Minerals Engineering 17 (2004) 69–75This article is also available online at:

www.elsevier.com/locate/mineng

Electrical separation of plastic materials using the triboelectric effect

Mihai Lungu *

Department of Physics, West University Timisoara, Blv. V. Paarvan No. 4, Timisoara 1900, Romania

Received 26 June 2003; accepted 14 October 2003

Abstract

This paper presents a study and some considerations regarding the electrical tribocharge and electrostatic separation of plastic

materials (polymers) using the triboelectric effect. The dependence of the electrical tribocharge on the relative humidity of the at-

mosphere in which the process takes place were analyzed, based on a theoretical model regarding the charge transfer between two

dielectric surfaces. The charge transfer takes place through an intermediate water layer interposed between surfaces belonging to

different polymer particles with different work functions. For this purpose a mixture containing two types of plastic materials (in this

case the polymers polyethylene and polystyrene) was separated using a free-fall separation system. The particles are separately

collected as a result of their electrical properties. Finally, the results and comments regarding the obtained vales for grade and

recovery are given.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Electrostatic separation; Environmental; Particles size; Recycling; Waste

1. Introduction

One of the most important problem that modern

economies have to deal with, is the recycling of reusable

plastic materials from different industrial or municipalwastes, i.e. wastes from car dashboards, plastic bottles

or wastes which result from packing for food products.

Such wastes contain various types of plastic materials

(e.g. polymers as polyethylene and polystyrene). To be

reused, they must be separated and concentrated. To

find the proper physical method for realizing this sepa-

ration was for a long time period a major problem. After

many attempts it has been found that, the most properphysical method is the one based on the triboelectric

effect.

The triboelectric effect (triboelectrification) consists in

the appearance of electric charges with opposite signs at

the contact of two surfaces belonging to different ma-

terials. Contact electrification can occur at solid–solid,

liquid–liquid or solid–liquid interfaces. For dissimilar

solids, which are initially uncharged and normally atearth potential, a transfer of a small electrical charge

takes place from one material to the other as they make

contact. The two materials become oppositely charged.

* Tel.: +40-56-194068; fax: +40-56-190333.

E-mail address: lmihai@physics.uvt.ro (M. Lungu).

0892-6875/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.mineng.2003.10.010

The surfaces acquire a net electric charge, which has as

result an electric field formed between them. This

method of charging is commonly referred to as tribo-

electrification or frictional charging.

In practice, rubbing surfaces together producescharging. When two insulating materials are rubbed

together, the surfaces acquire a net electric charge,

where one becomes negative and the other positive. For

many materials, it is only necessary to touch the surfaces

together, then separate them to transfer a measurable

charge. Most frictionally charged surfaces have both

positive and negative charge areas, but one polarity

predominates, determining the net charge of the surface.In order to build up an appreciable charge because of

the small contact areas, it is necessary to have repeated

contacts.

Experimentally it has been carried out that the charge

increases substantially if the two surfaces are rubbing

one to each other. The proper ‘‘friction’’ has the purpose

to increase the contact surfaces, because in the absence

of the relative movement of the two surfaces put intocontact, the electric charge transfer takes place only in

few small regions, the so-called contact points. The

charges, which are transferred between two solid bodies

put into contact, are in most case electrons, but in some

situations an ion transfer may also appear.

The purpose of this paper is to present a study and

some considerations about the dependence of the

70 M. Lungu / Minerals Engineering 17 (2004) 69–75

tribocharging on the humidity of the atmosphere in

which the process takes place. This work is based on a

theoretical model of the author regarding the charge

transfer between two insulating surfaces during the

tribocharge process. The surfaces belong to two different

polymer particles, namely polyethylene (PE) and poly-

styrene (PS), with different work functions, being incontact through an intermediate water layer.

In the case of insulators, the acquired electrical sur-

face charge density can be sufficient to determine the

deviation of a particle in a high electric field.

