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Part 2 Basics of Composite Construction
By Ron Alexander(originally published in EAA Sport Aviation, June 1999)
In the May issue of Sport Aviation, I presented the first part of a series of articles on the basics of composite construction. Workshop space, tools required, and the methods of working with core materials, cloth, etc. were discussed. In this part I will continue with fillers, safety issues and basic layups.
Many applications of composite construction require a filler material to thicken and/or reduce the density of the resin mixture for various purposes. The resulting mixture of the filler plus the resin is used to form a fillet to provide a radius where two composite pieces are joined together. Fillers are also used to seal the cells of foam. The slurry coat is used to fill the cells with a lower density material than that of pure resin. Fillers are also used to thicken a mixture so it can be applied without running, to enhance the strength of resin material for structural bonding, and to fill the weave of fabric during the composite finishing process. Mixtures may also be used to fill any gouges or dents in the foam core. Corners are also constructed using a filler material. Several different filler materials are used with resins. The more popular ones will be discussed.
Microballoons, as they are often called, are nothing more than very minute spheres of glass. Microscopic Christmas tree bulbs provide an accurate analogy. This material is very lightweight and very easily suspended in the air. Care must be taken when working with microballoons not to inhale any of these glass particles. Quartz "Q cells" is another type of microballoon called for in the plans of several kit aircraft. When either of these forms of filler is mixed with a resin material the resulting mixture becomes lighter in weight with less strength. This mixture is commonly referred to as "micro". Micro is usually mixed in three different thicknesses. First is a slurry consistency. This is usually a 1-to-1 mixture by volume of microballoons and resin. This provides a mixture that is almost the same viscosity as resin by itself. Slurry is used to fill the cells of the foam prior to applying the first layer of cloth. The second type of micro is usually termed "wet-micro." It is thicker than slurry and is used to join blocks of foam together. The mix ratio is approximately 2-3 parts of microballoons to one part of resin. The third type of micro is called "dry micro." This mixture requires about five parts of microballoons to one part of resin and it is used as a filler material.
Micro must NEVER be used between plies of a layup as the final strength will be severely decreased.
Flocked Cotton Fiber
This particular filler material, usually called cotton flox, is also mixed with resin. It consists of finely milled cotton fibers that provide an adhesive when properly mixed with a resin material. The mixture is termed "flox." Flox is usually mixed about two parts of filler to one part of resin. A popular use for flox is to reinforce a sharp corner to provide more strength within that area. It is used in filling sections that require structural strength. It has much higher shear qualities than micro but is much harder and heavier.
As the name implies, this filler material is made by milling fiberglass into a very fine consistency. Milled fibers have a higher strength than cotton flox. The mixture of milled fiber and resin is used as a structural filler. It is also often used to form a fillet that requires structural integrity. Milled fibers and resin are used to form a "hardpoint" on a fiberglass structure. The hardpoint is used to attach other structures to the fiberglass. Care must be taken when working with milled fiber due to the very fine particles of fiberglass that can penetrate the skin.
This material is the same as milled fibers, except it is available in different lengths. This allows its use as a filler for very specific areas where greater strengths are needed.
Cab-O-Sil is fumed silica that acts as a material to thicken a resin. Small amounts should be used. Larger amounts can act to inhibit the curing agents of some epoxies when used in concentrations greater than 15% by weight. Using Cab-O-Sil simply keeps a resin from running when you are applying it to a difficult area.
Poly-Fiber manufactures a substitute for dry micro called SuperFil. This filler material is mixed to the exact same consistency with each batch. In addition, it has talc added that facilitates the sanding operation.
SuperFil may be used as a filler for virtually any material including metal, wood, and fiberglass. The epoxy in SuperFil has been optimized for the filling process. Micro normally uses resin optimized for the laminating process. An important point-when you are mixing filler materials, always mix the resin and hardener thoroughly prior to adding the filler substance.
A review of the safety issues involving composite construction is in order. One of the most important issues regarding safety when working with composites is skin sensitization. Many people become sensitized to resins. This is more common with epoxy resin than with vinylester resin. Regardless of the type of resin you are using you must protect your skin. Wear long sleeve shirts and protect your hands using a form of glove. What type of glove to wear is controversial. Many people can simply use a latex type glove found in drug stores. However, a number of people are allergic to the powder often found inside the latex glove. Vinyl gloves are available and provide a very good alternative to latex. Rubber gloves are used by many
people who place a cotton liner inside the glove. Several builders use barrier creams such as Invisible Gloves with success. No matter what you use change gloves often or recoat with creams often. Never wash your hands with solvents. Always use soap and water.
