Customized polyurethane adhesives for sustainable facade cladding and thermal insulation

As the world strives towards a carbon-neutral future, the choices made in construction materials and design become pivotal in contributing to the overarching aim of reducing carbon footprints.
Specifically, in modern construction, the fusion of aesthetics and functionality plays a key role in shaping architectural landscapes. In this sense, stone facade cladding is an example of respectful construction practices that align with achieving environmental goals, as it has a natural origin.
Sandstone, in particular, is a yellowish characteristic stone, easily tailored in slabs or tiles, that captivates the eye and can preserve and enlarge the lifetime of traditional facade isolating materials, like expanded polystyrene panels, reducing the need for frequent replacements and minimise environmental impact.
Extruded polystyrene foam (XPS) is known for its lightweight and excellent insulation properties.
XPS panels in decorative facades contribute significantly to improving energy efficiency due to their outstanding thermal barrier roperties, helping to regulate indoor temperatures and reducing the reliance on artificial heating or cooling systems.
Therefore, these panels enhance the comfort of occupants and diminish energy consumption, also during transportation due to their lightweight (XPS ∼30 kg/m3) compared to other insulating materials like stone wool (70–220 kg/m3), two highimpact variables in the broader context of sustainable building practices.
Moreover, as it was stated before, sandstone-XPS synergy combines aesthetics and enhanced functionality.

Six PU adhesives were synthesised with two different polyols (PPG and PEG based) of three molecular weights, namely 400, 1000 and 2000 g mol−1, and MDI, to prepare sandstone-XPS isolating façade panels.
The chemical structure, thermal properties, and adhesion properties in the preparation of the panels were evaluated.

Both sandstone and XPS are commonly used materials in construction due to their excellent insulating properties, durability, flexibility, water resistance, and aesthetic appeal, making them environmentally attractive.
The industry trend is towards using sanded XPS in these panels, as better adhesion is achieved with typical adhesives compared to satin or unsanded XPS. However, this entails waste generation from sanding, a higher panel price and a difference in adhesive consumption from 309 g m−2 to 684 g m−2 for unsanded (or satin) and sanded XPS, respectively. In this study, all prepared panels exhibit good adhesion properties, even those made with satin XPS. Panels prepared from PPG-based adhesives show better adhesion performance than those made from PEG, reaching 0.52 ± 0.07 N mm−2 with the 1000 g mol−1 PPG polyol-based adhesive.
These highest adhesion properties are due to the optimal balance of flexible and rigid segments in the polymer material structure. The adhesive maintains its high performance even after harsh temperature and humidity conditions.
After accelerated ageing with heat, cold and water, the probes retain between 65 and 71 % of the original tensile strength (0.34 ± 0.03, 0.35 ± 0.07 and 0.37 ± 0.01 N mm−2, respectively).
This finding suggests an interesting starting point for cost savings associated with sanding and reduction in XPS waste generation, making them economically and environmentally appealing.