NIPAM polymer/polyurethane resin films and the moisture transport characteristics of film-treated fabrics

Abstract

Numerous studies have addressed the use of poly(N-isopropylacrylamide) or (poly-NIPAM) homopolymers in drug delivery systems. However, their application has not been extended to textiles owing to their high solubility in water. In this study, we investigated thermo-responsive, hydrophilic, breathable, waterproof films as well as fabrics treated with these films. To impart temperature-sensitivity, a poly-NIPAM polymer was used, with water solubility at or below the lower critical solution temperature (LCST). The poly-NIPAM used in this study had to be insoluble in water, even at the LCST or below it. Consequently, this study was performed using films wherein poly-NIPAM was copolymerized with hydrophilic polyurethane (PU). The change in water solubility of the homopolymer at temperatures greater than or less than the LCST could be verified from the water absorption coefficient of the films observed at various temperatures. Subsequently, we ascertained the functionality of the temperature-sensitive water absorption feature. In addition, we validated the functionality of the breathable, waterproof films and the condensation-inhibition capacity of film-treated fabrics. As a result, we postulated that the fabrics coated with poly-NIPAM/PU copolymer films could serve as functional materials for apparel with novel and improved moisture transport characteristics.

Keywords

polymers resin waterproof films poly-NIPAM fabrics lower critical solution temperature

For sportswear and rainwear, functional apparel materials have been researched and developed as breathable, waterproof, and insulating materials based on the mechanism relating to human thermoregulation.^1,2 Further advancements have been achieved in the performance of breathable and waterproof materials.³ For instance, improvements have been made in the breathable and waterproof functions to wick away perspiration from the body while preventing raindrops (from drizzle, thunderstorms, etc.) from permeating through the fabric.⁴

For breathable, waterproof materials, there is a general trend in which the moisture permeability performance improves in tandem with the condensation-inhibition performance. During short, intense exercise, or when there is a significant temperature difference between the inner and outer layers of the clothing, the generated water vapor cannot be completely transmitted by the moisture permeability property of the fabric. The water vapor remaining inside the clothing generates condensation, which results in uncomfortable sensations such as chills. The purpose of the high-moisture permeability of breathable, waterproof materials is to keep the humidity inside the clothes to a minimum. In order to impart condensation-inhibition performance to breathable, waterproof materials, high-moisture permeability, moisture absorbency, and water absorbency are also necessary. Thus, researchers have been extensively studying ways to improve condensation-inhibition performance through moisture transport characteristics, focusing on the aforementioned requirements.^5–7 In addition, a study was conducted on blending fine particles with high moisture absorption and desorption properties into a coated fabric,⁸ and a four-layer garment system was evaluated under seven conditions.⁹ In addition, the adsorption behavior and drying function of functional knits have been analyzed.¹⁰

In our most recent research,¹¹ the impact of temperature-sensitive, hydrophilic functionality on improving condensation-inhibition characteristics was examined with copolymerizing water-soluble poly(N-isopropylacrylamide) (poly-NIPAM) and hydrophilic polyurethane (PU). Specifically, poly-NIPAM with water solubility at temperatures at or below the lower critical solution temperature (LCST) was used. Generally, the LCST of NIPAM homopolymers is 32 ℃. Therefore, NIPAM homopolymers are temperature-sensitive polymers that dissolve in water at temperatures ≤32 ℃, but are largely insoluble above that temperature. In previous research, temperature-dependent sol-gel transitions have often been employed. Specifically, they have been applied in drug delivery systems and in attempts to control cell adsorption/desorption accompanying temperature changes when used in an immobilized enzyme carrier and a cell culture substrate.^12,13 Additionally, they have been applied in macromolecular electrolyte complexes.¹⁴ Although recent studies have used NIPAM in hydrogels,^15,16 none of these studies characterized the reversible properties below and above the LCST.

