The mechanical and physical properties of microcrystalline cellulose (MCC)/sisal/PMMA hybrid composites for dental applications

The study on polymethyl methacrylate (PMMA)-based composites in dental applications has gained much interest in recent years, resulting in many exciting studies worldwide. In those research, various filler types of reinforcing PMMA have been studied extensively. This study combines the microcrystalline cellulose (MCC) particles (0, 1, 2, 3, 5 vol.%) and sisal fiber to strengthen PMMA. We investigate their effects on the flexural, impact, hardness, compressive strength, water absorption, and thermal properties of (MCC)/sisal/PMMA hybrid composites. Scanning electron microscopy (SEM), universal testing machine (UTM), hardness Vickers, thermogravimetry analysis (TGA), and ANSYS Workbench 2022 R1 software are utilized to characterize the properties of the composites. X-ray diffraction (XRD) is used to characterize the degree of crystallinity of MCC and sisal fiber. Adding MCC to alkali-treated sisal/PMMA decreases the flexural and impact strengths but increases the hardness. Adding 1 and 2 vol.% MCC produces maximum flexural and impact strength and hardness values. Simulation on a composite added with 1% MCC by applying a full force load of 14.4 N yields compressive strength of 42.56 MPa. Thermal stability of all composites with and without MCC shows similarity until 250 °C but gradually degrades at over 250 °C, particularly for composites with MCC. Besides, as the addition of MCC increases, the water absorption also increases, with the lowest value of 37.54 μg mm−3 obtained by the composite added with 1 vol.% MCC, which is within the range of the standard dental materials.


Introduction
Polymethyl methacrylate (PMMA)-based resins in dental applications have been studied thanks to their excellent properties, such as biocompatibility, low density, appropriate aesthetics, and customizable mechanical and physical characteristics globally [1]. However, low fracture toughness is a well-known drawback of PMMA [2]. Therefore, many studies have attempted to improve the PMMA's properties. PMMA, as a transparent thermoplastic, has been developed to be a dental composite material reinforced with various fillers, such as particle, fiber, and particle-fiber. Different filler types have been reported in dental applications, such as TiO 2 [3], ZrO 2 [4], Al 2 O 3 [5], SiO 2 [6,7], and biomaterials of eggshell, seashell, and fish scale [8][9][10] for particle fillers, and natural and synthetic fibers [11,12]. However, there have been only a few studies on the particle-fiber combination, particularly the microcrystalline cellulose (MCC) particles-natural fiber. Some of them are described in the following paragraphs, primarily those related to flexural properties (i.e., strength and modulus) and hardness because they are considered important for dental material.
Antimicrobial graphene oxide nanosheets (nGO), TiO 2 nanoparticles (∼15 nm), and crystalline Curcumin (CUR) loaded nGO have been utilized as fillers of PMMA to study the flexural strength and hardness of the composites [13]. Among those fillers, they reported that TiO 2 nanoparticles are the most effective filler in Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
improving flexural strength from 24.94 ± 5.37 MPa to 52. 26 ± 5.48 MPa. The mixtures of all fillers drastically decrease the flexural strength to 31.59 ± 4.80 MPa, but hardness increases from 18.45 ± 1.04 HVN to 23.29 ± 0.80 HVN. A decrease in flexural strength is estimated due to excessive air bubbles being introduced into the specimen's body where the filler powder and liquid matrix are mixed. Another study on PMMA reinforced with TiO 2 nanoparticles (∼38.6 nm) at 0, 2, and 4 wt% resulted in the gradual improvement of flexural strength from 52.46 MPa to 57.88 MPa and hardness from 73.2 to 83.9 shore (D) [3].
Additionally, incorporating ZrO 2 nanoparticles and woven glass fiber with various ratios was investigated to reinforce PMMA [14]. The ZrO 2 nanoparticles/glass fiber ratio of 2.5%/2.5% had shown a maximum flexural strength of 94.05 ± 6.95 MPa and an impact strength of 3.89 ± 0.46 kJ m −2 that might be used to make a removable prosthesis.
Adding Alumina (Al 2 O 3 ) nanoparticles in various sizes and contents to PMMA [5] resulted in a significantly higher flexural strength than using TiO 2 nanoparticles described above [3]. 16 wt% Al 2 O 3 with the size of 40, 150, and 500 nm and a ratio of 1:1:1 had indicated the highest flexural strength of 168.45 MPa and fractured toughness of 2.36 MPa m −2 . Adding micro or nanoparticles can effectively improve the hardness of composite reinforced with particle fillers. However, a homogeneous dispersion of the particles makes enough separation between the nanoparticles. Therefore, the number and size of the particles are significant in enhancing the hardness of the composite.
In another study, the organic filler of chitosan micro and nanoparticles combined with kenaf and carbon fibers strengthened PMMA [15]. Chitosan is a natural polymer that is non-toxic and has an antibacterial property. Adding chitosan microparticles of 5 vol.% produces the highest flexural strength of the hybrid composites (133.57 MPa), while the composite added with chitosan nanoparticles had the maximum flexural strength of 123.90 MPa at one vol.%.
Cellulose is another organic filler potentially used in dental applications because cellulose is a non-toxic natural polymer. For instance, dental acrylic resin may be used with cellulose [16]. Unmodified and silanemodified celluloses have also been studied for PMMA's fillers. The problem of the natural hydrophilic property of pure cellulose could be handled using silane-modified cellulose, resulting in a higher flexural strength (∼ 87 MPa) of the composite with modified cellulose than that of unmodified ones (∼ 70 MPa) [17]. For its use as a dental material, the flexural strength and hardness of PMMA reinforced with nanocrystalline cellulose at various concentrations have been studied [18]. Adding 2.5% nanocrystalline cellulose particles yields the maximum flexural strength of 70.73 MPa and Vickers hardness of 23.76 HVN. In addition, the use of a 50% ratio of cellulose/hydroxyapatite (HAP) improves the mechanical properties of flexural strength/modulus (∼50 MPa/ ∼ 9 GPa), hardness (∼35 HVN), and compressive strength (∼97 MPa), making them appropriate to be used as fillers for dental restorative material [19].
Cellulose is one of the natural resources, especially in tropical climate countries, making it accessible and affordable. The research on using cellulose in dentistry is, nevertheless, still uncommon. Therefore, this study modifies the volume fraction of MCC particles by combining them with alkali-treated sisal fiber to strengthen PMMA. The integration of MCC, having a high degree of crystallinity, and the alkali-treated sisal fiber, which also contains crystalline cellulose, can reinforce PMMA. Suppose both MCC and sisal fiber are distributed homogeneously in the PMMA matrix. The composite's mechanical properties will improve due to good interfacial bonding between the sisal fiber and PMMA, as well as MCC and PMMA. We explored their influences on the flexural, impact, hardness, compressive, water absorption, and thermal properties of the (MCC)/sisal/PMMA hybrid composites. This research aims at producing economical dental composite materials that can compete with other composite materials and are included in the range of material standards for dental applications.

