Super Strong Thermaoplastic Takes On Metals

Dec. 13, 2001
Designers of high-performance plastic parts now have a new option: Europe's best kept secret - PA-MXD6.

Shawn Wedgeworth
Solvay Advanced Polymers LLC
Alpharetta, Ga.

Edited by Jean M. Hoffman

A semiaromatic polymer (PA-MXD6) called IXEF1032 replaces metal in an electric shaver from Braun GmbH, Kronberg, Germany. Braun chose the 60% glass-filled polymer because of its low-waterabsorption properties. The shaver heads are said to remain dimensionally stable even under highly humid conditions. The resin has good strength and rigidity. At glass loadings as high as 60%, the resin's as-molded, Class-A surface is easily metallized or painted.

Beneath the resin rich surface of a molded PA-MXD6 part is the skin layer where the fibers orient in the direction of the melt flow. The surface layer is strong due to the fiber orientation and resists sink marks even with localized thickness variations.

The inherent strength of the PA-MXD6 resin coupled with the high level of glass loadings positions it as one of the strongest and most rigid materials on the market.

A 60% glass-filled (GF) PA-MXD6 part can be as strong as a zinc alloy (Zamak 3) part but weigh 50% less.

The PA-MXD6 resin also has good creep resistance even at high stresses and elevated temperatures thanks to the location of the aromatic ring, the level of glass fillers, and the bond between those fillers and resin. A 50% glassfilled PA-MXD6 tested against a 30% glass-filled PA 6 exhibits less elongation over applied load even at 80°C

A Wohler curve shows residual strength of a material as a function of the number of cycles at a given frequency. Here a 50% glass-filled PA-MXD6 is compared to a standard zinc alloy (4% Al, 0.5% Mg) and an aluminum alloy (AG6). After 10 million cycles, the PA-MXD6 exhibits a higher residual strength when tested to an ISO 178 three-point bend test standard at 23°C and a frequency of 25 Hz.

European designers have long used a semicrystalline polyarylamide resin made from the polycondensation of meta-xylenediamine (MXDA) with adipic acid. But the polyarylamide or PAMXD6 resin may not be as well known to U.S. designers.

Automotive applications for this high-performance resin include fuel pumps, cam covers, rear-view mirror housings, door handles (painted and unpainted), and clutch parts. Electronic applications for PA-MXD6 are in connectors, chassis and housings for electrical and electronic equipment, safety switches, disk supports in CD players, and induction motor supports. In telecommunications and business machines, PAMXD6 is typically used for structural parts.

PA-MXD6 is a semiaromatic polymer. Unlike other aromatic polyamides (PA) or nylons, its aromatic, or benzene ring sits on the amide side of its chemical structure. The aromatic ring's location boosts strength, rigidity, creep resistance, and dimensional stability when compared to other aromatic PAs. Adding glass and mineral fillers at loadings up to 60% betters these properties even further.

The polymer's semicrystalline nature is responsible for good chemical and creep resistance. It also reduces the polymer's affinity to absorb water when compared to other PAs. At 50% relative humidity and 23°C, for example, a 50% glass-filled PA-MXD6 absorbs only 1% water whereas a 30% glass-filled GF nylon 6,6 absorbs around 2.2%.

Dimensional stability
Water absorption along with mold shrinkage and molding tolerances influence a part's dimensional stability. Water bonds to the amide groups in all PAs swelling the polymer chain structure.

This plasticizing action effectively lubricates the chain segments letting them move more freely. The polymer softens and becomes more pliable with increased water absorption. With PA-MXD6 there is a similar effect but to a much smaller degree. Additionally, the kinetics of the reaction is significantly lower. Therefore, while designers often shy away from nylons due to their hydroscopic nature, PAMXD6 is often used in high humidity applications.

The unique location of the aromatic ring in the polymer's molecular structure results in a higher glass-transition temperature, Tg, compared to conventional PAs.

The coefficient of linear thermal expansion of the polymer is similar to that of metals and metal alloys at ambient temperatures. Likewise, its tensile strength at ambient is equivalent or even superior to metals such as tin, bronze, brass, annealed zinc, and aluminum (99.6).

The strength and rigidity qualities naturally make PA-MXD6 a metal-alloy replacement in numerous applications. PA-MXD6 is one of a few engineered polymers that is actually as strong or stronger than common metal alloys such as Zamak 3, aluminum AG6, and Mg AZ91D.

Flowability and surface finish
The PA-MXD6 also has a 50°C spread between its melt and processing temperatures — 235 versus 285°C. This window between the melt and processing temperature coupled with low viscosity gives the resin high fluidity. This makes it easier to injection mold or add glass reinforcement and fillers. This includes parts with wall thicknesses of 0.5 mm (0.208 in.) and glass loadings as high as 60%. PAMXD6 often replaces liquid-crystal polymers (LCP) in applications where flowability is the number one concern.

The 50°C difference between melt and processing temperatures also helps produce a resin rich surface finish even with high filler levels. Fast injection cycles combined with heated tools create shearing forces at the interface between the tool and resin. This shearing force buries the glass or mineral fibers beneath the surface of the part leaving up to a 3 m-thick resin rich layer and a Class A finish.

The lack of glass fibers on the surface also improves plating or additional painting processes. Standard PA painting techniques can be applied to PA-MXD6 parts. Some plating techniques such as vacuum metallization and cathode sputtering work well, but electroless and electrolytic plating are more challenging due to the etching of the relatively inert surface.

However, with effective etching, plating for EMI/RFI and aesthetic plating are possible. The fast injection cycles coupled with low shrinkage and the inherent strength of the resin matrix helps eliminate sink marks on part surfaces as well.





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