Thermoplastic Soyprotein Films
Soyproteins are one of the most widely studied plant proteins to develop thermoplastic products mainly due to their larger availability, lower cost, and better properties of products obtained compared to other plant proteins. In addition, soyproteins are much purer (>90% protein) compared to wheat gluten (80% protein) that makes soyproteins easier to process and control the quality of the products. As seen from Table 2, thermoplastic films developed from soyproteins with strength as high as 40 MPa to as low as 1.5 MPa have been reported depending on the conditions used to process the films and type and amount of plasticizers. Zhang et al. developed soyprotein sheets by extrusion and have studied the effects of water, glycerol, methyl glucoside, zinc sulfate, epichlorohydrin, and glutaric dialdehyde on the mechanical and thermal properties of the extruded sheets. Glycerol added as plasticizer increased elongation but decreased strength and modulus. When glycerol content was changed from 10% to 50%, tensile strength at break decreased from 40.6 to 7.1 MPa, elongation increased from 3% to 185% and modulus decreased from 1226 to 144 MPa. Glycerol as plasticizer reduced the interaction between protein molecules and increased the flexibility, extensibility, and processability and Tg of soyproteins was also found to decrease with the increase in the concentration of glycerol. In the same research, methyl glycoside (10 parts) was included as plasticizer instead of glycerol and was found to increase tensile strength and toughness by 24.4% and 33.3%, respectively. Similar effect was also observed when the moisture content was increased from 2.8% to 26%. Among the various plasticizers studied, glycerol is most widely used mainly because it is cheaper, derived from a bio-based source and approved for food use and is biodegradable. Most studies on developing films have intended to use the films for food packaging applications and glycerol has been the most suitable choice. It may be beneficial to study the potential of using non-glycerol plasticizers for films intended for non-food applications.
Table 2. Tensile Properties of Thermoplastics Films Made from Soyproteins Under Various Conditions
| ||Tensile properties|| |
|Additives, modifications, and processing conditions||Strength (MPa)||Elongation at break (%)||Modulus (MPa)||References|
|Soyprotein with 10–50% glycerola||40.6–7.1||3–185||1226–144||7|
|0–10% methyl glycoside, 20–30 parts glycerol||12.5–14.7||105–125||305–380||7|
|2.8–26% water and 30 parts glycerol||2.4–41.1||13–159||17–1220||7|
|0–0.4% Epichlorohydrin, 30 parts glycerol||13.7–16.7||101–148||257–389||7|
|0–2% ZnSO4 and 30 parts glycerol||14.3–16.9||107–162||257–586||7|
|0–0.6% Glutaric aldehyde, 30 parts glycerol||14.3–17.3||119–148||257–550||7|
|Soyprotein with 0.5–10% SDS, 10% moistureb||5.3–26.5||1.4–2.4||758–1667||12|
|Soyprotein films with 20–40% glycerolc||15.8–2.6||4.2–74.5||–||17|
|Soyprotein films with 30–50% glycerold||7.8–2.9||132–137||7.8–2.9||18|
|Films with 30–40% glycerol, pH 1.4–10.0||1.5–7.5||10–160||–||18|
|Films with 0–30% stearic acid, 30% glycerole||9.0–6.0||168–25.6||120–221||19|
|Films with 5–15% bovine gelatin and 30–40% glycerolf||7.2–11.3||–||136–146||20|
|Acetylated soyprotein films||1.8–2.5||73–113||–||21|
|Steamed soyproteins with 15% glycerol||5.0 ± 0.8||14.5 ± 3.0||193 ± 60||22|
Because of the relatively poor properties of plant protein based films compared to synthetic polymer based films, external crosslinking agents are used to improve the mechanical properties and water stability. Crosslinking agents such as ZnSO4 and epichlorohydrin were used to improve the tensile properties of the films. ZnSO4 was found to increase the modulus but did not significantly change the elongation. Crosslinking with epichlorohydrin or glutaric aldehyde was found to increase the modulus of the films to a higher extent compared to films crosslinked with ZnSO4. Bovine gelatin was found to interact with soyproteins via hydrogen bonding, electrostatic, and/or hydrophobic interactions and improve the tensile properties and resistance to UV light. In addition to using chemicals for crosslinking, physical crosslinking using UV light, and heat have also been used to improve properties of the films. Chemical crosslinking provides better properties than physical crosslinking. However, use of chemical crosslinkers raises concerns on safety and some chemical crosslinkers also cause undesirable changes and may also be difficult to use. In addition, it may not be suitable to use chemicals for crosslinking thermoplastic films because of the high temperatures required for film formation.
