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Water Vapour Permeable Materials for Mattress Coverings
Journal of Tissue Viability 1998 Vol 8 No 1
12
WATER VAPOUR PERMEABLE MATERIALS FOR MATTRESS COVERINGS MICHAEL CLUBB Formerly a director of Penn Nyla I
1. The Development of Mattress Coverings
2. Water Vapour Permeability
It is not until the last decade or so, that the materials used to cover hospital mattresses and pressure relieving equipment have been the subject of careful study, evaluation and development. 'Out of sight- out of mind', might describe the level of profile a hospital mattress used to have in the not too distant past! The main requirement then, was for a cheap, flame retardant waterproof covering, with little consideration of its impact on patient comfort, or its interaction with the mattress itself and the bed frame. It was not too difficult to locate in hospitals, mouldy, stained covers, and rusty solid base beds. Progress to today's sophisticated two-way stretch coated materials, from the rubber or plastic sheeting, unyielding 'green stripe' type fabrics, and the rigid coated fabrics such as the much used 'marble print', seems to have been fairly slow.
A coating on a fabric that is water vapour permeable, sometimes referred to as moisture vapour permeable, has the ability to transmit water vapour molecules through itself, whilst at the same time remaining a complete barrier to liquid water. It is a concept that people have found difficult to understand. How can something that doesn't let water through, 'breathe'?! There are basically two mechanisms by which this is accomplished.
Theoretically, there could still be a place for the cheaper, cruder products of the past. Short stay beds for patients do not need to be particularly sophisticated. Such patients are not often incontinent, or present any level of pressure sore risk. However, the problem of movement of beds around a hospital, and the ever varying imbalance of patient need, really means that good quality mattresses and covers need to be available throughout a hospital, allowing for all eventualities to be accommodated. During the development of new cover fabrics in the mid 80's, regulatory changes were made to the flame retardancy requirements. New safety standards required foam cores to be flame retardant and for composites to be tested. This delayed the introduction of new materials, but had the effect of broadening the range of types of fabric that could be used for covering mattresses. Fabrics with more open substrate structures were developed. Different yams were considered and weights were varied, enabling new options on stretch characteristics to be introduced. The provision of stretch in a coated fabric has to take into account the extensibility of both fabric and coating. It is desirable to have slightly less potential stretch in the backing fabric, than that obtainable in the coating. This ensures that at points of major stress, it is the fabric that takes the load, and not the coating. A two-way stretch fabric is likely to extend at least 70%, depending on its structure. The degree of stretch required will depend on the properties of the supporting system. Coatings with varying degrees of water vapour permeability, suitable for use on the face of the fabrics were also evaluated.
a) Microporous materials These are membranes made from special polymers that have very tiny holes in them. Gortex is an example and they are mainly found in clothing applications. The holes are so small, that they will not allow liquid water molecules to go through, but they will allow the much smaller, water vapour molecules to pass through. Generally speaking, the membranes are somewhat fragile and are not suitable for use on the wearing surface of a fabric, which is the situation in a mattress cover. b) Hydrophilic materials (figure I) Hydrophilic means water attracting. A coating on a fabric that has hydrophilic properties, attracts moisture into itself and transmits the moisture through the coating by a chemical mechanism. It has no holes in it and is a complete barrier to liquid water. All polyurethanes are hydrophilic to some degree or another, and some can be formed into coatings that are tough, as well as being water vapour permeable. This makes them particularly suitable for mattress cover applications. Some polyurethanes are so hydrophilic, that they visibly swell as they absorb moisture. Moisture vapour can travel in either direction through the membrane or coating, the direction being from the atmosphere of highest humidity, to one of a lower humidity. The transfer is speeded up if there is a temperature differential as well. In practice, there normally is. There is a British Standard for water vapour permeable fabrics, BS7209, which sets out test methods and standards. It is however, directed at clothing applications and the minimum levels of permeability laid down, · are not achievable, and are not necessary for hospital mattress applications. Coated fabrics are sometimes just described as being 'breathable'. This can be a misused and misunderstood term. A fabric can be said to be 'breathable' in the sense that the
Journal of Tissue Viability 1998 Vol 8 No 1 Figure 1. Diagrammatic cross section ofDartex fabric.
