Please enable JavaScript.
Coggle requires JavaScript to display documents.
Week 2: Permeability of Plastic Packaging - Coggle Diagram
Week 2: Permeability of Plastic Packaging
Concept of Permeability
Gas or vapour (permeants) under steady state conditions diffuse across polymer at constant rate with constant pressure difference maintained across the polymer
Fick's first law below shows the relationship between rate of permeation and concentration gradient that can be used under steady state conditions:
J (rate of transport) of a permeant in a polymer: the amount passing though a plane of unit area normal to the direction of flow during unit time :
9 as :
:star:
In 8, permeability coefficient ,P, is found based on 4 assumptions
4 assumptions
D and S are independent of concentration of permeants
Concentration-distance relationship through the polymer is linear
Diffusion takes place in only one direction
Diffusion is under steady state conditions
not applicable to hydrophilic materials (e.g. EVOH or PA that interact with water vapour) or heterogeneous material e.g. laminated film
(thus uses) Transmission Rate (TR) : NOT the characteristic of polymer so it should only be used to compare orders of magnitude
2 more items...
(iFrom 9, n multilayer materialcommonly used to give more protection to foods esp. to increase barrier property )
Creates
Factors affecting permeability
Temperature: Diffusion coefficient (D) and Solubility coefficient (S) are affected by temperature which will affect permeability coefficient (P)
2 trends
D generally increase with temperature for both gases and vapours
In general, S increases with increasing temperature for gases and decrease for vapours
Thus, P of polymers determined at one temperature may not be the same in other temperatures
Moisture : Some packaging materials such as EVOH, PA and PVdC though have excellent gas barrier property they are sensitive to moisture
E.g. hydrophilic material EVOH absorbs significant amount of moisture when exposed directly to humid conditions, resulting an increase in its oxygen permeability
OH group that confers EVOH good oxygen barrier is the main cause for its moisture sensitivity as it results specific associations btw water molecules & polar OH groups
Small size water molecule is easily incorporated into the super molecular structure of the polymer resulting in plasticization. Plasticizing increases the distance btw neighbouring chains in the polymer chain causing an increase in free volume due to swelling. These free volume sites enable gas permeation reducing barrier property.
Pressure:
Generally, if there is no interaction btw polymer & diffusing materials, P is independent of the pressure of the diffusing gas
Increasing non thermal processing e.g. High Pressure Processing (HPP) where foods (inclusive of packaging) are subjected to pressure btw 100 – 1000 MPa for few seconds to minutes at ambient temp.
Some studies showed that barrier properties is not significantly modified by HPP. However, in some multi-layer film deformation leading to wrinkle or de-lamination occurred resulting in increase in permeability
Barrier properties: resistance to sorption or diffusion of substances such as gas, water vapour, organic vapours & low molecular weight compounds (e.g. flavour or aroma)
Gas and vapour could enter packaging through:
Pore effect: microscopic pores, pinholes or crack
Solubility-diffusion effect or true permeability: dissolve in polymer at one surface and diffuse through polymer by concentration gradient and evaporate at the other surface
Measurement of Permeability
Methods to measure gas permeability
Pressure Increase Method ~ manometric method sometimes known as isochoric or constant volume method
Volume Increase Method ~ standard volumetric method that measures the change in volume due to permeation of gas through film under constant pressure
Concentration Increase Method ~ Sensor permeability testing using Coulometric sensor or Coulometric detector
Detector Film Method ~ Rapid method using plastic detector film impregnated with gas sensitive reagent. The changes of absorption spectrum can be monitored using spectrophotometer
Methods to measure water vapour permeability:
ASTM E96 ~ Gravimetric procedure that measures weight gain by desiccant sealed in aluminium cup by the test film under controlled temp. & RH
ASTM F1249~ Commercial instrument using modulated infrared sensor based on isostatic method
