Effect of processing variables on the thermal and physical properties of poly(L-lactide) spun bond fabrics

Abstract

In this paper, the influence of the take-up velocity (V_t_–_u) of fibers on the molecular ordering and ά – α form transition of polylactide (PLA) non-woven fabrics during their manufacturing by spun-bonding is described. Non-woven samples were studied by wide-angle X-ray diffraction, differential scanning calorimetry and Fourier transform infrared spectroscopy. In addition, the physical and mechanical properties of the non-woven fabrics were determined. The results are discussed in terms of the structural changes of the PLA and the meso-phase content during the spun-bonded non-woven fabric forming process. This technological process includes preliminary molecular ordering of the PLA fibers in a downstream spinning block and crystallization on a calender system at a temperature higher than the glass transition. The molecular ordering of the investigated PLA fabric samples under different technological conditions was observed as follows: creation of the meso-phase and a disorder-to-order phase transition (ά to α form) during heating to approximately 110℃ and an increase in the degree of crystallinity for take-up velocities higher than 1400 m/min. The structural changes of the PLA explain the observed changes in the physical and mechanical properties of the non-woven fabrics obtained under different technological conditions.

Keywords

polylactide spun-bonded technology crystallization wide-angle X-ray diffraction Fourier transform infrared spectroscopy differential scanning calorimetry

In recent years, polylactide (PLA) has attracted a great deal of attention because it has excellent mechanical properties, comparable to those of other polyesters, and it can be produced from completely renewable resources, such as corn, potatoes and biomass waste.^1,2 This thermoplastic aliphatic polyester also exhibits special properties, including compostability and biocompatibility/bioresorbability.^3–5 Therefore, it is a promising alternative to petroleum-based polymers from an industrial applications point of view. PLA can be used to prepare foams, films, fibers and non-woven fabrics for many different applications, from medical to agricultural.^6–10 The material performance/property profile is strongly controlled by the semicrystalline morphology, that is, by the degree of crystallinity and the structure and habit of the crystals. The ability of PLA to crystallize depends strongly on the stereochemical form of the PLA, as the physical properties differ between isotactic poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA), syndiotactic poly(meso-lactide), atactic poly(meso-lactide) or poly(D,L-lactide), PLLA/PDLA stereocomplexes and copolymers with random contents of meso-, L- and D-lactide.¹¹

The homopolymer PLLA is a typical semicrystalline polyester with different crystal modifications, including α, β and γ, which are mainly related to the crystallization conditions.¹¹ The thermodynamically stable crystal modifications are the pseudo-orthorhombic (a = 1.06 nm, b = 0.61 nm, c = 2.88 and α = β = γ = 90°) and orthorhombic (a = 1.05 nm, b = 0.61 nm, c = 2.88 nm and α = β = γ = 90°) α forms, and they can be easily obtained from common sample preparation methods, such as solution casting, melt crystallization and/or annealing of the glassy state.¹² The trigonal (a = 1.052 nm, b = 1.052 nm, c = 0.88 nm and α = β = 90°, γ = 120°) or orthorhombic (a = 1.031 nm, b = 1.821 nm, c = 0.90 nm and α = β = γ = 90°) β forms of PLA are produced by stretching the α form at a very high drawing ratio and high temperature,¹³ while the orthorhombic (a = 0.995 nm, b = 0.625 nm, c = 0.88 nm and α = β = γ = 90°) γ form is produced through epitaxial crystallization.^14,15 Recently, it has been reported that PLLA exhibits another crystal modification that is very similar to the pseudo-orthorhombic (a = 1.06 nm, b = 0.61 nm, c = 2.88 and α = β = γ = 90°) α form with a disordered chain structure in the crystal cell. Wasanasuk and Tashiro¹⁶ defined this crystal modification as the “δ” form,¹⁶ but most researchers, such as Pan et al.¹⁷ and Zhang et al.,¹⁸ habitually defined this crystal modification as the ά form. This disordered form can be obtained when crystallization occurs at a melt temperature below 120℃.¹⁹ The ά form is a metastable crystal modification, and it is transformed into the stable α form by heating. Furthermore, it has been shown that stretching amorphous PLLA also induces a disorder–order transformation, which is related to the occurrence of mesomorphic ordering of the chains and the strain-induced ά form.²⁰ It is expected that the molecular ordering of PLA fabrics strongly depends on the stereoregularity of the PLA and the technological conditions applied during the fiber manufacturing process.^21–24 Therefore, it is important to analyze the molecular ordering and the formation of the ά or α forms of commercially available PLA during technological processes,²⁵ for example, the production of spun-bonded non-woven fabrics.²⁶

