Resins and Materials

This page is devoted to composite fabrication methods and materials. Some of the material in this page is general but important to understanding the limitations of different resins and materials.

All epoxy is not created equal. There are literally thousands of combinations of resins and curing agents available to the fabricator. The fabricator is not only responsible for making the part but also creating the material that the part is made from. There are many resin manufactures that have proven resin and curing agent combinations available. A good understanding of the cured and uncured properties of these resins and curing agents will allow the fabricator to make the right choice when choosing the resin. The choice of resin is dictated by the parameters that you want the resin to operate in.

Bellcrank and mold.

Resin Overview

Polyester is not a structural resin

Polyester is ok for car repair and non structural uses (tanks, bins, bathtubs) Lowest cost, highest production rates (very fast cure rates) Won't work on polystyrene foam (it softens the foam) Heavy, and hard to obtain proper fiber resin ratios due to wet out properties. Good secondary bonds are hard to obtain. Poor bond to Kevlar. Bondo and Featherfill are polyester-based.

Vinylester is similar to polyester but has better structural properties than polyester

Better bond to Kevlar and Carbon fibers than polyester. Gel time can be extended to 10 hours. Mid-cost, still less than epoxy.

Epoxy is the material of choice for structure

Cure cycles pace production work. Health hazards are present. Costly materials. Oven cure varieties are available to allow large lay-ups. Many varieties to match specific structural requirements.

General considerations for all resin systems

Health hazards. Workability. Cost. Chemical resistance (fuel proof). Compatibility with fibers and core materials. Required service temperature and moisture environment. Fabricator must understand the materials they are working with. Clean up issues and hazard waste.

Resin Matrices

Polyester

Cures by polymerization (long parallel molecular chains). Lowest cost resin. Unsuitable for structural lay-ups, low properties. Limited to low temperature applications. Insensitive to mix ratio (amount of catalyst affects cure rate not material strength) High shrinkage (unstable parts/tools, cloth print through) Polyester part will not bond to a epoxy part. Contains styrene; therefore cannot apply over polystyrene foam.

Vinylester

Improved version of polyester resin, better properties, higher cost. Less health risk than epoxy. properties are between polyester and epoxy. Extended pot life to allow larger lay-ups

Epoxy

Most common structural resin, many different varieties available. Cures by cross linking (three dimensional process). Very sensitive to proper mix ratio of resin to hardener. In the small batches that are used in modeling a Triple beam gram scale is needed. Highest cost. Room temperature or oven cure variants available. Absorbs moisture (hydroscopic). Oven cure variants have higher Tg and Heat Distortion Temperatures. Will bond to a polyester part. Multiple health issues. Lowest shrinkage (highest stability). Excellent adhesive properties (good secondary bonds). Face coats available for tooling surface finishing. Laminating and tooling resins available.

Epoxy Resin Specifications

	Mix Ratio: by weight (most accurate) or by volume. Typical ranges 100:44 to as low as 100:5 by weight. When mixing small quantities for model airplanes a triple beam gram scale that measures in 1/10 of a gram is required.
	Mixed viscosity, low viscosities required for laminating resins to ensure proper wet out, high viscosity for tools. A good viscosity for a hand lay-up is around 500 to 800 centipoise.
	Pot Life, 100 gram mass; larger masses cure faster, pot life characteristics limit the maximum laminate thickness possible per cure cycle. Oven cure resins have extended pot lives, allowing larger lay-ups to be accomplished. Most resins for a propeller or similar size part would require only 20 to 25 grams of resin. Mixing more than you need decreases the pot life and wastes resin.
	Pot Life for thin film: More representative of time available to wet out laminate and bag if required. 100 gram mass pot life is representative of a small batch mix according to the resin manufactures. Actual pot lives are even less then specified when thixotropics are used. Thixotropics include micro balloons, flox, Cab-O-Sil and chopped carbon fibers.

Gel Time: Similar to pot life; resin is too thick to wet out fibers once gelled.

Cured Hardness, Shore D: Cured resin can be hardness tested to assure full cure. When making a part it is important to save the left over epoxy in order to test the cure.

	Glass Transition Temperature (Tg): Maximum temperature at which resin properties diminish appreciably, sometimes referred to the resins "red line" temperature. When a cured polymer is heated, vast changes in thermal and mechanical properties occur. These changes are particularly large near the glass transition temperature, Tg. Below the Tg, the polymer is hard and glassy, and above the Tg it has a rubbery state. At this temperature, tensile strength, hardness, electrical properties and chemical resistance depreciate rapidly, while tensile elongation and flexibility increase markedly. Tg usually occurs over a range of temperature, but for simplicity a single temperature is selected as Tg.
	Heat Deflection Temperature (HDT): Temperature at which the resin begins to soften but still has good structural properties. The deflection temperature is commonly used as approximation of Tg. The method for measuring DT has been standardized by ASTM. The DT is determined on a casting which has been permanently stressed at (264 psi) by flexural loading and then heated at a constant rate until the casting deforms a specified amount. The DT method usually requires a larger sample than Tg methods.
	DT's and Tg's provide a measure of crosslink density of the polymer. Those polymers with higher DT's have higher crosslink densities, better performance at elevated temperatures and generally better solvent and chemical resistance. The choice of curing agent and the cure cycle (degree of cure of the polymer) are the largest factors affecting DT. You would want a higher Tg resin on a tuned pipe than on a wheel pant.
	Notch Sensitivity (Izod Impact): A measure of the resin's brittleness. A water ski would require a resin that is a little more flexible than a model airplane propeller blade. A plasticizer can be added to make the resin tougher and less prone to fracture.
	Post Cure: The manufacture's recommended elevated temperature cure cycle to be used to attain the best material properties. Post cures either follow a room temperature cure or an intermediate temperature oven cure for oven cure materials. Free standing post cures are typically successful if a gradual ramp up in temperature is used. High-temp assembly fixtures are required if a free-standing post cure cannot be accomplished.
	Peak exotherm, Fahrenheit: An indication of a resin's likelihood to exotherm uncontrollably. The chance of exotherm can be reduced by limiting mix batches to small quantities, proper disposal of leftover resin, and knowing your resins properties (testing). Exotherm is a term used to describe the internal heat generated by the cross linking of the resin to the hardener. On some resin hardener combinations a 50 gram mass is great enough to melt a plastic cup and become hot enough to burn your skin. Larger quantities create a fire hazard.
	Resin "Physicals" Include: Density, Hardness, Viscosity, Elongation, CTE or coefficient of thermal expansion, Tg, HDT, Pot life, Mix Ratio, Color, Peak Exotherm, Shrinkage, Izod Impact and others.

