Apparatus for injecting solid insoluble additives into polymerization streams

Process and apparatus are disclosed for improved additive systems for polymerization processes, which improved systems comprise a slurry additive system having a high shear mixer for mixing a carrier fluid and the solid additives and maintaining them in a suspension slurry prior to injecting them into a polymerization system.

FIELD OF THE INVENTION 
This invention relates to the field of polymerizing monovinyl aromatic 
compounds and more particularly discloses methods and apparatus for adding 
thermally-sensitive and oxidation-sensitive additives and anti-oxidants to 
the reactants in a monovinylaromatic polymerization system prior to or 
during the polymerization process. 
BACKGROUND OF THE INVENTION 
Of all the thermoplastics manufactured today, probably the most versatile 
and most widely used class of materials is polymerized monovinyl aromatic 
compounds such as polystyrene, polymerized alpha-methyl styrene, and 
polymers of ring-substituted styrenes. 
Virgin polystyrene manufactured by the polymerization of styrene monomer 
often requires the inclusion therein of additives such as pigments, 
stabilizers, anti-foaming agents, mold-release agents, plasticizers, and 
anti-oxidants. Plasticizers such as mineral oil and mold-release and 
stabilizer agents such as zinc stearate are necessary in the polymer to 
allow it to be formed in thermoforming equipment into the final consumer 
products. Anti-oxidants such as Irganox 1076, a hindered phenol 
manufactured by Ciba-Geigy Corporation of Greensboro, N.C., are necessary 
to prevent the polymer from degrading with age and from exposure to ultra 
violet light from sources such as sunlight and artificial lighting. 
As already mentioned, one of the most desirable, if not the most desirable, 
lubricant and mold-release agents added to polystyrene and other 
polymerized monovinyl aromatic compounds is zinc stearate. In conventional 
polymerization systems, zinc stearate is added to the process by first 
melting it in a closed heated vessel at 120.degree. to 130.degree. C. and 
then pumping it into the polymerization system at the desired location. 
The problems with this approach are many. 
First, the molten zinc stearates, as well as other additives, oxidize 
easily at temperatures above their melting points, and must be completely 
shut off from any traces of air to prevent oxidation of the material, 
which causes yellow discoloration of the finished polymer. This is 
normally achieved by maintaining the headspace in the melting vessel 
filled with nitrogen. 
Second, feeding problems often occur when trying to transport molten zinc 
stearate to the polymerization system. If any traces of air were allowed 
to leak in through the lines or fittings to the melt, the afore-mentioned 
oxidation will occur. Also, if the stearate supply piping were not 
maintained above the melt temperature of the stearate, the material would 
begin to solidify and precipitate out, clogging the lines or allowing 
solid chunks of the material into the polymerization process, adulterating 
the finished polymer. 
SUMMARY OF THE INVENTION 
The present invention discloses methods and apparatus for adding additives 
such as plasticizers, stabilizers, mold-release agents and anti-oxidants 
into polymerization processes, and more particularly discloses methods and 
apparatus for adding mineral oil, zinc stearate and hindered phenol 
compounds to polymerization systems by forming a slurry of the additives 
in an agitated vessel and injecting the slurry into the process at the 
desired location, or locations, at easily controlled temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the illustration of FIG. 1, this is a schematic diagram of a 
typical high impact polystyrene (HIPS) manufacturing process. Such a 
process is more particularly described in U.S. Pat. No. 4,857,587 in the 
name of Sosa et al, entitled "Continuous Process Including Recycle Stream 
Treatment for the Production of High Impact Polystyrene", which patent is 
hereby incorporated by reference in its entirety into the present 
application. In a typical high impact polystyrene process, such as that 
illustrated in FIG. 1, refined styrene monomer feed is fed through flow 
line F1 into a stirred tank reactor (CSTR1) which is a continuous stirred 
tank reactor. Styrene, polybutadiene, a free-radical initiator, and 
additional components such as solvents, anti-oxidants, dyes, and other 
additives are fed into the reactor through feed line F1. As used herein, 
the term "styrene" includes a variety of substituted styrenes, such as 
alpha-methyl styrene, ring-substituted styrenes, such as p-methylstyrene 
and p-chlorostyrene, as well as unsubstituted styrene. Typically, the 
mixture in polymerization reactor CSTR1 will comprise about 75 to 99% by 
weight refined styrene, about 1 to 15% by weight polybutadiene, and the 
remainder being free-radical initiator and additional components. 
