Gas and Vapor Permeation PDF

GAS - low-boiling-point molecules - Non-condensable - Exist only in gaseous state at ambient temperature and atmospheric pressure. - ‘permanent gases’: oxygen, nitrogen, hydrogen, carbon dioxide VAPOR – condensableat ambient temperature 2 major mechanisms of transport of gas and vap...

GAS - low-boiling-point molecules - Non-condensable - Exist only in gaseous state at ambient temperature and atmospheric pressure. - ‘permanent gases’: oxygen, nitrogen, hydrogen, carbon dioxide VAPOR – condensableat ambient temperature 2 major mechanisms of transport of gas and vapor: Permeation: the exchange of a gas and vapor (= permeant) through a plastic film or package wall Leak: the exchange of gas and or vapor through pinholes or channel leaks  Permeation of gas and vapor Permeation of gas and vapor can greatly influence the keeping quality of packed food. a) Oxygen ingress  lipid oxidation (e.g. in dehydrated meat, egg, cheese, fried food)  off flavors, loss of color & nutrient value. b) Penetration/ permeation of water vapor into the package  moisture gain  sogginess, microbial growth. c) Escape of water vapor from the package  moisture loss  undesirable textural changes in food. d) Desirable: exchange of oxygen, carbon dioxide and water vapor in MAP of fresh produce.  Permeation Permeation is inversely proportional to barrier. – E.g. a package which allows gas to permeate quickly is a package of low gas barrier. – To protect foods that are oxygen or moisture sensitive, high gas barrier is necessary. All packaging polymers are permeable to gas and vapor to various degrees. Glass and metal packaging materials are not permeable. Paper packaging materials are too permeable (unless coated with barrier layer).  Mechanism of gas transport through permeation Permeation of a permeant through a polymer film or a package wall is affected by concentration difference (from high to low concentration). Mechanism: – Adsorption of the permeant onto the high concentration side of the film surface. – Diffusion of the permeant across the film – Desorption of the permeant from the low concentration side of the film surface. Permeation through a polymer film In experiments: involve only gas and solid phases. In reality: permeation may also involve the liquid phase.  Diffusion of permeant Diffusion: movement of molecules from a region of high concentration to a region of low concentration. Fick’s First Law: J = - D (dc/dx) – J (mol cm-2 s-1) is diffusion flux, D (cm-2s-1) is diffusion coefficient (diffusivity), c (mol cm-3) is permeant concentration, x (cm) is distance in the flow direction. The negative sign shows the flow direction toward a lower concentration. – J = the amount of permeant diffusing per area per time. – dc/dx = concentration gradient = driving force of diffusion  net diffusion stops (it does not mean the molecules are no longer moving, but the rates of molecular movement on both sides of the film counterbalance each other). – D = a measure of the mobility of permeant molecules in the polymer film.  Diffusion of permeant Fickian behavior: concentration independent – Diffusion of permanent gases in polymers – e.g. D of oxygen in PE film remains the same when oxygen concentration is changed from 21% to 100%. non-Fickian behavior: concentration dependent – Diffusion of water vapor or organic vapors in polar polymers – e.g. nylon and EVOH, where there is a strong interactions between the permeant and the polymer matrix.  Adsorption and desorption of permeant Related to the solution or sorption behavior of the permeant molecules in the polymer film. Affected by the relative strengths of interaction between permeant/permeant, permeant/polymer and polymer/polymer. Henry’s Law: cs = S p – cs (mol cm-3) is permeant concentration at the solid-phase film surface – p (atm) is partial pressure of the permeant – S (mol cm-3 atm-1) is solubility coefficient If S is concentration independent (permanent gas), then the equation becomes a linear relationship  e.g. adsoprtion and desorption of permeant gases at atmospheric pressure, where the permeant/permeant and permeant/polymer interactions are weak compared to the polymer/polymer interactions.  Terminology and units for permeation -Transmission rate- TR = the quantity of permeant passing through a film per unit area per unit time at a steady state. TR (mol m-2 s-1)= quantity of permeant/(A.t) – Gas transmission rate (cm3(STP) m-2 day-1) GTR = volume of gas permeant (STP)/ (area.time) – Water vapor transmission rate (g m-2 day-1) WVTR = mass of water / (area.time) In reporting a TR value, the permeant, the polymer, as well as the test conditions (temperature, partial pressures on both sides of the film, and relative humidity) must also be stated.  