Materials
MCC was purchased from Sigma-Aldrich, whereas the sisal fiber was bought from a Research Institute for Sweeteners and Fiber, Indonesia. NaOH, CH 3 COOH, and PMMA were obtained from the local chemical supplier. Distilled water is required to dilute the chemical solution and rinse the fiber.

Alkali treatment
Sisal fiber was flushed with running water before being dried to clean the impurities on the fiber surface. The fiber was left to soak in a 6% NaOH solution at room temperature (RT) for 36 h and then neutralized in 1% CH 3 COOH for one hour to improve interfacial interaction with the matrix. It is then followed by immersing the fiber in distilled water for about 36 h before being washed off with flushing water and dried [20]. Subsequently, the treated sisal fiber was chopped to around 4 mm in length.

Composites fabrication
The mixture of the composite between fillers (MCC particle + sisal fiber) and PMMA matrix is at a ratio of 20:80 (vol.%). The MCC particle was varied by 0, 1, 2, 3, and 5 vol.%. The MCC was mixed with PMMA for around 20 s because PMMA is relatively easy to harden. Composites were made manually by randomly arranging the short sisal fibers in the mold, pouring PMMA liquid containing MCC particles onto the fibers, and mixing them to wet the entire fiber surfaces. The composite was then pressed at 80°C for ∼30 min [21]. The untreated sisal fiber (UTS) and alkali-treated sisal fiber (TS) were used. Table 1 shows the composition of the composite specimens.

