Trends in Food Science & Technology Volume issue 2013 [doi 10.1016_j.tifs.2013.10.009] Pankaj, S.K.;...

8/10/2019Trends in Food Science & Technology Volume issue 2013 [doi 10.1016_j.tifs.2013.10.009] Pankaj, S.K.; Bueno-Ferr

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Review

Applications of cold

plasma technology in

food packaging

S.K. Pankaja, C. Bueno-Ferrera,

N.N. Misraa, V. Milosavljevica,b,C.P. ODonnellb, P. Bourkea,

K.M. Keenera,c and P.J. Cullena,*aBioPlasma Research Group, School of Food Science

and Environmental Health, Dublin Institute ofTechnology, Cathal Brugha Street, Dublin 1, Ireland

(Tel.: D353 1 402 7595; fax: D353 1 402 4495;e-mail:[email protected])

bBiosystems Engineering, University College Dublin,Dublin 4, Ireland

cPurdue University, Nelson Hall of Food Science, Rm3215, 745 Agriculture Mall Drive, West Lafayette, IN

47907 2009, USA

Cold plasma technology is an emerging, green process offering

many potential applications for food packaging. While it was

originally developed to increase the surface energy of poly-

mers, enhancing adhesion and printability, it has recently

emerged as a powerful tool for surface decontamination of

both foodstuffs and food packaging materials. New trends

aim to develop in-package decontamination, offering non-

thermal treatment of foods post-packaging. This paper pro-

vides an overview of cold plasma theory, equipment and sum-

marises recent advances in the modification of polymeric food

packaging materials along with potential applications in the

food industry.

IntroductionFor the past few decades the trend of replacing traditional

materials such as glass, metals and paper by polymeric ma-

terials has been growing continually within the various pro-

cess industries, including the food industry. This is due to

the fact that physical and chemical characteristics of poly-

mers are on a par with conventional materials in terms of

functionality. In addition, polymeric packaging materials

provide greater flexibility, transparency, adequate chemical

inertness, have low specific weights and typically cost less.

However, in most cases polymeric surfaces are hydrophobic

in nature and are often characterised by a low surface en-

ergy (Medard, Soutif, & Poncin-Epaillard, 2002a; Vesel& Mozetic, 2012). This implies that these do not possess

the specific surface properties demanded in various applica-

tions. Moreover, the production of multi-layer structured

food packaging polymers is economically demanding. In

order to obtain polymers with the desired properties, in

most instances various surface treatments are employed.

Surface treatments of packaging can serve various pur-

poses including surface functionalisation, surface cleaning

or etching, and surface deposition. Surface functionalisa-

tion refers to the introduction of specific functional groups

onto the surface layer of a polymer. Surface functionalisa-

tion of polymers is usually carried out to improve its wetta-

bility, sealability, printability, dye up-take, resistance toglazing, or adhesion to other polymers or materials, without

compromising the desired bulk properties of the polymer

(Chou & Chang, 1994; Ozdemir, Yurteri, & Sadikoglu,

1999a). Surface functionalisation has additionally been

used to enhance barrier characteristics of food packaging

polymers and to impart antimicrobial properties

(Ozdemir, Yurteri, & Sadikoglu, 1999b). Surface treatments

can also be employed to clean or etch polymer surfaces by

removing unwanted materials and contaminants from poly-

mer surface layers. Additionally surface treatments can be

used for the deposition of thin layers of coatings on poly-

mer surfaces or for sterilisation.Surface modification of polymers can be performed

either by chemical or physical methods. Physical methods

have gained preference over chemical techniques, offering

greater precision, ease of process control, and environment

friendliness. Classical physicochemical methods for modi-

fying polymer surfaces include flame and corona treatment,

ultraviolet light, gamma-ray, ion-beam techniques, low-

pressure plasma and laser treatment (Adler et al., 1999).

However, flame and corona treatments are not well suited

to polymers due to the limited time scale of the improved

properties.

Cold plasma (CP) induces several chemical and physical

processes within the plasma volume and on the* Corresponding author.

0924-2244/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.tifs.2013.10.009

Trends in Food Science & Technology xx (2013) 1e13

Please cite this article in press as: Pankaj, S. K., et al., Applications of cold plasma technology in food packaging, Trends in Food Science & Technology

(2013), http://dx.doi.org/10.1016/j.tifs.2013.10.009
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plasmaepolymer interface, which modify the surface prop-

erties. This phenomena is exploited in surface functionali-

sation to impart selective and tuneable surface energies to

the packaging polymers for promoting adhesion or some-

times anti-adhesion (Poncin-Epaillard, Brosse, & Falher,

1999), improved printability, sealability, imparting anti-mist properties and improving the polymers resistance to

mechanical failure. Using plasma deposition of barrier

layers, the barrier properties of the packaging materials to-

wards gases (oxygen, carbon-dioxide) and chemical sol-

vents can be improved (Schneider et al., 2009). Gas

plasma reactions also establish efficient inactivation of

micro-organisms (bacterial cells, spores, yeasts and

moulds) adhering to polymer surfaces within short treat-

ment times. Packaging materials such as plastic bottles,

lids and films can be rapidly sterilised using cold plasma,

without adversely affecting their bulk properties or leaving

any residues (Muranyi, Wunderlich, & Heise, 2007).This paper reviews the state of the art for cold plasma

applications for modification and surface sterilisation of

polymers of importance to food packaging, following a

brief overview of the physics and chemistry of cold

plasmas. The polymers considered include polyethylene

(PE), polypropylene (PP) and polyethylene-terephthalate

(PET), which altogether account for more than 80% of

food packaging polymers (Plastics-the Facts, 2012). The re-

view also identifies research gaps and outlines the direction

for future research work in this area.

