Vacuum Process/Cycle - some science!

[zerolando]

After going around the Delta website I found this following answer in the FAQ:

"Q. Some windshield repair system manufacturers say that you need to vacuum all the air out of a break before injecting the windshield repair resin, is this true?

Of course not. Think of an empty pop can. It is full of air but you do not need to suck the air out of it before you fill it right? In fact if you did, the pop can would collapse. The same is true for a windshield repair. As the resin flows into the break the air is forced to the outside edge. When the vacuum cycle is initiated, the air is pulled up through the resin and into the injector. When the injector plunger is locked in the top position the air is released through the top of the injector. As the air is removed it is replaced by resin."

I just felt like commenting from a scicence point of view, on two issues which this FAQ Question/Answer seems to touch on.

A-As far as the initial vacuum process goes , as with other manufacturers, this step can help but is not sufficient in itself.
In HVAC , Vacuum is used not only to evacuate air but also to remove moisture.
There are two types of moisture, Liquid and Vapor. Vapor is easy to evacuate but liquid causes a problem. When moisture is in liquid state, it is much more difficult to remove. Usually when the vacuum pump starts removing the moisture , water will boil. This usually leads to a big volume of vapour to be removed.. I am not sure that the vacuum pumps with which other kits are equipped are capable of that.
Now if the break has been really well dried out before starting that so called initial vacuum cycle, then this vaccuum cycle is indeed helpful in the sense that most liquid moisture already turned into vapour and being in the vapour state, it is easy for the vacuum pump to evacuate them.
But then, you might have a problem of the vapour condensing somewhere in the pump itself if there no process similar to a cold trap.
Therefore this first vacuum cycle issue is really very volatile and depends on many variables which are different with each break and situation. Therefore not completely controllable. Sometimes it might help, sometimes it won't.
Even in the case where this initial vacuum cycle does help to a certain degree it still does not solve the issue of air bubbles which are inside the resin. The resins used being UV curable, and not 2-phase resins , it is not practical to be manually stirring them before application cause they will dry on contact with Ultraviolet Light. Therefore the resins are not being degassed , therefore they do contain a bit of air. Not performing pressure/vacuum cycles later on during the repair process might therefore be a problem.

Please also do not forget one thing. The glass break area is not a closed chamber, therefore there is always a leakage of air towards the break area whatever you do, vacuum or no vacuum. When resins settles in that break area, the resin being more dense then air, air is chased out and upward or in whatever direction it can escape.
So you're actual air related problem in the repair process, is air that is trapped inside the resin and not the air being present in the repair area before introduction of the resin.




B-Now concerning the other subject that I wanted to touch on it relates to the following sentence from the same FAQ Question/answer, and I quote:
"When the vacuum cycle is initiated, the air is pulled up through the resin and into the injector"

This statement is incorrect and incomplete and should be rephrased so that it won't continue to be used by competitors to discredit some kits which use an alternance of pressure/vaccuum cycles.
I'd like to also explain this process from a physics/chemistry point of view.

It is neither the pressure cycle by itself nor the vaccum cycle by itself which evacuates air trapped in between the resin. It is and this is really important, the alternance of pressure/vaccuum cycles which evacuates air bubbles out of the resin.
This process is called "Removal of Bubbles by Buoyancy Effects" by science.
Bubbles can usually be removed from melts(like the resin in this case) by physically rising to the surface, or by chemical dissolution of the gas into the surrounding melt.
Since the density of a bubble is less than that of the surrounding melt (resin in this case), a bubble will automatically rise to the surface and burst unless prevented from doing so by some external agent.
Buoyancy is given by Stock's Law. For the case of a gas buggle in a viscuous liquid , stocks law translates to:
Vb=3/2Vs=(g*Delta*Ro*r^2)/3 *n , Vb being the rate of rise of the bubble.

Therefore the velocity of rise of a bubble is inversely proportional to the viscoscity of the melt (resin in this case) i.e bubbles will rise faster in a more fluid met.
Additionally the velocity of rise of a bubble is proportional to the density of the melt(resin in this case) i.e bubbles rise more rapidly in a more dense melt that in a less dense one.

