MXPA97009000A - Improved process for resin transfer molding and useful formulations to practice elproc - Google Patents

Abstract

An improved adhesive for making bonded substrates containing: (a) at least one epoxy resin, and (b) at least one curing agent for the epoxy resin, but (c) no catalytic amount of catalyst for the reaction between the epoxy resin. epoxy resin and the healing agent. Adhered substrates are used to make compounds by: (1) placing one or more of the substrates adhered in a mold; (2) injecting into the mold a master resin formulation that is chemically similar to the adhesive, except that it contains a catalytic amount of catalyst for the reaction between the epoxy resin and the curing agent; and (3) the mother resin formulation. The adhesives are more stable in storage than similar adhesives containing catalyst, and the resulting compounds have and / or retain better physical properties than the compounds made using adhesives without curing agent.

Description

IMPROVED PROCESS FOR RESIN TRANSFER MOLDING AND USEFUL FORMULATIONS TO PRACTICE THE PROCESS The present invention relates to resin transfer molding processes and preforms useful in those processes. Resin transfer molding processes are used to make fiber reinforced composites. The layers of the reinforced material are loaded in a mold and a heat-setting resin is injected into the mold and cured by ordinary and accepted methods to provide a finished composite part. It is difficult and slow to load different layers of fiber into the mold. Previously it was necessary to join the fibers in order to provide preforms in the form of a network. A "thermoplastic-like" resin was sometimes used as an "adhesive" to harden the reinforced material and hold it in place before the molding process begins. See Heck et al., U.S. Patent 4,992,228 (February 12, 1991); and Flonc et al., U.S. Patent 5,080,851 (January 14, 1992). (The saying "similar to a thermoplastic" refers to the resin being a solid heat-setting resin exhibiting thermoplastic properties, such as glass transition temperature and / or a melting point, below the temperature that cures the resin, so that the resin is thermoformable.) In such a process, the individual fiber folds are sprayed with a solid powder of the adhesive. The powder is heated to melt on the surface of the substrate and then cooled to solidify it. The different folds can be stacked together, heated to melt the folds together and then cooled leaving a preform. The preformed can be placed in the mold and subsequently used in an ordinary resin transfer mold process. Other adhered substrates are described in Ko et al., U.S. Patent 5,369,192 (November 29, 1994); and Altognot et al., U.S. Patent 5,176,848 (January 5, 1993). It has been recognized that chemical differences between the adhesive and the mother resin can damage the physical properties of the compound. Consequently, it has been recommended to use an adhesive that is almost identical to the mother resin. See PCT Publication WO94-26493 (November 24, 1994). However, such adhesives may not be stable enough for long-term storage of the adhering substrate. What is needed is a material that is more stable for long-term storage, but that cures to form a product that is essentially identical to the cured mother resin. One aspect of the present invention is a bonded substrate comprising: (1) a fibrous substrate suitable for use in a parent compound; and (2) 0.25 to 15 weight percent of an adhesive containing: (a) an epoxy resin, characterized in that the adhesive further contains: (b) a curing agent for the epoxy resin, but (c) 0 less than 0.5 percent by weight of the following compounds, which act as catalysts for a curing reaction between Components (a) and (b): ammonium compounds and amines, phosphonium and phosphine compounds, aliphatic sulfonium compounds, and nitrogen-containing compounds heterocyclic and arsenic. A second aspect of the present invention is a process for using the bonded substrate comprising the steps of: (1) placing one or more of the substrates adhered, individually or as a preform, in a mold; (2) inject into the mold a formulation of mother resin containing. (a) an epoxy resin, (b) a curing agent for the epoxy resin, and (c) a catalytic amount of catalyst for the reaction between the epoxy resin and the curing agent; and (3) cure the parent resin formulation. A third aspect is a formulation, which is particularly useful as an adhesive in the preforms of the present invention and is useful as a master resin formulation in the process of the present invention. The formulation comprises: (1) 30 to 50 weight percent epoxy resin derived from a phenol-hydrocarbon resin; (2) 20 to 30 weight percent diglycidyl ether of a halogenated dihydric phenol; (3) 20 to 40 weight percent phenolic resin or polyhydric phenol; (4) optionally, up to 15 weight percent liquid epoxy resin; and (5) optionally, a catalytic amount of a catalyst for the reaction of the epoxy resins with the polyphenol resin.

