Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
1995-06-06
2002-04-09
Sergent, Rabon (Department: 1711)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S372200, C427S485000, C528S045000, C528S049000, C528S073000
Reexamination Certificate
active
06368669
ABSTRACT:
The present invention relates to novel blocked polyisocyanates, methods for making them, coating and other compositions e.g. paints and elastomers, containing them and methods of electrodeposition of the coating compositions.
Blocked polyisocyanates are commonly used in paints which also contain active hydrogen containing compounds e.g. amines and alcohols. Certain of these paints can be electrophoretically or conventionally e.g. spray deposited onto the article to be coated and then subsequently hardened by heating, often referred to as stoving. During stoving the blocked polyisocyanates dissociate so that the isocyanate groups become available to react with the active hydrogen containing compounds leading to crosslinking and hardening of the paint.
Blocked polyisocyanates reacting with any active hydrogen containing compound for the purposes of curing by chain extension or crosslinking are also used in cross-linking acrylic resins for automotive priming and finishing, formulating one-pack elastomers and surface coatings which contain the blocked isocyanate and as a chain extender in a single storage stable package which, when cast, can be cured by application of temperatures above the unblocking temperature.
Blocked polyisocyanates are polyisocyanates in which each isocyanate group has reacted with a protecting or blocking agent to form a derivative which will dissociate on heating to remove the protecting or blocking agent and release the reactive isocyanate group.
Compounds already known and used as blocking agents for polyisocyanates include aliphatic, cyclo-aliphatic or aralkyl monobydric alcohols, hydroxylamines and ketoximes.
Currently used blocked polyisocyanates dissociate at temperatures of around 160° C. If a blocked polyisocyanate could be used which dissociated at a lower temperature but was still stable at ambient temperatures, then heat sensitive materials could be utilised and energy savings could be made. The blocked polyisocyanates of the present invention dissociate at a significantly lower temperature than those currently used and are easily made. The presence of a catalyst is preferred in order to increase the rate of reaction between the liberated polyisocyanate and the active hydrogen containing compound, especially if the active hydrogen group is —OH. The catalyst can be any catalyst known in the art, e.g. dibutyl tin dilaurate or triethylene diamine.
The present invention comprises a compound of the formula:
R—Y
m
(I)
where R is an m valent aliphatic, cycloaliphatic , heterocyclic or aromatic residue and each Y, which may be the same or different, is
where R
1
is, or, when n is more than 1, each R
1
, which may be the same or different, is an alkyl, alkenyl, aralkyl, N-substituted carbamyl, phenyl, NO
2
, halogen or
group where R
2
is a C
1
-C
4
alkyl group,
n is 0, 1, 2 or 3
and m is an integer al, preferably 2-6.
When R
1
represents an alkyl or alkenyl group it preferably contains up to 4 carbon atoms. When it is an aralkyl group, it is preferred that the aryl portion is phenyl and that the alkyl portion contains 1 to 4 carbon atoms. When R
1
is a halogen, it is preferably chlorine or bromine.
The blocking agents used in the present invention are pyrazoles of the formula:
where R
1
and n are as defined above. Examples of the pyrazoles described include 3,5-dimethylpyrazole, 3-methylpyrazole, 4-nitro-3,5-dimethylpyrazole and 4-bromo-3,5-dimethylpyrazole.
The preferred blocking agent is 3,5-dimethylpyrazole.
Some of these pyrazoles can be made by converting acetylacetone (AA) into a derivative which will react with hydrazine to give the desired pyrazole e.g.
AA+Na+CH
2
═CHCH
2
Cl→Ac
2
CHCH
2
CH═CH
2
AA+Na+PhCH
2
Cl→Ac
2
CHCH
2
Ph
AA+PhNCO→Ac
2
CHCONHPh
The polyisocyanate which is to be blocked may be any organic polyisocyanate suitable for crosslinking compounds containing active hydrogen e.g. aliphatic including cycloaliphatic, aromatic, heterocyclic, and mixed aliphatic aromatic polyisocyanates containing 2, 3 or more isocyanate groups. The group R will normally be a hydrocarbon group but substitution e.g. by alkoxy groups is possible.
