Green chemistry approach to biodegradable flexible viscoelastic polyurethane foams from a new type of cellulose-based polyol

Abstract

This study aims to develop bio-based and biodegradable flexible polyurethane (PU) foams by applying green chemistry principles, a challenging yet essential goal for advancing sustainability. Additionally, these materials must comply with established product standards to ensure their practical viability. In this study, a new type of polyol (OH number: 273 mgKOH/g) was synthesized for the first time through the hydroalkylation reaction of cellulose and propylene carbonate. The synthesis of the cellulose-based polyol was simplified from a two-step to a one-step process, less materials were required and green solvent were employed aligning with green chemistry principles. This cellulose-based polyol was structurally characterized and used to produce flexible viscoelastic PU foams with 10, 20, 30, 40 and 50% of total polyol content. Physical, mechanical, structural, morphological and biodegradability properties of the viscoelastic PU foams were evaluated. While biodegradability of these open-celled foams increased to 35%, up to 40% cellulose-based polyol content, they also fulfill the final product standards (apparent density: 46–50 kg/m³, hardness: 70–75 N, tensile strength: 87–122 kPa, elongation: 300–390%, compression set: 3.3–5.2%, compression strength: 0.6–2.7 kPa, biodegradability: 4–35%). These next-generation materials serve as more sustainable alternatives to conventional flexible viscoelastic PU foams while preserving their quality.

Keywords

bio-based flexible polyurethane cellulose-based polyol hydroalkylation reaction biodegradability principles of green chemistry sustainability

Introduction

Global plastic waste is projected to quadruple by 2060, reaching nearly one billion metric tons.¹ PUs represent a significant portion of this consumption, with flexible PU foam alone accounting for over 67% of total PU production.² Although flexible PU foam is widely used in bedding, furniture, automotive interiors, packaging, agricultural plant nurseries, pressure sensors, and triboelectric nanogenerators for energy harvesting, it degrades only over very long time periods and is therefore classified as non-biodegradable. Its thermoset nature severely limits recycling options, restricting its reuse to small amounts as a filler material.

Bio-based products, derived from renewable resources like plants, can enhance sustainability and reduce fossil fuel reliance, offering benefits such as lower toxicity, reduced carbon footprint, and biodegradability.³ Biodegradable products can decompose naturally in soil without requiring chemical processes, preventing accumulation.⁴ Conventional PUs resist biodegradation in soil, since only selected microorganisms can break them down⁵ which requires processes that are both costly and labor-intensive. Consequently, research has shifted toward developing biodegradable PU foams from natural, bio-based resources that naturally degrade in soil ecosystems. PU biodegradation varies based on its type, chemical structure, and composition. Polyester PU’s, used in rigid PU formulations, are generally more prone to biodegradation, whereas polyether PUs, used in flexible PU formulations, exhibit higher resistance.^6,7 Thus, enhancing the biodegradability of polyether PU foams is especially crucial.

Several studies have explored the incorporation of bio-based components in PU formulations. Bio-based soy oil and lignin powder (comprising up to 25% of the polyol content) were utilized to produce rigid PU, improving biodegradability while preserving robust mechanical properties.⁸ Leszczyńska et al. developed rigid PU foams using a rapeseed oil-based polyol and eggshell waste. While the biodegradability of these foams was not reported, the incorporation of bio-based materials into the PU formulation yielded final products with comparable or enhanced properties.⁹ Sago starch, used as a bio-based and biodegradable filler in rigid PU formulations, acted as nucleation sites, increasing the number of nuclei and yielding smaller cells with a more uniform distribution.¹⁰ Succinic acid-based polyol with C black resulted in flexible PU foams with improved properties such as increased foam density and sound insulation performance.¹¹ Singh et al. prepared flexible PU foams using polyol containing ethylene oxide and propylene oxide based on glycerine. Compared with foams made from commercial polyols, those incorporating the bio-based polyol exhibited higher elongation, resilience, and compression set, as well as lower hardness.¹² Sohrabi et al. produced flexible PU foams using glycerine as a trifunctional chain extender and found that these foams exhibited superior compression‐set performance compared to those made with other chain extenders.¹³ Losio et al. produced flexible PU’s from bio-based epoxidized soy bean and linseed oils and caprylic acid or 3-phenyl butyric acid with 40–90 kg/m³ density and they obtained better mechanical properties than reference PU synthesized with propylene glycol.¹⁴ However, biodegradability of these foams were not reported either. Further studies are needed to comprehensively evaluate the impact of bio-based components on the properties of flexible PU foams, including their mechanical and morphological characteristics, as well as their biodegradability.

Cellulose, although the most abundant natural polysaccharide,¹⁵ remains underexplored for the production of biodegradable PU products. Studies include, esterification and depolymerization of microcrystalline cellulose to obtain a furan-rich bio-oil then reduction of this bio-oil to a type of cellulose-derived polyol¹⁶ that was used as a chain extender to synthesize polyurethane foams in presence of cellulose citrate as a thickening agent.¹⁷ Another chemical modification involves reacting cellulose only^18,19 or cellulose and metasilicic acid with glycidol and ethylene carbonate²⁰ to produce a bio-based polyol to obtain rigid PU’s with increased biodegradability. In another study, microcrystalline cellulose was employed, with castor oil serving as both an additional polyol and a dispersing agent in the reaction medium.²¹ In this context, cellulose was merely dispersed rather than chemically incorporated. Due to the extensive intra- and intermolecular hydrogen bonding within cellulose molecules, chemical modification is necessary to enable successful PU synthesis.

