Fabric Forms Get Sophisticated

Closed molding applications and demand for greater design flexibility are driving fabric suppliers to create a variety of products that mix and match fibers, fiber forms, core and resin.

Woven textile reinforcements gave composites parts manufacturers their first fabric forms with continuous fibers oriented along more than one axis. Although wovens are limited by loom design to interlaced fibers in the 0° and 90° (length or warp, and width or weft) directions, these constructions halved the labor required to lay up unidirectional tapes and reduced the risk of human error in terms of fiber orientation. Additionally, the interlacing of warp and weft fibers helped prevent part delamination.

The emergence of multiaxial fabrics in the mid-1980s spurred significant industry change. Unlike the chopped and continuous randomly oriented mat products that preceded them, this new type of nonwoven consisted, early on, of two, three or even four layers of continuous uni tapes stacked at differing angles of orientation and fixed together either by stitching thread or a polymer binder, or both. Almost all stitching systems use polyester thread, but the stitch itself can be varied considerably in tension, spacing and style. While a binder eliminates the cost of the stitching process from the fabric cost equation and completely eliminates crimp, stitch-bonding is the predominate method. Binders tend to stiffen the fabric, making it more difficult to lay up parts with complex contours. Stitched fabrics are more drapable. In addition, stitching can reduce multiaxial vulnerability to delamination, and stitch density can be varied in the number of stitch rows per inch (the gauge), and in the number of stitches per inch within each row (stitch length). However, stitching involves a trade-off: Denser stitching does a better job of maintaining fiber orientation, but it, too, can reduce drapability and retard resin flow.

Multiaxial nonwovens addressed several limitations inherent in the weaving process. First, the potential for more than two layers offered part fabricators greater labor savings. Second, unlike the mechanical over/under interlacing of the weaving process, stitching or adhesively bonding the uni layers significantly reduced or eliminated fiber crimp. While stitchbonding may cause a slight degree of “waviness” to warp fibers the much straighter fibers in multiaxial constructions were in-herently more resistant to stress and strain. As a result, two unidirectionals stacked in a 0°/90° orientation, for example, can be up to 50 percent stronger than 0°/90° woven fabric of the same weight. Further, uncrimped fibers provide a less torturous path for resin flow, which has made them attractive to those using resin infusion processes — and earned them the alternative designation of “noncrimp” fabrics. Since 2000, production of multiaxials has more than doubled as use of infusion processes has exploded.

A third advantage is that composite la-minates can be produced with greater fiber volume. In multiaxial constructions, fibers are nested more closely together than they are in wovens, leaving smaller spaces between fiber bundles where resin can pool. Taken together, these factors mitigate the higher up-front cost — 15 to 30 percent — of a multiaxial nonwoven vs. woven fabric of the same areal weight. When multiaxials replace wovens, a part with the same performance can be molded with less material, resin and labor.

From these beginnings, suppliers of textile reinforcements have gone on to refine, redesign and combine woven and nonwoven fabrics, pushed in part by composite part designers in search of new material combinations.

Designing With Fabric Forms

Trey Sawtelle, senior VP at fabric manufacturer Vectorply Corp. (Phenix City, Ala.), says the last few years have seen substantial change. Historically, multiaxial formats were restricted by limitations in manufacturing technology to layers oriented in one of four directions: length, width and plus/minus bias (0°, 90°, +45°, -45°). Standard configurations offered by multiaxial fabric suppliers were biaxial (0°, 90°), double bias (±45°), triaxial warp (0°, ±45°), triaxial weft (90°, ±45°) and quadraxial (0°, 90°, ±45°). “New equipment,” says Sawtelle, “allows variable angles of ±50°, ±65°, ±22.5° and more.”

Sawtelle points to increased regulation of styrene emissions and the consequent shift to closed molding as another driver toward multiaxials. “Infusion, RTM and light RTM has changed our customers’ ability to make better parts,” he says. Resin manufacturers have responded with low-viscosity polyesters, vinyl esters and epoxies that flow through a mold and wet out fibers much faster, without the aid of flow media. “Wet-out with some fabrics used to require flow aids, but with improvements in vacuum processing and with reduced resin viscosity, fabrication is more efficient,” says Will Warlick, sales manager at V2 Composites (Auburn, Ala.).

