BRPI0904295A2 - process of obtaining superabsorbent crosslinked nanocomposites - Google Patents

Abstract

PROCESS OF OBTAINING SUPERABSORVENT RETICULATED NANOCOMPOSITS The present invention relates to a process for obtaining superabsorbent crosslinked nanocomposites. More specifically, said nanocomposites are prepared by grafting a polysaccharide, in particular regular cornstarch, with acrylic acid. The resulting copolymer is cross-linked in the presence of small amounts of naturally sadistic montmorillonite clay (MMTNa), yielding a hydrogel endowed with improved mechanical strength and increased fluid absorption capacity. The obtained superabsorbent gels have low toxicity and ability to absorb large amounts of water at high absorption rates, making them suitable for use in the manufacture of toiletries.

Description

PROCEDURE FOR OBTAINING SUPERABSENT REINFORCED NANOCOMPOSITES

FIELD OF INVENTION

The present invention relates to the preparation of superabsorbent crosslinked nanocomposites with biodegradability characteristics which are the result of starch graft reaction with acrylic acid and crosslinking agents in the presence of clay.

BACKGROUND OF THE INVENTION

Superabsorbent gels (SAPs) are known as cross-linked polymers that absorb and retain water and aqueous solutions in large quantities, absorbing up to hundreds of times their own weight.

Usually, superabsorbent polymers are synthesized from acrylic monomers, such as acrylamide, acrylic acid and sodium acrylate, or by copolymerization between them, using a crosslinking agent. About 90% of all superabsorbent materials produced are used for the manufacture of disposable products. However, due to concerns about environmental issues, increasing efforts have been made to develop new products based on superabsorbent polymers that may be biodegradable.

Polysaccharides in general are known to have biodegradability characteristics, a fact that has aroused strong interest in the investigation of alternatives to obtain new superabsorbent materials. However, the fragility of polysaccharide gels may be a restrictive factor for their application for this purpose. Given their low cost and biodegradability characteristics, cellulose derivatives and starch have proved to be potentially suitable for use in this search for new superabsorbent materials.

RELATED TECHNIQUE

A gel is characterized as superabsorbent when it has capacity and high swelling rates and good mechanical properties. The specialized technical literature states that the presence of hydrophilic groups, flexible polymer chains and the availability of large free volume between polymer chains favor the absorption capacity of water and aqueous hydrogel solutions [Athawale, V.D. and Lele1 V., 2000, Applied Polymer Science Journal, Vol.77, 2480-2485].

It is also known that the monomers conventionally used to obtain this type of material are acrylamide, acrylic acid, acrylic acid salts (mainly sodium and potassium salts), while the techniques employed to obtain them comprise mass polymerization processes. , in solution and in inverse suspension, with thermal initiation and by oxy-reduction [Omidian, H; Rocca J.Ge Park, K., 2005, Journal of Controlled Release 102, 3-12].

Regarding the use of polysaccharides to obtain hydrogels, cellulose, carboxymethylcellulose, starch and modified starches, which may be biodegradable polymers, have been cited as an alternative.

Polymerization is generally done in stages, starting with the technique of polysaccharide graftization with synthetic monomers such as acrylonitrile, acrylamide and acrylic acid. Next, the copolymer is generally cross-linked with a hydrophilic crosslinking agent. US 5,470,964 and US 2007/0015878 may be cited as reference.

Early work on starch reports graftization with aacrylonitrile and later gamma radiation crosslinking. Gelatinized cassava starch was used, which was simultaneously grafted with acrylonitrile and cross-linked by application of gamma radiation. CuS04-5H20, Pb (NO3) 2 and AI (NO3) 3 ossals were used to minimize homopolymer deformation. However, the results revealed a homopolymer booster. The absorption capacity of the formed gel increased with decreasing homopolymer yield and was favored by the use of low radiation doses [Kiatkamjornwong, S. e Meechai, N., 1997, Radiation Physics Chemistry 49, 689-696].