The feed components fed through line F1 are stirred in the CSTR1 and 
reaction between the components is initiated therein. The components are 
then fed through flow line F2 into a second continuous stirred tank 
reactor CSTR2 for additional reaction and agitation by stirring. From 
there the HIPS components are transferred through flow line F3 into an 
initial polymerization reactor R1. A series of reactors R1, and R2, each 
comprising horizontal polymerization reactors may be used for the total 
polymerization process of the HIPS material. The polymerized 
styrene/butadiene mixture then exits reactor R2 and passes through 
flowline F5 to an optional static mixer SM and from there through flow 
line F6 into a preheater PH. From the preheater the polymerized product 
flows through line F7 into a devolatilizer DV where volatile components 
are transferred through line F10 to the recycle treatment vessel RTV. The 
finished HIPS material then exits DV through line F9 to the product 
finishing line where it may be pelletized or put into other transportable 
forms. The volatile elements removed in the devolatilizer DV are then 
passed through vessel RTV which usually comprises a filter bed such as 
clay to remove the acid components from the recycle stream. The refined 
recycle stream then moves through line F12 and may be recycled into the 
CSTR1 or CSTR2. 
The description given above is that of a typical high impact polystyrene 
manufacturing system described from a schematic or flow diagram viewpoint. 
The present invention involves the use of a slurry additive system for 
adding components such as anti-oxidants, stabilizers, mold-release agents, 
and other desirable compounds. The slurry additive system is more 
particularly described in FIG. 2 and is designated schematically at SAS 
and is shown with a feed input line F13 and a slurry supply line F14. Line 
F13, by means of manipulation of valves V6, V7, and V8, is arranged to 
provide recycled monomer from the RTV into the slurry additive system as a 
carrier for the additive to be injected into the polymerization reactor 
system R1-R2. 
The recycle stream entering the SAS vessel through F13 is slurried with the 
desirable additive, such as the previously mentioned zinc stearate and 
hindered phenol additives, to be injected into the polymerization system 
by manipulating valves V1 through V3. Injection of the additive slurry may 
be directed at any point in the polymerization process: by closing all 
valves except V1 the additive slurry may be injected prior to the 
polymerization reactor R1. Likewise, by opening valve V2, and V3 and 
closing all the other valves, the injection points may be moved to the 
various locations shown in the drawings. The opening of valve V3 and 
closing of valves V1 and V2 introduces the additive slurry into the system 
after the final reactor vessel R2. 
In this case, the optional static mixer SM must be utilized to thoroughly 
compound the additive slurry into the polymer stream. As previously 
mentioned, the static mixer SM is an optional element and is intended for 
the particular embodiment wherein the additive slurry is injected between 
the reactor system R1-R2 and the preheater PH. It is contemplated that if 
the injection point is at any other point in the system prior to reactor 
R2 then the static mixer SM will not be necessary and the output of R2 can 
be routed around the static mixer SM and into the preheater PH. 
Alternatively, if it is undesirable to utilize the recycle stream for a 
carrier material in the SAS, an alternate carrier fluid may be introduced 
into the slurry system through feed line F15 from an independent carrier 
material source (not shown). In one particular embodiment such a carrier 
material could be mineral oil which is often used as a plasticizer in 
polystyrene materials. In such a case, it would only be necessary to close 
valve V6 and open valves V7 and V8 as well as valve V9. 
Referring now to FIG. 2, there is illustrated a detailed schematic drawing 
of the slurry additive system SAS of FIG. 1. The SAS comprises a high 
shear mixer HSM located in a mixing vessel MV and having the feed inlet 
line F13 flowing thereinto. A zinc stearate supply ZS, which is added as a 
solid is indicated in the dashed line next to flow line F13. Zinc stearate 
may be added to the vessel by any conventional means such as a vessel 
hatch or gear pump or other means for adding solid material into a closed 
vessel. The carrier fluid entering line F13, which as previously mentioned 
can be either mineral oil or the recycle stream from the devolatilzer DV, 
which primarily consists of about 80 to 90% styrene monomer, 5 to 10% 
ethylbenzene, and 5 to 10% xylenes, toluenes, and propyl-benzene, is added 
to the agitator MV along with zinc stearate from a ZS supply and subjected 
to high shear through the action of the high shear mixer. This forms a 
finely divided slurry of zinc stearate in the carrier fluid which is then 
pumped through volumetric slurry pump SP and out flow line F16. A mass 
controller MC is located in flow line F16 and a recycle loop RL is 
branched off of line F16 upstream mass control of MC and feeds back into 
vessel MV. This type of system is commonly known as a "pump-around" 
system. Thus the action of mass control MC, which may be a conventionally 
known valving system, allows a constant control of the feed amounts 
through line F14 to the polymerization system. 
Any slurry that is not transported through line F14 is directed through 
return line RL back into the agitation of the high shear mixer HSM in 
vessel MV. This maintains a constant and consistent slurry of the zinc 
stearate in the carrier fluid and prevents settling out of the solids in 
the suspension. By controlling the amount of zinc stearate added to the 
mixing vessel MV and/or controlling the amount of recycle fluid, or 
alternate carrier fluid such as mineral oil, being added through lines F13 
and F15, the amount of additive slurry entering the polymerization system 
through flow line F14 can be very precisely controlled. Conventional 
ratios of the slurry additive material are known to those skilled in the 
art and can be adjusted precisely through the use of mass controller MC 
and slurry pump SP. Temperature of the slurry is maintained at a desirable 
constant value by the utilization of heat exchanger HEX located in return 
line RL. In one preferred embodiment the temperature was maintained at 
about 70.degree. F. 