Plasma physics and chemistry

The term plasma refers to a quasi-neutral ionised gas,primarily composed of photons, ions and free electrons as

well as atoms in their fundamental or excited states with

a net neutral charge. Plasma discharges are widely used

for processing and are indispensable for many technolog-

ical applications (Milosavljevic, Karkari, & Ellingboe,

2007). Through their wide variety of operational condi-

tions, plasma sources offer a tremendous freedom in the

generation of radiation and the creation of chemical com-

positions. As a result the field of technological and indus-

trial plasma applications is expanding strongly. Several

plasma applications have been identified in literature:

high-efficiency light sources (the rich plasma UV sourcefor surface sterilisation), material processing, such as depo-

sition, cleaning and surface modification (Lawet al., 2012),

spectrochemical analysis (analytical chemistry e plasma

spectral emission can be used for element detection with

very low detection limits) (Milosavljevic, Ellingboe, &

Daniels, 2011), waste treatment (e.g. detoxification e use

of thermal plasma torches, cascaded arc plasmas, or micro-

wave plasmas for the production of negative ions).

The ions and electrons from the plasma are generated at

an electrode by means of a radiofrequency (RF), micro-

wave (MW) or dielectric barrier discharge (DBD) power

source, and a biasing power source is applied to another

(packaging holding) electrode to create a significant ion

bombardment (remove-clean-deposit) component during

plasma treatment (Breen, Milosavljevic, & Dowling,

2011). The plasma process is a simultaneous deposition/

removing process in which loosely deposited species

over planar or topographical surfaces are sputtered off by

reactive ions and radicals during deposition.Plasma is an effective, economical, environmentally safe

method for critical cleaning. The vacuum ultraviolet (VUV)

energy is very effective in the breaking most organic bonds

(i.e., CeH, CeC, C]C, CeO, and CeN) of surface con-

taminants. This helps to break apart high molecular weight

contaminants (Donegan, Milosavljevic, & Dowling, 2013).

A second cleaning action is carried out by the oxygen spe-

cies created in the plasma (O2, O2

, O3, O, O, O, ion-

ised ozone, metastably-excited oxygen, and free electrons).

These species react with organic contaminants to form

H2O, CO, CO2, and lower molecular weight hydrocarbons.

The resulting surface is ultra-clean/sterilised. The plasmaactivated atoms and ions cause molecular sandblasting

and can break down organic contaminants.

Most of the cleaning process by-products are small

quantities of gases such as carbon-dioxide, and water

vapour with trace amounts of carbon monoxide and other

hydrocarbons (Prysiazhnyi, Zaporojchenko, Kersten, &Cernak, 2012). To put this in perspective, 10 min of auto-

mobile exhaust is approximately equivalent to one year of

plasma cleaning/sterilisation exhaust. Whether or not

organic removal is complete can be assessed by contact

angle measurements. When an organic contaminant is pre-

sent, the contact angle of water with the device will be high.

After the removal of the contaminant, the contact angle willbe reduced to the characteristic of contact with the pure

substrate. Plasma cleaning requires optimisation of a num-

ber of interrelated variables, most notably gas species, pres-

sure, time treatment, nature of substrate, and power. Thus, a

series of experiments designed to optimise processing con-

ditions should be carried out. The net result is a high degree

of day-to-day repeatability and improved yields.

Different treatment systems are being studied for appli-

cation to food packaging surfaces (Kowalonek, Kaczmarek,

& Dabrowska, 2010). A capacity coupled plasma (CCP)

sources is one of the most common types of technological

plasma sources (Milosavljevic, Ellingboe, Gaman, &Ringwood, 2008). It essentially consists of two metal elec-

trodes separated by a small distance, placed in a chamber.

The gas pressure in the chamber can be lower or equal to

atmospheric. A typical CCP system is driven by a single

RF power supply, typically at 13.56 MHz. One of two elec-

trodes is connected to the power supply, and the other one is

grounded. As this configuration is similar in principle to a

capacitor in an electric circuit, the plasma formed in this

configuration is called a capacitively coupled plasma.

CCPs have wide application including deposition, sputter-

ing and cleaning (Ryan, OFarrell, & Ellingboe, 2011).

An inductive coupled plasma (ICP) is a type of plasma

source in which the energy is supplied by electrical currents

2 S.K. Pankaj et al. / Trends in Food Science & Technology xx (2013) 1e13




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which are produced by electromagnetic induction, that is,

by time-varying magnetic fields (Milosavljevic, Faulkner,

& Hopkins, 2007). There are two types of ICP geometries:

planar and cylindrical. In the planar geometry, the electrode

is a coil of flat metal wound like a spiral. In cylindrical ge-

ometry, it is like a helical spring. When a time-varyingelectric current is passed through the coil, it creates a

time varying magnetic field around it, which in turn induces

azimuthal electric currents in the rarefied gas, leading to

break down and formation of plasma. The benefit of ICP

discharges is that they are relatively free of contamination

because the electrodes are completely outside the reaction

chamber. In a CCP, in contrast, the electrodes are often

placed inside the reactor and are thus exposed to the plasma

and subsequent reactive chemical species (Bauer, Schmuki,

von der Mark, & Park, 2013).