In this study, we synthesized a new type cellulose-based polyol to produce flexible viscoelastic PU foams. We employed water as a green solvent and unlike previous studies, we reacted microcrystalline cellulose solely with propylene carbonate, reducing both the number of chemicals and reaction steps, in line with green chemistry principles. Although the hydroxyalkylation reaction product of cellulose and propylene carbonate was used for other applications (i.e. regenerated cellulose films),²² to the best of our knowledge, this is the first time this material is used as a bio-based polyol to produce more environmentally-friendly flexible PU foams.

The bio-based polyol was structurally characterized using nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectrometers, and its hydroxyl number and water content were also evaluated. Structural, physical, mechanical, morphological and biodegradation properties of the resulting flexible PU foams with bio-based polyol loadings of 10, 20, 30, 40 and 50% w/w of the total polyol were investigated. PU foams with up to 40% bio-based content not only met final product standards including suitable morphology, density, hardness, tensile strength, elongation, compression set, and compression strength, all of which can be tailored by adjusting the cellulose-based polyol percentage, but also demonstrated enhanced bio-content and biodegradability (up to 35%). These attributes establish them as promising sustainable alternatives to conventional viscoelastic flexible PU foams.

Experimental

Materials

The following materials; propylene carbonate (Merck, Germany), DABCO 33LV as amine catalyst (99.9%, Evonik, Germany), Voranol 8322 (OH number: 48, viscosity (25°C): 700 mPa.s) (Dow Chemical, USA) as commercial polyether polyol and Caradol SA250-06 polyol (OH number: 250, viscosity (25°C): 270 mPa.s) (Shell Chemicals, USA) as commercial polyether polyol used in viscoelastic foam formulations, stannous octoate as catalyst (99.9%, Momentive, USA), silicone L620 as surfactant (Momentive, USA), toluene diisocyanate (TDI) Voranate T-80 (80:20 mixture of the 2,4- and 2,6- isomers of TDI, 99.5%, Dow Chemical, USA), acetonitrile (99.9%, ISOLAB, Germany), methanol (≥99.8%, ISOLAB, Germany), 2-propanol (≥99.5%, ISOLAB Germany), toluenesulphonyl isocyanate (TSI) (96%, Sigma Aldrich, Germany), microcrystalline cellulose (white powder, 65 μm, pH: 5–7, ρ: 0.26–0.31 g/cm³ (20°C), Tito, Türkiye), potassium carbonate (≥99,5% Tekkim, Türkiye), tetra-n-butylammonium hydroxide (TBAH) (96%, Sigma-Aldrich, Germany), Karl Fischer CombiTitrant 5 (ρ:1.19 g/cm³ (20°C), Merck, Germany) and anhydrous methanol (99.9%, Sigma-Aldrich, Germany) were used without further purification.

Synthesis of cellulose-based polyol

Water (250 g) and microcrystalline cellulose (53.95 g) were mixed at 80 °C (with a heating mantle) for 3 h in a three necked round bottom flask (1 L) equipped with a horizontally positioned water-cooled condenser and mechanical mixer (IKA Eurostar 60) which was set at 600 rpm. Propylene carbonate (542.79 g) and potassium carbonate (3.33 g) were added and further stirred for 6 h at 140 °C. The water in the reactor was removed after the reaction using a water-cooled condenser attached to a vacuum pump (Value VE 260N). The product was used without further purification.

Synthesis of partially bio-based PU

The synthesized bio-based polyol (0–50 g), commercial polyols Voranol (20 g) and Caradol (80–30 g), (where the combined content of all bio- and commercial polyols was fixed at 100 parts) (Table 1), water (2 g), amine catalyst (0.45 g), stannous octoate (0.05 g), silicone surfactant (2 g) and TDI (29-31 g) were mixed in a high speed mixer (1000 rpm) in an 800 mL plastic cup for 5 s and poured into an open box (45 cm × 45 cm × 20 cm) and flexible PU foam was obtained after 1.5 to 2 min. The TDI content was adjusted to maintain an isocyanate index of 0.9, a level shown to be optimal for flexible foam formulations.²³ PU foams were produced using bio-based polyol at 10, 20, 30, 40, and 50% (w/w) of the total 100 parts polyol. A reference formulation without bio-based polyol (BBP0) was included, while formulations containing 10–50% bio-based polyol were designated as BBP10, BBP20, BBP30, BBP40, and BBP50, respectively.

Table 1.

Proportions of each polyol in the total polyol formulation used for PU production.

	BBP0	BBP10	BBP20	BBP30	BBP40
Voranol	20	20	20	20	20
Caradol	80	70	60	50	40
Bio-based polyol	0	10	20	30	40

Characterizations of cellulose-based polyol and PU

Structural characterization of the polyol and PU was carried out using a Shimadzu IR Spirit-X Fourier Transform Infrared (FTIR) Spectrometer. ¹H nuclear magnetic resonance (NMR) spectroscopy of the polyol was performed using a Bruker Ascend 500 MHz Spectrometer.

The viscosity of the polyol was measured by using Brookfield Viscometer at 22°C (ASTM D4878). The hydroxyl number of the polyol was determined according to ASTM E1899-02 standard. In summary, 10 mL of acetonitrile was added to the 100 mL beaker containing the sample (0.16 g) and mixed for 20 min. The optimum mass was determined using the relation: sample mass (g) = 40/expected OH value. 20 mL of TSI solution (8% (v/v) in acetonitrile) was added and mixed for 5 min. Then, 0.5 mL of water was added to quench the excess TSI. After stirring for 1 minute, 50 mL of acetonitrile was introduced. After drying the electrode, titration was performed against 0.1 M TBAH solution prepared in 2-propanol/methanol by using SI Analytics Titrator TitroLine 7000 Automatic Titrator. Hydroxyl number was calculated according to Eq. 1 below where N, V₁ and V₂ correspond to normality and volume (mL) of TBAH at the second and first end point, respectively.

