Poly(lactic acid) fibers,yarns and fabrics: Manufacturing,properties and applications

Wet spinning

In wet spinning, PLA is first dissolved in a suitable solvent (e.g. chloroform) to prepare the spinning solution. The concentration of the PLA ranges from 6 wt% to 12 wt%, and is decided by the solubility and molecular weight of PLA polymer used and solution spinning pressure limitations. The prepared solution is then extruded into a coagulation bath from a spinneret which is submerged in the coagulation bath. The coagulation bath contains a non-solvent solution such as toluene. There may be one or more coagulation baths used for the PLA spinning. When the filaments are in the coagulation bath, the appearance and structure of the PLA fibers are formed. After spinning, the filaments are dried and removed. The production speed of wet spinning can be around 25–35 cm/min at laboratory scale.

Tsuji et al. ³⁸ reported the wet spinning of both PDLA and PLLA fiber and discussed the mechanical properties, appearance and cross-sectional structure of the PLA fibers. The PDLA and PLLA were dissolved in chloroform to make the spinning solution, respectively, with a concentration of 5–10 g/dL. Two coagulation baths were used. The first coagulation bath was carried out under 40°C, and the bath contained a mixture of ethanol and chloroform with a volume ratio of 10:3. Then the filaments were further coagulated in the second coagulation bath containing an ethanol/chloroform mixture with a volume ratio of 10:1 at 20°C. Finally, the filaments were wound up by a winder. Figure 7 presents the wet-spinning procedure and equipment used. The tensile property and appearance of the wet-spun PLA filaments were examined. It was found that the average diameter of the wet-spun PLA fibers was around 50 µm and the tensile strength was very low. It was also observed that the fiber surfaces were extremely irregular, and there were many pores of different diameters. The authors believed that the defective structure of the wet-spun PLA fiber was caused by the rapid desolvation of the PLA filament during coagulation.

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Figure 7.

Wet-spinning process and equipment. ³⁸

Dry spinning

For the dry-spinning process, the PLA solution is extruded into a thermally insulated chamber with hot air, where the solvent is removed by evaporation to produce PLA fiber. Several studies have shown that, for dry spinning of PLA, both good solvents such as chloroform, and poor solvents (e.g. toluene) are available.^39–42 The mixture of the good solvents and poor solvents can also be used for the production of PLA fiber, and the mixture even provides the PLA fiber with better tensile properties. In addition to the solvent used for dissolving PLA, the appearance, structure, properties of the dry-spun PLA fiber can be affected by many factors including extruding speed, winding speed, spinning temperature, hot-drawing and additives used, etc. Figure 8 shows the general manufacturing process of dry spinning and the equipment used.

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Figure 8.

Dry-spinning process and equipment. ⁴³

Leenslag and Pennings ³⁹ studied the manufacture of dry-spun PLA fiber using chloroform/toluene (40/60) solvent. The PLA was dissolved at 70°C, and then the solution was conditioned at the same temperature for 3 h before extrusion. The fibers were extruded through a stainless steel conical die and wound up to a bobbin with the extrusion speed of 3 m/min at 60°C. After the fiber collection, the whole bobbin was kept at 60°C for 10 min to completely solidify the fiber. Following this, the bobbin was dried at room temperature for 40 h until a constant weight of bobbin was reached. Then, the fibers were fully hot-drawn to the maximum draw ratio at 190°C.

Horáček and Kalíšek ⁴⁰ manufactured PLA fibers through a continuous dry-spinning/hot-drawing process. A mixture of chloroform and cyclohexane was used as solvent. The PLA polymer was dissolved by continuous stirring for 16 h at 50°C. Then the solution was cooled for 2 h to 33°C, and it was spun at ambient temperature with an extrusion speed of 6.5 cm/min. The filament was then continuously led on to a steel drum in drawing tube with a drawing temperature between 110 and 190°C. After the drawing process, the fibers were collected on another drum. In another work by Horáček and Kalíšek, ⁴¹ the PLA solutions were prepared using non-solvent or chloroform for producing PLA fibers. It was found that the PLA fibers manufactured with non-solvent exhibited better tensile strength. In addition, these authors ⁴² manufactured PLA fibers using various non-solvents including methanol, ethanol or petroleum ether as vapor in the extrusion chamber rather than in the solution. Their work indicated that the tensile strength, structure and degradability of manufactured fibers were highly affected by the concentration of the non-solvent vapor.

The influence of spinning temperature on dry-spun fiber was studied by Postema et al. ⁴⁴ They found that the tensile strength of the PLA fiber increased with the increase of ambient temperature up to 25°C. When the temperature is higher than 25°C, the tensile strength dramatically decreased with the increase of spinning temperature. The PLA fiber could obtain the highest tensile strength of 2.2 GPa when the spinning temperature was 25°C. Postema et al. ⁴⁵ also studied the effect of extrusion speed and winding speed on the tensile strength of PLA fiber and indicated that the tensile strength of PLA fiber decreased with the increase of extrusion rate. It was also found that there was no correlation between the extrusion rate and the crystallinity of the PLA fiber. The flow instability could lead to voiding and cavitation on the PLA fiber, damaging fiber properties.

PLA fibers manufactured with additives were studied by Leenslag et al. ⁴⁶ The PLA was dissolved in chloroform with different additives including toluene, camphor, poly(L-lactide)-polyurethane and commercial medical-grade polyurethane. After the fiber spinning, hot-drawing was carried out. The PLA fiber manufactured showed a loosened fibrillar structure.

Melt spinning

Melt spinning is the most studied and applied method for manufacturing PLA fibers. A number of researchers have studied the melt spinning of PLA fiber under various spinning conditions such as processing temperature, extruding speed, take-up speed and hot-drawing conditions, etc. ⁴ Figure 9 shows the schematic diagram of the manufacturing process of melt spinning.

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Figure 9.

Melt-spinning process and equipment. ⁴³

Eling et al. ³ manufactured PLLA fiber with high degree of orientation and crystallinity from low M_w (below 300,000 Da) PLLA polymer. The PLLA polymer was molten and extruded to form filaments with a diameter of 1 mm at 185°C. The take-up speed was 25–35 cm/min. After the spinning process, the fibers were hot-drawn in an electric tube furnace with an input speed of 20 mm/min. The highest tensile strength of the fibers was found to be around 0.5 GPa. Fambri et al. ⁴⁷ also reported the production of melt-spun PLLA fiber using polymer with a M_w of 330,000. To prevent the hydrolysis degradation, the PLLA polymer was dried at 50°C under vacuum condition for 48 h before the melting and extrusion process. A single-screw extruder with a diameter of 1 mm was used to spin PLLA monofilament under an inert atmosphere. A high take-up speed of 20 m/min was achieved. The monofilament was then drawn at 160°C using hot-plate drawing equipment with different drawing speeds and drawing cycles. It was found that the PLLA monofilament could reach the highest tensile strength of 0.87 GPa when it was drawn 10 times at 5 m/min.

