Polycarbonate processes

Injection molding. It is possible to injection-mold polycarbonate with as httle as 35 N/mm clamping force, but more commonly used pressures fall between 40 and 50 N/mm For complex, thin-wall components requiring fast injection speeds, combined with high injection pressures, a clamping force of up to 80 N/mm is required. Careful attention to the right tool and equipment combination is critical for such complex thin-waU parts. 

When one is injection-molding polycarbonate, it is important not to subject the melt to long residence times as this can cause material degradation and a subsequent drop in properties. The ideal maximum residence time is between 6 and 12 min, depending on the selected melt temperature. When one is processing standard polycarbonate grades on the upper limit of the melt temperature range, it is recommended that the shot size be 60 to 80% of the barrel capacity to minimize residence time. 

The fastest possible injection speed is desirable due to polycarbonate s fast setup times, especially when glass-reinforced grades are used. 

However, care must be taken to reduce overly excessive shear stresses that may be present, typically at gates and sharp corners. Adequate venting is essential when a fast injection speed is selected. High mold temperatures are desirable for optimum flow, minimum molded-in stress, and optimal surface appearance. For the greatest machine productivity, polycarbonate grades with mold release are used to reduce the amount of ejection force required to remove parts from mold cavities. Recent efforts with siloxane copolymers of polycarbonate have shown productivity improvements due to the reduction in surface friction. This allows for ejection at higher part temperatures due to reduced ejection forces. 

Standard injection-molding guidelines should be followed to maximize the quality and productivity that can be achieved while using versatile polycarbonate resins. 

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Phosgene addition is continued until all the phenoHc groups are converted to carbonate functionahties. Some hydrolysis of phosgene to sodium carbonate occurs incidentally. When the reaction is complete, the methylene chloride solution of polymer is washed first with acid to remove residual base and amine, then with water. To complete the process, the aqueous sodium chloride stream can be reclaimed in a chlor-alkah plant, ultimately regenerating phosgene. Many variations of this polycarbonate process have been patented, including use of many different types of catalysts, continuous or semicontinuous processes, methods which rely on formation of bischloroformate oligomers followed by polycondensation, etc. 

The toxicity of phosgene has spawned a lot of research into alternates for both MDI and TDI, as well as polycarbonates. In addition to safety, there are economic incentives for developing alternate routes. In the conventional MDI process, methylene diphenylmethane diamine (MDA) is formed by reacting aniline with formaldehyde. Separating excess aniline from crude MDA is an expensive operation. Also, by-product HCl formed in the conversion of MDA to MDI is an environmental issue. The final isocyanate product contains hydrolyzable chloride compounds that are difficult to separate and dispose of. The reactants must be kept bone dry to prevent corrosion, and the introduction of water can cause a runaway reaction. Similar concerns influence the search for nonphosgene routes for TDl. Conventional routes to polycarbonates also employ phosgene, which produces chlorine waste products, primarily sodium chloride, that present disposal problems. The elimination of chlorine from the polycarbonate process would constitute a major improvement. 

Quality of the Polycarbonate by Solid-state Polymerization Process. Asahi s new polycarbonate process is not only environmentally benign, but is also able to produce polycarbonates with very hi quality. 

T. Sakashita, T. Shimoda, and K. Kishimura, Method for the preparation of copolymeric polycarbonates process patent for the preparation thereof, U.S. Patent 5,401,826 (1995). 

EquUibrium constants are of the order of unity for polyesters and polycarbonates, and of the order of hundreds for polyamides. In order to obtain end-group conversions above 0.99, this implies a concentration of by-product [W] below 10" times the concentration of bonds in the former two systems. The high temperature of the processes increases the vapor pressure of the by-products and their equilibrium solubility decreases, but even so partial pressures of some millibars for polyester and polycarbonate processes are required they may be obtained by using a combination of vacuum and inert stripping gas. These low partial pressures and consequently nearly infinite dilution often justify the application of Henry s law to describe the vapor-liquid equilibrium according to Eq. (24), in which Pw is the equilibrium vapor pressure of by-product W, Hw is its Henry constant for the polymer melt (in the usual range of 10-1000 bar), ww is mass fraction in the melt, is the vapor pressure of pure W, and lw is its weight fraction activity coefficient. 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

Fig. 4. Diagram of the two-step process to manufacture nucleation track membranes, (a) Polycarbonate film is exposed to charged particles in a nuclear reactor, (b) Tracks left by particles are preferentially etched into uniform cylindrical pores (8).

Acrylic ESTER POLYMERS Acrylonitrile POLYMERS Cellulose esters). Engineering plastics (qv) such as acetal resins (qv), polyamides (qv), polycarbonate (qv), polyesters (qv), and poly(phenylene sulfide), and advanced materials such as Hquid crystal polymers, polysulfone, and polyetheretherketone are used in high performance appHcations they are processed at higher temperatures than their commodity counterparts (see Polymers containing sulfur). 

Polycarbonates are prepared commercially by two processes Schotten-Baumaim reaction of phosgene (qv) and an aromatic diol in an amine-cataly2ed interfacial condensation reaction or via base-cataly2ed transesterification of a bisphenol with a monomeric carbonate. Important products are also based on polycarbonate in blends with other materials, copolymers, branched resins, flame-retardant compositions, foams (qv), and other materials (see Flame retardants). Polycarbonate is produced globally by several companies. Total manufacture is over 1 million tons aimuaHy. Polycarbonate is also the object of academic research studies, owing to its widespread utiUty and unusual properties. Interest in polycarbonates has steadily increased since 1984. Over 4500 pubflcations and over 9000 patents have appeared on polycarbonate. Japan has issued 5654 polycarbonate patents since 1984 Europe, 1348 United States, 777 Germany, 623 France, 30 and other countries, 231. 

Transesterification. There has been renewed interest in the transesterification process for preparation of polycarbonate because of the desire to transition technology to environmentally friendly processes. The transesterification process utilizes no solvent during polymerization, producing neat polymer direcdy and thus chlorinated solvents may be entirely eliminated. General Electric operates a polycarbonate plant in Chiba, Japan which produces BPA polycarbonate via this melt process. 

An analogue of the transesterification process has also been demonstrated, in which the diacetate of BPA is transesterified with dimethyl carbonate, producing polycarbonate and methyl acetate (33). Removal of the methyl acetate from the equihbrium drives the reaction to completion. Methanol carbonylation, transesterification using phenol to diphenyl carbonate, and polymerization using BPA is commercially viable. The GE plant is the first to produce polycarbonate via a solventiess and phosgene-free process. 

Processing. Polycarbonates may be fabricated by ah. conventional thermoplastic processiag operatioas, of which iajectioa mol ding is the most common. Recommeaded operatiag coaditioas are stock temperatures of 275—325°C and mol ding pressures of 69—138 MPa (10,000—20,000 psi). 

Medical and health-care related appHcations consume about 21,000 t of polycarbonate aimuaHy. Polycarbonate is popular because of its clarity, impact strength, and low level of extractable impurities. Special grades have been developed to maintain clarity and resistance to yeHowing upon gamma radiation sterilization (qv) processes. Leisure and safety appHcations are many and varied, accounting for about 22,000 t of consumption aimuaHy. The... 

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