Blends of ground tire rubber devulcanized by microwaves/HDPE - Part A: influence of devulcanization process

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## Blends of ground tire rubber devulcanized by microwaves/ HDPE - Part A: influence of devulcanization process

Fabiula Danielli Bastos de Sousa 1* , Júlia Rocha Gouveia 1 , Pedro Mario Franco de Camargo Filho 1 ,
Suel Eric Vidotti 1 , Carlos Henrique Scuracchio 1 , Leice Gonçalves Amurin 2 and Ticiane Sanches Valera 2

_1Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas – CECS,_
_Universidade Federal do ABC – UFABC, Santo André, SP, Brazil_

_2Departamento de Engenharia Metalúrgica e de Materiais, Escola Politécnica,_
_Universidade de São Paulo – USP, São Paulo, SP, Brazil_

_*fabiuladesousa@gmail.com_

Abstract

The main objective of this work is the study of the influence of microwaves devulcanization of the elastomeric phase
on dynamically revulcanized blends based on Ground Tire Rubber (GTR)/High Density Polyethylene (HDPE).
The devulcanization of the GTR was performed in a system comprised of a conventional microwave oven adapted with
a motorized stirring at a constant microwaves power and at various exposure times. The influence of the devulcanization
process on the final properties of the blends was evaluated in terms of mechanical, viscoelastic, thermal and rheological
properties. The morphology was also studied.

Keywords: _elastomers, recycling, GTR, devulcanization, HDPE._

1. Introduction

The search for new materials has been a constant in
the human history. Similarly, solutions to the problem
of disposal of waste polymers, especially waste rubber,
that causes serious environmental problems and concern,
have been desired for many years. Rubber requires a long
period of time to degrade naturally due to its structure of
cross‑linkings and the presence of stabilizers and other
additives [1,2] .

The technique of devulcanization by microwaves is
currently one of the most promising ones, based on the good
properties of the devulcanized material and the possibility of
high productivity. The process takes advantage of volumetric
heating of the material by microwaves, promoting a more
uniform heating than that achieved by traditional methods of
heating, which depend on conduction and/or convection [3-5] .
Materials react differently when exposed to an electromagnetic
field, like the one generated by microwaves. In dielectric
materials, molecules or free ions are rearranged in dipole
momentum which results in the volumetric heating through
the volume of the material. These molecules vibrate at high
frequency tending to re-orient and align themselves with
the microwave field. Interaction between the material and
the microwave energy generates heat. The ability to convert
microwave energy into thermal energy depends on the
magnitude of the dielectric loss of the material [6,7] . Therefore,
in mixtures of materials, it is possible a selective heating of
specific regions, a property that has been exploited in the
processing of thermosets with mineral fillers [3-5] . Elastomers
such as natural rubber (NR), styrene-butadiene rubber
(SBR), and ethylene propylene diene monomer rubber
(EPDM) have low microwaves absorption due to their non
polar characteristic. This limitation can be overcome by the
addition of conductive filler like carbon black [8-10] .

According to Scuracchio et al. [3], the technique of
devulcanization by microwaves is able to generate a material
with properties quite different from the original vulcanized
rubber. Among the properties, the most remarkable is the
ability to flow and to be remolded. This feature, allied to
the possibility of its revulcanization, indicates the wide
applicability of the technique. Bani et al. [11] demonstrated
that microwaves can be applied easily and have many
advantages, such as high heating rate, without any need of
additional mechanical or chemical treatments.

On the other hand, thermoplastic vulcanized (TPVs)
are a kind of polymeric blend produced via dynamic
vulcanization of a dispersed elastomeric phase, i.e. the
selective cross‑linking of the rubber phase while mixed with
the molten thermoplastic [12] . The final morphology consists
of cross-linked rubber particles dispersed in a thermoplastic
matrix. The thermoplastic matrix is responsible for the
processability of TPVs, while the cross-linked elastomer
particles are responsible for the elasticity at room temperature.
The final morphology of this kind of material is the main
responsible for the rheological and physical properties
beyond being controlled by the processing conditions and
characteristics of the constituting materials [13] .

