Food Packaging and Shelf Life 6 (2015) 21–29

Permeation of water vapour, nitrogen, oxygen and carbon dioxidethrough whey protein isolate based films and coatings—Permselectivity and activation energy

Markus Schmida,b,*, Wiebke Zillingera,c, Kajetan Müllera,d, Sven Sängerlauba,b

aMaterials Development Department, Fraunhofer Institute for Process Engineering and Packaging IVV, Giggenhauser Straße 35, 85354 Freising, GermanybChair of Food Packaging Technology, Technische Universität München, Weihenstephaner Steig 22, 85354 Freising, Germanyc Food Technology Department, University of Applied Sciences Fulda, Marquardstraße 35, 36039 Fulda, Germanyd Faculty of Mechanical Engineering, University of Applied Science Kempten, Bahnhofstraße 61, 87435 Kempten, Germany

A R T I C L E I N F O

Article history:Received 29 March 2015Received in revised form 14 July 2015Accepted 9 August 2015Available online xxx

Keywords:Whey protein filmsPET coatingsPermselectivityOxygen permeabilityActivation energy

A B S T R A C T

The aim of this study was to evaluate the influence of relative humidity (RH) on the oxygen permeabilityand water vapour transmission rate (WVTR) of whey protein coated Polyethylene terephthalate (PET) orwhey protein monolayer films. Furthermore, the activation energies for the permeability of oxygen,carbon dioxide and nitrogen as well as the permselectivities under different set of temperatures weremeasured. The results showed that the permeability values through the whey protein coating and wheyprotein film increased with increasing RH. The water vapour permeability measured at 50% RH of(9.3 � 0.6)�10 cm3 (STP) cm cm�2 s�1 Pa�1 increased to (16.2 � 0.9)�10 cm3 (STP) cm cm�2 s�1 Pa�1 at 85%RH. An increase in temperature showed an expected increase of the permeability in both uncoated PETand the whey protein monolayer. The permselectivity of whey protein film was determined and thecalculated ratios of the permselectivity (P(N2)/P(O2)/P(CO2):1/(4–6/(34–35)) differed from the simplifiedratios given in the literature (P(N2)/P(O2)/P(CO2):1/4/16). Furthermore the calculated activation energyvalues for whey (51 kJ mol�1) were in agreement with other studies.

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1. Introduction

A multitude of different synthetic polymers were developed inthe last century and are now being processed into packagingmaterials andusedasor in laminates(Welle,2007). Increasingly, bio-based polymers are the focus of research and development work(Schmid et al., 2012; Belyamani, Prochazka, Assezat, & Debeaufort,2014; Kurek, Galus, & Debeaufort, 2014; Marcos et al., 2014; Peroneet al., 2014). An example is whey protein (WP) (McHugh & Krochta,1994; Schmid, Krimmel, 2014; Schmid, Sängerlaub, 2014). Thebarrier properties of WP and the influence of the processingconditions have been examined by various research groups (Fang,Tung, Britt, Yada, & Dalgleish, 2002; de Paula Herrmann, Yoshida,Antunes, & Marcondes, 2004; Hong & Krochta, 2004; Kokoszka,

* Corresponding author at: Materials Development Department, FraunhoferInstitute for Process Engineering and Packaging IVV, Giggenhauser Straße 35, 85354Freising, Germany. Fax: +49 (0) 8161 491 555.

E-mail addresses: markus.schmid@ivv.fraunhofer.de (M. Schmid),wiebke.zillinger@ivv.fraunhofer.de (W. Zillinger),kajetan.mueller@ivv.fraunhofer.de (K. Müller), sven.saengerlaub@ivv.fraunhofer.de(S. Sängerlaub).

http://dx.doi.org/10.1016/j.fpsl.2015.08.0022214-2894/ã 2015 Elsevier Ltd. All rights reserved.

Debeaufort, Lenart, & Voilley, 2010; Cinelli et al., 2014; Hammann, &Schmid, 2014; Schmid, Hammann, 2014). However, there is still alack of data about: (1) the permselectivity, (2) the activation energy(EA) for permeation and (3) the influence of the relative humidity(RH) on the oxygen (O2) and water vapour (WV) barrier. Informationabout these properties facilitate the design of tailor-made packagingmaterials for specific goods.

