Note: Descriptions are shown in the official language in which they were submitted.
~L2~$æ~
ABRASION RESISTANT THIN POLYURETHAN~ COATINGS
Background of the Invention
Field of the Invention
. .
The present invention relates generally to abrasion resistant
polyurethane coatings, and more particularly to thin polyurethane coatings
which are both hydrolytically and oxidatively stable.
Discussion of Current Technology
U.S. Patent No. 4,079,160 to Phillipson teaches an optical ele-
ment, such as an eyeglass lens, formed of a typical rigid polymeric sub-
strate of relatively low scratch resistance coated on at least one surface
with a relatively soft, resilient and tough, transparent polymeric material.
U.S. Patent ~o. 4,174,240 to Muller teaches laminates wherein
glass or glass-like plastics are coated ~ith a transparent film 0.1 to
5 millimeters thick formed from a polyurethane polyurea containing 1 to
20 percent by weight urea groups and 0.001 to 10 percent by weight lateral
-COOH groups.
Summary of the Invention
The present invention involves the use of relatively soft, resili-
ent, crosslinked polyurethanes as abrasion resistant coatings. The poly-
urethanes of the present invention are prepared from aliphatic diisocyanates,
straight chain polycarbonate diols, and crosslinking agents such as triols,
triamines and/or triisocyanates. The crosslinked aliphatic polycarbonate
urethane coatings of the present invention are less than 0.005 inch (0.13
3~
millimeter) thick9 preferably about 0.002 inch (0.05 millimeter), thick.
The polyurethane coatings of the present invention are especially useful
in aircraft transparencies as abrasion resistant coatings either on rigid
plastic layers such as polycarbonate or acrylic, or an resilient plastic
layers such as impact resistant polyurethane in a glass bilayer laminate.
Description of the Preferred ~mbodlments
An abrasion resistant, crosslinked aliphatic polyurethane is pre-
pared from a reaction mixture of an aliphatic organic diisocyanate, a poly-
carbonate diol, and an aliphatic triol, triamine and/or triisocyanate. The
polycarbonate diol preferably has a molecular weight below 500. Tetrabutyl
titanate catalyst is also preferred in accordance with U.S. Patent No.
4,160,853. The reaction mixture may also comprise a chain extender, such as
an aliphatic diol having up to about 12 carbon atoms.
The diisocyanate is preferably a cycloaliphatic diisocyanate such
as 4,4'-methylene-bis-(cyclohexyl isocyanate). Polyurethane prepared from
aliphatic isocyanates, particularly cycloaliphatic diisocyanates, have good
physical and optical properties over a wide temperature range, and are not
adversely affected by ultraviolet light. In addition to the most preferred
diisocyanate, 4'4'-methylene-bis-(cyclohexyl isocyanate), other suitable ali-
phatic diisocyanates include 1,4-cyclohexyl diisocyanate, 4,4-isopropylidene-
bis-(cyclohexyl isocyanate), and other dinuclear cycloaliphatic diisocya-
nates, preferably isophorone diisocyanate. Branched diisocyanates such as
trimethylhexamethylene diisocyanate may also be used, preferably in com-
bination with 4,4'-methylene-bis-(cyclohexyl isocyanate) or isophorone
diisocyanate.
3~
Polyols useful according to the present invention may be liquid
at room temperature or soluble in the reaction mixture. Preferred polyols
are polycarbonate diols having a molecular weight of about 200 to 500. The
polycarbonate diols are preferably synthesized in accordance with the
teachings of U.S. Patent No. 4,160,853. Polycarbonate diols are preferred
over polyether or other polyester polyols which may produce polyurethanes
with less oxidative and hydrolytic stability.
The aliphatic diisocyanate and polyol are preferably mixed together
under vacuum in a reaction kettle until the reaction mixture is slngle
phase and well degassed. The reaction mixture further comprises a catalyst,
preferably the tetrabutyl titanate which is present in the polycarbonate
diol synthesized by the preferred method disclosed above. The catalyst is
preferably present at a level of about 100 to 1000 parts per million.
The diisocyanate and polycarbonate diol are reacted in appropriate
proportions to form either an OH-terminated or an NCO-terminated prepolymer.
The prepolymer is then crosslinked to form an abrasion-resistant polyure-
thane by means of a triisocyanate, triamine or triol crosslinking agent. In
an alternatlve embodiment9 the polyol, diisocyanate and crosslinking agent
may be added together in a one-shot polymerization reaction mixture.
The polyurethane reaction mixture is preferably in solution in
any solvent for the reactants, preferably methyl ethyl ketone, propylace-
tate, toluene, cyclohexanone and mixtures thereof. The reaction solution
dries to form a coating less than about 0.005 inch (0.13 millimeter) thick,
pr~ferably about 0.001 to 0.002 inch (about 0.025 to 0.05 millimeter) thick.
The polyurethane is typically cured at temperatures from about 225 to 275F
(about 107 to 135C) 9 preferably about 250 to 275F (about 121 to 135DC),
for about 1 hour or l~nger to yield an abrasion resistant coating.
.~
3~
The present invention will be more fully understood from the
descriptions of specific examples which follow.
EXAMPLE I
A polycarbonate diol is prepared by reacting 1,6-hexanediol
with diethyl carbonate to for~ an average five unit oligomer. A hydroxy-
terminated prepolymer is prepared by reacting 57 weight percent of the
1,6-hexanediol based polycarbonate diol with 43 weight percent 4,4'-
methylene-bis-~cyclohexyl isocyanate), available as Hylene W from E. I.
du Pont de Nemours & Co., Inc., Wilmington, Delaware. A polyurethane is
prepared by reacting 81 weight percent of the prepolymer with 19 weight
percent of a triisocyanurate of isophorone diisocyanate, in a solvent com-
prising equal weights of toluene and cyclohexanone totaling about 3 times
the weight of the reactants. The reaction mixture is coated onto a rigid
polycarbonate sheet and cured for 1 1/2 hours at 250 F (about 121 C) to
form an abrasion resistant polyurethane coating.
EXAMPLE II
A polyurethane reaction mixture is coated onto a preformed
thermoplastic polyurethane sheet which is subsequently laminated to a glass
sheet to form a bilayer laminate. The polyurethane reaction mixture com-
prises 78 weight percent of the prepolymer as in ~xample I, 22 weight
percent of a triisocyanate biuret of 1,6-hexamethylene diisocyanate, and
three times the weight of these reactants of a solvent comprising equal
weights of toluene and cyclohexanone. The reaction mixture is dried and
cured during the lamination process to form a thin, abrasion-resistant
crosslinked polyurethane coating on the thermoplastic polyurethane surface
~ ~rad~ a~k
33~Bi;~
of the bilayer laminate. The lamination process is described in detail
in U. S. Patent No. 3,808,079 to Rieser et al.
The above examples are offered to illustrate the present inven
tion. Various modifications, such as the coating of other substrates,
the use of other components, applying and curing by other means, etc. are
included within the scope of the present invention, which is defined by the
following claims.
r,;~,p
