Chemical modification of leather is usually carried out to improve its properties with a view to satisfy end-user application. Desirable properties for leather are its hydrophobicity, stability, and mechanical properties as well as dyeability. In our study, glycidyl methacrylate was grafted onto leather with a graft yield of 26 % (ca. 0.07 mol/100gm epoxy content) using the emulsion polymerization technique. The grafted leather (GL) was treated with five different concentrations of dibutyl amines (DBA) using DBA/epoxy content molar ratios of 0.5, 1.0, 1.5, 2.0, and 2.5. Unmodified leather (BL), grafted leather (GL) as well as the five aforementioned GL treated with DBA (GLAm1, GLAm2, GLAm3, GLAm4 and GLAm5, respectively), were characterized and evaluated as detailed under.
It is well known that introduction of tert-amino group to polymeric material such as starch, cellulose and/or polyester would certainly affect their hydrophilic character to some extent [21-23]. The magnitude of the hydrophilic-hydrophobic property of the modified polymer greatly depends on the number and type of alkyl group attached to the nitrogen atom of tert-amino group. Our target in this work is focused toward getting a delicate balance between improving the dyeing, thermal, morphological and mechanical properties on one side, and maintains the hydrophobic character of leather on the other side.
Infrared spectra of leather
The IR Spectra of BL, GL and GLAm are shown in Figure 1. It is evident (Figure 1b) that the presence of the band at 906 cm-1 is due to the epoxy group of the glycidyl methacrylate which is only observed for GL thereby proofing occurrence of graft copolymerization of GMA monomer onto leather. In case of GLAm (Figure 1c), the presence of a broad band at the range between 3412-3484 cm-1 is characteristic for the tert-amino group of the GLAm. The presence of this band manifests the chemical interaction between the epoxy group of the monomer and DBA. On the other hand, the presence of a band in the range 1637-1658 cm-1 as well as a band in the range 2964-2968 cm-1 for all studied leather samples (Figure 1 a-c), could be related to the carbonyl group as well as C-H stretching, respectively, of the main leather structure.
Thermogravemetric analysis
Thermogravemetric analysis TGA is a simple and accurate method for studying the decomposition pattern and the thermal stability of the polymers. Figure (2) shows the primary thermograms and derivatograms for BL, GL and GLAm4. As it evident, for BL, there is an initial weight loss of 12.6% around 100OC, which is mainly due to the evaporation of absorbed moisture [24]. Also, the main decomposition for the unmodified leather sample proceeds in one step starts at 309OC and end at 362OC with the decomposition temperature (Td) of 342OC. The weight loss of this in this stage is about 35.8%. The thermal behavior for GL and GLAm4 show a similar trend to the native leather with only a difference in that the main decomposition proceeds in two processes. The initial weight loss decreases up to 7.9% and 8.2% for GL and GLAm4 and this could be mainly related to the deposition of the poly-(GMA) and/or the reaction products of the poly-(GMA) with the DBA (reaction scheme 1) on the surface of the leather, which reduces its capacity to absorb moisture. The main decomposition process, on the other hand, for the two modified leather samples started at a relatively lower temperature than that of ungrafted leather. The differential thermogravemetric (DTG) curve of the GL shows two peaks with Td of 317.2OC and 379.8OC, and weight losses of 12.2% and 35.3%, respectively. Meanwhile, the GLAm4 has the Td two peaks at 272OC and 363OC with weight losses of 14.9% and 45.6%, respectively. The observed lower main degradation temperature for GL and GLAm4 could be explained in terms of degradation effect caused by the action of pH medium, either for grafting bath for GL (as an acidic) or DBA chemical reagent for GLAm4 (as an alkaline).
Scanning electron microscope
The scanning electron microscope was used to study the morphology of some selected samples of unmodified leather, leather-g-polyGMA and grafted leather treated with DBA. From the micro-graphs (Figures 3), it is clear that the unmodified leather fibers are completely separated from each other, whereas, in case of grafted leather fibers aggregates of polyGMA can be noticed on the leather fibers which is a good evidence for the formation of polyGMA grafted on leather surface.
For the micrographs related to the grafted leather treated with DBA, it is observed that, the leather surface showed smooth fibers, soft grain and the fibers interfered with each other.
Nitrogen content
Table (1) shows the relation between the nitrogen contents and DBA concentration when the latter was reacted at different molar concentrations (0.5 -2..5) with leather-g-polyGMA. It is evident that the nitrogen content increases by increasing the DBA concentration from a ratio 0.5 up to 2.0, after which, the nitrogen content decreases. Logically, the increment of amine concentration leads to more interactions via opening the epoxy functional group of GMA according to the following equation:
After DBA/epoxy concentration of 2.0 molar ratio, the nitrogen content was noticeably decreased due to the fact that, the highly amine concentration confer suitable basic medium which affect the solubility of amino acids existing in the collagen macromolecule.
Mechanical properties:
Table (1) shows the mechanical properties expressed as the tensile strength (Kg) and elongation (%) as well as the hardness (shore) of the modified and unmodified leather samples. As is evident, the tensile strength and the elongation (%) increase after grafting with polyglycidyl methacrylate which could be related to the plasticizing effect of PGMA grafted in the matrix of the leather. Meanwhile, the tensile strength and elongation (%) of the grafted and DBA treated leather samples decrease at amine/epoxy molar ratio of 0.5 and this could be ascribed to the opening up of the leather structure after the interaction between the epoxy group of the monomer and the amino group of the DBA. Increasing the amine/epoxy molar ratio value over 0.5 induces an increment in the tensile strength and elongation (%) and reach a maximum at molar ratio value of 2.5. Logically, increasing the DBA bring about substantial improvement in the uniformity of the inter-fiber structure via more deposition of the reacted amine compound inside the fiber matrix. This finding's in accordance with that obtained by SEM study. The hardness values, on the other hand, of the grafted leather decrease by the addition of DBA and such decrease are directly proportional to the DBA concentration.
