The Aircraft Interior Dilemma: Phenolic vs. Epoxy Prepregs

There is no such thing as a perfect manufacturing material for aerospace engineering. But there are best-choice materials for specific applications. When it comes to designing aircraft interiors, engineers face a fundamental choice between phenolic and epoxy prepregs. Which one is better with the design currently on the table?

To be clear, the main difference between phenolic and epoxy prepregs is their respective resin systems. Epoxy is currently the dominant system for aircraft exterior structures. But for interior structures, the requirements are different. Interior components need less tensile strength but more resistance to fire, smoke, and toxicity (FST).

Our customers benefit from the fact that Axiom Materials specializes in both chemistries. We understand the nuances between phenolic prepregs and their epoxy counterparts in everything from safety to durability and overall weight.

Phenolic Prepregs: The Basics

Phenolic resins are among the oldest synthetic polymers on the market. They are created through a chemical reaction between phenol and formaldehyde. The resins can be utilized to impregnate a carbon fiber or glass fiber fabric at the factory, creating a ready-to-use material. That material is known as a phenolic prepreg.

In aviation, phenolic prepregs are desirable because of their FST properties. Those properties begin with the phenolic resin’s inherent flame resistance. In the event of an onboard fire, components made with phenolic prepregs:

• Self-Extinguish – Once the source of flame has been eliminated, combustion immediately ceases. This is because phenolic resins do not support combustion.
• Produce Less Smoke – Phenolic resins produce less smoke than almost any other polymer. The amount is significantly less compared to epoxy resins.
• Produce Minimal Toxicity – During combustion, phenolic resins produce fewer lethal gases compared to epoxies and polyurethanes.

These three properties make phenolic prepregs the industry standard for complying with FAA/EASA heat release regulations. That is why they are used so frequently in designing and manufacturing aircraft cabins.

Epoxy Prepregs: The Basics

We like to refer to epoxy resins as aerospace industry workhorses. The resins are formed by combining an epoxide with some sort of curing agent. The reaction between the two creates desirable mechanical performance and versatility properties.

Epoxy prepregs are manufactured by impregnating a woven fiber fabric (usually carbon or glass fiber) with an epoxy resin. Just as with phenolic prepregs, the impregnated fabrics are ready to use in layups out of the box. The aviation industry appreciates the following three characteristics:

• High Strength-to-Weight – Because epoxies offer superior fiber adhesion, they create materials with a high strength-to-weight ratio. Epoxy prepregs offer impressive structural properties.
• Better Surface Quality – Epoxy prepregs can be cured with a much smoother surface. That translates into less post-processing prior to finishing a part.
• Increased Toughness – Epoxy resins tend to be less brittle than their phenolic counterparts, making them more resistant to impact damage.

Based on the key characteristics of both types of prepregs, it is easy to understand why choosing between the two can be difficult. An aircraft cabin needs to be as safe as possible from the FST perspective. Still, the materials needed to construct interior components must be strong and durable.

Phenolic vs. Epoxy: A Direct Comparison

Making a choice between epoxy and phenolic prepregs often boils down to making a direct comparison based on the properties a designer wants to emphasize. To better understand why an engineer would choose one over the other, it is helpful to look at four critical performance categories:

1. Fire Safety

Phenolic prepregs offer superior thermal stability. Likewise, epoxy prepregs are not even close. Unless modified with additives, an epoxy resin will catch fire easily. It will produce heavy toxic smoke as it burns. On the other hand, phenolic prepregs will char before they melt. Charring creates a thermal barrier that protects the underlying structure. And remember, phenolics self-extinguish when the source of flame is removed.

2. Mechanical Properties

Parts that function as primary load-bearing structures are better when designed with epoxy prepregs. Epoxy-based composites offer a higher interlaminar sheer strength and superior fatigue resistance. On the other hand, certain interior parts do not require the strength epoxy brings to bear. They are components like ceiling panels, partitions, and sidewalls. Phenolic prepregs are stiff enough to be used for such components.

3. Manufacturing and Processing

Engineers must consider manufacturing and processing requirements when making their choices. Epoxy prepregs tend to be easier to process because the resins have a longer out-life. Epoxy resins also cure without releasing volatile byproducts.

Processing phenolics is a bit more complicated. During the curing process, they typically release water vapor through condensation. Fabricators must properly manage autoclave pressure, or they risk the curing parts developing voids between layers or excessive surface porosity. Both problems can significantly weaken a part.

4. Cosmetics and Aesthetics

When it comes to commercial and private passenger aircraft, interiors must be cosmetically and aesthetically pleasing. Phenolic resins have a disadvantage in this regard. They are naturally dark and make the fabric weave more visible. They also require a specialized film or fire-retarding primer.

Epoxies are clearer and smoother. That makes them easier to finish to a high-gloss standard. Proper finishing can all but eliminate the fabric weave from the visual field.

Phenolic vs. Epoxy: The Verdict

Unfortunately, there is no clear winner between epoxy and phenolic prepregs for aircraft interiors. Phenolic prepregs are preferred when:

• A part must meet strict FAA fire regulations.
• A part is deployed where high-temperature air is expected.
• Weight must be minimized by avoiding heavy fire-retardant paint systems.

Epoxy prepregs are preferred when:

• A part is a structural bracket or seat frame.
• A part does not have to meet the same FAA fire regulations.
• A part’s complex shape requires maximum airflow.

Axiom Materials do business based on the philosophy that one size does not fit all. Our phenolic prepregs are engineered to minimize the traditional disadvantages of phenol chemistry. Meanwhile, our modified epoxy systems are ideal for applications requiring the strength of epoxy alongside improved flame retardation. The bottom line for both types of prepregs is ‘safety first’.

FAQs

Are phenolic prepregs mandatory for aircraft interior systems?

For all intents and purposes, yes. FAA and other global aviation standards mandate strict FST compliance. Phenolic prepregs achieve those standards.

Can phenolic prepregs be deployed for structural parts?

Because phenolic resins are more brittle than epoxy, they are not appropriate for structural parts.

How does out-life impact phenolic resin curing?

Out-life is the amount of time a prepreg can be exposed to air before becoming unusable. Phenolic prepregs generally have a shorter out-life, meaning parts need to be prepared and cured in a shorter time frame.

Do phenolic resins support both carbon and glass fibers?

Yes, fiberglass/phenolic prepregs are the industry standard for sidewalls and floors, while carbon/phenolic is preferred for high-performance areas with greater stiffness and weight-saving requirements.

What are modified phenolics?

Modified phenolics are products that incorporate toughening agents to reduce brittleness. Axiom Materials offers a selection of modified phenolics for specific applications.