GB2075021A - Epoxy Resin Surface-coating Compositions - Google Patents

Abstract

A surface coating composition having a relatively low viscosity and curing to give a coating having good impact resistance comprises an epoxy resin, a cross-linking agent and a hydroxyl terminated polyester having the formula G-(O-A-H)x where G is an organic radical with x free valancies and A is a polymeric entity containing a plurality of polyester units, preferably polycaprolactone units. The composition may be produced by blending commercially available ingredients and may find application in paint primers, equipment coatings, appliance finishes, can, drum, tank and coil finishes and stoved finishes.

Description

SPECIFICATION Epoxy Resin Surface-coating Compositions Epoxy resins are widely used in combination with cross-linking resins in heat-curable surfacecoating compositions. Such compositions find application in, for example, paint primers, equipment coatings, appliance finishes, can, drum and tank coatings and stoved finishes.

Epoxy resin surface coating compositions have been subject to problems related to their viscosity.

In general, the higher the molecular weight of the epoxy resin in such a composition the higher the viscosity of the composition for a given resin solids concentration. To attain a suitable application viscosity it has been the practice to include solvent in epoxy resin surface coating compositions. The resulting decrease in the resin solids concentration sometimes being very substantial in compositions containing higher molecular weight epoxy resins say, for example, epoxy resins having molecular weights above about 2000 for example, particularly, above 2500. The solvent is removed from the epoxy resin composition prior to, or during, curing resulting in the need for solvent recovery equipment associated with the curing plant and resulting in hazards normally associated with the presence of solvent vapours.Low molecular weight epoxy resins are normally unsuitable for use in epoxy resin surface coating compositions unless subjected to resin modification and for this reason coating compositions containing higher molecular weight epoxy resins, despite the attendant disadvantages, have been relatively common.

As a result of the situation explained above attention has been directed to the development of "high solids" coating compositions which have an acceptable application viscosity while containing a lesser proportion of, or no, solvent. These "high solids" compositions still tend to have a somewhat high application viscosity and, as cured surface coatings, tend to have poor resistance to impact and flexing.

The formulation of some high solids epoxy resin coating compositions requires the carrying out of resin synthesis reactions. This, generally, entails disadvantage in requiring the use of specialised equipment and the exercise of close process control. High solids epoxy resin coating compositions requiring formulation in this manner are disclosed, for example, in US Patent No. 4163029. In the coating compositions disclosed in US Patent No. 4163029 a hydroxyl bearing polyester or a modified derivative thereof is blended with a methylolated melamine. According to one embodiment the modified hydroxyl bearing polyester is the product formed by pre-reacting a particular range of cyclohexene oxide epoxides with the hydroxyl bearing polyester and with a polycarboxylic acid anhydride.

According to one aspect thereof the present invention relates to new or improved epoxy resin coating compositions which may be produced directly by a simple blending operation from commercially available ingredients.

According to a further and different aspect thereof the invention relates to low viscosity high solids epoxy resin coating compositions capable of giving coatings having satisfactory properties.

According to yet a further and different aspect thereof the invention relates to coating compositions containing low molecular weight epoxy resins for example, particularly, unmodified low molecular weight epoxy resins.

The present invention provides an epoxy resin surface-coating composition comprising a blend of (a) at least one polyglycidyl ether epoxy resin, (b) at least one cross-linking agent and (c) at least one hydroxyl terminated polyester having the general formula G-(O-A-H), I wherein G is an organic radical with x free valencies, and A is a straight or branched chain polymeric entity containing a plurality of units, the same or different, selected from units having the general formula -(C0-B-CO-O-CH2-D-CH2-O)- II where B and D are straight or branched chain alkylene groups containing not more than 8 carbon atoms, or in the case of D is a chemical bond, and from units having the general formula -(C0-E-CH2-0)- 111 where E is a straight or branched chain alkylene group having a chain length of from 4 to 9 carbon atoms and containing not more than 9 carbon atoms in total.

Suitably, in the polyester used in accordance with the present invention, the entity A contains units having the general formula II which units may, suitably, be selected from ethylene glycol adipate polyesters, butylene glycol adipate polyesters and neopentyl glycol adipate polyesters and the corresponding sebacate and azelate polyesters.

