Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PREPARATION OF PAPER
The current invention concerns a process for the preparation of paper in
which an aqueous suspension of comminuted cellulose material is mechanically
treated and is subjected to a sheet-making process.
It is known that cellulose which is to be processed into paper must be milled
before sheet-making. According to DIN 54 360 (November 1977), milling means
mechanical treatment of the cellulose in a suspended state during which its
physical properties are so changed that it becomes suitable for the
preparation of paper. During the milling of the slurry formed by the
cellulose fibres and water, the fibres become cut, fibrillated and swollen.
Fibrillation means the splitting of the fibres parallel to the fibre axis. Aftermilling, the greater part of the fibrils remains hanging like frills on the
fibres. Swelling and fibrillation are of great importance in the preparation
of stron g paper.
The milling process is an energy-intensive process. Among products needing
highly-milled fibrous materials, the milling of cellulose is indeed one of the
most energy-intensive processes in paper-making. Polymeric saccharides,
monomeric and polymeric amines, amides and dyestuffs are recommended as
milling aids (M Milichovsky, Chemische Aspekte der Mahlung von Zellstoff:
Zellstoff und Papier, 38, page 17, (1989)). The degree of milling is given in
degrees Schopper-Riegler (SR). The strength of the resulting paper
generally increases as the degree of milling increases.
Machines such as for instance, Hollander beaters, Jordan mills, refiners, disc
mills and pulpers can be used for milling. The stock slurry is processed
between a moving part and a fixed part of the machine during the milling
process (Handbuch der Papier- und Pappenfabrikation, (Papierlexikon), 2nd
Edition, Vol II, L-Z, 1971).
It is moreover known that the strength of a paper can be further increased
with additives. Such materials are for example natural rubbers and
carboxymethyl cellulose. It is also known from US-A 3 447 956 that paper
which has had chemical treatment, wherein it is swollen at a temperature
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above 100 C in a molten tertiary amine-oxide, has a higher strength.
In US-A 3 503 700 it is said that paper which is soaked in a 25% aqueous
solution of a tertiary amine-oxide, dried and heated to above 200tand then
cooled and rinsed with water to wash out the tertiary amine-oxide again, has
a higher strength after such treatment.
On the other hand problems which occur in the processing of ~steE~per are
increasingly described in the literature. A phenomenon well known to the
paper-maker is that cellulose fibres which have already been used one or
more times for paper-making are no longer very suitable for renewed sheet-
making. In the specialist world, the opinion is held that fibres which have
already been milled once should not be milled any more (J Lumiainen, Refining
of Recycled Fibre - Advantages and Disadvantages, Papermakers Conference,
187, (1992)). Attempts to solve this problem have been made using special
milling techniques and by the addition of auxiliary materials which are
uncommon in paper. These auxiliary materials are supposed to increase the
strength of the paper, among other features.
The invention has as its object the further development of the conventional
process for paper-making, in order to allow the preparation of paper with a
higher strength without the use of conventional auxiliary materials, whereby
the milling time should be as short as possible.
The process according to the invention for~ the preparation of paper
commences with a process in which an aqueous suspension of comminuted
cellulose material, for example half-stuff, is mechanically treated and is then
subjected to a sheet-making process, characterised in that, the mechanical
treatment is carried out in the presence of a tertiary amine-oxide. The
mechanical treatment is preferably carried out in a refiner.
Tertiary amine-oxides are known to be alternative solvents for cellulose.
From US-PS 2 179 181 for example it is known that tertiary amine-oxides have
the property of dissolving cellulose without converting it into
derivatives, and that cellulose mouldings such as fibres can be obtained from
these solutions by precipitation. In US-PS 3 447 939, US-PS 3 447 956 and
US-PS 3 508 941 further processes for the preparation of cellulose solutions
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are described whereby the solvents used are preferably cyclic amine-oxides.
In all these processes however cellulose is physically dissolved at elevated
temperatu res.
It has been shown that with the process according to the invention, even waste
paper can be added to the cellulose material, for example cellulose, and that
even then the strength of the resulting paper is high. In this way therefore
~ste~r can be reused in the process according to the invention. It is
known that ~ste ~ or cellulose fibres which have been recycled many times
are not very suitable for renewed sheet-making. The given embodiment of
the process according to the invention is based on the finding that
comminuted ~ste ~:., which is subjected to a renewed milling process in the
presence of a tertiary amine-oxide together with another cellulose material, is
very suitable for use in the preparation of a high-grade paper.
A preferred embodiment of the process according to the invention consists of
N-methylmorpholine-N-oxide being used as the tertiary amine-oxide.
The best concentration of tertiary amine-oxide in the aqueous phase of the
suspension is between 55 and 72 wt%.
