Improving the mechanical properties and thermal stability of sodium alginate/hydrolyzed collagen films through the incorporation of SiO2.

Biopolymer-based films have become leading alternatives to traditional fossil-based packaging plastics. Among the countless types of biopolymers with potential for such applications, films containing hydrolyzed collagen in their composition were scarcely explored. This study determined the effect of different loads of nano-SiO2 (0, 2, 6, 8 and 10% w/w of sodium alginate) in the sodium alginate (SA) and hydrolyzed collagen (HC) blend films in terms of structure, thickness, mechanical properties, and thermal stability. The results indicated an improvement in the general mechanical and thermal behavior. Tensile strength increased from 18.2 MPa (control sample) to 25.4 MPa for the SA/HC film incorporated with 10% nano-SiO2. In the same condition, the film's elongation at break improved impressively (from 19.5 to 35.8%). Thermal stability improved slightly for all proportions of nano-SiO2. Therefore, the addition of nano-SiO2 can be an easy and simple strategy to improve crucial properties of SA/HC blend films, increasing its performance for future application as sustainable packaging.

Introduction

Synthetic plastic materials are widely employed in food packaging applications for their excellent properties (Marangoni et al., 2020). Although these polymers offer many advantages, such as protecting food from external conditions, the accumulation of these materials due to incorrect disposal remains a serious environmental problem (Moreira Gonçalves, Gomes Motta, Ribeiro-Santos, Hidalgo Chávez and Ramos de Melo, 2020). The development and application of packaging using raw materials from renewable sources is growing, since the play a fundamental role in reducing negative impacts on the environment. The most used renewable source polymer for film and coating manufacture are composed polysaccharides and proteins (Vianna et al., 2021).

Polysaccharides represent an attractive alternative for the production of films since they are abundant, inexpensive, and biodegradable (Nesic et al., 2020). Sodium alginate (SA) is a promising biomaterial with good film-forming properties for applications such as food packaging (Mousavi et al., 2021), and in the medical field (Abou-Okeil et al., 2018; Gheorghita Puscaselu, Lobiuc, Dimian and Covasa, 2020). SA is a natural anionic polysaccharide that is a by-product of the extraction of iodine and mannitol from brown algae. Its molecule is composed of β-d-manuronic acid (M) and α-l-guluronic acid (G) linked by a bond (1 → 4) (Kloster et al., 2017). SA contains abundant functional groups, such as hydroxyl and carboxyl, which can be easily modified to other functional groups (Zhang, Xu and Yang, 2021), and can help blending with other biopolymers, such as proteins, for obtaining better properties (Castiello et al., 2009).

Hydrolyzed collagen (HC) is a low-cost by-product protein from the tanning industry, and can be used in a mixture with SA, given the notable adhesive properties of proteins and their good processability (Marangoni Júnior, Rodrigues, da Silva, Vieira and Alves, 2021a; Seggiani et al., 2019). HC is a polypeptide compound made by additional hydrolysis of denatured collagen, with lower molar mass (3–6 kDa) than that of native collagen (285–300 kDa), low viscosity and better solubility in water (Denis et al., 2008; Zhang, Sun, Li, Liu and Su, 2006). Hence, incorporating HC in other bio-based films can be an easy strategy to take advantage of its properties and expand the use of this abundant raw material, often resulting in biodegradable (potentially even edible) packaging materials (Pei et al., 2013).

One of the most interesting biopolymer formulation strategies is adding nano-reinforcements, since they can alter the hydrophilic nature of polysaccharide/protein films and provide better mechanical properties (Condés et al., 2018). Nano-SiO2 is an amorphous white powder with a three-dimensional structure (Tian et al., 2020), forming non-toxic, stable, and durable multifunctional inorganic particles (Zhu et al., 2021). The distinctive characteristics of nano-SiO2, including small size, large specific surface area, high surface energy, unsaturated chemical bonds, and presence of hydroxyl group on the surface result in easy dispersion of nano-SiO2 throughout the polymeric matrix (Tabatabaei et al., 2018).

