Sulfur dioxide for vapor phase elimination of styrene and acrylonitrile popcorn polymer in bulk san production

A bulk or mass polymerization process is provided which involves bulk polymerization of vinyl monomers in a liquid phase in the presence of a nitrogen rich vapor phase containing amounts of sulfur dioxide effective to inhibit formation of popcorn polymer during the reaction. The process may be used as a step in the formation of polymer or may be used as the entire reaction process.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to bulk polymerization processes for making 
vinyl aromatic/vinyl cyanide copolymers, and more particularly relates to 
bulk polymerization processes having a vapor phase additive which inhibits 
popcorn formation. 
2. DESCRIPTION OF THE RELATED ART 
Mass or bulk polymerization techniques for making copolymers of 
monoethelenically unsaturated polar monomers and monovinylidene aromatic 
monomers are known, see U.S. Pat. Nos. 3509237; 3660535; 3243481; 4221833 
and 4239863, all of which are incorporated herein by reference. Such 
copolymers may be rubber modified graft copolymers or may be rubber-free 
rigid copolymer. Typical bulk processes, such as those involving a boiler 
reactor, typically involve a liquid phase reaction covered by a nitrogen 
(N2) atmosphere. In boiler reactors, heat is added to the reactor to cause 
boiling of the liquid monomeric composition. Boiled monomer then enters 
the nitrogen vapor phase, contacts the reactor dome, which is typically 
air or water cooled, condenses and returns to the liquid phase. Condensed 
monomer on the reactor dome will generate undesired, crosslinked popcorn 
polymer. Popcorn generation at the dome surface may be due in part to the 
nonvolatility of typical crosslink inhibitors in the liquid phase which do 
not volatilize with the monomeric composition, thereby necessitating that 
an inhibitor be present in the vapor phase if popcorn formation is to be 
inhibited. In the past, inhibitors such as oxygen have been incorporated 
into the vapor phase to prevent popcorn formation. It is believed, 
however, that oxygen may oxidize the polymer and may contribute to the 
formation of black carbonaceous material on the reactor walls. Analysis of 
the black material has indicated that the material has a high oxygen 
content which tends to support the proposition that the oxygen inhibitor 
is part of the cause of the formation thereof. Additionally, oxygen 
(O.sub.2) has a high solubility in many liquid organic monomers which 
tends to support the proposition that the oxygen is present in the liquid 
monomer phase during the bulk polymerization process. 
Accordingly, there is a need for vapor phase additives which will inhibit 
popcorn formation in bulk polymerization processes. 
SUMMARY OF THE INVENTION 
The present invention involves a bulk or mass polymerization process 
wherein vinyl monomers are reacted in a liquid phase which is blanketed by 
a vapor phase of diatomic nitrogen atmosphere comprising sulphur dioxide. 
The sulphur dioxide inhibits popcorn polymer formation during the bulk 
polymerization process.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention involves a bulk or mass polymerization process 
wherein a liquid phase comprising vinyl monomers is heated to cause 
polymerization of the monomers. The liquid phase is blanketed by a 
nitrogen rich vapor phase that comprises an amount of sulfur dioxide 
effective to prevent popcorn polymer formation. While the sulfur dioxide 
prevents popcorn formation, applicants also believe that it accelerates 
the polymerization of the monomers in the liquid phase. 
The vinyl monomers, in the absence of an effective inhibitor, are 
susceptible to the formation of a hard, brittle, highly crosslinked 
polymer, which is referred to as "popcorn polymer" in the prior art 
because of its physical appearance. 
Compounds suitable for homopolymerization and/or copolymerization in the 
process of this invention include vinyl monomers in the following classes: 
1. (Alkyl)acrylates which are polymerizable organic compounds containing a 
single ethylenic double bond conjugated with a carbon to oxygen double 
bond, i.e., compounds containing the structure: 
##STR1## 
2. Vinyl cyanide monomers which are polymerizable organic compounds 
containing a single ethylenic double bond conjugated with a carbon to 
nitrogen triple bond, i.e., compounds containing the structure: 
##STR2## 
3. Vinyl aromatic monomers which are polymerizable organic monomers 
containing a single ethylenic double bond conjugated with a carbon atom in 
an aromatic nucleus. 
Examples of monomers in class 1 include acrylic, methacrylic, ethacrylic 
and crotonic acid, and esters thereof, wherein the ester group contains 
one to 18 carbon atoms and wherein the alkyl group contains 1 to about 4 
carbon atoms. 
Specific examples of class 1 monomers are methyl acrylate, ethyl acrylate, 
isopropyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, dodecyl 
acrylate, octadecyl acrylate, methyl methacrylate, butyl methacrylate, 
2-ethylhexyl methacrylate, ethyl ethacrylate, octyl ethacrylate, methyl 
crotonate, heptyl crotonate, cyclohexyl acrylate, cyclohexyl methacrylate, 
benzyl acrylate, isobornyl acrylate and phenyl methacrylate. Preferably 
the alkyl acrylate monomer is methyl methacrylate. 
