Rigid synthetic fiber felts are widely used in indoor or automotive applications as acoustic panels, top roof, trunk carpets and linings. The possibility of thermoforming is essential for automotive applications where felts are shaped according to the design of the trunk. Polyester felts used for these applications typically need to be thermoformable at 200°C and to be dimensionally stable up to 130°C in order to resist to high summer temperatures. For this reason, the preferred binders are melamine-formaldehyde or urea-formaldehyde polymers.
However, the presence of formaldehyde has several safety issues. Formaldehyde is listed as a restricted substance by REACH and in particular, it is classified as a category 1B carcinogen. Polystyrene latexes find application as chemical binders for polyester felts but do not reach the thermal stability required by car interior applications.
To meet this requirement, the R&D laboratory of Icap-Sira Chemicals and Polymers SpA, Parabiago/Italy, developed Acrilem 3930 an evolution of Acrilem 3900 polystyrene water-based dispersion with enhanced thermal resistance.
The modification of Acrilem 3900 polymer started with an increase of the internal crosslinking mechanism without affecting the glass transition temperature, and with 2 subsequent increases of the glass transition temperature. The products were tested in padding a standard polyester felt for car trunks with a dry uptake of the polymer in the range of 85-92%. The rigid felts were placed in an oven at stable temperature with one side without supports. All the samples showed to be dimensionally and structurally stable at 60°C for 6 hours (Fig. 1).
Fig. 1 (Source: Icap-Sira)
After 1 hour at 120°C only the sample A and B began to fold. At 130°C after 1 hour the samples A and B were completely folded, sample C shows a slight bend while sample D (Acrilem 3930) kept its dimensional stability (Fig. 2).
Fig. 2 (Source: Icap-Sira)
At 150°C all the samples folded showing they maintain their thermoformability (Fig. 3). Acrilem 3930 allows the felt to be thermoformable, and the increased glass transition temperature of the polymer gives to the finished article the dimensional stability at high temperature required for car interiors. Moreover Acrilem 3930 has specific functional groups that allow the crosslinking with external crosslinkers like isocyanates or carbodiimides. External crosslinking increases the anti-dusting property of the polymer and enhances the mechanical resistance of the felt at usage temperatures.
Fig. 3 (Source: Icap-Sira)
Acrilem 3930 modified polystyrene dispersion is a valuable substitute of melamine formaldehyde and urea formaldehyde polymers for polyester felts chemical bonding. The product has a better toxicological profile, that avoids the limitations due to formaldehyde presence, and it guarantees the thermoformability of the felts while giving good mechanical resistance at usage temperatures.