Polyurethanes are produced in four different principal forms including elastomers, coatings, flexible foams, and cross-linked foams. Polyurethane foams are produced by reacting isocyanate compounds with polyol compounds generally in the presence of catalysts, surfactants, and other auxiliary agents. At the start of polyurethane foam production, the reactive raw materials are held as liquids in large, stainless steel tanks. These tanks are equipped with agitators to keep the materials fluid. A metering device is attached to the tanks so that the appropriate amount of the reactive material can be pumped out. Generally, the ratio of polyol to isocyanate is about 2:1; and the ratio of components is strictly metered to control the characteristics of the resulting polymers. The reacting materials are then mixed and dispensed. Reaction between the isocyanate and the polyol, usually referred to as the gel reaction, leads to the formation of a polymer of high molecular weight. This reaction increases the viscosity of the mixture and generally contributes to cross-link formation. The second major reaction occurs between isocyanate and water. This reaction produces carbon dioxide gas which promotes foaming causing the volume of the urethane polymer to grow. In some instances, auxiliary blowing agents are added to further increase the volume of the polymer.
Both the gel and blow reactions occur in foams blown partially or totally with carbon dioxide gas. In order to obtain a good urethane foam structure, the gel and blow reactions must proceed simultaneously and at optimally balanced rates. For example, if the carbon dioxide generation is too rapid in comparison with the gel reaction, the foam tends to collapse. Alternatively, if the gel reaction is too rapid in comparison with the blow reaction generating carbon dioxide, the rise of the foam will be restricted resulting in high density foam. In practice, the balancing of these two reactions is controlled by the characteristics of catalysts, temperatures, and auxiliary agents used in the process.
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 also thereby creating good tuft bind or fiber lock, and to bond secondary backing material to the greige (fibers/primary backing). For example, carpets having attached polyurethane 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, or the force required to delaminate or separate the secondary backing from the carpet. In order to achieve optimal annealing strength, the secondary backing must be in direct contact with the greige, and a sufficient amount of adhesive must be between the greige and the secondary backing to thoroughly wet the fibers. A second key property is fiber lock, or a measure of the force necessary to pull face fibers from the carpet.
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.
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 optimal, 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. Heating latex annealed carpet to achieve timely curing requires temperatures in at least the range of 70° C., and these temperatures may cause some carpet fibers and backing materials to shrink or change appearance and properties. To minimize the cost of latex adhesives, substantial quantities of filler material are added. The use of latex filler hinders the effective recycling of manufacturing remnants and used carpet at the end of its life cycle.
Alternatively, polyurethane adhesives have been employed to form carpet with superior annealing strength and other desirable physical properties. However, despite the advantages of polyurethane, cost and technical problems have kept it from widespread use in the industry as a coating, and even more rarely as a flexible foam.
Attempts to replace latex with polyurethane have resulted in a variety of new problems, requiring modifications to the usual latex annealing process. One striking example is the difficulty associated with placing polyurethane onto a greige material, while maintaining the necessary tack or adhesiveness to attach the second backing. After the pre-polymers have been mixed and polymerization begins, polyurethane soon begins to lose its adhesive properties. In addition, because the blow reaction substantially increases the volume of the polyurethane layer, even small irregularities in the application of the polyurethane components to the greige may result in unacceptable variations in the depth of the resulting polyurethane foam layer.
Loss of adhesiveness is generally not a problem with the use of latex. Conventional latex maintains its adhesiveness and viscosity during processing, even into 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's properties, good temporary adherence of the secondary backing to the greige is observed. In the curing oven, the latex's viscosity does not drop significantly as a significant portion of water is evaporated. Thus, the secondary backing satisfactorily adheres to the greige.
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 dispensed on the primary or secondary backing before the upstream edge of a doctor blade. However, unless the manufacturer guards against premature polymerization the delivery line tends to become clogged, thereby retarding the flow of polyurethane to the dispensing apparatus. The doctor blade will also tend to foul with polyurethane that adheres and cures, with a resulting loss of effectiveness. 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's attempts at controlling premature polymerization, the manufacturer has only a finite amount of time after the pre-polymers (polyol and isocyanate) have been mixed in which to apply the polyurethane and contact the greige to the backing before the polymer begins to lose its adhesive properties.
When chemical agents were previously added to control premature polymerization and maintain viscosity to enable the polyurethane to penetrate the fibers, achieve good tuft bond, and maintain adhesiveness to affix the secondary backing, the resulting mixture typically was not quickly cured without heat from ovens. If dry heat is applied to a polyurethane coated greige before marrying the secondary backing the polyurethane will tend to form a crust-like surface with little tack or adhesiveness. Oven curing adds time and cost to the finishing process, and can also adversely affect some fibers by matting or shrinkage.
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, combined with the inherent lack of adhesiveness associated with polyurethane, does not allow for the desired adherence between the greige and the secondary backing without the use of considerable and expensive quantities of the pre-polymers.
To extend the coverage of a given quantity of polyurethane pre-polymers, it is customary to add filler material to the mixture. However, filler materials are generally abrasive and complicate the application of the polyurethane mixture by either wearing on applicator parts or increasing the tendency of lines and applicators to clog or apply unevenly. Furthermore, frothing the pre-polymer mixtures containing substantial amounts of filler material generates heat, and at high filler levels frothing will activate heat sensitive catalysts and begin the polyurethane curing process.
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 of the secondary backing as the catalyzed polyurethane-foaming reaction begins to exert its effect. The greatest decrease in viscosity is often exhibited over the temperature range from ambient to about 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.
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. In U.S. Pat. No. 4,515,646, refrigerated isocyanate and polyol components were used without catalysts to prevent premature polymerization. WO 03/039869 proposed spraying a substantially water-free polyurethane on the greige and effecting the blow and curing with steam. None of these techniques was 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, or the failure of the techniques to work in a production environment. However, U.S. Pat. No. 7,368,008 disclosed an effective roller-based polyurethane foam carpet annealing process requiring only a single application of the polyurethane, while providing acceptable fiber lock and annealing strength, without using excessive quantities of polyurethane to provide sufficient annealing strength, and that can be cured without an oven.