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Industrial Crops and Products 36 (2012) 41–46

Contents lists available at SciVerse ScienceDirect

Industrial Crops and Products

journa l homepage: www.e lsev ier .com/ locate / indcrop

oy protein isolate/poly(lactic acid) injection-molded biodegradable blends forlow release of fertilizers

uciane Calabriaa, Nathália Vieceli a, Otávio Bianchib, Ricardo Vinicius Boff de Oliveirab,raja do Nascimento Filhoa, Vanessa Schmidtc,∗

Centro de Ciências Exatas e Tecnologia (CCET), Universidade de Caxias do Sul (UCS), Rua Francisco Getúlio Vargas, 1130 Caxias do Sul, RS 95070-560, BrazilInstituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Goncalves, 9500 Porto Alegre, RS 90040-060, BrazilDepartamento de Química, Universidade Federal de Santa Maria (UFSM), Av. Roraima, 1000 Santa Maria, RS 97105-900, Brazil

r t i c l e i n f o

rticle history:eceived 21 May 2011eceived in revised form 28 July 2011ccepted 13 August 2011vailable online 24 September 2011

a b s t r a c t

A slow release fertilizer system consisting of materials derived exclusively from biomass, and suitablefor i) production of injection-molded parts such as containers for growing plants, and ii) use as granules,is reported. Soy (Glycine max L. Merr.) protein isolate/poly(lactic acid) blends plasticized with triacetin(SPI/PLA–TA) were used as matrix for NPK fertilizer incorporation. Upon melt processing, this compos-ite material formed a highly ordered porous matrix of SPI in which PLA domains are homogeneouslydispersed with NPK salts. Dynamic conductivity measurements indicated good release properties as the

eywords:eleaseertilizeroy protein isolateoly(lactic acid)iodegradable blends

cumulative amount increased much slower with time as compared to pure NPK sample. Biodegradationwas accessed by examining weight loss and surface morphology as a function of incubation time in soil.

© 2011 Elsevier B.V. All rights reserved.

elt processing

. Introduction

Environmental and economic aspects related to the use ofertilizers in agriculture have prompted intense research on theevelopment of rational approaches to achieve substantial increase

n production of foodstuffs under stringent dosage limitationsJarosiewicz and Tomaszewska, 2002; Jin et al., 2011; Ni et al., 2010;eodorescu et al., 2009; Wu et al., 2008). In the case of conven-ionally formulated fertilizers, nearly half of the amount may turnneffective in terms of plant nutrient use depending on the methodf application and soil condition (Entry and Sojka, 2008; Vassilevnd Vassileva, 2003). The rapid dissolution of water-soluble inor-anic salts present in most fertilizers almost immediately leadso local nutrient concentration levels that are excessively highor effective absorption by crops, and eventually hazardous forurrounding ecosystems. The adverse effects may also extend toontamination of waterside and ground water reserves (Davidnd Gentry, 2000; Sharpley et al., 2000). Recent regulatory efforts

mposed by government agencies for water quality protection mayequire more efficient fertilization practices in the near futureHartz and Smith, 2009).

∗ Corresponding author.E-mail address: vschmidt@ufsm.br (V. Schmidt).

926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.indcrop.2011.08.003

Within this context, a number of strategies allowing for con-trolled (or slow) release fertilizers (CRF) have been devised inorder to improve the efficiency of fertilizers, and decrease thefrequency and concerns of their application (Jarosiewicz andTomaszewska, 2002; Jin et al., 2011; Ni et al., 2010; Teodorescuet al., 2009; Wu et al., 2008). Many successful approaches considereither the synthesis of novel compounds with low water solubilitythrough which slow nutrient release is attained simply by progres-sive dissolution (Bandyopadhyay et al., 2008; Bhattacharya et al.,2007), or the coating of fertilizers with insoluble organic materials(Jarosiewicz and Tomaszewska, 2002; Ni et al., 2010; Tomaszewskaand Jarosiewicz, 2002). In the latter case, the release rate ofnutrients can be controlled by different mechanisms includingdiffusion control through a protective barrier/film and biodegra-dation/erosion, which are established by the physical–chemicalproperties of the coating material. Ideally, the coating materialshould be obtained from renewable sources to ensure sustainabilityand low environmental impact of its production and application.

