Pressure sensitive hot melt adhesives are widely used. Most of such adhesives make use of thermoplastic rubbers which may be isoprene/styrene block copolymers or butadiene/styrene block copolymers. These rubbers are very hydrophobic. A drawback of such adhesives is that residues of such compositions become a very undesirable hydrophobic contaminant in water-based processes for recycling paper or fibreboard which has been treated with the adhesive. For instance if paper or board is treated with a hydrophobic pressure sensitive adhesive and used to make labels, the residue of the adhesive is a considerable obstacle to recycling any scrap material.
Pressure sensitive adhesives which are water dispersible are known. U.S. Pat. No. 4331576, Colon et al 1981, discloses hot melt pressure sensitive adhesives which contain vinyl pyrrolidone/vinyl acetate copolymers, ethylene/vinyl acetate copolymers, certain fatty acids, and rosin or tall oils. This patent explains that because of the water dispersability a label which is adhered with the adhesive can be removed on wetting the label.
Unexpectedly we have now found that it is possible to prepare water-soluble or water-dispersible hot melt pressure sensitive adhesives using the more conventional thermoplastic rubbers and the resulting compositions can give better tack, peel strength and cohesive strength than provided by the previously disclosed water-soluble or dispersible hot melt compositions. According to the present invention there is provided a hot melt composition comprising
3 to 50wt % of a thermoplastic rubber
10 to 50 wt % of a copolymer of vinyl pyrollidone and vinyl acetate
10 to 50 wt % of monocarboxylic fatty acid having 8 to 22 carbon atoms.
The balance of the composition can be provided by optional ingredients.
The thermoplastic rubber will generally be a copolymer of a monomer with one olefinic unsaturation and a monomer with two olefinic bonds especially a styrene/isoprene block copolymer or a styrene/butadiene block copolymer. An example of the former is the thermoplastic rubbers available from Shell under their registered trade mark Cariflex TR.
The amount of thermoplastic rubber is preferably from 4 to 20% by weight of the composition, and for higher peel strengths may be at least 6 wt %.
A suitable vinyl pyrrolidone/vinyl acetate copolymer is PVP/VA S-630 available from GAF Corporation.
This polymer contains vinyl pyrrolidone and vinyl acetate in approximate weight ratio of 6:4. These two monomers are randomly distributed in the polymer.
The amount of this copolymer ranges from 10 to 50 wt % of the composition, preferably 15 to 40 wt %.
The third essential main component is fatty acid in an amount from 10 to 50 wt %.
Commercially available fatty acids are generally mixtures with a range of carbon chain lengths present. The fatty acid which is used may have a preponderance of shorter chains, from 10 to 14 carbon atoms, or of larger chains from 16 to 20 carbon atoms. The latter may include unsaturated chains but need not do so. Shorter chain fatty acid may be coconut derived. A suitable larger chain fatty acid is isostearic acid.
An ingredient which will usually be included is a tackifying resin in a quantity from 10 to 50 wt %, better 20 to 40 wt % of the composition.
Materials which can serve as tackifying resins include wood rosins, tall oils, esters of such rosins, and tall oils, and polyterpenes. Specific examples which are commercially available include Foral 85E from Hercules BV which is hydrogenated rosin, Staybelite Resin E from Hercules BV which is partially hydrogenated rosin, Piccolyte polyterpene resins from Hercules BV and Imprez aliphatic hydrocarbon resin available from ICI.
Compositions according to the invention will generally include antioxidant in an amount from which is a fraction of a percent. The compositions will often also include from 2 to 10 wt % of an ester plasticiser, such as dialkyl phthalates or alkyl benzoates.
Compositions according to this invention can be prepared simply by mixing the ingredient materials with heating and stirring to form a melt of them all. It may be found convenient to mix tackifying resin and the thermoplastic rubber together in a high shear mixer before mixing with the other ingredients. The mixing operation may be carried out under a blanket of inert gas such as nitrogen. This is conventional practice for making other hot melt compositions.
The principal application envisaged for the invention is use as a hot melt pressure sensitive adhesive. For this application the compositions will generally include tackifying resin. They may be applied to a paper substrate by conventional methods for application of hot melt adhesive. The compositions may be used in particular for the production of pressure sensitive labels and pressure sensitive adhesive tapes. Each of these products consists of a substrate which may be paper, fibreboard or a plastic film, coated with the adhesive composition embodying the present invention.
Another application for the compositions of the present invention is as a thermoplastic material which may be used in various medical applications. A quantity of composition according to the invention can be made large enough to sustain its own shape when at body heat or below and can be adhered to the human skin. This could be done to adhere a probe or sensor to the skin. Alternatively the composition could be impregnated with a drug for topical application which will progressively migrate from the composition into the skin to which the composition is adhering.