CA1281476C

CA1281476C - Curable polyorganosiloxane compositions

Info

Publication number: CA1281476C
Application number: CA000532088A
Authority: CA
Inventors: Peter Otto Bentz
Original assignee: Dow Corning Ltd
Current assignee: Dow Silicones UK Ltd
Priority date: 1986-04-22
Filing date: 1987-03-16
Publication date: 1991-03-12
Anticipated expiration: 2008-03-12
Also published as: GB2189499B; GB2189499A; GB8704635D0; GB8610387D0

Abstract

Abstract of the Disclosure A composition comprising (A) a polyorganosiloxane having per molecule at least one substituent of the general formula -YZ, linked to a silicon atom through a Si-C bond wherein Y
denotes a divalent organic radical and Z denotes a radical containing a divalent olefinically polyunsaturated group wherein two olefinically unsaturated bonds are separated by not more than 3 carbon atoms, and (B) a photoinitiator, is useful for coating substrates and is curable by UV irradiation. The composition preferably also contains a mercapto-functional polysiloxane.

Description

2 ~ 14~7~

CU~ABLE POLYORGANOSILOXANE COMPOSITIONS

The present invention relates to curable compositions containing polyorganosiloxanes with substituents having olefinic polyunsaturation and to the use of such composi-tions for coating substrates.
Polyorganosiloxanes which are curable to a cross-linked material and which contain olefinically polyunsatu-rated substituents are known.
Published E~ropean patent application No. E.P. 152,630 and U.S.
Patent No. 4,526,954 (T.C. Willialr~) dated July 2, 19~5, ~^7hich corres~onds thereto, descr~s polyorganosiloxanes containin~, per mDl~le, at least one monovalent olefinically polyunsaturated organic radical containing at least one 1,3-pentadienylene group or ~ pentadienylene group bonded to silicon through silicon-to-carbon bond. These polyorganosiloxanes are said to be curable to a crosslinked material at ambient temperature upon exposure to gaseous oxygen. The curing of these poly-organosiloxanes is shown in the examples to be effected in a time which varies according to the formulation ~rom about 2 hours to more than 72 hours. In some cases these mate-rials had not yet cured fully within said 72 hours.
Thickened, but still liquid, material was obtained.
It is desirable to cause such polyorganosiloxanes to cure more rapidly, especially where coating processes for certain substrates are involved as for example in the coating of paper.
We have now found that if polyorganosiloxanes having substituents containing a divalent olefinically polyunsatu-rated moiety are exposed to ultra-violet radiation in the presence of a photoinitiator, a much fas~er cure rate can be obtained.
The invention accordingly provides in one of its aspects a composition comprising (A) a polyorganosiloxane having per molecule at least one substituent of the general s ~ 2o81~Jt7~.;

formula -YZ, linked to a silicon atom through a Si-C bond, wherein Y denotes a divalent organic radical consisting of C, H and O atoms and optionally includes N atoms and Z
denotes a monovalent hydrocarbon radical containing a divalent olefinically polyunsaturated group wherein two olefinically unsaturated bonds are separated by not more than 3 carbon atoms, and (B) a photoinitiator. In another aspect the invention provides a process for coating a surface which comprises applying thereto said composition 1~ and exposing the treated surface to ultra-violet radiation.
Polyorganosiloxanes (A), for use in a composition according to the in~ention, are known in the art and may be described as having units of the general formula (I) Ra R~b-siO4-a-b-c (I) (YZ ) c wherein R der~otes a monovalent hydrocarbon radical having from 1 to 9 carbon atoms, R' denotes ~, OH, an organic radical having from 1 to 30 carbon atoms, H atoms and, optionally, O, F or N atoms and Y and Z are as defined above; a, b and c have independently a value of 0, 1, 2 or 3 provided a + _ ~ c is not larger than 3 and at least one -YZ substituent is present per molecule.
Examples o~ the group R include alkyl groups e.g.
methyl, ethyl, propyl, isobutyl; aryl groups e.g. phenyl;
aralkyl groups e.g. phenylethyl and alkaryl groups e.g.
methylphenyl. Examples o R' include H, OH, hydroxy-functional hydrocarbon groups, alkoxy groups, aryloxy groups, alkoxyalkoxy groups, ~luorina~ed alkyl groups, amino~unctional or diamino-functional groups, groups containing mono-unsaturated moieties, epoxy-functional groups. Such groups are for example methoxy, ethoxy, _ 4 _ ~ 4 ~

