U.S. patent application number 11/075985 was filed with the patent office on 2005-07-21 for two-layer protective coating system for lcd glass.
Invention is credited to Allaire, Roger A., Baca, Adra S., Chien, Ching-Kee, Powell-Johnson, Andrienne M., Schissel, David N., Sever, Edward J., Shi, Youchun.
Application Number | 20050158565 11/075985 |
Document ID | / |
Family ID | 29710444 |
Filed Date | 2005-07-21 |
United States Patent
Application |
20050158565 |
Kind Code |
A1 |
Allaire, Roger A. ; et
al. |
July 21, 2005 |
Two-layer protective coating system for LCD glass
Abstract
Two-layer protective coating system for glass surfaces is
disclosed. The base coating comprises at least one polysaccharide,
and the top coating has a lower solubility in water than the base
coating. The coating system provides good protection against
contaminants and glass chips, good removability in mild cleaning
condition, and extra water resistance during process steps where
water is used as a cooling agent.
Inventors: |
Allaire, Roger A.; (Big
Flats, NY) ; Baca, Adra S.; (Corning, NY) ;
Chien, Ching-Kee; (Horseheads, NY) ; Powell-Johnson,
Andrienne M.; (Horseheads, NY) ; Schissel, David
N.; (Painted Post, NY) ; Sever, Edward J.;
(Corning, NY) ; Shi, Youchun; (Horseheads,
NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
29710444 |
Appl. No.: |
11/075985 |
Filed: |
March 9, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11075985 |
Mar 9, 2005 |
|
|
|
10165488 |
Jun 7, 2002 |
|
|
|
6896928 |
|
|
|
|
Current U.S.
Class: |
428/426 |
Current CPC
Class: |
Y10T 428/265 20150115;
C03C 2218/355 20130101; C09D 5/008 20130101; C03C 2217/42 20130101;
Y10T 428/31641 20150401; Y10T 428/25 20150115; C03C 2217/47
20130101; C03C 17/3405 20130101 |
Class at
Publication: |
428/426 |
International
Class: |
B05D 001/32 |
Claims
1-58. (canceled)
59. An article of manufacture comprising: (a) a glass sheet having
at least one substantially flat surface; (b) a first protective
coating on the substantially flat surface comprising at least one
polysaccharide, the coating having a thickness of at least 0.01
.mu.m; and (c) a second protective coating over the first
protective coating; wherein (i) the protective coatings protect the
surface from ambient contaminants, contaminants produced during the
processing of the glass and/or scratching; (ii) the second
protective coating is less soluble in water than the first
protective coating; and (iii) the protective coatings can be
removed through application of an aqueous cleaning composition to
result in a substantially clean surface.
60. An article in accordance with claim 59, wherein the glass
surface has a water contact angle of equal to or less than
8.degree. after the protective coating is sufficiently removed by a
cleaning composition.
61. An article in accordance with claim 60, wherein the glass
surface has a Rms surface roughness less than or equal to 0.40 nm
as measured by atomic force microscopy.
62. An article in accordance with claim 59, wherein the at least
one polysaccharide comprises straight chain polysaccharide
molecules and/or branched polysaccharide molecules.
63. An article in accordance with claim 59, wherein the at least
one polysaccharide comprises at least one starch.
64. An article in accordance with claim 63, wherein the at least
one starch comprises straight chain starch molecules and/or
branched starch molecules.
65. An article in accordance with claim 59, wherein the first
protective coating comprises a platicizer.
66. An article in accordance with claim 59, wherein the first
protective coating comprises a biocide.
67. An article in accordance with claim 59, wherein the first
protective coating has a thickness of less than 50 .mu.m.
68. An article in accordance with claim 59, wherein the second
protective coating comprises a continuous polymer film.
69. An article in accordance with claim 68, wherein the second
protective coating has a thickness of at least 0.01 .mu.m.
70. An article in accordance with claim 69, wherein the second
protective coating has a thickness of less than 100 .mu.m.
71. An article in accordance with claim 70, wherein the second
protective coating has a thickness of less than 50 .mu.m.
72. An article in accordance with claim 59, wherein the second
protective coating comprises at least one polymeric acid.
73. An article in accordance with claim 72, wherein the polymer
acid is selected from the group consisting of (i) homopolymers and
copolymers of carboxylic acid, phenols and acid anhydrides, salts
and partial salts thereof, and (ii) mixtures and other combinations
of the polymers.
74. An article in accordance with claim 73, wherein the at least
one polymeric acid is selected from the group consisting of (i)
homopolymers and copolymers of acrylic acid, methacrylic acid,
maleic acid and their anhydrides, salts and partial salts thereof,
and (ii) mixtures and other combinations of the polymers.
75. An article in accordance with claim 59, wherein the second
protective coating comprises polyvinyl alcohol.
76. An article in accordance with claim 59, wherein the second
protective coating comprises hydrophobically modified and/or
insolubilized polysaccharide.
77. An article in accordance with claim 76, wherein the
hydrophobically modified or insolubilized polysaccharide is a
polysaccharide modified by (i) glyoxal; (ii) octenyl succinic
anhydride; (iii) at least one water-insoluble resin; (iv) at least
one water-repellent additives; (v) at least one latex dispersion;
or (vi) any mixture of at least two of (i), (ii), (iii), (iv) and
(v).
78. An article in accordance with claim 76, wherein the
hydrophobically modified and/or insolubilized polysaccharide is
selected from glyoxal-crosslinked polysaccharide, octenyl succinic
anhydride modified polysaccharide and mixtures thereof.
79. An article in accordance with claim 76, wherein the
hydrophobically modified and/or insolubilized polysaccharide
comprises hydrophobically modified or insolubilized starch.
80. An article in accordance with claim 79, wherein hydrophobically
modified or insolubilized starch is a starch modified by (i)
glyoxal; (ii) octenyl succinic anhydride; (iii) at least one
water-insoluble resin; (iv) at least one water-repellent additives;
(v) at least one latex dispersion; or (vi) any mixture of at least
two of (i), (ii), (iii), (iv) and (v).
81. An article in accordance with claim 79, wherein the
hydrophobically modified and/or insolubilized starch is selected
from glyoxal-crosslinked starch, octenyl succinic anhydride
modified starch and mixtures thereof.
82. An article in accordance with claim 77, wherein: the at least
one water-insoluble resin is selected from copolymers of
unsaturated carboxylic acids with styrene, ethylene, alkyl vinyl
ethers and alkenyl fatty acid esters; the at least one
water-repellent additive is selected from alkenyl succinic
anhydride, alkyl ketene dimer, and stearylated melamine; and the at
least one latex dispersion is selected from polystyrene butadiene,
polyvinyl acetate and polystyrene acrylate.
83. An article in accordance with claim 59, wherein the second
protective coating comprises a wax.
84. An article in accordance with claim 83, wherein the wax is
petrolatum.
85. An article in accordance with claim 83, wherein the second
protective coating has a thickness of 0.01 .mu.m to 100 .mu.m.
86. An article in accordance with claim 59, wherein the second
protective coating comprises polymer particles.
87. An article in accordance with claim 85, wherein the polymer
particles are polymer beads having a particle size ranging from 0.1
.mu.m to 1 mm.
88. An article in accordance with claim 86, wherein the polymer
particles are selected from poly(styrene-divinylbenzene),
poly(methyl methacrylate), polyvinyl chloride, polyvinyl
dichloride, poly(styrene butadiene), polyvinyl acetate, and
mixtures thereof.
89. An article in accordance with claim 59, wherein the cleaning
composition is a basic aqueous solution having a pH of equal to or
above 10.
90. An article in accordance with claim 59, wherein the protective
coatings reduce the number per unit area of glass chips adhered to
the surface upon removal of the coatings by at least 90 percent
compared to the number per unit area of glass chips adhered to an
uncoated surface under comparable conditions.
91. An article in accordance with claim 90, wherein the number per
unit area of glass chips adhered to the surface is reduced by at
least 95 percent.
92. An article in accordance with claim 59, wherein the protective
coatings reduce the number of scratches on the glass surface per
unit area by at least 90% compared to the scratches per unit area
on an uncoated glass surface under comparable conditions.
93. An article in accordance with claim 92, wherein the number of
scratches per unit area is reduced by at least 95%.
94. An article in accordance with claim 59, wherein the glass
comprises at least two substantially flat surfaces, and both
surfaces have the first protective coating and the second
protective coating.
95. An article in accordance with claim 61, wherein the glass is
suitable for producing the substrate of a liquid crystal display.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the protection of glass
surfaces, and in particular, to the temporary protection of the
surfaces of glass sheets used in producing liquid crystal displays
(LCDs). The invention is useful, for example, in protecting the
glass sheets from being contaminated by ambient dirt or glass chips
produced during the processing of the sheets, such as cutting,
grinding, packing and transportation. In addition, the invention is
useful in protecting the glass sheets from scratching.
BACKGROUND OF THE INVENTION
[0002] Many uses of glass, including glass for producing LCDs,
require a very clean glass surface that is substantially free of
dust and other organic and/or inorganic contaminants. When exposed
to the environment, glass surface can quickly become contaminated
with dust and other inorganic and/or organic ambient contaminants,
with contamination being observed within a few hours.
[0003] Current procedures used to cut and grind glass surfaces and
edges often generate small glass chips. Such chips can have a size
in the range between about 1 and 100 microns. Some of these
particles irreversibly adhere to the clean glass surface, rendering
the glass useless for many applications. This is particularly a
serious problem for LCD glass surfaces.
[0004] LCD glass can be made by fusion draw process, which yields
flat and smooth glass surfaces. The glass sheets can be cut to the
desired size and then the edges ground. If water is actively
involved between the surface and the glass chips generated during
cutting and grinding, permanent chemical bonding may occur,
rendering the adhesion of the glass chips to the surface
irreversible.