13
which, various options have been evaluated, has resulted in the bulk of today's stretch cover fabrics having what seems to be an adequate level of permeability. This is usually about 500 g/m2/day, measured by Paynes Cup, or about 11 % by the index method. It is unwise to demand higher levels of permeability than this, unless for a particular situation it becomes an overriding priority. In delivering high permeability, the producer will be forced to compromise on other properties which under normal circumstances, are equally as important. An element of choice is there though, for both producer and end-user.
5. Essential Properties of a Mattress Cover
skin breathes. It does not mean that it can be breathed through, in the sense that air can pass through. They are not air permeable.
3. The Measurement Permeability
of Water
Vapour
There are many ways in which water vapour permeability can be measured. The two methods referred to in this paper, are the so-called upright Paynes Cup 2 , and the British Standard index method BS7209. In the Paynes Cup method, a 'cup' containing water is covered with the test fabric and sealed. It is placed in an oven at 40°C and periodically weighed to see how much water has evaporated through the fabric over a given time interval. The BS7209 method compares the test fabric with a 'standard' fabric. Water is weighed into two identical dishes. One dish is covered with the test fabric, and the other with the standard fabric. Both are then exposed to an atmosphere of controlled humidity and temperature, for fixed periods of time. The water that has evaporated through the two fabrics is then measured. The quantity that has passed through the test fabric, is then expressed as a percentage of the quantity that has passed through the standard fabric. In both test methods, water is never in contact with the test fabrics.
4. The Need for Water Vapour Permeability in Mattress Covers
Anyone who has lain on a rubber sheet for any length of time, or on a PVC (polyvinylchloride) cover, will kJ?.ow how uncomfortable it can be. Rubber and PVC have for all practical purposes, no water vapour permeability. A normal healthy body at rest, loses water vapour at a rate of about 450 g/m 2/day, and anything that inhibits this will cause discomfort in one form or another. Sheets and blankets cope with this rate of transpiration under normal circumstances, but the insertion of a waterproof interlayer, as has to happen in a hospital situation, interferes with the transfer. It is therefore obvious that a mattress cover offering good patient comfort, needs to have a degree of water vapour permeability. Experience gained over the last decade, during
Wouldn't it be nice to have a really cheap fabric with good permeability, and stretch, and long life, etc? It can't be done! The production of a modem cover fabric is a compromise of properties, all of which are necessary at some level or another. A cover fabric that was waterproof and capable of lasting for ever, would have to be fairly rigid, with a hard heavy coating, and with low permeability. A really breathable fabric on the other hand, would stain easily, have low resistance to cleaning agents and abrasion, and would probably be more expensive. However, a list of desirable features can be drawn up. For instance, a good cover fabric should have: a)
two-way stretch properties, balanced to the end-use, so as not to modify the properties of the support system being covered.
b)
resistance to abrasion damage.
c)
waterproofness to BS5455 3, for the life expectancy of the cover.
d)
flame retardancy to required standards4 .
e)
an appropriate degree of permeability, depending on application.
f)
resistance to microbiological attack.
g)
value for money.
while (c) and (d) are fixed by BS standards, other features can be adjusted to suit needs (except (g), I suspect!). Abrasion is measured in the laboratory by rubbing the coated fabric against a standard worsted cloth on a Martindale test machine, according to BS5455 (B2) 5. A minimum of 5000 'rubs' is required without damage to the coating. Given the relative thinness of polyurethane transfer coatings, they are remarkable resistant to abrasion damage, yet provide wonderful aesthetics compared with other systems.
