Process and apparatus for resin impregnation of a fibrous substrate

Abstract: An apparatus and a process are described for impregnating a fibrous substrate with a thermosettable resin. The process involves the use of resin application means comprising a moving surface on which is positioned a liquid-form thermosettable resin in essentially uncured state, passing a fibrous web in countercurrent contact with the thermosettable resin so as to transfer the thermosettable resin into the fibrous web, and passing the resin-containing fibrous web to a heating zone to partially cure the resin and form a prepreg. The process is particularly suited for application of a solventless resin formulation to a glass web in the preparation of a prepreg for an electrical laminate. The apparatus includes resin application means comprising a movable surface; means for applying a liquid-form thermosettable resin onto the movable surface; means for advancing, in a countercurrent direction with respect to the direction of motion of the movable surface, a fibrous web to the movable surface and in contact with the thermosettable resin thereon and thence to a resin cure zone; and means not in contact with the opposite side of the fibrous web at the point of resin transfer for maintaining tension in the glass web sufficient to promote transfer of the liquid resin film from the movable surface into the interior of the glass web.

Description: The present invention relates to a process for the preparation of prepregs as indicated in the preamble of claim 1 and an apparatus for impregnating a fibrous web as indicated in the preamble of claim 11. A process of the indicated type is known from AT-B-356 390; in said publication also the principle of a possible apparatus is described. The known process uses a solvent to formulate a resin solution. The present invention aims to provide a process in which the use of solvent can be avoided and to provide an apparatus with which such a process can be carried out. According to the invention the process of the described type includes the aspects as given in the characterizing part of claim 1. Further embodiments of above process are given in dependent claims 2-10. Further according to the invention the apparatus of the described type includes the aspects as given in the characterizing part of claim 11. Expedient embodiments of said apparatus include the aspects as given in dependent claims 12-13. The manufacture of the cured thermosettable resin base of an electronic circuit board begins with the impregnation of a fibrous glass substrate with a liquid thermosettabte resin system. The resin-impregnated substrate is then partially cured to form a "prepreg." A set of layered prepregs is then heated under pressure to fully cure the resin and to form a hard laminate, which serves as the base for electric circuitry. Although there exist thermosettable resins, such as low molecular weight epoxy resins, which are liquid at room temperature, current circuit board requirements make it necessary to use high-performance resins systems which are solids or viscous liquids at room temperature and to apply the resins to the substrate in melt or solution form. Attempts to process thermosettable resins in the melt, however, have not been successful because of the difficulty of achieving good "wet-out," or saturation of the fiber by the resin, and also because the high temperatures necessary to melt the resin cause the resin to cure prematurely, further adding to the wet-out problem. Current commercial processes for preparing prepregs apply the resin to the substrate using an organic solution of the resin. Solution processes must include a step, usually carried out in conjunction with partial curing of the resin, in which the solvent is removed from the prepreg by heating the solvent to its volatilization temperature. Such a process has a number of disadvantages: First, it requires the disposal or discharge of the organic volatiles. Second, volatilization of the solvent from the uncured resin can result in the presence of voids and irregularities in the prepreg and in the cured laminate. Furthermore, a considerable amount of time is required for the solvent removal step. A method for applying resin to the substrate which did not require solvents would thus have environmental, quality and efficiency advantages. Processes for applying liquid-form resins to the substrate include passing the substrate through a resin bath, as illustrated in U.S. patent No. 4,767,643, and coating a non-porous release sheet with liquid resin and then pressing the release film against the porous substrate to transfer the resin thereto, as described in U.S. patent No. 4,139,591. The former technique suffers from problems