[0005] One known method of protecting glass surfaces, specifically,
surfaces of LCD glass sheets, is to apply a pre-formed polymer film
on both major surfaces of the glass to protect the glass during the
scoring, breaking and beveling process. In a typical method, one
major surface has a polymer film attached with an adhesive, and the
other major surface has a film attached by static charge. The first
film is removed after the edge finishing (cutting and/or grinding)
of the sheet is completed, while the second is removed prior to the
finishing process. Although the adhesive-backed film protects the
surface from scratching by the handling equipment, it causes other
problems. Another problem with this film is that it may leave an
adhesive residue on the glass surface. A further problem with the
film approach is glass breakage during peeling of the film from the
glass surface, especially for large and/or thin glass sheets.
[0006] Many polymer coatings, such as polyvinyl alcohol, can offer
particle protection and scratch resistance capabilities. However,
few of them can be completely removed in a cleaning solution at a
temperature as low as 40.degree. C. in a typical manufacturing
process. One method of temporarily protecting glass surface,
especially LCD glass surface, involves applying an aqueous solution
of polysaccharides (e.g., a starch) to the glass surface, forming a
protective coating of the polysaccharides on the glass surface by
removing water from the solution, and then subsequently removing
the polysaccharide-containing coating from the surface using an
aqueous solution when desired to reveal the protected surface. The
removing aqueous solution may contain a detergent. The
polysaccharide coating formed on the glass surface offers particle
protection and scratch resistance capabilities. However, the high
water solubility of polysaccharides, especially starches,
constitutes a potential drawback of this method. Before the
cleaning step, glass sheets are usually subjected to other
finishing steps such as cutting and edge grinding, in which water
may be used as a cooling agent. Due to their high solubility in
water, the polysaccharide coatings may be diminished during such
stages, leading to reduced particle protection and scratch
resistance.
[0007] A desirable property of the temporary protective coating for
surface of LCD glass is its removability. Manufacturers of LCDs use
the glass as the starting point for complex manufacturing processes
in which semiconductors, e.g., thin film transistors, are formed on
the glass substrates. In order not to adversely affect such
processes, any protective coatings on the glass surface must be
readily removable prior to the beginning of the LCD production
process, without substantially changing the chemical and physical
characteristics of the glass surface.
[0008] Therefore, there remains a need for an improved method of
temporarily protecting the surface of glass using a coating system,
especially glass for producing LCDs, from being contaminated by
ambient contaminants, contaminants produced during the processing
of the glass and/or scratching. The coating system should be easy
to be removed and does not leave residue on the glass surface upon
removal, whereby a substantially clean and coating-free surface can
be restored for further use of the glass, e.g., for the production
of LCDs.
[0009] In view of the foregoing, there has been a need in the art
for a method for protecting surface of glass, especially glass
sheets for the production of liquid crystal displays, which has the
following characteristics:
[0010] (1) The method should be preferably one that can be easily
incorporated in the overall glass forming process, specifically, at
the end of the forming process, so that newly formed glass is
protected substantially immediately after it is produced. Thus, the
coating material should be able to withstand the environment of the
glass forming line (e.g., high temperatures). In addition, the
method should be safe to use in such an environment;
[0011] (2) The coating must offer sufficient protection to the
glass surface from being adhered to and contaminated by
contaminants produced during the processing of the glass sheet,
including cutting and/or grinding, and/or ambient contaminants,
organic and/or inorganic, that the glass surface typically may come
into contact with during packing, storage and shipment prior to
use;
[0012] (3) The coating must be sufficiently robust to continue to
provide protection after being exposed to the substantial amount of
water which typically comes into contact with the glass surface
during the processing of the glass, including cutting and/or
grinding. This requires that the coating system has a sufficiently
low solubility in water under the processing condition;
[0013] (4) The coating should preferably protect the glass sheet
from scratching during processing, handling, shipping, and storage
(as used herein, scratching includes abrasion). More preferably,
the coating should permit the glass sheets to be stacked very
closely with minimal spacing materials between them during
handling, shipping and storage;
[0014] (5) The coating should be substantially completely removable
from the glass prior to its ultimate use in, for example, producing
a liquid crystal display. Preferably, the removing condition should
be mild and environmentally friendly; and
[0015] (6) The coating should preserve the pristine glass surface
without substantially changing the surface's chemical composition
and physical properties, e.g., smoothness, as a result of the
coating process, the presence of coating on the surface during
handling, shipping, storage and the subsequent removal of the
coating from the surface.
[0016] The present invention addresses and satisfies this
long-standing need in the art.
SUMMARY OF THE INVENTION
[0017] In a first aspect, the present invention provides a method
for protecting a substantially clean surface of glass from being
contaminated by ambient contaminants and/or contaminants produced
during the processing of the glass and/or scratching. The present
inventive method comprises the steps of:
[0018] (A) forming a first protective coating on the surface of the
glass by (i) applying an aqueous first coating composition
comprising at least one polysaccharide to the surface, and
optionally (ii) removing at least part of the solvent from the
composition applied to the surface to leave a
polysaccharide-containing protective coating on the surface having
a thickness of at least 0.01 .mu.m;
[0019] (B) forming a second protective coating over the first
protective coating by (i) applying a second coating composition
over the first protective coating, and optionally (ii) removing the
solvent from the coating composition applied to the surface to
leave a second protective coating; wherein, (a) the second
protective coating is less soluble in water at or near ambient
temperature than the first protective coating, and (b) the first
and second protective coatings can be subsequently removed from the
surface using an aqueous cleaning composition, to result in a
surface which is substantially clean; and optionally
[0020] (C) subsequently removing the first and second protective
coatings from the surface of the glass using an aqueous cleaning
composition, to result in a surface which is substantially
clean.
[0021] In a second aspect of the present invention, it is provided
an article of manufacture comprising:
[0022] (a) a glass sheet having at least one substantially flat
surface;
[0023] (b) a first protective coating on the substantially flat
surface comprising at least one polysaccharide, the coating having
a thickness of at least 0.01 .mu.m; and
[0024] (c) a second protective coating over the first protective
coating;
[0025] wherein
[0026] (i) the protective coatings protect the surface from ambient
contaminants, contaminants produced during the processing of the
glass and/or scratching;
[0027] (ii) the second protective coating is less soluble in water
than the first protective coating; and
[0028] (iii) the protective coatings can be removed through
application of an aqueous cleaning composition to result in a
substantially clean surface.
[0029] The present inventive protection method can be used for any
glass surface that is in need of temporary protection. The present
inventive method can be advantageously employed in temporarily
protecting a surface of an LCD glass sheet, which is substantially
flat.
[0030] In the present invention, the at least one polysaccharide
can comprise straight chain polysaccharide molecules and branched
polysaccharide molecules. In certain preferred embodiments, the
polysaccharide comprises starches, including straight chain starch
molecules and branched starch molecules. In certain preferred
embodiments, the first coating composition and the first coating,
and/or the second coating composition and the second protective
coating, comprise a plasticizer and/or a biocide. Preferably, the
first protective coating has a thickness of less than 50 .mu.m.
More preferably, the thickness of the first protective coating is
between 0.1 and 20 .mu.m.
[0031] In certain preferred embodiments of the present invention,
the second coating composition is an aqueous mixture comprising at
least one polymer, such as a polymeric acid, and, preferably, the
second protective coating is applied by spraying the aqueous
mixture over the first protective coating.
[0032] In certain other preferred embodiments, the second coating
composition and the second protective coating comprise a wax, such
as petrolatum.
[0033] In still certain other preferred embodiments, the second
coating composition and the second protective coating comprise
polymer particles, and, preferably, the second protective coating
is applied by spreading the polymer particles over the first
protective coating before the first protective coating dries
up.
[0034] In certain preferred embodiments of the present invention,
the cleaning composition is a basic aqueous solution having pH of
equal to or above 10. More preferably, the cleaning composition is
an aqueous detergent solution, e.g., a commercially available
detergent package, preferably used in connection with brush washing
and/or ultrasonic cleaning. Preferably, the cleaning composition
for removing the polymeric acid-containing protective coating is
heated to a temperature in the range from 40.degree. C. to
75.degree. C.
[0035] In still certain other preferred embodiments of the present
invention, the first and second protective coatings are formed as a
part of the manufacturing process for the glass, such as a fusion
draw or a slot draw process, and the like, wherein the
manufacturing process produces newly formed glass at an elevated
temperature of above 150.degree. C. when it first comes into
contact with the first coating composition. Although it is
advantageous to integrate the present inventive method into the
glass manufacturing process, it can be operated off-line after the
glass is manufactured if so desired.
[0036] In other preferred embodiments of the present inventive
method, the first and second protective compositions are applied by
spraying onto hot glass surface. Other coating methods can be used
to carry out the step (A) of the present inventive method,
including, but not limited to, dip coating, flow coating, spin
coating, by equipment such as meniscus coaters, wick coaters,
rollers, and the like, where the coating compositions are in liquid
form.
[0037] The present inventive method can comprise the additional
steps between steps (B) and (C) of:
[0038] (a) cutting the glass; and
[0039] (b) grinding and/or polishing at least one edge of the
glass; wherein water or a water-containing composition is applied
to the coated glass surface during at least one of steps of (a) and
(b).
[0040] The present inventive method can also comprise the
additional steps between steps (B) and (C) of:
[0041] (c) packing the glass with the protective coating closely
with or without a spacing material; and optionally
[0042] (d) subsequently storing, shipping and/or unpacking the
glass.
[0043] The method and the coated glass of the present invention
result in a number of advantages over prior art. For example, the
protective coating system provides sufficient protection to the
surface of glass against ambient contaminants, contaminants
produced during the processing of the glass and/or scratching, thus
potentially allows the glass sheets to be packed closely with
minimal spacing material between them. In addition, the method of
the present invention can be conveniently integrated into the
overall glass manufacturing process, and the pristine surface of
the glass can be revealed by removing the protective coatings
sufficiently and conveniently without substantial change to its
chemical composition and physical properties. In particular, the
coating system of the present invention withstands water treatment
during the cutting and grinding steps of the glass where water is
used as a cooling agent.
