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Isovolta (Wiener Neudorf, Austria) is reported to be one of the world’s largest producers of laminates for aircraft cabin interiors and the largest prepreg supplier for interiors worldwide. The company has more than 1,500 employees across 16 locations on three continents and expertise in the fields of impregnation, laminating, compression molding, machining, polymer chemistry and process technology.

With decades of experience in high-performance epoxy and phenolic resins able to meet stringent fire, smoke and toxicity (FST) requirements for aircraft cabin/cockpit/cargo applications, Isovolta saw an evolving need for materials that could perform at even higher temperatures. “Most traditional aerospace thermoset resins fail below 300°C,” explains Peter Wagner, vice president technology for Isovolta. “With the growth in electric vehicles [EV] and new mobility and space applications, we could see a need for materials that could withstand up to 900°C, but without the high cost of traditional ceramic matrix composites [CMC], which are more difficult and time consuming to manufacture.”

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Isovolta developed CERAPREG as an alternative, a combination of silica fibers with a silica-aluminum oxide (alumina) matrix, enabling CMC parts that reduce weight compared to metal but are less expensive than Nextel (3M, Minneapolis, Minn., U.S.) or even more costly silicon carbide (SiC) fibers. “The mechanical properties and temperature resistance of parts made with CERAPREG are not as high as SiC/SiC or carbon/carbon CMC,” says Wagner, “but the material provides good performance in structures that need to operate consistently in harsh, high-temperature environments such as exhaust components or battery compartments for drones and automotive EVs.”

Silica versus SiC and quartz fiber

Silica fibers are primarily composed of silicon dioxide (SiO₂) while SiC fibers are a compound of silicon (Si) and carbon (C). The latter require lengthy processes for manufacture and are more expensive than silica fibers, which are easier to produce, typically being made using melt spinning similar to glass fiber production, says Wagner.

The silica fiber Isovolta uses in CERAPREG is also not quartz, but because the purity of the fiber is >95% silica, they have similarities. “The fibers we’re using actually have dielectric properties below 3.0 when tested with a frequency of 76.5 gigahertz,” notes Wagner. “Tests conducted with our Radom Epoxy System combined with silica fibers show similar results, and we are validating both combinations for use in radomes.”

He adds that although silica fiber can withstand a one-time exposure of up to 1600°C, it will start to degrade above 950°C. CERAPREG has been developed for long exposure at temperatures up to 900°C.

Ox-Ox system

While SiC/SiC materials operate at higher temperatures (for example, 1200-1600°C), alumina (aluminum oxide, Al₂O₃) fibers in an alumina matrix — referred to as Ox-Ox — commonly operate at up to 1200°C, but without the risk of degradation due to oxidation that SiC/SiC and carbon fiber/carbon matrix (C/C) materials can face.

CERAPREG is also an Ox-Ox system — producing oxide CMC (OCMC) — but one using silicon dioxide fibers with a silica-alumina matrix. “We have designed CERAPREG to offer a mix of properties,” says Wagner. “This includes long-time performance at 900°C with a more affordable cost, easier handling and parts manufacturing, as well as radar transparency. However, if long-duration service is needed at higher temperatures, then I would look at the traditional alumina/alumina CMC. There are new companies making alumina fibers, so availability and price should come down. We too have tested and produced alumina Ox-Ox CMC.”

Wagner describes the parts shown at right, which include a mixer structure for an aerospace exhaust system, a curved sandwich structure made with Eco ceramic honeycomb from Euro-Composites (Echternach, Luxembourg) and various CMC tubes. “These are test parts, but all have shown CERAPREG’s capability for complex shapes as well as good mechanical performance.”

The why and how of Ox-Ox prepreg

Why sell prepreg? “Because it enables companies to make parts more quickly,” says Wagner. “You can buy oxide fibers and fabrics, but you still need to have some kind of matrix to make the CMC, and there aren’t many companies that know how to do this. Prepreg makes it easier to proceed with making parts, but there are only a few companies that might sell such material. These companies also typically want to make the CMC parts, while Isovolta is purely selling materials. We tell our customers how to process the prepreg and provide training on how to work with the material and make simple parts, but they don’t have to share any details about what they will be producing or how.

