Thermal and mechanical properties of extruded LLDPE/wax blends

Abstract

The thermal and mechanical properties of linear low-density polyethylene (LLDPE)/wax blends, prepared in an extruder, were investigated, using differential scanning calorimetry (DSC) and tensile testing. DSC measurements indicated that blends consisting of 10 and 20% of wax are probably miscible in the crystalline phase. For 30% and more wax, phase separation of the two components were observed. The DSC curves of both pure LLDPE and blends show, beside the main exothermic peak, another small peak at about 70°C. Since this peak was observed for both pure LLDPE and the blends, it is probable that its origin is partly in the LLDPE structure, but it is also influenced by wax crystallization. An increase in Young’s modulus with an increase in wax content was observed. An increase in wax content causes a decrease in elongation at yield. A small increase in yield stress was observed for blends consisting of 10 and 20% of wax. For blends consisting of 30% and more wax, no yield point, but brittle rupture, was observed.

Full text

Thermal and mechanical properties of extruded LLDPE/wax blends
I. Krupa
a
, A.S. Luyt
b,
*
a
Polymer Institute, Slovak Academy of Science, 842 36 Bratislava, Slovak Republic
b
School of Chemical Sciences, University of the North (Qwa-Qwa), Private Bag X13, Phuthaditjhaba 9866, South Africa
Received 24 January 2001; accepted 27 February 2001
Abstract
The thermal and mechanical properties of linear low-density polyethylene (LLDPE)/wax blends, prepared in an extruder, were
investigated, using differential scanning calorimetry (DSC) and tensile testing. DSC measurements indicated that blends consisting
of 10 and 20% of wax are probably miscible in the crystalline phase. For 30% and more wax, phase separation of the two com-
ponents were observed. The DSC curves of both pure LLDPE and blends show, beside the main exothermic peak, another small peak
at about 70C. Since this peak was observed for both pure LLDPE and the blends, it is probable that its origin is partly in the
LLDPE structure, but it is also influenced by wax crystallization. An increase in Young’s modulus with an increase in wax content
was observed. An increase in wax content causes a decrease in elongation at yield. A small increase in yield stress was observed for
blends consisting of 10 and 20% of wax. For blends consisting of 30% and more wax, no yield point, but brittle rupture, was observed.
#2001 Elsevier Science Ltd. All rights reserved.
Keywords: LLDPE/wax blends; Thermal analysis; Tensile properties
1. Introduction
Blending of different polymers is regarded as an eco-
nomical alternative to the development of new polymers.
Materials with improved properties can be obtained by
blending two or more polymers having different chemi-
cal composition and physical properties [1]. In this
paper we investigate some thermal and mechanical
properties of LLDPE/wax blends.
Linear low-density polyethylene (LLDPE) has good
mechanical properties and is often used in industry. Gro-
cery bags, heavy duty shipping sacks, agricultural films,
pipes, liners for consumers, landfills and waste ponds are
only a few examples [2–4].
Paraffins are a class of aliphatic hydrocarbons, char-
acterized by straight or branched carbon chains, generic
formula C
n
H
2n+2
. Paraffin waxes (Fischer–Tropsch
synthesis) are white, translucent, tasteless and odourless
solids consisting of a mixture of solid hydrocarbons of
high molecular weight. Common properties are water
repellency, smooth texture, low toxicity, freedom from
objectionable odour and colour. They are combustible
and have good dielectric properties. They are used for the
preparation of candles, paper coating, protective sealant
for food products and beverages, glass-cleaning pre-
paration, hot-melt carpet backing, biodegradable mulch,
lubricants, stoppers for acid bottles, electrical insulation
and others [5].
In our previous studies we devoted attention to pre-
paration and characterization of cross-linked and uncross-
linked LLDPE, low-density polyethylene (LDPE)/wax
blends [6–8]. In that case, blends were only mechanically
mixed. In this case, blends were blended in an industrial
extruder. Very different behaviour was observed for all
physical properties. It is therefore clear that the morphol-
ogy of the blends strongly influences the final properties
[1]. This strongly depends on processing conditions.
2. Experimental
In this work, linear low-density polyethylene (MFI=3.5
g/10 min, density=0.938 g cm
3
, particle size — 90% less
than 600 m) and hard, brittle, straight hydrocarbon-chain
paraffin wax (carbon distribution C28–C120, average
molar mass 7.8510
1
kg mol
1
, density=9.410
2
kg
0141-3910/01/$ - see front matter #2001 Elsevier Science Ltd. All rights reserved.
PII: S0141-3910(01)00082-9
Polymer Degradation and Stability 73 (2001) 157–161
www.elsevier.nl/locate/polydegstab
* Corresponding author. Tel./fax: +27-58-713-0152.
E-mail address: luyt@uniqwa.ac.za (A.S. Luyt).

