PET properties

Table 2.14 Typical evolution of PET properties during polycondensation, according to ...

M. L. Drain, PET Properties and Performance Requirements, paper presented at the Polyester Packaging - The Critical Path Ahead Conference, sponsored hy Packaging Strategies, Inc., Newark, NJ, May 28-30 (1997). 

Experimental results obtained allow to mark a promise of this research direction. Developing new polymer composite materials based on natural layered silicates and polyethyleneterephthalate - the most needed polymer material - gives opportunity to modify PET properties and therefore expand the field of its application. 

The different effect of titanium catalysts on PET and PBT colour could be related to the thermal degradation. Polycondensation temperature for PET is significantly higher than that of PBT, and titanium(IV) can affect their thermal decomposition in different ways. However, the role of titanium catalysts on PET properties is not elear yet, but, all these problems, the stability, the discoloration, are correlated directly or indirectly to the catalyst composition. 

The effect of orientation on various PET properties is summarized in Table 33.3. 

This paper investigates the extrusion foaming behaviors of Polyethyleneterephthalate (PET) blown with supercritical CO2. The evolution of the foam s morphology once it exited from the die was recorded using a CCD camera and also characterized by SEM. Microcellular PET foams can be prepared by optimizing the PET properties as well as the processing conditions. 

The subsequent literature shows the rule to be generally valid, within a few pet cent, amongst systems which give Typje IV isotherms in the typical example of Table 3.1, the data refer to adsorptives differing widely in their physical and chemical properties, yet the deviation of the saturation volume y, from the mean is within 6 per cent. 

Blends of PET and HDPE have been suggested to exploit the availabiUty of these clean recycled polymers. The blends could combine the inherent chemical resistance of HDPE with the processiag characteristics of PET. Siace the two polymers are mutually immiscible, about 5% compatihilizer must be added to the molten mixture (41). The properties of polymer blends containing 80—90% PET/20—10% HDPE have been reported (42). Use of 5—15% compatbiLizer produces polymers more suitable for extmsion blow mol ding than pure PET. 

The elasticity of a fiber describes its abiUty to return to original dimensions upon release of a deforming stress, and is quantitatively described by the stress or tenacity at the yield point. The final fiber quaUty factor is its toughness, which describes its abiUty to absorb work. Toughness may be quantitatively designated by the work required to mpture the fiber, which may be evaluated from the area under the total stress-strain curve. The usual textile unit for this property is mass pet unit linear density. The toughness index, defined as one-half the product of the stress and strain at break also in units of mass pet unit linear density, is frequentiy used as an approximation of the work required to mpture a fiber. The stress-strain curves of some typical textile fibers ate shown in Figure 5. 

Standard polyester fibers contain no reactive dye sites. PET fibers are typically dyed by diffusiag dispersed dyestuffs iato the amorphous regions ia the fibers. Copolyesters from a variety of copolymeri2able glycol or diacid comonomers open the fiber stmcture to achieve deep dyeabiHty (7,28—30). This approach is useful when the attendant effects on the copolyester thermal or physical properties are not of concern (31,32). The addition of anionic sites to polyester usiag sodium dimethyl 5-sulfoisophthalate [3965-55-7] has been practiced to make fibers receptive to cationic dyes (33). Yams and fabrics made from mixtures of disperse and cationicaHy dyeable PET show a visual range from subde heather tones to striking contrasts (see Dyes, application and evaluation). 

Density, mechanical, and thermal properties are significantly affected by the degree of crystallinity. These properties can be used to experimentally estimate the percent crystallinity, although no measure is completely adequate (48). The crystalline density of PET can be calculated theoretically from the crystalline stmcture to be 1.455 g/cm. The density of amorphous PET is estimated to be 1.33 g/cm as determined experimentally using rapidly quenched polymer. Assuming the fiber is composed of only perfect crystals or amorphous material, the percent crystallinity can be estimated and correlated to other properties. 

Chemical Properties. The hydrolysis of PET is acid- or base-catalyzed and is highly temperature dependent and relatively rapid at polymer melt temperatures. Treatment for several weeks in 70°C water results in no significant fiber strength loss. However, at 100°C, approximately 20% of the PET tenacity is lost in one week and about 60% is lost in three weeks (47). In general, the hydrolysis and chemical resistance of copolyester materials is less than that for PET and depends on both the type and amount of comonomer. 

Other Properties. Polyester fibers have good resistance to uv radiation although prolonged exposure weakens the fibers (47,51). PET is not affected by iasects or microorganisms and can be designed to kill bacteria by the iacorporation of antimicrobial agents (19). The oleophilic surface of PET fibers attracts and holds oils. Other PET fiber properties can be found ia the Hterature (47,49). 

Terephthahc acid (TA) or dimethyl terephthalate [120-61 -6] (DMT) reacts with ethyleae glycol (2G) to form bis(2-hydroxyethyl) terephthalate [959-26-2] (BHET) which is coadeasatioa polymerized to PET with the elimination of 2G. Moltea polymer is extmded through a die (spinneret) forming filaments that are solidified by air cooling. Combinations of stress, strain, and thermal treatments are appHed to the filaments to orient and crystallize the molecular chains. These steps develop the fiber properties required for specific uses. The two general physical forms of PET fibers are continuous filament and cut staple. 

LOY is characterized by low spinning tension, mostiy rheological effects, Httie orientation, amorphous stmcture, low tensde strength, and high elongation. The spun filament must be drawn, usually three to six times its initial length, and heat-treated before it develops useful properties. Nearly all PET staple is spun this way. 

EOY speeds are the most recent development in PET spinning (78). Properties are similar to HOY and appear to be limited by the differential cooling rate from filament surface to filament core. This leads to radial distribution of viscosity, stress, and, consequentiy, molecular orientation (75). Eiber tensde strength is limited. Nevertheless, speeds up to 7000 m /min are commercial and forecasts are for speeds up to 9000 m /min by the year 2000 (79). Speeds to 9000 m/min have been studied (68,80,81). 

Polyester. Poly(ethylene terephthalate) [25038-59-9] (PET) polyester film has intermediate gas- and water- vapor barrier properties, very high tensile and impact strengths, and high temperature resistance (see Polyesters, thermoplastic). AppHcations include use as an outer web in laminations to protect aluminum foil. It is coated with PVDC to function as the flat or sealing web for vacuum/gas flush packaged processed meat, cheese, or fresh pasta. 

Physical Properties. Nitrobenzene is readily soluble in most organic solvents and is completely miscible with diethyl ether and benzene. Nitrobenzene is only slightly soluble in water with a solubiUty of 0.19 parts pet 100 parts of water at 20°C and 0.8 pph at 80°C. Nitrobenzene is a good organic solvent. For example, it is used in Friedel-Crafts reactions because aluminum chloride is soluble in nitrobenzene. The physical properties of nitrobenzene are summarized in Table 1. 

Poly(ethylene terephthalate). PET is a crystalline material and hence difficult to plasticize. Additionally, since PET is used as a high strength film and textile fiber, plasticization is not usually required although esters showing plasticizing properties with PVC may be used in small amounts as processing aids and external lubricants. Plasticizers have also been used to aid the injection mol ding of PET, but only at low concentrations. 

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