Development of Novel Soy Protein-Based Polymer Blends - ACS

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Chapter 4

Development of Novel Soy Protein-Based Polymer Blends Jinwen Zhang* and Feng Chen Composite Materials and Engineering Center, Washington State University, Pullman, WA 99163 *[email protected]

A novel approach was used in the preparation of soy protein concentrate (SPC) and biodegradable thermoplastic blends. In contrast to many other soy protein (SP) blends where SP functioned merely as a filler, in this study SPC was processed as a plastic in blending with poly(lactic acid) (PLA) and poly(butylene adipate-co-terephthalate) (PBAT), respectively. The plastication of SP and blending of the resulting SP plastic with PLA or PBAT were performed in the same extrusion process. The plastication involved gelation of SPC in the presence of water and subsequent plasticization of the gelated SPC by water and/or other plasticizer. The resulting PLA/SPC blends displayed a co-continuous phase structure and PBAT/SPC blends a percolated SPC network structure. Consequently, the blends demonstrated superior mechanical properties.

Introduction As a renewable polymer, SP has received great interest in non-food industrial applications. Bioplastics development is one of the high interest areas where SP may play an important role in the near future. Current plastic materials are almost entirely made from petroleum feedstock and consume a significant portion of limited petroleum resources. The demand for plastics is still growing steadily, which will further complicate the situation in view of an unstable oil supply and other associated problems. In addition, the disposal of traditional plastics such as polyethylene (PE) and polypropylene (PP) after use has posed a serious threat to © 2010 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


the environment due to their non-biodegradability. SP is an abundantly available and inexpensive plant polymer; as a result, SP-based plastics have received considerable attention in recent years. The use of SP as an ingredient in plastics can be traced back to the 1930s and 1940s when soy flour was incorporated into phenolic resins (1). In that case SP was used as a reactive component in phenolic resins and was hardened by formaldehyde, so the resulting resins displayed very low water absorption. Since then little progress has been made on SP plastics because of the advent of low-cost petroleum-based plastics; not until the 1990s did SP-based plastic regain research interest due to its environmentally benign advantages. In this chapter, the current status of SP plastics will be briefly reviewed, and the results from our recent new investigation of SP blends will be discussed in detail.

Current Status of SP Plastics Research In general, the role of SP in current bioplastics research can be classified into two types: as a thermoplastic for neat SP plastics or matrix polymer, and as filler for thermoplastics or thermosetting resins. In addition, chemical modification of SP has also studied to improve the physical and mechanical properties of SP plastics. SP as a Thermoplastic When heated, SP can undergo gelation in the presence of adequate water. The gelated SP can be further plasticized by water and other plasticizers and behave like a thermoplastic under shear stress. Compression molding is a simple method for processing SP plastics. By adding a small amount of extra water and/or plasticizers, native SP can be compression-molded (2–4). However, the strong intra- and intermolecular interactions of SP usually result in very high melt viscosity that makes melt processing such as extrusion and injection molding difficult unless a large amount of plasticizer is present. We previously demonstrated that in the presence of sufficient water and/or other plasticizers, soy protein isolate (SPI) could be extruded into clear sheets (5). Water is not only necessary for the gelation of SP, but is also the most efficient plasticizer for SP. However, water evaporates easily during storage and the products can become very brittle. Therefore, other plasticizers are needed for SP plastics. In 1939, Brother and McKinney (1) tested 70 commercially available plasticizers on formaldehyde-hardened SP and found that ethylene glycol performed better than the others. They also found that oleanolic acid and aluminum stearate in combination with ethylene glycol worked very well in reducing water absorption. Currently, glycerol, ethylene glycol, propylene glycol are usually employed in SP plastics (3, 5, 6). Table 1 shows the tensile property data of neat SPI sheets with varying concentrations of glycerol as plasticizer. The sheets containing 10% glycerol demonstrated fairly high strength but very low elongation. Large decreases in strength and Young’s modulus were noted when glycerol concentration increased from 20 to 30 parts. Further increase sin glycerol concentration resulted in 46 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table 1. Effect of glycerol concentration on tensile properties of SPI sheetsa Glycerol (parts)b

Strength (MPa)

Modulus (MPa)

Elongation (%)

10

40.6±1.0

1226±85

3±1

20

34.0±0.8

1119±70

74±8

30

15.6±0.4

374±70

133±10

40

9.1±0.2

176±12

159±12

50

7.1±0.4

144±16

185±15


a

Modified from Table 1 in the reference (5). weight.

