Effect of lignin on morphology, biodegradability, mechanical and thermal properties of low linear density polyethylene/lignin biocomposites

This research is purposed to study effects of lignin compositions on morphology, biodegradability, mechanical and thermal properties of low linear density polyethylene (LLDPE)/Lignin biocomposites. LLDPE/Lignin biocomposites has been manufactured by adding LLDPE, lignin and compatibilizer into rheomix at 200°C with a stirring speed of 70 rpm for 30 min. The composition of lignin added was 5, 10, 15, and 20 phr with compatibilizer 5 phr. LLDPE/lignin films has been made by using hydraulic hot press at 200-210°C with pressure of 6 bar for 20 min. Fourier Transform Infrared (FTIR) spectrum analysis was conducted to determine the functional groups of LLDPE/Lignin biocomposites. The surface morphology was observed by using Scanning Electron Microscope (SEM). The mechanical properties was measured as a tensile strength and thermal stability was measured by Thermogravimetric Analysis (TGA). In addition, biodegradation test was also conducted to determine the level of biodegradability. TGA results indicated that at 456°C LLDPE and lignin had similar thermal stability and the addition of lignin into LLDPE/lignin bicomposites can reduce the thermal stability up to temperature of 450-460°C. However, the thermal stability is increased at temperature over 460°C. The tensile strength and elongation at break of all LLDPE/Lignin biocomposites at various compositions is lower compared to those of LLDPE. The more lignin were added into LLDPE/Lignin biocomposites, the more the materials were biodegraded.


Introduction
Research on utilization and development of polymeric materials based on the use of renewable natural resources as raw material for the polymer industry has attracted the interest in the past few decades. Basically, the development of renewable materials for polymer comes from two main problems, which are the environmental problem associated with waste generated and the limitation of natural resources derived from fossil materials. Both of these problems has driven the efforts to find alternative materials based on renewable and biodegradable natural resources which can reduce the dependency on petroleum based material and simultaneously can solve the environmental problem caused by the generated waste.
Some of biomasses that can be used as raw material of polymers are starch, lignin, cellulose, protein, chitosan, and some vegetable oils. Among them, lignocellulose is considered as one of the most potential alternative material because it does not intersect with food crops. Lignocellulose is generally derived from wood containing cellulose, hemicellulose and lignin. Availability of lignin reaches 30% of all non-fossil organic carbon on earth and commonly produced by paper industry in a significant quantity. The utilization of lignin as a new renewable biomaterial may be a perfect candidate for modification and chemical reactions due to its functional properties (rich in phenolic and aliphatic hydroxyl groups) for the development of new biomaterials. However, there is a major challenge in polymer applications associated with unclearly defined structure and diversity based on their origins, separations and fragmentation processes. Chemical modification of lignin can be classified into lignin fragmentation into phenolic or other aromatic compounds, synthesis of new chemical active sites and functionalization of hydroxyl groups [1].
Research on lignin in polyurethane applications have also been conducted [5][6][7][8][9][10][11][12]. Studies on chemical modification of lignin as biobased polymers [1,13,14], biodegradable [15], green composites [16] and green hydrogel [17] have also been reported. Various efforts have been reported to combine lignin into other natural polymers to form a full bio-based materials. There have been also reports on addition of lignin into starch, proteins, polycaprolactone, poly(hydroxyl-butyrate) [3,18], gelatin [19], polybutyleneadipate-co-terephthalate [20], chitosan [21], and polylactic acid [22][23][24][25][26]. The number of hydroxyl groups in the lignin causes a relative polar characteristic thereby providing better affinity to the polar polymer. However it also raises a problem when composited with nonpolar material because it is not compatible and will result in composites with poor stress transfer in terms of mechanical properties. Therefore, a compatibilizer to improve the inter-phases adhesion so that the material be compatible becomes necessary. Moreover, the compatibility of lignin can also be improved by reducing the content of hydroxyl, even simple esterification or alkylation of hydroxyl groups can favor the compatibility of lignin with non-polar polymer matrices [3]. Recent studies on utilization of lignin are more focused on the production of lignin-based thermoplastics. In addition, studies on performance of polymers and composites by mixing lignin or lignocellulosic material with thermoplastic resin has been reported [27][28][29][30][31][32][33][34][35][36][37][38]. Studies of the addition of the lignin into a nonbiodegradable material are also intended to increase biodegradability of the resulted composites.
In this research, LLDPE/Lignin biocomposites were manufactured by adding various composition of lignin into LLDPE to study the effects of lignin addition on the morphology, biodegradability, and mechanical and thermal properties of LLDPE/lignin biocomposites with polyethylene-grafted maleic anhydride as the compatibilizer. Lignin used in this research was a pure alkali lignin without modification.

