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The Case of the Peeling Aluminum Siding Paint
By Rick Huntley, Coatings Consultant, KTA-Tator, Inc.
A manufacturer of aluminum products makes and sells wood grain embossed coated aluminum siding panels. The siding manufacturer, in conjunction with the coating manufacturer, offered a 30-year warranty against peeling. The coating consisted of a Plastisol made of plasticized polyvinyl chloride, provided by a major coating manufacturer.
Within six years of manufacture, complaints were received about the peeling paint on the aluminum siding. Eventually, the siding manufacturer contracted KTA to determine the cause of the peeling. The painted panels are made from coil stock.
At the beginning of the manufacturing process, bare aluminum coil is unwound through an accumulator. From the accumulator, the coil winds through a caustic cleaning and etching solution, and is then rinsed. The coil continues from the rinse through a chromate conversion coating. The coil enters the solution on one side of the chromate conversation bath, and exits out the other side of the bath. The chemical composition and temperature of the bath is continuously monitored. After the chromated aluminum coil exits the bath, it is rinsed, dried, and stamped with a batch number.
After the batch number is stamped onto the coil, the aluminum strip is looped back to an area near the start of the line. The strip then enters a room where the primer is applied using reverse urethane rollers. A reverse roller applies paint to the coil by rotating over the surface in a direction opposite the direction of the movement of the coil. (If the coil strip is moving left to right, a reverse roller on top of the strip rotates clockwise.)
In the same coil coating machine, a thin clear backer material is also applied. The strip then passes through a drying oven. After the strip passes through the drying oven, it is routed into another paint room where the topcoat is applied using similar urethane reverse rollers. After the topcoat is applied, the strip passes through another drying oven, proceeds to another accumulator, and is then re-wound. The aluminum coil is then transported to a location to manufacture the embossed aluminum siding. At that location, the painted aluminum coil is unrolled, bent, embossed and cut into its final form, before shipment to individual homes for installation.
A majority of the peeling problems occurred on the south side of the residences where there were numerous incidences of paint peeling and cracking. Peeling and cracking were also found to a lesser degree on the west wall of the residences, and to an even lesser extent on the east wall. The north wall of each residence was free of peeling or cracking.
Close examination of the coating revealed that the cracking occurred along the borders of the embossed wood grain (which was formed after the aluminum coil was coated). In some cases, small pieces of the paint could be removed along the crack. When the surface of the aluminum was examined beneath poorly adherent paint, it was found to be relatively clean and free of visible contamination. There was no evidence of oil, dirt, or other foreign material that may have contributed to the poor adhesion.
The back sides of the delaminated paint chips were a slightly yellowish color as compared to the pure white front. The delaminated paint was also relatively brittle as compared to newly applied Plastisol paint. The adhesion of the coating in exposed areas on the south wall ranged from poor to fair. In areas on the south wall that were continuously shaded from sunlight, the adhesion was found to be excellent. The adhesion was also found to be excellent in all areas on the north wall of the residences.
Representative samples of sound and failing coating were removed from the residences and returned to the KTA laboratory for analysis. The laboratory analysis consisted of Fourier Transform Infrared spectroscopy (FTIR) and High Pressure Liquid Chromatography (HPLC). Here, the infrared analysis revealed that the paint was based on polyvinyl chloride, as expected. The infrared spectra of both failing samples and non-failing samples were very similar, although the failing samples appeared to have much less resin near the outer surface of the coating. It was also noted that the surface of the failing samples had chalked much more than the non-failing samples.
HPLC was performed on several of the samples. The analysis revealed that all of the samples consisted of a high molecular weight polymeric material, and a portion of a lower molecular weight plasticizer, believed to be a phthalate. The analysis also revealed that the samples from peeling areas contained from 6.0 to 7.9% plasticizer and the samples from non-failing areas contained 8.7 to 14.3% plasticizer.
The field investigation and the laboratory analysis both indicated that the delamination of the polyvinyl chloride (PVC) Plastisol coating from the aluminum siding panels was caused by a degradation of the plasticizer in the paint.
The PVC portion of the resin is relatively brittle, and by itself, it is not suitable for coating aluminum siding or most other substrates. To make the coating suitable for coating aluminum siding, plasticizer is added to the paint. The plasticizer makes the coating softer and more flexible. The flexibility is required to allow the coating to bend and stretch when the substrate bends and flexes.
In this case, it was necessary for the coating to be flexible enough not only to bend and stretch with the aluminum siding after installation, but also to withstand the bending and embossing process during the manufacturing of the siding. The loss of plasticizer was caused by exposure to the sun.
The failures were found predominantly on the south side, which received the most sunlight. Even on the south walls, the coating had good adhesion directly beneath overhangs, or in other areas continuously shaded from sunlight. Since the plasticizer is expected to withstand heat and U.V. light exposure, the failure is considered to be a result of a defect in the plasticizer.
If a plasticizer in an exterior PVC coating lacks proper heat stability and U.V. light resistance, the coating slowly loses the plasticizer upon exposure.
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