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FUEL PROCESSING TECHNOLOGY
ELSEVIER Fuel Processing Technology 50 ( 1997) 87- 103
Characterization of naphtha and carbon black obtained by vacuum pyrolysis of polyisoprene
rubber
Christian Roy ap * , Hans Darmstadt a, Belhocine Benallal b, Carlos Amen-Chen b
a Dbpartement de Ghie Chimique, Universire' Lava1 Cite’ Universitaire, Qugbec, GIK 7P4 Canada b Instirut Pyrovac Inc., 1560 Ave. Du Part-Beauvoir Sillery, Qukbec, GIT 2M4 Canada
Received 13 December 1995; accepted 17 April 1996
Abstract
Pure polyisoprene and a commercial rubber sample containing 52% polyisoprene and 3 1%
carbon black were pyrolysed at 500°C and at a total pressure varying between 0.8 and 28.0 kPa. The yields of gas, oil and pyrolytic carbon black (CB,) changed little with the pyrolysis pressure. However, the oil composition and the CB, characteristics depended considerably on the pyrolysis pressure. For example, the amount of dl-limonene, a valuable compound in the naphtha fraction, decreased with increasing pyrolysis pressure. The CB, and the commercial carbon black initially present in the rubber sample were analysed by ESCA, SIMS and SEM. With decreasing pyrolysis pressure the surface chemistry of the CB, became similar to that of the commercial carbon black initially present in the rubber. Therefore, rubber pyrolysis should be performed at low pressures in order to obtain products with a higher commercial value.
Keywords: Carbon black; Limonene; Oil; Polyisoprene; Rubber; Tire; Vacuum pyrolysis
1. Introduction
Large amounts of waste rubber are produced every year. The most important source for rubber waste is scrap tires. A large fraction of the scrap tires is simply dumped in sites where they represent a hazard. Vacuum pyrolysis has the potential in an environ-
* Corresponding author. Fax: 1 418 656 2091; e-mail: [email protected].
0378-3820/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PII SO378-3820(96)01044-2
88 C. Roy er al/Fuel Processing Technology 50 (1997) 87-103
mentally friendly way to transform waste rubber to useful products. However, the success of this approach depends on the commercial value of the pyrolysis products. Rubber pyrolysis can be performed at atmospheric or reduced pressure. Atmospheric pyrolysis has been widely studied by different authors [l-8]. In this laboratory, the pyrolysis of tires and other carbonaceous materials is performed under vacuum. The tire pyrolysis process [9,10], the composition and potential commercial use of the pyrolytic oil [ 1 l-151 and the characteristics of the pyrolytic carbon black (CB,) [ 16-231 have been investigated. The tire vacuum pyrolysis process is a proprietary technology [24].
Polyisoprene is an important tire compound, especially in truck tires [25]. dl-Limonene (cyclic dimer of isoprene, C,,H,,) is a valuable compound present in oil from the vacuum pyrolysis of used tires, and is believed to be formed from polyisoprene decomposition products [ 111. Limonene was also found in small quantities in oil from tire pyrolysis conducted under atmospheric pressure conditions [ 11. However, vacuum pyrolysis has several advantages over atmospheric pyrolysis. For decomposition of elastomers at atmospheric pressure, higher temperatures are necessary than under vacuum. Furthermore, the residence time of the organic vapours formed by rubber decomposition is shorter during vacuum pyrolysis compared with atmospheric pyrolysis. The mild reaction conditions during vacuum pyrolysis limit secondary reactions. Hence, the formation of undesired products such as carbonaceous deposits on the CB, and the destruction of desirable intermediate products, such as dl-limonene, are minimized.
In this paper, product analysis of the vacuum pyrolysis of polyisoprene and polyiso- prene rubber is reported. The objective of this work is to study the potential of polyisoprene rubber pyrolysis products to replace commercial products.
