Download - CORPORATE TRAINING AND PLANNING POLYCARBONATE (PC)

CORPORATE TRAINING AND PLANNING

POLYCARBONATE (PC)


Polycarbonate (PC)

Polycarbonates are polyesters of carbonic acid. Although carbonic acid itself is not a stable compound, its derivatives (phosgene, urea, carbonates) are commonly available. The reaction of gaseous phosgene with bisphenol A gives rise the formulation of commercial polycarbonate.

Repeat unit of Polycarbonate


History of development

PC was first prepared by Einhorn in 1898 by reacting the hyroxy benzenes, hydroquinone and resorcinol separately with phosgene in pyridine solution. In 1920 similar kind of product was prepared by Bischoff and Von Heden storm with the use of diphenyl carbonate. In 1930 Carothers and Natta prepared a number of aliphatic polycarbonates using ester inter change reaction. In 1556 Herman Schnell of Farben Fabriken Bayer and D.W. Fox of GE independently produced PC from Bisphenol A and Phosgene. In 1958 production of Bisphenol, a polycarbonate was started simultaneously by GE Co. both in Germany and USA from Bisphenol A and phosgene. Today about 75% of the market is held by General Electric and Bayer. Other manufacturers are ANIC (Italy), Taifin Chemical Co., Mitsubishi Edogawa and Idemitsu Kasei in Japan and since in 1985, DOW (USA) do Polycarbonates in Brazil.


Monomer Ingredients for PC

The PC is made from Bisphenol A and Phosgene (COCl2).

Phosgene is prepared commercially from carbonmonoxide and chlorine.

Bisphenol A can be produced by the condensation of phenol with acetone under acidic conditions.

COCl2 Phosgene


Chemistry of Preparation of PC

Poly (bisphenol – A-carbonate) or PC, is a condensation product of bisphenol A, a carbonate precursor such as phosgene or diphenyl carbonate and a monophenol chain terminator such as phenol [11] or 6-butyl phenol. The structure of PC is given below :

Normally the following reaction takes place in the base medium.


Manufacturing of PC

Interfacial Polymerization

Transesterification Processes (melt polymerization)

Poly carbonates are commercially produced by either interfacial polymerization or melt polymerization.

The solution process, which was once a major commercial process is no longer used in industry because of its inferior economics.


Interfacial Polymerization

The interfacial polymerization of polycarbonates involves a reaction of BPA (Bisphenol A) with phosgene at the interface between an inert organic methylene chloride solution and an aqueous caustic solution. The reaction takes place in two steps. First, phosgene reacts with BPA to form monochloroformates. Then the poly condensation takes place between BPA hydroxyl groups and chloroformates in the presence of triethylamine as a catalyst, yielding the polymer that remains dissolved in the organic phase. During the polymerization, the by-product (hydrochloric acid) reacts with aqueous caustic phase to form sodium chloride. After the polymerization, the organic polymer solution is separated from the aqueous phase and purified. The polymer is recovered from the purified polymer solution by precipitation or evaporation. The solid polymer is crushed and extruded into pellets. The interfacial polymerization can be conducted by batch or continuous

reactors.


Schematic of a typical batch polycarbonate polymerization process is given below.

Batch polycarbonate polymerization process.


Gaseous or liquid phosgene is pumped into a well – stirred reactor containing BPA in an organic aqueous dispersion. Aqueous sodium hydroxide is added during reaction to maintain an alkaline PH. A molecular weight regulator, most often para-tert –butyl phenol or phenol is added to the reactor to inactivate some of the end groups in the oligomer carbonates. At the end of polymerization, the polymer containing organic phase is separated and washed to remove the catalyst and impurities such as sodium chloride. The washed solution is then concentrated by flashing and is passed through a devolatilizing extruder to produce a molten ribbon, which is then cooled and chopped into pellets. In the continuous process, multiple tubular reactors in series or a cascade of stirred tank reactor is used. In this process, oligomeric chloroformates are produced by the phosgenation of BPA prior to polycondensation in the presence of a catalyst.

Batch polycarbonate polymerization process.