Effect of processing conditions such as molding temperature and pressure on properties of soyprotein polymers were studied by Mo et al. Under the various conditions studied, it was found that the soyprotein polymers provided maximum strength and strain when the molding was done close to the phase transition temperature or about 40°C lower than the exothermic temperature (185–192°C). Another study had also reported that soyproteins molded at 140–160°C provided the strongest plastics. Although studies have suggested the optimum processing temperature, many other parameters including the composition of the proteins, moisture levels, condition and type of equipments, and analytical methods used significantly affect results. Therefore, researchers should use the suggested temperatures as a guide but develop their own conditions to obtain films with optimum properties.
Soyprotein isolates were mixed with sodium dodecyl sulfate to denature the soyproteins by disrupting the hydrophobic and electrostatic interaction leading to partial unfolding that would make the proteins thermoplastic. Addition of SDS (5%) was found to completely denature the proteins based on the denaturation temperature determined by DSC thermograms. Tg of the samples decreased from 87.2°C (0% SDS) continuously to −6.0°C with increase in SDS content (10%) and tensile strength of the samples increased at low levels of SDS but decreased when the SDS was higher than 5% due to the plasticizing effect of the surfactant. Chemical modifications of soyproteins have also been done to develop bioplastics with improved mechanical properties and water stability. Environmentally green plastics were developed from stearic acid modified soyprotein isolates. Addition of stearic acid was found to increase modulus, decrease strain, and fracture stress when higher than 25% stearic acid was used. Changes in the tensile properties due to the addition of stearic acid were attributed to lower moisture sorption of the films, plasticization, and crystallization. Soyprotein isolates were grafted with methyl acrylate and methyl methacrylate to develop a thermoplastic copolymer but no products were developed. In another study, two types of soyprotein isolates were acetylated and made into thermoplastic films without adding any plasticizers. Acetylation helped to improve the tensile strength and water stability of the soyprotein films. Chemical modifications will help to make the proteins thermoplastic and also to improve the properties. However, it should be noted that chemical modifications not only add to the cost but could also make the proteins less biodegradable depending on the type of chemical modification done.
Steaming of soyproteins was found to increase thermoplasticity by cleaving disulfide bonds and denaturing the proteins. Thermoplastics developed from steamed soyproteins were found to have better tensile properties than many soyprotein thermoplastics reported earlier. Treating soyprotein films with benzilic acid resulted in a lotus leaf like structure and films also exhibited higher mechanical properties and water stability compared to untreated films. Similarly, pH of soyproteins was also found to affect the tensile properties. Films prepared under acidic conditions and those prepared at pH 7.5 and 10 had the best mechanical properties. If similar properties can be achieved, modifying the properties of the proteins using steam, pH or other means is probably a better approach than chemical modifications in view of the simplicity, potential cost savings, and biodegradability concerns.
