Photocrosslinkable thermoplastic urethane coating system

The invention is directed to a coating system which is adaptable to being crosslinked subsequent to being embossed. The system comprises a linear polyurethane that contains sites of unsaturation only in its polymeric backbone and a photoinitiator. The polyurethane is produced by the reaction of a diisocyanate and an unsaturated polyester diol.

SUMMARY OF THE INVENTION 
Utilizing polyurethanes in photoreactive polymer systems is known in the 
art. Generally, such systems are unsaturated acrylate or methacrylate 
terminated polyurethanes obtained by the reaction of hydroxyalkyl acrylate 
or methacrylate esters or N-metholacrylamide with isocyanate end groups in 
the polyurethanes and require, in addition to the polyurethane, at least 
one added polymerizable monomer to cause a photocrosslinking reaction. 
Such prior art photoreactive systems are applied to a substrate in liquid 
form and, as such, cannot be effectively embossed. Coatings embossed in 
such a fluid state would not retain any dimensionality and would flow and 
level. Such coatings could only be embossed after being converted via 
irradiation to a cured, cross-linked, network and, as such, could not be 
embossed very sharply. In other photoreactive polymer systems, such as 
those used in imaging systems, a photoactive unsaturated side chain or a 
pendant substituent such as cinnamate or benzylideneacetone is required to 
produce the desired photocrosslinking. 
The present invention provides for tack-free, non-liquid, thermoplastic 
film that, unlike the prior art liquid photoreactive systems discussed 
above, can be embossed prior to being crosslinked and thus, can be 
embossed much more sharply than a film capable of being embossed only 
after crosslinking. Upon being crosslinked, the film will be converted to 
a product with a sharp, clearly defined, permanently embossed surface. 
Such desirable characteristics are realized by a photoreactive coating 
system that is comprised of (a) a linear polyurethane that contains sites 
of unsaturation, that is, carbon-carbon double bond moieties, only in its 
polymeric backbone and (b) a photoinitiator. The polyurethanes utilized in 
the coating system of the present invention are produced by a reaction of 
a diisocyanate and an unaturated polyester diol. Unlike prior art 
photoreactive polyurethanes, no added polymerizable monomer is required 
for crosslinking of the polyurethanes of the present invention. 
The present invention also relates to an embossed material and a method for 
producing such an embossed material, which method comprises the steps of 
applying to a substrate a coating of unsaturated polyurethane as defined 
above in combination with a photoinitiator, embossing the polyurethane 
coating and thereafter irradiating the coating with actinic radiation to 
crosslink the polyurethane. Prior to being embossed, it is preferred that 
the coating be subject to drying to evaporate any solvent carrier for the 
polymer and curing to chain extend the polymer. 
It has been found that embossed materials made by this method have sharper 
and deeper embossing than conventional embossed polyurethanes materials 
which are typically thermally crosslinked prior to embossing. 
The polyurethanes utilized in the present invention should contain a 
sufficient amount of unsaturated sites in their polymeric backbones to 
permit crosslinking. However, the ideal level of unsaturated sites in the 
polyurethane will depend to a great extend on the end use application of 
the coating. For example, when the materials are to be used as coating for 
resilient flooring, it has been discovered that typically from about 3 to 
about 6 mol percent of unsaturated sites will provide for a product with 
the optimum wear appearance properties. In fact, the lower point of this 
range (at about three mole percent of unsaturated sites) is the preferred 
minimum level of unsaturation. This minimum level is needed for use 
applications requiring better performance in wear characteristics such as 
durability, permanence of embossing, solvent resistance, stain resistance, 
gloss retention, abrasion resistance, etc. The lower levels of 
unsaturation (less than about 3 mole percent of unsaturated sites) will be 
utilized for applications and uses that can tolerate lower performance in 
these characteristics. 
The aromatic, aliphatic or alicyclic diisocyanates used to prepare the 
polyurethanes utilized in the coating systems of the present invention are 
of the general formula: OCN--R--NCO, wherein R is either R1 or R2-CH2-R3, 
wherein R1, R2 and R3 independently represent organic radicals selected 
from the group of alkyl, cycloalkyl and aryl. 
The term "alkyl" is used herein to represent both straight and branched 
chain alkyl groups having from 1 to about 6 carbon atoms. The term 
"cycloalkyl" refers to a cycloalkyl group having 3 to 7 carbon atoms. The 
term "aryl" represents phenyl, napthyl, toluyl or xylyl radicals. 