O H = \frac{(V_{2} - V_{1}) \times N \times 56.1}{s a m p l e, g}

(1)

The water content of polyol was determined by Karl Fischer (KF) method which uses iodometric reaction between the KF reagent and the trace amount of water present in the sample.²⁴ The volumetric KF instrument (Metroohm 915 KF Ti-Touch) comprises a titration vessel with a stirrer, a double platinum electrode, and a burette. The process began with pre-titration of the working medium to remove moisture, ensuring a stable endpoint. Anhydrous methanol (50 mL) dissolved the sample (0.5 g), and the burette delivered the KF reagent (CombiTitrant 5). The endpoint was detected potentiometrically by measuring the voltage between the platinum electrodes, which stabilized at a constant value. The water content is directly related to the amount of titrant used (Eq 2).

C_{H_{2} O} = \frac{V_{K F r e a g e n t} \times C_{K F r e a g e n t}}{m_{s a m p l e}} \times 100

(2)

Here, $C_{H_{2} O}$ , $V_{K F r e a g e n t}$ , $C_{K F r e a g e n t}$ and $m_{s a m p l e}$ correspond to water content (%), volume (mL) and concentration of the KF reagent (mg/mL) and sample mass (mg).

A Phillips XL30 SFEG Scanning Electron Microscope (North Billerica, MA, USA) was used for microstructural examination of the flexible PU foam sample surfaces. All samples were coated with a gold layer to prevent charging effects. Surface images were captured at magnifications of 50, 125, 500 and 2000, using an accelerating voltage of 5 kV. To obtain a representative image of the foam samples, at least six different regions were analyzed.

Density measurements were carried out according to ISO 845 standard. According to this standard, the middle section of the foam sample was cut into 40 × 40 × 5 cm and conditioned at 23 °C (±2 °C) and 50% (±5%) relative humidity for at least 24 h prior to measurement. The sample was weighed on a calibrated balance, and its dimensions were measured to determine its volume. The apparent density (ρ) was then calculated as ρ = m/V where m and V are the mass and volume of the sample.

Hardness was determined on the middle section of flexible PU foam specimens (35 × 35 × 5 cm) using a Devotrans Testing Machine in accordance with ISO 2439 Method B Indentation Force Deflection (IFD). IFD quantifies the force required to compress the foam to a specified percentage of its original height; higher IFD values correspond to harder foam.

The tensile and deformation behavior of flexible cellular materials are determined by subjecting them to a controlled extension rate until failure. The foams were conditioned at 23 °C (±2 °C) and 50% (±5%) relative humidity for at least 24 h prior to measurement and tested by using Devotrans universal testing machine (load cell 5 kN) according to ISO 1798 at 105 mm/min test speed. PU specimens were molded in a dog-bone geometry (165 mm overall length, 13 mm width, 50 mm gauge length) to concentrate fracture in the midsection and prevent failure at the clamping regions. Tensile strength represents the maximum tensile stress a test specimen can withstand before failure, while elongation at break denotes the percentage increase in length at the point of fracture.

For elasticity tests, ASTM D3574 ball rebound method was carried out. The test consisted of a 16 mm magnetic ball dropping freely onto a sample from a specified height of 500 mm. Foams were tested according to ISO 1856 for compression set, which shows the ability of the foam to rebound. For this, the foam samples were cut into 5 × 5 cm ( ×2.5 cm length) and 70% compressed at 70°C for 22 h. The compressive strength was measured for the samples with dimensions of 5 × 5 × 5 cm using a Instron 5569 Universal Testing Machine in accordance with ISO 844 standard. The test speed was 10 mm/min and the compression stress values were obtained at 40% deformation.

For biodegradability test, the samples were cut with dimensions of 5 × 5 × 1.5 cm. After drying the foam samples in an oven at 70 °C for 8 h to constant mass, they were weighed, wrapped in plastic mesh to prevent scattering, and buried in soil at a depth of 5 cm (Figure 1). The test was conducted at room temperature, with soil moisture maintained at 60% by adding 500 mL of distilled water every 2–3 days (pH: 6–7, electrical conductivity: 1–2 mS/cm). At weeks 1, 2, 3, 4, 6, 8, 10, and 12, samples were retrieved from the soil, lightly brushed, rinsed with water, and dried in an oven at 70 °C for 8 h to constant mass. The degree of biodegradation was calculated as the percentage mass loss, determined by the difference between the initial mass and the mass measured at each sampling interval.

Figure 1.

PU foam samples prepared for biodegradability test.

Results and discussion

In this study, a bio-based polyol was synthesized via the hydroalkylation reaction of microcrystalline cellulose with propylene carbonate, using potassium carbonate as a catalyst. Flexible PU foams were then produced with varying proportions of this polyol, and their properties were compared to those of a reference foam.

Physical properties of the cellulose-based polyol

The properties of the obtained cellulose-based polyol, in the form dark brown viscous liquid (Figure 2), are given in Table 2.

Figure 2.

General appearance of the bio-based polyol.

Table 2.

The properties of the synthesized bio-based biodegradable polyol.

Property	Unit	Value
Hydroxyl number	mg KOH/g	273 ± 2
Water content	%	0.11 ± 0.03
Viscosity	mPa.s (25 °C)	2500 ± 74

The average hydroxyl number of polyol mixtures in viscoelastic flexible PU foams ranges between 180 and 400 mg KOH/g^25–27 and the hydroxyl number of the synthesized bio-based polyol falls within this range. The water content of the polyol, determined by the Karl Fischer method was found to be 0.11%. The viscosity of the synthesized bio-based polyol was measured at 2500 mPa.s at 22 °C where similar viscosity ranges exist for polyether polyols used for flexible foam production.²⁸ Therefore, the synthesized polyol with these properties is a good candidate to obtain viscoelastic flexible PU foams successfully.

