|1.||A method of making blocks of polyurethane or other opencell foam arising from exothermic reaction of foam forming material, wherein once the reaction has reached a desired stage of completion air or other gas of suitable composition and temperature is passed through the body of the block to carry away the heat of reaction until a stable temperature is reached.|
|2.||A method according to claim 1, wherein reaction is taken to the stage of maximum block temperature (initial exotherm peak) and then passage of the cooling gas is begun.|
|3.||A method according to claim 1, wherein to reduce or eliminate false hardness cooling is begun before the stage set out in claim 2 has been reached.|
|4.||A method according to any preceding claim, wherein the cooling gas is passed through suitably porous skinned faces of the block.|
|5.||A method according to any preceding claim, wherein the cooling gas contains auxiliary reactants modifying the properties of the foam by chain termination or other means.|
|6.||A method according to any preceding claim, wherein the gas after passing through the block is treated to remove materials constituting pollutants if released into the atmosphere, or to remove materials, or heat, of value present in the gas.|
|7.||A method according to any preceding claim,wherein stages of block production, holding until the desired stage of reaction has been completed, and cooling, are performed continuously.|
|8.||Foam producing plant in which a foaming machine * for production of blocks of opencell foam by exothermic reaction of foam forming materials is associated with means for holding the blocks produced until the .reaction has reached the desired stage and with means for subsequent passing a gas of suitable temperature and composition through the body of the block to carry away the heat of reaction until a stable temperature is reached.|
|9.||Plant according to claim 8 wherein, a vertical foaming machine of the kind set out in claim 1 of European Patent Specification No. 0 058 553B is associated with a conveyor which constitutes the holding means and which passes the blocks to a cooling chamber wherein the cooling gas is passed .|
|10.||Plant according to claim 8 or 9 wherein the gas is passed through blocks from top to bottom while they rest on a permeable conveyor.|
|11.||Plant according to claim 9, 10 or 11 wherein the stages of block production, holding and cooling are carried out on continuously moving blocks.|
Processes for the production of polymeric foams by reactive chemical routes are varied and well known. An example is flexible polyurethane foam which is produced in blocks typically 2 metres x 2 metres x 1 metre. These large blocks can be produced either continuously on conveyor type machines, or discontinuously in moulds. In the case of flexible polyurethane foam the reacting mass achieves a high exotherm temperature within a very short time, typically between 5 minutes and 30 minutes.
Blocks once made therefrom have to be transferred to an intermediate "cure area" where they are carefully stacked with air space around each block until they have cooled. A large area is required for this purpose and the blocks typically need to be stored for a minimum of 10 hours before they can be restacked or loaded for transporting to the customer. This process of intermediate storage, to ensure adequate cooling of the blocks, is clearly inconvenient and costly in space requirements. Further, the intermediate storage area contains a large number of blocks of inflammable foam at high temperature, presenting a potential fire hazard. The building used for this intermediate storage needs to be specially constructed to meet fire regulations.
A further important factor is that certain of the foam forming reactions are reversible at high temperature, typically the allophanate reaction following the initial polyurethane bond formation. Typically, the blocks of foam in the intermediate storage area are at an internal temperature exceeding 140°C for several hours. If the ambient atmosphere in the intermediate area is not controlled, i.e. is of variable humidity, there is a potential for ingress of moisture into the block which will react with free isocyanate end groups and terminate them:-
This reaction t removing the isocyanate required for the allophanate " reaction ( results in an uncontrolled reduced level of cross linking in the foam and therefore a variable,
reduced, stiffness or compression hardness. In geographical locations where a high ambient humidity is common, it is known for foamers to deliberately increase the quantity of isocyanate in a given foam recipe, in spite of the cost penalty in doing so, so as to allow for the hardness loss that would otherwise be experienced. The Invention
The present invention provides a new approach of early cooling, and specifically a method of making blocks of polyurethane or other foam arising from exothermic reaction of foam-forming materials, wherein once the
reaction has reached a desired stage of completion a gas of suitable composition and temperature is passed through the body of the block to carry away the heat of reaction. The gas will normally be air, and the approach is the reverse of the present approach of slow cooling and minimum exposure to air while cooling takes place. The approach has to be seen in the light of a long standing problem in the polyurethane foam industry, namely autoignition of foam blocks due to excessive chemical exotherm. The problem occurs particularly with certain low density and high exotherm grades of foam, or foams containing additives which are included to render the foam resistant to small sources of ignition. Such foams can, after a period of two to three hours, and after they have started to cool, begin to increase in temperature again. This second exotherm is normally a self progressive type, eventually resulting in autoignition. Several factories have been burned down because of this phenomenom. One mechanism is thought to be the drawing in of air from the atmosphere as the block cools. The oxygen enriched atmosphere within the block then causes exothermic oxidation of the polyurethane polymer with a resulting temperature rise. The presence of air draughts around blocks has been shown to exacerbate the problem.
