Fibers, resins and foams: the different composite construction materials

In another article, we discussed the different composite structures suitable for shipbuilding, as well as the processes involved in applying them.

In this article, we'll look at the different component choices available to create this composite structure: resins, fibers and webs.

First: which resin?

In composite boat building, the choice of resin is crucial, as it affects not only the mechanical properties of the final product, but also in-service performance, durability and the manufacturing process. Here you'll find a comparison of three commonly used types of resin:

1. Polyester

• Features :
  • Cost : This is the least expensive resin, making it very popular for the mass production of pleasure boats.
  • Ease of use: Easy to work with, with good fiber impregnation and a wide range of catalysts to adjust curing time.
  • Chemical resistance : Less resistant to solvents and water than others. May be subject to osmosis (blistering) over time in seawater.
  • Adhesion : Good initial adhesion, but may deteriorate with prolonged exposure to moisture or water.
• Applications : Mainly used in pleasure boats, small craft, and in applications where cost is a determining factor.

2. Vinyl Ester

• Features :
  • Cost : More expensive than polyester, but generally less than epoxy. It offers a compromise between cost and performance.
  • Chemical Resistance: Superior to polyester, particularly in terms of resistance to water and blistering (osmosis), making it ideal for boat hulls.
  • Adhesion and durability : Better adhesion to fibers and more durable in marine environments than polyester.
  • Curing : The curing process is similar to that of polyester, but offers a better barrier against hydrolysis.
• Applications: Used for boat hulls requiring better protection against osmosis, in racing boats, and commercial vessels.

3. Epoxy

• Features :
  • Cost: The most expensive of the three, but justified by its superior performance.
  • Resistance : Excellent mechanical, chemical and moisture resistance. Very low shrinkage during curing, reducing the risk of delamination.
  • Adhesion : The best adhesion to fibers, enabling very strong, lightweight structures. It also has excellent fatigue resistance.
  • Workability : May be more difficult for amateurs to work with due to its viscosity and the need for good temperature and humidity control during curing.
• Applications : Used in high-performance boats, racing yachts, critical structures where weight and strength are paramount.

Points of comparison :

• Mechanical Performance: Epoxy > Vinyl Ester > Polyester
• Water resistance/Blistering: Epoxy > Vinyl Ester > Polyester
• Cost: Polyester < Vinyl Ester < Epoxy
• Easy to work with: Polyester > Vinyl Ester > Epoxy

Each type of resin has its own application in the marine industry, where choices are often guided by budget, performance requirements and the environmental conditions in which the boat will operate.

For ambitious or high value-added projects, epoxy resin is frequently preferred for its durability and excellent mechanical properties. On the other hand, polyester remains a favorite for general applications or tight budgets.

For our part, polyester resin was rejected out of hand. Although more economical, it's too vulnerable to osmosis and doesn't meet the robustness criteria we demand to guarantee the longevity of our boats.

Ideally, epoxy resin should be used for the entire structure. However, for cost reasons, we have opted for a mixed strategy: we use vinyl ester and epoxy resins depending on the parts being manufactured, their exposure to the elements and the stress levels they have to withstand.

Once the resin has been selected, it is used to impregnate fibers with varying properties.

Second: Which fiber to choose?

In composite construction, the choice of fiber is also crucial, as it largely determines the final properties of the composite material. Here's an overview of the three most commonly used fibers:

1. Fiberglass

• Features :
  • Strength and flexibility: Offers good tensile strength and flexibility. There are several types of fiberglass (E, S, R, etc.), each with specific properties, with E-glass being the most common in shipbuilding for its good cost-performance ratio.
  • Cost : Very affordable, making it the most common choice for applications where budgets are tight.
  • Weight : Heavier than carbon, but generally lighter than metal for equivalent strength.
  • Chemical resistance: Good resistance to most chemicals, but can be attacked by some acids and bases.

2. Basalt fiber

• Features :
  • Mechanical properties: Similar to glass fiber, with better thermal and chemical resistance. Basalt fiber can withstand higher temperatures before losing its mechanical properties.
  • Ecological: Considered more environmentally friendly than fiberglass because it requires less energy to produce and is derived from volcanic rock, an abundant natural resource.
  • Cost: More expensive than fiberglass but often less than carbon fiber, offering a good compromise between cost and performance.
  • Corrosion resistance: Excellent, making it ideal for marine environments.
• Applications: Used in applications requiring better fire, heat and chemical resistance than fiberglass, as well as in sectors where environmental impact is a concern.

