On use of trilayer low density polyethylene greenhouse cover as substitute for monolayer cover

Abstract

Few attempts are available in the literature on utilising the trilayer low density polyethylene (LDPE) film as new generation of greenhouse cover to substitute the monolayer film that is often used. This paper investigates and compares the degradation behaviour and durability of both covers. The covers are exposed to 7 months of natural withering. The results revealed that the degradation resistance of trilayer film is better than the monolayer film in terms of their mechanical and optical properties. The service lifespan, based on 50% reduction in the property criterion, of the trilayer film is found to be double the service lifespan of the monolayer film.

Keywords

LDPE greenhouse cover Monolayer film Trilayer film Degradation Natural aging Mechanical and optical properties Service lifespan

Introduction

Greenhouses are used extensively to control the environmental conditions. The main component of a greenhouse is its cover film. Commonly greenhouse covers are produced as a monolayer of low density polyethylene (LDPE) film. The authors are not aware of any systematic studies on the use of multilayered LDPE films used as greenhouse covers. This study looks at the influence of the climatic conditions on the physical and mechanical properties of polyethylene films. It has been reported by Dilara and Briassoulis¹ that the manufacturing process parameters of plastics that affect the mechanical properties of the film include temperature of the melt, die parameters, blow-up ratio, drawn ratio and frost line height and cooling conditions. Gruenwald² and Vishu³ have shown that molecular orientation during film blowing influences tensile properties, higher in the direction of the covalently bonded carbon–carbon chain than in the transverse direction that is dominated by weaker van der Waals bonds. The climatic conditions (the second category) such as solar irradiation, temperature, humidity, rain, wind loads and environmental pollution influence aging and mechanical properties of LDPE greenhouse covers. Polymeric materials undergo a series of reactions in the presence of sunlight, such as photolytic, photo-oxidative and thermo-oxidative that result in the degradation of the material leading to chemical degradation, with consequences such as brittleness, loss of the brightness, colour change, opacity and the formation of surface cracks.⁴ Activated molecules are formed in first step during UV irradiation of polymers and then various processes, such as chain scission, cross-linking and oxidation, occur in the second step. The main scission will first cause photodissociation and then radical formation. If the free radicals can migrate and recombine with other radicals or the main chain, cross-linking will occur.⁵ In addition to the reduction in molecular weight, a number of changes take place in the molecules during photodegradation with the formation of chemical groups like carbonyl, carboxylic acids and hydroperoxides. Carlsson et al., ⁶ Fechine et al.⁷ and Khan and Hamid⁸ have reported that the UV radiation (290 nm–400 nm) is the high energy part of the solar spectrum, and can be absorbed by the polyethylene. This leads to bond cleavage and polymerisation, causing photodegradation (oxidation) and thus mechanical degradation. Cyclic temperatures, high when in contact with metal greenhouse frames during hot and sunny days, but low at night might increase degradation.¹ Dilara and Briassoulis⁹ illustrated that the degradation of polymers is induced by different external weathering factors and mechanisms, including photo induced, thermal, mechanical, ultrasonics, chemical, biological and hydrolysis.

