High-Performance and Biodegradable Composites Based on Well-Characterized Cellulose and Poly (Butyleneadipate-Co-Terephthalate)

doi:10.21203/rs.3.rs-1243409/v1

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High-Performance and Biodegradable Composites Based on Well-Characterized Cellulose and Poly (Butyleneadipate-Co-Terephthalate)

https://doi.org/10.21203/rs.3.rs-1243409/v1

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As a flexible biodegradable polymer, polybutylene adipate-co-terephthalate (PBAT) is used for packing. However, high cost and limited properties restrict its applications. Due to the hydrophobic nature of poly butylene terephthalate (PBAT), low mechanical properties are especially showed when PBAT and the hydrophilic nature of cellulose blend. Here, in order to increase the interfacial adhesion between cellulose and PBAT, the γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560) was used as a reactive compatibilizer to modify cellulose. We demonstrate the method of one-step compounding and subsequent extrusion blowing, which is a simple and environmentally-friendly approach to fabricate a series of K-Cellulose/PBAT (KH560-Cellulose/PBAT) composites. The morphology and structure of the cellulose were measured by scanning electron microscopy, X-ray diffraction (XRD) and Fourier transform infrared spectrophotometer (FTIR). Meanwhile, thermal analysis of the hybrids showed an improvement of the thermal stability of the composites upon increasing the silanized cellulose content. In addition, the barrier properties of films are measured by WVP and OP. Finally, after adding reactive compatibilizer KH560, mechanical properties testing of the samples showed considerable improvement with increasing the cellulose content. Therefore, the composites prepared with these enhanced properties have great potential as substitutes for traditional commodity polymers.

Polymer Science

Cellulose

Poly butylene terephthalate

Biodegradable film

Extrusion blowing

The change in consumption habits in the last 50 years increased the volume of commodity polymers used in disposable or short-life items, as packing[1]. The plastic film market, for example, is dominated by these polymers, that have versatile properties, but are not biodegradable, taking hundreds of years to decompose in nature after disposal[2, 3]. Thus, to minimize environmental pollution from traditional plastics, many efforts have been made to develop environment-friendly biodegradable materials[3]. It is imperative to think about the replacement of such polymers by biodegradable ones in many applications.

Polybutylene adipate-co-terephthalate (PBAT) is a flexible biodegradable polymer, with the certification seal of biodegradable and compostable material, given by the European Bioplastics (EN13432 standard criteria) and by the Biodegradable Polymers Institute (ASTM D6400 standard specification)[4]. Nonetheless, to increase its range of use, it is important to improve some of its properties, like mechanical resistance and barrier properties. For instance, this improvement can be achieved through the cellulose of reinforcement fillers[5]. The employment of bio-nanoparticles as reinforcements in PBAT composites has been proved to be a valid method[6]. Recent studies reported that cellulose nanowhiskers (CNW), with a large aspect ratio[7], high strength, and elastic modulus reinforced PBAT composites and presented excellent thermal and mechanical properties[8]. Moso bamboo (Phyllostachys heterocycla) grows abundantly in many tropical and subtropical regions of the world, especially in Zhejiang Province, China[9]. It has been widely used in furniture manufacturing, construction materials and household items due to the advantages of fast growth, high strength, surface hardness and easy machinability[10]. However, a large amount of moso bamboo processing residue is underused. The cellulose content of bamboo is 40%-65%[11], which is comparable to wood[12]. Thus, bamboo residue is a good resource for renewable nanobiobased filler[13]. Some researchers[14, 15] prepared cellulose nanocrystals with a length of approximately 100 nm by the hydrolysis of bamboo fibers in the presence of sulfuric acid and phosphoric acid. Oliveira et al[16]. employed bamboo cellulose nanowhiskers (BCNW) as reinforcements in rubber composites and found that the BCNW with high length and aspect ratios improved the performance of composites[17]. Our study prepared bamboo cellulose from bamboo residues, and investigated the mechanism of alkali hydrolysis. The mechanical, barrier properties and thermal property about combination between cellulose, the γ-(2,3-epoxypropoxy) propytrimethoxysilane and PBAT have not been studied.

