Polymerization process with spraying step

Details Polymerization process with spraying step Polymerization process with spraying step

1998-03-09

2001-09-18

Henderson, Christopher (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C422S131000, C526S073000

Reexamination Certificate

active

06291605

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a spray polymerization process, an apparatus for producing a dry polymer and a polymer having novel physical characteristics. Particularly, the present invention relates to a polymerization process for the continuous production in a controlled atmosphere of a substantially dry polymer particle powder comprising polymer particles of desired size, shape and density from a liquid monomer source.
It is known in the art that polymers may be synthesized by step polymerization and chain polymerization processes. Chain polymerization is initiated by a reactive species produced by a compound or compounds referred to as an initiator. Generally, monomers show varying degrees of selectivity with regard to the type of reactive center that will cause chain polymerization. Monomers show high selectivity between anionic and cationic initiators, however, most monomers will undergo polymerization with a radical initiator, although at varying rates. Examples of the types of monomers which will polymerize to high molecular weight polymers in the presence of a radical initiator include: ethylene; 1,3-dienes; styrene; halogenated olefins; vinyl esters; acrylates; methacrylates; acrylonitrile; methacrylonitrile; acrylamide; methacrylamide; N-vinyl carbazole; N-vinyl pyrrolidone.
Essentially, radical polymerization conditions are either homogenous or heterogeneous, depending upon whether the initial reaction mixture is homogenous or heterogeneous. Some homogeneous systems however, may become heterogeneous as polymerization proceeds due to the insolubility of the polymer in the reaction media. Generally, mass and solution polymerizations are homogeneous processes, while suspension and emulsion polymerizations are heterogeneous processes. All monomers can be polymerized by any of the various processes however, it is usually found that for commercial considerations the polymerization of a particular monomer is best carried out by one or two of the processes.
Bulk or mass polymerization of a pure monomer offers the simplest process with a minimum of contamination of the product. Bulk polymerization, however, is difficult to control due to the characteristics of radical chain polymerization. The bulk process is highly exothermic, high activation energies are involved, and there is a tendency toward the gel effect. Such characteristics make the dissipation of heat difficult, therefor, careful temperature control is required during bulk polymerization processes. Additionally, the viscosity of the reaction system increases rapidly at a relatively low conversion, thereby requiring the use of elaborate stirring equipment. Localized “hot spots” may occur which damage, degrade and discolor the polymer product, and a broadened molecular weight distribution may result due to chain transfer between polymer molecules. There is also the risk in extreme cases that an uncontrolled acceleration of the polymerization rate can lead to disastrous runaway-type reactions.
Many of the disadvantages of bulk polymerization may be overcome by polymerizing a monomer in a solvent (solution polymerization). The solvent, which may be water, acts as a diluent and aids in the transfer of the heat of polymerization. The solvent can be easily stirred since the viscosity of the reaction mixture is decreased. Although thermal control of a solution polymerization process is easier than with mass or bulk polymerization, the purity of the polymer may be affected if there are difficulties in removing the solvent during and following polymerization.
Heterogeneous polymerization is used extensively to control the thermal viscosity problems often associated with homogeneous processes. Precipitation polymerization is a heterogeneous polymerization process which begins as a homogeneous polymerization but converts to heterogeneous polymerization. A monomer either in bulk or in solution (usually aqueous but sometimes organic) forms an insoluble polymer in the reaction medium. Precipitation polymerization can be referred to as powder or granular polymerizations because of the forms in which the final polymer products are obtained. The initiators used in precipitation polymerization are soluble in the initial reaction medium and polymerization proceeds following absorption of monomer into the polymer particles.
Suspension polymerization, also referred to as bead or pearl polymerization, is carried out by suspending the monomer (discontinuous phase) as droplets (50 to 500 &mgr;m in diameter) in water (continuous phase). The ratio of water to monomer typically will vary from about 1:1 to 4:1 in most polymerizations. The monomer droplets which are subsequently converted to polymer particles do not coalesce due to agitation and the presence of suspension stabilizers also referred to as dispersants or surfactants. Stabilizers may be. water soluble polymers or water insoluble inorganic powders. The suspension stabilizers are used typically in an amount that is less than 0.1 weight percent of the aqueous phase. The two-phase suspension system cannot be maintained in suspension polymerization without agitation.
Suspension polymerization initiators are soluble in the monomer droplets and are referred to as oil-soluble initiators. Suspension polymerization in the presence of high concentrations of water soluble stabilizers are used to produce latex-like dispersions of particles having small particle size. Such suspension polymerizations may be referred to as dispersion polymerizations. Inverse microsuspension polymerization involves an organic solvent as a continuous phase of a water soluble monomer either neat or dissolved in water. Inverse dispersion refers to systems involving the organic solvent as continuous phase with dissolved monomer initiator that yield insoluble polymer.
Emulsion polymerization involves the polymerization of monomers in the form of emulsions, i.e., colloidal dispersions. Emulsion polymerization differs from suspension polymerization in the type and smaller size of the particles in which polymerization occurs, in the kind of initiator employed, and in the dependence of polymer molecular weight on reaction parameters. For most polymerization processes there is an inverse relationship between the polymerization rate and the polymer molecular weight. Large decreases in the molecular weight of a polymer can be made without altering the polymerization rate by using chain transfer agents. Large increases in molecular weight can be made only by decreasing the polymerization rate, by lowering the initiator concentration, or lowering the reaction temperature.
Emulsion polymerization allows increasing the polymer molecular weight without decreasing the polymerization rate. Emulsion polymerization has the advantage of being able to simultaneously obtain both high molecular weights and high reaction rates. The dispersing medium is usually water in which the various components are dispersed by means of an emulsifier. Other components include the monomer, a dispersing medium and a water soluble initiator. Surfactants are typically used in emulsion polymerizations at from 1 to 5% weight. The ratio of water to monomer is generally in the range 70/30 to 40/60 by weight.
The polymerization processes discussed above involve additional steps either to dry the polymer formed, separate the polymer from the organic solvent used in the process, or to recover the organic solvent. The added steps require additional energy and time in preparing the final product, thereby increasing the cost of the polymer produced. Moreover, the polymers produced using these known processes typically are produced as an agglomeration which must, following drying, be pulverized or in some way broken up to yield a usable polymer product. Breaking up the polymer product by grinding or pulverizing produces a substantial amount of dust which raises environmental and health concerns to those having to work in and around the polymer dust.
Therefore, there remains a need for a polymerization process and an apparatus in which t

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