Polymers are a chain of numerous, smaller repeating-units called monomers. Polymer chains are very similar to paper clips; the repeat-units link together to make a long strand, and appear in varying lengths. They can be structured in different forms; they can be cross-linked, have branches, or become intertwined. Additionally, polymers can be linked by one or more types of monomers units, they can be joined by various kinds of chemical bonds (e.g. Ester linkages, amide linkages), and they can be oriented in different ways. The addition process for monomers is where all the atoms in the monomer are present in the polymer. The condensation process is when a small molecule by-product is also formed. Polyethylene Terephthalate polyester (PET or PETP) is the most common thermoplastic polyester. It is a polyester resin (sticky substance) formed via condensation of ethylene glycol and terephthalic acid with H2O as a by-product, this process is called esterification. This is one of two methods; the second method is called transesterification where the terephthalic acid is replaced with Dimethyl terephthalate to produce methanol as a by-product. The esterification process will be the main focal point. PET is used to make containers for beverages, foods and other liquids, as well as the application of other thermoforms. It is also one of the most important raw materials used in man-made fibers. During the manufacturing stage, the crystallinity can alter accordingly to display either an amorphous (transparent) or a semi-crystalline (opaque and white) material. PET is fully recyclable and very ecologically efficient. Some significant questions that will be explored throughout the report are:
How does the complex properties of PET compare to its main uses?
What are the risk factors associated with the manufacture and usage of PET?
How can PET be improved to become further effective to today's society?
What are the specifications when manufacturing PET?
Why does intrinsic viscosity have such an immense impact on the PET product?
The formation of condensation polymers is more intricate than the formation of addition polymers. It is the process in which two products result from the formation of a condensation polymer, unlike addition polymers. The two products are the polymer itself and another small molecule which tends to be H2O, most of the time. Condensation polymers can form from the linkages of a single kind of monomer, or, copolymers can form if two or more different monomers are involved.
Polyesters are a type of condensation polymer. They are called polyesters because the co-polymers are joined together by ester linkages. Dacron is probably the best known polyester. To attain this compound you must merge the two monomers: terephthalic acid (which has a carboxylic acid at both ends) and ethylene glycol (which has an alcohol, OH group), at both ends. When the OH group from the terephthalic acid molecule splits away and bonds with a hydrogen atom from the alcohol group, a water molecule is formed. This produces the polymer polyethylene terephthalate or PET. This polymer is most familiar to most people by its recycling number 1.
PET is made from the two raw materials: ethane-1,2-diol (ethylene glycol - EG) and purified terephthalic acid (PTA). The EG is mixed with the PTA in the presence of a condensation catalyst, in a paste, and fed into the esterification system. Selecting the most effective catalyst is essential. It would be most effective to use concentrated sulphuric acid (H2SO4) as it is most beneficial. It provides a means to remove the produced H2O molecules (disabling the chance of an equilibrium happening; yield increase) as well as speeding up the reaction. The catalyst system drastically reduces the polymerization time to produce PET without sacrificing colour and clarity of the polymer produced. This is very important when manufacturing a product, especially when it is to be used commercially; the highest yield possible is essential. The esterification takes place under atmospheric pressure by splitting off the water. After the esterification stage, the result enters the pre-polycondensation unit where the reaction takes place under a vacuum. The product is then dealt with increased temperature and vacuum in the final polycondensation reactor. When the polyester is melted, it can encounter two processes. It can be processed either into fibres/filaments or sent to the solid state polycondensation (SSP) unit to make bottle grade chips.
Research and technology have adapted to now use Antimony as a catalyst in the production of PET plastic. The reason for this modification is that the sulphuric acid tends to attack organic chemicals and also that antimony is generally less hazardous. Antimony oxide has a very low toxic content, is relatively inert, and does not have an effect on biological life. Long terms studies indicate that very little antimony oxide is released from PET; study says "less than five parts per billion" get released in liquid contents. Therefore, antimony oxide provides a very, very low risk because of its low toxicity and the low occurrence of it in this product. Its use in PET does not endanger workers, consumers, or the environment.
