Application of waterborne acrylic emulsions in coated controlled release fertilizer using reacted layer technology☆

Yazhen Shen,Cong Zhao,Jianmin Zhou,Changwen Du*

State Key Laboratory of Soil and Sustainable Agriculture,Institute of Soil Science,Chinese Academy of Sciences,Nanjing 210008,China

Keywords:Controlled release fertilizer Acrylic emulsion Siloxane Crosslinker Reacted layer technology

ABSTRACT Waterborne acrylic emulsions modified with organic siloxanes and aziridine crosslinker were synthesized and applied as coating of controlled release fertilizer.The free films were characterized and the nutrient release profiles of the coated fertilizers were determined.The results show that methyl silicone oil and methylsilanolate sodium could not improve water resistance performance and glass transition temperature T g of coatings,while the firmness is enhanced.Aziridine crosslinkerimproves the water resistance performance, firmness and T g.Incorporation of methyl silicone oil and aziridine crosslinker gives an excellent aqueous acrylic emulsion for coated controlled release fertilizer,with the 30-day cumulative nutrient release reduced to 16%and an estimated nutrient release duration over 190 days.Therefore,this waterborne coating is promising to meet the requirements for controlled release of nutrient and environmental protection.

1.Introduction

Controlled release fertilizers(CRFs)are preferred alternatives to soluble fertilizers.Due to proper nutrient release throughout the growing season,crops can uptake more nutrient,increasing nutrientuse efficiency and benefiting environmental protection[1-3].With their excellent controlled release effect,the polymer coated fertilizers are the most promising controlled release fertilizers[4].

Polymer coated fertilizers can be prepared by coating conventional fertilizer particles with various polymer materials,such as polyole fins[5],polyvinyl chloride[6-8],polysulfone[7],and polyacrylamide[9,10].These polymers are usually dissolved in specific organic solvent to obtain polymer solutions.The solution is sprayed on the surface of fertilizer granules and the solvent is removed through evaporation[10-13].This coating process demands costly volatile organic solvent and may lead to secondary pollution[14,15].Recently,waterborne coating technique has been used widely because of strict pollution regulation.Waterborne polyacrylate is considered as environmentally friendly materials.Polyacrylate is biodegradable because it contains susceptible ester bonds,does not influence the soil structure and soil microorganism community activity,and benefits soil fertility.In addition,the aqueous acrylic emulsions have excellent film-forming characteristics and appropriate viscosity,but its application in coated controlled release fertilizers is limited by the hydrophilicity of waterborne polymer[16].Siloxanes and aziridine crosslinker have been used to improve the water resistance performance of polymers by modifying the acrylic emulsion[17-22].The crosslinking reaction of aziridine is very weak at room temperature and will be rapidly accelerated with the increase of temperature[23-27],which provides a way for the crosslinking process concomitantly with film-forming(reacted layer technology)in the fertilizer coating using Wurster fluidized bed[28,29].

In our previous research,waterborne acrylic emulsion was used as the coating of controlled release fertilizers,but the release duration was too short for some crops with long growth periods[30].In this study,waterborne acrylic emulsion modified with organic siloxanes and aziridine crosslinker are synthesized to increase the nutrient release duration.The free films are characterized and the nutrient release profiles of the coated fertilizers are determined.

2.Experimental

2.1.Materials

The commercial granular NPK compound fertilizer(NPK 15-12-15)with 2-3 mm in diameter(Fulilong Guangdong Fertilizer Co.Ltd.)was used to prepare the coated fertilizers.Methyl methacrylate,butyl acrylate,and ethylene glycol(EG)were purchased from Nanjing Chemical Regent Co.,Ltd.;methacrylic acid was from Shanghai Ling Feng Chemical Reagent Co.,Ltd.;sodium dodecyl benzene sulfonate was from Chengdu Kelong Chemical Reagent Co.,Ltd.;alkyl phenol polyethenoxy(OP-10)was from Hebei Xingtai Kewang Auxiliary Agent Co.,Ltd.;and potassium persulfate was from Sinopharm Chemical Reagent Co.,Ltd.Methyl silicone oil(MSO)was from Boyang Industrial Co.,Ltd.and methylsilanolate sodium(MSS)was from Beijing Ruichen Chemical Auxiliary Agent Co.,Ltd.The chemical structures of MSO and MSS are demonstrated in Fig.1.Crosslinker(SaC-100)was from Shanghai UN Chemical Co.,Ltd.and the cross-linking reaction formula is shown in Fig.2.