How does this science relate to the Pressure /vaccuum cycles?
The purpose from the pressure cycle is and in addition to exerting pressure on the resin to penetrate small areas, is to compress the resin and therefore make it more dense, this as explained earlier will increase the velocity of the rise of the air bubbles trapped in the resin to the surface.
During the vaccuum cycle, these air bubbles which rose to the surface are then evacuated away from the repair area...

That is the purpose of the combined and sequential pressure/vaccum cycles. Compress resin to a higher density which increases the velocity of the rise of air bubbles and then evacuate that air away during the vaccuum cycle.

Please note the following though:
The rate of removal of bubbles is proportional to the square of the bubble radius or diameter. This means that Bubble rise is not a very efficient process for the removal of very small bubbles. There has to be other processes.
Vibration or manual stirring is one of these processes.
If bubbles do not rise sufficiently fast in a quiescent melt, the fluid itself can sometimes be moved by convection or stirring in such a manner that the bubbles are carried to the surface. Upward fluid motion can be obtained by mechanical stirring, by or
a) design of a glass tank floor to produce upward currents.
b) localized heating to produce a locally hotter and thus less dense region in the melt.
c) by bubbling with a gas introduced near the bottom of the melt.
These described processes are obviously impossible to implement in this case during the pressure cycle in a WSR repair. Well maybe not impossible, but i can't see how they can be implemented.

A common issue related to all kits should be to find a way to degass resin before using it. That would certainly help.

In my humble opinion , the future of WSR technology lays in the hands of improvements related to the resin material used rather then equipment used.
I think that nanotechnology can also be used to change the properties of repair resins as to become what Self consolidating concrete is to regular concrete. Self consolidating removes air bubbles by itself, while regular concrete needs to be manually consolidated through vibration.
Companies should team up with those companies in the market ( I think mostly european) who are producing nano-resins to reach the properties in resins which would help perfect glass repairs.

Hope will be seeing some more improvements in that area, and i hope this post has been helpfull.
All comments are welcome of course as I might be wrong somewhere.

Good day
Zerolando

[screenman]

I feel that in most types of breaks we encounter once the injector is situated over the break then the break is indeed a closed vessel. The injector being part of the said vessel.

Do you not feel it is possible to some degree to degas a resin by placing it in a vacuum ?

Whilst your knowledge of the science is very impressive, do you actually do WSR?

[zerolando]

Hello screenman.
Not really sure but i am not able to see some type of breaks like a star break or a long crack for example as being equivalent to a vaccuum sealed chamber due to the difference in the area of the injector head and the area covered by the break itself.

Glass artists degass resins through agitation.. some of them use the air compressor of a refrigerator hehe , but they usually use 2-part resins which are not UV cured, so they dont have to worry about the resin drying out due to UV. Their resins take a much longer time to cure(not really relevant but i thought i'd mention it).

And to answer your question, no i don't do WSR work yet. But i hope to start soon

[screenman]

Regardless of size most stone chip damage has a small impact point with the rest of the damage being subsurface, I say most as in rare instances you may get a surface break and most long cracks are also open to the surface. So if you see a 1 inch damage the chances are the only area open is about 1/8 or the pit as we in the trade call it.

Although UV curing it is possible to work with the resin keeping it away from UV. We have experimented with degasing using a vacuum pump placed over the top of the resin container, when doing this on a clear container it is possible to see a gas rising to the surface.

Nano resins have been tried and tested and until now found in most cases to be left wanting. One in particular I understand cures with natural light, bit of a problem trying to see in the dark I think.

Please bare in mind I am a very experienced WSR guy with over £1,000.000 worth of repairs behind me, however I certainly do not have your grasp of the physics of my industry, I hope when you do enter our trade you enjoy the success I have done with my limited knowledge.

[Brent Deines]

Zerolando,

I agree with many of your comments, and perhaps we some of our literature could be more detailed, however as I mentioned in a post to another topic, we have to try to put things into a language that the average person understands, and we cannot take 10 pages to explain something when we know most people will only read the first sentence. Still, your point is well taken, and the pop can illustration is an over simplification to say the least. However, if you are not a windshield repair technician, and have not developed windshield repair equipment or chemicals, then I think you should contact me before you make more posts telling those who have practical experience that they don't know what they are talking about. That may not be your intention, but I have received a few e-mails suggesting as much. Perhaps we can set up a time for you to visit and see how our equipment works. You can also see most of our competitor's equipment at our facility and I will give you the contact information for the engineer that we hired to study the vacuum pressure issue as I'm certain he can speak your language.

