materials testing | Weatherability
The Atlas Ci5000 tests the weatherability of materials to the D7869 standard
how tests apply to specific applications. The Vinyl Siding Institute (VSI), for example, certifies
its products based on outdoor weathering studies and uses comparative accelerated weathering as an interim step between required outdoor weathering studies as a check. “We did a number of studies comparing the results of different accelerated cycles with the results of outdoor weathering, and several were promising. But in the end we were not able to achieve a degree of confidence in the correlation between the two that would be needed to certify resistance to colour change based on accelerated weathering,” says Dave Johnston, senior technical director at VSI. “Manufacturers do use accelerated weathering for screening and internal quality control, but we do not yet use it for industry- wide standards.” Correlating outdoor weathering with accelerated lab
quantitative science,” says Quill. “It is, however, predictive and comparative. The new test is a tool that allows accurate prediction of what coating will perform better than another in the field.” Companies, such as Ford and Honda, have adopted the ASTM method into their material specifications, and other companies are considering it. “We have seen an unbelievable response to the new method. After a webinar, we gave away 1,000 copies of the method in just a few hours,” enthuses Quill. Because D7869 gives better approximations of
real-world weathering, it could lead to refinements in polymer stabilization systems, says Finnegan. He notes that another new standard, ASTM D7356-13, which BASF also helped create, simulates the effects of acid rain on automotive clearcoats using xenon-arc weather- ing devices. “This standard is interesting since many plastics are also exposed to acid rain not only in automotive applications, but other areas as well, such as building roofing membranes,” comments Finnegan. “We know that acid rain negatively impacts light-stabi- lizer performance in plastics, but haven’t had a standard way of demonstrating this effect. Now we may see this standard being adopted for application areas outside of automotive coatings, and formulations shifting towards less interactive light stabilizer formulas.”
Siding and decking Weatherability testing is a crucial part of formulation development in building and construction applications, and much work has been done to better understand
38 COMPOUNDING WORLD | June 2014
testing is particularly difficult for exterior products, such as siding profiles, agrees Chris Piazza, technical development director for plastics at Americhem. “Factors such as the freeze/thaw cycle and humidity affect weathering outdoors,” he notes. Americhem is currently compiling data comparing outdoor and lab tests for a range of polymer and colour combinations. “This study will give us a better idea of testing condi- tions to use to increase the confidence we can have in accelerated testing.” The compounder uses accelerated testing to screen and compare formulations. Wood-plastic composite (WPC) decking is another
application in which weathering is critical. The decking market has evolved to widespread use of coextruded structures, in which the top layer is designed to protect the board from weathering, notes Piazza. The top layer may be a weatherable acrylic, acrylic blend, or heavily stabilized PVC or polyolefin. Americhem introduced its eCap capstock compounds, which can be used with either PVC or polyolefins in such structures. The cap layer also helps prevent “moisture whitening,” in which voids in the deck boards absorb moisture, which then reflects light and appears whiter than other areas.
Weathering of ASA polymers Acrylonitrile styrene acrylate (ASA) polymers have a reputation for good weathering resistance, but still must be formulated to provide desirable weathering performance. “The appropriately fine-tuned combina- tion of polymer, UV stabilizer, colour, and other additives is what makes an ASA polymer weatherable,” says Steve Blazey, senior technology fellow at Network Polymers, now part of A. Schulman. ASA is used as capstock for sheet and profile, such as siding or decking, and also in injection-moulded applications, such as vehicle mirror housings. “Custom-
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