An Investigation Into PET Plastics and PET Recycling

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1.1Abstract:

The project is about the PET plastics and PET recycling. This is a process of recycling plastics that are recyclable from the environment. PET is a thermoplastic polymer resin belonging to the polyester family. It is basically used in the manufacture of synthetic fibres, food and beverage liquid containers. Other uses include the thermoforming applications and the engineering resins where it is often mixed with glass fibre.

On the methodology, research has shown that mixing starch with plastic will lower degradation, but it still doesn't lead to complete breakdown of the plastic. Bacteria have been genetically engineered to synthesize a completely biodegradable plastic, but this material is expensive at present.
Recycling of plastics reduces the trash deposited in landfills, and offers the opportunity for consumers to clean and reuse containers for everything from water spritzer bottles to holding excess amounts of shampoos, cleaning fluids or lotions. Recycling all plastic bottles that contain PET or HDPE labels somewhere on the bottle, most often found on the neck of the bottle to make separation of recyclable and non-recyclable plastic easier on consumers is a better remedy.

 It is recommended that the current bottle bills should also be expanded even to other countries. In 2005, 2 million tons of plastic bottles in the United States ended up in the trash instead of in recycling bins. The State container deposit law which is known as "bottle bills" are long overdue for an upgrade. They have proven to be the most effective approach to collecting bottles and cans. Currently, only 11 states have bottle bills in America.

2.1 Introduction:

Plastics are indeed everywhere, from the grocery to the shopping mall. They are important in the packaging industry, but a closer look at the plastics brings into focus the price to be paid by using and dumping it the wrong way. This, therefore, calls for a lot of care. Before you toss that plastic bag to the environment, then be sure of the impact you are creating. Plastics are very durable and this means that they will degrade very slowly, therefore, takes long in the environment. Decomposition by burning also creates serious toxic fumes in the atmosphere. The production as well, takes a significant amount of fossil fuels which pollutes the environment. Some plastic debris remains floating on the sea surface, and this creates danger to the marine life. Most sea animals have died due to the ingestion of plastic materials. Solar radiation degrades plastics into smaller particles, which eventually becomes plastic dust and goes to pollute the atmosphere.

The acronym PET stands for Polyethylene terephthalate. This is a thermoplastic polymer resin belonging to the polyester family. It is basically used in the manufacture of synthetic fibres, food and beverage liquid containers. Other uses include the thermoforming applications and the engineering resins where it is often mixed with glass fibre. The monomer can be processed in several ways, and these include: The esterification of terephthalic acid, and ethylene glycol with water as the by product.  It can also be synthesized through transesterification of ethylene glycol and dimethyl terephthalate with methanol as the by product. Finally it can be polymerized through polycondensation of the monomers with water as the by product.PET is commonly referred to as polyester and is the third most abundant plastic production behind polyethylene and poly propylene. It is widely used in the production of synthetic fibres.

Due to this damage to the environment, it is therefore imperative to develop ways of recycling the plastics and this brings us to the actual project that is PET plastics and PET recycling. The objective of the project is basically to reduce plastics in the environment through recycling. It also aims at developing appropriate measures to deal with the plastic menace. The project also focuses on achieving a clean environment which is safe to live in. It also aims at fostering recycling of plastics as the best option to keep them off the environment.

The recycling processes with polyester are as varied as the manufacturing processes based on primary pellets. Polyester can be used today in most of the polyester manufacturing processes as a blend with virgin polymer or increasingly as 100% recycled polymer. This will depend on the purity of the recycled materials. There are some exceptions like BOPET-film of low thickness, special applications like optical film or yarns through FDY-spinning at > 6000 m/min or microfilaments and micro-fibers that are produced from virgin polyester only.

There exist a number of internal recycling processes where fibre is reused directly to produce fibre and the same to both performs and films. In a nut shell the following general simple procedure is used in the process. First is the bale opening then followed by the sorting and selection for different colours, foreign polymers. Next is the pre-washing without cutting before the cutting process. Stones are then removed and other debris like glass and metal. Air sifting is done to remove film paper and labels. It is then ground and low density polymers removed. Hot wash is done followed by caustic wash to maintain intrinsic viscosity. After which the rinsing is done twice. It is then dried and followed by air sifting flakes before sorting the flakes automatically. Water circuit and water treatment technology is then employed to attain flake quality.