b

On the basis of 100-part dry SPC by

seriously detrimental effects on the overall mechanical properties. Hence, it seems that the formulation with 20% glycerol demonstrated optimal overall properties. Partial substitution of glycerol with methyl glucoside resulted in a higher Tg and therefore more rigid sheets, but reduced the flowability of the SP plastic melt (5). Other processing aids may be employed, such as sodium tripolyphosphate for interrupting soy protein ionic interactions (3, 7) or sodium sulfite as a reducing agent to break the disulfide bonds (8). These processing aids help to improve the denaturation of protein and slightly increase the moisture resistance of SP-based plastics (9). SP can also act as a matrix material in composites. For example, natural fibers such as kenaf (10), chitin whisker (11), ramie (12) and konjack (13) have been used to reinforce SP plastics. However, the already low flowability of SP plastic is further reduced by the incorporation of reinforcing fillers. Lubricant which facilitates the processibility of high molecular polymers in both molding and extrusion is also an important additive in soy plastic processing. Nevertheless, neat SP plastics and blends or composites with SP as matrix polymer still retain some serious problems including water/moisture sensitivity, narrow processing window, low impact strength and brittleness. SP as a Filler Blending is often the most efficient and economic means to address many of the problems SP-based bioplastics face. Blending SP with biodegradable thermoplastic polymers is particularly interested because the resulting blends retain complete biodegradability. These polymers include poly (butylenes succinate-co-adipate) (14), polycaprolactone (3, 15), poly (hydroxyl ester ether) (16) and poly (butylenes adipate-co-terephthalate) (17). Blending SP with non-biodegradable polymers including poly(ethylene-co-ethyl acrylate-co-maleic anhydride) (18), polyurethane (19) and styrene-butadiene latex (20) have also been studied. SP is a pressure-sensitive thermoset polymer; it will unfold/melt (paste-like) and cure under both elevated temperature and pressure. Most functional groups in SP are polar, while most thermoplastics are nonpolar (21). Because of the large disparity in molecular structures between SP and these thermoplastics, the interfacial adhesion in many blends is not strong enough to yield satisfactory mechanical properties. Improving interfacial adhesion by 47 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


adding appropriate compatibilizer or coupling agent in the blends has proved to be an efficient means to obtain fine phase structures and improved mechanical properties. However, SP was merely used as organic particulate filler dispersed in the matrix polymer in these blends. Although the blends generally displayed greater water resistance, processibility and toughness than neat SP plastics, and greater stiffness than the soft matrix polymers, they often exhibited less strength than both the neat SP plastics and polymers. Because the free hydroxyl and amino groups in SP can participate in the cure reactions of thermosetting resin, SP can be used as a reactive additive in thermosetting resins. These functional groups can also be utilized for chemical modification to improve physical and mechanical properties. For example, Wu et al. blended SP with polyurethane prepolymer based on polycaprolactone polyol/ hexamethylene diisocyanate (22); and the formation of urea-urethane linkages was detected in the cured resins, suggesting that the amino groups of SP were involved in the curing. The polyurethane prepolymer modified soy protein plastic demonstrated superior toughness and water resistance.

A Novel Method for SP Blends- Processing SP as a Plastic in Blending Since SP can be processed like a thermoplastic in the presence of sufficient water and/or other plasticizers, it is important to understand how this thermoplastic behavior will influence the morphology and properties of the resulting blends. Numerous studies have shown that the properties of a polymer blend are largely determined by its phase morphology. The advantage of processing SP as plastic rather than as a filler is that it allows a flexible design of the morphological structure of the blends, and hence the ability to manipulate their properties. In fact, processing SP as a thermoplastic involves the very same two fundamental requirements as does processing starch as a thermoplastic: gelation in the presence of sufficient water and subsequent plasticization of the gelated polymer by water and/or other plasticizers. Blends based on thermoplastic starch have demonstrated diverse morphological structures and hence varying performance. Thermoplastic starch blends have found some niche commercial applications. It is reasonable to consider that SP could also be similarly used as a plastic in blends with other thermoplastic polymers. In this study, two biodegradable thermoplastic polymers, PLA and PBAT, were selected to blend with soy protein concentrate (SPC). PLA exhibits high strength and modulus but low elongation, while PBAT displays low strength but high flexibility and high ductility. A one-step method was adopted for the preparation of blends, meaning that the plasticization of SPC in the presence of water and/or other plasticizers and blending between the resulting SPC plastic and polymer were accomplished in one extrusion process. One benefit of using a one-step method is that the blending can be carried out at a lower temperature than it usually takes to process the neat polymer because of the water and/or other plasticizers present in the pre-compounding SPC; another benefit is that there is no need for a large amount of water and other plasticizers in the plasticization of SPC because the thermoplastic polymer introduces good flowability. 48 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


