ORIGINAL CONTRIBUTION

Protein-based bioplastics: effect of thermo-mechanicalprocessing

Abel Jerez & Pedro Partal & Inmaculada Martínez &

Críspulo Gallegos & Antonio Guerrero

Received: 31 May 2006 /Accepted: 2 January 2007 / Published online: 16 February 2007# Springer-Verlag 2007

Abstract Bioplastics based on glycerol and differentproteins (wheat gluten, albumen, rice and albumen/glutenblends) have been manufactured to determine the effect thatprocessing and further thermal treatments exert on differentthermo-mechanical properties of the bioplastics obtained.Oscillatory shear, modulated differential scanning calorim-etry, dynamic mechanical thermal analysis, thermo-gravi-metric analysis and water absorption tests were carried outto study the effect of processing on the physical character-istics of the bioplastics. The protein-based bioplasticsstudied in this work present a high capacity for thermoset-ting modification because of protein denaturation that mayfavour the development of a wide variety of materials. Theuse of albumen or rice protein allows the reduction in bothprotein concentration and thermosetting temperature, lead-ing to linear viscoelastic moduli values similar to those ofsynthetic polymers such as LDPE and HDPE. Thehygroscopic characteristics of protein-glycerol bioplasticsmay lead to a decrease in the values of the linearviscoelasticity functions. However, hygroscopic propertiesdepend on the protein nature and may be used for industrialapplications where water absorption is required.

Keywords Protein . Bioplastics . Rheology .

Viscoelasticity . Calorimetry . Processing .Water absorption

The development of new materials to substitute syntheticpolymers has become an important challenge nowadays.Among these materials, biopolymers from agricultural sourcesare becoming an interesting alternative not only as biodegrad-able films suitable for food packaging but also as plastic stuffs,which require improved mechanical properties. Proteins, lipidsand polysaccharides have been used as biopolymer sources formany years (Irissin-Mangata et al. 2001; De Graaf 2000).Numerous vegetable proteins (corn, wheat gluten [WG], soyproteins, etc.) and animal proteins (milk proteins, collagen,gelatin, etc.) have been used to manufacture bioplastics(Pommet et al. 2003; Cuq et al. 1998). In addition, thebiodegradability of protein-based biomaterials have beenproved to be among the rates of fast-degrading polymers.Therefore, the use of proteins for non-food applications maybe a promising way to produce biodegradable materials witha large range of functional properties because of their uniquestructure (Domenek et al. 2004). These applications includematrices for enzyme immobilization or controlled-releasedevices (Chen and Tan 2006; Suda et al. 2000; Kinney andScranton 1997), as well as water absorbent materials inhealthcare, agriculture and horticulture applications, in whichwater absorbency and water retention are essential (Li et al.2006). Moreover, a number of advanced technologies arebeing applied to bioplastics to provide added value, includingactive packaging technology, natural fibre reinforcements,nanotechnology and innovative product design.

A protein-based material could be defined as a stable three-dimensional macromolecular network stabilised and strength-ened by hydrogen bonds, hydrophobic interactions anddisulfide bonds (Pommet et al. 2003). However, as proteinsthemselves do not have plasticity enough to handle, a

Rheol Acta (2007) 46:711–720DOI 10.1007/s00397-007-0165-z

This paper was presented at Annual European Rheology Conference(AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.

A. Jerez : P. Partal (*) : I. Martínez :C. GallegosDepartamento de Ingeniería Química,Facultad de Ciencias Experimentales,Campus el Carmen, Universidad de Huelva,21071 Huelva, Spaine-mail: partal@uhu.es

A. GuerreroDepartamento de Ingeniería Química, Facultad de Química,Universidad de Sevilla,41012 Sevilla, Spain

plasticizer is required to reduce intermolecular forces andincrease polymeric chain mobility, modifying the three-dimensional structure of proteins (Gennadios 2002). More-over, the plasticizer reduces the glass transition temperatureof thermoplastic proteins such as WG (Pouplin et al. 1999;Irissin-Mangata et al. 2001; Matveev et al. 2000).

