EVOH): a Review

Polymer Reviews

ISSN: 1558-3724 (Print) 1558-3716 (Online) Journal homepage: https://www.tandfonline.com/loi/lmsc20

Recent Updates on the Barrier Properties of Ethylene Vinyl Alcohol Copolymer (EVOH): A Review

Caroline Maes, Wout Luyten, Geert Herremans, Roos Peeters, Robert Carleer & Mieke Buntinx

To cite this article: Caroline Maes, Wout Luyten, Geert Herremans, Roos Peeters, Robert Carleer & Mieke Buntinx (2018) Recent Updates on the Barrier Properties of Ethylene Vinyl Alcohol Copolymer (EVOH): A Review, Polymer Reviews, 58:2, 209-246, DOI: 10.1080/15583724.2017.1394323 To link to this article: https://doi.org/10.1080/15583724.2017.1394323

Submit your article to this journal Article views: 3240

View related articles View Crossmark data

Citing articles: 14 View citing articles

Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=lmsc20 POLYMER REVIEWS 2018, VOL. 58, NO. 2, 209–246 https://doi.org/10.1080/15583724.2017.1394323

REVIEW Recent Updates on the Barrier Properties of Ethylene Vinyl Alcohol Copolymer (EVOH): A Review

Caroline Maes a,b, Wout Luytena, Geert Herremansa, Roos Peetersb, Robert Carleerc, and Mieke Buntinxb aKuraray–Eval Europe NV, Haven 1053 Nieuwe Weg 1, Bus 10, Zwijndrecht, Belgium; bHasselt University, Packaging Technology Center IMO-IMOMEC, Wetenschapspark 27, Diepenbeek, Belgium; cHasselt University, Applied and Analytical Chemistry IMO-IMOMEC, Agoralaan Building D, Diepenbeek, Belgium

ABSTRACT ARTICLE HISTORY The gas barrier properties of ethylene vinyl alcohol copolymer (EVOH) Received 26 June 2017 against oxygen, carbon dioxide and water vapor have been Accepted 15 October 2017 widely investigated in relation to different material characteristics, KEYWORDS environmental conditions and new processing technologies. Recently, Ethylene vinyl alcohol EVOH is gaining more attention as a barrier material against other gases copolymer; EVOH; and organic substances such as aromas, ﬂavors,fuels,chemicals(e.g., permeability; barrier BTEX), and as a functional barrier, e.g., to avoid mineral oil migration. properties; functional barrier; This review contains an update on permeability data of EVOH mineral oils; (volatile) organic emphasizing its potential as a barrier material for new and versatile compounds applications in food and pharmaceutical packaging, agriculture, construction, automotive, etc.

1. Introduction Ethylene vinyl alcohol copolymer (EVOH) was first commercialized by Kuraray under the trade- mark EVALTM in 1972. The barrier properties of EVOH against oxygen had been investigated for fifteen years and were found to be valuable. After Kuraray obtained the patent in 1971, they started producing EVOH commercially. Later, NipponGohseiandChangChunalsostartedtheir – commercial production of EVOH.[1 4] Kuraray is the largest producer of EVOH. Since 2001 they have more than doubled their global production capacity from 45,000 tons to 92,000 tons in 2016 – and a further expansion of 11,000 is planned in 2018, due a continued market growth.[5 7] EVOH is a random copolymer with a semi-crystalline structure, consisting of ethylene and vinyl alcohol monomer units. The synthesis of EVOH occurs in a two-step process as shown in Fig. 1.[8,9] Vinyl alcohol is unstable and it cannot be isolated, therefore the first step is a copolymerization reaction between ethylene and vinyl acetate, which results into the random copolymer ethylene vinyl acetate. In the second step polyethylene vinyl acetate is converted into EVOH through a transesterificiation with methanol and as a side product methyl acetate is produced. Nowadays, EVOH has gained worldwide recognition for its barrier properties against permanent gases such as oxygen (O2), carbon dioxide (CO2) and nitrogen (N2). Especially its

CONTACT Mieke Buntinx [email protected] Hasselt University, Packaging Technology Center IMO- IMOMEC, Martelarenlaan 42, 3500 Hasselt, Belgium. � 2018 The Author(s). Published by Taylor & Francis Group, LLC This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. 210 C. MAES ET AL.

Figure 1. Synthesis of EVOH is a two-step process: copolymerization of ethylene and vinyl acetate resulting in polyethylene vinyl acetate (a) followed by a transesterification with methanol, which results into EVOH and methyl acetate as a side product (b). outstanding oxygen barrier has made EVOH one of the most commonly used gas barrier – materials in multilayer food packages.[9 14] EVOH can also be used in clinical and pharmaceutical applications such as in parenteral nutrition bags and ampoules to increase shelf- life.[15] However, the applications for EVOH as an oxygen gas barrier material reach further than only food and pharma. Pipes for floor heating in the building industry, for example, contain an EVOH layer to keep oxygen out and thus preventing the oxygen from travelling to the boiler and causing corrosion.[16] In addition the radon barrier of EVOH is another application in the construction sector, where it is used to decrease radon exposure and improve the indoor air quality.[17] The excellent barrier properties of EVOH are attributed to the strong inter- and intra- molecular bonding caused by the polar hydroxyl groups present in the vinyl alcohol unit.[9,10,14] These hydroxyl groups are also responsible for the moisture sensitivity because water is easily absorbed due to the hydrophilic character of the polymer. Gradually, when more water is absorbed, EVOH will be plasticized and the inter- and intra-molecular bonds will weaken, causing a decrease in the barrier properties.[10] This plasticizing effect is the rea- son why EVOH is mainly applied in multilayer structures. This same effect is also known to occur with alcohols such as methanol and ethanol.[18] Nevertheless, even with its sensitivity to humidity, EVOH remains one of the best gas barrier materials amongst polymers.[9] More recently, there has been an increasing interest in EVOH as a barrier material against organic substances such as chemicals, hydrocarbons (e.g., mineral oils and fuel), aromas, and flavors. EVOH proved to be successful in fuel tanks to prevent fuel emission to the environment, in agricultural films to improve herbicide retention and in geomembrane films to – keep the methane release from landfills in check.[19 21] EVOH can also pose as a functional barrier in food packaging as it offers good chemical resistance and prevents the migration from chemical substances out of other materials into the packaging’s content.[22] In this paper the recent findings on the barrier properties of EVOH copolymer towards different molecules are summarized and reviewed.

2. Oxygen permeability of EVOH EVOH has one of the lowest oxygen permeability coefﬁcients amongst polymers, as shown ﬁ in Table 1. In this review, the oxygen permeability or oxygen permeability coef cient (PO2 ) POLYMER REVIEWS 211

Table 1. Oxygen permeability (PO2 ) of commonly used polymers. Based on data from Kuraray co. Ltd. (2017)[7], McKeen (2012),[21] Lange and Wyser (2003),[54] and Lagaron et al. (2004).[24] 3 m 2 Polymer PO2 [cm . m/(m .day.atm)]

PVOH 1.5 a EVOH32 6 b EVOH44 38 b PVDC 10–300 c PA 6 400–2,000 a PET 1,000–5,000 c PP 50,000–100,000 c PS 100,000–150,000 c LDPE 100,000–200,000 c HDPE 40,000–100,000 c aPermeability at 23C and 0% RH. bPermeability at 20C and 65% RH. cPermeability at 23C and 50% RH. is the thickness and pressure normalized rate at which O2 passes through a material and will 3 m 2 be expressed in cm . m/(m .day.atm), and the oxygen gas transmission rate (O2GTR) is the fi 3 2 actual amount of transmitted O2 measured for a certain package or lm in cm /(m .day. [21,23] atm) with no thickness normalization under a pressure difference of 1 atm. The PO2 of EVOH containing 32 mol% ethylene (EVOH32) and EVOH containing 44 mol% ethylene (EVOH44) measured at 20C and 65% relative humidity (RH) according to the ISO 14663-2 standard is respectively 6 and 38 cm3.mm/(m2.day.atm).[7] Poly(vinyl alcohol) (PVOH) has a lower oxygen permeability coefficient, i.e. 1.5 cm3.mm/ (m2.day.atm) at 23C and 0% RH,[24] however, this polymer is difficult to process because of its low thermal degradation point, which is about 150C, while its melting temperature is around 180–190C. Apart from this, PVOH is also soluble in water, explaining its limited use in packaging applications.[25] However, PVOH can be used as a coating on other plastics to increase the barrier properties. Other barrier polymers are poly(vinylidene dichloride) (PVDC), the polyamide nylon 6 (PA 6) and poly(ethylene terephthalate) (PET). These polymers have a PO2 that is 1 to 3 orders of magnitude higher than EVOH. When compared to the non-polar polymers such as low-density polyethylene (LDPE), high-density polyethylene

(HDPE), polypropylene (PP) and polystyrene (PS) the PO2 is over 3 orders of magnitude higher than EVOH.[9,26] In the following sections several factors that inﬂuence the oxygen permeability of EVOH will be discussed. These can be attributed to intrinsic factors of the material itself as well as extrinsic factors such as environmental conditions and permeant properties.[27]

2.1. Inﬂuence of material characteristics First, the intrinsic material characteristics of EVOH, including chemical structure, crystallinity, thickness, glass transition temperature and free volume that are related to the O2 barrier properties are discussed.

2.1.1. Ethylene content The structure of EVOH copolymer is key for understanding its outstanding barrier properties. EVOH consists of ethylene and vinyl alcohol monomer units. The vinyl alcohol units are responsible for the exceptional gas barrier properties due to the 212 C. MAES ET AL.

Figure 2. Oxygen permeability at 20C, 65% RH according to ISO 14663-2 and water vapor permeability at 40C, 0/90% RH according to ASTM E96-E as a function of the ethylene content in EVOH. Based on data from Kuraray co. Ltd. (2017).[7] inter- and intra-molecular bonding caused by the hydroxyl groups; although these units are also known to be soluble in water and to be quite difﬁcult to process. The ethylene units, on the other hand, display good water resistance, yet have one of the poorest gas barrier properties in polymers. The combination of both monomer units leads to a pro- cessable thermoplastic copolymer with excellent gas barrier properties, but it remains sensitive to water.[9,28] The oxygen barrier of EVOH improves at lower ethylene mol% content as shown in Fig. 2 by the dotted line.[10,29]

Oher material characteristics like the melting point (Tm) and the glass transition temperature (Tg), which are also important for the processability of EVOH, are dependent on the ethylene content. Both Tm and Tg decrease with an increasing ethylene content as can be seen in Table 2, making the polymer less brittle and easier to process. EVOH with a higher ethylene content is more suitable for thermoform, stretch and shrink applications, whereas EVOH with a lower ethylene content is used in ﬂexible and ultra-high barrier applications.[7] To achieve good barrier properties a particular EVOH composition range is required. EVOH is not considered a high barrier resin if the ethylene content is over 50 mol%, owing to char- [30] acteristic higher PO2 (Fig. 2).

2.1.2. Crystallinity The barrier properties of EVOH are also attributed to the inherent high degree of the polymer’s crystallinity. The semi-crystalline structure is typically represented by crystals or crystalline domains, which are dispersed in an amorphous matrix. The barrier properties of the

Table 2. Melting point (Tm) and glass transition temperature (Tg) of EVOH with different ethylene content. Based on data from Kuraray co. Ltd. (2017).[7]

Ethylene content (mol%) Tm ( C) Tg ( C)

27 190 63 32 183 60 38 172 56 44 165 53 48 157 50 POLYMER REVIEWS 213 polymer are also related to the size and distribution of the crystals.[31] When the ethylene content is below 40 mol%, the crystals are monoclinic similar to those in PVOH. At higher ethylene content (over 80 mol%) the crystals have an orthorhombic structure like in PE (polyethylene). Intermediate compositions show a mixture of both structures and have a minimal crystallinity level.[32] However, not only the polymer composition but also the crystallization conditions, such as the cooling rate, can affect the crystal structures. Samples with low ethylene contents also show orthorhombic crystals when they are quenched, opposed to slowly cooled samples, which present a monoclinic lattice. The degree of crystallinity also varies with the cooling rate. It decreases when the cooling rate increases and leads to crystal imperfections.[33] Armstrong (2008)[34] demonstrated that EVOH has a rapid crystallization rate as opposed to other thermoplastic polymers and therefore it is difficult to control the degree of crystallinity during cooling from melt state. The fastest crystallization rate for EVOH32 was observed at 140 C. To effectively decrease the PO2 by maximizing the degree of crystallinity slower cooling from the Tm close to 140 C, or long reheating at 140 C is recommended to allow the chains more time to arrange into a crystalline order. Alvarez et al. (2003)[35] reported an increasing degree of crystallinity (48%, 56% and 60% respectively) after isothermal crystallization of EVOH32, EVOH38 and EVOH44 with increasing ethylene content, but the crystallization rate was low. The samples were melted for 10 min at 250C, then cooled to the crystallization temperature (Tc) at 250 C/min and maintained here for 20 min to allow complete crystallization. Rwei et al. (2015),[36] on the other hand, found that the degree of crystallinity decreased with increasing ethylene content. The results for EVOH32, EVOH38 and EVOH44 were respectively 46.6%, 41.8% and 40.0%. However, the crystallization conditions of these samples were not mentioned. This decrease in crystallinity was earlier confirmed by Ketels (1989)[37] and demonstrates a correlation between the ethylene content and the PO2 . It is suggested that the permeation of low weight molecules such as permanent gases generally occurs through the amorphous regions of the copolymer; the crystalline regions in the polymer create a more irregular tortuous diffusion path, making it more difficult for permanent gases to pass through.[24,38] Fig. 3 illustrates the inverse rela- fi tionship between the degree of crystallinity and the PO2 of ve EVOH32 samples measured

Figure 3. Oxygen permeability of EVOH32 at 20C, 100% RH (after stretching and/or receiving a heat treatment of 140C), as a function of the crystallinity. Adapted with permission from Armstrong (2002).[86] 214 C. MAES ET AL. at 20C and 100% RH. A decrease of a factor of 17 is noted from 27% to 70% crystallinity. The variation in crystallinity is often related to the processing conditions to which the material is subjected and will be discussed later.[9,11,13]

2.1.3. Thickness The thickness of the EVOH layer is another factor which contributes to the barrier performance. EVOH is usually applied in multilayer structures as a thin barrier layer, its thickness depending on the application. In food packaging, an EVOH layer of only a few micrometers, usually less than 10 mm, is sufﬁcient as a barrier, whereas in ﬂoor heating pipes, the EVOH layer has a thickness of about 50–100 mm (lab data from EVAL Europe nv). In general, it can be stated that the thickness of the barrier layer is inversely proportional to its O2GTR (Fig. 4), although very thin structures may show inconsistent O2GTR values due to structural changes or surface phenomena.[23] The effect of the material thickness on the permeability is especially important in thermoformed packaging, because of the non-uniform material distribution during thermoforming. However, it should be noted that other parameters such as orientation and (re)crystallization can partly counteract the effect of thinning on the permeability of thermoformed packaging.[39] This will be addressed more in depth later in this paper.

