Synthesis and characterization of biobased polyurethane/SiO₂ nanocomposites from natural Sapium sebiferum oil

Abstract

Novel Sapium sebiferum oil-based polyurethane (PU) nanocomposites with different contents of nano-SiO₂ were successfully synthesized via an in situ polymerization method. Firstly, Sapium sebiferum polyol (SSP) was prepared in a continuous two-step process of epoxidation and hydroxylation, and nano-SiO₂ particles were modified with 3-aminopropyltriethoxysilane (AMEO). Furthermore, SSP and isophorone diisocyanate (IPDI) reacted with a certain amount of the modified nano-SiO₂ to enhance the physical properties of PU nanocomposites. The characterization of the structure and the properties of the surface modified nano-SiO₂ and the PU/SiO₂ nanocomposites were analyzed by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), solid-state ²⁹Si nuclear magnetic resonance (NMR), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results demonstrate that nano-SiO₂ particles can be homogeneously dispersed in the PU matrix with a loading amount less than 5 wt%. Compared with the pure PU, the glass transition temperatures and initial decomposition temperatures of the PU/SiO₂ nanocomposites were respectively enhanced by 20.5 °C and 43.5 °C with 3 wt% nano-SiO₂ content. Moreover, the tensile strength of the PU/SiO₂ nanocomposites was improved at 170%, and the water and toluene resistance properties of PU/SiO₂ were also significantly enhanced.

---

1. Introduction

Nowadays, modern chemical and fuel industries rely heavily on petroleum and its products. The use of petroleum feedstock in the chemical industry has led to a rapid increase of CO₂ emission and quick depletion of crude oil, which has seriously threatened the development of mankind.¹ In recent years, natural resources like plant oils and natural fats have been regarded as attractive feedstock for the preparation of resins and polymeric materials to replace or complement the traditional petro-chemical based polymers and resins.^2,3 As known, plant oils are considered to be one of the cheapest and most abundant biological sources, and are widely used as platform chemicals for polymer synthesis because of their availability, sustainability, low emission, inherent biodegradability, and low toxicity.^4–6

The increasing importance of polymeric materials from renewable resources has put plant-oil-derived polyurethanes (PUs) in the research spotlight, especially due to their simple preparation procedure and versatile properties potentially suitable for use in a variety of fields, such as footwear, machinery industry, coatings and paints, rigid insulations, elastic fibers, soft flexible foam, and medical devices.^5,7 Till now, plant-oil-based PUs have been reported to be actually applied in many diverse fields.^8–10 However, in most reported literatures, plant oils for the synthesis of PUs are food oils. As known, food oils are urgently needed to meet the tremendous demand of increasing world population. So, they are actually limited to PUs synthesis in practice, because of contradiction to food and farming land demands of the world.

Sapium sebiferum (SS) which originated in China belongs to the Euphorbiaceae family. The tree can be grown on marginal land such as alkaline, saline, droughty and acidic soils. This will not compete with existing agricultural resources and available arable land. Furthermore, the tree can grow rapidly to reach maturity within approximately 3–4 years and generate economic yields of 4–10 tons of seeds per hectare every year. The most advantage is that the tree can keep its productive life span for between 70 to 100 years. The seeds contain 45–60% oil which is non-edible plant oil due to the presence of some toxic components.¹¹ It eliminates competition for human food and feed. Meanwhile, the content of unsaturated fatty acids in Sapium sebiferum oil (SSO) excels 90%, and its double bonds per molecule attain 6.6, much higher than those of castor oil (3.0), soybean oil (4.5).¹² Its iodine value of 186.8 g of I₂/100 g, which is the relatively higher one in plant oils, corresponds to the higher degree of unsaturation in plant oils, resulting in the higher hydroxyl value of the synthesized polyols.¹³ Therefore, SSO as a potential feedstock is very suitable for polymer synthesis.

In addition, inorganic/organic nanocomposites are coming to light for polymer materials due to their extraordinary and unique combination of properties resulting from the synergism between the inorganic nanoparticles and the polymer.^14–16 Addition of a relatively small amount of the nanoparticles can dramatically change the properties of the resultant polymers.¹⁷ The combination of polymers and nanoscale inorganic fillers is potential pathways for engineering flexible composites that exhibit attractive mechanical, thermal, optical and electrical properties compared with conventional composites.^18,19 In the past several years, nano-SiO₂ has been widely introduced into PU matrix by physical mixture to improve the heat resistance, radiation resistance, mechanical and electrical properties.^20,21 However, they easily aggregate into large particles in the prepared nanocomposites and tend to accumulate in the dispersion media during the synthesis process owing to their nanometer dimensions and high specific surface area.²² The modification of nano-SiO₂ with 3-aminopropyltriethoxysilane (AMEO) before incorporating it into PU matrix can achieve good dispersion and result in good compatibility between nano-SiO₂ and PU matrix, leading to great property improvement of the resulted polymer nanocomposites.^23,24

Therefore, in this work, SSO was for the first time employed to synthesize novel PU/SiO₂ nanocomposites as renewable raw materials. The pure PU was synthesized with the polyol originated from SSO and isophorone diisocyanate (IPDI). Then, nano-SiO₂ modified with AMEO was used to synthesize novel PU/SiO₂ via in situ polymerization method. The microstructure and the properties of the PU/SiO₂ nanocomposites were further characterized and confirmed by FTIR, SEM, TGA and DSC.