Also included as useful ester monomers are hydroxyalkyl esters of acrylic, 
methacrylic, ethacrylic, ethacrylic or crotonic acids wherein the 
hydroxyalkyl group contains 2 to 10 carbon atoms and is, preferably, a 
beta-hydroxyalkyl group. Examples of hydroxyalkyl ester monomers are beta 
hydroxyethyl acrylate, beta hydroxyethyl methacrylate, beta hydroxyethyl 
crotonate, beta hydroxyethyl ethacrylate, beta hydroxypropyl acrylate, 
beta-hydroxy propyl methacrylate, gamma hydroxypropyl methacrylate, 
beta-hydroxybutyl acrylate, gamma hydroxybutyl methacrylate, delta 
hydroxybutyl crotonate, beta hydroxyoctyl acrylate and beta hydroxy-decyl 
methacrylate. 
Homopolymerizable and/or copolymerizable monomers in class 2 are 
acrylonitrile, methacrylonitrile, ethacrylonitrile, crotonic nitrile, etc. 
Preferably the vinyl cyanide monomer is acrylonitrile. 
Polymerizable monomers in class 3 include styrene, vinyl toluene, vinyl 
naphthalene, .alpha.-methyl styrene chlorostyrene, bromostyrenes and the 
like. Preferably the vinyl aromatic monomer is styrene. 
The process of the present invention may involve reacting a vinyl monomer 
feed stream in a batch, semi-batch or continuous process manner. The 
feedstream preferably comprises from about 20 to about 80 percent by 
weight of vinyl cyanide monomer based on the total weight of the liquid 
feed stream and from about 80 to about 20 percent by weight of vinyl 
aromatic monomer based on the total weight of the liquid feed stream and 
from about 0 to about 30 percent by weight of a diluent based on the total 
weight of the liquid feed stream. The diluent serves as a solvent for 
styrene and acrylonitrile monomers and as a diluent for a styrene and 
acrylonitrile containing polymer. Preferably liquid feed stream comprises 
from 65 to 85 percent by weight vinyl aromatic monomer based on the total 
weight of the feed stream and from 15 to 35 percent by weight vinyl 
cyanide monomer based on the total weight of the feed stream. Preferably 
the feedstream is passed into a polymerization zone in either a batch, 
semi batch or continuous manner wherein the feedstream becomes a liquid 
phase reaction mass, in the presence of a nitrogen vapor phase blanket, is 
maintained at a temperature of from about 120.degree. C., to about 
170.degree. C. under a pressure of from about 10 to about 300 pounds per 
square inch (hereinafter "psi") whereby at least a portion of the 
feedstream is thereafter subjected to heat and vacuum sufficient to remove 
at least a major portion of any residual unpolymerized components of the 
feedstream including the diluent. The improvement being the incorporation 
within the nitrogen vapor phase of a sulfur dioxide polymerization 
initiator at from about 0.05 to 5 parts by volume per hundred parts by 
volume of nitrogen over the feedstream. The attaining and maintaining of a 
level of sulfur dioxide in the vapor phase over the liquid phase reaction 
mass in the polymerization zone while the liquid phase reaction mass is 
being polymerized. Preferably the sulfur dioxide is not present at a level 
of less than 0.1 parts by volume per hundred parts by volume of the 
diatomic nitrogen in the vapor phase over the liquid phase reaction mass. 
The residence time of the liquid phase reaction mass within the 
polymerization zone being preferably from about 0.7 to about 1.4 hours. 
The vinyl aromatic monomer is preferably of commercial purity and is 
present at a level of from about 85 to 20 percent by weight based on the 
total weight of the feedstream and more preferably at a level of from 65 
to 85 percent by weight thereof. Correspondingly, the liquid phase 
reaction mass preferably initially has the vinyl aromatic monomer present 
at a level of from 85 to 20 percent by weight based on the total weight of 
the liquid phase reaction mass, and more preferably at a level of 65 to 85 
percent by weight thereof. 
The vinyl cyanide monomer is preferably of commercial purity and is present 
at a level of from 15 to 80 percent by weight based on the total weight of 
the feedstream composition, and more preferably at a level of from 15 to 
35 percent by weight thereof. Correspondingly, the liquid phase reaction 
mass preferably initially has the vinyl cyanide monomer present at a level 
of from 15 to 80 percent by weight based on the total weight of the liquid 
phase reaction mass, and more preferably at a level of from 15 to 35 
percent by weight thereof. 
