Startseite Transparent conductive far-infrared radiative film based on polyvinyl alcohol with carbon fiber apply in agriculture greenhouse
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Transparent conductive far-infrared radiative film based on polyvinyl alcohol with carbon fiber apply in agriculture greenhouse

  • Beiting Wang EMAIL logo , Tzer Hwai Gilbert Thio und Hock Siong Chong
Veröffentlicht/Copyright: 28. September 2022
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Open Agriculture
Aus der Zeitschrift Open Agriculture Band 7 Heft 1

Abstract

In this study, a transparent conductive film is developed using polyvinyl alcohol as the substrate material and carbon fiber is deposited for electrical conductivity. The two materials are mixed into a solution and then cast to form a transparent conductive film suitable for usage in agricultural greenhouses. The designed film with a dimension of 200 mm × 200 mm has an average edge-to-edge resistance of 560.87 + 118.17 Ω, block resistance (BR) of 1.4 + 0.29 Ω/cm2, light transmittance of 70.07% over a wavelength of 400 to 780 nm, and a heating capability of 72 W/m2 via far-infrared light over a wavelength of 25–1,000 µm. Being highly transparent, the film can be integrated into the structure of agricultural greenhouses as it allows adequate sunlight penetration for the necessary photosynthesis of crops while providing heating capability during cold climates in seasonal regions such as northern China, thus replacing the need for conventional electrical heaters. A proof-of-concept is conducted at an agricultural greenhouse in Shandong, China, in rural settings where electricity may not be available. The films were powered with 200 custom-made aluminum-air (Al-air) batteries rated for 12 V, 20 mA. The electrolyte used for the batteries is potassium sulfate (K2SO4), which is a kind of agricultural chemical fertilizer that is easily available in agricultural greenhouse settings. For 7 weeks, the films were successfully powered by the batteries and operated to provide constant heating to maintain the nighttime temperature inside the greenhouse at above 10.06°C with outside temperatures dropping as low as 3.8°C.

Keywords: transparent conductive film; conductive film; transparent film; PVA; carbon fiber

1 Introduction

In recent years, researchers have developed a series of new transparent conductive films that can replace indium tin oxide (ITO), such as conductive polymers, carbon nanotubes (CNTs), graphene, and metal nanostructures [1]. ITO is a highly rigid and fragile material and is not suitable for greenhouse applications that require a flexible or stretchable material. Graphene, CNTs and metallic nanowires are gaining significant interest as potential replacements for the brittle and costly ITO transparent conductive material. Carbon-based nanomaterials on the other hand have a great degree of flexibility and stretchability, but when fabricated thinly to achieve high transparency, the conductivity is too low for heating applications. Improving higher conductivity typically results in higher material costs. In recent years, various researchers have reported the use of metallic nanowires as transparent electrode replacements [2,3,4,5,6,7]. Metallic nanomaterials have remarkable optical–electrical properties, low cost, easy to manufacture, flexible, and widespread applicability. There are various other kinds of stable transparent electrodes such as nanocomposites of dielectric/metal/dielectric( materials that are both optically transparent and have good electrical conductivity [8,9]. The recent research on the application of materials as transparent heaters has attracted intense attention from both scientific and industrial sectors [10,11,12,13]. While transparent conductive oxides have dominated the field for the past five decades, a new generation of transparent heating elements based on nanomaterials has led to new paradigms in terms of applications and prospects in the past years [14].

In a modern agriculture facility, the environment is monitored and parameters such as temperature, lighting, air composition, water, and fertilizer are tightly controlled to form a conducive environment for growing crops. In agricultural greenhouses, the transmittance of sunlight is essential for crop growth. Light transmittance reflects the percentage of direct sunlight from the environment that is passed through the greenhouse walls to the interior. As this percentage is affected by the incident angle of the solar radiation in different seasons, the necessary light transmittance needed for various crops differs, and materials used for the outer structure of agricultural greenhouses vary depending on the crops. In general, the transmittance of plastic greenhouses is 50–60%, glass greenhouses is 60–70%, and solar greenhouses can reach more than 70% [15,16]. The main production season of greenhouses is spring, summer, and autumn. Through heat preservation and ventilation and cooling, the temperature of the greenhouses is maintained at 15–30°C for growth [17].

The concept of heating greenhouses was first recorded in Korea in the 1400’s, as residents in that cold country realized that they could artificially complement the sun’s heat and improve crop yields [18]. In winter times, cold temperatures are often a limiting factor for plant growth [19]. The commonly used agricultural greenhouse warming methods are as follows: (i) adding a few heating fans in the shed, temporarily heating the area where the temperature is relatively low, which usually results in low humidity conditions, (ii) transferring the heat produced from adjacent buildings such as brewery or bathhouses, which can introduce costly infrastructure construction, and (iii) adding insulation to the greenhouse structure such as grass felt, which can result in poor ventilate and blockage of sunlight [20]. Greenhouse heating is the most energy-consuming operational requirement during winter periods, having a significant impact on planting costs [13]. Modern greenhouse heating systems are typically automated, with digital sensors and controls, and with regulated air circulation [21]. Waddington in “7 Innovative Ways To Heat Your Greenhouse In Winter” specified that in places where winter temperatures regularly drop well below freezing point, some heating might be necessary to allow for year-round planting of food crops [22]. Around the world, 35% of the energy consumed in agricultural production is mainly due to greenhouse heating [23].