2. Experimental
2.1. Feedstock and vacuum pyrolysis
The feedstock consisted of natural polyisoprene (grade ‘ITR-20, Goodyear, Quebec City) and of a commercial rubber sample (TM 17343, Goodyear), containing 52.2 wt% polyisoprene (TTR-20), 31.0 wt% carbon black (N330), 5.4 wt% styrene-butadiene rubber (60% styrene), 4.4 wt% processing oil, 1.6 wt% ZnO, 1.1 wt% stearic acid, 1.1 wt% sulphur and 3.2 wt% of other compounds. The pyrolysis was performed under vacuum in a batch reactor (V = 15000 cm3). Feedstock particles with an average volume of 2 cm3 were heated at a rate of 15°C min- ’ from 25°C to a maximum temperature of 500°C. The total mass of rubber particles heated was 1 kg for each run. The gas atmosphere in the reactor consisted of the pyrolysis gases, and the total pressures were 0.8 kPa (run G42) and 28.0 kPa (run G45) for the two polyisoprene pyrolysis experiments and 0.8 kPa (run G47) and 6.4 kPa (run G58) for the polyisoprene rubber pyrolysis experiments. Gases were removed from the reactor with a vacuum pump. The pyrolytic oils were collected in three consecutive traps, placed between the reactor and the vacuum pump, which were cooled to - 15°C -78°C and -78°C respectively.
C. Roy et al./ Fuel Processing Technology 50 (1997) 87-103 89
2.2. Distillation and oil characterization
The combined pyrolytic oils from the last two traps were distilled under atmospheric pressure up to a temperature of 204°C in order to separate the naphtha fraction from the pyrolytic oil, following the ASTM D 86 procedure. The naphtha fraction was analysed by GC-MS. A detailed description of the GC-MS method can be found elsewhere [ 141.
2.3. Carbon black characterization
The nitrogen adsorption measurements (BET) were performed with an OMNISORP 100 apparatus from OMICRON. Before the adsorption experiment the samples were outgassed at 300°C in vacuum until the pressure was lower than 5 X 10e3 Pa. ESCA and Auger experiments were performed with an ESCALAB MK II spectrometer using non-monochromatized Mg X-ray radiation. The static SIMS spectra were recorded with the same spectrometer using Ar+ ions with an ion dose smaller than lOI ions cmm2. More details on the surface analysis methods can be found elsewhere [ 16,20,21]. The X-ray diffractograms were taken with a PW 1050 goniometer equipped with a PW 1011 generator from Philips. The CB, samples from the polyisoprene rubber pyrolysis were compared by using ESCA and SIMS with CB, samples obtained by tire pyrolysis. The tire pyrolysis was performed at 500°C and at 20 kPa and 100 kPa (atmospheric pressure). The production of the CB, sample from atmospheric tire pyrolysis has been described by Ledford [8]. Details of the production and the characterization of the CB, sample from vacuum tire pyrolysis have been reported by Darmstadt et al. [20].
3. Results and discussion
3.1. Product yields upon polyisoprene pyrolysis
Product yields from the pyrolysis of pure polyisoprene and polyisoprene rubber are given in Table 1. During pyrolysis the polyisoprene feedstock is decomposed to small hydrocarbons. These hydrocarbons may repolymerize to larger molecules, ultimately yielding solid carbonaceous residues on the CB,. Following cracking reactions, small uncondensable hydrocarbons may be formed, which remain in the gas phase. Such consecutive reactions are favoured by long residence times of the pyrolysis products in the reactor. The residence time decreases with decreasing pyrolysis pressure. Therefore consecutive reactions are limited by low pyrolysis pressure. This is confirmed by the pressure dependence of the pyrolysis yields. For pyrolysis of pure polyisoprene the yields of gas and solid residue, both products of consecutive reactions, increased when the pyrolysis pressure was raised from 0.8 to 28.0 kPa, whereas the oil yield decreased (Table 1). The amount of the naphtha fraction from the pyrolytic oil remained constant. However, the composition of the naphtha fraction changed considerably. In particular, the yield of the valuable dl-limonene decreased with increasing pyrolysis pressure. Therefore, in a next step of the investigation, the pyrolysis of a polyisoprene rubber at two low pressures was studied.
