WO1986002032A1

WO1986002032A1 - Process for manufacture of glued pressed products and arrangement intended for use therein

Info

Publication number: WO1986002032A1
Application number: PCT/FI1985/000075
Authority: WO
Inventors: Jarl-Gunnar Salin
Original assignee: Ekono Oy
Priority date: 1984-09-28
Filing date: 1985-09-13
Publication date: 1986-04-10
Also published as: FI72075B; EP0227675A1; FI72075C; FI843834L; FI843834A0

Abstract

Method for the preparation of glued formed pieces, such as chipboards and plywood, through drying of humid raw material, adding of glue and pressing of the raw material in order to form the glued product. It has now been observed that the presence of oxygen at those stages which take place at elevated temperature reduces the quality of the glued product. According to the invention these stages are carried out in essentially non-oxidizing atmosphere. The invention also relates to a device for cooling materials containing wood, wood fibers and cellulose, which have been dried under heating with hot steam in non-oxidizing atmosphere. Such a device comprises a closed chamber (3) with sluices for feeding (2) hot raw material and discharging (7) cooled raw material for the preparation of glued formed pieces, an inlet (8) for a non-oxidizing gas, a cooling device (5) communicating with the chamber (3), means (4) for creating a gas flow from the chamber (3) to the cooling device (5) and back therefrom as well as an outlet (6) for the condensate coming from the cooling device (5).

Description

Process for manufacture of glued pressed products and arrangement intended for use therein

The present invention relates to a process for the manu¬ facture of glued pressed products by drying of a humid raw material containing wood, wood fibre and/or cellulose, adding glue and pressing said raw material in order to form a glued product such as wood chipboard, plywood and such like. By means of the process improved properties are obtained in the finished product.

The present invention also relates to an arrangement for cooling of a raw material containing wood, wood fibre and/or cellulose which is dried under heating in a non-oxidizing atmosphere.

The manufacture of wood chipboard is carried out in several stages which can vary somewhat as to their arrangement in different factories. The main stages are the preparation of chips from the raw material, drying of the chips, screening and classification, preparation of glue, addition of glue to the chips, forming of the chips into a mat, pressing of the mat to a board and finishing of the board. The object of the first stage, the preparation of the chips, is to obtain a material with the desired particle size and particle shape. Depending on the raw material, which can be e.g. sawdust, low-grade timber, wood waste from plywood manufacture and such like, the subdivision of the wood material is carried out by cutting or grinding chippers. The processing is per¬ formed at room temperature or at the temperature of the out¬ side air depending on the arrangement of the factory. With e.g. grinding, however, the temperature of the chips can rise appreciably because of the significant input of mechan¬ ical power. In the following stage the chips are dried^' to a final mois¬ ture content of normally 1-5 % of the weight of the dry wood substance. A high dry-substance content is required partly in order to obtain good glue bonding and partly so that the pressing into boards can be carried out without the formation of steam bubbles in the board. Together with the glue a cer¬ tain amount of water is also added and this must be taken in¬ to account in determining a suitable moisture content for the dried chips. In practice two different methods of drying are used^'. One method is to dry the chips by means of direct con¬ tact with hot combustion gases or with air which has been preheated in e.g. steam radiators. In the other method the chips are dried by means of the indirect supply of heat to the chips from heating surfaces. In the combustion-gas drier and the hot-air drier the moist chips are brought into good contact with the hot drying gas and are transported- mainly with the current through the drying apparatus itself which is commonly of a drum type. The drying gas and the dried .. . chips are separated in a subsequent cyclone separator. The temperature of the drying gas at the beginning of drying is normally over 200°C but in certain combustion-gas driers can rise as high as 700°C. In the final phase of drying when the chips are almost completely dry and thus inflammable the tem¬ perature level is normally restricted to a maximum of 150- 160°C. In the indirect ^"method of drying the effective area of contact between the chips and the heating surface (e.g. a steam-heated tube packet) is relatively small so that a large part of the heat is transferred to the atmosphere which surrounds the chips (mainly air) . Consequently in prac_¬ tice also this type of drier operates as a direct hot-air drier but with the difference that the preheating of the air is incorporated in the drying section to form a single stage. The temperature of the chips is determined at the beginning of drying, when the chips still have a high moisture content, by the equilibrium with ^"the surrounding drying gas, i.e. primarily by the so-called wet-bulb temperature of the dry- . ing gas which is normally approx. 50-80°C. In the final phase of drying when the evaporation of water ceases and the hydroscopic properties of the wood appear the tempera¬ ture of the chips rises considerably and approaches the temperature of the drying gas. Thus at the end the tempera¬ ture of the chips approaches the level of 150°C. Because of the temperature restriction imposed by the inflammable chip material the combustion gas generator of a combustion-gas drier must be operated with a large surplus of air which results in a considerable oxygen content in the combustion gases. In spite of the consumption of oxygen by combustion the oxygen content of the combustion gases is therefore not appreciably different from the level of oxygen content in a hot-air drier. During the drying process the water vapour evaporated from the chips becomes mixed with the drying gas which is thereby diluted. In practice the amount of drying gas is large compared with the amount of evaporated water and the moisture ratio rarely rises above 0.3 kg H-O/kg of dry gas. The oxygen content is therefore in practice norm¬ ally above 10 % by volume in the drier. (The oxygen content of the air is 21 % by volume. ) This applies also to indirect driers.

