J. W. Vanderhoff

Semi-Continuous Emulsion Polymerization

There are three types of emulsion polymerization processes: 1. batch polymerization in which all ingredients are added at the beginning of the reaction; 2. semi-continuous or semi-batch polymerization in which the monomer is added continuously or in increments, neat or in emulsion; 3. continuous polymerization in which all of the ingredients are added continuously to one part of the reactor system and partially or completely converted latex is removed continuously from another part. All three processes comprise particle nucleation and particle growth stages, which may occur sequentially or concurrently. All three processes benefit by the use of a seed latex to obviate the particle nucleation stage. The seed latex may be prepared in a separate reaction or by polymerization in situ.

Monomer Distribution and Transport in Miniemulsion Copolymerization

Miniemulsions are oil-in water emulsions prepared using a mixed emulsifier system comprised by an ionic surfactant and a cosurfactant, such as a fatty alcohol or a long chain alkane1. The two main characteristics of the miniemulsions are their good stability and droplet size, ranging from 50 to 400 nm in diameter. From the latter characteristic arises the term miniemulsion, to distinguish them from the conventional emulsions or macroemulsions with droplets larger than 1 µm in diameter and from the microemulsions with droplets less than 0.1 µm in diameter.

The first products made in space - Monodisperse latex particles

The monodisperse polystyrene latexes widely used for calibration and other scientific uses are made by seeded emulsion polymerization, i.e., by polymerizing styrene in a previously prepared monodisperse latex, to grow the particles to larger size while maintaining their uniformity. The emulsifier concentration is critical: too little results in coagulation of the latex; too much, in the nucleation of a new crop of particles. Monodisperse latexes of 0.1-2.0 pm particle size have been available for some years. Larger sizes are difficult to prepare: the extent of coagulation increases with increasing particle size above 2 pm to complete coagulation at 10 pm. Brownian motion ceases for particles larger than 2 pm, and the large sticky monomer-swollen particles cream and the polymerized particles settle; this creaming or settling is offset by stirring the emulsion polymerization, but the monomer-swollen particles are sensitive to coagulation by mechanical shear, so that the amount of coagulum increases with increasing particle size. Polymerization in space eliminates the settling or creaming, so that the latex need be stirred only enough to give good heat transfer and mixing, thus alleviating or eliminating the coagulation by mechanical shear. Thus twenty monodisperse polystrene latexes were prepared in the MLR flight hardware on the STS-3, STS-4, STS-6, STS-7, and STS-11 flights of the Shuttle. Two polymerizations were small-particle-size controls. Of eighteen large-particle-size latex polymerizations, four on STS-4 failed owing to malfunction of the flight hardware, one on STS-6 owing to a broken heating wire, and one on STS-11 owing to a broken stirrer shearpin. The remaining twelve monodisperse latexes of 4-30 pm size had narrower particle size distributions (coefficients of variation 0.9-1.4%) than the ground-based control latexes (coefficients of variation 2-5%) and contained fewer offsize larger particles. The flight polymerizations produced only negligible amounts of coagulum; the ground-based control polymerizations produced increasing amounts with increasing particle size, and so were discontinued for latexes larger than 18 . The polymerization rates in space were the same as on earth wit tm in experimental error. The 10 pm STS-6 (coefficient of variation 0.9%) and the two 30 pm STS-11 (coefficients of variation 1.3%) latexes were accepted by the National Bureau of Standards as Standard Reference Materials, the first products made in space for sale on earth. Moreover, these particles were more https://ntrs.nasa.gov/search.jsp?R=19890010947 2020-01-08T06:49:57+00:00Z