Abstract:
Room temperature-storable, heat-curable organopolysiloxane compositions which comprise silicon-bonded vinyl radicals, silicon-bonded hydrogen atoms, a platinum-type compound and an unsaturated hydrocarbon compound of the conjugated ene-yen type provide a realease surface for tacky substances when coated onto the surface of a base material and heated, even mildly. The release surface possesses a time-stable release ability, particularly for pressure sensitive adhesive bearing articles such as tapes and labels.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a peelable film-forming organopolysiloxane composition. More specifically, this invention relates to a peelable film-forming organopolysiloxane which is storage-stable at room temperature and which can be rapidly cured at low temperatures after being coated on the surface of a sheetlike base material such as paper, synthetic resin film, fiber or aluminum. 
     It is known that a cured film of an addition reaction-curable organopolysiloxane composition which has been coated on the surface of a sheetlike base material such as paper, synthetic resin film, fiber or aluminum is peelable from a tacky substance. The coated organopolysiloxane composition must be thoroughly cured in order to impart a stable peelability to the surface of the sheetlike base material. When a film composed of an insufficiently cured composition is brought into contact with a tacky substance, the film will exhibit unstable peeling. Peeling will become difficult with time or the uncured organopolysiloxane composition will migrate into the tacky substance and alter its tacky nature. These are the drawbacks of prior art compositions which therefore cannot be used in practical application. The prior art addition reaction-curable organopolysiloxane compositions (Japanese Pat. No. 46-26798 [71-26798]) must be heated at 130° to 150° for 30 to 60 seconds in order to form an adequately cured film. However, such a high-temperature heat treatment cannot be applied to synthetic resin films such as polyethylene, polypropylene and polyester films or to poorly heat-resistant paper. When the above-mentioned composition is cured at ≧100° C., as long as 2 to 4 minutes are required for curing. The above-mentioned composition is therefore not practical from the standpoint of productivity. A peelable film-forming organopolysiloxane composition which can be more rapidly cured at lower temperatures has recently begun to be in strong demand in the peelable-paper industry in order to increase productivity and reduce costs. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a peelable film-forming organopolysiloxane composition which can be stored at room temperature and cured at a higher, but low, temperature. It is another object of this invention to provide a peelable film-forming organopolysiloxane composition which, after being cured, has a stable peelability from tacky materials. 
     These objects, and others which become apparent upon consideration of the following disclosure and appended claims, are obtained by the composition of this invention which result when one mixes 
     (1) 100 parts by weight of an essentially linear organopolysiloxane component having a viscosity of at least 100 centipoise at 25° C. and containing at least two silicon-bonded vinyl groups per molecule, 
     (2) 0.3 to 40 parts by weight of an organohydrogenpolysiloxane component having at least 2 silicon-bonded hydrogen atoms per molecule, there being at least a total of 5 silicon-bonded vinyl groups per molecule of component (1) and silicon-bonded hydrogen atoms per molecule of component (2), 
     (3) 0.01 to 7 parts by weight of an unsaturated hydrocarbon compound component having the formula 
     
         HC.tbd.CCR.sup.1 ═CR.sup.2 R.sup.3 
    
     wherein each R 1 , R 2  and R 3  denotes, independently, a hydrogen atom, a monovalent hydrocarbon group or a divalent hydrocarbon group with the requirements that the total number of carbon atoms in the R 1 , R 2  and R 3  groups has a value of from 2 to 6 and any R 1 , R 2  and R 3  divalent hydrocarbon group being bonded to another R 1 , R 2  or R 3  divalent hydrocarbon group by their second valences, and 
     (4) a catalytic amount of a platinum-type compound. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Component (1) of the compositions of this invention is an essentially linear organopolysiloxane expressed by the average unit formula 
     
