Study On Photostabilization Of Polyurethane Coatings Modified By Blending/Copolymerization

Polyurethane coatings were widely applied in many fields of national economy and defense industry because of their favorably comprehensive properties. In order to promote photostabilization of polyurethane coatings, the mechanism of photo-induced degration has been comprehensivly studied. And some ways and means of photostabilization have been developed successively. For instance, good performance of light could be achived with addition of pigments and photostabiliser into polyurethane coatings system. However, UV is the reason of coatings'photoaging, at the same time it is also the resource of energy for UV cured coatings. With the development of UV curing technique, it is possible to develop a new photostabilization method of polyurethane coatings based on the mechanism of UV cured coatings.In this dissertation, three kinds of UV curing monomers(A,B and C) and three kinds of UV curing resins (D,E and F) were used as modifier, and added into two different polyurethane coating systems(PU685 and MPU) respectively. The effect of polyurethane coating systems, the category and amount of modifier and QUV aging time on the photostabilization of PU coatings were investigated from the compatibility between modifiers and polyurethanes, the hardness, gloss, YI (yellow index) and aging evaluation by ways of thermal analysis, UV-Vis and FTIR. The results showed as follows.①All the six modifiers and the two selected polyurethane coating systems had good compatibility.②Before QUV aging, the pencil hardness of modified samples were 2H, which was not changed compared with the blank sample of polyurethane.③Comparing the hardness, gloss, YI, gloss reduction and apparent morphology of the blank PU685 and MPU sample after QUV aging for 1300h, it was found that there were remarkable differences between them. All of PU685 blank samples had cracks penetrated through the whole film, whose hardness descended from 2H to HB, gloss reduction was 49% and YI was 20.6%. However, there were no cracks after aging for MPU samples. The hardness only reduced from 2H to H, gloss reduction was 28%, and YI was 11.2%. These differences were possibly attributed to the different matrix resin and formulation.④The optimal modified results for PU685 were as follows: gloss reduction could be reduced to 28% with the addition of 2.5% C, and YI was reduced to 16.5% with 2.5% B added. To MPU, the optimum modified results were that gloss reduction could be reduced to 17.5% with the addition of 2.5% B and YI was reduced to 16.5% with the addition of 1.25% A. Therefore, the modifiers with good gloss retention and yellowing resitance were different for different polyurethane coating systems. Even for the same polyurethane coating system, the modifier of optimum gloss retention was inconsistent with that of yellowing resistance. So a simple formula of relative evaluation was presented based on the grade of crack ,rust, gloss reduction and yellow index, by which the optimum modifiers for PU685 and MPU were 3.75%A and 2.5%B, respectively.⑤The UV-Vis absorption peak of modified MPU were remarkably higher than that of the blank,which arranged in order by MPU-5%B > MPU-5%D > MPU-5%C > MPU- 5%F > MPU-5%E > MPU-5%A > MPU-blank. The ultraviolet radiation could accelerate not only cure reactions of modifiers but also degradation reactions of coatings. If the cure reaction was in ascendant during QUV aging, the degradation of the blank samples would be worse than the modified ones and vice versa. Thereby, it was estimated that the aging degree of MPU-5%B might be lightest and that of MPU-5%A be worst. This presume was basicaly proved by the comprehensive evaluation of the samples after QUV aging for 1300 hours, the result of which showed in order by MPU-5%B < MPU-5%E < MPU-blank < MPU-5%C < MPU-5%D < MPU-5%F < MPU-5%A.⑥The FTIR results showed that there were—OH and—C=O absorption peak after QUV aging which indicated that ageing degradation had occurred to some degree. A spot of =CH2 absorption peak of PU685 and MPU modified templet could be detected after aging for 1300h.⑦The possible mechanism which the modifiers should act as might be concerned with the following aspects. Firstly, there might be reactions between the modifiers during QUV aging. Secondly, some synergic effects might occur between the modifiers and the antioxidants. Moreover, there were reactions between the modifiers and the conjugate benzene imido, which might alleviate the yellowing of the films.