Effect of waterborne polyurethane coating on the durability and breathable waterproofing of electrospun nanofiber web-laminated fabrics

Abstract

An electrospun nanofiber web membrane is thinner, lighter and more moisture permeable than other types of membrane, but it has low waterproof and has poor abrasion resistance. To improve its waterproofness and abrasion resistance a waterborne polyurethane coating has been applied and the change in water vapor permeability and waterproofness, examined, together with its effect on the fabric structure. The effect of the hydrophilicity of the coating resin on these were properties was also assessed. Taffeta and taslan were used as the base fabrics, and the hydrophilicity of the coating resins was maintained at 10, 15, and 20%. Conclusions drawn from the study were that following the coating process the abrasion resistance and waterproofness improved; in taffeta the water vapor permeability decreased, but in taslan both properties decreased. Water vapor permeability and waterproofness were not significantly affected by the hydrophilicity of the coating resins. The surface flatness of the base fabric is shown to be an important factor in application of the waterborne polyurethane coating.

Keywords

electrospun nanofiber waterborne polyurethane abrasion resistance water vapor permeability waterproofness

Electrospun nanoweb membranes contain ultrafine pores as a result of their high surface area per unit volume and their layered structure. For this reason they are highly porous and form ultra-lightweight components. In clothing they may therefore be more comfortable, breathable, and waterproof than other types of membrane. However, during the formation of the web structure electrospun nanoweb membranes typically lack the binding force to retain the nanofibers within the web structure. Moreover, they exhibit poor physical characteristics;¹ for example when used in the manufacture of breathable waterproof materials the membrane surface is easily scratched or degraded. This unfortunately leads to a reduction in waterproofness, abrasion resistance and washability, which limits their commercial value.

The waterproof membranes developed so far exhibit good performance properties in terms of breathability and waterproofness; their generally poor mechanical properties have been dealt with by introducing micro-pores into the structure or by coating the membranes with a hydrophilic resin, and in practice a multilayered membrane structure is commonly employed. For instance, Gore-Tex® XCR has excellent breathability due to its microporous structure, but it tends to become contaminated with airborne dirt, as well as suntan lotion, skin exudates, detergents and surfactants, and condensed moisture tends to collect in the pores. This necessitates a pore-sealing process in which the polytetrafluoroethylene (PTFE) membrane surface is provided with a hydrophilic polyurethane layer; in effect the resin is used to consolidate the membrane layers, forming a multilayered structure.^2,3

The coating of nanoweb membrane fabrics may improve the mechanical properties, but the breathability and waterproofness, which are opposed in character, may be out of balance. The higher the hydrophilicity of the coating material, the more breathable it becomes; on the other hand too much hydrophilicity results in poor dimensional stability and abrasion resistance since the polymer tends to melt or shrink in contact with water.^4,5 For example, when coating a cotton fabric with waterborne polyurethane (WPU), the hydrophilic and hydrophobic properties of the resin are known to have a direct effect on the breathability and waterproofness of the coated fabric. As hydrophilicity increases the degree of fabric breathability, the waterproofness of the fabric is decreased.⁶ A process that utilizes both hydrophilic and hydrophobic materials is a thus candidate to be considered. In addition, the structural characteristics of the fabric can significantly affect the breathability, waterproofness and durability. This means that WPU treatment conditions may vary with the characteristics of the fabric. In a similar manner, the WPU coating treatment can also affect the structural characteristics of the fabric, such as its thickness and porosity, which in turn may affect its breathability and waterproofness.

In the present study multilayer nanoweb membrane functional fabrics were prepared by coating the laminated web with a WPU resin. As the nanoweb membrane was constructed from a hydrophobic PU, a hydrophilic PU was used to coat the surface of the nanoweb membrane in order to prevent it dissolving and disintegrating. Depending on the number of hydrophilic radicals attached and their chemical structure,⁷ WPUs with different characteristics can be formulated, covering a wide range of end uses.⁸ It was expected that modifications in breathability and waterproofness would depend on the ratio of the hydrophilicity and hydrophobicity of the coating material and on the structural characteristics of the base fabric.^9
10
11 Taffeta and taslan base fabrics with different nylon yarn sizes and textures were selected for laminating the nanoweb membrane. After coating the laminates with WPU resins the fabric was then assessed for changes in abrasion resistance, breathability, and waterproofness.