Fig.1.The chemical structures of methyl silicone oil and methylsilanolate sodium.

2.2.Synthesis of acrylic emulsions and preparation of free films

The emulsions were prepared by semicontinuous emulsion polymerization,carried out in a 1000 ml three-necked flask equipped with a reflux condenser,a stirrer,and a dropping funnel.The aqueous phase was prepared by dissolving 8.24 g OP-10,4.12 g sodium dodecyl benzene sulfonate,and 31 g EG in 248 g deionized water.The organic phase was prepared by mixing 110 g butyl acrylate,90 g methyl methacrylate and 3.5 g methacrylic acid.Both phases were vigorously stirred for 30 min and the temperature was raised to 80°C until the end of polymerization.25%(by mass)of the oilwater mixture in the flask was used as the initial charge.The rest mixture and the initiator solution(52 ml,0.013 g·ml?1K2S2O8)were fed alternately in 4 doses over 3 h.Siloxane(MSO or MSS)was then injected to the flask,and the polymerization was conducted under air atmosphere for another 3 h.One acrylic emulsion(A)and two silicone-acrylic emulsions(B and C)were obtained.0.75%(mass)of aziridine crosslinker was dissolved in deionized water,and then slowly added to acrylic emulsion at room temperature with continuous stirring for 15 min.Emulsions A,B and C were modified with crosslinking to emulsions Aa,Baand Cacorrespondingly,as shown in Table 1.The solid content of all emulsions was 40%.The free films were obtained by casting the prepared emulsion(2.0 g)onto a 9 cm2leveled plastic cap and dried in an oven at 60°C for 24 h.The free films were removed from the plastic caps for further use.

2.3.Characterization of the free films

The swelling degree is defined as(w2?w1)/w1×100%[26].The free films with the same thickness were immersed in 100 ml deionizedwater at 25°C for 72 h.The mass of films before(w1)and after(w2)immersion were recorded in triplicate.

Table 1 Compositions of emulsions,T g of free films and nutrient release durations of CRFs

For the glass transition temperature(Tg),8-10 mg of free films was weighed and differential scanning calorimetry(DSC,Perkin-Elmer Pyris 1,USA)was used at a heating rate of 20 °C·min?1.The thermal behavior was examined under the nitrogen atmosphere between?50°C and 150°C.Tgwas taken at the onset of the corresponding heat capacity jump.As a rule,two successive scans were made for every sample.All calculations were performed on the second heating cycle.

The surface morphology of free films was examined by optical microscope at a magnification of 1000×(Olympus,S83084).

A FTIR(Nicolet 380,USA)equipped with photoacoustic accessory(Model 300,MTEC,USA)was used for the spectra of free films,with a wave number range of 500-4000 cm?1and mirror velocity of 0.32 cm·s?1.32 successive scans were recorded with a resolution of 4 cm?1.

2.4.Preparation and release characteristics of coated fertilizers

The fertilizer granules were coated in a Wurster fluidized bed equipped with a bottom-spray pneumatic nozzle(LDP-3,Changzhou Jiafa Granulation Drying Equipment Co.,Ltd.).The process parameters were: fluidized bed temperature,45-50°C;spray rate of coating emulsions,2.5 g·min?1;atomization pressure,0.1 MPa.The amount of coating emulsions was 125 g per 500 g original fertilizer granules.The average coating thickness was about 100 μm.In all the cases,the coated pellets were tray-dried in an oven at 45°C for 24 h.

Ten-gram coated fertilizers were immersed in 100 ml of deionized water at25°C.100 ml was analyzed and replaced by 100 ml fresh deionized water,three replicates.The released nutrient content was evaluated by the solution conductivity[31],measured with an electrical conductivity apparatus(DDS-320,China).On the 30th day of study,the coated fertilizers were ground to determine the content of residual nutrient.The release characteristics were estimated as the cumulative release percentage versus time.The nutrient release duration of CRFs is denoted by the time for release of80%of the total nutrient content.The durations of A and B were found from the cumulative release curve,and those of other CRFs are calculated as follows:(80?Y30)/V10-20+30,where Y30stands for the fraction released at the 30th day,and V10-20is the average daily release rates from the 10th to the 20th day.