associated with the tendency of resin in the reservoir to "advance." or partially cure, if it is not immediately taken up by the substrate, and the latter method suffers from the inconvenience and expense of processing the release sheet. It would be desirable to develop techniques for resins application which do not involve the use of a resin bath or a release sheet. The process of the invention provides a technique for impregnating a fibrous substrate with a liquid-form thermosettable resin. The process of the invention is particularly suitable for impregnating a glass web with a solventless resin system in the preparation of a prepreg for ultimate use in electrical laminates. The process involves contacting an advancing tibrous substrate with a liquid-form resin positioned on a moving surface and transferring, by countercurrent contact, the resin from the moving surface to the fibrous substrate and into the interior thereof. As used herein, such a fiber application process involves impregnating the resin into the interior of the fibrous substrate and is to be distinguished from coating only the exterior surface of the substrate. The object of the resin application process of the invention is to achieve thorough wet- out of the substrate by the liquid resin and to thereby permit the fabrication of a high-quality cured laminate from the resin-impregnated fibrous substrate. In the resin application process according to the invention, a fibrous substrate is impregnated with a liquid-form thermosettable resin. The resin system used is one which does not contain an organic solvent, which includes both water-borne resin systems and solventless resin systems. For a solventless resin system, the liquid form can be achieved by use of a thermosettable resin which is a low-viscosity liquid at room temperature or which has been heated to a temperature effective to achieve sufficiently low viscosity for thorough wetout of the substrate. In the latter case, of course, the resin system (the thermosettable resin and any curing compounds used therewith) must not cure to any substantial degree at its melting temperature over the length of time of the substrate impregnation process. The resin application process and apparatus of the invention can be described by reference to Figure 1. Figure 1 is a cross sectional diagram of a preferred embodiment of an apparatus according to the invention. The substrate 2 in web form, generally any fibrous material in chopped, mat or woven form, preferably a web of woven glass fibers, is advanced from delivery means 1, which will generally include automatic means for advancing the web at a selected rate and with a selected web tension. The fibrous web is optionally heated by, for example, infrared heaters, prior to advancement to the resin application zone. Guiding means 3 is positioned to direct the web toward resin applicator roll 4 at a predetermined arc of contact α. Angle α can vary depending upon the overall configuration of the application scheme, but will generally be within the range of from 20 to 90 degrees, preferably of from 20 to 40 degrees, most preferably of from 25 to 34 degrees in the embodiment shown. Resin application means 4, which can be, for example, a roller and is shown here as a roller rotating in a counterclockwise direction, delivers liquid resin film 5 to a first surface of web 2 passing counterdirectional thereto. Applicator roll 4 is maintained at a temperature effective to keep resin film 5 in essentially uncured, liquidform. This temperature will vary depending upon the resin, butwill generally be within the range of from 50 to 200°C. The speed of rotation of applicator roll 4, the tension in web 2 as it contacts resin fi Im 5, and the speed at which web 2 is advanced to the applicator roll are coordinated to provide good wetout of the web. These specifications can vary widely depending, for example, upon the resin system, the type of web material, and the heating capacity of the downstream B-staging unit. In general, the speed of rotation of applicator roll 4 will be within the range of from 70 to 125 percent of web speed, preferably of from 90 to 100 percent of web speed; the tension in web 2 will generally be within the range of from 0.179 to 0.537 kilograms per linear centimeter, preferably of from 0.27 to 0.36 kg/cm; and the speed of advancement of the web through the resin application zone will be within the range of from 2.4 m/min to 61 m/min, preferably of from 15.3 to 45.8 m/min. Resin film 5 is applied to applicator roll 4 by means of resin delivery means, shown here as a combination of set gap roll 8 and nozzle 7 capable of applying a controlled quantity of liquid resin to the rotating surface of the applicator roll. Nozzle 