2.1.4. Glass transition temperature and free volume

Another parameter which impacts the barrier is Tg, referring to the temperature at which a transition from a “glassy” state to a “rubbery” state of the polymer occurs.[40] Below this temperature, the chain mobility is low, yet above the glass transition temperature there is enough thermal energy for the polymer to become more viscous by overcoming the intra- and inter- molecular bonds. It is argued that the permeation of gas molecules occurs due to cooperative movement of the gas molecules and the polymer chain segments.[41,42] EVOH has a high glass transition temperature between 50 and 63C, depending on the ethylene content (48 and 24 mol% ethylene, respectively) (Table 2), which is far above room temperature. Low oxygen barrier materials like PE and PP usually have a Tg below room temperature.

Figure 4. Oxygen transmission rate of EVOH27, EVOH32, EVOH38 and EVOH44 at 20C, 65% RH and atmospheric pressure as a function of the thickness. Based on data from Kuraray co. Ltd. (2017).[7] POLYMER REVIEWS 215

However, Lagaron et al. have shown that the Tg can strongly be reduced to below freezing point through the uptake of water, methanol or other molecules with a plasticizing effect on

EVOH. In addition to the high Tg, EVOH also has little free volume due to its strong cohe- sive energy.[9,18] Yeh et al. (2006) reported that the fractional free volume for EVOH32, EVOH44 and EVOH48 was respectively 6.1, 7.3, and 8.0%.[43] Ito et al. (2009) found that the free volume cavity size ranged from 0.04 to 0.07 nm3 for ethylene contents varying from 28 to 44 mol%.[29] Muramatsu et al. (2003) reported that the free volume cavity size for EVOH29 varied with the RH, ﬁrst decreasing when the RH increased from 0% to about 25% caused by the water molecules ﬁlling up the free volume cavities, and then increasing at higher RH to values above those of 0% RH due to the plasticizing effect of water on EVOH.[44]

2.2. Influence of environmental parameters 2.2.1. Temperature The temperature is an important environmental factor, which influences the permeability of polymers. When the temperature increases, the permeability will also increase, due to an increase in the diffusion rate of the gas molecules through the polymer. The solubility, however, decreases at higher temperatures, but the effect on the diffusion rate is significantly larger, resulting in an increase in permeability. The temperature dependence is expressed in fi the Arrhenius eq. (1), where permeability coef cient (P0) and activation energy (Ep) are characteristics attributed to a certain material and permeant pair. This equation is used to model the permeability coefficient P over a moderate temperature range as can be seen in Fig. 5 for the oxygen permeability of EVOH32 and EVOH44 at 0% RH.

¡ Ep 6RT P D P0e (1)

3 m 2 The units of P and P0 are usually cm . m/(m .day.atm). Ep is expressed in kJ/mol, tem- £ ¡3 perature (T) in K and R is the universal gas constant 8.3144 10 kJ/(mol.K). The Ep can

Figure 5. Oxygen permeability (0% RH) of EVOH32 and EVOH44 as a function of the temperature. Based on data from Kuraray co. Ltd. (2017).[7] 216 C. MAES ET AL. be determined from the graph and is 55.27 and 49.84 kJ/(mol.K) for EVOH32 and EVOH44, respectively.[21,45]

2.2.2. Relative humidity The second environmental factor is the RH, which is also one of the most significant key factors when it comes to permeability of EVOH. Due to its hydrophilic character, EVOH absorbs significant amounts of water from the environment when exposed to humid conditions. The water uptake increases with an increasing RH to about 8–13% for a EVOH32 film at 100% RH and 21C. The water uptake also depends on the crystallinity, e.g., a non-oriented EVOH32 film, with a crystallinity of approximately 58%, has a water uptake of 8.4% in the above mentioned conditions, while the biaxial oriented film, with a crystallinity of approximately 70%, has a water uptake of only 6.8%.[9,46] Lagaron et al. (2001) have shown in different studies that the water uptake causes a decrease in the Tg to below freezing point.[18,47] The effect of moisture on the barrier properties of EVOH depends on the RH of the environment. At lower RH the PO2 shows an initial decline, which increases after reaching a minimum at about 20–40% RH (Fig. 6). This phenomenon is usually explained by the change in free volume, where low RH has a positive effect on the barrier, because water molecules fill up the free volume between the polymer chains. A small amount of water in EVOH also induces an enthalpy relaxation, which could explain why the PO2 and the cavity size reach a – minimum around 20 40% RH. A further increase in RH coincides with a decrease in Tg,as the additional water molecules will act as plasticizer and weaken the inter- and intramolecu- lar hydrogen bonds between the chains, causing an increase in chain mobility and an increase in free volume due to swelling of the polymer, which leads to a higher oxygen permeability. The moisture sensitivity of EVOH is often seen as a limitation. However it should be noted that EVOH outperforms most barrier polymers even at 100% RH.[9,10,44,48] As was mentioned previously, the same effect is known to occur with methanol and ethanol. Cava et al. (2007) were able to use the swelling behaviour of EVOH as an advantage. Water, methanol and ethanol acted as swelling agents and led to a sorption-induced release

Figure 6. Oxygen permeability (20C) of EVOH32 and EVOH44 as a function of the relative humidity. Based on data from Kuraray co. Ltd. (2017).[7] POLYMER REVIEWS 217 of an antimicrobial component, which was added to the EVOH matrix, creating an active packaging material.[49]

2.2.3. Pressure Pressure can be seen as a third environmental factor. Here it is important to note the difference in total pressure and the partial pressure of one gas. In an ideal situation the total pressure has no inﬂuence on the solubility and diffusion and therefore on the permeability. However, plasticization and swelling effects may occur due to a change in pressure, leading to either an increasing or decreasing effect on the permeability or like in most cases with glassy polymers to an initial decreasefollowedbyanincreaseuponincreasing upstream pressure.[21,50] This needs to be taken into consideration when working with the differential pressure method, where a test specimen is mounted between two com- partments of a cell, the test gas is introduced at the upstream compartment and the downstream side is evacuated. The pressure increase is used to determine the permeability.[51] Apart from the total pressure difference in the system the partial pressure difference of the test gas can also be used to measure the permeability. Opposed to the differential pressure method, the total pressure on each side of the cell is at atmospheric pressure in the equal pressure method, yet the partial pressure of the test gas is different, creating a driving force from one side of the cell to the other. The gas molecules will per- meate from the side with the highest partial pressure or concentration to the side with the lowest partial pressure.[52]

2.3. Multilayer structures and blends

Due to the dependence of the PO2 on the RH (discussed in paragraph 2.3), EVOH is usually applied in multilayer structures to reduce the effect of moisture on the EVOH layer. The application of EVOH in multilayer structures also has other advantages as it improves the processability, heat sealability, mechanical properties like tensile, impact and tear strength of – the overall structure.[53 56] These structures are a cost-effective and light-weight alternative for traditional packaging materials such as glass and metal. Typical multilayer structures can be produced by either coextrusion or lamination and often consist of 5 to 9 layers, including the adhesive layers.[27]

2.3.1. Multilayers There are two different approaches to effectively control the RH inside the EVOH using multilayer structures. The first is the thickness of the layers protecting the EVOH. A thicker layer should be used on the side with the highest RH and a thinner layer should be used on the side with the lowest RH. A second approach is the choice of the materials protecting the EVOH layer. The most efficient way is to use a material with a higher water vapor barrier or lower PH2O on the side with the highest RH and a material with a higher PH2O on the side with the lowest RH. In Table 3 the PO2 of different multilayer structures using EVOH32, EVOH44 and PVDC as barrier layers sandwiched between different outer layers and PP measured at 100% RH inside and either 65 or 75% RH outside are shown. PP, which is considered a good moisture barrier material (Fig. 7) is used on the side with the highest RH. HDPE is a better moisture barrier than PP, but when used on the side with the lowest RH, fi the PO2 of the lm becomes worse than when PP is used on both sides. However, PET, 218 C. MAES ET AL.

Table 3. The impact of different inner and outer layers in multilayer structures on the oxygen permeability (PO2 ) with different barrier materials (EVOH32, EVOH44 and PVDC) at 20 C and various relative humidity settings. Based on data from Kuraray America Inc. (2007).[57] 3 m 3 PO2 [cm . m/(m .day.atm)]

65% RH out/100% RH in 75% RH out/100% RH in Multilayer structure (mm) outer/ barrier/inner (152/25/610) EVOH32 EVOH44 PVDC EVOH32 EVOH44 PVDC

PP/Barrier/PP 342 1184 1465 586 1587 1465 PET/Barrier/PP 183 1465 525 1465 PC/Barrier/PP 269 989 1465 427 1343 1465 PS/Barrier/PP 269 1465 427 1465 HDPE/Barrier/PP 427 1465 720 1465 PA/Barrier/PP 1013 1465 1343 1465 LDPE/Barrier/PP 1111 1465 1465 1465 polycarbonate (PC), PS, PA and LDPE are lesser moisture barrier materials than PP and ﬁ result in a more ef cient protection of the EVOH against moisture and thus in a lower PO2 . The increase in RH on the outside shows that even with the use of multilayer structures, the

PO2 of EVOH is still affected by the surrounding RH. PVDC is not affected by the RH, therefore different structures do not impact the barrier, but EVOH32 still easily outperforms PVDC at both conditions. However, the choice of materials also depends on the desired mechanical and physical properties.[57] Jakobsen and Risbo (2008)[58] developed a model to calculate the total oxygen resistance (R ) of multilayer structures containing EVOH in which R D 1 .Themodelwas t t O2GTR based on a PE/tie/EVOH/tie/PE structure, butthetielayerswereexcludedfromthemodel. To determine the Rt, it is important to know the oxygen resistance of the EVOH barrier layer (Rb), which is dependent on the RH or water activity (aw) inside the EVOH layer. This in turn depends on the inner and outer water activity (ai and ao)andthewatervapor resistance of the inner and outer layer (Rw;i and Rw;o). Using eq. (2) the asymmetry a, which can be interpreted as the fraction of water vapor resistance in the barrier layer

Figure 7. Oxygen permeability (EVOH at 20C, 65% RH; PA 6 at 23C, 0% RH and other materials at 23C, 50% RH) and water vapor permeability (38C, 0/90% RH according to ASTM E96-E) of EVOH and other barrier materials. Based on data from Kuraray co. Ltd. (2017),[7] Lagaron et al. (2004),[24] Yam (2009),[27] McKeen (2012)[21] and Lange and Wyser (2003).[54] POLYMER REVIEWS 219 mid-plane, can be calculated. 1 R ; C = R ; a D w i 2 w b (2) Rw;t

Rw;t is the total water resistance of the multilayer. This value can be used in eq. (3) to calculate the average water activity in the EVOH layer (ab). Note that the water resistance of EVOH (Rw;b) is also dependent on ab, but a thin layer of EVOH contributes little to the total water vapor resistance and therefore hardly inﬂuences a.

ab D aao C ðÞ1 ¡ a ai (3)

The Rb can now be calculated using eq. (4) by dividing the thickness of the EVOH layer ðÞ a (xb) by the PEVOH aw , which is the PO2 of the used EVOH material at the calculated b or RH from experimental data as presented in Fig. 6.

xb Rb D (4) PEVOHðÞaw

Lastly the Rt is determined by taking the sum of all oxygen resistances using eq. (5).

Rt D Ro C Rb C Ri (5)

The study also concluded that it is not sufﬁcient to sandwich the EVOH layer between thick layers of polymers with a low water vapor permeability (PH2O). The key to an optimal design lies in asymmetric multilayer structures, where a thin laminate can be more effective than a thick laminate.

2.3.2. Microlayers There is a lot of interest in microlayer extrusion technology. Microlayers are multilayer structures consisting of very thin alternating layers of two or more materials, which are coextruded through multiplication extrusion by using multipliers. In each multiplier the melt blend is split and recombined, multiplying the layers. There are claims that microlayer extrusion technology leads to improved physical and barrier properties of EVOH.[59,60] Li et al. (2009) prepared a material with alternating layers of PP and PP/EVOH blend through microlayer coextrusion. The morphology of the EVOH phase was changed from a zero-dimensional sphere to a one-dimensional fiber, then to a two-dimensional sheet with the increase of the layer number, which caused an improvement in the gas barrier properties when compared to a conventional PP/EVOH blend. Alipour et al. (2015) found that the PO2 for a 5- and 19-layer structure of EVOH, LDPE and a PE adhesive was the same, however an improvement in n-hexane uptake and improvement of mechanical strength were noted in the 19-layer structure compared to the 5-layer structure.[61] Su et al. (2015) prepared blends of EVOH32 and linear low-density polyethylene with 5% LLDPE grafted with 1% maleic anhydride (EVOH/LLDPE/LLDPE-g-MAH). These blends were coextruded with LLDPE using microlayer extrusion technology. The microlayer structure decreased the barrier percolation threshold from 50 wt% EVOH for a conventional EVOH/LLDPE/LLDPE-g-MAH blend to 220 C. MAES ET AL. about 5.6 wt% in a 16 layer microstructure.[62] Microlayer technology can also help to main- tain the barrier properties of films after flexing because of its improved flex crack resistance.[59]

2.3.3. Blends The production of multilayer structures is a complex and expensive process; next to this the end-material is often difficult to recycle. A good cost-reducing alternative for multilayer structures is blending two or more polymers to attain the desired mechanical and barrier properties depending on the application and the recycling process.[63] EVOH can be blended with other polymers either to improve the other polymers properties or to improve EVOH’s properties. Studies showed that the incorporation of 5–30 wt% of a barrier polymer into a base polymer led to a 2- to 10-fold increase in barrier properties depending on the polymer. However, unlike multilayer structures, the overall macroscopic property of a blended film is not equal to that of a single material, as it is a combined system of several components. This makes it more difficult to predict the barrier properties of a blended film. Ge and Popham (2016) found that effective prediction of gas permeability depends on the morphology, permeability ratio of matrix-to-disperse phase and the selection of an appropriate model.[64] In the following paragraph some examples of studies with EVOH blends are summarized.

