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Description  |
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FIELD OF THE INVENTION
The present invention relates to scratch-resistant surfaces and more
particularly concerns a photopolymerizable composition to be applied on a
substrate so as to produce thereon a translucent or transparent coating
resisting corrosion and abrasion. This coating is intended to protect said
substrate against shocks, bruises and other mechanical accidents as well
as against wear resulting from normal use. Such composition is very useful
in all industrial fields where it is desirable to avoid, as much as
possible, that sensitive objects exposed to shock and wear be
progressively damaged. This is particularly important when dealing with
transparent articles such as optical goods the surface of which must be
protected by all means against scratches not to lose its desirable optical
properties.
It has been definitely established by now that the manufacture of high
performance optical ware by using transparent organic materials is
possible the working of which, by casting or any other machining means, is
much easier and more economical than with corresponding articles of
ordinary glasses from metal oxides. On the other hand, such articles of
"organic glass" are relatively soft and poorly resist abrasion, wear and
corrosion by external agents. Thus, it is desirable to cover such articles
with an anti-abrasion and anti-corrosion protective film but thin enough
for not significantly altering the optical properties of the substrate.
THE PRIOR-ART
Very many coating compositions and application methods have already been
proposed for achieving the aforementioned object, this being with variable
success.
Among all these compositions of the prior-art, some are particularly
relevant that owe their properties to the presence of compounds from
elements other than the usual constituents of organic matter and, in
particular, to aluminum and silicon in the form of specific mineral or
organic compounds. With reference to silicon, for instance, some of the
techniques used involve the build-up of a protective coating on substrate,
this coating being obtained from the vapor phase deposition of glass or
silica evaporated under vacuum. Polysiloxane based protective coatings can
also be obtained, the structure of which resembles to some extent that of
cross-linked polysilicic acid, by the in-situ polymerization of
organo-silicon compounds previously partly hydrolyzed. During the
hardening (curing) of such coatings, polymerization occurs, either due to
the formation of Si-O-Si bridges (by the dehydration of silanol
functions), or due to the participation of polymerizable organic groups
belonging to substituents possibly present on the silicon atoms (olefins,
epoxy-, armino- groups, etc.), or by a combination of the said two
polymerization modes. From the references illustrating such techniques,
the followings can be cited: A. J. REEDY, Res. Discl. 1978, 171-6; Patents
USP 4,006,271; 4,098,840; 4,186,026, 4,197,335; JP (Kokai) 77, 101.235;
112.698; 152.426; 154.837; 79, 60.335; 62.267; 119.597; 119.599; 129.095
to 129.099; 133.600; 144.500; 148.100; 80, 05.924; and DOS 2.803.942;
2.805.552; 2.820.391; 2,831.220; 2.917.440. However, despite the
protection they impart to the substrate on which they are applied, these
coatings have some drawbacks. One of such disadvantages is related to the
relatively high temperatures needed for curing polysilicic type coatings
which can lead to substrate deformation. Another drawback is inherent to
the expansion coefficient of the polysiloxane coatings which is often
sufficiently different from that of the substrate for causing the
development of adhesion problems (for instance in the case of
polycarbonate or polymethacrylate organic glasses) and of cracks or
crazing after alternating hot and cold periods (particularly in the case
of articles subjected to weathering like automobile head-lights). Adhesion
problems were partially solved by interposing an intermediate bonding
sublayer between the coating and the substrate but, more generally, it has
been sought to remedy the above-mentioned drawbacks by replacing the
coatings from polymerized silicon compounds by compositions comprising,
dispersed within an organic or silico-organic matrix, fine particles of
silica or alumina. Thus, there were used in this context aqueous mixtures
of silicon compounds, colloidal silica and hydrocompatible solvents
(alcohols, glycols, etc.), with or without polymerizable organic monomers.
