Coating composition

Floor coverings include a wear layer made from a coat composition including a multifunctional polyurethane (meth)acrylate oligomer. The multifunctional polyurethane is made by reacting a polyisocyanate with a functionality equal or higher than 3, a polyester polyol, and a hydroxyalkyl-(meth)acrylate with a number average molecular weight of about 344 to about 472. The coating composition preferably includes a reactive (meth)acrylate diluent and a photoinitiator.

FIELD OF THE INVENTION 
The present invention relates to a coating composition including a 
multifunctional polyurethane (meth)acrylate oligomer and floor coverings 
made with the coating composition, and more particularly, to 
radiation-curable coatings which have superior gloss retention, abrasion, 
gouge and stain resistance surfaces. 
BACKGROUND OF THE INVENTION 
Coatings as wear layers on floor coverings are designed to provide 
durability and cleanability to the flooring materials. Durability in this 
case refers to the ability of the coatings to resist wear and tear. The 
wear and tear come in two forms: (a) abrasion in micro scales at the very 
top of the surface that results in changes of gloss and appearance, and 
(b) deep scratch and gouge damages from soils or other grit particles 
dragged under foot traffic. Cleanability refers to the ability of the 
coatings to resist stain and chemical attacks, and maintain their original 
appearance. 
Many coating systems have been used by the flooring industry to achieve 
durability and cleanability. Polyurethane-based coatings, in particular, 
are well known in the art for such purposes. 
U.S. Pat. No. 3,929,929 discloses an improved vinyl urethane resin 
comprising the reaction product of a diol, a polyisocyanate having a 
functionality equal to from about 2.1 to about 3.0, and a hydroxyl 
terminated ester of acrylic or methacrylic acid. The resin is prepared 
with either vinyl or allyl monomers and cobalt-activated peroxide-cured to 
form castings, coatings, and laminates. 
U.S. Pat. No. 4,034,017 discloses a solvent-soluble coating composition 
comprising polyisocyanate, diol, and a hydroxyalkyl ester of an 
ethylenically unsaturated carboxylic acid, one or more copolymerizable 
monomers and a curing agent. When cured, the composition is claimed to be 
durable, adherent, and highly flexible. 
U.S. Pat. No. 4,342,793 discloses a method for forming interpenetrating 
polyurethane and polyacrylate networks. The system is a two-component 
composition based on a radiation sensitive diluent, a saturated acrylic 
polyol and a polyisocyanate. Cure is accomplished by radiation and then 
heat. The fully cured composition is described as interpenetrated or one 
in which there is no crosslinking between the reactive diluent and the 
urethane linked copolymer. Although the interpenetrated resin composition 
is claimed to form tough and hard coatings on various substrates, 
including floor coverings, the coating has a limited pot-life which 
renders complications for the application of the coating. 
U.S. Pat. No. 5,003,026 discloses an improved UV-curable coating for a 
floor covering which comprises an oligomer which is the reaction product 
of a polyester polyol with a diisocyanate and a monohydroxymonoacrylate. 
The oligomer is diluted in mono- and diacrylate reactive monomers and 
photoinitiators. The coating when UV-cured provides good gloss retention, 
hydrolyric stability but poor stain resistance. 
These and other references are directed to polyurethane-based compositions 
which provide either gloss retention, stain resistance or gouge, but none 
of the references offer compositions that encompass all three properties. 
Consequently, the need exists for performance coatings which have the 
aforementioned gloss retention, stain, abrasion and gouge resistance for 
floor coverings. 
Accordingly, it is one of the objectives of the present invention to 
provide a novel radiation-curable composition which will demonstrate 
superior gloss retention, gouge, abrasion, and stain resistance. 
Another objective of the present invention is to provide a floor covering 
having a wear layer of the coating composition. 
Yet another objective of the present invention is to provide an oligomer 
which will yield a composition having the above-stated properties. 
These and other advantages of the present invention will become apparent 
from the detailed descriptions of preferred embodiments which follow. 
SUMMARY OF THE INVENTION 
The oligomer which compromises the coating composition of the present 
invention is the reaction product of a polyisocyanate with a functionality 
equal or higher than 3, preferably equal to or higher than 3.5, an 
aromatic polyester, preferably a phthalate polyester polyol, and a 
hydroxyalkyl(meth)acrylate with a molecular weight of about 344 to about 
472. The resulting oligomer will be liquid at room temperature, highly 
branched, and have multi(meth)acrylate functionality. As used herein, term 
"(meth)acrylate" and its variants mean "acrylate, methacrylate and 
mixtures thereof." 
The coating composition may consist essentially of the oligomer. However, 
the oligomer without a diluent is viscous and difficult to coat. Therefore 
the coating composition of the present invention preferably includes a 
diluent. The preferred diluent is a reactive (meth)acrylate diluent. 
The coating composition may be cured either by electron beam curing or UV 
curing. The preferred composition includes a photoinitiator and the 
composition is cured with both electron beam and UV light. 