An electron cyclotron resonance (ECR) plasma source

has a microwave input at 2.45 GHz and a magnetron gen-erates plasma (Milosavljevic, Macgearailt, Daniels, &

Turner, 2013). Electrons trajectory is spiral vertically along

the magnetic field lines. Magnetic field strength is 875

Gauss with a dome shaped contour. The electrode which

holds food packaging could be a RF power supplied and

is used to generate direct current (DC) bias independently

of plasma ionisation. In ECR electrons travel far enough

to gain sufficient energy to strike gas molecules and cause

ionisation. Electron density (ion flux) is over an order of

magnitude higher than for CCP or ICP plasma tools, and

therefore may be more efficient for surface treatments of

packaging, i.e. surface functionalisation, surface cleaning,

etching, and/or surface deposition.Dielectric-barrier discharge (DBD) is the electrical

discharge between two electrodes separated by an insulating

dielectric barrier (OConnor, Milosavljevic, & Daniels,

2011). The process uses high voltage alternating current,

often at lower RF frequencies, but recently even at micro-

wave levels. DBD devices can be employed in many config-

urations, typically planar, using parallel plates separated by a

dielectric or cylindrical, using coaxial plates with a dielectric

tube between them. In a common coaxial configuration, the

dielectric is shaped in the same form as common fluorescent

tubing. It is filled at atmospheric pressure with either a rare

gas or rare gas-halide mix, with the glass walls acting asthe dielectric barrier. Due to the atmospheric pressure level,

such processes require high energy levels to be sustained.

Common dielectric materials include glass, quartz, ceramics

and polymers (Liang, Jensen, Pappas, & Palmese, 2011).

Modification of food packaging polymersPolyethylene (PE)

Structurally PE is one of the simplest polymers used in

food packaging. PE of varying densities, characterized by

different WVTR (water vapour transmission rate), GTR

(gas transmission rate), tensile strength, heat sealing and

other properties are commercially available. This provides

freedom to food manufacturers to choose the package

type optimum for their needs (Pankaj, Kadam, & Misra,

2011). However, the low surface energy of PE, has driven

most of the research in cold plasma towards surface modi-

fications of PE. Surface characterisation of PE with CO2,

H2O and CO2/H2O plasma has been reported by Medard

et al. (2002a)and the proposed mechanism of CO2 plasmais described inFig. 1(Medard, Soutif, & Poncin-Epaillard,

2002b). Table 1 summarises the key findings from impor-

tant studies conducted on PE using cold plasma.

Polypropylene (PP)PP is a versatile polymer used in food packaging. Its low

density, low cost, high melting point, good heat sealability

and chemically inert nature have made it an obvious choice

as a packaging material for different food products (Pankaj

et al., 2011). The low surface tension of PP poses problems

in printing, coating and lamination, thereby requiring some

additional surface treatment to increase its surface energy.PP is a saturated hydrocarbon polymer with a carbon back-

bone containing hydrogen and methyl (CH3) groups ar-

ranged in an alternating fashion. The reactivity of the

hydrogen groups for surface reactions in PP depends on the

nature of the C atom towhich they are attached and in general

it varies as Htert > Hsec > Hpriwhere Htertrefers to H atom

bonded to three C atoms, Hsec refers to H atom bonded to

two C atoms and Hpri which is bonded to only one C. Exhaus-

tive work on modelling of modification of PP films in atmo-

spheric pressure plasma discharges has been done byDorai

and Kushner (2003)andWang and He (2006). The reaction

mechanism for PP treatment by air plasma has been

described byAkishevet al.(2008). The degradation of poly-propylene upon plasma treatment is mainly due to branch

scissions, formation of low molecular weight organic mole-

cules (LMWOM) and the degradation order is as follows:

N2 < He < Air0 O2 (Poncin-Epaillard et al., 1999).

Notable results of selected studies in cold plasma processing

of PP have been summarised inTable 2.

Poly(ethylene terephthalate) (PET)PET has many desirable properties, including good

strength, rigidity, high strength-to-weight ratio, transpar-

ency, thermal stability, gas barrier property, chemical resis-

tance and formability which make it a packaging materialof choice for a wide range of food products (Pankaj

et al., 2011). However, PET, like other synthetic polymers

has lower surface energy, which necessitates surface modi-

fication for good adhesion, printing and dyeing properties.

The crystallinity of PET film is an important factor which

determines the changes in surface energy upon CP treat-

ments (Jacobset al., 2011). Surface characterisation studies

for plasma treated PET film using oxygen, carbon-dioxide,

nitrogen and helium plasma have been reported by

Almazan-Almazanet al. (2005, 2006), Inagaki, Narushim,

Tuchida, and Miyazaki (2004). Table 3 summarises the

key findings of various studies conducted on PET using

cold plasma.

3S.K. Pankaj et al. / Trends in Food Science & Technology xx (2013) 1e13




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Ageing effectThe modification of packaging surfaces with cold

plasma processing may not be permanent over extended pe-

riods. Because of the minimisation of the free surface

enthalpy, dynamic processes are observed on all functional-

ised surfaces which fade the initial modification effect

(Adleret al., 1999). The loss of beneficial attributes derived

from CP processing of polymers over time is often calledageing. For example, a loss in hydrophilicity is observed

for CP treated polymeric films when stored. This is referred

to as hydrophobic recovery. Such effects are attributed pri-

marily to inward-diffusion, agglomeration or sublimation

of LMWOMs, the reorientation or reptation of polymer

chains, whereby covalently bonded polar groups become

buried beneath the outer surface; and migration of addi-

tives from the bulk towards the surface (Garcia, Fenollar,

Lopez, Sanchis, & Balart, 2008; Guimond, Radu,

Czeremuszkin, Carlsson, & Wertheimer, 2002; Poncin-Epaillard et al., 1999; Strobel, Strobel, Lyons, Dunatov,

& Perron, 1991). Ageing effects are significant when the

Fig. 1. Mechanisms of degradation, cross-linking and functionalisation occurring on polyethylene treated by CO2cold plasma. Adapted fromMedardet al. (2002b), with permission.