The manuscript applies ‘standard’ models and theories aboutpermeation based on Fick’s laws and Graham’s solution-diffusionmodel which are described elsewhere (Barrer, 1941; Crank, 1979;Fick, 1855; Graham, 1866; Koros William, 1990; Krevelen, 1990;Langowski, 2008; Vieth, 1991).

(1) Permselectivity. The term permselectivity describes thepermeability ratio of different permeates. The ratio of P(N2):P(O2):P(CO2) is for many polymers in the range of 1:4:16 (Müller, 2013).

(2) Activation energy. The permeability (Q) of a material is afunction of the temperature (T) and is described by Arrheniusequation Eq. (2) (Arrhenius, 1889). Q increases exponentially withT. The EA is a measure of the energy barrier that needs to beovercome for Q (McKeen, 2012). Eq. (1) is valid for polymers for agiven morphological state. Polymers are in a glassy state below theglass transmission temperature (Tg) and are in rubbery state above

22 M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29

Tg. The Tg of films made from the major component of WP(b-lactoglobolin) is reported to be approx. �47 �C at 43% RH. Thus,it can be assumed that plasticised WP based films are in rubberystate under ambient conditions (Sothornvit, Reid, & Krochta,2002).

Eq (1): Arrhenius’ equation

After logarithmical conversion (see Eq. (2)) and by given respectto the determinations, ln(P) against 1/T is plotted in a diagramwhich results in a linear curve (function of x). P0 represents the y-axis intercept from the regression line and is displayed by the tablecalculation software just like the value of the slope. Looking closer,it comes clear that Eq. (2) follows the form of a normal straight linein which x is represented by 1/T, the y-axis-intercept equals ln(P0)and the slope is EA/�R. As the universal gas constant and the slopecould be read from the diagram, EA could be calculated via Eq. (3).

Eq. (2): formation of Arrhenius’ equation

Eq. (3): activation energy

×

(3) Influence of relative humidity on barrier properties. Theinfluence of the water vapour (WV) partial pressure on Q can bedifferentiated into three cases. (1) The O2 and WV Q areindependent of the RH for polymers that do not interact withWV such as non-polar polymers (e.g. polyethylene). (2) Polarpolymers interact with WV, thus Q increases with higher WVpressure (Kucukpinar, & Doruker, 2004). This behaviour wasobserved for the WV permeability coefficient (P) of chitosan, ethylcellulose and polyamide 6 (Wellons, & Stannett, 1966; Marais et al.,2000; Cerruti, Laurienzo, Malinconico, & Carfagna, 2007). (3) Forsome polymers Q first decreases and then increases with higherWV pressure. Examples are the WV Q of glassy poly(acrylonitrile)(PAN), nylon 6,6 and some aromatic polyamides and the oxygenpermeability (O2Q) of ethylene-vinyl alcohol copolymer andpolyamide 6 (Barrie and Machin, 1969; Alfageme, Iriarte, Iruin,Etxeberria, & Uriarte, 1999; Zhang, Britt, & Tung, 2001; Hu et al.,2005; Muramatsu et al., 2003; Cerruti et al., 2007). Polyethyleneterephthalate (PET) is a promising substrate for WP coatings, theaforementioned properties tested on PET were therefore thesubject of this study.

2. Material & methods

2.1. Materials

In the present study a whey protein isolate (WPI) (BiProsupplied by Davisco Foods International Inc., Le Sueur, Minnesota,

USA, residual moisture �5 g/100 g), was used to build the base ofthe WP solution for the application. The plasticiser, glycerol (MerckKGaA, Darmstadt, Germany), was used to prevent brittleness of thefilms. The barrier properties were tested for (1) WPI coated on aPET substrate and (2) a monolayer WPI-film. The substrate wasneeded because the monolayer WPI film was more fragile andcould not be used for all Q testing devices. (1) PET, thickness(d) = 11.5 � 0.5 mm, Mitsubishi Polyester Film GmbH, Wiesbaden,Germany, was used as the substrate. The liquid WPI solutionwas applied on the surface. In combination with PET, the WPI was

applied as a part of a multilayer (hereafter called the “coating”). (2)Monolayer films were produced by coating poly(tetrafluoro-ethylene) (PTFE, d = 500 � 20 mm, Sahlberg GmbH & Co., KG,Feldkirchen, Germany). The WPI layer was removed after drying(hereafter called the “film”).

