2.3.8. Adherence Test

To test the adherence of ink on the prepared sand/polymer sheets, marker pens were used to write on the sheets. After writing on the sheets, the sheets were left to dry for 15 min. A piece of clear adhesive tape (1.5 cm wide) was then applied onto the surface where text was written. Any small bubbles that appeared while applying the clear tape were removed manually, and by using the rolling weight of the hand, the tape was firmly pressed. Consequently, the tape was pulled off each sample, implying that the smaller the amount of ink that was removed by the tape, the better the assessment. The test was repeated twice; once with permanent marker pen and the second time with removable marker pen. The adherence of ink to the selected sand/polymer composite sheet surfaces was assessed qualitatively as excellent, good, regular, or poor, as reported in the literature [13]. The removed tape was placed alongside the sheet to observe the difference. As a comparison, an adherence test was also performed on stone paper and regular A4 paper.

#### **3. Results and Discussion**

#### *3.1. Morphology*

In order to determine the size of the ground sand samples, SEM analysis was conducted to obtain an idea of the particle size as well as the shape of the particles. Figure 8a,b show the two sand samples, respectively. It can be clearly seen that the 25 μm sand particles have larger, more irregular-shaped distant particles, whereas the 5 μm sand particles have smaller, finer, and closer particles comparatively. The neat HDPE sheet, with 0 wt% filler, which was processed in the same way as the composite sheets was also analyzed using SEM. Figure 8c shows only a fine structure which is expected in a neat polymer film and is in agreement with the literature [37].

(**a**) (**b**)

**Figure 8.** *Cont*.

**Figure 8.** SEM images of (**a**) 25 μm sand particles and (**b**) 5 μm sand particles; (**c**) neat HDPE; (**d**) 50 wt%, 5 μm sand composite sheet; (**e**) 50 wt%, 5 μm sand compatibilized composite sheet.

Selected composite sheet samples were also chosen to conduct SEM analysis. At 35 wt% prepared from 5 μm, particles were visibly seen. However, after compatibilization, particles had decreased interparticle distance, suggesting improved dispersion and binding ability [21]. The literature reports good appearance at ~30 wt% for composite sheets as well [2]. Similarly, in the case of 50 wt% and 5 μm particle size, compatibilization shows increased presence of particles, thereby indicating decreased interparticle distance. An increase in the filler accumulation can result in agglomeration of particles, which can lead to a brittle material [13]. SEM images of both sets of composite sheets at 50 wt% are shown in Figure 8d,e, respectively.

#### *3.2. XRD Analysis*

The successful preparation of the composite sheets with sand as well as with the addition of the compatibilizer (C) was confirmed using XRD analysis. HDPE characteristic peaks at 2θ = ~21.5◦ and 2θ = ~23.5◦ were observed, which are supported in the literature as well [38,39]. Addition of the compatibilizer did not alter the characteristic HDPE peaks as shown in Figure 9; however, slight broadening of the peak was observed.

Just as a confirmation, XRD was also performed for selected sand/HDPE composite sheets. Figure 9 also shows that the presence of the 5 μm sand particles and compatibilizer did not alter the characteristic peaks of HDPE, as two distinct diffraction peaks of HDPE were observed for all the prepared sand/polymer composite sheets. As for the sand, two significant characteristic peaks were obtained at 2θ = 21.38◦ and 23.71◦, which are quite comparable to the literature values obtained for precipitated silica as well as sol–gelproduced silicon dioxide, ranging from 21.8◦ to 23◦, respectively [40,41]. The presence of these peaks was noticed for all the prepared samples at values close enough to the 2θ values (with slight shift to the right), showing that the addition of the filler (sand) does not significantly alter the basic structure of the prepared sand/polymer composite sheets at the molecular level as well.

**Figure 9.** XRD characteristic peak patterns: (**a**) neat HDPE before and after compatibilization; (**b**) sand/polymer composite sheets prepared from 5 μm sand particles; (**c**) sand/polymer compatibilized composite sheets prepared from 5 μm sand particles.