Previously, only the influence of the calender temperature on the meso and crystalline phase formation and an estimation of the critical temperature of the disorder-to-order transition were presented.²⁶ A more direct analysis of the technological process of spun-bonded non-woven production shows that other factors besides the calender temperature may affect the molecular ordering and crystallization. The present study focused on the influence of different technological conditions on the microstructure of PLA, especially the combined effect of the different pressures in the downstream spinning block resulting from the different air attenuator widths and the calender temperatures. The microstructures of the obtained materials were analyzed, that is, the degree of crystallinity, the crystallographic form of the crystals, the melting point and the glass transition. The influence of the polymer structure on the physical properties of the fabrics was also investigated. All of the studied non-woven fabrics were produced using a specially developed laboratory spun-bonded technological line. Their structures and properties were investigated using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and tensile tests.

Experimental details

Raw material

Non-woven fabrics were obtained from commercially available PLLA with a low content of D-lactide (1.4%), PLA 6251D (Nature Works LLC, USA), specifically designed for the manufacturing of non-woven fabrics by spun-bonding technology. The molar mass (M_n = 45,800 g/mol) and polydispersity M_w/M_n of 1.29 were determined by size-exclusion chromatography (SEC) with a multi-angle light scattering (MALLS) detector in methylene chloride. The glass transition temperature (T_g) and melting temperature (T_m) were determined by DSC at a heating rate of 10℃/min to be 61℃ and 168℃, respectively.

Spun-bonding technology details

The spun-bonded non-woven fabrics were prepared using a laboratory technological stand, which was previously described in detail.²⁶ Figure 1 shows a schematic of the spun-bonding process. The preliminary molecular ordering and crystallization are realized in the downstream spinning block, where the formed fibers pass through the air attenuator (1). The obtained air pressure produced the strain in the fibers and influenced the take-up velocity and physical properties. In the next steps, the fibers were laid on the belt (2) and were formed into a web and preliminary non-woven fabrics using the compaction roll (3). The spun-bonded non-woven fabrics were finally obtained by thermal bonding of the fibers on a calender (4) at temperatures above the glass transition. This bonding operation also induced an increase in the degree of crystallinity of the polymer.

Figure 1.

A schematic of the spun-bonded line.

In the present work, the air pressure was changed by changing the air attenuator size from 7 × 7 cm² to 21 × 21 cm². This parameter affected the fiber take-up velocity (V_t_–_u), which was calculated using the following formula:

V_{t - c} = \frac{W}{d π r^{2}}

(1)

where V_t_–_u is the take-up velocity, W is the polymer output (g/min/cap), r is the filament radius (µm) creating the web and d is the melt density of the polymer (g/cm³).

For each variant of web obtained at a given width of the air attenuator, the calculated take-up velocity is presented in Table 1. The non-woven fabrics were prepared at different calender temperatures: around the glass transition point and below and above the disorder-to-order structural transition temperature, that is, 65℃, 90℃ and 110℃, respectively.

Table 1.

Filament radius and calculated take-up velocities at different air attenuator widths

Width of air attenuator, cm²	7 × 7	9 × 9	11 × 11	13 × 13	15 × 15	17 × 17	19 × 19	21 × 21
Filament radius, µm	5.05	4.62	4,30	4.13	3,76	3,67	3,65	3,58
Take-up velocity, m/min	1052	1257	1451	1565	1897	1980	2013	2081

Physical characteristics

The values of mass per unit area and thickness were calculated according to European Union (EU) standards EN 29073-1:1992 “Methods of test for nonwovens. Determination of mass per unit area” and EN 29073-2:1992 “Methods of test for nonwovens. Determination of thickness”, respectively. According to the used standards in both tests, the specimen size was above 2500 mm². The fiber diameter of the investigated materials was analyzed using a scanning electron microscope, Nova NanoSEM 230 (FEI, Netherlands) (low-vacuum, HV 10 kV), with a Lucia 4 image analyzer.

