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 - 190 & + 260 °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 280°C), 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 204°C, 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 90°C. The absorbed water is rapidly given off again by warming the molding to temperatures between 120 and 175°C. 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 360°C.

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:345°C 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 260°C 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 260°C. 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 they’re 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 -50°C to 150°C. 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 (23°C, 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 4.2-3.9 4.4-6.5 Dissipation factor, tan d 50 Hz 0.026-0.03 0.022-0.023 Dielectric strength kV/mm 23 33 Fire performance to UL 94 class V-0 CORPORATE TRAINING AND PLANNING