Introduction

3D Printing, also called as additive manufacturing (AM), has gained widely acceptance and utilization in many fields. It possesses the ability to manufacture parts with complex inner and outer structure without using any mold and fixture1. 3D printing can reduce the material wastage, lighten the product weight and shorten the production cycle1. Fused deposition modeling (FDM) or Fused Filament Fabrication (FFF) is one of the most popular 3D printing technologies for processing polymer and its composites through simple electrical-heating and then extruding with a nozzle layer by layer.

Considering of the ever-growing globe energy crisis and environmental pollution, the renewable and biodegradable polymers have gotten more and more attention and utilization2,3. Polylactic acid (PLA) is the most prominent within the biodegradable polymers1. However, the brittleness and long-term degradation of PLA have limited its widespread applications4.

Natural fibers possess lots of merits over synthetic fibers, such as: low cost, light weight, non-toxic, recyclability and biodegradability5. Researchers have done valuable explorations about 3D printing of PLA composites reinforced with natural short or continuous fibers. Recent review papers summarized the natural short fibers being applied to enhance PLA, including nutshells (walnut, peanut, etc.), leaves (sisal, banana, etc.), basts (jute, ramie, etc.), cane (wood, bamboo, etc.) and seeds (kapok, coir, etc.)5,6,7. Natural short fibers enhanced the mechanical properties of polymers and their AM processing might be easier to carry out. However, the improvement extent of mechanical properties of polymers was limited by the ratio and size of fibers in polymer matrix, as well as fiber-matrix interphase adhesion8. Therefore, to achieve high performance polymer-fiber composites, continuous fibers might be the best candidate.

In 2016, Matsuzaki et al.9 first applied continuous jute fiber to enhance PLA with 3D printing processing, and improved the tensile strength and modulus of PLA from 3.25 GPa and 42.6 MPa to 5.11 GPa and 57.1 MPa, respectively. Since then, a growing number of researchers focused on 3D printing of natural continuous fiber-reinforced composites. Among these studies, 3D printed PLA/continuous flax fiber (PLA/CFF) composites got the most attention. Le Duigou et al.10 and Maximilian et al.11 studied the effect of the processing parameters on tensile properties. Le Duigou et al.10 found that the 3D printed CFF/PLA showed the similar tensile properties to the 3D printed continuous glass fiber/Polyamid composites. Zhang et al.12 investigated the flexural properties respectively. Long et al.13 and Kuschmitz et al.14 explored the overall mechanical properties, and improved the tensile strength and modulus of PLA by 325% and 570%. Additionally, other natural continuous fibers, including ramie1,15, hemp, pineapple leaf16, kenaf17, oil palm18 fibers also improved the mechanical properties of PLA.

Cotton is a plentiful natural fiber and the most favorite material in apparel due to its low price, softness, perspiration absorption and being comfortable in wear19. In the plastics industry, a small amount of cotton fiber has been used to reinforce polymers. Mu¨ller and Krobjilowski20 reviewed that natural fibers like ramie, hemp, flax, jute or cotton were well suited for reinforcing polymers. Foulk et al.21 compressed cotton/flax/high-density polyethylene (HDPE) composites, and found that with the increase of cotton ratio, the strength, density, and elongation of composites generally increased. Silva et al.22 prepared polyurethane (PU)/cotton fiber composites by using casting and found that composites exhibited a tensile stiffness and strength twice of pure PU. Zhao and Wu23 applied recycled polyester-cotton fiber (long and short) to composite with HDPE by using internal mixing and compression molding, and found that the long fiber performed 56.4%, 36.4% and 87.5% higher than the short one in terms of tensile, flexural and impact strength. Chen et al.24 also used polyester-cotton fiber on polyethyleneimine. Bodur et al.25 found that 30% of cotton improved the tensile and flexural properties of low-density polyethylene (LDPE). Tasgin et al.26 used cotton, sisal, coir and wool fibers to enhance epoxy, and concluded that the cotton composite obtained the highest tensile strength (52.81 MPa). Moreover, cotton fiber has also been used in PLA. De Macedo et al. 27 enhanced PLA with cotton (10 wt%) and starch (3 wt%) and concluded that higher ratio of natural cotton fibers resulted in a better overall performance. Oliveira et al.28 used cotton fibers to fill PLA and found the composites exhibited a reduced environmental impact when compared to PLA. The above literatures showed that cotton fiber performed better for polymer enhancement than other natural fibers, such as flax, sisal, wood and coir fiber. In term of 3D printing of PLA/continuous cotton fiber, Vishal et al.29 adopted in-nozzle impregnation method to 3D print CCT/PLA composites and revealed the effect of layer widths (0.5, 0.6, 0.7, and 0.8 mm) on the mechanical properties of the composites. They found that the tensile and flexural strength was improved by 1.2 times and 3.1 times respectively at 0.5 mm layer width when compared to that of PLA. They did not talk about the CCT/PLA composite filament and the impact properties of the 3D print CCT/PLA composites.