The study is based on the experimental results ob-

tained during the electrostatic separation process of

mixtures containing the two mentioned polymers. The

separation of the PE–PS mixture is realized using a free-fall separation system. After being tribocharged, the

particles are deviated under the action of a high electric

field inside the experimental installation and separately

collected according to the triboelectric charge they are

loaded with.

The different charging behavior of different polymers

opens the possibility to apply this phenomenon to sep-

arate mixtures containing various polymers.

2. Theoretical considerations

During contact between two materials, the electrical

charge moves from one to the other. Therefore, in order

to understand the phenomenon, it is necessary to con-

sider the energy of the electrons and ions at the surfacesof the two materials and the way in which they move

both within and between materials. There are many

hypotheses regarding the physical nature of this phe-

nomenon, but none of them explains it in a complete

way.

Helmholtz (1879) assumed that there exists an elec-

tron transfer between the two surfaces, leading to the

appearance of an electrical double layer, of about a fewatomic diameters width. As shown in Fig. 1, by putting

into contact two metals, for example, there appear

transient processes which lead finally to the establish-

ment of a balance by equalizing the Fermi levels (Eq.

(1)) of the materials in contact:

Fig. 1. The metal–metal contact.

EF1¼ EF2

: ð1ÞThe work function of the two materials in contact

are, in general, not equal, and the electrons are able to

flow from the metal with lower work function to the

metal with higher work function and cross the interfaceuntil the chemical potentials of the metals are at equi-

librium. The metal with lower work function becomes

positively charged. Thus it appears an electric contact

field, orientated from the metal with lower work func-

tion to the metal with higher work function, which

prevents further charge flow.

The electric contact field in the region between the

contacting surfaces is, as they separate, a function of therelative size of the capacitance between the surfaces.

The charge which has been transferred, creates a

potential difference between the two metals, Vc as in Fig.

1, known as the contact potential difference:

Vc ¼W1 � W2

e; ð2Þ

where W1 and W2 are the work functions of the first and

second metal, with W1 > W2, and e is the electronic

charge. The contact potential Vc is a characteristic pa-

rameter of a system being in thermodynamic equilib-

rium.The transferred charge depends also on the effective

capacity of the interface, which in its turn depends on

the state density at the interface and on the dielectricconstant of the same. When the metals are separated,

the capacitance between surfaces decreases.

If the interfacial charge transfer is assumed to be

analogue with the charging of a capacitor of capacity

C, then the process is characterized by the relaxation

time s ¼ RC ¼ qe, necessary for the process to reach an

equilibrium state. R is the electric resistance of the

contact zone, q the electric resistivity and e the inter-facial permittivity. For highly insulating materials, sbecomes very large and equilibrium may not be

achieved. If one or both materials are non-conductors,

the recombination of charge cannot take place com-

pletely and the separating materials retain their charge.

Regarding the contact between two insulators, none of

the existing literature on this subject explains the exact

mechanism of charge transfer. Usually the transfer ofelectrons is predominant, but we must also take into

account the transfer of ions, which can occur in certain

situations. Several charge mechanisms have been sug-

gested, for example, Cross (1986) proposed an electron

or ion transfer by surface properties and Seanor (1982)

the electron or ion transfer by bulk properties, and the

transfer related to mechanical dislodgment, respec-

tively. The polarity and magnitude of the electricalcharge can be influenced by many variables includ-

ing humidity, temperature, dust, external electric fields,

etc.