90 C. Roy et d/Fuel Processing Technology 50 (1997) 87-103
Table 1 Vacuum pyrolysis of polyisoprene rubber. Influence of the pressure on product yields
Feedstock
Pyrolysis pressure
Pyrolysis yields/wt% Solid residue Gas Pyrolytic oil
Pure polyisoprene Polyisoprene rubber
0.8 kPa 28.0 kPa 0.8 kPa 6.4 %Pa
0.1 3.8 36.8 35.1 2.6 5.9 1.0 3.2
97.3 90.3 62.2 61.7
100.0 100.0 100.0 100.0
Naphtha traction 30.3 30.7 25.7 21.4
Limonene yield/wt% based on: Total feedstock 16.6 11.9 5.0 4.2 Naphtha fraction 54.6 38.8 31.4 31.2
3.2. Product yields of the polyisoprene rubber pyrolysis
As for pure polyisoprene, the pyrolysis of polyisoprene rubber yields more gas and less pyrolytic oil with increasing pyrolysis pressure (Table 1). The solid residue consists of recovered carbon black filler and inorganic rubber components. Additionally, car- bonaceous deposits may be present on the recovered filler and the inorganic compo- nents. Carbon black filler, zinc oxide and sulphur represent approximately 34 wt% of the rubber feedstock. The yields of solid residue obtained by polyisoprene rubber pyrolysis at 0.8 and 6.4 kPa were 36.8 and 35.1 wt%, respectively. The slight difference between the expected and the experimental value for the yield of solid residue indicates the formation of small amounts of carbonaceous deposits on the recovered CB,. More accurate ESCA Cls measurements of the amount of carbonaceous deposits confirmed this assumption (see below).
3.3. GC-MS analysis of the naphtha fraction from pyrolysis of polyisoprene
The pyrolysis of polyisoprene has been investigated by different authors [26-281. It was observed that the monomer and the dimer (dZ-limonene) were the most important products. The concentration of other products, mostly olefins and to a small extent aromatics, increases with increasing pyrolysis temperature. In the proposed mechanism, dl-limonene is formed by a Diels-Alder reaction from isoprene; other products are formed by reactions of dl-limonene and isoprene in consecutive reactions. In contrast to some studies where mg or even Fg samples were used [26-281, this work was performed with 1 kg of polyisoprene feedstock. Therefore, mass transfer limitations were much more important in this study. Consequently, in this work consecutive reactions were favoured and more high-molecular-weight products were formed. How- ever, by reducing the pyrolysis pressure, consecutive reactions can be limited. In the naphtha fraction the most important compound was dl-limonene. The GC-MS spectra
C. Roy et al./Fuel Processing Technology 50 (1997) 87-103 91
0 5 10 15 20 25 30
Retention time [min]
Fig. 1. CC-MS spectra of the naphtha fraction from pyrolysis of polyisoprene and polyisoprene robber (PR) at 500°C.
showed that in the naphtha fraction, in addition to dl-limonene, a wide variety of other compounds was formed (Fig. I). At both pressures the same compounds were formed, but in different quantities. With increasing pyrolysis pressure the concentration of aromatic compounds [e.g. methylbenzene (peak A) and 1,2_dimethylbenzene (peak B)] increased, whereas the concentration of dl-limonene (peak C) decreased. After polyiso- prene pyrolysis at 28.0 kPa, most of the products in the naphtha fraction were olefins and only small amounts of aromatics and alkanes were formed.
3.4. GC-MS analysis of the naphtha fraction from pyrolysis of polyisoprene rubber
The pyrolysis of the polyisoprene rubber yielded higher amounts of aromatic compounds than the pyrolysis of pure polyisoprene. Part of this increase can be attributed to aromatic components in the polyisoprene rubber feedstock, such as processing oil and SBR. However, the product distribution of the polyisoprene pyrolysis is influenced by the other rubber compounds. At 0.8 kPa the pyrolysis of pure polyisoprene yielded 16.5 wt% dl-limonene, based on the initial feedstock. If one assumes that the pyrolysis of the polyisoprene portion in the rubber is independent of the other rubber components, the pyrolysis of a polyisoprene rubber sample containing 52% polyisoprene at 0.8 kPa should yield approximately 8.6% of dl-limonene. However, the observed limonene yield at this reaction condition was much smaller (5.0%). It is postulated that dl-limonene and/or dl-limonene precursors reacted with pyrolysis products of non-polyisoprene rubber components and, therefore, less dl-limonene was recovered. Nevertheless, because of the high price of dl-limonene, these yields are still significant for the process economics. The composition of the naphtha fractions from polyisoprene rubber pyrolysis at 0.8 and 6.4 kPa was similar (Table 2). However, as the
Tabl
e 2
Tent
ativ
e ch
arac
teriz
atio
n of
th
e py
roly
tic
naph
tha
fract
ion
(IB
P -2
04°C
) ob
tain
ed
from
py
roly
sis
of p
olyi
sopr
ene
(P>
and
poly
isop
rene
ru
bber
(P
R)
at v
ario
us
pres
sure
s
Yie
ld/%
Y
ield
/%
r,/m
in
P PR
PR
Te
ntat
ive
assi
gnm
ent
t,/m
in
P PR
PR
Te
ntat
ive
assi
gnm
ent
0.8
kPa
0.8
kPa
6.4
Wa
0.8
Wa
0.8
kPa
6.4
kPa
c1
2 2.
