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Protein-based Packaging Films, Sheets and Composites: Process Development and Functional Properties
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials. (Polymeric materials)
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The depletion of the petroleum resources and a number of environmental concerns led to considerable research efforts in the field of biodegradable materials over the last few decades. Of the diverse range of biopolymers, wheat gluten (WG) stands out as an alternative to synthetic plastics in packaging applications due to its attractive combination of flexibility and strength, high gas barrier properties under low humidity conditions and renewability. The availability of raw materials has also been largely increased with an increase in the production of WG as a low-cost surplus material due to increasing demand for ethanol as fuel. In this study, WG was processed into films, sheets and composites using some of the most widely used techniques including solution casting, compression molding, extrusion and injection molding, accompanying process optimizations and characterization of their functional properties. This thesis consists mainly of six parts based on the purpose of the study. The first part addresses the aging and optical properties of the cast film in order to understand the mechanisms and reasons for the time-dependant physical and chemical changes. The films plasticized with glycerol were cast from acidic (pH 4) and basic (pH11) solutions. The film prepared from the pH 11 solution was mechanically more stable upon aging than the pH 4 film, which was initially very ductile but became brittle with time. It was revealed that the protein structure of the pH 4 film was initially less polymerized/aggregated and the polymerization increased during storage but it did not reach the degree of aggregation of the pH 11 film. During aging, the pH 4 film lost more mass than the pH 11 film mainly due to migration of glycerol but also due to some loss of volatile mass. In addition the greater plasticizer loss of the pH 4 film was presumably due to its initial lower degree of protein aggregation/polymerization. Glycerol content did not significantly change the opacity and pH 4 films showed good contact clarity because of less Maillard reaction. In the second part, the heat-sealability of WG films was investigated, using an impulse-heat sealer, as the sealability is one of the most important properties in the use of flexible packaging materials. It was observed that the WG films were readily sealable while preserving their mechanical integrity. The sealing temperature had a negligible effect on the lap-shear strength, but the peel strength increased with sealing temperature. The lap-shear strength increased with increasing mold temperature and the failure mode changed. The third part describes the possibility of using industrial hemp fibers to reinforce wheat gluten sheets based on evaluation of the fiber contents, fiber distribution and bonding between the fibers and matrix. It was found that the hemp fibers enhanced the mechanical properties, in which the fiber contents played a significant role in the strength. The fiber bonding was improved by addition of diamine as a cross-linker, while the fiber distribution needed to be improved. The fourth part presents a novel approach to improve the barrier and mechanical properties of extruded WG sheets with a single screw extruder at alkaline conditions using 3-5wt.% NaOH with or without 1 wt.% salicylic acid. The oxygen barrier, at dry conditions, was improved significantly with the addition of NaOH, while the addition of salicylic acid yielded poorer barrier properties. It was also observed that the WG sheets with 3 wt.% NaOH had the most suitable combination of low oxygen permeability and relatively small time-dependant changes in mechanical properties, probably due to low plasticizer migration and an optimal protein aggregation/polymerization. In the fifth part WG/PLA laminates were characterized for the purpose of improving the water barrier properties. The lamination was performed at 110°C and scanning electron microscopy showed that the laminated films were uniform in thickness. The laminates significantly suppressed the mass loss and showed promising water vapor barrier properties in humid conditions indicating possible applications in packaging. The final part addresses the development of injection molding processes for WG. Injection-molded nanocomposites of WG/MMT were also characterized. WG sheets were successively processed using injection molding and the process temperatures were found to preferably be in a range of 170-200°C, which was varied depending on the sample compositions. The clay was found to enhance the processability, being well dispersed in the matrix. The natural clay increased the tensile stiffness, whereas the modified clay increased the surface hydrophobicity. Both clays decreased the Tg and increased the thermal stability of the nanocomposites. The overall conclusion was that injection molding is a promising method for producing WG items of simple shapes. Further studies will reveal if gluten can also be used for making more complex shapes.

