Leather split, the byproduct of leather manufacture, possesses low utility value because it has loose weave of collagen fibers and weak mechanical strengths. Herein, a practical and convenient method for increasing strengths of leather split was developed by one-step in situ polymerization. The structures and properties of polyacrylate/leather split composites were systematically investigated. The results suggested the monomers with an
Leather processed from hides and skins is a kind of favorable material used for production of shoes, bags, gloves, clothing, and so forth. To meet these uses, leather should be split to a proper thickness in tanneries. As a result, the so-called leather split, the lower reticular layer of tanned leather, was obtained as byproduct of leather manufacture. Leather split has loose weave of collagen fibers and weak mechanical strengths. Usually, the tensile and tear strengths of split are weaker than half of the strengths of top-grain leather. Therefore, the utilization of split is largely limited [
Through the past decades, many methods have been tried to improve the properties of leather split. The most commonly used methods are retanning and coating. The retanning agents used include amino resin, vegetable tannin, and acrylate resin. The strength enhancement of split is extremely limited by retanning, although the fullness and softness may be improved. Coating can strengthen the split, especially using a polyurethane film [
Polyacrylate (PA), one of the most common chemical families of polymers, can be easily prepared through the traditional free radical polymerization [
The applications of in situ polymerization of acrylate monomers on cotton, starch, cellulose, and wool [
In this study, we try to use in situ polymerization of acrylate monomers to increase the strengths of chrome-tanned leather split. The porosity of leather split is beneficial to the uniform dispersion of acrylate monomers or prepolymers into leather split, which may favor the in situ polymerization and may avoid considerable increase of thickness, hardness, and stiffness. The porosity of leather split also provides the possibility of the formation of bicontinuous structure. On the basis of these speculations, the optimized in situ polymerization conditions of acrylate monomers in leather split were investigated in this study.
Chrome-tanned cattle hide leather splits with an average thickness of 1.6 mm were obtained from a local tannery in China. Methyl acrylate (MA), ethyl acrylate (EA), n-butyl acrylate (nBA), methyl methacrylate (MMA), ethyl methacrylate (EMA), n-butyl methacrylate (nBMA), azodiisobutyronitrile (AIBN), acetone, and ethyl alcohol were all chemical pure and purchased from Kelong Chemical Reagent Factory, Sichuan, China; isobutyl methacrylate (iBMA), tert-butyl methacrylate (tBMA), n-amyl methacrylate (nAMA), n-hexyl methacrylate (nHMA), n-essien methacrylate (nEMA), isooctyl methacrylate (iOMA), isodecyl methacrylate (iDMA), lauryl methacrylate (LMA), and stearyl methacrylate (SMA) were all chemical pure and purchased from Aladdin Industrial Corporation, Shanghai, China. The drum (Ø30 cm) commonly used in leather processing trials was employed to stir acrylate monomer and leather split for penetration.
The chrome-tanned leather splits were somewhat uneven in thickness and tightness. To make the experiment results comparable, all the leather split samples (20 cm × 20 cm) were cut from butt area following back line. These selected samples were washed and wringed. Then, ethyl alcohol was used to dehydrate these samples three times (2 : 1,
Acrylate monomer with 3 wt% initiator AIBN (based on acrylate monomer weight) was dissolved in acetone to obtain 50 wt% acrylate monomer solution. The acrylate monomer solution and leather splits were put into drum (2 : 1,
The morphologies of the PA/split composites were observed by scanning electron microscope (SEM, JSM-5900, JEOL, Japan) with an accelerating voltage of 5 kV. The thickness of composites was measured with a dial thickness gauge (MY-3130-A2, MingYu, Dongguan, China). The mechanical properties of the composites were tested with a universal testing machine (AI-7000SN, GOTECH, Dongguan, China) according to Chinese national standards QB/T 2710-2005 and QB/T 2711-2005. The softness of composites was measured with a ball pressure softness tester (GT-303, GOTECH, Dongguan, China). The nitrogen content in composites was tested by nitrometer (K-06, ShenSheng, China) according to Kjeldahl. The polymer contents in composites were calculated as
Smashed leather split powder was used to simulate the chemical reaction possibly taking place in the permeation process of monomers in leather split during drumming. The reaction conditions were exactly the same as the composite preparation except that leather split was replaced by split powder. After solid-liquid separation, oxygen was fed to the reaction solutions to terminate the living chains. Then, the intrinsic viscosities of the solution were measured using Ubbelohde viscometer (diluted, 0.3~0.4 mm) to evaluate the prepolymerization degree of monomers during drumming approximately.
The
Physical properties and polymer contents of the PA/leather split composites.