The balance between break-up and coalescence of the
droplets of the elastomeric phase provides the final morphology
of a TPV during processing. In the special case of TPV
composed by devulcanized elastomer as dispersed phase, the
devulcanization acts increasing the break-up ability while
the revulcanization acts decreasing the coalescence, and
both effects contribute to the refinement of the morphology.
In addition, higher amounts of recycled rubber can be
added in the blend without properties degradation, since

the devulcanization process tends to increase the adhesion
of the particles on the thermoplastic phase.

In this work, the influence of microwaves devulcanization
of the elastomeric phase in the blends GTR/HDPE is
investigated. The results show that microwaves treatment
of the GTR in the blends can influence the mechanical,
viscoelastic, thermal and rheological properties.

2. Experimental

_2.1 Materials_

HDPE IA-59, a grade for injection molding, was kindly
supplied by Braskem (MFI = 7.3 g/10min). Ground waste
truck tire (GTR) separated from non elastomeric components;
rubber accelerator N-tert-butyl-2-benzothiazole sulfenamide
(TBBS) and sulfur were kindly supplied by Pirelli Pneus Ltda.

_2.2 Devulcanization of GTR and mixture with_
_vulcanization additives_

GTR was devulcanized in a system comprised of a
conventional microwave oven adapted with a motorized
stirring system with speed control. The devulcanization
process was done by using the maximum power of the oven,
i.e. 820W. The time at which the material was exposed to
microwaves ranged from 1 to 5 minutes and also 2-2, 2-2‑2,
and 3-3, where the numbers represent the exposure time
to microwaves (minutes) and the hyphen corresponds to
an interval of 10 minutes between consecutive treatments,
under stirring with the oven switched off.

The devulcanized GTR was mixed with the vulcanization
additives by using a laboratory two roll mill PRENMAR for
approximately 6 minutes at room temperature. To promote
the dynamic revulcanization during the processing with
the thermoplastic, 1 phr of accelerator TBBS and 1 phr of
sulfur were added.

_2.3 Preparation of the blends_

The blends were prepared in an internal mixer coupled
to a torque Rheometer Polylab 900 at 160°C and 80 rpm
for 15 minutes. The compositions and nomenclature used
for the blends are summarized in the Table 1.

_2.4 Characterization_

The revulcanization characteristics of the devulcanized
GTRs under different exposure times to microwaves were
studied by using an oscillatory dual cone Monsanto Rheometer
100, according to ASTM D1646-07. Curves of torque _versus_
time were obtained at 160°C. The nomenclature is type
GTRX+ad, where X represents the exposure time of GTR
to microwaves and “+ad” the presence of vulcanization
additives.

Thermal properties of the HDPE phase were analyzed
by Differential Scanning Calorimetry (DSC) in a DP Union
DSC Q200 under nitrogen atmosphere. The samples were
heated from room temperature to 190°C and were held at
this temperature for 3 min to eliminate their thermal history
and destroy the HDPE crystalline nuclei. They were then
cooled to –90°C and were subsequently heated to 200°C.
All the steps were performed at a rate of 10°C/min.

Mechanical properties of the blends were analyzed
by tensile tests in an Instron Universal Testing Machine
3369 with a 10 kN load cell at a crosshead speed of
50 mm/min. The samples were prepared in the shape of
plates by compression molding at 160°C in a hydraulic
press, and then the blends were cut into dumb-bell shaped
tensile test according to ASTM D412, type IV.

Rheological properties of the blends were analyzed
by small amplitude oscillatory rheometry in frequency
sweep mode, by using a parallel plate rheometer Anton
Paar CTD450 (diameter 25 mm, gap 1.3 mm, 0.5% strain
for the viscoelastic linear response at 170°C under inert
atmosphere).

Dynamic mechanical properties of the blends were
analyzed by using a Dynamic Mechanical Analyzer (DMA)
Q800 TA Instruments. The analyses were performed by
Single Cantilever mode, frequency of 1 Hz, temperature
ranging from –100 to 140°C and heating rate of 3°C/min.

A Jeol JMS-6701F Scanning Electron Microscope was
used to observe the morphology of the blends with working
distance of 5.5 mm. The samples were firstly pressed by
using a hydraulic press, cut, fractured just after being
immersed in liquid nitrogen and then coated with golden
by using a sputter coater.

Table 1. Nomenclatures and compositions of the blends produced in this work.