#### *3.3. Thermal Analysis*

#### 3.3.1. Melting and Crystallization Behavior

DSC analysis was conducted for all sand/polymer composite sheets prepared from both 25 μm sand particles and 5 μm sand particles at the same heating rate (10 ◦C/min) to understand the crystallization properties of the prepared sand/polymer composite sheets. Figure 10a shows the cooling (first cycle) and heating (second cycle) profiles for the neat HDPE. The observed peak melting temperature (Tm,peak) and peak crystallization temperature (Tc,peak) values for the neat HDPE were ~133.86 ◦C and ~117.50 ◦C, respectively, which is also well reported in the literature [38]. A trend of smooth transition temperatures is seen by the neat HDPE, which also shows the absence of any impurity in the sample.

Figure 10b,c show the cooling and heating profiles for the compatibilized sand/polymer composite sheets prepared from 5 μm sand particles, respectively. The thermal properties for all the sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles are reported in Table 2.

**Figure 10.** (**a**) DSC thermogram for neat HDPE. (**b**) Cooling profiles for compatibilized sand/polymer composite sheets prepared from 5 μm sand particles. (**c**) Heating profiles for compatibilized sand/polymer composite sheets prepared from 5 μm sand particles.

**Table 2.** DSC data for sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles, stone paper, regular A4 paper.


The obtained DSC results show that the crystallization temperature of HDPE is not influenced due to the addition of filler (sand) or the compatibilizer. All the samples exhibited a single crystallization exotherm and a corresponding melting endotherm. From the DSC thermograms, the onset crystallization temperatures (Tc,onset), peak temperatures for crystallization exotherms (Tc,peak), and melting endotherms (Tm,peak) for neat HDPE, all the prepared sand/polymer composite sheets, as well as stone paper and regular A4 paper, were evaluated. Determining the Tc,onset, Tc,peak and Tm,peak help in defining the processing temperature range of the polymer. Usually, the processing temperature of polymers is ±40 ◦C from the melting temperature, as reported by various studies [25,35]. Furthermore, by observing the changes that occur in the Tc,peak and Tm,peak, the values of their respective enthalpies can be calculated, which gives a wider idea of how much heat and energy is needed in the manufacturing process of such polymeric sheets.

Slight changes were observed for all the samples in the Tc,onset and Tc,peak in addition to the peak broadening/stretching during crystallization. For the compatibilized composite sheets prepared from 5 μm sand particles, the peak crystallization temperatures (Tc,peak) increased slightly from ~114 ◦C at 0 wt% to ~115 ◦C at 50 wt%. As for the peak melting temperatures (Tm,peak), the temperatures decreased slightly from ~133 ◦C at 0 wt% to ~132 ◦C at 50 wt%. Moreover, due to similar melting and crystalline temperatures obtained in all the prepared sand/polymer sheets, it can be inferred that filler or compatibilizer addition does not alter the thermal characteristics of HDPE and promotes their good thermal stability, as reported in literature as well [42,43]. The Tc,onset, Tc,peak, and Tm,peak for neat HDPE and for all the prepared sand/polymer composite sheets, as well as stone paper, are presented in Table 2.

The enthalpies of crystallization (ΔHc) and melting (ΔHm) were calculated by integrating the area under the cooling and heating curves, respectively. The percentage of crystallinity (Xc) in the prepared sand/polymer composite sheets was calculated using the following equation:

$$\text{\textsuperscript{\text{\textsuperscript{\text{\textsuperscript{\text{\%}}}}}} \color{\uresleftharpoons}{\text{\textsuperscript{\text{\footnotesize}}} \text{H} \text{m}}} \text{\textsuperscript{\text{\footnotesize}}} \text{H} \text{m} \text{m} \tag{1}$$

where Xc is the percentage of crystallinity of HDPE, ΔHm is the melting enthalpy, ΔH100% is the melting enthalpy of a 100% crystalline HDPE, taken as 293 J/g [43], and θ is the mass fraction of the filler (sand). For stone paper, the weight is considered as the manufacturer states, which is 70 wt% [7].

The observed melting enthalpy (ΔHm) in all the prepared sand/polymer composite sheets was always lower than for the pure HDPE (148.77 J/g). Moreover, a general fluctuating value of percent crystallinity (Xc) was observed throughout the prepared sand/polymer composite sheets, ranging from a crystallinity of 40–60%, which is well reported in several filler–polymer composite systems [36,44,45]. This variation is explained by either an insufficient amount of filler (sand) particles at the surface which can cause agglomerates, or an excess accumulation of filler particles which can form a soft layer at the interface present, which tend to decrease the nucleating effect [42].

