Note: Descriptions are shown in the official language in which they were submitted.
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THERMO~LASTIC SU3ST~RAT~HAVING IMPROVE~
WATER SPREADING ~HARACTE~ISTICS AND
METHOD OF MAKING
Backqround of the I~vention
The present invention relates to certain thermo-
plastic substrates, such as polycarbonates and polymethyl-
methacrylates, as well as glazing materials having improved
water spreading characteristics. More particularly, the pre-
sent invention relates to the impregnation of the surface ofa thermoplastic substrate with a colloidal inorganic oxide,
or by the use of an organic solvent coating composition in
the form of a blend of an acrylic resin and a colloidal inor-
ganic oxide whereby improved water wetting and water spread-
ing characteristics are imparted to the treated substratesurface.
Prior to the present invention, as shown by
Krautter et al., U.S. Patent 4,576,869, a method for treating
plastic material, such as a glazing material to improve its
water spreading characteristics was provided by initially
treating the plastic substrate with an adhesive layer of a
non-water soluble and an essentially non-swellable polymer
containing at least one polar group. For example, butyl
methacrylate can be mixed with a methyl methacrylate, gly-
cidyl methacrylate and methacrylic acid. After the adhesivelayer had dried, there was applied a colloidal inorganic ox-
ide, such as anionic aqueous silicic sol modified with alu-
minum oxide. The treated substrate was then dried for 5
minutes at ~emperatures up to about 80C. Although the
method of Krautter et al. provides treated plastic substrates
having improved wettability and water spreading characteris-
tics, it requires a multi-step procedure which often is in-
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convenient i~ a high production rate is to be maintained
during a commercial operation.
An anti-dimming coating for transparent synthetic
resin articles is shown in J.~.A. ~o. ?60~193 A mixture of
a colloidal silica and alumina in combination with a dis-
persion or emulsion of a hydrophobic acrylic resin is used to
treat the surface of a transparent synthetic resin film. It
is reported, however, that adhesion of the coating is com-
pletely unsatisfactory, particularly when moistened.
It would be desirable, therefore, to provide ther-
moplastic substrates having improved wetting characteristics
and water spreading characteristics in accordance with meth-
ods which would not require multi-step procedures and which
would provide coatings on thermoplastic substrates having a
satisfactory degree of adhesion.
The present invention is based on the discovery
that improved water spreading and wetting characteristics can
be imparted to thermoplastic substrates, such as a poly-
carbonate substrate or a polymethylmethacrylate substrate, if
treated with an organic solvent coa~ing composition of an
acrylic resin and a colloidal inorganic oxide, such as col-
loidal silica, as defined hereinafter. Although a heating of
the treated substrate until dry is required, no prior appli-
cation of an adhesive layer is needed. The adhesion of the
resulting applied coating to the thermoplastic substrate also
has been found to be satisfactory. It also has been discov-
ered that direct impregnation of the surface of the thermo-
plastic substrate, such as a polycarbonate or polymethyl-
methacrylate, can be achieved by the use of a mi~ture of a
strong solvent, such as diacetone alcohol, and a colloidal
inoxganic oxide, such as colloidal silica. The surface of
the treated thermoplastic substrate is then dried at elevated
temperatures~ Although the treated thermoplastic substrates
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have been found to have enhanced water wetting characte~is-
tics and water spreadability, novel properties can also be
lmparted to the plastic substrate by using diffe~ent metal
oxide colloids, e.g., conduction or antistatic properties
with colloidal tin oxide, UV absorbing property with col-
loidal titanium dioxide, flame retardancy with colloidal an-
timony oxide anA antireflection properties with colloidal
silica and alumina.
Statem~a~ the Invention
There is provided by the present invention, a sub-
stantially transparent thermoplastic substrate exhibiting im-
proved water wetting and water spreading characteristics re-
sulting from the treatment of the thermoplastic substratewith an organic solvent coating composition selected from the
class consisting of
~ A) a coating composition of organic solvent and
from l~ to 10% solids of a mixture consisting essentially or
by weight 3 to 50 parts of an acrylic resin, per lO0 parts of
a coll~idal inorganic oxide having an average particle size
of from 9 to 60 nanometers and, and
~ B) a coating composition having 0.1 to 2 parts by
weight of a colloidal inorganic oxide, per lO0 parts of or-
ganic solvent.