There exist some defects which are encountered during the process. They can be grouped into several categories. The reactive polyester –OH or –COOH end groups could be transformed into non reactive end groups like the formation of vinyl ester end group. The end group could also shift towards the direction of the –COOH end groups. This would be due to build up during thermal and oxidative degradation. The number of poly-functional macromolecules could also increase. The number, variety and concentration of non- polymer identical organic and inorganic compounds may increase.

They can be detected either chemically or physically through the following processes. Increase of –COOH end group, increase of color number B, increase of oligomer content, reduction in  filterability, increase of by products like acetaldehyde, increase of extractable foreign contaminants, decrease in colour L, decrease of intrinsic viscosity, decrease of crystallization temperature and increase in crystallization speed, decrease in mechanical properties like tensile strength, broadening of molecular weight distribution.

The statement of the problem to the project states that although plastics are very important to the users, proper disposal through recycling is required to avoid massive environmental degradation.

3.1 Literature Review:

Plastics have a major impact in the society due to poor disposal. Most marine lives, human lives and even the terrestrial animals’ lives have been lost due to this menace of plastics. This therefore calls for the appropriate measures to curtail the problem. The most appropriate method is then use of then recycling technology to keep the plastic materials out of the environment.

On the other hand there are scholars who claim the benefits of plastic materials outweigh their negative impact to the environment. Some of the benefits floated include:  In electronics, laptops could not function well in case they were made from glass. Plastics are used in building and construction as a cheaper material as compared to the others. They are indeed very useful in the packaging industry. Plastics have slowly taken over the normal paper in the packaging industry. Plastics are very durable and their manufacture is very cheap.  It can as well be recycled rather decomposing it. This can be achieved easily than producing a new one. Plastics can be reused over and over again as opposed to the aluminum materials. They do not corrode as does with the metals. They are not breakable as with the glass materials. Plastics are light in weight and usually they are odourless and thus can be used to carry most things.

PET is an acronym that stands for Polyethylene terephthalate. This is a thermoplastic polymer resin belonging to the polyester family. It is basically used in the manufacture of synthetic fibres, food and beverage liquid containers. Other uses include the thermoforming applications and the engineering resins where it is often mixed with glass fibre. The monomer can be processed in several ways and these include: The esterification of terephthalic acid and ethylene glycol with water as the by product.  It can also be synthesized through transesterification of ethylene glycol and dimethyl terephthalate with methanol as the by product. Finally it can be polymerized through polycondensation of the monomers with water as the by product.PET is commonly referred to as polyester and is the third most abundant plastic production behind polyethylene and poly propylene. It is widely used in the production of synthetic fibres. Due to the addition of polyvinyl alcohol, there is a reduction in oxygen permeability.

PET is considered an excellent barrier material as some health practitioners use gloves made of this material. It is usually used in tape applications like in the carrier for magnetic tape or the backing for pressure sensitive adhesive tapes due to its high mechanical strength. Thermoformed PET can be used in the storage of frozen dinners. This is because of their ability to withstand extreme temperatures. Nylon is a product of PET. It is naturally strong with high strength and ability to form several products. PET has a variable intrinsic viscosity range.

The process of manufacturing PET takes distinct stages. The initial stage is the Drying stage. This is achieved since PET is hygroscopic. Drying is achieved by the use a desiccant. This is basically achieved by the passage of a series of hot air through the material. It can also be done by the use compressed air. The next stage is the modification through copolymerization. An example is where the melting point can be lowered through the addition of cyclohexane dimethanol. Copolymerization is very important in the thermoforming and in the crystallization process like the material used in the seat belts. Crystallization of polymers occurs when a polymer chains fold to form some repeating pattern.

 PET   is considered a semi-crystalline polymer since only 40% of the polymer is amorphous. The solid state crystallization is a process in which PET is crystallized to glass like substance through a very rapid cooling process. For the process to make effectively, a catalyst must be employed in the form of Antimony trioxide (Sb2O3). Exposing PET to boiling or microwave could increase levels of antimony. This could be detrimental to the health status of individuals who consume products packaged with such material. Environmental Health Perspectives in April 2010 claims that PET might yield endocrine disruptions. Contrary to this Franz and Welle gave evidence based on mathematical modeling claiming that PET is unlikely to yield endocrine disruptions when consumed in mineral water.

