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Building Composite Aircraft Part 2
By Ron Alexander(originally published in EAA Sport Aviation, November 1997)
All resins, hardeners, catalysts, solvents, in short, all chemicals used in composite construction should be considered hazardous. Some of these are more hazardous than others but all pose a potential health problem. Absorption of these chemicals through the skin is a major hazard. Epoxies can be absorbed through skin contact and the effects are cumulative with extended use. You may use a certain epoxy for years with no adverse skin reaction and then you suddenly become sensitized and develop a painful rash or other problem. A wide variance of opinion exists among professionals concerning the best way to protect your skin (hands in particular). It is impossible to make an emphatic statement concerning how to protect your hands. It’s impossible because there are individual physiological differences. The bottom line is some people are much more sensitive than others. If you are just beginning to work with resins and your chance of contacting the chemicals is minimal, you can use Invisible Gloves, a skin barrier cream. The key to using Invisible Gloves is to recoat at least every hour. Barrier creams provide adequate protection when you have limited exposure. Latex gloves also offer protection and are widely used. Some people will use both Invisible Gloves followed by latex gloves. Sweating of the hands often contributes to an allergic reaction. To preclude this many people will use cotton glove liners followed by vinyl or butyl gloves. Overall, butyl gloves offer the best possible protection but they are expensive. You will need to decide which method works best for you. Avoid skin contact with epoxies. There are no safe epoxies.
Wear long sleeve shirts to protect your arms. Never wash your hands with solvents after you have been working with resins. Use only soap and water. A good cleaner for composite tools is ordinary apple cider vinegar. Denatured alcohol also works well. There is really no reason to use solvents with composite construction. Do not breathe the vapors emitted when using resins. Insure that you are in a very well ventilated area and use a charcoal-filtered respirator as an added precaution.
An additional hazard involved with using resins is the exothermic reaction that results from the curing process. A rapid increase in temperature results when the curing process of the resin system begins. Mixing large quantities of resins should be avoided. Often a large quantity of resins will exotherm to the point that the heat can potentially reach a temperature that will ignite a fire. To avoid this problem mix small quantities, no more than one quart.
Vinyl ester resins pose another type of problem. Skin sensitivity is often not as pronounced as with epoxies. However, vinyl esters must be catalyzed using MEKP (methyl ethyl ketone peroxide). This chemical is very hazardous if it contacts your eye. Be sure to wear eye protection if you are using a vinyl ester. Additional problems can be encountered if you are promoting vinyl esters. Usually a vinyl ester has been promoted when you receive it.
As I discussed last month, cutting the core materials can pose a safety problem. The only core material that we cut using a "hot-wire" device is polystyrene. All other foams emit a poisonous gas when burning. They must be cut using a saw or knife. Remember, do not burn the excess scraps of urethane foam. The gas emitted is cyanide. When cutting using a saw be sure to wear a dust mask to prevent breathing of the particles.
Sanding of reinforcement materials will release small airborne fibers into the air. To protect your lungs from these particles you should wear a dust mask or a respirator. Also, protect your skin from these small particles of glass. Mixing microballoons (small glass spheres) emits the spheres into the air. Do not breathe these glass spheres. Milled glass, Cab-O-Sil, and cotton flox also present the same problem. Do not breathe these particles or allow them onto your skin. Eye protection should also be used to prevent the particles from reaching your eyes.
Composite construction does have certain hazards. However, with every type of construction we are confronted with different types of safety problems. Proper knowledge and adequate preparation will protect you from the risks involved in building a composite aircraft.
Basic Building Techniques
A brief outline of each step involved in composite construction follows. This discussion is introductory in nature providing an overview. The actual steps involved require a more detailed analysis than space permits.
Cutting Foam Cores
If you are building an airplane from a set of plans you will be cutting the foam cores into the shape of an airfoil. Many kit airplanes come with premade parts precluding the necessity of learning how to shape a section of the airplane. Assuming you will need to cut the foam core I will briefly outline the procedure.
You will need a large work table on which to lay your foam pieces for shaping. If we are using polystyrene foam we will make a template the shape of our airfoil from our plans using masonite or aluminum as a backing (see figure 1). Duplex nails are used to secure the template to the foam. Notice that the template has numbers on one side. These numbers are used to insure uniform cutting by the two people necessary to hot wire the foam. One person calls the number where the actual wire is located and the other insures that the hot wire on their side is on the corresponding number. Our hot wire device is nothing more than an inconel wire mounted between two posts with a source of electricity providing current through the wire (see figure 2). The wire becomes hot and actually melts its way through the foam forming a very smooth, even surface. Hot wire devices can be up to about 60 inches wide. Anything longer than that is difficult to handle.
As you may ascertain, several pieces of foam will need to be cut and shaped then glued together to form a complete airfoil such as a wing. Final shaping of the piece is usually done by sanding. Once each piece has been properly shaped, all pieces are then glued together using a resin mixture. This completes the airfoil section. Usually additional shaping is necessary after the parts are glued. The entire foam structure is then prepared to accept the reinforcement material. Polystyrene foam has large cells that must be filled. If these cells are not filled the resin matrix will be absorbed into the foam through these cells. This will result in excess resin being used which adds to the overall weight. In addition, a poor bond with the reinforcement material may result due to voids that may be present. These cells are filled using a filler material. This can be a mixture of resin and microballoons mixed to the consistency of thick gravy. Another filler often used for this process is SuperFil that is a lightweight premixed material manufactured by Poly-Fiber. A thin layer of the filler is then placed on the core material using a rubber squeegee. Urethane and PVC foams usually require a different viscosity of microslurry because their cells are very small.
