Binary structured polypropylene-/propylene-based elastomer fibrous membranes with enhanced flexibility

Introduction

Polypropylene (PP) fibrous membrane is a class of commercial polymer membrane with variety of advantages such as liquid repellency, strength, and excellent chemical stability. Thus, PP fibrous membrane has been widely used in surgical drapes and gowns, cast paddings and diapers over the past decades [1,2]. To date, solution/melt electrospinning, solution blowing, flash spinning, centrifugal spinning and melt blowing have been reported as promising methods for fabricating PP fibrous membranes with desirable structures and characteristics [3–7]. Among these methods, electrospinning and melt blowing were reported to produce micro/nanofibrous membranes for many years [8,9]. However, PP cannot be dissolved in common solvents at room temperature [10] and is restricted for the PP fibrous membranes via electrospinning process due to its limitations such as low conductivity, complex equipment, and high melting temperature [11,12]. While, the melt blowing is a common method used in industry for producing micro/nanofibrous materials with the materials of thermoplastic polymer melt [13,14]. Therefore, melt blowing method has become an attractive strategy for preparing PP fibrous membranes owing to its advantages such as a simple preparation process, being environmentally friendly. However, pristine PP fibrous membranes show poor flexibility, which limits their potential for advanced functional applications.

To address these limits, a series of modified PP fibrous membranes were fabricated using a melt blowing process with raw materials of various blending polymers, such as thermoplastic polyurethane (TPU) [15], polyethylene (PE) [16], polyester (PET) [17], polylactic acid (PLA) [18,19], and propylene-based elastomer (PBE) [20]. Among these polymers, PBE is a copolymer that is synthesized by coordinative insertion polymerization using an organometallic single-site catalyst, and is regarded as a good candidate for producing elastic fibrous membranes owing to its attractive properties, such as compatibility with PP, chemical resistance, and low modulus [21,22]. However, publications on using and processing PBE fibrous products are mostly limited to the present literature, and few studies have focused on flexibility, which is the integrative reflection of elastic and softness [23,24]. Moreover, the softness of the fibrous products is not only related to structural parameters of such products, such as fiber diameter and mass per unit area, but is also influenced by the polymer properties [25].

It was reported that the smaller the fiber diameter, the more softness improved. Recently, many researchers have found that introducing microscale fibers into nanofibrous membranes is useful for improving the air permeability [26–29]. For example, Zhang et al. [26] designed and prepared a ripple-like polyamide-6 membrane with embedded scaffold PET filaments. This fibrous membrane presented bimodal fiber diameter distribution, leading to an air pressure drop of 95 Pa at airflow velocities of 5.3 cm/s. Meanwhile, some fibrous membranes with novel structures have also been researched for improving airflow resistance. Deng et al. [28] fabricated a novel polypropylene/polystyrene (PP/PS) membrane with multi-scale fiber diameter via a blending melt blowing process. They reported that the microscale fibers act as a skeleton support that can improve the permeability of nonwovens. From the above, it is obvious that the optimization of fiber diameter distribution for fibrous membranes would be an effective and necessary strategy for enhanced softness and air permeability.

Based on this, a PP/PBE fibrous membrane with a broad fiber diameter distribution was designed and prepared in this paper. Meanwhile, structural morphology and functional characteristics of the obtained fibrous membranes were investigated. Particularly, elasticity, softness, and air permeability were focused on for the purpose of application of the membranes in surgical gown cuffs, bandages, face mask ear loops, etc.

Experiment

Preparation

PP/PBE fibrous membranes were prepared using a melt-blowing machine (MB400-01A, Suzhou Doro New Material Technology Co., Ltd, China) (Figure 1). PP/PBE chips were firstly prepared by blending PBE chips and PP chips and the PBE mass fractions used were 0 (Pure PP), 15, 30, 60, and 80 wt%, respectively. PP/PBE chips then were fed into a screw extruder through a hopper, and melted into the blending melt. The screw extruder diameter was 25 mm, and extruder length-to-diameter (L/D) was 28:1. The blending melt passed through a filter and was pumped into the nozzles inside the spinneret by a gear pump. The nozzle diameter was 0.25 mm, and the spinneret temperature was 265°C. Meanwhile, compressed hot air with a temperature of 265°C and pressure of 0.1 MPa was ejected through the slits on both sides of the nozzles. Subsequently, the melt stream was attenuated and cooled into the PP/PBE micro/nanofibers. Finally, the PP/PBE micro/nanofibers were collected on a moving receiver and formed into PP/PBE fibrous membranes by the self-bonding between the fibers. Moreover, main melt blowing process setting was shown in Table S1.

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Figure 1.

Schematic of preparation of PP/PBE fibrous membranes in the melt blowing process.

