SPECIALTY PLASTICS Polyamideimide (PAI)
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Transcript of SPECIALTY PLASTICS Polyamideimide (PAI)
SPECIALTY PLASTICS Polyamideimide (PAI) Structure Polyamideimide
Polyamideimides were introduced in 1964 and the range includes
molding compounds, electrical insulating and stoving lacquers,
films and fibers. The original manufacturer of molding compounds of
interest to designers was Amoco (US). Manufacture This product
group consists of polyimides formed by polycondensation of imide
chains with aromatic diamines. Torlon is manufactured by
phosgenation of trimellitic anhydride and reaction of the acid
chloride with 4,4'-diaminophenyl methane in N-methyl pyrrolidone at
room temperature. The resultant polyamide acid is cyclized to
polyamideimide. General Description Normal molecular weight PAI
cannot be melt processed by injection molding/extrusion because of
very high viscosity To enable these processes to be used, the
material is supplied with reduced molecular weight. The melt
viscosity of PAI as supplied is so low that thin walled injection
moldings are possible. Because the moldings are still thermoplastic
after injection molding, i.e. prior to post treatment, scrap such
as sprues, rejects, etc. can be recycled. General Description PAI
melts polymerize further above 246 C. Since these molding compounds
are processed at about 350 C, post-polymerization occurs. Although
process ability is not generally affected adversely, the already
starting polymerization limits the residence time of the melt in
the plasticizing cylinder and recycling of scrap. Even post
heat-treated PAI is still somewhat thermoplastic but the melt
viscosity is so high that it cannot be plasticized again. Structure
and General Properties-1
The imide content imparts high stiffness, hardness and flame
retardance while the amide groups effect flexibility and ductility
and melt process ability of this polyamide imide. Structure and
General Properties-2
Polyamideimides are characterized by the following properties:
-high strength between & C, -high impact strength, high
dimensional stability (amorphous thermoplastic), -high fatigue
limit (JBW = 56 N/mm2), Structure and General Properties-3
-high heat distortion temperature (265 to 280C), low coefficient of
linear expansion (6x10-6 to 20x10-6 K -1), -very good dielectric
properties, high chemical resistance (except to strong alkaline
solutions, oxidizing acids, nitrogen containing solvents,
superheated steam above 160 C). -resistant to stress cracking
media, Structure and Properties -4
- flame retardant (V-0), low emission of smoke and toxic gases, -
resistant to oxidation, - resistant to high energy radiation, -
high UV stability, - low outgassing losses in high vacuum, - can be
bonded, - can be metallized by conventional methods. Designers and
processors must note the following:
-the relatively high melt viscosity limits the size of injection
moldings, -high pressures and injection speeds are required for
injection molding, -pellets must be predried, -injection molds must
be preheated to between 200 and 260 C, -moldings must be post heat
treated. Availability The range includes the unmodified grades and
other injection molding and extrusion compounds with fillers and/or
reinforcements. PTFE and graphite filled grades are available. PAI
is supplied as pellets for injection molding. Mechanical
Properties-1
Short-term Behavior at Low Rate of Deformation: At room
temperature, PAI do not exhibit a yield point. Even at 204C, the
flexural strength of the PAI grades selected is superior to that of
well known high temperature resistant thermoplastics and even
polyimides. The high level of mechanical properties of PAI is
retained even after extended annealing at 250 C. Mechanical
Properties-2
Creep Behavior Under Uniaxial Stress: PAI is very creep resistant.
It reacts to high mechanical stresses more like metals than
plastics. Glass fiber reinforced PAI are suitable for high
mechanical and thermal stresses. Behavior at High Rate of
Deformation: The impact strength of PAI is superior to that of
other high-performance plastics. Behavior Under Vibration: PAI
exhibits a high fatigue limit which is maintained up to about 170
C. Mechanical Properties-3
Friction and Wear Characteristics: The frequent use of PAI,
especially the graphite and PTFE-filled grades for manufacturing
bearings is based on the low wear of this material even in dry
running. All PAI moldings must be subsequently heat treated. The
wear factor K is significantly affected by this post-treatment.
Maximum wear resistance is achieved after more than eight days at
260 C. Electrical Properties
PAl exhibits excellent electrical & dielectric properties. The
conductivity of PAI can be increased by the addition of graphite.
Material reinforced with 30% w/w carbon fiber is used to shield
components from electromagnetic interference (EMI). Water
Absorption In humid atmospheres or when immersed in water, PAI
absorbs small amounts of moisture. The maximum amount of water
absorbed (5% w/w) is reached after about three months immersion at
90C. The absorbed water is rapidly given off again by warming the
molding to temperatures between 120 and 175C. The dimensions of the
molding alter on absorption of water and the dimensional stability
at high temperatures falls. Weathering Resistance
PAI exhibits excellent UV and thus weathering resistance.
Resistance to High Energy Radiation PAI is very resistant to
radiation. Flammability PAI is distinguished by low smoke emission
in fires. The flame temperature is 570 C, The oxygen index varies
between 44 and 52% depending on grade. Processing -1 PAI has a high
melt viscosity and is reactive in the melt state. This prevents the
use of increasing temperature to decrease viscosity. PAI is best
fabricated with heavy duty, high rate injection-molding equipment.
