- They are capable of being formed or shaped to precise and stable dimensions.
- The polymers like Polyamide 6, Polyamide 66, Polyethylene Terephthalate (PET), Polybutylene Terephthalate (PBT), Polycarbonate (PC), Polyphenylene Oxide (PPO), Polyacetal (POM) etc. represent this group of plastics.
Categorization of Engineering Plastics Based on Chemistry of Origin:
- Amide based engineering plastics
For example Polyamide 6, Polyamide 66 belong to this category.
- Ester based engineering plastics
PET, PBT and PC belong to this class of materials.
- Acetal based engineering plastics
Polyoxymethylene (Polyacetal) material is classified to this category.
- Ether based engineering plastics
Poly Phenylene Oxide (PPO) fall in this category. Since its chemical name is Poly (1,4 (2,6 di-methyle phenyl) ether).
Crystalline and Amorphous nature of engineering thermoplastics:
The engineering thermoplastics can be classified into two categories, based on their crystallization characteristics.
Crystalline Amorphous
- Polyamide 6 PC (Polycarbonate)
- Polyamide 66
- PET
- PBT
- POM
- PPO
Polyamide: Of the numerous methods of manufacturing polyamide, three are of commercial importance.
- Polycondensation of dicarboxylic acids and diamines via. an AH salt in aqueous medium, eg. the salt of hexamethylene diamine with adipic acid undergoes condensation to PA 66.
- Polycondensation or ω-amino acids.
- Ring opening and polymerisation (or polyaddition) of cyclic amide (eg. caprolactam) in the presence of water or in the absence of water by anionic and cationic high-speed polymerisation.
- The characteristics of polyamides are determined by the –CO–NH group.
- There are two groups of polyamides differentiated according to their structure.
- PA 6, PA 11 and PA 12 have molecular chains consisting of one basic units and two basic units lead for example to PA 6 6, PA 6 9 and PA 6 10.
Monomer Ingredients for polyamide 6: Cyclohexanone is normally prepared either from phenol or from cyclohexanone.
- Polyamide 6 is prepared from caprolactam in the presence of water (which acts as a catalysts) and acetic acid as a molecular weight regulator.
- The typical combination is charged into the vessel and reacted under a nitrogen blanket at 250°C for about 12 hours.
Manufacturing of Polyamide 6: The schematic diagrams of the continuous polymerization of caprolactam to produce Polyamide 6.
Flow diagram of continuous production of Polyamide 6.
- The so-called VK tube is used in the polyamide process.
- Reactive end groups are formed by hydrolysing the caprolactam to - amino caproic acid.
- A lactam melt with a relatively high water content (~15%) is fed to the top of the VK tube equipped with a stirrer and heating coil.
- The water vaporises at the top, when viscosity is still low, to give a residue of the desired composition.
- In the lower part of the tube, the equilibrium degree or polymerisation is reached with an increasing viscosity of the melt.
- The polymer is drawn off at the bottom and granulated. Its equilibrium content of caprolactam and oligomers is about 10% at a final temperature of 270°C.
- The monomer and oligomers are extracted from the chips with hot water, and the polymer is subsequently dried with hot gas in a ventricle cylinder hot dryer.
- Intensive drying can produce a further reaction in the solid state and according to the polycondensation equilibrium a higher degree of polymerization reached.
Relations of Structure and properties of Polyamide 6:
- The following structural variables affect the properties.
- The distance between the repeating –CONH- group
- The number of methylene groups in the intermediates
- The molecular weight
- N- substitution
- Co-polymerization
Characteristics of Polyamide 6 (For identification)
- The material is semicrystalline and having high water absorption capacity.
- It is milky white - yellow colour
- It is identified by the smell of burnt horn when burned, yellow flame with a blue halo, can be formed into a filament.
- Its melting point is 215°C
- Its short term and long term service temperature are respectively 140 - 160°C and 80 - 100°C.
Characteristics of Polyamide 6:
The general characteristics of Polyamide 6 is summarized below
- Hard and though thermoplastic
- Good abrasion resistance
- How co-efficient of friction
- High tensile strength
- Good dimensional stability
- Low tendency to work
- Smooth appearance of surface
- Average to high surface gloss
- Resistance to lubricants, engine fuels, grease etc.
- Good resistance to coolants, refrigerants, paints, solvent cements.
- Resistant to aqueous solution of many inorganic chemicals.
- Posses high HDT
- Attached by strong acids, phenols, cresol at devoted temperature
- Poor or resistance
- High temperature resistance
- Low co-efficient of linear thermal expansion
- High water absorption
Mechanical Properties
- The mechanical properties of moulded polyamide material depend on molecular weight, crystallinity and moisture content.
- The stress/ strain performance of the material is low and high strain rate gives a completely different impression.
- The useful information for design of the product from polyamide 6 can be derived from creep modulus and isochronous stress / strain curves which characterise the behaviour of mouldings under constant mechanical stress over long periods.