Mechanical and physical tests
All composite specimens were subjected to bending, impact, hardness, and water absorption tests, in which five samples were prepared for each test. After the specimen is prepared/cut to size according to the ASTM, the edges of the test specimen are slightly polished manually before the mechanical test is carried out. A three-point bending test was carried out using a universal test machine (UTM, Zwick/Roell Z020), following ASTM D790-03 with a specimen dimension of 127 mm×12.7 mm×3.2 mm using a maximum load of 0.2 MPa and a crosshead speed of 2 mm/min. Similarly, a UTM was used for the Charpy impact test. It used a specimen dimension of 127 mm×12.7 mm×3.2 mm, and the depth of the notch was 10.16 ± 0.05 mm with an angle of 45 ± 1°. The test was conducted according to the ASTM D6110 standard [22]. A hardness Vickers (Hardness Testing Machine Mitutoyo HM-200) was utilized for hardness testing on the specimen with a dimension of 30 mm×10 mm x 3.2 mm, in which the load was fixed at 50 g for 15 s [23]. The present study performed the compression test by simulation using ANSYS Workbench 2022 R1 software. The geometry for the compression test was made according to ASTM D 695.
A water absorption test was conducted following the ASTM D570 standard, with a specimen dimension of 76.2 mm×25.4 mm×3.2 mm [22]. Before immersing in distilled water at room temperature for 216 h, the weight of the specimens was measured. For every 12 h time interval, the weighing was carried out by removing samples from the water and wiping the excess water before measuring their weight.
A thermal test on the UTS, TS, MCC, UTS/PMMA, TS/PMMA, and MCC-1/TS/PMMA specimens was performed by thermogravimetric analysis (TGA, Hitachi STA200RV) to characterize the thermal property (thermal stability) of each specimen; and to determine the weight loss occurred in each specimen during heating. The specimen weighing 12 mg was heated at a maximum temperature of 550°C and a heating rate of 10°C min −1 in N 2 gas with a 100 ml min −1 flow rate. We then recorded and analyzed the weight loss of each sample.

Characterization
To evaluate their degree of crystallinity, MCC, UTS, and TS were examined by x-ray diffraction (XRD, SHIMADZU XRD-700). At the same time, scanning electron microscopy (SEM, Zeiss Evo 10) was utilized to characterize the morphology of MCC. SEM was also used to examine the impact fracture surface but not for the sisal fiber surface.

Results and discussion
3.1. Characteristics of MCC particle and sisal fiber The morphology of MCC particles (figure 1) shows particle sizes ranging from 40 to 250 μm. In this study, we used XRD analysis to determine the degree of crystallinity difference between MCC, UTS, and TS, knowing that it affects the mechanical properties of the related composite [24,25]. Figure 2 depicts the differences in UTS, TS, and MCC XRD profiles. Considering the cellulose property, their XRD peaks at 2θ around 22°reveal the difference in peak sharpness and intensity. The JCPDS (PDF # 030289) for the native cellulose showed the highest intensity (I 002 lattice reflection) at 2θ of 22.84°, which matched with the XRD pattern of the MCC but not with that of UTS and TS. Since cellulose and non-cellulosic hemicellulose and lignin make up most of the sisal fiber, the XRD peaks are not as sharp as MCC (figure 2). Alkali treatment often eliminated a portion of such non-cellulosic components, resulting in the increased proportion of the crystalline phase.
In this case, the cellulose crystallinity index (I c ) can be determined by the following empirical equation (1) [26,27].

= -
where I (002) and I (amorphous) are peak intensities at 2θ of approximately 22°and 18°, respectively, representing crystalline and amorphous celluloses. It results in I c of 49%, 65%, and 80% for UTS, TS, and MCC, respectively. Figure 3 shows the changes in the flexural properties (flexural strength and modulus) affected by alkali-treated sisal fiber and by adding MCC. It is described earlier [20] that modification of natural fiber surface by alkali solution can enhance the fiber/matrix interface bonding because of the surface roughness improvement [28].