In the present study, silanized cellulose/PBAT composite films with cellulose content were prepared using one-step compounding and subsequent extrusion blowing (Fig. 1). The silanized cellulose/PBAT composite films are also recorded as a K-Cellulose/PBAT composite film in the following article. The effects of K-Cellulose/PBAT weight proportions in the composite matrix on the microstructures, mechanical properties, barrier properties, thermal property and hydrophobicity of the films were investigated.

2.1. Material

Moso bamboo residues were supplied by a moso bamboo processing factory, Guangdong Province, China. The particles were screened through a mesh size of 400 and dried at 105 ℃ to constant quality for further use. The PBAT polymer used was an Ecoflex® C1200 grade (purchased from BASF) with melt flow index (MFI) of 2.5-4.5 (at 190°C; 2.16kg), density of 1.26±0.01g/cm³, and melting point of 115±5°C. The γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560) was obtained from Shanghai Yuanye Bio-Technology Co., Ltd. Sodium hydroxide, acetic acid and sodium chlorite were bought from Tianjin Sailboat Chemical Reagent Technology Co., Ltd. (Tianjin, China).

2.2. Preparation of the films

2.2.1. Purification of Bamboo Powder

First, put 25 g of bamboo powder into 250 mL4.0% of NaOH solution, suspension was mechanically stirred at 80℃ for 100 min. At room temperature overnight and then filtered, suspension was washed with distilled water to remove hemicellulose, resin and other substances until the NaOH is cleaned up to a PH value of 7 and finally put in 60℃ oven for 12 h. After this, we used the method of conventional sodium chlorite to removal of lignin , put 24 g of dried bamboo powder fiber after alkali treatment into 780 mL distilled water, with 9 g of sodium chlorite and 6 mL of glacial acetic acid, for 1 hour in a 75℃ constant temperature water bath. Subsequently, this process is repeat five times, and until the bamboo fiber turns white. In the meantime, the bamboo fiber was washed with distilled water to neutral with distilled water after each operation. Finally, the bamboo powder fiber was obtained by air drying.

In addition, to further study cellulose, we also prepared cellulose nanocellulose by bamboo slag. CNF is a sulfate-bleached cowhide pulp mediated by TEMPO.Sulphate bleached cowhide pulp (18g) was dissolved in deionized water (500mL) and soaked for 24h, and the pulp suspension was obtained with a distributor (30000r/min,10min).Tempo (10mg) and sodium bromide (100mg) were dissolved in deionized water and water bath for 15 min, then added to the pulp suspension for oxidation and kept pH in the 10–11 range (using 1wt% sodium hydroxide).Sodium hypochlorite solution (10wt%,59.6mL) was added to the slurry hybrid for the oxidation step, the sodium hydroxide solution (1wt%) was added at the same time and sonication dispersion was performed at 500W for 15min to keep the pH value of the hybrid solution at 10–11.

2.2.2. Silanization modification of Bamboo Powder

The pretreated bamboo powder was placed in 80° blast oven for 24 h and fully dried. Some amount of deionized water was adjusted with glacial acetic acid to form an aqueous solution with PH value of 3.5-5.5, and the coupling agent KH560 was diluted with the corresponding solvent to form a coupling agent solution at 1:5 ratio. At 60℃ high speed mixer, the coupling agent solution was evenly sprayed on the corresponding bamboo powder surface according to a certain proportion of ingredients, and stirred evenly.

After that, the above reaction mixture was centrifuged to separate the silanized cellulose at 11000 rpm for 10 min to remove KH-560.

Then the surface treated cellulose was placed in the blast oven and set aside.

2.2.3. Preparation of Composites

The fabrication of cellulose was performed using melt extrusion as per the procedure mentioned elsewhere[18]. Before processing, silanized cellulose was dried in vacuum oven (~80 °C) for 12 h for removing the moisture due to its hydrophilic nature. PBAT was placed in oven (~80 °C) for 12 h.

