Multifilament yarns of polyoxymethylene/poly(lactic acid) blends produced by a melt-spinning method

Abstract

Multifilament yarns of polyoxymethylene (POM) and poly(lactic acid) (PLA) blends were prepared using a melt-spinning method. The ratios of the POM/PLA fibers studied were 10/90, 30/70, 50/50, 70/30, and 10/90 by weight compared with that of the pure polymers. The extrusion of the dry blend polymers was carried out using a single-screw extruder at 180–210℃ with a winding speed of 800 m/min. The thermal and morphological analysis of the multifilament yarn confirmed the phase separation of the two polymers. However, the mechanical properties of the blends showed excellent elongation at break, which confirmed the good distribution of PLA in the POM matrix. POM/PLA with weight ratios of 70/30 and 90/10 showed high tenacity. The elongation of the POM/PLA blends shows excellent results, which is unusual for polymer blends with phase separation.

Keywords

polyoxymethylene poly(lactic acid)multifilament yarn melt-spinning process

The extensive use of plastic products has increased tremendously over many years. As a result, an enormous amount of plastic waste is being continuously disposed of in the environment.^1,2 It has been reported that great amounts of plastic waste have been disposed of in the sea.^3,4 Marine pollution has an obvious harmful impact on fish and sea animals due to the imbalance between the durability and degradation of plastic. Therefore, the amount of plastic waste accumulated in the sea is increasing rapidly.⁵ Furthermore, the conventional plastics used, including polyethylene (PE), polypropylene (PP), polystyrene (PS), polyester, and nylon, are nondegradable materials, and the recent problem of microplastics is the most important issue impacting a sustainable environment.^6,7 The most prominent solution for these problems is to use biodegradable plastics instead of petroleum-based polymers. Postconsumer bioplastics will not be a source of environmental pollution because they degrade rapidly. The final degraded materials are water and carbon dioxide.

The most widely used biodegradable plastic applied in industrial manufacturing is poly(lactic acid) (PLA). PLA is synthesized from renewable resources such as cassava starch, corn, and sugar cane. PLA has good mechanical properties, but some of its products have limited applications and need to be improved.^8–10 Copolymers of lactic acid, ethylene glycol, depsipeptide, and ɛ-caprolactone have been synthesized.^11–14 Polymer blends of PLA with biodegradable and nondegradable polymers have been reported.^15–18

Fishing nets (or seines) and strings have been reported to be one of the worst problems of marine waste.¹⁹ These materials are made from PP or nylon. Because polymers have a lower specific gravity than sea water, their wastes float on the sea surface and are harmful to fish and sea animals. It has been reported that plastics from marine litter have proved lethal to sea turtles in Australia. The turtles had swallowed balloons, plastic bags, nylon rope, Styrofoam, and thongs, among other things, possibly mistaking them for jellyfish.²⁰ Materials that are effective in replacing PP and nylon must consist of fibers that have a higher specific gravity than sea water and be friendly to the environment.

Polyoxymethylene (POM) is an engineering thermoplastic polymer with excellent chemical resistance and mechanical properties. POM has been widely used for automotive and electronic appliances.^21–25 Blends of POM and PLA have been reported to have high flexibility and toughness, although the two polymers are not compatible.²⁶ Another study has reported the excellent elongation of POM/PLA blends appropriate for producing injection products.²⁷ For extension to fiber fabrication, the modification of POM to produce a copolymer consisting of oxymethylene and oxyethylene monomers has been developed.^28,29 The POM copolymer/PLA fiber blends exhibited an excellent elongation property. However, the application of the POM/PLA blends to marine fishing nets has not been reported.

In this research, we studied the development of the melt-spinning process of POM/PLA blends to produce multifilament yarns for fishing net production. With the advantage of high density of POM and PLA, waste of marine fishing nets made from the polymer blends will not float in the sea, meaning they are not harmful to marine animals.

Experimental details

Materials

PLA grades 6100D and TP4000 were purchased from NatureWorks LLC, USA, and Unitika Co., Ltd, Japan, respectively. POM grade V20-HE (copolymer of oxymethylene and oxyethylene) was supplied by Mitsubishi Gas Chemicals, Japan. The tenacity and percent elongation at break of neat POM and neat PLA are shown in Table 1.

Table 1.

