Multifunctional and nanoreinforced polymers for food packaging

1

Multifunctional and nanoreinforced polymers for food packaging  N , Novel Materials and Nanotechnology Group, J.-M. LAGARO IATA-CSIC, Spain

Abstract: The packaging industry has been implementing at a rapidly expanding rate the number of packaging elements made of plastics over recent decades. Plastics, in contrast to more traditional packaging materials like glass and metals, (1) are permeable to the exchange of low molecular weight compounds such as gases and vapours, (2) undergo sorption, so-called scalping, of packaged food constituents, and (3) are amenable to migration into foodstuffs of packaging constituents. Despite these drawbacks, the availability of shapes and forms in which plastics can be conformed, their ease of processing and handling, their low price, their excellent chemical resistance, etc., have made them very attractive in packaging applications. Consequently, a lot of industrial and academic research has been devoted to understanding the mechanisms of mass transport in polymers in order to design new materials and composites with balanced physical properties in general and with improved barrier properties in particular, and to add additional functionalities which may take advantage of their permeability characteristics to positively actuate on the product. This chapter first highlights the factors that make polymers become more impermeable, putting special emphasis on nanotechnology approaches, and then reviews some of the general advances made in the field. Key words: nanotechnology, high barrier polymers/plastics, biopolymers/ bioplastics, packaging, food technology, transport properties.

1.1

Introduction

1.1.1

High barrier concept

High barrier is without doubt a highly desirable property of polymeric materials intended to be used in many packaging applications. The term high barrier usually refers to the low to very low permeability of a material to the transport of low molecular weight chemical species, like gases and vapours. Usually, the lower-limit definition for high barrier typically refers to the performance of PET polymers. However, this property has perhaps never attracted so much attention from industry as over the last decades, when it began to be pursued by some modern food and beverage packaging technologies making use of plastic materials.1±3 In this respect, high barrier has attracted a great deal of recent

ß Woodhead Publishing Limited, 2011

2

Multifunctional and nanoreinforced polymers for food packaging

attention from industry as it has become associated with primary objectives such as commercialization of perishable foods far away from their origin, food shelflife extension and maintaining food quality and safety. Furthermore, it has also become very relevant to a number of other applications including gas separation membranes, packaging of healthcare products, pharmaceuticals and chemicals, and housing of fuels and oxygenated fuels in fuel tanks and lines in the automotive field. The reason for the more recent interest in the development of high barrier polymers and polymer-based structures rests on a widespread trend to implement polymeric materials in an ever-increasing number of applications, in many cases aiming to substitute them for other, more traditional packaging materials. It is common knowledge that the attractiveness of plastics lies in their versatility and ability to offer a broad variety of properties and yet be cheap and easily processed and conformed into a myriad of shapes and sizes. However, polymers do have a number of limitations for certain applications when compared with more traditional materials like metals and alloys or ceramics. Among some of these limitations relevant to the purpose of this chapter are their permeability and comparatively low thermal resistance, and the strong interdependence between these two properties. The permeability of plastics to the exchange of gases and vapours imposes a number of challenges in those applications where high barrier, ideally impermeability, is required. These applications were, for instance, traditionally assumed by tinplate and glass in the food packaging field. However, polymer scientists, engineers and technologists in industry and academia have pulled together a great deal of effort and resources to push the limits of plastics performance towards impermeability, chiefly due to the overwhelming pressure exerted by the numerous other advantages associated with the use of plastics in high barrier applications. Table 1.1 gives typical oxygen permeability and water permeability values for a number of commercial polymers and structures used in food packaging applications.4

1.1.2

Functional packaging

The concept of functional or active/bioactive/intelligent packaging for food applications has been recently exploited, obtaining for the package an active role in the preservation, health-promoting capacity and provision of information concerning the products. Among these, active packaging is perhaps the area that has steered more research and industrial interest. Packages may be termed active when they perform some desired role in food preservation other than providing an inert barrier to external conditions. The opportunity of modifying the inner atmosphere of the package or even the product by simply incorporating certain substances in the package wall has made this group of technologies very attractive, representing an increasingly productive research area. Even though

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

3

Table 1.1 Water permeability (at 38ëC and 90% RH) and oxygen permeability (at 23ëC) of a number of commercial plastics and multilayer structures Material

PVOH EVOH PAN PAN (70% AN) PVDC PA6 aPA (amorphous) PET PP PC LDPE LCP PET/PVDC PA/PVDC PP/PVDC PET-met. PET/AlOx/PE PET/SiOx/PE PA/SiOx/PE PP/SiOx/PE PLA PLA PHB PHB PHBV PCL PCL PCL

Water permeability 1018 kg m/(m2 s Pa)

Oxygen permeability 1021 m3 m/(m2 s Pa) 0% RH

75% RH

485 000 17 000 2420 8250 30.53 20 600 2420 2300 726 19 400 1200 10 170 160 43 58 21 16 32 13 12 600

0.17 0.77 1.9 10.5 4.5 52 83 135 6750 10 500 21 500 0.42 17.5 18.2 25 3.5 7 4.9 7.7 81 2250

900 91

1689

230

6900 26 600

1590 4380 934 1960

31 225 60

15

2209 1750 5100 3010 7850

Reference of data source

4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 5 6 7 8 7 7 9 10

the first active packaging developments and most of the commercialized technologies consist of sachet technologies, which make use of a small permeable pouch (sachet) containing the active compound that is inserted inside the package, current trends tend towards the incorporation of active ingredients directly into the packaging wall. This strategy is associated with a number of advantages, such as reduction in package size, higher effectiveness of the active principles (which are now completely surrounding the product), and, in many cases, higher throughput in packaging production, since the additional step of incorporating the sachet is eliminated.7 Polymers, and in particular biomassderived polymers, are the preferred materials for active packaging because of their intrinsic properties, constituting an ideal carrier for active principles, with the advantage of being tuneable in terms of controlled release and the possibility

ß Woodhead Publishing Limited, 2011

4

Multifunctional and nanoreinforced polymers for food packaging

of combining several polymers through blending or multilayer extrusion to tailor the application. Active packaging has been used with many products and is under investigation for numerous others. These new food packaging technologies have been developed as a response to trends in consumer preferences towards mildly preserved, fresh, tasty, healthier, and convenient food products with prolonged shelf-life. These novel packaging technologies can also be used to compensate for shortcomings in the packaging design, for instance in order to control the oxygen, water or carbon dioxide levels in the package headspace. In addition, changes in retail practices, such as globalization of markets resulting in longer distribution distances, present major challenges to the food packaging industry, which finally act as driving forces for the development of new and improved packaging concepts that extend shelf-life while maintaining the safety, quality and health aspects of the packaged foods. The combinations of polymers and active substances that can be studied for potential use as active packages are in principle unlimited and it is forecast that the number of applications will increase in the near future. Among the existing active packaging technologies, oxygen scavengers and antimicrobial packaging stand out over the other developments. Both technologies were initially based on the sachet concept, using reducing and inhibitory substances, respectively. Lately, the growth in both areas has been enormous, especially in the case of antimicrobials. Other active packaging applications include systems capable of absorbing carbon dioxide, phase-changing materials, moisture, ethylene and/or flavour/odour taints; releasing carbon dioxide and/or flavour/odour. Traditionally, plastic food packaging has been related to negative food safety issues, due mainly to problems with migration of packaging components. In more recent trends, packaging is being designed more favourably to impact on consumer health by integrating functional ingredients in the packaging structure, through so-called bioactive packaging strategies.8 Novel active and bioactive packaging technologies, combined with bioplastics and nanotechnology, can best help do this. Therefore, proper combination of these technological cornerstones will provide innovation in the food packaging sector over the next few years. Furthermore, due to the shortage of oil resources and waste-management issues, research focus is shifting from synthetic oil-based plastics to biomassderived biodegradable and environmentally friendly polymers. The drawbacks that initially characterized these biopolymers in terms of poor barrier properties and high instability have, in turn, resulted in novel applications, making highly permeable and water-plasticizable biopolymers an ideal partner for active and bioactive packaging where the package is no longer a passive barrier, but actively contributes to the preservation of food by controlled release of the substances. Biopolymers are, thus, the ideal matrix for the incorporation and controlled release of a number of substances to be added to the food. Probably

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

5

the area that is evolving more quickly is the antimicrobial packaging one, but it is foreseen that biopackages will also serve as reservoirs for vitamins, antioxidants, and pre- and probiotics.

