Advances in Chemical Engi neering and Science , 2011, 1, 208-214
doi:10.4236/aces.2011.14030 Published Online October 2011 (http://www.SciRP.org/journal/aces)
Melt Rheology of Poly(Lactic Acid)/Low Density
Polyethylene Polymer Blends
Kotiba Hamad*, Mosab Kaseem, Fawaz Deri
Faculty of Science, Department of Chemistry, Laboratory of Materials Rheology (LMR),
University of Damascus, Damascus, Syria
Received June 28, 2011; revised July 26, 2011; accepted September 23, 2011
Abstract
In this work, rheological properties of poly(lactic acid) (PLA), low density polyethylene (LDPE) polymer
blends were investigated in the molten state. The experiments were carried on a capillary rheometer. The
effect of shear stress, temperature and blending ratio on the flow activation energy at a constant shear stress
and melt viscosity of the blends are described. The results showed that the PLA/LDPE polymer blends are
pseudo plastic in nature, where there viscosity decreases with increasing shear stress. Also it was found the
melt viscosity of the blends decreases with increasing PLA content in the blend.
Keywords: PLA, LDPE, Polymer Blends, Melt Viscosity, Flow Activation Energy
1. Introduction
Two or more existing polymers may be blended for
various reasons. One reason is to achieve a material that
has a combination of the properties of the constituents,
e.g. a blend of two polymers, on e of which is chemically
resistant and the other tough. Another reason is to save
costs by blending a high-performance polymer with a
cheaper material. Blending also benefits the manufac-
turer by offering: 1) Improved processability, product
uniformity, and scrap reduction. 2) Quick formulation
changes. 3) Plant flexibility and high productivity. 4)
Reduction of the number of grades that need to be
manufactured and stored. 5) Inherent recyclability, etc
[1].
Poly(lactic acid) (PLA) is well known aliphatic poly-
esters derived from corn and sugar beets, and degrades
into nontoxic compounds in landfill. It can be synthe-
sized from direct condensation of lactic acid or by
ring-opening polymerization of cyclic lactide [2], in this
way, intermediate lactide (cyclic dimer of lactic acid) is
formed in the first step, and after removing condensation
product water, the PLA oligomer is depolymerized to
lactide, in the second step, purified lactide converted into
corresponding high-molecular weight polyesters by cata-
lytic ring-opening polymerization. These new techniques
in synthesizing PLA, which allow economical production
of high molecular weight PLA polymer, have broadened
its uses. PLA was chosen for its high biocompatibility
and biodegradability. It has b ecome an alternative to tra-
ditional commodity plastics for everyday applications as
an environmental friendlily polymer due to its some
unique properties such as high strength, high stiffness
and resistance to fats and oil. However, brittleness and
other properties such as low viscosity, low thermal sta-
bility, high moisture sensitivity, medium gas barrier
properties, high cost (comparing with PE, PP, PS…) and
low solvents resistance (e.g., against water) of the pure
polymer are often insufficient for a lot of applications.
Properties of PLA can be modified by polymer blend-
ing techniques, where it was blended with several syn-
thetic and biopolymers in efforts to enhance its proper-
ties and also to obtain novel materials. PLA have been
blended with rubbers [3], thermoplastic starch (TPS)
[4-9], poly(butylene succinate) (PBS) [10], poly(butylene
succinate adipate) (PBSA) [11], poly(butylene adipate-
co-terephthalate) (PBAT) [12-15], acrilontryl- butadiene-
styrene (ABS) [16], polypropylene ( PP) [17-19], and po-
lystyrene (PS) [20,21], to obtain materials with lower
cost and improved properties.
Polyolefin’s industry is still a very dynamic business,
where low density polyethylene (LDPE) plays a key role
and it has a wide range of applications. LDPE has good
properties such as, low cost, processability, resilience
and moisture insensitivity. However, the use of LDPE
causes serious environmental concerns because it is not