The third group influencing mechanical properties of greenhouse polyethylene is microclimate conditions such as internal humidity and temperature, biological activity and agrochemicals. A combination of such harmful factors can eventually alter the properties of the polymer and affect adversely its mechanical and optical properties. It should be considered that various testing methods of researchers and approaches possibly introduce variations in data. The available tests for mechanical and physical properties of plastics used as greenhouse covering materials are mainly for general purpose and are not in European Union standards.¹ There is little information on the type of tests recommended for greenhouse covers. Mechanical degradation of greenhouse polyethylene can be of several forms that include rupture, tearing, shearing, penetration, impact or excessive elongation. Much effort is needed to standardise mechanical tests for greenhouse covers, especially to account for degradation due to aging (both natural and artificial). The effect of simulated sand wind for a duration of 4 h on a polyethylene film has been studied by Hassini et al.¹⁰ Briassoulis and Schettini¹¹ have utilised strain gauges as a mean for measuring the deformation in the film subjected to mechanical stresses. Salem et al.¹² have examined the effect of UV radiation on mechanical properties of LDPE films containing black carbon titanium oxide. The UV radiation has changed the elongation and shear stress of the samples. Pacini¹³ emphasised the importance of keeping the plastic greenhouse cover in good conditions and examined cover properties including total permeability to solar radiation, mechanical properties, service life, effect of climate and the homogeneity of film thickness and width. Shen et al.¹⁴ developed a simulation model to calculate the distribution of the molecular structure at the inner surface of the greenhouse cover and to calculate the yield and elasticity. This was to predict the maximum ice and storm loads on the greenhouse structure. Nijskens et al.¹⁵ discussed the effect of aging on the mechanical properties (maximum values of the tensile force, elongation and fracture force) of greenhouse polyethylene covers. Mourad et al.^16–21 have studied the mechanical, thermal and chemical stability of different grades of polyethylene under different aging conditions. Briassoulis et al.²² reported that, the final design of a greenhouse should fulfil a balance between three important issues: (1) the overall structural design of the greenhouse and the individual structural components characteristics; (2) the inherent mechanical and physical properties which determine the structural and the functional behaviour of the cladding materials; and (3) the specific sensitivity to light and temperature of the crop to be grown in the greenhouse along with other agronomic requirements. Therefore, the evaluation of the mechanical properties of the greenhouse covering material combined with their physical properties under the environmental factors involved is critical in assessing their quality and in determining the relevant requirements concerning the greenhouse structural design and construction details. Adam et al.²³ have studied the behaviour of polymeric cover made of a sandwich structure of three layers. One EVA19 layer inserted between two LDPE layers (LDPE/EVA19/LDPE). The film was manufactured by Prosyn-Polyan. The film thickness is ∼220 μm, each LDPE layer having a thickness of ∼20 μm. Optical, thermal, surface analysis and mechanical properties have been analysed on samples having undergone different thermal treatments associated with sand and wind simulation in order to test their performance when used in a desert environment. The data analysis is performed by comparing these results with those obtained on a monolayer polyethylene film. The performance of multilayers materials is found better than that obtained for the monolayer film, with regard to the mechanical, the optical properties and resistance to the abrasion effect due to sand/wind. Based on the above literature, UV radiation, external pressure, humidity and high temperature shortened the service life of the covers. There is little information on the influence of arid environmental conditions of aging on the behaviour of greenhouse polyethylene covering. Further, the previous studies have been conducted mainly on monolayer polyethylene films in moderate climates considering different amount of additives to improve their performance. Few authors were interested in long term behaviour of polyethylene films.¹¹ Yousef et al.^{24, 25} have studied the effect of natural aging on the trilayer LDPE films. They have shown that the trilayer film duration of use was estimated to be 10 months and the effect of sand wind at 40°C for 4 h deceases the optical transmission of a single layer of LDPE by 50%. Recently, the effects of artificial aging parameters (temperature and UV) on the LDPE film have been studied by Dehbi et al.²⁶ Other work includes the durability of the naturally and artificially aged trilayer been considered also by Dehbi and Mourad.²⁷ They have studied the effect of different arid conditions under long term (up to 5400 h) of natural and artificial aging on the behaviour of LDPE films as a greenhouse cover. The films were produced in Algeria by Agrofilm Company using a coextrusion process with some additives such as anti UV and, anti oxidising agents.

In general, the greenhouse covering should be durable, strong enough to resist loads due to snow, wind, crops and installation and with acceptable lifespan. The mechanical properties of greenhouse coverings are very important in relation to their mechanical behaviour under various loading conditions as well as to the overall structural behaviour of the greenhouse. Further, the function of the greenhouse cover is dependent to a far extent on its optical properties in addition to its mechanical ones. Therefore, the focal point of this work is to conduct a comparative study between monolayer film and the newly developed trilayer films in terms of the degradation in their mechanical and optical performance under different climatic conditions of natural aging.

Material and method

In this study, two different films, produced and supplied by Agrofilm SA (Sétif, Algeria), were employed. The first is extruded monolayer low density polyethylene LDPE film with a thickness of 160 μm. The second is three coextruded layers LDPE film with the same thickness of monolayer film (180 μm) and with the layer thickness proportions of ¼, ½ and ¼. The real film composition is kept confidential by the supplier, but it is established that the two exterior layers have different additives than those added into the medium layer. The various used additives are anti-UV/UV stabiliser, antioxidant/anti-O₂, Nickel stabiliser, plasticizer, etc. The density of LDPE before extrusion is 0·923 g cm⁻³ and the weight average molecular weight is in the range of 90 000–120 000. The initial colour of the film is milky yellow. The melt flow index MFI of the raw LDPE is 0·33 g/10 min.

