3.1. Thermal Degradation
Thermal gravimetric analysis was performed to determine the thermal stability of the SFM/PP blend, which is essential for melt-mixing and subsequent melt-spinning. Isothermal TGA scan was conducted for only 30/70 wt% SFM/PP composition because the highest soy content gave the highest resolution for weight loss during degradation.
Figure 2 displays the results for 30/70 wt% SFM/PP blend for different isothermal runs with holding temperatures ranging from 160 to 250 °C. As expected, the blend displays the most stable response at the lowest temperature of 160 °C, and weight loss observed at 160 °C for a holding time of 20 min is less than 0.1%. At higher temperatures, the blend has a weight loss of about 0.2 wt% at 190 °C and 0.6 wt% at 220 °C, for a processing time of 2 min. The weight loss increases to about 0.5 wt%, 1.8 wt%, and 5 wt% at 160, 190, and 220 °C, respectively, for a longer holding time of 20 min. At 250 °C, the weight loss increases significantly from 2 wt% for 2 min holding time to about 10 wt% for 20 min holding time.
Above results establish that SFM/PP has good thermal stability for processing at 160 °C and 190 °C, but only moderate stability at 220 °C. At 250 °C, there is a sharp increase in the thermal degradation level. In a previous study, we have shown that monoglyceride displays a drastic weight loss around 250 °C (Guzdemir et al. [
20]). Higher temperature and residence time accelerates the degradation of soy flour and also monoglyceride [
20] while PP stays stable at this temperature [
14].
3.3. Effect of Temperature on Fiber Spinnability and Properties
Based on thermal stability analysis, fiber spinning temperatures were chosen as 160, 190, and 220 °C. The mixing time and spinning time were determined as 2 min and 20 min, respectively. Limited fibers were also spun at 250 °C to confirm the degradation observed in TGA results. For 15 wt% filler content, SFM15-PP85 fibers were successfully produced by melt-spinning. At the lowest temperature of 160 °C, the SFM/PP blend was highly viscous as compared with those at higher temperatures, and the spinnability of the blend was limited due to the fact that polymers can be drawn-down (often called melt strength) less at lower temepartures [
24,
25]. At 190 °C and 220 °C, the flow was smooth, and extensibility of the blend increased, so it resulted in finer fiber diameters. This observation is consistent with independent rheological measurements, shown in
Figure 3. The shear viscosity of the blend at 160 °C is twice of that at 190 °C, i.e., as expected viscosity decreased as temperature increased. The viscosity behavior was fitted to a Power-law model:
where
(Pa·s) is viscosity, K is consistency index (Pa·s
n),
n is flow behavior index and
is shear rate (s
−1). The power-law parameters, displayed in
Table 3, indicate that shear-thinning is evident at all temperatures (all
n values less than 1). However, an increasing value of power-law exponent at increasing temperatures indicates that the extent of shear thinning decreases with increasing temperatures because it is know that Newtonian fluids display a power-law constant
n = 1.
Representative fibers produced from SFM15-PP85 are displayed in
Figure 4. The top row displays photographs of fibers starting with neat PP (left, for comparison) followed by fibers spun at temperatures increasing from 160 to 250 °C.The average fiber diameters were measured at 65 ± 11, 59 ± 4, 50 ± 6, 107 ± 27 µm at 160 °C, 190 °C, 220 °C, and 250 °C, respectively. This is consistent with prior literature results that confirm that at higher melt temperatures (160 to 220 °C), polymer blend has higher extensibility and easier draw-down [
26]. However, at 250 °C, soy flour degradation limited the extensibility of the blend, and the final product was filaments with large diameters. Starting with neat PP (white), fiber color turned light brown at 160 and 190 °C. At 220 and 250 °C, the fibers had a dark brown color because of significant soy flour decomposition. This result is in good agreement with the ones obtained by the thermal degradation test. The color change is likely due to Maillard reaction [
27,
28] where the soy sugars and soy protein react leading to a decrease in the content of hydrophilic groups and improvement of some properties of soy flour like bonding strength [
29]. Darker shades were produced at increasing spinning temperatures due to increasing extents of the reaction [
28,
30]. Also displayed in the bottom row of
Figure 4 are SEM micrographs of fiber cross-sections with black arrows pointing to visible soy particles. The micrographs indicate the clear presence of soy particles at 160 and 190 °C, but a significantly rough texture is evident at the higher temperatures of 220 and 250 °C due to significant thermal degradation of soy, which led to the formation of large voids as indicated by red arrows (
Figure 4h,j).
