Posted for "the colwells" <colwells@comcast.net>:

OK, I could not get the link in my e-mail to work either so I copied it.
This article covers a lot more than the subject but I think it relates.

Steve Colwell


Basic Composite Construction
Sport Aviation 8/99
By Ron Alexander

Over the past few months we have discussed most aspects of building a
composite airplane. This article will focus on a few specific items that
require explanation such as proper preparation of parts prior to
bonding, post-curing, blushing problems, etc.

Preparation of Composite Parts
In the last issue, I outlined a brief procedure for preparing composite
parts prior to bonding. This step is most important and needs to be
amplified. The quality of a bond is directly affected by the preparation
of the two parts being joined together. If contamination exists on
either part, the bond may be weakened even to the point of subsequent
failure. Let me emphasize that you should follow the directions found in
the kit manufacturer's manual regarding proper cleaning techniques.
However, the preparation procedure is important enough to warrant more
detailed discussion.

First of all, when bonding to an outside mold surface (such as many of
the parts you receive from the kit manufacturer) cleaning and sanding of
the parts is always required. When aircraft parts are molded, a release
agent is applied to the inside of the mold itself allowing the part to
be removed when cured. This mold release agent must be removed prior to
any bonding activity. The agent is barely visible. Water will usually
remove this agent. After removal of the agent and any contaminants,
sanding is then accomplished.

Any surface that is smooth because of being next to a mold must be
sanded prior to bonding. Any primer that may be present must also be
removed. Sanding is generally the accepted way to prepare the surface.
Opinions vary on the proper grit of sandpaper to be used. Usually 80
grit to 180 grit is recommended. Our workshop experience has shown that
180 grit sandpaper is usually satisfactory to prepare the surface. Use
of 180 grit will ensure the underlying fibers are not damaged or cut.
The surface should be thoroughly abraded (roughed) to completely remove
any glossy areas. Abaris Training, located in Reno, Nevada, instructs the military,
airlines and aerospace industry on composite construction and repair. I
consult with Mike Hoke, the President of Abaris, regularly concerning
composite construction. His company is considered to be one of the
leading composite training companies in the United States. The following
quote was taken directly from their training manual regarding surface
preparation. "High surface energy is the goal, not mechanical roughness.
One must shear up the top layer of molecules on the surface, creating
many broken bonds, without damaging or breaking underlying fibers. A
water break test can be used to determine surface energy. If surface
energy is high, clean distilled water will spread out in a thin uniform
film on the surface, and will not break into beads. If a water break
free surface can be maintained for 30 seconds, one has achieved a clean,
high energy surface suitable for bonding. If the surface is contaminated
or at low energy, the water will break into rivulets and bead up. "Note
that tap water will not work. It is dirty enough to contaminate the
surface itself, and one will never pass a water break test using it. "It
is important to note that the 'high energy"'condition, once achieved, is
short-lived. Within about 2-4 hours the effect is lost. In composites,
one should therefore wait as late as possible in the process before
surface abrasion is performed, so that all else is ready and the
adhesive can be quickly applied."

Dry the water off of the laminate with a hair dryer prior to applying
the adhesive. If it is wiped with a cloth it will likely contaminate the
area again. Do not use a heat gun for this process. The heat is too
intense and may damage the cured resin. This process also applies to
peel ply surfaces. Even though a peel ply surface fractures the top
layer of resin, it leaves a glossy, low energy surface in the weave
pattern of woven cloth. This must be abraded for proper bonding.

So, how should you clean parts prior to bonding? The best procedure is
to simply sand the surface, as discussed, and follow by a thorough
cleaning with soap and water. If you are using solvents, use them
initially to remove contaminants and then abrade the surface. Follow by
soap and water and then immediately dry using a hair dryer. Remember to
begin the bonding process within a few hours after preparing the
surface.