Ait-Kadi et al. (2007) blended PP with 16.5 wt% EVOH32, which led to a decrease of PO2 of 24% as compared to PP. The effect of different compatibilizers in PP or styrene-ethylene- butene-styrene copolymer (SEBS) grafted with maleic anhydride (polymer-g-MAH) and grafted with diethyl maleate (polymer-g-DEM) was investigated. While the polymer-g-

MAH was a better compatibilizer it led to an increase of the PO2 when compared to the PP/ EVOH blend without compatibilizer, due to the domain size of the EVOH phase leading to a ﬁbrillar rather than lamellar morphology, the polymer-g-MAH on the other hand [63] decreased the PO2 further as compared to the blend. Cerruti et al. (2007) found that for the blends PA 6/EVOH and PA 6/EVOH/EVOH-COOH, the presence of EVOH reduced the PO2 and the PH2O as compared to PA 6. Small amounts of EVOH-COOH improved the barrier properties even further especially the PO2 , due to stronger interfacial interactions with PA 6.[65] Lopez-Rubio and Lagaron (2008) investigated the retort improvement, when blending EVOH with amorphous PA (aPA) and a nylon-containing ionomer as blending additives. Only the binary blend EVOH/aPA showed a real improvement in PO2 immediately after retorting when compared to neat EVOH. The blends containing the ionomer performed even worse than the pure EVOH.[66] Ares et al. (2009) developed a PP/EVOH blend compatibilized with sodium ionomer. The PO2 of PP/EVOH blends containing more than 30 wt% decreased by 80% as compared to the pure PP. On the other hand the PH2O was increased more than 7 times upon addition of EVOH and less than 5 wt% sodium ionomer, due to its hydrophilic character. However, when more than 5 wt% of the sodium ionomer [67] was added the PH2O became even lower than pure PP, and the PO2 improved even further.

EVOH29 was blended with poly(lactic acid) (PLA) by Sanchez-Garcia et al. (2011). The PO2 of the PLA was improved at 0% RH, but performed slightly poorer at 80% RH. The PH2O increased opposed to neat PLA.[68] A blend containing poly(ethylene-co-methacrylic acid) (PEMA) and EVOH38 was produced by Seethamraju et al. (2014) for organic light emitting diodes (OLED) application. When an EVOH content of 5, 10 and 20 wt% was added to the blend, the PO2 , respectively, decreased 1.4, 2.0, and 3.3 times. For the PH2O the decrease was, respectively, 8 and 15 times for 5 and 10 wt% EVOH when compared to pure PEMA. Addition of montmorillonite (MMT) to create a nano-composite blend of PEMA/EVOH led POLYMER REVIEWS 221

[69] fi to a 25-fold reduction of the PO2 and a 12-fold reduction of the PH2O. A lm of ethylene vinyl acetate (EVA) copolymer was hydrolyzed on both surface sides by Puente et al. (2015), creating the structure EVOH/EVA/EVOH. When the hydrolysis time increased, the thickness of the generated EVOH layers was also increased, causing the PH2O to decrease due to the crystalline phase of the EVOH layers.[70] More recent studies show the effect of multiplication extrusion, also used for microlayer technology, on blends. Zhu et al. (2014) used an assembly of force-assembling elements (FAEs), similar to multipliers, combined with one extruder. In a FAE the melt blend is first sliced into a left and right part, then the melt is stretched biaxially, finally the two parts are combined. When one FAE was applied on a blend with 25 wt% of EVOH32 the PO2 was decreased with more than 1 order of magnitude compared with a non-FAE specimen. The PCO2 was hardly effected by the application of FAE.[71] Zhang et al. (2015) on the other hand found that a LLDPE/EVOH44 (50/50 wt%) blend extruded with up to 8 multipliers caused both the PO2 and PH2O to increase when the number of multipliers increased, because the multipliers broke the elongated and layer-like morphology of the blend. When no multiplier was used, the [60] LLDPE/EVOH blend had a PO2 , that was 1000 times lower than that of neat LLDPE.

2.4. Additives and nano-composites 2.4.1. Additives The addition of functional compounds can also change the barrier properties of EVOH, resulting in either strengthening or weakening the barrier. Beta-cyclodextrin (bCD) was incorporated into EVOH44 by Lopez-de-Dicastillo et al. (2010) as a scavenger for food aroma compounds such as terpenes. Even though the Tg and crystallinity were slightly increased, the barrier against O2,CO2, and H2O became worse due to an increase in free volume in the matrix.[72] This group also developed an antioxidant packaging film based on EVOH44 and green tea extract (GTE). The addition of 5 wt% GTE led to a decrease of the PO2 at 23 C and 50% RH and an increase at 90% RH. A similar effect was observed for the PH2O at 23 C, which decreased up to 50% at 50 and 75% RH, but increased at 90% RH as the film appeared to be more sensitive to water than the regular film.[73] Lagaron et al. (2012) Ò found that 4 wt% of a nano-additive oxygen scavenger (O2Block ) decreased the PO2 of EVOH29 by 29% at 21 C and 50% RH and by 42% at 90% RH. The PH2O even decreased by 71%.[74] Peter et al. (2014) showed that the addition of low concentrations of N,N’-bis

(2,2,6,6-tetramethyl-4-piperidyl)-isophthalamide caused a decrease in PO2 of approximately 30% through molecular interactions, however once concentrations were over 1.5 wt%, the

PO2 increased again. It is interesting to note that the haze decreased at low additive concentrations and increased above 2 wt%, indicating that at low concentrations both the barrier and optical properties improved due to the structural changes in EVOH.[13] Vannini et al. (2016) added different low-molecular-weight additives to EVOH32 and EVOH44 in order to improve the processability through a decrement in the crystallization temperature. How this affects the barrier properties was not investigated, but it is known that the crystalline phase notably inﬂuences the barrier so further research is needed.[75]

2.4.2. Nano-composites Apart from additives, the addition of silicates can also influence the barrier properties of EVOH. This can be achieved either through the creation of a tortuous path for gas diffusion 222 C. MAES ET AL. or through structural changes to the polymer itself. In case of the tortuous path hypothesis the silicate nano-platelets can be considered as impermeable inorganic crystals in the polymer matrix, causing the gas molecules to diffuse around them thus resulting in a longer diffusion path.[76,77] Liu et al. (2010) showed that by changing the blow film process, the orientation of EVOH/nano-SiO2 composites can be improved so that the grain size of the nano-composites becomes smaller and higher in number, which improves the crystallinity and the degree of molecular chain order, thus improving the barrier properties. The addition of 5 wt% SiO2 decreases thePO2 (23 C, 50% RH) by 64.4% and the PH2O (38 C, 90% RH) by 54.2%. However, the optical properties (haze and transparency) became worse with [78] increased nano-SiO2 concentrations. Nam and Kim (2010) used various contents of an inorganic silicate precursor tetraethoxyorthosilicate (TEOS) and a silane coupling agent 3- isocyanatopropyl triethoxysilane (IPTES) to prepare EVOH/SiO2 nano-composites, and revealed that an optimum content of both was required to attain a high barrier material. In the optimum range of TEOS and IPTES, the PO2 was respectively improved by approximately 70% and 50%.[79] Kim and Lee (2014) used graphene oxide (GO) and EVOH32 to create EVOH/GO nano-composites and found that 0.3 wt% of GO can reduce the PO2 (25C, 60% RH) to 63% compared to a pure EVOH32 film. However, the addition of GO also decreased the transparency to 84% at a wavelength of 550 nm.[80] Kim and Cha (2014) revealed that the incorporation of inorganic planar-structured nanoclay MMT can lead to a dramatic decrease of both PO2 at 23 C, 50% RH and water vapor permeability (PH2O)at40 C 90% RH: at 3 wt% the decrease was most prominent, respectively 59.4 and 90.1%. There was also an improvement in transparency and mechanical properties. However tensile strength deteriorated above 5 wt% due to stiffness of the nanoclay itself and the transparency became worse at 7 wt%.[14] Kim et al. (2014) added small concentrations of exfoliated graphite (EFG) to EVOH32 to create nanocomposites with improved barrier properties. At 23C and [81] 0% RH the PO2 was decreased by 97% and by 64% at 80% RH. Cerisuelo et al. (2014) coated EVOH29 nanocomposites containing 2 wt% bentonite on PP and PET, but found that this did not significantly improve the barrier properties as opposed to the PP and PET samples coated with regular EVOH29.[82] Sadeghi and Shahedi (2016) developed an EVOH/ chitosan polymer mixture film containing nano-ZnO. The incorporation led to a decrease in fi fi both PO2 and PH2O when the ZnO content was increased by lling pores in the lm matrix and the creation of a tortuous path.[83] The incorporation of MMT and phosphorylated soybean isolate protein (PSPI) into EVOH32 led to nanocomposites with enhanced mechanical and barrier properties. Wang et al. (2016) found that MMT-PSPI content of 3 wt% improved the PO2 (23 C, 0% RH) by 73.5% and the PH2O (25 C) by 61.3%. When the MMT-PSPI con- [84] tent became higher than 3 wt% the PO2 increased once more.

2.5. Impact of processing conditions and technologies The permeability can also be greatly influenced as a consequence of the polymer processing. Parameters such as processing temperature and the time to cool down amongst others, often have an effect on the material characteristics such as crystallinity. Processes like heat sterilization, radiation and antibacterial treatments can also have an impact on the barrier properties. The most accurate way of determining the actual oxygen permeability for a certain application is the measurement on the final item produced. In this paragraph the influence of several process parameters on the gas barrier properties of EVOH are discussed. POLYMER REVIEWS 223

2.5.1. Processing temperature Processing temperature has an important impact on the barrier properties. The data generated by Kuraray co. Ltd. were measured on samples that received a pre-heat treatment according to the ISO 14663-2 norm of 10 minutes at 20 C below the Tm (e.g., approximately 160C for EVOH32) to erase the thermal history of the films because samples often undergo a different heat treatment during their processing.[85] The processing temperature has influ- fi ence on the crystallinity. The non-heat treated EVOH32 lm had a PO2 which was 4 times higher than that of the EVOH32 film which received a heat treatment of 140C. In case of fi the uniaxially oriented EVOH32 lm the heat treatment of 140 C improved the PO2 8 times opposed to the non-heat treated film with uniaxial orientation (Fig. 3). This might explain why some values of different studies measured under the same conditions may vary. How- ever, more in-depth research is needed.

2.5.2. Stretching Films can be stretched in one or even two directions resulting in uniaxially or biaxially oriented films, respectively. The crystallinity of the film is on its turn directly related to the orientation. In case of the heat treated films (140C) the crystallinity of the uniaxially oriented film was 68% as compared to 58% for a non-oriented film, the biaxially oriented film had a fl crystallinity of 70%, which in uences the oxygen permeability; the PO2 of the uniaxially oriented film and the biaxially oriented film decreased respectively nearly 3 and 14 times opposed to the non-oriented film, as shown in Fig. 3.[9,86]

2.5.3. Thermoforming Thermoforming has a significant impact on the thickness distribution of the material. In this process the sheet or film material is softened using heat and shaped in a mould by mechanical stretching and/or pressure. The drawing depth and angle of the corners determine the thickness distribution and surface area, and thus the barrier properties of the final packaging. However, this effect can be partly counteracted as stretching of the film can affect the orientation of the polymer chains, which are possibly drawn closer to each other. Because of this the chain mobility in the amorphous zones can be restricted, making it more difficult for oxygen molecules to pass through, and thus resulting in lower oxygen permeability. Addi- tionally the cooling step to prevent shrinkage after production and the cooling rate affect the crystallization of the polymer and therefore also the permeability. Buntinx et al. (2014) also found that trays with straight corners performed better than round corners possibly due to the stretching of the EVOH.[39]

2.5.4. Retort processing and similar treatments Careful attention needs to be paid when multilayer structures containing EVOH are exposed to a retort process (e.g., sterilization of ready meal and pet food). The combination of high temperatures and steam can render the protective water barrier layer (e.g., PE, PP, etc.) in a multilayer ﬁlm ineffective for a limited time. The encapsulated EVOH layer will be exposed to the steam and take up water. Once the water barrier of the surrounding layers has restored itself the water is enclosed inside, resulting in a negative effect on the oxygen barrier of EVOH. This phenomenon is called retort shock. Some studies even report an increase of

PO2 with a factor of 100. The barrier usually restores itself after a period of time (e.g., days to 224 C. MAES ET AL.

Figure 8. Schematic representation of the oxygen permeability of EVOH before, during and after a retort process. weeks) depending on the retort conditions and the structure of the multilayer.[9,11,87] In

Fig. 8 a schematic overview of the PO2 before, during and after retort is shown. Mokwena et al. (2009) investigated the impact of microwave sterilization (MS) on the barrier properties of 2 films in comparison to a regular retort process. Film A was a laminated PET//EVOH32 (12 mm)//PP film and film B was a coextruded (represented by /) film laminated (represented by //) to an outer PET layer: PET//PP/tie/Nylon 6/EVOH27 m D (15 m)/Nylon 6/tie/PP. The retort procedure with a thermal treatment of F0 3 min led fi fi to an 11- and 48-fold increase of the PO2 , as compared to the untreated lm A and lm B, D respectively. On the other hand, MS with F0 3 min led to a 5- and 17-fold increase of the fi D PO2 as compared to the controls of lm A and B, respectively. Even a MS with F0 6 min resulted in a lower PO2 than the retort samples, which was 10 and 24 times higher for respectively film A and B. The higher values for retort heat treatment were attributed to the increased plasticising effect from water absorption by the films. After 2 months of storage at room temperature over 50% of the barrier was recovered, however beyond 2 months the barrier slowly deteriorated.[88] Galotto et al. (2010) found that high pressure processing (HPP), surprisingly slightly decreased the O2GTR of PE/EVOH/PE as compared to non-treated samples. This is probably due to a more compact structure formed by the high temperature, yet the changes were very small and the variation was large. It is also interesting to note that the water vapor transmission rate (WVTR) increased due to swelling.[89] Juliano et al. (2010) found that EVOH-based multilayer materials showed potential for HPP and are already used for commercial production. The variation in PO2 for HPP at high temperature (HP- HT) was lower than 12% compared to non-treated samples.[90] Dhawan et al. (2014) used the same films as Mokwena et al. (2009) to evaluate the influence of Pressure-Assisted Ther- mal Sterilization (PATS) also known as HP-HT of 680 MPa of 5 minutes at 100C on the fi fi PO2 at 23 C and 55% RH. For lm A this led to a 5-fold increase and for lm B this was 4 times higher. The PH2O (38 C, 100% RH) also increased by, respectively, 74% and 16% for films A and B. The difference between film A and B was explained by the overall change in crystallinity and the free volume.[91] These studies show that the impact of either MS or HPP on the barrier properties of the EVOH layer is less severe as compared to a regular retort process. POLYMER REVIEWS 225

2.5.5. Irradiation Lopez-Rubio et al. (2007) found that electron beam irradiation at doses of 30 and 90 kGy caused oxygen scavenging activity in an EVOH29 ﬁlm. Most likely free radicals formed during the irradiation process react with oxygen, causing an oxygen blocking effect. However, ﬁ after exhaustion of this capacity the lms showed a higher PO2 due to faster oxygen diffusion caused by a reduction in crystallinity.[92]

2.5.6. Antimicrobial gases Chlorine dioxide can be used as an antimicrobial gas in the headspace of fresh products like meat, poultry and seafood. However, Rubino et al. (2010) showed that this gas also exerts an effect on the material characteristics of polymers as it can change the chemical structure. ﬁ The PO2 of a multilayer EVA/EVOH/EVA lm was increased by nearly 50% and the CO2 6 [93] permeability (PCO2 ) even doubled, changing the permselectivity ratio (PCO2 PO2 ).