Moreover, the reduction of the sand/polymer composite melting enthalpies (ΔHm) and crystallinity (Xc) could be further explained by the reduction in the conformational changes available to the macromolecules during crystallization, which is due to the presence of the silica particles in the composites which are not densely packed. According to statistical thermodynamics, particles, in this case, silica, restrict the mobility of macromolecules and reduce the spaces available to be occupied by the macromolecules, thereby restricting the ability to form well-developed crystals. Additionally, the crystalline phase is not densely packed, which results in minimum intermolecular interactions and hence a decrease in the heat of fusion on melting which is also experienced by other HDPE systems such as carbon black/HDPE composites, wood/HDPE composites, and clay/HDPE composites [39,42].

Reduction of ΔHm in industrial terms can be translated to money and power savings during the extrusion/molding process, which encourages a positive attribute for the industrial process. ΔHm, ΔHc, and Xc for all the prepared sand/polymer composite sheets, as well as stone paper, are also presented in Table 2.

#### 3.3.2. Thermal Stability of Composite Sheets

The amount of filler can have a significant impact on the end use properties, for instance, thermal expansion, stiffness, etc., of a final product. The thermal stabilities of the prepared sand/polymer composite sheets were also analyzed by thermal gravimetric analysis (TGA) by observing their onset degradation temperatures (Td,onset), peak degradation temperatures (Td,peak), and their onset degradation temperatures at 10% weight loss (Td,onset @ 10% weight loss). By determining the Td,onset and Td,peak values, the upper range of processing temperature and the maximum temperature before degradation of the material can be confirmed, respectively. Hence, the range of processing temperature can be optimized to avoid any degradation of the material to occur which proves to be of substantial value for polymeric industrial scale processes. By noticing the changes occurring to the Td,onset @ 10% weight loss, the effect of filler percentages and compatibilizer can be evaluated on the initial degradation corresponding to the same weight loss occurring. This can suggest which composition has more impact on the onset of degradation.

The TG curves for 25 μm sand particles and 5 μm sand particles alongside neat HDPE and prepared sand/polymer composite sheets, as well as their compatibilized versions, are shown in Figure 11a–d, respectively. Figure 11 shows that the composites have two degradation steps; the first degradation step is for the polymer, the second degradation is for the sand particles, which is comparable to the neat HDPE, which has only one degradation step, and sand has degradation at higher temperatures [46,47]. HDPE was stable (no significant weight loss) up to 400 ◦C, confirming that the processing of HDPE and HDPE composites at 170 ◦C would not degrade the polymer. The complete weight loss was observed near 490 ◦C, where almost all of the HDPE was burned. The peak degradation temperature (Td,peak) for HDPE (obtained from derivative of weight loss curve (DTG)) was observed at ~489 ◦C.

The degradation steps became more evident at 50 wt% of filler addition. Additionally, it can be seen that increasing the amount of the filler increases the Td,onset and shifts the thermograms to the right, implying improved thermal stability, which is also reported for another study conducted on a SiO2/polymer composite system [47]. Moreover, in the case of sand/polymer composite sheets prepared from 25 μm sand particles, the Td,peak increased very slightly from ~491 ◦C at 20 wt% to ~492 ◦C at 35 wt%, and Td,onset @ 10% weight loss increased from 448 ◦C to 453 ◦C, respectively. Furthermore, there is also a slight increase in the Tonset for the same set from 20 wt% to 35 wt%, which is also explained in the literature as due to the presence of filler minimizing the permeation of heat [47–49].

In addition to that, it can also be observed that the Td,peak after compatibilization were always lower than their corresponding samples prepared without the addition of the compatibilizer (except for sand/polymer sheet prepared at 20 wt% from 5 μm sand particles). This can be explained as due to the presence of acidic groups of maleic anhydride in the compatibilized sheets possibly interacting with some parts of the filler, resulting in slightly faster degradation [50]. For instance, for the compatibilized sand/polymer composite sheets prepared from 5 μm, the Td,peak decreased very slightly from ~499 ◦C at 20 wt% to ~488 ◦C at 50 wt%. However, the overall Td,peak remained comparably close enough to the neat HDPE value (489 ◦C), hence promoting good thermal stability for the prepared sand/polymer composite sheets. A similar trend was observed for the case of sand/polymer composite sheets prepared from 25 μm sand particles.

