The search for sustainable solutions is visible in all sectors of human activity [1
]. It is increasingly recurrent for contemporary society to consider environmental issues in the developments in science and technology. The concern about the preservation of natural resources—as a consequence of the awareness of its finitude—makes itself present in several fields of academic research. Studies lead to new polymeric materials, which can provide a sustainable destination for synthetic waste with which nature cannot address [3
]. Transforming large scale waste into a commodity product would deal with both environmental and economic issues [7
]. In addition, upcycling materials can be more sustainable than recycling them [9
]. The construction industry is the second largest market of polymers as raw material [11
]. The numbers point out that 40%–75% of natural resources and 40%–44% of the energy produced on the planet are consumed by this activity [12
]. This could be an interesting field to develop solutions using recycled polymeric materials, as Conroy and co-workers noticed in 2006 [15
Regarding the Brazilian market, high density polyethylene (HDPE) is one of the most widely used polyolefins, and represents the biggest volume (37%) of urban plastic waste [16
]. In addition, sugarcane bagasse (SCB) is one of the many natural fibres considered a large-scale waste; 90% of this material is used in very primary ways (livestock feed, or fuel for thermoelectric generators). If ever used as input for a building material, this same fibre would gain more valuable applications. In a former work, our research group tried to develop a synthetic paper, using virgin HDPE and SCB [17
]. By that time, we noticed the high compressive resistance of those composites. Later, we developed another material using a different fibre (which resulted in inert filler) [18
]. From this point of view arose the idea of using recycled HDPE (rHDPE) and SCB to create a reinforced material for the construction industry. The choice for recycled HDPE led us to find in literature other interesting information about the use of this polyolefin as building material—more specifically, as a material resisting to compressive strength. As stated by Bozorg-Haddad and co-workers when studying compressive strength and creep of polymeric piling made of recycled HDPE [21
], in compressive loading, the cross-section of polymeric materials increases; consequently, the stress tends to decrease. Even if not used for structural purposes, a brick made of this polymer (HDPE with SCB) would tend to become tougher over time. This improvement on the mechanical properties of the brick would be obtained just due to the nature of the creep behaviour of HDPE, without further chemical or physical treatments in the polymer. Therefore, the aim of this work was to assess the feasibility of using rHDPE/SCB composites as material for masonry bricks.
3. Results and Discussion
The SEM images of the composites (Figure 1
) showed a fine dispersion between polymer and filler. We saw the same good dispersion between HDPE and SCB in a former work [17
]; we ascribed it to the very fine fibres—which passed through the #100 mesh sieve—equivalent to 55% of the total SCB used in the composites. The images showed that most of the SCB particles were fractured instead of pulled (Figure 2
). This can be taken as an indication of good adhesion between the materials. We believe that the presence of lignin in the SCB acted as coupling agent between polymer and filler [23
]. Considering that we did not use any delignification process (NaOh, H2SO4) or compatibilizing agent—as is usual in natural fibre reinforced composites (NFRC) literature—the interface resulted adequately for the intended purpose. The SEM images on Figure 1
and Figure 2
show broken fibres in the cryogenically fractured section, denoting a strong polymer/filler interaction. The increase in the filler content tends to decrease the adhesion—shown by some voids in the polymeric matrix in Figure 1
c. Most of them were caused by pulled fibres that were positioned parallel to the fracture section. As the intended application is compressive stress, this pulling effect on the SCB fibres will not be a problem.
and Figure 4
and Table 1
show the Tonset
and the residue content of the composites. As expected, the 100–0 composite showed single-step degradation, leaving 1% residue. The rest of the materials showed three different steps of mass loss due to the presence of SCB, as seen in a former work [17
]. According to literature, the composition of sugarcane bagasse is mainly cellulose (50%), hemicellulose (25%) and lignin (25%) [25
]. The first step of mass loss—until 295 °C—was ascribed to the hemicellulose present in the fibres. The second step (295–380 °C) was probably linked to cellulose mass loss. In addition, the third step was ascribed to the degradation of lignin; this last step caused a slight decrease (2%) in the Tonset
of the rHDPE. These steps were better seen through the changes in the DTG curves, where the Tmax
peaks usually show up. The first one was noticed by a slight “shoulder”—around 290 °C—which became more well defined as the filler content increased; the second one was around 347 °C, and also became sharper with more SCB in the composite. Despite the general decrease in the Tonset
of the neat rHDPE, the materials started to lose mass after the average processing temperature of HDPE (around 200 °C).