The term "thermoplastic substrate" as used in the
description of the present invention, means sheet having a
thickness of 1/16" to l/2" thick, film having a thickness of
30 mils or less and structured sheet as shown by Figure 2.
Some of the substantially transparent thermoplastic
substrate which can be utilized in the practice of the
present invention include, for example, LEXAN polycarbonate,
ULTEM polyetherimide, VALOX polyester, which are ther-
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moplastics manufactured by the General Electric Company,
i Mylar polyester and, in particular instances, polyethylene
and polypropylene sheet.
Some of the polyacrylic polymers which can be used
in the practice of the invention are polymethyl methacrylate
and its copolymers, polyethyl methacrylate and its copolymers
and polybutyl methacrylate and its copolymers. These materi-
als are available as Elvacite~ acrylic resins from the DuPont
de Nemours Company. These binders are thermoplastic and sol-
uble in organic solvents. In addition, a copolymer may con-
tain a functional group or groups such as hydroxyl, car-
boxylic or glycidyl which can be further crosslinked by using
a suitable crosslinking agent, such as melamine formaldehyde
resin. The binder system is not limited only to acrylics.
Polyurethanes and polyvinyl chloride and its copolymers also
can be used.
Another class of acrylic resins are water soluble
and dispersible in a water-based system. This class of mate-
rials, both thermoplastic and thermosetting, are available as
Carboset~ resins from B.F. Goodrich Company.
Another essential ingredient of the organic solvent
coating composition used in the practice of the present in
vention is the colloidal inoryanic oxide which can include
colloidal metal oxides, such as alumina, antimony oxide, tin
oxide, titanium dioxide, cerium oxide, silica and mixtures
thereof. Colloidal silica is preferred having an average
diameter of about 4 nm to about 60 nm.. Colloidal silica is
a dispersion of submicron-size silica ~SiO2) particles in an
aqueous or other organic solvent medium. Dispersions of col-
loidal silica are available from chemical manufacturers suchas DuPont de Nemours Company and Nalco Chemical Company.
Colloidal silica is available in either acidic or basic form.
Alkaline colloidal silica can be converted to acidic
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colloidal silica with acids, such as HCl or H2SO4 along with
high agitation.
An example of a satisfactory colloidal silica for
use in these coating compositions is Nalcoag 1034A, available
from Nalco Chemical Company, Chicago, Ill. Nalcoag 1034A is
a high purity, acidic pH aqueous colloidal silica dispersion
having a low Na2O content, a pH of approximately 3.1 and an
SiO2 content of approximately 34 percent by weight in water.
Another type of colloidal silica is Ludox AM available from
DuPont de Nemours Company, where the colloidal silica surface
is modified with aluminum oxide.
Another preferred source of colloidal silica is an
organosol available as a 30 wt % SiO2 of 20 nm average diame-
ter in ethylene glycol monopropyl ether solvent from Nalco
Chemical Company. This product is known as 84SS 258.
Organosols can also be prepared in other alcohol or glycol
ether type or dimethylformamide solvent. Both organosols and
aquasols can be used as a source of colloidal silica in this
system.
Organic solvents which can be used in the solvent
or organic coating composition of the present invention are,
for example, isoamyl alcohol, cyclohexanol, 2-ethylhexanol,
ethylene glycol, propylene glycol, ethylene glycol ethyl
ether, ethylene glycol butyl ether, propylene glycol me~hyl
ether, propylene glycol propyl ether, propylene glycol ~utyl
ether, diethylene glycol methyl ether, dipropylene glycol
methyl ether and tripropylene glycol methyl ether.