Please list your credentials and tell us why you are so interested in the windshield repair business.

[zerolando]

Dear Brent,

Please let it be understood , I am not criticizing any piece of equipment whether be Delta, GW, GT ,Novus (or whatever other brands are in the WSR business).
In addition please note that in my post, I have touched on the science of vacuum and buoyancy+ stock's law. These are scientific concepts which are in this case being used in equipments and kits. But lets not forget that these scientific concepts are not exclusive to the WSR business and in fact have existed and are being used in many different industries.
What I have touched upon are the scientific concepts in use. My aim was to scientifically explain these concepts and not attack or defame any company or piece of equipment, as I have already stated that I haven't really practically used any of them anyway .
So i am not talking from a technician's point of view, I am talking from an engineer's /scientist point of view.

As for giving me the engineer's contact information, that will not be necessary. After all my post was in support of the pressure/vacuum repetitive cycles which i think are being used in your kits? and many different kits too . In my post , I actually discredit those who have used your "average joe" type explanation to claim your product/process is scientifically incoherent. So I don't see how have I actually attacked you doing so?

As for the visit. Unfortunately , I don't currently reside in the USA so that is currently impossible. But If i decide to travel to the US, I'll keep your invitation in mind.

As for my credentials. I do feel I am being put under the microscope here? (misunderstanding).. But here it is anyway:
I am a holder of a Bachelor in Computer Engineering from the University of Knoxville Tennessee. I have done my Thesis in cooperation with the Oakridge National Labs on brain waves and EEG. This landed me 2 years at the Massachussetts Institute of Technology where I knew a couple of people working in the Nanotechnology Institute. After that I left the US for Germany, where I did a Masters in Computational Mechanical and Process Engineering at the Darmstadt University (near Frankfurt). In parallel I studied for a BS in Management in the London School of Economics and Political science (a collaborative program between LSE and Darmstadt). I worked in the Telecommunications industry when I was in the US. I have worked in Management, sales , even burgers (when I was a student hehe)... I have recently worked in the Construction industry as an engineer/Project Manager. And it is there where i started getting involved with Resins, epoxies, repairs and materials in general. This led me to starting research on the repair industry for glass after a debate with a colleague, which kind of landed me here.

One final thing though Brent. I am in no way trying to disrespect anyone and if my post was misunderstood I apologize. I don't have any intention to be harsh with anyone. Nor am I trying to claim technicians don't understand a thing. On the contrary, I have the outsmost respect for technicians especially in this repair industry because aside from your work being technical its also aesthetic.. Therefore not everyone can simply do it.
I have learned a lot and taken notes from the posts of many technicians who are involved in their forum.
I hope this has cleared things out
Good day
zerolando

[screenman]

Funny how someone as well educated as you are and well done for that, wants to end up doing the same thing as a left school at 15 married with kids person like me does now.

I am always amazed at the total cross section of guys in our industry.

[zerolando]

Hello screenman,

You might not know it, but what you are doing is actually pretty high tech and i find it cool. Add to it the fact that it has aesthetics into it, so its artistic in a sense.
The whole thing is about what you really want out of something. I know many well educated engineers who do the same monotone thing day in day out and there is nothing creative or interesting about what they do...(of course not all engineers are so).
I found WSR , especially that is unheard of locally where i am to be a good business opportunity. So i've decided to go for it aside other businesses that i plan to open in Solar technology and the construction business that we already own. I don't like to limit myself to one thing and one thing only. I have a big range of interests, with no necessary correlation between them.
Add to that, that i'm usually attracted to jobs that require hand manipulation (handy jobs?) cause i was never good at them

[Brent Deines]

zerolando,

Please don't misunderstand my post. I did not feel you were attacking me, Delta Kits, or Delta Kits technology. As I said, I agree with much of what you said, but frankly don't understand all of what you said. My point was simply that while the "science behind windshield repair" may be of great interest to some forum members, myself included, that phrase has been used as an advertising slogan to sell junk science in this industry so it touches a raw nerve with many windshield repair technicians. If you can put the science into layman's terms it will be much more beneficial to others. For instance, if you say that using a vacuum first system does not remove all the air from typical damage that a glass repair technician repairs, all you need to ask is why those systems require a second vacuum cycle. The obvious answer is that all the air was not removed in the first vacuum. For many of us that simple question sheds light on the science that we don't fully understand.