         R.sub.a SiO.sub.(4-a)/2 
    
     wherein a is 1.95 to 2.05, R represents a substituted or unsubstituted monovalent hydrocarbon group such as methyl, ethyl, propyl, octyl, vinyl allyl, phenyl or 3,3,3-trifluoropropyl and at least 50 mol% of R is preferably methyl. Component (1) has a viscosity at 25° C. ranging from 100 cp to that of the gum and possesses at least 2 silicon-bonded vinyl groups per molecule. &#34;Essentially linear&#34; as defined in this text denotes a straight chain or straight chain branched in part. The molecular chain end may be methyl, vinyl, hydroxyl or phenyl. The location of the vinyl groups is arbitrary and they may be present at the molecular chain ends only, along the molecular chain only or both at the ends and along the molecular chain. Component (1) may be a single type or a mixture of two or more types of essentially linear organopolysiloxanes which differ in vinyl group concentration and/or molecular weight. Preferably component (1) contains only methyl and vinyl organic groups. 
     Typical examples of essentially linear organopolysiloxanes include, but are not limited to, dimethylvinylsiloxy-terminated polydimethylsiloxane, phenylmethylvinylsiloxy-terminated polydimethylsiloxane and trimethylsiloxy-terminated polydimethylsiloxane-comethylvinylsiloxane. 
     Component (2) of the composition of this invention is an organohydrogenpolysiloxane having at least two SiH groups per molecule. The SiH groups can be located at terminal, internal or both terminal and internal locations of the organohydrogenpolysiloxane. Examples of terminal SiH groups include H(CH 3 ) 2  SiO 1/2  siloxy units and H(CH 3 )(C 6  H 5 )SiO 1/2  siloxy units. Examples of internal SiH groups include H(CH 3 )SiO 2/2  siloxy units, H(C 6  H 5 )SiO 2/2  siloxy units and HSiO 3/2  siloxy units. 
     Typical examples of component (2) include, but are not limited to, methylhydrogenpolysiloxanes such as tetramethyltetrahydrogencyclotetrasiloxane, methylhydrogenpolysiloxane and copolymers of methylhydrogensiloxane and dimethylsiloxane. However, component (2) is not limited to these examples and may contain alkyl groups other than methyl as well as phenyl groups. Its degree of polymerization can range from 2 up to the degree of polymerization obtained by polymerization. 
     The sum of the number of vinyl groups per molecule of component (1) and the number of SiH groups per molecule of component (2) must be ≧5 in order to obtain the formation of a cured film by the addition reaction of the vinyl groups of component (1) with the SiH groups of component (2). The quantity of component (2) to be added is usually 0.3 to 40 parts by weight per 100 parts by weight of component (1) from a consideration of the formation and peelability of the cured film. Preferably the ratio of the number of SiH groups to SiVinyl groups has a value of from 0.2/1.0 to 5/1, most preferably from 0.75/1.0 to 1.5/1.0. 
     Component (3) of the compositions of the present invention is an unsaturated hydrocarbon compound having the formula ##STR1## This component is a necessary component which provides storage stability (addition reaction inhibition) at room temperature and which does not inhibit the addition reaction at a higher temperature of ≦100° C., resulting in the rapid curability of this composition at low temperatures (low-temperature curability). R 1 , R 2  and R 3  in the above-mentioned formula represent hydrogen and/or hydrocarbon groups and R 1 , R 2  and R 3  must comprise a total of 2 to 6 carbon atoms. That is, the unsaturated hydrocarbon compound itself should comprise a total of 6 to 10 carbon atoms. When the unsaturated hydrocarbon compound contains 5 or fewer carbon atoms, its boiling point is so low that it evaporates rapidly at room temperature and ambient pressure and it cannot be employed in practical applications. On the other hand, when the unsaturated hydrocarbon compound contains more than 10 carbon atoms, it is not highly compatible with the organopolysiloxane with the result that the storage stabilizer undergoes separation or causes uneven curing. 
     Examples of the monovalent hydrocarbon groups R 1 , R 2  and R 3  include alkyl groups such as methyl, ethyl and propyl groups; however, aryl groups such as the phenyl groups may also be used. 
     Typical examples of the unsaturated hydrocarbons, component (3), having monovalent hydrocarbon groups are as follows: ##STR2## 
     Examples of divalent hydrocarbon groups R 1 , R 2  and R 3  include --CH 2  --, --CH 2  CH 2  --, --CH 2  CH 2  CH 2  --, --CH 2  CH(CH 3 )CH 2  --, --CH 2  C(CH 3 ) 2  CH 2  -- and --CH 2  CH(CH 3 )CH 2  CH 2  --. 
     Typical examples of component (3) which contain divalent hydrocarbon groups which are bonded together by their second valence include ##STR3## which may be alternatively considered as containing two --CH 2  CH 2  -- groups or one --CH 2  -- group and one --CH 2  CH 2  CH 2  -- group. Other examples include ##STR4## 
     Component (3) must possess a structure in which the terminal ethynyl group is conjugated with a double bond. The composition of this invention must contain the unsaturated hydrocarbon compound with the above-mentioned specific structure which will contribute to a room-temperature storage stability and a low-temperature curability upon heating. Compared with conventional addition reaction-curable silicone potting materials or with an alkynyl alcohol such as 3-methyl-1-butyne-3-ol or 3,5-dimethyl-1-hexyne-3-ol which is a known storage stabilizer for silicone resins, the effect of the above-mentioned unsaturated hydrocarbon compound varies significantly with the curing temperature. In particular, it exhibits an excellent curability at low temperatures. For this reason, when this composition is coated on the surface of a sheetlike base material and subsequently cured by heating, it can be cured at a temperature lower than the temperature employed for conventional compositions at a constant curing time or it can be cured in less time than the time required for conventional compositions at a constant curing temperature. 