butoxy, ethoxyethoxy, trifluoropropyl groups, aminopropyl groups, aminoethyl aminopropyl groups, vinylalkyl groups, glycidoxypropyl groups.
Z is a hydrocarbon radical. containing a divalent olefinically polyunsaturated group w~erein two unsaturated bonds are separated by no more than 3 carbon atoms. Such groups may have conjugated or unconjugated polyunsatu-ration. Examples of Z include such radicals as 8, 11 heptadecadienyl, 8, 11, 14 heptadecatrienyl and 8, 10, 12 heptadecatrienyl. To obtain polyorganosiloxanes (A) having the exemplified Z groups, polyorganosiloxanes having func-tional substituents may be reacted with e.g. linoleic acid, linolenic acid, eleostearic acid, or esters of these acids.
However, other compounds having the required divalent olefinically polyunsaturation may also be reacted with polyorganosiloxanes having functional substituents.
Examples of such o~her compounds include 2,4 hexadienoic acid and cyclohexadienoic acid.
Y is determined by the way polyorganosiloxane (A) is made. ~ifferent modes of producing these polyorganosilox-anes are possible. One such method involves the reaction of a polyorganosiloxane having an organic substituent with a functional group, e.g. a hydroxy, amino or diamino group or a Si bonded hydrogen atom with a compound of the general formula Z-Q, wherein Z is as defined above and Q is a reactive group which is capable of reacting with the func-tional group of the polyorganosiloxane. Examples of Q are -COOH and -COOR. If for example a polyorganosiloxane having an aminopropyl group linked to Si is reacted with Z-COOH, Y would be -(CH2)3-NH-C-. According to another method of preparing the polyorganosiloxanes (A) a siloxane with silicon-bonded hydrogen is reacted with a compound - 5 - ~Z ~

(OEI2=CH-CH2-0-C-Z). In the resulting product Y would be o the group -(CH2)3-O-C-. Yet another method for making polyorganosiloxanes (A) includes the reaction of previously prepared silanes e.g. of the general formula Z Y-Si-(R)C12 under hydrolysis conditions with e.g. dimethyldichloro-silanes or trimethylchlorosilanes and condensation of the hydrolysis products to the desired polyorganosiloxanes (A).
In the preferred polyorganosiloxanes (A), Y is the group o (CH2)3 NH C .
Polyorganosiloxanes (A) may vary from low viscosity fluids to high viscosity materials or resinous partially crosslinked materials. The number of units (I) may vary from 2 to 1000 or more. Values for a, _ and c and for a +
_ + c may vary from 0 to 3 as 1ong as the resulting polyor-ganosiloxanes (A) are sufficiently workable to be used in a coating process. Preferred polyorganosiloxanes (A) have an 20 average chain length of 20 - 600 Si atoms and are substan-tially linear polydiorganosiloxanes where at least 50% of the silicon bonded substituents are methyl groups, from 0.01 to 20% are -YZ groups and any remaining substituents are chosen from ethyl and phenyl groups.
Preferred polyorganosiloxanes (A) may also compris~ a number of R' substituents, which are unreacted functional groups, which were present on the polysiloxane precursor of (A) before reaction with for example the compound Z-Q as is defined above. Such groups R' will e.g. be present if less than stoichiometric amounts of said compound have been reacted with the polysiloxane precursor of (A).