The differential rate of weight loss (dW/dt) of all the prepared sand/polymer composite sheets was obtained from differential thermogravimetric analysis (DTG) at a set heating rate of 20 ◦C/min. Figure 11 also shows the DTG plots (as insets) for neat HDPE (0 wt%) and for sand/polymer composite sheets at varying compositions prepared from 25 μm sand particles and 5 μm sand particles without the addition of the compatibilizer and with the addition of the compatibilizer, respectively.

**Figure 11.** (**a**) TGA thermogram and DTG plot for sand/polymer non-compatibilized composite sheets prepared from 25 μm sand particles. (**b**) TGA thermogram and DTG plot for sand/polymer compatibilized composite sheets prepared from 25 μm sand particles. (**c**) TGA thermogram and DTG plot for sand/polymer non-compatibilized composite sheets prepared from 5 μm sand particles. (**d**) TGA thermogram and DTG plot for sand/polymer compatibilized composite sheets prepared from 5 μm sand particles.

A large fraction of the sand/polymer composite sheets decomposed between 300 ◦C and 600 ◦C, and this can be attributed to the decomposition of the HDPE. The thermal decomposition peak between 600 ◦C and 800 ◦C was assigned to the decomposition of sand particles which is more visible in samples prepared from 35 wt% composition and 50 wt% composition.

Both the DSC and TG analyses results indicated that the addition of sand particles of both 25 μm particle size and 5 μm particle size, as well as the addition of the compatibilizer, did not affect the thermal properties of the prepared sand/polymer composite sheets significantly, which promotes good thermal stability for the sheets. Comparable results for thermal stability are reported by numerous studies for various filler–polymer

composites [36,51]. Table 3 reports the onset degradation temperatures (Td,onset), peak degradation temperatures (Td,peak), and onset degradation temperatures at 10% weight loss (Td,onset @ 10% weight loss) for all the prepared sand/polymer composite sheets, stone paper, and regular A4 paper for comparison.

**Table 3.** TGA data for all the prepared sand/polymer composite sheets, stone paper, and regular A4 paper; 25 μm sand particles, 5 μm sand particles.


#### *3.4. Mechanical Properties*

The effect of filler and compatibilizer (C) on the elastic modulus and tensile strength for sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles was studied with the elastic modulus for both sets illustrated in Figure 12a,b, respectively. As seen in Figure 12, at 0 wt% filler the elastic modulus obtained for the pure HDPE sheet was ~1200 MPa with a corresponding yield stress of 35.15 MPa, respectively, which is comparable to the literature [2,45,52]. Generally, a decrease in the elastic modulus was observed with increasing filler concentration from 1298.33 MPa at 20 wt% to 905. 72 MPa at 50 wt% for the sheets prepared from 25 μm sand particles (in Figure 12a). Similar trends were observed in the case of 5 μm, where the elastic modulus dropped from 950.59 MPa at 20 wt% to 887.47 MPa at 35 wt% (except for an increase in the case of 50 wt%), as seen in Figure 12b.

Compatibilization lowered the elastic modulus for all cases compared to their noncompatibilized versions. For instance, compatibilized sand/polymer composite sheets prepared at 35 wt% experienced a sharp reduction in elastic modulus from 1182.23 MPa before compatibilization to 629.95 MPa after compatibilization, and from 887.47 MPa to 687 MPa for sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles at the same filler composition, respectively (Figure 12a,b). The effect of addition of fillers and compatibilizers on the decreasing value of elastic modulus is observed for many polymer composites and is believed to be caused by the random agglomeration of particles weakening the polymer matrix, causing uneven crystallization leading to their brittle nature, which also contributes to weaker adhesion between filler particles and the polymer matrix [34,47,51,53]. This also contributes to the fact that the compatibilizer (maleic anhydride) is not able to form strong bonds with the composite sheets, leading to a weaker matrix structure, as seen for other blends as well [54,55].