In addition to the placement of zinc stearate ZS into vessel MV, other 
additives can clearly be placed in the vessel to be slurried with the 
carrier fluid and the zinc stearate ZS. Such materials include those 
previously mentioned such as hindered phenols, anti-oxidants, solvents, 
initiators, and other such additives. The addition of other additives to 
the mixing vessel MV is indicated by a second dashed line designated at AA 
in FIG. 2. As another alternative, the carrier fluid for the slurry may be 
made up of virgin styrene monomer diverted from feedline F1, or can be a 
mixture of virgin monomer and recycle stream fluid, as well as other 
solvents compatible with the process, such as ethylbenzene. In addition, 
the high shear mixer may be utilized to disperse insoluble liquids in the 
chosen carrier fluid in place of or in addition to insoluble solids. 
OPERATION OF THE PREFERRED EMBODIMENT 
In typical operation, the slurry additive system SAS as illustrated more 
precisely in FIG. 2, is supplied with a carrier fluid such as a virgin 
styrene monomer, recycle styrene stream, or optionally, a mineral oil 
plasticizer, and one or more solid additives such as zinc stearate and 
anti-oxidants are placed in solid form into the closed vessel. There they 
are subjected to high shear and converted into a very finely divided 
suspension or slurry which is maintained by the constant action of a high 
shear mixer and a pump-around system. As the additives are needed, the 
slurry is pumped through a mass-controller into the polymerization system 
at any point prior to, in the middle of, or at the downstream end of the 
polystyrene polymerization reactor system. By utilizing the present 
invention, the need for heated zinc stearate vessels with nitrogen 
atmospheres are eleminated as well as the need for heated flow lines to 
prevent solidification of additives such as zinc stearate. The present 
invention provides a simple yet efficient means for injecting solid 
additives in a finely divided state into the styrene 
polymerization/copolymerization system as illustrated in FIG. 1. By 
controlling the amounts of solids added into the high shear mixer, 
slurries of known composition can be precisely obtained and injected into 
the polymerization system, very closely controlling the amount of 
additives and obtaining a fine, even distribution in the polymerizing 
sytrene. 
It should also be noted that the optional heat exchanger HEX in line RL 
keeps the slurry at the desired temperature, or within a desirable 
temperature range. There is no need for a nitrogen atmosphere in the 
mixing vessel since it is a closed vessel and the head-space is completely 
filled with the vapors generated from the recycle stream carrier fluid, 
but a nitrogen atmosphere can be utilized if desired. 
One particular additive utilized in styrene polymerization and added by the 
slurry additive system is solid zinc stearate. The agitation vessel MV was 
designed to maintain the particle size of the zinc stearate to less than 
200 microns. The slurry was delivered to the polymerization process 
utilizing a volumetric pump SP to precislely control the concentration of 
the additive. The concentration of additives was adjusted to maintain 
proper viscosity, for example, approximately ten weight percent zinc 
stearate and ten weight percent Irganox 1076 were dispersed and added to 
the slurry system. If the soluble anti-oxidant Irganox were not to be 
utilized, then higher levels of zinc stearate could be used to maintain 
the viscosity. Irganox 1076 is soluble in styrene and thereby increases 
the viscosity of the solution. It was also found that by adding the 
anti-oxidant and other additives late in the process, i.e. for example, at 
the static mixer location, improved properties in the finished product 
could be obtained. 
In summary, the slurry addition system is utilized to add heat-sensitive 
additives and additives that can possibly interfere in the early stages of 
the process into a monovinyl aromatic polymerization system. 
Although a specific preferred embodiment of the present invention has been 
described in the detailed description and drawings above, the description 
is not intended to limit the invention to the particular forms or 
embodiments disclosed therein since they are to be recognized as 
illustrative rather than restrictive, and it would be obvious to thos 
skilled in the art that the invention is not so limited. For example, 
instead of using the present system to slurry solid insoluble additives in 
a carrier fluid, the system could be utilized to add commercially 
available preformed emulsions or dispersions, such as silicon oil/water 
and zinc stearate/mineral oil, by insuring that "settling-out" does not 
occur in these formulations. It is also contemplated that the present 
invention can be utilized successfully in polymerizing monomers other than 
monovinyl aromatics, such as ethylene, propylene, polyesters, and others. 
Thus, the invention is declared to cover all changes and modifications of 
the specific examples of the invention herein disclosed for purposes of 
illustration which do not constitute a departure from the spirit and scope 
of the invention.