34
0.18
0.
13
0.39
2-
Met
hyl-
1,4-
pent
adie
ne
10.6
2 1.
60
3.89
4.
12
1 ,ZD
imet
hylb
enze
ne
Y
2.38
0.
47
0.41
0.
98
2-M
ethy
l-1,3
-pen
tadi
ene
12.3
3 2
n.p.
6.
77
7.97
1,
3,5,
7-C
yclo
octa
tetra
ene
D
2.43
0.
32
0.09
0.
71
1.3~
Cyc
lohe
xadi
ene
16.5
3 1.
13
1.21
1.
26
2,5-
Dim
ethy
l-3-m
ethy
lene
- 1,
5-
7 he
xadi
ene
3 2.
5 1
0.71
0.
38
0.99
3-
Met
hyl-
1.3.
pent
adie
ne
16.9
0 1.
25
1.38
1.
62
2,5,
6-Tr
imet
hyl-1
,3,6
-hep
tatri
ene
Z 4.
51
1.31
1.
41
2.13
4,
4-D
imet
hylc
yclo
pent
ene
17.2
2 5.
42
2.03
2.
41
I-M
ethy
l-5_(
1-m
ethy
l-eth
enyl
)cyc
lo-
?
hexe
ne
s 4.
74
0.49
0.
40
0.79
1,
2-D
imet
hylc
yclo
pent
ene
19.0
5 n.
p.
3.04
3.
48
I-Pr
open
ylbe
nzen
e E.
4.89
2.
64
2.72
4.
04
Met
hylb
enze
ne
19.6
6 1.
04
1.78
1.
73
Trim
ethy
lhep
tadi
ene
h
5.00
0.
21
0.20
n.
p.
Met
hylc
yclo
hexe
ne
20.6
6 0.
92
1.82
1.
09
Trim
ethy
lhep
tadi
ene
Y
5 5.
25
0.50
0.
17
n.p.
2,
4,4-
Trim
ethy
l-2-p
ente
ne
21.5
3 3.
89
3.26
1.
54
1 -M
ethy
l-4-c
1 -
met
hyl-e
thyl
kycl
o-
8 he
xane
F
6.38
0.
37
B
n.p.
n.
p.
1,4-
Dim
ethy
lcyc
lohe
xane
21
.83
n.p.
2.
69
2.44
1 -
Met
hyl-4
-(
1 -m
ethy
l-eth
yljb
enze
ne
7.46
1.
00
1.27
1.
39
2,3-
Dim
ethy
l-l
,Che
xadi
ene
22.1
6 54
.64
31.4
0 31
.22
dl-L
imon
ene
3 2 9.
58
0.39
0.
57
0.78
Tr
imet
hyl-
1,3-
cycl
open
ta-
25.7
6 n.
p.
1.38
1.
32
CEt
heny
l- 1,
2-di
met
hyl-b
enze
ne
S di
ene
2
9.98
n.
p.
2.64
2.
41
Ethy
lben
zene
?
70.3
5 51
.42
55.4
6 To
tal
olef
ms
S 3.
89
3.26
1.
54
Tota
l al
kane
s 4.
24
16.3
6 17
.81
Tota
l ar
omat
ics
78.4
8 71
.04
74.8
1 To
tal
iden
tifie
d
n.p.
: no
pea
k de
tect
ed.
C. Roy et al./ Fuel Processing Technology 50 (1997) 87-103 93
Table 3 Elemental analysis and ash of rubber feedstock, pyrolysis products and reference compounds
Sample
Polyisoprene rubber Pyrolytic oil at 0.8 kPa, 500°C Pyrolytic oil at 6.4 Wa, 500°C Crude oil Pyrolytic naphtha at 0.8 kPa, 500°C Pyrolytic naphtha at 6.4 kPa, 500°C Pyrolytic carbon black at 0.8 kPa, 500°C Pyrolytic carbon black at 6.4 Wa, 500°C Commercial carbon black N330
C H
91.1 7.0 86.3 11.8 85.5 13.4 n.d. n.d. 87.3 11.8 87.6 11.7 94.1 0.4 94.4 0.4 98.6 0.4
Element/w%
N S tOtal Ob Ash
0.6 1.1 0.2 1.6 0.9 0.9 0.1 n.d. 0.8 1.1 0.2 n.d. n.d. 0.5-1.5 a n.d. n.d. 0.6 < 0.03 0.3 n.d. 0.4 < 0.03 0.3 n.d. 0.2 3.3 2.0 6.8 0.3 3.0 1.9 6.7 0.0 0.6 0.4 0.4
n.d., not determined. 1 Concentration of sulphur in most crude oils [29].