3. Water vapor permeability of EVOH m Similar to the PO2 and O2GTR, in this paper the PH2O is a thickness corrected value in g. m/ (m2.day), while the WVTR is expressed in g/(m2.day). When compared to other polymers, EVOH has medium to poor (for EVOH with lower mol% ethylene) water barrier properties.[94] Other medium water barriers are PET, LDPE and PVDC (Fig. 7). Materials like PP and HDPE are preferred as moisture barriers whereas PS, polybutylene terephthalate (PBT), ﬂ PC and PA 6 have lesser water vapor barriers. The PH2O of EVOH is also in uenced by the [10,26,95] ethylene content (Fig. 2), showing a decreased PH2O at higher mol% of ethylene. The

PH2O increases at higher temperatures and higher RH. At lower RH (30-60% RH) this process is slow, but beyond 75% RH it becomes more pronounced.[10] Even though EVOH44 and EVOH48 show similar PH2O like LDPE and PET, these last two polymers are still used as outer barrier materials because they also provide good mechanical properties (e.g., heat seal, strength) and can be more easily applied in thicker structures, whereas EVOH is usually applied in thin structures.[9] Also, as was discussed previously the presence of water has a negative impact on the barrier properties of EVOH due to its moisture sensitivity, thus EVOH is not the best choice as water vapor barrier.[11] There are several techniques like the addition of nano-platelets, which can be used to increase the hydrophobicity and the water barrier of EVOH. This is especially interesting for [96] applications like encapsulating OLEDs, which require a very low O2GTR and WVTR. Several studies, which were already mentioned before, presented some effects of blending, and the addition of additives and nano-particles on the PH2O. Next to this, techniques like surface treatments can also improve the PH2O. Hong et al. (2009) treated EVOH32 with plasma to modify the surface, which enhanced the hydrophobic properties due to the formation of fluorine-containing functional groups.[97] Tenn et al. (2012) also used plasma treatment on EVOH29 and EVOH44 to generate hydrophobic surfaces. This reduced the PH2O up to 28% at 25C.[98] Another study showed the use of hydrolysis reactions on an EVA copolymer film. The surface of both sides of the film was hydrolyzed, creating the structure EVOH/EVA/EVOH. When the hydrolysis time increased, the thickness of the generated

EVOH layers was also increased, causing the PH2O to decrease due to the crystalline phase of the EVOH layers.[70] 226 C. MAES ET AL.

Figure 9. Comparison of gas permeability between EVOH32 and HDPE at 23C, 0% RH. Based on data from Kuraray co. Ltd. (2017),[7] The Nippon Synthetic Chemical Industry co. Ltd. (2013),[136] Armstrong (2011),[107] McKeen (2012)[21] and Maxwell and Roberts (2010).[137]

4. Permeation of other gases through EVOH EVOH is not only an excellent barrier against oxygen, but also for a broad range of other permanent gases such as carbon dioxide, nitrogen and hydrogen.[15,17] In Fig. 9, a comparison of the permeability of EVOH for several gases is made between EVOH32 and HDPE.

For the gases presented in Fig. 9, EVOH shows permeabilities for O2,N2,CO2,SO2,CH4 and Freon 12 between 0.4 and 12 cm3.mm/m2.day.atm at 23C and 0% RH, which are 3 to 5 orders of magnitude lower than those of HDPE. He and H2 show higher permeabilities, which are, respectively, 1–2 and 2–3 orders of magnitude lower for EVOH than HDPE. This might be explained by their small molecule size. However, the available data on these gases are usually limited to permeability measure- ments at 0% RH, making it more difficult to estimate the permeability under different conditions. Table 4 presents gas permeability data of EVOH32, EVOH38 and EVOH44. When the ethylene content increases, the gas permeability increases as well. The influence of temperature can be expressed by the Arrhenius equation for the other gas molecules in the same manner as was previously discussed for oxygen, meaning that an increase in temperature leads to a higher gas permeability. The data also show that the permeability for other gases is also influenced by the RH. In general the factors that influence oxygen permeability also influence the permeability for other permeants, but the gas permeability is also strongly dependent on the permeant itself. The permeability for nitrogen is lower than that of oxygen, while the permeability for carbon dioxide and helium is higher when measured at the same conditions. In general the gas permeability of polymers is lower for larger molecules. How- ever, carbon dioxide is a larger molecule than oxygen, yet it has a higher permeation rate. This phenomenon might be explained through the combination of the molecular size and the affinity for the polymer matrix. Carbon dioxide has a lower diffusion rate than oxygen through the same polymer due to its size, but it has a higher chemical affinity for the polymer matrix leading to a much higher solubility than oxygen. The permeability P can be calculated as the product of the diffusion coefficient D and the solubility coefficient S (P D DS), and is higher for carbon dioxide than for oxygen resulting in a higher POLYMER REVIEWS 227

Table 4. Gas Permeability (P) through different types of EVOH at various conditions. Based on data from Kuraray co. Ltd. (2017),[7] Yam (2009)[27] and Armstrong (2011).[107]

P [cm3.mm/(m2.day.atm)]

Gas Conditions EVOH32 EVOH38 EVOH44

N2 23 C, 0% RH 0.4 1.6 3.1 23C, 65% RH 0.8 2 3.1 25C, 0% RH 0.3 2.6 35C, 0% RH 0.8 3.1 5.8 O2 5 C, 0% RH 1.1 2.3 6.5 23C, 0% RH 5 10 23 23C, 65% RH 7.9 12 31 35C, 0% RH 12 24 48 Kr 50C, 0% RH 8 36 Ar 35C, 0% RH 32 50C, 0% RH 10 140 CO2 5 C, 0% RH 3.9 6.6 22 23C, 0% RH 12 26 83 23C, 65% RH 24 59 91 25C, 0% RH 16 142 35C, 0% RH 26 83 193 H2 20 C, 0% RH 600 4000 He 5C, 0% RH 1045 1783 2558 23C, 0% RH 3603 6433 9223 23C, 65% RH 7402 8661 9961 25C, 0% RH 3200 8200 35C, 0% RH 5308 9533 13795 permeability.[27] A more logical explanation for this is the kinetic diameter, which represents the smallest effective dimension of a given molecule. The kinetic diameter is smaller for carbon dioxide than that of oxygen, respectively, 0.330 and 0.346 nm, making it easier for carbon dioxide to diffuse through the polymer matrix[21,99]. Next to helium there is only limited permeability data for other noble gases such as krypton and argon at 35 and/or 50C and 0% RH. Unlike helium, which permeates easily as was mentioned before, the permeability of EVOH for krypton and argon is closer to that for oxygen due to their larger molecular size. Armstrong (2010) compared the radon diffusion coefﬁcient (DRa) of EVOH44 with other materials typically used in the construction sector to prevent radon intrusion and improve indoor air quality. The DRa of EVOH44 is several orders of magnitude lower than polyole- ﬁns, polyurethane (PU) coating and plasticized polyvinylchloride (PVC), and even lower than bitumen coated aluminium foil as shown in Fig. 10. Therefore the use of EVOH in a composite with commonly used materials such as HDPE, LLDPE and PP could dramatically reduce the diffusion of radon.[17]

5. Permeability of EVOH for organic substances 5.1. Aromas and flavors barrier Next to its outstanding gas barrier, EVOH is also extremely suitable to prevent aroma and flavor scalping in food packaging.[15] In Fig. 11 the permeability of EVOH, PVDC and LDPE for several aroma and flavor components is shown. The type of EVOH is not specified; however the barrier properties outperform both LDPE and PVDC for all components 228 C. MAES ET AL.

Figure 10. The radon diffusion coefﬁcient of EVOH44 compared to other materials. Based on data from Armstrong (2010)[17] and Jiranek et al. (2008).[138] under dry conditions. PVDC has a permeability (P) for ethyl hexanoate, ethyl 2-methylbuty- rate, hexanol, trans-2-hexanol, d-limonene and propyl butyrate of typically 1–3 orders of magnitude greater than EVOH, whereas the permeability of LDPE for these compounds is 5–7 orders of magnitude higher than EVOH with the exception of the P for trans-2-hexenal which is less than 1 order of magnitude higher for PVDC and about 4 orders of magnitude higher for LDPE[27]. Over the past years various studies have been performed on the permeability of aromas and ﬂavors, yet no standard procedure is recommended similar to the permanent gases. The simplest method is the gravimetric method similar to the ASTM E96 method used for WVTR, in which the permeation is determined by weight loss.[100] Another method is the sensory test, where trained panellists smell at different intervals until the aroma is detected. It is also possible to detect the permeant aroma compounds through gas chromatography (GC). There are many studies that focus on the sorption, which can be determined by the gravimetric method or analytical techniques like GC and high performance liquid

Figure 11. Comparison of aroma permeability between EVOH, PVDC and LDPE at 25C, 0% RH. Based on data from Yam (2009).[27] POLYMER REVIEWS 229

Table 5. Permeability of ethyl butyrate and a-pinene through EVOH32 at 25C and different RH. Based on data from Lopez-Carballo et al. (2005).[15]

P [g.mm/(m2.day.atm)]

RH (%) Ethyl butyrate a-Pinene

0 <0.00011 <0.000021 11 <0.00011 <0.0000009 23 <0.000042 <0.0000088 54 0.0024 0.00021 75 0.22 0.017 100 1.5 0.096 chromatography (HPLC).[27] Fukamachi et al. (1996) used a GC-based method and found that the sorption of flavors (ethyl hexanoate, n-octanal and n-octanol) from an ethanol solution into EVOH was influenced by the ethanol content in the solution. A maximum sorption for the three components was detected at 10–20 v% ethanol and was, respectively, 6.2, 3.5 and 4.1 times higer for ethyl hexanoate, n-octanal and n-octanol than at 0 v% ethanol.[101] Lopez-Carballo et al. (2005) determined the permeability, diffusion and sorption of ethyl butyrate and a-pinene through an EVOH32 film at different RH. The organic compound uptake was analysed by GC-flame ionization detector (FID) and Fourier transform infrared spectroscopy (FT-IR) was used to determine the mass transport of the aromas. The results are shown in Table 5. EVOH displays excellent barrier properties to both aroma compounds, which were reported to be 1000 times lower than those measured in LDPE, even at 100% RH. Remarkable is that the aroma permeability as a function of the RH shows a similar trend as the oxygen permeability (Fig. 6), a slight improvement at low RH is noted compared to dry conditions, which can be explained by the free volume theory.[15,102] Aroma permeation can also be measured by solid-phase micro-extraction (SPME), where a container filled with aroma compounds is placed in a glass container and a fused silica cap- illary is used to trap the aroma compounds that permeated through the container to the outside. The compounds are desorbed and analyzed by GC. Berlinet et al. (2008) used this method to investigate the permeation of orange juice aromas through a PET bottle and a HDPE cap. It was discovered that the permeation mainly occurred through the cap. By using a multilayer HDPE cap with an internal layer of LDPE/EVOH/LDPE, the permeation was considerably limited. The permeation of ethyl butyrate was reduced by a factor 30.[103] Zhou et al. (2004) developed an online measurement system to determine the permeation of aromas through films by coupling a permeation system to a purge-and-trap/fast gas chro- matographic system (P&T/fGC). With this method, the permeation of limonene and ethyl butyrate was measured through three multilayer HDPE films, which contained respectively EVOH, nylon and no barrier. The films containing EVOH and nylon had comparable aroma barrier properties for both limonene and ethyl butyrate and were superior to the film without barrier layer. This study also showed the effect of co-permeation, where one component’s presence can influence the permeability of the other component. While there was no effect on the permeability of limonene, the permeability of ethyl butyrate was more than doubled when limonene was present as a co-permeant for both films.[104] fl fl Just like the PO2 , different factors like RH in uence the aroma and avour permeability.

Apart from the factors mentioned in the section about PO2 , Leufven and Hermansson (1994) found that the pH also has an impact on the permeation of aromas due to the changes in 230 C. MAES ET AL. polarity and structure of the molecules. The sorption of trans-2-hexenol, 2-heptanone, 6-methyl-5-hepten-2-one, 6-methyl-5-hepten-2-ol and limonene increased when the pH increased from 4 to 7.[105,106]

5.2. Fuels and chemicals barrier EVOH can also be used as a barrier against fuels and other chemicals like liquid solutions of benzene, toluene, ethylbenzene and xylene isomers (BTEX). EVOH is already widely used as a barrier material in automotive applications like fuel tanks. EVOH also offers good chemical resistance to volatile organic compounds, hydrocarbons and organic solvents. The exposure to certain solvents can cause softening, swelling or environmental stress cracking in many polymers. However, EVOH retains its key physical properties in the presence of most organic solvents, acid and alkali solutions, and non-ionic surfactants. This is attributed to the combination of a highly ordered crystalline structure dispersed in disordered amorphous regions which give EVOH desirable properties of high resistance to the diffusion of gases and solvents.[107]

5.2.1. Fuels and alcohols Lagaron et al. (2001) determined the permeability of fuel using the gravimetric method and found that EVOH has outstanding barrier properties to fuel C (toluene and iso-octane 50/50 v/v) compared to PA 6 and HDPE (Table 6). However when methanol is added to create oxy- genated fuel as a clean alternative to standard fuels, the barrier deteriorates drastically, due to methanol’s plasticizing effect on EVOH, the same phenomenon can be observed in PA 6. HDPE is more resistant to methanol permeation than EVOH and PA 6, although EVOH still easily outperforms HDPE as a barrier against fuel CM15 (fuel C with 15 v% methanol). The plasticizing effect is less pronounced when 10 v% ethanol is added to fuel C (i.e., CE10).[18] Zhao et al. (2015) developed an automated permeameter using a GC-FID to measure the permeation of the different fuel components as low as 1 mg/m2.day. These results showed that the permeated ﬂux of fuel C mainly contained toluene, which was conﬁrmed with permeation experiments with pure toluene and iso-octane (no results were shown due to no detectable loss), whereas mixtures with ethanol mainly contained ethanol. This means that the higher permeability of fuel CE10 compared to fuel C is due to the high permeation of ethanol. This also corresponds to the permeability of EVOH32 for CM15, which is about 3

Table 6. Fuel permeability through EVOH, PA 6 and HDPE. Fuel C: toluene/iso-octane (50/50 v/v); Fuel CM15: Fuel C C 15% methanol; Fuel CE10: Fuel C C 10% ethanol. Based on data from Lagaron et al. (2001)[18] and Nulman et al. (1998).[135]

P [g.mm/(m2.day)]

