Material for modifying impact resistance

An emulsion polymer having at least three phases, adaptable as an agent for modifying the impact resistance of rigid thermoplastic synthetic resins such as polymethyl methacrylate, and consisting of PA0 A) a rigid core phase of a crosslinked homo- or copolymer, PA0 B) an elastomer phase, prepared in the presence of the core material and having a glass transition temperature not above -10.degree. C., comprising a crosslinked resin which includes an arylalkyl acrylate or arylalkyl methacrylate, and PA0 C) a rigid shell phase, prepared in the presence of the elastomer phase, having a glass transition temperature of at least 50.degree. C.

The present invention relates to multi-stage emulsion polymers primarily 
adaptable to admixture with rigid and relatively brittle synthetic resins 
in order to improve the impact resistance properties of the latter. Such 
polymers are often designated as agents for modifying impact resistance, 
although they can also be used alone as molding materials for the 
preparation of impact resistant shaped bodies, films, and the like. 
A fundamental requirement of the agents for modifying impact resistance is 
optical clarity, because only on this condition can an optically clear 
modified molding material be obtained. 
Their basic construction consists of a rigid core, an elastomeric 
intermediate stage comprising an acrylic ester polymer, and a rigid, 
non-elastomeric final stage. It is assumed that the polymer of the 
intermediate stage and of the final stage are arranged in the form of 
shells around the core. 
STATE OF THE ART 
Agents for modifying impact resistance prepared by emulsion polymerization 
from an elastomeric core and a rigid, non-elastomeric shell are known in 
large number in the prior art. To match the optical refractive index of 
the rigid phase, the elastomeric phase most often is made from an aromatic 
vinyl monomer, such as styrene or benzyl acrylate, in addition to acrylic 
esters (cf. FR-A 2 189 440). In this publication, benzyl acrylate is 
viewed as technically equivalent to styrene. However since it is 
considerably more expensive than styrene, it is not used in practice for 
this purpose. Comparative tests which are reproduced in detail at the end 
of the present specification have shown that in a two-stage agent for 
modifying impact resistance having an elastomeric core there is no 
influence detected o impact resistance when styrene is replaced by benzyl 
acrylate. 
According to U.S. Pat. No. 3,661,994, an improvement of this emulsion 
polymer is achieved when a rigid core is prepared as the first stage of 
the emulsion polymer and an elastomeric shell and a rigid shell are 
created by two subsequent polymerization stages. The elastomeric phase is 
crosslinked by polyfunctional vinyl monomers. 
If the elastomeric phase consists solely of alkyl acrylates and 
crosslinking monomers, it has, as a rule, a somewhat lower optical index 
of refraction than the rigid phases of the core and of the final stage. 
This leads to diffraction of light at the phase boundary surfaces and to 
the formation of scattered light, which has the consequence that the 
material appears cloudy to white if it is fused into a coherent mass. In 
order to avoid this undesired phenomenon, a small amount of styrene is 
built into the elastomeric phase in order to match the optical index of 
refraction of the rubbery phase to that of the rigid phase and in this way 
to avoid a clouding of the material by light scattering at the boundary 
surfaces of the particles of the rubbery phase.

PROBLEM AND SOLUTION 
The inventors have set themselves the goal of improving the notch impact 
strength of emulsion polymers of this kind, especially when cold, and also 
of assuring optical clarity. They ascribe the indistinct separation of the 
core phase and of the elastomeric phase in prior art particles t the 
different reactivities of acrylic esters and styrene in the emulsion 
polymerization. Styrene cannot be totally avoided because of the necessity 
of matching the indices of refraction. However, it has now been found that 
the substitution of an arylalkyl acrylate or methacrylate for styrene 
accomplishes both the desired matching of the indices of refraction as 
well as promoting a sharp separation of the core phase and elastomeric 
phase. This is not only recognizable in electron photomicrographs, but 
above all in the improvement of the notch impact strength. 
