Kerry J., Butler P. Smart Packaging Technologies for Fast Moving Consumer Goods

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(a)

−+

(b)

Figure 14.2 The direction of electrostatic force, F, acting on a point charge +q



in the ﬁeld

of a point charge q where (a) q is positive, or (b) q is negative.

yet electrically isolated, an image charge is induced by polarisation as the charged particle

approaches, as shownin Figure 14.3(b). The worst case scenario arises where the target to be

sprayed is insulating, yet here too electrostatics spraying can still deliver some improved

performance. No polarisation or image charge can be induced in an insulating surface.

Such surfaces can exhibit long-lasting surface charges arising through triboelectriﬁcation

with objects they have been in contact with, and this can result in coulombic forces of

attraction. Where a surface does not retain a surface charge opposite in polarity to the spray

droplets, deposition efﬁciency can still be increased where the two are at different electrical

potentials, and thus droplets will deposit on the surface to equalise this difference.

A sufﬁcient level of charge on the spray is a prerequisite for performance improvements

to be demonstrated. For the chargeon a particle to inﬂuence its trajectory in the air, the forces

of attraction or repulsion must overcome gravitational and inertial forces. It is generally con-

sidered that only above a charge-to-mass ratio of 100 μCkg

−1

will electrostatic forces start

to inﬂuence the trajectory of droplets (Gaunt et al., 2003a; Singh et al., 1978; Splinter, 1968).

The charge-to-mass ratio of typical off-the-shelf domestic aerosol products is in the

region of 0.01 – 1 μCkg

−1

, and in exceptional cases is as high as 10 μCkg

−1

(Gaunt

et al., 2003a,b). Similar charge levels are recorded for other household spray devices. This

−

(b)

−

(c)

−

−V

(a)

Figure 14.3 Attractive forces between a charged droplet or particle and a target surface. (a)

The target is maintained at the opposite potential to the charged particles. (b) The isolated

surface is polarised in the ﬁeld of the charged particle. (c) The target surface is connected to

earth so as the surface is polarised the negative charge is conducted away so as to reduce the

potential to zero.

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(b)

(a)

Figure 14.4 Electrostatic spraying (a), wrap-around, (b), Faraday cage effect.

charge level is insufﬁcient to inﬂuence the trajectory of spray droplets. However, sprays

incorporating designs that include passive charging technology or promote natural charge

separation phenomena, as discussed in the following sections, can produce higher levels of

charge, sufﬁcient to demonstrate the beneﬁts of electrostatic spraying.

The beneﬁts of electrostatic spraying arise when particles follow electric ﬁeld lines to a

target. This results in high deposition efﬁciency and particles reaching the back of the target,

which is usually shielded, as shown in Figure 14.4(a). However, because particles follow

ﬁeld lines it is difﬁcult to coat inside recesses due to a Faraday cage effect (Figure 14.4b).

Estimates from the paint spraying industry suggest that electrostatic spray systems deliver

improvements from 40 % efﬁciency to 70 % efﬁciency on ﬂat surfaces (Cross, 1987).

14.3 Natural Charge Separation Phenomenon

When liquids or solids come in contact with dissimilar materials, separation of positive and

negative charges readily occurs at the interface. Consequently, one of the materials becomes

positively charged and the other negatively charged. This process can occasionally lead to

signiﬁcant static problems but can also be exploited for industrial purposes. Industries that

rely on static charge to achieve a particular process require controlled sources of static

charge, so they generally make use of corona or induction charging, powered by an active

power supply. However, it is possible to exploit the natural charge separation phenomena

to good effect, giving reliable operation over long periods of time yet eliminating the need

for bulky, expensive power supply equipment. The process for charge separation in liquids

is known as ﬂow electriﬁcation and for solids it is called frictional or triboelectric charging.

The natural charge separation phenomena of tribocharging and ﬂow electriﬁcation are

rarely exploited in industrial processes. However, suitable levels of charge can readily

be attained by appropriate techniques and used to generate charged sprays. Employing

passive charging realises the advantages of electrostatic spraying whilst eliminating the

disadvantages associated with power supply equipment. In delivering passively charged

sprays, the spray device usually requires little modiﬁcation, or demands a novel design that

can be little more costly or complex than a conventional system. What can be necessary

is more careful attention to quality control and the need to earth components of the spray

device, usually through the user.

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−

Metal

−

Compact

layer

Diffuse layer

Figure 14.5 Schematic diagram of the electrical double layer of a solid–liquid interface.