The processing of films, coatings or other protein-basedmaterials requires the following three main steps: breakingof intermolecular bonds (non-covalent and covalent, ifnecessary) that stabilise polymers in their native forms byusing chemical or physical rupturing agents; arranging andorienting mobile polymer chains in the desired shape; and,finally, allowing the formation of new intermolecular bondsand interactions to stabilise the three-dimensional network(Gennadios 2002). The casting method, or physico-chemi-cal method, of film processing is based on the above-mentioned three steps using a chemical reactant to disruptdisulfide bonds, dispersing, solubilising proteins and finallydrying it (Gontard et al. 1993).

Another way of processing protein-based biomaterials isthe mechanical method, or thermoplastic processing, whichconsists of mixing proteins and plasticizer to obtain adough-like material (Attenburrow et al. 1990; Kokini et al.1994). Bioplastics can be processed using existing plastic-processing machinery, including thermoforming, varioustypes of injection moulding, compression moulding, extru-sion (films, fibres) and extrusion coating and lamination.

WG and soy proteins have been investigated as a bioplasticsource because they are low-cost raw materials, annuallyrenewable and readily available (Domenek et al. 2004).Although egg white/albumen (EW) bioplastics have beenpreviously developed by the casting method (Gennadios etal. 1996), the use of a thermo-mechanical method in themanufacture of EW- and rice (RC)-protein-based bioplasticshas not been previously investigated.

The aim of this work was to develop new bioplasticsbased on different proteins and glycerol. In this sense, animportant task is the characterisation of the effects thatprocessing and further thermal treatments exert on thethermo-mechanical properties of the bioplastics obtained.Consequently, the properties of gluten-based bioplastics,developed by casting and mechanical methods, have beencompared with those of the new protein-based bioplasticsobtained in this research (EW, RC and EW/gluten blends),all of them manufactured using thermoplastic processing.

Materials and methods

Three different protein sources have been used in this study.WG was provided by Riba S.A. (Spain); spray-dried EW byOvosec S.A. (Spain); and RC protein by Ferrer Alimenta-ción S.A. (Spain). Some compositional characteristics of

these protein concentrates, employed as raw materials inthis work, are shown in Table 1. Glycerol, from PanreacQuímica, S.A. (Spain), was used as a protein plasticizer.

WG biomaterials were prepared by the casting methodaccording to a procedure used by other authors (Cherian etal. 1995; Herald et al. 1995; Gontard et al. 1996; Micard etal. 2001; Lens et al. 2003). Plasticizer/gluten weight ratios(glycerol/WG, G/WG) ranged from 0.3 to 1.

The thermo-mechanical processing was carried out in atorque-rheometer (Rheocord, Haake, Germany), whichconsists of a batch mixer fitted with two counter-rotatingrollers, turning with different angular velocities (ratio 3:2).A detailed description of this equipment may be foundelsewhere (Dealy 1982; Gontard et al. 1996; Redl et al.1999). Both torque and temperature were recorded duringthe mixing process. The mixing chamber can be consideredadiabatic. The volume of the chamber (310 cm3) was filledwith approximately 245 g of sample, corresponding to 80%of its total volume, and the mixing process was alwayscarried out at 50 rpm (Jerez et al. 2005a).

Compression-moulded biomaterials were prepared bycompressing the dough-like materials obtained after themixing process at different gauge pressures in a 50×10×3-mm mould (Jerez et al. 2005b). The compression-mouldingprocess was carried out at temperatures comprised between50 and 160 °C, depending on protein. These temperatureswere above the maximum temperature reached during themixing process and below the temperature at which thedegradation of the blends takes place.

Frequency sweep tests from 0.01 to 100 rad/s inoscillatory shear, at a constant stress within the linearviscoelastic region, were conducted between 25 and 140 °Cin a controlled stress rheometer Rheoscope (Haake,Germany) using a parallel plate geometry (20-mm diameter;1-to 1.2-mm gap). Previously, stress sweep tests, at 6.28 rad/s, were carried out to characterise the linear viscoelasticregion of the material. All the samples were left for 30 min inthe measuring geometry before running any test, to allow thesample to achieve the temperature of measurement.

