Polylactide/cellulose acetate biocomposites as potential coating membranes for controlled and slow nutrients release from water-soluble fertilizers

https://doi.org/10.1016/j.porgcoat.2021.106255Get rights and content

Highlights

  • The Polylactide/cellulose acetate blend affords the combination of water absorbency and water barrier in one coating system.

  • The coating thicknesses influence the phosphorus release rate, that could be controlled by adjusting the coating process duration.

  • The proposed coating allowed to achieve the European standard for slow release fertilizers.

  • The introduction of cellulose acetate into the coating material increases its hydrophilic character thereby accelerates the hydrolytic cleavage of polylactide chains in soil.

Abstract

The unbalanced release of fertilizers influences the availability of nutrients in soil, and their leaching affects the ground water sources. Therefore, the coating of water-soluble fertilizers is considered as the main approach that improves their uses. Here, biodegradable coating based on polylactic acid (PLA) and cellulose acetate (CA) was proposed for the first time to encapsulate diammonium phosphate (DAP) granules using a rotary pan. The blends were prepared by vigorous mixing of PLA and CA in a solvent mixture using a homogenizer equipment and the prepared films of homopolymers and their blends were fully characterized (IR, DSC, TGA, SEM, EDS). Moreover, the coating thicknesses were controlled by adjusting the duration of the coating process. The delaying performance of the coatings meet the release rate required by the European standard (20 % of P released in the first 24 h). The introduction of CA decreases the PLA crystallinity and therefore accelerates the degradation of the coating in soil.

Introduction

The food production strongly depends on nitrogen (N) and phosphorus (P) as major nutrients, which are predominantly supplied from fertilizers amendment. However, conventional fertilizers are often characterized by the rapid release of nutrients, that may cause environmental problems [1,2]. The N and P losses cause negative impacts on the ecosystem, particularly on atmospheric greenhouse gas (GHG) concentrations, soil acidity and water resources contaminations [3]. In addition, soil acidity reduces the availability of phosphorus for the plants due to its interaction with iron and aluminum. The controlled and slow release fertilizers (CSRF) were developed to delay the dissolution of nutrients in soil and enable their availability in relation with plant’s nutrition requirements [4]. Indeed, previous studies have shown that delaying the phosphorus release rate may reduce its complexation [5,6].

The coating of water-soluble granular fertilizers by synthetic polymers constitutes a barrier to water penetration into the core, and consequently delays the nutrients release from the fertilizer granules [7,8]. Nevertheless, its major drawback is linked to the non-biodegradation of the coating material in soil. The resistance of synthetic polymers to the degradation by living organisms causes significant and continuous accumulation of polymer residues in soil after every fertilization process. It has been reported that around 50 kg per hectare of plastic residues were cumulated in arable land every year [9]. The accumulation of micro-plastics in soils are becoming a very alarming issue at the ecological context [10]. Therefore, biodegradable polymers were used to replace traditional synthetic materials in the field of fertilizer coatings [11,12]. In that regard, eco-friendly and cost effective polysaccharides are investigated in many studies. [11,[13], [14], [15], [16], [17]]. In the coating process, the formulation of polysaccharides in water instead of organic solvents makes them very attractive for such applications. However, the water solubility of some polysaccharides leads to uncontrolled nutrient losses during the coating process. This major limitation has generally been overlooked in published research articles. Furthermore, controlled and slow release criteria would be difficult to achieve using polysaccharides alone as a coating agent. On the other hand, the use of polysaccharides as a part of coating material will enable the increase of the water holding capacity of the coating system, in which the water absorbency seems to be a highly sought-after property, especially in the arid and semi-arid regions [18].

The fertilizers are defined as slow release ones by the European standard when they released less than 15 % of nutrient during the first 24 h [1]. For this purpose, the coating based on polysaccharide must contain a hydrophobic material. We have recently studied a coating system based on chitosan as inner coating and paraffin wax as an additional hydrophobic coating layer (outer coating), the release of phosphorus was significantly delayed in water and soil [17]. The double coating approach was repeated in many published papers and has achieved the combination of slow release properties and water holding capacity [14,19,20].

The structure of the coated fertilizer needs to be preserved during its transportation and storage. For that reason, the mechanical properties of coating materials should be enhanced by plasticizing, crosslinking steps or combining polymers in blends, composites or copolymers. Several investigations have been performed to develop sustainable composites material based on combining hydrophobic biopolymer and polysaccharides [[21], [22], [23], [24]]. Nevertheless, when hydrophilic polysaccharide is combined to hydrophobic polymer, their compatibility is restricted resulting in decrease of the desired optimal properties.

