Study on the improvement of water vapor permeability and moisture absorption of microfiber synthetic leather base by collagen

Abstract

The microfiber synthetic leather base pretreated by sulfuric acid was modified by collagen in which the organic phosphine FP was used as a cross-linking agent, for the purpose of increasing the active groups and improving its properties. Compared with the pretreated base, it was found that the amino content in the modified base was two times and the carboxyl content was three times. The water vapor permeability of the modified base increased by 65% and the moisture absorption increased by 181%. It was further found that the tensile strength of the modified base was 19.06 N/mm², the elongation at break was 54% and the tear strength was 112.34 N/mm. The test results of Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy, thermogravimetric analysis and water contact angle showed that the collagen molecules had evenly cross-linked with fiber. The modified base microfiber dispersion was greatly increased, the hydrophility was enhanced and the relative average roughness was decreased. Moreover, modification by collagen also affected the thermal properties of the base.

Keywords

collagen organic phosphine microfiber synthetic leather base water vapor permeability moisture absorption

Microfiber synthetic leather is a high-grade synthetic leather and its appearance and performance are fairly similar to that of natural leather, even presenting much better physical and mechanical properties. Made up of polyamide microfiber and polyurethane, it is an ideal substitute for natural leather. As the main component of synthetic leather, the properties of the microfiber synthetic leather base are the key factors to the properties of the finished synthetic leather.^1–3 Except for carboxyl and amino groups at the end of the molecular chain, there are fewer active groups in the polyamide microfiber than in natural leather, which has many side chain groups that are active. There are a large number of C-C bonds and amide bonds in the molecular chain, but no side chains exist. Consequently, the water vapor is hard to transfer and absorb between the fiber and outside the base. So microfiber synthetic leather has poor moisture absorption, water vapor permeability and wearing comfort.^4–8 Therefore, it is necessary to find ways to improve the properties of microfiber synthetic leather.^9–11

As a typical amphoteric polyelectrolyte, collagen contains many amino acids and active groups, and has outstanding hydrophilicity and biocompatible properties.¹² Being a country with many leather industries, China has to face the production of a large amount of leather waste every year. The proper application of collagen extracted from the leather waste can help to save resources and reduce solid waste pollution to a certain extent.¹³

In this research, collagen and a highly active cross-linking agent, tetrakis (hydroxymethyl) phosphonium chloride (organic phosphine FP), was used to modify the surface of a sulfuric acid pretreated synthetic leather base. The results of this modification were that the active groups on the surface of the base increased and the properties were fundamentally improved.

Experimental details

Materials and instruments

The materials and instruments used in this experiment are the following: gelatin (Hebei Cangzhou Gelatin Co., Ltd), alkaline protease (AR, Nanning Pangbo Biological Engineering Co., Ltd), polyamide microfiber synthetic leather base (Yantai Wanhua Synthetic Leather Co., Ltd), 98% sulfuric acid (Beijing Chemical Factory), organic phosphine FP (Beijing Fanbo Science and Technology Co., Ltd) and formaldehyde scavenger (Shenzhen Greg Weikang Environmental Protection Technology Co., Ltd).

Preparation of collagen

The 30% gelatin solution was placed in a water bath pot equipped to stir until the gelatin completely swelled. The temperature of the water bath was adjusted to 55℃, while the pH of the gelatin solution was 9.0. About 0.8% (based on the dry weight of gelatin) alkaline protease was added, and was stirred for 5 h under this reaction condition. In order to inactivate the enzyme, the reaction was kept for 10 min after the temperature was increased to 90℃. The collagen obtained was an orange liquid. The formaldehyde method¹⁴ was used to measure the prepared gelatin hydrolysate, and the amine content was about 1.2 mmol/g.

