*3.3. Three-Point Flexural Strength*

Table 5 presents the three-point flexural strength values of the artificial stones (AS-EP and AS-PU), commercial artificial stone, and the pure resins, epoxy, and natural vegetable polyurethane resin.

**Table 5.** Results of three-point flexural strength of the artificial stones developed (AS-EP and AS-PU), the commercial stone, the natural stone, and the pure resins.


The natural PU resin has lower flexural strength (23 MPa) compared to epoxy resin (93 MPa), which may be related to the polyurethane resin degree of polymerization. However, the study of the stoichiometric quantity between the NCO/OH ratio, responsible for the degree of polymerization, which involves the main reaction sites present in the prepolymer and polyol, was not carried out. Reactions with an excess isocyanate may occur along with parallel reactions that harden the material. On the other hand, reactions with an excess of hydroxyls cause the polymer to soften through the reduction in intercrossed bonds [36], which explains why AS-PU obtained a low flexural strength compared to the other tested stones.

In their study, Mileo et al. [37] classified polyurethane as a ductile material on account of the low flexural strength and large deformation without breakage at the maximum applied load. It may presuppose an inadequate cure or the bubbles excess caused by both the presence of moisture and a plasticizing effect. This validates the natural polyurethane non-rupture, the AS-PU at the maximum applied load, and the appearance of bubbles.

Figure 4 shows typical flexural stress x strain curves, obtained from three-point flexural tests. Comparing the behavior of the two polymeric resins and of the developed artificial stones, it is possible to observe that the addition of load contributed to the material hardening. This is an expected behavior, considering that the incorporation of rigid particles in a polymeric matrix generally increases the material's flexural modulus [4]. Maximum performance can be achieved if the polymer adhesion to the reinforcement is perfect. The stronger the matrix/particle interface, the better the mechanical properties of the developed artificial stone [38].

of the developed artificial stone [38].

(b)

of the developed artificial stone [38].

appearance of bubbles.

80

100

appearance of bubbles.

**Figure 4.** Mechanical behavior with 85% by weight of quartz residue, epoxy resin and natural polyurethane resin (AS-EP and AS-PU), natural stone commercially called "Cristallo" (NS), "Branco Aldan" commercial stone (CS), epoxy resin (EP), and polyurethane resin (PU), in three-point **Figure 4.** Mechanical behavior with 85% by weight of quartz residue, epoxy resin and natural polyurethane resin (AS-EP and AS-PU), natural stone commercially called "Cristallo" (NS), "Branco Aldan" commercial stone (CS), epoxy resin (EP), and polyurethane resin (PU), in three-point flexural strength tests. flexural strength tests. Figure 5 shows the confidence interval (average ± standard error) for the artificial

caused by both the presence of moisture and a plasticizing effect. This validates the natural polyurethane non-rupture, the AS-PU at the maximum applied load, and the

*Sustainability* **2022**, *14*, x FOR PEER REVIEW 10 of 18

flexural tests. Comparing the behavior of the two polymeric resins and of the developed artificial stones, it is possible to observe that the addition of load contributed to the material hardening. This is an expected behavior, considering that the incorporation of rigid particles in a polymeric matrix generally increases the material's flexural modulus [4]. Maximum performance can be achieved if the polymer adhesion to the reinforcement is perfect. The stronger the matrix/particle interface, the better the mechanical properties

Figure 4 shows typical flexural stress x strain curves, obtained from three-point

caused by both the presence of moisture and a plasticizing effect. This validates the

natural polyurethane non-rupture, the AS-PU at the maximum applied load, and the

flexural tests. Comparing the behavior of the two polymeric resins and of the developed

artificial stones, it is possible to observe that the addition of load contributed to the

material hardening. This is an expected behavior, considering that the incorporation of

rigid particles in a polymeric matrix generally increases the material's flexural modulus

[4]. Maximum performance can be achieved if the polymer adhesion to the reinforcement

is perfect. The stronger the matrix/particle interface, the better the mechanical properties

Figure 4 shows typical flexural stress x strain curves, obtained from three-point

flexural strength tests. Figure 5 shows the confidence interval (average ± standard error) for the artificial stone flexural strength test of artificial, natural, and commercial stones. stone flexural strength test of artificial, natural, and commercial stones.

**Figure 5.** Confidence interval (average ± standard error) of the three-point stone flexural strength test.

**Figure 5.** Confidence interval (average ± standard error) of the three-point stone flexural strength test. **Figure 5.** Confidence interval (average ± standard error) of the three-point stone flexural strength test. The ABNT/NBR-15844 Brazilian standard [28] stipulates that granite stones for The ABNT/NBR-15844 Brazilian standard [28] stipulates that granite stones for coating applications must have at least 10 MPa from the three-point flexural strength test. For coating applications, the ASTM C880 [39] standard indicates that, for granites and marble, the flexural strength must be ≥8.7 MPa, and the NBR 15845 standard [25] de-

coating applications must have at least 10 MPa from the three-point flexural strength test. For coating applications, the ASTM C880 [39] standard indicates that, for granites and marble, the flexural strength must be ≥8.7 MPa, and the NBR 15845 standard [25] demands

coating applications must have at least 10 MPa from the three-point flexural strength test.

marble, the flexural strength must be ≥8.7 MPa, and the NBR 15845 standard [25] demands

mands ≥10 MPa. Chiodi and Rodriguez [27] reported that ornamental stones with flexural strength above 20 MPa are classified as high quality for coating applications.

The AS-PU, despite the 10.7 MPa flexural strength, according to the referred standards and its specifications, can be used for civil construction. The "Branco Aldan" artificial commercial stone obtained better flexural strength, which can be attributed to the industrial and continuous manufacturing process as well as the type of additive, the granulometry, and the material used as aggregate, quartz powder, which is one of the hardest minerals (Mohs = 7).

Carvalho et al. [8] and Gomes et al. [9] developed artificial stone based in (quarry and red ceramic) wastes and epoxy resin with flexural strength values of 30 and 32 MPa, very close to those obtained in the AS-EP, of the same matrix, evidencing the influence of the polymeric binder in the final composite strength.

Gomes et al. [20] manufactured artificial stones using 85% granite particles and 15% polyurethane resin derived from castor oil, using the same methodology. The result was an 18 MPa maximum flexural strength, which was higher than AS-PU, possibly because this artificial stone may not have achieved a satisfactory cure.
