The optimum coating condition by response surface methodology for maximizing vapor-permeable water resistance and minimizing frictional sound of combat uniform fabric

Abstract

The objectives of this study are to investigate how the variables of the water-repellent coating condition, concentration of polyurethane (PU) and curing temperature, set up by response surface methodology, affect vapor-permeable water resistance and fabric frictional sound. Also it aims to analyze the relationship between tensile properties and the sound pressure level (SPL) of the fabric and, finally, to suggest the optimum coating condition for minimizing the fabric frictional sound and maximizing the vapor-permeable water resistance. It was observed that the higher PU concentration increased the water resistance and SPL, but decreased WVT (water vapor transmission). It was shown that higher curing temperature, the other variable of the coating condition, increased the water resistance and SPL but decreased WVT. The relationship between tensile properties and SPL was analyzed and it was found that tensile stress at break (R²= .716) and toughness (R²= .717) were highly related to SPL; however, tensile strain at break (R²= .508) was not. Finally, the optimum coating condition for minimizing fabric frictional sound and maximizing vapor-permeable water resistance was obtained at the PU concentration of 60% and the curing temperature of 149.7℃, and the predicted SPL and WVT were 72.27 dB and 8478.85 g/m² 24 h, respectively. The coefficients of determination (R²) were 0.82 and 0.85, respectively, which indicate that the model fit was highly significant (p < 0.05).

Keywords

Water resistant finish optimum coating conditions sound pressure level tensile properties response surface methodology central composite design combat uniform fabrics

It is well known that combat uniforms worn by soldiers in actual combat field support soldiers’ movement and protect their body against weather changes and risky situations. In order to be a suitable combat uniform, a series of functionalities are required. Among those, vapor-permeable water resistance is necessary. The combat uniforms transport water vapor from sweating to the outside atmosphere, and repel water drops of rain from the surfaces so that the microclimates of the uniforms can be maintained constant. Polyurethane (PU) is widely chosen for waterproof breathable property owing to a range of desirable properties, such as being elastomeric, resistant to abrasion and excellent hydrolytic stability.^1–3

However, it has been found that coating fabrics for this function generates significant frictional noise (sound pressure level: SPL) of over 70 dB,⁴ which means the magnitude of the sound. The fabric frictional sound was generated when the fabric is rubbed against another fabric because of wearer’s movement.⁵ According to US Army’s Field Manual, noise is one of the essential factors avoided for camouflage and survival. It warns that the noise generated from the movement can attract the enemy’s attention, so suggests slowing the pace as much as possible to minimize the sound and that using background noise to cover the noise from the movement. Even though there are lots of sources for the noise generating from the moving soldiers, such as hanging guns and water bottles, frictional sound generated from the combat uniform is not negligible, which is our only concern. When fabric sound generated by the movement reaches beyond visibility distance, detection and revelation will be unavoidable even if the combat uniform has a visual camouflage performance.⁶ Moreover, it could be a much more serious problem if the vapor-permeable water resistance coating is applied to combat uniform fabrics, because the fabric frictional sound could expose the wearer easily to the enemy when soldiers perform their military duties.

Previous studies^5,7 have been carried out to reduce the frictional noise of coated fabrics, but none of them have considered the effects of coating conditions such as resin concentrations and curing temperatures on noise reduction. The previous studies only showed that frictional sounds of various combat uniform fabrics had significant correlation with some mechanical properties measured by the Kawabata Evaluation System. Tensile properties such as tensile stress at break, tensile strain at break and toughness were not analyzed in depth as the dependent variables to find out how they affect the SPL of coated specimens. Therefore, it needs to be set in a proper material and coating conditions by suitable scientific methodology using the tensile properties as the dependent variables.

Response surface methodology (RSM) is a collection of statistical and mathematical techniques that are useful for modeling and optimization, and it helps to understand the interaction effects between factors and to reduce the total number of experimental points.⁸ As a result, RSM has been used to evaluate the effects of different factors and to optimize different process conditions.⁹ A central composite design (CCD) is a one of RSM for building a second-order (quadratic) model for the response variable without needing to use a full factorial experiment. Therefore, it is powerful and simultaneous in the way that it can attain same results with a lower number of experiments.¹⁰ For example, when the number of independent variables is two, it is possible to expect changes of dependent variables with nine experimental points. Because of the advantages of the CCD, it was intended to figure out the optimum coating conditions for this study.