[0044] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from the
description or recognized by practicing the invention as described
in the written description and claims hereof, as well as the
appended drawings.
[0045] It is to be understood that the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework to understanding the nature and character of the
invention as it is claimed.
[0046] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] In the accompanying drawings,
[0048] FIG. 1 is a schematic diagram showing the measurement of
water contact angle on the surface of the glass in the present
invention;
[0049] FIG. 2 is a diagram showing the morphology of a glass
surface coated with a starch base coating and a poly(methyl
methacrylate) bead top coating under a white-light interferometric
microscope before the glass is treated by immersion in water;
[0050] FIG. 3 is a diagram showing the morphology of the surface of
the glass in FIG. 2 under a white-light interferometric microscope
after it is treated by immersion in water.
DETAILED DESCRIPTION OF THE INVENTION
[0051] As used herein, "substantially clean" means sufficiently
clean in terms of number of contaminants per unit surface area,
water contact angle, surface roughness as measured by atomic force
microscopy (AFM), or other parameters, such that the glass can be
used for further applications as intended without the need of
further cleaning of the surface.
[0052] As embodied and broadly described herein, the present
invention provides a method for temporary protection of glass
surface by providing a removable coating system on the surface of
the glass.
[0053] Cleanliness of the surface of the glass substrate for a LCD
display is of vital importance for the quality of the thin-film
transistors formed on the surface of the substrate. The surface of
the substrate is required to be substantially free of ambient
contaminants and contaminants produced from the processing of the
glass, including cutting and grinding. As discussed supra, adhesion
of glass particles to the substrate surface is a long-standing
problem in the manufacture of LCD glass. In particular, scoring at
the bottom of draw (BOD) is a main source of adherent particles
during substrate manufacturing. Ultrasonic cleaning and brush
cleaning can remove some of the particles that deposited on the
glass surface for a short period of time. However, such cleaning
processes are not effective for particles deposited on the surface
for more than a few days, especially if the storage environment is
hot and humid, because permanent bonding between the particles and
the glass surface may have taken place.
[0054] Therefore, it is desirable to have a protective coating
system that can prevent particles from adhering to the LCD glass
surface at the bottom of draw. Additionally, it is also desirable
for the protective coating to provide resistance to scratching,
which may frequently occur during the processing, handling, storage
and shipping of the substrates. Excellent scratch resistance of the
coating allows the glass sheets to be packed closely to each other
with minimal use of spacing material between them. Besides
protecting the substrate surface from ambient dirt and glass
particle contamination and scratching, the coating should
preferably be removable with reasonable cleaning technique using
mild cleaning procedures, for example, a cleaning procedure that
includes an ultrasonic detergent wash at 40.degree. C. combined
with some brush cleaning steps. Although in principle organic
solvents can be used for cleaning and removing the protective
coating, they are not preferred due to health, environmental and
safety concerns. Rather, a mild cleaning procedure using aqueous
cleaning composition is preferred.
[0055] Many commercial polymer products can be applied to the glass
surface to form protective coatings, but they are not necessarily
sufficiently removable from the glass surface under the above
cleaning conditions due to their strong interaction with the glass
surface. For example, there are many organic coatings having good
water solubility at higher temperatures. However, the cleaning
temperature of 40.degree. C. is too low for many of them to be
sufficiently removed from the glass surface. Moreover, although
good aqueous solubility is desired, a coating should not be highly
hygroscopic because it must be able to withstand a hot and humid
environment without decreasing its coating effectiveness. In
addition, in order not to change the surface chemistry and major
physical characteristics, inter alia, smoothness, so that the glass
surface revealed upon removal of the protective coating is fit for
producing liquid crystal display without further surface treatment,
the coating composition, the protective coating per se and the
cleaning composition should not be chemically active or detrimental
toward the glass surface.
[0056] In search for a suitable protective coating for LCD glass,
the present inventors discovered the present inventive two-layer
coating and the coating method, the details of which are given
below. Generally, the two-layer protective coating of the present
invention comprise a first protective coating, which is applied
directly to the glass surface sought to be protected as a base
coating, and a second protective coating, which is applied over the
base coating. The first protective coating comprises at least one
polysaccharide, and the second protective coating has a solubility
in neutral water at room temperature lower than the first coating.
The coating system of the present invention offers good protection
to the glass surface, good removability under mild removing
conditions, and good water resistance during processing steps where
water is used as a cooling agent, such as the cutting and grinding
steps.
[0057] A. The First Protective Coating and the First Coating
Composition
[0058] As discussed above, the first protective coating and the
first coating composition of the present invention comprise at
least one polysaccharide. More particularly, the first protective
coating of the present inventive two-layer coating system consists
essentially of polysaccharide. As used herein, the term "consist
essentially of" means that the first protective coating may contain
ingredients other than polysaccharides, provided those ingredients
do not materially alter the novel and basic features of the
coating. Thus, "a coating consisting essentially of at least one
polysaccharide" contains at least one polysaccharide and may
comprise other ingredients, such as binders, biocides,
plasticizers, and the like, as long as the other components do not
materially alter the novel and basic feature of the first
protective coating of the present invention. The first protective
coating may be a homogeneous coating consisting essentially of a
single polysaccharide, or a mixture of a variety of different types
of polysaccharides, or a heterogeneous coating comprised of a
plurality of layers of different polysaccharides.
[0059] It has been discovered that polysaccharide protective
coating, used alone, can offer good protection to the glass
surface. Co-pending patent application Ser. No. 09/941182, entitled
"WATER REMOVABLE COATINGS FOR LCD GLASSES," has a description of
polysaccharide coating, which is incorporated herein by reference
in its entirety. However, this application did not describe the use
of polysaccharide coating as a base coating in conjunction with a
second protective coating over it.
[0060] A wide variety of polysaccharides are known in nature.
General discussions of polysaccharides and polysaccharide chemistry
can be found in the following references, the relevant portions of
which are incorporated herein by reference: T. M. Greenway,
"Water-Soluble Cellulose Derivatives and Their Commercial Use,"
Cellulosic Polymers, Blends, and Composites (R. D. Gilbert Ed.,
Hanser Publishers, New York, 1994) 173-88; R. B. Evans & O. B.
Wurzburg, "Production and Use of Starch Dextrins," Starch:
Chemistry and Technology, Volume 2 (R. L. Whistler & E. F.
Paschall Eds., Academic Press, New York, 1967) 254-78; S. Kitamura,
"Starch Polymers, Natural and Synthetic," Polymeric Materials
Encyclopedia (J. C. Salamone Ed., CRC Press, Boca Raton, Fla.,
1996) 7915-22; and "Polysaccharides I: Structure and Function" and
"Polysaccharides II: Chemical Modifications and Their
Applications," Essentials of Carbohydrate Chemistry (J. F. Robyt
Ed., Springer, New York 1998) 157-227 and 228-44.
[0061] Water solubility is low for most of the raw natural
polysaccharides, but manufacturers degrade and modify natural
products, e.g., by acid or enzymatic hydrolysis, to fit different
industrial applications, such as, food, paper, pharmaceuticals,
personal care, and paint. Depending on the modification,
polysaccharides having a variety of water solubilities and
viscosities are commercially available.
[0062] Starch derivatives, e.g., cornstarch derivatives, and
cellulose ethers, e.g., the METHOCEL cellulose ethers sold by Dow
Chemical, are the most common industrial polysaccharides. Low
molecular weight products have excellent water solubility. They
tend to form glossy, tough and flexible coatings. These materials
have been used in foods and in the pharmaceutical industry, e.g.,
in the manufacture of pills, and coatings made from these materials
have been proved to be non-toxic.
[0063] The METHOCEL family of polysaccharides includes two basic
types of cellulose ethers: methylcellulose and hydroxylpropyl
methylcellulose. Like starch, cellulose is composed of chains of
D-glucose units but it has a different glycoside linkage
configuration, i.e., its polymeric backbone is an all linear,
.beta.-1,4-glucosidic chain. This configuration results in various
property differences between cellulose and starches. For example,
naturally occurring cellulose is water insoluble with high
molecular weights up to 2,000,000.
[0064] Depending on how the cellulose is modified, cellulose ether
products can have a variety of different properties, such as, water
solubility, surface activity and thickening.
[0065] The METHOCEL cellulose ethers are water soluble, but when a
solution of this material is heated above a predetermined
temperature, a gel forms. METHOCEL films can be made by evaporating
water from a METHOCEL solution and the resulting films are clear,
tough, flexible and non-toxic. In a low temperature aqueous
solution, films can be rehydrated to form a gel and then go back
into solution.
[0066] In addition to the starches and cellulose products discussed
above, other polysaccharides that can be used in the practice of
the invention include degraded polysaccharides, hydroxyethyl
cellulose derivatives, exudate gums and their derivatives, and
alginates. A single polysaccharide or a mixture of polysaccharides
can be used in the practice of the invention, e.g., the coating can
comprise one polysaccharide or a mixture of two, three, or more
polysaccharides. Alternatively, a plurality of coating layers each
comprising a different polysaccharide may be sequentially applied
to the glass surface to form the first protective coating.
[0067] The mixtures and/or plural coating layers can be within one
class of polysaccharides (e.g., a mixture of starches) or can be
across classes (e.g., a mixture of a cellulose ether and a starch).
The most preferred polysaccharides for use in the invention are
starches. As used herein, the terms "starch" and "starches"
includes both water soluble starches, water soluble starch
derivatives and water soluble starch degradation products.
[0068] Starches are naturally occurring polymers. Potato, corn,
tapioca, wheat and many other plants are commercial sources of
starches. Some of the basic properties of starches are water
solubility and biodegradability, i.e., degradation by
microorganisms.