... , , . . ~

- 6 ~ 4~

In most preferred polyorganosiloxanes (A) from about 1 to 10% of the silicon-bonded substituents are Y~Z groups and substantially all other silicon-bonded substituents are methyl groups.
Component (B) of the compositions of this invention comprises one or more photoinitiator substances. A number of substances which function as photoinitiators are known in the art and include e.g. aromatic ketones e.g. acetophe-none, benzophenone and 4,4'-diaminobenzophenone, benzoin compounds e.g. benzoin, benzoin methyl ether and benzoin ethyl ether, quino~e and anthrone compounds e.g. hydroqui-none, anthraquinone, naphthoquinone and 3-methyl-1,3-diazo-1,9-benzanthrone, phenolic compounds e.g. 2,4-dinitrophenol and azo compounds e.g. azo-bis-isobutyronitrile. The preferred photoinitiators for use in the compositions of this invention are those which are freely miscible with the component (A) of the composition. The use of compatible photoinitiators avoids the difficulties which may arise due to separation of this component on storage.
Compositions according to the invention may be prepared by simply mixing component (A) and component (B).
The compositions according to the invention will, however, react in the presence of ultra-violet radiation and are therefore preferably stored in light-proof containers or storage areas. The photoinitia~or (B) may be employed in conventional amounts, from about 0.1 to about 5 percent by weight, based on the weight of polyorganosiloxane (A).
In a preferred embodiment of this invention the composition comprising components (A) and (B) also comprises another component (C) which is an organosiloxane having in the molecule at least 2 siloxane units of the general formula (II) 4~

X' I a HSX SiO3_a (II) wherein X represents a divalent saturated aliphatic hydrocarbon group having from 3 to 8 inclusive carbon atoms, X' represents a monovalent hydrocarbon group having from 1 to 6 inclusive carbon atoms and free of aliphatic unsaturation, an alkoxy group having from 1 to 4 carbon atoms or an alkoxyalkoxy group having from 2 to 6 carbon atoms and a has a value of 0, 1 or 2, any remaining ullits having the general formula X"bSiO~ b ~ (III) wherein X" represents a monovalent hydrocarbon group having from 1 to 6 carbon atoms and free of aliphatic unsaturation and b has a value of 0, 1, 2 or 3 at least 50 percent of the total X' and X" groups being methyl.
The organosiloxanes employed as component (C) of a composition according to this invention have in the molecule at least two mercaptoalkyl siloxane units falling within the general formula (II). In the general formula X
may be any divalent saturated aliphatic group having from 3 to ~ carbon atoms, for example -(CH2)3-, -CH~CH(CH3)CH2-, -(CH2)4- and -(CH2)6-. The substituent X', when present, ~` 25 may be for example methyl, ethyl, propyl, phenyl, methoxy, : ethoxy or methoxyethoxy. The organosiloxanes (C) may be ; homopolymers consisting only of units (II), or example as in the cyclic siloxanes, or they may be copolymers contain-ing both units (II) and units falling within the general formula (III). In the copolymeric units (III), X" may be any monovalent hy~rocarbon group having from 1 to 6 carbon atoms and free of aliphatic unsatura~ion, for example 8 ~ 4~

methyl, ethyl, propyl or phenyl. At least 50 percent, and preferably substantially all, of the to-tal X1 and X" subs-tituents should be methyl. It is therefore preferred that each X' and each X" is methyl.
The mercaptoalkyl substituents in organosiloxanes (C) may be attached ~o any of the silicon atoms in the molecule, that is they may be present in chain terminating units HSX(X')2SiOo 5, in HSX(X')SiO or in HSXSiOl 5 units.
The organosiloxanes (C) may vary in molecular size from the disiloxanes to high molecular weight polymers and may range in consistency from freely-flowing liquids to resinous solids. When the compositions of this invention are intended for the provision of coatings on paper and other flexible substrates the preferred organosiloxanes (C) are 15 polydiorganosiloxanes having from about 50 to about 500 siloxane units and a viscosity of from about 50 to about ln,000 cS at 25C, (i.e. 5 x 10 5 m2/s to 10 2 m2/s) at ; least three of the said siloxane units, and preferably from 5 to 20 percent of the total siloxane units of organosil-oxane (C) more preferably from 5 to 10 percent thereof having therein a HSX- group attached to silicon.
Organosiloxanes (C) are in general known substances and methods for preparing such organosiloxanes will be evident to those skilled in the art. For example, accord-ing to one method a silane bearing silicon-bonded hydroly-sable atoms or groups and a mercaptoalkyl group HSX- is hydrolysed and condensed to prepare a mixture of cyclic and linear siloxanes which is then mixed with cyclic and/or linear siloxanes having units X"bSiO4 b and the mixture equilibrated employing a suitable catalyst. The mixture preferably contains a source of endblocking units e.g.

