E.Vigueras-Santiago, S. Hernández-López, I. Linares-Hernández and K Linares-Hernández
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AZojomo (ISSN 1833-122X) Volume 6 December 2010
Topics CoveredAbstractKeywords IntroductionExperimental Leather Samples Cross-Linking Friction Wear Tensile StrengthResults and DiscussionConclusionsAcknowledgementReferencesContact Details
Mechanical properties as elastic module, friction and wear were evaluated on goat leather grafted with acrylate-epoxidized soybean oil (AESO) induced by UV-radiation at three different dosages. Results were dependent on the grafted face (inner or outer) and on the UV dosage (360 J/cm2, 720 J/cm2 and 2,700 J/cm2). Important changes of the above properties were more evident at higher dosages for both faces, all of them comparing with the ungrafted sample. There was a reduction in friction and tensile deformation, while the Young’s modulus and the protection against wear increased. These results are important because an environmentally friendly monomer was used in order to modify the properties of leather in a range of values which could be controlled by the graft dosage.
Leather, Photo Grafting, Acrylate-Epoxidized Soybean Oil, Mechanical Strength, Friction, Wear
Leather manufacturing is a technology which has developed over many centuries using goat, sheep and pig, among others. The properties of the leather end-product vary depending upon the kind of hide as well as the method used to make it, to treat it and to finish the hide. Finishing gives the leather the essential properties for its ultimate use. Leather is used in a wide range of applications as clothing, shoes, handbags and accessories and automotive applications. Mechanical, thermal and sun durability are the most demanded characteristics in all such applications. For many years, a lot of research on leather modification by radiation grafting processes with common volatile monomers as butyl acrylate [1-3] and methyl methacrylate [3-6] in emulsion systems have been reported. Unfortunately, the traditional addition of heavy polymer coatings to the surface of the leather has also altered the natural hand and feel of the leather, so that the most durable leathers also had the poorest aesthetic qualities. Reducing the number of polymer coating sand/or the amounts of polymer applied per layer can restore natural feel to the leather but then in turn reduces wear resistance and other strength properties. In a recently work  Acrylate-epoxidized soybean oil (AESO) was grafted on goat leather using three UV radiation dosages. In this work, their tension, friction and wear properties were studied and discussed as a possibility of replacement the volatile and toxic acrylic monomers. Typically, triglyceride-based polymers form gels, which can be hard or soft and exhibits a Young’s modulus of 1-2 GPa , depending on the level of functionalization  of the triglycerides, molecular weight, extent of polymerization, comonomer  type and content (in case of copolymerization) reaching a wide range of acceptable structural applications . Once polymerized, polyAESO is very stable under UV and gamma radiation exposition . That monomer is proposed as a good candidate for changing some mechanical properties by grafting it on goat leather using UV radiation.
The experiments were carried out with formaldehyde tanned skin from a young goat. According to the characterization method, pieces of leather were cut in different shapes and sizes (see Table 1). After, the pieces were washed with acetone under mechanical agitation for 15 min and dried under vacuum for 12 h. The skin had not received additional treatments; it was as crude as was possible. Its natural appearance was fairly uniform beige without visual physical damages. Percentage of grafting was gravimetrically calculated .
Grafting reaction was produced in a programmable UVP-shortwave cross-linker, CL-1000 series equipment with a maximum wavelength of 254 nm. The UV irradiance was 0.05W/cm2. Samples were exposed at that irradiance for 2, 4 and 15 h, receiving doses of 360 J/cm2, 720 J/cm2 and 2,700 J/cm2, respectively as described in . For an easy discussion, exposition time will be used.
Static and dynamic friction values were evaluated using a Q-TEST™/5 machine in the friction test mode, at 150 mm/min testing speed and a nominal weight of 0.4309 Kg. The reference was a Teflon surface with a static and dynamic friction Teflon-Teflon of 0.04 [12, 14].
Evaluations were made in a rotatory Abraser 5130  from Taber Industries using two wear discs H-18 of 500 g weight. Leather wear was estimated by the weight loss percent after 500 cycles of wearing.
Was recorded in an Instron Machine model 1185  at room temperature (25°C), a speed of 50 mm/min, and an acquisition frequency of 500 Hz. Leathersamples were cut as indicated in Table 1 and according to ASTM D 638-00 Test Method for Testing properties of Plastics.
Results and Discussion
It was founded that the presence of polyAESO on inner grafted face decreases both static/dynamic friction (Figure 1(a)) respect to ungrafted inner face. The higher friction for ungrafted inner face is due to its morphology described previously . The decreases on friction after grafting could be explained as follow: PolyAESO could be acting as glue that involves or covers the relatively long fibers of the inner face diminishing their direct contact and the free movement (friction) on a surface.