Preferably, in the polyester used in accordance with the invention, the entity A contains and, particularly preferably, consists of linked units of the general formula Ill. Preferably the entity A is selected from polylactones, particularly preferably, from poly (epsilon caprolactones). Preferably the poly (epsilon caprolactones) are derived from epsilon caprolactones having the genera formula

wherein R may represent an organic substituent such as, for example, a single ring aryl, an alkyl, an alkoxy or a single ring cycloalkyl substituent. Preferably, in at least 6 occurences, R is hydrogen.

Preferably, the total number of carbon atoms in all the occurences of R is not more than 1 2. Particularly preferably, the entity A is an unsubstituted poly (epsilon caprolactone) hereafter referred to merely as "polycaprolactone".

Polylactones may be obtained by the ring-opening polymerisation of lactones by means of an organic initiator compound having functional groups containing reactive hydrogen atoms. Such functional groups may, for example, be hydroxyl groups. It is particularly preferred in the practice of this invention that the reactive groups should consist solely of hydroxyl groups. Depending on the number of reactive groups on a single initiator molecule the polylactones will either be a straight chain or a branched chain material containing, attached to the chain, the residue of the polymerisation initiator molecule. In the general formula I above the said residue is represented by the radical G.

Preferably the polymerisation initiator is a diol such as, for example, ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, dipropylene glycol, 1,3-propylene glycol, polypropylene glycol or polyethylene glycol, a triol such as, for example, glycerol, trimethylol propane, triethanolamine, triisopropanolamine or 1,2,6-hexane triol or a tetrol such as for example, erythritol or pentaerythritol. Alternatively, the initiator may be a compound having a yet greater number of hydroxyl groups, for example up to 8, depending on the number of chain branchings desired in the polyester.

Suitably therefore, in the general formula I above, the radical G has from 2 to 8, preferably 2 to 4, valencies and the value of xis from 2 to 8.

As is well known in the art, the production of the polycaprolactone may be carried out at any temperature from about room temperature up to about 1 800C in the presence of a catalyst. The production of other polyesters which may be incorporated in the composition of the present invention is also well known in the art. One range of polycaprolactones suitable for incorporation in the compositions of the present invention is the CAPA series of polycaprolactones produced by Laporte Industries Limited.

Preferably, the polyester, for example the polycaprolactone, has an average molecular weight of from 200 to 2000 although, particularly preferably it is a liquid polycaprolactone having an average molecular weight of from 250 to 850.

Preferably, the polyglycidyl ether is a reaction product of epichlorhydrin with a di- or polyol.

Preferably the di- or polyol is selected from di- or polyhydric phenols such as for example, resorcinol, pyrocatechol, hydroquinone, 1 ,4-dihydroxy naphthalene, bis-(4-hydroxyphenyl) methyl phenyl methane, bis-(4-hydroxyphenyl) tolyl methane, 4,4' dihydroxy diphenyl, bis-(4-hydroxy phenyl) sulphone or, particularly preferably, 4,4' dihydroxy diphenyl dimethylmethane (Bisphenol "A"). Such ethers may be manufactured by known means involving a condensation between the epichlorhydrin and di- or polyol components under alkaline conditions.

The polyglycidyl ether may, alternatively, be modified with, for example, an amine or amideformaldehyde resin in which last case the resin may provide the cross-linking agent for the composition.

Preferably, the polyglycidyl ether has an average molecular weight of from 800 to 5000, particularly preferably from 850 to 2000. Preferably, the polyglycidyl ether has an epoxide equivalent (that is, the weight of polymer, in g, containing one g equivalent of epoxide) of from 400 to 4000.

Preferably the polyglycidyl ether contains at least 3, particularly preferably from 4 to 8 polyether linkages, on average, per molecule. One range of epoxy resins suitable for incorporation in the compositions of the present invention is the EPIKOTE series of resins produced by Shell based on Bisphenol A and epichlorohydrin.