Polymerization condition for batch / continuous processes


Transesterification Processes

Conventionally, the polymer is produced by, transterifying diphenyl carbonate with BPA in the presence of a catalyst such as a sodium salt of BPA. Monophenyl carbonate of BPA is formed first. It reacts further to produce oligomers having phenoxy and hydroxy end groups. The oligomers are then poly condensed to give poly carbonate. During the polymerization, the reaction temperature is kept above the melting point of the reaction mass, initially about 150°C. It is increased in steps to a final temperature of 30°C. Meanwhile, the pressure is reduced also in steps from an initial 760 torr to less than 1 torr at the final stage of the polymerization. During the high temperature low pressure polymerization, the viscosity of polycarbonates increases dramaticallyand specially designed extruder type reactors are required to finish the poly condensation.


Schematic process flow diagram of a continuous melt polycarbonate

polymerization process.

Continuous melt polymerization process of polycarbonate.


Other methods of melt polymerization of poly carbonate

An alternative process is to first prepare an oligomer, with an average molecular weight less than 3000, from BPA and phosgene by an interfacial process.

The oligomer is then transesterified with additional BPA to form the desired polycarbonate.

Another process is to prepare a prepolymer from BPA and phosgene, also by an interfacial polymerization.

The prepolymer with a molecular weight greater than 10,000 is polymerized by a melt polymerization to the finished polymer.


Relations of Structure and Properties of PC

A study of the molecular structure of bis-phenol A polycarbonates enables one to make fairly accurate prediction of the bulk properties of the polymer. The relevant factors to be considered are:

(a) The molecule has a symmetrical structure and therefore questions of stereo specificity do not arise.(b) The carbonate groups are polar but separated by aromatic hydro carbon groups.(c) The presence of benzene rings in the chain restricts flexibility of the molecule.(d) The repeating unit of the molecule is quite long.

Because of its regularity it would be expected that the polymer would be capable of crystallization. In practice, however, the x-ray pattern characteristics of crystalline polymer is absent in conventionally fabricated samples.


Characteristics of PC (For identification)

The characteristics of PC are,

The PC is amorphous material It is having good impact resistance and it is sensitive to stress cracking Its glass transition temperature is 145°C Short term and long term service temperatures are 135°C and 100°C respectively It is identified by transparency, orange yellow flame, soot forming, self extinguishing, melts but chars It smells phenolic during burning (ink smell)


Characteristics of PC

Good stain resistance Easy colourability Good bio-compatibility Chemically resistant to organic and inorganic liquids Non-toxic Good insulator Dielectric constant is independent of temperature High corrosion resistant High creep resistance over board temperature rangeExcellent toughnessDimensionally stableVery good impact resistance at small thicknessGood property retention upto-20°C

High clarity Low haze Transmission rate is between 86 to 89%High heat deflection temperatureFlame resistance Self extinguishing in natureLimited scratch resistanceNotch sensitive Hygroscopic in natureTurns yellow when exposed to UV light for a long period of timeCrazing occurs when exposed to water at elevated temperature


Salient features of polycarbonate

Rigidity upto 140°C

Toughness upto 140°C

Transparency

Very good electrical insulation characteristics

Virtually self-extinguishing

Physiological inertness.


Properties of Polycarbonate


Mechanical Properties

The material has

high strength,

high stiffness,

high hardness and

high toughness

over the range from –150 to + 135°C


Thermal Properties

PC is distinguished by its high deflection temperature.

135°C for unreinforced and

145°C for reinforced plastics.


Electrical Properties

The insulating properties of PC are almost independent of temperature and humidity.

It is indicated that the electrical property of PC are not particularly good when compared with PE(Polyethylene), its overall property profile (Deflection temperature, transparency, toughness, flame retardance) is that of a high performance material.

It has therefore found wide spread use in electrical engineerings.


Water Absorption

The water absorption of PC based on bisphenol A at 23°C and 65% RH is about 0.2% on immersion 0.38%.

The physical properties are not affected.

A water content above 0.01 % impair processability because of blistering.

The physical properties of PC are impaired by saponification.


Optical Properties

The refractive indices of PC lies in the range of 1.56 to 1.65, which is high for transparent plastics.

The refractive index of Bisphenol A based PC falls from 1.53 at - 20°C to 1.56 at +200°C.