Work also has progressed in the development of hybrid multiaxials, which mix oriented layers of, for example, S-2 Glass, aramid or carbon fibers in one ply, or combine plies of different fiber types. “Usually,” says Warlick, “hybrids are used to get the benefit of each fiber and to help with the cost factor.” In many applications, use of a 100 percent carbon or aramid fabric may prove not only prohibitively expensive but also may exceed design requirements. In such cases, a hybrid fabric can be constructed, with a percentage of glass fiber content, adjusted to meet but not exceed the spec.

For the designer of composite parts, this can be both a dream and a nightmare: Sawtelle says some customers come to Vectorply well schooled and well equipped to design with fabrics and understand how they should be used, but others need help in understanding first how nonwoven fabrics differ from woven materials, and then what strengths they bring to an application. “Sometimes,” says Warlick, “it’s cheaper to use one layer of a multiaxial — despite the higher cost — than to use multiple layers of lighter fabric.”

Erin Reagan, fabric development manager at Fiber Glass Industries Inc. (Amsterdam, N.Y.), says the development of cost-effective solutions using fabric is among her biggest challenges. “Sometimes the use of a more expensive fiber in a less-expensive fabric construction allows a processor to save money in labor or setup, which reduces overall cost.”

Ultra-thick parts

Overall, says Sawtelle, “our customers are trying to make parts larger, larger and larger for cost savings, because of parts consolidation and fewer fasteners.” And “larger” doesn’t just mean length and width. Boatbuilder Richmond Yachts (Richmond, British Columbia, Canada) uses multiaxial fabric for the construction of the mid-ship stabilizer struts installed inside the bottom of the hull on the firm’s 120-ft to 155-ft yachts. The stabilizer strut is, effectively, a massive block of fiberglass that measures 2 ft by 3 ft by 13 inches thick (0.61m by 0.91m by 330 mm). A vertically oriented axle passes through a hole cored in its center and is connected to a rudder that helps stabilize the craft.

Michael Bolton, Western Region sales manager at Vectorply, says Richmond, until recently, took up to 40 days to build the struts, laying the glass fiber by hand and then infusing four to five layers at a time to build the 13-inch thickness. Bolton, working with distributor Composites One (Arlington Hts., Ill.), started looking at methods of infusing the strut in fewer shots. “The main concern was to control exotherm in the plies,” says Bolton. He says he didn’t think one 13-inch shot was possible, but “we could do 7 inches [178 mm] pretty easily.”

Richmond turned to Vectorply’s E-3LTi 10800 glass fiber reinforcement, part of its VectorFusion product line. The stitched fabric, Vectorply’s heaviest, has an areal weight of 108 oz/yd2 and comprises six layers of unidirectional glass in a 0°/90°/0°/90°/0°/90° stacking geometry that keeps channels open to promote air escape and resin flow. Roving bundles reportedly sustain loft, ensuring the laminate will achieve maximum thickness with minimal resin voids. The stitching allows the fabric to be cut without unraveling. (Areal weights for the VectorFusion line range from 18 oz/yd2 to 108 oz/yd2.)

Richmond actually constructs and infuses the stabilizer strut in place at the bottom of the hull, and with the help of Vectorply and Composites One first did so by building up a 7-inch-thick block, infusing it under a vacuum bag for one hour, using Reichhold’s (Research Triangle Park, N.C.) HYDREX vinyl ester resin. Maximum exotherm was 180°F/82°C. This block was allowed to ambiently cure for a day, then the top layer was roughed and the remaining 6 inches/152 mm was built up and infused on top of the existing 7-inch block. The result was a 13-inch block of glass comprised of 108 plies of fabric. With six layers of glass fiber in each ply, this means that Richmond now can lay up and infuse 648 plies of fiber in about four days — a tenth of the time the manual method took.

Composite-for-Wood Replacement

Beyond applications like the stabilizer strut, Sawtelle says Vectorply is looking to capital-intensive markets for opportunities for material conversion. This includes development of a concept involving a foam core wrapped with multiaxial fabrics to create a replacement for wood 2×4s used in construction framing.

