Polyolefins glass transition temperature

The TPX experimental product of Mitsubishi Petrochemical Ind. (221) is an amorphous, transparent polyolefin with very low water absorption (0.01%) and a glass-transition temperature comparable to that of BPA-PC (ca 150°C). Birefringence (<20 nm/mm), flexural modulus, and elongation at break are on the same level as PMMA (221). The vacuum time, the time in minutes to reach a pressure of 0.13 mPa (10 torr), is similarly short like that of cychc polyolefins. Typical values of TPX are fisted in Table 11. A commercial application of TPX is not known as of this writing. 

Among the spectrum of melt-spinnable fibers such as polyolefins and nylons, PET stands at the upper end in terms of crystalline melt temperature and glass transition temperature. This provides superior dimensional stability for applications where moderately elevated temperatures are encountered, e.g. in automobile tires or in home laundering and drying of garments. The high thermal stability results from the aromatic rings that hinder the mobility of the polymer chain. 

The glass transition temperatures by DSC measurements vary in a broad range from 80°C up to 180°C, which leads to high HDT for a polyolefin, for example HDT B (0.46 MPa) in a range from 75°C to more than 170°C. Consequently, service temperatures are rather high for a polyolefin. 

Engineering Thermoplastics Polyolefins and Styrenics Table 2.3 Materials with High Glass Transition Temperatures (26)... 

Table 1 Typical physical and (room temperature) mechanical properties (melting point Tm, glass transition temperature Tg) Young s modulus E, Izod toughness, tensile yield stress av elongation at break b) and applications of commodity polyolefins...

The approximated lines at the right side of Fig. 3.14 correspond to an Arrhenius activation energy of approximately 120 kj/mol, which is significantly higher than the flow activation energy of the semi-crystalline polyolefin melts shown in Fig. 3.13. The temperature dependence of the melt viscosity for amorphous thermoplastics is substantially higher than that of semi-crystalline polymers and increases dramatically as the temperature approaches the glass transition temperature. 

It is well known from diffusion theory that different types of polymers have different diffusion behaviours. For example, the polyester type polymers like poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN) and polycarbonate (PC) as well as rigid poly(vinyl chloride) (PVC), which have a high glass transition temperature, are low diffusive polymers. The migration of potential contaminants in these polymers will result in low migration values. In contrast, polyolefins like high density polyethylene (HDPE), polypropylene (PP) or low density polyethylene (LDPE), which... 

Figure 11.13. Effect of side-chain branching on the melting point and glass transition temperature of polyolefins (—CHR—CH2—) — (R straight chain) (Ref 13)...

The well-known examples of blends are impact modified, toughened polymers, where polymers with different glass transition temperatures are blended, such as a rubber with a thermoplastic. Many other blends are known, such as barrier polymers for packaging, where specific polar or nonpolar polymers improve the properties of polymer blends, combined to increase the resistance against transport of water and a certain gas (oxygen, carbon dioxide, etc.), such as PA (barrier to oxygen) with a polyolefin (barrier to water vapor). 

Branched side chains, particularly if the branched point is located close to the main chain, increase the glass transition temperature. This is illustrated in Figures 13.29 and 13.30 fora series of polyaciylates and polyolefins with branched side chains, respectively. 

Polyolefin fibers, on the other hand, have some drawbacks when compared to common textile fibers such as polyester. These limitations include relatively lower resiliency, creeping due to their low glass transition temperature (Tg), poor... 

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