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Builders guide to safe aircraft materials
Plastics and thermosets
Rev. 4 — page content was last changed 23 January 2010
Macrogalleria or the Connecticut plastics learning center.
Polymers have a unique property called the glass transition temperature [Tg], the value of which varies with the polymer and its chemical condition. Tg is not the melting point but a transition temperature range below which the material is rigid, and to some extent, brittle. Above Tg polymers start to become softer and flexible, and able to be shaped without stressing the material. Some polymers have a Tg that is below normal room temperatures, so their normal useful condition is above the Tg when they are soft and flexible. Aircraft structural materials are designed for use at temperatures below the Tg. Such materials are significantly weakened if temperatures within the airframe are allowed to build up to that material's Tg; see thermosetting polymeric materials.
The thermoplastics commonly found in homebuilt airframes and coatings are the following:
Acrylic plastics or acrylics are any polymer or copolymer of acrylic acid or variants thereof. Plexiglas (one 's'), Perspex, Acrylite and Lucite are trade names for polymethyl methacrylate [PMMA] products. They are highly transparent to tinted or opaque, strong, light, and tough — a 30 mm thick sheet is bullet-resistant. PMMA can be sheared, sawn, turned, routed, milled and drilled if appropriate or modified tools or machine-tools are used and the material is prevented from overheating by air or water cooling. But if PMMAs are exposed to a direct flame they will melt and eventually burn. PMMAs have glass transition temperature ranges around 100° C.
Acrylic sheet may be purchased in an un-shrunk or a pre-shrunk condition. Un-shrunk sheet will shrink about 2% in the planar dimensions and expand in thickness during any subsequent heat-forming operation. The primary use for PMMA acrylics in aircraft is in canopies, windshields, windows and light covers because of its clarity and its abilities to be:
PMMA acrylic also provides some thermal insulation in an enclosed cockpit because it is a poor heat conductor.
Polycarbonates: Lexan, Makrolon/Hyzod and Tuffak are trade names for polycarbonate products widely used in sheet form for ultralight aircraft canopies, windshields and doors. Polycarbonate properties are similar to the PMMA acrylics but have very much greater impact strength to withstand bird-strike and are easier to bend at room temperature; the minimum bend radius is reduced to 100 times the sheet thickness. Polycarbonates will craze if exposed to fuel, some solvents used in paints and primers, and some alcohols used in cleaners and engine coolants; the crazing will lead to stress cracking. For more information on forming techniques, etc. see the fabrication guide for Makrolon polycarbonate sheet (375 KB pdf file).
laminated ply. Core material could be sheets of end-cut (sliced) balsa, Nomex, aluminium or other 'honeycombed' materials, or an expanded foam thermoplastic; the latter is the material most commonly used in homebuilt aircraft because it is easy to shape, very easy to bond to the outer layers and reasonably low cost.
Expanded foam materials are produced when voids created in the solid material by a chemical 'blowing' agent are filled with air. The easy-to-work foam blocks and sheets — in thicknesses starting at a few millimetres — provide low thermal conductivity and light weight, perhaps 10–20 kg/m³. Variations of the sandwich structure have application in many airframe parts — for example ribs, bulkheads, floors, fuselage skins, wing skins and even complete wings.
Polystyrene foams (expanded polystyrene) are low cost, available in many densities and thicknesses, usually blue in colour, and easy to work. Aerofoil section core forms are readily created when blocks of expanded polystyrene foam are cut with a shaped, electrically heated nichrome or stainless steel wire that vaporizes the foam without actually touching it — thus providing a very smooth cut surface. The shapes created are used as core material in control surfaces and wings where aerodynamic loads are transferred from the outer skins to a spar or some other internal structure. The core material also prevents the flexing of the outer skins under changing bending loads; such compressive buckling is generally not a particular problem with metal skins (where it is known as oil-canning) but can be with composites.
Polystyrene foam is quite compatible with epoxy resin but not with polyester or vinyl ester resins. Exposure to leaking fuel will dissolve it, so it should not be used in the vicinity of fuel tanks and lines.
Polyurethane foams are compatible with epoxy, polyester and vinyl ester resins, and are resistant to fuel (including those containing ethanol) and other solvents. But the vapour generated from hot-wire cutting of polyurethane foam contains a form of cyanide so it must be cut or shaped with a saw, file, knife or an abrasive means such as those normally used in shaping wood. The dust from sawing or sanding is hazardous. Usual colours are green or light brown.
There are other expanded foam compounds (e.g. polyvinyl chloride, polymethacrylamide, polyetherimide) available in a range of densities. Polyvinyl chloride [PVC] foam is difficult to cut, though it can be used with polyester resins and is suitable for mouldless structure core material. The other foams mentioned are high performance but perhaps over-expensive for homebuilt light aircraft applications and shaping may be hazardous. composite structures.
Manufacturers control various properties of their thermoset products by formulating the resin and the hardener/catalyst packages to include retarders, inhibitors, accelerators, adhesion promoters and plasticisers. These may adjust the viscosity, the time available between end-user mixing and initial curing (the pot life), the working time available, the cure temperature, the toughness, corrosion resistance and so on. The formulation combinations possibly run into tens of thousands. It is most important that users do not attempt to mix and match formulations or associated materials.
Many (most?) resin formulations are combustible and require fire-retardant additives in the formulation if the finished product is to be used in locations where materials must be flame resistant; engine cowlings and surfaces in proximity to the engine exhausts, for example. Even with the fire-retardant included, overheated and fully cured thermosets may char, emitting a heavy toxic smoke — so care must be taken to prevent leakage into the cockpit from the engine compartment.