The degradation process of PET takes different forms like the hydrolytic, thermal oxidation and the thermal which is the most important. This degradation process leads to discolouration, chain scissions due to reduced molecular weight and the formation of other products like the acetaldehyde and crosslinks. This interferes with the optimal requirements especially in the packaging industry. The best way to alleviate this menace is through the use of copolymers. These will lower the melting point and reduce the degree of its crystallinity. The use of stabilizers like phosphites which are mainly antioxidants can also stabilize the polymer. Acetaldehyde can cause foul taste in bottled water when formed onto the walls of the container.

There are two ways employed in the recycling of PET. They are the chemical and the mechanical. The chemical process destroys all the structure thereby forming new intermediates like cis-ß-hydroxyterephthalate. In mechanical recycling, the original properties of the polymer are being maintained. Chemical recycling comes in handy when large tones of the product are needed within a very short time. Mechanical recycling is used in small and medium scale industries. Apart from the general contaminants from the first stage of formation, mechanical impurities depreciate the quality of the recycling process. This calls for efficient sorting, cleaning and separation process in order to attain optimum results.

The industry consists of three major sections: The Waste Logistics section where collection and separation of wastes is done (Porter, 2002), the Flake production section where clean bottles are produced and the Flake Processing section where PET flakes are converted to final products. There exist a number of internal recycling processes where fibre is reused directly to produce fibre and the same to both performs and films. In a nut shell the following general simple procedure is used in the process. First is the bale opening then followed by the sorting and selection for different colours, foreign polymers? Next is the pre-washing without cutting before the cutting process. Stones are then removed and other debris like glass and metal. Air sifting is done to remove film paper and labels. It is then ground and low density polymers removed. Hot wash is done followed by caustic wash to maintain intrinsic viscosity. After which the rinsing is done twice. It is then dried and followed by air sifting flakes before sorting the flakes automatically. Water circuit and water treatment technology is then employed to attain flake quality.

The defects encountered during the process can be grouped into several categories. The reactive polyester –OH or –COOH end groups could be transformed into non reactive end groups like the formation of vinyl ester end group. The end group could also shift towards the direction of the –COOH end groups. This would be due to build up during thermal and oxidative degradation. The number of poly-functional macromolecules could also increase. The number, variety and concentration of non- polymer identical organic and inorganic compounds may increase.

These can be detected chemically or physically through the following processes. Increase of –COOH end group, increase of color number B, increase of oligomer content, reduction in  filterability, increase of by products like acetaldehyde, increase of extractable foreign contaminants, decrease in color L, decrease of intrinsic viscosity, decrease of crystallization temperature and increase in crystallization speed, decrease in mechanical properties like tensile strength, broadening of molecular weight distribution.

There are several examples for processing polyester. The recycling processes with polyester are as varied as the manufacturing processes based on primary pellets. Polyester can be used today in most of the polyester manufacturing processes as a blend with virgin polymer or increasingly as 100% recycled polymer. This will depend on the purity of the recycled materials. There are some exceptions like BOPET-film of low thickness, special applications like optical film or yarns -spinning at > 6000 m/min or microfilaments and micro-fibers that are produced from virgin polyester only.

The processes include simple re-pelletizing of bottle waste into flakes. This involves transforming bottle waste into flakes, by drying and crystallizing the flakes, by plasticizing and filtering, as well as by pelletizing. The product is an amorphous re-granulate of intrinsic viscosity in the range of 0.55–0.7 dl/g, depending on how complete pre-drying of PET flakes has been done. They have to be crystallized and dried before further processing. Processing is done in order to obtain the following a PET film for thermoforming, addition of PET virgin production, packaging film, PET bottle resin by SSP, Carpet yearn, Engineering plastic, Filaments, Non woven materials, Packaging stripes, Stable fibre. By selecting the re-pelletizing way means having an additional conversion process which is at the one side energy intensive, causes thermal destruction and cost consuming. On the contrary, pelletizing step is providing the following advantages: intensive melt filtration, intermediate quality control, modification by additives, product selection and separation by quality, increased processing flexibility, and quality uniformity.

