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A microscopic investigation of the surfaces of
Kraft Pulp papermaking fibres
Morag Weller, Chemistry,
McGill University, Montreal
September 2008
A thesis submitted to McGill University in partial fulfilment of the requirements
of the degree of Masters in Chemistry
© Morag Weller 2008
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Contents
Abstract Page 3
Abstract Page 4
Introduction Page 5
Chapter 1 Page 13
Comments on the interpretation of XPS (ESCA)
spectra of lignocelluslosic surfaces.
Chapter 2 Page 36
An atomic force microscopy investigation into the
physical properties of pulp fibre surfaces.
Chapter 3 Page 51
Transcrsytallisation of polypropylene at surfaces
of unbleached Kraft Pulp fibres.
Conclusion Page 66
Acknowledgements Page 67
References Page 69
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Abstract
To maintain its innovative and competitive edge the forestry sector is
focused on conducting research into more efficient ways of manufacturing
current products and generating new markets for by-products and
technologies. It is the surface of pulp fibres which are of fundamental
importance to the pulp and paper industry. A better understanding of the
chemistry and morphology of Kraft pulp fibres is the primary motivation for
this Master’s thesis study.
Through this work, we will show that simple and straight applications of
newer technologies (X-ray photoelectron spectroscopy, Atomic Force
microscopy, Optical Microscopy and Differential Scanning Calorimetry) could
be employed by the pulp and paper industry to determine the surface
chemistry of Kraft Pulp fibres (and therefore other lignocellulosic fibres). The
effect that variability in the surface composition of such fibres has on
industrial applications is commented on.
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Abstract
Pour maintenir son avantage innovateur et concurrentiel, le secteur de
sylviculture est concentré sur la recherché dans des manières plus efficaces
de produire des produits et des marchés courants pour des sous-produits et
des technologies. C’est l’extérieure des fibres de pâte qui est d’importance
fondamentale pour l’industrie de pâte et papier. Une meilleure
compréhension de la chimie et de la morphologie des fibres de pâte Kraft est
le raisonnement primaire pour cette thèse de maîtrise.
Dons çe recherche nous démontrons que des applications simple et
directes de nouvelles technologies (spectroscopie de photoélectron de rayon
X, microscopie atomique de force, microscopie optique, analyse entalbique
différentiel) peuvent être utilisés par l’industrie de pâte et papier pour
déterminer la composition extérieure des fibres de pâte Kraft (et donc d’autre
fibres lignocellulosique.) L’effect de la variabilité en composition extérieure
sur des applications industrielles sera aussi étudié.
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Introduction
The forestry sector in Canada is valued at $80 billion per annum, with
the pulp and paper industry making up approximately 40%. With the majority
of pulp and paper products destined for export markets it is the largest single
contribution to Canada’s balance of payments. The industry is also one of
the biggest and most high-tech employers in the country. Innovation and
continued development are of utmost importance if the industry is to maintain
its position as a dominant force in the global market. Industry directed
research within Canadian universities on subjects related to Pulp and Paper
is vast and varied, ranging from engineering paper mill solutions to bioactive
paper initiatives.
Cellulose is the most abundant organic substance occurring in nature.
Cellulose imparts the strength to wood and it is the cellulose in pulp fibres
that is the key to papermaking. It is the cellulose that provides the strength of
the individual fibres and it is the cellulose that forms the drying induced
bonding between the fibres in the sheet of paper. The useful properties of
paper result from the mechanical and surface properties of cellulose and the
interactions of cellulose and water. It is the surface of pulp fibres which are of
fundamental importance to the pulp and paper industry and this is the primary
motivation for this study.
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Wood is composed of cellulose, lignin, hemicellulose and extractives.
The relative ratio of the components varies between species and position
within the tree. Cellulose is a linear crystalline polymer of glucose units, as
depicted in Figure 1. It is the main structural component of plants, and the
most important component of papermaking. Lignin is a complex phenolic
cross-linked polymer network based on substituted phenylpropane units
bonded predominantly with ether linkages (Figure 2). Hemicelluloses are
polymers containing two or more sugar units often in the form of acetyl esters
or methyl ethers. They are often branched and are seldom crystalline.
Extractives are a wide variety of small molecules found in small amount in
wood.
Figure 1: Cellulose.
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Figure 2: Structural model of softwood lignin.1
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It is the wall of the fibre that is of particular interest in papermaking, as
this is where the fibre-fibre bonds that create paper occur. The location and
organization of the four wood components within the layer structure of the
wood fibre wall is well known (Figure 3). Cellulose is present as microfibrils of
extended cellulose chains, 10-30 µm in diameter and very long. These
microfibrils are oriented at different angles to the fibre long axis in each of the
fibre layers. Most of the cellulose is in the S2 layer, which makes up 70-90%
of the fibre wall thickness. The microfibrils are arranged spirally at an angle
between 0° and 30° to the fibre axis. In the S1 layer (10-20% of the wall
thickness) the fibril angle is higher, 50° to 70° and in the S3 layer
(approximately 10% of the wall thickness) the angle is even greater, 60° to
90°. However, in the thin primary wall the microfibrils are randomly oriented.
The lignin content of the fibre wall is also spread over the layers. The primary
wall is 70% lignin, although as it is very thin it contains just 10% of the total
lignin content. The S2 layer by contrast consists of 20% lignin but due to its
thickness contains 50% of the total lignin content of the fibre wall. The
hemicelluloses are intimately mixed with the other wood components
throughout the fibre wall and are in the same proportions as the cellulose, i.e.
most in the S2 layer and least in the primary layer. The extractives are found
in the parenchyma, in the resins canals, in the vessels and in the heartwood.
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Figure 3: A generic wood fibre model diagram2 showing the orientation and
location of the main components in the fibre wall.
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As a result of extensive research conducted by the global Pulp and
Paper industry for over a century, a branch of science with its own language
has developed. By many paper-making is still regarded as an art rather than
an exact science.
Due to its abundance in the Boreal Forest and its superior fibre quality
Black Spruce (Picea mariana) is the main wood source in Quebec. It is the
main source of newsprint papermaking fibres. Pulping may be defined as the
treatment of wood by chemical and/or mechanical means to produce a fibrous
material suitable for papermaking. The aim of pulping is to liberate the fibres
from the wood incurring as little damage as possible. The most widely
employed chemical pulping process is Kraft pulping. This is a sulphate
process, in which basic media are used to facilitate the removal of lignin from
the wood chips. H-factor and Kappa number are terms which are often used
to describe pulps.
The Kraft pulping process was invented by a German chemist, Dahl in
1879. The advantage of this process over existing methods was a much
faster delignification; this resulted in stronger pulps as the reduction in
cooking times corresponded to a reduction in carbohydrate degradation. It is
a full chemical pulping method that uses sodium hydroxide and sodium
sulphide in highly basic pH at temperatures of 160°C – 180°C to dissolve the
lignin from the wood.3
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As pulping apparatus and conditions can vary substantially the term H-
factor is employed to compare pulp cooks in a meaningful manner. The H-
factor is a Kraft pulping variable that combines the cooking temperature and
time into a single variable that indicates the extent of the reaction. The rate of
delignification approximately doubles for an increase in temperature of 8°C.
For example a cook of 1.5 hours at 170°C corresponds to 0.75 hours at
178°C or 3 hours at 162°C.3
The Kappa number is a measure of the lignin content of the pulp; the
higher the Kappa number, the higher the lignin content. The measurement is
based on the rapid oxidation of lignin (but not carbohydrate) by acid
permanganate at room temperature. It is defined as the number of millilitres
of 0.1M potassium permanganate consumed by one gram of pulp in 0.5N
sulphuric acid after ten minutes at 25°C under conditions such that one half of
the permanganate remain unreacted.3
Most applications of cellulosic fibres depend on the detailed surface
chemistry and morphology of the soft, often wet lignocellulosic surface.
Electron Spectroscopy for Chemical Analysis (ESCA or XPS) provides a
sensitive way to measure the surface composition of pulp fibres and paper.
Atomic Force Microscopy (AFM) facilitates the measurement of physical
properties on the surface under ambient papermaking conditions.
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X-ray photoelectron spectroscopy (XPS), was first
developed by Siegbahn in 1967.4 The technique of photoelectron
spectroscopy measures the ionisation energies of molecules
when electrons are ejected from different orbitals and uses the
information to infer the orbital energies.
