U.S. patent application number 09/813338 was filed with the patent office on 2001-09-27 for low-cost, user-friendly hardcoating solution, process and coating.
Invention is credited to Lewis, William.
Application Number | 20010024719 09/813338 |
Document ID | / |
Family ID | 27028515 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010024719 |
Kind Code |
A1 |
Lewis, William |
September 27, 2001 |
Low-cost, user-friendly hardcoating solution, process and
coating
Abstract
An improved, user-friendly silane/silica sol copolymer
hardcoating composition for protecting optical plastic and other
substrates (including wood, metals, glass, plastics and most coated
articles) from scratching is able to be manufactured at lower costs
and to provide much-improved ease of use in transportation,
storage, dipbath (or other coating tank) stability, and blush
resistance in ordinary cleanroom atmospheres. The silane/silica sol
copolymer is formed as a direct reaction product of an acidic
silica sol and monomethyltrialkoxysilane, preferably substantially
monomethyltriethoxysilane, in ratios of 30:70 to 70:30, most
preferably about 40:60. A tail solvent aids blush resistance,
reduces internal stress and permits adhesion to unprimed
polycarbonate. Optionally, colloidal silica sol dissolved in
water-miscible solvent may be reacted in a second stage with acidic
aqueous colloidal silica sol earlier silanized with
monomethyltrialkoxysilane.
Inventors: |
Lewis, William; (Chula
Vista, CA) |
Correspondence
Address: |
William Lewis
457 Rivercreek Ct.
Chula Vista
CA
91914
US
|
Family ID: |
27028515 |
Appl. No.: |
09/813338 |
Filed: |
March 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09813338 |
Mar 20, 2001 |
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09328725 |
Jun 7, 1999 |
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6265029 |
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09328725 |
Jun 7, 1999 |
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08771866 |
Dec 23, 1996 |
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08771866 |
Dec 23, 1996 |
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08430251 |
May 4, 1995 |
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5665814 |
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Current U.S.
Class: |
428/335 ;
427/387; 427/407.1; 428/412; 428/447 |
Current CPC
Class: |
C08J 7/0427 20200101;
G02B 1/14 20150115; Y10T 428/31663 20150401; C08J 7/043 20200101;
C08J 7/046 20200101; Y10T 428/264 20150115; C08J 2483/00 20130101;
C09D 183/04 20130101; Y10T 428/31507 20150401; Y10T 428/31667
20150401; C09D 183/04 20130101; C08L 2666/54 20130101 |
Class at
Publication: |
428/335 ;
427/387; 427/407.1; 428/412; 428/447 |
International
Class: |
B32B 027/36; B05D
003/02; B05D 001/36 |
Claims
I claim:
1. A method for imparting improved abrasion resistance to a solid
substrate, comprising the steps of: a. preparing or obtaining a
liquid thermosettable hardcoating composition comprising a
condensation-reaction copolymerization product of an acidic aqueous
colloidal silica sol, or at least functionally equivalent
solvent-based sol, or metal oxide particle suspension or metal
ester or mixture therewith, with [a silane ester mix consisting
primarily or exclusively of monomethyltrialkoxysilane] one or more
monomethyltrialkoxysilanes consisting of less than 50 mole percent
monomethyltrimethoxysilane, and said reaction product being with an
additive means of promoting cure, and at least one tail solvent,
and where said hardcoating composition can be stored at room
temperature for greater than 30 days without bulk gelation; b.
coating said substrate with said liquid thermosettable hardcoating
composition sufficient to apply 0.1-50 microns dry-film coating
thickness; c. drying to remove solvent and, if present, water; and
d. curing the coating.
2. A method of claim 1 wherein said liquid thermosettable
hardcoating composition has been maintained substantially at room
temperature for greater than 30 days without bulk gelation or loss
of basic properties upon curing on said substrate.
3. A method of claim 1 wherein said liquid thermosettable
hardcoating composition [has been] is applied while being
maintained substantially at room temperature.
4. A method of claim 1 wherein said liquid thermosettable
hardcoating composition [has been] is applied while being
maintained substantially at room temperature, and the steps of
coating and drying to remove solvent and water are conducted
without blush in the presence of ambient relative humidity greater
than 40% during drydown.
5. A method of claim 1 wherein said liquid thermosettable
hardcoating composition contains ["excess water"] excess water
beyond the stoichiometric amount needed for hydrolysis of silane to
and/or other metal esters, and with the additional step of said
substrate being coated at least a second time, after at least
partially drying to remove solvent and water after being coated a
first time, without substantially heat curing the coating to its
hardened state until said second layer is applied and at least
partially dried to remove solvent and water.
6. A method of claim 1 wherein said liquid thermosettable
hardcoating composition [has been] is applied directly onto
unetched and unprimed polycarbonate substrate, thus eliminating any
step of etching or applying and drying any liquid primer before
[dip]coating, such that a cured film of at least 4 microns
thickness of said liquid thermosettable hardcoating composition has
PSW scratch resistance surviving #000 steel wool, and 100%
cross-hatched adhesion.
7. A method of claim 1 wherein said liquid hardcoating composition
comprises a reaction product of an acidified aqueous dispersion of
colloidal silica sol having average particle size <100
millimicrons in diameter, or at least functionally equivalent
[solvent based] sol or ester, with [a monomethyltrialkoxysilane mix
consisting primarily of 67-100% monomethyltrialkoxysilane and
33-0%] one or more monomethyltrialkoxysilanes consisting of less
than 50 mole percent monomethyltrimethoxysilane, wherein said
reactants copolymerized in a ratio of not less than 30:70 and not
more than 70:30, and said reaction product being with a [latent]
condensation catalyst and at least one tail solvent; wherein said
[silane/silica sol copolymer] liquid thermosettable hardcoating
composition, after standing at room temperature for 30 days, does
not bulk gel nor lose basic properties on curing on said substrate;
wherein said coating is maintained substantially at room
temperature; and wherein said drying to remove solvent and any
water is carried out without blush in the presence of ambient
relative humidity greater than 40% during drydown.