7, represented here in cross-section, can be a tube whereof the axis is parallel to the length of the roll, having one or more exit ports for application of the liquid resin system to the roll. Blade 9, located closely adjacent the area of resin application to roll 4 and in contact with set gap roll 8, can be used to prevent movement of newly-deposited resin on the surface of roller 8 as it rotates and to define a small well or bead of active resin in the set gap area. Nozzle 7 can be associated with any means for continuous delivery of the resin in liquid form, at either ambient or elevated temperature. Delivery of the resin will be carried out at volume rates synchronized with the speed of the moving web so as to deliver a predetermined volume of resin to the web and to minimize residence time within the resin delivery system. Resin delivery means can include, for example, a temperature-controlled static blender or a mixing extruder with an outlet into nozzle 7. Metering means, shown here as a set gap roller 8 located between nozzle 7 and the point of contact of resin film 5 and the advancing web, is used, in conjunction with resin delivery means 7, to control the amount of liquid resin which is delivered to the web. Set gap roll 8 is preferably a smaller-diameter roll than applicator roll 4 so as to permit a generally vertical alignment of the set gap roll and the applicator roll and to minimize residence time of the liquid resin prior to application to the web. Control of the rate at which resin is applied to the web is achieved in the first instance by careful setting of the gap between set gap roll 8 and applicator roll 4 so as to maintain a uniform film thickness 5. Secondly, the rotational speed of the applicator roll is co-ordinated with web speed so as to achieve transfer of the resin film onto the moving web. In addition, because no backup roll is employed to control the contact of the moving web with the applicator roll, control of web tension is maintained to ensure stable operation of the resin application process. Resin removal means 10, shown here as a scraper blade located above discard trough 11, serves to remove from the applicator roll any resin which remains on the roll after contact between resin film 5 and advancing web 2, as any resin which has remained on the roll for a complete rotation is likely to have undergone excessive cure for application to the web. Resin-containing web 6 is advanced to optional second resin application means 12, shown here as a roller rotating in a direction counter to that of first roller 4. Second roller 12 can be used to smooth the second surface of the resin-impregnated web and can, if required, serve the optional function of applying additional liquid-form resin 11 to the web as desired to increase prepreg resin content. This second roller can optionally be heated. In the embodiment shown, liquid resin, the quantity of which is controlled by nozzle 15 and set gap roll 13, is applied to resin-containing web 6 to increase the resin content of the web and to fill in any interstices or voids in the resin-containing web. Roller 12 can serve the additional (or alternate, if no resin is applied to the web at this point) function of smoothing the resin on the surface of the web as the web is advanced toward cure zone 14. It is preferable to pass the resin-saturated web directly to the cure zone without cooling thereof by cooling means such as chilled rollers, for example. Second application means 12 is positioned between first application means 4 and cure zone 14. The web travel distance between the applicators can vary depending upon the other process variables, including the resin formulation, web porosity and web speed, but this distance will generally be within the range of from 0.30 to 1.52 meter, preferably of from 0.30 to 0.91 m. The resin application process is carried out in the absence of pressure applied to the web opposite the area of resin transfer from the applicator to the web. In certain conventional resin application processes, such pressure is applied by a backup roll contacting the web surface (or a release sheet in contact with the web surface) opposite the surface through which resin is being applied. In the invention process, such applicator backup rolls are undesirable, since said backup rolls may introduce too much strain in the web surface causing it to rupture. The use of a release sheet is an option with the single-roll embodiment of the invention process, but is not necessary or, in general, desirable in the practice thereof. The wetout of the web by the resin is achieved partially by pressure-driven flow at the applicator roll 4, but mainly by capillary action within the web. The capillary-induced flow of resin into