High-resistant condensation cross-linking silicon

Details High-resistant condensation cross-linking silicon High-resistant condensation cross-linking silicon

2001-01-19

2003-01-14

Dawson, Robert (Department: 1712)

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

C524S588000, C528S015000, C528S016000, C528S023000, C528S034000

Reexamination Certificate

active

06506279

ABSTRACT:

The present invention concerns a kit for the production of condensation cross-linking silicone mixtures of at least bifunctionally-terminated diorgano-polysiloxanes, acetoxysilane cross-linking agents, as well as possibly filling materials, suitable additives, pigments, colouring materials, oxidation, heat and light protection pigments, as well as solvents and plasticisers.
Such organopolysiloxane mixtures, also known as cold-vulcanising, monocomponent silicone rubbers, usually cross-link at room temperature with the take up of water from the surrounding atmosphere to give rubber-elastic polymers. As chain lengtheners and cross-linkers, there are used bi- and preferably higher functional acetoxysilane compounds which, by reaction with the polysiloxane or by hydrolysis, split off acetic acid and thus initiate the formation of a macromolecular mesh-work. After hardening has taken place, such masses are characterised by good inherent adhesion on the most varied material surfaces and by a generally high stability against the action of temperature, light, moisture, as well as chemicals.
The hardening of such monocomponent polysiloxane mixtures cross-linking at room temperature with the take up of moisture takes place comparatively slowly since the water necessary for the reaction must diffuse from the surrounding atmosphere into the interior of the mass. Therefore, the speed of the hardening through decreases with progressive reaction in the interior of the mass. In the case of low atmospheric humidity or in the case of an unfavourable ratio of surface to volume of the silicone mass, the reaction can become very slow or, as in vapour-tight closed off rooms, can also stop completely.
The per se multiple possibilities of use of such atmospheric moisture-hardening silicones as sealing or adhesive materials are, especially in the case of use in industrial fabrication, limited because of the slow hardening. Admittedly, bicomponent silicone rubber systems hardening rapidly at room temperature or also first at higher temperatures are known but their use fails frequently in the deficient inherent adhesion or also the comparatively low temperature resistance of these products. However, if one uses silicones hardening only slowly under the influence of atmospheric humidity, in the case of the short cycle times desired for economic reasons, large intermediate storage for sealed off or adhered parts are necessary in order to ensure the hardening. These intermediate storages must possibly be additionally climatised or moistened. Under certain circumstances, in this way already very large numbers of pieces are produced before, for the first time, a testing for freedom from faults and function of the goods produced is possible. Large-surface adhesions between diffusion-impervious surfaces are, in practice, just as little capable of being carried out with atmospheric moisture-hardening silicones as the production of formed bodies in closed moulds.
If one adds water in liquid form to the known acetoxysilane-containing and atmospheric moisture-hardening masses, in comparison with the cross-linking with atmospheric moisture, a certain acceleration of the hardening is achieved. However, this form of cross-linking does not lead to end products with material properties as are obtained in the case of pure atmospheric moisture cross-linking. On the contrary, masses result which, over the course of time, remain substantially softer, display poor inherent adhesion and are also swollen for a very long time with the acetic acid resulting as cross-linking fission product.
Similar results, thus only slight hardening acceleration bound up with comparatively poorer material properties and with long persisting smell of acetic acid are obtained when water is added in the form of water of crystallisation-containing neutral salts or also knowingly surface-moistened materials.
From U.S. Pat. No. 4,532,315, it is known that acetate-hardening silicone masses harden more quickly by addition of hydroxides of the alkali and alkaline earth metals and possibly addition of water. There are thereby used the water of crystallisation-containing forms of the hydroxides or water is additionally added to the mixture. The basic compounds react with the cleavage product, the acetic acid, with salt formation, whereby the reaction equilibrium is forced to the side of the products. The alkali added to the system and water initiates the hydrolysis reaction of the acetoxy groups of the cross-linker and, in this way, accelerates the formation of the polymer structure.
An important disadvantage of the known condensation cross-linking silicones is their limited strength. Whereas addition cross-linking systems achieve tear strengths of up to 7 N/mm
2
, the tear strengths of the condensation cross-linking systems lie at max. 2 N/mm
2
, mostly however distinctly thereunder. From this results a greatly limited possibility of use of the condensation cross-linked silicones to uses in which a high stretch-ability of the adhesion/sealing material is necessary. In the case of adhesions which are exposed to high, shock-like loadings or vibrations, hitherto no condensation cross-linking systems have been used.
Thus, the task forming the basis of the invention is to make available a kit for the production of sealing and adhesive masses based on condensation cross-linking acetoxysilane-polysiloxane mixtures, whereby an accelerated hardening after mixing of the kit is achieved and, furthermore, a high strength of the polymer material is achieved. The silicone masses produced from the kits according to the invention are to harden within a short time, i.e. within a few minutes up to a few hours and independently of the surrounding atmospheric humidity, whereby, besides the typical characteristics of the previously known vulcanisates, such as for example inherent adhesion, stability and mechanical properties, especially the tensile strength and the temperature stability of the acceleratedly hardened silicone is to be improved.
It has now been found that this task is solved when one replaces the hydroxides of the alkali and alkaline earth metals acting as accelerator according to U.S. Pat. No. 4,532,315 by water of crystallisation-containing acidic neutral salts of aluminium or iron with mineral acids, preferably by the alums but also, for example, aluminium sulphate or iron phosphate. In the case of use especially of the aluminium alums of ammonium and of the alkali metals, besides the accelerated action, in comparison with the air hardening, on the hardening, a simultaneously extraordinarily improved tensile strength or tear strength of the silicones polymerised under these conditions can be achieved. Amongst the acidic neutral salts, to which also belong the alums, are understood compounds in which theoretically all ionisable hydrogen atoms of the acid are replaced by other cations and only via the differing degree of dissociation of the underlying acids the aqueous solutions of the salts react acidicly, especially in the case of 1 molar solutions pH values of below 5, preferably below 4 are obtained. It is surprising that such compounds can be advantageously used instead of the alkalis and thereby produce an increased strength of the silicones. A certain difference consists in that the silicones accelerated in this way require a longer reaction time in comparison with, for example, basic salts.
The task forming the basis of the invention is solved by the features of the main claim and promoted by those of the subsidiary claims. Such kits for the production of the mixtures are characterised in that they contain at least the following components:
A) 100 wt. parts of an at least bifunctionally terminated diorganopolysiloxane, which is built up of a linear or branched chain of repeating units of the formula
 and—as illustrated in the following in the case of a linear chain—is terminated with functional end groups Z
 There hereby mean:
R
1
, R
2
: saturated or unsaturated hydrocarbon radicals with 1-15 carbon atoms, possibly substituted with halog

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