The energy of the incident photon is so great that the electrons are
ejected from the inner cores of the atom. The core ionisation energies are
characteristic of the individual atom rather than the molecule; giving lines
which are characteristic of the element rather than the compound. Whilst it is
largely true that the core ionisation energies are unaffected by bond
formation, small shifts can be detected and interpreted in terms of the
environments of the atoms. The shape of each peak and the binding energy
can be slightly altered by the chemical state of the emitting atom therefore
XPS can provide chemical bonding information.
XPS, also known as Electron Spectroscopy for Chemical Analysis
(ESCA) has been widely applied to the technologically important problem of
assessing the surface composition of lignocellulosic materials. The surface
compositions of solid wood products, pulps, paper and board are critical to
their end use performance. However, the materials are chemically and
morphologically complex and it is difficult to determine the amounts of
cellulose, hemicelluloses, lignin, extractives and additives at the surface of
these materials.
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Atomic force microscopy (AFM) was first introduced by Binning, Quate
and Gerber in 1986.5 Its success has led to the development of a family of
techniques based on the same principle grouped under the name scanning
force microscopy. These techniques have been applied to a variety of
research fields, including pulp and paper research.
One of the advantages to pulp and paper research is the fact that AFM
can be operated in both air and liquid environments. Consequently the
conditions of industrial papermaking can be replicated microscopically in the
laboratory. AFM has been used to study the topography of wood derived
samples, both wet and dry.6-30 The mechanism for AFM also facilitates the
measurement of physical properties on pulp samples.18,20,31-33
Combining the techniques of AFM and XPS to investigate the surface
of newsprint quality Kraft pulp fibres is the focus of this work.
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Chapter 1
Comments on the interpretation of XPS (ESCA) spectra of lignocellulosic
surfaces
INTRODUCTION
X-ray photoelectron spectroscopy (XPS), was first
developed by Siegbahn in 1967.4 The technique of photoelectron
spectroscopy measures the ionisation energies of molecules
when electrons are ejected from different orbitals and uses the
information to infer the orbital energies.
The principle of the conservation of energy states that the
total energy in any system is constant. Therefore, energy wil l be
conserved when a photon ionises a sample; the energy of the
incident photon, must be equal to the sum of the ionisation
energy of the sample and the kinetic energy of the ejected
electron, the photoelectron. An incoming photon carries energy,
a binding energy is required to remove an electron from the
orbital and the difference appears as the kinetic energy of the
electron (Figure 4). As the ejected electron cannot escape
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except from a few nanometres from the surface, this technique is
mainly l imited to the study of surfaces.
Figure 4: Schematic of the principle involved in XPS.
1s
Ene
rgy
Valence
2s
2p
K.E. = hν - Eb
hν
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The energy of the incident photon is so great that the electrons are
ejected from the inner cores of the atom. The core ionisation energies are
characteristic of the individual atom rather than the molecule; giving lines
which are characteristic of the element rather than the compound. Whilst it is
largely true that the core ionisation energies are unaffected by bond
formation, small shifts can be detected and interpreted in terms of the
environments of the atoms. The shape of each peak and the binding energy
can be slightly altered by the chemical state of the emitting atom therefore
XPS can provide chemical bonding information.
Nomenclature Approximate shift relative to C(1s) binding energy of 285 eV
C1
-1 ↔ 1
C2
1 ↔ 2.5
C3
2 ↔ 4
C4
3.5 ↔ 6
Table 1: A list of the chemical shifts that are present within the C (1s) peak.
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For example, the C (1s) peak in the XPS spectra of carbon (Figure 5)
can be broken down into four components, each one representing the
different possible carbon environments, as listed in Table 1. The C (1s) peak
itself is centred at 285eV. Any carbons atoms that are not bonded to any
oxygen atoms (C1) will show up between 284eV and 286eV. A carbon atom
bonded to one oxygen atom (C2) will have a slightly different electronic
configuration compared to that of a carbon atom bonded to other carbon
atoms only. The C2 peak to located between +1eV and +2.5eV of the centre
point. As the number of oxygen atoms bonded to the carbon increases the
corresponding peak moves further up field.34
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Figure 5: A typical C (1s) peak in XPS. The peak has been deconvoluted
into the four different carbon environments.
Binding Energy (eV)
Inte
nsit
y
C1
C2
C3
C4
285
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XPS, also known as Electron Spectroscopy for Chemical Analysis
(ESCA) has been widely applied to the technologically important problem of
assessing the surface composition of lignocellulosic materials. The surface
compositions of solid wood products, pulps, paper and board are critical to
their end use performance. However, the materials are chemically and
morphologically complex and it is difficult to determine the amounts of
cellulose, hemicelluloses, lignin, extractives and additives at the surface of
these materials.
The XPS method on solids gives, in essence the elemental composition
of a thin surface layer.4 An introduction to the method as applied to paper
surfaces was first given by Dorris and Gray.35 They measured the XPS
spectra for filter paper and samples of bleached Kraft and sulphite papers,
and for isolated lignins. The results were interpreted in terms of the ratio of
oxygen atoms to carbon atoms, (No/Nc), in the surface region, and the
observed chemical shifts of the carbon 1s XPS peaks.
Under suitable circumstances, the experimental XPS oxygen-carbon
ratio and the components of the carbon 1s peak after deconvolution35,36 may
be used to estimate the surface composition in terms of the individual wood
components. For example, lignin and especially extractives should have
much lower (No/Nc) values than cellulose, which has 5 oxygen atoms for
every 6 carbon atoms. This approach was quantified by Dorris and Gray to
estimate how much lignin was on the surface of hand-sheets made from
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mechanical pulps.37 This required careful extraction of the sheets to remove
resin and fatty acids from the sheet surface. In principal, XPS can also
quantify the surface extractives content; an attempt using stearic acid as a
model has been reported,34 but in general quantification of extractive
coverage is difficult.
The XPS method has been applied to a range of pulp and paper
samples25,26,38-40, but there have been continuing questions regarding both
the reproducibility of XPS measurements on lignocellulosics, and the validity
of interpretations. Recently, the results of a set of XPS measurements in four
laboratories in Scandinavia and Canada on identical paper samples have
been reported.41 The overall findings were that the experimental results were
in reasonable accord, providing care is taken to avoid or correct for carbon-
rich contaminants on cellulose-rich surfaces. However, methods of
interpretation in terms of surface lignin and extractives used by different
laboratories gave somewhat scattered results.28
The initial interpretation of XPS data in terms of surface composition
involved several problems. In contrast to most previously studied surfaces,
fibrous lignocellulosic surfaces were rough and chemically heterogeneous,
both across the paper surface and in the depth direction. The only XPS
observables were the ratio of oxygen atoms to carbon atoms, and the
deconvolution of the carbon peak into components with different chemical
shifts resulting from the numbers of oxygen atoms that were attached to the
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carbon atoms. Interpretation of this data in terms of the molecular
composition of the surface layer sampled by XPS required a number of
assumptions.
Thus, to interpret the XPS atomic composition data in terms of the
proportion of carbohydrates (cellulose and hemicellulose) and lignin content
of the surface layer, the following assumptions were employed or implied.35
The samples contained only carbohydrate and lignin. Wood extractives were
presumed to have been removed by suitable solvent treatments.
(i) The composition of the analysed volume of material close to the
surface was uniform.
(ii) The polysaccharide component of the surface layer (cellulose +
hemicellulose) was represented by the empirical composition, C6O5 ,
molar mass, 162.1.
(iii) The empirical formula for the lignin component was that for
Freudenberg lignin, namely C9.92O3.32 , with a molar mass of 183.5.
Dorris and Gray chose this model for lignin as it seemed an
appropriate approximation for the lignin in mechanical pulp.
To relate the observed oxygen carbon ratio, No/Nc, to the surface
composition, it was assumed that there were S anhydroglucose or sugar units
and L lignin phenylpropane segments per unit volume in the (uniform) volume
sampled by XPS at the surface of the fibres. Hence, from the empirical
formulae for S and L, the number of oxygen and carbon atoms in this volume
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were No = 5S + 3.32L and Nc = 6S + 9.92L, respectively. The segment mole
fraction of lignin in the surface, SL, was thus L/(L + S). From the empirical
formulae for S and L, the value for SL was derived in terms of the measured
No/Nc.
