O&P Library > POI > 1986, Vol 10, Num 3 > pp. 149 - 152

Technical note: The effect of water content on the stiffness of seating foams

A. M. Brown *
M. J. Pearcy *

Abstract

The chairbound, disabled person requires a supportive cushion to distribute pressure in order to reduce the risk of pressure sores and any alteration to the load carrying capacity of the cushion may have a deleterious effect on its ability to provide adequate support. The National Health Service supplies two basic grades of polyurethane foam for wheelchair cushions and this study investigated the effect of water content on their compressive load carrying capacity. Both foams became less stiff and exhibited greater than 20% increase in deformation when containing 20% water by volume at loads encountered in seating.

This decrease in stiffness may result in a dramatic change in the pressure distribution under a patient particularly if only a small section of the cushion becomes wet.

This result emphasizes the need to fit waterproof coverings to these foam cushions and to maintain the integrity of the covering.

Introduction

The chairbound person often requires a cushion to distribute the supportive force over the largest area possible in order to reduce the risk of the development of pressure sores, and any change in the ability of the cushion to support the individual may be a cause for concern. The aetiology of pressure sores is complex and, whilst pressure has been quoted as the main cause (Garber, 1985) there are other factors involved, such as shear forces arising out of pressure gradients, friction (Reichel, 1958), the microclimate at the interface between the cushion and the patient (Stewart et al, 1980), the state of nutrition of the patient and his or her level of motivation (Nelham, 1981).

Areas where thin layers of tissue overlie bony prominences, such as under the ischial tuberosities, are susceptible to pressure sores as the forces induced in the tissue when sitting on unyielding surfaces will cause deformations sufficient to restrict or cut off the blood supply, leading to necrosis. The use of a cushion provides a compliant surface which allows the forces to be distributed over larger areas such as the thighs. The most readily available cushions are made from polymer foams which are lightweight and relatively inexpensive.

In a previous study it was noted that polymer foams became more compliant when immersed in water (Fox, 1985). If an increase in compliance is caused by water in the foam then the load distributing property of the foams will be affected and as this would have major implications for their use in wheelchairs it was decided to investigate this phenomenon.

The National Health Service supplies two basic grades of polyurethane foams for seating cushions and the aim of this study was to evaluate the effect of water content on the compressive characteristics of these foams. To provide some form of comparative index between the two grades their void to solid ratio and permeability were measured. Following this, compression tests were conducted on samples of the foams from dry to saturated at different rates of deformation.

Materials and procedures

The two grades of polyurethane foams examined were—

Sample A—

A yellow coloured foam with a fairly soft feel and a density of 49 kg/m³.

Sample B—

A chip foam conglomerate with a stiffer feel and a density of 99 kg/m³.

Three cylindrical specimens of each sample were tested. All the specimens had a diameter of 70mm whilst their heights were 53mm for Sample A and 39mm for Sample B. The specimens were tested in loose fitting clear plastic film covers which prevented gross water loss during the tests but did not affect the compressive characteristics.

The compression tests were conducted using an Instron 1000 materials testing machine within which a small perspex bath was fitted. The specimens were compressed within the bath between two thin aluminium plates used to distribute the applied load over the end faces of the specimens.

Load compression curves were obtained at three different rates of deformation. 10,100 and 500 mm/min. At the beginning of each test at least three loading cycles were performed to pre-condition the specimens. Following this, repeated testing gave consistent results. The specimens were tested dry, then with 20% water by volume and finally immersed.

The void ratio of the samples was determined by immersing the specimens in a beaker of water The apparent increase in the volume of water, averaged from several readings, indicated the actual volume of polymer enabling the void ratio to be calculated from the measured volume of the cylindrical specimens. The percentage of water added for the compression tests was calculated as a percentage of the maximum volume of the voids within the foam.

The permeability of the samples was measured using a constant head permeameter which is suitable for materials which have a relatively high permeability giving a high flow rate of water.

Results

The results for the volumetric and permeability measurements given in Table 1 show that Sample A had slightly higher values of void ratio and permeability than Sample B.

