Abstract:
A method for preparing carpet by using polyurethane to anneal secondary backing to a greige, comprising fibers attached to a primary backing. The polyurethane monomers are mixed just prior to application to the greige primary backing allowing the relatively low viscosity polyurethane to penetrate the fibers of the primary backing to aid in fiber lock. The polyurethane coated greige is then contacted with the secondary backing before complete polymerization to insure sufficient tack to hold the backings together.

Description:
RELATED APPLICATION DATA  
       [0001]    The present application claims priority from U.S. Provisional Application Ser. No. 60/245,966 entitled POLYURETHANE COATING PROCESS FOR CARPET filed on Nov. 3, 2000, the entire specification of which is incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to improved methods for adhering secondary backing to tufted or woven carpeting employing polyurethane adhesive systems.  
         BACKGROUND OF THE INVENTION  
         [0003]    It is customary in the carpet and rug industry to use various forms of filled and unfilled latex or polyurethane to coat the back of carpet. The coating is used to bond the face fibers to the primary backing and to bond secondary backings to the greige (fibers/primary backing). For example, carpets having attached polyurethane foam layers as backing are described in U.S. Pat. Nos. 3,755,212; 3,821,130; 3,862,879; 4,022,941; 4,515,646; 5,604,267; 5,908,701; and 6,299,715. A key property of the carpet produced by these methods is annealing strength, the force required to separate the secondary backing from the carpet. In order to achieve suitable annealing strength, the secondary backing must be in direct contact with the greige, and a sufficient amount of adhesive must be interjacent the greige and the secondary backing to ensure complete wet out of the fibers. Another key property is, the fiber lock, which measures the force necessary to pull face fibers from the carpet.  
           [0004]    The most widely used annealing adhesive is latex. Latex is typically applied by methods involving roll over flatbed or roll over roll processes. Regardless, of the method used, the greige is coated with an adhesive precoat of latex, and the secondary backing, also coated with latex, is married to the greige and cured.  
           [0005]    Although, latex is a popular adhesive, carpet prepared from latex displays numerous shortcomings. For example, the strength and hydrolytic stability of latex is less than desired, and latex is less durable over time than alternative polymer systems such as PVC plastisol or polyurethane. Moreover, latex curing requires the evaporation of large amounts of water during cure, a process that is both expensive and energy intensive. The use of latex also hinders the effective recycling of manufacturing remnants and used carpet at the end of its life cycle. Alternatively, polyurethane adhesives are capable of forming carpet with superior annealing strength as well as other desirable physical properties. However, despite the advantages of polyurethane, technical problems have kept it from widespread use in the industry.  
           [0006]    Attempts to replace latex with polyurethane have faced a variety of problems, requiring numerous process modifications. One striking example is the difficulty associated with placing polyurethane onto a backing in single applications, while maintaining the necessary adhesiveness to attach a second backing. After the pre-polymers have been mixed and polymerization begins polyurethane soon begins to lose its adhesive properties. Loss of adhesiveness is generally not a problem with the use of latex. Conventional latex maintains its adhesiveness and viscosity during processing, even in the curing oven. Following application of latex adhesive to both the greige and the secondary backing the two components are married and as a result of the latex properties, good temporary adherence of the secondary backing to the greige is observed. In the curing oven, the latex viscosity does not drop significantly as water is evaporated. Thus, the secondary backing satisfactorily adheres to the greige, and little dripping of latex from the bottom of the carpet to the oven is observed.  
           [0007]    On the other hand, polyurethane application from bulk troughs, common in latex systems, is made very difficult due to premature polymerization in the delivery line. Typically, polyurethane is applied as “froth,” polymerized prior to application and sprayed onto the primary or secondary backing before the upstream edge of a doctor blade. However, unless the manufacturer guards against premature polymerization the delivery line becomes clogged retarding the flow of polyurethane to the spray apparatus. In addition polyurethane begins to lose its adhesiveness soon after polymerization begins unless the manufacturer controls the polymerization rate by using heat sensitive catalysts or other chemical agents designed to maintain the viscosity of the polyurethane. Regardless of the manufacturer&#39;s attempts at controlling premature polymerization the manufacturer has only a finite amount of time after the prepolymers (polyol and isocyanate) have been mixed in which to apply the polyurethane to the backing before the polymer begins to lose its adhesive properties. In an attempt to combat the rapid loss of adhesiveness manufacturers have applied one coating of polyurethane to the greige as fiber lock and a second coating of polyurethane just prior to contacting the secondary backing to insure sufficient adhesion between the backings. Even with the additional polyurethane, the slow advancement of most commercial carpet lines, and the inherent lack of adhesiveness associated with polyurethane, does not allow for the desired adherence between the greige and the secondary backing.  
