Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-02-09
2003-12-02
Dixon, Merrick (Department: 1774)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S433000
Reexamination Certificate
active
06656316
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns fiber reinforced compositions and articles and methods for producing such compositions and articles.
In particular, the present invention provides an apparatus and a method of prepregging reinforcing materials such as fibers with resinous or polymeric materials, especially with thermoplastic resin or polymer compositions, and the prepregs produced by such apparatus or method. The invention further provides a method of using such prepregs to form articles having highly desirable properties and to the articles produced thereby.
BACKGROUND AND SUMMARY OF THE INVENTION
Reinforced thermoplastic and thermoset materials have wide application in, for example, the aerospace, automotive, industrial/chemical, and sporting goods industries. Thermosetting resins are impregnated into the reinforcing material before curing, while the resinous materials are low in viscosity. Thermoplastic compositions, in contrast, are more difficult to impregnate into the reinforcing material because of comparatively higher viscosities. On the other hand, thermoplastic compositions offer a number of benefits over thermosetting compositions. For example, thermoplastic prepregs are easier to fabricate into articles. Another advantage is that thermoplastic articles formed from such prepregs may be recycled. In addition, a wide variety of properties may be achieved by proper selection of the thermoplastic matrix.
Fiber-reinforced plastic materials may be manufactured by first impregnating the fiber reinforcement with resin to form a prepreg, then consolidating two or more prepregs into a laminate, optionally with additional forming steps. Consolidation is typically necessary to remove voids that result from the inability of the resin to fully displace air from the fiber bundle, tow, or roving during the processes that have been used to impregnate the fibers with resin. The individually impregnated roving yams, tows, plies, or layers of prepregs are usually consolidated by heat and pressure, or with heat and vacuum as by vacuum-bag molding and compacting in an autoclave. The consolidation step has generally required the application of very high pressures or vacuums at high temperatures and for relatively long times. Alternatively, the prepreg fiber bundle is chopped or pelletized and then used in a molding or extrusion process, with or without other polymeric materials or reinforcements, to produce molded or extruded articles.
In the past, a thermoplastic composition has typically been heated, slurried, commingled, or diluted with solvents, plasticizers, or other low molecular weight materials in order to reduce the viscosity of the composition before it is used to impregnate the reinforcing material. These methods have suffered from serious drawbacks. In the case of using solvent to reduce viscosity, the solvent must be driven off after the impregnation step, resulting in an additional step in the process as well as unwanted emissions. Moreover, the desired thermoplastic composition may be insoluble in or incompatible the desired diluent.
In the case of heating the thermoplastic matrix composition to a temperature at which its viscosity is low enough for satisfactory impregnation of the fiber, the dwell time of the resin in the heated zone may result in degradation of the resin with attendant decrease in the desired mechanical properties. Furthermore, the molecular weight of the resin may need to be kept lower than would be desired for properties of the ultimate product in order to facilitate the impregnation step. Finally, as noted above, known processes for impregnating thermoplastic resin into reinforcing materials have required lengthy consolidation of the prepreg materials at high temperatures and pressures in order to develop the best physical strength and other properties and to minimize or eliminate outgassing during the consolidation or in later steps, e.g., finishing processes. Outgassing during consolidation results in voids within the composite that can cause microcracking or premature delamination that may adversely affect mechanical properties; outgassing during coating steps tends to cause pinholling or popping in the substrate or coating, resulting in an undesirably rough and blemished surfaces or finishes.
Cogswell et al., U.S. Pat. Nos. 5,213,889; 5,019,450; 4,559,262; and 4,549,926 (all of which are incorporated herein by reference), teach that impregnation of fibers with thermoplastic polymers requires (1) a relatively low molecular weight polymer to give sufficiently low melt viscosity (less than 30 Ns/m
2
; Table 1 showing increasingly poor fiber wetting as the 30 Ns/m
2
limit is approached), (2) having within the molten polymer bath a spreader with an external heat input to heat the polymer in the immediate vicinity of the spreader to a relatively high temperature, (3) slow rates of throughput (Table 2 showing significantly decreased fiber wetting at 60 cm/min, as compared to 36 cm/min), and (4) long dwell times of the fibers in the molten polymer bath (dwell time shown in Examples 1 and 5 to be 30 seconds). Conventional grades of thermoplastic material (i.e., having the molecular weights generally using in forming articles from thermoplastic materials) cannot satisfactorily wet out fibers in the Cogswell process, as is shown by patentee's Example 4.
The present invention provides the ability to form prepregs using higher molecular weight polymers in order to achieve the highest possible strength and to optimize other physical properties in articles formed therefrom. In addition, the present inventive process is lower cost and more efficient than previous processes because the present process allows a faster throughput rate and shorter dwell time in the molten polymer bath, while at the same time producing a prepreg in which the fibers are substantially fully wet out.
In another previously known process, Cogswell et al., U.S. Pat. Nos. 4,783,349; 4,735,828; and 4,624.886 (all of which are incorporated herein by reference), use a low molecular weight plasticizer to reduce the melt viscosity of a thermoplastic impregnating resin. The filaments of the fiber bundle are pre-wetted with the plasticizer before entering the molten polymer bath, which may include a further metal plasticizer. The plasticizer is removed by volatilization when it is not desired in the final product. Thus, production of materials with high strength and other physical properties for which plasticization is undesirable requires not only the pre-wetting step but also a step of evaporating or otherwise extracting the plasticizer following impregnation. The process is also illustrated for fabrics woven of reinforcing fibers, and patentees teach that the process does not require significant mechanical work input in the form of tensioning the fiber. Cogswell et al., in U.S. Pat. No. 4,541,884 (which is incorporated herein by reference), teach a process in which a plasticizer is incorporated in the molten thermoplastic polymer bath to decrease the viscosity of the molten bath. The plasticizer is volatilized from the prepreg in a further process step.
It is undesirable to plasticize the polymer in many applications, for example when higher tensile strength is important. Moreover, the extra steps of impregnating a tow or fiber bundle with a plasticizer and volatilizing or extracting the plasticizer after the impregnation step add expense and make the process cumbersome.
Cochran et al., U.S. Pat. No. 5,236,646 (which is incorporated herein by reference), disclose that a process using vacuum of up to about 28 inches of mercury below atmospheric pressure and temperatures above the melting point of the resin requires a shorter time for consolidation as compared to a process that uses high consolidation pressures of from about 100 to 300 psi. However, the consolidation step still requires a dwell time under vacuum of up to sixty minutes or more.
Because the length of time typically required to properly consolidate the prepreg plies determines the production rate for the part, it w
Dixon Merrick
Harness & Dickey & Pierce P.L.C.
LandOfFree
Method of prepregging with resin and novel prepregs produced... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of prepregging with resin and novel prepregs produced..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of prepregging with resin and novel prepregs produced... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3176930