GB2176198A - Flexible foams - Google Patents

Abstract

A method for making a high elongation, low modulus, flexible foam material from a foamed formulation comprises compressing said foamed formulation prior to cure to give the foam material a permanent set, and breaking the permanent set after cure by stretching. The foam material exemplified is a polyurethane.

Description

SPECIFICATION Flexible foams This invention relates to flexible foamed polyurethane material.

Densified polyurethane foam is known from US Patent 3 506 600.

The present invention provides a high extensibility, low modulus, flexible polyurethane foam material.

This invention comprises a method for making a high elongation, low modulus, flexible polyurethane foam material from a foamed formulation comprising compressing said foamed formulation prior to cure to give the material a permanent set, and breaking the permanent set after cure by stretching.

The compression may be relaxed substantially before cure, or maintained substantially to cure. It may be effected in a single direction, such as against the direction of foam rise, or in two directions at right angles, when one of said directions may be against the direction of foam rise.

The method may be carried out on a slab of foamed formulation or on a cylinder, when the compression may be axial.

The compressed, cured foam formulation may be cut into sheets or strips - from a slab, sheets or strips may be cut parallel to the sides, while from a cylinder, a sheet or strip may be peeled veneerfashion from it.

A material produced as described may have a complex load/extension relationship, having an initial inelastic extensibility under increasing load during said stretching, some of such extensibility being re-coverable, the material then having an elastic extensibility at least over the range in which it was initially extended at a lower modulus than the modulus effective during its initial extension.

Such material may also have a graduated foam structure by including portions of the compressed foamed formulation which have undergone different degrees of recovery from the compression prior to cure.

The material may be stretched so as to reticulate the foam structure. Such a reticulated structure may be used as a filter, for example an air filter for an automobile engine, and may advantageously have a graduated foam structure as described.

Embodiments of flexible foam material and methods for producing them according to the invention will now be described with reference to the accompanying drawings in which: Figure l is a cross-section through a block of just-produced foam before curing, Figure 2 is a cross-section through a block as shown in Figure 1 with a single direction compression applied to it, Figure 3 is a cross-section through a block as shown in Figure 1 with a two direction compression applied to it, Figure 4 is a section through the axis of a cylinder or cheese of just-produced foam before curing, Figure 5 is a section through a cylinder as shown in Figure 4 when compressed in the axial direction, Figure 6 is a perspective view of a compressed, trimmed block produced by a compression as illustrated in Figure 2, showing how it may be cut into sheets or strips, Figure 7 is a perspective view like Figure 6 of a block produced by a compression as illustrated in Figure 3, and Figure 8 is an axial view of a trimmed cylinder produced as illustrated in Figure 5 being peeled veneerfashion.

The Figures illustrate a method for making high elongation, low modulus, flexible foam material from a foamed formulation II comprising compressing said foamed formulation prior to cure to give the foam material a permanent set, and breaking the permanent set after cure by stretching.

A suitable formulation for producing a flexible polyether foam having a density of approximately 16Kg/ m3 (free rise) is as follows: Parts by Weight Polyol (3500 Mol Wt 48 OH value 100.00 Water 4.85 Silicone Surfactant (L.540 Union Carbide) 1.20 D.M.E.A. (Di-Methyl Amino Ethanol) 0.35 Stannous Octoate (Nuocure 28 Durham RM) 0.30 Trichloro-Fluoro-Methane (Freon 11 DuPont) 13.00 Pigment 0.01 Toluene Di Isocyanate (80:20) 110 Index This formulation may be processed in a conventional foam plant to produce a round topped continuous foam slab which is cut into blocks of required length. Such a block is shown in cross-section in Figure 1. A typical height H to the crown is 92 cms at a time five minutes after the formulation is mixed and poured onto the paper-lined conveyor of the plant.

The cut block is then compressed, as illustrated in Figure 2, as by a weight or a ram 12 to a height of about 16 cms, the compression taking place against the direction of foam rise, i.e. vertically, with the block in the same position as on the conveyor.

Different effects are obtained depending upon the time at which the compression is relaxed, e.g. by removing the weight or lifting the ram 12. If the compression is left on for five minutes and the block left to cure for 24 hours at ambient temperature, the block expands to a height of about 18cms. On examination, it will be seen that there are upper and lower layers of approximately 1.5 and 0.5 cms thickness respectively which are of apparently normal foam (i.e. what would be expected without compression) the remainder being evidently permanently compressed.

Clearly, the other faces of the block will also be irregular. After trimming the block all round, a foam slab of about 16cm thickness and substantially homogenous composition will be obtained with an overall density of about 90Kdg/m3.

Figure 6 shows such a trimmed slab 61 on which are marked lines 62, 63 indicating the way sheet or strip can be cut from the block (using any conventional cutting technique). The slab 61 has been compressed during its production process in the direction of arrow Z, and the resultant sheet or strip has a c#omplex load/extension relationship in this same direction.

When the sheet or strip is first stretched in the Z direction, it has a relatively high modulus and any substantial load is accompanied by an inelastic extension. It seems as if some inelastic or relatively inextensible bonds are broken in an initial stretching operation, so that on relaxation of an initial loadrthe material does not return to the initial length. By a sufficient degree of stretching, all of the inelastic extensibility of the material can be taken up.

It will then be found that the elastic properties of the material are changed as compared to the same properties before stretching. The material is now relatively highly extensible - for example up to four times its unstretched length - with relatively low force, as compared to the force required for the initial stretching. With a one-directionally compressed foam it is only the properties in that direction that are affected properties in the orthogonal directions are unaffected.