The mixtures were packaged in polyethylene bags[19]{Wei, 2021 #59} and stored for at least 24 h at room temperature to equilibrate all the components. The mixtures were compounded into BF/PBAT pellets by one-step extrusion using a Double Screw Extruder twin-screw extruder.

The barrel temperatures from the feed zone to the die zone were 125 ℃, 130 ℃, 135 ℃, and 120 ℃. The screw speed was 20-25 rpm/min. The films were prepared using an Extruded Film Blower film-blowing system (BL-6176 Floor Film Blower, Baolun Precision Testing Instrument Co., Ltd, China), and die diameter of 30 mm. The film-blowing conditions consisted of a screw speed of 30 rpm and extrusion temperature profiles of 110 ℃, 125 ℃, 135 ℃, and 145 ℃ for the four heating zones and 135 ℃ for the filmblowing die. Screw speed 15 rpm/min, traction speed 3.5 m/min. The film thickness was kept at 40–50 μm by carefully adjusting the blow-up ratio (ratio of the diameter of the blown bubble to that of the die) and take-up ratio (ratio of the take-up velocity to the film velocity at the die exit).

The silanized cellulose/PBAT composite films were achieved by adding silanized cellulose at ratios of 0wt%, 0.5wt%, 1wt%, 1.5wt%, and 2wt%.

2.2.4. Preparation of miscellaneous straws

The suspension containing 3g of cellulose (superfibers, nanofibers or hybrid fibers respectively) was fully hybridiated and filtered into a wet membrane using a 14cm diameter funnel vacuum.The hybrid straws were prepared with different mixed amounts of nanocellulose and superfine cellulose.

2.3. Characterization of the films

2.3.1. X-ray diffraction (XRD)

The crystal structure of cellulose and silanized cellulose was analyzed by X-ray diffraction using Cu Kα radiation (λ = 0.154 nm) with a scanning speed of 4°/min in scanning range of 10-80°(2θ)[20,21]. The basal spacing of the nanoclay was calculated using the Bragg’s diffraction equation, i.e., λ = 2d·sinθ[21], where λ is the wavelength of the X-ray radiation (0.1546 nm), d is the spacing between the diffraction lattice planes, and θ is the measured diffraction angle .

2.3.2. Fourier transformation infrared spectra (FT-IR)
FTIR measurements of modification of cellulose with different concentrations of coupling agents (0%, 2%, 4%, 6%, and 8%) and silane cellulose composite membrane with varying content (0%, 0.5%, 1%, 1.5%, 2%, and 2.5%) of the cross-linking agent were performed by means of a Nicolet 6700 spectrometer (Nicolet is5, America ThermoFisher) at ambient temperature. Data were collected over 16 scans at a 16 cm^-1 resolution from 500 to 4000 cm^-1 and analyzed using Origin 7.0[22].

2.3.3. Scanning electron microscopy (SEM)

The surface and cross-sectional morphology of the films was observed by scanning electron microscope (SEM, FEI Apreo, Thermo) at a voltage of 2 kV[23]. All samples were coated with gold prior to the investigation[24]. The film samples were frozen in liquid nitrogen and then cryofractured to obtain the cross-sectional samples. Images of the surface and cross section were taken at magnifications of 500× and 2000×, respectively.

2.3.4. Thermogravimetric analysis (TGA)

TG (TGA-Q50, USA) measurements were performed with a thermal analysis system under the protection of nitrogen atmosphere at a heating rate of 10 °C min^-1 from 30 to 800 °C. During the measurement, dried nitrogen was vented into the furnace at a constant flow rate of 50 mL min^-1[25]. The starting temperature of the thermal degradation of KH560-Cellulose samples was higher than pure cellulose, which suggested that the two-step modification method could enhance the thermal stability of cellulose.

2.3.5. Differential Scanning Calorimetry (DSC)

The glass transition temperature of the composite film was measured by DSC 204F1 differential calorimeter. The thermal history was first eliminated from 50℃ to 180℃ at a heating rate of 10℃/ min under the condition of N₂ protection; then the temperature was cooled to 50℃ at a rate of 10/ min; and finally warm up to 180℃ at a rate of 10℃/ min.