[0069] Chemically, starch is a polymer of D-glucose linked via
.alpha.-1,4- or .alpha.-1,6-glucosidic binding. There are two kinds
of starch polymer structures, namely, amylose structures and
amylopectin structures. Amyloses are linear starch molecules formed
via .alpha.-1,4-glucosidic binding. Amylopectins are branched
starch molecules in which several short linear amylose chains with
20 to 25 D-glucose units each are linked via .alpha.-1,6-glucosidic
bindings.
[0070] Native starches exist as cold water-insoluble granules. The
granules are composed of amylose and amylopectin molecules
associated by hydrogen bonding either directly or via water
hydrates to form oriented micelles or crystalline areas. When water
is added, starch granules absorb water and swell. With increasing
temperature, the granules swell further and the viscosity of the
solution increases. At the point of maximum viscosity, the starch
granule structure is destroyed. Thereafter, the viscosity decreases
gradually and a clear solution forms after the temperature passes
the point of maximum viscosity.
[0071] Compared to native starches, starches preferred for use in
the present invention have lower molecular weights and lower
viscosities as a result of acid or enzyme degradation of the native
material or of high molecular weight starch products derived from
native starches. These lower molecular weights and lower
viscosities result in coatings that dry much faster and are easier
to wash from the glass surface with an aqueous detergent solution.
These acid or enzyme degraded starches also have excellent
solubility in cold water. These degraded starch products are
sometimes called dextrins. They may include typical dextrin,
maltodetrin and corn syrup solid.
[0072] Water-based spray coating is the preferred method for
applying these coatings to glass substrates for particle and
abrasion protection. For such applications, the aqueous solution
preferably has a viscosity between 0.1 centipoise and 100
centipoise.
[0073] Examples of suitable starches are PURITY GUM 59, a waxy
maize starch product which has been degraded by an enzyme and
modified by propylene oxide, and CRYSTAL TEX 627, an
acid-hydrolyzed tapioca starch dextrin. Both of these products are
available from National Starch and Chemical Company, Bridgewater,
N.J. Another example is Maltrin M200, a corn syrup solid made by
Grain Processing Corporation, Muscatine, Iowa. They have low
solution viscosities, i.e., 1 to 2 cps at 2.5% concentration, they
form glossy and non-hygroscopic tough films, and they do not gel at
the temperatures normally used for coating removal, e.g.,
temperatures of around 40.degree. C.
[0074] Optional components can be added to the first coating
composition, thus to the protective coating, to adjust the coating
properties, solubility or dispersion of polysaccharide in the first
coating composition, or to inhibit growth of biological materials
in the protective coating and coating composition, and the like, in
suitable amounts such that they will not materially alter the novel
and basic features of the present inventive coating. Concentrated
coating compositions can be prepared, stored, and diluted to the
application concentration when desired.
[0075] As discussed above, starch coatings are biodegradable, which
means that they are attacked by microorganisms such as bacteria and
fungi. Under such circumstances, the first coating composition and
the protective coating of the present invention preferably contain
a biocide to inhibit growth and attack of biological materials
during the storage and shipment of the first coating composition
and coated glass. To this end, some commercial biocides, for
example, KATHON LX (Rohm & Haas) and DOWICIL 75 (Dow Chemical
Company) can be used. Sorbic acid and p-hydroxybenzoic acid esters
are the additional examples. Inclusion of boric acid in the coating
composition can also inhibit growth and attack of certain
microorganisms. A biocide may change the chemical and mechanical
properties of the coating. The amount of biocide in the coating
composition, which thus becomes a part of the protective coating,
should not exceed 20% by weight of the polysaccharide. Typically,
concentration of a biocide in the first coating composition is in
the range of 50 ppm and 1% by weight.
[0076] The polysaccharide first coating composition and the first
protective coating of the present invention may also contain one or
more plasticizers which may be a polyhydroxy compound. Examples of
suitable plasticizers include, but are not limited to, sorbitol,
glycerol, ethylene glycol, polyethylene glycol, and mixtures
thereof. Such components can reduce the probability of the coating
to become overly brittle at low humidity. Such plasticizers can
also enhance the physical properties of the protective coating in
terms of smoothness, mechanical strength which determines its
scratch resistance, as well the longevity of the coating.
Typically, concentration of plasticizers in the first coating
composition can range from 0 to 30% by weight of the
polysaccharide.
[0077] The above description of biocides and plasticizers as
optional components in first coating and the first coating
composition are not exhaustive, but are illustrative only. Other
components can be added and become a part of the first polymer
protective coating on the glass surface if desired, as long as they
do not alter the novel and basic features of the present inventive
protective coating.
[0078] The first protective coating is preferably applied to the
glass surface by spraying a solution of the polysaccharide and an
aqueous solvent (e.g., deionized water) onto the surface and
evaporating the aqueous solvent to form the coating. For example,
the coating composition can be applied to a glass surface having a
temperature in the range of 20-250.degree. C. using an air spray
gun with 20 to 60 psi pressure.
[0079] The coating is preferably applied to a newly formed sheet of
glass immediately after the forming process. In particular, the
aqueous solution is applied to the glass while its temperature is
above 100.degree. C., preferably above 150.degree. C., and most
preferably above 180.degree. C., where the temperature of the glass
is preferably measured with an infrared detector of the type
commonly used in the glass making art.
[0080] Polysaccharides and, in particular, starches start to
decompose above about 250.degree. C. Thus, the preferred glass
temperature at the point of coating application is less than
250.degree. C., a temperature which newly-formed glass reaches
quite quickly in, for example, a fusion draw glass manufacturing
facility. However, since a polysaccharide/water solution is being
applied and since water has a high evaporation heat, the
evaporation of water at the glass surface will cool the glass
quickly. Thus, the coating solution can be applied to, for example,
300.degree. C. glass without significant decomposition.
[0081] The temperature of the aqueous first coating composition
when applied is preferably in the range of 20.degree. C. to
85.degree. C., i.e., heated solutions can be used. One benefit of
using a heated solution is to help dry the coating when the glass
substrate temperature is less than 150.degree. C. Also, a heated
solution has a lower viscosity than at room temperature which can
be beneficial in achieving atomization of the solution. The
temperature of the aqueous solution, of course, should be below the
gel point of the polysaccharide used for those polysaccharides that
have a gel point.
[0082] Application of the coating as part of the glass
manufacturing process is advantageous because the glass is clean,
and the coating will protect the glass during the remainder of the
manufacturing process. Application of a coating to glass at
elevated temperatures means that the application time may need to
be relatively short depending on the rate at which the glass is
being formed and the desired minimum glass temperature at the end
of the application process.
[0083] The glass may be formed by several different processes,
including float processes, slot-draw processes, and fusion draw
processes. See, for example, U.S. Pat. Nos. 3,338,696 and
3,682,609, which are incorporated herein by reference in their
entirety. In the slot-draw and fusion draw processes, the
newly-formed glass sheet is oriented in a vertical direction. In
such cases, the aqueous solution should be applied under conditions
that do not result in the formation of drips since such drips can
interfere with cutting of the glass, e.g., the drips can cause the
glass to crack. In general terms, dripping can be avoided by
adjusting the spray level to keep the glass at a temperature above
100.degree. C. throughout the coating process. As the spray level
is adjusted, e.g., reduced, the concentration of polysaccharide in
the solution also needs to be adjusted, e.g., increased, to insure
that an adequate amount of polysaccharide reaches the surface to
fully cover the surface of the glass.
[0084] Rather than spraying, the coating can also be applied from a
flexible material impregnated with a solution of the coating
material. Other possibilities include dipping, meniscus coating,
rollers, brushes, spin-coating, or any other process which brings
the coating solution into contact with the glass surface. Spraying
is considered the most preferred since it readily accommodates
movement of the glass introduced by the glass manufacturing
process. Typically, both sides of the glass will be sprayed
simultaneously, although sequential coating of individual sides can
be performed if desired.
[0085] The coating thickness should be above 0.01 .mu.m and is
preferably less than 50 .mu.m. Most preferably, the coating
thickness is between 0.1 and 20 .mu.m. When the coating is too
thin, pin holes occur easily, i.e., a continuous layer of
polysaccharide is not formed. When the coating is too thick, it
takes too long to remove the coating from the glass surface and the
overall usage of coating materials is high.
[0086] B. The Second Protective Coating and the Second Coating
Composition
[0087] As discussed above, the two-layer coating of the present
invention comprises a second protective coating over the
polysaccharide base coating. The second protective coating is
designed, inter alia, to improve water resistance of the coating
system. Thus, the second protective coating, the second coating
composition, and the application process thereof should desirably
and advantageously have the following features:
[0088] (1) The second protective coating should have a lower
solubility in water than the first protective coating.
[0089] (2) The second protective coating should be compatible with
the first coating. The application process and existence of the
second coating should not destroy the integrity of the first
coating. The protective effect of the first coating should not be
substantially compromised or diminished by the second coating and
its application process. The second coating should advantageously
possess sufficient adherence to the first coating in order to offer
meaningful additional protection. The bonding between the first
protective coating and the second protective coating of the present
invention may be effected through chemical bonds and/or physical
forces, such as covalent bonds, hydrogen bonds, ionic bonds, or van
der Waals force, or other mechanism.
[0090] (3) The removability of the coating system of the present
invention should advantageously be sufficient to realize an easy
and convenient restoration of the pristine glass surface where
necessary notwithstanding the second protective coating. The second
coating should advantageously be removable by an aqueous cleaning
composition. Preferably, the whole coating system should be
advantageously removable using a single removing composition. More
preferably, the removing composition is a mild commercial cleaning
package.
[0091] (4) The chemical and physical properties of the second
protective coating are such that the application, presence and
removal thereof will not substantially change the chemical and
physical properties of the glass surface, as for the first
protective coating.
