Barrier polymers
Modern synthetic polymers have been used for over 50 years as barriers to mass transport of liquids and gases. Major applications of barrier polymers include food and pharmaceutical packaging. The growth in polymer-based packaging materials at the expense of metal and glass containers has seen the importance of barrier polymers continue to rise. Polymeric packaging offers the advantages of weight reduction, formability into useful and attractive shapes, reduced breakage, transparency, and cost savings. Recently, emphases on longer-term storage and convenience factors such as microwavability have become important. Research remains strong in industry and academia to offer improved polymer properties and fabrication methods and to address environmental issues including recyclability, source reduction, sustainability, and degradability.
Traditionally, the definition of a barrier polymer has been strongly attached to the oxygen permeability. Barrier polymers had oxygen permeabilities less than about 2.0 nmol/m s GPa ( = 1.0 cc(stp) mil/100 in.2 day atm). (See Table 1 for unit conversions.) This definition unnecessarily limits the range of barrier polymers. Some polymers with higher oxygen permeabilities are useful barriers for other molecules.
Permeability Unitsa with Conversion Factors Table 1.
aThroughout the Encyclopedia, cm3 (or mL) is used in preference to cc. However, the advantage of using cc here is an obvious visual aid in the complex units and there are further comments regarding cc versus cm3 in the text.
THE PERMEATION PROCESS
A basic understanding of the permeation process can help clarify the barrier characteristics of polymers. A permeant molecule moves through a barrier in a multistep process. First, the molecule collides with the polymer surface. Then, it must adsorb and dissolve into the polymer mass. In the polymer, the permeant ‘‘hops’’ or diffuses randomly as its own thermal kinetic energy keeps it moving from vacancy to vacancy as the polymer chains move. The random diffusion yields a net movement from the side of the barrier polymer that is in contact with a high concentration or partial pressure of permeant to the side that is in contact with a low concentration of permeant. After crossing the barrier polymer, the permeant moves to the polymer surface, desorbs, and moves away. In virtually every case, the permeation is controlled by the solution and diffusion steps. The diffusion coefficient, D, is a measure of the speed of molecules moving in the polymer. The solubility coefficient, S, is an indication of the number of permeant molecules that are diffusing. Together, the diffusion coefficient and the solubility coefficient describe the permeability coefficient, commonly called the permeability, P.
P = D x S (1)
A low permeability may result from a low diffusion coefficient or a low solubility coefficient or both. The permeability for a given polymer–permeant combination can be used to describe the steady-state transport. Equation (2) is Fick’s First Law adapted for packaging:
where ΔMx/Δt is the steady-state rate of permeation of permeant x through a polymer film with area A and thickness L. P is the permeability, and Dpx is the difference in pressure of the permeant on the two sides of the film.
Equation (2) shows why reliable tables of permeabilities are important. The packaging engineer has no control over the Δpx since the conditions of the environment and the contents are fixed. Mechanical, economic, and containment requirements limit the allowable ranges of area and thickness. Only P has a wide range of possibilities.
One caveat must be considered before applying equation (2). The permeation must be at steady state. With small molecules such as oxygen, steady state is usually attained in a few hours or less, depending the polymer and the thickness. However, with larger molecules in barrier polymers, especially glassy polymers, the time to reach steady state can be very long, possibly exceeding the anticipated storage time. The time to reach steady state, tss, can be estimated with equation (3):
tss ffi L2=4D ð3Þ
Equation (2) should only be used when tss is small compared to the storage time.
Water-vapor transmission is treated differently. The industry has arrived at a standard condition for reporting and comparing performance, 37.81C (1001F) and 90% rh difference. Equation (4) shows how the rate of water-vapor transmission can be calculated using the value of the water-vapor transmission rate (WVTR) and the package geometry:
DMH2O Dt ¼ WVTR A L ð4Þ
When the actual conditions differ from the standard, the WVTR can be adjusted with great care. Ideally, data have been reported at the actual conditions; otherwise, adjustments for both the relative humidity difference and temperature must be made. If the polymer is known to be insensitive to humidity, such as a polyolefin, the humidity adjustment is merely multiplication by the actual relative humidity difference on the two sides of the film divided by 90% rh. The temperature effect will be discussed later.
UNITS
The units for permeability are complex, and many correct combinations are used in the literature. Lamentably, many incorrect combinations are used too. Table 1 contains conversion factors for several common units for the permeability. Table 2 contains conversion factors for several common units for WVTR. In these units the quantity of permeant is a molar unit, typically a cc(stp). For the permeability of flavors and aromas, a unit using the mass of the permeant is useful. A modified unit, the MZU ( = 1020 kg m/m2 s Pa), can be converted to a molar unit according to equation (5), where MW is the molecular weight of the permeant in daltons (g/mol).