In our previous research, “Transparent Conductive Far-Infrared Radiative Film based on Cotton Pulp with carbon fiber (CF) in Agriculture Greenhouse,” we developed a transparent conductive film by depositing conductive CF cotton pulp substrate [24]. The developed film is a suitable energy-efficient replacement to the conventional electrical heaters used in current agricultural greenhouse to meet the temperature needs of crop growth. This study expands on the previous research to utilize polyvinyl alcohol (PVA) as the substrate. Figure 1 shows an example SEM image of the conductive network formed by CF in the PVA matrix.

Figure 1 SEM image of the conductive network formed by of CF in PVA matrix [29].
Figure 1

SEM image of the conductive network formed by of CF in PVA matrix [29].

PVA is environmentally friendly and can be broken down by microorganisms in the natural environment into carbon dioxide and water [25,26]. Using CF as the conductive material, the resulting film is flexible and highly transparent which allows for sunlight penetration into a greenhouse when used as the retaining structure. The film is also conductive, and when copper conductor strips are installed for electric power delivery to the film, enabling heating via radiated far infrared for greenhouse temperature control. The durability of such conductors in long-time water contact has been proven in studies involving various materials such as PVA, Polyaniline, and CNTs [27]. The studies have shown that the conductors maintained long-term electrochemical performance and flexibility after long-time water contact in supercapacitor applications. In another study, high-performance capacitive-type humidity sensors based on polymeric PVA/titanium carbon nanofibers displayed excellent stable long-term capacitive humidity response with good linearity and repeatability [28]. The transparent conductive film with high light transmittance and good electrical conductivity makes it suitable for integration into the enclosure structure of agricultural greenhouses while providing heating at the same time via far infrared during cold weather. The film constructed of the combination of CF and PVA is flexible, transparent, and conductive. In particular, the natural degradation yet durable characteristics in the field of agricultural applications shows unparalleled superiority.

2 Materials and methods

PVA film has a smooth surface and good gas permeability and is flexible, oil resistant, solvent resistant, and wear resistant. In general, PVA films have the following properties: refractive index of 1.49–1.52, thermal conductivity of 0.2 W/(m K), specific heat capacity of 1 to 5 J/(kg K), resistivity of (3.1–3.8) × 107 Ω cm, and soluble in water when heated to 65–75°C [30]. PVA is non-toxic, tasteless, harmless to the human body, has a good affinity with the natural environment and does not accumulate, and is pollution-free. After special treatment, PVA film is also water resistant and has a wide range of uses.

In this study, CF is added to a PVA film to enable infrared radiation functionality in the PVA film. The resultant film is also anti-static and has electromagnetic shielding properties. In the industry, PVA films are generally manufactured with the use of a casting machine with a roller press. For cost-effectiveness, this study adapts the wet method to fabricate the film.

In general terms, the wet method mainly involves the mixing of CF filament in diluted PVA and glycerin solution and cast into the film. The specific quantity of the mixture is shown in Table 1, and the process of producing a PVA-CF film is summarized in Figure 2.

Table 1

Experiment materials quantity for PVA-CF

Materials Quantity
PVA 30 g
CF 0.1–1.0 g
Water 385 mL
Glycerin 15 mL
Figure 2 Process of fabricating PVA-CF transparent conductive film.
Figure 2

Process of fabricating PVA-CF transparent conductive film.

2.1 Experimental setup

Figure 2 shows the wet method process of fabricating a PVA-CF film. First, CF filament is mixed in diluted PVA and glycerin solution. The addition of glycerin ensures that the resulting film is flexible and not rigid. The mixture is then slowly blended for up to 2.5 h to ensure the CF filament is dispersed evenly. The mixture is then cast in a mold and left for air-drying for up to 48 h in ambient room temperate. The resulting product is a thin and flexible PVA-CF transparent conductive film. Note that this method is only suitable for fabricating small-sized films.

For the application of the developed film for an agricultural greenhouse, two custom-made specialized fabrication machines are constructed to produce the film in larger sizes. The two fabrication machines are, respectively, shown in Figures 3 and 4. The first machine is used for the fabrication of the basic flexible conductive PVA-CF films, while the second machine is then used to apply metal copper foil strips for electric current delivery to the film and to laminate the film to provide electrical insulation and improve the durability of the film.

Figure 3 Fabrication machine for basic flexible conductive PVA-CF: (1) mixer drum for mixing and blending of PVA, CF, and glycerin; (2 and 4) pump; (3) vacuum cavitation cylinder; (5) horizontal slit funnel; (6) heated conveyor steel belt; (7) PVA-CF film; and (8) winding machine.
Figure 3

Fabrication machine for basic flexible conductive PVA-CF: (1) mixer drum for mixing and blending of PVA, CF, and glycerin; (2 and 4) pump; (3) vacuum cavitation cylinder; (5) horizontal slit funnel; (6) heated conveyor steel belt; (7) PVA-CF film; and (8) winding machine.