(1)
The weight fraction of lignin, WL, may be calculated from the segment mole
fraction and the molar masses of the segments:
(2)
A typical value for No/Nc for an extracted thermomechanical pulp sheet
was 0.56, giving a lignin weight fraction in the surface of 0.45, a value that
was somewhat higher than the bulk value.35
Since these early experiments, XPS has been applied to a variety of
lignocellulosic materials. For Kraft pulp fibres, a different way of interpreting
the No/Nc values,42 first suggested by Ström and Carlsson43 has been widely
accepted. For an extracted pulp, the surface coverage of lignin was taken as:
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(3)
where (NO/NC)Pulp sample is the measured oxygen/carbon atom ratio of the pulp
under study, (NO/NC)Lignin-free pulp is the measured oxygen/carbon atom ratio of
an appropriate lignin-free (fully-bleached) pulp measured under the same
conditions as the sample, and (NO/NC)Lignin is the corresponding value for a
sample of lignin. The resultant value for , the surface coverage of lignin,
is a dimensionless quantity that varies linearly with (NO/NC)Pulp sample and is
independent of the oxygen-carbon values for lignin and carbohydrate.
Recently, Li and Reeve44 took issue with the use of Equation 3,
pointing out, correctly, that the assumption of a linear relationship between
lignin content and (NO/NC)Pulp sample cannot be correct for simple algebraic
reasons. They provided a generalized form of the approach of Dorris and
Gray, where appropriate empirical formulae for the lignin and polysaccharide
components of the fibre may be used to estimate the surface lignin. For
carbohydrate formula CmOn and lignin CxOy, the segment mole fraction and
weight fractions of surface lignin given by Li and Reeve are44
(4)
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(5)
where and
This is not just a question of (very real) experimental problems such as
sample contamination or decomposition. The major discrepancy between
surface lignin contents calculated from Eq.2 and from Eq.3 occurs at low
lignin contents44 precisely the region where the effects of sample
contamination or degradation are most evident.
In this chapter, we consider these methods used to convert XPS
measurements of relative atomic composition to amounts of cellulose, lignin
and other components in the surface, and try to clarify the reasons for the
apparent discrepancies.
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EXPERIMENTAL
Black spruce chips, selected for uniformity (Paprican, Pointe-Claire)
were pulped under typical Kraft conditions (Active alkali – 18%, Sulphidity –
30%, Temp - 172oC, time to temperature - 90mins, Liquor/wood – 4.5:1, H-
factors – 1065, 1260, 1510, 1760 and 2050). Five pulp samples of
decreasing kappa numbers3,44 were obtained, ranging from a moderately high
value to a low value. The pH of 13 at the end of the cook was high enough to
avoid re-precipitation of dissolved lignin onto the fibre surfaces. For ESCA
analysis, two small hand-sheets were made from each sample. The hand-
sheets were extracted with acetone, air dried and pairs of the sheets were
placed between Whatman filter papers. The inside contacting faces of the
pulp sheets were used for XPS analysis. Two Whatman filter papers were
treated in the same manner and their surface composition was analyzed
along with the pulp samples.
The ESCA measurements were performed with a Kratos Ultra electron
spectrometer (Kratos Analytical) using monochromatic Al Kα X-ray source (15
kV, 15 mA). The low-resolution survey scans were taken with a 1 eV step
and 160 eV analyzer pass energy; high-resolution spectra were taken with a
0.1 eV step and 40 eV analyzer pass energy. The collected data was
analyzed using Vision software version 2.1.3 and CASA XPS version 2.3.
The analysis area was less than 1mm2 and measurements were taken at two
different locations on the each of the touching faces of the hand-sheets.
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RESULTS AND DISCUSSION
The bulk lignin contents of the five unbleached softwood Kraft samples,
expressed as Kappa numbers (see introduction), are listed in Table 2, along
with the results of the XPS elemental analysis of the surface of the samples.
The only XPS observables in this work are the ratio of oxygen atoms to
carbon atoms, and the deconvolution of the carbon peak into components
with different chemical shifts resulting from the numbers of oxygen atoms that
were attached to the carbon atoms.
The No/Nc value of 0.83 for cellulose is seldom observed experimentally,
presumably due to the presence on the surface of carbon rich contaminant,
degradation, etc. This is illustrated in Figure 6, which shows typical C(1s)
XPS spectra for a pure cellulose sample, including the carbon contamination
and for a lignocellulosic sample. Ideally, the contamination should be
minimized by taking the precautions outlined elsewhere.41 However the
conditions for absolute XPS measurements of atom ratios are difficult to
achieve, and many publications use explicit or implicit calibration against a
pure cellulose surface such as filter paper or low-yield bleached pulp. Thus
Equation 3 assumes a linear relationship between lignin content and (NO/NC),
with the zero lignin content assigned to the measured value of (NO/NC) for a
pure cellulose sample.
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Figure 6: C(1s) spectra for a lignocellulosic hand-sheet (left) and a pure
cellulose hand-sheet.
We propose the following way to correct for the excess carbon signal
observed in a series of XPS samples containing only lignin and carbohydrate
components, where a pure cellulose sample has been run under identical
conditions as the unknown lignocellulosics. The basic assumption is that the
excess carbon signal which results in the value less than 5/6 for (NO/NC) for
the pure cellulose sample is in the same proportion to the total signal (NO +
NC) for the unknown samples, measured under the same spectrometer
conditions.
(6)
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Writing the excess carbon signal (~ the C1 contribution) due to the
contaminating carbon as NC*, we first wish to estimate its value relative to the
total XPS signal, NC*/(NC + NO) from the measured oxygen to carbon
ratio,(NO/NC) for pure cellulose.
The following abbreviations are convenient. We write;
(NO/NC)Cellulose, theoretical = CT
(NO/NC)Cellulose, measured = CM
(NO/NC)Pulp, measured = PM
(NO/NC)Pulp, corrected = PC
This preserves the usual convention of reporting the ratios of XPS signals for
individual elements.
The measured oxygen to carbon atom ratio for cellulose contains an
added contribution from the contaminant carbon:
(7)
After some algebra,
(8)
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We then assume that the value for NC*/(NC + NO), obtained as above from the
experimental value for cellulose filter paper and Equation 8 will be the
approximate value of NC* /(NC + NO) for pulp samples measured at the same
time as the cellulose standard.
In order to correct for the unwanted contribution from the excess
carbon, NC*, the corrected value for the oxygen to carbon ratio for a pulp
sample is given by,
(9)
Again, after some algebra,
(10)
or, rearranging,
(11)
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Combining (9) and (11) and forgetting about abbreviations:
(12)
from which the corrected oxygen to carbon ratio for the pulp sample,
(NO/NC)Pulp, corrected can easily be evaluated. The (NO/NC)corrected values for the
five Kraft pulps along with the corresponding surface lignin content (Wt%
surface lignin) are shown in Table 2.
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Kappa
number
Wt% bulk lignin
(0.147 x kappa)
NO/NC
measured
(XPS)
NO/NC
corrected
(from Eqn 13)
Wt%
surface lignin
45 6.5 0.631 0.715 17.5
37 5.4 0.651 0.738 13.9
32 4.7 0.670 0.762 10.3
25 3.6 0.681 0.775 7.8
21 3.1 0.699 0.797 5.2
Table 2: XPS measurements of surface lignin content for softwood Kraft
pulps.
The surfaces of these pulps appear to be richer in lignin than the bulk
this is in agreement with many previous measurements. There is an as
expected decrease in the weight fraction of lignin content with decreasing
Kappa number, for both measured and corrected values. Li and Reeve noted
that the correct empirical formula for lignin should be chosen when
interpreting XPS spectra.44 Table 3 shows the corrected weight fraction
percentage of lignin calculated using the empirical formula for Freudenberg
lignin model, and also for a residual Kraft lignin with a molar mass of 189.46
and an empirical formula of C9H8.64O2.99S0.07(OCH3)0.73.45 In this case, the
final results are virtually identical for both formulae.
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Kappa
Number
Wt% surface
lignin
Residual Kraft
Softwood lignin
Wt% surface
lignin
Freudenberg
lignin
45 17.4 17.5
37 13.9 13.9
35 10.3 10.4
25 7.8 7.8
21 5.1 5.2
Table 3 : The calculated weight fraction percentage of surface lignin in five
different Kappa number Black spruce pulp hand-sheets.
The solvent extraction conditions also play a significant role in the
determination of excess carbon signal, and should be duly noted when
reporting XPS results. Table 4 shows the effect of different extraction
solvents and conditions on the same pulp fibre.
