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The Use of Glycolysis products of Poly (ethylene
terphthalate) waste as a Compatibilzer for SBR /
EPDM Blend. I- In Presence of Carbon Black
and Paraffinic Oil
S.F. Halim, S. H. El-Sabbagh, M. E. Tawfik
Abstract— Recycling of the polyethylene terphthalate (PET) waste does not only serve as a partial solution to the solid
waste but also contributes to the conservation of raw materials and energy. In our study, depolymerization process of post
consumer PET beverage bottles was carried out by glycolysis using cis-2-buten-1, 4-diol (2BD). The PET glycolysis
products (GP) were incorporated as a compatibilizer in 50/50 styrene butadiene rubber/ethylene propylene diene monomer
(SBR/EPDM) blends in different concentrations. The feasibility of the use GP as a compatibilizer was assessed by
calculating the heat of mixing, strain energy of the SBR/EPDM compounds. Swelling measurements were carried out to
evaluate the compatibilizer-rubber blend interaction and to calculate the molecular weight between two crosslink’s and
crosslink density. Differential scanning calorimetry (DSC) experiments were used to examine the compatibilizer efficiency
by detecting the glass transition temperature (Tg) of the SBR/EPDM compounds. Scanning electron microscope (SEM)
was used to study the effect of the compatibilizer on the morphology rubber blends. Moreover the effect of high abrasion
carbon black (HAF) and paraffinic oil on the compatibilizer efficiency were also studied.
Index Terms— compatibilizer, crosslink density, glycolysis, heat of mixing, mechanical properties, polyethylene
terphthalate waste, rubber blend, strain energy.
I. INTRODUCTION
Poly (ethylene terphthalate), more commonly known as PET in the packaging industry and generally referred to as
„polyester„in the textile industry. It is considered one of the easiest materials to recycle, and is second only to
aluminum in terms of the scrap values for recycled materials. Although PET does not create a direct hazard to the
environment, it has substantial fraction by volume in the waste stream and its high resistance to the atmospheric and
biological agents [1]. Thus, PET recycling has been one of the most successful and widespread among polymer
recycling [2], [3]. The recycling PET can be carried out in many ways. Four main approaches have been proposed
which are primary or „in-plant‟, secondary or mechanical, tertiary or chemical, quaternary involving energy
recovery [4],[5]. Chemical-recycling processes for PET are divided as follows: (i) hydrolysis, (ii) glycolysis, (iii)
methanolysis and (iv) other processes such as aminolysis and ammonolysis. The main depolymerization processes
that have reached commercial maturity are glycolysis and methanolysis. [1], [3]. In literature, glycolysis reactions
involved the insertion of ethylene glycol, diethylene glycol, propylene glycol, (1, 4)-butane diol, (1, 2)-butane diol,
cis -2-buten-1,4-diol, or mixtures of diols [6]-[11]. The main disadvantage is that the glycolosis reaction products
are not discrete chemicals but come along with higher oligomers, which are difficult to purify with conventional
techniques. However, the end products produced from the glycolysis of PET waste were used as a new raw
material for preparing other polymers such as epoxy resin, polyurethane, alkyd resins used as enamel paints or
coatings, unsaturated polyester used for the production of polymer composite, and water extended polyester for
manufacture of art figures [9], [12]- [14].
Polymer blends have always been considered to be interesting combinations for obtaining new high-performance
polymeric materials without synthesizing fully new polymers especially when the properties of two or more
polymers are synergistically combined. Unfortunately, most polymers are immiscible. Hence, their blends show
poor mechanical properties and unstable morphology. These problems can be overcome by using various kinds of
compatibilizers. [15]
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Ethylene propylene diene terpolymer (EPDM) is one of the most widely used and fastest growing synthetic
rubbers. It has general purpose applications, which usually imparts good aging characteristics, good weathering
oxidation, little moisture adsorption, good electrical properties and chemical resistance. Styrene–butadiene rubber
(SBR) has good mechanical properties; it does not break down easily. Also, it has good abrasion resistance, greater
ozone and weathering resistance, but its oil resistance is very poor [16]. EPDM like SBR can be vulcanized with
sulfur cure systems. However, the differences in the solubility of sulfur and the level of unsaturation in both
elastomers result in incompatibility in blends of these elastomers [17].
