DOI: 10.3303/CET25117090 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 1 January 2025; Revised: 2 March 2025; Accepted: 6 May 2025 
Please cite this article as: Silveira E.X., Schiavon M.I.R.B., Francisco B.A., Doubek G., Maciel Filho R., 2025, Bio-polyurethane Based 
Nanocomposites Membranes: Synthesis and Characterization, Chemical Engineering Transactions, 117, 535-540  DOI:10.3303/CET25117090 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 117, 2025 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto 

Copyright © 2025, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-17-5; ISSN 2283-9216 

Bio-Polyurethane based Nanocomposites Membranes: 

Synthesis and Characterization 

Elda X. S. Silveira*, Maria Ingrid R. B. Schiavon, Bruno A. B. Francisco, Gustavo 

Doubek, Rubens Maciel Filho 

University of Campinas, School of Chemical Engineering, Av. Albert Einstein, 500. Campinas, SP, Brazil, 13083-852 

elda.silveira1@gmail.com  

Biopolymer-based nanocomposites are an alternative to replace petroleum-based polymers due to their user-

friendliness, biodegradability, abundance, and non-toxicity. Additionally, polymers derived from natural 

resources have the potential to align with the principles of a circular economy. Castor oil, a natural, non-toxic, 

and inedible compound, offers numerous advantages, including low cost, abundant availability, and non-

competition with food production. In this study, bio-polyurethane (BPU) nanocomposite membranes were 

synthesized via in situ polymerization using castor oil, 4,4-diphenylmethane diisocyanate (MDI), 1,4-butanediol 

(BD), glycerine (Gly), and fumed silica (SiO2) nanoparticles, with varying silica concentrations (0%, 0.5%, and 

1%). The membranes were characterized in terms of surface morphology, cross-sectional structure, pore size 

distribution, surface area, water contact angle, and were compared with commercial polypropylene (PP) 

membranes. The results showed that while PP membranes exhibited a normal distribution of micropores, 

mesopores, and macropores, the BPU nanocomposite membranes only contained mesopores. Furthermore, 

the maximum pore size of the BPU nanocomposite membrane containing 1.0% SiO2 was reduced by more than 

80% compared to the membrane containing 0.5% nanoparticles. This work characterizes the structure of a new 

membrane synthesized from a renewable resource and assesses the effect of silica addition in the 

characteristics of the membrane as potential substitutes for fossil-derived membranes. 

1. Introduction 

Polymer nanocomposites demonstrate a broad spectrum of applications, including pharmaceuticals, agriculture, 

sports, and electronics. The incorporation of nanoparticles into polymer matrices not only improves key 

properties of pure polymers, such as elasticity, durability, barrier performance, solvent and thermal resistance, 

and flame retardancy, but also imparts novel functionalities depending on the specific nanomaterials employed 

(Khan et al., 2022). This approach addresses the inherent limitations of pure polymers, as nanostructures alter 

surface properties and fine-tune the physicochemical characteristics of the matrix, directly influencing the 

performance of the resulting composites (Assad et al., 2023).  

Silica nanoparticles have attracted significant attention due to their ease of production and diverse industrial 

applications. Their size and distribution greatly influence product performance, and studies show that 

incorporating small amounts of nanoparticles into polymers enhances mechanical, thermal, electrical, photonic, 

and gas barrier properties (Bakhtiar et al., 2022). 

Bio-based polymers are biodegradable materials derived from renewable biomass sources (Kumar et al., 2023) 

and offer ecological advantages, including reduced carbon footprint, waste management, and high recycling 

potential, playing a crucial role in the bioeconomy. These polymers are utilized in the development of biofilms, 

fibers, membranes, blends, composites, and nanocarriers (Zia et al., 2021). Among renewable resources, 

vegetable oils are highly promising, capable of direct chemical modification and functionalization. They consist 

of triglycerides, molecules with a triester of fatty acids (Rajput et al., 2023).  

Due to its simple structure, widespread availability, abundant active sites, low toxicity, cost-effectiveness, and 

industry suitability, castor oil has emerged as one of the rapidly growing commercial vegetable oils with several 

studies identifying castor oil as a potential polyol source to substitute petroleum-derived products (Ma et al., 

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2024). Castor oil distinguishes itself from other vegetable oils by being non-edible, non-competition with food 

and, as a natural polyol, is utilized in polyurethane synthesis (Nekhavhambe et al., 2019). Polyurethanes (PUs) 

can be synthesized through a polyaddition reaction between the hydroxyl groups (–OH) of polyols and the 

isocyanate functional groups (–N=C=O). By varying the types and amounts of polyol and isocyanate, 

polyurethanes with diverse properties can be obtained (Asare et al., 2022).  