Fourier transform infrared spectroscopy

The chain conformation evolution and the microstructure of the obtained fibrous samples were characterized using a Fourier transform infrared spectrometer Nicolet 6700 (Thermo Scientific, USA) in attenuated total reflectance (ATR) mode with a diamond crystal (Thermo Scientific, USA). The resolution was 2 cm⁻¹, and the range was 4000–600 cm⁻¹. All the measurements were carried out at room temperature (23℃).

Differential scanning calorimetry

The thermal characteristics of the studied materials, such as the glass transition temperature (T_g), the cold crystallization temperature (T_cc) and the melting temperature (T_m), were determined using a DSC Q2000 device (TA Instruments, UK) that was calibrated with indium. All measurements were made at a heating rate of 10℃ min⁻¹ over a temperature range of 0–200℃ in a dry nitrogen environment according to the PN-EN ISO 11357:2009 standard. The variation of the heat capacity (ΔCp) and the enthalpy of crystallization upon heating (ΔH_c_c) and of melting (ΔH_m) were estimated. The degree of crystallinity was calculated using the following equation:

χ_{C} (%) = \frac{Δ H_{m} - Δ H_{cc}}{H_{100 %}} \times 100

(2)

where ΔH is the difference between the melting enthalpy and the cold crystallization enthalpy and H_100% is the melting enthalpy of 100% crystalline PLA, which is equal to 93.1 J/g.²⁷

Wide-angle X-ray diffraction

The analysis of the PLA crystalline structure was performed using a wide-angle X-ray diffractometer by means of an X’Pert Pro System (PANalytical, Netherlands) with Cu Kα radiation (λ = 0.154 nm) operating at 40 kV and 30 mA. Prior to the measurements, the webs and non-woven fabrics were ground into powder specimens. The diffraction profiles were obtained in the 2θ range from 5° to 60° with a step size of 0.015°. The crystalline and mesomorphic phase contents were estimated using WAXSFIT²⁸ software based on Hindeleh and Johnson’s method.

Mechanical properties

The tensile strength and elongation of the studied spun-bonded fabrics were examined using the mechanical testing machine Instron 5511 according to EU standard EN 29073-3:1992 “Methods of test for nonwovens. Determination of tensile strength and elongation”. The specimen size was 50 × 30 mm², and the distance between clamps of test machines was 20 mm.

Results and discussion

Physical characteristics of PLA spun-bonded non-woven fabrics

The variable stabilizing and crystallization conditions used for manufacturing each variant of the spun-bonded non-woven fabrics, such as the calender temperature (T_c) and take-up velocity (V_t_–_u), and their physical characteristics (determined with a relative error of 5%) are provided in Table 2. The view of spun-bonded non-woven fabric is illustrated in Figure 2.

Figure 2.

Scanning electron microscopy images of spun-bonded non-woven fabrics.

Table 2.

Physical characteristics of the manufactured polylactide spun-bonded non-woven fabrics

