Newer surfacing films reduce surface on demolded composite parts
A wide variety of products reduces finishing costs in many applications.
By Susan S. Daggett Ph.D. | May 2004
We may never know for sure who first used surfacing films for composites. Early forms were simply structural film adhesives or non-structural prepregs, laid against the mold to cover the primary prepreg, since the resins used in structural prepregs often do not provide a sufficiently blemish-free surface. The extra layer provided a smoother surface than the prepreg alone and allowed for sanding and polishing and without cutting into the prepreg itself. While resin-rich, adhesive-based surfacing films, such as Hysol EA 9695, Cytec FM 300 and Cytec MB 1515, are still viable in many applications, a number of materials have been developed in recent years, designed specifically for surfacing. Several eliminate most, if not all, of the time, labor and materials required for the sand/fill/sand cycle.
Source: Raytheon
Surfacing films significantly improve productivity in the manufacturing of aircraft such as this from Raytheon Co.
A better look with less labor
In composite applications ranging from aerospace to automotive to sporting goods, aesthetics are critical and the smallest surface irregularities may be cause for part rejection. The most common imperfections are pinholes, caused by tiny pockets of gas released during the curing process. Some products may look glassy and smooth right out of the mold, but when scuff-sanded to enhance primer adhesion, pinholes appear. Other surface imperfections include print-through from honeycomb core and fabric weave patterns. Older molds may contain surface damage that is mirrored on the surface of the part, which needs to be smoothed out after cure, as well.
The traditional fix for these part surface blemishes is to go through a sand/fill/sand cycle, which sometimes must be repeated several times before the surface is blemish-free and ready to prime. The part sits idle while the filler cures, then waits for the next available technician for another sanding cycle. Moreover, the parts take up valuable space as well as labor, reducing overall production capacity. Surfacing films are designed to yield a blemish-free, ready-to-prime surface straight from the mold, with minimal surface prep, (e.g., only a quick clean to remove mold release agents).
Surfacing films need not have the mechanical properties of structural adhesives, which frees formulators to concentrate on the characteristics that are required for surfacing. Ray Cornwell, technical representative for SIA Adhesives Inc. (a division of Sovereign Specialty Chemicals Inc., Akron, Ohio, U.S.A.) explains, "In structural film adhesives the properties emphasized are peel, shear, and flatwise tension strength. With surfacing films, the emphasis is on gel, flow, drape, cyclic durability and paintability - things not typically looked at for a bonding film." Products also must pass paint adhesion tests and are sometimes tested for long-term microcracking resistance.
Generally, surfacing films weigh in between 0.15 to 0.17 kg/m² (0.030 to 0.035 lb/ft²), though heavier materials are available for some applications. Surfacing films tend to be ultra-low in volatiles, to minimize the off-gassing that causes pinholes. Often one-side tacky and repositionable for ease of use in a mold, the films possess flow-to-fill surface features on the prepreg side and are able to replicate joggles and other features on the mold side. Most work best in autoclave cures, but some are suitable for vacuum-bag processing. Some are compatible with a variety of resins, including epoxy, phenolic and polyester.
Most are available plain or with an embedded conductive metal mesh. Thin copper or aluminum wire meshes are used by some aircraft manufacturers for lightning strike protection. Historically, a layer of film adhesive was applied to the mold, and then the conductive mesh was applied and overlaid with structural prepreg layers. But the metal meshes are very thin and tear easily, so layup was difficult and time consuming. Most users of this scheme for lightning strike protection now purchase surfacing films with pre-embedded mesh, significantly reducing layup labor.
Highly filled, hard surface films
When Loctite Aerospace (Bay Point, Calif., U.S.A.) began work on a surfacing film in the early 1990s, most of the surfacing films on the market were resin-rich adhesives. While these products worked well in many applications, there were deficiencies when it came to thin-skinned honeycomb. When there are only a few plies of fiber over honeycomb core, the cured surface of bare prepreg tends to cure with a dimpled pattern. Surfacing films provide a layer that replicates the tool surface, however, core nodes tended to poke through, sometimes even cutting through prepreg, resulting in uneven thickness in adhesive-based surfacing films. Under repeated paint stripping operations over the life of an aircraft these thin areas would wear through, exposing the underlying prepreg.
Loctite's approach was to create a highly filled, relatively low-resin surfacing film that would replicate the tool surface with a constant thickness, resisting core dimpling. A further objective was to create something that resisted wear during paint stripping processes. The resulting product, SynSkin (short for "synthetic skin"), has a ready-to-prime surface requiring only a light scuff sand and solvent wipe to remove contaminants such as mold release agents. In fact, the material protects composites from all typical forms of stripping/sanding operations with the exception of wet sanding, which can be used to smooth out irregularities that transfer from a less-than-perfect mold surface.
Loctite Aerospace Lightning Strike Products offers SynSkin with embedded copper or aluminum conductive mesh. The rigidity of SynSkin has the effect of holding conductive meshes in position close enough to the surface to be effective, but protected by the SynSkin from being worn away by repetitive paint stripping.
Loctite was able to minimize gross porosity with SynSkin, though occasional pinpoint porosity may occur. Reportedly, a wipe-on/wipe-off product such as Loctite's 28C1 Static Conditioner can be used to fill remaining pinholes.
SynSkin's grips both the tool and the prepreg and its rigidity minimizes the "slip plane" that can lead to core crushing in steeply ramped areas.
Flight cycles can create microcracking in the surfacing film and overlying paint. Consultant Arthur D. Little (Cambridge, Mass., U.S.A.) helped Loctite evaluate the effects of load and thermal cycling on the entire laminate, including primer and paint coatings. SynSkin was found to resist microcracking better than adhesive-type surfacing films. The reason for this is not well understood, though current theories connect the effect variously to coefficient of thermal expansion (CTE), the type of paint primer used and/or the low porosity in the cured surfacing film.
SynSkin is a dual-cure (121°C/ 250°F and 177°C/350°F) epoxy-based material and can be out of freezer at room temperature for up to 21 days. Its cure cycle's ramp rate and dwell time are consistent with most aerospace-grade prepregs. While the current product is opaque grey, black and off-white versions are now being evaluated. The off-white is intended to eliminate the need for paint in dark areas such as the insides of baggage compartments, although the material has not been tested for burn resistance.
There are a few special handling procedures for SynSkin. The film replicates the mold surface exactly and is difficult to sand, so it is important that molds be free of resin buildup and surface imperfections. Technicians must learn several techniques for successful backing paper removal. Finally, Loctite advises use of a vacuum bag debulk cycle after the material is applied to a mold, to remove air trapped between SynSkin and the tool, and to smooth out wrinkles in contoured areas.
