WO1998022527A1

WO1998022527A1 - Epoxy resin composition suitable for sheet moulding

Info

Publication number: WO1998022527A1
Application number: PCT/EP1997/006507
Authority: WO
Inventors: Seetha Maha Lakshmi Coleman-Kammula; Geert Florizoone; Isabelle Anette Marie Madeleine Tiberghien
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 1996-11-18
Filing date: 1997-11-17
Publication date: 1998-05-28
Also published as: ZA9710293B

Abstract

A composition comprising (1) a liquid epoxy compound comprising on average more than one epoxy group per molecule, (2) a thickening agent comprising on average more than one carboxylic acid group per molecule or the anhydride thereof, which thickening agent effectively thickens the composition at ambient conditions, (3) optionally a catalyst for the reaction between the epoxy compound and the thickening agent, (4) a latent curing agent which effectively cures the epoxy compounds at elevated temperatures, (5) optionally a catalyst for the reaction between the epoxy compounds and the curing agent, (6) optionally reinforcement fibres and (7) optionally a filler compound.

Description

EPOXY RESIN COMPOSITION SUITABLE FOR SHEET MOULDING

The invention relates to an epoxy resin composition suitable for use in the Sheet Moulding Compound (SMC) moulding process. The composition can be made by mixing the ingredients, where after the composition thickens during storing for a number of days. The thickened product thus obtained may be cured under subsequently- imposed curing conditions.

The use of Sheet Moulding Compound (SMC) moulding has over the last years gained wide acceptance, especially in the automotive industry (bumpers, boot lids etc.) and further in the electrical industry (castings, low voltage applications etc.) . The most commonly used resins in this technique are unsaturated polyester resins. To cross-link the resin, a reactive monomer is used. Usually a vinylmonomer is used as monomer and in particular styrene monomer is used.

In the conventional process, a vinylmonomer (e.g. styrene) solution of the unsaturated carboxyl-terminated polyester is mixed with a peroxide (or other initiator) , a thickening agent such as magnesium oxide, and a filler such as calcium carbonate or clay. This liquid mixture is then mixed with e.g. chopped fibre or fibreglass between two sheets of (polyethylene) film, and air bubbles are removed by squeeze rolls. Over a few days, the viscosity increases from an initial value of usually between 0.01 and 100 Pa . s to a value usually in the range of 30.000 to 150.000 Pa.s. The increase in viscosity is caused by reaction of the carboxyl end groups of the polyester with the thickening agent. In the case of magnesium oxide as thickening agent polymeric magnesium carboxylates are formed. The viscosity reaches a plateau after the thickening agent is consumed. The semi-solid SMC composition then has a non-tacky, leathery consistency, suitable for draping into a mould. The viscosity of the thickened product should remain more or less constant for a period of preferably at least three months, which constitutes the "moulding window" of the SMC composition. If the viscosity is too low, liquid resin will squirt out of the mould during moulding. If the viscosity becomes too high, the SMC will be "boardy" and difficult to drape, and it may not have sufficient flow to fill the mould completely. Curing of the thickened SMC composition by polymerisation of the unsaturated bonds under influence of the peroxide, is typically done in 2 to

10 minutes at a temperature between 120 °C and 180 °C. A disadvantage associated with the above described state of the art polyester based SMC compositions is that use is made of styrene as a reactive diluent. As styrene may leak from the aged/thickened sheet, it might impose a health risk, as styrene nowadays is considered to be a relatively hazardous compound. The exposure to styrene problem may be overcome by installing extractors, which is, however, an expensive solution. Further, government regulations are expected to come into force to reduce the use of styrene. It has in the past been proposed to replace styrene by isocyanates, however, isocyanates are just slightly less hazardous than styrene and therefor cannot be regarded as the ideal solution of the problem. It has now been found that epoxy resin based SMC compositions can be made which have excellent viscosity properties for use in SMC moulding. The compositions comprise one or more liquid epoxy compounds, one or more acidic thickening compounds which react with a part of the epoxy groups at relatively low temperature resulting in a thickened composition, a latent curing agent which reacts with the remaining epoxy groups at relatively high temperature, resulting in a full cure of the composition, and optionally one or more catalyst for the reaction between the epoxy groups and the acidic thickening agent and/or the reaction between the epoxy groups and the curing agent, fillers and reinforcing fibres. Also reactive diluents comprising an epoxy group may be present. Further, additives may be added as releasing agents, pigments, flame retardants, low profile additives etc.