Figure 4 Modified food packaging machine for application of metal copper foil strips and protection laminate. Blue line indicates the feed path of the film: (1) current-carrying, (2) PVA-CF film roller, (3) top laminate, (4) glue applicator roller box, (5) guide roller, (6) forced convection oven, (7) button laminate, (8) hot drum, and (9) winding machine.
Figure 4

Modified food packaging machine for application of metal copper foil strips and protection laminate. Blue line indicates the feed path of the film: (1) current-carrying, (2) PVA-CF film roller, (3) top laminate, (4) glue applicator roller box, (5) guide roller, (6) forced convection oven, (7) button laminate, (8) hot drum, and (9) winding machine.

As shown in Figure 3, to produce the basic flexible conductive PVA-CF films, the mixture of CF filament in diluted PVA and glycerin solution is first blended in a mixer drum for 2.5 h. The mixture is then pumped into a vacuum cavitation cylinder that removes all air bubbles from the mixture solution before the mixture is pumped and squirted through a horizontal slit funnel onto a 10-meter length conveyor steel belt that is heated to 80°C. The mixture which is spread over the metal hot plate is then left to air-dry as it is slowly transferred along the heated steel belt for up to 5 min before being winded into a roll.

Figure 4 shows a modified food packaging machine used for the application of metal copper foil strips and protection laminate. First, the current-carrying metal copper foil strips are aligned and rolled onto the edges of the roll of conductive PVA-CF film. The top laminate is then applied onto the aligned copper foil strips and PVA-CF film to form a two-layer film. Next, the two-layer film is fed through a glue applicator roller box and then sent to a forced convection oven for drying. Finally, the bottom laminate is applied to the two-layer film and sent through two hot drums for the final pressing to obtain the finished laminated film. The result is a durable PVA-CF film that can be applied directly in humid and warm agricultural greenhouse settings.

2.2 Experimental method

As described in the previous section, the PVA-CF film is first cast, and then, the conductive metal strips and laminate are applied. The full process is shown in Figure 5.

Figure 5 Industrial process of fabricating PVA with CF transparent conductive film.
Figure 5

Industrial process of fabricating PVA with CF transparent conductive film.

Prior to the actual fabrication of the films for testing, an initial experiment to determine a suitable CF filament length is performed. 0.3 g of CF filaments of 3, 6, and 10 mm length are, respectively, cut and mixed with 30 g of PVA, 15 mL of glycerin, and 385 mL of distilled water. The solution is blended for 2 h and the left to settle for 2 h for the air bubbles to disperse. The uniformity of the CF filament dispersion in the mixture is then observed. Next, three samples were cast using the three mixtures, and the CF filament dispersion is further observed in the produced films.

To determine if the films are suitable for both heating and usage as building material in agricultural greenhouses, various film samples are fabricated at different CF-to-PVA ratios of 0.1:30 (0.33%), 0.2:30 (0.66%), 0.3:30 (1.0%), 0.6:30 (2.0%), and 1.0:30 (3.33%). The thickness, edge-to-edge resistance (EER), block resistance, light transmittance, and material resistivity are observed. While the resistance is measured using a regular multimeter, light transmittance is tested using LH-206 Optical Transmittance Meter made by Tianjing China.

Finally, to determine the effectiveness of the fabricated conductive film in the agricultural greenhouse, films were fabricated at CF-to-PVA ratio of 0.3:30 (1.0%). The film has the following properties: EER of 560.87 Ω, power rating of 100.83 W at 220 V, and dimension of 1,000 mm × 500 mm (Figure 6(a)). A custom 9 m × 4 m × 4 m agricultural greenhouse is used as the test bed of the films. A total of 26 sheets of the fabricated film are applied to the side walls of the greenhouse (Figure 6(b)), and the temperature difference is observed.

Figure 6 PVA-CF light transparent conductive film be installed on the greenhouse wall: (a) PVA-CF transparent conductive film; (b) installation of 26 sheets of PVA-CF to the custom-built agricultural greenhouse.
Figure 6

PVA-CF light transparent conductive film be installed on the greenhouse wall: (a) PVA-CF transparent conductive film; (b) installation of 26 sheets of PVA-CF to the custom-built agricultural greenhouse.

2.3 Aluminum-air batteries apply to agriculture greenhouse

Agricultural greenhouses are mainly located in rural areas and suburbs, where electricity and other forms of energy are in short supply. These rural areas are typically not serviced by national power grids [31]. Therefore, a custom-made aluminum (Al) air battery system is used to power the heating of the PVA-CF film.

The chemical reaction of Al-air cells is like Zinc-air cells. Typical Al-air cells use high-purity 99.99% Al as the negative electrode, oxygen as the positive electrode, and potassium hydroxide (KOH) or sodium hydroxide (NaOH) aqueous solution as the electrolyte [25]. The schematic diagram of an Al-air battery is shown in Figure 7. During operation, the Al electrode combines with the hydroxide solution to form aluminum hydroxide.

Figure 7 Schematic diagram of aluminum-air battery [16].
Figure 7

Schematic diagram of aluminum-air battery [16].

The biggest advantage of Al-air batteries is that they are self-sufficient and do not require regular electrical charging. The resulting aluminum hydroxide solution generated in the used batteries can be sent to a recycling unit to get 100% aluminum back, making the batteries 100% recyclable and thus safe for the environment. Another advantage of Al-air batteries is the use of water-based electrolyte that is free of toxins, as opposed to lithium-ion technology which uses highly flammable organic toxins-based electrolytes [32].