Have adequate ventilation so you are not breathing the fumes from resins. A small fan will assist in moving the air out of the area. You also should wear a respirator. This is important when doing layups and also when mixing fillers. Those tiny spheres of glass called microballoons will do a number on your lungs if inhaled. Particles of fiberglass resulting from sanding operations should not be inhaled.
Vinylester resins pose a different type of problem. They have chemicals that should not be mixed together outside of the basic resin chemical. The catalyst used with vinylester, MEKP, is destructive to the eye. A face shield is preferable to use when mixing MEKP with the vinylester resin. Again, skin sensitization is not as common when working with vinylester as when working with epoxies.
Always acquire and read the Material Safety Data Sheet for the material you are using. These MSDS sheets will explain the hazards of each type of resin or solvent you are using.
Finally, mixing too large a quantity of a resin can cause a problem known as exotherming. The exotherm process is a consequence of the chemical reaction that takes place as a resin hardens or cures. This chemical reaction causes heat to be generated which in turn speeds up the chemical reaction causing even more heat to be generated. If you mix a large batch of resin you can create an "out-of-control exotherm." The container holding the resin will get so hot from the chemical reaction that you cannot hold it. The resin may actually bubble or boil and you will see smoke rise from the substance. You can prevent this by mixing small quantities of resin (8-10 ounces by volume). If you see that you are getting an out-of-control exotherm you should immediately pour the resin onto a sheet of plastic. This will allow the heat to more readily dissipate into the air. The exotherm process can actually cause a fire if the container is thrown into the wrong place. A similar type problem can occur when putting foam blocks together if too large a micro joint is allowed. The foam is a good insulator and the heat will build without escaping. This can melt the foam and cause a core void.
Now that we have set the stage and we understand some of the basics, let's get to the fun part - doing an actual layup. First of all, what is a layup? It is probably more accurately defined as a laminate. A laminate is one layer of reinforcement material impregnated with resin and usually added to a core material or to another layer of reinforcement material. This process is commonly referred to as a layup.
If you are building a plans built airplane you will become very proficient in doing layups. In a plans built composite airplane you actually build most of the parts of the airplane and then bond them together. Building parts requires a lot of layup work. On the other hand, if you are building a kit aircraft you usually will only be required to bond the already completed parts together. However, you will still use the layup procedure for many activities on a kit aircraft. The most important thing I want to recommend prior to our discussion is for you to do practice layups before doing the real thing. Any experience you can acquire doing basic layups will enhance the quality of your work on the actual airplane. Attend one of the EAA/SportAir composite workshops and make all of your mistakes while learning in a classroom setting. No matter what - practice.
Before you actually begin the layup procedure you must be prepared. You should have everything on hand before you begin. This means gloves, respirator, mixing cups and sticks, scales or pump, squeegees, brushes, rollers, etc. Be sure the squeegees you are using have a smooth edge. If not, pass the squeegee over a sanding block to smooth it. The actual part itself must be ready for the layup. The cloth should be cut and ready to apply. The foam should be vacuumed clean of any debris. Temperature and humidity control is important. Begin by heating the shop, if necessary, and ensure the resin is warm (ideally 90 degrees F. or higher). The shop should be cleaned if you have been doing a sanding operation. Control of cleanliness is essential. If you are working on a large surface you may want to have someone to assist you. This is a good way to involve a member of your family. They can mix resins and maintain clean hands to move parts or do other activities that require cleanliness.
If you are bonding parts together you may encounter peel ply that was left in place by the kit manufacturer. Peel ply on a completed part is often difficult to see. You must remove this peel ply material prior to proceeding. The parts will not bond together if done over peel ply. The parts that are supplied with a kit have usually been manufactured in a mold and by the time you receive the part the resin has fully cured. This is important to the builder because the surface of a cured part must be prepared differently for an additional layup or bonding. This type of bond is called a secondary bond. Secondary bonding is the process of bonding together previously cured composite parts using a wet layup process. You should prepare the part according to the instructions provided by the kit manufacturer. This usually involves some type of sanding of the surface to remove any glossy areas. 180 grit sandpaper is often recommended to abrade the surface. Care must be taken to not damage any fibers.
Filling Cells of Foam
If you are doing a layup on a new piece of foam the cells of that foam must be filled to provide enough surface area for the cloth to stay in place and to achieve a strong bond. This also prevents excess resin from flowing into the core material and adding unnecessary weight. Polystyrene foam must be filled prior to application of the first layer of cloth. Some of the high-density foams do not require this filling step. Again, follow the directions of the designer. A slurry mixture of microballoons and resin is generally used to apply this first coat of material. SuperFil may be used very successfully to fill the cells on polystyrene foams. We will discuss the mixing procedure for slurry later in the article.