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Wt% surface lignin (Freudenberg) Extraction conditions
Acetone DCM
3% 70°C 11.6 13.8
3% 35°C 21.3 32.6
0.3% 50°C 15.7 15.0
Table 4: The weight fraction percentage of a Black Spruce Kraft pulp with
Kappa number 45 subjected to different extraction conditions.
Interpretation of the measured XPS data according to Equations 1 and
Equation 2 gave higher values for the weight fraction of surface lignin. These
values are based on the assumption that a pure cellulose surface gives a
No/Nc value of 0.83, as follows from the atom ratio of 5/6, and that the
empirical formula of lignin is given by the value accepted for Freudenberg
lignin in wood (C9.92O3.32). The higher values obtained highlight the
importance of correctly accounting for contamination and sample
degradation.
The experimental results for an inter-laboratory comparison of XPS
data for a range of pulps 41 were also re-examined in light of this suggested
method. The "corrected" (No/Nc)cor values were calculated and used, to give
a “corrected” weight fraction percentage of surface lignin and are listed in
Table 5. This data, calculated for each lab's results, gave quite good overall
agreement, with fairly clear assumptions in arriving at the results.
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Laboratory &
sample
No/Nc
measured
No/Nc
corrected
Wt % surface
lignin
corrected
Åbo Akademi University, Finland
TMP 0.66 0.66 26.7
UBK 0.78 0.78 7.6
ECF 0.81 0.81 3.3
Helsinki University of Technology, Finland
TMP 0.61 0.71 19.2
UBK 0.70 0.82 2.4
ECF 0.71 0.83 0.7
Chalmers University of Technology, Sweden
TMP 0.67 0.69 22.4
UBK 0.76 0.79 7.8
ECF 0.81 0.83 0.4
Universite de Québec à Trois-Rivieres, Canada
TMP 0.59 0.67 25.1
UBK 0.68 0.78 7.9
ECF 0.70 0.81 4.4
Interlab
TMP 0.63 0.68 23.8
UBK 0.73 0.79 6.4
ECF 0.76 0.83 1.6 Table 5: Attempt to summarize data for surface lignin, together with
recalculated data for weight % lignin, assuming uniform surface, and also
scaling O/C data to correct for lignin-rich contamination assuming that
measured O/C data for filter paper may be linearly scaled to expected values
for filter paper (O/C =0.83). TMP = Spruce Thermomechanical Pulp, UBK =
Unbleached Birch Kraft Pulp, ECF = Elemental Chlorine Free Bleached Birch
Kraft Pulp.
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CONCLUSION
We have suggested a simple method which may be used, with a high
degree of confidence (through the use of a pure cellulose reference and a few
logical assumptions), to convert XPS measurements of relative atomic
composition to amounts of cellulose, lignin and other components in the surface.
Page 36 of 75
Chapter 2
An atomic force microscopy investigation into the physical properties of
pulp fibre surfaces.
INTRODUCTION
Atomic force microscopy (AFM) was first introduced by Binning, Quate and
Gerber in 1986.5 Its success has led to the development of a family of
techniques based on the same principle grouped under the name scanning force
microscopies. These techniques have been applied to a variety of research
fields, including pulp and paper research.
One of the advantages to pulp and paper research is the fact that AFM
can be operated in both air and liquid environments. Consequently the
conditions of industrial papermaking can be replicated microscopically in the
laboratory. AFM has been used to study the topography of wood derived
samples, both wet and dry.6-27,29,30,46 The mechanism for AFM also facilitates the
measurement of physical properties on pulp samples.18,20,31-33
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The topographical imaging ability of AFM has been used to investigate
many surface properties of pulp fibres. Physical features such as surface
roughness,12,29 the existence and orientation of fibrils in the primary and
secondary cell wall,14 how extractives are located and adsorbed on the surface of
fibres and model surfaces,22 the effect of surface treatments,9,10,38 and the effect
of beating16 were acquired.8,11,13,46
There is another dimension to the challenge of obtaining accurate physical
measurements at the individual pulp fibre level. Wood is a very heterogeneous
material and this is evident even at the single pulp fibre level. There is a high
amount of variability in the properties of individual pulp fibres and the non-
uniformity of the pulping process.47 The processing conditions the fibres are
subjected to prior to any physical property measurements also plays an important
role. Bawden and Kibblewhite noted that the first drying the pulp fibre undergoes
causes the greatest change in fibre dimensions,48 and that this is not completely
reversed upon rewetting. Therefore, a large variability in the physical properties
obtained by the same method can be expected if the fibres are treated by
different pulping processing. The properties of never-dried, dried, and rewetted
fibres are also very different. There is even a vast range of modulus/flexibility
values in fibres derived from the same wood source depending on whether it
underwent a chemical or mechanical pulping process.49
Page 38 of 75
AFM has been perceived by many as a revolution in micro and nano scale
science. As with all techniques it has disadvantages and limitations. It is
theoretically possible to obtain atomic resolution on crystalline surfaces, and the
instrument can perform in a wide range of sample conditions. The practicalities
of achieving good quality and reproducible results are not always so easy. AFM
can produce force-distance curves between the tip and sample surfaces
approaching each other from distances of microns to nanometres, which can be
used to measure van der Waals,, double layer and, electrostatic forces.
From force-distance curves, indentation modulus values have been
calculated for many different materials. Unfortunately, many of the quoted values
have uncertainties arising from a lack of calibration of key components, such as
the use of manufacturers' nominal value for the cantilever spring constant and tip
radius, or the assumption of an incorrect model for calculation the modulus.50
Clifford and Seah50 found that these uncertainties could be as much as 40% for
homopolymers. With heterogeneous materials the uncertainties can be expected
to be even greater. The direct measurement of elastic modulus requires the
calibration of all the factors involved including the cantilever spring constant, the
tip radius, the tip shape and the piezoelectric scanner movement in the z
direction. The result is an accurate, traceable value for the indentation modulus
of the material.50
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A few groups have applied AFM force-distance measurements to the
problem of finding the modulus of a pulp fibre in the hope of understanding more
about fibre-fibre bonding.18,51-53 Other methods have also been investigated.
Zadorecki et al incorporated bleached unbeaten pulp fibres into a polyester
composite and calculated the modulus of the fibres from the modulus of the
composite and matrix.54 Page et al used the stress-strain curve taken directly
from individual dry fibres to calculate the axial and transverse modulus.55 Hartler
and Nyren also employed a technique using an individual fibre to obtain a load-
deformation curve from which the transverse modulus was determined.56 Wild et
al built a fibre compression instrument consisting of micro hammer and anvil set-
up and defined the modulus in such terms as to eliminate as much variation as
possible between the fibres.57 Tchepel et al used a single-fibre fatigue cell to
apply force and measure elongation and then deduce elasticity.58 Orso et al
employed standard beam theory to directly calculate the modulus of wood cell
wall in their focussed ion beam method.59
Previous work within Dr. Grays group by Furuta31 & Pang32 used AFM to
probe the surface properties of pulp fibres. They found that some of the complex
changes in the fibre properties could be directly observed in aqueous media by
AFM force-distance curves. The shape of the force-distance curve infers the
changes in fibre properties caused by the beating process, the higher the level of
beating the more readily compressible the surface of the wet fibre is. The levels
of fibrillation at different areas of the same fibre and the effect of salt solution on
Page 40 of 75
fibrillation is also very apparent form the corresponding force-distance curve.
Nilsson et al measured the surface stiffness of wet Kraft fibres from the force-
distance curves and used it to estimate the apparent local modulus of elasticity of
the fibre.53 Chhabra et al18 also determined the modulus from the force-distance
curves, the force-distance curve was converted into a load-depth plot using
Hook’s Law and then Sneddon’s equation was used to find the modulus. In this
case the tip was model as a conical indenter and the fibres were modelled as
purely elastic. Nanoindentation was used by Gindl et al on natural and
regenerated cellulose. They concluded that using nanoindentation absolute
values for modulus could only be applied to isotropic materials (regenerated
cellulose) and the values obtained for anisotropic materials (wood pulp fibres)
were relative values and valid only for comparison.52
In this chapter, we report on the use of AFM as a method of investigating
the surface properties of Kraft pulp paper-making fibres.
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EXPERIMENTAL
Black spruce chips, selected for uniformity (Paprican, Pointe-Claire) were
pulped under typical Kraft conditions (Active alkali – 18%, Sulphidity – 30%,
Temp - 172oC, time to temperature - 90mins, Liquor/wood – 4.5:1, H-factors –
1065, 1260, 1510, 1760 and 2050). Five pulp samples of decreasing kappa
numbers3,44 were obtained, ranging from a moderately high value to a low value.
