Silnylon glue bond strength
Introduction
This report is on silnylon glue bond strength using a bucket of water as the load. Ten samples of adventure fabrics were tested after RTV silicone rubber was used to glue was used to lap join fabric strips.
I have only had the experience of two batches of silnylon that I used for making DIY backpacking tents. One fabric formed excellent bonds with silicone rubber glueing while the other was weaker. I was interested to see how common this poorer adhesion problem was amongst the DIY crowd and sought small test samples from the Bushwalking Australia MYOG crowd for my testing.
My test samples were two silnylon remnant fabric samples from my own projects and five were from others who kindly donated a variety of other silnylons fabrics (and one silpoly) from their own DIY projects. I also sneaked in some other of my favourite adventure fabrics, silcotton and polyester, into the testing.
“And to the armchair DIY zealots, no this is not exhaustive testing of all silnylons in the known universe, I can not and do not generalise my findings to all silnylons. The information is just for those with a curiosity that may be like mine.”
Mothy The Elder
Silnylon glue bond strength load testing
Some samples were very small as you may expect, so all the test strips were made small. I cut 27mm wide strips (along the grain) of each fabric and glued two portions of the strip to each other having a glued overlap that was also 27mm deep. This made the glue contact surface a 27*27mm square for all samples. After glueing the test samples were left to cure for at least 48 hours. “The cure time was a damside longer for some samples as it has been on the back burner for some time. Procrastination naturally allows long cure times!”
Load-bearing hems. At each end of the glued test strip, I sewed on load-bearing hems made from polyester umbrella fabric. These hems were used to suspend the test piece on steel rods and also to suspend a loading bucket on the other end. Multiple rows of stitches were used to securely attach the hems. “However, as I discovered later this attachment method clearly compromised the strength of the fabric and an improved attachment was needed.”
Adding the load. To the load bucket (~1 kg) the water was added in ~1-Litre (1 kg) increments until the bond broke or the fabric broke or the bucket was full or some other strange thing happened.
Polyester test fabric with poor glue bond strength. Polyester umbrella fabric with a silver coating has become my favoured fabric for DIY tent-making. According to my field experience and testing, both polyester and silnylon shed rain wonderfully well and are wonderfully strong when new. However, I find that polyester has many benefits:
- Is hydrophobic,
- Has no waterproofing coating or treatment that can fail or decay,
- Has no wet-stretching that needs constant tension adjustment,
- Has minimal condensation problems because it is very permeable to moisture vapour,
- Stays drier and dries quickly for cold morning pack up,
- Sufferers less UV degradation (loss of strength) after long exposure to the elements (I speculate that this advantage comes from the UV protection by the silver coating rather than polyesters’ innately better UV resistance. This has been confirmed in another subsequent post).
One drawback is that polyester does not bond well with silicone rubber as do most silnylons (in my limited experience). However, this weaker bonding still is part of my tool kit for DIY gear and tent-making. So I included some polyester fabric in the load testing with no great expectations. I glued the silver to silver side and also non-silver to non-silver sides for the testing.
Silcotton. Woven cotton shirt fabric is another light, abundant and cheap fabric that can be impregnated, sealed and bonded with silicone rubber. It is now part of my DIY tinkerers tool kit. So I included a glued sample of shirt cotton in the tests, making a total of 10 fabrics that would be tested.
Confession: It is easy to rush to publish when a test is successful. It is much harder to do so when it is a failure. Nevertheless, sharing failures is important as it holds valuable lessons. Also, my hesitancy has caused me to lose some of the tested sampled, but it does not change my concluions.
The bent wire test rig shown below was used to suspend the test pieces from a tree while a bucket suspended below had 1 litre amounts of water added. An example of the bucket testing is shown in this little video.
Glued bond strength test results and discussion
For all samples, except for the polyesters and the grey silnylon (6, 7 & 8 respectively), the tests resulted in the glue bonds having a greater strength than the supporting fabric. “In other words, the fabric broke before the glue bond did and this was not what I expected”.
Glue strength
| 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|
| Breaking load (kg) for either the glue bond or the base fabric | |||||
| Trial with sewn loading hems | |||||
| Sample# | Description | D value | GSM | Glue bond breaking load | Base fabric breaking load |
| 1 | Silpoly(yellow) | 37? | intact | 4 | |
| 2 | Silnylon (light green) | 10 | intact | 8 | |
| 3 | Silnylon (????) | 20 | intact | 5 | |
| 4 | Silnylon (orange) | 10 | intact | 8 | |
| 5 | Silnylon (dark blue) | intact | 7 | ||
| 6 | Polyester (blue/blue) | 57 | 1 | intact | |
| 7 | Polyester (silver/silver) | 57 | 2 | intact | |
| 8 | Silnylon (grey) | 37 | 13 | intact | |
| 9 | Silnylon (orange) | 37 | intact | 5 | |
| 10 | Silcotton (white) | 118 | intact | 15 |
|
The grey silnylon (8) was the exception amongst the silnylons and I already knew that it formed suboptimal glued bonds when compared to my orange silnylon (9). However, I was somewhat surprised about the bond strength of this fabric reaching13 kg loading. It bodes well for the bond strength of the other silnylons, silpolys and silcotton if I can get an effective test method.
At first, this might seem to be a good result for the glue strength. However, the breaking load for the fabric test samples was much lower than I expected and this meant that the expected high breaking point for the glue bond could not be reached.
The supporting fabric usually broke along the line of sewing on the supporting hem before the glueing could be challenged. I speculate that this happened because somehow my hem sewing had caused a weakening of the fabric and or a stress concentration line along the first line of stitches.
At the fabric break line, there were usually long warp threads protruding beyond the last weft thread. This may mean that the warp threads could have stretched before breaking. Alternatively and more likely, it may mean that the warp threads had pulled out from the weft threads that were left trapped within the hem (See samples 1, 2 and 3 as examples of this). Perhaps the shortness of the sample strips in these cases contributed to this type of failure.
In another test, by chance, I caught a test sample 9 (orange silnylon) failing in a very unexpected way. The hems held much better (for some unknown reason) and the test sample stretched without the glue bond breaking or the fabric totally breaking.
Conclusion about silnylon glue bonding with RTV silicone rubber
The silnylon glue bond strength test method was a failure due to the use of sewn load-bearing loops or hems that compromised the fabric’s strength. However, it showed that glued bonds can be very strong, stronger than the stitching, for most silnylons used. Ironically, one of my own silnylons that had previously known had relatively poor bonding reached a loading of ~13kg when the bond broke.
The design of a load test piece that requires no sewing will be the subject of more procrastination and another post sometime.
Also, in future testing, the bonded area possibly should be ~10mm wide (or deep) so that it is more representative of the width that would typically be used in a sewn lay-flat seam on an ultralight tent etc.
Tim
Addendum 1
Since this experimentation, for the low-bonding fabrics, I have I have found that high glue bond strength can be achieved with multiple types of glue other than silicone rubber. “There seems to be a suitable glue for every adventure fabric.”