Materials and methods

Two very commonly used outdoor fabrics, taffeta and taslan, were selected as base fabrics. To these an electrospun nanoweb membrane was laminated to form a two-layer fabric. The fabrics were coated with WPU, and the characteristics of the finished fabrics were comparable to those of commercial Gore-Tex XCR. Table 1 summarizes the structural and treatment characteristics of the three sample fabrics.

Table 1.

Characteristics of the specimen fabrics

Fabric	Content (%)	Yarn count (D^a/F^b)	Fabric count (WXF/inch²)	Weave	Weight (g/m²)	Thickness (mm)	Finish
Taffeta	Nylon100	20/34 FDY^c× 20/34 FDY	202 × 192	plain	38	0.06	Ciré, WR^e
Taslan	Nylon100	70/24 FDY × 160/96 ATY^d	140 × 68	plain	136	0.22	WR
Gore Tex XCR	Nylon100	70/24 FDY × 160/68 ATY	136 × 64	plain	140	0.24	WR

Denier, ^b number of filaments, ^c fully drawn yarn, ^d air textured yarn, ^e water repellent finish.

The nanoweb membrane used in the study was produced by electrospinning hydrophobic polyurethane (Finetex, Korea). It had high porosity and a large pore diameter in a thin lightweight membrane as illustrated in Table 2.

Table 2.

Characteristics of the hydrophobic polyurethane nanoweb membrane

Property	Test method	Measurement
Pore diameter (µm)	Capillary flow porometer	0.65
Porosity (%)	Capillary flow porometer	86
Water penetration resistance (Pa)	ISO 15496	85900
Water vapor resistance (Ret)(m²Pa/W)	ISO 11092	0.15
Air permeability (mm/s)	DIN EN ISO 9237	8.5
Thickness (µm)	SEM	10
Weight (g/m²)	ISO 6741–1	5.2
Elongation (%)	JIS L 1096B	88 × 83
Tensile strength (kg f/in)	JIS L 1096D	0.27 × 0.24

Figure 1 shows the morphological characteristics of the polyurethane nanofiber web. The fibers were comparatively uniform in dimensions, 200–300 nm in diameter, and the web comprised layers of nanofibers, giving an even distribution of pores.

Figure 1.

SEM imge of polyurethane nanofiber web membrane.

Before the lamination process, both taffeta and taslan were treated at 17 m/min for water repellency using a fluorine-based chemical at a concentration of 4% in water. The taffeta and taslan gave wet pick-up rates of 1.2% and 3.0%, respectively (o.w.f). The taffeta was then ciré finished twice by rolling the back side of the fabric at 170℃.

Lamination of the base fabric with a nanoweb membrane

The nanoweb membrane was laminated to the base fabric by a hot-melt dot-lamination process using a gravure coating machine. A moisture-reactive hot-melt adhesive was used at a viscosity of 8,000 cps. The coater, with a 560–860 mm cell size, was run at 100℃ at a speed of 15 m/min. After bonding, the laminated fabric was cured for 24 h at 30℃ and 70% RH. The adhesive add-on was 9 g/m² for taffeta and 13 g/m² for taslan.

Coating of the membrane surface with waterborne polyurethane

Three types of WPU were prepared in order to determine the effect of polyurethane type on the breathability and waterproofness of the fabric. By varying the ratio of the hydrophilic polyethylene glycol (PEG) and the hydrophobic polyethylene terephthalate (PET) in the WPU, three types were produced and their characteristics are shown in Table 3. Hardener (Nanux NH–120) was mixed with an equal weight of water and added to each coating agent in the ratio 100 : 3. Thirty minutes before the coating process, a cross-linker was added to assist adhesion to the base fabric. The viscosity of the resin was within the range 3,000 ± 200 cps at 25℃.

Table 3.

Characteristics of waterborne polyurethane

Recipes	Waterborne polyurethane type
Recipes	A	B	C
Viscosity(cps)	3000	3000	3000
Hydrophobic (%)	90	85	80
Hydrophilic (%)	10	15	20
Wetting agent (phr)	0.5	0.5	0.5
Soft feel modifier (phr)	1	1.5	2
Anti-blocking agent (phr)	0.3	0.4	0.5
Cross-linker (phr)	1	2	3

To avoid the formation of thin irregular spots on the fabric surface, the coating process was repeated twice, giving a wet pickup rate of 6 ± 0.5 g/m². The gravure roll coater used had a 120 mesh. The coated fabric was dried at 100–120℃ at 8 m/min. The results are shown in Table 4, the 20N and 70 N samples being nanoweb-laminated fabrics and the 20A, 20B, 20C, 70A, 70B and 70C samples were nanoweb-laminated and WPU-coated.

