IE70083B1 - A process for continuous production of polyurethane foam blocks - Google Patents

Description

A Process for Continuous Production of Polyurethane Foam Blocks The invention relates to a process for continuous production of polyurethane foam blocks.

More particularly, the invention relates to a process of the type whereby reactants are metered separately and are pumped to a mix head at which they are mixed in a mixing chamber. The mixed reactants are delivered out of the chamber onto a moving conveyor belt which is lined at the bottom and sides. As the mixture is moved by the belt it reacts to form a polyurethane foam which rises to form a continuous length having a height of generally 1.0m to 1.5m. At the end of the conveyor belt, a cutting blade cuts separate blocks or slabs of the foam. These blocks are then cured by allowing dissipation of the heat formed by the reaction.

In a high pressure process of this type, the reactants are fed into the mixing chamber through nozzles under a high pressure.. There is a sharp pressure drop at the nozzles and the reactants mix by impingement of the streams. The primary reactants are generally a polyol and an isocyanate, and in particular toluene diisocyanate (TDI). Other reactants include catalysts, water and surfactants.

In many cases additives such as fire-retardant powders such as graphite are blended with the polyol.

British Patent Specification No's GB 1,520,542 (Metzeler Schaum GMBH) and GB 1,392,859 (Maschinefabrik Hennecke GMBH) disclose polyurethane foam block production processes whereby there is an upper covering sheet used to cover the foam as it forms. British Patent Specification No. GB 1,526,318 (Bayer) describes various formulations i for the reactants. PCT Patent Specification No. WO. - 2 86/01520 (Reeves Brothers) discloses a process for rapid curing of foam, while United States Patent Specification No. US 4,492,664 (Bruno) discloses a process where a composite web is placed over the foam as it is being ‘ produced.

The present invention is directed towards providing an improved process whereby there is an improvement in foam quality and whereby the blocks are formed in a uniform manner on the conveyor. Another object is that the cost of, equipment and operation of the equipment is kept to a minimum so that the process may be used for production of relatively small batches of polyurethane foam blocks.

According to the invention, there is provided a process for continuous production of polyurethane foam blocks, the process comprising the steps of:pumping controlled quantities of reactants to a mix head, including the sub-steps of:pumping a reactant requiring an additive to a mixing tank and mixing the reactant and the additive by action of an agitator, the resultant slurry being then pumped to a blending tank, in which the solids are maintained in suspension by action of an agitator, the slurry being pumped from the blending tank to the mix head as a reactant; pumping polyol under low pressure directly from a bulk storage tank or from the blending tank to a > valve in the mix head; and pumping other reactants to side valves of the mix head, in the range of 10% to 20% being under low pressure, and the balance under high pressure via a nozzle; agitating the reactants in the mix head; opening a two-way output valve of the mix head to deliver a controlled test quantity of mixed reactants to a test container; monitoring the foaming reaction in the test container and adjusting process parameters accordingly; opening the valve to deliver mixed reactants to a trough at the start of a conveyor having a lined base and lined side walls; spraying a water mist over the foam in the conveyor at a distance of between 3m and 7m from the mix head; and cutting blocks off the continuous foam piece at the end of the conveyor and curing the blocks.

Preferably, reactants fed under high pressure are initially re-circulated through the storage tank until the pump attains a desired operating speed for delivery to the mix head.

In one embodiment the base of the conveyor is lined with a lower paper-based film, and an upper polythene film extending between the side walls.

Ideally, the process comprises the further step of crushing a block of foam by passing it between two series of rollers mounted in a tapered configuration.

The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings in which:Fig. 1 is a schematic overview diagram showing a plant for carrying out a process of the invention; Fig. 2 is a front view of a mix head for use in the process; Fig. 3 is a diagrammatic side view showing foam being produced; Fig. 4(a) is a side view of a cutting station, Fig. 4(b) is an end view of portion of the conveyor, and Fig 4(c) is a perspective view showing two produced foam blocks; Figs. 5(a) and 5(b) are perspective and side views respectively of the cutting station; and Fig. 6 is a diagrammatic side view showing operation of a crushing device.

Referring to the drawings, a production plant 1 for production of foam blocks is shown. The plant 1 comprises a mix head 2 for the mixing of reactants supplied by a set of bulk storage tanks 3, a mix tank 4 and a blending tank 6. The mix tank 4 is connected to a hopper 5 for reception of an additive, namely, graphite and the blending tank 6 is connected to a mixing tank 7 which is in turn connected to a hopperJ 8 for reception of an additive, in this case melamine powder. The mix tank 4 and the blending tank 6 are connected to the mix head 2 by pump and valve assemblies 9 and 10 respectively. Each assembly 9 and 10 comprises a pump and a valve connected in a re-circulation circuit back to the relevant tank, flow of reactant being diverted either to the recirculation circuit or to the mix head 4 according to the position of the valve.