The polyester diols that are reacted with the above diisocyanate to prepare 
the polyurethanes utilized in the coating system of the present invention 
are random copolymers that generally have a number average molecular 
weight in the range of from about 1,500 to about 2,500, with a preferred 
molecular weight of about 2,000. These polyester diols are prepared by 
reacting at least one diacid with at least one diol. Of the diacids 
utilized, at least one will provide the unsaturated component in the 
backbone of the random polyester diol and, subsequently, in the backbone 
of the polyurethane itself. The preserred unsaturated diacid will be of 
the fumaric or maleic type such as, but not limited to, maleic acid, 
fumaric acid, mesaconic or citraconic acid, (which are the 2-methyl 
substituted derivatives of, respectively, fumaric and maleic acids). 
Additional diacids that may be utilized in the preparation of the polyester 
diol will be of the general formula: 
##STR1## 
where X can be an alkyl (straight, chain or branched), alkenyl, or aryl 
moiety. 
Diols suitable for use in the preparation of the polyester diol will be of 
the general formual: 
EQU HO--Z--OH, 
wherein Z is either Z1 or Z2-O-Z3, wherein Z1, Z2 and Z3 are independently 
selected from alkyl and cycloalkyl radicals. 
The reactants uitlized to produce the polyurethanes utilized in the present 
invention must be difunctional to provide for a linear, thermoplastic 
polymer. The use of trifunctional reactants, such as glycerol or a 
triisocyanate, would result in a premature crosslinking, i.e. prior to 
U.V. irradiation, which would not be desirable in the present system. 
The polyurethanes of the present invention are prepared by the reaction of 
the components specified above according to reaction conditions well known 
to those skilled in the art, i.e., the reaction will generally be run in a 
dry organic solvent medium in the presence of an appropriate tin catalyst. 
Typical reaction temperatures of about 100.degree. C. will lead to the 
desired polyurethane formation. 
In preparing a photocurable coating formulation in accordance with the 
present invention, the aforementioned unsaturated polyurethane is combined 
with a photoinitiator which is effective in initiating crosslinking of the 
polymer on exposure to radiation. For this purpose, the photoinitiator is 
advantageoulsy benzophenonone or -phenylacetophenone derivates such as 
benzil, benzoin and the benzoin ethers, e.g. benzoinmethyl, -ethyl, 
-n-propyl and -isopropyl ethers. While the amount and type of 
photoinitiator employed will vary with the intensity and dosage of radiant 
energy to be employed and the specific polyurethane being utilized, the 
photoinitiator is usually employed in amounts from about 1 to about 20 and 
preferably from about 5 to about 10 weight percent based on the weight of 
the resin making up the coating formulation. 
In addition to the foregoing photoinitiators, other additves such as impact 
modifiers (rubber polymers and elastomers), pigments and fillers, 
stabilizers, fire retardants, etc., can be employed in the coating 
composition of the present invention. The specific additives utilized will 
depend on the desired end use of the coating composition. 
After being embossed, the coating or film of the present invention is then 
exposed to sufficient actinic radiation to cure the coating or film to a 
material that is insoluble in aqueous or organic liquid media. For 
purposes of this invention, actinic radiation is defined as being any 
radiation which will cause the desired crosslinking reaction. 
Examples of actinic radiation advantageously employed include ultraviolet 
light; accelerated particulate (oinizing) radiation wherein particulates 
include electrons, protons, neutrons, etc.; X-rays; and the like, with 
ultraviolet light being preferred. In the case of ultraviolet light 
radiation, suitable intensity is supplied by mercury vapor lamps. 
The polyurethane coatings made according to one aspect of the present 
invention may be utilized to coat substrates such as resilient flooring, 
wood panels, paper, plastic sheets, sheet metal, structural foam and the 
like. The coatings are readily cast, sprayed or otherwise applied as films 
or coatings to substrates by conventional coating techniques employed by 
those skilled in the art. typically, the thickess of the film or coating 
will vary from about 0.025 mm to about 0.25 mm. The actual thickness 
utilized of course depends on the specific end use of the coating 
composition. 
The following examples are given to illustrate embodiments of the invention 
and should not be construed as limitng its scope. In these examples, all 
parts and percentages are by weight unless otherwise indicated. 
One skilled in the art will appreciate that the reaction system described 
herein and shown in the following examples will give a linear polymer 
having unsaturation only in its backbone; this means that the terminal 
ends of the linear polymer molecule are not unsaturated. The reaction 
system used here instead provides randomly distributed but internally 
located unsaturation. It should be appreciated that the terminal groups of 
the instant polymers are instead largely formed from the isocyanate group. 
