Polyurethanes (PUs) display a pivotal role in the polymer family [1], which belong to a particular class of polymers with unique qualities including low weight, increased durability, and resistance to absorption. PU is one of the versatile polymeric materials that find application in various fields such as elastomers, coatings, paint, adhesives, synthetic skins, foams (rigid or flexible), insulators, and so forth [2, 3]. In 1849, the first synthesis of urethane was reported by Wurtz [4] and afterward, in 1937, Otto Bayer synthesized PUs by reacting polyester diol with a diisocyanate [5]. Moreover, it is noteworthy that a relatively small number of PUs were first used as a super-coating on German aircraft during World War II and are now produced at an annual rate of more than 75 thousand tons in the United States [6]. PUs are polymers, typically synthesized by the reaction between the hydroxyl groups (-OH) of a polyol with an isocyanate functional group (-NCO), and the term refers to the resultant urethane linkage [2, 4, 7]. PU foams (either rigid or flexible) exhibit broad spectrum applications in building construction, bedding, automotive, packaging, and medical device industries because they offer numerous advantages which comprise their excellent strength, low thermal conductivity, lightweight nature, and high weight-carrying capacity [8, 9]. Indeed, rigid polyurethane foams (RPUFs) account for around 32% of all polyurethane manufacturing and are quite popular because of their low apparent density (10–70 kg/m3), excellent compressive strength, low brittleness, less thermal conductivity, and effective thermal insulation [10]. The potent insulating and stable properties of RPUFs become beneficial in tank insulation, refrigeration, and the construction industry [11]. In general, almost 95% of PU foams (PUFs) were made from petroleum sources of polyols and polyisocyanate in the presence of a catalyst, which creates the PUR matrix, and simultaneously isocyanate and water that produces urea and carbon dioxide which can act as a blowing agent to create the foam cells.
Over the past decade, fluctuating price of crude oil, depletion of fossil fuel reserves, low thermal resistance of the product, and increasing environmental concerns have triggered growing interest in the development of bio-renewable feedstocks to replace their petrochemically derived counterparts in the production of some polyurethane (including PUFs) materials [12–15]. Thus, there is a trend toward the use of hydroxyl derivatives derived from renewable raw materials to primarily replace petroleum polyol components [16]. In this context, vegetable oils are a viable and sustainable alternative to fossil fuels to produce polyols in the PUF industry because they are affordable, readily available, renewable, and even less expensive than petrochemical raw materials [17, 18]. In the manufacture of PUFs ranging from flexible to rigid, several vegetable oils have been used such as castor oil [19–21], soybean oil [14, 22–25], palm oil [26], rapeseed oil [27, 28], canola oil [29], and tung oil [30]. Moreover, vegetable oils become a viable alternative because of the presence of double bonds (odd or even) and hydroxyl groups in the triglyceride chains, which can be functionalized or directly utilized to prepare PUFs [17, 31].
In continuation, industrial hemp (Cannabis sativa L.) is known to be another oilseed crop with excellent potential in the chemical sector, and almost all parts of the hemp plant, including the fibers, seeds, and inflorescence, obtain an industrial application [32]. Hemp seed oil (HSO) is extracted by cold-pressing hemp seeds [33] and despite being a member of edible oil it was exempted from cooking due to its low smoke point. HSO is composed of large content of linoleic acid (55–60%), and linolenic acid (17–35%) with higher iodine value ranges from 140–175 g I2/100 g in comparison to soybean oil (128–143 g I2/100 g), sunflower oil (110–143 g I2/100 g), rapeseed oil (110–126 g I2/100 g) and palm oil (44–58 g I2/100 g) [33, 34]. Recently, Jariwala et al. reported the synthesis of hemp seed oil-based RPUFs and investigated their flame retardancy [35]. Nowadays, industry and academics have focused more on the synthesis and development of PUR composites with natural fillers [36, 37]. Polymer composites are essential to many industries, including the building, automotive, aerospace, and packaging sectors [38, 39]. Recently, researchers became interested in PUs as a polymer for the fabrication of composites due to their remarkable properties, such as resilience, low-temperature flexibility, durability, and great adhesion [40, 41]. An important class of multifunctional PU composite foam materials is stimulated by the combination of foaming and nanoparticles. Incorporation of natural fillers in PUs becomes advantageous because encapsulated fillers can also significantly improve composite PUFs’ properties indicating higher strength, higher stiffness, and certain special characteristics [42]. Moreover, polyurethane composite foams have also been exploited in the automobile industry as sound absorption material because of their high sound absorption efficiency applications. Concisely, PU composite foams have notable structural properties favorable for a wide range of applications.
In this context, the fabrication of HSO-based RPUF composites using bio-based natural filler is highly prestigious and quite acceptable. Cellulose is a plant-derived ubiquitous natural polysaccharide that is plenteously available in our environment [43, 44]. Microcrystalline cellulose has been used in several sectors, such as food, cosmetics, and pharmaceuticals, where it is mostly used as a filler [45]. Septevani et al. [16] reported the impact of cellulose nanocrystals on thermal conductivity and mechanical properties of RPUFs. In addition to filler, non-halogenated flame-retardant (FR) was added to PUR foam to improve its mechanical characteristics and increase its flame retardancy. Melamine is an effective flame retardant for polyurethane foams; however, to improve the mechanical properties of foams, it must be blended with other fillers. In 2020, Członka et al. demonstrated the effect of melamine and silica filler in RPUFs’ characteristics [46].
Herein, the main endeavor of our present research is to fabricate an HSO-polyol-based rigid polyurethane foam composite and investigate the impact of microcrystalline cellulose filler and melamine flame-retardant (FR) on foams' properties. The HSO-polyol was produced by epoxidation of HSO followed by oxirane ring-opening using methanol and tetrafluoroboric acid as catalysts. The resultant RPUF’s composite was subjected to scanning electron microscopy to assess their cell morphology and evaluated for apparent density, mechanical strength, closed cell content, and thermal behavior. The flame-retardancy of the RPUFs composite was also investigated in different loadings of melamine to gain insight into the flame resistance properties of the fabricated composite foam.