2.3.6. Static tensile test

Mechanical properties are tested according to GB/T 1040—2006 determination of tensile properties of plastics. The mechanical properties of the films were tested at room temperature using a Instron 3369 universal testing machine. The size of the strip is 100 mm×10 mm, and 8 samples are selected for testing. The distance between the upper and lower clamps is 30mm, and the tensile rate is 400 mm/min during the experiment.

2.3.7. Water-vapor permeability (WVP)

Test of moisture permeability of film: with reference to GB/T 1037—88 method of steam permeability test of plastic film and sheet-cup method, the moisture permeability of (silanized cellulose /PBAT) blend packaging film was tested.

2.3.8. Oxygen permeability (OP)

Film oxygen permeability system is determined by GB/T 1038—2000 pressure method of plastic film and sheet gas permeability test method. The size of the sample is 150 mm×150 mm, and the parallel sample is 3. The thickness of the film is measured by thickness gauge, and the average value of 5 points is taken and the test temperature is 23℃.

2.3.9. Water contact angle

The water contact angles of the films were measured by VCA Optima dynamic contact angle tester (Optima, AST) at room temperature. The films were first placed on the horizontal platform of the goniometer, and a drop of deionized water (approximate 7 μL) was then dribbled onto the film using a precision microsyringe. Simultaneously, multiple photographs of the water droplet onto the film were continuously taken within 150 s and stored. The valid values obtained were based on an average of at least five points. The change in the contact angle between the film surface and tangent of the droplet within 150s was analyzed using appropriate software.

3.1. XRD analysis

The intercalated or exfoliated structures of composites can be provided quantitative information by XRD analysis. In general, the intercalation of polymer molecules in the modified cellulose increases the d-spacing, resulting in a shift in the diffraction peak toward a smaller angle according to the Bragg’s diffraction equation. The crystallinity of cellulose is generally considered to be one of the most important factors in determining their mechanical and thermal properties.

XRD analysis was performed here to characterize the crystallinity of cellulose and silanized cellulose samples, as demonstrated in Fig.2. To all appearances, cellulose and silanized cellulose samples exhibite the well-known peaks at 14.9°, 16.7° and 22.9°, which were basically agreement with the characteristic diffraction peaks of cellulose. This result is consistent with the previous studies that silanized cellulose sample generally maintained the fundamental crystal structure of cellulose. However, some subtle differences in the diffraction patterns could still be seen for the above samples. In contrast to cellulose, the diffraction peak intensity of silanized cellulose was reduced, which indicated that the crystallinity of cellulose decreased after silanization modification. The reason of reduced crystallinity can be the fact that the surface of silanized cellulose was grafted with amorphous silane-based side chains.

3.2. FT-IR analysis

FT-IR analysis can be used to effectively study the molecular interactions that occur during reactive extrusion[23,26]. Fig.3a depicted the FTIR spectra of cellulose and silanized cellulose samples. As a group, for cellulose and silanized cellulose samples, a strong band appeared at approximately 3408 cm^-1, which was mainly attributed to the stretching vibration of O-H groups. After the coupling agent modification, the telescopic vibration peak of the O-H group is significantly weakened, indicating that the KH560 reacts with the O-H group in the cellulose and reduces the fiber polarity. The characteristic peak around 2890 cm^-1 was largely associated with the -C=O stretching vibrations. Moreover, an intense adsorption around 1648 cm^-1 originated from the absorbed water. Furthermore, the peaks at around 1374 cm^-1, 1065cm^-1 were related to -CH₂ and -C-H bending vibrations, respectively.

