**1. Introduction**

Replacing the traditionally used particles in particleboard production with particles of annual crops is still a current topic. The reasons that wood particles are being substituted are, among others, the high prices, storage problems, intense market competition, and lack of suitable quality materials [1]. In turn, annual products are available in abundant volume throughout the world. It is worth highlighting that the forecast for annual surplus straw in Poland in 2025 is circa 14 thousand tons [2]. The most widely available straws in Poland are cereal straws (73.5%) and rape straws (9.1%) [3].

The literature describes the use of various lignocellulosic materials as a substitute for wood in particleboard manufacturing. The particleboards were made of wheat straw, rape straw, rice straw, corn straw, reed stalk chips, rye straw, triticale straw or groundnut shell, and rice husk wastes [4–16]. The data in the literature also showed that white mustard can be applied as a good alternative to wood in particleboard production [17]. Moreover, evening primrose straw, kiwi, grape, and tomato stalks and coffee or tea waste are interesting options in the replacement of wood chips in the manufacturing of particleboards [18–21]. Although many studies have been conducted on the use of various lignocellulosic materials for the production of oriented strand boards (OSBs), not all of the obtained materials provide the physical and mechanical properties required in construction (type five according to the EN 310 [22]). In our previous research, we analyzed the

**Citation:** Banaszak, A.; Wo ´zniak, M.; Dziurka, D.; Mirski, R. Annual Plants and Thermoplastics in the Production of Polymer and Lignocellulose Boards. *Materials* **2023**, *16*, 4400. https://doi.org/10.3390/ ma16124400

Academic Editors: Marcin Małek and Janusz Kluczy ´nski

Received: 10 May 2023 Revised: 12 June 2023 Accepted: 13 June 2023 Published: 15 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

possibility of replacing pine chips with chips from annual plants (rye, triticale, rape, reed, and corn straw) in the outer layers of boards intended for the construction and furniture industry [11]. The use of annual plant straw was particularly favorable for the modulus of elasticity and smaller thickness swelling as compared with the pine boards. Moreover, the results indicated that, in specific conditions, all tested plants could serve as a partial substitute for wood chips in the external layers of particleboards [11]. Annual plants are an attractive alternative to wood chips in the production of chipboards due to, among other things, their low price, lower hygroscopicity, and specific gravity or better thermal and acoustic isolation [9,23–25]. On the other hand, straw has a larger amount of wax on the surface, which causes difficulties in using conventional adhesives. However, the modification of straws or adhesives such as pMDI (polymeric diphenylmethane diisocyanate) is the solution to this problem [6,26,27].

The particles of straw were also successfully used as a filler in a WPC (wood–plastic composite), using, among other straws, rape, sisal, hemp, jute fibers, and rice straw [28–30]. Mainly WPC is manufactured by methods used in the plastic industry. The exciting approach is flat pressing, which was used by numerous researchers [31–37]. The OSB or particleboard with polymer addition in inner layers was characterized by better physical properties, which maintained good mechanical properties. The plastic industry is very powerful, and the annual waste generated after using plastic products is still a big concern in our society. Using surplus straws and plastic polymers in particleboard is one idea to face the market's needs and gain a fully valuable product.

In our previous research, we investigated the effect of various types of straw (rye, triticale, and rape) and various type of thermoplastics (HDPE, low-density polyethylene (LDPE), polystyrene (PS), and PP) on the properties of five-layer boards [31]. The results showed that the properties of the obtained boards strongly depended on both kinds of the polymer and straw used [31]. The research described by Mihai et al. [38,39] indicated that triticale straw has grea<sup>t</sup> potential in the production of thermoplastic composites. Moreover, the triticale content, the presence of maleic anhydride grafted polypropylene as a coupling agent, and the addition of calcium oxide as a reactive additive have an influence on the properties of PP/triticale straw biocomposites [38,39]. Moreover, biocomposites based on triticale straw and PLA were characterized by good properties, and due to their biodegradability of both PLA and triticale, they are more environmentally friendly than synthetics based on thermoplastics [40]. Moreover, the PP and rye-husk-based composites showed good performance compared to softwood composites, including better elongation at break and better Charpy impact strength [41].

As shown above, the application of straw from various annual plants, including triticale and rye, as a substitute for wood in the production of composites has been reported in the literature [30–37]. In this paper, the characterization of particleboards made from straw (triticale and rye) with the addition of three polymers, namely PP, HDPE, and PLA, is reported. To the best of the authors' knowledge, this is the first report about the mechanical, physical, and thermal properties of three-layer polymer–straw boards produced based on triticale and rye and three different polymer matrices and with rape straw as an inner layer, with different moisture contents for the layers.

### **2. Materials and Methods**