Examples of such uses can be found in the following references: Belgian
Pat. Nos. 821.403; 877.372; U.S. Pat. No. 4,027,073; 4,188,451; 4,177,315;
GB Pat. Nos. 2,018,621; 2,018,622; DOS No. 2.811.072 and JP (Kokai) No.
79, 157.187. However, colloidal silica being essentially hydrophilic, as
are also the other types of silica such as amorphous, crystalline,
microcrystalline, precipitated and pyrogenic silicas, it is well
compatible, in general, only with hydrophilic polymers, for instance
organosilicon polymer, whereas it is much less or not miscible with
typical hydrophobic resins such as polyolefins, which very strongly
restricts its use as a filler in the film forming thermosetting or
photo-setting compositions. Moreover, adding hydrophilic silica to organic
polymerizable monomers leads to the formation, with relatively low
concentrations of solids, e.g. about 5 to 10% by weight, of highly
thixotropic masses (non-Newtonian rheologic behaviour) which are very
difficult to apply as thin layers on substrates. Hence, attempts were made
to remedy this disadvantage, i.e. to increase the level of silica in
organic resin coatings, while overcoming such application problems, by
treating the particles so as to make them organophilic. It should be
remarked at this stage that methods for imparting hydrophobic organophilic
properties to alumina or silica particles are already known, per se;
however it does not appear that there exists, by now, methods for giving
to silica or alumina particles sufficient organophilic properties to
enable them to be incorporated at high levels (of about 40% by weight or
more) into polymeric resin films, while maintaining suitable rheological
properties for application and nearly complete transparency of the films
formed. Yet, ensuring proper transparency of the protective coatings of
optical goods is a fundamental requirement, as will be seen hereinafter in
the description of the present invention. As pertinent references
regarding the methods for "treating" silica or alumina particles for
rendering them organophilic, South African Pat. No. 72.5180 and Japanese
Pat. (Kokai) No. 77, 138.154 can be cited. In the first of these
references, silica particles are treated with trimethylchlorosilane which,
by reaction with the silanol groups of said particles, generates
hydrophobic groups of formula
##STR1##
whereby said particles are rendered compatible with a mixture of olefinic
monomers (ethylenic and acrylic monomers). These particles are then
incorporated, to a level of about 5-10% by weight and together with a
proportion of alumina about 10 to 20 times greater, into a mixture of
polymerizable resins which, after curing, provides insulators for high
electric voltages. Such materials are however opaque and their resistance
to abrasion is not indicated. In the second of the two references cited
above, particles of alumina are coated with
--(glycidyloxy)-propyl-trimethoxysilane and a mixture containing about 25%
by weight of such treated alumina and an epoxy resin is used for coating a
polycarbonate article so as to obtain, after polymerization, an
abrasion-resistant film. Moreover, in the following references, there are
described methods for attaching organic groups such as vinyl, methacryl,
epoxy, glycidoxy to hydrophilic silica so as to impart thereto hydrophobic
properties: L. P. ZIEMJANSKI et al, Rubber World 163, 1 (1970); M. W.
RANEY et al, Meeting of the Div. of Rubber Chem., ACS Meeting, Cleveland,
Ohio (1971); M. W. RANEY et al, Meeting of the Div. of Rubber Chem., ACS,
Miami, Fla (1971); HI-SIL Bulletin 41, Jan. 1971, PPG Industries.
In addition to the above mentioned prior-art, some further U.S. patent
references can be cited in connection with the following subjects
pertinent to the invention:
1. SiO.sub.2 : U.S. Pat. Nos. 3,986,997; 4,177,315; 4,188,451; 4,242,403
1A. Treated SiO.sub.2, e.g. to make it hydrophobic: U.S. Pat. Nos.
2,610,167; 2,818,385; 3,652,379; 4,001,128.