Amine Blush
Sometimes when working with epoxy resins, you may encounter what is
referred to as an amine blush. The development of an amine blush is most
visible under high humidity conditions. An amine blush is a surface
effect resulting from the curing agent reacting with Carbon Dioxide
(CO2) in the atmosphere rather than the epoxy resin. The by-product of
this reaction is a compound that forms on the surface of the curing
resin and readily absorbs moisture from the air. Under high humidity
conditions, it will cause white streaks to appear on the surface of the
resin and the uncured laminate. During cure, the white streaks usually
disappear, but left behind will be a greasy or oily residue. Sometimes,
this residue appears in the form of sweat-like droplets. This residue is
water-soluble and will wash off with warm water. Depending on the
severity of the blushing event there may even be areas of surface
tackiness. This tackiness is only on the surface, and will not affect
the overall properties of the cured laminate.

Amine blush must be removed before any additional laminates are
initiated. Sanding will remove blush but it will also quickly gum up
your sandpaper. Wiping the surface with a warm wet rag prior to sanding
will reduce the gumming tendency. The best approach is to avoid amine blush altogether. Some resin systems
are inherently resistant to developing amine blush. And for others, it
may seem impossible to avoid it. But there are some things you can do to
minimize it greatly. Number one and foremost is, DO NOT use unventilated
combustion type heating sources to warm your shop. Gas or kerosene fired
salamander heaters produce copious amounts of CO2 and H2O. These are the
primary ingredients needed for producing an amine blush. So, use
electric heaters or ventilated exhaust type combustion heaters to keep
your shop warm. You should avoid mixing resins or doing any layups if the temperature is
less than 65 degrees F. If you do a layup at this temperature you should
immediately move the part into a warm room for curing. Purchase a
thermometer and a humidity indicator and place them in your work area.
Avoid mixing resins and working with resins if the temperature is below
65 degrees F or if the humidity rises above 80%. The best solution is to
place an air conditioning unit in your workshop area.

You can reduce the susceptibility to blush in the following ways:

. Work in the prescribed environmental conditions. . Use "dry" and ventilated heating sources . Use peel ply. Amine blush usually forms on the outer-most portion of a
layup. By using peel ply the amine blush is removed when the peel ply is
removed. . Cap all resins as soon as possible. This reduces their exposure to the
elements. . Use a resin with demonstrated blush resistance. Some resins are more
susceptible to blushing than others. Use of peel ply, purchasing a blush resistant resin, and working in the
right temperature and humidity will all work together to minimize amine
blush.

Hardpoints



Often you will be required to mechanically attach another piece to a
composite structure. One method of doing this is to fabricate a
"hardpoint". If you mechanically attach a piece to a fiberglass part,
the fiberglass must be reinforced in the area where it will be fitted to
accept the loads imposed by the attachment. An example of a hardpoint is
found on the GlaStar airplane. A welded fuselage frame is placed inside
a pre-molded fuselage shell. The two are attached using machine screws
that are placed through hardpoints fabricated in the fiberglass shell.
The most common method of fabricating a hardpoint is to route out a
small amount of foam core material between the inner and outer laminates
of the shell (see Figure 1). You must be sure not to remove any of the
reinforcement material on the outer and inner shells. A piece of piano
wire bent 90 degrees and placed in a drill works well for this step. The
core material may then be removed using a shop vacuum. After the core
material has been removed, a mixture of resin and milled fiber is
injected to fill the void. After the material is injected through the
drilled hole, a small piece of tape may be applied to keep the resin
mixture from escaping. After curing, this material provides the strength
needed to serve as an attach point. You must ensure that the entire area
is filled with material and no air bubbles are present. After the
material completely cures, a hole is drilled through the reinforced area
to receive the screw or bolt. This is one example of a hardpoint.
Various kit manufacturers use different methods. Complete instructions
on fabricating a hardpoint will be included in your assembly manual.



