THE EVALUATION OF THE SURFACE ENERGY OF THE LIGNOCELLULOSES COMPOSITE EXPOSED TO UV RADIATION

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1 The 14 th International Conference of the Slovenian Society for Non-Destructive Testing»Application of Contemporary Non-Destructive Testing in Engineering«September 4-6, 17, Bernardin, Slovenia More info about this article: THE EVALUATION OF THE SURFACE ENERGY OF THE LIGNOCELLULOSES COMPOSITE EXPOSED TO UV RADIATION Mariana Domnica Stanciu 1, Rozina Steigmann 2 and Cristina Valcea 3 1 Transilvania University of Brasov, Dept. of Mechanical Engineering 29 Eroilor Blvd, 536 Brasov, Romania, mariana.stanciu@unitbv.ro 2 National Institute of Research and Development for Technical Physics, Nondestructive Testing Department, 47 D. Mangeron Blvd, Iasi, 75, Romania steigmann@phys-iasi.ro 3 Transilvania University of Brasov, Dept. of Technical Linguistic, stejara76@yahoo.co.uk ABSTRACT Lignocelluloses composite have numerous advantages as good strength and rigidity, low density, biodegradable and annually renewable, but they recorded some problems on their durability against environmental factors: variations in humidity, temperature, UV radiations, chemical and biological agents. Exposure to ultraviolet radiations triggers changes in the chemical structure on the surface of the composite, a phenomenon known as photolysis. The paper evaluates the way in which the photo-degradation mechanism influences the surface energy of the lignocellulosic composite, thus identifying the hydrophilic/hydrophobic properties of them. The lignocelluloses specimens were obtained by mix polyester resin type 4- M888 POLYLITE, in 25% volume, with different oak wood particles classified by size class. The samples were analyzed before and after UV exposure. For the accelerated photolysis process, the specimens were exposed to UV-A radiations with a wavelength of 365 nm for 168 hours (7 days) in a room with 18 W Philips fluorescent lamps. The evaluation of the surface energy of the composite lignocellulose materials was carried out by the contact angle method - a thermodynamic equilibrium method resulting from superficial/ interfacial free energies appeared two media interfaces. Key words: surface energy, contact angle method, lignocelluloses composite 1. Introduction The natural fibers as hemp, sisal, jute, wood, flax are the most important lignocelluloses filling materials used in the biocomposite structure that have replaced traditional ones such as glass or carbon fibers. The development of natural fiber reinforced biodegradable polymer composites promotes the use of environmentally friendly materials [1-4]. The new materials derived from 117

2 biodegradable renewable sources are considered as partial solution to environmental pollution caused by the large waste volume [2, 6]. The use of lignocelluloses composite with polymer matrix in the environment with UV radiation can cause photo-degradation which lead to a weakening of the material and micro-cracking of the matrix. In multiple applications as washing, painting, printing, gluing, lamination is important to predict the type of interaction between that surface and the liquid. In addition, exposure to UV radiation can make the material more sensitive to other factors actions (i.e. humidity) [5-8]. The hydrophilicity is a disadvantage when using lignocelluloses fibers as reinforcement in polymer composites. Many approaches have studied the tendency of interfacial strength and dimensional stability of biocomposites as results of UV action [9-14]. The paper is focused on the photo-degradation mechanism that influences the surface energy of the lignocelluloses composite. The evaluation of the surface energy of the lignocelluloses composite materials was carried out by the contact angle method identifying the hydrophilic/hydrophobic properties of them. 2. Materials and method 2.1 Materials The materials taken into consideration are composite material containing hemp mat and polyurethane resin (RAIGITHANE 8274 / RAIGIDUR CREM), with 5% reinforcing natural fibers, oak particles classified by size class (<.4 mm;.4.1 mm;.1.2 mm;.2.4 mm;.4 1 mm) in 25% volume percentage. Thus, five types of composites and solid wood as control sample are obtained. In Table 1 are presented the physical characteristics of analyzed samples. Table 1: Characteristics of the lignocelluloses composites. TYPE OF LENGTH WIDTH THICKNESS WEIGTH CODE SAMPLES L[mm] B [mm] h [mm] m [g] C C Hemp fibers C C C L L Oak particles L L L L Solid wood W Method The evaluation of lignocelluloses composites surface energy was carried out by the contact angle method that is a thermodynamic equilibrium resulting from superficial/interfacial free energies. The value of the contact angle depends on three factors: - the morphology of the substrate expressed by the superficial tension between the solid - gas medium ( mn/m) - the nature of the liquid expressed by the superficial liquid - gas tension (, mn/m) - the nature of the liquid - substrate interactions expressed by the superficial tension between the solid - liquid medium (, mn/m), according to the relation (1). 118

