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De elopmen o albumen/soy biobased plas ic ma e ials
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p ocessed by injec ion molding
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M. Félix, J.E. Ma ín-Al onso, A. Rome o*, A. Gue e o
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Depa amen o de Ingenie ía Química, Uni e sidad de Se illa, Facul ad de Química, 41012
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Se illa, Spain
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Abs ac
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Biobased plas ics om enewable polyme s cons i u e a highly in e es ing ield o
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ele an indus ial applica ions such as packaging, ag icul u e, e c., in which
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he momechanical echniques (i.e. ex usion, comp ession molding, e c.) a e
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inc easingly being used. In spi e o he po en ials o injec ion molding in he
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manu ac u e o shaped p oduc s i is s ill sca cely used wi h biopolyme s. This s udy
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e alua es injec ion molding as an al e na i e o p oduce biobased ma e ials om
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blends p epa ed in a mixing heome e , using di e en albumen/soy a ios and glyce ol
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as he plas icize . Viscoelas ic measu emen s and DSC o p o ein/glyce ol blends we e
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used o selec sui able p ocessing condi ions. Physicochemical p ope ies o injec ion-
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molded p obes we e cha ac e ized h ough dynamic mechanical he mal analysis,
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ensile s eng h, wa e up ake and ansmi ance es s. Occu ence o shea -induced
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e ec s o e mixing was con i med by ex ac abili y analysis o p o ein concen a es and
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blends, pa icula ly o soy-based sys ems. Bo h p o eins and hei mix u es yield
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injec ion-molded bioplas ics, al hough showing lowe mechanical p ope ies han LDPE
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s anda ds.
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Keywo ds: Albumen; Bioplas ic; Dynamic Mechanical The mal Analysis; Soy P o ein;
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Tensile s eng h es ; T ansmi ance.
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_______________________
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*A. ROMERO
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Depa amen o de Ingenie ía Química,
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Uni e sidad de Se illa, Facul ad de Química,
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41012 Se illa (Spain)
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E-mail: al ome [email protected]
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Phone: +34 954557179; ax: +34 954556447.
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1. In oduc ion
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Plas ic ma e ials a e cu en ly conside ed e y impo an ma e ials due o hei
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excep ional p ope ies and pe o mance o e o he ma e ials such as me al and wood
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(Plas ic Eu ope, 2008). In ac , acco ding o a ecen epo , he demand o plas ic will
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con inue o ise ollowing a end ha has inc eased since 1950s (Plas ics Eu ope e
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al., 2008). Nowadays, he subs i u ion o pe oleum-based plas ics wi h bio-based
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plas ics is seen as a p omising al e na i e because i will educe he dependency o
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plas ics on ossil uels and he p essu e on land ills om plas ic solid was es (Al a ez-
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Cha ez e al., 2012). In ecen yea s he e has been a g ea in e es o u ilize
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enewable biomass in he manu ac u e o high-quali y, cos -compe i i e and
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biodeg adable consume goods as a means o educe he consump ion and he
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dependence on pe ochemical eeds ock and o diminish en i onmen al pollu ion
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(Rosen a e and O ieno, 2006; Tummala e al., 2006). In pa icula , packaging ilms
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and con aine s made o na u al biopolyme s ep esen a pa icula in e es due o hei
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biodeg adabili y, since mos o hese p oduc s ha e a ela i e sho se ice li e ending
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up in land ills. In his sense, p o ein-based ma e ials ha e been p o ed o be
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comple ely deg ading in 50 days when bu ied in a mland soils (Domenek e al., 2004).
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P o eins, lipids and polysaccha ides ha e been p oposed as biopolyme s sou ces
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o many yea s (A e ous, 2004; De G aa , 2000; He nández-Izquie do and K och a,
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2008; I issin-Manga a e al., 2001; Si acusa e al., 2008). Rega ding p o eins, mos
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s udies ha e used plan p o eins such as zein, whea glu en o soybean o manu ac u e
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bioplas ics (Cuq e al., 1998; Gomez-Ma inez e al., 2013; Je ez e al., 2005; Kim,
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2008; Zheng e al., 2003). Mo eo e , some wo ks ha e been ocused on animal
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3
p o eins such as milk p o eins, collagen, gela in, e c. (Cuq e al., 1998; Pomme e al.,
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2003).