2. Forming SiO.sub.2 in situ, e.g. hydrolyzing organic silicates: U.S. Pat.
Nos. 2,404,357; 2,404,426; 3,971,872; 4,049,868; 4,120,992; 4,186,026
3. Using siloxanes and/or silanes and the like: U.S. Pat. Nos. 2,610,167;
3,389,114; 3,801,361; 3,953,115; 3,986,997; 4,001,128; 4,006,271;
4,026,826; 4,027,073; 4,029,842; 4,049,868; 4,177,315; 4,186,026;
4,188,451; 4,197,335; 4,242,403
4. Combination of any of the above items with:
4A. Polymers: U.S. Pat. Nos. 2,404,357; 2,404,426; 2,610,167; 3,652,379;
3,801.361; 3,971,872; 4,001,128; 4,026,826; 4,049,868; 4,098,840;
4,120,992; 4,197,335; 4,242,403
4B. Prepolymers (oligomers or monomers): U.S. Pat. Nos. 3,819,562;
4,029,842; 4,197,335
4B1. Photopolymerizable monomers: U.S. Pat. Nos. 3,968,305; 3,968,309;
4,188,451
4C. Other chemicals, e.g. solvents, fillers cross-linking agents, to obtain
transparent abrasion-resistant coatings (as single or composite systems):
U.S. Pat. Nos. 3,986,997 (acidic alcohol H.sub.2 O solution); 4,001,128
(Al.sub.2 O.sub.3); 4,006,271 (solvent); 4,027,073 (acidic alcohol water
solution); 4,049,868; 4,186,026 and 4,120,992 (cross-links with
formaldehyde); 4,120,992.
5. Miscellaneous routes to such coatings: thus U.S. Pat. No. 3,645,779
provides a vacuum vapor deposited coating of B.sub.2 O.sub.3 -SiO.sub.2 on
organic glass; U.S. Pat. No. 4,051,297 discloses a sputtered film of
chromium silicide on smooth surfaces; in U.S. Pat. No. 4,242,403, there is
disclosed a polyethylene terephthalate sheet covered with an intermediate
layer of --(3,4-epoxycyclohexyl)-ethyltrimethoxysilane and an upper layer
of silica reinforced organopolysiloxane resin.
In spite of the progress achieved by the above mentioned techniques, it was
still desirable to have at hand a quick setting composition for providing
thin translucent or transparent films very resistant to abrasion by virtue
of a high level therein of hydrophobic silica. Thus, a first object of the
invention was to provide a composition for depositing transparent
protective films on substrates, such films being sufficiently mechanically
resistant to withstand normal wear or accidental abuses without impairment
of the surface properties.
A second object of the invention was to provide a composition for coating
protective transparent films on optical goods, the optical properties of
which will not be significantly modified by this film and which will keep
such properties for a significant period of time under adverse conditions.
Another object of the invention is to provide a composition for depositing
thin well adhering films on substrate, such adhesion not being affected by
weathering conditions even after a prolonged period of exposure.
Another object of the invention is to provide a film forming composition
that will strongly adhere to organic glass substrate and which can be
cured at room temperature, i.e. much below the softening temperatures of
the substrate.
Another object of the invention is to provide a composition for making
transparent scratch-resistant films, such films being coated on substrates
as one layer films, i.e. without the need of an intermediate bonding
layer.
Still another object of the invention is to provide a composition that can
be stored for prolonged periods at room temperature without hardening and
which can be cured on the substrates in a matter of seconds without the
use of elevated temperatures.
Another object of the invention is to provide industrial optical articles
made of relatively soft and easy moldable organic glasses protected with a
scratch resistant film that will withstand prolonged use under severe
weathering conditions without discoloration, crazing or significant
adhesion losses.
Other objects of the present invention will become apparent to people
skilled in the art from the description of the invention that follows and
from the disclosed preferred embodiments thereof.
SUMMARY OF THE INVENTION
The present invention enables to achieve the aforementioned objects.
