Solid acrylic resin using a continuous tube reactor

Details Solid acrylic resin using a continuous tube reactor Solid acrylic resin using a continuous tube reactor

2000-11-28

2002-11-05

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S064000, C526S227000, C526S230000, C526S318600, C526S347000, C524S556000

Reexamination Certificate

active

06476170

ABSTRACT:

FIELD OF THE INVENTION
The field of this invention is a process for making acrylic resins suitable as polymeric surfactants used in emulsion polymerization, as pigment grinding resins and for preparing dispersions used as overprint varnishes.
BACKGROUND OF THE INVENTION
Poly(&agr;-methyl styrene-co-acrylic acid-co-styrene) and poly(styrene-co-acrylic acid-co-methacrylic acid), acrylic resins, are used as a polymeric surfactant in emulsion polymerizations as a pigment grinding resin and for preparing dispersions used to make overprint varnishes. In use, the resins are suspended in water to form a solution and made into a dispersion, also known as a latex, by neutralizing them with a base such as 28% ammonium hydroxide. The base allows the acrylic resin to form polymeric surfactant micelles which have two chief advantages over solvent based systems. Firstly, they have lower viscosity, which is especially evident in high-solids systems. More importantly, however, is that being substantially solvent free, they are more environmentally friendly than solvent-based systems.
Typically, the acrylic resin has been made by bulk polymerization in a continuous-stirred tank reactor (CSTR). The CSTR is charged with styrene or styrene plus &agr;-methyl styrene, (meth)acrylic acid, a polymerization initiator and a solvent or just with styrene, &agr;-methyl styrene and (meth)acrylic acid. Reaction temperatures range from 180° C. to 300° C. and residence times are from 1 to 60 minutes. Of course, level control is very important. However, pressure is not controlled. The once-through percent conversion is on the order of 75%. The acrylic resin/unreacted monomer reaction product is sent to a devolatilizer for stripping of unreacted monomers for reuse. What emerges from the devolatilizer is the desired acrylic resin, suitable for flaking, pelletizing, pulverization, etc.
Heretofore, it has been believed that the reaction pressure appears to have no significant effect on the yield, and hence, pressure has not been controlled. Also, the use of tubular reactors for the bulk polymerization of styrenics has been taught away from because of problems encountered in thermal runaway reactions at 297° C., which resulted in resins having unacceptably large polydispersion. Past suggestions for avoiding this problem include the use of CSTRs with installed internal cooling coils.
The continuous tube reactor (CTR), also known as the linear flow reactor, has seen wide use in polymerizations because of its simplicity. No level controls are required, and because there is no stirring, there is no need for expensive, rotating seals capable of withstanding the pressure, temperature and solvent effects of the reaction. In the case of acrylics, it has been used in suspension polymerizations; the monomers employed are usually water soluble.
Note that all quantities appearing hereinafter, except in the examples are to be understood as being modified by the term “about.” Also, all percentages are weight percentages unless indicated otherwise.
SUMMARY OF THE INVENTION
The invention is a bulk polymerization process for preparing a solid acrylic resin, which comprises the steps of: charging into a continuous tube reactor, a feedstock of at least one vinylic monomer and a polymerization initiator; maintaining a flow rate through the reactor sufficient to provide a residence time of the feedstock in the reactor of from one minute to one hour; maintaining a pressure of 80 psig to 500 psig; maintaining the resulting molten resin mixture with a heat transfer medium within the range from 180° C. to a maximum of 260° C.; and devolatilizing the molten resin mixture exiting the reactor to remove unreacted monomers to provide a solid acrylic resin upon cooling. A preferred embodiment comprises the additional step of recycling the unreacted monomers recovered during the devolatilization step and charging them into the continuous tube reactor as a fraction of the feedstock.
Unexpectedly, the consequences of thermal runaway, mentioned as a concern in the prior art, may be avoided by limiting the reaction pressure and allowing vapor formation.
Another surprise is that the yield is a strong function of the pressure when acrylic resin is made in a CTR. Conversion can be made to vary from 60% to 99% by varying the pressure.
Unforeseen also, was that coatings derived from resin made with recycled monomer showed an improved property, gloss on white, when compared to those derived from virgin monomer, as well as when compared to the closest commercial alternate resin.
An environmental benefit of the invention is that, for many embodiments, no solvent is required to make the resin and coating systems made from it are predominantly water based, rather than solvent based.
DETAILED DESCRIPTION OF THE INVENTION
The monomers are polymerized using a single-pass flow-through tubular reactor. A monomer blend and a polymerization initiator blend are separately introduced and then combined via stainless steel tubing. Prior to combination, the monomer blend may be preheated by pumping through a preheating section of tubing which is dipped into an oil bath set for a preselected temperature. The preheating ensures that the temperature of the monomer blend will be increased to a desired initiation temperature level prior to entering the tubular reactor. The preheating step is not essential to the process. The combined flows then enter a static mixer where the two streams are homogeneously mixed. At this point a small amount of initiation may occur if the monomer blend is preheated. After exiting the static mixer, the combined flows then enter the tubular reactor. The reactor consists of a single tube or a series of tubes of increasing diameter bound in a coil with a single pass. The tubes are plain with no static mixer or other mixing elements therein or in combination therewith after the combined flows enter the tubular reactor. The coil is immersed into a circulating oil bath preset at the desired temperature. Initiation and polymerization occur as the combined flows enter the tubular reactor, conversion is high and the reaction is essentially complete as evidenced by the presence of polymerized resin. Unexpectedly, the single-pass flow-through tubular reactor will efficiently accomplish the desired result under the stated conditions.
The particular reactor used for the following examples is constructed of five 20 foot lengths of ½ inch outside diameter (O.D.) tube, three lengths of 20 foot ¾ inch O.D. tube and two lengths of 1 inch O.D. tube, all 18 gauge 316 stainless steel. They are joined in series and contained in a shell that is 21 feet long and 8 inches in diameter which contains recirculating hot oil as the heat transfer medium.
The design details are not particularly critical, and the reactor size can be scaled up or down within limits. However, the back pressure of the reactor is sensitive to the tube diameter, length and roughness, the number and radii of the connections as well as the changing rheological properties of the reaction mixture as it is converted to polymer as it travels the length of the tubing. These are computationally intractable and the optimal pressure control for each reactor design must be developed experimentally as the conversion rate, as will be seen, is a strong function of the pressure in a CTR. The minimum pressure, which is 80 psig, should be higher than the vapor pressures of the monomers at the heating oil temperature. The upper bound will depend on the hoop strength of the tubing used, the upper bound determined by economics and poor heat transfer, it may be reasonable to expect this to be 500 psig. For the reactor described, the optimal pressure range is from 100 to 300 psig. In this range, the conversion can vary from 60% to 99%.
In terms of mode of operation of the invention, it may be speculated that in a CSTR the pressure is not a variable independent of the temperature because a CSTR will have a headspace filled with vapor in thermodynamic equilibrium with the monomers. While not completely underst

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