Our group recently investigated concentration and dispersion thresholds of nano-SiO2 in pure sodium alginate composite films. We showed that good dispersions are obtained at nano-SiO2 concentrations lower than 1.5%, but saw no impact on mechanical properties (Júnior, da Silva, Anjos, Vieira and Alves, 2021). Therefore, this research aims to develop sodium alginate/hydrolyzed collagen films with different concentrations of nano-SiO2 (2–10% w/w of SA) to improve the properties. An investigation of the impact of nano-reinforcement on the physicochemical and morphological properties of the films is provided.

Material and methods

Materials

Sodium alginate (SA) and glycerol (Dinâmica Química Contemporânea Ltda, Indaiatuba/SP, Brazil), hydrolyzed collagen (HC) (NaturalLife, São José do Rio Preto/SP, Brazil), nano-SiO2 (SiO2), Aerosil® 200, with average particle size of 12 nm, and BET surface area of 200 m2/g (Evonik/Degussa Ltda, Americana/SP, Brazil) (Guha, 2010), were used as received.

Preparation of the films

The SA/HC/SiO2 composite films were prepared based on our previous work that aimed to evaluate the interactions of the material with water (Marangoni Júnior, Silva, Vieira and Alves, 2021b). In summary, sodium alginate solutions (4% w/w) were prepared by dissolving the biopolymer in distilled water containing glycerol as a plasticizer (30 g/100 g of sodium alginate), hydrolyzed collagen (10 g/100 g of SA), and different added concentrations of SiO2 nanoparticles (0, 2, 6, 8 and 10 g/100 g of SA). Film-forming solutions were heated to 80 °C with stirring for 20 min. Subsequently, the final solutions were treated in an ultrasound bath for 15 min, to ensure homogeneity and elimination of bubbles. After this step, 50 g of film-forming solutions were poured into polystyrene Petri dishes (diameter 14 cm) and dried at 40 °C for 24 h in a forced air circulation oven. Finally, after removing the films from the plates, all the samples were stored in an air-conditioning chamber (Weiss Technik, Reiskirchen, Germany) at 25 °C and 75% relative humidity (RH), prior to the characterization processes. Table 1 describes the composition and nomenclature of the different film samples.

Table 1

SA/HC/SiO2 film-forming solutions compositions (%).

Materials	SA/HC	SA/HC/2%SiO2	SA/HC/6%SiO2	SA/HC/8%SiO2	SA/HC/10%SiO2
Sodium alginate (SA)	4.00	4.00	4.00	4.00	4.00
Hydrolyzed collagen (HC)	0.40	0.40	0.40	0.40	0.40
Glycerol	1.20	1.20	1.20	1.20	1.20
SiO2	–	0.08	0.24	0.32	0.40
Water	94.40	94.32	94.16	94.08	94.00

Characterization of films

Film morphology

Images of surface (magnification of 1000 × ) and cross section (magnification of 2500 × ) of the composite films were obtained using scanning electron microscopy (SEM) (Leo 440i, LEO Electron Microscopy/Oxford, Cambridge, England). The samples were previously fractured with liquid nitrogen, then fixed in metallic support with the aid of double-sided carbon tape and covered with gold in a sputter coater (SC7620, VG Microtech, Kent, United Kingdom). The visualization was performed with a voltage of 15 kV and a current of 50 pA.

Fourier-transform infrared spectroscopy (FT-IR)

FT-IR analysis were recorded using a Thermo Scientific spectrometer (Nicolet Continuum, Madison, USA). For all composite films, it was used the attenuated total reflectance mode, ATR (Germanium crystal), employing the software Smart Omni Sampler, at 4000-650 cm−1, with a resolution of 4 cm−1, scan: 32–64. The measurements of each sample were performed for the same sample mass and strictly maintaining the same analysis conditions.

Thermogravimetric analysis (TGA)

Thermal stability testing was performed using a Mettler Toledo Thermogravimetric Analyzer (TGA), model TGA/DSC1 (Schwerzenbach, Switzerland). Measurements were performed in film samples of 10 mg heated from 25 to 600 °C, in a nitrogen flow of 50 mL min−1, at a heating rate of 20 °C min−1.