Quadrant Plastic Composites (Lenzburg, Switzerland) is working with German construction supply firm Hünnebeck on a similar front to develop a replacement for plywood in concrete soffit formwork panels. Plywood has been the material of choice in formwork applications, but has two weaknesses — a tendency to swell and rot, and rising cost — that forced Hünnebeck to look for alternatives. To meet the challenge, Quadrant combined two of its products to create a drop-in composite replacement for plywood called MultiQ, a sandwich of SymaLITE, Quadrant’s long glass fiber-reinforced polypropylene (PP) sheet, between two layers of GMTex, its glass mat thermoplastic (GMT).

SymaLITE features from 30 to 55 percent glass fiber content and an areal weight of 300 g/m2 to 2,000 g/m2. It’s coated on one side with a nonwoven PET scrim, and an abrasion-resistant PP film of similar thickness, or an adhesive film, on the other side. Nicola Adamo, Quadrant’s nonautomotive business line leader, says its x/y orientation is isotropic, and the production process results in z orientation for a significant portion of the fibers. The latter loft and create minute air pockets when preheated in infrared ovens, yielding a panel of low weight but high stiffness. Designed to fit directly into the same metal frames that are used for formwork, the colorable panels are 11 mm/0.43 inch thick and come in sizes of 1800 mm by 1800 mm (70.9 inches by 70.9 inches) and 900 mm by 1800 mm (35.4 inches by 70.9 inches). Tests done by Hünnebeck show that the panels can be nailed like wood but do not absorb moisture, can be used up to 200 times and reportedly are recyclable. Adamo says Quadrant is looking at applications in truck/trailer walls and floors as well as shipping containers.

Speedy Reinforcement Kits

To speed up lamination on its 34-ft to 47-ft sailing yachts, Poncin Yachts (La Rochelle, France) sought help to eliminate two time-consuming steps: infusion/RTM following lay-up of each fabric/core layer; and the spraying between layers of a bonding agent to secure laminates to core. SAERTEX (Arandon, France) devised the solution, says Frederic Pinan, an R&D engineer in the SAERTEX Technical and Marketing Dept.: a multiproduct combination of fabrics, cores and SAERfix, an unsaturated polyester- and vinyl ester-compatible adhesive that’s integrated with the fabrics to provide interlayer bonding. The materials are used throughout Poncin’s boats, but primarily in the deck, hull, and stringers. SAERTEX now delivers these and other materials to Poncin in prepackaged kits, sorted in order of assembly, with nonwoven fabrics of E-glass, either chopped strand mat (CSM), a bidirectional fabric, or a multiaxial depending on the spec. All are precoated with 6g/m2 of SAERfix, which provides a built-in bond with the core. Pinan says pre-application of the adhesive avoids manual overspraying of traditional adhesives and prevents inhibition during resin cure. These materials enable Poncin to infuse the entire lay up in one shot, using light RTM for the deck and vacuum infusion for the hull.

Flemings Textiles Ltd. (Kimarnock, U.K.) also offers a multiproduct combination for infusion processes in its Polymat Hi-Flow fabric. This mechanically stitch-bonded reinforcement consists of a deformable engineered thermoplastic fiber core sandwiched between two layers of chopped strand glass fiber that can be used with RTM, RTM light, VARTM and vacuum infusion. Top and bottom layers are comprised of chopped glass rovings stitch-bonded to the core, while the core has a weight of 105 g/m2 to 220 g/m2. Typical laminate weight is 705 g/m2 to 2,220 g/m2. Core options vary depending on end-product physical and aesthetic requirements. MO3 (105 g/m2) is designed for use in 2-mm to 4-mm (0.08-inch to 0.16-inch) cavities for applications that require good finish quality. MO5 (155 g/m2) is for use in 3-mm to 5-mm (0.12-inch to 0.20-inch) cavities where processing flexibility and finish are important. MO1 (220 g/m2) is for 3-mm to 7-mm (0.12-inch to 0.28-inch) cavities, reportedly offers good compression control, can be used with highly filled (more than 50 percent) and viscous resins and is suitable for vacuum bag infusion in high-strength applications. M15 (100 g/m2) is for 2-mm to 4-mm cavities and is highly conformable for cosmetic finish applications.