When the formulation is used as an adhesive, it is preferred that it does not contain Components (4) and (5). When the formulation is used as a mother resin formulation, it is preferable that it contain Components (4) and (5). The adhesive is useful for making bonded substrates of the present invention. The adhered substrates, the master resin formulation and the process of the present invention are useful for making parent compounds. In addition, the adhesive and the master resin formulation of the present invention may be useful for other purposes, and the substrates adhered and the process may be practiced using different adhesives and master resin formulations. The present invention uses a reinforced substrate. Suitable reinforcements are well known and familiar to persons skilled in the art. See, for example, Kirk-Othmer, Encvclopedia of Chemical Technoloqv-Supplement. "Composites, High Performance," in 260-281 (J. Wiley &Sons, 1984). The substrate generally contains fibers, such as quartz, aramid, boron, glass, carbon or twisted foam polyethylene fibers. The fibers can be unidirectional or multidirectional. They may be in the form of woven or non-woven mats, or in the form of random short fibers. Preferably, the substrate is in the form of woven or nonwoven fibrous material. The substrate is contacted with an adhesive under conditions such that the adhesive partially adheres to the fibers. The adhesive must be a solid and a glassy solid until temperatures of at least 40 ° C. It is preferable that it becomes fluid and sticky at a temperature of at least 40 ° C, more preferably up to at least 50 ° C and most preferably up to at least 60 ° C. An injectable liquid is preferable at a temperature of less than 200 ° C, most preferably at a temperature of at least 175 ° C and very preferably at a temperature of less than 150 ° C. (The same temperature criteria are preferably found individually for each component in the adhesive.) The adhesive contains; (a) at least one epoxy resin, and (b) at least one curing agent, which is capable of reacting with the epoxy resin, but (c) no catalytic amount of catalyst for the reaction between the epoxy resin and the healing agent. Epoxy resins are preferably polyglycidyl ethers of one or more polyhydric phenols, such as biphenols, bisphenols (such as bisphenol A or bisphenol F), novolac resins, phenol-hydrocarbon resins and halogenated variations of these resins. A more preferred epoxy resin mixture contains a mixture of: (1) a polyglycidyl ether of a phenol-hydrocarbon resin, and (2) a diglycidyl ether of a halogenated bisphenol. Examples of phenol-hydrocarbon resins, their glycidyl ethers and processes for making them are described in Nelson et al., U.S. Patent 4,390,680 (June 28, 1983); Nelson, U.S. Patent 4,394,497 (July 19, 1983); and Bogan et al., U.S. Patent 4,710,429 (December 1, 1987). The phenol-hydrocarbon resin is highly preferable a condensation product of dicyclopentadiene and phenol, as represented by the following formula: wherein "a" represents an average number of repetitive units, preferably 0 to 10, more preferably 0 to 5 and most preferably 0 to 2. The equivalent epoxy weight (EEW) of the phenol-hydrocarbon resin is preferably between 175 and 400, and more preferably between 200 and 260. Et diglycidyl ether of a halogenated bisphenol is preferably a diglycidyl ether of a brominated bisphenol, more preferably a diglycidyl ether of a brominated bisphenol A, and most preferably a diglycidyl. - Tetrabromobisphenol A Teater. The curing agent is preferably selected so that it will not substantially cure the epoxy resin at ambient temperatures without a catalyst. Examples of the preferred curing agents include: phenolic resins and / or polyhydric phenols, carboxylic acids, anhydrides of carboxylic acids and mixtures thereof. Phenolic resins and / or polyhydric phenols, which are useful as curing agents preferably contain on average more than 2 phenolic hydroxyl groups per molecule and more preferably, at least 3 phenolic hydroxyl groups per molecule. The maximum number of phenolic hydroxyl groups is not critical, but is limited by practical considerations, such as viscosity and glass transition temperature. In most cases, the phenolic resin or polyhydric phenol preferably contains on average no more than 12 phenolic hydroxyl groups per molecule and more preferably contains on average no more than 8 phenolic hydroxyl groups per molecule. The polyphenol is preferably a novolac resin and more preferably a cresol-novolac resin. The adhesive may optionally contain other non-catalytic components. For example, the adhesive may contain thermoplastic polymers, gums or elastomers, or other modifiers.