Temperature sweep tests from 25 to 170 °C were con-ducted in a controlled-strain rheometer ARES (RheometricsSci., USA) using a parallel plate geometry (25-mm

Table 1 Physico-chemical characteristics of the protein concentratesused as raw materials in this work

WG EW RC protein

Protein content (%) 83 73 79Lipids (%) 1.5–2 – 5.0Ashes (%) 0.7–0.8 6 2.0Moisture (%) 8 8 12Reducing sugars (%) – 0.1 –pH (10% solution) 6.88 7.10 –

712 Rheol Acta (2007) 46:711–720

diameter; 1- to 1.2-mm gap). Measurements were per-formed at a constant frequency (6,28 rad/s) and strain(within the linear viscoelastic region), selecting a 2 °C/mintemperature ramp.

Modulated differential scanning calorimetry (MDSC)experiments were performed with a Q100 (TA Instruments,USA), using 10- to 20-mg samples, in hermetic aluminiumpans. An oscillation period of 60 s, amplitude of ±0.5 °Cand a heating rate of 5 °C/min were selected. The samplewas purged with a nitrogen flow of 50 ml/min.

Dynamic mechanical thermal analysis (DMTA) experi-ments were developed with a Seiko DMS 6100 (SeikoInstruments, Japan), using 50×10×3-mm samples in adouble cantilever bending mode. All the experiments werecarried out at constant frequency (1 Hz) and strain (withinthe linear viscoelastic region), selecting a 2 °C/mintemperature ramp.

Thermo-gravimetric analysis (TGA) were carried out on10-mg samples, from room temperature to 400 °C, at aheating rate of 10 °C/min and under a nitrogen atmosphere(flow rate of 60 ml/min) in a TGA Q50 (TA Instruments,USA). The temperatures at which weight loss occurredwere determined directly from the thermograms.

Water absorption (Ab) was measured according to theASTM standard D570-81. Ab was calculated as follows:

Ab ¼ W1 �W0 þWsolð ÞW0

� 100 ð1Þ

where W1, W0 and Wsol are the weight of the specimen-containing water, the weight of the dried specimen and theweight of water-soluble residuals, respectively.

Results and discussion

Viscoelastic behaviour of gluten-based bioplastics

Figure 1 shows the frequency dependence of the linearviscoelasticity functions for a 0.4 G/WG blend in a range oftemperature comprised between 25 and 140 °C. A gel-likeviscoelastic behaviour, a characteristic of a highly structuredmaterial, is noticed. Thus, the storage modulus is alwayshigher than the loss modulus in the whole temperature rangestudied. Nevertheless, the polymer network, formed byproteins chains, is affected by temperature in differentmanners. Thus, a decrease in both moduli with temperatureis found between 25 and 70 °C, whilst quite similar storagemodulus values and a small plateau region are noticed above70 °C (Fig. 1a). On the contrary, a quite different behaviouris observed between 100 and 140 °C (Fig. 1b). Thus, themechanical spectrum demonstrates the development of aplateau region in G′, and the storage modulus increases withtemperature, which may be related to a protein denaturationprocess (Jerez et al. 2005a).

The linear viscoelasticity functions for a given G/WGblend can be empirically superposed onto a master curve(see Fig. 2) in the whole temperature range studied, (25–140 °C) by using two different shift factors (aT and bT).However, this superposition can been done with an uniqueshift factor, aT (and bT=1), in a temperature rangecomprised between 25 and 90 °C, which points out that amuch simpler thermal behaviour (thermoplastic behaviour)takes place below 90 °C. On the contrary, above 90 °C,both shifts factors are needed to obtain a master curve as a

10-1 100 101104

105

106

107

10-1 100 101104

105

106

107

a

G' G'' T, 25º C, 50º C, 70º C, 90º C

G',

G''

(Pa)

G' G'' T, 100º C, 110º C, 120º C, 140º C

b

G',

G''

(Pa)

(rad/s)

102

ω

Fig. 1 Frequency dependenceof the linear viscoelasticityfunctions for a 0.4 G/WG blendas a function of temperature

Rheol Acta (2007) 46:711–720 713

consequence of the protein denaturation process (seeFig. 3). In this way, bT would provide additional informa-tion about the effect of the denaturation process on thevalues of the linear viscoelasticity moduli.