Similar experiments were performed to obtain cassava-g-poly (acrylamide-co-acrylic acid) -amide, followed by crosslinking by the use of gamma radiation. Graphitized copolymers of acrylamide and acrylic acid led to better water-absorbing properties than those with only one monomer. Concentration of acrylic acid was considered as a parameter of major importance for water absorption. These polymers have been found to behave as pH sensitive superabsorbents, whose absorption capacity increases as a function of pH increase [Kiatkamjornwong, S .; Chomsaksakul, W. and Sonsuk, M. 2000, RadiationPhysics and Chemistry 59, 413-427],

On the other hand, more simplified polymerization techniques have also been reported to obtain starch-based superabsorbent gels. A hydrogel was obtained by grafting the acrylic acid starch using ammonium ceric nitrate (CAN) as the initiator.

Hydrogels were obtained after neutralization of the carboxylic groups with NaOH. The maximum water absorption achieved was 250 g / g [Athawale, V.D. and Lele, V. 1998, Carbohydrate Polymers 35, 21-27]. In a similar work, superabsorbent hydrogels were also obtained from the starch acrylic acid grafting, but in this case acrylamide was also used for the grafting. Maximum water absorption capacity reached 510 g / g [Athawale, V.D. and Lele, V., 2000, Applied Polymer Science Journal, Vol.77, 2480-2485].

US Patent 2006/0047068 describes a similar process for obtaining starch hydrogels. The cited monomers were also acrylic acid and / or acrylamide, the company of the ammonium persulfate initiator, and the ammonium ceric nitrate method was also indicated. The cross-linking of the obtained copolymers was done with Δ, Ν'-methylenobisacrylamide (MBA) and the obtained hydrogels neutralized with KOH. The materials obtained exhibited a 400 to 500-fold absorption of aqueous fluid.

The incorporation of clay into dams-based superabsorbent polymers to improve the strength and water-absorbing capacity of materials has also been reported. Superabsorbent composites of starch-g-poly (acrylamide) with tipobentonite, kaolinite and sercite clays were synthesized in the presence of ammoniacal ceric nitrate initiator and Ν, Ν'-methylenobisacrylamide as crosslinking agent. It was found that the addition of kaolinite and grouping -CONH2, -COONa, -COOH in the ratio of 8: 6: 3 conferred the highest water absorption capacity [Wu, J .; Wei Y; Lin, J. and Lin, S., 2003, Polymer44, 6513-6520].

As can be seen, acrylamide is generally used to obtain hydrogels. Polyacrylamide is known to have improved hydrophilic properties when associated with other materials [FR 9104600]. However, the use of polyacrylamide has a disadvantage. Hydrolysis of polyacrylamide leads to the release of ammonia, which is a toxic gas.

The addition of clay was also used in synthetic poly (acrylic acid) (PAA) or polyvinylamine (PVAm) based hydrogels with partially hydrolysed amino or carboxylic groups. The addition of clay is done before the neutralization step [WO 2004/018005]. Similarly, a synthetic hydrogel was obtained from the partially neutralized acrylic acid copolymerization in the presence of bentonite clay (also known as montmorillonite) with the use of β, metil'-methylenebisacrylamide as crosslinker and potassium persulfate initiator. Two types of bentonites (sodium and organophilic) were used. The results showed that the higher the debentonite concentration, the greater the swelling capacity of the hydrogel. [Santiago, F.; Muciente, A. E .; Osorio, M. and Rivera, C.F., 2007, European Polymer Journal 43, 1-9].

SUMMARY OF THE INVENTION

The present invention relates to obtaining superabsorbent crosslinked nanocomposites, said nanocomposites being prepared from the graftization of a polysaccharide, in particular regular maize starch, with acrylic acid. The resulting copolymer is crosslinked in the presence of small amounts of naturally sodium clay monomontmorillonite (MMTNa), yielding a hydrogeldotate of improved mechanical strength and increased fluid resorption capacity. The obtained superabsorbent gels have low toxicity and capacity to absorb large amounts of water, with high absorption rates. These characteristics make these materials especially useful for use in the manufacture of hygienic articles for personal use.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows the infrared spectra of superabsorbent nanocomposites, with and without the addition of clay.

Figure 2 shows the 13 C NMR spectrum of graftized starch in the presence of clay.

Figure 3 shows the X-ray diffractograms obtained for the samples and compares them to the diffractogram of the montmorillonite clay used in the invention.

Figures 4A and 4B present the analysis by scanning electron microscopy of the obtained hydrogels.

Figure 5 shows spectra of hydrogels when subjected to mechanical tests.

Figure 6 shows the percentage of organic matter converted to CO2 as a function of time.

