Recycling of PP/LDPE Blend: Miscibility, Thermal Properties

RECYCLING OF PP/LDPE BLEND: MISCIBILITY, THERMAL PROPERTIES,
RHEOLOGICAL BEHAVIOR AND CRYSTAL STRUCTURE
Chuanchom Aumnate1, Claudia Spicker1, Raphael Kiesel1, Majid Samadi2, Natalie Rudolph1,
1
Polymer Engineering Center, Mechanical Engineering Department,
2
Textile Science, Department of Design Studies and Materials Science Graduate Program,
University of Wisconsin-Madison, WI
Abstract
Blending of plastics used in packaging is an
interesting approach for recycling or upcycling.
Therefore, this study focused on the effects of processing
on the properties of recycled PP and PP/LDPE blends.
MFI measurements, Differential Scanning Calorimetry
(DSC) and hot-stage polarized optical microscopy
techniques were used to investigate the miscibility of
PP/LDPE blends based on the thermal properties, degree
of crystallinity, crystallization and morphology
development in the blends. The MFI indicates, that PP and
PP/LDPE blends are marginally sensitive to degradation
at common processing conditions. The degree of
crystallinity of the blends decreases with an increase of
the LDPE content. Furthermore, the spherulite growth
rate and crystal size of PP decrease with an increase of
LDPE content.
The shifts of crystallization temperatures from the
DSC measurement, in conjunction with the crystallization
kinetics, indicate that PP/LDPE (25 wt% LDPE) is
partially miscible.
Introduction
Plastics are used on a daily basis in a number of
applications. A huge amount of plastic is used in
disposable products. The amount of plastic consumed has
been growing steadily due to favorable properties such as
low density, high strength, ease of manufacturing and low
cost. As a result, both industry and private households
generate more and more plastic waste. In 2007,
polyolefins, in particular polyethylene (PE) and
polypropylene (PP), represented more than 40 wt% of the
total amount of plastic consumed in the world [1]. The
biggest concerns are single-use plastic items such as
packaging, bags and containers. Unfortunately,
appropriate waste management strategies are not
developing at the same rate as the increasing levels of
plastic wastes. A large amount of waste does not reach
proper disposal sites, instead littering the landscape, and
blowing or washing into the sea, which leads to serious
environmental problems. Continued increases in the use
of plastics lead to increasing amounts of plastics ending
up in the waste stream. In 2013, only 2.66 million tons out
of the total 39.3 million tons of plastic waste of the
American Municipal Waste Stream have been recycled.
Polyolefins, which have good recycling properties, make
up over 50% of the non-recycled plastic (17.80% HDPE,
19.60% LDPE, 13.90% PP). This has motivated the
interest in plastic reuse and recycling [2, 3].
Due to its desirable physical properties such as high
tensile strength, high stiffness and high chemical
resistance, polypropylene (PP) has been widely used as
packaging material for instance as margarine and yogurt
containers, bottle caps and microwavable food. However,
it shows poor impact strength at low temperatures and is
susceptible to environmental stress cracking [4]. LDPE
waste mostly results from bags and packaging films.
Owing to its low mechanical properties and easy
processability it is recycled as garbage bags. One
possibility to develop alternative applications is blending
it with other materials to improve the low mechanical
performance of recycled LDPE. The combination of
LDPE and PP is frequently found in polymer waste
streams. Because of their similar density, PP and LDPE
cannot be easily separated from each other in the
recycling stream [5]. In general, blends of PE and PP have
become a subject of great economic and research interest,
not only to improve the processing and mechanical
properties of PP, but also to expand opportunities to
recycle these mixed plastics [6, 7].
Since the 1980s, a large amount of work has been
focused on the study of mechanical properties of mixed
PP and PEs, which was mostly performed on virgin
materials. The effectiveness of blending largely depends
on the miscibility or immiscibility of the blended
components. PP and HDPE as well as PP and LDPE are
generally considered immiscible in any blending ratio and
show a remarkable phase separation during cooling and
crystallization. As a result, a theoretical model for the
properties of mixtures cannot be applied successfully [8].
The performance of PP/LDPE blends depends on the
ratio, the melt viscosities and the processing conditions.
Additions of other materials as compatibilizers can also
improve the performance of this blended system.
Borovanska et al. [9], and Penava et al. [10], improved the
mechanical properties of a LDPE/PP blends by adding
EPDM as a compatibilizer. Furthermore several technical
published reports confirmed that blends of virgin
polymers are significantly improved by adding
appropriate compatibilizers.