8. An improved coated article comprising a substrate coated with a
liquid thermosettable hardcoating composition comprising a
condensation-reaction copolymerization product of a colloidal
silica sol, or at least functionally equivalent solvent-based sol,
metal oxide particle suspension or metal ester or mixture
therewith, with [a silane ester mix consisting primarily or
exclusively of monomethyltrialkoxysilane] one or more
monomethyltrialkoxysilanes consisting of less than 50 mole percent
monomethyltrimethoxysilane, and said reaction product being with an
additive means of promoting cure, and at least one tail solvent;
said substrate coated with said coating having a cured film which
has 500 revolution Taber abrasion "delta haze" less than 5% and/or
which provides scratch resistance surviving steel wool at least as
coarse as #000 in the PSW test, wherein the improvement comprises
lower manufacturing cost and/or improved abrasion resistance.
9. An article of claim 8, wherein the coating thickness after
curing is 7 microns to 50 microns, yet said cured film shows
substantially no cracking, peeling or delamination from said
substrate.
10. An article of claim 8 wherein the coating thickness after
curing is 7 microns to about 15 microns yet said cured film shows
substantially no cracking, peeling or delamination from said
substrate.
11. An article of claim 8, wherein said substrate is polycarbonate,
acrylic, or other transparent plastic.
12. An article of claim 8 wherein said substrate is an
injection-molded polycarbonate spectacle lens or protective goggle
or safety eyewear.
13. An article of claim 8, wherein said substrate is an unetched
injection-molded polycarbonate spectacle lens or protective goggle
or safety eyewear having no layer of primer film between said
transparent injection molded polycarbonate substrate and said cured
hardcoating film of at least 4 microns thickness, said cured film
having 100% cross-hatched adhesion.
14. An improved article with PSW test resistance to at least #000
steel wool, comprising a substrate coated with a dried and cured
hardcoating composition comprising a reaction product of a
colloidal silica sol, or at least functionally equivalent
solvent-based sol, metal oxide particle suspension or metal ester
or mixture therewith, with [a silane ester mix consisting primarily
or exclusively of monomethyltrialkoxysilane] one or more
monomethyltrialkoxysilanes consisting of less than 50 mole percent
monomethyltrimethoxysilane, and said reaction product being with an
additive means of promoting cure, and at least one tail solvent,
said substrate optionally base coated and/or primed for smoothness,
visual depth, adhesion, or other functional or cosmetic
enhancement, wherein the improvement is enhanced abrasion
resistance and/or reduced cost or manufacturing environmental
impact versus the unimproved article.
Description
[0001] This is a divisional of Ser. No. 09/328,725, a
continuation-in-part application of Ser. No. 08/771,866, filed Dec.
12, 1996, now abandoned, which is a divisional of Ser. No.
08/430,251, filed May 4, 1995, now U.S. Pat. No. 5,665,814.
FIELD OF THE INVENTION
[0002] The chemical family of silane/silica sol copolymer
compositions finds commercial use in heat-curing scratch-resistant
hardcoats applied mainly to clear plastic sheets and molded parts.
The present invention makes improvements in lowering costs and
improving ease of use and performance of specific chemical
compositions of monomethyltrialkoxysilan- e copolymerized with
acidic aqueous silica sol. These compositions are to be coated onto
molded or cast optical lenses (including auto headlamp lenses and
spectacle lenses and sports eyewear), plastic extruded or cast
sheet (used for windows, protective covers, etc.), and other
substrates needing a scratch-and scuff-protective surface
finish.
II. BACKGROUND OF THE INVENTION
[0003] Clark (U.S. Ser. No. 3,986,997, issued Oct. 19, 1976; and
U.S. Pat. No. 4,027,073, issued May 31, 1977) is the basis for the
first conmercially useful silane/silica sol copolymer hardcoats for
optical plastics. Clark is still serving as the basis for the
commercially successful line of nontintable clear hardcoats for
optical plastics offered by SDC Coatings of Anaheim, Calif. SDC is
a joint-venture company of Swedlow (now re-named Pilkington
Aerospace) and Dow Corning; the latter is the assignee of the Clark
patents. SDC's commercial dipcoating formulation Silvue 101 is
believed to be a higher % solids version of a Clark formulation
(suited for dip coating); all Clark coatings are acidic pH.
[0004] Next came the GE Silicone entries into this field, as
embodied in Frye (U.S. Pat. No. 4,299,746, issued Nov. 10, 1981;
U.S. Pat. No. 4,324,839, issued Apr. 13, 1982; U.S. Pat. No.
4,413,088, issued Nov. 1, 1983) and Vaughn (U.S. Pat. No.
4,309,319, issued Jan. 5, 1982; U.S. Pat. No. 4,324,712, issued
Apr. 13, 1982; U.S. Pat. No. 4,414,349, issued Nov. 8, 1983). All
the abovementioned GE Silicone patents employ an alkaline pH
aqueous colloidal silica dispersion, and their resulting coatings
are alkaline pH, in contrast to Clark's. GE's commercial dipcoating
formulation SHC-1200 is a commercially useful reference point.
[0005] Both the Clark/SDC and the Frye/Vaughn/GE coatings form
heat-cured clear coating films of excellent scratch resistance
(commonly tested with steel wool pads under load with a rubbing
action, to simulate cleaning-type damage) and thereby offer
essentially equivalent protection to the clear plastic substrates,
when applied and cured at equal thicknesses. Both are sold at very
high prices (on 100% solids basis, typically $80-110 per pound or
more). Because of a greater propensity for autopolymerization
inherent in the alkaline aqueous dipcoating solutions of the
Frye/Vaughn/GE formulas, wherein polysiloxane bond formation via
condensation reaction is favored over maximizing silanol stability
(particularly as storage temperature or bath temperature is
increased towards room temperature), use of acidic aqueous
silica/silica sol copolymer, as in the Clark/SDC compositions, is
preferred and therefore is the chemical basis for the present
invention.
[0006] For users purchasing these liquid coatings, in addition to
paying high prices for the coating material, there is the special
handling required of refrigerated shipping and storage. Most
particularly when dipcoating tanks are of large capacity, it is
necessary to keep the contents chilled to minimize the rate of auto
polymerization. "For maximum solution stability, Silvue
abrasion-resistant coatings should be refrigerated at -18 to +4 C
degrees (0 to 40 F degrees)", from SDC 5 Product Information Sheet
#130-0, 03/93).