It will be appreciated that the curing agent involved should be latent at room temperature for a period of at least 7 days and should only become active at temperatures of 80 °C and higher.

In addition to the absence of styrene in the compositions of the present invention, there are a number of other advantages of the use of epoxy compounds in SMC moulding compositions. The cured products obtained by using epoxy based SMC compositions show better mechanical and thermal properties and an improved chemical resistance, especially against basic chemicals and acid chemicals, when compared with the traditional polyester based SMC compositions. It is furthermore observed that the compositions according to the present invention show viscosity properties closely corresponding to the conventional SMC compositions, as initial and final viscosity and time-viscosity profile, while also the curing rate is more or less comparable. Accordingly the compositions according to the present invention show initial viscosities of the total composition in the range of from 30 to 100 Pa.s, final viscosities of the total composition in the range of from 30,000 to 150,000 and a time-viscosity profile during thickening, characterized by the beforementioned viscosity increase within a period from 1 hour to 80 hours. In general, it has been found that with increasing diacid or multiacid concentration in the initial blend, the finally reached viscosity level of the thickened composition will increase too, whereas an increasing catalyst concentration will cause increase of viscosity increase rate. This makes the new compositions very suitable to replace the existing styrene based SMC compositions as no essential changes in the process cycle and/or processing equipment are necessary.

Thus, the present invention relates to a composition comprising (1) a liquid epoxy compound comprising on average more than one epoxy group per molecule, (2) a thickening agent comprising on average more than one carboxylic acid group per molecule or the anhydride thereof, which thickening agent effectively thickens the composition at ambient conditions, (3) optionally a catalyst for the reaction between the epoxy compound and the thickening agent, (4) a curing agent which effectively cures the epoxy compounds at elevated temperature, (5) optionally a catalyst for the reaction between the epoxy compounds and the curing agent, (6) optionally reinforcement fibres and (7) optionally a filler compound.

It will be appreciated that the present invention preferably relates to compositions free of vinylmonomer, and in particular styrene, and free of solvent.

With the term "at ambient conditions" as used throughout the present specification, is meant that a reaction takes place at temperatures in the range of from 10 to 50 °C and under atmospheric pressure.

The epoxy resins used as starting material in the present invention generally have an average of more than one, suitably at least 1.5, preferably at least 1.7, reactive 1,2-epoxy groups per molecule. These epoxy compounds generally have an average of up to 6, preferably up to 4, more preferably up to 2.5, reactive 1,2-epoxy groups per molecule. These epoxy compounds can be monomeric or polymeric, saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heteroaromatic and may be substituted, if desired, with other substituents in addition to the epoxy groups, e.g. hydroxyl groups, alkoxyl groups or halogen atoms, especially bromine atoms.

Suitable epoxy compounds are the reaction products of polyphenols and epihalohydrins, polyalcohols and epihalo- hydrins, amines and epihalohydrins, sulphur containing compounds and epihalohydrins, polycarboxylic acids and epihalohydrins or mixtures thereof.

Preferred epoxy compounds are the reaction products of polyphenols and epihalohydrins, of polyalcohols and epihalohydrins, polycarboxylic acids and epihalohydrins or mixtures thereof, the reaction products of polyphenols and epihalohydrins especially preferred. Illustrative examples of epoxy compounds are described in for instance The Handbook of Epoxy Resins by H. Lee and K. Neville, McGraw-Hill, New York (1967), Epoxy Resins, Chemistry and Technology, edited by CA. May, Marcel Dekker (1988) and Chemistry and Technology of Epoxy Resins, edited by B. Ellis, Blackie Academic & Professional (1993) . Epoxy compounds of particular interest in the practice of the present invention include diglycidyl ethers of bisphenol compounds, particularly those compounds represented by general formula I,

