ROHSTOFFE UND ANWENDUNGEN
RAW MATERIALS AND APPLICATIONS
Zinc oxide Rubber granulate Zinc leaching
Recently the reduction of the zinc oxide
level in rubber compounds has become an
important issue because of the harmful
effect of zinc ions to aquatic organisms. In
the present study the effect of reducing the
zinc oxide level in rubber compounds on the
amount of zinc leached from rubber granulate and powder was investigated.
Leaching tests were performed according to
DIN 18035-7 on rubber compounds used for
truck tyre re-treading containing standard
zinc oxide and active one, as well as on
commercially grinded rubber made from
truck and car tyres by ambient and cryogenic grinding.
It was found that the surface specific area
of the zinc oxide has no significant influence on the amount of zinc leached. The
amount of zinc leached increases with increase of zinc oxide level up to around 3 phr
and then levels off. The most influencing
factor is the particle size distribution of the
rubber granulate. Smaller amounts of zinc
are leached from cryogenic rubber granulate and powder than from ambient rubber
granulate and powder having the same
particle size.
Der Einfluss des Zinkoxidgehalts
in Kautschukmischungen auf die
Menge an ausgelaugtem Zink
Zinkoxid Gummimehl Auslaugen
Seit einiger Zeit hat die Verminderung des
Gehaltes an Zinkoxid in Kautschukrezepturen an Bedeutung gewonnen, weil von
gelösten Zinkverbindungen eine Aquatoxicität ausgeht. In der vorliegenden Untersuchung wurde der Effekt untersucht, den
ein verminderter Zinkoxidgehalt in Kautschukrezepturen auf die Menge an ausgelaugtem Zink ausübt. Die Gummigranulate und -mehle wurden Auslaugversuchen
nach DIN 18035-7 unterzogen. Dafür wurden für LKW- Runderneuerung typische
Rezepturen eingesetzt die jeweils ein
Standard-Zinkoxid bzw. ein aktives Zinkoxid enthielten. Darüber hinaus wurden
cryogen-gemahlene Granulate aus LKWund PKW-Reifen untersucht. Es wurde gefunden, dass die spezifischen Oberfläche
des Zinkoxids keinen signifikanten Einfluss
auf den Gehalt an ausgelaugtem Zink hat.
Der Einfluss steigt mit den Zinkgehalt der
Probe an und erreicht bei Dosierungen von
ca. 3 phr einen Grenzwert. Der wesentliche
Einfluss geht von der Korngrößenverteilung
des Gummimehls aus. Aus cryogen-gemahlenen Granulaten werden geringere
Mengen an Zink ausgelaugt als aus Granulaten die bei Umgebungstemperatur
gemahlen wurden und dieselbe Korngröße
haben.
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KGK Juni 2006
Influence of the Zinc Oxide
Level in Rubber Compounds on
the Amount of Zinc Leaching
Recently the reduction of the zinc oxide level in rubber compounds has become an important issue because of the harmful effect
associated to soluble zinc compounds to
aquatic organisms.
According to the Council Directive 2003/
105/EC, zinc oxide is classified as N „Dangerous for the Environment“ R50/53
„Very toxic to aquatic organisms, may cause
long term adverse effects in the aquatic environment“, S60 „This material and its container must be disposed of as hazardous
waste“ and S61 „Avoid release to the environment“ [1].
In June 2002 the German Standard DIN
18035-7 „Sports Grounds, Part 7“ „Artificial
Turf Areas“ was published [2]. According to
this standard two leaching test are required
for post- consumer tyre rubber granulate
used as infill material for artificial turf
and the following limits are set in leachates:
– 0.5 mg/l after leaching with deionised
water (DIN 38414-4)
– 3 mg/ l after leaching with water saturated with CO2
Particularly the second limit poses a threat
to using rubber granulate as infill material.
The Austrian expert Dr. Axel Begert considers that the conditions of the second leaching are not comparable with those in the
natural environment [3].
The aims of this paper were (i) to evaluate
influence of the zinc oxide level and grade
on the amount of zinc leaching from rubber
granulate and (ii) to describe the influence
of particle size distribution of rubber
granulate on the amount of zinc in the leachate.