The invention in contrast deliberately supplies air.
hile nothing of this kind has been proposed before, it is known for example that a method exists for lifting blocks of reconstituted bonded foam crumb which consists of a vacuum box having a perforated base. This vacuum box is applied to the top surface of the block of rebonded foam and sufficient suction is caused to be able to lift up the block. The vacuum lift technique serves a secondary purpose of removal of some of the moisture that was introduced during the rebonding process. No such concept has been applied to new foam, nor has cooling been the purpose. Moreover such a vacuum lifting process draws air through the block from the surrounding atmosphere, no attempt being made to control the composition of the air, and the air flow is not through the body of the block, the greater part entering- through the upper sides. Such a process would not give the desired results if applied to a hot block of open celled flexible foam as described above. In fact, the results would be most undesirable in that the foam hardness would be reduced unevenly throughout the block, due to reaction with the atmosphere moisture.
Other processes of gas cooling are even less relevant. For example U.K. 1 358 932 provides a skinned material for car seats and the like by heating one side and blowing air at the other side of a layer of foaming materials, while U.S. 4 435 523 injects a cold compressed gas into a cavity within an injection moulded body of foaming material, giving a heavily internally and externally skinned product. The present invention depends on passing gas through the body of a block of material, ordinarily from one face to another though it would be just possible to introduce cooled or even liquified gas by hollow needles into the body of a block for outward passage through the body of the material.
Details of Process The degree of control of the gas passed may vary according to circumstances and the class of foam being produced. It may be no more than to ensure that the ambient air is of satisfactory temperature and humidity and is free of dirt of other pollutants harmful to the foam. Preferably however, especially for polyurethane foam, the moisture content of ambient air is reduced and its cooling power raised by cooling the air and removing condensed moisture. Fcϊr special purposes liquified gases may be used for intense cooling, and/or inert gases such as nitrogen. Conveniently
air is recirculated allowing convenient removing of what would otherwise be waste heat and scrubbing of gases taken up from the blocks to prevent undesired release into the atmosphere of contaminating or valuable materials. The stage at which cooling is done is important. For reduction of block holding times and thus space requirements and other disadvantages or risks such as autoignition it is desirably as early as possible. It must not however be so early that the reaction forming the polymer of the block is not sufficiently completed. Normally the time/temperature curve of the block should have peaked, indicating substantially full reaction, but in some circumstances a somewhat earlier commencement of cooling (with polyurethane foam at least) will reduce the initial but not the in-service hardness of the foam. Polyurethane foam can feel undesirably stiff when new, for example in seat cushions, reaching after a period of service a hardness that it substantially retains thereafter. This 'false hardness 1 can be reduced or eliminated by early cooling, before the peak block temperature has been reached.
The gas may be drawn most readily through the cut faces of blocks made on continuous machines but it has surprisingly been found that at least on lightly skinned blocks such for example as those produced by the "VERTIFOAM"(Trade Mark)proCess of European Patent
Specification No. 0 058 553 air or other gas can be drawn through the skinned faces. Such a process is convenient in that the cut length of blocks is more likely to be varied in a production run than their other dimensions, and the cooling equipment can conveniently therefore be set up for passing the gas through a constant foam thickness.
