Previous research studies relating to the environmental sustainability of plastics in agricultural applications focused on two main uses (mulching and shading), as noted in the introduction. The utility and ecological sustainability of farming applications were influenced by the adjustment of the mechanical, optical, radiometric, and related properties. The effects of modifying material properties are reviewed in the next section.
2.1. Modification of Mechanical Properties of Shade Nets and Environmental Sustainability
The review of the mechanical properties of shade nets is critical to sustainability because of the following facts. One, the mechanical properties predict the useful life in the farm; exposure to extreme environmental conditions such as storms, rainfall, and prolonged sunlight and chemicals in pesticides impacts durability. For example, UV light degrades plastic materials. Therefore, the modification of the material properties to improve durability may contribute to sustainability considering the carbon footprint and ecological impact of synthetic plastics on the environment. The environmental impact is considered in
Section 2.6 (Life Cycle Analysis). In the present subsection, various methods used in the reinforcement of the mechanical properties of shade nets, including the addition of fibers to form fiber-reinforced polymers, blending, and the inclusion of biomaterials such as chitosan are discussed.
According to an experiment conducted by Black-Solis et al. [
24] the mechanical properties of agricultural nets can be reinforced by blending poly (butylene adipate-co-terephthalate) (PBAT), poly (butyl acrylate) (PBA), poly (butylene succinate-co-adipate) (PBSA), and polycaprolactone (PCL). The blended product had better mechanical and physical properties such as elongation and flexibility at break compared to the constituent materials. The phenomenon could be attributed to the unique features of the constituents, especially PBAT. The material exhibits optimal performance over a wide pH range. Additionally, it is ductile and compatible with other biodegradable polymers. Even though the blending of different types of biodegradable polymers was proven effective by Black-Solis et al. [
24], a criterion should be adopted in the selection of the constituent polymers to preserve the fundamental material properties and ensure that the materials were compatible. The unique features of other potential polymer blends, such as the tensile strength, elongation at break, glass transition temperature, and the melting points for different biodegradable materials, are presented in
Table 1.
Chitosan is an ideal additive material for enhancing the sustainability of plastics in farm environments because it forms extensive cross-linkages, substantial intra, and intermolecular hydrogen bonds, and has a compact crystalline structure [
25]. Additionally, the material has anti-microbial properties and can easily be extracted from crustaceans and insects [
26]. On the downside, the scope of application of chitosan is limited by solubility in different solvents. However, the limitations are not an impediment to the utilization of the material considering that biocompatibility, non-toxicity, and anti-microbial properties outweigh the limited solubility, which can be partly offset by modifying the chemical properties of the chitosan. Bashir and co-researchers noted that biodegradable plastics made from polyvinyl chloride, chitosan, mint, and grape seed extract materials had ultimate tensile strength values between 15 and 28 MPa and an elongation at break of 86% [
27].
Commercially available blends of chitosan containing polymers include Chitosan-Starch-Pectin, high methoxyl pectin (HMP), and low methoxyl pectin (LMP) have been developed for packaging agricultural produce [
14]. In brief, the utility of chitosan in enhancing the mechanical properties of bio-based polymers has been widely proven. Apart from improving the mechanical properties, chitosan is ideal for surface modification to enhance the pest-inhibition ability of shade nets and packaging films [
1].
Bamboo fibers have also been proven to be suitable alternatives for reinforcing the mechanical properties of polymers. The superior mechanical strength of the bamboo fiber reinforced polymers is derived from lignin, cellulose, and hemicellulose [
26]. On the downside, the material has been adopted in limited polymer applications compared to the manufacturing of composites. Beyond the addition of fibers, the strength of the polymers is augmented by modifying the manufacturing conditions and post-production pre-treatment.
Even though there are multiple practical methods for enhancing the mechanical properties of shade nets and other types of plastics used in farm environments, the researcher is cognizant of the fact that the enhancement of the tensile strength often involves a tradeoff with ductility, which in turn elevates the risks associated with brittleness. Another issue of concern is the potential costs associated with the modification of the mechanical properties and the life cycle analysis; there is limited data to establish if available methods were cost-effective and scalable and environmentally benign. Based on the gaps in the available body of knowledge, upcoming studies should focus on these issues.