3. Carbon fiber

• Features :
  • Strength and rigidity : Carbon fiber offers the highest tensile strength and greatest rigidity of the three, at a very low weight, making it the preferred choice for high-performance applications.
  • Cost: The most expensive, which may limit its use to applications where its exceptional properties justify the cost, such as aeronautics, racing cars and racing yachts.
  • Fatigue: Very good fatigue resistance, allowing repeated load cycles without significant loss of performance.
• Applications : In racing boat construction, critical yacht structural parts, and in any application where lightness and high performance are paramount.

Comparison :

• Strength: Carbon > Basalt > Glass
• Weight: Carbon < Basalt = Glass
• Cost: Glass < Basalt < Carbon
• Environmental impact: Basalt < Glass < Carbon.

Special Applications: Carbon fiber for applications requiring maximum strength-to-weight ratio, basalt fiber for aggressive environments or as a green alternative, and glass fiber for general and economical applications.

For our part, basalt is favored for its better overall resistance to the marine environment and its much lower ecological impact. Carbon is also a popular fiber at the shipyard, but its prohibitive cost doesn't always fit into the budget of the projects we are presented with.

In any case, we propose to work on all three fibers depending on the project.

Third: What about the core of the sandwich (ham/butter)?

In sandwich construction, the choice of core material is also important, as it influences the rigidity, weight, insulation and overall strength of the structure. Here's an overview of the types of foam commonly used:

1. PVC (Polyvinyl Chloride)

• Features :
  • Density : Available in a range of densities, offering design flexibility to balance weight and rigidity.
  • Chemical resistance: Very good resistance to chemicals, with the exception of organic solvents.
  • Insulation : Good thermal and acoustic insulation.
  • Cost: Relatively inexpensive compared to other core materials such as Nomex.
  • Water resistance: Waterproof, non-rotting and moisture-resistant, ideal for marine applications.
• Applications: Extremely popular in shipbuilding for hulls, decks and superstructures, also used in aeronautics and other industries requiring good rigidity for equal weight.

2. PET (Polyethylene Terephthalate)

• Features :
  • Eco-friendly: Often made from recycled materials and recyclable in its own right, making it a greener choice.
  • Performance: Good compressive and shear strength, with variable density.
  • Ductility: More ductile than PVC, which can be beneficial for absorbing impact energy.
  • Cost: Generally competitive, especially for recycled PET.

• Applications: Used in lightweight structures where durability and recycling are important factors. Found in building panels, windsurf boards, and other applications where a "greener" material is desired.
   

3. Other types of foam

• PU (Polyurethane) :
  • Characteristics: Used for its light weight and thermal insulation properties, but less resistant to water and chemicals than PVC or PET.
  • Applications: Mainly in industries where insulation is paramount, such as refrigerators, but less so in marine construction due to its susceptibility to hydrolysis.
• Nomex (Aramide) :
  • Features: Aramid honeycomb foam offers excellent fire resistance, low density and good mechanical strength, but at a much higher cost.
  • Applications : Used in aeronautics, racing yachts, and any application where weight, fire resistance, and performance are critical.
• Balsa :
  • Characteristics : A natural wood used as a core is very light and offers good rigidity for equal weight, but must be properly sealed to prevent water absorption.
  • Applications : Traditionally used in shipbuilding and increasingly in wind turbines for blades.

Points of comparison :

• Strength and rigidity: Nomex > PVC > PET > PU, with balsa offering good specific rigidity but requiring protection against moisture.
• Cost : PU and PET (especially recycled) can be competitive, PVC is a moderate standard choice, while Nomex is expensive.
• Environmental impact: PET (recycled) and balsa (natural) can be seen as more ecological, although balsa requires treatment.
• Insulation: PU for thermal insulation, PVC and PET for structural and acoustic insulation.

The choice of foam will depend on the specific requirements of the structure in terms of weight, cost, mechanical performance, insulation and environmental considerations.