In studies using poly-NIPAM as a breathable, waterproof film, the water-soluble poly-NIPAM must exhibit temperature-dependent transposability. However, because water-soluble poly-NIPAM is difficult to use, it is imperative to render it insoluble in water. Thus, either a monomer (with a different LCST) or a urethane reaction (with hydrophilic PU) was used in combination with a substrate NIPAM to form water-soluble poly-NIPAM, which was then copolymerized with hydrophilic PU. The objective of the second step was to make the water-soluble poly-NIPAM water resistant, thus also making it insoluble in water. This insolubility was necessary to render the poly-NIPAM functional as a temperature-sensitive, hydrophilic film material. In our most recent research,¹¹ the structure of the poly-NIPAM/PU copolymer film was observed and analyzed, and the basic contents were reported. The purpose of the current study is to create a laminated fabric using this temperature-sensitive poly-NIPAM/PU copolymer and polyester fabric and to evaluate the performance of the fabric with respect to its moisture transfer properties, such as dew condensation suppression. Therefore, we used poly-NIPAM/PU copolymer films, obtained as outlined above, and reconfirmed the LCST from the relationship between the film coefficient of water absorption and the water temperature using a different method from that used in the previous report. In addition, we confirmed the temperature-sensitive, breathable, waterproofing functionality and the condensation-inhibition performance of fabrics treated with these films and polyester fabric. As a result, considering the moisture transport characteristics, we report that the fabrics in this study could serve as functional apparel materials while preventing dew condensation in a temperature-sensitive manner; these properties have not been observed in conventional fabrics thus far.

Experimental methods

Adjustment of poly-NIPAM/PU copolymer

Poly-NIPAM polymers with water-solubility at or below the LCST were rendered insoluble in water by performing a reaction with hydrophilic PU in the same manner as in our previous report.¹¹ In stage 1 of the process, we synthesized three types of poly-NIPAM diols with adjusted LCSTs. The monomer compositions are shown in Table 1, wherein No. 1 to No. 3 were used for NPU1 to NPU3 in Table 2, respectively. These were then copolymerized with hydrophilic PU in stage 2. Then, dimethylformamide (DMF) solutions of the poly-NIPAM/PU copolymers were adjusted to a solid content concentration of 30 wt.% and a viscosity of 50 Pa·s (at 30 ℃); further, the three LCST-adjusted poly-NIPAM/PU copolymers, denoted by NPU1, NPU2, and NPU3, were synthesized at a ratio of 15:85. In addition, hydrophilic PU was synthesized from the above-mentioned PU component with a negligible weight percentage of hydrophobic polymer (0:100), and, hereafter, it is denoted by PU. Finally, as a comparative study resin, commercial styrene-acrylonitrile (SAN) resins (Daicel Polymer Ltd., Cevian-N) were incorporated into the hydrophilic PU as the hydrophobic resins in place of poly-NIPAM at the same ratio of 15:85, which is denoted by SAN/PU. The compounding ratios of all samples are listed in Table 2.

Table 1.

Compositional ratio of monomers and initial reagents

	No. 1	No. 2	No. 3
NIPAM (wt%)	80.0	60.0	40.0
MA	6.7	13.3	20.0
TBAA	12.0	25.4	38.7
HEA	1.3	1.3	1.3
AIBN (A/I)	73.0	73.4	73.8

AIBN: azobisisobutyronitrile; HEA: 2-hydroxyethyl acrylate; MA: methyl acrylate; NIPAM: N-isopropylacrylamide; TBAA: N-t-butyl acrylamide.

Table 2.

Polymer sample labels and compounding ratios of the films

Sample label	NPU1	NPU2	NPU3	PU	SAN/PU
Resin type	Poly-NIPAM/PU copolymer			Hydrophilic PU	SAN/Hydrophilic PU
Poly-NIPAM/PU ratio	15:85	15:85	15:85	0:100	–
SAN/PU ratio	–	–	–	–	15:85
LCST (℃)	25	20	0	–	–

NPU1–3: N-isopropylacrylamide/polyurethane with three different adjusted lower critical solution temperatures (LCSTs); PU: polyurethane; SAN/PU: styrene-acrylonitrile/polyurethane; Poly-NIPAM/PU: poly(N-isopropylacrylamide)/polyurethane.

