Patent Application
20070266534
Suitable nonwoven webs to be processed in accordance with the present invention include those prepared from thermoplastic polymers such as polyolefins, polyesters, polyamides and blends of these polymers. Nonwovens derived from polyesters are preferred and spunbonded or spunlaid polyester webs are particularly preferred. The nonwoven webs can be prepared using conventional methods and include dry-laid, wet-laid, spunlaid, melt-blown, spunbonded and spunlaced products.
Preferably, the nonwoven web is needled, heat-set and calendered before treatment with an elastomeric binder. Needling or needle-punching through the thickness of the nonwoven web creates fiber entanglement in the “Z” direction (i.e., through the thickness of the fabric) in addition to the normal thermal bonding in the “X” and “Y” direction (i.e., in the machine direction and cross-machine direction). The needling provides fiber bonding and entanglement in all directions, thereby increasing the opportunities for entanglement with the tufted yarn. Needling also provides additional loft to the fabric which results in a slightly thicker material for the same fabric weight and provides an additional grip on the tuft. The nonwoven web may be needled in one or both directions. Also, custom needling may be performed in a conventional manner to create patterns or grains in the web.
Needling (or needle-punching) can be performed using any commercially available needling apparatus. As is well known, the degree of needling affects the tensile strength of the web or fabric. The number of needle penetrations per square inch should be selected for optimum intermingling and entanglement of the individual filaments of the web.
Alternatively, fiber entanglement could be accomplished by other well-known techniques. These would include hydro-entangling using high pressure water jets instead of barbed needles.
The nonwoven web preferably is heat-set. This step improves dimensional stability and locks in the loft provided by a needle-punching step. The improved loft aids in reducing compression upon any subsequent calendering, and also preshrinks the web before locking in memory, thereby minimizing stretching or shrinking of the web which may occur during subsequent processing.
Operable heat-setting temperatures will depend in large part upon the nature of the polymer used to prepare the nonwoven web. Temperatures must be selected which are high enough to effect heat-setting but below the melting or decomposition temperatures of the polymeric materials. For spunbonded or spunlaid polyester nonwovens, a temperature range of about 190° C. to about 250° C. is preferred. A temperature of about 205°-210° C. is most preferred.
Any suitable heating apparatus can be employed. Drum ovens are particularly suitable. Heat-setting can be accomplished by exposing the web to pressurized saturated steam or by employing apparatus which provides dry heat.
The nonwoven web preferably is calendered after heat-setting by treating at temperatures and pressures sufficient to bond surface filaments and compact the web to a suitable thickness for further processing. Calendering may also be used to provide a smooth surface to the web, if desired. The temperature and pressure can be adjusted to provide a suitable thickness and surface texture to the web. Because the web was previously heat-set, the loft is unaffected and internal fiber entanglement is undisturbed.
The temperature and pressure conditions generally suitable for calendering range from about 100° C. to about 250° C. and from atmospheric up to about 500 lbs/in2. Conventional calender rolls or cylinders can be employed in the calendering process.
Preferably, the fabric is cooled after calendering, most preferably to room temperature. Cooling is believed to help set dimensional memory in the fabric. Cooling can be accomplished by air cooling or cooling jets or any conventional cooling means.
It is an important feature of the process of the invention to provide elastomeric properties to the web or fabric. This is accomplished by contacting the web with a liquid, curable, elastomeric binder formulation.
The elastomeric binder provides the fabric with an elastomeric property which enables the opening made with the tufting needle to shrink in size after the needle retracts. Shrinking of the opening after needle retraction increases the gripping action on the yarn tuft. The elastic nature of the binder also allows for multiple repairs of the fabric if the tuft yarn is removed from the opening for various reasons. In many backing fabrics, the piece of material between needle openings will tear when repairs are necessary. The elastic properties of nonwoven fabrics processed according to the invention allow the piece of material between needle openings to expand and stretch without tearing, thereby facilitating repairs.
Suitable elastomeric binder formulations include water-based and organic solvent-based elastomers containing conventional curatives and additions. Latexes are preferred for environmental reasons. Examples include curable polyurethanes, homopolymers and copolymers of dienes such as butadiene/styrene rubbers, acrylics, etc.
Conventional additives may be present in the elastomeric formulations. These additives include curing agents and curing adjuvants, fillers, lubricants, colorants, anti-microbials, water resists, etc. The amount of elastomeric binder and the solids content of the formulation can be adjusted for optimum performance. Generally, the solids content will range from about 10% to about 30% by weight, preferably about 15% to about 25%.
Conventional means may be employed to apply the binder to the web. Bath immersion, spraying or roller coating may be employed. Preferably, the nonwoven web is fully saturated by the elastomeric formulation. This can be accomplished by immersing the web in a dip tank containing the elastomeric binder.
After impregnating with the elastomeric binder, the web preferably is processed to remove excess binder. Simultaneous or subsequent to excess binder removal, the web is provided with holes, voids or depressions. The holes, voids or depressions are in a pattern consistent with, and in register with, the tufting needle pattern to be used when the finished web is subsequently tufted in a future carpet manufacturing operation. Preferably, the removal of excess binder and the application of holes or voids in the web occur simultaneously, for example, by feeding the web through a set of rollers, one having a smooth face and the other having a surface with raised protrusions of a predetermined height, size and pattern.
Any commercially available embossing apparatus may be employed to apply the voids (dimples), holes or depressions. Engraved heated rollers are particularly suitable.
The effect of this step is to provide a nonwoven web having little or no binder in the holes or depressions and having a hole which is smaller than the tufting needle containing the face yarn. Energy created when the hole is stretched during the tufting operation is, in effect, stored in the elastic binder which has been squeezed from the depression and surrounds the hole. When the needle retracts, the stored energy is released causing the stretched fabric around the hole to relax and hold the tuft firmly in place.
Following the treatment to apply voids or holes in the binder-treated fabric, the binder is allowed to cure. Preferably, the treated fabric is routed over drum heaters at a temperature high enough to dry the fabric and cure the binder without softening the fibers or changing the heat-set of the fibers. The drying and curing operation provides additional bonding at filament junctions and endows the fabric with elastomeric properties. A range of suitable temperatures for nonwoven polyesters for drying/curing is 100° C. to about 250° C.
The finished product is then wound up in rolls. Preferably, winding apparatus is used which is designed to drive the take-up roll at the core. Friction wheel winders may slip on a lubricated surface of the fabric and create poor packing on the roll. Core driven winders will pull the wraps tighter resulting in a much more stable package.
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Having described preferred embodiments of the invention, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit.