In this context, bio-polyurethane (BPU) nanocomposite membranes with different concentrations of SiO2 

nanoparticles were developed. These membranes were characterized in terms of surface morphology, cross-

sectional structure, pore size distribution, surface area, and water contact angle. Comparative analyses were 

performed against commercial polypropylene (PP) membranes to assess their potential as substitutes for fossil-

derived membranes. 

2. Method 

2.1 Synthesis of BPU nanocomposite membranes 

BPU nanocomposites were synthesized by in situ polymerization under an inert nitrogen atmosphere, using 

castor oil as the bio-polyol, 4,4'-diphenylmethane diisocyanate (MDI) as the isocyanate, and a chain extender 

blend consisting of 60:40 1,4-butanediol (BD) and glycerine (Gly) in a molar ratio of 2:1:1. Silica nanoparticles 

were incorporated at concentrations of 0.0%, 0.5%, and 1.0% by weight. Initially, castor oil, with a molecular 

mass of 933.4 g/mol, was dried at 70 °C under vacuum conditions. Subsequently, MDI was incorporated into 

the dried castor oil and stirred at 80 °C for 1 hour to facilitate the formation of a prepolymer. Afterward, silica 

nanoparticles were introduced into the prepolymer mixture, followed by the addition of 1,4-butanediol and 

glycerine. The resulting bio-polyurethane, synthesized in the presence of silica nanoparticles, was then applied 

onto a cellulose substrate (70 mm diameter, 80 g) using a Spin Coater (Chemat Technology, KW-4A, Northridge, 

USA) at a rotational speed of 5000 rpm to form the BPU nanocomposite membranes.  

2.2 Characterization of BPU nanocomposite membranes 

The morphology and thickness of the membranes were analyzed using scanning electron microscopy (SEM) 

(Quattro S, Thermo Fisher Scientific, Brno, Czech Republic). To enhance the image resolution, gold (Au) 

particles were sputtered onto the membrane surface using a sputter coater (Emitech K450, Kent, UK). The SEM 

equipment is coupled with a Leo 440i detector for energy-dispersive X-ray spectroscopy (EDS) (ANAX-60P-B, 

Thermo Scientific UltraDry, Brno, Czech Republic), which was employed to map the distribution of silica 

nanoparticles within the membranes. Nitrogen adsorption and desorption isotherms, surface area, and pore size 

distribution were determined using an Accelerated Surface Area and Porosimetry System (ASAP 2020 Plus, 

Micromeritics, Norcross, USA). The hydrophobic/hydrophilic properties of the membranes were evaluated using 

a Drop Shape Analyzer (DSA100, Krüss, Hamburg, German), with the contact angle measured at the moment 

the water droplet was first applied.  

3. Results and Discussion 

Figure 1 depicts the surface morphology (at different magnifications) and thickness of BPU nanocomposite 

membranes with different silica concentrations (0%, 0.5%, and 1%), compared to a commercial polypropylene 

(PP) membrane. The morphology of the BPU membrane without silica (Figure 1a, e) appears relatively smooth, 

indicating a homogeneous polymer matrix. The membranes containing 0.5% silica (Figure 1b, f) and 1.0% silica 

(Figure 1c, g) exhibited surface morphologies similar to that of the membrane without silica. The surface of the 

PP membrane (Figure 1d, h) shows a distinct fibrous, web-like structure, indicating a more porous or fiber-

oriented organization. The cross-sectional images (Figure 1i to l) reveal both membrane thickness and internal 

structure. The BPU membranes (Figure 1i, j, k) have a similar thickness of approximately 120 µm, but their 

internal microstructure appears more complex, with distinct layers and interfaces and the fibers appear to be 

more porous with the increase of silica content. In contrast, the PP membrane is significantly thinner (around 

23 µm) and presents a different internal structure, likely due to its fibrous, more oriented morphology (DeMeuse, 

2021) compared to the BPU membranes. 

The distribution of silica in the membranes (indicated by the green coloration in the images) is shown in Figure 

2. The BPU nanocomposite with 0.5% silica (Figure 2a) exhibited a uniform silica dispersion within the polymer 

matrix. In contrast, a denser silica distribution was observed in the BPU nanocomposite with 1.0% silica (Figure 

2b), suggesting that the higher silica concentration leads to the formation of larger silica agglomerates. 