In this study, Al-air batteries are made using potassium sulfate (K2SO4) as the electrolyte. K2SO4 is high-quality and efficient potassium fertilizer without chlorine. While Al-air batteries with K2SO4 electrolyte have a lower energy density than Al-air batteries with KOH or NaOH electrolyte, they are environmentally friendly while still achieving long battery life.

For the proof-of-concept in the agricultural greenhouse, 200 Al-air batteries using K2SO4 as the electrolyte are manufactured to provide 12 V at 20 mA. Each battery has a size of 18 mm diameter by 200 mm height and is rated for 0.6 V, 0.2 mA. The 200 Al-air batteries have been successful in powering the PVA-CF transparent conductive film for 7 weeks. The capacity of the batteries during the operation period is calculated as 235.2 Ah. Figure 8 shows the implementation of the 200 batteries to dive a 1,000 mm by 500 mm PVA-CF transparent conductive film with EER of 413.92 ± 118.16 Ω.

Figure 8 Aluminum-air batteries with K2SO4 electrolyte (a) compose in 200 single aluminum air batteries with 12 V and 20 mA for PVA-CF transparent conductive film; (b) to parallel connect 10 single battery and series connect 20 battery groups and all in one.
Figure 8

Aluminum-air batteries with K2SO4 electrolyte (a) compose in 200 single aluminum air batteries with 12 V and 20 mA for PVA-CF transparent conductive film; (b) to parallel connect 10 single battery and series connect 20 battery groups and all in one.

3 Results and discussion

Table 2 shows the ratio of the components used in the fabrication of the various film samples for determining the suitable CF filament length for fabricating PVA-CF films. Figure 13 shows the dispersion of the CF filaments in the PVA solution while Figure 9 shows the resulting film. The casting mold used for the test fabrication is shown in Figure 10(a).

Table 2

Mixture ratio for fabrication of films with CF of various lengths

Materials CF-3mm CF-6mm CF-10mm
CF (g):PVA (g) 0.3:30 0.3:30 0.3:30
Glycerin (mL/g) 15/18.95 15/18.95 15/18.95
H2O (mL) 385 385 385

Glycerin concentration is 1.26362 g/mL. Stir mixing time is 2 h.

Figure 9 Different length CF dispersing in PVA solution: (a) 3 mm, (b) 6 mm, and (c) 10 mm.
Figure 9

Different length CF dispersing in PVA solution: (a) 3 mm, (b) 6 mm, and (c) 10 mm.

Figure 10 Cast film with CF in PVA solution: (a) casting mold, (b) film with 3 mm CF filament, (c) film with 6 mm CF filament, and (d) film with 10 mm CF filament.
Figure 10

Cast film with CF in PVA solution: (a) casting mold, (b) film with 3 mm CF filament, (c) film with 6 mm CF filament, and (d) film with 10 mm CF filament.

In Figure 9, the dispersion of CF filament in PVA solution seems uniform regardless of fiber length. However, after casting, the dispersion of CF filament varies. From Figure 10(b) and (c), it is observed that 3 mm CF filaments disperse well in the PVA, while the distribution of the 6 mm filaments is very uneven, and lumpy agglomerations have formed throughout the casted film. Meanwhile, Figure 10(d) shows that the 10 mm filaments are hardly dispersed in PVA solution and the fibers are entangled into groups, which is expected to interfere with the process of forming thin conductive films.

As the films are applied to the agricultural greenhouse as heating films with high light transmittance, the film needs to be thin with uniform conductivity. Overall, the experiment result shows that the CF filaments of lengths 6 and 10 mm do not disperse uniformly in the PVA solution and are not suitable for the fabrication of thin transmittance film with uniform conductivity. In conclusion, the optimum CF length for film fabrication is 3 mm as it produced the film with good uniformity due to the good dispersion of the CF in the PVA solution. All other experiments were conducted using only 3 mm CF filaments.

Table 3 shows the results of the 12 samples of pure PVA films and PVA-CF films tested for light transmittance and EER. While the light transmittance is measured to be 75.58% over a wavelength of 400 to 780 nm, the EER is measured as 2144.16 + 597.25 Ω with a calculated block resistance of 5.173 + 1.4931 Ω/cm2. It is noted that in general while PVA film is superior to other films in glossiness and transparency when compared to common cellophane (PT) and PVC film, PVA film has reflectivity property and light transmittance that is higher by 20% and 50%, respectively [33].

Table 3

Properties of fabricated pure-PVA with PVA-CF film

No. Pure-PVA PVA-CF
LT (%) LT (%) EER (Ω)
1 91.02 77.80 3,140
2 87.33 74.92 2,140
3 89.76 75.15 2,220
4 89.12 80.18 1,750
5 88.83 74.92 2,140
6 90.60 78.49 1,410
7 90.73 76.63 1,950
8 88.43 79.87 3,170
9 90.73 69.13 1,340
10 90.57 75.66 1,840
11 90.30 72.94 1,900
12 88.73 71.30 2,730
Mean 89.68 75.58 2144.16
Average deviation (A.D.) ±1.169 ±3.33 ±597.25

Sample size is 200 mm x 200 mm. CF filament length is 3 mm, CF-to-PVA ratio is 0.1:30. LT denotes light transmittance while EER denotes edge-to-edge resistance. Pure PVA film is nonconductive and has an infinite edge-to-edge resistance. A sample calculation of using 220 V and Block Resistance of 419.9208 Ω gives a heating capability of 115.26 W/m2 via far-infrared light.

