Introduction

Poly(lactic acid) (PLA), a bio-based aliphatic polyester, is derived from agricultural sources like corn starch. It finds use in various textile applications such as household and industrial wipes, diapers, feminine hygiene products and disposable garments [1–3]. PLA fiber is commercially dyed with disperse dyes at low temperature in a short time (typically 110 °C for 30 min) to prevent the loss of mechanical properties due to the low-melting point and the low hydrolytic stability. Thus, the dyeing yield of PLA fiber strongly depends on the disperse dye structure [2, 4–11]. For increasing the dye uptake of PLA fiber with disperse dyes, the synthesizing of the special dye molecules, the modification of the dyeing procedure and the modification of PLA fiber during the melt spinning are widely preferred methods in the literature [12–18].

In the literature, various modifiers were used during the melt spinning to improve the dye uptake of synthetic fibers to anionic and cationic dyes [19–27]. In these studies, amine based modifiers [21, 22, 26], polyamide [23, 24], organoclay [19, 20, 27], and silver nanoparticles [25] were used as functional filler for increasing the dyeability properties of synthetic fibers including polypropylene [19, 21–23, 26, 27], polyamide 6 [20], polyethylene terephthalate [25] and polytrimethylene terephthalate [24]. No study was found in the literature to improve the dyeability behavior of PLA fiber with anionic dyes during the melt spinning.

The aim of this study is to produce acid dyeable PLA fiber using octaammonium POSS (OA-POSS) nanoparticle. OA-POSS nanoparticle bears eight quaternary ammonium cations. The molecular structure of the OA-POSS nanoparticle is shown in Fig. 1. This modification relies on the introduction of the cationic group as dye receptor sites into PLA fiber structure during the melt spinning. The produced PLA fibers are characterized using tensile testing, differential scanning calorimeter (DSC). The state of the OA-POSS dispersion in PLA matrix is investigated via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of the dyeing conditions including dyeing temperature and time, acid dye type and concentration, and OA-POSS concentration are investigated on the dyeability properties of PLA fiber samples. The wash and light fastness properties of the selected PLA fiber samples are also investigated.

Fig. 1
figure 1

The molecular structure of the OA-POSS nanoparticle

Full size image

Experimental

Materials

Fiber grade PLA with the trade name of 6202D was purchased from Cargill–Dow (Nebraska, USA). The density is 1.24 g/cm3 (ASTM D792) and the melt flow index is 15–30 g/10 min (2.16 kg, 210 °C), as provided by the supplier. OA-POSS was purchased from Hybrid Plastics Inc. (Hattiesburg, USA). Two different acid dyes, Acid Blue 193 and Acid Blue 62, were obtained from SETAS Colorcenter (Tekirdag, Turkey). Acid Blue 193 is a disulphonated 1:2 premetallised acid dye. Acid Blue 62 is a monosulphonated non-metallised acid dye. The chemical structures of acid dyes are shown in Fig. 2. Sandoclean PC, nonionic surfactant agent, was obtained from Clariant (Istanbul, Turkey). Analytical grade acetic acid was purchased from Sigma-Aldrich.

Fig. 2
figure 2

The chemical structures of the acid dyes

Full size image

Production of nanocomposite fibers

The nanocomposite fibers were produced in a two-step extrusion process and a one-step hot drawing process. To prevent the degradation caused by humidity, PLA was dried at 80 °C for 8 h under vacuum before the extrusion process [28, 29]. A corotating twin screw extruder (L/D: 12) (Gulnar Makina, Istanbul, Turkey) coupled to a drawing unit was used during the fiber spinning process. The diameter of the die was 0.5 mm. The first step of the extrusion process was conducted for obtaining the nanocomposite structure. The mixing PLA with OA-POSS (0.5, 1 and 3 wt%) was conducted at 100 rpm with a temperature profile of 120–160–180–200–210–210 °C. In the second step, the pellets produced from the first extrusion process were fed into the same extruder and the extruder was brought to force-controlled mode to ensure a uniform melt flow during the spinning process. The set force value was changed according to the type of formulation. It was adjusted as 5 g/min. The spinning speed was 300 m/min. No additional cooling was applied to the extrudate except for ambient conditions (at around 25 °C). The as-spun monofilaments were hot drawn with a draw ratio of 3 using heated rollers (80 °C). No heat setting was applied to the produced fiber samples. The obtained fibers had diameters of about 35 µm. For sample coding, the abbreviations PLA and OA-POSS terms are used. The sample coded as PLA/3 OA-POSS refers to the fiber sample containing 3 wt% OA-POSS.