The pH of 13 at the end of the cook was high enough to avoid reprecipitation of
dissolved lignin onto the fibre surfaces.
AFM measurements were made with a MFP-3D instrument from Asylum
Research (Santa Barbara, CA). Never-dried Kraft pulp fibres were secured to a
glass microscope slide with a temperature sensitive adhesive then immersed
completely in deionised water. The actual spring constant of the tip (Olympus
Biolever) was calculated prior to each experiment. Measurements were never
taken on the same spot in order to prevent damage to the fibre, to prevent
inaccurate force measurements being taken and to simulate the force felt by
another fibre in the bonding process. AFM topographies were obtained on dried
fibres in both contact and tapping modes. Individual fibres were placed on a
clean glass microscope slide and allowed to dry under ambient conditions. The
topographies were scanned perpendicular to the fibre axis.
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RESULTS AND DISCUSSION
The bulk lignin contents, expressed as Kappa number (see Introduction)
and the corresponding surface lignin content, from ESCA analysis are listed in
Table 6.
Kappa number
Wt% bulk lignin (0.147 x kappa)
Wt% surface lignin
45 6.5 17.5
37 5.4 13.9
32 4.7 10.3
25 3.6 7.8
21 3.1 5.2
Table 6: The Kappa numbers with corresponding bulk and surface lignin
contents of the five Black Spruce Kraft pulp fibres under study.
An AFM topography (deflection image in contact mode) of a typical dried
Black Spruce Kraft Pulp fibre is shown in Figure 1. The resolution of the AFM
imaging mode allows the ordering of the cellulose microfibrils around the pits and
within the S2 layer to be observed. The variation in surface structure of a Kraft
pulp fibre can be clearly seen. Although a large amount of information is
obtainable with AFM on dry Kraft fibres it does not accurately mimic the
conditions of the papermaking process.
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Figure 7: An AFM contact mode deflection topography of a typical dried Black
Spruce Kraft pulp fibre. The scanning direction is perpendicular to the fibre axis.
The bulk of the papermaking process is carried out in water. AFM has the
capability to acquire a variety of measurements in a liquid environment.
Therefore, the physical properties of wet Kraft pulp papermaking fibres can be
obtained. Several force-distance curves on never-dried pulp fibres were
obtained under water. Typical curves for the two extremes in pulping are shown
in Figure 8 and Figure 9.
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Figure 8: Typical deflection-distance curves for the two extremes in pulping
conditions. The red curve is the Kraft pulp with kappa number 45 and the blue
curve is the Kraft pulp with kappa number 21.
A lot of information can be inferred from the data collected by AFM.
Figure 8 shows the deflection of the cantilever as the tip is moved closer to the
sample and through into the contact region. In this case the “contact point” is
taken as the Burnham contact point,60 defined as the point when a repulsive
force is first detected by the cantilever. Overlaying the curves, defined in this
manner, the slope of the contact region to be clearly observed; the steeper the
slope of the contact region the harder that surface. From Figure 8 it appears that
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Kraft pulp 45 (red curve) is a harder surface than Kraft pulp 21 (blue curve). This
shows that the high lignin content fibre is more rigid than the fibre from which
most of the lignin has been removed by pulping.
Figure 9: Typical force-distance curves for the two extremes in pulping
conditions. The red curve is the Kraft pulp with kappa number 45 and the blue
curve is the Kraft pulp with kappa number 21.
Figure 9, in contrast to Figure 8, shows the force-distance curves, where
the force is determined from the deflection data and the actual tip spring
constant. The “contact point” in this case is calculated from the linear portions of
the curves. By overlaying the two pulping conditions curves it can be seen that
the pulp with less surface lignin (Kraft Pulp 21, blue) experiences a repulsive
Page 46 of 75
force between the tip at a distance around 10 times greater from the “true”
surface than the higher surface lignin content pulp (Kraft 45, red).
The earlier onset of the repulsive force observed as the AFM tip
approaches the Kraft 21 fibre is ascribed to surface fibrillation. The microfibrils
and the surface of the fibre are negatively charged due to the presence of some
carboxyl groups introduced during pulping, so mutual electrostatic repulsion
causes the microfibril tails extend away from the surface into the aqueous media,
where they interact the tip. The distance at which the AFM tip starts to sense the
repulsive force can be considered as an indication to the effective length of
dangling microfibrils. The higher the fibrillation, the greater the repulsive force.
From Goring’s Interrupted Lamella model61 it is evident that as lignin is
removed from the fibre, the fibre becomes more compressible. The results from
the AFM measurements are in agreement with this.
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Figure 10: Values for moduli of Kraft pulps with different lignin contents,
calculated by the method suggested by Nilsson.53
The data collected from the force-distance was used to calculate the
apparent modulus for all five Kraft pulps. The results are shown on Figure 10.
The average results is in agreement with the value calculated by Nilsson et al.53
The large range in apparent moduli for all five Kappa numbered pulps accurately
reflects the extent of variation found on the surface of pulp fibres. With such
diversity on the surface the sample population size must also be taken into
account.
0.000
0.500
1.000
1.500
2.000
2.500
20.00
25.00
30.00
35.00
40.00
45.00 Kappa
Number
Mod
ulus
(M
Pa)
Page 48 of 75
Pulp fibre Deformation Modulus Reference:
Crystalline Cellulose I Axial tensile 128 GPa Sakurada et al62
Kraft – dry In fibril direction 77 GPa Page et al55
Kraft – dry Transverse to fibril direction 8.8 GPa Page et al55
Kraft – dry Transverse compressibility 0.9 GPa Hartler and
Nyren56
Kraft – wet unbeaten
Transverse compressibility 0.56 GPa Hartler and
Nyren56
Kraft – wet beaten
Transverse compressibility 0.44 GPa Hartler and
Nyren56
Kraft – wet AFM Transverse microcompressibility 0.013 GPa Nilsson et al53
TCF bleached – wet
AFM Transverse microcompressibility 0.007 GPa Nilsson et al53
Kraft in composite Tensile of composite 16.1 GPa Zadorecki et al54
Kraft Tangent modulus 0.007 GPa Wild et al57
Spruce wood cell Standard Beam theory 28 GPa Orso et al59
Table 7: Some examples of moduli obtained for wood pulp fibres.
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The results of work conducted in this field are summarised in Table 7.
There are discrepancies in the moduli obtained by different means and the
processing conditions the fibre under question is subjected to also heavily
influences the modulus.
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CONCLUSION
It has been shown that although AFM is a great technique for providing
physical property information on a wide variety of surfaces and under different
environmental conditions, it is not suitable for measuring the modulus of wood
pulp fibres. The heterogeneous nature of wood pulp fibres would require a very
large sample population of measurements to reach valid conclusions as to the
effects of fibre source and pulping conditions. For the pulp and paper industry,
the valuable surface property information sought from wood pulp fibres is better
served by bulk methods.
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Chapter 3
Transcrystallisation of Polypropylene at the surfaces of Unbleached Kraft
Pulp Fibres
INTRODUCTION
For many years cellulose has been incorporated into synthetic polymer
composites in a variety of markets ranging from packaging to automotive
applications. Cellulose from wood sources is of increasing interest as it has a
very low cost per unit volume, is ‘green’ and can often match or improves upon
the mechanical and physical properties of existing reinforcing agents.63
Transcrystallisation is the preferential nucleation of polymer melts at
crystalline surfaces. The term often refers to enhanced nucleation of spherulitic
growth along fibres, typically observed for polymers such as isotactic
polypropylene. If the nucleation density is high the resultant spherulites crowd
together, producing a ‘transcrystalline’ layer in contact with the surface.63,64
Some time ago, a transcrystalline layer was observed at the surface of natural
Cellulose I fibres, but not at regenerated or mercerized (Cellulose II) surfaces.64
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Care is necessary in interpreting surface effects, as a transcrystalline morphology
can also be generated by shear at surfaces in the crystallising melt.65
Subsequent work on transcrystallisation of isotactic polypropylene (i-PP)
at cellulosic surfaces has focused on the nature of the cellulose surface and the
effect on composite performance. The presence of sizing agents on bleached
Kraft, ramie and microcrystalline cellulose surfaces inhibited
transcrystallisation.66 The transcrystallinity at Cellulose I surfaces was also
inhibited by esterification yet the effect was enhanced by beating.67 Fibres
treated with celllulase increased the nucleating ability of the i-PP/cellulose fibre
composite (matrix.)68 The presences of a transcrystalline layer improved
interfacial shear transfer between i-PP and cotton fibres.69 A thorough study of i-
PP transcrystallisation at flax fibre surfaces also showed enhanced interfacial
properties.70 In general, the higher the Cellulose I content at the fibre-melt
interface, the greater the transcrystalline layer observed, although there is one
report of transcrystallisation at a Cellulose II surface of NaOH-treated milled
woodpulp.68
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The reason why Cellulose I is a preferential nucleation site for isotactic
polypropylene is not fully understood. Felix and Gatenholm suggested that the
preferential nucleation occurs because in the helix of isotactic polypropylene the
distance between two methyl groups with the same spatial arrangement is
approximately the same length as the linear distance between glucosidic
oxygens in cellulose.69 This gives a good match of interaction sites and hence
nucleation occurs more readily.