SPE ANTEC™ Indianapolis 2016 / 81
However, there is no universal compatibilizer for all
kinds of mixtures. Besides, there are still discussions on
the formation of the structure and compatibility of
PP/LDPE blends [11]. Reprocessing of recycled material,
both virgin and blended polymers, leads to degradation,
and consequently, processability problems. Furthermore,
recycling of blended materials in unknown ratios can lead
to immiscible blends of varying and poor properties.
This study focuses on the blending of PP and PEs in
the ratios collected in the recycling stream in order to
eliminate the sorting process. In particular, it aims to
maintain or improve the properties of blends of recycled
materials with respect to their virgin materials. One of the
interesting applications is utilizing those recycled mixed
plastics to produce 3D printing filament. The advantage
would be to reuse plastic waste directly in a variety of
tailored products. This will not only aim at the reuse of
packaging waste in populated areas, but in remote
locations such as military bases or underdeveloped
regions, where the waste is produced. In this study, the
effect of processing parameters on the properties of
recycled materials such as the rheological behavior,
morphology and their miscibility as a function of different
blend compositions will be evaluated. In particular the
focus is on the rather immiscible blend of PP and LDPE,
which is inseparable in the recycling stream. In addition,
the thermal stabilities of PP after a number of
reprocessing cycles are investigated. The study will lead
to a scientifically based design methodology for
polyolefin blends, starting with PP/LDPE, that will allow
designers to select the appropriate blend to achieve the
desired properties.
Materials
The materials used in this study were divided into
two parts; the virgin and recycled materials. The
commercial virgin resins, which were used to study the
behavior of PP/LDPE blends are low-density
polyethylene, LDPE (DOW LDPE 132I) and
polypropylene, PP (PHR Polypropylene P9G1Z-047). For
the recycled part, the blends’ behavior and the effect of a
number of reprocessing cycles were studied using
recycled LDPE and regrind PP, manufacturing scrap
supplied by PLACON Incorporated.
Experimental
Processing
All materials were processed using a Leistritz
ZSE18HPe laboratory, modular, intermeshing, co-rotating
twin-screw extruder (screw speed: 150 rpm for recycled
materials and 200 rpm for virgin materials). The material
was subsequently pelletized with the appropriate
downstream equipment (a water-through, blown-air drier
and a rotary cutter).
Afterwards it was dried and, in the case of the
reprocessing study, reprocessed four more times under the
same conditions. Table 1 shows the temperature profile of
the extruder.
Table 1. Temperature profile of the extruder used to
reprocess and prepare the blends
Extruder
1
2
3
4
5
6
7 Die
Zone
Temperature
180 200 210 210 210 220 220 220
(ºC)
Melt Flow Index
The melt flow index (MFI) is a good technique to
determine the effects of reprocessing since it is an indirect
measurement of the melt viscosity of materials. It also
indicates the changes in molecular weight and is widely
used in the thermoplastic industry. The MFI
measurements were carried out in an extrusion
plastometer Series 4000 according to the ASTM D123810, using procedure A [12]. In case of the blends, the
tests were performed at the higher temperature of the two
polymers. For all materials, including virgin pellets and
blend samples, three tests were performed.
Differential Scanning Calorimetry (DSC)
The melting and crystallization behavior of PP,
LDPE and their blends were determined using a DSC 214
Polyma, NETZSCH Group) under a nitrogen atmosphere.
For crystallization and melting temperature measurement,
PP, LDPE and their blends were melted at 200°C, held
isothermal for 2 min, then cooled to -35°C and heated to
200°C again with the scanning temperature rate of
5K/min. For isothermal analysis, all blends and pure
polymers were quenched from 200°C to an isothermal
temperature between 100°C to 125°C.
Polarized Optical Microscopy
The phase transformation of PP, LDPE and their
blends was investigated using a Olympus IX71 polarising
optical microscope equipped with a 50x objective and a
hot-stage. All samples were prepared as 10 µm thick films
using a microtome. The films were heated between a glass
slide and cover slip to 200°C and kept for 2 min.
Subsequently, the samples were quenched to an
isothermal temperature between the crystallization
temperatures for PP (125ºC) and LDPE (100ºC).
Results and Discussions
Melt Flow Index
The effect of several processing cycles on the MFI
of PP is shown in Figure 1. It can be seen, that the MFI
increases moderately with every processing cycle. These
results imply a reduction of the melt viscosity and the
molecular weight during every processing cycle.
