Preparation and characterization of biodegradable flexible polyurethane foam containing acetylated tapioca starch and Castor oil

Abstract

This research project focused on developing flexible polyurethane foams with high biodegradability, suitable as substrates for growing vegetables or cereal sprouts. These foams incorporated acetylated tapioca starch as a filler and castor oil, and their properties were evaluated. Experiments assessed how varying concentrations of acetylated starch (0, 15, 30, 45, and 60 wt%) and castor oil (0, 10, 20, and 30 wt%) affected the foams’ density, tensile properties, compression set, moisture absorption, biodegradability, and microstructure. Results indicated that increasing acetylated starch concentration led to higher density, compression set, biodegradability, and closed cell percentage, while tensile strength, elongation at break, water absorption, and mean cell diameter decreased. Most changes were significant (p < 0.05), with density, compression set, biodegradability, tensile strength, elongation at break, water absorption, and average cell diameter being 21.7 kg/m², 11.4%, 9.2%, 101.4 kPa, 97.2%, 3820%, and 0.34 mm respectively, for foam without castor oil and acetylated starch compared to 33.1 kg/m², 19.7%, 37.7%, 62.3 kPa, 28.9%, 2600%, and 0.186 mm for foam with 60 wt% acetylated starch, respectively. The influence of castor oil concentration on the foams’ physical and mechanical properties was inconsistent. Additionally, the germination amount of grown green basil seeds on commercial flexible polyurethane foam was higher than that of selected prepared foam (84.7% vs. 81.3%), but the germination rate of grown green basil seeds on selected prepared foam was higher than that of commercial foam at all times, including the ninth day (4.7 vs. 4.2 cm).

Introduction

Hydrocarbon resources for petroleum-based synthetic polymers are limited and non-renewable. Meanwhile, disposing of non-biodegradable polymer waste can harm the environment and human health. Consequently, identifying renewable alternative raw materials for producing biodegradable polymers has gained significant importance and has been a focus of extensive research in recent years.

Polyurethanes are a diverse group of elastic polymers known for their lightweight nature, excellent strength-to-weight ratio, and energy absorption properties, including shock, vibration, sound, and heat. They are widely used in the furniture and seating industry, as well as for sound and thermal insulation. Recently, flexible polyurethane foams have also been utilized as substrates for growing vegetables and cereal sprouts^1–3. Due to the non-biodegradable nature of many polyurethane foams, there has been significant research into developing biodegradable alternatives using renewable resources. This includes the incorporation of organic and inorganic fillers as cost-effective biodegradable materials^4–6, as well as polyol compounds derived from processed vegetable oils which serve as natural alternatives to petroleum-based polyols^7–12, in polyurethane foam formulations. It is expected that incorporating the acetylated tapioca starch as highly biodegradable filler and castor oil as renewable and biodegradable biopolyol in polyurethane foam preparation formulation leads to polyurethane foam with added biodegrability.

In a study, petroleum-based polyether polyol was substituted with up to 30 wt% castor oil, and the influence of castor oil concentration on the physicomechanical properties of the resulting flexible polyurethane foams was examined¹³. The findings indicated that as the castor oil concentration increased, foam density showed a slight rise, while the compression set significantly increased, with the foam without castor oil exhibiting a compression set of 7.1% compared to 14.7% for the foam with 30 wt% castor oil. The tensile strength of the foam remained largely unchanged, except for the sample with 30% castor oil, which exhibited a tensile strength of 122.6 kPa compared to the sample without castor oil with a tensile strength of 104.17 kPa¹³.

A study by Zhou et al.¹⁴ prepared biodegradable polyurethane foam and examined the impact of varying starch-to-varnish weight ratios on its thermal stability, microstructure, and biodegradability. The results indicated that the foam modified with varnish and starch exhibited excellent mechanical properties, thermal stability, and biodegradability, with a 38% biodegradation observed after 120 days for a 5:2 starch-to-varnish weight ratio.

