1. Introduction
Depending on the type of food different barrier properties can be required. For example, a packaging film that enables retailers to market fresh foods with extended shelf-life without employing any major processing, preservatives or chemical additives would prevent a significant quantity of fresh products from spoiling [
1,
2].
Recently, due to the interest in environmentally friendly polymer composites, natural fibers have been used, enhancing mechanical properties and biodegradability of the polymer matrix. Cellulose, a natural fiber, is widely used as reinforcement for polymers due to its availability, great mechanical properties, low cost and biodegradability combined with unique characteristics such as low density, light weight, high specific area (that can interact more strongly with the matrix) and above all its, makes cellulose a potential eco-friendly additive [
3]. Besides these properties cellulose could also modify the barrier properties of the polymer matrix. However, their hydrophilic character causes a poor compatibility with hydrophobic matrices, such polyethylene [
4]. This problem can be easily solved since the presence of repetitive hydroxyl groups on the cellulose surface makes it suitable for several chemical modifications These changes are essential to increase the compatibility with a polymer matrix, which is an important requirement to achieve good mechanical properties [
3,
5,
6,
7]. PE filled with cellulose, can have a better compatibility through surface modification of cellulose by the addition of a coupling agent to the matrix. Intensive research on modification strategies of cellulose surface to improve the compatibilization degree between cellulose fibers and polymeric matrices has been performed [
7,
8,
9,
10]. Of all the coupling agents used, silane derivatives seem to be an excellent choice due to the diversity of functional groups and their commercial availability on a large scale. Moreover, the diversity of functional groups in silane coupling materials is a useful strategy that enhances the ability of covalent linkage between cellulose fibers and polymer matrices when two functional groups are presented. Usually, the general structure of silane coupling agents available are (RO)3-Si-R’-X, where alkoxy groups (RO) are capable to react with cellulose surface, rich in OH groups, intermediated by hydrolysis processes and the other group (R’-X) where R’ is an alkyl chain and X is an organofunctional group that can be used to react with polymer matrix by covalent linkage [
11,
12]. From all variety of silane coupling agents, 3-aminopropyltriethoxysilane (APTES), the agent used in this study, is frequently used in silane modification due to its high reactivity, simplistic structure and low cost resulting in a cellulose-silica composite [
13]. Nevertheless, the presence of an amine group on the APTES offers a good compatibilization by covalent linkage, via amine, with PE grafted with maleic anhydride. The modified PE (PE E226) used in this study is FUSABOND® E226 resin that is described by FDA as a material that can be used for packaging, transporting, or holding food, subject to the limitations and requirements therein.
Therefore, this work aims at developing a material that incorporates cellulose with good mechanical properties and barrier properties able to exchange gases for packaging application. First cellulose was modified with APTES and then the materials were obtained by reactive blending in a mixer. The structure, morphology and physical properties of the materials were characterized. Studies of surface hydrophobicity were also performed by contact angles measurement and permeability to water vapor and oxygen were carried out in to evaluate the barrier properties.
3. Results and Discussion
The reaction between MC and APTES was performed in solvent, LiCl and DMAc as a mixture of solvent is an important factor once the formed complex ([(DMAc)
2–Li]Cl) penetrate into the cellulose structure, acting as spacers in the MC packing chains, which facilitates the chemical modification [
18]. Therefore, the reaction with silane coupling agent, APTES, occurred successfully and could be confirmed by complementary analysis of ATR-FTIR
Figure 1 and EDX
Figure 2. The modification of MC with APTES was detected by the appearance of an additional peak at 1562 cm
-1, corresponding to the bending vibration of -NH
2 groups, indicating that they were successfully introduced onto the MC surface. The adsorption peak of Si–O–Si vibration, characteristic of the self-condensation occurred between the silane reaction with cellulose hydroxyl groups, at around 1000 - 1100 cm
-1 is overlapped with the C-O-C vibration bands of cellulose around 950-1200 cm
-1. Moreover, the band corresponding to the Si-O-Cellulose, around 1150 cm
-1, could not be observed due to the presence of the large and intense C-O-C vibration bands of cellulose [
13,
19,
20].