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Soy p o ein is he majo cop oduc o soybean oil and is one o he cheapes
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p o eins in na u e (Tian e al., 2012). In ac , soy p o eins ha e commonly been used
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o ood and animal eed o many yea s. Howe e , soy p o ein is a new polyme o
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biodeg adable esins. Soy p o ein polyme s a e mac omolecules ha con ain a numbe
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o amino acids and side chains ha can be used in he manu ac u e o plas ic (Sun e
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al., 1999). The mechanical p ope ies o soy p o ein based plas ics can be con olled
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and op imized by adjus ing he ini ial mois u e con en as well as some p ocessing
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pa ame e s such as he molding empe a u e and/o p essu e (Wang e al., 2007; Liang
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e al., 1999; Mo e al., 1999; Jane and Wang, 1996; Pa eau e al., 1994). Howe e , he
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applica ion o soy p o ein plas ics is limi ed because o i s low s eng h (Tummala e al.,
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2006) and high mois u e abso p ion (Liu e al., 2005). The e o e, i can be concluded
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ha he mos e ec i e me hod is o blend soy p o ein wi h ano he biodeg adable
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polyme . Cu en ly, soy-based blends o plas ic applica ions include polyphospha e
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(O aigbe and Adams, 1997), polyes e s (G ai e e al., 2004; Liu e al., 2004),
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polyu e hane (Tian e al., 2010) o na u al ibe s (Liu e al., 2005). On he o he hand,
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no epo s on he enhancemen o soy-based plas ic ma e ials by using a combina ion
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wi h o he p o eins ha e been ound.
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Egg whi e p o ein (albumen), adi ionally used by he ood indus y, has ecen ly
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p o ed he easibili y o p oduce bioplas ics (Je ez e al., 2007b). Mo eo e , i
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compa ed o o he common p o eins like glu en, egg whi e has p o ed o be an
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adequa e aw ma e ial in he ob aining o highly- anspa en bioplas ics wi h sui able
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mechanical p ope ies o he manu ac u e o biodeg adable ood packaging and o he
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plas ic p oduc s. Blends o his p o ein wi h o he p oduc s om ag icul u al sou ces,
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biodeg adable and o lowe cos han p o ein and syn he ic addi i es ha e been
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ecen ly p oposed (González-Gu ie ez e al., 2010).
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A p o ein-based ma e ial could be de ined as a s able h ee-dimensional
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mac omolecula ne wo k s abilized and s eng hened by hyd ogen bonds, hyd ophobic
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in e ac ions and disul ide bonds (Pomme e al., 2003). Howe e , as p o eins
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hemsel es do no ha e su icien plas ici y o be handled and b i le p ope ies a e
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ypically ound in bioplas ics, a plas icize is equi ed. The ole o plas icize s in
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educing he glass ansi ion empe a u e and p o iding mobili y o polyme ic chains
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has been ex ensi ely desc ibed (I issin-Manga a e al., 2001; Ma ee e al., 2000;
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Pouplin e al., 1999). Due o i s excellen p ope ies, glyce ol is among he mos
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commonly used plas icize s o biopolyme -based biodeg adable ma e ials.
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P o ein/plas icize blends (bioplas ics) can be p ocessed using exis ing plas ic
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p ocessing echnologies: om he physic-chemical o cas ing me hod (Gennadios,
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2002) o he mo-plas ic/mechanical me hod (comp ession molding o ex usion) (Je ez
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e al., 2007a; González-Gu ié ez e al., 2011). Howe e , a ele an echnique such as
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injec ion molding, which is among he mos common p ocessing me hods used wi h
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syn he ic polyme s, has no been ound o be used o p o ein-based bioplas ic
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applica ions. The po en ial use o p o ein-based injec ion molding o p oduce many
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kinds o shaped p oduc s will en ail new a gumen s in a o o conside ing hese
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biopolyme ma e ials as an al e na i e o syn he ic plas ics o di e en applica ions.
