Effects of plasma pre-treatment and polyurethane/IF-WS 2 nanocoating on mechanical and viscoelastic properties of UHMWPE yarns

Abstract

Ultra-high-molecular-weight polyethylene (UHMWPE) fiber is highly favored for fiber-based ballistic composites due to its high strength-to-weight ratio and flexibility. However, its poor adhesion to matrices largely limits its application in polymer composites. Also, the weight of ballistic armor produced from these fibers is still very high and there is a need to improve their performance to enable the use of lighter-weight armor. In this work, it was observed that applying plasma pre-treatment and waterborne polyurethane (PU)/inorganic fullerene-like tungsten disulfide (IF-WS₂) nanoparticle coating could significantly increase the toughness, energy to break, and maximum load of UHMWPE yarns. The corresponding properties of the plasma pre-treated and coated fibers increased up to 31.6%, 50.9%, and 33.3%, respectively. In addition, high wash durability of the coating was obtained with up to 92% weight retention after washing. Optimum properties were observed with the coating components of 15% PU (PUA type), 6% IF-WS₂, and water medium (SA3 nanocomposite). Moreover, plasma pre-treatment induced oxygen-containing functional groups on the fibers, which significantly improved the mechanical, viscoelastic, and wash durability properties of the final UHMWPE/PUA/IF-WS₂ composites.

Graphical Abstract

Keywords

UHMWPE fiber plasma pre-treatment polyurethane coating inorganic fullerenes fiber-based nanocomposite mechanical properties viscoelasticity

Introduction

Ultra-high-molecular-weight polyethylene (UHMWPE) fiber possesses a low density and exceptional mechanical properties such as high Young’s modulus, high tensile strength, and great work to fracture.^1,2 In recent years, it has attracted an increasing amount of interest in the application of fiber-based polymer composites for high-performance ballistic and impact-resistant products.^3
–8 However, UHMWPE fiber exhibits poor interfacial adhesion to polymer matrices due to its smooth fiber surface, low surface energy, and inert chemical structure.⁹ The poor fiber/polymer adhesion lowers the interlaminar shear strength of polymer composites and weakens their mechanical advantages, therefore, it largely restricts the applications of the UHMWPE fibers in composites.¹⁰ Various attempts have been made to improve the UHMWPE fiber/polymer adhesion such as chemical etching,^11,12 plasma treatment,^13
–15 irradiation treatment,^16,17 UV-initiated graft,¹⁸ and corona discharge. Among these, plasma treatment is extensively used for its high efficiency and simplicity while limiting damage to target materials.^19
–22 Huang et al.²³ studied the impact of plasma treatment with argon on UHMWPE fibers. The results revealed that the plasma treatment increased the micro-crazes and water contact angle on the textile surface and enhanced the adhesive peel strength of the UHMWPE fibers.

Waterborne polyurethane (PU) has been documented for elastomeric coatings and fabrication of polymer composites for anti-corrosion and mechanical strength improvements.^24
–27 Owing to its eco-friendly, easy-to-disperse, high elasticity, and excellent toughness benefits, waterborne PU has become a great polymer matrix candidate for developing nanofiller-reinforced high-strength composites.^28
–30 Based on their heat deformation abilities, PU polymers can be divided into thermoset and thermoplastic PUs, the former crosslinks during the curing process forming molecular chain networks.^31,32 Recent research has shown that incorporating inorganic nanoparticles into a polymer matrix improves thermo-mechanical characteristics significantly.³³ Among others, inorganic fullerene-like tungsten disulfide (IF-WS₂) demonstrates high stiffness, good compressive strength, exceptional thermal stability, and excellent shock-absorbing capabilities.^34,35 Due to its low cost and superior properties, IF-WS₂ has been used in research for making nanocomposites with improved mechanical properties and/or impact resistance.^36
–39 Therefore, incorporating IF-WS₂/PU with UHMWPE fibers appears to be a promising approach for fabricating lightweight and flexible fiber-based nanocomposites, of which enhanced mechanical properties, low environmental impact, and low cost are expected. Plasma pre-treatment of UHMWPE fibers has been shown to further strengthen the nanocomposites by increasing the interfacial bonding between UHMWPE fibers and PU matrices. However, in the literature, incorporating both plasma pre-treatment and PU/IF-WS₂ reinforcement to produce durable UHMWPE fiber-based nanocomposites has not been reported. Its effects on the mechanical and viscoelastic properties of UHMWPE fibers have not been investigated.

