Film production with flaxseed mucilage and polyvinyl alcohol mixtures and evaluation of their properties

Abstract

Flaxseed mucilage was extracted with distilled water, dried and used for film production with polyvinyl alcohol (PVA) (ratio 1:1) of different hydrolysis degrees (88.0 and 98.3%). The properties of the films were evaluated by determining the thickness, tensile measurements, moisture content, water vapor permeability, apparent opacity, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) analysis and thermogravimetric analysis. Flaxseed mucilage, when mixed with PVA, produces less resistant, less rigid, more-flexible films, has a higher thermal stability, and does not change the water vapor barrier properties compared with pure mucilage films. SEM revealed that films with mucilage and PVA mixtures formed a compact and homogeneous structure, corroborating the FTIR spectra that indicated a chemical interaction between these two biopolymers. In general, the degree of PVA hydrolysis did not influence the properties of the films when mixed with flaxseed mucilage extract. Therefore, films obtained from mixtures of flaxseed mucilage and PVA can be an interesting and advantageous alternative for producing bio-based packaging.

Introduction

The development of new biomaterials from agriculture by-products and wastes is an ineluctable global trend and has been one of the key challenges of the new millennium to promote environmental protection and the application of green products (Gheribi et al. 2018). In this context, plant gums and mucilages have attracted much attention as natural hydrocolloids in the food industry and as raw materials for the production of films, microparticles, and nanofibers because of their non-toxic, non-irritating nature, low cost, and other advantages (Beigomi et al. 2018; Raj et al. 2020).

Mucilage is defined as a gelatinous substance or hydrocolloid type that has strong interactions between polysaccharides and proteins (Lai and Liang 2012; Zeng and Lai 2016), and, when mixed with water, it forms a viscous solution. Usually, mucilage is part of the coating of some seeds (chia, linseed, yellow mustard, etc.) and, because of its highly hydrophilic nature, it can be easily extracted by soaking intact seeds in water. Mucilage extraction yield varies from 3.0% to 35.0% and depends on several parameters, such as botanical seed origin, seed and solvent ratio, medium pH, temperature and extraction process time (Soukoulis et al. 2018).

Flaxseed (Linum usitatissimum L.) contains approximately 3.0–9.0% mucilage, whose polysaccharide is heterogeneous and composed of galacturonic acid (21.0–36.0%), xylose (19.0–38.0%), rhamnose (11.0–16.0%), galactose (12.0–16.0%), arabinose (8.0–13.0%) and glucose (4.0–6.0%). The polysaccharide also contains neutral and acidic fractions, and the neutral fraction contains arabinoxylans with β-D-(1,4)-xylan chains with a molecular mass of 1200 kDa, while the acidic fraction contains two subfractions with molecular mass of 650 and 17 kDa (Fedeniuk and Biliaderis 1994; Oomah et al. 1995; Fabre et al. 2015).

In humans, flaxseed mucilage is valuable when consumed, because it can reduce diabetic glucose and cholesterol content and increase fecal fat excretion (Basiri et al. 2018). Industrially, prior extraction of flaxseed mucilage is performed positively to improve oil production and quality, and, consequently, decrease the fiber level in the bran that is intended for animal nutrition (Ziolkovska 2012). For non-ruminant animals, mucilage viscosity enables gum-like mass to form in the gut that can impair digestive enzyme activity and dietary nutrients (Francis et al. 2001). Removal of flaxseed mucilage may provide better flour digestibility. In this way, flaxseed mucilage is considered as a by-product and reinforces the importance of its reuse in different applications.

Mucilage extraction from different sources has been widely described, such as: chia, Dioscorea opposite, Dracocephalum moldavica seed, Alyssum homolocarpum seed and Cassia obtusifolia seed and these mucilages have been applied for coatings or film production. However, pure mucilage film does not have adequate mechanical properties when used for food packaging, and to improve this property, mucilage has been mixed with other biopolymers. Flaxseed mucilage mixtures with polyvinyl alcohol (PVA) were used for nanofibers by electrospinning (Hadad and Goli 2018), and cactus mucilage and PVA blends were employed in the production of the edible films (Gheribi et al. 2019). However, studies on the use of the blend of flaxseed mucilage and PVA with different hydrolysis degree for film production have not yet been reported.