Measurement of the coefficient of water absorption of poly-NIPAM/PU copolymer films

The coefficient of water absorption (WA%) of the films cast from the NPU1, NPU2, NPU3, PU, and SAN/PU polymers was measured by the following procedure. First, the DMF solutions of poly-NIPAM/PU copolymers NPU1 to NPU3, as well as PU and SAN/PU, were separately cast onto release papers. They were then prepared into solution layers of regular thickness, by draining, using a Baker Applicator. Thereafter, the solution layers were dried for 20 min in a drier (at 120 ℃), to obtain cast films (50 μm thick), hereafter labeled with the suffix -F to denote a film, i.e., NPU1-F to NPU3-F, PU-F, and SAN/PU-F.

Subsequently, the films were cut into squares (of 50 mm sides), and their dry mass a₁ (g) was measured at 20 ℃ and 50% relative humidity (RH). Thereafter, each of the films was soaked for 10 min in an ion-exchange water bath that was adjusted to various temperatures. The films were then removed, and the moisture that adhered to the film surfaces was blotted using filter paper at 20 ℃ and 50% RH. Immediately thereafter, the mass a₂ (g) of the film was measured. The water absorption (WA) at each temperature was determined using the following formula, and the average and standard deviation of three measurements were calculated:

WA (%) = a_{2} / a_{1} \times 100 .

Creation of the laminated fabrics

Using the aforementioned procedure, cast films of NPU1–NPU3, PU, and SAN/PU were fabricated and adjusted to a thickness of 10 μm. A plain weave, polyester fabric was then prepared as an outer fabric with dimensions of 78 dtex × 216 filaments (warp), and 156 dtex × 432 filaments (weft). The fabric was then used after it underwent water-repellent finishing. The manufactured fabric had a warp density of 140 threads/25.4 mm, a weft density of 83 threads/25.4 mm, and a basis weight of 116 g/m². A moisture-cured PU adhesive was arranged (at 16 meshes/dot) on the cast films (10 μm thick). The plain weave fabric manufactured as mentioned above was then applied to the cast films with the adhesive, thereby producing laminated fabrics, hereafter labeled with the suffix -LF, i.e., NPU1-LF to NPU3-LF, PU-LF, and AS/PU-LF. Figure 1 shows an SEM photograph of the NPU2-LF cross-section.

Figure 1.

SEM photograph of the NPU2-LF cross-section.

Evaluation of moisture transport characteristics of the laminated fabrics

Measurement of moisture permeability

The moisture permeability was measured using the JIS L 1099 B-1 potassium acetate method at three additional water temperature levels, 13 ℃, 33 ℃, and 43 ℃, as well as the 23 ℃ stipulated level. The moisture permeabilities of NPU1-LF to NPU3-LF, PU-LF, and AS/PU-LF were calculated.

Measurement of condensation values

Three sheets of each of the laminated fabrics (NPU1-LF to NPU3-LF, PU-LF, and SAN/PU-LF) were prepared as square test pieces (of 120 mm sides) to determine the condensation values. Figure 2 shows a schematic of the measuring device. A 500 cm³ cylindrical glass container (inner diameter: 60 mm) with thermal insulation capacity was used as the measuring device. This device was thermally insulated by enclosing the device with ∼10 mm thick polystyrene foam. The level of heat insulation was determined as follows. Water at a temperature of 40 ± 1 ℃ was poured into the measuring device until the level was 20 mm from the top (∼462 cm³). The device was then placed in a constant temperature/humidity chamber (for 1 h) at 10 ℃, 60% RH, and 0.8 m/s airflow, whereupon the water temperature (in the device) could be maintained at 30 ± 1 ℃. The condensation value was measured by pouring water (of 40 ± 1 ℃) into the measuring device, up to 20 mm from the top. The film surface that would be in direct contact with the human body when worn was then positioned on top of the device, facing the water surface. The periphery was subsequently affixed with a fastener and then placed in a constant temperature/humidity chamber at 10 ℃, 60% RH, and 0.8 m/s airflow (for 1 h). Moreover, the test pieces were placed in a constant temperature/humidity chamber at 10 ℃, 60% RH, and 0.8 m/s airflow (for 24 h) prior to measurement. The condensation value was calculated using the following formula, and the mean value of three measurements was used.