A two-step method was used by Cicero et al. to spin PLA fibers. ⁴⁸ , ⁴⁹ Before production, textile-grade PLA pellets were dried at 82°C to reduce the water level of the polymer to around 100 ppm. A Killion KL-125 single-screw extruder was adopted to produce PLA monofilament. After the filament was extruded out of the extruder, it was quenched in a water bath. Then the fiber was cold-drawn with a constant draw ratio of 40. The take-up speed was between 7.5 and 60 ft/min. The PLA fiber manufactured by this method was found to have fibrillar morphology with microfibril diameters in nanoscale. Yuan et al. ⁵⁰ also prepared PLA fibers using a two-step method. Pulverization and drying were carried out before the melt-spinning process. The PLA fibers were spun using a laboratory single-screw extruder which has three temperature zones with temperatures ranging from 200 to 240°C. A sandblasted glass drum was used to wind up the extruded filaments with a speed of 3.2 m/min. Following the extrusion, the filaments were drawn under a nitrogen environment at 120°C. The feed speed and take-up speed of the drawing process were 0.26 m/min and 1.09 m/min, respectively. After drawing, the filaments were kept under nitrogen at 120°C for another 10 min to produce the final hot-drawn fibers.

Ghosh and Vasanthan ⁵¹ studied the influence of spinning speed and drawing on fiber properties. PLA filaments were melt-spun at two spinning speeds of 500 mm⁻¹ and 1850 mm⁻¹, and the hot-drawing and heat-set processes were applied. The authors found that the PLA filaments produced at 500 mm⁻¹ had almost no crystal region, whereas the PLA filaments produced at 1850 mm⁻¹ had around 6% crystallinity. After drawing and heat-setting, the crystallinity of the PLA filaments was found to be further improved by around 60% compared with both 500 mm⁻¹ and 1850 mm⁻¹ as-spun filaments. The effects of draw ratio and temperature on the tensile and thermal properties and morphology of melt-spun PLA filaments were discussed by Persson et al. ⁵² Both monofilaments and multifilaments were manufactured and studied. These authors concluded that the melt draw ratio (ratio of extrusion speed and take-up speed during spinning), solid-state draw ratio (ratio of post-spinning drawing) and drawing temperature were the most important factors affecting the mechanical properties of PLA fibers. The orientation and crystallinity of the PLA fibers were found to be increased with the increase of solid-state draw ratio, which was dependent on the melt draw ratio. To manufacture PLA fibers with higher orientation, crystallinity and tenacity, a higher solid-state draw ratio was preferred, and the melt draw ratio should be lower accordingly. A drawing temperature of 90°C was proven to be suitable for manufacturing PLA fibers with the highest tenacity.

The degradation of PLA is one of the most critical problems of the melt-spinning method. It was observed that degradation could occur during the pulverization, spinning and post-spinning operations, which can cause 13.1–19.5%, 39–69% and around 9.1% degradation, respectively. ⁴ , ³⁶ , ³⁷ The degradation is believed to result from thermohydrolysis, depolymerization, intermolecular and cyclic oligomerization and transesterification. Specifically, the ester linkages of the PLA molecules are broken by water molecules and depolymerization processes due to hydrolytic cleavage. To minimize the degradation of PLA polymer during fiber spinning, dried PLA has to be used.

In addition to conventional PLA filament, PLA resin can be blended with other materials such as PBS and poly (hydroxybutyrate-co-hydroxyvalerate) (PHBV) to spin blended filament through the spinning process.^53–55 Hassan et al. ⁵⁵ fabricated PLA/PBS blended filaments with five different blending ratios for healthcare products. The proportions of PBS of the blended filaments were 0, 3%, 6%, 9% and 12%. PLA and PBS pellets were mixed according to the blending ratio in a container before being added into the extruder. The mixed polymer chips were then dried in a vacuum at 80°C for 2 days to remove moisture. A two-step melt-spinning/hot-drawing process was used to spin the blended filaments. The PLA/PBS blended filaments were found to have excellent miscibility and compatibility. The addition of PBS endowed the filament with improved melting temperature, crystallinity and elongation. Li et al. ⁵³ manufactured PLA/PHBV blended filaments and studied the properties of these blended filaments. Four types of blended filaments containing 10, 20, 30 and 40 wt% PHBV were manufactured for comparison using a melt-spinning/hot-drawing process. The PLA and PHBV pellets were mixed according to the weight ratio and dried at 80°C for 16 h before the melt-spinning process. The spinneret temperatures for spinning blended filaments with 10%, 20%, 30% and 40% PHBV were 227°C, 223°C, 220°C and 218°C, respectively. After spinning, the as-spun fibers were hot-drawn immediately with a draw ratio of 1.6. It was observed that the addition of PHBV can result in a blended filament with superb softness. However, the increase of PHBV content could result in the deterioration of other properties including the melt-spinnability, tenacity, modulus and elongation at break, etc.

PLA filament with special functionality can also be manufactured using a melting–spinning technique. Antibacterial PLA/zinc oxide (ZnO) filament was developed by Doumbia et al. ⁵⁶ The dried PLA pellets and nano ZnO were first mixed in a turbomixer. They were then added into a conventional co-rotating twin-screw extruder for further mixing and melt-compounding. The mixture was then extruded and pelletized for the melt-spinning process. The PLA filaments functionalized by nano ZnO were produced using a normal melt-spinning technique for PLA fiber. It was found that the nano ZnO-functionalized PLA had effective antibacterial performance, and also showed improved thermal and mechanical properties. In other research by Annandarajah et al., ⁵⁷ the PLA fiber with insect-repellent functionality was manufactured using melt spinning. The manmade insect repellent (N,N-Diethyl-meta-toluamide (DEET)) and the natural insect repellent (pyrethrum) were added to PLA fibers through the extrusion process. After manufacture of the functionalized PLA fibers, the insect-repellent performance and the mechanical properties of these PLA fibers were measured. It was observed that the addition of DEET and pyrethrum in PLA fibers could repel mosquitoes effectively, but it also caused depolymerization and reduction of the mechanical properties of the PLA fibers. In addition, the authors indicated that the PLA fiber with natural pyrethrum showed higher efficacy than PLA fiber with DEET in repelling mosquitoes.