Table 4.

Fabric specimens according to fabric type, hydrophilicity of the WPU and number of coatings

Specimen	Fabric type	Waterborne polyurethane type	Number of coating
20 N	20D Taffeta	Not treated	Not treated
20A		A	2
20B		B	2
20C		C	2
70 N	70D Taslan	Not treated	Not treated
70A		A	2
70B		B	2
70C		C	2
Gore Tex XCR	70D Taslan	–	–

Figure 2 illustrates the preparatory process used to obtain the various specimens.

Figure 2.

Preparation of the various specimens.

Physical properties

The abrasion resistance of the nanoweb membrane fabrics and the Gore-Tex XCR were measured using ASTM D3884 (the Taber method), employing a 250 g CS–10 abrasive at a weight of 25 N. After 20 abrasion cycles the weight before and after abrasion was measured and the percentage abrasion calculated using Equation (1):

Abrasion resistance % = \frac{{weight}_{before abrasion} - {weight}_{after abrasion}}{{weight}_{before abrasion}} \times 100

(1)

The water vapor transmission was measured using the ASTM E96 desiccant method. A circular cup of area 0.00283 m² and height 25 mm containing 33 g of CaCl₂ was placed in a chamber at 40 ± 2℃ and 90 ± 5% RH with a wind velocity of 0.5 m/s.

The weight of the cup was measured over 24 h and the water vapor transmission rate calculated according to Equation (2),

P = \frac{A_{2} - A_{1}}{S}

(2)

where

is the water vapor transmission rate over 24 h (g/m²),

A₂ – A₁

is the weight change (g) of calcium chloride in the test assembly in 24 h, and

is the area of the specimen (m²).

The waterproofness of the sample fabrics was measured according to ISO 811 (a low water-pressure method). The water temperature was maintained at 20 ± 2℃ and the rate of increase in water pressure was 600 mm/min.

To analyze the structural characteristics of the fabric samples, cross-sections were examined by scanning electron microscope (JSM 5410v, Joel, Japan) at magnification levels of 250× and 400×.

Results and discussion

Abrasion resistance of the nanoweb membrane after coating with waterborne polyurethane

The poor abrasion resistance of nanoweb membranes is an important issue since they are easily damaged by external scratching or during laundering, resulting in reduced waterproofness. To minimize this problem WPU was coated onto the membrane surface and its effect on the abrasion resistance was assessed.

As shown in Table 5, the two-layer fabric samples not treated with WPU (20 N taffeta and 70 N taslan), showed a greater weight decrease ratio than samples coated with WPU (20 C taffeta and 70 C taslan).

Table 5.

Abrasion resistance of nanoweb–membrane fabrics and Gore-Tex XCR

Specimen	Weight (g) (before abrasion)	Weight (g) (after abrasion)	Abrasion resistance (%)
20N	0.803	0.800	0.37
20C	0.870	0.869	0.11
70N	1.545	1.540	0.32
70C	2.096	2.092	0.19
Gore Tex XCR	1.783	1.779	0.22

As illustrated in Figure 3, considerable abrasion took place along the circular abrader track on the fabric surface of the untreated 20 N taffeta and 70 N samples, which resulted in fabric weight loss due to the low tensile strength of the nanosized fibers and insufficient binding force to hold the fibers together in the web structure.¹² On the other hand, on the surface of the WPU-treated samples, 20C taffeta and 70C taslan, fabric weight loss was hardly noticeable. The weight loss of the WPU-treated samples was less than that of the Gore-Tex material and showed better abrasion resistance (Table 5). In other words, the WPU coating protected the fabric surface and helped to counteract the weakness of the nanoweb membrane.

Figure 3.

Surface of specimens before and after abrasion.

Breathability and waterproofness of the nanoweb membrane after waterborne polyurethane treatment

(a) Breathability

Figure 4 shows how the water vapor permeability (WVP) of taffeta and taslan decreased after coating with WPU. The untreated taslan–nanoweb laminate (70 N) showed slightly higher WVP than the untreated taffeta–nanoweb laminate (20 N). After WPU coating, however, a greater difference was observed between the two. The lighter and thinner taffeta was expected to show a higher WVP rate than the taslan, but the result was in fact the opposite. The degree of moisture permeating is proportional to the diameter and number of pores and inversely proportional to the thickness of the membrane.¹³ The taffeta was ciré-finished, during which pressure was applied at a uniform rate, thus minimizing the micropores in the fabric structure. In addition, lamination of the nanoweb structure and the base fabric taffeta made the surface more uniform, resulting in a more consistent WPU coating. This caused the laminate to lose 44.2% of its WVP, as illustrated in Figure 4.

