Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in two or more physically distinct zones
Reexamination Certificate
2002-06-10
2004-02-17
Egwim, Kelechi C. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in two or more physically distinct zones
C526S067000, C526S068000, C526S078000, C526S079000, C526S080000, C526S081000, C526S087000, C526S088000, C526S201000
Reexamination Certificate
active
06693151
ABSTRACT:
This invention pertains to a method for semi-continuous emulsion polymerization, which produces a bimodal product in volumes greater than that of a single reactor, and which requires a minimum investment of additional equipment.
A bimodal polymer is a polymer wherein at least 90 percent of the polymer is present in two populations of particle mode, wherein each mode has a different particle size. The second mode creates a packing phenomena wherein the smaller mode fills the gaps between the larger mode, thus resulting in a polymer containing significantly higher solids than a unimodal polymer. Several patents describe the manufacture of bimodal polymers by various routes. However, because of the need to grow the second particle mode in the presence of an initial, or first mode, it has been generally accepted that the second particle mode must be grown in its entirety in the presence of the first mode, using a multi-step process. U.S. Pat. No. 4,254,004 discloses a two-step latex polymerization of ethylenically saturated monomer wherein the monomer feed rate exceeds the monomer polymerization rate in the first step until the Trommsdorff exotherm occurs, followed in the second step wherein the monomer feed rate is slower than the monomer polymerization rate to produce in-situ bimodal latex polymer particles. This process produces only a single reactor volume per batch.
In recent years, it has been discovered that unimodal emulsion polymerization can be carried out using a semi-continuous process. U.S. Pat. No. 4,946,891 discloses an aqueous copolymerization carried out on a semi-continuous basis using a single continuously stirred tank reactor in which, as is conventional, the materials being copolymerized (monomers) and the materials used in the copolymerization are supplied slowly to a reactor while maintaining conditions causing copolymerization to proceed until the total supply is in the range of from about 1.5 to about 5 reactor volumes. As materials are added to the reactor, a corresponding amount of the reactor contents spills out of the reactor into one or more vessels in which the copolymerization reaction is completed. This process produces only a unimodal polymerization product.
The problem addressed by the invention is the provision of a process for semi-continuous emulsion polymerization, which produces a bimodal product in volumes greater than that of a single reactor, and which requires a minimum investment of additional equipment.
In a first aspect of the invention there is provided a method of producing a bimodal polymer product, by semi-continuous, aqueous polymerization, composed of supplying to a single continuously stirred tank reactor, materials used in the polymerization, including water, and at least one ethylenically unsaturated monomer; creating in the reactor, conditions causing polymerization of a first polymer mode to proceed; adding a surfactant or an emulsion polymer having a diameter different from that of the particles in the first mode at that point in the reaction, to initiate growth of a second polymer mode; continuing the supply, under the conditions, producing a bimodal polymer product; continuing the supply to the reactor, while simultaneously removing part of the bimodal polymer product from the reactor; collecting the removed bimodal polymer product in at least one separate vessel, continuing the supply, and maintaining the conditions until the total supply is at least 1.05 reactor volumes and less than to 2 reactor volumes; completing the polymerization reaction in the separate vessel; and combining together all of the bimodal polymer product.
In a second aspect of the invention there is provided a bimodal composition, prepared by semi-continuous, aqueous polymerization composed of supplying to a single continuously stirred tank reactor, materials used in the polymerization, including water, and at least one ethylenically unsaturated monomer; creating in the reactor, conditions causing polymerization of a first polymer mode to proceed; adding a surfactant or an emulsion polymer having a diameter different from that of the particles in the first mode at that point in the reaction, to initiate growth of a second polymer mode; continuing the supply, under the conditions, producing a bimodal polymer product; continuing the supply to the reactor, while simultaneously removing part of the bimodal polymer product from the reactor; collecting the removed bimodal polymer product in at least one separate vessel, continuing the supply, and maintaining the conditions until the total supply is at least 1.05 reactor volumes and less than to 2 reactor volumes; completing the polymerization reaction in the separate vessel; and combining together all of the bimodal polymer product.
The process in this invention produces bimodal polymer particles. The bimodal product is made up of a mixture of polymer particles of two size distributions, wherein the diameter of the larger particles may be 3.5 to 30 times the diameter of the smaller distribution particles, and preferably 5 to 15 times larger than the smaller particles, as measured by weight average particle size.
The semi-continuous aqueous polymerization process is carried out by first supplying to a single continuously stirred tank reactor the materials used in the polymerization. These materials include water, and at least one ethylenically unsaturated monomer, and may include an emulsifier, catalyst and/or a polymer seed. By semi-continuous, herein is meant some of the reactants are charged to the reactor at the beginning of processing, and the remaining reactants are fed continuously as the reaction progresses while some of the product is simultaneously withdrawn from the reactor. By continuously stirred, herein is meant the reactants are agitated during processing, providing mixing and creating a substantially uniform composition within the reactor. By tank reactor, herein is meant a vessel with inlet and outlet pipes, equipped with some means of agitation and provisions for heat transfer (for example jacket, external or internal heat exchangers), and which can accommodate either batch or continuous operations over wide ranges of temperatures or pressures. By polymer seed, herein is meant, a polymer composition whose particle size predefines the diameter of the first polymer mode of the bimodal product. This invention contemplates that the addition of water and monomer to the reactor may include the addition of the water followed by neat monomer, or the addition of water followed by a monomer emulsion, or the addition of a mixture of water and a small amount of monomer followed by the addition of a monomer emulsion.
At least one ethylenically unsaturated monomer may be selected from amides such as (meth)acrylamide, propenamide, dimethylacrylamide; esters such as methyl acrylate, ethyl acrylate, butyl acrylate, propyl acrylate, 2-ethylhexyl acrylate, hydroxyethyl acrylate, methyl methacrylate, hydroxyethyl methacrylate, hydroxymethyl acrylate, hydroxymethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, allyl methacrylate, diallyl phthalate, 1,3-butylene glycol dimethacrylate, 1,6-hexanedioldiacrylate, vinyl acetate, vinyl proprionate, or other vinyl esters; nitriles such as acrylonitrile; and the like, and combinations of the foregoing. Other suitable ethylenically unsaturated monomers may include vinyl monomers such as vinyl chloride, vinylidine chloride, vinyl acetate and N-vinyl pyrollidone, divinyl benzene; styrene or alkyl-substituted styrene, butadiene; and the like, and combinations of the foregoing. Examples of ethylenically unsaturated carboxylic acid monomers that are useful in this invention include acrylic acid, methacrylic acid, fumaric acid, crotonic acid, maleic acid, itaconic acid, and combinations of two or more such acids. Preferably, the ethylenically unsaturated carboxylic acid is acrylic acid. Preferably, a stabilizing monomer, such as an acid containing monomer is used to stabilize the emulsion polymer. Examples of specific stabilizing monomers include the mo
Hallerud Owen Clifford
Throne Thomas George
Egwim Kelechi C.
Rohm and Haas Company
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