Pure PLA yarn

The staple spun pure PLA yarn can have unique properties such as fluffy handle and moisture conductivity that the PLA filament cannot have. To date, a number of studies have reported manufacturing techniques for ring-spun pure PLA yarn and discussed the influences of manufacturing parameters and finishing on yarn properties.

Li et al. ⁶¹ manufactured carded staple spun PLA yarn with a thickness of 9.7 tex. In the opening and cleaning process, the machines were set to process fibers with more opening effect and less cleaning effect in order to prevent fiber adhesion and to reduce fiber damage. Then, in the carding process, the gauge between the cylinder and flats was set to be larger than that in cotton spinning, and the production speed was relatively low to guarantee sliver formation as PLA fibers have low cohesive force. Three draw-frames were used in the drawing process. A high twist level was used for roving to increase the cohesive force between fibers, which was able to reduce fiber slippage during the spinning process. The PLA yarn manufactured was found to have more thin-place faults than cotton yarns due to the fiber slippage and unexpected sliver drafting.

Zhao ⁶⁰ produced carded staple spun pure PLA yarns with different thicknesses (9.8 tex, 11.5 tex, 14.7 tex, 19.7 tex, 27.8 tex) using a two draw-frames process. Anti-static agent and anti-lubricant was mixed with PLA fibers in the opening and cleaning process to prevent the accumulation of static electricity. The machine settings in all the processes from opening and cleaning to spinning followed the principle of large gauge, strong control of the fiber and low production speed.

Wang et al. ⁶² studied the influence of the twist level on the yarn strength and found that the breaking strength of 15 tex carded PLA yarn increased from around 91 cN to 140 cN when the twist factor increased from 280 to 400, and then decreased with further twisting. This trend fits the general features of staple ring-spun yarn. The critical twist factor of the staple spun PLA yarn was 405.9. Jiang and Gu ⁶³ illustrated the effect of heat finishing on the mechanical properties of PLA yarn. When the temperature of the heat finishing ranged from 60°C to 120°C, the breaking strength of the PLA yarn was found to increase with higher finishing temperature. However, the strength dropped dramatically once the temperature exceeded 120°C. This was because the molecular structure of the PLA was damaged under high temperature. In addition, the PLA yarn had higher elongation at yarn breakage when it was treated with higher temperature, as the higher temperature would reduce the intermolecular force and make it easier to create molecule displacement of PLA. These authors also demonstrated that the initial modulus of the PLA yarn decreased with the increase of heating temperature until 120°C.

The staple spun pure PLA yarn can also be manufactured by the Siro spinning technique. Li and Zhao ⁶⁴ studied the influence of spinning parameters on the strength, hairiness and unevenness of 23.9 tex Siro-spun PLA yarn. They found that the roving twist factor, the gap between two rovings on the spinning frame and the draft of the back-drafting zone of the spinning frame were the most important parameters affecting the properties of the Siro-spun PLA yarn. The optimized twist factor, gap and draft were 96, 6 mm and 1.3, respectively. The experimental results showed that the use of the Siro spinning technique could improve the yarn quality. The Siro-spun PLA yarn was found to have higher strength at break, less hairiness and fewer yarn faults. In addition, the yarn unevenness of the Siro-spun PLA yarn was improved to a large extent due to the doubling effect of the Siro spinning technique.

Blended PLA yarn

To date, most research on PLA yarn has focused on blended PLA yarn, as blended yarn can have many benefits such as combining the advantages of different fibers and reducing production costs. Zhang et al. ⁶⁵ manufactured 13 tex PLA/cotton blended yarn with a blend ratio of 65/35. In their method, the PLA fiber and cotton fiber were blended during the drawing process to reduce damage to the PLA fiber and to guarantee the right blending ratio. Before drawing, the cotton and PLA were processed separately. A combing process was applied to the cotton fiber to further reduce impurities, neps and short fibers. Three drawing processes were used to evenly blend the cotton fiber and PLA fiber. In the roving and spinning process, these authors used high twist level, low production speed and low tension.

The 14.7 tex carded PLA/cotton (50/50) blended yarn was manufactured by Cheng et al. ⁶⁶ via a manufacturing process similar to the process Zhang et al. ⁶⁵ used. They also manufactured polyester/cotton (50/50) blended yarn with the same thickness to examine differences between PLA/cotton blended yarn and polyester/cotton blended yarn. The PLA/cotton blended yarn was found to have more hairiness and higher extension than polyester/cotton blended yarn. In addition, the strength at breakage of PLA/cotton blended yarn was lower than that of polyester/cotton blended yarn.

In other research by Zhang et al., ⁶⁷ PLA staple fiber was blended with alginate fiber and chitosan fiber to produce “green” yarn for medical applications. The linear density of the yarn was 14.8 tex. The blending ratio of the alginate/chitosan/PLA was 40/40/20, which could effectively combine the unique properties of each fiber. They also adopted a carding process and sliver blending and three drawing processes. In addition to the normal preparing and spinning processes mentioned before, pretreatment of fibers was applied in which the fibers were mixed with oil to prevent static accumulation during the spinning process. In the ring spinning process, the Siro-compact spinning technique was applied resulting in less hairiness, higher strength and lower unevenness of the yarn.

Manich et al. ⁶⁸ used carded Siro-spun 55/45 PLA/wool blended yarn to make woven fabrics. The yarn had a thickness of 2 × 16.7 tex and a twist of 640 twist/meter. The PLA fiber used was specially designed for PLA/wool blended spinning. The length of the staple PLA fiber was 75 mm, and the fineness was 3 dtex. The wool was Australian wool with an average fineness of 19.5 µm.

