Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR REDUCING BRIGHTNESS REVERSION OF MECHANICAL
PULPS AND HIGH-YIELD CHEMICAL PULPS
Background of the Invention
Field of the Invention
The present invention relates to fibrous products. In particular, the present
invention
concerns a process for reducing the susceptibility of lignocellulosic material
to unwanted
brightness reversion, in particular to brightness reversion caused by light or
heat.
Descriution of Related Art
It is well-known in the art that light (UV light in particular), heat,
moisture and chemicals
can give rise to changes in the brightness of cellulose pulps. Usually, such
changes result
in reduced reflectivity, particularly in blue light. This phenomenon is known
as brightness
reversion or yellowing and can be caused by various factors depending on which
type of
pulp is concerned. Heat and damp are the main causes of the brightness
reversion of
chemical (lignin-free) pulps, whereas mechanical pulps mostly yellow when they
are
exposed to light. The brightness reversion of mechanical pulps also varies
depending on
the raw material (type of wood), production method (with or without chemical
pretreatment), and after-treatment (bleaching with different reagents) used.
Thus, for
instance, sulphonation and peroxide bleaching greatly increase the
susceptibility of pulp to
light-induced yellowing.
The brightness reversion of lignocellulosic pulps and products made from such
pulps can
be reduced or even prevented in various ways, for instance by means of
impregnation or
surface treatment using UV screens, antioxidants, or polymers, or by coating
the surface
with a coating layer or a layer of non-yellowing chemical pulp. Various
additives are
described in the patent literature. Thus, US 4,97,363 discloses a composition
and method
for treating fibers based on a mixture of an organopolysiloxane having at
least one amino-
substituted hydrocarbon radical directly bonded to a silicon atom and a higher
fatty
carboxylic acid. The carboxylic acid reacts with the amino radicals to reduce
yellowing
and oxidation of the fiber treatment. The composition and method provide non-
yellowing
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fibers and a treatment agent that does not gel during use, such as when
exposed to carbon
dioxide andlor used to treat carbon fibers.
US 6,599,326 discloses inhibition of pulp and paper yellowing using
hydroxylamines and
other coadditives. Chemical pulps or papers, especially kraft pulps or papers,
which may
still contain traces of lignin, have enhanced resistance to yellowing when
they contain an
effective stabilizing amount of a N,N-dialkylhydroxylamine, an ester, amide or
thin
substituted N,N-dialkylhydroxylamine or N,N-dibenzylhydroxylamine or an
ammonium
salt thereof. This performance is often further enhanced by the presence of
one or more
coadditives selected from the group consisting of UV absorbers, polymeric
inhibitors,
nitrones, fluorescent whitening agents and metal chelating agents.
Combinations of
hydroxylamines or their salts, benzotriazole or benzophenone UV absorbers and
a metal
chelating agent are, according to the cited patent, considered particularly
effective. As
specific examples, the patent mentions N,N-diethylhydroxylamine and N,N-
dibenzyl-
hydroxylamine.
Many of the additives that have been found to prevent yellowing are expensive
or
problematic from an environmental point of view; others are only effective
when
introduced in so large amounts that they may have a negative effect on other
properties of
the product or be uneconomical. Accordingly, there is still a need for methods
of
preventing yellowing
Summary of the Invention
It is an aim of the present invention to eliminate the problems of the prior
art and to
provide a new method of reducing or preventing yellowing. The method aims at
effectively
reducing both light- and heat-induced brightness reversion of mechanical pulps
and high-
yield chemical pulps.
The invention is based on the finding that the reactions that take place
during oxidation, in
particular enzymatic oxidation, of lignin appear to be similar to the
reactions that cause
brightness reversion. Therefore, the initial reaction causing brightness
reversion can be
activated by enzymatic or chemical means and simultaneously immediately
blocked by
targeted functionalization, by retarding or stopping the reactions.
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Thus, the present invention provides a method of modifying fibres by bonding
of new
compounds to the oxidized fibres via radical pathways. In particular, the aim
of the
bonding of the compounds is to stabilize the structure by forming a colourless
lignin
derivative unable to participate in yellowing reactions.
According to the invention, new fibrous products with modified properties are
produced by
activating the fibres of the matrix with an oxidizing agent capable of
oxidizing phenolic or
similar structural groups, which may undergo reactions conducive to the
formation of
coloured sites on the fibres, and attaching to the oxidized sites at least one
modifying agent
to block the reactivity of the oxidized sites. The activation is preferably
carried out
enzymatically although it is equally possible to use chemical agents for
achieving
oxidationlradicalization.