Condensationvalue (DC) = (m_{1} - m_{2}) / (0 . 03^{2} π)

where m₁ (g) represents the mass of the laminated fabric test piece after 1 h of the experiment, and m₂ (g) is the mass after removing water droplets from the surface of the test piece of laminated cloth with filter paper immediately after the measurement of m₁ (g).

Figure 2.

Schematic diagram of the apparatus to measure condensation values.

Measurement of vapor resistance (Ret)

The vapor resistance, or the resistance of evaporation of a textile (Ret), was measured in conformity with ISO 11092 using a T-H Permeability Tester (TIMS-1, made by Toyo Seiki Seisaku-sho Ltd.).

The test conditions involved varying the temperature (from 21 ℃ to 33 ℃) of the water vapor on the interior of the test material, which was the film-side surface of the fabric. The measurements were conducted on NPU2-LF, PU-LF, and SAN/PU-LF, and the ambient temperature and humidity were 20 ℃ and 65% RH, respectively.

Water resistance evaluation of laminated fabrics

The water-pressure resistance was measured based on a static, high water pressure method (JIS L 1092; Method B). for NPU2-LF, PU-LF, and SAN/PU-LF. Moreover, a water washing test (based on JIS L1092 6.2.1 C), using a household clothes washing machine, was repeated 10 times on these laminated fabrics. The items that were not subjected to wash testing were labeled HL-0, while those that had underwent wash testing 10 times were labeled with the number of washings, i.e., HL-10. The laminated fabrics were subsequently hung and line-dried naturally.

Evaluation of blocking resistance of the laminated fabrics

The blocking resistance of the fabrics on the film-side surface was evaluated based on the 7.8 Tack-free Test (JIS K 6772). NPU2-LF, PU-LF, and SAN/PU-LF samples of 60 mm width and 90 mm length were evaluated. First, the test fabrics were soaked (for 10 min) in an ion-exchange bath (at 80 ℃). The moisture that adhered to the outer and inner surfaces of the fabrics was then absorbed by filter paper. Next, the film-side surfaces were overlaid, facing each other, and the items stacked on glass plates (with 60 mm edges) were used as test pieces. Furthermore, a load of 29.43 N was applied to the test pieces at the 60 mm side of the base and then maintained for 2 h in a hot air dryer at 80 ± 2 ℃. Figure 3 shows a schematic of the measuring device. The blocking resistance of the fabrics was then evaluated. The test results were assessed according to the following five grades:

Grade 1: Breakage without peeling;

Grade 2: There is no material destruction, but it is very difficult to peel the superposed film surfaces;

Grade 3: It is difficult to peel the superposed film surface;

Grade 4: It is easy to peel the superposed film surface;

Grade 5: No blocking at all.

Figure 3.

Schematic diagram of the apparatus for evaluating the blocking resistance of the laminated fabrics.

Results and discussion

LCST of Poly-NIPAM/PU copolymer films

The relationship between water temperature and the coefficient of water absorption in poly-NIPAM/PU copolymer films is shown in Figure 4. The coefficients of water absorption of PU-F (made entirely from hydrophilic PU) and of SAN/PU-F (where hydrophobic SAN resin was mixed (with the hydrophilic PU) in 15 wt.%) both increased linearly as the water temperature decreased. The coefficient of water absorption of SAN/PU-F was presumed to be lower than that of PU-F across the entire range. This was because the hydrophilicity of the entire film was more inhibited by the hydrophobic SAN/PU-F resin than by the hydrophilic PU-F resin.

Figure 4.

Water absorption as a function of water temperature for poly-NIPAM/PU copolymer films.