The high rate is preferentially obtained by use of hydraulic
accumulators. PAI is shear sensitive and low compression screws are
recommended. Prior to injection, compression or transfer molding,
PAI must be dried for about 16 hours at 150 C or, in the case of
injection molding granules, for about 8 hours at 180 C.
Processing-2 At low shear rates, the viscosity of the PAI melt is
very high. At higher shear rates it approaches those of
polycarbonate and ABS. Thus complicated injection mold cavities can
be filled at high injection speed with relatively low injection
pressure. The viscosity is not particularly temperature dependent
in the processing range of 315 to 360C. Processing-3 Processing
conditions are: Injection molding: Melt temperature:336 to 360 C
Mold temperature:230 C Compression molding: Molding pressure:35 N
mm-2 Mold temperature:345C Preheating is required. Processing-4
Finally, post cure is an important step in processing PAI. PAI
moldings must be heat treated. The temperature is maintained at 245
C for 24 hours and then raised to 260C over 24 hours. Components
subject to wear should be kept at this temperature for 5 days to
increase wear resistance. Surface Finishing Of all the current
metallizing processes such as electroplating, plasma spraying, ion
plating and vacuum metallizing, only the last is unsuitable. The
moldings are first pickled, then washed, catalytically treated,
activated, chemically nickel plated, then electroplated and dried.
Electroplating is performed only prior to heat treatment Joining
Bonding PAI moldings can be bonded using adhesives based on
amide-imide. Such joins can be mechanically stressed and are
resistant to heat and chemicals. Suitable adhesives can be prepared
by, for example, dissolving PAI in n-methyl pyrrolidone (35%
solution). The surfaces to be joined must be free of grease and
clean and fit closely together. Roughening increases the bond
strength. After applying the adhesive and pressing the parts
together, they are kept at 175 to 190 C for 30 minutes.
Temperatures of 230 to 245 C are used for wall thicknesses greater
than 10 mm. epoxide resins and cyanoacrylate adhesives are also
suitable although their physical and chemical limitations should be
taken into account. Typical Applications-1
PAI glass reinforced resin grade is characterized by high strength
and high modulus. It has a very high strength to weight ratio
allowing it to replace metal in compressors and in aerospace
applications, including housings, structures, and equipment boxes.
PAI is used successfully for load bearing components exposed to
temperatures up to 260C. Stresses are mainly mechanical and/or
electrical. Typical Applications-2
Components subject to electrical/dielectric stresses include
connector insulating components of special grades of PAI for the
aerospace industry and coil formers and for seismographs. Moldings
subjected mainly to mechanical stresses include cam switches, vanes
for hydraulic & pneumatic motors, bearings and housings for
petrol consumption gauges for the automotive industry, cover frames
for office machinery, slide rings. Typical Applications-3
Parts for military aircraft, automotive transmissions and
off-highway equipment including hydraulic parts, seal rings,
washers and bushings can last longer when made from PAI because
these resins combine incredible wear resistance with other
long-life benefits like toughness, thermal performance and chemical
resistance. Plus theyre injection moldable, so fabrication can be
easier and less costly than machining metal parts. Seals Chip net
Bearing cage Electrical connector Can mandrel Typical
Applications-4
Precision components made from PAI are virtually indestructible,
making them a strong performer for demanding electronic handling
operations. Test sockets molded from PAI are used to protect
delicate devices during robotic handling and high-speed, high-force
compression into electrical test sockets. Typical
Applications-5
Testing units for printed IC boards must be sealed to maintain test
temperatures from -50C to 150C. Seal adapters machined from PAI can
provide better dimensional stability for a tighter seal fit and
offer longer part life than traditional materials. PAI resin is
also a major player in coating applications, due to its outstanding
surface adhesion to a multitude of materials including metals and
polytetrafluoroethylene. PAI dissolved in polar solvents is used to
make high temperature resistant wire lacquers and adhesives. Trade
Name Torlon (Solvay, US) Further Reading BRYDSON, J.A, Plastics
Material, Butterworth Heinemann, oxford, New Delhi (2005)
DOMININGHAUS.H, Plastics for Engineering, Hanser publishers,
Munich, New York (1998) CHARLESS A . HARPER, Modern Plastics Hand
Book McGraw Hill, New York(1999) CORPORATE TRAINING AND
PLANNING
Table -1 Guide values of properties of PAI Properties Units PAI
(Glass Filled) Density g/cm3 1.38 1.56 Water absorption (23C, 24h)
% 0.33 0.24 Mechanical Tensile strength N/mm2 190 205 Elongation at
break 15 7 Notched impact strength J/m 142 79 Service temperature
in air without mechanical loading short-term C 300 long-term 260
Heat deflection temperature 278 282 CORPORATE TRAINING AND PLANNING
CORPORATE TRAINING AND PLANNING
Table -1 Guide values of properties of PAI Properties Units PAI
(Glass Filled) Coefficient of linear expansion between 0 & 50 C
mm/mm/C 31 16 Thermal conductivity W/mK 0.26 0.37 Electrical Volume
resistivity Wcm 2 x1015 Surface resistance W 5 x1018 1x1018
Dielectric constant Dissipation factor, tan d 50 Hz Dielectric
strength kV/mm 23 33 Fire performance to UL 94 class V-0 CORPORATE
TRAINING AND PLANNING