Hot melt adhesive containing hydroxyl-functional ethylene-vinyl acetate and blocked polyisocyanate

A composition for a cross-linkable hot melt adhesive is provided and is formed by an ethylene-vinyl acetate copolymer containing primary hydroxyl functions and a blocked polyisocyanate. This composition when associated with a tackifying resin, which can also serve as blocking agent of the polyisocyanate, is a cross-linkable hot melt adhesive. The adhesive is used at a temperature at least equal to splitting temperature of the isocyanate functions of the polyisocyanate. The temperature at which such adhesives retains adherence can reach about 200.degree.-220.degree. C.

FIELD OF THE INVENTION 
This invention relates to a composition useful in a cross-linkable hot melt 
(thermofusible) adhesive and formed by an ethylene-vinyl acetate copolymer 
containing primary hydroxyl functions and a blocked polyisocyanate. This 
composition, when, combined with a tackifying (sticky) resin, makes it 
possible to obtain adherences which are retained at elevated temperatures. 
BACKGROUND OF THE INVENTION 
Adhesives generally appear in liquid form and solidify after being applied 
ensuring the bonding of the surfaces to be joined. The solidification 
habitually results from the polymerization or the polycondensation of the 
basic components of the adhesive. For several years it has been 
increasingly sought to provide adhesive compositions also in solid form. 
The adhesives in this case are hot melts having, as the base, a 
thermoplastic resin which is solid at room temperature and possessing 
adhesive properties. These adhesives are liquefied in the molten state by 
heating, the bonding between the surfaces to be joined being ensured when 
the adhesive becomes solid upon cooling. These thermofusible adhesives 
generally result from the combination of two basic constituents: a 
thermoplastic resin and a tackifying resin which can also include 
additives such as wasces, stabilizers, fillers, plasticizers, and the 
like. In the present state, the hot melt adhesives have good adhesive 
properties but have the disadvantage of exhibiting poor heat resistance, a 
resistance that does not exceed about 70.degree. to about 80.degree. C. 
The best known basic thermoplastic resins are the polyamides, the atactic 
polypropylene and in particular the ethylene-vinyl acetate copolymers. 
SUMMARY OF THE INVENTION 
This invention relates to a composition for a hot melt adhesive having a 
good heat resistance, the temperature of resistance of the adherence being 
capable of reaching from about 200.degree. to about 220.degree. C. The 
base of this composition is an ethylene-vinyl acetate copolymer having 
primary hydroxyl functions, hereinafter called EVA for the sake of 
simplicity, associated with a blocked polyisocyanate. This 
EVA-polyisocyanate composition is, according to the general rule, 
completed with a tackifying resin for obtaining the adhesive proper.

DETAILED DESCRIPTION OF THE INVENTION 
The EVA used in the composition according to the invention is in fact a 
copolymer of ethylene and vinyl acetate and an ethylenic monomer having at 
least one primary hydroxyl function. 
This EVA can be obtained in a manner known by grafting on the copolymer of 
ethylene and of vinyl acetate, the hydroxylated ethylenic monomer. The 
grafting is conveniently carried out by polymerization in solution, 
suspension, or in a mass in a molten state. The terpolymer can also be 
obtained by direct polymerization of three monomers, a known process 
constituting the most advantageous way. 
Among the ethylenic monomers having at least one primary hydroxyl function, 
there are preferably used: hydroxyethyl acrylate and hydroxyethyl 
methacrylate, but there can also be used the monomer of the formula 
##STR1## 
R.sub.1 representing a hydrocarbon residue or hydrogen 
R.sub.2 representing hydrogen or a methyl group 
R.sub.3 representing an ester, amide or an alkyl group 
Among these ethylenic monomers, there can be cited by way of example: 
allylic alcohol, oleilic alcohol, N-hydroxymethyl-acrylamide, and the 
like. 
The EVA composition is a combination by weight related to the initial 
monomers of from about 45 to 93% ethylene, from about 5 to 40% vinyl 
acetate and from about 2 to 15% of an ethylenically unsaturated monomer 
having at least one primary hydroxyl function. EVA preferably contains 
from about 2.times.10.sup.-2 to about 15.times.10.sup.-2 primary OH moles 
per 100 g of copolymer. 
The above described EVA is associated with a blocked polyisocyanate in the 
composition for adhesive. By polyisocyanate is understood the 
polyfunctional compounds having at least two --NCO functions per molecule. 
This polyisocyanate can be selected among the aliphatic, cycloaliphatic, 
aromatic and/or heterocyclic polyfunctional isocyanates; however, there 
are preferred diisocyanates such as, among the best known, toluene 
diisocyanate, diphenyl methane diisocyanate, hexamethyl diisocyanate, and 
also isophorone diisocyanate. The polyisocyanate is included in the 
composition in blocked form. By blocked polyisocyanate is understood a 
polyisocyanate whose --NCO functions are reversibly combined among 
themselves and/or by means of suitable compounds called blocking agents. 
