This disclosure relates to polysiloxane release coatings. Single sided liners, e.g., backing sheets for pressure sensitive adhesive labels, are usually adapted to temporarily retain the labels without affecting the adhesive properties of the labels. Double sided liners, e.g., interleaving sheets for double sided and transfer tapes, are utilized to ensure the protection and desired unwind characteristics of a double sided self-adhesive tape or adhesive film.

A substrate, e.g. a single sided liner, is coated by applying a polysiloxane based release coating composition from a source, e.g. a bath of the release coating composition onto the substrate and subsequently curing the composition, by, e.g. thermally initiated hydrosilylation. The release coating is required to adhere well to the liner while having relatively low adhesion to the adhesive so that the label can be removed from the liner by a predetermined peel force and then applied and permanently adhered to an appropriate substrate.

The substrates for both single and double sided liners have traditionally been paper, and the temperature at which the polysiloxane based release coating is cured is usually at least 130° C. Increasingly, the packaging industry wishes to use alternative substrates including thermally sensitive substrates such as low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), biaxially oriented polypropylene (BOPP), polyethylene coated Kraft paper (PEK) and temperature sensitive papers.

The required curing temperature for the polysiloxane based release coating on such thermally sensitive substrates is low, for example in the range 70° C. to 90° C., e.g. 85° C. It is desirable for polysiloxane release coating compositions to be capable of curing in an acceptable time at such temperatures, while being stable at ambient temperature to avoid premature curing during storage and transport.

A polysiloxane compound contains several Si—O—Si— bonds forming a polymeric chain, where the repeating unit contains —(Si—O)—. An organopolysiloxane, sometimes called a silicone, contains repeating —(Si—O)— units where at least one silicon atom bears at least one organic group i.e. containing at least one carbon atom. A polysiloxane comprises terminal groups and pendant groups. A terminal group is a chemical group located on a silicon atom which is at an end of the polymer chain. A pendant group is a group located on a silicon atom which silicon atom is not at the end of the polymeric chain.

A polysiloxane may comprise at least one of the following units: M unit (mono-functional), D unit (di-functional), T unit (tri-functional), Q unit (tetra-functional). M units typically have the formula R1R2R3SiO112. D units typically have the formula R2R3SiO2/2. T units typically have the formula R4SiO3/2. Q units typically have the formula SiO4/2. R1, R2, R3, and R4are each a substituent, typically an organic substituent. Each substituent R1, R2, R3, and R4can be selected for example from alkyl, aryl, alkenyl, acrylate, methacrylate and others. For example an unsaturated group may be alkynyl or alkenyl group, e.g. a vinyl group or a hexenyl group.

A linear polysiloxane typically contains only D units and terminal M units. A branched polysiloxane which may be a resin typically contains at least one T unit and/or at least one Q unit. Resins are 3 dimensional structures containing a plurality of T and/or Q units, for example, an MQ resin is an organopolysiloxane containing a plurality of interlinked Q units and terminal M units.

Hydrosilylation is an addition reaction where a compound containing at least one unsaturated group reacts with a compound containing at least one Si—H group. An unsaturated group which may alternatively be called an ethylenically unsaturated group is a group comprising a double or triple liaison between two atoms. The bonded atoms may be carbon atoms or carbon-heteroatom. For example an unsaturated group may be alkynyl or alkenyl e.g. a vinyl or hexenyl group. A hydrocarbyl group is a univalent radical derived from a hydrocarbon, such as alkyl, cycloalkyl or aryl.

A polysiloxane based release coating composition generally comprises a polysiloxane containing alkenyl groups, an organohydrogenpolysiloxane cross-linking agent and a hydrosilylation catalyst effective to catalyse the reaction between the alkenyl groups and the Si—H groups. The release coating composition usually requires an inhibitor to prevent the cure of the coating composition from occurring below a predetermined temperature. Whilst an inhibitor is not essential to the functioning of the coating composition itself, in the absence of an inhibitor the catalyst may initiate/catalyse the cure of the silicone based release coating composition at ambient temperature, once the three above constituents have been mixed together.