In the region of the glass transition temperature of about 145 °C a sharp bend occurs.

Transparency at 85 to 90% is reached in the region of visible light.


Permeability to Vapor and Gases

The Permeability values at 200 C/ 68 0 F for 1.5 mil film are given below.

* 1µm = 0.0394 mil


Chemical properties

PC is resistance to alcohol (except methanol), fats, oils milk, glycol, fruit juices, dilute acid and alkaline solution.

It is not resistant to Benzene ,Toluene, Xylene, chlorinated hydrocarbon ,methanol , numerous solvents, strong acid & base constant exposure to hot water.

After forming and cooling in the injection mold, internal stresses in PC molding may be so great that exposure to solvent and swelling media can lead to crazes.

Such media can also be used to detect internal stresses. Stresses which can cause crazes even after several years can be detected with the TNP test.


Chemical properties

The completely cooled molding is immersed in a moisture of 1 part by volume of toluene and 10 parts by volume of n-propanol (density =0.809 g cm –³ /0.0291 lb/in.3 at 22 C for 30 to 40 minutes.

Internal stresses can be removed by conditioning the injection molding in oil at 120 C .

Machined semi-finished products are conditioned in are at 120 C for 30 to 40 minutes.


Weathering Resistance

Weather resistant and light stabilized grades are suitable for many years of outdoor use.

The highest UV protection is provided by surface treatment.


Flammability

Polycarbonate burns with sooty luminous flame; after removing the ignition source it extinguishes.

The combustion gases smell of phenol. The after flame time and burnt length are short.

The results of flammability tests however frequently depend on wall thickness and additives.

Special flame retardant grads meet the requirements of the electromechanical and automotive industries.

Halogen-containing and halogen-free flame retardant grades have been developed.


Toxicity

PC produced from bisphenol A is free of taste and smell.

Plasticizers are not used in manufacture.

Suitably marked grades of PC meet the various regulations such as those in the Federal Republic of Germany and can be used in contact with foodstuffs.

Numerous grades satisfy the specification of the American Food and Drug Administration and the French positive list.


Additives of PC

Functional Additives

Fillers

Reinforcements


Functional Additives

UV absorbers are the only effective UV stabilizers

Colorants

Blowing Agents


Fillers

Graphite, MoS2 and PTFE are successfully used in PC to minimize abrasion and wear moldings.

Aluminum powder is used to increase the thermal and electrical conductivity.This provides protection against electromagnetic interference (EMI) in, for examples, data processing installations.


Reinforcements

The preferred reinforcement for PC as for many other plastics, glass fiber.

PC occupies third place amongst GF reinforced thermoplastics PA and PP.

The glass content varies between 10 and 40%. Chopped strand alkali-free E-glass surface-treated with silanes to promote adhesion is the main reinforcement.

A PC reinforced with 30% w/w glass fiber can compete in mechanical terms with non-ferrous metals or thermosets.

Wollastonite is used very occasionally to improve the stiffness of moldings.


Grades of PC

PC is available in the following grades.

Injection grades Extrusion grades Blow grades Thermoforming grades

In addition to that they are available in the following special grades, Transparent grades Tinted grades Unmodified grades Flame retardant grades


Processing considerations of PC

Drying in hopper dryer or in trays in an over for four hours at 120°C - 140°C will reduce the moisture level in the PC below 0.02%.

The apparent melt viscosity is also less dependent on the rate of shear than usual with thermoplastics. Because of high melt viscosities flow path ratio are in the range of 30:1 to 70:1 which is substantially less than for many more general purpose thermoplastics.

Processing temperatures are high at which thermal degradation occurs quite rapidly. Normally it is in the range of 280°C - 320°C

Polycarbonate adhere strongly to metal and if allowed to cool in an injection cylinder or extrusion barrel may on shrinkage, pull pieces of metal from the wall. It is therefore necessary to purge all equipment free of the resin with a polymer such as PE, after processing.


Processing techniques

Injection Moulding Extrusion Blow molding Thermoforming

Casting process is also used for making films. Like metals, polycarbonate can be cold formed by punching and cold rolling.