the web depends upon the viscosity and surface tension of the resin as well as the porosity of the web and surface tension of the web fibers. For impregnation of conventional glass web styles, resin formulations having viscosities within the range of from 0.05 to 0.6 Pa.s, typically 0.1 Pa.s, and resin surface tension within the range of 250 to 400 µN/cm, typically 32 µN/cm, can be used. Resin infusion times will vary of from 0.1 second to 0.5 second under typical conditions. For a web speed of 65 m/min, for example, the web will move approximately 0.5 m in 0.5 seconds. Therefore, to ensure complete wetting of the web before it reaches the second applicator roll 12, at web speeds of the order of 65 m/min, the second applicator roll 12 will typically be positioned so that the points of web contact at applicator rollers 4 and 12 are approximately 0.7 m apart. Of course, modification of the process conditions and resin characteristics will require modification of the applicator specifications, including the distance between applicator rolls. Resin delivery means designed for use in the resin application process and apparatus of the invention can be described with reference to the schematic drawing in Figure 2. Resin from reservoir 20 maintained, by optional temperature control means 21, at a predetermined viscosity is passed via conduit 22, pump 23 and conduit 24 to mixing means 31, shown here as a static blender with internal blending baffles. Control system 29 delivers the desired proportion of resin and curing agent to the mixer, which is coordinated with the line speed of the web through the resin application zone by variable speed electric drive system 37. Curing compound(s) in reservoir 25 maintained, by optional temperature control means 26, in liquid form are passed via conduit 27, pump 28 and conduit 30 to mixing means 31, wherein the resin component and the curing component are intimately mixed at a controlled temperature maintained by temperature control means 32. The mixed resin formulation passes via nozzle 33 to applicator roll 35 and set gap roll 34, shown here with associated temperature control means 36, of the resin application apparatus described above. The prepregging process of the invention can be described in general terms by reference to the flow diagram presented in Figure 3. Fibrous web 43 is delivered to resin application zone 44 by a suitable automated web advancement system 42 with means for measuring and controlling advancement speed and web tension. Web tension control devices are known in the art. For example, unwind roll 41 can include a brake which, in combination with a front-end dancer roll, maintains a preset web tension programmed into a pull-in unit located between the dancer roll and the resin application zone. Similarly, proper downstream web tension can be maintained by a dancer roll which moderates the speed of a variable-speed constant-diameter roll located downstream from the heating zone. The fibrous web is advanced through resin application zone 44, which is here shown in the preferred generally vertical orientation, in which the web passes in a generally upward direction as liquid resin is applied by the method described in detail above. Application zone 44 includes resin delivery means, including a mixing portion for blending the resin and curing system, and temperature control as necessary to maintain the resin system at the desired viscosity. Resin-saturated web 45 is advanced from the resin application zone to resin cure zone 46, typically a forced air heated treater, wherein the resin-saturated web is treated, by exposure to elevated temperature or UV radiation, for example, to partially cure the resin without gelation, a process known as "B-staging." The temperature in the treatment zone will vary depending upon the resin system and the degree of resin cure desired, but will generally be within the range of from 80 to 200°C, preferably of from 120 to 180°C. The resin-saturated web will be subjected to the B-staging treatment for a time sufficient to impart the desired degree of cure, generally of from 30 seconds to 8 minutes. The web is advanced from resin treatment zone 46 in the form of a prepreg 47, which is rolled at uptake roll 48 for storage or, alternatively, is passed directly to lamination. A laminate is fabricated by subjecting a set of layered prepregs to conditions effective to cure the resin and to integrate the prepregs into a laminated structure. The laminate can optionally include one or more layers of a conductive material such as copper. Laminating conditions generally include a time of from 30 minutes to 4 hours, preferably of from 1 hour to 2 hours, a temperature of from 160°C to 300°C, preferably of from 170°C to 200°C