The objectives of this study were to investigate how the variables of the water-repellent coating condition, concentration of PU and curing temperature, set up by RSM, affect vapor-permeable water resistance and fabric frictional sound. Also the study aimed to analyze the relationship between tensile properties and SPL of the fabric and, finally, to suggest the optimum coating condition for minimizing fabric frictional sound and maximizing the vapor-permeable water resistance by RSM.

Experimental details

Characteristics of combat uniform fabric

A combat uniform fabric with a digital pattern was used as a test specimen; the characteristics of the specimen are shown in Table 1. It is composed of polyester and rayon fibers and used as substrate fabric for coating. The combat uniform fabric was provided by SAMIL SPINNING CO., LTD.

Table 1.

Basic characteristics of combat uniform fabric

Fiber composition	Yarn size	Yarn structure	Fabric construction	Density (wxf/in²)	Thickness (mm)	Weight (g/m²)	Finish
Polyester/rayon 64/36	2/30’s Staple	2 ply Z-twist yarn	Plain	65 × 60	0.34	224.6	DTP

DTP: Digital Textile Printing.

Central composite design of RSM for coating condition

The coating conditions were determined by the CCD of RSM. Because the WVP and tensile properties of PU-coated fabric are affected by concentration of PU,¹¹ independent variables for coating condition were chosen as the PU concentration and curing temperature in this study. Five levels of PU concentrations and the curing temperatures were used: 60%, 65%, 70%, 75% and 80%, and 135℃, 140℃, 145℃, 150℃ and 155℃, respectively. These levels of the PU concentrations and curing temperature were coupled into nine combinations, which were set up by CCD (Table 2), and finally nine experimental points were determined, as shown in Figure 1.

Figure 1.

Plot of polyurethane concentrations and curing temperatures for nine coated specimens by central composite design of response surface methodology.

Table 2.

Determination of experimental points and specimen numbers by central composite design of response surface methodology

Conc. (%) Temp. (℃)	60	65	70
135	√ (1)
140	√ (6)	√ (4)
145	√ (8)	√ (2)	√ (9)
150	√ (7)	√ (5)
155	√ (3)

Coating procedure

The coating for vapor-permeable water resistance was done with a mixture of PU-based agent and PU adhesive (300 g) by the dry coating method under the nine coating conditions. The fabric (300 mm × 300 mm) was coated using a laboratory scale knife-coater, and the coated specimens were cured for 40 seconds under each of the curing temperatures. The agent was applied as a back coating, and it was a single coating in all cases. Accordingly, nine samples treated under the different coating conditions were prepared, and thickness change and add-on of treated specimens were calculated. The coating process was done in a pilot plant at YOUNG POOG FILLTEX CO., LTD.

Vapor-permeable water resistance tests of coated fabric

Generally, a fabric having a vapor-permeable water resistance has a negative relationship between the protection level and breathability. This is because a fabric with a more open structure generally has higher air permeability and lower water resistance.¹² Therefore, to evaluate whether the coated fabrics have a suitable vapor-permeable water resistance, water resistance (mmH₂O) and water vapor transmission (WVT) (g/m² 24 h) tests were conducted. The tests were conducted based on the ISO 881 (Hydrostatic pressure test) and the ISO 2528 (Gravimetric (dish) method), respectively.

Recording fabric frictional sound

Fabric sounds were generated on the ‘Simulator for Frictional Sound of Fabrics (Patent, No. 1011941720000)’¹³ with the speed of 0.6 m/s in an anechoic chamber. The generated sounds of fabrics were recorded using a microphone (Type 4190, B&K), and analyzed using a Pulse system (Type 7700, B&K).

Sound pressure level calculation

Recorded fabric sounds were saved as wave files via Sound Quality Program (ver. 3.2, B&K), and transformed into the fast Fourier transform (FFT) spectrum within a range of 0–20,000 Hz. The SPL was defined as

SPL (dB) = 10 log 10^{\frac{{BL}_{1}}{10} + \dots + \frac{{BL}_{n}}{10}}

where BL is the broadband sound level at each frequency of FFT spectrum, and n is the maximum frequency over frequency change in the 0–20,000 Hz range.