The modifying agent has at least one functional site or reactive structure,
which provides
for binding of the modifying compound to the lignocellulosic fibre material,
in particular at
the oxidized phenolic groups or corresponding chemical structures of the
fibres, which
have been oxidized during the activation step.
Based on the above, the present invention provides a process for producing a
fibre material
having increased resistance to brightness reversion, comprising a
lignocellulosic fibrous
matrix with phenolic or similar structural groups and a modifying agent
reducing the
susceptibility of yellowing, including the steps of
- reacting the lignocellulosic fibrous matrix with an oxidizing agent in the
presence of a catalyst capable of catalyzing the oxidation of phenolic or
similar
structural groups by said oxidizing agent to provide an oxidized fibre
material,
and
- contacting the oxidized fibre material with a modifying agent containing at
least
one first functional site, which is capable of bonding to oxidized fibre
material,
said modifying agent being capable of imparting to the lignocellulosic fibre
material improved resistance to brightness reversion caused by light or heat
or
combinations thereof.
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It should be noted that the term "catalyst" is to be given a broad
interpretation in the
present context, and it covers any agent capable of possibly - but not
exclusively - in
combination with a separate oxidation agent, of achieving oxidation of the
phenolic or
similar groups.
Another embodiment of the invention provides a method of reducing light or
heat induced
brightness reversion of mechanical or high-yield chemical pulp, comprising the
steps of
enzymatically or chemically oxidizing phenolic groups of the pulp and bonding
to the
oxidized phenolic groups a substance capable of forming a colourless lignin
derivative
unable to participate in yellowing reactions.
More specifically, the present invention is mainly characterized by what is
stated in the
characterizing parts of claims 1 and 18.
The present invention provides important advantages. Importantly, the
invention makes it
possible to produce novel kinds of fibrous materials having improved
brightness reversion.
By means of the process, the modifying agents can be reliable attached to the
fibres, and
the improved resistance to yellowing will not be significantly impaired by,
e.g., extensive
washing of the fibres prior to forming the material into a paper or cardboard
web.
~0
Further details and advantages of the invention will become apparent from the
following
detailed description and the appended working examples.
Brief Descriution of the Drawings
Figure 1 depict in graphical form yellowing of spruce TMP samples as function
of
irradiation energy.
Detailed Descriution of the Invention
As mentioned above, the invention generally relates to a method of producing
fibre
compositions with reduced susceptibility to yellowing.
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The fibre matrix comprises fibres containing phenolic or similar structural
groups, which
are capable of being oxidized by suitable oxidizing agents. Such fibres are
typically
"lignocellulosic" fibre materials, which include fibre made of annual or
perennial plants or
wooden raw material by, for example, mechanical, chemimechanical or chemical
pulping.
5 During industrial refining of wood by, e.g., refiner mechanical pulping
(RMP), pressurized
refiner mechanical pulping (PRMP), thermomechanical pulping (TMP), groundwood
(GW) or pressurized groundwood (PGW) or chemithermomechanical pulping (CTMP),
a
woody raw material, derived from different wood species as for example
hardwood and
softwood species, is refined into fine fibres in processes, which separate the
individual
fibres from each other. The fibres are typically split between the lamellas
along the
interlamellar lignin layer, leaving a fibre surface, which is at least partly
covered with
lignin or lignin-compounds having a phenolic basic structure
Within the scope of the present invention, also chemical pulps are included if
they are
susceptible to brightness reversion and have a residual content of lignin
sufficient to give at
least a minimum amount of phenolic groups necessary for providing binding
sites for the
modifying agent. Generally, the concentration of lignin in the fibre matrix
should be at
least 0.1 wt-%, preferably at least about 1.0 wt-%.
In addition to paper- and paperboard-making pulps of the above kind, also
other kinds of
fibres of plant origin can be treated, such as bagasse, jute, flax and hemp.
An essential feature of the invention is to block brightness reversion by
modifications of
phenolic hydroxyls, alfa-carbonyls and/or alfa-hydroxyls on the fibres. In
particular, by
subjecting lignin structures to enzymatic oxidation to yield oxidized groups
of the afore-
said kind, the normal reactions causing brightness reversion can be attained.
These
reactions are then stopped by bonding a desired compound to the activated,
oxidized
groups.