After the incorporation of modified cellulose in the polymer, a reaction occurs trough amide linkage between the amine group of cellulose-silane and maleic anhydride grafted onto PE, as illustrated on
Scheme 1.
According to ATR-FTIR results (
Figure 2a) both samples exhibited absorption peaks that are characteristic of cellulose, namely the peaks at 3318, 2859, 1428, 1315 and 1025 cm
-1, which are associated to the vibration of –OH, C–H, –CH
2 and C–O, respectively.
Films of the prepared materials and PE-g-MA were analyzed by FTIR in transmittance mode and are depicted in
Figure 1b. The covalent linkage between PE-g-MA and modified cellulose was confirmed by the disappearance of the bending vibration -NH2 at 1562 cm
-1, which demonstrate that the amino groups on the cellulose surfaces were converted to –NH–band at 3320 cm
-1 and amide band around 1613 cm
-1. Moreover, the bands intensity increases with increasing content of modified cellulose, and there is also a growth of the broad band related to the -OH groups of cellulose and to the consequent succinic ring opening.
The presence of silicon (Si) and oxygen (O) in the polymer matrix assessed by EDX are present in
Figure 2.
Figure 2a corresponds to the modified MC, where the O and Si peaks have a significant intensity. As expected, a lower intensity can be observed in
Figure 2b, which corresponds to the samples containing 5 wt.% of MC-APTES, respectively. Even though the peaks intensity increased with the amount of MC-APTES incorporated, they are almost undetected for the samples containing 1 wt.%. This can be due to the heterogeneous dispersion of the MC-APTES in the matrix that made the evaluation more complicated since only points are selected in this kind of analysis.
SEM micrographs of PE-
g-MA and PE-
g-MA containing 5 wt.% MC-APTES are represented in
Figure 3. Since the micrographs of the materials with 1 and 3 wt.% of MC-APTES are very similar to the one with 5% MC-APTES, only the latter is presented. The surface and cross-section of PE-
g-MA film,
Figure 3a, revealed smooth and homogeneous surfaces, whereas the film that incorporates MC-APTES exhibits a rough surface,
Figure 3b. Moreover, in the cross-section of the same samples
Figure 3b it is possible to noticed that MC-APTES located along the sample and at the surface. As expected from the chemical results a good adhesion between the modified MC and polymer was achieved.
The effect of MC functionalization with APTES on its thermal properties and incorporation in the PE matrix was evaluated (
Figure 4 and
Figure 5, respectively).
Figure 4a depicts the thermal decomposition of MC and modified MC, where it is possible to observe an initial weight loss (~5%) around T = 80 ºC, for MC, probably due to the vaporization of adsorbed water. The modification of MC surface with APTES increases both initial thermal degradation and temperature at the maximum degradation rate, with a difference around 10º C between MC (250.6 ºC) and MC-APTES (361.2 ºC), as it can be seen in
Figure 4b. This increase in the thermal stability of MC-APTES may be assigned to the good interaction amongst the APTES and MC and their consequent crosslinking reactions occurred during the functionalization. Moreover, the results also demonstrate that, for T= 500 ºC, MC-APTES have a higher residual mass value than unmodified MC, 26.9 and 14.6 %, respectively. This result can be associated the presence of siloxy moieties on MC-APTES product that remain as a residue. These results are in agreement with the results reported by H. Khanjanzadeh
et al. [
13].
TGA results of the prepared materials,
Figure 5, reveal that despite an earlier decomposition temperature between 320-343 ºC can be noticed, the thermal stability is slightly enhanced since the degradation peak of PE matrix shifts to higher temperatures. Whereas the curve for PE-
g-MA presented only one thermal degradation stage with mass loss of almost 100 %, the curve of the other samples displays two thermal degradation stages. The first degradation peak related to cellulose degradation (320-343 ºC), and as expected, increasing the MC-APTES amount increases the weight loss (around 4.5%) and a decrease on the decomposition temperature value,
Figure 5b. The same occurs for the degradation peak of PE (478-479 ºC), the sample with 1%MC-APTES seems to be the most thermal stable composite when compared with the other composites. This is in accordance with literature, Ch.V.Alexanyan
et al reported a study where it’s possible to verify that the presence of cellulosic materials translates in a slight increase in the degradation temperature. Moreover, the charcoal, resulted from cellulose degradation, contribute to the hydrogenation of the unsaturated products and, consequently, the hydrogenated products develop at higher temperature [
21].