Water absorption
Table (1) shows the water absorption (%) of BL, GL and GL treated with different DBA concentrations. It is evident that the water absorption firstly decreases after grafting with GMA. Treatment of grafted leather with different DBA concentrations is accompanied by an increase in the water absorption property till certain level after which it decreases. Increasing the water absorption by increasing the DBA concentration could be ascribed to the pronounced effect of the created tertiary amino group with its superior hydrophilic character in the molecular structure of leather. As the nitrogen content increases the magnitude of the butyl alkyl groups increases too causing creation of hydrophobic centers at the leather surface thereby preventing more water adsorption. This suggestion could be recognized via observing the variation in the leather morphological structure after DBA treatment.
Dye property
Seven sets of leather samples were independently dyed with acid and reactive dyes. Five sets of these samples are containing different DBA/epoxy content molar ratio of 0.5 – 2.5 ( GLAm1 – GLAm5 ) and the other two sets are GL and BL. The dyeing properties expressed as colour strength (K/S) are shown in Table (1). For acid dye, it is evident that, the GL acquires an improvement in the K/S value than that of unmodified one. Moreover, introducing tertiary amino groups in the molecular structure of leather by reacting GL with DBA up to a certain DBA concentration brings about considerable increase in the dye-uptake, as evidenced by the values of K/S. The colour strength (k/s) values for the acid dyed leather samples follow the following descending order:
GLAm2 > GLAm1 > GLAm3 > GL > GLAm4 > BL > GLAm5
The noticeable increment of the colour strength (k/s) values for GL and GLAm1-4 than that for unmodified leather could be associated with the following aspects: a) opening up structure of the collagen fibrous through grafting and/or DBA treatment, b) direct interaction between the acid dye and the poly-GMA containing leather via ring opening of the epoxide groups, and c) increasing the magnitude of acid dye accumulation and penetration through GLAm1-4 due to formation of hydrogen bonding between the acid dye and additional basic tertiary amino functional groups [25].
The gradual increase of the dyeing behaviour from GLAm1 to GLAm2 could be ascribed to the relatively higher nitrogen content of GLAm2 (Table 1), which in turn brings about a significant increase in the dye hydrogen bond interaction. On the other hand, the observed decrease in the colour strength values for GLAm3 and GLAm4 could be interpreted in terms of blocking of the leather intermiclle size by dibutyl amine causing a limitation of dye penetration. Needless to say that the higher alkaline medium treatment using DBA/epoxy content ratio of 2.5 causes a significant decrement in the leather amine content which, in turn, affect the extent of dyeing due to the lack of hydrogen bond interaction.
For reactive dye, it is also evident (Table 1), that the GLAm1-5 have higher colour strength values as compared with the untreated leather L-g- poly GMA exhibits the lowest colour strength value. This is because the dyeing operation was carried out without addition of any buffering alkaline medium. Hence the colour strength will rely greatly on the basic character of the reacted DBA. Also, the dyeing of reactive dye occurs via chemical bond interaction between the substrate and the dye [26] Increasing the leather dye uptake by increasing its nitrogen content from 8.65 for GLAm1 to 11.26 for GLAm4 could be simply attributed to the basicity of leather substrate, which leads to increase in the extent of the nucleophile substitution reaction between the dye molecules and the modified leather substrate [27]. The significant drop in the colour strength value for GL can be attributed to the repulsion force between the anionic epoxy content of leather substrate and the anionic nature of reactive dye, causing less affinity for dye-GL interaction.
Table (1): Dependence of colour strength, water absorption and mechanical properties of leather grafted with GMA and treated with DBA on DBA concentration.
Sample
Code
|
Amine/epoxy molar ratio
|
N
(%)
|
Color Strength
(K/S)
|
Water absorption
(%)
|
Mechanical Properties
|
Acid dye
|
Reactive dye
|
30 min
|
120 min
|
T.S*
(Kg/c)
|
E**
(%)
|
Hardness
(shore)
|
BL
|
__
|
7.6
|
26.33
|
17.88
|
78.0
|
83.0
|
6.72
|
15
|
95
|
GL
|
__
|
7.1
|
30.06
|
3.30
|
69.43
|
71.23
|
7.60
|
22
|
92
|
GLAm1
|
0.5
|
8.6
|
34.97
|
27.25
|
73.18
|
73.78
|
6.15
|
16
|
90
|
GLAm2
|
1.0
|
9.4
|
35.46
|
29.49
|
78.95
|
79.33
|
6.75
|
19
|
86
|
GLAm3
|
1.5
|
10.3
|
30.85
|
31.63
|
80.93
|
82.04
|
6.79
|
20
|
85
|
GLAm4
|
2.0
|
11.2
|
29.68
|
34.21
|
76.21
|
79.76
|
9.0
|
30
|
78
|
GLAm5
|
2.5
|
7.9
|
25.89
|
21.84
|
68.80
|
70.84
|
10.0
|
50
|
73
|
T.S* = Tensile strength; E** = Elongation