By difference.
yield of the naphtha fraction decreased with increasing pyrolysis pressure, the yield of dl-limonene, based on total feedstock, also decreased with increasing pyrolysis pressure (Table 1).
After a possible separation of the dl-limonene from the naphtha fraction, the rest would consist mostly of aromatics and branched olefins. A possible application for these compounds would be as an additive to gasoline. Nitrogen and sulphur are undesired elements for such an application. These elements, which are present in the pyrolytic oil, originate from rubber additives such as antioxidants and accelerators. The sulphur concentration of most crudes ranges from 0.5 to 1.5 wt% [29] and the sulphur concentration of the pyrolytic ‘oil was in the middle of this range. However, in the naphtha fraction of the pyrolytic oil the sulphur concentration was much lower ( < 0.03%, Table 3). In gasoline, a high concentration of olefinic compounds is undesirable because of their high propensities for oxidation and polymerization. However, in an earlier investigation it was shown that small quantities (2%) of crude naphtha from tire pyrolysis can be beneficially added to gasoline [ 141. By selective hydrogenation of the olefins in the pyrolytic naphtha the quality of the naphtha as gasoline additive can be further improved.
3.5. Characterization of the pyrolytic carbon black
3.5.1. X-ray di#raction An important difference between commercial carbon blacks and CB, is the presence
of inorganic material in the CB, [ 181. Inorganic components used in the formulation of rubber are ZnO as vulcanization activator and sulphur as vulcanization agent [25]. Most of the inorganic components present in the rubber will be found in the CB,. The bulk of the CB, was investigated by X-ray diffraction. The diffractogram of CB, obtained at 500°C and 0.8 kPa showed, in addition to the broad graphite lines, lines of other crystalline compounds. Comparison of the CB, diffractogram with the diffractograms of
94 C. Roy et al./Fuel Processing Technology 50 (1997) 87-103
(CBp)pR 0.8 kPa
30 40 50 60
20 Fig. 2. Diffractogramm of CB, recovered from N330-containing polyisoprene rubber (PR) at 0.8 Wa and 5OQ”C.
zinc compounds shows that the CB, contains (Y- and P-ZnS but no ZnO or other crystalline inorganic compounds (Fig. 2). The absence of ZnO is due to the reaction of ZnO with sulphur formed during the decomposition of organic sulphur compounds: ZnO + S + ZnS + l/20,. Formation of ZnS was also confirmed by ESCA and by Auger spectroscopy (see below). It has been shown in an earlier SEM investigation that ZnS can form individual particles at high pyrolysis temperatures [18]. As the density of ZnS is much higher than the density of the organic part of the CB, (4 vs. 2 g cme3), a physical separation of the undesired ZnS from the organic portion of the CB, should be easy. However, in the CB, samples of this investigation no individual ZnS particles were detected by SEM (not shown). Therefore, a post-pyrolysis heat treatment may be beneficial in order to promote the formation of easy-to-separate individual ZnS particles.
3.6. Specijc su$ace area (BET)
The specific surface area of the two CB, samples from polyisoprene rubber pyrolysis at 0.8 and 6.4 kPa were 75.1 and 69.9 m2 g-‘, respectively. This value is smaller than the surface area of the commercial carbon black N330 initially present in the rubber (80 m* g- ‘). However, the CB, samples contain approximately 6.7 wt% ash. Assuming that the surface area of the ash compounds is small compared with the organic portion of the CB,, the surface area of the organic fraction of the two CB, samples would be 80.5 and 74.9 m* g- ‘, respectively. This means that rubber pyrolysis at 0.8 kPa results in a
C. Roy et d/Fuel Processing Technology 50 (1997) 87-103 95
Table 4 Elemental surface composition, in at%, of commercial carbon black N330 and of CB, recovered from N330-containing polyisoprene rubber
Sample C 0 S N Zn N330 98.4 0.9 0.7 a a
(CB,),,, 0.8 Pa, 500°C 96. I 0.9 2.2 a 0.8 (CB,),,, 6.4 Pa, 500°C 97.0 1.1 1.3 a 0.6
a Not detected at acquisition time of IO min.
carbon black filler with virtually the same surface area as the initial carbon black. However, the recovered pyrolytic carbon black is “diluted” by ash components. The lower surface area of the CB, sample obtained at 6.4 kPa can be explained in two ways. It is known from ESCA and SIMS experiments (see below) that, after pyrolysis at 6.4 kPa, carbonaceous deposits are present on the CB,. These deposits either have a small surface area or they smooth the rough CB, surface, thereby reducing the CB, surface area. There are indications for the second possibility: in an earlier investigation it was indeed observed that the initially rough CB, surface was smoothed by carbonaceous deposits [22].