Structural properties of the cellulose-based polyol

Successful PU foam production requires that all reactants be liquids or dissolved in an appropriate solvent. Cellulose, however, is insoluble due to its extensive intra- and intermolecular H bonding; these bonds must be broken and replaced with chemical groups that make the molecule more soluble. Consequently, the hydroxyalkylation reaction of cellulose with propylene carbonate was conducted in the presence of potassium carbonate catalyst at 140 °C for 6 h (Figure 3).

Figure 3.

Hydroxyalkylation reaction of cellulose and propylene carbonate.

The reaction between microcrystalline cellulose and propylene carbonate was monitored through FTIR spectroscopy, with spectra recorded at hourly intervals throughout the reaction. The spectra of the reaction mixture at the initial stage, where all components were added including propylene carbonate and catalyst, as well as after 3 and 6 h, are presented in Figure 4. The absorption band at around 3350 cm^-1 is attributed to the O-H bending vibrations of cellulose and water in the reaction medium.

Figure 4.

FTIR spectra of microcrystalline cellulose and propylene carbonate reaction mixture at (1) t = 0, (2) t = 3 h and (3) t = 6 h.

As the reaction proceeds, the O-H band undergoes both a change in shape and a shift in its maximum from 3378 to 3324 cm^-1, indicating a reorganization of H bonding in the new structure.²⁹

The disappearance of characteristic peaks at 1178 cm^-1, associated with the asymmetrical stretching of C-O-C,³⁰ 1355 cm^-1 corresponding to the bending vibration of the ring and symmetric bending vibration of CH₃, and 1388 cm^-1 and 1784 cm^-1, linked to the wagging vibration of O-CH₂³⁰ and the stretching vibration of the carbonyl group (C=O) in propylene carbonate,³¹ further supports the occurrence of a successful hydroalkylation reaction. Furthermore, the decreasing intensity of the 1640 cm^-1 peak (O-H bending vibration of water) may also suggest a reduction in water bound to cellulose over time, indicating an increase in the material’s hydrophobicity.³² The C-O stretching band at 1046 cm^-1 remains unchanged in the spectra, confirming that the corresponding bond in the molecule is unaltered.¹⁸

The ¹H NMR spectrum of the cellulose-based polyol is presented in Figure 5. The peak observed at 1.30 ppm corresponds to the three protons in the CH₃ group of propylene carbonate³³ (a). The peaks at 4.00 ppm and the specific ratio at 4.50 ppm are attributed to the two protons in the CH₂ group (b), while the 4.90 ppm peak corresponds to a single proton in the CH group³³ (c). Additionally, the protons from cellulose backbone contribute to the peaks around 3.20–4.50 ppm, whereas OH protons are responsible for the peak observed at 1.00 ppm.³⁴ The disappearance of the characteristic OH peaks of cellulose at 4.30, 5.40, and 5.46 ppm indicates its successful reaction with propylene carbonate.³⁵

Figure 5.

¹H NMR spectrum of the synthesized cellulose-based polyol.

Structural properties of the partially bio-based flexible PU foam

Figure 6 shows the FTIR spectra of the synthesized flexible PU foam containing 0 and 40% bio-based polyol. The 3288 cm^-1 peak corresponds to N-H stretching of the urethane bond, while the peaks between 2968 and 2854 cm^-1 are attributed to asymmetric and symmetric C-H stretching of aliphatic chains. The 1723 cm^-1 peak represents the C=O stretching vibrations of the carbonyl group. Additionally, the peaks at 1530, 1222, and 1085 cm^-1 are associated with N-H bending, C-N stretching, and C-O stretching vibrations of the urethane bond, respectively.³⁶ Small peak observed in the FTIR spectrum of the PU’s include a 1597 cm^-1 peak, which corresponds to C=C vibrations in the aromatic ring of the PU synthesized with TDI, which contains an aromatic ring.³⁷ The only difference between the two PU samples is the 1640 cm^-1 band. This band, previously noted during bio-based polyol characterization, implies that residual cellulose particles likely remain in the foam, which is possible given that the bio-polyol was used without further purification.

Figure 6.

FTIR spectrum of flexible PU foam containing 0 and 40% BBP.

Physical and mechanical properties of the partially bio-based flexible PU foam

In this study, flexible viscoelastic PU formulations were prepared using a reference formulation with no bio-based polyol (BBP0) and formulations containing 10, 20, 30, and 40% (w/w) bio-based polyol (BBP), based on a total polyol content of 100% (Figure 7). While foams incorporating 10–40% BBP were successfully synthesized, the formulation with 50% BBP failed to achieve a suitable foam structure.

Figure 7.

The general appearance of the synthesized flexible viscoelastic foams: (a) BBP0, (b) BBP10, (c) BBP20, (d) BBP30, (e) BBP40, and (f) BBP50, where BBP0 contains no bio-based polyol and BBP10–BBP50 represent formulations containing 10%, 20%, 30%, 40%, and 50% (w/w) bio-based polyol, respectively, based on a total polyol content of 100%.