SPE ANTEC™ Indianapolis 2016 / 82
12
1.2
1,2
1.0
1,0
MFI (g/10min)
As the molecular weight is directly related to the
length of the polymer chain, a decrease in chain length
can be concluded. This chain scission can be caused by
thermal, thermal oxidative and mechanical degradation in
the extruder. Thermal degradation occurs due to the
thermal oscillation of the molecules at high temperatures,
leading to scission of the molecular bonds along the chain
and the formation of two free radicals. Mechanical
stresses like shear stress and tension have a similar effect
[13]. These results indicate that PP is marginally sensitive
to degradation at the used conditions.
0.8
0,8
0.6
0,6
0.4
0,4
0.2
0,2
0.0
0,0
100% vLDPE
75% vLDPE
50% vLDPE
25% vLDPE
100% vPP
Figure 3. MFI of virgin PP (vPP), virgin LDPE (vLDPE)
and their blends at different compositions (Temperature:
230°C).
MFI (g/10min)
10
8
6
4
2
0
Virgin
Regrind
Cycle 1
Cycle 2
Cycle 3
Processing Steps
Cycle 4
Cycle 5
Figure 1. Variation of MFI of virgin PP, manufacturing
regrind and recycled PP in dependence of the number of
extrusion cycles (Temperature: 230°C).
For the blends the processing temperature was set for
PP, which results in a higher processing temperature for
LDPE than recommended by the manufacturer. Figure 2
presents the MFI of recycled PP and LDPE as well as
their blends. The results show clearly that a higher amount
of LDPE is correlating to a higher MFI. Due to the lower
melting temperature of LDPE, it can be conclcuded that
the molecular weight and the melt viscosity of the LDPE
are decreasing much faster than those of the PP. As
described above, a lower molecular weight and a lower
melt viscosity are synonymous with a higher MFI. This
justifies the rise of the MFI with an increasing amount of
LDPE in the blend.
70
Differential Scanning Calorimetry (DSC)
PP and LDPE crystallize at different temperatures.
Both polymers can have a mutual effect on the process of
crystallization and the morphology of the blends [7]. The
melting temperature (Tm) and crystallization temperature
(Tc) of PP and LDPE in the blends are shown in Table 2.
Table 2. Melting temperatures (Tm) and crystallization
temperatures (Tc) of LDPE, PP and their blends of virgin
materials at different compositions.
LDPE Melting Temperatures
Crystallization
(wt%)
(°C)
Temperatures (°C)
LDPE
PP
LDPE
PP
0
25
50
75
100
60
MFI (g/10min)
Figure 3 represents the MFI of virgin PP, virgin
LDPE and their blends. It can be seen that the MFI of
LDPE and PP are nearly the same regarding the virgin
materials. In contrast to the MFI of the recycled materials,
the MFI of all blend compositions are increasing
compared to the individual material. These results indicate
that LDPE acts as a plasticizer for the PP molecules and
enhances the mobility and diffusion of PP in the blends.
Furthermore, due to the higher processing temperatures of
PP, all blends were prepared using a higher temperature
than the usual processing temperature for LDPE. This
might lead to a reduction in MFI of LDPE in all blends.
50
40
110.7 ± 0.2
111.1 ± 0.1
110.8 ± 0.2
110.9 ± 0.3
159.6 ± 0.2
158.4 ± 0.4
157.7 ± 0.2
158.6 ± 1.0
-
100.1 ± 0.3
99.8 ± 0.2
100.2 ± 0.3
99.6 ± 0.4
116.2 ± 0.1
115.6 ± 0.1
115.3 ± 0.9
113.7 ± 0.9
-
30
20
10
0
100% rLDPE
75% rLDPE
50% rLDPE
25% rLDPE
100% rPP
Figure 2. MFI of recycled PP (rPP), recycled LDPE
(rLDPE) and their blends at different compositions
(Temperature: 230°C).
Figure 4 shows the DSC heating curves of virgin
PP, virgin LDPE and their blends at different
compositions. The two endothermic peaks in the
temperature range correspond to the melting temperature
of the individual polymers. The melting temperature of
PP and LDPE does not significantly depend on the
composition of the blends.
SPE ANTEC™ Indianapolis 2016 / 83
∆𝐻! =
∆𝐻!
𝑤!
!
(1)
thus,
∆𝐻!!/!"#$ =
∆𝐻! !!