Figure 2 shows the resulting master curves, at areference temperature of 50 °C, of both storage and lossmoduli for three bioplastics prepared using three differentG/WG ratios (0.3, 0.4 and 0.8). As may be seen, lower

plasticizer/gluten ratios lead to higher values of bothmoduli. The blends show a plateau region in G′ and aminimum in G″ in the low frequency region. The plateauregion seems to be shifted to lower frequencies as theplasticizer content is reduced.

Figure 3 presents the values of the shift factors for thedifferent blends studied. These shift factors have been fittedto an Arrhenius-like equation. However, this fitting must be

10-9 10-7 10-5 10-3 10-1 101 103 105103

104

105

106

107

108

10-9 10-7 10-5 10-3 10-1 101 103 105103

104

105

106

107

108

E',,,

a

G'·b T

(Pa)

E''0.3 G/WG0.4 G/WG0.8 G/WG

b

G''

·bT

(Pa)

·aT

(rat /s)ω

Fig. 2 Master curves, at areference temperature of 50 °C,of the frequency dependenceof the linear viscelasticity func-tions for three different G/WGbioplastics (G/WG ratios, 0.3;0.4; and 0.8) obtained by thecasting method

20 40 60 80 100 120 14010-9

10-7

10-5

10-3

10-1

101

103

10-3

10-2

10-1

100

101

a T

T (ºC)

, 0.3 G / WG

, 0.4 G / WG

, 0.8 G / WG

Thermoplastic Denaturation

b T

Fig. 3 Evolution of the shiftfactors (aT and bT) with temper-ature for the different G/WGbioplastics (G/WG ratios, 0.3;0.4; and 0.8) studied

714 Rheol Acta (2007) 46:711–720

done in two different temperature ranges (25–90 °C and100–140 °C) as follows:

aT ¼ A expEa

RT

� �ð2Þ

bT ¼ B expk

T

� �ð3Þ

where A and B are pre-exponential factors, R is the gasconstant, Ea is an activation energy and k is a parameterrelated to the protein denaturation process. Table 2 gathersthe parameters obtained from the fitting of these equations.

The effect of the plasticizer concentration on themechanical properties of the blends can also be seen inFig. 3. Thus, at temperatures below 90 °C, the 0.8 G/WGblend has the lowest thermal susceptibility. In addition, thisblend shows the highest thermosetting potential because ofprotein denaturation. This fact can be deduced from the lowvalues of bT at temperatures above 90 °C.

The above-mentioned thermo-rheological behaviour maybe also confirmed by performing temperature sweep tests inoscillatory shear (see Fig. 4). Thus, between 20 and 90 °C,the complex modulus G* decreases down to a minimumvalue when an increase in protein–protein interactions takesplace. As may be seen, the minimum in G* tends to appearat a lower temperature as the glycerol content increases.Between 90° and 120 °C, the viscoelastic moduli undergo aremarkable increase, and tanδ a dramatic decrease (Fig. 5)that could be attributed to protein cross-linking reactions,which occur under severe thermal conditions (Gennadios2002). Furthermore, the minimum in loss tangent for thebiomaterials prepared by casting shows a linear dependenceon plasticizer content, being the temperature at which thisminimum appears lower as the glycerol content increases(inset in Fig. 5). On the other hand, it is worth pointing outthat the results obtained demonstrate that material thermo-setting (temperature range above the minimum in G*) takesplace in a short period of time, below 20 min (data deducedfrom Fig. 4 and the temperature sweep rate, 2 °C/min,selected).

Figure 4 also shows the temperature sweep results for a0.5 G/WG blend obtained by thermo-mechanical process-ing (Jerez et al. 2005a,b). Although both processingmethods yield similar values of G* at 30 °C, for samples

with identical content in plasticizer, the compression-moulded biomaterial shows a higher thermal susceptibility.Thus, the value of G* at the minimum is significantlylower. In addition, the minimum in tanδ appears at a lowertemperature for the compression-moulded bioplastic. Aftera further increase in temperature, both casting andcompression-moulded biomaterials undergo an increase inthe complex modulus, showing quite similar values at atemperature of around 140 °C.

Both casting and thermo-mechanical methods havedemonstrated to be interesting potential procedures toobtain a bioplastic. Moreover, the casting method seemsto provide biomaterials with higher thermosetting poten-tials. However, the simple mechanical mixing of proteinand plasticizer seems to be an easier and faster method toobtain the bioplastic. As a result, mechanical processing hasbeen selected to study the effect of thermal-treatment onselected protein-based bioplastics.

