A New Composition of High Heat General
Purpose Polystyrene (High Heat GPPS) Resin
K C Basavaraju1* , Darshan Jayanna1, Subodh Kumar Pal1, Roshan Jha1 and Bander Al Farhood2
1PETCHEM Department, SABIC Technology Center, India
2PETCHEM, SABIC, Riyadh, Saudi Arabia
Submission:February 15, 2021;Published: March 22, 2021
*Corresponding author: Basavaraju KC, PETCHEM Department, SABIC Technology Center, Bengaluru, India
How to cite this article:K C Basavaraju, Darshan J, Subodh Kumar P, Roshan J, Bander A F. A New Composition of High Heat General Purpose
Polystyrene (High Heat GPPS) Resin. Academ J Polym Sci. 2021; 4(5): 555650. DOI: 10.19080/AJOP.2021.04.555650
A new composition of high heat general-purpose polystyrene (High heat GPPS) resin synthesis is discussed. The new composition of resin
involves addition of cross-linker/comonomer in very low concentration during synthesis process. Commercially available vinyl cross-linkers like
divinylbenzene (DVB), Ethylene dimethacrylate (EDM), Tricycle (5.2.1.02,6) decanedimethanol diacrylate (TDDDA), Dicyclopentadiene (DCPD)
and vinyl comonomer like maleic anhydride, α-methylstyrene have been evaluated via bulk polymerization method. DVB showed enhancement
in the heat distortion temperature (HDT), vicat softening temperature (VST) & melt flow index (MFI) compared to the benchmark grade of GPPS,
whereas other mechanical and impact properties remained same or better than benchmark grade under identical methods of measurement.
Keywords: Divinylbenzene; Cross-linked; Branching; Polymerization; Heat Distortion Temperature (HDT); Melt Flow Index (MFI)
Polystyrene (PS) is a multipurpose polymer used in varied
applications in rigid and foamed forms. Based on orientation
of phenyl group on the polymer backbone, polystyrene can be
classified into isotactic, syndiotactic and atactic. Isotactic or
syndiotactic polystyrene prepared by metallocene catalyst can
give HDT/VST around 250 oC and shows lowest specific gravity
compared to any other engineering plastics in the market .
XAREC is world’s first syndiotactic polystyrene commercialized
by Idemitsu Corporation, used in electronic component
molding . The first two are commercially not important due
to complexity in the synthesis and processability compared to
atactic polystyrene. Atactic polystyrene is known as generalpurpose
polystyrene (GPPS). It is produced by simple thermal
initiated radical bulk polymerization . GPPS is clear, hard and
can be used in packaging, household items, and electronics. The
excellent physical and processing properties make GPPS suitable
for many applications as compared to any other plastics, but its
glass transition temperature (Tg) is only 100 oC that leads to limit
its use in certain applications. Tg dictates the continuous service
temperature that is a very important factor for polymer to evaluate
its processing and application performance. Therefore, there is a
demand for high Tg GPPS. Some efforts have been reported in this
direction by copolymerizing styrene with maleimide and imide,
which act as hydrogen-bond interaction site in the copolymer
. Although there is no direct relation between Tg and HDT in
polymers, but in amorphous polymer like GPPS, HDT is close to
HDT can be improved by increasing the interaction of chains
or by restricting the chain mobility at elevated temperature.
Although GPPS synthesis process is well established and matured,
the market requirement and customer application always demand
to develop new process or improve the efficiency of the process.
Vinyl based anhydrides, amides, maleimides and methylstyrenes
have been explored as comonomers in different loading to
enhance the heat resistance of polystyrene [4-5]. This is generally
achieved by introducing heteroatom containing monomer in the
polystyrene chain, which leads chemical irregularity or steric
hindrance in the polystyrene chain. GPPS has been widely used
in food packaging containers, disposable containers, kitchenware
& cutleries owing to its excellent transparency, water resistance
and colorability. Molded articles prepared by GPPS with lower
HDT/VST can undergo structural deformation especially when they are used in hot filled applications and lower HDT can affect
the production rate during molding due to the long cycle time.
