Introduction

Packaging from plastic synthetic widely used in daily life for: food industry, biomedical fields, electronic, and agriculture packaging, but it harms the environment [1]. The process of recycling or disposing of plastic waste by minimizing the side effects of residual waste on humans and environments is one of the main challenges today. Polypropylene (PP) is the most dominant packaging material due to the excellent processing performance and versatility with a melting point of 160 °C [1, 2] Polypropylene high hydrophobic, water repellence, and high molecular weight with the main contents are carbon and hydrogen, which are similar structures with polyolefin and polyethylene [3]. Although it has high mechanical strength, these materials cannot be degraded in a short time naturally. Therefore, recycling plastic waste to be useful and easily degradable is necessary to mix with natural polymers for bio-plastic packaging. [2].

Natural polymers are low mechanical properties, cannot stand at high temperatures, and brittle but easily decomposed by microorganisms. Therefore, the mixing of synthetic plastics with natural fibers is expecting to produce plastics that have high mechanical properties [1, 3]. Bioplastics from starch and chitosan are renewable natural polymers that pass through the thermo-mechanical process in suitable plasticizer mixtures such as glycerol. Starch is a glucose polymer, which is composed of two types of polysaccharides, namely amylose and amylopectin. This is an important ingredient for producing biofilms or bioplastics for the food and pharmaceutical industries. However, starch films are too fragile and need plasticization. The plasticizer affects the optical properties, structural properties, mechanical properties, and water vapor permeability [4]. Wood or cellulose potential to be used to increase the mechanical properties of composite. However, for industrial-scale leading to over-exploitation of forests, it negatively affects the climate and the environment. On the other hand, the destruction of these materials by burning or spoilage is a pollutant. A viable alternative is regenerative use materials obtained from plants or other plants with low pollution potential. Starch is one of the most promising material for this purpose because it can be easily obtained from corn, potatoes, or vegetables fully recyclable without toxic residues [5, 6].

The previous studies reported that using starch and chitosan reinforced polypropylene for high mechanical properties of bio-plastic [7, 8]. Reinforcing fillers such as cellulose from natural fibers is an alternative solution to improve mechanical properties. Sugar palm fibers (SPF) are obtained from the trunk of the tree, which is winding from the top down. All across the countries, natural fiber has become famous as an important ingredient in many industries applications. This opportunity has paid a lot of attention to scientists to learn more about SPF as a reinforcement in polymer composites [9, 10]. The color of SPFis brown to black, the diameters ranging between 99 μm and 311 μm, and the density about 1.05 g/cm3 which the tensile properties are increased depends on the size of the fiber due to the cellulose content [11]. As the researches development, palm fiber is proven to have many significant advantages to be considered; environmentally friendly, low price, and abundant in nature. While in terms of properties, palm fiber has low density, good mechanical properties, and good thermal properties [12].

Recently fiber from natural resources has been used for bio-plastic applications from oil palm empty fruit bunch, water hyacinth, cotton and sugarcane bagasse, and many other sources [13,14,15,16,17]. Several experiment techniques, plasticizers, and properties were reporting for producing the bioplastic composites starch-based [18,19,20,21].

The various concentration of SPF and ratio of starch/chitosan in the form of composite SPF/starch/chitosan/polypropylene affected the structural, mechanical, magnetic, optical, and degradation properties [7, 8, 22]. The relation between the structural properties and the optical phonon vibration for composite SPF/starch/chitosan/polypropylene to the mechanical properties and degradation performance has not been experimentally investigated adequately. These properties are fundamental knowledge to understand the mechanism and the relation between these properties to the processes of biodegradation performance. Hence, in this study, we focus on the quantitative analysis of the X-Ray diffraction (XRD) spectra for determining the structural properties (the percentage of amorphous and crystalline phase) and the Fourier transform infrared (FTIR) for determining the refractive index (n), extinction coefficient (k), and dielectric function (ε), and energy loss function (Im (−1/ε)). From these analyses, we continue to find the optical phonon vibration in the form of longitudinal (LO) and transversal (TO). We also find the relation between these types of optical vibration and the structural properties to the mechanical properties and biodegradation performance for the various amount of SPF in composite SPF/starch/chitosan/polypropylene to understand the effect of each other’s properties on the performance of these materials.

