3.1. Microstructural Observations
In the literature, there is ample evidence that achieving good dispersion of lignin in polyolefins using simply melt mixing is difficult. The application of a long mixing time in a twin-screw extruder may cause lignin degradation, reflected as the presence of air bubbles and microvoids, as reported by Toriz et al. [44
Images presented in Figure 2
, obtained using the optical microscope, show the local morphology of LDPE composites with MgO, lignin, and hybridized particles that contain 5% by wt. of additives, where the lignin content in hybridized structures varied from 20% up to 80% by wt. of the total weight of filler. Typical observation of morphology resulted in an expected conclusion, that no or minor particles in the agglomerated form are visible after the application of compatibilizer and configuration of screws in the plasticizing unit of the Zamak extruder to achieve uniform filler distribution. As expected, along with an increase of lignin content in bi-component fillers, particles became bigger, and they are visible as a brown/dark stain because of the lignin concentration. Increased amounts of lignin in the polyethylene matrix resulted in stronger interparticle affinity and occurred in an aggregated microstructure.
To verify the effectiveness of mixing efficiency and local dispersion of dual fillers in the presence of a higher lignin amount, the same sheets were analysed with a twofold higher magnification (Figure 3
). The images confirm that elongated voids appeared for the hybrid filler with a higher content of lignin and also for pristine lignin. Direct microscopic observations revealed that the mentioned pores are not created outside single lignin particles, but they are rather stretched into the form of elliptic voids between the particles. Therefore, the composites’ films do not exhibit regular porosities like foam or foamed microcellular thermoplastics. Those elliptic micro-voids resulted from the uniaxial stretching of viscous composites in the presence of hybridized fillers and lignin. This may suggest that the voids are created and localised around one single void and are continuously growing up to another neighbour particle. The mechanism of the void formation is less probable with filler degradation, but with lower adhesion on the polymer/filler interphase and local matrix deformation. To overcome such issues, a higher addition of the compatibilizer or surface modification of lignin is suggested based on a similar experience as it was examined in Reference [18
Contrary to the above description, it can be proposed that the evolution of composite morphology from classical dispersion to that which includes numerous micro-voids as a function of lignin content may possess some positive aspects. In general, the formation of voids in the material leads to poor mechanical properties, and therefore we can expect a shorter lifetime for such a foil, used for example as a short-term packaging material in contact with food. This could bring new aspects and properties associated with accelerated decomposition caused by increased water intake by lignin.
This idea is only a hypothesis which should be checked, but the association between lignin content and void appearance is a noteworthy and promising aspect in such a case. On the other hand, it should be also pointed out that numerous voids may contribute to the breathable properties of the foil, which can be used for fruit/vegetable transportation and as an antifogging material.
Further morphology investigations were carried out with scanning microscopy. Appropriate images are presented in Figure 4
. As expected, the SEM technique provided mostly topography, in case of which the polymer matrix covers incorporated filler particles. Therefore, as can be clearly seen in Figure 4
c,d, the inorganic phase is hidden and the assessment of morphology is difficult.
New information regarding microscopic morphology was obtained from a detailed observation of the LDPE/MgO-L (1:5 wt./wt.) composite surface. The application of a higher magnification allowed us to discover some unexpected micro droplets, visible in Figure 5
. The droplets are spherical and, for most of the observable droplets, the diameter does not exceed 1 micrometer. Moreover, they are randomly distributed on the film surface, not aggregated. The micrograph suggests that droplets do not swallow the polymer surface. They might result from water condensation on the film surface.
3.2. Mechanical and Technological Properties of Thermoformed Polyethylene/Hybrid Composites
Some of the selected mechanical results are summarised in Table 1
. As it was expected, the occurrence of micro-voids described in the previous subchapter, associated with increasing lignin content in composites, led to a deterioration of tensile strength. Relative to neat LDPE, the LDPE/MgO-L (1:5 wt./wt.) composite exhibited a 25% decrease in tensile strength, while in the case of the LDPE/MgO-L (5:1 wt./wt.) only a slight decrease was noted. This confrontation again shows the indirect relationship between the presence of lignin in compositions and the occurrence of voids, or in other words, tensile strength decrease. Interestingly, the increase of lignin content in hybrid fillers, as well as pristine lignin, resulted in a continuous increase of the Young’s modulus to almost twofold values compared to neat LDPE. Such a relationship between tensile strength and an increase of lignin concentration in LDPE as well as PP was also noticed by Iyer et al. [33
]. The authors reported that, in order to achieve a major improvement of Young’s modulus in LDPE, they had to use 30% by wt. of rigid lignin particles in the composition.
