POLYCARBONATE (PC):
The structure of polycarbonate is:
Polycarbonate is a linear polyester of carbonic acid in Which dihydric phenols are lined through carbonate groups. The commercial grades of polycarbonate is manufactured from bisphenol-A and phosgene.
1. Polycarbonates are polyesters of carbonic acid.
2. Although carbonic acid itself is not a stable compound, its derivatives (phosgene, urea, carbonates) are commonly available.
3. The reaction of gaseous phosgene with bisphenol A gives rise the formulation of commercial polycarbonate.
Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, tough materials, and some grades are optically transparent. They are easily worked, molded, and thermoformed.
History of development:
1. PC was first prepared by Einhorn in 1898 by reacting the hyroxy benzenes, hydroquinone and resorcinol separately with phosgene in pyridine solution.
2. In 1920 similar kind of product was prepared by Bischoff and Von Heden storm with the use of diphenyl carbonate.
3. In 1930 Carothers and Natta prepared a number of aliphatic polycarbonates using ester inter change reaction.
4. In 1556 Herman Schnell of Farben Fabriken Bayer and D.W. Fox of GE independently produced PC from Bisphenol A and Phosgene.
5. In 1958 production of Bisphenol, a polycarbonate was started simultaneously by GE Co. both in Germany and USA from Bisphenol A and phosgene.
6. Today about 75% of the market is held by General Electric and Bayer.
7. 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
1. The PC is made from Bisphenol A and Phosgene (COCl2).
2. Phosgene is prepared commercially from carbonmonoxide and chlorine.
3. Bisphenol A can be produced by the condensation of phenol with acetone under acidic conditions.
Chemistry of Preparation of PC:
1. 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:
1. Poly carbonates are commercially produced by either interfacial polymerization or melt polymerization.
2. The solution process, which was once a major commercial process is no longer used in industry because of its inferior economics.
- Interfacial Polymerization
- Transesterification Processes (melt polymerization)
Interfacial Polymerization:
1. 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.
2. The reaction takes place in two steps. First, phosgene reacts with BPA to form monochloroformates.
3. 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.
4. During the polymerization, the by-product (hydrochloric acid) reacts with aqueous caustic phase to form sodium chloride.
5. After the polymerization, the organic polymer solution is separated from the aqueous phase and purified.
6. The polymer is recovered from the purified polymer solution by precipitation or evaporation.
7. The solid polymer is crushed and extruded into pellets.
8. The interfacial polymerization can be conducted by batch or continuous reactors.
Batch polycarbonate polymerization process:
Schematic of a typical batch polycarbonate polymerization process is given below.
1. Gaseous or liquid phosgene is pumped into a well – stirred reactor containing BPA in an organic aqueous dispersion.
2. Aqueous sodium hydroxide is added during reaction to maintain an alkaline PH.
3. 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.
4. At the end of polymerization, the polymer containing organic phase is separated and washed to remove the catalyst and impurities such as sodium chloride.
5. 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.
6. In the continuous process, multiple tubular reactors in series or a cascade of stirred tank reactor is used.
7. In this process, oligomeric chloroformates are produced by the phosgenation of BPA prior to polycondensation in the presence of a catalyst.
Polymerization condition for batch / continuous processes:
Transesterification Processes:
1. 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.
2. It reacts further to produce oligomers having phenoxy and hydroxy end groups. The oligomers are then poly condensed to give poly carbonate.
3. During the polymerization, the reaction temperature is kept above the melting point of the reaction mass, initially about 150°C.
4. 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.
5. 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:
1. 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.
2. The oligomer is then transesterified with additional BPA to form the desired polycarbonate.
3. Another process is to prepare a prepolymer from BPA and phosgene, also by an interfacial polymerization.
4. 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:
1. 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.