14
Journal of Tissue Viability 1998 Vol 8 No 1
6. Production of a Water Vapour Permeable Fabric
Today's stretch mattress cover fabrics are made from knitted substrates, coated by the trans!er coating technique. Substrates can be warp knitted or weft knitted depending on the stretch characteristics demanded by the end-use. The waterproof coating is a type of Polyurethane. Different polyurethanes are used to provide different properties. A hard polyurethane will give good abrasion resistance, whereas a soft polyurethane will be very flexible and comfortable, but have reduced abrasion resistance. The degree of permeability provided, can be varied by the type of polyurethane chosen, and the thickness (or weight) applied to the fabric. In the coating process, a solution of the chosen polyurethane, is cast on to a moving, continuous sheet of special release paper. This coating, which is the one seen on the outside of the finished product, provides all the properties needed for the fabric to work properly in its chosen end-use. The permeability is provided by the characteristics of the polyurethane. Such things as colour, light resistance, fungal and microbiological resistance, are all provided by additives compounded into the solution before coating. After the coating has been dried, another layer of a different polyurethane is applied in a continuous operation, and this coating serves as an adhesive to bond the knitted substrate to the coating already applied to the paper. This polyurethane is also water vapour permeable. At the end of the process the release paper is stripped from the coated fabric, leaving a flexible, durable and waterproof film, bonded to the stretch fabric beneath. Only the transfer process enables a perfectly waterproof coating to be combined with fabrics with a wide range of structures and stretch characteristics.
7. The Permeable Cover in Use
As has been said above, a water vapour permeable cover breathes, by allowing moisture vapour to pass through it. Water vapour from the high humidity atmosphere surrounding a patient in bed, passes through the coating into the atmosphere within the mattress, this being at a lower level of humidity. It is essential that the water vapour accumulating inside the mattress can pass out into the surrounding environment. If it cannot, the humidity inside the mattress will rise and water vapour transmission rate will fall. Water vapour can only escape to the outside, if either the mattress or support system is ventilated, or, as in the case of a foam mattress, the mattress is on an open mesh bed base. On solid base beds, the transmission from inside to outside is much reduced, and water vapour can condense on the cooler surfaces of the bed base, as vapour levels inside the mattress reach saturation point. This then becomes an ideal bug growing environment and can have adverse effects on the properties of both the fabric and foam block. A foam mattress
and its cover, needs to be viewed as a composite in terms of component performance in this respect. Perversely, even though polyurethanes transmit water vapour, they are degraded by water in the longer term. It is essential that this is recognised and that measures are taken to ensure covers and foam are not allowed to remain damp. Turning cycle regimes, and other care instructions from manufacturers, need to be strictly adhered to. If they are, an average life of about three years is a reasonable expectation. However, there will be circumstances in which this will be greatly exceeded. On the other hand, covers that have been subjected to a harsher environment, for whatever reason, could fail earlier. Polyurethanes are generally resistant to most cleaning agents, and providing covers are wiped clean and dry, no degradation should occur. Cleaning agents that contain phenols should be avoided however.
8. Conclusion
Water vapour permeable fabrics, when combined with two way stretch substrates, are now recognised as being an essential component of any pressure relieving surface. Development work continues to accentuate the positive properties of polyurethane coated fabrics, and reduce the negatives. They provide greater patient comfort, complement carefully engineered support mechanisms, and when used with due consideration and understanding of their performance capabilities, give long and satisfactory service.
Address for Correspondence
Mr M Clubb, 55 Heronswood Drive, Spondon, Derby DE21 7AX.
Acknowledgements
The cross sectional diagram of Dartex fabric was provided by Penn Nyla.
References 1.
2.
3.
4.
5.
British Standards Institute. BS7209: 1990. Water vapour permeable apparel fabrics. Milton Keynes: The Institute, 1990. Payne HF. Permeability measurement. ASTM specification D1652. British Pharmaceutical Codex, 1973. British Standards Institute. BS5455:1977. Specification for polyurethane coated nylon fabrics for mattress covers. Milton Keynes: The Institute, 1977. British Standards Institute. BS6807:1996. Methods of test for assessment of ignitability of mattresses, upholstered divans and upholstered bed bases, with flaming types of primary and secondary sources of ignition. Milton Keynes: The Institute, 1996. British Standards Institute. BS5455:1977 (B2). Abrasion Resistance. Milton Keynes: The Institute, 1977.