K. HAMAD ET AL.
209
biodegradable; i.e., it is not biodegraded by the microor-
ganisms present in the environment. Even though recy-
cling has become an alternative, not all post-consumer
products may be recycled and, in some cases, this proc-
ess is not economically sustainable. The waste manage-
ment problem has prompted the development of, at least,
partially degrad able plastics, a fact that may be achieved
by blending with biodegradable fillers, which can effec-
tively reduce the volume of plastic waste by partial deg-
radation. Therefore, LDPE was blended with native
starch or thermoplastic starch (TPS) in efforts to enhance
its degradability [22], and it was found that LDPE’s de-
gradability can be improved when starch is added to it,
although the pure LDPE is not biodegradable. However,
the addition of TPS might negatively affect the proper-
ties of LDPE. In this case PLA, which has good me-
chanical properties, can be used as alternative to TPS.
Studies on PLA/PE blends were reported by many au-
thors [23-25].
Blends PLA with LDPE offer cost-performance bene-
fits, and enhance the degrad ability o f LDPE. In th is work
PLA/LDPE blends were prepared using a laboratory
scale single screw extruder and characterized in terms of
the rheological properties. Such works are very impor-
tant in the development of composites and blends from
biodegradable polyesters. No attention has been given in
the past to rheological characterization of PLA/LDPE
polymer blends.
2. Experimental
2.1. Materials
Poly(lactic acid) (PLA) (ESUN™ A-1001) [density =
1.25 g/cm3 (21.5˚C), MFI = 12.5 g/10 min (190˚C/2.18
Kg)] was supplied by Bright China Industrial Company.
Ltd (Shenzhen, China), the selected grade is an extrusion
material; it was dried at 70˚C for 6 h before using. Low
density polyethylen e (LDPE) (SABIC®LDPE2308TN00)
[density = 0.924 g/cm3, MFI = 7.5 g/10 min (190˚C/2.16
Kg)] was supplied by Sabic (KSA).
2.2. Blends Compounding
Blends of PLA/LDPE in different ratios were com-
pounded using a laboratory scale single screw extruder
(SSE) (D = 20 mm, L/D = 25) [SHAM EXTRUDER
25D Performance: Kreem Industrial Establishment, Da-
mascus-Syria], it could be operated at different speeds,
varied from 0 to100 rpm. The screw has a fluted type
mixing device located before the metering zone [26],
which can extend th e compounding ab ility of the SSE, in
this type of mixers the material is forced to pass at a high
shear stress. This brings in some level of dispersing ac-
tion besides reorienting the interfacial area and increas-
ing the imposed total strain. The flight depth of screw in
the metering zone was 1.5 mm, and the helix angle 17.7˚.
PT124G-124 melt pressure transducer (Shanghai Zhao-
hui Pressure Apparatus Co., Ltd-China) was located in
the die head for measuring the melt pressure.
The screw speed was set at 70 rpm in the blends
preparation, and the extruder temperature profile along
the barrel was 180˚C, 190˚C, 200˚C, 210˚C (from feed
zone to die). The blends were extruded through a multi
holes die (3 mm), the extrudates were then fed into a
granulator, which converted them into granules. The ob-
tained granules were dried at 70˚C for 6 h before study-
ing. The compositions of the blends are shown in Table 1.
2.3. Rheology
Rheological properties of the blends were studied using a
constant pressure circular capillary rheometer. The melt
is extruded by the use of dead weights (i.e. constant
pressure) rather than constant plunger speed. This in-
strument, popularly known as the Melt Flow Indexer
(MFI), is very popular in the thermoplastics industry due
to its ease of operation and low cost, which more than
compensates for its lack of sophistication. The parameter
measured through the melt flow indexer contains mixed
information of the elastic and viscous effects of the
polymer. The rheological experiments were carried out at
175˚C, 180˚C, 185˚C and 190˚C, and by using L/R = 8,
15, 25 and 36 capillaries. Bagley’s correction [27,28]
was performed by using the data from the four capillary
dies. The apparent shear rate (γa) is given by:
3
4
a
Q
R
  (1)
where R is the capillary radius, and Q is the volumetric
flow rate. The true shear rate (γr) is given by:
31
.
4
r
n
na






(2)
where n is the non-Newtonian index depending on tem-
perature, the term (


314nn) was the Rabinowitsch
correction factor. The apparent shear stress (τa) is given
by:
2
a
RP
L
  (3)
Table 1. Compositions of PLA/LDPE blends.
Sample PLA100PLA70 PLA50 PLA30PLA0
PLA (wt%)
LDPE (wt%) 100 0 70 30 50 50 30 70 0 100

210 K. HAMAD ET AL.
where P is the pressure at the capillary en trance, and L is
the capillary length. The true shear stress (τr) is given by:

2
r
P
LR e
  (4)
where e is the Bagley’s correction factor. The true vis-
cosity (ηr) is given by:
r
r
r



 (5)
The values of flow activation energy at a constant
shear stress (Eτ) were determined by using Arrhenius
equation form:
e
E
R
T
rA

  (6)
where A is the consistency related to structure and for-
mulation and R is the gas constant (8.314 J/mo lK).
3. Results and Discussion
3.1. Flow Curves
The flow curves, ie plots of shear stress versus shear rate
for PLA/LDPE blends covering the whole composition
range have been measured over a temperature range from
175˚C to 190˚C in steps of 5˚C. A typical result for the
plot at 175˚C is shown in Figure 1.
It can be seen that these curves apparently deviate
from linear relationship inclining to the axis of shear rate,
which means that the homopolymers of PLA, LDPE and
their blends are pseudo plastic non-Newton fluids similar
to most polymeric melts, and they obey the power law:
n
K

  (7)
where K is the consistency index and n is the non- New-
tonian index, which can be calculated from the slope of
Figure 1. Flow curves of the blends at 175˚C.
the lines in Figure 1.
dlog
dlog
n


