Comparative assessment of nano-morphology and properties of spray coated clear polyurethane coatings reinforced with different organoclays

• Processing combination for PU nanocomposite coatings.
• Hard soft segmental nano-morphology variably affected by clay type.
• Gloss, scratch, mar and impact resistance measured and analysed.
• Thermal stability was enhanced, the effect of clay dispersion was pronounced.

Two clays (Cloisite 30B and Cloisite 20A) with different organic modifications and cation exchange capacity values were used as fillers (between 0 and 3 wt%) for a two-pack polyurethane (PU) clear coating. A combination of processing equipments was used to homogeneously disperse clays in PU. Transmission electron microscopy showed the level of dispersion and platelet size for the respective clays in the polyurethane matrix. X-ray diffraction showed greater extent of exfoliation in Cloisite 30B filled PU. Atomic force microscopy studies demonstrated that the clays induced a nano-spherical (Cloisite 30B) or nano-cylindrical (Cloisite 20A) shape to the hard domains of PU on the coating surfaces. This unique morphology controlled the property attributes of both types of nanocomposite coatings. Rheological tests indicated differentiating degree of increase in viscosities for clay loaded suspensions. CIELab overall colour change parameter (ΔE*) was much less than 1 unit for PU-Cloisite 30B while it was 1.17 units for PU-Cloisite 20A (3 wt%) showing the effect of clay addition on colourless PU coatings. Scratch and mar resistance w.r.t. PU coatings for 3 wt% added clay content improved by 2H and 22% in PU-Cloisite 30B while incase of PU-Cloisite 20A the improvement was 0H and 11% respectively. PU-Cloisite 30B and PU-Cloisite 20A had an enhanced char percent of 4.0% and 3.8% respectively w.r.t. PU for 3 wt% clay addition. The varied chemical and physical interaction of nano-sized Cloisite 30B and Cloisite 20A platelets with PU chain segments especially hard domains led to dissimilar morphology and coating properties.

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