The results in Table 3 show that pure PVA films have an average light transmittance of 89.68%, and the PVA-CF films have a lower average light transmittance of 75.58%. This is easily explained by the added CF blocking light transmitted through the film. While the value is lower, it is still suitable for solar greenhouse. Table 3 shows that the average EER measured is 2144.16 + 597.25 Ω, which is suitable for the conversion of electrical energy to heat; the tradeoff of 14.1% light transmittance for the ability to perform heating is justifiable. A discussion of the generated heat will be presented in a later section.

Note that while electrical conductivity measurement can be affected by temperatures, at above standard room temperatures conductors generally show pure metallic behavior [34]. Polymers on the other hand have shown good optical and electrical properties in various applications at temperatures of room [35]. The studies confirm that the variation in electrical conductivity measurement such as resistance values due to temperature effects can be ignored.

Table 4 shows the effects of various CF-to-PVA ratios on the EER (Ω), block resistance (Ω), and light transmittance (%) properties of the PVA-CF film. The effect of solution volume on the achievable thickness of the final molded film is also presented in Table 4.

Table 4

Effects of CFs to PVA ratio on film properties

Film label CF:PVA H2O + Glycerin added TH EER Block resistance LT Material resistivity (ρ)
(%) (mL) (mm) (Ω) (Ω/cm2) (%) (Ω/mm)
Sample-01 0.1:30 80 0.097 945,000.00 2,362.50 84.89 58,590.00
0.33% 100 1.05 756,000.00 1,890.00 80.08 769,446.00
120 0.903 604,800.00 1,512.00 73.63 546,134.40
140 1.0535 483,840.00 1,209.60 70.7 509,725.44
Mean 110 0.77587 697,410.00 1,743.53 77.325 470,973.96
Average deviation (AD) ±25.8198 ±0.4759 ±199099.94 ±497.75 ±6.3865 ±297935.60
Sample-02 0.2:30 80 0.632 1,260.00 3.15 75.08 758.52
0.66% 100 0.7525 1,008.00 2.52 72.22 758.52
120 0.903 806.4 2.02 69.29 728.1792
140 1.0505 645.12 1.61 67.07 679.6339
Mean 110 0.8345 929.88 2.325 70.915 731.213275
Average deviation (AD) ±25.8198 ±0.1942 ±265.4665 ±0.6640 ±3.4868 ±37.2422
Sample-03 0.3:30 80 0.607 560.87 1.4 70.07 337.6437
1.00% 100 0.7505 448.696 1.12 60.72 325.6437
120 0.923 358.956 0.9 59.83 324.1379
140 1.0530 287.165 0.72 55.08 302.5287
Mean 110 0.833375 413.92175 1.035 61.425 322.4885
Average deviation (AD) ±25.8198 ±0.1942 ±118.1687 ±0.2932 ±6.2725 ±14.6144
Sample-04 0.6:30 80 0.605 430.25 1.08 62.36 259.0105
2.00% 100 0.7523 344.2 0.86 59.98 259.0105
Mean 120 0.913 275.36 0.69 58.74 248.65
140 1.0525 220.28 0.55 56.98 232.0734
110 0.8307 317.52 0.79 59.515 249.6861
Average deviation (AD) ±25.8198 ±0.1942 ±90.6512 ±0.2284 ±2.2610 ±12.7170
1.0:30 80 0.632 380.56 0.9514 51.04 229.09
Sample-05 3.33% 100 0.723 304.44 0.7611 45.25 229.09
120 0.913 243.55 0.608875 28.22 219.93
140 1.044 194.84 0.4871 23.08 205.27
Mean 110 0.828 280.85 0.70 36.89 220.85
Average deviation (AD) ±25.8198 ±0.1944 ±80.1822 ±0.2004 ±13.365 ±11.2454

TH denotes thickness of the film. LT denotes light transmittance under LED lightning at average 5395.5 Lux. EER (Ω) is comparable to block resistance (Ω) as the samples are square shaped. Specimen size is 200 mm × 200 mm, and CF length is 3 mm.

From the first set of results in Table 4 (Sample-01), it is observed that a low CF-to-PVA ratio of 0.1:30 (0.33%) produces film samples with average EER of 697.5 ± 199.09 kΩ and average achievable light transmittance of 77.325 ± 6.3865%. While the light transmittance is optimum, the low range of conductivity is not suitable for infrared applications in agricultural greenhouse settings.

On the other hand, from the last set of results in Table 4 (Sample-05), when a high CF-to-PVA ratio of 1.0:30 (3.33%) is applied the extreme opposite effects are observed. The EER average value is 280.85 ± 80.18 Ω, and the average achievable light transmittance is only 36.89 ± 13.3665%. While the resistance is suitable for the infrared application, the light transmittance is too low for greenhouse integration.