Experiment

Material

Sugar palm fibers (SPF) were taken from Makassar, South Sulawesi, Indonesia. Polypropylene composite synthesized from cup plastic waste, aquades, sago starch, chitosan (deacetylation rate of 94.88%; molecular weight, 200 Kda–500 Kda; 200–300 mesh particle size; viscosity 55.31 mPa), Glycerin (Merck), Acetic Acid (CH3COOH) (6%) (Merck), NaOH (5%).

Bioplastics Synthesis

SPF immerse in NaOH 5% solution for 1 h to remove dirt and surface hemicellulose at the fiber's surface. The SPF cleaned using distilled water and then dried at temperature 125 °C for 10 min [23], filtered using a ten mesh, and SPF ready for further use.

Polypropylene (plastics waste) was dissolved into glycerin and heated by hot magnetic stirrer at temperature of 350 °C for 2 h. Then, sago starch was a mixture with aquades by magnetic stirrer at temperature 100 °C. Acetic acid (CH3COOH) poured into chitosan and stirred until formed gel. The various ratio between starch and chitosan are 35/65, 50/50, and 65/35, with additional SPF for each composition; 1%, 3%, and 5%. Then, a polypropylene solution poured into the glass beaker containing SPF/starch/chitosan solutions. Subsequently, all solution materials were mixed and stirred at temperature 100 °C for 20 min with 600 rpm until homogenized and formed bio-plastic solution. After that, the bio-plastic solution poured into a sample container and dried at 85 °C for 16 h. The bioplastics synthesis process can be seen in Fig. 1, and the corresponding mass for each component of the biofilms are presented in Table 1.

Fig. 1
figure 1

Illustration of synthesizing processes for bio-plastics composite SPF/starch/chitosan/polypropylene with different ratio of starch/chitosan and various amount of SPF in this study

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Table 1 Corresponding mass from the Fig. 1 for each component of composite bioplastic SPF/starch/chitosan /polypropylene: starch, chitosan, polypropylene (PP), and additional sugar palm fiber (SPF): 1% (0.052 gr), 3% (0.156 gr), and 5% (0.260 gr) for each R(S/C) (Ratio: (Starch/Chitosan))
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Characterization of Bioplastic

The amount of crystalline and amorphous phase of bio-plastic composite SPF/starch/chitosan/ polypropylene analyzed by using powder X-Ray diffraction (XRD) Shimadzu 7000 with CuKα radiation sources (λ = 1.5405 Å) in the range diffraction angle 2Ө from 15 to 60°, operates at 30 kV and 10 mA. FTIR spectrometer (Shimadzu Corp.) IRPrestige-21 use for determining functional groups in bio-plastic composites SPF/starch/chitosan/ polypropylene at the wavelength 3800–800 cm−1. The mechanical properties are determining by using a texture analyzer (Texture Analyser AND MCT-2150). The size of bioplastic films is 2 × 5 cm for texture analyzer with crosshead speed setting is1 mm/min and crosshead return speed is10 mm/min. Biodegradability tests are performing by measure the rate of weight loss (Wloss) of bioplastic films after the sample burying in the soil for a specific time calculated by Wloss = (Wi−Wf)/Wi where Wi is initial weight and Wf is weight after biodegradability [24]. Bioplastic films with a size of 2 × 1cm are weighted (Wi) and then hiding in the ground for 5 cm below the soil’s surface. After 3 days, 7 days, 14 days, and 28 days, samples are dug out of the soil, washed with distilled water, then the sample was reheated in the oven at 85 ºC for 10 min and weighed (Wf) [24].

