| 摘要 |
1. A sheet of transparent plastics material of high optical quality comprising a layer based on polyurethane having energy-absorbing properties, characterised in that the layer having energy-absorbing properties is formed essentially of a polyurethane obtained by reactive casting on a flat horizontal support of a reaction mixture of an isocyanate component of viscosity less than 5 Pas at 40 degrees C and a polyol component, the isocyanate component comprising at least one aliphatic or cycloaliphatic di-isocyanate or an isocyanate prepolymer, the isocyanate component containing urea functions, the content of urea being up to 10% of the total weight of isocyanate component, the content of urea preferably being from 5 to 7%, the polyol component comprising at least one long difunctional polyol of molecular weight from 500 to 4000 and at least one short diol as a chain lengthening agent, the ratio of isocyanate group equivalents to hydroxyl group equivalents is about 1, the proportions of the different polyols are selected such that the number of hydroxyl group equivalents due to the short polyol represents from 20 to 70% of the total hydroxyl groups, and this layer having at a thickness of about 0.5 mm, a flow stress sigma y at -20 degrees C not exceeding 3 daN/mm**2, a rupture stress sigma R at 40 degrees C of at least 2 daN/mm**2, a lengthening at rupture vepsiln R at + 20 degrees C from 250 to 400%, and a tear resistance Ra at + 20 degrees C of at least 9 daN/mm. |