Experimental
A commercial SBR/NR/BR compound used
for retreading truck tyres was chosen to
evaluate the influence of the zinc oxide level on the amount of zinc leaching. Two
grades of zinc oxide were applied. The first
one was obtained by the so called French
process and was produce at Huta Olawa,
Poland. The product consists of large parti-
cles. Correspondingly this zinc oxide show a
small specific surface area of 5 m2/g only.
The second one was an active zinc oxide
produced in a wet process at Bayer. The
average particle size is considerably lower
(36 nm) and consequently the specific surface area is about 44 m2/g (Zincoxyd aktiv,
Bayer). The morphologies of both types of
zinc oxide are shown in Figure 1a and 1b.
The simple recipes used contains variable
amounts of zinc oxide (1, 2, 2.7 and
4 phr). For comparison: the original retreading compound contains 2.7 phr zinc oxide.
The total rubber content in the compound
is 54 %.
The rubber compounds were mixed in a laboratory Banbury mixer with the capacity
1.2 liter. They were vulcanized at 150 C according to time t90 (multiplied by coefficient
1.1) determined by means of ODR vulcameter. Vulcanisates were ground at first
on a rubber mill and next in a laboratory
grinder. Particle size distribution of granulates was determined by sieve analysis.
Leaching tests were performed according
to DIN 18035-7. The quantitative determinations of zinc in eluates were done using
atomic absorption spectroscopy (AAS).
Additional leaching tests were performed
using commercial rubber granulates made
from post-consumer truck and car tyres.
The characterization of the commercial
granulates is given in Table 1. The total rub-
Autoren
L. Pysklo, P. Pawlowski,
W. Parasiewicz, M. Piaskiewicz,
Piastow (Poland)
Corresponding author:
Leszek Pysklo
IPG Instytut Przemyslu
Gumowego „Stomil“
Kl. Harcerska 30
05-820 Piastow, Poland
Tel. þ 48/22/7 23/60 25
1
&
1 SEM pictures of zinc oxides: standard (a) and active (b)
&
1 Characteristics of tested commercial granulates
&
No.
Name
Nominal particle
size range, [mm]
Grinding method
Zinc oxide
content, [phr]
Total rubber
content, [%]
1
Truck tyre buffings
0.5 – 1
5.6
56
2
Car tyre buffings
0.5 – 1.5
4.2
58
3
Truck tyre buffings
0.5 – 1.5
5.3
59
4
5
6
Truck tyre 1–3
Car tyre 1–4
Mixed car and truck tyre
1–3
1–4
2–5
uffing and additional
ambient grinding
Buffing and additional
ambient grinding
Buffing and additional
ambient grinding
Ambient grinding
Ambient grinding
Ambient grinding
4.8
3.3
3.8
56
54
55
2 Vulcanisation characteristics of the commercial retreading compound containing various amounts of zinc oxide
&
Active 44 m2/g
ZnO content [phr]
Mmax [dNm]
Mmin [dNm]
DM [dNm]
t1 [min:s]
t90 [min:s]
Standard 5 m2/g
1
2
2.7
4
1
2
2.7
4
41
11
30
4:30
7:30
45
13
32
5:00
8:30
48
14
34
4:30
9:00
49
15
34
4:30
9:15
49
13
36
5:00
9:15
48
14
34
5:00
9:00
48
14
34
5:00
9:00
50
14
36
5:00
9:30
ber content was determined by TG analysis
and the zinc oxide content was obtained
from chemical analysis of every commercial
rubber granulate.
Results and discussion
As it can be shown from Table 2 the reduction of zinc oxide amount from 4 phr to
1 phr in the commercial truck tyre retreading compound has only minor influence on
the curing characteristics. In the case of the
standard zinc oxide the scorch time t1, the
optimum vulcanization time t90 and the increase in torque (DM ¼ Mmax – Mmin) which
is a simplified measure of crosslink density,
demonstrate stable values. Somewhat larger changes are observed in the case of
the active zinc oxide where the reduction
of the zinc oxide level results in a tendency
toward a decrease of t90 and DM.