12
WATER VAPOUR PERMEABLE MATERIALS FOR MATTRESS COVERINGS MICHAEL CLUBB Formerly a director of Penn Nyla I
1. The Development of Mattress Coverings
2. Water Vapour Permeability
It is not until the last decade or so, that the materials used to cover hospital mattresses and pressure relieving equipment have been the subject of careful study, evaluation and development. 'Out of sight- out of mind', might describe the level of profile a hospital mattress used to have in the not too distant past! The main requirement then, was for a cheap, flame retardant waterproof covering, with little consideration of its impact on patient comfort, or its interaction with the mattress itself and the bed frame. It was not too difficult to locate in hospitals, mouldy, stained covers, and rusty solid base beds. Progress to today's sophisticated two-way stretch coated materials, from the rubber or plastic sheeting, unyielding 'green stripe' type fabrics, and the rigid coated fabrics such as the much used 'marble print', seems to have been fairly slow.
A coating on a fabric that is water vapour permeable, sometimes referred to as moisture vapour permeable, has the ability to transmit water vapour molecules through itself, whilst at the same time remaining a complete barrier to liquid water. It is a concept that people have found difficult to understand. How can something that doesn't let water through, 'breathe'?! There are basically two mechanisms by which this is accomplished.
Theoretically, there could still be a place for the cheaper, cruder products of the past. Short stay beds for patients do not need to be particularly sophisticated. Such patients are not often incontinent, or present any level of pressure sore risk. However, the problem of movement of beds around a hospital, and the ever varying imbalance of patient need, really means that good quality mattresses and covers need to be available throughout a hospital, allowing for all eventualities to be accommodated. During the development of new cover fabrics in the mid 80's, regulatory changes were made to the flame retardancy requirements. New safety standards required foam cores to be flame retardant and for composites to be tested. This delayed the introduction of new materials, but had the effect of broadening the range of types of fabric that could be used for covering mattresses. Fabrics with more open substrate structures were developed. Different yams were considered and weights were varied, enabling new options on stretch characteristics to be introduced. The provision of stretch in a coated fabric has to take into account the extensibility of both fabric and coating. It is desirable to have slightly less potential stretch in the backing fabric, than that obtainable in the coating. This ensures that at points of major stress, it is the fabric that takes the load, and not the coating. A two-way stretch fabric is likely to extend at least 70%, depending on its structure. The degree of stretch required will depend on the properties of the supporting system. Coatings with varying degrees of water vapour permeability, suitable for use on the face of the fabrics were also evaluated.
a) Microporous materials These are membranes made from special polymers that have very tiny holes in them. Gortex is an example and they are mainly found in clothing applications. The holes are so small, that they will not allow liquid water molecules to go through, but they will allow the much smaller, water vapour molecules to pass through. Generally speaking, the membranes are somewhat fragile and are not suitable for use on the wearing surface of a fabric, which is the situation in a mattress cover. b) Hydrophilic materials (figure I) Hydrophilic means water attracting. A coating on a fabric that has hydrophilic properties, attracts moisture into itself and transmits the moisture through the coating by a chemical mechanism. It has no holes in it and is a complete barrier to liquid water. All polyurethanes are hydrophilic to some degree or another, and some can be formed into coatings that are tough, as well as being water vapour permeable. This makes them particularly suitable for mattress cover applications. Some polyurethanes are so hydrophilic, that they visibly swell as they absorb moisture. Moisture vapour can travel in either direction through the membrane or coating, the direction being from the atmosphere of highest humidity, to one of a lower humidity. The transfer is speeded up if there is a temperature differential as well. In practice, there normally is. There is a British Standard for water vapour permeable fabrics, BS7209, which sets out test methods and standards. It is however, directed at clothing applications and the minimum levels of permeability laid down, · are not achievable, and are not necessary for hospital mattress applications. Coated fabrics are sometimes just described as being 'breathable'. This can be a misused and misunderstood term. A fabric can be said to be 'breathable' in the sense that the
Journal of Tissue Viability 1998 Vol 8 No 1 Figure 1. Diagrammatic cross section ofDartex fabric.
