bk-2013-1144.ch027

Chapter 27

Converting Polysaccharides into High-Value Thermoplastic Materials Downloaded by MONASH UNIV on September 20, 2015 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1144.ch027

James H. Wang* and Bo Shi Kimberly-Clark Corporation, Corporate Research and Engineering, 2100 County Road II, Neenah, Wisconsin 54956, U.S.A. *E-mail: [email protected]

Modified starches including starch esters, pregelatinized Tapioca dextrin, and hydroxyalkylated starch were converted into thermoplastic materials. It was found that the rheology of thermoplastic modified starch (TPMS) was dependent on the type of modified starch, the type of plasticizers, and the amount of plasticizers present in TPMS. For the same modified starch, glycerol was a more effective plasticizer than sorbitol. The melt viscosity decreased as the amount of plasticizer amount increased. Binary polymer blends of TPMS and a biodegradable aliphatic-aromatic copolyester exhibited good film-forming properties and good water dispersibility, however, the mechanical properties of the films were poor. The binary polymer blend films had a laminar, layered microstructure. It was found that tertiary polymer blends comprising a majority of TPMS, a polyvinyl alcohol, and a biodegradable copolyester had good and balanced mechanical properties, film processability, and water dispersibility. The resulting water-soluble films had a high renewable content and properties needed for high-value, water-dispersible films.

Introduction Water-soluble and water-dispersible polymers are an important class of specialty functional polymers. Due to the unique application properties, such polymers command a high price on the market than commodity polymers (1). Among the water-soluble polymers, thermoplastic water-soluble polymers are of © 2013 American Chemical Society In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Downloaded by MONASH UNIV on September 20, 2015 | http://pubs.acs.org Publication Date (Web): November 22, 2013 | doi: 10.1021/bk-2013-1144.ch027

great industrial value due to the low process cost in making various articles such as films and fibers. The commonly practiced solution film casting and spinning processes have high operating costs and also high environmental footprint, primarily due to high energy requirements dealing with solution processing. Technical innovations are critically important to make water-soluble thermoplastic materials, for example, grafting-enabled thermoplastic polyethylene oxide (2–7) and rheologically engineered thermoplastic polyvinyl alcohol without use of a plasticizer (8–10). Although synthetic water-soluble polymers possess the desirable properties for water-dispersible applications, these polymers are often very expensive for large volume applications. From another perspective, these polymers are synthesized from 100% non-renewable monomers made from fossil resources (11), it is therefore highly desirable to improve the sustainability of existing water-soluble polymers. Since polysaccharides are the most abundant natural biopolymers in the world. Cellulose, hemicellulose, starch, chitins, alginates are the most common polysaccharides. Native starch can be extracted from a variety of plants including corn, cassava, potato, rice, wheat, etc. Both native and modified starches were utilized for many industrial applications including paper-making, adhesives, textiles, etc. However, starch is not thermoplastic and cannot be extruded; several technical approaches were developed to transform starch into thermoplastic starch (TPS). Although thermoplastic starch made from native starch is water sensitive, it is not water dispersible (12, 13). It was previously found that water-dispersible TPS can be made from certain modified starch (14, 15). This paper describes the conversion of non-thermoplastic modified starch into thermoplastic materials and the development of water-sensitive films with improved processability and mechanical properties.

Experimental Materials GlucoSol® 800 material is a hydroxypropyl starch supplied by Chemstar (Minneapolis, MN), its weight averaged molecular weight, as determined by GPC, is 2,900,000 with a polydispersity estimated at 28. The modified starch has a bulk density of 0.48~0.64 g/cm3, its particle sizes pass 98% minimum through 140 meshes, and it is supplied as off-white powders. Sorbitol was supplied by Archer-Daniel-Midland Co. (Decatur, IL), glycerol was from Cognis Corporation (Cincinnati, OH). Mono-diglyceride, Excel P-40S (Kao Corporation, Tokyo, Japan), was used as a surfactant. Aliphatic-aromatic copolyester, Ecoflex® F BX 7011 was purchased from BASF (Ludwigshafen, Germany). Polyvinyl alcohol, Elvanol® 51-05, was purchased from DuPont; it has a viscosity of 5.0 to 6.0 mPa.s (cP) as measured for a 4% solids aqueous solution at 20°C.

