Influence of Titanium Dioxide
Titanium dioxide is a white pigment used in many sealant formulas. In this study, different grades of titanium dioxide, which differ with respect to the surface treatment applied by the manufacturer (inorganic metal oxide and/or organic), were compared. As base sealants, a one-component polyurethane (PU-1) and a one-component modified silicone (MS-1) sealant formula were selected.
In the PU-1 model formula (Table 2), three different grades of titanium dioxide, which have a surface treatment of alumina (type A), alumina and silica (type B), and alumina, silica, and an organic coating (type C) were used. In each case, 4 % of titanium dioxide was used. Titanium dioxide type A and type В gave comparable weathering results, whereas type C, which has an organic coating in addition to the inorganic coating, gave a significantly better result.
Table 3 shows the influence of three different grades of titanium dioxide on the weathering performance of an acrylic MS-1 sealant. In this series titanium dioxide type F (alumina, silica and zirconia surface treatment), a grade optimized for weatherability, gave the best weathering performance in the early stages of exposure. After 1200 h of exposure the three different grades showed a similar behavior: severe cracking was observed in all samples.
TABLE 3—Influence of titanium dioxide grade on weathering behavior of an acrylic MS-1 sealant.
a3.3 % Titanium dioxide; thickness of film tested 0.2 mm; samples exposed in an open flame carbon arc weathermeter5.
bCracking 1: no cracks; 2: very small cracks; 3: small cracks; 4: large cracks; 5: entire surface covered with large cracks.
cChalking: 1: no chalking; 2: onset of chalking; 3; some chalking; 4: chalking; 5; severe chalking.
TABLE 4—Influence of various polypropylene oxide (PPO)-based plasticizers (acetate end-capped) on weathering behavior of an acrylic MS-1 sealant (cracking f’b.
“20 % Plasticizer. 25 % acrylic modified silicone polymer; samples exposed in an open flame carbon arc wealhermeter.
bCracking l: no cracks; 2; very small cracks; 3: small cracks; 4: large cracks; 5; entire surface covered with large cracks.
The data presented here are too limited to allow drawing general conclusions as to the best type of titanium dioxide grade or best surface treatment. The results demonstrate, however, that the type of titanium dioxide has a significant influence on the weathering behavior of a sealant and that the grade has to be selected carefully when high weatherability is needed.