Data Analysis

The crystalline phase \({X}_{c}\) obtained from XRD spectra by the empirical method [25] where \({I}_{am}\) and \({I}_{002}\) are the area of amorphous and crystalline phase (002), respectively, determined by the following equation:

$${X}_{c}=\frac{{I}_{002}-{I}_{am}}{{I}_{am}}\times 100\%$$
(1)

The optical properties, dielectric function, and energy loss function determined from the quantitative analysis of FTIR spectra. The transmittance spectra from the FTIR spectra is need to be converted to the reflectance spectra by the following equation [26]:

$$A\left(\omega \right)=2-\mathrm{log}[T\left(\omega \right)\%]$$
(2)
$$R\left(\omega \right)=100-[T\left(\omega \right)+A\left(w\right)]$$
(3)

The reflection spectra is used for determining the real part of refractive index \(n\left(\omega \right)\) and imaginary part of refractive index \(k\left(\omega \right)\) as follows [27, 28]:

$$n\left(\omega \right)= \frac{1-R(\omega )}{1+R\left(\omega \right)-2\sqrt{R\left(\omega \right)}\mathrm{cos}\varphi (\omega )}$$
(4)
$$k\left(\omega \right)= \frac{2\sqrt{R\left(\omega \right)}sin\varphi (\omega )}{1+R\left(\omega \right)-2\sqrt{R\left(\omega \right)}\mathrm{cos}\varphi (\omega )}$$
(5)

where \(\varphi (\omega )\) is the phase change between the reflection and the incident photon signals in the FTIR spectroscopy:

$$\varphi \left( \omega \right) = - \frac{\omega }{\pi }\int_{0}^{\infty } {\frac{{\ln R\left( {w^{\prime } } \right) - \ln R\left( \omega \right)}}{{\omega ^{{\prime ^{2} }} - \omega ^{2} }}}$$
(6)

By applying K-K relation, the equation (6) become:

$$\varphi \left({\omega }_{j}\right)= -\frac{4{\omega }_{j}}{\pi }x \Delta \omega x\sum_{i}\frac{\mathrm{ln}(\sqrt{R\left(\omega \right)})}{{\omega }_{i}^{2}-{\omega }_{j}^{2}}$$
(7)

j is a series of wavenumber, if j is an odd number so then i parts is 2,4,6,8,...,j−1,j+1 and while wavenumber j is an even, i parts is 1,3,5,7,...,j−1,j+1,... \(\Delta \omega\) = \({\omega }_{i+1}-{\omega }_{i}\).

The \(n\left(\omega \right)\) and \(k\left(\omega \right)\) are the input parameter in determining the real part \({\varepsilon }_{1}\left(\omega \right)\) and the imaginary part \({\varepsilon }_{2}(\omega )\) of the dielectric function from the relations:

$${\varepsilon }_{1}\left(\omega \right)={n}^{2}\left(\omega \right)-{k}^{2}(\omega )$$
(8)
$${\varepsilon }_{2}\left(\omega \right)=2n\left(\omega \right)k(\omega )$$
(9)

Degradation performance is determining in the form of the weight loss Is (%) of bio-plastic as follows [29]:

$${I}_{s}=\frac{{\mathrm{W}}_{o}-\mathrm{W}}{{\mathrm{W}}_{o}}\mathrm{ x }100\mathrm{\%}$$
(10)

where W0 is the weight before placement on the ground and W is the weight after a certain time on the ground.

Results and Discussion

XRD Analysis

The XRD pattern of bio-plastics are shown in Fig. 2a for various ratio starch/chitosan and SPF (a) 1%, (b) 3%, and (c) 5%. The crystalline phase increase with increasing the amount of SPF for the same ratio starch/chitosan (R(S/C)) in the composite, ex. for R(S/C) is 35/65 shows an increase from 7.89% for 1% SPF to 15.58% for 5% SPF as shown in Table 2. For the ratio R(S/C) is 50/50 with 5% SPF, the crystalline phase decreases sharply may due to starch and chitosan covering the SPF properly, consequently hindering the crystalline phase [30,31,32,33,34,35] that increase the amorphous phase. The intensity of diffraction peak influence to the crystalline phase indicated well-ordered structure due to the silica-containing fibers in the composite was removed [19, 36, 37]. Starch and chitosan have the hydrophilic, water molecules can be absorbed by the sample, and transferred to the interlayer domain for a period of moisture equilibrium [38].