These dependencies can be a result of the
poorer dispersion of the active zinc oxide.
The SEM pictures shown in Figure 2 demonstrates a different dispersion of the active
zinc oxide (Fig. 2b) than the standard one
in rubber matrix (Fig. 2a).
However, the reduction of the zinc oxide level has no significant influence on the tensile strength, the elongation at break and
the hardness of the vulcanizates (Table 3).
Particle size distribution
and leaching tests
To evaluate influence of zinc oxide level on
the amount of zinc leaching rubber powders of particle size below 0.7 mm were
used. As it can be shown from the sieve analysis about 80 % of particles were smaller
than 0.5 mm (Fig. 3). For comparison granulates prepared on the rubber mill having
about 50 % of particles larger than 1 mm
were also used.
From the extraction experiments it was
found that the specific surface area of the
zinc oxide incorporated into the rubber matrix has no significant effect on the amount
of zinc leached. As expected very high
amounts of zinc were leached from rubber
powders with water saturated with CO2.
The concentration of zinc in acidic eluates
increased proportionally with increase of
zinc oxide level to 3 phr and then leveled
off (Fig. 4).
KGK Juni 2006
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ROHSTOFFE UND ANWENDUNGEN
RAW MATERIALS AND APPLICATIONS
2
&
2 SEM pictures of zinc oxide agglomerates in rubber compounds: standard zinc oxide (a), active zinc oxide (b)
&
3
&
3 Sieve analysis of rubber granulate and powder prepared from the retreading compound
&
3 Vulcanisate properties of the commercial retreading compound containing various amounts of zinc oxide
&
Active 44 m2/g
ZnO content [phr]
Tensile strength [MPa]
Elongation at break [%]
Hardness [ Sh]
Standard 5 m2/g
1
2
2.7
4
1
2
2.7
4
21.0
336
70
21.1
346
68
19.21
336
70
19.0
341
70
21.0
375
69
22.0
383
70
21.3
389
70
19.3
352
68
4 Influence of particle size of rubber granulates on amount of zinc leached
&
No.
Name
Nominal particle
size range, [mm]
Grinding method
Zinc oxide
content, [phr]
Total rubber
content [%]
Amount of zinc
leached [mg/l]
1
Truck tyre buffings
0.5 – 1
5.6
56
1.69
2
Car tyre buffings
0.5 – 1.5
4.2
58
0.44
3
Truck tyre buffings
0.5 – 1.5
5.3
59
1.44
4
5
6
Truck tyre 1–3
Car tyre 1–4
Mixed car and
truck tyre
1–3
1–4
2–5
Buffing and additional
ambient grinding
Buffing and additional
ambient grinding
Buffing and additional
ambient grinding
Ambient grinding
Ambient grinding
Ambient grinding
4.8
3.3
3.8
56
54
55
1.39
0.69
0.04
By extracting granulates with 50 % of particles larger than 1 mm a significant reduction of the zinc leached (ca. 20 %) was observed. This result can be easily attributed
to the reduced rubber surface available in
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KGK Juni 2006
the extraction experiment. The zinc concentration in neutral water leachates of rubber
powders was also much higher than acceptable but half of that in acidic ones. Additional leaching tests with water were per-
formed on buffings from truck and car tyres
and commercial rubber granulates produced from end-of-life car and truck tyres.
The truck and car tyre buffings contained
5.3 and 4.2 phr zinc oxide respectively.
