This document discusses developing blends of high melt strength polypropylene (HMSPP) and polystyrene (PS) with high rigidity and impact strength for making rigid foam packaging. The effects of various compositions, processing conditions, and modifiers on the mechanical properties of HMSPP/PS blends were evaluated. A 70/30 blend of HMSPP and PS with nanoclay exhibited stiffness and impact strength similar to 100% PS, as well as improved flame resistance. Compatibilized blends containing 20-30% PS achieved stiffness comparable to 100% PS while improving impact strength over PS.

1. High melt Strength Polypropylene (HMSPP) / Polystyrene/ Nanoclay blends
With High rigidity and Impact strength
Amit Dharia
Transmit Technology Group, LLC, Irving, TX 75063
Abstract
Effects of compositions, type of PP, processing
conditions, and modifiers on flexural modulus, impact
strength, and HDT of High Melt Strength PP and GPPS
blends were evaluated. With a decrease in processing
temperature and MFR of PP stiffness, as well as impact
strength of PP/PS blends improved and values were
higher than predicted by the rule of mixture. 70/30
HMSPP/PS blend with 6 phr nano clay exhibited stiffness
and impact strength similar to 100% GPPS and better
Flammability resistance. High stiffness PP/PS blends are
proposed for making low density rigid foam for protective
packaging applications.
Introduction
Reduced amount of resin used in low density products
makes foam attractive for the environment. The US
market for foam is growing at 4.1% APR and estimated to
reach 8.6 billion lbs by 2017 (1). Protective packaging is
the largest volume application of foam. However, due to
low density and large volume, post-consumer foam waste
is difficult to collect and costly to transport.
PS accounts for 38% of the foam market and is the
leading thermoplastics resin used in rigid protective and
food packaging products. PS being amorphous can
imbibe a large amount of blowing agents at relatively low
pressures and high temperatures. Nominal Mw of PS is
also very high so it has the higher viscosity than PP.
Being amorphous, the viscosity of PS is less sensitive to
temperature than PP. The bulky benzene ring provides
necessary strain hardening and melt strength. This makes
it easier to make 20-30 kg/m3 low density PS foam
without resorting to tandem extruders. PS foam also has a
high thermal insulation value; 2 X to 3X higher stiffness
than PP and hence, suitable for hot beverages and food
containers. However, PS foam has very poor impact
strength crush resistance, top load resistance, contact
resistance, oil resistance, and oxygen barrier properties
and it is not microwaveable. EPS is good for single use
(2)
PP is relatively new in the rigid foam market with
less than 1% of the global foam market share. PP has
better impact, tear and crush strengths, higher resistance
to oil, low WVT, lower oxygen permeation, lower
deformation under dynamic load, higher thermal
resistance and flammability rating than EPS. However,
due to its linear structure, PP has poor melt strength and
lower stiffness than PS. Because of its lower stiffness,
poor processability and creep resistance, PP foam is not as
widely used as PS foam (3). Low melt viscosity, melt
strength and melt elasticity makes PP more sensitive than
PS to processing conditions. The introduction of long
chain branches reduce crystallinity, increase melt strength
and melt elasticity required for making low density foam
(4). Various in-reactor, post-reactor, and compounding
methods to enhance melt strength of PP are reported and
commercially practiced. Among various methods, PP
with long chain branches produced via post-
polymerization irradiation in a selective environment or
reactive extrusion has the highest melt strength and melt
elasticity (5,6,7,8). It is now possible to make very low
density thermoformable PP foam.
Use of PP in many applications where previously
EPS was a dominant player is on the rise. Polyolefin
foams are the fastest growing plastic foams globally,
expected to grow at 4.9% between 2014-2020. However, a
complete switch from PS foam to PP foam is cost
prohibitive due to inherent differences in processing and
pricing of PP and PS. Blending HMSPP with PS offers an
intermediate solution. PP and PS are not miscible and
their blends have poor mechanical properties.
Due to their large volume and complimentary
properties blends of PP and PS have been well studied and
reported (9,10,11). They are immiscible showing two-
phase morphologies. The blends showed poor mechanical
properties, especially elongation at break and impact
strength much lower than those predicted based on an
additive rule. In uncompatibilized blends, when
viscosities of two components are matched during
processing, minor phase size gets smaller and as the
contact area between two phases increase, mechanical
properties improve. In compatibilized blends, Styrene
block copolymers (SBC, SEBS, SIS, SEP), PP-b-PS, PP-
g-PS and even various organo clays are used as
compatibilizers. SBCs reduce interfacial tension and
hence reduce dispersed PS phase size which improves
impact strength. However, besides being expensive,
SBC are low modulus elastomers and even at 5% level,