Flexural properties
The figure shows the higher flexural properties of TS/PMMA than UTS/PMMA. Alkali-treated fiber surface improved flexural strength by around 12.70%. The trend of increasing flexural strength due to alkali-treated sisal fiber has also been reported in TS/PMMA composite by a fiber loading from 2.5% to 10% [29]. The flexural strength of around 55 MPa was achieved by 10% TS loading, which is lower than that of the present study (73 MPa) for 20 vol.% TS loading.  Adding 1 vol.% MCC to TS/PMMA decreases the flexural properties by ∼9.5% for flexural strength and ∼2.9% for flexural modulus. Those are still higher than that of UTS/PMMA and the composites by adding MCC higher than 1 vol.%. Although MCC has the highest crystallinity index compared to UTS and TS, the mechanical properties of the hybrid composites proposed in this work are reduced because the hydrophilicity of the cellulose might predominantly affect the mechanical properties of the composites.
This study used unmodified MCC and modified or alkali-treated sisal fiber. The natural hydrophilic cellulose leads to being easily aggregated. Increasing the number of added MCC particles reduces the contact surface area between the sisal fiber and the matrix. Aggregation of the particles caused voids formation and diminished stress transfer from the matrix to the particle, reducing its mechanical strength. The maximum flexural strength (65.90 MPa) and flexural modulus (4.30 GPa) were achieved by MCC-1/sisal/PMMA, which is within the range of dental material-based polymers recorded in ISO 1567, i.e., 60-65 MPa and 1500 MPa for minimum flexural strength and modulus, respectively. This flexural strength was higher than that of TiO 2 nanoparticles/PMMA at 2% TiO 2 [3] but lower than that of unmodified cellulose/PMMA (∼ 70 MPa) [17]. A study on TiO 2 -ZnO/PMMA nanocomposite for dentures has also reported a similar trend, i.e., maximum flexural (56.78 MPa), compressive (128.20 MPa), and impact (8.06 kJ/m 2 ) strengths obtained by the composite added with 1% TiO 2 -ZnO [30].

Impact strength
The impact strength of the TS/PMMA composite is significantly higher than that of the UTS/PMMA composite ( figure 4). However, we can observe that the impact strength decreases linearly with MCC concentration and the reduction of sisal fiber concentration. The influence of MCC higher than 1 vol.% on the impact strength differs from that on the flexural strength and modulus in figure 3, showing similar values. In this case, the higher the MCC content (the lower the sisal fiber content), the lower the fiber's energy absorbed. As a result, it decreases the  impact strength, as indicated by [31,32], in which the fiber content is one of the factors affecting the impact strength of polymer composite.
One possible cause of the different impact strengths could be the difference in interfacial adhesion between the fiber/matrix and particle/matrix interfaces. The alteration in the impact strength (figure 4) can be explained by the morphology of the fracture surfaces affected by the impact test, as observed by SEM in figure 5.

SEM analysis
The impact fracture surface of sisal/PMMA composites (figures 5(a) and (b)) shows that both have a uniform fiber distribution within the matrix with excellent fiber/matrix interfacial bonding. However, some debonding and fiber pull-out were found in the UTS/PMMA composite, which were not observed in the TS/PMMA composite, making the impact strength of TS/PMMA higher than that of UTS/PMMA.
As can be seen in figures 5(c)-(f), an increasing number of microvoids are developed because of the enhancement of MCC particle volume fraction, which tends to be clustering due to the hydrophilicity of cellulose. Besides, the fiber and the matrix interface adhesion is reduced because of the decreasing fiber content. These conditions play an essential role in reducing impact strength.

Hardness
The maximum value in terms of the hardness of the composite (figure 6) was achieved by MCC-2/sisal/PMMA. The high degree of fiber dispersion within the matrix and the efficient bonding of the fiber/matrix interface are considered the main factors affecting the increase of the polymer composite hardness. The hardness value determined from the composite specimen surface with a non-uniform fiber distribution will differ from a region with an even fiber distribution. In other words, the specimen surface with a compact or dense structure will provide a higher hardness than a porous structure.
Compared to the previous studies, the maximum hardness of 23.07 HVN obtained in this work was higher than the values reported by the composite added with 1% TiO 2 -ZnO [30], 1% Al 2 O 3 [33], and 1.5% ZrO 2 [34] nanoparticles. However, it was still lower than 30 HVN for (cellulose+HAP)/PMMA with a ratio of 40:60 and 35 HVN for (cellulose+HAP)/PMMA with a balance of 50:60. These results indicate that a higher concentration of (cellulose+HAP) makes the composite surface structure more compact than a lower (cellulose+HAP) content, letting the surface withstand a more significant load [19]. However, it was not shown in the present result ( figure 6). The addition of MCC higher than 2 vol.% led to the composite being more porous, as exhibited in SEM images (figures 5(e) and (f)), reducing the density of the composite surface and hardness value.

