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Builders guide to safe aircraft materials

Plastics and thermosets

Rev. 4 — page content was last changed 23 January 2010
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16.1 Polymers
A polymer (from the Greek words for 'many' and 'share') is a very large, strong molecule composed of many small molecules chain-bonded together in a chemical reaction process. Monomers are the small, light molecules capable of polymerisation; vinyl chloride and styrene, for example. When polymerised, these produce polyvinylchloride and polystyrene, respectively. A copolymer is a polymer made using two or more different monomers, acrylonitrile-butadiene-styrene [ABS] for example — although that is more correctly a 'terpolymer'. Polymers are often referred to as 'synthetic resins' (or 'resins') and can be classified as 'thermoplastics' (or 'plastics') and 'thermosetting' (or 'thermosets'), according to the effect of heat on their properties. Polymers are used extensively in homebuilt aircraft — as materials incorporated in primary and secondary structures, as fittings, as structural adhesives, and as surface sealers/primers and finishes. To learn more about the chemistry involved, and some of the terms appearing in the following text, visit 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.
16.2 Thermoplastic polymeric materials
Thermoplastics are generally solid materials at room temperature. Like metals, they can be heated (thermo) and shaped (plastic) then harden again when cooled. The heating–shaping–cooling process can usually be repeated as often as required without significant detrimental effect on the strength of the basic material. The chain-growth molecules are set up as linear or branched chains. Some thermoplastics, nylon for example, can be reinforced with chopped fibreglass and similar fibres.

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:
  • bent at room temperature without overstressing as long as the bend radius is greater than (roughly) 180 times the sheet thickness
  • shaped without stressing if heated and subsequently annealed
  • chemically bonded with methylene chloride solvent — and heat after the cement has dried — to produce a seamless (though slightly weakened) joint
  • buffed to remove scratches

PMMA acrylic also provides some thermal insulation in an enclosed cockpit because it is a poor heat conductor.

Polycarbon windscreen and acrylic canopy A completed acrylic cockpit canopy together with a polycarbonate (Hyzod/Makrolon) windscreen with the protective paper still attached. Note the Cleco clamps that hold through-drilled surfaces in place prior to riveting.

For more detail on the fabrication of this canopy see Lynn Jarvis's Sonex construction log.

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).

Windscreen, dashboard and instrument panel

Fitting the polycarbonate windscreen and cabin roof to Keith Manwaring's SkyRanger.

"The one piece 1.5 mm thick windscreen and cabin roof was marked out and cut from a roll of GE Lexan in accordance with a diagram in the manual — which indicated that three people were required to fit and rivet in place. I preferred to hold in place with alligator clips and secure the windscreen to the fibreglass cowling with small bolts instead of rivets. And, by the way, Loctite and polycarbonate are not compatible."

16.3 Core materials
Core materials are used in airframe three-layer sandwich structures consisting of a low-density core and two thin, high-density outer layers. The core material bonds the high stress load-carrying outer layers together so that the tension and compression forces are properly distributed, while the core carries the lesser shear loads. Such structures achieve high rigidity from the increased thickness and very efficient strength-to-weight ratios. Each outer layer could consist of more than one 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.
16.4 Thermosetting polymeric materials
Thermosetting compounds used in homebuilt aircraft structures are two-part formulations — a liquid (commonly vinyl ester, polyester or an epoxy) and a hardener or a catalyst (initiator) — finally mixed at room temperature by the end user. The catalyst initiates an irreversible chemical reaction — curing. When concluded, curing converts the two components of the liquid mix into another material — a solid polymer matrix of long-chain molecules where chains are cross-linked to adjoining chains in three-dimensional molecular bonds. Such molecular structures are substantially different from the linear or branched chains of thermoplastics. The initial curing process is exothermic; i.e. the heat released as the cross-links are established extends the curing process and thus increases the temperature. So if too much material is mixed in a batch, excessive heat generation may be a problem. The cured matrix can be machined, but it cannot be softened and reshaped. Such thermosets are used as a component of 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.
16.5 Post-curing
The thermosetting resin systems generally used by homebuilders are types that complete a substantial part of the cross-linking at room temperatures during the initial curing cycle. However, the cross-linking must be further extended throughout the matrix by post-curing — heat-soaking at higher temperatures for a particular time; i.e. a long period at a lower temperature, or a shorter period at a higher temperature. No part of the structure should be under load during post-cure; i.e. its weight should be supported. If post-curing of room-temperature systems is not done then all the possible links will not be established, and the matrix will not achieve its optimum mechanical properties and chemical resistance. The post-curing can be done in any rudimentary heating chamber constructed for the purpose using aluminium foil-backed building insulation material. In warmer climates, it can be done in the open air utilising solar radiation or in the roof space of the house/workshop. Care must be taken in temperature control particularly in avoiding thermal shock and ensuring that the breakdown temperature of any core material is not exceeded. (The temperature should be brought up reasonably slowly, maintained around the required level for the required time then brought down slowly.) Post-curing is usually done at least two weeks after initial curing and after the initial surface sanding and filling is completed. Priming and painting is carried out after post-curing.

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):

Post-curing time and temperatureTg achieved
24 hours at 25° C55° C
10 hours at 40° C75° C
10 hours at 50° C85° C
10 hours at 60° C100° C
8 hours at 80° C120° C

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.
16.6 Fillers
A number of filler materials of varying strength can be combined with resins for applications. These include creating hardpoints for attachment of other elements to a composite structure, forming internal corner radius fillets for subsequent application of structural reinforcing, sealing the surface of foam cores before building the laminate, joining foam blocks together, gap filling in various materials, used as a potting compound or filling dents, scrapes, hollows around countersunk rivets, in foam cores, cured composite surfaces, wood, metals or other materials. However, fillers are just that — they are not structural materials.

  • Chopped fibres are 5–10 mm long and mixed with resin to form very strong, but heavy, supporting fillets or hardpoints.
  • Milled or crushed fibres are very short lengths of fibreglass that, when mixed with resin, are used as a filler material; or they are perhaps used instead of chopped fibre in applications where lesser strength is needed.
  • Cotton flock or milled cotton fibres increase resin viscosity and strength, and are generally used in 2:1 filler/resin ratio as internal corner reinforcement. These are not as strong as milled fibreglass.
  • Hollow glass or phenolic microballoons in powder form — the glass looks like castor sugar and the phenolic is reddish — increase the viscosity of the resin while reducing density, thus providing a high strength-to-weight ratio and good sandability of the cured filler. 'Micro' naturally disperses throughout the mixture, so the edges and internal corners of the form tend to be stronger. Microballoons are used in various micro:resin mix ratios from 1:1 to perhaps 5:1. A 1:1 wet mix is used as foam core surface sealing slurry, a thicker 2:1 or 3:1 mix is used as filler or to bond non-porous materials while a 5:1 micro:resin dry mix is used as an adhesive when joining foam blocks or as the surface weave filler for glass-epoxy laminate. Q(uartz)-cells are similar in size to microballoons; microspheres are solid glass and probably no longer obtainable.
Preparing the cowling Extract from Lynn Jarvis's Sonex construction log.

Preparing the cowling

In order to get the shiny metallic finish I wanted on the polyester gel-coated fibreglass cowls, I spent hours filling with a West Systems microballoon:epoxy resin in a 2:1 filler mix then sanding, repeating again and again until the surface was perfect. Here you can see the filling of the rivets around the NACA ducts. I am sure you will like the white spots on the photo. The micro filler got everywhere, including the camera lens.

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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