Synthesis and charaterization of single hollow opaque polymer pigments

Karakaya, Ekin
In the present work opaque polymer pigments with single hollow structure were synthesized by osmotic swelling method. The hollow pigment is composed of an outer shell and an inner void space which can be filled with air or water. The osmotic swelling process involves synthesizing of a core polymer containing carboxylic acid group on the backbone, and encapsulating it with another polymer which functions as shell. The swelling of the polymer is provided by neutralization of the core with a base. Methyl methacrylate and methacrylic acid mixture is used as core monomer, and styrene is used as shell monomer. Sodium hydroxide is used for neutralization of polymer particles. The optimal experimental conditions such as alkali treatment time and the pH were investigated. The results indicated that pH plays an important role in void generation. Hollow morphology appeared when pH was above 10 but no hollow morphology was observed when pH is below 10. Also, it was seen that extending alkali treatment time was beneficial to get the swelling equilibrium of the polymer particles. The effect of the crosslinker content, acidity of the monomer, and the amount of surfactant on particle size and morphology of pigment were also investigated. When the carboxyl content of the core exceeded a certain value sedimentation of particles was observed. Furthermore, it was observed that low concentration of surfactant resulted in sedimentation of particles. The maximum pigment diameter was observed when crosslinker content was 2% w/w, methacrylic acid content 20% w/w, and the weight ratio of surfactant to water was 0.97. The surface morphology of the pigments was characterized by scanning electron microscope and the inner hollow structure was confirmed by transmission electron microscope. The analysis indicated that single hollow opaque polymer pigments and colored single hollow polymer pigments were successfully synthesized. The opacity, the L*a*b* color, and the gloss properties of polymer pigments were examined by color spectrophotometer and glossmeter. Single hollow polymer pigments without any other additive provided up to 93.8% opacity with 50% v/v solid content.