The Background to Rain Resistance in Clothing and Other Textiles
We have all seen claims for waterproof and showerproof and in more recent years, claims for hydrostatic head of anything up to 20m (2000cm or even 20,000mm). Marketing people do like big numbers, so they prefer to use mm instead of metres!
The question is: what does this actually mean and how is it measured?
Over the years there have been numerous attempts to provide answers to both questions with varying degrees of success.
In the days before microporous coatings and membranes and hydrophilic polymers, wax was used to keep the water out or rubber solutions such as the original Mackintosh. Rubber solutions were impermeable but wax treatments allowed the fabric to breathe.
In more recent times polymeric coatings either on the fabric face or the back have become more popular, and are lighter in weight than the rubber laminate and they don’t require retreating as with the wax.
Breathability remained a problem until it was discovered that if small holes were punched in the polymer coating, water vapour could escape from inside without letting liquid water in from outside. These small holes are microscopic and known as micropores and provided it is dryer outside than inside, moisture vapour from inside can pass to the outside thus keeping the wearer more comfortable.
The tests used to evaluate these properties have also been varied but two have remained the mainstays of water resistance testing. The hydrostatic head test (ISO 811 or ISO 1420) and the spray rating (ISO 4920) have become synonymous with waterproof and shower resistant at least in the eyes of the retail industry.
The term waterproof has been discouraged for many years but remains in common parlance by the retail trade and the consumer. In scientific terms, the word “proof” is an absolute term meaning that water cannot get through no matter what – this is clearly untrue because you can always increase the pressure and push harder. Eventually of course the fabric itself would burst and so in recent editions of the Textile Terms and Definitions, the fabric should not be called “waterproof” unless the hydrostatic head pressure is equal to the hydraulic bursting pressure of the fabric. So officially fabric that keeps water out, is said to be Water Penetration Resistant (WPR).
The hydrostatic head test is basically a clamped fabric specimen against which water is pumped at gradually increasing pressure until droplets are seen coming through. When the droplets are seen the pressure is recorded in cm (or mm) head of water or in kPa depending on which method you read. 100cm head of water is equivalent to 9.8 kPa.
There are possibly two shortcomings to this test at higher pressures:
- The fabric balloons in to a dome and stretches the micropores making them larger and letting water pass – something that would not happen in a garment no matter how hard the rain fell
- If the pores are very small, water can come through in such small droplets that the eye cannot detect them individually and so the surface just seems to get wet and detecting the end-point is therefore imprecise
The former could be countered by use of a fine, inextensible mesh clamped over the fabric to prevent distension, but the gauge of this mesh could detract from the ease of seeing the penetrating droplets. A suitable mesh is given in BS EN ISO 1420.
The spray test is often seen as a showerproof test but this is strictly not the case. The spray test provides a measure of surface wetting, but it is not a water penetration test. It will indicate how wet the coat will get, rather than how wet the wearer will get whilst wearing it.
Imagine for a moment a rainwear fabric which relies on a back coating to keep water out. When tested on the hydrostatic head, the water pressure will be increased until liquid water penetrates but this will depend on how resistant the back coating is. When assessed by the spray test, the face of the fabric will get wet depending on the absorbency of the fibres in the main fabric and the back coating should keep the wearer dry. So, it is quite possible to get a good hydrostatic head value if the back coating is sound but a poor spray test result if the face fabric is at all hygroscopic.
Applying the coating to the face of the fabric would enable the wearer to get the best result in this regard because the water would run off the coating without wetting. The aesthetic appearance however, may not be what was required. So, then we come to the nearly invisible Durable Water Repellent (DWR) finish being applied to the face side to repel liquid water and enable most of it to run off without wetting the surface.
The hydrostatic head test can be used for fabric with low WPR if the pumping speed is reduced. In fact, it was widely used for canvas fabrics and values of 30 to 80cm hydrostatic head were commonly accepted as with waxed cotton jackets, tents, tarpaulins, motorcycle coats and so on.
These days, using the hydrostatic head test for such low values is not seen as appropriate and something else is required. The spray test is often seen as the only alternative but there are and have been others which fill the gap between hydrostatic head which is seen as for “performance” fabric and the spray test which is seen as the lighter end. Tests such as the Wira Shower test (BS 5066), Bundesmann Water Repellency test (ISO 9865) and the Credit Rain Simulator. Each has its drawbacks and limitations but all filled that gap. Sadly, none is widely used. In fact the Credit Rain Simulator seems to have disappeared altogether whilst the Bundesmann and the Wira machines are difficult to find.
There was a time when retailers would define shower resistance and rain resistance as below when measured using either the Wira or the Bundesmann:
- Rain resistant: Penetration <25 mls after 10 minutes
- Shower resistant: Penetration more than 25mls but <50mls after 10 minutes
For tarpaulins, there is an old MOD test which is also mentioned in an Australian standard (AS 2001.2.18) which is known as the Cone Test – most appropriate these days for fabrics intended to protect loads where standing water on the tarpaulin might be a problem. A sample of fabric is formed in to a cone (in the manner of a filter paper in a funnel) and suspended over a beaker. The cone is filled with water and left overnight to see what water if any, makes its way through the fabric and in to the beaker.
AATCC gives a method (TM35) called the Rain Test which sprays water from a small rose against a vertically mounted specimen backed by blotting paper. The pressure of the water is regulated by a fixed column supplying the spray head, whilst the blotting paper absorbs any water penetrating the fabric and penetration is measured according to weight gain in the blotting paper.
In the European Union the specification for WPR for Protective Clothing against rain (BS EN 343) is given as a Hydrostatic Head pressure of between 8 and 13 kPa depending on the class of the fabric. This is equivalent to 82 cm to 133 cm water gauge.
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