M. Lungu / Minerals Engineering 17 (2004) 69–75 71

The electrical double layer is the most important

factor in electric charging of dissimilar particles through

friction in the separation process. Therefore it is im-

portant to know the factors which determine the nature

and type of the electric charges transferred from one

surface to the other, the sense of motion of the charges

as well as the value of the resulted charge density.Thus, separating two regions between which an

electrical double layer has been formed leads to a de-

crease of the interfacial capacity, fact that implies the

appearance of very high potential differences, even if the

initial potential difference was very small. These high

potential differences determine the appearance of very

intense local electric fields, which are in return respon-

sible for the passing of the electric charges from onesurface to the other, so that the surfaces become

charged. Theoretically, a perfect insulator has no elec-

trons in its conduction band; therefore this process

cannot take place and consequently no electric charges

on the two bodies in contact should appear. Yet, expe-

riences have shown that even when putting into contact

two insulators there appears an electrical double layer

and the two materials become electrically charged. Afterinterrupting the contact, the electric charges, which re-

main, have a very slow dumping in time. The explana-

tion of this phenomenon relies on the fact, that the

insulators are not perfect. There are many fouls in their

structures, leading to the appearance of some supple-

mentary levels in the forbidden zone, which act as traps

for the electrons, as shown in Fig. 2 (after Tanasescu

and Cramariuc, 1977). Some conduction electrons arecaptured in these traps, from where they can afterwards

reach the conduction levels.

The electron energies in an insulator are function of

the position, surface impurities and local atomic struc-

ture as well as on the chemical nature of the material

and work function of the insulator.

Referring to the charge to which the two insulators

put into contact are charged, Cohen (Lindley andRowson, 1997) has established a general rule according

to which the material with higher dielectric constant

becomes positively charged. Quantitatively, this can be

expressed through the surface charge density (qs) the

bodies are charged with:

Fig. 2. Capture level for electrons in the forbidden band.

qs ¼ 15� 10�6 er1ð � er2Þ ½C=m2�; ð3Þ

where er1 and er2 are the dielectric constants of the two

dielectric materials.

Water absorption influences the charging mechanism

(Lungu, 1998; Nemeth et al., 2003); most surfaces in airare covered by a water layer with a thickness which va-

riates from a monolayer to a microscopic thin film.

Where a continuous film is formed, the water provides a

medium for dissociation of ions, and ions of one polarity

will be preferentially attracted from one of the surfaces.

The triboelectric charging process of the polymers

can be considered (Nemeth et al., 2003) as an interfacial

phenomenon of interacting polymer surfaces which canbe described by two fundamental mechanisms. The first

mechanism explains the charging effect as the result of

an electron transfer between contacting polymer sur-

faces. Originally, this effect was observed (Seanor, 1982)

for polymers contacting metals with different work

functions. The second mechanism is linked to the pres-

ence of ionic species in the surface of the polymers and

the charge exchange takes place as a flux and exchangeof oppositely charged ions.

In the following a model proposed by the author re-

garding the triboelectric charge mechanism under the

absorbed water effect in the case of two different poly-

mer particles having different work functions will be

described.

The triboelectric charging of polymers is an extremely

complex phenomenon, with a series of randomize fac-tors. This mechanism depends on the physical charac-

teristics of the dielectric particles to be separated and

also on the environmental conditions in which the

tribocharging takes place. Therefore we start by some

simplifying hypothesis regarding this phenomenon,

based on our experimental results.

First we assume that the tribocharging of two types

of particles put into contact takes place due to thecharge transfer from one type of particle to the other.

The sign and magnitude of the charge depends on the

material type and on the relative humidity of the at-

mosphere in which the process takes place. According to

the Helmholtz hypothesis (Helmholtz, 1879), the charge

transfer is produced due to the difference between the

work functions of the two materials. This fact was

confirmed by experiments and pointed out by Seanor(1982), Cross (1986) and Schubert (1996) through the

perfect concordance between the triboelectric series and

the decreasing ordering, according to the work function

of the studied polymers.

Cross (1986) and Seanor (1982) have shown that the

magnitude of the transferred charge can be estimated

only in the case of the metal–metal contact or metal-

insulator contact by using the Helmholtz theory. In thecase of the insulator-insulator contact, the magnitude

is quite impossible to be established.

72 M. Lungu / Minerals Engineering 17 (2004) 69–75

According to Nemeth et al. (2004), different orders of

magnitude can be discussed as a result of different

sample chemistry, physic-chemistry, as well as different

experimental conditions.