           [0008]    Curing the backing to the greige is also complicated because of the considerable decrease in viscosity of the polyurethane prior to cure. The viscosity of the polyurethane, and likewise its adhesiveness, may decrease to only 10% of its initial value prior to application as the catalyzed polyurethane-forming reaction begins to exert its effect. The greatest decrease in viscosity is often exhibited over the temperature range of ambient to 70° C., where the polyurethane catalysts are not optimally active. As a result, if the initial adherence of the secondary backing to the greige is insufficient the secondary backing may separate during this period of low viscosity.  
           [0009]    In an attempt to address the problems associated with the use of polyurethane several changes to the underlying process have been disclosed. For example, U.S. Pat. No. 6,264,775 offers the addition of various chemical thickening agents to the polyurethane to maintain viscosity and adhesiveness. Another process provides for the use of multiple applications of polyurethane to the primary backing prior to joining the secondary backing. See e.g., U.S. Pat. No. 6,299,715. Still another technique disclosed in U.S. Pat. No. 6,299,715, is the application of both polyurethane to the primary backing and another tacky composition to the secondary backing prior to joining the two backings. None of these techniques have been favored over standard latex based carpet laminates, primarily due to the increased cost and complexity associated with building and using separate manufacturing lines to implement the new technologies.  
           [0010]    It would therefore be desirable to provide a polyurethane carpet annealing process requiring only a single application of the polyurethane, while providing acceptable fiber lock and annealing strength. It would also be desirable to provide a polyurethane annealing system which does not require excessive quantities of polyurethane to provide sufficient annealing strength. It would be beneficial to provide a polyurethane annealing system which does not require an oven for curing.  
         SUMMARY OF INVENTION  
         [0011]    The present invention pertains to a polyurethane carpet annealing system which discloses two unique advantages over the prior art. First it requires only a single application of polyurethane. The lower amount of polyurethane employed in a single application significantly decreases the weight of the carpet and decreases raw material costs. Second, the polyurethane is cured at ambient temperature. The absence of an oven-curing step markedly decreases the time from application of the polyurethane to product roll-up. The carpet produced by the process exhibits acceptable fiber lock and initial secondary backing adhesion without the use of large quantities of polyurethane and without the expensive heat curing step common in other annealing systems. In combination, the two advantages significantly increase the commercial utility of the product. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIG. 1 illustrates a typical prior art latex-based carpet annealing process.  
         [0013]    [0013]FIG. 2 illustrates a prior art single application polyurethane-based carpet annealing process.  
         [0014]    [0014]FIG. 3 illustrates a prior art dual application polyurethane-based carpet annealing process.  
         [0015]    [0015]FIG. 4 illustrates a prior art polyurethane-based process that coats primary backing with polyurethane and the secondary backing with a skip-coat adhesive.  
         [0016]    [0016]FIG. 5 illustrates one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The present invention is designed to improve the annealing of secondary backing to tufted or woven carpeting utilizing a polyurethane adhesive system. Referring now to the drawings in more detail, FIG. 1 illustrates a typical prior art commercial latex-based carpet annealing process. The greige  5 , with top carpet face  10  and bottom primary backing side  15  to which fiber lock adhesive is applied, is directed by rollers  20  to place the primary backing side  15  of the greige under trough  25 , which is supplied with latex through line  30 . The downstream edge of trough  25  serves as a doctor blade  35 , in conjunction with plate  40 . Adjusting the amount of latex applied also serves to force the latex through the primary backing  15  into the fibers. Further penetration is provided by pressure roller  45 . The carpet is then redirected by roller  50 . Secondary backing  55  is supplied from roll  60  and directed by rollers  65  across wheel roll  70 , which rotates in trough  75  filled with additional latex which coats the secondary backing  55 . The coated secondary backing  77  is pressed onto the latex-coated greige  80  by their travel through ligation rolls  85 . The carpet then passes through oven  90  where water is removed from the latex, and the latex cures, adhering fibers and secondary backing to form a lamellar carpet.  