Gel permeation chromatography of collagen

DIONEX BJ/U3000 gel permeation chromatography (GPC) was used to determine the distribution of the collagen molecular weight, employing sodium azide for universal calibration. The mobile phase was water, the velocity was 1 mL/min and the column temperature was 40℃. The result showed that two main peaks appeared in the GPC spectra. Mn was 883, Mw was 1374 and Mv was 1285. The GPC spectrum of collagen was as shown in Figure 1.

Figure 1.

Gel permeation chromatogram of collagen.

Microfiber synthetic leather base modification method

The modification method is given in Table 1. The 98% sulfuric acid was used to pretreat the microfiber synthetic leather base and the processing conditions were as given by Ren et al.¹⁵ Then the pretreated base was modified by collagen with organic phosphine FP and the collagen was added after the base fully reacted with FP. The combination schematic of the modified base by collagen is shown in Figure 2. Organic phosphonic contains four hydroxymethyls and is easily oxidized into three hydroxymethyl phosphine oxides, which then reaction with amino.

Figure 2.

The combination schematic of the microfiber synthetic leather base modification by collagen with organic phosphine FP.

Table 1.

Microfiber synthetic leather base modification method

Process	Material	Dosage (%)	Temperature (℃)	Time (min)	Remarks
Boiled	Water	5000	70	30	Drainage
Pretreatment	Water	5000	50
	98% sulfuric acid	15		240	Drainage
Washing	Water	3000		3×20	Drainage
Modification	Water	3000	35
	Organic phosphine FP	9		120	pH: 5.5
	Collagen	18		120
	NaHCO₃	0.3	55	60	pH: 6.5
	Formaldehyde scavenger	2	40	60
Washing	Water	3000		3×20	Drainage
Drying	Hanging the base to dry

The modification conditions were optimized via single-factor experiments in which the water vapor permeability and moisture absorption were used as indexes. The best modification conditions were as follows: the collagen dosage was 18%, the FP dosage was 9%, the modification temperature was 35℃, the time was 4 h and the pH was 5.5.

The operating instructions are summarized as follows:

the dosage percentage related to this experimental process was calculated by the quality of the base;

the modified base without FP of the tests was the collagen modified base for which the FP dosage was 0% in this method;

the formaldehyde scavenger was used to remove free formaldehyde in this method.

Determination of amino, carboxyl and formaldehyde contents

The amino and carboxyl contents were respectively tested using the salicylaldehyde method and phenolphthalein indicator hydroxide standard solution.¹⁶ The formaldehyde content of the modified base was tested according to GB/T 2912.1-2009.¹⁷

Determination of water vapor permeability

The water vapor permeability refers to the ability to make water vapor penetrate from air of the larger humidity to that of the smaller one. Owing to the water vapor permeability, the gas and water vapor of the wearers can be eliminated. Therefore, water vapor permeability is commonly used to characterize the properties of leather.¹⁸

The water vapor permeability of the base was tested according to QB/T 1811–1993.¹⁹ The base samples were placed into an oven (100℃), drying to constant weight. Then 30 mL measured distilled water was put into the water vapor permeability test dish, weighing it and marking it as W₁ . The test dish was placed into a dryer containing sulfuric acid of the relative density of 1.84. Then the dryer was placed into an environment with constant temperature of about 20℃ for 24 h, weighing it and marking it as W₂. The area of the specimen was 10 cm², and the water vapor permeability value was the quality difference of the samples before and after standing. The formula was as follows:

Water vapor permeability (mg / 10 {cm}^{2} • 24 h) = W_{1} - W_{2}

(1)

where W₁ and W₂ are the quality of the samples and the container (mg) before and after being dried, respectively.

Determination of moisture absorption

The moisture absorption of the base was tested according to GB/T 4689.21-1996.²⁰ The base samples were placed into an oven (100℃) drying to constant weight, weighing it and marking is as W₁, and then it was placed into environment with constant temperature of about 20℃ and relative humidity of about 65% for 24 h, weighing it and marking it as W₂. The moisture absorption amount was the ratio of the quality difference of samples before and after standing and the quality of samples before standing. The formula was as follows:

Moisture absorption = \frac{W_{2} - W_{1}}{W_{1}} \times 100 %

(2)

where W₁ and W₂ are the quality of the samples (mg) before and after standing for 24 h, respectively.

