Chinese J . Chern. Eng., 14(5) 654--659 (2006)

Surface Modification of Pyrolytic Carbon Black from Waste Tires and Its Use as Pigment for Offset Printing Ink*

ZHOU Jie(BJ $7)a*b, WANG Jingdai(3 *@)", REN Xiaohong(f3 $t YANG Yongrong( PEI & %)" and JIANG Binbo(%% &)",** a UNILAB Research Center for Chemical Reaction Engineering, Department of Chemical Engineering, Zhejiang University, Hangzhou 3 10027, China Department of Environmental Science and Engineering, Hangzhou Dianzi University, Hangzhou 3 10018, China

Abstract Pyrolysis has the potential of transforming waste into valuable products. Pyrolytic carbon black (PCB) is one of the most important products resulting from the pyrolysis of used tires. One of the most significant applica- tions of modified pyrolytic carbon black is its use as pigment for offset printing ink to obtain high added values. Inverse gas chromatography (IGC) results show that a large quantity of inorganic matters and carbonaceous deposit are removed by treating the pyrolytic carbon black with nitric acid solution. Plenty of active sites originally occu- pied by inorganic ash and coke are recovered. The surface ener y of pyrolytic carbon black (TWPC) modified by

ene-shows the strong interaction between the TWPC and the synthetic resins. The offset printing ink performance confirms the IGC prediction. And TWPC has the great potential of applications in printing ink industry as pigment. Keywords environmental engineering, waste tires, ink

titanate-coupling agent-especially the specific interaction ( P determined by the specific probe molecule, tolu-

1 INTRODUCTION Waste tires do not degrade in a short span of time;

they will bring accidental fire and emit poisonous gases with rich dioxins. The effective recycling and utilization of waste tires, and secondly, prevention of environmental pollution caused by them are new is- sues that the reuse of resources is faced with.

Gebauer et al."] found that the solid residues of pyrolysis of waste tires are the mixture of carbon black, high content of ash and gritty material (coke) formed by degradation of tire rubber. According to Merchant et ~ 1 . ' ~ ' the economic feasibility of pyrolysis of waste tires depends, to reat extent, on the value of solid residues. Yang et uZ?~-'] showed that there are large amounts of ester and short hydrocarbon grafts on the surface of pyrolytic carbon black. The carbona- ceous deposits mainly consisted of aromatic hydro- carbons. Although the deposits were of low polarity, similar to that of the synthetic resins in printing ink, the adequate space thickness could not be formed on the surface of the pyrolytic carbon black. The wetting and dispersion of the pyrolytic carbon black in synthetic resins were hindered, resulting in an increase in pul- verization times by three-roll mill in printing ink indus- try, low fluidity and instability of ink. The ash in the pyrolytic carbon black consists of zinc sulfide, zinc oxide, silica, e t ~ . ' ~ ' . These carbonaceous deposits and inorganic matters covered lots of active sites and thus hindered the recycle of the pyrolytic carbon black di- rectly. The offset printing ink is one of the most impor-

tant materials of publishing industry. The need of the domestic offset printing ink market continues to in- crease rapidly. One of the most significant applications of modified pyrolytic carbon black is its use as pigment in offset printing ink to obtain high added values.

Coupling agents have been commonly applied to improve the performance of particulate-fill com- pounds at presentL7]. Inverse gas chromatography (IGC) has become a powerful tool for the evaluation of solid surface properties in terms of surface energy, espe- cially for the solid powders. If suitable model com- pounds for polymers are used as probes, interactions between the synthetic resins and pigments can be es- timated using IGC in the printing ink system^'^'^].

In this study, firstly nitric acid was used to re- move lots of inorganic matters and coke to recover some active sites, and then the washed pyrolytic car- bon black was modified by the titanate-coupling agent to obtain the recycled carbonaceous material as pig- ment utilized in printing ink system. IGC was used to predict the interaction between the pigments and the synthetic resins by specific probe molecule. Further- more, in order to examine the application of the modi- fied pyrolytic carbon black in offset printing ink, the ink performance, such as pulverized times, fluidity, viscosity and blackness was studied.

2 EXPERIMENTAL 2.1 Materials preparation

The CHAOYANG brand 6 5 e 1 6 tire tread was

Received 2005-07-22, accepted 2006-03-02.

China (No.20490205). * Supported by the National Natural Science Foundation of China (No.20176051) and the Key Natural Science Foundation of

** To whom correspondence should be addressed. E-mail: jiangbb@zju.edu.cn

Surface Modification of Pyrolytic Carbon Black from Waste Tires and Its Use as Pigment for Offset Printing Ink 655

provided by Hangzhou Zhongce Rubber Co., Ltd. (China). About lOOg tire shreds (10cmX lOcmX 10cm) were set in a quartz tube reactor. The reactor was heated by a tubular furnace in Nz atmosphere from room temperature to 500°C at atmospheric pressure. Then the solid residue was collected and ground. Py- rolytic carbon black (PCB) was prepared.