14.4 Flow Electriﬁcation for Charge Separation

Flow electriﬁcation, sometimes known as double-layer charging, is a phenomenon whereby

liquids accumulate charge as they ﬂow. The phenomenon is observed during pumping and

transport of petroleum products, where it can be particularly hazardous if uncontrolled (as

reviewed by Klinkenberg and Van der Minne, 1958). Electriﬁcation in liquids can also be

exploited to generate naturally highly charged liquid sprays.

Charge reorganisation occurs on a microscopic scale in liquids at any interface, with a

solid, a gas or another liquid. One polarity of charge is held relatively tightly bound at the

interface, known as the compact layer, with ions of the opposite polarity held much more

weakly as a diffuse layer. The immobile ions of the compact layer and the diffuse layer

of counterions in the liquid together make up the electrical double layer, as represented

schematically in Figure 14.5. The nature of the electrochemical reaction at the interface

depends upon the interaction between the solid and liquid constituents, and this determines

the net ﬂux of electrical charge comprising the double layer. The distance that the diffuse

layer extends into the bulk of the liquid depends on the liquid’s conductivity. For water

based liquids where conductivity is greater than 10

−6

−1

the diffuse layer is only a few

molecules thick. For liquids where the conductivity is of the order of 10

−11

−1

, the

diffuse layer may extend a macroscopic distance from the interface. When the liquid is

moved across a surface, such as during spraying, the diffuse, weakly bound, layer of charge

is entrained within the liquid bulk, leaving the compact charge layer attached to the solid.

As charge from the diffuse layer is entrained, the electrical potential in the bulk liquid rises,

and when the liquid loses contact with the surface, such as on atomisation, the charge is

retained on the droplets. If the charging process is to continue, then the charge at the surface

must be replenished by moving through the walls of the pipe. In a metal pipe, current of

the order 10

−9

to 10

−14

A is recorded. Such low levels of current are able to ﬂow through

some insulating pipes and not limit charge separation. Alternatively, the more conducting

boundary layer at the solid–liquid interface can also provide a conduction path to earth and

thus allow charging to continue indeﬁnitely.

The amount of charge accumulated by liquid ﬂowing in a pipe is limited by liquid

conductivity, among other factors. A highly conducting liquid cannot accumulate charge

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252 Smart Packaging Technologies for Fast Moving Consumer Goods

as the rate at which ions diffuse to the pipe wall to be discharged is very rapid. A perfectly

pure insulating liquid will not charge because there are insufﬁcient dissociated ions to carry

the charge. The conductivity over which charging is observed is 10

−13

to 10

−7

−1

, while

the charge generation rate reaches a maximum at the order of 10

−10

−1

(Cross, 1987).

Flow factors are equally important in determining the charge levels attained. Liquid

charging increases with increasing ﬂow velocity and turbulence. During laminar ﬂow there

is little or no radial movement of the double layer. With the transition from laminar to

turbulent ﬂow, the diffuse layer of charge begins to be distributed across the bulk of the

liquid by turbulent transport, and as turbulence increases the thickness of the laminar

sublayer at the solid surface is reduced. The presence of a ﬁlter, valve or pump in the ﬂow

line can also enhance charging by several orders of magnitude, due to increased turbulence

and the increase in surface contact in relation to the volume of liquid.

14.5 Frictional Charging

The effects of contact and frictional charging can be easily observed when scraps of tissue

paper are picked up after rubbing a comb, from the small electric shocks delivered when

touching a door handle after dragging your feet across a carpet, or the crackle heard as a

woollen jumper is removed on a dry day. When two solids come into contact, charges are

exchanged as shown in Figure 14.6. This represents an over simpliﬁcation of the situation,

as both surfaces retain areas of negative and positive charge, yet one charge predominates.

Charge reorganisation at the interface brings the materials into thermodynamic equilibrium,

and as the surfaces are separated the excess charge is retained.

The amount of charge transferred during contact is determined by a number of factors.

In a simple, non-sliding, contact the amount of charge transferred to an insulator is gener-

ally proportional to the work function of the materials, and can be inﬂuenced by surface

imperfections and impurities (Cross, 1987). In practice charging is often achieved when

surfaces are rubbed together and in this case the energy of rubbing is more inﬂuential than

the nature of the materials. From knowledge of the materials it is seldom possible to pre-

dict how much charge will be transferred in a particular circumstance since the frictional

charging process is so complex. It is possible though, within limits, to predict the relative

Material A

Material B

−

(a)

(b)

(c)

Figure 14.6 The three stages of triboelectric charging: (a) two dissimilar materials are brought

into contact; (b) charge transfer occurs at the interface; (c) separation of the surfaces each with

its excess charge.