Relation to Temperature/Time in Block
The temperature time graph for the centre of a foam block cooled in the conventional manner is the curve marked A in Fig. 1. Typically the temperature rises very quickly in the first ten to thirty minutes and then levels out. After a time of one to three hours, depending on the density of the foam the temperature starts to fall. Typically a time of at least twelve hours must be allowed before the blocks may be safely stacked or loaded onto transport vehicles. A second exotherm is shown at B in Fig. 1, which is of the kind referred to earlier herein which if not speedily controlled leads to ignition. The graph also shows the effect of drawing through large amounts of cooled air of controlled composition, rapidly reducing the temperature of the block and thus preventing any possible onset of uncontrollable exotherm and autoignition, in curve C. The exact point at which the cooling according to
the invention begins depends on the effect required. It has been found that, in order to achieve a final foam hardness equivalent to that of a conventionally cooled block, it is generally necessary to delay the application of rapid cooling until the peak exotherm temperature has been reached. Alternatively, if it is required to achieve a foam block with reduced hardness, the rapid cooling process may be applied earlier.
Block Modification Thus there is provided a process for accelerating the cooling of a block of foam by passing through the block, a gas mixture of controlled composition and temperature. By this means the time for which a block of foam is in a vulnerable high temperature condition can be drastically reduced, from a matter of hours to one of minutes if required.
A further advantage is that, by modifying the composition of the gas mixture which is introduced or drawn through the block, so as to include material which will modify the foam by reacting with the basic foam constituents, the final properties of the foam can be influenced. For example, by introducing a monofunctional hydroxy compound such as methanol with the gas mixture whilst the internal block temperature is elevated, controlled termination of isocyanate groups
can be achieved, resulting in a block of foam having controllably reduced compression hardness or stiffness because of the reduced level of chemical cross linking.
Scorch Control A common problem and potential hazard in foam block manufacture is "scorch". This is manifested by a yellow or brown discolouration in the centre of the block which is undesirable in itself and in severe cases can develop into a thermal runway leading to a fire. One case of scorch is though to be oxidation of the hot foam and it is knownthat ingress of oxygen from the atmosphere, whilst the foam is at an elevated temperature, is a significant factor. Rapid cooling of the block to a safe intermediate temperature or even to ambient temperature minimises scorch and if done by introduction of a cool inert gas such as nitrogen incapable of supporting oxidation of the foam, eliminates any risk of heating at source.
Control of Factory Emissions It is becoming more and more a requirement that factories control the level of toxic substances in the factory and the quantity of fumes emitted into the atmosphere. During polyurethane production the machinery is normally contained within a ventilated
enclosure and the emitted fumes are exhausted to the atmosphere, sometimes after having been chemically treated (scrubbed). Some of the volatile materials however, are contained within the foam block and are liberated over a period of time in the curing area within the factory.
Because of the difficulty containing such a large area, the control of emissions at this point is very difficult. By rapid cooling of blocks the volatile materials within the block are removed during the cooling process, which enables more efficient treatment either by chemical scrubbing or by exhausting through a suitable chimney. The compact nature of the cooling process and the ready containment of the extracted volatiles from the block, also makes the process ideally suited to recovery of, for example, any fluorocarbon blowing agent which is used in the foam production. This recovery could be by means of, for example, an activated carbon filter.
Heat Recovery from Blocks.
A further advantage of the process is that the considerable amount of heat generated within a foam block may be conveniently recovered by the refrigeration unit. Each block of foam contains approximately 4,000 kilocalories of recoverable heat, depending obviously on the density and temperature of the block.
Other types of foam
The discussion is largely based on flexible polyurethane foam blocks, but it will be clear that the rapid cooling process is applicable to any open celled exothermic reaction foam material . Examples of such foams are, high resilience polyurethane flexible foam, phenol formaldehyde foams, silicone foams, polyimide foams, polyimidazole foams, epoxy foams, polyester urethane foams or any chemical combination of these types.