Good thermal properties like higher HDT/VST along with flow
are important requirement for the product performance with
better heat stability & shorter cycle time in the production line.
Therefore, our research was focused in first instant to increase the
Tg and HDT/VST, without affecting flow properties.
The molecular weight of intermediate samples and final
polymer was measured by GPC at 40 oC (Make-Shimadzu,
Class-VP). THF was used as mobile and diluent solvent, sample
concentration 2 mg/ml, column specification- PLgel 5 μm
MIXED-C, 300 x 7.5 mm, Detector- UV @254nm, standards-
Monodispersed polystyrene standards.
Differential scanning calorimetry (DSC) of the samples was
done with TA Instruments/DSC Q 1000 by heating the samples
from room temperature to 160 ºC and heating rate always
maintained at 10 ºC/min in nitrogen atmosphere.
Unreacted residual styrene monomer was detected and
quantified by high performance liquid chromatography (HPLC)
Agilent HPLC-1260 series. The chromatographic condition are as
follows, column- Agilent Zorbax,-C18, (4.6X150mm), 5μ; Mobile
phase: Reservoir A -0.02% Orth phosphoric acid in Milli-Q water,
Reservoir B-Methanol, Reservoir C - Acetonitrile at different
gradients. Flow rate 1 ml/min, detector-diode array detector,
wave lenth-254 nm. Column temperatue-40 oC.
Injection molded specimens are conditioned for 48 h at 23 oC
& relative humidity at 50%. Tensile strength was measured were
accordance to ASTM D638, on Zwick 2.5 RTI-UTM machine. Izod
impact notched was measured as per the ASTM D256.
The following comonomers/cross linkers were screened
in 100 g batch scale, the concentration of comonomer was
maintained at 0-5 wt. %.
Styrene with comonomer was transferred to the pre-heated
(128 oC) glass reactor, the residence time/reaction time and
temperature were increased at four different time intervals to
make a continuous process. Intermediate samples were collected
to monitor molecular weight, Tg and conversions at every 1 h
interval. Unreacted styrene monomer or residual monomer was
removed by precipitating polymer solution (in THF) in hexane
and then measured Tg.
Following three formulation were shortlisted for scale-up
in 1kg scale in the lab glass reactor setup (Figure 1 SI) based on
the screening experimental results and evaluated thermal, flow
& mechanical properties. The reaction conditions were kept
as in Table 1. The unreacted styrene was removed by applying
high vacuum (0-30 mbar) for 10-12 min at the end of the
a. High molecular weight GPPS (Mw > 300 kg mol-1)
b. Evaluation of cross linker DVB at 100 ppm loading
This approach is simple and straightforward; this has been
achieved by optimizing reaction time and temperature of the
synthesis process as shown in the Table 2. In general, polystyrene
molecular weight (Mw) can grow up to 340-350 kg mol-1 in
radical bulk polymerization technique at 3-3.5 h reaction time
(pre-poly), but later decreased due to thermal degradation and long residential time. In order to achieve Mw ≥300 kg mol-1, the
polymerization was stopped after 4 h reaction time, with ~ 80 %
conversion. At final stage high vacuum was applied for 10 min to
remove 17-18 % unreacted styrene, after double extrusion the
final residual was 1500 ppm.
From Table 3, MFI data shows that as molecular weight
increased the flow decreased, but HDT and VST increased by 2 oC.
Other mechanical properties like impact and tensile properties
are better or equal to the baseline material PS160. Due to the low
MFI, high molecular weight approach may have negative effect on
processability and cycle time during molding. Figure 2, the GPC
chromatograms shows that decreased molecular weight due to
thermal degradation at different stages of processing.