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Table 14.1 Triboelectric series. Materials at

the top of the table charge positively when

rubbed against materials below them in the

series (from Taylor and Secker, 1994).

Material

+ Asbestos

Glass

Human hair

Nylon

Wool

Lead

Aluminium

Cotton

Steel

Copper

Silver

Sulfur

Polyester

Polyurethane

Polypropylene

PVC

Silicon

− Teﬂon

polarities of the materials using a triboelectric series, such as given in Table 14.1. Materials

tend to charge positively against materials below them in the series, and this prediction is

more reliable the further apart the materials are from each other. The series does not give

any predictions relating to the magnitude of charge transferred, however. This is shown

when samples of identical materials are rubbed together, and substantial charge transfer

can occur. Many triboelectric series exist, and can differ from each other in some respects

due to small differences in the basic material and the test procedure used. However, it is

possible to identify trends. Contact between a metal and a plastic surface invariably leaves

the metal with a net positive charge and the plastic with a net negative charge. A rubbing

contact will impart several orders of magnitude more charge than will a simple, non-sliding

contact. Since charge transfer depends on the force and velocity of the contact, the charge

acquired by a particular material depends much on the energy of rubbing. As an example,

the level of charge generated during powder handling by grinding is in the order of 0.1 to

1 μCkg

−1

and by pneumatic transfer in the order of 1 to 100 μCkg

−1

14.6 Domestic Aerosol Sprays

Passively charging domestic aerosol sprays have recently been developed and success-

fully marketed for the household pest control sector. The technology (Gaunt and Hughes,

2003b), although challenging to implement, represents one of the most signiﬁcant technical

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Actuator

Valve Stem

Terminal Orifice

Valve

Gaseous

Propellant

Mounting Cup

Liquid

Formulation

Valve gasket

Spring

Dip tube

Figure 14.7 Schematic drawing of the main components of a domestic aerosol product and

a valve in the open position.

advances in aerosol technology since the invention of the pressure-pack spray in the late

1800s (Sanders 1970). Sprays with optimised formulation and pressure parameters ﬁtted

with an actuator that promotes shearing of the electrical double layer can produce higher

levels of charge, sufﬁcient to demonstrate the beneﬁts of electrostatic spraying. Charging is

achieved by exploiting ﬂow electriﬁcation. Formation and shearing of the electrical double

layer at the liquid/solid interface is maximised to promote the level of charge carried on

the formulation as it is atomised. Factors that affect the level of charge accumulated include

the electrochemistry of the liquids and solids interacting, ﬂow parameters such as turbulence

and velocity, and the contact area between liquid and solid (Gavis and Koszman, 1961).

The modern domestic aerosol comprises a propellant, a solvent and an active, ingredient

packaged in a container equipped with a valve for discharging the liquid formulation when

required and an actuator for delivering the desired spray characteristics of ﬂow rate, droplet

size distribution and cone angle. The main components of a typical dispenser are shown in

Figure 14.7. The propellants commonly used are liquid hydrocarbon gas (butane/propane

mix) or compressed gas. The formulation can be a homogeneous or a heterogeneous emul-

sion, the latter either having a water or oil continuous phase. Spray characteristics are

critical to good spray performance, and largely determined by actuator parameters such as

swirl chamber geometry, oriﬁce dimensions and also by propellant type and pressure.

The conductivity of the formulation is one important factor in successfully delivering

this electrostatic technology. It is the conductivity of the formulation that determines the

depth of the electrical double layer that forms at the interface of the liquid and solid,

and thus the level of electriﬁcation of the liquid. The range of conductivities over which

charging is observed is between 10

−7

and 10

−13

−1

, where the double layer may extend

a macroscopic distance from the interface (Cross, 1987). Where the formulation must

comprise a signiﬁcant percentage of water for reasons of economy and performance, this

seems impossible to achieve. The conductivity of tap water is typically in the order of 10

−2

−1

and deionised water is in the order of 10

−4

−1

, resulting in a double layer only

a few molecules thick. In overcoming this, the properties of emulsions can be exploited.

By enclosing the water in a water-in-oil emulsion, it is possible to obtain a formulation of

more suitable conductivity, such that formation of the double layer is promoted and a high

water content can be retained.

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To maintain formulation conductivity within suitable boundaries, it is also useful to use

liquid hydrocarbon gas (LPG) propellants rather than compressed gas. The conductivity of

a butane/propane mix, being of the order of 10

−8

−1

, makes LPG a suitable ingredient.