There are generally used a blocking agent of the polyisocyanate 
combinations with active hydrogen; to this effect there are preferably 
used phenols, alcohols, lactams, oximes, or also secondary amines. By way 
of example, there can be cited as blocking agents 
2-6-di-tert-butyl-4-methyl phenol, 4-tert-butyl-phenol, m-cresol, 
4-4'-thio-bis(3-methyl-6-tert-butyl phenol), as well as epsilon 
caprolactam or benzophenoxime. However, in order to reinforce the 
subsequent adhesive properties of the combination, it is preferred to use 
as a blocking agent, a tackifying resin. Said tackifying resin is selected 
among those having free functions capable of blocking the isocyanate 
functions. The tackifying resins are in general those having free phenolic 
functions and more particularly, terpene-phenolic resins. The 
terpene-phenolic resins, usually have a molecular weight between about 500 
and 5,000 and result from the copolymerization of phenolic compounds with 
terpenic derivatives. 
The blocked polyisocyanate is stable at room temperature. It can be admixed 
with the EVA at an elevated temperature while remaining always at a 
temperature below that of splitting of the --NCO functions without 
reaction. It is usefully admixed with the EVA in a proportion such that 
the ratio of the blocked --NCO of the polyisocyanate to the primary --OH 
of the EVA is between about 0.1 and 1. This pre-mixture constitutes the 
composition useful for the cross-linkable hot melt adhesives of this 
invention. The pre-mixture can be preserved for long periods. 
The composition according to the invention, associated with a tackifying 
resin, gives a hot melt adhesive having an excellent resistance to high 
temperature. The quantity of tackifying resin in relation to the 
composition is preferably on the order of from about 0 to 70 portions by 
weight to about 100 to 30 portions by weight of the composition 
EVA-blocked polyisocyanate. The tackifying resin imparts to the hot melt 
adhesive an improved tack. The compounds generally involved are of low 
molecular weight on the order of from about 400 to about 2500. As 
indicated by their name, they contribute tack: a property that is due 
either to the high degree of plasticization of the polymeric composition 
or to a limited compatibility with the polymeric composition. These 
tackifying resins, known per se, mainly belong to three large families: 
the rosins and the hydrogenated, maleinated derivatives thereof ester, 
terpenic resins and petroleum resins, for example: aliphatic, aromatic 
modified aliphatic, hydrogenated. 
When the polyisocyanate of the composition is blocked by a tackifying 
resin, it may not be necessary to add a tackifying resin and the 
composition can be directly used as an adhesive. 
However, it is not ruled out in this case, to add to the composition, a 
tackifying resin identical with, or different from, the one used as 
blocking agent of the --NCO functions of the polyisocyanate. 
It is obviously possible to incorporate into the adhesive the usual 
additives already cited. 
The adhesive obtained is used at elevated temperature, in particular at a 
temperature of at least about 130.degree. C. The temperature is in fact 
the one at which the blocked polyisocyanates begin to dissociate releasing 
their original functions, in other words, the free isocyanate functions 
re-form by unblocking. The reaction of the isocyanates with the primary 
hydroxyl functions of EVA produce the cross-linkage of EVA, which has as 
consequence, a clear improvement of the resistance to temperature of the 
adhesive in comparison to other known thermofusible adhesives. 
The blocking reaction of the isocyanates and the cross-linkage thereof with 
the hydroxyl groups can be favored by using a catalyst. This catalyst is 
usually selected from metallic catalysts like the tin salts or also among 
the tertiary amines like, for instance, the tin dibutyl dilaurate (DLDBSn) 
or the diazabicyclooctane (DABCO). 
The examples that follow illustrate the invention without limiting it. 
EXAMPLE 1 
In a 2-liter reactor provided with a stirring and heating means, there is 
prepared a 4-4' Diphenyl methane diisocyanate (MDI) blocked by a 
tackifying terpene phenolic resin (URAVAR 75205 of DSM). In the following 
order are introduced: 
480 g terpene-phenolic resin 
1.3 liter xylene 
0.1 liter toluene 
75 g MDI 
A temperature of 60.degree. C. is kept until the complete dissolution of 
the resin and of the polyisocyanate. The temperature is progressively 
raised to 120.degree. C. for two hours. After cooling the solution is 
poured into an aliphatic hydrocarbon. The precipitated product is filtered 
and then dried under vacuum. 
