Polyester silnylon, relative hydrostatic head test

Polyester and silnylon fabric for tent making are compared with regard to the relative hydrostatic water head that they can hold.

Introduction

General performance. Two experimental tents made from polyester umbrella fabric have been described; A simple square breathing polyester pyramid tent and a larger Breathing polyester pyramid tent with a vestibule entry. The polyester fabric performed well, the tents keep the rain out during heavy rain, there was negligible wet stretching and the condensation was absent or minimal when compared to a similar silnylon tent that was pitched under the same conditions. Field experience revealed that the polyester dried more quickly than silnylon when the tent had to be packed up on cold damp mornings.

Rapid ageing tests. Testing of the accelerated ageing of polyester and silnylon on my washing line showed that after 500 days of exposure to the elements, polyester had much more strength than silnylon did under the same conditions. This investigation begged the question (for me and others); What was the relative strength of these fabrics when new? I was also curious because there is a popular belief that polyester is generally weaker than silnylon but sufferers less from UV degradation.

Polyester and silnylon strength when new. I did crude load testing of polyester and silnylon as mentioned above (for the ageing test). I loaded a big bucket with approximately 10 litre of water and then added 2-litre increments from a jug until the bucket was full (~18,000g) or the fabric test sample broke.

The testing is described in this post [Add link] along with a video of the ‘bucket-testing’. Again, polyester came out on top when a silnylon test strip failed before the bucket was full.

Hydrostatic head. I have received considerable criticism for making a tent from fabric that is not waterproof. I am somewhat comforted to read a blog titled Tents 101 by Bogong Equipment  where they say:

Tent fly fabric doesn’t need to be extremely waterproof, because it doesn’t have to handle the same amount of pressure that a tent floor does (e.g. you won’t be kneeling on your tent fly) – the tent fly only has to deal with falling rain. What’s more, if tent fly fabric is tensioned, rain just bounces off it. (Think of how rain bounces off even crappy umbrella fabrics with very low waterproof ratings.)……So, it’s not the tent fly fabric, but the tent floor fabric that you should pay most attention to…..

Bogong Equipment

Practical experience and a hunch told me that silnylon, my previous go-to fabric for DIY tent making, did not have a very good hydrostatic head that may be no better than that of polyester without waterproofing. Consequently, I thought that it was time to do a simple comparative hydrostatic head test of polyester and silnylon.

Polyester and silnylon hydrostatic head testing

Making the hydrostatic head testing tubes. I made ~600mm long seam-sealed tubes of polyester umbrella fabric and silnylon. They were marked with calibration lines at 100 mm intervals from the bottom of the tube. “My intention was to only fill the tubes to the 500mm level to start the testing, but the water pissed out so quickly that I was glad that I had an extra bit of leeway to get the tests off to a fair and equal start for the photographic recording.”

The silnylon seam was first primed with DIY silicone rubber seam sealer (1 volume of silicone rubber + ~5 volumes of mineral turpentine). Then the tube was sealed with neat RTV silicone rubber, as described in DIY RTV silicone rubber uses for backpacking gear.

The polyester fabric does not form very strong bonds with TRV silicone rubber as silnylon does. Consequently, I glued the seam, leaving the silver side facing outwards. Then I immediately folded the seam and sewed it tightly together, through the four layers, while the silicone rubber was still in a liquid state. “In this application, I think of the weakly bonded silicone rubber as acting as a waterproof gasket, rather than providing mechanical strength.” Finally, I painted the stitches with DIY seam sealer from both sides along the stitch lines.

Visual assessment of the hydrostatic testing. I used still photos to describe the gross leakage through both fabrics. This leakage was immediate and the silnylon appeared to be leakier than the polyester without any waterproofing treatment. “And to the naysayers, no I did not let the silicone rubber spread from the seam area to the rest of the tube and thereby waterproof it. I used a mask to prevent the accidental spread. If only waterproofing could be that easy!”

This little video of the visual testing may better describe the situation than the photo below.

Gravimetric testing. To confirm my subjective assessment of the relative leakage rates of the two fabrics, I suspended the tubes over their own respective collection bowls and allowed the drips to accumulate over a 35-minute period. During this, time the leakage was 32g for silnylon and 24g for polyester.