Devulcanization time of Presence of
Nomenclature GTR amount (wt%) HDPE amount (wt%)

GTR (min) vulcanization additives

80GTR0/20HDPE 80 20 - 80GTR0+ad/20HDPE 80 20 - Yes
80GTR1+ad/20HDPE 80 20 1 Yes
80GTR2+ad/20HDPE 80 20 2 Yes
80GTR3+ad/20HDPE 80 20 3 Yes
80GTR4+ad/20HDPE 80 20 4 Yes
80GTR5+ad/20HDPE 80 20 5 Yes
80GTR2-2+ad/20HDPE 80 20 2-2 Yes
80GTR2-2-2+ad/20HDPE 80 20 2-2-2 Yes
80GTR3-3+ad/20HDPE 80 20 3-3 Yes

Devulcanization time of
Nomenclature GTR amount (wt%) HDPE amount (wt%)

GTR (min)

3. Results and Discussion

_3.1 Revulcanization characteristics_

Torque _versus_ time curves of devulcanized rubber
containing vulcanization additives are showed in the Figure 1.

According to Figure 1, the samples GTR5, GTR2-2‑2
and GTR3-3 presented reversion trend, i.e. the torque
measured by the equipment tends to decline at the end of the
analysis. This behavior happened probably as consequence
of degradation of rubber main chains, since these samples
were exposed to microwaves for long periods of time.

The revulcanization characteristics of the samples,
calculated from the curves presented in the Figure 1 are
summarized in the Table 2. In general, it can be observed
that the optimum cure time and scorch time, represented
by t90 and ts1 respectively, were lower for samples with the
highest exposure times to microwaves in one step as well
as in multistep treatments. This behavior is characteristic of
reclaimed rubber and it was observed by some other authors [14-17],
which probably happens due to the presence of residual
curatives from the first vulcanization. The devulcanization
process increases the freedom degree of the polymeric
chains, accelerating the reaction with the increasing of the
exposure time of the GTR to microwaves, possibly due to
the great amount of effective shocks during the process.

GTR5+ad, GTR2-2+ad and GTR3-3+ad presented
lower values of ML and MH, minimum and maximum
torque respectively, which demonstrates lower cross-linking

densities in comparison to the other samples. The values of
the subtraction of MH-ML did not present a trend, but they
were lower in relation to GTR0+ad, with exception of the
samples GTR1+ad and GTR3+ad. This value is related to
the cross-linking density of the sample and its reduction is
attributed to the breaking of reticulation as a result of the
devulcanization by microwaves.

It can be also observed that the samples with higher values
of CRA (Cure Rate Average) were exposed to microwaves
for 2 minutes, in two or three steps of treatment. CRA values
were calculated according to Equation 1 [18] :

where t90 is the optimum cure time and ts1 the scorch time.
The value is proportional to the average slope of torque
_versus_ time curve or, in other words, it is proportional to
the rubber revulcanization speed.

The devulcanization process reduced the ML values with
the increase of the exposure time to microwaves. However, a
clear trend was not observed. As the ML value is proportional
to the initial viscosity of the sample [19], the increase of the
exposure time to microwaves reduced the viscosity of rubber
induced by the breaking of the three‑dimensional network
of the vulcanized GTR. The sample GTR3 presented the
highest ML value probably due to the formation of new
bonds in the rubber, since during sample exposure to

1
_CRA_ =
_t_ 90 - _ts_ 1

(1)

Figure 1. Torque _versus_ time curves of devulcanized rubber containing vulcanization additives. The curves were separated for better
visualization and analysis of results, and GTR0 without vulcanization additives was also analyzed for comparison.

Table 2. Revulcanization behavior of the GTRs devulcanized by microwaves.

Sample t90 (min) ts1 (min) ML (dN.m) MH (dN.m) MH-ML (dN.m) CRA (min –1 )

GTR0+ad 2.90 2.16 27.40 34.00 6.60 1.35
GTR1+ad 2.45 1.59 24.10 30.90 6.80 1.16
GTR2+ad 1.97 1.89 18.50 20.80 2.30 12.50
GTR3+ad 2.28 1.48 29.20 38.00 8.80 1.25
GTR4+ad 2.04 1.52 22.50 28.20 5.70 1.92
GTR5+ad 1.95 1.25 16.80 23.00 6.20 1.43
GTR2-2+ad 1.88 1.61 15.30 17.90 2.60 3.70
GTR2-2-2+ad 2.06 1.59 22.20 27.00 4.80 2.13
GTR3-3+ad 1.85 1.34 14.40 17.70 3.30 1.96

microwaves, bonds can be broken and created at the same
time [20] . Therefore, depending on the time at which the
sample was exposed to microwaves, the former event can
exceed the later one or vice-versa. On the other hand, the
sample GTR3-3 presented the lowest ML value obtained,
which may be due to degradation of the rubber by the high
exposure time of the sample to microwaves.