The adhesive composition is prepared in the tank of 60 cm.sup.3 of a 
BRABENDER plastograph with cylinder of the type 50. The components are 
incorporated in the following order: EVA, the tackifying resin, then the 
blocked polyisocyanate. The whole is kneaded for 10 minutes at 100.degree. 
C. eventually in the presence of DLDBSn. 
The mixture is deposited between the two aluminium plates. The whole is 
compressed under a pressure of 0.1 MPa for 5 mm at 160.degree. C. for the 
tests 1, 3, 7 and at 180.degree. C. for the tests 2, 4, 5. 
The adhesive properties are evaluated in accordance with: 
T-peel test: standard ASTM D 1876 
Lap shear test: standard ASTM D 1002 
Shear adhesion failure temperature (S.A.F.T.) 
The S.A.F.T. measurement consists in suspending a load of 1 Kg with a test 
specimen prepared for lap shear test. The whole is placed in a temperature 
programmed over for a rise of 5.degree. C. per minute. The S.A.F.T. is the 
temperature at which the bond fails. This measure makes it possible to 
determine the upper service temperature limit of the adhesive. 
EXAMPLE 2 
Example 1 is reproduced with an MDI blocked by epsilon caprolactam under 
the conditions that follow. 
In a 1-liter reactor provided with a stirring and heating means there is 
prepared a 4,4' diphenyl methane diisocyanate (MDI) blocked by the 
caprolactam. 
There are introduced in the following order: 
118.65 g caprolactam 
95 g ethoxy ethyl acetate. 
A temperature of 60.degree. C. is kept until the dissolution of the 
caprolactam is complete: then the temperature is allowed to descend to 
room temperature. There is then progressively added under vigorous 
stirring, the solution of: 
131.25 g MDI 
394 g 2 ethoxy ethyl acetate. 
This is progressively heated to 70.degree. C. until the formation of the 
blocked isocyanate which precipitates. This precipitate is filtered, 
washed with hexane, then dried under vacuum at 40.degree. C. 
The compressions between the aluminum plates are effected at 160.degree. C. 
for tests 6, 8 and 9. 
The compositions and the results obtained from the tests of the two 
examples are summarized in the two tables that follow. 
TABLE 1 
__________________________________________________________________________ 
TESTS 
2 
1 EVA: 100 g 
COMATIVE DERTOPHENE T: 50 g 
EVA: 100 g MDI BLOCK A: 11.3 g 
Adhesive composition 
DERTOPHENE T: 50 g 
DLDBSn: 0.1 g 
__________________________________________________________________________ 
##STR2## 0 0.5 
Creep Resistance 
80 225 
(S.A.F.T.) 
in .degree.C. 
Lap Shear Test (MPa) 
at 20.degree. C. 
5.7 8.2 
at 60.degree. C. 
0.4 2.3 
at 80.degree. C. 
0.1 1.0 
T peel test (daN/cm) 
at 20.degree. C. 
1.4 3.0 
at 60.degree. C. 
0.3 0.7 
at 80.degree. C. 
0.1 0.3 
__________________________________________________________________________ 
EVA: Terpolymer by weight: ethylene 72%, vinyl acetate 23%, hydroxyethyl 
acrylate 5% 
Blocked MDI A: MDI blocked with a terpene phenolic resin 
DERTOPHENE T: Terphene phenolic resin of DRT 
3 TABLE 2 
TESTS 6 8 9 4 5 EVA 7 EVA 100 g EVA 100 g 3 EVA 100 g EVA 100 g 
MDI 100 g comparative DERTOPHENE T 50 g DERTOPHENE T 50 g Adhesive 
comparative MDI MDI blocked B 2,56 g EVA 100 g MDI blocked B 2,5 g MDI 
blocked B 5,1 g composition EVA blocked A 5,7 g blocked A 22,6 g DLDBSn 
0,1 g DERTOPHENE T 50 g DLDBSn 0,1 g DLDBSn 0,1 g 
##STR3## 
0 0,25 1 0,25 0 0,25 0,5 creep resistance (S.A.F.T.) 90 145 &gt;225 &gt;225 
80 140 &gt;225 in .degree.C. Lap Shear Test (MPa) at 60.degree. C. 1,2 
1,5 1,9 2,3 0,4 0,9 1,7 at 80.degree. C. 0,1 0,3 0,4 0,5 0,1 0,3 0,5 T 
peel test (daN/cm) at 60.degree. C. 0,7 0,85 1,3 1,0 0,3 0,8 0,4 at 
80.degree. 
EVA: Terpolymer by weight: Ethylene 72%, Vinyl acetate 23%, Hydroxyethyl 
acetate 5% 
MDI blocked A: MDI blocked with a terpene phenolic resin 
MDI blocked B: MDI blocked with a epsilon caprolactam 
DERTOPHENE T: Terpene phenolic resin of DRT