Dyeing procedure

All dyeing processes were made according to exhaustion technique in a laboratory-type dyeing machine (Termal, Turkey) at a liquor ratio of 1:20. The general representation of dyeing procedure is shown in Fig. 3. Pristine PLA and 1 wt% OA-POSS containing fiber samples were dyed at a constant dye concentration [1% owf (on weight of fiber)] and pH (6) with varying dyeing bath temperatures (40–110 °C) and times (5–40 min). The low-melting point and the low hydrolytic stability of the PLA fiber limit the dyeing temperature and time as 110 °C and 40 min [6, 8, 9, 11]. The other parameters, OA-POSS concentration (0.5, 1 and 3 wt%) and dye concentration (0.1–5% owf) were also examined under the dyeing conditions of 100 °C and 30 min. After the dyeing process, the resulting fiber samples were rinsed with water containing nonionic surfactant at 80 °C and finally allowed to dry at room temperature.

Fig. 3
figure 3

The general representation of the dyeing procedure

Full size image

Characterization methods

Morphology analysis

The nanomorphologies of the PLA fibers were examined using a transmission electron microscope (TEM) (Philips CM200 TEM) at an acceleration voltage of 120 kV. Ultrathin sections of 70 nm in thickness were cut with a diamond knife. All samples were trimmed parallel to the fiber forming direction. The surface morphologies of fiber samples were examined with SEM (LEO 440 computer controlled digital, 20 kV). All specimens were sputter-coated with Au/Pd before examination.

Differential scanning calorimeter

Differential scanning calorimeter analyses were used for assigning the melting point (T m) and χ c values of the PLA fiber samples. The percent crystallinity (χ c) and the crystal structure are the parameters affecting the mechanical and the dyeability properties of the synthetic fibers [30, 31]. DSC analyses were performed on a Perkin Elmer Diamond under 50 ml/min nitrogen flow. Ten milligrams of samples were sealed in aluminum pans and heated from 25 to 200 °C at a heating rate of 10 °C min−1. The following equation was used for the calculation of χ c values of the PLA fiber samples.

$$\chi_{\text{c}} \, \left( \% \right) \, = \, \left( {\Delta H_{\text{c}} /\left( {1 - \varPhi } \right) \times \, \Delta H_{\text{m}}^{\text{o}} } \right) \times 100$$
(1)

ΔH c is the measured enthalpy of crystallization, ΔH om is the enthalpy of 100% crystalline PLA (93 J g−1) [32], Φ is the weight fraction of OA-POSS in the fiber sample.

Tensile testing

The tensile measurements of monofilaments were made using a tensile testing machine (Shimadzu AG–X) with a load cell of 10 N according the ASTM D 3822 standard. The tests were done at room temperature (about 25 °C). The sample length and the deformation rate were 20 mm and 20 mm/min, respectively. All results represent an average value of twenty tests with standard deviations. The diameters of fiber samples were measured with digital microscope (Veho VMS-004) using imaging software to calculate the stress values. The diameters of monofilaments were measured at 20 different places on the fiber sample and the average values with standard deviations are listed in Table 1.

Table 1 Thermal and mechanical properties of pure PLA and OA-POSS containing fibers
Full size table

Color measurements

The colorimetric properties of the dyeings were determined using a spectrophotometer (Minolta 3600d, Japan) coupled to a PC under D65 illuminant/10° observer with a specular component included. The fibers were measured at three locations and the results were averaged. The depth of shade was assessed in terms of the color strength (K/S) values according to the Kubelka–Munk equation:

$$K/S = \, \left( {1 - R} \right)^{2} /2R$$
(2)

where K is the absorption coefficient, S is the scattering coefficient, and R is the reflectance.

Wash and light fastness

The wash fastness of the fiber samples was tested using the method given in ISO 105 C06 B1S. The light fastness was made according to the ISO 105 B02 standard.

Results and discussion

Fiber morphology

The surface images of the pure and OA-POSS containing PLA fiber samples are shown in Fig. 4 at a magnification of 2000×. As seen from Fig. 4, pure PLA fiber has smooth surface structure. Tiny particles embedded into the fiber structure are seen with the addition of the OA-POSS nanoparticles. The size of these particles increases owing to the increment in the aggregation tendency of OA-POSS nanoparticles as the added amount increases. Similar agglomeration trend is observed as POSS nanoparticle concentration increases in the literature [33–37]. To observe the nanoscale dispersion of the OA-POSS nanoparticles within the PLA matrix, the TEM micrographs were analyzed (Fig. 5). OA-POSS nanoparticles are detected as dark regions owing to their high electron density [38, 39]. Although the agglomeration tendency of the OA-POSS nanoparticles increases as the concentration increases, they are dispersed at submicron size at all concentrations. It is concluded from the morphological studies that the OA-POSS is well dispersed in the PLA matrix mostly at submicron level, even some micron-sized particles existed.