General Preparation Method for SP as a Plastic in Polymer Blends SPC (Arcon F) used in this study was obtained from Archer Daniels Midland Company, containing ca. 69% protein (on dry basis), 20% carbohydrate and 7.5% moisture as received. PLA (4032D) was a commercial product from NatureWorks. PBAT (Ecoflex F) was purchased from BASF. Poly(2-ethyl-2-oxazoline) (PEOX) with a weight average molecular weight of ca. 500 KDa was obtained from Aldrich, and was used as the compatibilizer for PLA/SPC blends. Maleic anhydride grafted PBAT (MA-g-PBAT) was prepared in our laboratory and used as the compatiblizer for PBAT/SPC blends. The reaction was conducted in melt reaction, and dicumyl peroxide was used as the initiator by forming free radicals. The unreacted anhydride was removed by sublimation at 80 °C in a vacuum oven. The degree of grafting was measured by the titration method and was found to be ca. 1.4%. Prior to blending, SPC was first mixed with a predetermined amount of water, plasticizer, Na2SO3 and other additives using a kitchen mixer. The formulated SP was sealed in a plastic bag and kept overnight at room temperature to equilibrate before compounding. Compounding extrusion was carried out using a co-rotating twin-screw extruder (Leistriz ZSE-18) equipped with 17.8 mm screws having an L/D ratio of 40. The extruder has eight controlled heating zones ranging from the zone next to the feeding segment to the die adaptor. The temperature profile of these heating zones ranged from 90 to 160 °C for blending SPC with PLA and from 90 to 145 °C for blending SPC with PBAT. The mixture of SP, polymer and compatibilizer was fed by a volumetric feeder, and the extrudate was cooled in a water bath and subsequently granulated by a strand pelletizer. Test specimens were prepared using a Sumitomo injection molding machine (SE 50D) with four barrel temperature zones from the feeding section to the nozzle. The pellets obtained were dried in a convection oven at 90 °C for at least 8 h before injection molding. Blending SPC with a Rigid Thermoplastic – PLA/SPC Blends In this part of study, the formulation of pre-compounding SP was: SPC (100 parts, dry weight), Na2SO3 (0.5 parts), sodium tripolyphosphate (1 part), lubricant (0.8 parts), glycerol (2 parts) and water (10 parts).

Phase Structure Figure 1 shows the phase morphology of PLA/SPC blends with different compositions. The samples for SEM analysis were prepared by etching the cryo-fractured surfaces of the injection-molded specimens with a buffer solution to remove the SP phase or with chloroform to remove the PLA phase. The SEM micrographs (Figure 1) show that blends with the SPC/PLA (w/w) ratio ranging from 30/ 70 to 70/30 all exhibited a fine co-continuous phase structure. This result demonstrates that SPC functioned like a plastic component in PLA/SPC blends, in contrast to most other polymer/SP blends in which SP performs merely as a particulate filler. The water added in pre-compounding SPC played a critical role 49 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 1. SEM micrographs of SPC and PLA phases in blends containing 3 phr PEOX. (a1) & (a2): SPC/PLA = 70/30 (w/w); (b1) & (b2): SPC/PLA = 50/50 (w/w); (c1) & (c2): SPC/PLA = 30/70 (w/w). Samples were fractured in the transverse direction; surfaces were etched with a buffer solution to remove SP or with CHCl3 to remove PLA.

50 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

in converting SPC into plastic. In the compounding extrusion process, gelation of SP occurred under heating and in the presence of sufficient water; subsequently the gelated SP was plasticized by water and other plasticizer(s). At certain water and plasticizer concentrations, the gelated SP possessed appropriate flowability (or rheological properties) as in the case where neat SPI was extruded and was able to flow under the processing conditions, therefore achieving a good blend with PLA.


Dynamic Rheological Properties In the terminal (low frequency) region, the PLA melt exhibited a typical liquid-like behavior like most other thermoplastic polymers. Logarithmic storage (G′) or loss (G′′) modulus versus logarithmic angular frequency (ω) showed a smooth linear relationship by G′ µ ω1.64 and G′′ µ ω (Figures 2a and b) which was close to the theoretical prediction (G′ µ ω2 and G′′ µ ω) for a typical linear polymer with narrow molecular weight distribution. PLA/SPC blends displayed not only much higher η*, G′ and G′′ in the terminal region than PLA but also a drastically different terminal behavior. In the terminal region, log(G′) and log(G′′)) vs. log(ω) of the blends tended to approach a (secondary) plateau, while log (η*) vs. log (ω) changed from a Newtonian (primary) plateau (Figure 2c) for the neat PLA to a continuous decrease with increasing ω for the blends. The deviation of the blends from the neat PLA at the terminal region increased with increasing SPC content in the blends. These changes in rheological properties suggest that a characteristic solid- or gel-like network structure existed in the melt (23). The rheological behavior of the blends corresponded to the evidence of SPC morphological structure in the blends. The rheological behavior of the blends indicates that the presence of SP phase greatly hindered the movement of polymer molecular chains in the melt state. Considering that both the residual moisture and glycerol concentrations were very low in the final blend products and crosslinking of SP also occurred to a certain degree during the compounding and molding processes, it is understandable that the SPC phase would possess very low flowability in the testing environment and behaved more like a solid domain.