DETAILED DESCRIPTION OF THE INVENTION 
It is well known that abrasion resistance and non-yellowing coatings can be 
obtained from radiation-curable polyurethane (meth)acrylate based on 
aliphatic diisocyanates such as isophorone diisocyanate (IPDI) or 
methylene-bis(4-cyclohexylisocyanate) (Bayer Corporation's Desmodur W). 
However, higher functionality polyisocyanates such as isocyanurate, biuret 
and allophanate have not been used in the radiation-cured coatings, nor 
are such uses disclosed in the prior art. 
When used, they are one component of a two component system and require 
thermal cure to achieve their useful properties. U.S. Pat. No. 4,342,793, 
for example, discloses the use of Desmodur N-100 (a biuret) in a thermally 
cured reaction with a hydroxy-functional acrylic resin. The NCO/OH 
reaction is a two-component system within a UV-cured coating matrix to 
produce a polyurethane-polyacrylate interpenetrating network. Because of 
their high reactivity, two-component coating systems such as this must be 
mixed just prior to the application. After the two-components are put 
together, there is a limited pot-life in which the mixture must be used up 
before it is rendered useless. Further, U.S. Pat. No. 4,342,793 does not 
disclose the reaction of the polyisocyanate with a long chain 
hydroxyalkyl(meth)acrylate or an aromatic (meth)acrylic polyol. 
It was discovered that reacting isocyanurate trimers such as Bayer's 
Desmodur N3300 or biurets such as Bayer's Desmodur N100 or N3200 with a 
long chain hydroxyalkyl(meth)acrylate and a polyester polyol could produce 
one-component radiation-curable oligomers which have a long shelf-life. 
The oligomers would produce radiation-curable coatings of superior stain, 
gloss retention, abrasion and gouge resistance. 
Preferred oligomers were prepared by reacting about 0.7 to 1.5 equivalents 
of Desmodur N-3300 with 1.0 equivalent of a mixture of 
hydroxyalkyl(meth)acrylate and phthalate polyester polyols (equivalent 
ratio of hydroxyalkyl(meth)acrylate to polyester polyol greater than about 
0.10) to produce a highly branched, multifunctional (meth)acrylate 
polyurethane product. The product was mixed with acrylate reactive 
diluents and photoinitiators to form a coating composition. Especially 
valuable properties were obtained when substantial quantities of high 
molecular weight, ethoxylated and propoxylated tri(meth)acrylates were the 
principle reactive diluents. 
An aromatic polyester polyol is defined as a polyester polyol of acid 
number of less than about 15, preferably less than about 5, comprising the 
reaction product of an equivalent excess of one or more polyols of 
equivalent weight less than 150 with at least one aromatic polycarboxylic 
acid. If the polycarboxylic acid is a phthalic acid derivative the result 
is a phthalate polyester polyol. Preferably at least 50 equivalent percent 
of the polycarboxylic acid is isophthalic acid, phthalic acid, 
terephthalic acid, phthalic anhydride, or dimethyl terephthalate. 
The preferred multifunctional polyurethane (meth)acrylate of the present 
invention can be represented by the following Formula 1. 
##STR1## 
The isocyanurates, biurets and allophanates of the present invention can be 
aliphatic, cycloaliphatic or aromatic such as those prepared from 1,6 
hexamethylene diisocyanate (Desmodur H); 
methylene-bis(4-cyclohexylisocyanate); 
1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane; 
2,2,4-trimethylhexamethylene diisocyanate; 2,4,4-trimethylhexamethylene 
diisocyanate; toluene diisocyanate; methylenediphenyl diisocyanate; and 
tetra methylxylene diisocyanate. The preferred polyisocyanates are the 
trimer isocyanates represented by R in Formula 1 (isocyanurate, biuret and 
allophanate). The most preferred trimers are those derived from 
1,6-hexamethylene diisocyanate which is commercially available as Desmodur 
N from Bayer Corporation. 
The preferred hydroxyalkyl(meth)acrylates are polycaprolactone based 
hydroxy(meth)acrylates according to the skeletal structures in Formula 2: 
##STR2## 
where 
R"=H or CH.sub.3, and 
p=2 or 3. 
The polycaprolactone based hydroxy(meth)acrylates are supplied commercially 
by Union Carbide as Tone M-100 (number average molecular weight of about 
344) and by San Esters Corporation as Placcel FA and FM series (number 
average molecular weights from about 230 to about 472). 
The preferred oligomer is prepared from Desmodur N-3300, Tone M-100 and a 
predominantly 1,6-hexylene phthalate polyester polyol. A predominantly 
1,6-hexylene phthalate polyester polyol is defined as a phthalate 
polyester polyol wherein at least 50 equivalent percent of the &lt;150 
equivalent weight (eq. wt. ) polyol is 1,6-hexanediol and wherein at least 
50 equivalent percent of the polycarboxylic acid is phthalic acid or 
phthalic anhydride. 