Fig. 4
figure 4

The surface images of pure and OA-POSS containing PLA fiber samples at a magnification of ×2000

Full size image
Fig. 5
figure 5

TEM micrographs of modified PLA fiber samples

Full size image

DSC analysis

The DSC graphs of as-spun and drawn PLA fibers are given in Fig. 6 and the DSC data of the drawn fiber samples are listed in Table 1. As-spun PLA fibers have percent crystallinity value below 10% owing to the low fiber spinning speed (300 m/min). Similar findings are observed in the previous studies [40–42]. One melting peak is observed in all as-spun fibers. Although pure PLA fiber shows one melting peak after the hot drawing process, OA-POSS containing PLA fibers have two melting peaks owing to the formation of thinner and/or less perfect crystals. Similar results were observed with the addition of the N-phenyl aminopropyl POSS (AP-POSS) and octaaminophenyl POSS (OAP-POSS) into PLA fiber in our previous studies [16, 17]. Pristine PLA fiber has a χc value of 43.9%. As the added amount of the OA-POSS increases, χ c value reduces due to the hindered formation of the ordered domains of the chains. Similar observations were found in the literature with different POSS nanoparticle matrix systems [43–46].

Fig. 6
figure 6

DSC graphs of as-spun and drawn PLA fibers

Full size image

Tensile properties

Tensile tests were performed for investigating the effect of the OA-POSS amount on the mechanical properties of the PLA fibers. The tensile strength (σ), elongation at break (ε) and Youngs’ modulus (E) values of the fiber samples are given in Table 1. The σ values of the modified PLA fibers decrease at about 5.4, 10 and 20.4% with respect to pure PLA fiber with the addition of 0.5, 1 and 3 wt% OA-POSS, respectively. The E values of the modified PLA fibers reduce as the added amount of OA-POSS increases. The possible reasons for the reduction in σ and E are the formation of the weak points along the fiber axis, the reduction in χ c and the formation of thinner and/or less perfect crystals. No significant difference is observed in the ε values of the modified PLA fiber with the addition of the OA-POSS when the standard deviations are considered.

Effect of dyeing temperature

The effect of dyeing temperature is examined using color strength (K/S) values of fiber samples. Pure and 1 wt% OA-POSS containing PLA fiber samples are dyed at different temperatures ranging from 40 to 110 °C with the increment of 10 °C for 30 min. The K/S versus temperature graphs of dyed fiber samples are shown in Fig. 7. The modified PLA fibers have higher K/S value than the pure PLA fiber at all temperatures because pure PLA fiber does not contain any dye sites to anionic dyes. The K/S value does not change when the temperature increases from 40 to 50 °C for both anionic dyes. The K/S value increases when the modified PLA fibers are dyed at 60 °C, which corresponds to the glass transition temperature of PLA, for both anionic dyes [34, 47]. The K/S value does not change when the dyeing temperature increases from 60 to 70 °C for Acid Blue 193, whereas the K/S value steadily increases as the temperature increases above 60 °C up to 100 °C for Acid Blue 62. It is thought that this trend arises from the molecular size difference between these anionic dyes. Acid Blue 193 has larger molecular size than Acid Blue 62 (Fig. 2). The larger dye molecules needs higher activation energy needed for diffusion and the larger free volume is required for dye diffusion.

Fig. 7
figure 7

K/S versus temperature graphs of dyed fiber samples

Full size image

Effect of dyeing time

To see the effect of dyeing time, pure and 1 wt% OA-POSS containing PLA fiber samples were dyed at 100 °C with different dyeing times of 5, 10, 20, 30 and 40 min. The K/S versus time graphs of the dyed fiber samples are shown in Fig. 8. For Acid Blue 193, K/S value increases as the dyeing time increases up to 40 min. For Acid Blue 62, K/S value increases up to 30 min. Further increment in dyeing time does not change the K/S value due to the maximum dye saturation value being reached. When the effect of dyeing time is compared according to dye type, the effect of dyeing time is more prominent with Acid Blue 193. It is thought that this observed trend arises from larger molecular size of Acid Blue 193. The time required for the diffusion of dye molecules into amorphous regions is higher for larger dye molecules.

Fig. 8
figure 8

K/S versus time graphs of the dyed fiber samples

Full size image

Effect of dye concentration

To see the effect of dyeing concentration, pure, 1 wt% OA-POSS containing PLA fiber samples are dyed with varying dye concentrations of 0.1, 0.5, 1, 3 and 5% owf at 100 °C for 30 min. The effect of dye concentration is examined using the K/S values of PLA fiber samples. The K/S versus dye concentration graphs of dyed fiber samples are shown in Fig. 9. Similar trend is observed with both anionic dyes. K/S value steadily increases up to 3% owf dye concentration. Negligible change is observed in K/S values of modified fibers with further increment in dye concentration. This finding implies that 3% owf dye concentration is enough for the saturation of modified PLA fibers with both anionic dyes.