Cutting the Cloth
This subject was discussed in the previous article. As a quick review, you should use a Sharpie pen to mark cloth. Cut the cloth according to the directions provided by the manufacturer. Usually this will involve cutting on a 45-degree angle. Remember to be very careful with the cloth as you are cutting it and while applying it to the structure. It is easily damaged or distorted.
Now that we have everything ready to go we will mix the resin material. Use only non-waxed cups, usually the 8- or 16-ounce size. Remember that you are only going to mix small quantities. If you do mix any large quantities the resin should be immediately poured into smaller containers. A large amount of resin will create an acceleration of the chemical reaction - hence an exotherm. Exotherm temperatures can easily exceed 200 degrees F. and may actually damage the foam core itself.
The total amount of resin to mix depends upon the weight of the cloth that you are applying. You should try for a 1-to-1 ratio by weight of cloth to resin. In other words, weigh the cloth you are applying and mix a corresponding amount of resin. You will usually mix somewhere between 50-100 grams of resin at a time. If the kit manufacturer states that you should use a resin pump then use that method to mix your resins. Be aware that you should be careful of clogging or air bubbles that sometimes can occur with a pump. Balance scales are also used to mix resins. The important fact to remember is that you must be accurate in your mixing. This is particularly true with epoxy resins. Do not adjust hardeners to change cure rates in epoxies. The cure rate of vinylester resins is easily adjusted during the mixing phase. Again, refer to the directions for the specific resin material.
(I want to clarify a procedure mentioned in last month's article. If you encounter a resin that has crystallized, you can use the following procedure to solve the problem. Put the can of resin in a container that will not melt. Remove the cap of the resin can and place the can in heated water to about 160 degrees for the length of time required to dissolve the crystals. You can then safely use the resin after it has cooled.)
Back to mixing. After you have carefully measured the resin and hardener, mix the two together for a minimum of two minutes. Take a mixing stick and cut the end at a 90-degree angle so it will reach the corners of the mixing cup. You must use a non-waxed mixing cup, otherwise the wax from cups will mix with the resin. Stir the mixture spending about 20% of the time scraping the sides and corners of the cup to ensure adequate mixing. Do not mix too aggressively as air bubbles will form. If any air bubbles form allow the resin to sit until the bubbles dissipate. Placing resin with bubbles in suspension on a layup can create a void of resin in the laminate. After you have completed mixing your resin, leave a small amount in a cup so it can cure. This will provide a good test to see if the resin is curing properly. After a couple of days scratch the resin in the cup with a knife. It should leave a white mark if it is suitably cured.
After the resin is completely mixed pour some of it over the surface you are working on. Use your squeegee and spread the resin over the surface. Then place the reinforcement cloth in place at the proper orientation called for in the plans. Be very careful not to distort the cloth. Use a squeegee and your protected hands to ensure the cloth is in the proper place. Then, using a squeegee begin to press gently from the center of the cloth making sure you move the squeegee in the same directions as the fibers of the cloth. Keep the fibers straight and press the fabric into the resin while working the resin up through the cloth. Be careful not to distort the fibers. You can use a brush and a roller to assist in this process. After you have worked most of the resin through the cloth pour on the remaining resin over the top of the cloth and work it into the fibers. When the layer appears to have a nice even sheen that is flat, you have a good layup. You do not want any air bubbles. Work air bubbles to the edge of the laminate to make them disappear. You can also use a brush that has been trimmed to stipple resin into areas that do not appear to have proper coverage or into problem areas.
If white spots appear in the laminate the cloth has not been properly wet out. A lighter color could also indicate an air bubble. Careful use of an ordinary hair dryer will the viscosity of the resin enough to allow it to flow into certain areas. Do not hold the hair dryer in one place for any length of time - keep it moving. Otherwise, it can create a void if you leave it in one place
When pulling the squeegee, excess resin will accumulate in front of it. Scrape this off into the mixing cup. Pressure applied to the squeegee varies with the type of resin, temperature, etc. Also, holding the squeegee at a 45-degree angle or less will move less resin. Holding it at 90 degrees or more will move more resin. Remember that the clock is running all the time on the working time of the resin. Normally, you will have 30 minutes or so to work until the resin begins to gel. This of course is dependent upon the type of resin, temperature, etc. Practice will make this entire process easy and understandable. Again, do several practice laminates prior to beginning on the actual structure. After doing this you will easily perfect your own technique of doing quality layups.
Inspection of Laminate
The laminate should be thoroughly inspected for air bubbles, any trapped air, excess resin, and of course dry areas or resin starved areas. Hold a light at different angles to observe any problems such as resin starved areas (not enough resin indicated by lighter color) or resin rich areas (too much resin indicated by darker or more glossy areas). When complete the laminate should have a nice even sheen. Have someone else inspect your work. They may see something you have overlooked. Inspect carefully for any delamination problems.