Bioplastic by thermo-mechanical processing

The results previously shown clearly demonstrate thatprotein-based bioplastics with very broad rheologicalcharacteristics can be obtained by combining formulationand thermal processing.

Figure 6 shows the results obtained from DMTA tests(conducted in bending mode) for a 0.3 G/WG blendsubmitted to different thermo-mechanical treatments. Ascan be observed, the elastic modulus E′ is always higherthan the loss modulus E″ in the whole temperature rangestudied (−80 to 160 °C). A low-temperature plateau region(E′≅1010) is noticed, more important as compression-moulding temperature increases. In this region, tanδ showsa maximum value at a temperature close to −50 °C, which is

Table 2 Activation energy values for the different G/WG blendsstudied

G/WG 0.3 0.4 0.8

aT Ea (25–90 °C; J/mol) 113.55 137.23 98.57Ea (100–140 °C; J/mol) 277.58 230.48 214.03

bT k (100–140 °C; K) 7.47 5.68 11.97

20 40 60 80 100 120 140 160 180104

105

106

107

Casting BiomaterialsG / WG G / WG

0.3 0.80.5 1.0

Compression-Moulded Biomaterial, 0.5 G / WG

G*(P

a)

Temp (º C)

Fig. 4 Temperature sweep tests in oscillatory shear (G*) for differentG/WG blends (G/WG ratios, 0.3–1.0) obtained by the casting method(solid symbols) and for a 0.5 G/WG blend obtained by thermo-mechanical processing (empty symbols)

Rheol Acta (2007) 46:711–720 715

a characteristic of a transition temperature Ta1. In addition,another plateau region seems to occur at temperatures above100 °C. As a result, a second maximum in tanδ, at atemperature close to 80–90 °C, is found, which correspondsto a second transition temperature, Ta2. Previous studieshave shown the existence of these transition temperatures inthe same temperature regions, associating Ta1 to themolecular motion of the free glycerol and Ta2 to the glasstransition of the plasticized gluten (Lim et al. 1999).

Moreover, the sample obtained by compression-mouldingat 90 °C shows a remaining thermosetting potential (Fig. 6),which may be related to an incomplete protein denaturationduring its previous thermo-mechanical processing. This factis confirmed by MDSC measurements (Fig. 7). Thus, anendothermic event at high temperatures, with a peak at ca.150 °C, is observed in the non-reversing heat flow curve ofany of the G/WG blends studied (not previously submitted toany thermo-mechanical processing). As may be seen, theendothermic peak becomes narrower and tends to vanish as

processing temperature increases. On the contrary, E′ doesnot present a further increases in the high temperature regionfor G/WG blends submitted to thermo-mechanical process-ing above 120 °C. Thus, bioplastics obtained at 120 and160 °C show lower thermal susceptibility because of ahigher protein thermal denaturation during their processing.At the same time, this higher thermal denaturation leads tomore structured systems with higher values of the linearviscoelasticity functions (Fig. 6).

Figure 8a displays the effect of the G/WG ratio on theevolution of the complex modulus values with a tempera-ture for bioplastics submitted to thermo-mechanical pro-cessing. A similar behaviour to that found for bioplasticsprepared by the casting method is noticed. Thus, thecomplex modulus values increase as the plasticizer contentdecreases, although they are always lower than those foundfor LDPE or HDPE. However, not only the plasticizerconcentration should be taken into account but also theplasticizer characteristics as well. Thus, by dehydrating

20 40 60 80 100 120 140 160 180

10-1

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

140

142

144

146

148

150

152

154

156

Tem

p(º

C)

G / WG Ratio

Casting Biomaterials

0.3 G / WG 0.6 G / WG0.4 G / WG 0.8 G / WG0.5 G / WG 1.0 G / WG

Compression-Moulded Biomaterial, 0.5 G / WG

tan

δ(P

a)

T (º C)Fig. 5 Temperature sweep tests in oscillatory shear (tanδ) for differentG/WG blends (G/WG ratios, 0.3–1.0) obtained by the casting method(solid symbols) and for a 0.5 G/WG blend obtained by thermo-

mechanical processing (empty symbols; inset: temperature dependenceof the minimum in tanδ with G/WG ratio for bioplastics obtained bycasting)