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The o e all objec i e has been o s udy plas icized albumen/soy biobased plas ic
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ma e ials p ocessed by injec ion molding. To achie e his objec i e, di e en
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albumen/soy a ios plas icized wi h glyce ol ha e been p ocessed. In addi ion, some
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a iables such as empe a u e and esidence ime in he p e-injec ion mixing chambe ,
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as well as he empe a u e o he mold ha e been also analyzed in o de o selec
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sui able p ocessing pa ame e s o p o ein-based injec ion molding.
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2. Ma e ial and me hods
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2.1. Ma e ials
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Comme cial sp ay-d ied albumen (AP) was p o ided by OVOSEC S.A and soy
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p o ein isola e (SPI) was supplied by P o ein Technologies In e na ional (SUPRO
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500E, Lepe , Belgium). The p o ein con en o bo h p oduc s was de e mined in
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quad uplica e as % N x 6.25 using a LECO CHNS-932 ni ogen mic o analyze (Leco
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Co po a ion, S . Joseph, MI, USA) (E he idge e al., 1998) being 83 w .% o AP and 91
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w .% o SPI. Glyce ol (GL), om Pan eac Química, S.A. (Spain), was used as p o ein
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plas icize .
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2.2. Sample p epa a ion
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Blends con aining 60 w .% p o ein, wi h di e en AP/SPI a ios as shown in Table 1,
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and 40 w .% glyce ol (GL) we e mixed in a wo-blade coun e - o a ing ba ch mixe
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(B abende Plas og aph, Ge many). Mixing p ocess was ca ied ou a 25ºC and 50
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pm o c.a. 10 min (Je ez e al., 2005) o ob ain a dough-like ma e ial a neu al pH.
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The inal pH alue was measu ed by a C ison pH 25 pHme e in combina ion wi h a
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punc u e elec ode (C ison Ins umen s S.A., Ba celona, Spain).
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The dough-like ma e ials ob ained a e he mixing p ocess we e subsequen ly
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p ocessed by injec ion molding using a MiniJe Pis on Injec ion Molding Sys em
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(The moHaake, Ka ls uhe, Ge many) o ob ain bioplas ic p obes. The mos sui able
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p ocessing a iables, such as injec ion empe a u e and p essu e, as well as esidence
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ime in he p e-injec ion mixing chambe , we e selec ed a e pe o ming empe a u e
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amp and ime sweep es s o he dough-like ma e ials. Two ypes o molds we e used
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o p epa e he p obes: a 60×10×1 mm ec angula shape mold o bo h DMTA
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expe imen s and anspa ency measu emen s and a Dumpbell ype p obe de ined by
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ISO 527-2:1993 o Tensile P ope ies o Plas ics.
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2.3. Cha ac e iza ion
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2.3.1. P o ein solubili y. P o ein solubili y a di e en pH alues was de e mined.
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Aqueous dispe sions (c.a. 1.00 g p o ein/40 mL) we e p epa ed and pH o di e en
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aliquo s was adjus ed o alkaline pH alues wi h 6 N NaOH, and o acid pH wi h 2 N
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HCl. Samples we e homogenized and subsequen ly cen i uged o 20 min a 10,000 x
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g a 10ºC. The supe na an we e collec ed o p o ein con en de e mina ion by means
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o he Ma kwell me hod (Ma kwell e al., 1978). Solubili y was exp essed as a
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pe cen age (g soluble p o ein/100 g isola e in sample).