A Multitude of Mixtures

Fiberex Glass Corp. (Leduc, Alberta, Canada) offers COMPASS multiaxial fabrics, made of two or more layers of unidirectional E-CR glass fibers, available in a variety of fiber orientations (0°, ±45°, 90°) and with or without a chopped mat backing.

Quadrant Plastics Composites, through its subsidiary Quadrant Natural Fiber Composites, specializes in the manufacture of nonwovens made of natural and synthetic fibers as well as glass and synthetic fibers used mostly in automotive applications. Nonwovens made from polypropylene (PP) and natural fibers and sandwich constructions are used to produce carrier components for auto interiors where low weight, stiffness, form stability and acoustic damping are critical. Fiber options include PP, PET, kenaf, hemp, flax, jute and combinations thereof in two- and three-layer sandwich constructions. Areal weights range from 200 g/m2 to 3,900 g/m2.

SAERTEX USA LLC (Huntersville, N.C.) offers four products in its fabric family: SAERuni provides unidirectional fiber in 0° or 90° orientation with a chopped strand mat (CSM) or fleece backing; SAERbid, bidirectional in 0° and 90° angles, with optional CSM or fleece stitched on the upper or lower side; SAERmax, a multiaxial fiber complex with various weights, and variable in the directions and arrangement of the individual layers (angles range from 22.5° to 90°); and SAERmat, chopped glass mat that is stitch-bonded and binder-free. Rounding out the offerings is SAERcore, which combines via stitch-bonding, needling or the use of a binder, a sandwich of SAERuni, SAERbid, SAERmax or SAERmat with any fibrous core material (e.g., PP, polyester or glass), which is used to create a resin flow zone). This product is said to be useful in boat superstructures and hulls, wall paneling and bath components (tubs, tub enclosures and doors).

Fiber Glass Industries offers 90° Unidirectional, a fabric that features single-end rovings in the 90° or weft direction and a lightweight polyester yarn in the 0° direction. With all reinforcement weight in the 90° direction, this material is designed for reinforcing tanks and pipes in the axial direction during hoop winding. Knitmat is a stitch-bonded fabric with 0° and 90° rovings, with or without chopped glass strand. It can be combined with polyester, other veils and CSM and can be used with hand lay up, SRIM, RRIM and SCRIMP. Flomat is a highly conformable closed mold reinforcing material for RTM and RTM light applications. This multilayer stitched material consists of a needled polyester core between two outside layers of binderless chopped fiberglass strand mat.

Introduced at IBEX 2006, 3TEX Inc.’s (Cary, N.C.) ZPlex features slender, tubular foam cores stitched between two layers of fabric, with Z-fibers fully integrating them with face skins. Integral resin channels reportedly speed infusion while the foam helps reduce print-through. The Z columns, once cured, become rigid and absorb loads. This material reportedly has proven particularly adapt at conforming to in-mold curves and tight angles that are difficult to negotiate with traditional core materials. It can be used with RTM and VARTM processes and readily accepts fasteners and rivets without separating the skins. ZPlex was used extensively in the construction of a new hovercraft developed by Green Cove Springs, Fla.-based All Terrain Land and Sea Hovercraft Inc. (see “Related Content,” at left).

V2 Composites has most recently developed V-Web, a unidirectional reinforcement fabric that, the company says, uses a proprietary technology to optimize the use of reinforcement fibers. It’s available with glass, carbon, aramid and other fibers. The supplier also makes noncrimp fabrics that can be stitch-bonded into multiaxial construction in 0°, 90°, ±45° angles that are also available with various backings. In addition to standard polyester stitching, V2 also offers a patented V-lock stitch, which results in an improved fabric edge that reportedly will not unravel, even with narrow fabric widths.

All of these fabric forms promise to streamline traditionally slow and labor-intensive composite fabrication. They offer attractive alternatives that can be easily integrated into existing processes — and make infusion processing even easier.