Note: Descriptions are shown in the official language in which they were submitted.
SPE C I F I CATI t)N ;~
LOU IS A . CUTTEF~
IMPACT MODI~IED METHYL
MET~ACRYLATE POLYMER
BA(~KGROUND OF_THE INYENTION
Polymethylmethacrylate is a strong, optically clear,
weather re~istant polymer, but as conventionally made by
free radical polymerization, either in the presence or absence of
cross~linkers, is quite brittle and deficient in impact strength
For that reason, its use is limited to applications in which only
very ~odest impact strength is required. To increase its range of
application, polymethylmethacrylate should have high impact
strength and should include qualities of resistance to ultraviolet
and visible radiation and oxidative degradation in order to provide
weatherability.
The highest quality polymethylmethacrylate sheet with the
smoothest surface is made by a casting process, in which the methyl
methacrylate monomer is polymerized either batchwise in a cell or
continuously between two continuous belts. A readily castable
high-impact polymethylmethacrylate composition of good
weath~rability is needed ~n ~he art.
There has been described the preparation of a
g~afted acrylic copolymer in emulsion
~ .
which can be dried, dispersed in a mixture of acrylic monomers, and
cast to produce a sheet with enhanced impact strength and good
weatherability. The amount of modifier that can be incorporated in
this way is severely limited by the maximum practical viscosity of
the casting mixture, and for that reason the impact strength
attainable is also limited. See also USP 3,793,402, which
describes preparation of impact-modified polymethylmethacrylate by
extrusion blending a graft emulsion polymer with a
polymethylmethacrylate/ethyl acrylate molding powder. It also
describes casting of the molding powder after sheeting a mixture of
emulsion polymer powder with a methyl methacrylate/ethyl acrylate
monomer mixture and initiator on a cold roll mill. This casting
procedure presents severe difficulties in implementation in casting
operations because of the very high viscosity of the monomer/graft
polymer mixture.
A second inherent limitation of conventional acrylic
modifiers is in maintaining impact strength at low temperatures,
because of their relatively high glass transition temperatures.
Other high-impact acrylic resins modified with
polybutadiene and/or styrene-butadiene have been described. See
USPs 21857~360r 3,029,222, and 3,261,887. Because of the
unsaturated carbon linkages, these are more susceptible to
oxidation and weathering than polymer modified with acrylic rubber,
as pointed out by Manson et al, ~Polymer Blends and Composites~,
pp. 117-119, Pl*num, New York (1976).
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,
Ethylene-propylene or ethylene-butylene rubbers offer
possibilities for combining glass transition temperatures lower
than those of acrylic rubbers with weatherability. A difficulty
with these materials is in making a stable dispersion of
appropriate particle size (0.2-5 microns) for maximum impact
streng~h, since they are not readily compatible with
polymethylmethacrylate, or soluble in the monomer.
In addition to the above-mentioned USP 3,793,402, the
reader may be interested in reviewing Yusa et al USP 3,922,321,
which recites improvements in weatherability as well as impact
strength; however, contrary to the solution to the problem offered
by the applicant herein, Yusa et al employ a methyl
methacrylate-grafted butyl acrylate/styrene, a different material
entieely. Also of interest will be various patents to Gergen and
Davison, i.e. 4,085,163 (see col. 18, line 58), 4,0~1,424 (col. 19,
line 50), 4,111,894; 4,110,303 and 4,102,854; however, those
disclosures which mention methacrylates utilize them only as more
or less incidental comonomers in the specifically described nitrile
barrier resins. The Lunk Patent 3,810,957 may also be of interest
for its recitation of various impact modifiers for use in
thermoplastics.
The use of styrene-(ethylene-butene-l)-styrene block
copolymer as an addition for scrap blends of polystyrene and
polypropylene is described in the December, 1981 issue of Modern
Plastics, pp. 60-62.
