Causes of Pattern Distortion
Most carpet is made by tufting into a lightweight, woven primary backing and then laminating the tufted fabric to a secondary backing using a latex binder. Although the base fabric is strong when pulled in the lengthwise and widthwise directions, it is easily distorted when pulled on the bias. The best starting point for a distortion-free pattern is a primary backing with a pick line that is perfectly straight and perpendicular to the warp yarns. Making a primary backing of that type requires attention to detail - insuring that the loom is properly aligned and fabric tensions are uniform. Similarly, during tufting, the pick line must coincide with the needlebar and let-offs and take-ups properly aligned. On the coater, the tufted primary must be pinned so that both sides advance at the same rate to avoid skew, and all rollers must be properly aligned and must contact the carpet in a manner that prevents localized stresses. Clearly, unless special care is taken throughout the entire process, pattern distortion is likely in the finished carpet.

Primary Backings for Patterned Carpets
For many years, Amoco Fabrics and Fibers Company has provided primary backings with features that minimize distortion in patterned carpets. In 1993, Amoco introduced straight pick PolyBac¿, the industry¿s first primary backing with consistent, very low pick line distortion. Over the last nine years, more than one billion sq yds of straight pick PolyBac¿ have been made and sold in North America.

A further refinement of the straight pick product was made in 1999 with the introduction of PolyBac¿ GL, a primary backing with a contrasting color filling yarn inserted at frequent intervals to allow easy alignment of the needlebar and make it easier sew-on of sequential pieces of backing or greige goods. Figure 1 shows needlebar/pick line alignment with PolyBac¿ GL.

In developing its full line of woven primary backings, Amoco evaluated several types of fill yarns and determined their impact on tuftability, carpet surface appearance, mendability, and resistance to distortion. In a test that measures skew resistance, results showed that a serrated (ridged) filling tape led to better skew resistance than a fibrillated (cut) tape (Figure 2). Other tests also showed better performance with serrated filling tapes. Consequently, Amoco¿s standard primary backings incorporate this feature.

Although straight pick and PolyBac¿ GL backings help prevent pattern distortion during tufting, they have only limited resistance to distortions on the bias. To address that need, Amoco introduced Matrix primary backing last year. Matrix backing, a composite made from PolyBac¿ GL and a tailored polypropylene nonwoven fabric, delivers significantly more skew resistance than comparable woven backings (Figure 2). Matrix has shown strong sales growth as more patterned carpet manufacturers specify it for critical styles.

Although Matrix offers a step change in backing performance, carpet styling trends such as finer gauge tufting and even more intricate patterns require that primary backing systems continue to improve. Consequently, Amoco is actively working to develop innovative, cost-effective backing solutions that will further benefit carpet makers, retailers, and installers.Hugh Gardner is Manager of Product Development for Amoco Fabrics and Fibers Company, a subsidiary of BP plc. He has been with Amoco for 16 years and is located in Austell, GA.