3.6.1. ESCA, survey spectra The surface elemental composition of the commercial carbon black N330 and the
CB, are shown in Table 4. On the surface of the commercial carbon black N330 only carbon, oxygen and sulphur were found. On the CB,, in addition to carbon and oxygen, zinc and higher concentrations of sulphur were detected. The surface concentrations of sulphur and zinc on the CB, surface decreased with increasing pyrolysis pressure. This is most probably due to the coverage of ZnS by carbonaceous deposits, which increases with increasing pyrolysis pressure [16,20]. The formation of carbonaceous deposits on the CB, surface can also explain the absence of nitrogen on the CB, surface, in spite of its presence in the bulk.
3.6.2. ESCA, carbon spectra The Cls spectra were fitted to a peak for aromatic, graphite-like carbons [C,, BE
(binding energy) 284.4 eV], a peak for carbons in aliphatic and/or small aromatic compounds (C,, BE = 285.0 eV>, three peaks for carbon with one, two and three bonds to oxygen (C,, C, and C,, BE = 285.9, 287.4 and 288.9 eV) and finally to a plasmon peak (C,, 291.4 eV> which originates as the C, peak from aromatic, graphite-like regions [31]. The C, peak had an asymmetric shape, which was kept constant during the fitting [16], whereas the other peaks had a symmetrical shape (Fig. 3).
The Cls spectrum of the commercial carbon black N330 showed, in addition to the C, and C, peaks, only very small other peaks. The CB, Cls spectra were similar to the spectrum of N330. However, in addition to the C, and C, peaks they also showed C, peaks. This indicates that the surface of N330 consisted nearly exclusively of aromatic, graphite-like compounds, whereas on the surface of the CB, aliphatic and/or small hydrocarbon compounds were also present. These small hydrocarbon compounds are
96 C. Roy et al./Fuel Processing Technology 50 (1997) 87-103
I A N330
280 285 290 295 Binding Energy [eV]
Fig. 3. Cls ESCA spectra of commercial carbon black N330, CB, recovered from N330-containing polyisoprene robber (PR) and CB, from tire pyrolysis;
components of the carbonaceous deposits formed from adsorbed hydrocarbons [16]. The effect of the pyrolysis pressure can be seen by comparing the C, peak area of the two CB, samples, which were obtained by vacuum pyrolysis, with that for the CB, sample which was obtained by pyrolysis of used tires at atmospheric pressure. In the spectrum of the CB, from vacuum pyrolysis at 0.8 kPa, the C , peak area was smaller than 1% of the total intensity. An increase of the.pyrolysis pressure to 6.4 kPa doubled the area of the C , peak. For the CB, from atmospheric tire pyrolysis, the relative C , peak area was
nearly 40% (Table 5). Application of vacuum significantly reduces the amount of carbonaceous deposits on the CB,, and a CB, is obtained which is close in its surface chemistry to the commercial carbon black initially present in the rubber.
3.6.3. ESCA, oxygen spectra The 01s spectra were fitted to three peaks for C=O type oxygen (C=O, -COOR,
BE = 53 1.8 +_ 0.3 eV, 0, ), for C-O type oxygen (C-OH, COOR, 534.0 &- 0.5 eVTOz)
Table 5 Relative area of Cl s ESCA peaks of commercial carbon black N330 and of CB, recovered from N330-con- taining polyisoprene rubber
Sample Area of the C 1 s peaks/%
C, C, C, C, C” C.
N330 90.4 a 1.6 a 0.9 7.1 (CB,),,, 0.8 kPa, 500°C 90.6 0.8 1.4 a 0.3 6.9
(CB,),,, 6.4 kPa, 500°C 89.7 1.9 1.3 a 0.2 6.9 (CB,),,,, 100.0 kPa, 500°C 54.6 38.4 5.0 a 1 2.0
a No peak detected.