40C, 12% RH 21C, 12% RH

Fuel/ Permeant EVOH32 PA 6 HDPE EVOH32

Fuel C 5 5,060 62,600 9 Fuel CM15 4,500–5,500 205,000 71,000 700 Fuel CE10 20 40 Methanol 24,000–61,700 360,000 1,400 7,300 Toluene 4 6 POLYMER REVIEWS 231 orders of magnitude higher than for fuel C at 40C and 12% RH due to the high permeability of EVOH32 for methanol, which is 4–5 orders of magnitude than fuel C. At 21C and 12% RH, this effect is less pronounced, yet the permeability for CM15 is still nearly 2 orders of magnitude higher than fuel C.[108] Gagnard et al. (2004) observed the co-permeation of toluene and methanol using a GC-FID. EVOH has a low permeability to toluene, but this is increased by the presence of methanol, which has a higher permeation. This makes structures with EVOH sandwiched between HDPE interesting in the automotive sector, since HDPE has a lower permeability to methanol (Table 6).[109,110] Inverse GC (IGC) was used by Cava et al. (2007) to measure the inﬂuence of RH on alcohol transport through EVOH38. Three alcohols, methanol, ethanol and 1-butanol were used. The methanol diffusion was most inﬂuenced by RH and 1-butanol the least, however for all three alcohols a sharp incre- ment could be noticed between 35 and 47% RH owing to the plasticization of the copolymer.[111]

5.2.2. BTEX In several studies the barrier properties of EVOH against BTEX have been investigated. BTEX contamination is especially important in groundwater and soils from where they can evapo- rate into building constructions and thus affect the indoor air quality.[17] Geomembranes with an EVOH barrier layer offer protection against these toxic vapors.[112,113] McWatters and Rowe (2015) found that the permeability coefﬁcients for BTEX and chlorinated hydrocarbons of HDPE and LLDPE monolayer geomembranes can be reduced by 2 to over 4 orders of magnitude by inclusion of an EVOH barrier layer in a LLDPE geomembrane.[114]

5.2.3. Other applications Sensor nodes in ubiquitous sensor networks (USN) require autonomous replacement of deteriorated sensors with reserve sensors. Lee et al. (2012) developed an encapsulation tech- nique to avoid poisoning of the reserve sensors and an autonomous activation method to replace the sensor through thermal decomposition of the encapsulation material. EVOH27 was found to be a suitable barrier material to protect the sensor against formaldehyde gas and also did not have a negative impact on the sensor during its thermal decomposition.[115] The permeation of flammable liquids through intermediate bulk containers (IBC), jerricans and drums during transport can lead to the creation of an explosive atmosphere. Bethke et al. (2013) found that when no barrier like EVOH was present, the lower explosive limit (LEL) for trichloroethylene, toluene and n-hexane was reached within a few hours at 40C. Whereas the incorporation of EVOH reduced the permeation with more than 1 order of magnitude.[116] Agricultural films containing a barrier like PA are used for three reasons: to decrease the dose of fumigants in soil disinfestations, to decrease the emission of fumigants into the atmosphere and to decrease the operator exposure. Fouillet et al. (2014) compared a new film containing EVOH to an existing film with PA as a barrier to dimethyl disulfide (DMDS) fumigant and found that EVOH is an alternative for PA.[117] The effect of blending EVOH with modified PA (mPA) on the permeation of turpentine (white spirit) was investigated by Yeh et al. (2006). Three different EVOH types (EVOH32, EVOH44 and EVOH48) were blended with mPA and bottles were prepared from these resins. The white spirit permeation decreased when the mol% ethylene in EVOH was lower and increased with an increasing mPA content in the EVOH/mPA blend. The white spirit permeation of neat EVOH32, EVOH44 and EVOH48 was, respectively, 882, 254 and 232 C. MAES ET AL.

208 times lower than PE.[43] Yeh et al. (2006) also investigated the permeation of gasoline. It was found that 22% of a gasoline mixture permeated through a PP bottle within one month. When 10 wt% EVOH32 was blended with PP, the permeation was reduced by a factor 8.6. A PP and 10 wt% mPA blend led to a decrease of a factor 10.1, which is better than the PP/ EVOH blend. Although EVOH is intrinsically a better barrier, this can be explained by the fracture of the lamina structure of the EVOH. When EVOH/mPA blend was added to PP the barrier improved as the mPA content increased, reaching a minimum permeation, which was 113 times better than neat PP, at 83.3 wt% mPA in the EVOH/mPA blend.[118]

5.3. Functional barrier Packaging materials intended for food contact, commonly referred to as food contact materials (FCM) must comply with Regulation (EC) N 1935/2004 and plastic materials additionally must meet with Regulation (EU) N 10/2011. These regulations state that FCM may not transfer their constituents to food in quantities that may endanger human health or bring about unacceptable changes in the composition of the food or bring about a dete- rioration of the organoleptic characteristics thereof; and contain a union list of authorized substances and their speciﬁc migration limit (SML) into food. However, non-authorised substances may be used, provided they are separated from the food by a functional barrier, which meets the criteria and limits the migration of these substances to below a given detection limit.[27,119]

5.3.1. Mineral oils In 2012 the European Food Safety Authority (EFSA) published a scientific report on the occurrence of mineral oil hydrocarbons in food,[120] in which possible health risks caused by exposure to mineral oil in food are brought to attention. One of the main causes for mineral oil contamination in food is recycled cardboard. Lorenzini et al. (2010) found that mineral oil compounds migrated from printing inks and recycled fibers in paperboard to dry food in concentrations far exceeding the acceptable daily intake (ADI). The highest measured concentrations were 1000 times higher than the ADI value. It was also found that higher molec- > ular weight compounds ( C24) seem negligible with regards to migration for both mineral oil saturated hydrocarbons (MOSH) and mineral oil aromatic hydrocarbons (MOAH). – < About 60 80% of the C24 fraction might be transferred into the packaged food when no functional barrier is present.[121] Vollmer et al. (2011) observed similar findings. The concentration of MOSH surpassed the ADI of 0.6 mg/kg bodyweight (using generally applied conventions of 60 kg bodyweight and the consumption of 1 kg contaminated food) with a factor of 10 to 100 and about 10–20% of the migrated compounds were MOAH of which the safety is still uncertain based on the available toxicological data. These results are quite alarming, especially because the samples were only 2–3 months old.[122] The 3rd draft document for the regulation of mineral oils from recycled fibres in paperboard was presented in 2014 by the German Federal Ministry of Food and Agriculture, in this document the limits were set on 2 mg/kg and 0.5 mg/kg for MOSH and MOAH, respectively.[123] In 2017 the 4th draft was published, which only mentions the migration limit for MOAH and obligates the use of a functional barrier.[124] This upcoming regulation makes the need for inclusion of functional barriers in paper-based packaging for food more urgent. Several studies have investigated functional barriers against the migration of mineral oils POLYMER REVIEWS 233 from paperboard packaging. A few studies mention EVOH as a possible functional barrier, but only limited data on permeation or migration rates have been published so far. One of the main reasons is that no standard method has been established yet, also the detection limit is challenging. Diehl and Welle (2015) summarized three different methods, which are currently used by different research institutes: migration experiments, permeation experiments and lag time experiments. Migration experiments are performed according to standard testing procedure DIN EN 14338, in which the concentration of migrants in the food or food stimulants is determined.[125] Pastorelli et al. (2008) performed migration experiments on benzophenone, which is commonly used as a photo-initiator in UV-cured inks. The migration was measured in cake at two different conditions. Cake packaged in PP contained over 30 times more benzophenone than when a structure of PP/EVOH/PP was used at 40C for 10 days. At 70C and 48 h the migration was twice as high for PP than for PP/EVOH/PP, the exact structures were not specified.[126] Permeation experiments are kinetic studies in which the permeation rate is derived by measuring the concentration of the permeated migrants at different time points. In most cases the migrants are spiked in much higher or the worst case concentrations either using real mineral oil mixtures or model compounds. Fiselier and Grob (2012) spiked a donor pack with real mineral oil mixtures at worst case concentrations on one side of the barrier material and a PE film was used on the other side as an acceptor material. At different time points a piece of the PE film was extracted with hexane for 2 hours at ambient temperature and analyzed for presence mineral oils by on-line HPLC-GC-FID to determine the breakthrough time of 1% of the initial concentration of the mineral oil at 60C. This could be recalculated to a time at 22C (ambient temperature) by the Arrhenius equation. Because mineral oil mixtures are complex and difficult to separate into single compounds only the total amount of migrants could be determined when using a real mineral oil mixture. The results are presented in Table 7. Whereas polyolefins such as PE and PP showed poor barrier properties towards the mineral oil, PA and PET did not show breakthrough of 1% up to 84 days at 60C, which translates to a shelf life of more than 3910 days or 10.7 year at 22C.[127] The same test was performed on two multilayer films containing only a few mmof either EVOH32 or EVOH44 (Table 7). EVOH32 can easily compete with PA and PET. There might have been a slight breakthrough of 2% at 84 days at 60C, this was close to the detection limit, however, after 56 days no breakthrough was detected, which still results in a shelf life of over 2576 days or 7.1 year at 22C. Considering that the initial spiked

Table 7. Comparison of breakthrough time of mineral oils through EVOH with other materials. Based on data from Fiselier and Grob (2012)[127] and Maes et al. (2017).[128]

Time to reach 1% breakthrough of MOH

Film sample Test at 60C [days] Recalculated to 22C [days]

50 mm LDPE/LLDPE 0.0033 0.15 60 mm LLPE/HDPE 0.010 0.44 50 mm OPP 0.13 5.9 20 mm PA cast > 85.0 > 3910.0 12 mm PET > 85.0 > 3910.0 PE/tie/EVOH32/tie/PE film (16/5/4.4/5/18 mm) > 84.0 > 3864.0 PE/tie/EVOH44/tie/PE film (16/6/5.2/6/18 mm) > 56.0 > 2576.0 234 C. MAES ET AL. concentration is far above the normal concentration present in cardboard, both EVOH films can be considered effective functional barriers.[128] Biedermann-Brem and Grob (2014) used a similar method, but instead of real mineral oil mixtures, the donor paper was spiked with four surrogate substances: dipropyl phthalate (DPP), 4-methyl benzophenone (MBP), triethyl citrate (TEC) and n-heptade- fi [129] cane (C17). Also a silicone paper was used as acceptor material instead of a PE lm. Richter et al. (2014) used this method to evaluate the barrier of different samples from the German and Swiss market, but added n-octadecane (C18)asafifth surrogate substance. The barrier efficiency was classified in five classes, where class 1 had no significant barrier, class 4 was a virtually tight barrier and class 5 included aluminum foil in the barrier. Multilayer structures with EVOH were classified in class 4 as being virtually tight barriers.[130] An automated permeation method was developed by Ewender et al. (2013). No acceptor material was used, instead the downstream of a permeation cell was flushed constantly with a permanent gas flow which led to a cryogenic sample trap coupled to a GC-FID. At certain time intervals the trapped permeants in the sample trap were desorbed by fast heating into the GC, allowing the permeation rate to be determined quite accurately by using 15 model substances (see Table 8)liken-alkanes (n-C12 to n-C24), substituted naphthalenes, substituted benzophenones, 2,7-diisopropyl naphthalene (DIPN) and 2,2,4-trimethyl-1,3-pentanediol-di-isobutyrate (TXIB) representing mineral oil chemistry. The paperboard was spiked with 750 mg/kg per substance, which can be considered as worst case scenario. The study also showed that there is a linear correlation between the logarithm of the experimental determined permeation rate at 40C and logarithm of the vapor pressure calculated at 25C, making it possible to predict other permeants. The study concluded that oriented PA (OPA) and oriented PET (OPET) are good functional barrier materials against mineral oil migration, whereas polyolefins provide insufficient barrier (see Table 8). It was also mentioned that a PP/ EVOH/PP multilayer structure reduced the permeation of benzophenone for about a factor of 2 compared to a PP monolayer film, however, no thickness or type of the EVOH were specified.[131] Ewender et al. (2015) performed the same experiment on different commercially available barrier films to find possible alternatives for OPA and OPET. A biaxially oriented PP (BOPP) multilayer film containing EVOH and a biaxally oriented PET film were found to be suitable alternatives for OPA and OPET. Unfortunately the thickness of the EVOH layer in the BOPP film was not mentioned.[132] According to the migration limits set in the draft document for the regulation of mineral oils from recycled fibresinpaperboardthetimetoreachtheSMLcanbecalculatedaccordingto Eq. (6), where mfood is the mass in kg of the packed food, PR is the permeation rate in mg/ dm2.day which can be found in Table 8 for the different films, and A is the area of the packaging in dm2 which is in contact with the food.[132]

SML : mfood tSML D (6) PR : A

Ewender and Welle (2014) adapted the previously described method by replacing the spiked donor material by a spiked gas stream. This enabled the possibility of preconditioning the cell with a non-spiked stream. Using this method the lag times at 70–120C could be experimentally determined, and allow a prediction of the lag times at ambient temperature POLYMER REVIEWS 235

Table 8. Permeation rate at 40C of model compounds for MOSH/MOAH through different types of ﬁlms. Based on data from Ewender et al. (2013)[131] and Ewender et al. (2015).[132]

Permeation rate [mg/dm2.day]

Boiling point 20 mm 14.9 mm 12.4 mm 12 mm 20 mm BOPP multilayer Substance [C] BOPP OPA OPET BOPET with EVOH

Dodecane (C12) 216 4900 <0.01 <0.01 0.01 0.02 Tetradecane (C14) 254 1390 <0.01 <0.01 0.008 0.014 Hexadecane (C16) 287 238 <0.01 <0.01 <0.006 0.008 Octadecane (C18) 317 33 <0.01 <0.01 0.009 0.01 Eicosane (C20) 343 5.51 <0.01 <0.01 <0.006 <0.006 Docosane (C22) 369 1.02 <0.01 <0.01 <0.007 <0.007 Tetracosane (C24) 391 <0.03 <0.01 <0.01 <0.007 <0.007 Naphthalene 218 693 <0.01 <0.01 0.01 0.009

1-Methyl naphthalene 243 1590 <0.01 <0.01 <0.006 <0.006

1-Ethyl naphthalene 260 1000 <0.01 <0.01 <0.006 <0.006

DIPN 279 88 <0.01 <0.01 <0.006 <0.006

TXIB 280 244 <0.01 <0.01 0.015 <0.009

Benzophenone 305 68 <0.01 <0.01 <0.007 0.008

4-Methyl-benzophenone 326 22 <0.01 <0.01 0.017 0.017

Phenanthrene 336 32 <0.01 <0.01 <0.006 <0.006 236 C. MAES ET AL. through the use of diffusion modelling. A functional barrier is sufﬁcient if the lag time is much higher than the shelf life of the packaged food. But so far no results on EVOH have been published using this method.[133]

5.3.2. Non-intentionally-added substances Several recent studies also investigated EVOH as a barrier against non-intentionally-added substances (NIAS) migrants coming from PU adhesives used in laminated multilayer films. Felix et al. (2012) found more than 63 volatile and semi-volatile compounds as potential migrants in the PU adhesives by using headspace solid-phase micro-extraction and gas chromatography coupled to mass spectrometry (HS-SPME-GC-MS). Two of these compounds were NIAS: 1,6-dioxacyclododecane-7,12-dione and 1,4,7-trioxacyclotridecane-8,13-dione. EVOH was shown to act as a barrier for polar migrants like caprolactam, triphenyl phos- phate, an unknown adipate and 1,4,7-trioxacyclotridecane-8,13-dione. Although the migration of 1,6-dioxacyclododecane-7,12-dione was not quantified, it was not found in the multilayer film containing EVOH between the PU adhesive and the food simulant, indicating that EVOH acted as a functional barrier[134] Isella et al. (2013) used ultra-performance liquid chromatography quadrupole-time-of-flight mass spectrometry to demonstrate the efficiency of EVOH as a functional barrier against non-volatile NIAS from PU adhesives. PE, even at higher thickness than normally used, showed no barrier for migration. Films containing EVOH, on the other hand, showed good properties to retain these NIAS, only one component migrated: N,N-dimethyllauramide[22] Carrizo et al. (2015) also found that 1,4,7-trioxacyclotridecane-8,13-dione easily migrated through PE, at even higher concentration than the 10 ppb limit (Regulation (EU) N 10/2011). EVOH successfully decreased the migration, but did not inhibit it completely[119] Further research in this field is necessary to confirm the usefulness of EVOH as a functional barrier.