Afterwards, any estimation about the magnitude of

the electrical charge left on the surface has to rely on

experimental results.Second, it is assumed that the tribocharging process

takes place in several stages, during the impact of the

two different polymer particles, between which the water

absorbed on the surface of one or both polymers is in-

terposed.

The first stage of the tribocharging process is con-

sidered to be the water absorption at the surface of one

or both polymers, depending if they are hydrophilic orhydrophobic.

In the second stage the collision between two polymer

particles is considered, namely polyethylene (PE) and

polystyrene (PS); polyethylene having a greater work

function than polystyrene. It is assumed that the colli-

sion between the two particles takes place through the

absorbed water on the surface of PS, which is hydro-

philic. Because no estimations of the way the impact isproduced can be made, only the central collision is

considered (see Fig. 3).

The electric contact field Ec which appears due to the

difference between the work functions of the collided

particles starts getting important when the distance

between the particles becomes sufficiently small.

The third stage is considered to be the polarization of

the intermediate water layer, which takes place due tothe field Ec. According to the author hypothesis, at the

interface water-PS (the dielectric material with lower

work function), due to the water polarization appears a

supplementary electric field, namely the interfacial field

E. This field penetrates the particle on the distance l as aresult of the water polarization (Lungu, 1998):

l ¼ 2eVceNd

; ð4Þ

where Vc is the contact potential between the water and

PS, e the dielectric constant and Nd the concentration of

the localized states of the electrons in the conduction

Fig. 3. The collision between a PE and a PS particle through the in-

termediate water layer.

band of PS. It has been taken into account that the di-

electric constant of water (ewater � 80) is much higher

than the one of the dielectric material, therefore the

interfacial field E has a very small value in water and

consequently stretches only in the dielectric material.

The depth l is relatively small (of order 10�1 lm), so that

the intensity of the interfacial field E in the dielectricmaterial can reach very high values, of about 103–106 V/

cm. These values depend on the water polarization,

which is conditioned by the distance between the parti-

cles.

The electrons, which are in the localized states from

the conduction band of the polymer with lower work

function, are extracted under the action of the interfacial

field E through Schottky and/or tunneling effects. In thisway the polymer acquire a positive surface charge in the

contact zone.

In the fourth stage the surface electrons are trans-

ferred to the particle with higher work function (PE),

which become negatively charged in the transfer region.

Finally, PS becomes positively charged and PE nega-

tively charged.

After the impact, the particles move away from eachother and the process will repeat. The interfacial ca-

pacity C increases with the diminishing distance between

the particles and determines thus the transfer of a higher

charge amount between the surfaces. The water layer

interposed between the particles increases the contact

area, determining in this way a raise of the interfacial

capacity, too.

After Lindley and Rowson (1997) the charging ratecan be considered in terms of the addition of the new

charge and the loss of existing charge:

dQdn

¼ Qc 1

�� exp

�� tbsb

��� Q 1

�� exp

�� tcsc

��;

ð5Þ

where Q is the total charge, n is the number of contacts,

tb refers to the time between contacts, tc refers to the

time of a contact, Qc ¼ C � Vc is the charge in the contact

region during contact time tb, sb represents the time

constant for the initial decay of charge and sc representsthe time constant for back flow of the charge.

If the particles are situated sufficiently close one to

another, the contact field Ec becomes very intense (>106

V/cm), able to produce the ionic dissociation of the

absorbed water molecules (H2O$OH� +Hþ) and to

determine a supplementary ionic charge of the particles

after the impact.

3. The experimental installation

Fig. 4 shows the outline of the experimental instal-

lation used to study the electrical tribocharging of

Fig. 4. The experimental installation.

M. Lungu / Minerals Engineering 17 (2004) 69–75 73

mixtures containing equal quantities of PE and PS

particles with sizes between 2–6 mm. The mixture sub-

jected to the separation process represents the feed.

