Current Trends in the Use of Biomass in the Manufacture of Rigid Polyurethane Foams: A Review

Abstract

This paper discusses methods of using biomass from the agriculture, forestry, food and aquaculture industries as potential raw materials for bio-polyols and as fillers in the production of rigid polyurethane (RPUR) foams. Various aspects of obtaining bio-polyols are discussed, as well as the impact of replacing petrochemical polyols with bio-polyols on the properties of foams. Special attention is paid to the conversion of vegetable oils and lignin. Another important aspect of the research is the use of biomass as foam fillers. Chemical and physical modifications are discussed, and important factors, such as the type and origin of biomass, particle size and amount, affecting the foaming process, microstructure and properties of RPUR foams are identified. The advantages and disadvantages of using biomass in foam production are described. It is found that bio-polyols can replace (at least partially) petrochemical polyols while maintaining the high insulation and strength of foams. In the case of the use of biomass as fillers, it is found that the shaping of their properties is largely dependent on the specific characteristics of the filler particles. This requires further research into process optimization but allows for the fine-tuning of RPUR foam properties to meet specific requirements.

1. Introduction

Materials based on the polyaddition of aromatic or aliphatic diisocyanates with polyols with an average functionality of two or more hydroxyl groups have been used in many industries for a long time. These materials, abbreviated as PURs, are characterized by very different physical and chemical properties. These properties mainly depend on the substrate composition and molecular weight, resulting in other applications [1,2,3,4,5,6]. The history of foams dates back to 1937, when Otto Bayer revealed the polyaddition process of polyurethanes. In contrast, the rapid industrial development of polyurethanes occurred in the 1960s, when polyurethane foam manufacturing equipment was invented.

In general, we can divide foams into flexible, semi-rigid and rigid. Flexible and semi-rigid foams are mainly used in the upholstery industry, while flexible foams are also used as sound insulation. In contrast, rigid polyurethane (RPUR) foams are primarily used as thermal insulation. These characteristics are related to the internal structure and, more specifically, the structure and shape of the cells. The shape of the cells in rigid foams was described by Dawson and Shortall [7]. They indicated that foams with a density of about 35 kg/m³ have cells with a shape that is best described by a pentagonal (regular) dodecahedron. In contrast, foams with a density of up to 300 kg/m³ have polyhedron-shaped cells with sharp or rounded edges. In contrast, the cells of foams with a density of 300–420 kg/m³ already have an isolated spherical structure.

The structure of a foam is referred to as an open or closed cell. The structure of a foam not only influences its rigidity but also the migration of gases and, therefore, water vapor or, ultimately, water permeability. For this reason, when foams are used to insulate buildings, closed-cell foams are used to insulate components from the outside, as these components are exposed to increased humidity. In contrast, open-cell flexible foams, which are usually of lower density and have a better thermal conductivity coefficient, are used to insulate the inside of a building. The deterioration of the insulating properties of foams is due to the diffusion of air into the foam structure [8,9,10]. This effect is also referred to as foam aging. According to Ostrogorsky et al. [11], it is possible to predict the effective diffusion coefficient using an analytical model they have developed. On the other hand, Cunningham et al. [12] indicated that the mechanism of heat transfer through rigid foams is based on three effects, i.e., gas phase, solid phase and radiation. Knowledge of this mechanism makes it easier to influence the improvement of the insulating properties of manufactured foams.

Another essential characteristic of RPUR foams is their mechanical durability. Most of the mechanical characteristics of foams depend on the temperature at which they are manufactured [13,14,15,16]. Temperature influences the density of the produced systems, and this, in turn, correlates very strongly with mechanical properties, including the modulus of elasticity [17,18,19,20,21]. In addition, the mechanical properties of polyurethane foams also depend on the temperature at which they are tested. This was confirmed by examining the failure mechanisms of foams and the tension and compression properties of foams at cryogenic, room and elevated temperatures [22,23,24]. Of course, the density of foams is also related to cell size. Thus, it is cell size that directly affects the mechanical characteristics of the foams produced [25,26,27], and density secondarily. Li et al. [28] confirm that cell shape and size change as density increases, which significantly impacts the observed values of the elastic modulus, shear modulus and limit of plasticity. Although low-density foams are not expected to transfer loads, the magnitude of load transfer depends on the direction of load application relative to the direction of growth [29]. The anisotropy of foams due to cell anisotropy has been pointed out previously by performing various static and dynamic tests [30,31,32]. Numerous studies have also been conducted for years, aiming at the most efficient numerical model for RPUR foams [32,33,34]. The starting point is often the linear-elastic deformation mechanism described by Menges and Knipschild [17]. More extended models have been analyzed by Ridha [35], Burbank and Smith [36], Pożarski [37] and He [38]. In many cases, the numerical models achieved agreement with the actual samples over 95%.

In addition to high strength, an important feature of rigid foams is to provide or enhance their fire resistance. An extensive study in this area was carried out by Anderson [39], indicating that reactive agents allow for better fire resistance characteristics than physical additives. This topic was developed in the following years and is still relevant today [40,41,42,43].

Although the development of RPUR foams began with the use of petrochemical substrates and many of the properties of foams directly depend on the characteristics of these substrates, it is possible to use substitutes of biological origin. Biomass can be used both for producing polyols and as fillers, affecting the physical and mechanical characteristics of the foams produced. Therefore, the aim of this paper is to review and identify the most promising biomass sources and evaluate their effects on the properties of prepared RPUR foams.

2. Diversity of Biomass Used in the RPUR Foam Manufacturing Process

The criteria for dividing biomass that can be used in the production of rigid polyurethane (RPUR) foams are a key starting point for understanding the variety of available raw materials and their impact on the properties of the resulting product. Figure 1 proposes a division of biomass based on its origin, as well as examples of biomass sources that have so far been considered in the manufacture of RPUR foams. The origin of biomass is one of the main aspects, taking into account a wide range of sectors, such as agriculture, forestry, food industry and aquaculture. Each of these sectors provides unique raw materials, from agricultural waste to processing waste, which can be used in RPUR foam production. The origin of biomass is important because it determines the availability of biomass, its cost and its impact on the environment and the community. The chemical composition of biomass is another important criterion when using biomass in RPUR foam production. Differences in the chemical composition of biomass, such as the content of cellulose, hemicellulose, lignin and other organic compounds, can affect the reactivity of the polyurethane system and, as a result, the properties of the final foams. Therefore, analysis of the chemical composition of biomass can be important for the optimal process of RPUR foam production and obtaining foams with the required physical and mechanical properties.

The structure and morphology of biomass is another aspect that has a significant impact on its application in RPUR foam production. Particle size, shape and the presence of particles/fibers can determine the ability of biomass to fill spaces in the foam material and affect its strength and flexibility. Biomass with different structures can be used to improve the mechanical and insulating properties of the material.

The degree of processing of biomass is another important factor to consider in its selection. Raw materials can occur in various degrees of processing, from their raw, natural form to industrial by-products or processing waste. The degree of processing can affect the purity, stability and yield of biomass in the RPUR foam production process and can have ecological significance. In addition, it is important to consider the biological potential of biomass, its ease of access in the market and its sustainable origin. Also, the impact of biomass on local communities and its ability to regenerate are criteria that can be considered when selecting optimal raw materials for rigid polyurethane foam production. With a full understanding of these criteria, it is possible to effectively adapt the raw material to the needs of RPUR foam production and achieve optimal properties of this material.

Taking the above into account, the following part of the paper will analyze the solutions developed so far for RPUR biocomposite foams produced with biomass of various types.