Mechanical Properties and Water Absorption Figure 3 shows the tensile strength and modulus of SPC/PLA blends. Tensile strength increased steadily with increasing PLA concentration in the blends. The blends containing 30 and 50% SPC-H2O retained 81 and 65% strength of neat PLA (63 MPa), respectively. On the other hand, modulus of the blends decreased slightly with increasing PLA concentration but to a lesser degree in comparison with the influence of PLA content on strength. Nevertheless, all blends displayed higher modulus than that of neat PLA (3.43 GPa), suggesting the SPC phase in the blend was more rigid than PLA. However, elongation at break of the blends was approximately half the already low elongation of neat PLA (3.8%), which leaves 51 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Dynamic frequency sweep of PLA and SP/PLA blends. strain = 5%, temperature = 175 °C. space for toughening and plasticization in the future investigation. The results of tensile property testing suggest that when SPC is used as a plastic, the resulting blends can still demonstrate high performance even at high SPC levels. Blending SPC with PLA not only improved the processibility of SP plastics because of reduced viscosity compared to that of neat SP plastic, it also greatly increased water resistance (Figure 4). Although the blend containing 70% SPC exhibited relatively high water absorption (~ 15%) in 2-h immersion in water compared to other blends, it still demonstrated impressively higher water resistance than neat SP plastics, which could uptake as much as 92% water in 2-h immersion (5). Increasing PLA content in the blends resulted in a progressive increase in water resistance. For example, the blend with 30% SPC displayed excellent water resistance, absorbing only 1.7 and 4.55% water in 24 and 72 h immersion, respectively.

Blending SPC with a Flexible Thermoplastic - PBAT/SPC Blends In this part of study, the formulation of pre-compounding SP was: SPC (100 parts, on the basis of dry weight), sodium sulfite (0.5 parts), glycerol (10 parts) and/or water. Two levels of water content in the SPC powder were selected: less than 1.0 wt% (vacuum dried at 70°C for 12 h) and 22.5wt% (by adding 15% extra water to native SPC), respectively. Hereinafter, the blends are coded as PBAT/ dry-SPC and PBAT/SPC-H2O. 52 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 3. Tensile strength and modulus of different SPC/PLA blends

Figure 4. Water absorption of SPC/PLA blends in immersion tests 53 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 5. Tapping mode AFM micrographs of the PBAT/SPC blends containing 3% MA-g-PBAT. (a): PBAT/SPC-H2O 70/30 (w/w) (b): PBAT/Dry SPC 70/30 (w/w); bright yellow area is the SPC phase and the dark red area is the PBAT phase

Phase Structure Figure 5 shows the high resolution tapping mode AFM micrographs of the blends phase structure. It was evident that SPC presented as fine threads in the PBAT/SPC-H2O blends, and the connection of SPC threads was clearly noted. On the other hand, SPC appeared as homogenously dispersed particulates in the PBAT/dry-SPC blends. These micrographs indicate that a percolated SPC network structure was formed in some blends. It is known that percolation threshold (volume fraction) depends on several characteristics of the filler component in the system, including shape and aspect ratio. If the SPC domains are approximately assumed to be ellipsoid in each blend, according to Garboczi et al. (24), who predicted the percolation threshold as a function of the aspect ratio of the ellipsoids for randomly oriented ellipsoids of revolution, dry SPC particles (aspect ratio ~ 1) would reach the percolation threshold at 28.5 vol%. Therefore, 30% dry SPC in the blend was probably in the vicinity of percolation threshold, while 50% dry SPC in the blend had already formed percolated structure. Because the formed SPC threads in the PLA/SPC-H2O blends had a much higher aspect ratio than the dry particles, the SPC concentration was well above the percolation threshold in both blends containing 30% SPC-H2O.

Dynamic Rheological Properties Dynamic rheological properties are correlated to phase structure and interactions within the polymer melt. Because the solid structure could be 54 In Green Polymer Chemistry: Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 6. Effect of water content in pre-compounding SPC on dynamic rheological properties of PBAT/SPC (70/30) blends (containing 3 wt% MA-g-PBAT). Strain = 3%, temperature = 160°C. Table 2. Tensile mechanical properties of PBAT/SPC blendsa Wet testd

Dry test PBAT/ SPC (parts) 50/50

70/30

85/15

Neat PBAT

H2O in SPC (%)b

Strength (MPa)

Modulus

Elongation

(MPa)

(%)

Strength (MPa)

Modulus

Elongation

(MPa)

(%)

22.5

24.5±0.7

937±77

11.6±1.6

--

--

--

Development of Novel Soy Protein-Based Polymer Blends - ACS

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