The yarn quality and properties of blended PLA yarns have been widely discussed and studied. For the PLA/cotton blended yarn, Shan and Li ⁶⁹ and Meng and Li ⁷⁰ demonstrated that the yarn unevenness and hairiness were strongly affected by the blending ratio between PLA fiber and cotton fiber. Yarns with PLA contents of 0 (pure cotton yarn), 20%, 35%, 50%, 65%, 80% and 100% (pure PLA yarn) were tested. It was found that the yarn unevenness was reduced with the increase of PLA fiber content, and the pure PLA yarn was the most even yarn. The relationship between blending ratio and yarn hairiness was similar to that between blending ratio and yarn unevenness. It was found that the yarn hairiness could be largely reduced when the PLA content increased from 0 to 65%. However, if the PLA content was higher than 65%, the yarn hairiness would increase again. However, with a higher proportion of PLA fiber, the yarn hairiness will devolve again. The influence of blending ratio of PLA/cotton yarn on mechanical properties was further studied in several studies. ⁷¹ , ⁷² The breaking strength of the PLA/cotton yarn decreased with the increase of PLA content from 0 to 50%. When the PLA content was over 50%, the blended yarn was found to be stronger with the increase of PLA fibers. The breaking elongation of the PLA/cotton blended yarn was found to be stable when the PLA content was under 50%, and it dramatically increased with the increase of PLA fiber when PLA fibers exceeded 65%. The reason is that in the blended yarn, the fiber with lower breaking elongation would break first and cause breakage of the whole yarn. When the PLA content was under 50%, the cotton fiber dominated the breaking elongation, as the cotton fiber had much lower breaking elongation compared with PLA fiber. When the PLA fiber was over 65%, the breaking elongation of the blended yarn was decided by the PLA fiber, and thus the breaking elongation was greatly increased. In addition, Zhao et al. ⁷³ proved that the blending ratio could also have huge effect on the specific work of breaking and initial modulus of the blended PLA yarn. It was found that the specific work of breaking of the PLA/cotton blended yarn increased with the increase of the PLA content and, on the contrary, the initial modulus of the yarn gradually decreased with the increase of PLA fiber. Tan et al. ⁷⁴ measured the stress-relaxing property of PLA/cotton blended yarns with different blending ratios. They observed that when the PLA fiber content was around 60%, the blended yarn could have more stable inner stress, leading to better fabric stability.

Jabbar et al. ⁷⁵ and Cao et al. ⁷⁶ revealed the correlation between the blending ratio and the mechanical properties of PLA/lyocell blended yarn. The breaking strength and tenacity of the yarn were found to be increased with the increase of lyocell fiber proportion in the blended yarn, whereas the elongation at break decreased with the increase of lyocell fiber. These trends can be explained by two reasons. First, the lyocell fiber was more than three times stronger than the PLA fiber and had much lower elongation at break compared with PLA fiber, thus more lyocell fiber could lead to stronger blended yarn with lower elongation. The second reason was that the lyocell fiber was finer than PLA fiber, and as a result there were more fibers in the cross-section of yarn with given thickness, which contributed to the increase of overall strength of the blended yarn. In addition, the authors pointed out that, for any PLA/lyocell blended yarn, the lyocell fiber contributed more to the overall tensile strength of the yarn. This is because the elongation at break of lyocell fiber was only around 10% whereas that of PLA fiber was around 50%. Therefore, at the breaking elongation of lyocell fiber the tensile stress on PLA fiber was low, which resulted in a smaller contribution of PLA fiber to the overall strength of the blended yarn. In other research, Yang et al. ⁷⁷ measured the initial moduli and specific works of breaking of PLA/lyocell blended yarns with various blending ratios. They showed that both initial moduli and specific works of the yarn at breaking increased with the increase of content of lyocell fiber.

The mechanical properties of PLA/modal blended yarn were studied by Yu et al. ⁷⁸ They demonstrated that the breaking strength of the blended yarn decreased with the increasing proportion of PLA fiber until the proportion of PLA fiber reached 70%. When the PLA content was under 70% of the yarn, the breaking strength increased with higher proportion of PLA fiber. The breaking elongation of the blended yarn also increased with the increase of proportion of PLA fiber, as the breaking elongation of PLA fiber was larger than that of modal fiber, and more PLA fiber could contribute more to the overall breaking elongation of the blended yarn. In addition, the specific works of PLA/modal blended yarns at braking remained stable when the blending ratios were 30/70, 40/60 and 50/50, but surged when the PLA content exceeded 70%. The initial modulus of the yarn decreased with higher proportion of PLA fiber. Tan et al. ⁷⁴ also studied the stress-relaxing property of the PLA/modal blended yarns. It was observed that, when the blending ratio of PLA/modal yarn was 70/30, the yarn could have smaller inner stress and better stability.

Pre-heat-setting of PLA fabrics

A pre-heat-setting step prior to any wet processing is necessary when PLA filaments, especially friction-twist textured PLA filaments, are used in the fabric because of their high shrinkage potential. It is not a must for fabrics containing only PLA spun yarns to be pre-heat set. ⁹¹ The pre-heat-setting can improve the dimension stability and crease resistance, and avoid stiff and harsh fabric. The optimum heat-setting condition for spun and false-twist textured PLA yarns was reported as 30–45 s at 130°C. The false-twist textured PLA yarns exhibited much higher shrinkage than spun PLA yarns after heat-setting, scouring and dyeing. ⁹³

The PLA fabrics were recommended to be set to a width 6–10% less than the “off loom” fabric width, and with 6–10% overfeed. ⁹¹ For PLA knitted pique fabric, the optimal pre-heat-setting process is the same as above-mentioned to treat the fabric at 130°C for 30–45 s to achieve dimensional stability with good tensile extension and formability. ⁹⁴ A study showed that tension is the most important factor in heat-setting of PLA knitted fabrics. It was found that the PLA fabric’s dimensional change increased with the increase of tension applied, because of the stress relaxation in the subsequent wet processes. Thus, 0% tension was suggested for PLA fabric heat-setting. The study also showed that heat-setting made the PLA fabric stiffer and more resistant to shearing movement, resulting in worse drape and worse bending recovery ability. ⁹⁵ Idumah et al. ⁹⁶ , ⁹⁷ compared the effects of heat-setting time on shearing properties of knitted PLA and PET fabrics. Shear rigidity and shear hysteresis of PLA fabric were significantly changed with the increasing time of heat-setting, whereas the PET fabric had little change in shear properties. PET fabric had higher shear hysteresis than PLA fabric before heat-setting and wet processing, whereas PLA fabric had higher shear hysteresis than PET fabric after the treatments. Liu et al. ⁹⁸ investigated the effect of heat-setting on thermal properties of PLA fiber. The melting temperature of PLA fiber increased when heat-setting temperature was raised from 120 to 150°C due to the increased orientation of PLA molecule chains. Thermal degradation of PLA was significantly increased when the heat-setting temperature is higher than 160°C.