DETAILED DESCRIPTION OF THE INVENTION

In order that the present invention may be better evaluated and understood, its detailed description will be illustrated by examples, whereby it will be apparent to those skilled in the art the advantages of the gels produced, as well as a list of possibilities of application of the materials obtained. It should be clear, however, that the examples set forth herein cannot be construed as limiting the invention.

The main raw materials used to obtain the hydrogel were regular maize starch, acrylic acid monomer and sodium montmorillonite sodium argil. These components, as presented and discussed above, strongly influence the properties and characteristics of the final superabsorbent product.

Starch is a biopolymer, found in nature in the form of granules. Two types of polysaccharides are present in the deamido granules, amylose and amylopectin, both consisting of α-D-glucose repeating units. The regular starch used in the present invention consists of about 70% amylose and 30% amylopectin. In naamylosis, the links between repetitive constitutional units are predominantly of the type [(1,4) -α-], which gives it a essentially linear structure. Amylopectin, in addition to the [(1,4) -a -] type bonds present between the D-glucose units in the main chain, has a structure characterized by a high degree of branching.

Gelatinization is known to be one of the most important properties of starches and deserves special attention. Gelatinization can be defined as the irreversible disruption of the molecular ordering of the granule, which occurs concomitantly with its swelling, lost crystallinity, and also increased viscosity and solubility. This process occurs in the presence of excess water. The degelatinization starting point, as well as the temperature range in which gelatinization occurs are governed by the following parameters: concentration, starch source, granule size and granule size distribution.

Clays are finely divided rocks made up of hydrated silicates (clay minerals). They have diameter particles in the range of microns and may have cation exchange capacity (CTC) between 3 and 150 meq / 100 g. Most mineral clays have a structural lamellar. Among the Iamelar clay minerals is the damontmorillonite group. Montmorillonite, hectorite and saponite are the most commonly used layered silicates and belong to the family of phyllosilicates. Its crystalline structure consists of two-layered tetrahedral coordinated silicon atoms or layers which are bonded to an octahedral layer of aluminum or magnesium hydroxide. The thickness of the layers measures approximately 1 nm and their side dimensions may vary from 30 nm to several micrometers depending on the type of silicate. The layers are wrapped on top of each other, giving rise to a regular distance between them, called degaleria or interlamellar distance. The cation exchange, such as Al3 + for Mg2 +, or Fe2 +, or Mg2 + for Li +, generates negative charges, which are neutralized by alkaline or earth alkaline cations, also located inside the galleries. This type of layered silicate is characterized by a surface charge known as cation exchange capacity (CTC).

In the present invention the naturally-naturally occurring montmorillonite (MMTNa) was applied. Its main characteristics are the high swelling capacity, up to 20 times its initial volume, with interplanar spaces up to 100 angstroms and the cation exchange capacity (CTC) in the range of 60 to 170 meq / 100 g. These characteristics give bentonite very specific properties, which justify its use for the formation of superabsorbent polymer nanocomposites. The clay used in the present invention, marketed by Southern Clay Products, Inc. under the name Algiers CN55, had a cationic exchange capacity, of 98.5 meq / 100g. Regular cornstarch is marketed by Com Products Brazil (Jundiaí, SP) under the name Amidex 3001.

Initially studies were carried out aiming to evaluate especially the properties and concentrations of starch, monomer and clay, which could lead to the formation of hydrogels with higher water absorption capacity. The hydrogels produced were later evaluated for the absorption capacity of aqueous solutions and the water absorption kinetics. Tests were also performed for structural, rheological and morphological characterization.

To further highlight the advantages of the process of the invention, the results will be presented in the form of examples.

Example 1: Obtaining crosslinked nanocomposites.

Briefly the process of obtaining the lattice nanocomposites object of the present invention comprises the following steps:

(a) gelatinization;

(b) graffiti and crosslinking; and,

(c) neutralization.

