1. Field of the Invention
The present invention relates to a crosslinkable polymer composition containing a crosslinkable polymer with hydrolysable silane groups and at least one silanol condensation catalyst.
2. Description of Related Art
It is known to crosslink different polymers by means of additives. Crosslinking improves such properties of the polymer as its mechanical strength and heat resistance. Polymers normally considered to be thermoplastics, and not crosslinkable, can also be crosslinked by introducing crosslinkable groups in the polymer. An example thereof is the crosslinking of polyolefins, such as polyethylene. A silane compound can be introduced as a crosslinkable group, e.g. by grafting the silane compound onto the prepared polyolefin, or by copolymerisation of the olefin and the silane compound. This technique is previously known, and further details may be obtained from U.S. Pat. Nos. 4,413,066; 4,297,310; 4,351,876; 4,397,981; 4,446,283; and 4,456,704, all of which are incorporated herein by reference.
The crosslinking of polymers with hydrolysable silane groups is carried out by so-called moisture curing. In a first step, the silane groups are hydrolysed under the influence of water, resulting in the splitting-off of alcohol and the formation of silanol groups. In a second step, the silanol groups are crosslinked by a condensation reaction splitting off water. In both steps, a so-called silanol condensation catalyst is used as catalyst.
Prior-art silanol condensation catalysts include carboxylates of metals, such as tin, zinc, iron, lead and cobalt; organic bases; inorganic acids; and organic acids.
Mention should here especially be made of dibutyl tin dilaurate (DBTDL), dibutyl tin diacetate, dioctyl tin dilaurate, stannous acetate, stannous caprylate, lead naphthenate, zinc caprylate, cobalt naphthenate, ethyl amines, dibutyl amine, hexylamines, pyridine, inorganic acids, such as sulphuric acid and hydrochloric acid, as well as organic acids, such as toluene sulphonic acid, acetic acid, stearic acid and maleic acid. Especially the tin carboxylates are much used as catalysts.
EP 0 475 064 further teaches the crosslinking of silane-containing polymer compositions with hydrolysable groups by means of a zeolite that has been modified by being reacted with at least one carboxylic acid and/or sulphonic acid at a temperature of at least 150.degree. C. Examples of sulphonic acids are toluene sulphonic acid and alpha- and beta-naphthalene sulphonic acid. The crosslink is said to be of essentially non-hydrolytic type.
WO 91/09075 further discloses the use of a blocked acid, such as an acid anhydride, e.g. stearic acid anhydride or benzoic acid anhydride, as silanol condensation catalyst. Such catalysts counteract undesirable premature crosslinking, commonly referred to as scorching or precuring, of the polymer composition.
GB 2,028,831 and EP 0,193,317 may be mentioned as examples of the prior art relating to the crosslinking of polymers containing hydrolysable silane groups by means of the above-mentioned silanol condensation catalysts. EP 0,207,627 also discloses the use of a special silanol condensation catalyst in the form of a tin-containing polymer.
Although the above silanol condensation catalysts, and in particular the tin carboxylates, are frequently used in the crosslinking of polymer compositions containing silanol groups, they are disadvantageous in some respects. Thus, efforts are being made to find silanol condensation catalysts reducing or obviating these disadvantages.
For instance, prior-art silanol condensation catalysts function satisfactorily only at elevated temperatures in the order of 80-100.degree. C. and give a poor performance at normal ambient temperatures and relative humidities, such as room temperature (about 20-25.degree. C.) and 50% RH. In many contexts, such as the production of cable insulations or water pipes, it is desirable that the silane-containing polymer composition can be crosslinked at room temperature without the use of water baths or steam cabinets. The degree of crosslinking of the polymer composition is measured as the gel content after crosslinking at a certain temperature for a certain period of time. It is desirable that crosslinking at room temperature for four days should result in a gel content of at least about 65% and a catalyst loading of about 1 mmole/kg composition. This is not achieved by existing silanol condensation catalysts, and it would therefore be a considerable step forward to provide a silanol condensation catalyst meeting this requirement.