1.1.3

Phenomenology of transport in polymers

According to the above, barrier properties in polymers are necessarily associated with their inherent ability to permit the exchange, to a higher or lower extent, of low molecular weight substances through mass-transport processes like permeation. The phenomenology of permeation of low molecular weight chemical species through a polymeric matrix is generally envisaged down to the molecular level as a combination of two processes, i.e. solution of the solutes and molecular diffusion.11 A permeating gas is first dissolved into the upstream face of the polymer film, and then undergoes a molecular diffusion to the downstream face of the film through typically the polymer amorphous phase, where it evaporates into the external phase again. A solution±diffusion mechanism is thus applied, which can be formally expressed in terms of permeability (P), solubility (S) and diffusion (D) coefficients as follows: P ˆ DS

1:1

This permeability coefficient derives from application of Henry's law of solubility to Fick's first law of diffusion as follows: Jˆ

q @c S
p
p ql ˆ ÿD ˆD ˆ DS ) P ˆ DS ˆ At @x l l At
p

1:2

The solubility coefficient S is thermodynamic in nature and is defined as the ratio of the equilibrium concentration of the dissolved penetrant in the polymer to its partial pressure (p) in the gas phase (Henry's law). In polymers, Henry's law is usually obeyed at low penetrant concentrations, i.e. when S is independent of concentration (or of the partial pressure). D characterizes the average ability of the sorbed permeate to move through the polymer chain segments and is typically governed by Fick's first law of diffusion, i.e. the flux of the permeant (J) is proportional to the local gradient of concentration (c) through the thickness of the polymer film (l). During sorption kinetic experiments, if Fickian transport (case I) is assumed, linear behaviour in the penetrant uptake vs. the t1/2 (t being time) curve at small times is usually observed.12 Case II diffusion is defined when linear behaviour is observed in the uptake vs. t curve. This behaviour is observed in a number of systems and is associated with large uptakes and plasticization of the structure by the penetrant. When complex sorption behaviours like sigmoidal shapes are observed it is usually assumed that an `anomalous' or non-Fickian transport occurs. Nevertheless, from recent works a better rationalization of these `anomalous' behaviours has been achieved, in which contributions from the effect of macroscopic elastic constraints arising

ß Woodhead Publishing Limited, 2011

6

Multifunctional and nanoreinforced polymers for food packaging

during the swelling process (geometrical effects) in adsorption experiments have been pointed out.13,14 Concerning the mechanisms of the mass-transport process through polymeric materials, two general approaches can be found, namely (1) molecular models studying the specific penetrant and chain motions in conjunction with the corresponding intermolecular forces, and (2) `free-volume' models which pay attention to the relations between the transport coefficients and the free volume existing in the polymeric matrix, without considering molecular-scale mechanisms. It is also relevant to emphasize here that the mass transport mechanisms, as well as their dependence on permeant partial pressure and testing temperature, are thought to be generally different depending on whether the polymer is in a rubbery or glassy state. Rubbery polymers are above their glass transition temperature (Tg) and, therefore, have very short relaxation times and respond quickly to physical changes. Thus, absorption of small molecules or penetrants causes immediate adjustments to a new equilibrium state and, consequently, there appears to exist a unique mode of penetrant transport for these polymers. Moreover, rubbery polymers are more amenable to show upwardly inflecting permeability responses with increasing penetrant partial pressure due to plasticization. This is typically the case in D-limonene, a common flavour component in fruit juices, in polymers like polyethylene and polypropylene. By comparison, glassy polymers are below their Tg and hence require on average long timescales to fully relax. Gas transport then typically occurs in glassy polymers under nonthermodynamic equilibrium conditions. In this case, penetrant molecules can allocate in holes or irregular cavities with very different diffusional mobility and, consequently, more than one mode of transport may be accessible. A `dual-mode sorption' model satisfactorily describes the dependence of transport properties on penetrant partial pressure in glassy polymers. This model postulates the existence of two different molecular populations dissolved in a glass: one dissolved by an ordinary dissolution process which can typically follow Henry's law (c ˆ Sp), and the other dissolved in a limited amount of fixed microcavities which can be described by a Langmuir-like isotherm: cˆ

cH bp 1 ‡ bp

1:3

In equation 1.3, cH is the hole saturation constant and b is the hole affinity constant. More complex sorption behaviours have also been postulated for other glassy materials. For instance, a modified dual-mode model requiring Langmuir and Flory±Huggings equations was suggested to explain the sorption of water in an amorphous polyamide.15 In what follows, we first overview some relevant structural factors defining and/or altering high gas barrier properties in polymers, and then comment on recent material developments in the field, i.e. blends, coatings and nanocomposites.

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

1.2

7

Structural factors governing barrier properties

The structural factors determining inherent high barrier properties in polymers are fundamentally the chemistry, but there are also other relevant factors making a significant impact on barrier properties for a given chemistry, including polymer morphology (crystallinity, thermal history, amorphous density, molecular orientation, etc.), polymer molecular architecture (branches, molecular weight and tacticity), polymer plasticization, temperature, penetrant type and chemistry, and others.

1.2.1

Polymer chemistry

Nowadays, very many chemical combinations and high throughput and selective catalyst technologies are accessible via cutting-edge polymer chemistry, to generate polymeric materials with tailor-made structures and properties. As would be reasonable to expect, then chemistry is the basic and main defining factor determining barrier properties in polymeric materials. Thus, by varying the chemistry of the macromolecule, often by just adjusting the pendant group along the polymer chain, a significantly large variation in barrier properties can be achieved (see Table 1.2). Some commonly employed abbreviations applied to both well-known and new commercial plastics are listed in the Appendix. Behind the significant changes in barrier properties resulting from variations of chemistry are, for instance, the introduction of apolar voluminous groups at the low barrier side of the permeability spectrum, or the incorporation of small and strongly self-interacting chemical groups at the high barrier side of the permeability spectrum. The permeability of a polymer can change by up to six orders of magnitude depending on the grafted chemical groups attached to the polymer backbone. As is well known, most polymeric materials comprise exceedingly long high molecular weight molecules (called polymer chains) which for the case of the most widely used plastics, the thermoplastics family, do not have intermolecular links in the amorphous state other than secondary forces of, for instance, the van der Waals type. Consequently, the presence of Table 1.2 Relative oxygen permeability of polymer materials based on the repetition of CH2±CHX Polymer PVOH PAN PVC PP PS PE

Pending X unit

Relative O2 permeability

±OH ±CN ±Cl ±CH3 ±C6H5 ±H

1 4 800 15 000 42 000 48 000

ß Woodhead Publishing Limited, 2011

8

Multifunctional and nanoreinforced polymers for food packaging

these different pendant groups can either disrupt or enhance the high intermolecular cohesion necessary to maintain high barrier efficiency against the transport of low molecular weight substances. Moreover, chemistry also defines the affinity between a potential permeant and the polymer matrix. As the process of permeation is a bimodal process comprising solution and diffusion, low solubility based on chemical disparity of a permeant and the polymer matrix will also result in low permeability, irrespective of whether the kinetics of diffusion are going to be favourable to the permeant transport. In this chapter, we will rather concentrate, due to their relevance and ease of generalization, on the barrier properties of non-interacting chemicals as is usually the case of the permanent gases. A physical magnitude called the cohesive energy density can be useful in helping to explain, quantify or even predict the behaviour in terms of barrier properties of polymeric materials. The cohesive energy of a substance in a condensed state is defined as the increase in internal energy per mole of substance if all the intermolecular forces are eliminated. For low molecular weight substances this energy can be experimentally calculated from the heat of evaporation. However, for polymers the cohesive energy density (defined as the cohesive energy per unit of volume) can be estimated using additive group contribution models like those devised by, for instance, Van Kreveland for cohesive energy and Traube for molar volume.16 These models propose contribution values for each of the chemical entities building up the polymer chain. Consequently, this parameter tells us about the strength of the interaction between molecules, and how this interaction changes when different chemical groups are added to the polymer chain. The cohesive energy density is often referred to as the square of the solubility parameter. Another important factor strongly associated with barrier properties is the free volume. The free volume comprehends the microcavities present in a polymeric material. Permeants make use of these cavities ± whether permanent or transient ± to diffuse through the polymer matrix. The transport properties of a permeant are therefore dependent on the number and size of these microcavities. This concept is usually expressed through the so-called fractional free volume parameter (Vf) and is indeed strongly related to chemistry (cohesive energy density), but it is also related to a number of other relevant factors having an impact on barrier properties like thermal history, polymer Tg, crystallinity and/or conformational order, etc. The fractional free volume Vf can be easily determined by the following simple equation: V ÿ V0 1:4 V where V is the specific volume of a particular polymer sample determined by density, and V0 is the specific volume at zero solubility (volume exclusively occupied by polymer chains). The latter parameter can be experimentally Vf ˆ