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Kelsey in USP 4,080,406 discloses transparent high-impact
compositions obtained by polymerizing a mixture of methyl
methacrylate, higher methacrylates and styrene with
styrene-butadiene block copolymer and/or polybutadiene. Here the
unsaturation in the rubber provides a source of good grafting
sites, helpful in stabilizing the rubbery phase during
polymerization and subsequent processing but detrimental to
weatherability. ~arlan, Jr. in USP 4,007,311 describes the
polymerization of acrylates or methacrylates with
styrene-butadiene-styrene block copolymers by polymerizing
methylmethacrylate in the presence of peroxides, also analogous
reactions when the butadiene portion of the block has been
hydrogenated, and their use in adhesives. Falk in USP 4,21~,958
discloses the preparation of graft/polyblend compositions by
polymerizing acrylic esters in the presence of saturated rubbers,
including ethylene-propylene rubber, hydrogenated styrene-butadiene
blocks, and hydrogenated polybutadiene with benzoyl peroxide in
emulsion and solution. The polymers were used as flow and impact
mo~ifiers for PVC. Kitagawa et al in USP 4,287,317 disclose a
continuous process for producing impact-modified
polymethylmethacrylate, using saturated or unsaturated rubbers
involving the preparation of a prepolymer syrup with dispersed
rubber particles, which can subsequently be cast to produce an
impact-modified polymethylmethacrylate. Moran in USP 4/097~555
discloses a method for producing transparent, high impact
compositions containing an alkenyl aromatic-alkenyl nitrile acrylic
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~atrix and styrene-b~tadiene-styrene bloc~ copolymer as a rubbery
phase (unsaturated).
_UMMARY o~ THE INVEN~ION
My invention includes cast acrylic forms, made
predominantly of polymethylmethacrylate, impact~modified with a
block copolymer of ABA structure where A is polystyrene and B is a
hydrogenated polybutadiene rubber, methods of making such polymeric
forms, and the syrups from which they are made. Specifically, the
casting syrup will include about 0.1% to about 2% styrene, about 69
to about 87~ methyl methacrylate, about 0.2% to about 2.0~ of a
peroxide initiator, and about 8 to about 14% of an ABA block
copolymer having a weight ratio of A:B monomer of about 1:2 to
about 1:3 wherein A is a chain of polystyrene, and B is a block of
hydrogenated polybutadiene, and up to about lQ% ethylene-propylene
rubber. The weight-average molecular weight of the block copolymer
is normally within the range of about 50,000 to about 90,000. The
syrup can also include up to about 3~ of a diunsaturated cross-
linking agent. The ABA block copolymers I use are presently made
by the complete or nearly complete hydrogenation of the
polybutadiene portion of polystyrene-polybutadiene-polystyrene
block copolymers containing a large 1-2 butadiene fraction, i.e.
having a weight ratio of styrene to rubber of about 1:2 to about
1:3 (see USP 4,107,130). The polybutadiene block should have
contained originally, prior to hydrogenation, at least about 10%
1,2 isomer of butadiene. Commercial examples of such polymers are
Kraton G1650*, Kraton G1651*, and Kraton G1652*. Of these, the
,
~ ~ ~ *Trade Mark
higher molecular weights, e.g. at least 70,000 are preferred.
Hydrogenation is accomplished catalytically by methods known to the
art, for example as described in USP 3,595,942. The amount of the
impact modifier is critical and, for the maximum impact strength
should be at least 8 percent by weight of the finished product and
preferably as much as can be conveniently and controllably
dispersed in the monomeric methyl methacrylate castlng mixture and
cast, e.g. up to about 14 percent using current commercial
techniques, preferably about 10 to about 14 percent or more.
Dispersion can be conducted at room
5a -
temperature, or by solution at about 60C. When dispersed in this
way, Kraton G1650 can yield a casting with a dispersion of rubber
particles in the 1-5 micron particle-size range, satisfactory for
impact strength. In addition to the styrene/hydrogenated butadiene
block copolymerl a different rubber containing little or no
olefinic unsaturation, such as ethylene-propylene rubber or
ethylene-propylene-butadiene rubber may be included in the casting
mixture to further increase impact strength. In the presence of
the graft copolymer, the rubber can be dispersed much more
effectively in the monomer mixture. Added rubber in the range 4-10
percent of the total casting mixture can materially improve the
impact strength. The casting mixture is predominantly methyl
methacryla~e but may contain other acrylates or methacrylates which
will copolymerize with it, as well as styrene, cross-linkers, chain
transfer agents, release agents, ultraviolet light screening
agents, and antioxidants, as is known in the art.