2. Materials and methods

2.1. Materials

SSO was purchased from a refinery factory of Dawu County, Hubei Province, P. R. China. Nano-SiO₂ with an average particle size of 30 nm, IPDI and AMEO were bought from Aladdin Chemistry Co. Ltd., China. Peracetic acid, acetic acid, methyl alcohol, fluoboric acid, chloroform, toluene, and acetone were commercially got from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The polyol and nano-SiO₂ were dried at 100 °C for 3 h under vacuum to remove moisture before use.

2.2. Synthesis methods

2.2.1. Preparation of Sapium sebiferum polyol (SSP). The polyol was generated from SSO in a continuous two-step process of epoxidation and hydroxylation to avoid unnecessary intermediate steps.^25,26 The preparation procedures of SSP are shown in Scheme 1. First, a certain amount of SSO was added in a 500 mL four-necked round bottom flask equipped with a thermometer, a dropping funnel, and a mechanical stirrer. The pretreated solution of peracetic acid dissolved in acetic acid was dropped into the oil at a temperature of 10 °C by an ice-bath. The mixture was stirred at 25 °C for 5 h, and then the epoxidized SSO production in acetic acid solution was gained. Methyl alcohol and fluoboric acid for the proportion of 24 to 1 were added into a 250 mL three-necked round bottom flask at room temperature. The prior prepared epoxidized SSO production was dropped into the methanol/fluoboric acid solution, and then was stirred at 50 °C for 1 h. After the solution cooled down, 100 mL water and 50 mL chloroform were putted into the round bottom flask. When left at rest for a certain time, the organic phase and water were layered. Then the mixture was washed with deionized water until it reached a neutral pH. The solvent was dried and filtered in a vacuum condition. Finally, the yellow oily matter was obtained. The polyol production was tested with –OH number of 195 mg KOH per g, and named as polyol-195.

Scheme 1 Preparation of SSP from SSO.

2.2.2. Surface modification of nano-SiO₂ particles. Nano-SiO₂ was modified with AMEO to introduce NH₂-groups as shown in Scheme 2.²⁷ Pre-dried nano-SiO₂ with average particle size of 30 nm was dispersed in toluene under nitrogen flow in a 250 mL two-necked round bottom flask equipped with a mechanical stirrer and thermometer. The mixture was firstly stirred for 30 min, and AMEO was added (1 g nano-SiO₂ corresponding to 2.5 mmol AMEO). Then, the obtained mixture was refluxed at 110 °C for 10 h. The solid was collected by filtration, washed with 100 mL toluene, and dried at 60 °C overnight.

Scheme 2 Surface modification of nano-SiO₂ particles.

2.2.3. Synthesis of PU/SiO₂ nanocomposites. PU was prepared from polyol-195 with IPDI via in situ synthesis as shown in Scheme 3.²⁸ In a typical run, a certain amount of the modified nano-SiO₂ and acetone were adequately mixed by sonic oscillation for 30 min. The mixture was placed into a 250 mL three-necked round bottom flask with a magnetic stirring and a water cooling condenser under N₂ atmosphere. Then, IPDI were added to the flask, and reacted at 80 °C for 2 h. After that, the temperature was reduced to 60 °C and polyol-195 (a 1 : 1.1 molar ratio of –OH and –NCO group) was added. After 2 h reaction, the obtained mixture was degassed under vacuum for 10 min. Finally, the product was poured onto a sheet of release paper and kept in an oven at 60 °C for 8 h. During this stage, acetone was added to reduce the systematic viscosity. The PU/SiO₂ with 1, 3, 5, 7 wt% loading of the modified nano-SiO₂ were produced, which were designated as PU1, PU3, PU5, PU7, respectively. A comparable sample of pure PU was also prepared via this procedure without adding nano-SiO₂, and named as PU0.

Scheme 3 Synthesis of PU/SiO₂ nanocomposites.

2.3. Testing and measurement

To examine the significant absorption bands of nano-SiO₂ and PU/SiO₂, Fourier transform infrared (FTIR) spectra of the sample powder pressed with KBr into pellets were observed in the wave number range from 4000 to 400 cm⁻¹ at a resolution of 4 cm⁻¹ using Bruker Vertex70 FTIR spectrophotometer at room temperature.

To investigate thermal stability and behavior of nano-SiO₂ and PU/SiO₂ nanocomposites, thermogravimetric analysis (TGA) was carried out with the aid of a Pyris1 TGA instrument. Nano-SiO₂ samples were examined under nitrogen flow from 30 to 900 °C at a heating rate of 10 °C min⁻¹, while the pure PU and PU/SiO₂ nanocomposites were examined in nitrogen atmosphere from 30 to 600 °C at a heating rate of 10 °C min⁻¹. The weight of the measured samples was about 5 mg.