- Polyamide 6 is widely used for bearings because of their good slip and dry running characteristics, high compressive strength and wear resistance and adequate shape retention at elevated temperatures.
- The polyamide articles are found in applications where dimensional stability is important.
Thermal Properties
- The thermal expansion is highly dependent on temperature
- The service temperature in air without mechanical loading for short- term 140 - 160°C and for long – term is 80 - 100°C
- The UL index value is 75°C (The temperature at which property values decreases to less than 50 percent of initial values)
- The change of specific volume of glass reinforced plastics as a function of temperature and pressure (PVT diagram) is significant.
Electrical Properties
- The utility of a plastic in electrical engineering is determined by its volume resistance, surface resistance, dielectric strength and tracking resistance as function of temperature and especially in the case of polyamide with moisture content
- The electrical properties can also change on heat ageing
- In contrast to other thermoplastics PA 6 is distinguished by high tracking resistance and dielectric strength under normal condition with continuous absorption of moisture
- The surface resistance is such that no dust deposits are formed as a result or electrostatic charge
- The dielectric properties are highly dependent on temperature and moisture
Water absorption
- Compared to other plastics, polyamide 6 absorb relatively large amount of water
- The resultant increase in volume affects the dimensions of the mouldings
- Conditioning ie., accelerated and defined absorption of water is thus an important final processing step which ensures that the molding has almost constant properties and dimensions in use
- Conditioning also increases toughness although hardness and stiffness fall
- Moldings are thus most effectively conditioned in hot water, a warm, humid atmosphere or in saturated steam.
Optical Properties
- Mouldings of crystalline polyamides are translucent to opaque depending on thickness, processing conditions and molding compound.
Permeability to water vapour and gases
- The low permeability of polyamide to gases and vapours is important for their use as packaging materials and it is tabulated in the Table- 3.
- The permeability to watervapour decreases in the order PA 6, PA 66, PA 610, PA 612, PA 11 and PA 12 while permeability to gases increases slightly
- Permeability increases considerably with increasing moisture content and temperature
- It is also affected by the processing method, degree of orientation and crystallinity.
Chemical Properties
- It is inherently resistant to lubricants, engine fuels, hydraulic fluids, coolants, refrigerants, paints, solvent, cleaners and aliphatic and aromatic hydrocarbons.
- They are also resistant to aqueous solution of many inorganic chemicals.
- Polyamide 6 is gradually attacked over a time by hot water, acids, phenols and few chlorinated hydrocarbons.
- The Polyamide present no toxicological problems because they are insoluble in body fluids and are biologically inert.
Weathering Properties
- The weatherability of polyamide 6 is poor unless a suitable stabilizer is incorporated
- PA 6 is resistance to boiling water and can be sterilized
- Deterioration can be expected only after months of exposure to hot water particularly if it contains high concentration of oxygen or oxidising substances
- Fault free, low stress, pigment or unpigmented articles generally have a service life in warm, dry atmosphere in excess of five years and of more than three years in warm humid atmosphere without affecting functional properties
- Moldings with high carbon black content (approx. 2%) donot become brittle even after ten years at outdoor weathering in warm dry or humid atmosphere
Resistance to high energy radiation
- This plastics exhibit average resistance to high energy radiation
- The properties of unreinforced grades are affected to varying degrees by exposure to high energy radiation
- Some properties are affected by medium dosages, others are almost unaffected by high dosages
- An energy dosage or 2000 KJ/Kg (200 Mrad) causes, for example a drop in impact strength of only 15 % to 30%
- The electrical properties are almost unaffected in the range upto 10000 KJ/ Kg
- Glass reinforced PA grades are extremely resistant to radiation
Flammability Properties
- Unmodified Polyamides are rated as (selfextinguishing class) UL-V2 according to UL. With modification it can be identified selfextinguishing class UL–V 0.
- Polyamide start to decompose slowly at temperatures greater than 300°C. Between 450 and 500°C, combustible gases are formed which continue to burn after ignition.
- Polyamide burns with yellowish orange blue edged flame
- They melt, drip and continue to burn after the ignition source is removed, giving a smell of burnt horn
- The decomposition product formed upto 400°C are less toxic than those formed from wood under the same conditions at higher temperature they are equally toxic.
- The heat of combustion of unreinforced grade is approx. 29000 to 32000 KJ/Kg
Toxicity
- Polyamide is odourless
- Taste problem encountered with polycondensation of caprolactam is overcome in recent years
- Neverthless food stuffs and beverages in aqueous phase should not be subjected to elevated temperature in polyamide vessels
- Work place where it is processed should be well ventilated
- Moulding compounds should need legislative requirement and it can be confirmed from the supplier also.
- All polyamides are physiologically inert
- They are compatible with tissue and can be used for artificial limbs
- Colorants are subject to various restrictions especially for toys
Sterilization
- γ-Radiation sterilization of 25 KJ / Kg (2.5 Mrad) does not affect the mechanical properties of Polyamide.
- Natural or pigmented parts turn slightly yellowish.
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