- Commercial production of this polymer for subsequent conversion into fibres was commenced by the DuPont company in December 1939, in Arlingcton, New Jersey.
- The first commercial Polyamide 66 (Polyamide 66) was produced in a pilot plant in Seaford, Delaware in 1940 and the first molding powder for the fabrication of engineering plastics was made available to DuPont customers in 1941.
- But the polymer became well known to customers in 1950.
Monomer Ingredients for Polyamide 66
Adipic Acid: The adipic acid and hexamethylene diamine are used for preparation of salt (Hexamethyelene Diammonium adipate).
Hexamethylene Diamine
Hexamethylene diamine is conventionally prepared from adipic acid via adiponitrile.
Chemistry of Preparation of Polyamide 66
The Polyamide 66 is prepared from Polyamide salt, which is prepared by reacting the hexamethylene diamine and adipic acid in boiling methanol.
A 60% aqueous solution of the salt is then run into a stainless steel autoclave together with a trace of acetic acid to limit the molecular weight
Manufacturing of Polyamide 66
The polymerization of Polyamide 66 is carried out in several different reactors connected in series.
- The starting material is an aqueous solution of polyamide salt (AH salt) containing equivalent quantities of hexamethylene diamine and adipic acid
- The solution with about 60% solid content is fed into the first horizontal cylindrical reactor then divided into several components where the water is drawn off as vapor and precondensate of low mol. wt. is formed.
- This is pumped into the second reactor, which is a heated tube reactor with a gradually increasing diameter.
- The Polycondensation proceeds here and vapor forms at falling pressure.
- The next step is the removal of water in a steam separator followed by feeding the polymer melt by means of a screw conveyor into the last reactor, which consists of a heated screw conveyor where water vapor is again withdrawn and the final poly-condensation equilibrium is attained.
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 66
- 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 264°C
- Its short term and long term service temperature are respectively 140 - 170°C and 80 - 100°C.
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. are respectively 140 - 170°C and 80 - 100°C.
- 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.
No comments:
Post a Comment