Thus, the object of the invention is a material, for modifying the impact 
resistance of thermoplastic synthetic resins, of a kind which consists of 
an emulsion polymer having at least three phases, namely 
A) a rigid core of a homopolymer or copolymer of ethylenically unsaturated 
free-radically polymerizable monomers; 
B) an elastomeric phase, created in the presence of the core material and 
having a glass transition temperature not above 10 .degree. C., which is 
synthesized from 
a) an alkyl ester of acrylic acid having 1 to 8 carbon atoms in the alkyl 
portion, 
b) at least one crosslinking comonomer having two or more polymerizable 
double bonds in the molecule, and 
c) at least one ethylenically unsaturated free radically polymerizable 
monomer having an aromatic group; and 
C) a rigid phase, prepared in the presence of the elastomeric phase, which 
is a homopolymer or copolymer of ethylenically unsaturated free radically 
polymerizable monomers having a glass transition temperature of at least 
50.degree. C. 
According to the invention, an arylalkyl acrylate or methacrylate is 
involved in the synthesis of the elastomeric phase as an ethylenically 
unsaturated free radically polymerizable monomer having an aromatic group. 
The emulsion polymer advantageously consists of 5 to 40 percent by weight 
of component A, 25 to 75 percent by weight of component B, and 20 to 60 
percent by weight of component C, all components totalling 100 percent by 
weight. 
DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 of the accompanying drawings reproduces an electron photomicrograph, 
in 50,000.times. magnification, of a section of a commercial polymethyl 
methacrylate molding compound ("PLEXIGLAS Y7N", Rohm GmbH. Darmstadt, 
Germany) modified by the addition thereto of spherical latex particles 
according to the state of the art (the commercial product "PLEXIGLAS ZK6A" 
Rohm GmbH). The added particles have polymethyl methacrylate as the rigid 
core and outer shell phase and an intermediate crosslinked elastomer 
phase, made from butyl acrylate and styrene, having a glass transition 
temperature of about 20 .degree. C. Treatment of the surface of the 
section with ruthenium tetroxide makes the elastomer phase (dark portions) 
visible under the microscope. (The third, outer, rigid phase has fused 
with the polymethyl methacrylate molding compound matrix and is not 
distinguishable on the photograph.) 
It is evident from the drawing that a considerable portion of the (dark) 
elastomer phase surrounding the (light) core is admixed with the core, a 
condition attributed--as earlier discussed--to swelling of the 
first-formed core polymer by the monomers of the elastomer phase prior to 
their polymerization. 
In contrast, FIG. 2 is a ruthenium tetroxide treated section of a 
polymethyl methacrylate molding compound to which have been added 
spherical latex particles according to the invention. Specifically, the 
particles are prepared according to Example 2 of the application and have 
polymethyl methacrylate as the rigid core and invisible outermost rigid 
shell phase, with an intermediate elastomer phase made from 72 percent by 
weight of butyl acrylate, 27 percent by weight of benzyl acrylate, and 1 
percent by weight of crosslinking monomers and having a glass transition 
temperature of -27 .degree. C. In contrast to FIG. 1, the elastomeric 
phase is clearly distinct from the core phase which it surrounds. This 
property is believed responsible for the improved impact strength of rigid 
polymers to which the modifying agent according to the invention is added. 
(The drawing includes particles which are uncut by sectioning or which 
have only a small slice of the spherical particle removed, rather than 
being cut diametrically.) 
In both cases, the indices of refraction of the rigid and elastomeric 
phases are matched to one another. The following properties were 
determined on modified molding compounds which were prepared, 
respectively, by admixture of a commercial polymethyl methacrylate molding 
compound ("PLEXIGLAS Y7N", Rohm GmbH) with the emulsion polymers of FIGS. 
1 and 2, described above. 
TABLE 1 
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Emulsion Polymers 
Prior art 
Ex. 2 
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Percent by weight butyl acrylate 
15 14 
units in the molding compound 
Vicat softening temperature (ISO 303)* 
98.degree. C. 
97.degree. C. 
Izod notch impact strength in kJ/m.sup.2 
(ISO 180 lA)* 
at +23.degree. C. 5.9 6.3 
at -10.degree. C. 3.6 4.7 
at -20.degree. C. 2.8 3.4 
Cloudiness (Haze value in percent) 
at +23.degree. C. 2.3 3.6 
at +40.degree. C. 3.1 3.4 
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*Test protocol of the International Organization for Standardization, 
Geneva, Switzerland. 
