Barrier Properties Of Cellulose Based Materials

Published Date: 02 Nov 2017

Introduction

Paper consists of a network of fibers entangling into each other. Generally the thickness of paper is far below the fibers length and no binding agent is applied to bond them together or to fill the spaces and pores between them. Consequently, it is not expected paper be a favorable barrier material whether the permeant is water vapor, grease, oil or gases like oxygen. However, studies show that cellulose derivatives reveal some good characteristics among them are barrier property.

At the same time, huge amount of synthetic barrier films consumption in everyday life (e.g. packaging applications) and their subsequent disposal and environmental problems, provoke concerns in replacing the synthetic barrier films with bio-based and bio-degradable ones.

In many papermaking operations and end-user applications, the mass transport is of the great importance. Examples range from the moisture and heat diffusion in drying, coating, calendaring and printing to the water vapor and organic compounds transmission in packaging applications (Hellén et al. 2002).

Fundamentals of permeability and barrier properties of papers

Migration of moisture in paper is described by a number of transport mechanisms which are defined depend on the state of the moisture and the means of transport: vapor-phase diffusion in the interfiber pore space, Knudsen diffusion in pores of diameters less than 100 Å, surface diffusion over fiber surfaces, bulk-solid diffusion within fibers, and capillary transport. The first two mechanisms occur in the gas phase, whereas the rest occur in the condensed state of the liquid (adsorbed, absorbed, and liquid water, respectively). Studies show that moisture transport in paper occurred via gas-phase diffusion at relative humidity (RH) levels below 58% rather than via the transport of absorbed water molecules. However, according to an earlier study, the transport of water in the condensed or bound state could be significant even at RH levels as low as 30−40% (Gupta and Chatterjee 2003).

Paper provides little barrier protection to the passage of water vapor. Accordingly, in the packaging applications a coating layer of a barrier material, which is usually a polymer, is applied on the paper surface. Two distinct mechanisms for permeation through coated papers are readily apparent. There are (a) flow through actual pores, capillaries, pinholes and other defects; (b) activated diffusion through intact polymer films. In principle, a combination of both mechanisms may be operative or one or the other may predominate according to the temperature and/or other conditions (Stannett 1973).

Flow through pores

Many mechanisms of pore flow (Stannett 1973) have been distinguished according to the pressure, pore diameter, barrier thickness, temperature and other parameters. In most practical barrier situations, the flow is normally of either the viscous, Poiseulle type or interdiffusion, when the pressure is similar on both sides of the barrier. The simplest of flow mechanisms is viscous flow, in which the volume of penetrant q passing through a capillary of radius r and length x in the unit time is given by Poiseuille equation (Eq. ):

Eq.

where is the viscosity of the permeant and p is the pressure drop across the capillary. The permeant flow per unit area of "capillary" and per unit time is therefore given by (Eq. ):

Eq.

where is a tortuosity factor that increases the effective length from x to x/. Hence, the membrane flux, q, can be given as equation Eq. , where  is the volume fraction of capillary in the membrane.

Eq.

Accordingly, the permeability coefficient, P, is defined by the following equation (Eq. 4):

Eq.

where  and r are independent of the penetrant and the permeability coefficients are inversely proportional to the viscosity of the penetrant.

When permeation occurs by flow mechanism, the pressure drop across the membrane is the driving force regardless of the phase of the penetrant, whether it is a gas or liquid.

Activated diffusion

The transmission of a gas or vapor through a polymer film in the absence of cracks or pinholes is called the activated diffusion (Stannett 1973). The process of permeation through a layer of a compact amorphous solid comprises a sequence of three stages: (i) Sorption (dissolution) of permeant at the surface layer of the solid in contact with the phase of higher permeant activity; (ii) Diffusion of permeant molecules through the bulk of the solid to the other side of the layer under the influence of the concentration gradient across it; (iii) Desorption (evaporation) of the permeant into the phase of lower permeant activity (Molyneux 2001). The rate of permeation depends on the solubility of the permeant in the film at a given pressure and on the diffusion constant of the dissolved gas in the polymer. Under steady state conditions, the rate of flow of the gas may be expressed by Fick’s law (Eq. ):

Eq.

where J is the amount of gas passing through a unit area of film in unit time, D is the diffusion constant and is the concentration gradient in the direction of flow. If the concentration changes linearly in the thickness direction, l, equationEq. may be integrated to (Eq. ):

Eq.

where C1 and C2 are the steady state concentrations of gas in the inflow and outflow surfaces of the film. The gas concentrations are usually expressed practically in terms of the partial pressures (p) of the gas above each surface and these quantities related by Henry’s law (Eq. ):

Eq.

where S is the solubility coefficient for the particular gas or vapor in the polymer. Substituting In the equationEq. , yields (Eq. ):

Eq.

where

Eq.

is the permeability constant. This value is a constant at a given temperature when Henry’s law holds and the diffusion constant is independent of the gas concentration. However, diffusion coefficients of water vapor are often concentration dependent, and this poses various challenges in the measurements and analyses of the results.