The cross-linking agent used in accordance with the present invention is, preferably, selected from aldehyde condensation products, for example condensation products with amines, amides or phenol, from amines, for example etherified polyamines, or from blocked isocyanates. The aldehyde used to form the aldehyde condensation products is, preferably, a saturated or unsaturated aliphatic aldehyde having from 1 to 6 carbon atoms, for example formaldehyde, acetaldehyde, acraldehyde or crotonaldehyde, an aromatic aldehyde, for example benzaldehyde, or a heterocyclic aldehyde, for example furfural. Formaldehyde is particularly preferred. The remaining constituent of the condensation product is, preferably, an aliphatic amide containing less than 6 carbon atoms, for example urea, an aromatic alcohol, for example phenol, an aromatic amine, a heterocyclic, amine, for example melamine, or an aromatic substituted heterocyclic amine, such as benzoguanamine. The amine cross-linking agent may be, for example, an aliphatic or aromatic amine, a tertiary amine, an amine adduct or may be a polyamine, for example an etherified polyamine. The etherified polyamide may be an etherified melamine, for example methoxylated, ethoxylated or butoxylated melamine such as hexamethoxymethyl melamine.

The blocked isocyanate cross-linking agent may be for example, a blocked aromatic or aliphatic di- or tri-isocyanate such a blocked toluene di-isocyanate or a blocked hexamethylene di-isocyanate.

The blocking agent is, preferably, selected for its ability to unblock at the curing temperature to be used. Where the curing temperature is to be, for example, above 1 700C the blocking agent may, suitably be diphenyl amine, phenol, isooctyl phenol, hydroxy biphenyl or pyrrolidone. The composition of suitable blocked isocyanate cross-linking agents is fully dealt with in "The development and use of polyurethane products" by E. N. Doyle, published by McGraw Hill 1971.

Particularly preferably the cross-linking agent is a phenol/formaldehyde resin, a urea/formaldehyde resin, a melamine/formaldehyde resin, a benzoguanamine/formaldehyde resin or an etherified melamine. A urea/formaldehyde cross-linking agent is particularly suitable for primer coatings. A melamine/formaldehyde cross-linking agent is particularly suitable for stoved finishes although a urea/formaldehyde cross-linking agent may be used as an alternative. Phenol/formaldehyde cross-linking agents are particularly suitable for industrial applications such as can coatings.

Benzoguanamine/formaldehyde or melamine/formaldehyde cross-linking agents are particularly suitable for appliance finishes.

The coating compositions according to the invention preferably contain no more than the quantity of organic solvent required to give the desired application viscosity having regard to the polyglycidyl ether epoxy resin used. Suitably, the solvent is selected from aromatic hydrocarbons, alcohols, ketones, glycols, esters and glycol ether esters.

The coating compositions may also contain a cross-linking catalyst which may be an acid such as, for example, hydrochloric acid, sulphuric acid, para-toluene sulphonic acid, salts of para-toluene sulphonic acid, such as the dimethyl ethanolamine salt, sulphonic acid, dodecylbenzene sulphonic acid, phosphoric acid, maleic acid, succinic acid or phthalic acid.

Additionally, the coating compositions may, subject to any overriding requirements such as, for example, the need for a clear coating, contain other organic or inorganic ingredients commonly used in the art such as, for example, fillers, extenders or pigments.

Suitable fillers may be, for example, mica, asbestos fines, silica, magnesium carbonate, ground chalk, asphalt, bitumen, cellulose, or glass fibres. Suitable pigments may be for example, inorganic pigment, such as titanium dioxide or lead chrome or organic pigments, such as a diarylamine yellow or phthalocyanine blue. Other additives such as, for example, corrosion inhibitors such as zinc chromate or zinc phosphate may also be included.

The cross-linking agent is preferably in compositions according to the invention in, preferably, from 5% to 40%, particularly preferably from 10% to 35% by weight of the resin components thereof that is, of the epoxy resin, the cross-linking agent and the polyester. Polyester is present in compositions according to the invention in, preferably, at least 1%, particularly preferably at least 5%, for example, particularly, from 10% to 40%, more particularly, from 10% to 30% by weight of the resin components thereof. The cross-linking catalyst may be present in the compositions according to the invention in, preferably, from 0. 1% to 5%, particularly preferably, from 1% to 3% by weight of the resin components thereof.

The compositions according to the invention may be formed by a blending of the constituents.

Preferably, the epoxy resin is firstly dissoived in any organic solvent used, and the epoxy resin or the solution thereby obtained is then, preferably, milled with any pigment or other solid ingredients used.