In the first stage of the present process, the lignocellulosic fibre material
is reacted with a
substance capable of catalyzing the oxidation of phenolic or similar
structural groups to
provide an oxidized fibre material. Typically, the substance is an enzyme and
the
enzymatic reaction is carried out by contacting the lignocellulosic fibre
material with an
oxidizing agent, which is capable - in the presence of the enzyme - of
oxidizing the
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phenolic or similar structural groups to provide an oxidized fibre material.
Such oxidizing
agents are selected from the group of oxygen and oxygen-containing gases, such
as air, and
hydrogen peroxide. Oxygen can be supplied by various means, such as efficient
mixing,
foaming, gases enriched with oxygen or oxygen supplied by enzymatic or
chemical means,
such as peroxides to the solution. Peroxides can be added or produced in situ.
According to an embodiment of the invention, the oxidative enzymes capable of
catalyzing
oxidation of phenolic groups, are selected from, e.g. the group of
phenoloxidases
(E.C.1.10.3.2 benzenediol:oxygen oxidoreductase) arid catalyzing the oxidation
of o- and
p-substituted phenolic hydroxyl and aminolamine groups in monomeric and
polymeric
aromatic compounds. The oxidative reaction leads to the formation of phenoxy
radicals.
Another groups of enzymes comprise the peroxidases and other oxidases.
"Peroxidases"
are enzymes, which catalyze oxidative reaction using hydrogen peroxide as
their electron
acceptor, whereas "oxidases" are enzymes, which catalyze oxidative reactions
using
molecular oxygen as their electron acceptor.
In the method of the present invention, the enzyme used may be for example
laccase,
tyrosinase, peroxidase or oxidase, in particular, the enzyme is selected from
the group of
laccases (EC 1.10.3.2), catechol oxidases (EC 1.10.3.1), tyrosinases (EC
1.14.18.1),
bilirubin oxidases (EC 1.3.3.5), horseradish peroxidase (EC 1.11.1.7),
manganese
peroxidase (EC1.11.1.13) and lignin peroxidase (EC 1.11.1.14).
The amount of the enzyme is selected depending on the activity of the
individual enzyme
and the desired effect on the fibre. Advantageously, the enzyme is employed in
an amount
of 0.0001 to 10 mg protein/g of dry matter fiber.
Different dosages can be used, but advantageously a dosage of about 1 to
100,000 nkat/g,
more advantageously 10-500 nkat/g.
In addition to enzymes, also chemical agents, such as alkali metal
persulphates and
hydrogen peroxide and other per-compounds, can be used for achieving
oxidization of the
phenolic groups and for forming phenoxy radicals. The dosage of the chemical
agent is,
depending on the chemical agent and on the pulp (i.e. on the amount of
phenolic groups
contained therein), typically in the range of about 0.01 to 100 kg/ton,
preferably about 0.1
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to about 50 kg/ton, e.g. about 0.5 to 20 kg/ton. In the case of chemical
agents, no separate
oxidation agent needs to be added. The per-compound will achieve the aimed
oxidation of
the phonolic groups.
The activation treatment is carried out in a liquid medium, preferably in an
aqueous
medium, such as in water or an aqueous solution, at a temperature in the range
of 5 to 100
°C, typically about 10 to 85 °C. Normally, a temperature of 20 -
80 °C is preferred. The
consistency of the pulp is, generally, 0.5 to 95 °lo by weight,
typically about 1 to 50 °lo by
weight, in particular about 2 to 40 % by weight. The pH of the medium is
preferably
slightly acidic, in particular the pH is about 2 to 10, in the case of
phenoloxidases. The
chemical agents are usually employed at slightly acidic conditions, such as at
pH 3 to 6.
Peroxidases are typically employed at pH of about 3 to 12. The reaction
mixture is stirred
during oxidation. Other enzymes can be used under similar conditions,
preferably at pH 2 -
10.
In the second step of the process, a modifying agent capable of reducing the
susceptibility
to yellowing of lignocellulosic fibres is bonded to the oxidized phenolic or
similar
structural groups of the matrix. Such a modifying agent typically exhibits at
least one first
functional site, which is compatible with the fibrous matrix, and at least one
second
functional site or structure providing for the above technical effect, as will
be explained in
more detail below.