For this study, two greenhouses of 32 m length, 8 m width and 3·5 m height have been built specially for studying the effect of the natural aging on the performance of the LDPE cover. Samples were taken every month over the duration of 7 months to determine the durability of the covers. Squares (30×30 cm) were cut from the LDPE sheets at particular aging periods for testing. The climatic conditions undergone by the roof are displayed in Table 1. The climatic condition in North Algeria was classical and standard while conducting the study.

UV-visible spectroscopy was conducted with a Shimadzu Model 3600, in transmission mode, with a resolution of 0·1 nm. The tensile tests were performed using a universal testing machine (Instron model 4301) with a load cell of 5 kN. All tests have been performed at room temperature with a crosshead speed of 50 mm min⁻¹ and displacement controlled conditions.

The presence of any chemical modifications of the polymeric surfaces has been examined by determining the free surface energy. The contact angle between a reference liquid and the film surface is firstly measured from which the film surface energy polarisation and energy dispersion are calculated. The summation of these two values gives the free surface energy for the different samples. Three reference liquids have been employed these are: (1) ultrapure water (milli-Q Water System, resistivity 18 Ω cm⁻¹); (2) glycerol (density 1·26 g cm⁻³ and viscosity at 20°C is 9·34 g cm⁻¹ s⁻¹) and (3) diiodomethane (density 3·325 g cm⁻³ and solubility in water is 14 g L⁻¹ at 20°C). All measurements were carried out at room temperature (23°C). Measurements were repeated five times for each liquid deposited on the sample to obtain the average value. A drop of 3 mL, deposited with a microsyringe, was photographed with a black and white CCD camera (500×500). Contact angle θ was determined from a computerised contact angle meter (NFT Communications Company). Free surface energy is calculated using the relationship proposed by Young et al. and with the Qwen Went method that was illustrated by Dehbi and Mourad.²⁷

Results and discussion

The UV-visible spectroscopy measurements have been conducted for six different aging conditions. Figure 1 shows the percentage of transmitted light as a function of the wavelength obtained for samples A to F. Sample A is for unaged monolayer film, sample B is for monolayer film aged for 3000 h at 50°C+UV-A, sample C is for monolayer film aged for 5000 h at 50°C+UV-A, sample D is for trilayer unaged film, sample E is for trilayer film aged for 3000 h at 50°C+UV-A and sample F is for trilayer film aged for 5000 h at 50°C+UV-A.

UV-visible light transmission for unaged and aged monolayer and trilayer LDPE films

For the whole domain of the UV-visible wavelength scanned in the present work, the greatest light transmission is found to be for the unaged monolayer film (sample A). Sample B of the monolayer film showed lower transmission than sample A. The light transmissions of the trilayers films (samples D, E and F) have been found to be less than that of sample B over the whole scanned wavelength domain. Similar observation has been reported by Adam et al.²³ for a sandwich structure single layer LDPE films. The least light transmission is found to be for sample C (monolayer film aged for 5000 h at 50°C +UV-A) above 600 nm. This illustrates that the aging period and exposure to UV-A radiation affect adversely the film transmission. Samples B and C of monolayer film lost 10 and 30% respectively of the light transmission of the unaged monolayer film. On the other side, samples E and F of the trilayer films lost 4 and 10% of the light transmission of the unaged trilayer film (sample D). The difference in the transmission of monolayer samples at 600 nm is approximately 23% compared to the trilayer samples which show a variability of 10% at 600 nm. Although, the transmission of the unaged monolayer is higher than that of the unaged trilayer film, the reduction in the light transmission of the monolayer is higher under exposure to UV radiation for 5000 h. This illustrates that the trilayer film maintains a better light transmission compared with monolayer film in the long term (after 5000 h of aging).

To obtain a quantitative estimation of the effects of natural aging on the mechanical performances of the monolayer and trilayer films, the tensile tests were conducted on unaged/virgin and naturally aged (up to a period of 7 months) samples. The load was applied on the specimen in a direction parallel to the average molecular orientation obtained during the film processing. The engineering stress as a function of the engineering strain has been recorded for each film. The tests were conducted to determine the tensile properties (modulus of elasticity E, yield strength σ_y, fracture stress and elongation at break) of the material before and after aging.