Such foams, in sheet or strip form, have good draping properties and can be fabricated into a variety of shapes. For example, a 5.00mm thick sheet having a width of 10cm (in the direction of compression) and a length of about 65cm can be joined (as by glue) along its ends to form a cylinder. The initial stretch can be performed across the diameter of the cylinder, which will then have an internal diameter of about 3.6cms. Such a foam cylinder can be fitted on a patient's arm from wrist to elbow, or a leg from the ankle to above the knee and conform to the various diameters there encountered with very little pressure effect.

The foam also, upon this initial stretching, becomes porous through reticulation, in which closed cell walls are selectively broken to greater or lesser extents depending on the degree of initial stretch. It can thus serve as a filter, particularly an air filter for an automobile engine. A thicker section of the foamed material can, after the initial stretching, be fabricated into a filter having similar dimensions to those of a conventional automobile filter, and a graded filtration effect can be achieved by bending the material into an annulus, so relatively stretching and compressing respectively the outer and inner faces of a cylinder of the material so that it is relatively open, i.e. has large intestices on the outer face and relatively closed i.e. has small intestices on the inner face.

Variations may be introduced by altering processing variables such as the degree of compression or the time for which the compression is applied. In general, the following can be said: 1. The lower the degree of compression, the greater the thickness of uncompressed foam top and bottom, the more variable is the foam structure and density in the "permanently" set core region and the lower the modulus after stretching. The permanent set of low compression foams is stretched out most easily.

2. The higher the degree of compression the smaller are the thicknesses of the top and bottom layers and the variability of foam structure in the core. The elongation after stretching is higher.

3. The longer the foam is held under compression, the less is the thickness of the top and- bottom foam layers and the more uniform is the foam structure in the core.

The following Table sets out some examples: Example Height of Time Under Block Block Density Comp Block Compression Height Height (trimmed (cmsj Untrimmed Trimmed rtgim3) 2 25 5 mins 28.0 23.5 59.5 3 25 16 hrs 25.5 24.7 60.0 4 20 5 mins 22.5 18.5 75.0 5 20 16 hrs 20.5 19.0 75.0 6 18 5 mins 21.0 17.0 80.0 7 18 16 hrs 18.5 17.5 80.0 After trimming the resultant blocks and cutting 5cm slices parallel to the compression direction, some samples were stretched to break the permanent set and allowed to relax. Examples 2 and 3 relaxed to about 110% of their length before the initial stretch, and had an elastic elongation to over three times their original compressed trimmed size.Examples 6 and 7 relaxed to about 120% of their length before the initial stretch and stretched to over four times their original length under a light load after the perma nent set had been removed.

The block could, of course, be compressed along a direction different to the direction of foam rise and then the stretch direction would be different.

Figure 3 illustrates compression in two orthogonal directions by means of a weight or ram 31 applied at the top and a ram 32 applying pressure at one side against a retaining wall 33 at the other side. The resulting product, treated as previously described displays the complex load/extension properties in both directions.

Figures 4 and 5 illustrate a like compression operation to that of Figure 2 performed on a cylinder or cheese 41 by a weight or ram 42.

Figure 7 is a drawing like Figure 6 showing how a two direction-compressed slab 71 is cut along lines 72.

Figure 8 shows a trimmed compressed cylinder or cheese 81 in axial view which is mounted for rota tion on a horizontal axis and peeled veneer-fashion by a knife 82 to produce a relatively long length of sheet having like complex load/extension properties as the other sheets or strips referred to.

Claims (17)

1. A method for making a high elongation, low modulus, flexible foam material from a foamed for mulation comprising compressing said foamed formulation prior to cure to give the foam material a per manent set, and breaking the permanent set after cure by stretching.

2. A method according to claim 1 in which the compression is applied at an early stage of curing.

3. A method according to claim 1 or claim 2 in which the compression is relaxed substantially before cure.

4. A method according to claim 1 or claim 2 in which the compression is maintained substantially to cure.

5. A method according to any one of claims 1 to 4 in which compression is effected in a single direc tion.

6. A method according to claim 5 in which said direction is against the direction of foam rise.

7. A method according to any one of claims 1 to 4 in which compression is effected in two directions at right angles.

8. A method according to claim 7 in which one of said two directions is against the direction of foam rise.

9. A method according to any one of claims 1 to 8 carried out on a slab of foamed formulation.

10. A method according to any one of claims 1 to 8 carried out on a cylinder of foamed formulation, the compression being axial.

11. A method according to any one of claims 1 to 10 in which the compressed, cured foam material is cut into sheets or strips.

12. A method according to any one of claims 1 to 11 in which the foamed formulation comprises a polyurethane foam.

13. A method substantially as hereinbefore described with reference to the accompanying drawings.

14. A flexible foam material produced by a method according to any one of claims 1 to 13.

15. A material according to claim 14 having a complex load/extension relationship, having an initial inelastic extensibility under increasing load during said stretching, some of such extensibility being re coverable, the material then having an elastic extensibility at least over the range in which it was initially extended at a lower modulus than the modulus effective during its initial extension.

16. A material according to claim 14 or claim 15 having a graduated foam structure by including por tions of the compressed foamed formulation which have undergone different degrees of recovery from the compression prior to cure.

17. A filter made from a material according to claim 16 which has been stretched so as to reticulate the foam structure.