GB2204872A - A psuedoplastic cyanoacrylate composition - Google Patents

Abstract

A cyanoacrylate composition which is filled with a hexamethyldisilazine treated fumed silica in amounts of about 2-20 per cent exhibits a high low shear rate to high shear rate apparent viscosity ratio and a high yield value. The composition was storage stable and showed unexpectedly low separation tendancies during storage.

Description

A PSUEDOPLASTIC CYANOACRYLATE COMPOSITION This number relates to gelled cyanoacrylate compositions.

Such compositions have gained wide acceptance in recent years, in particular as instant adhesives.

They are particularly desirable because their gel-like characteristics mean that there is less of a likelihood of a hazardous spillage during their application than with non-gelled adhesives. They are also more suitable for applications such as bonding porous substrates, applying to vertical surfaces, as a putty to fill large gaps or potting and embedding electronic components.

U.S. Patent No. 4533422 describes a process for the preparation of such a composition, and states that cyanoacrylate compositions containing polydimethylsiloxane-treated silicas exhibit suprisingly high thixotropy. This statement is perhaps somewhat confusing, since it is apparent from detailed examination of the text that what was measured was the thixotropic index or ratio which is not a measure of thixotropy. Thixotrophic index is simply defined as the quotient at two different shear rates or rotational speeds. This ratio is normally about 10 to 1 for a gelled adhesive. The thixotropic index of a Newtonian liquid is equal to 1. Values of greater than 3 indicate compositions with a high "sag resistance". So called "thixotropic index" is thus a measure of structural viscosity of pseudoplastic flow -behaviour.Pseudoplastic flow behaviour is characterised by a viscosity decrease with an accelerated shear rate or increased shear stress.

"Thixotrophy" is a term which is properly used to describe a non Newtonian flow behaviour that has a time dependent decrease in viscosity with increasing shear stress and shear rate.

This invention is concerned, as is U.S. Patent No. 4533422, with non-flowable gel compositions, which need not be thixotropic although the invention is not limited to non-thixotropic compositions. As used herein the terms "non flowable gel" is used to mean one which will not move when extruded on a vertical glass plate as a 1/8" diameter bead. Such compositions exhibit high yield values, that is, a high shear stress must be applied before the material can be made to flow.

It is know from U.S. Patent No. 4533422 that cyanoacrylate compositions filled with polydimethylsiloxane-coated fumed silicas exhibit high thixotropic ratios (i.e. are non flowable gels) at relatively low levels of incorporation. However they display substantial separation tendencies in storage. This shortcoming makes this type of composition unsuitable for many of the engineering and electronic etc. applications that require the homogeneous properties of a non flowable cyanoacrylate composition.

It is disclosed in U.S. Patent No. 4533422 that commercially available fumed silicas which have been treated with hexamethyldisilazine impart impractically low thixotropic ratios, and that at least some hexamethyldisilazine treated fillers tend to destabilise cyanoacrylate compositions.

Accordingly there exists a need for a cyanoacrylate gelling agent that does not promote polymerisation of the cyanoacrylate monomer, does not adversely affect bonding time, is substantially non separating in storage, and which is capable of producing an adhesive formulation with a high yield value, and a high thixotropic or apparent viscosity ratio.

The present invention provides a cyanoacrylate composition which employs as a filler a fumed silica coated with a trimethylsilane wherein the total content of free amine, ammonia or HC1 in the filler does not exceed 50 ppm (based on the weight of the filler). It has been unexpectedly found that this type of treated fumed silica incorporated in a cyanoacrylate composition does not promote premature polymerisation of the cyanoacrylate monomer, does not adversely affect the bonding time at levels up to about 10-20 per cent, and can provide an equivalent apparent viscosity ratio (thixotropic ratio) to that of cyanoacrylate compositions filled with polydimethylsiloxane-treated fumed silicas, whilst displaying substantially less tendency to separate than such prior art compositions.

The trimethylsilane coated filler may be prepared, for example by treating fumed silica with a methylchlorosilane, such as 3 dimethyl dichlorosilane or trimethyl chlorosilane, or more preferably with hexamethyldisilazine, followed, if necessary by treatment to reduce the levels of ammonia, amine, or HC1 to the desired level.

The composition in accordance with the invention preferably comprises an appropriately stabilized cyanoacrylate ester monomer and from 2 to 20 per cent of the fumed silica, based on the total composition weight. It is preferable that a small amount of organic polymer, for example, a polycyanoacrylate, poly(meth)acrylate, polyvinylacetate, ethylene-vinylacetate copolymer, or other polymers soluble in the monomer which do not adversely affect its bonding capability should be dissolved in the monomer in order to increase its viscosity. This increase in viscosity of the base material reduces the rate of separation of the silica filled composition.

The cyanoacrylate monomers used in the inventive cyanoacrylate compositions are compounds of the formula:

wherein R1 represents a straight chain or branched chain alkyl group having from 1 to 10 carbon atoms (which may be substituted with a substituent such as an alkoxy group or a halogen atom), a straight chain or branched chain alkenyl group having from 2 to 12 carbon atoms, an aralkyl group, cycloalkyl group, or an aryl group.

Specific examples of the groups for R1 are a methyl group, an ethyl group, and n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a pentyl group, a hexyl group, an allyl group, a methallyl group, an ethyl allyl group, a crotyl group, a propargyl group, a cyclohexyl group, a tertiary butyl cyclohexyl group, a benzyl group, a phenyl group, a cresyl group, a trifluoroethyl group, a 2-chloroethyl group, a 3-chloropropyl group, a 2-chlorobutyl group, a 2-ethoxyethyl group, a 2-methoxyethyl group, a 3-methoxybutyl group. Most preferred are methyl, ethyl and isobutyl cyanoacrylate.