In gelatinization step (a), regular cornstarch, with concentrations of 1% w / w, 2% w / w and 3% w / w, respectively, was dispersed in 180 ml distilled and deionized water in a glass flask, and kept under nitrogen bubbling and agitation. The reaction mixture was heated at 90 ° C for a period of approximately 10 to 30 min. To initiate graftization and cross-linking step (b), the temperature was lowered to a range of 40 ° C - 45 ° C and 10 mL of 1M nitric acid ammoniacal nitrate solution was slowly added. After about 10 to 30 min 0.1 g of α, Ν'-methylenebisacrylamide (MBA) solubilized in varying amounts of acrylic acid (0.92 to 1.48 mol L "1) was added to the reaction medium. Reaction conditions were maintained constant for a period of 3 hours under stirring and denitrogen flow During this time, the reaction temperature ranged from 40 ° C to 45 ° C. At the end of the reaction time, about 2 mL of ethyl alcohol was added. When the reaction medium reached 60 ° C, a varying amount of 1 M NaOH was added The amount of NaOH used was equivalent to neutralize about 70% of the carboxylic acid groups The heating was kept still for 40min, followed by cooling.

After cooling, the final product was precipitated in ethyl alcohol, and dried at 70 ° C for 24 h. The sieved montmorillonite clay (Algiers CN55), only sieved, was dispersed in 18 mL of distilled water and added to the reaction medium.

Two differentiated processes were used for clay addition, using clay concentrations of 0%, 1% 2% and 5% w / w relative to the monomer.

The obtained hydrogels were then evaluated for the absorption capacity of aqueous solutions and the absorption kinetics in water. The structural, rheological and material morphological characterization were also performed.

Example 2: Study of the influence of starch concentration on absorption capacity.

Reactions to obtain materials were carried out according to the method described in Example 1. The amounts used were 2.0 g, 4.0 g and 6.0 g. The monomer concentration was kept constant by the addition of 12.68 ml (0.92 mol L "1) of acrylic acid. The amount of NaOH was 140 ml. The addition of Algiers CN55 clay was made in two parts. The clay (0.03 g) was dispersed in 8 ml of distilled water and added in the starch graftization and crosslinking step.Nutalization of the product was cooled to 40 ° C, and 0.09 g of the dispersed clay in 8 ml of water. Distillate was added to the reaction medium.

In this case, the clay concentration was equivalent to 1% w / w relative to the monomer.

The results of the water absorption capacity evaluation tests for the reaction products in relation to the starch concentration are presented in Table 1.

TABLE 1

<table> table see original document page 11 </column> </row> <table>

Example 3: Study of the influence of acrylic monomer concentration.

For these reactions, the process for obtaining the materials was performed as described in Example 1. The added amount of the amide was 4.0 g (2% w / w).

The amounts of monomer used were 12.68 mL, 15.22 mL, 17.76 mL and 20.30 mL. The amounts of 1M NaOH were 140 mL, 166 mL, 195 mL and 222 mL.

The addition of Algiers CN55 clay was done in two steps. The clay (0.03 g) was dispersed in 8 mL of distilled water and added in the starch degrafting and crosslinking step.

At the end of the neutralization, the product was cooled to 40 ° C and 0.09 g of the clay dispersed in 12 mL of distilled water was added to the major reaction. In this case, the clay concentration was equivalent to 1% w / w in relation to the monomer. The results of the water absorption capacity assays for the reaction products in relation to the used demonomer concentration are shown in Table 2.

TABLE 2

<table> table see original document page 12 </column> </row> <table>

Example 4: Study of the influence of clay concentration on the meioreaction.

For these reactions, the process for obtaining the materials was performed as described in Example 1.

The added amount of starch was 4 g (2% w / w). Monomer concentration was kept constant by the addition of 12.68 mL (0.92 mol L -1) of acrylic acid.

The amount of 1M NaOH was 140 mL. The addition of ArgelCN55 clay was done in two steps.

The clay was dispersed in 13 mL of distilled water and added entirely to the starch graftization and crosslinking step.

Clay concentrations were equivalent to 0% w / w, 1% w / w, 3% w / w and 5% w / w relative to the monomer.

The results of the water absorption capacity evaluation tests for the reaction products in relation to the clay concentration are presented in Table 3. TABLE 3

<table> table see original document page 13 </column> </row> <table>

Absorption capacity in physiological solution and urine synthetic was also investigated. For this evaluation, hydrogels without clay addition (1% w / w) were evaluated. The addition of clay improved the absorption in both studied media, as can be seen in Table 4 below.

TABLE 4

<table> table see original document page 13 </column> </row> <table>

The morphological analysis performed on the nanocomposites obtained was grouped in order to facilitate its comprehension and will be presented and discussed below.