A conveyor 13 extends from the mix head 2 and this comprises a bottom lining 14 lying on the conveyor and a pair of side walls 15 which are lined by coated paper rolled onto reels 19. Foam being produced is indicated by the numeral 50 and for the initial stages, up to 5 to 7m of the conveyor 13, the leading end of foam being produced abuts against a movable barrier 18 so that waste at the leading end of the continuously formed foam is minimised. A trough 17 is mounted at the front end of the conveyor 13 for reception of the reactants as they are delivered from the mix head 2. The trough 17 is lined with a lining comprising an 80gm per square meter craft paper coated with 20gm per square meter polyethylene. A high temperature tape is used to close the folded seams of the liner. The trough liner, first cut flat to template is assembled over a mould of high hardness polyether foam whose cell structure creates a friction with the polyethylene coating of the disposable trough liner interior, reducing slippage and facilitating accurate and rapid taping of the seams to form a trough liner shape. The use of a new disposable trough liner in each production run ensures a smooth flow of chemicals from the trough to the conveyor with resulting optimum block shape and surface characteristics.

The shape of the trough 17 has been developed and refined so that the front wall of the trough is at an angle of 27.3 degrees from the vertical. This shape allows the smoothest flow of chemicals from the trough 17.

The end of the conveyor 13 is shown in Fig. 4(a) and this includes a cutting station 60 of the reciprocating blade type.

Referring now to Figs. 2 to 6 inclusive, operation of the plant 1 for a process of the invention is described in detail.

As shown in Fig. 2, the mix head 2 comprises a vertically mounted cylindrical mixing chamber 30, above which is mounted a motor 31 driving an agitating blade (not shown) within the mixing chamber 30. A primary input pipe 30 is connected to an upper portion of the mixing chamber 31 by an input valve and a number of secondary inlet pipes 33 and 34 are connected at lower positions. An outlet 34 of the mixing chamber 30 is connected to a manually operated valve 35 for routing of mixed reactants to a test outlet 36, or to a pair of outlet tubes 37 and 38, the ends of which are in the trough 17. As shown in Fig. 2, the side walls 15 of the conveyor 13 are lined by coated paper lining 39 which is held in position by rollers 40 mounted on the side walls 15.

The inlet pipes 33 and 34 which are used for high pressure delivery (as described below) are connected to the chamber 30 by fixed nozzles. This overcomes the disadvantages of using spring biased moving nozzles, which are prone to wear and tear, thus leading to a lack of reliability in the production process. ''i In the production process the primary reactant is the particular polyol for the type of foam being produced. i The polyol may be pumped directly- from a bulk storage tank 3, or alternatively it may be pumped from the mix tank 4 if there is a fire-retardant graphite additive, or from the blending tank 6 if there is a melamine additive. If pumped from the blending tank 6, the polyol is initially pumped from a bulk storage tank 3 to the mix tank 7 where it is mixed with the correct proportion of melamine powder delivered into the hopper 8. The mixing time is of the order of 40 minutes for most polyols and the mixture is then pumped into the blending tank 6 where the powder is held in suspension by intermittent operation of an agitator. Needless to say, the tanks 4 and 6 may be used for various other reactants which require additives.

Mixing of the reactants in the mix head 30 is achieved by pumping the polyol through the primary pipe 32 where it is fed at low pressure through the mixing chamber 30. In the region of seven to nine other reactants are supplied by the pipes 33 and 34. These include TDI, catalysts, water, surfactants and other reactants well known in the art. An important aspect of the invention is that in the region of 10% to 20% of these reactants i.e. one or two of the seven to nine reactants are supplied under low pressure through some of the pipes 33 or 34. The others are pumped under high pressure through nozzles in the other pipes 33 and 34 into the mixing chamber 30 so that there is a sharp pressure drop within the chamber 30. Action of the motor 31 also helps to mix the reactants together. Because not all of the reactants are fed at high pressure, the costs of equipment such as pumping, flow metering equipment and valves is considerably less than would otherwise be the case and maintenance of the equipment is also simpler. Another advantage is that flow of the reactants is easier to control.

High pressure pumping is carried out by pump and valve assemblies such as those indicted by the numerals 9 and . These act to re-circulate reactants back through the tank, and when the pump has attained the necessary speed, the valve is switched to route the reactant directly to the mix head nozzle? Thus, the pump encounters high pressure only when up and running at the operating speed.

When all reactants are being pumped to the mix head, the hand valve 35 is operated to divert a small quantity of the mixture to a test container. Foam forms in this container and this is tested in order to determine if any process parameters should be changed.

The mixed reactants are delivered into the trough 17 where they continue to react together and they spill over the leading edge of the trough 17. At this stage, the mixture is still in liquid form and flow is interrupted by the barrier 18 which helps to form a substantially vertical front wall for the continuous block which is produced in the process. The mixture is indicated by the numeral 50.