3. Biomass as a Raw Material for the Production of Bio-Polyols

Growing environmental awareness and the need to find alternative sources of raw materials are becoming more and more apparent. For this reason, the use of renewable materials, including biomass of various types, in the production of bio-based polyols as substitutes for petrochemical polyols needed in the production of RPUR foams is currently a leading topic in the polyurethane industry [44]. This will primarily reduce greenhouse gas emissions, reduce the use of non-renewable resources, sustainably manage raw materials and use recycled raw materials. This chapter analyzes the role of biomass in the polyol production process, considering different sources of biomass, types of biomass and their potential as raw materials for RPUR foams. The literature indicates that there is a high variability in the chemical composition of biomass of various types, which depends on many factors, such as the origin of the biomass, plant species, soil conditions, growing and harvesting periods, climate, cultivation methods and processing, among others [45,46,47,48,49]. This analysis will provide a better understanding of the benefits, challenges and prospects of biomass use in the RPUR foam polyol industry.

Polyurethane foams are obtained by exothermic reactions between alcohols that have two or more reactive hydroxyl groups –OH (diols, triols, polyols) and isocyanates that have more than one reactive isocyanate group –NCO (such as diisocyanates, polyisocyanates) [50]. A scheme of this reaction is shown in Figure 2.

However, it should be noted that the above reaction is the basis for obtaining many polyurethane materials, but there are important differences between the production and properties of RPUR foams and other polyurethane materials. A detailed analysis of the manufacturing process of various polyurethane materials is presented in the work of Janik et al. [52].

In the production of polyurethane adhesives and coatings, additives and modifiers are often used to improve adhesive or protective properties, such as flexibility, abrasion resistance and chemical resistance. In addition, lower-molecular-weight polyols and toluene diisocyanate (TDI) or other isocyanates are used as adhesives, tailored to specific adhesion needs. Polyurethane coatings are often based on higher-molecular-weight polyester or polyether polyols for flexibility and chemical resistance and on isocyanates such as hexamethylene diisocyanate (HDI) or isophorone diisocyanate (IPDI) for abrasion and weathering resistance [53]. For the synthesis of highly cross-linked and rigid polyurethanes, polyols with high functionality (3–8 OH groups/mol) and low molecular weight (in the range from 150 to more than 1000 Daltons) are used. Cross-linking density and stiffness can be increased by using polyisocyanates with a number of isocyanate groups from 2 to 6 NCO groups/mol as multifunctional monomers [52].

RPUR foams most often use polyester and polyether polyols, which can be combined with multifunctional polyols of low molecular weight and usually with methylene diphenyl diisocyanate (MDI) and its prepolymers, which provide high rigidity and good thermal insulation. In addition, the production of RPUR foams requires foaming agents that introduce gas bubbles into the reaction mixture, creating a cellular structure. It is this structure that gives RPUR foams their unique properties, such as thermal and acoustic insulation and high rigidity at a low density.

3.1. Oil-Based Bio-Polyols

Studies conducted to date have shown that it is possible to substitute petrochemical polyols with agricultural biomass, mainly oilseed crops, in the production of RPUR foams. Among oils, castor oil [54,55,56], rapeseed oil [50,57,58,59], soybean oil, sunflower oil, linseed oil, palm oil [60,61] and eucalyptus oil, as well as mustard seed oil [44] and fruit seed oil [62] can be used. Also useful are substances such as celluloses or lignins, as well as raw materials such as wood bark, woodworking lumber waste and algae.

Natural oils are complex triglyceride molecules that are esters of glycerol and fatty acids. Their disadvantage is their relatively low reactivity, making it necessary to convert them into compounds having at least two hydroxyl groups capable of reacting with diisocyanate [57]. The most commonly used methods in the functionalization of unsaturated vegetable oils include epoxidation and ring-opening reactions with halo-acids or alcohols, ozonolysis, hydroformylation, transesterification and thiol-ene coupling reactions [59,63,64]. Depending on the agents used to reactivate the oils, the resulting polyols can have primary hydroxyl groups, secondary hydroxyl groups or both at the same time, with the number and position of the OH groups affecting their reactivity [57].

An exception is castor oil, which naturally contains hydroxyl groups. It has the added advantage of being non-food grade, so it does not compete with food and can be obtained at a lower cost [65]. With a high hydroxyl number in the range of 160 mg KOH/g, it is suitable for direct use in the manufacture of flexible foams (required hydroxyl number of 28–56 mg KOH/g). However, for rigid polyurethane foams for which a higher hydroxyl number of polyols is required (about >200 mg KOH/g and a functionality of 3–6), its modification is necessary [66,67]. This is confirmed, among others, by the work of Ji et al. [68], who used polyols synthesized from epoxidized soybean oil by opening the oxirane ring with methanol, phenol and cyclohexanol to produce RPUR foams. Example diagrams showing the oil epoxidation reaction and ring opening are presented in Figure 3. The authors showed that depending on the agent used, the foams showed significant differences in microstructure, thermal conductivity and mechanical properties. For example, the foam produced with phenol-modified castor oil showed the highest density, smallest cell size and better mechanical and thermal properties while exhibiting higher thermal conductivity and lower dimensional stability. In contrast, the use of cyclohexanol did not improve the properties of the foam, as the aliphatic six-membered ring acted as a plasticizer and could not alleviate the drawbacks created by the dangling chains.

Another solution was proposed by Zhang and Kessler [69], who prepared a bio-polyol based on the ring-opening reaction of epoxidized soybean oil using castor oil fatty acid. By studying the effect of a mixture of different proportions of bio-polyol with petrochemical polyol, they also found that as the content of bio-polyols increased, the density of foams, as well as their thermal stability and thermal conductivity, increased, while compressive strength decreased.

Figure 3. Scheme: (a)—epoxidation reaction, (b)—ring-opening reaction. Adapted from Paciorek-Sadowska et al., 2018 [70] (Open Access).

Figure 3. Scheme: (a)—epoxidation reaction, (b)—ring-opening reaction. Adapted from Paciorek-Sadowska et al., 2018 [70] (Open Access).

Rapeseed oil is another oil whose derivative may find use in the production of bio-polyols, especially in Europe. It is a triglyceride of unsaturated fatty acids, containing about 61% oleic acid residues, 21% linoleic acid residues, 10% linolenic acid and 8% saturated fatty acid residues [71]. A study by Zieleniewska et al. [59] showed that the use of rapeseed bio-polyol improves the thermal stability of the foam and reduces the amount of easily volatilized products. On the other hand, there is a slight decrease in compressive strength caused by a reduction in the physical cross-linking of the material and a reduced glass transition temperature. It has been found that the optimal proportion of vegetable polyol, especially for the cosmetic industry, is 50% [59]. According to Prociak [72], bio-polyols based on canola oil, as well as on sunflower and linseed oil, also contribute to improving the thermal insulation of RPUR foams. However, the author suggests that polyols based on vegetable oils should not exceed 30% of the total weight of polyols, as higher concentrations result in the formation of an open-cell structure, which negatively affects the thermal insulating properties of the foam.

The use of such a solution also leads to a decrease in the reactivity of the polyurethane system. This is confirmed by Kurańska and Prociak [57], according to whom a higher proportion of bio-polyol requires an increase in the amount of catalyst; moreover, the higher the bio-polyol content, the lower the density of the foam. In addition, studies conducted by the authors have shown the effect of primary hydroxyls on the dielectric polarization and temperature of the PUR reaction mixtures involving rapeseed oil-based polyols. Polyols produced by three methods were compared: epoxidation/ring opening with diethylene glycol (PU-EPO), transesterification with triethanolamine (PU-TRE) and transamidation with diethanolamine (PU-TRA), with different hydroxyl numbers. The PU-TRA formulation showed the fastest decrease in dielectric polarization, indicating a rapid loss of the ability to form electric dipoles. In the case of PU-TRE and PU-TRA polyols, different effects of the presence of primary hydroxyls and free electron pairs on nitrogen had complex effects on dielectric polarization. In addition, it was shown that systems modified with bio-polyol containing amino groups are more reactive and show a faster decrease in dielectric polarization than formulations based on bio-polyol obtained by epoxidation and ring-opening reactions [73]. Also, Hu et al. [74], when producing rigid foams based on canola oil, found that the overall reactivity, expansion rate and gel time of the polyurethane system was lower than that of traditional petroleum-based foam. Similar conclusions were reached by Fan et al. [75], who investigating the effects of different proportions of high-viscosity soy polyols on the properties of RPUR foams, found that as the soy polyol content increased, the maximum foaming temperature of the foams decreased slightly. As the authors explain, the secondary OH groups and lower hydroxyl number in the soy polyol resulted in its lower reactivity and lower exothermic effect when reacting with isocyanate compared to the petrochemical polyol, which contains primary OH groups and a higher hydroxyl number.

