WO1988005372A1

WO1988005372A1 - Method of producing multi-layer fiber webs

Info

Publication number: WO1988005372A1
Application number: PCT/SE1988/000006
Authority: WO
Inventors: Bengt Johan Carlsson
Original assignee: Bengt Johan Carlsson
Priority date: 1987-01-15
Filing date: 1988-01-15
Publication date: 1988-07-28
Also published as: SE8700146D0; SE454337B; AU1188588A

Abstract

Method in producing multi-layer webs of fibers with additions of thermosetting binder by beating a lignocellulose containing material in an atmosphere of steam under pressure, adding binder, drying and forming webs in layers one above the other with mutually different composition of the layers with regard to binder and/or moisture ratio. In accordance with the invention, two or more fiber flows are produced which are each dried in its individual stage in a multi-stage dryer, whereby heat energy can be saved, and thermosetting binder is added to one or several fiber flows before drying and to one or several fiber flows after drying, whereby binder can be saved.

Description

METHΠD OF PRODUCING ULTILAYER FIBRE WEBS

The present invention relates to a method of producing multilarninated webs of fibers, which have been formed by beating a ligno_^pellulose-containing material such as wood chips in an atmosphere of steam under pressure. and which have been dried and had added to them a thermosetting binder, the composition of at least one of the layers deviating from the others.

The invention will be described here applied to the manu¬ facture of laminated wood fiber boards in three layers, using ureaformaldehyde glue as binder, the boards being formed by heat pressing fiber webs with two like surface layers having therebetween a layer deviating in its compo¬ sition from the surface layers.

When they are heat pressed, the webs are compressed between heated press platens , and are heated successively from the surfaces and inwards. The required press time is determined by the time for heating the central plane of the web to the curing temperature of the binder in the central layer and of the curing time for this.

The faces of the surface layers towards the press platens are heated so rapidly during pressing that the binder there has time to harden before full pressing pressure is attained. This results in that the pressed boards have porous surface zones, which are not desirable and there- fore must be ground off. A rapidly curing binder is used for the central layer and a slow curing binder for the surface layers is used to minimize the press time and the thickness of the porous surface zones.

The heating time for the central plane of the web can be shortened if the moisture ratio of the surface layers is high and that of the central layer low.

The fibers are produced by beating wood chips in a refiner pressurized with stean_jand are blown out suspended in steam in a blowing duct to a through-dryer comprising heating apparatus, e.g. a steam battery for heating drying air, a long drying duct, a cyclone and a fan for driving the drying air through these three components in the men- tioned order. The blowing duct is connected to the drying duct at its end facing towards the heating apparatus, and the fibers are separated after passage through the drying duct from the cooled and moistened air in the cyclone. The drying air is normally vented to the atmosphere from the cyclone.

In certain cases, he drying air is taken to a second dry¬ ing duct and from this to a second cyclone. This arrange¬ ment is known as a two-stage dryer and is used for saving heat energy such that the blower duct from the. refiner is connected to the inlet end of the second drying duct, that in the-second cyclone pre-dried fibers are separated From t_he drying alrv which is then vented to atmosphere, that the pre-dried fibers are taken to the inlet end of the first drying duct, and that after passage through it they are separated in a ready dried state from the drying air in the first cyclone, i.e. the one between the two drying ducts.

There is a physical relationship between the lowest moisture ratio which the fibers can be dried to on the one hand_j and the state of the drying air when the fibers are separated on the other hand.^" High temperatue and low relative moisture in the air promotes drying to a low fiber moisture ratio. To attain the same fiber moisture ratio, the state of the air departing from the cyclone for a single stage drying arrangement must correspond to the state of the air in a two-stage drying arrangement, when this air goes from the first cyclone to the second drying duct. When the fibers are pre-dried in the second drying duct, the required water vaporization in the first drying duct is decreased^ and consequently the air stream can be kept lower than in a single-stage drying arrangement. For a given air tem- perature after heating the drying air in the heating ap¬ paratus there is thusless use of heat energy in the two stage drying arrangement.