Thermoplastics from Wheat Gluten
Wheat proteins are composed of three main components gluten, glutenin, and gliadin which have all been made into thermoplastics films as seen from Figure 1 and with varying properties as seen from Table 3. Presence of distinct components with vastly different properties and relatively less purity make it difficult to process wheat gluten into films. However, the elasticity, binding ability, and oxygen barrier properties offered by wheat gluten are not found in other plant proteins. Also, the allergenic effects of gliadin in gluten make it a less desirable source of protein compared to soyproteins for edible applications and therefore more suitable to develop thermoplastic products for non-food industrial applications. Researchers have examined the role of heat, plasticizers, additives, crosslinkers, and effect of various wheat protein components on properties of the thermoplastics developed. A fundamental study on the molecular basis of processing wheat gluten to develop bioproducts was done by Lagrain et al. They reported that wheat gluten aggregates upon heating due to the direct covalent crosslinking in and between its protein components glutenin and gliadin. Oxidation of sulfhydryl groups and sulfhydryl/disulfide interchange reactions leading to the formation of disulfide crosslinks were reported to be responsible for the thermoplastic nature of wheat gluten. Using this approach, the authors claim to have developed gluten based materials with properties similar to materials made from polypropylene and epoxy. Similarly, unlike most other researches where the wheat gluten bioplastics developed had low tensile properties compared to commercially available synthetic bioplastics, sheets formed by chemically reductive thermoforming were claimed to have elasticity comparable to commercial polymeric materials. Incorporation of thiol-terminated star-branched molecules into wheat gluten was found to provide tough, plastic-like substance. In that research, incorporation of the thiol terminated star branch molecule increased work of fracture four folds and tensile strain doubled compared to unmodified gluten due to crosslinking. Interestingly, tensile properties of the samples were found to increase with time when stored under ambient conditions. In a similar approach, a multifunctional macromolecular thiol (TPVA) obtained by esterification of poly(vinyl alcohol) with 3-mercaptopropionic acid was used as reactive modifier for wheat gluten. The modified wheat gluten was compression molded into bars and had increased strength, elongation, and modulus unlike the plasticizers commonly used for wheat gluten. About 76% increase in fracture strength, 80% increase in elongation, and 25% higher modulus was observed.
Table 3. Comparison of the Tensile Properties of Thermoplastic Films Made from Wheat Proteins
| ||Tensile properties|| |
|Type of wheat protein and conditions used||Strength (MPa)||Elongation at break (%)||Modulus (MPa)||References|
|Wheat gluten with 36% glycerola||0.3–1.0||30–352||1.3–10.2||30|
|Gluten with 35% glycerolb||6.69||240||36||2|
|Glutenin reduced with sodium bisulfitec||1.54||87.8||–||34|
|Glutenin reduced with sodium sulfitec||1.67||100.2||–||34|
|Glutenin reduced with thioglycolic acidc||1.8||109.8||–||34|
|Gluten crosslinked with aldehydesd||2.5–3.3||110–115||–||33|
|Gluten crosslinked with L-cysteined||2.6||200||–||33|
|Gliadin with 10–40% glycerole||22–0.8||8.5–301||4.6–0.04||28|
|Gluten crosslinked with thiol-terminated moleculef||15–35||1.5–7.5||–||26|
|Gluten reacted with thiol modified PVAg||74–89||2–2.7||4–4.3||27|
|Gluten with 20% glycerolh||6.7||118||51||35|
|Gliadin with 20% glycerolh||2.2||46||33||35|
|Glutenin with 20% glycerolh||6.1||20||166||35|
Effect of glycerol and other plasticizers on properties of wheat protein films have also been studied by several researchers. Wheat gluten was mixed with glycerol and compression molded into films at different temperatures at a pressure of 9 MPa for compression time of 10 min. Sun et al. studied the effect of molding temperature on wheat gluten plastics plasticized with glycerol and reported that increasing molding temperature from 25°C to 125°C significantly increased the crosslinking density and therefore mechanical properties. The transport and tensile properties of wheat gluten plastics compression molded with 25–40% glycerol were studied by Gallstedt et al. It was reported that water vapor and oxygen permeability increased in the presence of a plasticizer. Tensile properties of the wheat gluten films were considerably influenced by humidity and storage time that was varied from 3 to 24 days. Bioplastics developed from wheat gluten were studied for their controlled release and hydrophilic properties. It was reported that addition of plasticizers with higher molecular weights results in stiffer and more elastic materials and that inclusion of polyethylene glycol reduced the release of potassium chloride (KCl) from the bioplastics. Influence of water, glycerol, 1,4-butanediol, lactic acid, and octanoic acid on the functional properties and reactivity of wheat gluten materials was studied. At the same molar content, the plasticizing effect of water, glycerol, and 1,4-butanediol were found to be similar whereas lactic acid had higher and octanoic acid had lower plasticizing effect. Water provided lower but lactic acid provided higher extensibilities to the thermoplastics. Mangavel et al. compared the properties and microstructure of wheat gluten films developed by compression molding and solution casting. Glycerol was added into both the solution cast and compression molded films and was found to have similar role but the compression molded films had higher tensile stress than solution cast films. Compression molded films were found to have large starch granules but had considerably less water uptake and swelling compared to the cast films. Considerably higher amount of research appears to have been conducted on understanding and developing wheat proteins into thermoplastic products mostly due to the complexity of wheat proteins and their unique behavior after chemical and physical modifications. However, the potential of chemically modifying wheat proteins and developing thermoplastic products has not been studied.
