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NEW INTERFERENCE FILM FOR CLIMATE CONTROL

I. Verlodt and P. Verschaeren Hyplast N.V.

Sint Lenaartseweg, 26, 2320 Hoogstraten, Belgium

Abstract : The new interference film Kool Lite Plus is a photoselective film, developed by Hyplast/Klerk's to reduce the extreme temperatures occurring in greenhouses in regions with temperate climates and hot summers. The Kool Lite Plus film contains a new type of pearlescent pigment, based on the same principle of interference as for the pigments used in the commercial Kool Lite/Astrolux® film. The new pigments have specifically been designed to reflect solar heat (NIR) without influencing the light transmission (PAR) too much.

During the winter 1996 and spring 1997, the new film was tested in the South of Tunisia, on a tomato crop grown under the standard tunnels used in this region. During this trial, temperatures and yields were registered under a reference tunnel covered by a standard thermic film, as well as under the Kool Lite/Astrolux®, and the new Kool Lite Plus film. The last results of April have shown an average difference of 7.5°F at noon and in the early afternoon.

Keywords : plastics, heat control, selective reflection, light quality, tomato

Introduction

In its continuous search for the most appropriate films for each climate and cultural method, Hyplast/Klerk's is focusing its research on the development of photoselective films. This research is mainly based on the use of the so-called interference films, jointly developed and patented by Hyplast/Klerk's and Merck KGaA, Germany. The theory of interference (4, 5) and the possible applications of the interference pigments in agricultural films have already been presented in the U.S. (1). The first step was the development of a film reflecting selectively a part of the near infrared (NIR) and green light (2, 6). Kool Lite/Astrolux® is now widely commercialized for climate control in regions with high irradiation (in the tropics). For the regions with hot summers, but lower irradiation during the winter (e.g. the Mediterranean), the Kool Lite/Astrolux® film has shown to be less adequate for certain crops. Therefore, a new interference film with a combined higher light transmission and at least the same cooling effect has been developed. Consequently, this film can be used throughout the whole year in these regions as well. This new experimental interference film has been tested with positive result on a tomato production crop in the South of Tunisia.

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Methods and Materials

Material design

The pearlescent pigments used in the so-called interference films consist of three single non absorbent layers including the central part of the pigments which serves as a substrate. The substrate is a fine mica platelet, and is covered with a fine coating of metal oxides (Ti02). The color of these pigments is essentially determined by the thickness of the coating. In the case of Kool Lite/Astrolux®, the layer thickness of Ti02 has been designed to reflect selectively a part of the green light and the NIR. The other wavelengths, corresponding for example to blue and red light are hardly influenced by interference and simply pass through the film. As a result, the film has a greenish color in reflection, and a purple one in transmission. This makes a big difference with the absorption pigments, which reflect and transmit the same color. Furthermore, the interference pigments make it possible to reflect solar heat (NIR) without affecting too much the transmission in the visible and PAR range.

In order to improve the performances of Kool Lite/Astrolux®, Merck has developed a new pearlescent pigment, based on the same technology. The goal was to obtain a stronger reflection of solar heat (NIR) combined with a higher light transmission in the PAR range, corresponding to a more selective reflection of NIR. To reach this, the amount of coated layers was increased to six, three on each side of the substrate. This new pigment design consists of two Ti02 layers with a high refraction index alternated with a Si02 layer with low refraction index. The new pigment has been processed into a masterbatch and then incorporated in a standard tunnel film formulation during extrusion, as it is normally done for Kool Lite/Astrolux®.

The gain in light transmission is illustrated in the transmission spectra of Fig. 1. These spectra of Fig. 1 refer to measurements done on a Variance Incidence Photometer (VIP), that is calibrated on a standard spectrophotometer. The VIP is characterized by an integrating sphere of one meter diameter, that enables measurements on samples of 0,5 m x 0,5 m, which are more closer to reality, especially for interference films. Furthermore, the light source is fixed on a movable arm so that all inclinations of the sun can be simulated. The VIP measures the total light transmission, including the diffuse and the direct light transmission. The figures of light transmission are given in Table 1, and refer to measurements on the VIP as well as on a standard spectrophotometer (Shimadzu UV 2101 PC). The spectrophotometer measures the total and the direct light transmission, making it possible to compare the light transmission, but also the diffusivity of the film. The light transmission of Kool Lite Plus on the VIP amounts to 84,5%, meaning an increase of 10.7% in comparison to Kool Lite/Astrolux®, and making the difference with a standard thermic diffusive tunnel film minor. The measurements on the spectrophotometer show the same trend. Kool Lite Plus has a 9% higher light transmission than Kool Lite/Astrolux® and 5.8% less than Hytifite (reference thermic diffusive film with 81.8% light transmission).