2.2 Demineralization and oxidation 20g PCB samples were mixed with 300ml nitric

acid [20% (by mass)]. The mixtures were heated at 80°C for 60min with vigorous stirring. Then the mix- tures were filtered, the solid residues were washed with distilled water till the pH value of filtered water was neutral and then were dried to obtain the washed pyrolytic carbon black (WPC). The recovered acids can be used three times consecutively.

2.3 Surface modification The commercial NDZ-3 1 1 titanate-coupling agent

was provided by Nanjing Shuguang Chemical Group Co., Ltd. (China). The quantity of titanate-coupling reagent was 1% (by mass) of that of WPC. NDZ-311 was dissolved in a certain quantity of isopropanol, and then WPC was added into the solution slowly with continual stirring. After stirring for about 30min, the mixtures were dried in oven to obtain the WPC modi- fied by titanate-coupling agent (TWPC).

2.4 Commercial pigments The master standard carbon black (MSCB) as

referential pigment was provided by Hanghua Ink Chemical Co. Ltd. (China).

3 CHARACTERIZATION 3.1 Sorption of nitrogen

The Brunauer-Emmett-Teller (BET) surface areas SBET of four kinds of carbon blacks were determined by N2 adsorption and desorption isotherms measured at 77K using an automatic adsorption apparatus (Me- termetric APAC 2 100).

3.2 Inverse gas chromatography The IGC experiments were performed with a

FULI9790 gas chromatography equipped with a highly sensitive flame ionization detector and a stainless steel cdumn having a length of 60cm and an inner diameter of 3.175mm. The four kinds of carbon blacks, in granular form, with the granule size be- tween 250 and 400pm were selected. The columns were conditioned at 200°C under nitrogen flow for 12h prior to the measurement to remove possible sur- face pollutants. The probes used were a range of n-alkanes ( c 5 - C ~ ) and alkyl-benzenes. Table 1 shows the characteristic of selected probes. The liquid vol- umes injected were in very small quantities-less than 0 . 1 ~ 1 of probes for the infinite dilution. Column tem-

perature was maintained at 180°C.

Table 1 Characteristic of the different probes Acid-base Molecular Boiling ap,

Probe property weight, g.rno1-I point,'C nm2 n-pentane neutral 72.15 36.12 0.43 n-hexane neutral 86.18 68.73 0.47 n-heptane neutral 100.2 98.43 0.51 n-octane neutral 114.23 125.68 0.54 benzene basic 78.11 80.09 0.36 toluene basic 92.13 110.63 0.41 ethyl-benzene basic 106.16 136.2 0.45

Note: The surface area occupied by a polar probe molecule (a,) on the carbon blacks is calculated from the molar mass and density"07'".

4 RESULTS AND DISCUSSION 4.1 Characteristics

In this study, four kinds of carbon blacks were utilized. Their different characteristics can be seen from Table 2. More than half of amount of the ashes in PCB were removed and a little quantity on the surface of PCB were recovered by washing with acid.

Table 2 Characteristics of carbon blacks Sample Ash content, % Surface area, m2.g-' MSCB 0.26 76.73 PCB 16.18 69.23 WPC 7.52 88.62 TWPC 7.45 78.56

4.2 Surface energy 4.2.1 The standard free energy for adsorption ( AG; )

The net retention volume (V,) was calculated ac- cording to Eq.(l). The flowrate measured with a bub- ble meter (Fo) was corrected for the pressure drop in the column [James Martin correction for gas com- pressibility'12], Eq.(2)], the surface tension of the water in the bubble meter [Eq.(3)], and the different tem- peratures in the column and the bubble meter [Eq.(4)]"31:

with v, = ( t, - tM ) JcTFo (1)

Po I (3)

(4) 1 meter

where tM is the zero retention reference time measured with a non-adsorbing probe (methane), Pi is the pres- sure at the column inlet, Po is the pressure at the

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656 Chinese J. Ch. E. (Vol. 14, NOS)

Table 3 Adsorption free energy of different probes on the surface of carbon blacks

MSCB PCB WPC TWPC Probe -AG;, -AGA SP I -AG;, -AGS\P, - A G ~ , - A G ~ P , -AG;, -AGA, SP

U-rno1-l ~ m o l - ' H.rno1-I H.rno1-I H.rno1-l H.rno1-l klernol-' H.rno1-l - - n-pentane 6.04 0.60 - 1.18 2.82 -

n-hexane 9.7 1 - 1.38 - 3.42 - 4.56 -

n-heptane 14.12 - 2.43 - 6.94 7.18 -

n-octane 18.60 3.82 - 8.18 - 9.13 -

benzene 8.89 11.36 2.91 4.74 7.22 10.88 10.33 11.44

toluene 14.04 10.29 5.83 5.45 9.09 9.11 12.82 12.22

ethyl-benzene 17.09 9.44 6.71 6.02 10.01 7.70 17.05 12.85

-

-

column outlet (atmospheric pressure), PHzo is the

water pressure in the bubble meter, Tcol is the column temperature and Tmeter is the temperature of the bubble meter.