The stress–strain diagrams for unaged and naturally aged monolayer films are demonstrated in Fig. 2, while the tensile curves for trilayers films are presented in Fig. 3. More details about the tensile test are given in Dehbi and Mourad.²⁷ Figures 2 and 3 illustrate the effect of different aging periods and climatic parameters on the properties of both types of films (monolayer and trilayer). The curves of unaged films are also included for comparison. The monolayer film tensile curves (Fig. 2) can be divided into four distinctive regions. In the first region (strain <10 mm mm⁻¹) the stress varies with strain following a linear relationship. In this region, Hook's law may be applicable and valid up to the proportional limit at which the curve deviates the linearity and the second region, nonlinear variation region, starts. In the third and forth regions, the stress varies almost linearly with the strain; however, the slope is less in the fourth region. All samples exhibit the same tensile behaviour trend regardless of the climatic condition parameters of aging. The stress–strain diagrams for the trilayer films increase almost linearly in the first (initial) region (Fig. 3). Then the curves become nonlinear in the second region (10 mm mm⁻¹<strain<25 mm mm⁻¹) with decreasing slope up to a point of zero gradient. Then the stress drops little to a constant drawing stress or becomes almost with zero gradient, in the third region, up to a certain engineering strain before the workhardening starts and the stress increases almost linearly with the strain in the fourth region up to fracture point. As for monolayer, all samples exhibit the same tensile behaviour trend regardless of the aging conditions. This trend of variation has been also observed by Mourad.^{16–21, 28} Figures 2 and 3 demonstrate that the unaged film has a higher level of stress than aged films. The curves reflect also the deteriorative effect of aging on both types of LDPE films. The deteriorative effect (in terms of the reduction in the yield and fracture strengths and elongation at brake) is very obvious after 7 months of aging. The aging also increases the stiffness of the films at the expense of ductility.

Stress–strain curves for unaged and aged monolayer LDPE films

Stress–strain curves for unaged and aged trilayer LDPE films

Figure 4 presents a comparison between the deteriorating effect of aging on the monolayer and trilayer films, each subplot shows two corresponding tensile curves plotted together at different aging periods. The levels of the stresses at the same aging period for trilayers films are quite higher than that for the monolayer. This is expected as the thickness of trilayers films (180 μm) is 12·5% higher than that of monolayer film (160 μm) or (1·125 times that of monolayer). Further, the trilayer films show a quite higher level of ductility. On contrast, the stiffness (elastic modulus) increases with aging, which is an indentation for degradation of the film flexibility rather improvement in its stiffness. This will be discussed from a quantitative point of view later in the subsequent paragraphs.

Comparison between stress–strain curves of monolayer and trilayer films at different aging periods

In the absence of a distinct yield point, yield strength can be defined/estimated by three different techniques.²⁷ In the first technique, the yield strength is defined as the stress at the point at which the curve deviates from its linearity (proportional limit stress σ_yp). In the second technique, the yield strength is defined as the stress at an offset strain of 0·002 mm mm⁻¹ (offset yield strength σ_y,0·2). In the third technique, the yield strength is defined as the stress at the point at which the curves has zero gradient/slope or the stress at the first peak. Based on the tensile curves trend of variation, of the two film types, the first and second concepts are more appropriate for the purpose of comparison and evaluating their yield strengths.

Figure 5 presents the evolution of the yield strength with aging time for both monolayer and trilayer films. The yield strength decreases with aging. The curves of trilayer films comprises of two regions with different constant rates of degradations. The rate of degradation is higher at the first region (up to 1500 h) for the trilayer films. On the other side, the curve of the monolayer shows almost a linear variation. The offset yield strengths based on the proportional limit technique are 2·6 and 0·45 MPa for unaged trilayer and monolayer films, respectively. That is, the yield strength of trilayers film is 5·8 times that of monolayer film (or 478% higher than that of monolayer film). After 7 months of aging the values are 1·9 and 0·20 respectively, which means the ratio of the yield strengths is 9·5 or the value for trilayer is 850% higher than that for monolayer. The trilayer film has lost 27% of its original strength (the value of the unaged film) after aging for 7 months; however, the monolayer film has lost 55% of the value of the unaged film. The yield strengths based on 0·2% offset strain concept are 4·8 and 1·1 MPa respectively. In sense, the yield strength of trilayer film is 4·36 times that of monolayer film. After 7 months of aging, the values have reduced to 3·2 and 0·52 for trilayer and monolayer films respectively, and the corresponding strength ratio is 6·15. The trilayers film lost 33% of its strength compared to that of unaged film, while the monolayer film lost 52%. The results reveal that the yield strength of trilayers films is much higher than that of monolayer films after aging and the diminishing in the yield strength for trilayers film is much less.

