Improved chelator for anaerobic sealants.

The present invention relates to an improved metal ion chelator for use in anaerobic sealants or adhesives. In particular the chelator of the present invention is suitable for counteracting copper contamination in fast-curing acrylate ester sealant formulations having as an accelerator 1-acetyl-2-phenylhydrazine (US Patent No. 4,180,640). Such adhesives contain latent accelerators of free radical polymerization which can lead to spurious polymerization in the bottle or elsewhere, prior to the time of intended use. It has been hypothesized that much of the unpredictability inherent in anaerobic compositions is due to metal contamination in the starting materials, or acquired during processing.

To overcome this problem the use of chelators in anaerobic sealants was suggested in U.S. Patent No. 4038 475 (Frauenglass & Werber).

The chelator which is currently used in anaerobic sealants is ethylenediaminetetraacetic acid tetrasodium salt or EDTA Na₄:- <chemistry id="chem0001" num="0001"><img file="EP0282292A2_D0001.tif" /></chemistry>

This chelator is very effective when used under appropriate conditions. However, the disadvantage with EDTA Na₄ is that it is precipitated from solution within about 24 hrs of addition due to its reaction with acidic components of the sealant formulation to produce acid analogues. In particular the tetraacid which may be formed shows a decreasing solubility with decreasing pH. This means that the added EDTA Na₄ is no longer able to act as a chelator to counteract trace metal contamination, and particulate material originating from the EDTA tetrasodium is formed in the sealant.

An object of the present invention is to provide a metal ion chelator for use in anaerobic sealants which is soluble in anaerobic sealant compositions, which can effectively chelate and deactivate contaminating metals and which does not seriously interfere with the cure profile of the sealant.

According to the present invention there is provided an anaerobic sealant composition containing N-hydroxyethylenediaminetriacetic acid trisodium salt (HEDTA Na₃) as a chelating agent. The HEDTA Na₃ should be present in an effective amount to act as a chelator. Preferably the sealant composition comprises between 1 and 1000 ppm of N-hydroxyethylenediaminetriacetic acid trisodium salt, a particularly preferred range being between 25 and 500 ppm.

Anaerobic sealant compositions are mixtures of polymerizable acrylate ester monomers and peroxy polymerization initiators therefor. Preferably at least a portion of the acrylate monomer is a di- or other polyacrylate ester. Suitable polyacrylate ester monomers are di-, tri- ­and tetraethyleneglycol dimethacrylate; dipropyleneglycol dimethacrylate; polyethyleneglycol dimethacrylate; polypropyleneglycol dimethyacrylate; di(pentamethyleneglycol) dimethacrylate; tetraethyleneglycol diacrylate; tetraethyleneglycol di(chloracrylate); diglycerol diacrylate; diglycerol tetramethacrylate; tetramethylene dimethacrylate; ethylene dimethacrylate; butyleneglycol dimethacrylate; neopentylglycol diacrylate; and trimethylolpropane triacrylate.

Monofunctional acrylate esters (esters containing one acrylate group) may also be used in anaerobic sealants, and preferably the esters are those with a relatively polar alcoholic moiety. Typical examples of compounds within this category are cyclohexylmethacrylate; tetrahydrofurfuryl methacrylate; hydroxethyl acrylate; hydroxypropyl methacrylate; t-butylaminoethyl methacrylate; cyanoethylacrylate and choroethyl methacrylate.

Another class of polyacrylate ester monomers utilized in anaerobic sealant compositions are the isocyanatemonoacrylate reaction products described in U.S. Patent No. 3424988 of Toback and Gorman, issued 4th February 1969.

The success of HEDTA Na₃ in meeting the objects of the invention could not have been predicted. The inventors have screened over thirty commercially available chelating agents in an attempt to find one which meets the above defined requirements for use in an anaerobically curable sealant.

Solubility Test

To test the solubility of potential chelators, model solutions of (a) polyethylene glycol dimethacrylate (PEGMA) containing 3% saccharin, and (b) polyethylene glycol dimethacrylate (PEGMA) containing 1% maleic acid were used. The chelator was added with stirring to the model solution to a level of 500 ppm and was heated to 55°C to aid dissolution, and then allowed to cool. The resulting mixture was filtered and any insoluble material collected on the filter paper was analysed by infra-red spectroscopy (IR). As shown in Table 1, fifteen of the chelators out of thirty one failed this solubility test, while twelve of the remaining sixteen caused rapid discolouration and polymerization of the model solution. <tables id="tabl0001" num="0001"><img file="EP0282292A2_D0002.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0282292A2_D0003.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0282292A2_D0004.tif" /></tables>

The chelators identified above as Henkel Proprietary products are available commercially from Henkel Corporation, Minerals Industry Division, 1844, West Grant Road, Tuscon, Arizona, USA.