_3.2 Processing behavior of the blends_

According to Shahbikian et al. [21], the advantage of
using internal mixer to produce TPV is the possibility
of monitoring the effect of each component on the
torque/temperature evolution of the blend. Some researchers
use this advantage to examine different phenomena such
as dynamic vulcanization [21-29], as performed in this work.

During the mixing, just after the addition of the matrix
phase and as soon as the torque measured by the equipment
was stabilized, GTR (containing or not vulcanization additives)
was added into the mixer, what permitted the analysis of
the dynamic revulcanization behavior of the blends, which
is shown in Table 3. The MFinal values represent the torque
measured by the equipment at the end of the mixing process.

In general, t90 and ts1 values of the blends were
much smaller than the values of the neat rubber obtained
by using a rheometer (Table 2), which shows that the
dynamic revulcanization reaction occurred with higher
rate. Consequently, the CRA values of the blends were also
higher in comparison to the neat rubber, which confirms,
as just verified through the results of t90 and ts1, that the
dynamic revulcanization reaction occurred more quickly
in comparison to revulcanization (exception: GTR2+ad,
in which the highest reaction rate happened). It happened
possibly due to higher shear rates generated within the
internal mixer during processing.

However, the values of ML, MH and the subtraction of
MH-ML were not analyzed in this section and compared with
the ones of the neat rubber (section 3.1), since they also take
into consideration the viscosity of the HDPE phase (among
other factors) and it may lead to erroneous conclusions.

In general, but with some exceptions, it was verified a
trend towards the reduction in the MFinal values of blends
(concerning the final viscosity of blends) as the exposure
time of GTR to microwaves got higher, what demonstrates
that the GTR devulcanization increased the fluidity of this
phase. The cross-linking density and a possible degradation
of the thermoplastic phase may also have influenced the

MFinal. The fluidity of rubber facilitates the processing, its
dispersion into other polymers to form a polymer blend, as
well as the revulcanization reaction.

_3.3 Oscillatory rheometry_

The storage modulus (G’) and complex viscosity (η*) of
the blends 80GTR/20HDPE, as function of the frequency,
are summarized in the Figure 2.

In order to facilitate the analysis of the results, a table
of the G’ at the minimum and maximum frequencies of all
the blends was created (Table 4).

According to the Figure 2, the complex viscosity
decreased with the increase of the frequency, which clearly
shows the pseudoplastic behavior of the blends, assuming
the Cox Merz rule [30-35] . η* of the dynamically revulcanized
blends are higher than the blend 80GTR0/20HDPE due the
increase of the cross-linking density of the GTR phase [35] .

Being G’ proportional to the stored energy [36], this value
is proportional to the elasticity or, in other words, to the
cross-linking density of the elastomeric phase of the blend.
G’ is influenced also by morphology of the blends [28,37] .
The morphology refinement and compatibility tend to
increase the G’ values. According to SEM micrographs, no
conclusion about the morphology refinement of the blends
can be made, but the mechanical properties results alert
to the poor adhesion between the phases. The blends that
presented the lowest elongation at break results were the
same that presented the lowest G’ at minimum frequency
(80GTR4+ad/20HDPE, 80GTR5+ad/20HDPE and
80GTR3‑3+ad/20HDPE), which can have been result of
the poor adhesion, occurring a possible particle detachment
from the matrix when applied an external stress.

_3.4 Dynamic mechanical properties_

The temperature dependence of tan δ of the blends is
shown in the Figure 3.

According to the Figure 3, there are two transitions
related to the phases of the blends: the first one around
-30°C refers to the glass transition (Tg) of the GTR and the
other refers to α transition of the HDPE phase (Tα) around
100°C. The existence of two distinct transitions confirms the
immiscible character of the blends. It can also be observed
that there is a trend towards the reduction of the area under
the peak related to GTR transition, as well as the reduction

Table 3. Dynamic revulcanization behavior of the of the blends 80GTR/20HDPE.