Figure 4.

Comparison of water vapor permeability before and after WPU coating of laminated fabrics and Gore-Tex XCR.

Before coating, minute empty spaces were noted between the nanoweb membrane and the taffeta fabric, as shown for the 20 N sample in Figure 5. Even after coating the 20 C sample, the WPU appeared to adhere tightly to the membrane. However, in case of the untreated taslan 70 N sample, air spaces were apparent among the air-textured yarns (ATY). In addition, the filling yarns formed a rounder curvature and the warp yarns appeared more irregularly packed than in the 20 N sample, causing the fabric to make fewer contacts with the nanoweb membrane and creating greater porosity in the membrane structure.

Figure 5.

SEM images of cross-sections before and after WPU coating on laminated taffeta and taslan. (a) 20N, (b) 20C, (c) 70N and (d) 70C.

In the coated taslan fabric 70C sample the weft yarns appeared rounder and the air spaces were more pronounced within the yarn, as well as between the base fabric and the nanoweb. Since the fabric count was unbalanced and the weft yarns were air-textured with one half of the warping, it appeared that the weft yarns had been affected by the coating process and had shrunk. This would explain why there was less reduction in the number of air pores and why the WVP rate of the coated fabric had decreased by only 26%. The reduction in the number of air pores between the warp and filling yarns caused a decrease in the permeability to both air and water.¹⁴ The coated taslan 70C sample had thicker yarns and hence a lower yarn density than the coated taffeta 20C sample. The coated taslan therefore contained larger air pores, both among and between the yarns. After texturing, the yarns adopted an irregular curved configuration and thus produced an uneven fabric surface, which in turn made uniform lamination difficult. Even after coating the WPU onto the nanoweb membrane, air pores remained between the base fabric and the membrane, giving a lower reduction in the WVP rate.

In comparison to Gore-Tex XCR, the treated taffeta 20C sample showed a lower WVP rate, whereas the treated taslan 70C sample had a higher WVP rate. In general, coating lowers the WVP rate of functional fabrics, and it is therefore important to minimize the WVP reduction as far as possible. The coating helped to decrease the WVP rate of the taffeta and taslan, but WPU-treated taslan still showed a higher WVP rate than Gore-Tex XCR after coating (Figure 4).

(b) Waterproofness

The surface flatness of the base fabric affects both WVP and waterproofness. In the ciré finish used to flatten the surface of taffeta, air pores between the yarns are reduced while the fabric count increases; this typically results in a reduced WVP rate but increases the waterproofness. In addition, the smoother surface provides a more uniform background for lamination and for coating. To sum up the effect of the base fabric structure on the lamination as well as the WPU coating processes, it was noted that the flatness of the fabric is an important factor, which in turn determines the WVP rate of the treated fabrics.

The waterproofness of the fabric specimens is shown in Figure 6. Although the untreated taffeta (20N) sample was thinner than the untreated taslan (70N) sample, the taffeta showed better waterproofness due to its compactness, with finer yarns in the fabric structure. The taffeta was also ciré-finished to flatten the surface, compressing the fine yarns and resulting in improved waterproofness.

Figure 6.

Comparison of hydrostatic pressure, before and after WPU coating on laminated fabrics and Gore-Tex XCR.

It was initially expected that the WPU coating would improve the waterproofness in every case, but this was in fact found to depend on the fabric type; for example taffeta 20C showed improved waterproofness, whereas the waterproofness of taslan 70C decreased after coating. The reason for the increased waterproofness of the coated taffeta was similar to the effect of the WVP rate: the flat taffeta surface not only provided uniform adhesion during lamination between the base fabric and the nanoweb membrane, it also provided a smooth and consistent background for coating, and the waterproofness of the fabric thus increased accordingly. On the other hand, the yarns in the taslan 70C sample were much thicker than those in the taffeta 70 samples.

Moreover, texturing the 70-denier yarns created bulky and irregular shapes, resulting in increased air pores in the nanoweb and between the nanoweb and the fabric during the lamination process. In general, the fabric thickness and porosity had a significant effect on the water vapor resistance of the fabric. After receiving the coating, however, the taslan still showed high air porosity. Moreover, the pores created an irregular coating of thick and thin portions with WPU through the engraved gravure rolls. Depending on whether there were background yarns or air pores in the fabric structure during the coating process, a pressure difference was created between the two portions, hindering the formation of a uniform coating. The difference in the internal pressure when water pressure was exerted was the ultimate factor lowering the waterproofness of the coated fabric.