∆𝐻! !"#$
+
𝑤 !!
𝑤 !"#$
(2)
The degree of crystallinity, 𝜒, of PP, LDPE and their
blends was calculated using the following equation;
𝜒=
Figure 4. Melting temperature (Tm) from the 2nd heating
scan for PP, LDPE and their blends of virgin materials at
different compositions.
where ∆𝐻! is the specific heat of melting of a completely
crystalline materials, ∆𝐻! = 147 𝐽/𝑔, and 293 𝐽/𝑔 for
PP and PEs, respectively [7].
As shown in Figure 6 and Figure 7, the melting
enthalpies of both materials tend to follow the Rule of
Mixtures. However, a deviation of 2-19% can be observed
between the melting enthalpy determined by the DSC
measurement and the Rule of Mixtures methods. The
maximum deviation (19%) was observed in PP/LDPE
(50wt% LDPE), while the melting enthalpy of PP/LDPE
(75wt%LDPE) fit well with the Rule of Mixtures
approach (2% deviation). This indicates that PP/LDPE
(75wt% LDPE) is partially miscible or compatible.
140
Enthalpy of Melting (J/g)
It can be seen in Figure 5 that the crystallization peak
of LDPE in all blend compositions shifts to higher
temperatures relatively to the crystallization peak of virgin
LDPE. In addition, the crystallization temperature of PP
shifts to lower temperatures in relation to its composition
in the blends, which indicates a reduction in the perfection
of the formed crystallites. Further, the shift of the
crystallization peak of PP (75% LDPE) overlaps with the
crystallization peak of LDPE, which might be associated
with a partial compatibility of LDPE and PP in the blend.
∆𝐻!
× 100 (3)
∆𝐻!
DSC Measurement
120
Rule of Mixture
100
80
60
40
20
0
0
25
50
75
100
LDPE Contents (wt%)
Figure 6. The Enthalpy of melting of LDPE (∆𝐻!"#$ ) in
PP/LDPE blends at different compositions determined
from the Rule of Mixtures in comparison with the DSC
measurements.
Figure 5. Crystallization temperature (Tc) of virgin PP,
virgin LDPE and their at different compositions.
According to the Rule of Mixtures, the crystallinity
of PP/LDPE blends was determined from the melting
enthalpy (∆𝐻! ). The actual melting enthalpy of a blend is
related to the enthalpy of the individual polymer and to
their weight fraction w in the blends:
SPE ANTEC™ Indianapolis 2016 / 84
Enthalpy of Melting (J/g)
120
DSC Measurement
100
Rule of Mixture
80
60
40
20
0
0
25
50
75
100
LDPE Contents (wt%)
From the DSC measurements, the degree of
crystallinity of PP and LDPE compositions were
determined and are shown in Figure 8. It can be seen that
the crystallinity of PP and LDPE in the blends is changing
depending on their compositions. It was found that the
increase of the LDPE content decreases the degree of
crystallinity of the blend (red line).
80
PP
70
Figure 9. DSC crystallization curves obtained at 125°C
for PP, LDPE and their blends of virgin materials at
different compositions.
1
Reduced Crystallinity
Figure 7. The Enthalpy of melting of PP (∆𝐻!! ) in
PP/LDPE blends at different compositions determined
from the Rule of Mixtures in comparison with the DSC
measurements.
0.8
0.6
0.4
vPP
0.2
PP/LDPE (25wt% LDPE)
Crystallinity (%)
LDPE
60
Total % Crystallinity
PP/LDPE (50wt% LDPE)
0
0
50
100
200
300
400
500
600
700
800
Time (sec)
40
The crystallization rates for virgin PP,
virgin LDPE and their blends at different
compositions measured as a function of time.
Figure 10.
30
20
10
0
100wt% vPP
25wt% vLDPE
50wt% vLDPE
75wt% vLDPE
100wt% vLDPE
Figure 8. % Crystallinity of PP, LDPE and their blends of
virgin materials at different compositions.
Figure 9 shows the isothermal crystallization curves
for PP, LDPE and their blends at 125°C, a temperature at
which only PP could crystallize. The higher the LDPE
contents, the broader peaks were observed. However, the
crystallization peaks for PP/LDPE with 75wt% LDPE and
100% LDPE was not observable at this temperature.
Based on Figure 9 the crystalline fractions for
all crystallized materials were plotted versus the
crystallization time as shown in Figure 10. The
crystallization rate decreased as the LDPE content
increased.
