Opaque polycarbonates

Hence, a great many applications for opaque polycarbonate make use of impact-modified products, including... 

Eyeglass frames made of cellulose acetate plasticized with diglycerol esters do not exhibit opaqueness at the frame-lens junction with polycarbonate plastic lenses (146,147). 

Crystallisable polymers have also been prepared from diphenylol compounds containing sulphur or oxygen atoms or both between the aromatic rings. Of these the polycarbonates from di-(4-hydroxyphenyl)ether and from di-(4-hydroxy-phenyl)sulphide crystallise sufficiently to form opaque products. Both materials are insoluble in the usual solvents. The diphenyl sulphide polymer also has excellent resistance to hydrolysing agents and very low water absorption. Schnell" quotes a water absorption of only 0.09% for a sample at 90% relative humidity and 250°C. Both the sulphide and ether polymers have melting ranges of about 220-240°C. The di-(4-hydroxyphenyl)sulphoxide and the di-(4-hydroxy-phenyl)sulphone yield hydrolysable polymers but whereas the polymer from the former is soluble in common solvents the latter is insoluble. 

Although it is called crystal clear polystyrene, atactic PS is amorphous and clear, as are PMMA and polycarbonate (PC). In contrast, hdpe and nylon 66 are highly crystalline and opaque. The relationship of packing efficiency to specific gravity and crystallinity is readily illustrated by ldpe and HDPE, which are 60 and 95% crystalline, respectively, and have specific gravities of 0.91 and 0.97, respectively. 

The density and n value of a polymer crystal are greater than those of an amorphous polymer. Many polymers are opaque because of the presence of ordered clusters of crystals called spherulites which have different n values. ptfe, which is highly crystalline, is opaque but amorphous polycarbonate (PC), PMMA, and PS are noncrystalline and clear. 

Pore size and density are also important in selecting a membrane. Some membranes such as polycarbonate have very high pore densities for maximum fluid transport, but are fragile and rather opaque. The pore size is also critical, depending on the experiment. For example, a 0.45- xm pore size will prevent cells from migrating through the membrane, while 3 jxm can be used for transmembrane... 

Sodium antimonate must be used with halogen containing compounds for it to act as effective fire retardant. The source of chlorine may come from polymer (e.g., PVC, chlorinated rubber, etc.) or other chlorinated or brominated material. The benefits of using sodium antimonate over antimony oxide include its low tinting strength and the acid scavenging capability. For these reasons, it is used in semi-opaque or dark colored materials and in polymers such as polyesters and polycarbonates which are acid sensitive. 

In order to improve both the thickness sensitivity and low temperature sensitivity of the impact strength, polycarbonate has been blended with a variety of low T, elastomeric impact modifiers. More important among these are the core-shell rubbers like PMMA-g-polybutadiene, PMMA-g-SBR (MBS), PMMA-g-n-butylacrylate (acrylic core-shell), all normally composed of 0.1/core particles. These modifiers improve both the thick-section (6.4 mm) and low temperature notched Izod impact properties of polycarbonate [Witman, 1981 Neuray and Ott, 1981 Bussink et al., 1977] (Figures 15.13 and 15.14). The blends are of course opaque. Impact modified polycarbonate also shows better retention of impact strength with heat-aging. 

Styrene-butadiene copolymers are often blended with other polymers. Transparent blends can be made with styrene, styrene-acrylonltrlle copolymers, or styrene-methyl methacrylate copolymers. Blends with styrene have low impact strength even at low styrene levels, while blends with styrene-methyl methacrylate copolymers can have greatly Improved impact strength. Blends with high impact polystyrene, polypropylene, and polycarbonate are opaque. 

As crystalline materials melt, their appearance transforms from opaque to transparent because the ordered structure is lost Highly amorphous polymers, including acryhcs, polycarbonate, and polystyrene do not form crystals, so are transparent (Figure 4.6). An exception is crystalline polyester poly (ethylene terephthalate) used in fizzy drinks botdes, which is transparent because its crystals are too small to interfere with hght waves. Fillers and additives usually decrease the light transmission of a plastic by scattering incident light. 

The influence of the solvent can be observed, for example, with a polystyrene/ polycarbonate-blend CH2CI2 allows a faster evaporation and leads to the expected turbid film consisting of the amorphous components PS and PC. Instead, THF and CHCI3 evaporate slower and, hence, allow the PC to partially crystallize, which results in an opaque film. 

Phase-separated PEI blends have been investigated. Combinations of PEI with polycarbonate (PC) or polycarbonate ester (PCE) copolymers have a fine, laminar two-phase morphology [36]. Combinations of PEI with polycarbonate or polyester carbonates yield a family of two-phase opaque systems that have reduced heat capability versus PEI, but show improved impact and better melt flow [37, 38]. 

These copolymers, containing a high level of ITR, have also been commercialized in a highly weather-resistant film that has the appearance of a paint film. The film is available in transparent and opaque colors. In addition, the film can be made with a metallic look. The film can be used with in-mold-decoration (IMD) (see the section Processing Polycarbonates ) molding processes to produce class A automotive exterior parts. Because the film contains very high levels of the ITR resin, it has an excellent balance of scratch resistance, weatherability, chemical resistance, and thermal performance. 

Water-clear transparency is probably the single unique and most important attribute of polycarbonate. It is of such importance because there are very few transparent polymers—especially with high optical properties. The vast majority of polymers are opaque, with a few families providing degrees of translucency. And while it is certainly true that polycarbonate is of interest in applications other than those requiring clarity, it is also true that approximately one-half of all the polycarbonate sold is formulated... 

By first segregating polycarbonates into transparent and opaque applications and then reviewing other combinations of properties—in conjunction with transparency or opacity—that make this resin appropriate for a given application, its breadth of use begins to become clearer in the sections below. Then, by subsequently looking at blends of polycarbonate and other polymers, some unique applications will also be covered. 

Aside from clarity, the same combination of properties that helps polycarbonate succeed in transparent applications also helps these materials be selected for a large number of applications where opacity is not an issue. Since most polymers are naturally opaque, there is potentially a greater pool of viable competitors for each opaque application. Nonetheless,... 

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