The polyurethanes in the following examples were moisture cured at room 
temperature. With polyurethane in the presence of water, moisture curing 
occurs whereby a condensation reaction occurs with the isocyanate end 
groups. In this reaction, the polymers join together. With the instant 
system, the polymers are not branched and therefore longer linear 
molecules are formed. It will also be readily appreciated that while 
moisture curing can be done merely at room temperature using moisture from 
the air as shown in the examples, more frequently, moisture curing is done 
using steam and, if desired, heat.

EXAMPLE 1 
This example sets forth the procedure for preparing a polyurethane utilized 
in the present invention. The polyurethane is obtained by the reaction of 
a cycloaliphatic diisocyanate, 4,4'-methylenebis(cyclnexyl isocyanate), 
(22 wt-%) and a partially aromatic unsaturated polyester diol (78 wt-%) 
with an acid number 0.4 and a hydroxyl number 56 of the following 
formulation: 
______________________________________ 
Isopthalic Acid 3.0 equivalents 
Azelaic Acid 10.5 equivalents 
Fumaric Acid 6.2 equivalents 
Cyclohexanedimethanol 
18.2 equivalents 
Ethylene Glycol 4.5 equivalents 
______________________________________ 
The reactants and excess toluene are charged into a reactor equipped with a 
stirrer and distillation head. Sufficient toluene (100 ml.) is distilled 
off under a dry air stream to remove any residual moisture from the 
reaction mixture, and 0.06% by weight of a dimethyltindineodecanoate 
catalyst is added. The reaction mixture is heated at 105.degree. C. for 20 
minutes and then cooled to room temperature. 
There was formed a polyurethane (4.50% solids in toluene) containing 5 
mol-% unsaturation of the fumarate type. 
EXAMPLE 2 
The polyurethane of Example 1 was moisturecured at room temperature. The 
thermoplastic coating material can be readily embossed to give fine 
embossing detail. Addition of a photoinitiator prior to air curing 
(Irgacure 651R, a trademark of Ciba Geigy Corp. for 
dimethoxyphenylacetophenone photoinitiator) at 10 weight percent based on 
polymer solids provided a composition which was crosslinked by exposure to 
an ultraviolet light source to give a largely insoluble network. The 
crosslinked composition was compared in a series of tests to a 
non-irridiated, non-crosslinked composition to demonstrate the changes in 
Tg and percent insolubility in the coating. The results are set forth 
below: 
TABLE 1 
______________________________________ 
P.I. IRR. Time.sup.a 
Tg $ Insol.sup.b 
______________________________________ 
None -- +2.degree. C. 
0.2 
10% 5 minutes +4.degree. C. 
84.9 
10% 30 minutes +18.degree. C. 
92.8 
______________________________________ 
.sup.a Hanovia 450 W Hg lamp 
.sup.b Toluene at 100.degree. C. 
EXAMPLE 3 
This example shows the preparation of an unsaturated polyurethane and the 
utilization of that polyurethane in the process of the present invention. 
This composition is the reaction product of 4,4'-methylenebis(cyclohexyl 
isocyanate) (22 wet-%) and a lower fumarate content polyester diol (78 
wt-%) than that of Example 1, said polyester having an acid number 0.2 and 
a hydroxyl number 56 and the following formulation: 
______________________________________ 
Isopthalic Acid 4.0 equivalents 
Azelaic Acid 12.75 equivalents 
Fumaric Acid 3.25 equivalents 
Cyclohexanedimethanol 
18.2 equivalents 
Ethylene Glycol 4.6 equivalents 
______________________________________ 
The resulting polyurethane (52.5%) solids in toluene containing 3 mole-% 
fumarate-type unsaturation was moisture-cured at room temperature to give 
a thermoplastic coating which was readily thermally embossable to give 
sharp, embossed images. Benzophenone was added in an amount of 5% by 
weight of the resin total solids, as a photoinitiator prior to air curing 
gave a crosslinkable film on exposure to a U.V. radiation source. 
Properties of the non-crosslinked and crosslinked films are tabulated 
below: 
TABLE 2 
______________________________________ 
P.I. IRR. Time.sup.a 
Tg $ Insol.sup.b 
______________________________________ 
None -- -0.5.degree. C. 
0.4 
5% BZP 5 minutes +1.5.degree. C. 
85.7 
5% BZP 30 minutes +10.5.degree. C. 
89.7 
______________________________________ 
.sup.a & b same as Table 1. 
The unexpectedly high levels of insolubility noted in this example is based 
on the use of a different photoinitiator (benzophenone) which appears to 
be highly efficient. 
