PORTABLE CELL FOR QUALITATIVE ASSESSMENT OF VAPOR PERMEABILITY OF FILM MATERIALS
A portable device for high-quality express measurement of vapor permeability of film materials has been developed. The paper describes in detail the design and electrical features of the device, as well as a universal method of qualitative measurement of vapor permeability. The functional of the cell is shown by the example of HDPE films deformed in supercritical carbon dioxide (SC-CO2) to various tensile strain. The article presents a number of direct experimental data (relative humidity in RH%, temperature in Celsius degrees), as well as the result of renormalization of relative humidity into absolute humidity units (vapor pressure/density) for use as a quantitative characteristic.
Porous polymeric materials have found wide application in our everyday life. They are used as breathable components for sportswear and footwear ; membranes for separation of liquids and gases ; medical dressings with incorporated medication or silver nanoparticles , etc. Some porous polymer films allow small molecules of liquid or gas to migrate across their surface. In this case we can speak of vapour- or gas-permeable materials.
To date, a number of standard methodologies have been developed to characterise vapour permeability, using sportswear materials as an example.
The water vapour transmission rate (WVTR), which is a diffusion rate measured in g/(m2 · 24 h), is commonly used in tests of porous materials for water vapour permeability. There are various methods for its determination (American vertical cup test ASTM E 96 , inverted cup test ASTM E BW, Japanese vertical cup test with dry desiccant (A1) JIS L 1099 A1 and others) ). Almost all of them are based on measuring the mass dependence of the substance on time. However, a significant drawback of these techniques is measurement of the WVTR under different conditions (such as wind strength, temperature, ambient humidity, etc.). The WVTR calculated in this way is not completely objective. Often manufacturers carry out a series of standard tests, resulting in the best WVTR, which does not guarantee objectivity at all . Therefore, it is not possible to objectively compare WVTR values for fabrics from different manufacturers due to the use of different techniques.
The method proposed in this paper is based on a qualitative measurement of vapour permeability using a special HYT 939 capacitive sensor, which is synchronised with an Arduino Nano board to automatically record the data obtained in Excel program. The measurement result presents the qualitative vapour permeability (above the film) in units of relative humidity RH%, subsequently converted to absolute using table values, and the steady-state temperature (°C) in a cell.
In this paper a demonstration of cell operation is given using porous HDPE polymer films as an example. These films are obtained by uniaxial deformation in supercritical carbon dioxide. The obtained films are hydrophobic and do not swell or permeate water.
A device has been developed to measure vapour permeability of polymer porous films (Fig.1). The function of the cell was demonstrated by means of HDPE films (Mw = 200 kDa, Tmelt= 130 °C, degree of crystallinity 70%, thickness 75 µm) that were preliminary uniaxially deformed in supercritical carbon dioxide (SC–CO2) at 10 MPa and 35 °С to different tensile strain. The obtained porous films were characterized by the value of effective volume porosity, which was defined as the ratio of incremental volume of the samples in the process of stretching to the final volume:
where W is the volumetric porosity, V0 is the initial and V is the final (after stretching) volume of the sample.
Initially, a special device No. 1  was developed to measure vapour permeability, which was further developed and improved through a series of experiments to obtain device No. 2 (Fig.1 a–d, right). The overall dimensions of the cell are 90 × 70 × 80 mm3.
The vapour permeability measurement cell consists of the following components: container for liquid (1), holder (exchangeable unit) for the film clamps (2), cover (3) with internal sensor limiter (4) to collect relative humidity and temperature readings from the same sample area, and the main electronic component – an Arduino Nano board (5) in a protective housing with the preset HYT 939 relative humidity and temperature sensor (6).
All structural elements are made by the Maker Bot Replicator 2X 3D printer from ABS plastic. The layers of the cell are connected by magnets that are embedded into its corners. Complete abutment of the surfaces is ensured by a careful surface finish with sandpaper.
The maximum volume of the liquid container is 8 ml (working volume is 5 ml). The inner surface (walls and bottom) is coated with a thin layer of waterproof paraffin.
The limiter area (4) around the perimeter of the sensor is 3.14 cm2. For this unit, the relative humidity and temperature sensor HYT 939 in a metal casing with a splash-proof filter for liquid and chemical agents with an outer diameter of 9 mm was selected.
The developed cell has the following features:
Multilayering. It makes it possible to develop and design new interchangeable units (2) for film clamps whose shape can be varied, without significant changes to the overall cell design.
Shape. The triangular shape and magnets allow the cell to be partially "opened", making it possible to replace the fluid without completely removing all layers of the design (Fig.1. c), and to make quick changes to the sample.
Design of the electronic components. The Arduino Nano board and the HYT 939 sensor are separated by about 2–3 cm, which prevents the sensor from heating up when the board is in operation for a long time. The enclosure grid prevents overheating of the operating electronic components. The internal architecture of the upper casing prevents leakage of liquid vapour.
Portability. This cell is easy and safe to transport, with all electronic components protected by an outer casing.
Below (Fig.2) is a schematic cross-sectional view of the liquid-film cell and the sensor specifications. The film was placed in the cell in such a way that it was not in close contact with the surface of the liquid and the sensor (at a distance of 1–2 cm). The applied HYT 939 sensor has good technical characteristics: high accuracy, short response time, repeatability, etc., as well as the minimal drift of the indicated values (Table 1).