Desizing of PLA fabrics

As PLA yarns may have insufficient strength in weaving, sizes are usually applied to improve PLA yarn strength and resilience, and abrasion resistance. Desizing is a process to remove the sizes to avoid uneven dyeing and finishing. Sizes for synthetic fibers are normally composed of starches or synthetic chemical agents, such as PVA, carboxylcellulose, and polyacrylic acids. ⁹⁹ As PLA fiber is sensitive to strong alkaline and high temperature, traditional chemical desizing including alkali steeping, acid steeping and oxidative treatment will cause high strength loss in PLA fabrics. Thus, PLA yarns are commonly sized with water-soluble PVA or polyester sizes, ⁹ which can be removed by hot water. In addition, biochemical enzymes are often used to decompose the starch-based sizes on PLA fabric.

Desizing conditions are recommended as chemical bath pH less than 7.5, and applied temperature below 90°C. ⁹¹ Different desizing methods for PLA woven fabrics with PVA–acrylic size have been studied. ¹⁰⁰ The results showed that PLA fabrics had some fiber damage when treated with hydrogen peroxide with sodium hydroxide and sodium carbonate, causing 7.0% and 3.8% strength loss, respectively. Desizing with warm water, hot water, non-ionic surfactant, and anionic surfactant resulted in little fiber damage with strength loss less than 1.0%. Desizing would be completed using warm water without any chemical, which removed size effectively and retained strength. The application of ultrasonic technology in enzyme desizing of PLA/cotton blended fabric has been investigated. ¹⁰¹ , ¹⁰² The results showed that compared with conventional enzyme desizing, ultrasonic enzyme desizing achieved a higher desizing rate and whiteness, but reduced the fabric breaking strength. Furthermore, the desizing process could be combined with scouring, removing sizes during aqueous scouring.

Scouring and bleaching of PLA fabrics

The impurities of PLA fabrics are mainly removed in the scouring process. The main impurities of PLA fabric include lubricants or anti-static agents added to assist in spinning the yarn or knitting a fabric, dirt and stains acquired during manufacturing, and wax applied on spun yarns. As mentioned above, PLA fibers are sufficiently white for many purposes, but bleaching is still needed for extra whiteness or for the cotton component of PLA/cotton blended fabrics. The key point for PLA fabric pretreatment is that alkaline-oxidative conditions are deleterious to the integrity of PLA polymer at high temperature, although there is little damage at low temperature. The acidic-oxidative conditions do not significantly affect bulk or surface properties of PLA fiber even at high temperature. ¹⁰³ To minimize the risk of hydrolysis of PLA fibers, strong alkaline conditions at high temperature for scouring and bleaching should be avoided.

According to the suggestion from NatureWorks, ⁹¹ scouring for PLA fabrics can be carried out with non-ionic scouring agents at 60°C for 10 min, using soda ash and mono sodium phosphate to adjust the pH in a range of 4.5–7.5. PLA fabric was not damaged during scouring by sodium carbonate up to 5 g/l at 60°C. ¹⁰³ However, alkaline treatment with sodium hydroxide is not a suitable process for PLA fabric pretreatment, due to high strength loss at low treatment temperature. Enzyme scouring can effectively prepare PLA fabrics for dyeing. Lipase and esterase were used in pH 8 at 40°C, which improved the moisture regain and dyeing ability of PLA fabrics with little strength loss. ¹⁰⁰ , ¹⁰⁴

For PLA/cotton blended fabrics, cotton fiber requires intense scouring and bleaching conditions, which cause damage to the PLA fiber. The bleaching process has a greater effect on reducing the strength of the PLA/cotton fabric than simple scouring. ¹⁰⁵ A study showed that the molecular weight of PLA and ball burst strength of PLA/cotton fabric reduced slightly after alkaline scouring in a bath containing 2 g/l soda ash and non-ionic surfactant at 60°C for 20 min, but the bursting strength of fabric was significantly reduced after hydrogen peroxide bleaching in strong alkaline condition at 95°C for 30 min. ¹⁰⁶

The pretreatment conditions for the PLA/cotton blended fabric were optimized to avoid fiber damage. Enzyme scouring (10% owf pectinase, 60°C, 60 min) was also recommended for PLA/cotton blended fabrics which would go through deep-shade dyeing to obtain satisfactory water absorbency without causing any adverse effect on the fabric strength. However, for pale-shade fabrics, bleaching was required to achieve the correct shade on the fabrics in the dyeing process. A one-bath scouring/bleaching process with 7% owf hydrogen peroxide (100°C, 60 min) was found to be a suitable pretreatment for pale-shade PLA/cotton blended fabrics to obtain adequate absorbency and whiteness and maintain the fabric strength. ¹⁰⁷ The cold pad batch technique has been applied to hydrogen peroxide bleaching, resulting in little PLA fiber damage, whereas the exhaust technique caused holes and slit formation in the fiber structure. Sodium chlorite and sodium hypochlorite also caused less damage to the mechanical properties of PLA in the bleaching process when compared with hydrogen peroxide. ¹⁰³ , ¹⁰⁸ An ozone treatment was carried out on PLA fabric at room temperature. A 6% increase in whiteness was observed after 10 min of treatment, which caused no surface damage to the PLA fiber. ¹⁰⁹ UV/ozone irradiation could also modify the PLA hydrophilicity and dyeability. ¹¹⁰ The color depth of the PLA fabric was increased by pretreatment with UV/ozone irradiation. ¹¹¹

Overall, the temperature of PLA fabric scouring has been controlled at 60°C around its glass transition temperature, and enzyme scouring is recommended to provide a mild condition. Although bleaching needs to be carried out at high temperature, a one-bath scouring/bleaching process, and cold pad batch technique has been used to reduce fiber damage.