[0007] Another indirect cost to the user is the need to apply these
water-based silane/silica sol copolymers--including dip, flow or
spray coating operations and coating solvent-drydown areas--at very
low humidity, typically a maximum of 35-40% R.H. Reference is made
to GE Silicones product literature titled "SHC 1200 Optical 11
Grade Abrasion-Resistant Silicone Hard Coat & SHP Primer" #CDS
4532 (5/90), stating . . . "relative humidity controlled to 35% or
less", and the previously-referenced Silvue literature, stating . .
. "relative humidity of less than 40% is recommended. Variations
from these conditions may result in blushing of the coating or a
poor film formation". Thus, for optically and cosmetically
satisfactory hardcoatings of spectacle lenses and other optically
useful products, not only are the usual clean room conditions with
HEPA-filtered, laminar airflow desirable, but the mandated maximum
relative humidities are below the range which can be readily
attained by ordinary HVAC interior air conditioning systems during
year-round use. Supplemental dehumidification is needed, calling
for expensive regenerative-desiccant-bed air-handling equipment
which can easily exceed $100,000 capital, just to handle the volume
of clean air recirculating within a fully-enclosed room or machine
used for applying such coatings.
[0008] "Blush" can be defined as a hazy or foggy appearance within
the otherwise-transparent cured film of the coating, which appears
during drydown of the coating. Early versions of low-solids,
high-water-content Clark coatings when applied, dried, and cured in
ordinary, ambient conditions had very poor "blush" resistance; 30
grains of moisture or less was then specified as necessary in the
air of the coating drydown area. Subsequently, the added step of to
azeotropic stripping of those 20% nominal solids was found to
remove both excess alcohol and part of the excess water, so that
the resulting stripped 35% solids Clark solutions had greater
"blush" resistance & humidity tolerance. However, this
stripping operation adds substantially to manufacturing costs.
[0009] Since the time of the Clark and Frye/Vaughn inventions,
solvent-based colloidal silica sols have become commercially
available, Potentially, they represent an alternative way of
reducing the sensitivity of these silane/silica sol copolymer
hardcoats to ambient levels of humidity, for increased "blush"
resistance. Excess water in the liquid coating composition is
defined as that which exceeds 100% stoichiometric amounts needed
for hydrolysis of the alkoxy groups on the silane (3 moles of water
per 1 mole of trialkoxysilane), in order to form the siloxane bond
between the silane and the silica sol. Use of a solvent-based
silica sol may actually require adding water to meet stoichiometric
requirements for proper formation of the copolymer. However, the
economic impact of replacing the aqueous silica sols with
solvent-based silica sols is extremely negative, by at least a
five-fold to ten-fold price factor. For example, Clark's preferred
aqueous silica, Nalco 1034A (from Nalco Chemical; Naperville,
Ill.), costs about $3 per pound on 100% solids basis, in drum
quantities. By comparison, an alcohol-based sol of equivalent
particle size from Nissan Chemical (New York City) costs over $75
per pound on equivalent 100% solids basis. A more recent one
commercialized by Nalco still costs about $17 per pound on 100%
solids basis so for lowest costs, solvent-based silicas need to be
minimized or eliminated in the coating formulation.
[0010] Notwithstanding this cost factor, solvent-based silicas have
been copolymerized with various organofunctional (most commonly,
epoxide attached by propyl to silicon) trialkoxysilanes, to make
tintable dipcoatings for protecting plastic ophthalmic prescription
spectacle lenses. Such coated lenses can be readily dip-dyed for
desirable colorations. In these formulations, the organofunctional
group attached to the silane is chosen for dye receptivity. Such
coatings can be crosslinked by heat (examples in which Applicant
was co-inventor are U.S. Pat. No. 5,013,608 and 5,102,695) or by
ultraviolet radiation (examples in which Applicant was co-inventor
are U.S. Pat. Nos. 5,221,560 and 5,296,295). These specialized
tintable Rx lens coatings are used on lower volume, high priced
lenses, so they can tolerate higher-priced silicas and silanes than
the general-purpose Clark hardcoatings.
[0011] Athough the Clark/SDC and the Frye/Vaughn/GE coatings
provide "excellent" scratch resistance compared to prior art
coatings, further improvements are desired and needed, not only for
clear plastic substrates but also to protect the surfaces of
nontransparent plastics, metals, woods, and even painted or
finished articles. The deficiencies of the SDC/GE coatings are
readily seen when they are rubbed with coarser steel wools than the
finest, #0000, typically used in demonstrating their scratch
resistance.
[0012] A convenient and effective embodiment of such a test is the
Progressive Steel Wools (PSW) test. Successive grades of steel
wools are rubbed 10 strokes (5 double rubs, back and forth) under
very firm (at least 10 pounds) thumb pressure on the article to be
tested, cross-wise of the alignment of the wool fibers. Depending
on the hardness of the coating, and of the substrate it is on,
there may be no visible scratches using the finest steel wool(s).
However, coarser wools will produce marks--fine, medium, or coarse
scratches. A coating is considered to have survived testing with a
given grade of steel wool if there are a half-dozen or fewer
scratches visible to the naked eye after such rubbing.
[0013] Another method of testing the hardness of coatings,
unfortunately only applicable to flat substrates, is the Taber
abrasion test. Using CS-10 wheels, Clark/SDC and Frye/Vaughn/GE
coatings typically increase in haze about 5-7% after 500
revolutions of the grit-filled, abrasive wheels, which scuff the
surface tested in a rolling/skidding motion. Harder coatings that
are user-friendly are sought after.
[0014] A third method of testing the hardness of coatings, which is
particularly used in the ophthalmic (prescription eyeglass lens)
industry, is the AO tumble test. In this test, coated or uncoated
articles, usually lenses, are tumbled with abrasive pads, grit,
sawdust, etc. to simulate in-use wear. The test (details available
from American Optical Lens Corp, Southbridge Mass.) was correlated
at the time of its development with 1- and 2-year eyeglass lens
wear tests using several materials and coatings. After tumbling and
cleaning, the samples are compared against standards, and ranked,
with a score of 10 being the highest.
III. OBJECTIVES OF THE PRESENT INVENTION
[0015] It is one objective of the present invention to make an
acidic silane/silica sol copolymer hardcoating with the lowest
possible manufacturing costs, by employing mostly or all low-cost
aqueous silica sols with minimal or no solvent-based sols, and
further, by using the copolymer "reaction products" without any
subsequent step of azeotropic stripping of excess solvent and
water.