Both substrates were corona pre-treated to a surface energy>40 mN/m in order to achieve suitable wettability for the WPIsolution for even spreading. Regarding the PTFE surface, the effectof the enhanced surface energy diminishes during the time ofstorage of the samples. Furthermore, due to the marked non-adhesiveness of PTFE, it was possible to remove a coherent WPIfilm from that substrate.

2.2. WPI solution preparation

A standard denatured WPI solution was used for all samples,since denaturation of the proteins is a prerequisite for buildingcoherent films. An aqueous solution of WPI (10% w/w) was heatedto 90 �C and stirred (Thermomix 31-1 heating stirrer, VorwerkDeutschland Stiftung & Co., KG, Wuppertal, Germany) for 30 min toensure denaturation of all protein fractions (Schmid, Eib, & Reinelt,2015). The solution was then filled into a glass flask and placed in

M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29 23

an ultrasonic water bath (DT 514H, Bandelin electronic GmbH &Co., KG, Berlin, Germany) to degas the solution and cool it to roomtemperature (23 �C). Then, the plasticiser was added (66.7% basedon WPI content) and the mixture was stirred for another 30 min.The degassing was then repeated to remove any air bubbles thatcould cause defects in the subsequent coating process. The solutionwas usable for 48 h.

2.3. WPI coating on the substrate

During the coating process, the WPI solution was spread evenlyon the different substrates (PET & PTFE) using a coating unit (CUF 5,Sumet Messtechnik, Denklingen, Germany.) The same parameterswere used for all the samples: speed of application 20 mm/s;sample length 330 mm; contact pressure 20 N; drying temperature105 �C; drying interval 8.5 min. After the coating process, allsamples were stored horizontal side by side, not stacked, at 23 �Cand 50% RH for at least five days. Within the storage time, thesamples gained equilibrium moisture. The samples of the coatedPET were used for investigations of the bilayer, whereas the layeron the PTFE was removed from the substrate and used as a samplefilm.

2.4. Barrier properties

For testing the barrier properties, three methods were applied:(1) a manometric method for O2, nitrogen (N2) and carbon dioxide(CO2), (2) an O2—specific carrier gas method and (3) the water

vapour transmission rate (WVTR) was determined using agravimetric method.

(1) Manometric method. The determinations were performedaccording to DIN 53380-2 (GDP/E measuring device, BruggerFeinmechanik GmbH, München, Germany) at three temperatures:23 �C, 35 �C and 40 �C and with three different dry gases: N2, O2 andCO2, respectively. An uncoated PET film was compared to a WPIfilm of approximately 20 mm thickness.

The measurement was based on an absolute pressure differencebetween the two sides of the sample. While the gas floats on theupper side, there is vacuum on the other side fitted with thepressure sensor. The measuring area of the above device issandblasted. Due to the softness of the WPI film and the vacuumduring the measuring process, the film is placed in the spaces of thesandblasted area and therefore alters the measuring area andresults. Therefore, a piece of thin silk paper (0.33 � 100 m roll,

Table 1Performed experiments.

Sample

WPI film (24.15 � 4.24 mm) WPI coat

O2, manometric method (23, 35, 40) �C; 0% RH –

CO2, manometric method (23, 35, 40) �C; 0% RH –

N2, manometric method (23, 35, 40) �C; 0% RH –

O2, O2-specific carrier gas method 23 �C, (35, 50, 75)% RH 23 �C, (35WVTR 23 �C; 85 ! 0; 50 ! 0% RH 23 �C; 85

Hahnemuhle FineArt GmbH, Dassel, Germany) with a size ofmeasuring area was placed between the sandblasted surface andthe smooth WPI film. These measurements were performed andreported as mean value of two replications.