The adhesive must not contain a catalytic amount of any catalyst for the reaction between the curing agent and the epoxy resin. Examples of catalysts include: ammonium compounds and amines, phosphonium and phosphine compounds, aliphatic arsenic and suifonium compounds, and heterocyclic nitrogen containing compounds. More specific examples of catalysts are described in: Bertram et al., U.S. Patent 4,594,291 (June 10, 1986) (Column 8, line 59 to Column 9, line 11); and Bertram et al., U.S. Patent 5,134,239 (July 28, 1992) (Column 2, line 15 to Column 23, line 29). The adhesive preferably contains less than 0.05 percent by weight of catalyst, more preferably less than 0.01 percent by weight of catalyst and most preferably about 0 percent by weight of catalyst.

Epoxy resins, curing agents and other components are mixed to make the adhesive. They are preferably mixed by melting, cooled to solidify the melt and then crushed to a powder. The average particle diameter of the adhesive is preferably not more than 150 μm, more preferably not more than 100 μm and most preferably not more than 75 μm. The minimum average particle diameter is not critical and is limited by practical considerations, such as handling and application. In most cases, the average particle diameter is preferably at least 45 μm and more preferably at least 50 μm. In theory (without pretending to limit), during the curing stage, the catalyst in the mother resin formulation can initiate healing reactions on the surface of the particle, which will continue towards the center of the particle. Accordingly, it is advantageous to use a small particle size.

The adhesive must be applied to the substrate in an amount large enough to hold the fibers in the desired shape and position, but small enough to leave the resulting preformed porous, so that subsequently the mother resin formulation can be emptied through of the substrate. The amount of adhesive is preferably at least 0.25 percent by weight of the substrate, more preferably at least 1 percent by weight and more preferably at least 3 percent by weight. The amount of adhesive is preferably not more than 15 percent by weight of the substrate, more preferably not more than 8 percent by weight and most preferably not more than 5 percent by weight. The adhesive can be applied by known methods for powders, such as spraying or electrostatic application. Preferably it is applied substantially uniformly through the substrate. The adhesive is heated above its glass transition temperature to cause it to adhere to the fibers and to adhere together to the fibers. This step can be carried out either when the adhesive is applied or subsequently. The temperature is preferably low enough that the adhesive does not cure substantially. The temperature is preferably between 40 ° C and 150 ° C, more preferably between 80 ° C and 110 ° C. Preferably, the individual folds of the adhered substrate are formed and / or laminated together to preform after the adhesive has been applied and before the compound is made. For example, multiple folds can be pressed together at a temperature above the glass transition temperature of the adhesive. Also, the laminated preforms or individual folds can be molded or formed at a temperature that is above the glass transition temperature of the adhesive and which does not completely cure the adhesive. The temperature is preferably as previously described to adhere the adhesive. Examples of forming and laminating different preforms are described in: Heck, U.S. Patent 4,992,228 (February 12, 1991); and Flonc, U.S. Patent 5,080,851 (January 14, 1992). The preforms are used for resin transfer molding by: (1) loading the preform into a mold; (2) injecting a master resin formulation into the mold; and (3) cure the mother resin formulation. (Theoretically, the adhesive cures simultaneously with the mother resin, although direct observation is difficult to verify simultaneous curing.) The master resin formulation contains at least one epoxy resin and at least one curing agent. The description and preferred embodiments of the epoxy resin and the curing agent are the same as previously described for the adhesive. However, the master resin formulation optionally contains some lower molecular weight resins, which reduces the melt viscosity of the formulation. Moreover, the mother resin formulation must contain a catalytic amount of a catalyst for the reaction between the epoxy resin and the curing agent. The epoxy resin (s) and curing agent (s) in the mother resin formulation are preferably essentially identical to the epoxy resin (s) and curing agent (s). in the adhesive, so that the adhesive and the mother resin formulation cure to provide substantially equivalent cured polymers. For example, the components of the epoxy resin and the curing agent of the adhesive and the formulation of the master resin: (a) preferably contain essentially the same reactive sites as the curing agent; (b) preferably have in common at least 50 percent of the units of the backbone linking the reactive sites, more preferably at least 80 or 90 percent, and most preferably, essentially all; (c) preferably have weight average molecular weights that differ from each other by no more than a 5: 1 ratio, more preferably no more than a 3: 1 ratio and most preferably no more than a 2: 1 ratio; (d) preferably have average equivalent weights that differ from each other by no more than a 5: 1 ratio, more preferably no more than a 3: 1 ratio and more preferably no more than a 2: 1 ratio; and most preferably no more than a 1.5: 1 ratio; and (e) preferably differ by no more than 50 percent, more preferably no more than 25 percent and most preferably no more than percent in its concentration in the formulation. Suitable catalysts for the parent resin formulation have already been described and are well known in the art. The catalyst is preferably an amine, a phosphonium salt or an imidazole. It is more preferable that it contain a 2-aiquilimidazole, a tetraalkylphosphonium salt or an alkyltritolylphosphonium salt. The catalyst concentration is preferably at least 0.05 weight percent and more preferably at least 0.1 weight percent. The maximum concentration of catalyst is not critical and is limited mainly by practical considerations, such as cost, time of cure and properties of the cured resin. In most cases, the catalyst concentration is preferably not more than 5 percent by weight and more preferably not more than 2 percent by weight. The amount of the master resin formulation should be sufficient to keep the fibers together, to maintain the alignment of the fibers and, preferably, to transfer loads around the broken fibers. It is generally preferable to minimize the amount of the parent resin formulation in the compound. After curing, the master resin formulation and the adhesive should be at least 25 percent by volume of the compound and more preferably be up to 35 percent by volume. The masterbatch formulation and the adhesive are preferably not more than 75 percent by volume of the compound, more preferably not more than 45 percent by volume and most preferably not more than 40 percent by volume. The mother resin formulation is preferably applied under sufficient pressure to force it through the preform. So that the preform is impregnated with the resin and the voids within the cured compound are minimized. The mother resin formulation is preferably applied at vacuum pressures of 42.18 kg gauge / cm 2. Frequently there is a pressure drop through the mold, so that the pressure at the inlet can be as high as 28.12 kg / cm2, while the pressure at the outlet is 3.515 kg / cm2 or less. The cured compound preferably contains no more than 5 percent by volume of voids, more preferably no more than 2 percent by void volume and most preferably no more than 1 percent by void volume.