~ 7~
hexamethyldisiloxane but such source may be omitted e.g.
where a high molecular weight organopolysiloxane is requi-red. Another, less preferred method comprises reacting a hydroxyl-terminated polydiorganoslloxane with a silane HSXSiRa(OAlk)3 a wherein OAlk is an alkoxy group and a is 0 or 1.
Compositions comprising components (A), (B) and (C) exhibit a faster cure than those compositions which do not include component (C). The compositions of this preferred embodiment of the invention may be prepared by simply mixing components (A), (B) and (C) in any order. In the undiluted state components (A) and (C) react together in the presence of (B) and ultra-violet radiatlon. They may, however, be stored in the mixed state in the absence of such radiation, for example in lightproof containers or storage areas. For maximum storage stability it is pref-erred to provide the compositions as a two package system, the organosiloxanes (A) and (C) being packaged separately and the photoinitiator (B) being present with component (C). The amounts of component (C) which may be used may vary from traces up to such quantities which will give at least a ratio of one -SiXSH group in (C) per unsaturated bond in (A). An excess of the former will also work, but is unlikely to give an improved cure rate. Preferably, however, the ratio of SH groups in (C) present in the composition over the number of wnsaturated bonds present in Z of (A) is from about 1/5 to 1/1.
The cure rates of the compositions of this invention when exposed to ultra-violet radiation are found to be superior to the cure rate of polyorganosiloxanes (A) when exposed to oxygen. Even when the compositions of this invention are exposed to ultra-violet radiation in the absence of oxygen, for example under a nitrogen blanket, - lO ~ 4~ ~

cure rates still exceed those o~ polyorganosiloxanes (A) under oxygen. Compositions comprising components (A), (B) and (C) are seen to cure within minu-tes when exposed to ultra-violet radiation.
Curing of compositions comprising polydiorganosi-loxanes bearing mercaptoalkyl radicals and polyorganosi-loxanes having substituents with unsaturation by exposure to ultra-violet radiation, is known in the art and is described for example in U.S. patent 4 052 529. However, an advantage of compositions according to the present invention is that they may be cured by exposure to ultra-violet radiation or by exposure to oxygen or a combination of both. This property makes it possible, for example in the case of coating three dimensional objects where certain areas are inaccessible or less accessible to the ultra-violet light rays, to continue curing the coating after the expos-ure to ultra-violet radiation has ceased as long as the object is kept in contact with oxygen; for example by exposure to -the normal atmosphere. Advantage may also be ~ 20 taken of this dual cure property to obtain tack free films ; by brief exposure to UV and subsequent more complete cure ~ on exposure to oxygen.
; In order to accelerate the secondary cure by exposure to oxygen, metal compounds which are known in the paint and varnish industry as driers, may be incorporated into the compositions of this invention according to known techniques in such industries. Examples of such driers are carboxylic acid salts of cobalt, copper, tin, zinc etc.
Such driers do not adversely affect the cure of the composition when exposed to UV light. However, driers which are reactive with mercaptoalkyl groups are preferably omitted from compositions of this invention in which organoslloxane (C) is present. In the event of reactlon ~ 4~ ~