Variation of yield strength (proportional limit yield strength and 0·002 offset strain yield strength) with aging period

The variation of the fracture stress (the stress at break) with the aging time is presented in Fig. 6. The fracture stress reduces with aging time and the trend of variation resembles that of the yield strength. The maximum achieved fracture stress for unaged trilayer and monolayer films are 16·2 and 1·63 MPa respectively; hence, the fracture stress of unaged trilayer film is 9·9 times greater than that of monolayer film. The corresponding values for trilayer and monolayer are 12·51 and 0·5 MPa respectively after 7 months of aging. The trilayer lost about 22% of its value for the unaged film and the monolayer lost 69%. The results again demonstrate that the fracture stress of the trilayers film is essentially higher than that of the monolayer one and effect of aging climatic parameters on monolayer is more pronounced.

Variation of fracture stress with aging time

The influence of aging on the strain at break is shown in Fig. 7. The trend of variation is similar to that of the yield strength (Fig. 5) and fracture stress (Fig. 6) for both types of films. The unaged trilayer film exhibits a remarkable ductility (420% elongation). Though the monolayer exhibits also a ductile behaviour (259% elongation), yet the ductility of trilayer is 1·63 greater than that of monolayer. This ductile behaviour diminishes with aging. The percentage elongations of both types of films are 258 and 118% respectively, at the end of the total aging period. Moreover, the percentage elongation of the trilayer film to the monolayer percentage elongation ratio is 2·18. It is worth noting here that the percentage elongation of trilayer film after 7 months of aging is approximately of the same order of that of the unaged monolayer film (258%), which again reflects the notable ductility of trilayers film compared to the monolayer film.

Variation of fracture strain with aging time

The variation of the modulus of elasticity E with aging time for the trilayer and monolayer films is given in Fig. 8. Since all tensile curves of monolayer and trilayer films exhibit almost an initial linear relationship, the modulus of elasticity E is determined as the slope of the first portion of the tensile curve. The values of the moduli of elasticity were found to be of 343 and 177 MPa for unaged trilayer and monolayer films respectively. The values increase with aging time and they are of 445 and 355 MPa after 7 months of aging. The modulus for the aged monolayer is of the same order of that of the unaged trilayer after the total aging duration (Fig. 8). The value for the trilayer increased by 23% of that of the unaged film and the corresponding value for the monolayer is 50%. The rate of degradation in the flexibility is higher after 1500 h of aging and the rate of increase in the value of modulus of elasticity or decrease in the flexibility is higher for monolayer film. This reflects the adverse impact of aging on the flexibility, especially for the monolayer film, as the increase in the stiffness is an undesirable property for greenhouse cover films. It is worth noting that the trend of variation of stiffness/flexibility is opposite to that of the yield strength, fracture stress and fracture strain for both films.

Variation of modulus of elasticity E with aging time

The polymeric films that are used as greenhouse covers should not possess only high initial ductility, but also should have high initial strength, resilience and toughness. Both resilience and tensile toughness are calculated from the area under the elastic region of the tensile curve and the entire area under the curve respectively. The measured resilience and toughness are plotted in Figs. 9 and 10. The resiliences for the unaged trilayer and monolayer films are 135 and 5·5 N mm mm⁻³ respectively, with a ratio of 24·5. The values decrease with aging time. At the end of the aging period, the resilience values are 90 and 2·1 N mm mm⁻³ for both films respectively, and the ratio is 43. In sense, the trilayers film loses 33% of its initial resilience; however the loss is 61% in the case of monolayer. Similarly, the toughness of the unaged trilayers and monolayer films are 4800 and 340 N mm mm⁻³ respectively. The toughness of trilayer is 14 times that of the monolayer. The values decrease with aging time. The toughness values after the 7 month aging are 2600 and 80 N mm mm⁻³ for both trilayer and monolayer films respectively (the toughness ratio is 32·5). The trilayer and the monolayer films lost 45 and 76% of their initial toughness respectively, In the light of the results, though the thickness ratio of trilayers film to monolayer film is only 1·125, the trilayers film has much higher toughness (i.e. strength and ductility) and flexibility (elasticity) than the monolayer film before and after aging and the monolayer film is more susceptible to degradation. Therefore, the trilayer films are recommended for using as greenhouse covers in place of the commonly used monolayer films.