Determination of mechanical properties

The tensile strength and elongation at break of the base were tested (refer to QB/T 2710-2005)²¹ via a tensile tester (PT-1171, Dongguan Baoda International Co., Ltd). The tear strength of the base was tested (refer to QB-T 3812.6-1999)²² via a tensile tester (PT-1171, Dongguan Baoda International Co., Ltd).

Instrument characterization

Fourier transform infrared spectroscopy analysis

The test samples were placed into an oven (100℃), drying to constant weight and then prepared by the attenuated total reflectance (ATR) method. The samples were prepared with a Bruker VECTOR-22 Fourier transform infrared (FT-IR) spectrometer.

Scanning electron microscope measurement

The test samples were placed into an oven (100℃), drying to constant weight. After metal sputtering of each sample sheet side of about 10 nm, the surface morphology of the base was observed with the aid of a TM-1000 scanning electron microscope.

Atomic force microscope measurement

The surface structure of the base sample was tested using the contact method with Japan’s SPI3800N/SPA400 atomic force microscope. The test range was 500 nm, the maximum of the two-dimensional range was 20 µm × 20 µm, the minimum were 10 nm × 10 nm and the surface roughness maximum was 2 µm.

Thermal analysis measurement

The thermal weight loss of the base sample was tested under the condition of nitrogen protection, with the heating rate of 10℃/min, using a TGA Q500 thermogravimetric analyzer in the range of 25–600℃. Each sample was about 5–10 mg.

Contact angle measurement

Each base sample was glued on the glass slide. Then it was gently rolled over and over again until the surface became smooth. With the aid of an OCA 20 contact angle measurement instrument, the water contact angle was tested.

Results and discussion

Amino, carboxyl and formaldehyde contents

As shown in Table 2, the active groups of the modified base without FP increased slightly. The amino group content in the modified base by collagen was two times and the carboxyl group content was three times compared with that of the pretreated base. This was because collagen contained a large amount of carboxyl and amino active groups, and more active groups were added into the modified base by collagen. In addition, formaldehyde may be produced by FP, so it was necessary to detect the formaldehyde content. According to the national standard GB/T 2912.1 2009,¹⁷ the formaldehyde content in close-fitting textiles cannot exceed 20 mg/kg, and the formaldehyde content in this research was lower than the national standard (see Table 2). Therefore, this cross-linking modification method was safe and qualified.

Table 2.

Amino, carboxyl and formaldehyde contents comparison of the base under different treatment conditions

Group content	Untreated base	Pretreated base	Modified base without FP	Modified base by collagen
Amino content (mmol/g)	0.10	0.25	0.36	0.53
Carboxyl content (mmol/g)	0.12	0.30	0.48	0.89
Formaldehyde content (mg/kg)	—	—	—	16.96

Water vapor permeability, moisture absorption and mechanical properties

Both water vapor permeability and moisture absorption are not only important properties of materials, but also essential factors affecting the application of microfiber synthetic leather as clothes, shoes and so on.

As shown in Table 3, the water vapor permeability in the modified base by collagen increased by 65% and the moisture absorption increased by 181% compared with that of the pretreated base. It was because when carboxyl and amino groups in the base increased after being modified by collagen, the water vapor permeability and moisture absorption were improved. The facts indicated that the modification was successful.

Table 3.