Furthermore, with elastic moduli of 603.54 MPa and 687 MPa, the compatibilized sand/polymer composite sheets prepared with 5 μm sand particles at 20 wt% and 35 wt%, respectively, gave results closest to the stone paper, which yielded an elastic modulus of 596.32 MPa. Moreover, another study on composite sheets also suggested that optimum results were obtained from sheets prepared at 30% by weight [2].

The tensile strength did not change significantly with increasing the filler concentration before and after compatibilization, which is reported in the literature as well [56]. However, the tensile strength decreased significantly with the addition of the compatibilizer for both sets of sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles. Even though compatibilizers are expected to increase the tensile strength, the decreasing trend could possibly be explained due to the compatibilizer limiting the stress transfer and swamping the surface [57]. Furthermore, as increasing the filler decreases the mechanical properties (such as the elastic modulus), the decreased tensile strength can also be a combined effect of the addition of the compatibilizer and filler, as reported in another study on a filler–polymer composite material [53].

A wide variation of mechanical properties was observed for the case of sheets prepared at 50 wt%, irrelevant of the particle size, which is also reported to occur in thermoplasticbased films subjected to extensive molecular orientation [2]. For filler content of 50 wt%, the lowest tensile strength values were obtained, thereby suggesting increased brittleness for sheets prepared at higher filler percent by weight, which is also supported by the literature [45]. Due to possible formation of hydrogen bonding, crack propagation at weak phase interfaces can be facilitated, resulting in a lower tensile strength of the blend, which is also reported in the literature [54]. Comparatively, one of the highest tensile strength values (15.66 MPa) was measured for the regular A4 paper, which can be corresponded to its fibrous network structure with strong hydrogen bonding [58]. Table 4 reports all the mechanical properties of sand/polymer composite sheets and compatibilized sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles, as well as stone paper and regular A4 paper.

#### *3.5. Wettability Performance*

Surface wettability is one of the most essential properties to determine the use of the material in a specific application, such as in this case for printing-based applications. Wettability is determined based on the contact angle measured on the surface of the material. Typically, any non-polar surface would measure a contact angle of 90◦ or above. Anything below 90◦ suggests increased hydrophilicity with the liquid on the surface and results in increased absorption of the liquid in contact. Increasing the filler concentration increased the contact angle slightly from 86.62◦ at 20 wt% to 94.72◦ at 50 wt% and 88.88◦ at 20 wt% to 94.6◦ at 50 wt% for sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles, respectively, suggesting improved anti-wetting performance with water for both particle sizes. These results indicate that the nonpolar, hydrophobic blocks of the compatibilizer units may have been arranged on the surface of the composite sheets, as reported for other blends in the literature as well [54]. Contact angles of above 90◦ were obtained for all the compatibilized sheets. Figure 13 illustrates this data.


**Table 4.** Mechanical properties of all the prepared sheets, stone paper, and regular A4.

**Figure 13.** Contact angles of printing ink on sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles.

In the case of the non-polar solvent, benzene, decreased contact angles were obtained (ranging between 20–35◦) for all the sand/polymer composite sheets prepared, suggesting their absorption ability to a certain extent if they were to come into contact with any organic solvents. This can be excepted, as the surface is mainly non-polar and any contact with a non-polar liquid would promote the widely known "like dissolving in like" phenomena. Since maleic anhydride is an organic compound, the oxygen in its functional group binds to the chemical structure of benzene. Values of ~90◦ were obtained when the sheets were tested with a water–benzene mixture. In addition to that, regular A4 paper also exhibited a relatively higher value of contact angle (82.27◦) with water–benzene mixture.

Contact angle values of lower than 45◦ were obtained with printing ink. Irrespective of the particle size used, a general trend of decreased contact angles was observed with the addition of the compatibilizer compared to their non-compatibilized versions, indicating improved ink-wetting performance. For example, as seen in Figure 13 for sand/polymer composite sheets prepared from 25 μm sand particles, for both at 35 wt% and 50 wt%, the contact angles decreased from ~44◦ to ~38◦. Similar contact angles for ink-wetting have

been reported [59]. Moreover, the values obtained were relatively close to the commercial stone paper, which resulted in a contact angle of 32.50◦ with printing ink. Contact angles below 50◦ correspond to surface free energies of below 45 dyne/cm, which is preferred for printing [13,60]. A contact angle of 0◦ was obtained for regular A4 paper with printing ink, which is expected due to its increased hydrophilic surface. The wettability performances with printing ink for all the sand/polymer composite sheets prepared from 25 μm sand particles and 5 μm sand particles are illustrated in Figure 13.