The principal thermosets used in aircraft construction are:
Polyester resins provide moderate strength, are easily mixed (with inaccuracies in the amount of catalyst added not affecting the strength developed, only the time to achieve that strength) and cure at room temperatures. Polyester resins are lower cost and the most common resins used in industrial applications. But a high shrinkage (perhaps 8% by volume) during cure must be allowed for. Although most of the shrinkage occurs during the initial cure period of 24 hours or so, some additional shrinkage will continue during the full cure period of perhaps 60 to 90 days. This has a detrimental effect on the surface finish; in laminations it is called 'print through' — the fabric weave shows through. Polyester resins are not recommended for aircraft structural use or for use as a filler because of the possibility of cracking as shrinkage occurs. Polyester resin only adheres well to itself.
Polyesters (and vinyl ester resins) emit styrene (a solvent) during cure and are thus not compatible with polystyrene foam cores; also the emissions are evil-smelling and noxious. The initiator generally used is methyl-ethyl-ketone-peroxide [MEKP], which is very dangerous to the eyes.
Vinyl ester resins are more expensive than polyester but cheaper than epoxy, easy to use and the cure time can be reduced by adding more catalyst. They have better mechanical and corrosion resistance properties than polyester though still a high cure shrinkage — 5–10% greater than epoxy. However, if the manufacturer has not already done so, it may be necessary to add a 'promoting' agent to the resin supplied. The promoting agent is cobalt napthenate [CONAP] and a hazardous situation could develop if CONAP is inadvertently mixed with the vinyl ester catalyst MEKP outside the resin. CONAP must be added to the resin before the catalyst. The ratio of catalyst to resin is very low, perhaps 1:750 by volume.
Vinyl esters will not bond satisfactorily to cured epoxies, but epoxies will bond to fully cured vinyl esters.
The term epoxy resins covers a wide range of complex cross-linked polymers with a corresponding range of mechanical properties and of applications. The room-temperature curing varieties (as opposed to oven-temperature curing varieties) can be post-cured at higher temperature, have low shrinkage (2–3%0, good chemical resistance, excellent adhesive and hand-laminating properties, provide a better matrix in terms of strength, and are generally the recommended resins for homebuilders. Downsides are the higher cost, a need for an accurate resin/hardener ratio (epoxies use a hardener not a catalyst, and the ratio will vary) when mixing and the need for skin protection when using the resins. Epoxies have many applications but the homebuilder is generally concerned with their use as laminating resins in composites, as structural adhesives for bonding composite components, wood to wood, metal to metal, fabric to metals or wood, and surface coatings to substrates. The formulation of, and the additives to, available epoxy and other resins will vary according to the particular user need that the manufacturer/supplier perceives. One advantage that epoxy resins offer to the homebuilder is that the hardeners available allow a wide range of choice in work times.
Protective measures must be taken before mixing and using any of the thermosetting resins. A material safety data sheet [MSDS] should be available for every formulation and the procedures and protective measures outlined in the MSDS should be complied with. Apart from minimising fire risk, the personal protective measures include skin, eye and breathing protection.
Thus there are three stage points in the resin curing process: the A stage where the resin parts have been mixed but are still liquid; the semi-cured B stage where the matrix is stiff but soft and tacky to touch; and the fully cured C stage where the matrix is a solid. Cured epoxy is a honey colour.
Also the heat resistance or glass transition temperature is dependent on the level of curing; Tg is at its lowest following initial cure and will rise with post-curing. There is another temperature value, the heat deflection temperature [HDT], which may be stated by a manufacturer in lieu of Tg. HDT is the temperature at which a specified test piece of the material, subject to a specified load, softens enough to allow deflection through a specified distance; it is also called the 'deflection temperature under load' [DTUL]. HDT will be a few degrees (around 5–10° C) less than Tg and possibly a more relevant means of comparing epoxy resins. Here are some post-curing and Tg figures for one room-temperature curing epoxy system (others will differ):
The temperatures that can develop within an airframe structure when parked and exposed to solar radiation are very high; see the table in the module 'Surface coatings and finishes'. Obviously post-curing is necessary; it can be seen that even with a white finish (the norm for all composite aircraft) the 68° C temperature reached is well above the initial room-temperature curing Tg.
Heat build-up will also damage foam cores
Note that unless a UV inhibitor has been included in the formulation, such as in polyester resin gel coatings, the cured resins are UV-sensitive and will eventually weaken with exposure to sunlight; the resins will need protection with a UV blocking surface coating.
Fully-cured thermoset matrices are medium weight, have reasonable rigidity and dimensional stability, high compressive strength, high thermal stability and good thermal and electrical insulating properties, but comparatively low tensile strength. Thus reinforcing fibres must be used with the matrix material to produce useful composite structural materials.
There are other fillers/epoxy thickeners commonly used, the colloidal (fumed) silica-based epoxy thickener products are probably best known by trade names — CAB-O-SIL for example. Poly-Fiber's SuperFil is a light, non-shrinking, epoxy-based fully prepared mix, light blue in colour, applied with a paint scraper or squeegee and in common use to fill surface indentations and imperfections on wood, aluminium or composite structures. With all fillers, check that they are compatible with the material being worked, and with the primers and top coats to be used. Be aware that most filler materials are readily inhaled, thus hazardous to the lungs; respiratory and eye protection should be worn, particularly when sanding.
Note: if an excessive amount of an exothermic compound is used to bond foam blocks together, the trapped heat will lead to voids within the foam near the joint.
The next module in this fabrics, composites and coatings group is 'Reinforcing fibres and composites'
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