The water vapor permeability, moisture absorption and mechanical properties comparison of the base under different treatment conditions

Properties	Untreated base	Pretreated base	Modified base without FP	Modified base by collagen
Water vapor permeability (mg/10 cm²·24 h)	421.8	478.1	602.3	789.7
Moisture absorption (%)	2.0	2.1	3.9	5.9
Tensile strength (N/mm²)	18.34	15.78	17.64	19.06
Elongation at break (%)	52	47	51	54
Tear strength (N/mm)	102.21	105.14	108.89	112.34

Compared with the untreated base, the tensile strength, elongation at break and tearing strength of the base had different degrees of enhancement after modification. To some extent, the pretreated base would break the amide bonds of the polyamide fiber and loose microfiber, and reduce its strength, so hydrolyzing the base using acid would reduce its physical and mechanical properties in general. However, physical and mechanical properties can be strengthened by adding organic phosphine, which would re-cross-link the broken fibers. Furthermore, with added collagen, the amino groups of the base were enhanced, and it would form ion bonds and hydrogen bonds that also made a contribution to strengthen physical and mechanical properties. In summary, compared with the untreated base, the water vapor permeability, moisture absorption and physical and mechanical properties of the modified base were all improved dramatically, showing the good performance of this modified method.

Instrument characterization

Fourier transform infrared spectroscopy analysis

Figure 3 refers to the FT-IR spectra of collagen, the pretreated base, the blank base, the modified base without FP and the modified base by collagen. The amide stretching vibration peaks of N-H appeared at ∼3300 cm⁻¹. The carboxyl stretching vibration peaks belonged to C = O at ∼1732 cm⁻¹, and the amide stretching vibration peaks and the amide bending vibration peaks of C = O and C-N-H appeared at ∼1636 and ∼1536 cm⁻¹, respectively.^23,24 As shown in Figure 3, the FT-IR spectra (b), (c), (d) and (e) were similar, because there was little change of the general structure of the modified base, except the content of the active groups. Stretching vibration peaks appeared at ∼1732 cm⁻¹ of Figures 3(d) and (e), and the peak at ∼1732 cm⁻¹ was found more evident in Figure 3(e), indicating the stretching vibration peaks of C = O in collagen molecules. It can be observed that the characteristic peak at about ∼3300 cm⁻¹ N-H of Figure 3(e) became intensified. Compared with collagen in Figure 3(a), it may result from the change of collagen, which showed that the application of FP can realize the modification to the pretreated base to some extent.

Figure 3.

Fourier transform infrared spectra of (a) collagen, (b) the pretreated base, (c) the blank base, (d) the modified base without FP and (e) the modified base by collagen.

Surface morphology comparison

Figure 4 shows the scanning electron microscopy (SEM) images magnified by 1000 times of the pretreated base, blank base, modified base without FP and base modified by collagen. Figure 4(a) shows the pretreated base and the polyamide fiber adhered to the bundles. As shown in Figures 4(b) and (c), the base fiber bundles were loosened slightly. Figure 4(d) shows the base modified by collagen, and the degree of looseness of the base fiber bundles were increased obviously. The reason for this was that the collagen penetrated inside the base fiber by the modification, and the hydrogen bonds between fibers were broken, which was advantageous to the scatter of base fiber bundles, transmission and absorption of water vapor. It illustrated that using FP can realize the modification to the pretreated microfiber synthetic leather base by collagen and the improvement of properties of the modified base.

Figure 4.

Scanning electron microscopy images of (a) the pretreated base, (b) the blank base, (c) the modified base without FP and (d) the base modified by collagen.

Surface structure comparison

Figure 5 shows to the three-dimensional contour atomic force microscopy (AFM) images of the pretreated base, blank base, modified base without FP and base modified by collagen, which reflected the height characteristics of the fiber surface morphology. Such images with height data can be directly used to evaluate the roughness of fiber surface.²⁵ As shown in Figure 5, the surface of pretreated base was not flat, and there was a big difference in height, and the R_a (relative average roughness) was 19.93 nm. The R_a of the blank base was 18.98 nm, which changed slightly compared with the pretreated base. The R_a of the modified base without FP was 15.31 nm, showing a slight change. However, after modification by collagen with FP, the surface of the base became relatively smooth. The relative average roughness became much lower, and the R_a was 7.85 nm. The reason for this was that the modified base by the collagen fiber surface combines with a large amount of collagen, filling the empty space between the fibers, leading to the reduction of the surface roughness. From this, it showed that the collagen molecules have cross-linked with the fiber surface base equably.