PVA has been highlighted because of their water solubility, biocompatibility, and biodegradability (Limpan et al. 2012; Gaikwad et al. 2015). PVA is obtained by partial or complete hydrolysis of polyvinyl acetate to remove acetate groups, and their properties depend on its molar mass and degree of hydrolysis (DH). The partially hydrolyzed grades contain residual acetate groups, and considering the degree of hydrolysis, the PVA are classified as partially hydrolyzed (above 88%) or highly hydrolyzed (98–99%) (Tang and Alavi 2011). Carvalho et al. (2019) studied the effect of the different DH and degrees of polymerization (DP) of PVA on the properties of the films, and concluded that DH was most important for PVA films properties than DP effect.

In this sense, the objective of this work was to produce films with brown flaxseed mucilage mixed with PVA of different hydrolysis degrees and to evaluate the properties of the films. The films were characterized in terms of thickness, tensile properties, moisture content, water vapor permeability (WVP), apparent opacity, morphology, and thermal properties.

Materials and methods

Materials

The brown flaxseed was purchased from a local market in Londrina, Paraná, Brazil. Polyvinyl alcohol with hydrolysis degrees of 98.30% (Selvol 107) and 88.04% (Selvol 540) for film production was purchased from Selvol (Sekisui Chemical Co. Ltd, Japan) and glycerol, as a plasticizer, was acquired from Dinâmica, Brazil.

Flaxseed mucilage extraction

Flaxseed mucilage was extracted according to the procedure described by Paynel et al. (2013) with modifications. Flaxseed grains were mixed with distilled water (22.5 g/150 mL) and mechanically shaken (Solab, SL222 model, Brazil) at 200 rpm for 2 h at 25 °C. This procedure was repeated three times. After stirring, the mixture was sieved (Tyler screen, 120 mesh) to retain the flaxseed grains. The filtrates were dried in an air circulation oven (Quimis, Brazil) for 24 h at 50 °C. The dried flaxseed mucilage was ground and stored in hermetically sealed hermetic bottles.

Film production with flaxseed mucilage and PVA

The films were produced by a casting technique using a filmogenic solution containing 1.5% (w/v) solids (1.2% biopolymer and 0.3% glycerol) and water as solvent. The solids content of the filmogenic solution was defined by preliminary test, considering the ability to form continuous and homogeneous films that are easily removed from the Teflon-coated plate. The tested concentration of the solids ranged from 1.0 to 2.0% (w/v), and glycerol concentration was fixed. The following mucilage formulations were prepared: (1) pure flaxseed mucilage (MUC), (2) pure PVA 107 (PVA₁₀₇), (3) pure PVA 540 (PVA₅₄₀), (4) mixture (1:1) of flaxseed mucilage and PVA 107 (MPVA₁₀₇), and (5) mixture (1:1) of flaxseed mucilage and PVA 540 (MPVA₅₄₀). All components were weighted, solubilized in distilled water, and heated with magnetic stirring until the temperature reached 90 °C. The obtained filmogenic solution was placed in a Teflon-coated plate (20 cm in diameter) and dried in a forced convection oven (Cienlab, Brazil) for 24 h at 40 °C. The films obtained were placed in desiccators containing saturated magnesium nitrate solution (53% RH) and kept until use for characterization.

Flaxseed mucilage and PVA films characterization

Scanning electron microscopy (SEM)

The morphology of the films was evaluated by SEM. The films were previously dried for 14 days in a desiccator containing silica gel and, later, fractured in liquid nitrogen and fixed on stubs with carbon ribbons. The samples were covered with gold in a sputter coater (BAL-TEC, SCD-050, Balzers, Liechtenstein) for visualization of the surface and fracture area with the scanning electron microscope (Philips, FEI Quanta 200, Japan) at an acceleration power of 20 kV. Magnifications of 1600 times for the fracture area and 800 times for the surface were used (de Souza et al. 2020).