Solid-state ²⁹Si nuclear magnetic resonance (NMR) spectra were collected with a Varian Infinity Plus-400 spectrometer. ²⁹Si spectra were recorded at 79.48 MHz, with a pulse width of π/4, a pulse delay of 8 s, a spinning rate of 5 kHz.

To explore various thermal transitions, the glass transition temperatures (T_gs) of the pure PU and PU/SiO₂ nanocomposites were tested by differential scanning calorimetry (Diamond DSC) with a heating rate of 10 °C min⁻¹ from −20 to 120 °C under nitrogen flow. 6 mg of the sample was placed in an aluminum pan for each run.

Scanning electron microscopy (SEM) and backscattered electron microscopy (BSE) (Nova NanoSEM 450) were employed to observe the morphology of PU, PU/SiO₂ nanocomposites, and the dispersion of nano-SiO₂ in the polymer matrix. The freeze-fractured surfaces of PU and PU/SiO₂ nanocomposites were obtained at liquid nitrogen temperature, and then, all these specimens were coated with gold before SEM observation.

Mechanical properties of the pure PU and PU/SiO₂ films (100 mm × 10 mm × 4 mm) were examined on a CMT4104 universal testing machine (Shenzhen SANS Testing Machine Co. Ltd., Shenzhen, China) at a speed of 50 mm min⁻¹. The results were averaged over at least five specimens.

Water swelling value (the water absorption degree of the pure PU and PU/SiO₂ films) was assessed as follows. The films (20 mm × 20 mm × 1 mm) were dipped in distilled water at ambient temperature for 14 days and then taken out. Subsequently, surface water was absorbed with a filter paper. Water swelling was calculated according to eqn (1):

Swelling (%) = (w₁ − w₀)/w₀ × 100% (1)

where, w₁ is the weight of swollen film, w₀ is the weight of the original dry film.

Toluene resistance was assessed by weight loss after drying the swollen films. The films (20 mm × 20 mm × 1 mm) were dipped in toluene for 24 h and taken out to be placed in oven at 110 °C. The weight loss of toluene resistance was obtained by eqn (2):

Weight loss (%) = (w₀ − w₂)/w₀ × 100% (2)

where, w₂ is the weight of the dried films after being dipped in toluene, w₀ is the weight of the original dry film.

3. Results and discussion

3.1. Preparation of SSP via epoxidation and hydroxylation

The FTIR spectra of SSO, ESS and SSP are shown in Fig. 1. After epoxidation, the characteristic peak at 3011 cm⁻¹ attributed to the C–H stretching of C=C–H in the SSO disappeared, while a new peak at 824 cm⁻¹ attributed to the epoxy group emerges in the spectrum of ESS. The further hydroxylation treatment resulted in the opening of epoxy group in the ESS, which is confirmed by the disappearance of the 824 cm⁻¹ peak in the spectrum of SSP. At the same time, a new characteristic absorption peak at 3447 cm⁻¹ in the spectrum of the SSP emerges, which is attributed to the –OH group. Moreover, the emergent peak at 1096 cm⁻¹ can be ascribed to the stretching of the C–O bond associated with the ether groups.²⁹ These change of absorption peaks indicates that a successful reaction between methanol and ESS which results in a complete hydroxylation. With SSO as the raw materials, the synthesis of SSP can be achieved by the same method for soy-based polyols.²⁹ However, unsaturated fatty acids of SSO add up to more than 90% and its double bonds per molecule reach up to 6.6. The composition and iodine values of fatty acids in common plant oils are presented in Table 1.^30,31 Seen from Table 1, SSO has the highest iodine value of 186.8 g of I₂/100 g, which means to correspond to the highest degree of unsaturation in plant oils. Therefore, it can be concluded that SSO is the most suitable platform chemical for different polymers production, such as polyurethanes.

Fig. 1 FTIR spectra of (a) SSO; (b) ESS; (c) SSP.

Table 1 Fatty acids composition (mass%) and iodine values of common plant oils

Name	Palmitic	Stearic	Oleic	Linoleic	Linolenic	Iodine value
						(g of I2/100 g)
Soybean	14.0	4.0	23.3	52.2	5.6	128.7
Linseed	5.0	4.0	22.0	17.0	52.0	180.0
Sunflower	6.5	2.0	45.4	46.0	0.1	120.2
Peanut	11.6	2.2	46.5	32.2	—	97.6
Cottonseed	22.1	2.8	19.4	53.5	2.3	109.4
Palm	41.8	3.4	41.9	11.0	—	43.3
Rapeseed	4.0	2.0	56.0	26.0	10.0	102.0
Castor	1.5	0.5	5.0	4.0	0.5	102.2
Corn	10.0	4.0	34.0	48.0	—	123.5
Sapium sebiferum	7.1	2.0	14.2	29.8	42.7	186.8

