The present invention relates to a method of manufacturing a rigid plate material of straw or a similar raw material, based on a bonding together of the raw material by means of a foaming binder agent. Many attempts of achieving such a bonding have been made in connection with straw, which is an increasingly interesting material for use as a basis in building and construction elements, but in spite of large and expensive efforts the attempts have not so far resulted in a suitable combination of good economy and practical usability of the produced materials. This can be explained in more detail, but at this place it is deemed sufficient to note the fact that the rich sources of applicable straw have not so far been used to any noticeable degree in the building industry, where the need for cheap materials is even pronounced.

It has been proposed to bond the straw with binders other than the foaming agents, inasfar as the straw material, when pressed, may well be bonded by various non-foaming glues, but is has already been recognized that with the use of foaming binding agents the consumption of the agents can be minimized for achieving a good economy, it being a main condition that the binding agent be distributed very evenly in the material. A foaming from the inside will promote such a good distribution, and still it is possible to subject the material to a pressing treatment as required so far, such that plate members having even surfaces and a relatively low specific weight can be produced.

With the present invention it has been recognized that it is possible to entirely avoid the said expensive pressing treatment when a foaming binding agent is used, and it has been found that it is possible hereby to

obtain quite surprising practical results, both economically and qualitatively.

The desired material can of course hardly be produced without the use of pressure, but according to the invention the pressing treatment will not have to be effected in an active manner, but can be effected passively when a foaming binding agent is used, insofar as an interior foaming produces a compression pressure, if the expanding workpiece is confined between opposed rigid moulding walls. Then it will be sufficient to introduce the loose raw material with admixed foaming binder agent in a production tunnel, in which the material expands so as to contact respective bottom and ceiling surfaces in the tunnel, such that the expansion will be purely structurally limited to far less the potential free expansion, i.e. with an associated building up of an interior compression force in the workpiece without any corresponding and energy consuming pressing operation on the workpiece being required.

However, when the starting material is loose straw it will be actual to lay up a straw layer having a thickness well larger than the desired plate material thickness and then cause the layer to be pressed together into the latter thickness, by which the straw material should be reasonably compact. This pressing together, however, can easily be worked out without any substantial power, and it is not comparable to a real production pressing. Upon this compaction the straw layer will be ready for easy introduction into the rigid production tunnel.

It is important to use only a small amount of the foaming binder agent, because preferably there should be no free foam in the final product. This is a known recognition, as the foam substance acts as a binder and not as an independent strength giving component, such that any free foam will incur the possibility of breaking

areas. When thus only a small amount of the foaming agent is to be used it will be that more important to arrange for an extraordinary even distribution of the agent in the matrix, and there are several indications of exactly this condition being responsible for the general failure of the prior art proposals.

In the prior art the required even distribution of the binding agent has been arranged by adding the agent to the base material under thorough agitation of the latter. This inevitably takes some time, and for common, cheap foaming agents this will imply that the agent will start expanding already prior to the mixing agitation being concluded. Thereby, during some few minutes, a certain free foaming up will occur on the surface portions of the base material, and therewith the effect of the expansion of the foaming agent during the following pressing phase will be noticeably reduced. Ideally the binding agent should not start any significant foaming until the expansion can take place against an outer pressure, because then it will be achieved that the expansion will also be directed 'inwardly in the material, for a sort of impregnation thereof. It is already known that in the straw material a strengthwise advantageous cellulose/pectin-reaction will result from the intrusion of a strong base, which just happens to be present in the considered foaming agents, but an efficient intrusion of the agent seems to fail, if a pressing force is not established until the agent has started to carry out a noticeable free expansion.