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

9

determined by, for instance, extrapolation of experimental data17 or can be estimated from additive group contributions models. A very useful concept for free volume is that proposed by Cohen and Turnbull18,19 and Fujita20 through a general expression as follows: D / eÿBd =Vf

1:5

In this expression, D is the diffusion coefficient and Bd is a constant that depends only on the size of the penetrant molecule. This model has been shown to adequately describe the transport kinetics of organic vapours and small gas molecules in a number of polymers. More recent efforts have led to the development of an experimental methodology based on a technique called positronium annihilation spectroscopy. This methodology provides an experimental approach to determining free volume, as it enables one to measure hole size on a nanoscale and its fraction.21 Nevertheless, the absolute value of the fractional free volume cannot be directly obtained from only positron lifetime measurements. In spite of that, a study making use of positronium annihilation spectroscopy showed that there exists an excellent correlation between the oxygen permeability and a relative fractional free volume parameter as determined by this technique in a number of EVOH copolymers.22 From the experiments, it was clear that the fractional free volume in these materials does mainly concern the free volume size, as only the free volume size and not the orthopositronium o-Ps lifetime intensity, i.e. the number of holes, varied across composition in these polymers. It is, therefore, relevant to realize that high barrier polymers are the result of a permeable structure (amorphous phase) with a high cohesive energy density and very low fractional free volume. Figure 1.1 plots the oxygen permeability of a number of plastics, superimposed with the performance of bioplastics, vs. the ratio of the cohesive energy density to the fractional free volume. From this figure, it can be seen that EVOH copolymers (with 32 mol% ethylene) are one of the most efficient oxygen barrier materials due to their high intermolecular cohesion and low fractional free volume. Consequently, this material is being increasingly introduced in packaging applications where high barrier properties to gases are required. On the contrary, polymers like HDPE have much lower gas barrier properties due to low intermolecular cohesion and large fractional free volume. High intermolecular cohesion can, however, be distorted by for instance chemical alterations in the material (polymer degradation) due to thermal treatments.23 Polymer chain rigidity or polymer Tg also plays a relevant role in barrier properties since, as explained earlier, penetrant transport mechanisms are greatly altered depending on whether the permeation process occurs above (rubbery state) or below (glassy state) the polymer glass transition temperature. There is a very general trend that indicates that the higher the polymer Tg the lower the gas permeability and the better the permselectivity. However, this does not apply to common polymers like PS or PC which are very rigid glassy materials with

ß Woodhead Publishing Limited, 2011

10

Multifunctional and nanoreinforced polymers for food packaging

1.1 PO2 (cm3 mm/m2 day atm) vs. the fractional free volume/cohesive energy density ratio for a number of polymers typically used in food packaging applications. References to the typical oxygen barrier properties of biopolymers are also included.

values of Tg above 100ëC and very high permeability. This is of course a consequence of the voluminous side groups which indeed reduce chain segment mobility due to steric hindrance but in turn generate large fractional free volumes. On the other hand, polymers like EVOH copolymers, PK copolymers or PVDC have lower values of Tg than for instance PS, PC or other materials like PET and yet have outstanding barrier properties. This is again due to the very high cohesive energy density and low fractional free volume exhibited by the former materials.

1.2.2

Polymer morphology

An important issue that has been implicit in all the previous considerations is the well-known characteristic that polymers are not able to fully crystallize due to metastability, some being in fact totally amorphous. Many polymers used in packaging applications have, therefore, a semicrystalline nature and hence are, from a structural viewpoint, heterogeneous materials. These polymers contain, under the most simplistic two-phase model visualization, both a fraction of chain segments constituting highly packed and conformationally ordered threedimensional structures ± polymer crystalline fraction () ± and another fraction in an amorphous state without conformational regularity and lateral order. As a large body of experimental evidence suggests that polymer crystals are impermeable to the transport of most low molecular weight substances, it is broadly accepted that the amorphous phase is the only phase available for permeation of these substances.

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

11

It is therefore this particular structural feature, i.e. polymer crystallinity, together with a low intermolecular cohesion between polymer chains in the amorphous phase that best defines many of the most characteristic polymer properties, including permeability. However, polymer crystals not only fill the molecular structure of semicrystalline materials with microscopic impermeable blocks but also affect the surrounding amorphous phase. To begin with, the presence of crystallinity, its morphology (for instance, crystal width-to-thickness ratio) and orientation bring in additional considerations in terms of permeability as the penetrant molecules have to circumvent the crystallites, and thereby travel through a more tortuous diffusive path than in a fully amorphous material. This effect is usually accounted for in the calculations of the transport coefficients (see equation for diffusion below) by the so-called tortuosity or geometrical impedance factor ( ). Thus, the tortuosity factor is in essence the path length that a permeant has to travel across a film thickness divided by its actual thickness. Furthermore and as commented above, the presence of these crystalline blocks also affects the surrounding conformationally disordered amorphous phase. The constraining effects imposed by crystals to the chain segments in the amorphous phase typically depend on factors like crystal surface area and penetrant size. This phenomenon is substantiated from extensive mechanical and transport data, which clearly indicate that the segmental mobility of the non-crystalline fraction is much less than that in the fully amorphous polymer.24,25 This effect is accounted for in the calculations of the transport coefficients (see equation below) by the so-called chain immobilization factor (): Dsemicrystalline ˆ

Damorphous …1 ÿ † 

1:6

As a result of this, being aware of the implications of the crystallinity and its morphology on the barrier properties is, as a matter of fact, a relevant issue, because by adequate processing (thermal history) of polymers these parameters can be optimized to obtain specimens, based on the same chemistry, with enhanced permeability. Polymer molecular orientation due to drawing or processing generally leads to an increase in barrier properties. This is usually attributed to (1) orientationinduced crystallization, (2) fractionation and alignment (perpendicular to the permeant transport) of the crystals in the straining direction (increase in the tortuosity factor), and (3) densification (reduction in free volume) of the amorphous phase due to an increase in conformational order in the non-crystalline chain segments. The oxygen permeability, diffusivity and solubility parameters have been found to decrease with the amount of uniaxial orientation in PET due to conformational transformations of glycol linkages from gauche to trans. However, for a given uniaxial orientation in PET, biaxial drawing results in increased permeability, reducing the barrier performance. Orientation is then generally seen as the process of decreasing excess free volume bringing the nonequilibrium glassy polymer closer to the equilibrium condition.

ß Woodhead Publishing Limited, 2011

12

Multifunctional and nanoreinforced polymers for food packaging

A special case in barrier properties is that of liquid crystal polymers and PVDC. These materials can have gas barrier properties as good as those of high barrier EVOH copolymers. Liquid crystal polymers are often termed `mesomorphic' because they have structures between those of amorphous polymers with no regular order and those with a three-dimensional crystal lattice. The unique packing arrangement of these polymeric systems has raised some fundamental questions about the permeation mechanisms of low molecular weight molecules, i.e. whether they behave more like glasses or conventional crystals. PVDC also shows high barrier properties to gases and water vapour, attributed to high lateral molecular order and hence density. Although the barrier properties of PVDC are somewhat inferior to those of dry EVOH, the former has the advantage that unlike EVOH it is not plasticized by sorption of moisture in medium to high humidity ranges due to its high molecular lateral packing.