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Materials

Carbon Fibers

	Carbon fibers, Though known since Thomas Edison's development of the incandescent light in the 1870s, were not made in large quantities until the late 1960s. At that time it was found that carbonizing several fibrous materials resulted in a continuous fiber with relatively low density and high Young's modulus of elasticity. Modulus of elasticity is a parameter indicating a material's stiffness. Young must have been the one who came up with a mathematical way to measure this. High modulus materials are stiffer than low modulus materials.
	If you have ever handled carbon tow in the raw state without sizing it is very soft. Fabricators rely on its tensile strength for the parts rigidity. It is of great importance to keep the fibers in line with the loads being applied to it for the best strength properties. Even the crimp in a carbon tow that allows the tow to be woven into a cloth weakens the material. This is why unidirectional carbon lay-ups tend to be the strongest. While talking to Hiroshi Kiyomoto who fabricated Kaz Minato's carbon wing of his latest Blue Max at last years Nationals he stated that he used unidirectional carbon in the wing. One layer was from root to tip and the other was from the trailing edge to the leading edge. By doing this he could obtain a stronger lighter wing than would have been possible with a carbon cloth. Carbon cloth holds excess resin in between the tows if not compacted with a vacuum bag. Carbon cloth greatly speeds up the production process though.
	While carbon fiber is considered a light weight material in the full size world, it still remains a great challenge to save weight on a model airplane with this material. If you are trying to replace 4 to 6 pound per cubic foot balsa with carbon fiber the results are usually disappointing. The tools to achieve the proper compaction are out of the hobbyist's range. These tools are autoclaves and composite ovens. It's still hard to beat a good piece of 4 1/2 pound balsa for formers, ribs and the like. Carbon fiber excels on the materials normally constructed from hardwoods, metal, plastic etc. or high stress areas such as spars and the like.
	One of the biggest obstacles to overcome when designing a part from composite materials is the natural tendency to copy a part exactly as if it were made from its previous material. The part should be redesigned to take advantage of the formability and strengths of the composite material. A composite part should not be limited to the shape of a metal or plastic part.
	There are three types of carbon fiber, Rayon, Polyacrylonitrile (PAN) and Pitch.
	Rayon precursors, which are derived from cellulosic materials, were one of the earliest precursors used to make carbon fibers. The processing disadvantage was a high weight loss, or low conversion yield to carbon fiber. Typically only 25% of the initial fiber mass remains after carbonization, which means that carbon fiber made from these materials is comparatively more expensive than carbon fibers made from other materials.
	Polyacrylonitrile (PAN) precursors are the basis for the majority of carbon fibers commercially available today. They provide a carbon fiber conversion yield that is 50 to 55%. These precursors can be thermally rearranged before thermal decomposition, which allows them to be oxidized and stabilized before the carbon fiber conversion process, while maintaining the same filamentary configuration. The chemical composition of PAN precursors defines the thermal characteristics that the material displays throughout the oxidation/stabilization portion of the conversion process. These thermal characteristics influence the processing sequences that are used to convert PAN precursors to carbon fiber. Carbon fiber based on a PAN precursor generally has a higher tensile strength than a fiber based on any other precursor. This is due to a lack of surface defects, which act as stress concentrators and, hence, reduce tensile strength. Carbon propellers and bellcranks that are made by Winship Models utilize PAN based carbon fiber.
	Pitch precursors based on petroleum asphalt, coal tar, and polyvinyl chloride can also be used to produce carbon fiber. Pitches are relatively low in cost and high in carbon yield. Their most significant drawback is nonuniformity from batch to batch.

Glass Fibers

	There are literally thousands of fiber glass fabrics and tows available to the fabricator. For modeling use the varieties of fabrics under 2 ounces per square yard are the most common.
	E- Glass is the most common (and least expensive) grade of glass fiber.
	S -Glass is a special grade of glass that is much stiffer than E-glass and somewhat stronger.
	The use of fiber glass on a stunt ship is usually limited to the center section of a foam wing or around the nose section, either on the outside or for reinforcement around the nose formers. If using fiber glass to strengthen the center section of a foam wing keep the glass fibers running at a 45 degree angle to the center wing joint. This will allow both the warp and the fill fibers of the cloth to span the wing joint giving double the strength with no weight penalty. The warp direction is the direction of the long fibers as the cloth is pulled off of the roll. The fill fibers are the fibers that run side to side as the cloth is pulled off of the roll.
	Almost all glass cloth has sizing applied to it to aid in resin wet out and adhesion. Some sizings are for polyester resins and some are for epoxy resins. If the glass cloth or tow has the wrong sizing for the type of resin used, then the bond will be weak between the resin and cloth or tow. While this might not be of great concern on a pair of wheel pants it might prove to a problem if a speed prop was constructed without the proper sizing applied to the tow. Usually a fiber glass that is not properly wet out will have silver of white streaks or spots in the laminate.

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