Physical and mechanical test results of the PU foams containing up to 40% BBP are given Table 3. As the BBP ratio in the foam increased, the hardness of the foam decreased from 75.0 to 69.8 N. BBP10 had the lowest density, and as the BBP content in the foam increased, the foam density increased, approaching the density of the standard foam. Tensile stress values decreased from 116 kPa (BBP0) to 87 kPa at BBP20, then increased beyond the reference sample (BBP0) to 122 kPa at BBP30, before returning to the reference value at BBP40. An increase in BBP content enhanced the foam’s flexibility, with a notable rise observed for BBP10 and BBP20, reaching up to 388%. However, at BBP30, the flexibility decreased to levels similar to the reference sample (305%) before increasing again at BBP40 to 320%. The elasticity of all samples was comparable, with only a slight decrease observed at BBP40. According to the compression set, the ability of the foam to turn to its original thickness was lowered as the BBP content increased. As BBP content increased from 0% to 30%, the compression strength of PU foams remained relatively stable, showing only a slight decrease from 2.7 kPa to 2.4 kPa. At 30% and 40% BBP content, the value gradually declined to 1.1 kPa and 0.6 kPa. The bio-based polyol exhibits considerably higher viscosity than the commercial polyols employed in these PU foams. As its proportion in the formulation increased, the overall viscosity of the reacting mixture also rose, which can result in higher density at greater bio-based polyol contents. The crosslinking efficiency of this polyol is likely lower, leading to greater chain mobility and enhanced softness.³⁸ Consequently, as the bio-based polyol content increased, the resulting foams showed decreased hardness and compression strength and increased elongation, reflecting improved flexibility and cushioning performance of the foam products.³⁹ For applications where high compressive strength is critical, BBP20 appears to be the most suitable formulation. Conversely, in cases where tensile strength is the primary requirement, BBP40 may also be considered a viable option. Consequently, the findings indicate that the physical and mechanical properties required for a specific flexible PU foam application can be tailored by adjusting the cellulose-based polyol content.

Table 3.

The physical and mechanical properties of the flexible viscoelastic PUs (mean ± std dev).

	Unit	Standard	BBP0	BBP10	BBP20	BBP30	BBP40
Apparent density	kg/m³	ISO 845	50.1 ± 2.0	46.5 ± 1.8	47.2 ± 2.0	48.1 ± 1.9	50.3 ± 2.1
Hardness	N	ISO 2439	75.0 ± 5.0	73.6 ± 4.8	72.0 ± 4.5	70.6 ± 4.9	69.8 ± 4.6
Tensile strength	kPa	ISO 1798	116 ± 5	89 ± 2	87 ± 0	122 ± 3	116 ± 4
Elongation at break	%	ISO 1798	305 ± 24	388 ± 20	373 ± 10	305 ± 16	320 ± 16
Elasticity	%	ASTM D3574	5.0 ± 0.6	6.0 ± 0.6	5.0 ± 0.6	6.0 ± 0.6	4.0 ± 0.6
Compression set	%	ISO 1856	3.3 ± 0.2	3.4 ± 0.2	4.1 ± 0.2	4.7 ± 0.2	5.2 ± 0.2
Compression strength (at 40% deformation)	kPa (×-1)	ISO 844	2.7 ± 0.1	2.5 ± 0.2	2.4 ± 0.0	1.1 ± 0.1	0.6 ± 0.0

Several studies support the findings presented in this study. For instance, Ugarte el al. Synthesizing flexible PU foam with castor oil- and corn-based polyols found that increasing the bio-based polyol content raised foam density, reduced hardness, and enhanced elasticity.⁴⁰ Singh et al. produced flexible PU foams incorporating a glycerine-based polyol containing ethylene oxide and propylene oxide. These foams exhibited also increased elongation at break and compression set, as well as reduced hardness, compared to those formulated with commercial polyols.¹² Lumcharoen et al. synthesized a palm oil-based polyol and incorporated it into flexible PU foam formulations at levels up to 50%. Increasing the palm-based polyol content resulted in a smaller pore structure, higher density, and lower hardness, tensile strength, and elongation at break.³⁹ A study using natural-oil-derived polyol found that increasing the bio-based content up to 50% of the total led to higher foam deformation values.⁴¹ The results reported here further demonstrate that the physical properties of the foams are comparable to those of standard foam, with certain properties retained even at higher bio-based polyol percentages. Despite some variations, all formulations remain viable candidates for the production of commercial flexible viscoelastic foams.

Morphological properties of the partially bio-based flexible PU foam

SEM images in Figure 8 demonstrated that BBP10, BBP20, BBP30, and BBP40 exhibited an open-cell structure comparable to the standard foam BBP0, which represents the preferred morphology for flexible PU foams. The SEM analysis indicated a reduction in pore size with increasing bio-based polyol content, a trend that correlates with the observed increase in density from the physical tests. Additionally, an increased number of connections between the cells were observed as the bio-content increased. Similarly, Ling et al. found that, in rigid PU foams synthesized from a cellulose-derived bio-based polyol, increasing the bio-based polyol content resulted in smaller pore sizes and higher foam densities.⁴² As discussed in the previous section, the higher viscosity of the bio-based polyol increased the overall system viscosity, which may have hindered bubble expansion during the foaming process, leading to smaller pore sizes.⁴³ In a study on flexible PU production using fir sawdust, higher filler content resulted in smaller pores was explained as incomplete expansion from increased reaction medium viscosity.⁴⁴ In addition, in this study, some cellulose particles may remain in the foam product (as discussed in the FTIR analysis of the BBP40 foam) and this cellulosic content may have acted as a nucleating agent during cell formation, triggering the simultaneous initiation of many cells.¹⁰ This rapid nucleation can reduce the available gas for cell expansion, ultimately leading to smaller cell sizes. Decreasing the pore size in an open-pore structure generally increases airflow resistance, thereby enhancing thermal insulation, sound absorption and noise reduction performance. PU foams with smaller pore sizes are ideal for pressure sensors, as they can reliably detect pressures above 3 kPa.⁴⁵ Accordingly, higher density values are preferred for flexible PU foams used in triboelectric nanogenerators for energy harvesting in self-powered devices, as they facilitate higher voltage generation.⁴⁶ These enhancements in physical properties and morphology support the development of environmentally friendly thermal-insulation and sound-absorption materials, pressure sensors, and triboelectric nanogenerators.