It is preferred that the filler in accordance with the invention has a specific surface area of at least 140 m2/g, more preferably 200 m2/g, more preferably still at least 250 m2/g when measured by the method of DIN 66131.

A preferred type of hexamethyldisilazine treated fumed silica which has been found to impart the desired pseudoplastic flow behaviour properties to the composition is Aerosil R812 (TM of Degussa Limited) which is a hydrophobic silica having a BET surface area of 260 + 30 M2/gm and an ignition loss (2 hours at 1000 degrees C) of 1-2.5 per cent. This silica is formed by treating Aerosil 300 (TM of Degussa Limited) with hexamethyldisilazine, at high temperatures approximately 400 degrees C, under Nitrogen, to form a bound coating of trimethylsilane. Ammonia is released in the process. However the level of free ammonia and amines in Aerosil R812 is less than 50 ppm. It has been found that if substantial amounts of ammonia, silazine or moisture are present in the filler they promote premature polymerisation of the cyanoacrylate composition.

The filler utilized in the composition of the present invention contains not more than 50 ppm, preferably not more than 10 ppm, of ammonia and/or free amine. The amounts of these destabilizing agents present can be reduced by heating the filler, preferably at a temperature of at least 193 degrees C under high shear in an inert gas such as nitrogen.

Temperatures of greater than about 200 degrees C, particularly greater than 205 degrees C may adversely affect the filler by altering or partially removing the surface treatment, if carried out in an atmosphere of air.

The invention is illustrated by the following examples.

EXAMPLES An ethyl cyanoacrylate formulation having a viscosity (Brookfield RVT, spindle 2, 20 degrees C) of 40 mPa.s was filled with a variety of fumed silicas. Viscosity ratios stability and percentage separation were measured, and the results are shown in Table 1.

Table 1 Brookfield RVT Spindle TB Measurement at 20 degrees % % Stability Separation Level of at 82 After 1 hour 2.5 rpm 20 rpm Ratio Yield Addition Degrees C at 800 rpm Comparative Example 1 Cabosil N70TS (1) 1 hr 24,000 5,500 4.4 7 5.6 < 5 days 2 24 hrs 20,000 5,400 3.7 6 Comparative Example 2 Aerosil R805 (2) 1 hr 36,000 7,500 4.8 7.5 5.6 < 5 days 1 24 hrs 32,000 12,000 2.6 7.0 Comparative Example 3 Tullanox 500 (4) 1 hr 30,400 9,000 3.37 7 7.4 > 7 hrs (untreated) 24 hrs 36,800 13,000 2.8 8.5 Example 1 Aerosil R812 (3) 1 hr 43,200 9,000 4.8 15 7.4 < 5 days 0 24 hrs 38,400 9,600 4.0 12 Example 2 Aerosil R809 (3) 1 hr 30,400 6,800 4.47 9.5 20.0 < 5 days 0 24 hrs 32,000 7,600 4.20 7.0 Example 3 Tullanox 500 (5) 1 hr 27,200 7,800 3.5 6.5 7.4 < 5 days (treated 10 hrs 24 hrs 27,200 8,000 3.4 6.5 at 200 C) Example 4 HDK 2000 (6) 24 hrs 36,800 7,200 5.1 11.5 16 < 5 days - (1) (Trade Mark of Cabot Corporation) polydimethylsiloxane - treated silica (2) (Trade Mark of Degussa Corporation) trimethoxyoctylsilane - treated silica (3) (Trade Mark of Degussa Corporation) hexamethyldisilazine - treated fumed silica (4) Trade Mark of Tulco Incorporated - hexamethyldisilazine - treated fumed silica (5) As (4) - but after heat treatement at 200 C for 10 hours (6) Trade Mark of Wacker Chemie - hexamethyldisilazine - treated fumed silica As can be seen from Table 1, Examples 1 and 2 show equivalent high yield values, high apparent viscosity ratio and substantially less separation tendency than comparative Examples 1 and 2.

Relative yield value was determined on the Brookfield viscometer as the maximum dial reading that can be reached and maintained by rotating the vessels containing the composition very slowly i.e.

0.1 rpm.

Percentage level of addition is based on the total composition weight.

Accelerated heat ageing stability was carried out at 82 + 1 degrees C in a stoppered glass test tube.

Separation tendencies were determined by centrifugation at 800 rpm for one hour. 0.5% Carbon Black (Flammruss 101 T.M. Degussa Limited) was added to each composition as a visual aid.

Claims (9)

1. A cyanoacrylate composition comprising a filler, which is a fumed silica coated with a trimethylsilane, wherein the total content of free amine, ammonia or HC1 in the filler does not exceed 50 ppm based on the weight of the filler.

2. A composition as claimed in Claim 1, wherein the filler is one which has been prepared by treating a fumed silica with hexamethyldisilazine.

3. A composition as claimed in Claim 1, wherein the filler is one which has been prepared by treating a fumed silica with a methylchlorosilane.

4. A composition as claimed in Claim 3, wherein the methylchlorosilane is 3 dimethyl dichlorosilane, or trimethyl chlorosilane.

5. A process as claimed in any one of Claims 2 to 4, wherein the filler has been subjected to a treatment to reduce the amine, ammonia, or HC1 level therein.

6. A composition as claimed in any one of the preceding claims, wherein the total content of free amine and/or ammonia in the filler does not exceed 10 ppm based on the weight of the filler.

7. A composition as claimed in any one of the preceding claims, wherein the specific surface area of the filler is at least 140 m2/g, when measured according to DIN 66131.

8. A composition as claimed in Claim 7, wherein the specific surface area of the filler is at least 200 m2/g, when measured according to DIN 66131.

9. A composition as claimed in Claim 7, wherein the specific surface area of the filler is at least 250 m2/g, when measured according to DIN 66131.