408 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Converting Modified Starch into Thermoplastic Modified Starch


A mixture of the modified starch, sorbitol, and Excel P-40S was fed by a gravimetric feeder (K-Tron America, Pitman, NJ) at the feed throat of a ZSK-30 twin screw extruder (Werner and Pfleiderer Corporation, Ramsey, NJ). ZSK-30 extruder is a co-rotating, twin screw extruder. The extruder diameter is 30 mm with the length of the screws up to 1328 mm. The extruder has 14 barrels, numbered consecutively 1 to14 from the feed hopper to the die. The first barrel No. 1 received the mixture of starch and plasticizer at 19 lbs/hr. The temperature profile on the extruder was 100, 110, 124, 124, 124, 110, 105, 115oC, respectively, for the 7 heating zones and polymer melt. The melt pressure ranged from 2.76 to 3.45 MPa. The torque ranged from 50 to 60%. The screw speed was 160 rpm. Melt Rheology The rheological properties of polymers were determined using a Göttfert Rheograph 2003 capillary rheometer with WinRHEO version 2.31 analysis software. The setup included a 2000-bar pressure transducer and a 30/1 L/D round capillary die. Sample loading was done by alternating between sample addition and packing with a ramrod. A 2-minute melt time preceded each test to allow the polymer to completely melt at the test temperature (usually 150°C to 220°C). The capillary rheometer determined the apparent viscosity (Pa·s) at various shear rates, such as 100, 200, 500, 1000, 2000, and 4000 s-1. The resultant rheology curve of apparent shear rate versus apparent viscosity gave an indication of how the polymer would run at that temperature in an extrusion process. Tensile Properties The strip tensile strength values were determined in accordance with ASTM Standard D638-99. A constant-rate-of-extension type of tensile tester was employed. The tensile testing system was a Sintech 1/D tensile tester, which is available from Sintech Corp. (Cary, NC). The tensile tester was equipped with TESTWORKS 4.08B software from MTS Corporation to support the testing. An appropriate load cell was selected so that the tested value fell within the range of 10-90% of the full scale load. The film samples were initially cut into dog-bone shapes with a center width of 3.0 mm before testing. The samples were held between grips having a front and back face measuring 25.4 millimeters x 76 millimeters. The grip faces were rubberized, and the longer dimension of the grip was perpendicular to the direction of pull. The grip pressure was pneumatically maintained at a pressure of 0.275 MPa. The tensile test was run using a gauge length of 18.0 millimeters and a break sensitivity of 40%. Five samples were tested by applying the test load along the machine-direction and five samples were tested by applying the test load along the cross direction. During the test, samples were stretched at a crosshead speed of about 127 millimeters per minute until breakage occurred. The modulus, peak stress, and strain at break (peak elongation) were measured in the machine direction (“MD”) and cross-machine directions (“CD”). 409 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Water Disintegration The rate of film disintegration in tap water was tested using a “slosh box”, which has a physical dimension of a 35.6 cm x 45.7 cm x 30.5 cm plastic box on a hinged platform. The one end of the platform is attached to a reciprocating cam. The typical amplitude is 5.1 cm (10.2 cm range), with sloshing occurring at 0.5 ~1.5 sloshes per second. The preferred action is 0.9~1.3 sloshes per second. During a test, the slosh box rocks up and down with the water inside, “sloshing” back and forth. This action produces a wave front and intermittent motion on a sample susceptible to dispersing in water. To quantify a measurement of sample film disintegration in water, without image analysis, simply timing is sufficient. Three liters of tap water were added into the slosh box and resulted in ~ 14.0 cm water depth in the box. A frequency of 3.5 was selected for the testing. Each film sample was cut into 2.5 cm x 7.6 cm size. Three pieces were dropped into the slosh box. The time to disintegrate the sample under the defined conditions was recorded twice for each sample. The average of the time to the sample disintegration is then reported.