Using the inflection points on the graph, we confirmed that the LCSTs of NPU1-F and NPU2-F were 25 and 20 ℃, respectively. There was no inflection point visible in NPU3-F; however, we determined that the LCST existed near 0 ℃ by using the utilization ratio of the monomer during synthesis. The LCST confirmed in Figure 4 was consistent with the results of the previous study,¹¹ which were obtained by measuring the water swelling ratio at different film temperatures. Furthermore, the confirmed LCST also corresponded to the monomer composition shown in Table 1. When the No. 1 blend is copolymerized with NPU1, the total ratio of the hydrophobic monomers, MA and TBAA, is 18.7 wt.% with respect to the monomer ratio of NIPAM, which is 80.0 wt.%.

Table 1 shows that this ratio increased from 18.7 wt.% for sample No. 1 to 58.7 wt.% for sample No. 3. It is known that the LCST decreases when the proportion of hydrophobic monomers increases. In the case of the Poly-NIPAM/PU copolymer film of this time, when the hydrophobic monomer ratio exceeds 50 wt.%, as is the case with sample No. 3, the LCST is considered to be approximately 0 ℃. In addition, in the three poly-NIPAM/PU copolymer films, we were able to confirm that the behavior of the film at temperatures above the LCST exhibited similar tendencies to PU-F and SAN/PU-F. Based on this result, it is possible that under these conditions, poly-NIPAM/PU copolymer films are in a state in which the hydrophilic PU is mixed with poly-NIPAM, thereby exhibiting dehydration and hydrophobic behavior. Alternatively, in the temperature ranges below the respective LCSTs observed in NPU1-F and NPU2-F, it is presumed that the hydrophilicity of the entire poly-NIPAM/PU copolymer film was enhanced, and that the water-absorbing capacity increased in tandem with the decline in temperature. The enhanced hydrophilicity is a function of the poly-NIPAM constituent, which was made hydrophilic by hydrating and binding with the water molecules. These behaviors were consistent with the results of our previous report,¹¹ which were obtained by confirming the structural analysis by transmission electron microscopy (TEM), the water swelling ratio at each temperature, and the drying property of the film. The structure of the poly-NIPAM/PU copolymer film was determined via TEM analysis, which showed that poly-NIPAM was micro-dispersed in PU in a sea-island manner. In addition, we confirmed that water molecules that hydrated into the poly-NIPAM domain at a temperature below the LCST desorbed at a temperature above the LCST. This phenomenon is similar to the behavior of a nonionic surfactant having a cloud point. A similar tendency was confirmed in this study.

Moisture transport characteristics of laminated fabrics

Moisture permeability

The relationship between temperature and the B-1 moisture permeability in laminated fabrics using poly-NIPAM/PU copolymer films is shown in Figure 5. The PU-LF that exclusively used hydrophilic PU had greater moisture permeability than the other fabrics across the entire temperature range. In contrast, the moisture permeability of SAN/PU-LF, which used hydrophobic SAN resin mixed at 15 wt.% with the hydrophilic PU, was confirmed to be lower at all temperatures. This tendency was considered to be the result of the difference in the amount of moisture absorption of the moisture-permeable film used for the fabric and was correlated with the water absorption rate in Figure 4. However, NPU3-LF, which was determined to have an LCST near 0 ℃, was assumed to exhibit behavior largely similar to that of SAN/PU-LF, as the poly-NIPAM constituents all exhibited hydrophobic properties in the measurement range of B-1 moisture permeability. On the other hand, in NPU1-F and NPU2-F – where the LCST was confirmed to be 25 and 20 ℃, respectively – their respective behavior had changed in relation to the LCST. The B-1 moisture permeability in the temperature ranges below the LCST exhibited a similarly high value to that of PU-LF and exhibited a trend similar to that of SAN/PU-LF at temperature ranges above the LCST. This finding is assumed to be due to the poly-NIPAM constituents of the respective poly-NIPAM/PU copolymer films becoming hydrophilic, the moisture absorption and water absorption increasing in temperature ranges below the LCST, and the hydrophobicity increasing in temperature ranges above the LCST. This principle is thought to be similar to the water absorption principle in the poly-NIPAM/PU copolymer film described above. These results confirmed that the moisture permeability of the fabric was imparted with temperature sensitivity owing to the hydrophilic-hydrophobic phase transition of the poly-NIPAM component below and above the LCST.