m
3
, melting point 104C) from Schu
¨mann–Sasol were
used. All blends were blended in an industrial extruder
(Bandera film blower at 100 r.p.m.) at 180C and then
pressed for 3 min at 180C.
Differential scanning calorimetry was carried out on a
Perkin Elmer DSC7 thermal analyzer in nitrogen atmo-
sphere. Samples were heated from 25 to 160C at a heat-
ing rate of 10C min
1
and then cooled at the same rate.
Thermal properties, like melting and crystallization tem-
peratures and enthalpies, were determined from the
second scan.
For the determination of mechanical properties a sim-
ple tensile tester (Hounsfield W5K ROM rev M73) was
used. The speed of deformation was 50 mm/min. The
final mechanical properties were evaluated from at least
six different measurements.
Scanning electron microscopy photos were taken of the
break interface of samples cooled in liquid nitrogen and
broken. A Philips XL30 DX4I scanning electron micro-
scope was used at 10 kV voltage.
3. Results and discussion
3.1. Differential scanning calorimetry
The results obtained from differential scanning calori-
metry (DSC) are summarized in Table 1. Fig. 1 shows
DSC heating curves for the blends. We see a difference
in the behaviour for different concentrations of wax.
For 10% wax only one endothermic peak is observed.
For 20% wax a very small indication of other peaks is
observable (it is seen more clearly on the original curves),
but they are not significant. In these cases (at least for
10% wax), we can say that LLDPE and wax are mis-
cible in the crystalline phase [9]. For 30% and more wax,
three significant peaks are observable — one of them
corresponds to that of LLDPE and the other two (at
about 80 and 109C) correspond to that of wax. In this
concentration region, LLDPE and wax are, therefore,
not miscible with each other in the crystalline phase. T
o
and T
m
also decreases with an increase in wax portion
(Table 1, Fig. 1).
In our previous work, where blends were only mechani-
cally mixed [6], we observed different behaviour. The DSC
heating curves showed only one endothermic peak for all
the wax concentrations. This peak corresponded to the
LLDPE melting peak and the T
o
and T
m
temperatures of
the blends were not influenced by the amount of wax in
the blends.
The DSC cooling curves of the samples are shown in
Fig. 2. The crystallization of the blends is different. The
DSC curves of all the samples, both pure LLDPE and
blends, show beside the main exothermic peak, another
small peak at about 70C (peak at about 70Cis
obvious and reproducible in the original curves). Since
this peak was observed for both pure LLDPE and the
blends, it is probable that its origin is partly in the
LLDPE structure. Generally, LLDPEs themselves are
not simple materials; there is some evidence that they
may phase separate in the melt. The LLDPEs are linear
Table 1
The parameters obtained from DSC measurements for LLDPE/wax
a
Sample T
o,m
(C) T
p,m
=T
m
(C) H
m
(kJ kg
1
)T
o,c
=T
c
(C) T
p,c
(C) H
c
(kJ kg
1
)
LLDPE 121.7 130.0 155.90 114.2 110.6 174.49
90/10 120.8 127.0 161.67 (161.62) 112.8 109.8 184.28
80/20 121.0 125.4 168.68 (167.33) 115.4 107.8 187.72
70/30 119.9 124.9
b
172.13 (173.04) 111.0 108.1
c
194.92
60/40 118.8 123.0
b
173.34 (178.76) 110.2 107.7
c
205.56
50/50 116.9 123.0
b
184.23 (184.48) 109.1 106.3
c
204.19
Wax 60.1 77.2
d
213.06 95.2 91.8
e
211.21
a
T, is temperature; H, is specific enthalpy; m, melting; c, cooling; o, onset; p, peak. Notation x/y means weight portion of LLDPE and weight
portion of wax.
b
The DSC heating curves of LLDPE/wax blends show three endothermic peaks. This one is the main peak. The others are at about 80 and
109C. See Fig. 1.
c
The DSC cooling curve of pure wax shows two exothermic peaks. This one is the main peak. The other one is at 80C. See Fig. 2.
d
The DSC heating curve of pure wax shows three endothermic peaks. This one is the main peak. The others are at about 90 and 101C. See Fig. 1.
e
The DSC cooling curve of pure wax shows two exothermic peaks. This one is the main peak. The other one is at 70C. See Fig. 2.
Fig. 1. DSC heating curves of LLDPE, wax and different LLDPE wax
blends prepared in an extruder.
158 I. Krupa, A.S. Luyt / Polymer Degradation and Stability 73 (2001) 157–161