The DSC experimental curves of PE-
g-MA_MC-APTES composites obtained from the first heating and cooling cycle are displayed at
Figure 6 and
Table 2.
Crystalline polymers are characterized by a melting transition at a certain temperature, the melting temperature (Tm) and enthalpy (∆H) for melting. The crystallinity of the PE was in the range of 40–44% for all studied composites, where the crystallinity of neat PE-g-MA is 42.9%. The presence of MC slightly shifts the melting point of PE-g-MA for lower temperatures, narrowing the peak. This could be evidence that the presence of cellulose in the matrix induces less stable crystals. On the other hand, cellulose can act as a nucleating agent during the cooling cycle, whereas the crystallization peak starts at higher temperatures. Although this results, no significant changes on crystallinity are detected, which cannot be related to the following characterization of the film properties.
Dynamic mechanical results of all materials exhibited an increase in Storage Modulus as the amount of modified MC content in the polymer increases. This is associated the reinforcement effect of the MC. Moreover, the shift of Tan Delta to lower temperatures, this in agreement with E’ enhancement. As the shift to lower temperatures, indicates a better compatibilization between modified MC and the polymer matrix.
Polyethylene is known as a hydrophobic polymer, meaning that wetting ability is very low, which can be a disadvantage in food applications. Therefore the incorporation of a more hydrophilic materials, such as, cellulose and/or cellulose-silane composites, could increase the wettability capacity [
22].
Distilled water (volume: 3 μL; rate: 2 μL/s) was dropped on the film surface with a precision syringe using the sessile drop method. The image of the initial drop (taken at 0 seconds) was recorded with a video camera, the contact angles along with the drop image is depicted in
Figure 8. As expected, the increase of MC-APTES content on PE matrix increases the surface wettability and decreases the contact angle. The CA of PE-
g-MA is around 90º, due to the hydrophobic nature of PE, while for the materials with 1, 3 and 5% of MC-APTES, the CA are approximately 83º, 81º and 79º, respectively. This means that the surface of the film became more hydrophilic with increasing cellulose content, as already reported in literature [
22,
23,
24]. Thus, these results are in agreement with the obtained surface SEM image,
Figure 3b, where is possible to observe the presence of modified cellulose on film surface. Thus, it is visible the effect of cellulose on the hydrophilic character of the prepared materials, since the hydroxyl groups present in cellulose are able to form strong hydrogen bonds with the water molecule. Therefore, it´s possible to change the hydrophilicity/hydrophobicity on the material changing the MC content.
Water vapor barrier property is vital for packaging, which can prevent or allow the transmission of this gas from the atmosphere to the food. Therefore, is crucial to control the transmission of gases/moisture from the environment to the food to extend the shelf life and quality of food [
25]. The incorporation of a more hydrophilic materials, as cellulosic derivatives, can change barrier properties to gases. Although cellulose displays an effective barrier to gases, when it is in a humid environment, cellulose swells, to overcome this drawback and to afford a hydrophobic character, chemical functionalization have been carried out on the cellulose structure [
26]. Despite the silanization of the cellulose surface, a smaller number of hydroxyl groups still available to linkage water molecules promoting a path for water vapor.
The WVTR characterizes the capability of moisture to penetrate and pass through the film and it was assessed to understand the effect of MC-APTES content on films water vapor transmission,
Figure 9. The results of WVTR demonstrate that the addition of modified cellulose in the PE matrix increased the WVTR of the films from 0.13± 0.030 g.h
-1m
-2 (PE-
g-MA) to 0.29±0.026; 0.37±0.038 and 0.69±0.015 g.h
-1m
-2 of the films with 1, 3 and 5%MC-APTES, respectively. The presence of modified MC results in a lower barrier to water molecules when compared PE-g-MA, and an increase from 3 to 5% MC-APTES raises the WVTR almost twice, 0.37 and 0.69 g.h
-1.m
2, respectively. This agrees with literature results, where it is reported that cellulose increases barrier properties due to their solid web-like architecture.