Place, publisher, year, edition, pages
Stockholm: KTH , 2009. , 81 p.
Keyword [en]
wheat gluten, solution casting, compression molding, extrusion, injection molding, aging, migration, opacity, heat sealability, hemp fiber, laminate, polylactic acid, clay, nanocomposites.
National Category
Other Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-10567ISBN: 978-91-7415-358-3 (print)OAI: oai:DiVA.org:kth-10567DiVA: diva2:219235
Public defence
2009-06-12, H1, Teknikringen 33, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100708Available from: 2009-06-04 Created: 2009-05-26 Last updated: 2011-03-23Bibliographically approved
List of papers
1. Aging properties of films of plasticized vital wheat gluten cast from acidic and basic solutions
Open this publication in new window or tab >>Aging properties of films of plasticized vital wheat gluten cast from acidic and basic solutions
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2006 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 7, no 5, 1657-1664 p.Article in journal (Refereed) Published
Abstract [en]

In order to understand the mechanisms behind the undesired aging of films based on vital wheat gluten plasticized with glycerol, films cast from water/ethanol solutions were investigated. The effect of pH was studied by casting from solutions at pH 4 and pH 11. The films were aged for 120 days at 50% relative humidity and 23 C, and the tensile properties and oxygen and water vapor permeabilities were measured as a function of aging time. The changes in the protein structure were determined by infrared spectroscopy and size-exclusion and reverse-phase high-performance liquid chromatography, and the film structure was revealed by optical and scanning electron microscopy. The pH 11 film was mechanically more stable with time than the pH 4 film, the latter being initially very ductile but turning brittle toward the end of the aging period. The protein solubility and infrared spectroscopy measurements indicated that the protein structure of the pH 4 film was initially significantly less polymerized/ aggregated than that of the pH 11 film. The polymerization of the pH 4 film increased during storage but it did not reach the degree of aggregation of the pH 11 film. Reverse-phase chromatography indicated that the pH 11 films were to some extent deamidated and that this increased with aging. At the same time a large fraction of the aged pH 11 film was unaffected by reducing agents, suggesting that a time-induced isopeptide cross-linking had occurred. This isopeptide formation did not, however, change the overall degree of aggregation and consequently the mechanical properties of the film. During aging, the pH 4 films lost more mass than the pH 11 films mainly due to migration of glycerol but also due to some loss of volatile mass. Scanning electron and optical microscopy showed that the pH 11 film was more uniform in thickness and that the film structure was more homogeneous than that of the pH 4 film. The oxygen permeability was also lower for the pH 11 film. The fact that the pH 4 film experienced a larger and more rapid change in its mechanical properties with time than the pH 11 film, as a consequence of a greater loss of plasticizer, was presumably due to its initial lower degree of protein aggregation/ polymerization. Consequently, the cross-link density achieved at pH 4 was too low to effectively retain volatiles and glycerol within the matrix.

Keyword
hydrogen-ion equilibria, barrier properties, mechanical-properties, secondary structure, chemical properties, protein-composition, dope solution, transport, time, increase
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-14024 (URN)10.1021/bm0600973 (DOI)000237593600036 ()2-s2.0-33744517880 (Scopus ID)
Note
QC 20100708Available from: 2010-07-08 Created: 2010-07-08 Last updated: 2010-11-03Bibliographically approved
2. Heat-sealing properties of compression-molded wheat gluten films
Open this publication in new window or tab >>Heat-sealing properties of compression-molded wheat gluten films
2007 (English)In: Journal of Biobased Materials and Bioenergy, ISSN 1556-6560, Vol. 1, no 1, 56-63 p.Article in journal (Refereed) Published
Abstract [en]

The impulse heat-sealing properties of wheat gluten films were investigated. Films containing 30 wt% glycerol were compression molded at 100-130 degrees C and then sealed in a lap-shear or peel-test geometry at 120-175 degrees C. The tensile properties of the pristine films and the lap-shear and peel strength of the sealed films were evaluated and the seals were examined by scanning electron microscopy. Glycerol was added to the film surfaces prior to sealing in an attempt to enhance the seal strength. It was observed that the wheat gluten films were readily sealable. At a 120 degrees C sealing temperature and without glycerol as adhesive, the lap-shear strength was greater than or similar to that of polyethylene film, although the peel strength was poorer. The sealing temperature had a negligible effect on the lap-shear strength, but the peel strength increased with sealing temperature. The lap-shear strength increased with increasing mold temperature and the failure mode changed, especially in the absence of glycerol adhesive, from a cohesive (material failure) to an adhesive type. From previous results, it is known that the high-temperature (130 degrees C) compression-molded film was highly cross-linked and aggregated, and this prevents molecular interdiffusion and entanglement and thus leads to incomplete seal fusion and, in general, adhesive failure. The presence of glycerol adhesive had a beneficial affect on the peel strength but no, or only a minor, effect on the lap-shear strength.