Polysaccharides and proteins are suitable starting materialsfor the manufacture of a variety of products (Orliac et al., 2003).Indeed, a number of investigations demonstrated the advantages of

using sunflower (Helianthus annuus L.) (Orliac et al., 2003), wheat(Triticum aestivum L.) gluten (Irissin-Mangata et al., 2001), corn(Zea mays L.) (Gennadios and Weller, 1990), maize zein (Lai andPadua, 1997), and soy proteins (Chabba et al., 2005; Kumar et al.,

42 L. Calabria et al. / Industrial Crops and Products 36 (2012) 41–46

Table 1Composition and thermal properties of SPI/PLA–TA blends and SPI/PLA–TA–NPK composite materials.

Entry Sample Composition Thermal properties

SPI/PLA (w/w) TA (phr) NPK(phr) Tg (DSC) Tm (DSC) Tg (DMA) T˛ (DMA)

Blend componentsSPI – – – 148 – –PLA – – – 57 145 61 –

SPI/PLA–TA (w/o fertilizer)1 60/40–5 60/40 5 – 57 149 55 742 60/40–10 60/40 10 – 36 142 45 653 60/40–20 60/40 20 – 29 142 29 774 70/30–5 70/30 5 – 59 150 54 695 70/30–10 70/30 10 – 57 149 44 666 70/30–20 70/30 20 – 56 149 40 837 80/20–5 80/20 5 – 58 150 ND ND8 80/20–10 80/20 10 – 58 149 ND ND9 80/20–20 80/20 20 – 58 151 ND ND

SPI/PLA–TA–NPK10 60/40–5–10 60/40 5 10 58 149 54 7011 60/40–10–10 60/40 10 10 46 145 46 6612 60/40–20–10 60/40 20 10 43 144 30 7213 70/30–5–10 70/30 5 10 59 149 ND ND14 70/30–10–10 70/30 10 10 58 149 37 6815 70/30–20–10 70/30 20 10 56 148 ND 6916 80/20–5–10 80/20 5 10 59 149 ND ND17 80/20–10–10 80/20 10 10 58 149 ND ND18 80/20–20–10 80/20 20 10 58 148 ND ND

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002; Ly et al., 1998; Schmidt et al., 2005b; Swain et al., 2004) inhe fabrication of polymeric materials. Special interest is hereinlaced on soy protein isolate (SPI) (Kumar et al., 2002; Ly et al.,998), which has attracted huge interests because it can produceels and undergo conventional melt processing similar to a ther-oplastic, specially in presence of secondary components such as

olyesters and plasticizers responsible for the reduction of meltiscosity imparted by strong intra- and intermolecular interac-ions (Ly et al., 1998; Park and Hettiarachchy, 2000). Indeed, SPIas been successfully combined with synthetic and natural poly-ers, bringing its biocompatibility and environmentally friendly

haracteristics to a wide range of materials produced for differentpplications (Kaplan, 1998).

In this study, a new class of CRF systems was developedsing materials derived exclusively from biomass, and suitableor melt processing techniques such as extrusion and injection

olding. Nitrogen, phosphorous and potassium fertilizer (5:20:20)as dispersed in a matrix consisting of SPI (hydrophilic naturalolymer), poly(lactic acid) (PLA, hydrophobic biopolymer), andriacetin (TA, plasticizer synthesized from glycerol – a byprod-ct of biodiesel production). The resulting fertilizer-containingiodegradable composite material can be injection-molded intoiverse shapes such as containers for growing plants, as well assed in the form of granules.

. Materials and methods

.1. Materials

Poly(lactic acid) (PLA, NatureWorks D2002), triacetin (TA, Dowhemical), and NPK fertilizer (NPK 5:20:20; Piratini Fertilizantestda) were used as received. Nitrogen, phosphorous and potas-ium elements were present in the form of ammonium nitrate,

mmonium sulfate, ammonium dihydrogenphosphate, ammo-ium monohydrogenphosphate, calcium dihydrogenphosphate,alcium monohydrogenphosphate, calcium phosphate, potas-ium chlorine, potassium sulfate and potassium nitrate. The

corresponding dissolved anions and cations were identified by ion-exchange chromatography during release experiments. Soy proteinisolate (SPI, Supro 500E) was kindly donated by Solae do Brasil, andwas also used as received.