While in Europe, a polyol made from rapeseed and sunflower oils is used in the production of bio-based polyurethane foams, in Asia, it is mainly palm oil, which has the significant advantage of being cheaper than, for example, castor oil. It is one of the most widely used oils in the food and chemical industries. As early as the 1990s, Chain and Gan [60] showed that it was suitable for making RPUR foams with high strength parameters. To this end, they developed a bio-polyol derived from refined, bleached, deodorized palm oil by derivatizing the oil with the inclusion of chain extenders such as diols or diamines.

In a later study, Gomez et al. [61] used residual palm oil from palm oil mill waste water mixed in various proportions with oil extracted from algae to produce foam. According to the authors, a 50% proportion of algae oil results in an increase in the hydroxyl number of the polyol by up to 49%, an increase in thermal stability and a significant increase in compressive strength (by up to 60%). In addition, the thermal degradation profile of foams with such a high proportion of algae oil is comparable to that of RPUR foams with fire-resistant and insulating properties. Based on this, the researchers concluded that the foams they produced have great potential as materials for industrial applications, such as cores in sandwich panels or other insulation materials. It is noteworthy that such good performance of the produced foams is due to the specific properties of algae oil, in particular the microalgae Clorella sp., which contains about 48% fats in its dry weight, most of which are unsaturated fatty acids, including about 60–80% monounsaturated and about 30% polyunsaturated fatty acids, such as ω-3, ω-6 and ω-9 acids [76].

Apart from the commonly used oils, other oils like those from cold-pressed mustard seeds [44,77], biennial evening primrose [70], flaxseed, corn, olive, canola, rice bran and grape seed [78] have also received attention. As pointed out by Paciorek-Sadowska et al. [70], some of these oils, such as flax or mustard, are characterized by a higher content of unsaturated bonds. For this reason, the bio-polyols formed on their basis are characterized by a higher hydroxyl number, and the resulting PU foams are stiffer. In addition, as claimed by Borowicz et al. [44], bio-polyols from mustard seeds, when blended with petrochemical polyols, caused a slight increase in processing time but had a favorable effect on the foams’ absorbability and their thermal conductivity and, together with the addition of fire retardants, caused a flame-retardant effect through a synergistic interaction between the two. In addition to better fire resistance, such foams were also characterized by lower brittleness [77].

A bio-polyol extracted from pomegranate seeds is also an interesting alternative. Studies conducted by Malewska et al. [62] demonstrated the possibility of producing RPUR foams with 75% of this type of bio-polyol and reported that the polyurethane system had a higher reactivity, as confirmed by a faster decrease in dielectric polarization. Similar to polyols from edible oils, polyols from pomegranate seeds also contribute to the demonstration of increased thermal conductivity of foams, which is related to the change in foam morphology due to the presence of an amine in the polyol structure, which also accelerates foaming reactions.

Besides edible and fruit seed oils, researchers are also showing a growing interest in by-products of crop processing and other renewable feedstocks. An example of such a feedstock is glycerol, which is produced as a by-product of canola oil processing in biodiesel production [79]. This is advantageous because according to statistics for 2022, biodiesel production in the European Union (excluding the United Kingdom), which accounts for 25% of total production, was 13.7 million tons [80]. Taking into account that each 1 m³ of biodiesel produces about 0.1 m³ of crude glycerol and its density is in the range of 860–900 kg/m³, it can be estimated that glycerol production is about 1.57 million tons [81,82,83]. By-products such as glycerol offer the added benefits of raw material efficiency, waste reduction and increased economic value of processing. Hejna et al. [79], by polymerizing crude glycerol and then condensing the resulting polyglycerol with castor oil, obtained rigid polyurethane–polyisocyanurate foams with cell structures containing a higher number of closed cells, lower thermal conductivity and significantly higher compressive strength (by more than 90%). In addition, the use of glycerol-based polyol caused changes in the thermal degradation of the foam, which was manifested by a reduction in the maximum value of the heat release rate and a greater amount of residual charring after combustion (by up to 24%) and at the same time reduced by 35% the carbon monoxide emitted during the burning of the foam. Also, Ma et al. [84] used crude glycerol in the synthesis of bio-polyols, where about 45% of the glycerol components present in crude glycerol underwent esterification or transesterification reactions to form polyhydroxy compounds. The resulting polyol was characterized by parameters suitable for the production of rigid foams, i.e., hydroxyl number (406 mg KOH/g), viscosity (1092 mPa·s) and acid number (1.9 mg KOH/g).

Less common is the use of marine biomass in obtaining polyols. However, an example of this type of research is the work of Pawar et al. [85], who utilized algae oil derived from chlorella microalgae having an iodine value of 120 g I₂/100 g of oil to prepare bio-polyols. Natural polyols from algal oil were synthesized by oxidizing the oil with hydrogen peroxide and acetic acid and then opening the epoxy ring using lactic acid or ethylene glycol. The resulting foams had a higher content of closed cells and similar thermal stability compared to standard foams. However, it was noted that such a method of obtaining polyols, despite satisfactory results, is time-consuming and multi-step, and it requires the consumption of significant amounts of solvents, which is not environmentally friendly [86]. Similar applications of marine biomass have also been investigated by other researchers like Petrović et al. [87], who subjected crude algal oil to ozonolysis, epoxidation and hydroformylation. In their tests, they found that both epoxidation and hydroformylation of the oil were effective, as an OH number of 150 mg KOH/g was obtained; however, in the case of hydroformylation, the polyol was black. Research in this area was also conducted by Arbenz et al. [88] and Olejnik et al. [89].

3.2. Bio-Polyol Synthesis in the Biomass Liquefaction Process

Another direction for obtaining polyols from biomass is through the liquefaction process, which is a key process that enables the conversion of these wastes into useful chemical compounds. According to Członka et al. [90], unlike bio-polyols from vegetable oils, the main advantage of many lignocellulosic raw materials is their non-food application, which eliminates competition with the food industry. These raw materials consist mainly of cellulose, hemicellulose and lignin and contain two or more –OH groups in their structure. As a result, their liquefaction provides an excellent source for the production of biopolymers necessary in the processing of RPUR foams. During the biomass liquefaction process, component degradation, esterification and polycondensation reactions occur, and the structure of the resulting products (polyols) depends primarily on the solvent used and the process conditions [91]. Figure 4 illustrates the proposed reaction scheme occurring as a result of the liquefaction of biomass.

