Technical field The present invention relates to elastic ad- hesive compositions.

Background Art The long term performance of bonding is de- pendent on the elastic behavior of the adhesive. The first-generation meth (acrylic) adhesives are brittle, with very low elongation at break. Many approaches are known to increase the flexibility of methacrylic adhe- sives.

One of said efforts to improve flexibility has led to the addition of solid non-reactive elastomers that are dissolved in the meth (acrylic) monomers. Such compositions are called rubber-toughened adhesive compo- sitions. The elastomers are solids at room temperature, and commercially available as large particles or granu- lates.

This approach is described in several patent documents, such as: US 3,890,407, US 4,106,971, US 4,263,419, US 3,725,504, US 4,200,480, US 3,994,764, EP 0 641 846. The disadvantage of such formulations is that they can only be mixed homogeneously with special equip- ment like extruders or kneaders due to the high viscosity of the polymer that is formed in the monomer solution.

Moreover, this type of compositions suffers from the limitation that the monomer, or the monomer mixture, must be chosen such that the non-reactive thermoplastic poly- mer is soluble therein. Practically, methyl methacrylate is the only monomer with high dissolving properties, and thus the only monomer that allows a polymer-in-monomer composition with up to 30% polymer content. Such low mo- lar mass monomers, like methyl methacrylate, have a strong odor and are highly flammable. Polymer-in-monomer compositions resulting from this approach have a rubbery, stringy consistency and a high viscosity that makes their handling difficult. Due to the high viscosity of the mix- ture, only a limited amount of fillers can be used making the formulation expensive. A high viscosity is also lim- iting the adhesion of the formulation, as it limits the wetting of the substrate.

A further approach is the addition of liquid, low-molar mass elastomers that dissolve in the monomers.

A number of patent documents describe such approach to increase the flexibility of the systems by adding liquid reactive elastomers to the reactive monomer (mixtures) to increase the flexility of the adhesives, namely US 4,769,419; US 4,331 ; 765 and EP 0,561,352 disclosing mix- tures of monomers and liquid rubber.

DE 2,610,423; US 4,439,600; EP 0,640,672; DE 2,319,637 and US 4 223 115 disclose mixtures of monomers and acrylic functionalised polyurethanes. In US 4,223,115 the composition optionally contains a dissolved elas- tomer, e. g. NBR (nitrile-butadiene-rubber), polychloro- prene.

Flexibility is measured in terms of elonga- tion at break. During deformation, however, all the above mentioned prior-art adhesives show a visco-elastic behav- ior with a significant viscous component. This means that during the deformation the binder matrix of the adhesive shows plastic flow and is being damaged. The visco- elastic behavior of the material is reflected in the

shape of the stress-strain curve, measured at an elonga- tion speed relevant to the practical application. The plot of the prior-art adhesives is not linear, and this even for the compositions of US 4,439,600 (see above) al- though elastic behavior is claimed. The plastic flow com- ponent during deformation reduces the number of deforma- tion cycles an adhesive can perform before failing. This significantly reduces the life span of an adhesive bond under dynamic load.

It was therefore an object of the present in- vention to provide meth (acrylic) adhesive compositions with a high elasticity, measured in terms of elongation at break, and an elastic behavior approaching the ideal Hook law in the stress-strain diagram.

Disclosure of the Invention Thus one object of the present invention is an adhesive composition comprising (a) at least one monofunctional meth (acrylic) monomer A whose homo-polymer or co-polymer exhibit a glass transition temperature (Tg) between 40°C and 140°C, (b) at least one monofunctional meth (acrylic) monomer B of the following structure (B): wherein R is H or CH3, R'is H or (CH2) nCH3, with n = 0 to 2, in particular CH2CH3 or H, and

R"is C3-C20-alkyl or phenoxy or 0- (CH2) n- CH3, or 0- [(CH2) 2~o] n-cH2-cH3 with n = 0 to 2, in par- ticular (CH2) nCH3, wherein n is 3 or 10-13, and c) at least one liquid elastomer C in the mo- lecular weight range of 1000 to 9000 with (meth) acrylic derivative groups, preferably (meth) acrylic ester groups, whereby the components A, B, and C are pres- ent in the following amounts: B based on the total weight of A+B+C = 5 to 20 % by weight, preferably 5 to 12 % by weight, C based on the total weight of A+C = 30 to 70 % by weight, preferably 40 to 60 % by weight.

Much preferred components of structure (B) are those wherein R'is CH2CH3 and R"is (CH2) 3CH3 or R'is H and R"is (CH2) nCH3 with n = 10-13.

