Ultraviolet absorbing film

Paints are commonly divided into oil-base, or solvent systems, and latex, or water-base, paints. These paints are essentially a suspension of inorganic pigments and a resin, or vehicle, in a suitable solvent. Paints protect wood surfaces by forming an ultraviolet absorbing film that adheres strongly to the surface. 

To improve the resistance to ultraviolet light carbon black is often useful as a light screen. Its use in fibres and films is clearly very restricted and in these instances ultraviolet absorbers and/or quenching agents are used. Recent developments include the greater use of hindered amine and nickel compounds. 

Because polycarbonates are good light absorbers, ultraviolet degradation does not occur beyond a depth of 0.030-0.050 in (0.075-0.125 cm). Whilst this is often not serious with moulded and extruded parts, film may become extremely brittle. Improvements in the resistance of cast film may be made by addition of an ultraviolet absorber but common absorbers cannot be used in moulding compositions because they do not withstand the high processing temperatures. 

Hindered amines, such as 4-(2,2,6,6-tctramcthylpiperidinyl) decanedioate, serve as radical scavengers and will protect thin films under conditions in which ultraviolet absorbers are ineffective. Metal salts of nickel, such as dibutyldithiocarbamate, are used in polyolefins to quench singlet oxygen or electronically excited states of other species in the polymer. Zinc salts function as peroxide decomposers. 

In less than 50 hours, the unstabilized film cracked when flexed 90°. The film containing 0.25% PPP was protected for 152 hours, while the combination of 0.4% DLTDP 4 0.2% PPP failed after 274 hours. The commercial hydroxybenzophenone at 0.5% prevented embrittlement for 580 hours, while at 0.25% this compound protected the film for approximately 300 hours. By adding 0.25% PPP and reducing the amount of hydroxybenzophenone to 0.1%, the film lasted for 525 hours before embrittlement. The two commercial benzotriazole ultraviolet absorbers at 0.25% protected the polypropylene films for 290 hours and 425 hours, respectively. 

In general, the substituted benzophenone and substituted benzo-triazole ultraviolet absorbers were equally effective in protecting vinyl films during outdoor exposure in the absence of biocide. In the presence of the organic sulfur biocide failure was evident the arsine-epoxy adduct produced no adverse effect. The organic tin biocide was only slightly less effective (Table VI). 

Kawabata T. Transparent ultraviolet absorbing protective Film. Patent Application 1994 JP 6297630. 

Measurement of acetic acid accumulation at ultraviolet irradiation of stabilized and unstabilized CDA film were carried out for quantitative evaluation of stabilizing activity of PAC as ultraviolet absorbers. Relation of the rates of acetic acid in the absence (Wo) and presence (Wst) of additives was recognized as characteristic of phototransformation [10]. 

In contrast, the o-hydroxybenzophenone had no noticeable effect on the decomposition chonistry. Though it has been conmonly referred to as an ultraviolet absorber, it actually behaves more like a deactivator since the screen film containing it had a lower oxidation rate in Fig. 11 than the underlying clear film devoid of additives. Absorption can hardly account for the protective action of the stabilizer in thin films which are almost completely transparent to 300nm radiation. Neither can the hydroxybenzo-phenone be an important terminator of radical chain reactions because it is much less effective than other hindered enols in inhibiting oxidation in the dark. 

Tocker [143] has observed a better light stability of polyethylene films with copolymerized ultraviolet absorbers than with dispersed ultraviolet absorbers. This effect is probably due to the absence of diffusion out of... 

The mechanism of photodegradation of certain hydroxyphenyl benzotriazole based ultraviolet absorbers has been investigated and a new polymerizable ultraviolet absorber in this group has been synthesized. The photoreactivity is entirely confined at the surface of polymethylmethacrylate films containing the ultraviolet absorbers as pendant groups. A mechanism involving sensitized photooxidation has been proposed to interpret the data. 

In conclusion, we have investigated the mechanism of sensitized photooxidation of ultraviolet absorbing clear acrylic films containing pendant ultraviolet absorber groups. The main conclusions of the mechanistic study indicated that propenyl derivatives of ultraviolet chromophores, copolymerization of which would lead to development of methyl groups on the backbone would be more appropriate candidates for outdoor applications requiring long service life. Synthesis of the first such comonomer has been reported here. 

Ultraviolet absorbers were among the first organic stabilizers used. They are colorless compounds that strongly, but selectively absorb ultraviolet radiation and harmlessly dissipate it as heat so that it does not lead to photosensitization. They are also characterized by their very good stability to the absorbed radiation. However, based on the UV absorption mechanism alone, they can only provide limited protection to surface layers and thin samples, for example fibers and films. In accordance with the Beer-Lambert absorption law, the amount of radiation reaching any particular layer diminishes exponentially with the distance from the exposed surface. Thus, the effectiveness of protection via screening of the actinic radiation from the polymer by the UV absorber increases with sample thickness. Protection by UV absorbers is most effective when the additive is concentrated on the surface, such as when it is incorporated in a thin film coextruded over the polymer [75]. 

This expression is simply the ratio of the radiant energy absorbed by an unprotected polymer to the energy absorbed by the polymer in a system protected with an ultraviolet absorber. The denominator in this expression will always be less than the numerator since the absorber will prevent part of the incident radiation from being absorbed by the polymer. The more effective the absorber in this respect, the greater will be the value of the protective efficiency . By means of Eq. (46) it is possible to compare the protective capacity of the various absorbers, to estimate which pol3uners are better protected by an absorber, and to determine the effect of film thickness and absorber concentration on the efficiency of the UV absorber. 

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