Post Curing
Post curing is a process used to obtain increased strength from a resin.
If an epoxy resin is allowed to cure only at room temperature, its
ultimate strength is rarely achieved. Post curing will increase two
critical performance properties of an epoxy, chemical resistance and
heat resistance. Fuel tanks constructed using an epoxy will benefit
considerably from post curing. Post curing the entire airplane will
increase overall resistance to the heat build-up inside the airplane
resulting from the high temperatures found on any ramp in the summer.
This build-up of heat can reach the glass transition temperature causing
a weakened state of the resin itself. To understand post curing, it is
necessary to define the term glass transition temperature or Tg. The
glass transition temperature is the point where the physical properties
of a resin material start to decrease as temperatures are elevated. The
temperature at which the resin "transitions" (T) from a hard, glassy
state (g) to a soft rubbery state is called its Tg. At the Tg the
tensile strength, chemical resistance, and hardness are significantly
reduced while the flexibility is increased. As you might imagine, we do
not want our completed airplane to reach the Tg temperature. To prevent
this from occurring, one method is to post cure the resin. Another way
is to paint our airplane a light color (usually white) to preclude the
temperature on the inside of the airplane from being excessive. On a 90
degree F day, it is not unusual for the temperature inside your airplane
structure to reach 180 degrees F plus. This is why you see most
composite airplanes painted white. The white color helps reflect the
heat keeping the temperature inside the airplane component parts as low
as possible.

Another term often used is referred to as the Heat Deflection
Temperature (HDT). The value of this number provides us with an idea of
the upper service temperature limit for a plastic. This is the
temperature at which a resin will begin to soften if placed under a
load. The HDT is usually about 20-30 degrees C lower than the Tg of a
resin. The reason this is true is because the test to determine this
value is accomplished under a load. For this reason, HDT is often a
better indicator of the true upper service temperature limit for a given
resin. Regardless, it may be difficult for you to find the value of the Tg
and/or the HDT of a resin. Resin manufacturers sometimes display one or
both of these values within their instructions but many do not. You will
have to seek out this information and determine the temperature and time
required at that temperature for a post curing operation. Should you
post cure? Post curing is not absolutely necessary but it certainly is
advantageous for all epoxy resins. Some resin manufactures require a
post cure as standard practice. Basically, post curing your component
parts and your composite airplane will ease your mind concerning the
quality of your layups and bonds. If you are somewhat unsure about
whether or not the resin properly cured on a particular layup or bond,
post curing will likely solve that problem. If you are using epoxy to
construct a fuel tank, you should definitely post cure that area. Post
curing will ensure adequate fuel resistance not only for today's fuel
compositions, but tomorrow's as well. Without post curing, you may
encounter a gummy substance in your fuel tank that can plug gascolator
screens and filters.

The bottom line in discussing this issue with Gary Hunter - an
acknowledged expert on resins who works for Shell Chemical Company (a
major manufacturer of epoxy resins) and EAA Technical Counselor - Gary
recommends post curing a composite airplane. In his opinion, it takes
all of the worries out of the construction process as it pertains to
resins. It is a little more insurance that you are getting the maximum
performance available from your resin system.

What about vinyl ester resins - do they require post curing? It is not
necessary to post cure vinyl esters but it is helpful. Room temperature
cured vinyl ester resins develop a larger portion of their ultimate
properties, than most room temperature cured epoxies, and as such, they
tend to be more resistant to chemicals overall. Therefore, the benefits
of a post cure are not as significant. However, post curing simply
improves these attributes even more.

How do we post cure? Raising the temperature of a typical laminate above
standard room cure temperature performs post curing. Again, most resin
systems will not reach their full strength unless they are cured at a
temperature considerably above room temperature. Usually this
temperature is about 40 degrees F below the Tg specified for the resin.
The post cure temperature should never surpass the maximum temperature
of another material in the laminate such as the foam. (As an example,
polystyrene foam swells at a temperature around 165 degrees F.) Without
post curing the Tg of a resin used on your airplane will only be
approximately 40 degrees F above the temperature at which the resin was
cured. On a hot day the temperature of a structure can exceed the Tg.
That could result in the entire composite matrix softening. This
softening can result in the matrix of the heated portion being weakened
and pulled away. The once smooth surface now exposes the weave of the
fabric. High temperatures inside structures that have not been post
cured can also affect structural integrity.