In view of the about-equal glass transition temperatures and the comparable 
content of butyl acrylate units, the two emulsion polymers can be 
considered comparable from the point of view of commercial use. This is 
true also of the values of haze and of white-break (Table 3 infra), which 
are at a favorable low levels. Products having a haze value below 10, 
particularly below 6, measured at 23.degree. C. with a haze measuring 
apparatus like the Hunterlab No. D 29-9, are considered to be optically 
clear. However, a surprising improvement in notch impact strength is shown 
for the emulsion polymer according to the invention, particularly in the 
cold. In the so-called white break properties, i.e. the formation of white 
places--because of increased light diffusion--upon impact stressing, the 
molding compounds according to the invention have shown themselves easily 
superior to the known compounds. 
The Emulsion Polymer 
An emulsion polymer having at least three phases is the basis of the impact 
strength modifying agent of the invention, which polymer per se or in 
admixture with another molding compound can be worked into formed 
products. The inclusion of further phases to achieve additional effects is 
possible only to the extent that it does not negate the special advantages 
of the products according to the invention. 
The effect of the arylalkyl esters on the impact resistance is a 
peculiarity of the emulsion polymer having at least three phases. 
Comparative tests have shown that the exchange of styrene for benzyl 
acrylate in two-phased emulsion polymers consisting of an elastomeric core 
and a rigid shell has no effect on the impact resistance. 
The core and the rigid shell of the emulsion polymer consist of rigid 
polymers which at room temperature are still clearly below their elastic 
or plastic state. Their glass temperatures are as a rule clearly above 
10.degree. C., preferably above 50.degree. C., especially above 70.degree. 
C. Apart from the fact that the core is optionally crosslinked and the 
rigid shell is not crosslinked, both polymers may be synthesized from the 
same or similar ethylenically unsaturated free-radically polymerizable 
monomers or mixtures thereof. Methyl methacrylate is preferably present in 
an amount of at least 50 percent by weight or is used alone-- in the core 
optionally together with a crosslinking monomer. As comonomers, lower 
alkyl acrylates, particularly those having 1 to 7 C-atoms in the alkyl 
portion can optionally be present up to 30 percent by weight of the core 
or rigid shell. If the core is crosslinked, the crosslinking monomer as a 
rule is from 0.1 to 10 percent by weight of the core. 
The elastomeric phase is as a rule made up of at least 50, preferably more 
than 60, percent by weight of alkyl acrylates or methacrylates. The 
acrylates permit attaining lower glass transition temperatures for the 
elastomeric phase and are preferred for this reason; Examples are ethyl-, 
propyl-, n-butyl-, or 2-ethylhexyl acrylate. The amount of the arylalkyl 
acrylate or methacrylate is chosen at such a value that the optical 
indices of refraction of the rigid and elastomeric phases are approximated 
one to another, which is recognizable when the emulsion polymer has a haze 
value which does not exceed 10. The difference between the refractive 
indices of the phases is as a rule less than 0.001. Naturally it is a 
prerequisite that the measured haze is not attributable to causes other 
than the difference in refractive index between the phases and not, for 
example, to impurities. An amount of 15 to 45 percent by weight of the 
arylalkyl acrylate or methacrylate can be necessary for adaptation of the 
refractive index. Suitable arylalkyl acrylates are, e.g. benzyl-, 
phenylethyl-, phenylpropyl-, phenylpentyl-, or phenylhexyl acrylate. 
Both monomer components (a) and (b) essentially determine the glass 
transition temperature of the elastomer phase, which is not above 
-10.degree. C., preferably between -15.degree. C. and -40.degree. C. To 
the extent that the required glass temperature is achieved, other 
free-radically polymerizable aliphatic comonomers which can be 
copolymerized with alkyl acrylates and the arylalkyl acrylate or 
methacrylates can optionally also be used, as is known according to the 
state of the art. However, other aromatic comonomers, such as styrene, 
.alpha.-methyl styrene, or vinyl toluene, should be excluded as much as 
possible. 