Ureaformaldehyde binder is often added dissolved in water in a 50 % concentration. The addition can take place after fiber drying in a special bindermixer. Known such mixers give an uneven distribution of the binder on the fibers, which leads to fibers that are coated with too much glue forming wads. If the fibers for the outer layers are treated in this way, the wads show up on the surfaces of the finished boards in the form of dark patches, which decreases the value of the boards. Alter¬ natively, the addition can take place before drying the fibers, e.g. by pumping into the blowing duct. The sur- face patches can be avoided in this way, but the binder is affected when the fibers are transported in the hot air in the dryer, so that to achieve the desired strength values of the boards, the amount of binder must be in¬ creased by about 20 % at normal drying temperatures. This increases the total production cost by about 5 %.

It is known to produce three-layer webs where the fibers for the outer layers are supplied with binder before dry¬ ing and the fibers for the central layer after drying. Since the amount of fibers of each kind is approximately the same, the extra cost for avoiding surface patches in this way can be halved. For carrying out this there has been used one set of equipment for beating, drying and adding the binder for fibers intended for the central layer and another set for fibers intended to the surface layers. The refiners^with associated auxiliary equipment constitute a large part of the total investment cost of the plant.

To reduce this, most medium sized and smaller plants are laid out with only one refinerand one dryer, the binder being added before drying, whereby patchy surfaces are avoided, but there is no possibility of differentiating the amount or sort of binder or the moisture ratio over the thickness of the formed webs. There is a least amount of binder which is consumed for achieving the desired strength properties of the ready boards, and this is often different for the outer and inner parts of the board. For homogeneous webs the higher additive level must be selected for the entire fiber flow. With three-layer webs, if the dry substance ratio between binder and fibers in a given case for surface and central layer fibers must reach 0.080 and 0.090^ respectively^for addition after drying, then the ratios need to be increased to 0.100 and 0.110 for addition before drying. The corresponding ratios for addition to the surface layer fibers before drying and to the central layer fibers after drying will consequently be 0.100 and 0.090, respectively, or an average of 0.095 if both fiber flows are equally as great. On the other hand, a dry substance ratio of the order of magnitu_de- 0.1-10 is required for homogenous webs.

The necessary increase of the additive magnitude when addi¬ tion of binder takes place before drying could be re_duce_d con- sinderably by lowering the drying air temperature at the inlet in t_he drying duct. For the drying air state to give the desired fiber moisture ratio when the fibers are separated the drying air flow must be increased. This requires a bigger and more expensive dryer and more blower energy, but above all more heat energy is used for drying. It is therefore preferred to keep the inlet temperature as high as possible, taking into account the risk of spontaneous combustion of the fibers.

Methods are known in which fibers produced in a pressur¬ ized refiner and suspended in steam are supplied to indi¬ vidual dryers via two or more blowing ducts.

After the fibers have been dried and have had binder added to them they are usually collected in tanks, one for the surface and one for the central layer of fibers, from which they are fed to a web-forming station where they -are spread in layers above each other. Two flows are fed out from the tank for surface layer fibers, one being used for forming the lower and the other the upper of the surface layers.

The present invention relates to a method in the produc¬ tion of multilayer webs of fibers mixed with a thermo- setting binder and having a given moisture ratio, with at least one of the layers having a composition deviating from that of the others, of saving binder and heat energy. In addition, investment costs can be reduced by an exten¬ sion of the invention.

The object of the invention is primarily obtained by using a two-stage dryer, in the first stage of which a fiber flow is dried at a high temperature and thereafter has a binder added to it, a second fiber flow containing binder being dried at a low temperature in the second stage of the dryer. The object of the extension of the invention is achieved by using a refiner from which a fiber flow is blown to the first stage of the dryer_j and another fiber flow with an addition of binder is blown to the second stage of the dryer.