[0016] It is another objective of the present invention to make
said acidic silane/silica sol copolymer compositions which
heat-cure to transparent films of equal or preferably better
hardness (e.g., Taber abrasion resistance) and steel-wool scratch
resistance as the competitive SDC and GE coatings.
[0017] It is another objective of the present invention to make
said silane/silica sol copolymer compositions with improved
resistance to autopolymerization/gelation and thereby eliminate any
need for refrigerated transportation, and to provide improved
stability in dipbath (or other coating method) operating conditions
and/or in inventory storage.
[0018] It is another objective of the present invention to
eliminate any need to use desiccant air-drying equipment in the
area where the liquid coating is applied and devolatilized, and be
able to successfully dry down these coating films without "blush"
using relative-humidity-control conditions (typically 40-55% R.H.)
achievable with normal HVAC air-conditioning, or even up to about
75% R.H without air conditioning.
[0019] It is another objective of the present invention to make a
silane/silica sol copolymer in a "self-priming" formulation which
successfully adheres to polycarbonate and other
difficult-to-adhere-to substrates without priming or any special
pre-treatment other than standard washing to remove surface
impurities.
[0020] It is another objective of the present invention to provide
coatings which can be cured thicker without cracking or crazing
compared to prior art coatings, to provide improved resistance to
severe abuse, represented by scratching with relatively coarse
steel wools.
[0021] It is another objective of the present invention to provide
abrasion resistant hardcoatings which can be practically and
economically applied, with minimal operator training, to a wider
variety of substrates than prior art coatings--such as wood,
furniture or paneling, polished or chemically- or
electrochemically-treated metals, painted (or varnished, powder
coated, etc.) articles, plastic films, glass, etc.
[0022] It is another objective of the present invention to provide
relatively low-cost, user-friendly hardcoatings which can be used
as part of a multicoat system to lower overall costs of coated
articles and/or improve environmental impact, in which a
less-expensive and/or more environmentally friendly base coat
system replaces a more-expensive and/or less environmentally
friendly complete-coat system, and is topcoated with a coating of
the present invention, to provide overall properties and
performance which exceeds the prior art coating system in benefits
compared to costs and environmental impact,
IV. SUMMARY OF INVENTION
[0023] The present invention comprises an improved acidic
silane/silica sol copolymer hardcoating composition and process for
protecting optical plastic and other substrates, a method for
improving abrasion resistance of many types of articles, including
plastics and consumer and industrial goods, and the resulting
improved articles. In a preferred embodiment, the acidic
silane/silica sol copolymer is formed as a direct reaction product
of an acidic aqueous silica sol and an monomethyltrialkoxysilane
other than methyltrimethoxysilane, in dry weight ratios of 30:70 to
70:30, most preferably about 40:60.
[0024] Preferably, no distillation off nor vacuum stripping of
excess solvent and/or excess water is required, just control of
temperature and time. The formulation also contains a "tail
solvent", which is not water. Herein, a "tail solvent" is defined
as "the last to leave" during devolatilization of the wet coating
film. That is, the tail solvent has lower volatility than water and
the lowest evaporation rate and/or highest boiling point of all the
solvents present. It can be expected to have a B.P.>100.degree.
C.
[0025] Surprisingly, it has been found that the
monomethyltrialkoxysilane of choice in the cited Clark &
Frye/Vaughn patents, methyltrimethoxysilane ("Z6070", in Dow
Corning product nomenclature) contributes greatly to the problems
previously listed. Both of the SDC and GE conmercial hardcoatings
mentioned previously also can be logically deduced to substantially
employ this silane as the monomethyltrialkoxysilane constituent (as
seen by their % solvent breakdown in their respective MSDS data
sheets, which show very high % methanol) The only exception to this
is an example found in Frye U.S. Pat. No. 4,324,839 wherein tiny
amounts of monomethyltriacetoxysilane are used with predominantly
methyltrimethoxysilane to buffer the alkaline sol pH in Examples
1-3 (thus using the tiny amounts of monomethyltriacetoxysilane as a
latent source of acetic acid).
[0026] Applicant has found that higher-alcohol substituents for the
alkoxy group are preferred over methoxy, for reasons mentioned
herein, and specifically, ethoxy is preferred over methoxy. All
silane/silica sol copolymer prior art is silent on any such
advantages, nor even consideration of ethoxy-based silanes in any
examples. Therefore, the central element of the present invention
is that Applicant employs mostly monomethyltriethoxysilane (using
ethanol as the alkoxy group) in Applicants choice for
monomethyltrialkoxy silane, with minor 50%) or, preferably, no
methyltrimethoxysilane in the formulation.
V. DETAILED DESCRIPTION OF INVENTION
[0027] The silica sol constituents are preferably dispersions of
colloidal silica of average particle size <100 millimicrons in
diameter, preferrably acidified and predominantly aqueous (although
some blends with solvent-based sols have special benefits which may
offset their higher costs), and more preferably, an average
particle size of 10-30 millimicrons. In one preferred embodiment,
the chosen acidified silica sol is Nalco 1042 (from Nalco Chemical;
Naperville, Ill.), having nominal 34% SiO2 solids and about
15-millimicron average particle size. Nyacol 2034DI, made by the PQ
Corporation, Valley Forge, Pa., was a functional equivalent to
Nalco 1042. See Example 1 & others. In another preferred
embodiment, a solvent-based sol (Nalco 1057, 30% solids in glycol
ether (2-propoxyethanol)) is chosen along with the acidified
aqueous silica sol Nalco 1042, so the resultant coating is a
copolymer of both. See Example 2 & others. A similar
solvent-based silica is now available as Highlink OG-402 from
Clarion Corporation. Alternatively, the silica sol may be partly or
entirely prepared in situ, by reaction of a tetraalkoxysilane ester
with water (equal, less, or greater than the theoretical
stoichiometric ratio). Other metal sols and/or their precursors may
also be used as functionally equivalent minor components of the
mixture, for specific purposes such as increasing refractive index
or modifying the hardness or flexibility of the coating.