X-ray diffraction (XRD)

X-ray diffraction was recorded on an X-ray analyzer (X'Pert-MPD, Philips, Almelo, Netherlands). The XRD measurements of composite films were operated with Cu Kα radiation (λ = 1.54056 Å) at a scan rate of 0.033333°/s (step = 0.04° and time per step = 1.2 s), with the accelerated voltage of 40 kV and the applied current of 40 mA, varying from 5 to 60°.

Film thickness

The thicknesses of the composite films were determined at five random points from five specimens per formulation sample using a system composed of a flat granite base and comparator clock (Mitutoyo Co., Kawasaki-Shi, Japan), with 0.1 μm resolution (ISO-4593, 1993).

Mechanical properties

The samples were cut with a 15 mm width in high precision equipment (RDS-100-C, ChemInstruments, OH, USA). Then, they were conditioned, for 48 h, at 25 ± 2 °C and 75 ± 5% RH. Tensile strength (MPa), elongation at break (%) and modulus of elasticity (MPa) were determined following a standard method ASTM-D882 (2018), using a universal testing machine (Instron, 5966-E2, Norwood, USA), with a 100 N load cell, speed of 12 mm min−1 and with initial grip separation of 50 mm. Each film formulation was tested at least 4 replicated and their average reported.

Statistical analysis

The results were statistically evaluated by analysis of variance (ANOVA). Comparisons among mean values were determined by Tukey's tests (p < 0.05). The results were expressed as the mean ± standard deviation.

Results and discussions

Film morphology

Fig. 1 (a) and (b) show the surface and cross-section of the control films. Although the films apparently presented a uniform and smooth surface, a more accurate evaluation in the cross section indicates the occurrence of numerous micrometric-sized pores that may indicate low compatibility between alginate and hydrolyzed collagen. A slight surface roughness (Fig. 1 (c)) appears with the addition of 2% nano-SiO2, but Fig. 1 (d) shows a relatively appropriate dispersion throughout the polymer matrix in the cross-section image. However, small domains of agglomerated particles appear even in this concentration. With the gradual increase in the proportion of nano-SiO2 added, surface and cross-section roughness increased, indicating poor dispersion and the formation of a composite material instead of a nanocomposite (Fig. 1 (e)–(j)). Despite the difficulty in obtaining a uniform morphology, these higher concentrations positively influenced the mechanical behavior, reported in the following topics of this work.

Fig. 1

Evaluation of the composite films morphology, in which (a), (c), (e), (g) and (i) refer to surface images with 1000 × magnification; and (b), (d), (f), (h) and (j) refer to the cross-sectional images, with 2500 × magnification. (a) and (b): SA/HC, (c) and (d): SA/HC/2%SiO2, (e) and (f): SA/HC/6%SiO2, (g) and (h): SA/HC/8%SiO2, and (i) and (j): SA/HC/10%SiO2.

FT-IR analysis

Fig. 2(a) shows that all films scanned showed the typical functional groups of sodium alginate and hydrolyzed collagen. In the control film (SA/HC), the wavenumber of 3342 cm−1 is associated to the –OH stretching. Characteristic C–H bending is noted at 2933 cm−1 (Pereira et al., 2011). At the wavenumbers of 1598 and 1413 cm−1 are the asymmetric and symmetric stretching vibrations of COO-, respectively (Santos et al., 2020), which are present in SA. The polypeptide backbone C–O stretching vibration of hydrolyzed collagen, and the formation of hydrogen bond between N–H stretch and C–O can also be found in the range of 1600–1700 cm−1 (Riaz et al., 2018), causing overlapping of signals. The sharp peak registered at 1030 cm−1 is assigned to the elongation of C–O groups (Pereira et al., 2011). The FT-IR spectra of the films remained unchanged despite the SiO2 load, except for the 1088 cm−1 peak, which is associated with the Si–O–Si bending vibrations (Mohammadi et al., 2019), that increased in intensity with the increment of SiO2 percentages in the films. The low concentrations of nano-SiO2 and the overlap of this characteristic bands with those of alginate justify the few changes to the spectrum (Júnior et al., 2021).