The ‘so-called’ thermoforming ability was checked using shaping in a positive-forming single plug assist. In this case, a male (positive) plug assist is pushed into the heated sheet before the vacuum is applied. This method allows for a better distribution of material, and deeper shapes can be formed, in case of which the depth-to-width ratio of more than 1:1 is possible, contrary to the negative-forming mode. Positive-forming plug-assisted forming applies a pre-stretch to the plastic sheet heated above the softening point and, therefore, improves the performance of the material and improves wall thickness distribution. In order to determine the effect of fillers on wall thinning of the product during thermoforming, eleven measuring points of the thickness of thermoformed films have been chosen. The wall thickness was measured by using a micrometer with an accuracy of 0.001 mm. Approximate locations for measuring points on the cross-section of the thermoformed shape are presented in Figure 6
. The measuring points with numbers 1, 7, 8, and 11 were set at a 5 mm distance from the bottom, while measuring points 2, 6, 9, and 10 were established at 40 mm from the bottom. Finally, measuring points 3 and 5 were chosen at 70 mm from the basis of the shape. The results of the wall thickness of the thermoformed sheets with the positive forming method is presented in Table 1
To compare the role of the fillers in the film thickness distribution in positive-formed shapes, the authors decided to show the relative wall distribution. In that case, the reference point and initial wall thickness were taken into account. After making all the necessery measurements for all tested shapes, including initial wall thickness, the thicknesses measured from point no. 4 corresponded closely to the value of the initial film thickness. Therefore point no. 4 was chosen as a reference point for the estimation of wall thinning. We think that during positive forming and film stretching, this is the place of the first contact with the male mold face. As a consequence, this area is not subjected to stretching. Moreover, we consider that after the first contact between the hot film and the male mold face, the coefficient of friction became high and the film cannot be moved and, therefore, massive wall thinning is avoided. That was the basis for the decision to choose point no. 4 as a reference point in the case of wall thickness comparison. The described procedure gives results of the wall thinning related to the initial thickness of the film. That assumption is correct if we agree that point no. 4, which represents the reference wall thickness, is only slightly elongated at the very beginning of the thermoforming process. In Figure 7
, there is a picture of a real shape, formed in positive mode with an indication of the place where the reference point was established.
The results of the wall thicknesses measurements taken for shapes produced in the positive forming process and recalculated as a percentage contribution of initial film thickness, represented by point no. 4, are presented in Table 2
The most visible percentage of wall thinning occurred for the LDPE/MgO-L (5:1 wt./wt.) composite film. The increase of the lignin amount in such a dual-component filler has a very positive influence on wall thickness distribution, which resulted in decreased thinning. Undoubtedly, the shape formed with LDPE/MgO-L (1:5 wt./wt.) composition is characterized by the best material arrangement and the highest mean wall thickness percentage. One of the possible explanations for such a coincidence is that the low molecular weight lignin polymer, which was spread out into the polyethylene matrix, may act as a softener that enables the achievement of more uniform sheet softening. On the other hand, thermal decomposition of lignin was restricted by the presence of magnesium oxide into the bi-component dual phase filler. Our previous studies confirmed the better thermal stability of the MgO-L systems against pristine lignin filler [26
]. Moreover, a high number of aggregates visible in LDPE/lignin morphology, presented in Figure 2
e, may effectively reduce the ability of such material to achieve uniform film thinning during stretching of the initial sheet.