---

3.2. Grafting AMEO onto the nano-SiO₂ particles

The comparison of the FTIR spectra between the original nano-SiO₂ particles and the surface-modified particles confirms the interaction between nano-SiO₂ and AMEO, as shown in Fig. 2. In the spectrum of the original nano-SiO₂ particles (Fig. 2a), the vibration peaks at 471 and 800 cm⁻¹ are assigned to the Si–O bond rocking and bending, respectively. The vibration band of 1103 cm⁻¹ is attributed to the Si–O asymmetric stretching. In the spectrum of the surface-modified nano-SiO₂ (Fig. 2b), besides the peaks belonging to the original nano-SiO₂, the characteristic peaks at 2930 and 2856 cm⁻¹ are attributed to the C–H asymmetric and symmetric stretching vibration of –CH₂. These new peaks indicate a successful grafting of the nano-SiO₂ particles with AMEO.

Fig. 2 FTIR spectra of (a) original nano-SiO₂; (b) modified nano-SiO₂.

Furthermore, the TGA analysis can also reflect the efficiency of AMEO grafting on nano-SiO₂. The difference of TGA curves between the original and surface-modified nano-SiO₂ shown in Fig. 3 confirms the grafting of AMEO on the nano-SiO₂. For the original nano-SiO₂, the weight loss is very slight, about 2.8 wt%, which is probably assigned to dehydration of the nano-SiO₂ particles. While for the surface-modified nano-SiO₂, the curve shows a sharp weight loss from 270 °C to 660 °C. The weight loss is more than 12 wt%, which can be attributed to the large scale oxidative thermal decomposition of AMEO grafted on the surface of the modified nano-SiO₂.³²

Fig. 3 TGA curves of (a) original nano-SiO₂; (b) modified nano-SiO₂.

In addition, the solid ²⁹Si NMR spectra of the samples are displayed in Fig. 4. The nano-SiO₂ exhibits in its non-CP spectrum (Fig. 4a) a sharp peak around −112 ppm, which is attributed to the Q⁴ site, and a broad peak in the region of −103 to −106 ppm, part of which is corresponding to the Q³ site, i.e., Si(OH)(OSi)₃ or Si(OH)(OAl)(OSi)₂.³³ The weak peak around −90 ppm in the spectrum is ascribed to the small amount of Q² groups, i.e., Si(OH)₂(OSi)₂, Si(OH)₂(OAl)(OSi), etc.³⁴ In the non-CP spectra (Fig. 4b), after grafting treatment, the peak intensity between −103 to −106 ppm and at around −90 ppm of the modified nano-SiO₂ strongly decreases in comparison with original nano-SiO₂. However, there is little change of the peak intensity at −112 ppm after grafting treatment. Therefore, the decreased concentration of the silanol groups (Fig. 4b) should be the result of the consumption of silanol groups of nano-SiO₂, which reacted with –NH₂ group of AMEO. These results indicate that AMEO has been successfully grafted on the surface of nano-SiO₂ nanoparticles.

Fig. 4 Solid state ²⁹Si MAS NMR spectroscopy (a) original nano-SiO₂; (b) modified nano-SiO₂.

3.3. Synthesis of pure PU and PU/SiO₂ nanocomposites

The representative FTIR spectra of the pure PU and PU/SiO₂ nanocomposites are shown in Fig. 5, which indicates the changes after polymerization between SSP and IPDI. The disappearance of stretching vibration band at 2270 cm⁻¹ of –NCO group for the IPDI suggests that –NCO groups have been consumed by –OH group of the polyol after reaction. The characteristic bands occurring at 3346–3352 cm⁻¹ (N–H stretching vibration), 2926–2932 cm⁻¹ (–CH₂ asymmetric stretching vibration), 2853–2890 cm⁻¹ (–CH₂ symmetric stretching vibration), 1708–1718 cm⁻¹ (the carbonyl C=O stretching vibration), 1541–1553 cm⁻¹ (N–H out-of-plane bending and C–N stretching vibrations) and 1242–1250 cm⁻¹ (the ester C–O stretching vibration) confirm the formation of urethane groups (–NH–(C=O)–O–) after reaction. The occurrence of urethane group indicates the successful preparation of PUs. From the comparison of the spectra of the pure PU and PU/SiO₂ nanocomposites, it is very hard to find any new peak except for the slight peak at 476 cm⁻¹ belonging to SiO₂ in PU/SiO₂ nanocomposites, which means that the segmented structure of PU is not affected by the presence of nano-SiO₂. This phenomenon suggests the reservation of the PU chemical structure after the dispersion of nano-SiO₂ in the polyurethane matrix.²⁸

Fig. 5 FTIR spectra of PU and PU/SiO₂ nanocomposites.