Unlike the common approach of using glycerol and other plasticizers, wheat gluten was plasticized with fatty acids containing 6–10 carbon chain lengths and it was found that the longer the length of the fatty acid chain, poorer was the compatibility with proteins. Water vapor permeability was highest for glycerol and lowest for palmitic acid plasticized wheat gluten suggesting that fatty acids were able to provide better water resistance to the films. Mechanical properties of the films were not studied in that research.
Wheat gluten films have also been crosslinked using crosslinking agents such as L-cysteine, glutaraldehyde and formaldehyde to improve mechanical properties. It was found that crosslinking with L-cysteine resulted in higher glass transition temperature (59°C) compared to 38.8 and 23.6°C for glutaraldehyde and formaldehyde crosslinked samples due to high degree of phase separation. Crosslinking with aldehydes was found to increase strength but decrease elongation whereas cysteine crosslinking improved both the strength and elongation. Most studies on developing thermoplastic films from wheat proteins have focused on reporting the dry mechanical properties. However, properties of the films when treated in water or under high humidity conditions are also important. It will be necessary to understand the ability of the films to withstand actual use conditions before the films can be developed for commercial applications.
Instead of using commercially available gluten, glutenin-rich fractions were compression molded into bioplastics by Song and Zheng. When added into glutenin, reducing agents such as sodium bisulfite and sodium sulfite were found not to affect tensile strength but increased water vapor permeability and decreased elongation. Similarly, gliadin extracted from wheat gluten was also compression molded to form plastics. Samples with strength ranging from 0.8 to 22 MPa were obtained depending on the concentration of glycerol which decreased the Tg in the gliadin rich and glycerol rich domains. A higher activation energy (227–356 kJ mol−1) was necessary to process the gliadins into thermoplastics in this research compared to glycerol plasticized wheat gluten. We have recently studied the properties of thermoplastic films developed from wheat gluten, gliadin, and glutenin with and without starch. Under the optimized conditions for each protein, gluten, and pure glutenin (without starch) had higher tensile strength than gliadin and glutenin with starch. However, gluten had much higher elongation (118%) compared to 46% for gliadin when the proteins were compression molded with 15% glycerol. Gliadin films were unstable and disintegrated when immersed in water whereas gluten and glutenin were stable but retained only 10% of their tensile strength when immersed in 21°C water for 24 h. Gliadin is toxic to humans and therefore using gliadin for non-food applications seems to be a sensible approach. However, films developed from gliadin have poor mechanical properties, especially water stability.
