Mythbusting – Silicone

There are actually two silicone myths that we see in the DIY section on a regular basis:
1. New silicone will not stick to old silicone.
2. Silicone will not stick to plastic.

The first has been reportedly debunked by a Dow chemist that said that silicone will bond seamlessly to old silicone. The catch is that the old silicone has to be cleaned first; a thin bio-film is enough to prevent bonding.

Both myths were easy to test in one shot. The silicone product used in this test is Permatex 66B. It is reasonable to expect one product to be able to represent most silicone brands; however each product is formulated independently and for different properties. Therefore, the results can vary from brand to brand. On the test piece of pvc pipe I drew a line and cleaned one side with pvc cleaner; the other side I left alone. When the piece was dry, I laid a bead of silicone on each side of the line. I let the first bead cure for 24 hours before adding the second bead.

In the video, the first silicone bead to be tested is on the side of pipe that was not cleaned. This is testing both myths at the same time. The side that was cleaned had surprising results. Actually I was surprised at what happened on both sides.

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The bond on the first side was very strong between both the pipe and silicone, and between the two beads of silicone. The failure of the bond between the beads shows that they do not bond perfectly seamlessly. However, the strength in the seam bond and the strength within the bead are incredibly close.

The bond on the second side that had been cleaned was still very strong even though it looked weak in the video. I had put most of the stress on the leading edge of the bead and peeled it off rather than pulling it off perpendicularly. The tensile strength is measured in psi (pounds per square inch) and by concentrating the force to the leading edge, I quickly exceeded the tensile strength. Importantly thought, the strength of the unclean side was still stronger. I think that this is because my pvc cleaner was contaminated from excess usage. The cleaned side should have been stronger and this deserves further investigation. In my next test, I’ll be using that brand that we hear so much of.



From my observations in this test, I am calling the myths as follows:

1. BUSTED (for now) – The new silicone bonded significantly well to the cured silicone. If the bond strength is not strong enough for a given application then a single new bead is not going to provide enough safety factor either. In applications of resealing a tank, the bond strength is beyond sufficient.

2. BUSTED – The silicone in the first test bonded to the plastic better than it bonded to itself. In most silicone to plastic sealing applications, tensile strength is not important. For adhesive applications, silicone is not the best choice anyway. There are many high strength glues on the market that surpass silicone as an adhesive.


http://www.chemicaldesigncorp.net/PDF_Files/GE_Silicone.pdf
In this link for GE silicones the tensile strength is given as 200psi and the peel strength as 27 lb/inch.

IKeepPacu;2002891; said:

wow, you may have just saved my sons favorite hex. tank from being thrown away.

has a small leak, but i was always under the impression old wont bond to new. you just gave me the courage to go and do it

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It is imperative to clean the old silicone first. Any film left on the old silicone will prevent it from bonding. Alcohol reportedly is a sufficient cleaner. I have not tried it yet but will soon.

BushFishRox;2002894; said:

over time the two beads of silicone will separate, it has been seen many times in showers and sinks, where ever one might try and patch up cocking etc.... so I think you should try the test again but wait longer then 24/48 hours and in a situation where it will be exposed to water, either a shower or fully submerged. Also most of the time silicone that you want to connect to or go over is more then 24 hours old...

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The sink/shower thing underscores the need for cleaning the old silicone before adding the new. I am going to test this on an old tank. It is 10+ years old. The results should be interesting to see.