The first functional site comprises in particular functional groups, which are
capable of
contacting and binding to the fibre at the oxidized phenolic or similar
structural groups or
at its vicinity. The bond formed between the oxidized phenolic or similar
residue can be
covalent or ionic or even based on hydrogen bonding. Typical functionalities
of the first
functional site include reactive groups, such as hydroxyl (including phenolic
hydroxy
groups), carboxy, anhydride, aldehyde, ketone, amino, amine, amide, imine,
imidine and
derivatives and salts thereof, to mention some examples. Also electronegative
bonds, such
as carbon-to-carbon double bonds, carbon-to-hetero atom (e.g. C=N, C=O) as
well as oxo
or azo -bridges can provide for bonding to the oxidized residues.
Tt is essential that the modifying agent is chemically or physically bonded to
the fibre
matrix to such an extent that at least an essential part of it cannot be
removed. One
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criterion, which can be applied to test this feature, is washing in aqueous
medium, because
often the fibrous matrix will be processed in an aqueous environment, and it
is important
that it retains the new and valuable properties even after such processing.
Thus, preferably,
at least 10 mol-%, in particular at least 20 mol-%, and preferably at least 30
mol-%, of the
modifying agent remains attached to the matrix after washing or leaching in an
aqueous
medium.
According to an embodiment of the invention, the modifying agent is activated
with an
oxidizing agent.
The interaction of the oxidized lignocellulosic material and the modifying
agent, resulting
in bonding of the modifying agent to the lignocellulosic material, typically
takes place in
liquid phase, usually in water or in another aqueous medium. The pulp or other
lignocellulosic fibrous matrix is suspended in the medium and it is contacted
with the
modifying agent or a precursor thereof, which is dissolved or dispersed in the
same
medium. The conditions can vary freely, although it is preferred to carry out
the contacting
under mixing or stirnng. The temperature is generally between the melting
point and the
boiling point of the medium; preferably it is about 5 to 100 °C.
Depending on the
modifying agent or its precursor, the pH of the medium can be neutral or
weakly alkaline
or acidic (pH typically about 2 to 12). It is preferred to avoid strongly
alkaline or acidic
conditions because they can cause hydrolyzation of the fibrous matrix. Normal
pressure
(ambient pressure) is also preferred, although it is possible to carry out the
process under
reduced or elevated pressure in pressure resistant equipment. Generally, the
consistency of
the fibrous material is about 0.5 to 95 % by weight during the contacting
stage.
According to a particularly preferred embodiment, the first and the second
stages of the
process are carried out in the same reaction medium, without separating the
fibrous matrix
after the oxidation step. The conditions (consistency, temperature, pH,
pressure) can,
though, even in this embodiment be different during the various processing
stages.
The first and the second stages of the process are carried out sequentially or
simultaneously. However, it should be noted that the first step of the process
aims at the
formation in the fibrous substrate of phenoxy radicals, which are capable of
binding
modifying agents. Some modifying agents will form substrates for the oxidative
enzymes
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used in the invention, and in that case, it is preferred to first add the
oxidative enzymes and
to allow the enzyme interact with the fibrous substrate containing phenolic or
similar
groups, e.g. for 0.1 to 180 minutes, in particular about 1 to 30 minutes to
achieve oxidation
of the phenolic groups, and to add the modifying agents after the enzymatic
oxidation.
The same observations are true for the chemical oxidation agents mentioned
above. As
Example 3 shows, reasonably good results are obtained with the simulataneous
application
of oxidation agent and modifying agent, although the best results are attained
when steps
one and two are carried our sequentially.
According to one preferred embodiment, the modifying agent is an aliphatic or
aromatic,
monocyclic, bicyclic or tricyclic substance. The aliphatic compound can be an
unsaturated
carboxylic acid, advantageously a monocarboxylic unsaturated fatty acid,
having 4 to 30
carbon atoms. In particular, the modifying agent can be a monocarboxylic,
unsaturated
fatty acids containing a minimum of two double bonds, preferably two
conjugated double
bonds. Such fatty acids have an even number of carbon atoms, typically in the
range of 16
to 22. It is also possible to use lower alkanols, i.e. alcoholic compounds
comprising 1 to 6,
in particular 1 to 4 carbon atoms. Examples include n- and i-propanol and n-
and t-butanol.
Examples of particularly suitable compounds are constituted by linoleic and
linolenic acid.
It would appear that the unsaturated fatty acid bonds to the oxidized groups
or structure via
one of the double bonds.
Other suitable compounds include antioxidants, such as tocopherol and beta-
carotene.
The compound can have special properties, such as capability to trap radicals
and form
colourless substituents.
The above two steps can be carried sequentially or simultaneously. Also other
compounds,
such as papermaking chemicals may be present during the reaction steps.