Blend t90 (min) ts1 (min) CRA (min –1 ) MFinal (dN.m)

80GTR0/20HDPE 107.00
80GTR0+ad/20HDPE 1.17 0.75 2.38 108.00
80GTR1+ad/20HDPE 0.95 0.75 5.00 106.00
80GTR2+ad/20HDPE 0.65 0.48 5.88 111.00
80GTR3+ad/20HDPE 1.15 0.95 5.00 92.50
80GTR4+ad/20HDPE 1.13 0.72 2.44 93.90
80GTR5+ad/20HDPE 1.01 0.68 3.03 63.10
80GTR2-2+ad/20HDPE 0.80 0.45 2.86 103.00
80GTR2-2-2+ad/20HDPE 0.75 0.50 4.00 96.20
80GTR3-3+ad/20HDPE 1.05 0.68 2.70 54.30

of the height of the same peak, which is due to mobility
restriction generated by cross-linkings of this phase [32,38,39] .

Tg values of the devulcanized rubber and rubber phases
of the blends were obtained from the values of maximum
peaks of the curves tan δ _versus_ temperature. These values
are shown in the Figure 4.

According to the Figure 4, three zones of distinct Tg
behaviors can be determined. They were divided into
continuous, dotted and dashed line zones, which are
described below.

Table 4. G’ at the minimum (0.01 rad/s) and maximum (300 rad/s)
frequencies of the blends.

G’ (Pa) G’ (Pa)
Blend
at 0.01 rad/s at 300 rad/s

80GTR0/20HDPE 1.52x10 5 5.75x10 5

80GTR0+ad/20HDPE 2.16x10 5 5.52x10 5

80GTR1+ad/20HDPE 1.82x10 5 5.19x10 5

80GTR2+ad/20HDPE 1.76x10 5 4.95x10 5

80GTR3+ad/20HDPE 2.34x10 5 6.69x10 5

80GTR4+ad/20HDPE 1.98x10 5 5.82x10 5

80GTR5+ad/20HDPE 1.75x10 5 5.84x10 5

80GTR2-2+ad/20HDPE 2.27x10 5 5.87x10 5

80GTR2-2-2+ad/20HDPE 2.34x10 5 6.69x10 5

80GTR3-3+ad/20HDPE 2.22x10 5 6.73x10 5

Continuous line zone: the GTR was not exposed to
microwaves.

Dotted line zones: the final temperature of the GTRs after
the exposure time to microwaves probably was not enough
to provide high degree of devulcanization in the samples.
Due to the low degree of devulcanization, there was not a
significant change in Tg of the rubber, which behaved just
like a vulcanized one.

Dashed line zones: the final temperature of the GTRs after
the exposure time to microwaves was enough to generate high
degree of devulcanization in the sample. During processing
of the blends, due to the devulcanization degree reached
by the elastomeric phase of the samples, the rubber chains
acquired some mobility, demonstrated by the increase in the
Tg values. In other words, the devulcanization level of the
elastomeric phase influenced the dynamic revulcanization
reaction, changing the Tg value of this phase.

_3.5 Thermal properties by DSC_

The results of the DSC obtained from the second heating
cycle of the blends are shown in the Table 5. The crystallization
degree was calculated according to Equation 2 [40] :

χ _c_ = ∆ _Hm_ (∆ _Hm_ 100. _WHDPE_ ) .100

(2)

Figure 2. G’ and η* _versus_ frequency of the blends. The curves were separated for better visualization and analysis of results.

Figure 3. Tan δ _versus_ temperature of the blends 80GTR/20HDPE. The curves were separated for better visualization and analysis of
the results.

where _χc_ is the crystallization degree, Δ _Hm_ is the enthalpy
of melting (J/g), Δ _Hm100_ is the enthalpy of melting of the
HDPE 100% crystalline (293 J/g) [41] and _WHDPE_ is the mass
fraction of HDPE in blend.

The melting temperatures of the HDPE phase did not
present large variations in function of the exposure time
of the GRT to microwaves. However, the crystallization
degree of the HDPE phase was affected by the presence
of the rubber phase.

