Adhesive/sealant composition comprising a rubber component

A hot-vulcanizable, pumpable rubber-based adhesive/sealant where the rubber component is a mixture of butadiene solid rubber and butadiene or isoprene liquid rubber, and wherein the liquid rubber comprises carboxylic acid groups. The adhesive/sealant compositions of the invention are particularly useful for bonding metal sheeting in the construction of automobile shells.

BACKGROUND OF THE INVENTION 
The present invention relates to a hot-vulcanisable, pumpable rubber-based 
adhesive/sealant suitable for bonding metal sheeting in the construction 
of automobile shells. 
The bonding of metal sheeting in the construction of automobile shells has 
the objective of sealing cavities and suppressing any vibration which 
occur. The sheeting used is coated with a number of grams of 
corrosion-protection oils and drawing oils per square meter, and should be 
able to absorb these oils. In order to simplify processing and to improve 
oil absorption, the adhesive/sealant is pumped at temperatures of from 
about 50.degree. to 80.degree. C. and applied to one side of the sheeting 
to be bonded. The second sheet is then pressed against the first sheet. It 
should also be ensured here that the second sheet is not applied until the 
adhesive/sealant applied has cooled and that, nevertheless, good oil 
absorption and adhesion are achieved. The adhesive/sealant must be stable 
for a relatively long time at the processing temperatures. Curing should 
not take place at temperatures below from about 150.degree. to 200.degree. 
C., as are usual in the baking ovens used in automobile production. 
For the above-mentioned purpose, the use of solvent-free, pumpable, 
rubber-based adhesives/sealant is known, for example in EP-A-0 097 394, 
where the rubber component comprises a liquid polybutadiene, to which 
small amounts of solid polybutadiene can be admixed. However, such 
adhesives/sealants do not achieve the good adhesion required in automobile 
production. As used herein, the term "pumpable" refers to a composition 
that may be pumped, by means of conventional pumps for hot-melt 
substances, at elevated temperatures such as about 60.degree. C. 
EP-A-0 256 316 describes adhesives/sealants having a rubber component 
comprising polybutadiene liquid rubber and chemically depolymerised solid 
rubber. The depolymerisation of the solid rubber is said to give better 
rheological properties. On the other hand, depolymerisation of the solid 
rubber has the disadvantage of an additional process step. 
DE-A-38 34 818 describes an adhesive/sealant whose rubber component 
comprises polybutadiene liquid rubber containing terminal hydroxyl groups 
and cis-1,4-polybutadiene solid rubber. 
DE-A 27 58 222 discloses pressure-sensitive adhesives for self-adhesive 
tapes containing tackifying resins, for example polyterpenes or 
polybutene, and a maleic anhydride-modified liquid polybutadiene. Curing 
is carried out at room temperature by means of polyamines or 
poly-epoxides. 
Finally, EP-A 0 181 441 discloses cold-curing sealants/adhesives which 
contain, as binders, hydroxyl-containing homopolymers or copolymers of 
1,3-butadiene which carry lateral succinic anhydride groups. Curing is 
carried out using organic nitrogen bases selected from amines and 
N-alkylimidazoles. 
SUMMARY OF THE INVENTION 
Against the background of the abovementioned prior art, the object of the 
invention is now to provide a hot-vulcanisable, pumpable adhesive/sealant 
for bonding metal sheeting in the construction of automobile shells which 
has improved overall properties. 
This object is achieved by an adhesive/sealant, where the rubber component 
is a mixture of butadiene solid rubber and butadiene or isoprene liquid 
rubber, characterised in that the liquid rubber contains carboxylic acid 
groups. 