Table 1. Shape and sizes of leather samples for each characterization
||1.5 cm diameter
||Static and Dynamic friction|
||10.0 cm diameter
Figure 1. Effect of polymerization time on both static and dynamic friction for (a) outer grafted leather and (b) inner grafted face
Taking as reference the ungrafted inner face with static/dynamic frictions of 0.28/0.14, they were lowered 13%/17% at 2h, 29%/32% at 4h and 33%/32% at 15 h of irradiation. Outer face showed the opposite behavior (Figure 1(b)). At 2 h of grafting, static/dynamic frictions are 21%/32% higher than ungrafted side; however as the time of grafting increases at 4 and 15 h, those values are now 19%/38% and 8%/34% respectively, respect to the ungrafted outer face. We have observed the same tendency for pure AESO polymerized at different gamma radiation doses  due to an increasing in the polymerization degree making it a harder thermoset polymer. We could deduce for 2 h of photo grafting that a low polymerization extent of AESO is producing a soft and sticky polymer with a high adhesive power and also a high friction. Polymerization continues at longer UV dosages reducing the friction.
These results are in agreement with those of wear. As we can see in Figure 2, AESO graft produces a covering against wear effect being higher as grafting time increases.
Figure 2. Weight loss after abrasion proof for inner and outer grafted faces respect to grafting time.
Ungrafted inner face has the highest weight loss (8%) due to its long, free and more vulnerable fibers to be peeled off by contact with the abrasive stone. However, the protective effect of AESO graft was not so important: after 2 h of grafting it still lost 7% weight and after 4 and 15 h it lost 6% weight. For outer face the effect was more interesting considering that the ungrafted side lost 2% weight. After 2 h, the weight loss was lightly higher, 2.5% due to the presence of a poor and soft cross-linked ASEA, not enough bonded to the leather. But as the cross-linking time increases, the more resistant and harder ASEA graft improves the wear. For 4 and 5 h UV-grafting the wear weight lost diminished to 1.3%, meaning 39% of improvement.
Deformation results show (Figure 3) that the natural leather elongated 43% at break point indicating that it is a little deformable material, but not the grafted ones. Grafted outer face showed a higher deformation than natural leather at 2 h (27%) and 4 h (12%) of grafting and a diminishing for 15 h (11%). Similar behavior was observed for inner face having smaller changes: for 2 h deformation increased 19%, for 4 h it was 17% and for 15 h only 7%. Graft on inner face never decreases the deformation respect to ungrafted one. It´s well known that ASEA increases its cross-linking density by exposition time to radiation  allowing it a harder consistence, less deformable and more cross linked structure. As was demonstrated previously  grafting thickness on outer face was deeper than on inner face and the consequence of this is evident in the higher elongation effect for the grafted outer face than for the grafted inner face. At 2 h, the AESO polymer is soft and sticky while at 15 h is hard, brittle and not deformable.
Figure 3. Elongation percent at break for inner and outer face respect to grafting time
Natural leather had a Young´s module of 23 MN/m2 whereas grafted inner face (Figure 4) show that at 2 h it increased to 13% whereas after 4 and 15 h the modulus increases 36% and 52%, respectively. The Young´s module for grafted leather by outer face after 15 h had the smallest Young´s modulus with a 12% decreases respect to natural leather. This is related to a high cross-linked graft which tends to rigidity and fragility. Samples grafted at 2 and 4 h presented a better resistance showing increases of 4% and 19% respectively, meaning that graft with some micrometers deep  contributes to reach a better opposition to deformation which could have interesting applications as a finishing treatment.
Figure 4. Young’s modulus in natural and grafted faces according to grafting times.
Mechanical properties as Young´s module, friction and wear were evaluated on goat leather grafted with acrylate-epoxidized soybean oil (AESO) induced by UV-radiation at three different dosages (360 J/cm2, 720 J/cm2 and 2,700 J/cm2). Results were dependent on the UV dosage and on the grafted face (inner or outer) which have different morphologies and surface properties. Important changes of the above properties were more evident at higher dosages for both faces, all of them comparing with the ungrafted sample. The polyAESO graft render to the volume sample almost the same tendency in the mechanical and surface properties but in different magnitudes. There was a reduction in friction from 13 to 33% for inner face and an increasing from 8 to 21% for outer face. Tensile deformation decreases at the highest dosage 11% and 7% for outer and inner faces, respectively. Young’s modulus and protection against wear increased as follow: From 4% to 52% and from 12 to 39%, respectively. These results are important because an environmentally friendly monomer was used in order to modify the properties of leather in a range of values which could be controlled by the graft dosage. The study of these properties at the graft conditions established in this work could render many possibilities to use the monomer and to apply the photo grafting technique in order to changing or improve some properties on leather articles.
This research was supported by UAEM and SEP under the projects 1981/2004 and PROMEP/103.5/05/0204.
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E.Vigueras-Santiago, S. Hernández-López, I. Linares-Hernández and K Linares-Hernández
Laboratorio de Investigación y Desarrollo de Materiales Avanzados (LIDMA), Facultad de Química-UAEM.
Paseo Colón esquina con Paseo Tollocan, Toluca, Estado de México. C.P. 50000, México.
E-mail : [email protected]
This paper was also published in print form in "Advances in Technology of Materials and Materials Processing", 11 (2009) 43-48.