Results and Discussion Water-Dispersible Thermoplastic Materials from Modified Starch Even though native starch is not water-soluble or water-dispersible, some chemically modified starches through etherification or esterification are water-dispersible. In this study, three modified starches: Tapon ND (a starch ester); INSTANT N-OIL® (a pregelatinized Tapioca dextrin), and GlucoSol® 800 (a hydroxypropyl starch, a starch ether) were investigated for developing water-dispersible polymer applications. Since the modified starches are not thermoplastic, the first step of this study was to convert the “non-thermoplastic” modified starches into “thermoplastic” materials. This was accomplished by plasticization inside a twin screw extruder. Two types of plasticizers, glycerol and sorbitol, were used in this study. The effects of plasticizer type and level on the melt rheological properties of thermoplastic modified starches (TPMS) were investigated. It was found that melt rheology of TPMS was highly dependent upon the type of modified starch. At the same level of plasticizer, the melt viscosity of the three types of modified starches was: Tapon ND < INSTANT N-OIL® < GlucoSol® 800 (16). For the same modified starch, the melt rheology was determined by the type and amount of plasticizer present. As shown in Figure 1, thermoplastic hydroxypropyl starch (GlucoSol® 800) had a much lower melt viscosity with glycerol as the plasticizer than with sorbitol at the level of 30% of plasticizer. For the TPMS with different amount of sorbitol, the melt viscosity decreased as the amount of sorbitol increased from 30% to 40%. Although thermoplastic modified starch is extrudable in a manner similar to typical thermoplastic synthetic polymer, the resulting materials had poor mechanical properties as exhibited by its highly brittle nature, the material was also weak and hygroscopic to be useful for any practical applications. 410 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 1. Melt rheology of thermoplastic hydroxypropylstarch. Improving the Properties of Thermoplastic Modified Starch by Creating Binary Polymer Blends To improve the mechanical properties of TPMS, a polymer blending approach with a biodegradable polymer having good mechanical properties was pursued. There are many of such biopolymers suitable for blending with TPMS, including polyalkylene alkanoate such as polybutylene succinate and aliphatic-aromatic copolyester. In this paper, the results on using an aliphatic-aromatic copolyester-polybutylene adipate terephthalate (PBAT) are discussed. The specific PBAT used was Ecoflex™ copolyester from BASF. PBAT had excellent tensile strength and ductility similar to polyethylene. It was well known that good films could be made from polymer blends of thermoplastic starch and PBAT when TPS is a minority component, typically less than 45% by weight. However, this study was focused on making water-dispersible films, TPMS has to be the majority component in order to retain water-sensitivity. A number of binary polymer blends containing from 60% to 90% of TPMS and from 10 to 30% PBAT copolyester were prepared. 411 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


In contrast to the lack of processability of TPMS for making thin films, the polymer blends of TPMS and PBAT had excellent processability, thin films were made from these blends even though TPMS was the majority component, this showed significantly improved melt strength of the blends in the presence of PBAT. The mechanical properties of binary blends containing a thermoplastic starch ester (Tapon ND) and PBAT copolyester were determined. The peak stress as a function of the blend composition is shown in Figure 2. The thin films had good tensile strength in the machine direction (MD), ranging from about 26 MPa to about 47 MPa, this is comparable to typical polyethylene films. The peak stress increased as the amount of PBAT copolyester increased, this is expected based on high peak stress of PBAT copolyester. However, the peak stress in the cross direction (CD) was quite low, showing a strong anisotropy between MD and CD. The ductility of the thermoplastic starch ester at different compositions is illustrated in Figure 3. In the machine direction, the ductility increased with the amount of PBAT copolyester in the blends. In general, these films were quite brittle with only 2 to 6% of strain-at-break in MD. The low ductility resulted from the majority of TPMS in the blends. There was no trend in the cross direction (CD).

Figure 2. Peak stress of thermoplastic modified starch blends. 412 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 3. Ductility of thermoplastic modified starch ester blends.

The mechanical properties of thermoplastic starch ether (Glucosol 800) and PBAT copolyester were also characterized. The ductility of the blends is shown in Figure 4. As compared to the ductility of TPMS ester in Figure 3, the TMPS ether had much better ductility in MD, ranging from about 31% to about 95% in strain-to-break. The ductility was higher at a higher PBAT level, except from 10% to 20% of PBAT. But the ductility in CD was also very low, exhibiting a strong anisotropy between MD and CD. The morphology of the binary TPMS/PBAT was studied by scanning electron microscopy (SEM). The microimage of 70/30 TPMS ether/PBAT blend film in machine direction is shown in Figure 5. The blend film had a laminar, layered structure.