Brief Description of the Drawings The invention will be better understood and objects other than those set forth above will become ap- parent when consideration is given to the following de- scription thereof and the annexed Figures, wherein: Figure 1 shows a stress-strain diagram for the inventive composition of Example 1.

Figure 2 shows a stress-strain diagram for the inventive composition of Example 2.

Figure 3 shows a stress-strain diagram for the inventive composition of Example 3.

Figure 4 shows a stress-strain diagram for the comparative composition of Example 4.

Figure 5 shows a stress-strain diagram for the state of the art composition of Example 5.

Modes for Carrying Out the Invention

In a preferred embodiments of the inventive adhesive composition, the monofunctional meth (acrylic) monomer A or mixture of meth (acrylic) monomers A whose homo-polymer or co-polymer exhibit a glass transition temperature (Tg) between 40°C and 140°C is a methacrylate ester, preferably an alkyl ester with a linear or branched or cyclic C1-C6 alkyl, or a heterocyclic or aro- matic ester, much preferred a methacrylate ester selected from the group consisting of methyl methacrylate (MMA), tetrahydrofurfuryl methacrylate (THFMA), cyclohexyl- methacrylate (CHMA), cyclic trimethylolpropane formal ac- rylate (CTFA), isobornylmethacrylate (IBMA), benzyl- methacrylate (BMA), dicyclopentadienyloxyethyl- methacrylate (DCPOEMA), t-butylmethacrylate (tBMA), iso- bornylacrylate (IBA), dihydrodicyclopentadienylacrylate (DHDCPA), and mixtures thereof.

In another preferred embodiment of the inven- tive composition, the monofunctional (meth) acrylic mono- mer B of formula (B) is a (meth) acrylic ester, such as an ester selected from the group consisting of linear and branched C6-Cl5-alkyl esters, in particular an acrylic ester such as lauryl acrylate, 2-ethyl hexyl acrylate, and mixtures thereof. Preferred (meth) acrylates lead to homopolymers and copolymers with a preferred Tg of <40°C.

In yet another preferred embodiment, the in- ventive adhesive composition contains a liquid elastomer C with ethylenically unsaturated groups which is chosen from the group consisting of - (meth) acrylic functionalized butadiene, isoprene based polymer or block-copolymer, - PU- (meth) acrylate obtainable through the syntheses of a polyethylene polyol or polypropylene polyol, a diisocyanate and a hydroxy functionalyzed eth- ylenically unsaturated monomer, whereby said PU (meth) acrylate is preferably obtainable through a synthe- sis using polyols with low unsaturation and narrow mo- lecular weight distribution as obtainable through double

metal cyanide complex catalysis (such polyols are e. g. known as Acclaim polyols).

The compositions of the present invention usually also comprise at least one initiator and/or cata- lyst, as well as preferably also at least one organic or inorganic filler or thixotropic agent. The compositions can also comprise further substances, such as stabiliz- ers, additives, toughening agents, adhesion promotors, impact modifiers, core-shell polymers, defoaming agents, thickeners, plasticizers, wetting agents, wax com- pounds, cross-linking agents, inhibitors etc. Such addi- tional substances are known to the skilled person. Exam- ples for free radical initiators are organic peroxides, in particular benzoylperoxide, and examples for catalysts are tertiary amines and/or salts and/or complexes of transition metals. Examples for tertiary amines are e. g N, N-dimethylaniline, N, N-dimethyl-p-toluidine, N, N- diethylaniline, N, N-diethyltoluidine, N, N-bis (2- hydroxyethyl)-p-toluidine, N-ethoxylated p-toluidine, N- alkylmorpholine or mixtures thereof, and examples for the salts and complexes of transition metals are salts and complexes of cobalt, nickel and/or copper. Examples for inhibitors are hydrochinone, methylhydrochinone, t-butyl- p-cresol and for thixotropic agents e. g. Aerosil.

A further advantage of the compositions of the present invention is that they can be easily manufac- tured, namely by simply mixing monomer (s) A, monomer (s) B and liquid elastomer and an initiator and/or catalyst.

Said mixture is preferably obtained at ambient tempera- ture for a time sufficient to get a homogeneous liquid (usually about 10 to 20 minutes). Obtaining a homogeneous liquid, e. g. at 40°C, usually takes about 15 minutes.

Then, preferably also present filler (s) are added and the composition is mixed at high revolutions per minute until the material shows thixotropic behavior.

