Air bubbles problem

I read something about this but I can't remember where. Basically, they would heat the things before putting the plastic and then cool it. The coldness dropped the pressure and the air bubbles just collapse. I'm not exactly sure if that's what actually happened (it's been a long time).

I was going to suggest doing the operation in a vacuum but that sounds terribly complicated and impractical.

There is no easy way out. Vaccuum impregnation is the usual way to draw air out of a porous substrate. The other way is capillary fill from beneath.

If the material is low in viscosity you can submerge it and draw a cyclic partial vaccuum(on for 1-2 seconds off for 1-2 seconds, just 2-3 psi vaccuum will expand the bubbles, and they rise and you drop the vacuum and fill their space. Experiment to see how many cycles are needed) that will not wortk with viscous materials

Hi everybody,

Dear Aurizon your second suggestion seems good

Our material dispersion on base water we're dipping porous substrate into dipping tank and are drawing about 1-2 meter/min. speed. This partial vacuum (as you mention 1-2 second on and off),is it applied to substrate in dip tank or just after dipping while drawing vertically ? Could you tell me how to make this ? I mean is it expensive ?

and I would like to share my opinion, if we heat this substrate and then press in passing from between two rolls just before dipping. can it be useful for removal of air

Thanks

you would have to seal the tank and then use a large vaccuum type centrifugal blower to draw a few inches of water vacuum, and then relax it. The tank mut be made to tolerate this partial vaccuum without collapse.

the easy thing to do is to make a small 6" square tank out of clear plastic and silicone RTV with a hole in it for a vaccuum pump and a gauge. put in some liquid, immerse a 5"x5" piece of your material in your liquid and observe what happens when you draw a vaccuum. If you see bubbles expanding and rising from the substrate.

Another method is to place the material in the tank and then gradually flood from below so the water displaces the air as it rises through the material. Air being a lot less viscous than water and you will not have surface tension trapped bubbles.

Dear Aurizon, Thanks for your kind useful explanation As far as I understand in your last paragraph, you suggest that simple way to be pumped this water dispersion onto surface of porous substrate in dip tank. In this case, it will be forced to replace air to water. It seems logical this flooding system than making vacuum. Because our process speed is about 1 Mt/min. I think it can be difficult to be synchronized vacuum 1-2 second on/off with substrate speed What's about may opinion mentioned before the following if we heat this substrate and then press in passing from between two rolls just before dipping. can it be useful for removal of air Looking forward to receiving your kind response Thanks

well, if you pour the water from the top you trap air bubbles under the fluid. If you allow the porous material to slowly sink beneath the surface so the water starts at the bottom and works it's way to the top and pushes the low viscosity air out by upwards displacement. That means the material does need to be relatively porous to allow this. What is the material?.

Do you have competitors and can you see how they solve the same problem?...if you can see their process?

Thanks Aurizon,

Our substrate is glass woven fabric in thickness 0,20 mm

Fluid is PTFE dispersion on base water (% 40 Water + % 60 PTFE )

I can't understand your last writing exactly. we don't pour our dispersion from top.

This glass fabric is dipped at fix speed (such as 1 mt/min) into PTFE dispersion tank and then

is drawn vertically to enter vertical oven for drying and sintering.

it can be there chemical treatment for removal of this air on surface?

Kindly requesting you to reply me

I see. I had thiught this was flat plates of material. A cloth is far easier to penetrate.

a fine powder of PTFE in water. Is there any surface active agent to help wet the glass? Do you have a prior step to remove and grease or oil from the glass to aid in wetting? What is the reason you apply this PTFE to the glass cloth? Whay are the bubbles a problem? what is done with the dried cloth afterwards?

If a vertical descent into the suspension and slow enough the bubbles should be displaced.

There are so questions I will try to reply for each ones

PTFE are coated on glass fabrics for industrial purposes Air bubbles are problem because this fabric is first dried and then sintered ad cured at the high temperature to make strong structure. This coating process continue 5-6 times till getting satisfying result. for each time 5 -10 micron are coated

If air are trapped by PTFE during coating then when dried and so sintered up to 270 -300 C these air bubbles cause "pin holes" problem on surface. this is unwanted thing, because fabric should be smooth and shiny.

If we can remove these air compressed in fabric before or while coating then I think we had solved this problem completely. Because there isn't any air to make trouble at high temperature.

Do you have any suggestion after these my explanations?

I see this is a complex situation, which is beyond my ability to offer more suggestions as I do not have a complete understanding. Suggest you work with the fabric supplier and the coating supplier to establish the variables on a test bed and then work to optimize/eliminate these problems.