The other process is the manufacture of PET pellets or flakes for bottles. This is a bottle to bottle process. It is similar to the one described above in principle; however, pellets are directly crystallized and then subjected to a solid-state polycondensation (SSP) in a tumbling drier or a vertical tube reactor (Piringer, 2000). The corresponding intrinsic viscosity of 0.80 – 0.085 dl/g is re build again and, at the same time, the acetaldehyde content is reduced to 1 ppm in this process. Besides its’ approval it is nevertheless important that any user of such processes has to constantly check the limits for the raw materials manufactured by himself for the process.

There is also the direct conversion of bottle flakes. This is done to minimize costs.  Polyester intermediate producers like spinning mills, strapping mills or cast film mills are working on the direct use of the PET-flakes, from the treatment of used bottles, with a view to manufacturing an increasing number of polyester intermediates. It is possibly necessary to reconstitute the viscosity through polycondensation in the melt phase or solid-state polycondensation of the flakes in order to adjust the viscosity. The application of twin screw extruders, multi-screw extruders or multi-rotation systems and coincidental vacuum degassing to remove moisture and avoid flake pre-drying are indeed the current PET flake conversion processes. They allow the conversion of non-dried PET flakes without substantial viscosity decrease caused by hydrolysis. About 70% is of PET bottle flakes are converted to fibers and filaments.

In direct secondary materials such as bottle flakes in spinning processes, there are a few processing principles to obtain. The first being the high speed spinning process for the manufacture of POY. This needs viscosity of 0.62–0.64 dl/g. The viscosity can be set via the degree of drying beginning from the bottle flakes. For full dull or semi dull yarn the use of TiO2 must be added. An efficient filtration of the melt is necessary for the protection of spinnerets. Staple fibres may also be spun in an intrinsic viscosity range which lies lower and which should be between 0.58 to 0.62 dl/g. The viscosity can be adjusted via drying or vacuum adjustment in case of vacuum extrusion. However, the addition of chain length modifiers like ethylene glycol or diethylene glycol may be used. The basic spinning non-woven in the field for textile applications as well as heavy spinning non-woven as basic materials, for example in roof covers or in road building where it can be manufactured by spinning bottle flakes. The spinning viscosity is again within a range of 0.58–0.65 dl/g. The manufacture of high tenacity packaging stripes and monofilaments is another area of great interest. The initial raw material is a recycled material of higher intrinsic viscosity. The monofilaments as well as the high tenacity packaging stripes are then manufactured in the melt spinning process.

The recycling of PET back to the initial raw materials takes three major processes. The first one is the glycolysis or partial glycolysis process. The polyester is transformed into an oligomer by adding ethylene glycol or other glycols during thermal treatment. The advantage of this process is the possibility of separating the mechanical deposits directly and efficient through a progressive and stepwise filtration (Mantia, 2002). The decisive effect on the quality of the end product depends on the filteration finess of the last step. This demonstrates how bottle waste can successfully be recycled in a continuously operating polyester line. The quality of the bottle pellets which is manufactured on the line are maintained with 10-25% bottle flakes feeding into the processor.

Temperature is brought to the lowest possible limit. The possibility of chemical decomposition of the hydro peroxides is possible by adding a corresponding P-stabilizer directly when plasticizing. Treatment by adding H3PO3 inthe last step helps in the destruction of hydro peroxide groups. The finely filtered recycled and partially glycolyzed materials are continuously fed to the esterification or prepolycondensation reactor. Dosing quantities are adjusted accordingly. Japan has used the treatment of waste through total glycolysis to convert polyester to Cis-B hydroxyl-terephthalate in experimental production.

The next process is through hydrolysis which operates under very high pressures and supercritical conditions. PET-waste is directly hydrolyzed for example by applying supercritical water steam. Re-crystallization in acetic acid will help in the purification of crude terephthalic acid.

The final process is the methanolysis which is basically for the large scale production. Polyester waste is transformed with methanol into DMT, under pressure and in the presence of catalysts. Filtration of the methanolysis product is then applied. Crude DMT is finally purified by vacuum distillation. Methanolysis is only rarely carried out in industries today since polyester production based on DMT shrunk tremendously and with this DMT producers disappeared step by step during the last decade.