Figure 5.

Atomic force microscopy images of (a) the pretreated base, (b) the blank base, (c) the modified base without FP and (d) the base modified by collagen.

Thermal analysis comparison

Figure 6 shows the thermogravimetric analysis (TGA) curves of collagen, the pretreated base, the blank base, the modified base without FP and the base modified by collagen, which reflected the thermal properties of the base before and after modification. As shown in Figure 6, it can be seen that the base began to gradually disappear at 230℃, and the base modified by collagen with FP disappeared faster. This probably resulted from the fact that the collagen decomposition curve was different from that of the base,²⁶ where the thermal weight loss of the modified base was two times that of the thermogravimetric result of the base and collagen, leading to the difference of the modified base. When the temperature was 280–460℃, the primary weight losses were caused by degradation of the produced water, carbon dioxide and other volatile substances. When the temperature reached 500℃, all the bases almost reached a steady trend; in particular, the residual ash of the base modified by collagen almost disappeared entirely. This was because the degree of looseness of the base fiber bundles and the amide bond increased obviously by collagen modification, and it was decomposed easily, showing that the collagen modification to the base was successful, and the modification affected the thermal properties of the base.²⁷

Figure 6.

Thermogravimetric analysis curves of (a) collagen, (b) the pretreated base, (c) the blank base, (d) the modified base without FP and (e) the base modified by collagen.

Water contact angle

Figure 7 shows the water contact angle images of the pretreated base, blank base, modified base without FP and base modified by collagen. As shown in Figure 7, the water contact angle of the acid hydrolyzed base was 99.2°, the water contact angle of the blank base decreased slightly, the water contact angle of the modified base without FP reduced to some extent, and the water contact angle of the base modified by collagen reduced obviously, which means that the hydrophility of the modified base was greatly improved compared with pretreated base. This resulted from the following factors: the collagen modification of the base was successful, the content of carboxyl and amino groups greatly increased, the surface tension was changed, the water contact angle became smaller and the water absorption ability increased. Then, it can be concluded that the collagen modification to the base was successfully performed, and the collagen modification improved the hydrophility of the base.

Figure 7.

Water contact angle images of (a) the pretreated base, (b) the blank base, (c) the modified base without FP and (d) the base modified by collagen.

The water penetration rate was measured by water penetration time. The water penetration time refers to the time required from the beginning to complete penetration when a needle drop of water was dropped to the base on the water contact angle measurement instrument.

It was found that the hydrophility of modified base by collagen with FP was good and the water penetration rate was fast. Therefore, it was necessary to detect the water penetration time on this basis for comprehensive analysis. The determination result showed that the water penetration time of the modified base by collagen was 4 s, shortened by 78% compared with that of the pretreated base, further indicating that the collagen modification to the base was successful.

Conclusions

The sulfuric acid pretreated microfiber synthetic leather base was modified by collagen, and the organic phosphine FP was used as a cross-linking agent, for the purpose of increasing the active groups and improving its properties. Compared with the pretreated base, the amino content in the modified base by collagen was two times and the carboxyl content was three times. The water vapor permeability of the based modified by collagen increased by 65% and the moisture absorption increased by 181%. It was further found that the tensile strength, elongation at break and tearing strength of the base had different degrees of enhancement after modification. The test results of FT-IR spectroscopy, SEM, AFM, TGA and water contact angle showed that the modified base microfiber dispersion was greatly increased, the hydrophility was enhanced and the relative average roughness was decreased. In addition, the modification also affected the thermal properties of the base, showing that the collagen modification to the microfiber synthetic leather base was successful and the properties of the modified base were improved obviously.