While the invention lies primarily in the process it extends also to foam producing plant in which a foaming machine for production of blocks of open-cell foam by exothermic reaction of foam forming materials is associated with means for holding the blocks produced until the reaction has reached the desired stage and with means for subsequently passing a gas of controlled temperature and composition through the body of the block to carry away the heat of reaction until a stable temperature is reached.
Conveniently in particular a vertical foaming machine of the kind set out in Claim 1 of European Patent Specification No. 0 058 553B is associated with a conveyor which constitutes the holding means and which passes the blocks to a cooling chamber wherein the
cooling gas is passed.
Conveniently in any event the gas is passed through blocks from top to bottom while they rest on a permeable conveyor. The concept of rapid cooling of a block in minutes rather than hours is particularly attractive when considered as above in conjunction with a process which is producing blocks of foam at a modest rate. An example of such a process is the vertical foam process as drescribed in European Patent No. 82 300748.9 (publication No. 0 058 553), where typically blocks are cut off at a rate of one every 2 minutes. " An economically very attractive arrangement is to site a rapid cooling unit adjacent to the vertical foam machine. " Subject to a minimum holding time where necessary for sufficient reaction, blocks can be individually cooled as they are produced, whereupon they can be immediately stacked in an efficient manner or even loaded directly onto transport. By this means the block inventory is kept to an absolute minimum and the need for the customary large and costly intermediate storage area, with its inherent problems, is eliminated.
It will be appreciated that for a continuous foam production process it is convenient to cool blocks on a continuous cooling unit rather than one by one. The blocks are conveyed on an open mesh conveyor which passes
through a pressure/suction zone where the pressure difference across the block causes a flow of air through the foam. The speed of a conveyor is ajusted so that the blocks have cooled sufficiently by the time they emerge from the other end. The process can be conveniently monitored by means of a temperature probe at the far end of the low pressure zone.
Drawings and Examples
The invention is illustrated by detailed examples below and by discussion of the drawings which are: Fig. 1 Temperature/time curves for flexible polyurethane blocks;
Fig. 2 Temperature/time curves for the thermocouples referredto in Example 1; Fig. 3 Perspective view of single-block cooling device used as referred to in Examples 1 to 6; Fig. 4 Schematic plan view of same; Fig. 5 Sectional longitudinal elevation of a continuous cooling plant; and Fig. 6 Section transverse elevation of same.
Description of Device used in Examples
The cooling equipment of Figs...3 and 4 consists of an enclosure (1) into which can be inserted a block (2) of foam of dimensions approxiamtely 1.7 metres wide by
1.1 metres high by 1 metre long. The block of foam may be produced on any conventional continuous foam process or for example by the "VERTIFOAM" (Trade Mark) process as described in European Patent Specification No. 0 058 553. The block is positioned in such a manner that the circulated air flows in through one and out through the other of the cut faces of the block. Leakage of the circulated air is minimised by an adjustable seal (3) which may be fitted tightly against the foam surface.
Further sealing of the block of foam into the enclosure is effected by a cover (not shown) placed over
* the enclosures and sealed against it.
Air, or any other chosen cooling gas, is circulated through the foam block and around the enclosure through a refrigerator-cooled radiator (5) contained within the enclosure by means of a centrifugal fan (6). Pressure of the air entering the block and leaving the block is measured by means of water filled 'U 1 tubes (4) at each side of the enclosure. The volume of air flow is calculated from air velocity measurements made by means of an anenometer (7) placed in the air stream.
Alternatively, air or any chosen cooling gas may be drawn through the block and then exhausted without recirculation. The air may not be required to be cooled below ambient. An advantage of cooling to lower than
ambient temperature is that water vapour content is reduced as the temperature is reduced.
Temperature of the cooled radiator is monitored by a thermocouple (8). Temperature of the foam block is monitored by means of thin wire thermocouples T2, T3 and
T4, inserted in various positions within the foam block. This apparatus was used in the following Examples.