3.3. Analysis of Wettability for Obtained LDPE Films
The wettability is an important property of synthesized polymers films because it allows us to determine the interactions between the solid and the liquids, and hence control the structure of the surface. The results of the contact angle measurements for analysed films are presented in Table 3
The wettability analysis for the LDPE film indicated that the contact angle for the water was equal to approximately 102°, which is a similar value to that presented in the scientific literature [45
]. To determine the influence of the addition of magnesium oxide and lignin, reference samples such as LDPE/MgO as well as LDPE/lignin were investigated. For the LDPE with magnesium oxide addition, a decrease of the water contact angle (92.08°) and hence surface free energy compared to LDPE was observed. For the second sample (LDPE/lignin) the contact angle for diiodomethane and water was equal to 44.91° and 74.06°; additionally, the SFE was 41.19°.
In the next step of wettability analysis, the hybrid materials, LDPE with the addition MgO-lignin, were analysed. In the case of LDPE/MgO-L (5:1 wt./wt.), the contact angle for water was similar to the reference sample LDPE/MgO; however, the contact angle for diiodomethane was decreased (48.8°), and hence the SEF has increased to a value 34.25 mN/m2
. With the increase of the lignin content (sample LDPE/MgO-L (1:1 wt./wt.)), a similar value of the contact angle for water relative to the above-mentioned hybrid material was observed. Additionally, a change of the contact angle for diiodomethane (52.77°) and surface free energy (36.92 mN/m2
) was noted. In the case of the LDPE/MgO-L (1:5 wt./wt.) hybrid material, the contact angle for water and diiodomethane was respectively equal to 53.91° and 32.23°. Furthermore, it should be noted that this material exhibited the highest SEF of all synthesized LDPE films, including pristine LDPE. This fact may be associated with the synergic effect between LDPE and the MgO-lignin hybrid material. Both for the pristine LDPE and LDPE with lignin, the surface free energy is high (37.19 and 41.19 mN/m2
, respectively). Based on the literature reports, it is commonly known that some properties of synthesized materials can be improved in the chemical systems such as oxides or hybrid systems. Wysokowski et al. [46
] indicated that the synthesis of chitin-POSS hybrid systems influenced the wettability. In this case, the improvement of hydrophobic properties was observed. Moreover, Sulym et al. [47
] reported that by changing the mass ratio of multi-walled carbon nanotubes and poly(dimethylsiloxane), the wettability of the surface can be changed. Therefore, the synthesis of LDPE/MgO-lignin hybrid systems is indicated as the main factor influencing the wettability of the obtained LDPE films. Finally, according to scientific knowledge, the analysed LDPE films, in particular LDPE/MgO-L, have been recognized as materials with high wettability.
The research of Kasalkova et al. [48
] and Parizek et al. [45
] proved that in the case of polymer materials, their wettability is unsuitable for a wide range of applications such as tissue engineering, printing, and coating. Howarter and Youngblood [49
] reported that the modification of various polymers surfaces with 3-aminopropyltriethoxysilane improves the hydrophobic properties of the analysed surface. However, the use of the above-mentioned organosilicon compound generates high costs, while in the green chemistry strategy, it is important to use low-cost materials. Therefore, there is a need to study new groups of fillers for polymers based on hybrid materials [27
]. Moreover, Notley and Norgren [50
] reported that the lignin had a high surface free energy at a low contact angle for water (about 50°), and hence it is an interesting material for modifying the polymer surface. Based on the mentioned literature review, the use of lignin has been justified; however, the authors also emphasize the use of magnesium oxide. The available scientific literature does not indicate any influence of MgO on wettability because the MgO in hybrid systems mainly improves the thermal stability [27
] as well as increases of weld strength and the force needed for tear [26
]. Although the effect of MgO on contact angle for water as well as surface free energy is not currently described in literature reports, it cannot be excluded. Based on the presented results, it should be noted that the sample LDPE/MgO-L (1:5 wt./wt.) is characterized by the best wettability, an additionally exhibits the highest surface free energy within all the analysed materials. The improvement of wettability can be explained by the synergic effect with lignin, and additionally, the MgO has numerous hydroxyl groups on the surface [51
], which can allow for a decrease of the contact angle.
The contact angle measurements for LDPE/MgO-L films are presented in Figure 8