Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1997-05-29
2001-04-17
Gorr, Rachel (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C528S059000, C528S075000, C528S081000
Reexamination Certificate
active
06218471
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to adhesive compositions based on ethylene-unsaturated acid ester copolymers and containing hydroxyl functional groups.
It relates more particularly to crosslinkable hot-melt adhesive compositions in the form of prepolymer containing free isocyanate functional groups; the said prepolymer results from the reaction of a polyisocyanate and of a copolymer (A) of ethylene and of at least one unsaturated carboxylic acid ester and containing hydroxyl functional groups.
BACKGROUND OF THE INVENTION
This composition, generally associated with an adhesive (tackifying) resin makes it possible to produce adhesive bondings which resist elevated temperature after crosslinking of the free —NCO functional groups, usually under the effect of atmospheric moisture.
The adhesives are generally in solid form and are applied as a melt phase; they become rigid after application while providing the bonding of the surfaces to be joined. This rigidification usually results from the polymerization or polycondensation of the base components of the adhesive by postcrosslinking. For a number of years attempts have increasingly been made also to introduce adhesive compositions in solid form. In this case these are hot-melt adhesives which have, as base, a thermoplastic resin which is solid at ambient temperature, possessing adhesive properties on postcrosslinking. These adhesives are fluidized when hot, the bonding of the surfaces to be joined being ensured when the adhesive becomes rigid again on cooling. These hot-melt adhesives generally result from the combination of two base constituents: a thermoplastic resin and an adhesive (tackifying) resin, which can be used in combination with additives such as wax, stabilizer, filler, plasticizers and the like. The best-known base thermoplastic resins are polyamides, atactic polypropylene and, in particular, ethylene-vinyl acetate (EVA) copolymers. In the present state these hot-melt adhesives have good adhesive properties but have the disadvantage of exhibiting poor heat resistance, a resistance which barely exceeds 70 to 80° C.
DESCRIPTION OF THE INVENTION
To overcome this disadvantage, a new generation of crosslinkable hot-melt products: crosslinkable polyurethane hot-melt adhesives, has been developed. These products are prepared in a conventional manner by reacting polyisocyanate with polyols of the polyester type, at least one of which is solid at ambient temperature. These adhesives are applied at high temperature as a melt phase. They have the disadvantage of being incompatible with the adhesive (tackifying) resins, and this greatly restricts the formulation possibilities. This is then reflected in a limited bondability to some well-defined substrates. In addition, these products have assembly times of the order of a few minutes, which is too long for some high-output rate applications. The assembly time of a hot-melt adhesive is the time available for performing the bonding, between the instant when the adhesive is applied in the molten state onto the first substrate to be bonded and the instant when the hot-melt adhesive is no longer sufficiently fluid to allow the second substrate to be assembled correctly.
Another type of crosslinkable hot-melt adhesive has been developed from hydroxylated ethylene-vinyl acetate copolymers reacting with a blocked polyisocyanate in stoichiometric quantity, as described in European Patent EP 294 271. These hot-melt adhesives, which are compatible with the adhesive (tackifying) resins, have good adhesive properties but have the disadvantage of requiring subsequent heat treatments at temperatures of at least 120° C. for several minutes to ensure their good crosslinking. Such bonding conditions are not acceptable in the case of some thermally sensitive substrates.
EP 380 379 describes crosslinkable hot-melt adhesive compositions which are in the form of a prepolymer containing free isocyanate functional groups; the said prepolymer results from the reaction of a copolymer of ethylene and of vinyl acetate containing hydroxyl functional groups with a polyisocyanate. This prior art describes the copolymers obtained by direct copolymerization of ethylene, of vinyl acetate and of hydroxyethyl acrylate, as well as the ethylene-vinyl acetate-vinyl alcohol copolymers obtained by partial hydrolysis of ethylene-vinyl acetate copolymers.
The compositions of the present invention have a better strength in the SAPT test and in T peeling than the compositions of the prior art based on ethylene/vinyl acetate/hydroxylated monomer copolymers.
The hydroxyl functional groups of the copolymer (A) can be introduced:
by grafting or by copolymerization of an unsaturated monomer containing at least one hydroxyl functional group,
by grafting or by copolymerization of an unsaturated monomer and then reaction of this monomer with a product introducing at least one hydroxyl functional group,
by modifying a grafted or copolymerized monomer to create at least one hydroxyl functional group.
The present invention also relates to these copolymers (A).
As an example of an unsaturated carboxylic acid ester forming the base of the copolymer (A) with ethylene there may be mentioned alkyl (meth)acrylates, the alkyls containing from 1 to 24 carbon atoms.
Methyl, ethyl, n-butyl, isobutyl and 2-ethylhexyl (meth)acrylates may be mentioned.
The copolymer (A) may therefore be an ethylene copolymer of an unsaturated carboxylic acid ester and of a grafted or copolymerized monomer chosen, for example, from hydroxyethyl (meth)acrylate or of a carboxylic acid anhydride, allyl alcohol or N-hydroxymethylacrylamide.
The copolymer (A) may also be an ethylene copolymer of an unsaturated carboxylic acid ester and of a carboxylic acid or of a carboxylic acid anhydride (grafted or copolymerized) neutralized with a diol or a polyetherdiol such as ethylene glycol, polyethylene glycol, polypropylene glycol or polytetramethylene glycol.
The copolymer (A) may also be an ethylene copolymer of an unsaturated carboxylic acid ester and of a saturated carboxylic vinyl ester (grafted or copolymerized) which has been hydrolysed.
The copolymer (A) is advantageously chosen from:
copolymers of ethylene, of an alkyl (meth)acrylate and of hydroxyethyl (meth)acrylate, which are obtained by direct polymerization of the three monomers,
copolymers of ethylene, of an alkyl (meth)acrylate and of polyethylene glycol (meth)acrylate.
The copolymer (A) advantageously contains, by weight, 40 to 95% of ethylene, 4 to 40% of unsaturated carboxylic acid ester and 1 to 15% of monomer containing at least one hydroxyl functional group. The melt index of the copolymer (A) according to ASTM D 1238-73 is advantageously between 1 and 1000 (g/10 min). The melt viscosity of the copolymer (A) is advantageously between 100 and 10 000 Pa s.
The copolymer (A) preferably contains from 2×10
−3
to 15×10
−2
mol OH per 100 g of copolymer.
This copolymer (A) reacts with a polyisocyanate, preferably a diisocyanate, advantageously aliphatic, cycloaliphatic or aromatic. An excess of polyisocyanate is employed in order that free NCOs should remain.
2,4-Tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), hexamethylene diisocyanate (HMDI) and isophorone diisocyanate (IPDI) may be mentioned among the preferred diisocyanates. MDI is preferably employed.
Free isocyanate functional groups remain in the present compositions. The content of free NCO functional groups is preferably between 1 and 5% by weight of free NCO functional groups, relative to the total weight of the adhesive. This content is preferably from 2 to 3%. This content makes it possible to produce a good compromise between the speed of crosslinking of the adhesive (after bonding) and its stability when hot (before bonding).
The prepolymer is advantageously prepared in the presence of a monoalcohol.
The compositions according to the present invention may additionally contain: (for simplicity they will be called “additives” in the text which follows)
one or several tackifying resins,
Chenard Jean-Yves
Lebez Jean
Pierrot Jean-Michel
Elf Atochem S. A.
Gorr Rachel
Smith , Gambrell & Russell, LLP
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