Therefore, this study aims to enhance PLA with continuous cotton thread (CCT) and prepare the composites with 3D printing technology. The main contents are: alkali treatment of CCT; introducing the lab-made device for PLA/CCT filament preparing; investigating effects of tension device, dragging device; diameter of CCT and immersion temperature on the property of PLA/CCT filament; exploring how the processing parameters (nozzle temperature and printing speed) influence the flexural property of 3D printed PLA/CCT composites; and finally investigating the impact property with the optimal parameters.

Experiments and methods

Materials

The used Poly (lactic acid) (PLA) powder, with trade name 2002D, was manufactured by NatureWorks LLC (USA) with density of 1.24 g/cm3. Continuous cotton thread (CCT) purchased from Liyang Kailai Thread Company with diameter of 0.6, 0.8 and 1.0 mm, shown in Fig. 1a. The CCT was soaked in an aqueous sodium hydroxide with 5 wt% concentration at room temperature for 2 h (Fig. 1b) and then washed with deionized water until pH7. The alkali treatment is able to remove the hemicellulose, pectin, and dust from the surface and to rough the surface of cotton, thus enhance the interfacial compatibility between CCT and PLA. The treated CCT was shown in Fig. 1c and named as TCCT.

Fig. 1
figure 1

Cotton thread treating process.

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Device design for preparing of PLA/TCCT and PLA/CCT filaments

Figure 2 illustrates the device and processing procedure of PLA/CCT and PLA/TCCT filaments. The device consists of the main equipment and auxiliary system. In Fig. 2a, the PLA powder passed through the hopper and came into barrel, and then moved forward with the rotation of the single screw driven by a motor, and gradually melted under the heat from heaters around the barrel, and finally entered into the die. Meanwhile, the CCT or TCCT went into the die from the direction vertical to the screw axis, and then immersed into the PLA melts for impregnation. Finally, the CCT or TCCT went out from the die with immersed PLA (Fig. 2b).

Fig. 2
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Device and processing of the PLA/TCCT and PLA/CCT filaments.

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For ensuring the filaments smoothly and straightly pass through the die, an auxiliary system was designed and shown in Fig. 2c. A series of rollers with position-adjusting function (tensioning equipment) were provided to make sure the thread keeping in line. A dragger machine (Fig. 2d) was adopted to make certain a line movement of the filaments, following behind the cooling process. Therefore, PLA/CCT and PLA/TCCT filaments (Fig. 2e) were obtained.

Additionally, a series experiments were carried out to investigate the effect of processing parameters on the quality of filaments, including temperature (160–168 ℃, 163–172 ℃, 167–180 ℃, 170–178 ℃, and 170–185 ℃), dragging speed (10, 12, 14 and 16 r/min), diameter of the CCT (0.6, 0.8 and 1.0 mm), tensioning equipment (with or without) and alkali treatment.

3D printing of PLA/TCCT and PLA/CCT filaments without cutting off during printing

The PLA/TCCT and PLA/CCT filaments were submitted to a single-nozzle 3D printer (Allct Yinke, Wuhan, China) to prepare these samples. According to the GBT 9341–2008 standard, rectangular-shaped specimens with a dimension of 80 mm × 10 mm × 4 mm were designed for the flexural test (Fig. 3a). Based on the GB/T 229–1994 standard, samples in a dimension of 55 mm × 10 mm × 10 mm were manufactured for the impact experiment.

Fig. 3
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Route planning of the flexural samples.

