Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Patent
1996-02-29
1999-10-19
Szekely, Peter A.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
524274, 525143, 525149, 525218, 525221, 525227, 156327, 156334, C08L 9304, C08L 3306, C09J13306, C09J12504
Patent
active
059690259
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a water-based adhesive composition which is suitable for bonding polar and non-polar substrates and which shows good bond retention in the presence of water. The adhesive composition is particularly suitable for use in the manufacture of absorbent pads, in particular diapers and sanitary napkins.
Water-based adhesives are known to have a number of advantages over solvent based and hot melt adhesives, in particular the following: application and can be applied at room temperature; content even in the case of high molecular weight polymers.
In the bonding of two dissimilar materials, the more diverse the chemical nature of the materials to be bonded, the more difficult they are to bond. One example of the need to bond two materials which differ chemically is in the manufacture of absorbent pads, in particular diapers and sanitary napkins, in which it is necessary to bond a polar material such as cellulose to a non-polar material such as a polyolefin, e.g. polypropylene or polyethylene. Many adhesives are suitable for bonding either polar or non-polar materials but not the one to the other.
In many cases, particularly in the case of absorbent pads, it is necessary that the bond retain its strength when wet and retention of a substantial fraction of the bond strength is desirable even after contact with water for several hours at body temperature. At present, the manufacture of absorbent pads often uses hot melt adhesives and although these form a true adhesive bond with non-polar substrates such as polyolefin, they probably form only a mechanical bond with cellulose by interlocking with the cellulose fibres. As a result the bond formed by hot melt adhesives between polyolefin and cellulose is relatively easy to debond when wet.
Some water based adhesives are known to be capable of adhering to a wide variety of substrates, both polar and non-polar, provided that the adhesive itself has the correct balance of polar and non-polar characteristics. Known adhesives of this sort include: esters and ranging from homopolymers to copolymers with ethylene and/or acrylic monomers (vinyl acrylics). In all vinylic emulsions vinyl esters, and especially vinyl acetate, are the major component. that whilst adhesion to polar substrates (such as cellulose) is good, adhesion to non-polar substrates such as polyolefins can be unsatisfactory.
Water resistance of the bond formed by water based adhesives is typically poor, particularly in the case of vinylics. Water resistance is generally measured as percent bond retention after immersion in water at room temperature for a standard time, e.g. 1 hour. Water resistance ranges from zero (quick, self-debonding of the sample after a short immersion period) to values of 10 or 15% which have, in the past, been regarded as satisfactory.
The manufacture of absorbent pads requires that a non-polar substrate such as polyolefin be bonded to a polar substrate such as cellulose using an adhesive capable of providing a bond with high water resistance, e.g. bond retention of at least 30% of the dry value even after several hours immersion in water at body temperature. It has generally been thought that bond retention at this sort of level when wet can only be achieved with a water-borne adhesive by cross-linking of the adhesive. Cross-linking is generally achieved by including a reactive co-monomer (e.g. a monomer containing carboxyl, hydroxyl, epoxy, amide, isocyanate or the like functionality), which reactive co-monomers are capable of cross-linking the polymer themselves (e.g. carboxyl groups reacting with hydroxyl, epoxy or isocyanate groups) or by reaction with an external cross-linker (e.g. urea-formaldehyde resin, isocyanates, polyols, epoxides, amines and metal salts, especially zinc).
Many of these cross-linking systems are unsuitable for use in products in the hygiene field for toxicological reasons, for example products which might contain unreacted active monomers or zinc or other metals cannot be used safely in contact with the human skin. Furthermore, cross-
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Szekely Peter A.
The Procter & Gamble & Company
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