The compositions of the present invention are especially suitable for binding applications, in particu-

lar bonding applications with materials having different thermal expansion coefficients as they are e. g. found in motor vehicles, e. g. trucks, and rail cars. Examples of such applications are bonding of side panels of trailers or direct glazing.

The ideal elastic behavior over a wide range of elongations obtained with the compositions of the pre- sent invention increases significantly the performance of the bond under dynamic load. Furthermore, the composi- tions show an excellent adhesion on many substrates with- out pre-treatment, and thus are easy to manufacture due to their low viscosity. The exceptionally high degree of elasticity imparts them excellent impact resistance at low temperatures, even without impact modifiers. Also their close to ideal elastic behavior imparts them an ex- cellent recovery behavior. Their very high elasticity can be maintained even when the formulations are filled with commonly known fillers.

Examples In the compositions used in the examples 1 to 5, the abbreviations have the following meanings: MMA = methyl methacrylate THFMA = tetrahydrofuryl methacrylate LA = lauryl acrylate PUE = prepolymer PUA p-Tol = N, N-bis (2-dihydroxyethyl)-p- toluidine BPO paste = a past of 40 % benzoyl peroxide in phthalate plasticizer The compounds used to formulate the adhesive compositions of examples 1 to 5 and their amounts are listed in Table 1.

Preparation of Prepolymer PUA To 800 g of a diol with mw 4000 (e. g. Acclaim 4200 from Bayer), 88.6 g of IPDI (e. g. Vestanat IPDI from Bayer) together with 0.05% of dibutyltin dilaurate (DBTL) are added. The reaction mixture is stirred at 80°C for about 4 hours under nitrogen. Then 64 g of 2-hydroxy- ethyl methacrylate (HEMA) are added at once, and the mix- ture is stirred again at 80°C for another 45 minutes. Af- ter cooling the NCO value of the reaction product is mea- sured as 0.01.

Preparation of the adhesive compositions of Examples 1 to 5 Methyl methacrylate, prepolymer PUA, lauryl methacrylate and N, N-bis- (2-hydroxyethyl)-p-toluidine were mixed in a dissolver at 40°C for 15 minutes, until a homogeneous liquid was obtained. Then Aerosil was added

and mixed at high revolutions per minute until the mate- rial showed thixotropic behavior.

The adhesives were cured by adding 4 % of the BPO paste.

The test results obtained for the cured adhe- sives are also listed in Table 1.

The adhesives were tested as follows: Tensile strength and Elongation at Break (DIN 53504) The adhesive was cured to form a sheet of ap- proximately 2. 5 mm thickness from which tensile test dumbbells were cut. The stress-strain tests were perfor- med using a rate of 200 mm per minute for measurements at room temperature.

Tensile Shear Strength (TSS) (DIN EN 1465) Substrates with a dimension of 100 mm x 25 mm and 2 mm thick in case of aluminium and 4 mm thick in case of ABS were cleaned with isopropanol. The bond thickness is 1.5 mm and the overlap 11.5 mm. The tensile shear strength was measured at 10 mm/min.

Table 1:

Designa-Compo-Example 1 Example 2 Example 3 Example 4 Example 5 tion nents [w/w%] [w/w%] [w/w%] [w/w%] [w/w%] A MMA 40 38 20 A THFMA 40 70 BLA8 10 8 10 C PUA 40 42 41 60 18 -Tol 2 2 2 2 2 Aerosil 10 9 8 10 200 Tensile 9. 5 7. 8 13 9. 8 Strength [MPa] Elongation 243 224 293 224 140 at Break % TSS 6.4 n. d. n. d. n. d. n. d. ALMg3 [MPa] TSS ABS 4.5 n. d. n. d. n. d. n. d. [MPa] n. d. = not determined Examples 1 to 4 all have a high tensile strength and a high elongation at break. Only examples 1 to 3 exhibit an elastic behaviour which approaches the ideal Hook law. The stress strain curves shown in Figures 1 to 3 are linear. Example 4 contains a larger amount of liquid elastomer C than is described in the present in- vention. The composition of example 4 has a stress strain curve which is only linear up to about 5 %. Although this composition has an elongation at break of up to 200%, it does not exhibits an elastic behaviour. Example 5 repre- sents a state of the art composition. The stress strain plot of Example 5 is not linear, and therefore, this ad- hesive is not elastic.

While there are shown and described presently preferred embodiments of the invention, it is to be dis- tinctly understood that the invention is not limited thereto but may be otherwise variously embodied and prac- ticed within the scope of the following claims.