Table 1. Summary of reported studies on cold plasma processing of polyethylene (PE).

Polymericpackagingmaterial

Plasma source Treatment conditions Key findings References

LDPE film RF discharge(13.56 MHz, 100 W)

Ar plasma (15e90 s,25e100 W, 15 ml/min)

Contact angle (Y),Crystallinity (Y),Roughness ([)

Ataeefard, Moradian, Mirabedini,Ebrahimi, and Asiaban (2009)


O2plasma (15e90 s,25e100 W, 15 ml/min)

Contact angle (Y),Crystallinity (Y),

Roughness ([)

Ataeefard et al. (2009)

HDPE film RF discharge (13.56 MHz) Ar:O2 9:1 (150 W,30 sccm, 0.01 torr)

Contact angle (Y) Banik et al. (2002)

HDPE film RF discharge (13.56 MHz) Ar:O2 1:9 (150 W,30 sccm, 0.01 torr)

Contact angle (Y) Banik et al. (2002)

LDPE film RF discharge (13.56 MHz) O2plasma (150 W,0.02 torr)

Contact angle (Y) Bronco, Bertoldo, Taburoni,Cepek, and Sancrotti (2004)

LDPE film RF discharge (13.56 MHz) N2plasma (150 W,0.02 torr)

Contact angle (Y) Broncoet al. (2004)

LDPE film RF discharge (8 W, 50 mTorr) Ar (2 sccm), Ar:O2(1:1 sccm), Ar:H2O(1:1 sccm) plasma

Contact angle (Y) Gilliam and Yu (2006)

PE film Microwave plasma (2860 MHz) Air plasma (140 mA,0.04 mbar, 15e60 s)

Contact angle (Y) Kaminska et al. (2002)

LDPE film RF discharge (2 kV, 0.1 mA,

13.56 MHz)

O2, N2and Air plasma

(60 s, 26 Pa)

Surface energy ([) Novak et al. (2007)





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power input to the plasma and process times are both low.

For the aforementioned example, this signifies insignificant

changes in the surface roughness, i.e. less etching (Carrino,

Polini, & Sorrentino, 2004; Mirabedini, Arabi, Salem, &

Asiaban, 2007). Conversely, where intermediate to high

doses of plasma discharges are employed, a further post-processing decrease in contact angle occurs (Kaminska,

Kaczmarek, & Kowalonek, 2002; Upadhyay, Cui,

Anderson, & Brown, 2004). Selection of suitable operating

gas mixtures for plasma, such as use of an organic gas

(CH4) with a highly reactive gas (O2) can considerably

reduce the ageing process with respect to hydrophobic re-

covery (Garcia et al., 2008). The mechanism of ageing

and approaches to delay the hydrophobic recovery is a sub-

ject of active research.Table 4provides a summary of the

research works conducted to study the ageing effects in

cold plasma treated polymeric surfaces. The ageing behav-

iour of plasma treated polymers depends on different pa-rameters, such as the medium, temperature, crystallinity

and humidity (Vesel & Mozetic, 2012).

ApplicationsFood packaging surface sterilisation

Most regulatory guidelines specify microbiological re-

quirements for food packaging materials and in many cases

the packaging process is an important critical control point

in a hazard analysis critical control point (HACCP) system

(Mittendorfer, Bierbaumer, Gratzl, & Kellauer, 2002). Food

packaging materials are intended to preserve food quality

along the distribution and storage chain and also to protectit from deterioration, damage or outside contamination. If

food packaging is not properly sterilised this may cause

further contamination of the food from the packaging sur-

face and consequently lead to health risks and economic

losses (Misra, Tiwari, Raghavarao, & Cullen, 2011). Steri-

lisation methods such as dry heat, steam, UV light and

chemicals like ethylene oxide and hydrogen peroxide

have been traditionally used for medical instruments and

implants as well as packaging materials in food industry,

but certain limitations have motivated the search for new

approaches (Lerouge, Wertheimer, & Yahia, 2001;

Schneider et al., 2005). The main drawback associatedwith such conventional sterilisation techniques is the gener-

ation of liquid effluents, which add to the overall cost of the

process. On the contrary, cold plasma sterilisation is a

chemical free, fast and safe approach, applicable to a

wide range of packaging materials and does not result in

any residues. However, its adoption for mass-production

in the food packaging sector is limited by the treatment

times, which often extend to minutes; extended sterilisation

periods are not affordable by the food industry. Schneider

et al. (2005)investigated the scalability of a plasma array

system (Duo-Plasmaline) for industrial applications, and

compared the performance to a laboratory scale system

(Plasmodul) using PET foil substrates and common

treatment times of 5 s. The spore reduction kinetics for

both systems suggests scalability of the approach.

Muranyi and co-researchers have reported on the use of

cold plasma treatments for sterilisation of PET foils, poly-

styrene, as well as multi-layer packaging based on PET/

PVDC/PE-LD (Muranyi, Wunderlich, & Heise, 2008;Muranyi, Wunderlich, & Langowski, 2010). The group

has identified an increase in relative gas humidity as a

key factor to achieve a minimum of 2log10 inactivation in

Aspergillus niger and Bacillus subtilis for 1 s treatments.

