Lignin as Sustainable Antimicrobial Fillers to Develop PET Multifilaments by Melting Process

2.2.1 Extrusion

The preparation of PET/fillers compounds by extrusion, requires a drying stage in a vacuum oven at 80°C during 12 h for removing any absorbed water and for avoiding the hydrolyze phenomena of PET. Lignins or TiO₂ incorporation into the PET, with different weight percentages (1 and 2 wt.%), was realized thanks to a co-rotating intermeshing twin-screw extruder from Thermo Haake (L/D = 25). For each blend, the rotational speed was 100 rpm and the five heating zones temperatures of the extruder were fixed ranging from 252 to 260°C. A granulator cut the extruded rod into cylindrical pellets which were dried again for the spinning step.

2.2.2 Melt spinning

Multifilament yarns were obtained by a melt spinning process using the Spinboy I manufactured by Busschaert Engineering spinning device. The extruded blend (PET + fillers) pellets passed through a single-screw extruder heated from 255 to 265°C which brought them in molten state. The spinning temperatures of extruder zones were defined in Table 1. They were injected through two dies, containing each 40 holes with a diameter of 1.2 mm, thanks to a volumetric pump. Two bundles of monofilaments were obtained, cooled down by air and then covered with a spin finish oil to ensure multifilament cohesion along the process. The multifilament continuous yarn was hot drawn between two rolls with varying speeds (S1 and S2 defined in Table 1) before winding. The theoretical drawing of multifilament was given by the draw ratio (DR), Eq. (1):

Two draw ratios were applied, 2 and 4 by keeping the first roll at the same speed (200 m/min). DR = 4 was obtained when rollers reach the maximum speed at which the multifilaments were spinnable, for optimal drawing. The blend PET/KL was not spinnable with DR = 4 because the heap of LK powder blocks the dies which prevents the formation of filaments.

2.2.3 Knit fabric

Developed multifilaments (PET + lignin; PET + TiO₂) were used for the development of A4 format textile structures by manual knitting machine. The knit fabric had to have a large pore structure for the antibacterial test (agar), so that bacteria can breathe underneath the knit. Indeed, bacteria used in this study, multiply in the presence of oxygen, they are called aerobic. All samples were knitted in rib 1 × 1 with 69 needles and E8 gauge. For antibacterial test, the knitting structures were desired for removing the presence of oil and surfactant at the yarns surface. Three steps were required for the desizing; the first one is several cycles in a soxhlet containing petroleum ether during 4 h. Fabrics were dried during at last 12 h, and then underwent a second soxhlet in ethanol during the same time as before. Three cleanings in distilled water under ultrasound (37 Hz) for 20 min released residual products from the sizing. After drying the knits during 12 h, they could be characterized.

2.3.1 Thermal properties

Thermogravimetric analyses (TGA) under air were performed using a TA 2050 Instruments. A sample of 10 ± 0.5 mg of filler or pellet was placed in an open platinum pan. Loss weight measurements were carried out from 20 to 700°C at a heating rate of 10°C/min in a 50 ml/min flow of nitrogen. Degradation temperature was studied after 5 wt.% loss weight. Differential scanning calorimetry (DSC) characterizations of multifilaments were performed on a 2920 Modulated DSC (TA Instruments) with typically 10 ± 0.2 mg of dry material. The manipulation carried out under nitrogen atmosphere (with a flow of 50 ml/min) consisted in two identical cycles, the first one being devoted to the elimination of the thermal history of the nanocomposite previously extruded. The first and the second cycle had the same temperature variation: from −20 to 200°C at 10°C/min, an isotherm at 200°C during 3 min and a cooling stage at 10°C/min to return to −20°C. Analyses were made on the second cycle. The crystallinity degree (χ) of PET in blend was calculated according to Eq. (2):

where mfilleris the theoretical content (wt.%) of fillers introduced in PET; ΔHm0is the reference enthalpy defined as heat of a 100% crystalline sample (117.6 J/g); ΔHmis the melting enthalpy.

2.3.2 Mechanical properties

Mechanical properties were tested to control that the multifilaments with fillers keep sufficient mechanical properties for undergoing to a textile transformation. The mechanical tests were realized on monofilaments extracted from the multifilament yarn. The count of 10 samples was measured in decitex (dTex) by Vibroskop LENZING INSTRUMENTS and each monofilament was tested to define its tensile strength. These tests were carried out following the standard NF EN ISO 5079 on a tensile testing machine from Zwick (1456). The force sensor used was 10 N. All the tests were made at standard atmosphere (20 ± 2°C; 65 ± 5%). The length of the sample was 20 mm and the deformation rate 20 mm/min. All the results represented an average value of 10 tests.

