For a radical polymerizing compound which cures to an elastic body, acrylic acid esters are well known and acrylic rubbers or coatings having excellent elasticity is obtained by the homopolymerization of these esters or copolymerizations thereof with other copolymerizable monomers. However, since acrylic acid esters have very easily polymerizing property, it is difficult to stably store the esters in a state of containing a polymerization initiator and further, since these esters give severe stimulative property to skins and bad smell, they are unsuitable for the uses such as adhesives or sealants which are stored as liquid for a long period of time and are required good working environment.
On the other hand, since methacrylic acid esters show relatively good storage stability even in the presence of a polymerization initiator and give less irritating property and are less in odor, they have been frequently used as curing compositions in the aforesaid field. (See, for example, Japanese Patent Publication Nos. 6564/68, 17,520/68, 19,390/70, etc.). However, the cured products are poor in elasticity or poor in strength and hence they are unsuitable for the purposes of requiring elasticity.
On the other hand, a liquid gasket is generally a viscous liquid and when a liquid gasket is coated onto a screw portion or a flange surface and it is clamped, the liquid gasket is dried or causes chemical reaction after a definite period of time to form a coating or a film having elasticity, whereby the leakage of liquid or gas from the joint is easily prevented. A typical example of the liquid gasket is a solvent type mainly composed of polymers such as a synthetic rubber, a phenol resin, a petroleum resin, etc., containing a filler, a plasticizer, a solvent, etc., and there are a drying type gasket and a nondrying type gasket.
Since such a gasket is liquid different from a sheet like gasket such as a paper, an asbestos, etc., the gasket is only coated followed by clamping, whereby it fills fine uneven portions of the joint to exhibit an excellent air-tightness. However, since the residual solvent of the gasket gradually volatilizes even after drying, to reduce the thickness of the coating, it given a disadvantage that leakage occurs if clamping is not periodically applied. Also, the strength of coating itself is insufficient, so that the use thereof under severe operating condition is difficult.
For overcoming the difficulties, a non-solvent type gasket such as a moisture curing type liquid gasket mainly composed of an isocyanate compound or a silicone and curing by the action of moisture in the air and an anaerobic liquid gasket mainly composed of a methacryl monomer and quickly curing when it is isolated from air contact has been developed. Such a non-solvent type gasket has excellent properties that shrink of the coating does not occur and a coating having high strength is formed, whereby the above-described disadvantage can be eliminated. However, the moisture curing type gasket has a disadvantage that it takes a long period of time to accomplish curing and thus is unsuitable for the use requiring quick curing. Also, the anaerobic curing type gasket shows quick curing property, but since the methacryl monomer which is the main component for the gasket is generally polyfunctional, the cured film or coating is hard but brittle and poor in elasticity and also there is a disadvantage that in the case of using such a gasket for the portions which are liable to be attacked by thermal change, vibration, impact, etc., leakage may occur. Accordingly, the improvement of the elasticity of cured films or coatings has been strongly desired.
For the purpose, a method of dissolving a large amount of synthetic rubber has been proposed as disclosed in, for example, Japanese Patent Publication (Unexamined) Nos. 140,538/75 and 4243/76, Japanese Patent Publication No. 28,177/79, Japanese Patent Publication (Unexamined) Nos. 65,065/81 and 199,784/84) but the composition thus obtained has a very high viscosity or is putty like and hence is practically unsuitable for liquid gasket.
The inventors have investigated the properties of cured products of various well-known methacrylic acid esters solely or as various combination thereof. As the results thereof, it has been discovered that in the case of curing a simple methacrylic acid ester, a hard cured product lacking in elasticity or a soft cured product lacking in strength is obtained but when monomethacrylic acid esters having specific structures are mixed at a specific ratio and the mixture is cured in the presence of an organic peroxide, a soft cured product having elasticity together with toughness is obtained without need of the addition of a softness providing agent such as a synthetic rubber, etc.
Also, it has been discovered that a mixture of methacrylic acid esters capable of giving cured product having excellent elasticity is used as a base monomer for an anaerobic liquid gasket and the mixture is compounded with a filler and small amounts of an anaerobic curing accelerator and a stabilizer, the composition thus obtained has good storage stability and gives a soft cured product having elasticity in the case of curing the composition at a gap of junction, whereby the composition can be used as an anaerobic liquid gasket having excellent sealing property and pressure resistance capable of sufficiently standing dynamic changes such as thermal change, vibration, impact, etc., as well as having improved detachability required as gasket.