Damage to the DNA of Bacillus atrophaeus endospores

and vegetative cells as a consequence of synergistic combi-

nation of UV radiation and direct plasma from cascaded

dielectric barrier discharge (CDBD) have also been re-

ported (Muranyi et al., 2010). This treatment combination

suggests effective sterilisation with very short treatment

times, whereby changes in packaging materials are

restricted and functionality remains uncompromised. Otherstudies (Yang, Chen, Gao, & Guo, 2009) report the effect of

O2 plasma excited by 13.56 MHz RF sterilisation of PET

sheets depending on their position in the discharge area,

afterglow area or remote area in the reaction equipment.

Respectively decreasing germicidal effect was found for

the three areas in Pseudomonas aeruginosa with less time

exposure than other traditional methods.

The immobilisation of bioactive functional compounds

like lysozyme, nicin, vanillin, sodium benzoate, glucose

oxidase or antimicrobial peptides into the packaging mate-

rial by plasma treatment has been extensively studied in the

emerging field of antimicrobial and active packaging

(Appendini & Hotchkiss, 2002; Buonocore et al., 2004;Fernandez-Gutierrez, Pedrow, Pitts, & Powers, 2010;

Ghanem & Ghaly, 2004; Lee, 2010; Lerouge et al., 2001;

Mastromatteo, Lecce, De Vietro, Favia, & Del Nobile,

2011; Misra et al., 2011). Other antimicrobial substances

like chitosan, silver and trichlosan have been immobilised

on films by plasma treatment (Joerger, Sabesan, Visioli,

Urian, & Joerger, 2009; Nobile et al., 2004; Popelka

et al., 2012; Vartiainen, Ratto, & Paulussen, 2005; Zhang

et al., 2006). In the study ofJoerger et al. (2009), chitosan

and chitosan/silver films were obtained by a relatively sim-

ple coating process by means of corona treatment showing

good antimicrobial activity in Escherichia coli and Listeriamonocytogenes. A recent study of Popelka et al. (2012)

demonstrated the successful immobilisation of triclosan

and chlorhexidine on LDPE via polyacrylic acid (PAA)

grafted on LDPE by low-temperature barrier discharge

plasma (Fig. 2). It was found that both substances were

properly grafted and met the required antibacterial specifi-

cations. Triclosan coated samples were more active against

the two micro-organisms tested (E. coli and Staphylococcus

aureus).

PrintingSurface activation and functionalisation by atmospheric-

pressure plasma enables the processing of different





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materials and coatings that are very thin; as for example in

the production of composite packaging. Whether labelling

jam jars, printing on glass containers, or sealing liquid

packages, a key concern in the packaging industry is the

ability to process materials reliably and at low cost

(Banu, 2012; Ozdemir & Sadikoglu, 1998).

Imprints on packaging in the food and pharmaceutical

industries are diverse (e.g. best-before dates or European

Article Number codes -EAN-) and it is essential that such

imprints are secure against abrasion. Plasma treatment

can fulfil the requirements of precise colour matching and

high pixel accuracy, when applying decoration to glass bot-

tles or jars. In addition, it enables exclusion of air bubbles,

improvement of coating adhesion and high scratch resis-

tance, without posterior damage.

Several techniques are in practice for quantification of

surface changes resulting from plasma treatment that affect

printability. However, surface contact angle is a widespread

Table 2. Summary of reported studies on cold plasma processing of polypropylene (PP).



PP film AC discharge (50 Hz,

2 electrodes, 1000 Pa)

Air plasma

(30 kV, 20 dm3/h, 120 s)

Contact angle (Y),

Adhesion ([)

Carrino, Moroni,

and Polini (2002)PP film Jet plasma DC discharge

(35 W, diffusive-filamentarymode)

Air plasma (20 m/s, 6 W/cm2) Contact angle (Y) Akishevet al. (2008)

PP film Jet plasma DC discharge(35 W, diffusive-filamentarymode)

Nitrogen plasma(15 m/s, 3e5 W/cm2)

Contact angle (Y) Akishevet al. (2008)

IsotacticPP film

Microwave plasma(433 MHz, 0e250 W)

CO2 plasma(60 W, 20 sccm, 0.75 mbar)

Degradation yield ([),Roughness ([),Total surface energy ([)

Bertrand andPoncin-Epaillard (2003)

PP film Air corona Air plasma (30 kHz, 1.7 J/cm2) Contact angle (Y),Ink Adhesion ([)

Dixon and Meenan(2012), Strobelet al. (1991)

PP film RF plasma(13.56 MHz, 150 W)

CH4eO2 plasma [80:20](100 cm3/min, 31e32 Pa)

Contact angle (Y),Increase in weight,Oxygen content ([),Nitrogen content (w),Roughness([)

Garcia et al. (2008),Lopez, Sanchis, Garca,Fenollar, and Balart (2009)


Ar plasma(20 sccm, 23.33 Pa, 8 min)

Contact angle (Y),Roughness ([)

Gomathi and Neogi (2009)

BiaxiallyOriented PP(BOPP) film

Air corona Air plasma (1 kHz) Contact angle (Y),Roughness ([)

Guimond et al. (2002)

BOPP film APGD N2 plasma (1e6 kHz) Contact angle (Y),Roughness ([)

Guimond et al. (2002)

BOPP film RF plasma(13.56 MHz, 10e50 W)