Free radical initiators are required for the
polymerization to take place. The concentration of free radical
initiator and temperature is quite critical to the impact
properties of the cast sheet and should be chosen so that the
maximum exothermic heat of reaction is not delayed beyond about 80
minutes; the ~ixture should contain at least 0.2~ peroxide or
peroxyester initiators with a 10-hour half-life in the range of
25-105C. and a l-minute half-life in the range 75-160C. Suitable
initiators include for example t-butyl peroxyneodecanoate, t-butyl
peroxypivalate, decanoyl peroxide and lauroyl peroxide, but are not
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restricted to them. By half-life, I mean the ti~e required for
half of the initiator to undergo thermal decomposition at a given
temperature.
The presence of styrene copolymerized with the methyl
methacrylate makes it possible to match refractive indices between
the rubber phase and the polymethylmethacrylate ma~rix to make a
clear sheet. Styrene also causes the impact strength to increase
dramatically. For clarity, the styrene content should be in the
range 0.1-20 percent, preferably 4-8 percent, of the casting
mixture (syrup). Impact strength is enhanced when 5-11 percent
styrene is copolymerized. Thus, my preferred concentration of
styrene is about 5-8~ for a clear sheet with high impact
properties.
Cross-linkers such as polyethylene glycol dimethacrylate
may be added to enhance hardness and tensile modulus. A mercaptan
such as n-dodecyl mercaptan may be included to achieve a
lower-molecular-weight moiety in the matrix. Inclusion of
n-aodecyl mercaptan, about 0.15 percent, in the formulation,
however, results in a product of very low strength; the amount
should therefore be kept to the minimum dictated by processing
requirements.
The incluslon of butyl acrylate in the copolymer lowers
impact strength and tensile strength, tensile modulus, hardness,
heat distortion temperature, and Vicat. Its level should be kept
to the minimum consistent with thermoformability.
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:s
The inclusion of ultraviolet light screening agents and
antioxidants is advantageous to improve the thermal stability and
weatherability of the sheet. Suitable light screening agents
include substituted benzotriazoles such as 2t2-hydroxy 5 methyl
phenyl) benzotriaæole, 2(2-hydroxy 5-t-octyl phenyl benzotriazole)
and substituted benzophenones. These may be present in amounts up
to about 1 percent of the formulation. Antioxidants include
hindered phenols which confer protection without serious
discoloration on heating: for example, 2,6 ditert butyl 4 methyl
phenol. Also more highly substituted derivatives of lower vapor
pressure such as octadecyl 3-(3',5' ditert butyl - 4' hydroxyphenyl)
propionate, may be present in amounts up to about 1 percent of the
cast sheet.
In the following examples, the preparation of the cast
sheet involves three processing steps:
1. Dispersion of the polymer in the monomers
a. All monomers are mixed in a stirred vessel at room
temperature and the block copolymer added and dispersed until the
mixture is uniform and free of lumps~ ~t this point the free
radical initiators are added to the casting mixture and stirred
until well mixed.
b. Alternatively, the monomers (not including
cross-linkers), graft polymer, and a small amount of phenolic
polymerization inhibitor (.005-.02%) are heated to a temperature
where the graft polymer will dissolve in the monomer mixture and
significant polymerization does not occur (50-70~C.) and stirred
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i
~%~
until the graft polymer is dissolved. If additional saturated
rubber is to be dispersed in the casting mixture, it is added at
this point, and stirred until dispersion is complete. The mixture
is cooled to 30C. or less, and difunctional monomeric
cross-linkers, antioxidants, light stabilizers, release agents, and
free radical initiators, are added and mixed thoroughly.
2. Degassiny
The mixture is then degassed under a vacuum (absolute
pressure not more than 125 Torr), for at least 15 minutes. (This
step is necessary only if it is desired to avoid the formation of
bubbles in the sheet.)