Indeed, the invention provides a photo-polymerizable composition
comprising one or more photo-polymerizable monomers, at least one
phto-initiator and SiO.sub.2 or Al.sub.2 O.sub.3 particles having, grafted
on some of the oxygen atoms thereof, substituents of the formulae A.sup.1
(I) or SiA.sup.1 A.sup.2 A.sup.3 (II) wherein A.sup.1 represents R or OR
groups, R being a saturated or unsaturated substituted or unsubstituted
hydrocarbon radical and A.sup.2 and A.sup.3 either represent oxygen atoms
for connecting the Si atom in formula (II) to neighboring silicon or
aluminum atoms of the silica or alumina particle, or they have the same
definition as for A.sup.1. Naturally when, by virtue of the aforesaid
definition, the Si atom in (II) bears more than one R or OR groups, the
R's can be the same or they can be different. The detailed nature of the
R's will be explained in a moment.
One distinctive feature of the composition of the invention is that the
total number of carbon atoms which are included in formulae (I) or (II),
i.e. in A.sup.1, or in A.sup.1 plus A.sup.2 and/or A.sup.3 in case more
than one of the A's on the Si atom of (II) are R and/or OR groups, should
always be four or more in order to obtain rheological properties of the
coating compositions containing high concentrations of coated particles
that allow satisfactory practical application of the compositions to
organic glass substrates. For example, as will be cited later, suitable
coatings were not obtained with compositions containing silica treated
with silicon compounds having less than four carbon atoms, while other
compositions involving four or more carbon atoms gave satisfactory results
(see data in table VIIa versus those in tables VI and VII).
Another distinctive feature of the composition is that the refraction index
"n" of the organic phase of the composition should be as near as possible
to that of the particles used. Furthermore, if the refraction index in the
protective film of the organic matrix which is composed of the various
organic constituents of the composition is not near that of the mineral
particles, then said protective film is not perfectly clear but only
translucent, this effect being particularly significant with high levels
of mineral fillers, for instance of the order of 10 or 20 to 40% by
weight. Thus, it was noticed that if the index "n" of the organic mixture
is between 1.45 and 1.48, there is obtained with for instance a pyrogenic
silica of index "n"=1.475, even at high concentration levels, excellent
clear coatings even for thicknesses thereof of the order of several
microns. In the case of alumina (n=1.60-1.76), such index values for the
organic phase are nowadays impossible to achieve and, for this reason, the
coatings containing high proportions of alumina are translucent and not
transparent. In general, it is preferred within the scope of the invention
to use particles with 1.40<n<1.50 and an organic phase the "n" of which
lies in the same range.
PREFERRED EMBODIMENTS OF THE INVENTION
It should be noted that the size of the particles is important with respect
to the optical properties of the present protective coating. Thus, using
relatively large particles, i.e. having a diameter of about the same order
of magnitude as of the thickness of the film produces at the surface
thereof microscopic prominences not visible with the eye but being
detrimental to the optical properties thereof (undesirable light
reflection and diffraction effects) and may impart thereto a milky
appearance. To be perfectly clear, the film should have a flawless,
smooth, mirror-like surface. Consequently, there will preferably be used
particles of a size about one order of magnitude less than the coating
thickness. Thus, for instance, with coatings having a thickness of the
order of one micron or less, there are advantageously used particles sizes
of 0.007 to 0.05.mu. (pyrogenic SiO.sub.2 : AEROSIL (Degussa, Germany),
CAB-O-SIL (Cabot Corp. USA); precipitated silica: Hi-SIL (PPG Industries,
USA), etc.). For thicker coatings, larger size particles are possible, for
instance 0.02 to 0.1.mu. (precipitated silica). The same is true for
alumina, corresponding requirements for this mineral filler being however
less, since films loaded with Al.sub.2 O.sub.3 are usually not transparent
per se. As suitable alumina for the present composition, there can be
mentioned a product called ALON (Alcan, Canada), the particles of which
have a size approximately 0.006.mu.. The silicas used or tried within the
limits of this invention are the followings:
______________________________________
Name and type Specific area
Particle size
of silica (m.sup.2 /g)
(.mu.m)
______________________________________
Pyrogenic silica
CAB-O-SIL
EH-5 390 .+-. 40
0.007
H-5 325 .+-. 25
0.007
M-5 200 .+-. 25
0.012
L-5 50 0.05
AEROSIL
380 380 .+-. 30
0.007
300 300 .+-. 30
0.007
200 200 .+-. 25
0.012
130 130 .+-. 25
0.016
Precipitated silica
Hi-SIL
233 -- --
215 150 0.02
SILENE EF 90 0.03
Organophilic silica*
AEROSIL R-972 120 .+-. 30
0.016
______________________________________
*This silica was made organophilic by reacting with trimethylchlorosilane
the number of carbon atoms per grafted silicon atom is thus only three
which does not correspond to the standards required for embodying the
invention. Indeed, under testing, this hydrophobic silica did not provide
compositions with properties suitable for achieving protective coatings
according to the invention.