Ar plasma(15 ml/min, 0.35 bar, 0e300 s)


Mirabedini et al. (2007)

BOPP film RF plasma(13.56 MHz, 10e50 W)

O2 plasma(15 ml/min, 0.35 bar, 0e300 s)


Mirabedini et al. (2007)

PP film Glow discharge (DC)(400 V, 10 W, 25 mA)

Air plasma(0.2 mbar, 2e20 min)

Contact angle (Y),Adhesion work ([),Polarity ([),Degradation yield ([),Oxygen content ([),Roughness ([)

Navaneetha Pandiyarajet al. (2008)

PP film Diode plasma discharge(3.1, 8.3 W)

Ar plasma (10 Pa, 0e240 s) Contact angle (Y),Oxygen content ([),Roughness (w)

Slepicka et al. (2010)

PP film DC plasma (1e30 kV) O2 plasma (5e120 s,0.5e2 kPa)

Contact angle (Y) Sorrentino, Carrino,and Napolitano (2007)

PP film DBD plasma (3e20 kV,25e50 kHz)

Air plasma (upto 6.7 J/cm2) Contact angle (Y),O/C ratio ([and thensaturates)

Sorrentino et al. (2007)

PP film Microwave plasma

(2860 MHz)

Air plasma

(140 mA, 0.04 mbar, 15e

135 s)


PP film DBD plasma (15 kV,300e1000 W, 30 kHz)

Air plasma (1.2e60 kJ/m2) Contact angle (Y), (Y),O/C ratio ([and thensaturates), Roughness ([)

Leroux, Campagne,Perwuelz, andGengembre (2008)





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technique to investigate wettability and surface energy,

which is closely related to ink adhesion (Dixon &

Meenan, 2012; Navaneetha Pandiyaraj, Selvarajan,

Deshmukh, & Gao, 2008; Strobel et al., 1991). Notably,

the wetting of a surface is not only determined by the

magnitude of its surface energy, but also by the free energy

of adhesion between the solid and liquid as well as the sur-

face tension of the liquid (Bardos & Barankova, 2010). This

aspect must be taken into consideration for all practical ap-plications. The benefits of plasma processing in regards to

printing on packages can be visually appreciated from

Fig. 3, which compares the effects on a PE surface (a)

before and (b) after a 5 s exposure to a Ne-FHC plasma

treatment. Clearly, post-plasma processing, the spread-

ability of ink on PE is dramatically enhanced.

Mass transferTreatment of food packaging materials with cold plasma

can enhance food packaging barrier properties. Whether

used as a sterilisation method or for surface modification

of packages, cold plasma treatment can affect mechanical

and mass transfer (barrier and migration) properties. This

has remained an under researched topic and future work

should consider e (i) permeation of gases or vapours (ox-

ygen, water vapour, aroma compounds, etc.) through the

packaging materials from the external atmosphere into

the food or the headspace and vice versa; and (ii) migration

of low-molecular weight substances from the packaging

into the food (e.g. monomers, plasticisers, solvents) that

need to be evaluated for legislation and toxicological eval-

uation (Guillard, Mauricio-Iglesias, & Gontard, 2010).Alteration of barrier properties in materials for food con-

tact applications has been one of the most studied applica-

tions in polymers treated by cold plasma (Lee, 2010;

Ozdemir & Sadikoglu, 1998) since it is crucial factor to

control shelf life of fresh produce. Tennet al. (2012)eval-

uated the water vapour permeability of plasma treated

ethylene vinyl alcohol (EVOH) films with different percent-

ages of ethylene content. They reported that the hydropho-

bicity was significantly improved after plasma treatment for

all films and consequently water permeability was

decreased by up to 28% in some cases. Furthermore, the ef-

fects varied with content of ethylene and hydroxyl groups

of film, following cross-linking reactions. Literature also

Table 3. Summary of reported studies on cold plasma processing of PET.



PET film Jet plasma DC discharge

(35 W, diffusive-filamentary mode)

Air plasma (20 m/s, 6 W/cm2) Contact angle (Y) Akishev et al. (2008)

PET film Jet plasma DC discharge(35 W, diffusive-filamentary mode)

Nitrogen plasma(15 m/s, 3e5 W/cm2)

Contact angle (Y) Akishev et al. (2008)

PET film Microwave plasma (200 W) CO2 plasma(4 and 15 min, 1.33 mbar)

Surface energy ([),Roughness ([)

Almazan-Almazanet al. (2005)

PET film Microwave plasma (200 W) O2plasma(4 and 15 min, 1.33 mbar)

Surface energy ([),Roughness ([)

Almazan-Almazanet al. (2005)

PET fibre RF plasma (13.56 MHz, 50 W) O2plasma (40 Pa, 5e100 s) Contact angle (Y),Average tensilestrength ([)

Cioffi, Voorwald,and Mota (2003)

PET film DBD plasma(3e20 kV, 40e80 kHz)

Air plasma(9.6, 14, 21.9 W/cm2)

Contact angle (Y),O/C ratio ([)

Cui, Upadhyay, Anderson,Meenan, and Brown (2007)

PET film RF plasma(150e300 W, 15 kV, 30 kHz)

Air plasma(43.4, 73.4, 105.4 J/cm2)

Reflectivity ([),Roughness ([)

Esena, Zanini,and Riccardi (2007)

PET film(biaxiallyoriented)

Jet plasma (285 V, 6 A, 16 kHz) Air plasma (0.16e0.81 m/s) Contact angle (Y) Gotoh, Yasukawa, andTaniguchi (2011)