Terminology and units for permeation -Permeance- Permeance: transmission rate normalized for pressure difference. P’ (mol m-2 s-1 Pa-1) = TR / ∆p – O2 permeance: cm3(STP) m-2 day-1 Pa-1 – Water vapor permeance: g m-2 day-1 Pa-1 Like TR, permeance may be used for either homogenous or heterogenous films.  Terminology and units for permeation -Permeability- For homogenous films whose properties are characteristics of the bulk material: P (mol m-1 s-1 Pa-1) = P’.L = (TR / ∆p). L Since permeability is normalized for thickness, its use is appropriate only if the transmission rate decreases linearly with thickness. Total permeability of a multilayer film: has to be calculated from the permeability of individual layers.  Permeability of food packaging polymers Polymers offer a wide choice of barrier properties. – Low oxygen barrier, e.g. PE. – High oxygen barrier, e.g. EVOH. Polar polymers, e.g. EVOH, deteriorates in gas permeability under high relative humidity conditions. Most important: oxygen permeability and water vapor permeability  the food’s sensitivity to oxygen and moisture have to be considered in designing the packaging system. Permeability values of polymers Permeability of food packaging polymers  Factors affecting permeation -Nature of polymer- Gas and vapor barrier of polymers is improved with: – Increasing polarity – Increasing regularity of molecular structure – Closer chain-to-chain packing in the polymer matrix Polar functional group, e.g. OH, Cl and CN, decrease O2 and CO2 permeabilities under dry conditions due to strong polymer interactions. – High polarity results in high cohesive energy between the polymer chains  lower diffusion and permeability.  Effect of interactions on diffusion paths a) PE: there is no or very weak interaction between the nonpolar polymer chains  permeant molecule may diffuse through the polymer matrix via a shorter and more direct path. b) EVOH: strong intermolecular interactions e.g. hydrogen bonding between the polymer chains  block the passage of the permeant molecule, requiring it to diffuse through the polymer matrix via a much longer path  deceased permeability.  Factors affecting permeation -Nature of polymer- Polymers with regular molecular structure and close chain-to- chain packing tend to have higher degrees of crystallinity and are more easily oriented in molecular structure  lower solubility of permeant in the polymer matrix  lower permeability. Permeability generally increases with addition of additives, fillers and plasticizers in the polymer matrix. – Inert fillers, e.g. CaCO3 and ceramic powder  used to increase permeability in polyolefin films for fresh produce packaging applications.  Factors affecting permeation -Nature of permeant- Permeability depends on molecular size of the permeant and its chemical affinity to the polymer matrix. Permeabilities of most inert gases are independent of permeant concentration. Permeabilities of organic vapors e.g. aroma, flavors and solvents, are dependent on the concentration, since they have strong interaction with the polymer.  Larger permeant molecules generally have lower diffusivity and higher solubility compared to smaller molecules.  Solubility also depends on the chemical similarity between the polymer and the permeant.  Interesting fact: CO2 permeability is about 3-7 times of 02 permeability even though CO2 molecules are larger than O2 molecules. So, how come?  Even though CO2 diffuses slower than O2, but the solubility of CO2 is much higher than O2  the combined effect causes CO2 to permeate faster than O2.  Therefore, although CO2 molecules diffuse slower, but there are more of them diffusing through the polymer matrix.  