Tensile property tests

For the tensile property test, tensile stress at break (N/mm²) and tensile strain at break (%) were measured according to ISO 1421 under the standard condition (temperature 20℃; humidity 65%). Measuring toughness (J/m³) was carried out by calculation of the area under the stress strain curve from the tensile test. Five replications were measured and averaged for the three tensile properties.

Data analysis

A first-order regression analysis was conducted to investigate the relationship between SPL and tensile properties using PASW statistics package (ver. 18.0). The CCD of the RSM was used for the optimization of the coating condition, which has a minimum SPL and maximum vapor-permeable water resistance at that same time. To achieve this, regression models were invested with regard to SPL, water resistance and water vapor permeability. Then the optimum condition was derived based on the regression model of SPL and vapor-permeable water resistance, which had a high R² (0.85) value. SAS statistics package (Ver. 9.1) was used in this process.

Results and discussion

Effect of the coating condition on vapor-permeable water resistance

The specimens’ vapor-permeable water resistance, water resistance and WVT were measured and are given in Figure 2. To investigate the effect of PU concentration on water resistance at the constant curing temperature, the water resistances of specimen 6 and 4, specimens 8, 2 and 9, and specimens 7 and 5, that were treated at 140℃, 145℃ and 150℃, respectively, and is shown in Figure 2(a). In this manner, the water resistances of specimens 6 and 7, specimens 1, 2 and 3, and specimens 4 and 5 are presented in Figure 2(b) to establish the effect of curing temperatrue at the same PU concentration.

Figure 2.

Vapor-permeable water resistance changes according to the coating conditions: (a) water resistance according to polyurethane (PU) concentration; (b) water resistance according to curing temperature; (c) water vapor transmission according to PU concentration; and (d) water vapor transmission according to curing temperature.

Coated specimens in the study showed much better water resistance than uncoated specimens, the average resistance value being 4557.22 mmH₂O. Generally, water resistance levels of waterproof fabrics are classified as low water resistant (300–800 mmH₂O), middle water resistant (1000–2500 mmH₂O) and high water resistant (5000–30000 mmH₂O), based on the water resistance values.¹⁴ The specimens of this study showed values in the range of middle-to-high water resistant. According to this, the specimens could be used in mild water exposure situations. The highest water resistance was observed in specimen 9, coated with 80% of PU concentration at 145℃; this was probably because of the compact coating layer, which had the greatest concentration of coating mixture, thickness and weight.¹⁵

As can be seen from Figure 2(a), the overall water resistance of specimens had a tendency to increase with the increasing of PU concentrations, which indicated that the adding of PU attributed to the enhancement of water resistance. However, the opposite trend was observed at 150℃ of curing temperature. This might be because the add-ons of the specimens included in this temperature, specimens 7 and 5, were much lower than that of other coated specimens; thus, PU concentration might have less of an effect on the water resistance in this case (Table 3). In addition, that specimen 7 coated with 60% of PU concentration showed a rather higher add-on than that of specimen 5 coated with 75% of PU concentration might have acted as one factor of this result. The water resistance of specimens exhibited an obvious decrease toward the increase of curing temperature, especially over 140℃ (Figure2(b)).

Table 3.

Thickness change and add-on of coated specimens

Specimen no. (conc., temp.)	Thickness change	Add-on
Specimen no. (conc., temp.)	%	%
1 (70, 135)	0	6.06
2 (70, 145)	–11.76	12.87
3 (70, 155)	–14.71	6.19
4 (75, 140)	–5.88	14.60
5 (75, 150)	–5.88	6.77
6 (65, 140)	0.00	26.63
7 (65, 150)	–8.82	9.71
8 (60, 145)	–5.88	11.84
9 (80, 145)	0.00	11.53

Thickness change = (T_coated – T_untreated)/T_u_n_treated × 100; add-on = (W_coated – W_untreated)/W_untreated × 100.

WVT of specimens was measured in terms of the amount of water vapor passed in grams per 24 hours per square meter of the specimen’s surface area, shown in Figure 2(c) and (d). WVT of all coated specimens in the study was lower than that of uncoated specimens, which means WVT of all the coated specimens decreased after coating. The lowered WVT is described in Figure 3, which shows the scanning electron microscopy (SEM) micrographs of the untreated and coated specimens. The surface of uncoated specimen shows the woven structure, which has many open spaces; on the other hand, the surfaces of coated specimens show the PU-coated layer on the woven structure. In spite of the presence of the PU-coated layer, a number of pores are observed underneath the coated layer (Figure 3(d)). This might have resulted in the greatest WVT to specimen 8 treated with 60% at 145℃. The average value of WVT of coated specimens was 6117.44 g/m² 24 h, which can be classified as having a high WVT level.¹⁴ Also, all specimens showed higher WVT than 4000 g/m² 24 h, which is the minimum requirement of WVT during strenuous exercise; in particular, specimen 8 had the highest WVT among the coated specimens.