Tenacity and percent elongation at break of neat polyoxymethylene (POM) and poly(lactic acid) (PLA)

Polymer	Code	Tenacity (gf/d)	Elongation at break (%)
POM	V20-HE	10.90	1416
PLA	6100D	6.06	342
PLA	TP4000	6.48	558

Preparation of POM/PLA blends

POM (V20-HE) and PLA (6100D and TP4000) were dried at 80℃ for 12 h. The polymers were subjected to a melt-spinning process using a single-screw extruder with a 24-hole spinneret having a hole diameter of 0.32 mm. The process temperatures were set up at 180℃, 190℃, 200℃ (extruder zone), 210℃ (connecter), and 210℃ (die) with a screw speed of 8 rpm. The winding speed was controlled for 800 m/min. The ratios of the POM/PLA blends were 0/100, 10/90, 30/70, 50/50, 70/30, 90/10, and 100/0 by weight.

Thermal property analysis

The POM/PLA fibers produced were subjected to thermal property analysis using the differential scanning calorimetry (DSC) method at a heating rate of 10℃/min from 20℃ to 250℃.

Mechanical property analysis

A tensile test (ASTM D 3822 01) was performed using a 50 kN gate length of 250 mm and a pulling speed of 50 mm/min. The samples were prepared by arranging the fiber samples with a C-clamp with a length of 2.3 cm.

Morphological analysis

The morphology of the fiber samples was analyzed by a scanning electron microscope (JEOL, JSM-S410 LV, Japan) at 15 kV using gold as an electrical conductor and magnifications of 1000 times.

X-ray diffraction analysis

X-ray diffraction (XRD) analysis of POM, PLA, and POM/PLA (10/90 and 70/30) was performed on an X-ray diffractometer (PW3040/60 Panalytical X'Pert Pro) with Cu Kα radiation (λ = 1.54Å) generated at a voltage of 40 kV and current of 30 mA. Scanning was done at the 2θ angle of 5–80° with a scan step size and time per step of 0.01° and 0.5 s, respectively.

Results and discussion

The specific gravities of POM (V20-HE), PLA (6100D), and PLA (TP4000) are 1.40, 1.28, and 1.25 g/cm³, respectively. In addition, the POM/PLA blend fibers will have higher density than sea water (1.03–1.10 g/cm³). Therefore, the application of POM/PLA polymers to fishing nets will have the advantages of being environmentally friendly materials and they will not be dangerous to fish and sea animals because the wastes of the fishing nets will sink to the bottom of the sea, followed by the degradation of PLA. POM will gradually degrade, and the microscopic amount of degraded product will be slowly released. The degradation of POM to gaseous formaldehyde must be performed through thermal and photolytic degradation.³⁰

Thermal property

The thermal analysis of neat POM and neat PLA fibers produced by melt spinning at 800 m/min speed is shown in Table 2. The DSC thermogram shows the melting temperature (T_m) of POM (V20-HE) at 162.5℃, PLA (6100D) at 178.2℃, and PLA (TP4000) at 181.5℃. The glass transition temperatures (T_g) of PLA (6100D) and PLA (TP4000) are 69.5℃ and 68.5℃, respectively. The degree of crystallinity (X_c) of POM and PLA was determined by using Equation (1), where

Δ H_{m}

is the measured heat of fusion of the POM or PLA sample and

Δ H_{m}^{0}

is the heat of fusion of 100% crystalline POM (326.3 J/g)³¹ or PLA (93 J/g).³² The crystallinity of PLA (6100D) is higher than that of PLA (TP4000)

X_{C} = \frac{Δ H_{m}}{Δ H_{m}^{0}} \times 100 %

(1)

Table 2.

Thermal analysis of neat polyoxymethylene (POM) and neat poly(lactic acid) (PLA) fibers

Polymer	Code	T_m (℃)	T_g (℃)	ΔH (J/g)	Crystallinity (%)
POM	V20-HE	162.5	–	67.6	20.7
PLA	6100D	178.2	69.5	46.8	50.3
PLA	TP4000	181.5	68.5	35.9	38.6

The thermal analysis of the POM/PLA blend fibers is shown in Tables 3 and 4. The ratios of the POM/PLA studied were 10/90, 30/70, 50/50, 70/30, and 90/10 wt%. In each sample, two peaks of T_m were observed, which confirmed the phase separation of POM and PLA. The POM used was grade V20-HE, while two types of PLA (6100D and TP4000) were used. The average T_m, T_g, and enthalpy of fusion (ΔH) of 10 measurements are reported.

Table 3.