Measurement of permeation properties -Isostatic method - Equipment: a permeability cell with 2 cylindrical stainless steel compartments separated by a sample film. Continuous stream of permeant gas and an inert carrier gas (e.g. nitrogen or helium) through the compartments. The same total pressure is achieved on both sides of the film by balancing the 2 gas flows so that the film is not stressed  isostatic. A partial pressure difference (Δp) is maintained  to provide the constant driving force for a portion of the permeant gas to move across the film. Isostatic method for measuring permeability  Measurement of permeation properties – Isostatic method Data generated from the detector connected to the outlet of the lower compartment are permeation rate as a function of time. – Initial period of time for conditioning the film  permeation rate increases with time. – The permeation rate then reaches steady state at later time when it is no longer changing. – With A and ∆p known, permeance can be calculated. – If the film is homogenous, permeability can also be calculated. – If D is independent of permeant concentration, so: D = L2 / (7.205 t1/2) D = diffusion coefficient, t1/2 is time required to reach one half of steady state permeation rate Q, and L is film thickness. – Solubility coefficient can be estimated as S = P / D Measurement of permeation properties – Quasi-isostatic method Similar to the isostatic method except that the flow of carrier gas in lower compartment of the permeability cell is zero. – The lower compartment is first purged with a carrier gas, then the end ports are closed to leave behind a stagnant volume of carrier gas. – The upper compartment is filled with a stream of permeant gas creating a driving force for the permeant molecules to permeate toward and accumulate in the lower compartment. – The permeant must be totally outgassed from the film before time zero is defined. Diffusion coefficient: D = L2 / 6 τ τ is lag time estimated from the intersection by the steady state straight line and the x-axis. Solubility: P = D S Quasi-isostatic method for measuring permeability  Measurement of permeation properties – measuring permeation rate of finished packages Similar method to the isostatic method that involves continuous flows of test gas and carried gas. – O2 (test gas) and N2 (carrier gas) is sweeping the outside and the inside of a plastic bottle  creates an O2 pressure difference of approximately 1 atm across the bottle walls. A detector will measure the amount of O2 permeates through the package. The enclosure (a chamber or a plastic bag) is needed to ensure that the O2 concentration surrounding the outer walls of the bottle is approximately 100%. Permeation rate in cm3 (STP)/day is obtained once steady state is reached. Permeability is usually not calculated since the bottle does not have uniform thickness. Measuring O2 permeation of a plastic container  Measurement of permeation properties – permeation of plastic containers Similar to the quasi-isostatic method that Estimating gas permeabilities of a hermetic plastic also involves periodic withdrawal of gas container samples. – The lid has 3 silicon ports: vacuum pump port & injection pump port that are used to flush CO2 throughout container, and sampling port that is used to periodically withdraw gas samples for analysis. – Once the container is completely filled Plots for estimating package permeation with CO2, the sampling port is used to periodically withdraw headspace gas samples for O2, CO2 and N2 analysis. As permeation occurs across the container, the composition of container’s headspace changes with time. Package permeability (Pi.A/L) can be obtained from the slope of the plotted line.  Measurement of permeation properties – gravimetric method Gravimetric method for measuring WVTR Dessicant: calcium chloride or calcium sulfate  to maintain a low water vapor pressure inside the cup. Storage: in temperature-controlled cabinet at RH around 90% WVPTR is obtained from the slope of the line (weight vs. time) after steady state condition has been achieved. Water vapor permeability: Pw = WVTR. L / (A (0.9 Ps)) – L = film thickness; A = surface area of film; Ps = saturated water vapor pressure; 0.9Ps = water vapor pressure difference between 90% and 0%.  Gas transport through leaks Types: pinholes and channel leaks. – Pinholes: on package walls, e.g. on very thin aluminum foils of less than 1 mil thick. – Channel leaks: minute channels in defective seal areas caused by improper sealing conditions. Channel leaks: larger depth than pinholes  Slower leak rate.  Gas transport through leaks Gas transport through a pinhole involves diffusion of gas molecules through a column of stagnant air inside the pinhole – NO adsorption and desorption. Diffusion through leak occurs in air, while diffusion in permeation occurs in solid. Diffusion velocity in gases, liquids and solids: 0.00001, 0.5 and 10 cm/min  faster transport of gas through leak than permeation. Both channel leaks and pinholes should be prevented since they can significantly Pinhole and channel leak compromise the gas barrier of the package.