Note: Descriptions are shown in the official language in which they were submitted.
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This inve~tio~ relates to a proceQs for manufacturing
lignocellulosic material fibreboard~ comprising the ~teps o~
preparing a bind~r mixture, proviai~g a lignocellulo~ic
material~ shredding said material, dige~ting ~aid shredded
material, grinding the resulting product, drying the fibre,
impregnQting the fibre with said bi~aer mixture and pressi~g it.
The currcnt practice is ~or the m~nu~acturer o~ this type
of ~ibreboard to u~e a binder mixture obtainea from a
urea-formaldehyde resin prepared i~ turn from a co~mercially
available urea-~ormaldehyde preco~densate.
Urea-~ormaldehyde precondensates are commercial high
ooncentratio~ formaldehyde solutions, stabilised with urea.
A resin is prepared from the precondensate 9 by reaction
with urea (or urea and melamine) and this re~n ~s used to
prepare a binder mix~ure with which the ligno¢ellulosic materia1
fibre is impregnated (normally soft woodq are u~ed, such as
pine, eucalyptus, poplar9 etc., but other ~oods and products
such a~ bagasse, straw, etc. may be u~ed) and the impregnated
fibre i~ trans~erred thereafter to a preQ~, where the fibreboard
i~ prepared by heat ( by hot fluids flo~ing through the pre S3
platen or by electrical system3: elements, high frequency) and
pressure. Ne~ertheless, ~everal disadvantageY are encountered
in this proces~.
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In the first placef there i~ a high degree of instability,
sI~ce the res m remai~ ~table for a period o~ less tha~ 48
hours.
In the second place9 the fibre is not correctly i~pregnatea,
since the instability of the re~in ~ea~s that its degree of
polymerization increa~es with time, ~aking it impossible to
obtain correct, ~table fibre impregnstion conditions. This
incorreet i~pregnation gives rise to the appearance of binder
- mixture stai~s on the fibreboard surface~ with the disadvantage
(apart from the irregular visual appearanoe of the board
surface) of causing problems at the time of applying ~inishes
such as pa mt, Yarnish, plastified papers, etc., by the
different degree of absorption in the areas showing dry resin
stains relative to the unstained areasO
In the third place, there are problems i~ the tra~sfer of
the impregnated fibre, since the incorrect fibre impregnation
~nd the high degree of tack of the resin used cause~ the
formation of lumps of bonded fibre (balls of bonded fibre)
giving rise to serious problems in the transfer 9y9tem9 (air or
mechanical) of the impregn~ted fibre to the press.
To solve the above problem~, the invention provides a
proces~ of the above mentioned type characteri~ed essentially
in that said binder mixture is prepared ~rom a tack free
urea-~ormalaehyde resin having a solid~ content lying between
60~ and 70%, preferably 65%, having at 25C, for a 65% solids
content9 a vi8c08ity lying betwee~ 150 and 300 Cp8 and a free
formaldehyde ~slue o~ less than 1~, prefersbly.lying be-t~een
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0.45% and 0.65%~ to which resin th~re i9 added water at a rate
o~ 25% to 30% by weight of the re~in and about 0~2~o of
ammonium chloride or sulphate a~ catalyst. The re ulting binder
mi~ture ha~ a resi~ solids content o~ about 50% and a YiSCosity
at 25~C of less than 30 cps.
In a ~urther development, from 1 to 15 ~ of the urea
in the resin is replaced by a product having a similar
chemical behaviour, preferably melamine or dicyandiamide.
The fibreboard produced by the process a~ the i~ve~tion
provides important Improvements over the hitherto known
~ibreboards7 particularly in comparison ~ith particle board.
The following advantages may be pointed out:
A homogenous appearance of the surface and the inside of
the board. ~his homogenousness even allows mouldings to be
made in it.
Greater bending strength.
Greater resistence to removal of screws in the board.
A~ saia above, the resin used for the preparation of the
bi~der mixture h~s a solid~ content lying bet~een 60% and 70
pre~erably 6~o and at 25C, for a 65~ solids content, it has a
~isco~ity of 150-300 cps.
I~ certain cases, there may be an economic intere~t in
prepar m g a similar resin~ but with a lower solid~ content ~e.g.
wh~n the resin is to be used at a point close to where it i~
menu~actured, the slight increase in the carriage costs
resulting from the greater product weight may be compensated by
the reduction o~ the energy required for concentrating the
resin). Such a re~in ~ith a 60% solid~ content ha~ a viscosity
at 25 QC 0~ 80 to 140 cps.