716 Rheol Acta (2007) 46:711–720

some of these samples (i.e. 0.2 and 0.3 G/WG), theircomplex modulus values underwent a remarkable increase,showing values of the 0.2 G/WG to be higher than thosefound for HDPE at temperatures below 65 °C; whereas the0.3 G/WG blend shows values higher than those found forthe LDPE below this temperature. In this sense, thehygroscopic characteristics of protein-glycerol bioplasticswere confirmed by TGA experiments (Fig. 9). As can beobserved, TGA curves show that the bioplastic weight loss

is about 5% between 100 and 150 °C, which is probablybecause of a slow vaporization of water and small amountsof glycerol (Zheng et al. 2002).

An essential issue in protein-based bioplastics develop-ment is the type of protein used. Some results concerning theviscoelastic properties of new protein-based bioplasticsobtained by thermo-mechanical processing are displayed inFig. 8b. As may be seen, higher complex modulus valuesthan those shown by G/WG bioplastics can be obtained by

-100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 180 200 220

Thermo-mechanical Processingno thermal processing

140 º C200 º C

Non

reve

rsin

gH

eatF

low

0.02

(W/g

)

T (º C)

Fig. 7 Non-reversing heat flowsignals obtained by MDSC for0.5 G/WG blends processed atdifferent temperatures (not sub-mitted to previous thermo-me-chanical processing, 140 and200 °C)

-80 -40 0 40 80 120 160 200106

107

108

109

1010

1011

tan

δ

E',

E''

(Pa)

T (ºC)

Bioplastic G / WG = 0.3E' E'' tan δ

, , TC 90ºC, 880 bar, , TC 120ºC, 880 bar, , TC 160ºC, 880 bar

0.1

0.2

0.3

0.4

0.5

0.6

0.7Fig. 6 Dynamic mechanicalthermal analysis results forthree different gluten-based bio-plastics processed at differenttemperatures during compres-sion moulding

Rheol Acta (2007) 46:711–720 717

0 50 100 150 200 250 300 350 40020

30

40

50

60

70

80

90

100

110

50 100 150

Wei

ght

(%)

T (º C)

1.97 %

5.20 %

4.93 %

0.5 G/WG 120 º C 880 bar0.5 G/EW 120 º C 880 bar0.5 G/RC 120 º C 880 bar

Wei

ght(

%)

T(ºC)Fig. 9 Thermo-gravimetric analysis results for three different protein-based bioplastics

0 40 80 120 160106

107

108

109

1010

0 40 80 120 160 200106

107

108

109

1010

a

E* Thermo-moulding0.5 G /WG 140 º C 880 bar0.3 G /WG 140 º C 880 bar0.3 G /WG 140 º C 880 bar Dehidrated0.2 G /WG 140 º C 880 bar0.2 G /WG 140 º C 880 bar DehidratedHDPE LDPE

E*

(Pa)

b Bioplastic, G / Protein = 0.5E* Protein Thermo-moulding

WG 120 º C 880 barEW 120 º C 880 bar1:1 WG:EW 120 º C 880 barRC 120 ºC 880 bar

HDPE LDPE

E*(P

a)

T(º C)

Fig. 8 DMTA results for adifferent G/WG bioplastics andb different protein-basedbioplastics

718 Rheol Acta (2007) 46:711–720

using EWor RC proteins in the bioplastic formulation. Thus,using lower protein concentration and thermosetting temper-ature, EW and RC proteins lead to complex modulus valueshigher than LDPE and similar to those found for HDPE.

The hygroscopic characteristics of these new protein-based bioplastics may be also observed in Fig. 9. EW-basedbioplastic (G/EW) presents a weight loss similar to thatfound for gluten-based bioplastic (5%). On the contrary, RCprotein-based biomaterial (G/RC) shows a much lowerweight loss (about 2%).

