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Open Access 12-06-2022 | Original Research

Exploring the mechanical and bacterial prospects of flexible polyurethane foam with chitosan

Authors: A. A. Maamoun, A. A. Mahmoud

Published in: Cellulose | Issue 11/2022

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Abstract

In this work, chitosan (CT) with different ratios (0–5 wt.%) was utilized as a bio filler in polyurethane flexible (PUF) foam to increase mechanical performance and bacterial inhibition characteristics. The chemical structure of CT and polyurethane flexible foam/ chitosan (PUF/CT) composites was examined using FTIR spectroscopy. Furthermore, the cross-link density of PUF/CT composites was examined using the Flory and Rhener equation. Moreover, the PUF/CT composites’ thermal stability was observed utilizing TGA analysis. Further, the morphology and phase behavior of the PUF/CT composites were investigated using SEM and DSC techniques, respectively. The results showed that the cross-link density, thermal stability, cavities sizes, and the glass transition temperature of soft segments T_g(ss) increased with increased CT wt.% content. Besides, characteristics like apparent density, compressive strength, elongation at break, and tensile strength were tested. The results indicated that the density and compressive strength increased by 128.00% and 305.64% for PUF/CT5%, respectively, compared with unfilled PUF foam. At the same time, the tensile strength and elongation at break enhanced 162.50% and 174.30% for PUF/CT4%, respectively, compared with unfilled PUF foam. Finally, the antibacterial test was carried out for PUF/CT composites using the broth dilution procedure. The results revealed that the bacterial growth was inhibited by increasing CT wt.% content. Thus, the obtained composites are promising for industrial biological applications such as packaging and medical intensive care units.

Graphical abstract

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Introduction

Recently, polyurethanes (PUs) have been among the world's most popular and versatile materials (Akindoyo et al. 2016). They are synthetic polymers used in various biological applications (Zia et al. 2014). The repeating moiety in PUs is a urethane linkage (NH-COO) developed by the polyaddition reaction of an alcohol (OH) with an isocyanate (NCO). Other units may be produced in the chemical composition, such as urea, ester, ether, and aromatic (Chattopadhyay and Webster 2009). PU products include flexible and rigid foams, adhesives, sealants, elastomers, and coatings.

Among PU products, polyurethane flexible (PUF) foam is an indispensable component of PU products, accounting solely for around 31% of their market share (Gama et al. 2018). It is commonly used on furniture, footwear, mattresses, aerospace, architectural decoration, packaging, textiles, and vehicle interiors, owing to its strong tensile strength, low density, low thermal conductivity, and superior gas permeability (Gama et al. 2018; Nabipour et al. 2020; Zia et al. 2016). The phase separation of soft and hard segments causes the elasticity of PUF foam. The soft segment provides elasticity through stretchable chains, while the hard segment contributes to strength and rigidity through physical cross-linking points (Lan et al. 2014). Therefore, PUF foams can be tailored by adjusting these segments' composition and ratio. The biological application of PUs is expanding due to the biocompatible property of polyurethane. However, in humid and oxygen-rich environments, polyurethane foams allow aerobic bacteria to appear and grow, thus destroying the foams' color, odor, and elastic properties. The increasing amount of polyurethane warrants extending its use to a preferred field where antibacterial activity is also essential. Infections are known to generate severe public health problems, and therefore, the prevention of bacterial growth is crucial in all fields of application (Czél et al. 2021).

The most significant chitin derivative is chitosan (CT), one of nature's most abundant natural polysaccharides (Aranaz et al. 2009). It is a linear amino polysaccharide copolymer of 1,4-d-glucosamine and N-acetyl glucosamine (No and Meyers 1989). CT contains two OH groups at the C₃ OH and C₆ OH positions and one amine (NH₂) group at the C₂-position (Zia et al. 2014).

This biopolymer's hydroxyl and amine functionalities enable physical interaction with the PU matrix. Chitosan-based polyurethanes may be a more appealing option because they provide exceptional mechanical and biological flexibility in various applications (Crini 2006).

