Note: Descriptions are shown in the official language in which they were submitted.
CA 02218802 1997-10-22
SOLID ACRYLIC RESIN USING A CONTINUOUS TUBE REACTOR
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of. and claims priority of, prior
application Serial No. 08/686,860 filed on July 26. 1996, the contents of which
are incorporated herein by reference.
5 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 (~-methyl styrene-co-acrylic acid-co-styrene) and poly ( styrene-co-
acrylic acid-co-methacvrylic 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 and made into a dispersion, also known as a latex, by
15 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
20 than solvent-based systems.
Typically, the acrylic resin has been made by bulk polymerization in a
. CA 02218802 1997-10-22
continuous-stirred tank reactor (CSTR). The CSTR is charged with styrene or
styrene plus a-methyl styrene, (meth)acrylic acid, a polymerization initiator and
a solvent or just with styrene, ~-methyl styrene and (meth) acrylic acid. Reaction
temperatures range from 1 80~C to 300OC and residence times are from 1 to 60
5 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
15 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,
20 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
CA 02218802 1997-10-22
are weight percentages unless indicated otherwise.
SUMMARY OF THE INVENTION
The invention is a bulk polymerization process for preparing a solid acrylic
5 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
10 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
15 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
20 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
' CA 02218802 1997-10-22
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.
5 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
10 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
15 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
20 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.
' CA 02218802 1997-10-22
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 1/2 inch outside diameter (O.D.) tube, three lengths of 20
foot 3/4 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
' CA 02218802 1997-10-22
because a CSTR will have a headspace filled with vapor in thermodynamic
equilibrium with the monomers. While not completely understood, the pressure
in a CTR may be a variable that is at least partially independent of the
temperature. While the formation of at least some vapor phase has been
observed through a transparent tube reactor as transient foaming at the initiation
of polymerization, it has been suggested that perhaps the continuous dynamic
phase change in the CTR inhibits the establishing of thermodynamic equilibrium
within the reactor. Beyond this, it can only qualitatively be stated that lower
pressures increase vapor fraction and therefore reduce the residence time,
hence the conversion. In order to obtain acceptable conversion and properties,
the pressure should be simultaneously optimized with both temperature and
residence time.
If the heat transfer fluid temperature is controlled to a maximum of 260~C
(500~F), then it is possible to use a CTR to make acrylic resin without the danger
of thermal run-away. Nor is there need for internal cooling coils and their
inherent thermodynamic inefficiency. The lower bound for the temperature is
180~C. At this temperature, conversion is so slow that residence times become
uneconomically long and the viscosities are too high to handle. The preferred
temperature range for this reactor and monomer/initiator mixture is from 204~C
(400~F) to 260~C (500~F); more preferred is 210~C (410~F) to 246~C (475~F).
It may reasonably be expected that a longer tube will require lower temperature
for equal conversion, while larger O.D. or thicker walls might necessitate higher
temperatures. Note that while the heat transfer fluid is limited to 260~C, the
stream at the reactor exit can be as high as 271 ~C.
' CA 02218802 1997-10-22
The residence time lower limit is bounded by 1 minute, conversion being
low. On the upper end there are diminishing returns on percent conversion as
well as economic waste for needless dwell time; this limit is 1 hour. Also,
polymer properties suffer at higher residence times. The preferred dwell time for
5 this reactor is optimized simultaneously with the pressure and temperature, as
described above, and is 150 to 250 seconds.
While no solvent is required, solvent can, of course, be added. Glycol
ethers are the class of solvents most commonly encountered; diethylene glycol
monoethylether and diethylene glycol dimethyl ether are examples and they are
I0 typically used at levels of up to 25%.
The feedstock should comprise at least one unreacted vinylic monomer.