Application of Reinforcement Material
Recalling our composite structure, we have basically three materials. One is the core (usually foam), the second is the reinforcement material (usually fiberglass), and the third is the resin matrix (usually epoxy) which binds the materials. The three together form a very strong part.
After the foam has been properly sealed, we now are ready to "lay-up" the layers of reinforcement material. The type of material and the number of layers are determined by the aircraft designer. Be sure to follow the manufacturer or designer’s plans. The fiberglass is usually placed on the foam in layers with the strength required determining the number of layers.
The work area should be clean with the ideal temperature being 70 to 80 degrees F. Cut your pieces of fiberglass using shears designed for cutting this type of material. Keep the pieces clean. As a goal to minimize the overall weight of the airplane, the weight of the resin should equal or be slightly less than the weight of the fiberglass you are laying up. If you strive for 50-50 weight distributed between the resin and the glass you will usually achieve your objective. It is essential that you wet out the fabric thoroughly while being careful not to use too much resin. Excess resin is wasted and simply adds additional weight. So, weigh the fiberglass or material you are bonding and mix that amount of resin material. The most accurate way to mix resins is with a simple postal scale pictured in figure 3. These scales are fairly inexpensive and they provide both ounces and grams as units of measurement. Prepare yourself for mixing resins by protecting your skin. Using a measuring cup weigh the proper amounts of resin and hardener as noted on the container. Mix the two together by stirring with a mixing stick for a period of at least 2 minutes to insure adequate blending. At a temperature of 70 degrees you will usually have a working time of about 45 minutes depending on the resin system used. Place the fiberglass on the foam surface orienting the fibers according the design and then pour a small amount of resin on the fiberglass. Use the rubber squeegee to spread the resin onto the glass. Brushes and grooved laminate rollers are often used in the laminate process. Be sure to cover the glass uniformly with the resin mixture. Clean up your tools using apple cider vinegar. Points to remember -- proper mixing of the resin is essential to insure adequate bonding strength mix small amounts to avoid the exotherm problem, thoroughly wet the fabric without using excess resin, and don’t forget to protect your skin.
Use of Peel Ply
Peel ply is nylon or polyester fabric (similar to the fabric used on airplanes) which is used after a layup has been completed to remove excess resin and to insure an adequate bond between layers of glass. This material is placed on the resin before it has cured. It is squeegeed into place actually wicking up resin from underneath the peel ply itself. The resin is then allowed to cure and then the peel ply is removed from the laminate. The result is a very smooth surface, derived without sanding, which will result in greater adhesion of subsequent layers of material. The use of peel ply on laminates (layers) of material has the following advantages: (1) peel ply causes the fibers to lay flat, (2) it reduces the amount of sanding necessary, (3) peel ply increases the adhesion in subsequent bonding and the adhesion of primers, and (4) it reduces the amount of resin used on the structure.
This term is familiar to many builders but often not understood. Vacuum bagging , very simply, is a more sophisticated method used to remove excess resin and to improve laminate quality. Vacuum bagging a process using a vacuum pump to "draw" a vacuum on several parts of a laminate. This draws the parts very tightly together forcing out all voids and excess resin. The process also serves to hold reinforcements, resins, and core materials in close conformity to complex shapes. Without a doubt, vacuum bagging increases the time and materials cost of a laminate. However, it offers significant advantages when optimum strength to weight is essential. While specific materials may vary depending on the particular application, the basic components of a vacuum bag assembly include laminate (layer of glass), peel ply, bleeder ply, bagging film, sealant, connector and vacuum pump (see figure 4). As noted, peel ply is also used with this application. The vacuum pump is attached through the connector valve into the bagging film. The bagging film contains the vacuum and applies pressure to the laminate. It must be able to stretch and conform without rupturing. Bleeder ply absorbs the excess resin and communicates the vacuum evenly over the entire surface. A perforated release sheet allows excess resin to transfer from the part being bagged to the bleeder ply. Peel ply separates the cured laminate from the bag assembly allowing removal after curing. The removal of the peel ply is usually not done until the surface is ready for painting or secondary bonding. Keeping it in place will protect the laminate surface from dirt and oil. A tremendous amount of pressure can be applied using this process. As an example, a vacuum of 15 inches of mercury will produce a force exceeding 1000 pounds per square inch. As you can see, this is a very efficient means of removing excess resin and eliminating voids.
Post curing is a process used to obtain maximum strength from a resin. To understand post curing it is necessary to define the term Glass Transition Temperature or Tg. The transition temperature of a resin from a hard glassy state 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. Raising the temperature of the laminate above standard room cure temperature performs post curing. 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 any maximum temperature of another material in the laminate such as the foam. Without post curing the Tg 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 which could cause the entire matrix to soften. This softening can result in the matrix of the heated portion being softened and pulling away. The once smooth surface now exposes the weave of the fabric. High temperatures in 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 your part or the entire airplane. Introduce the heat gradually to the temperature specified by the resin manufacturer. Usually this will be between 140 degrees to 180 degrees F. Again, care must be taken to not exceed the break down temperature of other components such as the foam.
The above discussion will provide you with a basic understanding of composite construction. Most composite kit aircraft do not require shaping the airfoil section from foam. Instead, you are provided sections of the airplane that have to be bonded together. Next month I will conclude the discussion of composite construction by presenting information concerning bonding techniques and finishing composite surfaces. Hopefully, at the conclusion of these articles you will have a basic understanding of composite airplanes and how they are assembled. At that point you will be prepared to decide which airplane you want to build.