and a pressure of from 3.4 to 34 atm. The laminate can optionally be "post-cured" by heating at a temperature of from 200 to 230°C at ambient pressure for 1 to 6 hours to improve thermal properties. Thermosettable resins which can be used in preparing electrical laminates include epoxy resins, imide resins, cyanate resins, propargyl ethers, and blends and reaction products thereof. The currently favored resins, because of their low cost and cured properties, are epoxy resins, alone or blended with another resin. Suitable epoxy resins for electrical laminates include glycidyl ethers of dihydric and polyhydric phenols. Exemplary die-poxy resins include those which are prepared by the base-catalyzed reaction of a bisphenol and an epichlorohydrin and can be represented by formula I: in which n is zero or a number greater than zero, commonly in the range of 0 to 10, preferably in the range of 0 to 2, and R is methylene or 2,2-propylene. An example of a suitable epoxy resin component is EPIKOTE 1123 (EPIKOTE is a trademark), a brominated diglycidyl ether of bisphenol-A having a molecular weight of 800. Also suitable as the epoxy resin component are multifunctional glycidyl ethers of the tetraphenol of ethane, as represented below by formula II. Such multifunctional epoxy resins are available commercially as EPON Resin 1031 from Shell Chemical Company. Other suitable resins can be prepared by the reaction of epichlorohydrin with mononuclear di- and trihydroxy phenolic compounds such as resorcinol and phloroglucinol, selected polynuclear polyhydroxy phenolic compounds such as bis(p-hydroxyphenyl)methane and 4,4'-dihydroxybiphenyl, or aliphatic polyols such a 1,4-butanediol and glycerol. The epoxy resin component of the composite can also be novolac-based epoxy resins ("novolac epoxy resins"), which are the glycidyl ethers of the product of reacting a phenol, such as phenol, cresol, resorcinol or bisphenol-A, with formaldehyde in acid solution. An example of a suitable class of bisphenol-A novolac epoxy resins is represented below in formula III. Otherthermosettable resins, alone and in combination with epoxy resins, can be processed into laminates by the process of the invention. Such thermosettable resins include, for example, cyanate esters, propargyl ethers, and vinyl esters, and blends of such resins with epoxy resins. Highly suitable thermosettable resins for electrical applications include imides such as bismaleimides and trismaleimides. Preferred bismaleimides include N,N'-bisimides of unsaturated carboxylic acids which can be represented by the formula IV: in which Y is a substituted or unsubstituted divalent radical containing at least 2 carbon atoms, preferably 2 to 6 carbon atoms, and a carbon-carbon double bond, and Z is a divalent radical containing at least 1, generally of from 1 to 40 carbon atoms. Z can be aliphatic, cycloaliphatic, aromatic or heterocyclic. A preferred class of bismaleimides is derived from an aromatic diamine and can be represented by the formula V: in which each R 1 is selected independently from H, C 1-2 alkyl or halide; R 2 is selected from divalent hydrocarbon radicals containing of from 1 to 6 carbon atoms, -O-, -SO 2 -, -COO-, -CONH-, -CO- and -S-S-; and each R 3 is selected independently from H, C 1-3 alkyl and halide. Examples of such bismaleimides include N,N'-4,4'-methylene-bismaleimide N,N'-4,4'-ethylene-bismaleimide N,N'-hexamethylene-bismaleimide N,N'-meta-phenylene-bismaleimide N,N'-p-phenylene-bismaleimide N,N'-4,4'-diphenylmethane bismaleimide N,N'-4,4'-diphenylether bismaleimide N,N'-4,4'-diphenylsulphone bismaleimide N,N'-4,4'-dicyclohexylmethane bismaleimide N,N'-4,4'-(3,5-diphenylpyridine) bismaleimide N,N'-pyridinidi-2,6-Y bismaleimide N,N'-α,α'-4,4'-dimethylenecyclohexane bismaleimide N,N'-meta-xylelene bismaleimide N,N'-4,4'-diphenylcyclohexane bismaleimide N,N'-meta-phenylene bisdichloromaleimide N,N'-4,4'-diphenylmethane biscitraconimide N,N'-4,4'-(1,1-diphenylpropane) bismaleimide N,N'-4,4'-(1,1,1-triphenylethane) bismaleimide N,N'-4,4'-triphenylmethane bismaleimide N,N'-3,5-(1,2,4-triazole) bismaleimide, and various N,N'-bismaleimides disclosed in U.S. patent Nos. 3,562,223, 4,211,860 and 4,211,861. Bismaleimides can be prepared by methods known in the art, such as described in U.S. patent No. 3,018,290, for example. The imide can also be a trifunctional maleimide reaction product of a bis(aminobenzyl)aniline with maleic anhydride. For laminating applications, the imide will preferably be blended with an epoxy resin in an amount within the weight ratios of from 1:9 to 9:1, preferably of from 1:1 to 9:1. An epoxy resin-containing laminating composition will include a curing agent. Effective curing agents for epoxy resins are known to