Polyethylene terephthalate cooling

In addition to the desired polymerization reaction, the dialcohol reactants can participate in deleterious side reactions. Ethylene glycol, used in the manufacture of polyethylene terephthalate, can react with itself to form a dialcohol ether and water as shown in Fig. 24.4a). This dialcohol ether can incorporate into the growing polymer chain because it contains terminal alcohol units. Unfortunately, this incorporation lowers the crystallinity of the polyester on cooling which alters the polymer s physical properties. 1,4 butanediol, the dialcohol used to manufacture polybutylene terephthalate, can form tetrahydrofuran and water as shown in Fig. 24.4b). Both the tetrahydrofuran and water can be easily removed from the melt but this reaction reduces the efficiency of the process since reactants are lost. 

Both polyethylene terephthalate and polybutylene terephthalate exhibit partial crystallinity in the solid state. The molecular weight of the polymer and the time permitted for cooling define the degree of crystallinity of the polymer. Very slow cooling results in high crystallinity and opacity, while fast quenching creates low crystallinity, high clarity material. 

Polyester fibers, similar to polyamide fibers, represent another important family of fiber. Polyester fiber was discovered in England in 1941 and commercialized in 1950. Two common trade names of polyester are Dacron in the US and Terylene in the UK. The term polyester fiber represents a family of fibers made of polyethylene terephthalate. Dimethyl terephthalate is reacted with ethylene glycol in the presence of a catalyst, antimony oxide, to produce polyethylene terephthalate or polyester. The chain repeat structure of PET is given in Fig. 4.6. Although polyesters can be both thermosetting and thermoplastic, the term polyester has become synonymous with PET. Note that the PET chain structure is different from the simpler structure of nylon or polyethylene. In PET, the aromatic ring and its associated C-C bonds provide a rigidity to the structure. The polyester structure is also bulkier than that of nylon or polyethylene. These factors make polyester less flexible than nylon and polyethylene, and the crystallization rate of PET slower than that of nylon or polyethylene. Thus, when polyester is cooled from the melt, an appreciable amount of crystallization does not result. 

Plastic polymers make up a high proportion of waste and the volume and range used is increasing dramatically. The two main types of plastic are thermoplastics which soften when heated and harden again when cooled and thermosets which harden by curing and cannot be remoulded. The six main plastics in municipal solid waste are, high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC) and polyethylene terephthalate (PET). In addition there are... 

Fig. 46. Change of the small angle scattering during stepwise heating and cooling of unoriented polyethylene terephthalate...

Fig. 47. Some small angle scattering curves obtained during heating and cooling of oriented polyethylene terephthalate...

Thermoplastics are polymers that can be melted and then molded into shapes that are retained when the polymer is cooled. Although they have high Tg values and are hard at room temperature, heating causes individual polymer chains to slip past each other, causing the material to soften. Polyethylene terephthalate and polystyrene are thermoplastic polymers. 

Figure 10.7 DSC curves of polyethylene terephthalate)-poly(acrylonitrile-butaliene-styrene) (PET-ABS) blends (a) conventional DSC first and second heating curves with heating and cooling rate of lOKmin-1 and (b) temperature modulated DSC (TMDSC) first heating curves with /3=2Kmin 1, p = 60s and 5= 1K. Tg, glass transition temperature. (Reproduced with permission from T. Hatakeyama and F.X. Quinn, Thermal Analysis Fundamentals and Applications to Polymer Science, 2nd ed., John Wiley Sons Ltd, Chichester. 1999 John Wiley Sons Ltd.)...

Fleece can be made from polyethylene terephthalate (PET) bottles. The first step is to wash and then mechanically crush the bottles, shaping the plastic into small chips. The chips can then be heated and forced through tiny holes in a metal plate (called a spinneret), which forms fibers that harden as they cool to room temperature. These fibers are wound onto a spool as they are formed, and they can subsequently be stretched to improve their strength. Machines can then be used to texturize and cut the fibers to their desired length and be used to make fleece cloth for clothing, blankets, etc. 

