Polyurethane adhesive

Details Polyurethane adhesive Polyurethane adhesive

2000-05-24

2003-10-07

Yao, Sam Chuan (Department: 1733)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C524S591000

Reexamination Certificate

active

06630050

ABSTRACT:

This application is filed under 35 U.S.C. 371 and based on PCT/EP98/005841, filed Sep. 15, 1998.
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to an adhesive based on a polyurethane of at least one diisocyanate and at least one polyethylene glycol of low molecular weight, to its production and to its use.
Discussion of Related Art
An adhesive based on these components is known. Thus, a high molecular weight nonionic water-soluble polyurethane is described in WO 94/13726. The high molecular weight is reflected in the specific viscosity of at least 0.4 (1% aqueous solution at 20° C.) and in the melt viscosity of more than 3 Pas at 175° C. (Epprecht). The high solubility in water is reflected by the virtually unlimited mixing ratio at 20° C. Another important characteristic is its melting point of 60 to 80° C. and its degree of crystallization of 20 to 80%, as measured by DSC and based on the degree of crystallization of commercially available, relatively long-chain polyethylene glycol, particularly polyethylene glycol 6000. The known polyurethane can be produced from a diisocyanate and a polyethylene glycol with a degree of polymerization n of 8 to 500. The polyethylene glycol may also be partly replaced by hydrophobic diols, for example by polypropylene glycol. The known polyurethane may be used as an adhesive whether in the form of a hotmelt or in the form of an aqueous dispersion or solution. Above all, it may be used to bond paper, particularly labels and wall coverings. The bonds obtained are strong and can be redissolved by exposure to water or heat (ca. 60 to 80° C.). The properties required for numerous applications are thus satisfied.
However, there is also a need for adhesives which are permanently tacky and remain tacky and which adhere to almost all substrates under light pressure. In particular, there is an increasing demand for such contact adhesives for polyolefin-based packs, more particularly for polyethylene- and polypropylene-based packs. Adhesives such as these should also be soluble in water in the interests of better disposal. In addition, particular economy and easy handling are of course expected.
DESCRIPTION OF THE INVENTION
The solution provided by the invention is defined in the claims and consists essentially of an adhesive based on a polyurethane of
A) at least one diisocyanate and
B) at least one low molecular weight polyethylene glycol, the polyethylene having a molecular weight M
z
of up to 2,000 and the polyurethane having a degree of crystallization of less than 20%. The degree of crystallization is preferably less than 15%, based on the degree of crystallization of commercial polyethylene glycol 6000 as standard. The degree of crystallization is determined by the DSC method (see Examples). The degree of crystallization can be influenced through the type and quantity of diols and diisocyanates used, more particularly through the molecular weights of the polyethylene glycol units.
The viscosity of a 50% by weight aqueous solution is above 10,000 and, preferably, above 20,000 mPas at 22° C., as measured by the Brookfield method (for further particulars, see Examples).
For the polyurethanes according to the invention, the Epprecht melt viscosity at 110° C. is above 5 and, more particularly, above 30 Pas.
The adhesive according to the invention is permanently tacky, at least for 6 months, under normal climatic conditions (20° C./65% relative air humidity) in the same way as a contact or pressure-sensitive adhesive. However, the adhesive according to the invention may also be used like a dispersion or solvent-based adhesive, i.e. the water or the solvent need not be removed before bonding. In contrast to those adhesives, however, it remains tacky, i.e. for example the bond can be dissolved and then re- established simply by pressing the substrates together without the adhesive having to be activated.