T_c	65℃			90℃			110℃
V_t–u, m/min	Mass per unit area, g/m² (cv, %)	Thickness of fabric, mm (cv, %)	Fiber diameter, µm (cv, %)	Mass per unit area, g/m² (cv, %)	Thickness of fabric, mm (cv, %)	Fiber diameter, µm (cv, %)	Mass per unit area, g/m² (cv, %)	Thickness of fabric, mm (cv, %)	Fiber diameter, µm (cv, %)
1052	54.7 (7.5)	0.33 (2.1)	11.1 (14.4)	62.2 (2.9)	0.27 (1.8)	9.5 (15.9)	56.5 (3.9)	0.31 (6.4)	9.3 (10.4)
1257	55.1 (6.3)	0.31 (3.8)	9.2 (10.8)	59.0 (2.0)	0.26 (4.1)	9.5 (14.3)	55.5 (3.0)	0.26 (5.4)	8.7 (11.4)
1451	55.1 (6.5)	0.31 (2.6)	9.1 (13.8)	56.5 (2.0)	0.26 (5.1)	8.7 (13.3)	53.1 (0.7)	0.26 (6.3)	8.4 (9.9)
1565	53.9 (11.3)	0.31 (4.5)	8.5 (11.7)	54.6 (3.9)	0.24 (3.8)	8.4 (9.9)	52.5 (4.2)	0.32 (2.7)	8.6 (9.9)
1897	53.8 (8.6)	0.30 (4.3)	8.5 (11.4)	51.6 (2.2)	0.22 (4.9)	8.2 (9.4)	51.6 (1.8)	0.28 (5.1)	7.9 (9.0)
1980	53.2 (3.8)	0.29 (4.1)	8.2 (10.1)	48.0 (2.1)	0.21 (3.3)	8.3 (9.4)	49.4 (3.4)	0.27 (6.8)	7.9 (9.1)
2013	52.9 (6.9)	0.29 (3.4)	8.2 (14.4)	50.4 (2.68)	0.22 (6.4)	7.7 (11.5)	51.7 (4.8)	0.28 (5.3)	7.8 (10.4)
2081	52.1 (9.7)	0.30 (4.0)	7.8 (11.2)	48.5 (7.4)	0.21 (6.9)	7.6 (12.4)	46.4 (0.5)	0.26 (4.50)	8.2 (10.5)

The physical parameters of the non-woven fabrics, such as the mass per unit area, thickness and fiber diameter, show an insignificant decrease with increasing take-up velocity. The observed changes are mainly the result of the varying fiber diameter obtained at variable V_t_–_u. It is clearly seen that the fiber diameter decreases with increasing take-up velocity, which confirms the creation of strain in the fibers in the air attenuator. What is also worthy of note is the fact that the reduction of fiber diameter by increasing take-up velocity happens insignificantly higher for calendering at 65℃ compared with 90℃ and 110℃. In our opinion, this phenomenon is a result of the influence of temperature on the various types of oriented structure of PLA. At 65℃ the polymer structure could be amorphous or preliminary oriented and fibers are less stiff and more susceptible to deformation during calendering. At higher temperature the stiffness of fibers is higher and this effect is less visible. The stiffness of the fibers depends on the molecular orientation and degree of crystallization. The structural changes of PLA during the technological process will be described in the next section.

In addition, for all studied materials, the coefficients of variation for the estimated mean values are rather large. This observation indicates the inhomogeneous morphology of the non-woven fabrics obtained in these technological regimes, which could affect the mechanical properties.

Formation of molecular ordering and crystalline forms

The evolution of the microstructure of PLA during the spun-bonded non-woven fabric manufacturing process was investigated using FTIR. The results are presented in Figures 3 and 4. As shown in Figures 3(a) and (b), for the web and non-woven fabrics calendered at a temperature around the glass transition point (65℃), the intensity of the absorption band at 921 cm⁻¹ increases insignificantly with increasing take-up velocity, while the intensity of the absorption band at 957 cm⁻¹ is constant or changes insignificantly. A similar phenomenon is observed for non-woven fabrics after calendering at 90℃ and 110℃ (Figure 3(c) and (d)), but the increase of the 921 cm⁻¹ band is definitely more visible. According to the current state of knowledge, the bands at approximately 921 and 957 cm⁻¹ are assigned to the crystalline α (or ά) form with a 10₃ helix conformation and to the amorphous phase in the PLA sample, respectively.^29,30 The increased intensity of the absorption band at 921 cm⁻¹ indicates the increase of the crystalline component in the sample. The band ratio between the crystalline component intensity (A₉₂₁) and the sum of the crystalline component intensity and the amorphous component intensity (A₉₂₁ + A₉₅₇), that is, (A₉₂₁)/(A₉₂₁ + A₉₅₇), usually corresponds to the relative fraction of the crystalline component in the sample.³¹ In Figure 4, the changes in the (A₉₂₁)/(A₉₂₁ + A₉₅₇) factor for the investigated materials are presented. In the case of the web and non-woven fabrics calendered at 65℃, the changes in the relative fraction of the crystalline component are insignificant. It is worth noting that the increase of the crystalline component with increasing V_t_–_u is visible. A more interesting result can be seen for the non-woven fabrics calendered at a temperature higher than the glass transition of PLA. The (A₉₂₁)/(A₉₂₁ + A₉₅₇) factor increases with increasing take-up velocity. Furthermore, a significant increase of the crystalline components is clearly visible for V_t_–_u in the range of 1257–1565 m/min. Therefore, the relationship between the take-up velocity and the crystallization for calender temperatures greater than the glass transition temperature is not linear. Most likely, the range of 1257–1565 m/min could be the optimal take-up velocity to achieve molecular pre-ordering, which is interesting from a technological point of view.