Figure 3 illustrates the nitrogen adsorption-desorption isotherms and the surface areas calculated using the BET 

(Brunauer-Emmett-Teller) method for the membranes. The BPU nanocomposite with 0.0% silica isotherm 

(Figure 3a) exhibited behavior typical of mesoporous materials, with a gradual increase in the adsorbed quantity 

at intermediate relative pressures, followed by a sharp increase at high relative pressures (P/P₀ ≈ 1) (Nishi & 

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Inagaki, 2016). The presence of hysteresis between the adsorption and desorption curves suggests the 

existence of pores that are not fully accessible, characteristic of cylindrical or bottle-neck-shaped pores (Kyzas 

& Mitropoulos, 2018). The behavior of the membrane with 0.5% silica (Figure 3b) is similar to the membrane 

without silica, including the adsorbed volume (<0.5 cm³/g) and the BET surface area (~0.6 m²/g). The hysteresis 

loop persists, indicating that the porous morphology remains relatively unchanged. For the membrane with 1.0% 

silica (Figure 3c), the adsorbed quantity is noticeably lower (<0.2 cm³/g), suggesting that the higher 

concentration of silica may have led to partial pore blockage or the formation of silica aggregates (as noted in 

Figure 2b), reducing the accessible surface area (BET surface area with 1.0% silica = 0.3052 m²/g). The 

hysteresis loop appears slightly different when compared to the other membranes, which may indicate a change 

in pore structure, possibly with fewer mesopores or a different pore shape. The polypropylene isotherm (Figure 

3d) shows a distinct shape, with higher adsorption at high relative pressures, typical of macroporous materials 

(Anovitz & Cole, 2015). The almost vertical isotherm at maximum relative pressure (P/P₀ ≈ 1) indicates limited 

adsorption at lower relative pressures and rapid capillary condensation in macropores at high pressures (Zhang 

et al., 2016). Furthermore, the BET surface area is higher than that of the BPU nanocomposites (27.6170 m²/g), 

which can be attributed to its more porous structure (as noted in Figure 1h). 

 

 
 

Figure 1: Images at low and high magnifications (x5 k and x70 k) and thickness of the membranes: (a, e, i) BPU 

nanocomposite with 0.0% silica; (b, f, j) BPU nanocomposite with 0.5% silica; (c, g, k) BPU nanocomposite with 

1.0% silica; and (d, h, l) polypropylene (PP), respectively. 

  

 
 

Figure 2: Map of silica distribution in the membranes. (a) BPU nanocomposite with 0.5% nanosilica and (b) BPU 

nanocomposite with 1.0% silica. 

 

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Figure 3: Nitrogen adsorption-desorption isotherms at 77 K and the BET surface area. (a) BPU nanocomposite 
with 0.0% silica. (b) BPU nanocomposite with 0.5% silica. (c) BPU nanocomposite with 1.0% silica. (d) 
polypropylene (PP). 
 
 
The BJH (Barrett-Joyner-Halenda) method was applied to determine the pore size distribution of the membranes 

(Figure 4). Data were obtained from the BJH Desorption Pore Distribution Report generated by the ASAP 2020 

Plus system. The pores of the BPU nanocomposite membranes (Figures 4a to 4c) fall within the mesoporous 

range. However, the BPU nanocomposite without silica (Figure 4a) and the composite with 0.5% silica (Figure 

4b) exhibit a maximum pore diameter of approximately 36 nm, with a gradual decrease in pore volume as the 

diameter reduces to around 15 nm. This suggests that low silica content induces minimal changes in the overall 

porosity distribution. In contrast, the BPU nanocomposite with 1.0% silica (Figure 4c) shows pores concentrated 

between 2 to 4 nm, with a reduced pore volume. This indicates that higher silica concentrations either fill or 

disrupt pore formation, leading to a denser matrix with fewer accessible pores, potentially decreasing the 

porosity of the membrane (Alothman, 2012). The polypropylene membrane (Figure 4d) presents a distinct 

structure, with a broad pore size distribution spanning micro, meso, and macropores, and a significantly higher 

pore volume, particularly in the meso and macropore regions. This suggests that polypropylene exhibits a 

predominantly macroporous structure, contrasting with the denser matrix of the BPU nanocomposites. 

Figure 5 shows the water contact angle measurements for the membranes. The incorporation of 0.5% silica into 

the polyurethane matrix increased the hydrophobicity of the composite material (Figure 5b) compared to the 

BPU nanocomposite without silica (Figure 5a). This enhancement in hydrophobicity is attributed to the silica 

nanoparticles reducing the surface energy of the polymer matrix, thereby decreasing its affinity for water (Singh 

& Singh, 2022). The improved hydrophobicity properties arise from the surface characteristics of the silica 

nanoparticles and their interaction with the polymer matrix, resulting in changes in surface roughness and 

increased water repellency (Tripathi et al., 2024). However, the hydrophobicity of the BPU nanocomposite with 

1.0% silica (Figure 5c) decreased. This observation suggests that the dispersion of silica within the polymer 

matrix significantly affects this property. As highlighted in Figure 2, higher silica concentrations in the 

polyurethane matrix result in particle agglomeration, which may reduce the effective interaction between silica 

and the polymer matrix, thereby diminishing the hydrophobic effect. The polypropylene membrane (Figure 5d) 

exhibited higher hydrophobicity compared to the other membranes, attributed to its chemical and physical 

properties, including non-polar hydrocarbon chains with strong C-H and C-C bonds (Gilbert, 2017). 