Regarding the photopolymerizable monomers that fit the requirements of the
present invention, one can use most monomers or mixtures of monomers
generally known to photopolymerize and the photopolymerization of which is
fast enough under usual conditions to be completed shortly (i.e. with
exposure times from about a few seconds to a few minutes) and the "n"
indexes of which fall within the aforementioned limits. Examples of such
monomers (olefinic and preferably acrylic) can be found in the following
reference: UV Curing by S. Peter PAPPAS, Science & Technology, Technology
Marketing Corp., USA (1978).
Among the monomers usable in the present invention, there can be mentioned
also some olefinic prepolymers with a photopolymerizable function which
possess, at the start, a significant intrinsic viscosity. This feature is
valuable when it is wished to deposit with the present composition a
relatively thick film but with sufficient flow stability during the period
before the photopolymerization not to collapse and spread out or run away
from the substrate before curing. Such prepolymers are known in practice
most often under generic commercial names such as UVITHANE (Thiokol
Corp.), EBECRYL (Union Chimique Belge), UCAR-X (Union Carbide), SETAROL
(Kunstharsfabrick Syntehse NV, Holland). The structure of such prepolymers
which fit well in the invention, provided they have the proper refraction
indexes, are generally not disclosed publicly except for the fact that
they are mainly polyol-acrylates (polyesterglycols) or
polyurethane-glycols. In practising the invention, one should use either
monomers the "n" index of which is intrinsically close to that of the
mineral filler used or, and this is the most frequent case, mixtures of
photopolymerizable monomers and/or prepolymers the mixture index of which
comes as near as possible to that of said mineral filler. By suitably
varying the proportions of the two or more monomeric constituents the
respective indexes of which are above and below the desired value, the
latter can be approximated close enough for eventually obtaining, with the
composition according to the invention, a practically transparent
protective film with silica contents of up to 40% by weight or more. As
non limiting examples, Tables I and II below give a list of such possible
monomeric ingredients in the form of individual constituents or of
mixtures (proportions of constituents in the mixtures are given), the
refraction indexes thereof as well as viscosities under standard
conditions.