The SEM photographs (Fig. 6) and BSE images (Fig. 7) illustrate the morphological characterization of PU and PU/SiO₂ and the distribution of nano-SiO₂ in the PU/SiO₂ nanocomposites. As shown Fig. 7b and c, the (white) bright spots in the images represent nano-SiO₂ particles in the PU/SiO₂ nanocomposites and homogeneous distribution of nano-SiO₂ in the matrix can be achieved with nano-SiO₂ at 3 wt% loading level.^35,36 When the loaded nano-SiO₂ increases up to 5 wt%, a slight agglomeration appears (see Fig. 6d and 7d), and a serious agglomeration and poor distribution of nano-SiO₂ are observed with nano-SiO₂ amount more than 5 wt% (Fig. 6e and 7e). The aggregation of the nano-SiO₂ particles in PU/SiO₂ is probably attributed to strong hydrophilicity and high surface free energy of the small particle size nano-SiO₂.³⁷ Hence, this grafting polymerization method can provide homogeneous dispersion of the modified nano-SiO₂ in the polyurethane matrix, which enhances the overall properties of PU/SiO₂.

Fig. 6 SEM images in 10 μm of PU and PU/SiO₂ nanocomposites of the sample (a) 0% nano-SiO₂; (b) 1% nano-SiO₂; (c) 3% nano-SiO₂; (d) 5% nano-SiO₂; (e) 7% nano-SiO₂.

Fig. 7 BSE images in 500 nm of PU and PU/SiO₂ nanocomposites of the sample (a) 0% nano-SiO₂; (b) 1% nano-SiO₂; (c) 3% nano-SiO₂; (d) 5% nano-SiO₂; (e) 7% nano-SiO₂.

3.4. Characterizing the properties of pure PU and PU/SiO₂ nanocomposites

3.4.1. Glass transition. The DSC experiments revealed the glass transition trends for the pure PU and PU/SiO₂ nanocomposites of different nano-SiO₂ contents. The T_gs of the pure PU and PU/SiO₂ nanocomposites are presented in Fig. 8. It can be seen that the T_gs of all PU/SiO₂ samples are higher than that of the pure PU. Furthermore, the variation tendency for T_gs of PU/SiO₂ with reference to the nano-SiO₂ contents presents a volcano curve, which has its maximum of 69.9 °C at 3 wt% nano-SiO₂ in the PU/SiO₂. As known, T_g represents the mobility level of polymer chains and network in the matrix at the molecular level. Therefore, the increase of T_g value indicates a decrease of molecular movement in the polymer.³⁸ It has been reported that well-dispersed nano-SiO₂ can restrict the molecule motion of the polymer chains and lead to the increase of T_g value. However, the overloading of nano-SiO₂ enhances the micro-phase separation of nano-SiO₂ and PU/SiO₂ matrix, which results in the aggregation of nano-SiO₂ and the decreased T_g value of PU/SiO₂.³⁹ As shown above, the aggregation of nano-SiO₂ has been confirmed by SEM (Fig. 6) and BSE (Fig. 7) mapping graphs. Therefore, with the increase of nano-SiO₂ contents in PU/SiO₂, the well distribution of the nano-SiO₂ into the PU matrix can efficiently increased the maximum value of T_gs.

Fig. 8 DSC curves of PU and PU/SiO₂ nanocomposites.

3.4.2. Thermal properties. TGA and derivative thermogravimetry (DTG) curves of the pure PU and PU/SiO₂ nanocomposites are shown in Fig. 9. The initial decomposition temperatures (IDT), which are assigned to be the temperature at 5% weight loss, are listed in Table 2. The IDT of PU/SiO₂ samples are all higher than that of the pure PU. For the PU/SiO₂ samples with different content of nano-SiO₂, the IDT first increases and then decreases with the increment of nano-SiO₂ contents. When loaded nano-SiO₂ amount is up to 3 wt%, the IDT reaches its highest value of 286.2 °C, which are 45.5 °C higher than that of the pure PU.

Fig. 9 TGA (a) and DTG (b) curves of PU and PU/SiO₂ nanocomposites.

Table 2 TGA and DTG results for pure PU and PU nanocomposites

Sample	IDT (°C)	Tmax		Residue at 600 °C (%)
		1st step (°C)	2nd step (°C)
PU0	242.7	321.5	404.6	1.9
PU1	272.3	326.2	406.8	3.1
PU3	286.2	323.3	444.6	4.4
PU5	256.7	322.6	426.4	4.5
PU7	246.9	322.2	420.4	6.1

---

It is known that the process for thermal decomposition of PU can be divided into two stages.⁴⁰ According to the DTG curves (Fig. 9b), the maximum of degradation temperatures (T_maxs) in the two stages for the pure PU and PU/SiO₂ can be determined, as shown in Table 2. At each stage, T_maxs of PU/SiO₂ samples are all higher than that of the pure PU. There is a maximum value with the nano-SiO₂ contents in the range of 0% to 7%. Moreover, the decomposition residues of the samples at 600 °C increase monotonously with the increase of the nano-SiO₂ contents (Fig. 9a and Table 2).

