Light as a feather — stiff as a board

Glass-mat thermoplastics (GMTs) are becoming the material of choice for automotive engineers designing high-impact, structural parts. Compared to metals, GMTs have higher strength-to-weight ratios and impact resistance, offer greater design flexibility, and better resistance to chemicals and corrosion. They are also electrically insulating and have relatively high stiffness and good dimensional stability.

GMTs are challenging traditional sheet-molding compounds (SMCs) as well. GMTs have shorter molding cycle times and an infinite shelf life — thermoset (TS) SMCs need controlled storage and have a limited life. GMTs are also lighter and more ductile than their SMC counterparts and better resist impact at both high and low temperatures. Composite maker Azdel Inc., Shelby, N.C., recently added two new GMT systems for automotive use.

One is a low-density, long-fiber blend that is 50 to 85% lighter than conventional GMTs. Called Superlight, it is currently geared toward nonstructural interior parts such as headliners, load floors, sunshades, and trunk liners. Superlight is also a more environ-mentally friendly, recyclable alternative to conventional PVC under-body corrosion barriers. The other GMT from Azdel is a first-generation, mineral-filled reinforced composite that can replace SMCs in nonappearance structural applications.

Traditional GMT composites are generally flow molded in a compression press and weigh 4,000 to 4,900 gm/m ² for thicknesses on the order of 2.5 mm. According to Enamul Haque, global technology manager at Azdel, the new lowdensity (LD) GMT offers the same advantages as conventional GMTs but weighs in at only 600 to 2,000 gm/m ² for the same composite thickness.

LD-GMT is made in a process similar to that for making paper. Raw materials (chopped fiber-glass, polypropylene (PP), and additives) disperse as an aqueous foam in a mixing tank. From the tank, the mixture pumps onto a forming belt. A uniform fibrous web results as the slurry passes through a vacuum. The wet web goes through a dryer to remove moisture and melt the PP. From the dryer, the web is laminated to special films and spunbond plies. A scrim is attached to make the glass-reinforced material easier to handle. The LD-GMT is then continuously cut to size.

Heading a different direction
LD-GMT can be direct molded giving numerous advantages over competing headliner materials such as polyurethane (PU) laminates, corrugated cardboard, and fiberglass. Here, a thermoforming process melds individual sheets of LD-GMT with a film of hot melt. Sheets are robotically placed in a carrier and heated in an oven to 190 to 205°C. The liner fabric shuttles on to the cooled mold, then hot LD-GMT is sandwiched between the fabric and mold cavity. The mold closes and the LD-GMT bonds to the fabric. After removal from the cooled die, the liner goes to a laser cutter that trims and cuts access holes for sunvisors and dome lights. Lasers are able to make smoother cuts on LDGMTs than on traditional PU materials, Azdel says.

LD-GMT substrate is reported to have better acoustical properties than PU and corrugated cardboard headliners as well, especially above 2,500 Hz (ASTM E1050 test procedures). Frequencies higher than 2,500 Hz are usually tougher to control.

"Filler up" for added stiffness
The other GMT developed by Azdel uses mineral fillers and long chopped fiberglass in a TP matrix of polypropylene (PP). This technology differs from traditional SMC that has a TS resin matrix. SMCs use fiber-glass and mineral-filler additives but the TS resin remains uncured until it's heated in a hot mold. The heat initiates polymer cross-linking that cures or hardens the resin. For this reason, SMCs don't recycle as readily as mineral-filled GMTs.

Azdel has developed three mineral-filled GMTs from different standard glass mats borrowed from traditional GMT composites. The first uses continuous-strand, randomly oriented glass mats to give the composite a good balance of stiffness and strength in all three axes. The second version uses unidirectional long-glass-fiber mats to boost stiffness and strength in a single axial direction. The last uses chopped fiberglass mats to improve flow and energy management with only a small stiffness penalty. All three mineral-filled GMT types have improved impact resistance as well.

Additionally, because the thermal-expansion coefficient (CTE) of the mineral fillers is lower than that of the PP matrix the parts see less mold shrinkage and have fewer surface defects than standard GMT composites. The mineral fillers also have higher thermal conductivity that both improves melt quality and decreases differential shrinkage and warpage. Higher thermal conductivity also helps cut cycle times.

A variety of fillers are available to improve mechanical properties or lower composite cost. They include talc, calcium carbonate, mica, wollastonite, glass beads, and barium sulfate.

To form mineral-filled GMT sheets, the mineral-filled PP and glass mats are combined under temperature and pressure through a double belt system. Pressure from double rollers improves material flow and helps eliminate air pockets and voids. The laminate is cooled under pressure.

Mineral-filled GMTs offer several advantages over standard SMC, reports Azdel. They include a lower specific gravity, infinite shelf life, better sound dampening, and lower scrap rates. On the other hand, standard SMCs have good surface quality and dimensional stability as well as a lower CTE. They are also stiffer and have a higher heat deflection temperature.

Information for this article came from Enamul Haque, Global Technology Manager, Azdel Inc. Azdel is a joint venture company of GE Plastics, Pittsfield, Mass., and PPG Industries Inc., Pittsburgh.