Plastic films
Three film laminates were selected in this study for measuring their oxygen permeability and OTR: a well-known MIA laminate, ESCAL™, and a local laminate in Japan, Cross-Barrier™, and a local laminate in Egypt, Nylon.
ESCAL™
ESCAL™ is manufactured by Mitsubishi Gas Chemicals Inc., Japan, and is distributed worldwide. It is produced in sheet and tube rolls and sachets. ESCAL™ is ceramic-coated polyvinyl alcohol (PVAL). The basic structure for this barrier film is orientated polypropylene (OPP) as a surface layer, silica deposited polyvinyl alcohol (SiO)x / PVAL as an intermediate layer, and linear low-density polyethylene (LLDPE) as an inner layer. It has superior oxygen and vapor barrier properties and its OTR is less than 0.1 cm3 m−2 d−1 atm−1 at 25 °C and 60% RH [31]. It is optically very yellow and structurally thick (114 μm) (Fig. 2a).
Cross-barrier™
The laminate is manufactured by Nissin Kasei Co., Ltd., Saitama, Japan. It is produced in sheet rolls. Its thickness is 145 μm and its weight is 115 g m−2. It was selected because it is a typical laminate consisting of three layers: a surface layer, an intermediate layer, and a back-side layer. The surface layer consists of aluminum oxide (ALO)x deposited on polyethylene terephthalate (PET). The intermediate reinforcement layer is typically made of polyethylene (PE). The back-side layer consists of linear low-density polyethylene (LLDPE). The composition ratio is approx. 13% PET, approx. 87% PE, and approx. 0.05% (ALO)x (Fig. 2b).
Local commercial nylon film
The laminate is manufactured by M2Pack Co., Cairo, Egypt. It is produced in different size sachets. The sachet used in this experiment measures 25 × 30 cm. Its thickness is 92 μm and its weight is approx. 77.33 g m−2. It was selected because it is a low-cost, widely locally available laminate consisting of two layers, a surface layer, and an inner layer. The surface layer consists of polyamide (PA). The inner layer is typically made of linear low-density polyethylene (LLDPE). The interesting fact about this laminate is that it is recyclable (Fig. 2c).
Test chamber
The test system consisted of two well-sealed chambers: a source chamber and a collecting chamber. The chambers were long and cuboid to suit all grades of permeable films (high to low) [27, 28]. The adopted design from ASTM [30] was modified by constructing the chambers from 6 mm-thick plexiglass (Spiroplastic S.A., Cairo, Egypt) sheets for the availability and workability of the material in conservation laboratories in museums. The chamber walls were glued using chloroform. Each chamber had an internal volume of 3 L (10 × 10 × 30 cm), allowing a surface area of 100 cm2 to attach the laminate membrane. One end of each chamber was closed by gluing it to a 10 × 10 cm piece of plexiglass. Holes with an 11-mm diameter were drilled in the test chambers at the mid-point height as shown in Fig. 3 to create the inlet and sampling ports. The holes were fitted with one ¼ in brass male O-seal connector (Swagelok® Company, Ohio, United States) and sealed off to the outer surface of the acrylic sheet with a washer. The inlet port was connected to a ¼ in two-way brass ball valve (Swagelok® Company, Ohio, United States) to allow or shut off the gas purge process and enhance sealing after purge (Fig. 4). At the sampling ports, two septa (Ametek Mocon, Inc., Minnesota, United States) were fixed and plugged with a ¼ in brass plug (Swagelok® Company, Ohio, United States) for sealing off the port (Fig. 5).
Selection and source of inert gas
Nitrogen was selected as the inert fumigant in this experiment because it is the most utilized MIA gas and the most difficult to maintain at the same time. The gas was obtained from a high-pressure prepurified T-size cylinder equipped with a Mujelli OX – AC one-stage regulator (GCE Group, Malmö, Sweden) with a built-in flow meter [11].
Experimental procedure
Installation of laminates
The tested film was glued to the open side of one chamber using epoxy glue mixed with its hardener and spread as a thin layer onto the 6-mm rim of the open side. After drying, a small amount of epoxy glue was evenly spread onto the rim of the other chamber that was directly placed onto the test film aligning and joining the two chambers together with the test film in the middle. The contact line between the two chambers was later tapped with a 2-cm wide strip of high-pressure aluminum tape to guarantee tight sealing.
After installation, the film laminates were conditioned for a minimum of 40 h at standard laboratory conditions (23 ± 2 °C and 50 ± 5% RH) immediately before testing according to ASTM D618-05 [32]. The conditions were controlled and monitored using the heating ventilation and air conditioning (HVAC) system of the fumigation laboratory, where the tests were performed.
Gas purge sampling and analysis
The source chamber was purged with pure, dry N2 gas, following the dynamic purge process described by Elkhial [11], through the inlet port. After reaching 100% N2, the septa and plug were installed at the sampling port, followed by a quick shutdown of the ball valve then the gas regulator to avoid pressure build-up and guarantee an O2-free gas path.
The concentration of N2 was measured by monitoring the concentration of O2 inside the bag using Dansensor® Checkmate® 3 Headspace O2 analyzer (Ametek Mocon, Inc., United States), measuring range 0—100%, accuracy ± 0.01%, and measuring three numbers after the decimal point. The utilized O2 sensor uses the latest zirconia O2 sensor, which has the inherent benefits of low cost, fast response, high sensitivity [33], and feedback control [34, 35]. The analyzer was calibrated using fresh air at 20.946% O2.
Calculations and data processing
The oxygen permeability can be measured by a series of mathematical equations as given in many publications [27, 28, 30]. However, the standard provided a Windows-based software program, FilmPC (U.S. Department of Agriculture, California, United States), to facilitate the calculations by automatically solving the associated mathematical equations using a nonlinear least squares algorithm described by Marquardt [36] to obtain the parameters and statistics. To obtain the results in this study, FilmPC software, version 3.0.4, 2011 [37], was provided with the sampling time and concentration for each chamber sampled at the same time of the day. The initial concentration of oxygen in the source chamber was 0%. The initial value of oxygen in the collecting chamber was 20.9%, normalized to 100% in the graphs, and the loss of oxygen over time due to the transmission was recorded as the collecting chamber concentrations. After obtaining the results from the software, the OTR was calculated as it represents the permeability of oxygen over time through 1 m2 area of the film at specified conditions of temperature and relative humidity following the equation:
where Q is the amount of permeant passing through the polymer (cm3), A is the area (m2) and t is the time (d) [38].