2.2. SPI/PLA–TA and SPI/PLA–TA–NPK blend preparation

The title blends were obtained by mechanical mixture usinga Haake high shear mixer. A mixture containing SPI, PLA, and TAplasticizer was processed during 5 min at 165 ◦C and screw speedof 60 rpm. In the case of SPI/PLA–TA–NPK blends, the NPK fertil-izer was introduced in the mixer after PLA melting, which wascharacterized by stabilization of torque. In the sequence, sampleswere cryogenic milled using an IKA M20 apparatus, and injectionmolded at 165 ◦C using a Haake MiniJet II system in order to pre-pare 120 mm × 10 mm × 3.3 mm specimens. Using this approach,samples containing NPK fertilizer and different amounts of biopoly-mers (SPI/PLA (w/w) – 60/40, 70/30, and 80/20) and plasticizer(5–20 phr – parts per hundred parts of blend) were obtained. Hereand throughout the text, the composition of blends is noted as inSPI/PLA–TA–NPK 60/40–10–10, where 60/40 indicates the relativeSPI/PLA mass ratio, and –10–10 refers to the amount (in phr) of TAplasticizer and NPK fertilizer. Table 1 shows the composition andselected physical–chemical properties of blends prepared using theabove-described method.

2.3. Thermal analysis

Thermal degradation of SPI-based blends was evaluated using aShimadzu TGA-50 thermogravimetric analyzer within the temper-ature range of 25–900 ◦C. Differential scanning calorimetry (DSC)studies were carried out on a Shimadzu DSC-60 differential scan-

ning calorimeter in the temperature range of 25–190 ◦C. DSC curvescorrespond to the second heating/cooling cycle (except for PLA –first cycle). In both cases, the experiments were conducted undernitrogen flow of 50 mL/min at heating rate of 10 ◦C/min.

rops and Products 36 (2012) 41–46 43

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and melting occurred as the temperature increased. The compositematerials, on the other hand, were characterized by two thermallyinduced transitions that were not affect by NPK fertilizer. In Fig. 2,the first abrupt increase in the loss tangent around 55 ◦C is ascribed

L. Calabria et al. / Industrial C

.4. Scanning electron microscopy (SEM) and energy dispersion-ray microanalysis (EDX)

Surface and cross-section morphologies were determined usingShimadzu SSX-550 scanning electron microscope operating at

5 keV. SPI based blends were fixed on a sample holder with doubledhesive carbon tape and gold metalized. For cross-section anal-sis, specimens were fractured under cryogenic conditions. Theurface chemical composition was determined by an EDX equip-ent coupled to the Shimadzu SSX-550 microscope.

.5. Release of NPK fertilizer

In order to understand the release profile of NPK salts fromhe composite materials, conductivity measurements reflecting theotal concentration of electrolytes in solution (i.e.: this methodoes not differentiate each source of N, P and K) were performeds a function of time in aqueous media (250 mL of Milli-Q water;esistivity = 17.3 mohms) after the immersion of blend specimensmass = 1.60 g). Experiments were carried out at 25 ◦C under gen-le magnetic stirring, and the conductivity values were collectedvery hour. Control experiments were performed using free NPKalts (same amount present in blends) and blend matrix (withoutPK).

.6. Biodegradation tests

Biodegradation was evaluated according to ASTM D-6400-4 standard (6400-04-99, 1993) for 0.3 mm × 10.0 mm × 10.0 mmpecimens maintained at 32 ± 2 ◦C and 80% relative humidity inriplicate. Periodically, samples were removed from soil, washednd dried at 40 ◦C for subsequent weight loss measurements, SEMmaging and DSC thermal characterization.

. Results and discussion

.1. Blend processing and properties

SPI/PLA blends compatibilized using a polyoxazoline were foundo be suitable for melt processing as previously described (Zhangt al., 2006). In the present study, however, SPI/PLA blends wererepared without addition of compatibilizing agents. The influencef the agrochemical on thermal, mechanical and morphologicalharacteristics was evaluated in order to better understand theevelopment process and controlled release applications of thesenvironmentally friendly materials.