In addition to the above conventional solvents,
there can be used strong polymer solvents or mixtures thereof
with the aforementioned alcohols or glycol ethers. Strong
solvents can be selected from a number of classes of materi-
als, such as ketones, aromatic hydrocarbons, chlorohydrocar-
bons, esters, ethers, amides, etc. For example, methyl ethyl
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ketone, methyl isobutyl ketone, methyl amyl ketone,
isophorone, diacetone alcohol, cyclohexanone, toluene, methy-
lene chloride, glycol ether acetates, tetrahydrofuran, ethy-
lene glycol methyl ether, carbitol (Union Carbide Corp.),
propylene glycol methyl ether and dimethyl formamide.
The solvent blend should provide a right combina-
tion of polymer solubility, evaporation and processing char-
acteristics. The solvent blend can consist of a blend of the
conventional solvent and strong solvent having from 5 to 50%
~y weight of the strong solvent. The amount of strong sol-
vent can be ad~usted so that crazing or hazing of the thermo-
plastic substrate is minimized while providing optimum poly-
mer solubility and evaporation characteristics.
In addition, optionally the coating composition may
contain with small amounts of silane coupling agents or zir-
conia aluminum metallo organic complexes and non-ionic sur-
factant or wetting agents.
In order that those skilled in the art will be bet-
ter able to practice the present invention, reference is made
to the drawings. There is shown at Figure l side views at
l(a), l(b) and l(c) of a thermoformable or thermoplastic sub-
strate having a water drQplet on its surface at various con-
tact angles. In Figure 2 there is shown an isometric view of
a typical structured plastic sheet and a side view.
More particularly, there is shown at Fig. l(a) a
water droplet at lO on an untreated plastic sheet at ll. The
droplet exhibits a hi~h contact angle. In Fig. l(b) there is
shown a plastic substrate at 20 coated with a cured inorganic
oxide-polyacrylate coating at 21 having a thickness of from
about 0.01 to 1 micron in accordance with the practice of the
invention, and a water droplet at a low contact angle at 22.
In Fig. l(c) there is shown a water droplet at 30 which on a
treated plastic substrate which has spread completely.Figure
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RD-19089
2b shows a UV absorbing layer at 40, an inorganic oxide-
acrylate coating at 41 and a water layer at 32.
In the practice of the method of the present inven-
tion, the thermoplastic sheet can be treated with the col-
S loidal inorganic oxide by either solvent impregnation of thecolloidal inorganic oxide or by the use of a colloidal inor-
ganic oxide and polyacrylate blend which can be applied onto
the surface of the thermoplastic sheet in the form of the
organic solvent coating composition. After applying the in-
organic oxide coating, the treated thermoplastic sheet can beallowed to air dry at room temperature followed by the heat
drying in an air circulated oven for 5 to 20 minutes at
temperatures in the range of from 90C to 130C.
The thermoplastic sheet, which includes film and
structured sheet as previously defined, also can be treated
with the colLoidal inorganic oxide and thereafter the treated
substrate can be dried.
Treatment of the thermoplastic sheet can be ef-
fected by direct solvent impregnation of the thermoplastic
sheet surface with the colloidal inorganic oxide, or the col-
loidal inorganic oxide can be applied as a colloidal inor-
ganic oxide-acrylate coating. The colloidal inorganic oxide
coating can have a thickness of about 0.01 micron to 1 micron
and preferably 0.05 to 0.5 micron.
Treatment of the thermoplastic sheet or substrate
can be achieved by typical means such as flow coating, spray-
ing, roller coating, dipping or curtain coating.
Strong organic solvents, such as diacetone alcohol
alone or in mixture with glycol ether solvents can be used to
directly impregnate the thermoplastic substra~e. Effective
results can be achieved if there is used a solution having
from about 0.1% to 2~ by weight colloidal inorganic solids.
After impregnation~ the treated substrate can be heated to
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from 90C to 130C utilizing heating means such as an air cir-
culating oven or an IR lamp bank.
In instances where an organic solvent coating com-
position of an inorganic oxide-acrylate mixture is used, a
solids concentration of from 1% to 10% can be used having
from 3 parts to 50 parts by weight of polyacrylate resin, per
lO0 parts of colloidal inorganic oxide. Baking temperatures
in the range of from 90~C to 130C can be employed using
equipment as previously described.
