must be crystallized and dried before further processing.

can be use on:

Amorphous polyester sheet for thermoforming

PET polyester raw material additives

Biaxial PET polyester packaging film

Solid phase polycondensation PET bottle resin

Carpet yarn

Engineering plastics

Filament

Non-woven fabric

Packing stripes

Staple fiber.

Choosing the method of regranulation means that an additional conversion process is required. On the one hand, this process is energy-intensive and cost-intensive, and will cause thermal damage. On the other hand, the granulation step has the following advantages:

Enhanced melt filtration

Intermediate product quality control

Additive modification

Product selection and separation based on quality

Increased processing flexibility

The quality is homogenized.

Manufacture of PET polyester pellets or flakes (bottle to bottle) and amorphous PET polyester for bottles

This process is basically similar to the process described above; however, the pellets produced are crystallized directly (continuously or discontinuously) and then undergo solid-phase polycondensation in a drum dryer or a riser reactor. In this processing step, the corresponding intrinsic viscosity of 0.80-0.085 dℓ/g is restored, and the acetaldehyde content is reduced to less than 1 ppm.

In fact, some machine manufacturers and production line builders in Europe and the United States (such as BePET[33], Starlinger[34], URRC, BÜHLER) are working hard to provide independent recycling processes, such as the so-called bottle-to-bottle ( B-2-B) Process, the purpose is to apply the so-called challenging test method to provide general proof of the "existence" of the required extraction residues and the removal of simulated pollutants. According to the requirements of the US Food and Drug Administration (FDA), it must be processed This test is necessary for the used polyester to be used in the food industry. In addition to the process requiring approval, users of any such process must frequently check the restrictions set by the US Food and Drug Administration (FDA) for the production of raw materials for their process.

Direct conversion of bottle flakes

In order to save costs, more and more polyester intermediate manufacturers, such as spinning mills, cable tie factories or cast film manufacturers, are committed to directly using polyester flakes recovered from second-hand plastic bottles. Produce more polyester intermediates. In order to adjust the necessary viscosity, in addition to effective drying of the flakes, the flakes may also be subjected to a melt polycondensation or solid phase polycondensation process to reconstruct the viscosity. The latest PET polyester flake conversion process uses twin-screw extruders, multi-screw extruders or multiple rotation systems and supporting vacuum degassing equipment to remove moisture and avoid pre-drying of the flakes. These methods can be converted into non-dried polyester flakes and will not significantly reduce viscosity due to hydrolysis.

For the consumption of PET PET bottle flakes, 70% of its main part is converted into fibers and filaments. When using second-hand materials such as bottle flakes directly in the spinning process, there are several processing principles that need to be followed.

The high-speed spinning process for making pre-oriented yarn (POY) usually requires a viscosity of 0.62-0.64 dℓ/g. Starting from the bottle flake raw material, the viscosity can be set by the degree of dryness. The addition of titanium dioxide is necessary for full matte or semi matte yarns. In order to protect the spinneret, effective filtration of the melt is necessary in any case. Currently, the number of pre-oriented yarns (POY) made from 100% recycled polyester is still very small, because this process requires high-purity spinning melt. In most cases, a mixture of primary particles and recycled particles is used.

Short fibers can be spun in a relatively low intrinsic viscosity range, and the required viscosity range is between 0.58 and 0.62 dℓ/g. In this case, the required viscosity can be controlled by drying or vacuum adjustment (when using vacuum extrusion). In order to adjust the viscosity, a long-chain regulator such as ethylene glycol or diethylene glycol can also be added.

Spunbond non-woven fabrics-textiles used in the field of fine denier and as heavy-duty spunbond non-woven fabric substrates, such as roof coverings or road construction-can be manufactured from textile bottle sheets. The textile viscosity is also in the range of 0.58-0.65 dℓ/g.

The manufacture of high-tenacity packaging stripes and monofilaments require the use of recycled materials, and this field is attracting more and more attention. In both cases, the initial raw materials are mainly recycled materials with higher intrinsic viscosity. Then high tenacity packaging stripes and monofilaments are produced in a melt spinning process.

10.3 Recovery into monomer

Polyethylene terephthalate can be depolymerized to form monomer components. After purification, the monomer can be used to prepare new polyethylene terephthalate. The ester bond in polyethylene terephthalate can be cleaved by hydrolysis or transesterification. These reactions are completely opposite to those in the production process.

Partial depolymerization of ethylene glycol

Partial depolymerization of ethylene glycol (transesterification reaction with ethylene glycol) converts rigid polymers into short-chain oligomers, which can be melt-filtered at low temperatures. Once the impurities are removed, the oligomer can be returned to the production process for polymerization.

This task needs to supply 10-25% of the bottle flake feed, while maintaining the quality of the bottle ball products on the production line. The way to achieve this goal is to degrade the PET polyester bottle flakes to an intrinsic viscosity of about 0.30 dℓ/g by adding a small amount of ethylene glycol (it has been in the first plasticization process, which can be used in a single-screw or multi-screw extruder In the process), and after plasticization, the low-viscosity melt stream is directly subjected to effective filtration treatment. In addition, the temperature can be reduced to a minimum. In addition, through this treatment method, the corresponding phosphorus stabilizer is directly added during the plasticization process, and the hydroperoxide may be chemically decomposed. Same as other processes (such as H3P.