The other components of the compositions such as the cross-linking agent and the polyester may then be blended in. Alternatively, the cross-linking agent may be included with the epoxy resin as a copolymer. Such copolymers are available commercially. By virtue of the method of formation compositions according to the invention at the moment when curing is initiated, contain functional epoxy groups.

The invention will now be illustrated by means of Examples 1 to 26 of which Examples 1, 3, 5, 7, 9 to 1 3, 1 5, 1 7, 19, 21, 23 and 25 are according to the invention and Examples 2, 4, 6, 8, 14, 16, 18, 20, 22, 24 and 26 are not according to the invention and are inserted for comparative purposes only.

In each example the formulation indicated in the following tables was made up by blending the commercially available ingredients at room temperature for 30 minutes the epoxy resin being first dissolved in the solvent of the formulation. In the case of the pigmented compositions the pigment was ball-milled with the epoxy resin solution before the other ingredients were blended in.

The ingredients used in the formulations were: Epoxy resin Epkote 1001,1004 or 1009 (Numberaveragemol.wt. 900,1400 or 3750 respectively) Shell or Epoxy melamine copolymer (BE647) BIP or Epoxy dehydrated castor oil fatty acid ester (ED4X) Croda Crosslinking Agent Hexamethoxymethyl melamine (HMMM) (BE 370) BIP or Urea/formaldehyde (U/F) resin (BE 610) BIP or Benzoguanamine/formaldehyde (B/F) (BE 659) BIP or Phenol-formaldehyde (PR285) Resinous Chemicals Ltd.

Melamine/formaldehyde (M/F) resin (BE 683) BIP Polyester Polycaprolactone (Capa 200) Laporte Industries Limited Solvents Xylene and 2-Ethoxy ethyl acetate. N-butanol (present as a result of the formulation of the cross-linking agent or the catalyst Catalyst Para-toluene sulphonic Solution acid/dimethyl ethanolamine salt (PTSsDMEA salt) at a 1:1 mole ratio 25% in N-butanol Pigment Titanium dioxide (TIONA) Laporte Industries Limited.

The formulations were stable at ambient temperature and had the resin solid content (wt %) and the viscosity (centipoises) indicated in the following tables.

The formulations were applied to Bonderized (Trade Mark) steel panels or (Ex 23, 24) tin plate with a wire-wound bar coater.

The coated panels were heated at 1 800C for 30 minutes, were allowed to stand at room temperature for 24 hours and were subjected to tests for film hardness (by attempting to scratch the film with Staedtler pencil leads of increasing hardness until the hardest lead which did not penetrate the film was determined) reverse impact resistance (by using a Sheen Tester to allow a 4 Ib (1.8 Kg) weight to fall onto the back of the panel the force, expressed in Kg. cm units, required to cause flaking of the coating being noted) solvent resistance (by rubbing with a methyl ethyl ketone-soaked cloth for 200 double rubs), and flexibility (by the 1800 T bend test recognised by the European Coil Coaters Association-the lower the value given the greater the flexibility). The films manufactured according to the invention showed excellent solvent resistance and film hardness. The results of the reverse impact tests and the flexibility tests are set out in the following Tables.

