Silicone Sacs: Are They Really Silicone?

[hbquikcomjamesl]

For my own part, I'd discovered the evidence (well, first the allegation, then the evidence to back it up) that most of the sacs sold as silicone were actually PVC over a week ago, when I started looking for repair parts for my Wearever. And I promptly modified the parameters of what I was looking for.

 

I stopped buying PVC notebooks years ago, when better ones (polypropylene, I think) came out, because PVC, and the plasticizers in it, eats xerographic toners.

 

Something analogous happened in the organ world, many years ago: a material called Perflex was introduced as a cheaper, supposedly more durable substitute for leather in organ bellows, reservoirs, and "pouches" (i.e., small bellows used as pneumatic actuators). It turned out that the stuff disintegrated after a few years, turning into crud that was difficult to remove.

 

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In terms of securing sacs, I find myself thinking of some sort of tiny hose clamp.

Edited March 4, 2013 by hbquikcomjamesl

[Vintagepens]

Though I have been able to make shellac work with silicone sacs, I'm now recommending using silicone adhesive to attach them instead. The attachment is much stronger, and on balance I do not think that the adhesive is significantly more reactive than shellac's alcohol base.

 

David

[jkingrph]

Most plastics and rubber products are based on carbon chemistry. Substitute the silicon molecule for carbon and we have silicon chemistry. It has been too many years since organic chemistry, which studies the chemistry of compounds containing carbon for me to remember any specifics, like how many silicon molecules have to be present to be considered a silicon compound vs a carbon compound.

[Ron Z]

Only silicone really sticks to silicone. But a lot of your adhesion with silicone sacs is the tension of the sac on the sac nipple. i.e. the sac is streteched on the sac nipple. The shellac fills in the irregularities and so provides sealing and perhaps some adhesion.

 

I had a rather long chat with one of the manufacturers of silicone adhesives about what to use in this application given the materials involved. They suggested using a non-corrosive silicone RTV instead of the standard RTV, which releases acetic acid when it cures (the vinegar smell). The alcoxy type releases small amounts of alcohol, the thinner for shellac. Cure time is 24 hours at room temperatuer, but again, most of the "stick" comes from the tightness of the sac on the sac nipple.

 

A self leveling, so thinner silicone sealant, is recommended. Loctite 5040 was suggested. If you want a paste, use 5045.

[mhosea]

If the alkoxy type hasn't been freshly opened, I'd suggest testing it the day before to see if it cures as expected. Some silicone adhesives store better/longer than others. I fear that the ideal adhesive may be a bit expensive for us regular folk on account of this, unless you like to go hunting around the house for stuff to glue/seal.

[Ron Z]

 

 

I fear that the ideal adhesive may be a bit expensive for us regular folk on account of this,

 

Which is why you would depend more on the tightness of the sac on the nipple (which you do anyway) than the strength of the adhesive.

[Plusfoursmax]

I would like to report an amazing response from The Pendragons in the UK (www.vintagepensacsandpart.com) where I got some 'silicone' sacs for my RHR Parker Duofold Senior.

Having scraped and sworn the old sac out, I thought it would make sense to insert a non-reactive material instead, so ordered some 'Silicone' sacs through their ebay site, Chilipea.

On receipt, I noticed some of the telltales that these were not silicone, but PVC (not moulded, permanent flattening, low melting point, acrid smoke and a black, not white residue). Thank you David @Vintagepens for your very informative video

).

On raising this with them they gave a full refund and have pulled all stock until they sort it with their supplier. Go consumer power!

 

Max