Moisture

The triplicate (2 × 2 cm) films were weighed and dried in the oven for 24 h at 105 °C. Subsequently, they were weighed again and the moisture content was calculated based on the amount of water removed on drying (de Moraes Crizel et al. 2018).

Water vapor permeability (WVP)

The WVP was determined in triplicate using the gravimetric method and according to the American Society for Testing and Materials (ASTM E-96-00 2000). The films were previously stored at 25 °C and 53% relative humidity (RH) for 48 h and then fixed in the 60 mm diameter aluminum cylindrical cells. Calcium chloride (0% RH) was added to the cells, and the device was stored at 25 °C in a desiccator containing a saturated sodium chloride solution (75% RH). Capsule weight gain was monitored every 6 h and up to 10 measurements. WVP was calculated based on Eq. (1) and expressed in g m⁻¹ h⁻¹ Pa⁻¹.

$$WVP=\frac{w}{t}\times \frac{\delta }{A\times \mathrm{\Delta }P}$$ 1

where w/t is the angular coefficient determined graphically by plotting the weight gain (w) as a function of time (t) (g/h), δ is the mean film thickness (m), A is the film permeation area (m²), and ΔP is the difference in water vapor pressure through the film for pure water at 25 °C (Pa).

Opacity

The opacity of the films was determined in triplicate in a UV–visible spectrophotometer (Biochrom, Libra model, Cambridge, England) according to De Moraes Crizel et al. (2018). The films were fixed to the inner wall of the quartz cuvette (10 mm optical path), and the transmittance reading was taken at 600 nm. The opacity of the films was calculated by Eq. (2).

$$Opacity=\frac{-log{T}_{600}}{x}$$ 2

where T₆₀₀ is the fractional transmittance at 600 nm, and x is the equivalent film thickness (mm).

Thickness

The thickness of the films was measured at 10 different points and using a digital micrometer (Starrett, Brazil), with an accuracy of ± 0.001 mm, at 10 different points.

Tensile measurements

The tensile measurements of the films were performed using a texturometer (Stable Micro Systems, TA-TX2, England). The maximum tensile strength (σ), elongation at break (ε), and modulus of elasticity or Young’s modulus (YM) were determined according to ASTM D882-00 (2002). For each measurement, 10 samples with a size of 5 × 2 cm were prepared. The following test conditions were used: crosshead speed set at 0.8 mm/s and an initial jaw distance of 30 mm.

Fourier transform infrared spectroscopy (FTIR)

The FTIR spectra were obtained in a FTIR spectrometer (Perkin Elmer, SpectrumTwo, USA) with an attenuated total reflectance (ATR) accessory. The FTIR-ATR spectra were recorded in the wave number range of 500–4000 cm⁻¹ with a resolution of 2 cm⁻¹ and using 32 accumulated scans. The samples were conditioned in a desiccator containing silica for 14 days before the analysis to remove the residual moisture of the films, and minimize the interference in the spectrum (de Souza et al. 2020).

Thermogravimetric analysis (TGA)

The thermal stability of the films was evaluated by TGA (Shimadzu, TGA-50, Japan) according to Gheribi et al. (2019) with minor modification. Approximately 10 mg of the sample was scanned from 25 to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere (20 mL/min).

Statistical analysis

The means of the results were evaluated using the analysis of variance, and the means of the treatments were compared using the Tukey test at a level of 5% significance (P < 0.05) using the Statistica 10 software (Stat Soft, Tulsa, OK, USA).

Results and discussion

Flaxseed mucilage extraction and yield

The color of the dry flaxseed mucilage was light brown. The flaxseed mucilage extraction yield was 6.3–8.5% and was higher than the extract yield obtained with flaxseed by Paynel et al. (2013), which was 5.3–6.2%. However, the yield of mucilage extraction was lower than quince seeds, basil seeds mucilages and Alyssum homolocarpum seed reported by Jouki et al. (2013a, b), Khazaei et al. (2014) and Mohammadi Nafchi et al. (2017) who obtained yields of 13.9%, 14% and 20%, respectively. According to Gheribi et al.(2018), because of the high availability of raw materials, a simple and safe extraction method using non-toxic solvents and obtaining an acceptable yield for mucilage extraction can be profitable for industries.