CH₂OCH-CH₂-0-C₆X4-A-C₆X4-0- (CH CH (OH) CH₂OC₆X4-A- CgX4θ)_n-CH₂-CHOCH₂ I wherein each A independently is a divalent hydrocarbon group having 1 to 8 carbon atoms, preferably methylene or isopropylidene, -C(0)~, -0-, -S- , -S-S-, -S(O)-, -S(0)2- or a covalent bond, each X independently is hydrogen, an alkyl group having 1 to 4 carbon atoms, preferably methyl, or halogen, preferably chlorine or bromine, and n has an average value of 0 to 12, preferably 0 to 2. More preferably A is isopropylene, X is hydrogen or bromine, especially hydrogen, and n is up to 0.2. The average epoxy equivalent weight is from 140 to 3000 and preferably from 170 up to 950, more preferably from 180 to 450.

Especially preferred examples of the epoxy compounds are those derived from bisphenol A and bisphenol F type epoxy compounds and preferably bisphenol A type epoxy compounds, having an average epoxy equivalent weight of from 160 to 200. They are commercially available from Shell Chemicals Europe under the trade names EPIKOTE 826, 828, 862 and 806. According to an alternative embodiment of the present invention epoxy compounds derived from brominated. bisphenol A compounds can be used and more in particular those which have an epoxy equivalent of from 200 to 800. Further useful epoxy compounds are epoxy novolac resins. The epoxy novolac resins can be obtained by reacting, preferably in the presence of a basic catalyst, e.g. sodium or potassium hydroxide, an epihalohydrin, e.g. epichlorohydrin, with the resinous condensate of an aldehyde, e.g. formaldehyde, and either a monohydric phenol, e.g. phenol itself, or a polyhydric phenol. Further details concerning the nature and preparation of these epoxy novolacs resins can be obtained in Handbook of Epoxy Resins and the other references mentioned above. The above-mentioned epoxy compounds are obtainable by means known for the preparation of epoxy resins from a compound containing hydroxyl groups by reacting such compounds with an epihalohydrin in the presence of a suitable catalyst and reacting the resultant intermediate halohydrin ether with a basic acting substance such as an alkali metal hydroxide. It has to be observed that many of the usual epoxy compounds to be used for the production of the modified resins of this invention will be commercially available.

The thickening agents used as starting material in the present invention generally have on average two or more carboxyl groups per molecule and more in particular from 2 to 6 on average . The thickening agent can be monomeric or polymeric, saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heteroaromatic and may be substituted, if desired, with other substituents in addition to the carboxyl groups, e.g. hydroxyl groups, alkoxyl groups, alkyl groups and in particular those having from 1 to 4 carbon atoms, or halogen atoms, especially bromine atoms.

It will be appreciated that also thickening agents can be used wherein a part of the total present carboxyl groups have been transformed into anhydride groups. Suitably the thickening agent is a saturated or unsaturated aliphatic or cycloaliphatic dicarboxylic acid, optionally substituted by one or more alkyl groups, especially Cl-4 alkyl groups, more especially methyl groups, or the corresponding anhydride. The aliphatic or cycloaliphatic part suitably comprises 4 to 9 carbon atoms, preferably 5 to 7 carbon atoms. Examples are hexahydrophthalic acid, tetrahydrophthalic acid, dihydro- phthalic acid, methyl hexahydrophthalic acid, methyl tetrahydrophthalic acid, methyl endomethylene tetrahydrophthalic acid or the corresponding anhydrides. Preferably the anhydrides are used. Another type of suitable thickening agents are the dicarboxylic reaction products of the above described anhydrides and aliphatic dialcohols. Suitable dialcohols comprising between two and eight carbon atoms, optionally substituted by one or more Cl-4, especially methyl groups, can be used. Examples are methyl or dimethyl pentanediol, methyl or dimethyl hexanediol, especially 2-methyl-2, 4-pentanediol . It will be appreciated that also mixtures of diacids and/or anhydrides can be used. Preferably the thickening agent is a liquid at ambient temperature, or is a low melting compound, e.g. melting below 50 °C, especially below 30 °C.