(2) Oxygen-specific carrier gas method. This method was onlyapplied for the PET films and WPI coatings, because the WPI filmswere relatively thin and therefore mechanically sensitive todamage. With this method the influence of the RH on O2Q wascharacterized. The barrier properties of this WPI coating werecalculated from the multilayer. For the O2Q tests, a processfollowing the instructions of DIN 53380-3 (Ox-Tran, Mocon Inc.,Minneapolis, Minnesota, USA) was chosen. During these measure-ments, one side of the sample was exposed to flowing O2, whereasthe other side faced a flowing carrier gas such as N2. The adjustedRH values were 35%, 50% and 75%. The method was not suitable fortesting WPI coatings at 0% RH because the measuring device wasnot designed for long measurement times at low RH, in accordancewith DIN 53380-3, as this would damage the sensor. Thesemeasurements were performed with at least two fold determi-nations.

(3) Gravimetric determination of water vapour transmission(WVT). The gravimetric method was applied for PET films and WPIcoatings and films (WPI layer thickness circa 20 mm). The barrierproperties of the WPI coating was calculated from the multilayerand compared with a WPI film of the same thickness (Eq. (4)). Thetemperature during the measurements was set to 23 �C for alldeterminations.

Eq. (4): total permeability of a multilayer

The WVT determinations were performed according to DIN53122-1. In this method a metal screw cup (Fraunhofer IVV,Freising, Germany) with adjusted RH of 0% (silica gel) is sealed withthe sample and placed in a room having a specific RH. Because ofthe pressure gradient, WV permeates into the shell, which can berecorded as an increase in weight of the shells. The pressuregradients for these determinations were set at 50% RH ! 0% RH and85% RH ! 0% RH. For each sample at least a threefold determina-tion was performed. The experiments are summarised andpresented in Table 1.

2.5. Thickness measurements

The thicknesses of the films were measured with a thicknessmeasuring device (Precision Thickness Gauge FT3, RhopointInstruments, Queensway, East Sussex, UK). A mechanical device

ing (24.15 � 4.24 mm) on PET (11.40 � 0.26 mm) PET (11.40 � 0.26 mm)

(23, 35, 40) �C; 0% RH(23, 35, 40) �C; 0% RH(23, 35, 40) �C; 0% RH

, 50, 75)% RH 23 �C, (35, 50, 75)% RH ! 0; 50 ! 0% RH 23 �C; 85 ! 0; 50!0% RH

24 M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29

measures the thickness at five points on the sample. Thecorresponding analyse system calculates the arithmetic meanand the standard deviation of the five measurements. To gain thedry film thickness (dft) of certain WPI coatings on PET, the values ofuncoated PET were subtracted from the values determined for themultilayer samples.

2.6. Statistical analysis

Statistical analyses were performed using the statisticalsoftware Visual Xsel 12.1 (CRGraph, München, Germany). Thedeterminations concerning the influence of changing RH werestatistically evaluated by calculation of the arithmetic mean andstandard deviation. For statistical comparison, Student’s t-test wasperformed with a significance level p � 0.05. Due to quantityrestrictions, the characterisations concerning the EA and thepermselectivity were finished after two fold determinations.Statistical evaluation was not possible with less than three values.In order to gain a base for comparison, the arithmetic mean of

these values was calculated, and the error bars were representedwith the minimum and maximum value.

3. Results and discussion

3.1. Permselectivity

Figs. 1 and 2 show the Q of N2, O2 and CO2 through uncoated PETand through a WPI film at different temperatures and 0% RH. It canbe seen that the N2Q shows the lowest values and the CO2Q showsthe highest values. The O2Q lies between these values. This orderrepresents the permselectivity.

Fig. 1. Q of N2, O2 and CO2 through uncoate

The results also show that the Q of all permeants through PET ishigher than through the WPI film of approximately 20 mmthickness. This means that the barrier properties of WPI towardsthe chosen test gases are higher than that of PET.

