Preparation and Characterization of Bio-based Degradable Plastic Films Composed of Cellulose Acetate and Starch Acetate
Bio-based degradable plastic is superior to petroleum-based plastic in terms of environmental protection and preservation of resources. A novel cross-linking system using tributyl citrate (TBC) as a cross-linking agent was used to create a bio-based degradable plastic via acetylation modified starch acetate (SA) and cellulose acetate (CA). Fourier transform infrared spectroscopy of the new plastic films revealed that a cross-Claisen ester condensation reaction had indeed occurred among TBC, CA and SA in the cross-linking system. Thermogravimetric analysis, X-ray diffraction, Scanning electron microscopy and mechanical properties tests confirmed that the fabricated films had mechanical properties and thermal stability that were comparable to conventional plastic films. A degradation-in-soil test showed that the fabricated films have controllable biodegradation properties by adjusting the amount of SA added. The tensile stress of two films—one with mass ratio of SA to CA1:3 and the other with mass ratio of 2:3—reached 3.5 and 3.0 MPa, respectively, the corresponding tensile strains were 22.3 and 17.9 %. The highest thermal stability temperature for both films was 205 °C. After being buried in soil for 35 days, the first film was 36 % degraded, and the second film was 41 % degraded. This paper is expected to promote the study and fabricate of bio-based degradable plastic film.
KeywordsStarch acetate Cellulose acetate Bio-based degradable plastic film Controllable degradation
In recent years, the pollution problems associated with traditional petroleum-based plastics have become increasingly prominent , much attention has been paid to developing naturally degradable materials to replace the traditional plastic, in a cost-effective manner [2, 3, 4]. The development of degradable plastics has become a highlight of the sustainable development and circular economy movements . Bio-based degradable plastics are promising materials because during biological degradation, they produced only water, carbon dioxide and inorganic compounds without leaving any toxic residues .
Bio-based plastics  are high molecular weight polymer materials with plastic properties directly synthesized by living organisms (i.e., animals, plants and microorganisms), such as polyhydroxyalkanoate (PHA)  or biological polymers, such as starch, cellulose, lignin, chitin, proteins, peptides and polysaccharides. Bio-based plastics can also be blends of compounds with the above polymer materials serving as the main component, such as polylactic acid, poly amino acids, starch-based plastics, plant fiber molding products, modified cellulose, modified protein and bio-based polyamides.
Starch-based plastics  constitute a new type of biodegradable plastics with potential for further development. There have been many studies on the preparation of thermoplastics using modified starch, such as starch acetate (SA) [9, 10, 11, 12, 13, 14]. With the addition of ester groups, the association of the hydroxyl group in the starch macromolecule is weakened, so the poor water resistance and heat resistance of starch are reduced to some extent, and the film-forming property of the material is enhanced. However, compared with other polymers, the disadvantages of low degree of polymerization, high crystallinity, and poor solvent blending properties limit the application of starch-based materials in plastic industry.
Cellulose , a naturally occurring linear polymer of 1,4-linked ß-d-glucose, has become one of the most abundant natural polymer and an important substitute for traditional plastics [15, 16, 17]. Cellulose acetate (CA) , the acetate ester of cellulose, is an environmental friendly product from sustainable resources that is available at a low price and has good biocompatibility. As a result, CA has become one of the most important bio-based plastics and is widely used in the manufacture of paint, textile fiber, packaging materials, film, artificial kidney and reverse osmosis membrane [1, 4, 16, 19, 20]. However, when compared with SA, CA has the disadvantage of degrading slowly in the natural environment [21, 22].
The aim of the present work is to develop a simple and cost-effective route to fabricate bio-based degradable plastic film. SA and CA are employed as raw materials, a novel cross-linking system was designed to thoroughly dissolve them and cross-link them into biodegradable plastic films using TBC as a cross-linking agent. The films were characterized by means of Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), a universal material test machine and a soil burial test. The results show that the fabrication procedure is a simple method for producing plastic films at a low price that also have a controllable extent of degradation under natural conditions.
Corn starch, refined cotton fiber and TBC were purchased from Guangdong Shangjiu Bio-degradable Plastics Co., Ltd. (Dongguan, China). Acetic anhydride and concentrated sulfuric acid were purchased from Guangzhou Chemical Reagent factory (Guangzhou, China). Glacial acetic acid, methanol and anhydrous ethanol were purchased from Baishi Chemical Industry Co. Ltd. (Tianjin China). Dibutyltin dilaurate (DBTDL) was purchased from Tianjin FuChen Chemical Reagent factory (Tianjin China). Acetone and dichloromethane were purchased from KaiXin Chemical Reagent Co. Ltd. (Hengyang, China). Sodium ethoxide (SE) was purchased from KeLong Chemical Reagent factory (Chengdu, China).
Preparation of Films
The compositions of casting solution of bio-based degraded films
Characterization of the Fabricated Films
The Degree of Substitution and Polymerization of CA and SA
The degree of substitution (DS) values were obtained using a acid–base titration method . The degree of polymerization (DP) was determined by viscosimetry  with dichloromethane/methanol as solvent. Each film was tested at least three times and an average taken.
The Thickness and Mechanical Properties of the Films
The thickness of the fabricated films were measured by micrometer (0–25 screw-thread micrometer, Cangzhou Jilu experiment instrument Co. Ltd. China), and the mechanical properties were determined using the 5,565 Electronical Universal Material Test instrument (Instron Corporation, USA). The samples were 50 mm × 10 mm (width × length) in size, the stretching velocity was set as 5 mm/min, and each sample was tested lengthways three times and an average taken.
