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Milichovská Svatava, Milichovský MiloslavVII. International Symposium Selected Processes at the Wood Processing,

Banská Štiavnice, 12th – 14th September 2007

Univerzita Pardubice, fakulta chemicko-technologická University of Pardubice, Faculty of Chemical Technology Katedra dřeva, celulózy a papíru Department of Wood, Pulp and Paper

Degradation 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

Cellulose aging is evoked by

• Acidific and enzymatic hydrolysis

– well known

• Chemical and photochemical oxidation

– a little information

• Combination of both reactionsAs 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

Cellulose

Selectively oxidized cellulose, PAGA

Degraded PAGA

Oxidation

Depolymerization

Destructed products of PAGA and cellulose

Acid hydrolysis

Destruction

N2O4

We found (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

• called as general DP-peeling model in mathematical form,

c• DP = 1/ (a + b.t )

whereas an oxidation by simple model for first order kinetics.

d[CelOH] d[CelOOH]

• _ ------------ = -------------- = kCOOH . [CelOH] ,

dt dt

We can introduce schematically the general DP-peeling model of acid hydrolysis of native cellulose as follows

• Before degradation

Chain of cellulose

Repulsive hydration forces

Attractive hydration forces

db

Oriented domain    of cellulose

Acid hydrolysis of native cellulose

• After degradation a cellulosic fibre is more rigid because increased concentration of oriented crystalline fragments of cellulose

Degraded chain of cellulose

da

da < db

Oriented fragment of           cellulose

All these results are based on measurements of carboxylic groups content by use of classical titrimetric method and DP measuring 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 i or

DP = ∑ DPi . xi ; ∑ xi = 1 i i

where DP = M/ Mmonomer and DPi = Mi/ Mmonomer are degree of polymerizations of the average polymer and the polymer with the molecular weight M i and the mass concentration xi (w/w),

respectively.

y = 0,9898x + 47,865

R2 = 0,9976

0

500

1000

1500

2000

2500

3000

0 500 1000 1500 2000 2500

DPmeasured

DP

calc

ula

ted

• Recently investigations have demonstrated (Evtuguin,D.V.; Daniel A.I.D.; Neto P. Determination of Hexenuronic Acid and Residual Lignin in Pulps by UV Spectroscopy in Cadoxen Solutions.

Journal of Pulp and Paper Science 2002; 28: 189-192.) that UV-VIS spectroscopy of cadoxen solutions are a potentially useful method, especially in the range of wavelength 210-350 nm.

• Moreover, other substances appearing during of PAGA aging and indicating the PAGA destruction it is possible to observe by UV-VIS spectroscopy of cadoxen solution at wavelength band 350-450 nm.

• By connection all of these measurements, i.e. COOH groups content and viscosimetric measurements with UV-VIS spectroscopy of cadoxen solution, we have received a new method for analysing OC giving the new and more detailed characteristics of OC (Milichovsky M., Milichovska Sv.: Characterization of Oxidized

Cellulose by using UV-VIS Spectroscopy. JAPS, prepared for publication) .

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.

New qualitative parameters and their determination

There are defined new qualitative parameters:

• Characterisation of polydispersity in OC samples by help of parameter – PDP 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); If PDP = 0 then the polydispersity of the DP is monodispersive

• Content of free glucuronic acid in the OC - xGA ; calculated by use of spectrophotometric data (see Fig.)

• Degree of substitution COOH groups in OC and share of COOH groups in PAGA - DSPAGA DSPAGA = (3,955. xCOOH - 0,91752. xGA)/ (1 – xGA); xCOOH concentration of COOH groups in OC measured by titrimetric method

• Content of destabilising components in OC - xGA-PAGA ; measured by use of UV-VIS spectroscopy (see Fig.) The absorption band with maximum at λmax = 396 nm is the most important result of UV-VIS spectrophotometrical measurement, because an appearance of this one is characteristic for a non-stabile OC sample. It is appeared only if OC is composed by components initiating its destabilization and finishing as well as by their destruction

UV VIS spectra of the OC samples cadoxen solutions – the different times of a cellulose oxidation

0

0,5

1

1,5

2

2,5

200 250 300 350 400 450 500

Wavelength, nm

Ab

so

rba

nc

e

Sample no.A - reaction time = 45hoursSample no.B - reaction time = 24hoursSample no.C - reaction time = 17hours

GA

Glucose

PAGA-GA

Qualitative parameters of the OC samples prepared in the different times of oxidation

OC, sample No.