In this work, the UHMWPE fibers were plasma pre-treated and coated with several IF-WS₂/PU nanofluids using a continuous dip-coating method. Different amounts of IF-WS₂ and waterborne PUs (thermoset or thermoplastic) were added to the selected dispersion medium to prepare the nanofluids. The effects of the plasma treatment on the chemical structure and surface morphology of UHMWPE fibers were investigated. The coating components were screened according to the viscoelastic, mechanical, and coating durability properties of the UHMWPE/PU/IF-WS₂ nanocomposites. Also studied was the impact of plasma pre-treatment on the aforementioned properties of the nanocomposites.

Materials and methods

Materials

The UHMWPE yarn (untwisted multifilament yarn, trade name: Spectra^® 1600) was provided by Honeywell, USA. Their physical properties include linear density (measured) of 1557 denier, filament diameter of 34 μm, and filament number of 360. Two types of aqueous PU dispersions were supplied by Lubrizol, USA. PU product information is listed in Table 1. The water-based IF-WS₂ additive was obtained from Nanotech Industrial Solutions, USA (trade name: IW-4000, particle size: 20–80 nm as reported,⁴⁰ measured solid content: 48.8%).

Table 1.

Product information for the two waterborne PU products used.

PU type	Trade name	Polyol type	Solid content (%)	Self-crosslinking
PUA	Aptalon™ M8100	Polyamide polyol	37	Yes
PUB	Permax™ 202	Polyether polyol	41	No

Plasma pre-treatment and composite preparation

The continuous direct plasma treatment of Spectra^® yarns was carried out by Pillar Technologies, Inc (USA). The treatment was conducted with 80%N₂/20%O₂ gas using 700 W of power. The yarns were treated with a linear speed of 6.1 m/min. The distance between the electrode and sample was 1.0 mm. The width of plasma electrode used in this experiment was 0.3556 m and both sides of the yarn were treated during plasma charging. In addition, the energy applied per unit area of the yarn surface was 161.46 Wmin/m² per side. To reduce the risk of contamination, all samples were immediately placed in a clean sealed plastic bag following plasma treatment.

The plasma-treated (PT) yarns were coated with different PU/IF-WS₂ mixtures (SA1-4, SB1-4) as described in Table 2. Several factors with different levels were considered for preparing the mixtures based on preliminary trials. These factors include PU type, PUA amount, PUB amount, IF-WS₂ amount, and type of solvent medium. A control specimen SA3-U was also fabricated with untreated Spectra^® yarns. The amounts of PU and IF-WS₂ in coating dispersions were calculated based on total liquid weight. A water/ethanol mixture (weight ratio: 1:5) was used as the dispersing medium to prepare SA4 and SB4 composites, due to the high stability of the IF-WS₂ nanoparticles in ethanol and the good dispersibility of the PU products in water.⁴⁰ Thus, the PU amounts for SA4 and SB4 were designed based on the water weight. During the preparation, a required amount of IF-WS₂ suspension was added to the selected dispersion medium, followed by sonication in a Kendal ultrasonic bath for 30 min to form a homogeneous fluid. Then, the corresponding amount of PUA or PUB was added to the mixture, which was then sonicated for another 30 min. The cap of the container (centrifuge tube) was opened every 15 min for 1–2 s and the water bath was replaced every 30 min during sonication to prevent PU from solidifying by the heat generated during mixing.

Table 2.

The abbreviations and fabrication conditions of the Spectra^®/PU/IF-WS₂ composites.

Abbreviations	Yarn	PUA (wt%)	PUB (wt%)	WS₂ (wt%)	Medium
SA1	PT	10	-	2	W
SA2	PT	15	-	2	W
SA3	PT	15	-	6	W
SA4	PT	15	-	2	WE
SB1	PT	-	9	2	W
SB2	PT	-	14	2	W
SB3	PT	-	14	6	W
SB4	PT	-	14	2	WE
SA3-U	UT	15	-	6	W

PT: plasma pre-treated; UT: untreated; W: water; WE: water/ethanol mixture (weight ratio: 1:5).