         [0018]    [0018]FIG. 2 illustrates a single application polyurethane-based carpet annealing process. The greige  5  with carpet face  10  downward is fed onto a metering plate  95 . Here polyurethane “froth”  100  is placed on the backing side  15  using a mixing head  105 . After the polyurethane  100  is applied, its thickness may be adjusted by doctor blade  35  and metering plate  95 . A secondary backing  55  is then applied to the surface of the uncured polyurethane  100  using a ligation roller  85 . The coated and backed greige  78  is then passed through oven  90  to cure the polyurethane layer  100 . After the carpet has been cured, it may then be advanced through a trimmer  115  and finally to an accumulator  120  or roll-up area.  
         [0019]    [0019]FIG. 3 illustrates a dual application polyurethane-based carpet annealing process. To the primary backing side  15  of the greige  5 , polyurethane  100  is applied from first line  30 . Doctor blade  35 , in cooperation with plate  40 , limits the applied weight of polyurethane  100  and forces it into the greige  5 . Following the first line  30  is second line  32 , which supplies additional frothed polyurethane  100 . Doctor blade  36 , in conjunction with plate  41 , adjusts the thickness of the deposited polyurethane  100 . While the first doctor blade  35  substantially contacts the primary backing  15 , the second doctor blade  36  is spaced a distance away to allow enough polyurethane  100  to be deposited to temporarily bind the secondary backing  55  before curing. The secondary backing  55 , supplied from roll  60 , contacts the coated greige  80  below ligation roller  85 . Prior to entry into the oven  90  for curing, press rollers  125  assist in maintaining stable contact between the secondary backing  55  and the greige  5 .  
         [0020]    [0020]FIG. 4 illustrates a polyurethane-based process that coats the primary backing with polyurethane and the secondary backing with a polyurethane adhesive. Following application of the polyurethane  100  from line  30  and its impregnation onto and into the primary backing side  15  of the greige  5  between doctor blade  35  and plate  40 , the carpet is contacted with a wheel-coated secondary backing  55 . The secondary backing  55 , supplied from roll  60 , has been directed by rollers  65  past wheel roll  130  which rotates in trough  135  filled with wheel coating agent. Rotation of wheel roll  130  in the trough  135  coats the roll with wheel coating agent which is then transferred to the secondary backing  55  to form a wheel-coated secondary backing  140 . Wheel roll  130 , supplies a tacky polyurethane based coating agent which is not “self-curing” and therefore does not substantially increase in viscosity over the life of the composition in the trough  135 . Wheel-coated secondary backing  140  mates with the polyurethane adhesive coated greige below roller  85 , which may be supplemented with a roller directly below, or a plate, to form a pair of ligation rollers or equivalent devices. The carpet then passes between press rollers  125  into oven  90  for curing.  
         [0021]    [0021]FIG. 5 illustrates the carpet manufacturing process according to the present invention. The greige  5  with carpet face  10  downward is fed onto a tenter roll  12  that directs the greige  5  to metering plate  95 , the carpet face  10  is advanced over the metering plate  95  using pulling rollers  21 .  
         [0022]    In FIG. 5, a layer  100  of polyurethane polymer is deposited onto the primary backing side  15  as it passes over metering plate  95  using a mixing head  105 . Preferably, the polyurethane is applied to the primary backing  15  at an area concentration of about 1.0 g/m 2  to about 30 g/m 2  or in a more preferred concentration of about 5.0 g/m 2  to about 12 g/m 2 . In an even more preferred embodiment of the present invention, the polyol and isocyanate pre-polymer components are mechanically blown through a mixing head  105  to form the polyurethane layer  100 . This prevents the pre-polymers from contact until just prior to application to the primary backing  15 . According to this method, the polymerization process does not begin to occur until just prior to the exit of the polyurethane components from the mixing head  105 . This process allows the relatively low viscosity polyurethane to deeply penetrate the fibers in the primary backing, resulting in excellent fiber lock characteristics. A preferred method of blowing the polyurethane layer is by feeding separate lines of polyol and isocyanate components and a stream of air into a Gusmer Corporation spray attachment (model GX7-400). If the need arises a third line is available for the introduction of catalyst. The spray attachment combined with the air stream provides sufficient turbulence to provide a homogenous solution of the urethane-forming components.  