Characterizations
Analysis on functional groups of LLDPE, Lignin and LLDPE/Lignin bicomposite was conducted using FTIR at wave number of 4000-400 cm -1 . A total of 40 scans were accumulated in transmission mode with a resolution of 4 cm -1 . The morphological analysis of the LLDPE/Lignin biocomposite was investigated using a Scanning Electron Microscope (SEM) HITACHI SU3500. Thermal analysis was conducted using STA Linseis STAPT 1600. Biocomposites was heated up to 500°C with heating rate of 10°C/min in sample of small pieces with a weight of ± 20 mg. A universal testing machine (UTM) Strograph 50, Toyoseiki was used to evaluate the tensile strength dan elongation at break of LLDPE/Lignin biocomposites films according to ASTM D638. Biodegradation test was performed according to ASTM G 21-96. The medium for the fungi was prepared by dissolving KH2PO4 (0.7 g), MgSO4.7H2O (0.7 g), NH4NO3 (1.0g), NaCl (0.005 g), FeSO4.7H2O (0.002 g), ZnSO4.7H2O (0.002 g), MnSO4.7H2O (0.001 g) and agar (15 g) into 1L of water. Next step is the sterilization of agar medium by autoclaving at 121°C for 90 min. The sterilized agar medium is then poured into a sterile petri dish with depth of 3-6 mm and left until condensed. Samples biocomposites with size 2x2 cm 2 were planted on an agar medium, then added suspense fungus Aspergillus niger spores. Further, the samples were incubated for 68 days. The sample was then sterilized with alcohol 70%, then soaked in distilled water. Finally, the samples were dried at 50°C and then their dry weight were measured after biodegradation.

Results and Discussion
The composition of lignin added to LLDPE affects the color of LLDPE/Lignin biocomposites produced. The effect of lignin composition to the visual color biocomposite film is shown in Figure 1.
The more lignin were added to the biocomposite then the darker the color of the produced LLDPE/Lignin biocomposites films. This is because the color of LLDPE used was translucent, whereas the lignin was dark brown. Therefore, the resultant composite tends to get darker with increasing lignin composition is added. biocomposites that indicate certain functional groups are presented in Table 1.

Thermal Properties
The effect of addition of lignin composition on the thermal stability of LLDPE/lignin biocomposites was studied by thermogravimetric analysis (TGA). TGA curves of LLDPE, lignin and LLDPE/lignin biocomposites are shown in Figure 4.
LLDPE, lignin and LLDPE/lignin biocomposites have a single degradation step. LLDPE starts degrading at 250-300°C, while the lignin starts degrading at 40°C and a major degradation occurs at 300-500°C related to fragmentation of inter-linkage units [25]. At temperature below 450-460°C, LLDPE/lignin biocomposites are more susceptible to degrade compared to LLDPE, however, they are more resistant to thermal degradation when compared with lignin. On the contrary, at temperature over 460°C, LLDPE/Lignin biocomposites are more resistant to thermal degradation than LLDPE, however, more susceptible to thermal degradation compared to lignin. The high thermal stability of lignin is caused by the presence of aromatic phenyl groups. In addition, the presence of hydroxyl groups also contributes to the thermal stability and improves the stability of aromatic structures in lignin and prevents damage due to temperature effects [22]. The increase in the residual mass is caused by the chemical structure of lignin, which shows a less contribution to the flammability of materials due to the high charring ability and the low heat release when burned [23,50]. This shows that the increase of lignin composition in the LLDPE/lignin biocomposites will increase the char content [24,34,51] and the residual mass and will increase its thermal resistance as well. TGA results indicate that the addition of lignin into composite LLDPE/lignin biocomposites can reduce the thermal stability up to temperature of 450-460°C compared to LLDPE. The thermal stability curve of intersection between LLDPE (Figure 4a) and lignin (Figure 4f) shown at 456°C. This intersection curve show that at temperatures below 456°C LLDPE is more resistant to degradation than lignin, whereas the temperature above 456°C LLDPE more susceptible than lignin. Therefore, the composite of LLDPE and lignin will result in LLDPE/lignin biocomposites with a thermal stability which is lower than LLDPE, however, higher than lignin. The degradation thermal stability of LLDPE/lignin biocomposites is also allegedly due to the empty space in the LLDPE/lignin biocomposites which is filled with oxygen or air trapped. Subsequently, it will initiate the degradation process resulting in lower thermal stability [49]. However, the thermal stability LLDPE/lignin biocomposites is increased at temperature over 460°C compared to LLDPE. The increase in thermal stability of LLDPE/lignin is due to the appearance of complex phenylpropanoid unit containing aromatic phenyl groups from lignin. In addition, the presence of multiple hydroxyl groups also contribute in the improvement of the stability of aromatic structures within the structure of lignin so that it will increase the mass of the remaining. Furthermore, the presence of a hydroxyl group also contributes to the thermal stability because it increases the stability of aromatic structures present in lignin. The increase in the residue is due to the chemical structure of lignin indicating a less contribution to the burn-ability due to the high ability to become charcoal and low heat release when burned to prevent it from degradation because of the influence of temperature [22,23,50].