To better understand the mechanism of the biomass liquefaction process, many researchers are investigating the extraction of bio-polyols from individual biomass components such as cellulose and lignins. The conversion of lignins into polyols is performed by solvothermal liquefaction, hydroxymethylation, oxypropylation and phenolation [92]. Olszewski et al. [93] indicated that the biomass solvothermal liquefaction process is one of the most interesting processes for the polyurethane industry. This process, which occurs at 120–250 °C under normal pressure and with solvents, leads to the cleavage of biomass chains into lower-molecular-weight compounds rich in hydroxyl groups [94]. The most common solvents used for biomass liquefaction are polyethylene glycol (PEG), ethylene glycol (EG) and glycerol. A catalyst in the form of strong acids or bases is used to accelerate the reaction. The resulting compounds include glycerol derivatives, glycols, esters, ethers, carbohydrates and carboxylic acids. The product also contains water, which is discharged by drying under reduced pressure. As a result, polyols are obtained that can successfully replace petrochemical polyols (at least in part) in the production of polyurethanes, including RPUR foams. The authors carried out the process of biomass liquefaction of cellulose in a mixture of glycerol and PEGs with different molecular masses (PEG 200, PEG 400, PEG 600), thus obtaining bio-polyols with different chain lengths, which they then used to prepare PUR-PIR foams. Consequently, the resulting polyol increased the foam growth time by up to 112%, which the authors explained by the presence of less reactive secondary hydroxyl groups and the slightly acidic pH of the bio-polyol. In addition, the introduction of bio-polyols with reduced reactivity and different viscosities disrupted the foaming process, resulting in a change in average pore size and cell aspect ratio. Nevertheless, there was an increase in compressive strength, which was attributed to an increase in the number of cross-linked bio-polys with higher hydroxyl numbers and functionalities. The use of higher-molecular-weight bio-polyols increased the foams’ resistance to thermal degradation, although combustion studies indicated that their flammability was slightly impaired.

Significant findings were reached by Kosmela et al. [95], who, in a study on the liquefaction of lignocellulosic biomass and, specifically, cellulose to obtain bio-polyols for RPUR foams, determined the effect of time and temperature on the efficiency of the liquefaction process and the properties of the synthesized polyols. In the course of the study, it was determined that the optimal parameters for bio-polyol production, guaranteeing a yield of 94%, could be obtained at 150 °C for a reaction time of 6 h. Polyols obtained under such conditions are characterized by a high hydroxyl number of 643 mg KOH/g and enhanced thermal and oxidative stability compared to polyols obtained at lower temperatures. According to the authors, bio-polyols prepared in this way can be classified as pseudoplastic liquids with viscosities similar to commercially available products. The partial substitution of petrochemical polyol with obtained polyols of natural origin allowed for the preparation of RPUR foams characterized by a slightly increased apparent density and average cell size compared to unmodified foams. It was concluded that the best solution was a 35 wt.% share of bio-polyol in the polyol mixture, which indicates a synergistic effect between the polyols used.

In turn, Zhang et al. [96] carried out an optimization of the liquefaction of wheat straw, corn straw, rice straw and rapeseed straw using a mixture of ethylene glycol (EG) and polyethylene glycol 400 (PEG 400), with sulfuric acid as a catalyst. The authors studied the effects of liquid-to-solid ratio, straw particle size, PEG-to-EG ratio, catalyst dose, temperature and reaction time. Based on their analysis, they found that temperature and catalyst concentration had a greater effect on the properties of the bio-polyol than the other parameters mentioned above. As a result, the synthesized polyols made it possible to produce a foam with better thermal stability and good mechanical properties.

The specific liquefaction parameter of biomass (bamboo shoot husks) for the production of polyols was determined by Ye et al. [91], who showed that the highest percentage of liquefaction of bamboo shoot husks of 99.79% could be obtained using optimal process conditions such as a liquid-to-solid ratio of 6:1, temperature of 150 °C, reaction time of 80 min, size of the crude material greater than 40 mesh, mass percentage of the catalyst in the solvent of 4% and solvent volume ratio of 3:1. Also, Zhang et al. [97], carrying out liquefaction of the three basic components of biomass, i.e., cellulose, hemicellulose and lignin, with a mixture of polyethylene glycol and glycerin, found that for the best efficiency, the liquefaction of cellulose should be carried out at a temperature of about 160 °C.

An interesting solution was proposed by Kosmela et al. [86], who produced and studied rigid foams using a bio-polyol synthesized in the process of liquefaction of biomass from the Baltic Sea, which consisted of Enteromorpha macroalgae and Zostera marina seagrass at a ratio of 1 to 9. As shown, the addition of bio-polyol up to 30 wt.% increased the reactivity of the polyol mixture and improved the mechanical, thermal and insulating properties of the foams, without a significant change in the chemical structure and cell size. According to the authors, the increase in reactivity of the system, manifested by a reduction in rise time with the increase of bio-polyol content, is due to its the chemical structure, i.e., higher hydroxyl number LOH = 650 mg KOH/g and higher viscosity in comparison with the petrochemical polyol. The addition of the bio-polyol also resulted in a decrease in the average cell size and porosity of the foams, as well as reduced values of the sol fraction, which resulted in a decrease in thermal conductivity and an increase in compressive strength of the produced foams. In contrast, the higher isocyanate index caused a slight decrease in the processing times of the foams and their mechanical properties and increased the concentration of polyisocyanurate groups.

Apart from the above-mentioned works, the process of obtaining polyols through the liquefaction of biomass has also been dealt with by other researchers, who have used corn bran [98], southern pine wood meal [99], walnut shells [90], cashew nut shells [100], sugarcane bagasse [101], oil palm empty fruit bunch fiber and empty fruit bunch cellulose [102], bamboo residues [103], coffee grinds [104] and others for this purpose.

3.3. Lignin-Based Bio-Polyols

Previous studies have shown that for RPUR foams, particularly those with densities >60 kg/m³, unmodified lignin can replace up to 30 wt.% of the petrochemical polyol. Although lignin-based polyols have many advantages, there are several challenges associated with their use, which limits their application in RPUR production [90]. This is due, among other things, to the fact that the reactivity of the hydroxyl groups in lignin toward isocyanates can be limited by steric hindrances due to lignin’s complex structure and self-association, making it difficult to access these groups. Its solubility is also a problem, as lignin must be soluble in a suitable solvent or other polyols. Lignins obtained by biomass delignification are partially or completely soluble in many organic solvents, increasing in solubility as the polarity of the solvents increases [105]. In addition, lignins may contain sulfur, which causes yellowing of the final products and odor problems and may not be cost-effective to remove [90,106]. A study by Pan and Saddler [107] showed that replacing polyol with lignin can also have a negative effect on the strength and structure of rigid polyurethane foams, which, according to the authors, is due to the low content of hydroxyl groups in lignins and their poor availability. The authors compared the properties of RPUR foams made by replacing 19–30% of traditional polyol with lignins from hardwood organosolv and kraft. On this basis, they found that foams made with hardwood organosolv had higher compressive strengths than the foams made with hardwood kraft lignin due to the better miscibility of the organosolv lignin in the petroleum-based polyol. Thus, as noted by Mahmood et al. [108], there are some limitations on the degree of substitution of the petrochemical polyol with crude lignin. Therefore, in order to increase the functionality of lignin, it is subjected to depolymerization or modification.

The depolymerization of lignin involves its transformation into smaller fragments such as monomers, dimers and oligomers involving acids, bases or oxidants (chemical depolymerization); high temperature (thermal depolymerization); enzymes or microorganisms capable of breaking down lignin (biological depolymerization); and catalysts that accelerate the breakdown of lignin bonds (catalytic depolymerization). Quinsaat et al. [109] subjected native lignin from Pinus radiata wood to a hydrogenolysis process to obtain a depolymerized native lignin oil consisting of monomers, dimers and oligomers of predominantly dihydroconiferyl alcohol and 4-propyl guaiacol. When used in amounts of 33% or 50%, the monomer/dimer-rich fraction of lignin oil allowed for a significant increase in the compressive modulus, which the authors believe should be attributed to the presence of dimeric polyols in the lignin oil. This is in line with earlier findings by Mahmood et al. [110], who concluded that the hydrolytic depolymerization of kraft lignin and the hydrolysis of lignin can be a viable method for producing depolymerized products with low molecular weight and sufficiently high hydroxyl numbers. These products can be used to produce rigid foams, even at higher substitution rates (up to 50%), without significantly degrading the properties of the produced foams. All depolymerized lignins prepared by the authors had aliphatic-hydroxyl numbers in the range 236–352 mg KOH/g, making them potential bio-polyols for RPUR foam synthesis.

