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Maziero, R., Soares, K., Filho, A. I., Franco Jr., A. R., and Rubio, J. C. C. (2019). "Maleated polypropylene as coupling agent for polypropylene composites reinforced with Eucalyptus and Pinus particles," BioRes. 14(2), 4774-4791.

Abstract

Waste from the processing of hardwood and coniferous wood generated in the timber industries is difficult to dispose of and can cause considerable environmental impacts, such as soil and groundwater contamination. In this context, composites with varying concentrations of polypropylene, maleated polypropylene, and particulate Eucalyptus and Pinus waste were produced in a twin screw extruder and injection molded as test bodies for tensile and flexural tests. The morphology of the composites was investigated via scanning electron microscopy. The thermal properties were identified through differential scanning calorimetry. The tensile and flexural results for the two waste formulations indicated that the addition of vegetable fillers increased the modulus of elasticity and bending, and the compatibilizer provided increased resistance to stress and maximum deflection. The scanning electron micrographs illustrated the wetting of the cellulosic charge by the thermoplastic polymer with the compatibilizer, which corroborated the possible occurrence of an esterification reaction and hydrogen bonding interactions in the matrix-particle interface. The incorporation of waste in the composite resulted in the reduction of the degree of crystallinity of polypropylene, regardless of the use of the compatibilizer. This was explained by the barrier capacity of the charge, which prevented the growth of the crystals.


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Maleated Polypropylene as Coupling Agent for Polypropylene Composites Reinforced with Eucalyptus and Pinus Particles

Rômulo Maziero,a,b,* Kinglston Soares,c André I. Filho,b Adonias R. Franco Jr.,b and Juan C. C. Rubio a

Waste from the processing of hardwood and coniferous wood generated in the timber industries is difficult to dispose of and can cause considerable environmental impacts, such as soil and groundwater contamination. In this context, composites with varying concentrations of polypropylene, maleated polypropylene, and particulate Eucalyptus and Pinus waste were produced in a twin screw extruder and injection molded as test bodies for tensile and flexural tests. The morphology of the composites was investigated via scanning electron microscopy. The thermal properties were identified through differential scanning calorimetry. The tensile and flexural results for the two waste formulations indicated that the addition of vegetable fillers increased the modulus of elasticity and bending, and the compatibilizer provided increased resistance to stress and maximum deflection. The scanning electron micrographs illustrated the wetting of the cellulosic charge by the thermoplastic polymer with the compatibilizer, which corroborated the possible occurrence of an esterification reaction and hydrogen bonding interactions in the matrix-particle interface. The incorporation of waste in the composite resulted in the reduction of the degree of crystallinity of polypropylene, regardless of the use of the compatibilizer. This was explained by the barrier capacity of the charge, which prevented the growth of the crystals.

Keywords: Composites; Polypropylene; Maleated polypropylene; Solid waste; Wood

Contact information: a: Post-Graduate Program in Mechanical Engineering, Federal University of Minas Gerais, 6627 Av. Antonio Carlos, 31270-901, Belo Horizonte, Minas Gerais, Brazil; b: Post-Graduate Program in Metallurgical and Materials Engineering, Federal Institute of Education, Science and Technology of Espirito Santo, 1729 Av. Vitoria, 29040-780, Vitoria, Espirito Santo, Brazil; c: Post-Graduate Program in Sustainable Technologies, Federal Institute of Education, Science and Technology of Espirito Santo, 1729 Av. Vitoria, 29040-780, Vitoria, Espirito Santo, Brazil;

* Corresponding author: maziero@ufmg.br

INTRODUCTION

Timber industry solid waste accumulation is a continuous process that requires storage spaces that do not pose a risk of fire hazard. However, part of this volume of waste is used by small farmers in breeding and soil fodder crops, as well as by briquette industries, fertilizer companies, and potteries in the generation of energy. Wood waste can also be used as reinforcement in the manufacturing of wood-plastic composites, commercially known as WPCs.