Polarized Optical Microscopy
The crystal structures and morphology of virgin PP,
virgin LDPE and their blends at different compositions
were captured at appropriate times using the polarized
optical microscope. From the DSC results, it is known that
PP starts crystallizing at 125°C while LDPE starts at
100°C (Figure 5). In this study, we monitored crystal
growth in the blends in order to get more information and
understanding of their morphology. Each blend was
crystallized isothermally at 125°C, where only PP could
crystallize.
PP is a semi-crystalline thermoplastic polymer with
high degree of crystallinity due to its regular chain
structure. In general, PP can crystallize in a wide range of
spherulite dimensions (from 10-50 µm to 280-370 µm)
depending on the crystallization conditions as well as the
presence of nucleating agent [14, 15].
SPE ANTEC™ Indianapolis 2016 / 85
LDPE crystals, in general, consist of ethylene units
in a chain fold structure. Thus, the segment length of an
ethylene unit can limit the lamella thickness. PP crystals
consist of compacted spherulites, as it can be observed in
Figure 11(b). It is clearly visible that LDPE crystals are
much smaller in size compared to PP crystals.
(a)
(b)
Figure 11. Polarizing optical microscopy of (a) 100%
LDPE after isothermal crystallization at 100°C for 20 min
and (b) 100% PP after isothermal crystallization at 125°C
for 20 min.
Figure 12 shows the nucleation and growth process
of PP/LDPE blends during the isothermal crystallization
at 125°C from 4 to 10 min. The spherulites of PP in
blends were not as sharp as in the virgin PP but they could
still be easily distinguished. During crystallization, the PP
in the blend crystallizes at almost the same rate as virgin
PP.
Figure 12. Polarizing optical microscopy of the nucleation and growth process of PP/LDPE blends during
isothermal crystallization at 125°C: (a) PP/LDPE (25 wt% LDPE); (b) PP/LDPE (50 wt% LDPE) and (c)
PP/LDPE (75 wt% LDPE).
SPE ANTEC™ Indianapolis 2016 / 86
A larger LDPE content resulted in a reduction of the
average spherulite size of PP. This might imply an
increase in the PP nucleation density due to blending.
Thus, smaller crystals were observed. Although PP should
be somewhat soluble in the melted LDPE, is seems to be
consuming the remaining PP dissolved in the matrix,
preventing bridging growth [16]. This can be also
concluded from the shifting of Tc of PP to a lower
temperature with larger LDPE contents.
The crystallization rate of PP was found to be
slightly slower at larger LDPE contents. As can be seen
from Figure 12 at 4 min, the PP spherulite in the 75 wt%
LDPE blend developed slower than that of PP spherulites
in the 50 wt% LDPE and 75 wt% LDPE blends,
respectively.
Even though the PP crystals in the blend of 25wt%
LDPE nucleated faster than that of the blend with 50wt%
LDPE, it should be noted that once the PP crystals in the
blend of 50 wt% LDPE were nucleated, they grew faster.
This can be seen at 8 min. Some crystals nucleated at a
much lower rate and then grew faster afterwards. In our
continued study, we will focus on the crystallization
kinetics of the blends, which could supportive explain the
crystallization behavior of our blend system more clearly.
1.
2.
3.
4.
5.
6.
7.
Conclusions
The effects of processing on the properties of
recycled PP, recycled LDPE and their blends were
investigated by MFI measurements. Based on the
moderate increase of the MFI, it can be concluded that PP
is marginally sensitive to degradation at the used
conditions, which was also observed in the PP/LDPE
blends.
The thermal properties, crystallinity and morphology
development of polymer blends, which strongly depend
on the miscibility between blending components, can be
investigated using DSC measurement and the hot-stage
polarized optical microscopy technique. In this study, the
addition of LDPE decreases the degree of crystallinity of
the PP/LDPE blends. The blends with 25 wt% LDPE
exhibit a partial miscibility and compatibility between PP
and LDPE phases. The crystallization kinetics of all blend
compositions, both isothermal and dynamic, and their
applicability in processing will be further studied to
evaluate the ideal conditions to produce miscible blends.
The effect of thermal, thermal oxidative and mechanical
stress will be investigated separately to determine their
contribution to the observed degradation. Furthermore, the
study will be expanded to HDPE and HDPE/PP blends to
later on work with mixtures of all three materials as they
can be found in municipal waste.
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