Date of Award
MS in Polymers and Coatings
Chemistry & Biochemistry
Raymond H. Fernando
Plastics have become a universal material for use in a myriad of commercial and consumer products. One such product, exterior siding, is the focus of this project. Although siding products were originally made from wood, vinyl siding, which offered superior performance, was introduced in the 1950’s. More recently, polypropylene (PP) siding has been introduced; PP provides a stronger product, which allows for deeper patterns and better edge detailing. PP siding, compared to traditional wood siding, doesn’t warp, crack, or degrade as easily with extended exposure to the elements, and is cheaper to maintain. However, even plastic siding must be coated. The requirements of a coating for siding are good adhesion, durability, and a suitable appearance. However, polypropylene, like many plastics, has a low surface energy, making wetting and coating adhesion difficult.
One of the many ways to increase the surface energy of polypropylene, thus increasing wettability and adhesion, is plasma treatment. The primary focus of this project was to study how plasma treatment improved adhesion of a water-reducible coating. This coating represented a product used in commercial siding. The surface tension of the panels was increased from ~30 dynes/cm to 60+ dynes/cm with plasma treatment. This increased the adhesion of the coating to the polypropylene panel from virtually no adhesion to almost perfect adhesion. Adhesion was tested according to ASTM D3359, the crosscut adhesion test. Pull-off adhesion testing (ASTM D4541-09) was also conducted, using a Deflesko PosiTest AT-A automatic adhesion tester. The average force needed to remove a dolly from a plasma treated panel was 233 ± 47 psi (1,605 ± 325 kPa), compared to 92 ± 26 psi (634 ± 179 kPa) for non-treated and corona treated panels.
The sponsor of the project provided Cal Poly with 16 different compositions of polypropylene containing different amounts of UV stabilizers, adhesion promoters, and lubricants. The effect of substrate composition on coating adhesion and performance was measured. Crosscut adhesion testing results revealed all polypropylene compositions improved from virtually no adhesion to perfect adhesion after plasma treatment. Pull-off adhesion testing revealed the adhesion force of all compositions improved from less than 100 psi to greater than 200 psi. One polypropylene composition, Category 16, resulted in unusually high pull-off forces. This composition was investigated using X-ray Photoelectron Spectroscopy (XPS) and FT-IR spectroscopy. XPS was used to examine the surface composition between non-treated and plasma treated PP panels. It was observed that plasma treatment provides a larger amount of oxygen species and nitrogen when compared to untreated panels. The category 16 panels did not reveal any significant surface differences compared to the category 7 panels (which represented the standard production material). FT-IR spectroscopy of the category 16 panels also showed no unusual characteristics.
The secondary focus of this project was to study the durability of coated, plasma treated polypropylene siding. Accelerated weathering testing was conducted on 12 of the 16 different compositions of polypropylene. Changes in gloss and the LAB colorspace of coated, plasma treated polypropylene panels of different compositions, upon exposure to long-term weathering conditions, were monitored via ASTM G53 using a Q-Panel lab product QUV/se weathering tester. After 2400 hours, all PP compositions tested shared negligible changes in color, but the gloss of each category panel showed a steady increase.
An approach to improve durability of siding is to apply a clearcoat over already coated PP panels. This approach was tested in a limited manner by adding a clearcoat to coated, plasma treated polypropylene panels. These panels were then exposed to a variety of common, household cleaning agents using a modified double rub test (ASTM D4752 and ASTM D5402). The samples with a clearcoat showed improved cleaning agent resistance compared to samples without the clearcoat.