Patent Application
20050004304
This application claims the priority of European patent application No. 01 105 351.9, filed Mar. 8, 2001, the disclosure of which is incorporated herein by reference in its entirety.
The present invention relates to elastic adhesive compositions.
The long term performance of bonding is dependent 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 adhesives.
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 compositions. The elastomers are solids at room temperature, and commercially available as large particles or granulates.
This approach is described in several patent documents, such as: U.S. Pat. No. 3,890,407, U.S. Pat. No. 4,106,971, U.S. Pat. No. 4,263,419, U.S. Pat. No. 3,725,504, U.S. Pat. No. 4,200,480, U.S. Pat. No. 3,994,764, EP 0 641 846. The disadvantage of such formulations is that they can only be mixed homogeneously with special equipment 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 polymer 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 molar 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 mixture, only a limited amount of fillers can be used making the formulation expensive. A high viscosity is also limiting 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 U.S. Pat. No. 4,769,419; U.S. Pat. No. 4,331;765 and EP 0,561,352 disclosing mixtures of monomers and liquid rubber.
DE 2,610,423; U.S. Pat. No. 4,439,600; EP 0,640,672; DE 2,319,637 and U.S. Pat. No. 4,223,115 disclose mixtures of monomers and acrylic functionalised polyurethanes. In U.S. Pat. No. 4,223,115 the composition optionally contains a dissolved elastomer, e.g. NBR (nitrile-butadiene-rubber), polychloroprene.
Flexibility is measured in terms of elongation at break. During deformation, however, all the above mentioned prior-art adhesives show a visco-elastic behavior 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 elongation speed relevant to the practical application. The plot of the prior-art adhesives is not linear, and this even for the compositions of U.S. Pat. No. 4,439,600 (see above) although elastic behavior is claimed. The plastic flow component during deformation reduces the number of deformation 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 invention 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.
Thus one object of the present invention is an adhesive composition comprising
Much preferred components of structure (B) are those wherein
The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following description thereof and the annexed Figures, wherein:
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 aromatic ester, much preferred a methacrylate ester selected from the group consisting of methyl methacrylate (MMA), tetrahydrofurfuryl methacrylate (THFMXA), cyclohexylmethacrylate (CHMA), cyclic trimethylolpropane formal acrylate (CTFA), isobornylmethacrylate (IBMA), benzyl-methacrylate (BMA), dicyclopentadienyloxyethylmethacrylate (DCPOEMA), t-butylmethacrylate (tBMA), isobornylacrylate (IBA), dihydrodicyclopentadienylacrylate (DHDCPA), and mixtures thereof.
In another preferred embodiment of the inventive composition, the monofunctional (meth)acrylic monomer B of formula (B) is a (meth)acrylic ester, such as an ester selected from the group consisting of linear and branched C6-C15-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 inventive adhesive composition contains a liquid elastomer C with ethylenically unsaturated groups which is chosen from the group consisting of
The compositions of the present invention usually also comprise at least one initiator and/or catalyst, as well as preferably also at least one organic or inorganic filler or thixotropic agent. The compositions can also comprise further substances, such as stabilizers, additives, toughening agents, adhesion promoters, impact modifiers, core-shell polymers, defoaming agents, thickeners, plasticizers, wetting agents, wax compounds, cross-linking agents, inhibitors etc. Such additional substances are known to the skilled person. Examples 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 manufactured, 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 temperature 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 particular 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 present invention increases significantly the performance of the bond under dynamic load. Furthermore, the compositions show an excellent adhesion on many substrates without 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 excellent recovery behavior. Their very high elasticity can be maintained even when the formulations are filled with commonly known fillers.
In the compositions used in the examples 1 to 5, the abbreviations have the following meanings:
The compounds used to formulate the adhesive compositions of examples 1 to 5 and their amounts are listed in Table 1.
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 mixture is stirred again at 80° C. for another 45 minutes. After cooling the NCO value of the reaction product is measured as 0.01.
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 material showed thixotropic behavior.
The adhesives were cured by adding 4% of the BPO paste.
The test results obtained for the cured adhesives are also listed in Table 1.
The adhesives were tested as follows:
The adhesive was cured to form a sheet of approximately 2.5 mm thickness from which tensile test dumbbells were cut. The stress-strain tests were performed using a rate of 200 mm per minute for measurements at room temperature.
Substrates with a dimension of 100 mm×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.
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 FIGS. 1 to 3 are linear. Example 4 contains a larger amount of liquid elastomer C than is described in the present invention. 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 represents a state of the art composition. The stress strain plot of Example 5 is not linear, and therefore, this adhesive is not elastic.
While there are shown and described presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 011053519 | Mar 2001 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB02/00649 | 3/4/2002 | WO |
