SOLID-STATE STAMPING OF FIBER-REINFORCED THERMOPLASTIC SEMI-CRYSTALLINE SHEET COMPOSITES

Abstract: Method for solid-state stamping of fiber-reinforced thermoplastic composites. Such method comprises: (a) restricting said thermoplastic to be a semi-crystalline thermoplastic polymeric material; (b) heating said composite in an oven to a temperature of less than the peak melting temperature of said polymeric material; (c) transferring said heated polymeric material from said oven to a mold and permitting said polymeric material to cool during said transferring; and (d) stamping said composite in said mold, the temperature of said composite at the commencement of stamping being less than the peak melting temperature but greater than the crystallization onset temperature of said composite, said temperatures determined by differential scanning calorimetry of said composite.

Description: SOLID-STATE STAMPING OF FIBER-REINFORCED THERMOPLASTIC SEMI-CRYSTALLINE SHEET COMPOSITES Background of the Invention The present invention relates to thermoplastic sheet composites and more particularly to the stamping of such sheet composites to make three- dimensional parts. Composite materials represent a fast growing segment of the plastics industry because of their capability of providing the high strength and stiffness exhibited by metals, but at a greatly reduced part weight. Composite materials additionally can display resistance to chemicals and provide a cost savings in many applications. Parts fashioned from fiber- reinforced polymeric sheet composites can exhibit high mechanical properties due to the fiber reinforcement with the polymer matrix providing the appropriate shape, resistance to chemicals, and like properties. Most currently-available composites are formed from thermosetting polymers including polyesters, epoxies, vinyl esters, polyimides, and the like. Thermosetting polymers, while providing adequate performance in many applications, do suffer from a number of disadvantages. Because thermosetting polymers are formed from reactive materials, shelf life and storage stability are a prime concern to the manufacturer and to the user. Additionally, scrap cannot be reused once the components have been reacted to form the ultimate thermosetting polymer. Often too, thermosetting polymers suffer from brittleness and insufficient impact resistance. Thermoplastic polymers are not reactive which eliminates the storage problems encountered when dealing with thermosetting polymers. Additionally, scrap can be reused and mechanical properties, chemical resistance, and the like often can be equivalent to such properties of thermosetting polymers. The major disadvantage in using thermoplastic polymers is their relatively high melt viscosity which makes penetration of the polymer melt into the fiber bundles difficult. Thermosets, on the other hand, are typified by low viscosity components which readily flow among the fibers. Additionally, long term creep properties of thermoplastic matrix composites are unknown since their use in the marketplace has not found wide acceptance. Heretofore, some thermoplastic polymers have been utilized in injection molding processes to make parts; however, such parts typically are not highly reinforced. In forming stamped parts from thermoplastic polymers, the art traditionally has taught that the polymer should be heated to a temperature just above the melt temperature of the polymer followed by stamping of the resulting polymeric melt. Such proposals can be found, for example, in U.S. Pats. Nos. 3,713,962; 4,098,943; and 4,269,884. U.S. Pats. Nos. 4,399,085 and 4,479,998 propose to foam the thermoplastic polymer melt and stamp such polymer melt for forming a foamed part. U.S. Pats. Nos. 4,263,364; 4,379,801; and 4,379,802 propose to rapidly quench heated thermoplastic polymeric material for forming an amorphous polymer sheet. The resulting amorphous polymer sheet then can be stamped by heating the sheet to a temperature which is above the glass transition temperature of the polymer but is less than the melting point of the polymer. Finally, U.S. Pat. No. 4,014,970 proposes to solid-state stamp thermoplastic polymer composite sheets by heating the polymer sheets to a temperature just below that at which the material is no longer solid and subjecting the heated material to high compressive strain to cause an abrupt flow of the material throughout a mold cavity. A relative velocity of one portion of the mold to another portion of the mold of at least 6.4 cm/sec is taught in this patent. This patent is important in that it teaches that solid- state stamping of thermoplastic polymers is possible. Unfortunately, many practical difficulties arise by practicing such process. For example, on an assembly line it is quite difficult to properly gauge accurately and reproducibly the precise temperature control required to maintain the thermoplastic material at a relatively high temperature whereat it is almost, but not quite, at its melt point. Further, practical problems arise as to how the material is going to be maintained in a heated condition for placing in the mold which must be kept at room temperature in accordance with such disclosure. Note, for example, in Example 2 that a 5°F temperature range for molding polypropylene is taught to be necessary. Clearly, implementation of such process on a commercial scale would not be without substantial difficulty. Broad Statement of the Invention The present invention relates to a method for solid-state stamping of fiber-reinforced