The isocyanate compound may be, for example, ethylene diisocyanate, propylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, decamethylene diisocyanate, dodecamethylene diisocyanate, 2,4,4-trimethylhexamethylene-1,6 diisocyanate, phenylene diisocyanate, tolylene or naphthylene diisocyanate, 4,4′-methylene-bis(phenyl isocyanate), 4,4′-ethylene-bis (phenyl isocyanate), &ohgr;,&ohgr;′-diisocyanato-1,3-dimethyl benzene, &ohgr;,&ohgr;′-diisocyanato-1,4-dimethyl cyclohexane, &ohgr;,&ohgr;′-diisocyanato-1,4-dimethyl benzene, &ohgr;,&ohgr;′-diisocyanato-1,3-dimetbylcyclohexane, 1-methyl-2,,4-diisocyanato cyclohexane, 4,4′-methylene-bis (cyclohexyl isocyanate), 3-isocyanato-methyl-3,5,5-trimethyl cyclohexyl isocyanate, dimer acid-diisocyanate, &ohgr;,&ohgr;′-diisocyanato-diethyl benzene, &ohgr;,&ohgr;′-diisocyanatodimethyl toluene, &ohgr;,&ohgr;′-diisocyanato-dietbyl toluene, fumaric acid-bis (2-isocyanato ethyl) ester or triphenyl-methane-triisocyanate, 1,4-bis-(2-isocyanato-prop-2yl) benzene, 1,3-bis-(2-isocyanato prop-2yl) benzene, but is preferably free from isocyanate groups directly attached to aromatic nuclei.
Use can also be made of polyisocyanates obtained by reaction of an excess amount of the isocyanate with a) water, b) a lower molecular weight polyol (e.g. m.w.≦300) or c) a medium molecular weight polyol, e.g. a polyol of greater than 300 and less than 8000 m.w., eg sucrose, or by the reaction of the isocyanate with itself to give an isocyanurate.
The lower molecular weight polyol comprises, for example, ethyleneglycol, propyleneglycol, 1,3-butylene glycol, neopentyl glycol, 2,2,4-trimethyl-1,3-pentane diol, hexamethylene glycol, cyclohexane dimethanol, hydrogenated bisphenol-A, trimethylol propane, trimethylol ethane, 1,2,6-hexane triol, glycerine, sorbitol or pentaerythritol.
The di- or poly-isocyanate obtained by the above reaction may have a biuret structure, or an allophanate group.
The blocked polyisocyanate of the formula I is formed by admixing the polyisocyanate
R(NCO)
m
(III)
with a sufficient quantity of a pyrazole of the formula:
such that the reaction product contains no free isocyanate groups and is a urea of formula I. This reaction is exothermic and since the reaction product will dissociate if the temperature is raised sufficiently, cooling may be required to keep the temperature of the reaction mixture down, preferably to 80° C. or less.
One use of blocked polyisocyanates is in electrophoretically deposited paints. The invention also comprises a paint composition comprising a pigment carrier containing active hydrogen groups, a pigment and a compound of the formula I.
These paints are usually composed of a pigment dispersed in an aqueous dispersion of a resin containing active hydrogen which is to be crosslinked by the polyisocyanate. Preferably the paint contains 0.5 to 2 blocked isocyanate groups per active hydrogen containing group. Suitable active hydrogen containing resins include polyamide-polyamine resins, e.g. the product from a dimer fatty acid and an aliphatic polyamine, carboxylic acid group containing acrylic resins, and tertiary amine group containing hydroxyacrylic resins and polymers thereof.
The total concentration of the dispersed solids will, of course, depend upon the process for which the paint is to be used. Various standard additives such as surface active agents, catalysts and anti-oxidants may also be incorporated.
The invention also comprises a method of electrodepositing onto substrates a paint composition as described above and then heating the deposited paint to cross link the pigment carrier.
The electrophoretic deposition process is well known and involves the use of a cathode and an anode in contact with a bath containing the paint. The surface to be coated is one of the electrodes. On applying a
Hughes Anthony H
Topham Arthur
Nixon & Vanderhye
Sergent Rabon
The Baxenden Chemical Co.
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