Figure 3.

Fourier transform infrared spectra of the obtained polylactide samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

Figure 4.

Plots of the absorbance ratios between the crystalline component and the sum of the crystalline and amorphous bands (A₉₂₁/(A₉₂₁ + A₉₅₇)) for the obtained web and non-woven fabrics calendered at different temperatures.

Additional analysis of the absorption band at approximately 870 cm⁻¹ confirms the influence of the take-up velocity on the molecular ordering. The 870 cm⁻¹ band at 1052 m/min shifts to a higher wavenumber, approximately 874 cm⁻¹, at velocities higher than 1565 m/min because of the increased dipole–dipole interactions and intermolecular packing. The presented results suggest the influence of the preliminary molecular ordering obtained during fiber formation on the quantity of the crystalline components of the non-woven fabrics after the calendering process.

The FTIR measurements also allowed a preliminary assessment of the formation of α or ά crystalline forms during the technological process. The presence of the α or ά form of PLA can be seen by the variations in the FTIR spectrum. Generally, the carboxyl stretching (1700–1800 cm⁻¹) region is sensitive to the conformation of the chain, and it can be used to differentiate the crystalline forms of PLA. In this region, the ordered α-form of PLA exhibits splitting bands, while the disordered ά-form exhibits only one intense band at this region.¹⁹ As shown in Figure 5, broadening of the band at 1700–1800 cm⁻¹ is observed, but the splitting bands are still inconspicuous. Therefore, it can be concluded that the ordered crystalline structure occurs only in the non-woven fabrics calendered at 110℃.

Figure 5.

Fourier transform infrared spectra (black) and their second derivatives (gray) of the obtained polylactide samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

The FTIR measurements show the changes in the molecular ordering and the quantity of crystalline components of the PLA during the spun-bonding process. A more insightful analysis of the crystallization process based on the thermal properties of the polymer was carried out by DSC. The variation of the corresponding thermal properties as a function of the technological parameters is seen in the first heating DSC thermograms (Figures 6 and 7). As shown in Figure 6, the glass transition temperature (T_g) changes slightly depending on the technological parameters and is approximately 65℃. The web and non-woven fabrics calendered at 65℃ exhibited a significant endothermic event at T_g, followed by a pronounced exothermic peak. For these samples, a cold crystallization peak (T_cc) is observed at approximately 95℃ at 1057 m/min, and its maximum peak position tends to decrease with increasing take-up velocity, suggesting that the air pressure in the air attenuator promotes the occurrence of cold crystallization at relatively lower temperatures during the DSC heating process. This phenomenon is mainly attributed to the formation of a locally ordered structure during the fiber stretching process and the increased amount of locally ordered structure at higher air pressure in the air attenuator. During the DSC heating process, the locally ordered structure acts as a nucleus and promotes the cold crystallization of PLA at relatively lower temperatures. This phenomenon is also observed for the samples calendered at higher temperature and low take-up velocity. For V_t_–_u higher than 1451 m/min, the endothermic effect at T_g is insignificant, and the cold crystallization peak is not clearly visible. The influence of the calender temperature on the crystallization has been reported before, where the crystallization was seen to significantly increase with increasing calender temperature. The pre-ordered structure that occurs during the fiber stretching process in the downstream spinning block facilitates the thermally induced crystallization of the polymer using the calender. The increasing crystallinity of the polymer processed at different technological parameters results in a slight shifting of the melting temperature to lower values. For example, for the sample prepared at 1057 m/min and stabilized at 90℃, the melting temperature is calculated as 166.4℃, and for a take-up velocity of 2081 m/min, the melting point was shifted to 163.7℃ (Figure 7). This phenomenon is also observed for the web, but the temperature range is different. For V_t_–_c equal to 1057 m/min, T_m is calculated as 170.1℃, and it decreases with an increase in the take-up velocity up to 167.7℃. This observation is interesting because it suggests that most of the crystallites detected by DSC originate from the locally ordered structure obtained during the fiber stretching process in the air attenuator. For each of the non-woven fabric samples, the melting point is observed at a lower temperature, which could be caused by the rebuilding of the polymer structure during the calender process.