The amount of thickening agent to be used will be less then the stoichiometric amount with respect to the epoxy groups. Suitably up to 0.75 equivalents of carboxylic groups with respect to epoxy groups will be used, especially less then 0.50 equivalents. Preferably the amount is between 0.05 and 0.25, more preferably about 0.15 equivalent. The amount of thickening agent should always result in an effective thickening of the composition, i.e. the composition should be converted in a mouldable composition in a practical period of time. A suitable conversion period is between 1 and 40 days, more suitably between 2 and 30 days, preferably between 3 and 20 days. Especially a period between 10 and 15 days is used. The thickening of the composition is suitably carried out at temperatures between 0 °C and 60 °C, more suitably between 10 °C and 50 °C, especially between 15 °C and 40 °C, preferably between 20 °C and 30 °C. It will be appreciated that when the thickening agent has been consumed, no further increase in viscosity will occur due to the reaction between the thickening agent and the epoxy compound. The viscosity of the mouldable composition is suitably between 20,000 and 300,000 Pa.s, more suitably between 25,000 and 250,000 Pa.s. Preferably the viscosity is between 30,000 and 200,000, more preferably between 50,000 and 150,000.

The latent curing agent to be used in the compositions of the present invention may be each suitable curing agent to cure epoxy resins at elevated temperature, i.e. a temperature above 80 °C, suitably above 120 °C. Curing agents which reacts at lower temperature will result in products which have an insufficiently long shelf live. In order to be commer- cially attractive the shelf live of the thickened composition should be at least 10 days, suitably at least 20 days, preferably at least 30 days, more preferably at least two months. Very attractive are shelf lives of three months or more, e.g. up to a year. Suitable curing agents are polyamine, polyamide, polyamidoamine and polyaminoadduct, a polycarboxylic acid or anhydride thereof or a polyphenolic compound. In this respect also reference is made to the three general references mentioned above. Suitable examples include primary and secondary aliphatic polyamines, modified imidazoles, carboxylic acids and anhydrides thereof, phenolic hydroxyl-containing compounds, guanidines, biguanidines, polyamides, Mannich bases, ketimines, oxazolines and combinations thereof. Particularly suitable curing agents include, for example, ethylene diamine, diethylenetri- amine, triethylenetetramine, dicyandiamide, modified imidazoles, diaminocyclohexane, adipic acid, phosphoric acid or combinations thereof. Many of the suitable curing agents are commercially available. The curing agents are employed in an amount which will effectively cure the composition containing the modified epoxy resin. These amounts will depend upon the particular epoxy resin and the curing agent employed. Suitable amounts are from 0.4 to 1.2, suitably from 0.6 to 1.1, more suitably from 0.8 to 1.0, most suitably about 1 equivalent of curing agent per epoxide equivalent (remained after thickening) for those curing agents which cure by reacting with the epoxy group of the epoxy resin. The Handbook of Epoxy Resins and the other two references mentioned above contain various discussions concerning the curing of epoxy resins as well as a compilation of suitable curing agents.

Curing is suitably performed at temperatures between 90 and 250 °C, especially between 120 and 200 °C. Curing times depend on curing agent and temperature, but should be such that substantially all (i.e. more than 95 percent, preferably more than 98 percent) reactive groups have reacted. Usually the curing time is between 1 and 120 minutes, suitably between 2 and 60 minutes, preferably between 3 and 30 minutes, more preferably between 4 and 20 minutes.

Optionally a catalyst may be used for the thickening of the composition. Suitable catalysts are basic polyalkanoates, containing metal ions selected from chromium, vanadium or iron (III) and alkanoic acid residues having from 6 to 12 carbon atoms and preferably from 7 to 10 carbon atoms having the form of trinuclear clusters around three metal atoms arranged around a central oxygen and six bridging alkanoate moieties, while one alkanoate anion compensates the monopositive charge of the cluster, e.g. Cr3θ (RC00)7, wherein R=C7Hi5- Further, in order to promote the curing reaction a cure accelerator may be used. Many suitable accelerators, such as ureas, tertiary amines, imidazoles, phosphenes, octoates, and boron trifluorides are known in the literature. In this respect reference is also made to the three general references mentioned above. The preferred accelerators are imidazoles, for example 1-methyl imida- zole, 2-ethyl 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 from 0.01 to 7, preferably from 0.05 to 3 weight percent, based on the weight of the composition.