The invention will now be described in more detail with reference to Figures 1 and 2. Since only apparatus well known to one skilled in the art are required for working the invention, these are only illustrated schematically for the sake of clarity.

Figure 1 is a flow diagram for two refiners 1, 2 each connected to a blowing duct 3, 4. Figure 1 also shows heating apparatus 5 for air, a blower 6, a first drying duct 7, a cyclone 8, a second drying duct 9 and a second cyclone 10, connected to each other in the order mentioned. The cyclones 8,10areprovided with fiber dischargers 11, 12. The blowing ducts 3, 4 are each connected to a drying duct 7, 8 at the duct end illustrated in Figure 1. The fiber discharger 12 at the cyclone 8. is connected to a binder mixer 13, to which a dosing devicel4 is also connected for adding binder. The binder mixer 13 is connected to a tank 15 for intermediate storage of fibers intended for the central layer of the webs.

A second dosing devicel6 for binder is connected to the blowing duct 3, opening out in the second drying duct 9. The fiber discharger 11 at the second cyclone 10 is con- nected to a tank 17 for intermediate storage of fibers intended for the surface layers of the webs.

Figure 2 illustrates a variant of the arrangement in Fig¬ ure 1, with a refiner 18 connected to two blowing ducts 3, 4, identical with those illustrated in Figure 1 and op.n- _ _

ing out in the same way in their respective drying ducts 7, 9. In the same way, a dosing device for binder is con¬ nected to the blowing duct 3, which opens out in the sec¬ ond drying duct 9. For the remainder, the flow diagram illustrated in Figure 2 corresponds entirely with the one illustrated in Figure 1.

Discharge of fibers from the tanks 15, 17 to the forming station, and their spreading there in three layers one above the other, is well known technique for one skilled in the art and therefore does not require any closer de¬ scription.

The function of the invention will be explained in the following in example 1, and a comparison will be made with two conventional methods in examples 2 and 3.

Example 1

Fresh air is taken in by the blower 6 through the heating apparatus 5, where it is heated to 160 C and is blown out to the atmosphere after passage through the drying ducts 7, 9 and cyclones 8, 10. Two approximately equally as great fiber flows with the moisture ratio 0.87 are blown through the ducts 3, 4 suspended in about 0.55 kg steam per kg dry fiber, to their respective drying ducts 9, 7. 0.085 kg binder dissolved in an equal amount of water per kg dry fiber is supplied to one duct by the dosing device 16 and the mixture is lead to the second drying duct 9, where it accompanies the drying air to the second cyclone 10. The second half of the fibers is blown in as they are into hot air at 160°C in the first drying duct 7 and ac¬ company the air to the cyclone 8, where the fibers are separated and discharged via the fiber discharger 12 to the binder mixer 13. For a normal state of the fresh air, and if the drying duct has sufficient length, the air is cooled when it leaves the cyclone 8 to 110°C and its moisture ratio is under^'5 %. In this final state of the air in the first dry- ing stage, the moisture ratio of the fibers is reduced to under 0.01, i.e. they are practically completely dry when they are supplied to the mixer 13.

The air departing from the cyclone 8 is taken through the second drying duct 9 and the second cyclone 10, thus being cooled by the accompanying mixture of fibers and binder to 67°C, the moisture ratio adjusting itself to 40 %, which is the state in the departing drying air. Simultaneously, the drying at the moisture ratio 0.13 stops before the fibers in the fiber discharger 11 are supplied to the surface fitπurs tank 17 with the aid of the fiber discharger 11.

What has been said above applies for 19 kg dry air per kg dry fiber.