For each determination in part, at a certain value ofthe humidity, the mixture is introduced in pot 1 in order

to be tribocharged. For this purpose, the pot is vibrating

on the vertical direction for a certain time span. It has

been found out experimentally that the tribocharge de-

pends strongly on the relative humidity of the atmo-

sphere in which the charge takes place. Therefore, only

the charge dependence on the humidity was studied, the

other parameters (i.e. temperature, time span and thevibration amplitude of the pot) being kept constant.

Due to the friction and the repeated contacts between

the particles, the materials become tribocharged. Af-

terwards, the particles are let to fall free through the

funnel tube 2, along the median line between the high-

Table 1

Experimental results

Relative humidity (%)

20 30 40 50 60

m1 (g) 99 94 98 104 118

m2 (g) 198 203 198 187 179

q1 (nC) 393 295 354 416 562

q2 (nC) 951 809 873 954 998

MPS (g) 93 89.7 93.5 97.6 113

MPE (g) 138.5 146 144 143.5 145

GPS (%) 92 93.4 93.5 93.8 95.8

RPS (%) 63 63.5 65 70 75.4

GPE (%) 69.6 71.6 72 73.2 79.7

RPE (%) 94.7 95 95.5 95.6 96.7

voltage electrodes 3. Under the action of the electric field

between the electrodes, the charged particles are devi-

ated and separately collected in the compartments of

system 4. The particles, which are positively charged, fall

into the compartment designed to the product m1, while

the particles that are negatively charged in the com-

partment for the product m2. Because some particleschange their charge due the collisions with other parti-

cles or with the walls 1 and 2, the product m1 will con-

tain particles of a certain kind (PS in this case) mixed

with a small amount of particles of the other kind (PE in

this case). Just the same, the product m2 will be also

contaminated with a small amount of particles of the

other kind (PS in this case). Finally, the problem is to

establish the optimal conditions in order to obtain ineach compartment a maximum amount of a product

with a maximum purity of the collected kind of material.

4. Experimental results

The experimental results for the electrical positive q1and negative charge q2 of the collected particles corre-sponding to the quantities m1 and m2 (m1 and m2 being

mass of the particles collected in the two compartments

of the system 4) versus relative humidity of the atmo-

sphere in which the tribocharging process took place are

given in Table 1. As mentioned above, product m1

contains a quantity MPS of PS and a small amount of

PE, respectively, product m2 contains mass MPE of PE

and a small amount of PS. The problem is to establishthe optimal conditions to recover maximum quantities

of PS and PE in compartments designed to the product

m1 and m2 with a high grade of purity. In this sense the

quantities grade G (ratio between mass of a material in

the product, i.e. the quantity of PS or PE collected in

one of the useful compartments, and the product m1 or

m2) and recovery R (ratio between mass of a material in

the product and mass of the same material in the feed)have been determined for the two types of polymers, PS

and PE, versus the relative humidity.

70 80 90 92 95

148 148.6 147.5 148 141

148 147.6 147.5 148 155

1579 1864 2318 2270 604

1695 1898 2110 2113 660

147.3 149.8 148 147.6 144

147.3 148.8 149 147 149.5

98.2 99.2 98 98.4 99.4

98 99.8 98.7 97.7 96

98.2 99.9 97.3 98 96.5

98.2 99.2 96.7 98.2 99.4

Fig. 5. Electrical charge of the collected particles versus the relative

humidity.

Fig. 6. Variation of grade and recovery with the humidity in the case

of PS.

Fig. 7. Variation of grade and recovery with the humidity in the case

of PE.

74 M. Lungu / Minerals Engineering 17 (2004) 69–75

The magnitude variation of the electrical charges of

the particles corresponding to the quantities m1 and m2

function of the relative humidity is given in Fig. 5 (the

resulted electrical positive charge q1 correspond to the

quantity m1 and the resulted negative charge q2, corre-spond to the quantity m2).