Figure 4. Ductility of thermoplastic modified starch ether (TPSE)/copolyester blends.



Figure 5. SEM image of TPSE/PBAT 70/30 film.

Water-Dispersible Films with Balanced Mechanical Properties It was clear the mechanical properties of the binary thin films were deficient, especially the weak properties in cross directions. To overcome this issue, it was discovered that adding another component-thermoplastic polyvinyl alcohol, was critical to achieve the desired balance between MD and CD properties. Polyvinyl alcohol (PVOH) is a water-soluble polymer, but most commercial polyvinyl alcohol is not thermoplastic, previous work by Wang, et. al. invented thermoplastic polyvinyl alcohol having certain compositions and molecular weights (8–10). In this study, the previously identified thermoplastic polyvinyl alcohol was used, 20% sorbitol plasticizer was added to the PVOH to further achieve high ductility and improved thermoplastic processability.

Water-Dispersible Tertiary Blend Films A number of the tertiary polymer blends of the water-dispersible polymers were prepared and compared with the control water-dispersible films. The Young’s moduli of these materials are shown in Figure 6. The pure thermoplastic modified starch ether (Material No. 1, TPSE) and plasticized PVOH (Material No. 2, p-PVOH) both had high moduli. The moduli in MD are significantly higher than those in CD. This shows that both the control materials are fairly stiff. However, the tertiary blends, Material 3 (TPSE/p-PVOH/copolyester 60/30/10) and Material 4 (TPSE/p-PVOH/copolyester 70/20/10) both had significantly lower moduli; therefore, the tertiary blend films were very soft. 415 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 6. Modulus of water-dispersible tertiary blend films: 1. thermoplastic modified starch ether (TPSE: Glucosol 800/sorbitol 70/30), 2. plasticized PVOH/sorbitol 80/20 (p-PVOH), 3. TPSE/p-PVOH/copolyester 60/30/10, 4. TPSE/p-PVOH/copolyester 70/20/10.

Figure 7. Peak stress of water-dispersible tertiary blend films: 1. TMSE, 2. p-PVOH, 3. TPSE/p-PVOH/copolyester 60/30/10, 4. TPSE/p-PVOH/copolyester 70/20/10. 416 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


The peak stress of the water-dispersible tertiary blend films are shown in Figure 7. The pure TPSE film (Material 1) had decent tensile strength in MD, but its peak stress in CD was unusually low. The plasticized PVOH (Material 2) film was strong with high peak stress values in both MD and CD (42 MPa and 37 MPa respectively). The films from the tertiary blends (Materials 3 and 4) both had balanced peak stresses in MD and CD. This is quite different from the anisotropic MD and CD tensile properties found on the binary blend films as discussed previously. The ductility of these films is plotted in Figure 8. Pure TPSE films were very brittle. The plasticized PVOH films had strain to break values of 180% and 130% in MD and CD, respectively. However, the water-dispersible tertiary blend films were significantly higher, at 250% and 220% for Material 3 and 240% and 210% for Material 4, respectively. Therefore, the tertiary blends of TPSE, p-PVOH, and copolyester had good ductility, this is critically important for practical applications.

Figure 8. Ductility of water-dispersible tertiary blend films: 1. TMSE, 2. P-PVOH, 3. TPSE/p-PVOH/copolyester 60/30/10, 4. TPSE/p-PVOH/Copolyester 70/20/10.

Effects of Copolyester Level To determine the impact of the composition of the tertiary blend films, three tertiary blends at 10%, 20%, and 30% of copolyester levels were prepared and processed into melt extruded films. These blends had excellent processability for cast film processing. The relationship between the peak stresses of these films with compositions are shown in Figure 9. All the three films had very close tensile strength. In MD, the peak stress ranged from 16 to 19 MPa, while the peak stress varied from 17 to 21 MPa in CD. The material with the highest level of copolyester (Material 3) had the highest tensile strength. 417 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 9. Peak stress of water-dispersible tertiary blend films: 5. TPSE/p-PVOH/copolyester 63/27/10, 6. TPSE/p-PVOH/copolyester 56/24/20, 7. TPSE/p-PVOH/copolyester 49/21/30.