Keyword
wheat gluten; compression molding; mechanical properties; sealing properties; lap-shear strength; peel strength
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-7475 (URN)10.1166/jbmb.2007.006 (DOI)000253201400007 ()
Note
QC 20100708Available from: 2007-09-19 Created: 2007-09-19 Last updated: 2010-09-15Bibliographically approved
3. Use of Industrial Hemp Fibers to Reinforce Wheat Gluten Plastics
Open this publication in new window or tab >>Use of Industrial Hemp Fibers to Reinforce Wheat Gluten Plastics
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2009 (English)In: Journal of polymers and the environment, ISSN 1566-2543, E-ISSN 1572-8900, Vol. 17, no 4, 259-266 p.Article in journal (Refereed) Published
Abstract [en]

The next generation of manufactured products must be sustainable and industrially eco-efficient, making materials derived from plants an alternative of particular interest. Wheat gluten (WG) is an interesting plant material to be used for production of plastic similar materials due to its film-forming properties. For usage of plastics in a wider range of applications, composite materials with improved mechanical properties are demanded. The present study investigates the possibilities of reinforcing WG plastics with hemp fibers. Samples were manufactured using compression molding (130 A degrees C, 1600 bar, 5 min). Variation in fiber length, content (5, 10, 15 and 20 wt%) and quality (poor, standard, good) were evaluated. Mechanical properties and structure of materials were examined using tensile testing, light and scanning electron microscopy. Hemp fiber reinforcement of gluten plastics significantly influenced the mechanical properties of the material. Short hemp fibers processed in a high speed grinder were more homogenously spread in the material than long unprocessed fibers. Fiber content in the material showed a significant positive correlation with tensile strength and Young's modulus, and a negative correlation with fracture strain and strain at maximum stress. Quality of the hemp fibers did not play any significant role for tensile strength and strain, but the Young's modulus was significantly and positively correlated with hemp fiber quality. Despite the use of short hemp fibers, the reinforced gluten material still showed uneven mechanical properties within the material, a result from clustering of the fibers and too poor bonding between fibers and gluten material. Both these problems have to be resolved before reinforcement of gluten plastics by industrial hemp fibers is applicable on an industrial scale.

Keyword
Composite materials, Hemp, Plastics, Renewable raw materials, Wheat gluten, NATURAL FIBERS, GREEN COMPOSITES, SALICYLIC-ACID, BIOCOMPOSITES, PROTEIN, FILMS, CAST
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-14025 (URN)10.1007/s10924-009-0147-6 (DOI)000272912000006 ()2-s2.0-73349126995 (Scopus ID)
Note
QC 20100708Available from: 2010-07-08 Created: 2010-07-08 Last updated: 2011-02-04Bibliographically approved
4. Properties of Extruded Vital Wheat Gluten Sheets with Sodium Hydroxide and Salicylic Acid
Open this publication in new window or tab >>Properties of Extruded Vital Wheat Gluten Sheets with Sodium Hydroxide and Salicylic Acid
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2009 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 10, no 3, 479-488 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a novel approach to improve the barrier and mechanical properties of extruded glycerol-plasticized vital wheat gluten sheets. The sheets were extruded with a single screw extruder at alkaline conditions using 3-5 wt % NaOH. Salicylic acid (SA), known to improve the extrudability of wheat gluten, was also added alone or in combination with NaOH. Oxygen transmission rate and volatile mass measurements, tensile tests, protein solubility, glycerol migration, infrared spectroscopy, and electrophoresis were used to assess the properties of the extrudate. Electrophoresis showed that the gluten/glycerol sheet and the sheet with 3 wt % NaOH and I wt % SA contained the same building blocks in terms of proteins and protein subunits, although the protein solubility in these samples was different. The oxygen barrier, at dry conditions, was improved significantly with the addition of NaOH, On the other hand, the addition of salicylic acid yielded poorer barrier properties. The extrudate was placed on a blotting paper and its aging properties were investigated during the first 120 days. It was observed that the extrudate with 3 wt % NaOH had the most suitable combination of properties (low oxygen permeability, large strain at break, and relatively small aging-induced changes in mechanical properties); the reason is probably due to low plasticizer migration and an optimal protein aggregation/polymerization.