3. Casting
The mixture from step 2 is filtered to remove lumps and
cast between two smooth surfaces, typically .05-0.5 inches apart
depending on the desired thickness of the sheet, preferably highly
polished stainless steel. The casting operation may be batch or
continuous, with the sheet maintained under conditions of
temperature and pressure (approximately one atmosphere) so that
essentially complete polymerization occurs in the time allotted and
at the same time, the monomer mixture is prevented from boiling,
and bubbles do not appear in the finished sheet. Heat is added to
the sheet to bring the syrup to polymerization temperature and
removed during the polymerization to control the temperature in the
desired range. Water baths, water sprays and hot air are
con~eniently used as heat transfer media in the polymerization,
applied to the outer wall of the polymerization space. ~n a batch
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operation syrup (preferably degassed) is poured into a chamber
consisting of a pair of parallel plates separated by a gasket, with
heat added or removed through the outer wall of the plates by a
heat transfer fluid (usually air, water or both).
In a continuous process, the syrup is placed between two
care~ully spaced, essentially horizontal, moving endless belts
gasketed at the edges. Heat is added to and removed from the sheet
through the belts by a heat transfer medium in contact with their
outer surface~ Equipment for continuous casting is described in
USP 3,371,383 and British Patent l,3009400.
Free-radical initiators are incorporated in the casting
mixture to decompose thermally at the polymerization temperature
an~ provide free radicals for the initiation of the polymerization
of the monomers, all of which polymerize by a free-radical chain
mechanism. To be useful the initiators must decompose nearly
completely at the polymerization temperature within the
polymerization time. Excess initiator after polymerization is
undesirable for economic reasons, and can be a source of
instability for the polymer in ~ubsequent thermal processing. Some
initiator must be present at all times during the polymerization to
provide a source of free radicals. Initiators which are usable
include organic peroxides and peroxyesters with 10-hour half-lives
in the range 25-105C. and one minute half-lives in the range
75-160C.
Initiator concentrations and polymerization temperatures
~hould be adjusted so that the polymerization time to maximum
exothermic heat of reaction is kept under 80 minutes, both for
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economic use of the equipment and for optimization of the impact
strength with control of particle size in the 1-5 ~icron range.
Very low initiator concentrations, coupled with low reaction
temperature can lead to unsatisfactory impact strength and
larger-than~desired rubber particles as well as a very slow
polymerization. An increased initiator level increases the rate of
polymerization so that there is less opportunity for the rubber
particles precipitating during the polymerization to coalesce.
Also it is quite possible that a limited degree of hydrogen
abstruction grafting takes place in the presence of peroxide, which
tends to stabilize the smaller particles, preventin~ their
coalescence during polymerization. One or more peroxide initiators
totaling at least 0.2~ and up to about 2.0~ should be present to
assure good impact strength and particle size control in the range
l-S microns with minimal risk of latent activity in the finished
product.
A mixture of the following was made up in a 2-litre glass
reactor and ~tirred at 400 rpm for four hours:
Methyl Methacrylate 428 g
Styrene 48 g
Butyl Acrylate 36 ~
Triisooctyl Phosphite 2.4 9
Polyethylene Glycol *
Dimethacrylate, (RChemlink 600~) 9.0 g
Kraton G1650 72 ~
Then, 102 g decanoyl peroxide and 2.0 g 75 percent t-butyl
peroxypi~alate were added and stirred for five minutes.
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*Trade Mark
The viscosity at this point was 560 centipoise. The
mixture was cast between two 14 in. x 14`in. stainless-steel plates
spaced 0.145 in. apart after degassing at 3-5 Torr for twenty
minutes. The plates were immer~ed in a water bath at 77C. for
44 minutes. The mixture was transferred to an oven at 120C. for
36 minutes. An exotherm was observed in the oven on heat-up. A
casting of thickness averaging 0.119 in. was obtained. The Gardner
impact was 19.8 in.-lb and the haze was 10.9 percent. The particle
size of the rubber appeared to be about 3 microns by electron and
light microscopy.
A second casting made with the same formulation as the
first was cured in the bath at 77C. for 65 minutes. .~n exotherm
was noted when the casting temperature rose 7C. above the bath
temperature at 62 minutes. No exothermic heating was observed
during the 23-minute oven cure at 120~C. The following physical
properties were obtained:
Tensile Strength at Break 2325 p9i Rockwell R 77
Elongation at Break 22.4% DTUL (264 psi) 145F.
