1. Introduction
Polyolefins consist of the most economically important family of polymers. Globally, the most produced polyolefins are polyethylene (PE) and polypropylene (PP) [
1]. Isotactic PP presents higher melting temperatures compared to PE, low density, and chemical inertia and is a versatile material that allows processing in various forms, i.e., dense solid parts, films, and fibers. However, about one-third of PP’s annual production is related to polymer fiber applications [
2]. PP fibers are used as raw materials for fabric production (woven or non-woven) and also for the packaging, agriculture, and construction sector [
3].
Isotactic polypropylene (iPP) is the most common commercial form of PP. It is a semicrystalline material presenting 40 to 70% crystallinity [
4]. The most common crystals that are found in the iPP matrix are
α-crystals.
β-crystals are more rarely found and are produced under shear stress [
5,
6], using
β-nucleating agents, or under specific crystallization conditions (e.g., by crystallizing iPP in the temperature range of 115–135 °C) [
7].
β-crystals melt at lower temperatures and present lower density compared to
α-crystals, while they may exhibit higher crystallization rates, depending on the crystallization temperature [
8,
9]. Upon heating, the conversion of
β-crystals to
α-crystals is observed. This effect is known as
βα-recrystallization, which may also occur under tensile stress, e.g., during the process of drawing [
7,
10]. The existence of
β-crystals in non-drawn samples facilitates the drawing process [
10] and results in enhanced mechanical properties after drawing [
11].
Polymer chain orientation obtained via the biaxial drawing of polymer films or the uniaxial drawing of polymer fibers has a considerable impact on various properties, such as crystallinity, tensile strength, and fragility at low temperatures [
4]. Under appropriate process conditions, drawing can result in a tremendous increase in tensile strength. For example, a uniaxial drawing process with a drawing ratio equal to 7 may increase more than ten times the tensile strength of PP, from around 30–40 MPa to around 400–500 MPa [
12,
13].
Besides drawing, inorganic fillers are typically used to enhance the mechanical and thermal properties of PP. The drawing of polymer composite fibers containing needle-shaped additives (such as wollastonite or carbon nanotubes) tends to align the filler particles to the drawing axis parallel with the chain orientation, resulting in a further increase in mechanical properties [
14,
15]. In this direction, PP–single wall carbon nanotube composite drawn fibers, produced using a drawing ratio equal to 21, exhibited tensile strength up to 802 MPa [
16]. Also, PP–wollastonite composite drawn fibers produced, using a drawing ratio of 7–9, exhibited a tensile strength of up to 527 MPa [
13].
PP, as a polyolefin, is characterized by a non-polar and hydrophobic nature, contrary to the polar nature of common additives such as montmorillonite, talc, wollastonite, etc. Thus, intermolecular interactions of PP macromolecules with most inorganic additives are rather poor. In order to increase the favorable intermolecular interactions, the surface modification of the additive particles, the modification of the polymer matrix, or both of them are required [
12]. Although one of these procedures may be enough for polar polymers, for PP, both of them are usually utilized [
17,
18,
19]. The grafting of PP chains with polar groups, such as maleic anhydride, results in more thermodynamically favored interactions with hydrophilic fillers, which, in turn, may result in better dispersion of the filler particles inside the polymer matrix, causing a significant improvement of composite’s properties [
12,
19,
20,
21,
22,
23,
24,
25]. In this direction, using PP-g-MA as a compatibilizer in PP–wollastonite composites increased the yield strength of PP-silane modified wollastonite from 25.3 to 31.8 MPa [
19].
Wollastonite is an inosilicate mineral that belongs to pyroxenoids. It is commonly found in the form of micro-sized needle-shaped particles. However, besides its natural occurrence, nano-sized wollastonite particles can be synthesized [
26]. Wollastonite has been utilized as an additive for PP in order to enhance the mechanical properties [
19,
26,
27,
28,
29,
30,
31,
32,
33], increase the crystallinity [
28,
29,
30], and improve the thermal stability [
26,
34] of the polymer matrix. However, most of such literature studies do not refer to drawn or fibrous PP composites. Since drawing has a severe effect on mechanical properties, further increase of tensile strength by the addition of inorganic particles in composite drawn fibers is a difficult task and less studied compared to PP non-drawn composites. Consequently, despite the fact that around 30–35% percent of the produced wollastonite is used in polymer applications [
35], there are only a few studies on PP–wollastonite composite drawn fibers [
12,
13,
36], rendering the effect of the needle-shaped fillers on the alignment of polymer chains as an issue that requires further investigation.