1.2.3

Polymer molecular architecture

Some relevant routes to modifying the molecular architecture of polymers, and hence their barrier properties, are copolymerization, i.e. introducing a few side groups or branches along the main chain, and modification of the molecular weight or the stereoisomerism. Linear polyethylene (HDPE) is more crystalline than both branched polyethylenes (e.g. LLDPEs and LDPE) and ultra-high molecular weight polyethylenes and is, therefore, found to be more dense, less permeable and stiffer, albeit less tough. Moreover, the homogeneous or heterogeneous character of the incorporation of the branches along the polymer backbone has a large impact on properties, including barrier properties.26,27 The more recently developed polyolefins obtained by single site catalyst technologies can lead to very low density materials with unprecedented very low barrier properties, which in thin film form can serve as excellent packaging materials for products that have breathing necessities like fruits and vegetables. A significant effect is also the stereoisomerism (tacticity). This is due to the different stereochemical arrangements that can be present along the polymer backbone and that cannot be changed by rotation along the C±C bond. A polymer for which the pendant groups contain the same configuration is said to be isotactic. Polymers for which alternate carbon atoms have the same configuration are called syndiotactic and when the configuration is at random are called atactic. The atactic configuration is in principle more permeable as it usually yields amorphous polymers (e.g. PS or PMMA).

1.2.4

Polymer plasticization

In this context, it is relevant to add here that polymer plasticization (Tg depletion) due to polymer/permeant interactions or due to polymer and surrounding media chemical interactions has very detrimental effects, which

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

13

usually lead to losses in intermolecular cohesion and decrease in overall barrier performance.28 Relative humidity has a tremendously detrimental impact on the outstanding gas barrier capacity of EVOH polymers, proteins and polysaccharides. This is also the case, albeit to a lesser extent, for other polar polymers like those in the polyamide family. Thus, it is often the case that polymers that are high barrier to gases have very low barrier performance to polar solvents like water, except PVDC. This behaviour is associated with the disruption by moisture of the existing polymer intermolecular self-association promoted by, for instance, hydrogen bonding in EVOH, PVOH and PA.29±31 As opposed to this behaviour, polymers like polyolefins, PE and PP have low barrier properties to gases due to weak self-association but are extremely good barrier materials to water due to their olefinic hydrophobic character. An exceptional case is that of the amorphous polyamide (aPA) and some polyimides, for which oxygen permeability decreases with increasing relative humidity.11 For this aPA, even though the presence of moisture greatly decreases the polymer Tg, the oxygen permeability does not decrease but surprisingly increases (see Table 1.1). Recent spectroscopic work suggests that moisture has a specific interaction with this particular polymer.32 The results indicate that moisture molecules do not disrupt the originally existing hydrogen bonding intermolecular interactions between amide groups, but rather link to the few remaining free amide groups, and most of the sorbed water molecules selfassociate forming clusters, which altogether act as a free volume blocking mechanism to the diffusion of oxygen molecules. This behaviour also occurs in EVOH copolymers but in the low humidity range. For these copolymers, dry EVOH at 0% RH is a lower barrier than EVOH at 30% RH, due to sorbed moisture at low water activities acting as adsorbed blocking elements to the solubility and diffusion of gas molecules.

1.2.5

Temperature

It is well known that temperature affects many of the properties of polymers. Temperature-induced changes in barrier properties are of an exponential nature. In the case of diffusion, the D value increases exponentially with temperature, in agreement with the Arrhenius law (equation 1.7), since activation energies (ED) are always positive. This phenomenon is related to the greater mobility of polymer chains at higher temperatures, which reduces the energy needed by the permeant molecules to jump to the next active site, and with an increase in the free volume of the polymer:33 D ˆ D0 eÿED =RT

1:7

In the case of the solubility coefficient, the exponential dependence on T is described by Van't Hoof's Law (equation 1.8). The enthalpy of solution (
H S)

ß Woodhead Publishing Limited, 2011

14

Multifunctional and nanoreinforced polymers for food packaging

values is usually positive, although negative values have also been reported.34 In this case, in spite of the larger number of molecules that can be accommodated in the active sites produced by the greater mobility of the polymer chains and the bigger free volume size, the volatility of the sorbates also affects their partition equilibrium between the polymer and the outer medium.35 S ˆ S0 eÿAHS =RT

1:8

Finally, as permeability combines sorption and diffusion, its changes with temperature depend on the values of ED and AHS as shown in equation 1.9. Since the values of ED are usually greater than the absolute value of AHS, the permeation equation is considered to be an Arrhenius-type expression, the temperature dependence being described through the activation energy of permeation (EP):    P ˆ D0 eÿED =RT S0 eÿAHS =RT ˆ D0 S0 e…ÿED ÿAHS †=RT ˆ P0 eÿEP =RT 1:9 The temperature also affects the state of the polymer, the transport properties of the polymer being affected by it. In the melted polymer, the crystalline regions disappear and transport takes place across the entire matrix, which behaves like a liquid. In this case, all the polymer volume is available for the permeant, which increases its solubility, and the blocking effect of the crystals disappears, which reduces tortuosity and makes diffusion easier. Also, the polymer chains are in constant movement, which facilitates the jumps of the permeant molecules. Changes associated with the glass transition, i.e. with the passage of the polymer from the glassy to the rubbery state, take place as a result of the relaxation or increased mobility of the chain segments in the amorphous phase of the polymer. Above the glass transition temperature (Tg) the amorphous phase of the polymer is in the rubbery state; below this temperature it is in the glassy state. In the rubbery state, relaxation times are shorter and, after the sorption of permeant molecules, a new equilibrium state is reached more quickly. As a result, diffusion is faster when the polymer is in the rubbery state.

1.2.6

The permeant

Characteristics of the permeant like molecular size, shape and chemical nature usually affect its transport properties. Increasing the molecular size in homologous series of permeants (alkanes, esters, aldehydes or alcohols) generally reduces the diffusion and solubility coefficient values of the permeants, mainly for steric reasons. Only when solutes are in the form of vapour do the higher solubilities correspond to the larger molecules, as a consequence of their greater condensabilities.36 The shape of the permeant molecules is also important, as flattened or elongated molecules will diffuse more quickly through the polymer

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

15

than spherical ones with the same molecular volume.37 The nature of the permeant also affects its transport properties, as described above in the effect of chemistry. If the affinity between the permeant and the polymer is very high it can sometimes cause plasticization of the polymer. In this case, sorption leads to a decrease in the self-association between adjacent macromolecules in the amorphous region. The initial hydrogen bonding and van der Waals forces are replaced by polymer±sorbate interactions, increasing chain mobility and free volume, reducing the Tg and raising the diffusion and solubility coefficients of the solute. Plasticization depends on the penetrant concentration, which has to be above a certain limit for it to take place. However, while outstanding affinity between the sorbate and the polymer and large uptakes are necessary, sometimes they are not sufficient to produce plasticization of the polymer, as described in the case of aPA. When a complex matrix like a foodstuff is placed inside a polymeric package, the polymer will be in contact with a large number of solvents simultaneously and the transport properties of one solute are often affected by the presence of the other co-solvents. Water is the main component of many foodstuffs and also the most frequently reported co-solvent. In hydrophilic polymers like the EVOH copolymers, waterinduced plasticization at high moisture levels has been reported to increase the permeability to hydrophobic and apolar solvents like limonene and oxygen.38 However, as described before in the case of the aPAs, the presence of water can also have a positive effect on the barrier properties of the material. Another co-solute whose effect has been widely described in the literature is limonene, the main component of orange juice flavour. The effect of this terpene on the barrier performance of apolar polyolefins is similar to that of water on polar EVOH copolymers. The presence of high concentrations of limonene has been reported to double the permeability of ethyl-butyrate through HDPE and to increase that of ethyl acetate through biaxially oriented polypropylene by up to 40 times.39 The simultaneous transport of a group of co-solvents with similar transport properties has usually been described as a competition between them for the active sites, resulting in the transport of certain compounds being reduced and that of the rest increased.40 However, positive synergistic effects have also been reported, as in the case of toluene/methanol mixtures.41

1.3

Novel polymers and blends

Novel developments in high barrier plastics mainly come from three sources, namely (1) new polymers including biopolymers, (2) polymer blends including nanocomposites, and (3) inorganic coatings such as aluminium obtained by vacuum deposition technologies and oxides (AlOx or SiOx). Polymeric materials