Following cross linkers and comonomers were screened at
different loading for high heat GPPS (Table 4) at 100 g scale. All
the cross linkers were screened in the range of 50-500 ppm to
avoid gelling in the reactor, whereas comonomers were tested in
1-5 wt. % loading. DVB was shortlisted for scale-up in 1kg scale
and evaluated for thermal and mechanical properties as it showed better Tg (107 oC, when residual styrene ~100 ppm). Moreover,
it is commercially available in bulk quantities, FDA approved and
reactivity is almost similar to the styrene monomer [6-7]. The
same formulation was scaled up to 5 kg scale in batch reactor in
similar reaction conditions. Tricyclo (5.2.1.02,6) decane dimethanol
diacrylate and N,N-Hexamethylenebis (methacrylamide) as
crosslinkers did not show any improvement in Tg in the final
polymer. MA showed best Tg among comonomers at 4-5 wt. %
loading and was scaled up to 1 kg in lab scale. DVB loading was
optimized to 100 ppm, higher loading showed low MFI (< 3.0 g/10
min) because of high molecular weight and more crosslinking.
Intermediate sample analysis showed that molecular weight
increased up to 3rd hour and then started decreasing (Figure 3).
This is may be due to thermal degradation under dynamic reaction
conditions. Styrene-DVB copolymer always gave broader PDI
(≥3.0) due to partial crosslinking or chain extension/branching,
which leads to higher MFI. The chain extension was envisaged by
lower slope/slower relaxation in the lower frequency region in
rotational rheometer study as shown in Figure 4.
Solid content and viscosity at prepoly stage was almost
similar to the homopolymer (Table 5). From Figure 7 it is very
clear that residual styrene can affect Tg, that is higher the residual,
lower the Tg, hence HDT/VST can also change accordingly (Table
6). It is required to maintain optimum level of (< 500 ppm, more
preferably < 200 ppm as in example 2) residual in the final
polymer to hit HDT > 102 oC or VST >106 oC as showed in table
6, example 2-4. The mechanical properties as if tensile strength &
impact were better than the baseline PS160 (Table 6). There is no
change in the thermal stability of styrene-DVB copolymer at 100
ppm DVB loading in compared to baseline GPPS PS160 (Figure
2 SI). Comonomers were evaluated in 1-5wt.% loading along
with styrene, under similar reaction conditions as mentioned
in the Table 2. Maleic anhydride (MA) showed HDT-102 oC, and
MFI ~ 10-12 at 5 wt. % loading, but imparts haziness to the
final polymer and more brittle than baseline PS 160. MA cannot
homopolymerize, however in the presence of styrene it always
forms alternative copolymer. This is may be due to higher activity
of MA (0.02 time higher than styrene) .
Since, we are restricted ourselves to MA content to 5 wt. %
maximum in the formulation, MA was added slowly over 1 h time
in the prepoly stage and anticipated fair distribution throughout
the chain. As MA content increased in the formulation the
copolymer molecular weight decreased below critical molecular
weight (~240 kg mol-1) as showed in the Table 7 & Figure 9. As 5
wt. % loading showed Tg~ 108 oC (Figure 10), the same was scaled
up to 1kg in the lab reactor and evaluated for the mechanical and thermal properties. In styrene-MA copolymerization, the
propagating chains were styrinic in nature because the maleic
anhydride radicals undergo chain transfer reaction in this
highly reactive system . The styrene-MA system was highly
exothermic compared to styrene homopolymerization and
styrene-DVB system. This is because hydrogen abstraction from
the maleic anhydride radical is 40 kJ/ mol more exothermic than
that of styrene radical. This finally leads to inferior mechanical
properties compared to the baseline as shown in the Table 7.
Other comonomers like alpha-methylstyrene and DCPD did not
show considerable improvement in thermal or flow properties up
to 5 wt. % loading.
Divinylbenzene (DVB) as a crossliker at 100 ppm loading
showed balance of thermal and flow properties as compared to
baseline PS160. The controlled crosslinking of polystyrene chains
by DVB play major role in broad distribution of molecular weight
and hence better flow (MFI). There is an increment in the HDT/
VST by 2-3 oC and MFI from 3.3 g/10 min to 7 g/10 min without
compromising any other properties of GPPS.