It is further preferred that the feedstock comprises a blend of vinylic monomers
containing at least one acrylic monomer and at least one monoalkenyl aromatic
monomer. Monoalkenyl aromatic monomers that can be used include vinyl
15 toluene, para-methyl-styrene, tert-butyl-styrene and chlorostyrene, but preferred
are styrene and a-methyl-styrene. Acrylic monomers that may be used include
acrylic, methacrylic acid, crotonic acid and their esters and derivatives and
maleic anhydride. Among them are butyl acrylate, 2-ethylhexylacrylate, 2-
ethylhexylmethacrylate, methylmethacrylate, hydroxyethylmethacrylate and the
20 like. Acrylic and mthhacrylic acid are preferred. The preferred blends comprises
styrene, ~-methyl styrene and acrylic acid and styrene, acrylic acid and
methacrylic acid. Styrene and a-methyl styrene are hydrophobic, while
(meth)acrylic acid is hydrophilic, especially when neutralized to a salt. In order
to produce a dispersible polymeric surfactant, the hydrophobic portion must have
' CA 02218802 1997-10-22
a certain balance with the hydrophilic portion. Pure acrylic acid would result in
polyacrylic acid, which forms a true solution in water, rather than a dispersion.
Pure polystyrene or poly(styrene-co~-methyl styrene) will not disperse in water
because it has no hydrophilic functionality, nor an acid group whose
S hydrophilicity can be increased via neutralization. The composition range of
styrene plus a-methyl styrene versus (meth)acrylic acid that has produced
successful dispersions is 50 to 80 wt.% styrene plus a-methyl styrene and 18
to 40 wt.% (meth)acrylic acid, balance being initiator and any solvent. The ratio
of styrene to a-methyl styrene is broader, as both are of the same character,
lo hydrophobic1 and may vary from 2.5:1 to 20:1. In terms of mole % of the
monomers, the preferred ranges are 25 to 60 % styrene, 2 to 35 % alpha-
methyl styrene and 25 to 50 % acrylic acid.
With respect to the styrene, acylic acid and methacrylic acid embodiment,
ratios of 1:2:1, 1:1:1 and 13:1 mole ratios are possible, giving a range of
15 styrene:(methacrylic acid) of 1:2 to 1:4 moles.
Recycling of the monomers recovered from the reaction mass exiting the
reactor as distillate from the devolatilization step is a preferred embodiment. As
will be seen in the examples, the properties of the acrylic resins so produced,
especially glossiness of the coatings made therefrom, are significantly improved.
20 Typically, 10 wt.% of the feed consists of recycled monomer, however, at least
80 wt.% of the recovered monomers can be recycled. The recycled monomers
may require pre-processing such as purification.
The polymerization initiator is of the free radical type with a half-life
rangingfrom 1 to 10hoursat 90to 100~C. Preferredareinitiatorswithhalf-
CA 02218802 1997-10-22
lives of 10 hours at 1 00~C. Initiators of this sort may be azo-type, such as azo-
bis isobutyronitrile (AIBN), 1-tert-amylazo-1-cyanocyclohexane and 1-tert-
butylazo-1-cyanocyclohexane. They may also be peroxides and hydroperoxides
such as tert-butylperoctoate, tert-butylperbenzoate. dicumyl peroxide and tert-
butyl hydroperoxide. Preferred are di-tert-butyl peroxide and cumene
hydroperoxide. The quantity of initiator typically used ranges from 0.0005:1 to
0.06:1 moles initiator per mole monomer. When di-tert-butyl peroxide is used it
is preferred that it is at 0.002 to 0.05 mole ratio, preferably from 0.003 to 0.04
mole ratio. For the styrene, acrylic acid and methacrylic acid embodiment, 1 part
by weight per hundered monomer of di-tert-butyl peroxide has been found useful.
Once the reaction product exits the CTR, it is devolatilized to separate the
molten acrylic resin, which can be flaked or pelletized after cooling. This thencan be used to prepare dispersions. A typical formula would be prepared as
follows:
lS Charge #1 is 201.52 g acrylic resin, 102.16 9 de-ionized (Dl) water, 4.99 9
Dowfax2A1 and 5.17 9 Triton X-100 surfactant; adjust to a pH of 8.71 with ca.
3 9 ammonium hydroxide.
Charge #2 is 146.58 9 styrene and 27.60 9 2-ethylhexylacrylate.
Charge #3 is 1.99 9 ammonium persulfate and 5.30 g Dl water.