2.3.3 Morphology

The longitudinal view of multifilaments was observed with an optical microscope. The Nikon eclipse LV100POL with advanced research image analysis was used to analyze the dispersion of the fillers into PET. The morphology of PET with 1 and 2 wt.% KL and DL blends was studied using a Field Emission Gun-Scanning Electron Microscope (FEG-SEM) to analyze the distribution of lignin in the polymer. The technology used was Jeol JSM7600F with Oxford EDX (energy dispersive X-ray spectrometry) analysis.

2.3.4 Interfacial energy

The interfacial energy between the PET and the fillers were determined from the surface energy of each components by the sessile drop technique. Lignins films were created and were tested with a drop of water and diiodomethane which were polar and apolar liquid with a known surface tension (Table 2). Surface energy could be expressed as a sum of several components as in Eq. (3): polar (p), dispersive (d), ionic (i), covalent (c), metallic (m), etc. [30].

Only two components, polar and dispersive, are taken into account; the other terms are defined as negligible.

γLdis the dispersive component and γLpis the polar component of liquids.

θ was considered as the angle of the liquid on the solid. The Fowkes method allows to determinate the polar and dispersive components of the surface energy from the contact angles obtained on this solid with various liquids [31, 32].

The equation of Owen and Wendt [33] (4) with two unknowns allows to define the surface energy of the solid:

γSis the surface tension of lignins (mN/m); γLis the surface tension of the liquid (mN/m); γpis the polar component (mN/m); γdis the dispersive component (mN/m).

Surface energy of lignins and PET allow to measure interfacial energy with Eq. (5) between x/PET and xis the lignin KL or DL [34]:

xis the lignin KL or DL.

2.3.5 Antibacterial activity

Antimicrobial tests have been run on each lignin powder and desizing knitted structures. First of all, knits were cut and autoclave steam sterilization was carried out at 120°C during 20 min to ensure other bacteria were not going to grow during the test. In test tubes, nutrient broth and bacteria were mixed and then cultured for 24 h at 37°C, which was the optimal temperature for bacteria growth. The culture solution was diluted and 250 μL of this solution with a concentration of ∼105 CFU/ml was poured onto sterilized petri dishes with the solid culture medium (agar) that was adapted to the bacteria. Then 50 mg of each lignin powder was spread or knit fabrics (4 × 3 cm) were positioned in different petri dishes. Qualitative test was made with the Agar diffusion test. During this test, two different types of bacteria, described lower, were studied. This difference could be detected by staining technique: Gram-negative has an outer membrane. Indeed, antibacterial effect depends on the gram and the genus of the bacteria. The first was a Gram-positive bacterium, genus Staphylococcus epidermidisATCC^® 12228™, it is a bacterium that is found on the skin and is therefore constantly in contact with clothing textiles. The second bacterium tested was Gram-negative, genus Escherichia coliATCC^® 25922™. It is found mainly in the intestines of mammals and will serve as a reference for all Gram-negative bacteria.

Once the knits or the powders were positioned in contact with bacteria in petri dishes, they were placed in an oven for 24 h at 37°C for incubation. The antibacterial halo around the powder could be defined but for the textile, there is no diffusion of agent out of the fiber. Therefore, knits have been removed for defining antibacterial activity by contact underneath the knit.

3.1.1 Thermal stability properties

The results in Table 3 showed that the fillers presented thermal stability with the processing temperature of the PET (264°C) during melt spinning. In Table 3, degradation temperature (Td) of KL and DL were close to the PET processing temperature at 264°C whereas TiO₂ was very resistant up to over 700°C. The degradation at 264°C for both lignins was <8 wt.%. Therefore, KL and DL could be used by melt spinning as filler in the PET.

3.1.2 Antibacterial properties

Lignins powder were first tested with the agar diffusion test and qualitative test as explained in Section 2.3, antibacterial activity to confirm the antibacterial properties of lignins and the results are shown in Figure 1.

The black line delimits the halo which means that no bacteria are present around the lignin thanks to its antibacterial activity. Two kinds of bacteria were tested: S. epidermidisand E. coli, as explained. Against Gram-positive bacteria, halos were created around powders, so KL and DL were efficient. However, there was less diffusion of lignins with Gram-negative bacteria but a small halo could be noticed. Their effectiveness after an exposition at high temperature during 5 min were verified. The powders still demonstrated antibacterial activity with the same inhibition zone against bacteria after this thermal treatment.

3.2.1 Mechanical properties

Count (dTex) was measured on monofilament extracted from multifilament yarns and the same monofilament was used for tensile test.