The standard free energy for adsorption ( AG: )

of one mole for the gas phase at 1.013X 105Pa (PO) relative to a standard state on the surface is given by Refs. [ 14,151 :

AG; = -RT InV, + B ( 5 )

where B is a constant, S is the specific surface area of the carbon black, XDB is the surface ressure of the standard state of De Boer (3.38 X 10 N.m ') and M is the mass of the carbon black in the column. Table 3 shows the adsorption free energy of different probes on the surface of carbon blacks. 4.2.2 S u ~ a c e energy y,

The solid surface has a set of different accessible active sites of different nature and heterogeneous character. The surface energy ys is the sum of the free energy of all of those active sites. Generally, ys can be

split in two components-dispersive component ys and specific interaction

-B -

d

Y S = d +f: s toluene (7)

(1) Dispersive component of surface energy ( y: )

ysd expresses the potential of a solid to undergo London- or dispersive-type interactions. These inter- actions are also termed "non-specific" because they will always take place when two surfaces are brought into contact. And ysd can be obtained from the slope

( AGCH, ) of a graph AG; vs. number of carbon at-

oms for different n-alkanes (Fig. 1):

October, 2006

yCH, = 36.76 - O.O58T,,, (9) where N is Avogadro's number, a is the area of a -CH2- group (0.06nm2) and yCHZ is the surface

energy of a solid made entirely of --CH2- group, i.e. poly(ethy1ene).

20,

5.0 5.5 6.0 6.5 7.0 7.5 8.0 number of carbon atoms

Figure 1 Determination of the dispersive, non-specific interaction

16

0.25 0.30 0.35 0.40 0.45 0.50 0.55 a(probe), nmz

Figure 2 Free energies of adsorption of different probes on carbon blacks,

determination of the specific interaction

Surface Modification of Pyrolytic Carbon Black from Waste Tires and Its Use as Pigment for Offset Printing Ink 657

(2) Specific interaction ( dp ) f p determines the potential of the surface to ex-

change specific interactions, such as polar, acidhase, hydrogen bonding, etc., i.e. all types other than dis- persive interactions. It can be obtained from the dif- ference of the free energy of adsorption ( AGip ) be- tween the polar probe and a hypothetical n-alkane with the same surface area (Fig.2):

AGip = (-AGi) - (-AGi,alkane)

AGF ZP =7 The dispersive components of surface energy

( d ) of the different carbons are listed in Table 4.

PCB has a low value because of the formation of carbonaceous deposits on the carbon black during the pyrolysis. The surface energy is the excess of the total energy that a solid has over the value it would have if the surface were in the same thermodynamic state as the interior. To minimize the surface energy, surface atoms may rearrange or substance with a lower sur- face energy may be adsorbed on the surface. The hy- drocarbons and inorganic matters formed from the decomposing rubber may adsorb on active sites of the carbon black surface, thereby lowering the surface energy. It is found that WPC has a much higher f value than PC. Washing with nitric acid removed a large quantity of inorganic matters and carbonaceous deposits, which results in the recovering of the active sites. The value of TWPC is decreased because a small quantity of titanate-coupling agent adsorbs on the active sites of WPC.

Table 4 Surface energies (non-specific component and specific interaction) of carbon blacks

d SP Sample Ys ' Ys,toluene ys

m J d mJ.m-' mJ.m-*

MSCB 324.16 41.69 365.85

PCB 20.98 22.08 43.06

WPC 109.96 36.91 146.87

TWPC 84.94 49.5 1 134.45

It can be seen from the data presented in Fig.2 that the free energies of adsorption of alkyl-benzenes on carbon blacks were higher than that of n-alkanes with the same surface area. This suggests that there were active sites on the carbon surfaces, which inter- act selectively with the .n-system of the aromatic rin .