After the above processing, the modified fibre having new properties is
generally separated
from the liquid reaction and further used in target applications.
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The following non-limiting examples illustrate the invention:
Example 1
5 A 5 g portion of bleached spruce TMP was suspended in water. The pH of the
suspension
was adjusted to pH 4.5 by addition of acid. The suspension was stirred at RT.
Lactase
dosage was 1000 nkatlg of pulp dry matter and the final pulp consistency was
7.5 %. After
30 minutes lactase reaction, 0.15 mmol linoleic acid/g of pulp dry matter was
added to the
pulp suspension. After 1 h total reaction time, the pulp suspension was
filtered and the pulp
10 was washed thoroughly with water. Handsheets were prepared. For comparison
purposes,
reference treatments were carried out using the same procedure as described
above but
without addition of lactase or linoleic acid or both. The light-fastness on
the pulps was
tested with Xenotest 1505 light exposure and weathering test instrument using
"window
glass" filter. The brightness of the handsheets was measured as function of
irradiation
dosage. The results are presented graphically in Fig. 1.
From the results presented in Fig. 1, it is apparent that the addition of
linoleic acid and
lactase was found to decrease the yellowing tendency of the pulp. In other
words, addition
of a modifying agent in the presence of an oxidizing agent and a suitable
catalyst, the
yellowing tendency of pulp was decreased.
Example 2
Bonding of new compounds to TMP
A 5 g portion of spruce TMP was suspended in water. The pH of the suspension
was
adjusted to pH 4.5 by addition of acid. The suspension was stirred at RT.
Lactase dosage
was 1000 nkatlg of pulp dry matter and the final pulp consistency was 7.5 %.
After 30
minutes lactase reaction the new compound was added to the pulp suspension.
After 1 h
total reaction time, the pulp suspension was filtered and the pulp was washed
thoroughly
with water. Handsheets were prepared. For comparison purposes, reference
treatments
were carned out using the same procedure as described above but without
addition of
lactase or the new compound. The light-fastness on the pulps was tested with
Xenotest
150S light exposure and weathering test instrument using "window glass"
filter. The
changes in the ISO brightnesses after irradiation are summarized in Table 1.
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Table 1
Treatment Irradation a Brightness
(Whm2) (as ISO-
Brightness)
TMP Reference 1260 10
TMP + laCCase + ferulic acid (0.151260 3
mmol/g)
TMP + lactase + vinyl laurate (0.31260 2
mmol/g)
Example 3
Sample A: Peroxide bleached aspen-CTMP-pulp was treated with sodium
persulphate
(dosage 5 kg/ton of pulp) and linoleic acid (5 kg) at 80 °C, at pH 5
for 60 minutes. The
treatment was carried out at a consistency of 10 %.
Sample B: The pulp sample was treated in the same way as Sample A except that
ammonium persulphate (5 kg) was used instead of Na-persulphate.
Sample C: The pulp sample was treated in the same way as Samples A and B
except that
hydrogen peroxide was used instead of persulphate. The pH of the test was 4.
Sample D: The pulp sample was treated as Sample A but t-butanol (5 kg) was
used instead
of linoleic acid.
Sample E: The pulp sample was treated in the same way as Sample A, but no
linoleic was
added. After the treatment with persulphate, a separate treatment was made
with linoleic
acid (5 kg) at 80 °C at a consistency of 10 %. The duration of the
treatment was 30 min,
and the pH was 5
Sample F: The sample was prepared as Sample D, but without using any t-
butanol. After
the persulphate treatment, a separate treatment (30 min, pH 5) with t-butanol
was carried
out at a Consistency of 10 % and a temperature of 80 °C, the dosage
being 5 kg/ton of pulp.
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Sheets were manufactured from the pulp samples and their brightness stability
was tested
with a Xenotest S 150 using a "window pane" filter. The radiation of the
Xenotest -
apparatus corresponded to that of sunlight through a window pane, but the
intensity of the
radiation was much stronger (accelerated test). The brightness of the samples
was
determined after a 2 h radiation (corresponds to 1260 wh/m2)
The results are indicated in Table 2 below:
Table 2
Sample Brightness reduction (d Brightness, °IoISO)
Reference (untreated) 10.4
A 6.9
B 7.2
C 6.8
D 7.2
E 6.1
F 6.1
As apparent from the above results, the brightness stability of the samples
treated by the
present invention has been improved by even more than 4 units.