Keyword
RHEOLOGICAL PROPERTIES, PROTEIN-COMPOSITION, BARRIER PROPERTIES, WATER-VAPOR, FILMS, TEMPERATURE, FLOUR, PERMEABILITY, CONFORMATION, AGGREGATION
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-14026 (URN)10.1021/bm800691h (DOI)000264059400003 ()2-s2.0-65249159003 (Scopus ID)
Note
QC 20100708. Tidigare titel: Properties of plasticized vital wheat gluten sheets with sodiumAvailable from: 2010-07-08 Created: 2010-07-08 Last updated: 2010-09-15Bibliographically approved
5. Effects of glycerol content and film thickness on the properties of vital wheat gluten films cast at pH 4 and 1
Open this publication in new window or tab >>Effects of glycerol content and film thickness on the properties of vital wheat gluten films cast at pH 4 and 1
2010 (English)In: Journal of Applied Polymer Science, ISSN 0021-8995, E-ISSN 1097-4628, Vol. 117, no 6, 3506-3514 p.Article in journal (Refereed) Published
Abstract [en]

This study deals with the optical properties and plasticizer migration properties of vital wheat gluten (WG) films cast at pH 4 and 11. The films contained initially 8, 16, and 25 wt % glycerol and were aged at 23 degrees C and 50% relative humidity for at least 17 weeks on a paper support to simulate a situation where a paper packaging is laminated with an oxygen barrier film of WG. The films, having target thicknesses of 50 and 250 mu m, were characterized visually and with ultraviolet/visible and infrared spectroscopy; the mass loss was measured by gravimetry or by a glycerol-specific gas chromatography method. The thin films produced at pH 4 were, in general, more heterogeneous than those produced at pH 11. The thin pH 4 films consisted of transparent regions surrounding beige glycerol-rich regions, the former probably rich in gliadin and the latter rich in glutenin. This, together with less Maillard browning, meant that the thin pH 4 films, in contrast to the more homogeneous (beige) thin pH 11 films, showed good contact clarity. The variations in glycerol content did not significantly change the optical properties of the films. All the films showed a significant loss of glycerol to the paper support but, after almost 9 months, the thick pH 11 film containing initially 25 wt % glycerol was still very flexible and, despite a better contact to the paper, had a higher residual glycerol content than the pH 4 film, which was also more brittle.

Keyword
biopolymers, diffusion, films, proteins, transparency
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-14027 (URN)10.1002/app.32235 (DOI)000279958200051 ()2-s2.0-77955957759 (Scopus ID)
Note

QC 20100708. Updated from submitted to published, 20120315. Previous title: Comparison of properties between vital wheat gluten films, cast at pH 4 and pH 11: effects of glycerol content and film thickness

Available from: 2010-07-08 Created: 2010-07-08 Last updated: 2016-01-13Bibliographically approved
6. Effects of fiber blending and diamines on wheat gluten materials reinforced with hemp fiber
Open this publication in new window or tab >>Effects of fiber blending and diamines on wheat gluten materials reinforced with hemp fiber
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2010 (English)In: Journal of Materials Science, ISSN 0022-2461, E-ISSN 1573-4803, Vol. 45, no 15, 4196-4205 p.Article in journal (Refereed) Published
Abstract [en]

Wheat gluten (WG) is a promising base material for production of "green" plastics, although reinforcement is needed in more demanding applications. Hemp fiber is a promising reinforcement source but difficulties exist in obtaining desired properties with a WG-based matrix. This study aimed at improving fiber dispersion and fiber-matrix interactions using a high speed blender and a diamine as a cross-linker. Samples were manufactured using compression molding, two types of blenders and addition of diamine. Mechanical properties were assessed with tensile testing. Tensile-fractured surfaces were examined with scanning electron microscopy (SEM). Protein polymerization and fiber-protein matrix interactions were examined using high performance liquid chromatography (HPLC) and confocal laser scanning microscopy (CLSM). The results showed that a higher-speed grinding yielded a more even distribution of fibers and a more polymerized protein structure compared to a lower-speed grinding. However, these improvements did not result in increased strength, stiffness, and extensibility for the higher-speed grinding. The strength was increased when the grinding was combined with addition of a diamine (Jeffamine(R) EDR-176). HPLC, SEM, and CLSM, indicated that diamine added samples showed a more "plastic" appearance together with a stiffer and stronger structure with less cracking compared to samples without diamine. The use of the diamine also led to an increased polymerization of the proteins, although no effect on the fiber-protein matrix interactions was observed using microscopical techniques. Thus, for future successful use of hemp fibers to reinforce gluten materials, an appropriate method to increase the fiber-protein matrix interaction is needed.