Fig. 2
figure 2

X-Ray diffraction (XRD) spectra (a) and Fourier transform infrared (FTIR) spectroscopy spectra (b) of composite bio-plastic SPF/starch/chitosan/ polypropylene for different ratio starch/chitosan and various amount of SPF (1%, 3%, and 5%)

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Table 2 Optical phonon vibration mode for transversal (TO), longitudinal (LO), and the difference between LO and TO (Δ) determined from quantitative analysis of FTIR spectra in Fig. 3
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FTIR Analysis

Figure 2b shows FTIR spectra of composite bio-plastic SPF/starch/chitosan/polypropylene for various SPF and the ratio starch/chitosan. There are four main absorption peaks: the bonding vibration of the O–H stretching, C–H stretching, C=C stretching and C–O stretching [7, 39]. O–H bonding at 3381 cm−1 which indicated polysaccharides and alcohol. C–O stretching at 1037 cm−1 of ribbon is along with the addition of the starch and SPF containing lignin and cellulose. All peaks are found in the same range indicated that the SPF is not significantly affected the bonding configuration in composite [40]. The absorbance band located at 3100–3700 cm−1 identified as O–H vibration.

The cellulose can be identified through the absorptions peak at 2933 cm−1 (C–H alkane vibrations), and 1037 cm−1 (stretching C–O ester) [7, 8]. For the absorbance at 1643 cm−1 is stretching aromatic ring of C=C from the SPF in the lignin or starch [41]. However, these peaks were decrease when the ratio starch/chitosan is increasing in the composite probably due to the starch covering and bonding the SPF properly, consequently hinder the lignin and hemicellulose. Besides that, the absorbance peak 1000–1300 cm−1 indicated the presence combination of C–O and C–H stretching groups [31, 42].

The optical properties, dielectric function, and energy loss function as a function of wavenumber determined from the quantitative analysis of FTIR spectra by applying K-K relation [43,44,45,46]. The K-K relation also usually used in quantitative analysis of electron spectroscopy spectra as a function of energy loss [47,48,49,50,51,52,53,54,55]. The quantitative analysis’s optical properties are in the form refractive index \(n\left(\omega \right)\) and extinction coefficient \(k\left(\omega \right)\), complex dielectric function, and the energy loss function.

The optical properties is presented in the top row of Fig. 3a for 1% SPF, Fig. 3b for 3% SPF, and Fig. 3c for 5% SPF. The cross point between n and k is related to the optical intersection with the lattice indicated by TO at a lower wavelength for transverse optical and LO at a higher wavelength for longitudinal optical phonon vibration mode for more detail see Table 2. The difference between LO and TO (\(\Delta\) = (LO−TO)) is increases with increasing the ratio between starch and chitosan (R(S/C)) as can be seen for 1% SPF and 3% SPF but not for 5% SPF may due to the cross-linking is not bonding properly between SPF and starch/chitosan.

Fig. 3
figure 3

Quantitative analysis of FTIR spectra in determining optical properties (first rows), dielectric function (second rows), and energy loss function (third rows) of composite bio-plastic SPF/starch/chitosan/polypropylene for various ratio starch/chitosan with 1% SPF (a), 3% SPF (b), and 5% SPF (c)

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Dielectric Function

Figure 3 shows the results of quantitative analysis of FTIR spectra for 1% SPF (a), 3% SPF (b), and 5% SPF (c). The first rows of Fig. 3a–c shows the refractive index (n) and extinction coefficient (k), second rows is the real (\({\varepsilon }_{1}\left(\omega \right)\)) and imaginary parts (\({\varepsilon }_{2}(\omega )\)) of the dielectric function, and the third rows is the energy loss function (Im (−1/ε \(\left(\omega \right)\)). The main peak of \({\varepsilon }_{2}(\omega )\) also used to identified of the TO phonon vibration mode. The high confidence analysis result for TO phonon vibration mode wavenumber position if the wavenumber position between the cross point of n and k (first rows) consistence with the main peak of \({\varepsilon }_{2}(\omega )\) (second rows). For the ratio starch/chitosan is 35/65, the main peak position of \({\varepsilon }_{2}(\omega )\) shifted to the higher wavenumber position with increasing the SPF from 1 to 5% in the composite may due to the cross-linking bonding rearranged to form a new structure [43, 56]. Another ratio that shows fluctuation may due to the effect of cohesion forces between the SPF and the plasticizer from polypropylene. It’s phenomena caused by the non-uniformity cross-linking and less stable structure in the composite.

