Polycarbonates

Polycarbonate resins offer a combination of properties that extends the usefulness and fields of application for thermoplastic materials. This relatively new plastic material is characterized by very high impact strength, superior heat resistance, and good electrical properties.

In addition, the low water absorption, high heat distortion point, low and uniform mold shrinkage, and excellent creep resistance of the material result in especially good dimensional stability. Of value for many applications is the its transparency, shear strength, stain resistance, colorability and gloss, oil resistance, machinability, and maintenance of good properties over a broad temperature range from less than -73°C to 132 to 138°C. The fact that polycarbonate resin is self-extinguishing is important in many applications.

Polycarbonates are amorphous engineering thermoplastics that offer exceptional toughness over a wide temperature range. The natural resins are water-clear and transparent.

Polycarbonate resins are available in general-purpose molding and extrusion grades and in special grades that provide specific properties or processing characteristics. These include flame-retardant formulations as well as grades that meet Food and Drug Administration regulations for parts used in food-contact and medical applications. Other special grades are used for blow-molding, weather and UV-resistance, glass-reinforcement, EMI, RFI, ESD-shielding, and structural-foam applications. Polycarbonate is also available in extruded sheet and film.

Composition

The polycarbonate name is taken from the carbonate linkage that joins the organic units in the polymer. This is the first commercially useful thermoplastic material that incorporates the carbonate radical as an integral part of the main polymer chain.

There are several ways polycarbonates can be made. One method involves a bifunctional phenol, bisphenol A, which combines with car-bonyl chloride by splitting out hydrochloric acid to give a linear polymer consisting of bisphenol groups joined together by carbonate linkages. Bisphenol A, which is the condensation product of phenol and acetone, is the basic building block used also in the preparation of epoxy resins.

Other members of the polycarbonate family may be made by using other phenols and other ketones to modify the isopropylidene group, or to replace this bridge entirely 1)’N’ other radicals Subatttutiotr on the benzene ring offer further possibilities for variations.

Properties

Polycarbonate is a linear, low-crystalline, transparent, high-molecular-weight plastic. It is generally considered to be the toughest of all plastics. In thin sections, up to about 0.478 cm, its impact strength is as high as 24 kg.m. In addition, polycarbonate is one of the hardest plastics. It also has good strength and rigidity, and, because of its high modulus of elasticity, is resistant to creep. These properties, along with its excellent electrical resistivity, are maintained over a temperature range of about -170 to 121°C (see Table P.2). It has negligible moisture absorption, but it also has poor solvent resistance and, in a stressed condition, will craze or crack when exposed to some chemicals. It is generally unaffected by greases, oils, and acids. Polycarbonate plastics are easily processed by extrusion, by injection, blow, and rotational molding, and by vacuum forming. They have very low and uniform mold shrinkage. With a white light transmission of almost 90% and high impact resistance, they are good glazing materials. They have more than 30 times the impact resistance of safety glass.

Other typical applications are safety shields and lenses. Besides glazing, the high impact strength of polycarbonate makes it useful for air-conditioner housings, filter bowls, portable tool housings, marine propellers, and housings for small appliances and food-dispensing machines.

Humidity changes have little effect on dimensions or properties of molded parts. Even boiling water exposure does not change dimensions more than 0.30 mm/mm after parts are returned to room temperature. Creep resistance is excellent throughout a broad temperature range and is improved by a factor of 2 to 3 in glass-reinforced compounds.

The insulating and other electrical characteristics of polycarbonate are excellent and almost unchanged by temperature and humidity conditions. One exception is arc resistance, which is lower than that of many other plastics.

Polycarbonates are generally unaffected by greases, oils, and acids. Nevertheless, compatibility with specific substances in a service environment should be checked with the resin supplier. Water at room temperature has no effect, but continuous exposure in hot (65°C) water causes gradual embrittlement. The resins are soluble in chlorinated hydrocarbons and are attacked by most aromatic solvents, esters, and ketones, which cause crazing and cracking in stressed parts. Grades with improved chemical resistance are available, and special coating systems can be applied to provide additional chemical protection.

Fabrication

Polycarbonate resin has been molded in standard injection equipment using existing molds designed for nylon, polystyrene, acrylic, or other thermoplastic materials. Differences in mold shrinkage must be considered. And, in fabrication, the polycarbonate does have its own unique processing characteristics. Most important among these are the broad plastic range and high melt viscosity of the resin. Production runs in molds designed for nylon or acetal resin are not recommended.

Like other amorphous polymers, polycarbonate resin has no precise melting point. It softens and begins to melt over a range from 216 to 227°C. Optimum molding temperatures lie above 271 °C. The most desirable range of cylinder temperatures for molding the resin is in the area of 275 to 316°C.

