10 Permeability of multilayer materials and meansto measure permeability

1. INTRODUCTION:

The transport behaviour of substances in polymeric materials has become progressively more important in recent years with the widespread use of plastic films and rigid plastics for food packaging. The selection of plastic materials for food packaging applications with strict design specifications relating to their transport behaviour requires knowledge and appreciation of the many features which affect those phenomena.

There are many examples of foods packaged with an obvious lack of proper consideration of the effects of the environment on properties, or of limitations forced on performance due to unfavourable transport characteristics. The plasticization of polymers by sorption of ambient vapours or liquids causing subsequent decrease in mechanical properties and the loss of beverage components are few of many examples which may be cited. An objective of research in this field is to establish mechanisms and expressions relating solubility and transport with the molecular properties and characteristics of the components.

2. PERMEATION

Permeation through a film is a three-part process:

1.      Solution/absorption of penetrant into the polymer surface

2.      Migration/diffusion of penetrant through polymer(s)

3.      Emergence/desorption of penetrant from opposite surface of polymer.

Absorption and desorption depend on the solubility of the permeant, and solubility is greatest when penetrant and material have similar properties. Other relevant theory comprises:

Graham’s Law (1833) states that the velocity of diffusion of a gas is inversely proportional to the square root of the density of material.

Fick (1855) stated that the quantity of diffusing gas is proportional to concentration and time and inversely proportional to the thickness of the material through which it is diffusing.

Henry’s Law (1803) states that amount of gas absorbed by a particular volume of a liquid at a specified temperature is directly proportionate to the partial pressure of the gas.

In practice, the film of packaging material may comprise more than a single polymer, and there may be discontinuities in coatings, pinholes in films, variations in molecular structure and degree of crystallinity. The penetrant molecular size, shape and degree of polarity and ambient conditions are relevant. These are all factors which affect diffusion and solubility, which in turn have a direct impact on permeability.

The permeability of plastic films to water vapour and common gases like oxygen, carbon dioxide and nitrogen has been measured by standardised test methods. Oxygen, can cause oxidative rancidity in oil or fat containing food products. Water vapour permeation into a product may cause a loss of texture, and the outflow of water vapour, from a product through the packaging may cause dehydration, textural changes and loss of weight. An example of the latter would be a plastic film-wrapped cakes or bread which would lose moisture in storage prior to sale, where a negotiation has to be made in balancing weight loss in storage with initial weight and the water vapour barrier protection provided by the plastic film. In this example, in addition to flavour retention and texture, the actual weight at the point of sale would also have to meet appropriate regulations.

The results of permeability tests give direction with respect to the choice of material(s) for the packaging of specific food products. Some other possible penetrants and the effect of the presence of polymer additives can lead to unexpected results. It is still necessary to carry out shelf life tests to establish performance of packaging material in practice with the food under consideration.

Under steady state conditions, a gas or vapour will diffuse through a polymer at a constant rate if a constant pressure difference is maintained across the film. The diffusive flux, J, of a permeant in a polymer can be defined as the amount passing through a plane surface of unit area normal to the direction of flow during unit time, i.e:

J = Q / A x t

Where, Q is the total amount of permeant which has passed through area A during time t.

2.1 Desired Properties of a Good Barrier Material

To be a good all-round barrier material, the polymer must possess the following properties:

1. Some degree of polarity as found in nitrile, chloride, fluoride, acrylic or ester groups;
2. High chain stiffness;
3. Inertness to the permeant. Polymers containing polar groups, can absorb moisture from the atmosphere. This has the effect of swelling or plasticizing the polymer and reducing the barrier properties.
4. Close chain-to-chain packing ability got by molecular regularity, crystallinity or orientation. Linear polymers with a simple molecular structure lead to good chain packing and lower permeability than polymers in which the backbone has a bulky side groups leading to poor packing ability. The higher the degree of crystallinity, the lower the permeability, because crystalline regions are less permeable compared with the amorphous regions. Orientation of amorphous regions decreases permeation by about 10 to 15%, while in crystalline polymers reductions of over 50% can be observed.
5. Some bonding between chains. Cross-linking of polymers restricts their mobility and thus decreases permeability, due to the decrease in the diffusion coefficient. For example, in the case of polyethylene, one cross-link about every thirty monomer units leads to a one half the reduction of the diffusion coefficient. The effect of cross-linking is more prominent for large molecular sized permeants.