Protein Films from Corn Zein
Unlike most plant proteins, corn zein is a prolamin that dissolves in aqueous ethanol. Therefore, most of the attempts on developing films from zein have used the solution casting approach. Nevertheless, some reports are available on thermo-processing of zein to develop plastics as seen from Table 4. Thermoplastic zein films were developed by blow molding using poly(ethylene glycol) as the plasticizer. Tensile properties of the films were considerably influenced by the processing conditions and also by the amount of α-helix content. Oleic acid plasticized zein was extruded using a single and twin-screw extruder and also blow-molded into ribbons and later compression molded into sheets and the tensile properties were studied. No major difference was observed in tensile properties for the single or twin screw extruded samples but heat treatment (80°C) increased the tensile properties for samples obtained by twin screw extrusion. Films were also developed from zein after addition of polyethylene glycol (PEG), lactic acid, lauric acid, and stearic acid as plasticizers and the effect of mixing temperature and temperature on tensile properties were investigated. Mixing process and temperature during mixing were found to play a critical role on film properties and a processing temperature between 60–100°C was found to be most suitable for zein.
Table 4. Comparison of the Tensile Properties of Thermoplastic Films Made from Corn Zein
| ||Tensile properties|| |
|Processing parameters||Strength (MPa)||Elongation at break (%)||Modulus (MPa)||References|
|Blow molded zein filmsa||0.04–3.6||42–270||4.1–383||36|
|Extruded zein sheets with 70% oleic acid||7.1 ± 1.0||50.9 ± 7.0||145.6 ± 16.3||37|
|Blow molded zein sheets with 70% oleic acid||3.3 ± 0.2||79.2 ± 7.3||81.3 ± 8.4||37|
|Zein films coated with oilsb||2.9–6.0||19–78||75–200||41|
|Films crosslinked with 1–8% glutaraldehydec||23.2–42.5||19.8-30.7||291–423||39|
|Zein plasticized with 25% poly(ethylene glycol)d||0.7–18.0||–||24–866||38|
Zein crosslinked with glutaraldehyde was compression molded into films at 99°C. Increasing concentration of glutaraldehyde from 1% to 4% increased the tensile strength and elongation of the films. Crosslinked zein films were found to have good resistance to soaking and boiling. After soaking in room temperature water for 24 h, the tensile strength decreased by 73% whereas elongation of the films increased nearly 3.5 folds. Films boiled in water for 10 min also had similar decrease in strength but had an even higher increase in elongation by about five folds.
Several studies have also been done on developing zein films blended/coated with other proteins, biopolymers and synthetic polymers. Soyprotein films were coated with zein to decrease the water vapor permeability. It was also found that coating zein provided soyprotein films higher strength and modulus but lower elongation. A single coat of zein increased strength of the soyprotein films from 1.7 to 3.3 MPa, modulus from 45 to 101 MPa and the elongation decreased from 79% to 43%. Double coating of zein further increased the strength and modulus to 6.2 and 316 MPa, respectively, and significantly decreased the elongation to 2.6%. Lower molecular weight and ductile nature of zein coated on the surface were attributed for the decreasing tensile properties after coating of zein. Coating of zein sheets with vegetable oils (tung, linseed, and soybean) was found to increase tensile strength and elongation but decreased water vapor permeability.
Blends of zein and starch were compression molded into films with 20–40% glycerol as plasticizer. Modulus and strength of films were found to increase with increasing zein content whereas elongation at break decreased sharply, a phenomenon also observed by other researchers. Zein was blended with poly(ε-caprolactone) in an effort to reduce cost and improve biodegradability. Blends of PCL/zein were found to be incompatible resulting in lower tensile strength and elongation at break but had increased modulus compared to films made from neat polymers. Unlike soyproteins and wheat gluten which are inevitably generated during processing, zein is deliberately extracted from corn and is available to a lesser extent and also has much higher cost ($15–18 per lb) compared to soyproteins ($1–1.20) and wheat gluten ($0.80–1.20). Based on the reports so far, there are no specific properties that make zein preferable over the other plant proteins for thermoplastic applications. However, zein (up to 20%) can be extracted from distillers dried grains (DDG) which are the coproducts of ethanol production and are available at about $100–125 per ton which would make zein price-wise competitive to soyproteins and wheat gluten. Ability of zein extracted from DDG to be thermoprocessed and properties of the products developed in comparison to zein extracted from corn need to be explored.