 (8)
Figure 2 shows the values of n for PLA/LDPE blend
melts over blending ratio at 175˚C. It could be noted
from Figure 2, th at the values of n were less than 1, im-
plying that PLA/LDPE blend melts were pseudo plastic;
similar to most polymer melts. Also it could be noted
from Figure 2, that the n values of the blends are less
than those of the homopolymers, PLA and LDPE. The
value of n describes the deviation from the Newtonian
fluids about flow behavior, so it is also called the flow
behavior index. A higher value of n reveals less influ-
ence of shear rate on flow behavior. In other words, the
changes in viscosity upon shear rate are not obvious, so it
could be said that the flow behavior of the blend melts is
more sensitive to shear rate as compared with the ho-
mopolymers.
Also it should be noted from Figure 2, that the
PLA/LD PE blends f ound to be incompatib le, wher e they
show a drop in n values and PLA50 is the most non-
Newtonian (n = 0.52). The compatible polymer blends
show high non-Newtonian indexes which are closed to
that of their major phase (PLA and LDPE).
3.2. Melt Viscosity
Figure 3 shows plots of the melt viscosity versus shear
stress for homopolymers PLA, LDPE and the blends at
different temperatures. It shows that the viscosity of PLA,
LDPE and the blends decreases with increasing shear
stress, showing a typical property of pseudo-plastic
non-Newton fluids, this behavior was attributed to the
alignment or arrangement of chain segments of polymers
in the direction of applied shear stress [29].
Figure 2. Non-newtonian inde x of the blends at 175 ˚C.

K. HAMAD ET AL.
211
In order to observe how the melt viscosity varies with
the blending ratio at different temperatures, cross plots of
Figure 3 are given in Figure 4. The high viscosity at a
low shear rate provide the integrity of the extrudate dur-
ing extrusion, and the low viscosity at a high shear rate
enables low injection pressure, high injection speed and
less time of the injection cycle.
It could be noted from Figure 4 that the melt viscosity
of the blend decreases with increasing PLA at 175˚C,
and this behavior can be attributed to the low melt vis-
Figure 3. Variation of melt viscosity with shear stress for PLA/LDPE blends.
Figure 4. Variation of melt viscosity with blending ratio for PLA/LDPE blends.

K. HAMAD ET AL.
212
cosity of PLA co mparing wi th LDPE. The same behavi or
can be noted at the different temperatures (180, 185,
190˚C). This behavior can be explained according to the
flow activation energy of the blends.
The low melt viscosity of the blends makes them suit-
able to be used in injection molding process. The good
processability (low processing temperature or low pres-
sure) of this blend was detected during injection process
of the blends by using NEGRI BOSSI (NB 25) injection
machine (Lessona Corporation, Italy). Figure 5 shows
samples of the blends and their processing condition
(temperature, pressure and cooling time in the mold). It
could be noted from Figure 5 that the injection tempera-
ture of the blend decreases with increasing PLA content,
which was attributed to the decreasing of the melt vis-
cosity with increasing PLA content in the blend.
3.3. Flow Activation Energy
The true viscosity variation of the blends with tempera-
ture at τ = 10, 20 and 40 KPa is shown in Figures 6. The
true viscosity of PLA, LDPE and all of the blends de-
creases with increasing temperature. However, the melt
viscosity of PLA changes with temperature much more
sharply than that of LDPE. The plots of true viscosity
versus 1/T shown in Figures 6, give a group of straight
lines, which indicates that the relationship between the
true viscosity and temperature follows Arrhenius equa-
tion (Equation (6)). With a rise of temperature the mo-
tion ability of polymer chains enhances, and the resis-
tance between the melt layers decreases relevantly, lead-
ing to reduction of the melt viscosity. Flow activation
energy at a constant shear stress (Eτ) could be calculated
from the slopes of these lines.
Figure 7 shows the effect of the blending ratio on the
flow activation energy, it is clearly seen that the flow
activation energy of PLA is greater than that of LDPE,
which indicates that the flow behavior of PLA is more
sensitive to temperature compared with LDPE. Also, it
could be noted from Figure 7, that the flow activation
energies of PLA/LDPE blends at τ = 104 Pa are between
those of PLA and LDPE, whereas at τ = 4 × 104 Pa, the
flow activation energy of PLA50 is lower than that of
LDPE which indicates that the sensitivity of PLA50 vis-
cosity to temperature sharply decreases with increasing
shear stress.
Figure 5. Injected samples and their processing condition.
Figure 6. True viscosity versus 1/T of the sample melts.

K. HAMAD ET AL.
213
Figure 7. Flow activation energy at a constant shear stress
versus PLA content (wt%).
4. Conclusions
In this work, rheological properties of PLA/LDPE poly-
mer blends were studied. The blends were prepared using
a single screw extruder. Rheological results showed that
PLA/LDPE polymer blends are pseudo plastic in nature;
their viscosity decreases with increasing shear rate simi-
lar to most polymer melts. The true viscosity of PLA
decreases sharply with increasing temperature, whereas
the true viscosity of LDPE varies slightly only with
temperature. Also it was found that the true viscosity of
the blends decreases with increasing PLA content which
was attributed to PLA low viscosity. The rheological
results indicate that the prepared blends are suitable to be
used in injection molding process, where they have low
melt viscosity. Such blends are very important in devel-
opment new biodegradable materials with low cost.
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