3. PERMEABILITY OF MULTI-LAYER MATERIALS

Multi-layer materials can be considered as a number of membranes in series. Consider the case of three layers in series. The total thickness XT = X1 + X2 + X3. Assuming steady state flux, the rate of permeation through each layer must be constant, i.e.

QT = Q1 = Q2 = Q3

Likewise, the areas will also be constant so that

AT = A1 = A2 = A3

Therefore, by substituting in equation:

then

By rearranging above equation and writing it for the case of permeation through the multi-layer material:

Thus if the thicknesses and permeability coefficients are known for each layer, and provided that the permeability coefficients are independent of pressure, then above can be used to calculate the permeability coefficient for any multi-layer material. If they are not, then differing permeability coefficients will be obtained depending on the positioning of the layers.

The standard methods for gas permeability measurements through plastic materials specify dry gas. But, in practice the films are almost used in humid conditions, and for materials such as ethylene-vinyl alcohol copolymers, the oxygen permeability is dependent on the humidity. In such a case the above equation cannot be used directly since P2 (the permeability coefficient of the centre layer) will depend on the average partial pressure at the centre.

An equation for forecasting the average partial pressure at the centre of a multi-layer material containing a water sensitive centre layer can be derived as follows. Consider again the case of three layers in series, but this time assume that the oxygen permeability of the centre layer is moisture dependent and that the direction of water vapour flux is from the outside to inside.

Since the partial pressure of water vapour will not be constant across the multi-layer, the equation must be modified to include a term for the partial pressure difference and the thickness:

Now since the area A and time t will be the same for all three layers, the equilibrium WVTR between the outside and centre layers can be calculated as:

Similarly, the equilibrium WVTR between the centre and the inside layer is

The average partial pressure of the centre layer (pc) will be:

Simultaneous linear solution of equations for p2 and p3 and substitution in above equation:

By the data of pc, the permeability coefficient Pc of the centre layer can be determined at this partial pressure and equation for PT can be used to calculate the overall permeability of the multi-layer material.

4 MEASUREMENT OF PERMEABILITY

4.1 Gas Permeability

There are many methods for measuring permeability; the some important methods will be considered here. For a complete understanding of the principles behind permeability measurements it is important that the meaning of two terms which are constantly used – total and partial pressure of gases in a mixture is clearly appreciated.

In constant volume, total pressure exerted by gases present is the sum of the partial pressures of each of the gases, a discovery made by John Dalton and known as Dalton’s law. The partial pressure of any one of the constituent gases is the pressure which would result if that particular gas occupied the same volume by itself. That is, each gas in a gas mixture behaves independently of the other gases.

The permeation rate of a gas through a polymeric material is function of the partial pressure difference of that gas across the material and not of the total pressure difference between the two sides.

4.1.1. Pressure Increase Method

The ASTM standard method for measuring gas transmission rates and permeability of flat films is labelled as D 1434. It includes the manometric method which utilizes the Dow gas transmission cell. The film is backed with a filter paper and sealed with an O-ring. The pressure in the receiving compartment is measured with an open-ended mercury manometer. Detailed descriptions of the calibration and testing procedures are given in the ASTM method. The corresponding British Standard is BS 2782 Part 8 Method 821A.

On condition that the pressure on the high-pressure side remains much larger than on low-pressure side, the pressure difference remains constant and the permeability coefficient can be calculated as follows. The slope of the straight line portion of the plot of the pressure (in mm Hg) on the low-pressure side versus time (ΔpL / t) is determined and substituted into the following equation:

Where VL is the volume of the low-pressure side, X the thickness of the film, T the absolute temperature and A the film area.