Biodegradable flexible polyurethane foam was developed in the work of Wang and Zhou¹⁵, which examined the impact of soy protein isolate concentration on the foam’s physico-mechanical properties and biodegradability. The findings revealed that increasing soy protein isolate concentration to 30 wt% of petroleum-based polyether polyol led to higher density and compression set. Additionally, the biodegradability of the foam with 30 wt% SPI was 15% after 28 days. Olcay and Kocak¹⁶ studied the effect of surface-modified artichoke stem waste fibers concentration as a natural filler, up to 51.7 wt% of the petroleum-based polyol blend, on the physicomechanical properties of flexible polyurethane foam. Their results indicated that higher filler concentrations decreased both tensile strength and elongation at break. The effect of proportional blend of periwinkle and African star apple seed shell as bio-fillers in flexible polyether foam was studied by Onwuka and coworkers¹⁷. The researches claimed that since the fillers are of organic origin, readily available, cheap and eco-friendly, they provide a means of making biodegradable foam. The results of the experiment showed that the density of the foam samples progressively was increased as the quantity of the filler increased also the tensile strength and elongation at break of the foam was decreased on addition of the filler. This study developed flexible polyurethane foams using acetylated tapioca starch filler and castor oil as renewable and biodegradable materials, and assessed their physico-mechanical properties and suitability for growing green basil seeds.

Materials and methods

Materials

Petroleum-based polyether polyol (E3500, China) with a hydroxyl number of 47 mg KOH/g, viscosity of 500–700 mPas, molecular weight of 3500, functionality of 3, and moisture content below 0.05% was purchased from an Iranian supplier. Food-grade castor oil (PACIFIC, India) with a molecular weight of 933 and a hydroxyl number of 164 mg KOH/g was also acquired from an Iranian supplier. Toluene diisocyanate (20/80, Karun Petrochemical Company, Iran), amine catalyst (DABCO 33-LV, Evonik, Germany), tin octoate catalyst (DABCO T-9, Evonik, Germany), silicone oil (Momentive, Germany), commercial-grade methylene chloride, and distilled water as a foaming agent were all purchased from an Iranian supplier. Additionally, acetylated tapioca starch with 13% moisture content, a pH of 5–7, and an acetyl content of 2.5 g acetyl/100 g was obtained from an Iranian supplier.

Foam Preparation

Firstly the castor oil (CO) in the range of 0, 10, 20, and 30 part per hundred of polyol mixture (pphp) and the petroleum-based polyether polyol in an amount according to the relevant experiment were mixed well together. Next, the acetylated tapioca starch (ATS) in the range of 0, 15, 30, 45, and 60 pphp and in an amount according to the relevant experiment was added to the polyol mixture and the resultant mixture mixed well for 15 min at room temperature at a speed of 3000 rpm. The concentration ranges of ATS and CO were selected based on pre-tests as well as values ​​used in previous researches. The resulting compound was left to rest for 24 h so that the ATS particles were completely soaked into the polyol mixture. Then 1.2 part per hundred of polyol mixture (pphp) of silicone oil, 4.5 pphp of distilled water, 3 pphp of methylene chloride, 0.3 pphp of amine catalyst, and 0.2 pphp of Tin-based catalyst were added to the mixture of polyol compound and ATS respectively and mixed well for 2 min at ambient temperature at a speed of 2000–3000 rpm. While stirring (3000 rpm) the previous mixture at ambient temperature, a calculated amount of toluene diisocyanate (at an amount of 57.9, 59.5, 61.5 and 63.5 pphp according to castor oil concentration of formulation of 0, 10, 20 and 30 pphp respectively) was added to it for about 7 s and then the resulting viscous mixture was discharged into a previously prepared cardboard mold (with a polyethylene inner liner). After 7 days and the completion of the curing reaction at ambient temperature, the prepared foam samples were cut to the appropriate dimensions for each test.

Foam characterization

Density

The density of foam samples measuring 50 mm in length, width, and 25 mm in thickness was measured following ASTM D3574-17¹⁸. The density, calculated in kg/m² using Eq. 1, was based on three test repetitions, and the average value was reported.

1

where, W is weight of sample in g, and V is its volume in mm³.

Tensile properties

The test was conducted on a 10 mm thick dumbbell-shaped sample cut (in the direction of foam rising) from a foam sheet according to ASTM D412 Die A. Tensile properties were measured at ambient temperature using a 5KN servo-electromechanical testing machine (STM-5, Santam, Iran), following ASTM D3574-17. A 60 N load cell was employed, with a jaw separation of 100 mm and an upper jaw movement speed of 500 mm/min. The test included 4 repetitions, and the average values for tensile strength and elongation at break were reported.