To fabricate Spectra^®/PU/IF-WS₂ composites, Spectra^® yarns were continuously coated with prepared PU/IF-WS₂ dispersions using a lab-assembled setup as shown in Figure 1(a). A continuous (plasma pre-treated or untreated) yarn was stretched and oriented under tension, dip-coated with prepared PU/IF-WS₂ dispersions, nipped through two pressure rollers, pre-dried in the furnace, and wound on a 3D printed rod attached directly to the DC motor. The nip pressure between the rollers and the winding speed was kept constant. The measured temperature within the furnace tube was 196°C ± 7°C. After continuous coating, each composite yarn on the winding rod was transferred and evenly spread on a hollow aluminum (Al) frame with high heat resistance (Figure 1(b)). Then the coated yarn was further dried and cured in a lab oven at 100°C (PUA) or 60°C (PUB) for 30 min. A higher temperature was used for PUA-incorporated composites since PUA requires a high curing temperature to achieve good film formation, that is, uniform, and durable films, as low temperature leads to inconsistent and brittle PUA films. Figure 1(c) illustrates the complete fabrication process for the Spectra^®/PU/IF-WS₂ composites.

Figure 1.

Continuous yarn dip-coating: (a) image of the lab-assembled setup, (b) coated yarn wound on a hollow aluminum frame, and (c) illustration of the fabrication process for Spectra^®/PU/IF-WS₂ specimens.

Attenuated total reflectance-Fourier transform infrared spectroscopy

Infrared spectrums of absorbance for the neat and plasma-treated yarns were obtained using an attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectrometer (ThermoFisher Nicolet 6700). The effect of plasma treatment on the chemical structure of the Spectra^® fibers was studied. The spectrums range from 750 to 4000 cm⁻¹ and the scan number of 128 was applied. FTIR peaks on the spectrums were analyzed.

Measurement of surface energy

The surface wettability of the UHMWPE fibers (single fiber) before and after plasma treatment was determined using Dynamic Contact Angle measuring device and Tensiometer DCAT (DataPhysics Instruments GmbH, Germany). The sample (single fiber, fiber diameter was input for the measurement system) was dipped automatically by the instrument into distilled water (around 5 mm) and pulled back again. Single fiber holder FH 12 was used for holding fibers while dipping into water. The software DCATS 32 was used for controlling measurement. The software calculates the dynamic contact angles using the recorded force data.

Scanning electron microscopy

Scanning electron microscopy (SEM) was performed on the neat and plasma-treated yarns using an FE-SEM Thermo Fisher Teneo. Before testing, the specimens were coated with gold to avoid charging the samples. The magnifications of 500× and 200× and acceleration voltage of 5 kV were used. The effect of plasma treatment on the surface morphology of the Spectra^® fibers was observed.

Linear density and add-on percentage

Ten neat yarns with a length of 125 cm were cut sharply and weighed. The average sample weight was used to calculate the linear density, D (denier), of the neat yarn for reporting with equation (1). Seven yarns with a length of 30 cm were cut from each coated yarn and weighted individually to estimate the add-on percentage of each sample with equation (2). The add-on percentage for each sample can be calculated by averaging the results from all replicates.

D = \frac{W}{L} \times 9000

(1)

Where D is the linear density of a yarn in denier, $W$ is the weight of a yarn in grams, $L$ is the length of a yarn in meters

A d d o n (%) = \frac{Y a r n w e i g h t a f t e r c o a t i n g - Y a r n w e i g h t b e f o r e c o a t i n g}{Y a r n w e i g h t b e f o r e c o a t i n g} \times 100 = \frac{W_{a c} - W_{b c}}{W_{b c}} \times 100

(2)

Where $W_{a c}$ is the weight of a yarn after coating in grams, and $W_{b c}$ is the weight of a yarn before coating in grams, estimated with the neat yarn linear density and sample length of 30 cm.

Dynamic mechanical analysis

A dynamic mechanical analysis (DMA) instrument (PerkinElmer DMA 8000) in tension mode with 0.005 mm strain and 1.2 force multiplier was used to test the viscoelastic properties of the neat, PT, and PU/IF-WS₂ coated yarns. Test samples with a length of 22 mm were used. The DMA temperature increased from 30°C to 100°C at a heating rate of 3°C/min and the frequency was 1 Hz. The storage modulus, loss modulus, and tan delta (δ) of the samples were analyzed.