The master resin formulation and the adhesive are then subjected to temperature and other suitable conditions to cure them completely. Again, the optimum temperature and time vary widely depending on the master resin formulation and the adhesive used, and will be familiar to those skilled in the art. The final cure step is preferably 30 to 120 minutes at a temperature of 90 ° C to 180 ° C. However, for some mother formulations, the best cure temperature can be as high as 250 ° C or more. The final cure step is preferably completed in a mold that can provide a compound of the desired form. The resulting cured compound can be cooled, finished and used in the ordinary manner for parent compounds. A preferred resin formulation for use in adhesives and mother resins contains: (1) 30 to 50 weight percent epoxy resin derived from a phenol-hydrocarbon resin, more preferably 35 to 45 weight percent and most preferably 40 to 43 percent in weigh; (2) 20 to 30 weight percent diglycidyl ether of a halogenated dihydric phenol and more preferably 25 to 30 weight percent; (3) 20 to 40 percent by weight of polyphenol resin, more preferably 25 to 35 percent by weight and most preferably 27 to 33 percent by weight; and (4) optionally, up to 15 weight percent liquid epoxy resin, more preferably up to 10 weight percent and most preferably up to 4 weight percent.

The above percentages exclude the weight of the solvent, if present. The formulation can also contain, organic solvent when used as a mother resin formulation, but preferably not. The concentration of solids is preferably at least 50 percent by weight, more preferably at least 75 percent by weight and most preferably at least 99 percent by weight. When the formulation is used as an adhesive, Element (4) is preferably not used. When the formulation is used as a mother resin, Element (4) is preferably used. Formulations containing liquid epoxy resin - Element (4) - preferably contain at least 1 percent by weight. The formulation may also contain catalyst, optionally. The preferred minimum amounts of catalyst were previously discussed. The process of the present invention is illustrated more fully in the following examples.

Examples The following example is for illustrative purposes only and should not be taken as limiting the scope of either the Specification or the Claims. Unless indicated otherwise, all parts and percentages are by weight. The following materials were used in the example: Epoxy Resin A - is a diglycidyl ether of tetrabromobisphenol having an epoxide equivalent by weight (EEW) of 305 to 355, which is commercially available from The Dow Chemical Company as epoxy resin DER * 542 .

Epoxy resin B - is the polyglycidyl ether of a dicyclopentadien-phenol oligomer.