Keyword
GREEN COMPOSITES, NATURAL FIBERS, SALICYLIC-ACID, BIOCOMPOSITES, FILMS
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-14029 (URN)10.1007/s10853-010-4514-4 (DOI)000278580100034 ()2-s2.0-77954863799 (Scopus ID)
Note
QC 20100708Available from: 2010-07-08 Created: 2010-07-08 Last updated: 2010-07-08Bibliographically approved
7. Properties of Wheat Gluten/Poly(lactic acid) Laminates
Open this publication in new window or tab >>Properties of Wheat Gluten/Poly(lactic acid) Laminates
2010 (English)In: Journal of Agricultural and Food Chemistry, ISSN 0021-8561, E-ISSN 1520-5118, Vol. 58, no 12, 7344-7350 p.Article in journal (Refereed) Published
Abstract [en]

Laminates of compression-molded glycerol-plasticized wheat gluten (WG) films surrounded and supported by poly(lactic acid) (PLA) films have been produced and characterized. The objective was to obtain a fully renewable high gas barrier film with sufficient mechanical integrity to function in, for example, extrusion-coating paper/board applications. It was shown that the lamination made it possible to make films with a broad range of glycerol contents (0-30 wt %) with greater strength than single unsupported WG films. The low plasticizer contents yielded laminates with very good oxygen barrier properties. In addition, whereas the unsupported WO films had an immeasurably high water vapor transmission rate (WVTR), the laminate showed values that were finite and surprisingly, in several cases, also lower than that of PLA. Besides being a mechanical support (as evidenced by bending and tensile data) and a shield between the WG and surrounding moisture, the PLA layer also prevented the loss of the glycerol plasticizer from the WG layer. This was observed after the laminate had been aged on an "absorbing" blotting paper for up to 17 weeks. The interlayer adhesion (peel strength) decreased with decreasing glycerol content and increasing WG film molding temperature (130 degrees C instead of 110 degrees C). The latter effect was probably due to a higher protein aggregation, as revealed by infrared spectroscopy. The lamination temperature (110-140 degrees C) did not, however, have a major effect on the final peel strength.

Keyword
Wheat gluten, poly(lactic acid), glycerol content, laminate, oxygen permeability, water vapor transmission rate, biopolymer, GLUTEN FILMS, BARRIER PROPERTIES, SALICYLIC-ACID, PLASTICIZERS, EXTRUSION, GLYCEROL, PROTEINS, CORN
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-14030 (URN)10.1021/jf1003144 (DOI)000278704000034 ()2-s2.0-77953624673 (Scopus ID)
Note
QC 20100708Available from: 2010-07-08 Created: 2010-07-08 Last updated: 2010-07-08Bibliographically approved
8. Injection-molded nanocomposites and materials based on wheat gluten
Open this publication in new window or tab >>Injection-molded nanocomposites and materials based on wheat gluten
2011 (English)In: International Journal of Biological Macromolecules, ISSN 0141-8130, E-ISSN 1879-0003, Vol. 48, no 1, 146-152 p.Article in journal (Refereed) Published
Abstract [en]

This is, to our knowledge, the first study of the injection molding of materials where wheat gluten (WG) is the main component. In addition to a plasticizer (glycerol), 5 wt.% natural montmorillonite clay was added. X-ray indicated intercalated clay and transmission electron microscopy indicated locally good clay platelet dispersion. Prior to feeding into the injection molder, the material was first compression molded into plates and pelletized. The filling of the circular mold via the central gate was characterized by a divergent flow yielding, in general, a stronger and stiffer material in the circumferential direction. It was observed that 20-30 wt.% glycerol yielded the best combination of processability and mechanical properties. The clay yielded improved processability, plate homogeneity and tensile stiffness. IR spectroscopy and protein solubility indicated that the injection molding process yielded a highly aggregated structure. The overall conclusion was that injection molding is a very promising method for producing WG objects.

Keyword
Injection molding, Nanocomposites, Wheat gluten, Montmorillonite, Mechanical properties
National Category
Biochemistry and Molecular Biology
Identifiers
urn:nbn:se:kth:diva-31626 (URN)10.1016/j.ijbiomac.2010.10.012 (DOI)000287279700025 ()2-s2.0-78650245588 (Scopus ID)
Funder
Swedish Research Council Formas, 2006-621
Note

QC 20110323

Available from: 2011-03-23 Created: 2011-03-21 Last updated: 2016-12-21Bibliographically approved

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