The diluent is advantageously a solvent in which the styrene and 
acrylonitrile monomers are soluble and is ultimately used to adjust the 
viscosity of the liquid phase reaction mass. More advantageously, the 
diluent is selected from the group consisting of ethylbenzene and 
N,N-dimethylformamide in an amount of from about 0 to about 30 parts by 
weight, based upon the total weight of the feedstream. Beneficially, the 
diluent is selected from the group consisting of ethylbenzene and 
N,N-dimethylformamide in an amount of from about 15 to about 25 parts by 
weight, based upon the total weight of the feedstream. Preferably, the 
diluent consists of from about 15 to 25 parts by weight, based upon the 
total weight of the feedstream of ethylbenzene. 
The feedstream preferably consists of from about 85 to about 15 parts by 
weight of styrene, from about 20 to about 80 parts by weight of 
acrylonitrile, and from about 0 to about 30 parts by weight of a diluent 
such as ethylbenzene. Optionally, other vinyl monomers may be used. Also, 
optionally, the process may be used for making a rubber modified polymer, 
for example an ABS graft copolymer or an ASA copolymer as set out below by 
including in the feedstream a rubbery polymer such as polybutadiene or 
polybutylacrylate. Bulk polymerization of a rubber modified polymer 
typically involves dissolving of the rubbery polymer in the monomeric 
composition followed by polymerization of the monomers to yield a grafted 
copolymer followed by phase inversion and completion of the reaction to 
build molecular weight and crosslink the rubber. 
In practicing the process of this invention, one may use a recirculating 
coil apparatus, a recirculating tube train apparatus or a boiling reactor. 
The polymerization initiator is added to the feedstream after mixing and 
prior to being passed into a polymerization zone. Advantageously, the 
feedstream is subjected to temperatures of from about 120.degree. C. to 
about 170.degree. C. and to pressures of from about 10 to about 300 psi. 
Preferably the feedstream is subjected to temperatures of from about 
135.degree. C. to about 155.degree. C. and to pressures of from about 100 
to about 200 psi for the recirculating coil apparatus and the 
recirculating tube train apparatus or from about 10 to about 60 psi for 
the boiling reactor. At temperatures in excess of about 170.degree. C., 
cooling of the reactor becomes very difficult. At temperatures below about 
120.degree. C., the polymerization rate is too low for the purposes of 
commercial or practical application, thereby necessitating an increase in 
the amount of time which the feedstream must remain within the 
polymerization zone which in turn may lead to undesirable products, some 
of which may have excessive molecular weights. In addition, when using 
either the recirculating coil apparatus or the recirculating tube train 
apparatus, the pressures must be maintained above the vapor pressure of 
the component with the lowest boiling point in order to prevent 
polymerization within the vapor phase, but below the preferred maximum of 
about 300 psi as pressures in excess of about 300 psi may cause structural 
changes in the polymer which may cause the process to become uneconomical. 
When using the boiling reactor, the rate of reaction within the boiling 
reactor will be difficult to control at pressures in excess of 60 psi. At 
pressures of less than 10 psi, the process would be uneconomical for 
commercial or practical purposes as it would lead either to excessive 
molecular weight polymers or to the use of uneconomical amounts of 
additives such as catalysts and chain-transfer agents. 
The rate of feed into a polymerization vessel is determined by the length 
of residence time of the feedstream in the polymerization zone within the 
polymerization vessel which in turn is determined by the portion of the 
feedstream which is to be polymerized. The residence time is preferably 
from about 0.7 to about 1.4 hours. Most preferably, the residence time is 
from about 0.9 to about 1.1 hours. If a residence time of less than about 
0.7 hour is selected, the heat of reaction under the preferred range of 
operating conditions would be too great and would require too much diluent 
to control the reaction, thereby making the process uneconomical. If a 
residence time in excess of 1.4 hours is selected, the color is 
detrimentally affected and the process becomes uneconomical. 
The product of the polymerization process is subjected to heat and vacuum 
sufficient to remove at least a major portion of any residual 
unpolymerized components of the feedstream, such as the diluent, the 
styrene and acrylonitrile monomers, and the polymerization initiator. 
Any of the well known solvents can be used as a reaction medium for 
conducting the process of this invention, such solvents being aromatic, 
cycloaliphatic and aliphatic hydrocarbons, ketones, esters, ethers, 
alcohols and the like. However, as stated herein before it is preferred to 
carry out the polymerizations as bulk polymerizations wherein no solvent 
is used. 
The process of the present invention preferably involves mass 
polymerization of styrene and acrylonitrile in a liquid phase blanketed by 
a nitrogen atmosphere containing 0.1 to 1 weight percent Sulfur dioxide as 
an inhibitor of crosslinked SAN popcorn formation in the reactor vessel. 