An essential component of the elastomer phase is a sufficient quantity of 
crosslinking monomer units. Crosslinking is sufficient if the crosslinked 
polymer is not essentially swollen by the monomer added during the 
polymerization of the subsequent stage. A distinction is made here between 
crosslinkers and graftlinkers. Monomers which have at least two readily 
copolymerizing groups, e.g. acryl- or methacryl groups, belong to the 
group of crosslinkers. Monomers which, in addition to an acryl- or 
methacryl-group, have yet another ethylenically unsaturated group of 
clearly smaller tendency to polymerize, as a rule an allyl group, are 
characterized as graftlinkers. For the goal of the invention, an amount of 
graftlinker in the elastomer phase of at least 0.5 percent, better from 
0.8 to 4 percent, by 1 weight of the elastomer phase, is advantageous. 
However, the graftlinker can be replaced with equally good effect by 
crosslinking monomers which contain three or more readily copolymerizing 
groups, e.g. acryl- or methacryl-groups, in the molecule. On the other 
hand, other crosslinkers have proved superfluous, even although in some 
cases they are advantageous in an amount from 0.05 to 2 percent by weight 
of the elastomer phase. 
The amount of the graftlinker, or of the crosslinker having three or more 
ethylenically unsaturated free-radically polymerizable double bonds which 
can be used in its place, is preferably so chosen within the limits from 
0.5 to 5 percent by weight that in the finished emulsion polymer at least 
15 percent by weight of the rigid phase is covalently bonded with the 
elastomer phase. The degree of bonding is evident on dissolving an aliquot 
amount of the emulsion polymer in a solvent for the rigid phase. In this 
case, the elastomer phase and the portion of the rigid phase covalently 
bonded therewith remain undissolved. The weight of the undissolved portion 
should be greater than the calculated sum of the weights of the core and 
the elastomer phase of the aliquot sample by at least 15, and preferably 
30 to 80, percent by weight of the calculated weight of the rigid phase. 
As graftlinkers, the allyl esters of acrylic or methacrylic acid are 
preferred, but also other graftlinkers mentioned in U.S. Pat. Nos. 
3,808,180 and 3,843,753 are suitable. Triallyl cyanurate, 
trimethylolpropane triacrylate and trimethacrylate, pentaerythritol 
triacrylate and trimethacrylate, and related compounds, of which further 
examples are given in DE-A 33 00 526, are crosslinking monomers having 
three or more ethylenically unsaturated free-radically polymerizable 
groups. 
The three or multiphase emulsion polymer is prepared in an aqueous phase in 
the usual way by three- or multi-stage emulsion polymerization. In the 
first stage, the core is created. It should have an average particle size 
from 100 to 300 nanometers (nm). Methods for adjusting the desired 
particle size are known to the skilled artisan. Advantageously, control of 
particle size is according to the seed latex method. 
After conclusion of the first polymerization stage, the elastomer phase is 
prepared in the second polymerization stage in the presence of the core. 
Finally, in the third stage, after the second polymerization stage is 
concluded, the final rigid phase is created in the same way in the 
presence of the emulsion polymer of the second stage. 
The emulsion polymerization is suitably carried out in the presence of 
anionic emulsifiers. Among these are, for example, sulfonates, alkyl 
sulfosuccinates, and alkoxylated and sulfated paraffins. 
As the polymerization initiator, 0.01 to 0.5 percent, for example, by 
weight of the aqueous phase, of alkali metal or ammonium peroxidisulfates 
are added and the polymerization is initiated at temperatures from 
20.degree. C. to 100.degree. C. Preferably redox systems are used, for 
example of 0.01 to 0.05 percent by weight of organic hydroperoxides and 
0.05 to 0.15 percent by weight of rongalite at temperatures from 
20.degree. C. to 80.degree. C. In the polymerization of the rigid phase, 
as a rule a suitable amount of a chain transfer agent, e.g. of a 
mercaptan, is used in order to approximate the molecular weight of the 
rigid phase polymer to that of the molding compound which is to be 
modified with the three-phase emulsion polymer. 
