Perspex surface tension problem 2

[simonsk]

Hello all.

I am currently looking into a problem for a client.

They have a vertical perspex sheet the they are running water over the surface to form a waterwall feature. The water is separating into rivulets but the desired effect is to have uniform flow across the whole surface.

We have witnessed a similar problem on stainless steel in the past but an alkaline cleaner has removed the surface coating (left behind by the protective film) , and the flow has been uniform.

We have tried solvents, mild acidic, and hot alkaline cleaners but still the water separates.

Can anybody shed any light on the problem?

Thanks in advance

Simon

 

[Demon3]

You need to increase the surface energy of the Perspex (PMMA). I see four ways to do that.

1. Corona treat the sheet to oxidise it

2. Flame treat the sheet to oxidise it

3. Use strong base to hydrolyse the surface to give acid groups

4. Coat the PMMA. For example try some polyurethane varnish.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[btrueblood]

Along the same lines,

- coat the surface with a hydrophilic compound (might require periodic recoating), or

- add a surfactant (wetting agent) to the water (may cause foam).

- Possibly TiO2, see:

http://en.wikipedia.org/wiki/Super_hydrophilicity

Dunno if TiO2 will stably bond to perspex...Pat?

- Use a textured surface (brushed, pebble-grained, sanded) to force a more turbulent, dispersed flow pattern.

- Increase the water flow rate or decrease the angle or length of the run.

 

[simonsk]

Thank you for your suggestions.

I have purchased some more development sheets to try some of these ideas on.

I have just applied a varnish to one sheet and will hopefully trial it tomorrow.

The perspex needs to remain clear as there is an etched pattern on the reverse that needs to be visible.

I have no experience with Ti02, how is it applied, will it be clear?

I have had more repsonse from this forum in a couple of hours than I have had from ICI in a couple of days!

 

[Demon3]

Also try other transparent polymers. You can try polycarbonate and MABS to see if they wet. I just tried MABS and it wets pretty well depending what contact angle you need.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[simonsk]

Thanks Demon3.

Unfortunately we cannot change the material as the feature is already installed. We have been called in to try to sort it out.

I have toyed with the idea of fixing a sheet of alternative material to the front of the existing PMMA.

We have had similar features working with other manufaturers acrylic, for some odd reason 'Perspex' seems to be different.

 

[Demon3]

Then I think that surfactant plus some anti-foam if needed is your best bet. Sure to work and no modification to the installation required.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[btrueblood]

Titanium dioxide is the white powder pigment in paint, but supposedly it can be deposited (vapor deposited in a vacuum I would suspect) as a thin film on window glass and is therefore presumably transparent. Note that the article talks about it becoming super-hydrophylic under UV light (that could look pretty cool though, with some bubbles in the water!).

Google super-absorbent polymers and talk to the companies that come up from that search about the possibility of using the polymers as a coating. These are also acrylic compounds, and so it might be feasible to get a coating made of them, or their precursors, to bond quite nicely to perspex. I would think that Demon3's idea of hydrolyzing the surface with a strong base might work too, but I would try that on a seperate test piece first, as the material might develop crazing from the exposure (the crazing may dissapear when submerged, dunno).

I would sure try just wet-sanding a piece of the stuff to test, using say 600 grit or finer wet/dry sandpaper. Tell me: is the side of the perspex sealed with silicone rubber caulk? It's likely the surface got contaminated with silicone; it may not respond to any of the things we've come up with so far, except maybe the flame-polish that Demon3 talked about.

 

[simonsk]

Thanks btrueblood.

I will look into the super-absorbent polymers.

There are 5, 1 metre wide panels that are joined using silicone sealant.

You will have to excuse my ignorance over this! Do you think the silicone has contaminated the surface via the water flow?

We have tried wet & dry on a sample piece but it leaves the surface too 'foggy' (and the flow is still not great).

What is the process for flame polishing?

Many thanks

Simon

 

[patprimmer]

flame polishing is just what it says. Wave a flame over it at intensity and speed so the surface just melts, but but extensive melting or a fire does not start.

The only really viable answer I have seen is use a surfactant. There are low foam surfactants. I used to know about effect on surface energy and foam formation characteristics of a range of wetting agents, but I have been out of that for well over 30 years. Some people who supply to the textiles wet processing industry should be able to advise.

TiO2 is a very powerful white pigment. It could only be transparent if the particle was only a few nones big.

Regards

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[simonsk]

Feedback:

I would like to thank everybody that has suggested solutions to my problem.

Wet and dry paper left the surface too opaque and the flow was less than perfect.

Varnish also left the surface opaque and the flow was still poor.

I then flame polished a new sheet and it solved the problem instantly. I was very sceptical about the process (through ignorance!), but it works.

The surface looks exactly the same but the flow is perfect (uniform across the length and width of the acrylic sheet).

Thanks again

Simon

 

[pierdesign]

flame polishing depolymerizes the plastic at the flame contact zone. Over time, days to months, the material will craze / crack, sometimes only slightly but often grossly.

 

[btrueblood]

Silicone oils are weird stuff, as soon as you get some near a surface, it seems to get everywhere. Then again, I'm pretty messy with it...

There are other cements that work well with acrylic (are waterproof), and contain no silicone. Even a clear epoxy would work.

Flame polishing might help convert silicone oils on the surface to oxides, or it might just migrate it into pores under the surface, only to bubble back up later. According to paint shops, the best way to avoid silicone from causing problems is to not apply it in the first place; i.e. keeping any silicone greases, oils, sealants out of the shops supplying parts to be painted.

 

[Demon3]

Silicones do indeed get everywhere. They wet and spread spontaneously due to their very low surface tension. Silicones are in common household sprays like Pledge so that may be how your PMMA got contaminated or it may be that the protective release paper that PMMA ships in used a silicone adhesive.

There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[patprimmer]

I have never seen any crazing problems attributed to flame polishing.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.