Process for coating substrates

Details Process for coating substrates Process for coating substrates

2001-10-31

2004-09-14

Tsoy, Elena (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S385500, C427S558000, C427S595000

Reexamination Certificate

active

06790485

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for coating a substrate in which certain liquid coating compound compositions, after being applied to a body (substrate), are cured by irradiation with ultraviolet (UV) light and thermal post-reaction.
2. Description of the Prior Art
Curing coating compositions or lacquers, by UV light, is very efficient due to the low energy and solvent consumption and the high speed. A disadvantage often cited, however, is that of inadequate adhesion of the coatings to certain substrates, inter alia, metal. For example, EP-A 0 928 800 describes a coating system of urethane (meth)acrylate isocyanates, which contains free isocyanate (NCO) groups and groups which react with NCO groups. For this reason the coating system is prepared as two components which are mixed only shortly before application of the coatings. After mixing the pot life is limited to a few hours.
A one-component system (with regard to the storage of the coating composition) is described, e.g., in U.S. Pat. No. 4,961,960. The system contains a) a light-curing monomer or polymer having a plurality of ethylenically unsaturated groups and b) an adduct of a polyisocyanate, a blocking agent for polyisocyanates and a photo-polymerizable alcohol having at least one ethylenically unsaturated group. Moreover, the requirement is made that component a) is free from NCO groups or blocked NCO groups and that the sum of the ethylenically unsaturated groups contained in a) and b) is at least 4. It is described that the use of polyisocyanates b) that do not contain ethylenically unsaturated group(s), without thermal post-curing, leads to non-resistant films, and with thermal post-curing to films with an inadequate appearance.
Because polyisocyanate products that contain both ethylenically unsaturated groups and blocked NCO groups are difficult to prepare, an object of the present invention is to provide a process for coating which, even with the use of blocked polyisocyanates in combination with UV-curing oligomers or polymers, leads to coatings which are resistant after UV curing and, after additional thermal curing, to optically perfect coatings.
This object may be achieved according to the present invention as described hereinafter.
SUMMARY OF THE INVENTION
The present invention relates to a process for coating a substrate by applying to a substrate a coating composition containing
A) at least one compound which contains at least two (meth)acrylate groups and at least one isocyanate-reactive group and which is free from isocyanate groups and blocked isocyanate groups,
B) at least one blocked polyisocyanate, which does not contain any ethylenically unsaturated groups and
C) at least one photoinitiator,
curing the coating composition by the action of UV light and post-curing by increasing the temperature of the coating.
DETAILED DESCRIPTION OF THE INVENTION
The term “(meth)acrylate”, within the meaning of the invention, relates to esters of acrylic acid and/or methacrylic acid.
Component A) is selected from known radiation-curing binders, provided that the binders contain at least isocyanate-reactive group, preferably at least one hydroxyl group. Examples of these binders include epoxy acrylates, polyester acrylates, polyether acrylates or partially (meth)acrylated trifunctional or higher functionality alcohols having a molecular weight below 600. The use of epoxy acrylates preferably having an average of 1.5 to 3 hydroxyl groups is preferred. Component A) may preferably contains 2 to 4 ethylenically unsaturated groups.
The preparation of epoxy acrylates is described, for example, in P.K.T. Oldring (Ed.), Chemistry & Technology of UV & EB Formulations For Coatings, Inks & Paints, Vol. 2, 1991, SITA Technology, London, p. 37 -68 and in EP-A 816 412. Glycidyl ethers are usually reacted with carboxylic acids and the resulting products contain secondary hydroxyl groups. These products are preferred for the process according to the invention.
Polyester acrylates containing hydroxyl groups and having an OH number of 60 to 300 mg KOH/g may also be used as component A). To prepare the hydroxy-functional polyester acrylates, the following monomers may be used:
1. (Cyclo)alkane diols (i.e., dihydric alcohols with (cyclo)aliphatically bound hydroxyl groups) having a molecular weight of 62 to 286, such as ethane diol, propane 1,2- and 1,3-diol, butane 1,2-, 1,3-and 1,4-diol, pentane 1,5-diol, hexane 1,6-diol, neopentyl glycol, cyclohexane-1,4-dimethanol, cyclohexane 1,2- and 1,4-diol, 2-ethyl-2-butylpropane diol. Also suitable are diols containing ether oxygens such as diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol, polyethylene, polypropylene or polybutylene glycols having a maximum number average molecular weight of 2000, preferably 1000, and more preferably 500. Reaction products of the above-mentioned diols with &egr;-caprolactone or other lactones may also be used as diols.
2. Trihydric and higher functionality alcohols having a molecular weight of 92 to 254 such as glycerol, trimethylolpropane, pentaerythritol, dipentaerythritol and sorbitol. Also suitable are polyethers started on these alcohols such as the reaction product of 1 mole of trimethylolpropane with 4 moles of ethylene oxide.
3. Monoalcohols such as ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol, hexan-1-ol, 2-ethylhexanol, cyclohexanol and benzyl alcohol.
4. Dicarboxylic acids having a molecular weight of 104 to 600 and/or the anhydrides thereof, such as phthalic acid, phthalic anhydride, isophthalic acid, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, cyclohexane dicarboxylic acid, maleic anhydride, fumaric acid, malonic acid, succinic acid, succinic anhydride, glutaric acid, adipic acid, pimelic acid, suberic acid, sebacic acid, dodecanedioic acid and hydrogenated dimer fatty acids.
5. Higher functionality carboxylic acids and the anhydrides thereof such as trimellitic acid and trimellitic anhydride.
6. Monocarboxylic acids such as benzoic acid, cyclohexane carboxylic acid, 2-ethylhexanoic acid, caproic acid, caprylic acid, capric acid, lauric acid, natural and synthetic fatty acids.
7. Acrylic acid, methacrylic acid and dimeric acrylic acid.
Polyester acrylates containing hydroxyl groups contain the reaction product of at least one compound from group
1
or
2
with at least one compound from group
4
or
5
and at least one compound from group
7
.
It is also possible to react a portion of (excess) carboxyl groups, particularly (meth)acrylic acid, with mono-, di- or polyepoxides. This reaction may be used particularly to increase the OH number of the polyester acrylate since one OH group is produced during the epoxide-acid reaction. The acid number of the resulting product is less than 20 mg KOH/g, preferably less than 10 mg KOH/g and more preferably less than 5 mg KOH/g.
The preparation of polyester acrylates is described, for example, in P.K.T. Oldring (Ed.), Chemistry & Technology of UV & EB Formulations For Coatings, Inks & Paints, Vol. 2, 1991, SITA Technology, London, p. 123-135.
It is also possible to use reaction products of the above-mentioned epoxy acrylates, polyester acrylates, polyether acrylates or partially (meth)acrylated trifunctional or higher functionality alcohols having a molecular weight below 600 with di- or polyisocyanates as long as the reaction products contain at least one isocyanate-reactive group, preferably at least one hydroxyl group. Suitable di- or polyisocyanates include (cyclo)aliphatic, araliphatic and aromatic compounds, such as butylene diisocyanate, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), trimethylhexamethylene diisocyanate (2,2,4 and/or 2,4,4-trimethylhexamethylene diisocyanate), bis(isocyanatocyclohexyl)methane, toluene diisocyanate, diphenylmethane diisocyanate, isocyanatomethyl-1,8-octane diisocyanate and derivatives of these diisocyanates containing urethane, isocyanurate, allophanate, biur

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