Figure 8.

SEM images of flexible PU foams (a) BBP0, (b) BBP10, (c) BBP20, (d) BBP30, (e) BBP40 where BBP0 contains no bio-based polyol and BBP10–BBP40 represent formulations containing 10%, 20%, 30%, and 40% (w/w) bio-based polyol, respectively, based on a total polyol content of 100%. Scale bar: 500 μm.

Biodegradability of the partially bio-based flexible PU foam

Figure 9 illustrates the biodegradability% of foam samples over time. Viscoelastic PU foam with no BBP content had a maximum of around 4% biodegradability at week 12. As soon as BBP content was increased to 10%, biodegradability values increased more than three times (14.39%) for the same time period. As the BBP content in PU foam increased, its biodegradability improved across all levels. While there was a consistent increase in biodegradability as BBP content increased, interestingly, a more than two-fold increase in biodegradability was observed between BBP30 and BBP40 until 10^th week. The foam containing 40% BBP had the highest biodegradability rate in all weeks compared to other foams, with a biodegradability of 35% at 12^th week.

Figure 9.

Biodegradability percentages as a function of non-bio-based and partially bio-based flexible PU samples.

Biodegradation values reported in the literature vary significantly, largely depending on the type and composition of the raw materials used. The biodegradability of bio-based PU foams synthesized from liquefied wheat straw (LWS)–castor oil polyols depended on both the castor oil:LWS ratio and the isocyanate type (MDI or TDI). At a 50:50 ratio using TDI, the foam achieved 12.25 % biodegradation after 60 days.⁴⁷ Lignin and soy oil-derived polyols polymeric methyldiphenyl diisocyanate produced PU foams with a biodegradability of around 10% at 16 weeks.⁸ Wang et al. demonstrated that adding soy protein isolate to flexible PU foam enhanced biodegradability, reaching approximately 16% after 28 days with 30% soy protein content.³ Olivito et al., reacted polyethylene glycol and L-lysine ethyl ester diisocyanate using a bio-based chain extender bis(hydroxymethyl)furan and the resulting flexible PU foam had a biodegradability of 45% after 12 weeks.⁴⁸ Rigid PU foams, synthesized using polyols prepared in a similar manner to this study (via hydroalkylation of cellulose with glycidol and ethylene carbonate) resulted in to 83% biodegradation within 4 weeks under laboratory conditions.¹⁸ This is likely due to the study’s exclusive use of cellulose-based polyol to produce rigid PU, which, as previously noted, is more susceptible to biodegradation than flexible PU.^6,7 Additionally, our study employed TDI, whereas the previous work used polymeric diphenylmethane-4,4′-diisocyanate; this difference may affect PU foam biodegradation rates, as the aromatic structure of TDI inherently slows its degradation. Moreover, TDI degradation yields aromatic amines that are potentially toxic and thus require careful management.²⁷ Besides that, cellulose-based polyols break down into non-toxic, naturally occurring metabolites, such as simple sugars and oligosaccharides, and hydroxyalkylated cellulose-derived polyols have been reported to achieve complete biodegradation within approximately 1 month.¹⁹ Propylene carbonate is classified as a green chemical due to its non-toxic profile and its derivation from renewable resources.⁴⁹ Accordingly, this polyol is highly likely to degrade into non-toxic metabolites that do not inhibit soil health or plant growth.

PU foams derived from bio‐based components biodegrade faster and produce less toxic byproducts, making cellulose‐based polyols a more environmentally favorable alternative to petroleum‐based materials. For future work, the isocyanate component in PU could be also replaced with biodegradable or non-isocyanate containing alternatives to further increase the sustainability and biodegradability of the flexible PU foam. Nevertheless, increasing biodegradability from a few percent to 35% represents a significant advancement in the sustainability of flexible PU foams, with minimal compromise to their functional properties.

Conclusions

In this study, flexible viscoelastic PU foam that contains 10–50% of cellulose-based polyol, which was synthesized for the first time by hydroalkylation reaction of microcrystalline cellulose and propylene carbonate, was physically, structurally and morphologically characterized. The number of reaction steps and required chemicals for polyol production was minimized in line with green chemistry principles. Flexible viscoelastic PU foams, which are inherently challenging to biodegrade, incorporated up to 40% cellulose-based polyol successfully demonstrated a 35% enhancement in biodegradability while preserving the properties of conventional petroleum-based foams. The foams exhibited an open-cell structure, aligning with the desired flexible foam morphology. Pore size, apparent density (46–50 kg/m³), hardness (70–75 N), tensile strength (87–122 kPa), elongation (300–390%), compression set (3.3–5.2%), compression strength (0.6–2.7 kPa) and biodegradability (4–35%) could be tuned by varying the percentage of cellulose-based polyol. These materials are recommended for use in any flexible (viscoelastic) PU foam applications as a more environmentally-friendly material alternative.

Footnotes

Acknowledgements

The authors would like to acknowledge Safaş Saf Plastik San. Tic. A.Ş. for the provision of resources, Adem Şen for compression tests and Ahmet Nazım for SEM analysis.

ORCID iD

Merve Mocan

Ethical considerations

This study does not involve any human or animal subjects, and therefore, ethical approval is not required.

Author contributions

E.B.A. Methodology, Investigation, Data curation, Visualization, Writing – original draft; M.M. Conceptualization, Methodology, Supervision, Data curation, Visualization, Writing – original draft, Writing – review and editing, Writing – Proofreading.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References

OECD . Global Plastics Outlook: policy scenarios to 2060. : OECD Publishing, 2022.