The influence of chitosan on the thermal and mechanical characteristics of PU was investigated (Lin et al. 2007). The results revealed that increasing the CT concentration improved tensile strength and thermal stability. Chitosan/curcumin-based PU with better thermal and mechanical characteristics than unfilled PU was prepared (Zia et al. 2016). The effect of chitosan/modified halloysite nanotubes on the mechanical properties of PU was examined (Fu et al. 2015). The findings showed that tensile strength and elongation at break increased when the CT content was less than 2 wt.%. Chitosan-based PU foam with different CT concentrations was synthesized (Piotrowska-Kirschling et al. 2021). The results showed that adding CT to the PU matrix improves the prepared material's thermal stability. These materials could be used as adsorbents in treating oil spills. Carboxymethyl chitosan-based PU foam (CMCTS-PUF) that successfully adsorbs methyl blue from wastewater was developed (Ren et al. 2021). When compared to PUF, Young's modulus and tensile strength of CMCTS-PUF 5% were raised by 252% and 97%, respectively. Javaid et al. (2018) produced CT and montmorillonite clay-based PU nanocomposites, and results showed that antibacterial activity improved with an increase in the CT loading. Kang et al. (2011) prepared polyurethane/CT blend nanofiber using an electrospinning method for tissue engineering application. The outcomes exhibited good inhibition against bacterial growth. Indumathi and Rajarajeswari (2019) produced PU/CT/nano ZnO composite film for packaging purposes. The produced film demonstrated superior antimicrobial effects against both gram-positive and gram-negative bacteria.

There is great interest in the literature on the effect of CT on different types of polyurethanes, but there is no focus on the effect of CT on polyurethane flexible foams toward bacteria growth. This study aimed to prepare new formulations from polyurethane flexible foam/chitosan (PUF/CT) composites to improve the antibacterial properties of PUF for potential biological applications, especially for hospitals mattresses in which infections transfer among people. The PUF foam in this study is based on toluene diisocyanate and polyether polyol. The exotherm of the reaction and the foaming level are controlled with catalysts, silicone surfactant, and a water-blowing agent.

The selected strains for the antibacterial experiment were Bacillus sphaericus, Enterobacter aerogenes, Pseudomonas aeruginosa, and Staphylococcus aureus. Furthermore, the effect of CT on the chemical structure, morphology, and mechanical properties of PUF foams was investigated.

Experimental section

Materials

Konix KE1990 (Styrene-acrylonitrile copolymer-based polymeric polyol, solid content of 10wt.%, for polyurethane flexible foam production, KPX Chemical, Korea) with a hydroxyl number of 41 mg KOH/gm, an average molecular weight of 4100 g/mol, and a viscosity of 950 mPa s at room temperature. Lupranate T80 (a blend of 20% 2,6- and 80% 2,4- toluene diisocyanate (TDI), BASF polyurethanes, Germany). Amine catalyst (Dabco 33-LV, 33% diethylenediamine, 67% dipropylene glycol, Air product, UK) and stannous-based catalyst (Dabco T-9, Air product, UK). Silicon surfactant (Dabco DC5933, Evonik, Germany). Chitosan (CT) powder (deacetylated chitin with a degree of deacetylation of ≥ 75% was purchased from Merck chemicals). Dimethylformamide (DMF) solvent (purchased from EL Gomhorya Chemical Co.). Distilled water was obtained from our laboratory. All materials are used without additional purification.

Preparation of polyurethane flexible foam/chitosan (PUF/CT) composites

The PUF foam involves two main reactions: gelling and blowing reactions. The former is the reaction between the polyol and isocyanate, and the latter is between isocyanate and water. The schematic illustration of the PUF/CT composite synthesis was exposed in Scheme 1. PUF/CT composites are manufactured utilizing the one-shot free-rising polymerization process. The reaction stoichiometry was calculated based on the following equation:

$${\text{m }}\left( {{\text{TDI}}} \right) = \left[ {\frac{{{\text{I }}\left( {{\text{NCO}}} \right)}}{100}} \right]\left( {{\text{Eq}}\,{\text{TDI}}} \right)\left[ {{ }\frac{{{\text{m }}\,{\text{polyol}}}}{{{\text{Eq }}\,{\text{polyol}}}} + { }\frac{{{\text{m }}\,{\text{H}}_{2} {\text{O}}}}{{{\text{Eq}}\,{\text{H}}_{2} {\text{O}}}}{ }} \right]$$

(1)

where: m (TDI) and m polyol are the TDI and polyol masses, respectively; I (NCO) is the isocyanate index; m H₂O is the water mass; and Eq TDI, Eq polyol, and Eq water are the TDI, polyol, and water equivalent weights, respectively.

