A comprehensive review of the academic and industrial aspects of the preparation, characteriza-tion, materials properties, crystallization behavior, melt rheology and processing of PCNs was given by Ray and Okamoto (2003), with a special emphasis on biodegradable polymers. Pavlidou and Papaspyrides (2008) reviewed recent advances in the field of PCNs.
The three most common methods to synthesize PCNs are intercalation of a suitable monomer and subsequent in situ polymerization, intercalation of polymer from solution and polymer melt intercalation (Ray and Easteal, 2007):
1. In situ intercalative polymerization: The layered silicates are allowed to swell by absorp-tion of a liquid monomer, or a monomer soluabsorp-tion. The monomer migrates into the galleries of the layered silicate, so that polymerization can occur within the intercalated sheets.
Polymerization can be initiated either by heat or radiation, by diffusion of a suitable initia-tor or by an organic initiainitia-tor.
2. Intercalation of polymer from solution: This approach is similar to that used in the in situ intercalative polymerization method. First, the organoclay is swollen in a nonpolar solvent, for example, toluene. Then the polymer, dissolved in the same solvent, is added to the solu-tion and intercalates between the clay layers. The final step is removal of the solvent by evaporation, usually under vacuum.
3. Polymer melt intercalation: The layered silicate is mixed with the polymer matrix in the molten state; no solvent is required.
For most technologically important polymers, both in situ polymerization and intercalation from solution are limited because neither a suitable monomer nor a compatible polymer–silicate sol-vent system is always available. Furthermore, they are not always compatible with current poly-mer processing techniques. Melt processing allows PCNs to be formulated directly using ordinary compounding devices such as extruders or mixers, without the necessary involvement of resin production. It therefore shifts PCN production downstream, giving end-use manufacturers many degrees of freedom with regard to final product specifications. At the same time, melt processing is environmentally sound since no solvents are required. For these reasons, the direct melt intercalation method has become the most common method of preparing PCNs (Ratto et al., 2009).
Layered silicate Polymer
(b) (c)
(a)
FIGURE 5.7 Illustration of different types of composites that can arise from the interaction between layered silicates and polymers. (a) Phase separated (microcomposite), (b) intercalated (nanocomposite) and (c) exfoli-ated (nanocomposite). (From Alexandre, M. and Dubois, P., Mater. Sci. Eng., 28, 1, 2000.)
5.6.4 BARRIER PROPERTIES
Generally, PCNs show an improvement in barrier properties. Although the enhancement in bar-rier properties of PCNs is well known, the dependence on factors such as the relative orientation and dispersion of the clay (whether intercalated, exfoliated or some intermediate stage) is not well understood. The clays or impermeable nanoparticles increase the barrier properties by creating a maze or tortuous path that retards the progress of the permeating molecules through the PCN by creating a longer diffusive path that the penetrants must travel as shown in Figure 5.8.
The barrier properties of a PA-12/organo-modified MMT nanocomposite were shown by Alexandre et al. (2010) to depend not only on the degree of exfoliation of the clay particles, but also on the interactions of diffusing molecules with the clay/matrix interface, leading to percola-tion paths. While a reducpercola-tion in permeability for N2 was mainly due to an increase in tortuosity, for water and toluene the permeation kinetics revealed many concomitant phenomena responsible for the permeation behavior. Despite the tortuosity effect, the toluene permeability of the nanocom-posite increased with increasing levels of clay. The plasticization effect of water and toluene in the matrix involved a concentration-dependent diffusion coefficient. They suggested a new approach for relative permeability modeling, not only based on the geometrical parameters such as aspect ratio and orientation, but also including phenomenological parameters deduced from structural characterization and permeation kinetics.
5.6.5 APPLICATIONS
An increasing number of investigations have shown that PCNs have a lower permeability than that of the corresponding virgin polymer. PET is one of the most widely studied polymers in nanocom-posite applications. Decreases in O2 permeability of about 55% in melt-mixed nanocomposites of PET with 5 wt% nanoclay have been reported. In other examples, a PA-6 with 2 wt% nanoclay could achieve three times the O2 barrier of unfilled PA-6, while 4 wt% nanoclay conferred a sixfold improvement, retaining most of its existing favorable characteristics like toughness, clarity and oil/
grease resistance. It also showed almost double stiffness, higher HDT (heat distortion temperature) and improved clarity, thus making it an ideal barrier layer in multilayer PET bottles. By combining nanocomposite and O2 scavenger technologies, a new family of barrier PAs was developed for use in multilayer packaging structures, particularly in multilayer, co-injection stretch blow molded PET bottles for extended shelf life packaging of O2-sensitive foods and beverages. In another commer-cial application, the addition of nanoclay to amorphous PA-MDX6 as the core of a three-layer PET bottle could reduce the OTR by a 100-fold compared with that of neat PET. An EVOH copolymer nanocomposite has achieved the O2 barrier requirement for meals ready to eat (MRE).