In general, for infrared applications, an EER below 1000 Ω is suitable, while for agricultural greenhouse applications, a light transmittance value of 50–70% is comparable to plastic and glass greenhouses. For the results shown in Table 4, it can be seen that film samples with CF-to-PVA ratios of 0.2:30 (0.66%), 0.3:30 (1.0%), and 0.6:30 (2.0%) have EERs suitable for infrared applications while having acceptable light transmittance values.

3.1 Relationship of block resistance (Ω/cm2) with film thickness (mm)

Figures 11–15 show the relationship of block resistance (Ω/cm2) with film thickness (mm) for the various samples fabricated with varying CF:PVA ratios (Table 4). The analysis of this relationship will facilitate the selection of suitable film for use as a heating element for agricultural greenhouses. It is observed that in general, films with a CF:PVA ratio of at least 0.3:30 (1.0%) such as those of Sample-03, -04, and -05 have suitable EERs of 1,000 Ω and below across all film thickness range, while films with a CF:PVA ratio of 0.2:30 (0.66%) such as those from Sample-02 are only suitable when the thickness is greater than 0.7525 mm, and films with a CF:PVA ratio of 0.1:30 (0.33%) such as those from Sample-01 have extremely high EERs and are not suitable across all film thickness range.

Figure 11 Sample-01 (CF:PVA = 0.1:30) block resistance (Ω/cm2) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.
Figure 11

Sample-01 (CF:PVA = 0.1:30) block resistance (Ω/cm2) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.

Figure 12 Sample-02 (CF:PVA = 0.2:30) block resistance (Ω) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.
Figure 12

Sample-02 (CF:PVA = 0.2:30) block resistance (Ω) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.

Figure 13 Sample-03 (CF:PVA = 0.3:30) block resistance (Ω) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.
Figure 13

Sample-03 (CF:PVA = 0.3:30) block resistance (Ω) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.

Figure 14 Sample-04 (CF:PVA = 0.6:30) block resistance (Ω) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.
Figure 14

Sample-04 (CF:PVA = 0.6:30) block resistance (Ω) versus thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.

Figure 15 Sample-05 (CF:PVA = 1.0:30) block resistance (Ω) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.
Figure 15

Sample-05 (CF:PVA = 1.0:30) block resistance (Ω) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted curve.

3.2 Relationship of light transmittance (%) with film thickness (mm)

Figures 16–20 show the relationship of Light Transmittance (%) vs Film Thickness (mm) for the various samples fabricated with varying CF:PVA ratios (Table 4). The analysis of this relationship will facilitate the selection of suitable film for use as a building material for agricultural greenhouses. As shown, light transmittance reduces with an increase in film thickness. In other words, the transmittance of the film is inversely proportional to the thickness of the film. To achieve suitable light transmittance, thinner films are necessary. From the results, the best film thickness is 0.602 mm when the light transmittance is 70.07%, and the EER is 560.870 Ω (Sample-03).

Figure 16 Sample-01 (CF:PVA = 0.1:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 16

Sample-01 (CF:PVA = 0.1:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 17 Sample-02 (CF:PVA = 0.2:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 17

Sample-02 (CF:PVA = 0.2:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 18 Sample-03 (CF:PVA = 0.3:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 18

Sample-03 (CF:PVA = 0.3:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 19 Sample-04 (CF:PVA = 0.6:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 19

Sample-04 (CF:PVA = 0.6:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 20 Sample-05 (CF:PVA = 1.0:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 20

Sample-05 (CF:PVA = 1.0:30) light transmittance (%) versus film thickness (mm) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

3.3 Relationship of light transmittance (%) with block resistance (Ω/cm2)

Figures 21–25 show the relationship of light transmittance (%) with block resistance (Ω/cm2) for the various samples fabricated with varying CF:PVA ratios (Table 4). Films of Sample-01 with a CF:PVA ratio of 0.1:30 (0.33%) can achieve light transmittance comparable to a solar greenhouse across all film thickness ranges, while films of Samples-02 with a CF:PVA ratio of 0.2:30 (0.66%) are able to accomplish light transmittance of solar and glass greenhouses as the thickness varies from a low of 0.6 mm to a high of 1.05 mm. On the other hand, films from Sample-03, -04, and -05 with CF:PVA ratios of 0.3:30 (1%) and higher have wide-ranging light transmittance from 70% down to 23%. Designing films at these high CF:PVA ratios would require close monitoring of the achievable light transmittance at different desired film thicknesses.

Figure 21 Sample-01 (CF:PVA = 0.1:30) light transmittance (%) versus block resistance (kΩ) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.
Figure 21

Sample-01 (CF:PVA = 0.1:30) light transmittance (%) versus block resistance (kΩ) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.

Figure 22 Sample-02 (CF:PVA = 0.2:30) Light Transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.
Figure 22

Sample-02 (CF:PVA = 0.2:30) Light Transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.

Figure 23 Sample-03 (CF:PVA = 0.3:30) light transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.
Figure 23

Sample-03 (CF:PVA = 0.3:30) light transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.

Figure 24 Sample-04 (CF:PVA = 0.6:30) light transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.
Figure 24

Sample-04 (CF:PVA = 0.6:30) light transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.