However, the above treatment of lignin has a disadvantage, as after the removal of solvents, the depolymerized lignin still remains in powder form, with limited availability of hydroxyl groups. Therefore, chemical modification by oxypropylation with alkylene oxide is advantageous, as it increases the availability of hydroxyl groups and transforms the lignin from a solid to a liquid polyol with exposed hydroxyl groups and an elongated chain [95,107,108]. This is an example of the process of lignin oxyalkylation in the presence of alkylene oxides, which was first used in the 1980s by Glasser and Leitheiser [111]. It is used, among others, in the processing of lignin, which is characterized by the low reactivity of OH groups with isocyanates, and involves the reaction of OH groups of lignin with propylene oxide, leading to the grafting of poly(propylene glycol) onto lignin [112]. The oxypropylated lignin polyol is a viscous liquid, and the phenolic OH groups of lignin are converted to more reactive aliphatic OH groups with less spatial hindrance [113]. Cateto et al. [112] found that the optimal oxypropylation conditions for polyols used in RPUR foams are a hydroxyl number of 300–800 mg KOH/g and a viscosity below 300 Pa·s. However, it should be kept in mind that propylene oxide is highly toxic and carcinogenic, with a low boiling point (about 34 °C) and potentially explosive vapors. In addition, the oxypropylated reaction requires a high temperature (above 150 °C) and pressure (above 10 bar) and therefore requires the use of pressurized reactors, making it difficult to apply this method on an industrial scale [114]. Nevertheless, as demonstrated by Nadji et al. [113], the produced rigid foams based on polyols obtained by this method are characterized by good dimensional stability and insulating properties, even after aging.

An interesting solution was proposed by Duval et al. [114] when developing lignin-based syntheses of liquid polyols. The authors performed the reaction of organosolv lignin with ethylene carbonate in PEG, which led to the conversion of phenolic OH to primary aliphatic OH groups. In the course of the study, they found that by modifying the molar mass of PEG, a wide range of hydroxyl (IOH) values could be taken into account, and polyols containing up to 30 wt.% lignin had viscosities suitable for preparing RPUR foams. According to the authors, their proposed method has significant advantages over oxypropylation, as it is carried out under ambient pressure without the use of toxic chemicals, requires no purification or post-treatment and allows for the production of polyols with tunable properties. The polyols obtained by this method have high reactivity, allowing the catalyst content in the polyurethane formulation to be reduced by 75%. RPUR foams prepared with a 25% substitution of petrochemical polyol showed properties in the range of commercial foams, with a well-formed structure containing more than 90% closed cells and a low thermal conductivity of 25 mW/m·K, which allows for their use in thermal insulation.

A novel solution was proposed by Perez-Arce et al. [115], who obtained a group of lignin-based polyols in a reaction involving the cationic ring-opening polymerization reaction of oxiranes in the presence of tetrahydrofuran as both the solvent and co-monomer and an organosolv lignin at atmospheric pressure and room temperature in acidic media. The result was a lignin with a hydroxyl number in the range of 53 to 253 mg KOH/g showing reactivity with diisocyanates.

To conclude this section, it is worth mentioning that the process of lignin oxypropylation described above is also a frequent topic of other research using various types of biomass for this purpose. Examples also include the oxypropylation of cork particles [116], rapeseed cake residue [117], starch [118], wheat straw [119] and others.

4. Effectiveness of Using Biomass as a Filler for RPUR Foams

The use of biomass as a filler for polyurethane foams has a significant impact on the properties of the final products, determining their physical, mechanical and chemical properties. Natural (vegetable) fillers are characterized by a variety of structures, which include the degree of porosity, arrangement of layers and shape, size and distribution of individual units. They mainly consist of cellulose, hemicellulose, lignin, fats, proteins, tannins, dyes, mineral salts and water [58,120]. Lignin, which is a hydrophobic substance, limits the ability of plant fibers to bind water and swell, while increasing their resistance to microorganisms. Cellulose, on the other hand, which is the main structural component of cell walls, is made of linear D-glucose chains linked with β1-4 glycosidic bonds and is non-toxic, hydrophilic and biodegradable [58,121].

The use of fillers in the form of solid biomass or biomass ash in the synthesis of polyurethane foams leads to a reduction in the use of petrochemicals, which contributes to the creation of greener materials [122]. However, it should be noted that the addition of fillers to the polyurethane matrix can affect the reaction between the polyol and the isocyanate. This is because the hydroxyl groups on the surface of lignocellulosic fillers are prone to react with the isocyanate (–NCO) groups of the polymer matrix, so they can change the isocyanate ratio and affect the consumption of NCO groups [123]. In addition, the filler can affect the processing times of the foaming process. This is due to changes in the viscosity of the reaction mixture and the interactions of the filler surface groups with the isocyanate, polyol and water, which acts as a blowing agent. Thus, the introduction of these raw materials to the polyurethane system can affect the final properties of foams, contributing to changes in their strength and stability.

4.1. Application Perspectives for Agricultural Biomass as RPUR Foam Fillers

From a review of the literature, it was seen that the most commonly used filler for rigid polyurethane foams is agricultural biomass. Agriculture produces 140 billion tons of biomass every year [124]. Instead of leaving it on the fields as waste, it can be turned into raw materials for RPUR foams, which is an economically and environmentally sustainable solution. Through this use of agricultural biomass, pressure on the environment can be reduced by reducing the amount of agricultural waste, reducing the consumption of petrochemical raw materials, and contributing to a reduction in greenhouse gas emissions by reducing the burning of agricultural waste for energy extraction.

From the work published to date, cereal straw shows significant potential for applications. It should be noted, however, that its choice may be dictated by the seasonality of crops and, therefore, the local climatic conditions and economic aspects of agricultural production. Tao et al. [125] investigated the properties of biocomposite RPUR foams with relatively high densities of the order of 100 kg/m³ and filled with wheat and rice straw fibers of 1–3 mm in length in amounts of 5, 10, 15 and 20 php. The authors evaluated the effect of filler addition on the foams’ structures (SEM analysis), sound absorption coefficients, thermal conductivities and compressive properties. The results presented in the paper showed that the addition of filler fibers to the polymer matrix adversely affected the microstructures of the foams, leading to an increase in the number of open cells and a consequent decrease in compressive strength. In contrast, the use of filler in the amount of 5 and 10 php contributed to a significant improvement in the insulating properties of the foams. An increase in the average sound absorption coefficients and a reduction in thermal conductivity of 25% and 50%, respectively, compared to pure foam, were noted. The authors concluded that the use of larger amounts of straw fibers, i.e., above 10 php, leads to an overall deterioration in the insulating properties of the foams and a further reduction in their strength.

Besides improving thermal and acoustic insulation, the introduction of lignocellulosic filler into the structure of a foam also improves its dielectric properties, which is beneficial for applications in electrical insulation. Tests of biocomposite foams filled with kenaf fibers have shown that the dielectric constant and loss tangent increase with increasing fiber content (from 0 to 15 php), with the dielectric constant being high at low frequencies and decreasing at high frequencies. The use of kenaf fiber rich in -OH groups as a filler increased the total number of polar groups in the foam, leading to dipole orientation or orientational polarization. At the same time, higher temperatures led to a higher dielectric constant and loss tangent, especially above 120 °C [126].

This correlates with the work of Głowacz-Czerwonka et al. [127], who found that among the tested fillers, i.e., sunflower husks, rice husks and buckwheat hulls at various weight ratios, i.e., 5–15 wt.%, the best results in terms of compressive strength were obtained for a 10% filler addition of sunflower husks. In addition, the method of modifying the foams adopted by the authors reduced their absorbability and significantly improved their thermal stability. It turned out that while pure foam began to degrade at 171 °C, foams with the addition of z fillers began to degrade from 237 °C. Despite this, flammability tests of the manufactured foams showed a negative effect of the bio-fillers used. The foam containing 5 wt.% rice husks showed the highest flame resistance.