This possible explanation of the previous disappointing results has already been fortified by the achievement of clearly improved results already with the use of foaming agents, the foaming of which is noticeably prolonged in time by addition of retarding agents, whereby also the initial foaming up is retarded,

such that prior to that there will be time to carry out the said mixing in a suitable effective manner preparatory to the pressing treatment. Hereby, however, it will be necessary to accept a prolonged processing time, but what is more decisive is that the achievement of the instantly improved results becomes uninteresting due to the fact that the improved results will not be durable, because with the use of all retarding agents known so far there is connected the drawback that in the material a continuous hardening will take place, without stopping before after some time a state of 'brittle-hardening' is reached, this result being absolutely unacceptable. For just that reason the discussed problem, in practice, has not either been overcome by the use of foaming agents holding retarding substances.

On this background it is recognized by the present invention that for achieving a perfect result it will be imperative that the binding agent is added to the basis material without any use of retarding agents and so briefly before the introduction of the material into the said production tunnel that prior to this introduction there has been no noticeable expansion of the binding agent. The time in question will be a matter of only 2-3 minutes, and from a mixing technical point of view the difficulty arises that it is hardly possible in practice to obtain an even admixturing ^" of a very small amount of the added material, below 1%, during such a short time.

This is taken into account with the present invention, by which, on the other hand, it is recognized that it is possible to utilize the expansion effect of the binding agent to promote an even admixture based on an uneven admixture, when the expansion can take in a non-expanding space as represented by the said production tunnel. Thus, in the material layer to be introduced into this tunnel it is possible to inject the

binding agent in mutually separated, local injection areas, which should just be located relatively close to each other, and such injections can be carried out quite rapidly, such that the material layer may be placed in the production tunnel correspondingly rapidly.

By the subsequent initial expansion of the binding agent the material is brought to fill out the space between the bottom and the ceiling of the production tunnel, and by its further expansion this agent will build up an interior pressure in the material, whereby the foaming agent already in the initial phase will get an opportunity to intrude into and forwardly through the material portions located between the injection areas, such that the binding agent so to speak distributes itself evenly in the material by its own expansion effect. This distribution will take place both in the width direction, between the injection areas, and in the depth direction, inwardly into the basis material, such that both the desired even distribution and the desired impregnation effect will be obtained.

Thus, for the invention it is important that a layer of loose material is arranged on a carrier surface and is compacted into a layer having a thickness approximately corresponding to the height of the production tunnel, preferably 15-20 cm, and that this material layer is subjected to a multispot injection of the foaming binding agent immediately prior to the layer being introduced into the tunnel. However, this may be effected in a rather simple manner, viz. by arranging of a spear injection of the binding agent through several spears mounted in a transverse row across the layer of the loose material, while his layer is advanced longitudinally, the spears being moved up and down such that during the advancing of the layer they will bring the binding agent into the layer in transverse rows with a small mutual distance. In this manner it is possible

to supply the material layer including the injected binding agent to the production tunnel less than 2-3 minutes after the injection of the agent, such that the foaming up of the binding agent will not really start until the material has been brought into the tunnel.

It is important that the material layer is laid up based on entirely loose straw, e.g. raked from a straw bale and optionally deposited on a moving belt, such that a good bond in the material and a uniform material filling can be ensures throughout the workpiece. The injection of the binding agent may be effected in the loose material or with the material in a more or less compacted condition, optionally by introduction of the spears through holes or slots in one or both of the pressed walls.

By the subsequent expansion of the binding agent inside the production tunnel the agent will intrude into the base material, both in the depth and in the width thereof, such that a uniform distribution will be achieved. Of course, the produced foam will fill out all cavities in the base material, but as described it will also penetrate into the material itself and be uniformly distributed therein, as it will not be able to produce any substantial expansion of the base material, when the latter is held between the floor and the ceiling side of the production tunnel. Thus, the required expansion force is created internally.

After 15-20 minutes the binding foam will be fully set, and the plate member may then (or even shortly before) be taken out of the production tunnel.

The invention also comprises the described means for carrying out the disclosed method.

In the following the invention is explained in more detail with reference to the drawing, in which, by way of example, a production system according to the invention is shown in a schematic perspective view.