P inMZU ð10=MWÞ ¼ P in nmol=m s Gpa ð5Þ
Water Vapor Transmission Rate Units with Conversion Factors Table 2.
PERMEABILITY DATA
Table 3 contains oxygen, nitrogen, and carbon dioxide permeability data for several polymers at 201C and 75% rh. Generally, the permeabilities of N2, O2, and CO2 are in the ratio 1:4:14. The polymers are ranked roughly in order of increasing permeability. This list contains polymers that meet the traditional criterion for barrier polymers and several that do not. The range of permeabilities here is more than four orders of magnitude. Table 4 contains diffusion coefficients and solubility coefficients for oxygen and carbon dioxide in many of the same polymers. Note that these values are useful for comparison with the flavor, aroma, and solvent permeation that is presented later in the chapter.
Permeabilities of Selected Polymersa Table 3.
a Reference 1; see Table 1 for unit conversion.
b Trademark of Mitsubishi Gas Chemical Co.
c Trademark of E. I. du Pont de Nemours & Co., Inc.
Barrier polymers are typically used as discrete layers, as coatings, or in blends. Depending on the packaging application, oxygen barrier polymers are frequently selected from the group consisting of ethylene–vinyl alcohol copolymers, polyvinyl alcohol, polyvinylidene chloride, polyacrylonitrile, polyesters, and various types of polyamides. Moisture barrier polymers include polyethylene (particularly high density polyethylene), polypropylene, polyvinylidene chloride, cyclic olefin copolymers, and polychlorotrifluoroethylene. OTR and WVTR data for selected polymers is shown in Table 5. A comprehensive list of permeability properties of polymers can be found in reference 4.
Diffusion and Solubility Coefficients for Oxygen and Carbon Dioxide in Selected Polymers at 231C, Drya Table 4.
a Reference 1.
b For unit conversion, see equation (5).
c 42 mol% ethylene.
Oxygen and Water-Vapor Transmission Rates of Selected Polymers Table 5.
a At 23°C and 0% rh unless otherwise noted.
b At 38°C and 90% rh unless otherwise noted.
c Measured at 25°C. Reference (4).
d Measured at 38°C and 100% rh. Reference (4).
e Aluminium oxide coated 48ga polyester film. Units cm3 or gm/(100 in2 day atm).
f Measured at 23°C and 85% rh. Reference (4).
g Measured at 40°C and 90% rh. Reference (4).
h PAA-coated 48ga polyester film. Units cm3/(100 in2 day atm). Measured at 23°C and 80% rh after retort.
i Measured at 24°C. Reference (4).
The increased demand for transparent packaging has seen a number of organic, inorganic and ceramic barrier coatings introduced as alternatives to metallized coatings and aluminum foil (3). Examples of commercially available barrier coating materials include polyacrylic acid, polyvinylidene chloride, polyvinyl alcohol, nitrocellulose, epoxy-amine and polyamino ether coatings, and nanocomposites thereof. In addition to high-barrier, transparency, and cost-savings drivers, retort and microwave packaging requirements contributed to the introduction of thin silicon oxide (SiOx) and aluminum oxide (AlOx) coatings. These coatings are typically applied to bottles and oriented polyester films via vapor deposition processes. While SiOx and AlOx coatings are not polymeric, they offer an excellent combination of oxygen and moisture barrier properties to polymer-based packaging.
Multilayer barrier structures are commonly used for both flexible and rigid applications. These structures can be the result of coextrusion, lamination, or coating. Typically, one of the layers provides most of the barrier while other layers provide inexpensive mechanical integrity, printability, opacity, sealability, formability, adhesion, or merely a place to locate reground scrap. The total barrier performance of a multilayer structure can be estimated with equation (6):
Lt Pt ¼ L1 P1 þ L2 P2 þ L3 P3 þ ð6Þ
where L1, L2, and L3 are the thicknesses of layers and P1, P2, and P3 are the respective permeabilities of the layers. Lt is the total thickness, and Pt is the effective permeability of the multilayer structure. The quantity Lt/Pt is the ‘‘permeance.’’ Lt and Pt may be used in equation (2) to calculate the expected performance. An example of a seven-layer barrier film is shown in Figure 1.
NANOCOMPOSITE APPROACH TO BARRIER PERFORMANCE
Polymer nanocomposites containing nanometer-sized layered silicates (1nm thick, 100- 500-nm-diameter platelets) have been the focus of considerable research interest in recent years (5). This is because significant improvements in mechanical, thermal, and barrier properties have been reported, particularly in nylon systems.