Figure 6.

Differential scanning calorimetry thermograms recorded during the first heating (in the temperature range around T_g and T_cc) for the studied samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

Figure 7.

Differential scanning calorimetry thermograms recorded during the first heating (in the temperature range around T_m) for the studied samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

The influence of the take-up velocity on the crystallization is presented in Figure 8, where the calculated degree of crystallinity for each sample is compared with the variation of the heat capacity (ΔCp). The heat capacity decreases as the crystallinity of the sample increases with increasing take-up velocity. It is clearly seen that the web and non-woven fabrics calendered at 65℃ are semicrystalline samples. This observation confirms the possibility of pre-ordering of the polymer in the downstream spinning block and the stability of the locally ordered structure after calendering at a temperature around the glass transition. This phenomenon is also observed for non-woven fabrics calendered at higher temperature. For each sample, the influence of the take-up velocity on the crystallization of the polymer is clearly significant. Despite the applied temperature being higher than the cold crystallization temperature, the crystallization of the polymer strongly depends on the pre-ordered structure of the polymer. The presence of a locally ordered structure to act as a nucleus promotes the crystallization on the calender at temperatures higher than T_g.

Figure 8.

Relationships between the calculated degree of crystallinity and the change in heat capacity (ΔCp) for the studied samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

Detailed analyses of the crystalline structures of the investigated samples were carried out using WAXD. The variation of the WAXD profiles versus the technological parameters is presented in Figure 9. The content of the crystalline structure is seen to increase with increasing take-up velocity. For the web obtained at a take-up velocity higher than 1980 m/min, the diffraction profiles are dominated by single peaks at 16.5° superimposed on the amorphous halo, with only weak traces of peaks at 18.8°, which suggests that they originate predominantly from the mesomorphic form rather than from the well-developed crystalline phase. At lower V_t_–_c, only the amorphous halo and meso-phase are observed. However, the pre-ordered structure formed during calendering at a temperature of approximately 65℃ could be rebuilding; only weak traces of the peaks at 16.5° and the amorphous halo are visible at V_t_–_c higher than 1980 m/min. The obtained WAXD results are supported by the FTIR and DSC results, which suggest the creation of a locally ordered structure during fiber formation. This conclusion is supported by the FTIR spectra, which clearly show an increase in the band at 921 cm⁻¹, assigned to the crystalline α (or ά) form with a 10₃ helix conformation, and a shift of the 870 cm⁻¹ band to a higher wavenumber, approximately 874 cm⁻¹, as a result of increased intermolecular packing. In addition, this conclusion is also supported by the DSC thermograms, in which a marked post-T_g endothermic event and a subsequent exothermic peak were observed. This behavior is, in turn, attributed by Stoclet et al.²⁰ to the partial melting and recrystallization of the meso-phase.

Figure 9.

