Blends of PET and PEN

Blends of these starting materials were obtained by coprecipitation from solution in hexafluoroisopropanol. Amorphous films were then obtained from the precipited powder by melt pressing in vacuo for different times varying from 0.2 min to 45 min followed by quenching in ice-water. In this way PET/PEN blends with weight compositions 90/10,70/30,60/40,44/56,30/70 and 10/90 were prepared. 

In order to explain the variation of H vs tm it is convenient to analyse microhardness as a function of composition. It is found that for tm 0.2 min, H increases 

Let us recall that when the melt-pressing time is about 0.2-0.5 min two TgS are observed, indicating that there are two phases present. In case of tm 2 

One may ask at this stage, why does H then gradually decrease with increasing tm if the molecular weight and the viscosity remain practically constant as shown by additional measurements One possible explanation is that at the beginning of the transesterification process the copolyester has a rather block-like character. Only after longer times does it become a statistical copolymer as found for other similar blends (Fakirov Denchev, 1999). The results, therefore, indicate that the microhardness of the block copolyester is larger than that of the statistical copolymer. The existence of blocks may lead to a microphase separation between PEN and PET blocks. It seems, then, reasonable to assume that parallel packed sequences of blocks with the same chemical compositions would yield mechanically less easily than parallel copolymer sequences of statistical composition. 

In conclusion, in order to obtain the optimum mechanical properties of these blends, one should use melt-pressing times in the range 5-19 min. Otherwise, the mechanical properties represented by microhardness may be reduced by 10-15%. 

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Transesterification. In 1 1 blends of PET and PEN, for transesterification levels higher than 23%, the blends tend to transform into a one-phase system and the cold crystallization of PET is strongly inhibited due to the significant reduction of the PET segment length. For lower levels of transesterification the blends are phase separated. 

It is possible to control both the rate of change of intrinsic viscosity (IV) and the rate of transesterification of a blend of PET and PEN during solid state polymerization. The method comprises providing PEN with a first IV and a PET with a second IV. The PEN and PET are reacted in the presence of an ethylene glycol compound in an amount sufficient to achieve a desired final IV and final level of transesterification in the copolymerized PEN/PET product. Due to the improved thermal resistance, the materials can be used for hot fill containers. 

In order to improve the heat resistance and the gas barrier properties of the PET polyester resins, the usage of blends of PET with PEN has been proposed. Blends of PET and PEN generate acetaldehyde when melt kneaded at elevated temperatures to improve the compatibility. This causes problems, such as change of taste of the contents filled in the container, and the lowering of transparency. 

Montserrat, S. Roman, F. Colomer, P. Study of the crystallization and melting region of PET and PEN and there blends by tmDSC. J. Therm. Anal. Calorimetr. 2003, 72, 657-666. 

This chapter covers fundamental and applied research on polyester/clay nanocomposites (Section 31.2), which includes polyethylene terephthalate (PET), blends of PET and poly(ethylene 2,6-naphthalene dicarboxy-late) (PEN), and unsaturated polyester resins. Section 31.3 deals with polyethylene (PE) and polypropylene (PP)-montmorillonite (MMT) nanocomposites, including blends of low density polyethylene (LDPE), linear low density polyethylene (LLDPE), and high density polyethylene (HDPE). Section 31.4 analyzes the fire-retardant properties of nanocomposites made of high impact polystyrene (HIPS), layered clays, and nonhalogenated additives. Section 31.5 discusses the conductive properties of blends of PET/PMMA (poly (methyl methacrylate)) and PET/HDPE combined with several types of carbon... 

A polyester composition having better gas barrier properties with less acetaldehyde ejection consists in a blend of prepolymers of PET and PEN, or a copol5uner. This blend is subjected to the solid state polymerization process. " ... 

Polyester fibers with improved properties are prepared from blends of PET with PEN, or a blend of PET with a copolymer having terephthal-ate and naphthalate units. In addition, recycled PET can be used, thus providing a valuable use for recycled polyester materials. 