Figure 25 Sample-05 (CF:PVA = 1.0:30) light transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.
Figure 25

Sample-05 (CF:PVA = 1.0:30) light transmittance (%) versus block resistance (Ω) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted line.

To determine a film with acceptable light transmittance for use as a building material for an agricultural greenhouse, while having good EER for infrared applications requires further study of the relationship between light transmittance and CF content. Light transmittance increases with block resistance. Higher block resistance is a result of lower CF content, which increases the film’s transparency as the ratio of the PVA material, which has high light transmittance property, increases.

3.4 Relationship of material resistivity, ρ (Ω/mm), with light transmittance (%)

Figures 26–30 show the relationship of Material resistivity, ρ (Ω/mm), with light transmittance (%) for the various samples fabricated with varying PVA:CF ratios (Table 4). In general, material resistivity increases with the increase in light transmittance. This is in agreement with the results shown in Figures 26–30, as material resistivity is a function of the carbon percentage content of fiber content which blocks light transmission through the PVA-based films.

Figure 26 Sample-01 (CF:PVA = 0.1:30) material resistivity ρ(Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 26

Sample-01 (CF:PVA = 0.1:30) material resistivity ρ(Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 27 Sample-02 (CF:PVA = 0.2:30) material resistivity ρ (Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 27

Sample-02 (CF:PVA = 0.2:30) material resistivity ρ (Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 28 Sample-03 (CF:PVA = 0.3:30) material resistivity ρ (Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 28

Sample-03 (CF:PVA = 0.3:30) material resistivity ρ (Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 29 Sample-04 (CF:PVA = 0.6:30) material resistivity ρ (Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 29

Sample-04 (CF:PVA = 0.6:30) material resistivity ρ (Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

Figure 30 Sample-05 (CF:PVA = 1.0:30) material resistivity ρ(Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.
Figure 30

Sample-05 (CF:PVA = 1.0:30) material resistivity ρ(Ω mm) versus light transmittance (%) with equation for PVA-CF. The dotted lines represent the polynomial fitted curve and the linear fitted.

3.5 Simple equations on the CF:PVA of 0.3:30 (1%) film

In Figures 15–30, the various design equations obtained through regression analysis are also presented. The equations allow us to match corresponding film thickness and required CF:PVA ratios to desired light transmittance and resistance values. As the light transmittance and resistance properties of a film are highly dependent on the film thickness, which in turn is affected by the CF:PVA ratios, a set of equations is required for each film with its own specific CF:PVA ratio.

As an example, below is a compilation of the equations for CF:PVA of 0.3:30 (1%) film samples:

(1) BR TH , 03 = 1.1143 * TH 03 2 − 3.357 * TH 03 + 3.0192

(2) LT TH , 03 = − 455.67 * TH 03 3 + 1038.8 * TH 03 2 − 839.65 * TH 03 + 298.47

(3) LT 03 = 16.121 * BR 03 2 − 13.925 * BR 03 + 57.517

(4) MR LT , 0 . 3 = − 0.1997 * LT 03 2 + 27.325 * LT 03 − 596.7

where BR is the block resistance in Ω/cm2, TH is the film thickness in mm, LT is light transmittance in %, and MR is material resistivity in Ω/mm.

Equations (1) and (2) allow for the calculation of light transmittance and block resistance given the film thickness, while equations (3) and (4) relate Light Transmittance to Block Resistance and Material Resistivity to Light Transmittance. As the thickness of a film can be controlled in an industrial manufacturing setting, the equations can be used to determine the achievable block resistance and light transmittance. Table 5 shows a design using this set of equations for a desired thickness of 0.8 mm.

Table 5

Sample design calculations for Sample-03 CF:PVA = 0.3:30 (1%) at TH = 0.8 mm

Relationship Equation R 2 Calculate Value
BR vs TH BR TH , 03 = 1.1143 ⁎ TH 03 2 − 3.357 ⁎ TH 03 + 3.019 (equation (1)) 1 BRTH,03 = 1.0465 Ω/cm2
LT vs TH LT TH , 03 = − 455.67 ⁎ TH 03 3 + 1038.8 ⁎ TH 03 2 − 839.65 ⁎ TH 03 + 298.47 (equation (2)) 1 LTTH,03 = 58.2790%
LT vs BR LT BR , 03 = 16.121 ⁎ BR 03 2 − 13.925 ⁎ BR 03 + 57.517 (equation (3)) 0.9878 LTBR,03 = 60.599%
MR vs LT MR LT , 03 = − 0.1997 ⁎ LT 03 2 + 27.325 ⁎ LT 03 − 596.7 (equation (4)) 0.9885 MRLT03 = 311.5902

BR – block resistance; TH – film thickness; LT – light transmittance; and MR – material resistivity. R 2 is an indicator of the fitting degree of the equation to the data set when performing linear regression. A value of 1 indicates a fit of high reliability.

In Table 5, it is observed that the light transmittance calculated using equations (2) and (3) differs by 2.32%. This is due to the less-than-ideal R 2 value in the linear regression when obtaining equation (3).

In general, films with CF/PVA ≤ 0.33% has high EER (which translates to low conductivity) and are not suitable for far-infrared heating applications. However, films with CF/PVA ≥ 3.33% have light transmittance values that are too low for greenhouse building integration.