Figure 5.

Moisture permeability as a function of water temperature for poly-NIPAM/PU copolymer laminated fabrics.

Condensation values

The condensation values in laminated fabrics, using poly-NIPAM/PU copolymer films, are shown in Table 3. NPU3-LF and SAN/PU-LF exhibited higher condensation values than the other three types. For this reason, it is deduced that moisture absorption and water absorption are characteristic of poly-NIPAM/PU copolymer films. The LCST of the poly-NIPAM/PU copolymer film used in NPU3-LF is assumed to be ∼0 ℃. Consequently, the poly-NIPAM constituents of the poly-NIPAM/PU copolymer film comprising the NPU3-LF were presumed to exhibit hydrophobicity under the environmental conditions (10 ℃) in which the condensation value was measured. Therefore, it is assumed to have exhibited similar behavior to that of the SAN/PU-LF in which the hydrophobic resin was mixed beforehand. As a result, it had a lower degree of hydrophilicity than PU-LF, where only a film of hydrophilic PU was used. On the other hand, in NPU1-LF (of 25 ℃ LCST) and NPU2-LF (of 20 ℃ LCST), it is deduced that the poly-NIPAM constituents exhibited hydrophilicity under the environment (10 ℃) where the condensation value was measured. As a result, it was postulated that the degree of hydrophilicity increased together with the PU-LF and that the condensation value decreased. These laminated fabrics were therefore expected to exhibit the condensation-inhibition ability required in breathable, waterproof fabrics.

Table 3.

Dew condensations of poly-NIPAM/PU copolymer laminated fabrics

Sample label	NPU1-LF	NPU2-LF	NPU3-LF	PU-LF	SAN/PU-LF
Dew condensation (g/m²·h)	4.07	8.52	23.22	8.35	12.41

Poly-NIPAM/PU: poly(N-isopropylacrylamide)/polyurethane; SAN/PU: styrene-acrylonitrile/polyurethane.

Vapor resistance (Ret)

The relationship between the water vapor temperature on the interior of the test material and the Ret in laminated fabrics using poly-NIPAM/PU copolymer films is shown in Figure 6. In all the processed fabrics, Ret exhibited an increasing trend as the water vapor temperature on the interior of the test material decreased. SAN/PU-LF demonstrated a higher rate of Ret increase along with the temperature decline than did PU-LF. As for the source of the increase, we determined that it was triggered by differences in the degree of hydrophilicity of the films. The difference in the degree of hydrophilicity of the film was confirmed by the difference in the water absorption shown in Figure 4. Furthermore, NPU2-LF, with an LCST of 20 ℃, exhibited a different behavior with regard to Ret when the steam temperature inside of the test material was 25.5 ℃. Between 32 and 25.5 ℃, NPU2-LF and SAN/PU-LF showed the same increasing trend. However, at temperatures below 25.5 ℃, NPU2-LF tended to exhibit a trend similar to that exhibited by PU-LF. When the water vapor temperature inside the test material was 25.5 ℃, the surface temperature of the laminated fabric measured at the same time was 19.5 ℃. This temperature closely matched the 20 ℃ LCST of the poly-NIPAM / PU copolymer film used in NPU2-LF. From these results, it was inferred that the tendency of increase in Ret changed owing to the change in the degree of hydrophilicity of the poly-NIPAM / PU copolymer film of the laminated fabric.

Figure 6.

Relationship between water vapor temperature on the inside of the test material and vapor resistance.

Water resistance of laminated fabrics

Figure 7 shows the water-pressure resistance of laminated fabrics before and after testing. The initial water pressure resistance of NPU2-LF, PU-LF, and SAN/PU-LF was ≥130.0 kPa. The water-pressure resistance remained high, even after wash testing. Hence, we confirmed that the laminated fabrics generated from this study are practical with regard to water resistance.

