Use of grafted ethylene-vinylacetate copolymers for the production of hotmelt adhesives

The invention relates to the use of grafted ethylene-vinylacetate copolymers (EVA-copolymers) for the production of hotmelt adhesives.

This invention relates to the use of grafted ethylene-vinylacetate 
copolymers (EVA copolymers) for the production of hotmelt adhesives. 
Many different homopolymers, copolymers and graft copolymers have been 
proposed as polymers for the production of adhesives. Mixtures thereof 
with natural or synthetic resins and waxes are suitable for hotmelt 
adhesives. It is also known that certain ethylene-vinylacetate copolymers 
are suitable for the production of hotmelt adhesives of the type used for 
glueing wood or paper. 
However, advances in mechanization are imposing increasingly more stringent 
demands upon the adhesion properties of adhesives, with the result that 
adhesion to a variety of different substrates, for example to metals, is 
of considerable interest. In this connection, it would be desirable to 
combine the advantages of a hotmelt adhesive, for example absence of 
solvents, quick setting and simple application, with firm adhesion to 
metals. 
To this end, it has already been proposed to introduce acid groups into 
ethylene-vinylacetate copolymers through the incorporation by 
polymerization of acrylic acid, maleic acid or maleic acid anhydride. 
However, although some improvement in adhesion to metals is obtained in 
this way, it is still not entirely satisfactory. 
The grafting of monomers containing carboxyl groups onto 
ethylene-vinylacetate copolymers has also been described. 
Thus, DE-OS No. 23 51 155 and DE-OS No. 24 16 614 for example propose graft 
polymers of carboxyl group-containing monomers on ethylene-vinylacetate 
copolymers for use in hotmelt adhesives. The graft copolymers are produced 
using a radical initiator. 
It has now been found that ethylene-vinylacetate copolymers, which 
following the introduction of peroxide groups into the polymer are grafted 
with polymerizable carboxylic acids, may be used for the production of 
hotmelt adhesives. 
Accordingly, the present invention relates to the use of graft copolymers 
composed of 90 to 99.5% by weight of a graft base, consisting of a 
copolymer of 50 to 85% by weight of ethylene units and 15 to 50% by weight 
of vinylacetate units together with optionally 0 to 10% by weight of units 
of other unsaturated compounds and of 0.5 to 10% by weight of monomer 
units grafted on by the process according to DE-OS No. 27 34 105 and 
consisting of 80 to 100% by weight of methacrylic acid units and 
optionally of 0 to 20% by weight of units of other unsaturated monomers, 
for the production of hotmelt adhesives. 
The graft copolymers used in accordance with the invention are composed of 
90 to 99.5% by weight of a graft base, consisting of a copolymer of 50 to 
85% by weight of ethylene units, preferably 55 to 80% by weight of 
ethylene units, and of 15 to 50% by weight of vinyl-acetate units, 
preferably 20 to 45% by weight of vinyl-acetate units, together with 0 to 
10% by weight of units of other unsaturated compounds, and of 0.5 to 10% 
by weight of grafted-on monomer units consisting of 80 to 100% by weight 
of methacrylic acid units and 0 to 20% by weight of units of other 
unsaturated monomers, grafting being carried out by the process according 
to DE-OS No. 27 34 105. 
Graft copolymers of the type obtained by graft polymerization of the 
unsaturated acids in the conventional monomer in the presence of the graft 
substrate and a radical former do not show these favourable properties 
when used for the production of hotmelt adhesives. 
In the graft polymers used in accordance with the invention, the peroxide 
groups in the graft substrate are introduced by perioxygenation in the 
melt using the process according to DE-OS No. 27 34 105, the reaction 
being carried out either in a reaction screw or in kneaders. 
The ethylene-vinylacetate copolymers used should contain from 15 to 50 % by 
weight of vinylacetate units and ethylene-vinylacetate copolymers 
containing from 20 to 45% by weight of vinylacetate units are preferred. 
In addition to vinylacetate and ethylene, the graft substrate may also 
contain up to 10% by weight of other unsaturated compounds, for example 
acrylic acid or acrylates, methacrylates or other copolymerizable 
unsaturated compounds in copolymerized form. 
Ethylene-vinylacetate copolymers are preferably used as the graft 
substrate. 
Ethylene-vinylacetate copolymers having a melt index of from 0.5 to 1000 
g/10 mins (as measured in accordance with DIN 53 735 at 190.degree. C. 
under a load of 2.16 kg) are preferably used for producing the graft 
copolymers used in accordance with the invention. Ethylenevinylacetate 
copolymers having melt indices of from 1 to 500 g/10 mins (as measured in 
accordance with DIN 53 735 at 190.degree. C. under a load of 2.16 kg) are 
preferably used. 
The graft polymers used in accordance with the invention are produced by 
the grafting process described in DE-OS No. 27 34 105. If, for example, 
the ethylene-vinylacetate copolymers used are peroxygenated with 
atmospheric oxygen under pressure in the melt, products containing from 50 
to 3000 ppm of active oxygen are obtained. During this reaction, the graft 
base may even undergo oxidative degradation. This can be advantageous 
because the melt index of the graft copolymer and the application 
properties of the hotmelt adhesive are favourably influenced in this way. 
The peroxide-containing polymers thus obtained are then used in the same 
process step for polymerizing the graft monomers. In this way, a large 
number of grafted-on side chains are introduced into the graft substrate. 
From 80 to 100% by weight of the grafted-on monomer units consist of 
methacrylic acid. They may contain from 0 to 20% by weight of other 
unsaturated compounds, for example butylacrylates and styrene. 
The adhesives used in accordance with the invention are produced by mixing 
the ethylene-vinylacetate graft copolymers with tackifying resins, 
optionally in the presence of, for example, antiagers, light stabilizers, 
plasticizers, waxes, fillers and pigments. 
The tackifying resins used are, for example, modified or unmodified 
colophony resins, such as rosin, hydrogenated rosin or dimerized rosin or 
esters thereof with alcohols, such as glycerol or pentaerythritol, 
hydrocarbon resins, such as coumarone or indene resins, or other resins 
which tackify the mixture. 
Mixing of the components is carried out in known manner in mixers, kneaders 
or extruders at elevated temperatures, normally at temperatures in the 
range of from 100.degree. to 250.degree. C. However, the components may 
also be combined at room temperature and subsequently homogenized by 
increasing the temperature. 
The hotmelt adhesives produced in accordance with the invention are 
distinguished by their favourable adhesion to metals. They are 
particularly suitable for metal-to-metal bonding and for bonding metals to 
other substrates, such as plastics or natural materials. Other materials 
may of course also be bonded using the adhesives according to the 
invention. 
The formation of bonds of the type in question is known and is carried out, 
for example, by applying the molten adhesive to the substrate, followed by 
pressing, or by making a film from the hotmelt adhesive and heating it 
between the parts to be bonded.