W. Stephen tait, ph.D. Chief Science Officer & principal Consultant, pair O Docs professionals, LLC Corrosion Corner Why do the coatings and laminate films on spray package metals corrode? Figure 1. Dry coating/laminate inside a new spray package 30 Spray December 2015 a polymer’s glass transition temperature is one type of polymer corrosion. Thus, Figure 1 illustrates an important property of polymer coatings and laminate films: Polymers are not perfect barriers between the environment (your formula) and the substrate package metal. Figure 2 depicts a polymer shortly after spray package filling. The various colored circles represent formula ingredients and the light blue area represents the formula’s matrix material (solvent or solvents). Initially, the formula matrix material and ingredients have not significantly absorbed into the polymer. However, osmotic forces cause liquid diffusion into the polymer micropores as indicated by the dashed circle. How fast your formula diffuses into the micropores is determined by the chemical composition of your formula; size, shape and length of the pores; and type of package metal. The interaction between the rate of diffusion through the micropores, chemical composition of your formula and type of spray package metal determines if polymer and/or metal corrosion occurs. Figure 3 illustrates one of many mechanisms for steady-state corrosion of the polymer with metal corrosion. The polymer is saturated with the (light blue) matrix liquid and select formula ingredients, represented by circles, inside the polymer and at the metal surface. The dashed lines near the center of Figure 3 represent delaminated polymer chains and branches. Coating/laminate corrosion in the form of delamination also begins when the polymer is at steady-state saturation. The time it takes for a polymer to become saturated is determined by the polymer chemistry and morphology, the polymer thickness and the chemical composition of your formula. Notice in Figure 3 that liquid has accumulated in an area around pitting corrosion in the metal. The circles with the plus signs represent metal ions created by corrosion. Hello, everyone. Last month we discussed why coated and uncoated spray package metals corrode. Corrosion is broadly defined as the degradation of materials. Therefore, polymer coatings and laminate films also corrode. Polymers are a complex array of large linear molecules (referred to as the backbone chains) with branches on each backbone and branches that bond separate backbone chains together. The chains and branches can bond with the spray package metal. Figure 1 provides a two dimensional model for a polymer bonded to a spray package metal. The polymer is represented by the yellow area with the meandering lines (backbone chains), and shorter lines representing branches on backbone chains and branches that bond separate chains together. A polymer is typically not a perfect film on top of the package metal. There are areas where the polymer does not wet or cover the package metal, causing small holes in the coating. In addition, there are microscopic tunnels through polymers that terminate at the metal, referred to as pores—indicated by the dashed arrow in Figure 1. Liquid from your formula can diffuse through polymer’s microscopic pores to the metal underneath and into microscopic caves inside the polymer. However, microscopic pores and small holes in coatings are not the only ways for liquid to diffuse into/ through polymers. Formula ingredients—particularly water—are also able to open channels through the polymer by wending their way through the complex array of polymer chains and branches and accumulating in caves. Diffusion of liquid through a polymer and accumulation of liquid in caves causes swelling that opens new pores through the polymer. Accumulation of liquids in polymers changes their physical properties, such as the glass transition temperature. Changing Figure 2. Spray package polymer coating/laminate shortly after filling.
Spray December 2015
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