Figure 7.

Water resistances measured by the JIS L1092 method.

Blocking resistance of laminated fabrics

Table 4 shows the blocking resistance grade of the laminated fabrics. PU-LF was assessed to have a blocking resistance grade of 2 (“There is no material destruction, but it is but very difficult to peel the superposed film surfaces”), in contrast with NPU2-LF (with a 20 ℃ LCST) and SAN/PU-LF, which were both evaluated to have a grade of 4 (“It is easy to peel the superposed film surfaces”). PU-LF exhibited inferior blocking resistance than either NPU2-LF (with a 20 ℃ LCST) or SAN/PU-LF. This finding was presumably related to the water absorbance at the test temperature of 80 ℃. NPU2-LF, with an LCST of 20 ℃, is concluded to exhibit the hydrophobicity of the poly-NIPAM constituent (of the poly-NIPAM/PU copolymer film) at 80 ℃. As a result, NPU2-LF had a low water absorption volume, similar to that of AS/PU-LF, while its blocking resistance was higher than that of PU-LF. This blocking resistance is an important consideration in the practical performance of breathable, waterproof fabrics. When fabrics made into clothing are transported as cargo, they are often exposed to high temperatures and humidity. Hence, this evaluation is frequently performed in order to confirm practicality. We subsequently determined that PU-LF, which exclusively used a hydrophilic PU film, had low practicality.

Table 4.

Blocking resistances of select laminated fabrics measured by the JIS K 6772 method

Sample label	NPU2-LF	PU-LF	SAN/PU-LF
Blocking resistance (Grade)	4	2	4

Poly-NIPAM/PU: poly(N-isopropylacrylamide)/polyurethane; SAN/PU: styrene-acrylonitrile/polyurethane.

Conclusions

In this study, we investigated laminated fabrics synthesized from a poly-NIPAM/PU copolymer, which can alter the coefficient of water absorption, below and above the LCST. The following conclusions were summarized in consideration of the moisture transport characteristics.

The measurements performed with regard to the moisture-transport characteristics suggest the following conclusions in cases where a NIPAM/PU copolymer film is used in clothing applications, such as cold-weather clothing, clothing for winter sports, rainwear, and the like with a breathable, waterproof film as a raw material.

First, under conditions of low external air temperature, and assuming that exercise is performed while wearing clothing, the temperature within the clothing rises to be higher than that outside. The saturated water vapor pressure also rises, and therefore, the water vapor generated within the clothing is released from the inside to the outside of the clothing by the moisture permeability of the material. Higher B-1 moisture permeability is desirable, and on this basis, NPU1-F, NPU2-LF, and PU-LF were thought to be superior to SAN/PU-LF.

Next, the amount of water vapor generated within the clothing further increases with exercise, and when the material reaches the limit of its moisture permeability, discomfort (such as stuffiness) is experienced by the wearer. When exercise stops and the temperature inside the clothing decreases, the water vapor remaining inside the clothing generates condensation phenomena on the inner surface of the breathable, waterproof film material of the clothing. Because the interior of the clothing represents the interface between the wearer and the outside air, the aforementioned phenomena generates discomfort to the wearer, such as chills. The condensation phenomenon is unfavorable; hence, items with a low condensation value are preferable. For this reason, PU1-F, NPU2-LF, and PU-LF were more desirable than SAN/PU-LF.

When condensation phenomena from vapor resistance are further considered, the low Rets of NPU2-LF and PU-LF are presumably more favorable than the high Ret of SAN/PU-LF in low temperature ranges.

In addition, the fabric using the poly-NIPAM/PU copolymer film obtained in this study had sufficient water resistance and blocking resistance for practical applications.