A block of flexible polyurethane foam was produced by the "VERTIFOAM" continuous foam process as described in
European Patent Specification No. 0 058 553. The chemical formulation was as follows:
Table 1 Parts by weight
Polyether polyol (3500 molecular weight 48 hydroxyl no.) 100
Silicone surfactant • 0.9
Amine catalyst - "Dabco"(Trade Mark) 0.2
33 V Stannous octoate catalyst. 0.2
(Trade Mark) 11 1.5
Toluene diisocyanate (80:20 TDI) 55.6
Approximate time from the chemicals being mixed in the mixing head, to the block being cut off was eight minutes. The time of cut off was noted.
Dimensions of the foam block were, 1.7 metre wide, 1.1 metres high by 1 metre long (cut face, to cut face).
The block was positioned in the cooling enclosure so that the air flow direction through the block was from one cut face to the other.
At thirty minutes after cut off time (just past the exotherm peak), the refrigerator was switched on. When the radiator temperature reached -10°C (after about one minute) the circulating fan was switched on. Pressure measurements at the block inlet face were, + 14 mm of water and at the outlet face -36 mm of water. Air flow volume rate was calculated at 23.6 cubic metres per minute.
Temperatures were monitored by thermocouples placed as follows:
Reference Position Tl In the airflow at the cooled radiator
T2 Inserted to a depth of 10 mm into the centre of the inlet face of the block
T3 Threaded by means of a long "needle" into the centre of the block T4 Inserted to a depth of 10 mm into the centre of the outler face of the block.
About one minute after the fan had been switched on, it was noticed that a small quantity of fume was escaping from leaks around the block/enclosure seal. These fumes subsided after a further two minutes. Eleven and a half minutes after switching on the circulating fan, when the temperature indicated by the thermocouple number 4
dropped to +40 °C, the circulating fan switched off and the foam block removed. Temperature at the centre of the block at this time was 16°C. This temperature was monitored for a further four hours to check for any temperature rise due to chemical exotherm but no significant change was noted, the temperature slowly rising to ambient temperature which was 18 °C.
The four thermocouple monitored temperatures were plotted against time and are shown in Figure 2. It was noted that the temperature at the centre of the block was 164 °C before the rapid cooling commenced.
The curves show that the block does not cool evenly throughout the mass. Instead, because of the efficient heat exchange between foam and gas, the effect is of a "cold front" moving through the foam block.
The following day the physical properties of the foam were evaluated by testing to BS.3379 : 1975 "Flexible Urethane Foam For Load Bearing Applications", and compared with similar tests carried out on a conventionally cooled block of foam from the same production run. Results are tabled below.
Table 3 Physical Test Results - Example 1
Block A Block B Normal Cooling Rapid Cooling Procedure Procedure
Piece density, Kg/m 3 . 20.8 20.8.
Sample density, Kg/m 3
Top of block 20.8 21.1
Middle of block 20.2 20.1 Bottom of block 20.9 20.5
Sample of hardness, Newtons
Top of block 120 120
Middle of block 140 135
Bottom of block 130 130 Tensile strength, Kpa 107 100
Elongation at break, % 250 200
Fatigue test (Constant Load pounding test)
Hardness, % 35.7 34.1 Height loss, % ' 5 5
A block of the same formulation and process as Example 1 was taken and was subjected to the same procedure as Example 1 except that the rapid cooling was commenced at only ten minutes after the cut off time. It was noted that the temperature in the centre of the block, in this case was only 150°C when the circulation fan was started. Physical tests of the foam were again measured after twenty four hours and it was found that the hardness was
lower than that of a conventionally cooled block from the same production run.
A block of foam of the same formulation and process as Examples 1 and 2 was subjected to the same procedure as Example 1, except that 400 ml of water was spread over the inlet of the block of foam prior to inserting it into the enclosure. Time interval between block cut off and rapid cooling was thirty minutes as for Example 1. Foam density and hardness was measured the following day, comparing them with conventionally cooled foam from the same production run. Samples were also subjected to the constant load pounding fatigue test, BS.3127 which gives an assessment of service performance. Lower hardness loss indicates superior performance. The . results are shown in below:
Conventional .Rap:id cooled cooled block block
Density 20.9 20.0
Hardness, Newtons 126 110
Hardness after fatigue tes ;tt
Newtons 87 85
% loss 31 23
It was noted that the foam samples that had been rapid cooled had a better fatigue performance as measured by the constant load pounding test.