The NCO index (NCO equivalents/OH equivalents) was adjusted to 1.05 to ensure a complete reaction. At ambient conditions, polyol, CT with different ratios (0–5 wt.%), silicon surfactant, catalysts, and water blowing agent were charged into a polypropylene cup and vigorously stirred mechanically until complete homogeneity at 1500 rpm for 60 s. Afterward, TDI was quickly added to the reaction mixture and agitated for 7 s at 2000 rpm. Finally, the whole mixture is poured into an open stainless mold (200 × 200 × 200 mm³) and left to cure for 24 h. The PUF/CT composite formulations are manifested in Table 1.

Table 1

PUF/CT composite formulations

Compound	Formulation (Pphp*)
Grafted polyol	100
Chitosan	0–5
Dabco 33-LV	0.2
Dabco T-9	0.15
Dabco DC5933	2
Distilled water	5
TDI	56

*Pphp Part per hundred polyol

Characterization and testing

Particle size determination

Mastersizer 2000 was used to determine the particle size distribution (Malvern Co., UK) of the original CT powder. CT powder was ultrasonically dispersed in water for 2 min before being measured.

Scanning electron microscopy (SEM)

The morphology of CT, PUF/CT0%, and PUF/CT composites was studied using field-emission scanning electron microscopy (FESEM, FEI Quanta FEG 250, USA). The assessments were carried out at a constant magnification of 100 $\times$ and an accelerating voltage of 20.00 kV.

Fourier transform infrared (FTIR) spectroscopy

The chemical structure of the CT bio filler and obtained composite materials were investigated using Fourier transform infrared (FTIR) (JASCO FTIR 460 PLU Spectrometer, Pike Technologies, Madison, USA). In transmission mode, seven samples were scanned in the range of 400–4000 cm⁻¹ using the ATR method at a resolution of 4 cm⁻¹.

Apparent density

The apparent density of the manufactured composites was calculated as the ratio of the mass and volume of cube specimens cut from the middle of the composites with dimensions of 50 × 50 × 40 mm³. Each composite's average values were provided based on four specimens.

Cross-link density

The equilibrium swelling method was used to estimate the cross-link density and swelling properties of the developed PUF/CT composites, following the procedures published in the literature (Storey and Hoffman 1992; Trzebiatowska et al. 2018). Dimethylformamide (DMF) was used as a swelling solvent in this test. After 72 h, the equilibrium point was achieved, and the average value of three measurements was taken for each sample. At equilibrium, the samples were removed from the solvent, patted with tissue, and the weight of the swollen samples was recorded. The samples were dried in the oven at 60 °C for 24 h, and the weight of the dried samples was measured. The following equations, introduced by Flory and Rhener Eq (Flory and Rehner 1943), were used to estimate the cross-link density (V_c) (mol cm⁻³).

$${\text{V}}_{c} = \left[ {\frac{{ - {\text{ ln}}\left( {1 - V_{r} } \right) + V_{r} + { }\upchi \,{\text{V}}_{r}^{2} }}{{{\text{V}}_{1} \left( {V_{r }^{\frac{1}{3}} - \frac{1}{2}V_{r } } \right)}}} \right]$$

(2)

where V₁ is the molecular volume of the solvent, $\chi$ is the interaction parameter between the polymer and the solvent (0.49) (Ni and Thring 2003), and V_r is the volume fraction of the polymer in the swollen network in equilibrium with the pure solvent and measured using the following relationship:

$$V_{r} = \frac{Volume \,of\,unswollen\,polymer\,matrix }{{Volume\,of\,swollen\,polymer\,matrix }}$$

(3)

$$= \frac{{\left( {\frac{Weight \,of\, polymer - Weight \,of \,filler }{{density\, of \,polymer }}} \right)}}{{\left( {\frac{Weight\, of \,dry\, polymer - Weight\, of \,filler }{{density\, of \,polymer }}} \right) + \left( {\frac{Weight\, of\, solvent}{{density\, of\, solvent }}} \right) }}$$

(4)

where the density of solvent was 0.948 g/mL for DMF.

Thermogravimetric analysis (TGA)

Material thermal deterioration trends were investigated using thermogravimetric analysis (TGA). Using a TA Instruments Q500 device, samples were placed in platinum pans and heated in nitrogen gas at a constant rate of 10 °C/min in a temperature range of 25–1000 °C, the differential thermogravimetric analysis (DTG) data were also reported.