Representative tensile stress-strain curves for SFP15-PP85 fibers spun at four different temperatures are reported in
Figure 5, and various material properties are summarized in
Table 4. As the spinning temperature increased from 160 to 220 °C, the yield stress decreased from 35 ± 6 MPa to 19 ± 4 MPa, which is an acceptable value for nonwoven fabrics. It is noted that when fibers are converted into fabrics, yield strength is the more relevant material property as compared with ultimate strength because fibers start to undergo plastic deformation at yield stress, so that is the upper limit of how fast the fibers can be pulled during fabric formation. At 250 °C, all fibers properties were reduced due to significant degradation of soy flour. In comparison, neat PP displayed a tensile modulus, yield strength, tensile strength, and strain-to-failure values of 1224 ± 136 MPa, 37 ± 3 MPa, 104 ± 10 MPa, and 260 ± 35%, respectively. Although lower than those of neat PP fibers, SFM/PP fibers processed at 160 and 190 °C had similar tensile properties, but fiber spinning was easier at 190 °C. Properties deteriorated significantly at 220 and 250 °C. The drastic reduction of yield strength and strain-to-failure (over 50% reduction) for fibers spun at 250 °C is consistent with the presence of voids, as observed in
Figure 4j. Therefore, for further studies, 190 °C was chosen for subsequent mixing/spinning trials.
3.4. Effect of Filler Composition on Mechanical Properties
Next, fibers filled with 5, 15, 30 wt% SFM were spun at 190 °C. SEM micrographs of the lateral (left) and cross-sectional (right) surfaces of neat PP and soy-PP fibers containing 5, 15 and 30 wt% SFM are displayed in
Figure 6a–d.
The nominal circular cross-sectional shape of neat PP fibers was retained in soy-PP fibers, although the presence of soy particles imparted more texture. As the soy content increased, the lateral surfaces clearly display increasing texture, i.e., soy particles protruding from the fiber surface. This is a consequence of soy not being miscible in PP, which results in only partial dispersion of soy particles within the PP matrix. Contrary to the thinking that rough surface is undesirable, this rough surface is actually desirable because it provides such blend fibers with a tactile feel similar to natural fibers such as cotton, which have a crenulated and irregular cross-section (i.e., not smooth and circular that pure PP has). In prior studies, we have established that polyolefin bi-component fibers with an outer sheath containing an immiscible blend, with the minor component protruding out of the surface, provide a desirable tactile feel [
31].
Representative tensile stress-strain characteristics of various fibers are displayed in
Figure 7 and the material properties are summarized in
Table 5. The yield strain and strain-to-failure of SFM/PP fibers are not statistically different from those of neat PP fibers and are not affected by the soy composition because the continuous PP phase dominates the strain behavior of the composite.
The yield strength of soy/PP fibers decreased to 32 MPa, 29 MPa, and 18 MPa for 5, 15, and 30% SFM/PP fibers, respectively, as compared with neat PP yield strength of 37 MPa. A similar decreasing trend was observed for tensile strength; values of 97, 74, and 44 MPa were measured for fibers containing 5, 15, and 30% soy, respectively. The reduction of strength with increasing soy content is consistent with prior literature results of Sailaja et al. [
13] for bulk soy/PP composites. Athough it was more difficult to spin soy-PP fibers relative to their PE counterparts (due to the higher temperatures for PP where soy degrades), these tensile properties are similar (or slightly better) to those for soy-PE fibers reported in our earlier study [
20]. For instance, the yield strength and modulus of soy-PP fibers at 30 wt% SFM was 18 and 674 MPa, respectively, when the values were slightly lower at 15 and 615 MPa for PE-based fibers (also at 30 wt% soy). However, due to the slightly higher degradation of soy at the higher spinning temperature of 190 °C for PP-based fibers, as compared with only 140 °C for PE-based fibers, the ratio of properties for the blend fibers relative to their neat polymer counterparts was slightly lower at 0.5 for PP-based as compared to 0.6 for PE-based fibers.
It is noted that while clays and hard inclusions lead to an increase in composite modulus [
32,
33,
34,
35], inclusion of soy flour led to a reduction in tensile modulus for 15 and 30 wt% composites, as compared to that of neat PP fibers. The decrease in tensile modulus is explained by the lower stiffness of SFM. The reduced stiffness provides a softer feel to the composite fibers as compared to that of neat PP fibers. These results indicate that fibers containing up to 30 wt% SFM are potential as fibers for disposable nonwoven fabrics.
Figure 8a,b displays the dynamic storage moduli and tan δ for various fibers. The storage moduli were measured in the range of 1047–1570 MPa, 988–1270 MPa, 664–1130 MPa, and 450–706 MPa for neat PP, SFM5PP95, SFM15-PP85, and SFM30-PP70 fibers, respectively. The storage moduli decreased with increasing SFM content, consistent with the trend discussed earlier for static moduli. The storage moduli and tan δ for SFM15PP85 fibers at three different temperatures are presented in
Figure 8c,d. As expected, fibers showed a decrease in the storage moduli with increasing temperature. Also, a slight increase in moduli is observed with increasing frequency due to a relatively greater elastic response typically observed for all viscoelastic materials. Also, as expected, tan δ increased with increasing temperature consistent with the fact that polymeric materials display greater viscous (as compared with elastic) response at elevated temperatures.