thermoplastic composites and to a method for predicting suitable, relatively broad temperature ranges whereat the thermoplastic composites can be stamped in the solid state. The method for solid-state stamping of fiber reinforced thermoplastic composites includes restricting the thermoplastic material to be a semi-crystalline thermoplastic polymeric material. A sheet of the thermoplastic composite is heated in an oven to a temperature which is less than the peak melting temperature of said polymeric material and such heated polymeric material then is transferred from the oven to a mold. The polymeric composite sheet is permitted to cool during the transferring of the sheet from the oven to the mold. Finally, the composite sheet is stamped in the mold. The temperature of the composite sheet at the commencement of the stamping operation is less than the peak melting temperature of the semi-crystalline thermoplastic material, but is at a temperature which is greater than the crystalization onset temperature of the composite. The peak melting temperature and the crystalization onset temperatures are determined by differential scanning calorimetry of the composite. The ability to accurately predict a temperature stamping range for semi-crystalline thermoplastic composites has many advantages. One advantage is that the part need not be cooled excessively prior to its removal from the mold. This translates into another advantage which is the dramatically increased throughput which the present invention permits in molding thermoplastic parts. A further advantage is that special precautions for handling the heated thermoplastic composite from the oven need not be undertaken because the cooling rate of the sheet from the oven can be predicted accurately. Such accurate temperature prediction results in the ability to set up an assembly line with known distance, i.e. time, between the oven and the mold for reliably stamping at a predetermined temperature of the part. A further correlary advantage is that the temperature of the mold no longer is important and no special cooling of the mold is required, e.g. mold temperatures from room temperature up to 80°- 100° C or higher, depending upon the composite material, may be used to advantage. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein. Brief Description of the Drawings Fig. 1 graphically portrays the cooling rate of several thermoplastic composite sheets which were heated prior to stamping to various .oven temperatures; Fig. 2 is a composite differential scanning calorimetry curve for polybutylene terephthalate; Fig. 3 is a composite differential scanning calorimetry curve for glass fiber-reinforced polybutylene terephthalate; Fig. 4 is a composite differential scanning calorimetry curve for polypropylene; Fig. 5 is a composite differential scanning calorimetry curve for glass fiber-reinforced Nylon 12; Fig. 6 is a composite differential scanning calorimetry curve for polyethylene terephthalate; Fig. 7 is a composite differential scanning calorimetry curve for polyphenylene sulfide; and Fig. 8 is a composite differential scanning calorimetry curve for polyether ether ketone. The drawings will be described in detail in connection with the working examples which follow. Detailed Description of the Invention The present invention is unique in several respects. Initially, the reinforcing fiber content limitation suffered by injection molding (about or 25% or less) thereabouts) is not a limitation suffered by the stamping process of the present invention which can function effectively at fiber contents in excess of 40% and on up to 70% or more, depending upon the fiber length, composition, weaving, aspect ratio, and like factors. A further advantage is that the stamper need not operate with a thermoplastic composite sheet which has been heated to a temperature whereat the continuous thermoplastic phase is molten. Moreover, the unexpected discoveries upon which the present invention is based even do away with the notion taught in U.S. Pat. No. 4,014,970 that solid-state stamping of thermoplastic composites can only take place when the composite is heated to a temperature which is just below the melt temperature of the composite. In fact, following the heating regimen set forth herein, the thermoplastic composite can be stamped at temperatures as much as 25°- 30° C below the stamping temperature taught in the '970 patent. Another conventional wisdom which has been dispelled is the need for special handling procedures to convert the semi-crystalline thermoplastic material to its amorphous state in order to stamp it as taught in U.S. Pat. No. 4,263,364 and its progeny. The solid-state stamping process of the present invention retains conventional benefits of solid-state drawing, much like the benefits retained in analogous metal stamping processes. In the case of fiber-reinforced composites, fiber orientation certainly is one such advantage retained by the solid-state stamping process of the present invention. Moreover, it is axiomatic that the less energy that needs to be inputted in the stamping operation, the less energy is required to be removed during the stamping procedure. This translates into much shorter molding times being required by the inventive process with concomitant higher throughput of parts. The present invention has its origins in experiments conducted whereat thermoplastic composites were heated to a temperature just short of the melt temperature of the thermoplastic composite followed by cooling of the composite to