Examples 4/5 Two foam blocks of the same formulation and process as Examples 1-3 were subjected to the same procedure as Example 3 except that for Example 4 industrial methylated spirit was subsitituted for the water. In Example 5 0.880 ammonia solution was substituted for the water used in Example 3.
In both cases the foam was of a lower hardness than the foam blocks from the same run that had been cooled in the conventional manner.
Example 6 A flexible polyether urethane foam block was produced by a continuous foam machine of the well known "MAXFOAM" (Trade Mark) "trough and fall plate" type. This block was of similar dimensions to the foam blocks used in the previous Examples 1-5. The block was subjected to the same rapid cooling procedure as in Example 1. Physical test results were as follows:
Density, Kg/m 3 = 16.5 Hardness, Newtons = 60 The foam was within specification for this grade.
The rapid cooling enclosure, as earlier described and as used for Examples 1-6, was modified so as to enlarge the inlet and outlet orifices to an area of 2.1 metres x 1.6 metres. A foam block weighing 58 Kgs and of dimensions 2.2 metres long x 1.7 metres wide x 1.1 metres high, produced by the "VERTIFOAM" (Trade Mark) process (European Patent Specification No. 0 058 553 was, after a time of thirty minutes following cut off, inserted in the enclosure in such a manner that the air flow direction was through the skinned surfaces of the 2.2 x 1.7 metre dimensions. The volume air flow rate measured during rapid cooling was 28 cubic metres per minute. The time taken to cool the block to ambient temperature was six minutes.
A flexible polyurethane foam block of 22 Kg per cubic metre density was produced on the"MAXFOAM"process using a formulation containing fire retardant additives. This grade of foam normally produces internal discolouration during the conventional cooling process due to the well known phenomenon "scorch". The foam block, after a delay of fifteen minutes following cut off, was rapid cooled in twelve minutes. The following day the block was cut in half. No scorch was discernable. A block of foam from the same production run was similarly " "
A production machine for putting the invention into practice is shown in Figs. 5 and 6. An enclosure 10 defines an air inlet zone 11 and an air exit zone 12 from which air is drawn by a centrifugal fan 13. The fan has 5000 cu.ft./per. in. capacity at 5 inch water gauge suction (2500 litres per second at 120mm water gauge).
The fan feeds the air to a refrigerating coil unit 14 of which the inlet and outlet for refrigerant, circulated by a compressor (not shown) are schematically indicated by arrows. The refrigeration unit has a capacity of 600,000 BTU per hour (2500 K.Cal per minute) working temperature -20°C.
The enclosure 10 is surrounded by a fume extraction enclosure 15 so that any leakage from the cooling enclosure of undesired fumes is prevented from escaping into the working area.
Foam blocks are fed to the machine on a roller conveyor 16 of such a length that the blocks have reached the first exotherm peak before cooling starts. The length of this conveyor depends of course on the production rate of the foaming machine itself and on the speed of transfer through the cooling zone which is conveniently approximately 1 metre per minute. A similar conveyor 17 carries the cooled blocks away. The cooling enclosure
dimensions are such that the resident time of the block in the zone is sufficient to achieve cooling and for example at the 1 metre per minute speed a cooling zone length of 2.5 metres (three times the average block width) is convenient.
Within the cooling zone the blocks are supported on an open-mesh conveyor 18 driven by rollers 19 and passing over supports 20. The conveyor may for example be a belt of open wire mesh or open hinged sections capable of taking a load of for example 0.2 Ibs/sq.in. (0.014 Kg/sq.cm.) Just below the conveyor at the outlet end of the cooling zone is a temperature probe 21 by means of which the temperature of the circulating air emerging from the block can be monitored just before it leaves to ensure that the cooling process has been completed. As may be seen in Fig. 6 the supports for a wire mesh conveyor are conveniently longitudinal bars. The effective width of the cooling zone may be for example 2 metres which can cope with a block size of 2 to 3 metres.
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