2. Because of its regularity it would be expected that the polymer would be capable of crystallization.
3. 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,
1. The PC is amorphous material
2. It is having good impact resistance and it is sensitive to stress cracking
3. Its glass transition temperature is 145°C
4. Short term and long term service temperatures are 135°C and 100°C respectively.
5. It is identified by transparency, orange yellow flame, soot forming, self extinguishing, melts but chars
6. It smells phenolic during burning (ink smell)
Characteristics of PC:
Salient features of polycarbonate:
1. Rigidity upto 140°C
2. Toughness upto 140°C
3. Transparency
4. Very good electrical insulation characteristics
5. Virtually self-extinguishing
6. Physiological inertness.
Properties:
1. High impact strength even at sub zero temp.
2. Remains flexible down to -150 ℃
3. Good creep resistance in dry conditions up to 115℃
4. Non-flammable, self extinguishing
5. High heat deflection temperature
6. Good dimensional stability
7. Resistance to weak acids, hydrocarbons and oils
8. Good electrical properties, stable over wide range of temperature, frequency and humidity
9. Can be injection moulded, compression, transfer and blow moulded, extruded and easily machinable.
10. Excellent transparency
11. High strength and toughness up to 140℃
12. Impact and break resistant
13. Very good heat resistance
14. Weather Resistance
15. Good processing characteristics
Applications of PC:
1. Appliances
2. Automotive
3. Electrical & Electronics
4. Food contact articles
5. Medical
6. Optical
7. Miscellaneous
Mechanical Properties: The material has:
1. high strength
2. high stiffness
3. high hardness
4. 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.
Applications of Polycarbonate
Traffic And Automotive
1. Housing and colored lenses for traffic signals
2. Reflectors
3. Ventilation and radiator Grills
4. Bumpers Wind shields for two
5. Wheelers
6. Headlight
7. Lenses
8. Tail lamps
9. Fuse box covers
10. Wind screen
11. Wiper bracket
ELECTRICAL / ELECTRONICS ENGINEERING
- 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.
1. Housing for computers
2. Coil former and housing
3. Telephone housings for mining operations
4. Compact discs
5. Plug and socket
6. Battery covers
7. Terminal blocks
MEDICAL INDUSTRIES
1. Dispenser for inhalers
2. Tissue culture Dishes
3. Surgical lights
4. Blood bottles
5. Sterilizable lab wares
6. Housing for blood cleaning filters
Water Absorption:
1. The water absorption of PC based on bisphenol A at 23°C and 65% RH is about 0.2% on immersion 0.38%.
2. The physical properties are not affected.
3. A water content above 0.01 % impair processability because of blistering.
4. The physical properties of PC are impaired by saponification.
HOUSEHOLD AND DOMESTIC WARES
1. Baby feeding bottles
2. Coffee filter
3. Table wares
4. Mixers and blenders
5. Water containers
6. Vacuum cleaner
7. Housings
Optical Properties:
1. The refractive indices of PC lies in the range of 1.56 to 1.65, which is high for transparent plastics.
2. The refractive index of Bisphenol A based PC falls from 1.53 at - 20°C to 1.56 at +200°C.
3. In the region of the glass transition temperature of about 145 °C a sharp bend occurs.
4. 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)
MISCELLANEOUS
1. Safety helmets
2. Goggles
3. Safety glazing
4. Green house
POLYCARBONATE BLENDS
PC/ABS BLENDS
1. Moulding characteristics
2. High impact strength at low temperature
PC/ASA BLENDS
1. Rigidity and dimensional stability
2. Outstanding resistance to high temperature ageing
3. Good resistance to UV radiation
PC/SMA BLENDS
1. Superior toughness
2. Heat resistance
3. Outstanding processability
Chemical properties:
1. PC is resistance to alcohol (except methanol), fats, oils milk, glycol, fruit juices, dilute acid and alkaline solution.
2. It is not resistant to Benzene ,Toluene, Xylene, chlorinated hydrocarbon ,methanol , numerous solvents, strong acid & base constant exposure to hot water.
3. 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.
4. 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.
5. 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.
6. Internal stresses can be removed by conditioning the injection molding in oil at 120 C .
7. Machined semi-finished products are conditioned in are at 120 C for 30 to 40 minutes.
Weathering Resistance:
1. Weather resistant and light stabilized grades are suitable for many years of outdoor use.
2. The highest UV protection is provided by surface treatment.
Flammability:
1. Polycarbonate burns with sooty luminous flame; after removing the ignition source it extinguishes.
2. The combustion gases smell of phenol. The after flame time and burnt length are short.
3. The results of flammability tests however frequently depend on wall thickness and additives.
4. Special flame retardant grads meet the requirements of the electromechanical and automotive industries.
5. Halogen-containing and halogen-free flame retardant grades have been developed.
Toxicity:
1. PC produced from bisphenol A is free of taste and smell.
2. Plasticizers are not used in manufacture.
3. 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.
4. Numerous grades satisfy the specification of the American Food and Drug Administration and the French positive list.
No comments:
Post a Comment