The characteristic DSC profiles of SPI, PLA, and selected SPI/PLAystems containing NPK are presented in Fig. 1, and the data forll blends are given in Table 1. The SPI has an amorphous struc-ure, showing only one glass transition temperature (Tg) at 148 ◦CSchmidt et al., 2005a). However, PLA is a semi-crystalline poly-

er that exhibits a Tg at 57 ◦C followed by melting (Tm) at 145 ◦C.he latter is hardly observed in the second heating scan due toack of structural re-orientation during cooling as a consequencef high molecular weight and low dispersity of macromolecularhains (Signori et al., 2009). SPI/PLA blends plasticized with TA toncrease processability featured the thermal events of individualomponents. The first transition corresponds to the Tg of PLA, ands followed by an exothermic peak attributed to its cold crystal-ization that is induced and accelerated by SPI (Zhang et al., 2006).ubsequently, an endothermic peak associated with the meltingf PLA crystals was observed, which overlaps the Tg of SPI in the

ame temperature range. The glass transition of PLA was the onlyhermal event clearly influenced by plasticizer concentration. Inter-

ediate TA amounts (10–20 phr TA) caused a noticeable decreasen the Tg (from 57 down to 43 ◦C) and Tm (from 149 down to 144 ◦C)

Fig. 1. DSC thermograms of pure components and SPI/PLA–TA–NPK blends, as indi-cated.

(Table 1), because of the higher free volume of macromolecularchains imparted by TA. The same behavior was identified in absenceof NPK, but the decrease in Tg and Tm was more pronounced. There-fore, the effects of NPK salts and the plasticizer are opposed, asusually observed for inorganic fillers (Osswald and Menges, 2003).

An interesting feature observed in the DSC curves is the changein the melting enthalpy values that reflect the crystallinity of thematerial (not shown). In general, the highest degree of crystallinity(on the second heating/cooling cycle) was observed for blends con-taining high amounts of PLA and TA. On the other hand, SPI/PLA–TA60/40–05 blends presented very low crystallinity, essentially mir-roring the behavior of the PLA polymer. These results confirm thatSPI/PLA–TA blends can have their crystallinity tailored by adjust-ing the SPI/PLA ratio and TA amount. Importantly, the degree ofcrystallinity did not change upon NPK addition.

The viscoelastic behavior of PLA and SPI/PLA blends in the pres-ence of NPK was investigated by DMA (Fig. 2). The loss tangent(tan ı) of neat PLA presented a damping peak at 61 ◦C related to its�-transition. Above this transition, a severe deformation (bended)

Fig. 2. Loss tangent for PLA and selected SPI/PLA–TA–NPK blends.

44 L. Calabria et al. / Industrial Crops and Products 36 (2012) 41–46

F rtilizer

tPSetse

cat

Fa

ig. 3. SEM micrographs (cross-section) showing the effect of plasticizer and feespectively. Scale bars: 10 �m; magnification: 1000×.

o the TA plasticizer concentration-dependent glass transition ofLA, whereas the second process is attributed to an �-transition ofPI, in agreement with previous work (Zhang et al., 2006). Plasticiz-rs such as moisture, but not TA, modify the temperature at whichhe latter transition occurs. SPI may undergo up to three glass tran-itions depending on the relative humidity during storage (Tangt al., 2007).

In general, SPI/PLA–TA and SPI/PLA–TA–NPK blends consisted ofomplex coarsened phase structures for both SPI and PLA domains,s determined by SEM imaging analysis shown in Fig. 3. Soy pro-ein isolate domain forms the polymeric matrix, in which the PLA

ig. 4. SEM micrographs (cross-section) of SPI/PLA–TA 80/20–10 (left) and SPI/PLA–TA–Nselective solvent for PLA and TA. Scale bars: 10 �m; magnification: 1000×.