Disperse dye structures

The structures of disperse dyes affect the dyeability of PLA fiber, as shown in Table 2. The majority of disperse dyes are azo dyes, and they are often applied to PLA fabric. The dyeing and spectroscopic properties of azo disperse dyes with different substituents on PLA fabric have been studied and compared with those of PET fabric. Most of the dyes exhausted well into both PLA and PET fabrics (exhaustion >90%), except those having the -C₂H₄OH group. The shade of a given dye on PLA was deeper (higher K/S values), yellower (lower λ_max) and slightly brighter than that on PET, when applying the same dye concentration. ¹¹² Azo disperse dyes containing β-sulfatoethylsulfonyl group have been applied in PLA fabric dyeing, and were temporarily solubilized and applied without dispersant. The dyes showed higher color yield and lower chemical oxygen demand levels when compared with commercial disperse dyes. However, the dyeing properties of these azo disperse dyes containing β-sulfatoethylsulfonyl group and formed with different structures were different. Some should be used in acid condition and could obtain good to very good wash fastness, whereas others should be used in alkaline condition with very poor to poor wash fastness. ¹¹³ , ¹¹⁴ In addition to azo disperse dyes, aminoanthraquinone dyes with strong hydrophobicity have been applied in PLA dyeing. Good exhaustion (higher than 85%) and wash fastness between 4 and 5 were obtained. The dyes with aromatic amino or cyclo-fatty amino groups had higher light fastness than dyes with fatty amino groups, while the substitution had little effect on wash fastness. ¹¹⁵

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Table 2.

The effects of disperse dye structures and substituents on PLA dyeing

Disperse dye structures	Substituents	Results
Azo disperse dyes	β-sulfatoethylsulfonyl group	Higher color yield; Good to very good wash fastness
Aminoanthraquinone dyes		Wash fastness between 4 and 5; the substitution had little effect on wash fastness
	Aromatic amino or cyclo-fatty amino groups	Higher light fastness
Disazo pyrazole disperse		High color yield; high K/S values
Acylamide disperse dyes	Phenylazopyrazolone	Highest dye sorption of 96.8%; good wash and rub fastness of level 4–5 and 5
Sulfonamide disperse dyes	Phenylazo-β-naphthol	High affinity on PLA
	–N(C2H4OCOCH3)2, –(CO)2NC3H6OCH3, –SO2NHC6H5, –NO2, –CN(NH)C6H4, and –CH(CO)2C6H4	Strongest interactions with PLA
	–Br and –Cl	Weakest interactions with PLA

New disperse dyes have been developed as dyestuffs for PLA fiber coloration. Novel disazo pyrazole disperse dyes with two pyrazole rings were synthesized by diazotizing and coupling reactions. The absorption ability of these dyes substituted with electron-withdrawing and electron-donating groups at their o-, m- and p-positions was examined in detail. ¹¹⁶ These synthesized yellow-red dyes were applied to PLA, PET, and polyamide (PA) 6.6 fibers, resulting in quite high color yield on PLA and PET fabrics with high K/S values leading to medium to heavy depth of shades at only 2% dye concentration. PLA and PET fabrics presented higher color strength and darker shades than PA 6.6 fabrics, while the light fastness of PET fabric was better than PLA and PA 6.6 fabrics. The dyed PLA fabrics displayed better sublimation fastness than dyed PET and PA 6.6 fabrics. ¹¹⁷ Novel phenylazopyrazolone-containing acylamide disperse yellow dyes were designed and synthesized for PLA, based on the prediction that some polar groups build stronger force between dyes and PLA macromolecular chain. The dye sorptions changed depending on the types of acylamide dyes. The highest dye sorption of 96.8% was achieved by acylamide dye with two –(CH₂)₃CH₃ groups. All the dyes exhibited good build-up properties, with color strength reaching saturation at 3.0% owf, and good wash and rub fastness of level 4–5 and 5. ¹¹⁸ A series of phenylazo-β-naphthol-containing sulfonamide disperse dyes were designed and synthesized for PLA fabric dyeing. The introduction of the tertiary sulfonamide groups gave suitable water solubility to the disperse dyes, so that the dyes had high affinity on PLA, improving dye exhaustion (98.6% was achieved) and fastness properties of PLA fabric. ¹¹⁹

Fundamental research on dyeing kinetics and prediction of dye sorption on PLA fiber brought better understanding of the dyeing process. The diffusion coefficient of the disperse dyes on PLA fabric was calculated, and the kinetic dyeing model was established, which was proved by the dyeing results obtained from three different disperse dyes. This model set a theoretical basis for the dyeing process of disperse dyes on PLA fabric, which guided the optimization of parameters in PLA fabric dyeing and color matching. ¹²⁰ A linear equation was developed to predict the percentage sorption of disperse dyes with different structures onto PLA based on the interaction energies, which were calculated using molecular modeling. The prediction between various disperse dyes and PLA has been shown to agree with the actual percentage sorption. Dyes with stronger interactions had higher sorption on PLA fiber. The results showed that the substituents that form the strongest interactions with PLA are –N(C₂H₄OCOCH₃)₂, –(CO)₂NC₃H₆OCH₃, –SO₂NHC₆H₅, –NO₂, –CN(NH)C₆H₄, and –CH(CO)₂C₆H₄, and the substituents that form the weakest interactions with PLA are –Br and –Cl. ¹²¹

Another explanation for disperse dye sorption on PLA has been made through solubility parameters. Disperse dyes with solubility parameters close to that of PLA tend to give better sorption on PLA in dyeing at 100–110°C. The chemical structure of these dyes gives low cohesive energy and high molar volume. They are mainly azo dyes and other dyes that contain more –NHR, –NR₂, –NHCOR, –COR, –OR, or –COOR groups than –NO₂, –NH₂, –OH, –CN and halide groups, and the R groups are –CH₃, –(CH₂)_nCH₃, or a phenyl. Most anthraquinone dyes do not have similar solubility parameters with PLA, except these with large –(CH₂)_nCH₃ groups. ¹²²

Polymer properties

In addition to dye structures, the dye exhaustion is affected by the polymer properties of PLA (Table 3). PLA fiber generally has a moderate affinity with disperse dyes. PLA fiber with high crystallinity (50%) was found to have lower exhaustion than PLA fiber with low crystallinity (21%). The fiber with high crystallinity had a lower degree of amorphous and fewer interstices that dye molecules could penetrate, which limited the dye saturation capacities. The dyes with multiple carbonyl groups did not exhaust well on the high-crystallinity PLA, whereas their exhaustion on low-crystallinity PLA was high. ¹²³

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Table 3.