For LO vibration modes, firstly identified from the intersection between \(n(\omega )\) and \(k(\omega )\) (first rows) of Fig. 3a–c and the second, by the main peak of energy loss function (Im (−1/ε \(\left(\omega \right)\)) = (\({\varepsilon }_{1}\left(\omega \right)\)) / ((\({\varepsilon }_{1}^{2}\left(\omega \right)\)+\({\varepsilon }_{2}^{2}\left(\omega \right)\)) (third rows) of Fig. 3a–c [43]. The best results indicated by the same wavenumber positions for these two methods. Energy loss function usually determined from the quantitative analysis of electron spectroscopy spectra [57,58,59] and the main peak position consider as the plasma frequency. Similar to the quantitative analysis from the FTIR spectra for composite cement/BaSO4/Fe3O4, for geopolymer fly ash-metal [60] and composites Fe/CNs/PVA [61] shows that the main peak position of (Im (−1/ε \(\left(\omega \right)\)) is the plasma frequency [43].

Tensile Strength

To clarify the effect of SPF to the tensile strength, we determined the tensile strength of composite bio-plastic SPF/starch/chitosan/polypropylene for various SPF and the different ratio of the starch/chitosan. Figure 4 shows the tensile strength increases significantly with the increase in the SPF amount in the composite. Indicated that the SPF could effectively enhance the tensile strength of the composite bio-plastic SPF/starch/chitosan/polypropylene. The probability that the SPF has better dispersion and interaction in the starch/chitosan/polypropylene matrix. The strength of SPF come from the cellulose, as reported for Ibrahim, Sapuan, Zainudin, Zuhri [62] due to the some of the lignin and hemisellulose of SPF remove after immersed in the NaOH [40].

Fig. 4
figure 4

Tensile Strength for bioplastics composite SPF/starch/chitosan/ polypropylene with different ratios of starch/chitosan and various amount of SPF in this study

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The tensile strength is increase with increasing the ratios of the starch/chitosan (R(S/C)), for 1% SPF increases from 25.02 to 40.01 MPa, for 2% SPF increases from 55.23 to 59.02 MPa, and for 5% SPF increases from 63.24 to 78.00 MPa, respectively from lower R(S/C) to higher R(S/C). It shows that the tensile strength increase with increasing the amount of SPF in the composites bioplastic. The lowest range tensile strength is for 1% SPF and the highest is for 5% SPF as shown in Fig. 4 and corresponding data is presented in Table 2. The tensile strength increases with increasing the starch/chitosan ratios due to the interface formation of hydrogen bonding from the NH3+ cation from chitosan with OH− backbones-anions from the starch [63].

Biodegradation Test

Biodegradation of composite bio-plastic SPF/starch/chitosan/polypropylene in this study for various amount of SPF (1%, 3%, and 5%) as shown in Fig. 5. The degradation for all composition at 28 days grounded is > 80% which is in the soil may the microorganisms and moisture play an important role for degradation process [7, 8]. Water also can accelerate biodegradation by absorption processes of the bio-plastic causes swelling [64]. For the starch content in bio-plastic up to 50% of their weight, then they can easily absorb water, consequently will be very rapid biodegradation [65, 66]. For the effect of SPF can be seen for 28 days in Table 2 shows for ratio starch/chitosan 50/50 increase for 1% SPF is 87.23%, for 3% SPF is 92.59%, and for 5% SPF is 94.12%. For ratio starch/chitosan is 65/35 shows decreases degradation with increasing the amount of SPF may due to the crystalline phase (ordered structure) increase, which affected to the ability in absorbing water [63].