The goal of our present study is to use the recycling process of PET wastes; not only as a partial solution to the
solid-waste problem but also to conserve raw materials and energy. This will be achieved by depolymerization of
post-consumer PET by glycolysis. The reaction products (GP) will be used as a compatibilzer for EPDM/SBR
blends. The compatibilizing effect of these compounds will be evaluated via various measurements and techniques.
II.
METHODOLOGY
A. Materials
Post-consumer soft-drink clear bottles were used as a source for PET. The caps and bottom parts as well as the
labels and the adhesives had been removed. The bottles were cut into flakes at a maximum size 2-3 mm, cleaned
thoroughly by washing with water and soap and then with distilled water. The clean flakes of PET wastes were
dried at 80 °C to constant weight.
Chemicals used for glycolysis are: cis-2-buten-1, 4-diol, (2-BD) a liquid with a boiling point of 234 ºC and a
density of 1.07 g/cm3 from Aldrich; zinc acetate from Merck was candidate as catalyst for the proposed PET
recycling process.
Styrene butadiene rubber (SBR 1502) with a specific gravity of 0.945, styrene content of 23.5% and Mooney
viscosity ML (1 + 4) at 100°C is 48–58. Ethylene-propylene–diene monomer (EPDM) with density 0.86 g/cm3,
ethylene weight content of 70%, an unsaturated ratio NB/100, 8 and 22% propylene and Mooney viscosity ML (1
+ 4) at 100°C is 85. SBR and EPDM were purchased from Transportation & Engineering Company (TRENCO) –
Alexandria, Egypt.
Chemicals used for rubber compounding: stearic acid with specific gravity 0.9–0.97 is 15°C; zinc oxide (ZnO) with
specific gravity = 5.55–5.61; elemental sulfur (S) with specific gravity = 2.04–2.06; N-cyclohexyl-2-benzothiazole
sulphenamide (CBS) with specific gravity = 1.27–1.31, melting range = 95–100 °C; all these chemicals are Aldrich
company products and used as received without further purification. High abrasion furnace carbon black (HAF)
N-330 and paraffin oil were purchased from Transportation & Engineering Company (TRENCO) – Alexandria,
Egypt.
B. Experimental techniques
Glycolysis of PET waste: predetermined weight of waste flakes were added in the reactor to cis-2-buten-1,4-diol
in a molar ratio 1: 2.185 (PET: 2-BD). Zinc acetate was added as transesterification catalyst. The reactor was
immersed in an oil bath and heated for 5 h at 230 ºC. The reaction was carried out under reflux in nitrogen
atmosphere. After the 5 h period, the reactor was allowed to cool to room temperature and the reaction product was
filtered. . The glycolysated PET products (GP) were previously examined in our laboratory [18]. It consists mainly
of unsaturated hydroxyl-terminated oligomers with average molecular weight of 628 g/mol and polydispersity of
1.22 and could be represented by the following structures:
4, 4’-di (2, 3-butenhydroxy) terephthalate monomer (GP)
Preparation of the rubber compounds: The compounding of rubber blends were carried out according to ASTM
–D3182-07 on a two-roll mill. The GP was incorporated in SBR/EPDM, at different loadings, in absence and
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presence of paraffinic oil and carbon black (HAF). The compounds formulations are shown in Table 1. The other
additives of the recipe were added in parts per hundred (phr) as follows: 5 phr zinc oxide, 1.5 phr stearic acid, 0.8
phr CBS, and 2 phr sulphur. An oscillating disc rheometer (MDR 2000), supplied by Alpha Technologies, UK,
was used for measuring the optimum cure time of the different mixes at 150 ◦C. An electrical press was used for
vulcanization of the mixes, at 4 MPa and 1500C, each according to the corresponding cure time obtained from the
rheometer.
