Stock material or miscellaneous articles – Structurally defined web or sheet – Including variation in thickness
Reexamination Certificate
1997-11-24
2002-06-11
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including variation in thickness
C428S167000, C428S169000, C428S174000
Reexamination Certificate
active
06403196
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to shaped fiber network structures. More particularly, this invention relates to three-dimensionally shaped fiber network structures which are rigid and have improved post-yield dimensional recovery. Furthermore, this invention relates to improved methods of making such structures and to articles incorporating such structures.
Three-dimensionally shaped fiber network structures and methods of making them are known in the art.
For example, such structures have been made by multi-step processes involving impregnating a fabric with a high level of thermoset resin, deforming the resin-impregnated fabric into the desired network shape, and then curing the thermoset resin so as to lock the structure in such desired shape. These methods are taught, for example, in U.S. Pat. Nos. 4,631,221; 4,890,877; 5,158,821; and 5,447,776.
The resin-based network structures formed by the process described above have several disadvantages. For example, both the compression properties and the stiffness properties of the resin-based network structures (which are derived from conventional textile-type yarns) are determined by the type and amount of resin present in the structure. In fact, resin loading is the limiting factor in network stiffness. Increasing the stiffness of the network structure requires progressively higher loadings of resin. A typical resin-based network will contain more than 50% by weight of thermoset resin. Very stiff network structures are usually in the form of composites constructed by nesting single network structures.
The prior art resin-based networks are composed of multifilament yarns and a stiff but brittle matrix material. When the dome structures in these networks are compressed, the network elements are bent. Because all of the fiber crossover points are tightly bonded, bending is highly localized, i.e., bending occurs in the short lengths between the fiber crossover points. Even at small dome compressions, some network elements are highly strained while others are under no strain. Kinking (brittle failure) of the most highly bent elements occurs at low overall network compressions (less than 30%). Once an element kinks, it behaves like a hinge and offers no further resistance to bending. The network has yielded and offers reduced resistance to further compression. Since the kinking or buckling is permanent, the network cannot recover its original height or shape when the kink-inducing compression is removed. Because the network structure cannot recover its original height or shape when its yield load has been exceeded, the network structure is described herein as having “low post-yield dimensional recovery”.
Another drawback of the prior art resin-based network structures is that their maximum stiffness tends to be limited by the natural tendency of textile yarns to flatten, thus presenting the thinnest, softest cross-section when bent.
Because the prior art resin-based network structures are usually stiff and brittle and suffer permanent deformation when compressed beyond 10% to 20% of their thickness, the use of such network structures is generally limited to lightweight structural applications.
The prior art process described previously herein for making the resin-based network structures discussed above also has drawbacks. For example, the process requires a separate costly initial resin treatment. In addition, the fabric used in the process is not particularly stable since, until its deformation, the fabric must be maintained at a temperature below the curing temperature of the resin. Furthermore, the deformation process is time-consuming since it is controlled by the amount of time required to heat up the mold, the fabric and the resin and the amount of time required to cure the resin. Thus, although the prior art resin-based network structures have found use in a number of applications such as, e.g., building panels, automotive doors, flooring systems, and geotextiles, use of these network structures is limited primarily by the high cost of making them.
To overcome the difficulties associated with the above-described resin-based process, methods of making resin-free three-dimensionally shaped fiber network structures were introduced. For example, resin-free network structures have been formed using multifilament yarn textile fabrics consisting of high melting temperature reinforcing filaments and lower melting temperature thermoplastic matrix filaments, wherein the network structure is formed by melting the matrix filaments, forming the desired network shape, and re-solidifying the matrix material prior to demolding. Such a method and resin-free structure are disclosed, e.g., in U.S. Pat. No. 5,364,686.
The properties of the resin-free network structure formed by the process described in U.S. Pat. No. 5,364,686 are similar to those of the aforementioned resin-based network structures. Although simpler and cleaner than the methods for making the resin-based network structures, the method described in U.S. Pat. No. 5,364,686 for making resin-free network structures is extremely slow because the matrix polymer must be melted, shaped and then cooled below its melting temperature and allowed to harden sufficiently so that the network shape can be maintained prior to demolding.
A drawback to both the resin-based and resin-free prior art processes described hereinabove is that before the deformed network structure can be removed from the mold the thermoset resin must be cured in the resin-based process or the low melting thermoplastic must solidify in the resin-free process. This is time-consuming.
Another drawback to the resin-based and resin-free prior art processes described hereinabove is that both processes use multifilament yarns to form the network structures therein. The use of multifilament yarns to form such network structures has several disadvantages. For example, multifilament yarns generally cannot support their own weight unless the individual fibers therein are bonded together (i.e., the multifilament yarns are “limp”). However, bonded multifilament yarns are also disadvantageous in that they can delaminate along weaknesses when they are flexed and consequently become dramatically softer. In addition, multifilament yarns tend to flatten to form the softest cross-section, i.e., a ribbon, during the network-forming process. This limits the achievable compression modulus.
Both the resin-based and resin-free network structures produced by the prior art processes described above are rigid, quasi-brittle structures. Both types of structures are stiff and can be deformed only a limited amount before yielding and acquiring a permanent deformation.
More recently, resin-free three-dimensionally shaped fiber network structures have been formed using large-diameter thermoplastic polymer monofilaments having a diameter of at least about 0.1 millimeter. Such monofilament-based structures are disclosed, for example, in copending, commonly assigned U.S. patent application Ser. No. 08/577,655 to Kim et al., filed Dec. 22, 1995.
In the monofilament-based network structures disclosed in the Kim et al. application, the limp multifilament yarns and brittle resins are replaced with large diameter monofilament yarns. When these network structures are compressed, the stiff monofilament yarns are bent. However, since the fiber crossover points are not bonded, the total bending strain is distributed over longer lengths of yarn. The resistance to compression can still be significant but the local fiber strains are much lower than in the rigid networks. These networks can sustain much greater total compression, e.g., 60% or more, without any fiber kinking. Consequently, these networks are intrinsically softer than the prior art rigid networks but are highly resilient. Recovery from repeated 50% compressions is typically 95% to 100%.
Because of the bending stiffness of the large-diameter monofilaments used therein, the network structures formed by the Kim et al. method exhibit a nearly Hookean resistance to compression and exhib
Bessey William E.
Felton Clinton Dale
Haas Joseph S. W.
Rumierz Gerald Peter
Dixon Merrick
Jenkins & Wilson, P.A.
North Carolina State University
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