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2.3.2. P o ein ex ac abili y. Samples we e ex ac ed in di e en ex ac ion media (2.5
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mL): dis illed wa e ; a dena u ing agen solu ion (Me hod A); a solu ion o dena u ing
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and educing agen s (Me hod B). All he ma e ials we e ex ac ed o 2h a 20ºC by
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magne ic agi a ion (app oxima ely 400 pm). Me hod A used a 0.086 mol L-1 T is-base,
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0.045 mmol·L-1 glycine, 2 mmol·L-1 EDTA, 10 g·L-1, 5 g·L-1 Sodium Dodecyl Sul a e
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(SDS) pH 8 bu e . In me hod B, p o eins we e dissol ed in he same bu e con aining
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10 g·L-1 Di hio h ei ol (DTT). Dispe sions we e cen i uged a 10,000 x g o 15 min a
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15ºC and p o ein con en was de e mined by a LECO CHNS-932 ni ogen mic o
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analyze . Simila me hods we e p e iously used by he au ho s wi h p o ein gels
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(Rome o e al., 2011). P o ein solubili y was de e mined om supe na an and
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exp essed as 100x p o ein con en in he supe na an / o al p o ein con en . Th ee
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independen ex ac ions we e ca ied ou wi h each sol en . A e age alues (±s anda d
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de ia ion) a e epo ed.
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2.3.3. F ee and o al sul hyd yls. F ee and o al sul hyd yl g oups o p o ein samples
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we e de e mined using he me hod de eloped by Be e idge e al. (1974) and
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Thannhause e al. (1984), espec i ely. Samples we e suspended (1 mg/mL) in 0.086
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mol/L T is-HCl – 0.09 mol/L glycine – 4 mmol/L EDTA – 8 mol/L u ea – pH 8 bu e .
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Dispe sions we e s i ed a 25 ºC du ing 10 min a 500 pm in a he momixe and hen
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cen i uged a 15,000g (10 min, 10 ºC). Supe na an was incuba ed wi h Ellman’s
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eagen (4mg DTNB/mL me hanol) and 1 mL NTSB was used in he case o he o al
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sul hyd yls. Abso bance a 412 nm was measu ed in a Genesis-20 spec opho ome e
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(The mo Scien i ic, USA). The mola ex inc ion coe icien o NTB (13,600 L·mol-1·cm-1)
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was used. P o ein concen a ion o ex ac s was de e mined by he B ad o d me hod
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(B ad o d, 1976).
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2.3.4. Su ace hyd ophobici y o p o eins (H0). Su ace hyd ophobici y (H0) o soluble
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p o eins om p o ein ex ac s (pH 8) was measu ed acco ding o Ka o and Nakai
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(1980), using he luo escen p obe 1-anilino-8-naph alene-sul ona e (ANS). P o ein
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ex ac we e dilu ed wi h pH 8-0.05 M phospha e bu e o ob ain p o ein concen a ions
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anging om 5 o 0.005 mg/mL. Then, 40 μL o ANS (8.0 mM in he same bu e ) we e
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added o 2 mL o sample. Fluo escence in ensi y (FI) was measu ed wi h a Pe kin-
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Elme 2000 luo escence spec ome e (Pe kin-Elme Co p. No walk, CT, USA), a
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wa eleng hs o 365 nm (exci a ion) and 484 nm (emission). The ini ial slope o
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luo escence in ensi y e sus p o ein concen a ion plo was used as an index o
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p o ein hyd ophobici y (H0).
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2.3.5. Di e en ial Scanning Calo ime y (DSC). DSC expe imen s we e pe o med wi h
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a Q20 (TA Ins umen s, USA), using 5 o 10 mg samples, in he me ic aluminum pans.
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A hea ing a e o 10 ºC/min was selec ed. The sample was pu ged wi h a ni ogen low
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o 50 mL/min.
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2.3.6. Rheological measu emen s. Dough-like ma e ial was cha ac e ized by Small
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Ampli ude Oscilla o y Shea (SAOS) measu emen s, using a con olled-s ain
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heome e (ARES), in o de o selec he op imum condi ions o injec ion molding. The
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geome y used has been a pla e and pla e geome y (dia: 25 mm) wi h a ough su ace
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and a gap be ween pla es o 1 mm. Low iscosi y Dow Co ning 200 luid has been
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used as sealan o a oid sample d ying. S ain sweep SAOS es s we e also pe o med
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in o de o es ablish he linea iscoelas ici y ange. Tempe a u e amp es s we e
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ca ied ou a 5 ºC/min om 20 o 100 ºC and ime sweep es s we e pe o med o
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1800 s a a selec ed cons an empe a u e. In hese measu emen s, complex iscosi y
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(

*) was moni o ed a a cons an equency o 2 ad/s. All he sys ems s udied had he
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same he mo heological his o y be o e pe o ming any heological es .