The effects of polymer properties on PLA dyeing

PLA Polymer properties	Results
High crystallinity	Lower dye exhaustion
High D-isomer PLA	Greater dye exhaustion and color strength
Addition of nanoparticles in PLA	Higher dyeability

The concentration of D-isomer in PLA affects its crystallinity, and further influences its dyeability. PLA fibers with higher D-isomer concentration have greater fiber entropy, more amorphous and less crystalline regions in the polymer, with respect to low D-isomer fibers. Therefore, high D-isomer fibers presented greater dye exhaustion and color strength for all kinds of dyes and concentrations. Beside, high D-isomer fibers could be dyed to an excellent black shade, whereas low D-isomer fibers appeared very brown. The wash fastness of both fibers was very similar. ¹²⁴

Nanoparticles were added in PLA fiber to improve its dyeability. The addition of N-Phenylaminopropyl polyhedral oligomeric silsesquioxane (AP-POSS) during melt spinning is effective for increasing the dyeability of PLA fiber regardless of the type of disperse dye. The effectiveness of AP-POSS rose with the increase of the amount added, and it was more effective in disperse dyes with low dye exhaustion. The main reason for the dyeability enhancement is the huge surface area of AP-POSS, which is essential for good sorbent. The polar (secondary amine structure) and aromatic (benzene ring) functional groups of AP-POSS also improved the attractions between PLA fiber and dye molecules. ¹²⁵ The improvement of dyeability on PLA fiber was also investigated by addition of octa (aminophenyl) polyhedral oligomeric silsesquioxane (OAP-POSS) ¹²⁶ and octaammonium polyhedral oligomeric silsesquioxane (OA-POSS) (dyeing with anionic dyes) ¹²⁷ nanoparticles during the melt spinning.

The dyeing efficiency on PLA could also be enhanced by plasma treatment and coating with polyethylene glycol, because the surface roughness and hydrophilicity of PLA fiber were increased. ¹²⁸

Dyeing conditions

The effects of dyeing temperature, time and pH on dye exhaustion, shade depth and color fastness have been studied in a great number of research works. Scheyer and Chiweshe ¹²³ studied nine disperse dyes on PLA fabric, and four of them had exhaustion values greater than 60% at 2% owf dye concentration and 100°C. The dye exhaustion percentages for disperse dyes increased dramatically, while the temperature rose from 70°C to 100°C. Maximum dye exhaustion occurred within 30 min for most dyes. The dyed PLA fabrics had acceptable wash fastness with ratings higher than 4, but low staining fastness with ratings lower than 3. Another study ¹²⁹ showed that two out of the 10 disperse dyes had exhaustion higher than 80% on PLA with 2% owf dye at 110°C and pH 5, while all 10 dyes had more than 90% exhaustion on PET at 130°C. Although PLA had lower disperse dye exhaustion than PET, it had 30% higher color yield, which was attributed to the lower reflectance of PLA. A similar apparent shade depth could be achieved on PLA as for PET. The dyed PLA fabrics had lower washing and crocking fastness, and similar light fastness compared with PET. An energy-saving dyeing process was developed in which the PLA fabric was kept in dye bath at 60°C for 10 min and then dyed at 110°C for 10 min. The same color fastness was achieved by this two-stage dyeing process when compared with a conventional dyeing process at 110°C for 30 min. ¹³⁰ In terms of the shades, Choi and colleagues ¹³¹ , ¹³² found that the yellow and blue dyes were greener and orange/red dyes were yellower in hue on PLA than on PET, while PLA has brighter shades and greater values for hue angle (h) than PET for the same dye.

Ultrasound was applied to enhance the color strength of disperse dyeing on PLA, which was attributed to dye disaggregation. The color strength of three (out of six) disperse dyes was improved by ultrasound when dyeing at 70°C, but dyeing at 80°C in the presence of ultrasound resulted in pale, dull shades because of the breakdown of the dye dispersions at this particular temperature. ¹³³

Non-aqueous dyeing was developed for disperse dyeing on PLA fiber to reduce the hydrolytic degradation of PLA fibers under the conventional aqueous dyeing conditions. PLA fabric was dyed with disperse dye in supercritical carbon dioxide (SC-CO₂) at pressure of 17 MPa and 120°C for 60 min. The breaking strength loss of SC-CO₂-dyed PLA fabric was 9.22%, which was much improved compared with the 44.66% loss of fabric dyed in aqueous condition. The dye sorption of PLA in SC-CO₂ increased faster and reached dyeing equilibrium in 10 min, whereas it took 40 min in water. The wash fastness and light fastness of SC-CO₂-dyed PLA fabric and aqueous dyed fabric were comparable. ¹³⁴ Solvent disperse dyeing using liquid paraffin without water and auxiliaries at 130°C achieved high-quality dyed PLA fabrics. The solvent dyed fabrics had color fastness comparable to aqueously dyed fabrics. After post-heat-setting, the degree of orientation increased from 80.6% to 85.6%, and the breaking strength loss decreased from 59.1% to 16.7%. The solvent dyeing did not require high-pressure equipment as for SC-CO₂ dyeing. ¹³⁵

Clearing

In the disperse dyeing process, dye molecules are deposited on PLA fibers. The unfixed dye molecules should be removed to improve color fastness and shade by a clearing step using a detergent or reductive agent. The azo disperse dyes can be decomposed by a reductive agent (sodium dithionite) in alkaline condition. Reductive clearing of dyed PET fibers is usually carried out using 2–3 g/L sodium dithionite and 1–3 g/L sodium hydroxide, at 70–80°C for 10–20 min. However, PLA is sensitive to alkaline condition, and the milder alkali sodium carbonate is used to substitute sodium hydroxide for clearing. Preferred conditions for PLA fabric were clearing with 2 g/L sodium carbonate and 2 g/L sodium dithionite at 60°C for 15 min. The effect of reductive clearing on wash fastness depends on the chemical structures of the dyes. A high level of wash fastness was achieved by alkali-sensitive disperse dyes. ¹³⁶ It could also be achieved by lower air content in the sealed reduction-clearing equipment and a greater amount of sodium dithionite in the clearing bath. ¹³⁷ Clearing of dyed PLA fabric under acid condition was also carried out. Reductive agents including zinc formaldehyde sulphoxylate, a sodium salt of a sulphinic acid derivative, and a sulphinic acid derivative were applied at pH 5.0 and pH 4.0, respectively, which obtained commercially acceptable high levels of wash, alkaline perspiration and wet rubbing fastness. ¹³⁸

ECE detergent clearing on disperse-dyed PLA fabric had comparable effectiveness in terms of fastness and color in comparison to the traditional reduction clearing with sodium carbonate and sodium dithionite. This alternative after-clearing reduced the risk of hydrolytic damage of PLA fibers and chemical consumption. ¹³⁹ Water clearing on disperse-dyed PLA fabric showed little improvement on fastness and little effect on color. ¹⁴⁰ , ¹⁴¹ Both wash and rub fastness were improved when clearing temperature rose from 50°C to 60°C, but little improvement on clearing effectiveness could be made when the temperature was further increased to above 60°C. ¹³⁹ , ¹⁴¹

