Recommended Preparation Techniques for Secondary Bonding

Carsten Sundin from Lancair posted a response in the Lancair Mail List addressing the issue of how Lancair builders should prepare surfaces for secondary bonds in Lancair aircraft construction. This is a very controversial topic, so I am posting the content of the message below unabridged and unedited.

Fri, 22 Mar 2002 18:12:12 -0500
From: "Carsten" <carstens@lancair.com>
To: <lancair.list@olsusa.com>
Subject: Lancair's response

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<< Lancair Builders' Mail List >>
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We have been following the recent debate on bonding techniques. On a regular basis we consult with experts in the industry on recommended procedures. Expert advice and structural testing is what we base the procedures recommended by Lancair.

Surface preparation for bonding is obviously very important in our business. Our methods have changed slighty over time and we believe our current bonding practices work the best for us (Lancair constrution that is). We used to recommend MC, however switched away from this. Without going into too much detail on the chemical process, a good acetone does the job and is gentler on the composites. Also health hazards are considered, another reason to stay away from MC.

When the discussion regarding bonding techniques emerged again on the web site I contacted Louis at Abaris to review our current procedures. I also asked him to write up the basics of their recommendations- please see below. I would like to point out that Lancair does recommend a slightly courser grit sandpaper than the 320 or 400 grit Abaris recommends. Lancair recommends 40 to 80 grit sandpaper. A big misunderstanding in chemical bonding is that rough sandpaper provides a better bond. In fact, you will see virtually no difference in bond strengths in the grit ranges from 40 to 400 grit. Our selection of 40 to 80 grit sandpaper is mostly from a practical standpoint: it is easier to abrade the larger bonding surfaces as well as inspect for proper abrasion.

The current bonding procedures is what we continue to recommend. Here is Louis Dorworth's of Abaris Trainings input:

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From Louis C. Dorworth
Chief Technical Instructor
Abaris Training Resources, Inc.

I have been involved in the fabrication and repair of advanced composite structures for over 20 years, having worked on such projects as the Lear Fan, the B-2 bomber, the Beech Starship, the C-17 cargo transport, and the Boeing 777. All of these aircraft rely heavily on advanced composite
structures that are both bonded and fastened together.

Currently I am the Chief Technical Instructor at Abaris Training in Reno, NV. We offer training in bonding advanced composite and metal structures and I want to pass along the current standard practice for preparing a composite surface for bonding for your builders to ponder.

I must say that the methods of surface preparation for composite surfaces vary from one manufacturer to another but there are some common ideas amongst all of the major composite aircraft component manufacturers.

First, if the surface is not covered with a (non-release coated) peel-ply that can be removed to reveal a clean surface, then the surface should be cleaned free of contaminants prior to abrading. Acetone, isopropyl alcohol, MEK, or MIBK is usually specified for this task. (Methylene Chloride (MC) is not used by any of the big commercial manufacturers for numerous reasons.) I prefer acetone, as it is the least toxic of these solvents and is adequate for reducing most contaminants. Wear the appropriate type of gloves when handling solvents! An organic vapor respirator is recommended for more extended exposure. (The 3M #8577 nuisance-level organic mask is affordable and protects the user well.) The surface to be prepared is “double-wiped” with a clean solvent-soaked cloth in one hand and a clean, dry cloth immediately behind that. The dry wipe behind the solvent wipe helps pick-up the residual contaminants from the surface. Repeat as often as necessary with clean, fresh wipes to ensure good cleanliness. Allow the surface to dry thoroughly before abrading. This usually takes 15-20 minutes with good ventilation.

Next, abrade the surface with an appropriate abrasive that will raise the surface-free energy of the matrix resin but will not damage the fibers at the surface. Usually 3M’s ScotchBrite® # 7447 pads are used for this purpose. Optionally, #320 or #400 grit sandpaper may be used on non-textured surfaces. A thorough abrasion is best accomplished by abrading in a multi-directional manner. It takes a bit of work to do a good job by hand. The surface should be uniform in appearance all along the length of the joint.

Clean the dust off of the surface with a clean, dry piece of Rymplecloth®, or an equivalent (non-chemical treated) cheesecloth material. Repeat the dry-wipe procedure with fresh cloths several times until no dust residue remains. (This non-solvent approach is becoming more mainstream in the aerospace industry as the risk of introducing additional contaminants or moisture, or changing the surface energy with the solvent is of major concern.)

If you must use a solvent, than reagent grade acetone is probably the best choice. Purchase small pint-size cans and keep the lid tightly secured when not using the stuff. This will reduce the potential for moisture uptake in the stored solvent. Be aware that any solvent used at this point may bring
moisture onto the surface; so follow the double-wipe method as described earlier. If there is a “milky” look to the surface after wiping, it is probably moisture. This is usually noticeable when the relative humidity is greater than 50%

After cleaning, it is suggested that the adhesive be applied and the joint closed and clamped as soon as possible. The longer you wait, the more the surface has a chance to stabilize, thus reducing your surface-free energy and diminishing your covalent exchange between the surface and the adhesive. If anybody is interested in learning more about this, or other composite related subjects, I invite you to come take a class at our Reno, NV, or our Griffin, GA facilities. It may be well worth your time. You can visit our web site for more information: www.abaris.com.

Louis C. Dorworth
Chief Technical Instructor
Abaris Training Resources, Inc.
5401 Longley Ln. #49
Reno, NV. 89511
Phone: (775) 827-6568
Fax: (775) 827-6599
e-mail: lou@abaris.com

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