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In spite of the~e low viscosity values which would indicate
(for resin~ ha~i~g ~imilar solids content3 a~d form~ldehyde and
urea contents) that they are resins with a lo~ level of
polymerization and having high ~ree form~ldehyde le~el~, there
are obtained, surpris~ngly, ~ree for~aldehyde ~alues of le88
than ~, ~ore precisely, ~ying between 0.45 and 0.65%
(determination ef~ected with the traditional "cold sulphite'
method).
When impreg~ati~g the ~ibre, the binder mixture behaves as
though it had no tac~, such that there i~ avoided the ~ormation
of l~mps or "balls" of bo~ded fibre~, giving ri~e to serious
problems in the transfer (air or mechanical) of the bonded
fibres, and there i~ a~oided also the appearance m the finished
fibreboard of irregularities in the mechanical properties and
appearance, which irregularities are caused by the arrival at
the press of some of the~e lumps or ~ball9~ of bonded fibres.
To impregnate the dry fibre with re in, metering me~ns are
required to regulate the proportions o~ the fibre and resin.
The weight ratio normally used is:
~eight dry resi~
- . . = 0.07 - 0.11
Weight dry fibre
The impregnation i~ nor~ally effected at the outlet from
the dryer using a continuously or intermittently operating
~ apparatus in whi~h the fibre i9 mechanic~lly (or pneumatically)
stirred and the previously catalysed resin is sprayed thereon
using sprayer~ operating by direct pres~ure9 c~mpressed air, etc
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XAMPI.~ 1
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The b mder mixture was prepared with a urea-formaldehyde
resin having a 301id~ content of 65%, a visc09ity at 25~
of 225 cps and a ~ree formaldehyde value of 0.50%, in a mixing .
vessel equipped with 3tirring means, in the way and
proportio~s gi~en belo~.
100 parts m ~eight o~ the abo~e resi~ were poured into the
p~eparation ve~el. Thereafter 28 parts in weight of water and
2 parts m weight of 10% ammonium sulphate were added (the
same effects are achieved with 1.8 parts i~ weight oi 10
ammonium chloride) and stirred. The thus prepared resin
solution has a gelling time at 100C of 2 minutes and good
stability on the shelf at 20QC. The viscosity of the binder
mixture i~ 30 cp~ and the resin solids content i9 50%.
EXAMp~E 2
Impregnation of dry fibre with the mixture prepared accordin~
to Example 1.
The fibre used was a mixture of pine and poplar wood chip~,
at a rate of 90~ and 10~ respectively.
~ he fibre was pretreated with a paraf~in emulsion. The
amount of paraffin ~upplied over the dry fibre wa3 1% paraffin
solids .
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A fibre charge of 6,000 g with a 5% moisture level was
prepared. ~he corresponding amount of binder mixture was 912 g,
6,912 g o~ bonded ~ibre were obtained.
EX~PLE 3
~ he fibre impregnatea accordi~g to the previous example
(fibre dried be~ore impregnation) was used for the manufacture
o~ fibreboard ~ith the following proces~.
The impregnated fibre was moulded in a 500 x 500 mm
laboratory ~ormer. It was prepressed cold in a 500 x 500 mm
SrEMPELKAMP laboratory press for 60 sec at a pressure of
15 kg/cm2. ~he edges of the block were trimmed to 450 x 450 mm
prior to hot pressing.
The 6.912 g of impregnated ~ibre were ~ufficient to prepare
a finished board measuring 450 x 450 mm by 17 mm thick, with a
density of 720 kg/m3.
The pressing of the cold prepressed fibre block~ wa~
effected in a SIEMPE~KAMP laboratory press, ha~ing both platens
electrically heated to a temperature of 180QC, under a pressure
of 23 kg/cm 9 with automatic ~entilation device.
; The pressing time was 6 minutes.
After sanding the boards, the re~ults were as follo~:
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SPECIFICATIONS OF THE BOARD
Thick~ess mm 16.1
Density K ~m3 715
Bending strength kg/cm2 345
Tensile strength k ~cm2 8.2
Absorptio~ in ~ater
at 20QC % 10.5
Swell~ng in water
at 20QC ~ 3.7
Resistence to screw
removal on sur~ace kg 145
Resi~tence to screw
remo~al m edge kg 115
The above texts were ef~ected with fibreboards manufactured
with den~ities set to a particular value. They may al~o be
m~nufactured with densities other than those chosen, there `~
~arying in this case mainly the tensile strength values and
the bending 3trength values to a lesser extent. These
properties go down at lower den~ities and increase at hi~her
den8itie8 .
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