A thermoplastic sheet treated in accordance with
the practice of the invention can be used as roofing for
greenhouses, skylights and arcades. In instances where film
is used, improved sunglasses and ski goggles also are pro-
vided.
It has been found that the cured coating of col-
loidal silica and polyacrylate, as previously defined can
contain from 65 to 95 weight ~ silica based on solids. It
has been found that a contact angle of 20 or less requires
greater than ôO weight ~ silica.
In order that those skilled in the art will be bet-
ter able to practice the present invention, the following ex-
ample is given by way of i}lustration and not by way of limi-
tation. All parts are by weight.
Example l
Lexan polycarbonate sheets (6" x l~" x l/8") was
surface impregnated with organic solvent coating compositions
containing 0.3 and 0.6 Phs (parts per hundred solvent) col-
loidal silica in diacetone alcohol, referred to hereinafteras coating composition A and B. The treated polycarbonate
sheet was then allowed to air dry at room temperature for 2
to 3 minutes and then heat dried in an air circulated oven
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for 20 minutes at 126. Colloidal silica sol 84SS258 con-
sists of a 30 weight percent SiO2 having an average diameter
of 20 nm. in ethyleneglycol monopropyl ether which was ob-
tained from the Nalco Chemical Company. Coating compositions
C and D also were used which were obtained by mixing
polymethylmethacrylate Elvacite 2041 (E . I . DuPont de Nemours
Companyj and its copolymer Elvacite 2014 which were mixed
with colloidal silica in the same solve~t medium. These
coated Lexan polycarbonate sheets were also flow coated and
dried in accordance with the same procedure. The following
results were obtained.
Table 1
15 Coating Compositions
A B C D
(1) Diacetone alcohol (gms) 100 100 100 100
(2) Elvacite 2041 (3 pbw in 100 pbw ~ 0.66 ---
Dowanol PM)
(Polymethyl methacrylate)(gms)
(3) Elvacite 2014 (3 pbw in 100 pbw --~ -- 0.66
Dowanol PM)
(PMMA copolymer, acid No. 13)(gms)
(4) 30% Colloidal Silica Organosol 1.0 2.0 2.0 2.0
84SS258 (gms)
Amount of Colloidal SiO2 0.3 0.6 --- ---
(phs, parts per hundred solvent)
Wt % SiO2, based on solids--- --- 97 97
Contact angle of water, ~ol22 14 12 17
1) Contact angle of untreated Lexan sheet 75
qo ,
The above results show that improved contact angle
of w~ter was obtained on a Lexan polycarbonate sheet treated
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in accordance with the practice of the present invention as
compared to an untreated Lexan polycarbonate sheet having a
contact angle of 75. It was also found that Lexan polycar-
bonate sheet treated with coating composition C continued to
show a 12 contact angle after 31 days of continuous immer-
sion in hot water at 165C indicating good adhesion and re-
sistance to water.
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Following in the procedure of Example 1, additional
coating compositions (E, F, G and H) were prepared. The
coating compositions had consisted of 75/25, 50/50 and 25/75
Pbw diacetone alcohol and Dowanol PM (Dow Chemical Company).
These coating compositions were used to treat Lexan polycar-
bonate structured sheet, which is shown in Figure 2. The
following results were obtained.
Table 2
Coating Compositions
E F G
Diacetone alcohol tgms) 75 75 50 25
Dowanol PM (gms) 25 25 50 75
Elvacite 2041 (3 pbw in 100 -- 1.2 1.2 1.2
pbw Dowanol PM)(gms)
30% Colloidal Silica- 4.0 4.0 4.0 4.0
Organosol 84SS258 (gms)
Contact angle (H2O), ~ol 27 20 17 14
~ after 19 days D~ hot 14 12 -- --
water immersion at 65C
1) Contact angle of untreated Lexan profiled sheet 75.