ß Woodhead Publishing Limited, 2011

16

Multifunctional and nanoreinforced polymers for food packaging

for high barrier applications are challenged today by a broad range of stringent property requirements including ease of processing, higher barrier properties to permanent gases, to moisture and to low molecular weight organic compounds, excellent chemical resistance, permselectivity, low relative humidity dependence for the barrier performance, and ease of recycling and biodegradability. Among the novel high barrier polymers that have been more recently developed are materials like the PK copolymers (aliphatic polyketones).42,43 These semicrystalline materials have an outstanding range of mechanical, thermal and high barrier properties (comparable to some EVOH copolymers, see Fig. 1.1), chemical resistance and reduced relative humidity dependence for barrier properties, which give them significant commercial potential in a broad range of engineering, barrier packaging, fibre and blend application. Another novel, extremely high barrier material that has been recently developed is polyglycolic acid (PGA). This biodegradable polymer is claimed to have very low O2 and CO2 permeabilities, one hundredth that of PET (see Fig. 1.2). Additionally, and as opposed to EVOH and PVOH, the barrier properties of commercial PGA resins are said to be largely insensitive to humidity conditions, making it ideally suited for a variety of beverage and perishable food packaging applications.44 Another family of resins that have been recently developed and are currently making their way into the market are the amorphous vinyl alcohol resins (AVOH).45 Water-soluble but melt-compoundable AVOH is said to have, in addition to excellent gas barrier properties and good chemical resistance compared to PVOH and EVOH, superior extrusion properties, orientability, shrinkability and transparency. This polymer can be used in all extrusion processes such as melt-spinning, oriented film, transparent container and injection, and because it is biodegradable, it lends itself to a variety of applications such as new packaging materials that reduce the burden on the environment.

1.2 OTR/WVTR of some polymers vs. the properties claimed for PGA.

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

17

Aromatic polyamides such as Ny-MXD6, i.e. polyamide resins produced from meta-xylenediamine and adipic acid, are currently being considered in packaging applications since they provide a transparent high gas barrier at high humidity properties (see Fig. 1.2 and Chapter 9) and can be functionalized to achieve oxygen scavenging properties. Another new range of promising materials that have already been developed and in some cases marketed with success in packaging applications are a number of resins derived from biomass and, therefore, to a higher or lower extent easily biodegradable or compostable.6,46 Among these materials, it is possible to find (1) polymers synthesized from bio-derived monomers such as polylactic acid resins (PLA); (2) polymers produced directly by microorganisms like PHAs, bacterial cellulose, etc.; and (3) polymers extracted directly from biomass such as polysaccharides (plant cellulose, starch, chitosan), proteins (soy protein, gluten, zein) and lipids. These biopolymers can have excellent barrier properties to gases such as for instance plasticized chitosan, although their barrier performance is dramatically reduced in the presence of moisture. However, other polymers like PLA and PHAs have relatively good water barrier properties and their relatively good oxygen barrier, lower than for PET, is largely insensitive to moisture sorption. So in principle, one could devise a bio-based derived high barrier multiplayer system where an inner layer of plasticized chitosan could be sandwiched between high moisture barrier PLA or PHA layers. An interesting property of some of these bio-based polymers, e.g. PLA and starch, is that their permeability to carbon dioxide compared to oxygen (permselectivity) is higher than that of most conventional mineral oil based plastics. This is, for instance, of interest for some food packaging applications where a high barrier to oxygen is required, but CO2 generated by the product should be allowed to exit the package headspace to avoid package swelling. These materials, however, still suffer from high production costs compared to polyolefins but are now competitive with, for instance, PET. An interesting development based on cellulose has been recently published.47 In this study, softwood and hardwood celluloses were oxidized by 2,2,6,6tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation. The TEMPO-oxidized cellulose fibres were converted to transparent dispersions in water, which consisted of cellulose nanofibres 3±4 nm in width. Films derived from this material were seen to consist of randomly assembled nanofibres, were transparent and flexible, and had extremely low coefficients of thermal expansion caused by the high crystallinity. Moreover, the oxygen permeability of a polylactic acid (PLA) film drastically decreased by a factor of about 750 by forming a thin layer of the cellulose material on the PLA film. Hydrophobization of the originally hydrophilic films was achieved by treatment with alkylketene dimer. Blending polymers is a feasible route to accessing the desired balance of properties by controlling the polymer phase interaction and/or the morphology

ß Woodhead Publishing Limited, 2011

18

Multifunctional and nanoreinforced polymers for food packaging

1.3 Modelling of oxygen permeability for various dispositions of EVOH/aPA blend components facing the transport of oxygen gas and as a function of the volume fraction of EVOH. Experimental data (see arrow) for 80/20 EVOH/PA and EVOH/ionomer melt-mixed blends recently developed in our labs are also provided.

in monolayer barrier systems.48 The most commonly used case is to blend polymers with other polymers that have higher barrier properties. The barrier properties of these blends seem to follow a relationship (see equation 1.10) in good general agreement with that proposed by Maxwell and extended by Roberson (see equation 1.1049) for spheres of a low oxygen barrier phase (aPA in Fig. 1.3), but with higher water resistance, dispersed in a high oxygen barrier (EVOH in Fig. 1.3) continuous matrix which has a lower water resistance.50 This simple model would appear to closely reflect, albeit with a slight positive deviation (due to orientation, see Fig. 1.3), the case of the dispersed morphology found for this EVOH/PA blend. The EVOH/ionomer blend even presents a considerably better barrier than is predicted from equation 1.10 due to the fact that the morphology of the particles is elongated (higher aspect ratio) in the machine direction and normal to the permeation direction.   PaPA ‡ 2PEVOH ÿ 2VaPA …PEVOH ÿ PaPA † 1:10 PEVOH=aPA ˆ PEVOH PaPA ‡ 2PEVOH ‡ VaPA …PEVOH ÿ PaPA † The permeability of blends following the above equation would then approach the permeability of a co-extruded multilayer (see equation 1.11) system comprising two layers, one made of a lower barrier disperse phase and the other of a high barrier matrix; therefore, the overall permeability will be close to the

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

19

permeability of the neat high barrier matrix for a sufficiently high volume fraction of the matrix (VEVOH). Equation 1.11 presents a very favourable situation in terms of permeability for a non-miscible blend. PEVOH=aPA ˆ

PEVOH PaPA VaPA PEVOH ‡ VEVOH PaPA

1:11

The circles on the graph in Fig. 1.3 represent the values of permeability obtained by application of a simple additive rule (layers parallel to permeant flow: see equation 1.12). This case would clearly represent a very unfavourable situation in terms of permeability for blends. PEVOH=aPA ˆ PEVOH VEVOH ‡ PaPA VaPA

1:12

Figure 1.3 shows, as an example, some modelling for the barrier properties of EVOH/aPA blends as a function of blend composition and the orientation of the blend constituents in relation to the direction of oxygen transport. High barrier blends of EVOH with an ionomer and an amorphous polyamide have also been developed.30,31 These blends show excellent barrier properties to gases compared to neat EVOH (see experimental values for EVOH 80/20 blends in Fig. 1.3), and yet much better thermoformability than EVOH alone for the production of thermoformed multilayer rigid food containers. Curiously, the EVOH/aPA blends, that under dry conditions present a lower barrier to oxygen, when submitted to typical packaged food water vapour sterilization (at 120ëC for 20 minutes) processes, have a better oxygen barrier than EVOH due to the decreased water sensitivity of the system. There are also a relatively large number of blends reported in the literature in which a high gas barrier polymer like EVOH was added to improve the barrier properties of a low gas barrier material and, conversely, in which a high water barrier polymer is added to a high gas barrier material to reduce relative humidity dependence in the barrier properties of the latter. In a recent paper, a PVOH-based interpolymer complex stabilized by hydrogen bonding with enhanced gas barrier was reported.51 Thus, hydrogen bonding between poly(methyl vinyl ether-co-maleic acid) (PMVE±MA) and PVOH resulted in films with lower oxygen transmission rates (OTR) than pure PVOH. In the range 20±30% (w/w) PMVE±MA, complexation between the two polymers was maximized. The improved oxygen barrier properties were believed to result from a combination of the relatively intact PVOH crystalline regions and a higher degree of hydrogen bonding in the amorphous regions of the PVOH and PMVE±MA films. This leads to denser amorphous regions that reduce the rate of gases diffusing through the polymer film, hence reducing oxygen permeability. Some other successful blending routes are achieved by blending PET with polyamides. Thus, in a recent study52 PET was blended with an aromatic

