pinholes and surface defects of any kind 3

[avior]

I’m interested to know the mechanism of formation of pinholes on a composite structure surface. Surface porosity is a problem that gets worse when adding lightning strike materials such as metal mesh.

 

[Compositepro]

The pinholes you refer to are due mainly to air entrapped between the adhesive layer and tool surface. The air bubbles will be expanded by ay volatiles present but volatiles alone will not usually cause the problem. The problem is caused by the tack of the adhesive which requires the adhesive to "wet" the surface that it is sticking to. This forms a seal that will not allow any air to move to the vacuum. Vacuum-bagging applies the same pressure to the seal as to the trapped air so air will not move. Surface tension during cure will cause the air-resin interface area to be minimized and the air bubbles will change from very thin large areas to small round bubbles that become pits. You can simulate the problem with clear adhesive tape. Try sticking two pieces of tape together or to a piece of glass without trapping air bubbles.

The solution is an appropriate texture on the adhesive surface that consists of grooves that allow air to flow to the edges of the part where there is vacuum. The rheology of the resin is critical also. It has to be viscous enough that the grooves do not close before the air is removed. The easiest way to get these grooves is with a fine diamoned embossed pattern on the polyethylene separator film on the adhesive. All the adhesive manufacturers that I'm aware of have no controls in how this poly is applied. But the texture of the poly is very different on each side. one side will imprint grooves into the adhesive and the other will imprint dimples which will actually increase the air entrapment problem.

Another solution that is very effective for stopping air entrapment is with a very thin spunbonded polyester (Remay). This acts as a breather against the tool surface. The big drawback is that it also eliminates tack to the tool. The Remay can be partly inbedded in the adhesive and this is called "one side tacky" or OST by some.

With lighting strike materials the bubbles will be broken up by the screen so you will get a uniform pattern of pits.

 

[CompositeGeek]

I have used sticky backed teflon tape on the surface of tools where surface pinholes were a large problem with great success. Particularly when I could not introduce a skin surface resin like Synskin, etc.

You simply cover the tool with the material and butt splice all darts and splices necessary due to contour. We used off the shelf products and did not have to ask for any changes in processing specifications or add new materials. I also recommend a good time under vacuum prior to cure (12 to 24 hours).

It helped to provide a gas path for air and volitiles that otherwise were trapped and caused pin holes, voids etc.

The film was reusable for several tool cycles as well. Particularly useful when the part size or geometry caused a lot of trapped surfaces.

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[avior]

Another two questions:
1 – what do you think about Surface Master 905 – Cytec’s surfacing film, Sprint – SP’s asymmetric surfacing film and L-316 - JD Lincoln’s surfacing film?
2 – does exist a permanent tool surface coating that gives the same properties as teflon tape surface finish?

Thank you Compositepro for your lesson,
Thank you CompositeGeek for technical advice.

 

[Compositepro]

Surfacing films like 905 contain a lot of particulate fillers that tend to break-up the bubbles so you can't see them as easily but they are not usually 100% effective, and they add weight. CompositeGeek's suggestion will work very well and is great for flat surfaces but is difficult to apply to contoured surfaces. I'm not sure if he uses fiberglass reinforced Teflon tape or solid Teflon. Fiberglass reinforced Teflon is actual somewhat porous (even though it is called non-porous) and acts as a breather. You can watch acetone wick down the length of the fiberglass threads. Solid teflon is more conformable but less effective. Another product I came across but have not tried is Tool Surface Breather

http://www.nfgsales.com/data sheets/Miscellaneous/tsbsysap.htm

 

[avior]

What do you think about a "precure" at 150F for 30 min under vacuum to eliminate a part of volatile?

 

[Compositepro]

Volatiles are often a "red herring" in void problems. They do enlarge air bubbles that are present and that can actually be useful for stripping air out of a part. In an autoclve you can heat the part without pressure so that bubbles can expand and "burp" out to vacuum. That quickly removes residual air leaving bubbles that are only volatile vapor. When pressure is applied in the autoclave these bubbles will consense and completely disappear, whereas air bubbles will only get smaller.

So with the right cure cycle you can often get better parts when volatiles are present than when they are not. To degass viscous resins in a vacuum chamber quickly and completely the trick is to add a few drops of solvent. But, remember that a vacuum-bag is NOT a vacuum chamber. There is always bag pressure on the part.

However, as a general rule, particularly in non-autoclave processing, the lower the cure temperature the easier it is to reduce voids. You do have to know the cure chemistry of your resin as well. You won't necessarilly get the same properties with a different cure cycle because there are often different reaction that occur at different temperatures.

 

[CompositeGeek]

Avior,
re your questions 1 and 2.