TABLE I
______________________________________
Refractive index
Monomer "n.sub.D 20" Viscosity cP
______________________________________
Methyl acrylate 1.4040 max. 10
Methyl methacrylate
1.4142 max. 10
Ethylene glycol diacrylate
1.4550 max. 10
(EGDA)
1-6-Hexanediol diacrylate
1.4574 max. 10
(HDDA)
1,4-Butanediol diacrylate
1.4567 max. 10
(BUDA)
Neopentylglycol diacrylate
1.4515 max. 10
(NPGDA)
Diethyleneglycol diacrylate
1.4621 max. 10
(DEGDA)
Tripropyleneglycol
1.4495 max. 10
diacrylate
(TPGDA)
Tetraethyleneglycol
1.4616 max. 10
diacrylate
(TEGDA)
Bisphenol-A diacrylate
1.5415 1000 .+-. 20%
(EBECRYL-150)
Trimethylolpropane triacrylate
1.4738 70 .+-. 20%
(TMPTA)
Pentaerythritol triacrylate
1.4871 650 .+-. 20%
(PETIA)
Pentaerythritol tetraacrylate
1.4855 800 .+-. 20%
(PETEA)
Dipentaerythritol pentaa-
1.4932 4400 .+-. 20%
crylate
EBECRYL-210 (Acrylic
1.4980 125.10.sup.3 .+-. 20%
prepolymer)
EBECRYL-220 (Acrylic
1.5030 18.10.sup.3 .+-. 10%
prepolymer)
EBECRYL-230 (Acrylic
1.4646 6.10.sup.4 .+-. 30%
prepolymer)
EBECRYL-240 (Acrylic
1.4743 3.10.sup.4 .+-. 50%
prepolymer)
EBECRYL-270 (Acrylic
1.4755 15.10.sup.4 .+-. 13%
prepolymer)
UVITHANE-782 (Acrylic
1.5024 paste
prepolymer)
UVITHANE-783 (Acrylic
1.5264 paste
prepolymer)
UVITHANE-788 (Acrylic
1.5085 paste
prepolymer)
UCAR X-117 (Acrylic
1.4816 135.10.sup.2 .+-. 1%
prepolymer)
UCAR X-118 (Acrylic
1.4898 17.10.sup.2 .+-. 5%
prepolymer)
UCAR X-125 (Acrylic
1.4978 106.10.sup.2 .+-. 1%
prepolymer)
EBECRYL-600 (epoxy-
1.53 4-8.10.sup.2 (60.degree. C.)
acrylate)
EBECRYL-601 (epoxy-
1.55 2.10.sup.5 .+-. 10%
acrylate)
EBECRYL-830 (acrylic
1.5005 45.10.sup.3 .+-. 10%
polyester)
EBECRYL-810 (acrylic
1.4675 500 .+-. 40%
polyester)
SETAROL-3625 (olefinic polyester)
-- solid
Ethylene glycol dimetha-
1.4527 (25.degree. C.)
max. 10
crylate
(EDGMA)
Diethylene glycol dimetha-
1.4580 (25.degree. C.)
max. 10
crylate
(DEGDMA)
Triethylene glycol dimetha-
1.4595 max. 10
crylate
(TRIGDMA)
Tetraethylene glycol di-
1.4609 max. 10
methacrylate
(TEGDMA)
Bis-phenol-A dimethacrylate
1.5412 1600 .+-. 20%
1.6-Hexandiol dimetha-
-- max. 10
crylate
(HDDMA)
Trimethylolpropane tri-
1.4700 (25.degree. C.)
35 .+-. 20%
methacrylate
(TMPTMA)
Pentaerythritol tetrametha-
solid M.P. 52-55.degree. C.
crylate
______________________________________
TABLE II
______________________________________
Monomers or mixtures Index
(% by weight) "n.sub.D 20"
Viscosity cP
______________________________________
Trimethylol-propane triacrylate
(100) 1.4740 75 .+-. 15
Pentaerythritol triacrylate
(50)
Diethylene-glycol diacrylate
(50) 1.4742 70 .+-. 15
UCAR-X 118 (49,2)
Diethylene-glycol diacrylate
(50,8) 1.4748 290 .+-. 10
UCAR-X 118 (11,0)
Diethylene-glycol diacrylate
(89.0) 1.4670 max. 30
UCAR-X 118 (18)
Diethylene-glycol diacrylate
(82) 1.4670 45 .+-. 5
EBECRYL-600 (33,3)
Diethylene-glycol diacrylate
(66,6) 1.4915 75 .+-. 5
EBERCRYL-600 (16,7)
Diethylene-glycol diacrylate
(83,3) 1.4765 max. 30
EBECRYL-830 (33,3)
Diethylene-glycol diacrylate
(66,6) 1.4742 65 .+-. 5
SETAROL 3625 (16,7)
Diethylene-glycol diacrylate
(83,3) 1.4735 100 .+-. 10
Methyl methacrylate
(38,46)
Pentaerythritol triacrylate
(38,46)
EBECRYL 600 (23,08) 1.4732 max 30
______________________________________
There is further noted that, especially for some applications to be
described hereinafter, the adhesion of the film toward glass substrates
should preferably be weak or nil and, in such cases, the mixture of
photopolymerizable monomers will include no hydrophilic monomer such as
acrylic acid or glycol acrylates and methacrylates.