taking place the effectiveness of (C) in accelerating the cure rate can be significantly diminished.
Such drying agents are also not recommended for use with those compositions comprising polyorganosiloxane (A) in which Y is a group of the formula -(CH2)3-NH-(CH2)2-NH-C- . The drying agents are believed to form a complex with these polyorganosiloxanes (A) and inhibit their cure when exposed to oxygen.
The compositions of this invention are comparatively easy to prepare and may be applied to a variety of subs-trates. They may be cured thereon by exposure to ultra-violet radiation to produce adherent coatings. Although curing will take place slowly in the presence of normal daylight it is preferred to accelerate the cure rate by exposure to lamps which emit U.V. light, preferably with a wavelength in the range from 250 to 450 nm for example medium pressure mercury lamps. The compositions may be applied to substrates such as metals e.g. aluminium, iron, steel and copper, plastics e.g. nylon, polyester, polyethy-lene and polypropylene, siliceous materials e.g. cement, textiles e.g. cotton and synthetics and cellulosic materials such as paper, plastics coated paper and paper board. They are particularly useful for the formation of release coatings or for conformal coatings e.g. on elec-trical and electronic circuits. The compositions may be applied to the substrate employing any suitable means such as dip coating, spraying, doctor blade or grawre roll.
In addition to components (A) and (B) or to compo-nents (A), (B) and (~) of the preferred embodiment of thecompositions of this invention the compositions may also contain ingredients normally present in curable coating - 1 2 ~L~8~47~i compositions. For example, the composi~ions may contain dyes or colorants and may be diluted with organic solvents to facilitate application to some substrates. However, when a significant amount of solvent is present it may be necessary to subject the coated substrate to elevated temperatures prior to curing in order to effect solvent removal therefrom. Other ingredients which may be present include fillers, pigments and additives for modifying the release properties of the coating.
The following examples wherein Me denotes a methyl radical and parts and percentages are expressed by weight, unless otherwise specified, illustrate the invention.
Example 1 3.94g (54.9 mmol~ hexamethyldisiloxane, 115~g (15 15 mol) (Me2SiO)n, 37.8g (322 mmol) (H2N(CH2)3SiO)n and 12.0g K-silanolate (contains 20 mmol K) were placed in a 2 ltr flask wi~h stirrer and ~hermometer and heated under nitrogen atmosphere to 130C for 2 hours and thereafter to 170C for 4 hours. The viscous mixture was cooled to 20 100C, 94g (338 mmol) linseed oil acid was added and the mixture reheated to 140C. A distillation bridge was connected to the flask and under slowly reducing pressure ; first the reaction water and then low boiling siloxanes were distilled off (150C/3mm Hg). After 3~ hours a total of 138g distillate had been collected. The cloudy viscous product was diluted with ether and filtered through diato-maceous earth. The solvent was removed from the filtrate by distillation under reduced pressure (150C/3mm Hg) and finally 1091g of a pale yellow viscous liquid remained in the flask. This liquid was a trimethylsiloxy terminated copolymer of Me2SiO units and Me(AC(O)NHC3H6)SiO units, where A has the average formula C16 gH30 2.

- 1 3 ~ 4i7~i _ample 2 3 samples were prepared from the copolymer product from Example 1 as follows. A first comparative sample (Sample A) was poured onto a glass plate to form a film of about 0.5mm thickness. A second sample according to the invention (Sample B), was prepared by heating 99 parts of the product of experiment 12 and l part benzophenone to 50C and stirring vigorously to obtain a homogeneous solution, a part of which was poured onto a glass plate to a thickness of about 0.5mm. A third sample according to the preferred embodiment of the invention (Sample C), was prepared by heating 66 parts of the product of Example 1, 33 parts mercapto functional polysiloxane (DP 125, 8 mol %
S-H) and 1 part benzophenone to 50C and stirring vigor-ously to provide a homogeneous viscous liquid, from which apart was poured onto a glass plate to a thickness of ca.
0.5mm. These films were exposed to sunlight. Within two weeks Sample (A) was cured to a soft, tack free rubber with good adhesion to the glass plate. Within a few hours Sample (~) was cured to a tack free rubber with good adhesion to the glass plate. Within 15 to 30 minutes the surface of Sample (C), and after 1 hour the total sample was cured to a tack free rubber with good adhesion to the glass plate.
Example 3 Samples (A), (B) and (C) as prepared in Example 2 were coated onto Super Calendered Kraft paper using a blade coater at a level of about O.~g per square metre.
The coated paper was exposed to light from a medium pressure mercury vapour lamp rated at 80 w/cm, held at a distance of 50mm and focused by an elliptical reflector.
The coating was exposed for 0.4 seconds. All samples showed varying degrees of smearing after exposure. Sample 14 ~ ~,8~ 7~;;