1. Generally speaking I am not a fan of films used to eliminate surface porosity. Mostly due to the increase in material costs (low usage at high price per pound), and weight impact. I tend to use them after exhausting other methods or to reduce paint prep and finish time after cure, when financially justifiable.

2. I do not know of a particular tool coating that would do what teflon tape does. In fact it seems to me it would make cleaning and prep of the tool surface between cures far more difficult if the surface was permanent. Thats the nice thing about the teflon, you can pull it off and replace it without a lot of fuss and bother.

Compositepro - we used fiberglass "non-porous" reinforced tape - should have been clearer - And we were very successful on contoured as well as flat parts but you had to be very patient and use more small pieces to get the tape to cover the contour without bridging. Additionally butt splices could gaps in splices lead to resin ridges after cure and demold.

I am a HUGE fan of long slow debulks under vacuum at low temps. Usually in an Oven but I have used heat blankets as well. Never higher than 120-130 degrees F, on a 350 cure system for example, rarely used an autoclave for this step. I have used this type of "Extra Long Debulk" after the first ply with some success to reduce surface porosity as well.

I agree with Compositepro that the discussion of "volatiles" is usually a red herring. You are really dealing with trapped air and moisture not the gas by products of the resin cross linking. There is far more trapped air in a honeycomb part than volatiles!

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[avior]

I’m using prepregs /epoxy 350F and 260F cure, certified by our clients and cured by spec, (90psi, 50 psi or non-autoclave 15psi) so I haven’t too much room to play in.

Do you have any idea how can I collect a few mgrams of resin from prepreg surface for some DSC tests as Compositepro suggested?

 

[CompositeGeek]

Avior,
You probably don't need to run a full DSC to understand the chemistry and behaviour of the resin. The material vendor should be able to tell you at what temp the resin starts to flow and what the viscosity is at that time. Some resins stay pretty gell like, others reach the consistancy of water.

I think what C-pro was saying is that depending on how the resin changes state through the cure you may need to manipulate the cure cycle (within spec of course) such that you optimize the length of time the resin is in the liquid state depending on your needs. By manipulating I mean how fast you build up pressure, raise temp, and whenyou vent vacuum in the cure cycle. I have spent many hours and days trying to find the optimal mix that reduces voids, skin and interlamite porosity.

Last but not least; the reason that I think this is so critical is if you can influence how the resin is flowing, (direction etc), in your cure you are influencing how the gases are flowing as well!



Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

 

[ArturoJones]

As always, excellent information from CompositeGeek and CompositePro. I have a question on a related note. I'm not sure if this a generally observed phenomenon, but in my own experience, pinholes and other surface defects invariably manifest themselves on the tool side as opposed to the bag side. The situation I'm thinking of is in vacuum only processing with RFI or semi-preg materials. It would seem to me that both tool and bag side are served by the same air / resin flow paths within the laminate. Both sides have impermeable boundaries - metal tool on one side and release film on the bag side. So what is different about the boundary conditions that makes the bag side less prone to having voids than the toolside?

 

[Compositepro]

As in most engineering problems it is often misleading to say something like one material is permeable and another is impermeable. Or one is a conductor and one is an insulator. All materials are permeable (or conductors) and it is only a question of degree of permeability. Granted that sometimes the difference is is so many orders of magintude that it for practical purposes it doesn't make a difference, but in this case it is the answer to your question.

If you look at the datasheets for any plastic film you will find the permeability which is often in cc/square meter/24 hours. Release films will also have a breather (i.e, vacuum) on the other side from the part. So all fluid flow (liquid or gas) is outward, and the rate is controlled by the permeability. Permeability is strongly affected by things like pinholes.

For the tool-side, permeability is often thought of as zero, but that is wrong. In addition, on the other side of the tool fron the part is pressurized gas rather than vacuum. There can be leaks in the tool, porosity in the tool that is not all the way though but contains gas,or absorbed moisture. A galvanic reaction between aluminum tooling and carbon fiber during cure, with resin as the electrolyte, is something that I have measured. This generates hydrogen gas and it takes very little reaction to generate significant volumes.

 

[rerig]

2 cents worth: Check your Temperature Rate-of-rise.
Most people use the max. ROR to get the process finished quicker. If the tool isn't at the same temp as the outer surface of the laminate (Non-tool side) at resin flow, the non-tool side will cure first and trap residual air bubbles against the tool surface.
Most composite cure cycles will have a dwell just prior to resin flow to allow the tool to be at cure temperature when the prepreg is.
Process specs will usually state .5 -7 Deg F per minute rate of rise. Production managers will usually state - make that 7 Deg F and ship those parts!

Just one side of a multi-sided problem. But don't forget about it.