The butadiene or isoprene liquid rubber, also referred to below as 
polybutadiene oil or polyisoprene oil, present in the adhesive/sealant 
according to the invention contains carboxylic acid groups, which are 
preferably in the form of carboxylic anhydride. Although the use of a 
polybutadiene liquid rubber modified in this way is already known from the 
sealants/adhesives of DE-A 27 58 222 and EP-A 0 181 441 mentioned above, 
these are cold-curing systems, where curing takes place by reaction of 
carboxylic acid groups with polyamines. By contrast, the adhesive/sealant 
of the invention is a hot-vulcanisable system, where vulcanisation or 
crosslinking takes place by the reaction of the double bonds of butadiene 
or isoprene, for example by means of a sulphur/accelerator system, or by 
means of free-radical initiators, preferably peroxides. 
DETAILED DESCRIPTION OF THE INVENTION 
The carboxylic acid-modified butadiene or isoprene liquid rubbers used 
according to the invention have a molecular weight M.sub.n of from about 
500 to 20,000, preferably from about 1,000 to 10,000 and more preferably 
from about 1,500 to 5,000 and preferably have a viscosity at 20.degree. C. 
of from 1 to 250 Pa.s. 
In order to achieve low viscosity and good adhesion, it is advantageous to 
employ liquid rubbers having a high content of cis-1,4 double bonds, 
preferably more than about 70 mole percent of cis-1,4 double bonds, based 
on the total double bonds of the molecule, and having not more than about 
2 mole percent of vinyl double bonds, based on the total double bonds of 
the molecule. The term vinyl double bond refers to to a unit of a liquid 
rubber molecule which is polymerized through the double bond in the 
1-position, resulting in a vinyl double bond attached to the polymer 
backbone. Thus, a vinyl double bond refers to a 1,2-polymerized butadiene 
in a polybutadiene liquid rubber molecule. The polybutadiene of the 
following formula (I) illustrates such groups, where the unit [x] depicts 
a cis-1,4 double bond, the unit [y] depicts a trans-1,4 double bond, and 
the unit [z] depicts a vinyl double bond. 
##STR1## 
The carboxylic acid groups of butadiene or isoprene liquid rubber generally 
contain from 3 to 10, preferably 4 to 6, carbon atoms and are derived, for 
example, from acrylic acid, methacrylic acid, maleic acid, fumaric acid, 
citraconic acid, itaconic acid or the anhydrides thereof, so long as the 
acids are able to form anhydrides. They are randomly distributed over the 
polymer molecule, each polymer molecule containing from 1 to 10, 
preferably from 2 to 6, carboxylic acid groups or from 1 to 6, preferably 
from 1 to 3, carboxylic anhydride groups. 
In a particular embodiment, the adhesives/sealants of the invention may 
also contain, in addition to the carboxylic acid-modified liquid rubber, a 
non-modified liquid rubber, provided that a 50/50 weight ratio should not 
be exceeded. However, since the desired good adhesion properties fall off 
with decreasing amounts of carboxylic acid-modified liquid rubber, this 
embodiment is less preferred. 
The butadiene solid rubber is suitably either polybutadiene or a 
butadiene-styrene rubber having a molecular weight M.sub.n of from about 
50,000 to 500,000, preferably from about 80,000 to 200,000. The 
polybutadiene contains predominantly cis-1,4 double bonds, preferably at 
least about 95 mole percent of cis-1,4 double bonds, based on the total 
double bonds of the molecule. The styrene-butadiene solid rubber is a 
random copolymer containing from about 15 to 25 mole percent of styrene. 
The adhesives/sealants of the invention may contain conventional fillers, 
other additives and processing assistants in conventional amounts. 
Examples of suitable fillers are coated calcium carbonate, calcium oxide 
and carbon black. It is particularly preferred to use a mixture of these 
three fillers which contains from about 23 to 70% by weight, preferably 
from about 30 to 60% by weight and in particular from about 50 to 60% by 
weight, of coated calcium carbonate, from about 1 to 5% by weight, 
preferably from about 2 to 5% by weight and in particular from about 3 to 
5% by weight, of calcium oxide and from about 1 to 10% by weight, 
preferably from about 1 to 5% by weight and in particular from about 3 to 
5% by weight, of carbon black. 
Conventional plasticisers may also be used; it is preferred to use 
paraffinic, aromatic or in particular naphthenic oils. 