Thermoplastic Films from Lesser Known Plant Proteins
In addition to soy, wheat, and corn, proteins from lesser known cereal grains have also been studied for their potential to be made into thermoplastic films. A comparison of the properties of thermoplastic protein films obtained from barley, sunflower protein isolates, peanut, and sorghum proteins are provided in Table 5. Although these cereal crops or their proteins are not available at quantities and cost comparable to soyproteins or wheat gluten, these lesser known proteins are regionally based and are the primary crops in specific region. Researchers have therefore explored the possibility of developing films from these proteins with a view of finding exquisite properties and/or to add value to the crops. Barley protein films were prepared by compression molding with 20–40% glycerol as the plasticizer. Barley proteins were reported to have good cohesive and elastic properties similar to wheat gluten and are hydrophobic with about one-third of hydrophobic amino acids. Films made from barley proteins were reported to have water vapor permeability values similar to wheat gluten films and the films were also found to be biocompatible to Caco-2 cells which are the intestinal cells used as models for studying the absorptive and defensive properties of the intestinal mucosa.[44, 45] Sunflower protein isolates containing 11S globulin and 2S albumin were extruded into films at 160°C. Films were sensitive to water and swelled about 180% when soaked in water for 24 h. In another report, the effect of crosslinking agents, plasticizers, and other additives on the mechanical properties and water stability of compression molded sunflower protein isolate films were studied. Aldehydes used to crosslink the films provided higher strength than tannins and gallic acid because of their covalent interactions with the proteins. Octonal, decanol, or dodeconal added to increase the surface hydrophobicity and decrease solubility of the films was found to improve the plasticity of the films without affecting the tensile properties.. In another report, it was found that addition of various plasticizers (40–60%) resulted in sunflower protein films with strength ranging from 6.2 to 9.6 MPa and elongation ranging from 2% to 140%.. Similar to compression molding, sunflower protein isolates were injection molded on a single screw extruder at 85–160°C with 10–70 parts glycerol. Sheets obtained after extrusion had relatively low tensile strength ranging from 0.6 to 1.9 MPa and elongation of 9.3–36%.
Table 5. Comparison of the Tensile Properties of Thermoplastic Films Made from Peanut, Sorghum, Sunflower, and Barley Proteins
| ||Tensile properties|| |
|Type of protein||Strength (MPa)||Elongation at break (%)||Modulus (MPa)||References|
|Peanut proteina||8.0 ± 0.6||63.0 ± 13.5||147 ± 17||48|
|Kafrin (sorghum) with 25% plasticizersb||6.3–9.0||–||126-752||38|
|Sunflower proteins, 50% glycerol, 0–30% waterc||2.2–1.5||43–36||–||45|
|Sunflower proteins with plasticizers (40–70%)d||6.4–9.6||2–140||–||47|
|Barley proteins with 20–40% glycerol*||65–17||5–97||500–1840||44|
We have recently reported the development of compression molded films from peanut proteins that were extracted from peanut meal. Similar to soymeal, peanut meal is the coproducts after processing the seed for oil. Oil-meals contain up to 50% protein and are inexpensive sources of protein compared to extracting proteins directly from the grains. However, the proteins in the meal may have to undergone physical and/or chemical modifications during extraction of oil and it is therefore necessary to understand the properties of the proteins in the meal before using the proteins for various applications. Peanut proteins were mixed with 20% glycerol and compression molded into films at 150–175°C. It was also found that compression molding provided better films than solution cast peanut protein films. Peanut protein films had considerably higher strength than similar protein films developed from wheat gluten or soyproteins. Proteins extracted from sorghum referred to as Kafrin proteins were compression molded into films using different plasticizers. It was found that the tensile properties of the kafrin protein films were similar to that of zein films.