This is most clearly seen in Fig. 3. The bottom lining of the conveyor 13 is formed by a coated paper lining, above which there is a lining of polythene film. These are indicated by the numeral 62 in Fig. 4(b). When the foam has travelled 5m from the mix head 2, a set of three nozzles 51 mounted transversely above the foam 50 pump out a spray of water at approximately 700 psi to form a fine mist over the foam 50. The distance of the nozzles 51 from the mix head may be varied in teh range of 5m to 10m from the mix head. This is important to ensure optimum usefulness for each particular reactant mixture.

This water reacts with TDI gas being emitted from the foam t, and neutralises it and also helps to form a less tacky upper surface of the foam 50 so that it may be cut more f easily at the cutting station.

The cutting station 60 is shown in Fig. 4(a) and this comprises a vertical blade mounted on a carriage which moves in synchronism With the conveyor 13 to cut across the foam to form blocks 61 shown in Fig. 4(c).

The polythene film forming the upper part of the bottom lining of the conveyor 13 often adheres to the bottom of these blocks and may easily be peeled off. It will be seen from Fig. 4(c) that the bottom corners of the blocks 61 are regular shapes. This is achieved because the polythene lining helps to seal the comers with the side lining 39 in the conveyor 13.

Referring to Figs. 5(a) and 5(b), an alternative cutting station 70 for use in the process is shown. The cutting station 70 includes pulleys 71 connected to a control unit 72 to control speed of operation of a blade 73 running in the pulleys as it cuts through the foam. The control unit 72 includes a lenze inverter speed control device for the motor driving the pulley 71 to allow manual step-less speed control of the blade. This is particularly important where quite hard foams are being produced and it is important to avoid wear and tear of the blade 73 by cutting more slowly. It also allows optimum blade speed control for optimum power consumption. The cut foam blocks are delivered at side rails 74 of the cutting station 70.

After the foam blocks have been cut, they are cured at a curing bay for over 24 hours. In order to break cells of some of the foam blocks so that they are more suitable for production of furniture, as shown in Fig. 6, the foam is delivered through a crushing station 80. This comprises upper and lower series of rollers mounted in a tapered configuration so that as the foam block is driven by the rollers forwardly through the narrower gap between the rollers, it is gradually crushed. After exiting from the rollers, the foam blbck reverts to its original shape.

However, it has a softer texture. ' It has been found that by starting up high-pressure pumping of reactants to the mix head 2 by initially re5 circulating the particular reactant through the storage vessel until the pump achieves its operating speed, there is much less wear on the pump and it has a longer life and is more reliable. This is particularly important in achieving-reliability of the production process. Also, in response to the initial test carried out, various other parameters such as conveyor speed may be adjusted. This is an important aspect of the invention as the test container gives a much more accurate reflection of the actual process than a laboratory test using the reactants would give. Further, it has been found that by having a mix of both high and low pressure pumping of reactants to the mix head good mixing is achieved, whereas at the same time the equipment used is less expensive and more reliable than would otherwise be the case. The water mist which is generated above the foam at between 5m - 10m from the mix head helps to ensure that gases are neutralised quickly and help provide for clean cutting of the foam and a longer life for the cutting blade. Because a polythene lining is used at the bottom of the conveyor, a less expensive grade of coated paper may be used underneath the lining and also the polythene lining is of considerable help in sealing the bottom comers so that an even block is formed.

The invention is not limited to the embodiments hereinbefore described but may be varied in construction v and detail.

Claims (6)

1. A process for continuous production of polyurethane foam blocks, the process comprising the steps of:pumping controlled quantities of reactants to a mix 5 head, including the sub-steps of:pumping a reactant requiring an additive to a mixing tank and mixing the reactant and the additive by action of an agitator, the resultant slurry being then pumped to a blending tank, in 10 which the solids are maintained in suspension by action of an agitator, the slurry being pumped from the blending tank to the mix head as a reactant; pumping polyol under low pressure directly from a 15 bulk storage tank or from the blending tank to a valve in the mix head; and pumping other reactants to side valves of the mix head, in the range of 10% to 20% being under low pressure, and the balance under high pressure via 20 a nozzle; agitating the reactants in the mix head; opening a two-way output valve of the mix head to deliver a controlled test quantity of mixed reactants to a test container; 25 monitoring the foaming reaction in the test container and adjusting process parameters accordingly; opening the valve to deliver mixed reactants to a trough at the start of a conveyor having a lined base and lined side walls; spraying a water mist over the foam in the conveyor at a distance of between 3m and 7m from the mix head; and cutting blocks off the continuous foam piece at the end of the conveyor and curing the blocks.

2. A process as claimed in claim 1, wherein reactants fed under high pressure are initially re-circulated through the storage tank until the pump attains a desired operating speed for delivery to the mix head.

3. A process as claimed in claims 1 or 2, wherein the base of the conveyor is lined with a lower paper-based film, and an upper polythene film extending between the side walls.

4. A process as claimed in any preceding claim comprising the further step of crushing a block of foam by passing it between two series of rollers mounted in a tapered configuration.

5. A process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A polyurethane foam block whenever produced by a process as claimed in any preceding claim.