Bending Properties of Wood Flour Filled Polyethylene in Wet Environment

Bending Properties of Wood Flour Filled Polyethylene in Wet Environment

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Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000

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Procedia Engineering 200 (2017) 68–72

3rd International Conference on Natural Fibers: Advanced Materials for a Greener World, ICNF 2017, 21-23 June 2017, Braga, Portugal

Bending Properties of Wood Flour Filled Polyethylene in Wet Environment V. Mazzanti*, F. Mollica Università degli Studi di Ferrara, via Saragat , Ferrara 4412, Italy

Abstract Wood polymer composites (WPCs) are made of a mixture of a thermoplastic or thermoset polymer, wood fiber or sawdust and small amount of additives. These materials represent an increasingly growing area in polymer industry, in particular they are frequently used as substitutes of wood especially for outdoor products in wet environment. This particular application is due to the hydrophobicity of the matrix that protects the natural fiber reinforcement and increases the durability of the final product. The present work investigates the bending properties and the water uptake of wood flour filled polyethylene in a range of temperatures, spanning from 10°C up to 40°C. The tests show a remarkable decrease of the mechanical properties, in particular strength and stiffness, and an increase in ductility with increasing water temperature. The water uptake tests have shown the influence of temperature on the characteristics of diffusion. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 3rd International Conference on Natural Fibers: Advanced Materials for a Greener World. Keywords: WPC; mechanical properties; wet environment, water uptake, wood fibers

1. Introduction Wood fibers are more and more used as fillers for thermoplastic polymers due to their low cost, high filling level and possibility of increasing the matrix mechanical properties [1,2]. Indeed, the strength and stiffness increase * Corresponding author. Tel.: +39 3297506547. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 3rd International Conference on Natural Fibers: Advanced Materials for a Greener World.

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 3rd International Conference on Natural Fibers: Advanced Materials for a Greener World 10.1016/j.proeng.2017.07.011



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conferred by wood fibers is less significant with respect to more traditional reinforcement such as glass fibers, nevertheless their reduced hardness makes them less abrasive against the surface of processing machineries and this allows longer lifespan and reduced amortizing costs. Wood fiber filled polymers or wood plastic composites (WPCs) have also some important advantages over natural wood. Besides their reduced environmental impact [3], the presence of the hydrophobic polymeric matrix enables them to withstand wet environment better than natural wood, thus WPC profiles can be successfully employed in structures such as docks, piers and boardwalk decking that must perform in close contact with water and moisture. Nevertheless, natural fibers show a relatively high moisture absorption, which leads to dimensional changes, accelerated ageing which affect mechanical properties of WPCs [4]. In literature there are several studies that link the effect of moisture or water exposition with the loss of impact and creep properties [5], the loss of modulus of elasticity [6] and with bending properties [7]. Some authors [8,9] explained that the decrease can be due to internal stress developed inside the composite because of the water-swollen filler. This swelling effect breaks the interfacial adhesion formed between the filler and the matrix, reducing the mechanical properties of the material. However, only few papers [8, 10] have connected the effect of moisture with temperature, even though this aspect can be particularly interesting because these materials are mainly used in outdoor applications where temperature can vary in a quite large range. The aim of this study is to investigate the influence of water exposition at various operating temperatures (from 10°C up to 40°C) on the mechanical performance, in particular in bending after 4 hours preconditioning in water, on a high density polyethylene based WPC filled with 50 wt.% wood flour. In order to evaluate the water content of the specimens during preconditioning, water absorption tests have also been performed. 2. Experimental 2.1. Materials The materials used in this study are a polyethylene matrix (PE) (Eraclene BC-82 by Versalis) and wood flour obtained from wood planning residues. A commercially available maleic anhydride grafted polyethylene (Licocene PE MA 4351 by Clariant) has been used as a coupling agent in the amount of 4 wt.% to improve compatibility and adhesion between matrix and fibers. 2.2. Sample preparation Wood fibers have been initially dried at 80°C in an air–circulation oven for 24h before mixing process to reduce the moisture content. The PE, the wood fibers at 50% by weight and the coupling agent have been mixed together using a compounding-extrusion unit composed of a twin screw co-rotating extruder (50 mm screw) directly feeding into a single screw extruder (80 mm screw) for profile extrusion. The specimens have been obtained by cutting the profiles in bars of 10 mm x 15 mm rectangular cross section. 2.3. Mechanical testing All specimens have been preconditioned for 4 hours in water at the selected temperature. Bending tests have been performed with the three point bending method using a length span of 100 mm at a cross head speed of 1 mm/min in water kept at 10°C, 20°C, 30°C and 40°C. An Instron 4467 with a 500 N load cell was used to measure the stiffness and the strength of the samples. A minimum of five specimens have been tested for each condition. 2.4. Water absorption Before testing, all samples have been dried in an air –circulation oven for 24h and subsequently weighed with a precision scale. Three samples (10 x 15 x 60 mm) for each temperature (10°C, 20°C, 30°C and 40°C) have been immersed in different thermal baths of distilled water. Periodically the specimens were removed from water and

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wiped with a tissue paper before weighing. The weight has been monitored to determine the absorption of water up to saturation. The percentage of water absorption Wa at any time, t, was calculated using the following equation:

Wa (%)  100 

Wt  Wo Wo

(1)

where Wt is the weight of the sample at time t, and Wo is the initial weight of the sample (at t = 0). 3. Results and discussions The results from the bending tests are shown in Tab. 1 in terms of stiffness and strength at various temperatures in wet environment. Representative stress-strain curves are shown in Fig. 1. As it is clear from the results, stiffness and strength decrease with temperature and the WPC becomes more ductile. The loss in modulus is comparable with the loss in strength. Table 1. Three point bending tests. Number in parentheses are the standard deviations 10°C

20°C

30°C

40°C

Stiffness (GPa)

3.42 (0.89)

2.89 (0.86)

2.19 (0.15)

1.78 (0.12)

Strength (MPa)

21.23 (0.77)

18.65 (0.44)

16.83 (1.40)

12.18 (1.49)

Fig. 1. Bending test results.