Compression set

The compression set of the samples was measured following ASTM D3574-17. A foam sample measuring 50 mm in length, width, and 25 mm in thickness was cut and placed between the device plates. It was then compressed to 50% of its initial thickness (12.5 mm). The device was subjected to 70 °C for 22 h in an oven with approximately 5% relative humidity. Afterward, the device was removed, and the sample was left for 40 min at room temperature and atmospheric pressure. The final thickness was measured, and the compression set was calculated using Eq. 2. The test was conducted with three repetitions, and the average of the results was reported.

2

where, t₀ and t_f are initial (2.5 mm) and final thickness of the sample, respectively.

Porosity

To calculate the porosity of each sample, the formulation density was first determined using the weight percentages and densities of the components, which are as follows: petroleum-based polyol polyether (1.1 g/cm²), CO (0.96 g/cm²), ATS (1.5 g/cm²), toluene diisocyanate (1.2 g/cm²), and other materials (1 g/cm²). The porosity for each foam sample was then calculated by dividing the formulation density by the measured density.

Water absorption

Water absorption for each sample was measured following ASTM D570-98 (2010)¹⁹ standard by comparing the initial dry weight to the weight after 1 h of absorbing distilled water. The water absorption value was calculated using Eq. 3 (ASTM, 2010). The test was conducted in triplicate, and the average value of the results was reported.

3

where, m_f and m_i are final and initial weight of the sample, respectively.

Scanning electron microscopy (SEM)

Scanning electron microscope images of selected samples were captured using a Tescan VEGA-II (XMU, Czech Republic) at a voltage of 20 kV. A sample measuring approximately 50 mm in length, 50 mm in width, and 5 mm in thickness was cut from the prepared foam and coated with gold prior to imaging. Also cell morphological parameters including mean cell diameter and cell number density of various foam samples were determined from SEM images using Image J software version 3.4.1.

FTIR

FTIR analysis was conducted on samples measuring approximately 20 mm in length, 20 mm in width, and 5 mm in thickness with a Thermo Nicolet (Avatar 37, USA) spectrophotometer to examine functional groups and possible interactions among raw materials in the synthesized samples. The absorbance spectra were measured over the wavenumber range of 400–4000 cm^− 1 at a resolution of 2 cm^− 1 with 100 min^− 1 consecutive scans.

Biodegradability under composting conditions

The biodegradability of selected samples and the ATS, positive control, under composting conditions was assessed following the ASTM D5988-18 standard (ASTM, 2018). This test measures biodegradability based on the CO₂ released by microorganisms as they degrade the samples over time at approximately 58 °C.

Comparative study of the growth of green basil seeds grown on selected prepared and commercial foam

The growth of green basil seeds on selected prepared foam was compared to that of commercial flexible polyurethane foam by measuring germination amount, germination rate, and sensory attributes of green basil leaves.

Statistical analysis

Statistical analysis and variance analysis were performed using Minitab software (version 18). Means were compared using Duncan’s test at a 5% significance level (P < 0.05).

Results and discussion

Density

Density is a crucial characteristic of flexible polyurethane foams, indicating their load-bearing capacity and pricing within the industry. Figure 1 illustrates how ATS and CO concentrations affect foam density. As ATS concentration rises at each CO level, foam density significantly increases (p < 0.05), with densities of 21.7 kg/m³ for sample ATS0CO0 and 33.1 kg/m³ for sample ATS60CO0. Similarly, ATS0CO30 and ATS60CO30 exhibited densities of 22.6 kg/m³ and 42.5 kg/m³, respectively. Higher ATS concentrations lead to more pore filling, increased weight, and decreased porosity so resulting in greater density^17,20,21. Additionally, higher concentrations of fillers raise the viscosity of the reaction mixture, affecting the cell growth process and change the cell geometry²². Table 1 presents foam properties, showing that higher ATS concentrations correspond to a decrease in final rising height for all CO levels; for instance, ATS0CO0 and ATS60CO0 reached heights of 15.4 and 12.8 cm, while ATS0CO30 and ATS60CO30 had 13.5 and 8.8 cm height, respectively. Thus, as the final rising height—and therefore, the foam volume—decreases, its density increases.

Fig. 1.

Density variations in foam samples with ATS and CO concentrations. Different capital letters on bars in a column indicate the significant effect of ATS concentration on foam density. Different lowercase letters on bars in a row indicate the significant effect of CO concentration on foam density.

Table 1.

Some features of foam obtained from different formulations.