Fig.3b shows the FT-IR spectra of the composite films with various weight proportions of silanized cellulose and PBAT. PBAT belong to the polyester family, C-O stretching vibration peaks near the 1265 cm-1, which is characteristic of esters; near 1459 cm-1 is the absorption peak of C-H asymmetric and symmetrical bending vibration. The broad peak around 3400 cm⁻¹ occurred because of the presence of a large number of hydroxyl groups (−OH) in the starch chains (Fig.3b). Compared with the pure PBAT, the position of the characteristic absorption peak of the composite is basically unchanged, but the intensity of some absorption peaks changed obviously. And the strength of the stretching vibration peak near the 1459 cm^-1 of the composite becomes stronger. This is because of the stretching vibration peaks of C-O-C and Si-O-C formed by silanized cellulose near 1459 cm^-1[26]. The absorption peak of C-H in alkanes is obviously enhanced, especially in the 1.5wt% K-Cellulose/PBAT, the corresponding absorption intensity increases more obviously, because it's 0.5wt% K-Cellulose/PBAT, compared 1.5wt% K-Cellulose/PBAT silanization was highest, it contains more alkanes.

3.3. Morphology -SEM analysis

To intuitively describe and analyze the surface and cross-section morphology of the composites[27], and the compatibility of the two phases of the composites was further studied by SEM.

It could be seen that the surface of the cellulose was smooth, while the k-cellulose powders were rougher (Figure.4). The surface shows distinct grooves which are able to form obvious pores that can produce a strong capillary effect[28] and in turn can improve the infiltration effect of the plastic matrix, thus enhancing the interfacial forces between the PBAT and cellulose. Furthermore, the geometric average diameter of cellulose was around a few microns, respectively, while the diameter of k-cellulose powders was increased to around 40um. These results were attributed to the effect of the interactions through hydrogen bonding, which leads to severe aggregation. After KH560 modification, the diameter of cellulose was further increased. With the dimensional changes, it was shown that the average diameter gradually increased for KH560-cellulose due to the introduction of grafting molecular chains onto the surface of cellulose.

Fig.5 is a SEM diagram of liquid nitrogen embrittlement section of PBAT and its composites. The section of the PBAT is relatively flat, indicating PBAT brittle fracture occurred after liquid nitrogen treatment. There was little difference in cross-section morphology of 5 composites (Fig.5), 0.5wt%K-Cellulose/PBAT, 1wt%K-Cellulose/PBAT, 1.5wt%K-Cellulose/PBAT, 2wt%K-Cellulose/PBAT and 2.5wt%K-Cellulose/PBAT. The cross sections of the composites are flat, which indicates that the dispersion of K-Cellulose in PBAT is better, because silanization modifies the hydrophilic hydroxyl groups on cellulose molecular chains to be replaced by hydrophobic Si-O-C bonds[29], which improves the interdependence of the two phases.

3.4.TGA analysis

The thermal properties of the obtained cellulose and silanized cellulose were assessed by TG-DTG analysis, as illustrated in Fig. 6(a,b). It can be observed in Fig. 6a that there was a slight weight loss up to 300 °C for silanized cellulose, followed by a drastic loss at 300-500 °C. These results were similar to those reported in the previous works[30]. For silanized cellulose, there was a slight weight loss up to 350 °C, the vast majority of weight loss occurred within the range of 350-500 °C. Furthermore, DTG peak temperature can be employed to evaluate the thermal stability of samples as well. Based on the DTG curves in Fig. 6b, the thermal decomposition peaks of the initial weight loss appeared at 300 °C and 380 °C for cellulose and silanized cellulose, respectively. The above results provided direct evidence that the thermal stability of silanized cellulose samples was higher than that of cellulose. In addition, the amounts of the char residues for cellulose and silanized cellulose were 18 % and 23 %, respectively (Fig. 6a). The the presence of alkoxysilane group might be the main cause to increase char residue for silanized cellulose samples.