Epoxy resin B contains on average about 3.2 epoxy groups per molecule and has an EEW of about 255. Epoxy resin C - the polyglycidyl ether of a dicyclopentadien-phenol oiigomer. Epoxy resin C contains on average around 2.2 epoxy groups per molecule and has an EEW of 215 to 235. It is commercially available from The Dow Chemical Company as TACTIX * 556 epoxy resin. Epoxy resin D - is a liquid diglycidyl ether of bisphenol A that has an EEW of 172 to 176. Novolac resin E - is a novolac-cresol resin containing about 6 phenolic hydroxyl groups per molecule, which is commercially available from Schenectady Chemical, Inc. As CRJ-406. Catalyst F - is a product obtained by mixing a complex of tetrabutylphosphonium acetate-acetic acid and fluoroboric acid. Example 1 - Process using the epoxy resin mixture of phenol-hydrocarbon and hatogenated epoxy resin. An adhesive was made by the following procedure. A quantity of .663 of epoxy resin A and an amount of 3.215.5 g of epoxy resin B were melted separately at 150 ° C. Both resins were mixed in a reactor at 145 ° C. 7,014.5 g of additional B-flakes were added to the molten mixture, and 6.770 g of Novolac E flakes were added. The mixture was stirred for 50 minutes and then emptied into a sheet and cooled to room temperature to solidify. The solid mixture was broken, milled and sieved at an average particle diameter of not more than 106 μm.

An adhered canvas was made by spraying the adhesive on a carbon canvas, heating the adhesive to 93 ° C to melt it and then cooling the adhesive. The canvas contained 4 percent by weight of adhesive at the end of the procedure. A preform was made using 8 folds of attached canvas. The folds were stacked in a vacuum bag apparatus and evacuated to set 1 atmospheric pressure in the stack of folds. The folds were then heated to 93 ° C and cooled to room temperature to make a preform. A composite was made using the preform. The preformed was placed in a tongue-and-groove detroquel mold, which was sealed and placed under vacuum. A master resin formulation was injected into the mold. The formulation contained: 26 weight percent Epoxy Resin A, 42 weight percent Epoxy Resin C, 3 weight percent Epoxy Resin D, 30 weight percent Novolac E Resin and 1 weight percent Catalyst F. The panel He was cured for three hours at 177 ° C. The product is a composite panel.

Claims (7)

1. A bonded substrate comprising: (1) a fibrous substrate suitable for use in a parent compound; and (2) 0.25 to 15 weight percent of an adhesive containing: (a) at least one epoxy resin; characterized in that the adhesive substrate further contains: (b) at least one curing agent for the epoxy resin, wherein the curing agent is a polyhydric phenol, phenolic resin, carboxylic acid or mixtures thereof; and (c) 0 or less than 0.5 weight percent of the following compounds, which act as catalysts for a curing reaction between Components (a) and (b): ammonium compounds and amines, phosphonium and phosphine compounds, compounds of sulfonium, aliphatic and arsenic, and compounds containing heterocyclic nitrogen.

2. A bonded substrate as described in Claim 1, wherein the curing agent of Component (b) contains any one or more than one: phenolic resin or a polyhydric phenol, a carboxylic acid or a carboxylic acid anhydride.

3. The bonded substrate of Claim 2, wherein the adhesive becomes fluid and tacky at a temperature between 40 ° C and 200 ° C.

The bonded substrate of Claim 2, wherein at least one epoxy resin is a glycidyl ether derivative of a phenol-hydrocarbon resin.

5. A bonded substrate as described in Claim 2, wherein the curing agent is a phenolic resin or polyhydric phenol.

6. A bonded substrate as described in claim 2, wherein the adhesive contains: (1) 30 to 50 weight percent epoxy resin derived from a phenol hydrocarbon resin; (2) 20 to 30 weight percent diglycidyl a halogenated dihydric phenol ether, and (3) 20 to 40 weight percent phenolic resin or polyhydric phenol

7. A method for using a bonded substrate as described in any one of Claims 1 to 6, comprising the steps of: (1) placing one or more of the substrates adhered as described in any of Claims 1 to 6, individually or as a preformed, in a mold; (2) injecting into the mold one or more of the formulations of mother resin containing: (a) an epoxy resin, and (b) a curing agent for the epoxy resin wherein the curing agent is a polyhydric phenol, phenolic resin, carboxylic acid or mixtures thereof; ) a catalytic amount of catalyst p for the reaction between the epoxy resin and the curing agent; and (3) cure the parent resin formulation.