Test number	Sample code	Final rising height (mm)	Final rising time (s)	Calculated density (kg/m3)	Measured density (kg/m3)	Porosity (%)
1	ATS0CO0	15.4	120	1130	21.7	52.1
2	ATS0CO10	14.5	150	1119	23.4	47.8
3	ATS0CO20	14.0	190	1111	21.6	51.4
4	ATS0CO30	13.5	234	1102	22.6	48.8
5	ATS15CO0	14.9	140	1157	22.5	51.4
6	ATS15CO10	13.5	220	1147	25.3	45.3
7	ATS15CO20	12.8	270	1092	27.0	40.4
8	ATS15CO30	10.1	390	1131	25.5	44.4
9	ATS30CO0	13.3	220	1180	28.3	41.7
10	ATS30CO10	13.2	230	1173	31.1	37.7
11	ATS30CO20	12.6	350	1163	29.4	39.6
12	ATS30CO30	9.8	453	1155	30.5	37.9
13	ATS45CO0	12.9	270	1206	29.6	40.7
14	ATS45CO10	11.1	321	1210	32.8	36.9
15	ATS45CO20	10.0	383	1185	34.7	34.1
16	ATS45CO30	9.5	475	1176	37.7	31.2
17	ATS60CO0	12.8	320	1230	33.1	37.2
18	ATS60CO10	11.5	346	1214	38.4	31.6
19	ATS60CO20	9.1	470	1203	41.0	29.3
20	ATS60CO30	8.8	650	1195	42.5	28.1

Some researchers investigated the concentration effect of a mixture of equal weights of periwinkle and African star apple seed shell as natural fillers (0–50 wt%) in a petroleum-based polyol blend on the physicomechanical properties of flexible polyurethane foam¹⁷. Their findings showed that foam density increased with higher filler concentration, from 19.20 kg/m³ for the foam without fillers to 26.45 kg/m³ at 50 wt%. Also chicken egg shells (ES) in the range of 0–15 wt% was incorporated in polymeric polyol and produced flexible polyurethane foams were caracterized²³. According to results, density of the foam without filler and foam with 15 wt% filler, was 20.87 kg/m³ and 22.39 kg/m³, respectively. Similar trends have been reported by other studies regarding organic or mineral fillers’ impact on flexible polyurethane foam density^{15,20,21,24–26}.

Figure 1 indicates that the relationship between castor oil concentration and foam density varied according to ATS concentration; specifically, at high concentrations of ATS, foam density increased with increasing CO concentration. For instance, ATS60CO0 and ATS60CO30 had densities of 33.1 and 42.5 kg/m³, respectively. In summary, the higher hydroxyl number of CO compared to petroleum-based polyol polyether results in a greater number of functional groups in the triglycerides of this vegetable oil. As the proportion of CO in the mixture increases, the polymerization reaction intensifies, leading to higher viscosity. This increased viscosity slows the movement of gas produced during foaming in the reaction mixture so reducing its ability to raise the foam, which results in a slightly lower final foam height, decreased foam volume, and increased density eventually. Furthermore, Ogunfeyitimi et al.¹³ studied the replacement of petroleum-based polyether polyol with up to 30% castor oil in the polyol mixture¹³. Their results indicated a slight increase in foam density with higher castor oil concentrations, from 21.1 kg/m³ for foam without castor oil to 22.2 kg/m³ at 30 wt%. This finding is consistent with previous research on replacing petroleum-based polyether polyol with castor oil^27,28.

Tensile properties

Figure 2a,b illustrate the effects of ATS and CO concentrations on the tensile strength and elongation at break of the foams. The data indicates that, for each CO concentration, both tensile strength and elongation at break decreased significantly (p < 0.05) with increasing ATS concentration. For instance, ATS0CO0 exhibited tensile strength of 101.4 kPa, while ATS60CO0 showed 62.3 kPa. Similar results were observed with ATS0CO20 and ATS60CO20, which had tensile strengths of 164.4 kPa and 70.8 kPa, respectively. The elongation at break for ATS0CO0 was 97.2%, compared to 28.9% for ATS60CO0, and 111.2% for ATS0CO30 versus 22.3% for ATS60CO30.

Fig. 2.

Tensile strength (a), elongation at break (b) changes of foam samples with ATS and CO concentration. Different capital letters on bars in a row indicate the significant effect of ATS concentration on foam tensile strength and elongation at break. Different lowercase letters on bars in a column indicate the significant effect of CO concentration on foam tensile strength and elongation at break.