The calculated permselectivities are shown in Table 2. The errorranges are calculated by error propagation according to Gauß(Eden & Gebhard, 2011). For both measured samples, the ratio ofthe permselectivity differed from the simplified ratio of 1:4:16 (N2:O2:CO2) (Müller, 2013). However, the results are in the range for theresults of another study on crystalline PET.

The permselectivity O2/N2 of the WPI film increases withincreasing T. This behaviour can be explained with the help of theEq. (5) that displays the calculation of the ratio of permeabilitybetween the two gases O2 and N2: an increasing permselectivityO2/N2 implies a higher activation energy for O2 compared to N2. Incontrast, the permselectivity CO2/N2 of the WPI film is nearlyconstant with increasing T and therefore the activation energy forCO2 and N2 should be in the same range.

Eq. (5): calculation of permselectivity O2/N2

3.2. Activation energy

By taking the above-mentioned results for Q and plotting ln(P)against the reciprocal temperature (1/T [1/K]), as already describedin the introduction, a linear curve is obtained as shown in Fig. 3.The error range of these values is calculated with the help of R2,which is also shown in the figure.

By plotting ln(P) against 1/T, the values of the slopes and theintercepts of the lines should be in the same order as thepermeabilities. Looking closer at Fig. 3 it comes clear that the O2Qfunction of the WPI film has a greater slope and higher intercepts

d PET (11.40 � 0.26 mm) at different T.

Fig. 2. Q of N2, O2 and CO2 through a WPI film (dft = 24.15 � 4.24 mm) at different T.

M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29 25

than the functions of the other gases. For both samples (theuncoated PET and the WPI film) the EA is calculated as described inthe introduction and the values are compared in Table 3.

Generally, a higher EA indicates a higher change of Q when Tchanges. This was verified with regards to the values of EA for PET.

The measured values were compared to the determinations ofHong and Krochta (2003). They investigated the O2 barrierproperties of WPI coatings on polypropylene films (PP). Theircalculated EA of the WPI coating was 47.73 � 3.77 kJ mol�1 andtherefore verifies the values calculated in this study.

When the EA for the PET used is compared to that of otherstudies it can be concluded that the results of this study are slightlylower than the values published in other studies. This could beexplained by the fact that other studies did not use the same PETgrade and the relative error of the manometric measurementmethod.

Table 2Permselectivity of N2, O2 and CO2 through uncoated PET and a WPI film at different T.

PET

This study Other studies (Brandrup & Imm1963)

23 �C 35 �C 40 �C 25 �C

N2Q: N2Q 1.00 � 0.19 1.00 � 0.14 1.00 � 0.13 1

O2Q: N2Q 7.40 � 1.33 6.92 � 0.95 6.52 � 0.93 2.23–5.39

CO2Q:N2Q

32.74 � 5.87 27.57 � 4.37 28.21 � 3.93 23.10–26.15

Fig. 3. ln(P) against on 1/T of N2, O2 and CO2 Q m

3.3. Influence of relative humidity on the water vapour transmissionrate and on the effective water vapour permeability coefficient

Fig. 4 visualises the measured values of the WVTR throughcoated and uncoated PET with different RH gradients at 23 �C. Thethickness of the coating is shown in parentheses. As four folddeterminations were performed, statistical evaluation was possi-ble. The error bars represent the standard deviation of the values.The results were used to calculate P, which is presented later.

The results showed that WVTR of both WPI coated anduncoated PET increases with increasing RH. The differencebetween the samples, measured with the different humiditygradients, is significant due to the given significance level, whereasthe samples among each other show no significant difference.

Though, the influence of coating seems to be negligibly small.Considering Fig. 4, it can be seen that the WVTR for the coated PET

WPI film

ergut, 1975; Michaels, Vieth, & Barrie, This study

23 �C 35 �C 40 �C

1.00 � 0.17 1.00 � 0.19 1.00 � 0.134.22 � 0.71 5.66 � 0.77 5.72 � 0.5933.52 � 4.08 34.55 � 4.74 34.07 � 3.31

easurements of WPI film (Arrhenius plot).