Figure 20.10 The effect of the blending time as a function of sequence length of the PET ( ) and PEN ( ) segments for the PHB/PEN/PET (40/30/30 (mol%)) blends [31]. From Park, J. K., Jeong, B. J. and Kim, S. H., Pseudo liquid crystallinity and characteristics of PHB/PEN/PET melt blend, Polym. (Korea), 24, 113-123 (2000). Reproduced by permission of The Polymer Society of Korea...

The morphology of the spherulites was in the form of a Maltese Cross , which was confirmed by the Avrami exponent value in the DSC study. The spherulite size of the binary blends was smaller than that of pure PET and PEN. 

The PET and PEN blends are completely amorphous after quenching from the melt as revealed by the DSC experiments (Zachmann et al, 1994). Figure 5.8 shows in detail the variation of the microhardness with melt-pressing time tm for the PET/PEN composition 44/56. For all compositions, H shows first a rapid initial increase with exhibiting a maximum just before = 10 min and, then, for longer times, a gradual decrease down to values which can be even lower than the starting ones. 

Table 20.1 The content of the hetero sequence (/ten), sequence lengths of the PET segment (L pet) and PEN segment (L pen) and degree of randomness (B) of the PHB/PEN/ PET blends [21]. From Kim, S. H., Kang, S. W., Park, J. K. and Park, Y. H., J. Appl. Polym. ScL, 70, 1065-1073 (1998), Copyright (1998, John Wiley Sons, Inc.). This material is used by permission of John Wiley Sons, Inc...

PET and PEN form immiscible mixtures. Improved miscibility can be obtained by performing a transesterification reaction between both ingredients to produce copolyesters, which act as compatibilizers in the interface of the blend. This reaction, when carried out in a melt extruder, depends strongly on temperature and residence time, in particular, within 50-80 wt% PEN content [63]. Physical and mechanical properties of the resulting blend depend on the degree of transesterification and also on the resulting copolymer... 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

In many cases blends of PET with other polymers are only partially blends and partially copolymers, due to transesterification reactions, which take place during extrusion. This is the case, for example, for the PET/PEN blends which will be discussed in Section 4.9.3. 

The EDA approved PEN for food contact appiications in Aprii, 1996. PEN and PET can be blended together to make useful materials, and also PEN/PET copolymers can be produced. When PEN/PET blends are used, one is really making copolymers in the extruder. Analysis of the material that is extruded shows that transesterification reactions occur in the extruder, forming molecular bonds between PET and PEN molecules. Therefore, processing conditions are important to the quality of product one makes when using blends. Blends, as well as copolymers manufactured by polymer suppliers, can open up markets for this polyester that are not accessible to PEN alone because of its high price. While these materials have not yet been approved for food contact applications in the U.S., there appear to be no technical barriers to their approval. 

PEN and PET copolymers fall into two groups. Low-NDC copolymers contain less than 15% NDC, and high NDC copolymers have 85% or more NDC. Copolymers with intermediate ranges of NDC are not used because they cannot crystallize and therefore have inferior properties. Because homopolymer PET and PEN are immiscible, blends require special mixing techniques to cause sufficient transesterification to occur. This amounts to the production of a copolymer during the extrusion process, as mentioned earlier. Blends of homopolymers with copolymers are easier to process than blends of the homopolymers themselves. Usually low-NDC copolymers are blended with PET, and high-NDC copolymers with PEN. 

Silvi et al. (1997) have compatibilized an immiscible blend of LCP and similar PEI through copolymer formation in the presence of a coupling agent. Representative coupling agents included PE-co-GMA and the o-cresol novolak reaction product with epichlorohydrin. Mechanical properties and HDT were compared to properties for blends without polyepoxide. PEI could be diluted with polyarylate, polyestercarbonate, PET, or PEN. 

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