Adhesive compostions based on alpha-cyanoacrylate esters belong to a class of adhesives known as reactive liquid adhesives. These cyanoacrylate adhesives are single-part, low viscosity adhesives which polymerize rapidly at room temperature without the use of an added catalyst when pressed between two substrates. Application of the cyanoacrylate adhesives merely involves spreading a small amount of the adhesive between two substrates, pressing the substrates together, and allowing the resultant bond to cure. The adhesive develops sufficient strength after a short period of time to hold the substrates together until the adhesive completely polymerizes and builds up to its maximum bonding strength.
Initiation of polymerization, or cure, is generally believed to proceed through an anionic mechanism. The cyanoacrylate adhesives have such a great tendency to polymerize that water is a sufficiently active initiator. When the adhesive is applied to a substrate and exposed to atmospheric and surface moisture, polymerization usually begins within a relatively short period of time, generally less than one minute, and on many surfaces within a matter of a few seconds.
For many years a need has existed for improved bonding of polymeric substrates to other materials, e.g., metals, wood and ceramics, or to themselves with cyanoacrylate adhesives. Untreated polymeric substrates, particularly polyolefin and polyfluorocarbon substrates, when bonded with cyanoacrylate adhesives, generally exhibit less shear strength than is desired for many end uses.
Modification of polymeric surfaces has been used as a means of increasing the adhesion of various adhesives to polymeric surfaces. Surface modification techniques which have been examined are corona discharge treatment, flame treatment with, for example, helium gas plasma or oxygen gas plasma, and chemical treatment with, for example, chromic acid, potassium permanganate, or peroxydisulphate solutions. Such treatments are described by D. Briggs in "Surface Treatments for Polyolefins," Surface Analysis and Pretreatment of Plastics and Metals, D. M. Brewis, Ed., MacMillan, N.Y., 1982, pp. 199-226. These treatments and other surface modification procedures have a common shortcoming in poor durability. Light rubbing of the surface causes a decrease in the effect, the altered surface being easily abraded.
Irradiation of polyolefin substrates, such as with an electron beam, to improve the adhesion of various coatings is also known and has been disclose, for example, in U.S. Pat. Nos. 4,041,192 (Heger et al.), 4,148,839 (Fydelor), 3,252,880 (Magat et al.) and 4,179,401 (Garnett et al.).
Application of a precoat, or primer, on surfaces of various materials has also been used to improve the adhesion of cyanoacrylate adhesives to various substrates. Primers which have been used include alkyl monohydric alcohols (U.S. Pat. No. 2,768,109, Coover, Jr.), monomeric epoxides (U.S. Pat. No. 3,259,534, Wicker, Jr. et al.), organic amines such as secondary amines, tertiary amines, N-substituted alkanolamines, acrylated N-substituted alkanolamines, diamines and certain heterocyclic amines (U.S. Pat. No. 3,260,637, Von Bramer), organometallic compounds (European Patent Publication No. 0 129 069, Toagosei), a solid mixture of alkaline or basic accelerator, such as sodium or potassium hydroxide or sodium borosilicate spheres of micro-size, combined in a chalkable calcareous binder (U.S. Pat. No. 4,215,173, Hubbard), alkyl 2-cyanopenta-2,4-dienoate (U.S. Pat. No. 4,425,471, Millet), and tannins, such as digallic acid, tannic acid, or other hydrolyzable or condensed tannin (U.S. Pat. No. 4,511,686, Millet). However, many of these priming materials are easily removed from the substrate to which they are applied by evaporation or abrasion during handling of the primed substrate.
Improved adhesion of cyanoacrylate adhesives to various substrates through the use of adhesive additives has also been described. Various additives which have been suggested include plasticizers, such as alkyl esters of aliphatic monocarboxylic acids, alkyl esters of aliphatic dicarboxylic acids, alkyl phosphates, triaromatic phosphates, polyfunctional aliphatic esters, and aliphatic and aromatic phosphonates (U.S. Pat. No. 2,784,127, Joyner et al.), itaconic acid anhydride (U.S. Pat. No. 3,948,794, Eberhard), acetic acid (U.S. Pat. No. 4,125,494, Schoenberg et al.), compounds having the formula ##STR1## wherein R' is hydrogen or an alkyl, aryl, or cycloalkyl group having 1 to 10 carbon atoms, such as gallic acid monohydrate, methyl gallate, propyl gallate and hexyl gallate (U.S. Pat. No. 4,139,693, Schoenberg), a combination of polyethylene glycols having a degree of polymerization of at least 3 and non-ionic surface active agents having a poly(ethyleneoxy) moiety therein with the poly(ethyleneoxy) moiety having a degree of polymerization of at least 3 (U.S. Pat. No. 4,170,585, Motegi et al.), trichlortrifluoroethane (U.S. Pat. No. 4,200,549, Okamura et al.), esters compounds such as acrylates, methacrylates, and crotonates of glycols, glycol monoethers, and monoesters (U.S. Pat. No. 4,307,216, Shiraishi et al.), a combination of (1) at least one aliphatic polyol and/or polyether and derivatives thereof and (2) at least one aromatic polyol and/or carboxylic acid and derivatives thereof (U.S. Pat. No. 4,377,490, Shiraishi et al.), and phthalic anhydride (U.S. Pat. No. 4,450,265, Harris).
Polyolefin substrates have been provided with a treated surface for improved adhesion of pressure-sensitive adhesives. U.S. Pat. No. 3,628,987 (Nakata et al.) discloses a pressure-sensitive adhesive film wherein the film surface to which the adhesive is adhered has graft-polymerized thereto a vinyl monomer or diene monomer, the adhesive having a solubility parameter near that of polymers of the vinyl or diene monomer. U.S. Pat. No. 4,563,388 (Bonk et al.) discloses a polyolefin substrate having graft-polymerized thereto at least one monomer selected from the group consisting of acrylic acid, methacrylic acid and ester thereof; acrylamide; methacrylamide; sterically non-hindered tertiary alkyl acrylamides and methacrylamides having three or less carbon atoms in the alkyl group; and N-vinyl pyrrolidone, and firmly adherently bonded to the graft-polymerized monomer, an acrylic-type, normally tacky and pressure-sensitive adhesive.
A series of articles, "Surface Modification of Polyethylene by Radiation-Induced Grafting for Adhesive Bonding. I. Relationship Between Adhesive Bond Strength and Surface Composition," (S. Yamakawa, J. Appl. Polym. Sci., 20, 3057-3072 (1976); "II. Relationship Between Adhesive Bond Strength and Surface Structure," (S. Yamakawa et al., Macromolecules, 9, 754-758, 1976); "III. Oxidative Degradation and Stabilization of Grafted Layer," (S. Yamakawa et al., J. Appl. Polym. Sci., 22, 2459-2470, 1978); "IV. Improvement in Wet Peel Strength," (S. Yamakawa et al., J. Appl. Polym. Sci., 25, 25-39, 1980), and "V. Comparison with Other Surface Treatments," (S. Yamakawa et al., J. Appl. Polym. Sci., 25, 40-49, 1980), disclose grafting of methyl acrylate (followed by saponification), vinyl acetate, acrylic acid, acrylamide, and methylolacrylamide to polyethylene by vapor-phase mutual grafting or liquid-phase preirradiation at thickness of grafted monomer of more than 10 micrometers to improve adhesion of epoxy adhesives.