The limited interaction between ATS as a filler and the urethane matrix, as confirmed by FTIR test results, led to poor stress distribution and ineffective stress transfer from the matrix to the filler. This ultimately resulted in reduced tensile strength and elongation at break with higher ATS concentrations.

Olcay and Kocak examined the impact of surface-modified artichoke stem waste fibers as a natural filler at concentrations up to 51.7 wt% in a petroleum-based polyol blend for flexible polyurethane foam¹⁶. Their findings showed decreased tensile strength and elongation at break with increasing filler concentration: a foam with 10.9 wt% filler had a tensile strength of 265 KPa and 98% elongation at break, while one with 51.7 wt% had 150 kKPa and 83%, respectively. Also in a recent study the concentration effect of Camelina sativa (L.) Crantz pomace as a green filler (0–15 wt%) in a petroleum-based Commercial blend of short– and long–chain polyether polyols on the physicomechanical properties of polyurethane foam was investigated²⁹. According to the results obtained, the foam with 15 wt% filler showed a 10.7% and 6.4% decrease in tensile strength and elongation at break, respectively, compared to foam with 2.5 wt% filler. Similar trends have been reported by other researchers regarding the effects of filler concentration on the tensile properties of polyurethane foams^20,26,30,31.

According to Fig. 2a,b, the CO concentration also affected tensile strength and elongation at break, though the results were somewhat significant (p < 0.05) without a consistent trend. This irregularity has been noted in other studies examining the relationship between CO concentration and foam tensile properties^13,27.

Compression set

The compression set quantifies the irreversible thickness loss of flexible foam after removing a static load. Figure 3 illustrates how ATS and CO concentrations affect the foam’s compression set. For each CO concentration, the compression set increased with higher ATS concentration; for example, ATS0CO0 and ATS60CO0 had compression sets of 11.4% and 19.7%, respectively, while ATS15CO20 and ATS45CO20 had compression sets of 22.9% and 26.7%.

Fig. 3.

Compression set changes of foam samples with ATS and CO concentration.

The ATS filler particles, being neutral and non-interactive with the polyurethane matrix (as confirmed by FTIR tests), merely occupied space between polymer chains and foam pores. Consequently, by increasing the ATS concentration as the testing conducted under pressure, more foam deformation and increase in compression set was resulted. Another factor contributing to the increased compression set with higher ATS concentration is the conversion of mechanical energy stored in the filler particles into heat during testing. Research shows a similar trend, where the compression set of flexible polyether foam increased with the concentration of an equal weight mixture of eggshell and groundnut husk powder as natural fillers, from 7.5% (no filler) to 35% (40 wt% filler)³². Other studies also indicate increased compression sets in polyurethane foam with higher filler concentrations^15,20.

Figure 3 illustrates that foam compression set increased with higher CO concentrations at each ATS level. Specifically, ATS0CO0 and ATS0CO30 had compression sets of 11.4% and 20.5%, while ATS60CO0 and ATS60CO30 showed compression sets of 19.7% and 29.8%, respectively. The higher hydroxyl number of CO compared to petroleum-based polyol polyether increases polar urethane groups, which reduces polymer chain mobility and reversibility of foam thickness. This leads to greater deformation after load removal, resulting in higher foam compression sets. Ogunfeyitimi et al.¹³ similarly reported that substituting up to 30 wt% CO with petroleum-based polyether polyol in flexible foam resulted in increased compression set, with values rising from 1.7% for foam without CO to 14.7% for foam containing 30 wt% CO.

Water absorption

Water absorption is an important functional property of foams and can be either beneficial or detrimental. For applications like substrate for growing vegetables or grains sprouts or as a water absorbent, a foam with high water absorption is ideal, while a low water absorbing foam is preferred for other uses. Figure 4 illustrates the impact of ATS and CO concentration on foam water absorption. It shows that for each CO concentration, increasing ATS concentration significantly decreased water absorption (p < 0.05). Specifically, ATS0CO0 had a water absorption of 3820%, while ATS60CO0 measured 2600%. Similarly, ATS0CO20 and ATS45CO20 had water absorption levels of 3700% and 1890%, respectively.

Fig. 4.

Water absorption changes of foam samples with ATS and CO concentration.