Other types of drier also have been proposed for drying chips but these have not in practice come into use.

After drying a classification of the chips is made normally into two fractions, i.e. an outer-layer fraction with fine material and a core-layer fraction with course material. These fractions are kept separate and processed separately up to the formation of the chip mat when the fractions form the outer and core layers of the board respectively. Classi¬ fication can also be carried out prior to drying in which case drying is carried out separately for each fraction. In the next stage of manufacture glue is added to the chip fractions with effective mixing so that the glue is uni¬ formly distributed over the surfaces of the chips. Of the various types of glue carbamide glue is the most usual. In connection with the addition of glue hardener for the glue and other possible^' additives are also added.

This is followed by the actual board manufacture which con¬ sists of forming the boards and subsequent pressing. In forming the boards the aim is to achieve a chip mat of uni¬ form thickness comprising outer/core/outer chip layers and with the desired areal density and otherwise homogeneous composition. The chip mat thus produced is pressed between hot press plates. The pressure brings the chips into close contact with each other and the heat hardens the glue bon¬ ding the chips into a uniform chipboard. The board assumes its final shape in the press.

It is clear that the strength properties of wood chipboard depend entirely on how well the glue is able to bind the chips together, since the chip assembly itself does not display any cohesive forces.

The manufacture of plywood displays, despite the superficial differences, several production phases corresponding to those in the manufacture of chipboard. The actual manufacturing process begins with the cutting of the veneer in which a long mat of veneer with a thickness of 1-3 mm, usually 1.5 mm, is peeled from a log in a veneer lathe. This is followed by dry_¬ ing of the veneer. Drying may, however, be preceded by clip_¬ ping of the veneer mat into sheets and sorting of the sheets. The drier is accordingly arranged either for drying long mats of veneer or for drying sheets of veneer. The first-named is generally arranged so that the veneer mat is transported be_¬ tween metal nets in a back-and-forth motion through the dry¬ ing chamber, a so-called belt drier. In a sheet drier the transport through the drier is performed with the aid of a pair of rotating cylinders, a so-called roll drier. In both types the drying is carried out with hot air which is heated by steam radiators. In modern driers the drying has been made more efficient by blowing hot air onto the veneer from nozzle boxes whereby an internal recirculation and reheating of the air is achieved. In exceptional cases the drier has been designed as a combustion-gas drier.

For the same reasons as were put forward for the drying of chips, a final moisture content in the veneer of approx. 2-7 % is aimed for. The temperature of the drying air is normally kept at 150-190°C. The temperature of the veneer follows the same pattern as the chip temperature in chip drying, i.e. approx. the wet-bulb temperature of the drying air, 60-85°C, at the beginning and up to 150°C at the end of drying. Very considerable amounts of leakage air enter the drying chamber and as a result the maximum moisture ratio of the drying air is in practice 0.8-1.0 kg H-O/kg dry air (on average considerably lower). This means that the oxygen content of the humid drying air is in practice normally above 10 % by volume.

The drying of the veneer is followed by clipping into sheets, sorting of the sheets, joining of sheets and parts of sheets together at the edges, and glue spreading. The desired num¬ ber of glue-coated sheets are laid up so that the grain directions of adjacent layers are at right-angles to each other. This pile of veneer is then pressed between hot press plates whereon the glue bonds the sheets of veneer to each other. After edge trimming the final plywood produce is finished.