[0028] In the preferred 30:70 to 70:30 dry weight ratios
(calculated as SiO2: CH3SiO3/2) of the present invention for silica
sol : monomethyltrialkoxysilane, there may be a stoichemetric
excess of water already present. Mixing monomethyltriethoxysilane
and silica sol constituents in the presence of acetic acid (or
formic acid, or similar weak organic acid) at the desired pH range
causes hydrolysis of the alkoxy groups on the silane and start of
condensation-reaction copolymerization between the silica sol and
silane constituents, which is optionally promoted by warming the
reaction mixture to a moderately elevated temperature.
Copolymerization rate (sometimes called "silanization") is a
function of the specific acid, concentrations of reactants, time,
temperature and pH. Generally, a pH from 4.5 to 5.5 appears to be
optimum for storage stability of the liquid coating. Less acid
during hydrolysis can speed up the aging or "bodying" of the
coating (with more acid added later) but can increase undesirable
branching of the siloxane oligomers formed. Only small amounts of
strong organic mineral acids can be tolerated by the coatings
without rapid bulk gellation (extremely low pH is to be avoided
when maximizing shelf/bath life). The weak organic acids are
strongly preferred, in the previously mentioned preferred pH range.
These weak organic acids, such as acetic acid, are typically
buffered by the added catalyst and can be further buffered by
volatile amines, whether or not they act as cure catalysts.
[0029] Coatings of the present invention are
thermosettable--crosslinking after evaporation of the solvent and
water by well-known siloxane condensation reactions--so in theory
any of those siloxane condensation catalysts commonly known to
those skilled in the art could be used in the cure reaction.
However, some work better at the lower temperatures which are
tolerated by plastic substrates (typically 70-130 C), and others
lack miscibility, or upset the pH range needed for coating solution
stability. So, like the Clark acidic aqueous silica sol:
monomethyltrimethoxysilane copolymers, cure reactions can employ
buffered latent catalysts such as alkali metal salts of carboxylic
acids, amines or quaternary amines (amine carboxylates), and
ammonium carboxylates. For example, sodium acetate acts as an in
situ latent buffered catalyst, such that, after all solvent and
water is removed and heating occurs, the catalyst breaks down to
give the high-pH conditions known to promote siloxane condensation.
Applicants also have prior experience with metal complexes such as
aluminum acetylacetonate, but find the quaternary amines (amine
carboxylates) are preferred as most active. Typical concentrations
are 1.0% or less, measured "solids on solids" ("s/s"), based on the
total dry weight in the formulation. It is possible to cure the
coatings at ambient or elevated temperature, using any convective
or radiant heat source. However, there is usually a preferred
temperature range in which cracking or crazing is minimized, and
adhesion maximized, as in the range noted above for plastics--which
works well for many substrates.
[0030] In addition to the methyltriethoxysilane or other
methyltrialkoxysilane used as a major component of the coatings of
the present invention, smaller amounts of other organofunctional
silanes such as methacryloxypropyltrimethoxysilane,
glycidoxypropyltrimethoxy-(or -ethoxy-) silane,
chloropropyltrimethoxysilane, glycidoxypropylmethyidiet-
hoxysilane, etc., may be added. Typically, such silanes would be
added to increase adhesion to a substrate or to a subsequently
applied overcoating (especially via vacuum deposition) or to
increase flexibility of the coating. To maximize cured coating film
hardness and scratch resistance, the resulting silane mix should
still be mostly monomethyl silane. Tint-uptake enhancing additives
(see previously-cited U.S. Pat. Nos. 5,013,608 and 5,102,695),
tints/pigments, UV absorbing, anti-static, or antifog-promoting
additives and/or flattening agents (for matte coatings) are also
contemplated.
[0031] When the desired degree of copolymerization (sometimes
called "bodying") has been reached, cooling and/or dilution of the
reaction mixture with cold solvent helps stop the copolymerization.
Among common solvents, diacetone alcohol or (blends of)
lower-aliphatic alcohols are preferred. Up to about half of the
aqueous silica may be replaced with an alcohol-based silica (or
equivalent substitute), if desired, to further improve the coating
stability. When a solvent-based silica is used in conjunction with
an aqueous silica, it is important that the aqueous silica be at
least partially silanized before adding the solvent-based silica
(and additional silane), or else compatibility problems and/or
precipitates can result. Likewise, silanization should be far along
toward completion before the latent catalyst ingredient is added,
whichever type of silica is used, or else precipitates and gel
particle formation will be excessive. It is preferred to react the
solvent-based silica with silane in the presence of the (silanized)
aqueous silica, so that the latter is the source of water for
hydrolysis of the silane reacted with the solvent-based silica.
[0032] In a specially preferred embodiment, diacetone alcohol is
selected as the "tail solvent". It is typically present at a level
of 3-20% of the total coating formulation in its "ready-to-use"
state. Diacetone alcohol was found to be preferred over ordinary
aliphatic alcohols such as butanol, 1-methoxy-2-propanol, or
Cellosolve (alkoxy ethanol) alcohols or their esters, or commonly
available ketones, for its desirable balance of hydrogen bonding,
polarity, and miscibility with the siloxane copolymer backbone. It
also seems to contribute to primerless adhesion on
difficult-to-adhere-to substrates such as polycarbonate and to
minimizing blush problems during drydown.
[0033] The coating compositions of the present invention can be
applied by any conventional process, including dip, flow, spin, and
spray. For high-volume automated coating of both sides of optical
plastic or other substrates at minimum costs, dipping is usually
preferred. Flow coating is advantageous for smaller volume
operations, since equipment costs and quantity of liquid coating in
recirculated use can be minimized. Spin coating usually gives the
most uniform coating thickness, when substrate shape permits its
use.
[0034] A unique and surprising feature of the coating solutions of
the present invention is that they can be prepared at ambient
temperature in a simple process using extremely simple hardware. In
the place of expensive custom-engineered reactors, wiped-film
evaporators and/or distillation systems for stripping under reflux
which are used with some prior art technologies known to Applicant,
a balance and a manual or mechanical means of chemical transfer
(e.g., transfer pump, tubing, and optional filter) and of solution
agitation are all that are required in simplest embodiments of the
present invention. A drum heater to accelerate the initial aging or
"bodying" of the coating solution is optional. Thus, the capital
costs for equipment can be reduced to {fraction (1/10)}th or less.
Chillers, as would be required for making--or
using--methyltrimethoxysilane-based coatings, are also
unnecessary.
