Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-05-23
2004-09-07
Niland, Patrick D. (Department: 1714)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S372200, C428S423100, C524S591000, C524S839000, C524S840000
Reexamination Certificate
active
06787190
ABSTRACT:
This invention relates to water based paint compositions which are useful for improving chipping resistance and corrosion resistance, etc, of multi-layered coating formed of layers of cationic electropaint, intermediate coating and top coating, which are formed on metallic objects, in particular, car bodies and the like; and also to a process for forming multi-layered coating using such a water based paint composition.
Improvement in resistance properties of multi-layered coating composed of cationic electropaint, intermediate coating and top coating, which is applied onto outer shell portions of metallic car bodies, in particular, prevention of peeling or chipping due to impact exerted by collision with pebbles or gravel upflung by wheels of running cars, and prevention of thereby caused deterioration in corrosion resistance of the coating and progress in corrosion of the metal (coated object or substrate), is being attached increasing importance. For example, in cold districts of Europe and the United States, gravel mixed with a large amount of rock salt is often spread on road to prevent surface freezing, and cars running on such road frequently are hit with rock salt grains or small pebbles upflung by their wheels. Impacts so exerted on coated surfaces of car bodies frequently induce a phenomenon of local peeling of the coating, which is called “chipping”. When the metallic substrate surfaces at the hit spots are exposed as the coating thereon is peeled off, rusting starts from those spots and corrosion progresses easily. Other problems also occur such that chipping of top coating markedly impairs aesthetic appearance of cars. Stripping of coated films by chipping usually takes place more frequently at cars' base or bottom portions and lower part of shell bodies around wheels, but it can occur also at hood and roof portions and local corrosion appreciably progresses within about a half to one year.
For the purpose of preventing such progress in corrosion of metal substrate caused by chipping, for example, U.S. Pat. No. 4,755,435 proposed to apply a barrier coating composed chiefly of modified polyolefin resin between an electrodeposited coating and intermediate coating. The modified polyolefin resin, however, is given a relatively high molecular weight and a large amount of organic solvent is required to dilute it to a viscosity suitable for application. This leads to such defects as adverse effects on safety and health of operators at the application time, environmental pollution after the application and danger of fire, explosion and the like during the application and baking. Moreover, when this barrier coating is left uncured before application of intermediate coating and top coating thereon, finished appearance factors of the multi-layered coating such as flow—and—leveling property are liable to be impaired and, therefore, it is the current practice to heat and dry the barrier coating before applying an intermediate coating thereon.
We have engaged in concentrative studies with the view to solve those problems as described above, and now discovered: a water based paint composition containing a polyurethane resin obtained by reaction of an isoiyanate-containing prepolymer with monohydric alcohol having at least 5 carbons, said prepolymer having been formed by urethanation reaction of polyisocyanate compound with polyalkylene glycol; and a water-soluble organic solvent having boiling point not higher than 150° C., excels in chipping resistance; and furthermore, application of this water based paint composition between a cationic electropaint coating and an intermediate coating enables to form multi-layered coating exhibiting excellent chipping resistance. The present invention is whereupon completed.
Thus, according to the present invention, water based paint compositions (“present paint compositions”) are provided, which are characterized by containing
(a) polyurethane resin obtained by reacting isocyanate-containing prepolymer with monohydric alcohol having at least 5 carbon atoms, said isocyanate-containing prepolymer having been obtained by urethanation reaction of polyisocyanate compound with polyalkylene-glycol, and
(b) water-soluble organic solvent having boiling point not higher than 150° C.
Hereinafter the present paint composition is explained in further details.
The present paint composition can be formulated by, for example, causing conventional water based paint to contain: (a) polyurethane resin which is obtained by reacting isocyanate-containing prepolymer with monohydric alcohol having at least 5 carbon atoms, said prepolymer having been formed by urethanation reaction of polyisocyanate compound with polyalkylene glycol; and (b) a water-soluble organic solvent boiling at no higher than 150° C.
As such water based paints in which the polyurethane resin (a) and the water-soluble organic solvent (b) are incorporated following the present invention, those paints known per se can be used. For example, water based paints which contain, as vehicle component, base resin such as water-dispersible or water-soluble acrylic resin, polyester resin, alkyd resin, urethane resin and the like which have crosslinkable functional groups, e.g., hydroxyl groups, and crosslinking agent such as hydrophilic or hydrophobic melamine resin; and as individual occasions demand, further contain such paint additives as coloring pigment, extender, antisettle agent, organic solvent and the like, can be used, which are formed by mixing and dispersing those components with, and in, water.
Polyurethane resin (a) is obtained by forming an isocyanate-containing prepolymer by urethanation reaction of polyisocyanate compound with polyalkylene glycol, and further reacting the prepolymer with monohydric alcohol having at least 5 carbons.
Polyisocyanate compounds are those containing at least two, preferably two, isocyanate groups per molecule. As such, aliphatic, alicyclic and aromatic diisocyanate compounds are conveniently used, in particular, aliphatic and alicyclic diisocyanates are preferred. Examples of aliphatic and alicyclic diisocyanates include C
2
-C
12
aliphatic diisocyanates such as hexamethylene diisocyanate, 2,2,4-trimethylhexane diisocyanate and lysine diisocyanate; C
4
-C
18
alicyclic diisocyanates such as 1,4-cyclohexane diisocyanate, 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (isophorone diisocyanate), 4,4′-dicyclohexylmethane diisocyanate, methylcyclohexylene diisocyanate and isopropylidenedicyclohexyl-4,4′-diisocyanate; and modifications of those diisocyanates (e.g., carbodiimide-, urethodione- or urethoimine-containing modified products). They may be used either singly or as combination of more than one.
Polyalkylene glycols to be reacted with these polyisocyanate compounds include polyether glycols which are obtained by (block or random) polymerization or copolymerization of at least one compound selected from alkylene oxides (for example, C
2
-C
7
alkylene oxides such as ethylene oxide, propylene oxide, butylene oxide and the like) and cyclic ethers (for example, tetrahydrofuran and the like). Specific examples include polyethylene glycol, polypropylene glycol, polyethylene-propylene (block or random) glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol and polyoctamethylene ether glycol. It is generally preferred for these polyalkylene glycols to have number-average molecular weight ranging 500-5,000, in particular, 1,000-3,000.
In the occasion of preparing such polyurethane resin (a), it is permissible to use, concurrently with those polyisocyanate compound and polyalkylene glycol, low molecular weight glycol(s) having number-average molecular weight less than 500, such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol and bishydroxymethylcyclohexane. Use rate of such low molecular weight glycol is usually not more than 20% by weight, preferably 0.1-20% by weight, inter alia 0.5-10% by weight, based on the weight of the polyalkylene glycol used.
Urethan
Kansai Paint Co. Ltd.
Niland Patrick D.
Wenderoth Lind & Ponack LLP
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