4.1.2 Concentration Increase Method / Isostatic method

In this method a partial pressure difference through the film with respect to the test gas is created without change in total pressure, thus avoiding the need for rigid support of the film. A partial pressure difference is maintained by sweeping one side continuously with the test gas and keeping an inert gas on the other side into which the test gas diffuses. The concentration of the diffusing gas can be measured by chemical analysis, gas chromatography, thermal conductivity or special electrodes. As the method of measuring the concentration of the test gas can be specific to that gas, equipment can be developed in which the relative humidity of both the test and inert gases can be controlled. This is of main importance when measurements are carried out on films whose gas barrier properties are related to humidity or moisture.

Instrument in extensive commercial use for the estimation of oxygen permeability by the isostatic method is MoCon Ox-Tran (Modern Controls, Inc., Minneapolis, Minn., U.S.A.). An advantage of this instrument over the permeability cells is that the permeability of not only flat film but also containers, bottles, pouches, tubes, etc. can be measured, thus allowing the analysis of possible adverse effects of machine processing, printing and distribution. The use of this instrument is included in ASTM D 3985.

The Ox-Tran has a two-chamber measuring cell between which the test film is placed. Gas stream of known oxygen partial pressure is passed through one of the chambers; oxygen-free carrier gas is passed through the other chamber to a coulometric detector. A separate part of the instrument is fitted with two openings for the carrier gas over which containers can be fixed and sealed. A glass dome is located over this arrangement into which oxygen flows by means of a filling tube.

Modern Controls, Inc. has designed an instrument for the measurement of carbon dioxide permeability, it is known as the Permatran C, it is similar in construction to the Ox-Tran, but it uses a pressure-modulated infrared detector.

4.1.3. Volume Increase Method

In this method, the change in volume at constant pressure, because of permeation of gas through the film is measured. Variable volume permeation cells are often used for rapid estimation of relatively high steady-state permeation rates. Though, the volume increase method is simpler to implement but less sensitive than the pressure increase method, it is rarely used for high-pressure time lag measurements. Volumetric methods are used relatively infrequently compared with the use of the pressure increase or concentration increase methods.

4.1.4. Detector Film Method

A method for measuring permeability of films which requires little equipment and is both rapid and accurate, has been devised. The principle of the method is a plastic detector film saturated with a reagent which is sensitive to the measured gas. The film, having an absorption spectrum that changes with the gas or vapour when absorbed, is thus suitable for spectrophotometric measurements. The detector film is sealed between two pieces of test film in a simple cell so that the permeation rate of the penetrant gas or vapour can be readily measured. The detector film can measure much less than the minimum detectable quantity of oxygen determined by most other methods, and therefore allows the use of either smaller film samples or more rapid permeability determinations.

The oxygen detector consist a cast film of ethyl cellulose containing dimethylanthracene (DMA) and erythrosine. On absorbing blue light, the erythrosine can trigger oxygen dissolved in the ethyl cellulose to form singlet oxygen, a reactive form of oxygen. Singlet oxygen diffuses to a neighbouring DMA molecule and reacts with it. Thus, the disappearance of DMA is monitored in the UV, which is a measure of the oxygen consumed. As the ethyl cellulose detector is highly permeable to oxygen, it is capable of measuring very low rates of oxygen permeation.

4.2 Water Vapour Permeability

The standard method to determine water vapour transmission rates is to place a quantity of desiccant in an aluminium dish which is covered with a sheet of the material being tested and sealed in the same position with wax. The dish is then placed in a closely controlled atmosphere (Either 25±0.5°C & 75±2% RH for temperate conditions, or 38±0.5°C & 90±2% RH for tropical conditions) and the increase in weight noted as a function of time. If the points are plotted out they should fall more or less on a straight line since Dp is constant throughout the test. To convert WVTR into permeance (P/X), it should be divided by the driving force Dp.

This method has several disadvantages, including the length of time needed to make a determination of 2 and 14 days and the lower limit of the useful range of about 1g / m2 /day for a typical packaging film. Another disadvantage is that, depending on the desiccant, Dp may not remain constant during the test period. When using anhydrous calcium chloride, the partial pressure of water vapour in the dish remains below 2% of the vapour pressure of water at the test temperature, while in case of silica gel the partial pressure of water adsorbed on it increases with coverage.