Surface Finishing of PC

PC can be easily polished to a high gloss. Only alkali-free polishing pastes should be used so as not to damage the surface.

Suitable products are available for printing ,coating, hot embossing etc.

PC moldings can also be vacuum metallized.


Machineability of PCMachineability of PC

Machining can be possible in PC with much precaution because it has stress-cracking tendency.


Weldings

Moldings and semi-finished products can be joined by vibration, friction, heated tool and hot gas welding in recent years, welding and riveting with ultrasonics have been preferred.

Heated tool welding is preferable to heated gas welding.

The tool temperature is 400 C while air temperatures of 450 to 500 C are required for hot gas welding.


Bonding

Suitable adhesives are solvents such as methylene chloride which dissolve the surfaces of the parts to be joined.

Two pack adhesives based on epoxide and silicone resins and polyurethane adhesives are suitable for joining PC to PC and other materials.

The adhesives must be free of component which are incompatible with PC.


Applications of PC

Appliances

Automotive

Electrical & Electronics

Food contact articles

Medical

Optical

Miscellaneous


Applications of PC in appliances

Coffee filters, shaver housings, chocolate moulds, blenders, tablewares, kitchen mixer bowls, grinder bowls, housing for ball point and fountain pens, rim heater grills, motor bracket and housing, camera, binocular casings, housings for hair dryers and coffee makers, water tank for steam iron, films for labels and memory switches, fruits juicer parts, high impact vacuum sweeper housing, mixers and power tools, bobbins for textile industries, baby feeding bottles and cutlery

Hair dryer housing Business machine covers


Applications of PC in Automotive

Wind screen wiper brackets, car interior moulded trims, instruments glazing, indicator lamps, wind shields for two wheelers, door handles, tail and side marker lights, PC blends in instruments, panels as well as bumpers, wheel cover and body panels protective hoods, fan wheels, components for sewing machines, chaises, levers, valves, control cams, directional signs, heating grill, ventilators and radiator grills, overrides, fuse box & covers and housing for automobile & aerial motors.

Cover for Tail lamps


Applications of PC

Instrument panels Car Tail lamp


Applications of PC

Automotive Head Lamp


Applications of PC in Electrical & Electronics

Wiring devices, insulators panels, plugs and socket terminal blocks, coil formers, Slater enclosures, housing and cover for distributor boxes. Panel light covers, battery boxes fuses, electric meter covers, connectors, breaker boxes, gears, printers housings, slide window,, telephone housing for mining operations, telephone dails, coil formers and housings, winding supports, switch plates, fuse boxes, housing for computers, calculation machine and magnetic disk packs.

Power distributor box Automobile Lamp covers


Applications of PC in Food Control articles

Mineral water bottles, microwave oven wares, beermugs table wares and food storage containers.

Water Bottle


Applications of PC in Medical

Blood bottles, dispensers for inhalers, sterilisable lab-wares, tissue culture dishes, posts for IV fluids, sterilizable container and packaging materials, surgical lighting, disposable, diagnostic cardio-vascular and intravenous devices, drug delivery systems and housings for blood cleaning filters.

Medical Equipment's


Applications of PC in Optical

Diffusers, lenses for lighting, vacuum metallised reflectors, housing for steel lamps and traffic signals, lamp holders, bulk heads, light fixture, lenses and safety glasses, window panels ( for out door lighting) sunglasses, ski goggles and face protective wires, audio compact discs, film and slide cassettes

Compact Disc


Miscellaneous Applications of PC

Sporting goods, stencils, ink ducts, slide rules components and rulers

Helmets


Blends of PC

PC-ABS Alloys of a Bisphenol A poly carbonates with ABS and MBS resins have been known for many years. These alloys retain their impact strength down to -50 C .They are non-splintering .The hardness of the alloys is comparable to that of PC. Because of the above properties, ability to mould to close dimensional tolerances , low warpage , low shrinkage, surface finish, PC-ABS alloys are widely used in automotive industry for electrical application and for housings of domestics and business equipments.PC-PBTThis is particularly notable for its high levels of toughens and resistance to petrols and oils. PC-PETThis is recently introduced for incorporating advantages of both resin

in the alloys .


List of Manufacturers/Suppliers of PC


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