**8. Polymers for herbicide encapsulation**

Generally, carriers are polymeric materials that are employed for the encapsulation of herbicides to develop a smart delivery system. There are numerous carrier materials (natural, synthetic, and semisynthetic polymers) available for herbicide encapsulation. However, synthetic polymer has less significance than natural and semisynthetic polymer since it is not degradable in nature and remains as a contaminant in the soil. In contrast, natural carrier materials are advantageous since they are eco-friendly, biocompatible, cost-effective, easily available, and biodegradable in nature [96, 97]. Alginates, chitosan, starch, pectin, lignin, Arabic gum, cyclodextrin, cellulose, and gelatin are the biopolymers employed for herbicide encapsulation [90, 98–101]. Commonly used synthetic polymers are polycaprolactone, polyurethane, polyvinyl alcohol, and polystyrene sulfonate [99, 102, 103]. Semisynthetic polymers are natural polymers with side-chain modification through the replacement of hydrogen from hydroxyl group of glucose repeating units with ethyl, methyl, carboxymethyl, and carboxyethyl moieties.

## **8.1 Natural polymers**

Alginate is an anionic linear polysaccharide polymer that naturally exists in the cell walls of brown seaweed *viz*. *Ascophyllum nodosum*, *Laminaria hyperborea,* and *Macrocystis pyrifera* [104, 105]. Alginate has been explored for the controlled release of active compounds via ionotropic gelation method [92]. Leaching potential of

#### *Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*

sulfentrazone herbicide was reduced by developing sustained release of herbicide by exploiting alginate polymeric system. Tebuthiuron was encapsulated using alginate as carrier material to impede leachability of herbicides in agroecosystem [106]. Similarly, starch is a homopolysaccharide that is made up of two distinct molecules of amylose and amylopectin [90].

Starch is extensively found in cereal grains, roots, tubers, and fruits, which is also employed as a carrier for smart release of herbicide. Herbicides such as 2,4-D and 2,4,5-T were encapsulated with corn, wheat, potato, and cassava starches [107]. Encapsulation with wheat and potato starches exhibited slower release of herbicide because of higher amylose content and molecular weight of starch in wheat and potato starch. Sulfentrazone herbicide was encapsulated using starch *via* solvent evaporation method for season-long weed control by reducing leaching potential of herbicide [108]. Atrazine was encapsulated by utilizing starch as carrier and resultant formulation minimize the volatilization loss over the conventional formulation [93].

Chitosan is a nontoxic, biodegradable, and biocompatible polymer obtained through the deacetylation of chitin, which is usually found in the cell walls of fungi and bacteria. Chitosan is a cationic linear polysaccharide, which is highly efficient carrier system for agrochemicals [89, 109]. Paraquat-loaded chitosan/tripolyphosphate nanoparticles reduced the soil sorption of paraquat, thus improving the stability of herbicide [99]. Cellulose and its derivatives are explored as a carrier system for the smart delivery of active compounds. The formulation was developed by mixing chitosan and glyphosate at different molar ratios in water for the mart delivery of glyphosate, where chitosan polymer plays a dual role as eco-friendly adjuvant and polymeric carrier of glyphosate facilitating prolonged release of herbicide [110].

Cellulose is a polysaccharide that is biodegradable in nature and available in abundance at a lower cost. Alginate/cellulose-based delivery system containing imazethapyr offered the extended-release of active material [100].

Pectin is a polysaccharide and an anionic biopolymer, which is abundantly present in higher plants' cell walls. Pectin is a biodegradable, nontoxic, and easily available natural polymer. Pectin is composed of D-galacturonic acid units, which are linked by α-(1-4) glycosidic linkage [111]. Nowadays, pectin is also explored as a carrier system for the controlled release of the active ingredient due to its characteristics *viz*. more stability at acidic and high-temperature conditions, gelation property, non-toxicity, biocompatibility, and easily available at a cheaper cost [112]. Six percent pectin and two percent calcium chloride were found as optimum concentration for smart release of herbicides *via* ion gelation technique [113]. Metsulfuron-methyl loaded in pectin nanoparticles were found to be effective with higher herbicidal activity at a lower application rate as compared to the commercial herbicide [87].