r concentration in SPI/PLA 60/40 (left) and SPI/PLA–TA 60/40–10 blends (right),

domains are dispersed. As the morphology of polymeric blendsdepends on many variables, including interfacial adhesion, vis-cosity and processing parameters for example (Cho and Rhee,2002), the antagonist properties of the hydrophilic protein (SPI)and the hydrophobic polymer (PLA) lead to poor interfacial adhe-sion. According to Signori et al., the high viscosity of SPI can leadto the formation of a continuous blend, as indeed observed in the

present study (Signori et al., 2009). This is clearly evidenced inFig. 4, which shows SEM images obtained from the cross-sections ofSPI/PLA–TA blends after chemical etching (extraction) with CHCl3– a selective solvent for PLA. Such a procedure revealed a highly

PK 80/20–10–10 (right) materials following 60 min of immersion in chloroform –

L. Calabria et al. / Industrial Crops and Products 36 (2012) 41–46 45

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ig. 5. Nutrient release monitoring by conductivity measurements a function ofime at 25 ◦C for NPK fertilizer, SPI/PLA–TA 60/40–10 blends, and SPI/PLA/TA–NPK0/40–10–10 composite material (dynamic tests).

rdered porous matrix of SPI, in which the PLA domains are homo-eneously dispersed as a consequence of its percolation throughhe SPI phase. Similar results were reported earlier for blends of soyrotein concentrated (SPC), PLA, and glycerol compatibilized witholy(2-ethyl-2-oxazoline) (Zhang et al., 2006). SEM micrographsaken using such an extraction technique to reveal SPI domainsndicated that the thickness of this structure is proportional to thePI content in the blend. Indeed, SPI/PLA 60/40 samples (Fig. 4) con-isted of porous membrane whose wall was clearly thinner than forPI/PLA 80/20 samples (Fig. 4, inset).

Fig. 4B shows the cross-section micrograph of thePI/PLA–TA–NPK blend, on which a remarkably different morphol-gy is evidenced. This is caused by the presence of NPK salts (whiterystalline structures on the image) that change the interactionith the solvent. These salts are dispersed in the low viscosity PLAhase during melt processing, meanwhile the SPI tended to remains a continuous phase due to its high viscosity. Importantly, theertilizer is therefore homogenously distributed within a phasehat percolates a porous biopolymer membrane.

.2. Fertilizer release

Dynamic conductivity tests carried out in aqueous media as aunction of time at 25 ◦C are shown in Fig. 5. Conductivity values

easured in control experiments for NPK fertilizer (same con-ent as in the blend) immediately attained a plateau region dueo the rapid dissociation of soluble salts. In the case of SPI/PLA–TA0/40–10 matrix blend (without NPK), on the other hand, a lowoncentration of electrolytes was slowly released to the mediums a function of time. These species might originate from the pro-ein, which was separated from other soy components employingechniques that make use of inorganic acids and bases.

Sustained release of NPK nutrients was achieved usingPI/PLA–TA–NPK composite materials. Fig. 5 exhibits the conduc-ivity curve recorded for SPI/PLA–TA–NPK 60/40–10–10 samples.he corresponding profile indicated that the cumulative releasedmount increased much slower with time (especially duringhe first week) as compared to pure NPK sample. For this sys-

em, the experimental data could be fitted neither according tourely diffusion-controlled mechanisms nor considering anoma-

ous transport (diffusion from the matrix and polymer relaxation),s judged from the discrepancy between experimental data and the

Fig. 6. Weight loss estimations as a function of the biodegradation time forSPI/PLA–TA 60/40–10 blends and SPI/PLA/TA–NPK 60/40–10–10 composite materialin simulated soil conditions.

theoretical curve plotted on the basis of the Krosmeyer–Peppasequation which is defined as C(t) = C(0) + Kktn, where C(t) is theamount of probe released at time t, C(0) is the initial probe con-centration in solution, Kk is the release constant, and n the releaseexponent which characterizes the mechanism of probe release(n = 0.45 for the dotted line in Fig. 5 – Fickian diffusion) (Korsmeyeret al., 1983). In fact, the release rate at the beginning of the exper-iment was higher than expected for Fickian or case II models,probably because of the rapid dissolution of salts near the surfaceregions of samples (Fig. 4, top right panel).

