## 1 Introduction
The structure was weakened due to the lower miscibility of the bio polyol with petroleum-based polyol and its slower reaction rate. The properties of PUR foams can be varied, and some drawbacks can be mitigated by incorporating various fillers to develop novel composite PUR foam materials. The most interesting fillers for thermal insulating PUR foams are those capable of improving thermal conductivity, water absorption, and mechanical performance. For instance, [PERSON] et al. [26] investigated nanoclays as promising fillers for enhancing the microstructural features and thermal insulating properties of PUR foam composites. Authors have con cluded that a 22-26% improvement in thermal insulation was achieved. Additionally, unmodified expanded vermiculite (Verm) and perlite (Per) were incorporated into the PUR foam matrix. [PERSON] et al. [27] suggested that Verm did not react with the PUR matrix, thus causing the formation of an irregular structure and weakening the interfacial adhesion. Although Verm and Per are relatively similar, [PERSON] et al. [28] demonstrated that it is possible to obtain thermal insulating PUR foam composites with reduced thermal conductivity while maintaining unchanged mechanical performance. However, none of these studies touched on the impact of water on the resulting foams. In particular, vermiculite and perlite are lightweight, sound-absorbent materials with excellent thermal insulating properties. Due to their structure, these materials offer intensive volume growth after high-temperature impact, resulting in expansion. Thermal expansion occurs perpendicularly to Verm and Per sheets, thus obtaining a highly porous structure [29]. Thanks to their expansion ability, these materials can be applied to various purposes, such as structural, thermal insulation, acoustic, agricultural, and as a filler for polymer-based products [30; 31]. Furthermore, numerous studies have shown that Verm and Per are promising for reducing flammability in various composites [32; 33; 34]. However, various fillers lacking proper functional groups on their surface can sometimes lead to the formation of an improper interface between the filler and the polymer matrix. Therefore, research on the functionalization of fillers is currently of great interest to the research community. For instance, [PERSON] et al. [35], [PERSON] et al. [36], and [PERSON] et al. [37] applied silane coupling agents to modify the surface of metal and clay fillers. The results demonstrated outstanding compatibility with dispersing media, enabling the avoidance of particle agglomeration and imparting chemical reactivity to the particles. [PERSON] et al. [38] proposed a more environmentally friendly approach by modifying the ball-milling process to incorporate hydroxyl functional groups into casein, chitosan, and potato protein on the surface of vermiculite. It was determined that proteins and saccharides applied to vermiculite result in PUR foam composites with reduced average cell size, improved mechanical performance, and nearly unchanged resistance to water impact. However, the reaction of the prepared premixes was slowed down with all the surface modifiers used. In addition, the same surface modification method with casein was applied to natural fillers--apricot stones [39]. The same observation about reaction kinetics was noted. Moreover, structural parameters, such as closed cell content, deteriorated, leading to increased thermal conductivity values. Although the suggested surface modification method and materials used are more sustainable, the granted performance characteristics for PUR foam composites are not entirely satisfactory. Given the bio-based origin and availability of dextrin (Dex), its application as a sole filler surface modifier is promising for producing PUR foam composites with improved microstructural parameters and thermal insulation properties. According to [PERSON] et al. [40], the hydroxyl groups in Dex are more easily exposed and have higher reactivity, making it suitable as a surface modifier for Verm and Per fillers.