     When the quantity of addition of component (3) is less than 0.01 parts by weight per 100 parts by weight of component (1), the resulting composition exhibits poor room-temperature storage stability with a resulting rapid gel formation after the viscosity begins to increase. On the other hand, when the above-mentioned quantity exceeds 7 parts by weight, the composition will exhibit an excellent room-temperature storage stability with the result that it will not undergo a change in viscosity for a long period of time; however, the curability declines. Due to this, the curing temperature must be increased or the curing time prolonged. For this reason, this composition cannot exhibit advantageous characteristics. Therefore, the quantity of component (3) to be added should be 0.01 to 7 parts by weight per 100 parts by weight of component (1). 
     The platinum-type compound comprising component (4) is the usual compound which is used for the addition reaction of silicon-bonded vinyl or allyl groups with silicon-bonded hydrogen. Examples are chloroplatinic acid, alcohol-modified chloroplatinic acid, platinum-olefin or platinum-vinylsiloxane complexes, extremely fine platinum powder supported on a carrier such as alumina or silica, palladium catalysts and rhodium catalysts. A platinum-containing compound is preferred. The quantity of addition of platinum compound is arbitrary as long as it is sufficient to cure the composition of this invention; however, from the standpoint of economics it is usually 1 to 1,000 ppm as platinum based on the weight of component (1) in order to obtain a good cured film. 
     The compositions of the present invention can further comprise non-essential component which will not adversely affect their curing characteristics and adhesive release characteristics. For example, when a composition of this invention is to be coated on the surface of a sheetlike base material, it may optionally be diluted with an organic solvent which can dissolve the organopolysiloxane. Such an organic solvent includes aromatic hydrocarbons such as benzene, toluene and xylene; aliphatic hydrocarbons such as heptane, hexane, and pentane; halogenated hydrocarbons such as trichloroethylene and perchloroethylene; ethyl acetate and methyl ethyl ketone. 
     Additional examples of non-essential components that can be added to the compositions of this invention include diorganopolysiloxane which do not contain vinyl groups, fume silica, wet-process silica, siloxane resins composed of R 3  SiO 1/2 , RSiO 3/2  and SiO 4/2  siloxane units disclosed in U.S. Pat. No. 4,293,671 and siloxane resins composed of R 3  &#39;SiO 3/2  and SiO 4/2  siloxane units in a ratio of from 0.1/1.0 to 1.1/1.0 and having a silicon-bonded hydroxyl content of 6% by weight or less, R&#39; being a monovalent hydrocarbon group. 
     The compositions of this invention can be prepared by any suitable method; however, they are preferably prepared by mixing, until homogeneous, the various components thereof, component (4) being mixed with component (2) in a final step. 
     The peelable film-forming organopolysiloxane composition produced by the method of this invention exhibits good storage stability and can rapidly form an extremely well adhered cured film on the surface of a sheetlike base material such as paper, synthetic resin film, fiber or aluminum at a low temperature of ≦100° C. On the other hand it will exhibit an extremely stable peelability against a tacky substance. For this reason, it is effectively applicable to synthetic resin films which are easily deformed at elevated temperatures and poorly heat-resistant paper. Moreover, since it exhibits an excellent coatability on the above-mentioned base materials, a small quantity is adequate. The short curing process at low temperatures significantly increases the productivity. 
     This invention will be explained using demonstrational examples. &#34;Parts&#34; in the examples denote &#34;parts by weight&#34;. The viscosity was measured at 25° C. The pot life, curability, peel resistance and residual adhesion were measured as follows. 
     Pot life--A composition (450 ml) produced by the method of this invention is place in a 600 ml glass bottle equipped with a cooling coil and allowed to stand at 25° C. with occasional agitation (usually with a stirrer but in an agitator when the viscosity was high). Samples are collected at prescribed intervals (usually every 24 hours for 1, 2 or 3 days) and the viscosity is immediately measured. 
     Curability--The prescribed quantity of a composition produced by the method of this invention is coated on the surface of a sheetlike base material. The time (sec) required for the formation of a completely cured film in a hot-air circulation oven at a specified temperature is measured. The completion of curing is judged by rubbing the coated surface with a finger in order to determine the point at which the coated surface does not peel off or smudge. 
     Peel resistance--The surface of a sheetlike base material is thinly coated with a composition produced by the method of this invention and subsequently cured at a specified temperature for a specified period of time. The surface of the resulting cured film is coated with the acrylic pressure-sensitive adhesive Olibain BPS5127 (Toyo Ink Mfg. Co., Ltd.) or with the rubber pressure-sensitive adhesive Olibain BPS2411 (Toyo Ink Mfg. Co., Ltd.) and then heated at 70° C. for 2 minutes. Two sheets with this treated film are prepared, adhered to each other and then aged at 25° C. under a load of 20 g/cm 2  for a specified period of time (usually 1, 30 or 60 days or 1 year). The aged sample is cut into 5  cm wide pieces. The adhered sheets are peeled off from each other at an angle of 180° at a tensile rate of 30 cm/min using a tensile tester in order to measure the force (g) required to peel the two sheets. 
     Residual adhesion--A film is formed on the surface of a sheetlike base material by the method used in the peel resistance test and adhered with Nitto Polyester Tape 31B (Nitto Electric Ind. Co., Ltd.). It is heated at 70° C. under a load of 20 g/cm 2  for 20 hours. The tape is then peeled off and subsequently adhered to a stainless steel plate. The force (g) required to peel the treated tape from the stainless steel plate at an angle of 180° at a rate of 30 cm/min is then measured. This force (g) is expressed as a percentage of the force (g) required to peel off the fresh standard tape from the stainless steel plate. 
    