3.3. Biodegradation tests

The effect of NPK fertilizer on the biodegradation of SPI/PLA–TAblends in simulated soil under controlled temperature and humid-ity was accessed by weight loss, and microscopic and thermalanalyses. In the weight loss experiment, the variation of samplemass is due to both NPK release and polymer biodegradation. Eventhough the contribution from each of these phenomena cannotthe separated, the decrease in the residual mass as a functionof time (Fig. 6) was faster for unloaded matrix SPI/PLA–TA thenfor composite material SPI/PLA–TA–NPK, suggesting the NPK saltsdelay the biodegradation process in spite of the high experimen-tal error typical of such estimations. This is indeed confirmedby visual inspection of samples removed at different time inter-vals. In contrast, morphology examination by SEM shows theopposite behavior, with NPK-containing samples featuring a moreeroded/degraded surface as compared to the matrix for the dura-tion of the test (Fig. 6). Also evident on these images, and inparticular on those recorded after 15 days, is the formation ofbiofilms that adhere to the surface and play an important role inpolymer degradation (Kaplan, 1998).

4. Conclusions

Biodegradable protein isolate/poly(lactic acid) blends plasti-cized with triacetin are suitable for the production of slow releasefertilizer systems. The blends can be processed in the melt state inpresence of NPK fertilizer to obtain composite materials consistingof a highly ordered porous matrix of soy protein isolate in which

poly(lactic acid) domains are homogeneously dispersed with NPKsalts that are source of mainly nitrogen, phosphorous and potas-sium. Such nutrients can then be released to the environment in asustained manner. Polymer/fertilizer composites can be used in the

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6 L. Calabria et al. / Industrial C

orm of granules, as well as injection-molded into diverse shapesuch as containers for growing plants.

Importantly, the slow release fertilizer system developed usinghe herein reported approach consists of polymeric carriers derivedntirely from the biomass.

cknowledgments

Financial support was provided by CNPq and CAPES/Brazil. Theuthors are grateful to Solae do Brasil Indústria e Comércio de Ali-entos for kindly supplying the SPI samples.

eferences

merican society for testing and materials (ASTM), 1993. Annual book of ASTMstandards. In: Standard Practice for Evaluating Microbial Susceptibility of Non-metallic Materials by Laboratory Soil Burial. ASTM D 6400-04-99.

andyopadhyay, S., Bhattacharya, I., Ghosh, K., Varadachari, C., 2008. New slow-releasing molybdenum fertilizer. J. Agric. Food Chem. 56, 1343–1349.

hattacharya, I., Bandyopadhyay, S., Varadachari, C., Ghosh, K., 2007. Developmentof a novel slow-releasing iron–manganese fertilizer compound. Ind. Eng. Chem.Res. 46, 2870–2876.

habba, S., Matthews, G.F., Netravali, A.N., 2005. ‘Green’ composites using cross-linked soy flour and flax yarns. Green Chem. 7, 576–581.

ho, S.Y., Rhee, C., 2002. Sorption characteristics of soy protein films and their rela-tion to mechanical properties. Lebensm. Wiss. Technol. 35, 151–157.

avid, M.B., Gentry, L.E., 2000. Anthropogenic inputs of nitrogen and phosphorusand riverine export for Illinois, USA. J. Environ. Qual. 29, 494–508.

ntry, J.A., Sojka, R.E., 2008. Matrix based fertilizers reduce nitrogen and phosphorusleaching in three soils. J. Environ. Manage. 87, 364–372.

ennadios, A., Weller, C.L., 1990. Edible films and coatings from wheat and cornproteins. Food Chem. 44, 63–69.

artz, T.K., Smith, R.F., 2009. Controlled-release fertilizer for vegetable production:the California experience. Hort Technol. 19, 20–22.

rissin-Mangata, J., Bauduin, G., Boutevin, B., Gontard, N., 2001. New plasticizers forwheat gluten films. Eur. Polym. J. 37, 1533–1541.

arosiewicz, A., Tomaszewska, M., 2002. Controlled-release NPK fertilizer encapsu-lated by polymeric membranes. J. Agric. Food Chem. 51, 413–417.

in, S., Yue, G., Feng, L., Han, Y., Yu, X., Zhang, Z., 2011. Preparation and properties of a

coated slow-release and water-retention biuret phosphoramide fertilizer withsuperabsorbent. J. Agric. Food Chem. 59, 322–327.

aplan, D.L., 1998. Introduction to biopolymers from renewable resources. In:Kaplan, D.L. (Ed.), Biopolymers from Renewable Resources. Springer, Berlin, pp.1–29.

nd Products 36 (2012) 41–46

Korsmeyer, R.W., Gurny, R., Doelker, E., Buri, P., Peppas, N.A., 1983. Mecha-nisms of solute release from porous hydrophilic polymers. Int. J. Pharm. 15,25–35.

