Unstable viscosity after HPMC dissolution
January 15th 2025
Vinyl acetate and ethylene are copolymerized in an emulsion process to first produce VAE compositions containing between 10 and 40 percent ethylene in the copolymer. Ethylene is a soft, hydrophobic monomer unit in the copolymers, functioning as an internal “plasticizing” moiety. In copolymers, ethylene effectively reduces the glass transition temperature (Tg) of the hard, polar vinyl acetate monomer, which has a Tg of 30 °C. Thus, we can achieve copolymers with Tgs ranging from +15 °C to minus 15 °C while converting the base emulsion resins used in the production of the VAE into redispersible powders. By using ethylene monomer to adjust the Tg, the formulator does not require any external plasticizers.
We’ve depicted the manufacturing process for redispersible powders below. Starting with a VAE emulsion produced using a colloid stabilizer like polyvinyl alcohol, the emulsion is mixed with a fine mineral anti-caking agent and then spray-dried. This affords a stable, free-flowing powder which can be redispersed when mixed with water. The RDP’s minimum solid’s content is 98 percent; the level of anti-caking agent content is up to 8-12 percent of the total weight. These VAE’s have very low amounts of volatile organic compounds (VOCs) and low odor levels.
The picture below shows the free-flowing nature of the VAE-RDP. The SEM micrograph shows a VAE solid particle. The anti-caking agent or partitioning agent also appears in the SEM. The graph on the right shows the particle size dimensions (um) and number average distribution of the original VAE emulsion (red line) and the powder when redispersed into water (blue line). Noteworthy is the retention of the dimensions of the original VAE emulsion particle, after redispersing the powder. The blue hump in the graph is associated with the anti-caking agent.
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