Compressive strength
The compressive strength is assessed based on the simulation results by ANSYS Workbench 2022 R1 software. In this study, we used the software to investigate the utilization of the MCC/sisal/PMMA composite for dental material applications. The model used for the simulation in this study is the maxillary incisors because they are the most fractured teeth in the primary and permanent dentition [35,36]. In this case, the compressive strength of the material used for central incisors is considered higher than that for lateral incisors.
The Finite element analysis on various post materials, such as ZrO 2 , stainless steel, and glass fiber for dental applications, has been reported in [37]. In that study, the authors performed the simulation on the maxillary  central incisor with some models to investigate the rigidity of post-core systems on stress distribution. Therefore, the focus of stress distribution analysis is different from this study. The results show that stainless steel's maximum von-Mises stress (∼ 55 MPa) is higher than others.
The compressive strength obtained from the proposed method in the present study composite by simulation was significantly lower than some previously studied composites, such as TiO 2 -ZnO/PMMA [30], microlignin/PMMA, and ZrO 2 nanoparticles/PMMA [12]. For denture base material, therefore, post-material may be required. Figure 8 demonstrates the water absorption of all composites. We can observe that the water absorption of UTS/ PMMA is lower than that of TS/PMMA. In this case, the alkalization of sisal fiber caused the fiber to be more hydrophilic because more hydroxyl groups were exposed on the fiber surface, making the water molecules easily bound and increasing the weight gain.

Water absorption
In addition, the MCC-5/sisal/PMMA hybrid composite achieved the highest weight gain (∼ 42.5 μg m −3 ) in water absorption. It is attributed to the increase in the number of hydrophilic cellulose derived from MCC and sisal fiber. The water uptake of all composites except the MCC-5/sisal/PMMA is less than 40 μg m −3 , which is within the range for polymer base filling, restorative, and luting materials according to ISO-4049 standards. However, under the ISO-1567 standard, denture base polymer requires water absorption of at least 32 μg m −3 .
According to the base filler material and water absorption results in the proposed composite, the MCC/ sisal/PMMA is suggested as appropriate for the denture base or bridge framework.
3.8. Thermal analysis TGA curves of UTS and TS fibers, MCC particles, and UTS/PMMA, TS/PMMA, and MCC-1/TS/PMMA composites obtained from 30°C to 350°C are shown in figure 9. Slightly different patterns for UTS fibers and TS fibers, and MCC particles ranging from 50°C to 100°C indicated the presence of mass loss (2.37%, 3.11% and 1.89% for UTS, TS, and MCC, respectively) (table 2) because of the evaporation of water molecules contained in the sisal fibers and MCC [38][39][40]. The mass of UTS and TS are relatively stable until approximately 200°C, but that of MCC is stable until around 250°C. Their mass then decreases drastically up to around 350°C. According to [41,42], it is caused by (i) the breakdown of hemicellulose and pectin and (ii) the thermal decomposition of hemicellulose, lignin, pectin, and the glycosidic linkages of cellulose [43].
Furthermore, when UTS, TS, and MCC were used as the composite's fillers, their mass reduction was much better with relatively low mass loss up to 250°C, then decreased continuously until 350°C. Thermal degradation on the composite differs when using UTS, TS, and MCC as the fillers because the PMMA matrix covers them.
Based on the evaluation of the degree of crystallinity in section 3.1. that the degree of crystallinity of MCC is higher than that of UTS and TS, leading to increased resistance toward heat [24]. However, the thermal degradation of the MCC-1/sisal/PMMA hybrid composite occurred first, followed by UTS/ PMMA and TS/ PMMA. Therefore, the influence of MCC′s high degree of crystallinity did not play a crucial role. It is possibly due to the low volume fraction of the added MCC.

Conclusions
We have successfully fabricated the MCC/sisal/PMMA composites and analyzed the characterization of their mechanical and physical properties. The analysis shows that the flexural strength, impact strength, and hardness of the MCC-1/sisal/ PMMA hybrid composite can compete with existing dental composite materials, such as TiO 2 nanoparticle/PMMA, ZrO 2 /glass fiber/PMMA, and cellulose/PMMA. Moreover, the flexural properties and water absorption are within the range of dental material values according to the ISO 1567 standard. We believe these findings can be the basis for developing new composite materials in dentistry and producing high-quality yet economical products.

Data availability statement
The data cannot be made publicly available upon publication due to legal restrictions preventing unrestricted public distribution. The data that support the findings of this study are available upon reasonable request from the authors.