The significant variations in the weather patterns should be taken into consideration in the production of shade nets and mulching films; this is because they influence exposure to hail, frost [
29], severe storms, and excess solar radiation. A high level of solar radiation exposure is capable of inducing photodegradation/light-induced damage to plastic materials [
30]. Even though the risks associated with photodegradation can be mitigated by the inclusion of photo stabilizers and antioxidants [
31], UV damage of plastic shade nets and mulching materials is a common problem in tropical areas. Such areas receive intense sunlight compared to the subtropical/temperate zones. Additionally, the duration of global solar radiation exposure is longer, as shown in the appendix section.
The observation is supported by experiments conducted in the arid areas of Saudi Arabia [
27]. Arid and desert regions have sand storms, with pronounced tensile and shear stresses, which degrade the mechanical properties of the material. Based on the geography-specific challenges reported by Abdel-ghany et al. [
30] and Briassoulis et al. [
32], the weather patterns influence the sustainability of the material in agricultural applications. Apart from UV degradation, the starch content in the biodegradable polymers predicts the rate of natural deterioration. Luckachan and Pillai [
33] reported a 30% weight reduction after 8 months of exposure to Baltic Seawater. On the downside, such risks are not adequately taken into consideration during the selection of greenhouse materials.
2.2. Modification of Optical Properties of Shade Nets and Environmental Sustainability
Similar to the modification of the mechanical properties, optical properties have an impact on environmental sustainability because they determine a plant’s IR absorbance and transmittance, heat transfer, UV and radiation control [
2]. The regulation of these variables determines agricultural yields and plant health. Poor/low yields have a domino effect on sustainability considering that commercial agriculture contributes to global warming [
34]. From a practical point of view, it is hypothesized that poor yields would require intensification of farming (increasing the acreage under cultivation) to meet market demands.
The optical properties of different shade net materials were modified by adjusting the intensity of the shade nets and the surface color. According to the data presented in
Table 2, shade nets with green and black strips had a shading intensity of 34% and 40%, respectively. Even though the variations in the shading intensity were non-significant, they impacted the transmission of photo-synthetically active radiation (PAR), light transmission in the near-infrared, and the total transmission band. The transmission of light beyond the acceptable values impacted fruit/vegetable yields and exposure to pest and disease and physiological disorders such as cat face, skin cracking, sun scalding, and blossom end rot. Following the review of the material properties, the selection of the most suitable material does not involve a tradeoff between the optical properties, and the marketable properties of the fruits.
B49 (black shade net with a shading intensity of 49%) had the lowest PAR transmission rate, and highest marketable yield, mean fruit mass, and physiological conditions [
34]. The combination of the optical properties and reduction in plant’s exposure to pest and diseases makes material number B49 most ideal compared to Gr34 (green with a shading intensity of 34%) and B&Gr40 (black and green with a shading intensity of 40%) [
34]. The impact of shade net colors on optical properties reported by Kittas et al. [
34] is consistent with findings published by Milenkovi [
35], who reported optimal tomato fruit yield in red and pearl shade nets with 40% shade. The variations in the performance of the shade nets illustrate that there was no standard net color or shading intensity, which is appropriate. The color suitability is influenced by the type of plant under the shade net conditions and the local weather patterns and the grade of shade nets.
2.3. Plastic Covers/Films for Greenhouses
Plastic covers for greenhouses complement shade nets, especially when creating a local micro-climate is necessary to protect plants from the external environment. Plastic films also prove effective in solarization, better heating efficiency, and facilitate soil nitration [
36,
37]. However, soil solarization involves a tradeoff between the destruction of pests and disease and the reduction in the richness of bacteria and fungi, which is beneficial to plant growth [
38]. Additionally, there are sustainability concerns relating to the utilization of synthetic films in solarization and the limited efficiency of biodegradable plastics [
39]. According to [
32], the plastic covers provided an insect-proof screen, which in turn, eliminated or reduced the frequency of pesticide applications. The surface properties of the plastic covers can be modified to slowly release pesticides, as noted by [
33]. The slow release of pesticides is an effective form of integrated pest management (IPM) because constant flow of pesticides limits the proliferation of pesticides and reduces waste and soil toxicity. From an environmental point of view there are also contraindications: resistance to insecticides and diseases, contamination during installation, release of toxic substances into the environment. The use of substances compatible with organic agriculture would be perfect.