Table 1: Rubber formulations of the SBR/EPDM blend compounds.
Ingredients, phr*
K0
K1
K2
K3
K4
K5
K6
50
50
50
50
50
50
50
50
50
50
50
50
50
50
-
5
-
5
7.5
10
15
-
-
5
5
5
5
5
-
-
20
20
20
20
20
SBR
EPDM
PET
Paraffinic oil
HAF
*
phr: part per hundred.
Determination of the heat of mixing: Schneier [19] stated that the heat of mixing may be an approximate measure
of the polymer-polymer compatibility. He proposed the following equation for calculating the heat of mixing for a
two component blend system.
ΔHm
(1)
Where X, ρ, M and δ are the weight fraction of polymer, polymer density, the molecular weight of monomer unit
and solubility parameter of the polymer respectively.
The mechanical properties Tensile testing machine Zwick Z010, Germany was used to determine the tensile
strength (T.S), modulus at 100 % strain ( M@100%) and elongation at break (E%) according to ASTM D
412-06a.
Determination of the strain – energy. It had been stated before that the energy absorbed per unit volume (W) is
expected to increase in deforming rubber blends; thus the energy absorbed can be written as [20],[21]:
(2)
where 𝝈 is stress as a function of the strain 𝜺. Strain-energy values (energy absorbed per unit volume) were
obtained for all samples by integrating the area under the stress-strain curve obtained from the tensile testing
machine.
Swelling measurements. The interaction between rubber and GP has been investigated using a swelling
technique. The equilibrium swelling of the blend vulcanizates was carried out according to ASTM D 471-10.
Assuming that swelling is completely restricted at the rubber- compatibilzer interface, the equation of Lorenz and
Parks [22] has been applied to study the rubber- compatibilzer interaction.
(3)
Where Q is defined as grams of solvent per gram of rubber hydrocarbon, c and g refer to compatibilzer and gum
mixes. Z is the weight fraction of compatibilzer in the vulcanizates, a and b are characteristic constants of the
system. Q was calculated by:
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(4)
The swelling data were also utilized to determine the molecular weight between two crosslink‟s (Mc), by using
Flory-Rehner relation equation [23]:
(5)
where Mc is the average molecular weight between two successive crosslink‟s, ρ is the density of rubber, V0 is the
molar volume of swelling solvent absorbed (toluene V0=106.3cc/mol), and VR is the volume fraction of the swollen
rubber that can be obtained from the mass and densities of rubber samples and the solvent, µ is the interaction
parameter between the rubber and toluene which is 0.446 for SBR and 0.49 for EPDM [24]. The cross linking
density υ can be calculated from the equation:
(6)
Differential scanning Calorimetric measurements (DSC). A Netzsch STA 409C/CD (Boston, USA) was used.
The experiments were carried out under nitrogen atmosphere and operated at a heating rate 10˚C /min within the
temperature range from -100 to 200˚C.
Scanning electron microscopy (SEM). SEM was done using (JXA- 840 A) electron probe micro-analyzer Japan
Jeol. Rubber specimen was broken in liquid nitrogen; cross section surface was covered with a very thin layer of
gold to avoid electrostatic charging during examination.
III. RESULTS AND DISCUSSION
A. Heat of mixing
Heat of mixing can be taken as approximate measure of the free energy of mixing and may be used as an indicator
of possible compatibility [25]. Singh et al [26] calculated the heat of mixing of some polymer blends using the
Schneier equation, eq.1, and reported that the heat of mixing with values below a limiting value of 41.85 x 10 -3
J/mol indicates compatible blends while the incompatible blends have mostly higher values.
Fig 1: Heat of mixing of rubber blends: a) SBR/EPDM blends; b) SBR/EPDM blends in presence of 5phr GP.
