This invention relates to adhesive compositions containing as the major polymerisable constituent one or more esters of 2-Cγanoacrylic acid, such as Ethyl 2-cyano— aerylate. ^' The improvement of cyanoacrylate adhesive compo¬ sitions,- particularly as regards the stability of the adhesive bonds formed therewith under exposure to high temperatures and 'moist conditions, is an area of wide- ranging and energetic research activity. The present invention adds one compound to the list of additive substances known to improve these compositions in the said respect.

There follows a short outline of the closest prior art known to us. 1. Federal German Patent No. 24 29 070 (1975:

Bostik GmbH) discloses the use of Itaconic anhydride as an additive in alkyl and allyl cyanoacrylate compositions to impart improved heat resistance to the adhesive bonds formed. 2. Patent No. 34874 (1972:Loctite (Ireland)

Laimited) and its U.S. equivalent Patent No. 3 832 334 disclose the use of aleic anhydride and derivatives thereof as additives .in alkyl cyanoacrylate compositions to impart improved heat resistance to the adhesive bonds

formed.

3. Japanese Patent No. 78 110 635 (1978iToa ^* Gosei) discloses* the use of hydroxyalkyl and hydroxy aloalkyl esters of α,S-unsaturated carboxylic acids as additives in alkyl cyanoacrylate compositions to impart, improved heat resistance to the adhesive bonds fo^rmed.

4. Japanese Patent No. 77 80 336 (1977:Toa Gosei) discloses the use of dicarboxylic acids and their anhydrides as additives in ethyl cyano¬ acrylate compositions to impart improved impac res-istance to the adhesive bonds formed.

5. Japanese Patent No, 77 78 933 (1977:Toa Gosei) discloses the use of aromatic pσlycarboxylic acids and their anhydrides as additives in

Ethyl cyanoacrylate compositions to impart improved impact resistance to the adhesive bond ^' s formed. The above references also provide a good .indicatio of progress up to their respective dates .in the improvemen of heat and impact resistance in adhesive bonds formed fro alkyl cyanoacrylate compositions by the use of additives.

The invention provides an adhesive composition containing as the major polymerisable constituent at leas one ester of 2-Cyanoacrylic acid, characterised in that th composition additionally comprises a proportion of Phthal anhydride effective for favourably influencing the streng ■and/or durability of adhesive bonds formed from said composition, under exposure to moisture or elevated te pe ature.

.An effective proportion of Phthalic anhydride for the above recited purpose is selected from the range 0.1% to 5.0% by weight of the composition. A preferred propor is selected from the range 0.3% to 0.7% by weight, a specially preferred proportion being about 0.5% by weight the composition.

It has been found that an Ethyl cyanoacylate

solution containing 0.5% by weight Phthalic anhydride yields adhesive bonds having a heat resistane superior to that of bonds made with an otherwise identical composition containing 1.0% by weight Maleic anhydride in place of the Phthalic anhydride. Some of the bonds were formed between grit blasted steel lapshears, others between grit blasted aluminium lapshears. The bonds made with the invented composition were also found to possess significantly improved moisture resistance as compared with bonds made with the Maleic anhydride composition.

It may be remarked that the Maleic anhydride composition is itself known to outperform a composition containing Ethyl cyanoacrylate alone, in both of the above mentioned respects•- It will be noted that various organic anhydrides have been proposed for the general purpose of the present invention. However none of the prior proposals known to us, either in the -art cited above or elsewhere, embraces the use of Phthalic anhydride. Some of the prior art goes so far as to exclude Phthalic anhydride specifically, indicating perhaps some misconception or prejudice prevalent in the art about the suitability of this compound Hence the present invention may be said to rest on the surprising discovery of a hitherto unsuspected utility of Phthalic anhydride.

The invention will be appreciated in greater detail from the following examples of specific and preferred embodiments thereof. Each test result given in these examples is the mean of ten determinations. Example 1

^• Grit-blasted stainless steel strips 2.54 cm wide by circa 1 mm thick were prepared, aligned face to face, and adhesively secured together on an overlap length of 1.27 cm to give a bonded area of 3.22 cm . The bonds were allowed to cure at room temperature for 3 days, following which about one third of their number were tested. Half of the remaining number were thereupon subjected to

ageing at 120°C fo ^' 48 hours -and tested. The other half of said rema.inder (one third of the original number of bonds) were subjected to ageing at 40°C in conditions of 95% relative humidity for 7 days, after which they too were tested-

The tests were destructive measurements of the tensile shear strength of the adhesive bonds, the results

2 being expressed in kg/cm and (in parentheses) as a percentage of that initial strength which was determined after 3 days at room temperature. The procedure was carried out separately using three different adhesive compositions one of which was a composition of the invention, the other two being included for comparison. The polymerisable monomer of the composition in eve.ry case was Ethyl 2-cyanoacrylate. The results are set out in Table 1.

Table 1

Tensile shear strength kg/cm 2

2 days -+* 48 hours + 7 days

Additive RT 120°C 40 ^Q C 95% RH

0.5% Phthalic anhydride 155 109 (70%) 130 (84%) 1 % Maleic anhydride 178 110 (62%) 85 (48%) none 158 59 (37%) 88 (56%) Example 2

The procedure of Example 1 was repeated, but using grit-blasted aluminium strips as the adhesive substrate in place of stainless steel. The results are set out in Table 2.

Table 2

2

Tensile shear strength kg/cm

Additive -RϊT ^{da S +} J12*0°C ^{5 +} 40 ⁷ °C ^d 9 ^Y 5 ^S % .RH

C 5.%. Phthalic anhy-dride 95 69 (73%) 63 ( 66% )

1 % Maleic. anhydride 159 • 51 (32%) 57 (36% ) none 123 28 (23%) 61 (50%)

The utility of Phthalic anhydride i≤ apparent from a comparison of its results with those where no additive was used. Its superiority to Maleic anhydride is likewise evident, though less pronounced in the case of ^* stainless steel lapshears than in those of aluminium. The adhesive compositions of the invention are found, additionally, to exhibit excellent stability in the unpolymerised state when stored for periods of three months or more. Thus they can be presented as one-part cyanoacrylic compositions without the known disadvantage associated with the use of Maleic anhydride. Maleic anhydride must, as a rule, be mixed into the Ethyl cyanoacrylate just prior to use. Storage of 1% Maleic anhydride in Ethyl cyanoacrylate leads to loss of the adhesive properties of the composition after about the third month, due ^' to over tabilisation by Maleic acid resulting from in situ hydrolysis of the anhydride.