FIGURE 5.8 Nanolayered clay platelets force low MW components to follow a tortuous path through a polymer, thus improving its barrier properties.
Less spectacular improvements have been reported for other polymers. For example, LDPE con-taining 4 wt% nanoclay exhibited a decrease in O2 permeability of 24% compared with the pure material. PP nanocomposites containing 5 wt% nanoclay reportedly showed a reduction in O2 per-meability of 57% and CO2 permeability of 48%. A PS nanocomposite with 7 wt% clay exhibited a decrease in O2 permeability of over 60%.
The improvements in properties seem to plateau at levels of about 4 wt%, although in PA-6, levels of 7 wt% have been reached because of hydrogen bonding between the amide groups and the nanoclay particles (Nguyen and Baird, 2006). It is clear that the expected improvements are not always attained. Reasons for this include incomplete exfoliation of the nanoclay and incompat-ibility between the clay and the polymer. In general, there is still a need for a deeper understand-ing of the composition-structure-processunderstand-ing properties relationships in PCNs at both a laboratory and an industrial scale. Moreover, because most studies related to nanoclays have been carried out using few nanoclay grades (most are based on MMT), there is still a lot of room for variation and maturation in the PCN area. The main kinds of nanoparticles that have been studied for use in food packaging, as well as their effects and applications, were reviewed by De Azeredo (2009).
5.6.6 BIONANOCOMPOSITES
Bionanocomposites has become a common term to designate those nanocomposites involving a naturally occurring polymer (biopolymer) in combination with an inorganic moiety, and showing at least one dimension on the nanometer scale. These were discussed in Section 3.3.12 and reviewed by Lagarón and Sanchez-Garcia (2008) and Lagarón (2011). The biopolymers typically considered are PLA, PCL, PHA and starch. Reductions in O2 and water vapor permeability range from 12% to 65% and 11% to 80%, respectively.
Ray and Bousmina (2005) reviewed the use of biodegradable polymers in PCN. An interesting aspect of nanocomposite technology is the enhancement in biodegradability after nanocomposite formation. A recent article described the mathematical modeling of mechanical and barrier proper-ties of bionanocomposites using analytical micromechanics (Kumar et al., 2011).
5.6.7 FUTURE DEVELOPMENTS
Most applications of nanocomposites in plastics have made use of laminar clays and, in some cases, of carbon nanotubes. However, other types of reinforcing elements such as biodegradable cellu-lose nanowhiskers (CNW) and nanostructures obtained by electrospinning are promising in several application fields. The electrospinning method is a simple and versatile technology that can generate ultrafine fibers that have large surface-to-mass ratios (up to 103 higher than a microfiber), excellent mechanical strength, flexibility and lightness. The procedure is not mechanical but electrostatic and is applied to the polymer in solution or to polymer melts. As a result of the latter, it is a suitable technique for the generation of ultrafine fibers of biodegradable materials that are, in general, easy to dissolve. It has been reported that around 100 different polymers (including biopolymers) and polymer blends have been nanofabricated by electrospinning (Sanchez-Garcia and Lagarón, 2009).
Recently, Compton et al. (2010) reported the exceptional ability of a graphene-based nanofiller to limit both O2 permeation and light transmission in polymer films. At only 0.02 vol%, crumpled graphene nanosheets significantly densified PS films, thus lowering the free volume within the polymer matrix. This resulted in an unprecedented reduction in O2 solubility, which manifested as a considerable decrease in O2 permeability. At such a low concentration, crumpled graphene sheets were as effective as clay-based nanofillers at ∼25 to 130 times higher loadings. In addition, this low loading of graphene significantly reduced the light transmittance of ∼0.028 cm-thick PS films at 350 nm from 94% to 31%. Thus, the addition of low concentrations of graphene nanosheets offers a simple, inexpensive means to significantly enhance the barrier properties of polymer-based packag-ing materials for air- and light-sensitive products.
An alternative approach to improving the barrier properties of polymers using nanoclays is layer-by-layer (LbL) assembly, an aqueous coating technology that is capable of building multifunctional thin films. These films are produced through alternate exposure of a charged (or polar) substrate to water-based solutions (or mixtures) containing charged (or polar) ingredients. Each pair of complemen-tary layers is referred to as a bilayer, which is typically 1–100 nm thick (Jang et al., 2008). Thin films of sodium MMT clay and cationic polyacrylamide were grown on a PET film using LbL assembly.
