Rubber-based adhesive/sealant

Abstract: A hot-vulcanisable, pumpable rubber-based adhesive/sealant where the rubber component is a mixture of butadiene solid rubber and butadiene or isoprene liquid rubber. The liquid rubber contains carboxylic acid groups. Use of the adhesive/sealant for bonding metal sheeting in the construction of automobile shells.

Description: The invention relates to a hot-vulcanisable, pumpable rubber-based adhesive/sealant 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 metre, 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 to 80 ° 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 to 200°C, as are usual in the baking ovens used in automobile production. For the abovementioned 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. 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-mofified 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. 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 butadiene or isoprene liquid rubber contains carboxylic acid groups and has more than 70 mole % of cis-1,4 double bonds and no more than 2 mole % of vinyl double bonds, and in that the butadiene or isoprene liquid rubber represents from 10 to 20% by weight of the total weight of the adhesive/sealant whereas the butadiene solid rubber represents from 1 to 15% by weight of the total weight of said adhesive/sealant. 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. The carboxylic acid-modified butadiene or isoprene liquid rubbers used according to the invention have a molecular weight 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°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, more than about 70 mole % of cis-1,4 double bonds and having not more than about 2 mole % of vinyl double bonds, based on the total double bonds of the molecule. The carboxylic acid groups of butadiene or isoprene liquid rubber are generally introduced by copolymerisation of carboxylic acids containing from 3 to 10, preferably 4 to 6, carbon atoms, for example, 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 either polybutadiene or a butadiene-styrene rubber having a molecular weight M 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 % of cis-1,4 double bonds. The styrene-butadiene solid rubber is a random copolymer containing from about 15 to 25 mole % 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 to 200°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. 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°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°C. After cooling, the shear strength is measured in accordance with DIN 53281 on a bonded area of 2.5 X 0.2 cm and the T-peel is measured in accordance with DIN 53283 on a bonded area of 2.5 X 7.5 X 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 n 1 700 Viscosity 6 Pa.s (at 20°C) 1.1 Anhydride groups per molecule more than 70 mole % of cis-1,4 double bonds, less than 2 mole % of vinyl double bonds. Maleic anhydride-modified polybutadiene oil 2 Molecular weight M n 1 700 Viscosity 220 Pa.s (at 20 °C) 2.3 Anhydride groups per molecule more than 70 mole % of cis-1,4 double bonds, less than 2 mole % of vinyl double bonds. Polybutadiene oil Molecular weight M n 3 000 Viscosity 3 Pa.s (at 20°C) 75 mole % of cis-1,4 double bonds, 1 mole % of vinyl double bonds and 24 mole % of trans-1,4 double bonds Hydroxyl-modified polybutadiene oil Molecular weight M n 2 800 Viscosity 4 Pa.s (at 20 °C) 2.4 OH groups per molecule 60 mole % of trans-1,4 double bonds, 20 mole % of cis-1,4 double bonds, 20 mole % of vinyl double bonds. Butadiene solid rubber Molecular weight M n 95 000 Mooney viscosity ML (1 + 4) at 100°C : 46 95 mole % of cis-1,4 double bonds, 3 mole % trans-1,4 double bonds, and 2 mole % of vinyl double bonds. Butadiene-styrene solid rubber Molecular weight M n 100 000 Mooney viscosity ML (1 + 4) at 100°C : 50 Styrene content 23.5 % Vulcanisation accelerator Dibenzothiazole disulphite Aliphatic carboxylic acid Stearic acid Oxidation stabiliser Octadecyl 3-(3,5-di-ter.butyl-1,4-hydroxyphenyl)-propionate Constituent % by weight preferred Liquid rubber containing carboxylic acid groups 10 to 20 12 to 18 Butadiene solid rubber 1 to 15 2 to 10 Fillers 40 to 70 50 to 70 Plasticisers 4 to 15 6 to 10 Adhesion promoters 0 to 8 0 to 5 Oxidation stabilisers 0.5 to 2 0.5 to 1 Sulphur 1 to 5 1 to 3 Vulcanisation Accelerator 0.5 to 3 0.5 to 2 Aliphatic carboxylic acid 0.2 to 0.8 0.2 to 0.5 Zinc oxide 2 to 8 2 to 5 or Free-radical initiators 0.05 to 2 0.1 to 1 Zinc oxide 2 to 8 2 to 4

Claims: A hot-vulcanisable, pumpable rubber-based adhesive/sealant where the rubber component is a mixture of butadiene solid rubber and butadiene or isoprene liquid rubber, characterised in that : the butadiene or isoprene liquid rubber contains carboxylic acid groups and has more than 70 mole % of cis-1,4 double bonds and no more than 2 mole % of vinyl double bonds, and in that the butadiene or isoprene liquid rubber represents from 10 to 20% by weight of the total weight of the adhesive/sealant whereas the butadiene solid rubber represents from 1 to 15% by weight of the total weight of said adhesive/sealant. A hot-vulcanisable, pumpable rubber-based adhesive/sealant according to claim 1, characterised in that the carboxylic acid groups in the liquid rubber are in the anhydride form. A hot-vulcanisable, pumpable rubber-based adhesive/sealant according to claim 1 or to claim 2, characterised in that the liquid rubber has a molecular weight M n of from 1.000 to 10.000. A hot-vulcanisable, pumpable rubber-based adhesive/sealant according to anyone of claims 1 to 3, characterised in that the liquid rubber is a butadiene liquid rubber. A hot-vulcanisable, pumpable rubber-based adhesives/sealant according to claim 4, characterised in that the butadiene liquid rubber contains maleic anhydride groups. A hot-vulcanisable, pumpable rubber-based adhesive/sealant according to anyone of claims 1 to 5, characterised in that each liquid rubber molecule contains from 1 to 3 carboxylic anhydride groups. A hot-vulcanisable, pumpable rubber-based adhesive/sealant according to anyone of claims 1 to 6, characterised in that it comprises a vulcanisation system comprising sulphur and a vulcanisation accelerator. Use of a hot-vulcanisable, pumpable rubber-based adhesive/sealant according to anyone of claims 1 to 7 for bonding metal sheeting in the construction of automobile shells.

Inventors: KOCH MARKUS, KOCH, MARKUS
Applicants: JOINT FRANCAIS, LE JOINT FRANCAIS SNC
Application year: 1992
Main classification: C09J109
IPC codes: C09J121, C09K3, C09J109, C09J113