According to a review of the literature, smaller amounts of filler are also capable of improving the strength of foams, as demonstrated, among others, by Paberza et al. [128], who obtained an improvement in the compressive strength of foams with densities in the range of 45–60 kg/m³ for a variant containing 1.2% (wt.%) lignin from wheat straw.

A number of studies have shown that a key process for using different types and sizes of filler particles is surface modification. This is intended to remove impurities from the surface of filler particles/fibers and improve the compatibility of the filler with the polymer matrix, leading to a better dispersion of the filler in the polymer. In addition, modified filler surfaces can increase adhesion between filler and matrix, resulting in better mechanical properties of foams, such as compressive strength and stiffness. An appropriate modification can reduce the hydrophilicity of vegetable fillers, reducing moisture absorption and improving the thermal stability of foams. Finally, surface modification can introduce additional functionality, such as fire-resistant or antibacterial properties, expanding the range of applications for RPUR foams.

An example of work in this area is a study by Wang et al. [129], who used an alkaline treatment of jute fiber inserted into RPUR foam. The results showed that the fiber treatment reduced the amount of impurities on the filler surface and etched many cracks and grooves, which increased the strength of the interfacial bond. This increased the compressive strength of the foams by about 8% to 20%.

One popular method of filler modification is silanization. This process involves coating the surface of the fillers with a layer of silanes, which improves their compatibility with the polymer matrix. With silanization, adhesion between the filler and polymer is increased, leading to improved mechanical and thermal properties of the composites. Silanization can also reduce the hygroscopicity of fillers, which is particularly beneficial for composites used in humid environments. This method was used, among others, by Miedzińska et al. [130], who used powdered plum seeds modified with 3-isocyanatopropyltriethoxysilane in their study. As expected, the addition of such prepared fillers to polyol systems increased their viscosity and significantly increased the processing times characteristic of foaming RPUR foams. As explained, the addition of organic fillers affects the stoichiometry of the reaction of hydroxyl groups with the isocyanate of polyurethane systems. The introduction of a filler modified with a silane containing isocyanate groups capable of reacting with hydroxyl groups increases the efficiency of the reaction [131]. However, this addition increases the viscosity of the polyol, and thus of the entire reaction mixture, by increasing the expansion time of the foams. This is confirmed by the authors’ results, presented in Figure 5.

Moreover, the introduction of 1% unmodified ground plum particles into the polymer matrix resulted in increased compressive strength of the foams and better flexural strength. On the other hand, the addition of silanized filler improved the thermal stability and the hydrophobicity of the obtained RPUR foams, as confirmed by water uptake and contact angle tests (Figure 6).

The silanization of walnut shell particles used to fill RPUR foams also had beneficial effects [132]. In this case, the use of modified walnut shells with 3-aminopropyl (diethoxy)methyl-silane led to PU foams with more regular structures and improved physical-mechanical properties. Foams reinforced with 1% silanized filler showed higher compressive strength and tensile strength, as well as better impact strength. In addition, its use improved the thermomechanical stability of the foams. Most researchers have attempted the chemical modification of fillers.

Członka et al. [133] investigated physically treated hemp shives, which they then incorporated into a polyurethane matrix. Hemp shives (HSs) were modified with sunflower oil and tung oil. The produced biocomposite RPUR foams showed higher compressive strengths than pure foam. However, the impregnation of the filler particles improved the thermal stability and fire resistance of the foams. Indeed, it was shown that a reduction in peak heat release (pHRR), total smoke release (TSR) and limiting oxygen index (LOI) values was recorded for both oils used to impregnate hemp shives. Moreover, the produced foams were characterized by increased hydrophobicity and reduced water absorption. An important observation is that the introduction of non-impregnated filler did not affect the conductivity of the foams, while the effect of treating the clamshells with sunflower and tung oil was an increase in thermal conductivity by up to 24%. As the authors explained, this was a result of the reaction of the functional groups of the filler particles with isocyanate groups, which increase the degree of cross-linking of RPUR molecular chains (Figure 7). This resulted in foams with smaller cells and higher apparent density, which further increased the λ_solid value.

An interesting solution, significantly improving the physical and mechanical parameters of foams, was proposed by Kairytė et al. [134,135], who conducted a series of studies on the use of sunflower press cake filler as a filler for RPUR foams. Initially, they determined the effect of the addition of its particles on the properties of the finished foams, and in further work, they carried out vacuum-based impregnation with liquid glass. The filler was added to the foam structure in significant amounts, i.e., 10–30%. Despite the significant filling of the foams, the authors found that the addition of 10–20% by weight of the filler led to a more uniform foam structure, which was manifested by an increased content of closed cells (from 81% by volume to 87% by volume) and decreased cell size (by 9% and 36%), as shown in Figure 8. The increased closed-cell content was modified.

RPUR foams may be related to sufficient interfacial adhesion between the filler surface and the polymer matrix. In addition, changes in cell morphology may be related to the ability of the fillers to act as gas nucleation sites during foaming. In further work, the authors carried out the impregnation of sunflower press cake filler with liquid glass. This filler treatment, in addition to further increasing the compressive strength of the foams, resulted in a reduction in thermal conductivity and short-term water absorption. In turn, ignitability tests, cone calorimetry and char layer analysis showed that the water glass led to a reduction in the negative effect of bio-fillers on the fire resistance properties of polyurethane foam composites. According to the authors, foams modified in this way are suitable for use in building structures.

4.2. Food Industry Waste as RPUR Foam Fillers

Another example of work on the use of food industry waste in the manufacture of PUR foams is a study by Zieleniewska et al. [136], who used ground eggshells (5–25 wt.%) with a particle size of less than 63 µm as a natural filler for foams. As in many other studies, this type of filler increased the apparent density of the foams, their compressive strength in the direction of foam growth and their dimensional stability in aqueous environments and reduced their absorbability and brittleness. Since the developed composite (optimal amount of filler 20%) lacks toxicity toward human skin cells and shows resistance toward bacterial adhesion, it can find applications in the cosmetic industry.

One more example showing the potential of recycling food industry waste in the production of RPUR foams is potato protein. This the main product of the thermal and acidic coagulation of potato juice obtained from potatoes during starch production, was also used to reinforce RPUR foams [137]. It turned out that the addition of 0.1 wt.% potato protein improved the compressive strength of the foams due to its reinforcing effect. In addition, it provided low thermal conductivity and about 14% less water absorption compared to pure PUR foam.

Członka et al. [138] used keratin feathers to reinforce foams based on a soybean oil bio-polyol. The study showed that replacing 10 wt.% of the petrochemical polyol with soybean oil and adding 0.1 wt.% keratin feathers improved the thermal insulation and mechanical properties of the foams, increasing compressive strength by about 20% and reducing thermal conductivity by about 9% compared to pure foam. Adding 0.5% by weight of feathers did not significantly affect the properties of the composite, while using them in the amount of 1.5 wt.% negatively affected the morphology of the cells, which consequently worsened the physical-mechanical properties of the foams.

It should be noted that the use of food industry products or wastes has also been investigated by other researchers, who have used, among others, brewers’ spent grain [139], wheat slops [140], rapeseed cake [141], casein/apricot filler [142], nutmeg [143], POSS-impregnated sugar beet pulp [144], cinnamon extract, green coffee extract, cocoa extract [145] and others.