Table 5 summarizes all the measured contact angles for the prepared sand/polymer composite sheets with water, benzene, water–benzene mixture, and printing ink. Contact angles for commercial stone paper and regular A4 paper are reported as well.


**Table 5.** Contact angle measurements.

#### *3.6. Printing Test*

The printing tests showed the capability of the prepared sand/polymer composite sheets of absorbing printer ink. The sheets were printed on and left to dry for over 24 h. It was observed that some parts were still able to wipe off for all the prepared sand/polymer composite sheets, and some parts were able to stay on the surface. Pure HDPE showed the least absorption of the printing ink and was unable to retain the printing ink on the surface. The selected 35 wt% sheet prepared from 25 μm sand particles showed almost similar printability to its compatibilized version. However, for the case of the 35 wt% prepared from 5 μm sand particles, improved printability with the addition of the compatibilizer was noticed. Moreover, by observing the printing tests closely, ink had the ability to form drops yet not spread on the surface of the sand/polymer composite sheets prepared with either 25 μm sand particles or 5 μm sand particles, resulting in a blurred, non-graphic quality, which could be easily smudged. Similar results have been reported in the literature [13]. Figure 14 shows all the sheets after the printing tests (enclosed within the red highlights are the surfaces of the respective sheets).

On the stone paper, slight removal was observed after 24 h in contrast to the regular A4 paper, where the ink was completely dried and absorbed almost immediately. It can be concluded that the quality of the printing improved with the addition of the filler for the case of sand/polymer composite sheets prepared from 25 μm sand particles (compared to pure HDPE), and after compatibilization in the case of sand/polymer composite sheets prepared from 5 μm sand particles.

**Figure 14.** Printing tests after 24 h for (**a**) stone paper, (**b**) regular A4 paper, (**c**) HDPE, (**d**) 35 wt%, 25 μm (**e**) 35 wt% + C, 25 μm, (**f)** 35 wt%, 5 μm, and (**g**) 35 wt% + C, 5 μm.

#### *3.7. Adhesion Test*

Selected sand/polymer composite sheet surfaces that were studied for the adherence test using a permanent marker pen are shown in Table 6. Amongst all the sand/polymer composite sheets, including the compatibilized sand/polymer composite sheets, the sand/polymer composite sheet prepared from 5 μm sand particles at 35 wt% showed relatively better adherence. Stone paper and regular A4 paper showed excellent adherence comparatively.

Table 7 shows the sand/polymer composite sheet surfaces before and after the adherence test using removable marker pen. Only stone paper showed excellent adhesion compared to all the other tested sheets, which is reported in the literature as well [13]. For all the sand/polymer composite sheets prepared, the ink was easily removed with the tape irrespective of the particle size used or compatibilizer added. Moreover, the ink could be wiped out manually by swiping of the hand as well, which could be an advantage for applications that require such kind of erasable surfaces.

#### *3.8. Comparative Analysis of Prepared Sand/Polymer Composite Sheets to Stone Paper*

In this study, several properties varying from thermal to mechanical and wettability properties with several liquids were analyzed. Printing and adhesion tests were also tried on selected composite sheets as well as on the stone paper. To obtain a complete understanding of which sheets resulted in properties closest to the commercial stone paper, a comparative analysis was conducted.

In terms of percent crystallinity (%Xc), the compatibilized sand/polymer composite sheet prepared from 25 μm sand particles at 50 wt% yielded an almost similar value, of 41%, to stone paper, which resulted in 41.03%. With a peak degradation temperature (Td) of 490 ◦C, the sand/polymer composite sheet prepared from 25 μm sand particles at 50 wt% resulted in the closest value to stone paper, which had a peak degradation of 489.5 ◦C. In terms of mechanical properties, the compatibilized sand/polymer composite sheet prepared from 5 μm at 20 wt% resulted in an elastic modulus (E) of 603.54 MPa, whereas stone paper resulted in an elastic modulus of 596.32 MPa. For the tensile strength, with a value of 4.36 MPa, the compatibilized sand/polymer composite sheet prepared from 25 μm sand particles at 50 wt% gave the closest value to stone paper, which had a tensile strength (TS) of 6.17 MPa.