Among the available polysaccharides, cellulose stands out as the most abundant biomass on Earth and have already overwhelmingly dominated the research revolving around the design on new bio-based materials. Native cellulose is insoluble in water and the most organic solvents whereas some of its derivatives are soluble and have a wide range of applications in coatings [25]. Cellulose acetate (CA) with low degree of substitution (DS∼1) is a relatively hydrophilic polysaccharide. It is one of the most commonly used cellulose derivatives since it is soluble in common solvents such as acetone, dioxane, and other organic solvents. The CA might be an attractive component in composites material not only for its non-toxicity and biodegradability but also due to its low cost. On the other hand, hybrid material based on blending CA with higher DS (up to 1.9) and aliphatic polyester was achieved and good adhesion and miscibility was stated without compatibilizers [26]. Indeed, a high degree of substitution (DS) of CA leads to hydrophobic polysaccharides with better physical properties [[27], [28], [29]]. However, this highly substituted cellulose (CA, DS > 1.9) is still much more expensive for coating fertilizer application.

Polylactide (PLA) is a biodegradable biopolymer characterized by high elastics modulus, high mechanical strength and hydrophobic character. PLA is basically synthetized through direct condensation of the free acid monomer or by the ring opening polymerization (ROP) of lactic monomer [30]. PLA gains interest principally in agricultural and medical applications due to its biodegradability and processability [31]. However, the relatively high cost of PLA remains the major limitation of its application such as coating fertilizers. Despite this drawback, few studies on PLA as coating material of granular fertilizers were reported in the last two decades. Jintakanon and coworkers have used PLA as a coating for urea granules. High molecular weight of PLA showed the best urea-retaining efficiency [32]. Also, the PLA was blended with functionalized softwood kraft lignin to obtain urea coating material for the controlled release option [33]. Recently, PLA was physically blended with cellulose acetate, polyethylene glycol and SiO2 nanoparticles to obtain adjustable porous coating membranes of urea pellets for controlling the nutrient release [34]. Indeed, the PLA based membrane protected the urea granules from fast solubilization in water and delayed the nutrients release. Furthermore, the polysaccharide introduced into the PLA matrix reduces the hydrophobic character of the coating system and leads to its water uptake tendency, which is highly required in arid and semi-arid region. In addition to enhance their efficiency, these coating agents could be considered as water reservoir for the plants. On the other hand, PLA undergoes hydrolytic degradation in water medium, that is strongly affected by its molecular weight and the degree of crystallinity [35]. In fact, water enters preferentially the amorphous phase and catalyzes the hydrolytic mechanism of PLA chains. It has been reported that a high degree of crystallinity limits the hydration of the ordered chains [36]. Furthermore, any actions that increase the penetration of water accelerate the hydrolysis rate and lead to faster degradation of the coating system based on PLA [37]. In other words, controlling the kinetic of PLA chains hydrolysis could open a large perspective on controlling the release of active principle through the coating material.

The main objective of this work is to prepare a coating formulations based on physically blending of PLA and CA (DS ≤ 1) for the granular fertilizers coating. In addition to the complementary properties of those biopolymers in term of biodegradation and processability, the introduction of CA into PLA may influence its hydrolytic ester cleavage and consequently could affect the rate of degradation of the proposed coating agent. This eventual opportunity must influence the phosphorus release rate in water through PLA-CA membrane. Furthermore, the material prepared in solvent mixture of THF-acetone was fully characterized and the spherical granules of fertilizer (DAP) were selected for the coating step using rotary pan process. A THF-acetone solvent mixture was selected to avoid the loss of nutrients during the coating process and to assure the polymers adhesion. The coating thickness was controlled by the duration of the coating processing and evaluated by the scanning electron microscopy (SEM). Moreover, the phosphorus release was examined by tracking its dissolution rate in water using spectrophotometric methods. At the end, the kinetic of biodegradation under aerobic conditions of the coating material was investigated.

Section snippets

Materials

Polylactide (PLA) pellets with an average molecular weight of Mn: 60,000 g/mol were purchased from, Nature Works LLC, (USA), Cellulose acetate (CA) DS ∼ 0.9 and (Mn: 30.000 g/mol), acetone and tetrahydrofuran were purchased from sigma Aldrich. All the compounds were used as received.

Coating materials preparation (PLA-CA)

The polylactic acid (PLA) composite films were prepared by casting method, in which a blended solution (5%.wt) of different PLA-CA weight ratios (PLA-CA 100:0, 80:20, 50:50, 20:80 and 0:100) were solubilized and

Preparation and characterization of PLA/CA blends (coating materials)

The hydrophobic property of the coating materials plays an important role in delaying the dissolution rate of nutrients from the granular core, therefore the hydrophobic/hydrophilic balance of the neat CA, neat PLA and their blends, was determined by measuring the contact angle of drop of distilled water on their surfaces. Clearly, the contact angle measurements display the hydrophobic character of the prepared films (Fig. 2).