Examples 1 to 4 Formulation 1 2 3 4 Epoxy resin (1001) 55 75 50 70 HMMM 25 25 - - Urea/Formaldehyde - 44.8 44.8 Polycaprolactone 20 - 20 Xylene 75 75 67.6 67.6 2 Ethoxy Ethyl Acetate 75 75 67.6 67.6 Catalyst solution 1.9 1.9 1.9 1.9 Properties Weight Solids 39.8 39.8 39.8 39.8 Viscosity 30.0 42.5 65.0 80.0 Reverse Impact > 190 < 11.9 > 190 < 11.9 Examples 5 and 6 Formulation 5 6 Epoxy resin (1004) 60 80 HMMM 20 20 Polycaprolactone 20 Xylene 75 75 2 Ethoxy Ethyl Acetate 75 75 Catalyst Solution 1.5 1.5 Properties Weight solids 39.9 39.9 Viscosity 72.5 180.0 Reverse Impact > 190 < 11.9 Examples 7 and 8 Formulation 7 8 Epoxy resin (1009) 65 85 HMMM 15 15 Polycaprolactone 20 Xylene 75 75 2 Ethoxy Ethyl acetate 75 75 Catalyst Solution 1.1 1.1 Properties Weight solids 39.9 39.9 Viscosity 580.0 1962.0 Reverse impact 166 < 11.9 Examples 9 to 12 Formulation 9 10 11 12 Titanium dioxide 100 100 100 100 Epoxy resin (1004) 60 55 55 60 HMMM 20 - - Urea/Formaldehyde - 37.3 35.7 Benzoguanamine/Formaldehyde - - 42.8 Melamine/Formaldehyde - - - 26.7 Polycaprolactone 20 20 20 20 Xylene 43 35.6 36.6 38.8 2 Ethoxy Ethyl Acetate 43 35.6 36.6 38.8 Catalyst solution 9 11 11 9 Properties Reverse impact > 190 > 190 > 190 > 190 Examples 13 Off 14 Formulation Titanium Dioxide 100.0 100.0 Epoxy/Melamine (Be 647) 116.7 166.7 Polycaprolactone 30.0 Xylene 30.0 20.0 2-Ethoxy Ethyl Acetate 30.0 20.0 Catalyst Solution 5 5 Properties Reverse Impact 1 50 10 Examples 15 s 16 Formulation 15 16 Pigment 100 100 Epoxy Ester (ED4X) 91.6 125 Urea/Formaldehyde 37.3 37.3 Polycaprolactone 20.0 Xylene 20.0 13.0 2-Ethoxy Ethyl Acetate 20.0 13.0 Catalyst Solution 7 7 Properties Reverse Impact 150 50 It can be seen from the above examples that the formulations according to the invention showed greatly reduced viscosity, particularly when high molecular weight epoxy resins were used thereby enabling relatively high solids compositions to be applied while still attaining satisfactory coating properties. It can also be seen that the formulations according to the invention incorporating low molecular weight epoxy resins showed satisfactory coating properties.

Examples 1 7 to 26, summarised in the following Tables, were conducted to illustrate the differences between pairs of formulations, of each of which only one is according to the invention, having equal viscosity. The formulations of Examples 17/18, 19/20, 21/22,23/24 and 25/26 were made up to equal viscosities. In each Example the formulation was coated onto Bonderized (Trade Mark) steel panels or tin plate (Ex. 23, 24) to the film thickness indicated and were stoved for 30 minutes at 1 800C. The cured films were found to have equal solvent resistance and pencil hardness between the pairs of Examples. The reverse impact resistance tended to be slightly higher in the Example of each pair which is according to the invention as is seen from the following Tables.The flexibility of the film produced in the Example in each pair which is according to the invention is seen from the Tables to be considerably improved.

Examples 17 to 20 Formulation 17 18 19 20 Pigment 29.35 26.84 29.02 26.59 Microtalc Epoxy resin (1004) 19.07 22.81 15.95 19.94 Epoxy resin (1009) Zinc Chromate Benzoguanamine/Formaldehyde 10.29 9.52 Urea/Formaldehyde Phenol/Formaldehyde Hexamethoxy methyl 4.40 4.03 melamine Polycaprolactone 5.87 5.82 Xylene 20.40 22.92 18.95 21.56 2 Ethoxy Ethyl Acetate 20.40 22.92 18.95 21.56 Catalyst solution 0.51 0.48 0.92 0.83 % wt solids 58.8 53.8 58.3 53.4 Properties Reverse impact > 190 166 > 190 > 190 Flexibility 2 66 1 T 4 Examples 21 to 24 Formulation 21 22 23 24 Pigment 27.63 25.41 Microtalc Epoxy resin (1004) 15.18 19.06 24.12 27.20 Epoxy resin (1009) Zinc Chromate Benzoguanamine/Formaldehyde Urea/Formaldehyde 10.33 9.50 2.33 2.30 Phenol/Formaldehyde 18.76 21.20 Hexamethoxy methyl Melamine Examples 21 to 24 Formulation 21 22 23 24 Polycaprolactone 5.54 - 7.63 Xylene 20.22 22.61 23.58 24.65 2 Ethoxy Ethyl Acetate 20.22 22.61 23.58 24.65 Catalyst solution 0.88 0.81 % wt solids 55.4 51 43.6 40.4 Properties > 190 166 > 190 > 190 Flexibility 2 5 i 2 Examples 25 and 26 Formulation 25 26 Pigment 10.61 5.99 Microtalc 10.61 5.99 Epoxy resin (1004) 17.11 Epoxy resin (1009) 12.48 Zinc Chromate 2.36 1.33 Benzoguanamine/Formaldehyde Urea/Formaldehyde 9.69 6.20 Phenol/Formaldehyde Hexamethoxy methyl Melamine Polycaprolactone 5.92 Xylene 21.31 33.70 2 Ethoxy Ethyl Acetate 21.31 33.70 Catalyst solution 1.08 0.61 % wt solids Properties Reverse impact > 190 166 Flexibility 1 T 3