4.3. Forest Biomass as PUR Foam Fillers

Forest biomass, in the form of wood, bark, needles or other forest waste, as well as by-products from primary wood processing, can also play an important role in RPUR foam production. Its use can have an impact on sustainable forest management and environmental protection. A series of studies in this direction were conducted by Delucis et al. [146,147,148], among others, who used six fillers from forest waste, i.e., wood, bark, cones and needles from young pine trees, kraft lignin and recycled paper sludge from industry waste as fillers in PUF foams based on castor oil and crude glycerin. The authors studied the effects of different filler contents (1, 5 and 10% by weight) and NCO/OH ratios (0.6, 0.9 and 1.2) on the properties of the finished foams. It was found that the addition of such fillers reduced water absorption and yielded foams with a more homogeneous cell structure. It was also shown that the most effective filler was wood in the amounts of 1% and 5% because foams obtained with its participation were characterized by better mechanical and hygroscopic properties. As the authors claim, this was probably due to the high compatibility of wood with the polyurethane system, which promotes urethane bonds between the filler and isocyanate. The incorporation of wood, bark, kraft lignin and paper sludge into the structure of the foams also resulted in the coloring of the foams, and their photodegradation yielded a decrease in L*, specular gloss and an increase in a* and b*, especially over the first 20 days of UV exposure. Foams with bark and sulfate lignin fillers, characterized by a darker color, showed greater resistance to photodegradation. There was also an increase in surface wettability and a weakening of the foam structure [146]. In addition, foams with wood chips showed improved thermal and dimensional stability and comparable thermal conductivity and flammability to pure foam [148].

Bradai et al. [149] evaluated the effects of different wood particles on the reinforcement of RPUR foams. In the study, the wood chips used were ground to form three different fractions, i.e., fine (<0.106 mm), medium (0.1–0.3 mm) and coarse (0.3–0.5 mm), in amounts of 10, 20 and 30 php. In addition, they used kraft chemical pulp fibers, microcrystalline cellulose and black spruce wood fibers treated by acetylation with acetic acid and hydrogen peroxide as foam fillers. The authors also evaluated the effects of the methods of homogenizing the reaction mixture (hand and mechanical mixing) on the properties of the finished foams. They found that regardless of the proportion and size of the particles, mechanical mixing better homogenized the reaction mixture, which provided a stronger strengthening effect of the wood flour. In addition, they showed that the use of larger filler particles reduced the compressive strength of the foams compared to smaller filler particles. The use of smaller particles promoted an increase in the strength of the foams and their density, especially with the addition of filler in the amount of 10 php. The introduction of wood particles into the polyurethane matrix, regardless of their particle size, had a slightly negative effect on the insulating properties of the foams. This amount of filler also turned out to be optimal when using kraft chemical pulp fibers, microcrystalline cellulose and acetylated spruce fibers. The authors noted that smaller fibers and microcrystalline cellulose showed a tendency to aggregate. Therefore, even with a strong interaction with isocyanate, these fibers acted as nucleation points and did not participate in cross-linking reactions, hence the average mechanical properties. The chemical modification of the fibers did not improve the mechanical properties of the foam, mainly due to the aggregation of the fibers, thus causing a disruption of the foam’s microstructure. However, it was found that the fiber contribution of 10 php improved the compressive strength of both polyurethane and polyisocyanurate foams. In addition, wood fibers slightly improved the thermal stability, thermal conductivity and fire resistance of the foams.

Similar results were obtained by Dukarska et al. [150], who used by-products of elemental wood processing in the form of sawdust of a 0.315–1.25 mm fraction in amounts of 5–20% as filler for RPUR foams. It was found, as a result of the study, that with an increase in the amount of wood filler, the density of the produced foams increased and significant changes occurred in their microstructure, especially when filler was added above 10%, as illustrated in Figure 9a. As the authors explained, such an amount of filler did not significantly affect the kinetics of the foaming process or the foam cellular structure. In fact, they observed a significant reduction in the average cell size and disruption of the structure of the foams produced, mainly in the interfacial areas, i.e., where filler particles were clearly attached to the foam cell walls, weakening the cell structure and leading to cracks (Figure 9b). Such changes in the structure of the foams contributed to a decrease in strength properties and an increase in water absorption, and also to an improvement in thermal insulation.

In addition, irrespective of the amount of sawdust introduced, a decrease in the brittleness of the foams was also noted. However, the authors emphasize that despite the noted decreases in the values of these selected parameters, the results obtained were satisfactory and in line with the thermal insulation parameters of RPUR foam-based materials currently available on the market. The greater filler additions (15 and 20%) resulted not only in an increase in thermal conductivity but also in a significant deterioration of the strength and required dimensional stability of the foams, which practically disqualified them from application.

An interesting solution was proposed by Luo et al. [151], who synthesized foams using a one-pot, self-rising method without any blowing agent, dispersing 0–25% lignin into soybean oil-based polyol. Lignin was extracted from forest and non-food agricultural materials such as pieces of wood, agricultural residues and switchgrass, using bacteria and yeast to produce bioethanol. The inclusion of lignin helped to reduce the density of the foams but at the same time improved the biodegradability, mechanical and thermal properties of the resulting foams. Moreover, according to the authors, the foaming process did not require a blowing agent (it ran on its own), which saves time. In addition, the inclusion of lignin in the polyurethane matrix improved the foams’ ultraviolet (UV) stability and antioxidant properties [152].

The additions of these fillers can contribute to a reduction in the reactivity of the reaction mixture, as noted by Luo et al. [153], who introduced wood particles at 10% and 20% and kraft lignins at 5% and 10% into the polyurethane matrix. Similar to the work mentioned above, the addition of wood filler increased the density of the foams, decreased the compressive strength and increased the water absorption of the foams. In turn, the addition of lignin had a favorable effect on these parameters. Overall, the authors found that the foam with a 5% lignin addition had the most optimal properties. For this variant, there was an increase in compressive strength by 74%, and daily water absorption decreased by 28%. Thermogravimetric analysis showed no effect of the fillers on the level of thermal degradation of the foam; only an increase in the mass of residual carbonization was noted.

Nevertheless, Mosiewicki et al. [154] showed that the chemical reaction between the wood flour and isocyanate significantly affected the thermogravimetric results of RPUR foams. The authors also found that the compressive modulus and yield strength decreased as the wood flour content increased. In addition, Augaitis et al. [155] proved that a ratio of polyurethane binder to filler particles (pine sawdust) equal to 0.4 is insufficient to adequately wet the filler particles, while increasing it to 0.7 makes it possible to obtain foams with the most suitable strength and thermal conductivity parameters.

Important conclusions, relevant from a practical point of view, were reached by Aranguren et al. [156], who, in a study of polyurethanes based on tung oil and wood flour composite, showed that these materials are not easily biodegradable by microorganisms. As the authors suggest, hydrolytic degradation of the tested foams proved to be a key mechanism of decomposition in aggressive environments. When the materials were exposed to degrading agents, their glass transition (T_g) moved toward higher temperatures, which was attributed to the removal of free or dangling pendant chains that plasticized the material. However, with prolonged exposure, T_g shifted back to lower values due to continued degradation. The wetting angle, which decreased after exposure to soil and vermiculite, suggested an increase in the polarization of the material. Electron microscope (SEM) observations showed the presence of holes and cracks on the surface of the samples after exposure to degrading media. Thermal degradation was evident in the TGA curves, which was related to the lower concentration of dangling chains in the materials exposed to the degrading factor.

5. Environmental Benefits Arising from the Use of Biomass and Challenges Associated with Biomass Utilization in the RPUR Foam Industry

5.1. Positive Environmental Aspects of Biomass Application

Global plastics production reached over 400 million tons in 2022. The market is worth more than €400 billion in turnover. Although the share of polyurethane materials in global plastics production appears to be only 5.3%, due to the very low densities of these materials, by volume, they represent a sizeable environmental load [157]. Bio-based derivatives account for only 0.5% of global plastics production. Introducing materials of natural origin into the production process, even in only a fraction of the share, therefore has a positive impact on the environmental aspect of the production and use of these products. The main benefits of using biomass in the production of both polyols and fillers used in the manufacture of foams include, in particular, a reduction in the use of petrochemicals in RPUR synthesis, the creation of environmentally friendly materials and an improvement in the final properties of the foams. Notably, the biomass used or that can be used in manufacturing RPUR foams is most often of negligible economic importance. This is because, most often, it is agricultural or horticultural biowaste, which may have other industrial uses but is very often composted due to the amount and availability of different technologies in the area [158]. Agricultural biowaste is used in biogas production.