Conversely carbon dioxide, a commonly used compressed gas propellant, dissolves in and

increases the conductivity of the water phase. The use of liquid hydrocarbon propellants is

also important in maintaining a high and constant pressure, ensuring consistent charging

for the lifetime of the product. LPG propellants also contribute to generating an aerosol

product with a small and even droplet size distribution. This is important as a higher level

of charge would be required to impart electrostatic beneﬁt to larger diameter droplets.

Selection of the dispenser components primarily inﬂuences shearing of the double layer.

Promoting ﬂow velocity, ﬂow turbulence and increasing the contact area between liquid

and solid can increase the charge imparted to the spray on atomisation. As charge accu-

mulated in the liquid bulk always has an opportunity to relax further along the system, it

is the modiﬁcations made at the actuator level that are most inﬂuential in determining the

ultimate charge imparted. Thus, selection of actuator oriﬁce diameter and swirl chamber

conﬁguration can affect the charge-to-mass ratio by over two orders of magnitude. To in-

crease further the contact area between liquid and solid at the point of atomisation, novel

terminal oriﬁce designs have been developed. A standard circular oriﬁce has the lowest

possible area of contact, so any deviation from this can be expected to promote charge

levels. For example, Figure 14.8 shows the commercialised novel actuator oriﬁce design

along side a conventional oriﬁce delivering the same ﬂow rate.

Through combinations of suitable formulations and actuator selections, charges in excess

of 100 μCkg

−1

can be attained. Such systems consistently generate this level of charge

for the lifetime of the product due to the stability of an optimised assembly. Variations

in ambient temperature and relative humidity affect charging very little. Once generated,

aerosol droplets in the air do not lose their charge. As a droplet evaporates, charge is retained

on the droplet and thus the charge-to-mass ratio actually increases, enhancing electrostatic

effects.

(a) (b)

Figure 14.8 Actuator oriﬁce shapes: (a) a conventional circular oriﬁce; (b) a charge-promoting

segmented oriﬁce with the same cross-sectional area as (a). The grey shaded area represents

the solid parts.

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14.7 Induction Charging for Charge Separation

Exploitation of the ﬂow electriﬁcation phenomenon is not suitable for all spray applications.

Where very high water content formulations are to be used, or where a liquid hydrocarbon

propellant in a pressurised devicecannot be used, an alternativecharging method is required.

In industrial applications, charging is usually achieved by corona or induction charging.

Corona charging is not appropriate for a passive charging technology, as the high electrical

potential required to drive a corona discharge can realistically only be achieved using a

conventional power supply. A system using induction charging can be designed, exploiting

only natural charge separation phenomena. Real charges induce image charges in adjacent

conductors, as shownin Figure 14.3(c). This can be used in spray applications by atomising a

conducting liquid in an electric ﬁeld, as shown in Figure 14.9. The liquid will be earthed and

act as the earth electrode to create the electric ﬁeld. As the liquid ﬁlament enters the electric

ﬁeld region, charges ﬂowinto the liquid from earth. Once the liquid is atomised into droplets,

the excess charge is retained and a charged aerosol is produced. The principle determinants

of charge are the liquid’s conductivity and the voltage between the two electrodes. This

technique is suitable for devices such as pump- and trigger-actuated sprays of the type used

for some domestic cleaning products and beauty and personal care products.

In conventional induction charging spray devices, the voltage on the induction electrode

is achieved with a power supply. In a passive system however, it can be charged by triboelec-

triﬁcation (Gaunt and Hughes, 2003a,b). The charge imparted to spray droplets is highly

dependent upon the design of the induction electrode, but a charge in the region of 1–100

nC on the induction electrode is sufﬁcient to achieve charge levels on the spray in excess of

100 μCkg

−1

. 1–100 nC of surface charge can be readily attained by tribocharging between

two materials. The induction electrode when appropriately designed and constructed from

a conducting material allows intensiﬁcation of an electric ﬁeld at the point where a liquid

ﬁlament fragments into droplets. The induction electrode must be electrically isolated to

minimise leakage of static charge. Contact or rubbing with a suitable polymer or other

insulating material then imparts high levels of surface charge to the induction electrode.

−

+ + +

−−

−

Cylindrical induction

electrode

Continuous with

liquid reservoir

Spray nozzle

−

+ + +

Figure 14.9 Schematic representation of induction charging of a liquid aerosol.