Differential scanning calorimetry (DSC)

The PUF/CT0% foam and PUF/CT composites' phase behavior was investigated using a differential scanning calorimeter (DSC 2500, TA instrument, USA). The foam samples (3–6 mg) were sealed in aluminum pans and purged with nitrogen gas at a 10 mL/min scanning rate. In the first scan, the samples were heated from 25 to 200 °C to eliminate thermal memory. Then, they were cooled to − 90 °C and heated to 200 °C. All tests were carried out at a scanning rate of 10 °C/m.

Mechanical properties

Compression strength

The compression strength and elastic modulus of PUF composites were demonstrated using a universal testing machine (ZwickRoell/Z100, Germany) with a load cell of 1 kN following the American Society for Testing and Materials - ASTM D-3574-17 test C. The sample dimensions were 50 × 50 × 40 mm³. The test was run at a 50 mm/min crosshead movement and a deformation of 50%.

Tensile strength

The ASTM D-3574-17 test E was used to determine the tensile strength and elongation at the break of PUF foam composite materials. The test was performed on a universal testing machine (a ZwickRoell/Z010, Germany) with a 1 kN load cell. Tensile tests were carried out on three type C dumbbell test specimens at a 500 mm/min crosshead rate. Both compressive and tensile properties are evaluated in the direction of foaming rise.

Antimicrobial activity

A broth dilution procedure was applied to evaluate the antibacterial properties of the prepared PUF/CT composites. To begin, 100 µL of preserved bacteria in glycerol tubes were put onto nutrient agar plates, streaked, and incubated for 24 h at 37 ± 2 °C. Following incubation, numerous bacterial colonies were implanted in brain heart infusion broth and incubated at 37 ± 2 °C until they attained the turbidity of 1/2 McFarland standard (≈ 1 × 10⁶ CFU/mL) before being seeded on Muller Hinton agar plates (Shehabeldine et al. 2021). A loopful of inoculum was taken from a pure culture of the respective bacteria/ grown on slants and inoculated into 10 ml of nutrient broth. The broth suspension was then incubated at 37 °C for 24 h. The growth so obtained was used as inoculum for the sensitivity assay. In each tube were put pieces of PUF/CT composites (100 mg) coded from one to five and incubated for 24 h under aerobic conditions. An ELISA reader was used to determine the optical density (OD) at 620 nm (Tecan Elx800, USA). Negative control was employed, which was the sterile nutritional broth. The trials were performed three times, and the OD was averaged (Farag et al. 2020; Shehabeldine et al. 2020).

Results and discussion

Characterization of chitosan (CT)

The filler particles' size and specific surface area are two of the most critical characteristics determining their behavior in a polymer matrix. The dispersion may be challenging if the particles are too small and have a high surface area, as they can build agglomerates that act like massive particles. The foaming process and other materials' features may be harmed if the particles are too big or have a low surface area (Członka et al. 2018). Figure 1 shows the average particle size distribution of CT bio filler. It was reported at 476 µm; the measured specific surface area was 0.0231 m²/g. CT's larger size distribution and smaller surface area are due to its stronger aggregation proclivity due to its inter-chain hydrogen bonding (Mohamed et al. 2015).

Morphological characterization

The SEM image of the CT filler is shown in Fig. 2. The utilized CT was found to contain flakes layers and be permeable in certain spots. Fibrillar structures may be identified in certain regions of CT (El-Naggar et al. 2020).

The effect of the CT bio filler on the prepared polyurethane flexible foam morphology was investigated. The SEM micrographs of the obtained PUF/CT0% and PUF/CT composites are presented in Fig. 3. It is clear that the composites contain cavities and pores of different sizes. The pore structure mainly contains three of pores: closed, partially open, and open. The mechanism of cell formation can be described as follows: Water, a chemical blowing agent, reacts with isocyanate to produce CO₂ gas (Zhang et al. 2017). CO₂ molecules expand the walls of cavities until they collide with neighboring cavities. Pores form if the narrow hollow walls cannot withstand pressure from both sides. The thickness of the wall cavity and the drainage flow rate affect the pore types. The presence of open pores may be due to low wall strength and high drainage flow rate in thin cavity walls. Partially open pores occurred at high wall strength and low drainage flow rate. However, the pores remain closed when the gelling reaction is completed before the breakage of cavity walls (Baek and Kim 2020; Sung et al. 2016).