Lignin is another important polymer that is obtained as a byproduct in pulp and paper industries. Lignin exhibits UV shielding property and antimicrobial activity, which attracted lignin to explore as a polymer for the delivery of herbicide. Moreover, lignin is relatively available in abundance at a lower cost [114, 115]. Dicamba herbicide was encapsulated in lignosulfonate carrier system for sustained release [116]. Ligninpolyethylene glycol-based chloridazon and metribuzin were synthesized for minimizing leachability of herbicides in light-textured soils [117].

Cyclodextrin is a cyclic oligosaccharide consisting of glucose units, derived through enzymatic conversion of starch. β-Cyclodextrin is a highly preferred molecule for encapsulation of active molecules since it is easily available at a lower cost [118]. Cyclodextrin has a unique structure, which enables it to form inclusion complex with hydrophobic active molecules. Terbuthylazine herbicide molecule was encapsulated using cyclodextrin, which showed improved solubility and bioavailability of herbicide molecule [119].

Guar gum is a neutral polysaccharide made up of the main chain of D-mannopyranose residues linked together by β-(1,4) glycosidic bonds and a secondary chain of D-galactopyranose residues linked together by α-(1,6) glycosidic bonds. Solubility of guar gum in cold water rises in proportion to the galactose/ mannose molar ratio [120]. Herbicide formulation of guar gum-g-cl-polyacrylate/ bentonite clay hydrogel composite was employed for pre-emergence application, while guar gum-g-cl-poly N-isopropylacrylamide nano hydrogel was used for the post-emergence application [121]. The encapsulation efficiencies of imazethapyr into guar gum-g-cl-polyacrylate/bentonite clay hydrogel composite ranged from 75.99 to 98.96% and guar gum-g-cl-poly N-isopropylacrylamide nano hydrogel ranged from 67.98 to 80.90%. The time to release 50 percent of the loaded imazethapyr (t1/2) was between 0.06 and 4.8 days in CGNHG, while it was from 4.4 to 12.6 days in GG-HG system, Encapsulation of bioherbicides were also attempted using natural carrier materials such as Arabic gum, Persian gum/gelatin and gelatin [101].

#### **8.2 Synthetic polymers**

Polycaprolactone is biodegradable and hydrophobic polyester belonging to the aliphatic family. Polycaprolactone is utilized as a smart delivery vehicle for various active ingredients since it is biocompatible, cost-effective and possesses unique mechanical properties [122, 123]. Encapsulation of pretilachlor in polycaprolactone polymer enhanced the stability and herbicidal activity of herbicide [124]. Encapsulated atrazine and paraquat herbicides in poly-ε-carpolactone carrier system minimized the environmental impacts associated with the use of herbicides [125]. Similarly, poly-εcaprolactone based atrazine nanocapsules reduced the soil mobility of herbicide and showed higher weed control efficiency at a lower application rate [94, 126, 127].

Polyurea is a product derived from the reaction of isocyanates and amines. Polyurea is used as shell material for herbicide encapsulation since it has high thermal stability and is available at a lower cost. Polyurea was utilized as a polymer for encapsulation of oxyfluorfen to reduce the phytotoxic effect on non-target plants [128]. Polyurea-based pretilachlor microcapsule formulation was synthesized through polymerization, which was found to be efficient in controlling weeds [129]. Polyurea-based pendimethalin encapsulated formulation reduced the usage of organic solvents during the manufacture of emulsifiable concentrate formulation eliminating the environmental pollution due to its application [130]. Pendimethalin was encapsulated using shell material made up of polyurethane urea to improve weed control efficiency [131].