The stronger NIR reflection of the Kool Lite Plus film containing the new pearlescent pigments is illustrated in the transmission spectra of Fig. 2. For this purpose the different films have been measured on an extended spectrophotometer, which enables to measure the transmission over a larger range (250 -2500 nm), including not only the PAR, but also the NIR. The Kool Lite Plus film shows a much lower transmission in the NER range than the reference and the Kool Lite/Astrolux® film, and this thanks to its sharp reflection of this part of the spectrum, meaning less heat going through the new interference film.

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Table 1. Characteristics of the 3 tested films, Hytifite (standard thermic diffusive film used as a reference), Kool Lite/Astrolux® (commercial interference film) and Kool Lite Plus (new interference film). The light transmission mentioned as VIP refers to a measurement carried out on the Variance Incidence Photometer (cf. above). The direct and total light transmission are measured on a Shimadzu UV-VIS spectrophotometer. The diffusivity is calculated as the percentage diffuse light related to the total light transmission of each film.

Hytifite Kool Lite® Kool Lite Plus Light VIP 89,9* 73,8 84,5 transmission direct 25.3 8.9 36.2 (%) total 81,8 67 76

diffusivity 69 86.5 52.5 Thermicity 7-14 pm 16,1 20 18,4 (%) 7-25 pm 25 25,7 26,5

* Hytilux

Transmission spectrum measured with extended spectrophotometer

4G0 430 300 SSO eOO 630 TOO 730

Wavelength (nm)

| Hytilux KL Pius — Astrolux [

20

500 1000 1500 2000 2500 X (nm)

Transmission spectrum measured with V.I.P. (with D65 correction)

Fig. 1. Transmission spectra of Kool Lite Plus in comparison with a reference film (standard thermic diffusive film) and Kool Lite/Astrolux® as measured on the V.I.P.

Field experiment

Fig. 2. Transmission spectra of Kool Lite Plus in comparison with a reference film (standard thermic diffusive film) and Kool Lite/Astrolux® as measured on the extended spectrophotometer.

In order to test the expected positive effects in practice, a field test was started in El Hamma (South-Tunisia). The experimental film Kool Lite Plus is being compared to a reference thermic diffusive film (Hytifite) and Kool Lite/Astrolux® as cover for 8 m wide and 60 m long tunnels. The three tunnels were positioned parallel next to each other. The tomato variety 'Elena' was sown on 25th September 1996 in the tunnels on a sand substrate. In each tunnel, the plants were treated in the same way with regard to fertigation, ventilation and heating (geothermal water). Temperature measurements were performed with air temperature sensors connected to a data logger. The sensors were positioned at lm height, and in between the plants so that the sensors were never irradiated with direct light. The outside temperature was also registered with an extra sensor connected to the same data logger. Fruit production started around 15-20 January and the yield was registered on a weekly basis for each tunnel.

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Results and Discussion

The temperature registrations performed during the winter 1996-97 have already been presented (7). The measurements of spring 1997 now available are confirming the earlier report. At night, and more precisely from 7 p.m. on, the air temperature was highest in the Kool Lite Plus tunnel. The minimum and average night temperature (Table 2) in the Kool Lite Plus tunnel were on average respectively 1°F higher than in the Hytifite tunnel for the period from March, 28th and April, 19th. During the winter period the difference was higher, namely on average 2.5°F. The difference in thermicity as measured in the broader range (7-25 pm), which is the most accurate, shows negligible differences between the three films (Table 1). The higher temperature under Kool Lite Plus can possibly be explained by the thermicity of the pearlescent pigments in the range > 25 pm (not measured). In the morning, the Kool Lite Plus tunnel remains the warmer one, as the temperature rises faster under the new film than under the reference and the Kool Lite/Astrolux® film (Fig. 3). At 11 a.m, the temperature curves are crossing each other, so that at the warmest moment of the day, the Kool Lite Plus tunnel is the coolest one (Fig. 4). For this, we can also refer to the average maximum temperature registrations for the considered period, given in Table 2. However, the small difference in average maximum temperature between Kool Lite Plus and Hytifite (3°F) is not representative for the strong cooling effect of the new interference film, but is better illustrated in Fig. 3 and 4, representing the temperature course of different days.

Table 2. Average night, day, overall 24 hours, minimum and maximum temperatures (°C and °F) under the new interference film Kool Lite Plus, the existing Kool Lite/Astrolux® and the reference film Hytifite (standard thermic diffusive film) for the period between March, 28th and April, 19th .