various temperatures for stamping of the composite. When a ladder series of composites were stamped at various stamping temperatures, it was noted that some stamping temperatures resulted in unacceptable parts, unacceptability being defined as insufficient tensile strength, though other criteria certainly are applicable, e.g. bulk modulus of elasticity, flexural modulus, impact strength, and like properties. The difficulties presented by this data involve the correlation of the data to a provable theory whereby prediction of properties and stamping conditions, and their correlation was feasible. The next step in the saga involves the investigation of the various thermal properties and thermal responses which various semi-crystalline thermoplastic materials evoked. Differential scanning calorimetry was resorted to as a method for determining such thermal responses. It is known that a thermoplastic material exhibits an exotherm about its melt temperature when heated. A semi-crystalline material undergoes a transformation from solid-state to liquid state over this temperature range with the concomitant release of energy. It also is known that semi- crystalline thermoplastic substrates when cooled from such melt temperature range exhibit a second exotherm which commences at about the onset temperature of crystalization, i.e. that temperature at which energy is released due to crystalization of the polymer. Quite unexpectedly, it was discovered that the temperature stamping range empirically determined coincided with the temperature range bounded by the melt exotherm and the crystalization exotherm of the thermoplastic material, as such exotherms are determined by differential scanning calorimetry (DSC). With respect to the temperature range or window for solid-state stamping in accordance with the inventive process, it will be appreciated that the upper temperature limit to which the composite sheet must be heated is less than the melt point of the thermoplastic material and often is several degrees below such temperature. That is, there is a given temperature range over which the exotherm commences and prior to which it maximizes at the melt temperature of the thermoplastic material. It was even more unexpectedly discovered that the thermoplastic composites need only be heated to a temperature at about the mid point of such exotherm and even somewhat less (e.g. about 40%) in order for solid-state stamping to be efficacious. For example, for PBT (see Fig. 2) the melt exotherm commences at about 205° C with a peak melt temperature being recorded at about 222° . The PBT thermoplastic sheet need not be heated to about 220° C, but to only about 210°-215° C (and then cooled to a temperature down to 185° C) for solid-state stamping. It should be recognized that reinforcing fiber typically acts to seed crystallization so that the lower temperature limit (crystallization onset temperature) will vary accordingly. Further, it will be appreciated that the crystallization temperature can be lowered by controlled rapid cooling of the thermoplastic provided that the cooling rate is not so rapid as to form the amorphous form of the thermoplastic. The composite sheet may be stamped at such upper temperature limit or any temperature lower until the crystalization onset temperature is encountered. Over this temperature range or window properties of the ultimate part may vary so that optimization of particular properties of the part may be correlated to particular stamping temperatures within the window. Determination of such optimum stamping temperatures as correlated and related to various properties of the stamped parts is well within the skill of those in this art field. Now, for the first time, composite sheets of known fiber reinforcement kind and content, and known semi-crystalline thermoplastic continuous phase composition, can be evaluated in accordance with another aspect of the present invention and the stamping temperature or stamping temperature window controlled for such composite sheet. Moreover, this aspect of the present invention further enables the artisan to design an assembly line layout whereat oven heating time, placement of the oven with respect to the mold with concomitant handling times therebetween, mold closing time, and part removal from the mold can be designed with a high degree of predictability and certainty. That is, a particular stamping temperature can be determined ahead of time and correlated to a particular property or group of properties which the part desirably should possess. Once this stamping temperature or range of temperatures is known, it will be known how much the heated sheet can be cooled prior to its entry into the mold following its removal from the oven. Solid-state stamping simplifies handling procedures and the predictive power of this aspect of the present invention enables the sheet to enter the mold at the correct temperature by permitting natural cooling of the part to take place between the oven and the mold. This natural cooling phenomena which is not to be avoided, but may be quite desirable in order that the part be cooled to the appropriate stamping temperature, also can be predicted and determined ahead of time. Further tests conducted during the course of investigation on this invention revealed the fact that filled and unfilled thermoplastic composite sheets exhibit a predictable and reproduceable cooling rate upon removal from the oven, transfer in air to the mold, and during the molding operation itself. Based upon these cooling curves, the