Charge #4 is 1.24 9 t-butyl hydroperoxide (70%).
Charge #5 is 0.70 9 sodium ascorbate and 8 00 g Dl water.
At t=0 min., T= 23~C, apply a nitrogen blanket, charge #1 and start heating.
Note that the reactor is blanketed with nitrogen to quench any free radicals
present; nitrogen is not involved in the actual resin chemistry in any way.
CA 02218802 1997-10-22
At 38 min. 80~C, charge 16.79 g #2.
At 43 min., 82~C, charge #3.
At 53 min., 85~C, start monomer #2.
At 118 min., 84~C, complete addition of monomer #2.
At 183 min., 84~C, charge #4 and 1/3 of #5.
At 188 min., 84~C, charge 1/3 of #5.
At 193 min., 84~C, charge remainder of #5.
At 198 min., remove and allow to cool.
Normally, the above would be augmented with preservatives, dyes,
pigments, thixotropes, perfumes, wetting agents, antifoams, coalescing agents,
slip aids and the iike prior to use.
Examples 1 through 10 explore variations of the reaction parameters,
particularly pressure, on the percent conversion (one-pass yield) and the
properties of coatings made from dispersions prepared from the acrylic resins
produced by the CTR. Example 11 describes a preferred embodiment, wherein
the monomers are recycled and surprisingly form a product not only better than
that had from virgin monomers, with respect to gloss on white, but also superiorto the nearest commercial equivalent, Joncryl 678. Joncryl's properties as a
control are shown in example 1. Example 12 shows the effect of varying
monomer ratios on the yield obtained, as well as the highest yield obtained. Allpercents are weight percents and all molecular weights are weight average.
EXAMPLE 1
29.1% styrene, 40.9% a-methyl styrene, 29.5% acrylic acid and 0.5% di-
CA 02218802 1997-10-22
tertiary butyl peroxide (the "feedstock") was passed through a continuous tube
reactor. The residence time was 200 seconds, the pressure was 140 psig and
the temperature was 232CC (450~F). The conversion was 77.6%. The acrylic
resin/unreacted monomer blend was devolatilized. The resulting resin had a
S weight average molecular weight of 7913. an acid value of 253 and a glass
transition temperature (Tg) of 11 7~C. A control sample of Joncryl 678
(trademark, S.C. Johnson Co.) was measured and found to have a weight
average molecular weight of 9000, an acid value of 224 and a glass transition
temperature (Tg) of 117~C. The composition of Joncryl 678 is believed to be
30% styrene, 40% ~-methyl styrene and 30% acrylic acid. A dispersion was
made from the experimental resin by the technique described above but
neutralizing to pH 9.32. The final dispersion was 49.02% solids. Its viscosity
was 530 cps, the particle size was 94.1 nm, while the gloss on black was 91.3
and the gloss on white was 82. The coatings were evaluated for gloss by
conventional means, i.e. simply measuring within a Macbeth Novo-Gloss meter
the visible light reflected from the surface at the same angle (i.e. 60 degrees) as
the incident angle of the light. The values expressed are for an average of
several measurements. A similar dispersion made from the Joncryl 678 to
49.13% solids and a pH of 8.40. The dispersion had a viscosity of 340 cps, a
particle size of 55.7 nm, gloss on black of 92.0 and a gloss on white of 102Ø
EXAMPLE 2
The same feedstock was used as in experiment 1. The residence time
was 250 seconds, the pressure was 150 psig and the temperature was 238~C
' CA 02218802 1997-10-22
(460~F). The conversion was 81.8%. The resulting resin had a weight average
molecular weight of 7582, an acid value of 249 and a glass transition
temperature (Tg) of 114~C. The dispersion was made as above, neutralized to
pH 9.34. The final dispersion was 49.98% solids. Its viscosity was 510 cps, the
S particle size was 83.2 nm, while the gloss on black was 95.8 and the gloss on
white was 75.1.