The above-mentioned ability of protein-based bioplasticsto retain water makes them potentially interesting for differentindustrial applications, such as active food packaging,absorbents (i.e. hygienic products, agriculture, horticulture,etc.), drug-delivery systems or water-blocking tapes (Zhang etal. 2006). Table 3 gathers the water-absorption values for theprotein-based bioplastics studied and several thermo-me-chanical treatments. As may be observed, water-absorptionvalues range from 40 to 320%, depending on protein natureand the thermal treatment selected to manufacture thebioplastic. RC protein shows the lowest water absorptionvalues. These results are in good agreement with thoseobtained from the TGA tests. On the other hand, althoughgluten and EW usually show similar water-absorptionvalues, much higher values of this parameter (up to 300%)can be obtained when EW-based bioplastic is processedunder gentle thermo-mechanical conditions.

Moreover, it is well-known that tanδ-temperature curvescan be used to reveal information concerning molecularand/or segmental scale motions in polymers (Zheng et al.2002). In this sense, the second transition temperature, Ta2,obtained from DMTA tests (see Fig. 6) has been used torelate the values of the elastic modulus E′ (at Ta2) andwater absorption (Fig. 10). A general trend has been foundfor different proteins and thermo-mechanical treatments.Thus, E′ Ta2ð Þ decreases as water absorption increases (upto a certain value of around 100%, then it remains

constant). These results are in good agreement with thoseobtained by other authors (Zheng et al. 2002; Buonocore etal. 2003) who found that the swelling ratio of a polymericmatrix decreases as the degree of cross-linking increases.As described above, bioplastic thermo-mechanical treat-ment leads to protein denaturation and, therefore, leads toan increase in the degree of cross-linking between mole-cules (Buonocore et al. 2003).

Conclusions

Oscillatory-shear measurements performed on G/WG bio-plastics obtained by a casting method have demonstrateddifferent rheological responses depending on the tempera-ture range studied. Thus, between 25 and 90 °C, bioplasticsbehave like thermoplastic materials with a decrease in thevalues of the linear viscoelasticity moduli as the tempera-ture increases. Above 100 °C, the linear viscoelasticitymoduli increase with temperature because of protein

50 100 150 200 250 300 350105

106

107

108

109

1010

E'(

Pa)

% Water Absortion

WGEW

1:1 EW:WGRC

Fig. 10 Relationship between the elastic modulus at the secondtransition temperature and bioplastic water absorption

Table 3 Water absorptionvalues for the protein-basedbioplastics studied and manu-factured with differentthermo-moulding conditions

Thermo-moulding conditions Water absorption (%)

Protein

WG EW EW/WG (1:1) RC

Temperature (°C) Pressure (bar) – – – –50 1 – – 97.7 69.450 880 125.2 – 110.2 77.560 1 – 322.5 – –60 880 88.4 – –90 1 – – 178.5 53.790 880 122.2 – 106.4 54.7120 1 – – 82.9 42.7120 880 94.3 97.28 79.9 45.4140 1 – – 69.2 –140 880 – – 73.9 –

Rheol Acta (2007) 46:711–720 719

denaturation. This thermosetting effect may be used toimprove bioplastic properties by applying a further thermo-mechanical treatment. The observed complex rheologicalbehaviour can be empirically superposed onto a mastercurve by using two shift factors (aT and bT) in the wholetemperature range (25–140 °C) studied.

Both processing methods (casting and thermo-mechanical)have demonstrated to be interesting potential procedures toobtain a bioplastic. Moreover, the casting method seems toprovide biomaterials with higher thermosetting potentials.However, the simple mechanical mixing of protein andplasticizer makes easier and faster bioplastic manufacture. Inaddition, compression-moulded biomaterials show a widerange of mechanical characteristics and bioplastic propertiesdepending on the thermal and mechanical treatment selected.The use of EW or RC protein allows the reduction in bothprotein concentration and thermosetting temperature, leadingto complex modulus values higher than LDPE and similar tothose found for HDPE.

The water-absorption values for the different bioplasticsstudied have been obtained. They range from 40 to 320%,depending on protein nature and the processing conditionsselected. A general trend has been found concerning theevolution of the values of the elastic modulus, E′ (at Ta2)and water absorption for different proteins and thermo-mechanical treatments. Thus, E′ Ta2ð Þ generally decreasesas water absorption increases.

Acknowledgements This work is part of a research projectsponsored by the MEC programme (HP2005-0127) and Junta deAndalucia programme (P06-TEP-02126). The authors gratefullyacknowledge its financial support.

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