Apart from covering greenhouses, the films are ideal mulching materials compared to organic matter such as hay/dried grass. Zhang et al. [
11] argue that plastic films are suitable because they suppress weeds, facilitate fertigation and drip irrigation, regulation of soil temperatures, regulation of soil temperatures, and converse moisture. The observations are in line with Briassoulis and Giannoulis’s [
10] assessment of the functionality of bio-based polymers in mulching applications. However, the tensile strength of the commercially available bio-based films (Ecovio and Mater-Bi) was lower relative to the control (LLDPE) in both the transverse and machine directions, as shown in
Figure 1 [
10]. The tensile strength is a critical factor in predicting aging and failure. The failure of bio-based polymers results in the perforation of the film by weeds and free-falling objects. The susceptibility to mechanical damage limits the durability of the bio-based films.
The main challenge is to achieve similar capabilities using bio-based polymer films/covers. The data reported by Antón, Torrellas, Raya, and Montero [
39] and Seven, Tastan, Tas, Ünal, and Ince [
40] is based on polycarbonate and LLDPE films, which are not biodegradable. However, the degradation process can be augmented by UV light and oxidizing agents [
41]. Alternatively, the chemical structure can be modified to integrate carbonyl groups in PE, which are easily cleaved by microorganisms [
41]. The insights drawn from anti-microbial methyl-cellulose, organoclay, and coffee grounds based packaging films [
34] could help reduce the environmental effects of polycarbonate and LLDPE films in commercial farming. New materials with the potential to kill weeds have been developed such as de-oiled pomace (DOP) and their application has been proven in oil orchards [
42]. In addition, biodegradable sprays and drip irrigation systems have been investigated in the cultivation of ornamental shrubs and greenhouse plants [
43,
44]. The mulching sprays were effective in preventing the growth of
S. asper but less effective for
E. montanum.
Recent LCA analyses show that synthetic plastic films pollute the environment during the end of life treatment (landfilling, incineration, and recycling) [
45]. The long-term effects on the environment offset the superior material properties of the synthetic plastics.
Considering that bio-based plastic films offer near similar performance as synthetic plastics in reducing weeds, control of soil temperatures and moisture, and augment drip irrigation and facilitate fertigation, the main question is whether carbon footprint considerations should be a priority compared to durability and cost. The bio-based polymers are less affordable due to limited supply and commercial adoption. The selection of the materials should be based on context-specific factors such as the plant growth cycles, weather, and farming practices (such as organic farming). The economics of bio-based films are reviewed in the next section.
Potato Starch
The performance of plastics has also been augmented by the inclusion of organic materials. For example, potato starch blended with poly(hydroxybutyrate) (PHB) can be used to enhance the thermal stability and glass transition temperature of the biopolymers [
28]. The materials formed using these methods exhibit a glassy thermoplastic behavior that is comparable to HDPE and PP. Additionally, the potato peel waste is an ideal raw material for bio-composites and biopolymers [
46]. The materials can be used in the development of bio-based plastics for greenhouses and mulching films, and potato starch-graft-poly (acrylonitrile), chemical grafting is an ideal method for modifying the chemical properties of biopolymers [
47].
2.5. Degradation and Compostability
Apart from the modification of the microstructural properties to achieve better mechanical properties, optical reflectance, and customizable transmission of UV radiation (optical and radiometric properties), the rate of natural degradation and compostability predicts the environmental sustainability of plastics in agricultural settings. Synthetic plastics such as HDPE, PET, and PP plastics are not biodegradable; this is demonstrated by the low specific surface degradation rate (SSDR) of 10
2 and 10
3 μm/year [
50]. The limited biodegradability of these structures is attributed to the unique chemical properties and bonding. The synthetic plastics have strong C-C bonds, which are resistant to photo-oxidative degradation. Additionally, the bonds are not easily hydrolyzed. However, the challenge can be offset by modifying the structure to include functional groups with C=C, and C=O bonds indirectly facilitate photo-oxidation because the functional groups are capable of absorbing UV radiation. Considering that the SSDR is lowest on land, landfilling waste plastics (shade nets and mulching materials) is not a practical option because there is a risk of micro-plastic and plastic debris accumulation [
51]. Additionally, incineration or recycling is limited by cost considerations because the procedure generates significant volumes of waste. The degradation of plastics in farms can be induced by microbial activity (enzymatic activity and microbial metabolism) [
39] since landfilling is not a viable ecological approach.