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Traditionally, those samples were 3D printed with a path created automatically by a slicing program of the 3D printer, shown in Fig. 3b. Every layer of the automatic path consists of outline and inner section. The former was printed along the route A-B-C-D and a-b-c-d. The later was divided into three zones and finished according to the sequence of zone 1, 2 and 3. The nozzle finished the zone 1 with the path 1–2-3–4-…-5–6, and then jumped to zone 2 and printed along the route 7–8-…-9–10, and finally skipped to print zone 3 with the path 11–12-…-13–14. After finishing one layer, the platform descended a layer height, and then the nozzle began to print the next layer. This program was repeated layer by layer until the sample was finished.

However, the continuous filament had to be cut off when the nozzle jumped from one zone to another. In order to realize the non-cutting off function, G-codes produced by 3D printer automatically were reedited. As shown in Fig. 3c, d, the nozzle moved along the route A-B-C-D-E to finish one layer until the sample was finished (Fig. 3e). The flexural and impact samples were designed to be printed three layers and six layers, respectively, and six lines for each layer. Therefore, the volume percentage of samples (\({P}_{CCT})\) were calculated with Eq. (1).

$${\text{P}}_{CCT}=\frac{3.14\times {{(CCT}_{d})}^{2}\times {CCT}_{n}}{4\times {S}_{w}\times {S}_{t}}$$
(1)

where \({CCT}_{d}{ and CCT}_{d}\) is the diameter and line number of CCT in the PLA/CCT composites; \({S}_{w} and {S}_{t}\) denotes the width and thickness of the PLA/CCT composites. Therefore, the ratio of CCT in the PLA/CCT composites was calculated in Table 1.

Table 1 The volume ratio of CCT in the PLA/CCT composites.
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The Taguchi experiment possessed significant advantages in the number of experiment, high efficiency, and reliability over other methods, for example gradient decent algorithm. Therefore, this method were taken to investigate the relationship between the 3D printing parameters and flexural properties of the 3D printed PLA/CCT samples by using L9 (33) array. The CCT diameter (D, mm), nozzle temperature (T, ℃) and printing speed (V, mm/s) were chosen as the studied factors, and were assigned to three levels, respectively, presented in Table 2. The flexural strength and strain at the yield point were selected as the outputs. According to the average analysis (ANOVA), the experimental results were processed. Range analysis was performed with the outputs for obtaining optimal parameters. Kij denotes the average of experimental results, and Rij denotes the range and can be obtained with Eq. (2):

$${R}_{ij}=\text{max}({K}_{ij})-\text{min}({K}_{ij)} i, j=1, 2, 3$$
(2)

where i and j denotes the level and sequence of three factors, respectively.

Table 2 Arrangement of the levels to the factors.
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Characterization techniques

In order to investigate the tensile tests of PLA/TCC and PLA/TCCT filaments, a fixture was home-designed and 3D printed by using flexible TPU filament. The fixture consists four parts with the same structure, each of which was L-shape with an arc-shape groove on one side for holding filaments (Fig. 4b, c) and another side for fitting with the electronic universal testing machine (UTM5105, SUNS, China)machine (Fig. 4a). Then tensile tests were carried out according to GB/T 228.1-2010 standard. The length and diameter of each filament section equaled to 124 mm and 1.75 mm, respectively. The tensile speed was set as 2 mm/min.

Fig. 4
figure 4

Flexible fixture used for the tensile testing of the PLA/TCCT and PLA/CCT filaments: (a) fixed on the machine clamps, (b,c) structure of the home-made fixture.

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The flexural properties of the 3D printed samples were obtained by using a universal testing machine (UTM5105, SUNS, China) with a rate of 5 mm/min, based on the standard GB/T 1449-2005. According to the standard GB/T 229-199, the impact properties were investigated with a pendulum impact testing machine (PTM2452, SUNS, China), and an impact notch with 2 mm depth were carved by using a notching machine (QTM3000, SUNS, China).

The morphology of the fractural section of filaments was characterized with a scanning electron microscope (SEM, VEGA3, America). The samples were gold coated prior to the testing with an ion sputtering instrument.

