## Materials and Methods
### Life Cycle Assessment (LCA)  
LCA is a well-established method for multi-criteria evaluation of environmental impacts of products (see, e.g., [43]). It is especially useful in comparative studies, where it provides a complex basis for the decision-making process [44]. Its general framework is defined in ISO 14040 [45]. This ISO standard is rather vague for the purposes of the presented study. Therefore, the boundary conditions and specifications for building-material-related LCAs described in European standard EN 15804 [46] were also applied.  
The presented research utilized LCA to support decision making during the development of the prototype absorber composition and production process. The LCA presented in this paper shows environmental impacts related to the production of the final absorber prototype (see scheme in Figure 2) and compares them with environmental impacts of the production of S3, R2, and R3 absorbers (as representatives of the S1-S3 and R1-R3 absorbers). The selection of production process corresponds with \"cradle-to-gate\" system boundaries (modules A1-A3) defined as one of the options for product LCA in EN 15804. Installation, maintenance, or disposal (modules A4-D according to EN 15804) of the absorbers were omitted at this stage of the absorber development because they are the focus of ongoing research. The addition of hypothetical scenarios representing these life cycle stages would needlessly reduce the accuracy of the comparison. However, previous work [47] clearly shows that environmental impacts of these stages could overshadow the production stage. They can even justify the initial increase in environmental impacts by operational energy savings. Future LCAs will therefore cover the whole life cycle of the absorber.  
The LCA presented in this paper considered the following boundary conditions and simplifications:  
* The environmental impacts were calculated using 13 impact categories defined in EN 15804 (with amendment A2). The results are presented using both the impact category indicators and normalized indicators (transformed to \"person equivalents\") to increase clarity.
* The environmental impacts were calculated in the GaBi software with the ecoinvent 3.6 database.
* The declared unit of the LCA is 1 m\({}^{2}\) of the absorber. This equals 42 g of P1, 300 g of S3, 1795 g of R2, and 813 g of R3 absorbers. The selection of the declared unit is based on information in EN 15804 and following standards.
* Production losses of 5% were considered during production of the P1 absorber based on information from the operator of the production line. Landfill is considered the worst-case production waste management scenario.
* Only primary raw materials were considered to provide the worst-case scenario results.
* No packaging was considered.
* The hypothetical final production site for the P1 absorber was set to Brno, Czech Republic. This was reflected in transport of raw materials from suppliers (transport of MPET from Germany, pigment from Norway, and LDPE from Czech Republic).
* Data on the production of the absorbers and related raw materials were assigned to suitable ecoinvent datasets. The database however does not contain datasets representing pigment and MPET production. Pigment production was therefore modelled based on data from a Norwegian producer and MPET production was modelled based on [48]. Both of their models include raw material supply, production process, and (major) waste management.