Resilience variation with aging time

Toughness variation with aging time

It is important to set a criterion to evaluate the performance of the films over time and to study the effect of aging on the loss factor of the property. The lifetime of the cover should, of course, be based on the loss of its physicochemical and mechanical properties. Briassoulis²⁹ has shown that if the greenhouse cover film loses 50% of one of its original/initial properties (i.e. for unaged film), it becomes unusable. Figure 11 compares the normalized mechanical properties and lifetimes of monolayer and trilayer films. Each property is normalised with the corresponding property of the unaged film.

Variation of normalised mechanical properties with aging time for a monolayer and b trilayer films

Figure 11a demonstrates that the aged monolayer film loses 50% of its mechanical properties compared to its initial value (the value of the unaged film) after exposure time between 3200 and 4200 h. On average, the lifetime is around 3500 h (5 months), which is just sufficient for only one agricultural season. It should be noted that the 50% increase in the modulus of elasticity (which is undesirable increase in the stiffness) means 50% loss in the flexibility. The evolution of the normalised mechanical properties with aging exposure time for trilayer films is demonstrated in Fig. 11b. Unlike the monolayer LDPE cover, the 7 month exposure time to the natural conditions were insufficient to cause a loss of 50% in any of the mechanical properties. Work is currently in progress to cover a longer period of exposure. Therefore, a fitting curve technique is used here to predict the aging time at which the 50% loss in the property occurs. This technique has been used satisfactorily in previous work by Dehbi et al.²⁶ and Dehbi and Mourad²⁷ in which some fitting equations have been tried to fit the experimental data. The following equation (equation (1)) has been found to achieve the best fit and used to extrapolate the experimental data beyond the exposure time at which the 50% loss in the property takes place. 1 where y(t) is the property/parameter as a function of time t, a and b are the constants of normalisation and τ is the time beyond which the rate of degradation becomes faster. The variation of three normalised experimental properties with exposure time (as representative cases) along with the predicted results based on the fitting equation is presented in Fig. 12. The plot shows that the predicted results are in good agreement with the experimental results. The same accurate prediction has been observed for all properties. Table 2 presents the normalisation constants for each property. Table 1

Variation of predicted and measured normalised properties of trilayer film with aging time

Table 1

Average temperature and average moisture during aging

Month	1 (March)	2 (April)	3 (May)	4 (June)	5 (July)	6 (August)	7 (September)
T/°C	32·8	33·6	37·6	41·6	44·2	43·6	41·1
H/%	76·2	69·3	62·5	60·58	58·7	59	63·7

Table 2

Normalisation constants

Property/parameter	Normalisation constants
	a	b	τ
Proportional limit based yield strength	0·55	0·44	2804
0·2% offset strain based yield strength	0·64	0·21	2060
Fracture stress	0·76	0·23	2073
Fracture strain	0·55	0·44	2084
Elastic modulus	0·987	0·013	1989
Resilience	0·57	0·44	3075
Toughness	0·43	0·52	2281

Figure 11b indicates that all normalised properties degrade with time. The rate of degradation of each property increases rapidly at a specific value of τ as presented in Table 2. Figure 11b shows also that the 50% loss of the initial value of various parameters occurs between 7200 and 8700 h. On average, all normalised properties loses 50% of their initial values at ∼8000 h (∼10 months). It means that the trilayer film can be used as a cover for almost two agriculture seasons (cultivation of two crops). Further, the trilayer film can serve efficiently for a lifetime double the lifetime of monolayer film (3500 h/5 months).