Monomer | Tensile strength (MPa) | Tear strength (N/mm) | Elongation at break (%) | Softness index | Polymer content (wt%) |
---|---|---|---|---|---|
None | 14.86 (±4.12) | 45.43 (±8.74) | 40.17 (±6.43) | 5.28 (±0.36) | 0 |
MA | 36.80 (±1.17) | 81.59 (±3.31) | 18.90 (±2.24) | 2.17 (±0.20) | 20.73 (±0.65) |
MMA | 31.90 (±2.21) | 72.66 (±4.44) | 21.10 (±2.10) | 3.19 (±0.21) | 19.91 (±0.58) |
EA | 24.68 (±2.33) | 79.24 (±4.28) | 31.17 (±4.85) | 3.08 (±0.17) | 20.06 (±0.71) |
EMA | 22.54 (±1.98) | 63.50 (±5.01) | 36.21 (±4.32) | 4.51 (±0.18) | 19.53 (±0.33) |
nBA | 20.23 (±1.62) | 74.97 (±3.15) | 32.61 (±3.11) | 3.21 (±0.19) | 17.94 (±0.89) |
nBMA | 18.72 (±1.41) | 62.79 (±3.77) | 46.02 (±3.48) | 5.41 (±0.17) | 17.36 (±0.60) |
Meanwhile, it can be observed that, as for monomers with the same ester group, those without
The mechanical properties and polymer contents of PA/leather split composites prepared by methacrylate monomers with different branches are shown in Table
Physical properties and polymer contents of the PMA/leather split composites.
Monomer | Tensile strength (MPa) | Tear strength (N/mm) | Elongation at break (%) | Softness index | Polymer content (wt%) |
---|---|---|---|---|---|
None | 14.86 (±4.12) | 45.43 (±8.74) | 40.17 (±6.43) | 5.28 (±0.36) | 0 |
MMA | 31.90 (±2.21) | 72.66 (±4.44) | 21.10 (±2.10) | 3.19 (±0.21) | 19.91 (±0.58) |
EMA | 22.54 (±1.98) | 63.50 (±5.01) | 36.21 (±4.32) | 4.51 (±0.18) | 19.53 (±0.33) |
nBMA | 18.72 (±1.41) | 62.79 (±3.77) | 46.02 (±3.48) | 5.41 (±0.17) | 17.36 (±0.60) |
iBMA | 16.59 (±2.43) | 67.01 (±4.83) | 42.43 (±3.92) | 5.18 (±0.19) | 19.58 (±0.58) |
tBMA | 18.10 (±1.68) | 68.29 (±4.21) | 38.65 (±4.25) | 4.19 (±0.16) | 19.91 (±0.35) |
nHMA | 18.76 (±1.71) | 67.46 (±3.73) | 46.88 (±3.61) | 5.22 (±0.22) | 16.88 (±0.29) |
iOMA | 19.04 (±2.03) | 56.66 (±4.67) | 48.98 (±3.77) | 4.53 (±0.14) | 15.27 (±0.48) |
iDMA | 19.23 (±1.69) | 52.41 (±4.05) | 51.10 (±4.12) | 4.35 (±0.20) | 13.85 (±0.56) |
LMA | — | — | — | — | 35.45 (±0.29) |
SMA | — | — | — | — | 37.66 (±1.33) |
Before the acrylate monomers and leather splits were mixed in drum, the polymerization inhibitor in monomers had been removed, and the initiator had been added. So the monomers might polymerize slowly during the drumming even at room temperature, leading to the generation of a series of prepolymers. Leather split is a kind of three-dimensional structural material with porosity. Hence, it may take time for acrylate monomers to fully penetrate into leather split. The penetration of generated prepolymers during the drumming would be slower. Therefore, the prepolymerization degree of monomers in drum may largely affect the distribution of polymers after heating, as well as the properties of PA/leather split composite obtained.
In order to prove the occurrence of prepolymerization of acrylate monomers during drumming, the mixture of leather split powder and nBMA was used to simulate the reaction between leather split and acrylate monomer in drum, and viscosity change of the solution with drumming time was determined to characterize the degree of prepolymerization. There are many ways to determine the molecular weight of polymers. Among them, viscometry is a relatively quick and simple method. The viscosity-average molecular weight can be calculated by the Mark-Houwink-Sakurada (MHS) equation:
However, the viscosity-average molecular weight of nBMA prepolymer in the reaction solution was too small to test at the standard concentration (0.1 mol/L), and the standard
Figure
The relationship between
Therefore, the prepolymers instead of acrylate monomers might be the main reactants inside leather split during following heating process. The prepolymers with different molecular weights penetrated into leather split and reached different depths of the three-dimensional porous structure, which would affect the properties of PnBMA/leather split composite.
Figure
Effect of drumming time on physical properties and polymer contents of the PnBMA/Split composites.
Figure
SEM photos of the cross-sections of the PnBMA/split composites (×2000).
Furthermore, to detect the distribution of PnBMA in composites, the PnBMA/split composites were horizontally cut into 3 layers (cutting a 0.5 mm sheet from each face), and polymer content in each layer was determined. As shown in Figure
Tensile and tear strengths of leather split can be remarkably improved by in situ polymerization of acrylate monomers in split collagen fibers in proper conditions, probably because the interpenetrating network structure between PA and collagen fibers is constructed. The comprehensive performances of the modified leather split, including tensile strength, tear strength, softness, and elongation at break, largely depend on the monomers adopted. The monomers that have
The authors declare that they have no competing interests.
The financial support of this study was from the National Natural Science Foundation of China (Subject no. 21506129). The authors wish to thank all the testers of National Engineering Laboratory for Clean Technology of Leather Manufacture (Sichuan University) for their help and efforts in completing this research.