Wood-plastic composites are widely marketed in the United States and Europe in construction, automotive, and structural applications. In Brazil, the use of this product is still low, although Brazil is one of the largest producers of planted forests in the world, generating large volumes of wood waste that can be used in the preparation of WPCs. Approximately 47 million tons of wood waste from the wood processing and forest harvesting industries were generated in 2015, of which 33 million tons (70.5%) were generated by forestry activities and 13.8 million tons (29.5%) by industrialists (IBÁ 2017). However, the distance between the sawmills and the processing industries, as well as the lack of knowledge of the characteristics of the waste generated in the mechanical processing of wood, prevents the increase of the production of WPCs (Clemons 2002). Therefore, knowing the properties of the waste and using them properly is important to produce ecologically responsible products with reduced cost. These materials can have advantages in the production of components with greater dimensional stability, lower weight, and easy processing (Malkapuram et al. 2009; Leong et al. 2014).

Waste from species such as Eucalyptus spp. and Pinus spp. are interesting alternatives for reinforcements in polymer matrix composites. The genus Eucalyptus presents a great diversity of qualities, including high forest productivity, adaptability to diverse environments, and fast growth. The genus Pinus has low density, fast and easy growing cycles, and a shorter processing time compared to other species. The automotive sector was first to use vegetal wastes of the genera Eucalyptus and Pinus in composites for the manufacture of composite pieces with an approximate weight reduction of 40% when compared with fiberglass (Leão et al. 2009). The use of wood waste as reinforcement in thermoplastic resins has been studied by many authors (Yamaji and Bonduelle 2004; Bledzki et al. 2005; Paes et al. 2011; Dai and Fan 2014). The inclusion of particulate wood waste in a thermoplastic matrix provides consistent results for commercial use (Hillig et al. 2008) and low cost in relation to inorganic loads, such as fiberglass, talc, CaCO3, and mica, among others. According to López-Manchado et al. (2002) and Parente and Pinheiro (2008), the addition of lignocellulosic charges in a thermoplastic matrix increases the modulus of elasticity and attenuates time-dependent phenomena such as creep and relaxation.

Today, it is common to manufacture WPCs with polypropylene (PP) resin thermoplastic and wood, using a compatibilizer to improve adhesion between the matrix and the load. Polypropylene is widely exploited in a variety of applications that when combined with favorable economic conditions and excellent thermal and mechanical properties has encouraged a rapid expansion in the use of this material (Rosário et al. 2011). The maleic anhydride-modified polypropylene additive (PPMA) is one of the most widely used in WPCs because it presents an excellent adhesion between the matrix-particle interface.

Results from the literature show that the use of this compatibilizer improves the mechanical properties of the composites produced (Stark 1999; Albano et al. 2001; Keener et al. 2004; Kim et al. 2008; Yeh et al. 2013; El-Sabbagh 2014; Haq and Srivastava 2017). In this context, the main objective of this work is to compare the mechanical, morphological, and thermal properties of reinforced composites with planted forest waste (Eucalyptus spp. and Pinus spp.) in a PP matrix with and without PPMA coupling agent. In the literature no work was found comparisons between these species no treatment using PPMA in the same matrix PP.

This research contributes to the development of new wood-plastic composites and the reduction of environmental impact caused by the disposal of Eucalyptus and Pinus waste.

EXPERIMENTAL

Materials

The matrix used was the virgin isotactic homopolymer polypropylene (PP-Ho) in granules, grade H 301, flow index IF = 10 g/10 min (230 °C/2.16 kg), and a density of 0.905 g cm-3(Braskem, Sao Paulo, Brazil). The compatibilizer used was PPMA in granules, IF = 112 g/10 min (190 °C/2.16 kg), a density of 0.910 g cm-3, and 1% (w/w) maleic anhydride (MA) (Addivant, Danbury, USA). In its dispersed phase (ASTM D1921-18 and ISO 13322-14) as unmixed waste, granulometries were used in the range of 0.15 mm to 1.20 mm of Eucalyptusspp. (aspect ratio 0.4) and Pinus spp. (aspect ratio 0.5) without chemical treatment, a density of 0.11 ± 0.01 g cm-3 and 0.20 ± 0.01 g cm-3 with moisture contents of 9.9 ± 0.9% and 7.0 ± 0.2% for Eucalyptus and Pinus, respectively.