time interval between the oven and the mold can be determined for any particular composite being subjected to the molding operation. The predictability and the reproduceability of results realized in accordance with the present invention clearly is a significant advancement in the composite stamping art. Precise details on these various aspects of the invention will be more fully set forth in the Examples which follow. A variety of semi-crystalline thermoplastic materials find use in the present invention. Such materials include a variety of homopolymers and copolymers including, for example, olefins, polyethers, aromatic polyesters, polyamides, polyacetals, and the like. The thermoplastic material need only be semi-crystalline in order for use in accordance with the precepts of the present invention. Specific preferred semi-crystalline thermoplastic materials which have found efficacy in the present invention include, for example, polypropylene, Nylon resins, polybutylene phthalates, polyethylene terephthalates, polyphenylene sulfides, and polyether ether ketones. While the process works, efficiently and effectively on unfilled thermoplastic materials, the true value of the present invention involves the fibrous reinforcement of thermoplastic materials to form a variety of single or multiple-ply composites or laminates. Fibrous reinforcement is not a limitation in that conventional glass, carbon, various polymers (e.g. polyamides), metal fibers, and the like find utility in the stamping process of the present invention. Moreover, the fibers may be in the form of continuous or chopped mats, woven fabric (e.g. plain weave, twill weave, etc.), optionally surface treated with various coupling agents or wetting agents for improving impregnation, wetting, thermoplastic adhesion, and the like properties. As noted above, fibrous reinforcement includes contents from as low as a few percent on up to about 70%. The composite sheets subjected to the stamping process can be formed by a variety of conventional and unconventional techniques. One conventional technique involves melt impregnation which may be accomplished statically or by a roll impregnation technique. Another conventional technique involves ply consolidation utilizing pressed prepegs as is well known in this art field. Wet forming processes also find utility in forming the composite sheets to be stamped and preferably that wet forming process disclosed in commonly-assigned application of Hiscock entitled Wet-Laid, Non-Woven, Fiber-Reinforced Composites Containing Stabilizing Pulp and Process of Making Same, USSN 820,485 , filed January 17, 1986 (Attorney's Docket No. p-p 2681 ), the disclosure of which is expressly incorporated herein by reference. The Hiscock wet forming process is very similar to a conventional Fourdrinier paper making process wherein a dilute water slurry of reinforcing fibers and polymer powder are filtered out on a fine mesh screen to produce a mat of fibers and polymer which then can be consolidated under heat and pressure to form a composite sheet. The inventive wet forming process utilizes a minor proportion of fibrous pulp in order to substantially augment the wet forming process. Such wet forming process can be used to advantage to form composite sheets for stamping in accordance with the precepts of the present invention. The following examples show how the present invention has been practiced but should not be construed as limiting. In this application, all percentages and proportions are by weight and all units are in the metric system, unless otherwise expressly indicated. EXAMPLE 1 Characterization of the polymers utilized in these examples is set forth below. TABLE 1 Glass Melt Tensile Modulus of Polymer Transition Temp Temp Strength Elasticity Density (Tg, ° C) (Tm, ° C) (Tp, MPa) (E, GPa) ( , g/cc) Polypropylene (PP) -20 162 31 1.50 0.91 Nylon 12 (N 12) 42 177 55 1.24 1.02 Polybutylene . 40 223 52 2.50 1.32 Terephthalate (PBT) Polyethylene 80 257 70 3.10 1.38 Terephthalate (PET) Polyphenylene 93 275 70 3.80 1.34 Sulfide (PPS) Polyether Ether 143 338 95 3.85 1.36 Ketone (PEEK) Additional characterization of the polymers as used in the composites included a determination of the rate of cooling of the samples in indoor ambient air while being transferred from the oven to the mold for stamping. Before stamping, the composite samples were dried at 80° C for four hours and then placed in a small oven located near the mold for heating to a predetermined temperature. The actual temperature of the sample was monitored by embedding a thermocouple in the middle ply of a five-ply laminate at a point one cm. interiorly of the center of one side of the 5 cm. x 7.6 cm. sample. The heated sample was removed from the oven, placed in the mold, and the mold closed. The molds were composed of tool steel and could be closed in less that 2 seconds for stamping. One mold was hat-shaped (trapezoidal) with a depth of 13 mm of an angle of 60° to the horizontal plane of the part. The second mold was hemispherical in shape with a radius male half of 15.9 mm. Shims were used to accommodate various sheet thicknesses. The overall stamping cycle time from removal of the sample from the oven to opening the mold was approximately 10 seconds. The thermal responses of the various thermoplastic composite parts evaluated is depicted graphically in Fig. 1. The results set forth in Fig. 1 for the four different polymers evaluated confirm the predictibility of the uniform manner in which the samples cooled upon removal from the