The thermal properties of PLA in various states were investigated by using a simultaneous thermal analyzer (WCT-121, Beiguang Hongyuan, China). The testing was carried out under a nitrogen flow rate of 20 ML/min, with a program as the following: heating from the ambient temperature to 500℃ with a heating rate of 10 ℃/min, and then holding for 5 min, finally cooling to room temperature. With the testing, the thermogravimetry (TG) data, and the temperature at the glass transition (Tg), crystallization (Tc) and melting point (Tm) were obtained, respectively.

Results and discussions of properties of the PLA/CCT filaments

Effects of dragging speed on quality of the PLA/CCT filaments

During the processing of PLA/CCT filament, the impregnation and coating of CCT were finished in the cavity of the extrusion die. Four dragging speeds were investigated to obtain a suitable diameter, shown in Fig. 5. With the decreasing of the dragging speed from 16 to 10 r/min, the diameter of the PLA/CCT filament increased from 1.53 to 1.81 mm. This result is similar to the study of Fan et al.30, where the filament diameter increased from 0.3 to 1.2 mm as the straggling speed decreased from 25 to 5 r/min. In this study, the PLA/CCT filament was produced for the 3D printer without any refit, so that which should possesses the standard diameter, 1.75 mm. In Fig. 4, only when the dragging speed was set as 12 r/min, the filament’s diameter equaled to 1.75 mm.

Fig. 5
figure 5

Diameter of the PLA/CCT filament with various dragging speed: (a) 16 r/min, (b) 14 r/min, (c) 12 r/min and (d) 10 r/min.

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Effect of tensioning equipment on quality of the PLA/CCT filaments

In order to make sure the continuous fiber straightly went through the extrusion die, a series rollers (tensioning equipment) was adopted. As shown in Fig. 6a, the cotton thread inclined to the lower direction due to the gravity, which would affect the quality of the 3D printed part. Under the function of the tensioning equipment, the cotton threads almost remained at the center of the filament (Fig. 6b). The center position of the thread in filament is beneficial to the homogeneity of mechanical property of the 3D printed part.

Fig. 6
figure 6

CCT distribution in filament: (a) without and (b) using tensioning equipment.

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Effects of temperature on tensile properties of PLA/CCT filaments

Figure 7 and Table 3 show the tensile properties of PLA/CCT filaments under various temperatures of extrusion die (Tdie, 160–170 °C) and barrel temperature (Tbarrel, 168–185 °C). All the tensile procedure included two or three stages: elastic deformation of PLA, unsticking between CCT and PLA, deformation of CCT. Here the sample 5 was taken to illustrate the tensile process.

Fig. 7
figure 7

Relationship between tensile strength and strain of PLA/CCT filaments affected by temperature.

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Table 3 Tensile properties of filament affected by immersion temperature.
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During the immersion procedure, PLA melts came into CCT bundle firstly, and then coated on and immersed in CCT. Therefore, the PLA coats were stretched together with the PLA immersed CCT under tensile strength during the elastic stage. However, the higher brittleness of PLA is much larger than CCT, resulting in the fracture of PLA coats (point A). It can be seen that the strength at point A reached to the climax value (47.09 MPa) when the Tdie and Tbarrel was 163 and 172 °C respectively, indicating that the liquidity of PLA was the optimal at this temperature combination. The tensile strain at point A almost kept level around 1.9% when the Tbarrel was lower than 180 °C. This phenomenon indicated that the interfacial compatibility between the coated PLA and CCT was not well. As the temperature climbed up, the strength and strain at point A declined to 18.1 MPa and 1.03% rapidly, indicating that the PLA melts took excess heat, and degraded thermally.

With the tensile force continuously increase, the tensile came into the second stage (process II), unsticking between CCT and PLA. The values at point B showed that the tensile strength and strain performed a small rangeability when the Tbarrel was lower than 185 °C. This indicated that the immersion degree of PLA on CCT was weak and shallow. When the Tdie and Tbarrel was 163 and 172 °C respectively, the tensile strength and strain climbed dramatically to 27.66 MPa and 18.46%, indicating a perfect immersion of CCT. It was worth to notice that the tensile strength went up-down in small range, creating a zigzag curve at the process II. This phenomenon indicated that the unsticking between CCT and PLA consumed tensile energy continuously, thus improving the tensile strain. Finally, the tensile came to the third process, where the CCT was stretched until breaking (point B).