Methods

Preparation of WPCs

The composites were prepared in a Werner-Pfleiderer ZSK-30 L/D 35 corrotative interpenetrating modular twin screw extruder (Coperion GmbH, Stuttgart, Germany) with a 100 rpm screw speed. Gravimetric feeders (Coperion K-Tron, Salina, USA) were used at a feed rate of 1.5 kg h-1 (side hopper) feed and the feed rate of the polymer (main funnel) was varied to maintain the wood particles at concentrations of 10%, 20%, 30%, and 40% w/w. The use of PPMA at 1%, 5%, and 10% was determined according to the substitution of the equivalent quantity of polypropylene. For some formulations, only the concentrations of 20% and 40% w/w wood for 1% and 10% w/w of PPMA were tested, in order to evaluate the behavior of the largest intervals of each fraction. Figure 1 shows the mounting of the thread profile and the temperatures used in the six heating zones.

Fig. 1. Schematic of the thread profile and extrusion temperatures

Subsequently, specimens were molded from the extruded granules and oven-dried under vacuum at 80 °C for 12 h in an injector Arburg Allrounder (270V/300-120 L/D 30; Arburg GmbH, Loßburg, Germany) with the temperature profile of the feed to the injection nozzle at 210 °C, 215 °C, 225 °C, 230 °C, and 235 °C with an injection pressure of 450 bar.

Mechanical tests

The tensile and flexure tests were performed in an EMIC universal test machine (DL 10000/700; Instron, Sao Jose dos Pinhais, Brazil). The tensile test followed the ASTM D638-14 (2014) standard using specimens with ‘Type I geometry’, a 1000 kgf load cell, 50-mm extensometer, and constant speed for 5 mm min-1 assay. The flexural test followed the ASTM D790-10 (2010) standard using specimens with ‘Procedure A’ geometry, a 200 kgf load cell, and a deformation rate of 1.36 mm min-1.

Scanning electron microscopy (SEM)

The adhesion between the matrix-particle interfaces was verified by means of the fracture surface of samples after tensile test. The observations were confirmed using a ZEISS® scanning electron microscope (EVO MA10; Carl Zeiss AG, Feldbach, Switzerland). The samples of the composites were fixed to a support with the help of double-sided self-adhesive carbon tape, metallized with a gold film, and analyzed at an acceleration voltage of 25 kV.

Differential scanning calorimetry (DSC)

The samples were extracted from fractured traction bodies for thermal analysis. The tests followed ASTM D3418-12 (2012) standard and were conducted on DSC equipment, specifically a NETZSCH STA 449 F3 Jupiter® (NETZSCH-Gerätebau GmbH, Selb, Germany), at a heating rate of 10 °C min-1 in the temperature range of 25 °C to 230 °C in an alumina crucible, inert argon atmosphere as the purge gas (60 mL min-1), and protection (20 mL min-1) at 0.5 bar. Subsequently, the samples were cooled to 25 °C and heated again under the same conditions. The samples were submitted to two heating cycles. The first cycle was to eliminate the thermal history of the material from extrusion and injection and the second was to analyze the thermal characteristics without influence of the processing conditions. The percentage of crystallinity of the polypropylene (Eq. 1) in the composites was calculated using 100% = 209 J g-1as reference (Hirayamaa et al. 2017),

 (1)

where 100% is the melt enthalpy (J g-1) for crystalline PP and x represents the weight fraction of the particle in the composite. The values of melt temperature (Tm), melt enthalpy of samples (ΔHm), and percent crystallinity (Xc) were taken from the curve of the second heating cycle.

RESULTS AND DISCUSSION

The mean values obtained in the tensile tests for the PP/PPMA/Eucalyptus and PP/PPMA/Pinus composites are shown in Figs. 2, 3, and 4.