PET film Microwave plasma (2860 MHz) Air plasma (140 mA,0.04 mbar, 15e135 s)


PET film Glow discharge (DC)(400 V, 10 W, 25 mA)

Air plasma (0.2 mbar,2e20 min)

Contact angle (Y),Adhesion work ([),Polarity ([),Degradation yield ([),Oxygen content ([),Roughness ([)

Navaneetha Pandiyarajet al. (2008)

PET film Corona discharge (0.14e1 kW) Air plasma (5e25 m/min) Contact angle (Y),Oxygen content (w),

OHare et al. (2002)

PET film Glow discharge (400 V) Air plasma (0.2 mbar,2e25 min)

Contact angle (Y),Degradation yield ([),Roughness ([),Crystallinity ([),Oxygen content ([)

Pandiyaraj et al. (2008)





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Table 4. Summary of reported studies on the ageing effects in cold plasma treated polymeric surfaces.


Plasma source Treatment conditions Storageperiod

Observations

BOPP film RF plasma

(13.56 MHz, 10e50 W)

Ar and O2 plasma

(15 ml/min, 0.35 bar, 0e300 s)

30 days Aging effect on samples treated

for a longer time is less than samtreated for shorter time

BOPP film Air corona and APGD Air and N2 plasma(1 kHz; 1e6 kHz)

3 months Similar aging kinetics for both tre


CH4eO2 plasma [80:20](100 cm3/min, 31e32 Pa)

3 weeks Reduction in wettability is low thother gas plasma,Storage temperature and RH are cfor hydrophobic recovery process

PP film AC discharge(50 Hz, 2 electrodes,1000 Pa)

Air plasma(30 kV, 20 dm3/h, 120 s)

10 days Wettability decrease not significafirst few hours treatment, but is reafter one or more days,Wettability decay is not influencecold plasma parameters like tenstreatment time and air flow rate

PP film Diode plasma

discharge (3.1, 8.3 W)

Ar plasma (10 Pa, 0e240 s) 7 days Independent to plasma discharge

full surface relaxation and contacrestoration to saturated value waafter about 70 h of aging

PP film DC plasma (1e30 kV) O2plasma(5e120 s, 0.5e2 kPa)

30 days Decrease of 5% wettability in onafter the treatment while it achie18% after 30 days

PP film DBD plasma(3e20 kV, 25e50 kHz)

Air plasma (upto 6.7 J/cm2) 30 days Lower doses: slight recovery of cangle; intermediate and high dosdecrease in contact angle


Ar plasma (15e90 s,25e100 W, 15 ml/min)

7 days Non-linear decrease in contact a

HDPE film RF discharge(13.56 MHz)

Ar:O2 1:9/9:1 plasma(50e150 W, 30 sccm, 0.01 torr)

30 days Decrease in aging effects by incrcrystallinity

LDPE film Corona discharge(1 kW, 50 Hz)

Air plasma (600 W, 15 m/min) 21 days Partial hydrophobic recovery

PET film DBD plasma(3e20 kV, 40e80 kHz)

Air plasma (9.6, 14, 21.9 W/cm2) 3 months Partial hydrophobic recovery

PET film(biaxiallyoriented)

Jet plasma(285 V, 6 A, 16 kHz)

Air plasma (0.16e0.81 m/s) 14 days Partial hydrophobic recovery

PET film Glow discharge (400 V) Air plasma(0.2 mbar, 2e25 min)

20 days Increase in contact angle, no sigdifference in 10 and 20 days stor

Pleasecitethisarticleinpressas:Pankaj,S.K.,etal.,Applicationsofcoldplasmatechnologyinfoodpackaging,TrendsinFoodScience&

Technology

(2013),http://dx.doi.org/10.1016/j.tifs.2013.10.009


9/13

reveals numerous studies for improvement of barrier prop-

erties through deposition of thin layers of SiOx

on PET foils

by plasma enhanced chemical vapour deposition (PECVD)

(Deilmann, Grabowski, Thei, Bibinov, & Awakowicz,

2008; Deilmann, Thei, & Awakowicz, 2008; Plog,

Schneider, Walker, Schulz, & Stroth, 2011). In general, a

reduction by more than a factor of two in water vapour

flux has been observed in coated PET foils. Films havethe advantage of being colourless, thereby permitting cus-

tomers to have a clear view of the packaged food. Fortu-

nately, PECVD does not compromise transparency of

packaging films. Novel green bio-polymers such as poly(-

lactic acid) (PLA), chitosan and arabinoxylans (AXs) in-

tended for food packaging applications, are under

development. Of these, only PLA enjoys commercialised

status. Unfortunately, barrier properties of these materials

are usually inferior to traditional polymers. Cold plasma

treatment has been shown to enhance gas permeability

through cross-linking PLA with tetramethoxysilane

(TMOS) (Uemuraet al., 2006), or deposition of hydropho-

bic silicon coating onto chitosan polysaccharide film

Fig. 2. AFM surface changes for Sample 1e5: 1 e untreated LDPE; 2 e plasma-treated; 3 e AA grafted; 4 e triclosan coated; 5 e chlorhexidinecoated. Adapted fromAnton Popelka et al. 2012, with permission.

Fig. 3. Effect of FHC plasma activation on PE web surface. Testing inkballs up and forms drops on an untreated surface (surface energyb34 mN/m), while on the plasma treated surface (surface energy56 mN/m) the ink forms a continuous film. Adapted from Bardos

and Barankova, 2010, with permission.