Working up the emulsion polymer into molding compounds 
The emulsion polymer occurs in the form of an aqueous dispersion having a 
solids content of 30 to 60 percent by weight. The emulsion polymer can be 
isolated by spray drying, coagulation by freezing, precipitation by the 
addition of electrolytes, or by mechanical or thermal stressing, such as 
can be carried out according to DE-A 2 750 682 or U.S. Pat. No. 4,110,843 
using a degassing extruder. The spray drying method is the most common, 
although the other mentioned methods have the advantage that in them the 
water soluble polymerization auxiliaries are at least partially separated 
from the polymer. 
The material for modifying impact resistance according to the invention 
acts to improve the impact resistance of rigid thermoplastic synthetic 
resins which are compatible with the rigid phase, preferably of polymethyl 
methacrylate. Too, rigid copolymers of methyl methacrylate with acrylic 
esters, acrylonitrile, or with maleic acid anhydride and styrene, as well 
as polyvinyl chloride, come under consideration. As a rule, 10 to 60 parts 
of the material modifying impact resistance are admixed with 100 parts of 
the molding compound to be modified. 
Mixtures of this sort can be prepared in different ways. For example, the 
dispersion of the emulsion polymer prepared according to the invention can 
be mixed with an aqueous dispersion of the component to be admixed 
therewith and the resultant mixture coagulated, the aqueous phase 
separated, and the coagulate melted into a molding compound. By this 
process, a particularly uniform mixing of the two compounds can be 
achieved. The components can also be prepared separately and isolated, 
mixed in the form of their melts or as powders or granules, and then 
homogenized in a multi-screw extruder or on a rolling mill. 
Appropriate conventional additives can be admixed in each stage of 
processing. Among them are dyes, pigments, fillers, reinforcing fibers, 
lubricants, UV-protective agents, etc. 
Admixtures of the agent modifying impact resistance, in particular with 
polymethyl methacrylate, are adaptable, for example, to the preparation of 
shaped bodies having a wall thickness greater than 1 millimeters (mm), 
such as extruded webs 1 to 10 mm in thickness, which can readily be 
stamped and are, for example, useful for the preparation of printable 
screens for electrical apparatus, or for the preparation of injected 
shaped bodies of high quality, such as windows for automotive vehicles. 
Thin films, for example 50 microns thick, can also be prepared therefrom. 
In the following Examples, given by way of illustration for a better 
understanding of the invention and of its many advantages, the following 
abbreviations are used for the starting materials: 
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MMA methyl methacrylate 
BA n-butyl acrylate 
EA ethyl acrylate 
BzA benzyl acrylate 
ALMA allyl methacrylate 
PPA 1-phenyl-propyl-3-acrylate 
S styrene 
BHP tert.-butyl hydroperoxide 
PPS potassium peroxydisulfate 
EM emulsifier of tri-isobutylphenol + 6 mols 
ethylene oxide, sulfated, Na salt 
RON Na-hydroxymethyl sulfinate (rongalite) 
PSN C.sub.15 -paraffin sulfonate - Na 
pbw part(s) by weight 
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EXAMPLES 1-3 
257 pbw of water, 0.15 pbw of EM, 0.004 pbw of II)sulfate 7 H.sub.2 O; 109 
pbw MMA, and 0.66 pbw ALMA are introduced into a polymerization reactor 
having a stirring arrangement. The mixture is emulsified by stirring and 
heated to 50.degree. C. 4.17 pbw of a seed latex containing 30 percent by 
weight of PMMA, particle size 75 nm ("Nanosizer" measurement), are added 
to the emulsion. An aqueous solution of 0.05 pbw of PPS and 0.076 pbw of 
Na disulfite is gradually added for carrying out the polymerization. If 
the temperature maximum is exceeded, 0.6 pbw of PSN and then 0.26 pbw of 
Na-disulfite are added. 
Over the course of two hours, an emulsion I (cf. Table 2) is then uniformly 
added for formation of the elastomer phase. 
Then 0.16 pbw of RON is added and, in the course of the next two hours, 
Emulsion II, consisting of 96.7 pbw of water, 216 pbw of MMA, 4.4 pbw of 
EA, 0.22 pbw of PSN, 0.16 pbw of RON, 0.12 pbw of BHP, and 0.5 pbw of 
dodecyl mercaptan, is gradually added for formation of the rigid phase. 
The dispersion obtained is cooled and filtered. 