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2.3.7. Dynamic Mechanical Tempe a u e Analysis (DMTA). DMTA es s we e ca ied
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ou wi h a RSA3 (TA Ins umen s, New Cas le, DE, USA), on ec angula p obes using
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dual can ile e bending. All he expe imen s we e ca ied ou a cons an equency
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(1Hz) and s ain (be ween 0.01 and 0.3%, wi hin he linea iscoelas ic egion). The
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selec ed hea ing a e was 3ºC min−1. All he samples we e coa ed wi h Dow Co ning
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high acuum g ease o a oid wa e loss.
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2.3.8. Tensile s eng h measu emen s. Tensile es s we e pe o med by using he
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Insigh 10 kN Elec omechanical Tes ing Sys em (MTS, Eden P ai ie, MN, USA),
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acco ding o by ISO 527-2:1993 o Tensile P ope ies o Plas ics. Tensile s ess and
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elonga ion a b eak we e e alua ed om a leas h ee duplica es o each p oduc
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using ype IV p obes and an ex ensional a e o 100 mm·min−1 a oom empe a u e.
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2.3.9. Wa e abso p ion capaci y. Wa e up ake o bioplas ics was de e mined ollowing
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he ASTM D570 no m (ASTM D570-98, S anda d es Me hod o Wa e Abso p ion o
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Plas ics) (ASTM, 2001) using a leas h ee 60×10×1 mm specimens imme sed in
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dis illa e wa e o 2h o 24 h a oom empe a u e.
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2.3.10. T anspa ency Measu emen s. T anspa ency measu emen s we e done by
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means o a Genesis-20 spec opho ome e (The mo Scien i ic, USA). T ansmi ance
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(%) o ec angula specimens, 3 mm hick, was measu ed using a wa eleng h o 600
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nm. Ai was used as blank (100% ansmi ance). A ansmi ance index (IT) was
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de ined in o de o compa e he anspa ency o he di e en bioplas ic.
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𝐼𝑇=𝐵𝑖𝑜𝑝𝑙𝑎𝑠𝑡𝑖𝑐 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑎𝑛𝑐𝑒 (%)
𝐴𝑙𝑏𝑢𝑚𝑖𝑛 − 𝑏𝑎𝑠𝑒𝑑 𝐵𝑖𝑜𝑝𝑙𝑎𝑠𝑡𝑖𝑐 𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑎𝑛𝑐𝑒 (%) ∙100 (1)
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2.4. S a is ical analysis
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A leas h ee eplica es o each measu emen we e ca ied ou . S a is ical analyses
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we e pe o med using - es and one-way analysis o a iance (ANOVA, p0.05) by
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means o he s a is ical package SPSS 18. S anda d de ia ions om some selec ed
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pa ame e s we e calcula ed.
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3. Resul s and discussion
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3.1. P o ein solubili y
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Fig. 1 shows he solubili y-pH p o ile ob ained o bo h AP and SPI sys ems. AP
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sys em displays high solubili y alues wi hin he whole expe imen al ange o pH
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alues, showing alues highe han 80 w .%. These esul s suppo he high hyd ophilic
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cha ac e o AP sys em which is clea ly domina ed by i s high con en in highly soluble
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albumin p o eins. Thus, almos 70% o p o ein in egg whi e is o oalbumin and
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conalbumin (Che el e al., 1989). This high solubili y akes place e en a he minimum
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shown in Fig.1 obse ed a pH 5.8-6, which should co espond o he isoelec ic poin
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(IEP). Howe e , he IEP o egg whi e has been epo ed o be sligh ly lowe since Loeb
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(1923) epo ed a alue o 4.8, while he IEP is loca ed a 5.4 acco ding o Riddick
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(1968). These alues a e consis en wi h he esul s ob ained om z-po en ial
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measu emen s (da a no shown) pe o med o his AP sys em ha exhibi ed a null
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alue a pH a ound 5.