C. Roy et al./Fuel Processing Technology 50 (1997) 87-103 97
525 530 535 540
Binding Energy [eV]
Fig. 4. 01s ESCA spectra of commercial carbon black N330 and of CB, recovered from N330-containing polyisoprene rubber (PR).
and to a shake-up peak (537.5 eV, 0,) (Fig. 4). No signals of oxide oxygen at a BE of 530.0-53 1 .O eV were detected in the CB ,, spectra, confirming the absence of zinc oxide on the CB, surface because of the conversion of ZnO to ZnS (see Section 3.5.1. In all the 01s spectra the most intense peak was the 0, peak, followed by the 0, peak (Table 6). However, only in the spectrum of N330, not in the CB, spectra, was a shake-up peak detected. Therefore, on the surface of the commercial carbon black only, at least a part of the oxygen was close to the aromatic system.
3.6.4. ESCA, sulphur spectra The S2p spectra were fitted to a doublet for sulphides (S ,), for organic sulphur
without bonds to oxygen (S,), for organic sulphur with one bond to oxygen (S,), for sulphates (S,) and a shake-up peak (S,). The S2p spectra of the CB, samples were similar (Fig. 5). Both spectra showed S, and S, doublets with a comparable intensity ratio (Table 7). However, in contrast to the spectrum of CB, obtained at 0.8 kPa, in the
Table 6 Relative area of 01s ESCA peaks of commercial carbon black N330 and of CB, recovered from N330-con- taining polyisoprene rubber
Sample Area of the 0 1 s peaks/%
0, 02 03
N330 57.9 30.9 11.4 (CB&.a, 0.8 kPa, 500°C 58.6 41.4 a (CB,),, ,6.4 @a, 500°C 77.5 22.5 a
’ No peak detected.
98 C. Roy et al./ Fuel Processing Technology 50 (1997) 87-103
160 16.5 170 175
Binding Energy [eV]
Fig. 5. S2p ESCA spectra of commercial carbon black N330 and of CB, recovered from N330-containing polyisoprene rubber (PR).
spectrum of the CB, sample obtained at 6.4 kPa a very weak sulphate doublet was found. The most important difference between the spectrum of the commercial carbon black N330 and the CB, was the absence of the sulphide doublet in the spectrum of N330. As already mentioned, in the CB,, ZnS is formed during the pyrolysis. A further difference between the commercial carbon black and the CB, is the high concentration of sulphur-oxygen compounds on the surface of N330, which results from oxidation of sulphur compounds at high temperature during the production of commercial carbon blacks.
3.7. Auger spectroscopy
The transformation of ZnO to ZnS on the CB, surface was also investigated by Auger spectroscopy. Discrimination of ZnO from ZnS is not possible by Zn ESCA, as the BE of the Zn2p peaks of ZnO and ZnS differ only very slightly [30]. However, the
Table I Relative area of S2p doublets of commercial carbon black N330 and of CB, recovered from N330-containing polyisoprene rubber
Sample Area of the S2p doublets/%
N330 a 74.1 10.5 4.0 9.4 2.0 (CB,),, , 0.8 Wa, 500°C 78.3 21.7 a a a L
(CB,),,, 6.4 kPa, 500°C 76.3 22.8 a a 0.9 =
a No peak detected.
C. Roy er d/Fuel Processing Technology 50 (1997) 87-103 99
Fig. 6. zn Auger spectra
I I I I I
1000 990 980
Kinetic Energy [eV]
of CB, recovered from N330-containing polyisoprene robber (PR).
Auger spectra of different Zn compounds, such as Zn, ZnO and ZnS, differ in their shape. Therefore, a discrimination between the different zinc compounds is possible (Fig. 6). Comparison of the ZnAuger spectra of the two CB, with those of the reference compounds showed that, as in the bulk, on the surface of the CB, samples all ZnO was transformed to ZnS.
