A scratch-resistant polyurethane coating for safety windows is made by curing a sheet of a liquid composition comprising (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing 10.5%-12.0% free hydroxyls and (2) a biuret of 1,6-hexamethylene-diisocyanate, having 21-22% isocyanate groups, the weight ratio of said biuret to said polyglycol ether being about 0.9-1.1.
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1. A safety window comprising at least one glass sheet having bonded thereto an adherent solid transparent layer of a scratch-resistant polyurethane formed by curing a liquid mixture containing (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing about 10.5-12% by weight free hydroxyls together with (2) a biuret of 1,6-hexamethylene diisocyanate having about 21-22% by weight isocyanate groups, the weight of said biuret being between about 0.9 and 1.1 times the weight of said polyglycol ether.
2. A safety glass laminate of a plurality of solid glass sheets having disposed between adjacent sheets an adherent, transparent solid interlayer of polyvinylbutyral and having bonded to at least one outer glass sheet of this laminate an adherent solid transparent layer of scratch-resistant polyurethane formed by curing a liquid mixture containing (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing about 10.5-12.0% by weight free hydroxyls together with (2) a biuret of 1,6-hexamethylene diisocyanate having about 21-22% by weight isocyanate groups, the weight of said biuret being between about 0.9 and 1.1 times the weight of said polyglycol ether.
3. A two-layered article comprising a layer of a scratch-resistant polyurethane cast onto a casting support, wherein said polyurethane layer is formed by curing a liquid mixture containing (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing about 10.5-12% by weight free hydroxyls together with (2) a biuret of 1,6-hexamethylene diisocyanate having about 21-22% by weight isocyanate groups, the weight of said biuret being between about 0.9 and 1.1 times the weight of said polyglycol ether. 4. The two-layered article of
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glass glazing on the surface facing the interior of a vehicle.
The curves of FIG. 4 show clearly the advantage obtained by the process of this invention. It is evident that products with W2 /W1 substantially exceeding unity have significantly diminished scratch-resistance even without high heat treatment (Curve C) and that heat treatment at greater than 90°C substantially decreases this resistance by amounts which make the product commercially unacceptable. In particular a ratio of W1 /W1 =1.28, corresponding to stoichiometric equivalence, is completely unsatisfactory.
The overall thrust of the information in these figures is that the weight ratio of polyglycol ether to biuret components of the composition of this invention should be selected between 0.9 and 1.1, preferably close to unity. in order to make a laminate of the cured polyurethane on glasssafety glazing which can be subjected to high temperatures. It may be confidently predicted that this region of composition is also advantageous even when the layer is not subjected to a high temperature treatment, inasmuch as it is well known that the deterioration process which takes place at elevated temperatures involves an acceleration of aging phenomena which take place more slowly at lower temperatures.
FIGS. 5 and 6 illustrate assemblies in which the scratch-resistant coating of this invention is bonded to a glass sheet. In FIG. 5 there is exemplarily one glass sheet 1 and one layer 2 of the polyurethane coating of this invention adhered thereto. In FIG. 6 there is illustrated an assembly of at least two sheets 3 and 4 of glass having disposed between them sheets of an adherent transparent interlayer 5 of polyvinylbutyral and having bonded to at least one outer glass sheet of this laminate an adherent solid transparent layer 6 of the scratch-resistant polyurethane of this invention.
In preparing the polyurethane coating composition from a mixture of components 1 and 2, it is advantageous to include also an accelerator. Substances which are particularly suitable as accelerators are the dialkyltin carboxylates and stannous carboxylates, including exemplarily dibutyltin di-(2-ethylhexoate), dibutyltin dilaurate, stannous oleate and stannous octoate. Zinc carboxylates such as zinc octoate or zinc naphthenate can also be used. Dibutyltin is preferred.
Since the accelerator can also catalyze the aging of the cured film, the weight proportion of accelerator, based on the polyglycol component 1, should not be greater than about 0.1%. It is preferred that the weight ratio of accelerator to component 1 should be between about 0.01 and 0.07%. The most suitable value of such ratio is about 0.045-0.055%.
It is also suitable, as known, to add along with the accelerator an antioxidant stabilizer, for example one of the polyalkylphenols having sterically hindering substituent groups in one or both of the ortho positions. A particularly suitable stabilizer is 2,6-di(tert.butyl)-p-cresol. The weight percentage of stabilizer, based on the polyglycol component 1 can be between 0 and 5%, suitably between 1 and 5%, the preferred percentage being about 2-2.5%.
To achieve a polyurethane coating composition according to this invention, a mixture is made, at a temperature between about 20° and 50°C, as follows: To the mixing vessel there is first added 1000 grams of the polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane, having a molecular weight about 450 and containing 10.5-12% free hydroxyls. To this polyglycolether there is added 23 grams of 2,6-di(tert.butyl)-p-cresol as stabilizer and 0.5 grams of dibutyltin dilaurate as accelerator. When these ingredients are well-mixed, there is further added 1000 grams of the biuret of 1,6-hexamethylene diisocyanate having 21-22% isocyanate groups.
A homogeneous mixture is obtained by stirring carefully to avoid the formation of air-bubbles or blisters.
This liquid mixture is then cast into a film by pouring on to a substrate or support exemplarily of glass, where it polymerizes or cures to acquire its characteristic film structure. The curing or drying time, which is 24 hours at 20°C, can be reduced to 15 minutes by heating at 140°C If it is desired to remove the layer of cured film from its support, the surface of the glass or other support is advantageously first treated with a release agent such as magnesium stearate.
The cured coating or film prepared in this manner has the following properties:
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| Breaking strength 100-140 kg/cm2 |
| Elongation at break 100-135% |
| Tear Resistance 15 kg/cm |
| Scratch Resistance 25-30 grams |
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A particular advantage of the liquid composition of this invention is its relatively low and manageable viscosity before curing.
If, contrary to the teaching of the instant invention, the weights W1 and W2 respectively of components 1 and 2 are used in proportion so that W2 /W1 =1.28 (i.e. 128 parts by weight of component 2 are mixed with 100 parts by weight of component 1, it is extremely difficult to operate without addition of solvent because the viscosity of this mixture is 4,600 centipoises at 20°C On the other hand when the ratio of W2 /W1 =1.00 as in this example of the instant invention, the liquid composition has a viscosity of only 2,800 centiposies at the same temperature.
It is of course possible to decrease the working viscosity of prior-art compositions by adding solvents. However, this gives rise to other disadvantages. The time required for drying and curing is prolonged. More elaborate drying equipment is necessary, including installations for solvent recovery. Additional expenses are involved both for building the equipment and for operating with the elevated output of heat energy needed.
Furthermore, in addition to these disadvantages in handling, it is extremely difficult to eliminate solvent residues completely, giving rise to deficiencies in the final curing at elevated temperatures such as blistering and non-uniformity of adhesion.
By avoiding the necessity of resorting to solvents, the compositions of the instant invention are substantially improved over the prior art.
Raedisch, Helmer, Schafer, Wolfgang
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Jan 30 1985 | Saint-Gobain Industries | (assignment on the face of the patent) | / |
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