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SPI exhibi s much lowe solubili y alues han AP sys em, showing a maximum
227
solubili y (ca. 36%) a pH 8 and a minimum (ca. 7.2%) a pH 6. Howe e , a wide low-
228
solubili y ange (lowe han 10%) may be no iced be ween pH 3 and 6. Molina-O iz e
229
al. (2004) epo ed highe solubili y alues, pa icula ly a acidic pH, wi h a much
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na owe minimum a pH 4.5 o SPI p epa ed om a non he mally ea ed soy lou ( o
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a oid p o ein dena u a ion). Thus, he ex ension o he low solubili y p o ile o he 3-6
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pH ange migh be a ibu ed o some p o ein dena u a ion du ing SPI manu ac u e.
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I is also in e es ing o men ion ha he solubili y p o ile shows a second minimum a
234
pH 9. This minimum, al hough being less appa en , has been also de ec ed by F anzen
235
and Kinsella (1976) o a SPI showing much highe solubili y a alkaline pH.
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3.2. Disul ide and sul hyd yl g oups
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p o ein-based bioplas ic ma e ials (Je ez e al., 2007a, 2007b; Zá a e-Ramí ez e al.,
401
2011), indica es ha some he mose ing po en ial s ill emains a e injec ion molding
402
a he expe imen al condi ions. These esul s con i m ha he condi ions selec ed o
403
injec ion molding, summa ized in Table 5, a e mo e sui able o SPI-based p obes. As
404
o he AP/SPI-based bioplas ics, he beha io is much close o ha one ound o he
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SPI/GL sys em. The AP/SPI mixed sys ems show an in e media e beha io , being only
406
appa en a empe a u es highe han 60ºC.
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The highe iscoelas ic p ope ies shown by he sys em con aining AP as he only
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p o ein may be ela ed o some o he physicochemical p ope ies p e iously shown.
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Thus, he lowes empe a u e o he DSC peak shown in Fig. 3 (much lowe han he
410
p ocessing empe a u e) would lead o a high deg ee o dena u a ion o his sys em.
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This e ec would lead o a highe su ace hyd ophoboci y ha in any case is highe o
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AP (Table 3) ha would p oduce a highe inc ease in p o ein agg ega ion. In addi ion,
413
he highe alue o ee sul hyd yls shown by AP (Table 2) would also yield a highe
414
densi y o c osslinking by o ming hyd ogen bonds. Simila esul s ha e been epo ed
415
by Bounoco e e al. (2003) ha also explained his beha io in e ms o he mal
416
induced c osslinking.
417
All he p obes s udied display simila loss angen p o iles (Fig. 4B) showing one
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single peak, which is ela ed o a glass-like ansi ion o he plas icized p o ein-based
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ma e ial, wi h an 𝛿 alues anging om 0.1 o 0.5, excep ing o he AP/GL sys em ha
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shows a much lowe an 𝛿 peak a ca. 0.34. These unimodal p o iles indica e a good
421
compa ibili y be ween p o ein and glyce ol, ob ained o all he sys ems a e he
422
injec ion molding p ocess, ega dless o he p o ein used (AP, SPI o AP/SPI mix u es).
423
The alue o he empe a u e a he peak o he AP/GL sys em is ca. 63ºC, which is
424
a he coinciden wi h hose alues p e iously ound o albumen-based bioplas ic
425
p epa ed by comp ession molding wi h he same p o ein/glyce ol a io (Je ez e al.,
426
2007a, González-Gu ie ez e al., 2010). As may be obse ed in he plo included in
427
Fig.4B, his empe a u e a he peak inc eases asymp o ically wi h he ela i e
428

17
p opo ion o SPI in he p o ein mix u e up o a alue o ca. 78.5ºC. This ea lie
429
ansi ion ound o AP/GL bioplas ics is also consis en wi h he ea lie Tg ob ained o
430
AP/GL blends om DSC measu emen s.
431
3.8. Uniaxial ensile s eng h measu emen s
432
The esul s ob ained om uniaxial s eng h measu emen s a e shown in Fig. 5. Fig.