The strength of the single veneers is good in bending at right-angles to the grain direction but very poor in ben- ding along the grain direction. By means of assembling the veneers so that the grain directions are alternately at right-angles to each other the plywood obtains good strength properties in all directions. A precondition is naturally that the strength of the glue joint is sufficient. Thus it is important that the glue joint and the bonding of the glue are as good as possible even if this:; requirement is not as pronounced as for chipboard.

It is generally known that if wood is heated up to tempera¬ tures above 200-250°C various chemical decomposition reac¬ tions start to occur and the chemical and physical properties of the wood are altered. This applies, then, to temperatures of the wood higher than those which occur in the manufacture of e.g. wood chipboard and plywood. It is further known that if drying of veneer in conventional apparatus is carried too far ("over drying") the glueing properties of the veneer are impaired with an effect also on the strength properties of the plywood. It can thus be said that it is known that in the processing of wood at elevated temperatures (as e.g. in • the manufacture of chipboard and plywood) a number of changes take place in the wood which have a harmful effect on the glueing properties of:^".the wood and the strength properties of products made from the wood.

It has now been surprisingly discovered that these undesirable changes can be reduced or completely eliminated by preventing contact with oxygen in the process stages in which the tem_¬ perature of the wood is elevated.

The object of the present invention is accordingly to achieve a process for the manufacture of glued pressed products by drying of a humid raw material containing wood, wood fibre and/or cellulose, adding glue and pressing the raw material to form a glued product with improved properties compared with similar products which are dried, glued and pressed in a similar manner. The present invention is based on the surprising discovery that the above-named undesired changes in the wood can be reduced or completely eliminated by preventing contact with oxygen in the process stages in which the temperature of the wood is elevated.

In the stage or stages in which the temperature is elevated the prevailing atmosphere suitably contains less than 10 % by volume of oxygen, advantageously not more than 5 % by volume, e.g. less than 1 °/_ by volume of oxygen. The tem¬ perature is suitably maintained lower than 200°C and pre¬ ferably higher than 60°C. Further it is advantageous to add the glue to the raw material while the latter has an elevated temperature in order to avoid contact with oxygen between drying and the application of the glue.

The inert atmosphere in the stage or the stages in which the temperature is elevated is suitably achieved and main¬ tained by isolating the raw material from the air by means of displacing the air with non-oxidizing gas.

In a preferred embodiment of the invention the raw material is heated by e.g. hot steam, after which the hot, dry raw material is fed through an air lock and brought into con¬ tact with cooled, substantially dehumidified non-oxidizing gas before discharge through an air lock into the surround¬ ings. Cooling can also be carried out at reduced pressure.

As examples of material suitably employed as raw material in the manufacture of glued pressed products according to the foregoing process may be named wood chips and veneer.

The invention also relates to an arrangement for cooling of raw material containing wood, wood fibre and/or cellulose, which is dried under heating in a non-oxidizing atmosphere. According to the invention such an arrangement comprises a closed chamber with air locks for feeding in hot raw material and discharging cold raw material for the manufacture of said glued pressed products, an inlet for non-oxidizing gas, a cooling device cpmmunicating with said chamber, means for creating a flow of gas from said chamber to said cooling de¬ vice and back therefrom as well as an outlet for the conden¬ sate from said cooling device.

The process according to the invention can be implemented in practice according to e.g. the following principles.

Of the various phases of treatment in which the temperature of the wood is elevated the drying processes are the most important. Elevated temperatures can also occur in heat- treatment processes and e.g. grinding processes where the temperature rises as a consequence of the high input of mechanical energy. It should also be noted that when hot wood material is discharged from a process stage it easily comes into contact with the surrounding air.