    
     EXAMPLE 1 
     A dimethylsiloxane-methylvinylsiloxane dimethylvinylsilyl-terminated copolymer (100 parts; vinyl group content, 2 mole%; viscosity, 400 cp) was combined with a trimethylsilyl-terminated methylhydrogenpolysiloxane (5 parts; viscosity, 20 cp) and then with 3-methyl-3-pentene-1-yne (1 part) as the storage stabilizer. The resulting mixture was then blended to homogeneity. The mixture was then combined with platinum-vinylsiloxane complex (corresponding to 150 ppm platinum based on the quantity of the above-mentioned dimethylsiloxane-methylvinylsiloxane copolymer) and subsequently thoroughly blended in order to produce the composition of this invention. 
     As Comparison Example 1, a composition was produced as specified above with exception that 1 part 3-methyl-1-butyne-3-ol was used instead of the above-mentioned storage stabilizer. 
     The pot life, curability, peel resistance and residual adhesion of these compositions were measured by the above-mentioned methods. The curabilities were measured at 90° C. and 100° C. using a polyethylene-laminated kraft paper which had been coated with 0.8 g/m 2  of the composition. Polyethylene-laminated kraft paper was coated with 0.8 g/m 2  of the composition and then heated at 100° C. for 20 seconds in order to examine the peel resistance and residual adhesion. The acrylic pressure-sensitive adhesive Olibain BPS5127 (Toyo Ink Mfg. Co., Ltd.) was used for the peel resistance measurement. 
     Another comparison example was attempted in which no storage stabilizer was used; however, the composition gelled during the mixing process after the addition of the platinum compound catalyst. It cured so rapidly that it could not be coated on the polyethylene-laminated kraft paper. 
     The test results for Example 1 and Comparison Example 1 are reported in Tables 1 and 2. 
     The pot lives of the compositions of both this invention and Comparison Example 1 were scored as satisfactory because they rarely underwent an increase in viscosity at 25° C. in 1 day. However, the composition of this invention efficiently formed a film when heated at 100° C. for 20 seconds and its peel resistance and residual adhesion were stable and satisfactory while the composition of Comparison Example 1 was not sufficiently cured upon heating at 100° C. for 20 seconds with the result that its peel resistance was unstable and its residual adhesion was very poor so that it could not be used in practical applications. 
     