Night Day Overall 24 h Minimum Maximum °C °F °C op °C op °C op °C op

Hytifite 17.2 63 27.8 82 22 72 14.9 58.8 33.5 92.5 Kool Lite/Astrolux® 17.2 63 26.3 79 21.4 71 15.1 59.2 31.1 88 Kool Lite Plus 17.8 64 27.3 81 22.2 72 15.3 59.5 32 89.5 Outside 13.8 56.8 21.8 71 17.4 63 10.1 50 25.6 78 Hytifite - Kool Lite Plus -0.6 -1 0.5 1 -0.2 0 -0.45 -0.7 1.5 3 Hytifite - Kool Lite 0 0 1.5 3 0.7 1 -0.2 -0.4 2.4 4.5 Kool Lite Plus - Kool Lite 0.6 1 1 2 0.9 1 0.25 0.3 0.9 1.5

The new interference film induces the coolest climate in the greenhouse during the hottest period of the day (11 a.m. till 5 p.m.) in comparison to the reference and the Kool Lite/Astrolux® film. For the considered period of March 28th till April, 19th, the average hour per hour difference in temperature between Kool Lite Plus and Hytifite increases from 1.8°F at 12 a.m. to 7.5°F at 3 p.m. (Fig. 4). Kool Lite Plus is also 1.6°F to 2.3°F cooler than Kool Lite/Astrolux® between 2 p.m. and 4 p.m.. These are the averages for 3 weeks of registrations, which include also some cooler days. During these cooler days with an outside temperature beneath 65°F, the temperature in the new interference tunnel is equal to the reference film (e.g. April, 8th, cf. Fig. 3 right). When considering only a hot day, the great cooling effect of Kool Lite Plus is more revealed. Taken as an example in Fig. 3 left, the registrations of April, 4th, illustrate that the new interference film can induce till 13°F lower temperatures at noon and the early afternoon (1 p.m. till 4 p.m.).

2 0

Tim« (h)

Kool Lite — Hytifite — Kool Lite Plus —- Outside]

Comparison of different types of tunnel film Temperature course at 4/4/97

Time (h)

Kool Lite — Hytifite — Kool Lite Plus — - Outside

Comparison of different types of tunnel film Temperature course at 8/4/97

Fig. 3. Temperature registrations in April : left, the temperature course of a representative hot day, and right, the temperature course of a representative cold cloudy day.

Comparison of different types of tunnel film Average temperature course over the period

Time (h)

| Kool Lite — Hytifite Kool Lite Plus —- Outside [

Fig. 4. Average temperatures courses for the period from March, 27th to April, 19th : left, the average temperature, and right, the average difference in temperature.

The quicker warming up during the morning under the Kool Lite Plus film, combined with the lower temperatures at noon and the afternoon, only result in a very small difference in average day and overall 24 hours average temperature (Table 2). During the winter period, the overall 24h average temperature was 1.6°F higher under Kool Lite Plus than under the reference film, due to the higher average night temperature (cf. above and 7). This brings the temperature of the tunnel closer to the optimal 66°F to 70°F for the tomato crop and resulted in a higher fruit development rate or a shorter fruit development time as defined by de Koning (3). This formula for the round tomato type gave a theoretically earlier production of 8 days, which corresponded quite well to reality. The production data only were registered the first 10 weeks of harvest, and are given in Fig. 5. The more optimal climate in the Kool Lite Plus results in a slightly earlier production start. Furthermore, the higher yields under the new interference film persist in the following weeks of production, with a slight increase in production difference between Kool Lite Plus and Hytifite. However, these data are just illustrative, and still need to be confirmed by other trials.

Time (h)

1 KLP-Kool Lite — Hytifite-Kool Lite —Hytifite-KLP

Comparison of different types of tunnel film Difference in average temperature over the period

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Compar ison of different tunnel films Evolution of the tomato production

J < S 6 7 8 1 10

Time (week)

I Kool Liu Plus — Hytifite]

Fig. 5. Evolution of the tomato production in the first weeks of harvest, from week 3 to week 10, 1997.

Conclusions

The lower temperatures under the Kool Lite Plus film at noon and in the afternoon can be related to the lower transmission of NIR rays, reducing the extreme temperatures. The new interference film Kool Lite Plus moderates the extreme temperatures, so that the climate is more favorable for the crop. The results presented here confirm the first results collected during the winter 1996/97.

Literature Cited

1. Daponte, T. & P. Verschaeren. 1994. Multilayer greenhouse films in Europe. Ace-SYS Greenhouse Systems - Automation, Culture and Environment International Conference - July 20-22, 1994 - New Brunswick, New Jersey, USA.

2. Daponte, T. & P. Verschaeren. 1994. New photoselective films for use in horti- and agriculture. 13th International CIPA Congress - March 8-11, 1994 - Verona, Italy.

3. de Koning, A.N.M. 1994. Development and dry matter distribution in glass-house tomato : a quantitative approach. Thesis Wageningen, ISBN 90-5485-322-8, 240 p.

4. Maisch, R. & M. Weigand. 1992. Pearl lustre pigments. Physical principles, properties, applications. Verlag Moderne Industrie.

5. Schmidt, C. & M. Friz. 1992. Optical physics of synthetic interference pigments. Kontakte (Merck-Darmstadt), 1992 (2): 15-24.

6. Verlodt, I., T. Daponte & P. Verschaeren. 1995. Interference pigments for greenhouse films. Plasticulture n° 108, 1995(4) 13-26.

7. Verlodt, I & P. Verschaeren. New developments in photoselective films. 14th International Congress for Plastics in Agriculture CIPA - March 9-14, 1997 - Tel Aviv, Israel.

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