The variation of grade (GPS) and recovery (RPS) withthe humidity in the case of PS is given in Fig. 6 and the

variation of grade (GPE) and recovery (RPE) with the

humidity in the case of for PE is given in Fig. 7.

5. Discussions

It follows from Figs. 6 and 7 that both grade andrecovery for PS and PE increases with the relative hu-

midity, reaching maximal values for relative humidity

between 70% and 90%, and decrease afterwards. As

shown in Fig. 5, this behavior is due to the accumulated

tribocharge of the particles which increases slightly for a

humidity up to 65% and strongly afterwards. The

maximum is reach for a value of humidity of about 90%.

The explanation of this behavior relies in the fact thatthe size of the contact surface mediated by the water

increases with the humidity. Therefore, a higher charge

quantity can be transferred. Moreover, when the value

of 65% for the humidity is exceeded, an increase of the

density of ionic charges obtained by dissociation of

water due to the increase of the contact field may be

possible. This possibility is determined by the union of

the absorbed water drops and forming on certain zonesof a water film with a smaller thickness than the one of

the drops. At the same time, the forming and extension

of the water film implies an increase of the contact field

and also an increase of the contact surface, making a

greater charge transfer possible.

After reaching the maximum, the charge decreases

suddenly. This, probably due to the increase of the

thickness of the water film interposed between the inci-dent particles, which diminishes the contact field and

consequently leads to a decrease of the charge transfer.

6. Conclusions

The main conclusion can be stated is that, as was

shown, the optimal values of the grade and recovery areincluded between the interval of 70–90%, namely at the

atmospheric values of the relative humidity. This sim-

plifies the separation equipment; being not necessary to

use a device to condition the humidity of the atmosphere

in which the process takes place, the cost of the instal-

lation decrease.

A disadvantage of the presented free-fall separation

system is the missing of controllability of the gravita-tional force, which acts upon the particles when they fall

M. Lungu / Minerals Engineering 17 (2004) 69–75 75

between the two electrodes. So, if the particles are very

small or the charges are very low, the separation area

between the electrodes has to be very long.

Generally, the disadvantage of the tribocharging

separation methods is that real surfaces, particularly the

surfaces of polymers and highly insulating materials, in

the presence of adsorbed gases and water vapor, areeven less well understood.

On the other hand, there is not easy to separate dif-

ferent insulating particle contained in a mixture one

from each another. The methods usually used for sep-

arating plastic materials (e.g. separation through mag-

netic liquids, air currents or through flotation) rely on

the mass density differences between the materials to be

separated. These methods are very expensive and theobtained results are not really satisfying. Using the tri-

boelectric effect in the separation process of insulating

particles turned out to be much cheaper and the ob-

tained results are much better than those obtained when

using the above mentioned classical separation methods.

All these phenomena determined by the existence of

many randomize factors make the tribocharge to be a

very complex dynamic process, which probably cannot betheoretically modeled in a complete way. Work function

theory provides a means of predicting the polarity of

charge just on pure surfaces. The magnitude of charge

transfer remaining on the surfaces after separation is

difficult to predict and thus commercial applications must

be based on results from experimentation because the

estimations, which can be made, are just qualitative ones.

It is important to ensure, that test work is carried out incontrolled environmental conditions which exactly reflect

those in which the process will be used.

The theory of charge transfer is still in its infancy and

evaluation of the various models requires a much better

understanding of the nature of the surfaces of polymers

and other insulators, including the electron energy levels

and the nature of the impurity layers.

Finally, the example has shown, that the triboelectric

separation method can be very helpful for processing

recycling, with a big potential in the recycling industry.

Acknowledgements

The experimental part of this work has been carried

out at the TU Bergakademie Freiberg/Germany. The

author wishes to acknowledge Prof. Dr. Ing. G.

Schubert for interesting discussions and for the help

granted in executing the experimental measurementsat his department.

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