Water absorption curves as a function of time at each operating temperature are shown in Fig. 2a. After 4 hours preconditioning, water absorption is very low, being 0.54%, 0.56%, 0.78% and 0.97% at 10°C, 20°C, 30°C and 40°C respectively. The equilibrium sorption point is achieved when the value of the sample weight stops increasing. In this case this equilibrium point is reached at about the same time for each temperature (around 700 h) but at different saturation values that increased with temperature. In case of Fickian diffusion behavior, the coefficient of diffusion can be obtained at small times from the following equation:



V. Mazzanti et al. / Procedia Engineering 200 (2017) 68–72 Author name / Procedia Engineering 00 (2017) 000–000

Mt 4  M 

Dt r2

71 4

(2)

where Mt is the water content at time t, M∞ is the water content at equilibrium, t is time, D is the coefficient of diffusion and r is the equivalent radius that can be obtained as

r

2A p

(3)

where A is the area of the cross section and p is the perimeter of the sample. By Equation (2), the water diffusion coefficient can be obtained from the slope of the linear part of the plot of Mt/M∞ as a function of square root of time as shown in Fig. 2b. This plot shows that all curves collapse onto a single one and therefore the water diffusion coefficient is nearly independent of temperature and its value is 4.328E-08 cm2/s. An indication for the explanation of this behavior can be found in [4]. These authors assert that the water absorption within composites filled with natural fibers is led by three different mechanisms: diffusion within the interstices of the polymer chains, capillary transport through defects and weaknesses at the interface between polymer and matrix and micro-cracks in the matrix. Now, the effect of temperature on the material is an increase in ductility, as can be seen from Fig. 1, thus the third mechanism does not appear to be dominant. The first mechanism seems to be excluded by the temperature independence of the diffusion coefficient pictured in Fig. 2b. Thus, the second mechanism seems the most probable one for explaining the water absorption behavior shown in Fig. 2a.

Fig. 2. (a) Water absorption results; (b) Water content at time t divided by water content at equilibrium vs. root of time.

4. Conclusions In this work we have analyzed the influence of water absorption and temperature on the behavior of a high density polyethylene based WPC filled with 50 wt.% wood planning residues. From the bending test it can be concluded that wet environment and temperature lead to a decrease in the mechanical performance of the WPC. From the water uptake tests, it comes out that the coefficient of diffusion is independent of temperature. This may indicate that most probably the dominant water transport mechanism is capillarity through defects at the interface. This aspects must be further investigated with specific tests, such as SEM analysis of fractured specimens. This tests are underway and are to be considered as a future work.

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Acknowledgements The authors gratefully acknowledge the company Iperwood S.r.l. located in Ferrara (Italy) for compound and specimens preparation. References [1] V. Mazzanti , F. Mollica, Rheological and mechanical characterization of polypropylene-based wood plastic composites. Polymer Composite 37 (2016) 3460 – 3473. [2] K. L. Pickering, M. G. A. Efendy, T. M. Le, A review of recent developments in natural fibre composites and their mechanical performance. Comp. Part A 83 (2016) 98 – 112. [3] O. Adekomaya, T. Jamiru, R. Sadiku, Z. Huan, A review on the sustainability of natural fiber in matrix reinforcement – A practical perspective, J Reinf Plast Comp 35 (2016) 3-7. [4] S. K. Saw, R. Purwar, S. Nandy, J. Ghose, G. Sarkhel, Fabrication, Characterization, and Evaluation of Luffa cylindrica Fiber Reinforced Epoxy Composites 8 (2013) 4805-4826. [5] A. K Bledzki, O. Faruk, Creep and impact properties of wood fibre–polypropylene composites: influence of temperature and moisture content, Composites Science and Technology 64 (2004) 693-70. [6] J. S. Machado, S. Santos, F. F.S. Pinho, F. Luís, A. Alves, R. Simões, J. C. Rodrigues, Impact of high moisture conditions on the high moisture conditions on the serviceability performance of wood plastic composite decks, Materials & Design 103 (2016) 122-131 [7] A. K. Bledzki, M. Letman-Sakiewicz, M. Murr, Influence of static and cyclic climate condition on bending properties of wood plastic composites (WPC), eXPRESS Polymer Letters 4 (2010) 364–372. [8] S. Panthapulakkal, S. Law, M. Sain, Effect of Water Absorption, Freezing and Thawing, and Photo-Aging on Flexural Properties of Extruded HDPE/Rice Husk Composites, J. of App. Polym. Sci. 100 (2006) 3619–3625. [9] A. Espert, F. Vilaplana, S. Karlsson. Comparison of water absorption in natural cellulosic fibres from wood and 1 year crops in polypropylene composites and its influence on their mechanical properties, Composites Part A 35 (2004) 1267–1276. [10] S. Kuciel, P Jakubowska, P. Kuzniar, A study on the mechanical properties and the influence of water uptake and temperature on biocomposites based on polyethylene from renewable sources, Composites: Part B 64 (2014) 72–77.