Univerzita Pardubice, fakulta chemicko-technologická University of Pardubice, Faculty of Chemical Technology Katedra dřeva, celulózy a papíru Department of Wood, Pulp and PaperDegradation and destruction of cellulose– a principal reason of aging of cellulosic materials NEW KNOWLEDGE OF CELLULOSE AGING DETERMINATIONS BY HELP OF METHOD UV-VIS SPECTROSCOPY IN CADOXEN SOLUTIONS
Milichovská Svatava, Milichovský Miloslav
VII. International Symposium Selected Processes at the Wood Processing, Banská Štiavnice, 12th – 14th September 2007
– well known
– a little information
As typical,due to these reactions cellulose and cellulosic materials are getting more fragile and yellow.
Nitroxide-mediated oxidation, in which dimeric nitroxide is applied in acidific aqueous media, is well known method to oxidise cellulose to get PAGA (1 4)-linked β-D-glucuronan (C-6-oxycellulose).
Schematic representation of chemical reactions taking place during PAGA preparation
Selectively oxidized cellulose, PAGA
Destructed products of PAGA and cellulose
We found during PAGA preparation (Milichovsky M., Sopuch T., Richter J.:Depolymerization during nitroxide-mediated oxidation of native cellulose. JAPS, Aug.30,2007, Published Online by A Wiley Company, DOI 10.1002/app.24540) that the depolymerization of cellulose during its nitroxide-mediated oxidation can be described very well by model
whereas an oxidation by simple model for first order kinetics.
Oriented domain of cellulose during PAGA preparation
Repulsive hydration forces
Chain of cellulose
Attractive hydration forcesWe can introduce schematically the general DP-peeling model of acid hydrolysis of native cellulose as follows
Oriented fragment of cellulose during PAGA preparation
Degraded chain of cellulose
da < dbAcid hydrolysis of native cellulose
All these results are based on measurements of carboxylic groups content by use of classical titrimetric method and DPmeasuring in cadoxen solutions. Correctness of DP measurement by viscosimetric method in cadoxen solution both for the cellulose and the oxycellulose was confirmed by proportionality according to a distribution of their molecules (see Figure), i.e.M.c = ∑ Mi . ci; c = ∑ ci i ior DP = ∑ DPi . xi ; ∑ xi = 1 i iwhere DP = M/ Mmonomer and DPi = Mi/ Mmonomer are degree of polymerizations of the average polymer and the polymer with the molecular weight Mi and the mass concentration xi (w/w), respectively.
However, OC prepared by oxidation of naturally cellulose is a mixture of PAGA (poly 1,4 β-D- anhydroglucuronic acid) and other products initiating destruction of PAGA having a different supramolecular structure in oriented and amorphous part of cell-wall.
There are defined new qualitative parameters:
PDP = (DPH-DP)/(DP) where
DP – polymerisation degree of OC sample measured by viscose cadoxen – method;
DPH – calculated by use of Eq. DPH = (DP-xm)/(1-xm); xm(0;1);IfPDP = 0 then the polydispersity of the DPis monodispersive
DSPAGA = (3,955. xCOOH - 0,91752. xGA)/ (1 – xGA);
xCOOH concentration of COOH groups in OC measured by titrimetric method
With the shorter oxidation time:
PDP - value for characterisation of OC polydispersity gets drop, i.e. OC gets more
XDS - value of a share of COOH groups in PAGA gets higher, i.e. more COOH groups
are bounded on PAGA molecules.
xGA – content of free glucuronic acid in OC gets fall, i.e. the OC destruction is lower.
xGA-PAGA – content of destabilizing components in OC gets drop dramatically. As will
be shown below, a decrease of this parameter in the OC leads to the longer stability
during its storage.
Knowledge of the carboxylic acid content in OC and the DP value are
insufficient for the determination of OC properties, as it is shown.
Comment of results:
OC sample No. A, having the highest content of destabilising components (1, 2 mmolGA-
PAGA/g OC) on the start watching (after preparation), shows very low stability. After storage 2 months at 40 oC, appearance of sample changed from the white elastic gauze to the yellow
fragile material giving yellow powder during its manipulation. The content of destabilising
components and the content of free glucuronic acid grew rapidly.
are formed by reaction between PAGA and free GA contained in the OC sample (see Figures). It was proved that this
reaction is equimolecular.
The GA-PAGA substance represents an intermediate following up by their finally destruction. We have not identified yet the substance or a kind of substances GA-PAGA. However, as it is documented in Figure below, the origin of GA-PAGA is initiated as soon as GA concentration in OC achieves a critical concentration of GA in the sample, xGAkritic = 1,4 mmol GA/g of OC.
Formation of nitrogen oxides in atmosphere (Tropospheric oxidants)(Becker K.H.: Atmospheric Pollution by Photooxidants over Europe. 5-th CEC-European Symposium Proceedings, 1993)
It is well known that
irradiation of air + VOC + NOx → O3
However, at constant VOC with increasing NOx the irradiation leads to O3 maximum and above a NOx –limit concentration suddenly decreases.
NO2 + hν → NO + O. at wavelengths below 410 nm
O + O2 → O3
Reverse reaction in excess of O3
NO + O3 → NO2 + O2
Formation of “excess ozone”
by transport from areas of high O3 concentration
by efficient VOC oxidation via an OH. radical chain in presence of NO2 where NO is oxidized to NO2 by RO2. or HO2. radicals and not by ozone.
RO2. + NO → RO. + NO2
HO2. + NO → OH. + NO2
Termination competing reaction (important is peroxyacetyl radical)
RO2. + NO2 → RO2NO2 - formation of peroxynitrates
RO2. + RO2. → ROOR + O2- formation of peroxycompounds
RO2NO2→ NO2 + RO2. - formation of hydroperoxydes and organic
RO2. + H2O → ROOH + OH.acids athigh humidity, particuraly in forest areas
Decrease of NO2 by competing reactions
NO2 + OH. → HNO3
NO2 + NO2 → N2O4- selective oxidation agents of cellulose to PAGA
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