After 30 clay–polymer layers were deposited with a thickness of 571 nm, the resulting transparent film had an OTR below the detection limit of commercial instrumentation (<0.001 mL m−2 day−1).
This low OTR, which is unprecedented for PCN, is believed to be due to a brick wall nanostructure comprised of completely exfoliated clay in a polymeric mortar. With an optical transparency greater than 90% and potential for microwavability, this thin composite is a good candidate for foil replace-ment in food packaging (Jang et al., 2008).
More recently, Priolo et al. (2010) reported LbL assembly of a three-component system to further increase the space between clay layers as the film is deposited. The resulting PCN thin films also had unprecedented barrier performance, with O2 permeability below that of SiOx at a thickness of just 51 nm, produced with only eight bilayers (or four quadlayers). Coupling high flexibility, trans-parency and barrier protection, these films are good candidates for a variety of food packaging applications.
Another recent development was the report by Park et al. (2011) of gravure-ink-containing nano-clays as an alternative O2 barrier material for OPP film. Barrier properties of OPP films coated with three different types of gravure-ink-containing nanoclays (1 wt%) reduced OTR by 12%–37%.
However, no remarkable differences were evident in the water vapor barrier performance of control and coated OPP films. As nanoclay content and dispersion time increased, OTR values of the coated OPP films decreased.
As the secrets of nature’s methodology to optimize material properties by nanolevel construction are unlocked (biomimetics), translation of these findings to PCNs should allow for further advances (Paul and Robeson, 2008).
5.7 ORIENTATION
Orientation of polymer films is a means of improving their strength and durability in order to broaden their scope of application and make them serviceable in thinner gauges. Films may be ori-ented in either one direction (uniaxial orientation) or, more commonly, in two directions, usually at right angles to each other (biaxial orientation [BO]). Virtually all thermoplastics can be oriented to some extent, but amorphous films can be more readily oriented than crystalline films.
Orientation of thermoplastic film involves stretching the material in such a manner so as to line up the molecular chains in a predetermined direction. Once lined up, the ordered arrangement is frozen in the strained condition. BO films possess superior tensile and impact strengths, improved flexibility, clarity, stiffness and toughness and increased shrinkability.
It is difficult to generalize the effect of orientation on the gas and water vapor permeability of polymers; permeability in some polymers is unaffected by orientation, while, in others, increases or decreases are observed depending on the type of polymer, and the degree and temperature of orientation. Gas and water vapor permeabilities for amorphous polymers (e.g., PS and PET) tend to decrease by about 10%–15% when oriented. Crystalline polymers (e.g., PP and PVdC copolymer) show significant reductions in permeability of over 50% when oriented. This difference is greatest at low degrees of crystallinity (10%–15%) and gradually becomes less as the degree of crystallin-ity increases, until, at 40%–50% crystallincrystallin-ity, no differences are discernible (see Table 4.7). The gas permeabilities are largely dependent on the amorphous content, which outweighs any effect introduced by orientation. The permeability of an amorphous polymer below or not far above its Tg depends on the degree of orientation of the molecular segments; it is normally reduced compared to that at high temperatures, although sometimes some small strains increase the permeability.
Orientation generally has a detrimental effect on elongation, ease of tear propagation and the sealability of the film. The heat sealability range is narrowed and the film may vary in properties with age. Oriented film cannot be easily heat sealed because it shrinks and puckers at temperatures below the sealing temperature. A suitable solution is to apply a surface coating of some thermo-plastic having a lower melting point. For example, OPP may be coated with a dispersion of PVdC copolymer, or a copolymer of PP with a small quantity of LDPE.
Among the more common commercially oriented films are PET, PA, PVdC copolymers, PP and LDPE, the latter commonly being irradiated before blowing into film. Because radiation cross-links the molecules (see Section 5.8), the film can be stretched without becoming fluid at the melting point of a nonirradiated film, resulting in greatly improved tensile strength and shrink tension compared to nonirradiated LDPE. HDPE is not oriented because its very rapid rate of crystallization limits the extent to which it can be stretched. When the resin is blended 70:30 with LDPE, the rate of crystal-lization is slowed. The crystallinity imparts properties similar to those achieved through radiation cross-linking. The largest application of orientation techniques is in the manufacture of OPP, which results in a considerable improvement in its barrier properties.
In the case of crystalline polymers, the action of orientation induces additional crystallization, with the crystalline structure aligned in the direction of stretching. The induced crystallinity is general and does not occur in spherulite form; therefore, oriented films usually have a high degree of clarity, because of the relative absence of spherulites that cause light scattering. Orientation decreases the permeability of PET to almost one-third that of the unoriented, amorphous polymer because of decreases in both the diffusion coefficient and the solubility coefficient. Crystallization by heat setting above the Tg does not dramatically affect the permeability.