Kumar, R., Choudhary, V., Mishra, S., Varma, I.K., Mattiason, B., 2002. Adhesives andplastics based on soy protein products. Ind. Crops Prod. 16, 155–172.

Lai, H.-M., Padua, G.W., 1997. Properties and microstructure of plasticized zein films.Miscellaneous 74, 771–775.

Ly, V.T.-P., Johnson, L.A., Jane, J., 1998. Soy protein as a biopolymer. In: Kaplan, D.L.(Ed.), Biopolymers from Renewable Resources. Springer, Berlin, pp. 144–176.

Ni, B., Liu, M., Lu, S., Xie, L., Wang, Y., 2010. Multifunctional slow-releaseorganic–inorganic compound fertilizer. J. Agric. Food Chem. 58, 12373–12378.

Orliac, O., Rouilly, A., Silvestre, F., Rigal, L., 2003. Effects of various plasticizers onthe mechanical properties, water resistance and aging of thermo-moulded filmsmade from sunflower proteins. Ind. Crops Prod. 18, 91–100.

Osswald, T.A., Menges, G., 2003. Materials Science of Polymer for Engineers, 2nd ed.Carl Hanser Verlag, Munich.

Park, S.K., Hettiarachchy, N.S., 2000. Degradation behavior of soy protein–wheatgluten films in simulated soil conditions. J. Agric. Food Chem. 48, 3027–3031.

Schmidt, V., Giacomelli, C., Soldi, M.S., Soldi, V., 2005a. Soy protein isolate basedfilms: influence of sodium dodecyl sulfate and polycaprolactone-triol on theirproperties. Macromol. Symp. 229, 127–137.

Schmidt, V., Giacomelli, C., Soldi, V., 2005b. Thermal stability of films formed by soyprotein isolate–sodium dodecyl sulfate. Polym. Degrad. Stab. 37, 25–31.

Sharpley, A.N., Foy, B.H., Withers, P.J.A., 2000. Practical and innovative measures forthe control of agricultural phosphorus losses to water: an overview. J. Environ.Qual. 29, 1–9.

Signori, F., Coltelli, M.-B., Bronco, S., 2009. Thermal degradation of poly(lactic acid)(PLA) and poly(butylene adipate-co-terephthalate) (PBAT) and their blendsupon melt processing. Polym. Degrad. Stab. 94, 74–82.

Swain, S.N., Biswal, S.M., Nanda, P.K., Nayak, P.L., 2004. Biodegradable soy-basedplastics: opportunities and challenges. J. Polym. Environ. 12, 35–42.

Tang, C.-H., Choi, S.-M., Ma, C.-Y., 2007. Study of thermal properties and heat-induceddenaturation and aggregation of soy proteins by modulated differential scanningcalorimetry. Int. J. Biol. Macromol. 40, 96–104.

Teodorescu, M., Lungu, A., Stanescu, P.O., Neamtu, C., 2009. Preparation and proper-ties of novel slow-release NPK agrochemical formulations based on poly(acrylicacid) hydrogels and liquid fertilizers. Ind. Eng. Chem. Res. 48, 6527–6534.

Tomaszewska, M., Jarosiewicz, A., 2002. Use of polysulfone in controlled-releaseNPK fertilizer formulations. J. Agric. Food Chem. 50, 4634–4639.

Vassilev, N., Vassileva, M., 2003. Biotechnological solubilization of rock phosphateon media containing agro-industrial wastes. Appl. Microbiol. Biotechnol. 61,435–440.

release NPK compound fertilizer with superabsorbent and water-retention.Bioresour. Technol. 99, 547–554.

Zhang, J., Jiang, L., Zhu, L., Jane, J.-l., Mungara, P., 2006. Morphology and propertiesof soy protein and polylactide blends. Biomacromolecules 7, 1551–1561.