The compression results for the specimens of each sample were very similar. Typical graphs for the nominal stress/strain relationships for the two samples at a deformation rate of 100 mm/min are given in Fig. 1. In air both samples showed little response to change in the rate of deformation, whilst both showed some stiffening at the highest rate when immersed. However, the change was small in comparison to that caused by water content and so results for the 100 mm/min rate of deformation only are presented. The graphs have a sigmoidal shape with an initial stiffer linear part followed by a plateau region (especially with Sample A) and then a third stiffer region.

The black dots on the curves are the recommended maximum pressures to which the following areas of the body should be subjected (Nelham, 1981)—

Posterior thighs—

80-100mm Hg (10.7-13.3 kN/m²)

Sub-trochanteric shelf—

60mm Hg (8.0 kN/m²)

Ischial tuberosities—

40mm Hg (5.3 kN/m²)

Coccyx/sacrum—

14mm Hg (1.9 kN/m²)

It can be seen from the graphs that Sample A was more compliant than Sample B. The increase in deformation that occurred with the addition of water is demonstrated by comparison of the values of strain at the recommended pressures (Table 2). The values for Sample A with 20% water approach those for saturation very early whilst those for Sample B only approach the saturation values at high strains.

Discussion

Polyurethane foam has an open cell structure. It can be considered as a network of fibres corresponding to the edges of the original bubble faces. When the structure is compressed the fibres will support the load by compression along their length. Resistance to loading should be fairly high in this mode until a critical value is reached and the fibres begin to buckle. As the fibres will not all lie in the same direction there should be a smooth transition to a more compliant response. This is seen in all the graphs where an initial linear, relatively stiff region leads into a more compliant plateau region. After this a stiffer region appears when the void spaces have been closed and the bulk material is compressed. Sample A, the lower density foam, with a higher void ratio, showed a clearer plateau region as might be expected, there being less actual material forming the fibres. Also, the chip foam has extra internal support from the adhesive holding the small pieces of foam together.

The effect of adding water to the foams was quite dramatic. There are two main implications for the use of the foams as cushion materials designed to distribute the load in a particular way. First, if one part of a cushion became wet then the load taken by that part would be greatly reduced and the support lost would be taken up by other areas, with the danger of exceeding the recommended pressures. Secondly, if the whole cushion became damp it might not be able to support the patient completely with the risk of bottoming out creating very high pressures, particularly under the ischial tuberosities. Sample B, the chip foam with its lower void ratio and permeability, was less affected at the lower values of strain due to the less pronounced plateau region, but at the higher strains there was little difference between the actual increase in strain for either sample. However, when looked at in terms of percentage increase in strain from the dry state Sample B is seen to be more affected. For example, at a stress of 8 kN/m² the percentage increases to % strain from dry to containing 20% water were 22% for Sample A and 60% for Sample B (Table 2). The lower permeability of Sample B together with the adhesive holding the chips together may result in the water being retained throughout the foam, whereas it is possible that the water drains to the bottom of Sample A, consequently having less effect.

The reason why water reduces the stiffness of these foams is unclear. Some form of molecular lubrication is the most likely explanation but further work is required to confirm this.

In conclusion, this study has demonstrated clearly that small volumes of water within polyurethane foams markedly reduces their stiffness and highlights the necessity for a waterproof covering when used for wheelchair cushions and the maintenance of the integrity of the cover.

References:

1. Fox, K. R. (1985). The deformation of highly porous materials. Final year honours project report. Durham, England: University of Durham, School of Engineering and Applied Science.
2. Garber, S. L. (1985). Wheelchair cushions: A historical review. Am. J. Occup. Ther. 39, 453-459.
3. Nelham, R. L. (1981). Seating for the chairbound disabled person-a survey of seating equipment in the United Kingdom. J. Biomed Eng. 3, 267-274.
4. Reichel, S. M. (1958). Shearing force as a factor in decubitus ulcers in paraplegics. J. Am. Med. Assoc. 166, 762-763.
5. Stewart, S. F. C, Palmieri, V., Cochran, G. V. B. (1980). Wheelchair cushion effect on skin temperature, heat flux and relative humidity. Arch. Phys. Med Rehabil. 61, 229-233.

O&P Library > POI > 1986, Vol 10, Num 3 > pp. 149 - 152