2. reduce overall rigidity and HDT (12, 13, and 14). Organo
clays degrade PS and hence, resulting blends have poor
mechanical properties and color (15,16, 17). PP-b-PS di-
block copolymer made via two stages in a reactor is a very
effective compatibilizer for PP-PS blends but long
gestation time for imbibing styrene monomer in porous
PP beads commercially is not practiced. PP-g-PS graft
copolymer is more effective and less expensive than PP-b-
PS (18,19,20,21). PP-PS interpenetrating network (IPN)
made with scCO2 as a solvent and DVB as a crosslinking
agent for PS is reported (22). However, no commercial
grades of PP-b-PS, PP-g-PS, PP/PS IPN are available.
In previous studies, linear PP is used and has shown
that as the amount of PS in PP/PS blend increases, the
morphology of dispersed phase changes from globules
and fibrils to striated layers. Blends with greater than 30%
PS in PP were found to have poor mechanical properties.
The purpose of this study is to develop HMSPP-PS blends
with less than 30% PS but with PS like high stiffness,
improved impact strength, HDT and flame resistance
suitable for making rigid foam. Rigid foams made from
such compositions will have opportunities in food and
multi-use protective packaging.
Experimental Procedure
Materials
Molding grade crystal GPPS (Sp. Gravity 1.04 and MFR
10 @ 230 C, 2.16 kg), linear PP homopolymer (Profax
6301, Specific Gravity 0.905. MFR 12 g/10’ at 230 C/
2160 gm weight, powder) and high melt strength
Polypropylene with long chain branches made by Borealis
(Daploy 135 WB, I2 3.2 g/10’ and I5 16 g/10’, and
Daploy WB 180 with I2 3.8 g/10’, I5 33 g/10’, pellets)
were used. Properties of PP and GPPS are summarized in
Table-1. Low Mw organic additive and SEBS (Kraton
1651) styrene block copolymer were used as modifiers to
compatibilize PP and PS. Nanoclay 50% MB in PP was
provided by Nanocor.
Mixing, Molding and Testing Procedures
Two different methods of mixing were used. In direct
injection molding, PS and PP (and stabilizer) were dry
blended in a blender at low speed for five minutes and
molded at different temperatures and high screw speeds to
see the effect of temperatures on properties.
In melt-mixing, dry-mixed PP, PS, and compatibilizers
were first melt-mixed using ZSK25, 40:1 L/D lab twin
extruder at 350 rpm and 175 C. Melt mixed blends were
then molded at an optimum temperature.
MFR were measured at 230 C and 2.16 (I2)and 5 kg
weights (I5) as per ASTM D 1238. Flexural properties
were measured as per ASTM D 790 method B at 12.5
mm/minute speed. Notched and unnotched Izod Impact
strengths were measured as per ASTM D 256.
When blends were incompatible, specimens broke while
ejecting molded specimens.
Results and Discussion
Effect of molding Temperature in uncompatibilized
20/80 PS/PP blends:
Dry mix of 20% PS and 80% Daploy 135 WB HMSPP
was molded at molding temperatures of 175 C, 200 C and
225 C. Properties of molded parts are shown in Table-2
and Figure-1. With a decrease in molding temperature,
flex modulus, flexural strength, notched, unnotched
impact, strengths increased. Similar findings are reported
in various earlier studies. Properties of immiscible blends
are highly dependent on morphology. Morphology
depends on relative viscosities and hence, on processing
conditions. With a decrease in temperature at a fixed
RPM, shear rates increased. As can be seen from I5/I2
ratio for PP and PS (Table-1), PS is not as shear thinning
as PP. i.e. even though MFR of PS used in this study is
higher than that of PP, under processing conditions the
viscosity of PS melt is higher than that of PP melt. When
dispersed phase has higher viscosity and deformability,
morphology tends to be globules or fibril.
It is also interesting to notice that I5/I2 (shear sensitivity)
of 80/20 HMSPP/PS blends are similar to PP when mixed
at high temperatures but closer to PS when mixed at lower
temperatures. Higher temperatures processing resulted in
continuous PP phase with dispersed PS globules. Low
temperature mixing resulted in co-continuous morphology
with PS domains extended into fibril shape. It appears
that some “mechanical grafting” occurred in blends
processed low temperatures even when no compatibilizer
is added.
II. Effect of viscosity Ratio, 80% PP-20% PS:
80% of HMSPP (WB135 and WB180) of different MFR
were dry-blended with 20% PS and molded at 225 C to
see effect of PS/PP viscosity ratio. Results are shown in
Table-3 and Figure-2. Based on the similar MFR and
smaller starting particle size of PP 6301 (powder), one
would expect properties of melt-mixed blends of linear PP
6301 and PS to be better.
However, irrespective of MFR or type of PP, flex
modulus of 80/20 PP/PS blends molded at 225 C were
close but impact strengths were lower than expected based
on additive rule of mixture.

