Heat-Activated Adhesive Coating

This invention relates to an adhesive coating which has a dry, tack-free surface convertible to a permanently pressure-sensitive adhesive layer by the application of heat. The invention is useful for labeling, sealing, adhesive layer transfer, decalcomania, and other packaging applications.

Non-tacky coated products which can be converted by heat to a tacky adhesive product have been described in prior art. Perry in U.S. Pat. No. 2,462,029 describes adhesive compositions containing discrete solid particles of a plasticizer dispersed in a potentially adhesive film. Heating the film activates the adhesive by causing the plasticizer to melt and merge with the polymeric components of the film. This results in an adhesive product which will retain its tack over an extended period of time. Typically this merging of the plasticizer and polymeric material results in increased tack or viscidity, and also lowers the temperature at which the composition solidifies to a dry, non-tacky film and conversely, at which temperature such dry film, after solidification, softens or activates upon a subsequent heating. The compositions of Perry have proved to be extremely useful in providing adhesive films and articles having such films, for example tapes and labels, which are dry and non-tacky at normal room and storage temperature conditions, but are heat activatable to adhesive tackiness and retain such adhesive tackiness when subsequently cooled to temperatures below their initial activation temperature. After initial activation, these films can be allowed to cool to temperatures appreciably below the activation temperature and can subsequently be applied to the desired surface. Heat need not, therefore, be maintained after the initial activation to keep the adhesive ready for use. Compositions of the Perry invention are limited to the use of solid crystalline plasticizers which due to their solid nature prior to initial activation remain inert within the composition.

Lawton et al. U.S. Pat. No. 3,104,979 describes an improvement over the Perry composition wherein liquid plasticizers are used in the place of the solid plasticizers of Perry. These liquid plasticizers are incorporated as solid inclusion compounds with urea. Heat forces the liquid plasticizer from the channels in the solid urea crystal and this liqiud plasticizer merges with the thermoplastic polymeric components of the film. The potentially adhesive film is dry and non-tacky when the plasticizer is in the included state and becomes tacky when the plasticizer is released from urea.

Both systems have a disadvantage in that both systems will detackify on cooling or ageing. The Perry plasticizers recrystallize and the Lawton plasticizers will recomplex with urea. There have been reports of numerous examples of the Perry type labels falling off the substrates during cold winter conditions. The adhesion of these systems is restricted to a limited number of substrates primarily due to lack of flexibility in adhesive formulations.

It has been discovered that a thin, dry, non-tacky barrier coating can be formed on and in the surface layer of any conventional pressure-sensitive coating by a simple procedure. The barrier properties of this coating can be destroyed by heat to restore the original properties of the pressure-sensitive adhesive layer. Thus, the wide choice of performance characteristics of conventional pressure-sensitive adhesive products can be obtained from a dry, tack-free heat activatable composition.

The barrier layer is obtained by in situ formation of molecular complexes or compounds within and/or on the surface portion of the adhesive composition. Molecular compounds are not new in themselves, but are generally described by P. Pfeiffer, Organische Molekularverbindungen (Ferdinand Enke, Stuttgart, 1927) and by F. Cramer, Einschlussverbindungen (Springer-Verlag, Berlin 1954). Molecular compounds have the unique property of being dissociable by heat to yield the original components from which such molecular compounds were prepared.

The barrier coat can be easily formed by several methods. The complex barrier can be obtained by wash-coating a conventional adhesive composition containing one component of a molecular compound with a solution of the co-reactant or other component of the molecular compound. The two components of the complex then react in or on the surface portion of the adhesive to give a dry, tack-free barrier coating of the molecular compound or complex. The wash-coat can be a simple solution of the co-reactant or can also contain other materials such as polymers, UV absorbers, and the like. This barrier coat is only a minor part of the adhesive layer and heat dissociation of the molecular compounds causes volatilization and/or migration of the components through the bulk of the adhesive layer resulting in the generation of the original properties of the adhesive. Since the molecular compounds make up only a small portion of the total composition, future recombination of the components in the bulk layer have little if any effect on the properties of the adhesive. Either component may be used in the wash coat and the choice is at the convenience of the processsor.