Mechanical Properties
The effect of the addition of lignin composition to the mechanical properties of LLDPE/lignin biocomposites was studied by analyzing the tensile strength and elongation at break. The analysis results of tensile strength and elongation at break are shown in Table 2.  The decrease in tensile strength and elongation at break is considered as a result of non-uniformly distributed lignin in LLDPE/lignin biocomposite which causes an agglomeration affecting its stress distribution [26]. The elongation at break of LLDPE/lignin biocomposites was drastically decreased

Morphology
SEM micrographs of the fractured surface for LLDPE/lignin biocomposites films are shown in Figure   5. Figure

Biodegradability
The effect during degradation proses is greatly depended on hydrophilicity or hydrophobicity and particle dispersion. Moreover, the delay in biodegradability is caused by the increase in barrier properties of the material which can prevent water diffusion through the bulk [55]. The effect of lignin addition to the biodegradability of LLDPE/Lignin biocomposites is studied using ASTM G21-70. The results are shown in Table 3. hydrophilic functional groups that promote biodegradation. The more lignin were added into LLDPE/lignin biocomposites, the more the materials were biodegraded. This is because as the number of lignin added increases, the biocomposites become more hydrophilic, and therefore, the material become more biodegradable. However, at the lignin composition LDPE/lignin 20, the biodegradation result is lower than LLDPE/lignin 15. It was allegedly because of lignin has structural complexity and high molecular weight so the optimal biodegradability of fungi especially aspergillus niger to degrade the lignin is not more than 15 phr (LDPE/lignin 15). For lignin degradation requires specific enzyme, possibility some of the enzyme from aspergillus niger is non-spesific for lignin so its ability to degrade lignin is not optimal for the large composition [56].

Conclusion
The FTIR spectrum of all LLDPE/lignin biocomposite showed similar absorption peak with lignin and also show absorption peaks at wavenumber of 1730 cm -1 related to C=O from the compatibilizer. It was indicated that absorption peak of LLDPE/lignin biocomposite is mixture between absorption peak of LLDPE and lignin and shows that the LLDPE/lignin biocomposites was successfully manufactured.
The composition of lignin added to LLDPE affects the color of LLDPE/lignin biocomposites produced. The degradation thermal stability of LLDPE/lignin biocomposites is allegedly due to the Subsequently, it will initiate the degradation process resulting in lower thermal stability. However, the thermal stability LLDPE/lignin biocomposites is increased at temperature over 460°C compared to LLDPE. The increase in thermal stability of LLDPE/lignin is due to the appearance of complex phenylpropanoid unit containing aromatic phenyl groups from lignin. In addition, the presence of multiple hydroxyl groups also contribute in the improvement of the stability of aromatic structures within the structure of lignin so that it will increase the mass of the remaining. The addition of lignin into LLDPE made the surface structure of the resulted LLDPE/lignin biocomposites become rougher and distributed non-uniformly in LLDPE/lignin biocomposites. This rough surface also possibly initiated faster biodegradation. The agglomeration can result in the lack of interface adhesion, thus, leads in a poor distribution of stress when pressured. Furthermore, it will result in the degradation of tensile strength and elongation at break. The addition of lignin results in enhancement of biodegradability properties of LLDPE/lignin biocomposites. These LLDPE/lignin biocomposite degradations were associated with the presence of lignin which has many hydrophilic functional groups that promote biodegradation.