One of the most important characteristics of polyethylene terephthalate is referred to as intrinsic viscosity. This is a measure of the polymer's molecular weight; therefore indicating the material's crystallinity, melting point and tensile strength. This clearly provides an obvious trend, demonstrating that the properties of PET are very relative to the molecular size. So as the molecular size of the polymer increases, the properties begin to change accordingly and intentionally. The intrinsic viscosity of the material is measured in decilitres per gram (dL/g). It is the length of the polymer chains which determines the measure of this. As the chain gets longer, the more entanglements there are between the chains; the viscosity will increase as that continues. The average chain length of a particular batch of resin can be controlled during polycondensation:
This data provides an apparent consistency of variation of viscosity range. It is noticeable that the textile of the fibre grade has a very low viscosity (soft, pliable) and the monofilament has a very high viscosity (stiff, rigid). Essentially, during the manufacturing process, end stoppers are added to the polymer when it has a high enough viscosity (long enough chain). These end stoppers are molecules that can only form one bond and can stop the growth of the polymer. By regulating the ratio of moles of the compound that forms one bond to the moles of the polymer builders, the average length of the chain can be manipulated.
The intrinsic viscosity range of PET:
Fibre grade
0.40 - 0.70 dL/g Textile
0.72 - 0.98 dL/g Technical, tyre cord
Film grade
0.60 - 0.70 dL/g BoPET (biaxially oriented PET film)
0.70 - 1.00 dL/g Sheet grade for thermoforming
Bottle grade
0.70 - 0.78 dL/g Water bottles (flat)
0.78 - 0.85 dL/g Carbonated soft drink grade
Monofilament
1.00 - 2.00 dL/g
This also presents the anomaly that it is ever-intriguing how such a defined material could be available in such different composites. The quality of PET can be very diverse in such means that it can be produced into a soft synthetic fibre as well as a rigid, strong bottle container. It seems so unimaginable how a material can change its properties so differently and still remain as the same base polymer.
PET in its natural state is a crystalline resin. By rapidly cooling a molten polymer to form an amorphous solid, a clear product can be produced. Amorphous PET forms much like glass; if its molecules are not given enough time to arrange themselves appropriately whilst the melt is cooling. Solid-state crystallization is when enough heat energy is put back into a frozen molecule to allow mobilization, and then the crystals can nucleate and grow.
PET has good chemical resistance to mineral oils, acids and solvents but not to bases. It is absolutely vital that the users of this product do not use a PET container as a means of alkaline chemical containment because it will hydrolyse the PET and it could be very perilous. Melted PET will produce thermal burns; other than that PET is not considered to be hazardous or dangerous. Consumers should be aware that, once any packaging has been opened, bacteria can grow if the conditions are suitable for bacterial growth. Therefore, before re-using the container it is crucial that it be thoroughly cleaned with hot soapy water and dried.
SCENARIO (1): Polyethylene terephthalate consists of barrier properties very close to those of glass. For instance, milk filled in a PET-package will never smell of products placed nearby in the refrigerator (common polyethylene is incapable to provide it). The oxygen from outside does not penetrate into a closed bottle, and as a result the bottle contents do not turn sour. It is possible to fill PET-packages with liquids subject to pasteurizing and aseptic filling (beer, juices, ice tea, dairy products). Thus it is clear that use of polymer material for bottle production has a number of advantages as compared with production of glass bottles.
SCENARIO (2): Pet has always been considered unsuitable for beer due to the material's permeability and sensitivity to oxygen and carbon dioxide. This was until a barrier to minimise oxygen and carbon dioxide permeation was developed by the PET bottle manufacturers. This assisted the bottle to be able to preserve the beer's flavour characteristics for up to six months in PET containers. By incorporating an activating metal into a container wall of stretched plastic material, it is able to exert high oxygen barrier properties. The preferred metal is a transition metal which can be derived from a salt, such as a halide or acetate. This metal is either put into the mixture, or contained in one of both of the polymers. To produce the container wall with the high oxygen barrier properties, the material is stretched and aged.
SCENARIO (3): Although many different resins are considered and actively evaluated, PET should be chosen for some automotive car bodies because it best meets the four critical elements; specifically, PET is affordable, recyclable, has an existing infrastructure and most importantly, has a substantial global supply. Having met the criteria for availability, PET could be the ideal material for large, structural automotive parts. Chrysler have been able to modify the base resin for UV stability, add pigment for mould-in-colour capability, and add glass fibre and chemical modifiers for structural integrity and improved flow. The material proved to be highly impact-resistant as well. The end result inspires a new product line called Impet Hi, which has many of the properties of Chrysler's proprietary resin.
The data in the following graph confirms that the demand for polyethylene terephthalate is drastically increasing every year because research is developing to make it a more substantial and useful material. The world's PET production has the most dominance in the synthetic fibers industry (in excess of 60%) with bottle production short of that at around 30% of global demand. If they persist on enhancing the manufacturing system, developing a method which allows a PET material to have practically the same properties as glass, it would become a necessity around the world because it would be much more costly beneficial. Overall, PET is a success and is definitely a beneficial product commercially and industrially.