For the proof-of-concept conducted in greenhouse settings, films fabricated with CF/PVA of 1.0%, with a block resistance of 1.4 Ω/cm2 and light transmittance of 70.07% are selected due to the optimum balanced conductivity and light transmittance values (Sample-03 in Table 4). The films are supplied with an input power of 2,600 W (72 W/m2) and monitored at 12 h interval at 10am and 10 pm daily. The results are shown in Table 6. From the results, it is observed that while the ambient temperature outside the greenhouse is measured to be 3.8 to 14.23 + 4.69°C, the PVA-CF film is able to maintain an indoor temperature of 10.06 to 12.33 + 2.50°C.

Table 6

Data of Test warming temperature in greenhouse at morning 10:00 and night 10:00

Date (11/2011) In outside greenhouse air temperature (°C) Test warming temperature in greenhouse (°C)
The highest The lowest 10:00 am 10:00 pm
1 15 6 12 8
2 16 9 12 9
3 20 9 15 10
4 22 11 16 11
5 21 14 16 16
6 19 12 16 12
7 14 −3 12 8
8 5 −1 10 8
9 10 2 8 8
10 12 3 10 8
11 11 2 9 8
12 13 2 10 8
13 17 3 12 12
14 17 4 12 12
15 15 4 12 12
16 15 5 13 12
17 18 7 14 12
18 20 6 16 14
19 18 8 16 14
20 18 10 16 12
21 11 0 12 12
22 3 −3 10 8
23 9 −1 7 8
24 13 −2 12 8
25 14 −1 12 8
26 12 −1 12 8
27 13 4 12 8
28 14 5 12 10
29 16 2 14 10
30 6 −2 10 8
Mean 14.23 3.8 12.33 10.06
Average deviation (AD) ±4.63 ±4.69 ±2.50 ±2.33

Warming time night 10:00 to next morning 10:00 every day.

This illustrates that the PVA-CF transparent conductive film is suitable for greenhouse applications during wintertime

4 Conclusion

This study has presented the design of transparent conductive films with PVA as substrate material and CF deposits for enabling far infrared heating in agriculture greenhouse settings for temperature control to promote crop growth.

The suitability of CF lengths has been studied and is found that a shorter length of 3 mm provides a most homogenous CF and PVA mixture when casting the film. In comparing between pure PVA and PVA-CF films, it is observed that the light transmittance has dropped by 14.1%, which is an acceptable tradeoff for the added heating capability introduced to the film. The testing of various films with a CF/PVA ratio of 0.33–3.33% has shown that a low CF/PVA ratio produces films with good light transmittance values that meet the specifications of solar greenhouses, while a higher CF/PVA ratio produces films that are suitable for far infrared heating purposes.

An optimum CF/PVA ratio of 1.0% was finally used to develop the film for use in the proof-of-concept application in greenhouse settings. The film has an EER of 560.87 + 118.17 Ω, block resistance of 1.4 + 0.29 Ω/cm2, light transmittance of 70.07% over a wavelength of 400–780 nm, and has a heating capability of 72 W/m2 via far-infrared light over a wavelength of 25–1,000 µm. The proof-of-concept experimental work has also demonstrated the durability of the PVA-CF films in heated and humid conditions. Generally, polymers suffer from degradation, especially when under prolonged heating. In the PVA-CF films, the PVA molecular chain contains many hydroxyl groups and the strong intermolecular hydrogen bond interaction results in a high melting point close to the decomposition temperature of 262.25°C [28]. As the PVA-CF films are operated at temperatures below 30°C, no pyrolysis phenomenon will occur.

In summary, this study has provided a deep insight into the development of PVA-CF film via casting and the results have showed that in greenhouse settings PVA-CF film has good light transmittance allowing sunlight penetration for the photosynthesis process of crops, has good conductivity for far infrared heating capability, and is flexible and durable for use as an integration building material in agriculture greenhouse.

Acknowledgments

The authors would like to thank the Faculty of Engineering, Built Environment and Information Technology (FoEBEIT), SEGi University for supporting the research work. The authors would also like to thank Xiaofei Wang from Beijing Biyan Special Materials Co., LTD for her help and support in fabricating the CF-cotton pulp conductive film under industry manufacturing conditions. The authors would also like to thank Shuting Yu and Mingqing Lu from Shangdong Hengtian Special Materials Co., LTD for their help and support in building the equipment for the transparent conductive process. The authors would also like to thank the Yuquanwa National Demonstration Center of Efficient Agriculture for providing the use of the agricultural greenhouse site for the practical testing of the transparent conductive film. Tzer Hwai Gilbert Thio would like to acknowledge the support of research fund from SEGi University (SEGiIRF/2018-11/FoEBE-18/81).

  1. Funding information: The study was financed with sources of SEGi University (SEGiIRF/2018-11/FoEBE-18/81).

  2. Conflict of interest: The authors state no conflict of interest.

  3. Data availability statement: All data generated or analyzed during this study are included in this published article

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Received: 2022-07-05
Revised: 2022-08-25
Accepted: 2022-09-03
Published Online: 2022-09-28

© 2022 Beiting Wang et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/opag-2022-0139
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transparent conductive film; conductive film; transparent film; PVA; carbon fiber
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Artikel in diesem Heft

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