The adhesive strength is at least sufficient to hold writing paper on vertical walls, even in the dry state, when the adhesive is applied in a quantity of at least 50 g/m
2
.
The isocyanates are compounds with the general structure O═C═N—X—N═C═O, where X is an aliphatic, alicyclic or aromatic radical and the aliphatic or alicydic radical contains 4 to 18 carbon atoms. An aromatic diisocyanate, preferably MDI, should be used. Up to 50% by weight may be usefully replaced by aliphatic or alicyclic diisocyanates, more particularly TMXDI. Examples of suitable isocyanates are 1,5-naphthylene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), 2,4′-diphenylmethane diisocyanate or isomer mixtures of MDI, hydrogenated MDI (H
12
MDI), xylylene diisocyanate (XDI), tetramethyl xylylene diisocyanate (TMXDI), 4,4′-diphenyl dimethyl-methane diisocyanate, di- and tetraalkyl diphenylmethane diisocyanate, 4,4′-dibenzyl diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diiso-cyanate, the isomers of toluene diisocyanate (TDI), 1-methyl-2,4-diiso-cyanatocyclohexane, 1,6-diisocyanato-2,2,4-trimethyl hexane, 1,6-diiso-cyanato-2,4,4-trimethyl hexane, 1-isocyanatomethyl-3-isocyanato-1,5,5-trimethyl cyclohexane (IPDI), chlorinated and brominated diisocyanates, phosphorus-containing diisocyanates, 4,4′-diisocyanatophenyl perfluoro-ethane, tetramethoxybutane-1,4-diisocyanate, butane-1,4-diisocyanate, hexane-1,6-diisocyanate (HDI), dicyclohexylmethane diisocyanate, cyclo-hexane-1,4-diisocyanate, ethylene diisocyanate, phthalic acid-bis-isocyanato-ethyl ester; diisocyanates containing reactive halogen atoms, such as 1-chloromethylphenyl-2,4-diisocyanate, 1-bromomethylphenyl-2,6-diisocyanate, 3,3-bis-chloromethylether-4,4′-diphenyl diisocyanate. Sulfur-containing polyisocyanates are obtained, for example, by reaction of 2 moles of hexamethylene diisocyanate with 1 mole of thiodiglycol or dihydroxydihexyl sulfide. Other important diisocyanates are trimethyl hexamethylene diisocyanate, 1,4-diisocyanatobutane, 1,12-diisocyanatododecane and dimer fatty acid diisocyanate. Particularly suitable diisocyanates are tetramethy-lene, hexamethylene, undecane, dodecamethylene, 2,2,4-trimethylhexane, 1,3-cyclohexane, 1,4-cyclohexane, 1,3- and 1,4-tetramethyl xylene, isophorone, 4,4-dicyclohexanemethane and lysine ester diisocyanate. Mixtures may also be used.
The solubility of the polyurethane in water is largely attributable to the groups of the polyethylene ether —[CH
2
CH
2
—O—]
n
—, where n is a number which corresponds to a molecular weight M
z
of up to 2,000, preferably in the range from 150 to 1,500, more preferably in the range from 200 to 900 and most preferably in the range from 300 to 600 g/mole. These structural units are derived from the polyethylene glycols used as diols. However, polyethylene glycol in the context of the invention not only encompasses polyadducts of ethylene oxide with water or ethylene glycol as starting molecule, but also polyadducts with other dihydric alcohols, for example butanediol, hexanediol, 4,4′-dihydroxydiphenyl propane and 2,4,7,9-tetramethyl-5-decine-4,7-diol. Several polyethylene glycols with different average molecular weights may also be used.
In a preferred embodiment, the polyethylene glycol with a molecular weight of up to 2,000 is partly replaced by at least one polyol from the following group:
a) to a level of up to 50%, by a block or statistical copolymer or ethylene oxide,
b) to a level of preferably up to 20%, by a hydrophobic aliphatic or cycloaliphatic diol alone,
c) by hydrophobic homopolymeric polyalkylene glycols, the alkylene group containing more than 2 carbon atoms; by up to 10% for C
3
alkylene groups and by up to 20% for C
4
alkylene groups,
d) to a level of up to 40%, by polyethylene glycol with a molecular weight of more than 2,000 to 40,000,
e) to a level of up to 5%, by alcohols of relatively high functionality and
f) to a level of up to 5%, by ion-forming diols.
The percentages shown represent % by weight, based on the diols. Hydrophobic aliphatic or cycloaliphatic diols alone are preferably added.
Besides the polyet

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