PBT-polycarbonate blends

This combination gives a very useful blend of mechanical properties. The PBT phase provides melt flow, solvent resistance and the ultimate heat performance of the blend (Tm). The PC phase provides reduced shrink, better dimensional stability, higher heat capability under low load (66 psi HDT) and improved impact strength. Interestingly, the PC also provides improved paint adhesion by being present as a very thin outer layer in molded parts. PBT, by virtue of its solvent 

The extent of the benefits of adding PC to the blend will depend on the PC/PBT ratio. Very low levels ( 5 %) of PC or PBT may be totally miscible in the other resin and act like a slightly modified PC or PBT resin. Most PBT blend products use from 10-60% PC. High PC content will improve impact and lower shrink but reduce flow and solvent resistance. More PBT gives better flow and solvent resistance with more shrink and loss of some impact. A range of PBT-PC blends covering the spread of properties is commercially available. 

Studies have shown that this reaction is a result of transesterification between PBT and PC. Transesterification is influenced by many factors, including PBT end groups and catalyst residues. While a little copolymer formation is not a bad thing for the performance of the blend, uncontrolled reaction is unacceptable since the same material could never be made twice. Fortunately, methods to control this chemistry were developed. Generally, addition of certain phosphites is used to quench the transesterification and related reactions [45, 46], Since phosphites are also used as antioxidants and color stabilizers, the quencher was often 

Overall, the PC blends improve the HDT and impact, especially in the absence of fillers such as glass fiber. The PC reduces shrink and, in glass-filled blends, reduces warp (the amorphous PC resin shrinking less than the PBT). The PC also improves PBT paint adhesion and other secondary operations such as gluing. 

PBT-PC blends show increased melt strength allowing them to be easily processed by blow molding and profile extrusion. The PBT-PC blends have been extruded into sheet and thermoformed into parts. Enhanced melt strength allows PBT-PC blends to be foamed. Structural foam grades for injection molding (10-30% density reduction) are commercially available. 

---

PBT/polycarbonate blend, first introduced in 1980 by General Electric (Xenoy ) has enjoyed a fast growth in automotive applications, particularly for bumpers. The blend was developed to meet the low temperature impact strength, dimensional stability and paintability requirements of rigid bumper fascias [Bertolucci and Delany, 1983], which accounted for the bulk of the current market volume for the PBT/PC blend (estimated 25 kton/y in the USA). The development of commercial PET/PC blends followed shortly after the initial success of PBT/PC blends. Currently there are several commercial blends of both PBT/PC and PET/PC blends available. 

Table 15.26. Properties of some commercial grades of PBT/polycarbonate blends...

Blends of PBT with polycarbonates have been widely used for car bumpers. Interest in PBT/PET blends and PBT/ASA has arisen because of the good surface finish possible even with glass-reinforced grades. Copolyesters based on PBT but with some longer chain diol or acid are also now produced. 

Figure 3. Flame Retarding Polycarbonate/PBT Polyester Blends (12% Flame Retardant)...

Pellow-Jarman, M. and Hetem, M., The effect of the polybutylene terephthalate constituent on the reactions occurring in PBT-polycarbonate polymer blends below their decomposition temperature, Plast. Rubber Composites Proc. Appl., 23, 31-41 (1995). 

Most of the discussion has been centered around just the neat polymer or the polymer plus a modifier or additive. Coloring issues further escalate when polymer blends are used or when multiple additives are incorporated. Would it be difficult, and would the color gamut be somewhat restricted, if one was to color an impact modified, flame retardant, glass reinforced PBT/polycarbonate alloy The answer now better be a resounding YES ... 

Using micro-Raman imaging three blends consisting of PP/PE/EPR copolymer, PBT/polycarbonate/LDPE, and SAN copolymer/styrene-maleic anhydride copolymer/polydimethylphenylene oxide were studied with regard to compositional and morphological heterogeneities (221). 

The second most important class of commercial polycarbonate blends is derived by blending with commercial thermoplastic polyesters such as polybutylene tere-phthalate (PBT) and polyethylene terephthalate (PET). Both PBT and PET are crystallizable polymers and hence offer the expected chemical resistance advantages of the crystalline polymers in blends with polycarbonate. Among the thermoplastic polyester/polycarbonate blends, the PBT/PC blend has the major commercial volume, followed by the PET/PC blend. A copolymer of 1,4-cyclohexanedimethanol, ethylene glycol, and terephthalic acid (PCTG) forms a miscible blend with polycarbonate. PCTG/PC blend was earlier offered by Eastman (Ektar ) for specialty applications, but it is no longer commercial. 

Arguably, the most remarkable property of polycarbonate Is its very low notch sensitivity in thin sections. As thickness goes up, the notched Izod impact goes down, PBT/PCO blends exploit this behavior effectively, ameliorating the notch sensitivity which is a drawback of plain PBT. These blends also Improve other mechanical properties over those of the base polymer, especially stiffness and heat deflection temperature. Of course, there Is no "free lunch" here either, but the price is modest - Increased creep with PCO content, associated with an apparent drop in the glass transition temperature [24]. 

Another blend, also invented at General Electric, is Xenoy . This is an immiscible blend of polybutylene terephthalate (PBT) and polycarbonate (PC) [5,6]. This blend is immiscible it has separate phases of PBT and of PC. An immiscible blend is characterized by two distinct Tgs and thermal analysis of this blend shows the Tg of PBT and of PC as well as a sharp melting point for the crystalline PBT. The blend also includes an impact modifier. It exhibits the good impact associated with PC and the solvent resistance associated with PBT. Because of these properties, it is suitable for applications such as automotive bumpers requiring high impact and gasoline resistance. 

Tribological systems typically do not contain pure polycarbonate, but polycarbonate blends (PC/ABS and PC/PBT) are used often in the automotive industry. It is known that ester oils or polyglycols cause stress-cracking in polycarbonate [836]. 

PBT polymer blends combine the properties of partially crystalline PBT with those of amorphous thermoplastics such as polycarbonate or ABS. The amorphous partner improves the impact strength and warpage behaviour, while the PBT ensures the chemical resistance of the blend. 

---