Thermal analysis of the polyoxymethylene (POM)/poly(lactic acid) (PLA) (6100D) blend fibers

POM/PLA	T_m (℃)		T_g (℃)	ΔH (J/g)
POM/PLA	Peak 1	Peak 2	T_g (℃)	ΔH₁	ΔH₂
90/10	159.8	177.5	76.4	79.0	5.1
70/30	158.9	177.4	69.2	52.8	20.0
50/50	155.8	179.5	72.6	14.6	34.5
30/70	160.1	179.6	69.9	6.8	35.8
10/90	156.5	179.9	71.0	5.7	40.1

Table 4.

Thermal analysis of the polyoxymethylene (POM)/poly(lactic acid) (PLA) (TP4000) blend fibers

POM/PLA	T_m (℃)		T_g (℃)	ΔH (J/g)
POM/PLA	Peak 1	Peak 2	T_g (℃)	ΔH₁	ΔH₂
90/10	154.7	168.2	54.3	76.5	1.5
70/30	154.2	170.0	56.6	55.1	6.8
50/50	153.0	172.0	68.2	18.6	24.5
30/70	156.5	171.8	66.5	14.3	33.6
10/90	151.9	171.9	69.3	9.5	34.5

The melting temperatures of POM and PLA confirmed the phase separation of the two polymers. It has been reported that the polymer blends of POM and PLA at low temperature are miscible in the melt state. However, at elevated temperatures, phase separation is observed.^31,32 The immiscibility of POM and PLA is accounted for by the weak interactions between the carboxyl groups of PLA and the methylene groups of POM.

Comparing the two types of pure PLA (Table 2), TP4000 has a higher T_m (181.5℃) than 6100D (178.2℃). However, in the thermal analysis of the POM/PLA blend fibers (Tables 3 and 4) the T_m of PLA, TP4000 (ca. 168–172℃) in the blend is lower than that of 6100D (ca. 177–180℃). The opposite results may be due to the molecular orderly and crystallinity of polymer molecules, which affects the melting temperature of PLA. For the neat PLA, TP4000 has low crystallinity (38.6%) while the neat 6100D has higher crystallinity (50.3%); crystallinity indicates the tendency to arrange the molecules of the latter polymer in an orderly fashion. Comparing the enthalpy of fusion (ΔH₂) of PLA in Tables 2 and 3, it can be clearly seen that the ΔH₂ of 6100D is higher than that of TP4000 at the same ratios of POM/PLA. The crystallinity of the polymer has a direct effect on increasing the melting temperature. Therefore, the T_m of 6100D is higher than that of TP4000.

Mechanical property analysis

The tensile strength and elongation of POM (V20-HE) and PLA (6100D and TP4000) fibers drawn at 800 m/min are shown in Table 1. The average tensile strength (tenacity) and percent elongation at break of 10 samples of POM/PLA blend fibers drawn at 800 m/min are shown in Figures 1 and 2, respectively. POM is an engineering polymer with high tensile strength, while POM/PLA shows satisfactory tensile properties of the fibers. POM is a copolymer of oxymethylene and oxyethylene in which the latter monomer exhibited good toughness. The elongation at break of POM is higher than that of PLA, which behaved like a slightly brittle polymer.

Figure 1.

Tensile strength (tenacity) of polyoxymethylene (POM)/poly(lactic acid) (PLA) blend fibers.

Figure 2.

Elongation at break of polyoxymethylene (POM)/poly(lactic acid) (PLA) blend fibers.

From Figure 1, the tendency of tensile strength (tenacity) for POM/PLA (6100D and TP4000) follows the relationship of POM content. When the content of POM becomes a matrix (more than 50 wt%), interpenetration of PLA in POM is more orderly and homogeneous. However, when the POM content is less than that of the PLA low tenacity of both POM/PLA blends (10/90 and 30/70 w/w) is observed, which may be due to the different distribution behavior of POM in the PLA matrix. The melt flow of PLA 6100D (21.0 g/10 min at 210℃) is closer to that of POM (15.9 g/10 min at 210℃) than that of the PLA TP4000 (9.4 g/10 min at 210℃). Therefore, the mixing behavior of POM with two types of PLA may be different when one polymer is a matrix. At high temperature, distribution and molecular order may be more difficult for the low melt flow PLA TP4000 in the POM matrix (POM/PLA ratio 90/10 w/w). A high tenacity of the POM/PLA (TP4000) ratio 30/70 is observed, which confirmed the good distribution of both polymers.

The elongation at break of POM/PLA blends shows excellent results, which is unusual for polymer blends having separated phases (Figure 2). The phase structure of POM/PLA was analyzed using a previously reported transmission electron microscopy (TEM) method.^27,32,33 POM is easily crystallized, and the crystallinity may increase to 80%. A stacked structure of the POM crystals was intercalated by PLA. At low POM content, POM and PLA randomly interpenetrated each other. However, the stacked structure became more orderly and homogeneous with increasing POM content. The stacked structure showed a nearly parallel lamellar arrangement when the POM content was higher than 50 wt%.