Example 5: Structural Analysis of Superabsorbent Nanocomposites

Infrared spectra (FTIR) of the hydrogels obtained without the addition of clay are shown in Figure 1. The spectra are similar, indicating that the presence of clay did not interfere with the starch modification reaction under the conditions used. In the spectrum of both products, the main absorptions occurred in the region between 3400-2500 cm "1 and 1600 cm" 1. The band observed in the 3400-2500 cm-1 region can be attributed to the absorption (axial deformation) of the hydroxyl group. The intense absorption at 1600 cm-1 occurs when the carboxylate ion is present, which indicates that acrylic acid crushing occurred.

Figure 2 shows the 13 C NMR spectrum of the clay enterprise graft starch. In this spectrum, starch peaks (C1, C2, C3, C4, C5 and C6) were observed in the region between 101 to 59.7 ppm. The characteristic peak of the carboxyl carbon atom (C = O) can be observed in the lower field of the spectrum at approximately 185 ppm. The characteristic peaks of the absorption of the acrylic acid carbonode atoms, [-CH2-] and [-CH], appeared in the higher field at approximately 39 and 48 ppm, respectively.

Samples with different amounts of clay, 1% w / w, 3% w / w and 5% w / w, respectively, were also evaluated by X-ray diffraction. Figure 3 shows the diffractograms obtained for these samples and those compared to the diffractogram of the sample. sodium montmorillonite clay (MMTNa) Algiers CN55. It was possible to observe an intense peak at the diffraction angle (2Θ) at approximately 7.9 °, corresponding to an interplanarbasal distance (d00i) of 11.2 ° A. The X-ray results for the streaking products showed that for the synthesized hydrogels, exfoliation of the clay occurred regardless of the clay concentration used in the experiment. In diffractograms for hydrogels, it can be observed that the characteristic clay peak, at angle 2Θ = 7.9 °, was not detected. This result represents a strong indication of clay exfoliation during the preparation process of superabsorbent hydrogels.

Example 6: Morphological analysis of the materials obtained. Figures 4A and 4B below show the results of the analysis by scanning electron microscopy performed to separate the morphology of hydrogels. Examples are given at 200x and 2500x magnifications for the hydrogel with 2% w / w starch, 1% w / w clay and 12.68 ml acrylic acid.

Figure 4A (200x magnification) shows that the hydrogel is composed of particles of different shapes and sizes. The particles are smooth or porous. The porous particles were enlarged up to 2500 times (Figure 4B). These particles have large pores, which show that this structure allows water to be absorbed faster to the drier center. This behavior increases the absorption capacity [Omidian, H .; Rocca, J.G .; Park, K., 2003, Journal of Controlled Release 102, 3-12].

Example 7: Rheological analysis of the obtained nanocomposites.

The rheological properties of the hydrogels without the addition of clay (3% w / w) were evaluated. For these samples, the deamid concentration was maintained at 2% w / w and 12.68 mL of added acrylic acid. The samples without clay and clay presented water absorption capacities of 75 g / g and 97 g / g, respectively.

The rheological assays were performed in a TA Instruments AR-G2 controlled tension rheometer. The type of geometry used for the realization of the tests was rough plate, stainless steel and 40mm in diameter. The tests were performed with a 1500μm interval. The temperature was kept constant at 25 ° C.

Initially, the sample was submitted to a stabilization step for a period of up to 30 min, under a frequency of 6.283 rad.s "1. Then, a mechanical spectrum was performed, with the amplitude of deformation at 0.1% and frequency variation in the frequency. range 0.06283 to 62.83rad.s "1. Finally, a strain amplitude test was performed at a range of 0.01 to 10%. The objective of the stabilization step was only to accommodate the gel in the geometry used. The mechanical spectrum obtained for the samples is shown in Figure 5.

It can be observed that G '(co) is greater than G "(g>) in all studied frequency range. This behavior characterizes the structure of a strong gel, also formed by covalent bonds. It can be observed that the clay sample showed modulus. slightly larger than the sample without clay.This result may indicate that clay contributes to a slight increase in gel strength.This increase can be explained by the types of interactions that occur between the polymer and the clay particles, namely:

■ interaction by hydrogen bonds between the mineral surface hydroxyl groups and the dopolymer carboxylic groups

■ gel stabilization by electrostatic forces. These interactions probably increase the crosslinking of the gel which provides the increase in its mechanical properties.