Effects of diameter of CCT on tensile properties of PLA/CCT filaments

Table 4 and Fig. 8 demonstrate how the diameter of CCT affected the tensile property of PLA/CCT filaments. It can be seen that PLA processed the lowest yield strength (43.66 MPa, point A) and the highest fractural tensile strength (38.61 MPa, point B) and strain (8.39%, point B). With the increase of the diameter of CCT, the yield tensile strength (point A) increased steadily to 52.81 MPa at diameter of 1.0 mm, indicating that the addition of CCT improved the resistance to tension of PLA. The yield tensile strain at point A performed a similar tendency to the tensile strength. It was interesting that the fractural strength and strain of PLA/CCT filaments went to the valley (6.84 MPa, 6.82%) at 0.8 mm diameter of CCT (point B). This probably contribute to the ratio between PLA and CCT, either the PLA or the CCT took the main function during the tensile process II for sample 2 and sample 4. Here the interfacial compatibility between PLA and CCT was not well, resulting in the low performance at point B when PLA and CCT took function half-half.

Table 4 Tensile properties of filament affected by diameter.
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Fig. 8
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Relationship between tensile strength and strain of PLA/CCT filaments affected by diameter of filaments.

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Effects of alkali treatment of CCT on tensile properties of filaments

Figure 9a and Table 5 show the tensile properties of PLA/CCT filaments affected by alkali treatment. Sample 1 and 2 demonstrated a similar performance to sample in Figs. 7 and 8, ascribing to that the CCT was not well surface compatible with PLA. Under the function of NaOH, the tensile properties of PLA/TCCT filaments performed excellent toughness (sample 3, 4 and 5), indicating a dramatic interfacial compatibility. This result can be proved from the fracture morphology in Fig. 10, the CCT (Fig. 10a) separated obviously with PLA, but TCCT mixed totally with PLA (Fig. 10b). Furthermore, Fig. 11 shows that the incoming of CCT just slightly reduced the contact angle of PLA from 73.8 to 66.9°. While PLA/TCCT performed marvelously with a contact angle of 36.9°, indicating an excellent wettability and interfacial compatibility between PLA and TCCT.

Fig. 9
figure 9

Relationship between tensile strength and strain of PLA/CCT filaments affected by alkali treatment: (a) stress–strain curves, (b) end of stretch of sample 4, (c) end of stretch of sample 5 and (d) comparison of sample 5 between before and after stretch.

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Table 5 Tensile properties of filament affected by alkali treatment.
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Fig. 10
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Fractural morphology of composite filament in Table 4: (a) sample 1 and (b) sample 3.

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Fig. 11
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Contact angles of PLA, PLA/CCT and PLA/TCCT.

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There was only once fracture for the treated samples, indicating that the PLA and CCT broke at the same time (sample 3 and 4, Fig. 9b). The yield tensile strength and strain (point A) of composite filaments with treated CCT in diameter 0.6 mm and 0.8 mm increased slightly from 44.26 MPa, 1.88% (sample 1) and 46.63 MPa, 2.18% (sample 2) to 52.78 MPa, 3.14% (sample 3) and 52.99 MPa, 3.16% (sample 4). Meanwhile, the fractural tensile strength and strain (point B) of composite filament rocketed up to 63.75 MPa, 19.99% and 66.16 MPa, 20.53%, respectively. Sample 5 performed a marvelous toughness, which kept unbroken state after being stretched, shown in Fig. 9c, d.

These results indicated that the treated CCT can be used to enhance the tensile properties of polymers, and the composite filaments with excellent tensile properties are suitable to 3D printing.

Results and discussions of properties of the 3D printed PLA/CCT

Mechanical properties of the 3D printed PLA/CCT composites

(1) Taguchi experiment of the 3D printed PLA/CCT composites

In order to investigate the diameter of CCT and the printing parameters on the flexural properties, Taguchi experiment were carried out. It can be seen from Table 6 and Fig. 12 that the printing speed affected the flexural strength mostly, following by the diameter of CCT. Only the changing of printing speed affected the flexural strain remarkably.

Table 6 Range analysis of the flexural strength and strain at the yield point.
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Fig. 12
figure 12

The 3D printed fractural samples of PLA/CCT composite in Taguchi experiment: (a-i), sample 1–9 in L9 (33) array after being bent, and pure PLA sample.