Thermogravimetric analysis was also carried out in order to characterize the influence of KH560 on the thermal stability of the composites. It was found that the TG decomposition temperature of the PBAT film is 363.9℃. After adding KH560, the TG decomposition temperature of the composite increased from 363 to 395℃, improving by 30 ℃, indicating that addition of silanized cellulose can efficiently improve the thermal stability of the silanized cellulose/PBAT composite. From Fig. 6d, it can be seen that there are two peaks. The double peaks are due to the degradation of the silanized cellulose/PBAT composite. The first is degradation of cellulose, about 356.7℃, the second is degradation of PBAT, about 410.1 ℃. The two peaks are due to the uneven molecular structure, which contains some small molecular substances. In the process of heating, some crosslinked small molecules are destroyed, leading to the loss of some raw materials. After adding a certain amount of silanized cellulose, the temperature of the two thermal degradation processes almost has no obvious change. Because of the heterogeneity of molecular structure, the thermal degradation rate of DW/DT value of weight-loss rate changes greatly with the addition of KH560.

KH560 plays the role of chain extender in the mixing process[31], thus, it not only accelerates the thermal movement between molecules, increases the molecular weight, but also increases the binding force between chemical bonds, making it not easy to be destroyed at high temperature.

3.5.DSC analysis

The crystallization and melting behavior of the K-Cellulose/PBAT composites were studied using DSC as already described. A heat-cool-heat program was used to first remove the thermal history of the samples followed by crystallization during cooling and subsequent heat run to study the melting behavior. The crystallization exotherms in the cool runs are shown in Figure 7(a) and the second heat runs (melting endotherms) are shown in Figure 7(b).

Obviously, the incorporation of the silanized cellulose in the PBAT matrix showed that the crystallization temperature of the matrix gradually increased towards higher temperatures. The observed Tc of 96°C increased to 102°C with incorporation of silanized cellulose particles. This clearly shows that the presence of the cellulose whiskers gave a heterogeneous nucleation effect[32,33] by increasing the nucleating sites for promotion of crystallization of the PBAT where silicate platelets provided nucleating sites for the polymer matrix crystallization. In the second heating cycle (Figure 7(b)), it can be seen that while the pure PBAT showed an endothermic melting peak at 117°C, with increasing content of silanized cellulose, the melting temperatures showed a gradual increase up to the maximum temperature of 117°C in the case of 1.5% silanized cellulose sample.

3.6. Mechanical properties

Cellulose and silane coupling agent are esterified to connect nonpolar ester group to cellulose molecular chain, which will improve the interfacial compatibility between cellulose and PBAT and further affect the mechanical properties of the composites. In general, as the compatibility of the two phases is improved, the mechanical properties of the composites will be enhanced.

The mechanical properties of the composites were assessed by the mechanical analysis, as illustrated in Fig.8. The tensile strength, elongation at break, and elastic modulus of the K-Cellulose/PBAT composite with 2 wt % K-Cellulose content were increased by 1.8, 1.3, and 3.8 times respectively over the cellulose /PBAT blend without the added KH560. The higher toughenin effect of KH560 in cellulose /PBAT composites can be mainly ascribed to the improved interface bonding between cellulose and PBAT[34]. The γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560) can react with the hydroxyl group on the cellulose surface. The compatibility of the composites with different modified cellulose dosage is different, so it is necessary to test the mechanical properties of the composites with different modified cellulose dosage, select the appropriate blending ratio, and ensure the mechanical properties of the composites. Reduce the cost of the material, so that the material can be widely used in real production and life.

The decline in tensile strength of high-content of silined cellulose of composite films can be mainly ascribed to the agglomeration of an excess of K-Cellulose, resulting in weak interaction between the particles and PBAT[35]. Moreover, the alkyl groups between the KH-560 molecules polycondensed and entwined on the K-Cellulose surface reduced the contact area with the PBAT molecules. As a result, the effective interface layer was reduced, and the mechanical properties of the composite films were lowered.

3.7. Barrier properties

The influence of silanized cellulose on barrier properties of films was investigated[36], and the WVP and OP values of all the film samples are illustrated in Table1.

It can be observed that, the PBAT film exhibited maximum WVP compared with the K-Cellulose/PBAT composite films. The K-Cellulose/PBAT composite films exhibited a significant reduction in WVP with the increase in the silanized cellulose content. The WVP of the 1.5wt% K-Cellulose/PBAT composite film decreased to approximately one half that of the 0.5wt% K-Cellulose/PBAT composite film. Cellulose is well known to be a hygroscopic material and has much great raffinity for water than PBAT. This property causes an increase in the permeability of water vapor through the films under high silanized cellulose content. Furthermore, the more intercalated structure sand better compatibility in the film matrix with higher silanized cellulose content can contribute to a lower WVP.