but have a significant number of branches introduced by
using co-monomers, such as butene-1 or octene-1. The
co-monomer content is about 8–10% [10]. Marabella et
al. [11] and Deblieck and Mathot [12] have shown that,
when highly branched materials are quenched from the
melt, regions of differing crystallinity are manifested.
Various authors have analysed LLDPE materials by tem-
perature-rising elution fractionation (TREF) [13–15] and
have shown that LLDPEs contain molecules of very
different molecular weights and branched content. This
probably explains the existence of the second peak for
LLDPE. Since this peak is more intense for 30% and
more wax, it means that it is influenced by wax crystal-
lization. In this case we also observe a slightly decreas-
ing crystallization temperature with increasing wax
content. We observed similar behaviour during the
investigation of only mechanically mixed blends.
The specific enthalpy of melting of LLDPE/wax blends
increases with an increase in wax portion. The experi-
mental results (Table 1) and the values calculated accord-
ing to the additive rule [Eq. (1)] are in excellent agreement.
Hadd
m¼Hm;PEwPE þHm;wwwð1Þ
H
m,PE
,H
m,w
,H
m
add
are the specific enthalpies of
melting of LLDPE, wax and blends, and w
PE
,w
w
are
weight portions of LLDPE and wax in the blends.
3.2. Mechanical properties
An increase in Young’s modulus with an increase in
wax content was observed, indicating that the modulus
of this wax is higher than the modulus of LLDPE. It is
probably associated with its higher degree of crystal-
linity. The degrees of crystallinity of LLDPE and wax
are respectively 53.2 and 72.7%. These values were cal-
culated according to Eq. (2):
Xc¼Hm=Hþ
mð2Þ
where X
c
is the degree of crystallinity, H
m
is the specific
enthalpy of melting and H
+
m
is the specific enthalpy of
melting for 100% crystalline polyethylene. We used the
value H
+
m
=293 kJ kg
1
[16].
An increase in wax portion causes a decrease in elonga-
tion at yield. This is to be expected, since the wax is harder
than the LLDPE. As far as yield stress is concerned, a
small increase was observed for blends consisting of 10
and 20% of wax. For blends consisting of 30% of wax and
more, no yield point, but brittle rupture was observed.
Stress at break depends on the wax concentration.
Three different types of behaviour were observed. This is
shown schematically in Fig. 3, where a logarithmic scale
was used, because the differences in elongation between
the different samples were too large. Materials, which
undergo strain hardening during stretching, have higher
strength at break than materials which do not undergo
strain hardening [17]. In this case only pure LLDPE
undergoes significant strain hardening (Fig. 3), and there-
fore the value of stress at break for pure LLDPE is the
highest — even higher than the yield stress (Table 2).
Samples, which consisted of 10 and 20% wax, reached
yield point, then underwent strain softening, but when
they reached draw stress (the minimum stress reached
during stress softening), the orientation hardening is very
small. In these cases, stress at break is much smaller than
yield stress (Table 2). Samples, which consisted of 30%
and more wax, does not have a yield point. They exhibit
brittle rupture and do not undergo strain softening. In
these cases, an increase in stress at break was observed.
An increase in wax content results in a decrease in
elongation at break at all concentrations investigated.
This decrease is the sharpest when the wax content is
higher than 30% (Table 2). In this case the material loses
its drawability and is very brittle.
The above behaviour is closely associated with the mis-
cibility of components, as we have discussed above. The
DSC measurements indicated that blends containing 10
and 20% of wax are miscible in the crystalline phase and
Fig. 2. DSC cooling curves of LLDPE, wax and different LLDPE wax
blends prepared in an extruder.
Fig. 3. Stress–strain curves of LLDPE and two LLDPE/wax blends
prepared in an extruder.
I. Krupa, A.S. Luyt / Polymer Degradation and Stability 73 (2001) 157–161 159