Mechanical test

A tensile tester (Instron 4400R) was used to test the mechanical properties of the neat, PT, and PU/IF-WS₂ coated Spectra^® yarns as per ASTM 2256 standard. To reduce yarn slippage during the test, capstan yarn grips were installed. Besides, both ends of the yarn were glued onto cardboard chips (dimension of 1.6 cm × 2.9 cm) with ELMER’S clear glue.⁴⁰ The glued samples were dried overnight. For some samples, 12.5 full twists in 105 mm were applied between the cardboards to further increase the valid breaking rate as directed in the standard. All samples were tested with a crosshead speed of 100 mm/min and a gage length of 100 mm. The “work of rupture” – total energy to break (J) of an individual yarn was calculated as the integrated area under its load-displacement curve. Then the corresponding gravimetric toughness (J/g) can be calculated using equation (3)⁴¹ it shows the weight normalized tensile energy absorption capability of the yarns for potential lightweight bullet-proof and anti-impact applications. At least three replicates were tested, and the averages were compared.

T o u g h n e s s (\frac{J}{g}) = \frac{t o t a l e n e r g y t o b r e a k}{t e s t e d y a r n w e i g h t} = \frac{E B}{W_{t}}

(3)

Where EB is the energy to break of a yarn in joules and $W_{t}$ is the tested yarn weight, estimated with the corresponding yarn linear density and tested length of 100 mm.

Wash durability

To study the influence of PU type and coating components on the wash durability of the PU/IF-WS₂ coatings, the Spectra^®/PU/IF-WS₂ yarns (length of 30 cm) were washed using an Atlas Launder-ometer as per AATCC Test Method 61 (condition No. 1A). The samples were treated at (40 ± 2) °C temperature for 45 min (one washing cycle), with total liquor volume 200 ml. In addition,10 steel balls were used during treatment. The wash durability of a coating was evaluated based on its estimated weight retention on Spectra^® fibers after washing. The calculation for the weight retention (%) of a sample is shown in equation (4).

C o a t i n g w e i g h t r e t e n t i o n (%) = \frac{C o a t i n g w e i g h t a f t e r w a s h i n g}{C o a t i n g w e i g h t b e f o r e w a s h i n g} \times 100 = \frac{W_{a w} - W_{n}}{W_{b w} - W_{n}} \times 100

(4)

Where W_bw and W_aw are the coated yarn weights before and after washing respectively in grams, W_n is the neat yarn weight in grams, calculated based on the neat yarn linear density and sample length of 30 cm.

Results and discussion

Chemical structure

Figure 2 illustrates the FT-IR spectrums of the neat and plasma treated Spectra^® yarns. From both spectrums, the characteristic peaks of UHMWPE fibers can be identified. The peaks at 2916, 2849, and 1473 cm⁻¹ are assigned to the symmetric stretching of CH₂, asymmetric stretching of CH₂, and bending vibration of CH₂, respectively.¹⁵ After plasma treatment, new absorption peaks appear at 3250 and 1720 cm⁻¹, which are attributed to O-H and C=O stretching vibrations, respectively. O-H bending at 1360–1370 cm⁻¹ and C-O bonds at 1050–1310 cm⁻¹ are also observed in the plasma-treated sample.^15,42 Meanwhile, the peak at 1122 cm⁻¹ shows significantly enhanced intensity in the PT specimen due to C-O stretching in secondary alcohol. These characteristic peaks indicate that new oxygen-containing polar groups (-OH, C=O, C-O) were introduced into the Spectra^® yarn through the plasma treatment.^42,43

Figure 2.

FT-IR spectrums of the Spectra^® yarns: (a) neat and (b) plasma treated (PT).

Surface morphology

SEM was utilized to observe the surface morphologies on neat and PT Spectra^® fibers and study the effect of the plasma treatment. As shown in Figure 3(a), the Spectra^® fibers in the neat yarn show longitudinal striations and uneven surfaces, which are formed from manufacturing. After surface modification using plasma treatment, no obvious surface morphological changes (e.g. etching pits, deepened gullies, cracks, or increased roughness) were shown on the fibers (Figure 3(b)). This revealed that the plasma treatment process was well-controlled to prevent severe fiber damage.