Morphological analysis

The morphology of the POM/PLA fiber blends is shown in Figures 3 and 4.

Figure 3.

Morphology of polyoxymethylene/poly(lactic acid) (6100D) at weight ratios of (a) 30/70 and (b) 70/30.

Figure 4.

Morphology of polyoxymethylene/poly(lactic acid) (TP4000) at weight ratios of (a) 30/70 and (b) 70/30.

PLA has been known as a brittle polymer.³⁴ The percent elongation at break of PLA fibers was 342%, while that of POM is 1416%. Blends of POM/PLA show more ductile fracture behavior than neat PLA due to its higher toughness, and increases with the addition of higher contents of POM.

The fracture surface of POM/PLA (6100D and TP4000) fiber ratios 30/70 and 70/30 (wt%) showed irregular surfaces resulting from incompatible polymers (Figures 3 and 4). The fracture surface of POM/PLA ratios 30/70 (wt%) showed a brittle fracture behavior because more PLA was present in the blends. In contrast, the fracture surface of the blended fibers with POM/PLA ratios of 70/30 (wt%) showed a ductile fracture behavior, which was in agreement with the elongation behavior of the blends with high POM contents. Both POM/PLA (6100D) and POM/PLA TP4000) show similar morphology of the fracture surface.

X-ray diffraction analysis

Figures 5–7 respectively illustrate the XRD analysis of POM, PLA, and POM/PLA blends to examine their polymer crystallinity.

Figure 5.

X-ray diffraction analysis of neat polyoxymethylene (POM) and neat poly(lactic acid) (PLA) (6100D and TP4000).

Figure 6.

X-ray diffraction analysis of polyoxymethylene (POM)/poly(lactic acid) (PLA) (6100D) blend fibers.

Figure 7.

X-ray diffraction analysis of polyoxymethylene (POM)/poly(lactic acid) (PLA) (TP4000) blend fibers.

In Figure 5, the XRD analysis of the crystalline structure of POM and PLA shows distinct diffraction peaks. For POM, a trigonal form consisting of chains in helical conformation is common, which indicates an orthorhombic arrangement. The positions of the peaks are 23.2°, 29.6°, 47.8°, and 48.7°. The main diffraction planes at 23.2° have Miller indices of 100.³⁴ PLA shows two diffraction peaks detected at 2θ = 16.4° and 22.6°, indicating an amorphous state.³⁵ PLA (6100D) shows a higher XRD peak intensity than PLA (TP4000), which is in agreement with the thermal analysis results. The percent crystallinity of PLA (6100D, 49.9%) is higher than that of PLA (TP4000, 35.6%).

Figures 6 and 7 show XRD patterns of POM/PLA blend fibers of PLA (6100D) and PLA (TP4000), respectively, at POM/PLA ratio of 30/70 and 70/30 by weight. Similar patterns are observed for both PLA fibers (6100D and TP4000). For the POM/PLA ratio of 30/70, a high PLA content shows peaks at 16.4° for PLA and 23.2° for POM, while the PLA peak at 22.6° could not be clearly seen due to overlapping with the POM peak. POM/PLA ratio 70/30 shows a small peak at 16.4° and a very high-intensity peak at 23.2°.

Conclusion

Multifilament yarns of POM, PLA, and POM/PLA blends were produced and their mechanical and physical properties were analyzed. POM containing high content of oxyethylene copolymer (grade V20-HE) could be spun using a melt-spinning process. PLA (grades 6100D and TP4000) was blended with POM to compare the properties of the blended fibers with those of the suitable conditions of the melt-spinning process. The POM/PLA blends showed phase separation of the two polymers, but interestingly, excellent toughness of the blended multifilament yarns was observed. The blended fibers are suitable for application as fishing nets and strings due to their density being higher than that of sea water. Therefore, their wastes are expected to be not float at the sea surface level, which will not be harmful to fish and marine animals. Further study on the drawing effect on crystallinity and tenacity of the blend fibers compared with traditional polymers (nylon, PP) is under investigation.

Footnotes

Acknowledgements

We would like to acknowledge the Thai Polycarbonate Co., Ltd (TPAC), Mitsubishi Engineering Plastic (MEP) Technical Center Asia, Ltd, and the Department of Advanced Fibro Science, Kyoto Institute of Technology, Japan, for materials and experimental support.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Research Council of Thailand (NRCT).

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