ß Woodhead Publishing Limited, 2011

20

Multifunctional and nanoreinforced polymers for food packaging

polyamide, either poly(m-xylylene adipamide) (Ny-MXD6) or a copolyamide based on Ny-MXD6 in which 12 mol% adipamide was replaced with isophthalamide (Ny-MXD6-12I). Incorporating a small amount of sodium 5-sulfoisophthalate into the PET matrix was needed to compatibilize the blends and was seen to reduce the polyamide domain size to 100±300 nm. Blending PET with 10 wt% Ny-MXD6 or Ny-MXD6-12I reduced oxygen permeability of PET by a factor of about 0.8 (P/PPET) when measured at 43% relative humidity (RH), in accordance with the Maxwell model prediction. However, after biaxial orientation, oxygen permeability of blends with 10 wt% Ny-MXD6 was reduced by 0.3 at 43% RH, and permeability of blends with 10 wt% Ny-MXD6-12I was reduced by 0.4. Even at 85% RH, oxygen permeability was reduced by 0.4 and 0.6 for blends with Ny-MXD6 and Ny-MXD6-12I, respectively. The blends were even more effective in reducing carbon dioxide permeability of oriented PET. Transformation of spherical polyamide domains into platelets of high aspect ratio was thought to cause the barrier increase. The platelet aspect ratio predicted by the Nielsen model was confirmed by atomic force microscopy. The higher aspect ratio of Ny-MXD6 domains was ascribed to a lower Tg compared to Ny-MXD6-12I. More interestingly, similar reduction in oxygen permeability was achieved in bottle walls blown from PET blends with Ny-MXD6 or NyMXD6-12I. A very interesting blending technique with high potential is the `layer multiplying co-extrusion' technique, which enables the production of layered films with tens to thousands of alternating layers of two or three different polymers with individual layer thicknesses in the 10 nm to 100 m range and various arrangements.53 Using this technology, polymers with widely dissimilar solid state morphologies and properties can be combined into unique layered and gradient structures. Micro- and nanolayers with up to 4096 layers and individual layer thicknesses less than 20 nm have been successfully produced with the technology. As the layer thickness approaches the micro- and nanometre length scales, useful and interesting changes in gas transport, mechanical and optical properties occur. This technology therefore offers an attractive approach for creating designed architectures from particulate-filled polymers such as alternating filled/unfilled layers with varying thickness and composition. Coupling of carefully chosen inorganic/organic barrier systems with multilayering technology offers the potential for generating tens or hundreds of individual, high aspect ratio barrier domains through which oxygen, carbon dioxide, water vapour or any permeant of interest would have to traverse. Finally, inorganic coatings or nanocoatings such as metallized layers, silicon oxide (SiOx) and aluminium oxide (Al2O3) layers are also being used or developed to reduce permeability in packaging structures. Thus, coating plastics with vacuum-deposited aluminium seeks to increase barrier properties to gases, moisture and organic vapours, and results in better flexibility, greater consumer appeal and lower environmental impact due to reduction in metal consumption

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

21

and better recyclability than conventional lamination with aluminium foil.54 On the other hand, the metal coating of polymeric films imposes reductions in flexibility, stretchability and thermoformability compared to the performance of the polymer films alone. SiOx coatings possess highly desirable properties, such as transparency, recyclability, retortability and microwave use, and are superior in these regards to the thin metal (generally aluminium-based) coatings currently employed commercially on various polymer substrates. For the SiOx coatings to compete effectively against more established, as well as concurrently emerging barrier technologies, they must demonstrate time and temperature stability and promote substantially reduced oxygen and water vapour permeability. Recent studies of SiOx coatings produced by different processing routes have, in fact, shown that these criteria are usually satisfied. One of the benefits of SiOx coatings lies in the flexibility by which they can be deposited on polymer surfaces. Thus far, sputtering, electronbeam deposition, and plasma-enhanced chemical vapour deposition (PECVD) have all been utilized successfully to produce SiOx barrier coatings on polymer substrates. Of these methods, the last one has become the most popular due to its operational ease and efficacy.55 Thin aluminium oxide (Al2O3) layers have also been considered as high barrier coatings and were trialled on various uncoated papers, polymer-coated papers and boards and plain polymer films using the atomic layer deposition (ALD) technique.56 This study demonstrated that such ALD-grown Al2O3 coatings efficiently enhanced the gas-diffusion barrier performance of the studied porous and non-porous materials against oxygen, water vapour and aromas.

1.4 Nanocomposites Over the last few years there has been a significant increase in the number of research works devoted to enhancing relevant polymer properties, mainly mechanical and barrier properties, but also surface hardness, control released, active and intelligent functionalizations, UV±Vis (ultraviolet±visible light) protection, thermal stability and fire retardancy, in existing polymers by means of nanotechnology. Nanotechnology is by definition the creation and utilization of structures with at least one dimension in the nanometre length scale, typically below 100 nm, that creates novel properties and phenomena otherwise not displayed by either isolated molecules or bulk materials. Among the various existing nanotechnologies available such as metallic antimicrobial and UV light protecting nanoparticles,57 carbon nanotubes and nanofibres,58 the very recently developed grapheme-based materials,59 cellulose nanowhiskers,60 electrospun nanofibres and nanocapsules,61 the one that has attracted more attention in the food packaging field is the use of inorganic layered nanoclays. It has been broadly reported in the scientific literature that the addition of low loadings of nanoclay particles, with thickness in the nanometre scale and with high aspect ratios, to a raw polymer to generate the so-called

ß Woodhead Publishing Limited, 2011

22

Multifunctional and nanoreinforced polymers for food packaging

1.4 Typical modelling examples of permeability reductions in nanocomposites as a result of the application of the Nielsen and Fricke models to layered particles.

nanocomposites can have a profoundly enhancing effect over some material properties, such as mechanical properties, thermal stability, UV±Vis protection,62 active properties, conductivity and gas and vapour barrier properties. Figure 1.4 shows typical modelling examples of permeability reductions in nanocomposites as a result of the application of the Nielsen and Fricke models to layered particles. The model of Nielsen63 (see equation 1.13), and other ulterior refinements such as that of Fredrickson and Bicerano,64 describe systems in which the layered, i.e. thin, flat and squared, particles are perfectly oriented with length and width perpendicular to the permeant transport direction and are homogeneously diluted in the polymer matrix: 1 ÿ Vclay Pnano ˆ Pneat 1 ‡ …L=2W †
Vclay

1:13

In the above equation, L=W is the aspect ratio of the platelets, Vclay is the volume fraction of the clay filler, Pnano is the permeability of the nanocomposite, and Pneat is the permeability of the pure material. A more realistic system to consider is one in which a discontinuous lowpermeability phase is present in a high-permeability matrix. Maxwell developed a model to describe the conductivity of a two-phase system in which permeable spheres are dispersed in a continuous permeable matrix.50 Fricke extended Maxwell's model to describe the conductivity of a two-phase system in which permeable ellipsoids are dispersed in a more permeable continuous matrix.65

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

23

This model describes the conductivity of a two-phase system in which lower permeability elongated ellipsoids (Pd) are dispersed in a more permeable continuous matrix (Pm). According to this model, the permeability of a composite system consisting of a blend of the two materials in which the dispersed phase (2 is the volume fraction of the dispersed phase) is distributed as ellipsoids can be expressed as follows:48 Pˆ