“We’ve designed the prepreg to be nontoxic and easy to handle with no special equipment required except an oven,” he continues. (Parts can also be made using a press as shown at right.) “Also, staff familiar with polymer composite prepreg should be able to use it without special training, but we do provide as much training as required — either at our facilities or at customer locations.

“Another issue is that if such prepregs are solvent-based, it can lower their shelf life,” says Wagner. “Our prepreg is water-based — we do have a polymer binder, but it’s less than 5% of the material. This offers an extended shelf life — which is currently 6 months but could be increased — and no additional extraction equipment or measures are needed to deal with volatile organic compounds.”

The material starts as a water-based ceramic slurry. “We then combine that with the silica fiber fabric and use a short thermal processing cycle to create the prepreg,” says Wagner. “Usually, you have a horizontal or vertical prepreg line, but with ceramics, and because it's a water-based slurry, we actually don’t want to dry it. Instead, we want to keep it as moist as possible to ensure it doesn’t dry out during transportation to the customer but remains moldable and formable. To achieve this, we modified the production line to ensure that the necessary water content and mechanical properties are maintained. Then we package it in a form that facilitates the customer processing it.” He notes CERAPREG should be stored at 5-20°C but must not be frozen.

Performance, parts, future developments

CERAPREG is currently available in two standard products, one with a fabric weight of 600 grams/square meter (gsm) and thickness of 0.7-0.8 millimeters and the other with a fabric weight of 300 gsm and thickness <0.6 millimeter. Their resin density, thermal and dielectric properties are the same as shown below. “For both materials, we achieve a tensile strength of at least 40 megapascals,” says Wagner, “but when tested in the direction of the fiber, we achieve more than 60 megapascals with the 300 gsm material and have one parts manufacturer who has achieved 75 megapascals.”

Although maximum temperature for continuous service is listed as 900°C, Wagner says Isovolta has tested CERAPREG parts to 2000°C. “We are continuing to work with the Fraunhofer Institute and other institutes in Austria to fully characterize the mechanical properties of the material at 700-1000°C,” he explains, “and we are sharing that data with our customers.”

The maximum temperature that CERAPREG parts can withstand depends on the combination of time and loading, says Wagner. “Do you have very little load and just want to have a thermal barrier? Then you can increase the temperature. For more structural parts, the maximum temperature also depends on the type of load that must be sustained, for example, cyclic or vibration or shock loading. You can model these, but you also have to build and test the CMC parts.”

Isovolta is not just testing materials, but also CERAPREG parts, for example, acting as thermal barriers for automotive EV applications. “We’re also working with eVTOLs, because in both cases, they need not just temperature resistance but also light weight and strength,” notes Wagner. “We are doing tests where we simulate a thermal runaway where metal powder from the battery is ejected as high-temperature projectiles. And in this case, even if you have very good insulation against the heat and flame, these projectiles can impact the material and create holes which then disable the insulation.”

Isovolta has also completed flame testing of hybrid parts, for example, where a 1-millimeter-thick carbon fiber-reinforced polymer (CFRP) laminate was bonded to a 2-millimeter-thick CERAPREG laminate. “We tested this at 1200°C and only reached 600°C on the backside after 5 minutes, but with no visible damage,” says Wagner. “The CERAPREG boosts the CFRP thermal performance, but if you want to increase the mechanical properties of CERAPREG, it's easy to bond it to cyanate ester or epoxy resin CFRP.” Isovolta has also tested aerogels and mica laminates with 1- and 3-millimeter-thick CERAPREG laminates.

Isovolta sees many possibilities with CERAPREG and many ways to take advantage of its CMC properties, says Wagner. “We are also continuing to explore new manufacturing options, including hot-in, hot-out pressing.”