3 A liquid placed on a solid surface, in the absence of gravitational forces, will take the shape corresponding to the minimum energy of the system [8, 11]. This occurs when = + (1) where solid gas superficial tension, solid liquid interfacial tension, the superficial tension of the liquid in contact with its vapors, all in [mn/m], θ - contact angle (the angle of the tangent to the liquid-gas surface in the contact point of the liquid-solid interface). The contact angle may take values between and 18. If the contact angle θ <9 (polar surface), the material is hydrophilic (it absorbs the liquid), and if θ> 9 (scattered surface), the material has a hydrophobic character (it does not absorb liquid) (Figure 1). The type of interaction between that surface and the liquid used in the analysis can be estimated in function of θ angle [9, 11, 14]. Fig. 1: The measurement of the contact angle The contact angles were measured by the hanging drop method using System OCA- equipment (Data Physics Instruments - Fig. 2, a). A syringe with H 2 O and one with NaCl in 3.5% concentration were successively placed on top of all specimens. One drop with a volume of 15 L of sample liquid was dropped from each syringe. The angle formed by droplet and specimen surface has been measured. The sample is placed between a light source and a recording camera (Fig. 2 b). The measurements have been carried out at room temperature and normal humidity conditions(t = 22.7 C and RH=65%). Taking images at every second, dynamic studies of the contact angle changes allow the evaluation of absorption/ adsorption capacity of the specimens. a) b) Fig. 2: The experimental determination of the contact angle. 3. Results and discussion 3.1 Surface energy As the contact angle between droplets and the specimen surface defines the surface changes of the composites due to photolysis, the absorption / adsorption capacity of the specimens taken into consideration has been evaluated from the image recorded during the measurements. 119

4 The total surface energy is given by the addition of two components - U d the dispersive component and U p the polar component, that were measured (Table 2). U U U (2) tot d p Table 2: Experimental results H 2 O NaCl (3.5%) U SAMPLE tot U [mn/m] d [mn/m] U d [mn/m] CONTACT ANGLE [ ] S Initial UV Initial UV Initial UV Initial UV Initial UV C L L L L L The surface energy increased after the exposure to UV radiations (Figure 3), by changing the contribution of the two components: the value of the polar component E p increased while the value of the dispersant component E d decreased. The total surface energy U tot, [mn/m] 7 6 5,5 36,8 34,4 34,1 29,2 28,828,9 61,6 54,6 56,2 31,7 31,1 34,3 52,5 Fig. 3: The variation of the surface energy before and after the artificial weathering Before the artificial weathering, the composites reinforced with wood particles have contact angles with values close to the NaCl solution and distilled water H 2 O (Figure 4). After the exposure to UV radiations, a decrease in the values of the contact angle is noticed, due to the structural and morphological changes, the analyzed surfaces changing from polar into less polar ones. Significant modifications of the contact angles have been noticed when the sodium chloride solution (NaCl - 3.5%) have been employed, comparatively with measurements obtained with distilled water (H 2 O), when samples are exposed to UV. When water has been employed, it can be observed that UV radiations have influenced the superficial structure of the lignocelluloses composites, the least affected and at the same time more chemically stable were the specimens of polyester resin and the wood particles of.4 mm and.1 mm in size (Figure 4, a). When particle sizes are increased (L.4; L1.), the lignocellulose composites behave more like solid wood. It can be observed that the material becomes hydrophilic after exposure to ultraviolet radiations, thus increasing the capacity of the lignocelluloses material to absorb liquids, which leads to structural, geometrical and mechanical changes of the material. In order to stop this phenomenon and implicitly the increase of the lifetime, after the manufacturing of the specimens, a final cross-linking is recommended, that 1

5 might lead to stable chemical bonds between the molecules of the polymer or the use of some additives against UV radiation. Contact angle [ ] determined with H 2 O a) b) Fig. 4: The variation of the contact angle when indicating the size of the reinforcing particles of the lignocelluloses composites in the case of two liquids H 2 O (a) and NaCl (b), before and after exposure to UV radiations. 3.2 Structural and morphological changes The behavior determined above is confirmed by the topographical evaluation with atomic force microscope indicating a homogeneous surface of samples before UV exposure compared to increases of surface roughness after artificial ageing (Figure 5). The UV radiation leads to a porous surface due to migration of chemical compounds. This chemical and physical modification affects the rheological behavior of lignocelluloses composite along their use. Contact angle [ ] determined with NaCl Sample reinforced with hemp fiber C3 Sample reinforced with wood particle L.4 (.4 mm) 121

