Polyols for flexible foam — A guide to formulation and performance tuning
Polyurethane foam manufacturers strive to optimise comfort, durability, and support in their products.
But these performance qualities aren’t decided at the moulding stage. They’re built into the formulation itself.
A clear understanding of how polyol characteristics shape foam behaviour empowers formulators to tailor performance precisely to end-use demands and customer expectations.1
Six polyol traits that influence foam behaviour
Every polyol property plays a specific role in determining how a foam behaves during production and in use. Mastering these characteristics helps manufacturers engineer foams that meet targeted structural, aesthetic, and regulatory needs.
Molecular weight controls feel and flexibility
Foams made with lower molecular weight polyols tend to be firmer and more supportive. In contrast, higher molecular weight polyols enhance softness and elasticity. This makes them well-suited for high-resilience (HR) applications.2
Formulators may increase molecular weight to improve cushioning in impact-prone areas or decrease it when firmer support and reduced sag are priorities.
Solids content determines hardness and load-bearing capacity
Polymer polyols that contain solids — such as styrene-acrylonitrile (SAN) — boost foam hardness and structural integrity.3
Adjusting solids content gives formulators a way to control how foam resists indentation and recovers under load, which is especially important for products requiring long-term performance.
Hydroxyl number and functionality support structural stability
Higher functionality polyols support greater crosslinking, resulting in foams that are more dimensionally stable and resistant to compression over time.
Viscosity affects processability and cell uniformity
Lower-viscosity polyols flow more easily and mix efficiently, aiding in faster and more uniform production. Higher-viscosity polyols support tighter cell structures but may require more precise handling.4
A formulator might choose lower viscosity for high-throughput slabstock operations or higher viscosity to enhance foam structure.
Reactivity profile shapes production efficiency
Fast-reacting polyols enable rapid, continuous production cycles, while slower-reacting formulations allow better flow and more consistent rise profiles in complex moulds.5
Formulators can adjust reactivity based on the needs of the production environment, such as whether they require speed or greater control.
Low VOC polyurethane foams for compliance and safety
Foams made with low-VOC polyols help improve indoor air quality and may be necessary to meet safety and emissions guidelines, such as those that fall under the EU Ecolabel program.6
Choosing low-VOC polyols can enable compliance as well as brand sustainability claims.
With an understanding of how polyol characteristics shape foam performance, formulators can begin aligning those properties with end-use requirements.
Optimise polyurethane foam performance
Flexibility, firmness, and durability all begin with the right polyol selection. Shell CARADOL® polyols help manufacturers achieve product strength, comfort, and resilience, without compromising processability or VOC targets.
Polyurethane foam formulation by application
Many flexible foam formulations rely on blending polyols to achieve the desired performance balance.7 The right combination depends on desired performance characteristics, such as comfort, durability, or resilience. The following examples illustrate how polyol properties are tuned for specific markets:
Mattresses
Foams in mattresses must provide comfort, support, and low emissions.
- High molecular weight: Delivers softness and flexibility
- Moderate solids content: Helps maintain structure and prevent sag
- Low-VOC formulations: Enable compliance with indoor air quality standards.8
Formulators aim to balance softness and firmness while ensuring durability and regulatory alignment.
Automotive seating
Seating foams must retain shape under extended load and fluctuating temperatures.
- High functionality: Supports crosslinking for dimensional stability9
- Fast-reacting polyols: Enable efficient moulding cycles10
For seating applications, resilience, precision moulding, and lifecycle performance are paramount.
Footwear midsoles
These foams must withstand repeated stress while maintaining comfort and shape.
- High molecular weight: Increases flexibility and rebound
- Moderate reactivity: Helps balance energy return and form retention11
Formulators optimise both cushioning and compression properties depending on the shoe’s intended function.
| Application | Polyol consideration | Performance goals |
|---|---|---|
| Mattresses | High MW, moderate solids, low VOC | Softness, support, and low emissions |
| Automotive seating | High functionality, high solids, fast reactivity | Shape retention, consistent support, and comfort |
| Footwear midsoles | High MW, low viscosity, moderate reactivity | Flexibility, comfort, and rebound |
Design better foams from the molecular level with polyether polyols
Foam performance starts with smart formulation. By selecting polyether polyols with tailored molecular weights, solids content, reactivity profiles, and other key attributes, formulators can design products that meet both technical and regulatory requirements.
Shell CARADOL polyols include high-resilience, low-VOC, and co-polymer options to support desired performance across flexible foam applications.
Learn more about how Shell CARADOL can meet your formulation needs
Sources:
1 https://link.springer.com/article/10.1007/s10924-019-01527-7
2 https://link.springer.com/rwe/10.1007/978-0-387-30160-0_9089
3 https://www.sciencedirect.com/science/article/pii/S0142941821002178
4 https://pubs.acs.org/doi/pdf/10.1021/bk-2021-1380.ch001
5 https://www.sciencedirect.com/science/article/pii/S0014305721004079
6 https://www.sn-tin.com/2025/04/30/tin-octoate-the-key-to-precise-control-of-viscosity-in-polyurethane-systems/
7 https://ec.europa.eu/environment/ecolabel/documents/User_manual_mattresses.pdf
8 https://www.linkedin.com/pulse/blended-polyols-key-tailored-polyurethane-performance-resty-nuegas/
9 https://www.sciencedirect.com/topics/engineering/polyether-polyols
10 https://pubs.acs.org/doi/pdf/10.1021/bk-2021-1380.ch001
11 https://www.sn-tin.com/2025/04/30/tin-octoate-the-key-to-precise-control-of-viscosity-in-polyurethane-systems/
12 https://www.organotin.org/archives/33734