E)CAMPLE 3
The same feedstock was used as in experiment 1. The residence time
was 150 seconds, the pressure was 130 psig and the temperature was 238~C
(460~F). The conversion was 61.8%. The resulting resin had a weight average
molecular weight of 8098, an acid value of 255 and a glass transition
temperature (Tg) of 127~C. The dispersion was made as above, neutralized to
pH 9.10. The final dispersion was 49.48% solids. Its viscosity was 875 cps, the
particle size was 74.5 nm, while the gloss on black was 93.5 and the gloss on
I S white was 94.6.
EXAMPLE 4
The same feedstock was used as in experiment 1. The residence time
was 250 seconds, the pressure was 130 psig and the temperature was 238~C
(460~F). The conversion was 75.4%. The resulting resin had a weight average
molecular weight of 8096, an acid value of 254 and a glass transition
temperature (Tg) of 120.57~C. The dispersion was made as above, neutralized
to pH 9.34. The final dispersion was 49.98% solids. Its viscosity was 440 cps,
CA 02218802 1997-10-22
the particle size was 90 1 nm, while the gloss on black was 93.5 and the gloss
on white was 75 0
EXAMPLE 5
The same feedstock was used as in experiment 1. The residence time
s was 150 seconds, the pressure was 150 psig and the temperature was 238~C
(460~F). The conversion was 65 9%. The resulting resin had a weight average
molecular weight of 7518, an acid value of 257 and a glass transition
temperature (Tg) of 124~C. The dispersion was made as above, neutralized to
pH 9 48. The final dispersion was 51 24% solids. Its viscosity was 1350 cps,
the particle size was 65 nm, while the gloss on black was 95.8 and the gloss on
white was 71.6.
EXAMPLE 6
The same feedstock was used as in experiment 1. The residence time
was 250 seconds, the pressure was 130 psig and the temperature was 227~C
lS (440~F). The conversion was 82%. The resulting resin had a weight average
molecular weight of 8339, an acid value of 250 and a glass transition
temperature (Tg) of 128~C. The dispersion was made as above, neutralized to
pH 8 70. The final dispersion was 47.85% solids. Its viscosity was 303 cps, the
particle size was 71.9 nm, while the gloss on black was 95.1 and the gloss on
white was 88.6.
EXAMPLE 7
CA 02218802 1997-10-22
The same feedstock was used as in experiment 1 The residence time
was 150 seconds, the pressure was 150 psig and the temperature was 227~C
(440~F). The conversion was 72 2%. The resulting resin had a weight average
molecular weight of 7093, an acid value of 256 and a glass transition
temperature (Tg) of 118~C. The dispersion was made as above, neutralized to
pH 8 44 The final dispersion was 50 38% solids. Its viscosity was 535 cps, the
particle size was 65 0 nm, while the gloss on black was 95.1 and the gloss on
white was 85.9.
EXAMPLE 8
lo The same feedstock was used as in experiment 1. The residence time
was 250 seconds, the pressure was 150 psig and the temperature was 227~C
(440~F). The conversion was 86.4%. The resulting resin had a weight average
moiecular weight of 7809, an acid value of 249 and a glass transition
temperature (Tg) of 111 ~C. The dispersion was made as above, neutralized to
l 5 pH 8 35. The final dispersion was 48 61 % solids. Its viscosity was 323 cps, the
particle size was 79.6 nm, while the gloss on black was 94 8 and the gloss on
white was 76 0.
EXAMPLE 9
The same feedstock was used as in experiment 1. The residence time
was 150 seconds, the pressure was 130 psig and the temperature was 227~C
(440~F). The conversion was 65 0%. The resulting resin had a weight average
molecular weight of 7783, an acid value of 258 and a glass transition
CA 02218802 1997-10-22
temperature (Tg) of 125~C. The dispersion was made as above, neutralized to
pH 8.49. The final dispersion was 48.85% solids. Its viscosity was 595 cps, the
particle size was 65.5 nm, while the gloss on black was 92.5 and the gloss on
white was 99.5.