Pm ‡ Pd F 1‡F

where  Fˆ



1:14 2

3

6 7 2 1 6  7 4 5 P 1 ÿ 2 d 1 ‡ …1 ÿ M† ÿ1 Pm

1:15

M ˆ ‰cos =sin 3Š‰ ÿ 12 sin 2Š and cos  ˆ W =L where W is the dimension of the axis of the ellipsoid parallel to, and L the dimension perpendicular to, the direction of transport, and  is in radians. In this regard, gas and water vapour permeabilities have been found to decrease, in some cases, to a large extent in the nanocomposites due to, among other factors, increased tortuosity factors.66 For example, an EPDM±clay nanocomposite with a 4 wt% loading was found to decrease N2 permeability by 30% compared to EPDM alone.67 Oxygen permeability decreased by a factor of 3 in polyester±clay nanocomposites at 2.5 wt% loading. A 60% reduction in the water permeability was measured in a 5 wt% loaded poly(vinyl alcohol)/sodium montmorillonite nanocomposite and the material still retained its optical clarity.68 In EVOH nanocomposites, reductions in oxygen permeability of more than 70%, over a range of relative humidity values, have been reported69,70 and reductions in water permeability beyond 90% in some proteins and polysaccharides have also been reported.71 Table 1.3 reports the interesting behaviour of EVOH nanocomposites containing a recently developed kaolinite-based grade complying with food contact legislation,72 in which the oxygen permeability reduction due to the nanoclay is higher with increasing relative humidity with minimum impact on transparency. EVOH resins are known to be strongly sensitive to moisture sorption and hence EVOH nanocomposites are the only efficient technology that can overcome this drawback while retaining transparency and film integrity. Additionally, a higher retorting, i.e. humid heat sterilization resistance is observed in EVOH nanocomposites compared to EVOH alone (see Fig. 1.5). This may have considerable implications in retortable packaged foods, where thick layers of hydrophobic

ß Woodhead Publishing Limited, 2011

24

Multifunctional and nanoreinforced polymers for food packaging

Table 1.3 PO2 of extruded films of EVOH29 and of EVOH29 nanocomposites as a function of relative humidity Material

PO2 (cm3mm)/(m2day)

EVOH29 EVOH29 EVOH29 containing 4 wt% nanoclay EVOH29 containing 4 wt% nanoclay

4.2 (50% RH) 1470.6 (90% RH) 3.0 (50% RH) (28% reduction) 427.8 (90% RH) (71% reduction)

polymers are needed to protect EVOH from significant barrier and structural deterioration. In fact, reducing the water sensitivity of EVOH by blending without significant losses in transparency, with higher barrier properties and with enhanced retorting resistance can only be achieved, to the best of our knowledge, by the nanocomposites technology. Moreover, nanocomposites containing specific nanoclays can also be used as UV-light barrier materials for protection of UV-sensitive packaged products.73 A very recent development is the use of nanoclays as carriers of novel functionalizations such as for the controlled release of antimicrobials, antioxidants and oxygen scavengers of value in, for instance, active food packaging technologies.74,75 Notwithstanding the above, in general, the experimentally measured reductions in permeability have not been in full agreement with the values expected from modelling work for most systems, due to lack of complete exfoliation, insufficient compatibility, morphological alterations, solubility effects and other factors.

1.5 Retorting (humid heat sterilization) resistance experiments at 120ëC for 20 minutes of similar food packaging multilayer systems containing in the intermediate layer (a) pure EVOH and (b) an EVOH nanocomposite with 4 wt% nanoclay.

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

1.5

25

Future trends

Great efforts have been made by researchers in multidisciplinary fields over the last decades to develop new, high-performance polymeric materials or novel technological solutions for existing materials. The overall objective has been to extend the shelf-life of packaged foodstuffs, retaining or even enhancing their quality and safety attributes. The technological `holy grails' have been both (1) to procure glass-tight barrier performance and to make plastics more functional and versatile while retaining their positive attributes, and (2) to provide property-tailoring solutions for the newly developed and poorly performing renewable and biodegradable first generations of biopolymeric resins. To do so, new materials, but more importantly selected nanotechnology and functionalization tools, have been implemented from simple research ideas into fully functional commercial applications. In the years to come, new nanomaterials and functionalities with property-balancing capacity will continue to make their way from research centres across application fields into the food packaging area to additionally provide more efficiency for innovative food packaging strategies such as emerging preservation, active, bioactive and intelligent technologies. Thus, several cutting-edge nanotechnologies and novel functionalities are currently being trialled by an increasing number of material manufacturers and packaging converters. Nevertheless, for their wide commercial implementation and success they need to comply with current and future legislation and be specifically designed to reach specific targets in materials and properties. It is also clear that there is still a lot of missing information in the food packaging sector regarding their use and potentialities in finished articles and we, the authors and editors, really hope that this book can help steer the mind of the readers towards filling this gap.

1.6

References

1. G. Bureau, J.L. Multon, editors (1995). Food Packaging Technology, Vols 1 and 2, VCH Publishers, New York. 2. A.L. Brody, K.S. Marsh, editors (1997). The Wiley Encyclopedia of Packaging Technology, 2nd edn, John Wiley & Sons, New York. 3. J.F. Hanlon, R.J. Kelsey, H.E. Forcinio (1998). Handbook of Package Engineering, 3rd edn, Technomic Publishing Co., Lancaster, PA. 4. Permeability and Other Film Properties of Plastics and Elastomers (1995). PDL Handbook Series, Plastics Design Library, New York. 5. M.D. Sanchez-Garcia, E. Gimenez, J.M. LagaroÂn (2007). J. Plastic Film and Sheeting, 23, 133±148. 6. K. Petersen, P.V. Nielsen, M.B. Olsen (2001). Starch, 53, 356. 7. A. LoÂpez-Rubio, J.M. LagaroÂn, M.J. Ocio (2008). Active polymer packaging of nonmeat food products. In Smart Packaging Technologies for Fast Moving Consumer Goods, ed. J. Kerry and P. Butler, John Wiley & Sons, Chichester, UK, pp. 19±32. 8. A. LoÂpez-Rubio, R. Gavara, J.M. LagaroÂn (2006). Bioactive packaging: turning

ß Woodhead Publishing Limited, 2011

26

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

Multifunctional and nanoreinforced polymers for food packaging foods into healthier foods through biomaterials. Trends Food Sci. Tech., 17, 567± 575. S.K. Young, G.C. Gemeinhardt, J.W. Sherman, R.F. Storey, K.A. Mauritz, D.A. Schiraldi, A. Polyakova, A. Hiltner, E. Baer (2002). Polymer, 43(23), 6101±6114. I. Olabarrieta, D. ForsstroÈm, U.W. Gedde, M.S. Hedenqvist (2001). Polymer, 42, 4401±4408. J. Crank, G.S. Park (1968). Diffusion in Polymers, Academic Press, New York. Z. Zhang, I.J. Britt, M.A. Tung (1999). J. Polym. Sci.: Part B: Polym. Phys., 37, 691. M.A. Samus, G. Rossi (1996). Macromolecules, 29, 2275. G. Rossi (1996). Trends Polym. Sci., 4, 337. R.J. Hernandez, J.R. Giacin, E.A. Grulke (1992). J. Membrane Sci., 65, 187. D.W. Van Krevelen (1990). Properties of Polymers, Elsevier, New York. R.Y.F. Liu, D.A. Schiraldi, A. Hiltner, E. Baer (2002). J. Polym. Sci., Part B: Polym. Phys., 40, 862. M.H. Cohen, D. Turnbull (1959). J. Chem. Phys., 31, 1164. D. Turnbull, M.H. Cohen (1970). J. Chem. Phys., 52, 3038. H. Fujita (1961). Fortschr. Hochpolym. Forsch., 3, 1. K. Tanaka, T. Kawai, H. Kita, K. Okamoto, Y. Ito (2000). Macromolecules, 33, 5513. K. Ito, Y. Saito, T. Yamamoto, Y. Ujihira, K. Nomura (2001). Macromolecules, 34, 6153. J.M. LagaroÂn, E. Gimenez, J.J. Saura (2001). Polym. International, 50, 635. R.H. Boyd (1979). Polym. Eng. Sci., 19, 1010. M. Hedenqvist, A. Angelstok, L. Edsberg, P.T. Larsson, U.W. Gedde (1996). Polymer, 37, 2887. J.M. LagaroÂn, S. Lopez-Quintana, J.C. Rodriguez-Cabello, J.C. Merino, J.M. Pastor (2000). Polymer, 41, 2999. B. Neway, M.S. Hedenqvist, V.B.F. Mathod, U.W. Gedde (2001). Polymer, 42, 5307. S Aucejo, R. Catala, R. Gavara (2000). Food Sci. Technol. Int., 6, 159±164. J.M. LagaroÂn, A.K. Powell, J.G. Bonner (2001). Polymer Testing, 20/5, 569. J.M. LagaroÂn, E. Gimenez, R. Gavara, J.J. Saura (2001). Polymer, 42, 9531. J.M. LagaroÂn, E. Gimenez, J.J. Saura, R. Gavara (2001). Polymer, 42, 7381. J.M. LagaroÂn, E. Gimenez, R. Catala, R. Gavara (2003). Macrom. Chem. Phys., 202(4). L. Nicolas, E. Drioli, H.B. Hopfenbergand, D. Tidone (1977). Polymer, 18, 1137. J. Brandrupand, E.H. Immergut (1989). Polymer Handbook, 3rd edn, John Wiley & Sons, New York. Q. Zhou, K.R. Cadwallader (2004). J. Agric. Food Chem., 52, 6271. G.W. Halek, J.P. Luttmann (1991). In Food Packaging Interactions 2, ed. J.H. Hotchkiss, ACS, Washington, DC. A.R. Berens, H.B. Hopfenberg (1982). J. Membrane Sci., 10(2±3), 283. Z. Zhang, I.J. Britt, M.A. Tung (2001). J. Appl. Polym. Sci., 82, 1886. T.J. Nielsen, J.R. Giacin (1994). Packaging Technol. Sci., 7(5), 247. J. Letinski, G.W. Halek (1992). J. Food Sci., 57(2), 481. C. Gagnard, Y. Germain, P. Keraudren, B. Barriere (2004). J. Appl. Polym. Sci., 92, 676. J.G. Bonner, A.K. Powell (1997). Proc. 213th National American Chemical Society Meeting, ACS Materials Chemistry Publications, Washington, DC. J.M. LagaroÂn, A.K. Powell (2000). Patent WO 0042089. http://www.kureha.com/pdfs/Kureha-KUREDUX.pdf