In this chapter, we examine the effect of pulp lignin content on the amount
of transcrystalline morphology generated at Kraft fibre surfaces.
Page 54 of 75
EXPERIMENTAL
Black spruce chips, selected for uniformity (Paprican, Pointe-Claire) were
pulped under typical Kraft conditions (Active alkali – 18%, Sulphidity – 30%,
Temp - 172oC, time to temperature - 90mins, Liquor/wood – 4.5:1, H-factors –
1065, 1260, 1510, 1760 and 2050). Five pulp samples of decreasing kappa
numbers3 were obtained, ranging from a moderately high value to a low value.
Isotactic polypropylene (i-PP) pellets (Nominal Mn, 67,000,Mw 250,000)
were purchased from Sigma-Aldrich. Thin (~200 µm) polypropylene discs were
prepared by melting single pellets of i-PP between microscope slides on a Kofler
hot bench (Reichert) at ~200oC. A fibre of the Kappa number under study was
placed on the melt, a second layer of i-PP was laid over the fibre and a
microscope cover slip was then placed on top. The samples were melted at
~200oC for a few minutes, then transferred rapidly to a Mettler FP82 hot stage
set at 136oC, and viewed with a Nikon Microphot-FXA polarized light microscope.
Images were captured with a Nikon Coolpix 990 digital camera.
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Differential Scanning Calorimetry measurements of i-PP/pulp fibre
composites were performed under nitrogen on a TA Instruments DSC Q1000.
The samples were melted at a rate of 10°C min-1 to 200°C and maintained at this
temperature for 5 minutes in order to eliminate any thermal history. The sample
was cooled to 80°C at a rate of 5°C min-1 and then the heating procedure
repeated. In each experiment there was a pure i-PP sample in the reference
pan, which was the same weight as the i-PP/pulp fibre composite under study.
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RESULTS AND DISCUSSION
Crystalline polypropylene is birefringent and therefore easy to observe
with a polarising microscope. The transcrystalline layer growth along several
Kraft pulp fibres in isotactic polypropylene was observed. After a few minutes
spherulitic growth could be detected along the fibre where few spherulites were
apparent in the bulk. As the spherulites on the fibre grow they impinge upon their
neighbours and form a transcrystalline layer around the fibre. The fibre becomes
completely encased in the transcrystalline layer long before the bulk is fully
crystalline. This effect is clearly shown in Figure 11; the rapid development of a
crystalline layer around the fibre contrasts with the lack of normal spherulites in
the bulk of the sample, indicating enhanced nucleation at the fibre surface.
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Figure 11: Transcrystallisation layer growth about a Kraft pulp fibre (Kappa
number 45) at a) 0 minutes, b) 2 minutes, c) 4 minutes, d) 6 minutes, e) 8
minutes, f) 10 minutes, g) 12 minutes, h) 14 minutes, i) 16 minutes.
The bulk lignin (calculated from Kappa numbers) and surface lignin (from
XPS measurements in chapter 2 of this thesis) of the five unbleached softwood
Kraft samples are listed in Table 8. The main difference between the five
different Kappa number pulps is the number of nucleation sites after a short
period at 136°C. Figure 12 shows growth of transcrystalline layer at the surfaces
of a low Kappa number Kraft fibre and a high Kappa number Kraft fibre.
Page 58 of 75
Kappa number Wt% bulk lignin
(0.147 x kappa) Wt% surface lignin
45 6.5 17.5
37 5.4 13.9
32 4.7 10.3
25 3.6 7.8
21 3.1 5.2
Table 8: Bulk and surface lignin (as calculated in chapter 2 of this thesis)
contents of the Kraft Pulp samples.
Page 59 of 75
Page 60 of 75
Figure 12: Transcrystallisation along Kraft pulp fibres with Kappa number 45
(left) and Kappa number 25 (right). The scale bar corresponds to 200µm.
Even at the early stages of the crystallisation, nucleation along the fibre
with less lignin and hemicellulose (Kappa number 25, Figure 12, right) is dense.
The amount of nucleation along the higher Kappa number fibre (Figure 12, left) at
6 minutes closely resembles the lower Kappa number fibre at 4 minutes.
The fibres with higher Kappa numbers take longer to obtain the same
amount of nucleation sites. Note that the presence of the fibre does not alter the
growth rate of the spherulites but rather changes the onset of nucleation. All
fibres are fully covered in a transcrystalline layer before normal bulk spherulitic
growth is significant. The diameter of the encapsulating transcrystalline layer at a
given time interval may be taken as a rough indication of the nucleating tendency
of the fibre. This is shown in Figure 13.
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Figure 13: Plot of the mean transcrystalline layer thickness along a Kraft Pulp
fibre. (Kraft pulp kappa number 21, blue diamonds and Kraft pulp kappa number
45, pink squares.)
It has been shown that natural cellulose fibres appear to lower the degree
of super-cooling necessary to induce crystallisation. Thus the heat evolved on
crystallisation near the fibres should be detectable at slightly higher temperatures
than the heat of crystallisation for a melt containing no fibres. Likewise, a fibre
with a higher Cellulose I surface content should have a heat of crystallisation that
is detectable at slightly higher temperatures than a fibre with more lignin and
hemicellulose on the surface.
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Figure 14: DSC curve comparison for the heat of crystallisation of i-PP in contact
with Kraft fibres, prepared by the two extremes of pulping (Kraft pulp kappa
number 21, red curve and Kraft pulp kappa number 45, green curve.)
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Figure 14 shows the Dynamic Scanning Calorimetry curves for the two
extremes in pulping conditions. Kraft pulp 21 (Figure 14, red curve) has a
surface lignin content of 5.2% and the Kraft pulp 45 (Figure 14, green curve) has
a surface lignin content of 17.5%. From the above curves it is hard to clearly see
what the difference in heats of crystallisation actually is. Air is commonly used as
the reference material in DSC experiments, in our experiments i-PP was used as
the reference. This allows us to evaluate the effect of the transcrystallisation
layer around the Kraft pulp fibre.
Figure 15: Differential of the DSC curve comparison for the heat of crystallisation
of i-PP in contact with Kraft fibres, prepared by the two extremes of pulping( Kraft
pulp kappa number 21, red curve and Kraft pulp kappa number 45, green curve.)
Page 64 of 75
If we take the differential of the curves in Figure 14 with respect to
temperature we can observe the differences between the two curves more easily.
This is shown in Figure 15. The heat of crystallisation associated with the
transcrystalline layer around the Kraft pulp 21 fibre (Figure 15, red curve, surface
lignin content 5.2%) is first detected at 125°C. Three degrees lower, the
transcrystalline layer around the Kraft pulp 45 fibre (Figure 15, green curve,
surface lignin content 17.5%) appears. As the Kraft pulp 21 fibre has a lower
surface lignin content and hence higher surface Cellulose I content it would be
expected to induce crystallisation at a slightly higher temperature than the Kraft
pulp fibre with a lower amount of Cellulose I on the surface.
Page 65 of 75
CONCLUSION
The rate of transcrystalline layer growth along a fibre is dependent upon
the surface chemistry of that fibre. The greater the amount of Cellulose I on the
fibre surfaces the faster the transcrystalline layer forms. The transcrystalline
layer is always completed before crystallisation in the bulk melt. As the size of
the transcrystalline layer is constant at one spherulite diameter, the physical
properties of the i-PP/pulp fibre composite should be unaffected by the subtle
variation of the surface chemistry on the Kraft pulp fibres.