13
which, various options have been evaluated, has resulted in the bulk of today's stretch cover fabrics having what seems to be an adequate level of permeability. This is usually about 500 g/m2/day, measured by Paynes Cup, or about 11 % by the index method. It is unwise to demand higher levels of permeability than this, unless for a particular situation it becomes an overriding priority. In delivering high permeability, the producer will be forced to compromise on other properties which under normal circumstances, are equally as important. An element of choice is there though, for both producer and end-user.
5. Essential Properties of a Mattress Cover
skin breathes. It does not mean that it can be breathed through, in the sense that air can pass through. They are not air permeable.
3. The Measurement Permeability
of Water
Vapour
There are many ways in which water vapour permeability can be measured. The two methods referred to in this paper, are the so-called upright Paynes Cup 2 , and the British Standard index method BS7209. In the Paynes Cup method, a 'cup' containing water is covered with the test fabric and sealed. It is placed in an oven at 40°C and periodically weighed to see how much water has evaporated through the fabric over a given time interval. The BS7209 method compares the test fabric with a 'standard' fabric. Water is weighed into two identical dishes. One dish is covered with the test fabric, and the other with the standard fabric. Both are then exposed to an atmosphere of controlled humidity and temperature, for fixed periods of time. The water that has evaporated through the two fabrics is then measured. The quantity that has passed through the test fabric, is then expressed as a percentage of the quantity that has passed through the standard fabric. In both test methods, water is never in contact with the test fabrics.
4. The Need for Water Vapour Permeability in Mattress Covers
Anyone who has lain on a rubber sheet for any length of time, or on a PVC (polyvinylchloride) cover, will kJ?.ow how uncomfortable it can be. Rubber and PVC have for all practical purposes, no water vapour permeability. A normal healthy body at rest, loses water vapour at a rate of about 450 g/m 2/day, and anything that inhibits this will cause discomfort in one form or another. Sheets and blankets cope with this rate of transpiration under normal circumstances, but the insertion of a waterproof interlayer, as has to happen in a hospital situation, interferes with the transfer. It is therefore obvious that a mattress cover offering good patient comfort, needs to have a degree of water vapour permeability. Experience gained over the last decade, during
Wouldn't it be nice to have a really cheap fabric with good permeability, and stretch, and long life, etc? It can't be done! The production of a modem cover fabric is a compromise of properties, all of which are necessary at some level or another. A cover fabric that was waterproof and capable of lasting for ever, would have to be fairly rigid, with a hard heavy coating, and with low permeability. A really breathable fabric on the other hand, would stain easily, have low resistance to cleaning agents and abrasion, and would probably be more expensive. However, a list of desirable features can be drawn up. For instance, a good cover fabric should have: a)
two-way stretch properties, balanced to the end-use, so as not to modify the properties of the support system being covered.
b)
resistance to abrasion damage.
c)
waterproofness to BS5455 3, for the life expectancy of the cover.
d)
flame retardancy to required standards4 .
e)
an appropriate degree of permeability, depending on application.
f)
resistance to microbiological attack.
g)
value for money.
while (c) and (d) are fixed by BS standards, other features can be adjusted to suit needs (except (g), I suspect!). Abrasion is measured in the laboratory by rubbing the coated fabric against a standard worsted cloth on a Martindale test machine, according to BS5455 (B2) 5. A minimum of 5000 'rubs' is required without damage to the coating. Given the relative thinness of polyurethane transfer coatings, they are remarkable resistant to abrasion damage, yet provide wonderful aesthetics compared with other systems.