Figure 3.

SEM images of the Spectra^® fibers: (a) neat and (b) plasma treated.

Wettability of single fiber

The water contact angle of the control (untreated) and the plasma treated UHMWPE fibers were measured by using a dynamic contact angle measuring device. The plasma treated sample was found to have statistically significant (p = 0.0018 at 95% confidence level) lower contact angles (80.45°) than the untreated sample (100.18°). For each sample, five measurements were taken for statistical analysis. The reduced contact angle (decrease in hydrophobicity) of plasma treated UHMWPE is attributed to surface roughening and activation of the fiber surface by plasma modification. This finding was supported by previous studies.^23,44,45

Add-on percentage

After nanocomposite coating, the add-on percentages of different PT yarn/PU/IF-WS₂ specimens were measured. As shown in Table 3, the average add-on percentages for the PT treated and PU/IF-WS₂ coated samples (SA1-4, SB1-4) were maintained in a small range from 4.2% to 12.1%. According to the ANOVA test conducted, the add-on amounts for SA1, SA2, SA4, SB1, SB2, and SB4 are not significantly different at a 95% confidence level (F ratio = 2.239, p = 0.0714). The same confidence level was used in all statistical analyses in this research unless stated otherwise. With a 6% IF-WS₂ amount, SA3 and SB3 obtained slightly higher add-on percentages compared to SA2 and SB2 with 2% IF-WS₂. Besides, the add-on amount for SA3-U increased slightly compared to its counterpart SA3 (F ratio = 5.610, p = 0.0355). The toughness presented in this work was calculated based on sample weight to enable direct comparison among samples with different add-ons.

Table 3.

The add-ons (mean and SE) for the PU/IF-WS₂ coated Spectra^® yarns.

Samples	SA1	SA2	SA3	SA4	SB1	SB2	SB3	SB4	SA3-U
Add-on (%)	8.9 ± 1.6	7.7 ± 1.3	12.1 ± 1.4	4.6 ± 1.0	4.2 ± 1.0	5.5 ± 1.2	11.2 ± 1.8	4.9 ± 1.3	16.6 ± 1.3

Viscoelastic properties

DMA temperature sweeps (30°C–100°C) at 1 Hz frequency were conducted on the plasma pre-treat/coated specimens including SA1-4 and SB1-4. In addition, neat and PT Spectra^® yarns were tested as controls. Figures 4 and 5 show their dynamic mechanical behaviors (storage modulus, loss modulus, and tan delta). Both moduli of the samples show approximately linear decrease upon heating due to the rising molecular kinetic energy and mobility of Spectra^® fibers, which lead to a softening effect on the fibers.^46,47 As observed in Figure 4(a)–(c), both dynamic storage and loss moduli of the PT sample decreased slightly, compared to those of the neat yarn. The order of the storage modulus for SA1–SA4 composites is observed as SA1 > SA2 > SA3 > SA4, while their tan δ (loss to storage modulus ratio) shows the opposite order as SA4 > SA3 > SA2 > SA1. Specimens SA1, SA2, and SA3 showed improved storage and loss moduli in comparison with the neat and PT yarns. The increased storage modulus indicates their enhanced elasticity and energy storage ability under stress, while the higher loss modulus reflects better viscosity and energy dissipation capability of the composite yarns.⁴⁸ These property enhancements are desired for materials used in ballistic and anti-impact applications. The poor viscoelastic properties of SA-4 could be attributed to the combined effects of the PUA type, low coating add-on, and the mixture medium used. Similarly, Figure 5(a)–(c) shows the viscoelastic properties of the SB samples, and small improvements in storage modulus were observed from SB1-SB4 compared to the PT yarn. Both SB3 and SB4 exhibited higher loss modulus than the neat and PT yarns, while SB1-SB2 showed slightly improved loss modulus compared to the PT yarn. Tan δ of SB2-SB4 increased to different degrees with the highest value obtained from SB3 indicating its largely enhanced damping ratio. Both moduli of the PT and composite yarns were collected at 30°C and listed in Table 4. As seen, all PT Spectra^®/PU/IF-WS₂ composites, except SA4, showed increased storage modulus (up to 51.9%) and loss modulus (up to 53.2%) in comparison to the PT yarn.

