Process for forming densified tufted carpet tiles by shrinking primary backing
The present invention relates to fabric materials, and more particularly to densified tufted carpet tiles. 
     BACKGROUND OF THE INVENTION 
     Fabric materials, and particularly carpet fabrics, are usually prepared by selecting a primary backing material and tufting the backing using a needle tufting process. Standard gauge carpets are generally produced to have approximately 8 to 13 tufts per inch in the across machine direction (AMD) and 7 to 11 tufts in the machine direction (MD). Although it is often desirable to obtain more densely tufted fabrics, these materials are extremely difficult to prepare. The problems which are typically encountered include a reduction in production speed because, for example, as the tufts are brought closer together, an inherently slower shift mechanism must be used to stagger the tufts. Such complex equipment is inherently more difficult to use than conventional equipment, and overall operating efficiency is often reduced. 
     The textures which may be obtained are also limited because of the necessary use of smaller yarn sizes and narrower tufting gauges. Aside from being more expensive, these materials also cause difficulties with non-woven backings because of the number of perforations which are placed in small areas of the backing. Thus, because of the physical disruption to the backing, such materials are more difficult to hand-repair in place, a procedure which can readily be achieved with conventionally tufted materials. Accordingly, it is seen that previous attempts to produce densely tufted products have often led to slower production times, increased incidences of mechanical problems and more expensive products. 
     PRIOR ART 
     The aforementioned problems have long been documented in the art; thus, it is known that the denser the weave of the backing material, the more difficult it is to obtain high quality, uniformly tufted materials. One reference which relates to an alternative tufting procedure is U.S. Pat. No. 3,238,595. That reference discloses that backing materials having a loose, open-weave construction may be needled more efficiently than tightly woven carpet backings. Therefore, the reference teaches that one may tuft a loosely woven backing and then induce the backing to shrink by applying a latex secondary backing or a thermoplastic coated scrim as a secondary backing, and then heating the combined materials such that the hot latex or the hot thermoplastic induces the loosely woven primary backing to shrink. This procedure is not suitable, however, to produce highly densified carpet tile materials. 
     Accordingly, one objective of the present invention is to provide a process for preparing highly densified tufted carpet tiles wherein the tuft density increases by at least 130% over the initially needled tuft density. 
     Another objective of the present invention is to provide densified tufted carpet tiles which comprise a secondary backing and which are not subject to curl. 
     These and other objectives of the present invention will become apparent from the detailed description of preferred embodiments which follow. 
     SUMMARY OF THE INVENTION 
     A heat-shrinkable backing material, which preferably is a spun-bonded polyolefin backing material, is tufted with a relatively heat stable yarn and the tufted backing is subjected to a temperature of not less than 300° F. The applied temperature induces the tufted backing to shrink, thereby increasing the tuft density by not less than about 130% so as to produce an unbacked, densely tufted carpet tile. A secondary backing is then applied to provide a stable structure. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In one embodiment, the present invention relates to a process for forming a densely tufted carpet tile, said process comprising the steps of selecting a heat-shrinkable primary backing material, said backing material being capable of shrinking to not more than 85% of its initial dimensions when subjected to a temperature of not less than 300° F.; tufting said backing with a yarn which will not be adversely affected by the shrinkage conditions; placing the tufted backing on a tenter frame; subjecting the tufted backing to a temperature not less than 300° F., thereby inducing said backing to shrink so as to increase the tuft density by at least 130%; and applying a secondary backing to the densified tufted backing. 
     In a second embodiment, the present invention relates to a densified tufted carpet tile comprising tufting, a primary backing and a secondary backing, said tile being obtained from the heating on a tenter frame of a heat shrinkable primary backing at a temperature of not less than about 300° F. such that said backing shrank to not more than 85% of its initial dimensions, said backing having been pretufted with a yarn which was not adversely affected by the heating conditions, said heating having caused an increase in tuft density of at least 130% relative to the initial tuft density, said densified primary backing having been provided with a secondary backing after densification of said tufting. 
     The primary backing materials which may be used to practice the present invention comprise heat sensitive backings of standard construction; however, because of the purpose of the present invention, the backings must be capable of shrinking to not more than 85% of their initial width and length dimensions. These backings may comprise standard weave backings such as those having from about 24 warp strands per inch and about 13 weft strands per inch. Loosely woven backings are not preferred because excessive heat treatment is necessary to achieve the high densification desired according to the present invention. Preferably, the primary backing material will comprise a spun-bonded polyolefin material, and most preferably, a spun-bonded polypropylene material. An example of such material is Typar® which is sold by du Pont. 