These findings suggest that incorporation of nano-SiO₂ in the PU matrix enhances the thermal stability of PU/SiO₂ nanocomposites. The enhanced thermostability can be ascribed to the interaction between nano-SiO₂ and macromolecular chains, which limits the movement of the molecular chain of PU. On the other hand, after grafting nano-SiO₂ of high melting point onto the polymer matrix, the nano-SiO₂ particles can serve as a good thermal cover layer and prevent the direct thermal decomposition of polymer matrix by heat.⁴¹ However, the thermostability can be weakened with higher silica loading, which is probably due to formation of the irreversible aggregation of nano-SiO₂ particles.

3.4.3. Mechanical properties. The mechanical properties of PU/SiO₂ with different contents of nano-SiO₂ were investigated by tensile testing. The tensile strength and elongation at break of the pure PU and PU/SiO₂ are presented in Table 3. The results indicate that incorporating a certain amount of nano-SiO₂ particles can improve the mechanical properties of the polyurethanes. It is known that active –NH₂ groups on the surface of the modified nano-SiO₂ can react with the –NCO groups of IPDI. Hence, more chemical interactions occur between nano-SiO₂ and PU matrix with the increase of nano-SiO₂ content, resulting in more networks in the nanocomposites. The network structure in nanocomposite is favorable for improvement of mechanical strength. It is worth noting that, with 3 wt% nano-SiO₂ loading, the tensile strength of the nanocomposites reaches its maximum of 12.4 MPa, compared with 4.6 MPa of the pure PU. When the content of nano-SiO₂ is above 3 wt%, the tensile strength of the nanocomposites starts to decrease. On the other hand, the elongation at break feebly decreases for all the tested PU/SiO₂ films compared with the pure PU film, with a maximum decrease at 3 wt% loading level. This phenomenon can be explained by the fact that the construction of rigid filler network structure is responsible for the enhanced effect. The decrease of the mechanical strength of PU/SiO₂ with more than 5 wt% may be attributed to the aggregation of excess loading of the nano-SiO₂ in PU matrix. Furthermore, the aggregation behavior increases with the increment of nano-SiO₂ contents, indicating an increase in the incompatibility of the PU/SiO₂ and excess nano-SiO₂.

Table 3 Mechanical properties of pure PU and PU nanocomposites

Sample	Tensile strength (MPa)	Elongation at break (%)
PU0	4.6 ± 0.2	263.4 ± 20.6
PU1	7.1 ± 0.3	188.6 ± 14.3
PU3	12.4 ± 0.8	100.9 ± 10.5
PU5	11.3 ± 0.6	107.3 ± 11.8
PU7	9.5 ± 0.7	112.1 ± 9.7

---

In combination with the result of SEM in Fig. 6 and BSE in Fig. 7, it can be concluded that an optimal amount of nano-SiO₂ incorporation (3% nano-SiO₂) effectively enhances the mechanical properties of PU/SiO₂. As a result, an ideal reinforcement can be achieved with the homogeneous dispersion of nano-SiO₂ in the PU/SiO₂.

The thermal and mechanical properties comparison of the synthesized PUs from common plant oils is presented in Table 4.^42–48 The properties of PUs depend on the reactivity of plant oils, the properties of fatty acids, and their relative percentages.³¹ PUs belonging to various types of plant oils show a wide range of variations in properties, and this makes them useful in many application-based products, including footwear, coatings, rigid insulations, elastic fibers, soft flexible foam, and medical devices.⁵ From Table 4, the properties of SSO-based PU, such as thermal stability, tensile strength and elongation at break well match with other types of plant-oil-PUs. Furthermore, DSC results illustrate T_g of SSO-based PU is higher than those plant-oil-PUs. It may be because there are more hydroxyl numbers of the polyols used in SSO-based PU synthesis and higher cross-linking density in its chemical structure.¹³

Table 4 Properties comparison of synthesized PUs from common plant oils

Sample	DSC (°C)	TGA			Tensile strength (MPa)	Elongation at break (%)
		IDT (°C)	Tmax1 (°C)	Tmax2 (°C)
Soybean	20.9	242	—	—	4.5 ± 0.6	329.6 ± 47.2
Linseed	35.9	150	267	312	3.8 ± 1	256 ± 6
Sunflower	41	—	284	408	23.18 ± 0.8	750 ± 5
Peanut	−9.2	—	—	—	2.27 ± 0.03	343 ± 10
Cottonseed	—	—	—	—	3.26 ± 0.50	2.44 ± 0.27
Palm	39.7	191.9	275	447	1.5 ± 0.3	—
Rapeseed	43.9	256	282	423	11.7 ± 0.8	420 ± 10
Castor	−40.2	281	354	434	6.3 ± 0.3	389.1 ± 12.3
Corn	−6.6	—	—	—	3.40 ± 0.23	322 ± 12
Sapium sebiferum	49.4	242.7	321.5	404.6	4.6 ± 0.2	263.4 ± 20.6

---

3.4.4. Water swelling and toluene resistance properties. As a function of the nano-SiO₂ contents, Fig. 10 shows water swelling and toluene resistance of the pure PU and PU/SiO₂ films. It is clear that incorporation of nano-SiO₂ significantly decreases the value of water swelling. It decreases from 9.6% to 4.7% with the increase of nano-SiO₂ contents from 0% to 7%. The result demonstrates that the presence of impermeable nano-SiO₂ in PU matrix can reduce water swelling and enhance the resistance to moisture. The probable reason is that the homogeneously dispersed nano-SiO₂ could enlarge the mean free path of water molecules to pass through the PU/SiO₂ network structure.⁴⁹

Fig. 10 Water swelling and toluene resistance of PU and PU/SiO₂ nanocomposites.