Figure 4.

DMA temperature sweeps of the neat Spectra^®, PT, and PT yarn/PU/IF-WS₂ nanocomposite yarns (SA1–SA4): (a) storage modulus, (b) loss modulus, and (c) tan delta.

Figure 5.

DMA temperature sweeps of the neat Spectra^®, PT, and PT yarn/PU/IF-WS₂ nanocomposites (SB1–SB4) yarns: (a) storage modulus, (b) loss modulus, and (c) tan delta.

Table 4.

Summary of storage and loss moduli of the PT and PT yarn/ PU/IF-WS₂ nanocomposite yarns at 30°C.

Properties	PT	SA1	SA2	SA3	SA4	SB1	SB2	SB3	SB4
Storage modulus (GPa)	32.29	49.06	46.69	43.26	31.89	34.65	34.68	38.65	41.85
Loss modulus (GPa)	8.96	9.89	10.13	10.68	8.97	9.83	10.81	13.73	13.47

Mechanical properties

The tensile tests were conducted for the neat Spectra^®, PT, and PT/coated yarns. As per the ASTM 2256 standard, 12.5 twists were applied to each glued sample before testing to prevent yarn slippage under tension. From the test result, total energy-to-break and toughness were calculated for the controls and composite samples (SA1-4, SB1-4). As seen in Figure 6, plasma treatment had no significant effect on the mechanical properties (energy-to-break and toughness) of Spectra^® yarns. This result agrees with the intact fiber surfaces after plasma treatment.

Figure 6.

The total energy to break and toughness (mean and SE) of the neat Spectra^® yarn, PT yarn, and PT yarn/PU/IF-WS₂ nanocomposites (SA1–SA4, SB1–SB4).

However, a one-way ANOVA test shows the energy-to-break values of composites SA1–SA4 and SB1–SB4 were significantly different (F ratio = 3.8570, p = 0.0025). The maximum mean energy to break was obtained from specimen SA3, which showed 26.7% and 29.9% increases compared to the neat yarn (p = 0.0187) and PT yarn (p = 0.0482), respectively. The gravimetrical toughness of the composites (total energy absorbed per unit weight of materials before rupturing) shows a similar trend to the energy-to-break values due to the small range of add-on amounts. Similarly, the ANOVA analysis result indicates a significant difference in the toughness among coated samples (F ratio = 2.7409, p = 0.0195). SA3 and SB1 are the samples that have the highest toughness values from the PUA and PUB groups, respectively. Their toughness improved by 11.4% (SA3) and 9.7% (SB1) in comparison to neat yarn, and by 14.2% (SA3) and 12.4% (SB1) compared to PT yarn. The test results confirm the effectiveness of plasma pre-treatment and nanocomposite coatings in enhancing tensile energy absorption of the Spectra^® yarns while retaining their lightweight qualities.

Coating wash durability

Wash durability of the coated yarns is important for their potential applications in protective garments and textiles. To test the coating wash durability, all composite yarns were washed as per AATCC method 61 to simulate the home laundry procedure. The sample weight for each composite was measured before and after washing, and an estimated percentage of retained weight was calculated. As presented in Figure 7, the weight retentions of the tested samples are significantly different (F ratio = 11.6246, p < 0.0001). The retention means of the SA samples are higher than those of the SB specimens. On average, the coatings on SA1-SA4 were retained 62.1%–92.0% while the coatings on SB1-SB4 remained only 10.3%–42.3% after washing. Figure 8 exhibits the images of unwashed (a, c) and washed (b, d) composite yarns. The color change of the samples after washing matches the weight retention result. In Figure 8(b), SA1–SA4 retained most of their IF-WS₂/PUA coating after washing with little color change. However, specimens SB1–SB4 in Figure 8(d) lost most of their coatings and exposed the original white fiber color after washing. This difference is due to the self-crosslinking property that PUA has over PUB, which leads to higher durability of the IF-WS₂/PUA coatings. This finding was supported by the literature.⁴⁹ Through this process, PUA forms water-resistant and durable thermoset films on^®Spectra^® fiber surface leading to high weight retention of PUA/IF-WS₂ coating after washing. In contrast, waterborne PUB forms thermoplastic films without crosslinks on the fiber surface, making them more susceptible to damage by water swelling and agitation during washing. Thus, the coatings on SB1-SB4 (PUB-based) samples showed lower water durability and weight retention in the wash. Among all tested samples, SA3 obtained the highest weight retention (92%) after washing, which is significantly higher than that of all SB samples according to the Tukey test. Its high wash durability is also confirmed by its appearance after washing, the SA3 replicates maintained their light gray colors and united-filament structures. Furthermore, Figure 9 shows the surface morphologies of unwashed (a, c) and washed (b, d) SA3 samples. No obvious loss of the nanocomposite coating was observed on the washed sample, its fiber shape and appearance were well retained after washing.