Interestingly, it was found that CT bio filler affects the size of the cavity and pores and, consequently, the cell structure of the obtained PUF/CT composites, as shown in Table 2. There is an obvious trend that both cavity and pore size increase with increased CT wt.% compared with unfilled foam; this trend is the opposite of polyurethane composites reported in the literature (Członka et al. 2018; Saint-Michel et al. 2006). The type of the filler surface feature is intimately connected to the steady increase in cavity and pore diameters. The presence of CT particles disturbs the formation of pores, which become larger and irregular. Also, the CT is a hydrophilic polymer by nature (Ye et al. 2014), and the PU matrix, including aliphatic polyether polyols, exhibits hydrophilic characteristics (Sung and Kim 2017), leading to good compatibility between the CT and the PU matrix. The greater the hydrophilicity of the filler surface, the bigger the cavity and pore diameters of the cell shape (Sung and Kim 2017). Besides, this compatibility minimizes the cell rupturing phenomenon in polyurethane foams, and the cavities later develop into larger ones during the secondary blowing reactions between the isocyanates and H₂O molecules (Harikrishnan et al. 2006).

Table 2

Cavity and pore sizes of PUF/CT0% and PUF/CT composites

Composites	Cavity size (µm)	Pore size (µm)
PUF/CT0%	745 ± 70	287 ± 62
PUF/CT1%	764 ± 92	312 ± 84
PUF/CT2%	912 ± 178	381 ± 146
PUF/CT3%	923 ± 44	429 ± 86
PUF/CT4%	962 ± 288	463 ± 106
PUF/CT5%	1144 ± 431	471 ± 93

FTIR characterization

FTIR spectroscopy was conducted to characterize CT and PUF/CT composites using ATR mode. Figure 4a presents the FTIR spectra of CT bio filler, while Fig. 4b displays the FTIR spectra of PUF/CT0% and PUF/CT composites. The CT spectra showed predominant broadband at 3350 cm⁻¹, assigned to the O–H stretch and N–H stretch overlapping. Other noticeable peaks in the spectra are 2870 cm⁻¹, 1636 cm⁻¹, 1160 cm⁻¹, and 1021 cm⁻¹ attributed to C–H stretching, NH₂ deformation, C–O–C stretching, and C–O stretching, respectively, as described in other studies (Wang et al. 2016; Zheng et al. 2011).

All the well-defined bands of PUF foams are noticed in Fig. 4b. The main peak at 3300 cm⁻¹ corresponds to the N–H stretching of the urethane group (Maamoun et al. 2019). The peaks at 2967 cm⁻¹ and 2866 cm⁻¹ are associated with aliphatic –CH₂ stretching (Carriço et al. 2017; Leng et al. 2019). The bands at 2273 cm⁻¹ and 1088 cm⁻¹ were attributed to unreacted NCO and C–O–C stretching of soft segments, respectively (Da Silva et al. 2013). The characteristic peaks at around 1720 cm⁻¹, 1640 cm⁻¹, and 1595 cm⁻¹ were associated with carbonyl groups C=O of urethane, monodentate H-bonded of C=O of urea groups, and bidentate H-bonded of C=O of urea groups, respectively (Chen et al. 2013; Cinelli et al. 2013). Thus, these outcomes proved that CT filler contributed to physical interaction with the PU matrix.

Apparent density

Table 3 discloses the density of the manufactured PUF/CT composites. The density is a significant parameter regarding foam support and comfort, which relies primarily on fillers, usually improving density. Compared with PUF/CT0% foam, the density increases with increased CT bio filler percentages, as shown in Table 3. This is due to the increased physical networking between CT and PU matrix (Zhang et al. 2021), as a consequence of which the polymer chains are more compactly arranged (Gómez-Fernández et al. 2016). Also, CT filler content increases the material weight per unit volume, thus increasing the foam density.