The process of the present invention may also be used to prevent popcorn 
formation in the production of bulk vinyl aromatic/vinyl cyanide/rubber 
graft copolymers such as bulk ABS or a rubber modified polymer such as 
vinyl aromatic/vinyl cyanide/butyl acrylate copolymers such as bulk ASA. 
The "ABS resin prepared by bulk polymerization" employed in this invention 
means a product obtained by dissolving a butadiene type rubber, such as 
polybutadiene or a butadiene-styrene copolymer, into a mixture of a vinyl 
cyano compound represented by acrylonitrile and a vinyl aromatic 
hydrocarbon (including a halogenated compound) represented by styrene, 
polymerizing the mixture substantially under bulk polymerization 
conditions, under such a high agitation as to shear the rubber being 
precipitated as polymerization advances, until the polymerization is 
substantially completed. The amount of the rubber to the total amount of 
the monomer mixture (styrene plus acrylonitrile) is 1-50 wt. percent, 
preferably 2-20 wt. percent. The monomer mixture preferably contains 85 to 
65 percent by weight of styrene to 15 to 35 percent by weight of 
acrylonitrile based on the total combined weight of the styrene and 
acrylonitrile. 
The present process may also be employed as a step in a multistep 
polymerization process including, for example, a bulk suspension process 
such as for the production of vinyl aromatic/vinyl cyanide/rubber graft 
copolymers wherein the bulk process is employed, until 10-40 percent by 
weight of the vinyl cyano compound and vinyl aromatic compound are 
polymerized, and then adding water and a suspension stabilizer to the 
polymerization system and continuing the polymerization under the 
suspension polymerization conditions until the polymerization is 
substantially completed. 
The following examples illustrate the improvements which result from the 
process of this invention but are not in limitation thereof. 
The invention was experimentally examined on a 2-gallon reactor facility in 
a continuous process. There are three basic units in this facility 
(monomer feed unit, reactor unit, and devolatilization/pelletization unit) 
for SAN synthesis in a bulk process. Two different control experiments are 
carried out. The first one is the control experiment that produces 
significant amount of SAN popcorn polymer (about 20 grams) in the vapor 
head space of the 2-gallon reactor over a given reaction time length (40 
hours), using only nitrogen (&gt;99.95%) as the reaction atmosphere. The 
second one is also the control experiment that produces no SAN popcorn 
polymer over the same reaction hours, using 2% oxygen in nitrogen (by 
volume). The copolymerization reactions for all the experiments are set at 
the same reaction conditions except that the reaction atmospheres are 
different from one to another. The experiments using 2% sulfur dioxide in 
nitrogen (by volume) show that sulfur dioxide eliminates the formation of 
SAN popcorn polymer in the vapor head space. Table 1 shows conditions and 
results for some of these experiments. 
Kinetic data of SAN copolymerization under sulfur dioxide/nitrogen mixture 
and characterization data of the corresponding SAN polymer suggest that 
sulfur dioxide may also be used as a dual-function catalyst. That is, the 
sulfur dioxide/nitrogen atmosphere will not only accelerate rates of 
copolymerization of ST and AN but also increase molecular weights (number 
average and weight average) of SAN polymer comparing with the SAN polymers 
obtained from control ones. 
TABLE 1 
______________________________________ 
Conditions and Results for SAN Popcorn Polymer Prevention 
Experiments Using a 2-Gallon Reactor Facility* 
Expt. ST/AN/NOM Rx. Temp. Rx. SAN 
No. (Wt.)** .degree.C. 
Rx. Atm. 
Hrs. Popcorn 
______________________________________ 
A 69/31/0.15 120 N.sub.2 
40 22 g 
B 69/31/0.15 120 2.0% O.sub.2 
40 0 
in N.sub.2 
1 69/31/0.15 120 2.0% SO.sub.2 
40 0 
in N.sub.2 
______________________________________ 
*The reactor top lid was cooled by compressed air and was kept at 70 to 
75.degree. C. 
**ST/AN/NOM: styrene/acrylonitrile/noctyl mercaptan feed ratio by weight. 
Rx. Temp. means reaction temperature. 
Rx. Atm. means vapor phase atmosphere. 
Rx. Hrs. means reaction time. 
SAN Popcorn means the amount of SAN popcorn formed during the reaction. 
As mentioned above, one possible advantage of using sulfur dioxide is its 
ability to accelerate the polymerization of the monomers in the liquid 
phase while inhibiting popcorn formation in the vapor phase. The process 
may be employed as a step in bulk-suspension processes, or may be used as 
the entire polymerization process. A suitable vapor phase comprises less 
than 0.1 percent by volume diatomic oxygen based on the total volume of 
the vapor phase. Preferably the vapor phase comprises less than 0.05 
percent by volume diatomic oxygen based on the total volume of nitrogen in 
the vapor phase.