Bacterial activity is primarily confined to bio-based plastics, which contain organic materials that can be converted to water and carbon dioxide. However, the effectiveness of this method is also dependent on the prevailing meteorological conditions (primarily the intensity of solar radiation, relative humidity) and geography. Rudnik and Briassoulis [
52] noted that the hydrolyzation of the plastic materials was enhanced in high humidity environments. Additionally, optimal microbial action was observed in warm/hot weather.
Even though biodegradable plastics are susceptible to environmental degradation, the phenomenon is not ubiquitous, especially given that the polymers are blended with standard non-biodegradable materials and additive to enhance the desired properties [
39]. The decline in the rate of biodegradability is correlated with the ratios of the biopolymer blends. A higher proportion of biopolymer blends translates to better standards of biodegradation. However, the ecological effects associated with blending can be offset with the recent advances in material development. For example, Black-Solis et al. [
24] noted that the physical properties of PLA biopolymers could be enhanced by blending with novel biodegradable materials including poly (butylene adipate-co-terephthalate) (PBAT) and poly (butylene succinate-co-adipate) (PBSA) to improve the physical properties of the polymers. The latter approach eliminates the need for non-biodegradable polymer blends. On the downside, there is minimal evidence of the commercialization of these innovations.
Commercially available shade nets and mulching materials are blended with non-biodegradable materials—a process that involves a tradeoff between environmental pollution and better mechanical strength [
53]. The compostability of plastics in farms is distinct from biodegradability. The process of compostability is defined by fragmentation in the environment, the rate of conversion to CO
2 and biomass, the presence of metals and other impurities, and the ability to support plant growth [
54]. Following the review of these parameters, compostable plastics exhibit unique behaviors in the environment compared to biodegradable plastics. On the downside, nearly all plastics used in agricultural applications are not compostable, except for twines and clips used to support greenhouse plants. The environmental benefits associated with compostability versus biodegradability can be contested based on the following facts. First, compositing bio-based plastics is time-intensive and requires elevated temperatures (58 °C). Additionally, moisture and airflow should be optimized. The complexity of such procedures limits the compositing of plastics in farm environments. From an ecological point of view, the biodegradability of plastics is a fundamental criterion compared to compositing.
2.7. Spread of Micro-Plastics in the Environment
Micro-plastics are small plastic fragments with a size that varies between 0.5 and 5 mm [
50], which originate from plastic additives and plastic debris [
61]. In 2020, there were at least, 250,000 tons and 51 trillion pieces of micro-plastics [
61]. Additionally, the micro-plastics are the most common forms of plastic pollutants in oceans—94.6% of plastics in the Mediterranean Ocean were micro-plastics in 2019 [
62]. The spread of micro-plastics within the environment is a limiting factor in the utilization of plastic materials in farm environments. For example, micro-plastics pollute the marine environment [
63]. However, the exact mechanism through which the plastics impact marine and freshwater ecosystems largely remains unknown. One school of thought suggests that isotropic motion (a defining attribute of micro-plastics) coupled with low aspect ratios result in unique fragmentation behavior in the oceans. The unique fragmentation behaviors also limit the formation of biofilms, and by extension the probability of decomposition. In contrast to biofilms, which are degraded naturally in the environment, micro-plastics exhibit oxygen incorporation, which translates to an increase in the mass and the attraction of micro-organisms [
50], which may latter aid the degradation process. However, since the degradation process is slow the risk of ingestion by marine species remains high [
62]; this would impact human health considering that seafood is a staple diet in coastal areas.