Improved barrier performance in nanocomposites can result from the high-aspect-ratio platelets providing a tortuous path for the permeant to travel through, or by the filler inclusions altering the diffusivity of the host matrix—for example, by decreasing the free volume of the polymer—or by changing the polymer crystal type/orientation. Improvements in barrier performance upon filler addition do not always occur.
A number of companies currently manufacture polyolefin and nylon nanocomposites for packaging applications. However, the use of these materials for barrier packaging has been limited to niche applications. At present, research is ongoing to overcome the challenges that are preventing widespread commercialization. These challenges include processing methods and inexpensive surface modifications to ensure excellent particle dispersion and particle alignment, which are required in order to optimize property performance and reproducibility.
FLAVOR/AROMA/SOLVENT BARRIER
In addition to functioning as gas and moisture barriers, polymers are also utilized in packaging applications that require flavor, aroma, or solvent barriers. Table 6 contains data for the transmission of flavor, aroma, and solvent (F/ A/S) molecules in a few polymers. These data are for 251C at 0% rh and very low activity (or partial pressure) of the permeant. This list represents only a small fraction of the virtually limitless combinations of F/A/S compounds and polymer films. Reliable data at low activities, as typically encountered in foods, are difficult to find. However, the data in Table 6 are consistent with some general rules. First, polyolefins are not good barriers for F/A/S compounds. Second, vinylidene chloride copolymers and ethylene– vinyl alcohol copolymers are good barriers. A third rule is not apparent from Table 6. A polymer below its glass-transition temperature (Tg)—that is, glassy polymer— is an excellent barrier for F/A/S compounds. Data are extremely rare; hence, only a few data are given for an ethylene–vinyl alcohol copolymer. The problem is that the diffusion coefficients are so low that the experiments take too long to do with accuracy. The previous author and associates were unsuccessful in many attempts to test polystyrene, PET, and nylons.
Examples of Permeation of Flavor and Aroma Compounds in Selected Polymers at 25ºC,a Dryb Table 6.
a Values for vinylidene chloride copolymer and ethylene–vinyl alcohol are extrapolated from higher temperatures.
b Permeation in the vinylidene chloride copolymer and the polyolefins is not affected by humidity; the permeability and diffusion coefficient in the ethylene–vinyl alcohol copolymer can be as much as 1000 times greater with high humidity (1).
c MZU= (10–20 kg m)/m2 s Pa); see equation (5) for unit conversions.
In these tables of data, different polymers occupy the top barrier positions. For oxygen, vinylidene chloride copolymers and ethylene–vinyl alcohol copolymers are the best barriers. For water vapor, vinylidene chloride copolymers and the polyolefins are the best barriers. For F/A/S compounds, vinylidene chloride copolymers remain good. Some glassy polymers that are not given are the best barriers. Yet of all these polymers, only a few meet the traditional definition of a barrier polymer.
FACTORS AFFECTING PERMEABILITY
The permeability increases with increasing temperature for all known cases. A plot of logarithm P versus 1/T in kelvin yields a straight line with a slope proportional to the activation energy for permeation. Usually the slope is steeper above Tg than below Tg. Hence, knowledge of the permeability at two temperatures allows calculation of the permeability at a third temperature, provided that Tg is not in the range. For many polymers the oxygen permeability increases about 9% per 1C above Tg and about 5% per 1C below Tg. The temperature sensitivity is greater for largerpermeant molecules. The temperature sensitivity for the WVTR is theoretically a littlemore complicated; however, it is about the same as for the oxygen permeability.
Humidity can affect the permeability of some polymers. When a polymer equilibrates with a humid environment, it absorbs water. The water concentration in the polymer might be very low as in polyolefins or it might be several weight percent as in ethylene–vinyl alcohol copolymers. Absorbed water does not affect the permeabilities of some polymers including vinylidene chloride copolymers, acrylonitrile copolymers, and polyolefins. Absorbed water increases the permeabilities in some polymers including ethylene–vinyl alcohol copolymers and most polyamides. A few polymers show a slight decrease in the oxygen permeability with increasing humidity. These include PET and amorphous nylon. Since humidity is inescapable in many packaging situations, this effect cannot be overlooked. The humidity in the environment is often above 50%rh, and the humidity inside a food package can be nearly 100% rh.