As photopolymerization initiators, there can be used in the present
composition most substances generally suitable for this purpose and being
compatible with the contemplated monomers and fillers. For example, the
following photo-initiators suitable for the present invention can be:
benzophenone, Mischler's ketone, ethyl 4-dimethylamino benzoate, benzil,
2-ethylanthraquinone, diethoxyacetophenone, (DEAP, Union Carbide) UVECRYL
P-36 (U.C.B.), IRGACURE-651 (Ciba), SANDORAY-1000 (Sandoz), FI-4 (Eastman
Kodak), VICURE-10 and -30 (Stauffer Chemicals), TRIGONAL-14 and P-1
(Noury), UV-HARTER Nos 1173 and 1116 (Merck), 2-chlorothioxanthone, etc.
Using diethoxyacetophenone is appreciated as, being a liquid, it dissolves
particularly well in the present photopolymerizable composition. Another
excellent photoinitiator is UV-HARTER No. 1116 (Merck). Generally, there
can be used advantageously from 0.5 to 5% by weight of the photo-initiator
depending on the selected mixture, on the amount of filler and on the
polymerization rates which are desired. Using 1 to 2% by weight of
diacetophenone or other initiators is advantageous.
The nature of the radical R which intervenes in the formulae (I) and (II)
can be much varied and its range is essentially dictated by the
requirement of mutual compatibility with the organic phase components. In
general, alkyl, alkenyl, cycloalkyl and cycloalkenyl of about 1 to about
12 carbon atoms are suitable, provided of course that the total number of
C's in (I) or (II) is 4 or more, i.e. for instance, if only one organic
radical per grafting site is involved, then it should be at least a four
carbon radical while if more than one organic radical are involved, say
three for instance, two of such radicals can be methyl and the third be
ethyl or the like. The organic radicals can be unsubstituted or
substituted with functions containing oxygen or heteroatoms (N, S, etc.).
Oxygen functions can be hydroxy, keto, ester, ether functions and the
like. Unsubstituted radicals can include photopolymerizable functions that
will participate to the overall photopolymerization of the composition and
provide thus photo-copolymers in which some of the copolymerized groups
will actually bond to the silica particles by virtue of the fact that the
photopolymerizable R was included in the compounds of formulae (I) or (II)
for grafting to said silica particles. Other definitions for the R
radicals will appear from further details hereinafter. Preferably, for
optimal properties of the scratch-resistant coatings of this invention,
the weight of the organic substituents used for grafting the silica
particles relative to the weight of the SiO.sub.2 of said particles will
be at least 20% and can be more.