The tubular-film process is unsuitable for polymers with a very low melt strength such as polyethylene terephthalate. It is also not suitable for polypropylene films for packaging because the films are too crystalline, opaque and brittle due to too slow cooling. However, these films are often used as a precursor for fibrillated film fibres. 

In melt spinning, thermoplastic polymers (i.e. polymers which soften and melt when heated) such as polyamide or polyethylene terephthalate are made molten or liquefied in an extruder and are forced through the spinneret by a spinning pump. The filaments are then soUdilied by air-cooling. 

The behavior in the liquid state is essentially the same as amorphous polymers. In the transition region, an abrupt change in slope occurs as crystallization begins to take place. This is the crystallization temperature T, . If the material is cooled very rapidly, the crystallization rate can be depressed, depending on the crystallization kinetics. In fact, in some materials with sufficientiy slow crystallization kinetics, the crystallization can be almost completely suppressed by rapid cooling. A well-known example is polyethylene terephthalate (PET). If PET is rapidly quenched, it is almost completely amorphous with a density of about 1.33 g/-cm. If it is cooled slowly, it will crystallize with a resulting higher density of about 1.40 gZ-cm. ... 

Polyethylene terephthalate (PET) is obtained by reacting purified terephthalic acid (PTA) and monoethylene glycol (MEG) and melting the reaction product to initiate the polycondensation. The molten polymer is then extruded, cut into chips and cooled. PET main use is in the soft drinks and water bottles, other applications include thick-walled containers for cosmetics and pharmaceuticals. 

Figure 3.21 shows the profiles of concentration of the contaminant developed through the thickness of the bottle cooled in the mould whose external surface is kept at 8 °C. Thickness of the polyethylene terephthalate (PET) bottle = 0.03 cm thickness of the mould =1.5 cm temperature of the PET at injection = 280 °C time of residence of the PET bottle in the mould = 1 s. 

INFORMATION. The TLC analysis is carried out using Eastman Kodak silica gel-polyethylene terephthalate plates with a fluorescent indicator. Activate the plates at an oven temperature of 100 °Cfor 30 min and then place them in a desiccator to cool until needed. After spotting, elute the plates using methylene chloride as the solvent. Visualize the spots with a UV lamp. The course of the reaction is followed by removing small samples (2-3 drops) of solution from the hot test tube at set time intervals with a Pasteur pipet and placing them in separate -dram vials. See Technique 6Afor the method of TLC analysis and the determination of Rf values. Approximate Rf values trans = 0.72 cis = 0.64. 

INFORMATION. Good results have been achieved by conducting the TLC analysis with Eastman Kodak silica gel-polyethylene terephthalate plates ( 13179). Tktivate the plates at an oven temperature of 100 °C for 30 min. Place them in a desiccator for cooling and storing until used. Elute the plates using pure methylene chloride as the elution solvent. Visualization of unreacted ferrocene can be enhanced with iodine vapor. See Prior Reading for methods of TEC analysis and determination o/Rf values. 

Many pol3nners, including polyethylene terephthalate, also crystallize if they are cooled slowly from the melt. In this case we may say that they are crystalline but unoriented. Although such specimens are unoriented in the macroscopic sense, i.e. they possess isotropic bulk mechanical properties, they are not homogeneous in the microscopic sense and often show a spherulitic structure under a polarizing microscope. 

The largest volume flame retardant is alumina trihydrate, AI2O3 3H2O. In the flame, water is released and this cools the plastic, preventing further combustion. Alumina trihydrate is used in polymers such as polyolefins, PVC, polyacrylates, and thermoset polyesters. It cannot be used in polymers such as polycarbonate, nylon 6,6, or polyethylene terephthalate because these are processed at temperatures that will cause the evolution of water during processing. 

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