[0092] (5) Preferably, the second protective coating should be easy
to apply and possible to integrate into the glass manufacturing
process so that the glass surface can be protected since the
earliest stage.
[0093] (6) Like the first coating, the second coating should be
advantageously environmentally friendly and pose as few as possible
safety and health concerns in its application and removal.
[0094] The second protective coating can be a continuous film
covering the whole outer surface of the first coating, a continuous
porous network covering part of the outer surface of the first
coating, or a layer formed by discrete or partly adjoined patches,
particles or beads. Where the second protective coating is a
continuous layer covering fully the first protective coating, the
first protective coating is shielded from water during cutting and
grinding if the second coating is not removed in the process. Where
the second protective coating partially covers the first protective
layer by, for example, forming a porous network over the first
protective coating, it shields part of the first protective coating
from water and holds the polysaccharide coating that may become
loose or detached from the glass surface in place. Either way, the
higher hydrophobicity or water insolubility of the second
protective coating helps to prevent the first coating from being
diminished, improved water resistance of the coating system, and
enhances the coating system's protection against glass chips and
other contaminants.
[0095] The second protective coating can be in solid state, or in
the form of a gel, a viscous fluid or a liquid. The second coating
composition, depending on the particular coating material, can be
in solid form, an aqueous solution, a suspension or an emulsion
containing the materials that form the second protective coating.
The methods of forming the second coating, some detailed infra,
vary for different coating compositions.
[0096] I. Polymer as the Second Protective Coating
[0097] The second protective coating may comprise at least one
polymer. More particularly, the second protective coating system of
the present inventive two-layer coating consists essentially of at
least one polymer. "A coating consisting essentially of at least
one polymer" contains at least one polymer and may comprise other
ingredients, such as binders, solvents, biocides, plasticizers, and
the like, as long as the other components do not materially alter
the novel and basic feature of the second protective coating of the
present invention. The polymer used for the second protective
coating can be thermoplastic or thermosetting.
[0098] (i) Polymeric Acid
[0099] One example of the polymers that the second protective
coating may comprise is polymeric acid. Polymeric acid has been
discovered to be useable as a protective coating for LCD glass.
Co-pending, co-assigned patent application Ser. No. 10/109463 has a
description of using polymeric acid protective coating, which is
incorporated herein by reference in its entirety. A polymeric acid
is a polyelectrolyte having at least one group capable of producing
a proton upon contacting water, such as a --COOH group (carboxylic
acid), a hydroxyl group in phenol and its derivatives, an anhydride
group, and the like, and/or a salt and/or partial salt thereof.
Polyelectrolytes are polymers with ionizable groups on their chain,
and therefore, tend to ionize in aqueous solutions. The degree of
ionization of polyelectrolytes varies depending on the number and
properties of the ionizable groups on the polymer chains, polymer
chain structure and pH of the solution. The polymeric acid used in
the present invention can be an acidic homopolymer, or copolymer,
including random, alternate and block copolymer, or a combination
thereof. The number of ions on the polymeric acid chain varies as a
function of the pH of the aqueous solution. Without intending to be
bound by any particular theory, applicants believe that at higher
pH, the acidic groups tend to dissociate better to form more ions
and thus more electrical charges on the chain, leading to a high
solubility of the polymeric acid in the aqueous solution, and vice
versa. Thus, the polymeric acid second protective coating of the
present invention can provide resistance to neutral water used as
cooling agent in the cutting and/or grinding steps because of its
relatively low solubility at neutral pH, and accordingly offer
robust protection to the glass surface from contaminants during
such processing steps of the glass, inter alia, glass chips. In the
meantime, a polymeric acid second protective coating can be readily
removed in a typical cleaning composition, which normally has pH
higher than 10, where the polymeric acid has a higher solubility.
This variable and controllable solubility of the polymeric acid in
aqueous media provides the coating a combination of robust
protection during cutting and grinding when neutral water is used,
and sufficient removability in a cleaning composition, which
preferably has higher pH.
[0100] A wide variety of polymeric acids are known. General
discussion of polymeric acid and chemistry of polymeric acid can be
found in the following references, the relevant portion of which
are incorporated herein by reference: E. A. Berkturov, L. A.
Bimendina & S. E. Kudaibergenov, "Polyelectrolytes," Polymeric
Materials Encyclopedia, Volume 8 (J. C. Salamone Editor-in-Chief,
CRC Press, 1996) 5800; Polyelectrolytes and Their Applications (A.
Rembaum & E. Selegny Eds., Reidel: Dordrecht, Germany, 1975);
C. A. Finch, Chemistry and Technology of Water-soluble Polymers
(Plenum: New York, N.Y. 1983); and F. J. Glavis, Poly(acrylic acid)
and Its Homologs in Water-Soluble Resins (R. L. Davidson & M.
Sittig Eds., Chapman & Hall, Ltd., London 1962) 133.
Non-limiting examples of polymeric acid suitable for the second
coating composition and the second protective coating of the
present invention are homopolymers, copolymers, mixtures and other
combinations of acrylic acid, methacrylic acid, maleic acid and
their anhydrides, and polymers containing an acidic hydroxyl group
as in the case of phenol and their derivatives. Many polymeric
acids suitable for use in the present invention are commercially
available, for example, polyacrylic acid and poly(methyl vinyl
ether-alt-maleic acid) from Aldrich.
[0101] As defined supra, polymeric acid as used herein includes
polymers having at least one group capable of producing a proton
upon contacting water, and/or a salt or partial salt thereof. A
partial salt is a polymeric acid with a part of the acidic groups
on its chain neutralized by a base. For example, a partial ammonium
salt of a polymeric acid is a polymeric acid partially neutralized
by ammonia. As long as the polymeric acid-containing protective
coating formed over the first protective coating demonstrates
sufficiently low solubility in water and sufficiently high
solubility in the cleaning composition under an acceptable
condition, the polymeric acid used in the coating composition and
the formed coating of the present invention can be neutralized by
one or more bases in any suitable proportion. Thus, the polymeric
acid in the coating composition and the formed coating may take
various forms in various proportions. The salt and/or partial salt
can be an ammonium salt, a sodium salt, a potassium salt, and the
like, or a combination thereof. Preferably, the salt and/or partial
salt, if contained in the polymeric acid, is ammonium salt or an
alkaline metal salt. More preferably, the salt and/or partial salt
is an ammonium salt. The proportion of salt to unneutralized
polymeric acid can range from 0% to 100%.
[0102] (ii) Polyvinyl Alcohol
[0103] Another example of the at least one polymer that the second
protective coating and the second coating composition of the
present invention may comprise is polyvinyl alcohol. The polyvinyl
alcohol suitable for the second protective coating has an average
molecular weight of at least 50,000 g/mol, preferably at least
100,000 g/mol, more preferably 150,000 g/mol, and a degree of
hydrolysis of at least 90%, preferably at least 95%, more
preferably at least 97.5%. Generally, solubility of polyvinyl
alcohol with a high average molecular weight and high degree of
hydrolysis in neutral water at room temperature is very low (almost
insoluble), while its solubility in hot water (80.degree. C., for
example) is quite high. Thus, the polyvinyl alcohol second
protective coating provides good hydrophobicity and water
resistance to the coating system of the present invention during
the grinding and cutting processes where room temperature water is
generally used as a cooling agent because of its lower solubility
in neutral water at room temperature. The polyvinyl alcohol can be
removed by using a heated aqueous cleaning composition.
[0104] (iii) Hydrophobically Modified or Insolubilized
Polysaccharides
[0105] Hydrophobically modified or insolubilized polysaccharides
are a category of polymers suitable for the second protective
coating as well. By "hydrophobically modified or insolubilized," it
is meant that the polysaccharide is modified by reaction with
chemical modifiers or by admixture with more hydrophobic additives
to make the coating more water resistant or essentially insoluble
in neutral water at or near ambient temperature, or by both. A
typical chemically reactive hydrophobic modifier of polysaccharide
is glyoxal, which is a highly reactive dialdehyde. For example,
cornstarch or potato starch can easily react with glyoxal to form
hemiacetals followed by further reaction to form acetals, which are
more water resistant than unmodified starches. Formaldehye, like
glyoxal, is also an effective crosslinking agent for starch and
other polysaccharides, but is less preferred than glyoxal because
of environmental and health concerns. A typical hydrophobic starch
derivative is a starch modified by octenyl succinic andydride. The
reaction between the starch and octenyl succinic andydride produces
starch octenyl succinic ester. The hydrophobic octenyl chains thus
attached to the starch molecule improve the hydrophobicity to the
starch molecule. Addition of water-insoluble resins (such as resins
of copolymers of unsaturated carboxylic acids--such as maleic acid
anhydride or acrylic acid--with styrene, ethylene, alkyl vinyl
ethers, alkenyl fatty acid esters, or other monomers),
water-repellent additives (such as alkenyl succinic anhydride,
alkyl ketene dimer, and stearylated melamine), latex dispersion
(such as polystyrene butadiene, polyvinyl acetate and polystyrene
acrylate), to the polysaccharide coating, for example, a starch
coating, can enhance its water repellence. Such chemically and/or
physically hydrophobically modified or insolubilized polysaccharide
coating can conveniently serve as the second protective coating.
They tend to have good compatibility with the first polysaccharide
coating because of the structural similarity therebetween.
[0106] (iv) Other Polymers
[0107] Other polymers, so long as they have the features listed
above for the second protective coating, may be employed in the
second protective coating. Such other polymers include, but are not
limited to, polyolefins, polysulfones, polyesters, polyethers,
polyamides, polysiloxanes, polysilicone ethers, polyurethanes, or
copolymers and mixtures thereof. The polymers may be natural,
modified natural or synthesized products. They may be thermoplastic
or thermosetting.