Fig.2.Mechanism of cross-linking reaction.

3.Results and Discussion

3.1.Characterization of the free films

3.1.1.The swelling properties of free films

The swelling degree of free films is an important evidence of water resistance performance,and lower swelling degree indicates better water resistance performance.The swelling degree of the free films is shown in Fig.3.The water resistance performance changes less after modified with MSO.Ethylene glycol was used in silicone-acrylic emulsions B and C to restrain hydrolysis and self-condensation of siloxane,butitis a good absorbent for water.In addition to the hydrophilicity of EG,MSS is a short chain siloxane that is easily hydrolyzed(Fig.1).Thus hydroxyl group content increases and water resistance performance is worse after being modified with MSS.It can be concluded that siloxane does not improve the water resistance performance of the coating as expected.The crosslinker significantly improves the water resistance performance of coating with acrylic emulsion and silicone-acrylic emulsion.The swelling degrees of Aa,Baand Cawere respectively decreased by 25%,67%and 90%compared with A,B and C.Aziridine crosslinker could react with carboxyl of the acrylic emulsion[25](Fig.2),so the hydrophilic carboxyl group content is reduced and water resistance is improved.In addition,crosslinking reaction results in a network in the structure,restricting the swelling of hydrophilic group.The decrease extent depends on emulsion type in the presence of aziridine crosslinker.The crosslinker reduces the swelling degree more with silicone-acrylic emulsion than with acrylic emulsion.The possible reason is that more hydroxyl from EG and hydrolysis of siloxane provide much more active sites for aziridine crosslinker in the silicone-acrylic emulsion.

Fig.3.The swelling degrees of free films prepared with acrylic emulsion(A),siliconeacrylic emulsion modified with MSO(B)and MSS(C).

3.1.2.The glass transition temperature of free films

Tgof the coating is related closely with the film formation state.If Tgis too low,the fertilizer granules would stick to each other,which would hamper particle fluidization and encapsulation;if Tgis too high,the coating would be hard and vulnerable to rupture,which leads to a sudden nutrient release[30].The glass transition temperatures of free films are presented in Table 1.The values of Tgfor B and C are lower than those of A due to low glass transition temperature of siloxanes[31]and plastication of ethylene glycol.The coating modified with aziridine crosslinker presents higher Tg,because the crosslinking reaction strengthens molecular interactions and restricts molecular movement.Higher Tgcan prevent CRFs from softness.

3.1.3.Surface morphology of free films

The film morphology is closely related to the film-forming process,and the morphology of these free films is strongly influenced by the composition of emulsion as displayed in Fig.4.The surfaces in Fig.4B and C are more smooth and uniform than those in Fig.4A,since MSO and MSS behave as a general crosslinking agent with the formation of Si-O-C link and physical interpenetration between Si-O-Si and Si-OC networks.Compared with A,B and C,Aa,Baand Cagive firmer and more homogeneous surfaces.It suggests that a high density crosslinking network forms through crosslinking reaction,which benefits the film property.

3.1.4.FTIR-PAS analysis

Fig.5 demonstrates the FTIR-PAS spectra for the free films.A broad band due to O-H stretching vibration at 3250-3550 cm?1and another band due to aliphatic C-H stretching vibration at 2800-2950 cm?1are observed.The ratio of absorption band denoting hydrophilic group(O-H)to that representing hydrophobic group(C-H)indicates the hydrophilicity of films[30].The absorption intensity of O-H is similar to that of C-H in A,while the absorption intensity of O-H is stronger than that of C-Hin both B and C.Compared with A,B and C,the intensity of C-H relative to O-H is strengthened in Aa,Baand Ca.The absorption intensity ratios of CH/OH for A,B and C are 0.81,0.83,and 1.03,respectively,while those are 1.33,1.22,and 1.05 for Aa,Baand Ca,respectively.Such a change demonstrates that water resistance performance is worse after being modified with siloxanes(MSO and MSS)and is improved after being modified with aziridine crosslinker(SaC-100).The results are in good agreement with that of the swelling degree.The spectra of B and C in the range of 990-1136 cm?1are broader compared to those of A because of the Si-O-Si and Si-O-C bands at 1030 and 965 cm?1,respectively[32,33].Besides,the peaks at 1446 cm?1are detected in all free films except for A.For B and C,the peak at 1446 cm?1is ascribed to asymmetric deformation vibration of CH3in Si-CH3[34].For Aa,the peak is assigned to the distortion vibration of CH2due to stronger interaction from aziridine crosslinker[35].For Baand Cathe peak appears due to both asymmetric deformation vibration of CH3in Si-CH3and distortion vibration of CH2.