The invention further concerns the use of a tertiary amine-oxide as an
auxiliary material in the mechanical treatment of comminuted material
containing cellulose for the preparation of paper. The use of N-methyl-
morpholine-N-oxide has proved to be especially advantageous.
The invention is further described by the following Examples.
Examples
In order to reveal the effect of tertiary amine-oxides, cellulose from both
sulphate and sulphite wood pulping processes were subjected to a
standardised milling treatment at room temperature, in accordance with DIN 54
360 (November 1977) of the Standards Committee for Paper and Cardboard of
the German Institute for Standards (registered Society). The reason for such
milling treatment is to set standards for assessing the strength properties of
the cellulose.
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This is only possible when test sheets are prepared under given and
reproducible conditions, and their strength properties are measured.
In accordance with the process described in DIN 54 360, suspensions of
prehydrolysed sulphate cellulose ('Buckeye V5 high-grade pulp'), a sulphite-
cellulose (beechwood digested-cellulose) and another beechwood digested-
cellulose which had been mechanically premilled (sieve size 350 llm) were milledin a Jokro mill to varying SR degrees, whereby the consistency of the
suspensions was standardised at 12%. These celluloses are not classical
celluloses for paper-making; they were chosen so that the beneficial effects
of amine-oxide could be better demonstrated. Using classical paper
celluloses, still better effects would have been achieved without exception.
Not only water but also an aqueous solution of NMMO (60 wt% NMMO, 40 wt%
water) was used as the suspension medium in every test.
Sheet-making, using the respective cellulose suspensions which were obtained,
was carried out according to DIN 54 358 T1 (February 1981). Strength
testing (tear length) of the sheets obtained was carried out according to
the standardised process described in DIN 53 112 (October 1981). The results
are g i ven i n th e fol low i n g Tab l es.
Table 1: Pre-hydrolysed sulphate-cellulose
(Southern pine, Buckeye V5, Proctor & Gamble)
Milling time (min) 10 20 30
Suspension medium - water
Milling degree ( SR): 8 9 10
Tear length (m): 700 800 1000
Suspension medium - 60% NMMO, 40% water
Milling degree ( SR): 10 11 12
Tear length (m): 900 1300 1500
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Table 2: Sulphite cellulose
tBeechwood, Lenzing)
Milling time (min) 10 20 30
Suspension med i um - water
Milling degree ( SR): 32 42 47
Tear length (m): 1800 2100 2500
Suspension medium - 60% NMMO, 40% water
Milling degree ( SR): 37 47 54
Tear length (m): 2100 2500 3100
Table 3: Sulphite cellulose (premilled)
(BKZ premilled, Lenzing)
Milling time (min) 10 20 30
Suspension medium - water
Milling degree ( SR): 51 63 72
Tear length (m): 1100 1700 1600
Suspension medium - 60% NMMO, 40% water
Milling degree ( SR): 57 67 67
Tear length (m): 1400 1600 1900
It can be seen from all the Tables that the tear length of a given paper
increases with increasing milling time, and that in every case the presence of
NMMO gives the paper a greater tear length. This effect is especially
marked with prehydrolysed sulphate-cellulose (see Table 1) where the
presence of NMMO during milling of the cellulose fibres increases the tear
length of the paper obtained by up to 50%.
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It can also be seen from all the Tables that the presence of NMMO leads to
more efficient milling because higher degrees of milling are attained in a
shorter period of time. This is naturally reflected in a saving of energy.
It is astonishing that the increase in the degree of milling is not proportionalto the tear length. Thus for example, the presence of NMMO during the
30 minute milling of prehydrolysed sulphate-cellulose effects a 20% increase
in the degree of milling but a 50% increase in the tear length.
It can also be seen that the premilled sulphite-cellulose (Table 3), which on
account of its poor fibre-forming properties after 30 minutes milling gives a
paper with a tear length of only 1600 m, gives with the addition of NMMO
a higher quality paper with a tear length of 1900 m; this latter paper
is comparable with a paper prepared from a higher grade sulphite-cellulose
(Table 2) using a suspension milled for 10 minutes without the addition of
NMMO. This means that a material which can be processed by conventional
means to give only a low grade paper, can be processed to give a high
quality paper by milling in the presence of NMMO.
The process according to the invention is suitable for all types of cellulose
and is independent of the chosen digestion process.
With the process according to the invention moreover, even waste paper
consisting of fibres which have already been subjected several times to a
conventional milling process and to sheet-making, can be processed together
with cellulose to give high quality paper without additional auxiliary materialshaving to be added.
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SUMMARY
Process for the preparation of paper in which an aqueous suspension of
comminuted cellulose material is mechanically treated and is subsequently
subjected to a sheet-making process, characterised in that, the suspension
contains a tertiary amine-oxide. The comminuted cellulose material can come
partly from waste paper.