Gama

Ferreira

Barros-Timmons

. Polyurethane foams: past, present, and future. Materials 2018; 11: 1841. https://doi.org/10.3390/ma11101841

Wang

Zhou

. Soy protein based biodegradable flexible polyurethane foam. Adv Mater Res 2011; 152-153: 1862–1865. https://doi.org/10.4028/www.scientific.net/AMR.152-153.1862

Moshood

Nawanir

Mahmud

, et al.

Sustainability of biodegradable plastics: new problem or solution to solve the global plastic pollution?

Curr Res Green Sustain Chem 2022; 5: 100273. https://doi.org/10.1016/j.crgsc.2022.100273

Howard

. Biodegradation of polyurethane: a review. Int Biodeterior Biodegrad 2002; 49: 245–252. https://doi.org/10.1016/s0964-8305(02)00051-3

Skleničková

Abbrent

Halecký

, et al. Biodegradability and ecotoxicity of polyurethane foams: a review. Crit Rev Environ Sci Technol 2020; 52: 157–202. https://doi.org/10.1080/10643389.2020.1818496

Kemona

Piotrowska

. Polyurethane recycling and disposal: methods and prospects. Polymers 2020; 12. https://doi.org/10.3390/POLYM12081752

Luo

Xiao

, et al. Development of high-performance biodegradable rigid polyurethane foams using all bioresource-based polyols: Lignin and soy oil-derived polyols. Int J Biol Macromol 2018; 115: 786–791. https://doi.org/10.1016/j.ijbiomac.2018.04.126

Leszczyńska

Ryszkowska

Szczepkowski

, et al. Cooperative effect of rapeseed oil-based polyol and egg shells on the structure and properties of rigid polyurethane foams. Polym Test 2020; 90: 106696. https://doi.org/10.1016/j.polymertesting.2020.106696

10.

Ruamcharoen

Phetphaisit

Chanlert

, et al. Sago starch and esterified sago starch as eco-friendly fillers for rigid polyurethane foams. J Cell Plast 2024; 60: 23–40. https://doi.org/10.1177/0021955x231224765

11.

Oliviero

Verdolotti

Stanzione

, et al. Bio-based flexible polyurethane foams derived from succinic polyol: mechanical and acoustic performances. J Appl Polym Sci 2017; 134: 45113. https://doi.org/10.1002/app.45113

12.

Singh

Gopalasamudram

Maitra

. Investigating the physico-mechanical and thermal properties of flexible polyurethane foams made with standard and special polyols produced by the variable pressure foaming technique. J Cell Plast 2025; 61: 81–102. https://doi.org/10.1177/0021955x251314036

13.

Sohrabi

. Effects of di- and trifunctional chain extenders on the properties of flexible cold-cure polyurethane foams. J Cell Plast 2024; 60: 41–57. https://doi.org/10.1177/0021955x231224776

14.

Losio

Cifarelli

Vignali

, et al. Flexible polyurethane foams from bio-based polyols: prepolymer synthesis and characterization. Polymers 2023; 15: 4423. https://doi.org/10.3390/polym15224423

15.

Seddiqi

Oliaei

Honarkar

, et al. Cellulose and its derivatives: towards biomedical applications. Cellulose 2021; 28: 1893–1931. https://doi.org/10.1007/s10570-020-03674-w

16.

Romeo

Olivito

Tursi

, et al. Totally green cellulose conversion into bio-oil and cellulose citrate using molten citric acid in an open system: synthesis, characterization and computational investigation of reaction mechanisms. RSC Advances 2020; 10(57). https://doi.org/10.1039/D0RA06542K

17.

Maiuolo

Olivito

Algieri

, et al. Synthesis, Characterization and Mechanical Properties of Novel Bio-Based Polyurethane Foams Using Cellulose-Derived Polyol for Chain Extension and Cellulose Citrate as a Thickener Additive. Polymers 2021; 13(16): 2802. https://doi.org/10.3390/polym13162802

18.

Szpiłyk

Lubczak

. The biodegradable cellulose-derived polyol and polyurethane foam. Polym Test 2021; 100: 107250. https://doi.org/10.1016/j.polymertesting.2021.107250

19.

Lubczak

Kus-Liśkiewicz

Lubczak

, et al. Biodegradable polyurethane foams based on polyols obtained from cellulose and its hydroxypropyl derivative. Materials 2024; 17: 5490. https://doi.org/10.3390/ma17225490

20.

Lubczak

Chmiel-Bator

, et al. Polyols and polyurethane foams obtained from mixture of metasilicic acid and cellulose. Polymers 2022; 14: 4039. https://doi.org/10.3390/polym14194039

21.

Porto

Cassales

Ciol

, et al. Cellulose as a polyol in the synthesis of bio-based polyurethanes with simultaneous film formation. Cellulose 2022; 29: 6301–6322. https://doi.org/10.1007/s10570-022-04662-y

22.

Nguyen

Kragl

Barke

, et al. Coagulation using organic carbonates opens up a sustainable route towards regenerated cellulose films. Commun Chem 2020; 3: 116. https://doi.org/10.1038/s42004-020-00360-7

23.

Olszewski

Kosmela

Piasecki

, et al. The impact of isocyanate index and filler functionalities on the performance of flexible foamed polyurethane/ground tire rubber composites. Polymers 2022; 14: 2022/12–24. https://doi.org/10.3390/polym14245558

24.

Rivera-Quintero

Patience

, et al. Experimental methods in chemical engineering: Karl fischer titration. Can J Chem Eng 2024; 102: 2980–2997. https://doi.org/10.1002/cjce.25295

25.

Magnin

Pollet

Phalip

, et al. Evaluation of biological degradation of polyurethanes. Biotechnol Adv 2020; 39: 107457. https://doi.org/10.1016/j.biotechadv.2019.107457

26.