a–d shows the DSC curves of the materials. Considering the second heating cycle (Table 2
and Figure 6
), there was no significant variation in the Tm
and the Tc
of the rHDPE in the composites (133 and 120 °C, respectively). This could mean that there was no change in the average size of the crystals of the polymer. The Xc
increased non-linearly, raising 18% in the 80–20 composite, 11% in the 70–30 and 20% in the 60–40. The anomalous behaviour of 70–30 can be explained as a consequence of the very small mass of the tested sample in this kind of analysis (5 to 7 mg). There is also the fact that polymeric composites filled with natural fibres are not homogeneous materials; despite of the efficiency of the compounding process, each separate small portion of the material does not have the exact polymer/filler ratio desired.
decreased according to the filler content, showing a progressive interference of the filler in the crystalline packing of the polymer. As seen in former works [17
], we assume that some transcrystallization has happened because the Xc
raised, while the Tc
remained the same. This phenomenon happens under homogeneous nucleation, and in composites can be considered a separate case of crystallization (heterogeneous nucleation induced by the fibre surface) [28
]. The increase in the Xc
of the polymer is probably related to the transcrystallization furthered by the SCB over regions of uncrystallized polymer in the matrix.
According to ASTM D695—Standard Test Method for Compressive Properties of Rigid Plastics [22
], section A1.3: “In the case of a material that does not exhibit any linear region (…), any attempt to use the tangent through the inflection point as a basis for determination of an offset yield point may result in unacceptable error”(page 6). As the compressive strength curves of our materials did not exhibit any linear region (Figure 7
), we calculated the secant compressive moduli (Ecs
) of each composite (Table 3
). The results were calculated using the median curve of seven tested specimens because, this way, we obtained the moduli directly from an actual experimental curve—instead of plotting a theoretical curve and applying the standard deviation of the seven specimens over it. With 20% of SCB, the Ecs
of the polymer increased 46%, raised a little less (37%) with 30% SCB and reached 63% of increase with 40% SCB. According to the Brazilian standard for masonry bricks [30
], the values of Ecs
for all materials (between 5 and 15 MPa) were enough to make closing wall (non-structural) bricks.
In the rheological measurements, we initially performed the deformation variation tests of the composites, in order to determine the strain interval of the linear viscoelasticity. The superimposition of the moduli versus deformation curves showed that, for all composites, the 1% strain was inside the linear viscoelastic plateau. Then, we made the frequency variation (fv
) tests for all materials. The data obtained—angular frequency and module at the crossover point—are shown in Figure 8
. The composites showed an increase in the modulus at the crossover point, showing that the presence of SCB in the polymeric matrix has raised the resistance to deformation of the rHDPE. The crossover point—point where the values of elastic and viscous contributions of the material are equal—can show variations in the molecular weight and molecular weight distribution of polymers; and may be used as an indication of the relaxation time of a material. We can make an analogy between the changes in the crossover point of a neat polymer and composites of the same polymer, in an attempt to understand variations of the melt flow behaviour of the materials [31
]. The values of G’ and G” at the crossover point were shifted left and up, increasing 34% (80–20), 91% (70–30) and 240% (60–40). Compared to the neat polymer, this change to the left can be interpreted as an increase in the hydrodynamic volume of the SCB particles covered by the polymer. In addition, the variation to higher values could mean the raise in the homogeneity of the material as a whole (rHDPE+SCB), as well as more homogeneity in the SCB fibres. The shearing that occurred in the material may have furthered this improvement on the uniformity of the composites, especially on the 60–40 one—where the modulus at the crossover point raised 240%.
The overlay of the complex viscosity curves (η*) of the composites (Figure 9
) confirms the trends shown by the results of fv
curves. The viscosity rose with the increase of SCB content, and decreased with the increase of frequency, showing the pseudoplastic behaviour of the materials. The presence of fibres has changed the average flow of the rHDPE, and restrained the mobility of the molten polymer. The composites became more resistant to deformation than the neat polymer in the melt state. According to Lozano et al. [32
], the decrease in viscosity at higher frequencies can be a plus because it means that it will probably not be a problem to process these composites at high shear rates (average processing conditions of polyolefin).