Results and discussion

Crystal structure

The WXRD curves of samples of the PP/PBE fibrous membranes with PBE mass ratio of 0 (Pure PP), 15, 30, 60, and 80 wt% were displayed in Figure 2. The curves showed that all the samples have a broad diffraction peak in the range of 12°–24° in 2θ, and all the curves showed four peaks, which correspond to (110), (040), (130), and (131) of the lattice planes of the monoclinic α structure, respectively [30]. A strong peak was also observed at 16.0° where reported as the monoclinic β structure in the curves of samples with different PEG mass ratios of 5, 10, and 15 wt% [31]. This indicated that α-form and β-form PP crystals coexist in the PP/PBE fibrous membranes. It should be noted that the intensity of the diffraction peak gradually decreased as the proportion of PBE increased from 15 to 80 wt%. This may be because the excessive PBE retards the crystallization of PP in the PP/PBE blends and introduces the defects in crystals due to the formation of entanglements between PBE and PP [32]. From the above, the introduction of PBE disturbed the crystalline nature of PP and enlarged the amorphous region, thus reducing the crystallinity of the PP/PBE fibers from 27.9 to 14.1% [33,34].

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Figure 2.

WXRD curves of PP, POE, and PP/POE blends.

Morphology analysis

Figure 3(a) to (e) shows the surface morphology of PP/PBE fibrous membrane samples with PBE mass ratios of 0, 15, 30, 60, and 80 wt%, respectively. A typical porous fibrous membrane morphology composed of randomly distributed and overlapped smooth fibers was observed on the SEM images [35]. It is clear that fiber diameter size ranged from sub-micrometers to a few 10 s of micrometers, which exhibited in a binary structure [36]. Furthermore, fiber diameter distribution of the samples displayed that the non-uniform fiber diameter distribution of PP/PBE fibrous membranes ranged from 0.4 to 15 µm. More interestingly, the fiber diameter distribution exhibits two sections, the first diameter distribution section of fine fibers was in the range of 0.4–2 µm, and the second diameter distribution section of large fibers was in the range of 8–15 µm. For this binary structure, the large fiber acted as a framework thus improving strength and air permeability, while the fine fiber worked as a bridge enhancing the softness [37,38]. Both SEM images and fiber diameter analysis revealed that the frequency of fine fibers with a diameter <3 µm increased from 48.6 to 61.2% with the increase in PBE mass ratio from 15 to 80 wt% at the same spinning temperature and nozzle. In addition, the “fiber strand” phenomenon of two or three fibers interfused closely together and fine fibers with 3D twisted structures were observed and both increased with the increases in PBE mass ratio from 15 to 80 wt%, which enhanced the binarization structure. The twisted-fibers and the “fiber strand” phenomenon may be attributed to the flexibility of PBE and the miscibility behavior of the two polymers has significant effects on the formation of twisted structures [39–41]. Furthermore, revealed that the PP/PBE fibrous membrane retained the typical porous fibrous morphology consisting of overlapped and staggered micro/nanofibers in the horizontal direction and the binary structure was a 3D porous network that was formed as multilayer thin fiber webs in the thickness direction.

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Figure 3.

Surface SEM images of the PP/PBE fibrous membranes with different PBE mass ratios including (a) 15, (b) 30, (c) 60, and (d) 80 wt%. (e) Cross sectional SEM images of samples with PBE mass ratio of 60 wt%.

Tensile strength

As illustrated in Figure 4(a) and (b), the machine direction (MD) elongation at the break was 23.7, 34.1, 42.2, 102.9, and 158.3% and the cross direction (CD) elongation at the break was 17.9, 34.9, 41.2, 125.8, and 204.7% for the PP/PBE fibrous membrane samples with different PBE mass ratios of 0, 15, 30, 60, and 80 wt%, respectively. The results indicated that addition of PBE significantly enhanced the elongation at the break. Furthermore, the comparison of the elongation–strength curves of the samples with the addition of PBE and that of pure PP indicated that the former presented much more obvious gentle changes and smaller slopes. This phenomenon may be clarified by a broken mechanism and as illustrated in Figure 4(c). The binary structured PP/PBE fibrous membrane was assumed as a fiber web consisting of two kinds of self-bonded fibers, one being fine fibers with a diameter of 1 µm and the other being large fibers with a diameter of 5 µm [42–44]. Thus, fine fibers are extended first because they are easy to stretch, and the large fibers then slip and align under an suppose of each fiber has similar behavior [22,45]. Then fine fibers begin to generate plastic deformation and fracture, and thus gradually lose their reinforcing effect on the fiber membrane. Meanwhile, further slipping of the large fibers causes the breakage of the adhesion point. Finally, the fibrous membranes break due to the cumulative results of single fiber breakage and broke at self-bonding point [46]. Furthermore, an interesting phenomenon is that the elongation at the break in the MD was bigger than that in the CD when the PBE mass ratio was less than 60 wt%, which is similar to typical melt blown fibrous membranes [35]. However, as the PBE mass ratio increased from 60 to 80 wt%, the elongation at the break in the CD was bigger than that in the MD. This may be due to the fact that the main deformation of the samples was the slipping between the fibers when the PBE mass ratio was less than 60 wt%. These results illustrated that deformation ability was enhanced by further increasing the PBE mass ratio.

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Figure 4.

Strength – elongation curves of the PP/PBE fibrous membranes in (a) MD and (b) CD. (c) Diagrammatic for the broken mechanism of the binary structured fibrous membranes.