The hydrophilic character of cellulose acetate (CA) shows a contact

Conclusion

A physical blending of polylactide (PLA) and cellulose acetate (CA) was carried out to prepare different coating agents of granular fertilizers (DAP) using rotary drum process. The coating formulation was solubilized in 50:50 mixture of acetone and tetrahydrofuran to avoid the precocious release from the granules during the coating process, when water was used as solvent. The film based on PLA-CA 80:20 presents a homogenous surface as revealed by SEM images, which is probably due to a partial

Authors statement

Taha El Assimi: Investigation, Characterizations, Formal analysis, Writing – original draft, Writing - review & editing.

Roko Blažic: Characterizations, Investigation.

Elvira Vidoviĉ: Investigation, Review & editing.

Mustapha Raihane: Supervising, Visualization and Resources.

Abdellatif El Meziane: Investigation, Review & editing.

Mohamed Hassen V Baouab: Investigation, Formal analysis.

Mehdi Khouloud: Resources and Validation.

Redouane Beniazza: Investigation and characterizations.

Hans Kricheldorf:

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The Authors would like to acknowledge the support through the R&D Initiative—Appel à projets autour des phosphates APPHOS—sponsored by OCP (OCP Foundation, R&D OCP, Mohammed VI Polytechnic University, National Centre of Scientific and technical Research CNRST, Ministry of Higher Education, Scientific Research and Professional Training of Morocco MESRSFC) under the project entitled “Stabilisation et liberation contrôlée des fertilisants par enrobage des engrais phosphatés par de nouvelles

References (56)

  • J. Li et al.

    Structural functionalization of industrial softwood kraft lignin for simple dip-coating of urea as highly efficient nitrogen fertilizer

    Ind. Crops Prod.

    (2017)
  • I. Castilla-Cortázar et al.

    Hydrolytic and enzymatic degradation of a poly(ε-caprolactone) network

    Polym. Degrad. Stab.

    (2012)
  • M.A. Paul et al.

    Polylactide/montmorillonite nanocomposites: study of the hydrolytic degradation

    Polym. Degrad. Stab.

    (2005)
  • H.R. Kricheldorf et al.

    High: T m poly(L-lactide)s via REP or ROPPOC of l-lactide

    Polym. Chem.

    (2020)
  • L. Wu et al.

    Preparation and properties of chitosan-coated NPK compound fertilizer with controlled-release and water-retention

    Carbohydr. Polym.

    (2008)
  • M. Peesan et al.

    Preparation and characterization of hexanoyl chitosan/polylactide blend films

    Carbohydr. Polym.

    (2005)
  • C. Xing et al.

    Mechanical and thermal properties of eco-friendly poly(propylene carbonate)/cellulose acetate butyrate blends

    Carbohydr. Polym.

    (2013)
  • G. Arthanareeswaran et al.

    Synthesis, characterization and thermal studies on cellulose acetate membranes with additive

    Eur. Polym. J.

    (2004)
  • R. Quintana et al.

    Enhancement of cellulose acetate degradation under accelerated weathering by plasticization with eco-friendly plasticizers

    Polym. Degrad. Stab.

    (2013)
  • V. Dornburg et al.

    Economics and GHG emission reduction of a PLA bio-refinery system - combining bottom-up analysis with price elasticity effects

    Resour. Conserv. Recycl.

    (2006)
  • M.E. Trenkel

    Slow- and Controlled-Release and Stabilized Fertilizers: An Option for Enhancing Nutrient Use Efficiency in Agriculture

    (2010)
  • M. Calabi-floody et al.

    Smart Fertilizers as a Strategy for Sustainable Agriculture

    (2018)
  • D. Davidson et al.

    Materials for sustained and controlled release of nutrients and molecules to support plant growth

    J. Agric. Food Chem.

    (2012)
  • R. De Niro Gazola et al.

    Efeito residual da aplicação de fosfato monoamônio revestido por diferentes polímeros na cultura de milho

    Rev. Ceres.

    (2013)
  • D.W. Rindt et al.

    Sulfur coating on nitrogen fertilizer to reduce dissolution rate

    J. Agric. Food Chem.

    (1968)
  • T. El et al.

    Poly (ε ‑ caprolactone)‑ g ‑ guar gum and poly (ε ‑ caprolactone)‑ g ‑ halloysite nanotubes as coatings for slow ‑ release DAP fertilizer

    J. Polym. Environ.

    (2020)
  • Z. Majeed et al.

    A comprehensive review on biodegradable polymers and their blends used in controlled-release fertilizer processes

    Rev. Chem. Eng.

    (2015)
  • K. Lubkowski

    Coating fertilizer granules with biodegradable materials for controlled fertilizer release

    Environ. Eng. Manage. J.

    (2014)
  • Cited by (28)

    View all citing articles on Scopus
    View full text