                       TABLE 1______________________________________          Time required          for curing (sec)Composition      90° C.                    100° C.______________________________________Example 1         30     15Comparison       180     60Example 1______________________________________ 
    
     
                       TABLE 2______________________________________       Peel Resistance                   Residual       (g/5 cm) after                   AdhesionComposition 1 day       60 days %______________________________________Example 1   25           30     96Comparison  39          600     46Example 1______________________________________ 
    
     EXAMPLE 2 
     A dimethylsiloxane-methylvinylsiloxane copolymer gum (100 parts; vinyl group content, 1.5 mol%), a trimethylsilyl-terminated methylhydrogenpolysiloxane (3.5 parts; viscosity, 5 cp) and 3,5-dimethyl-3-hexene-1-yne (0.8 parts) as storage stabilizer were dissolved and mixed in toluene (241 parts). Immediately prior to application, the mixture was combined with toluene (1,345 parts) and platinum-vinylsiloxane complex (corresponding to 120 ppm platinum based to the above-mentioned copolymer gum) in order to obtain the composition of this invention. 
     As Comparison Example 2, 3,5-dimethyl-1-hexene-3-ol (0.8 parts) was used instead of the above-mentioned storage stabilizer and, as Comparison Example 3, 3-methyl-1-pentene-3-ol (0.8 parts) was dissolved and mixed by the above-mentioned method. 
     The compositions of this invention and Comparison Examples 2 and 3 were tested by the methods specified in Example 1. To measure the curabilities, 0.7 g/m 2  of the composition was coated on a polyethylene-laminated kraft paper and subsequently cured at 90° C. or 100° C. With regard to the peel resistance and residual adhesion, 0.7 g/m 2  of the composition was coated on a polyethylene-laminated kraft paper and then heated at 100° C. for 30 seconds in order to form a film. The pressure-sensitive adhesive employed for the peel resistance test was Olibain BPS2411 (Toyo Ink Mfg. Co., Ltd.). The results for these tests are reported in Tables 3 and 4. 
     The pot lives of the compositions of this invention and Comparison Examples 2 and 3 were concluded to be excellent because the compositions rarely underwent an increase in viscosity at 25° C. in 2 days. However, the composition of this invention formed a sufficiently cured film under heating 100° C. for 30 seconds to give a satisfactory peel resistance and residual adhesion. The compositions of Comparison Examples 2 and 3 were not adequately cured at 100° C. for 30 seconds so that their peel resistance and residual adhesion were very poor with the result that they were inappropriate for practical applications. 
     
