Note: Descriptions are shown in the official language in which they were submitted.
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~~ 93/2S358 ~13 6 5 3 7 PC~r/SE93/003g8
Manu~acture of board
This invention relates to the manufacture of board of
lignocellulose-containing material, such as fiber board.
Board of this kind is manufactured by disintegrating the
material to particles containing fiber bundles and fibers. Glue
is added to the disintegrated material and the material is dried
and a web i5 formed which finally is pressed at increased
temperature for curing the glue so that a hGiard is formed. Fiber
board is made of material which is disintegrated by defibering,
i.e. of more or less free fibers.
The binding agent normally used is urea formaldehyde
glue~ whloh is admixed to the disintegrated material. The amount
of glue can be 6% or more. The ~lue is relatively expensive, and
it is therefore desired to minimize the glue addition. A smaller
amount of glue, however, deteriorates the strength of the
completed board. In view thereof, the glue addition must be
optimized~
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~` ~ ;At the drying of the glued material, a great or small
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~i,! ~ ~ ~' amcunt o~ water soluble formaldehyde is given off from the glue.
After the separation o~ the drying gas from the fiber material,
~h' ~ for example in a cyclonio, the liquid content is condensed ou~ of
the gas. In conventional cleaning proc~ses th~ water soluble
formaldehyde then causes problems. The formaldehyde following
along with the material to the final pressing also can caus~
problems when it is successively given off from the completed
board. When glue is added to the material after its drying, the
problem with formaldehyde separation arises at the pressing as
~we~ll as from the co~pleted board.
;~Fiber board manufactured according to the dry method
~; ~normally i$ called ~DF ~Medium Density Fiberboard~ and~is a board
praduct made of wood fiber. The yield exceeds 95~. The starting
material preferably is wood chips, which are defibered with
;relatively low energy input, 100-500 kWh/ton fibers, for freeing
the ~i~ers. The defibered fiber material is glued, dried, formed
',J~to a web and pressed in a hot press. Alterna~ively, the material
can be dried prior to the gluing. The material normally is
preheated prior to the defibering to above 150C whereafter the
defibering is carried out under pressure at this high temperature
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W O 93/25358 ~ 1 3 6~ 2 PCT~SE93/00388
in order to facilita~e the freeing of the fibers. At such a high
temperature, however, the material can be degrada~ed and
~ miscoloured due to occurring hydrolysis. A short heating time
j~ reduces this shortcoming, but miscol~uring yet cannot be avoided,
and at the same time the energy consumption increases.
The urea for~aldehyde glue normally used can be mixed
with the fiber material on the conveying line (blow line) from
the defibering apparatus or in a separate mixing device. The
amount of admixed glue can be 6~ or more.
When the fiber material subsequently is dried in a tube
drier by means of gas at high temperature, a great or small
amount of water soluble formaldehyde is given off from the glue.
~s stated above, the formaldehyde following along with the fiber
mate~ial to the final pressing also can cause problems when it is
given off successively from the completed board. This applies
especially to gluing after the drying of the material, which can
be carried out at the manufacture of fiber board.
The aforesaid problems c~n be reduced by the present
d~: ~ invention, which simultaneously offers further advantages.
According to the invention, the fiber material is treated with an
impregnation liquid containing fiber surface activating chemicals
and preheated be~ore it is finally disintegrated.
Examples of~such chemicals are such which affect the
lignin, such as Na2503 and NaHS03. These chemicals can be added in
an amount of 1-30 kg/ton material expressed as Na2S03. According
;~ to the invention, it is possible to increase the ISO-brightness
of the completed product, to reduce the energy consumption at th~
defibering, to reduce the glue amount at maintained strength or
to increase the strength at maintained glue amount. Furthermore,
al~o ~he formaldehyde separation can ~e reduced substantiaily.
The inYention is described in greater detail in the
ollowing with reference to some embodiments thereof.
Fiberboard is manufactured by i~pregnating the raw
material in the form of chips of, for example, softwood with
lignin affectlng chemicals, such as Na2S03 and NaHS03, in an
amount of 1-30, preferably 10-20 kg/ton fiber material expressed
3~ ~ as Na~S03.
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~ ~093~25358 ~13 6 5 ~ 7 PCT/SE93/00388
The impregnation, which has to be carried out rapidly,
can last some minute. Thereafter the temperature is increased to
150-200C, preferably 160-180C, for 2-3 minutes. However, the
preheating can of course be performed prior to or at the same
time as the impregnation. The pH value is kept within 2-12. This
implies, that an impregnation solution with ~a2S03 and NaHSO3 will
contain sulphite in the form of HSO3- and/or S032-. The pH value
suitably can be controlled by NaOH. Due to this chemical
treatment ~sulphonation~, the subsequent defibering can be
carried out to the desired defibering degree with an energy input
lower than normal. The energy consumption is in the interval 50-
400 kWh/ton fiber material~ The energy saving can amount to about
10% compared with conventional defibering.
Owing to the impregnation, also the brightness of the
completed fiber board increases.
As regards the fiber disintegration in the defibered
material, it was found that both the fine material content and
the shives content were reduced, i.e. the proportion of prima
fiber has increased.
It~was also found at the impregnation according to the
invention tha~ the strength properties of the complited board
have improved substantially. The tensile~strength as well as the
elasticity modules and the fiber bond have increased by at least
10%. Alternatively, thi~ effect can be utilized for reducing the
amount of~glue addition at main~alned s~rength prop~rties. A 10%
increase in the fiber bond corresponds to a 1% decrease in glue
addition. The sulphonation according to the invention also
implies, th3it the separation of formaldehyde can be reduced, due
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~,!,', ~ ~ ~ to the fact, that the sulphite ions in the impregnation solution
s ~ l~ reac~with the formaldehyd~ so that it is bound in thel fiber
material instead of bein~ given off from the completed board.
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W093/25358 21 36 53 7 4 PCT/SE93/00388
EXAMPLE
Raw material in the form of chips was exposed to
different impregnation treatment. One batch was treated with
~l 10 kg Na2SO3 per ton chips, and one batch with 10 ~g NaHSO3 per
ton chips. A reference batch was not impregnated.
The different batches were preheated to about 170C and
`~ thereafter defibered. In the blow line after the defibering, urea
~,j formaldehyde glue of the type E2 was added in an amount of lO~
dry glue cal ulated on dry fiber. The material was thereafter
dried to a dry matter content of about 90%. The fibers were dry
formed to a mat, which was prepressed with a pressure of about
1.5 MPa and hot pressed for 5-6 minutes at 170C. The board thus
manufactured was tested on strength, ISO-brightness and
formaldehyde content ~perforator value). The relative energy
consumption for the different batches was also measured. The
results were as follows:
Property Reference Na2SO3 NaHSO3
batch atch batch_
; Bending strength
MPa 38.4 43.8 40.0
Internal bond strength
MPa 1.~ 2.1 2.0
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ISO-brightness
% 24 26 26
Formaldehyde content
':d::
mg CH20/aOO g 27 ~0 18
Relative energy consumptionl.O 0.95 0.95
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The results confirm the technical effects stated above.
'~ The invention, of course, is not restricted to the
embodiments described above, but can be varied within the scope
of the invention idea.
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