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The yield flexural strength and strain varied between 115.80 and 135.02 MPa and 4.34–4.39% respectively at speed in ranges of 6–10 mm/s. When the printing speed was too fast, the molecular chain of PLA had not enough time to unfold and bonded between lines, resulting in cracks (blue rectangle) after being bent. When the PLA/CCT changed direction at corner, the CCT was buckled (Fig. 12b, c, green rectangle) or deviated from the planned track (Fig. 12e, g, green rectangle). The faults were worse at bigger printing speed (Fig. 12b, c, g). This result was similar to study of continuous carbon fibers reinforced EP polymer31 and the continuous glass fiber reinforced PLA32.

The nozzle temperature (200–220 °C) mainly influenced yield flexural strength, and created the max strength (131.16 MPa) at 210 °C and fewer flaw in sample e and h (Fig. 12e, h).The higher temperature was beneficial to the movement of molecular chain and interface bonding of two adjacent deposition lines33. However, when the temperature rose up to 220 °C, the PLA melts overflowed and created flash to the sample (Fig. 12c, f, blue arrow), which strongly lowered the appearance quality of samples. In literatures about tensile property investigation of continuous glass fiber reinforced PLA, Wang et al.32 found that 210/220 °C of printing temperature was moderate, whereas Chen et al. concluded that the printing temperature was directly proportional to the flexural strength and modulus34.

Increasing the diameter of CCT (0.6–1.0 mm) obviously increased the ratio of CCT (12.72–35.33%, Table 2) in composites, resulting in enhanced flexural strength of PLA/CCT composites. When being compared to PLA, CCT possessed higher strength along length. Therefore, when the diameter of CCT increased from 0.6 to 1.0 mm, the flexural strength was improved from 117.54 to 135.49 MPa. That can be used to explain why the sample a, b, c, d was half-fractured under bending force (Fig. 12a–d, black rectangle).

Take a comprehensive consideration of the three factors, the flexural strength of samples (Fig. 12h) reached to the peak at 1.0 mm of diameter, 210 °C of nozzle temperature and 6 mm/s of printing speed. The sample h (Fig. 12h) demonstrated perfect quality almost with no crack. Moreover, nozzle temperature and printing speed were strongly interacted each other. Increasing the temperature took a similar function to reducing the speed, because both extend the heating time of polymer. Therefore, sample f (Fig. 12i) kept unbroken due to its printing condition (220 °C of nozzle temperature and 6 mm/s of printing speed). Whereas sample g (Fig. 12g) cracked at 200 °C of nozzle temperature and 10 mm/s of printing speed), in spite of 1.0 mm of diameter of CCT.

In order to compare the flexural properties of pure PLA and PLA/CCT composites, PLA samples were prepared with the optimal parameters (210 °C of nozzle temperature and 6 mm/s of printing speed). It can be seen that PLA sample was totally broken (Fig. 12 PLA). The testing result showed that the flexural strength and strain of PLA was 100.85 MPa and 1.6%, which obviously was lower than that of composites. Therefore, the inputting of CCT in PLA composites enhanced the flexural properties of PLA.

(2) Impact resistance of the 3D printed PLA/CCT composites

The impact resistance experiments of the 3D printed PLA/CCT composites and pure PLA were carried out to investigate the toughness property. As shown in Fig. 13, the impact strength of PLA was 2.75 J/cm2, indicating that the PLA is brittle material. This can be proved by its fracture picture (Fig. 14a, b). For improving the toughness of PLA, CCT with diameter of 0.6, 0.8 and 1.0 mm was adopted (Fig. 14c). The inputting of CCT dramatically enhanced the impact strength of PLA to 18.87 J/cm2 (1.0 mm diameter of CCT). The fracture pictures (Fig. 14d–f) showed that all the composite samples were only broken and did not fracture. The broken degree reduced with the diameter of CCT increased. The fractural depth of composites with 0.6, 0.8 and 1.0 mm diameter of CCT was 3/4, 1/2 and 1/4 of the thickness, respectively (Fig. 14d–f).

Fig. 13
figure 13

Impact property of the 3D printed PLA/CCT composites.

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Fig. 14
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Impact samples of the 3D printed PLA and PLA/CCT composites.