(A) showed the most and Sample (C) showed the least smearing. After storing the exposed samples under ambient conditions for l to 2 hours the smearing was reduced and in the case of Sample (C) almost disappeared. After a few days at room temperature the smearing of Samples (B) and (C) had disappeared, while Sample (A) still showed some smearing.
Example 4 2g of Sample (B) as prepared in Example 2 were poured into a shallow aluminium dish and kept under an inert atmosphere during the experiment by covering the dish with a plate of quartz and purging the dish with a constant flow of nitrogen. The sample was then irradiated with W light (75 watt; distance = 15cm) for 10 minutes. A thin non-tacky film had been formed on the surface of the sample.
Example_5 99 parts of the product from Example 1, 1 part ofbenzophenone and 1 part of copper siccative (copper salt of mainly octanoic acid containing 8 weight % Cu) were heated to 50C and stirred until a homogeneous solution was obtained. A glass test tube was filled with this composi-tion and placed in a flask. This flask was purged with a constant flow of nitrogen and which was covered with a quartz plate. The sample was then exposed to UV light (75 watt, distance = 15cm) for 15 minutes. A thin, almost colourless, non-tacky film was formed on the surface of the sample during the irradiation. The sample was then stored in a glass flask under nitrogen a~mosphere to prevent any contact with air but to allow observation of the sample during storage. After one week there was no indication ~ 7 that the sample had undergone any further cure since the irradiation. When an identical sample was stored in a dark place, but in contact with air, af~er having been exposed to UV irradiation further curing of the sample was observed.

Claims

1. A composition comprising (A) a polyorganosiloxane having per molecule at least one substituent of the general formula -YZ, linked to a silicon atom through a Si-C bond, wherein Y
denotes a divalent organic radical consisting of C, H and 0 atoms and optionally includes N atoms and Z denotes a monovalent hydrocarbon radical containing a divalent olefinically polyunsaturated group wherein two olefinically unsaturated bonds are separated by not more than 3 carbon atoms, and (B) a photoinitiator.

2. A composition as claimed in Claim 1 wherein polyorgano-siloxane (A) is a substantially linear polydiorganosiloxane selected from those having from 20 to about 600 silicon atoms, wherein at least 50% of the silicon-bonded substituents are methyl groups.

3. A composition as claimed in Claim 1 wherein in polyorgano-siloxane (A) 0.01 to 20% of all silicon-bonded substituents are -YZ substituents.

4. A composition as claimed in Claim 3 wherein 1 to 10% of all silicon-bonded substituents of polyorganosiloxane (A) are -YZ
substituents.

5. A composition as claimed in Claim 1 wherein photoinitiator (B) is compatible with polyorganosiloxane (A).

6. A composition as claimed in Claim 1 which also comprises (C) an organosiloxane having in the molecule at least 2 siloxane units of the general formula (II) wherein X is selected from the group consisting of divalent saturated aliphatic hydrocarbon groups having from 3 to 8 inclusive carbon atoms, X' is selected from a group consisting of monovalent hydrocarbon groups free from aliphatic unsatu-ration and having from 1 to 6 inclusive carbon atoms, alkoxy groups having from 1 to 4 carbon atoms and alkoxyalkoxy groups having from 2 to 6 carbon atoms and a is an integer smaller than 3, any remaining units when present having the general formula (III) wherein X" is selected from a group consisting of monovalent hydrocarbon groups having from 1 to 6 carbon atoms being free of aliphatic unsaturation and b is an integer smaller than 4 at least 50% of the total amount of X' and X" units being methyl groups.

7. A composition as claimed in Claim 6 wherein organosiloxane (C) is selected from a group consisting of polydiorganosiloxanes having from 50 to about 500 siloxane units wherein 5 to 20% of the total number of siloxane units have a HSX- group attached to silicon.

8. A composition as claimed in Claim 6 wherein the ratio of the number of -SH groups in organosiloxane (C) over the number of unsaturated bonds present in Z of polyorganosiloxane (A) is from about 1/5 to 1/1.

9. A process for coating a surface which comprises applying thereto a composition as claimed in Claim 1 and exposing the treated surface to ultra-violet radiation.