Table 1 WVP and OP of the PBAT/ K-Cellulose composite films.

Samples	WVP(g/(m².day))	OP(cm³.m²d bar)
PBAT	72.2408	4040
0.5% K-Cellulose /PBAT	58.3105	2150
1% K-Cellulose /PBAT	48.1428	1700
1.5% K-Cellulose /PBAT	29.8968	1370
2% K-Cellulose /PBAT	59.9336	1970
2.5% K-Cellulose /PBAT	76.5052	1440

In contrast to the WVP of the films, the OP values reduced with increasing silanized cellulose content. In general, the factors that affect the OP of the films include microstructure, void volume, alignment of polymer chains, and adhesion of film matrix. The results were demonstrated in Table1. When the silanized cellulose addition increased from 0 to 1.5wt%, the air permeability decreased from 4040 cm³.m²d bar to 1370 cm³.m²d bar. The results were probably due to the fact that PBAT was composed of non-polar molecules, and non-polar oxygen molecules are liable to pass through the film matrix. A well-dispersed mixture allowed the formation of a uniform and tight network structure of the composite membrane, which may be partly the cause of the reduced air permeability. The addition of silanized cellulose promoted the formation of annitercalated structure, which could prevent oxygen molecules from passing through the film matrix.

3.8. Surface hydrophobicity

Water contact angle is a primary indicator for hydrophobic characterization of polymer materials. The contact angle of the PBAT/ K-Cellulose composite films were observed to further study the durability of hydrophobic characterization of polymer materials. The dynamic water contact angles of the film samples within 150s are shown in Fig. 9. The water contact angles of the films decreased as a function of time due to the reorientation of the polar groups on the film surface[37,38]. The images of the water droplet taken on the films explicitly reflected this phenomenon. The K-Cellulose/PBAT composite films exhibited also smaller drop. In particular, the water contact angle of the 1.5wt% K-Cellulose/PBAT composite film was only reduced by 11° within 150s after the water drop. The water contact angle of the 2wt% K-Cellulose/PBAT composite ﬁlm measured at 150s was 39.8°, whereas the 1.5wt% K-Cellulose/PBAT composite film exhibited higher water contact angles, especially the 1wt% K-Cellulose/PBAT (76.2°) and 1.5wt% K-Cellulose/PBAT (72.1°) composite ﬁlms. Therefore, although the addition of silanized cellulose reduces the hydrophobic of the composite film, the 1.5wt% K-Cellulose/PBAT composite film did not decrease much compared to pure PBAT.

In the present work, bamboo cellulose whiskers were employed as filler for PBAT biocomposites. K-Cellulose/PBAT composite films were successfully prepared using one-step compounding and subsequent extrusion blowing. All formulations exhibited excellent processability. K-Cellulose/PBAT composites with 1.5wt% silanized cellulose content had a relatively high level of transparency and tensile modulus; excessive silanized cellulose contents lowered the tensile property. The SEM images of the films with higher silanized cellulose content showed continuous and uniform appearance. The K-Cellulose/PBAT composite films can have a great application potential as food-packaging materials, which can reduce the consumption of non-biodegradable plastics and alleviate environmental issues. We also explored a fully natural cellulose straw prepared from cellulose hybrid fibers as an alternative to petroleum-based plastic straws. It provides reference for developing alternative plastic straws with degradation, good mechanical performance and good water stability. This work presents a green and feasible route to produce cost-efficient biodegradable materials with controlled mechanical and highly-barrier properties for packaging application.

Acknowledgements

This work was supported by the Tianjin Enterprise Science and Technology Commissioner Project (21YDTPJC00570).

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High-Performance and Biodegradable Composites Based on Well-Characterized Cellulose and Poly (Butyleneadipate-Co-Terephthalate)

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