Polyvinyl alcohol is a water-soluble polymer being widely explored for herbicide encapsulation. Glyphosate, a non-selective herbicide was encapsulated using polyvinyl alcohol and polystyrene sulfonate to minimize the herbicide loss in the environment [103]. Further, polyurethane polymeric systems are also utilized for the controlled delivery of active ingredients. Polyurethane is a synthetic polymer composed of urethane units, which is biocompatible and biodegradable in nature [132]. Trifluralin loaded in polyurethane network through interfacial polymerization protected the active ingredient from volatilization and photodegradation [133].

Polylactic acid is a biodegradable polymer derived from renewable sources such as corn, wheat, and rice. Polylactic acid is an aliphatic semicrystalline polyester which is hydrolyzable, eco-friendly, and biocompatible in nature [134, 135]. Microparticles

*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*

of metazachlor herbicide were synthesized with low molecular weight polylactic acid for the controlled release of active molecules [136, 137]. Encapsulation of metolachlor herbicide was also attempted using a high molecular weight of polylactic acid for smart delivery of herbicide [138]. Similarly, Poly (lactic-co-glycolic acid) is a biopolymer composed of monomers of lactic and glycolic acids [139], exploited as carrier system for the smart delivery of atrazine herbicide to reduce the environmental impacts associated with application of herbicide [140].

#### **8.3 Semi-synthetic polymers**

Cellulose and its derivatives are exploited as a carrier system for the smart delivery of active compounds in agriculture. The two primary classes of cellulose derivatives are cellulose ethers and cellulose esters, which have varied levels of mechanical and physicochemical properties.

Ethyl cellulose is a derivative of cellulose in which the hydroxyl group of cellulose is substituted with the ethyl ester group [141]. Ethyl cellulose is a hydrophobic polymer utilized for improving the stability of the active ingredient to achieve higher use efficiency. 2,4-D herbicide was loaded in ethyl cellulose microspheres to achieve sustained release of herbicides [142]. Ethyl celluloseloaded alachlor formulation reduced the soil mobility of herbicide which achieved prolonged weed control at a lower application rate [143]. Norfluazon based controlled release system using ethyl cellulose reduced the soil mobility of herbicide and protected the active ingredient from photodegradation [144, 145]. Solvent evaporation method was utilized to introduce atrazine, a broadleaf weed control herbicide, into ethyl cellulose-controlled release formulations [146], to sustain the release of herbicide.

Carboxymethyl cellulose is a cellulose derivative that is anionic in nature with high solubility in water. Carboxylmethyl cellulose readily forms gel in solutions of multivalent cations, such as aluminum or iron cations, to generate hydrogels. Controlled release formulations of acetochlor were synthesized with various modified forms of clay/carboxymethyl cellulose (**Figures 2**–**4** and **Table 10**) [147].