The blends that presented the highest crystallization
degree of the HDPE phase were the ones in which the GTR
phase was exposed to microwaves for longer periods of
time (with exception of the samples 80GTR0+ad/20HDPE
and 80GTR3+ad/20HDPE). This fact is probably due to

the more pronounced refinement of the morphology of
the blends with the increase of the exposure of the GTR
to microwaves, which could not be observed by SEM
micrographs. According to Utracki (p. 248) [42], “[...] the
finer the amorphous droplets are dispersed, the larger the
total interfacial contact surface, and thus the higher is the
possibility of nucleation at these interfaces.”

_3.6 SEM_

The morphologies of some blends are shown in the
Figure 5.

According to the SEM micrographs, it could be observed
that the blends containing GTR with longer exposure time to
microwaves presented a less coarse surface in comparison
to the other blends as a result of a lower fracture resistance
to the external force applied on the blends. This tendency
was also observed in the results of mechanical properties
(see section 3.7). Regarding the morphology refinement,
no conclusions can be made because the impossibility to
distinguish the phases from the presented SEM micrographs.
In the blend 80GTR3-3/20HDPE some voids can clearly
be observed (arrows in the Figure 5f) probably due to the
degradation of the rubber phase and poor degree of interfacial
adhesion between the phases of the blend.

_3.7 Mechanical properties_

The main results of the tensile tests of the blends are
presented in the Table 6.

On the whole but with some exceptions, the values of
stress at break and elongation at break reduced as the exposure
time of GTR to microwaves got higher, while the values of
Figure 4. Tg values of the devulcanized rubber and rubber phases Young’s modulus presented an opposite behavior. The tensile
of the blends as determined by DMA.

Figure 5. SEM micrographs of the blends: (a) 80GTR0/20HDPE, (b) 80GTR0+ad/20HDPE, (c) 80GTR3+ad/20HDPE, (d)
80GTR4+ad/20HDPE, (e) 80GTR2-2+ad/20HDPE, (f) 80GTR3-3+ad/20HDPE.

Table 5. Values of melting temperature, enthalpy of melting (ΔHm) and crystallization degree (χc) of the HDPE phase of the blends.

Sample Tm ΔHm χC Sample Tm ΔHm χC
( o C) (J/g) (%) ( o C) (J/g) (%)

HDPE 141.74 183.72 62.70
80GTR0/20HDPE 134.86 52.84 90.17 80GTR4+ad/20HDPE 133.81 39.02 66.59
80GTR0+ad/20HDPE 132.12 33.57 57.29 80GTR5+ad/20HDPE 135.79 52.03 88.79
80GTR1+ad/20HDPE 133.93 41.38 70.61 80GTR2-2+ad/20HDPE 133.46 51.75 88.30
80GTR2+ad/20HDPE 133.53 40.43 68.99 80GTR2-2-2+ad/20HDPE 136.65 54.56 93.10
80GTR3+ad/20HDPE 133.14 51.29 87.52 80GTR3-3+ad/20HDPE 133.77 46.66 79.63

Sample Tm
( o C)

ΔHm
(J/g)

χC Sample Tm
(%) ( o C)

ΔHm
(J/g)

Table 6. Mechanical properties of the blends.

Young’s modulus
Blend

(MPa)

Stress at break

(MPa)

Tensile strength

(MPa)

Elongation at break

Blend

(MPa) (MPa) (MPa) (mm/mm)

80GTR0/20HDPE 36.02±7.97 60.61±5.45 2.72±0.24 0.40±0.06
80GTR0+ad/20HDPE 33.33±1.99 92.93±7.30 4.30±0.34 0.59±0.10
80GTR1+ad/20HDPE 27.98±0.67 54.87±8.36 3.01±0.46 0.25±0.06
80GTR2+ad/20HDPE 30.60±1.98 80.59±6.56 3.67±0.34 0.53±0.10
80GTR3+ad/20HDPE 33.94±3.95 79.98±5.02 3.73±0.19 0.40±0.06
80GTR4+ad/20HDPE 37.30±3.54 54.48±8.35 2.99±0.46 0.15±0.05
80GTR5+ad/20HDPE 39.49±1.33 64.36±15.95 3.53±0.87 0.16±0.05
80GTR2-2+ad/20HDPE 25.01±0.86 79.25±2.91 4.26±0.94 0.47±0.06
80GTR2-2-2+ad/20HDPE 48.70±2.81 53.63±10.62 2.94±0.58 0.19±0.05
80GTR3-3+ad/20HDPE 57.02±3.37 40.67±4.70 2.23±0.18 0.09±0.01

(MPa)

strength of the blends did not vary significantly with the
increase of the exposure time of the GTR to microwaves.