[0035] "Bodying" of the coating solution, i.e., heating to elevated
temperature or aging at room temperature to increase viscosity,
presumably by fostering oligomerization and increasing molecular
weight of the resulting silane/sol copolymer thermosettable resin,
is commonly carried out to improve flow-out of the coating. The use
of leveling or wetting agents also improves flow-out and smoothness
of the coating. Such surfactants are preferably Fluorad PC-430 (a
non-ionic fluorinated alkyl ester made by 3M Co., St. Paul, Minn.),
used at a typical concentration of 0.01-0.1 PHR (parts hundred
resin), or alternatively a silicone-glycol copolymer such as DC 190
(made by Dow Corning Corp., Midland, Mich.) or BYK-300 (made by
Byk-Chemie USA; Wallingford, Conn.) or other surfactant. Use of
such surfactants is preferred because of the good surface
smoothness obtained, even on imperfectly cleaned substrates, and
because reducing the need for a bodying step can improve the
subsequent shelf life of the coating. The storage and tank or bath
stability of the coating compositions of the present invention
depends, as expected from the prior art cited and ordinary chemical
knowledge, on a variety of factors including pH. co-solvents or
tail solvents used, loss of solvent and/or water due to
evaporation, and ambient temperature. Nevertheless, under normal
use and storage, the coating compositions of the present invention
will be essentially stable (i.e., useful) for at least several
weeks at room temperature, and may be useably stable for months.
Stability--or the lack thereof--is determined by large-multiple
increases in viscosity and/or haze or rapid plugging of filters due
to the increasing rate of micro-gelation (producing gel particles
in the coating) or onset of bulk gelation; or the loss of
initially-obtained basic properties in the cured coating:
adhesiveness to the coated substrate, scratch resistance, or
clarity. On polycarbonate, for example, the coatings may require
longer cure at a given temperature (typically 121-130 C) to achieve
100% adhesion as the coating ages. This increasingly reluctant
adhesion with increased aging of the liquid coating continues until
even an overnight cure (nominal 16 hours at 250 F) is insufficient
(unless freshly-prepared coating is blended in). Such a loss of
adhesiveness despite overnight cure is an example of a loss of a
basic property. In contrast, with fresh coating, 100% adhesion on
polycarbonate is achieved typically in 2-6 hours at 121-130 C (and
on acrylics in 2-4 hours at 160-180 F), as measured by the
well-known cross-hatched tape adhesion test (ref: ASIM-D-3359,
modified by substitution of 3M #600 tape instead of #710 for use in
this test, due to its greater adhesiveness, thus providing a
relatively more "severe" test). Excellent adhesion is defined as at
least 90% retention of coating in the crosshatched area after 3
rapid pulls with firmly-rubbed-down adhesive tape.
[0036] A useful reference for abrasion resistance loss in cured
coatings made from aged liquid coating is the PSW test result
before vs. after the aging. Using the same operator, coarseness of
wool, and technique, an average tripling of visible scratches in
the range of 0.5 to 5 dozen can be taken as a loss of this basic
property. In evaluating haze in clear coatings made from aged
hardcoating solution, an increase to more than 3% haze in the cured
coating, as measured using a commercial haze meter, is considered
to signify a lost basic property. However, this does not apply with
non-clear coatings, or when the haze can be shown to be due to
another cause, such as a specific interaction with the substrate
(e.g., attack on an undercured primer or base coat).
[0037] A surprising result obtained with the coatings of the
present invention is the lack of crazing, cracking and/or adhesion
loss when extra-thick coatings are applied and cured. While not
wishing to be bound to a particular theory, we believe that,
consistent with their lower tendency to gel in solution, the
molecular structure of these compositions of the present invention
cure, cross-link, and/or set up more slowly, after the wet coating
has to been applied onto a substrate, in a way that retains
critical flexibility as the coating dries down and cures, reducing
the residual stress within the coating film. By contrast, Applicant
believes that methyltrimethoxysilane-based coatings "set up" too
quickly due to higher inherent reactivity, probably while there is
still residual solvent in the coating. Thus, although from a
theoretical viewpoint the final cured coating made from
methyltrimethoxysilane ("MTMS") or methyltriethoxysilane ("MTES")
would be expected to be the same, we find our cured coatings are in
fact demonstrably different from prior art based on
methyltrimethoxysilane, regarding Taber abrasion and steel wool
test resistance and the lack of crazing, cracking and/or adhesion
loss when extra-thick coatings are applied and cured.
[0038] A further advantage of coatings of the present invention
over coatings of the prior art is the potentially higher flash
point and lower vapor toxicity resulting from the effective
replacement of methanol. The MSDS sheet for GE's SHC-1200 shows 17%
methanol, which has 200 ppm TLV, and 19 C (67 F) flash point. In
contrast, Applicants' Example 1 has 0% methanol, with ethanol
instead having 1000 ppm TLV, and 25 C (78 F) flash point. A flash
point of 23 C (73 F) or higher is required for expedited shipment
by air freight.
[0039] In coatings of the present invention with "excess water",
improved adhesion (or rate of development of adhesion at a given
cure temperature) can be obtained by dipping twice to achieve the
desired coating thickness (instead of dipping once at a faster
withdrawal rate), by "pre-curing" to a "B stage" tack-free state
(e.g., at 60 C for 10 minutes--sometimes used for pre-inspection
for coating flaws before final cure), and/or by ramping up to final
cure temperature or stepping up, e.g., 93 C for 10 minutes, 110 C
for 10 minutes, 121 C for 5 minutes, then cure at 130 C; instead of
placing lenses in a preheated 130 C oven directly. We believe that
each of these 3 optional steps helps to selectively volatilize the
water in (and released during initial cure of) the coating, while
the tail solvent remains (sufficiently) to reduce initial dry-down
or curing stresses and to promote bonding to the substrate.
[0040] The largest anticipated use of coatings and methods of the
present invention is to provide previously unattained, economical
(and environmentally friendly) abrasion resistance for a wide
variety of articles of commerce, including large surface areas such
as storage tanks, walls (including exterior surfaces, especially on
land vehicles, boats and planes), and machinery. Although prior art
hardcoatings might possibly be used for some of these applications,
they are so user-unfriendly, expensive, capital-equipment
intensive, and prone to problems that they have not seen any such
broad usage. In contrast, coatings of the present invention are
within the reach of thousands of small shops which could not afford
the investment in equipment, expertise and up-front licensing fees
and/or ongoing costs of use of the prior art coatings. Now such
shops can, with a gallon or so of coating, a small recirculating
pump and filter, and a clean-air booth, flow coat articles of
almost any size or shape. On some substrates, a silane primer,
usually applied by the same techniques, may be necessary. A
home-made oven will usually suffice for curing. No worries about
refrigeration of the coating or dehumidification of the coating
application/drydown are needed. The user-friendliness and relative
storage/handling stability built into these coatings makes the
difference in practicality.