Cellulose-based materials and their barrier properties

Cellulose

Cellulose is considered to be the most abundant organic compound derived from biomass. In addition to wood, which cellulose is its main building material, there are other major sources for cellulose such as plant fibers, marine animals, or algae, fungi, invertebrates, and bacteria. Cellulose is an unbranched homopolysaccharide composed of -D-glucopyranose units linked by (14) glycosidic bonds (O'Sullivan 1997). The rings are believed to be in the chair conformation 4C1, where the hydroxyl groups are positioned in an equatorial places. The degree of polymerization (DP) for Cellulose chains are approximately 10,000 glucopyranose units in wood and 15,000 for native cellulose cotton. Basic chemical structure of cellulose is a dimer called cellobiose (Figure ).

Figure . Repeating unit of cellulose chain (O'Sullivan 1997)

Cellobiose consists of three hydroxyl groups. Presence of these hydroxyl groups facilitates the monomer to create strong hydrogen bonds which give cellulose unique properties: Multi-scale microfibrillated structure, which means cutting down the microfibrillated cellulose structure retains the properties of the original cellulose, in addition, offers new properties that are specially present in smaller size scales; duality in characteristics (crystalline vs. amorphous regions), which donates strength and flexibility at the same time; and highly cohesive nature (with a glass transition temperature higher than its degradation temperature (Lavoine et al. 2012).

Microfibrillated cellulose and nanocellulose

Microfibrillated celluloses (MFC), which is assembled into cellulose fibers, are the largest unit of elementary fibrils or microfibrils who are generated through combination of about 36 individual cellulose molecules. MFC, which is also called cellulose microfibril, microfibrillar cellulose, or more currently, nanofibrillated cellulose (NFC), is cellulosic product with greatly expanded surface area that is obtained from mechanical disintegration of cellulosic materials. Different methods are already developed for producing MFC. Turbak was the first who introduced homogenizer method. The method consists of successive disintegration of very dilute suspension of wood pulps in high-pressure in order to obtain a viscous and shear thinning aqueous gel. In an alternative method, so called Microfluidizer, the wood pulp is forced through thin z-shaped chambers under pressures as high as 30,000 psi. Consequently, it is possible to produce more uniform nanocellulose that has thinner dimensions. In this method, the mechanical treatment is repeated with different chamber sizes to increase the degree of fibrillation. This latter process produces more uniformly sized fibers.

The diameter of elementary fibrils is about 5 nm whereas the MFC has diameters ranging from 20 to 50 nm. The microfibrils which are several micrometers in length are forming after biosynthesis of cellulose. Microfibrils can be considered as strings of ordered cellulose crystals linked along the microfibril axis by disordered amorphous domains. The cellulose chains are tightly packed together and stabilized by strong and complex network of hydrogen bonds (Lavoine, Desloges et al. 2012).

Cellulose nanocrystals (CNC) might be called with different names: rod-like colloidal particles, nanocrystalline cellulose, cellulose whiskers, cellulose microcrystallites, microcrystals, microfibrils. CNCs are high purity single crystals of cellulose structure. They are produced mainly by acid hydrolysis under controlled conditions of temperature, agitation, and time. Acid hydrolysis dissolves the amorphous regions, which are acquired weak bonds, leaving the crystalline regions, the more resistant domain, intact. Afterward, the resulting suspension is washed several times with distilled water to remove any remained acids (Lavoine, Desloges et al. 2012).

Barrier properties

The interests of the barrier properties of bio-based materials are increasing in order to develop environmentally friendly and efficient materials for different purposes. MFC, as one of the promising bio-based materials, is explored in the forms of films, nanocomposites, and paper coating. 