Figure 7.

Coating weight retention percentages (mean and SE) of the plasma pre-treated Spectra^® yarn/PU/IF-WS₂ nanocomposites after washing.

Figure 8.

Images of unwashed and washed plasma pre-treated Spectra^® yarn/PU/IF-WS₂ nanocomposites: (a) unwashed SA1–SA4, (b) washed SA1–SA4, (c) unwashed SB1–SB4, and (d) washed SB1–SB4.

Figure 9.

Surface appearances of the unwashed (a and c) and washed (b and d) SA3.

Effect of plasma pre-treatment

As shown in Sections 3.5 and 3.6, composite SA3 has the highest tensile energy absorption, toughness, and wash durability among all fabricated nanocomposites. Thus, the effect of the plasma pre-treatment on specimen SA3 was further investigated. SA3-U was fabricated as a control using the same PUA/IF-WS₂ coating component for untreated Spectra^® yarn. The mechanical properties, viscoelastic properties, wash durability, and surface morphologies of both specimens were tested and compared. Table 5 shows the mechanical properties of SA3-U and SA3 from tensile tests. During the test, 12.5 twists were applied to each sample before testing to reduce yarn slippage under tension. Although applying twists may limit yarn slippage, its effects on the mechanical properties of Spectra^® yarn are unclear. To study this, the mechanical properties of these specimens and neat yarns were also tested without applying yarn twists (Table 6). The weight retention percentages of their coatings are also included in the table. By comparing the results in both tables, it was found that applying yarn twists had no significant effect on the toughness and energy to break values of the yarns. However, it significantly increased the breaking extension for both SA3-U (F ratio = 27.5042, p = 0.0004) and SA3 specimens (F ratio = 35.4744, p = 0.0002). In addition, the maximum load of SA3 with twists showed a significant decrease (F ratio: 7.3070, p-value: 0.0243). To match the untwisted pristine yarn condition, the mechanical results from untwisted yarns were further analyzed. The result shows SA3 achieved significantly higher toughness (F ratio: 6.2823, p-value: 0.0311), energy to break (F ratio: 9.8663, p-value: 0.0105), and maximum load (F ratio: 6.1193, p-value: 0.0329), compared to SA3-U. It indicates that the applied plasma pre-treatment significantly improved the mechanical properties of SA3 by 21.2% (toughness), 16.2% (energy to break), and 9.9% (maximum load).

Table 5.

Mechanical properties (mean and SE) of SA3-U and SA3 were measured with 12.5 twists.

Samples	Toughness (J/g)	Energy to break (J)	Maximum load (N)	Elongation at break (%)
SA3-U	362 ± 25	0.73 ± 0.05	497 ± 19	4.36 ± 0.09
SA3	409 ± 31	0.81 ± 0.06	509 ± 15	4.78 ± 0.08

Table 6.

Mechanical properties (mean and SE) of SA3-U, SA3, and neat yarn measured with no twists along with the coating weight retention of SA3-U and SA3.

