Zinc coating—modified polyphenylene oxide

Coatings can interact with the substrate as shown with zinc on modified-polyphenylene oxide structural foam. Examination of zinc on modified-polyphenylene oxide has been undertaken to ascertain the origin of the synergism and is reported separately. The presence of zinc shows an increase in char, and a decrease in low molecular weight volatiles at pyrolysis temperatures versus the uncoated substrate. 

An initial experiment involved determination of Arapahoe Smoke Chamber results for samples with and without the zinc coating present. Data are presented in Table II. Depending upon orientation of the sample, an increase in char occurred for some samples with zinc present, while no change in smoke formation was seen. Initial pyrolysis GC/mass spectroscopy results at 90CPC in helium showed no difference in volatiles formed with or without zinc. These results suggested enhanced char formation as the origin of the Radiant Panel results for zinc on modified-polyphenylene oxide (m-PPO). Zinc oxide is a known, effective thermal stabilizer in the alloy. The next work then focused on DSC/TGA studies. 

Two of the zinc surface samples show results that might intuitively be expected for metal coated surfaces. The polycarbonate and RIM-polyurethane substrates show much lower Fs values with the zinc coating as expected, but Q values are very similar to that of the uncoated substrates. For modified-polyphenylene oxide, however, a very low Q value is obtained suggesting flame... 

Table III. Analysis of Carbon and Hydrogen Content of Char Samples for Modified Polyphenylene Oxide With and Without Zinc Coating...

Nelson [136] has reported studies of zinc, zinc oxide, and zinc borate in coatings on or as a filler in modified polyphenylene oxide (m-PPO). Zinc arc spray, or zinc, zinc borate, and zinc/zinc borate in epoxy coatings showed a substantial reduction of flame spread index (ASTM E-162) (I,) for m-PPO. Zinc oxide in epoxy, however, showed a dramatic increase in I, on m-PPO. Zinc arc spray on m-PPO led to enhanced stability in the 500-600°C range in both isothermal and GC/MS experiments. It was speculated that since zinc melts at 420OC, just at the early stage of decomposition of m-PPO, this could allow intimate contact with the charring substrate. As in pure polystyrene, char formation is enhanced in air in m-PPO, and this was thought to be enhanced further by the presence of zinc. Indeed it was observed that volatilization of small molecules is reduced for m-PPO with zinc present at temperatures under 700°C, with preference for volatilization of the triaryl phosphate flame retardant, styrene trimer, and PPO dimers. 

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