         [0023]    After the polyurethane components are applied polymerization accelerates and foaming begins. The thickness of the polyurethane foam may be adjusted by various means known in the art such as a doctor blade  35  or air knife. The polyurethane coated greige  79  is then optionally passed across a steam box  150 . Steam box  150  adds water to the reaction, to aid in accelerating the polymerization rate. In addition the steam serves to enhance blooming of the yarns passing through the primary backing for deeper penetration and assures a more level coverage of polyurethane. A Fume hood  155  is placed above the steam box  150  to capture escaping volatiles released with the steam. Conspicuously absent at this stage of the process is an oven curing step. An oven is not necessary in the process because the polyurethane components are essentially kept separate until they are applied to the primary backing  15 , therefore there is no need to dope the polyurethane components in order to control the rate of polymerization. In a preferred embodiment of the present process temperatures are generally maintained within 30° F. of ambient temperature or in a more preferred embodiment within 15° F. of ambient temperature during the carpet manufacturing process. In contrast, prior art polyurethane curing processes are required to utilize heat sensitive catalysts and other chemical additives to maintain sufficient viscosities to apply the polyurethane, from various holding lines and troughs, to the backing surface. These additives necessitate a heat curing stage.  
         [0024]    The independence gained by controlling the mixing of polyol and isocyanate has lead to the removal of a formal curing stage. The excision of this step has dramatically improved the processing speed of laminated carpets, cutting manufacturing time by as much as 50-60% without sacrificing the annealing strength or fiber lock properties of the carpet.  
         [0025]    After applying the polyurethane coating, and steaming the coated greige  79 , the coated greige  79  is rapidly pulled to merge roll  160  where the secondary backing  55 , supplied from roll  60 , contacts the coated greige  79  downstream from roller  87 . Due to the short time that elapses between the initial application of polyurethane and the merger of the secondary backing  55  with the coated greige  79 , the polyurethane possesses sufficient tackiness to anneal the secondary backing in place. The annealed carpet can then be cooled briefly  190  and rolled for storage and transport.  
         [0026]    In all of the foregoing embodiments, the Figures have been simplified for clarity and to ease viewing and understanding. In commercial embodiments, additional devices, e.g. drive motors, tension devices, etc. will be required.  
         [0027]    Polyurethane prepolymers useful in the practice of the present invention are prepared by the reaction of active hydrogen compounds with any amount of isocyanate in a stoichiometric excess relative to active hydrogen material.  
         [0028]    The prepolymer formulations of the present invention include a polyol component. Active hydrogen containing compounds most commonly used in polyurethane production are those compounds having at least two hydroxyl groups or amine groups. However, any active hydrogen containing compound can be used with the present invention.  
         [0029]    In the practice of the present invention, preferably at least 50 weight percent of the active hydrogen compounds used to prepare the polyurethane is a polyol having molecular weight of from about 100-400.  
         [0030]    The polyisocyanate component of the formulations of the present invention can be prepared using any organic polyisocyanates, modified polyisocyanates, isocyanate based prepolymers and mixtures thereof. These can include aliphatic or aromatic isocyanates. Preferably the isocyanate used to prepare the prepolymer formulation of the present invention is methyl diisocyanates such as Bayer&#39;s 142L or Dow p901 or blends of equal type.  
         [0031]    Catalysts suitable for use in preparing the polyurethanes of the present invention include tertiary amines, and organometallic compounds and mixtures thereof. For example, suitable catalysts include stannous octoate, triethylenediamine, N-methyl morpholine, like compounds and mixtures thereof. The catalysts do not necessarily need elevated activation temperatures or other promoters to initiate polymerization.  
         [0032]    Surfactants can be useful for preparing a stable dispersion or froth of the present invention. Surfactants useful for preparing a stable dispersion can be cationic, anionic, or non-ionic surfactants. Preferably the surfactants used to prepare the prepolymer formulation of the present invention are silicone surfactants such as Dow Corning DC-194 or Union Carbide&#39;s L-540. A surfactant can be included in a formulation of the present invention in an amount ranging from about 0.01 to about 7 parts per 100 parts by weight of polyurethane component.  
         [0033]    A compound of the present invention optionally includes a filler material. The filler material can include conventional fillers such as milled glass, calcium carbonate, aluminum trihydrate, barium sulfate, fly ash, dyes and pigments or fire retardants (aluminum trihydrate and Tris polyolefin glycol). Preferably the filler can be present in an amount ranging from 0 to 300 parts per 100 parts of the polyurethane component.  
         [0034]    Generally, any method known to one skilled in the art of preparing polyurethane froths can be used in the practice of the present invention to prepare a polyurethane froth suitable for preparing, for example, a carpet of the present invention.  
         [0035]    Although a preferred embodiment of the present invention has been disclosed herein, it will be understood that various substitutions and modifications may be made to the disclosed embodiment described herein without departing from the scope and spirit of the present invention as recited in the appended claims.