Characterization of films produced with flaxseed mucilage and PVA

Film appearance and morphology

The color of the film obtained with pure flaxseed mucilage was light brown and opaque and different from the transparent film obtained with pure PVA. All films produced were easily removed from the Teflon-coated plate and had homogeneous and easy-to-handle characteristics, and no apparent glycerol migration.

The use of SEM to examine the structural characteristics of films can provide a better understanding of the relationships between water vapor transmission, mechanical properties, and optical properties (Jouki et al. 2013a, b). Figure 1 shows SEM images of the surfaces and fractures of films produced with flaxseed mucilage and their mixture with pure PVA. It was observed (Fig. 1a–e) that all films presented a smooth surface without cracks or micropores. Fracture images of the pure mucilage and PVA films (Fig. 1f, g, i) were compact and uniform without noticeable cracks, breaks, or openings on the surfaces similar as observed by Jouki et al. (2013a) in cress seed gum film. The mucilage and PVA blended films showed slight roughness but remained intact suggesting that there was no phase separation between the polymers and glycerol. The integrity of the structure of films made with flaxseed mucilage and mixing with pure PVA can be explained by the fact that flaxseed mucilage contains large amounts of xylose that can interact with the hydroxyls present in the PVA structure (Wang et al. 2014).

Fig. 1.

Surface (left, 1600 times magnification) and fracture (right, 800 times magnification) SEM images of flaxseed mucilage and PVA films: MUC (a, f), PVA₁₀₇ (b, g), MPVA₁₀₇ (c, h), PVA₅₄₀ (d, i), and MPVA₅₄₀ (e, j)

Water vapor permeability, moisture and opacity

The WVP (Table 1) of the films showed no significant difference (P < 0.05) and is probably caused by the hydrophilic characteristic of the mucilage and PVA that show a good interaction between the chains. Similar WVP values were observed in films made with basil, balangu, Dracocephalum moldavica seed and Alyssum homolocarpum seed mucilages (Jouki et al. 2013b; Mohammadi Nafchi et al. 2017; Beigomi et al. 2018; Sadeghi-Varkani et al. 2018).

Table 1.

Water vapor permeability, moisture content, and opacity of films produced with flaxseed mucilage, pure PVA, and mixtures

Film	WVP (× 107) (g m−1 Pa h)	Moisture (%)	Opacity (T600/mm)
MUC	1.57ª ± 0.15	31.17ab ± 1.19	3.57b ± 0.23
PVA107	3.04ª ± 1.30	32.72bc ± 2.15	1.24a ± 0.10
MPVA107	1.44ª ± 0.93	35.28c ± 1.06	3.31b ± 0.17
PVA540	1.60ª ± 0.40	27.61a ± 1.47	1.28a ± 0.10
MPVA540	1.49ª ± 0.36	33.54bc ± 1.21	3.86b ± 0.09

WVP, water vapor permeability; MUC, pure flaxseed mucilage film; PVA₁₀₇, pure PVA₁₀₇ film (DH = 98.3%); MPVA₁₀₇, mucilage and PVA₁₀₇ blended film; PVA₅₄₀, pure PVA₅₄₀ film (DH = 88.0%); MPVA₅₄₀, mucilage and PVA₅₄₀ blended film

Means in the same column followed by different letters showed a significant difference (P < 0.05) according to Tukey’s test

The results obtained by Gheribi et al. (2019) were different from the results obtained in this work. They observed that blending cactus mucilage and PVA led to a significant decrease in WVP by 25% and 18% at mucilage:PVA 90:10 and 80:20 ratios, respectively. According to those authors, this decrease in WVP may result from the formation of hydrogen interactions between PVA and cactus mucilage, reducing the availability of the hydrophilic groups and leading to a decrease in water vapor affinity.

The moisture content of the films was similar, except for the PVA₅₄₀ film which had lower content (Table 1), possibly because of the lower degree of hydrolysis (88.04%). The moisture content was higher than that of the balangu seed mucilage film (Sadeghi-Varkani et al. 2018), Dracocephalum moldavica seed mucilage film (Beigomi et al. 2018) and Opuntia ficus-indica mucilage film (Gheribi et al. 2018), whose differences may be to the result of the chemical composition, molecular weight of each mucilage source, and glycerol concentration in the film formulation.