include, for example, amines, acids, anhydrides, phenols and imidazoles. The presently-preferred curing agents for imparting optimum laminating properties to epoxy compositions are phenolic compounds which have a phenolic functionality greater than 1.75. The preferred phenolic curing agents are phenolic novolacs prepared by reacting a dihydroxy phenol such as resorcinol or bisphenol-A with formaldehyde in acid solution. The preferred phenolic novolac resin curing agents are bisphenol-A novolacs having a weight per phenolic group (WPP) of from 60 to 500, preferably of from 60 to 300, and, on the average, more than 2 phenolic hydroxyl groups per molecule, preferably of from 3 to 5. Such phenolic novolacs are available under the tradename Epikure DX-175 from Shell International Chemical Company (Epikure is a trademark). The phenolic novolac curing agent will be present in the composition in an amount effective to cure the epoxy resin, which will generally be a stoichiometric amount of from 0.75 to 1.25 equivalents per equivalent of epoxy resin. In terms of weight percent, the curing agent will be present in an amount generally from 10 to 70 weight percent, preferably of from 15 to 50, most preferably of from 15 to 40, based on the combined weight of epoxy resin and curing agent. The curing agent, for flame-proof applications, can be a mixture of the phenolic resin curing agent and a brominated phenolic curing agent. The brominated phenolic curing agent can be any monomeric or polymeric compound having at least one free phenolic hydroxyl group and one or more bromine atoms on the aromatic ring. Examples of suitable brominated phenolic curing agents include for example brominated bisphenol-A novolac, brominated phenolic novolac, brominated polyphenylene oxide, brominated bisphenol-A and brominated bisphenol-A carbonate. The brominated bisphenol-A will be present in an amount effective to increase flame retardancy, generally an amount up to 40 weight percent, usually of from 2 to 15 weight percent, based on the combined weight of epoxy resin and curing agent(s). In order to promote faster and/or lower temperature cure of the resin components of the composition, an optional cure accelerator may be used. Many suitable accelerators, such as ureas, tertiary amines, imidazoles, phosphenes, octoates and boron trifluorides, for example, are known in the art. The presently preferred class is imidazoles such as 1-methyl imidazole, 2-ethyl imidazole, 2-methyl imidazole, 2-methyl-4-ethyl imidazole and isopropyl imidazole. Because of its availability and performance characteristics, 2-methyl imidazole is the preferred accelerator. The accelerator will be present in the composition in an amount effective to increase the cure rate and/or lower the cure temperature of the composition, generally in an amount of from 0.01 to 7, preferably of from 0.05 to 3 weight percent, based on the weight of the composition. The thermosettable resin system must be designed within certain specifications dictated by the resin application process parameters. The resin formulation must be a liquid at a temperature at which the resin does not undergo cure over the time necessary for application of the resin to the substrate. The resin system must be of sufficiently low viscosity that it achieves good "wetout," or saturation of the web, without the use of a backup roll at the point of application. Once applied to the substrate, however, the resin system must have sufficient viscosity that it does not drop from the resin-containing web before it reaches the heating zone. Resin formulations having viscosities in the range of from 0.1 to 1.0 Pa.s, preferably of from 0.1 to 0.6 Pa.s are most suitable. The currently preferred resin system is a blend of a diglycidyl ether of bisphenol-A having an Epoxy Equivalent Weight (EEW) of 175-185, a brominated diglycidyl ether of bisphenol-A having an EEW of 310-350 and a bromine content of 30-50%, a phenolic novolac curing agent, and 2-methylimidazole accelerator. Example 1 This example describes the preparation and testing of a solventless theremosettable resin system to determine its suitability for use in the invention prepreg preparation process. The resin component was prepared by charging 41.0 g brominated diglycidyl ether of bisphenol-A and 59.0 g liquid diglycidyl ether of bisphenol-A (EEW 178-186) to a heating vessel and stirring under nitrogen at 120°C for 30 minutes. The curing agent component was prepared by heating 98.3 g of a phenol-formaldehyde novolac (HRJ-1166 from Schenectady Chemicals, WPP 103-105) to 120°C, adding 1.73 g of 2-methyl imidazole, and mixing at 120°C for 30 minutes. The resin formulation was prepared by blending the resin component and the curing agent component