3.7.1. SIMS, negative ions SIMS, like ESCA, is a surface specific characterization technique. However, the two
techniques differ in the depth of analysis. By ESCA the surface and near surface regions up to a depth of about 20 A are analysed, whereas with static SIMS only information from the first one to two atomic layers is obtained. In the SIMS spectra of commercial carbon blacks the C,H- and CT peaks were assigned to aromatic carbons with and without bonds to hydrogen, respectively [32]. Following this assignment, the C,H-/C; ratio is an indication for the inverse size of the aromatic system on the carbon black surface. This assignment is confirmed by the C,H-/C; ratios of the reference compounds (Fig. 7). For graphite, the model compound for an extended aromatic system, the C,H-/C; ratio is 0.28, whereas for 3,4-benzofluoranthene, consisting of five condensed rings, a C,H-/C, ratio of 2.2 was found (Table 8). The spectrum of an asphaltene sample, consisting of aromatic carbon and a high proportion of aliphatic groups, showed a smaller C,H-/C; ratio than for 3,4-benzofluoranthene. Therefore, an important contribution of aliphatic carbons to the C,H- peak can be ruled out and the C,H-/C; ratio is not, or is only slightly, influenced by aliphatic groups. For the carbon black samples the SIMS &H-/C; peak ratio increased in the same order as the ESCA C, peak: N330 > (CB,),, 0.8 kPa > (CB,),, 6.4 kPa > (CB,),,, 100 kPa. A very good correlation was found between the two surface spectroscopic methods (Fig.
100 C. Roy et al./ Fuel Processing Technology 50 (1997) 87-103
22 24 26 28
m/z L-1 30
Fig. 7. SIMS spectra of commercial carbon black N330, CB, recovered from N330-containing rubber (PR) and CB, from tire pyrolysis, C,H; region.
8). This figure also includes two CB, samples from tire pyrolysis at 20 and 100 kPa. The data show that by ESCA only small differences were detected between the commercial carbon black N330 and the two CB, samples from the polyisoprene rubber pyrolysis at 0.8 and 6.4 kPa, whereas the SIMS spectra differ considerably. This means that these samples differ significantly only in their first atomic layers probed by SIMS, whereas in the near surface region probed by ESCA the chemistry was similar. On the two CB, samples from tire pyrolysis at 20 and 100 kPa more carbonaceous deposits were formed. Therefore, the differences in their surface chemistry with respect to N330 could easily be monitored by ESCA. It can be concluded that SIMS, in contrast to ESCA, is a very sensitive method for the detection of small amounts of carbonaceous
Table 8 Intensities of negative SIMS peaks of commercial carbon black N330, of CB, recovered from N330-contain- ing polyisoprene rubber and of reference compounds
Sample C,H-
Peak intensities relative to C; = 1 .O
C2H; C- CH- CH;
Graphite 0.28 0.06 0.06 0.12 0.02
N330 0.45 0.07 0.02 0.02 p (CB,),,, 0.8 kPa, 500°C 1.04 0.49 0.04 0.05 0.01 (CB,),, ,6.4 kPa, 500°C 1.23 0.45 0.07 0.11 0.03 (CBPjTirc. 100 kPa, 500°C a 2.89 0.69 0.30 0.07 0.01 3,CBenzofluoranthene 2.20 0.08 0.67 1.22 0.07 Asphaltenes 1.88 0.19 1.44 3.88 0.68
a No peak detected.
C. Roy et al./ Fuel Processing Technology 50 (1997) 87-103 101
0.8
Ratio of SIMS Peaks C,H-/C,-
Fig. 8. Correlation between the amount of carbonaceous deposits on carbon blacks measured by SIMS (C,H-/C; peak ratio) and Cls ESCA (C, /CO peak ratio).
deposits present on CB, samples obtained at low pyrolysis pressures. In an earlier investigation it was observed that the amount of carbonaceous deposits on CB, decreased with increasing pyrolysis temperature [16]. Therefore, following a post-pyrol- ysis heat treatment, the amount of carbonaceous deposits on the CB, may be reduced and the surface CB, chemistry may become even closer to that of a grade N330 commercial carbon black.
4. Conclusion
Vacuum pyrolysis of polyisoprene rubber at low pressures generated a good quality pyrolytic carbon black (CB,) and pyrolytic oil. The pyrolytic oil-derived naphtha fraction contains considerable amounts of valuable dl-limonene. The concentration of this compound increased with decreasing pyrolysis pressure. The rest of the naphtha fraction consists mostly of aromatics and branched olefins, which can be used as a gasoline additive. The CB, is a mixture of inorganic components of the rubber and the recovered carbon black filler. The surface chemistry of the organic portion of the recovered carbon black obtained at low pyrolysis pressures is close to that of the commercial carbon black N330 initially present in the rubber. Only a small amount of carbonaceous deposits was formed on the surface of the recovered CB, during pyrolysis at low pressures. The surface area of the recovered carbon black is close to that of N330. Therefore, after a separation of the inorganic components, the CB, may have the potential to replace commercial carbon blacks for some applications.
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