Table 3Activation energy of Q for N2, O2 and CO2 for different commodities.

Activation energy/kJ mol�1

PET WPI film

This study Other study (Michaels et al., 1963; Brandrup, & Immergut, 1975; Polyakova, Liu, Schiraldi, Hiltner,& Baer, 2001)

This study Other study (Hong, & Krochta, 2003)

Crystalline Amorphous

N2 34.26 32.7 26.4 36.01O2 28.81 32.3 37.6 30; 32.5 50.73 47.73 � 3.77CO2 23.06 18.4 27.6 33.35

Fig. 5. WVT through a WPI film with different moisture gradients (columns withdifferent letters are significantly different (p < 0.05).

26 M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29

is lower compared to the uncoated sample with a gradient of50% ! 0% RH. It can be hypothesised that this effect might beattributed to the enhancement of the barrier due to the WPI layer.In contrast, the coated sample shows a higher WVTR with a biggerRH gradient. This could be explained by the interaction of the WPIcoating with humidity. However, due to the fact that there is nosignificant difference compared to the uncoated sample, theslightly higher increase of the WV permeation could be related tomeasuring uncertainties.

Due to the higher WVTR of the coated sample compared to theuncoated sample for the 85% ! 0% RH gradient, the barrierproperties of the WPI coating could not be calculated from themultilayer and so these values were not considered further.According to the water vapour sorption isotherms of whey proteinfilms (unpublished data) the water content in WPI based films canincrease by approximately 30% (w/w) at 85% RH compared to theequilibrium moisture content at 50% RH. This leads to increasedpermeability due to the higher water content in the WPI basedcoatings. Thus, the substrate is limiting the permeability at highRH.

Fig. 5 shows the effective P (Peff) through a WPI film measuredwith different moisture gradients at 23 �C. The WPI film was on thehumidified side (50% RH or 85% RH) whereas the PET substrate wason the dry site. For this reason, it was assumed that the WPI had RHin the layer of 50% RH or 85% RH. These values were taken for thecalculation of the Peff. As these values were measured with fourfold determination, statistical evaluation was performed. The errorbars represent the standard deviation.

The higher the RH the samples were exposed to, the higher theWVP. There is a progressively increased WV absorption by WP

Fig. 4. WVT through WPI coated and uncoated PET with different moisture gra

films and coatings at increasing RH (Berlin, Anderson, & Pallansch,1968; Berlin, Kliman, Anderson, & Pallansch, 1973). The results forthe WVP for WPI at 50% RH of (9.3 � 0.6)�10 cm3 (STP) cm cm�2

s�1 Pa�1 and at 85% RH of (16.2 � 0.9)�10 cm3 (STP) cm cm�2 s�1

Pa�1 were in the range of another study. The results for PET at 50%RH of (2.0 � 0.2)�11 cm3 (STP) cm cm�2 s�1 Pa�1 and at 85% RH of(2.1 �0.1)�11 cm3 (STP) cm cm�2 s�1 Pa�1 were also in the range ofother studies which measured values of (1.3–3.4)�11 cm3 (STP) cmcm�2 s�1 Pa�1 (Marais et al., 2000; Yasuda, & Stannett, 1962).

dients (columns with different letters are significantly different (p < 0.05)).

Fig. 6. O2Q through coated and uncoated PET at different RH.

M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29 27

3.4. Influence of the relative humidity on the oxygen permeability

The O2Q through PET is also affected by the RH. Fig. 6 displaysthe O2Q of uncoated and coated PET films (the thickness of thecoating is written in parentheses) with increasing humidity at23 �C. Due to the fact that there was only two fold determination,statistical evaluation was not possible. The error bars display themeasured minimum and maximum values.

The results show that the influence of increasing RH on O2Q isdifferent for uncoated and coated PET films. While the Q throughuncoated PET decreases at higher RH, the Q through the coatedsample increases.

PET is a polymer that interacts with humidity. The anomaly ofthe barrier properties of PET at higher RH has been observed byAuras, Harte, and Selke (2004).