[0108] The thickness of the second polymer protective coating is at
least 0.01 .mu.m, preferably at least 0.1 .mu.m to offer sufficient
protection, and generally less than 100 .mu.m, preferably less than
50 .mu.m, and most preferably less than 20 .mu.m to offer
sufficient removability.
[0109] The second polymer coating composition is preferably an
aqueous solution, suspension or emulsion of the coating material
due to concerns of health, environment, safety and economy.
However, organic solvents may be used alone or in addition to water
to dissolve the coating materials to form the coating composition.
Nonlimiting examples of organic solvents include alcohols, ketones,
tetrahydrofuran and ethers. Concentration of the polymer in the
second coating composition is not crucial to the present invention.
For coating compositions with a higher concentration, coating can
be effected with fewer application cycles and less application
time. For coating compositions with a lower concentration, the
protective coating of sufficient thickness can be obtained by
multiple application cycles. Spray coating of aqueous coating
composition is a preferred method for applying the second
protective coating. Viscosity of the coating composition may vary
as a function of the concentration of the coating material in the
second coating composition. For applications of spraying coating of
aqueous solution, the viscosity of the coating composition is
preferably between 0.1 and 100 centipoise.
[0110] The polymer second coating composition can be prepared by
dissolving the coating material in deionized water and/or other
solvents. Optional components can be added to the second coating
composition, thus to the protective coating, to adjust the coating
properties, solubility or dispersion of the coating material in the
solution, or to inhibit growth of biological materials in the
protective coating and coating composition, and the like, in
suitable amounts such that they will not materially alter the novel
and basic features of the present inventive coating. Concentrated
coating compositions can be prepared, stored, and diluted to the
application concentration when desired.
[0111] Where the second protective coating is a hydrophobically
modified or insolubilized polysaccharide coating, it can be
produced by application of a modifier solution over the surface of
the first protective coating, or by application of a pre-formed
coating composition containing chemically and/or physically
modified polysaccharide. For example, one embodiment of the present
invention involves application of 40% by weight glyoxal aqueous
solution over the first protective coating, either when the first
coating has partially dried up or after it dries up completely. In
this embodiment, essentially part of the first coating was
transformed into the second protective coating, which has higher
water resistance property than the largely un-modified first
protective coating. In another embodiment of the present invention,
the second protective coating was prepared by application of a
pre-formed modified aqueous polysaccharide coating composition. The
aqueous modified polysaccharide coating composition can be the
typical polysaccharide coating composition used for the first
coating composition added with (i) glyoxal; (ii) octenyl succinic
anhydride; (iii) water-insoluble resins (such as resins of
copolymers of unsaturated carboxylic acids--such as maleic acid
anhydride or acrylic acid--with styrene, ethylene, alkyl vinyl
ethers, alkenyl fatty acid esters, or other monomers); (iv)
water-repellent additives (such as alkenyl succinic anhydride,
alkyl ketene dimer, and stearylated melamine), (v) latex dispersion
(such as polystyrene butadiene, polyvinyl acetate and polystyrene
acrylate), or (vi) any mixture of at least two of (i), (ii), (iii),
(iv) and (v).
[0112] Some of the polymer second coating compositions of the
present invention may be bio-degradable, which means they may be
attacked by microorganisms such as bacteria and fingi. Under such
circumstances, the second coating composition and the protective
coating of the present invention preferably contain a biocide to
inhibit growth and attack of biological materials during the
storage and shipment of the coating composition and coated glass.
To this end, some commercial biocides, for example, KATHON LX (Rohm
& Haas) can be used. Sorbic acid and p-hydroxybenzoic acid
esters are the additional examples. Inclusion of boric acid in the
coating composition can also inhibit growth and attack of certain
microorganisms. A biocide may change the chemical and mechanical
properties of the coating. The amount of biocide in the coating
composition, which thus becomes a part of the protective coating,
should not exceed 20% by weight of the coating material. Typically,
concentration of a biocide in the coating composition is in the
range of 50 ppm and 0.1% by weight.
[0113] The polymer second coating composition and the second
protective coating of the present invention may also contain one or
more plasticizers which may be a polyhydroxy compound. Examples of
suitable plasticizers include, but are not limited to, sorbitol,
glycerol, ethylene glycol, polyethylene glycol, and mixtures
thereof. Such components can reduce the probability of the coating
to become overly brittle at low humidity. Such plasticizers can
also enhance the physical properties of the protective coating in
terms of smoothness and mechanical strength, which determines its
scratch resistance, as well the longevity of the coating.
Typically, concentration of plasticizers in the coating composition
can range from 0 to 30% by weight of the coating material.
[0114] The above description of biocides and plasticizers as
optional components in the second protective coating are not
exhaustive, but are illustrative only. Other components can be
added to the second coating composition and become a part of the
protective coating if desired, as long as they do not alter the
novel and basic features of the present inventive protective
coating.
[0115] (II) Wax as the Second Protective Coating
[0116] Wax as a group of hydrophobic materials can be used for the
second protective coating as well. Wax as used herein refers to a
substance that is a high viscosity fluid or a plastic solid at
ambient temperature and that, when subjected to a moderately
elevated temperature, becomes a low viscosity fluid. A wax may
contain various components, including naturally occurring esters of
fatty acids and certain alcohols as well as natural and synthesized
products having resembling properties of the natural waxes.
Depending on the source, waxes can be categorized to naturally
occurring waxes and synthesized waxes. Many waxes are already known
in the art. A general description of waxes can be found in William
P. Cottom, "Waxes," Polymer Material Encyclopedia, volume 17 (Eds.
Herman F. Mark et al. John Wiley & Sons, 1989), 614-626, the
relevant portions of which are incorporated herein by
reference.
[0117] Naturally occurring waxes suitable for the second protective
coating include insect and animal waxes, such as beewax,
spermaceti, and the like; vegetable waxes, such as camauba,
candelilla, Japan wax, ouricury wax, rice-bran wax, jojoba, castor
wax, bayberry wax, and the like; mineral waxes, such as montan wax,
peat wax, ozokerite and ceresin waxes, petroleum waxes, and the
like. Petroleum waxes are preferred because of their stability and
consistency in quality and composition.
[0118] Petroleum waxes are derived from petroleum and are generally
hydrocarbons of three types: paraffin, semicrystalline or
intermediate, and microcrystalline. A paraffin wax consists
principally of normal alkanes. Microcrystalline wax contains
substantial proportions of branched and cyclic saturated
hydrocarbons, in addition to normal alkanes. Semicrystalline wax
contains more branched and cyclic compounds than paraffin waxes,
but less than microcrystalline. Any wax in these three categories
can be used for the second protective coating owing to their good
hydrophobicity. A mixture of waxes of different categories, such as
petrolatum, may be conveniently used as well. Petrolatum, often
referred to as petroleum jelly, is an amorphous mixture of
microcrystalline wax, mineral oil and paraffin wax. Many
commercially available petroleum waxes can be used directly for the
second protective coating, or used after being formulated into the
second coating composition. Non-limiting examples of commercial
petroleum waxes are M 0745 and R 7132 from Moore & Munger,
Inc., Shelton, Conn.
[0119] Synthetic waxes suitable for the second protective coating
include, but are not limited to, polyethylene waxes,
Fischer-Tropsch waxes, chemically modified waxes, substituted amide
waxes, and polymerized .alpha.-olefin waxes.
[0120] Waxes can be applied directly as the second coating
composition over the first protective coating using a brush, a
roller, and the like, if it is in vicious liquid form at ambient
temperature, such as petrolatum. If the wax has a relatively higher
melting temperature zone, it may be heated moderately to liquid
state and then coated by a brush or roller or other conventional
means, such as hot-melt spray. Where the glass is still hot after
the first protective coating is dried, ground solid wax particles
may be sprinkled, sprayed or otherwise dispensed onto the glass
surface, which, heated to above its melting temperature by the
residual heat of the glass, will liquefy and form a coating over
the first protective coating.
[0121] The waxes can be formed into an aqueous suspension or an
emulsion as the second coating composition. Surfactants may be
added to the aqueous suspension or emulsion to stabilize the wax
dispersion. The second coating composition, an aqueous wax
dispersion, can then be conveniently applied to the surface of the
first protective coating by brush coating, roller coating, and
preferably, spray coating. Upon removing the solvent from the
second coating composition by drying, a second wax protective
coating is formed over the first protective coating.
[0122] (III) Solid Particles as the Second Protective Coating
[0123] The second protective coating may be formed by solid
particles. The particles attach to the surface of the first
protective coating, or are partially embedded in the first
protective coating layer. Adjacent particles may or may not be
bonded with each other. The particles are made of materials that
have a lower solubility in neutral water than the underlying first
protective coating. The hydrophobicity of the particles shield part
of the first protective coating layer from water during cutting and
grinding, and thus improves the water resistance of the coating
system of the present invention. Also, the presence of the
particles on the surface of the glass increases the distance
between the surface of glass sheets packed adjacent to one another,
making it possible to pack the glass sheets without or with minimal
spacing materials therebetween. The particles can be inorganic or
organic, and are preferably organic or inorganic polymers,
thermosetting or thermoplastic. The particles may be comprised of,
inter alia, polyamides, polyesters, polyethers, polysulfones,
polyaldehydes, polyketones, polyolefins, polydienes, polysilicone
compounds, polyurethanes, or copolymers or mixtures thereof.
Hardness of the particles is preferred not too high to cause
scratches on the glass surface during the application and removal
of the coating, as well as packing, grinding and transportation of
the glass. The surface of the particles may be regular and smooth,
for example, spherical or ellipsoidal (polymer beads), or irregular
and rough, such as the surfaces of scraped plastic and rubber.
Preferably, the surface of the particles are regular and smooth
where they have a high hardness so that there are few sharp edges
that would scratch the glass surface during the removal of the
coating. However, particles with irregular shapes and surfaces may
be used as well when hardness is low or when the particles are
elastic or flexible, as in the cases of polyurethane foam
particles, polydiene rubbers, polysilicone rubbers and the like.