[0041] In many cases, previously uncoated articles will be coated.
In other cases, a currently-used coating will be replaced or
topcoated, to provide a more abrasion resistant finish. In yet
other cases, hardcoatings and methods of the present invention will
be used for topcoating over a less-expensive and/or more
environmentally friendly base coat than the coating previously used
on the article to be coated, and the net result will be an
enhanced-performance article with lower overall cost of
manufacture. The substrate (article) may optionally be base coated
and/or primed for smoothness, depth, adhesion, or other functional
or cosmetic enhancement. The hardcoating (i.e., topcoat, optionally
over a reduced-cost/environmentally friendly basecoat) provides
improved abrasion resistance and/or reduced cost or environmental
impact versus the unimproved article.
[0042] Although coatings of the present invention contain a
substantial percentage of volatile organic compounds (VOC's), in
terms of the square footage of surface covered, their environmental
impact is quite low, since they are applied much thinner than
typical paint/varnish type coatings. Also, the enhanced durability
vs. ordinary paints will lead to much less re-painting, with
further economic and environmental savings.
[0043] Preparation of some specific formulations of the present
invention are now given in the examples below
EXAMPLE 1
[0044] A two-liter reaction vessel was charged with 533 g Nalco
1042 acid pH (34% solids in water) aqueous silica sol, 745 g
methyltriethoxysilane and 51 g glacial acetic acid, heated to 60 C,
and agitated until it exothermed to reflux at 84 C. It was cooled
to 60 C. maintained one hour, then allowed to cool to room
temperature. Four hours later, a latent condensation catalyst
(0.65% s/s) was added, and the hazy solution was heated overnight
(nominally 16 hours) at about 77 C, then allowed to cool down to
ambient. After standing at room temperature for 7 days, 239 g of
diacetone alcohol was added as tail solvent.
[0045] The resultant coating was used to dip coat acrylic and
polycarbonate sheets at 26 C and 39% R.H., followed by oven curing
at 93 and 121 C, respectively. At 4-5 microns coating thickness,
the cured coatings had 100% cross-hatched tape adhesion and showed
few or no scratches when rubbed with # 000 steel wool, using 5
double strokes under firm thumb pressure,
[0046] Equivalent results were obtained from analogous samples dip
coated at up to 55 R.H., and yet no hazing "blush" of the coating
was observed, and primeness polycarbonate adhesion and scratch
resistance were excellent Analogous samples coated after a month of
solution aging at room temperature still gave 100% adhesion to
polycarbonate and excellent scratch resistance.
EXAMPLE 2
[0047] A mixture of 19.5 g Nalco 1042, 17.5 g diacetone alcohol,
3.9 g glacial acetic acid, and 27.2 g methyltriethoxysilane was
stirred vigorously and exothermed to 32-33.degree. C. After 11/2
hours, 22.1 g Nalco 1057 and an additional 27.2 g
methyltriethoxysilane were added, and the mixture exothermed to
25-26.degree. C. After another 21/2 hours, a latent buffered
catalyst (0.65% s/s) was added, giving a slightly hazy solution,
which was then heated 8 hours at 79 C,
[0048] This formulation not only gave good adhesion on unprimed
polycarbonate, as freshly prepared, but also after 5 months aging
at room temperature, at which time it was still useable. Scratch
resistance at 4-5 microns coating thickness was excellent, as in
Example 1. Addition of fluorinated surfactant (3M's FC 430 in
previously mentioned small concentrations) improved the coating
flow-out/leveling for dip coating. An analogous formulation without
the diacetone alcohol also gave 100% adhesion to unprimed
polycarbonate--the 2-propoxyethanol solvent present in the Nalco
1057 was acting as substitute tail solvent, in that case.
EXAMPLE 3
[0049] Coatings propared analogously to those of Examples 1 & 2
were applied to injection-molded polycarbonate lenses. Adhesion and
scratch resistance were excellent. Scratch resistance was
noticeably dependent on coating thickness, with the best resistance
to 000 steel wool obtained at >4 microns coating thickness. The
formulation like Example 2 developed 100% adhesion to PC faster
than the coating like Example 1, even at a lower catalyst level. It
was also more tolerant of high humidity while dip coating.
EXAMPLE 4
[0050] The coating solution of Example 2 was left at room
temperature for 5 months, incompletely sealed, so that about 20% of
the original volume evaporated. Used as-is for dip coating a
polycarbonate piano safety spectacle lens, at 5-inch-per-minute
withdrawal rate, the cured coating was about 12 microns thick, but
was smooth and still had 100% adhesion and outstanding scratch
resistance, without cracking or crazing. Five double rubs under
firm pressure with very coarse #3 steel wool only produced a few,
mostly fine scratches. After dilution with isopropanol, similar
results were obtained by flow coating.
EXAMPLE 5
[0051] Coatings analogous to Examples 1 & 2, but prepared with
diacetone alcohol added prior to the methyltriethoxysilane and/or
optionally without heat-accelerated aging or "bodying", and
especially with a surfactant such as FC-430, were used on
polycarbonate safety lenses, and gave excellent cured coatings
equivalent to those of Examples 1 & 2.
EXAMPLE 6
[0052] A coating prepared from 13.2 parts Nalco 1042, 24.6 parts
methyltriethoxysilane, 1.7 parts glacial acetic acid, 7.9 parts
diacetone alcohol, 1% (s/s) quaternary anmonium catalyst, and 0.006
parts FC-430 was used to coat a polycarbonate lens, giving a cured
coating with 100% adhesion and with scratch resistance barely less
than the coating of Example 1, despite the use of 25% less silica
on a relative basis. The coating developed 100% adhesion in a
faster time with more catalyst, but some crazing appeared in the
cured coating at 1.5-2.0% catalyst.