6 Solid wood Fig. 5: The phase contrast of different studied samples obtained with AFM. Two distinctive cases can be observed concerning roughness, (Figure 6) a) roughness increased after the exposure to UV radiations, which shows an oxidative degradation of the respective surfaces as for the specimen reinforced with hemp fibers (C1), the sample reinforced with wood particles of.4 mm in size (L.4) and the solid wood. b) roughness decreased after the exposure to UV. Because the decrease in roughness is of tens of nanometers (nm), it can be considered that the specimen is morphologically unstable, as for the specimens reinforced with wood particles (L.1, L.2 and L.4). The surface roughness RMS [nm] ,26 34,3 32,28 23,2 24,5 22,2 19, ,3 13,9,63 11,48 5,48 7,45 4. Conclusions Fig. 6: The variation of surface roughness before and after UV exposure. The paper aims to assess structural and morphological changes of lignocelulosis composites surface after UV exposure. The determination of structural surface changes in lignocelulosis composites due to exposure to UV radiation is influenced by the hydrophilic behavior of cellulose. The contact angle method provides the information about surface energy in terms of the capacity of the lignocelluloses material to absorb liquids or to act as a barrier for liquids. The size and type of lignocelluloses reinforcement in the presence of ultraviolet radiation influence the 122

7 contribution of the two components of surface energy: the value of the polar component U p increased while the value of the dispersant component U d decreased. 5. Acknowledgement This paper was supported by Program partnership in priority domains - PNIII under the aegis of MECS -UEFISCDI, project no. PN-III-P2-2.1-BG-16-17/85 SINOPTIC. 6. References [1] Sahari J., Sapuan S.M.: Natural fibre reinforced biodegradable polymer composites, Rev.Adv.Mater. Sci., Vol., 11, [2] Stanciu M.D., Curtu I., Groza M., Savin A.: The evaluation of rheological properties of composites reinforced with hemp, subjected to photo and thermal degradation, International Congress of Automotive and Transport Engineering, Brasov, 16, DOI.7/ _62. [3] Essabir H., Bensalah M.O., Rodrigue D., Bouhfid R., Qaiss A.: Structural, mechanical and thermal properties of bio-based hybrid composites from waste coir residues: Fibers and shell particles. Mechanics of Materials, Vol. 93, 16, [4] Cristaldi G., Latteri A., Recca G., Cicala G.: Composites Based on Natural Fibre Fabrics., Woven Fabric Engineering P. D. Dubrovski, Croația, Published by Sciyo,. p [5] Han H.: Study of Agro-composite Hemp/Polypropylene: Treatment of Fibers, Morphological and Mechanical Characterization, PhD Thesis, Troise University of Technology, France, 15. [6] Belgacem M.N., Gandini A.: The surface modification of cellulose fibres for use as reinforcing elements in composite materials, Compos Interfaces. Vol. 12, 5, [7] Peng, Y., Liu, R, Cao, J.: Effects of Antioxidants on Photodegradation of Flour/Polypropylene Composites during Artificial Weathering. BioResources Vol. 9(4), 13, [8] Dufresne, A., Belgacem M.: Cellulose reinforced composite: from micro to nanoscale, Polimeros, Vol. 23(3), 13, [9] Guo G., Shi Q., Luo Y., Fan R., Zhou L, Qian Z.,Yu J.: Preparation and ageing-resistant properties of polyester composites modified with functional nanoscale additives, Nanoscale Research Letters 14, 9:215. [] Beg M.D.H., Pickering K.L.: Accelerated weathering of unbleached and bleached Kraft wood fibre reinforced polypropylene composites, Polymer Degradation and Stability, Vol. 93, 8, [11] Cosnita M., Cazan C., Duta A.: Interfaces and mechanical properties of recycled rubber polyethylene terephthalate wood composites, Journal of Composite Materials, Vol. 48 (6), 14. [12] Romanzini D., Lavoratti A., Ornaghi Jr.H.L., Amico S.C., Zattera A.J.: Influence of fiber content on the mechanical and dynamic mechanical properties of glass/ramie polymer composites. Materials and Design, Vol 47, 13, [13] Butylina S., Hyvärinen M., Kärki T.: A study of surface changes of wood-polypropylene composites as the result of exterior weathering. Polymer Degradation and Stability, Vol 97, 12, [14] Azwa Z.N., Yousif B.F., Manalo A.C., Karunasena W.: A review on the degradability of polymeric composites based on natural fibres. Materials and Design, Vol 47, 13,