3. The shear sensitivity (I5/I2) of all three blends processed
at higher temperature was also about the same. When
PP/PS blends are processed at higher temperatures, PP
becomes continuous phase and properties are mainly
governed by PP phase.
Thus far, blends of Daploy WB 135 (HMSPP) with GPPS
processed at lower temperatures produced better stiffness
and impact properties so in following experiments WB135
was first melt mixed with GPPS using ZSK25 at 175 C.
III. Effect of amount of GPPS in compatibilized
blends:
20, 25 and 30% of GPPS was melt mixed with Daploy
WB135, in the presence of 10-20% low Mw linear HC
modifier and 12 phr of 50% Nanoclay MB (in GPPS) at
175 C using ZSK25 lab twin screw extruder. ASTM test
specimens were molded also at 175 C. Results are as
shown Table-4.
As expected, with an increase in the amount of GPPS
from 20-30%, the flex modulus and strength increased.
Flexural modulus of compatibilized blends is greater than
that predicted based on the simple additively rule.
Compatibilized blends containing only 30% GPPS has
similar flex modulus as 100% GPPS. In all cases, notched
Izod impact strength remained close to that of GPPS
while the unnotched Izod impact strength was
significantly higher than 100% GPPS. The addition of
nanoclay markedly improved stiffness and flame
resistance but reduced impact strength. The addition of
20% modifier seems to have been effective but due to its
low Mw and mobility reduced HDT by 3 C.
Adding 5% SEBS Styrene block copolymer, on the other
hand, reduced flex modulus from 3103 Mpa to 2020 Mpa
with very little improvement in notched Izod impact
strength.
Conclusions
The processing temperature is found to be the more
important than the viscosity ratio in mixing PP with
PS. When processed at higher temperatures PP
became continuous phase irrespective of the type of
PP used and had minimal effect on stiffness.
HMSPP due to LCB and lower crystallinity seems to
blend better with GPPS than linear PP. The addition
of a low molecular weight organic modifier and
nano-clay in PP-PS blends containing only 20-30%
GPPS resulted in stiffness, strength, HDT similar to
100% GPPS but higher unnotched impact strength
and improved flame resistance than GPPS.
Table-1 Properties of PP and PS raw materials
Type of PP WB135 WB180 PP6301 GPPS
Sp. Gravity 0.905 0.91 0.91 1.04
Flex Modulus Mpa 1641 1689 1241 3103
Flex Strength Mpa 48 46 37 70
Notched Izod j/m 59 70 56 19
Unnotched Izod J/m NB* NB* NB* 107
MFR, I2 g/10' 3 4 12 10
MFR, I5 g/10' 16 33 60 36
I5/I2 5.3 8.3 5 3.6
NB – No break, CB – Complete break
Table-2 Effect of Temperature on properties of 20/80
PS/HMSPP blends
80% Daploy WB 135 +20% GPPS
Temperature, C 225 200 175
Flex Modulus Mpa 1834 1999 2068
Flex Strength Mpa 47 54 58
Notched izod j/m 41 41 54
Unnotched Izod J/m 214 117 342
MFR, I2 g/10' 3 4 8
MFR, I5 g/10' 17 28 36
I5/I2 5.6 7 4.50

4. Table-3 Effect of MFR ratio on flex and impact
Properties of 80/20 PP/PS blends melt mixed and
Molded at 225 C
Type of PP WB135 WB180 PP6301
MFR ratio PS/PP 225 C 4 1.5 0.83
Flex Modulus Mpa 1834 1806 1558
Flex Strength Mpa 47 53 47
Notched izod j/m 41 49 48
Unnotched Izod J/m 214 226 256
MFR, I2 g/10' 3 10 15
MFR, I5 g/10' 17 47 61
I5/I2 ratio 5.66 4.7 4.6
Figure-1 Effect of Processing Temperature on properties
of 80/20 PP/PS blends
Figure-2 Effect of PS/PP MFR ratio on properties of
80/20 PP/PS blends

5. Table-4 Effect of % PS on properties of melt mixed and compatibilized PP/PS blends
% GPPS 100 0 20 25 30 30 30 30 30
Daploy Wb135 0 100 60 55 70 50 50 50 60
HC Modifier 0 0 20 20 0 20 20 20 10
Kraton 1651 0 0 0 0 0 0 0 5 0
MMT 50% MB, PHR 0 12 12 0 0 12 12 12
Sp. Gravity 1.04 0.905 0.99 0.99 0.938 0.980 0.99 0.971 0.974
Flex Modulus Mpa 3103 1641 2544 2772 2732 2220 3000 2020 2530
Flex Strength Mpa 70 48 60 62 58 64 66 61 61
Notched izod j/m 19 59 19 18 32 17 18 19 16
Unnotched Izod J/m 107 NB 133 107 182 167 107 160 NA
HDT, 264 psi C 70 90 57 60 81 NA 66 67 70
Vertical burn test Heavy
Shoot, CB*
Drip,
CB
No drip,
CB
No Drip
CB
Heavy
Drip, CB
Drip,
Black
shoot
No drip
CB, low
smoke
No Drip,
CB
No drip,
CB
CB* continue to burn
Table-5 Comparison of Properties of PS with compatibilized and uncompatibilized PP/30% PS blends
PS
70/30 PP/PS
Uncompatibilized
Compatibilized
PP/PS +Nanoclay
+modifier
Sp. Gravity 1.04 0.938 0.99
Flex Modulus, Mpa 3103 2732 3000
Notched Izod, J/m 19 32 18
Unnotched Izod, J/m 107 182 107
HDT, 264 psi, C 70 81 67
Burning drip
Heavy
Shoot, drip
heavy
drip No drip

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