In terms of the wettability performance, the sand/polymer composite sheet prepared from 5 μm sand particles at 20 wt% gave almost the exact same contact angles (θ) with water to stone paper, each yielding value of 105.61◦ and 105.62◦, respectively. The contact angle (θ) with printing ink that was closest to the stone paper (32.5◦) was of the compatibilized sand/polymer composite sheet prepared from 5 μm sand particles at 35 wt% (37.55◦).

The printing performance of the sand/polymer composite sheet prepared from 25 μm sand particles at 35 wt% showed the best results amongst all the tested composite sheets. In terms of the adhesion property using permanent marker, the sand/polymer composite sheets prepared from 5 μm sand particles at 35 wt% showed relatively better adherence compared to the other tested composite sheets. As for the adhesion property using a removable marker, all the tested sand/polymer composite sheets showed the same results. The sheets that resulted in values closest to the stone paper with respect to the analyzed properties are tabulated in Table 8.

Before

After After

**Stone Paper Regular A4 Paper 0 wt% 35 wt%, 25** μ**m 35 wt% + C, 25** μ**m 35 wt%, 5** μ**m 35 wt% + C, 5** μ**m** adherence test with permanent marker pen (sheet surface) adherence test with permanent marker pen (sheet surface) adherence test with permanent marker pen (removed adhesive tape)

**Table 6.** Adherence test using permanent marker pen. **Table 7.** Adherence test using removable marker pen.



**Table 8.** Comparison of sheets resulting in values closest to stone paper.

#### **4. Conclusions**

In this work, sand/polymer composite sheets prepared from sand and HDPE were successfully manufactured via melt blending and compression molding at varying filler compositions. Thermal characterization revealed that the crystallization temperatures remained almost constant at ~113–115 ◦C, and the melting temperatures remained steady at ~132–135 ◦C for all the prepared sand/polymer composite sheets, promoting their good thermal stability. Moreover, no significant degradation (visual or on the bases of weight loss) at the optimized processing conditions were observed. The maximum degradation temperature was almost constant, ranging from ~489–493 ◦C for all the prepared sand polymer composite sheets, and similar crystallization, melting, and degradation temperatures were also obtained for the commercial stone paper, giving the prepared sand/polymer composite sheets decent thermal results. The mechanical characterization of all the sand/polymer composite sheets showed a decrease in their strength, as the elastic modulus values decreased significantly with the addition of the compatibilizer. The wettability analysis suggested that increasing the filler composition, as well as addition of the compatibilizer, led to an increase in the contact angles corresponding to the improved anti-wetting performance. Additionally, in the case of contact angles with printing ink, the observed angles were almost half the value obtained for the pure HDPE, varying in the ranges of 30–45◦, which was also comparable to the stone paper, suggesting good potential for their ink wettability for printing and paper-based applications. The printing test showed that the prepared sand/polymer composite sheets gave comparatively better results than the pure HDPE sheets, implying some potential for printing with filler and compatibilizer addition. The prepared sheets showed good adherence with the use of permanent marker pen, suggesting potential for possible ink-based applications with such kind of ink. Furthermore, the sheets showed weaker adherence with a removable marker pen, but could possibly be used for any application that requires erasable surfaces (such as erasable sheets/boards for student learning, educational toys, etc.). The results obtained in this research provide information about the potential of the production of local sand/polymer composite sheets and their use in paper-based applications.

Key achievements:

• Local sand was used as a filler to develop composite sheets, which were investigated.


**Author Contributions:** S.S.: formal analysis, investigation, resources, data curation, writing—original draft preparation. A.H.A.-M.: conceptualization, visualization, writing—review and editing, project administration, funding acquisition, supervision. M.Z.I.: methodology, software, validation, supervision, resources. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by UAE University, Grant number 21N221.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not required.

**Acknowledgments:** The authors would like to thank the University Innovation Program (UIP'18) of Expo Live 2020 for partially funding this research under Grant Number 21N221.

**Conflicts of Interest:** The authors declare no conflict of interest.