Example 7
[0053] A coating like Example 2, but without diacetone alcohol, and
only using {fraction (1/4)} as much glacial acetic acid, gave
excellent results like Example 2. It was free of haze from drydown
at up to 75% RH. On polycarbonate, the cured coating had
outstanding Taber abrasion resistance using CS-10 wheels, such that
the change in haze after 500 revolutions was 2-3%. This is much
better than typical results for Clark or Frye/Vaughn coatings, in
the range of 5-7% change in haze after 500 revolutions.
EXAMPLE 8
[0054] To show the advantage of using methyltriethoxysilane
("MTES") instead of methyltrimethoxysilane ("MTMS") in the coatings
of the present invention, a series of mixtures of Nalco 1042,
silane(s) and acetic acid was prepared. To each of the seven vials
were added in order the ingredients listed in Table 1, followed by
capping, shaking 15 seconds twice (with a 15-second delay), letting
stand {fraction (1/2)} hour, and tumbling 1 hour. Sample 1, with
only methyltrimethoxysilane present, became homogeneous and quite
warm immediately upon shaking. The other samples through Sample 7
were progressively slower to become homogeneous and became less
warm, but all were homogeneous after tumbling. The vials were then
let stand at ambient temperature until they gelled, to demonstrate
the relative stability of scratch resistant coating resins made
from colloidal silica and either or both methyltriethoxysilane
and/or methyltrimethoxysilane. As shown in Table 1, there is a 5-50
fold increase in the stability of the mixture when half or more of
the methyltrimethoxysilane is replaced with methyltriethoxysilane.
Although dilution is known to increase the stability of these
mixtures, the effect with methanol by itself is small (2.5.times.
for a dilution equivalent to the greater volume of Sample 7 versus
Sample 1), as confirmed in a separate analogous test.
1TABLE 1 Sample No.; 1 2 3 4 5 6 7 MTMS: MTES mole ratio 6:0 5:1
4:2 3:3 2:4 1:5 0:6 MTMS, grams 1.08 0.90 0.72 0.54 0.36 0.18 --
MTES, grams -- 0.24 0.47 0.71 0.94 1.18 1.41 Acetic Acid, drops* 2
2 2 2 2 2 2 Nalco 1042, grams 1.01 1.01 1.01 1.01 1.01 1.01 1.01
Days to gel 3 5-7 11 18-23 44 134 >230 *2 drops glacial acetic
acid = 0.035-0.04 grams
COMPARATIVE EXAMPLE A
[0055] A mixture of 43.7 g Nalco 1042 and 19.7 g diacetone alcohol
(added at this point to reduce viscosity) was chilled in an ice
bath, and a mixture of 46.6 g methyltrimethoxysilane and 2.3 g
glacial acetic acid was added and stirred in vigorously at a rate
to limit exotherm to -15 C (5 F). After standing 12 hours in the
ice bath, a mixture of 2.1 g acetic acid, 6.1 g isopropyl alcohol,
and a latent condensation catalyst (0.65% s/s) were added and mixed
in.
[0056] After standing about a week at room temperature to develop
"body" (increased molecular weight & viscosity), the coating
was used to dip coat acrylic and polycarbonate sheets, followed by
oven curing at 93 and 121 C respectively, at the same time as the
samples coated with the formulation of Example 1. At 4-5 microns
coating thickness, the cured coatings had 100% cross-hatched tape
adhesion only on acrylic. Adhesion to polycarbonate was 0-20%, even
after curing overnight at 121 C. On either substrate, finest steel
wool resistance was comparable, but not equal to that for Example
1. Coarser steel wools stripped the poorly-adhered coatings.
COMPARATIVE EXAMPLE B
[0057] A commercial coating offered by SDC Coatings Inc., of
Anaheim, Calif., named "Silvue 121", believed to comprise a
reaction product of acidic aqueous colloidal silica and
methyltrialkoxysilane but containing no "tail solvent" as defined
herein, gave 0% adhesion when coated and cured on unprimed
polycarbonate under conditions analogous to those for Example 1.
(Apparently recognizing that limitation, SDC sells various primers
to be used on polycarbonate substrates with such coatings.)
COMPARATIVE EXAMPLE C
[0058] SHC-1200 from GE Silicones, Waterford, N.Y., was used to
prepare a 4-5 micron coating on unprimed polycarbonate, by
dipcoating and curing as for Example 1. Steel wool testing showed
that the coating according to Example 1 had equal or slightly
better abrasion resistance than this GE coating, which contains
diacetone alcohol. This GE competitive coating had 100% adhesion on
unprimed polycarbonate, but can be very unstable in an open-topped
diptank operating at room temperature, even with continuous pump
circulation and filtration.
EXAMPLE 9
[0059] Coatings like Example 2, but using all aqueous silica
(replacing the Nalco 1057), or like Example 7 (optionally
containing a second tail solvent such as benzyl ether,
hydroxyacetophenone, or dichlorotoluene) were coated and cured on a
variety of substrates, giving excellent adhesion and abrasion
resistance on wood (bare or primed with sanding sealer or acrylic
coating as an alternative to a more-expensive polyurethane, or on
so-called faux finishes on wood in a positive-results direct
comparison test to a polyurethane); melamine-cured and epoxy
coatings or primers; silane-primed brass copper, mild &
stainless steel (including anti-corrosion primed steel) aluminum
(including anti-corrosion treated aluminum); glass (with or without
added epoxy silane); caustic-etched CR-39 plastic; primed, painted
plastic, a section of a clear-coated auto body panel; several types
of polished stone tiles and tile grout; sanded polyurethane coating
on carbon fiber shafts; eyeglass frames; cd's; acrylic watch
crystals; and colored acrylic panels. The coatings were applied by
flowing, brushing dipping or spinning at ambient temperature and
humidity (typically 40-65% RH) without .box-solid.
blushing..box-solid. The cured coatings with 100% adhesion survived
10-40 vigorous rubs with #2 steel wool under at least 10 pounds
applied (downward) force with 0-6 visible scratches (except on bare
wood and polycarbonate, due to the substrate softness). On
polycarbonate, the coatings survived 10 such rubs with #1 or #2
steel wool. By comparison, a recently introduced ophthalmic coating
on polycarbonate said to have tiger-like properties was scratched
by #0000 steel wool. A diamond-like film on a polycarbonate lens,
produced by the now-defunct Diamonex company, suffered slight
scratching with #2 steel wool.
* * * * *