The ductility of these films is illustrated in Figure 10. In general, these films had comparable ductility. With the film at 20% of copolyester (Material 6) had slightly higher strain-to-break value than the other two materials, all around 200%. For these mostly thermoplastic modified starch based films, the ductility values are fairly impressive, considering the fact that pure TPSE had strain to break of less than 10%. Therefore, the objective to develop highly ductile thermoplastic films from renewable starch based materials was achieved.

Figure 10. Ductility of water-dispersible tertiary blend films: 5. TPSE/p-PVOH/copolyester 63/27/10, 6. TPSE/p-PVOH/copolyester 56/24/20, 7. TPSE/p-PVOH/copolyester 49/21/30. 418 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Water-Dispersibility of Tertiary Water-Dispersible Films


A water disintegration test was used to evaluate the water dispersibility of the films developed. Figure 11 shows the picture of the TPSE/p-PVOH/copolyester 60/30/10 (Material 3) film after placing in water for 30 seconds. The film was disintegrated and dispersed into small pieces. The image of the film in water at 60 seconds is showed in Figure 12, more finely dispersed film residues were formed after the extended exposure in water.

Figure 11. Water disintegration of TPSE/p-PVOH/copolyester 60/30/10 (Material 3), 30 seconds in water.

Figure 12. Water disintegration of TPSE/p-PVOH/copolyester 60/30/10 (Material 3), 60 seconds in water.

In the water-dispersible tertiary blends, there is a water-insoluble copolyester component, the concentration of the copolyester could substantially influence the water dispersibility of the resulting films. The results on the effects of copolyester on water-dispersibility are summarized in Table 1. Thermoplastic modified starch ether (Material 1) was the most water-dispersible materials; it had a water dispersion time of only 10 seconds. Thermoplastic PVOH had slightly longer water dispersion time of 20 seconds. The tertiary blend films had water dispersion time dependent upon the amount of insoluble copolyester, when copolyester content was 10 and 20%, the water-dispersion times ranged from 30 to 60 seconds. However, when 419 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

copolyester content increased to 30% of the composition, the water dispersion time significantly increased to more than 1 hr. This indicated that the increased amount of copolyester could have formed an internal barrier for water penetration and thus substantially retarded the dispersion process in water.


Table 1. Water Disintegration Test Results of Water-Dispersible Films Materials

Water Dispersion Time (Seconds)

1

10

2

20

3

30

4

30

5

60

6

60

7

>1 hr.

Materials: 1. Thermoplastic modified starch ether (TPSE: Glucosol 800/sorbitol 70/30). 2. Plasticized PVOH (p-PVOH). 3. TMSE/p-PVOH/copolyester 60/30/10. 4. TPSE/pPVOH/copolyester 70/20/10. 5. TPSE/p-PVOH/copolyester 63/27/10. 6. TPSE/p-PVOH/ copolyester 56/24/20. 7. TPSE/p-PVOH/copolyester 49/21/30.

Conclusion This article described water-dispersible films from renewable, modified starch. Starch ester and starch ether were converted into thermoplastic modified starch (TPMS). The rheology of TPMS was dependent on the type of modified starch (e.g. etherification or esterification), the type of plasticizers, and the amount of plasticizers. The films made from TPMS were too brittle for practical applications. Binary polymer blends comprising of TPMS and an aliphatic-aromatic copolyester (polybutylene adipate terephthalate, PBAT) were prepared and characterized, it was found that the binary blends had good processability for making thin films; the films were very anisotropic, i.e. with a large property difference between the machine and cross directions (MD and CD). Further experiments led to the invention of tertiary blend films. It was found that the addition of a synthetic water-soluble polymer, polyvinyl alcohol (PVOH), to the binary systems was critical to obtain thin films with balanced mechanical properties in MD and CD. The tertiary blend films had good ductility (~200% strain to break) even in the presence of brittle thermoplastic modified starch in majority. It was further determined that the water dispersion time of the tertiary blend films depended on the amount of water-insoluble copolyester present. This article has demonstrated the challenges and success in developing high-value functional materials from renewable starch-based polymers. 420 In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Acknowledgments The authors would like to thank Irene Hall for study on modified starches, Gregory Wideman for assistance in extrusion process, and Mike Shlepr for scanning electron microscopy (SEM).

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