The system shown comprises a belt conveyor 2 having mounted above its inlet end a unit 4 for dropping loose straw onto the conveyor belt. The unit 4 is shown fully schematically, as it is unimportant how it is designed; it may be outlet of a machine for rasping straw bales and delivering the straw in a fully loose, optionally very coarsely cut condition.

The straw is deposited on the belt 2 in a layer 6, e.g. with a height of 1-1,5 m, and this layer is advanced towards an overlying, downwardly inclined conveyor belt 8, which will thus act to press together the loose straw layer for delivery thereof with a thickness of e.g. 15-20 cm.

This delivery takes place to a tunnel 10 in a press tube 12, into which the compacted straw layer can be introduced merely by the action of the conveyor belts 2 and 8, though if required these belts may be provided with e.g. knoblike protrusions.

Between the units 4 and 8 there is mounted an injector unit 14 comprising a carrier beam 16 for a number of downwardly projecting tube spears 18, connected with a supply hose 20 for a foaming binding agent. The beam 16 is supported by cylinders 22 which are operable to rapidly move the spears 18 into the straw layer 6 and retract them therefrom. Optionally the cylinders 22 may be mounted for a limited longitudinally reciprocating movement, whereby they can carry out their injection movement while the straw layer is being moved on the conveyor 2.

During the advancing of the straw layer 6 the actuation of the spears 18 is repeated, such that the spears will be introduced into the layer with a longitudinal spacing corresponding roughly to the spacing between the spears. By means of a control equipment (not shown) the dosing of binding agent through the spears may be controlled such that the agent

is injected e.g. only during each retraction movement of the spears.

The supplied amount of binding agent should be kept very low, as in the final product the expanded foam should be present with a volume percentage of 2-3%. Inasfar as each volume unit of the liquid, foaming agent may expand to a volume some 35 times as big it really is required to keep the injection of the foam liquid very low.

Upon the introduction of the compacted straw layer in the press tube 12 rapidly after the injection of the binding agent into the material the said expansion effect of the binding agent will commence, such that the material inside the rigid press tunnel 10 will be subjected to an inner expansion force, which acts to distribute the binding agent -in the material and to produce an intrusion of this agent into the sub parts of the material already from the initial expansion phase of the binding agent.

Just because it is possible to introduce the material into the press tunnel very quickly after the injection of the binding agent there will be no need to make use of a binding agent in admixture with an agent for retarding the foaming up of the binding agent, and it is possible, therefore, to achieve a rapid and lasting, highly efficient setting or curing of the foamed binding agent.

As shown, it is possible to make use of more press tubes 12, which may be successively filled from the unit 2,8 by a relative transverse displacement, such that the supply unit may operate almost continuously while the expansion and setting process in the single tubes is going on, e.g. during 10-15 minutes. Thereafter the finished plate elements may be removed from the press tubes, preferably by these being openable for an easy release of the plate elements.

In small size systems the supply unit may well be adapted to operate intermittently, e.g. with the inclined conveyor 8 replaced by a horizontal piston plate, which by each operation may compress the entire length of the loose straw layer that is to be introduced into an applied press tube.

The injection of the binding agent may be effected even after the initial compaction of the loose straw layer, e.g. by a horizontal introduction of the spears underneath the inclined conveyor 8.

As a further possibility the foaming binding agent liquid may be applied in an atomized condition already by the laying out of the loose straw material on the belt 2, such that a good distribution of the agent is achieved briefly before the material is introduced into the press tunnel 10.

It has been mentioned that the volume percentage of the binding foam should be 2-3%, which will be optimal. The invention, however, will comprise an addition within somewhat broader limits, viz. 1-5%.

As an alternative to the method described the compaction of the loose material may be achieved by displacing downwardly the upper press plate, which is thereafter locked in its lower position so as to thereafter act as a ceiling in the press tunnel as thus integrally connected therewith. The plate is raised again, when the plate material is ready for delivery, whereby also the delivery is facilitated.

With the use of the principle of the invention straw plates with highly surprising positive qualities have been manufactured, but it is deemed unnecessary to specify this in more detail in the present connection.