4
&
5
&
4 Zinc concentration in H2O/CO2 extracts
&
5 Sieve analysis of mixed car and truck tyre granulates
&
5 Amount of zinc leached with water from cryogenic granulates of different particle size and cryogenic powder produced from mixed car and truck waste
&
tyres
Powder
Granulate I
Granulate II
Granulate III
Nominal particle
size range, [mm]
Zinc oxide
content, [phr]
Total rubber
content, [%]
Amount of zinc
leached, [mg/l]
0.18 – 0.6
0.6 – 1.4
1 – 2.4
2.4 – 7
4.1 (2.3 %)
4.2 (2.3 %)
4 – 4.5 (2.3 %)
2.9 – 3.9 (1.8 %)
56.5
56.5
50.7 – 57.0
46.5 – 63.0
0.37
0.29
0.14
0.16
The amount of zinc leached from buffings
by water was two orders lower than for
the powders made of vulcanisates shortly
after vulcanisation. The zinc content in leachates of the truck tyre buffings was three
times higher than acceptable. The effect of
particle size distribution was also visible.
Only car tyre buffings met the requirement
of DIN 18035-7: 0.5 mg Zn/l (Table 4). The
commercial granulates produced from truck
tyres, car tyres and mixed ones contained
4.8, 3.3 and 3.8 phr of zinc oxide respectively. The car and truck tyre granulates
had similar particle size distribution. The
particle sizes were in the 1.4 – 3.2 mm
range. The mixed granulate comprised particles in the 2 – 4.3 mm range (Fig. 5).
The mixed granulate characterized very low
zinc leaching so it indicates clearly that possibility of zinc leaching by water from large
rubber particles of small specific surface
area is low. Larger amounts of zinc were leached from the truck tyre granulate than car
tyre one. It is connected with higher level of
zinc oxide in truck tyres and probably different composition of byproducts of crosslinking reactions and products created during
use of tyres on roads and production of
granulate.
It is worth to remember that not only the
particle size distribution but also the shape
and the surface area of rubber particles are
important. There is a significant difference
between the shape and surface area of
granulates produced by the ambient and
cryogenic grinding and also tyre buffings
[4, 5]. The specific surface area of ambiently
ground rubber granulates is greater than
that of cryogenically ground ones with
the same particle size distribution. Therefore, it is reasonable to conclude that smaller amounts of zinc will be leached from
cryogenic granulates. So additional tests
were done on cryogenic granulates and
powder produced from mixed car and truck
waste tyres.
The results of the leaching test are shown in
table 5. The smaller amounts of zinc are leached from cryogenic rubber granulate and
powder than ambient granulate and powder. The amount of zinc leached decreases
with increase of particle size. It was also
found that even very fine cryogenic powder
fulfils the DIN-Standard requirements.
Conclusions
1. The amount of zinc leaching from rubber
granulates produced from end-of-life
tyres depends mainly on zinc oxide level
in virgin tyre rubber compounds, particle
size distribution and surface area of rubber granulates. The specific surface area
of zinc oxide has no special effect on
amount of zinc leached.
2. The amount of zinc leaching increases
proportionally to zinc oxide concentration in tyre compounds up to certain level
and then levels off. This level of zinc oxide
concentration is probably different for
truck and car tyres.
3. The particle size distribution and specific
surface area of rubber granulate have
very significant influence on amount of
zinc leaching. Fine powder should be accurately removed from rubber granulate
to decrease amount of zinc leaching.
4. Larger amounts of zinc are leached from
truck tyre granulate than from car tyre
one with the same particle size distribution. In our tests the amount of zinc leached from truck tyre ambient granulate
with water exceeded significantly the acceptable limit 0.5 mg/l.
5. Smaller amounts of zinc are leached from
cryogenic rubber granulate and powder
than from ambient rubber granulate
and powder having the same particle
size.
6. In the case of granulate and powder produced from mixed car and truck tyres
more samples should be tested to
evaluate their susceptibility to zinc leaching.
References
[1] A. Chapman, T. Johnson, Proceedings Fall Rubber
Colloquium. November 10–13, 2004, Hannover.
[2] Translations of prestandard: DIN V 18035-7, Part
7: Synthetic turf areas, June 2002.
[3] A. Begert, Technical Opinion on the Application of
Rubber Pellets for the Building of Football Pitches
with Artificial Turf, Bachmanning, April 2003.
[4] J.M Hausman, Rubber Technology International’97, p. 223, UK & International Press 1997.
[5] L. Pysklo and W. Parasiewicz, R. Stepkowski, Elastomery 4 (2000) 15.
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