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In order to study the effect of GP as a compatibilzer between SBR/EPDM, the heat of mixing for the SBR/EPDM
blends over the entire range of weight percent composition were calculated using eq.2. Heat of mixing of
SBR/EPDM blend in absence of GP is shown in Fig.1a. The heat of mixing of SBR and EPDM, over the entire
range of compositions, is higher than the limiting value of compatibility indicating the incompatibility of
SBR/EPDM blend. On the other hand, the heat of mixing of SBR and EPDM in presence of 5 phr GP, Fig. 1b, is
below the limiting value of compatibility indicating a compatible blend. That means that GB increases the
miscibility between the two rubbers.
B. Mechanical properties and strain energy
When a compatibilizer is added to a binary blend, it concentrates at the interface and induces chemical or physical
interactions. These interactions reduce the interfacial tension between the rubber blend phases leading to formation
of smaller dispersed phases. The size of dispersed phases decrease as the concentration of compatibilizer increases
until the interface between the two phases is saturated. At this point an equilibrium particle size is observed.
However, as the concentration of compatibilizer reaches its saturation point it can no longer act as an interfacial
agent [27] - [29]. The mechanical properties, shown in Fig. 2, can be used to assess the compatibilizing action of
GP for 50/50 SBR/EPDM blends. Comparing the mechanical properties of the K2 to that of K3-K6, containing 5,
7.5, 10 and 15phr GP, it is clear that the increase in the GP loadings in the rubber blends leads to an increase in the
tensile strength (T.S) and modulus at 100% elongation (M@100%) properties. The increase in the T.S values may
be attributed to the improvement of the interfacial bonding between SBR and EPDM rubbers in presence of GP.
This in turns leads to an increase in the crosslink density in the SBR/EPDM rubber matrix which is ensured by the
high M@100% values for SBR/GP/SBR compounds. Moreover, it is noticed from Fig. 2 the decrease in the
elongation at break (E %) values of K3-K6. This again can be referred to the increase in the interfacial bonding
between SBR and EPDM rubbers and consequently increase in the crosslink density which restricts the
macromolecules stretching in the rubber matrix.
Fig 2: The tensile strength, modulus at 100% elongation and elongation at break,%, of the different SBR/EPDM
compounds.
Another evidence for the acting role of GP as a compatibilzer for SBR/EPDM blend can be obtained by studying
the strain energy of the rubber blends. According to eq. 2, the energy absorbed per unit volume (W) is expected to
increase in deforming rubber blends. Therefore the higher the area under stress-strain curve, the higher the energy
absorption capacity. It is clear from Fig. 3 that the strain energy of the SBR/EPDM blends containing different
concentrations of GP, K3-K6, is higher than that of without GP, K0 and K2. This indicates that the compatibility of
SBR/EPDM blends has been improved in presence of GP especially at 7.5 and 10 phr loadings.
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Moreover, from Fig.s 1 and 2 it is worthy to note that the addition of HAF carbon black and paraffinic oil enhances
the role of GP as a compatibilzer for SBR/EPDM blends. This can be observed on comparing the mechanical
properties and strain energy of the sample control sample K0 to that of K1, K2 and K3 containing 5phr GP, HAF/
paraffinic oil and GP/ HAF/ oil respectively.
Fig 3: Strain energy values (MJ/m3) of the different SBR/EPDM compounds.
C. Swelling measurements
Fig 4: The swelling parameters of the different SBR/EPDM compounds.
The swelling measurements were used to determine the values of (1/Q), (Qc/Qg), molecular weight between
cross-links (Mc) and crosslink density ( ) for each composite. The effect of interaction between SBR/EPDM and
the compatibilzer can be determined by the value (1/Q) as previously shown by Kranbuehl et al. [30]. Higher
values of (Qc/Qg) lead to lower interaction between the compatibilizer and the rubber matrix. Comparing the 1/Q
and Qc/Qg values of K1 and K3, Fig. 4, it is obvious that the presence of HAF and paraffinic oil in K3
(SBR/EPDM/5phr GP) tremendously increase the rubber-compatibilzer interaction and leads to an increase of the
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crosslink density to nearly four times that of K1 (SBR/EPDM/GP) as shown in Fig. 5. This may be due to that the
presence of the paraffinic oil and HAF facilitates the role of the compatibilzer in the formation of smaller dispersed
rubber phases due the decrease of the interfacial tension between them. Meanwhile, as the GP content increase the
(1/Q) values increase and the (Qc/Qg) decreases indicating stronger rubber compatibilzer interaction; in other
words lead to formation of more crosslink‟s.