As a result, we confirmed that the fabrics using the poly-NIPAM/PU copolymer films in this study were capable of serving as a functional material for apparel, wherein the condensation-inhibition function was improved beyond that of conventional items; this was achieved by incorporating a new function of temperature sensitivity. In the future, a higher-range wear test will be needed to fully determine the wear comfort of the Poly-NIPAM/PU copolymer film manufactured in this study, as the results of this study alone are insufficient for a complete evaluation. Furthermore, the fabric obtained in this study did not only consist of PU; rather, it consisted of Poly-NIPAM copolymerized with PU. It can therefore be assumed that the solvent resistance of the resin film surface is lower than that of the conventional product made of PU. Resistance to dry cleaning solvents is an issue to be investigated in the future. Similarly, dye transfer from the polyester cloth to the exterior fabric or to the surface of the resin film may be considered; nevertheless, caution is required in dyeing and finishing the polyester cloth.

Footnotes

Acknowledgements

We would like to thank Editage () for English language editing.

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 received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Masao Enomoto

References

Morimoto

. The temperature regulation of the human being (in Japanese). Jpn Res Assn Text End-Uses 2003; 44: 256–262.

Kanosue

Nagashima

Taniguchi

, et al. What is thermal comfort/discomfort(hotness/coldness)? (in Japanese). Jpn Res Assn Text End-Uses 2002; 43: 159–164.

Shibata

. Development of the breathable materials of Toray Industries (in Japanese). Jpn Res Assn Text End-Uses 2011; 52: 366–370.

Harada

Tuchida

. Wear comfort and microclimate-within-clothing (in Japanese). Soc Fiber Sci Technol Japan (Sen'i Gakkaishi) 1990; 46: 97–101.

Enomoto

Suehiro

Muraoka

, et al. Physical properties of polyurethane blend dope-coated fabrics. Text Res J 1997; 67: 601–608.

Enomoto

Suehiro

Muraoka

. Effect of composition and coagulation structure of coating polymers made from hydrophobic and hydrophilic polyurethane resin on moisture transporting properties on waterproof/moisture-permeable fabrics. J Text Mach Soc Japan 1997; 50: T233–T241.

Enomoto

Muraoka

. Vapor transport properties of polyurethane coated nylon fabrics. J Text Eng 2008; 54: 57–62.

Nishimoto

Omote

Takahashi

Nakagawa

. Development of moisture permeable, waterproof fabric “AIRDRIVE” (in Japanese). Soc Fiber Sci Technol Japan (Sen'i Gakkaishi) 2003; 59: P344–P347.

Kim

. Assessment of heat and moisture transfer in a multilayer protective fabric system under various ambient conditions. Text Res J 2015; 85: 227–237.

10.

Fangueiro

Filgueiras

Soutinho

, et al. Wicking behavior and drying capability of functional knitted fabrics. Text Res J 2010; 80: 1522–1530.

11.

Enomoto

Omote

Miyajima

, et al. Structure and water permeability and resistance characteristics of N-isopropylacrylamide-based polymer/polyurethane resin films. Adv Polym Tech 2018; 37: 3737–3746.

12.

Bae

Okano

Hsu

, et al. Thermo-sensitive polymers as on-off switches for drag release. Macromol Chem Rapid Commun 1987; 8: 481–485.

13.

Okano

Yamada

Sakai

, et al. A novel recovery system for cultured cells using plasma-treated polystyrene dishes grafted with poly(N-isopropylacrylamide). J Biomed Mater Res 1993; 27: 1243–1251.

14.

Teramoto

Kohno

Abe

. Phase transition for aqueous solution of polyelectrolyte complex containing N-isopropylacrylamide (in Japanese). Japan J Polym Sci Technol (Kobunshi Ronbunshu) 1997; 54: 477–482.

15.

Xiao

Tan

Wang

Yue

Huang

. Fast swelling behaviors of thermosensitive polyN-isopropylacrylamide-co-methacryloxyethyltrimethyl ammonium chloride/Na₂WO₄ cationic composite hydrogels. J Appl Polym Sci 2018; 135: 463–475.

16.

Shimizu

Wada

Okabe

. Preparation and characterization of micrometer-sized poly(N-isopropylacrylamide) hydrogel particles: effect of initiator concentration (in Japanese). Japan J Polym Sci Technol (Kobunshi Ronbunshu) 2008; 65: 751–755.