It is also possible for the adhesive/sealant of the invention to contain 
adhesion promoters so long as these do not undergo chemical reactions with 
the other components of the composition. Examples of suitable adhesion 
promoters are phenolic resins and silanes. 
The adhesives/sealants according to the invention are cured by hot 
vulcanisation at baking temperatures of, for example, from about 
150.degree. to 200.degree. C. which are conventional in the construction 
of automobile shells; it is preferred to use a sulphur/accelerator 
vulcanisation system, but it is also possible to use free-radical 
initiators. 
Examples of suitable accelerators are benzothiazoles, thiurams, 
dithiocarbamates, thioureas or mixtures thereof. Benzothiazoles, for 
example dibenzothiazole disulphide, are preferred. 
Examples of suitable thermal free-radical initiators are organic peroxides, 
in particular those having high decomposition temperatures. Preferred 
examples are dicumyl peroxide and 2,5-dimethyl-2,5-bis (t-butylperoxy) 
hexane. 
Table I below shows the composition of some typical adhesives/sealants 
according to the invention wherein amounts are expressed as percents by 
weight of the mixture. 
All documents mentioned herein are incorporated herein by reference.

The following examples illustrate the invention without limitation. 
The compositions are prepared in a Pfleiderer compounder. First, the 
butadiene solid rubber is kneaded with the fillers, the oxidation 
stabiliser and the plasticiser until the mixture is homogeneous, the 
carboxylic acid-modified liquid rubber is then added, and finally the 
vulcanisation chemicals (zinc oxide, sulphur, accelerator and aliphatic 
carboxylic acid) are incorporated into the composition with cooling. 
Oiled steel is used to test the adhesives/sealants. 
For processing the adhesive/sealant is first warmed to a temperature of 
60.degree. C. and then applied to the oiled steel sheeting in a bead 
thickness of 4 mm. The second sheet is pressed on, the adhesive/sealant 
bead is compressed to a layer thickness of 2 mm, and curing is carried out 
for 30 minutes at a temperature of 180.degree. C. After cooling, the shear 
strength is measured in accordance with DIN 53281 on a bonded area of 
2.5.times.0.2 cm and the T-peel is measured in accordance with DIN 53283 
on a bonded area of 2.5.times.7.5.times.0.2 cm. The failure mechanism is 
assessed visually as follows: 
C=cohesive 
C/A=predominantly cohesive, partly adhesive 
A/C=predominantly adhesive, partly cohesive 
The results are shown in Table II, the following starting materials being 
used: 
Maleic Anhydride-Modified Polybutadiene Oil 1 
Molecular weight M.sub.n 1,700 
Viscosity 6 Pa.s. (at 20.degree. C.) 
1.1 Anhydride groups per molecule 
more than 70 mole percent of cis-1,4 double bonds (based on total double 
bonds of molecule), less than 2 mole percent of vinyl double bonds (based 
on total double bonds of molecule). 
Maleic Anhydride-modified polybutadiene oil 2 
Molecular weight M.sub.n 1,700 
Viscosity 220 Pa.s. (at 20.degree. C.) 
2.3 Anhydride groups per molecule 
more than 70 mole percent of cis-1,4 double bonds (based on total double 
bonds of molecule), less than 2 mole percent of vinyl double bonds (based 
on total double bonds of molecule). 
Polybutadiene oil 
Molecular weight M.sub.n 3,000 
Viscosity 3 Pa.s. (at 20.degree. C.) 
75 mole percent of cis-1,4 double bonds (based on total double bonds of 
molecule), 1 mole percent of vinyl double bonds (based on total double 
bonds of molecule), and 24 mole percent of trans-1,4 double bonds (based 
on total double bonds of molecule). 
Hydroxyl-Modified Polybutadiene Oil 
Molecular weight M.sub.n 2,800 
Viscosity 4 Pa.s. (at 20.degree. C.) 
