Original Article ISSN (Online): 2350-0530 ISSN (Print): 2394-3629 International Journal of Research - GRANTHAALAYAH December 2021 9(12), 273-285 How to cite this article (APA): Križan, P., Matúš, M., and Beniak, J., (2021). Hay-Plastic Composites – Effect of Hay Particle Size on The Water Absorption. International Journal of Research - GRANTHAALAYAH, 9(12), 273-285. doi: 10.29121/granthaalayah.v9.i12.2021.4445 273 HAY-PLASTIC COMPOSITES – EFFECT OF HAY PARTICLE SIZE ON THE WATER ABSORPTION Peter Križan 1 , Miloš Matúš 1 , Juraj Beniak 1 Institute of Manufacturing Systems, Environmental Technology and Quality Management, Faculty of Mechanical Engineering, Slovak University of Technology in Bratislava, Slovak Republic. Received 2 November 2021 Accepted 14 December 2021 Published 31 December 2021 Corresponding Author Peter Križan, peter.krizan@stuba.sk DOI 10.29121/granthaalayah.v9.i12.2021 .4445 Funding: This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. Copyright: © 2021 The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. ABSTRACT This paper presents the results of experimental investigations into the effects of raw material composition and properties on water absorption in hay-plastic composites. It is possible to identify important parameters of the raw materials used in HPCs production such as the size of the hay sawdust particles, hay/plastic ratio, and type or plastic matrix. The goal of this research was to produce HPCs with acceptable and comparable quality. This is determined by the final mechanical properties. The particle size of the hay sawdust used to produce HPC has a significant impact on the mechanical properties of composites as well as other important properties like water absorption, hardness and frost resistance. This paper examines the relationship and impact of the input hay- sawdust particle sizes, hay/plastic ratio, and water absorption on composites. The authors also intend to explore the potential for waste materials usage. These experimental research results were made using a semi-operational molding press, where injection is done by a screw. Meadow hay, HDPE resin matrix, and recycled HDPE were used as input materials. The lids made from PET bottles served as the containers. Based on a combination of default levels of hay/HDPE concentration and using recycled HDPE rather than virgin HDPE, the effect of hay sawdust particles on water absorption was determined. Keywords: Hay Sawdust, HPC, Hay-Plastic Composites, Particle Size, Water Absorption, Recycling 1. INTRODUCTION To improve the physical and mechanical properties of products made from biomass-plastic composites, fillers are used. Today, environmental protection is a major concern. This is why there is increasing interest in organic fillers such as wood, straw, corn stalks, or rice husks replacing inorganic fillers. Due to their easy recycling and ecological benefits, the development of renewable raw materials composites has been a major success. Natural fibers can be easily recycled, are carbon dioxide neutral, and are readily available in large quantities Migneault et al. (2009), Kuo et al. (2009) The scope of use of biomass-plastic composites (BPCs) today is mainly in the automotive, engineering and electrical engineering industries. There are also various research studies Zhang et al. (2008),Yang et al. (2007),Smith and Wolcott (2006) that examine the effects of material and technological variables on production. BPCs can be made by mixing ground biomass particles with heated thermoplastic resin Soury et al. (2009),Yam et al. (1990) ,Carrino et al. (2011). Although extruding the material