ß Woodhead Publishing Limited, 2011

Multifunctional and nanoreinforced polymers for food packaging

27

45. http://www.g-polymer.com 46. C.J. Weber, V. Haugaard, R. Festersen, G. Bertelsen (2002). Food Additives and Contaminants, 19, 172. 47. H. Fukuzumi, T. Saito, T. Iwata, Y. Kumamoto, A. Isogai (2009). Biomacromolecules, 10, 162±165. 48. D.R. Paul, C.B. Bucknall, editors (2000). Polymer Blends, Volume 2: Performance, John Wiley & Sons, New York. 49. H.B. Hopfenberg, D.R. Paul (1978). In Polymer Blends, ed. D.R. Paul and S. Newman, Academic Press, New York. 50. J.M. LagaroÂn, E. Gimenez, V. Del-Valle, B. Altava, R. Gavara (2003). Macromolecular Symposia, 198, 473. 51. P.W. Labuschagne, W.A. Germishuizen, S.M.C. Verryn, F.S. Moolman (2008). Eur. Polym. J., 44, 2146±2152. 52. Y.S. Hua, V. Prattipatia, S. Mehtab, D.A. Schiraldia, A. Hiltnera, E. Baera (2005). Polymer, 46, 2685±2698. 53. M. Gupta, Y. Lin, T. Deans, E. Baer, A. Hiltner, D.A. Schiraldi (2010). Macromolecules, 43, 4230±4239. 54. R.S.A. Kelly (1992). I+D Packaging Conference, Sevilla, Spain. 55. A.G. Erlat, R.J. Spontak, R.P. Clarke, T.C. Robinson, P.D. Haaland, Y. Tropsha, N.G. Harvey, E.A. Vogler (1999). J. Phys. Chem. B, 103, 6047±6055. 56. T. Hirvikorpi, M. VaÈhaÈ-Nissi, T. Mustonen, E. Iiskola, M. Karppinen (2010). Thin Solid Films, 518, 2654±2658. 57. A. Travan, C. Pelillo, I. Donati, E. Marsich, M. Benincasa, T. Scarpa, S. Semeraro, G. Turco, R. Gennaro, S. Paoletti (2009). Biomacromolecules, 10(6), 1429±1435. 58. M.D. Sanchez-Garcia, J.M. LagaroÂn, S.V. Hoa (2010). Comp. Sci. Technol., 70(7), 1095±1105. 59. T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso, R.D. Piner, D.H. Adamson, H.C. Schniepp, X. Chen, R.S. Ruoff, S.T. Nguyen, I.A. Aksay, R.K. Prud'homme, L.C. Brinson (2008). Nature Nanotechnology, 3, 327±331. 60. M.D. Sanchez-Garcia, J.M. LagaroÂn (2010). Cellulose, 17, 987±1004. 61. A. Fernandez, S. Torres-Giner, J.M. LagaroÂn (2009). Food Hydrocolloids, 23(5), 1427±1432. 62. M.D. Sanchez-Garcia, J.M. LagaroÂn (2010). J. Appl. Polym. Sci., 118(1), 188-199. 63. L.E. Nielsen (1967). Models for the permeability of filled polymer systems. J. Macromol. Sci. (Chem.), A1, 929±942. 64. G.H. Fredrickson, J. Bicerano (1999). Barrier properties of oriented disk composites. J. Chem. Phys., 110, 2181±2188. 65. M. Krook, G. Morgan, M.S. Hedenqvist (2005). Barrier and mechanical properties of injection molded montmorillonite/polyesteramide nanocomposites. Polym. Eng. Sci., 45, 136±140. 66. R.K. Bharadwaj, A.R. Hehrabi, C. Hamilton, C. Trujillo, M. Murga, R. Fan, A. Chavira, A.K. Thompson (2002). Structure-property relationships in cross-linked polyester-clay nanocomposites. Polymer, 43, 3699. 67. A. Usuki, A. Tukigase, M. Kato (2002). Polymer, 43, 2185. 68. K.E. Strawhecker, E. Manias (2000). Chem. Mater., 12, 2943. 69. J.M. LagaroÂn, D. Cava, L. Cabedo, R. Gavara, E. Gimenez (2005). Food Additives and Contaminants, 22(10), 994±998. 70. L. Cabedo, E. GimeÂnez, J.M. LagaroÂn, R. Gavara, J.J. Saura (2004). Polymer, 45/15, 5233±5238. 71. J.M. LagaroÂn, E. Gimenez, M.D. SaÂnchez-GarcõÂa, M.J. Ocio, A. Fendler (2007).

ß Woodhead Publishing Limited, 2011

28

72. 73. 74. 75.

Multifunctional and nanoreinforced polymers for food packaging Food Contact Polymers, Rapra Conference Proceedings, Chapter 19, ISBN 978-184735-012-1. www.nanobiomatters.com J.-M. LagaroÂn-Cabello, M.D. Sanchez-Garcia, E. Gimenez-Torres (2009). Patent WO/2009/065986. M.A. Busolo, P. Fernandez, M.J. Ocio, J.-M. LagaroÂn (2010). Food Additives and Contaminants: Part A, 27(11), 1617±1626. M.A. Busolo, A. Aouad, J.-M. LagaroÂn (2010). Conference Proceedings, ANTEC2010, 2044±2047.

1.7

Appendix: Abbreviations

aPA AVOH EPDM EVOH HDPE LCP LDPE LLDPE Ny-MXD6 PA PA6 PAN PC PCL PE PET PGA PHA PK PLA PMMA PMVE±MA PP PS PVC PVDC PVOH

Amorphous polyamide Amorphous vinyl polymers Ethylene propylene diene monomer Ethylene±vinyl alcohol copolymers High density polyethylene Liquid crystal polymer Low density polyethylene Linear low density polyethylene Aromatic polyamide, poly(m-xylylene adipamide) Polyamide Polyamide 6 (Nylon) Polyacrylonitrile Polycarbonate Polycaprolactone Polyethylene Polyethylene terephthalate Polyglycolic acid Polyhydroxyalkanoates Aliphatic polyketone copolymers Polylactic acid Polymethyl methacrylate Poly(methyl vinyl ether-co-maleic acid) Polypropylene Polystyrene Polyvinyl chloride Polyvinylidene chloride Polyvinyl alcohol

ß Woodhead Publishing Limited, 2011