The methods which can be advantageously used for rendering organophilic the
particles of the mineral fillers that are incorporated into the
composition of the invention are selected among the known methods the
references of which are listed in the introduction. Among these methods,
the four methods (A to D) described hereinafter suit the invention to
various extents. In the following schemes the sign
##STR2##
represents one of the peripheral silicon atoms (with a silanol function)
of a hygrophilic silica particle which is to be made hydrophobic. It will
remain understood that the free Si bonds represented in the schemes mean
that this Si atom is bonded to the general polysilicic acid network of the
particle as follows:
##STR3##
It should be further remarked that the particles of silica thus treated,
even the smallest, each have a relatively large number of oxygen and
silicon atoms. For instance, a particle of 0.02.mu. diameter has a weight
of about 10.sup.-17 g assuming a value of 2.3 for the average density
which corresponds to about 10.sup.-18/6 mole of SiO.sub.2. Since the
number of molecules in a mole is 6.10.sup.23, said particle will have
about 10.sup.5 atoms of Si. The particles are therefore aggregates of
relatively high molecular weight and the mixtures therefrom in liquid
media are indeed micellar dispersions or colloidal solutions and not true
solutions of organo-silicon compounds as in the majority of prior-art
compositions mentioned hereinbefore. It is thus all the more remarkable
that the composition of the invention does provide, in the case of silica
particles, transparent films even with very high levels of such mineral
fillers.
In the case of alumina particles, the above discussion will apply by
analogy since peripheral alumina molecules also bear reactive OH
functions.
The grafting methods which were experimented in the scope of the invention
are listed below schematically. They are given for illustration and
evidently they do not limit the invention as other method could be
contemplated or even preferred as far as they may be more economical or
more efficient.
A. The conversion of some OH functions of the mineral particles (silanol
functions in the case of silica particles) into reactive functions; e.g.
by chlorination as in the schemes below:
##STR4##
Then alkylation of the silicon atom with elimination of the chlorine atom:
##STR5##
In the above schemes, R' and R" (organic radicals) can be the same as R or
be different from R. They can have (taken individually) less than four
carbon atoms since for having the grafting conditions within the scope of
the invention to be satisfied, it is sufficient to have only one of the
organic substituents brought up during grafting at one site with at least
four C atoms or, otherwise, the total of the carbon atoms of substituents
R, R' and R" put together in accordance with the definition of the
aforesaid formula (II) should be at least four.
C. The condensation promoted by heat with silanols (R--Si(OH).sub.3):
##STR6##
It should be noted with regard to reaction 7 that the remaining OH
functions can still react after grafting by further dehydration with other
silanol molecules (chain extension by grafting) or with an OH on a
neighbor Si atom in the polysilicic acid backbone of the particle under
reaction (cross-link bridges). It should also be noted that the silanols
used generally result from the hydrolysis of trialkoxysilanes according to
reaction 8:
8. RSi(OMe).sub.3 +3H.sub.2 O.fwdarw.RSi(OH).sub.3 +3MeOH
D. A reaction of "physisorption" with trialkoxysilanes. This route is a
"complexation" reaction providing a product in which the bonds to the
silicon atom to be grafted are not covalent. It is carried out by boiling
in an organic solvent like xylene:
##STR7##
It should be remarked that the "complex" thus obtained (electrostatic type
of bonds) is not very stable and that a dispersion made from particles
grafted as such has characteristics different from that of dispersions
made from particles grafted by methods A to C above. In particular,
dispersions obtained from particles treated according to 9 have a
rheologic behavior that is sometimes non-Newtonian in character and are
more difficult to use in the present composition.
In the above described grafting methods, the group R will preferably be a
radical such as n-butyl, n-hexyl, n-heptyl, n-octyl, oleyl, 3-butenyl,
decanyl, etc. Also functional groups are suitable that result from the
use, when alkylating activated mineral particles, of glycol acrylates or
methacrylates. Thus, in the substituent formulae R can be
--(CH.sub.2).sub.n OCO--CH.dbd.CH.sub.2 where n can be for instance an
integer between 1 and 6. When the group R has an olefinic moiety, that
function can copolymerize with the other monomers of the composition when
under irradiation, in which case the particles are then immobilized by
chemical bonds within the coating organic matrix.
Grafting method C is preferred in the methods described hereinabove because
it is relatively simple and because no halogenated intermediates are
necessary, the handling and the disposal of which are undesirable
regarding safety and environmental problems. Further, compounds of the
formula R--Si(OR').sub.3 where R' is an easily hydrolyzable lower alkyl
are commercially available, the range of the vario | | |