                       TABLE 3______________________________________          Time required          for curing (sec)Composition      90° C.                    100° C.______________________________________Example 2        35      20Comparison       160     70Example 2Comparison       190     80Example 3______________________________________ 
    
     
                       TABLE 4______________________________________     Peel Resistance                   Residual     (g/5 cm) after                   AdhesionComposition 1 day       60 days %______________________________________Example 2   27           25     95Comparison  47          680     53Example 2Comparison  56          710     49Example 3______________________________________ 
    
     EXAMPLE 3 
     A dimethylsiloxane-methylphenylsiloxane-methylvinylsiloxane copolymer gum (100 parts; vinyl group content, 1.5 mol%; phenyl group content, 5 mol%), trimethyl-terminated methylhydrogenpolysiloxane (7 parts; viscosity, 20 cp) and 3-methyl-3-pentene-1-yne ((A), 0.1 parts; (B), 1 part; (C), 3 parts; (D), 8 parts) were dissolved in toluene (241 parts). Immediately prior to application, the composition was combined and blended with toluene (1,345 parts) and with a platinum-vinylsiloxane complex (corresponding to 180 ppm platinum based on the above-mentioned copolymer gum) in order to prepare compositions (A), (B), (C) and (D) of this invention. 
     A composition comprising Comparison Example 4 was prepared by the above method with the exception that 3,5-dimethyl-1-hexyne-3-ol (1 part) (to be compared with (B) of this invention) was used instead of the above storage stabilizer. 
     Compositions (A), (B), (C) and (D) of this invention and Comparison Example 4 were tested by the methods of Example 1. The curability was measured by coating 0.4 g/m 2  of a composition on a polypropylene film and subsequently curing at 90° C. or 100° C. With regard to peel resistance and residual adhesion, 0.4 g/m 2  of a composition was coated on a polypropylene film and cured by heating at 100° C. for 30 seconds in order to form a cured film which was subsequently tested. The pressure-sensitive adhesive used for the peel resistance test was Olibain BPS5127 (Toyo Ink Mfg. Co., Ltd.). 
     The test results are reported in Tables 5 and 6. Although the (A) of this invention exhibited a slight increase in viscosity at 25° C. in 1 day with respect to pot life, the others (compositions (B), (C) and (D) of this invention and Comparison Example 4) did not undergo any increase in viscosity at 25° C. in two days. Composition (D) of this invention and the composition of Comparison Example 4 did not adequately cure at 100° C. in 30 seconds while compositions (A), (B) and (C) of this invention were completely cured under these conditions with the result that they exhibited excellent peel resistance and excellent residual adhesion. 
     
                       TABLE 5______________________________________            Time required            for curing (sec)Composition        90° C.                      100° C.______________________________________This Invention (A) 25      15This Invention (B) 30      15This Invention (C) 40      15This Invention (D) 120     50Comparison         165     60Example 4______________________________________ 
    
     
                       TABLE 6______________________________________        Peel Resistance                    Residual        (g/5 cm) after                    AdhesionComposition   1 day      60 days %______________________________________This Invention (A)         26         27      95This Invention (B)         25         25      96This Invention (C)         26         28      95This Invention (D)         42         189     62Comparison    45         283     55Example 4______________________________________