On the other hand, the differences are very small and may be related to the intrinsic error of the experiment, or to small differences in the masses used [Gümgor, N .; Karaoglan, S. (2001), Materials Letters48, 168-175],

However, incorporating small amounts of glycyl into the hydrogel improved mechanical strength and significantly increased fluid absorption capacity.

The superabsorbent gels obtained, besides low toxicity, were able to absorb large amounts of water, with high absorption rates. These characteristics make them particularly suitable for use in the manufacture of hygienic articles for personal use, such as female pads and disposable diapers. These articles have the main purpose of absorbing water and biological fluids such as urine, secretions and blood, even when subjected to pressure.

In addition to this application, superabsorbent gels have also been of great interest for use in the dehydration of fuel oils.

In agriculture, initially these materials were also employed as water retention agents. Currently, they have been used as additives for the controlled (or prolonged) release of pesticides and fertilizers.

Example 8: Biodegrability of superabsorbent and commercial sample polymeric nanocomposites.

The biodegradability of hydrogels was evaluated. Comte hydrogels other than acrylic acid have been subjected to ground degradation tests in accordance with ASTM D 5988-96.

Figure 6 shows the percentage of organic matter converted to CO2 as a function of time over 180 days of incubation on land for cellulose samples (Figure 6 - a) of hydrogels obtained from deamid, with the addition of 12.68 mL ( 0.92 mol L "1) of acrylic acid (Figure 6-b), with addition of 15.22 ml (1.11 mol L" 1) of acrylic acid (Figure 6 - c); efor the commercial hydrogel (Figure 6 - d).

The experiments were performed according to Example 3.The first sample (b), with water absorption capacity of 182 ± 2 g / g, was obtained from 2% (w / w) starch, with addition 12.68 mL (0.92 mol L "1) of acrylic acid.

The second hydrogel sample (c), with a water absorption capacity of 475 ± 8 g / g, was obtained from 2% (w / w) starch with the addition of 15.22 mL (1.11 mol L "1) of acrylic acid.

The results showed that for emam based hydrogels, a higher evolution of CO2 occurs in the first 20 days of incubation. For the commercial sample, CO2 evolution was practically linear throughout the incubation period. Increasing the concentration of acrylic acid in the reaction mixture led to hydrogel with greater resistance to biodegradation.

Claims (9)

1.The process of obtaining superabsorbent cross-linked nanocomposite, characterized in that it comprises the grafting of a polysaccharide, in particular milhoregular starch, with a copolymer of acrylic acid, which is cross-linked in the presence of small amounts of naturally occurring sodium montmorillonite clay. a hydrogel with characteristics of biodegradability, low toxicity, capable of absorbing large amounts of water, at high absorption rates and mechanical resistance. A process for obtaining superabsorbent nanocomposite bonds according to claim 1, characterized in that the acrylic acid copolymer comprises a reaction product of acrylic acid and Ν, Ν'-methylenebisacrylamide (MBA) in the presence of ammonium ceric nitrate. 3. A process for obtaining superabsorbent nanocomposite bonds according to claim 1, wherein the regular maize starch is used at a concentration of 1% w / w to 3% w / w. 4. A process for obtaining superabsorbent nanocomposite substrates according to claim 1, wherein the sodium montmorillonite clay (MMTNa) is used at a concentration of up to 5% w / w relative to the acrylic acid monomer. 5. A process for obtaining superabsorbent nanocomposite bonds according to claim 1, wherein the acrylic acid is used at a concentration of 1 to 1.5 mol Γ1. 6. A process for obtaining superabsorbent nanocomposite bonds according to claim 1, wherein the hydrogels obtained are applied in the manufacture of hygienic articles for personal use. 7. A process for obtaining superabsorbent nanocomposites according to claim 1, characterized in that the hydrogels obtained are applied to the dehydration of oils and fuels. 8. A process for obtaining superabsorbent nanocomposite bonds according to claim 1, characterized in that the hydrogels obtained are applied in agriculture as water retention agents. 9. A process for obtaining superabsorbent recycled nanocomposite according to claim 1, characterized in that the hydrogels obtained are applied in agriculture as additives for the controlled release of pesticides and fertilizers.