Table 1 presents the porosity of the different foam samples, highlighting that porosity decreased as ATS concentration increased for each CO concentration. The reduction in water absorption can be linked to the diminished porosity obviously. A study exploring the effect of cassava starch on the water absorption of polyurethane foam based on petroleum-based polyether polyol found that foams with 20% and 30% cassava starch exhibited water absorptions of 256.5% and 251%, respectively³³. Furthermore, Fig. 4 indicates that the effect of CO concentration on foam water absorption varied irregularly across different ATS levels, although higher ATS concentrations generally correlated with decreased water absorption. A separate study evaluated polyurethane flexible foams made from a CO and polyethylene glycol blend, examining the influence of CO weight percentages ranging from 77.2 to 88.5% on various foam properties²⁸. Results showed that increased CO percentages led to reduced water absorption, with foams containing 77.2% and 88.5% CO having water absorptions of 1508% and 540%, respectively.

Microstructure

Figure 5a,b,c display scanning electron microscopy images of foam samples without CO and containing 0, 30, and 60 wt% ATS, while Fig. 5d,e,f show SEM images of samples without ATS but with 0, 10, and 30 wt% CO. The results indicate that increasing ATS and CO concentrations reduced the mean cell diameter of the foam also the cellular structure transitioned from open to semi-open cells, with a higher percentage of closed cells. This can be attributed to the findings in Table 1, which reveal that the foam’s final rise time increased with higher ATS and CO concentrations. The increased reaction mixture viscosity from CO (due to the higher viscosity of CO than that of petroleum-based polyol polyether) and the increased concentration of ATS as a filler slowed the growth rate and movement of gas molecules in the mixture, resulting in smaller cell sizes and a denser closed cell structure. Mean cell diameter and cell number density parameters were determined from SEM images using Image J software version 3.4.1, with results presented in Table 2. This table shows that as ATS concentration increased, the mean cell diameter decreased while cell number density increased: ATS0CO0, ATS30CO0, and ATS60CO0 had mean cell diameters of 0.340, 0.236, and 0.186 mm, and cell number densities of 8.3, 11.2, and 13.2 mm^− 3, respectively. Similarly, for increasing CO concentrations, ATS0CO0, ATS0CO10, and ATS0CO30 had mean cell diameters of 0.340, 0.323, and 0.297 mm, and cell number densities of 8.3, 9.9, and 11.1 mm^− 3, respectively.

Fig. 5.

SEM images of (a) ATS0CO0: 0 pphp ATS and 0 pphp CO; (b) ATS30CO0: 30 pphp ATS and 0 pphp CO; (c) ATS60CO0: 60 pphp ATS and 0 pphp CO; (d) ATS0CO0: 0 pphp ATS and 0 pphp CO; (e) ATS0CO10: 0 pphp ATS and 10 pphp CO; (f) ATS0CO30: 0 pphp ATS and 30 pphp CO.

Table 2.

Cell morphological parameters of various foam samples.

Test No.	Sample code	Mean cell diameter (mm)	Cell number density (mm− 3)
1	ATS0CO0	0.340	8.3
2	ATS30CO0	0.236	11.2
3	ATS60CO0	0.186	13.2
4	ATS0CO10	0.323	9.9
5	ATS0CO30	0.297	11.1

Flexible polyurethane foams were produced using a polyol mixture of equal weights of CO and petroleum-based polyether polyol and the impact of glass bead filler concentrations of 10% and 15% by weight on foam microstructure was examined³⁴. Results indicated that the foam with 10% filler had a mean cell diameter of 0.467 mm and a cell number density of 18 mm^− 3, while the foam with 15% filler had a mean cell diameter of 0.213 mm and a cell number density of 196 mm^− 3. Additionally, Chris-Okafor et al.³² investigated the effect of increasing concentrations of a natural filler mixture (An equal weight ratios of eggshell and peanut shell powder, 0–50% by weight) on polyether-based foam microstructure, finding that higher natural filler concentrations reduced foam pore size. Similar findings have been reported regarding organic and inorganic filler concentrations affecting the microstructure of polyurethane foams^33,35.