Tensile Modulus 101,000 psi Vicat Softening Point 200F.
A piece o commercial acrylic w~thout an impact modifier cast in a
si~ilar manner as a control (see U. S. Patent 3,371,383 for a
description of the endless belt casting machinery) had a Gardner
impact of 3.0 in.-lb and a haze of 2.2 percent.
Other impact castings were prepared, and data on them are
presented in Table I.
Typical molecular weights of useful Kraton products may be
seen in Table II.
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~6~
~b1- I
~rDton C1650: _ E _ 12
0uty~ Acr~to: 0 ~ 0 6
~ Stvrene
G~r~ner, 1n.-7b 7.2 7.0
Z .015
n~m Ver~
Ver~ Hb2~ hb~y
5.0-S.9
C-rdner, 1n.-lb 29.~ 9.9
~e, ~ 2.7
7,5-8.6
Cdrdner, lr.. -lb 5.3 3.3 25.6 t9.D
k~e, ~ 11.0 16.~ ~5.6 10.9
T-nsilo, p-l ~770 ~550 3455 2~25
~4~u~u~, p-~ 279,00024~,000 1~9,000lOl,OOû
Elong~t1On, ~ 91.~ lS.9 a~.l 22.4
Rock~ell R 105.3 3b.3 77
DTUL, F 105 16b 145
V1c~t, ~F 22S 22~ 200
11
C~rdn~r, ln.-lb 19.0
~2~, 3 13.2
ft~ct nt Kr~ton
Styr~no5,6~ 9ut~1 Acr~l~to 0~ ~r~ton 12.0~
Kr~ton G16S Kr~on C1651 Kroton 9165?
C~rdnor, 1n.-t~ 29.q 15.0 a.3
Hb~ 4.~ 17.6 ~.~
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TABLE II
Molecular Weight Data on Kraton G
Styrene
Chain Length Rubber
_ _ Styrene/ 2/molecule Chain Length
MwMn Rubber Ratio on Mw on Mw
Kraton G1650 88500 75900 28/72 12390 63720
Kraton G1651 129900 96300 33/67 21433 87033
Kraton G1652 67900 55400 29/71 9845 48209
Determined in triclorbenzene at 143C. by gel permeation
chromatography and calibrated with polypropylene standards.
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A further example was prepared with ethylene-propylene
rubber in addition to ~raton.
Example II
C ting with Ethylene-Pro~ylene Rubber
Methyl Methacrylate 447 g
Styrene 30 g
Kraton ~1650 72 g
Eugenol* .06 g (inhibitor)
Mixture heated to 60Co ~ Kraton dissolved.
*
Then 36 grams Epcar 405 ethylene-propylene rubber were added and
dispersed or three hour~ Solution did not occur but most of the
ethylene-pr~pylene rubber dispersed~ The mixture was cooled to
about 30C. At this point the viscosity was 500 centipoises. The
following were then added:
Triisooctyl Phosphite 2.4 g
Polyethylene Glycol Dimethacrylate 9.0 g
Mw 600
t-Butyl Peroxypivalate 75% 200 g
Decanoyl Peroxide 1.2 g
The mixture degassed under vacuum and cast between two stainless
stePl plates 14" x 14" held about 50 minutes in a water bath at
77C. with a maximum exotherm at 45 minutes and in an air oven
23 minutes at 121C. After cooling to room temperature, a casting
0.137 inches thick was obtained which wa~ opaque with a Gardner
impact of 63 in.-lb.
*Trade Mark
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Exa~ple III
Casting mi~tures were made up as follows (nu~bers refer to
parts by weight):
Methyl Methacrylate 78.5
Styrene 8.0
Trii~ooctyl Phosphite 0.4
Polyethylene Glycol 0.01
Dimethacrylate Cyasorb 5411 0.0 or 0.3
Irganox 1076 * 0.0 or 0.3
Zelec NE * 0.01
Eugenol * 0.004
The~e materials were heated under ni~rogen to 50C. with
agitation and 12 parts Kraton added as indicated in Table III,
either Kraton G1650 (hydrogenated) or Kraton DllO1
(unhydrogenated). After one hour of agitation, solution was
complete and the mixture was cooled to 27C., and initiators added.
t-Butyl Peroxypivalate (75~) 0.2
t-Butyl Peroxyneodecanoate (75~) 0.