The particles may be beads derived from suspension polymerization,
or may be obtained by scraping bulk polymer materials or by other
means. Particle size and particle size distribution are not
critical for the present invention. Generally, the particle size is
within the range of 0.01-50 .mu.m, preferably within the range of
0.1-20 .mu.m. If the particle size is overly large, for example,
over 50 .mu.m, good adhesion between the particles and the first
protective coating will be difficult to obtain. Finer particles
adhere to the first protective coating better, and are easier to
cover more areas of the surface of the first protective coating. Of
course, a mixture of particles with both large and small particles
is conducive to forming a dense particle coating as they can pack
more closely to each other, and thus increase water resistance of
the coating system. Non-limiting examples of polymer particles
suitable for the second protective coating of the present invention
include poly(methyl methacrylate) beads,
poly(styrene-divinylbenzene) beads, polyvinyl chloride beads,
polyvinyl dichloride beads, poly(styrene butadiene) and polyvinyl
acetate beads.
[0124] The solid particles can be used as the second coating
composition per se. Application of the polymer particles can be
done by spreading, for example, sprinkling the particles onto the
surface of the first protective coating before the first protective
coating dries up. After the application of the particles, the
coating system is subjected to further drying treatment. Once the
first coating completely dries, the particles will adhere to the
first coating or be partially embedded in the first coating,
forming a second protective coating. Where the particles have a
relatively low melting temperature, they may partially melt as a
result of the residue heat of the first coating and the glass
sheet. The melted particles harden as the temperature lowers to
ambient temperature to form a bond with the first coating and
possibly with the adjacent particles. Where the particles are made
of relatively high melting temperature polymers or thermosetting
polymers, they will not melt after application to the first
protective coating, thus the second protective coating is formed by
discrete particles.
[0125] Alternatively, the particles may be formed into an aqueous
dispersion thereof, such as a suspension or an emulsion before
application thereof to the first protective coating. Surfactants
may be added to the dispersion to stabilize it. Where a dispersion
is used as the second coating composition, it can be coated onto
the surface of the first protective coating using conventional
means, such as brush coating, roller coating, dip coating, flow
coating, and spray coating, with spraying coating being preferred
for its ease of application. Upon removal of solvent from the
second coating composition, a second protective coating comprising
the particles is formed over the first protective coating.
[0126] The application of the second protective coating is
advantageously integrated into the overall glass manufacturing
process as well. The second protective coating can be applied
immediately after the application of the first protective coating.
Of course, the second protective coating may be applied onto the
first protective coating long after the first protective coating is
formed where necessary. Preferably, the second protective coating
is applied before any cutting or grinding steps where water is used
as a cooling agent in order to take advantage of the water
resistance of the second protective coating. However, the second
coating composition may be applied multiple times, for example, one
before cutting and grinding, one after cutting and grinding, where
necessary, in order to impart sufficient protection to the glass
surface during further processing and handling steps.
[0127] The Cleaning Composition and Removal of the Coatings
[0128] It is desired for a successful protective coating system to
withstand the manufacturing process and still be sufficiently
removable when necessary. The two-layer coating system of the
present invention can be applied to the surface of glass before it
is scored for the first time and is strong enough to survive the
rest of the manufacturing process. The protective coating system of
the present invention can be readily removed by a cleaning
composition, usually in combination with application of additional
cleaning technique, such as mechanical brushing, ultrasonic wave
energy, and the like. Other alternative techniques for the removal
of the coating, such as oxidation, e.g., ozone-based oxidation,
CO.sub.2 cleaning, CO.sub.2 snow cleaning, O.sub.2 plasma and
pyrolysis cleaning can be employed either alone or in combination
with other removing techniques, although the use of an aqueous
cleaning composition coupled with ultrasonication and/or brush
cleaning is preferred.
[0129] The cleaning composition for use in the present invention
should advantageously be of a mild nature, which provides
sufficient removability of the protective coating without
substantially altering the chemical composition and physical
properties, inter alia, smoothness, of the glass surface. The
application of brushing and energy should meet this requirement as
well. Though cleaning compositions based on or comprising organic
solvents such as alcohols, tetrahydrofuran, ketones and ethers can
be used for removing the protective coating in the present
invention, an aqueous cleaning composition is preferred for
environmental, health and safety concerns. Aqueous cleaning
composition employed generally has alkaline pH, usually at least
10, preferably at least 11, more preferably around 12.5. However,
very strongly basic solution should be avoided because they may
react with the glass surface and change the chemical composition
and/or physical parameters thereof. Any reactive component that
will change the chemical and physical nature of the glass surface
should be avoided. Typically, a mild detergent with various
compositions is a part of the cleaning solution, which facilitates
removal of the protective coating and other oily materials and
particles. Where a detergent is present, its concentration in the
cleaning composition is in the range of 2-8% by weight, and the
cleaning composition will have alkaline pH. Removal of the
protective coating can be conducted at a temperature in the range
of 20-75.degree. C., with higher temperatures normally resulting in
more efficient removal of the coating, particles and organic
contaminants. Cleaning time is normally between 1 and 20
minutes.
[0130] It should be noted that the removal of the coating can be
done by the manufacturer of the glass or by the end user of the
glass, such as a manufacturer of liquid crystal devices, after the
glass is shipped with the protective coating thereon to the end
user. Upon removal of the protective coating system, a pristine
glass surface is revealed and can be used for further application,
for example, the production of LCDs.
[0131] To verify removal of a coating, the wettability of the glass
surface before and after the removal of the glass can be measured
and compared. Water contact angle is a good indicator of
wettability, which can be obtained using a variety of known methods
in the art. A schematic diagram of the contact angle measurement is
shown in FIG. 1, wherein .theta..sub.c is the contact angle, also
referred to as the sessile drop contact angle in the art.
Advantageously, the water contact angle of the glass surface upon
removal of the protective coating has a value of less than or equal
to 8.degree., indicating the glass surface is substantially clean.
Other methods that can be used to determine coating removal include
XPS (X-ray photoelectron spectroscopy) and TOF-SIMS (Time of Flight
Secondary Ion Mass Spectrometry), which can be used in combination
with water contact angle measurement. The surface upon removal of
the coatings should advantageously have a Rms surface roughness as
measured by atomic force microscopy of less than or equal to 0.40
nm.
[0132] D. Representative Benefits of the Present Invention
[0133] The present inventive coating system possesses all the
technical advantages of a single polysaccharide coating by using a
polysaccharide coating as the base coating, coupled with improved
water resistance during stages where water is used as the cooling
agent, such as in cutting and grinding of the glass. Thus, better
protection to the glass surface is achieved.
[0134] One of the benefits of this invention is its ability to
protect glass sheets from ambient contaminants which the glass may
be exposed to during, for example, storage or transportation.
[0135] Another benefit is the ability of the invention to reduce
chip adhesions when a glass sheet is cut or ground. As discussed
above, glass chip adhesions present a significant problem in the
manufacture of cut or ground glass, particularly in the manufacture
of LCD glass.
[0136] In particular, the present invention reduces the formation
of chip adhesions by providing a stable removable coating on the
surface of the glass sheet. As used herein, the phrase "stable
removable coating" means a coating that is bonded to the glass and
that is not removed or significantly degraded during handling,
storage and shipping, but is removable during the cleaning stage.
The coating adheres to the glass via interactions with the silica
on the glass surface, and acts as a barrier between the surface of
the glass and the glass chips. Because the coating reduces or
prevents glass chips from coming into contact with the surface of
the glass sheet, the occurrence of chip adhesion is reduced.
[0137] A further advantage of the invention is that the surface of
the glass sheet after removal of the coating has substantially the
same chemistry and smoothness as it had prior to application of the
coating. For example, the glass surface preferably has a RMS
surface roughness less than or equal to 0.40 nanometers as measured
by atomic force microscopy (AFM) after removal of the coating.
[0138] Moreover, the present invention makes it possible for the
glass sheets to be packed closely to each other without or with
minimal spacing material therebetween, because of the robust
protection provided by the protective coating system of the present
invention. And the extra second coating layer compared to a
mono-layer coating increases the scratch resistance of the coating
system.
[0139] The following examples provide further illustration of the
present invention, and are not intended to limit the scope of the
present invention to the specific embodiments described
therein.
E. EXAMPLES
[0140] In the following examples, glass sheets used for the testing
were 1737 LCD glass samples (5".times.5".times.0.7 mm) produced by
Corning Incorporated, Corning, N.Y. Each sheet was covered on one
side with a polymer film attached with an adhesive, and the other
major surface had a film attached by static charge. Both coatings
were removed from glass sheets followed by pre-cleaning. All glass
sheets were pre-cleaned before application of the coating
compositions. Pre-cleaning of the glass sheets in the examples were
carried out in accordance with the following procedure: (1) 2%
SEMICLEAN KG was sprayed on the substrates or coatings and
hand-scrubbing performed using a clean-room cloth; (2) the
substrates or coatings were subjected to ultrasound cleaning (40
kHz, 2% SEMICLEAN KG, about 40.degree. C.) for 15 minutes; and (3)
the substrates or coatings were subjected to brush cleaning with 2%
SEMICLEAN KG and deionized water, and spin-drying using a brush
cleaner (ULTRATECH 605 Photomask/Substrate Cleaner).
[0141] Water contact angles were measured to evaluate cleanness of
the glass surface and removability of coatings in the examples. It
has the advantages of being quick and easy. The coatings of the
present invention have lower surface energies than glass surface,
thus higher water contact angle are observed when coating residues
are present on the glass surface. For a substantially clean glass
surface free of polymer residues and contaminants, the water
contact angle should be extremely low due to the high surface
energy of the clean glass surface.