Figure 2 represents counts of multifilaments realized, for 1 and 2 wt.% of TiO₂, KL and DL and for virgin PET. The monofilament with KL was less regular and the standard deviation of monofilament count with KL was higher than others. The count with DR = 4 was always slightly smaller than DR = 2, indeed multifilaments at DR = 4 were more stretched. Virgin PET was used as reference, and the introduction of TiO₂ or DL did not significantly modify the average count of the monofilaments extracted from the multifilament. On the other hand the incorporation of KL increased the count which could be explained by the agglomeration of fillers in PET.

Figure 3(a) represents the tenacity and (b)the elongation at break according to the fillers content. The dotted line is for DR = 4 and the continuous line for DR = 2. In Figure 3(a), the incorporation of KL and DL fillers decreased the tenacity of the PET by almost 40% no matter the type of lignin. Both lignins weakened the monofilament but thanks to the good mechanical properties of PET, the multifilament could be still transformed into textile structure. The fillers content had low effect on the elongation at break except for KL, the tenacity continue to reduce with 2 wt.%. Moreover, when DR = 4 the material was more resistant than at DR = 2, because the macromolecules had been more oriented in the direction of the stress during drawing in the glassy state. In Figure 3(b), all values of elongation at break were between 50 and 80%, the latter being weakly impacted by the adding of fillers. Elongation at break increased slightly with 1 wt.% TiO₂ and DL (DR = 2) but not significantly according to standard deviation. Indeed, the standard deviation is higher for 1 wt.% DL in comparison with the other measures. The incorporation of 1 and 2 wt.% KL decreased the elongation at break and this value decreased with the draw ration leading to the orientation of the macromolecules. The more content of KL was, the weaker and less elastic the filament was. For DL there was no general trend, there was a decrease in mechanical properties with 1 wt.% and the properties were stable at 2 wt.%.

3.2.2 SEM and optical microscope

Figures 4 and 5 are respectively the SEM pictures and images from optical microscope. In the SEM pictures KL caused asperities on the surface of filaments and showed that it is not regular, unlike the wire with DL which is longitudinally regular. Microscopy analyzes show that KL forms agglomeration while DL is homogeneously dispersed. In Figure 5(a) and (b), the optical microscope showed many agglomerates of KL in PET, especially with 2 wt.%. The higher the filler content, the more defects there were. Most of agglomerates were larger than 100 μm and some of them appeared to the naked eye as in Figure 6. DL presented a better dispersion in PET as seen in Figure 5(c) and (d). The count of the yarn with KL was higher because of the poor dispersion and the heap of powder that increased the thickness. Agglomerates were arranged randomly along the filament which was explained the big standard deviation of the count in Section 3.2.1.

3.2.3 Interfacial energy

Interfacial energy was calculated with formulas in Section 2.3.3, Interfacial energy for explaining the difference of homogeneity between lignins in the PET. Tables 4 and 5 show respectively the surface energy of DL, KL and PET and the interfacial energy between KL/PET and DL/PET.

Table 5 had showed that the interfacial energy for Dl in PET was smaller than for KL, which means that the affinity between DL and PET is better than KL. A higher interfacial energy (γKL/PET)shows a low affinity which caused agglomerates as seen in the optical microscope images. Omar Faruk et al. [24] have explained that when lignin was pure the blending was better, but in this case, it was the opposite because as seen in Section 2.1, KL has slightly higher purity than DL. The difference could be explained by the side-chain of each lignin, which account for the difference of the interfacial energy with PET.

3.2.4 Thermal properties

Knowing the thermal properties of the filament allowed to determine the crystallinity content of the polymer, to understand its behavior at different temperatures. DSC measured the crystallinity content, melting and glass temperature. The crystallinity content has been calculated as explain in Section 2.3, differential scanning calorimetry and are presented in Figure 7.

Figures 7 and 8 introduce the results obtained for the filaments with DR = 2 because results were similar for both DR. Tc is the crystallinity temperature measured on the second cycle of curves obtained by DSC analysis. Figures 7 and 8 show the values according to the percentage of fillers in the PET. For all fillers, the incorporation enhanced the crystallinity content but for KL and TiO₂, the percentage of filler had no impact. For both lignins, crystallinity temperature increase and the crystallization start beforehand. The crystallinity content and temperature of DL increased with the percentage; therefore, DL can be defined as nucleating agent and its effect increase slightly with the fillers content.

Integration of lignin at 1, 2 wt.% or TiO₂ did not modify the thermal stability of the PET and the degradation temperatures were always around 400 ± 2°C (which was the accuracy of TGA device). These fillers did not have any influence on thermal transition, nor on the glass transition and melting temperature. Whatever the draw ratio, fillers and their content, the glass temperature was always the same, close to 78 ± 2°C and the melting temperature is 256 ± 2°C. The fillers did not impact the thermal properties except DL which behaves as a nucleating agent.