When compared with Gore-Tex XCR for waterproofness (Figure 6), the coated and laminated taffeta (20C) sample showed lower hydrostatic pressure than that of the coated and laminated Gore-Tex XCR, whereas the coated and laminated taslan (70C) sample showed a pressure slightly above half that of the Gore-Tex XCR value. Although the 20C sample showed lower hydrostatic pressure, it had excellent hydrostatic pressure relative to its thickness. In Figure 7, the coated and laminated 70C sample and the pore-sealing laminated Gore-Tex XCR are compared, in order to determine the effect of membrane thickness on the degree of waterproofness. Both face fabrics had a similar structure and density. It is seen that the membrane thickness clearly affected the waterproofness. It was shown that the thickness of the pore-sealed treated PTFE membrane of the Gore-Tex XCR was about 35 µm, while that of the WPU-coated and laminated nanoweb was approximately 13 µm. This demonstrates that greater membrane thickness leads to better waterproofness and lower water vapor permeability.¹⁵

Figure 7.

Comparison of nanoweb laminated and WPU-coated taslan vs. Gore-Tex XCR. (a) Coated and laminated 70C and (b) Pore-sealing laminated Gor-tex® XCR.

Effect of the ratio of hydrophilic to hydrophobic waterborne polyurethane

(a) Breathability

The ratio of the hydrophilic and hydrophobic components of the WPU affected the fabric breathability of the coated fabrics in terms of the WVP rate. The ratios of hydrophobic and hydrophilic polyurethane resins investigated in the present study were (A) 90/10%, (B) 85/15%, and (C) 80/20%. As shown in Figure 8, the 80/20% ratio gave the greatest degree of breathability for both taffeta and taslan samples, in other words the highest level of the hydrophilic polyurethane component provided the greatest degree of breathability.

Figure 8.

Comparison of water vapor permeability of nanoweb laminated fabrics with three different WPU coatings.

A hydrophilic resin absorbs moisture, and the absorbed moisture evaporates. It appears that higher hydrophilicity of the resin leads to better breathability. In an earlier study, in which WPU was coated directly onto a cotton fabric,¹⁶ it was found that breathability depended on the molecular length of the polyol in the WPU. In addition, as the hydrophilic component increased, the breathability increased and the waterproofness decreased.

In the present study, however, the high-to-low ranking order of the WVP rate as a measure of breathability by hydrophobic/hydrophilic PU components was C > A > B for taffeta and C > A = B for taslan. Although the most hydrophilic PU component showed the greatest breathability, the ratio of hydrophobic : hydrophilic PU components was not consistent in the two fabrics. Other factors, such as the fabric structure and the laminating and coating processes, and their combined variability, seemed also to affect the degree of fabric breathability.

(b) Waterproofness

Figure 9 shows the effect of the WPU coating on fabric waterproofness, measured in terms of the hydrostatic pressure in mm H₂O. The degree of waterproofness did not differ significantly with the type of WPU, A, B, or C. In the taslan, however, the 70C sample showed the highest degree of waterproofness, but there was no significant difference between the 70A and 70B samples.

Figure 9.

Comparison of hydrostatic pressure of nanoweb laminated fabrics with three different WPU coatings.

The hydrophobic components of the three WPU resins (A, B, and C) were 90%, 85%, and 80%, respectively. It was therefore expected that resin A would give the greatest waterproofness. For taffeta, however, the three resins showed similarly high levels of waterproofness. For taslan, resin C gave the greatest waterproofness, but it was still lower than that of the taffeta fabrics; resins A and B gave similar low levels of waterproofness (Figure 9). Overall, no pattern to the change in the waterproofness could be observed depending on the resin type. The fact that the 20-denier taffeta showed greater waterproofness than the 70-denier taslan indicated that waterproofness depended more on the evenness and uniformity of the coating treatment than on the characteristics of the resin.

