Bio-resin: Comparison of bio-based casting resins

Bio-resins – bio-based casting resins – replace some of the petroleum-based raw materials with plant-based resources such as vegetable oils, cashew nut shells, or tree resins. The market for sustainable resin systems is growing rapidly, especially in the arts and crafts sector (resin art, river tables), but also in technical applications such as composites and mold making. This guide explains the different types of bio-resins, compares them with conventional resins, and helps you choose the right system.

What is bio-resin?

Bio-resin (also known as bio-based casting resin or ecological casting resin) refers to resin systems in which some of the petrochemical raw materials are replaced by renewable, plant-based sources. The bio-based content typically varies between 20% and 56%, depending on the manufacturer and formulation.

Important to understand: Bio-resin does not automatically mean biodegradable or compostable. The cured resin is a thermosetting plastic and behaves chemically similar to conventional resin. The ecological advantage lies primarily in the reduction of the CO₂ footprint and the use of renewable raw materials in production.

Plant-based raw materials for bio-resins

Overview of bio-resin types

Bio-based epoxy resins

The largest and most important category. Some of the bisphenol A (BPA) or epichlorohydrin is replaced by plant-based raw materials.

Typical products:

• Sicomin GreenPoxy 56: 56% bio-based carbon content. One of the highest bio-based contents on the market. Used in boat building, surfboard lamination, and composite manufacturing.
• Entropy Resins Super Sap: 18–37% bio-based content. Developed by surfer engineers for surfboards and composites. Laminating and casting resin versions available.
• EcoPoxy: Bio-based epoxy resins for art and wood. Known for river tables, jewelry, and decorative castings. Lower VOC emissions than conventional systems.
• Cardolite (CNSL-based): Industrial hardeners and reactive thinners based on cardanol. Used as an additive in conventional epoxy systems for improved flexibility.

Bio-based polyester resins

Partially renewable polyester resins in which dicarboxylic acids or diols are derived from plant sources. Less common than bio-based epoxies.

Application: Boat building (GRP laminates), sanitary ware production (bathtubs, shower trays), general mold making.

Bio-based polyurethane resins (PU)

PU resins based on castor oil polyols. The organic content can be particularly high here (up to 70%), as the polyol component can be entirely plant-based.

Application: Flexible castings, foams, coatings, wood impregnation systems.

SILITECH Bio-Resin range: Biothan & Acrystal

SILITECH AG offers two established bio-resin product lines: Biothan/Biodur (bio-based PU casting resins) and Acrystal (water-based acrylic resins). Both systems are based on renewable raw materials and are non-toxic.

Biothan / Biodur – Bio-based polyurethane casting resins

Manufacturer: Bioresin Products (Germany), founded in 2001. Over 35 standard products, all based on renewable raw materials (olive biopolyols, caprolactones), all non-toxic and without hazard labeling.

Key advantages of all Biothan products:

• Basis: Renewable raw materials (olive biopolyols + caprolactones)
• VOC content: 0.0% (verified)
• Non-toxic – no safety labeling required
• UV-stable (resistant to yellowing)
• Schrumpf <0,1 %
• Pot life adjustable: 1–120 minutes
• Crystal clear transparency
• Very easy to dye

Acrystal – Water-based acrylic resin system

Manufacturer: Acrystal (France). 2K system consisting of water-based acrylic resin + mineral powder. Non-toxic, odorless, washable with water, no solvents.

Acrystal replaces conventional polyurethane, epoxy, or polyester resins in many applications—it is odorless, VOC-free, flame-retardant, and can be cleaned up with water.

Bio-resin vs. conventional casting resin: a comparison

Conclusion: Bio-resins achieve 90–95% of the performance of conventional resins in most mechanical properties. The glass transition temperature (Tg) is slightly lower in some bio-formulations, which can limit the operating temperature. Processing is largely identical.

Organic resin for art and hobbies (resin art)

The arts and crafts sector is the fastest-growing market for organic resins. The reasons:

• Less odor: Reduced VOC emissions make working indoors more pleasant
• A clear conscience: Partially plant-based for environmentally conscious creatives
• Comparable results: Clarity, gloss, and hardness are comparable to conventional resins.
• UV stability: Many bio-resins offer good UV resistance (protection against yellowing).