[0142] In the following examples, the first coating compositions
used were all Crystal Tex 627, which is a starch-containing aqueous
product available from National Starch & Chemical Company, with
0.1% boric anhydride added. Thus the first protective coatings
consisted essentially of starch. The first coating composition was
sprayed onto a surface of a glass sheet pre-cleaned and pre-heated
to about 200.degree. C.
[0143] To obtain coating thickness data, part of the coating
measured in the examples was removed using a piece of sharp razor
blade to reveal the glass surface. The coating was then measured
against the exposed glass surface using a Zygo white light
interferometric microscope (New View 5000 from Zygo Corporation,
Middlefield, Conn.). Multiple spots of the coating were measured
and the results were averaged to calculate the final coating
thickness.
[0144] In the examples, all coatings were subjected to the
following coating removal procedure: (1) hand-scrub the glass sheet
with 2% Semiclean KG at room temperature; (2) clean the glass sheet
in a countertop ultrasonic cleaner (Fisher Scientific Model 140)
with 2% Semiclean KG at 40-45.degree. C. for 15 minutes at 40 kHz;
(3) thoroughly rinse the glass sheet with deionized water and then
load the glass sheet in a water bucket; (4) brush-wash the glass
sheet using Ultratech 605 Photomask Cleaner according to the
following sequential program: 3 brush washer cycles with 2%
Semiclean KG at room temperature; 4 deionized water brush cycles; 5
deionized water jet spray cycles; and 10 air jet spray cycles.
Example 1
[0145] Two coating systems were tested in this example, both using
polymers that are more hydrophobic than starch as the second
protective coating over a starch first protective coating.
[0146] In this example, the first coating composition had a
concentration of starch of 10%. The two second coating compositions
used in this example were (i) a styrene/acrylic co-polymer based
emulsion (20% solids) available from Johnson Wax Professional,
Sturtevant, Wiss., and (ii) Chempeel WB high solids (55-60%)
strippable coating product available from PPG Industries, One PPG
Place, Pittsburgh, Pa., respectively.
[0147] The first protective coating was formed as discussed above.
After the first coating dried up, the second coating composition
was applied to the surface of the first protective coating layer
and dried with an IR heater to form a second protective coating
layer. Water contact angle was then measured on the surface of the
second protective coating layer thus obtained and recorded as the
first water contact angle .theta..sub.1 in TABLE I. Subsequently,
the coatings were subjected to removal procedures as described
above. Water contact angle was then measured again on the cleaned
glass surface and recorded as the second water contact angle
.theta..sub.2 in TABLE I.
[0148] As shown in TABLE I, water contact angle on the second
protective coatings obtained from coating compositions (i) and (ii)
were over 50.degree. and 40.degree., respectively, both higher than
the water contact angle on a starch coating, which is normally
around 30.degree.. Thus these two coatings were more repellent to
water than a single starch coating. The second water contact angle
data .theta..sub.2 for the two second protective coatings obtained
from coating compositions (i) and (ii) were both lower than
8.degree. C., demonstrating that the glass surfaces were
substantially clean upon removal of the coating systems and that
the coatings were substantially completely removed under the mild
cleaning condition.
Example 2
[0149] One coating system was tested in this example, using
petrolatum as the second protective coating over a starch first
protective coating.
[0150] In this example, the first coating composition has a
concentration of starch of 5%. The second coating composition used
in this example was petrolatum gel available from Fisher
Scientific, Pittsburgh, Pa.
[0151] The first protective coating was formed as discussed above.
The thickness of the dried first protective coating was about 1
.mu.m. After the first coating dried up, the second coating
composition (petrolatum gel) was applied to the surface of the
first protective coating layer by smearing to form a second
protective coating layer. Water contact angle was then measured on
the surface of the second protective coating layer thus obtained
and recorded as the first water contact angle .theta..sub.1 in
TABLE I.
[0152] Half of the glass sheet was then subjected to water
treatment by dipping and immersion in deionized water for 1 minute.
Coating thickness of the immersed side and the non-immersed side
were measured using a Zygo white light interferometric microscope
as discussed above and recorded in TABLE II, with the thickness of
coating not subjected to water treatment reported as T.sub.1, and
the thickness of coating subjected to water treatment as
T.sub.2.
[0153] Subsequently, the coatings were subjected to removal
procedures as described above. Water contact angle was then
measured again on the cleaned glass surface and recorded as the
second water contact angle .theta..sub.2 in TABLE I.
[0154] As shown in TABLE I, water contact angle on the second
protective coating obtained from petrolatum gel was over
100.degree., much higher than the water contact angle on a starch
coating, which is normally around 30.degree.. Thus the
starch-petrolatum coating system was far more repellent to water
than a single starch coating. The second water contact angle data
.theta..sub.2 for the starch-petrolatum coating was lower than
8.degree. C., demonstrating that the glass surface was
substantially clean upon removal of the coating system and that the
two-layer coating was substantially completely removed under the
mild cleaning condition.
[0155] The coating thickness data in TABLE 2 show that for the
starch-petrolatum coating system, T.sub.2 as measured was even
greater than T.sub.1 as measured. This might be caused by the
unevenness of the coating. Nonetheless, the proximity between
T.sub.1 and T.sub.2 is indicative that the coating thickness
reduction, if any, was negligible during the water treatment of the
coating system, showing the starch-petrolatum coating of the
present invention had very good water resistance.
Example 3
[0156] Two coating systems were tested in this example, both using
polymer beads as the second protective coating over a starch first
protective coating.
[0157] In this example, the first coating composition has a
concentration of starch of 10%. The second coating composition used
in this example was (iii) poly(styrene-divinylbenzene) (PSDB, 20%
divinyl benzene, average molecular weight 250,000 g/mol) beads and
(iv) poly(methyl methacrylate) (PMMA, average molecular weight
25,000 g/mol) beads, respectively, both available from
Polysciences, Inc., Warrington, Pa.
[0158] The first protective coating was formed as discussed above.
Once the first coating composition was applied to the hot glass
surface and before it dried up, the glass sheet was placed
horizontally and the second coating composition (iii) or (iv) was
applied to the surface of the first protective coating layer by
sprinkling. After the coating dried up, excessive loose polymer
beads were removed by flipping the glass sheet. Water contact angle
was then measured on the surface of the second protective coating
layer thus obtained and recorded as the first water contact angle
.theta..sub.1 in TABLE I.
[0159] Half of the glass sheet was then subjected to water
treatment by dipping and immersion in deionized water for 1 minute.
Coating thickness of the immersed side and the non-immersed side
were measured using a Zygo white light interferometric microscope
as discussed above and recorded in TABLE II, with the thickness of
coating not subjected to water treatment reported as T.sub.1, and
the thickness of coating subjected to water treatment as
T.sub.2.
[0160] FIG. 2 is a diagram showing the morphology of the glass
surface coated with a starch base coating and a PMMA top coating
under the interferometric microscope before the glass was treated
by immersion in water; The rugged left side of the diagram shows
the surface with the coating system, and the flat right side shows
the surface of the glass after the coating was removed using a
sharp razor blade.
[0161] FIG. 3 is a diagram showing the morphology of the surface of
the glass in FIG. 2 under a while-light interferometric microscope
after it is treated by immersion in water. Likewise, the rugged
left side of the diagram shows the surface with the coating system
after being treated by immersion in water, and the flat right side
shows the surface of the glass after the coating was removed using
a sharp razor blade.
[0162] Subsequently, the coatings were subjected to removal
procedures as described above. Water contact angle was then
measured again on the cleaned glass surface and recorded as the
second water contact angle .theta..sub.2 in TABLE I.
[0163] While FIGS. 2 and 3 as well as the T.sub.1 and T.sub.2 data
in TABLE II indicate that the thickness of the starch/PSDB and
starch/PMMA coating systems of the present invention might have
diminished during the water treatment, the thickness reduction was
acceptable for the purpose of protecting the glass surface,
especially when the small T.sub.1 data are taken into
consideration. The residual submicron-thickness coating after water
treatment would still provide sufficient protection against further
contamination and scratching. Thus the very thin initial coating
(as indicated by the T.sub.1 data in TABLE II) provided sufficient
protection during the water treatment. Where a thicker coating is
desired after the water treatment, the original starch coating can
be formed with a larger thickness, or additional starch coating can
be applied after the water treatment.
[0164] As shown in TABLE I, water contact angles on the second
protective coatings obtained from coating compositions (iii) and
(iv) were similar to the water contact angle on a single starch
coating, which is normally around 30.degree.. This was caused by
surface roughness as shown in FIG. 2 and FIG. 3 and incomplete
coverage of the beads over the starch coating. It is known that
water contact angle on a flat surface of a bulk polymer of either
PSDB or PMMA should be much higher than 30.degree.. For example,
water contact angle on a flat PMMA surface is about 80.degree..
Thus, the presence of polymer beads on the surface of the starch
coating should nonetheless have improved the overall hydrophobicity
of the coating system compared to a single starch coating. The
second water contact angle data .theta..sub.2 for the
starch-petrolatum coating was lower than 8.degree., demonstrating
that the glass surfaces were substantially clean upon removal of
the coating system and that the two-layer coating was substantially
completely removed under the mild cleaning condition.
1 TABLE I* Second Coating Composition .theta..sub.1 (.degree.)
.theta..sub.2 (.degree.) i 51 .ltoreq.8 ii 42 .ltoreq.8 petrolatum
108 .ltoreq.8 iii 28 .ltoreq.8 iv 31 .ltoreq.8 *The sensitivity of
the instrument for measuring water contact angle was 8.degree..
[0165]
2 TABLE II Second Coating Composition T.sub.1 (.mu.m) T.sub.2
(.mu.m) petrolatum 79 80 iii 0.1-5 0.15 iv 0.2-5 0.7
[0166] It will be apparent to those skilled in the art that various
modifications and alterations can be made to the present invention
without departing from the scope and spirit of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *