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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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