Jin

Dong

Guo

, et al. Current advances in polyurethane biodegradation. Polym Int 2022; 71: 1384–1392. https://doi.org/10.1002/pi.6360

27.

Trhlíková

Vlčková

Abbrent

, et al. Microbial and abiotic degradation of fully aliphatic polyurethane foam suitable for biotechnologies. Polym Degrad Stabil 2021; 194: 109764. https://doi.org/10.1016/j.polymdegradstab.2021.109764

28.

Ionescu

. Chemistry and technology of polyols for polyurethanes. : Rapra Technology, 2005, p. 586.

29.

Seo

Cho

Ohga

. Possibility of hydrogen bonding between AKD and cellulose molecules during AKD sizing. J Fac Agric Kyushu Univ 2008; 53: 405–410, Article. https://doi.org/10.5109/12849

30.

Ikezawa

Ariga

. In situ FTIR spectra at the Cu electrode/propylene carbonate solution interface. Electrochim Acta 2007; 52: 2710–2715. https://doi.org/10.1016/j.electacta.2006.09.050

31.

Członka

Strakowska

Kairyte

. Application of walnut shells-derived biopolyol in the synthesis of rigid polyurethane foams. Materials 2020; 13. https://doi.org/10.3390/ma13122687

32.

Selvaraj

Subramanian

. A comparative study of smart polyurethane foam using RSM and COMSOL multiphysics for acoustical applications: from materials to component. J Porous Mater 2023; 30: 547–563. https://doi.org/10.1007/s10934-022-01362-7

33.

Mandal

Monkowius

Chakraborty

. Synthesis and structural characterization of titanium and zirconium complexes containing half-salen ligands as catalysts for polymerization reactions. New J Chem 2016; 40: 9824–9839. https://doi.org/10.1039/c6nj02148d

34.

King

AWT

Mäkelä

Kedzior

, et al. Liquid-state NMR analysis of nanocelluloses. Biomacromolecules 2018; 19: 2708–2720. https://doi.org/10.1021/acs.biomac.8b00295

35.

Guo

Liu

Chen

, et al. Direct grafting modification of pulp in ionic liquids and self-assembly behavior of the graft copolymers. Cellulose 2013; 20: 873–884. https://doi.org/10.1007/s10570-012-9847-5

36.

Haryńska

Kucinska-Lipka

Sulowska

, et al. Medical-grade PCL based polyurethane system for FDM 3D printing—characterization and fabrication. Materials 2019; 12: 887. https://doi.org/10.3390/ma12060887

37.

Tang

Gao

. Structure analysis of polyether-based thermoplastic polyurethane elastomers by FTIR, 1H NMR and 13C NMR. Int J Polym Anal Char 2017; 22: 569–574. https://doi.org/10.1080/1023666X.2017.1312754

38.

Andersons

Cabulis

Kirpluks

. The effect of crosslink density on the physical and mechanical properties of bio-based polyurethane foams. Macromol Symp 2022; 404: 2100329. https://doi.org/10.1002/masy.202100329

39.

Lumcharoen

Saravari

. Preparation and characterization of flexible polyurethane foams from palm oil-based polyol. Adv Mater Res 2014; 911: 352–356. https://doi.org/10.4028/www.scientific.net/AMR.911.352

40.

Ugarte

Saralegi

Fernández

, et al. Flexible polyurethane foams based on 100% renewably sourced polyols. Ind Crop Prod 2015; 62: 545–551. https://doi.org/10.1016/j.indcrop.2014.09.028

41.

Guillame

Khalil

Misra

. Green and sustainable polyurethanes for advanced applications. J Appl Polym Sci 2017; 134: 45646. https://doi.org/10.1002/app.45646

42.

Kai

Aung

Rayung

, et al. Performance of ionic transport properties in vegetable oil-based polyurethane acrylate gel polymer electrolyte. ACS Omega 2019; 4: 2554–2564. https://doi.org/10.1021/acsomega.8b02100

43.

Jiang

Chen

Liu

, et al. Effect of bio-based polyols and chain extender on the microphase separation structure, mechanical properties and morphology of rigid polyurethane foams. Eur Polym J 2022; 179: 111572. https://doi.org/10.1016/j.eurpolymj.2022.111572

44.

Tiuc

A-E

Borlea

Nemeş

, et al. New composite materials made from rigid/flexible polyurethane foams with fir sawdust: acoustic and thermal behavior. Polymers 2022; 14: 3643. https://doi.org/10.3390/polym14173643

45.

Nabeel

Varga

Kuzsella

, et al. The effect of pore volume on the behavior of polyurethane-foam-based pressure sensors. Polymers 2022; 14: 3652. https://doi.org/10.3390/polym14173652

46.

Maamoun

Mahmoud

Naeim

, et al. Effect of density and thickness of flexible polyurethane foam on the performance of triboelectric nanogenerators. Mater Adv 2024; 5: 6132–6144. https://doi.org/10.1039/D4MA00304G

47.

Serrano

Rincón

García

, et al. Bio-degradable polyurethane foams produced by liquefied polyol from wheat straw biomass. Polymers 2020; 12: 1–14. https://doi.org/10.3390/polym12112646

48.

Olivito

Jagdale

Oza

. Synthesis and biodegradation test of a new polyether polyurethane foam produced from PEG 400, L-Lysine ethyl ester diisocyanate (L-LDI) and bis-Hydroxymethyl furan (BHMF). Toxics 2023; 11: 698. https://doi.org/10.3390/toxics11080698

49.

Akil

Lorenz

Lehnen

, et al. Safe and non-toxic hydroxyalkylation of xylan using propylene carbonate. Eur Polym J 2016; 77: 88–97. https://doi.org/10.1016/j.eurpolymj.2016.02.010