Typical resin art applications

Bio-resin in technical applications

Composite manufacturing and boat building

Surfboard and boat building was one of the first areas in which bio-resins were used commercially. Sicomin (GreenPoxy) and Entropy Resins (Super Sap) were developed specifically for glass fiber and carbon fiber laminates.

Advantages in composite construction:

• Fiber wetting and soaking behavior comparable to conventional systems
• Infusion and vacuum methods possible
• Reduced styrene gas content in bio-polyesters
• Marketing advantage: "Green" label for end product

mold making

Bio-epoxides are suitable for mold making as casting resins for prototypes and small series. The slightly lower glass transition temperature should be taken into account when using molds for hot hardening processes.

Processing bio-resin: practical tips

The processing of bio-resins is largely identical to conventional resin systems. Some special features:

Mixing

• Adhere strictly to the mixing ratio (typically 2:1 or 100:45 by weight).
• Stir thoroughly for at least 3–5 minutes.
• Repot and remix for bubble-free results
• Processing temperature: 20–25 °C ideal

watering

• Pour thinly and build up in layers if the pouring depth is >2 cm (unless using a deep pour system).
• Remove blisters with a heat gun or gas burner (briefly and carefully).
• Dust-free environment for clear surfaces
• Apply release agent to mold (silicone oil or wax)

curing

• Room temperature curing: 24–72 hours (depending on the system)
• Post-curing at 60–80 °C improves Tg and mechanical properties by 20–30%.
• Avoid curing below 15 °C (significantly prolongs curing time, may lead to cloudy results)

Buying organic resin: What to look out for?

The market for bio-resins is confusing. Pay attention to the following points:

1. Bio-based content (%)

Reputable manufacturers specify the bio-based carbon content in accordance with ASTM D6866. Values of 20–56% are realistic. Products that claim to be "100% bio" or "completely natural" should be viewed with skepticism—there is currently no such thing as a fully bio-based, high-performance casting resin.

2. Certifications

• USDA BioPreferred: US standard for bio-based products
• ASTM D6866: Measurement of bio-based carbon content using the radiocarbon method
• ISO 16620: International standard for bio-based plastics

3. Intended use

• Casting resin (deep pour): Low exothermicity, high casting height
• Laminating resin: Low viscosity, good fiber wetting
• Coating resin: Self-leveling, UV-stable
• Structural casting resin: High mechanical strength, defined Tg

4. Value for money

Bio-resins typically cost 20–50% more than conventional resins. For arts and crafts applications, this additional cost is less significant (small quantities). For large industrial quantities, the additional cost can be considerable.

Frequently asked questions (FAQ)

Is bio-resin biodegradable?

No. Cured bio-resin is a thermoset and behaves like conventional epoxy or polyester resin. It does not decompose under normal environmental conditions. "Bio" refers to the raw material base (plant-based), not to disposal.

Is bio-resin as strong as regular epoxy resin?

For the most part, yes. Mechanical properties typically reach 90–95% of conventional resins. The glass transition temperature (Tg) may be slightly lower (50–80°C vs. 60–120°C), which limits the maximum operating temperature. For most arts and crafts applications, this is not a disadvantage.

Can I use Bio-Resin for river tables?

Yes, there are special deep-pour bio-resins with low exothermicity that allow casting heights of 5–10 cm per layer. Make sure you choose a system with low exothermicity (low heat generation) to prevent cracking and yellowing in large casting volumes.

What is the actual organic content?

Typically 20–56% bio-based carbon content (measured according to ASTM D6866). The remaining 44–80% still comes from petrochemical sources. There is currently no 100% bio-based high-performance casting resin available.

Can I color Bio-Resin with normal pigments?

Yes. Bio-resins are compatible with standard resin pigments (epoxy pigment pastes, mica powder, alcohol inks). Processing is identical to conventional systems.

Where can I buy organic resin in Switzerland?

SILITECH AG stocks the Biothan/Biodur range (bio-based PU casting resins) and the Acrystal system (water-based acrylic resins) in Switzerland. All products are available in the SILITECH online shop or on request.