Improving adhesion of acrylate resins

Details Improving adhesion of acrylate resins Improving adhesion of acrylate resins

2000-07-18

2002-12-31

Berman, Susan W. (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C522S104000, C522S134000, C522S146000, C522S170000, C522S173000, C522S174000

Reexamination Certificate

active

06500878

ABSTRACT:

The invention relates to a process for improving the adhesion of radiation-curable acrylate resins comprising amino-functional acrylates on substrates by admixing and amine-hardening a polyepoxide compound.
Radiation-curable coating materials based on acrylate resins are known and are often used for coatings on substrates such as metals or polymer moldings. The polymerization shrinkage which occurs in the course of radiation curing of the coating film applied to the substrate has an adverse effect on the adhesion of the coating material to said substrate. It is known that the adhesion of the radiation-curable coating film may be improved by increasing the molecular weight of the film-forming resins, reducing their double bond density, or adding non-reactive polymers. This, however, leads to a sharp and unwanted increase in the viscosity of the coating materials. Without doubt there exists a need for radiation-curable coating materials which are based on acrylate resins and possess good adhesion to metal or plastics substrates.
We have found that the adhesion of radiation-curable coating materials based on acrylate resins as binders to substrates is improved if the coating materials comprise at least one compound having amine groups and unsaturated acrylate groups and also comprise a small amount of polyepoxide compounds and if the coating film applied to the substrate is cured with high-energy radiation and stored, or thermally conditioned, at a temperature of above 50° C., in particular of from about 50 to 120° C.
The present invention therefore provides a process for improving the adhesive strength of radiation-curable acrylate resins or mixtures thereof, comprising a compound containing at least one amine group and at least one radiation-curable unsaturated acrylate group, to substrates, which comprises admixing the acrylate resins with at least one polyepoxide compound having an epoxide value of from 1 to 15 mol/kg, curing the film of the resultant acrylate resin, applied to the substrate, using high-energy radiation, and conducting at least partial amine hardening of the polyepoxide compounds by treatment at a temperature of above 50° C.
The acrylate resin mixture obtained by admixing the polyepoxide compound(s) is sometimes referred to below as the coating material or coating mixture.
It was surprising that the process not only led to improved adhesive strength between the substrate such as a metal sheet or polymer film and the cured coating film but also brought about a reduction in cracking in the coating films and a reduction in the yellowing of the coating materials. A further surprise was the good storage stability of the uncured mixtures in comparison to mixtures which comprise epoxy resins and low molecular mass aliphatic amines as hardeners.
Acrylate resins are understood to be known reaction products, in resin form, of (i) methacrylic acid and/or acrylic acid with (ii) at least dihydroxy polyesters, polyethers, polyurethanes or epoxy resins (which comprise at least two functional groups which react with (meth)acrylic acid), and also reaction products of (i) hydroxyalkyl (meth)acrylates with (ii) compounds containing isocyanate groups. Radiation-curable acrylate resins of this kind are customary in commerce and are described, for example, in P. K. T. Oldring, Chemistry & Technology of UV & EB Formulation for Coatings, Inks & Paints, Vol. II: Prepolymers & Reactive Diluents, J. Wiley and Sons, New York and Sita Technology Ltd., London 1997, and in H. Kittel, Lehrbuch der Lacke und Beschichtungen [Textbook of Paints and Coatings], Volume VII: Processing of Paints and Coating Materials, pp. 240-245 etc., Verlag W. A. Colomb, Berlin 1979. The acrylate resins, which are subdivided, in accordance with their preparation and the repeating structural units in the molecule chain, into polyester acrylates, polyether acrylates, urethane acrylates, epoxy acrylates and melamine acrylates, are, as what are known as radiation-curable prepolymers, of relatively low molecular mass, generally having an average molecular weight M
n
of from 300 to 15,000 and preferably from 400 to 3000 g/mol, as determined by gel permeation chromatography (GPC) using polystyrene as the standard and tetrahydrofuran as the eluent. The resins contain generally from 0.1 to 1.0 and preferably from 0.1 to 0.5 mol of polymerizable C—C double bonds per 100 g of prepolymer. Very suitable (meth)acrylate resins contain from 2 to 20, in particular from 2 to 10, and preferably from 2 to 6, methacryloyl and/or acryloyl groups in the molecule. Among the acrylate resins, particular suitability is possessed by those derived from polyfunctional aliphatic alcohols having no functional groups other than the hydroxyl groups, except for ether, ester, and urethane groups. Examples of alcohols are dihydric, trihydric and higher polyhydric alcohols such as propylene glycol, diethylene glycol, triethylene glycol, butanediol, hexanediol, neopentyl glycol, cyclohexanediol, glycerol, trimethylolpropane, ditrimethylolpropane, pentaerythritol, dipentaerythritol, and sorbitol. Compounds suitable for preparing polyester acrylates are primarily aliphatic polyester polyols. Polyester acrylates may be prepared in one or more stages from polyols, polycarboxylic acids, and (meth)acrylic acid. They are described, for example, in EP-A 279303. Alcohols suitable for preparing polyether acrylates are, in particular, alkoxylated, preferably ethoxylated and/or propoxylated, polyhydric alcohols, in which the degree of alkoxylation per hydroxyl group may be from 0 to 10.
In accordance with the process of the invention, the acrylate resins in the mixture comprise compounds having at least one amine group and at least one radiation-curable unsaturated acrylate group, especially amine-modified acrylate resins having a molecular weight M
n
of at least 300 and preferably at least 400 g/mol. The mixtures may be of amino-free acrylate resins with compounds having at least one amine group and at least one radiation-curable unsaturated acrylate group, such as an amine-modified acrylate resin, although exclusively amine-modified acrylate resins may also be used with advantage as binders. By amine-modified acrylate resins are meant here acrylate resins which comprise Michael adducts of aliphatic amines with primary and/or secondary amino groups. These may be amine-modified polyether, polyester, epoxy and urethane acrylates, with polyether and polyester acrylates being preferred. Highly suitable acrylate resins are those in which from 0.5 to 60, and in particular from 0.5 to 30, mol % of the (methlacrylic groups are present in the form of Michael adducts of an amine having a primary and/or secondary amino group. The preparation of amine-modified acrylate resins is described, for example, in Patent Applications DE-A 2346 424, DE-A 4007 146, EP-A 211 978, EP-A 280 222 and EP-A 731 121. Amine-modified acrylate resins particularly suitable for the process of the invention are those which have an amine number of from 5 to 450 and preferably from 20 to 250 mg KOH/g. The mixture of the coating components should contain an amine number of from 5 to 250, in particular from 5 to 100, and preferably from 20 to 50 mg KOH/g. Amine synergists based on multifunctional monomers may also be used in some cases.
In addition to the compound having amine groups, the coating mixtures of the invention comprise at least one amine-hardenable polyepoxide compound having an epoxide value of 1-15 and especially 3-8 mol/kg. Preference is given to aliphatic polyepoxides and also aliphatic or aromatic glycidyl ethers and glycidyl esters having at least 2 glycidyl groups. Highly suitable polyepoxides are aliphatic glycidyl ethers such as pentaerythritol triglycidyl ether, trimethylolpropane triglycidyl ether, glycerol triglycidyl ether, and neopentyl glycol diglycidyl ether. The epoxide groups of the polyepoxide compounds may also have been partially reacted with (meth)acrylic acid, subject to the proviso that two intact epoxide groups remain in the molecule. The molecular wei

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