FT-IR analysis

In the FTIR spectrum of ATS0CO0 sample, the N-H functional group from the amide formed by polyol polyether and diisocyanate reaction exhibits two absorption bands: one at 3304 cm^− 1 (broadband associated with N-H stretching) and another at 1511 cm^− 1 (narrow band related to N-H bending). The spectrum also features absorption bands for methyl (C-H) and methylene (CH₃) stretching vibrations in the 2900–3000 cm^− 1 range, as well as a band between 1750 and 1700 cm^− 1 for C = O bond stretching vibrations. Additionally, there is an absorption band around 1000 to 1200 cm^− 1 typically related to C-O stretching vibrations. For the ATS0CO20 sample, which includes 20 wt% castor oil, the FTIR spectrum indicates no significant structural changes compared to the foam without starch or castor oil; there are no shifts in absorption bands, only a slight reduction in peak intensity. Similarly, the FTIR spectrum of the ATS30CO0 compound with 30% acetylated tapioca starch also shows no significant changes or shifts in absorption bands compared to the ATS0CO0 sample. In summary, adding ATS to the foam did not create new bonds or interactions. The FTIR spectrum of the ATS30CO20 compound, which contains 20% castor oil and 30% acetylated tapioca starch, further confirms that no new bonds were formed; instead, the intensities of the absorption bands related to the foam were significantly reduced, without any observed shifts in the bands (Fig. 6).

Fig. 6.

FTIR spectra of castor oil, acetylated tapioca starch, and ATS0CO0 (0 pphp ATS and 0 pphp CO), ATS0CO20 (0 pphp ATS and 20 pphp CO), ATS30CO0 (30 pphp ATS and 0 pphp CO), and ATS30CO20 (30 pphp ATS and 20 pphp CO) foam samples.

Biodegradability

Figure 7 illustrates the biodegradation of selected foam samples over time. The ATS, positive control, exhibited the highest biodegradation rate, reaching 4% per day up to day seven, and a total biodegradability of 91.6% by day 91. The Fig. 7 also indicates that the biodegradability of the foams increased with higher concentrations of CO and ATS in their formulations. Specifically, the biodegradability on day 91 for ATS60CO0, ATS60CO10, ATS60CO20, and ATS60CO30 were 37.7%, 42.8%, 44.5%, and 48.6%, respectively. In contrast, ATS0CO0, ATS30CO0, and ATS60CO0 had biodegradability of 9.2%, 22.5%, and 37.7%, respectively. The polyurethane degradation begins with physical destruction, such as microbial colonization, followed by enzymatic catalysis that breaks down the polymeric chains³⁶. This enzymatic action leads to the catabolism of depolymerized products, ultimately resulting in the polymer’s complete degradation. The specific mechanisms of PU biodegradation vary due to the distinct functional groups in the polymer’s soft segments. Ester bonds within PU are prone to hydrolytic degradation, catalyzed by enzymes like esterases, while ether bonds are more resistant to hydrolysis and primarily undergo oxidation, facilitated by reactive oxygen species^36–38. The extensive hydrogen bonding between the hard and soft segments in PU contributes to significant phase separation, further complicating the biodegradation process³⁹. Thus, PU biodegradation involves a combination of biological oxidation and hydrolysis, with the degradation rate depending on the polymer’s structural characteristics. Also incorporating fully biodegradable components like ATS and CO in polyurethane formulation can obviousely accelerate polyurethane foam biodegradation process. Research by Ganji et al.⁴⁰ involved preparing polyurethane foams using a CO and polyethylene glycol polyol blend, evaluating the impact of CO weight percentages (83.5%, 93.8%, and 97.9%) on biodegradability under enzymatic degradation conditions. Results showed that higher CO percentages correlated with increased enzymatic degradation, with biodegradability on day 28 at 11.2%, 30.8%, and 36.2% for the respective concentrations.

Fig. 7.

Biodegradability over time for some foam samples: ATS60CO30 (60 pphp ATS and 30 pphp CO); ATS60CO20 (60 pphp ATS and 20 pphp CO); ATS60CO10 (60 pphp ATS and 10 pphp CO); ATS60CO0 (60 pphp ATS and 0 pphp CO); ATS30CO0 (30 pphp ATS and 0 pphp CO); ATS0CO0 (0 pphp ATS and 0 pphp CO).