Footnotes

Funding

This work was supported in part by the National Science Foundation Projects of China (No: 51103082), by the Scientific Research Group of Shaanxi University of Science and Technology (No: TD12-04) and by the Key Scientific Research Group of Shaanxi Province (No: 2013KCT-08).

References

Liao

. Vigorously developed Chinese synthetic leather industry. Sci Technol Inform Dev Eco 2003; 13: 123–125.

Qian

Chen

Fan

. The present situation and development of synthetic leather base. J Ind Textil 2005; 4: 5–9.

Ren

Zhao

Wang

. Synthesis of amino-terminated hyperbranched polymers and their application in microfiber synthetic leather base dyeing. Textil Res J 2013; 83: 382–395.

Tang

Wang

Liu

. Study on the water vapor permeability of leathers. Chin Leather 2002; 31: 17–19.

. Synthetic leather technology, Beijing: Chemical Industry Press, 2010, pp. 1–2.

Sarmadi

Kwon

Young

. Wettability of nonwoven fabrics. Ind Eng Chem Res 1993; 32: 279–287.

Zhang

Ren

. Structure comparison between collagen fiber and microfiber. Indian Leather 2008; 41: 49–58.

Xing

. Preparation, structure and properties of multi-functional silk via ATRP method. Appl Surf Sci 2012; 258: 3208–3213.

Kazuo H. Synthetic leather having excellent moisture permeability. JP2566799 (B2), Japan, 1996.

10.

Nobuo O and Yoshiyuki S. Highly moisture-permeable synthetic leather. JP4082975 (A), Japan, 1992.

11.

Haruo I, Toshikatsu M and Masuhiro M. Production of synthetic leather. JP6093571 (A), Japan, 1994.

12.

Jiang

. Collagen and collagen protein, Beijing: Chemical Industry Press, 2006.

13.

Santhy

Selvapathy

. Removal of reactive dyes from wastewater by adsorption on coir pith activated carbon. Bioresour Technol 2006; 97: 1329–1336.

14.

GB 18186-2000. Fermented soy sauce.

15.

Ren

Sun

Qiang

. Study on the effect of hydrolysis on physical properties of microfiber synthetic leather base. Leather Sci Eng 2013; 23: 16–23.

16.

Zhang

. Quantitative analysis of organic compounds, 1st ed. Beijing: Chemical Industry Press, 1990, pp. 316–319.

17.

GB/T 2912.1-2009. Textiles determination of formaldehyde.

18.

Ding

Luo

Zhou

. Analysis and test of leather products, Beijing: China Light Industry Press, 2010.

19.

QB/T 1811-1993. Leather-determination of water vapour freely.

20.

GB/T 4689.21-1996. Leather-determination of absorption of water.

21.

QB/T 2710-2005. Leather-physical and mechanical tests-determination of tensile strength and percentage extension.

22.

QB-T 3812.6-1999. Leather-determination of tearing load.

23.

Chen

Zhou

Chen

. Analyses of structures for a synthetic leather made of polyurethane and microfiber. J Appl Polym Sci 2007; 103: 903–908.

24.

Liu

Zhang

Chen

. Thermal behavior and fire performance of nylon-6, 6 fabric modified with acrylamide by photografting. Polym Degrad Stab 2010; 95: 1842–1848.

25.

Shen

Wan

. Effect of the enzyme treatment on the performance of secondary fiber. Paper Chem 2011; 23: 12–19.

26.

Yang

. Study of collagen self-assembly mechanism of regulating mineralization. J Fujian Univ Technol 2010; 8: 205–208.

27.

Ibrahim

Salmawi

Ibrahim

. Electron-beam modification of textile fabrics for hydrophilic finishing. Appl Surf Sci 2005; 241: 309–320.