Heat shrink tubing. 3

[mhallin]

Hello everyone,

For an application I´m currently working on a heat shrinkinig function would solve my problems, much like heat shrink tubes in electrical applications.
I have seached the web and came across tubes made from both elastomers(type not specified)and Viton.

My question is; does anyone have any input on how these recepies are designed to acchieve the shrinking function?

Thank you very much!

/Mathias

 

[Compositepro]

Heat shrink tapes are plastics that are stretched hot and cooled so they cannot shrink back while cold. I'm not aware of rubber heat shrink tapes. Rubber would be used as a stretch wrap where you stretch the tape as you wrap your part. It is a common misnomer to call it shrink wrap. There are also many plastic stretch wraps or shrink tapes that are not heat shrink.

 

[mhallin]

Thank you for your reply,

I got this info from wikipedia...

"
According to the exact material used, there are two ways that heat shrink may work. If the material contains many monomers, then when the tubing is heated the monomers polymerise. This increases the density of the material as the monomers become bonded together, therefore taking up less space. Accordingly, the volume of the material shrinks.

Heat shrink can also be expansion-based. This process involves producing the tubing as normal, heating it to just above the polymer's crystalline melting point and mechanically stretching the tubing (often by inflating it with a gas); finally, it is rapidly cooled. Later, when heated, the tubing will relax back to the un-expanded size.
"

The second method is as you mention a mechanical stretch which returns when reheated. However it´s the first method that caught my interest, how does this work and for what kind of materials?

Also, neoprene is mentioned as a material used. Is it a CR-TPE?

/Mathias

 

[GrahamBennett]

As far as I'm aware, Neoprene TPEs have yet to be marketed (if they have been developed). TPEs are an alloy of an elastomer (usually a grade of EPDM) and a plastic (usually polyethylene or polyproylene). The Malaysian Rubber Board has developed, and is marketing, an NR-based TPE, which has superior fatigue resistance compared to others such as the Santoprene grades.

I would not trust Wiki's input into this subject. (I for one would not want to be around many of the monomers used for very long as they are dangerous to life and limb.) I suggest reading the Q&As in this link:

http://van.physics.illinois.edu/qa/listing.php?id=519

 

[mhallin]

Well, I actually didn’t really trust wikipedia and so I posted the question on this forum (which was wise as it seems... )

Never the less, wholesalers are supplying "Neoprene Heat Shrinkable Tubing" (

http://www.buyheatshrink.com/heatshrinktubing/neoprene-heat-shrink.htm).

Are these made with the above mentioned stretch technique as well, and if so, how does one get a neoprene tube to behave in this way?

(I was unsure of the existance of CR-TPE, but now I know... thanks. ;-))

 

[Compositepro]

Almost all thermosets shrink on cure. This is normally a problem to be worked around rather than used to any benefit, except to get easier release from molds. This is cure shrinkage and not heat shrinkage.

 

[mhallin]

In my experience cure shrinkage is limited to about 1-3%. The neoprene heat shrink tubes are stated to have a shrink ratio of 2:1 or about 50% on the diameter.

Also, this suggests that the tubes are delivered uncured and then applied and shrunk (cured) with a hot air gun or such...

/Mathias

 

[Metten]

In the electrical industry there is also the alternative of "cold shrink" joints. I know EPDM is used to produce these. The products is strechted (+/- 300%) and then stored. By removing the inner part, the rubber will revert to its natural condition and seal the tube.