As mentioned above, the surface modification of hydrophilic inorganic fillers is needed to improve their interactions with the PP matrix. Wollastonite modification is typically achieved via a reaction with carboxylic acids and, more specifically, via the interaction between the calcium ions present on the surface of wollastonite and the carboxyl group of organic acids [
29,
31,
32,
37,
38]. In this direction, it has been observed that the dispersion of wollastonite in the PP matrix was improved by modifying the surface of wollastonite with malonic, pimelic, and stearic acid [
29,
32,
37]. Besides the improvement of particle dispersion, such a surface modification of wollastonite alters the crystallization behavior of PP since wollastonite modified with malonic acid [
32] or pimelic acid [
29,
31,
38] has been reported to induce
β-crystals formation. The same nucleating effect has been reported for calcium carbonate salts modified with malonic, glutaric, pimelic, and suberic acid, which showed a strong
β-crystal nucleating effect for PP upon isothermal crystallization at 130 °C [
39].
A factor that has not been thoroughly studied in the literature is the effect of organic solvents used for the surface modification of the filler. In literature studies, the solvents used for the modification of wollastonite with carboxylic acids are usually ethanol [
29,
32,
40] and acetone [
31,
38,
41]. Rao et al. used different types of solvents to modify wollastonite with stearic acid. In more detail, acidic, basic, and non-polar solvents were tested, and the authors concluded that non-polar solvents resulted in higher acid absorption on the surface of the particles, while the absorption decreased with increasing acidity or basicity of the solvents [
40]. In some cases, hybrid processes to modify wollastonite have been investigated, such as grinding in the presence of organic acids [
31,
38,
42,
43].
Regarding the mechanical properties of composite PP–wollastonite materials (non-drawn materials), it has been reported that for wollastonite content up to 2.5 wt%, tensile strength was increased for composites containing modified wollastonite with pimelic acid, while filler contents higher than 2.5 wt% were found to have a negative effect on such property [
29]. Also, for the same wollastonite content (2.5 wt%), the impact strength was considerably improved for the composites containing modified wollastonite with malonic acid, but no significant improvement was observed regarding the tensile strength [
32].
Nevertheless, the findings mentioned above are related to non-fibrous and non-drawn materials. Thus, the aim of this work is to study the effect of the modification pathway (type of acid and solvent used for the modification) of wollastonite on the mechanical and thermal properties of PP/modified wollastonite composite drawn fibers.
2. Materials and Methods
2.1. Materials
Isotactic PP (Ecolen HZ42Q) with melt flow index equal to 18 g/10 min, tensile strength equal to 33 MPa, and melting point of 168–171 °C was obtained from Hellenic Petroleum S.A., Thessaloniki, Greece. A masterbatch (Bondyram® 1001) with PP grafted with maleic anhydride (PP-g-MA), with MA content of 1%, melt flow index equal to 100 g/10 min, and melting point of 160 °C, obtained from Polyram Plastic Industries Ltd., Gilboa, Israel, was used as a compatibilizer. Wollastonite in the form of needle-like particles (NYAD M9000) with aspect ratio equal to 3 (9 μm length to 3 μm diameter) was kindly supplied by Imerys Minerals Ltd., Paris, France.
Two different organic solvents were used, namely, ethanol and carbon tetrachloride, for the modification procedure of wollastonite. Ethanol was purchased from Honeywell, Charlotte, NC, USA (purity ≥ 99.8%), while carbon tetrachloride was purchased from Fluka Chemie AG, Buchs, Switzerland (purity > 99.5%).
Myristic, malonic, glutaric, pimelic, and suberic acid were purchased from Sigma-Aldrich, St. Louis, MO, USA, while maleic acid was purchased from Alfa Aesar, Haverhill, MA, USA. Potassium bromide (KBr, >99.5%) was purchased from Chem-Lab, Zedelgem, Belgium and was dried at 170 °C for 3 h prior to use. All other chemicals were used as received (unless otherwise noted).
2.2. Wollastonite Modification
Prior to the modification process, pristine wollastonite was dried at 110 °C for 6–8 h in order to remove excess moisture. The dried wollastonite–organic acid mixture (10 g) and 50 mL of the solvent (ethanol or carbon tetrachloride) were added to a beaker, which was placed on a hotplate stirrer inside a fume hood. The modification took place at 80 °C until all the solvent was evaporated. Organic acids and wollastonite weighing were carried out using a Sartorius B 120 S scale (accuracy ± 0.1 mg). After the modification process, the obtained modified wollastonite was once more dried in an oven at 80 °C for about 24 h. Finally, the dried modified wollastonite was ground in powder form using a mortar. The modification process is shown in
Figure 1.