Page 66 of 75
Conclusion
Through this work we have shown that simple and straight forward means
may be employed to determine the surface chemistry of Kraft Pulp fibres (and
therefore other lignocellulosic fibres) and the effects that variability in surface
composition has on industrial applications for lignocellulosic fibres. We have also
highlighted the challenges of quantifying a naturally highly heterogeneous
material.
It has been shown that although AFM is a great technique for providing
physical property information on a wide variety of surfaces and under different
environmental conditions, the heterogeneous nature of wood pulp fibres would
require a very large number of measurements to reach valid conclusions as to
the effects of fibre source and pulping conditions. The relative atomic
composition of the amounts of cellulose, lignin and other components in the
surface of a lignocellulosic fibre can be confidently obtained through the use of
XPS and straight forward algebra. The rate of transcrystalline layer growth along
a Kraft pulp fibre is governed by the subtle variation in surface chemistry of the
fibre. The size of the transcrystalline layer is constant at one spherulite diameter,
therefore the physical properties of an i-PP/pulp fibre composite should be
unaffected by this variability in surface chemistry.
Page 67 of 75
Acknowledgements
The author wishes to express her thanks to the following; without their
contributions, though all differing in magnitude and prowess, this project would
not have been the adventure that it was.
To the project supervisor, Dr. Derek G. Gray for his advice, support and
guidance throughout the duration of this project and beyond. For allowing me the
freedom to foster my true talents and encouraging me in seeking opportunities to
fully utilise them.
For their technical expertise I am indebted to Nilgun Ulkem primarily for
her expertise in pulping but also for her friendship and understanding and Agnes
Lejeune (Université du Québec à Trois-Rivières) for her talents in running XPS
on paper samples.
My thanks to the members of the Gray research group who cultivated a
creative environment in which problems were halved, shoulders were cried on
and shoes were bought, especially to Tiffany Abitbol and Emily Dawn
Cranston. My friendship with Emily would not have flourished had fate not
placed us both in the Gray group, for that I and my family are eternally grateful.
Page 68 of 75
A debt of gratitude is owed to the Third Floor Tea Club; the institution it
has become and its members past, present and future. No glory is so great and
no failure so bad that it cannot be made better with cake.
The staff, faculty and students at the Pulp and Paper Research Centre,
McGill University, who made my time among them a most pleasant experience. I
would especially like to thank Colleen McNamee, for always lending her ear
generously and her never ending supply of chocolate.
My husband, John for his unfoundering belief in me, his unequivocal
support and his immaculate timing with a glass of wine and a bar of chocolate.
My parents and extended family who never doubted I would succeed and never
stopped telling me how proud they were of all I accomplished. I am indebted to
Jenny Warrington the original and the best supportive friend.
The most important thanks go to Amy Megan Weller and Jack Peter
Weller for putting it all into perspective.
Page 69 of 75
References
1. Sakakibara A. A structural model of softwood lignin. Wood Science and Technology 1980;14(2):89-100.
2. Kollman FFP. Principles of Wood Science and Technology. Kollman FFP, Cote WA, editors. Berlin; Heidelberg ; New York: Springer-Verlag; 1968.
3. Biermann CJ. Handbook of Pulping and Papermaking. Biermann CJ, editor. San Diego: Academic Press; 1996.
4. Siegbahn K, et al. ESCA [Electron Spectroscopy for Chemical Analysis]. Atomic, molecular, and solid state structure studied by means of electron spectroscopy. Nova Acta Regiae Societatis Scientiarum Upsaliensis 1967;20:282 pp.
5. Binnig G, Quate CF, Gerber C. Atomic Force Microscopy. Physical Review Letters 1986;56(9):930-933.
6. Hanley SJ, Gray DG. Atomic force microscope images of black spruce wood sections and pulp fibers. Holzforschung 1994;48(1):29-34.
7. Hanley SJ, Gray DG. Atomic force microscopy (AFM) images in air and water of kraft pulp fibers. Journal of Pulp and Paper Science 1999;25(6):196-200.
8. Groom L, Pesacreta T. Atomic force microscopy of wood fiber surfaces. International Conference on Woodfiber-Plastic Composites, 4th, Madison, Wis., May 12-14, 1997 1997:26-31.
9. Mahlberg R, Niemi HEM, Denes F, Rowell RM. Effect of oxygen and hexamethyldisiloxane plasmas on morphology, wettability and adhesion properties of polypropylene and lignocellulosics. International Journal of Adhesion and Adhesives 1998;18(4):283-297.
10. Mahlberg R, Niemi HEM, Denes FS, Rowell RM. Application of AFM on the adhesion studies of oxygen-plasma-treated polypropylene and lignocellulosics. Langmuir 1999;15(8):2985-2992.
Page 70 of 75
11. Boras L, Gatenholm P. Surface properties of mechanical pulps prepared under various sulfonation conditions and preheating time. Holzforschung 1999;53(4):429-434.
12. Snell R, Groom LH, Rials TG. Characterizing the surface roughness of thermomechanical pulp fibers with atomic force microscopy. Holzforschung 2001;55(5):511-520.
13. Maciel AM, Wilkins CP. AFM ultrastructural studies of chemical softwood tracheids and secondary fines generated by various refining treatments. Paper Technology (Bury, United Kingdom) 2002;43(6):25-33.
14. Fahlen J, Salmen L. Cross-sectional structure of the secondary wall of wood fibers as affected by processing. Journal of Materials Science 2003;38(1):119-126.
15. Gustafsson J, Lehto JH, Tienvieri T, Ciovica L, Peltonen J. Surface characteristics of thermomechanical pulps; the influence of defibration temperature and refining. Colloids and Surfaces, A: Physicochemical and Engineering Aspects 2003;225(1-3):95-104.
16. Kang T, Paulapuro H, Hiltunen E. Fracture mechanism in interfibre bond failure - microscopic observations. Appita Journal 2004;57(3):199-203.
17. Chakraborty A, Sain M, Kortschot M. Structural development of wood microfibres as biodegradable reinforcing agents for composites. Abstracts of Papers, 229th ACS National Meeting, San Diego, CA, United States, March 13-17, 2005 2005:CELL-193.
18. Chhabra N, Spelt JK, Yip CM, Kortschot MT. An investigation of pulp fiber surfaces by atomic force microscopy. Journal of Pulp and Paper Science 2005;31(1):52-56.
19. Matsumura H, Glasser WG. Cellulosic nanocomposites. II. Studies by atomic force microscopy. Journal of Applied Polymer Science 2000;78(13):2254-2261.
20. Sasaki T, Okamoto T, Meshitsuka G. Influence of deformability of Kraft pulp fiber surface estimated by force curve measurements on atomic force microscope (AFM) contact mode imaging. Journal of Wood Science 2006;52:377-382.
Page 71 of 75
21. Westbye P, Svanberg C, Gatenholm P. The effect of molecular composition of xylan extracted from birch on its assembly onto bleached softwood kraft pulp. Holzforschung 2006;60(2):143-148.
22. Oesterberg M, Schmidt U, Jaeaeskelaeinen A-S. Combining confocal Raman spectroscopy and atomic force microscopy to study wood extractives on cellulose surfaces. Colloids and Surfaces, A: Physicochemical and Engineering Aspects 2006;291(1-3):197-201.
23. Wistara N, Zhang X, Young RA. Properties and treatments of pulps from recycled paper. Part II. surface properties and crystallinity of fibers and fines. Cellulose (Dordrecht, Netherlands) 1999;6(4):325-348.
24. Fardim P, Gustafsson J, von Schoultz S, Peltonen J, Holmbom B. Extractives on fiber surfaces investigated by XPS, ToF-SIMS and AFM. Colloids and Surfaces, A: Physicochemical and Engineering Aspects 2005;255(1-3):91-103.
25. Maximova N, Osterberg M, Koljonen K, Stenius P. Lignin adsorption on cellulose fibre surfaces: effect on surface chemistry, surface morphology and paper strength. Cellulose (Dordrecht, Netherlands) 2001;8(2):113-125.
26. Koljonen K, Osterberg M, Johansson LS, Stenius P. Surface chemistry and morphology of different mechanical pulps determined by ESCA and AFM. Colloids and Surfaces, A: Physicochemical and Engineering Aspects 2003;228(1-3):143-158.