Table 3

The typical apparent density of PUF/CT composites

Composites	Density (kg/m³)
PUF/CT0%	20.40 ± 0.43
PUF/CT1%	22.20 ± 0.34
PUF/CT2%	24.40 ± 1.25
PUF/CT3%	24.56 ± 0.37
PUF/CT4%	25.45 ± 0.44
PUF/CT5%	26.12 ± 1.16

Cross-link density

The swelling parameters of PUF/CT0% and PUF/CT composites are presented in Fig. 5. The type and amount of filler employed significantly impact the swelling ratio, volume fraction, and, therefore, the cross-link density. The swelling ratio is estimated by dividing the mass of solvent uptake per mass of material. The swelling ratio decreased with increased CT wt.% after 72 h when equilibrium was reached, as shown in Fig. 5a. This is proof that the PUF/CT composites are physically cross-linked and there is less free space for solvent uptake than PUF/CT0%, which is wholly dissolved in DMF solvent and, as a result, not cross-linked.

The cross-link density of the produced PUF/CT composites was investigated by applying the Flory–Rehner equation. The estimation of cross-link density is relevant and can be thought of as a structural parameter that can explain other properties of produced composites, such as thermal or mechanical behaviors (Trzebiatowska et al. 2018). It was noticed that the cross-link density enhanced as the CT wt.% content increased, as illustrated in Fig. 5b. This is because CT bio filler assists the formation of physical networking with PU matrix, which is enhanced with increased CT loading. This could lead to a decrease in polymer chain mobility and a smaller amount of DMF entering the cellular structure of the polymer.

TGA analysis

The thermal behavior of the CT and PUF/CT composites are displayed in Figs. 6 and 7, respectively. The thermal degradation of the CT used in this research showed that the one major region of weight loss at a maximum temperature of 296 °C is attributed to the decomposition of CT moieties (Fu et al. 2015). At the same time, the thermal decomposition of PUF/CT composites illustrated two main weight loss regions. The first event degradation lies in the range of 250–360 °C and is ascribed to the decomposition of urea and urethane linkage, related to hard segments (Izarra et al. 2021; Kerche et al. 2021). Urethanes are thermally unstable materials, and the urethane bond's breakdown temperature varies depending on the polyurethane structure (Lapprand et al. 2005; Rueda-Larraz et al. 2009). The urethane linkage degrades by three mechanisms: dissociation of the initial isocyanates and polyol precursors, creation of carbamic acid and olefin with subsequent carbamic acid dissociation to primary amine and carbon dioxide, and formation of a secondary amine and carbon dioxide (Septevani et al. 2015).

Moreover, the second event degradation, which occurred fast, lies in the range of 360–450 °C and is attributed to soft segments (Mahmoud et al. 2021). The thermal decomposition values of CT, PUF/CT0% foam, and PUF/CT composites obtained from TGA analysis are shown in Table 4, where T_max(1) and T_max(2) are the temperatures of maximum degradation in the first and second weight loss, respectively. The results revealed that the maximum decomposition temperature T_max(1) increased from 278.95 °C for PUF/CT0% to 287.68 °C for PUF/CT5%, indicating thermally formed stable urethane linkages. In addition, the weight loss in the first degradation step for PUF/CT0% foam was 53 wt.% and decreased to 32 wt.% in the case of PUF/CT composites. The increasing CT filler has no significant effect on the weight loss in the first degradation step. Thus, the thermal stability of PUF/CT composites increases with increased CT loading; this is because CT acts as a physical cross-linking agent, increasing the system's cross-linking density (Fu et al. 2015). Moreover, the second degradation step with T_max(2) depends on the nature of the soft segments and their three-dimensional arrangement. The difference in T_max(2) values may be due to differences in the three-dimensional arrangement of soft segment polyol with increasing CT contents (Piotrowska-Kirschling et al. 2021).

Table 4

Exhibits thermal data of CT and PUF/CT composites

Composites	T_max(1)	T_max(2)
Composites	°C	°C
CT	296	–
PUF/CT0%	278.95	371.47
PUF/CT1%	282.62	367.71
PUF/CT2%	283.44	358.83
PUF/CT3%	284.78	372.19
PUF/CT4%	285.10	392.00
PUF/CT5%	287.68	373.87

DSC analysis

Figure 8 illustrates the DSC thermograms of the prepared PUF/CT0% and PUF/CT composites. The observed endothermic peak generally relates to melting the PU matrix's soft segments, whereas the exothermic peak is associated with PU matrix domain rearrangements and ordering (Raftopoulos et al. 2011). In addition, the prepared foam composites have two main glass transition temperatures (T_g), one in the low-temperature region attributed to soft segments (SS) or polyol, and the other in the high-temperature region attributed to hard segments (HS) or styrene-acrylonitrile solid particles in the polyol system (Corcuera et al. 2010; Lenges et al. 2021).