Generally, it can be concluded that the trilayer film have a longer lifespan and more tolerable to the normal aging climatic conditions of North of Africa. It has been confirmed from the Agrofilm, Algeria company that the various important additives are anti-UV (UV stabiliser), anti-O2 (antioxidant), nickel stabiliser, plasticizer and yellow colour additives. These various additives are added during the film production (to protect LDPE against irradiation) and are able to diffuse from the bulk to the surface. In fact, yellow additives are also not stabilised and can migrate easily to the film surface. This is the main change that takes place in the material during aging. Over time, as the anti-UV additives diminish, the UV radiation and the associated oxidisation reaction takes place and accelerate and dominate the aging process at the surface and through the thickness.³⁰ It has been well known that there is a formation of radicals on the LDPE film surface due to the light and/or heat effects. These radicals lead to three different reactions which are responsible for degradation. These are reactions of reticulation of chains, reactions with oxygen in air and reactions of scission of chains. These three types of reactions can coexist. The prevalence of a mode of degradation with respect to another depends on the nature and the stability of the radical formed in the course of the reaction of the photo-oxidation.¹ In the case of LDPE the tertiary carbons (20–40 carbon atoms/1000) can lead to stable radicals. They are likely to react with other radicals and to lead to reticulations of the chains. The secondary carbons lead to very reactive radicals and cause reactions of chains scission.³¹ Tidjani³² showed clearly the degradation model of polyethylene. This result supports the possible formation of vinyls by an intramolecular decomposition on sec-hydroperoxides. Vinyls can also be produced from the Norrish type II reaction on ketones. UV fractions of the solar radiation are absorbed by the yellow dye additives or the additives of anti-UV (that present in the film) penetrates and progresses in the thickness of film.³³ Such UV penetration causes a progressive degradation not only on the surface, but also through the thickness of the film. In the current work, the observed aging and deterioration in all mechanical properties (yield strength, fracture stress and strain, elastic modulus, resilience and toughness) of LDPE greenhouse cover are attributed to the climatic conditions such as exposure to solar radiation (with wavelengths of 290–1400 nm), temperature, humidity, rain and wind loads, and environmental pollution. The major part of the observed deterioration is due to the direct relationship between the degradation of the mechanical properties and the UV radiation which is in the range 290 nm to 400 nm. This part is known as the high energy part of the solar spectrum that can be absorbed by the polyethylene. It causes bond cleavage and depolymerisation in the polymer that leads to photodegradation (i.e. free radicals formation). Such produced free radicals react freely with the atmospheric oxygen (this is said to be photooxidation) leading to further mechanical degradation to the polymer. This has been reported by several investigators for monolayer film (e.g. Dilara and Briassoulis,¹ Khan and Hamid,⁸ Salem,¹² Amin et al., ³⁴ Hamid et al., ³⁵ Yanai et al.³⁶). The degradation in all mechanical properties has been observed even after the first month of exposure to natural aging as shown in Fig. 11. This is in contrast with the observation of Al-Madfa et al.³⁷ They have shown that during the natural aging, the mechanical properties of polyethylene are improved during the first months of exposure and that after they collapse significantly.

Salem¹² examined the effect of UV radiation on mechanical properties of monolayer LDPE films containing black carbon titanium oxide. He has shown that the UV radiation has changed the elongation and shear stress of LDPE films. The findings of this work revealed that, the monolayer and trilayer lost 50% of there mechanical properties after exposure to natural aging of 5 and 10 months respectively. The study of Ram et al.³⁸ showed that the unprotected monolayer LDPE films failed (50% retained elongation) after the equivalent of <2 months exposure. Qureshi et al.³⁹ have reported that linear LDPE monolayer films had a 50% reduction in tensile strength at break after only 3 months of natural exposure in a hot region.