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Induction electrode

Spray orifice

Induction charging

zone

Figure 14.10 Schematic drawing of the passive-charged trigger-actuated spray device.

In pilot studies, an induction electrode of aluminium was electrically isolated using

a polymer-supporting base. The induction electrode surrounds the components making

up the spray device, such that the electric ﬁeld is intensiﬁed around the point of liquid

atomisation. The induction electrode is conducting so that charge is mobile in the material,

allowing the electric ﬁeld to intensify in the desired location. A prototype device is shown

in Figure 14.10. The induction electrode is charged by rubbing with an insulating material.

It is preferred that this material be relatively insulating in order to maximise the level of

charge imparted. A charge of 20 nC on the induction electrode charges a spray of water to

a charge-to-mass ratio of 140 μCkg

−1

, which compares with a conventional charge level

of 5 μCkg

−1

when there is no static charge on the electrode. Higher electrode charges

generate higher spray charge-to-mass ratio values.

The design of the electrode with respect to the point of liquid atomisation is critical in

maximising aerosol charging. Droplets must break away from the grounded liquid ﬁlament

at the point of highest electric ﬁeld in order to maximise charge levels. One advantage of

this device is that charge is retained on the induction electrode rather than being lost or

given up during spray generation. Surface charge will decline with time due to relaxation

and surface leakage, and thus spray charge levels will similarly decline. This is reﬂected

in Figure 14.11, which shows how successive atomisations from a trigger-actuated spray

are charged without the level of charge on the induction electrode being topped up. The

implications for the technology here are that for each successive spray achieved by trigger

actuation, the charge on the induction electrode only needs to be topped up and not totally

generated in each actuation.

This technology is not currently restricted from commercial applications by the ability

to impart charge to the droplets, which can readily be achieved. Instead it is the pump- and

trigger-actuated spray technology which presently limits application of this technology.

The droplet size distribution of the aerosol produced from these types of spray in relatively

large, having a MMD (mass median diameter) in the region of 50 to 150 μm compared with

20 to 50 μm for LPG-pressurised aerosols. In order to realise the beneﬁts of electrostatic

spraying, droplets of this size would require a charge-to-mass ratio exceeding that rea-

sonably achievable by passive charging techniques. As better spray technology inevitably

becomes available, two-stage passive charging by triboelectriﬁcation and induction will

become worthwhile.

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0.0E+00

2.0E−05

4.0E−05

6.0E−05

8.0E−05

1.0E−04

1.2E−04

1.4E−04

1.6E−04

First spray Second spray Third spray

Spray charge (C/kg)

Figure 14.11 Charge-to-mass ratio of successive trigger sprays of water. The induction elec-

trode is charged once by triboelectriﬁcation to 0 C  or to 20 nC .

14.8 Realised Beneﬁts

Modern domestic aerosols are widely used to deliver air fragrances, insecticides, deodor-

ants, household surface care and other products. In the majority of these applications the

purpose is to deliver an active ingredient onto a surface, and therefore electrostatic spray-

ing will achieve a performance enhancement as a result of attraction of the spray onto the

surface, and a more even coating and deposition of droplets onto surfaces not in the direct

line of ﬁre by wrap-around.

A case study of charged domestic insecticide aerosols provides evidence for the level

of efﬁciency improvements attainable. In this example, the bioefﬁcacy of a commercially

available insecticide aerosol with a charge-to-mass ratio of 20 μCkg

−1

was compared

with the same product, which had been modiﬁed according to the principles described

above to give a charge-to-mass ratio of 200 μCkg

−1

. In two bioassays using house ﬂies

and mosquitoes the rate at which the insecticide spray took effect was increased by 25 %

(Gaunt et al., 2003a). The bioassay mimics the use of an insecticide spray against insects

in ﬂight, so involves releasing 50 insects into a room in which a spray of insecticide has

recently been introduced. A typical result for the rate of knockdown (a state of paralysis in

which the insect is incapable of coordinated movement) in house ﬂies is shown in Figure

14.12. The charged aerosol consistently achieves faster knockdown than the conventional

spray, signifying a faster rate of deposition onto insects in ﬂight.

Insects in free ﬂight are electrically isolated, conducting, targets. Charged droplets in

close proximity to an insect will cause local polarisation of charges, as indicated in Figure

14.3(b). Attraction of droplets to the insect in this way will increase deposition efﬁciency.

Space charge forces within the cloud of charged aerosol droplets will disperse the spray to

a greater degree into all areas of a space. Typical behaviour of insects means that they tend