Polymers 2022 , 14 , 3309
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a small natural tendency for aggregation. The reference synthetic commercial retention agent (CPAM) presents a curve distinct from those using CCs. CPAM resulted in the best performance for dosages below 5 mg/g of PCC, also showing the fastest flocculation kinetics up to a dosage of 2 mg/g of PCC (16.5 μ m), from which the size of the flocs starts to stabilize, leveling off at ca. 40 μ m. These results indicate that a dosage of CPAM above 2 or 3 mg/g of PCC does not bring benefits in terms of flocculation. With the addition of the original BEKP, no changes were observed in the flocculation of PCC, with the flocs achieving a size of 5.6 μ m after the final addition of 15 mg/g of PCC (as with the PCC suspension alone). Similar results were detected for the samples CH0.08, GT1.06_P and GT0.36_FP, with flocs reaching a final size of 6.0, 5.7 and 5.3 μ m, respectively. Samples GT0.02_F, GT0.32 and CH0.16 resulted in floc sizes that were slightly bigger than those of the PCC without a retention agent, with the final sizes ranging between 6.6 and 7.4 μ m. However, with samples GT0.04_F and GT0.16_F, an increase in flocculation perfor- mance was observed, resulting in a final floc size of 11 and 20 μ m, respectively; sample CH0.13_F led to the best flocculation performance, exhibiting an average d 50 of 554 μ m. Contrary to the other samples, the increased tendency of this latter sample to promote PCC flocculation also resulted in a high standard deviation of d 50 , clearly visible after an addition level of 5 mg/g of PCC and is indicative of more heterogeneous flocculation. Comparing the curves obtained for the distinct cellulose samples and considering their morphological and chemical properties, some major tendencies can be derived. Grouping the samples composed of intact cellulose fibers (BEKP, CH0.08 and CH0.16), it is possible to observe a small increase in flocculation performance with an increasing level of cation- ization. Sample GT0.32, wherein the cellulose fibers start showing signs of degradation, led to a slight decrease in the average size of the flocs. This behavior is an indication that the cationic charge of the fibers (with a DS of 0.32) is becoming excessive (compared to CH0.16, with a DS of 0.16) and, since the PCC used in this study presents a slightly positive charge, this can lead to charge repulsion. An increase in cationization (GT1.06_P) resulted in the complete absence of flocculation, further strengthening the hypothesis of excessive cationicity. The positive effect of HPH treatment and the difference between the two distinct cation- ization methods can be observed by comparing sample CH0.16 with samples GT0.16_F and CH0.13_F. With the fibrillation of the cellulose fibers and the subsequent increase in the available specific surface area, the final d 50 increased by ca. 3 times for GT0.16_F and ca. 75 times for CH0.13_F. Although both samples present a similar DS and were subjected to the same HPH treatment, they led to very distinct flocculation performances. Such behavior can be explained by the length difference of the obtained fibrils, with sample GT0.16_F presenting a more degraded structure and, even, partially solubilized cellulose, a consequence of the harsher cationization reaction, namely, the sodium periodate oxidation that leads to the opening of the glucose rings, as explained elsewhere [23]. The fibrillated samples obtained by cationization with GT presented an increased flocculation behavior from sample GT0.02_F up to sample GT0.16_F with higher DS. An increase in DS up to 0.36 (GT0.36_FP) resulted in the complete solubilization of the sample and complete loss of the flocculation potential. The two best-performing CCs (CH0.13_F and GT0.16_F) and CPAM were subjected to further flocculation tests. The growth of the PCC flocs was recorded over time (15 min) for a fixed dosage of the retention agent (1, 2, 4, 8 and 10 mg/g of PCC), added all at once at 0 min. The obtained flocculation curves are plotted in Figure 6.
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