As described above the oxygen content in the drying atmos¬ phere is at least 10 % in the apparatus currently employed for drying of wood chips and veneer. The oxygen content can be appreciably reduced if drying is carried out in a steam atmosphere in such a manner that the oxygen is displaced. Two possibilities can be distinguished. On the one hand in¬ direct drying in which the heat is introduced via a heating surface and in which the vapour phase is formed by the water evaporated from the wood. On the other hand direct drying in which the heat is introduced in the form of superheated steam and the vapour phase is formed by the steam thus in_¬ troduced together with the water evaporated from the wood. The drying apparatuses for these drying methods are known per se, e.g. the so-called counter-pressure drier (FI Pat¬ ent No. 60072) . Contrary to what has previously been the case' special attention should be paid to the feeding in and discharging of the wood material to the actual drying chamber where an elevated temperature prevails. Feeding in should take place in such a manner that the smallest possible amount of air (oxygen) accompanies the wood material into the drying cham¬ ber. By means of letting a minor amount of steam flow in counter-current to the material being fed in a displacement of the oxygen is obtained at the same time as a gradual heating of the material takes place. In discharging the wood material from the drying chamber it should be ensured that the material is cooled sufficiently before contact with with oxygen (air). This can be achieved e.g. with an appara¬ tus as shown in Fig. 1. The figure illustrates diagram- matically and by way of an example an embodiment for chips which can, however, readily be modified for other materials. The material flows from the drying chamber via a cyclone separator 1 through a rotary-vane feeder 2 to a cooling chamber 3. The material at this stage has an elevated tem¬ perature and is accompanied by a certain amount of steam, partly as such and partly in the form of water which can be evaporated in the cooling chamber. In the cooling chamber 3 an oxygen-free inert gas atmosphere is maintained which contains minor amounts of water vapour. The inert gas is circulated by the aid of a fan through a cooling battery 5 and is thereafter brought into good contact with the wood material in the cooling chamber 3. The inert gas is cooled by the battery 5 through which flows e.g. cold water and at the same time a part of the water vapour in the inert gas is condensed in said battery and the condensate is run out through a separate pipe 6. The cooled and dry wood material is discharged by a rotary-vane discharger 7. The cooling chamber 3 can be furnished with fittings to obtain a good contact with the inert gas and a sufficient transit time through the apparatus. Losses of inert gas are made up by addition of inert gas (e.g. nitrogen) via a pipe 8. A slight excess pressure is suitably maintained in -ehamber 3 in order ^• to prevent leakage of air into the chamber. In the apparatus a cooling of the wood material is obtained in an oxygen-free atmosphere without any appreciable rise in the moisture con¬ tent of said material since the partial pressure of the water vapour is kept continuously low by the dehumidifying effect of the cooling battery.

The apparatus can also be arranged so that a reduced pressure is maintained in the cooling chamber by means of condensing out steam with the cooling battery and removing inert gas by a vacuum pump. The temperature of the chips is then de¬ termined by the equilibrium at the pressure in question and cooling can thus be obtained without contact with oxygen.

In batch drying freedom from oxygen at an elevated temperature can be obtained if the oxygen in the drying chamber is dis¬ placed with oxygen-free inert gas prior to heating up and if the steam content of the drying chamber is displaced by oxygen-free inert gas after drying whereon the wood material can be cooled without recondensation of the water.

In process stages other than drying, such as e.g. heat treat¬ ment and grinding in which elevated temperatures occur in the wood material it is possible to prevent contact with oxygen by displacing the oxygen with oxygen-free inert gas or steam. In e.g. grinding of moist wood material the apparatus can be arranged so that the water vapour formed during grinding displaces oxygen from the apparatus.

If an elevated temperature occurs in the final process stage prior to the application of glue then these stages can be combined so that the glue is applied before the material is discharged into an atmosphere containing oxygen. The present invention is described below in greater detail with the aid of the following comparative example.

Example

Two samples of chips were taken at a wood chipboard factory from the same batch of chips prior to drying and were treated in different wayβ. One sample (the reference sample) was dried in a laboratory drier by hot air to a normal final moisture content and from the dried chips test chipboards were^"made by the normal laboratory procedure and the pro¬ perties of these boards were tested. The other sample of chips was dried in a steam drier by means of superheated oxygen-free steam. The drying took place in the following manner. The chips are fed by a rotary-vane feeder without prewarming into a circulating stream of steam under pressure (in the present case approx. 0.2 MPa abs.) .in which the speed of the steam is sufficient to transport the chips through the drier. The steam-chip suspension thereon passes through tubes which are heated from the outside by steam (approx. 1.0 MPa abs.). The carrying steam is thus kept continuously superheated (in the present case the steam temperature was 150-170°C) whereby the water in the chips is evaporated and the chips dried. Following the drying apparatus the chips are separated from the steam by a cyclone separator and discharged by a rotary-vane device. Most of the steam is recirculated by a fan but part, corresponding to the water evaporated from the chips, is removed from the apparatus.