10/13

surface (Assis & Hotchkiss, 2007) and grafting omega-3

fatty acids onto AX polymeric chains (Peroval et al., 2003).

Although permeation of gases can affect shelf life and

quality of packaged food and beverage, the second type

of mass transfer, namely migration, could be a potential

risk to consumers health. Only a few studies have reportedon migration in food packaging materials resulting from

cold plasma processing (Audic, Poncin-Epaillard, Reyx,

& Brosse, 2001). The potential migration of low-

molecular substances when plasma is applied with other

objectives (surface sterilisation, increase in adhesion or

printability, etc) has not been investigated to date. In the

case of poly(vinyl chloride) (PVC)-based commercial

wrap films or gaskets in metal lids for glass jars, various

processing aids like plasticisers and/or stabilisers may

exude from the packaging material during storage or can

be extracted by the foodstuff. This migration phenomenon

causes chemical contamination of the packaged food anda decrease of chemical and physical properties of the plas-

tic material.Audic et al. (2001) have investigated the cold

plasma modification of PVC flexible films with respect to

the migrational properties. They employed different gases

for PVC films with conventional (bis(2-ethylhexyl)adipate

-DEHA-, and epoxidised soybean oil -ESO-) and non-

conventional (permanent elastomeric EVACO) plasticisers.

The group reported a decrease in overall migration when

DEHA and ESO were substituted by EVACO, but an in-

crease in specific migration of DEHA and ESO. After treat-

ment with Ar plasma, a significant reduction in migration

from all plasticised PVC films was also noted.

Despite the numerous efforts made by food safety re-searchers to control migration of polymer additives from

food packaging materials to food products, the problem re-

mains unresolved. In light of the aforementioned studies for

safety assessment of packaging materials following plasma

treatment to avoid migration or the extent of effects in food

packaging materials, adjustment and optimisation of the

technique should be addressed as a function of the targeted

effects in future works.

In-package plasma technologyRecently DBDs have been employed for generation of

plasma inside sealed packages containing bacterial samples(Connolly et al., 2013; Leipold, Schultz-Jensen, Kusano,

Bindslev, & Jacobsen, 2011; Misra, Ziuzina, Cullen, &

Keener, 2012), fresh produce (Klockow & Keener, 2009),

fish (Chiper, Chen, Mejlholm, Dalgaard, & Stamate,

2011) and meat (Rd, Hansen, Leipold, & Knchel,

2012). The in-package plasma decontamination of foods

and biomaterials relies on use of the polymeric package it-

self as a dielectric and has been studied using several pack-

aging materials such as LDPE, HDPE, polystyrene (PS),

Tyvek etc. (Keener et al., 2012). All these works have

demonstrated significant reduction in microbial population

on food products. Moreover, this approach is easy to scale-

up to continuous industrial processing and could prevent

post-packaging contamination (Misra et al., 2011). For a

complete assessment of the technology, it is essential to

quantify all possible changes to the packaging, induced

by the cold plasma. For example, the migration limits of ad-

ditives, monomers, oligomers and low molecular weight

volatile compounds from the packaging material into thefood (following in-package plasma) should be evaluated

for food safety reasons, as well as water vapour and oxygen

permeability.

ConclusionsBesides modification of chemical and physical states of

material surfaces (without altering the bulk properties),

cold plasma treatment of polymeric surfaces is an important

technique for achieving surface sterilisation. The numerous

works reported to date for the characterisation of surface

modifications in cold plasma treated polymeric materials

of importance to food packaging have been consolidatedin this review. The quantitation of bulk and mass transport

properties (WVTR, GTR, chemical migration) of cold

plasma treated films is an under-researched area. These

properties are essential for design of packages suitable for

both respiring and non-respiring foods, and also the assess-

ment of product safety.

Future trendsPlasma modification of polymeric surfaces has evolved

as an alternative to wet chemical surface modifying treat-

ments due to its many important advantages such as unifor-

mity, reproducibility, short reaction time and environmental

safety. Although our review focused on cold plasma assis-ted surface modification of food packaging materials, the

technique can also be used for suitably modifying the bio-

responsive properties of food contact surfaces, including

metals. For example, when stainless steel is deposited

with ethylenediamine (EDA) it decreases the microbial

attachment and creates bacterial anti-fouling surfaces

(Sen, Bagc, Gulec, & Mutlu, 2012).

Fernandez-Gutierrez et al. (2010) demonstrated the appli-

cation of cold gas plasma to apples for the deposition of

vanillin film, the antimicrobial nature of which against bac-

teria, yeasts and fungi is well established (Cerrutti &

Alzamora, 1996; Fitzgeraldet al., 2004). Thus, cold plasmaaided deposition of bioactives and antimicrobials (Orhan,

Kut, & Gunesoglu, 2012; Popelka et al., 2012) can add a

new dimension in the emerging field of edible films and

active packaging of foods. Future studies should be directed

towards assessment of the efficacy of antimicrobials after im-

mobilisation on cold plasma grafted food contact surfaces.

AcknowledgementsThe authors would like to acknowledge funding from the

European Communitys Seventh Framework Program (FP7/

2207-2013) under grant agreement number 285820 and the

Food Institutional Research Measure, administered by the

Department of Agriculture, Food & the Marine Ireland.





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Trends in Food Science & Technology Volume issue 2013 [doi 10.1016_j.tifs.2013.10.009] Pankaj, S.K.;...

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Transcript of Trends in Food Science & Technology Volume issue 2013 [doi 10.1016_j.tifs.2013.10.009] Pankaj, S.K.;...