TABLE P.2

Properties of Polycarbonates

Mold design must take into consideration the high melt viscosity of the material. Large sprues, large full-round runners, and generous gates with short lands usually give best results.

Tab gating is good for filling large thin sections. The gate to the tab should be large.

In injection molding polycarbonate, the following conditions are desirable:

1. Heated molds. Generally, hot water heat is adequate for heating molds, with typical mold temperatures ranging from 77 to 93°C. Molds for large areas, thin sections, or complex shapes or multiple cavity molds with long runners may require higher temperatures. Some molds have run best at 110 to 121°C.

2. Cylinder temperatures. The most usual cylinder temperature for molding polycarbonates is in the range of 275 to 316°C. Few parts require cylinder temperatures above 316°C. A heated nozzle and adequate mold temperature are helpful in keeping cylinder temperatures below 316°C. In most cases rear cylinder temperatures higher than front cylinder temperatures give best results.

3. Heated nozzle. In general, nozzle temperature equal to front cylinder temperature gives good results.

4. Adequate injection pressure. Injection pressures used in molding polycarbonate resin range from 80 to 204 MPa. Most usual range is in the 103 to 136 MPa range. Typical pressure setting is 3/4 to full pressure capacity of the press.

5. Fast fill time. For most parts molded, a fast ram travel time has been found desirable for thick as well as thin sections. For very thick sections, it is better to utilize somewhat slower ram speed.

Polycarbonate must be well dried to obtain optimum properties in the molded part. For this reason the resin is packaged in sealed containers.

Preheating pellets in the can to 121°C for 4 to 8 h and using of hopper heaters at 121°C are recommended for production operations to prevent moisture pickup.

Although polycarbonates are fabricated primarily by injection molding, other fabricating techniques may be used. Rod, tubing, shapes, film, and sheet may be extruded by conventional techniques. Films and coatings may be cast from solution. Parts can readily be machined from rod or standard shapes. Cementing, painting, metalizing, heat sealing, welding, machining operations, and other standard finishing operations may be employed. Film and sheet can be vacuum-formed or cold-formed.

Applications

The properties of polycarbonate resin make this new plastic suitable for a wide variety of applications. It is now being used in business machine parts, electrical and electronic parts, military components, and aircraft parts, and is finding increasing use in automotive, instrument, pump, appliance, communication equipment, and many other varied industrial and consumer applications.

One of the applications is in molded coil forms, which take advantage of the electrical properties, heat and oxidation resistance, dimensional stability, and resistance to deformation under stress of the resin.

A transparent plastic with the heat resistance, the dimensional stability, and the impact resistance of polycarbonate resin has created considerable interest for optical parts, such as outdoor lenses, instrument covers, lenses, and lighting devices.

Housings make use of the impact resistance of the material and its attractive appearance and colorability. In many cases, also, heat resistance and dimensional stability are important.

An interesting application area for plastic materials is the use of polycarbonate resin for fabricating fasteners of various types. Such uses as grommets, rivets, nails, staples, and nuts are in production or under evaluation. The ability of polycarbonate parts to be cold-headed has developed considerable interest in rivet applications.

Terminal blocks, connectors, switch housings, and other electrical parts may advantageously be molded of polycarbonate resin to take advantage both of the electrical properties of the material and the unusual physical properties, which give strength and toughness to the parts over a range of temperatures. Because of the heat resistance of polycarbonate, and the fact that it is self-extinguishing, molded parts can be used in current-carrying support applications.

Another application is in bushings, cams, and gears. Here, dimensional stability is important as is the high impact strength of the resin. Good physical properties over a broad temperature range, low water absorption, resistance to deformation under load, and resistance to creep suggest its use for many applications of this type. However, the resin has a higher coefficient of friction and a lower fatigue endurance limit than do some other plastics used in these types of applications. For this reason, it should not be considered a general-purpose gear and bearing material, but might be considered for applications subject to light loading, or to heavier but intermittent loading.

In most heart-bypass operations, the saphe-nous vein from a patient’s leg replaces blocked blood vessels in the heart. The separate surgical procedure, performed during the heart-bypass operation, involves removing the vein through a long incision. Following the surgery, patients frequently complain of ongoing leg pain, potentially leading to reduced mobility and delayed rehabilitation while the large incision heals.

A new generation of surgical instruments not only makes the procedure less invasive, it helps speed the patient’s return to normal activities. The system uses endoscopic techniques to harvest the vein. This, in turn, requires smaller incisions. The potential benefits are less postoperative pain, fewer wound-healing complications, minimal scarring, and quicker recovery.

The materials chosen for these applications have provided several significant benefits and, from the medical side, the materials were bio-compatible. The materials resist chemicals and withstand gamma sterilization. The balloon mount is made with polycarbonate. On the orbital dissection cannula, the end piece embodies a polycarbonate resin.