14
Journal of Tissue Viability 1998 Vol 8 No 1
6. Production of a Water Vapour Permeable Fabric
Today's stretch mattress cover fabrics are made from knitted substrates, coated by the trans!er coating technique. Substrates can be warp knitted or weft knitted depending on the stretch characteristics demanded by the end-use. The waterproof coating is a type of Polyurethane. Different polyurethanes are used to provide different properties. A hard polyurethane will give good abrasion resistance, whereas a soft polyurethane will be very flexible and comfortable, but have reduced abrasion resistance. The degree of permeability provided, can be varied by the type of polyurethane chosen, and the thickness (or weight) applied to the fabric. In the coating process, a solution of the chosen polyurethane, is cast on to a moving, continuous sheet of special release paper. This coating, which is the one seen on the outside of the finished product, provides all the properties needed for the fabric to work properly in its chosen end-use. The permeability is provided by the characteristics of the polyurethane. Such things as colour, light resistance, fungal and microbiological resistance, are all provided by additives compounded into the solution before coating. After the coating has been dried, another layer of a different polyurethane is applied in a continuous operation, and this coating serves as an adhesive to bond the knitted substrate to the coating already applied to the paper. This polyurethane is also water vapour permeable. At the end of the process the release paper is stripped from the coated fabric, leaving a flexible, durable and waterproof film, bonded to the stretch fabric beneath. Only the transfer process enables a perfectly waterproof coating to be combined with fabrics with a wide range of structures and stretch characteristics.
7. The Permeable Cover in Use
As has been said above, a water vapour permeable cover breathes, by allowing moisture vapour to pass through it. Water vapour from the high humidity atmosphere surrounding a patient in bed, passes through the coating into the atmosphere within the mattress, this being at a lower level of humidity. It is essential that the water vapour accumulating inside the mattress can pass out into the surrounding environment. If it cannot, the humidity inside the mattress will rise and water vapour transmission rate will fall. Water vapour can only escape to the outside, if either the mattress or support system is ventilated, or, as in the case of a foam mattress, the mattress is on an open mesh bed base. On solid base beds, the transmission from inside to outside is much reduced, and water vapour can condense on the cooler surfaces of the bed base, as vapour levels inside the mattress reach saturation point. This then becomes an ideal bug growing environment and can have adverse effects on the properties of both the fabric and foam block. A foam mattress
and its cover, needs to be viewed as a composite in terms of component performance in this respect. Perversely, even though polyurethanes transmit water vapour, they are degraded by water in the longer term. It is essential that this is recognised and that measures are taken to ensure covers and foam are not allowed to remain damp. Turning cycle regimes, and other care instructions from manufacturers, need to be strictly adhered to. If they are, an average life of about three years is a reasonable expectation. However, there will be circumstances in which this will be greatly exceeded. On the other hand, covers that have been subjected to a harsher environment, for whatever reason, could fail earlier. Polyurethanes are generally resistant to most cleaning agents, and providing covers are wiped clean and dry, no degradation should occur. Cleaning agents that contain phenols should be avoided however.
8. Conclusion
Water vapour permeable fabrics, when combined with two way stretch substrates, are now recognised as being an essential component of any pressure relieving surface. Development work continues to accentuate the positive properties of polyurethane coated fabrics, and reduce the negatives. They provide greater patient comfort, complement carefully engineered support mechanisms, and when used with due consideration and understanding of their performance capabilities, give long and satisfactory service.
Address for Correspondence
Mr M Clubb, 55 Heronswood Drive, Spondon, Derby DE21 7AX.
Acknowledgements
The cross sectional diagram of Dartex fabric was provided by Penn Nyla.
References 1.
2.
3.
4.
5.
British Standards Institute. BS7209: 1990. Water vapour permeable apparel fabrics. Milton Keynes: The Institute, 1990. Payne HF. Permeability measurement. ASTM specification D1652. British Pharmaceutical Codex, 1973. British Standards Institute. BS5455:1977. Specification for polyurethane coated nylon fabrics for mattress covers. Milton Keynes: The Institute, 1977. British Standards Institute. BS6807:1996. Methods of test for assessment of ignitability of mattresses, upholstered divans and upholstered bed bases, with flaming types of primary and secondary sources of ignition. Milton Keynes: The Institute, 1996. British Standards Institute. BS5455:1977 (B2). Abrasion Resistance. Milton Keynes: The Institute, 1977.