Waterborne, Chlorine-Free Adhesion Promoter For TPO
Effect of Crystallinity on Adhesion and Gasoline-Resistance Performance Properties
Due to their high chemical stability, low price, excellent balance of physical properties, possible recycling, etc., the amount of polypropylene (PP) and TPO consumed by automotive parts, household electrical appliances and molded general goods businesses is increasing. However, PP and TPO are materials with low surface energy that make painting and adhesion problematic, hence chlorinated polyolefin (CPO) has found wide use as an adhesion promoter. Solventborne CPOs have been traditionally used, and many studies have been conducted to clarify the adhesion mechanism of the solventborne CPO to TPO substrates.1-8 Excellent adhesion between TPO substrates and CPO can be obtained as the result of good wetting and higher dispersion interaction, which are affected by the properties of CPO's chlorine content, crystallinity, melting temperature, molecular weight and its polydispersity.1-5 Solvent composition is also one of the most important factors that determines the adhesion property by dissolving CPO and swelling rubbers near the surface of TPO, leading to morphological changes in the TPO surface and in the possible entanglement between CPO molecules and swelled rubber.6,7 In addition, baking time and temperature are also believed to be important, which affect the surface wettability of the CPO layer and the extent of penetration and interdiffusion of CPO into TPO. Baking may also rearrange the surface morphology of TPO, which can affect the surface adhesion.9,10
In recent years, waterborne coatings applied to TPO substrates have been widely studied for use as both interior and exterior automotive coatings as a result of increasing environmental awareness. Waterborne coatings consist of a colloidal phase in the form of individual spheres dispersed in water, and have a different film formation mechanism from solventborne coatings.11 As the water evaporates, the spherical polymer particles are closely packed where a so-called "honeycomb" structure is formed after further evaporation of water and loss of the voids. The continuous coating film is finally obtained through deformation and coagulation of the particles by further heat treatment or use of coalescing agents. Traditional waterborne coatings used to be baked at 120 ºC (248 ºF) to compensate for the slow evaporation rate of water; high baking temperatures also helped produce good deformation and coagulation of the particles to obtain a continuous film coating.