As expected, the increased Q through the coated sample is dueto the applied WPI layer and its residual moisture content. The factthat even the highest Q of the coated sample is lower than thelowest value of the uncoated samples justifies a closer look at theWPI coating itself. Therefore, the Q values of the WPI coating werecalculated from the multilayer and are shown in Fig. 7 along withthe values for a WPI film. Both values (coating and film) arepresented after transformation of the Q values to a thickness of100 mm (Q100) in order to have a standard base for comparison.

The error bars display, via error propagation, the calculatederror ranges of the specific values. Therefore, these values could becounted as the minimum O2 permeability, WPI films could possiblyperform. The calculation of the error ranges considered the film

Fig. 7. O2Q100 comparison of W

thickness and its error range. It can be clearly seen that Q increaseswith increasing RH. This can be explained by the water vapoursorption isotherms of whey protein films (unpublished data). Thewater content in WPI based films can increase by approximately30% (w/w) at 85% RH compared to the equilibrium moisturecontent at 50% RH. This leads to increased permeability due to thehigher water content in the WPI based coatings. In addition,previous studies of Bugnicourt et al. (2013) showed similarmoisture-sensitive behaviour of WPI layers. With increasing RH,the WPI layer also showed decreasing barrier properties towardsO2 measured according to DIN 53380-3 (DIN, 1998). Consideringthe difference in Q between the coating and the film, no clear trendwas visible. All O2Q100 values, measured at different RH, of thecoatings were higher than these of WPI films. This indicates aslightly higher moisture resistance of the WPI film than the coatedWPI layer on PET. As the film has obviously better barrierproperties towards O2 with increasing RH than a coated WPIlayer, adhering of a WPI film to a substrate should be consideredrather than applying it via coating.

In summary, it can be stated that the Q of either a WPI coating ora WPI film is affected by the surrounding RH. For greater insight,higher numbers of determinations are recommended.

4. Conclusions

Due to denaturation, WPI is able to form coherent films havingexcellent barrier properties towards non-polar gases. In contrast,

PI coating and WPI film.

28 M. Schmid et al. / Food Packaging and Shelf Life 6 (2015) 21–29

the WVTR through these films is relatively high because WPI formsmoisture-sensitive polymers.

The goal of this study was to study the fundamental propertiesof WPI layers, either as films or applied as coatings. Differenttemperatures (23, 35, 40 �C) and different RH (0, 35, 50, 75, 85% RH)were studied using WPI films (monolayers) and coatings (part of amultilayer).

The permselectivity of a WPI film was described for the firsttime. The calculated ratios of permselectivity (P(N2)/P(O2)/P(CO2):1/(4–6/(34–35)) differed from the simplified ratios forstandard polymers found in the literature, namely (P(N2)/P(O2)/P(CO2):1/4/16). These observations are important for the selectionof suitable headspace gas compositions in modified atmospherepackaging (MAP). Due to the significantly higher CO2 permeabilityin comparision to O2 and N2, a headspace gas composition withhigh CO2 concentrations could potentially lead to collapsing of thepackage. Therefore, these new data gained in this study are of highindustrial relevance and should be considered when WPI basedbarrier materials are used for MAP concepts.

Additionally, the different EA values for the permselectivity ofpermeants were determined with respect to varying T. An increasein T resulted in a definite increase of Q through both uncoated PETand the WPI monolayer, which points to a dependency of Q on thesurrounding T. Regarding the calculated EA of both polymers,statements about the degree of that dependency can be made. Therelatively high EA values of the WPI layer cause greater dependencyof Q on T.

The measurements at different RH showed clear results. Qvalues for both O2 and WV through the WPI coating and the WPIfilm increased with increasing RH. However, the coating showed agreater increase than the film, indicating that the substrateinfluences the properties of the coating. The denatured WPI layer isstill not soluble, but is strongly affected by moisture. Therefore itshould be used in combination with hydrophobic polymers.

For further investigations it would be interesting to develop aprocess for determining the breakthrough time for a WPI coatingfor different permeates, which would make it possible to calculatethe diffusion coefficient and the solubility coefficient for gases andwater vapour.

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