Fig. 2

FT-IR spectra (a), XRD pattern (b), TGA (c) and DTGA (d) of SA/HC/SiO2 composite films.

Thermogravimetric analysis (TGA)

Fig. 2 (c) and (d) show the TGA of sodium alginate-hydrolyzed collagen films added with nano-SiO2. Note that the degradation behavior of all composite films occurred in two main distinct thermal events, which is typical for sodium alginate-based films (Fig. 2 (c)) (Aloui et al., 2021; Ho et al., 2020; Liu et al., 2017). Table 2 shows the initial degradation (Tonset) and maximum decomposition (Tmax) temperatures for the mentioned thermal events. The first thermal event for all samples analyzed was registered around 80 °C, and corresponds to the evaporation of free water in the films.

Table 2

Initial degradation (Tonset) and maximum decomposition (Tmax) temperatures of SA/HC/SiO2 composite films.

Sample	First thermal event (°C)		Second thermal event (°C)
	Tonset	Tmax	Tonset	Tmax
SA/HC	56.18	83.92	217.51	230.07
SA/HC/2%SiO2	46.15	75.32	218.32	230.34
SA/HC/6%SiO2	51.72	79.59	221.18	231.93
SA/HC/8%SiO2	57.37	83.56	221.54	232.86
SA/HC/10%SiO2	55.03	79.61	222.48	233.06

The second thermal event is associated with the decomposition of the polymeric elements of the composites (Liu et al., 2017), the Tonset and Tmax (second thermal event) of the control film were 217.51 and 230.07 °C, respectively (Table 2). These findings are very similar to those reported for pure sodium alginate films since hydrolyzed collagen thermal degradation occurs in a range close to that of sodium alginate (Aloui et al., 2021; Sadeghi and Hosseinzadeh, 2013). The addition of nano-SiO2 on the sodium alginate-based films slightly increased their Tonset and Tmax compared with the sodium alginate-hydrolyzed collagen control. This thermal behavior was especially noted for the second event of the SA/HC/10%SiO2 film. Agreeing with the results presented in this work, the literature reports the general behavior of thermal stability enhancement of polymeric films due to SiO2 incorporation. Zhu et al. reported a noticeable increase in the Tmax of starch-based extrusion-blown composite films incorporated with nano-SiO2 (Zhu et al., 2021).

X-ray diffraction (XRD)

To investigate the crystallinity, Fig. 2 (b) shows the XRD spectra of the composite films based on sodium alginate and hydrolyzed collagen produced in this study. The diffraction peaks at 2θ for the composite films reveal semi-crystalline structures, predominantly amorphous. The spectra of the control sodium alginate-hydrolyzed collagen film registered a small peak at 2θ ≈ 14° and a broad diffraction peak at 2θ ≈ 22°, which indicates low crystallinity (Bhagyaraj and Krupa, 2020). All formulated composites showed a coinciding pattern. With the addition of nano-SiO2, the intensity of the crystalline peaks of the sodium alginate-based composites increased slightly. However, although it is not possible to make an accurate comparison on the diffractogram, the XRD patterns are of similar height, appearing to be mostly unchanged. Our results are different from those of Yang et al., who verified the addition of nano-SiO2 to pure alginate films increased the amorphization of the polymer (Yang et al., 2016).

Thickness and mechanical properties

Table 3 shows thickness and mechanical properties of the composite films. The control film (SA/HC) had the lowest thickness among all films (160.6 ± 40.2 μm). However, the incorporation of up to 8% of nano-SiO2 in the SA/HC films did not significantly influence the thickness results. On the other hand, note that the thickness of the SA/HC/10%SiO2 film increased significantly compared with the SA/HC control film, which is attributed to the higher concentration of solids in the polymer matrix.

Table 3

Thickness and mechanical properties of SA/HC/SiO2 composite films.