Conversely, the horticultural industry’s biowaste is often condensed and used as animal feed. However, existing processing methods still need to utilize existing biomass fully. Polyols use about 50% rPET recyclate and about 32% vegetable oils. Thus, while the production of polyols can already be considered environmentally friendly, managing at least a significant proportion of this 50% is still possible. Assuming that, in line with the Green Deal, the share of petroleum derivatives in producing products for storing and keeping food products will fall, the share of plant materials in the production of polyols should increase, as PET derivatives will disappear from the market. The Swedish company Perstorp Holding AB (Malmö, Sweden) has already introduced polyols made from 100% renewable sources. Thus, their products already have a negative carbon footprint. Using these polyols to produce RPUR foams will improve their CO₂ balance across the LCA.

Studies by Ge et al. [159] have shown that the use of cellulose pulp or its derivatives in the production of polyols makes it possible to produce RPUR foams with a density of about 40 kg/m³, a compressive strength of 150 kPa and a modulus of elasticity of 4 MPa, which, according to the authors, is comparable to the properties of conventional foams. Zakrzewska’s study [122] found that adding biomass ash increases the reactivity of the mixture during the polyurethane foaming process. A significant increase in the temperature of the foaming process was observed, but the mechanism of this phenomenon was not explained. Research should be continued. The use of soy flour in the production of RPUR panes resulted in an increase in the properties of the foams, including dimensional stability [159]. This is important as soy flour is available in many markets; thus, producing foams can become more environmentally friendly and cheaper. Another very accessible raw material, which is a problematic waste from the cellulose extraction process, is lignin. Only in the past few years have technologies been developed that allow lignin to be used to produce plastic components or ingredients, including RPUR foams [106]. Currently, non-isocyanate polyurethanes (NIPUs) are also being developed, which can further significantly improve the environmental aspect of polyurethane foam production [160,161,162,163,164].

5.2. Barriers and Difficulties in the Application of Biomass in the Production of RPUR Foams

Despite the potential environmental benefits of using biomass in the production of RPUR foams, the solution also carries a number of negative aspects and challenges that can affect production efficiency and the quality of final products. One of the main challenges is the higher cost of biomass production and processing compared to conventional petrochemical raw materials. Biomass often requires more complex harvesting, transportation and processing processes, which increase overall production costs. It should also be noted that currently, the continuing lack of access to advanced biomass processing technologies in some regions hinders their use in polyurethane foam production. In addition, adapting existing production lines to process biomass can require significant investments in new technology and equipment, further increasing costs.

Another problem is the greater variability in the chemical composition of biomass compared to petrochemical raw materials, which makes it difficult to maintain consistent production efficiency and quality of the final product. The efficiency of biomass processing, especially for obtaining renewable polyols, varies depending on the type, composition, structure and morphology of the lignocellulosic biomass [165].Variability in the composition and properties of biomass can also result from soil and climatic conditions, which affect the stability of production processes and the quality of the resulting products. This makes RPUR foams made from biomass exhibit great variation in physical properties, mechanical properties and stability compared to traditional foams, requiring additional research and the modification of production processes.

Biomass is also characterized by its seasonality and typically low bulk density, which pose challenges in the supply chain, increase transportation costs and require large areas for storage [166]. To prevent biomass degradation, it is necessary to maintain proper storage conditions, such as humidity and temperature control.

A major challenge for the RPUR foam industry is also the transition from the research and experimental phase to the production phase. While the use of biomass or biomass ash as fillers does not present many difficulties, the extraction of polyols and, even more so, non-isocyanate polyurethanes often requires a rather cumbersome synthesis process. As can be seen from the analysis, the processes for obtaining bio-polyols, for example, by the liquefaction of biomass, are complicated, with a large number of chemical reactions competing simultaneously. Another challenge for the process of obtaining bio-polyols is the significant consumption of solvents, which make up a large part of the reactants during liquefaction processes. It is expected that the use of bio-based solvents may provide a solution for the mitigation of these problems [165]. In addition, as stated by Kaikade and Sabnis [66], in the previous decade, most of the research was on the use of vegetable oils in the production of foams, but with current modifications, it is not possible to 100% replace petroleum-based polyols with bio-based polyols, as this causes some deterioration of properties, mainly in rigid foams.

Overcoming these challenges requires investments in research and development, as well as cooperation between raw material producers, technologists and the foam industry. It is worthwhile to continue research into improving the efficiency of production processes, the quality of raw materials and the development of sustainable methods for obtaining biomass in order to fully realize its potential in the production of polyurethane foams. It is important to note that intensive research into the use of biomass in the production of PUR panes has only been carried out for less than ten years, while the history of foams is nearly 90 years old.

6. Conclusions

Research into the use of biomass in the production of rigid polyurethane (RPUR) foams is promising and could bring many benefits, both from an environmental and practical point of view. The main directions for the use of biomass in the production of RPUR foams are the preparation of natural polyols and their use as fillers. A review of the literature made it possible to identify a number of methods for obtaining natural polyols from various types of biomass, as well as techniques for the chemical and physical modification of fillers, which have a significant impact on the properties of finished foams.

The study showed that bio-polyols obtained from biomass are a promising alternative to traditional petrochemical polyols in the production of RPUR foams. Their main advantage is that they are derived from renewable sources, which contributes to reducing the dependence on petroleum feedstocks and lowering the carbon footprint of the production process. In addition, the use of biomass can help reduce waste and environmental pollution. One of the main limitations is their varying type, quality and chemical composition, which can affect the stability of the RPUR foam production process and the final material properties. In addition, compared to petrochemical polyols, biomass polyols can exhibit different reactivities due to differences in structure and chemical composition. Some natural polyols may be as reactive as their petrochemical counterparts, while others may exhibit lower reactivity, which will require the use of larger amounts of catalysts. Therefore, it is important to appropriately adapt the production process and select the degree of petrochemical polyol substitution with a bio-polyol for a specific application to ensure optimal reaction activity and obtain the required properties of RPUR foams. Nevertheless, studies have shown that the substitution of petrochemical polyols with newly developed bio-polyols can help improve the final properties of RPUR foams, such as thermal insulation and mechanical strength. However, in order to achieve the desired effects, it is necessary to take into account the differences in reactivity between bio-polys and petrochemical polyols. Thus, it can be concluded that the conversion of biomass, particularly of vegetable oils and lignin to natural polyols, is an interesting research direction, the potential of which is worth pursuing in order to develop more efficient and environmentally friendly methods of producing RPUR foams.

The use of biomass as fillers in the manufacture of RPUR foams is also an important issue. The introduction of biomass in the production of these foams can help reduce CO₂ emissions and reduce the consumption of fossil raw materials, which are important steps toward the sustainable development of the polyurethane industry. It is also important to consider the various factors affecting the foaming process, such as the type and amount of filler, its chemical composition and the quality and method of modification, which can significantly affect the microstructure and properties of foams. From the studies conducted so far, it appears that the introduction of different types of filler into the polyurethane system generally increases its viscosity, which consequently delays the processing times of foams, affecting the microstructure and, as a result, the properties of finished RPUR foams. The size of the particles, their quantity and the method of preparation of the filler (modified or without chemical-physical modification) have a significant impact on these parameters. The improper selection of these parameters not only disturbs the foaming process but also adversely affects such foam properties as compressive strength, bending and tensile strength, thermal conductivity and acoustic insulation, as well as other parameters.

Thus, these findings suggest that the further research and development of technologies for producing biomass-based RPUR foams are key to optimizing their manufacturing processes, improving their properties and expanding their range of applications.