433
5A displays he esul s o s ess-s ain cu es ob ained o p o ein-based bioplas ics
434
using AP, SPI and 50:50 AP/SPI mix u es.
435
All he cu es exhibi an ini ial linea elas ic beha io o high cons an s ess-s ain
436
slope yielding high alues o he Young’s Modulus (E), ollowed by a plas ic
437
de o ma ion s age wi h a con inuous dec ease in he s ess-s ain slope a e he elas ic
438
limi . A second cons an slope is eached a he end o his plas ic de o ma ion s age.
439
All he cu es e en ually each a maximum alue o he s ess (max) and he s ain
440
(εmax), which is immedia ely ollowed by a sudden dec ease in s ess ha co esponds
441
o he up u e o he sample. Fig. 5B shows he alues o he h ee pa ame e s (E, max
442
and εmax) om ensile es s pe o med on AP/SPI/GL bioplas ic p obes, as a unc ion o
443
he albumen con en in he AP/SPI mix u e. Pa ame e s (max) and E show a
444
p og essi e inc ease wi h inc easing albumen con en , wi h an o e all g ow h in he
445
o de o 50% o bo h pa ame e s. Pa ame e εmax also unde goes an inc ease up o
446
50% AP concen a ion. Howe e , a u he inc ease up o 100% does no lead o any
447
signi ican di e ence. The e o e, i may be gene ally s a ed ha AP-based bioplas ics
448
exhibi be e ensile p ope ies han SPI-based sys ems in acco dance o he highes
449
alues also ound o he elas ic modulus om DMA measu emen s and he lowes Td
450
ob ained wi h DSC es s.
451
In any case, all he P o ein/GL bioplas ic p obes exhibi ensile p ope ies lowe han
452
syn he ic polyme s such as LDPE. Thus, pa ame e s max, εmax and E show alues ha
453
each as much as 15%, 18% and 10%, espec i ely, o he alues o ASTM
454
no malized LDPE (ASTM D638).
455
18
3.9. Wa e up ake capaci y
456
Fig. 6 shows he esul s om wa e up ake measu emen s ob ained a e imme sion
457
o bioplas ic samples o 2 and 24 h, as well as he wa e -soluble ma e loss, as a
458
unc ion o AP con en .
459
No signi ican di e ences be ween wa e up ake pe cen ages a e 2 and 24 h a e
460
ound, ega dless o he AP/SPI a io used in he bioplas ic p epa a ion by injec ion
461
molding. This ac indica es occu ence o ela i ely as e wa e abso p ion kine ics as
462
compa ed o glu en-based bioplas ics p ocessed by comp ession molding (unpublished
463
esul s). In addi ion, an inc ease in AP con en induces a p og essi e dec ease in wa e
464
abso p ion up o hal he alue co esponding o he AP ee sample. I may be poin ed
465
ou ha his las alue is much lowe han hose p e iously epo ed (Je ez e al.,
466
2007a) o simila AP/GL samples. Howe e , hese samples used highe AP/GL a io (2
467
ins ead o 1.5) and we e p epa ed by comp ession molding. Al hough he o me ac
468
may induce some changes, he abo e-men ioned di e ence in wa e up ake capaci y
469
may be mainly a ibu ed o he di e en he momechanical p ocessing condi ions used.
470
In ac , as epo ed by Je ez e al. (2007a), an inc ease in p essu e led o a ema kable
471
educ ion in wa e abso p ion.
472
On he o he hand, he ac ha AP-based bioplas ic display lowe abso p ion
473
capaci y han SPI-based samples may be ela ed o a highe s uc u e deg ee o he
474
AP p o ein ma ix. This is consis en wi h he abo e-men ioned highe p o ein
475
c osslinking ob ained o AP/GL bioplas ic p obes as deduced om DMA and ensile
476
es s. This beha io is in ag eemen wi h hose esul s ound by o he au ho s
477
(Buonoco e e al., 2003; Zheng e al., 2003), who epo ed ha he swelling a io o
478
polyme ma ices dec ease wi h inc easing p o ein c osslinking.