2.3. Fiber Production and Drawing
Weightings of the polymer pellets and the wollastonite powder were carried out with a KERN PLS 1200—3A scale (±0.003 g). In all cases, pellets of isotactic polypropylene were mechanically pre-mixed with pellets of the compatibilizer’s masterbatch (PP-g-MA) and wollastonite in powder form, either modified or not. Initially, the mixture of PP and additives (compatibilizer and wollastonite) was inserted in a twin screw (screw rotating speed = 25 rpm) extruder (HAAKE Rheodrive 5001) with four heating zones (190, 210, 215, and 220 °C from feed to nozzle). The produced filament was cut into pellets and fed into a single-screw (screw rotating speed = 15 rpm) extruder (Noztec Xcalibur) with three heating zones (215, 225, and 210 °C from feed to nozzle). A winding machine (Noztek Filament Winder 2.0) was used to collect the produced fiber into a drum. Finally, solid state drawing was performed at 140 °C, keeping the drawing ratio constant and equal to 7. More details can be found in previous studies [
12,
13].
2.4. Characterization
Thermal properties of the samples were studied via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using Shimadzu DSC-50 and Shimadzu TGA-5 instruments, respectively. DSC measurements were performed with a heating rate of 10 °C/min up to 210 °C under nitrogen atmosphere, while TGA measurements were performed with a heating rate of 20 °C/min up to 550 °C under nitrogen atmosphere. Heat of fusion was calculated using the standard procedure. The degree of crystallinity was estimated by dividing the measured heat of fusion by the heat of fusion of 100% crystalline PP, which was considered equal to 209 J/g [
44,
45] and 185 J/g [
10] for
α and
β form, respectively. The onset decomposition temperature was considered as the temperature at which the remaining mass is equal to 97% of the initial mass.
Mechanical properties were studied via tensile tests using Hans Schmidt & Co. GmbH (Waldkraiburg, Germany) Universal Testing Machine ZPM equipped with a Pacific PA6110 loadcell (headspeed 100 mm/min). For each sample, at least 15 tensile tests were performed, and the values presented in the next sections are the average values of all measurements. It should be noted that in our previous studies [
12,
13,
16], the strain (reduced elongation) was expressed as the length of the specimen divided by the initial length (l/l
o), while in this study, the strain is expressed as the length of the specimen minus the initial length divided by the initial length ((l−l
o)/l
o).
Lastly, Fourier Transform Infrared (FTIR) spectroscopic measurements were performed in KBr pellets using a Biorad FTS 175 spectrometer. For each sample, 64 scans were collected in the range of 400–4000 cm−1 with a resolution of 2 cm−1. KBr pellets were prepared by mixing the sample and KBr in 1:200 mass proportion and grinding in a mortar in order to obtain a fine powder. The powder was processed into pellets in a hydraulic press (applying pressure equal to 100 bar).
4. Conclusions
In this study, the thermal and mechanical properties of PP-modified wollastonite composite drawn fibers were investigated via DSC, TGA, and tensile tests. Modification of wollastonite was performed via organic acid treatment. Various organic acids were studied. Also, the effect of the type of solvent used to modify wollastonite with organic acids was studied. It was shown that the modification of wollastonite in a non-polar solvent, such as carbon tetrachloride, slightly enhances the adsorbed acid on the surface of the mineral, compared to ethanol. Among the six different acids that were used for the modification of wollastonite, only myristic, maleic, malonic, and pimelic acids showed that they can interact with wollastonite’s surface. In samples containing wollastonite modified with malonic acid, β-crystals were formed after the second extrusion. Those crystals were not present after drawing as tensile stress induces βα-recrystallization. In addition, the onset decomposition temperature increases by about 5–10 °C with the addition of 2% wollastonite, modified or not. Regarding mechanical properties, the composite samples exhibited poorer mechanical properties compared to the neat PP, possibly due to inadequate dispersion of the filler in the polymer matrix and, to some extent, due to the high crystallinity of the fibers before drawing. However, the crystallinity of PP–wollastonite composite fibers increased by around 10% after drawing. The use of the compatibilizer did not show any significant effect on the mechanical and thermal properties of composite drawn fibers.
Future research on this topic includes the use of less hydrophobic polymers for testing the dispersion of the investigated modified wollastonite particles and the use of particles with a less diameter-to-length ratio.