4.1 Background Information:

Plastics have had an impact on our culture. However, it has become obvious that there is a price to be paid for their use. A controversy arose in the late 1950s and early 1960s where there were a number of incidents where small children crawled into plastic bags used by launderers to cover clothing, and suffocated to death.  Plastics industry fended off the trouble by launching a massive public-education campaign.  Most local authorities now offer collection facilities forplastic bottles either from your kerbside collection scheme or at recycling centre. By late 1960s plastics were seen as the symbol for backwardness. However, this was just a fashion statement, since plastics remained in widespread use anyway, and in many cases were much more effective and environmentally benign than alternative materials. It also came with problems of litter and waste disposal. The invention is however credited to Nathaniel Wyeth of Du Pont according to the American Patent No. 3733309 of 1973.

Plastic was generally good, as it was durable and degraded very slowly.  Burning of the plastic material could release toxic fumes. The manufacture of plastics has generally managed to create massive quantities of nasty chemical pollutants, and depleted the Earth's bounded supply of fossil fuels. By the 1990s, Plastic recycling programs became common in the 1990s (Lundquist, 2000). Thermoset plastics can be ground and used as filler, thermoplastics can be re-melted and reused, though the purity of the material tends to degrade with each reuse cycle.  Automobile machines are now being redesigned to make recycling of their large plastic parts much easier and cheaper. The Plastic Bottle Institute of the Society of the Plastics Industry devised the now-familiar scheme to mark plastic bottles by plastic type in order to assist in the recycling of plastic disposable items.  A container using this scheme is marked with a triangle with three "chasing arrows" inside of it, which enclose a number giving the plastic type: PET, HDPE, PVC, LDPE, PP, PS, and OTHER as shown in the table below.

 

Plastic container code system: 

MATERIAL

PERCENT OF TOTAL

Polyethylene Terephthalate (PET)

20-30 percent

High Density Polyethylene

50-60 percent

Vinyl/Polyvinyl Chloride (PVC)

5-10 percent

Low Density Polyethylene

5-10 percent

Polypropylene

5-10 percent

Polystyrene

5-10 percent

All other resins

5-10 percent

 

4.1.2 Research questions and Hypotheses:

The assumptions of the study and the research questions are as follows:

  1. Are plastics generally a menace to the environment and society at large?
  2. Can plastics be eradicated completely from our environment?
  3. What are the major cost benefit analyses of the plastic materials?
  4. What are the benefits of using PET plastics?
  5. Are there other materials that can replace in the PET plastics in the common market?
  6. Recycling of PET is much better than producing new plastics.
  7. Recycling of PET reduces environmental pollution.
  8. Recycled PET is the material of choice to be used.
  9. Recycling PET comes with its’ demerits.
  10. Recycling of PET reduces costs and is much cheaper than the virgin type.

4.2 Methodology:

Recycling plastics has proved to be very difficult. It is difficult to Automating the sorting of plastic waste is very difficult, and so it is labor-intensive (Harper, 2000). Consumer toy like a cellular phone may be made of many small parts consisting of over a dozen different types and colors of plastics yet containers are usually made from a single type and color of plastic, making them relatively easy to sort out. As the value of the material is low, Recycling plastics is unprofitable and for this reason, the percentage of plastics recycled is very small, that is around 5%.


Research is being conducted on "biodegradable" plastics that break down with exposure to sunlight. By mixing starch with plastic degradation will be lowered, but it still doesn't lead to a complete breakdown of the plastic. Bacteria have been genetically engineered that synthesize a completely biodegradable plastic, but this material is expensive at present.

Critics claim that their only real problem to be addressed is roadside litter, which is regarded as a secondary issue.  When plastics are dumped into landfills, they can become "mummified" and persist for decades even if they are supposed to be biodegradable.
The Courtald, the original producer of rayon, came up with a revised process for the material in the mid-1980s to produce tencel which has superior properties to rayon, but is still produced from "biomass" feedstock, and its manufacture is extraordinarily clean by the standards of plastic production. I believe that effective and consistent education encourages consumers to recycle. When both adults and children understand how recycled containers are re-processed and re-manufactured into new items, it helps them understand and take ownership of the process.

For a successful recycling operation to be achieved, the following requirements must be put into place. First of all acquire a very steady source of very high grade and competitive materials. After which, establish a cost effective transportation and collection medium. Create a recycling processing technology and indeed equipment that can handle the material efficiently and economically. Ensure you develop the ability to market both continuous and quality in the quantity of materials. Finally develop proper markets for end products.

 

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