With this in mind, it is important that you follow a post curing
procedure. You can do this yourself by introducing the proper amount of
heat into a fireproof tent-like structure containing a specific part or
the entire airplane. Introduce the heat gradually to raise the
temperature to that specified by the resin manufacturer. Usually this
will be between 140 degrees to 180 degrees F. Let it warm up slowly and
evenly. The resin manufacturer will specify the amount of time required
at this temperature. An excellent method of post-curing is to rent a
paint booth from a local car painter. These booths are usually heated
and you can place your parts or the entire airplane in the booth. Put a
couple of fans within the booth to circulate the air for even heating
rates. Another built-in area to post cure is your attic. The temperature
of most attics will reach 140 degrees F. Granted, you have little
control over the heating but small parts can be

post cured in an attic area. A regular oven can be very effectively used
to post cure parts. You can purchase foil back insulation material and
construct a small post cure booth. The insulation can be taped together
using duct tape (see Figure 2). You can then place a thermostat
controlled electric heater in the booth with a couple of thermometer
probes placed through the insulation to indicate the temperature. It is important that you properly support parts to prevent any
distortion. This does not mean that you have to place a wing back in a
jig. This is assuming the resin has cured for at least a week. (If you
are immediately post curing then you should leave the wings in the jig.)
Regardless, you must provide adequate support. This means positioning a
wing on a flat surface with the leading edge down, as an example.
Cowlings should be in place on the airplane or set on the floor with the
forward edges down.After the part has been heated for the required
amount of time, slowly cool the temperature. Do not simply pull the part
out of the heated area. Again, care must be taken to not exceed the
break down temperature of other components such as the foam. Many kit planes are manufactured from heat cured prepregs and as such,
they are essentially post cured as delivered. However, the adhesive bond
lines and tape layups the builder makes to assemble the prefab pieces
will only have a room temperature cure. It only makes sense to post cure
these bond lines and layups so the properties will better match the
prefab parts from the manufacturer. This can be accomplished by
introducing heat into a closed-up fuselage or wing area for a certain
amount of time. After all, being made from foam or honeycomb cored
composites, they are naturally insulating structures. One way to do this
is to use the exhaust from a vacuum cleaner as a mild source of heat.
Many builders have used this procedure to introduce heat into a fuselage
area for a period of time. All of the bulkheads that have been bonded
and other resin applications will be post cured. When to post cure is another question. It really does not matter when
you post cure. It is usually best to wait at least two weeks after you
have completed your layup or bonding to allow the resin to cure as much
as possible at room temperature. Even if you have completed the work 6
months ago or longer you will still derive benefits from post curing.
Similarly, the fillers and faring compounds used to smooth and contour
your airplanes painted surfaces will benefit from a post cure. Fillers
inherently shrink as they cure, and after a few months in the hot sun a
show quality finish can literally shrink away exposing the weave of the
reinforcing fabric and other unsightly discontinuities. This is commonly
referred to as "Print Through". Post curing your airplane after the
filling work but prior to priming and painting will essentially
pre-shrink these fillers and allow you to see and re-fill any resultant
print through prior to final painting.As you can see, there are many
ways to post cure. There is nothing absolutely critical about the
method. The slow introduction of heat up to the desired level followed
by the proper time at that temperature is important. Again, slowly lower
the temperature when you are through. As Gary Hunter states, "Post
curing is not absolutely necessary, but the results are always
comforting on that first encounter with clear air turbulence." Next
month I will conclude this series on composite construction. That
article will focus on forming and proper finishing techniques.