The "laminate film" method for measuring the permeation of volatile organic compounds through polymer films : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Packaging Technology at Massey University
The use of barrier polymers for packaging foods has created a need to test the transmission of organic penetrants, such as flavour and aroma compounds, that can be lost from the food to the polymer. Many studies have been carried out on organic penetrants but due to the large number of possible penetrant-polymer combinations and the interactions between these plus the factors that can affect these interactions, there are many conflicting results. In addition, the tests were generally carried out at high penetrant concentrations and the results were not applicable to the low concentrations found in food packages. Traditional methods for measuring permeation of organic compounds are slow and capital intensive, with no standard test method or test conditions existing. A simple, inexpensive method, known as the "laminate film" method was developed by Holland and Santangelo in 1988 in which the transmission of a penetrant is measured by the change in absorbance in a polymer film.
This "laminate film" method was used to measure the sorption of d-limonene and vanillin by low density polyethylene, nylon 6 and polyvinylidene chloride coated nylon 6 at 10, 25, and 40°C and in ethylene-vinyl alcohol films at 25°C and 90% relative humidity. Different thicknesses of the polymers were also tested. The sorption of d-limonene and vanillin was studied over long periods of time up to 8000 hours.
The amounts of vanillin sorbed and the rate of uptake of vanillin were lowest in low density polyethylene and polyvinylidene chloride coated nylon 6. Nylon 6 and moist ethylene-vinyl alcohol films rapidly picked up large amounts of vanillin. Increasing the temperature increased the rate and amount of vanillin sorbed.
Both orientation of the film and increasing the proportion of ethylene in the ethylene-vinyl alcohol films decreased the amount of vanillin sorbed, but increasing the relative humidity increased it.
Increasing the thickness of the films increased the amount of vanillin sorbed but did not affect the rate or method of uptake. Therefore, more layers can be used to
accurately estimate small diffusion coefficients without affecting the permeation of the penetrant.
The results became less reproducible with time due to effects such as oxidation of the penetrant, degradation of the polymer film and concentration effects.
Diffusion, partition (or solubility) and permeation coefficients were calculated for all films with vanillin. For low density polyethylene, these were of the order of
3 12 2
12 2 2
10-13cm2/s; polyvinylidene chloride coated nylon 6 10-14cm2/ s, 10-2 10-15cm2/s and
ethylene-vinyl alcohol films 10-13cm2/ s, 10-2 10-13cm2/s. The units used for the
coefficients allow for the direct comparison of results for other penetrants as no conversion is necessary when comparing solid, liquid or gaseous penetrants.
D-Limonene was not an appropriate compound to be used with this method as it had a low extinction coefficient and the absorbance was not easily measured.
From this study, there are certain requirements that need to be met ,if this method is to be used as a standard test method. The compound to be tested needs a large extinction coefficient so that sorption of the penetrant can be accurately measured, and the polymer must be transparent. Temperature and relative humidity, (if the film is moisture sensitive), must be controlled as these factors can have dramatic effects on the permeation.
The "laminate film" method would be useful in industry, where a simple and inexpensive method is needed to measure transmission of flavours and aromas in polymer films in order to enable the best barrier film for a packaging application to be chosen.