In thebinder mixer . the heavily dried fibers have binder added to them with the aid of the device 14, the mix¬ ture then being transferred to the tank for central layer fibers. The binder is added in an amount of 0.090 kg dissolved in the same amount of water per kg dry fiber. The moisture ratio increases then to 0.090-0.100. The moisture ratios achieved promote rapid through-heating of the fibers during hot pressing. A rapidly thermosetting binder can be used without inconvenience for the central layer of fibers, since it does not partake in drying, which still further decreases the press time, which decides how large and expensive a press is required. Since the binder for the surface layer fibers is added be- fore drying, patchy board surfaces are avoided. The desired, extensive, drying out of the central layer fibers cannot be achieved without the air departing from the cyclone 8 being hot and dry, the invention thus results in that the surface layer fibers are dried without cost for heating drying air.

The circumstance that the drying air at the input to the second drying duct 9 is cooled in combination with using a slow-curing binder for the surface layer fibers enables the binder to be affected only to a minor degree on pass¬ age through the dryer. The ratio between binder and fibers can therefore be kept at a low level.

In the extension of the invention illustrated in Figure 2, the two fiber flows are beaten in a common refiner with associated auxiliary equipment and are subsequently divided for individual processing. The refiner must then, in principle, have double as large a capacity. The pro¬ curing cost per unit only increases slightly with capacity, however, and such an arrangement cheapens the plant.

Costs for maintenance are reduced at the same time, and larger refiners give better fibers for the same applica¬ tion of beating energy.

Example 2

In most fiber_^board plants the desired production capaci¬ ties can be achieved with the use of a single refiner. Taking into account what has been said above, such a solu¬ tion is generally selected, and homogeneous boards are produced. Since patchy surfaces reduce the value of the boards, the binder is added before drying and accompanies the fibers through the dryer. In consideration of drying costs the drying air is heated to 160 C. The binder is thus effected to such a degree that to achieve the desired strength in the inner portions of the board the dry sub- stance ratio between binder and fibers must be kept at 0.105-0.110, and throughout the entire web thickness. Binder consumption will thus be considerably higher than in Example 1. More drying air is used to reach a desired moisture ratio.

Example 3

With very large capacities, the fiber requirement must be covered by using two refiners, which are each connected to a dryer. If both fiber flows have binder added after drying, it is required that the dry content ratios in the surface layers and the central layer are 0.080 and 0.090. To avoid patches binder is added to the surface layer fibers before drying.. The dry content ratio for the sur- face layers must then be raised. If low temperature dry¬ ing is applied here as in Example 1, the ratio for the surface layers must be raised to 0.085 and the binder con¬ sumption will be the same as that in Example 1. However, the heat used in the low temperature dryer will so great that it is preferred to increase the input air temperature to 160 C, in spite of then having to increase the dry substance ratio to 0.095-0.100. This method is thus inferior to the one in Example 1, regarding costs for binder as well as costs for dryer heating energy.

The invention is not limited to the number of web layers, drying stages, refiners used or the number of blowing lines from a single refiner, but may be varied within the scope of the claims. Within the scope of the invention the binder for the- surface layers can be added to the ma¬ terial for beating in front of or in the refiner instead of in the blowing ducts.

The invention is not restricted to drying each flow in a single drying stage, and a flow may be pre-dried in one drying stage and finally dried in another, for example, optionally adding binder between the drying stages.

The invention can also be varied so that in using multi- component binder, one or more components or other sub¬ stances can be added separately and in a way other than that described.

Claims

1. A method in producing multi-layer webs of fibers with additives of thermosetting binder by beating lignocellu- lose-containing material in an atmosphere of steam under pressure, drying the fibers in individual partial flows and forming webs in layers one above the other with mutu¬ ally different compositions in the layers with regard to either binder or moisture ratio or both, characterized in that at least two partial flows are dried individually,each in its own particular stage or stages in a multi-stage dryer.

2. Method as claimed in claim 1, characterized in that at least one partial flow has binder added before drying and at least one other partial flow has binder added after drying.

3. Method as claimed in claim 1, characterized in that at least one partial flow is dried in more than one stage and that binder is added between one of the stages.

4. Method as claimed in claims 1-3, characterized in that at least two of the partial flows are formed by beating in a refiner common to them, followed by division into the partial flows.