only one melting peak is observed. If the concentration of
wax is 30% and more, the components are more or less
separated. It has an influence on some mechanical prop-
erties: the material loses drawability, a yield point does
not exist, and elongation at break strongly decreases. In
our previous work we observed much higher values for
elongation at break [7].
The change in the mechanism of rupture is demon-
strated in Fig. 4. From these SEM photos it is clear that
an increase in wax portion causes LLDPE/wax blends
to rupture along different lines.
4. Conclusions
DSC measurements indicated that blends consisting
of 10 and 20% of wax are probably miscible in the crys-
talline phase. Only one peak was observed in the DSC
Table 2
Mechanical properties of LLDPE/wax blends
a
Sample "
y
Se
y
(%) 
y
Ss
y
(MPa) "
b
Se
b
(%) 
b
Ss
b
(MPa) ES
E
(MPa)
LLDPE 24.53.2 16.2 0.9 1249132 22.21.4 123 9
90/10 21.51.3 21.1 0.3 730 192 15.12.1 145 11
80/20 16.03.5 19.0 0.9 142 37 11.20.9 199 12
70/30 X X 186 18.72.1 225 16
60/40 X X 103 17.43.3 283 14
50/50 X X 72 17.13.3 29516
a
"
y
,
y,
"
b
,
b
,Eare elongation at yield, yield stress, elongation at break, stress at break, Young’s modulus of elasticity; Se
y
,Ss
y
,Se
b
,Ss
b
,S
E
are
their standard deviations; X, brittle rupture, no yield point was observed. The notation x/y means weight portion of LLDPE/weight portion of wax
in the blend.
Fig. 4. SEM photographs of samples of ruptured (a) LLDPE, (b) 90/10 LLDPE/wax, and (c) 60/40 LLDPE/wax.
160 I. Krupa, A.S. Luyt / Polymer Degradation and Stability 73 (2001) 157–161

curve. For 30% and more wax two other peaks were
observed. It indicates phase separation of components.
It also has an influence on the mechanical properties. T
o
and T
m
decrease with an increase in wax content.
The DSC curves of both pure LLDPE and blends
show, beside the main exothermic peak, another small
peak at about 70C. Since this peak was observed for
both pure LLDPE and the blends, it is probable that its
origin is partly in the LLDPE structure. On the other
hand, for blends consisting of 30% and more wax, this
peak is more intense, which means that it is influenced
by wax crystallization. In this case we also observe that
an increase in wax content slightly decreases the tem-
perature of crystallization.
An increase in Young’s modulus with an increase in
wax content was observed, indicating that the modulus
of this wax is higher than the modulus of LLDPE. It is
probably associated with its higher degree of crystallinity.
An increase in wax content causes a decrease in elongation
at yield. This is to be expected, since the wax is harder than
the LLDPE. A small increase in yield stress was observed
for blends consisting of 10 and 20% of wax. For blends
consisting of 30% and more wax, no yield point, but brit-
tle rupture, was observed.
The influence of wax content on stress at break depends
on its concentration. Since pure LLDPE undergoes sig-
nificant strain hardening, its value of stress at break is
the highest — even higher than yield stress. Samples
containing 10 and 20% wax underwent strain softening
after their yield points. Stress at break is therefore much
smaller than yield stress. Samples, which consist of 30%
and more wax, do not have yield points, and exhibit
brittle rupture — increase in stress at break. An increase
in wax content results in a decrease in elongation at break
in the whole concentration region. This decrease is the
sharpest for 30% and more wax in the blends. In this
case, the material loses its drawability and is very brittle.
The SEM photos also confirm that an increase in wax
content causes LLDPE/wax blends to rupture along
different lines.
We also discussed these results in comparison with our
previous work. We showed that the route of sample
preparation — mechanical mixing versus blending in the
melt — has a marked (yet unexplained) influence on all
properties investigated.
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