Pure PVA films were transparent as indicated by their low opacity when compared with pure mucilage films (Table 1). Considering the degree of hydrolysis PVA, no significant difference was observed. In addition, mixing mucilage with PVA increased opacity, and this change in transparency was associated with the presence of fiber, pigments and phenolic compounds in the mucilage. Comparing these results, a similar opacity (3.38–4.49) was observed in glycerol plasticized chia mucilage films (Dick et al. 2015) and lower opacity (0.22–1.10) in the film of gelatin made with papaya peel microparticles (de Moraes Crizel et al. 2018). Moreover, compared with commercial films used for packaging purposes, it was observed that the opacities of MPVA₁₀₇ and MPVA₅₄₀ films were closer to that of low-density polyethylene film (4.26).

Mechanical properties and thickness

The mechanical properties and thickness of the films are presented in Table 2. The film thickness ranged from 35.5 to 47.05 μm, and there was a significant difference between the formulations, whose films made with PVA₁₀₇ and MPVA₅₄₀ were thicker. This may be associated with slight inclinations that occurred during drying and that possibly changed the final film thickness. However, the thickness of the films produced in this work was similar and close to that of the films obtained with chia seed mucilage (Dick et al. 2015), quince seed (Jouki et al. 2013b), basil seed (Khazaei et al. 2014), balangu seed (Sadeghi-Varkani et al. 2018), and flaxseed mucilage and cellulose nanocrystals (Prado et al. 2018).

Table 2.

Mechanical properties and thickness of films produced with flaxseed mucilage, PVA, and mixtures

Films	Thickness (µm)	σ (MPa)	ε (%)	YM (MPa)
MUC	36.9a ± 0.9	14.8b ± 2.3	19.7d ± 3.8	279.0d ± 38.4
PVA107	43.6b ± 2.8	13.1ab ± 2.9	224.1b ± 48.8	65.2b ± 10.7
MPVA107	35.6a ± 3.0	11.5ab ± 1.5	43.0c ± 7.7	97.1c ± 15.6
PVA540	35.5a ± 0.9	19.7c ± 4.5	416.2a ± 65.1	14.4a ± 2.1
MPVA540	47.0b ± 4.7	10.3a ± 0.7	46.9c ± 3.3	74.9bc ± 9.3

σ, tensile strength; ε, elongation at break; YM, Young’s modulus; MUC, pure flaxseed mucilage film; PVA₁₀₇, pure PVA₁₀₇ film (DH = 98.3%); MPVA₁₀₇, mucilage and PVA₁₀₇ blended film; PVA₅₄₀, pure PVA₅₄₀ film (DH = 88.0%); MPVA₅₄₀, mucilage and PVA₅₄₀ blended film

Means in the same column followed by different letters showed significant difference (P < 0.05) according to Tukey’s test

The mechanical properties of food packaging are relevant and of great interest to the packaging industry, as they reflect the ability to protect the material from food integrity during storage and marketing. For this reason, the strength and flexibility of packaging are necessary to maintain this integrity (Beigomi et al. 2018; Gheribi et al. 2018). Film produced with pure PVA₅₄₀ had a higher σ value (Table 2), and it is therefore more resistant and flexible when compared with film produced with flaxseed mucilage extract and PVA₁₀₇. The highest σ and ε values could be related to their larger chain size and higher viscosity (49.4 cP), resulting in more resistant, flexible and deformable materials (Carvalho et al. 2019). In addition, the mechanical properties of the pure PVA films were influenced by DH. PVA with high DH results in greater crystallinity due to increased free hydroxyls able to interact between them through hydrogen bonding, resulting in more organized regions (crystal), resulting in materials with higher Young’s modulus (Tang and Alavi 2011; Carvalho et al. 2019), as observed for PVA₁₀₇ film.

