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Cellulose (2018) 25:1353–1364
Table 2 XPS results of silver washing efficiency from AgFAU (PZAg ? ) EDTA concentration, mol/dm 3 Portions of eluent, cm 3
XPS signal after spectra normalisation, a.u.
1st run
2nd run
–
–
–
1.05 ± 0.05
150 H 2 O
150 H 2 O 150 H 2 O 150 H 2 O 150 H 2 O
1.19 ± 0.06
–
0.01
150 EDTA
1.25 ± 0.07
0.1
150 EDTA
0.29 ± 0.23
0.1
300 EDTA
0.31 ± 0.02
The pH of paper
Corresponding with analysis of eluents, the amount of silver at the surface stabilises after treatment with the EDTA solution of higher concentration ( [ 0.01 M), and is around four times lower than for the initial sample. Neither the 0.01 M EDTA nor the water were able to remove external silver oxide particles from the zeolite samples. The remaining silver will be called stable silver attached to Al–O- groups in zeolite, impossible to remove by treatment with a chelating agent. The content of silver present in the paper samples is shown in Table 1. For the final PZAg ? _EDTA, the amount of silver was measured as 1.5 wt%. It can be noted that, due to the washing procedure, the silver content in the PZAg ? _EDTA sample was 27% lower (1.5 wt% drops to 1.1 wt%) than in the PZAg ? untreated sample. In the PAg ? and the PAg 0 control samples, Ag content was from two to three times lower compared to the paper modified with the zeolite. The silver content in the paper was not controlled other than by the affinity of different additives (silver cations, silver NPs or AgFAU particles) to cellulose fibres, which varies from sample to sample. As can be inferred from the optical images of the final paper sample PZAg ? _EDTA, the zeolite filler dispersed evenly within the paper pulp (Fig. 2a, b). The zeolite particles can be noted as black spots in the transmission image of the sample (Fig. 2a). What is more, SEM analysis confirmed that zeolite is presented on the cellulose fibres. The back-scattered electron mode allowed silver-exchanged zeolite to be shown as brighter spots (Fig. 3). In the picture, there are also marked (as numbers) places where EDX analysis was performed. The average values of element content (Table 3) confirm the presence of elements typical for zeolite (Al, Si, O), as well as for silver which was introduced to zeolite.
The pH values (Fig. 4) of the paper samples contain- ing silver in any form were lower than for the reference samples without silver; for P and PZ0, pH amounted to 8.8 ± 0.25, 9.4 ± 0.08, respectively. The lowest value was observed for sample PAg ? (4.7 ± 0.11). For samples which contained zeolites, both unmodi- fied and modified, the pH values were higher, 5.9 ± 0.03; 7.00 ± 0.04, respectively. Although, the addition of zeolite with trapped silver causes a decrease in the pH value, the decrease is much lower than for materials in which the active agent was presented in unbound form. It seems that the higher acidity of paper material designed for packaging is not an obstacle. In fact, in the long run the acidity of paper is responsible for the loss of the mechanical strength of paper or cellulose-based materials caused by hydrol- ysis of cellulose fibres (Chu 1981; Łojewski et al. 2010). Low pH of cellulose-based materials may result from, amongst other things, the manufacturing pro- cess, the presence of acids in the raw materials, or as products of cellulose degradation. In the presence of water molecules (atmospheric moisture, bound water) an acid hydrolysis reaction occurs. Therefore, hydro- lytic cleavage of b (1-4) glycosidic bonds leads to shortening of cellulose chains. This process will be reflected in the consecutive decrease of average DP value, and possibly in loss of mechanical strength of cellulose fibres.
Mechanical properties and DP
According to the results of zero-span breaking strength of the paper samples, of all tested materials the PZAg ? _EDTA was around 20% stronger than unmod- ified paper, and stronger than the rest of the samples
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