Figure 3.

Scanning electron micrographs of the specimens (×100). PU: polyurethane.

The coated specimens’ WVT decreased with the increasing of PU concentrations (Figure 2(c)), and this was probably because of a reduction of micropores on the coated fabrics with increase in PU concentration.¹⁶ Also, it could be related to the increasing of water resistance under the same conditions as demonstrated in Figure 2(a). The increase of curing temperature, like PU concentration, also resulted in the decrease of WVT, as shown in Figure2(c). However, specimens 4 and 5, coated with 80% of PU concentration, had the opposite tendency. In general, WVT values decreased with increase in quantity of coating materials because of the increase in coating add-on.¹⁶ Thus, that the significantly greater add-on of specimen 4 (14.67%) than that of specimen 5 (6.77%) might have resulted in a decrease of WVT, and it also meant that the increase in add-on caused by the increase of concentration of PU was possibly below 80% of curing temperature.

Effect of the coating condition on sound pressure level

The SPL of the specimens is shown in Figure 4. All coated specimens’ SPL increased an average of 11% after coating, which demonstrated that the coating caused fabric frictional sound to be louder. Overall, the SPL increased with the increasing of the PU concentrations, especially when the PU concentration increased from 70% to 80% (Figure 4(a)). In the range of 65–75%, however, the SPL decreased rather than increased when the curing temperature was 140℃, and this result might be related to tensile properties of specimen 6. The increase of curing temperature, like PU concentration, also resulted in increase of SPL, as shown in Figure 4(b).

Figure 4.

Sound pressure level (SPL) changes according to the coating conditions: (a) SPL according to polyurethane (PU) concentration and (b) SPL according to curing temperature.

Specimen 3, coated with 70% of PU concentration at 155℃, had the highest SPL among other coated specimens. This can also be seen with the SEM micrographs of coated fabrics with PU shown in Figure 3(c)–(f). Compared with Figures 3(c) and (d), Figure 3(e) and (f) show a thicker coated layer on the surface. This might cause the fabric to sound louder.

Relationship between sound pressure level and tensile properties

To find out the relationship between SPL and the tensile properties of specimens, Pearson’s correlation analysis was conducted; the results are shown in Figure 5. In this figure, the level of SPL is plotted along the Y-axis, and the degree of tensile stress at break and toughness of the specimens are plotted along the X-axis. These results mean that the SPL was related to the tensile stress at break (R²= .716) and toughness (R²= .717); however, tensile strain at break (R²= .508) was not.

Figure 5.

Relationship between sound pressure level (SPL) and tensile properties: (a) relationship between SPL and tensile stress at break and (b) relationship between SPL and toughness.

The tensile stress at break of specimens increased 17.51% after coating, and SPL increased with the increasing of tensile stress at break (Figure 5(a)). This was probably because the adhesion between coating solution and the fabric was attributed to the increase of the tensile stress at break, and it made the SPL of the coated specimen higher, as shown in many previous studies,^3,4 which found a positive relationship between tensile properties and SPL.

The relationship between SPL and toughness, which reflects the mobility of the fabric under deformation calculated as the area under the stress–strain curve, is shown Figure 5(b). The toughness of all coated specimens increased an average of 52.2% after coating, especially specimen 3, which showed relatively low water resistance among the specimens that had the highest toughness. There may be two reasons for this: firstly, the coating materials on the specimens, being more stretchable than fabrics, led to resisted tensile force in comparison with non-coated specimen and, secondly, it was reported that PU-coated fabrics that have a relatively higher water resistance lead to a lower tensile strength because of hydrogen bonds between moisture and hard segments of PU bonds.^17–20

These results meant that SPL was determined according to the tensile properties of the fabrics, especially tensile stress at break and toughness, and was able to be controlled by varying these tensile properties.