Fig. 9
figure 9

K/S versus dye concentration graphs of dyed fiber samples

Full size image

Effect of POSS concentration

To examine the effect of POSS concentration (0.5, 1, 3 wt%) on the dyeability of PLA fiber, samples were dyed at 100 °C for 30 min at 3% owf dye concentration. K/S versus POSS concentration graphs of dyed fiber samples are shown in Fig. 10. The related CIELAB color coordinates (L*, a*, b*, and C*) of OA-POSS containing PLA fibers dyed are given in Table 2. The photographs of related PLA fiber samples dyed with OA-POSS are shown in Fig. 11. It is easily seen in the photographs of dyed fiber samples that pure PLA fiber has no affinity to both anionic dyes owing to the lack of dye sites for anionic dyes.

Fig. 10
figure 10

K/S versus POSS concentration graphs of dyed fiber samples

Full size image
Table 2 CIELAB color coordinates of pure and OA-POSS containing PLA fibers
Full size table
Fig. 11
figure 11

The photographs of PLA fiber samples dyed with acid dyes

Full size image

According to the dyeing results, K/S value increases and L* value decreases as the added amount of OA-POSS nanoparticle increases. The increase in K/S values and decrease in L* values clearly shows that the OA-POSS nanoparticle is effective for increasing the dyeability of PLA fiber with acid dyes. Acid dyes bearing one or more sulphonate groups are water-soluble dyes and behave anionic character in dye bath. They migrate from aqueous dye bath and diffuse into polymer matrix. It is known that the fixation of acid dyes towards the textile fibers arises primarily from ion–ion attractions [19, 25, 48]. To favor the ion–ion interaction between acid dye and PLA fiber, the cationic group bearing POSS nanoparticle (OA-POSS) containing PLA fiber is produced. Accordingly, it is thought that the enhanced dyeability arises from the ionic interaction between the terminal ammonium groups of OA-POSS nanoparticle and the sulphonyl groups of dye molecules. The schematic representation of the proposed interaction is given in Fig. 12.

Fig. 12
figure 12

The schematic representation of proposed interaction between OA-POSS and acid dyes

Full size image

Fastness properties

The wash and light fastness of 1 wt% OA-POSS containing fibers dyed at 100 °C for 30 min with 3% owf dye concentration are shown in Table 3. The shade change of all PLA fibers are good (4) and fair (3) for Acid Blue 193 and Acid Blue 62, respectively. Acid Blue 62 shows lower shade change and staining on values to wool and nylon due to small molecular size. The added amount of OA-POSS does alter shade change of PLA fiber for both dyes. However, the staining on values of wool and nylon reduces as the OA-POSS concentration increases. It is thought that the reduction in staining on values arises from the high dye uptake values. The light fastness of PLA fibers does not change with the addition of OA-POSS nanoparticles regardless of their amount and remain high. The moderate or low wash fastness properties are the inherent characteristics of these dyes. In the literature, post-treatment were applied to improve the wash fastness properties [49, 50].

Table 3 Wash and light fastness of pure and OA-POSS containing PLA fiber samples (dyeing conditions: pH 6, 3% owf dye concentration, 100 °C, 30 min)
Full size table

Conclusion

In the current study, the dyeability of PLA fiber with anionic dyes is tried to improve with the addition of OA-POSS nanoparticles during the melt spinning. Two different anionic dyes, a disulphonated 1:2 premetallised and monosulphonated non-metallised, are used. According to the results of morphological studies, OA-POSS is dispersed mostly at submicron level in the PLA matrix. Pure PLA fiber samples have smooth surface structure and tiny particles embedded into the fiber structure are seen with the addition of OA-POSS nanoparticles. According to the DSC results, the addition of OA-POSS promotes the formation of thinner and/or less perfect crystals after the drawing process and decreases the χ c value as the added amount of OA-POSS increases. According to the tensile test results, both the σ and E values of OA-POSS containing PLA fibers decrease with respect to pure PLA fiber as the added amount of OA-POSS increases. It can be deduced from the dyeing results that OA-POSS is effective for increasing the dyeability of PLA fiber regardless of anionic dye type used in this study. The effectiveness of OA-POSS increases as the added amount increases. According to fastness properties, the added amount of OA-POSS does not alter the shade change and light fastness of modified PLA fiber. Whereas the staining on values of nylon and wool decrease as the OA-POSS concentration increases.