The invention relates to a radiation-curable liquid resin for a secondary coating of lightwave guides. The invention further relates to a secondary coating produced from the liquid resin and a lightwave guide with this secondary coating.
Lightwave guides in general have two coatings: a soft primary coating and a firm secondary coating. Currently, primary coatings are predominantly radiation-cured urethane acrylates. As secondary coatings, radiation-cured epoxy acrylates or urethane acrylates are predominantly being used (see, for example: "Polym. Mater. Sci. Eng.," Vol. 55, 1986, pages 536 to 539). The resins used as coatings must be radiation-curable so as to permit rapid and cost-effective curing. The resins must also be liquid so that it is possible to process them essentially solvent-free in order to meet the requirements regarding environmental protection.
The epoxy and urethane acrylates used as secondary coatings have a sufficiently high glass transition range (Tg.gtoreq.+60.degree. C.), and they are compatible with a primary coating based on urethane acrylate. In the manufacture of coatings, it is also important that the appropriate liquid resins have high radiation sensitivity in order to permit high curing rates.
For example, "Chemical Abstracts," Vol. 106 (1987), page 7, No. 161368y, discloses a coating material for optical glass fibers on a polyoxyalkylene base which can serve for the production of secondary coatings. This coating material contains oligomers which are obtained through the conversion of hydroxy-terminated aliphatic linear compounds, such as polypropylene glycol, with diisocyanates and trishydroxyalkylisocyanurate-di(meth-acrylates. Such resins have more than two radiation-curable groups per molecule so that they should have a high curing rate. However, the coupling of the radiation-curable groups via bis-urethane structures (using diisocyanates) results in resins having a high viscosity. It is therefore required to add to these resins large quantities of reactive thinners in order to be able to apply them as coating materials. This thinning step not only entails additional production costs but it is also accompanied by other disadvantages due to the high vapor pressure of the reactive thinners.
It is an object of the invention to provide a radiation-curable liquid resin which has a high radiation sensitivity.
It is another object of the invention to provide a radiation-curable liquid resin with a viscosity which, at most, requires the addition of only relatively small amounts of thinning agents, to be used for the manufacture of a secondary coating of lightwave guides.
It is a further object of the invention to provide secondary coatings for lightwave guides which have a glass transition range of .gtoreq.60.degree. C. and which are compatible with the primary coatings, especially primary coatings based on urethane acrylates.