Most synthetic resins used in offset contain some func- tional groups. In addition to alkanes, alkyl-benzenes

These active sites may be surface oxygen groups , I 6 .

were used as analogs for synthetic resins with aro- matic rings and double bonds, such as phenolic resin, polyethylene resin etc. Consequently, toluene was chosen as model probe for polymers to estimate the interactions between the synthetic resins and pigments in the printing ink systems because of its appropriate gasification temperature and residence time in inverse gas chromatography. The specific interaction for tolu- ene ( f:oluene ) is shown in Table 4. WPC has a higher

f:oluene value than PCB because there are few hy-

droxyl and other oxygen groups on the surface of WPC after demineralization and oxidation of PCB. TWPC has a higher fLluene value than MSCB, indi-

cating the stronger interaction between the TWPC and the synthetic resins than that between the MSCB and the synthetic resins.

4.3 Application in offset printing ink as pigment Printing ink is a colloid dispersive system com-

posed of pigment, filler, linking stuff and auxiliary agent. The synthetic resins are the main components of the linking stuff in modern ink industry. Thus, the dispersion of pigment particles in synthetic resins af- fects the quality of the offset printing ink greatly. In our research, the primary four parameters were exam- ined to evaluate the dispersion of four kinds of car- bons in synthetic resins. Hanghua Ink Chemical Co. Ltd. (China) carried out the determination of the ink performances. The typical offset printing ink formula- tion is shown in Table 5.

Table 5 Formulation of printing ink Ingredient

phenolic varnish polyethylene varnish bentonite colloid calcium pigment (carbon blacks) wax B.H.T petrolatum

Content, %

44 14 1.5 9 22 0.5 5

(5.0) ~~

Table 6 shows that the needed pulverized times in three-roll mill and viscosity of the ink containing TWPC are lower than that of the ink containing MSCB. The fineness of ink was determined by grind-0-meter. A TV-20H rotary viscosimeter was used to measure the viscosity of ink at a rotating speed of 50rmin-'. The lower pulverized times and viscos- ity indicate the good dispersion of TWPC in the syn- thetic resins. And that also can improve the productiv- ity in ink industry. The fluidity is an important process parameter and guideline in ink industry. The fluidity

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658 Chinese J. Ch. E. (Vol. 14, NOS)

of the ink containing TWPC is better than that of the ink containing MSCB. Although the blackness of the ink containing TWPC is increased, the intensity is still lower than that of the ink containing MSCB because of the high ash content of the TWPC.

Table 6 Performances of offset printing ink

Pulverization Viscosity times Fluidity, (25 C ,

(ink fineness mm 50r.min-’), Pigment Blackness

G 10pm) P a s MSCB 4 2.52 27 2450 PCB 5 2.17 26.5 3150 WPC 4 2.32 27 3010 TWPC 3 2.40 30 2100

The chemical structure of NDZ-311 titan- ate-coupling agent as shown in Fig3 allows the for- mation of a strong van der Waals attraction and/or the chemical interaction between the organic chains of titanate and the long chains of the synthetic resins. The possible coupling mechanism is shown in Fig.4. There are two main coupling forms. The polarity of the organic chain of the selected NDZ-311 titan- ate-coupling agent may be similar to that of the syn- thetic resins used in above typical offset printing ink formulation. That results in the improvement of the ink performance.

0 0 H,C - 0,

H,C I - O/ )2

OH

titanate-coupling agent Figure 3 Chemical structure of NDZ-311

Figure 4 Possible coupling mechanism of NDZ-311 titanate-coupling agent on the surface of WPC

NOMENCLATURE area of a -CH2- group, 0.06nm2 surface area occupied by a polar probe mole- cule, nm’ constant surface tension of the water in the bubble meter flowrate measured with a bubble meter, mlmin-l

standard free energy for adsorption, kJ.mol-’

free energy of adsorption a hypothetical n-alkane,

H.mol-’

difference of the free energy of adsorption between the polar probe and a hypothetical n-alkane with the same surface area, kJ.mol-’ James Martin correction for gas compressibility mass of the carbon black in the column, g Avogadro’s number, 6.02 X 1 OZ3

water pressure in the bubble meter, MPa

pressure at the column inlet, MPa pressure at the column outlet (atmospheric pressure), 0.101325 MPa specific surface area of the carbon black, m2.g-’ Temperature correction column temperature, K temperature of the bubble meter, K zero retention reference time measured with a non-adsorbing probe (methane), min retention volume, min net retention volume, ml

surface energy of a solid made entirely of

+H2- group, i.e. poly(ethylene), m l d surface energy, mJ.m-’

dispersive component of surface energy, m . J d

specific interaction for toluene, mJ.m-’

surface pressure of the standard state of De Boer ,3.38 X 10-3N.m-’

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Surface Modification of Pyrolytic Carbon Black from Waste Tires and Its Use as Pigment for Offset Printing Ink 659

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Chinese J. Ch. E. 14(5) 654 (2006)