Phthalate polyester polyols suitable for the present invention can be 
prepared by a procedure similar to example 1 of U.S. Pat. No. 4,138,299 by 
reacting acids such as isophthalic, terephthalic, phthalic (or phthalic 
anhydride), adipic, azelaic, and/or sebacic with polyols such as 
1,6-hexanediol; trimethylol propane; triethylene glycol; neopentyl glycol; 
cyclohexane dimethanol; 2-methyl-1,3-propanediol; 
trihydroxyethylisocyanurate; and glycerol in the presence of a catalyst 
such as dibutyltin bislauryl mercaptide or stannous oxalate. 
Suitable (meth)acrylate reactive diluents include (meth)acrylic acid, 
isodecyl (meth)acrylate, N-vinyl formamide, isobornyl (meth)acrylate, 
tetraethylene glycol (meth)acrylate, tripropylene glycol (meth)acrylate, 
hexanediol di(meth)acrylate, ethoxylate bisphenol-A di(meth)acrylate, 
ethoxylated neopentyl glycol di(meth)acrylate, propoxylated neopentyl 
glycol di(meth)acrylate, ethoxylated tripropylene glycol di(meth)acrylate, 
glyceryl propoxylated tri(meth)acrylate, tris (2-hydroxy ethyl) 
isocyanurate tri(meth)acrylate, pentaerythritol tri(meth)acrylate, 
pentaerythritol tetra(meth)acrylate, dimethylol propane tri(meth)acrylate 
dipentaerythritol monohydroxypenta(meth)acrylate, and trimethylol propane 
tri(meth)acrylate and its ethoxylated and propoxylated analogues of the 
skeletal structures in Formula 3: 
##STR3## 
where 
R"=H, or CH.sub.3, and 
q=0, 1, 2, . . . , 9 or 10. 
The preferred (meth)acrylate reactive diluents are the multifunctional 
acrylates with number average molecular weights of about 226 to about 
2000. Examples of such are tetraethylene glycol diacrylate with a 
molecular weight of about 302, ethoxylated bisphenol-A diacrylate with a 
number average molecular weight of about 776 (SR602 from Sartomer 
Company), trihydroxyethyl isocyanurate triacrylate with number average 
molecular weight of about 423 (SR368 from Sartomer), trimethylol propane 
triacrylate with a number average molecular weight of about 296 (SR351 
from Sartomer), and ethoxylated trimethylol propane triacrylates with 
number average molecular weights from about 400 to about 2000 (SR454, 
SR499, SR502, SR9035, and SR 415 from Sartomer Company and Photomer 4155 
and Photomer 4158 from Henkel Corporation). 
To practice the present invention, it is preferred to employ 
photoinitiators by themselves or in combinations and/or in combination 
with photoactivators and photosensitizers. Suitable photoinitiators 
include benzophenone; 4-methylbenzophenone; benzyl dimethyl ketal; 
diethoxy acetophenone; benzoin ethers; thioxanthones; 1-hydroxycyclohexyl 
phenyl ketone (Irgacure 184 from Ciba Corp); 
2-hydroxy-2-methyl-1-phenol-propane-1-one; 
4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-methylpropyl) ketone; 
2,4,6-trimethylbenzoyl diphenylphosphine oxide; bis 
(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide; 
2,2-dimethoxy-2-phenyl acetophenone; 
2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone; and 
2-methyl-1-4-(methylthio)phenyl!-2-morpholino propan-1-one. A preferred 
photoinitiator is benzophenone, which may be used in conjunction with 
other photoinitiators, photoactivators and/or photosensitizers. A 
preferred combination is about two parts benzophenone per one part of 
1-hydroxycyclohexyl phenyl ketone. 
In addition, other components which may be useful can also be included in 
the present invention. These include flow additives, free radical 
inhibitors, thermal stabilizers, light stabilizers, dyes, pigments, 
optical brighteners, flatting agents, plasticizers, and others as would be 
obvious to one skilled in the art. 
In preparing the oligomer of the present invention, a catalyst such as 
dibutyltin dilaurate may or may not be included to aid the urethane 
formation between the polyisocyanate and the hydroxy-containing 
components. However, it is advantageous to prepare the oligomer in a clean 
vessel free of moisture or water. The vessel is equipped with dry-air 
blanket and a heating jacket, and has continuous mixing. It is further 
advantageous to prepare the oligomer in the presence of substantial 
amounts of the reactive acrylate diluents to help control heat of 
reactions which may otherwise cause undesired side reactions. The reaction 
temperature is maintained at about 75.degree. C. to about 85.degree. C. 
Higher temperatures can initiate undesired free radical polymerization. On 
the other hand, lower temperatures will retard or slow down the NCO/OH 
reaction. 
The compositions of the present invention can be applied by brushing, 
roller, gravure, spin, flow, or curtain coating at room temperature to an 
elevated temperature of up to about 75.degree. C. The composition of the 
present invention has an excellent high temperature stability. Prolonged 
exposure to elevated temperature does not have any adverse effect on its 
superior properties. The radiation cure is achieved with ultraviolet or 
electron beam radiation. Although no photoinitiator is required when the 
composition is cured by electron beam, including a photoinitiator can 
further enhance the properties of the present invention.