To the best of the authors’ knowledge, there are few studies available in the literature addressing the effect of aging on the greenhouse trilayer LDPE cover^{25, 26} and Dehbi and Mourad.^{27, 40–42} As discussed in the preceding sections, the newly developed trilayer film has more resistance to aging than the monolayer one and consequently has more durability and lifespan. The results show that natural aging induces sever degradation on the external surface of film due to the sunlight radiation which is major element of degradation. The migration of the additives such as UV stabiliser, antioxidant and nickel stabiliser to the surface of the LDPE film during aging is a crucial parameter in the degradation process. Therefore, such degradation resistance of the trilayer due to sunlight radiation can be interpreted based on its composite structure. It has been confirmed from the manufacturer that the coextruded three layers are adhered together by heating to a temperature level of 70°C. Such structural design allows forming of two boarders between the intermediate film and the upper and lower films at the meeting surfaces. For monolayer film, the migration process of the additives is easier than in the trilayer in which the extra layers block the additives in the intermediate layer and slow down the migration process of the additives towards the outer surface. Moreover, the rate of diminishing of the protective additives in the trilayer film is slower than that in the monolayer film due to the presence of a separating layer at each two mating surfaces. Such two separating layers act as a barrier for migration of the protective additives to the exposed surface of the film. This reduces the penetration or the deteriorative attack of UV through the film thickness. While photodegradation can influence the whole thickness of the monolayer film, as it is transparent, photo-oxidation can only take place in the near surface region of the trilayer film. Furthermore, the severity of the oxidation process is determined by the diffusion of oxygen inside the material. Therefore, the destructive effect of the photo-oxidation is expected to be less in the trilayer due to its structure. Undoubtedly, the photodegradation process of the greenhouse covering material is extremely complex.

As mentioned in the preceding discussion, the impact of the UV radiation is combined with synergetic effects of many interacting factors, such as exposure to varying and high temperature, water absorption, etc. High temperature accelerates the rate of reaction for photo-oxidation and thus, higher mechanical degradation rate. Also the humidity and/or rainfall may lead to the gradual wash out of the performance enhancing additives reducing the resistance to oxidation and accelerating degradation. The humidity may also contribute to the degradation of the cover through photochemical reactions, which can result in the production of free radicals or other reactive species that can promote various free radical reactions. It is worth noting here that these interacting affects again depends on the stability of the stabilising additives, which is more in trilayer covers than monolayer.

In the light of the above discussion the observed enhanced mechanical properties and durability of the trilayer film can be interpreted based on its composite structure. From economical point of view, it has been confirmed from the suppliers that the costs of monolayer and trilayer films are 0·88 and 0·98 Euros per 1 m² respectively. The cost of trilayer film is 11·36% more. The greenhouse consumes about 250 m² which means that there is a difference of only 28·4 Euros if the cover will be made of trilayer film. Based on the results of this work from both durability and economical perspectives, the trilayer film is highly recommended for using as a cover for the greenhouse. However, the effect of some more parameters that have adverse effect on the performance of polymeric materials in general should be studied such as process parameters, high strain rate effects, etc.^43–47 The effects of all these parameter are the main objectives of our ongoing research project on greenhouse covers. Further work will also involve studying the performance of the multilayers (five or more) and compare with the trilayer films.

Conclusions

In this study, the durability of both LDPE monolayer against the newly developed trilayer greenhouse covers has been discussed. The objective was to study the impact of natural aging (up to 5040 h/7 months) on the mechanical behaviour and light transmittance of both monolayer and trilayer films. The tensile tests have been conducted to explore and compare the mechanical properties (e.g. yield strength, elasticity modulus, fracture stress/stress to break, fracture strain/strain to break and toughness) of unaged and aged films. The results of this work revealed that the degradation of the mechanical and optical properties is interrelated to the weathering and aging process. There is marked decrease in mechanical properties leading ultimately to material disintegration. As far as the resistance to weathering and to the effect of UV radiation, high temperature, etc. is concerned, trilayer cover is considered to be inherently more stable and can withstand a 10 month exposure without exceeding the 50% deterioration in the mechanical properties. On the other hand, the monolayer film deteriorates rapidly within only 5 months on continuous exposure to aging. Although the transmission of the unaged monolayer is higher than that of the trilayer film, the reduction in the light transmission of the aged monolayer is higher. This illustrates that, in the long term (after aging period of 5000 h), the trilayer film has a better light transmission if compared to monolayer film. Furthermore, the presence of separating layers at each two mating surfaces of the trilayer film acts as barrier for migration of the protective additives to the exposed surfaces of the film and for diffusion of oxygen through the mid-film layer. This improves its mechanical and optical properties, durability and lifespan. Based on the mechanical and optical characteristics, lifespan and cost analysis, the trilayer covers are more superior to monolayer covers.

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