It was noticed that the T_g(SS) increases from − 55.81 °C for PUF/CT0% to − 48.38 °C for PUF/CT4%; this is owing to chain extension occurring during the first stage of polymerization, also indicating a relatively small amount of HS that is linked with SS, which restrict the molecular motion (Rueda-Larraz et al. 2009). However, the slight increase in T_g(SS) for PUF/CT5% is insignificant.

Besides, unlike styrene-acrylonitrile particles' hard segments present in the polyol, the addition of CT resulted in increased phase-mixing, which results in even more restriction of the mobility of amorphous soft segments (Lenges et al. 2021). Furthermore, T_g(HS) values for all composites increases compared with PUF/CT0% with increasing CT wt.% contents, even though the percentage of the styrene-acrylonitrile in the polyol system remains constant in all formulations. This is due to an abnormal chain alignment due to the presence of CT particles and possibly a stiffening of the chains due to physical interactions between the CT particles and the PU matrix.

Mechanical properties

Compression strength

The compressive stress–strain curve for PUF/CT0% and PUF/CT composites is elucidated in Fig. 9. All the prepared foams possess the typical behavior of PUF foam when subjected to compression tests. The graphs show three distinct regions, each dominated by a different deformation mode. The first region represented linear elastic response, located at low strain (Li et al. 2019); in this region, the rigidity of the struts prevents the sample from collapsing. The polymer wall buckling and microcell compression at medium strain are reported in the second stage with a lengthy plateau. Densification occurs in the last step, under high strain when the polymer walls begin to compress against the surrounding walls. As the strain mounted, so did the tension (Kharbas et al. 2017).

It was noticed that the compressive strength of PUF/CT composites was elevated with increased CT wt.% filler, as shown in Fig. 9. The data revealed that the compressive strength increased by 187.1%, 191.00%, 136.30%, 258.87%, and 305.64% for PUF/CT1%, PUF/CT2%, PUF/CT3%, PUF/CT4%, and PUF/CT5%, respectively, compared with PUF/CT0%; this is due to the increased cross-link density of the PUF/CT composites with increasing CT wt.% content. This enhancement is also due to enhanced physical interaction between the CT bio filler and the polymer matrix. Besides, the higher density is associated with more compact cellular structures; consequently, extra filler wt.% per unit area develops the foam's compressive strength (Bernardini et al. 2015). In addition, the elastic modulus of PUF/CT0% and PUF/CT composites is presented in Fig. 10. The slope of the stress–strain curve in the linear elastic area is used to calculate the elastic modulus. The observed trend in the elastic modulus of PUF/CT composites increased with CT wt.% than PUF/CT0%. This increment is due to increased foam stiffing with increasing CT bio filler content, as observed by DSC analysis (Rueda-Larraz et al. 2009).

Tensile strength

Figure 11 reveals the tensile strength and elongation at the break of PUF/CT0% and PUF/CT composites. The incorporation of CT possessed a significant influence on both tensile strength and elongation at break. It is noticeable that the PUF/CT4% composite recorded the maximum values at 0.26 MPa and 170.58%, while PUF/CT0% has 0.16 MPa and 97.86% for tensile strength and elongation at break, respectively. This enhancement is because the tensile strength and elongation at break mainly depend on interfacial adhesion between the filler and the PU matrix (Fu et al. 2015). The CT makes strong H-bonds with PU matrix, producing stiffer foam (Chattopadhyay and Webster 2009). Thus, the tensile strength and elongation at break improved with increased CT to 4wt.%. However, there was a slight decrease in tensile strength and elongation at break for the PUF/CT5% composite; it may be due to neck deformation formed at high CT wt.% content.

Antimicrobial activity

Table 5 summarizes the biological activity results of the PUF/CT0% and PUF/CT composites. The composites were tested for antibacterial activity against two gram-positive bacteria, Bacillus sphaericus and Staphylococcus aureus, two gram-negative bacteria, Enterobacter aerogenes, and Pseudomonas aeruginosa. The effects of CT with different doses (0.5, 1, and 2% w/w) on the physico-chemical, mechanical, and biological activity of polycaprolactone/PU mixture was investigated (Arévalo et al. 2016). The biological activity results revealed that the antimicrobial activity against E. coli and S. aureus increases with increased CT concentration. The antibacterial activity against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, and Bacillus subtilis of the prepared chitosan/water-borne PU was evaluated (Naz et al. 2018). The results showed that the antibacterial activity of materials was enhanced with an increased CT ratio.

