Лако-красочные материалы — производство

With the considerations noted above, we can now look at the performance of UV stabilizers in several illustrative examples. Table 1 shows the effectiveness of BTZ-2 and HALS-1 at preventing the degradation of a natural rubber/hydrocarbon tackifier water — based adhesive when exposed to UV light. Natural rubber undergoes chain scission upon oxidation, resulting in a loss of molecular weight and hence a loss of cohesive strength. Samples were coated on polyester film, adhered to a glass plate, and exposed in a carbon — arc weatherometer (CAW). The extent of degradation was monitored by following dis­coloration, and by peeling the tape from the glass and observing cohesive failure. As shown by the data and as would be predicted from the arguments above, a HALS is more effective than the BTZ at protecting the adhesive. The BTZ/HALS combination

was as effective as the HALS alone, which may indicate that a lower concentration of HALS would be sufficient under these exposure conditions.

In the second example (Table 2), two acrylic adhesive formulations are compared. The first contains a hydrogenated rosin ester tackifier, which shows relatively good UV stability. This tackifier can be replaced by one of lower stability (and lower cost), such as the nonhydrogenated rosin ester, by the incorporation of the appropriate light stabilizers. As these data show, some improvement in stability is seen when a BTZ is used at 0.5% (or a combination of BTZ/HALS at 0.25%/0.25%), but the best stability is achieved when the adhesive is stabilized with a HALS at the same level (0.5%). It should be pointed out that BTZs are commonly used in films and coatings (e. g., automotive clear coats) when the objective is to protect the material below the film. Similar applications can be imagined where UVA-containing adhesive films are used to screen and protect other substrates.

Conversely, in cases where the adhesive is very difficult to stabilize, the UVA can be incorporated into the substrate exposed to UV light to protect the adhesive.

To illustrate the effect of sample thickness, we can compare adhesives to sealants, which in many cases can be viewed as thick adhesives. As the sample thickness is increased, the benefit of the BTZ is clearly demonstrated. In Table 3 a SEBS/hydrogenated hydrocarbon tackifier sealant formulation was prepared as a hot melt and poured into shallow petri dishes. Although both of these polymers have good inherent stability, sealant applications may require extended exposure to UV radiation. The discoloration data show that the BTZ prevents yellowing of the sealant. However, examination of the sealant surface shows surface crazing and cracking when not protected by incorporation of the HALS. The combination of the two classes of light stabilizers provides the best overall performance.

As shown above regarding acrylic adhesive formulation, the use of light stabilizers may allow the incorporation of a less stable (and less expensive) tackifier in sealant formulations. The data in Table 4 compare two SEBS sealants, one containing a

hydrogenated hydrocarbon tackifier and the second containing a mixture of the same hydrocarbon tackifier with a rosin ester. Comparable stability to the ‘‘stable tackifier’’ can be achieved in a mixed formulation with a combination of BTZ and HALS.