6. Conclusion EVOH copolymer has excellent barrier properties to gases, aromas, fuels, chemicals and even offers a good functional barrier for mineral oils and NIAS, making it useful in a wide range of applications. Its barrier properties are inﬂuenced by various parameters that are either intrinsic material properties or extrinsic factors like environmental conditions or permeant properties. The combination of these factors determines the barrier of EVOH.

A higher ethylene content in EVOH causes an increase in the PO2 and a decrease in the

PH2O, additionally it also improves the processability. EVOH also has a high degree of crystallinity and the barrier improves when the crystallinity is increased. The high Tg and low free volume lead to low chain mobility, which inhibits the permeation of molecules through the polymer. Addition of additives and nano-particles can either strengthen or weaken the barrier of EVOH. In addition, material processing can change the material properties and therefore have an impact on the barrier as well. The temperature, RH and pressure of the environment also influence the barrier properties of EVOH. Generally a higher temperature will increase the permeability coefficient. Plas- ticization and swelling effects may occur due to a change in pressure, which also affect the barrier. The most important environmental parameter is RH due to water sensitivity of EVOH. At low RH the permeability is initially reduced compared to dry conditions, which reaches a minimum at about 20–40% RH. But at higher RH the barrier becomes worse owing POLYMER REVIEWS 237 to the plasticization effect of water molecules. The same effect is also caused by alcohols and other molecules containing polar groups. EVOH also offers less resistance to permeation of these molecules. Because of this, EVOH is usually applied in multilayer structures or blended with other materials to protect the EVOH layer from moisture and alcohols and it also improves the processability of the final material. However, there is still a lot of demand for in-depth research concerning the permeability of other gases than oxygen, volatile organic compounds and the effect of different components (e.g., in co-permeation) at different conditions. This will lead to deeper understanding of the EVOH barrier mechanisms and create new possibilities and markets for EVOH as a barrier layer material. Online permeation measurement coupled to a GC is a powerful tool to determine the permeability for different kinds of organic substances like aromas, flavours, fuel, mineral oils and other chemicals.

List of abbreviations

ADI acceptable daily intake aPA amorphous polyamide BOPP biaxially oriented polypropylene BTEX benzene, toluene, ethylbenzene and xylene isomers CE10 fuel C with 10 v% ethanol CM15 fuel C with 15 v% methanol DIPN 2,7-diisopropyl naphthalene DRa radon diffusion coefficient EFSA European Food Safety Authority EVA ethylene vinyl acetate EVOH ethylene vinyl alcohol copolymer EVOHxx EVOH containing xx mol% ethylene FAE force-assembling element FCM food contact material FID flame ionization detector GC gas chromatography g-MAH grafted with maleic anhydride GO graphene oxide GTE green tea extract HDPE high-density polyethylene HP-HT high pressure high temperature processing HPLC high performance liquid chromatography HPP high pressure processing IPTES 3-isocyanatopropyl triethoxysilane LDPE low-density polyethylene LLDPE linear low-density polyethylene MMT montmorillonite MOAH mineral oil aromatic hydrocarbons MOSH mineral oil saturated hydrocarbons mPA modified polyamide 238 C. MAES ET AL.

MS microwave sterilization NIAS non-intentionally-added substances

O2GTR oxygen gas transmission rate OLED organic light emitting diodes OPA oriented polyamide OPET oriented poly(ethylene terephthalate) P permeability (coefficient) PA polyamide PA 6 polyamide nylon 6 PBT polybutylene terephthalate PC polycarbonate fi PCO2 carbon dioxide permeability (coef cient) PE polyethylene PEMA poly(ethylene-co-methacrylic acid) PET poly(ethylene terephthalate) fi PH2O water vapor permeability (coef cient) PLA poly(lactic acid) fi PO2 oxygen permeability (coef cient) PP polypropylene PS polystyrene PSPI phosphorylated soybean isolate protein PU polyurethane PVC polyvinylchloride PVDC poly(vinylidene dichloride) PVOH poly(vinyl alcohol) RH relative humidity SML specific migration limit TEOS tetraethoxyorthosilicate

Tg glass transition temperature Tm melting point TXIB 2,2,4-trimethyl-1,3-pentanediol-di-isobutyrate WVTR water vapor transmission rate

Acknowledgments

The authors acknowledge support from the agency Flanders Innovation & Entrepreneurship via the VLAIO Baekeland mandate of Caroline Maes. The authors also acknowledge Cynthia Teniers, Didier Houssier and Bruno Steenssens of EVAL Europe nv for their valuable input.

ORCID

Caroline Maes http://orcid.org/0000-0001-7226-7933

References

[1] Wagner, J. R. Multilayer Flexible Packaging; Elsevier Science: Oxford, United Kingdom and Cambridge, MA, USA, 2016. POLYMER REVIEWS 239

[2] Kable. EVAL Production Expansion, Antwerp, Belgium:http://www.chemicals-technology. com/projects/eval-expansion/(last accessed 6 June 2016) [3] Kuraray co. Ltd. The Story of the Birth of New Products (EVAL):http://www.kuraray.co.jp/en/ company/history/story/eval.html (last accessed 9 June 2016) [4] Finch, C. A. Polyvinyl Alchol Developments; John Wiley & Sons Ltd.: Hoboken, NJ, USA, 1992. [5] ICIS Chemical Business. Eval to Build EVOH Plant to Meet Global Demand:https://www.icis. com/resources/news/2001/06/25/141585/eval-to-build-evoh-plant-to-meet-global-demand/ (last accessed 13 March 2017) [6] Kuraray America Inc. Kuraray Americas EVAL Plant to Increase Production by More than 17 Million Pounds:http://www.kuraray.us.com/kuraray-americas-eval-plant-to-increase-produc tion-by-more-than-17-million-pounds/ (last accessed 13 March 2017) [7] Kuraray Co. Ltd. Kuraray – EVALTM:http://www.evalevoh.com (last accessed 28 February 2017) [8] Iwanami, T.; Hirai, Y. Ethylene vinyl alcohol Resins for Gas-Barrier Material. Tappi J. 1983, 66, 85–90. [9] Mokwena, K. K.; Tang, J. Ethylene vinyl alcohol: A Review of Barrier Properties for Packaging Shelf Stable Foods. Crit. Rev. Food Sci. Nutr. 2012, 52, 640–650. doi: 10.1080/ 10408398.2010.504903. [10] Zhang, Z. B.; Britt, I. J.; Tung, M. A. Permeation of Oxygen and Water Vapor Through EVOH Films as Inﬂuenced by Relative Humidity. J. Appl. Polym. Sci. 2001, 82, 1866–1872. doi: 10.1002/app.2030. [11] Mokwena, K. K.; Tang, J.; Laborie, M. P. Water Absorption and Oxygen Barrier Characteristics of ethylene vinyl alcohol Films. J. Food Eng. 2011, 105, 436–443. doi: 10.1016/j. jfoodeng.2011.02.040. [12] Lopez-Rubio, A.; Lagaron, J. M.; Gimenez, E.; Cava, D.; Hernandez-Munoz,~ P.; Yamamoto, T.; Gavara, R. Morphological Alterations Induced by Temperature and Humidity in ethylene¡vinyl alcohol Copolymers. Macromolecules 2003, 36, 9467–9476. doi: 10.1021/ma035346j. [13] Peter, Z.; Kenyo, C.; Renner, K.; Krohnke, C.; Pukanszky, B. Decreased Oxygen Permeability of EVOH Through Molecular Interactions. Express Polym. Lett. 2014, 8, 756–766. doi: 10.3144/ expresspolymlett.2014.78. [14] Kim, S. W.; Cha, S. H. Thermal, Mechanical, and Gas Barrier Properties of ethylene- vinyl alcohol Copolymer- Based Nanocomposites for Food Packaging Films: Effects of Nanoclay Load- ing. J. Appl. Polym. Sci. 2014, 131, 8. doi: 10.1002/app.40289. [15] Lopez-Carballo, G.; Cava, D.; Lagaron, J. M.; Catala, R.; Gavara, R. Characterization of the Interaction Between Two Food Aroma Components, Alpha-Pinene and Ethyl Butyrate, and ethylene-vinyl alcohol Copolymer (EVOH) Packaging Films as a Function Of Environmental Humidity. J. Agric. Food Chem. 2005, 53, 7212–6. doi: 10.1021/jf051041n. [16] Shimo, H.; Yamashita, T. Pipe For Hot-Water Circulation. Google Patents. U.S. 6447858 B1, 2002. [17] Armstrong, R. B.; Asme, M.; Spe, M.; Ig, M. Improving Indoor Air Quality by Reducing Radon and Vapor Intrusion Through the Use of ethylene vinyl alcohol (EVOH). International Radon Symposium, Columbus Ohio, 2010. [18] Lagaron, J. M.; Powell, A. K.; Bonner, G. Permeation of Water, Methanol, Fuel and Alcohol- Containing Fuels in High-Barrier ethylene–vinyl alcohol Copolymer. Polym. Test. 2001, 20, 569–577. doi: 10.1016/S0142-9418(00)00077-5. [19] Armstrong, R. Reduction in Landﬁll Emissions of Hydrocarbons and Greenhouse Gasses Through the use of EVOH. Global Waste Management Symposium: Arizone Grand Resort – Phoenix, AZ, USA, 2012. [20] Armstrong, R. B. Improving Performance of Geosynthetics for Containment of Volatile Organic Compounds Through the use of ethylene vinyl alcohol (EVOH). Geo-Frontiers: Shera- ton Dallas Hotel; Dallas, Texas USA, 2011. [21] Mckeen, L. W. Permeability Properties of Plastics and Elastomers; William Andrew: Waltham, USA and Oxford, UK, 2012. 240 C. MAES ET AL.

[22] Isella, F.; Canellas, E.; Bosetti, O.; Nerin, C. Migration of Non Intentionally Added Sub- stances from Adhesives by UPLC–QTOF/MS and the Role of EVOH to Avoid Migration in Multilayer Packaging Materials. J. Mass Spectrom. 2013, 48, 430–437. doi: 10.1002/ jms.3165. [23] Stevens, M. Permeation Basics. 14th International MOCONÒ Conference: Permeation Mea- surement Today: Montabour, Germany, 2015. [24] Lagaron, J. M.; Catala, R.; Gavara, R. Structural Characteristics Defining High Barrier Properties in Polymeric Materials. Mater. Sci. Technol. 2004, 20,1–7. doi: 10.1179/026708304225010442. [25] Holland, B. J.; Hay, J. N. The Thermal Degradation of Poly(vinyl alcohol). Polymer 2001, 42, 6775–6783. doi: 10.1016/S0032-3861(01)00166-5. [26] Zhang, Z. B.; Britt, I. J.; Tung, M. A. Water Absorption in EVOH Films and its Influence on Glass Transition Temperature. J. Polym. Sci. Part B-Polym. Phys. 1999, 37, 691–699. doi: 10.1002/(SICI)1099-0488(19990401)37:7%3c691::AID-POLB20%3e3.0.CO;2-V. [27] Yam, K. L. The Wiley Encyclopedia of Packaging Technology; John Wiley & Sons Ltd: Hoboken, NJ, USA, 2009. [28] Robertson, G. L. Food Packaging: Principles and Practice; Taylor & Francis Group: Boca Raton, FL, USA, 2012. [29] Ito, K.; Saito, Y.; Yamamoto, T.; Ujihira, Y.; Nomura, K. Correlation Study Between Oxygen Permeability and Free Volume of ethylene-vinyl alcohol Copolymer Through Positronium Life- time Measurement. Macromolecules 2001, 34, 6153–6155. doi: 10.1021/ma001813a. [30] Artzi, N.; Narkis, M.; Siegmann, A. Review of Melt-Processed Nanocomposites Based on EVOH/Organoclay. Journal of Polymer Science Part B-Polymer Physics 2005, 43, 1931–1943. doi: 10.1002/polb.20481. [31] Ezquerra, T. A.; Sics, I.; Nogales, A.; Denchev, Z.; Balta-Calleja, F. J. Simultaneous Crystalline- Amorphous Phase Evolution During Crystallization of Polymer Systems. Europhys. Lett. 2002, 59, 417–422. doi: 10.1209/epl/i2002-00211-3. [32] Artzi, N. Why Nano-Composites are in? A Glimpse into Polymer Based Nanocomposites. Rev. Chem. Eng. 2005, 21, 307–345. doi: 10.1515/REVCE.2005.21.5.307. [33] Cabedo, L.; Lagaron, J. M.; Cava, D.; Saura, J. J.; Gimenez, E. The Effect of Ethylene Content on the Interaction Between ethylene-vinyl alcohol Copolymers and Water—II: Influence of Water Sorption on the Mechanical Properties of EVOH Copolymers. Polym. Test. 2006, 25, 860–867. doi: 10.1016/j.polymertesting.2006.04.012. [34] Armstrong, R. B. Crystallization Kinetics of Semi-Crystalline Thermoplastic: Ethylene Vinyl Alcohol; Topcon; 2008. [35] Alvarez, V. A.; Kenny, J. M.; Vazquez, A. Isothermal Crystallization of Poly(vinyl alcohol-co- ethylene). J. Appl. Polym. Sci. 2003, 89, 1071–1077. doi: 10.1002/app.12255. [36] Rwei, S.-P.; Jue, Z.-F.; Chiang, W.-Y.; Chen, J.-H.; Chen, Y.-H. Characterization of melt spinn- ability of ethylene vinyl alcohol copolymers. Text. Res. J. 2015, 86, 1191–1201. doi: 10.1177/ 0040517515603805. [37] Ketels, H. H. T. M. Synthesis, Characterization and Applications of ethylene vinylalcohol Copolymers. Technische Universiteit Eindhoven Eindhoven, 1989. [38] Michaels, A. S.; Barrie, J. A.; Vieth, W. R. Diffusion of Gases in Polyethylene Terphthalate. J. Appl. Phys. 1963, 34,13–&. doi: 10.1063/1.1729054. [39] Buntinx, M.; Willems, G.; Knockaert, G.; Adons, D.; Yperman, J.; Carleer, R.; Peeters, R. Evalua- tion of the Thickness and Oxygen Transmission Rate before and after Thermoforming Mono- and Multi-layer Sheets into Trays with Variable Depth. Polymers 2014, 6, 3019–3043. doi: 10.3390/polym6123019. [40] Aharoni, S. M. Increased Glass Transition Temperature in Motionally Constrained Semicrystal- line Polymers. Polym. Adv. Technol. 1998, 9, 169–201. doi: 10.1002/(SICI)1099-1581(199803) 9:3%3c169::AID-PAT740%3e3.0.CO;2-Z. [41] Adam, G.; Gibbs, J. H. On Temperature Dependence of Cooperative Relaxation Properties in Glass-Forming Liquids. J. Chem. Phys. 1965, 43, 139–146. doi: 10.1063/1.1696442. [42] Eyring, H. Viscosity, Plasticity, and Diffusion as Examples of Absolute Reaction Rates. J. Chem. Phys. 1936, 4, 283–291. doi: 10.1063/1.1749836. POLYMER REVIEWS 241