3.2.Nutrient release characteristics of polymer coated fertilizers

Controlled release characteristics are essential to evaluate whether the coatings are suitable for CRFs.The nutrient release characteristics of CRFs coated with different emulsions are demonstrated in Fig.6.The 30-day cumulative nutrient release of B and C is decreased by 7%and 16%,respectively,in comparison with that of A,and the nutrient release durations of B(29 days)and C(33 days)are longer than those of A(23 days).It was observed that coated fertilizers of B and C were stuck in the release test,which prevents moisture penetrating through the coating and slows nutrient release.The adhesion could be explained by lower Tgof B(5.5 °C)and C(8.93 °C)than that of A(10.4 °C).After being modified by MSO and MSS,free films are more homogeneous,as shown in Fig.4.It was reported that such a structure could control the nutrient diffusion better[36].

Fig.4.Optical microscope pictures of free films(magnification 1000×).A:acrylic emulsion;B and C:silicone-acrylic emulsions modified with methyl siloxane oil and methylsilanolate sodium,respectively;Aa,Ba and Ca:with crosslinker(SaC-100)added to A,B and C.

Modified with aziridine crosslinker,coated fertilizers exhibit a much slower nutrient release.The 30-day cumulative nutrient release of A,B and C was reduced from 87%to 39%,80%to 16%and 71%to 38%,respectively.The nutrient release durations of Aa,Baand Cawere substantially prolonged(as shown in Table 1).On the one hand,the coating presents better water resistance performance after being modified with aziridine crosslinker,as shown by the swelling degree and FTIR-PAS analysis.On the other hand,crosslinking reaction forms a three dimensional network structure,so the coating is firm and more homogeneous(Fig.4).It can be concluded that nutrient release characteristics of CRFs are associated with both coating hydrophobicity and film property.Although the hydrophobicity of film A is stronger than that of C,its film surface is rough,so that the film property is relatively poor,its influence dominates the nutrient release,and the release duration is relatively shorter.Incorporation of organic siloxanes(MSO)and aziridine crosslinker(SaC-100),Tgof the coating is enhanced,avoiding the coated pellets sticking to each other.The water resistance performance and firmness are both improved,reducing nutrient diffusion rate.In practice,the nutrient release of the controlled release fertilizers is mainly affected by rainfall,temperature and soil types.It is difficult to model the real release rate in soil since the soil condition varies.Anyway,the release of the polymer coated fertilizers in soil is significantly slower than that without coating.

4.Conclusions

Fig.5.FTIR-PAS spectra of free films without(a)and with(b)crosslinker.A:acrylic emulsion;B and C:silicone-acrylic emulsions modified with methylsiloxane oil and methylsilanolate sodium,respectively;Aa,Ba and Ca:with crosslinker(SaC-100)added to A,B and C.

Fig.6.The cumulative release of polymer coated fertilizers.a:acrylic emulsion;b and c:silicone-acrylic emulsions modified with methyl siloxane oil and methylsilanolate sodium,respectively;Aa,Ba and Ca:with crosslinker(SaC-100)added to A,B and C.

In order to increase the nutrient release duration,the waterborne acrylic emulsions were modified with siloxanes(methyl silicone oil and methylsilanolate sodium)and aziridine crosslinker(SaC-100)based on reacted layer technology.After modified with two siloxanes,the water resistance performance was not improved and Tgwas decreased,but denser coatings were achieved.Aziridine crosslinker improved the water resistance performance, firmness and Tg.Incorporation of methyl silicone oil and aziridine crosslinker(SaC-100)produced excellent aqueous acrylate emulsions for fertilizer coating,and the 30-day cumulative nutrientrelease was reduced to 16%compared to that of acrylic emulsion(87%).The nutrient release duration was estimated as 190 days.Methyl silicone oil and aziridine crosslinker modified waterborne acrylic emulsion can be successfully used as fertilizer coating to decrease the nutrient release rate and is promising in the industry of polymer coated controlled release fertilizer.