**Figure 2.** *Solvent evaporation technique.*

#### **Figure 4.**

*Preformed polymerization technique.*


**Herbicide Polymer Encapsulation technique Characteristics of formulation Authors** Tebuthiuron Sodium alginate Ion gelation technique Controlled release carrier system for tebuthiuron [106] Chloridazon and Metribuzin Lignin and ethyl cellulose Reduced leaching and photo degradation of herbicides [117] Terbuthylazine β-cyclodextrin Kneading method Improved herbicide solubility [119] Atrazine Poly ε-caprolactone with chitosan as coating agent Modified interfacial deposition of preformed polymer Improved adhesive property of herbicide on foliage of target weeds [98] Paraquat Chitosan/ tripolyphosphate Ionic gelation Encapsulation efficiency of polymeric system was 65% with stability of 60 days. Reduced Soil sorption of herbicide in soils [99] Tribenuronmethyl Zein Solvent evaporation Encapsulation efficiency was 81± 3% with enhanced solubility, controlled release formulation improved weed control [148] Sulfentrazone Sodium alginate Ionotropic gelation Minimized herbicide leaching into the soil [92] Metazachlor Polylactic acid/ polyethylene glycol Solvent evaporation Controlled release system for the delivery of herbicides for prolonged weed control [137] Pendimethalin Starch Solvent evaporation Slow release system depends on soil moisture availability and non-toxic to earthworms [88] Metsulfuronmethyl Pectin Emulsification Encapsulation efficiency of 63 ± 2% with increased herbicidal activity at lower dose [87] Tebuthiuron Sodium alginate Ionotropic gelation Reduced herbicide loss due to leaching [149] Imazapic and Imazapyr Alginate/chitosan and Chitosan/ tripolyphosphate Ionotropic gelation Enhanced herbicidal activity and less toxic [150] Imazethapyr Alginate and Alginate/cellulose Ionotropic gelation Extended release of Imazethapyr for 30 days of application [100] Atrazine Poly ε-caprolactone Interfacial deposition of preformed polymer Improved post-emergence activity at lower dose (ten-fold lower than recommended levels) in controlling target weeds. Reduced soil mobility [126, 127, 151] Norflurazon Ethyl cellulose Solvent evaporation Prolonged release and reduced soil mobility and offered protection from [144]

photo degradation

*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*


*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*


#### **Table 10.**

*Brief overview of herbicide encapsulation.*

#### **9. Release profile of encapsulated herbicides**

Herbicide encapsulation protects the active compound from different losses *viz*. leaching, volatilization, adsorption, photodecomposition, etc. The loss of herbicides is controlled by altering the release rate of active ingredients from the polymeric systems. Therefore, herbicide encapsulation serves as a platform to design herbicide formulation with varying release patterns of active molecules. Encapsulated formulation modified the herbicide release profile. The encapsulation of herbicides minimizes the adverse consequence in soil environment due to use of herbicides. Similarly, encapsulation technique offers an extended period of weed control at a lower dosage.

The particle size of the formulation greatly influences the release rate of active ingredients into the environment [142, 154–156]. Polymer-solvent ratio, water diffusion rate, pH of the releasing medium, molecular weight of the polymer, nature of interaction between shell and core materials (active molecules), polymers, methodology, and preparation conditions also govern the release profile of active molecules [143, 153, 157–159].

Encapsulation of metribuzin and chloridazon in lignin-polyethylene glycol system coated with ethyl cellulose (20%) and dibutyl sebacate (2.25%) resulted in the controlled release formulation and time taken for the delivery of 50% corresponding herbicide were 16.94 and 65.39 h, respectively [117]. Release kinetics study of paraquat loaded in pectin/chitosan/tripolyphosphate nanoparticles revealed that polymeric system sustained release of paraquat compared to that of conventional formulation where a significant amount of paraquat was not released until 30 min of incubation [160]. Similarly, alginate/ chitosan-based paraquat nano-formulation modified the release profile of paraquat, which achieved 100% herbicide release in eight hours of incubation. The release of paraquat was extended for two hours compared to that of free form of paraquat in water medium [161]. Conventional paraquat released 92% of active molecules after 350 min of incubation, while paraquat from chitosan/tripolyphosphate nanoparticles diffused only 72% during the same period [99]. The commercial formulation of imazethapyr released more than 76% of active herbicide molecules in less than one day, whereas the time taken for fifty percent release of the active molecule from alginate and alginate/cellulose beads were 11.30 and 43.73 days respectively [100]. Laboratory studies on the release profile of starch-encapsulated atrazine revealed that 70% of active ingredients were delivered in three days, while the remaining quantities of herbicides were released over 16 days of incubation. However, the maximum release was noticed after 15 days of application under field conditions as against the peak release of herbicide, which was observed in three days of incubation in

**Figure 5.** *Overview of encapsulated herbicide formulation.*

*in vitro* study [162]. Moreover, multilayer encapsulation of active ingredients resulted in the reduction of burst release and extended the release period (**Figure 5**) [152].