oven. The temperature of the sample during molding, therefore, can be pedicted from these curves. Additionally, the temperature of the sample at the moment the mold is closed can be predicted from these curves. This can be important for an assembly line operation where plant configuration, handling equipment, handling procedures, and production rates often dictate the relative location of the oven and the mold and their operation. EXAMPLE 2 Experiments to determine a temperature range of stampability of PBT-twill glass fabric composites (48 wt-% fiber loading) involved stamping temperatures ranging between 150° C and 260° C. The mold was maintained at a temperature of 87.7° C (190°F). Visual inspection of the parts revealed that acceptable parts could be stamp-formed at temperatures of between 185° C and 220° C. Below 185° C, the parts would break because the thermoplastic could not flow or deform, but could only fracture. Above 220° C, the polymer would flow excessively and thereby severely distort the fabric. In order to understand this data, resort to Differential Scanning Calorimetry (DSC) was made in order to ascertain any relationship between the stamping data and the thermal response of the polymer (or composite). DSC runs involved heating the sample to above its melt point followed by cooling the sample at about its ''natural" cooling rate (that cooling rate determined in Example 1 and displayed in Fig. 1). The DSC curve for PBT is depicted in Fig. 2 which is a composite of two separate curves. That is, the second peak corresponds to the thermal response of the thermoplastic when heating to a temperature above its melt point. Upon cooling of the heated sample, the first peak is encountered from the high temperature side. The DSC curve for PBT in Fig. 2 shows an exotherm at about 205° C with a melt point of PBT determined at about 222° C. Note that the DSC melt point corresponds closely with the upper temperature limit for solid-state stamping of PBT. Next, upon cooling of the sample, a second exotherm commencing at about 185° C occurs which exotherm corresponds to the onset of crystallization of PBT. This crystallization onset temperature also corresponds to the lower temperature limit for solid stamping of PBT. Thus, within the temperature range bounded by the thermoplastic melt temperature and the crystallization onset temperature (as determined by DSC), the thermoplastic material possesses sufficient molecular mobility to deform during stamping. An important advantage of this solid-state stamping temperature range is that no special cool-down of the sample in the mold is required. As soon as the sample exhibits a temperature of below its crystallization onset temperature, the mold can be opened. Importantly, the crystallization onset temperature can be predicted from Fig. 1 for timing of the stamping cycle. Faster cycle times now are possible with concomitant higher throughput. EXA MPLE 3 Next, the effect of fiber reinforcement on the stamping temperature window determined in Example 2 was investigated. The PBT-glass fiber composite consisting of continuous fiber glass fabric of 8 harness satin weave, as supplied by J.P. Stevens, style 1581 (44.1 wt-% glass fiber) was subjected to DSC analysis. The results thereof are graphically set forth in Fig. 3. The effect of fiber reinforcement is clear in Fig. 3. The presence of the glass has shifted the crystallization endotherm to a higher temperature range. Thus, the lower temperature limit for stamping has been increased to about 195° C, apparently due to seeding effects displayed by the glass fibers. DSC curves for PP, glass fiber reinforced Nylon 12 lactam of 12- aminododecanoic acid, furnished by Atochem, Paris, France) PET, PPS, and PEEK are set forth in F igs. 4-8, respectively. Based on these data, recommended solid-state stamping temperature ranges for the various polymers investigated are set forth below. TAB: E 2 Thermoplastic Lower T emp. Upper Temp_3tampin g Window (°C) (°C) ( °C) PP 115 160 45 Nylon 12 (reinforced) 150 175 25 PBT 185 220 35 PET 220 255 35 PPS 240 275 35 PEEK 305 335 30

Claims: CLAI S 1. In a method for solid-state stamping of fiber-reinforced thermoplastic composites, the improvement which comprises: (a) restricting said thermoplastic to be a semi-crystalline thermoplastic polymeric material; (b) heating said composite in an oven to a temperature of less than the peak melting temperature of said polymeric material; (c) transferring said heated polymeric material from said oven to a mold and permitting said polymeric material to cool during said transferring; and (d) stamping said composite in said mold, the temperature of said composite at the commencement of stamping being less than the peak melting temperature but greater than the crystalization onset temperature of said composite, said temperatures determined by differential scanning calorimetry of said composite. 2. The method of claim 1 wherein reinforcing fiber ranges up to about 70% by weight. 3. The method of claim 1 wherein said fiber is composed of glass, carbon, or polymer. 4. The method of claim 1 wherein the cooling rate of said composite during said transferring and said stamping is determined. 5. The method of claim 1 wherein said thermoplastic material is selected from the group consisting of olefins, polyethers, aromatic polyesters, polyamides, and polyacetals.

Inventors: PRESTON JOSEPH R, PRESTON, JOSEPH, R.
Applicants: BATTELLE MEMORIAL INSTITUTE
Application year: 1987
Main classification: B29C43
IPC codes: B26F1, B29C43, B29K105