Table 5

Antimicrobial activity as indicated by broth dilution methods of PUF/CT0%, PUF/CT1%, PUF/CT2%, PUF/CT3%, PUF/CT4%, and PUF/CT5% against different strains of bacteria

Composites	B. sphaericus	S. aureus	E. aerogenes	P. aeruginosa
Composites	Mean OD
PUF/CT0%	1.82 ± 0.06	1.76 ± 0.02	1.62 ± 0.04	1.82 ± 0.03
PUF/CT1%	1.25 ± 0.16	1.31 ± 0.10	1.53 ± 0.11	1.65 ± 0.08
PUF/CT2%	1.03 ± 0.07	0.89 ± 0.02	0.96 ± 0.05	1.34 ± 0.10
PUF/CT3%	1.02 ± 0.16	0.75 ± 0.06	0.84 ± 0.04	0.86 ± 0.05
PUF/CT4%	0.84 ± 0.06	0.55 ± 0.04	0.64 ± 0.04	0.62 ± 0.03
PUF/CT5%	0.73 ± 0.04	0.31 ± 0.02	0.55 ± 0.04	0.44 ± 0.04

As shown in Table 5, the inhibition of bacterial growth by PUF/CT composites is superior to that of PUF/CT0%. This means that the antibacterial activity of composites increases as the CT wt.% increases. This is because CT polymers contain amino groups that inhibit bacterial metabolism by absorbing and stacking the CT chain on the bacterial cell wall and blocking DNA transcription (El-Sayed et al. 2010). Also, the increased pore sizes facilitate the contact of the CT particles trapped therein with the bacterial solution. Thus, the prepared PUF/CT composites are considered a promising material for potential biomedical applications.

Conclusion

The chemical structure of fabricated PUF/CT composites and the physical interaction between the CT and the PU matrix were successfully investigated using FTIR spectroscopy. The measured swelling parameters showed the formation of cross-linked PUF/CT composites compared with PUF/CT0%. The TGA results revealed that the thermal properties of PUF/CT composites were improved by the hydrogen-bonding between CT and PU matrix. The SEM images disclosed that cavities and pore sizes increased with CT content. The measured compressive strength exhibited an improvement of 305.64% for PUF/CT5% compared with PUF/CT0%. In addition, the tensile strength and elongation at break increased to 0.26 MPa and 170.58%, respectively. The antibacterial activity study revealed that PUF/CT composites had remarkable activity against four of the most common causes of infections: B. sphaericus, S. aureus, E. aerogenes, and P. aeruginosa. These results indicate that PUF/CT composites are potentially used in many industrial biological applications.

Acknowledgments

The authors would like to thank Royal Foam Egypt Co. for their support.

Declarations

Conflict of interest

The authors declare no competing interests or other interests that might be perceived to influence the results and/or discussion reported in this paper.

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Title: Exploring the mechanical and bacterial prospects of flexible polyurethane foam with chitosan
Authors: A. A. Maamoun
A. A. Mahmoud
Publication date: 12-06-2022
Publisher: Springer Netherlands
Published in: Cellulose / Issue 11/2022
Print ISSN: 0969-0239
Electronic ISSN: 1572-882X
DOI: https://doi.org/10.1007/s10570-022-04655-x

Springer Professional

Abstract

Graphical abstract

Publisher's Note

Introduction

Experimental section

Materials

Preparation of polyurethane flexible foam/chitosan (PUF/CT) composites

Characterization and testing

Particle size determination

Scanning electron microscopy (SEM)

Fourier transform infrared (FTIR) spectroscopy

Apparent density

Cross-link density

Thermogravimetric analysis (TGA)

Differential scanning calorimetry (DSC)

Mechanical properties

Compression strength

Tensile strength

Antimicrobial activity

Results and discussion

Characterization of chitosan (CT)

Morphological characterization

FTIR characterization

Apparent density

Cross-link density

TGA analysis

DSC analysis

Mechanical properties

Compression strength

Tensile strength

Antimicrobial activity

Conclusion

Acknowledgments

Declarations

Conflict of interest

Publisher's Note

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