in a weight ratio of 72:28. A laminate was prepared under laboratory conditions simulating conditions of resin delivery and application to a glass web to confirm that the viscosity and cure characteristics of the resin formulation met processing requirements for the invention resin application technique. The viscosity of the above blend at 100°C was 0.9 Pa.s. The blend was mixed at this temperature for 3 seconds to simulate mixing in an extruder barrel. The resulting blend had a gel time (measured at 140°C) of 89 seconds and a viscosity at 120°C of 0.6 Pa.s. After mixing at 120°C for 3 seconds to simulate one-side web application, the gel time (140°C) was 85 seconds. The blend was applied by a hand squeeze-rolling technique to woven glass (style BFC 7628) and partially cured in an oven for 2 minutes at 163°C for simulation of a conventional treater oven B-staging operation. The dust gel of the B-staged resin (140°C) was 40 seconds, confirming advancement of the resin. A prepreg produced in this manner was made into an eight-ply laminate by pressing at a pressure of 13.6 atm and holding at a temperature of 175°C. Heat-up and cool-down rate for this operation were maintained at 14°C/min. The laminate had the following properties: Flexural strength (23°C, atm) 4.9 . 10 3 Flexural modulus (23°C, atm) 2.41 . 10 5 Tg (°C, DSC) 152 Dielectric constant (23°C, D-24/23) 4.47 Volume resistivity (.10 13 ohm-cm) 290 Water absorption (1 hr 1.02 atm steam, %w) 0.22 Copper peel (28.4 g Cu, kg/cm) 1.56 Flammability UL-94 VO Example 2 This example demonstrates application of a solventless resin system to a glass web using the process of the invention on a laboratory scale. A single-side coater was constructed using an adjustable experimental coating apparatus. The applicator roll diameter was 20.3 cm. A 2.54 cm Meyer rod was used to define a gap through which the resin was metered prior to contact with the web. The applicator roll was rotated at a surface speed of 2.44 m/min, and the Meyer rod was rotated at 0.305 m/min. The applicator roll surface was heated to 120°C. A flexible, porous woven glass fabric web (1080 glass style) 20.3 cm in width was fed to the roller at a rate of 3.05 m/min. The resin formulation described in Example 1 was deposited onto the applicator roll using a hot-melt dispensing pump operating at 100°C. Resin was applied to the cloth to provide a resin content of 67 %wt. The resin-containing web was passed through a 4.57 m oven maintained at 140°C to B-stage the resin. The dust gel (measured at 171°C) for B-staged resin removed from the prepreg was 34 seconds. The prepreg was used to make a 16-ply laminate by pressing at the following conditions: rate of rise (°C/min) 2.8 maximum temperature (°C) 175 pressure (atm) 6.8 time at temperature (hrs) 1 cooldown rate (°C/min) 22 The laminate had the following properties: Flexural strength (23°C, atm) 3.62 . 10 3 Flexural modulus (23°C, atm) 1.43 . 10 5 Tg (°C, DSC) 135 Volume resistivity ( . 10 13 ohm-cm) 240 Laminate appearance clear, uniform, good adhesion to glass, some voids

Claims: A process for the preparation of prepregs, comprising a fibrous substrate impregnated with a partially cured thermosettable resin, by impregnating the fibrous substrate with a solventless thermosettable resin composition, the process comprising: (a) providing resin application means comprising a rotating applicator roll (4); (b) applying to said rotating applicator roll (4) a liquid form thermosettable resin composition comprising an essentially uncured thermosettable resin; (c) passing a fibrous web (6), having first and second web surfaces, in contact with said thermosettable resin composition on said rotating applicator roll (4), so as to transfer said thermosettable resin composition to the first surface of said fibrous web (6); (d) passing the resulting resin-containing web to a heating zone maintained at a temperature effective to partially cure said thermosettable resin, characterized in that prepregs for electric or electronic circuit boards are prepared by: (i) applying a liquid solventless thermosettable resin composition to the applicator roll (4) by means of a combination of a set gap roll (8) and a nozzle (7), capable of applying a controlled quantity of liquid resin to the rotating surface of the applicator roll (4), wherein the liquid-form, solventless thermosettable resin composition, comprises an essentially uncured thermosettable resin, a curing agent and optionally an accelerator, and having a viscosity in the range of from 0.05 to 1.0 Pa.s, in the absence of a vessel for maintaining the resin system in a standing bath prior to application of said resin composition to the fibrous web, (ii) passing the fibrous web (6), in countercurrent