2.4 OH groups per molecule 
60 mole percent of trans-1,4 double bonds (based on total double bonds of 
molecule), 20 mole percent of cis-1,4 double bonds (based on total double 
bonds of molecule), 20 mole percent of vinyl double bonds (based on total 
double bonds of molecule). 
Butadiene Solid Rubber 
Molecular weight M.sub.n 95,000 
Mooney viscosity ML (1+4) at 100.degree. C.: 46 
95 mole percent of cis-1,4 double bonds (based on total double bonds of 
molecule) and 3 mole percent trans-1,4 double bonds (based on total double 
bonds of molecule), 2 mole percent vinyl double bonds (based on total 
double bonds of molecule). 
Butadiene-Styrene Solid Rubber 
Molecular weight M.sub.n 100,000 
Mooney viscosity ML (1+4) at 100.degree. C.: 50 
Styrene content 23.5 mole percent 
Vulcanisation Accelerator 
Dibenzothiazole disulphite 
Aliphatic Carboxylic Acid 
Stearic acid 
Oxidation stabiliser 
Octadecyl 3-(3,5-di-ter.butyl-1,4-hydroxyphenyl)-propionate 
TABLE I 
______________________________________ 
% by weight 
Particularly 
Constituent General Preferred preferred 
______________________________________ 
Liquid rubber 
8 to 30 10 to 20 12 to 18 
containing carboxylic 
acid groups 
Butadiene solid rubber 
1 to 20 1 to 15 2 to 10 
Fillers 25 to 75 40 to 70 50 to 70 
Plasticisers 0 to 20 4 to 15 6 to 10 
Adhesion promoters 
0 to 10 0 to 8 0 to 5 
Oxidation stabilisers 
0 to 2 0.5 to 
2 0.5 to 
1 
Sulphur 1 to 10 1 to 5 1 to 3 
Vulcanisation 
0.5 to 5 0.5 to 
3 0.5 to 
2 
Accelerator 
Aliphatic carboxylic 
0 to 1 0.2 to 
0.8 0.2 to 
0.5 
acid 
Zinc oxide 2 to 10 2 to 8 2 to 5 
or 
Free-radical initiators 
0.05 to 3 0.05 to 
2 0.1 to 
1 
Zinc oxide 0 to 10 2 to 8 2 to 4 
______________________________________ 
TABLE II 
______________________________________ 
Comp 
(Ger. 
Constituents, Pat. No. 
% by weight 
1 2 3 4 38 34 818) 
______________________________________ 
Maleic anhydride- 
12.0 12.0 -- 15.0 -- 
modified poly- 
butadiene oil 1 
Maleic anhydride- 
-- -- 6.0 -- -- 
modified poly- 
butadiene oil 2 
Polybutadiene oil 
-- -- 6.0 -- -- 
Hydroxyl- -- -- -- -- 12.0 
modified poly- 
butadiene oil 
Butadiene solid 
5.0 -- 5.0 10.0 5.0 
rubber 
Butadiene-styrene 
-- 5.0 -- -- -- 
solid rubber 
Plasticiser 
10.0 10.0 10.0 6.0 10.0 
Chalk 55.0 55.0 55.0 50.0 55.0 
Carbon black 
5.0 5.0 5.0 5.0 5.0 
Calcium oxide 
5.0 5.0 5.0 5.0 5.0 
Oxidation 0.8 0.8 0.8 0.8 0.8 
stabiliser 
Sulphur 2.0 2.0 2.0 2.0 2.0 
Vulcanisation 
0.8 0.8 0.8 0.8 0.8 
accelerator 
Zinc oxide 4.0 4.0 4.0 4.0 4.0 
Aliphatic 0.4 0.4 0.4 0.4 0.4 
carboxylic acid 
Properties: 
Shear strength 
1.3 1.3 1.3 1.9 0.9 
[N/mm.sup.2 ], oiled 
steel 
Failure C C C/A C A/C 
mechanism 
T-peel, oiled steel 
148/48 143/62 129/52 
200/50 
122/48 
[N/25 mm] 
commencement 
of cracking/mean 
force 
Failure C C C/A C A/C 
mechanism 
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