Prediction of optimum coating conditions

To investigate the optimum coating condition for minimizing fabric frictional sound and maximizing the vapor-permeable water resistance at the same time, the regression models of SPL, water resistance and WVT were investigated (Table 4). These models present a change of each variable with respect to two variables of the coating condition, concentration of PU and curing temperature, as shown in Table 4. The predicted regression models of SPL (R²= .85) and WVT (R²= .82) had high R² values; however, the regression model of water resistance (R²= .68) showed relatively low R² values. Therefore, the optimum coating condition was derived only based on the regression models of SPL and WVT.

Table 4.

Regression models predicted by coating conditions

Response variables	Regression models	Adj. R²
Water resistance (Y₁)	Y₁ = –67,326 + 861.33X₁ + 591.75X₂ + 26.15X₁X₂ + 22.47 $X_{1}^{2}$ + 3.82 $X_{2}^{2}$	.68
Water vapor transmission (Y₂)	Y₂ = 698,826 – 6789.97X₁ – 143.00X₂ + 17.64X₁X₂ + 28.98 $X_{1}^{2}$ + 16.68 $X_{2}^{2}$	.82
SPL (Y₃)	Y₃ = 1952.02 – 13.32X₁ – 19.77X₂ + 0.13X₁X₂ – 0.04 $X_{1}^{2}$ + 0.04 $X_{2}^{2}$	.85

X₁: concentration of polyurethane; X₂: curing temperature; SPL: sound pressure level.

Because it is not possible to obtain the exact optimum condition that satisfies two conditions analytically, the top five optimum conditions were obtained by checking conditions across a grid of values in the range of interest (Table 5). They indicated that optimal values of the coating conditions were 60% for concentration of PU and around 148.56℃ for curing temperature. In this study, specimen 1 was selected as the optimum coating condition because of the lowest SPL (72.27 dB) around 8478.85 g/m² 24 h of WVT. Under this condition, the predicted WVT and SPL were 8478.85 g/m² 24 h and 72.27 dB, respectively (Figure 6). This meant that it would be possible to obtain less noisy combat uniform fabrics with a high performance of vapor-permeable water resistance. The coefficients of determination (R²) were 0.82 and 0.85, which indicates that the model fit was relatively significant (p < 0.05).

Figure 6.

Predicted optimum coating condition for minimizing sound pressure level (SPL) and maximizing vapor-permeable water resistance: (a) predicted minimum SPL and (b) predicted maximum water vapor transmission (WVT). PU: polyurethane.

Table 5.

Top five optimum coating conditions and response variables under the condition

No.	Optimum coating conditions		Response variables
No.	Conc. of PU (%)	Curing temp. (℃)	Water vapor transmission (g/m² 24 h)	SPL (dB)
1	60	149.7	8478.85	72.27
2	60	149.5	8497.44	72.38
3	60	149.3	8517.37	72.48
4	60	148.5	8610.41	72.96
5	60	145.8	9082.11	74.93

PU: polyurethane; SPL: sound pressure level.

Conclusion

This study aimed to investigate how the variables of the water-repellent coating condition, concentration of PU and curing temperature, set up by RSM, affect vapor-permeable water resistance and fabric frictional sound. Also it analyzed the relationship of tensile properties and SPL of the fabric, and suggested the optimum coating condition for minimizing the fabric frictional sound and maximizing the vapor-permeable water resistance. It was observed that the higher PU concentration increased the water resistance and SPL, but decreased WVT, and the higher curing temperature, the other variable of the coating condition, increased the water resistance and SPL but decreased WVT. Analysis of the relationship between SPL and tensile properties showed that tensile stress at break (R²= .716) and toughness (R²= .717) were highly related to SPL; however, tensile strain at break (R²= .508) was not. Finally, according to the results of RSM, the predicted optimum coating conditions were a 60% PU concentration and a curing temperature of 150℃. Under this condition, the predicted SPL and WVT were 72.27 dB and 8478.85 g/m² 24 h, respectively.

However, the predicted optimum coating conditions were slightly out of the center of interest regions and were attained based on statistical techniques. In future study, new treatment condition ranges of the CCD matrix are required for reliability. Lastly, coating of fabric according to the optimum condition of this study, and actual SPL and WVT measurements, are required. In addition, durability and launder ability tests need to be conducted.

Footnotes

Funding

This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2011-0015658).

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