With regard to MFC films, WVTR and WVP were the first barrier properties that were studied. One objective was to use these films in barrier packaging to replace the current packaging materials, in a so called modified atmosphere packaging (MAP). The influences of the types and chemical compositions of wood sources on the WVTR were studied in details. Comparisons between the original pulps and MFC showed the reduction in WVTR. Among the MFC, derived from different sources of wood, MFC from bleached hardwood has the highest water vapor barrier property (200 gm-2day-1). As compared with the WVTR of low density polyethylene (20 gm-2day-1), MFC shows 10 times lower barrier property (Lavoine, Desloges et al. 2012). It was also shown that the water absorption increases with lignin content in the original pulps, however, after pre-treatment and after homogenization, water absorption is not a function of lignin content anymore. This was explained based on the structural difference between original pulp and MFC films, where the MFC films are believed to be more compact, and less water can penetrate into the film even in high lignin content samples (Spence et al. 2010).

Barrier property is greatly affected by the physical structure of MFC. The degree of crystallinity strongly influences the barrier properties of a cellulosic material. Generally, it is believed that higher crystallinity is associated with a lower permeability (Aulin et al. 2010). The degree of crystallinity increases from fibers to MFC and CNC (Lavoine, Desloges et al. 2012). Comparing the barrier properties of cellulose whiskers films (a typical of CNC structure) with MFC films showed that cellulose whiskers films absorbed as much water as MFC films. It is also accompanying with higher diffusion coefficient of cellulose whiskers films comparing with MFC films. However, it was expected from cellulose whiskers films to provide more resistance against water absorption. This suggests that, there might be other important parameters such as nanoporosity and tortuosity of diffusion pathway that exert a more significant influence on water barrier than crystallinity (Belbekhouche et al. 2011). Several studies focused on the influence of the modifications of MFC by pre-treatments and post-treatments, on the water vapor permeability. Despite of some improvements, the WVTRs of MFC films still remain high as compared to those of other polymer films (Lavoine, Desloges et al. 2012).

In food packaging industry, oxygen barrier property has a key role. The recommended threshold for Oxygen Transmission Rate (OTR) value is as low as 10-20 mL m-2 day-1. In one study for barrier properties of MFC films made from fully bleached spruce sulfite pulp (Syverud and Stenius 2009), it was reported that the OTR values for MFC films is in the range 17-18 mL m-2 day-1, for a thickness of between 20 and 30 µm. Compared to films made from synthetic polymers of approximately the same thickness, the MFC films are comparable with the best synthetic polymers with respect to OTR (9-15 mL m-2 day-1 and 3-5 mL m-2 day-1, for PVdC coated, oriented polyester and EVOH, respectively). It was reported that carboxymethylation pre-treatment of MFC has a positive impact on oxygen permeability: carboxymethylated MFC films exhibited approximately 5 times better oxygen barrier as compared to non-pretreated MFC films at 50% RH (Lavoine, Desloges et al. 2012). The influence of number of passes through the homogenizer on oxygen permeability was studied by Aulin. It was shown that by increasing the number of passes the components of the film microstructure became finer and smoother, however the OTR found to be very similar (Aulin, Gällstedt et al. 2010).

One important aspect is the effect of RH on oxygen barrier property in MFC, either in a film form or for paper coating. Studying the MFC films in different RHs revealed that the OTR increases significantly with RH and in higher than 70% RH values, the OTR increases sharply. Comparisons with hydrophilic polymers and edible films exhibit similarities in this behavior, which can be explained by recalling the abrupt changes in polymeric chains mobility in high RH values. Amorphous domains in the MFC structure are plasticized by absorbing water molecules, and consequently the hydrogen bonds and fibril-fibril joints become weak. Hence the fibril network mobility increases. As a result new sites for permeation of oxygen are created in the polymer network. Moreover, in addition to structural modifications of the fibril network, oxygen solubility is also increased in presence of higher amount of moisture (Aulin, Gällstedt et al. 2010).

The study of MFC films in presence of additives were also the focus of a number of researchers. In a recent study, clay mixed with MFC produced a favorable gas barrier property. At 0% RH and 50% RH, low oxygen permeability values were confirmed for a nanocomposite composed of half MFC and half clay (0.001 and 0.045 mL mm m-2 day-1 atm-1, respectively). At 95% RH, the oxygen permeability value increased up to 3.5 mL mm m-2 day-1 atm-1. Comparing with the results from pure MFC films, one finds the inhibiting effects of clay particles for oxygen permeation in medium and high RH values (6×10-5, 0.085 and 17.5 mL mm m-2 day-1 atm-1, for 0%, 50% and 95% RH, respectively) (Lavoine, Desloges et al. 2012).