Time of oxidation, hours DP PDP xCOOH,% XDS, %

xGA,

mmolGA/g OC

xGA-PAGA,

mmol GA-PAGA/g OC

A 45 22.5 2.0 17.7 30.6 3.5 1.2

B 24 27.3 0.6 17 66.9 2.6 0.4

C 17 20.4 0.5 17.7 78.7 1.9 0.1

With the shorter oxidation time:PDP - value for characterisation of OC polydispersity gets drop, i.e. OC gets more

      homogenously. 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.

Aging of oxidised cellulose during its storage at 40oC

OC, sample No.

Time of oxidation, hours

Measured after

Storage at 40o C

Appearance of sample

Colour of sample DP PDP

xCOOH,

% XDS, %

xGA,

mmolGA/g OC

xGA-PAGA, mmol GA-PAGA/g

OC

A 45 Prep.   elastic gauze white 22.5 2.0 17.7 30.6 3.5 1.2

A 45   2 months powder yellow    -   - 5.5 2.1

B 24 Prep.   elastic gauze white 27.3 0.6 17 66.9 2.6 0.4

B 24   2 monthsloss elasticity, fragile slight yellow   3.7 - 4.1 1.5

B 24   3 months powder yellow   4.5 - 4.2 1.6

C 17 Prep.   elastic gauze white 20.4 0.5 17.7 78.7 1.9 0.1

C 17   2 months elastic gauze white   1.4   49 3.1 0.6

C 17   3 months elastic gauze slight yellow   1.4   50.9 3.1 0.7

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.

Non-stabile substances GA-PAGA (starting the PAGA destruction)

are formed by reaction between PAGA and free GA contained in the OC sample (see Figures). It was proved that this

reaction is equimolecular.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

-0,0012 -0,0002 0,0008 0,0018 0,0028 0,0038 0,0048 0,0058

mol GA/ g of PAGA

Abs

orba

nce

DP=22.5 DP=65.3 DP=46,6

Eo(λ=396 nm)cGA-PAGA

y = 559.6x

R2 = 0.9521

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0 0,0005 0,001

mol GA-PAGA/ g of OC

Abs

orba

nce

Non-stabile substances GA-PAGA (starting the PAGA destruction) - comment of received results

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.

y = 0,4842x - 0,6786

R2 = 0,8094

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

2

0 1 2 3 4 5 6

mmol GA/g oxycellulose

mm

ol

GA

-PA

GA

/g

Hypothetical mechanism of degradation and destruction of PAGA

Conclusion• Cellulose is easy oxidized in water medium with nitrogendioxide on OC

predominantly PAGA followed up by its degradation and destruction• During PAGA degradation, the GA is formulated firstly followed by further

acceleration of this process due to strong acidific behaviour of GA • Firstly, the cellulose in non-oriented amorphous part of cellulose matter are

reacted followed up step by step by reaction in more and more deeply part of oriented crystallised part of cellulose matter

• At suitable reaction condition, the GA initiates an origin of GA-PAGA intermediate as predecessor of totally PAGA destruction

• Suitable reaction conditions are represented by formation of glucuronic end groups of PAGA reacting further with free GA to form a substance GA-PAGA

• GA-PAGA is formulated by two forms, i.e. by formation of glycosidic and ester O-bonds in heptacyclic and di-ester O-bonds in decacyclic end- structures of PAGA-GA

• Heptacyclic end-structures of PAGA-GA are split off into heptacyclic glucuronic ester structure (HCGE ) and shorter PAGA molecule in the first step followed by destruction of HCGE into low molecular organic compounds at the second step of the destruction reaction

• As typical for all of these cellulose harmful reaction, a dehydration is taking place

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.

Photolysis NO2 + hν → NO + O. at wavelengths below 410 nm

O + O2 → O3

Reverse reaction in excess of O3NO + 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 at high humidity, particuraly in forest areas

Decrease of NO2 by competing reactionsNO2 + OH. → HNO3

NO2 + NO2 → N2O4 - selective oxidation agents of cellulose to PAGA

THANK YOU FOR ATTENTION

University of Pardubice Faculty of Chemical Technology Department of Wood, Pulp and Paper Studentská 95CZ 532 10 PardubiceCzech Republic

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