Ultrasound was also applied to assist the clearing of disperse-dyed PLA fabric, resulting in an enhancement of the rub fastness of both reduction clearing and ECE detergent clearing, but with little change on color. ¹⁴⁰ , ¹⁴¹

Post-heat-setting

As a type of synthetic fiber fabric, after dyeing PLA fabrics are heat-set at 125°C for 25–45 s. Many factors should be considered when dealing with the post-heat-setting conditions including the desired width, density, smoothness, drape, hand feel and bulk of PLA fabrics. Experiments were suggested to be carried out to set the final condition for each type of fabric. ⁹¹

Dyeing properties are also affected by the post-heat-setting through thermal migration. A dyed PLA fabric was reported to exhibit 0.5–1.0 levels lower in wash fastness (particularly the stain on a nylon adjacent) compared with PET fabric, because of the higher degree of thermal migration of the disperse dyes to the surface of the PLA fiber. There was no loss of disperse dye due to sublimation after heat-setting at 130°C for 30 s. ¹⁴²

Natural dyes, vat dyes and cationic dyes

In addition to disperse dyes, the application of natural dyes, vat dyes, and cationic dyes on PLA fabrics has been investigated. Many natural dyes have been used to dye PLA fiber including curcumin, emodin, indigo, and so on. The application of natural dyes on PLA has environmental advantages. It is suggested that natural dyes that have similar solubility parameter to the fiber could have good dyeing properties. As most natural dyes are soluble in water, they can readily dye on hydrophilic textile fibers. ¹⁴³ As curcumin is hydrophobic in nature, it was found to be dyeable on PLA fiber. It was applied on PLA at 110°C, providing a bright yellow color. It had higher build-up and color yield on PLA than on PET. ¹⁴⁴ Natural emodin dye was also used on PLA fabric at 100°C and gave the fabric a brilliant yellow color and excellent soaping and crocking fastness at levels 4–5 and 5. ¹⁴⁵ Other natural yellow dyes from turmeric and cassumunar had good dyeing properties and exhibited medium-deep shade and fluorescence emission on PLA fabric. ¹⁴³ Natural indigo dye was applied on PLA fabrics by exhaustion in the presence of 5 g/L sodium dithionite and 0.2 g/L sodium hydroxide at 80°C for 60 min to obtain the maximum color strength. ¹⁴⁶ Many other natural dyes were compared including marigold petals, rhubarb rhizomes, garcinia barks, turmeric rhizomes, sappan barks and catechu barks. The results showed that only the turmeric dye performed well on PLA. ¹⁴⁷ Like disperse dyes, the addition of POSS nanoparticles enhanced the dyeability of natural dyes. ¹⁴⁸

PLA fabrics have also been dyed with vat dyes. Three indigoid vat dyes were applied, showing increasing color strength and fastness with the rise of dyeing temperature from 60°C to 110°C. The vat-dyed PLA fabrics had inferior wash fastness to dispersed-dyed PLA and vat-dyed PA fabrics, mainly because no clearing process had been carried out to remove surplus dyes on vat-dyed PLA fabrics. ¹⁴⁹ , ¹⁵⁰

Cationic dyes were used to dye modified PLA films which were irradiated by UV/O₃. The UV/O₃ irradiation increased the hydrophilicity of PLA and introduced anionic and dipolar sites to electrostatically interact with the cationic dye. The modified PLA films were successfully dyed with 2% owf cationic dye at 60°C and pH 5.5 for 1 h. ¹¹⁰

Dyeing of blended PLA fabrics

As PLA/cotton blended fabric involves hydrophobic polyester PLA fiber and hydrophilic cellulose cotton fiber, like PET/cotton blended fabric, it should be dyed with a two-bath, two-stage process in which the PLA component is dyed using disperse dyes, followed by the cotton component being dyed with reactive dyes. However, reactive dyeing for cotton fiber is normally carried out in strong alkaline condition, which may cause high hydrolysis of PLA. To avoid severe strength loss of PLA, low temperature (lower than 60°C) and low alkali concentration (less than 3 g/L) were applied in reactive dyeing for the cotton component in PLA/cotton blended fabric, while the PLA component was first dyed with disperse dyes at 110°C with pH 5 for 20 min. The fixation rate, which refers to the percentage of dyes forming covalent bonds with cotton fiber, was investigated in terms of reactive dye structures. The results showed that, in this low-temperature and low-alkali dyeing system, the reactive dyes containing monofluorotriazine and monofluorotriazine/sulphatoethylsulphone groups gave higher total fixation rate than conventional monochlorotriazine and monochlorotrizaine/sulphatoethylsulphone groups-based dyes. ¹⁵¹ A reactive-disperse dye was synthesized to dye the PLA/cotton blended fabric in one bath. Its dyeability for PLA and cotton was studied separately. The findings indicated that synthesized dye performed better on PLA than on cotton. Using this synthesized dye, the optimal dyeing condition for PLA was at pH 6 and 110°C, while for cotton it was at 80°C with high alkali concentration. ¹⁵²

PLA/silk blended fabric was dyed with Everzil ED reactive dyes in weak alkali condition using sodium bicarbonate at 80°C. The reactive dyes exhibited a good dyeability on the PLA/silk blended fabric, showing quick fixation, good build-up properties and wash and rub fastness between 4 to 5. ⁸⁹

For PLA/wool blends, the wool fibers are usually dyed with acid dyes in acidic condition, which conforms with the sensitivity of PLA to alkali treatments. ⁵

Recently, PLA has been spun with PHBV and PBS into textile fibers. The dyeing of PLA/PHBV fabric was optimized using C.I. Disperse Orange 30, Red 74, and Blue 79 at 100°C with pH 5 for 10 min. Satisfactory build-up properties could be obtained using 1% owf dye concentration. The color fastness to laundry and rubbing of the dyed PLA/PHBV fabrics reached the commercially acceptable levels of 5 or 4–5. ¹⁵³ The dyeability of PLA/PBS fabric was also investigated, which provided evidence that the addition of PBS to PLA improved the dye exhaustion and shade depth of PLA, with good wash and rubbing fastness rating 5 or 4–5. ⁵⁵