To summarize, MFC shows a good barrier property for oxygen; however it is not yet an attractive alternative among other water vapor barriers. On the other hand, oxygen barrier property is drastically affected in an environment of higher relative humidity.

The methods to improve barrier properties

Application methods

Extrusion coating operation is a versatile means for producing highly uniform coated products. This coating operation can handle fluids with quite different rheological properties; for example, both dilute aqueous solutions and molten polymers can be coated with this method. An extrusion coater can be considered as a premetred coating device (Ruschak, 1976); therefore the coating thickness is usually predetermined before coating is performed.

Extrusion coating operation is widely employed for the production of adhesive tapes, labels, photographic films, packaging materials and various paper products.

Aqueous coating

Electro-spinning

Material modifications

Nano-coating

Composite coating

Multi-layer coating

Methods for measuring permeability

There is a growing need for reliable methods for measuring water vapor transmission rate. It is generally more difficult to make accurate sorption and transport measurements for water vapor than for most other penetrants due to the following characteristic properties of water:

A tendency to adsorb on high energy surfaces such as glass or metal;

A relatively high heat of vaporization;

A low saturation vapor pressure;

High solubility in many polymers;

A tendency to plasticize polymers, with the level of plasticization being a strong function of activity level; and

A tendency to cluster in the polymer at high activities.

To obtain useful data, these properties must be considered, and they necessitate careful design of the experimental apparatus for such measurements (Turbak et al. 1983).

Gravimetric tests

Standard test techniques for measuring H2O permeation rates was first developed based on weight gain or loss with time, and consequently classified as Gravimetric tests (Demorest and Mayer 1996). Gravimetric tests are the most extensively used method for measuring the water vapor transmission rate of papers predominantly due to easiness of experiment setup. However, there are a number of challenges regarding the accuracy of the method. Hence, sources of resistance to water vapor transmission other than the test film have been considered and corrections have been proposed for: (1) resistance from the layer of still air in the cup; (2) resistance from the surface of the specimen inside the cup; and (3) resistance from the surface of the specimen outside the cup (Hu et al. 2001).

The prescribed procedures of gravimetric method suggested by different standards, which are essentially the same, include the humidity and temperature controlled environment and a light, shallow non-permeable test dish (or cup). The sample film is placed to the opening of the dish which is well sealed using rubber ring or wax. Pressure difference of water vapor in the cup in contrast with the controlled environment, provides the driving force for transferring the water vapor through the barrier and after reaching a steady state condition, the weight change of the dish during the specified time intervals is recorded. The slope of diagram of weight change with time divided by the exposed sample area, gives the water vapor transmission rate (WVTR) of the sample (Eq. ):

Eq.

Water vapor permeability (WVP) could be calculated by dividing WVTR by the pressure gradient across the film thickness (Eq. ):

Eq.

The WVTR depends upon the thickness, composition and permeability of the constituent materials and upon the conditions of temperature and relative humidity under which the test is carried out ((ISO) 1995). Standard descriptions for this method can be found in TAPPI T 448 or T 464, ISO 2528, ASTM E96-90, AFNOR NF H00-030, etc.

The standard explanations for gravimetric method are based on the assumption that the resistance to mass transfer within the stagnant air layer inside the cup in contact with the sample is negligible; therefore, the relative humidity under the film is assumed to be 100% when water is inside the cup. Thus the WVP values for films tested using different air gap heights should be equal. The WVP correction method accounts for the water vapor partial pressure gradient in the stagnant air gap, alleviating the effect of air gap height on sample WVP values (McHugh et al. 1993). According to McHugh et al., WVP values are calculated using experimental WVTR data using different stagnant air gap heights and similar film thicknesses. In this correction method, the effect of diffusion of water vapor through the air layer is taken into consideration using a classical equation (Eq. ):

Eq.

where P is the total pressure; D is the diffusion coefficient of water through air at 25oC; R is the gas law constant; T is absolute temperature; z is the mean stagnant air height (); p1 is the water vapor partial pressure at solution surface and p2 is the corrected water vapor partial pressure at film inner surface in cup. It is proved that the assumption of 100% RH at the film underside was inappropriate for WVP measurements of hydrophilic samples. At the same time, it is shown that WVP correction method is not necessary for synthetic polymer films, in which the WVP is very low. If the film has relatively low permeability so that water vapor flux through the film is low, the RH at the inner surface of the film is close to 100% (P2=P1), and pressure difference across the film (P=P2-P3) is maximum (Figure ).

