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Cellulose (2018) 25:1353–1364
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surface of the fibres due to the enrichment with –OH groups (He et al. 2003). Indeed, it has been docu- mented that hydrogen bonding is responsible not only for the crystallinity of cellulose, but also for tensile strength (Chu 1981). Let us note that, even in the sample containing zero valent silver NPs (PZAg 0 ), the presence of Ag ? cannot be excluded as the sonication process used for silver reduction proceeded with high but limited efficiency. In the paper sample PZAg ? , which was not eluted, the crystallites of silver oxide are still present. However, it has to be taken into account that the conclusions drawn from the results of the measurements of mechanical properties are not very firm, due to high experimental error. The results of paper mechanical resistance are consistent with paper acidity (compare Figs. 4, 5). This means that paper acidity can be correlated with cellulose fibre strength and used as a gauge for predicting mechanical properties. In brief, paper sample PAg 0 , containing silver NPs, demonstrated both much lower pH and breaking strength than the final material prepared by us containing silver-ex- changed zeolite PZAg ? _EDTA. It can also be noted from the comparison of the PZAg ? andPZAg ? _EDTA samples that washing with EDTA improves both the acidity and mechanical properties of the modified paper material. The degree of polymerisation (DP w ) of the initial unaged and aged samples was assessed based on the SEC measurements of molecular mass distribution of cellulose. When comparing the unaged samples, all the initial values were similar and fell in the range 1200-1400 DP w , except the unaged sample containing zeolite PZAg ? , for which DP w was about 1070. An important observation is also that the DP w of the final sample (PZAg ? _EDTA) is the highest among the studied samples – even up to around 100 units higher than for the reference paper sample (see Fig. 6). The same observation can be made for the initial PAg 0 paper containing silver NPs. Upon thermal ageing, the decrease in DP w values can be noted for all samples. The lowest degradation was measured for P samples, and there is almost no degradation at all for the pure zeolite-enriched paper sample PZ0. The highest drop in DP w values was shown by the PAg ? and PAg 0 samples with silver additives, indicating highly degraded cellulose chains of the 200 DP w value. There is a noteworthy difference between the PAg 0 sample with silver NPs and the final sample of
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PZAg ? _EDTA, which confirms the conclusion about the positive influence of faujasite exchanged with silver on the condition of both cellulose chains and fibres in the paper material. Fig. 6 Degree of polymerisation (DP w ) of cellulose in initial paper materials (white bars) and ageing for 12 days (indicated by grey bars)
Visual properties
The parameter L* from the CIELab colour model was used as the measure of brightness—L* with a range from 0 (black) to 100 (white). The total colour change ( D E) was calculated from the L*, a*, b* values following the equation described in (Gulrajani 2010). For the initial samples, regardless of the filler composition, no colour differences were noted among any sample containing silver, by either colour change indicator (L* and D E) in comparison with the refer- ence paper sample P. The results presenting the differences that occurred after 2 months of storage at
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Fig. 7 The brightness of the paper samples after 3 months of natural aging
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