First, WSTM was prepared using a twin-screw extruder as this extruder is an actual machine used for the industrial production of plastics. Table I (a) summarizes the composition of the as-prepared WSTM. Talc was applied as stone. Notably, without talc, forming resin pellets with a twin-screw extruder was difficult. In addition, when the content of wood was less than a certain ratio, the resin pellets were not formed using the twin-screw extruder. On the other hand, when the content of wood is greater than a certain ratio, the necessary force for pelletization becomes extremely strong such that wood-based pellets cannot be produced. Therefore, the contents of wood and talc need to be optimized to prepare WSTM pellets. NADES comprising lactic acid and glucose reportedly extracted botanical chemicals, such as anthraquinone, from plants (Wu et al. 2018). On the other hand, lactic acid based NADES especially prepared with choline chloride is often examined for the dissolution of wood-based material (Smink et al. 2019). In our study, chloride was not used for preparing NADES because hydrochloric acid may be formed during the twin-screw extruder process at high temperatures. However, notably, even chloric-based chemicals were not used, the corrosion of the metal parts in the twin-screw extruder was observed. Therefore, metal parts should be coated with an anti-corrosion coating (e.g., fluoride-based coating), or Hastelloy should be used because of its corrosion resistance. Acidity possibly originating from lactic acid could be one of the reasons for possible corrosion. Nevertheless, it was able to make test specimen for measuring mechanical strength when compositions were optimized by using an injection molding machine, which is a key advantage as injection molding is the most common process for preparing plastic molding products among various molding processes, including blow molding, film and sheet molding, and vacuum molding.
However, water resistance is one of the drawbacks. Considering our goal to produce wood-based thermoplastic materials with 100% natural ingredients without petroleum-based chemicals, we selected only plant-biomass-based chemicals or bioplastics as additive materials, e.g., starch, PLA, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), rosin, shellac, glue, and chitin. Starch was reported to be also weak in water (Abdullah et al. 2019). PLA does not exhibit marine biodegradability. Shellac, glue, and chitin are not plant-based biomass; hence, rosin and PHBV are selected as natural additives to render water resistance. As a result, water resistance was rendered by the addition of rosin and PHBV. With an increase in the rosin or PHBV content, the mechanical strength of the plastics increased. Moreover, the improvement in the bending strength was more obvious upon the addition of rosin instead of PHBV. Aldas et al. (2021) reported that when rosin was added into a PLA/PBAT composite, the mechanical strength was improved because of compatibility enhancement and plasticizer effects. In addition, WSTM was mixed with conventional petroleum-based plastics such as PE and PP (Table I (b)). WSTM exhibited good compatibility with these petroleum-based plastics, and its mechanical strength and water resistance were improved. Although these plastics still contain petroleum-based chemicals, they contribute to partially reducing CO2 emissions and plastic pollution by using wood and stone as the raw materials. Hence, WSTM can be used as a bulky agent for the plastic industry because of its cost-effectiveness.
Table I.
(a) Composition of as-prepared WSTM with 100 % natural composition without petroleum-derived chemicals.
(b) Composition of as-prepared WSTM combined with petroleum-based plastics (PE, PP).
Figure 2 shows the SEM image of the as-prepared WSTM. Parts of wood and stone (talc) were observed. Even NADES was reported to dissolve wood; hence, unreacted or undissolved wood was observed. The percentage of dissolved cellulose, lignin, or wood critically depends on the DES composition and percentage with respect to the wood weight as well as the reaction temperature (Wu et al. 2020). The dispersibility of wood and stone can be improved by further mixing using a twin extruder although excessive blending may deteriorate the quality of the WSTM; hence, mixing conditions should be optimized.
Figure 3 shows the TG–DTA results of as-prepared WSTM. The majority of the lactic acid and glucose (NADES composition) should be decomposed at temperatures of less than 300°C, which was not observed in our experiment (Komesu et al. 2017, Schmidt el al. 2012). A small peak at 368°C in the DTA curve exhibited a relatively good agreement with the DTA peak assignment in a wood thermal decomposition study reported previously. This is despite that we observed a more obvious peak at 423–445°C that corresponds to a slightly higher decomposition temperature than that reported in previous wood decomposition studies (Wu et al. 2014). The reason for the increase in the decomposition temperature of WSTM is not clear at this stage. It could be suggested that the physical properties of compressed and solvated wood changed during WSTM preparation using a twin-screw extruder at high temperatures. At 700°C, the remaining weight ratio was approximately 33%, corresponding to the weight of talc in the WSTM.
Figure 4 shows the IR spectra of WSTM. A wide absorption at 900–1200 cm−1 corresponded to the O–H group in the glucose structure, and an absorption corresponding to the H2O of cellulose was observed at 1650 cm−1. The peak at 1034 cm−1 corresponded to the C–O stretching vibration of cellulose ether bonds. Furthermore, the C–O stretching vibrations of lignin and hemicellulose were observed at 1243 cm−1, whereas the aromatic nucleus skeleton vibration was observed at 1505 cm−1. In addition, the C–H stretching vibration and O–H stretching vibration were observed at 2925 and 3375 cm−1, respectively (Wu et al. 2020). From these observations, the high-temperature twin-screw extruder process promoted wood dissolution; hence, these IR absorption peaks characteristic of cellulose and lignin were observed. Wang et al. also claimed that DESs dissolved lignin via the cleavage of a lignin–carbohydrate complex and β-O-4 linkages (Wang et al. 2020). Furthermore, Zhang et al (2021). also reported that DESs increased the hydrolysis efficiency of cellulose. Therefore, wood was speculated to be dissolved to show cellulose and lignin in our preparation procedure.
Figure 5 shows the 1H NMR spectrum of WSTM. The peak at 1.2–1.4 ppm corresponded to the protons in aliphatic and aromatic acetates. The methyl group peak should be observed at approximately 1.8 ppm although a clear peak was not observed. These peaks were thought to be derived from lignin. In addition, the signal observed at approximately 2.4–2.5 ppm corresponded to protons from aliphatic acetates and aromatics or the acetyl group of hemicellulose (Maheswari et al. 2020). The peaks observed at approximately 3.5 and 4.0 ppm were thought to be the Hγ of β-O-4, Hβ, or the methyl protons of 4-O-methyl-α-D-glucuronic acid (Peng et al. 2012). The peak at approximately 5.0 ppm could be assigned to Hα of α-O-4 (Sun et al. 2005). All of these peaks confirmed the presence of cellulose, lignin, and hemicellulose, which were consistent with the IR spectral data.
Wang et al. (2023) reported that DES is effective in removing some of lignin and hemicellulose from wood, while the C–C bonds in lignin were unaffected, allowing most of the characteristics and activities of natural lignin to be maintained. In contrast to traditional methods such as strong acid or alkali treatment, DES treatment is a physical dissolution process rather than a chemical decomposition process such that DES delignification treatment does not damage the cellulose structure, and the stiff cell wall becomes more flexible. Therefore, this physical wood dissolution process by DES is thought to render thermoplastic characteristics, which was effective in the twin-screw extruder process for obtaining pellet-like morphology.