Decanoyl Peroxide 0.1
l'he solution was degassed under vacuum for 20 minutes,
(10 Torr or less total pres~ure), and poured between two stainless
steel plates for casting. Water bath times at 82C. were variable:
all were fini~hed in an oven at 121C. for 26 minutes.
Samples were aged 1000 hours in an Atlas Xenon arc oven
weathermometer, with borosilicate glass inner and outer filters and
water sprays vn 18 minutes out of 120 minutes.
Results are pre3ented in Table III. It will be seen from
Table III that the impact Rtrength of the hydrogenated rubber-
containing material~ ha~ outstanding endurance, particularly as
compared ~o the unhydrogenated rubber containing formulations~
Moreover, the Qheet wa~ clear, while the DllOl~containi~g
for~ulation had a bluish haz~.
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*Trade Mark
TABLE III
Kraton Type G1650 G1650 G1650 DllOlDllOlDllOl
Cyasorb 5411*0.3 0-3 - 0.3 0~3
Irganox 1076*0.3 . . 0-3
Visco~ity ~p11.5 9 11 198 260 180
Time to Max.
Exotherm Min.45 42 40 39 38 36
Max. Exother~ DC. 97 113 116 94 95 96
Time in Water
Bath Min. 54 50 45 44 55 47
Particle Size
Microns 1-1.5 2-5 1-3 1-5 1-5 1-2
Gardner Impact
n.-lb.
0 hr.54.7 56.0 59.5 88.8110.9 105.1
250 hr.42.0 62.4 63.0 77.152.0 44.0
500 hr.30.9 59.3 58.2 7S.041.5 11.2
1,000 hr.38.0 60.6 32.7 19.021.0 9.0
Haze % 0 hr.14.7 10.2 14.1 11.6 8.7 11.4
250 hr.19.8 16.4 14.3 10.5 8.1 9.5
500 hr.21.2 13.3 12.6 11.1 9.0 10.9
1,000 hr.27.7 15.3 11.4 12.711.9 10.3
Thickness in..178 .196 .193 .183.179 .184
Appearance Clear Clear Clear BlueBlue Blue
HazeHaze Haze
(no change on aging)
Example IV
Castings were prepared a~ in Example III except as noted.
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s~A *Trade Mark
~2~B~,
TABLE IV
Effect of Initiator Concentration and Temperature
on Impact Strength and Particle Size
Methyl Methacrylate79.4
Styrene 5 9 6
Triisooctyl Phosphite 0.4
Polyethylene Glycol Dimethacrylate 0.01
Base ~ Zelec NE 0.01
Cyasorb 541l 0.3
Irganox 1076 0.3
Kraton G1650
(dis~olved at 50C.3 12.0
ugenol 0.01 0.01 0.004
t-Butyl Peroxyneodecanoate0.0 0.0 0.20
75%
t-Butyl Peroxypivalate 0.12 0.12 0.20
Variables 75%
Decanoyl Peroxide0.06 0.06 0.20
Bath Temperature C.77 77 82
Time to Max. Exotherm Min. 95 96 31
Max. Polymer Temperature C. 85 87 100
.
~Gardner Impact in.-lb.10.518.0 55O5
¦ 'Fhickness in. 0.1730.181 0.188
Results ~ Particle Size Microns5-10 4-10 1-3
Haze % 8.0 7.3 30.7
~Appearance Blue Blue Blue
Haze Haze Haze
Table IV illuRtrate~ the importance to the impact strength
and particle ~ize of maintaining initiator concentrations of at
least 0.2 percent peroxide initiator, and of maintaining
polymerization conditions 80 that the maximum exotherm may be
reached in 80 minutes or less. In the first two cases, where the
to~al active initiator level was 0.15% and the time to maximum
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~2~
exotherm was about 95 minutes, a product with much larger rubber
particles and much lower impact strength than in the third case
where the polymerization conditions were within the recommendations
of this specification, with a total initiator level of at least 0.5
percent by weight of the ~otal mixture and a time to maximum
exotherm of 31 minutes.
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