Comparative study of the growth of green Basil seeds

Images of green basil seeds grown on selected prepared foam (ATS45CO10) and commercial foam (pink) on the third, fifth, seventh, and ninth days are shown in Fig. 8 and the growth parameters extracted from these images, including germination amount and germination rate (green basil stem length), are given in Table 3. Also the results of sensory evaluation of green basil is given in Table 4. According to the results of Table 3, the germination amount of green basil seeds grown on commercial foam is higher than that of ATS45CO10, but the germination rate of green basil seeds grown on ATS45CO10 is higher than that of commercial foam at all studied days. Also, the results of Table 4 shows that green basil grown on commercial foam got a higher score than that of ATS45CO10 in terms of aroma and smell, while green basil grown on ATS45CO10 scored higher than that of commercial foam in terms of color and appearance, texture crispness and overall acceptance.

Fig. 8.

The images of grown green basil seeds on ATS45CO10 and commercial (pink) foam on (a) and (b) third day, (c) and (d) fifth day, (e) and (f) seventh day, (g) and (h) ninth day.

Table 3.

The growth parameters for green Basil seeds grown on ATS45CO10 and commercial foam.

Growth parameter	ATS45CO10 sample	Commercial foam
Germination amount (%)	81.3 a	84.7 a
Germination rate on third day (cm)	2.9 a	2.7 a
Germination rate on fifth day (cm)	3.5 a	3.2 a
Germination rate on seventh day (cm)	4.4 a	3.9 a
Germination rate on ninth day (cm)	4.7 a	4.2 a

Different lowercase letters on data in a row indicate the significant effect of the type of foam on growth parameter.

Table 4.

Sensory attributes of green Basil grown on ATS45CO10 and commercial foam.

Sensory attribute	ATS45CO10 sample	Commercial foam
Aroma and smell	0.8a ± 4.4	0.5a ± 4.6
Color and appearance	0.4a ± 4.8	0.7ab ± 4.0
Texture crispness	0.8a ± 4.2	0.0a ± 4.0
Overall acceptance	0.4a ± 4.8	0.4ab ± 4.2

Different lowercase letters on data in a row indicate the significant effect of the type of foam on sensory attribute.

In this study, we developed biodegradable flexible polyurethane foams using renewable resources, specifically acetylated tapioca starch (ATS) and castor oil (CO), to replace petroleum-based feedstocks. The results of this study demonstrate that the incorporation of acetylated tapioca starch and castor oil into flexible polyurethane foams significantly affects their physical and mechanical properties, including density, tensile strength, and biodegradability. The results show that increasing the concentration of acetylated tapioca starch directly correlates with higher foam density. This emphasizes the efficacy of starch as a filler material that enhances the foam’s overall mass and structural integrity. Our findings reveal a decrease in tensile strength and elongation at break with higher concentrations of acetylated starch. This indicates that while density improves, the mechanical stability may be compromised, highlighting the need for optimal formulation adjustments to balance these properties. Our biodegradability assessments indicate that foams with the most ATS and CO concentrations achieved up to 48.6% biodegradability by day 91, highlighting the environmental benefits of biobased materials in foam production and their role in reducing non-biodegradable waste, thus enhancing sustainability in the polymer industry. Additionally, a comparative study of green basil seed growth on our biodegradable foams versus commercial options showed favorable germination rates and sensory attributes, confirming their suitability for agricultural applications. In conclusion, this work emphasizes the value of integrating renewable materials into polyurethane foam formulations to promote environmental sustainability and functional versatility. Future research should focus on optimizing ATS and CO concentrations to further enhance mechanical properties and biodegradability, as well as exploring additional renewable fillers to expand bio-based polyurethane foams. This progress contributes to combatting global plastic waste and promotes sustainable practices in material development and application.

Author contributions

Mohammad Reza Abdollahi Moghaddam: Conceptualization, Methodology, Formal analysis, Data Curation, Project administration, Funding acquisition, Software, Writing - Original Draft, Writing - Review & Editing; Maryam Mahdavi: Investigation, Formal analysis; Adel Beigbabaee: Methodology, Resources, Data Curation; Mohsen Heidary: Methodology, Resources, Data Curation; Mohammad Ali Hesarinejad: Conceptualization, Methodology, Formal analysis, Data Curation, Project administration, Funding acquisition, Writing - Review & Editing.

Funding

The authors would like to acknowledge the Iran National Science Foundation (INSF) for supporting the research grant number 4020860.

Data availability

All data generated or analyzed during this study are included in this published article.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval and consent to participate

This article does not contain any studies with human or animal subjects.

Consent for publication

All authors have read and agreed to the published version of the manuscript. All authors read and approved the final manuscript.