Due to its uniform surface structure, an even coating was formed across the surface of taffeta and gave a similar degree of waterproofness regardless of the nature of the coating resin. Similarly, due to its uneven surface and hence an irregular coating taslan exhibited lower waterproofness. The thick irregular yarns in the latter fabric made the surface rough and uneven. Moreover, the irregularity appeared to be the cause of the weak and irregular composition of the nanoweb membrane. Accordingly, when water exerted pressure on a weak spot in the membrane it was able to pass through and these areas, giving poor waterproofness.¹³

When the nanoweb and Gore-Tex XCR were compared Gore-Tex XCR was seen to be thicker and denser, with a porosity of 82%, whereas the membrane had a porosity of 86%. This meant a higher degree of breathability for the nanoweb, whereas Gore-Tex XCR showed greater waterproofness. On the other hand the results of the present investigation did not enable us to draw the conclusion that the degree of waterproofness was related to the type of coating resin, and further research will be required.

Conclusions

The electrospun nanoweb membrane had a larger surface area and a greater degree of micropores in its structure than the other types of membrane examined. When used in functional fabrics these unique structural characteristics provided excellent breathability and air porosity, without compromising the thinness and low weight of the fabric. The drawbacks of the membrane, however, are poor abrasion resistance and a general lack of durability against external deforming forces, which in turn can give rise to problems with fabric care during use. We therefore attempted to improve the abrasion resistance of the nanoweb membrane by creating a multilayer structure in the membrane and base fabric, and by coating the membrane with WPU. Since such treatment is likely to affect fabric breathability and waterproofness, these characteristics were also investigated, specifically whether the structural characteristics of the base fabric and coating resin properties would influence the breathability and waterproofness of the treated fabrics.

It was found initially that the WPU coating gave improved abrasion resistance and durability against external deforming forces. Providing a multilayer structure to the membrane and the base fabric, and by coating the membrane, gave an improvement to the structural integrity of the fabric, thus improving its physical and mechanical properties in terms of breathability and waterproofness.

Secondly, a thinner fabric of lower density was found to reduce water vapor permeability.¹⁹ On the other hand, in regard to the effect of the structural characteristics of the fabric on its breathability and waterproofness, the 20-denier yarn taffeta sample – thin, dense, and ciré-finished – gave reduced breathability after WPU coating, but improved waterproofness. Due to its high fabric count taffeta had relatively low porosity per unit area. After ciré-finishing, however, its surface was flattened and thus provided much lower porosity and a smooth background for lamination with the nanofiber web or for coating with WPU. The uniform lamination and coating led to lower breathability but greater waterproofness in the finished functional fabric.

On the other hand the 70-denier taslan sample was comprised of air-textured yarns and gave a relatively low fabric density of much higher porosity than taffeta. It therefore showed less reduction in the degree of breathability, but its waterproofness was lower after WPU coating compared with that of the 20-denier taffeta sample. It is thus concluded that in this instance porosity is a more important factor than fabric thickness.

Since taslan is composed of thick and textured yarns, pore spaces were developed during the lamination process. When the fabric was coated with WPU on a gravure roll these spaces eventually caused unevenness in the surface of the fabric. From these results we conclude that the structural characteristics of the fabric were important during the lamination and coating treatments, since they directly affected the breathability and waterproofness of the fabric. On the other hand, these characteristics differed significantly according to the pore size and flatness of the finished fabric. In comparison, the commercial fabric Gore-Tex XCR had a base fabric similar to that of taslan but showed greater waterproofness, apparently due to differences in the membrane thickness and porosity of the two fabrics.

Thirdly, the breathability and waterproofness of the WPU-coated functional fabrics did not differ with the type of coating resin used. In both taffeta and taslan the most hydrophilic coating resin produced the greatest breathability, but no relationship was established between breathability and the hydrophilicity of the resin type. The same was true of the waterproofness of the finished fabrics. It was concluded in the first place that a multilayer structure was not easy to create in a thin nanoweb membrane. Its irregular construction led in turn to changes in water vapor permeability and hydrostatic pressure and this might have nullified any trend in the change in these characteristics due to the type of coating.

This investigation has shown that the limitations of poor abrasion resistance and durability of the nanoweb membrane may be overcome by coating the membrane surface with WPU. The structural characteristics of the base fabric are important, as they may influence the effect of both lamination and coating processes. In addition, the breathability and waterproofness of the finished fabric differ with the structural properties of the base fabric, such as its density and yarn and fabric thickness. The base fabric appears to be crucial in sustaining the resin within the fabric structure and for maintaining the mechanical strength of the coated fabrics.

Further investigation is required to investigate the effect of different types of WPU coating resins, variations in the lamination and coating processes, and in calculating the appropriate amount of resin for these membrane surfaces.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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