Additives are blended into polymers to improvemechanical or chemical properties such as flexibility, cling, and thermal stability. When the additive is a small molecule and it is soluble in the polymer, the polymer is likely to be plasticized. This effect increases the diffusion coefficient for permeant molecules. The solubility coefficient is unaffected. The effect can be small if the additive is included at less than about 1 wt%. However, at larger concentrations the effect can be large. The oxygen permeability of poly (vinyl chloride) increases by about 10 times when enough plasticizer is added to make the resulting film flexible. The phenomenon of antiplasticization is under investigation. The potential exists that, for some small molecule additives, the diffusion coefficient can be decreased.
If the additive is not soluble in the polymer, the result is more complicated. For an inorganic filler, the permeability might increase if the polymer does not wet the filler. The permeability will decrease if the polymer wets the filler. However, the effect is likely to be small unless the loading of filler is greater than about 20 wt%. Loadings this high are avoided since these composites are typically difficult to handle.
Filler in the form of platelets can lower the permeability more than filler with a more compact shape. If the platelets tend to lie in the plane of the film, permeant molecules must make wide detours (tortuous path) while traversing the film. This gives a greatly reduced effective diffusion coefficient.
Crystallinity is an overrated contributor to barrier in polymers. First, if a polymer has crystallinity, the level of crystallinity typically exists within a narrow range with only modest variation allowed from fabrication variables. Hence, crystallinity is not a strong design parameter. Second, the same properties that lead to crystallinity also lead to efficient packing in the amorphous phase. Efficient packing in the amorphous phase gives a low diffusion coefficient. Polyolefins are glaring exceptions because they have considerable crystallinity and high diffusion coefficients.
Orientation is frequently cited as a contributor to barrier in polymer films. If the polymer molecules truly are oriented in the plane of the film, either uniaxially or biaxially, the permeability is probably lower than that in an unoriented film. However, sometimes the word ‘‘oriented’’ merely means that the film has been stretched. If the polymer molecules relax during stretching, little orientation results and little effect is expected. A few cases have been noted where stretching has created microfissures in the polymer and the permeability has increased. Table 7 contains permeability data for elongated films. The results vary. A practitioner is wise to test permeability before concluding that a fabrication involving elongation will lower the permeability. When elongation does lower the permeability, the causes are a combination of better packing among parallel molecules, difficulty in moving perpendicular to the alignment of the polymer molecules, and, when crystallinity is present, the tendency for crystallites to act as platelets aligned in the plane of the film. When performed correctly, orientation is an effective means of improving barrier properties and reducing material requirements.
Effect of Orientation on Oxygen Permeability for Certain Polymersa Table 7.
a Reference (2).
b At 231C.
c See Table 1 for unit conversions.
POLYMER COMPOSITION
Although a case has been made for a situational definition for barrier polymers, common practice still focuses on a rather small set of polymers. Figure 2 contains schematic chemical structures of polymers with low permeabilities to permanent gases, especially oxygen.
Chemical structures of barrier polymers. (a) Vinylidene chloride copolymers; (b) hydrolyzed ethylene– vinyl acetate (EVOH); (c) acrylonitrile barrier polymers; (d) nylon-6; (e) nylon-6,6; (f) amorphous nylon (Selar PA 3426), y = x + z; (g) nylon-MXD6; (h) poly(ethylene terephthalate); and (i) poly(vinyl chloride). Figure 2.
Finding common traits among this diverse group is difficult. However, each has some polarity which leads to chain-to-chain interactions that give good packing in the amorphous phase. Also, frequently, sufficient symmetry exists to allow crystallinity to develop. Again, this can lead to good packing in the amorphous phase.
AVAILABILITY
Barrier polymers are available as resins for extrusion, resins for dissolution and coating, and latices for coating. Each form has its own advantages and disadvantages. Resins for extrusion can be made into monolayers or multilayers with thicknesses to give adequate barrier for demanding applications. However, extrusion can give a severe thermal stress to the polymer which could lead to degradation. For semicrystalline polymers the extrusion temperature must exceed the melting temperature. When more modest total barrier is needed, a coating may be adequate. Typically, coatings more than 3 mm (0.1mil) are difficult to achieve. However, semicrystalline polymers can be used in solvents well below the melting temperature. Hence, polymers with marginal thermal stability may be used. Solvent recovery and management must be considered. For latices, the particles are too small to allow crystallinity to develop; hence, semicrystalline polymers remain amorphous until after coating and drying. Latices cannot be stored indefinitely and must be used before coagulation occurs.
CONCLUSION
The choice of barrier polymer(s) for any given application is determined by economics, performance requirements, and the package format. A wide variety of barrier polymers are available for use, and the number of material options will continue to grow. In extreme cases, barrier requirements can be met without the use of foil by combining the correct barrier polymers, with technologies such as gas flushing and active packaging scavengers.