Elasticity

Elasticity tests further quantitatively demonstrated the resilience and deformation capacity of the PP/PBE fibrous membrane samples. As shown in the corresponding stress–strain curves (Figure 5(a) to (d)), the stress first increased nonlinearly with the strain up to the 100%, and was then followed by a fast unloading along a different stress–strain path, which was responsible for the release of the deformation energy [47]. It was noted that the area of the hysteresis loops decreased with the increase in the PBE ratio from 0 to 80 wt% in the strain-release cycles, which suggested that the introduction of PBE was helpful to improve elasticity recovery. In addition, the slope of stress–strain curves of the samples with a PBE mass ratio of 60 wt% was smaller than that of pure PP. This phenomenon implied that adding PBE content was useful for developing fabric with fine elasticity and the higher PBE content led to easier deformation of samples under stress. Moreover, the plastic deformation rate in the MD decreased from 37.1 to 19.8% with the increase in the PBE ratio from 0 to 80 wt% and the elastic recovery rate in the MD increased from 62.9 to 80.3% under the same conditions, and the characteristic parameters of samples were shown in Tables S2 to S4. This change may be attributed to two factors: slipping and aligning between the fibers under tension leading to the formation and extension of fibers displayed in the deformation mechanisms as identified in previous studies [48,49]; and fibers with high PBE content exhibiting high deformation ability. From the above, excellent elasticity should be attributed to the introduction of PBE and the binary structure of the fibrous membranes.

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Figure 5.

Elastic recovery curves of PP/PBE fibrous membranes varying with PBE mass ration: (a) cycle 1 in MD, (b) cycle 2 in MD, (c) cycle 1 in CD, (d) cycle 2 in CD.

Softness

The softness scores of samples with varying PBE ratios and mass per unit areas are shown in Figure 6(a) and (b). Generally speaking, the higher the softness score, the softer the fabric will be [50], i.e., they deform more easily under force. It can be seen that the softness values of the samples increased from 15.9 to 54.3 with the increases of PBE mass ratio from 15 to 80 wt%. Compared to the samples of pure PP fibrous membranes with softness value of 15.3, a significant increase in the softness of the fibrous membranes. This may be attributed to the following reasons. (a) Decreased crystallinity (Figure 2) is beneficial to improve deformation ability. (b) A higher frequency of sub-micrometer fibers with a diameter <3 µm. The softness of the fibrous membranes increased with the increase in sub-micrometer fiber frequency. Moreover, the softness score increased from 37.7 to 73.7 as the mass per unit area of the samples decreased from 129.89 to 40.12 g/m². This is mainly because as the mass per unit area decreases, the entanglement strength between the fibers is reduced, making the fibrous membranes softer [15]. From the above, it can be concluded that adding PBE mass ration ranged from 20 to 80 wt% and the decrease in the mass per unit area of the fibrous membrane were both helpful for improving softness.

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Figure 6.

Softness scores of PP/PBE micro-nanofibrous membranes varying with (a) PBE mass ratios at mass per unit area ranged from 131.48 to 127.68 g/m², (b) mass per unit area at PBE mass ratio of 60 wt%.

Air permeability

Air permeability, which is related to the degree of moisture in the environment and oxygen permeability, is very important for wound dressing and personal hygiene [51]. The effects of PBE mass ratio and mass per unit area on the air permeability of the PP/PBE micro-nanofibrous membranes are shown in Figure 7(a) and (b). It was noted that air permeability decreased from 1292 to 844 m³/m² · h as the PBE mass ratio increased from 15 to 80 wt%. The main reason for this may be that the binary structure of the fibrous membrane consisted of fibers with diameters ranging from sub-micrometers to tenth-micrometers. From the SEM images, the addition of PBE appeared to produce a lot of sub-micrometer fibers, which decreased air permeability. Meanwhile, air permeability increased from 856 to 2257 m³/m² · h as the mass per unit area of the samples decreased from 129.9 to 40.1 g/m². This may be attributed that the number of fibers decreased in the process of the airflow penetrating the membrane, and the air permeability was improved because of the decrease in air resistance.

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Figure 7.

Air permeability of PP/PBE fibrous membranes varying with (a) PBE mass ratios at mass per unit area ranged from 131.48 to 127.68 g/m² and (b) mass per unit area at PBE mass ratio of 60 wt%.

Conclusion

A binary structured PP/PBE fibrous membrane with high flexibility was prepared via a melt blowing process in this study. A non-uniform fiber diameter distribution of PP/PBE fibrous membranes in the range of 0.4–15 µm was acquired, and the frequency of fibers with a diameter <3 µm increased from 48.6 to 61.2% with an increase in PBE mass ratio from 15 to 80 wt%. Benefiting from the binary structure morphologies, the resulting 80 wt% PP/PBE fibrous membranes possessed a fine elastic recovery rate of 80.25% in the MD, air permeability of 844 m³/m² · h and high softness values of 54.31. Moreover, the binary structure of the fibrous membrane plays an important role in improving its performance in many applications, including bandages, elastic bands for surgical masks and surgical gowns.

Supplemental Material