Rather than use a wash-coat solution of the co-reactant to form the complex barrier layer the co-reactant may be in the form of a vapor or gas in contact with the adhesive layer or may be a simple frangible coating on another substrate wherein the complex barrier layer is formed by bringing the adhesive and co-reactant coatings in contact.

Two classes of molecular complexes have been found to be particularly useful in this invention.

The hydrogen bond complexing of hydroxyaromatic compounds with amines has provided an effective method for obtaining the complex barrier coat. Compositions which give solid complexes are readily obtained by reacting stoichiometric amounts of the compounds in methanol. The solid complex will separate and after filtering and drying, the dissociation temperature can be determined. The dissociation temperature of the complex relates to the activation temperature of the adhesive. It has been found that the dissociation temperatures can be lowered by including additives which cause a combined heat-solvent effect for dissociation of the complex. The following table lists some of the molecular complexes along with the co-reactants and dissociation temperatures.

The inclusion of long-chain (8 or more carbons in the chain) hydrocarbon derivatives in urea is another effective means for obtaining the barrier coat. Certain additives will decompose the urea complex at lower than normal activation temperatures for urea inclusion compounds. Some compounds which are readily complexed by urea are:

Long chain normal aliphatic hydrocarbons

Long chain normal aliphatic alcohols

Long chain normal aliphatic fatty acids

Long chain normal aliphatic fatty amides

Long chain normal aliphatic paraffin amines

Long chain normal aliphatic fatty acid esters

Long chain normal aliphatic fatty nitriles

Long chain normal aliphatic ketones

Long chain normal aliphatic ethers

Long chain normal aliphatic halides

Long chain normal aliphatic aldehydes

Long chain normal aliphatic nitro compounds

Long chain normal aliphatic sulfides

Long chain normal aliphatic mercaptans

Long chain normal unbranched olefins

Similar results may be obtained by substituting thiourea for urea and using branch-chained hydrocarbon derivatives in place of the normal or straight chained derivatives listed above.

The versatility of the invention also allows the use of hot-melt, water, or solvent base compositions for the adhesive coating.

1. An adhesive composition (commercial or experimental) is chosen to give the final adhesive characteristics desired in the finished product. These compositions are readily developed by those who are skilled in the formulation of pressuresensitive adhesives.

2. One of the co-reactants (either the amine or the aromatic hydroxy compound for hydrogen bonded complexes or either the hydrocarbon derivative or urea for the inclusion complex) is dissolved in the adhesive composition and the total adhesive formulation is coated onto the desired substrate at a coating thickness normal for pressure-sensitive adhesives and allowed to dry.

3. The co-reactant necessary to form the molecular complex is dissolved in a volatile medium which acts as a solvent for both reactants but which does not dissolve the molecular complex. This solution is applied to the surface of the adhesive layer by wash-coating, spraying, dipping, or any other convenient method which will permit the co-reactants to come in contact. A thin layer of the molecular complex is formed at and in the surface of the adhesive coating which, when dry, acts as a non-tacky barrier masking the adhesive properties of the coating.

4. The original adhesive characteristics may be regenerated by heating to cause dissocation of the complex and migration of the components into the bulk or body of the adhesive layer.

Specific illustrations demonstrating the working of this invention are contained in the following examples.

The adhesive coating is prepared by incorporating 4 parts of amine in 96 parts Adhesive Base, applying this mixture onto conventional base paper stock such as Kromekote with a 6 mil Bird applicator and allowing to dry at room temperature to give a conventional tacky pressure-sensitive adhesive.

A seven-percent solution of the hydroxyaromatic compound is prepared using methanol as the solvent and the adhesive coating is dipped into this solution to give a wash coat. Upon evaporation of the methanol, the surface of the adhesive layer is dry and essentially tack-free due to the in situ formation of the hydrogen bonded molecular complex between the amine and the aromatic hydroxy compoound.

When these non-tacky potentially adhesive coated papers are heated for 10 seconds on a hot arch at a temperature of about 135.degree. to 143.degree. C., adhesiveness is similar to the properties of the original adhesive coating without wash-coat treatment. The adhesive tack is still present two weeks after heat activation.