Aurizon mentioned "surface active agent" or surfactant, and I didn't see a response. But it is possible that some kind of surfactant (wetting agent) could be sufficient to solve your problem (unless you already have optimum wetting). There are two possible routes for this: simply add a surfactant to the bath. Or, pre-coat the web with a very thin layer of a material that wets the fibres and is itself wetted by (or miscible with) the final coating fluid (you may need to do this in between each existing dip). Whatever you try, it will be wise to consult manufacturers of the dip solution first (to avoid adverse interactions). Good luck

Fyz

You mentioned that you are dipping the material at 1 meter/minute. Is there any possibility of slowing it down a little? Aurizon said that by dipping from the top, the air may have time to escape before the substrate is completely submerged. If air is still being trapped, the dipping speed might be a little too fast.

It might also be possible that the substrate has varying density. What I mean is that some areas may have bigger cavities and some areas have smaller cavities. The solution may be having difficulty getting into the smaller cavities due to high viscosity.

One last thought. Could you possibly impart a very gentle stirring of the substrate as you are dipping it? The flow action may dislodge any air bubbles that do get formed. Or maybe squeeze the substrate while it's dipped. I'm giving unqualified suggestions since I don't know your process or product. These are just off the top of my head.

It is possible that an ultrasonic transducer in the liquid phase will shake bubbles loose from their attachments. This would be easy to test

Dear Vulcan, Yes you're right the slow speed is essential for this application we can reduce down speed but it also should be an optimum level We also shake substrate by blowing hot air just after dipping We don't observe so much air bubble problem when coated less porous substrate and as this regards I think the problem is due to porosity. how much porous then much more air bubble I haven't knowledge about surface technology more . but I believe to optimize these bubbles if we can obtain quite good wetting properties and I think it should be a tips or simple method except of slow dipping that air isn't trapped while substrate is being wetting by dipping, Suggestion of Aurizon is coming me quite good, pumping this dispersion onto substrate in the dip tank then maybe dispersion drops replace air baubles Nobody answer my suggestion, if we heat this substrate and then press in passing from between two rolls just before dipping. can it be useful for removal of air Aurizon hi, Yes I hear and read some articles about using UV for dipping application. But I think it may be expensive application. do you have any knowledge about this how can I test it easy as you said

Years ago I worked at Lever Brothers and they were experimenting with ultrasound to increase agitation. I recall when we immersed the U/S wand into the water and turned it oh there was a liberation of trapped air bubbles from the cloth that rose as a mist of white from the cloth (which was previously immersed in warm water + detergent). This was a one time release, and we concluded it was interstitial air.

A small square of the cloth immersed in water and placed in an U/S cleaning bath should tell you if this will release some portion of the trapped air?

Aurizon hi,

That means if we dip this glass fabric into ultrasonic bath then air bubbles collapse and substrate will wet better. Could you tell me what frequency and what time does it need for this?

I do not think they collapse. What happens is the bubbles are displaced from mechanical lodgements within the fibers by the agitation. You could probably do the same with a mechanical beater that shook the bubbles loose. In addition, the PTFE is not a well wetted surface and so the water based PTFE may not readily 'wet' the fibers. The addition of a wetting agent and the vibration could enhance wetting and release of air. What is good is a small bath 6" square can be used to try the degree of ultrasound. often in the 50-200 Kilohertz range, you may be able to find a U/S supplier who will test the idea for you in their facilities at minimal cost. If it workd they install the U/S. If it does not work, little is wasted in terms of money or time

Aurizon's suggestion of using ultrasound is good. I've used them before though not on substrates such as yours. They work well for cleaning but I've seen them "de-bubble" objects that are dipped into them (we use an ultrasonic tank). The ones we use operate at 100kHz. Can't remember the wattage but I think it's 100W or so. Aurizon also mentioned an ultrasonic wand. Same purpose, different method. You just dip it into the solution or hang it over the side.

Sorry for not answering your question about heating and pressing. As I said, I don't know your material or process. I can only guess.

Ultrasonics have been used in photographic materials manufacture for many years. One suggested mechanism is dispersal to the surface and the other is dispersal within the fluid. The ultrasound eliminates bubbles from fluids before they are passed to the coating machinery, as the last thing one wants on the photograph of someone's face is "spots"!

It is so interesting that I what to know more about ultrasound, but used in medical domain, for my finally project on faculty, if I don't ask you to much. Actually I just want to give me some ideas because it is huge domain and my project most to have also an concret machine and I am very confused. I am sorry for disturbing you...

It is so interesting that I what to know more about ultrasound, but used in medical domain, for my finally project on faculty, if don't ask you to much. Actually I just want to give me some ideas because it is huge domain and my project most to have also an concret machine and I am very confused. I am sorry for disturbing you...