To investigate the nature of interactions among the three components in SFM-PP blend, FTIR analysis was conducted. Spectra for neat monoglyceride, soy, and PP, and SFM-PP are displayed in
Figure 9. Monoglyceride has two strong peaks at 3000–2850 cm
−1 due to C-H stretching and one peak at 1715 cm
−1 due to the carbonyl (C = O) [
36]. For the protein component of soy, the amide I bond (C = O) stretching in the protein secondary structure, appears at 1630 cm
−1, whereas soy carbohydrate bands are located between 1200 and 1000 cm
−1 due to C-O, C-C, and C-O-H stretching/bending [
20]. For PP, the main absorption bands are at 2951, 2839, 1450, and 1375 cm
−1 attributable to alkyl groups (C-H stretching) [
37]. SFM-PP fiber spectrum consists of a combination of these aforementioned peaks, with broad band at 3300 and 3307 cm
−1 attributable to O-H from soy protein and monoglyceride, and N-H from soy protein [
20,
38,
39]. Finally, a numerically superposed spectrum was created by combining the spectra of neat components (soy/monoglyceride/PP). When the actual spectrum of SFM-PP fibers is compared with the numerically superposed spectra, it is found that there is not any new peak that is formed during melt processing of SFM-PP fibers. This indicates that the interaction between soy, monoglyceride, and PP is primarily physical in nature. This is consistent with the absence of any additional phase in the SEM micrographs.
3.5. Hydrophilicity/Coloring
For use in disposable nonwoven fabrics, other properties of fibers are also important and were investigated next. Some disposable nonwoven fabrics come in contact with water and body fluids, e.g., sanitary pads, diapers, and band-aids. Therefore, contact angles with water were measured for the three different soy contents of 5, 15, and 30 wt%. These measurements were performed on films obtained from pressing appropriate extrudates into films. The contact angles were measured at 101 ± 3°, 83 ± 3°, 53 ± 7° and 34 ± 5°, respectively, for the blends having soy contents of 0 (PP as control), 5, 15, and 30 wt%. Contact angles larger than 90° indicate a hydrophobic surface. The lower angles measured for SFM/PP surface revealed that presence of soy particles on the surface improved the hydrophilicity of the composite.
Figure 10 displays the moisture absorbed after 1 h exposure time for SFM/PP fibers at different compositions, with neat PP fibers included as a control. One hour exposure time was chosen to mimic how such fibers might behave when used in disposable sanitary nonwovens. It is evident that neat PP fibers (i.e., without soy flour) have the lowest moisture absorption capacity with no measurable uptake (~0 wt%), consistent with the hydrophobic nature of PP. At 5 wt% soy content, the moisture gain by fibers was measured at 8 wt%. This is an indicator that even a small soy content can impart some hydrophilic character to PP-based fibers. At 15 wt% SFM content, the fiber weight gain was 13 wt%, increasing to 18 wt% for composite fibers containing 30 wt% soy. This hydrophilic fiber property is desirable in many nonwoven fabrics that come into contact with human skin, such as in disposable/sanitary applications. Further, this moisture gain is similar to that observed for soy-PE fibers (about 20 wt%) reported in our earlier study [
20]. This is consistent with the fact that the soy particles present on the surface are primarily responsible for moisture gain as PE and PP matrices by themselves are both highly hydrophobic.
To determine the extent of deterioration of fiber properties, tensile testing was also conducted on fibers exposed to moisture. Also, displayed in
Table 6, the fibers preserved almost 90% of their tensile properties. Therefore, the fibers are suitable for applications where the fabrics may be exposed to moisture.
The other desired property for any textile product is its colorability.
Figure 11 displays neat and filled PP fibers before and after coloring in a water-soluble red food dye (McCormick
TM, Baltimore, MD, USA). While control sample (neat PP fibers) could not absorb any color after immersion in colored water for 10 min, SFM/PP fibers turned light pink on the matrix; the red dots on the fiber represent soy agglomerates. This observation is consistent with the studies show that neat PP is hard to dye with organic colors by classical methods because of its non-polar (purely aliphatic) structure as well as high crystallinity [
40,
41]. Increased hydrophilicity provides ease of coloring to the fibers. In addition, even without any coloring, SFM/PP fibers have a tan color that resembles some natural fibers like flax that have the potential for disposable nonwovens.