This case study details the development of a recyclable, bio-based polyurethane (PU) window frame designed to replace conventional Polyvinyl chloride (PVC), addressing key environmental and circularity challenges in the construction industry.
Challenges
The reference product for this case study is a standard PVC window frame. While widely used, PVC presents several sustainability challenges:
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Environmental Hotspots: The primary environmental impacts of the PVC window's lifecycle come from the production of the PVC material itself and the steel reinforcements used within the frame.
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Recycling Difficulties: Conventional PU and PVC have low recycling rates. Solid polyurethane waste is generated during window assembly and production, and there is a lack of available infrastructure for its recycling.
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Hazardous Substances: PVC formulations often contain potentially harmful additives like plasticisers and stabilisers. Hazard assessments identified potential ecotoxicological hazards for many ingredients used in both reference and alternative PU formulations.
Objectives
The primary goal was to design a new PU window frame that significantly improves on the performance of PVC. The specific Key Performance Indicators (KPIs) were:
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Performance: Achieve superior insulation properties (Uf value of 0.5-0.8 W/m²k), increase durability to 35-45 years, and reduce weight by 25-35% compared to PVC.
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Safety: Eliminate the use of potentially harmful additives and reduce the release of Intentionally Added Substances (IAS) by over 75%.
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Sustainability: Reduce the carbon footprint by over 40% through the use of bio-based raw materials and an energy-efficient manufacturing process.
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Recyclability: Enable at least three recycling loops with minimal loss of material properties by designing an intrinsically recyclable material.
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Cost: Ensure the final material cost is less than 20% higher than PVC.
Our SSRbD Approach & Key Findings
To meet these objectives, the SURPASS project developed a novel bio-based polyurethane formulation incorporating vitrimer chemistry. Vitrimers are a class of polymer that can be reprocessed like a thermoplastic, enabling true recyclability for a thermoset material.
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Environmental Performance (LCA): The assessment revealed that, at a low Technology Readiness Level (TRL), the new PU solutions have a higher environmental impact than the mature, mass-produced PVC reference. The main hotspots for the PU window are the polyol formulation and the window production stages. Notably, the bio-based PU had a higher impact than the fossil-based version, largely due to the agricultural production of castor seeds for the polyol. However, the recyclable vitrimer route (SSRbD#2) showed a significant environmental improvement over the non-recyclable PU alternatives.
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Economic Viability (LCC): The Life Cycle Costing assessment showed that direct costs are the main driver of the final price. The assembly cost for the innovative PU window was found to be 24% higher than for PVC, which is close to the project's target of <20%. The analysis indicates that increasing production capacity is the key lever for achieving cost parity.
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Hazard & Release Assessment:
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Hazard: The hazard assessment of individual ingredients found that the alternative bio-based polyols were, in some cases, more hazardous than the fossil-based reference, showing effects on cytotoxicity and oxidative stress.
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Release: The recycling process (thermal compression) of the vitrimer PU generated higher Volatile Organic Compound (VOC) emissions than the benchmark, mainly due to the release of solvents and by-products from the vitrimerization agent. However, after ageing, the release of specific compounds was strongly reduced for all polymers, and the bio-based PU materials led to significantly lower release of organic compounds into water.
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Key Takeaways
This case study demonstrates the complexities of early-stage sustainable innovation. While the novel PU material shows great promise for recyclability, its environmental and hazard profiles at a low TRL are not yet superior to the highly optimised PVC benchmark. This highlights the importance of a full lifecycle perspective and the need to account for process maturity when comparing materials.