Release sheets for laminated materials
This invention relates to an improvement of resin saturable release sheets used in the art of laminate manufacture. More particularly, the invention relates to an improved release sheet for releasing from one another in a press pack a plurality of heat and pressure consolidated resin impregnated laminates. The release sheet comprises an unsized cellulosic-based paper substrate to which an aqueous solution of water-soluble multivalent salt (or salts) is applied during formation of the substrate, and wherein the substrate is subsequently coated after formation on at least one salt-treated side with a film of a salt of alginic acid.
1 . A release sheet for use in a laminate formation process comprising an unsized saturating kraft paper substrate at least one side of which is coated with a from about 0.01% to about 3.0% by weight based on the dry weight of the substrate of a water-soluble multivalent salt, which coating is in turn covered by a film comprising at least one salt of alginic acid.
2 . The release sheet of claim 1 wherein the multivalent salt coating has a solids content of about 0.05% to about 1.0% by weight based upon the dry weight of the substrate.
3 . The release sheet of claim 2 wherein the water-soluble multivalent salt is selected from the group consisting of salts of aluminum, salts of barium, salts of beryllium, salts of calcium, salts of chromium, salts of copper, salts of iron, salts of magnesium, salts of strontium, salts of zinc, salts of zirconium, and combinations thereof.
4 . The release sheet of claim 1 wherein the alginic acid salt is a member selected from the group consisting of ammonium alginate, iron alginate, lithium alginate, potassium alginate, sodium alginate, and combinations thereof.
5 . An improved method of preparing stacked laminate release sheets which comprises coating a cellulosic substrate with at lease one multivalent salt, which coating is in turn coated with at least one salt of alginic acid, wherein the improvement comprises the cellulosic-based paper substrate is an unsized saturating kraft paper and the salt-treatment of at least one surface of said substrate during formation of the substrate is by the application to said surface of an aqueous solution comprising an amount of at least one water-soluble multivalent salt sufficient to provide a dried solids content of about 0.01% to about 3.0% by weight based upon the dry weight of the substrate.
6 . The improved method of claim 5 wherein the aqueous solution of water-soluble multivalent salt is applied during formation of the substrate in an amount sufficient to provide a solids content of about 0.05% to about 1.0% by weight based upon the dry weight of the ubstrate.
7 . The improved method of claim 5 wherein the aqueous solution of water-soluble multivalent salt is applied during formation of the substrate in an amount sufficient to provide a solids content of about 0.1% to about 0.5% by weight based upon the dry weight of the substrate.
8 . The improved method of claim 5 wherein the water-soluble multivalent salt is selected from the group consisting of salts of aluminum, salts of barium, salts of beryllium, salts of calcium, salts of chromium, salts of copper, salts of iron, salts of magnesium, salts of strontium, salts of zinc, salts of zirconium, and combinations thereof.
9 . The improved method of claim 8 wherein the water-soluble multivalent salt is selected from the group consisting of salts of aluminum, salts of calcium, salts of magnesium, salts of zirconium, and combinations thereof.
10 . The improved method of claim 8 wherein the water-soluble salt is calcium propionate.
11 . The improved method of claim 5 wherein the alginic acid salt is a member selected from the group consisting of ammonium alginate, iron alginate, lithium alginate, potassium alginate, sodium alginate, and combinations thereof.
12 . The method of preparing the release sheet of claim 1 comprising treating an unsized saturating kraft paper substrate during formation of said substrate with an aqueous solution of at least one water-soluble multivalent salt in an amount sufficient to provide a dried solids content surface coating of about 0.01% to about 3.0% by weight based upon the dry weight of the substrate, and subsequently applying said substrate after formation on at least one salt-treated surface with a film comprising at least one salt of alginic acid.