Claims (21)

Claims

1. An epoxy resin surface-coating composition comprising a blend of (a) at least one polyglycidyl ether epoxy resin (b) at least one cross-linking agent and (c) at least one hydroxyl terminated polyester having the general formula G(OAH)x wherein G is an organic radical with x free valencies and A is a straight or branched chain polymeric entity containing a plurality of units, the same or different, selected from units having the general formula --(C O--BB-CCOO-O-C H,D--CH,,-OO)- where B and D are straight or branched chain alkylene groups containing not more than 8 carbon atoms or, in the case of D, is a chemical bond, and from units having the general formula --(COO--E-CH,,O)- where E is a straight or branched chain alkylene group having a chain length of from 4 to 9 carbon atoms and containing not more than 9 carbon atoms in total.

2. A composition as claimed in claim 1 wherein the polyglycidyl ether epoxy resin is a reaction product of epichlorohydrin with a di- or polyol.

3. A composition as claimed in claim 2 wherein the di- or polyol is a di- or polyhydric phenol.

4. A composition as claimed in any preceding claim wherein the polyglycidyl ether epoxy resin is a reaction product of epichlorohydrin and Bisphenol A.

5. A composition as claimed in any preceding claim wherein the polyglycidyl ether epoxy resin has an average molecular weight of 800 to 5000.

6. A composition as claimed in claim 5 wherein the polyglycidyl ether has an average molecular weight of from 850 to 2000.

7. A composition as claimed in any preceding claim wherein the polyglycidyl ether epoxy resin has an epoxide equivalent of from 400 to 4000.

8. A composition as claimed in any preceding claim wherein the polydiglycidyl ether epoxy resin has, on average, from 4 to 8 polyether linkages per molecule.

9. A composition as claimed in any preceding claim wherein the polydiglycidyl ether is unmodified.

10. A composition as claimed in any preceding claim wherein the cross-linking agent is an aldehyde condensation product with an amide or a phenol, or is an etherified polyamine or is a blocked isocyanate.

11. A composition as claimed in claim 10 wherein the cross-linking agent is a benzoguanamineformaldehyde, urea-formaldehyde, phenol formaldehyde or melamine-formaldehyde condensation product, or is an etherified melamine.

12. A composition as claimed in any preceding claim wherein the hydroxy terminated polyester is an ethylene glycol, butylene glycol or neopentyl glycol adipate, sebacate or azelate.

13. A composition as claimed in any one of claims 1 to 11 wherein the hydroxy terminated polyester is a polylactone.

14. A composition as claimed in claim 13 wherein the polylactone is polyepsilon caprolactone having the general formula

wherein R may represent an organic substituent but is, in at least 6 occurrences, hydrogen the total number of carbon atoms in all occurrences of R not exceeding 12.

15. A composition as claimed in claim 14 wherein the polylactone is "polycaprolactone".

16. A composition as in any one of claims 1 to 1 5 wherein the hydroxyl terminated polyester has an average molecular weight of from 200 to 2000.

1 7. A composition as claimed in claim 16 wherein the hydroxyl terminated polyester has an average molecular weight of from 250 to 850.

1 8. A composition as claimed in any preceding claim also containing a cross-linking catalyst and one or more solvents and having a resin solids content of at least 40%.

19. A composition as claimed in any preceding claim also containing pigment or filler.

20. A composition as claimed in any preceding claim wherein the cross-linking agent is present in from 5% to 40% and the hydroxyl terminated polyester is present in from 1% to 40% by weight of the cross-linking agent, the hydroxyl terminated polyester, and the polyglycidyl ether.

21. A surface coating, the cured product of a composition as claimed in any preceding claim.