contact with said resin composition on said rotating resin applicator roll (4) so as to transfer said resin composition to the first surface of said fibrous web and thence to the interior thereof, said transfer being carried out in the absence of external pressure applied to said second web surface opposite the area of resin transfer to said first web surface. A process as claimed in claim 1, wherein the fibrous web is a fibrous glass web. A process as claimed in claims 1 and 2, wherein the liquid form solventless thermosettable resin composition has a viscosity in the range of from 0.1 to 0.6 Pa.s. A process as claimed in claims 1 and 2, wherein the applicator roll is maintained at a temperature within the range of from 50 to 200 °C. A process as claimed in claims 1-4, wherein the liquid form solventless thermosettable resin composition comprises a brominated diglycidyl ether of bisphenol A and a phenol-formaldehyde novolac curing agent. A process as claimed in claims 1 to 5, which further comprises passing a first stream comprising a liquid-form solventless thermosettable resin composition and a second stream comprising a curing agent into a mixing chamber wherein said first and second streams are blended to form the liquid-form solventless thermosettable resin composition, which is directly applied to the applicator roll. A process as claimed in any one of the claims 1 to 6 in which the surface speed of the applicator roll is within the range of from 70 to 125 percent of the speed of the web. A process as claimed in any one of the claims 1 to 7 in which the fibrous web is passed in contact with the rotating applicator roll at a speed within the range of from 15.3 m/min to 45.8 m/min. A process as claimed in any one of the claims 1 to 8 in which the arc of contact of the fibrous web on the rotating applicator roll is within the range of from 20 to 40 degrees. A process as claimed in any of the claims 1-9 in which the curing agent is a mixture of a phenolic resin curing agent and a brominated phenolic curing agent. An apparatus for impregnating a fibrous web (6) with a thermosettable resin composition, which apparatus comprises a resin applicator roll (4), means for controlling the speed of the resin applicator roll (4), a resin delivery system for continuously applying the resin composition to the resin applicator roll (4), characterized in that it comprises a set gap roll (8) arranged near the resin applicator roll (4) and means for rotating the set gap roll (8), means for advancing a fibrous web (6) in contact with the resin applicator roll in a direction which is opposite to the rotation of the resin applicator roll (4), and means for maintaining sufficient tension in the fibrous web (6) to enable the fibrous web (6) to absorb resin composition from the resin applicator roll (4), said tension-maintaining means positioned so as not to apply pressure to the opposite surface of said fibrous web (6) as the web contacts the thermosettable resin on the applicator roll (4) surface, wherein the resin delivery is carried out at volume rates synchronized with the speed of the moving web so as to deliver a predetermined volume of resin to the web and to minimize residence time within the resin delivery system, including a temperature controlled static blender or a mixing extruder with an outlet into a nozzle (7), which is capable of applying a controlled quantity of liquid resin to the rotating surface of the applicator roll (4), and in that the set gap roll is so positioned that it is located between the nozzle (7) and the zone where the advancing web is in contact with the resin film on the resin applicator roll (4). Apparatus according to claim 11, wherein the means for supplying resin composition to the nozzle (7) comprises a mixer of which the outlet is in fluid communication with the nozzle, a first pump for delivering resin to the mixer, a second pump for delivery of the curing agent to the mixer, a system to control the delivery of the desired proportion of resin and curing agent to the mixer in an amount which is co-ordinated with the line speed of the fibrous web (6) through the resin application zone by a variable speed electric drive system. Apparatus according to claim 12, wherein the mixer is provided with a heater to allow delivering resin composition to the nozzle at a predetermined temperature.

Inventors: CHEVAKO DONALD MICHAEL, HARTLEY JOHN JOSEPH, STELLING WALTER ROBERT, TAIT ROBERT ANDREW, CHEVAKO, DONALD MICHAEL, HARTLEY, JOHN JOSEPH, STELLING, WALTER ROBERT, TAIT, ROBERT ANDREW
Applicants: RESOLUTION RES NEDERLAND BV, RESOLUTION RESEARCH NEDERLAND B.V.
Application year: 1991
Main classification: B29B15
IPC codes: D06M15, D06B1, D06M101, H05K3, D06B3, B29B11, D06M13, B29B15