S EXAMPLE 10
The same feedstock was used as in experiment 1. The residence time
was 200 seconds, the pressure was 140 psig and the temperature was 232~C
(450~F). The conversion was 75.4%. The resulting resin had a weight average
molecular weight of 7944, an acid value of 253 and a glass transition
temperature (Tg) of 114~C. The dispersion was made as above, neutralized to
pH 8.54. The final dispersion was 49.15% solids. Its viscosity was 475 cps, the
particle size was 72.9 nm, while the gloss on black was 95.6 and the gloss on
white was 86Ø
EXAMPLE 11
The feedstock used was 90% that of experiment 1, plus 10% of the
monomers recycled from the devolatilization step. The residence time was 150
seconds, the pressure was 130 psig and the temperature was 216~C (420~F).
The conversion was 69.7%. A cut at the beginning of the run and again at the
end of the run was taken. The resulting resin from one cut had a weight
average molecular weight of 7913, an acid value of 253 and a glass transition
temperature (Tg) of 117~C. The dispersion was made as above, neutralized to
pH 8.48. The final dispersion was 48.00% solids. Its viscosity was 720 cps, the
CA 02218802 1997-10-22
particle size was 66.6 nm, while the gloss on black was 91.92 and the gloss on
white was 101.83. Note that the resin made with recycled monomers resulted
in significantly higher gloss on white than that made with neat feedstock.
The resulting resin made from the other cut had a weight average
molecular weight of 7582, an acid value of 249 and a glass transition
temperature (Tg) of 114~C. The dispersion was made as above, neutralized to
pH 8.13. The final dispersion was 48.30% solids. Its viscosity was 385 cps, the
particle size was 75.5 nm, while the gloss on black was 93.24 and the gloss on
white was 106.33. ~ote that the resin made with recycled monomers resulted
in significantly higher gloss on white than that made with neat feedstock. Note
also that the gloss on white, as well as the gloss on black, is superior to thatobtained with the closest commercial equivalent, Joncryl 678.
EXAMPLE 12
The monomer portion of the feedstock consisted of 30% ~-methyl styrene,
while the ratio of acrylic acid:styrene (M:Styrene) of the balance of the monomer
was varied. Reactor pressure was 220 psig, while the residence time was 240
seconds and the heat transfer fluid temperature was 246~C (475~F). The results
were:
M:Styrene Tg (~C) Acid value Mol. Wt. % Conversion
1.31 95 292 1910 81
1.17 89 261 1940 84
1.05 94 265 1923 85
0.93 90 246 1899 76
0.83 93 242 1930 91
16
CA 02218802 1997-10-22
EXAMPLE 13
The reaction conditions for this, and the remaining examples, are a
reaction temperature of 410~F, a hot oil temperature set at 440~F and pressure
of 120 psig and a residence time of 3.3 minutes. Conversion was greater than
90% for this and th efollowing examples. Styrene:acrylic acid:methacrylic acid
was run at a 1 :2:1 molar ratio (31.1 :43.2:25.7 weight ratio) with 1 part by weight
di-tert-butyl peroxide as the feedstock. The resulting polymer had a Tg of 112~C,
a softening point of 162~C, a theoretical AV of 500. an experimental AV of 395,
1.6% residual monomer and a molecular weight of 10,000. A dispersion of the
above resin was prepared by using 2 times the theoretical ammonia and then
boiling off excess ammonia to pH 8-8.5 and adding Dl water to compensate for
loss.
EXAMPLE 14
Styrene:acrylic acid:methacrylic acid was run at a 1:3:1 molar ratio
(25.6:53.2:21.2 weight ratio) with l part by weight di-tert-butyl peroxide as the
feedstock. The resulting polymer had a Tg of 129~C, an experimental AV of 372,
5.06% residual monomer and a molecular weight of 10,400.
EXAMPLE 15
Styrene:acrylic acid:methacrylic acid was run at a 1:1:1 molar ratio
(39.7:27.5:32.8 weight ratio) with 1 part by weight di-tert-butyl peroxide as the
feedstock.
' CA 02218802 1997-10-22
Although various embodiments of the invention are shown and described
herein, they are not meant to be limiting, those of skill in the art may recognize
various modifications to the embodiments, which modifications are meant to be
covered by the spirit and scope of the appended claims.