Sample	Thickness (μm)	Tensile Strength (MPa)	Elongation at Break (%)	Modulus of elasticity (MPa)
SA/HC	160.6 ± 40.2b	18.2 ± 3.1b	19.5 ± 5.3b	424.0 ± 150.1a
SA/HC/2%SiO2	178.9 ± 37.5ab	22.4 ± 1.0ab	35.3 ± 5.1a	502.5 ± 134.4a
SA/HC/6%SiO2	161.5 ± 35.1b	20.2 ± 2.9ab	30.2 ± 3.8a	469.8 ± 25.9a
SA/HC/8%SiO2	180.6 ± 36.2ab	21.8 ± 2.7ab	30.3 ± 4.1a	557.0 ± 39.5a
SA/HC/10%SiO2	195.0 ± 42.9a	25.4 ± 0.2a	35.8 ± 5.1a	453.8 ± 55.8a

The results are expressed as mean ± standard deviation.

a, b, c The means, followed by the same letter, in the column, do not differ at the 95% confidence level (p < 0.05).

The tensile tests of SA/HC/SiO2 composite films were carried out to evaluate the influence of SiO2 nanoparticles on the mechanical properties. The results of tensile strength (TS) and elongation at break (EB) of the SA/HC control film were 18.2 ± 3.1 MPa and 19.5 ± 5.3%, respectively (Table 3). The addition of nano-SiO2 in the SA/HC films increased TS values. However, although TS increased in the SA/HC/2%SiO2, SA/HC/6%SiO2 and SA/HC/8%SiO2 films, it was not significantly different from the control sample. Only the addition of 10% nano-SiO2 significantly increased the TS of the film compared with the control. For all concentrations of nano-SiO2, the EB presented significantly higher results compared with the film without nano-SiO2. Finally, the incorporation of nano-SiO2 did not significantly influence the modulus of elasticity (ME), that is, the material's stiffness remained unchanged.

The improvement in TS for SA/HC/10%SiO2 may be related to the strong interactions (hydrogen bonds) formed by the hydroxyl groups of the nano-SiO2 with the carboxylic groups of the film polymeric matrix (Voon et al., 2012), reinforcing the material at high filler concentrations. The nano-SiO2 concentrations below 10% had no significant influence in the TS, which corroborates the possible plasticizing effect of nano-SiO2, justifying the large increase in elongation at break (EB). The increase in EB with the increase in the concentration of nano-SiO2 has been attributed to a possible plasticizing role of SiO2 nanoparticles (Salarbashi et al., 2021). The dispersion of nano-SiO2 through the molecular matrix influence the interactions between polysaccharide-protein chain, and consequently improves the flexibility of the film (Hou et al., 2019). Furthermore, the agglomeration of filler particles may also have influenced this result, since such nano-SiO2 agglomerates may have acted as internal lubricants, reducing friction forces in the polymer matrix. These results are in agreement with those found for composite films of gelatin/k-carrageenan/SiO2 (Tabatabaei et al., 2018), and agar/sodium alginate/SiO2 (Hou et al., 2019).

Conclusions

This study investigated the effects of different nano-SiO2 loads as fillers in films based on sodium alginate and hydrolyzed collagen. The results showed SiO2 nanoparticles concentrations ranging from 2 to 10% can be easily incorporated into SA/HC blends by casting, with significant improvement in mechanical properties and thermal stability. Only the highest concentration of nano-SiO2 (10%) evaluated significantly improved the tensile strength of the films, from 18.2 MPa (control sample) to 25.4 MPa. However, all investigated nano-SiO2 proportions increased elongation at break, suggesting a plasticizing effect. The films were much more flexible and resistant with 10% nano-SiO2. Thermal stability improved slightly with the addition of nano-SiO2, with the second Tonset increase from 217.51 °C (control) to 222.48 °C). Therefore, the use of this ratio could represent an easy and effective strategy for possible future applications of these materials as food packaging.

CRediT authorship contribution statement

Luís Marangoni Júnior: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Writing – original draft, Writing – review & editing. Plínio Ribeiro Rodrigues: Data curation, Investigation, Writing – original draft. Renan Garcia da Silva: Formal analysis, Investigation, Writing – review & editing. Roniérik Pioli Vieira: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing. Rosa Maria Vercelino Alves: Formal analysis, Funding acquisition, Investigation, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.