Because of the good chemical stability of nano-SiO₂ particles, the incorporation of nano-SiO₂ in the PU matrix maintains good chemical resistance properties of PU/SiO₂. It can also be seen from Fig. 10, toluene resistance properties of PU/SiO₂ are greatly improved in comparison to the pure PU.

4. Conclusion

In this study, polyol-195 was synthesized from SSO, and a novel PU was further prepared by reaction of this polyol with IPDI via in situ polymerization. AMEO was then successfully grafted onto the surface of nano-SiO₂ particles, and the modified nano-SiO₂ particles were used to enhance the properties of the SSO-based PU/SiO₂ matrix. The structure–property relationships with different loading of nano-SiO₂ were evaluated. SEM and BSE mappings reveal that nano-SiO₂ particles with less than 5% content are more easily dispersed in the PU matrix, the excess of nano-SiO₂ loading leads to irreversible agglomerates. The glass transition temperatures of the prepared PU matrix also increases from 49.4 to 69.9 °C, indicating adding small amounts (3 wt%) of nano-SiO₂ to the PU matrix significantly improves its thermostability. Meanwhile, the tensile strengths of PU/SiO₂ are enhanced from 4.6 to 12.4 MPa in comparison with the pure PU. In addition, water swelling and toluene resistance properties of the prepared PU/SiO₂ nanocomposites are greatly improved.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (no. 31070089, 31170078 and J1103514), the National High Technology Research and Development Program of China (2011AA02A204, 2013AA065805), the Innovation Foundation of Shenzhen Government (JCYJ20120831111657864) and the Fundamental Research Funds for HUST (no. 2014NY007). Many thanks are indebted to Analytical and Testing Center of HUST for their valuable assistances in FTIR, SEM, TGA and DSC measurement.