If desired, the thermosettable compositions of the present invention can be blended with other materials such as (reactive) diluents, pigments, dyes, flow modifiers, thickeners, anti-foamers, fire retarding or suppressing agents and combinations thereof. These additives are added in functionally amounts, e.g. the pigments and/or dyes are added in quantities which will provide the composition with the desired colour. Suitably the amount of additives is from 0 to 50 percent by weight, especially 2 to 20 percent, based upon the combined weight of the epoxy resin and the curing agent.

The composition usually will contain reinforcement fibres such chopped fibres, woven- or non woven mats etc. The fibres are suitable made from glass or mineral wool, but may also be nylon fibres, aramide fibres, carbon fibres etc.

Further, fillers may be applied in the compositions of the present application. Suitable fillers are aluminium oxide, silica, modified montmorillonites, calcium carbonate and clay.

The invention is illustrated by the following examples, however without restricting its scope to these specific embodiments. Examples 1 and 2

Two SMC compositions according to the present invention were prepared, comprising the ingredients as listed in the following Table 1, which also contains the respective relevant physical properties of the cured compositions .

Table 1

* AJICURE and DICY are trademarks. These respective compounds (blocked imidazoles and dicyandiamide) are used as curing agents for the thickened composition. ** An esterification catalyst.

Example 3 and Comparative Example a

Two SMC compositions were prepared. One of them, derived from an epoxy resin is to be regarded as a typical representative of the compositions according to the present invention (Ex. 3), and the other, derived from a polyester resin, as a typical representative of the usual prior art compositions (Co p. Ex. a) .

The composition ingredients and the relevant properties have been listed in Tables 2 and 3.

Table 2 Compositions of the polyester and epoxy SMC formulations

^aReaction product of 2.2 mole methylhexahydrophthalic anhydride and 1.0 mole 2-methyl 2, 4-pentanediol Table 3

Neat resin casting properties after

30 minutes cure at 130 °C

^aMidpoint of DSC scan at 10 °C/min

^Measured on 4 mm thick casting according to ASTM 178 ^cWeight increase after 500 hours immersion in different chemicals

Claims

C L A I M S

1. A composition comprising (1) a liquid epoxy compound comprising on average more than one epoxy group per molecule, (2) a thickening agent comprising on average more than one carboxylic acid group per molecule or the anhydride thereof, which thickening agent effectively thickens the composition at ambient conditions, (3) optionally a catalyst for the reaction between the epoxy compound and the thickening agent, (4) a latent curing agent which effectively cures the epoxy compounds at elevated temperatures, (5) optionally a catalyst for the reaction between the epoxy compounds and the curing agent, (6) optionally reinforcement fibres and (7) optionally a filler compound.

2. A composition according to claim 1, in which the liquid epoxy compound has an average of between 1.5 and

4.0, preferably between 1.7 and 2.5, 1,2-epoxy groups per molecule .

3. A composition according to claim 1 or 2, in which the epoxy compound is the reaction product of a poly-phenol and an epihalohydrin .

4. A composition according to claim 3, in which the epoxy compound has the general formula I

CH₂0CH-CH₂-O-C₆X4-A-C₆X4-0- (CH₂CH (OH) CH OC₆X -A- C₆X 0)_n-CH -CHOCH₂ I in which A is a divalent hydrocarbon group having 1 to 8 carbon atoms, -C(O)-, -0-, -S-, -S-S-, -S(O)-, -S(0)2- or a covalent bond, each X independently is hydrogen, an alkyl group having 1 to 4 carbon atoms, chlorine or bromine, n has an average value of 0 to 12.

5. A composition according to claim 4, in which A is methylene or isopropylene, X is hydrogen or bromine and n has an average value from 0 to 2.

6. A composition according to claim 5, in which A is isopropylene, X is hydrogen and n is up to 0.2.

7. A composition according to claim 6, in which the epoxy compound is an epoxy novolac resin.

8. A composition according to any of claims 1 to 7, in which the thickening agent is a saturated or unsaturated cycloalkane compound comprising five to eight carbon atoms in the ring.