479
As ega ds he wa e -soluble loss ma e he AP/GL sys em shows a alue o 40%
480
and all he SPI-con aining p obes exhibi alues ha a e sligh ly highe . These esul s
481
sugges ha he loss o soluble ma e co esponds basically o he highly hyd ophilic
482
glyce ol. In ac , all he samples con ain 40% GL. The ex a-loss ma e obse ed o
483
19
SPI-based samples should co espond o mois u e con en as well as o some p o ein
484
ha is no s ongly associa ed o he ne wo k s uc u e.
485
3.10. T anspa ency measu emen s
486
Fig. 7 shows he e olu ion o he ansmi ance index (IT) wi h inc easing AP
487
pe cen age in he o al p o ein con en o he injec ion molded bioplas ic p obe. Images
488
o each p obe a e also included in Fig. 7. The e olu ion o colo and anspa ency wi h
489
inc easing AP con en is appa en , leading o a ema kable inc ease in he alue o IT.
490
The alue ob ained o AP-based p obe is 46.8  0.4, being sligh ly lowe han he
491
alue co esponding o LDPE (54.0 ± 0.3) and 52% highe han ha o he SPI-based
492
p obe. Mo eo e , as may be no ed, addi ion o 25% AP causes he highes inc ease in
493
IT alue ep esen ing abou 63% o he o al inc ease in ansmi ance.
494
4. Concluding ema ks
495
AP concen a e shows highe solubili y han SPI. AP also shows highe sul hyd yl
496
g oups and disul ide bonds, as well as highe hyd ophobici y. The e o e, AP
497
concen a e e idences highe po en ials o he momechanical p ocessing han SPI.
498
Acco ding o he ex ac abili y analysis o p o ein concen a es and p o ein/GL
499
blends, some shea -induce e ec s ook place du ing he mixing p ocess al hough being
500
less no iceable o AP han o SPI sys ems. As a esul o he mixing p ocess, glyce ol
501
educed he glass ansi ion and dena u a ion empe a u es o a highe ex en when
502
SPI p o ein was used ins ead o AP. The combina ion o empe a u e amps and ime
503
sweep es s a selec ed empe a u e has demons a ed o be sui able echniques in
504
o de o selec ope a ion condi ions o injec ion molding. These p ocessing condi ions
505
ook in o accoun he mixed he moplas ic and he mose cha ac e o he p o ein
506
sys ems s udied whe e he con ibu ion o he la e is mo e ele an .
507
Bo h p o ein sys ems used alone o in combina ion yield bioplas ics ha can be
508
he momechanically p ocessed by injec ion molding, al hough showing lowe
509
20
heological and mechanical p ope ies han LDPE s anda ds, unde he selec ed
510
p ocessing condi ions.
511
A compa ison be ween AP and SPI-based injec ion molded bioplas ic specimens
512
e ealed ha he o me show highe iscoelas ic bending p ope ies and highe
513
uniaxial ensile p ope ies. Howe e , unde he selec ed condi ions, SPI s ill showed a
514
emaining he mose ing po en ial. In any case, as a consequence o he lowe
515
heological and mechanical p ope ies o he la e , SPI showed a highe wa e up ake
516
capaci y.
517
The esul s o ansmi ance pu o wa d ha injec ion molding may yield highly
518
anspa en p o ein-based polyme ic ma e ials by adding a mode a e p opo ion o AP
519
o he aw p o ein/plas icise blend. Some o hese ma e ials showed anspa ency
520
p ope ies ha we e e en compa able o LDPE plas ic ma e ials. These esul s a e
521
consis en wi h hose epo ed by o he p e ious esea che s ha used egg whi e-
522
based comp ession molded ma e ials.
523
Acknowledgemen s
524
This wo k is pa o a esea ch p ojec sponso ed by Andalousian Go e nmen ,
525
(Spain) (p ojec TEP-6134) and by “Minis e io de Economía y Compe i i idad” om
526
Spanish Go e nmen (Re . MAT2011-29275-C02-02/01). The au ho s g a e ully
527
acknowledge hei inancial suppo .
528
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