  

 

  

 

 

  

  

  

  

   

   

   

   

   

   

               
 
 
 
  
  

  
 
 
 
  

 
 
  
 
 

  
 
  

 
 
 

                        

          

          

   

                             

 

    

   

    

   

    

   

    

   

    

   

               
 
 
 
  
  

  
 
 
 
  

 
 
  
 
 

  
 
  

 
 
 

                        

          

          

   

                            
 

    

   

    

   

    

   

    

   

    

   

               
 
 
 
  
  

  
 
 
 
  

 
 
  
 
 

  
 
  

 
 
 

                        

          

          

   

                            

 

    

   

    

   

    

   

    

   

    

   

               
 
 
 
  
  

  
 
 
 
  

 
 
  
 
 

  
 
  

 
 
 

                        

          

          

   

                            

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Figure 4: Pore size distribution at BJH desorption. (a) BPU nanocomposite with 0.0% silica; (b) BPU 
nanocomposite with 0.5% silica; (c) BPU nanocomposite with 1.0% silica; and (d) polypropylene (PP). 
 
 
 

 
 
Figure 5: Contact angle taken at the first instant the water dropped. (a) BPU nanocomposite with 0.0% silica; 
(b) BPU nanocomposite with 0.5% silica; (c) BPU nanocomposite with 1.0% silica; and (d) polypropylene (PP). 

4. Conclusion 

This study demonstrated the potential of castor oil-based polyurethane nanocomposite membranes as a 

promising alternative to fossil-derived polymers. The in-situ synthesis and incorporation of fumed silica 

nanoparticles enabled control over the morphology and physicochemical properties of the membranes, including 

pore distribution and hydrophobic characteristics. Notably, the 1.0% silica concentration led to a significant 

reduction in the maximum pore size (>80%) compared to the membrane containing 0.5% nanoparticles, 

highlighting the critical influence of silica loading on pore structure. Furthermore, nitrogen adsorption-desorption 

analysis revealed that BPU membranes exhibited behavior characteristic of mesoporous materials, whereas 

commercial polypropylene demonstrated macroporous characteristics. Water contact angle evaluation showed 

that hydrophobicity initially increased with the addition of silica (0.5%), but decreased at higher concentrations 

(1.0%) due to silica agglomeration, which impaired homogeneous dispersion and effective interactions within 

the composite. These findings underscore the viability of BPU membranes as functional materials for specific 

applications requiring tailored porosity and surface properties. Thus, integrating renewable materials into 

separation systems represents a significant contribution to the development of technologies aligned with circular 

economy principles and sustainability. 

Acknowledgments 

The authors gratefully acknowledge the support provided by grant #2015/20630-4 and grant #2017/11958-1 

from the São Paulo Research Foundation (FAPESP), grant #140933/2023-9 from the Brazilian National Council 

for Scientific and Technological Development (Conselho Nacional de Desenvolvimento Científico e Tecnológico 

      

 

    

    

    

    

   

    

    

    

                  

  
 
  

 
 
 
  

 
 
 
  

 
 
  

 
 
  
 
 

  
 
 

                     

   

  

   

   

          
       

         
           

          
         

 

       

       

       

       

      

       

         

  
 
  

 
 
 
  

  
 
 
  

  
 
  

 
 
  
 
 

  
 
 

                     

   

  

    

  

          
       

         
           

          
         

 

       

       

       

       

      

       

         

  
 
  

 
 
 
  

 
 

 
  

 
 
  

 
 
  
 
 

  
 
 

                     

   

  

    

  

          
       

         
           

          
         

 

       

       

       

       

      

       

         

  
 
  

 
 
 
  

  
 
 
  

  
 
  

 
 
  
 
 

  
 
 

                     

   

  

    

  

          

       

         

           

          

         

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- CNPq), and Prof. Dr. Celso Aparecido Bertran and Humberto Machado from the Functional Materials and 

Interfaces Laboratory (FMILab) at the Institute of Chemistry (Instituto de Química – IQ), UNICAMP, for their 

assistance with the DSA100 Drop Shape Analyzer. 

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