The remaining four chelators which passed the solubility test were again tested in the model solution and this time the concentration of the chelator was quantified by titrimetric analysis as a function of time elapsed since addition. The first of these was EDTA Na₄ described above. The other three are N-hydroxyethylenediaminetriacetic acid trisodium salt (HEDTA Na₃) of formula:- <chemistry id="chem0002" num="0002"><img file="EP0282292A2_D0005.tif" /></chemistry> ethylenediaminetetra (propyl alcohol) or tetrakis-(2-hydroxy-propyl)-­ethylenediamine (EDTPA) of formula:- <chemistry id="chem0003" num="0003"><img file="EP0282292A2_D0006.tif" /></chemistry> and diethylenetriaminepentaacetic acid pentasodium salt (DTPA Na₅):- <chemistry id="chem0004" num="0004"><img file="EP0282292A2_D0007.tif" /></chemistry> All of these chelators are commercially available.

The results of this test are given in Table 2. <tables id="tabl0004" num="0004"><img file="EP0282292A2_D0008.tif" /></tables>

Table 2 shows that EDTA Na₄ and DTPA Na₅ have a very low solubility in the model formulation whereas HEDTA Na₃ and EDTPA have a greater solubility, particularly EDTPA. Therefore, only HEDTA Na₃ and EDTPA, along with EDTA Na₄ as a control were tested in the next stage.

STABILITY OF SOLUBLE CHELATORS WITH OR WITHOUT ADDED COPPER.

A sensitive curable system based on the following formulation was prepared and subdivided to give a series of solutions with differing concentrations of the relevant chelators (as detailed in Table 3):- Polyethylene glycol dimethacrylate      96.88% Cumene hydroperoxide      0.60% 1-acetyl-2-phenyl-hydrazine      0.30% Benzoic suphimide      1.15% Maleic Acid      1.00% 1,4 Naphthoquinone      0.01% Chelator*      0.05%* NOTE*: Chelator used was selected from Table 3 with 0.05% representing upper level utilized.

A standard 80 × 100 mm glass tube was half filled with the anaerobic composition containing the chelator under test and placed in a metal heat-block at 82°C. The time taken to the first gellation was noted and used as a measure of stability of the composition.

Experience with similar formulations has shown that a formulation which remains stable for longer than 3 hours at 82°C should have a lifetime (i.e. it should remain liquid in the non-cured form) exceeding 12 months at ambient temperature. <tables id="tabl0005" num="0005"><img file="EP0282292A2_D0009.tif" /></tables>

The stabilities in Table 3 are an average of three determinations and were tested when fresh and after standing for one week at room temperature (of which only the supernatant liquid was tested). These stabilities show that all formulations were similar, whichseems to indicate that these chelators do not interfere with the chemistry of the cure system.

However, in Table 4 the formulations were "spiked" with copper when fresh and after one week at room temperature. (In the case of solutions aged for one week, only the supernatant was tested). The fresh formulation containing EDTA Na₄ showed excellent stability down to 50 ppm while the week old samples failed in all cases. A similar trial in which the supernatant liquid was tested 24 hrs after the addition of the chelator yielded similar results, i.e. the supernatant liquid ex EDTA Na₄ formulation gelled rapidly on addition of copper (0.5 ppm).

The EDTPA and HEDTA Na₃ showed similar stabilities to EDTA Na₄ when fresh, but there was no loss of chelation efficiency on aging. Therefore, HEDTA Na₃ and EDTPA appear to offer a significant improvement in the retention of chelating capability on aging.

Fixture Time Test

Table 4 shows that in the case of HEDTA Na₃ and EDTPA, 300 ppm of chelator was able to counteract 0.5 ppm of added copper. A series of formulations were therefore prepared, with and without copper, with a level of 200 ppm HEDTA Na₃ and EDPTA and curespeed was determined using standard M10 degreased black oxide nuts and bolts. A thin coating of the anaerobic sealant formulation was applied to the nuts and bolts and these were assembled. At the intervals shown in Table 5 they were checked for fixture strength using an electronic torque tension analyser. All tests were carried out in triplicate with five nuts and bolts per test.

Experience has shown that the strength obtained after standing for 1 and 2h enables the speed of cure to be assesed in relative terms i.e. comparison of modified and standard formulations. The strength obtained after 24h is a measure of the ultimate strength to be expected from such a formulation. <tables id="tabl0006" num="0006"><img file="EP0282292A2_D0010.tif" /></tables><tables id="tabl0007" num="0007"><img file="EP0282292A2_D0011.tif" /></tables>

The cure speed profiles reveal that EDTPA has a detrimental effect on cure speeds, even at levels as low as 200 ppm, whereas HEDTA Na₃ retains speed of cure when compared to EDTA Na₄ at about 200 ppm.

EDTPA is therefore unsuitable for use as a chelator in anaerobic sealants whereas HEDTA Na₃ has been shown to be a much superior chelator to EDTA Na₄ in such formulations.