Fig. 2. Maximum tension variation for polypropylene and compatibilized and not compatibilized composites of PP/Eucalyptus (a) and PP/Pinus (b)

Particle-particle interaction, which is a combination of particle agglomeration and matrix-particle interaction, determines macroscopic behavior and material performance of a composite (Pukánszky 2005). Thus, it is observed in Fig. 2 that the maximum stress was reduced with the addition of wood particles in the polymer matrix, due to the possible propagation of cracks inside the composite caused by clusters of particles that acted as stress concentrators and, in this way, resulted in low mechanical strength of the material. Another indication was related to the weak matrix-reinforcement interaction caused by the lack of wettability between the reinforcement, which has a polar nature, and the polymer matrix, which has an apolar nature. It was noted that the PP/10%Pinus formulation in comparison with the thermoplastic matrix showed no decrease with the incorporation of the waste. This may have been related to the distribution of the charge in the matrix.

It was also observed that the same behavior of decline in the maximum tension with the addition of waste with 1% and 5% compatibilizer was observed for the PP/Eucalyptus composite. Unlike the PP/Eucalyptus and PP/Pinus composites with 10% compatibilizer, no decrease in tensile strength was observed with the waste concentration, in addition to a marked increase in the maximum stress supported by the material, in relation to the polypropylene matrix.

The addition of the PPMA compatibilizer led to a remarkable increase in the maximum stress when compared to the PP/Eucalyptus and PP/Pinus composites for the same mass waste concentration. This was associated with the possible improvement in the adsorption and interfacial adhesion as a function of the decrease in the surface tension between the phases that allowed a better transfer of tension between the matrix and the reinforcement. Thus, if the reinforcement percentage in the matrix is higher, the stress level supported by the composite will be higher as a consequence of the larger compatibilized matrix-particle interfacial area. Another possible factor for the improvement in wettability of solid surfaces and the consequent increase in maximum composite stress was due to the probable reaction of esterification and hydrogen bonding of cellulose with maleic anhydride modified polypropylene. According to Correa et al. (2003), this is due to a possible occurrence of mechanical anchoring or chemical interactions between the anhydride groups and the hydroxyl groups of the cellulose at the polymer-wood interface, promoting the effective transfer of forces from the matrix to the dispersed phase.

The PP/10%PPMA/20%Eucalyptus and PP/10%PPMA/20%Pinus showed an approximate 25% increase in the maximum stress, and PP/10%PPMA/40%Eucalyptus and PP/10%PPMA/40%Pinus exhibited an approximate 55% increase in the maximum stress, compared to the same not compatibilized formulations. The results for the PP/Eucalyptus and PP/Pinus composites with 1% and 5% of PPMA in relation to the not compatibilized composites and those with 10% PPMA presented interposed values. It was observed that the use of 10% compatibilizer was enough to avoid losses in the tensile strength of the polymer, even when 40% of wood particles were added in the PP/PPMA/Eucalyptus and PP/PPMA/Pinusformulations.

The elongation of the polymer is not shown in Fig. 3 because no rupture of the test specimens occurred at the extension limit of the claws of the test equipment. For all formulations, the ductility of the matrix decreased with the addition of waste, as the cellulosic load had high stiffness and low deformation. Lopez-Manchado et al. (2002) found that the addition of organic charges in a composite reduces the elongation at rupture because the fillers act as a reinforcing agent in the polypropylene, reducing the mobility of the polymer chains of the matrix, making the composite hard.

Fig. 3. Variation of elongation at break for polypropylene and compatibilized and not compatibilized composites of PP/Eucalyptus (a) and PP/Pinus (b)

In the composite materials, voids were formed during phase processing, which, in addition to acting as voltage concentrators, narrowed the cross-section in which the force was applied. The concentration of particles in the composites also acted as stress concentrators, causing material rupture with a lower deformation.

The addition of compatibilizer had no remarkable influence on the elongation and also did not produce noticeable effects on the modulus of elasticity of some formulated composites, but the increase in the modulus was evident with the increased concentration of the load, as shown in Fig. 4.