EXAMPLE 4 
This composition is a totally aliphatic polyurethane obtained by the 
reaction of 4,4'-methylenebis(cyclohexyl isocyanate) (21 wt-%) with an 
unsaturated polyester diol (79 wt-%) based on maleic acid as the 
unsaturated component, said polyester having an acid number 0.2 and a 
hydroxyl number 54 and the following formulation: 
______________________________________ 
Azelaic Acid 14.0 equivalents 
Maleic Anhydride 6.0 equivalents 
Cyclohexanedimethanol 
22.8 equivalents 
______________________________________ 
The resulting polyurethane (40.5% solids in toluene) contained 5 mol-% 
maleate-type unsaturation. After adding 10% by weight of Irgacure 
651.RTM., a photoinitiator, the polyurethane was moisture-cured at room 
temperature to give a thermoplastic coating material which will have 
detailed, embossed images upon thermal embossing. The polyurethane was 
then crosslinked by exposure to a U.V. light source. Properties of the non 
crosslinked and crosslinked films are tabulated below: 
TABLE 3 
______________________________________ 
P.I. IRR. Time.sup.a 
Tg $ Insol.sup.b 
______________________________________ 
None -- +4.5.degree. C. 
0.3 
10% 5 minutes +2.degree. C. 
72.9 
10% 30 minutes +4.degree. C. 
84.4 
______________________________________ 
.sup.a & b same as Table 1. 
EXAMPLE 5 
This composition is a totally aliphatic polyurethane obtained by the 
reaction of 4,4'-methylenebis(cyclohexyl isocyanate) 21 wt-%) with an 
unsaturated polyester diol (79 wt-%) based on maleic acid as the 
unsaturated component, said polyester having an acid number 1.3 and a 
hydroxyl number 56 and the following formulation: 
______________________________________ 
Azelaic Acid 14.0 equivalents 
Maleic Anhyride 6.0 equivalents 
1,6-Hexanediol 5.0 equivalents 
Cyclohexanedimethanol 
17.8 equivalents 
______________________________________ 
The resulting polyurethane (40.5% solids in toluene) contained 4.3 mole-% 
maleate-type unsaturation. 
EXAMPLE 6 
To the toluene solution of the polyurethane of Example 5 was added, 10% (by 
weight of resin solids) of dimethoxyphenylacetophenone photoinitiator, and 
the resulting photocrosslinkable coating composition was applied to a bond 
paper substrate at an application rate of 3.5 grams/ft.sup.2. The coated 
substrate was dried in an impingement air oven at 250.degree. F. for three 
minutes to give a 0.5 mil thick tack-free coating. The coated paper was 
embossed at 230.degree. F. for ten seconds in a flat bed press to give a 
20 mil deep decorative texture. Exposure of the embossed coatd paper to 
1.0 J/cm.sup.2 UV irradiation from a medium pressure mercury lamp 
crosslinked the coating, resulting in a permanent embossed texture. 
EXAMPLE 7 
The photocrosslinkable coating composition of Example 6 was applied with a 
wire wound rod to a 4 mils thick poly(ethyleneterephthalate) film at a 
rate of 3.5 g/ft.sup.2 and the coated film was dried in an oven at 
250.degree. F. for three minutes. The coated substrate was embossed at 
250.degree. F. in a flat bed press for ten seconds to give a lenticular 
image of 100 dots/in.sup.2. Ultraviolet irradiation at a dose of 3.0 
J/cm.sup.2 resulted in a permanently embossed coated polyester film. 
EXAMPLE 8 
The photocrosslinkable coating composition of Example 6 was applied to a 
woven nylon fabric at an application rate of 3.5 g/ft.sup.2 and the 
saturated web was dried in an oven at 250.degree. F. for three minutes to 
give a tack-free, flexible coated fabric. Embossing in a flat bed press at 
225.degree. F. for ten seconds gave a multi-level textured image which was 
made permanent by exposure of the web to 1.0 J/cm2 UV irradiation from a 
medium pressure mercury lamp. 
PERFORMANCE AFTER CROSSLINKING 
Comparing polyurethane samples at different levels of unsaturation, it can 
be noted that as crosslinking increases with higher levels of 
unsaturation, the following particularily desired characteristics improve: 
(a) abrasion resistance 
(b) gloss retention 
(c) stain resistance 
(d) percent insolubility 
Since in many cases, the instant product compositions are desired for use 
as flooring materials, these characteristics are particularily important, 
and, the minimum preformance levels required, understandably are high. 
As the unsaturation level decreases, the performance level in these desired 
characteristics will also decrease. Samples having had only 3 mole percent 
unsaturation will give test values, showing approximately the minimum 
acceptable performance level for compositions to be used in flooring 
materials. Samples having more unsaturation and a higher level of 
crosslinking will show better performance for these characteristics.