     The primary backing may be tufted by means well known in the art using yarns which will be stable at the temperatures used to induce shrinkage of the primary backing. Suitable tufting materials may include nylon yarns, polyester yarns, cellulosic yarns and acrylic yarns. However, because the shrinkage conditions will utilize temperatures which are not less than 300° F., materials such as PVC yarn and polyolefin yarn are often unsatisfactory. 
     The tufted primary backing is placed on a tenter frame which can control the dimension to which the tile ultimately shrinks. The tenter frame may be fixed at a final dimension, as illustrated in the following examples, or an adjustable tenter frame which can apply an adjustable tension can be used, the selection of the frame being within the skill of an ordinary artisan. 
     The objective of the invention is to cause shrinkage of the primary backing such that the tuft density is increased to at least 130% of the initial density, and preferably to at least 150% of the initial density. It is also preferred that the densification occur uniformly in both a machine direction and an across machine direction. 
     Once the shrinkage is complete, the densified fabric will be provided with a secondary backing. The secondary backing may be any type which is well known in the art, including woven secondary backings which are applied using adhesives, foamed secondary backings, hot-melt secondary backings and latex secondary backings. 
    
    
     The advantages of the present invention will become more apparent from the following examples which are provided by way of illustration and not limitation. 
     EXAMPLES 
     The following tufted primary carpet backings having a dimension of 26 inches AMD by 46 inches MD were placed on a 20-inch AMD by 36-inch MD tenter frame such that the edges of the samples were hooked and the samples hung loosely in the frame. Each sample was subjected to a temperature of about 350° F. for about 10 minutes, thereby inducing the backing to shrink and densification to occur. After heating, each sample was cooled to room temperature and examined to determine the amount of densification. Information concerning the primary backing used for each sample is set forth in Table I, and the shrinkage information is described in Table II. 
     After evaluating each sample, two of the densified backings (City Line and Verdict) were back-coated with a preformed conventional 1/16-inch polyurethane secondary backing which was acquired from Textile Rubber Company. Adhesion was achieved through the use of a water-based pressure sensitive adhesive, F-114, from Franklin Chemical Company. Two samples (Elegant Expressions and Engravers Mark) were also backed with jute using a conventional thermoset adhesive. None of the samples showed evidence of curl. 
     
                       TABLE I                                                     
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INITIAL NYLON TUFTED BACKING CHARACTERISTICS                              
                            Type   Weave                                  
Commercial                                                                
         Primary  Weight    Con-   (Tufts/in                              
Name     Backing  (Oz./Yd..sup.2)                                         
                            struction                                     
                                   Warp × Weft)                     
______________________________________                                    
Sculptured                                                                
         Typar    3.2       Spun   --                                     
Touch                       Bonded                                        
Elegant  Polypro- 3.9       Woven  24 × 15                          
Expressions                                                               
         pylene                                                           
Commenda-                                                                 
         Polypro- 3.5       Woven  24 × 13                          
tion     pylene                                                           
City Line                                                                 
         Polypro- 4.0       Woven  24 × 15                          
         pylene                                                           
Windward Polypro- 3.6       Woven  24 × 13                          
         pylene                                                           
Engravers                                                                 
         Typar    4.2       Spun   --                                     
Mark                        Bonded                                        
Very     Typar    4.2       Spun   --                                     
Impressive                  Bonded                                        
Print                                                                     
Diamo    Polypro- 3.3       Woven  24 × 11                          
         pylene                                                           
Verdict  Polypro- 3.9       Woven  24 × 15                          
         pylene                                                           
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                                           TABLE II                                
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SHRINKAGE DATA                                                            
                              Initial Tuft                                
                                      Final Tuft                          
                                              Percent                     
Commercial                                                                
        Initial    Final      Density (Tufts                              
                                      Density (Tufts)                     
                                              Density                     
Name    Inch AMD                                                          
              Inch MD                                                     
                   Inch AMD                                               
                         Inch MD                                          
                              Per Inch.sup.2)                             
                                      Per Inch.sup.2)                     
                                              Increase                    
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Sculptured                                                                
        12.8  10.5 20    16   134.4   320     238                         
Touch                                                                     
Elegant 10    10   14      12.5                                           
                              100     175     175                         
Expressions                                                               
Commendation                                                              
        8     10.5 11    14   84      154     183                         
City Line                                                                 
        10    9    15    11   90      165     183                         
Windward                                                                  
        12.8  10.5 17    15   134.4   255     190                         
Engravers                                                                 
        12.8  9.5  18    11   121.6   198     163                         
Mark                                                                      
Very    12.8  9.5  17    14   121.6   238     196                         
Impressive                                                                
Print                                                                     
Diamo   5.3   7     8    11    37.1    88     237                         
Verdict                                                                   
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 (Not measured; heating converted a 24.5 inch by 24.5 inch piece to a     
 dimension of 20 inches by 20 inches)