Newer type of film detector to measure rate of transmission of water vapour has also been developed. It comprises of transparent cellulose film which becomes bright blue when soaked in cobalt chloride solution and dried over calcium chloride but rapidly turns pink on exposure to high humidity. A humidity cabinet is used to provide the partial pressure gradient through the test film, which is sealed in the same way and in a cell of similar design to that used to measure oxygen permeability. The change in absorbance of the detector film is measured at 690 nm, and from this the quantity of water absorbed by the detector film, and hence WVTR of the test film, can be calculated. Good results have been obtained using this method.

4.3 Odour Permeability

The permeability of packaging materials to organic vapours is of substantial interest, predominantly where the contents of the package has to be protected against foreign odours or where there is a prerequisite that volatile flavouring materials are not lost from the package. The major off-flavours found in some food products may result from the packaging material itself, or may permeate via the packaging material from the outside environment. In other situation, foods may contain highly desirable but volatile flavour compounds whose loss from the packaged food will reduce its quality. In both situations, suitable tests must be undertaken to select materials which have the desired odour barrier properties.

There are no standard methods for the measurement of odour permeability. A number of methods have been described for vapour permeability measurements, although many of them are only suitable for use with saturated vapours only. A sophisticated instrument for studying the transport of aromas in polymer films has been described, which utilizes a mass spectrophotometer to detect the permeant. Temperatures up to 150°C and relative humidity from 0 to 100% can be used, making it possible to obtain data on the likely aroma, flavour and odour permeation of polymeric materials used in retortable pouches. A method for the quantitative evaluation of the aroma barrier of packaging materials has been developed which uses a permeation cell similar to that described for the concentration increase method. Nitrogen gas is bubbled through the liquid permeant and then passed with the permeant vapours through the cell. The concentrations of the permeating vapours and related humidity are monitored by gas chromatography.

A common, odour penetration test involves packaging various odoriferous substances in pouches made from the test materials. The pouches are then placed in clean glass bottles and sealed by crimping with aluminium foil. After storage for a fixed time, the bottles are sampled, either by gas chromatography and mass spectroscopy, or by sniffing using a sensory evaluation panel. By these results it is possible to rank a range of packaging materials according to their odour barrier properties.

5. Conclusion

There are various methods for determination of permeability of multilayer packaging materials. But, particular methods have often been lost and then rediscovered, their origins are forgotten. For example, the isostatic method has been reinvented many number of times, most recently in 1973, since Mitchell used it in 1831. Although the permeability of the permanent gases and of water vapour through many packaging materials are well known, there is a lack of data for the permeation of organic vapours. Much of the published work has involved the use of saturated solvent vapours, and although this data is useful in estimating how well a packaging material will withstand accidental high-level contamination, it is not generally valid to use such data to estimate permeation rates at the very much lower levels of vapour encountered in typical retailing situations. So there is a need of work to be done on measurement of permeability of organic vapours or odorous compounds in the actual retail situations.

Reference:

• Ashley, R.J. 1985. Permeability and plastics packaging. In Polymer Permeability, J. Comyn (Ed.), Elsevier Applied Science Publishers Ltd., Essex, England
• Brown, R.P. (Ed.). 1981. Permeability. In Handbook of Plastics Test Methods, 2nd Edn., George Godwin Ltd., Essex, England
• Hernandez, R.J., Giacin, J.R. and Baner, A.L. 1989. The evaluation of the aroma barrier properties of polymer films. In Plastic Film Technology, K.M. Finlayson (Ed.), Technomic Publishing Co., Inc., Lancaster, Pennsylvania, USA
• Paine, F.A. and Paine, H.Y. 1992. A Hand Book of Food Packaging. Blackie Academic & Professional, London, England
• Pauly, S. 1989. Permeability and Diffusion Data. In Polymer Handbook, 3rd Edn., J. Brandrup and E.H. Immergut (Eds.), Wiley-Interscience, New York, USA
• Robertson, G.L. 1993. Food Packaging Principles and Practice. Marcel Dekker INC. New York, USA