Wide-angle X-ray diffraction profiles recorded for the studied samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

However, for each of the non-woven fabrics calendered at a temperature higher than T_g, the diffraction profiles are dominated by the diffraction peaks located at 2θ = 16.5° and 18.8°, corresponding to the (110)/(200) and (203) lattice planes of the α or ά forms of PLA. In addition, traces of small diffraction peaks at 22.3° and 28.8°, assigned to the reflection from the (015) and (216) crystallographic planes, are discernible and intensify with increasing V_t_–_c. For calender temperatures equal to 110℃, an additional peak at 14.9° becomes visible, and it is assigned to the reflections from the (010) α planes. This conclusion is supported by the FTIR spectra, where the increase in the band at 921 cm⁻¹, which is assigned to the crystalline α (or ά) form, and the splitting of the bands in the carboxyl stretching (1700–1800 cm⁻¹) region are clearly visible.

A more precise structural analysis of the investigated samples was obtained by deconvoluting the diffraction profiles into the amorphous halo and the crystalline and mesomorphic peaks. For this analysis, the experimental data were fitted by a composite of the Gauss and Lorentz functions calculated using Hindeleh and Johnson’s method.²⁸ The shapes of the amorphous halo and the mesomorphic and crystalline peaks were selected according to the model proposed by Stoclet et al.²⁰ The crystalline and mesomorphic phase contents were calculated using WAXSFIT software as a function of the take-up velocity and are presented in Figure 10. The meso-phase of PLA was detectable for the web and non-woven fabrics calendered at 65℃ and dominated the structure of the investigated materials. As shown in Figures 10(c) and (d), for each sample calendered at 90℃ and 110℃, the degree of crystallinity increased up to 55% with increasing V_t_–_c, as seen in Figure 9 as reflections of other (hkl) planes. Numerical analysis of the WAXD results makes it possible to detect two different forms of PLA, the crystalline and meso-phase forms, and their changes with increases in the take-up velocity and calender temperature. The conducted analysis confirmed all of the structural changes in the investigated samples illustrated in Figure 10. It should be noted that the degree of crystallinity determined from the WAXD diffraction profiles agrees with that calculated from the FTIR data assuming that the meso and crystalline phases are the same for the FTIR technique. Agreement between the WAXD and DSC results is only seen for non-woven fabrics calendered at temperatures higher than the glass transition. For the web and non-woven fabrics calendered at 65℃, the locally ordered structure acting as a nucleus and promoting the crystallization of the polymer is most likely more easily detected using DSC.

Figure 10.

Analysis of the crystalline and meso-phase contents calculated for the studied samples: web (a) and non-woven fabrics calendered at 65℃ (b), 90℃ (c) and 110℃ (d).

The creation of ordered or disordered crystalline forms of PLA of the studied fabrics was visible in the FTIR spectra and was also verified by an analysis of the d-spacing (lattice length), calculated according to Bragg’s equation.²⁸ The d-spacing was calculated for the two most intense diffraction peaks, corresponding to the (110)/(200) and (203) planes, which were visible only in the diffraction profiles obtained for the non-woven fabrics calendered at 90℃ and at 110℃. The changes in the d-spacing of the crystalline forms of the studied samples are presented in Figure 11.

Figure 11.

Changes in the d-spacing (lattice length) of the polylactide crystals of the non-woven fabrics calendered at 90℃ (a) and 110℃ (b) at different take-up velocities.

For the non-woven fabrics calendered at 90℃, the mean values of the d-spacings are 0.536 and 0.472 nm for the (110)/(200) and (203) lattice planes, respectively. The cell units were equal to a = 1.073 nm, b = 0.619 nm and c = 2.967 nm. When the calender temperature is 110℃, the diameters of the lattice changed, and the mean d-spacings were approximately 0.533 and 0.469 nm (a = 1.067 nm, b = 0.617 nm and c = 2.953 nm). The obtained results agree with those presented in a previous report, confirming the crystallization of PLA in the ά form below the critical temperature of 100℃ and the formation of α crystals at higher temperatures.²⁶ It is worth noting that the d-spacing depends insignificantly on the take-up velocity up to 1257 m/min.