Addition of 4 parts-per-hundred parts of the adhesive formulation of a third co-reactant comprising any one of the following: benzil, hydroquinone-monomethylether, p-hydroxy-acetophenone, 4-methoxybenzil, t.butyl hydroquinone, 1,5-diphenylpentenedione, or dehydroacetic acid and wash-coating as previously described results in dry, tack-free coatings which can be activated to full adhesiveness by heating for 10 seconds at 110.degree. C. Without the use of these additives, full adhesiveness is not obtained under the same activation conditions.

The adhesive coating is prepared by dissolving 10 parts of urea in 100 parts of the previously described Adhesive Base. This coating mixture is applied with a 6 mil Bird applicator to a suitable substrate and allowed to air-dry.

The wash-coat is prepared by dissolving 15 parts hydrocarbon derivative in 85 parts methanol. This wash-out is applied in the manner described earlier and allowed to airdry 24 hours. The resulting product is a dry,tack-free potentially adhesive composition which can be activated to its adhesive state by heating 10 seconds at 177.degree. C. The adhesive properties are still present 2 weeks after activation. If 8 parts of dehydroacetic acid or glycollic acid are added to 100 parts of the adhesive formulation, activation can be obtained by heating for 10 seconds at 138.degree. C.

The adhesive coating is made by incorporating 4 parts of amine into 100 parts adhesive and applying at a thickness of 6 mils wet to a conventional pressure-sensitive adhesive base paper. The resulting coating is air-dried and wash-coated with a 5-percent solution of the aromatic hydroxy compound. Air-drying results in a dry, tack-free barrier coat of the complex on the surface of the adhesive layer.

When these dry, non-tacky potentially adhesive products are activated for 10 seconds at 149.degree. C., the original adhesive character of the adhesive coating is regenerated. The adhesive properties still persist 8 days after activation.

The activation temperatures of formulas 8, 10, 12, and 14 are lowered to 116.degree. C. or below by incorporating any of the following additives into the adhesive formulation: 1,5-diphenyl-3-pentenedione, benzophenone, 2,3,4-trihydroxydecaphenone, 4,4'-dimethylbenzophenone, dehydroacetic acid, benzil, 4-methoxybenzil, ethyl-p-hydroxybenzoate, resorcinol monobenzoate, or p-bromoacetophenone.

The adhesive coating is prepared by dissolving 10 parts of the hydrocarbon derivative in 100 parts Adhesive I. The mixture is then coated at a thickness of 6 mils (wet) onto conventional pressure-sensitive paper stock and air-dried. The tacky adhesive layer is then washed in the normal manner with a saturated solution of urea in methanol. The resulting tack-free air-dried coatings can be activated by heating for 10 seconds at 177.degree. C. The activation temperature can be lowered to below 138.degree. C. by using additives such as diglycollic acid or dehydroacetic acid to the adhesive layer or wash coat.

Products which can be used for adhesive transfer are obtained by coating with the previously described adhesive formulations (Ex.1 to 25) silicone treated release papers normally used to prevent sticking of the conventional pressure-sensitive adhesive products. These coatings are washed with solutions containing the co-reactants described above to give a dry, tack-free surface. The coated product is activated by heat as already described to give a pressure-sensitive adhesive surface. When the adhesive surface is pressed against a substrate such as metal, wood, glass, paper, plastic, or the like, the silicone release sheet can be pulled free to leave the adhesive coating at any location or surface desired.

If so desired the dry, non-tacky complex barrier surface on the adhesive can be printed by letterpress, gravure, off-set, or other conventional means. The printed product can be heat-activated as already described and applied to the desired surface. Upon peeling off the silicone release paper, the transferred printed decal is clearly readable and the print is protected by the adhesive layer covering its surface.

The foregoing detailed description of my invention has been made with respect to several preferred embodiments. It is to be understood that many of the specific materials and temperatures stated herein may be varied by persons skilled in the art without departing from the spirit and scope of my invention.