The solid material is isolated from the dispersion by freeze coagulation. 
For this, the dispersion is cooled to -25.degree. C., then thawed and 
filtered. The residue is dried at 80.degree. C. In each instance, the 
elastomer phase has a glass transition temperature less than -10.degree. 
C., while the rigid shell in each case has a glass transition temperature 
above 50.degree. C. 
For preparing an impact resistant molding compound, the dried emulsion 
polymer is mixed in an extruder with such an amount of a commercially 
available PMMA molding compound ("PLEXIGLAS 7", Rohm GmbH) that the 
content of polymerized butyl acrylate in the total mixture amounts to 14 
percent by weight, and then is shaped thermoplastically into the requisite 
test bodies. 
TABLE 2 
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Composition of the Emulsions I: 
Example No. 1 2 3 
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Water 96.7 96.7 96.7 
PSN 0.37 0.37 0.37 
PPS 0.16 0.16 0.16 
BA 158 158 147 
BzA 59.4 59.4 -- 
PPA -- -- 70.4 
ALMA 2.6 2.2 2.6 
Na disulfite 0.26 0.26 0.26 
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In following Table 3, the properties of the impact resistant molding 
compound are entered. As a comparison, in column V the properties of a 
molding compound which is obtained in the same way using a commercially 
available agent for modifying impact resistance ("PLEXIGLAS ZK6A", Rohm 
GmbH) are given. 
TABLE 3 
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Example No. 1 2 3 V 
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Vicat Softening 
98 97 97 98 
Temp. (.degree.C.) after 
16 hours at 80.degree. C. 
Notch impact 
strength (Izod) 
in kJ/m 
at +23.degree. C. 
7.0 6.8 6.2 5.9 
at -10.degree. C. 
4.7 4.7 4.7 3.6 
at -20.degree. C. 
-- 3.4 3.6 2.8 
Haze (percent) 
at 23.degree. C. 
3.9 3.6 4.4 2.3 
at 40.degree. C. 
3.7 3.4 5.8 3.1 
White-break 7 8 7 10 
after loading 
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Comparison tests with a two-phase emulsion polymer having an elastomeric 
core, using styrene and benzyl acrylate 
An aqueous phase of 125 pbw of water and 0.025 pbw of EM is introduced into 
a stirred vessel and warmed to 80 .degree. C. After the addition of 0.15 
pbw of PPS, Emulsion I is uniformly added over 2.5 hours and then Emulsion 
II is uniformly added over a further 1.5 hours; their composition is given 
in Table 4. Then the batch is stirred for 1 hour at 80 .degree. C., 
cooled, and filtered. The emulsion polymer is obtained by freeze 
coagulation as in Examples 1-3 and worked up with PMMA molding compound 
into test bodies. 
TABLE 4 
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Emulsion I 
Emulsion I 
Emulsion II 
Test A Test B Tests A + B 
______________________________________ 
Water 225 225 150 
EM 0.62 0.62 0.31 
PPS 0.31 0.31 0.26 
BA 215.4 247.4 -- 
BzA 81.0 -- -- 
S -- 48.9 -- 
MMA -- -- 196 
EA -- -- 4 
ALMA 3.6 3.6 -- 
Dodecyl mercaptan 
-- -- 0.5 
______________________________________ 
The properties of the modified PMMA molding compounds according to 
comparative tests A and B, as well as of a molding compound C prepared 
according to Example 1 of FR-A 2 189 440, are contrasted in Table 5 with 
the molding compound of Example 3 according to the invention. 
TABLE 5 
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Comparison or Example 
A B C Ex.3 
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Vicat Softening 96 96 98 97 
Temperature (.degree.C.) after 
16 hours at 80.degree. C. 
Notch impact 
strength (Izod) 
in kJ/m 
at +23.degree. C. 
3.2 3.6 4.9 7.0 
at -10.degree. C. 
1.8 1.7 2.3 4.7 
at -20.degree. C. 
1.7 1.8 1.6 3.4 
Haze (percent) 
at 23.degree. C. 2.0 3.8 3.3 3.6 
at 40.degree. C. 7.5 11.7 10.0 3.4 
White-break 48.5 48.3 -- 8.0 
after loading 
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