During this testing, I also observed a general slowing of the leakage rate, particularly for the silnylon. This slowing could be due to the nylon fibres adsorbing water and swelling so that the leakage holes became smaller. “I have seen cotton canvas tents behave this way in my childhood.”

Conclusion

These primitive hydrostatic head tests indicate that both polyester and silnylon were quite leaky with a pressure head of about 500mm of water. As expected, the leakage was the greatest at the bottom of each tube, there was also leakage up the entire length of the water column, as far as I could tell. “So there was no magical dry hydrostatic head threshold.” The unproofed polyester was no worse than silnylon and actually appeared to be noticeably better both visually and gravimetrically. The polyester’s better performance is probably due to the strongly hydrophobic properties of the polyester polymer when compared with the hydrophilic nature of nylon.

There is considerable discussion about the hydrostatic head of outdoor fabrics deteriorating with repeated use. I have no doubt about this (considering how badly new fabric leaks). “I wonder if ageing and usage are blamed for the poor hydrostatic head performance of some fabrics when perhaps the performance was ‘crap’ when the fabric was new and unused?”

Anyway, the simplicity of the polyester alternative fabric means that there is no waterproof coating or DWR to deteriorate or blame for the loss of performance with use.

In my experience, both fabrics perform well as a means of providing a rain shelter while camping. So a high hydrostatic head rating is apparently not so important for a tent canopy. On the other hand polyester’s hydrostatic head rating (poor, but adequate) can be added to its: low wet stretch, low condensation, better strength (when new), better strength when aged, low price and faster drying for backpacking.

Collectively, these attributes make unproofed polyester umbrella fabric an ideal alternative to silnylon.

“On a rainy and windy night, I find it very comfortable sleeping in a polyester tent with out waterproofing that theoretically (according to armchair experts) should leak. My mind is at rest. It is far better than sleeping in a silnylon tent that will most certainly rain down condensation under the same conditions.”

Mothy the elder

Tim

Addendum 1

The summary table below may help to better summarise the tangled mess of my tent fabric and cord testing for backpacking tents.

Nylon and polyester strength after extended sun exposure

Table 1. Breaking loads of various tent fabric samples and cords before and after extended sun exposure. Also subjective assessment of the performance of the materials for backpacking pyramid tent. The fabric test samples were sewn tubes of fabric that were 30mm wide. The samples were supporting a suspended bucket/s to which 2L jug loads of water was added. The weights shown (~kg) included the tare weight of the bucket/s and were based on the equivalence of one litre of water=one kg
Sample type	Polyester umbrella fabric silver coated	Silylon ripstop fabric 1 (grey)	Silylon ripstop fabric 2 (orange)	Polyester cord (brickies line, yellow)	Nylon cord (brickies line, yellow)
Material property
Estimated material weight	~57gsm	~37gsm	~37gsm	xx/m	xxg/m
Fabric strength in as-new condition (breaking load kg) Post link	>40kg	~38kg	~16kg	38kg	38kg
Fabric strength after 500 days of weather exposure (breaking load kg) Post link	~9kg	~3kg	NA	NA	NA
Fabric strength after 540 days of inside-out exposure Post link	~3kg	NA	NA	NA	NA
Cord strength in as new condition before weather exposure (breaking load kg) Post link	NA	NA	NA	38kg	38kg
Cord strength after 540 days of exposure Post link	NA	NA	NA	19kg	29kg
Other performance characteristics
Fabric permeability to prevent tent condensation	Good	Poor	Poor	NA	NA
Fabric resistance to wet stretching	Excellent	Poor	Poor	not yet tested	not yet tested
Speed of drying for cold morning pack ups Post link	Excellent	Poor	Poor	NA	NA
Bond strength when glued with silicone rubber. [Add post link when available]	Very poor	Good	Excellent
Fraying/unraveling [Add post link when available]	NA	NA	NA	Poor	Good
Simple knot holding [Add link when available]	NA	NA	NA	Poor	Good
The meaning of life [Add post link when available]	NA	NA	NA	NA	NA