According to Prut et al. [36], Young’s modulus depends
on the crystallinity development during the quenching.
With the increase of the crystallinity, the matrix became
tougher, which led to the increase of the Young’s modulus.
The increase of the crystallinity of the matrix could also
generate less perfect crystals, which also may have resulted
into an increase of the Young’s modulus values.

Another observation is that, in general, the results obtained
were not so good, especially the results of elongation at
break, for blends which are supposed to be TPVs. According
to Hong and Isayev [19], adhesion between the GRT and
polymer matrix is one of the major factors controlling the
mechanical properties of such blends. Also, according to
some authors [43,44], deterioration on the elongation at break
is due to the poor interfacial adhesion between the phases.
According to the presented results, adhesion between the
phases was not sufficient to promote good stress transference.

The blends containing GTR0 and vulcanization additives
presented higher mechanical properties in comparison to the
same ones without additives. These results showed that the
dynamic revulcanization improves the mechanical properties,
as also observed by other authors [39,45-51] .

Luo and Isayev [46] studied the properties of the blends
polypropylene (PP)/GTR devulcanized by ultrasound using
different curing systems and processing routes, and all
the blends presented low elongation at break. Some other
authors [19,52] also achieved the same results. The reason for
this behavior is the large size of rubber particles and the
premature curing of this phase when the curatives are poorly
distributed in the rubber [46] . Also, according to Antunes et al.

[22], when the dynamic vulcanization happens, the curatives are
not well distributed by the batch mixer, resulting in different

levels of cross-linkings. In the present work, this problem
was avoided, since the vulcanization agents were previously
added in the rubber phase and mixed by using a two roll
mill. So, the reason for the poor mechanical properties is
probably the lack of adhesion between the phases.

One of the qualifying standards for a blend to be deemed
as a TPV is to present typical elastomeric elongation, which
has also not been verified in the obtained results. However,
these blends have high concentrations of GTR phase (80% in
mass), a recycled material, what may have deteriorated
the mechanical properties. Grigoryeva et al. [53] produced
dynamically vulcanized blends using GTR, and in some of
the production methods used by the authors, TPVs were also
not obtained. According to the authors, in these cases there
was not an effective interfacial stress transference between
the phases, and not an entanglement of the GTR rubber
chains into the surrounding matrix. These facts could also
have happened in this work. Therefore, additional studies
must be performed, taking into consideration the use of
compatibilizer additives or a way to improve the interfacial
characteristics of the blends like using nanofillers.

4. Conclusions

The dynamically revulcanized blends based on GTR
devulcanized by microwaves (under different exposure
times) and HDPE were analyzed by different techniques.
According to the torque development during the mixing
process, dynamic revulcanization was faster than the
revulcanization of the neat rubber, due to the high shear rates
generated during the processing. The oscillatory rheometry
results showed that the lack of adhesion between the phases
influenced the rheological properties of the blends, which
resulted into poor mechanical properties, especially in the
blends containing GTR exposed to microwaves for longer

exposure times. The dynamic mechanical properties showed
that there were differences in the Tg values of the elastomeric
phase, depending on the exposure time to microwaves.
However, no conclusion about the morphology refinement
of the blends can be made based on the SEM micrographs.

Summarizing, devulcanization process of GTR can change
completely the final properties of the revulcanized blends
80GTR/20HDPE, since it changes the fluidity of rubber
during processing. The process parameters like exposure
time can be analyzed based on the final properties of these
blends. Devulcanization by microwaves can be a strong
alternative to solve the problem of disposal of waste rubber.

5. Acknowledgements

The authors would like to thank Braskem and Pirelli
for the material donation, Departamento de Engenharia de
Materiais - DEMa, Universidade Federal de São Carlos
for the laboratory facilities, FAPESP (process number
2010/15799-6) and CNPq (process number 201891/2011-5)
for the financial support.

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_Received: Apr. 14, 2014_

_Revised: Oct. 07, 2014_
_Accepted: Nov. 11, 2014_

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