Also, Fig. 5 illustrates the decrease of Mc values and the increase of cross-linking density υ as the GP content
increases in the compound. This is new evidence to the formation of additional physical and chemical crosslink‟s as
the compatibilizer acts as a bridge to bond the SBR/EPDM rubber chains within the matrix [31]. Thus, we can say
that the GP can act as a compatibilizer for SBR/EPDM blends.
Fig 5: The Molecular weight between cross-links (Mc) and crosslink density (υ) of the different SBR/EPDM
compounds.
D. Differential scanning calorimetry studies (DSC)
Fig 6: DSC graphs of SBR/EPDM blends: K0 (control sample); K1 (EPDM/SBR/5phr GP); K3 (EPDM/SBR/5phr GP/
HAF/ Oil; K7 (EPDM/SBR/15phr GP/ HAF/ Oil).
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DSC was used as an additional evidence to support proving the former assumptions. The thermal behavior of
SBR/EPDM rubber blends with and without the compatibilizer was analyzed by DSC. Figure 6 shows the DSC
traces of the samples. The glass transition temperature (Tg) was recorded as the half height of the corresponding
heat capacity jump. It can clearly be seen that, for SBR/EPDM rubber blends without the compatibilzer K0 there
are two distinct glass transitions, the lower glass transition is due to the SBR rubber phase (- 53.48ºC) while the
higher is due to EPDM rubber phase (- 37ºC). On adding 5phr GP to SBR/EPDM blend (K1) and
SBR/EPDM/HAF/oil (K3) only one Tg appears at -30.6 & -40ºC respectively. These data illustrate that Tg of
SBR/EPDM rubber blends become one glass transition upon incorporation of the GP. This indicates that GP
improved and increased the miscibility between SBR and EPDM rubber due to the reduction of interfacial energy
and the increase of interaction between rubber phases; which takes place by segmented interaction rather than
polymeric interaction as previously stated [32] - [35]. At higher compatibilizer content, 15 phr, the Tg of the
sample K7 is shifted to the large temperature (-9.5ºC). These findings are in good agreement with Al Malaika et al.
who reported that compatibility of polymers causes shift of their glass transition temperatures [36].
E. Scanning electron microscopy
Fig 7: SEM graphs of different SBR/EPDM blends: K0 (control sample); K1 (EPDM/SBR/5phr GP); K3
(EPDM/SBR/5phr GP/ HAF/ Oil); K7 (EPDM/SBR/15phr GP/ HAF/ Oil).
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SEM has been used by to study the effect of the compatibilzer on the morphology of polymer blends. It is easy to
see the coarse agglomerates which are most probably the SBR domains separated from the continuous phase of
EPDM in absence of the compatibilizer for the control sample K0, Fig. 7 a. By introducing 5phr compatibilizer,
Fig. 7 b, one can see better phase dispersion; the agglomerates size becomes smaller and embedded to some extent
in the EPDM continuous phase. Figure 7 c shows a remarkable reduction in the domain size for the sample K3
containing (5phr GP/HAF/paraffinic oil). Fig.7d showed a uniform dispersion and distribution of the dispersed
phase in presence of 15phr GP/ carbon black / paraffinic oil as evidenced from Fig. 7d. In other words, the addition
of the compatibilizer has improved the SBR/EPDM blend morphology and consequently the compatibility of the
blends due to the reduction of the interfacial tension between SBR and EPDM.
ACKNOWLEDGMENT
The authors express their cordial thanks to the Administration of the National Research Center for providing funding
and facilities to carry out this work. Also, the authors thank Polymer Metrology and Technology Department, National
Institute of Standards, NIS, for their cooperation.
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