For many applications, shrinkage is not desirable and a greater degree of heat stability is required.
Films can be annealed by the application of heat to partially relax the forces while maintaining the film in a highly stretched condition. It is then cooled to room temperature and the restraint on the film released. Such a film is referred to as heat set and will not shrink if heated to below the annealing temperature. The procedure of annealing does result in some reduction in dimension in the stretched direction or directions.
5.7.1 ORIENTATION PROCESSES
The most common method used to orientate a thermoplastic film is to stretch it after it has been heated to a temperature at which it is soft. This temperature is below the flow temperature at which the molecules would readily glide past one another when the material is stressed, but above the Tg. As a result of this stretching, the direction of the molecules changes toward that in which the mate-rial is stressed, and the molecules are extended like springs. The temperature is then dropped below the softening point of the material, while the molecules are held in this configuration so that the molecules are frozen in the strained position.
Films can be oriented using two processes: tenter frame and double-bubble. In the tenter frame process, thick (500–600 μm) cast film is fed to a system of differential draw rolls that are heated to bring the film to a suitable temperature below its melting point. The film is stretched in the machine direction (MD) and the extent of orientation is determined by the ratio of the width of the film enter-ing to the width of the film leaventer-ing the system. The film is then fed to a tenter frame (Figure 5.9), where a series of clips (mounted side by side on endless chains that diverge at constant angle) grasp both edges of the film and draw it in the transverse direction (TD) as it travels forward at an increas-ing speed. Stretch rate is determined by the chain speed, divergence angle and extent of orienta-tion. Draw ratios in both directions normally vary between 4:1 and 10:1. After tentering, the film is annealed, passed over a cooling roller and reeled up.
A new simultaneous BO system called LISIM® (linear motor simultaneous stretching system) has been developed. It features individually driven clips with linear motors to drive the clips and imparts MD stretch as the film travels down the diverging tenter rails (Breil, 2010). This has replaced
the mechanical complexity of the earlier simultaneous systems with electronic control complexity that is readily handled by computer. This new tenter design has been implemented commercially and results in improved mechanical properties due to higher stretching ratios.
In the double-bubble process (Figure 5.10), molten polymer is extruded as a tube from an annular die and then quenched and gauged in cold water. The tube is flattened by passing through nip rolls and reheated to a uniform temperature. The air pressure in the tube is increased to expand the film transversely, the draw ratio being varied by adjusting the volume of entrapped air. Pinch or collaps-ing rolls at the end of the bubble are run at a faster speed than rolls at the beginncollaps-ing of the bubble, thus causing drawing of the film in the MD. After trimming the edges, the film is separated into two webs and wound up.
Wind up Cooling
roll Casting
rolls Extruder
Die
Acceleration tenter clips
FIGURE 5.9 Flat sheet orientation process using a tenter.
Extruder
(External and/or internal cooling) (Die and bath)
Quench zone Temperature
control zone
Simultaneous stretch in machine and
transverse directions
Biaxial orientation
zone
Fast nip rolls Collapsing
frame
Wind up Wind up FIGURE 5.10 Orientation by the double bubble process.
The amount of orientation imparted to a film depends on the stretching temperature, the amount of stretching, the rate of stretching and the quench. Quenching is carried out either by extruding the web onto a chill roll or by passing it through a quench tank prior to orientation. Generally, orientation is increased by decreasing the stretching temperature, increasing the amount of stretch, increasing the rate of stretch and increasing the amount of quench. Films such as PVdC copolymer and PP that have Tgs below room temperature show an appreciable crystallization rate even at room temperature and, therefore, have to be quenched and oriented immediately after extruding.
The potential energy stored in the extended molecules is the elastic memory, characteristic of oriented, nonheat-set thermoplastics. When such a film is reheated to its orientation temperature, it shrinks as the molecules tend to return to their original size and spatial arrangement. At elevated temperatures, but below the orientation temperature, some shrinkage will occur but to a lesser extent. BOPP is typically oriented 700%–800%, while other films are oriented 200%–1000% in
The potential energy stored in the extended molecules is the elastic memory, characteristic of oriented, nonheat-set thermoplastics. When such a film is reheated to its orientation temperature, it shrinks as the molecules tend to return to their original size and spatial arrangement. At elevated temperatures, but below the orientation temperature, some shrinkage will occur but to a lesser extent. BOPP is typically oriented 700%–800%, while other films are oriented 200%–1000% in