Fig. 5
figure 5

Biodegradation for bioplastics composite SPF/starch/chitosan/polypropylene with different ratios of starch/chitosan and various amount of SPF in this study

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Figure 6 shows the illustration of the degradation process naturally. The step processes of biodegradation are divided by; first, catalyzed biologically, in this phase, enzymes and decomposition chemicals act as catalysts for plastic biofilm. During this time, aerobic microbes are formed, and water vapor builds up in the garbage. The moisture absorption increases the ability for swelling and weakening the polymeric bonds. The size and component of compounds that make up the substrate are factors that affect degradation. The second, bacteria, algae, and actinomycete, in collaboration with water, affected the structure of fibers and macromolecular adhesives break down. The third, destruction of cementing points and erosion on fiber and resin connections, provides more degradation sites for further attacks of microorganisms. The increase in temperature causes the kinetic energy of the substrate molecules and enzymes to increase, which also contributed for accelerating the degradation process [67]. However, high temperatures cause damage the enzymes called denaturation, while low temperatures can inhibit enzymes working well. The final process of this degradation process is the formation of urea, which can fertilize plants.

Fig. 6
figure 6

Illustration of the degradation process of bio-plastic naturally

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Figure 7 shows the relation between the amount of crystalline phase, the distance wavenumber between two optical phonon modes \(\Delta\)(LO−TO), tensile strength, biodegradation ability for various amount of SPF (1%, 3%, and 5%). By this method, there are two relations: first, the relation between \(\Delta\)(LO−TO) and crystalline phase and the second, the biodegradation with crystalline phase and tensile strength for various amounts of SPF. Figure 7 shows clearly crystalline phase increase the tensile strength also increase may due to the formation ordered structure as the effect of interfacial complexion between the SPF with starch/chitosan/polypropylene as a matrix [68].

Fig. 7
figure 7

The relation between structural properties and optical properties with the tensile strength and biodegradation performance

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Future work, understand the effects of weather, decomposition of materials under thermal technology, and degradation kinetics. In many cases, the level of soil nutrition, carbon, and nitrogen content are the main factors causing the shift in soil structure and microbial community network. Organic amendments can affect microbial diversity as well as the relative abundance of copiotrophic and oligotrophic bacteria.

Conclusion

Composite bioplastic SPF/Starch/Chitosan/Polypropylene has been synthesized for the various ratio between starch and chitosan are 35/65, 50/50, and 65/35 with additional SPF for each ratio is 1%, 3%, and 5%. The X-Ray diffraction (XRD) spectra used for determining the amorphous and crystalline phase and Fourier transform infra-red (FTIR) spectroscopy used for determining optical properties. In the form of refractive index (n), extinction coefficient (k), dielectric functions (ε), and energy loss function (Im (−1/ ε)) by using Kramers-Kronig (K-K) relations. The crystalline phase is increased with increasing the amount of SPF in composite bioplastic SPF/Starch/Chitosan /Polypropylene for ratio starch/chitosan is 35/65, which consistent with the distance between the wavenumber (Δ) of transversal and longitudinal optical phonon vibration mode. For ratio starch/chitosan is 35/65 shows tensile strength for 1% SPF is 25.02 MPa increase to 63.24 MPa for 3% SPF, similar for 50/50, and 65/35%. The highest tensile strength is 78 MPa for ratio starch/chitosan is 65/35 with 5% SPF indicated that the SPF has better dispersion and interaction in the starch/chitosan/polypropylene matrix. For the effect of SPF on the degradation performance, shows for ratio starch/chitosan 50/50 increase for 1% SPF is 87.23%, for 3% SPF is 92.59%, and for 5% SPF is 94.12%. For composite bioplastic SPF/Starch/Chitosan /Polypropylene in this study, we found that a good correlation between optical properties, structural properties, mechanical properties, and degradation performance for ratio starch/chitosan 35/65 and 50/50 with SPF 1% and 3% in the composite. This study clearly shows, the FTIR spectra could be useful for determining the optical phonon vibration, dielectric function, and energy loss function of composite bioplastic SPF/Starch/Chitosan /Polypropylene. The degradation performance indicated that the bioplastic with additional polypropylene from plastic waste shows a potential solution for decreasing plastic waste in the world.