Figure . Schematic representation of water vapor pressure in the wet cup test of a film with low WVTR. (Hu, Topolkaraev et al. 2001)

In another study (Hu, Topolkaraev et al. 2001), it is showed that the driving force for water vapor transport through a film (P2 in Figure ) is considerably reduced if the flux through the film is high. Accordingly, the measured WVTR is lower than it would be if P2=P1. A larger film area, which increases the flux, magnifies the effect. By requiring the largest film area, the ASTM condition is most likely to give misleading results, especially in comparing the WVTR of materials with vastly different water vapor permeabilities. Decreasing the film area relative to the water surface area lessens the effect. For the most permeable materials the flux through even a 1 cm2 film was high enough to affect P2. This means that even the smallest area used in the study did not produce a low-enough flux to meet the condition P2=100% RH. Hence, rather than base comparisons on a constant area WVTR value, a more satisfactory approach is to compare materials under conditions of constant flux.

Variable-pressure constant-volume method

In variable-pressure constant-volume method (Baker et al. 2000), a vacuum is applied at the permeate side of the membrane and a water vapor is present at the feed side of the membrane. The water vapor permeability is determined from the pressure increase in the calibrated permeate volume. However, corrections have to be made for adsorption of the permeated water vapor on the equipment. As an example of necessity of such corrections, other study (Turbak, Snyder et al. 1983), showed that there is significant difference in calculated permeability of the same operating condition but different receiving volume materials (Figure ). They used two different materials for the receiving volume and support disk, i.e. stainless steel and high density polyethylene (HDPE). The calculated permeability, when the stainless steel volume is used, was much lower than the expected permeability. This implies that a significant amount of water vapor is adsorbed on the components surfaces of permeate side.

Figure - Measured water permeation as a function of time normalized by the film area and thickness, at steady state, using stainless steel receiving volume with a sintered metal support disk and high density polyethylene receiving volume with a polyethylene support disk (Turbak, Snyder et al. 1983).

Quartz crystal microbalance (QCM)

Quartz crystal microbalance (QCM) was developed to study chemical and biological processes in thin films. The quartz microbalance measures mass uptake or release in the sub-milligram solid film sample when gases interact with the film in an isothermal surrounding. A flat quartz disc with electrodes on both surfaces can be forced to oscillate by a radio frequency voltage applied at the resonance frequency, f, of the plate. This device is called a transverse shear mode (TSM) quartz plate resonator. A TSM resonator whose frequency is continuously monitored when sample is deposited on its surface is known as quartz crystal microbalance (Park et al. 1993). If a foreign mass is attached uniformly to one of the electrode’s surface, then the change of frequency can be calculated and converted into weight change (Syverud and Stenius 2009) . With the same principle, it is possible to measure the weight change in a deposited film which is interacting with a reactive gas, e.g. water vapor. Accordingly the solubility and the diffusion coefficient and consequently the permeability of water vapor (Eq. ) in a thin polymer film applied to the surface of a QCM can be calculated. Plotting the mass change with time determines the absorption property of sample film within a specified condition of RH and temperature, and diffusion coefficient of water vapor into the sample film is calculated. The solubility of water vapor in the film is obtained by plotting the equilibrium concentration of water in the film versus relative humidity. The permeability is calculated as the product of diffusion and solubility.

QCM method has the advantage of working with very low quantities of sample materials. Applying the sample film directly on the QCM electrode makes it possible to study the sample barrier properties directly. However, it is not possible to study the applied barrier film on the paper substrate. Comparative studies (Aulin, Gällstedt et al. 2010) of the permeability calculated by the QCM method with the similar measured permeabilities of cup method and variable-pressure constant-volume method, shows significant difference which requires further studies.

Dynamic moisture permeation cell (DMPC) Phillip W. Gibson 2000

In the dynamic moisture permeation cell (DMPC) method (Minelli et al. 2010), nitrogen streams consisting of a mixture of dry nitrogen and water saturated nitrogen are passed over the top and bottom surfaces of the sample. The relative humidity of these streams is varied by controlling the proportion of the saturated and the dry components. By knowing the temperature and water vapor concentration of the entering nitrogen flows, and by measuring the temperature, water vapor concentration, and flow rates of the nitrogen flows leaving the cell, one may measure the fluxes of gas and water vapor transported through the test samples. DMPC is the most applicable method (O'Sullivan 1997) for accurately measurement of the water vapor permeability in highly permeable barriers at different humidity conditions. Most of available commercial equipments for WVP measurements are made based on this method.

Conclusion