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(3) Mechanical properties comparison of the 3D printed continuous fiber/PLA composites

The mechanical properties of CCT reinforced PLA obtained in this work were compared to those of other continuous fiber/PLA composites prepared with 3D printing from other literatures, shown in Table 7. In term of flexural strength, it can be seen that the continuous carbon fiber ranked to the first in improving the flexural strength of PLA (335 MPa at 27 vol% of carbon fiber) 33, following by the flax fiber (197 MPa at 44 vol% of flax) 35. The continuous cotton fiber in our work performed better than continuous Kevlar36 and ramie1 fibers in other works. When speaking of the impact strength, the CCT performed excellently and created a significant improvement for PLA to 18.87 J/cm2, by 5.86 times. At the similar volume percentage of continuous fiber, the impact strength of CCT/PLA was 8.09 J/cm2, which was more than twice times than that of the carbon/PLA composites (3.45 J/cm2) 33.

Table 7 Mechanical properties comparison of 3D printed continuous fiber/PLA composites.
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Thermal properties PLA in various states

As the matrix material, the thermal stability of PLA is important. Once PLA degrade, the mechanical strength of PLA will be affected. PLA is a kind of low stability material38 with a lower glass transition temperature (Tg), about 69 °C (Fig. 15a). In order to investigate if PLA material degraded during the immersing CCT and the printing process, thermal properties of PLA in various states (PLA powder, PLA from the PLA/CCT composite filaments and PLA from the 3D printed PLA/CCT composites) were characterized, shown in Fig. 15. It can be seen that the thermal values (Tg, Tm and Tc) changed in small ranges (Fig. 15a), and the TG curves of all samples (Fig. 15b) kept only one degrade step, indicating the molecular structure of PLA in three states stayed the same.

Fig. 15
figure 15

Thermal properties of PLA: (a) DSC, (b) TG.

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During the immersing process, PLA in form of powder was melted for the first time to immerse CCT and to prepare the PLA/CCT composite filaments. When the filaments were provided for 3D printing, PLA from the PLA/CCT composite filaments was melted for the second time in nozzle. The PLA from PLA/CCT composite filaments possessed the highest Tc when compared to the other two, indicating that the molecular chain of PLA rearranged more regularly (Fig. 15a). The TG curve of PLA from the PLA/CCT composite filaments and PLA from the 3D printed PLA/CCT composites slightly moved to the left of that of PLA powder, showing a negative increase in thermal stability. The mass retaining of PLA powder was smaller than that of the other two, contributing to the less regular arrangement.

Conclusions

This paper investigated the properties of the prepeg continuous cotton fiber (CCT)/PLA filaments and the mechanical properties of 3D printed Poly (lactic acid) (PLA) reinforced with natural fiber CCT. Firstly, a set of device equipped with a tension and dragging mechanism was home-made to immerse CCT with PLA melts. The dragging speed was optimized to 12 r/min in order to obtain the standard 3D printed diameter (1.75 mm). The immersion temperature of the die and barrel was optimized to 163 °C and 172 °C respectively in term of the tensile strength of the PLA/CCT filament. Increasing of the CCT diameter improved the tensile properties of PLA/CCT filament by adding the volume ratio of CCT in PLA/CCT composites. The fractural morphology and contact angle showed that the alkali treatment enhanced the wettability and interfacial compatibility between PLA and CCT, and further improved the tensile properties of PLA/CCT filament. Secondly, in order to keep the CCT uncutting during the PLA/CCT composites 3D Printing, a route was planned. With this route, the flexural and impact properties of 3D printed PLA/CCT composites were investigated. The tensile strength and strain of PLA were improved by using Taugchi experiment to 135 MPa and 4.39% with 3D printing parameters (210℃ of nozzle temperature and 6 mm/s of printing speed). The impact strength of PLA was enhanced dramatically from 2.75 to 18.87 J/mm2 by 5.86 times. When being compared to other literatures, although the PLA/CCT obtained inferior flexural strength than that of carbon/PLA and flax/PLA, it performed better than ramie/PLA and Kevlar/PLA. Even more, the impact strength in our work was much higher than that of carbon/PLA. Finally, the thermal experiments showed that PLA did not degrade during the different processing and its molecular structure stayed the same at various states. The results showed that the CCT can be used to enhance PLA and the PLA/CCT composites have the potential to be applied in fields of 3D printing of light-weight parts.