Samples	Toughness (J/g)	Energy to break (J)	Maximum load (N)	Elongation at break (%)	Weight retention after washing (%)
SA3-U	364 ± 13	0.74 ± 0.03	513 ± 15	3.72 ± 0.08	83.9 ± 1.9
SA3	441 ± 21	0.86 ± 0.04	564 ± 14	3.82 ± 0.13	92.0 ± 1.9
Neat yarn	335 ± 44	0.57 ± 0.08	423 ± 14	3.50 ± 0.48	-

Meanwhile, the coating weight retention of composite SA3 after washing is 9.7% higher than that of SA3-U (F ratio: 8.8374, p-value: 0.0410). This revealed the plasma pre-treatment enhanced the coating durability of the composite by inducing oxygen-containing groups to the fiber surface and enhancing Spectra^® fibers/PUA adhesion. Overall, in comparison to neat Spectra^® yarns, the PT yarn/PUA/IF-WS₂ composite yarn SA3 showed significantly higher toughness (31.6%), energy-to-break (50.9%), and maximum load (33.3%). Their corresponding ANOVA test results are (1) F ratio: 6.4254, p-value: 0.039, (2) F ratio: 13.6722, p-value: 0.0077, and (3) F ratio: 40.3593, p-value: 0.0004, respectively.

To determine the influence of plasma pre-treatment on the viscoelasticity of the composite, DMA tests were carried out on SA3-U and SA3. As depicted in Figure 10, both storage and loss moduli showed similar decreasing trends upon heating for nanocomposites SA3-U and SA3. However, in the tested temperature range, both moduli of SA3 were higher than those of SA3-U. This indicates the reinforcing effect of plasma pre-treatment on the PUA/IF-WS₂/Spectra^® composite SA3, which leads to its improved ability to store (elasticity) and dissipate (viscosity) dynamic energy. At 30°C, its storage and loss moduli increased by 30.0% and 25.8%, respectively, compared with those of SA3-U. Figure 11 shows the coating appearance on SA3-U and SA3. A significant amount of PU clusters is observed on SA3-U. The coating uniformity of PU matrix on SA3 is much higher than on SA3-U due to the increased wettability of Spectra^® fibers after plasma treatment, which results in better spreading and bonding of waterborne PU on the fiber surface, thus better mechanical and viscoelastic properties for the composite.

Figure 10.

DMA results of SA3-U and SA3 samples: (a) storage modulus and (b) loss modulus.

Figure 11.

SEM photographs showing the surface features of: (a) SA3-U and (b) SA3 samples.

Conclusions

The poor interfacial adhesion between UHMWPE fibers and polymer matrix restricts the application of UHMWPE fibers in fiber-based polymer composites for ballistic protection gears. In this work, Spectra^® multifilament yarns were surface-modified using plasma treatment. Oxygen-containing groups (-OH, C=O, C-O) were introduced onto the fiber surface. After incorporating waterborne PU and IF-WS₂ coatings, most Spectra^® fiber-based nanocomposites showed improved storage and loss moduli, compared to the PT yarn. Tensile test results revealed that the coatings significantly affected the energy to break and toughness of the nanocomposites. Among all the specimens, SA3 (15% PUA, 6% IF-WS₂, water medium) showed the highest energy to break and toughness on average. The PUA/IF-WS₂ coatings exhibited better wash durability than the PUB/IF-WS₂ coatings on Spectra^® yarns due to the higher water resistance and self-crosslinking function of the PUA product. The maximum coating weight retention after washing (92%) was also obtained for SA3. Compared to its counterpart SA3-U with untreated fibers, SA3 with PT fibers exhibited higher toughness (21.2%), energy to break (16.2%), and weight retention after washing (9.7%). It indicated that pre-treating Spectra^® fibers with plasma discharge enhanced the energy storage and dissipation capabilities of the fiber-based nanocomposites. The toughness, energy to break, and maximum load of the SA3 sample significantly improved by 31.6%, 50.9%, and 33.3%, respectively, compared to the pristine yarn. Overall, the nanocomposites showed promising enhancements in mechanical and viscoelastic properties and high coating durability. Meanwhile, it retains the lightweight and flexibility of UHMWPE fibers. In the future, PUA/IF-WS₂ nanocomposite yarns can be further studied for their potential application in soft and lightweight body armor and anti-impact materials.

Footnotes

Acknowledgements

The authors of this paper would like to thank Dr. Smriti Rai for guiding the design and assembly of the yarn coating setup, Honeywell for providing Spectra fiber samples, and Lubrizol for providing PU samples.

Declaration of conflicting interests

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

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Avik Kumar Dhar

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