27. Koljonen K, Oesterberg M, Kleen M, Fuhrmann A, Stenius P. Precipitation of lignin and extractives on kraft pulp: effect on surface chemistry, surface morphology and paper strength. Cellulose (Dordrecht, Netherlands) 2004;11(2):209-224.
28. Fardim P, Hulten AH, Boisvert J-P, Johansson L-S, Ernstsson M, Campbell JM, Lejeune A, Holmbom B, Laine J, Gray D. Critical comparison of methods for surface coverage by extractives and lignin in pulps by X-ray photoelectron spectroscopy (XPS). Holzforschung 2006;60(2):149-155.
29. Medeiros RG, Silva LP, Azevedo RB, Silva FG, Filho EXF. The use of atomic force microscopy as a tool to study the effect of a xylanase from Humicola grisea var. thermoidea in kraft pulp bleaching. Enzyme and Microbial Technology 2007;40(4):723-731.
Page 72 of 75
30. Simola J, Malkavaara P, Alen R, Peltonen J. Scanning probe microscopy of pine and birch kraft pulp fibers. Polymer 1999;41(6):2121-2126.
31. Furuta T, Gray DG. Direct force-distance measurements on wood-pulp fibers in aqueous media. Journal of Pulp and Paper Science 1998;24(10):320-324.
32. Pang L, Gray DG. Heterogeneous Fibrillation of Kraft Pulp Fibre Surfaces Observed by Atomic Force Microscopy. J. pulp and paper science 1998;24(11):369-372.
33. Brancato A, Walsh FL, Sabo R, Banerjee S. Effect of Recycling on the Properties of Paper Surfaces. Industrial & Engineering Chemistry Research 2007;46(26):9103-9106.
34. Takeyama S, Gray DG. Surface analysis of some sulfite pulps by ESCA. Transactions of the Technical Section (Canadian Pulp and Paper Association) 1980;6(3):TR61-TR64.
35. Dorris GM, Gray DG. The surface analysis of paper and wood fibers by ESCA (electron spectroscopy for chemical analysis). I. Application to cellulose and lignin. Cellulose Chemistry and Technology 1978;12(1):9-23.
36. Gray DG. The surface analysis of paper and wood fibers by ESCA. III. Interpretation of carbon (1s) peak shape. Cellulose Chemistry and Technology 1978;12(6):735-43.
37. Dorris GM, Gray DG. The surface analysis of paper and wood fibers by ESCA. II. Surface composition of mechanical pulps. Cellulose Chemistry and Technology 1978;12(6):721-34.
38. Gustafsson J, Ciovica L, Peltonen J. The ultrastructure of spruce kraft pulps studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Polymer 2003;44:661-670.
39. Johansson L-S, Campbell JM, Koljonen K, Stenius P. Evaluation of surface lignin on cellulose fibers with XPS. Applied Surface Science 1999;144-145:92-95.
40. Hulten AH, Basta J, Larsson P, Ernstsson M. Comparison of different XPS methods for fiber surface analysis. Holzforschung 2006;60(1):14-19.
Page 73 of 75
41. Johansson L-S, Campbell JM, Fardim P, Hulten AH, Boisvert J-P, Ernstsson M. An XPS round robin investigation on analysis of wood pulp fibres and filter paper. Surface Science 2005;584(1):126-132.
42. Laine J, Stenius P, Carlsson G, Strom G. Surface characterization of unbleached kraft pulps by means of ESCA. Cellulose (London) 1994;1(2):145-160.
43. Stroem G, Carlsson G. Wettability of kraft pulps - effect of surface composition and oxygen plasma treatment. Journal of Adhesion Science and Technology 1992;6(6):745-61.
44. Li K, Reeve DW. Determination of surface lignin of wood pulp fibres by x-ray photoelectron spectroscopy. Cellulose Chemistry and Technology 2004;38(3-4):197-210.
45. Toven K, Gellerstedt G. Structural changes of softwood kraft lignin in oxygen delignification and prebleaching. 1999 7-10 June 1999; Yokohama, Japan. p 340-345.
46. Li K, Tan X, Yan D. The middle lamella remainders on the surface of various mechanical pulp fibres. Surface and Interface Analysis 2006;38(10):1328-1335.
47. Niskanen K. Paper Physics. Helsinki: Fapet Oy; 1998.
48. Bawden AD, Kibblewhite RP. Effects of Multiple Drying Treatments on Kraft Fibre Walls. Journal of Pulp & Paper Science 1997;23(7):J340 - J346.
49. Tam Doo PA, Kerekes RJ. The flexibility of wet pulp fibres. Pulp and Paper Canada 1982;83(2):46-50.
50. Clifford CA, Seah MP. Quantification issues in the identification of nanoscale regions of homopolymers using modulus measurement via AFM nanoindentation. Applied Surface Science 2005;252(5):1915-1933.
51. Gindl W, Gupta HS, Schoeberl T, Lichtenegger HC, Fratzl P. Mechanical properties of spruce wood cell walls by nanoindentation. Applied Physics A: Materials Science & Processing 2004;79(8):2069-2073.
52. Gindl W, Konnerth J, Schoberl T. Nanoindentation of regenerated cellulose fibres. Cellulose 2006;13:1-7.
Page 74 of 75
53. Nilsson B, Wagberg L, Gray DG. Conformability of wet pulp fibres at small length scales. 2001 2001; Oxford, UK.
54. Zadorecki P, Karnerfors H, Lindenfors S. Cellulose fibers as reinforcement in composites: Determination of the stiffness of cellulose fibers. Composites Science and Technology 1986;27:291-303.
55. Page DH, El-Hosseiny F, Winkler K, Lancaster APS. Elastic modulus of single wood pulp fibers. TAPPI Journal 1977;60(4):114-117.
56. Hartler N, Nyren J. Transverse Compressibility of Pulp Fibers II. Influence of Cooking Method, Yield, Beating and Drying. TAPPI Journal 1970;53(5):820-823.
57. Wild P, Omholt I, Steinke D, Schuetze A. Experimental characterization of the behaviour of wet single wood pulp fibres under transverse compression. Journal of Pulp & Paper Science 2005;13(3):116-120.
58. Tchepel MV, McDonald JD, Dixon T. The effect of peroxide bleachingon the mechanical properties of black spruce fibres. Journal of Pulp & Paper Science 2006;32(2):100-104.
59. Orso S, Wegst UGK, Arzt E. The elastic modulus of spruce wood cell wall material measured by an in situ bending technique. Journal of Material Science 2006;41:5122-5126.
60. Burnham NA, Colton RJ, Pollock HM. Interpretation of force curves in force microscopy. Nanotechnology 1993;4(2):64-80.
61. Favis BD, Goring DAI. A model for the leaching of lignin macromolecules from pulp fibers. Journal of Pulp & Paper Science 1984;10(5):139-143.
62. Sakurada I, Nukushima Y, Ito T. Experimental determination of the Elastic Modulus of Crsytalline Regions in Oriented Polymers. Journal of Polymer Science 1962;57:651-661.
63. Zadorecki P, Michell AJ. Future prospects for wood cellulose as reinforcement in organic polymer composites. Polymer Composites 1989;10(2):69-77.
64. Gray DG. Polypropylene transcrystallization at the surface of cellulose fibers. Journal of Polymer Science, Polymer Letters Edition 1974;12(9):509-15.
Page 75 of 75
65. Gray DG. Transcrystallization induced by mechanical stress on a polypropylene melt. Journal of Polymer Science, Polymer Letters Edition 1974;12(11):645-50.
66. Quillin DT, Caufield DF, Koutsky JA. Crystallinity in the polypropylene/cellulose system. I. Nucleation and crystalline morphology. Journal of Applied Polymer Science 1993;50(7):1187-94.
67. Lenes M, Gregersen OW. Effect of surface chemistry and topography of sulphite fibres on the transcrystallinity of polypropylene. Cellulose (Dordrecht, Netherlands) 2006;13(4):345-355.
68. Son S-J, Lee Y-M, Im S-S. Transcrystalline morphology and mechanical properties in polypropylene composites containing cellulose treated with sodium hydroxide and cellulase. Journal of Materials Science 2000;35(22):5767-5778.
69. Felix JM, Gatenholm P. Effect of transcrystalline morphology on interfacial adhesion in cellulose/polypropylene composites. Journal of Materials Science 1994;29(11):3043-9.
70. Zafeiropoulos NE, Baillie CA, Matthews FL. A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials. Composites, Part A: Applied Science and Manufacturing 2001;32A(3-4):525-543.