Mixing mucilage with PVA significantly influenced the mechanical properties, providing less rigid films by reducing YM and more flexibility with increasing elongation at break (Table 2). For example, an approximate increase of more than 200% in elongation at break and a 300% decrease in YM were obtained by mixing flaxseed mucilage with pure PVA. In the work of Gheribi et al. (2019), however, in the films based on cactus mucilage and PVA, increasing PVA content beyond 30% led to a significant decrease in σ, and only the composite at the ratio of 80:20 (mucilage:PVA) showed a significant increase in percentage ε. The degree of hydrolysis and viscosity of PVAs (MPVA₁₀₇ and MPVA₅₄₀) showed that these mechanical properties were similar and did not interfere when mixed with the mucilage in the proportion used.

The use of mucilage has produced brittle and fragile films and according to Paynel et al. (2013), this material would therefore not be suitable for film production. This fact was also observed in the present study, where the values of ε (19.7%) and YM (279.0 MPa) were low (Table 2). In comparison with other films prepared with mucilage extracts by Espino-Díaz et al. (2010) and Dick et al. (2015), it was observed (Table 2) that the mixture of flaxseed mucilage extract with PVA₁₀₇ or PVA₄₅₀ significantly improved the ε and YM characteristics of the films. Glycerol and mucilage concentration in the filmogenic solution of balangu seed mucilage films also influenced mechanical properties (Sadeghi-Varkani et al. 2018). Increasing the content of cellulose nanocrystals also improved the mechanical properties of linseed mucilage films (Prado et al. 2018).

FTIR

FTIR analysis was performed to determine the functional groups of films made with flaxseed mucilage and to elucidate the interactions between mucilage and PVA. Figure 2 illustrates the FTIR spectra of flaxseed mucilage films, PVAs and their respective mixtures. In all spectra (Fig. 2), a wide range of absorbance and characteristic of the hydroxyl (O–H) stretch in the region of approximately 3300 cm⁻¹ was observed. For the pure mucilage film spectrum, peaks at 2932 cm⁻¹ are related to the C–H bonds of the CH₂–CH₃ groups of aliphatic chains, at 1604 cm⁻¹ to the vibration I of the amide, at 1411 cm⁻¹ to C–OH uronic acid, and at 1030 cm⁻¹ to the C–O–C or C–OH bonds of pyranose (Hadad and Goli 2018; Prado et al. 2018).

Fig. 2.

FTIR spectra of flaxseed mucilage and PVA composite films: MUC, pure flaxseed mucilage film, PVA₁₀₇, pure PVA₁₀₇ film; MPVA₁₀₇, mucilage and PVA₁₀₇ blended film; PVA₅₄₀, pure PVA₅₄₀ film; MPVA₅₄₀, mucilage and PVA₅₄₀ blended film

In the spectra of the films (Fig. 2) produced with pure PVA₁₀₇ and PVA₅₄₀, a band in the range of approximately 820 cm⁻¹ could be attributed to the C–C vibration; bands located in the 1040 to 1260 cm⁻¹ region may be of C–O vibration, bands near 1420 cm⁻¹ may be from C–OH vibration, and bands at 1330 and 1375 cm⁻¹ may be related to CH₂ bending vibration (Hadad and Goli 2018). The band at 1715 cm⁻¹ (Fig. 2) can be attributed to the stretching of the carbonyl groups present in the PVA acetate groups only in PVA₅₄₀, because it presented a lower degree of hydrolysis and had residual acetate groups.

A peak at 1603 cm⁻¹ was observed only in pure mucilage film and was linked to COOH groups (Guadarrama-Lezama et al. 2018). The appearance of this peak in MPVA₁₀₇ and MPVA₅₄₀ films indicates the formation of glyosidic bonds in blends networks (Monjazeb Marvdashti et al. 2017; Gheribi et al. 2019). Furthermore, a reduction in the OH band was observed in the MPVA₅₄₀ film and possibly a hydrogen bond interaction between PVA and mucilage. These facts suggest that there was a weak interaction between mucilage and PVA and these results are consistent with the uniform and compact morphology observed in SEM images (Fig. 1). In addition, significant improvement in the mechanical properties was obtained when mucilage was blended with PVA, independent of their DH.