A ready-made Excel macro, PLX-DAQ, was used to record and transmit the experimental temperature and relative humidity values from the sensor to a personal computer.
RESULTS AND DISCUSSION
Operation of the device was demonstrated with the use of porous HDPE polymer films deformed to tensile strain in SC-CO2. The technique for obtaining samples can be found in papers [6, 7] which show that under these conditions an open-porous structure is formed in the polymer film, containing anisotropic pores 20–30 nm in width and up to 200 nm in length.
Temperature (°С) and relative humidity (RH, %) data versus time were obtained for the following systems: empty cell; cell with water; cell with water and films with 0%, 75%, 140%, 185%, 200% and 400% strain rate. All the films to be tested were fixed in a holder so that the water cell was placed on one side and the sensor on the other. Distilled water in a volume of 5 ml was chosen as the experimental liquid. The surface area read was 3.14 cm2. This area value was chosen depending on the size of the limiter 4 placed around the perimeter of the sensor (Fig.1, b).
Figure 3 shows a graph of the relative humidity-temperature time dependence for an empty cell. In a closed cell in the absence of liquids and films, these values tend to be constant.
The temperature varied only slightly throughout all experiments. However, the data underwent a slight drift when the sample was replaced (Fig.4a), which caused scattering of the initial values. The final temperature reaches a plateau 2400–3000 seconds after the start of the experiment (40–50 minutes). The average temperature value is <T> = 27.0±0.5°C, which is consistent with the average ambient temperature at which measurements were taken. The temperature error was calculated from the plateau section of the graph. This amounts to approximately 1,000 measurements. Similarly, the relative humidity value for the indicated systems (empty cell, cell with water, cell with water and films with a tensile strain ranging from 0 to 400%) reached a plateau in 40–50 minutes (Fig.4, b).
For the cell with water, the expected 100% RH was not achieved. This is presumably due to unsaturated vapour inside the cell. Therefore, at this stage the vapour permeability estimation can be of a purely qualitative nature. Nevertheless, for a better understanding of the % RH data obtained, we provide an approach for converting relative humidity units to absolute humidity units.
To convert the relative humidity data to absolute humidity expressed in pressure units (mmHg), the tabulated values of saturated vapour pressure versus temperature (in the range 21 to 30 °С) were used .
The tabulated values between 21 and 30 °C were approximated by a second-degree polynomial of the following form:
where p0(T) is a function of the saturated vapour pressure over the liquid and T is the temperature in degrees Celsius.
According to the definition, the relative humidity is calculated using the following formula:
Where RH is relative humidity in %, p(T) is absolute humidity (vapour pressure, mmHg), p0(T) is saturated vapour pressure over liquid, mmHg.
By substituting (2) for (3), the value of absolute humidity (vapour pressure over liquid) can be found:
Similarly, the tabulated data (saturated vapour density) can be used to convert absolute humidity into density units (g/m3). Further calculations were carried out for the temperature value T = 27 °С.
Relative humidity vs. time (Fig.4, b) and absolute humidity vs. time (Fig.6) curves have similar character. This is due to the constant for conversion of relative humidity to units of absolute humidity calculated for a temperature of 27 °С (Table 2).
According to the obtained data, the absolute humidity content was plotted as a function of the tensile strain of the HDPE films (Fig.7).
With increasing strain, permeability of the films increases non-linearly and reaches a plateau in the region of values for the case where there is no film but the cell is filled with water. Up to a strain rate of 200%, there is a symbiotic increase in vapour permeability and volume porosity of the polymer films. However, at higher tensile strain, when rearrangements of HDPE structure start occurring, accompanied by some reduction in porosity (Fig.7, curve 2), the formed interpenetrating open-porous structure of the film and its high vapor permeability are maintained. Similar features of the porous structure formed with the use of orientational stretching in SC-CO2 medium have been previously described in  for the case of liquid ethanol permeability. The experimental results obtained with the developed cell indicate good agreement with the results obtained using traditional techniques for determining the value of WVTR, previously used also for the films with similar porous structure .
A portable apparatus for qualitative assessment of the vapour permeability of film materials has been developed. The device was tested with HDPE films uniaxially deformed in SC-CO2. The response time of this system was shown to be less than 10 seconds. The time required for the measured values to reach the plateau is 40–50 minutes. The system accuracy and reliability are ensured by the design and software features of the cell (layer fixing, data synchronization, etc.) and high precision sensor equipped with a waterproof metal case that protects the sensor against contamination and mechanical damage. It was possible to establish the equilibrium value dependence of the relative and absolute humidity of water vapour after the HDPE film has passed through as a function of the degree of uniaxial deformation of the film in SC-SO2 medium.
This device and the developed technique can be recommended as an express method for qualitative changes in the vapour permeability of porous materials. Modularity of the system makes it possible to extend the measurement horizon and to use objects of different sizes (from 20 to 55 mm in diameter) with little modification of the design.
The study was completed with the financial support of the RSF, project No. 20-13-00178.
PEER REVIEW INFO
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.