[43] Yeh, J.-T.; Chen, H.-Y.; Tsai, F.-C. Blending and White Spirit Permeation Properties of the Blends of Modified Polyamide and ethylene vinyl alcohol with Varying vinyl alcohol contents. J. Appl. Polym. Sci. 2006, 102, 1224–1233. doi: 10.1002/app.24425. [44] Muramatsu, M.; Okura, M.; Kuboyama, K.; Ougizawa, T.; Yamamoto, T.; Nishihara, Y.; Saito, Y.; Ito, K.; Hirata, K.; Kobayashi, Y. Oxygen Permeability and Free Volume Hole Size in Ethyl- ene-Vinyl Alcohol Copolymer Film: Temperature and Humidity Dependence. Radiat. Phys. Chem. 2003, 68, 561–564. doi: 10.1016/S0969-806X(03)00231-7. [45] Botos, J.; Muller, K.; Heidemeyer, P.; Kretschmer, K.; Bastian, M.; Hochrein, T. Rapid Perme- ation Measurement System for the Production Control of Monolayer and Multilayer Films. Proceedings of Pps-29: The 29th International Conference of the Polymer – Conference Papers 2014, 1593,64–67. [46] Aucejo, S.; Marco, C.; Gavara, R. Water Effect on the Morphology of EVOH copolymers. J. Appl. Polym. Sci. 1999, 74, 1201–1206. doi: 10.1002/(SICI)1097-4628(19991031)74:5%3c1201:: AID-APP17%3e3.0.CO;2-8. [47] Lagaron, J. M.; Gimenez, E.; Gavara, R.; Saura, J. J. Study of the Influence of Water Sorption in Pure Components and Binary Blends of High Barrier Ethylene–Vinyl Alcohol Copolymer and Amorphous Polyamide and Nylon-Containing Ionomer. Polymer 2001, 42, 9531–9540. doi: 10.1016/S0032-3861(01)00496-7. [48] Yamamoto, T.; Kanda, T.; Nishihara, Y.; Ooshima, T.; Saito, Y. Correlation Study among Oxy- gen Permeability, Molecular Mobility, and Amorphous Structure Change of Poly(ethylene- vinylalcohol copolymers) by Moisture. Journal of Polymer Science Part B-Polymer Physics 2009, 47, 1181–1191. doi: 10.1002/polb.21723. [49] Cava, D.; Sammon, C.; Lagaron, J. M. Sorption-Induced Release of Antimicrobial Isopropanol in EVOH Copolymers as Determined by ATR-FTIR Spectroscopy. J. Appl. Polym. Sci. 2007, 103, 3431–3437. doi: 10.1002/app.25532. [50] Minelli, M.; Sarti, G. C. Permeability and Diffusivity of CO2 in Glassy Polymers with and With- out Plasticization. J. Membr. Sci. 2013, 435, 176–185. doi: 10.1016/j.memsci.2013.02.013. [51] ISO “ISO 15105-1”, In Plastics – Film and sheeting – Determination of gas-transmission rate – Part 1: Differential-pressure methods, 2007. [52] ISO “ISO 15105-2”, In Plastics – Film and sheeting – Determination of gas-transmission rate – Part 2: Equal-pressure method, 2003. [53] Lee, S. Y.; Kim, S. C. Morphology and Oxygen Barrier Properties of LDPE/EVOH Blends. Int. Polym. Process. 1996, 11, 238–247. doi: 10.3139/217.960238. [54] Lange, J.; Wyser, Y. Recent Innovations in Barrier Technologies for Plastic Packaging—A Review. Packag. Technol. Sci. 2003, 16, 149–158. doi: 10.1002/pts.621. [55] Gaucher-Miri, V.; Jones, G. K.; Kaas, R.; Hiltner, A.; Baer, E. Plastic Deformation of EVA, EVOH and Their Multilayers. J. Mater. Sci. 2002, 37,2635–2644. doi: 10.1023/A:1015872129472. [56] Ebnesajjad, S. Plastic Films in Food Packaging: Materials, Technology, and Applications; William Andrew: Waltham, MA, USA, 2013. [57] Kuraray America Inc. Gas Barrier Properties of EVAL resins – Technical Bulletin no. 110. Supplier technical report.:http://evalevoh.com/media/36916/tb_no_110.pdf (last accessed 1 August 2016) [58] Jakobsen, M.; Risbo, J. A Simple Model for the Interaction Between Water Vapor and Oxygen Transmission in Multilayer Barrier Materials Intended for Food Packaging Applications. Packag. Technol. Sci. 2008, 21, 207–216. doi: 10.1002/pts.799. [59] Lee, P. C.; Dooley, J.; Robacki, J.; Jenkins, S.; Wrisley, R. Flex Barrier Property Enhancement in Film Structures Using Microlayer Coextrusion Technology. ANTEC: Duke Energy Convention Center; Cincinnati, Ohio, 2013. [60] Zhang, G. J.; Xu, H.; MacInnis, K.; Baer, E. The Structure-Property Relationships of LLDPE- EVOH Blend Films Fabricated by Multiplication Extrusion. Polymer 2015, 57, 117–124. doi: 10.1016/j.polymer.2014.12.025. [61] Alipour, N.; Gedde, U. W.; Hedenqvist, M. S.; Yu, S.; Roth, S.; Bruning,€ K.; Vieyres, A.; Schneider, K. Structure and properties of polyethylene-based and EVOH-based multilayered films with layer thicknesses of 150 nm and greater. Eur. Polym. J. 2015, 64,36–51. doi: 10.1016/ j.eurpolymj.2014.12.011. 242 C. MAES ET AL.

[62] Su, H. J.; Xue, J.; Cai, P. L.; Li, J.; Guo, S. Y. Structure and Oxygen-Barrier Properties of (linear low-density polyethylene/ethylene-vinyl alcohol copolymer)/linear low-density polyethylene Composite Films Prepared by Microlayer Coextrusion. J. Appl. Polym. Sci. 2015, 132, 7. doi: 10.1002/app.42211. [63] Ait-Kadi, A.; Bousmina, M.; Youseﬁ, A. A.; Mighri, F. High Performance Structured Polymer Barrier Films Obtained from Compatibilized polypropylene/ethylene vinyl alcohol Blends. Polym. Eng. Sci. 2007, 47, 1114–1121. doi: 10.1002/pen.20794. [64] Ge, C. F.; Popham, J. A Review and Evaluation of Prediction Models of Gas Permeation for a Blended Flexible Packaging Film. Packag. Technol. Sci. 2016, 29, 247–262. doi: 10.1002/ pts.2200. [65] Cerruti, P.; Laurienzo, P.; Malinconico, M.; Carfagna, C. Thermal Oxidative Stability and Effect of Water on Gas Transport and Mechanical Properties in PA6-EVOH ﬁlms. J. Polym. Sci. Part B-Polym. Phys. 2007, 45, 840–849. doi: 10.1002/polb.21095. [66] Lopez-Rubio, A.; Lagaron, J. M. Improving the Resistance to Humid Heat Sterilization of EVOH Copolymers Through Blending. J. Appl. Polym. Sci. 2008, 109, 174–181. doi: 10.1002/ app.27991. [67] Ares, A.; Silva, J.; Maia, J. M.; Barral, L.; Abad, M. J. Rheomechanical and Morphological Study of Compatibilized PP/EVOH blends. Rheol. Acta 2009, 48, 993–1004. doi: 10.1007/s00397-009- 0381-9. [68] Sanchez-Garcia, M. D.; Nordqvist, D.; Hedenqvist, M.; Lagaron, J. M. Incorporating Amylopectin in Poly(lactic acid) by Melt Blending Using Poly(ethylene-co-vinyl alcohol) as a Thermoplastic Carrier. II. Physical Properties. J. Appl. Polym. Sci. 2011, 119,3708–3716. doi: 10.1002/app.32891. [69] Seethamraju, S.; Ramamurthy, P. C.; Madras, G. Performance of an Ionomer Blend-nanocomposite as an Effective gas Barrier Material for Organic Devices. Rsc Adv. 2014, 4, 11176–11187. doi: 10.1039/c3ra47442a. [70] Puente, J. A. S.; Fatyeyeya, K.; Marais, S.; Dargent, E. Multifunctional Hydrolyzed EVA Mem- branes with Tunable Microstructure and Water Barrier Properties. J. Membr. Sci. 2015, 480, 93–103. doi: 10.1016/j.memsci.2015.01.008. [71] Zhu, J. M.; Shen, J. B.; Guo, S. Y. In Situ Biaxial Stretching for the Platelet Formation of an Eth- ylene-Vinyl Alcohol Copolymer Through Multistage Stretching Extrusion and Its Effect on the Gas-Barrier Properties of Polyethylene. J. Appl. Polym. Sci. 2014, 131, 7. doi: 10.1002/app.40221. [72] Lopez-de-Dicastillo, C.; Gallur, M.; Catala, R.; Gavara, R.; Hernandez-Munoz, P. Immobiliza- tion of Beta-Cyclodextrin in Ethylene-Vinyl Alcohol Copolymer for Active Food Packaging Applications. J. Membr. Sci. 2010, 353, 184–191. doi: 10.1016/j.memsci.2010.02.049. [73] de Dicastillo, C. L.; Nerin, C.; Alfaro, P.; Catala, R.; Gavara, R.; Hernandez-Munoz, P. Develop- ment of New Antioxidant Active Packaging Films Based on Ethylene Vinyl Alcohol Copolymer (EVOH) and Green Tea Extract. J. Agric. Food Chem. 2011, 59, 7832–7840. doi: 10.1021/ jf201246g. [74] Lagaron, J. M.; Nunez, E.; Onishi, H. Novel EVOH Compound with Enhanced Moisture Resis- tance for High Oxygen Barrier Packaging. 18th Iapri World Packaging Conference 2012, 225– 228. [75] Vannini, M.; Marchese, P.; Celli, A.; Lorenzetti, C. Strategy To Modify the Crystallization Behavior of EVOH32 through Interactions with Low-Molecular-Weight Molecules. Ind. Eng. Chem. Res. 2016, 55, 3517–3524. doi: 10.1021/acs.iecr.5b04191. [76] Duncan, T. V. Applications of Nanotechnology in Food Packaging and Food Safety: Barrier Materials, Antimicrobials And Sensors. J. Colloid Interface Sci. 2011, 363,1–24. doi: 10.1016/j. jcis.2011.07.017. [77] Nielsen, L. E. Models for the Permeability of Filled Polymer Systems. J. Macromol. Sci.—Chem. 1967, 1, 929–942. doi: 10.1080/10601326708053745. [78] Liu, Y. W.; Liu, Y. J.; Wei, S. S. Processing Technologies of EVOH/Nano-SiO2 High-Barrier Packaging Composites. Sci Res Publ, Inc-Srp: Irvin, 2010. [79] Nam, J.-D.; Kim, S. W. Phase Compatibility and Barrier Properties of Ethylene/Vinyl Alcohol Copolymer Based Hybrid Materials. J. Appl. Polym. Sci. 2010, 115, 1663–1670. doi: 10.1002/ app.31159. POLYMER REVIEWS 243