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The above results show that a significant reduction
in contact angle was achieved using the coating composition
of the present invention as compared to the contact angle of
5 7~ on untreated structured Lexan polycarbonate sheet. It is
further shown that the coating composition using diacetone
alcohol Dowanol PM can be varied considerably without any
significant effect on the contact angle. In addition, it was
found that coating compositions E and F continued to show a
contact angle of 12 and 14 after 19 days of continuous im-
mersion in hot water at 65C. In accordance with the proce-
dure of Examples 1 and 2, a study was conducted to detersnine
the effect of colloidal silica level and contact angle. The
following results were obtained.
Table 3
Coating Compositions
Diacetone alcohol~gms) 25 25 25 50 25
Dowanol PM (gms) 75 75 75 50 75
Elvacite 2041 (3 pbw in100 15 10 7.5 3.6 1.2
pbw Dowanol PM~ (gms)
30% Colloidal Silica- 4.5 4.5 4.5 4.5 4.0
Organosol 84SS258 (gms)
Wt % SiO2 based on 75 82 86 92 97
solids
Contact angle of water, ~ 50 30 19 16 14
The above results show that an optimum level of
weight percent SiO2 based on solids is about 85 weight per-
cent to provide a water contact ang~e of 20 or less. It was
further found that othex metal oxides such as tin oxide also
40 can be used to reduce the contact angle of water. For exam-
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ple, one coating composition having solids containing 90
weight percent tin oxide and 10% Hycar ~of the B.F. Goodrich
Company) a thermoset acrylic polymer emulsion exhibited a
contact angle of 27 on Lexan sheet.
Example 4
Following the procedure of Example l, additional
coating compositions were prepared which were used on Lexan
polycarbonate structured sheet for the purpose of determining
the degree of adhesion of the colloidal silica coating on the
substrate. A Scotch tape adhesion test was used employing
one inch wide 3M-610 tape which was applied to the treated
Lexan polycarbonate after the coating was dried. The tape
was then pulled and the substrate was washed with isopropanol
and water and dried followed by ~easuring the ccntact angle.
A substantial increase in the contact angle to about 1.5 to
two times the initial value was considered a failure of the
adhesion test. The following results were obtained where " t"
indicates that no failure occurred when testing was
discontinued.
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Table 4
Diacetone Alcohol (gms) 75 75 75 25 25
Dowanol PM ~gms) 25 25 25 75 75
Elvacite 2041 (3 pbw in -- 1.2 5.0 5.0 7.5
100 pbw ~owanol PM)tgms)
30~ Colloidal Silica- 4.0 4.0 4.5 4.5 4.5
Organosol 84SS258 (gms)
Wt % SiO2 in the coating -- 97 90 90 86
15 based on solids
Initial contact angle with 19 14 19 19 19
water, ~
20 Initial tape adhesionPass Pass Pass ~ass Pass
Tape adhesion after3 14 14+ 25 30+
hot water immersion at
65C, Number of days
25 for failure
The above results show that improved adhesion was
obtained in instances where the coating composition contained
Elvacite acrylic resin in combination with colloidal silica.
A coating composition was prepared using Nalcoag
2326 (Nalco Chemical Company) which is a 14.5% colloidal sil-
ica of 5 nanometer size in water. The colloidal silica solwas mixed with a mixture of Dowanol PM and diacetone alcohol.
The following results show the coating composition and the
results obtained.
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Table 5
Dowanol PM ~gms) 50
s
Diacetone alcohol (gm.e) 50
Nalcoag 2326 sol (14.5% 2.
SiO2 of 5 nm size in water )
10 (gms)
Initial contact angle 21
with water, ~
15 Initial tape adhesion Pass
Tape adhesion after hot 20+
water immersion at 65C,
number of days for failure
The above results show that improved adhesion can -
also be achieved with surface impregnated colloidal silica
using colloidal silica free of solvents, such as ethylenegly-
col monopropyl ether. The adhesion obtained was greater than
20 days after a hot water immersion at 65C.
Although the above examples are directed to only a
few of the very many variables which can be used in the prac-
tice of the method of the present invention as well as the
treated thermoplastic substrates obtained therefrom, it
should be understood that the present invention is directed
to a much broader variety of thermoplastic substrates as well
as coating compositions as set forth in the description pre-
ceding these examples.
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