References

1. Y. Yan, X. Li, G. Wang, X. Gui, G. Li, F. Su, X. Wang and T. Liu, Appl. Energy, 2014, 113, 1614–1631.
2. A. Gandini, Macromolecules, 2008, 41, 9491–9504.
3. X. Yao, G. Wu, L. Xu, H. Zhang and Y. Yan, RSC Adv., 2014, 4, 31062–31070.
4. U. Biermann, W. Friedt, S. Lang, W. Lühs, G. Machmüller, J. O. Metzger, M. Ruesch Gen Klaas, H. J. Schaefer and M. P. Schneider, Angew. Chem., Int. Ed., 2000, 39, 2206–2224.
5. Z. S. Petrović, Polym. Rev., 2008, 48, 109–155.
6. G. Lligadas, J. C. Ronda, M. Galia and V. Cádiz, Biomacromolecules, 2010, 11, 2825–2835.
7. D. V. Palaskar, A. Boyer, E. Cloutet, C. Alfos and H. Cramail, Biomacromolecules, 2010, 11, 1202–1211.
8. E. Sharmin, D. Akram, F. Zafar, S. M. Ashraf and S. Ahmad, Prog. Org. Coat., 2012, 73, 118–122.
9. M. Desroches, S. Caillol, V. Lapinte, R. Auvergne and B. Boutevin, Macromolecules, 2011, 44, 2489–2500.
10. X. Kong, G. Liu and J. M. Curtis, Eur. Polym. J., 2012, 48, 2097–2106.
11. Q. Li and Y. Yan, Appl. Energy, 2010, 87, 3148–3154.
12. A. Zlatanić, C. Lava, W. Zhang and Z. S. Petrović, J. Polym. Sci., Part B: Polym. Phys., 2004, 42, 809–819.
13. X. Kong, G. Liu, H. Qi and J. M. Curtis, Prog. Org. Coat., 2013, 76, 1151–1160.
14. J. Liu, G. Chen and J. Yang, Polymer, 2008, 49, 3923–3927.
15. C. Yuan, J. Zhang, G. Chen and J. Yang, Chem. Commun., 2011, 47, 899–901.
16. Z. Hu and G. Chen, Adv. Mater., 2014, 26, 5950–5956.
17. P. A. Charpentier, K. Burgess, L. Wang, R. R. Chowdhury, A. F. Lotus and G. Moula, Nanotechnology, 2012, 23, 425606.
18. Y. Lin, A. Böker, J. He, K. Sill, H. Xiang, C. Abetz, X. Li, J. Wang, T. Emrick and S. Long, Nature, 2005, 434, 55–59.
19. M. Q. Zhang, G. Yu, H. M. Zeng, H. B. Zhang and Y. H. Hou, Macromolecules, 1998, 31, 6724–6726.
20. G. Chen, S. Zhou, G. Gu and L. Wu, Colloids Surf., A, 2007, 296, 29–36.
21. G. Leder, T. Ladwig, V. Valter, S. Frahn and J. Meyer, Prog. Org. Coat., 2002, 45, 139–144.
22. E. Amerio, P. Fabbri, G. Malucelli, M. Messori, M. Sangermano and R. Taurino, Prog. Org. Coat., 2008, 62, 129–133.
23. F. Dolatzadeh, S. Moradian and M. M. Jalili, Corros. Sci., 2011, 53, 4248–4257.
24. Y. Liao, X. Wu, Z. Wang, R. Yue, G. Liu and Y. Chen, Mater. Chem. Phys., 2012, 133, 642–648.
25. I. Javni, Z. S. Petrović, A. Guo and R. Fuller, J. Appl. Polym. Sci., 2000, 77, 1723–1734.
26. Z. S. Petrović, A. Guo, I. Javni, I. Cvetković and D. P. Hong, Polym. Int., 2008, 57, 275–281.
27. J. Chen, S. J. Halin, E. A. Pidko, M. Verhoeven, D. M. P. Ferrandez, E. J. Hensen, J. C. Schouten and T. A. Nijhuis, ChemCatChem, 2013, 5, 467–478.
28. X. Gao, Y. Zhu, X. Zhao, Z. Wang, D. An, Y. Ma, S. Guan, Y. Du and B. Zhou, Appl. Surf. Sci., 2011, 257, 4719–4724.
29. C. S. Wang, L. T. Yang, B. L. Ni and G. Shi, J. Appl. Polym. Sci., 2009, 114, 125–131.
30. A. E. Atabani, A. S. Silitonga, H. C. Ong, T. M. I. Mahlia, H. H. Masjuki, I. A. Badruddin and H. Fayaz, Renewable Sustainable Energy Rev., 2013, 18, 211–245.
31. M. R. Islam, M. D. H. Beg and S. S. Jamari, J. Appl. Polym. Sci., 2014, 131 DOI:10.1002/app.40787 , online.
32. R. Hashemi-Nasab and S. M. Mirabedini, Prog. Org. Coat., 2013, 76, 1016–1023.
33. F. Jin, Y. Cui and Y. Li, Appl. Catal., A, 2008, 350, 71–78.
34. S. Maheshwari, E. Jordan, S. Kumar, F. S. Bates, R. L. Penn, D. F. Shantz and M. Tsapatsis, J. Am. Chem. Soc., 2008, 130, 1507–1516.
35. C. A. Heck, J. H. Z. Dos Santos and C. R. Wolf, Int. J. Adhes. Adhes., 2015, 58, 13–20.
36. S. Lee, Y. B. Hahn, K. S. Nahm and Y. Lee, Polym. Adv. Technol., 2005, 16, 328–331.
37. M. Parvinzadeh, S. Moradian, A. Rashidi and M. Yazdanshenas, Appl. Surf. Sci., 2010, 256, 2792–2802.
38. C. Wang, X. Chen, J. Chen, C. Liu, H. Xie and R. Cheng, J. Appl. Polym. Sci., 2011, 122, 2449–2455.
39. H. Xia and M. Song, Soft Matter, 2005, 1, 386–394.
40. X. Liu, K. Xu, H. Liu, H. Cai, J. Su, Z. Fu, Y. Guo and M. Chen, Prog. Org. Coat., 2011, 72, 612–620.
41. Y. Chen, S. Zhou, H. Yang, G. Gu and L. Wu, J. Colloid Interface Sci., 2004, 279, 370–378.
42. Y. Lu, Y. Xia and R. C. Larock, Prog. Org. Coat., 2011, 71, 336–342.
43. C. Chang and K. Lu, Prog. Org. Coat., 2013, 76, 1024–1031.
44. B. Das, U. Konwar, M. Mandal and N. Karak, Ind. Crops Prod., 2013, 44, 396–404.
45. T. F. Garrison, M. R. Kessler and R. C. Larock, Polymer, 2014, 55, 1004–1011.
46. Z. He, D. C. Chapital, H. N. Cheng, K. Thomas Klasson, O. M. Olanya and J. Uknalis, Ind. Crops Prod., 2014, 61, 398–402.
47. A. Fridrihsone, U. Stirna, B. Lazdiņa, M. Misāne and D. Vilsone, Eur. Polym. J., 2013, 49, 1204–1214.
48. T. Gurunathan, S. Mohanty and S. K. Nayak, Prog. Org. Coat., 2015, 80, 39–48.
49. T. Chen, Y. Tien and K. Wei, Polymer, 2000, 41, 1345–1353.

---