Thermal properties

TGA was performed to verify the thermal stability of mucilage and PVA composite films and the TGA curves are shown in Fig. 3. According to Gheribi et al. (2018), during thermal degradation of polysaccharides and molecules of the polyol type, two phenomena usually occur: the stripping of chains produced by the removal of water molecules (dehydration) followed by splitting and decomposition of chains. Thus, it was observed (Table 3) that the TGA of the films occurred in three steps of degradation, and the degradation temperature of the all sample showed a significant (P < 0.05) difference for each stage. The first step started at less than 100 °C. It was verified that water evaporation occurred, and the weight loss ranged from 4.0 to 6.2%. In the second step, there was thermal degradation of mucilage and PVA, with an increase of the main chain and degradation temperature of MPVA₁₀₇ and MPVA₅₄₀ films, compared with pure mucilage film. Finally, in the third step of the TGA analysis, a continuous decomposition of film with mucilage and pure PVA occurred, and again the highest degradation temperature occurred with MPVA₁₀₇ and MPVA₅₄₀ films. Higher residue levels were observed in mucilage films, and these levels may be related to the presence of minerals in the mucilage that precipitate as salts (Prado et al. 2018).

Fig. 3.

TGA curves of flaxseed mucilage and PVA composite films. MUC, pure flaxseed mucilage film, PVA₁₀₇, pure PVA₁₀₇ film; MPVA₁₀₇, mucilage and PVA 107 blended film; PVA₅₄₀, pure PVA₅₄₀ film; MPVA₅₄₀, mucilage and PVA₅₄₀ blended film

Table 3.

TGA data of flaxseed mucilage and PVA composite films

Sample	Decomposition stage	Temperature peak (°C)a*	Weight loss (%)	Residue (%)
MUC	1	62	6.2	6.5
	2	258	35
	3	654	87
PVA107	1	68	7	0
	2	299	50
	3	552	87
MPVA107	1	56	4	3
	2	288	40
	3	714	90
PVA540	1	72	4.4	0
	2	379	50
	3	557	96
MPVA540	1	78	4.4	4
	2	331	55
	3	680	88

MUC, pure flaxseed mucilage film; PVA₁₀₇, pure PVA₁₀₇ film (DH = 98.3%); MPVA₁₀₇, mucilage and PVA₁₀₇ blended film; PVA₅₄₀, pure PVA₅₄₀ film (DH = 88.0%); MPVA₅₄₀, mucilage and PVA₅₄₀ blended film

*All temperature peak showed significant difference (P < 0.05) according to Tukey’s test for each decomposition stage

^aTemperature peak correspond to the values of the derivative thermograms obtained by the TGA curve

Considering the derivative (dm/dT) of the mass loss of the first curve in relation to temperature, it can be concluded that the addition of PVA in the mucilage improved the thermal properties and increased the heating stability of the films. This positive aspect was probably to the result of a good interaction between flaxseed mucilage and PVA that can also be observed (Fig. 1) in SEM images and FTIR spectra (Fig. 2). Hadad and Goli (2018) also observed these thermal properties in flaxseed mucilage and PVA nanofiber and Gheribi et al. (2019) in the cactus mucilage and PVA films. According to Dick et al. (2015), Gheribi et al. (2018), and Prado et al. (2018), the thermal stability of other investigated and mucilage films was influenced by the concentration and type of plasticizer.

Conclusion

Flaxseed mucilage is an interesting raw material for film production. When blended with PVA, produces less resistant, less rigid, more-flexible films with better thermal stability, and it did not change water vapor barrier properties compared with pure mucilage films and pure PVA films. Morphological analyze revealed that films with mucilage and PVA mixtures formed a compact and homogeneous structure, corroborating the FTIR spectra that indicated a chemical interaction between these two biopolymers. In general, the degree of PVA hydrolysis did not influence the properties of the films when mixed with flaxseed mucilage. These films obtained with mixtures of flaxseed and PVA mucilage extracts can be an interesting and advantageous alternative for producing bio-based packaging for different application. Also, it is relevant to perform in vitro toxicity assay aimed application as food packaging.