Mechanical properties of the PLA spun-bonded non-woven fabrics

The mechanical properties of polymeric materials depend strongly on the molecular ordering and the crystalline form. In the case of the studied samples, the mechanical properties were also analyzed.

The studied PLA non-woven fabrics were characterized by two parameters, tenacity and strain at break, measured in the machine direction (MD) and transverse direction (TD). The tenacity was defined as the ratio of measured force at break to the apparent density of the fabric on account of the changes of mass per unit area and fabric thickness as a function of the take-up velocity.

The changes in the mechanical properties of the non-woven fabrics manufactured under different technological conditions are illustrated in Figures 12 and 13.

Figure 12.

Changes in the tenacity values of the non-woven fabrics manufactured under different technological conditions.

Figure 13.

Changes in the strain at break values of the non-woven fabrics manufactured under different technological conditions.

In Figure 12, the tenacity is plotted as a function of the take-up velocity. The values of the ratio of the force at break to the apparent density are higher when measured in the MD than in the TD. This result suggests the anisotropy of the mechanical properties of the PLA non-woven fabrics. In addition, the maximum value of the tenacity measured in the MD and TD is located at 1565 m/min for different calender temperatures. This effect is also observed for the measurements in the TD

However, the strain at break measured for the non-woven fabrics calendered at 65℃ and 90℃ is similar, approximately 3% regardless of the take-up velocity. The influence of V_t_–_u on the strain at break is clearly seen in the non-woven fabrics calendered at 110℃. At take-up velocities higher than 1565 m/min, it increases to 30% (Figure 13).

The observed changes of the mechanical properties as a function of V_t_–_u mainly result from two physical properties of the studied materials: the morphology of the non-woven fabrics and the molecular ordering of the polymer. The presence of polymer chain ordering depreciated the mechanical properties despite the high degree of crystallinity. It thus appears that the optimal crystalline form is the disordered ά form. The high take-up velocity also causes changes in the morphology of the non-woven fabrics, for example the thickness of the fabrics and the fiber diameter, resulting in a reduction in the tenacity. It is worth noting that the measured maximum values of the ratio of the force at break to the apparent density measured in the MD and TD for all the materials tested is located at approximately 1565 m/min, which confirms the influence of both the morphology of the non-woven fabrics and the molecular ordering of the polymer on the mechanical properties of the studied non-woven fabrics.

The inhomogeneous non-woven fabric structure is responsible for the observed high strain at break of the samples calendered at 110℃. This phenomenon resulted from the thermal degradation of the thin fibers at the point of contact between the non-woven fabric and the calender. It is likely that local destruction of the sintered points occurred, as previously described in detail.

Conclusions

A detailed analysis of the influence of the technological conditions on the microstructure of PLA showed the importance of the molecular ordering and the ά – α form transition of PLA during the spun-bonded non-woven fabric forming process.

The most important conclusions include the following:

preliminary molecular ordering and the creation of meso and crystalline structures of PLA are realized in the downstream spinning block under the influence of increasing take-up velocity;

rebuilding of the preliminary molecular ordering occurs after calendering at a temperature around the glass transition;

there is an increasing degree of crystallization of the PLA non-woven fabrics with increasing preliminary molecular ordering of the fibers in the downstream spinning block for samples calendered at temperatures higher than the glass transition;

no effect of the preliminary molecular ordering of the fibers in the downstream spinning block was observed on the temperature of the disorder-to-order phase transition (ά to α form) of PLA;

the disorder-to-order phase transition (ά to α form) of PLLA with a low content of PDLA (1.4%) was observed only for the samples calendered at approximately 110℃;

the web forming in the optimum take-up velocity range of 1451–1565 m/min made it possible to reach high values of tenacity;

an increase of the strain at the breaking point was observed for the ordered crystalline phase of PLA obtained at V_t_–_u above 1980 m/min.

Footnotes

Funding

This work was performed within the framework of the key project titled “Biodegradable fibrous products” (acronym: Biogratex) supported by the European Regional Development Fund (Agreement No. POIG.01.03.01-00-007/08-00).

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