The temperature of the chips in the drier was thus initially (after the heating-up phase) equal to the boiling point of water at the pressure in question (approx. 120°C) and at the end of the drying equal to the temperature of the steam, i.e. approx. 150 C. The chip temperature was thus within the same range as in conventional drying. The oxygen content in the drying.apparatus, on the other hand, was -very low. The small amounts of air which unavoidably accompanied the chips when they were fed in were removed in conjunction with the continual drawing off of steam. The oxygen content in the steam was therefore very consid^'erably under 1 %. The chips contained small quantities of water on discharge from the apparatus and part of this water was evaporated by the rapid reduction in pressure. By this means a rapid cooling of the chips took place and the chips were at the same time sur¬ rounded by steam. The chips were collected in sacks and be¬ cause of the small quantities a relatively rapid cooling of the chips down to room temperature was obtained. This chip drying experiment was thus characterized by the fact that the chips did not come into contact with oxygen at elevated temperature. The chips so dried with a moisture content of 2-3 % were also made into- test chipboards having the same thickness and same quantities of glue and additives as for the reference sample. The test boards, which were pressed to

3 the same densities (688 and 689 kg/m respectively) and which had the same moisture content (6.5 %) were tested according to the standard procedure with the following results:

Unit Reference Oxygen-free

Strength in bending N/mm² 11.4 16.8

Modulus of elasticity N/mm² 1931 2346

Transverse tens, strength N/mm² 0.56 0.93

Swelling in water, 2 h % 10.6 8.1

Swelling in water, 24 h 2 5 x 25 mm % 37.7 21.8

Swelling in water, 24 h 100 x 100 mm % 33.4 19.9

Additionally it was noted that the board was somewhat lighter in colour compared with the reference board. The values ob- tained for- the reference board correspond^'"to ^'the values which are normally found with conventional production.

Thus with oxygen-free drying of the chips the values ob¬ tained for strength were 21-66 % and those for swelling 24-42 % better. These differences are considerably greater than the normal spread of measured values.

The improved properties discovered here can be utilized in e.g.^' any of the following wayst

a) The amount of glue in the board can be reduced without impairing the properties of the board compared with conven¬ tional production.

b) The board can be pressed to a lower density without impairing the properties of the board compared to conven¬ tional technology.

c) Boards intended for certain purposes can be made thinner.

In this way an appreciable saving can be made in glue con¬ sumption or consumption of wood raw material or both.

From the purely chemical point of view the observed quality changes can be considered to be explained in the following manner. Good glue adhesion requires that a bond of a chemical nature is formed between the glue and certain molecules in the wood. The strength of the joint thus depends on the num¬ ber of chemically reactive groups in the surface of the wood. If at elevated temperatures these groups react with oxygen the possibility of obtaining good adhesion is diminished. If there is no oxygen available then the reactivity of these molecule groups is preserved. Once the glue has been added the chemical bond between the glue and the-active groups in the raw material has been formed and thus subsequent treat¬ ment at elevated temperatures can possibly be carried out also in an oxidizing atmosphere.

Claims

MENDED CLAIMS[received by the International Bureau on 19 February 1986 (19.02.86); original claims Ml replaced by new claims 1-9 (2 pages)]Claims

1. Process for.manufacture of glued pressed products by drying of humid raw material containing wood, wood fibre and/or cellulose, addition of glue and pressing of said raw material to form the glued product, c h a r a c t e r ¬ i z e d i n that the process is carried out at least in part at elevated temperature and while at such temperature constantly in an essentially non-oxidizing atmosphere comprising a steam phase with an oxygen content of less than 10 per cent by volume.

2. Process according to claim 1, c h a r a c t e r ¬ i z e d i n that the process is carried out at least in part at elevated temperature in an atmosphere containing at most 5 per cent by volume, e.g. less than 1 % by volume of oxygen.

3. Process according to claim 1 or 2, c h a r a c t e r ¬ i z e d i n that the process is carried out so that the o temperature of the raw material is kept below 200 C and o preferably above 60 C.

4. Process according to any of the preceding claims, c h a r a c t e r i z e d i n that the glue is added to the raw material without intermediate cooling of the latter after drying.

5. Process according to any of the preceding claims, c h a r a c t e r i z e d i n that prior to being heated the raw material is isolated from oxygen by means of dis¬ placing the oxygen with a non-oxidizing gas.

6. Process according to any of the preceding claims, c h a r a c t e r i z e d i n that the hot raw material is fed through an air lock and brought into contact with cooled, essentially dehumidified non-oxidizing gas prior to being discharged through an air lock into the surroundings.

7. Process according to any of the preceding claims, c h a r a c t e r i z e d i n that prior to cooling of the raw material steam is displaced by another non- oxidizing gas.

8. Process according to claim 6 or 7, c h a r a c t e r i z e d i that cooling is carried out at reduced pressure.

9. Process according to any of the preceding claims, c h a r a c t e r i z e d i n that wood chips of veneer are used as raw material.