[80] Kim, H. M.; Lee, H. S. Water and Oxygen Permeation Through Transparent ethylene vinyl alcohol/(graphene oxide) Membranes. Carbon Lett. 2014, 15,50–56. doi: 10.5714/ CL.2014.15.1.050. [81] Kim, D.; Kwon, H.; Seo, J. EVOH Nanocomposite Films with Enhanced Barrier Properties under High Humidity Conditions. Polym. Compos. 2014, 35, 644–654. doi: 10.1002/pc.22707. [82] Cerisuelo, J. P.; Gavara, R.; Hernandez-Munoz, P. Natural Antimicrobial – Containing EVOH Coatings on PP and PET Films: Functional and Active Property Characterization. Packag. Technol. Sci. 2014, 27, 901–920. doi: 10.1002/pts.2078. [83] Sadeghi, K.; Shahedi, M. Physical, Mechanical, and Antimicrobial Properties of ethylene vinyl alcohol copolymer/chitosan/nano-ZnO (ECNZn) Nanocomposite Films Incorporating Glyc- erol Plasticizer. J. Food Meas. Charact. 2016, 10, 137–147. doi: 10.1007/s11694-015-9287-7. [84] Wang, H. L.; Zhang, H.; Niu, B. C.; Jiang, S. W.; Cheng, J. F.; Jiang, S. T. Structure and Proper- ties of the poly(vinyl alcohol-co-ethylene)/montmorillonite-phosphorylated Soybean Protein Isolate Barrier Film. Rsc Adv. 2016, 6, 29294–29302. doi: 10.1039/C6RA03158G. [85] ISO “ISO 14663-2”, In Plastics — Ethylene/vinyl alcohol (EVOH) copolymer moulding and extrusion materials – Part 2: Preparation of test specimens and determination of properties, 2006. [86] Armstrong, R. B. Effects of Polymer Structure on Gas Barrier of ethylene vinyl alcohol (EVOH) and Considerations for Package Development. PLACE; TAPPI, 2002. [87] Fraunhofer IVV Oxygen-scavengers combat retort-shock. [88] Mokwena, K. K.; Tang, J.; Dunne, C. P.; Yang, T. C. S.; Chow, E. Oxygen Transmission of Mul- tilayer EVOH Films after Microwave Sterilization. J. Food Eng. 2009, 92, 291–296. doi: 10.1016/ j.jfoodeng.2008.11.011. [89] Galotto, M. J.; Ulloa, P.; Escobar, R.; Guarda, A.; Gavara, R.; Miltz, J. Effect of High-Pressure Food Processing on the Mass Transfer Properties of Selected Packaging Materials. Packag. Technol. Sci. 2010, 23, 253–266. [90] Juliano, P.; Koutchma, T.; Sui, Q.; Barbosa-Canovas, G. V.; Sadler, G. Polymeric-Based Food Packaging for High-Pressure Processing. Food Eng. Rev. 2010, 2, 274–297. doi: 10.1007/s12393- 010-9026-0. [91] Dhawan, S.; Varney, C.; Barbosa-Canovas, G. V.; Tang, J.; Selim, F.; Sablani, S. S. Pressure- Assisted Thermal Sterilization Effects on Gas Barrier, Morphological, and Free Volume Proper- ties of Multilayer EVOH films. J. Food Eng. 2014, 128,40–45. doi: 10.1016/j. jfoodeng.2013.12.012. [92] Lopez-Rubio, A.; Lagaron, J. M.; Yamamoto, T.; Gavara, R. “Radiation-Induced Oxygen Scav- enging Activity in EVOH Copolymers. J. Appl. Polym. Sci. 2007, 105, 2676–2682. doi: 10.1002/ app.26502. [93] Rubino, M.; Netramai, S.; Auras, R.; Annous, B. A. Effect of Chlorine Dioxide Gas on Physical, Thermal, Mechanical, and Barrier Properties of Polymeric Packaging Materials. J. Appl. Polym. Sci. 2010, 115, 1742–1750. doi: 10.1002/app.31244. [94] Aucejo, S.; Catala, R.; Gavara, R. Interactions Between Water and EVOH Food packaging films. Food Sci. Technol. Int. 2000, 6, 159–164. doi: 10.1177/108201320000600211. [95] Cava, D.; Cabedo, L.; Gimenez, E.; Gavara, R.; Lagaron, J. M. The Effect Of Ethylene Content On The Interaction Between Ethylene–Vinyl Alcohol Copolymers And Water: (I) Application of FT-IR Spectroscopy to Determine Transport Properties and Interactions in Food Packaging Films. Polym. Test. 2006, 25, 254–261. doi: 10.1016/j.polymertesting.2005.09.018. [96] Seethamraju, S.; Ramamurthy, P. C.; Madras, G. “Flexible poly(vinyl alcohol-co-ethylene)/modified MMT Moisture Barrier Composite for Encapsulating Organic Devices. Rsc Adv. 2013, 3, 12831–12838. doi: 10.1039/c3ra41557k. [97] Hong, S. I.; Kim, K. B.; Lee, Y.; Cho, S. Y.; Ko, J. A.; Hong, S. K.; Park, H. J. Surface Modification of Ethylene-Vinyl Alcohol Copolymer Treated with Plasma Source Ion Implantation. J. Appl. Polym. Sci. 2009, 113, 2988–2996. doi: 10.1002/app.30383. [98] Tenn, N.; Follain, N.; Fatyeyeva, K.; Valleton, J.-M.; Poncin-Epaillard, F.; Delpouve, N.; Marais, S. Improvement of Water Barrier Properties of Poly(ethylene-co-vinyl alcohol) Films by 244 C. MAES ET AL.

Hydrophobic Plasma Surface Treatments. J. Phys. Chem. C 2012, 116, 12599–12612. doi: 10.1021/jp302223j. [99] Vandewijngaarden, J.; Murariu, M.; Dubois, P.; Carleer, R.; Yperman, J.; Adriaensens, P.; Schreurs, S.; Lepot, N.; Peeters, R.; Buntinx, M. Gas Permeability Properties of Poly(3-hydroxybutyrate-co- 3-hydroxyhexanoate). J. Polym. Environ. 2014, 22,501–507. doi: 10.1007/s10924-014-0688-1. [100] MOCON Inc. Permeation Measurement of Organic and Volatile Compounds. Minneapolis, MN 55428 USA, 2015. [101] Fukamachi, M.; Matsui, T.; Hwang, Y. H.; Shimoda, M.; Osajima, Y. Sorption Behavior of Fla- vor Compounds into Packaging Films from Ethanol Solution. J. Agric. Food Chem. 1996, 44, 2810–2813. doi: 10.1021/jf950707i. [102] Brody, A. L. Flavor Scalping: Quality Loss Due to Packaging. Food Technol. 2002, 56, 124–125. [103] Berlinet, C.; Brat, P.; Ducruet, V. Quality of Orange Juice in Barrier Packaging Material. Packag. Technol. Sci. 2008, 21, 279–286. doi: 10.1002/pts.802. [104] Zhou, Q. X.; Guthrie, B.; Cadwallader, K. R. Development of a System for Measurement of Cou- pling Dynamic Vapor Sorption with Purge-And-Trap/Fast Gas Chromatography. Packag. Technol. Sci. 2004, 17, 175–185. doi: 10.1002/pts.648. [105] Sajilata, M. G.; Savitha, K.; Singhal, R. S.; Kanetkar, V. R. Scalping of Flavors in Packaged Foods. Comprehensive Rev. Food Sci. Food Saf. 2007, 6,17–35. doi: 10.1111/j.1541-4337.2007.00014.x. [106] Leufven, A.; Hermansson, C. The Sorption of Aroma Components From Tomato Juice by Food-Contact Polymers. J. Sci. Food Agric. 1994, 64, 101–105. doi: 10.1002/jsfa.2740640115. [107] Armstrong, R. B. Improving Performance of Geosynthetics for Containment of Volatile Organic Compounds through the Use of Ethylene Vinyl Alcohol (EVOH). Geo-Frontiers 2011: Advances in Geotechnical Engineering, Dallas, Texas, 2011. ISBN: 978-1-61782-594-1. [108] Zhao, J.; Kanaan, C.; Clement, R.; Brule, B.; Lenda, H.; Jonquieres, A. Permeability of EVOH Barrier Material Used in Automotive Applications: Metrology Development for Model Fuel Mixtures. Oil Gas Sci. Technol.-Revue D Ifp Energies Nouvelles 2015, 70, 353–366. doi: 10.2516/ ogst/2013167. [109] Gagnard, C.; Germain, Y.; Keraudren, P.; Barriere, B. Permeability of Semicrystalline Polymers to Toluene/Methanol Mixture. J. Appl. Polym. Sci. 2004, 92, 676–682. doi: 10.1002/app.20184. [110] Clement, R.; Kanaan, C.; Brule, B.; Lenda, H.; Lochon, P.; Jonquieres, A. An Original Auto- mated Desorption Apparatus for Measuring Multi-Component Sorption Properties of Barrier Polymer Films. J. Membr. Sci. 2007, 302,95–101. doi: 10.1016/j.memsci.2007.06.031. [111] Cava, D.; Lagaron, J. M.; Martinez-Gimenez, F.; Gavara, R. Inverse Gas Chromatography Study on the Effect of Humidity on the Mass Transport of Alcohols in an Ethylene-Vinyl Alcohol Copolymer Near the Glass Transition Temperature. J. Chromatogr. A 2007, 1175, 267–274. doi: 10.1016/j.chroma.2007.10.050. [112] Armstrong, R. Performance of Model Geosynthetics for Containment of Volatile Organic Com- pounds in Landﬁll Applications. Global Waste Management Symposium: Arizona Grand Resort – Phoenix, AZ, USA, 2012. [113] McWatters, R. S.; Rowe, R. K.; Wilkins, D.; Spedding, T.; Jones, D.; Wise, L.; Mets, J.; Terry, D.; Hince, G.; Gates, W. P.; Di Battista, V.; Shoaib, M.; Bouazza, A.; Snape, I. Geosynthetics in Ant- arctica: Performance of a Composite Barrier System to Contain Hydrocarbon-Contaminated Soil After Three Years in The Field. Geotext. Geomembr. 2016, 44, 673–685. doi: 10.1016/j. geotexmem.2016.06.001. [114] McWatters, R. S.; Rowe, R. K. Permeation of Volatile Organic Compounds through EVOH Thin Film Membranes and Coextruded LLDPE/EVOH/LLDPE Geomembranes. J. Geotech. Geoenviron. Eng. 2015, 141, 14. doi: 10.1061/(ASCE)GT.1943-5606.0001209. [115] Lee, H.-K.; Yang, W. S.; Choi, N.-J.; Moon, S. E. Encapsulation of Semiconductor Gas Sensors with Gas Barrier Films for USN Application. Etri J. 2012, 34,713–718. doi: 10.4218/etrij.12.0112.0266. [116] Bethke, J.; Goedecke, T.; Jahnke, W. Permeation Through Plastic Dangerous Goods Packaging During Transport in Freight Containers – Detection of Potentially Explosive Mixtures in Con- tainers Under Normal Conditions of Carriage. Packag. Technol. Sci. 2013, 26,1–15. doi: 10.1002/pts.994. POLYMER REVIEWS 245

[117] Fouillet, T.; Rebizak, R.; Descamps, S.; Ciesla, Y.; Guery, B. Development of a New Gastight Film Containing EVOH. Viii International Symposium on Chemical and Non-Chemical Soil and Substrate Disinfestation 2014, 1044, 301–305. [118] Yeh, J.-T.; Chen, H.-Y.; Tsai, F.-C. Gasoline Permeation Resistance of Polypropylene, Polypro- pylene/Ethylene Vinyl Alcohol, Polypropylene/Modified Polyamide, and Polypropylene/Blends of Modified Polyamide and Ethylene Vinyl Alcohol Containers. J. Polym. Res. 2006, 13, 451– 460. doi: 10.1007/s10965-006-9066-6. [119] Carrizo, D.; Maccagnan, A.; Felix, J. S.; Nerin, C.; Bosetti, O. The Barrier Effect of EVOH versus 1,4,7-Triaxocyclotridecane-8,13-Dione, a Non-intentionally Added Compound from Polyure- thane Adhesives in Multilayer Food Packaging. Packag. Technol. Sci. 2015, 28, 1039–1046. doi: 10.1002/pts.2182. [120] EFSA “EFSA Panel on Contaminants in the Food Chain (CONTAM). Scientific Opinion on Mineral Oil Hydrocarbons in Food. EFSA Journal 2012, 10, 185. [121] Lorenzini, R.; Fiselier, K.; Biedermann, M.; Barbanera, M.; Braschi, I.; Grob, K. Saturated and Aromatic Mineral Oil Hydrocarbons from Paperboard Food Packaging: Estimation of Long- Term Migration From Contents in the Paperboard and Data on Boxes from the Market. Food Additives Contaminants: Part A 2010, 27, 1765–1774. doi: 10.1080/19440049.2010.517568. [122] Vollmer, A.; Biedermann, M.; Grundbock,€ F.; Ingenhoff, J.-E.; Biedermann-Brem, S.; Altkofer, W.; Grob, K. Migration of Mineral Oil From Printed Paperboard Into Dry Foods: Survey of the German Market. Eur. Food Res. Technol. 2010, 232, 175–182. doi: 10.1007/s00217-010-1376-6. [123] Bundesministerium fur€ Ern€ahrung und Landwirtshaft: Entwurf – Stand: 24.07.2014 – Zweiundzwanzigste Verordnung zur Anderung€ der Bedarfsgegenst€andeverordnung. 2014. [124] Bundesministerium fur€ Ern€ahrung und Landwirtshaft: Entwurf – Stand: 24.02.2017 – Zweiundzwanzigste Verordnung zur Anderung€ der Bedarfsgegenst€andeverordnung. 2017. [125] Diehl, H.; Welle, F. How to Determine Functional Barrier Performance Towards Mineral Oil Contaminants from Recycled Cardboard. Food Packag. Shelf Life 2015, 5,41–49. doi: 10.1016/j. fpsl.2015.05.003. [126] Pastorelli, S.; Sanches-Silva, A.; Manuel Cruz, J.; Simoneau, C.; Paseiro Losada, P. Study of the Migration of Benzophenone from Printed Paperboard Packages to Cakes Through Different Plastic Films. Eur. Food Res. Technol. 2008, 227, 1585–1590. doi: 10.1007/s00217-008-0882-2. [127] Fiselier, K.; Grob, K. Barriers Against the Migration of Mineral oil from Paperboard Food Pack- aging: Experimental Determination of Breakthrough Periods. Packag. Technol. Sci. 2012, 25, 285–301. doi:10.1002/pts.982. [128] Maes, C.; Teniers, C.; Luyten, W.; Herremans, G.; Peeters, R.; Carleer, R.; Buntinx, M. Card- board Packaging with Ethylene Vinyl Alcohol Copolymer (EVOH) as a Functional Barrier Against Mineral Oil Migration. In 28th IAPRI Symposium on Packaging 2017: Unlocking the full potential of packaging across the value-chain: The Olympic Museum in Lausanne, Switzer- land, 2017. [129] BiedermannBrem, S.; Grob, K. Barriers Against the Migration from Recycled Paperboard Into Food: Measuring Efficiency by Surrogate Components. Packag. Technol. Sci. 2014, 27, 713–726. doi: 10.1002/pts.2062. [130] Richter, L.; Biedermann-Brem, S.; Simat, T. J.; Grob, K. Internal Bags with Barrier Layers for Foods Packed in Recycled Paperboard: Recent Progress. Eur. Food Res. Technol. 2014, 239, 215–225. doi: 10.1007/s00217-014-2208-x. [131] Ewender, J.; Franz, R.; Welle, F. Permeation of Mineral Oil Components from Cardboard Pack- aging Materials Through Polymer Films. Packag. Technol. Sci. 2013, 26, 423–434. doi: 10.1002/ pts.1990. [132] Ewender, J.; Langowski, H.-C.; Welle, F. Functional Barriers Towards Mineral Oil Contami- nants. Verpackungs-Rundschau 2015, 50–51. [133] Ewender, J.; Welle, F. Determination and Prediction of the Lag Times of Hydrocarbons through a Polyethylene Terephthalate Film. Packag. Technol. Sci. 2014, 27, 963–974. doi: 10.1002/ pts.2085. [134] Felix, J.; Isella, F.; Bosetti, O.; Nerın, C. Analytical Tools for Identification of Non-Intentionally Added Substances (NIAS) Coming from Polyurethane Adhesives in Multilayer Packaging 246 C. MAES ET AL.

Materials and their Migration into Food Simulants. Anal. Bioanal. Chem. 2012, 403, 2869–2882. doi: 10.1007/s00216-012-5965-z. [135] Nulman, M.; Olejnik, A.; Samus, M.; Fead, E.; Rossi, G. State of Alternative Fuel Technologies. In Paper 981360; Society of Automotive Engineers, Inc Warrendale, PA, 1998. [136] The Nippon Synthetic Chemical Industry Co. Ltd. High gas-barrier resin SoarnolTM:www.soar nol.com/ (last accessed 27 July 2016) [137] Maxwell, A. S.; Roberts, S. J. Review of Data on Gas Migration through Polymer Encapsulants: Report to NDA – Radioactive Waste Management Directorate; Serco, 2010. [138] Jiranek, M.; Rovenska, K.; Fronka, A. Radon Diffusion Coefﬁcient – A Material Property Deter- mining the Applicability of Waterproof Membranes as Radon Barriers. In American Associa- tion of Radon Scientists and Technologists 2008 International Symposium: Las Vegas NV, September 14–17, 2008.