U.S. Pat. No. 6,806,339 describes a release coating composition comprising a branched polysiloxane containing alkenyl or alkynyl units, an organohydrogenpolysiloxane cross-linking agent in an amount such that the ratio of the total number of Si—H groups in the composition to aliphatically unsaturated hydrocarbon groups in the composition is from 0.9:1 to 3:1, and a hydrosilylation catalyst effective to catalyse the reaction between the branched siloxane and the cross-linking agent. The branched siloxane consists of one or more Q units of the formula (SiO4/2), 15 to 995 D units of the formula Rb2SiO2/2, and M units of the formula RaRb2SiO1/2, wherein each Rasubstituent is an alkyl, alkenyl or alkynyl group having from 1 to 6 carbon atoms, at least three Rasubstituents in the branched siloxane being alkenyl or alkynyl units, and each Rbsubstituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an aryl group, an alkoxy group, an acrylate group and a methacrylate group.

U.S. Pat. No. 4,530,989 describes room temperature stable organopolysiloxane compositions containing organosilicon compounds having Si-bonded atoms and aliphatic multiple bonds, a platinum catalyst, and an inhibitor which is a maleinimide or a maleic acid derivative of the formula R1—C(O)—CH═CH—C(O)—R2, where R1is a hydroxyl or trimethylsiloxy group and R2is a hydrocarbonoxy radical.

U.S. Pat. No. 7,238,755 describes crosslinkable silicone coating compositions comprising organosilicon compounds having radicals containing aliphatic carbon-carbon multiple bonds, organosilicon compounds containing Si-bonded hydrogen atoms, catalysts which promote the addition of Si-bonded hydrogen onto aliphatic multiple bond, and optionally inhibitors, and an anti-misting additive which is a liquid branched polysiloxane containing alkenyl groups formed by reaction of α,ω-dialkenylsiloxane polymers with organosilicon compounds containing at least 3 Si-bonded hydrogen atoms per molecule.

U.S. Pat. No. 4,609,574 describes a curable release coating composition comprising an olefinic polydiorganosiloxane, a metal hydrosilylation catalyst, a methylhydrogenpolysiloxane, and an inhibitor for the hydrosilylation catalyst. The olefinic polydiorganopolysiloxane is made up of diorganosiloxane units and chain terminating units. It contains unsaturated radicals containing higher alkenyl radical containing at least 6 carbon atoms.

WO 03/050174 describes polyaddition-curable polyorganosiloxane compositions including a mixture based on a platinum group metal catalyst, an inhibitor of platinum group metal catalysts, and a pyridylamine ligand.

BRIEF SUMMARY OF THE INVENTION

There is provided herein, according to a first aspect of this disclosure, a curable polysiloxane release coating composition (“composition”). The composition comprises (A) an organopolysiloxane containing at least two (2) ethylenically unsaturated groups. The composition further comprises (B) an organopolysiloxane containing at least 2 Si—H groups per molecule. If component (A) contains only 2 ethylenically unsaturated groups, component (B) contains on average more than 2 Si—H groups per molecule. The composition further comprises (C) a hydrosilylation catalyst. Component (C) comprises a platinum group metal or a complex or compound of a platinum group metal. The composition further comprises (D1) a hydrosilylation inhibitor. Component (D1) comprises a maleimide of the general formula (I):


wherein A3represents a hydrogen atom or a hydrocarbyl or substituted hydrocarbyl group having 1 to 18 carbon atoms, and A1and A2each represent a hydrogen atom or a hydrocarbyl or substituted hydrocarbyl group having 1 to 18 carbon atoms. The composition yet further comprises (D2) a second hydrosilylation inhibitor. Component (D2) is selected from acetylenic compounds, ethylenically unsaturated isocyanates, acetylenically unsaturated silanes and unsaturated dicarboxylic acid diesters or a maleate compound or a mixture thereof.

Also provided herein, according to second aspect of this disclosure, is a process of applying a polysiloxane release coating on the surface of a substrate. Further provided herein, according to a third aspect of this disclosure, is use of the hydrosilylation inhibitors (D1) and (D2) in a curable polysiloxane release coating composition.