When you speak of air bubbles of different sizes I think what you are saying is that no windshield repair system removes "all" the air from the damage. This is absolutely correct. We attempt to remove all of the "visible air" from the damage, but as you stated there is air in the resin, and sometimes air coming in from a break that has surface cracks as well as air in the injector that cannot be completely removed in the absence of a perfect vacuum. Since doing repairs in space or in sealed laboratory conditions is not economically feasable for a windshield repair technician, removing "all" the air is simply not possible with current technology, which brings us back to the removal of all visible air as a reasonable goal. Frankly I steer clear of this subject whenever possible because no matter how many engineers we hire to substantiate the science behind this issue, some forum members will be convinced that because it is on a Delta Kits website it is only a sales pitch. While we often disagree, I still respect the opinions of other forum members and don't wish to offend them. The fact is that some windshield repair technicians prefer a vacuum first system and some prefer a pressure first system. Which one works best will be an ongoing debate, but frankly I think the success or failure of any particular system is highly dependent on training. Knowing how to properly use a windshield repair system is the only way to see what its true capabilities are. That said, credible information from someone with your credentials who has no vested interest in any particular windshield repair system is something that I would think all windshield repair forum users appreciate. You just have to convince them you have no vested interest.

You also must understand that this site is primarily a forum for windshield repair technicians to share information and opinion. Therefore if you are not a windshield repair technician but wish to be the resident scientific expert you will most certainly be "under the microscope", and should publish your name, title, etc., so members can verify your credentials. Normally a forum such as this offers anonymity, but if you profess to be an expert you need to be willing to sign your name to your work. The engineers I have hired sign their name to the reports they provide me so they are open to public criticism. As a professional I think you have to open yourself up to the same, and if you do not do so you lose credibility. Feel free to remain anonymous, but understand the implications of doing so.

I respect your education and your opinion and appreciate the fact that you are willing to share them here, but I'm surprised you do not list your sources when you attempt to explain the scientific principles used in the windshield repair industry. Again this would lend credibility to your statements and is something that chemists and engineers I have worked with typically make a point of doing.

I'm sorry you don't feel it is worth the trip to visit Delta Kits. When I used to work in the concrete industry we would always try to get engineers to come to the job site to work side by side with us for awhile. Doing so proved to be very beneficial to the engineers as well as to the workers. Sometimes theory just does not have a practical application and we were often able to prove that to the engineers. On the other hand, engineers were often able to show us how to incorporate a scientific theory into a practical application. It was a win-win situation.

Again, I'm sorry if I came off as not appreciating your input. Nothing could be further from the truth. I just need you to understand where the members of this forum are coming from so your posts do not end up sparking a fire that gets out of control. I'm sure that is not your intention, but moderating this forum is a balancing act and it is sometimes impossible to keep everyone happy. Frankly there are just not enough hours in the day, so I have to be a bit more protective than I would like to be. I hope you understand and do not take offense.

[GLASSTIME]

Mother Mary of God! My eyeballs literally hurt.
As I am flattered that someone can shed some science on "our" profession. I'm also disturbed that this type of conversation is being displayed here on the forum.
I agree with Brent as most people don't understand what yor alking about as they donot speak your language. In fact most would do exactly what I did and read the first 2 paragraphs and skip to the next reply.
Because its simply to complicating. If I visited Delta' website back when I was interested in purchasing my own kit and I had to read some non-sense worth of 5 pages of some scientific solution,remedy or formula I would not have purchased anything from them.
So on that note thank yo Delta for speaking 99% or language. LOL.


Now I'm going to go read my Bible as it has more interesting science in it!