Polyester strength for backpacking tents
Polyester strength is tested. This cheap breathing umbrella fabric is compared with traditional silnylon in the context of making DIY backpacking tents.
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.
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.
“While DIY treating or doping silnylon with diluted RTV silicone rubber I could see that the doping liquid easily passed through the fabric from one side and out the other!” I describe simple comparative tests of polyester and silnylon that show that although they both can provide great protection from rain, the fabrics are far from waterproof.
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.
Polyester and silnylon strength when new. The above 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. The following ‘simple bucket tests’ put this conjecture to an objective test.
Polyester strength testing
For the testing, I used scraps of polyester umbrella fabric and two silnylon fabrics from my sewing room. The fabrics were in new condition and had not been significantly exposed to the elements.
Test fabric strips. Straight grain strips of the fabric were sewn so that they formed double-layered lay-flat tubes that were 30mm wide. These were turned inside out and open load-bearing hems were formed on each end by folding and then sewing the hem with three lines of stitches.
The polyester strength testing rig. Bent fencing wire was used to form U-shaped mounting wires with hooks formed at the tops of the ‘U’. One pair of hooks were used to hold the testing rig up in a tree. The other pair of hooks were used to suspend a 20-litre bucket.
Results. The polyester test strip and the grey silnylon passed the strength test and supported about 18,000g of water load. The orange silnylon test strip failed at about 15,000g of water as added (total load of ~16,000g including the bucket).
These little videos of the testing may help to better describe how this crude but fair testing was done.
“Although breaking the test strips is always on the cards, the shock of it happening always takes me by surprise. I have edited out my exclamation and substituted the singing of my backyard bird chorus. Thanks, birds.”
More strength testing a competition to destruction
I considered that supporting and surviving a load of 18kg was a fair test, when new, for the polyester and the grey silnylon. However, some followers have asked questions about the breaking strength of these two surviving samples. My largest plastic bucket weighed about 1kg and could hold about 26L. I had a feeling that this might not be big enough to break the polyester. So I suspended two additional 10L buckets (0.5kg each) on the sides of the first bucket. Consequently, the tare weight of the three buckets was a nice round 2kg. Again I added 10L of water from a bucket and then added additional loads of 2L from a jug until the sample broke or I ran out of space in the buckets.
The record of testing was recorded in these two videos; Silnylon38 and Polyester40. The silnylon broke after the addition of about 38kg and the polyester survived at 40kg when the rig was full. The ultimate breaking load of the polyester is still unknown.
Conclusion
I consider that these crude tests show that polyester is right up there with silnylon with regard to strength when new. Polyester was way better (~9kg) than silnylon (~2kg) after the fabrics had been exposed to the elements for 500 days on my washing line.
Polyester’s superior strength (even without a ripstop grid)) should also be considered in the context of polyester’s other advantages over silnylon; negligible wet stretch, low condensation, better strength when aged and faster drying for backpacking. “Maybe faster drying is the wrong term. Perhaps I could say that polyester becomes dry sooner because its hydrophobic properties allow it to get less wet than silnylon.”Lastly, according to my testing, both fabric have equivalent hydrostatic head ratings (poor, but adequate for a rain shelter).
Collectively, these attributes make unproofed polyester umbrella fabric an ideal alternative to silnylon for lightweight backpacking tents especially considering its relatively low price.
Silpoly may address the wet stretching issue, but probably not the condensation problem of its cousin silnylon.
“On a rainy and windy night, I find it very comfortable sleeping in a polyester tent with out waterproofing even if it is theoretically (according to armchair experts) weaker than silnylon and should leak. My mind is at rest. It is far better than sleeping in an equivalent silnylon tent that will probably survive the night and most certainly rain down condensation or ‘tent snow’ under the same conditions.”
Mothy the elder
Addendum 1
Having speculated about the silver coating possibly providing UV protection to the silver-coated polyester umbrella fabric, I realised that I had the inside-out test sample still on my washing line on March 10 2023.
So I hastily prepared to load test it. While settling the test rig down with an empty (~1kg) bucket suspended from the sample I estimate that I applied an extra one or two kg load to the bucket and the fabric started to rip. It was very weak and I think I could safely conclude that the silver coating was the major contributor to the superior survival of the polyester when it was used as a sun shield. This also confirms the wisdom of putting the silver coating on the outside of my tents.
Addendum 2
My investigations of fabric strength have been a bit ‘topsy-turvey’. They started with testing strength after 500 days of exposure. Then they included strength when new, then ultimately breaking loads when new and now silver-coated polyester when exposed for 540 days with the silver coating on the inside of the test sample. Consequently, the summary table below might help to make things a little clearer.
Tim
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 |