FT-IR, XRD, SEM and TGA Measurements
A Fourier transform infrared spectrophotometer (FT-IR, Vector 33, Bruker Optics, Germany) was used to investigate the chemical composition and functional groups in the raw material and prepared films. Each spectrum was captured by 32 scans at a resolution of 2 cm−1 in the wavenumber range of 350–4,000 cm−1.
The crystal structure of the raw material and prepared films were analyzed by X-ray diffractometer (XRD, D8 ADVANCE, RIGAKU, Japan), under the following conditions: Copper target, LynxExe array detector,40 kV–40 mA;at the scanning rate of 0.02º/s in the range of 2θ between 4° and 60°.
The cross-section and surface morphologies of the fabricated films were observed by scanning electron microscopy (SEM, EVO LS10, Zeiss Corporation, Germany).
The thermal stability of the fabricated films was studied using thermogravimetry (TGA, TG 209 F1, Netzsch Corporation, Germay) at a heating rate of 20 °C/min over a temperature range of 30–600 °C under a nitrogen atmosphere.
Soil Burial Test
Result and Discussion
Characterization of the Fabricated Films and its Raw Materials
The thickness of the fabricated films were in the range of 60–180 μm. For CA, the DS was 2.85, the DP was 245, while for SA, the corresponding values were 2.82 and 165.
The result of cross-Claisen ester condensation reaction can also be seen in Fig. 3, where an hydrogen atom was replaced from the α carbon atoms in the acetic ester of the glucose ring of both SA and CA molecules, thereby decreasing the content of H3C–, while increasing the content of the –OH with the TBC involvement. So inspecting the characteristic peak around 1,220 cm−1, it is evident that the characteristic deformation peaks in the region around 1,030 cm−1 (–OH) were strengthened, while the deformation peaks around 1,369 cm−1 (H3C–) were weakened, indicating that the cross-Claisen condensation reaction had occurred between TBC and SA and/or CA.
There are two board diffraction peaks around 2θ of 8° and 20° in the fabricated films that are obvious weaker than those of CA and SA. Based on the spectra, it is clear that after cross-Claisen ester condensation reaction, the crystallinity of the fabricated films decreased, and the films had better flexibility and mechanical properties, because the molecular orientation of SA and CA were broken, most of the crystalline substance of the SA and CA had turned into a more amorphous form, and the intertwining of the molecular chains reduced the mobility of the cross-linked product, inhibiting the crystallization process, the granule of the films become refined. Compared with CA or SA, the fabricated films had a more uniform microstructure, furthermore, the fabricated films have the same raw materials, so the peaks around 8° remained basically unchanged regardless of the SA: CA ratio and smaller than that of either CA or SA alone, the peaks of film#1, #2, #3 and #4 around 20° were in the same situation. While the peak of film #5 around 20° became close to that of SA slightly, it may be because SA became a majority in film #5 with the SA: CA ratio up to 7:3, so the films demonstrate more like SA in XRD patterns, then the peak at 20° became close to SA.
Performance of the Fabricated Films
For the fabricated films, there is little difference in their TGA curves; i.e., their thermal degradation started around 205 °C and ending around 450 °C (Fig. 6a), showed two maximum weight loss rate peaks around 275 and 340 °C (Fig. 6b). The films have the same raw materials SA and CA, but differ in their relative composition, so it is reasonable that the thermogravimetric properties are similar. While the thermal degradation of SA and CA strated at different temperatures; i.e., 230 and 330 °C, respectively. Therefore, the thermal degradation of the fabricated films started with the CA component first, then the SA component. This behavior is why the TGA curves present two thermal degradation weight loss regions.
On the other hand, in the cross-linking system, the cross-linker TBC with a boiling point of 234 °C has a relative poor thermal stability, as part of the films, it will very likely decrease the thermal degradation temperature of the fabricated films. Furthermore, the polymer with the lower crystallinity can be more easily thermal degraded , so that the thermal degradation temperature of the films was lower than that of either of CA and SA. Even so, the films barely decomposed below 205 °C. Such thermal performance can meet the needs of and industrial applications.
Degradation Properties of Films of Different Compositions
The degradation rate of all the tested films slowed after 50 days, with the greatest degradation ratio after 120 days being 52.43 %, during this period, the films’ integral structures were degraded due to chemical hydrolysis and biological action of microorganism in soil. The un-decomposed polymer contained a large amount of acetyl esters, which have no toxic effects in either soil or water after disintegration. Theoretically, they too, will degraded after few months via deacetylation and de-esterification produced by microorganism . The important feature from the soil burial test is that the biodegradation of the film increases with increasing amounts of SA.
In this work, a novel cross-linking system has been shown to be effective in preparing bio-based degradable plastic film, using the cross-Claisen ester condensation reaction between TBC and mixtures of SA and CA. The films combine the advantages of CA and SA, while reduce their disadvantages. For example, the SA film is fragile and easily torn, the CA film degrades slowly when buried in soil. However by changing the proportion of SA, the mechanical properties of the film improve and the degradation properties can be controlled. The film #2 (25 wt% SA) and film #3 (40 wt% SA) have a smooth surface and tight structure. Their tensile stress can reach 3.5 MPa and 3.0 MPa; their tensile strain can reach 22.3 and 17.9 %. The soil burial degradation ratio at 35 days can reach 40.37 and 35.82 %, respectively. The highest thermal stability temperature for both films is above 200 °C.
These films can be produced at a low price from readily available stocks of both starch and cellulose. As a result, they not only have high economic and industrial value but also have considerable environmental benefits to the extent that they replace petroleum-based, non-degrading plastics.
This research was financially supported by the National Natural Science Foundation of China (Grants No. 51378217 and No. U1360101), Guangdong Natural Science and Foundation (No. S2012020010887), Research Project of Guangdong Provincial Department of Science and Technology Department (Grants No. 2012A010800006).
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