Performance of polypropylene–wood fiber composites

Polymer Testing 18 (1999) 581–587

Material Properties

Performance of polypropylene–wood fiber composites Fernanda M.B. Coutinho*, Thais H.S. Costa Instituto de Macromole´culas Professora Eloisa Mano, UFRJ, P.O. Box 68525, 21945-970, Rio de Janeiro, RJ, Brazil Received 5 May 1998; accepted 15 July 1998

Abstract Polypropylene–wood fiber composites were prepared in the optimal mixture conditions determined in a previous work (180°C, 60 rpm, 10 min). Tensile, impact and three-point bending tests were performed in order to evaluate the adhesion between matrix and wood fibers. Other than mixture conditions, drying temperature of treated wood fiber is also an important factor to obtain good performance composites as shown in this work. Tensile properties of composites submitted to two extreme conditions (immersion in water at ambient temperature for 90 days and immersion in boiling water for 1 h) were determined. Heat deflection temperature and thermal analysis of composites were evaluated.  1999 Elsevier Science Ltd. All rights reserved.

1. Introduction Recycling, together with the use of cleaner technology, greatly contributes to reduce the impact of industrialization upon the environment [1]. According to Zaikov et al. [2], the production of synthetic polymers in 1996 plus the level of production of natural polymers (cellulose and its derivatives, natural rubber etc) was estimated as 250–260 million tons which represents 240 million m3. Since figures predicted that the volumes of production of polymers will increase two fold for the period from 1995 to 2010, if no massive reutilization of these materials is adopted, serious ecological and economic problems are likely to endanger our society. Simionescu [3] emphasized the overwhelming responsibility of scientists, not only towards the present society * Corresponding author. Tel.: ⫹ 1-502-123-456; fax: ⫹ 1-502-789-101; e-mail: [email protected] 0142-9418/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 1 4 2 - 9 4 1 8 ( 9 8 ) 0 0 0 5 6 - 7

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but mostly, towards future generations. Estimations are being made that at least half of the energy related to the world’s vegetal biomass are lost during harvesting and conversion, part of it returning to the soil as melioration agents, while other parts (straw, leaves, brushwood) are lost from an economic point of view [3]. Reutilization may be viewed as an economical practice but most importantly as a way to curtail the modern trend of a never ending use of disposable products. The use of lignocellulosic materials in thermoplastic composites may contribute to reduce the waste of vegetal biomass. Recently, Guillet [4] showed that plastics have the lowest energy costs of all comparable materials and cause less environmental pollution in their production and fabrication. The technology of photodegradable plastics and experimental studies of the biodegradation of conventional and photodegradable polyethylene, polypropylene and poly(ethylene terephthalate) may lead to thermoplastic/cellulosic filler composites still more environmentally friendly since these reinforcing fillers derive from renewable source, are biodegradable and can reduce squandering of many valuable cellulosic fibers [4]. In this work, good performance polypropylene–wood fiber composites [5] were prepared with the use of wood fiber treated with a silane coupling agent and further coated with maleated polypropylene in a polypropylene matrix. Being recyclable, polypropylene contributes to reduce the polymeric waste and the composites may make use of Brazil’s potential for, on one side the availability of wood fiber, and on the other the production capacity of polypropylene.

2. Experimental 2.1. Materials Polypropylene (PP) and maleated polypropylene (MAPP) were donated by OPP Petroquı´mica S.A, molecular weight (Mw) and molecular weight distribution (Mw/Mn) are respectively 196.000, 106.000, 3.8 and 3.1. Chemithermomechanical (CTMP) aspen pulp (steam cooked at 120°C for 10 min; impregnation solution with 8% Na2S03) was donated by Centre de Recherche en Paˆtes et Papiers (Universite´ du Quebec a` Trois-Rivie`res—Canada). After defibration and grinding, the pulp was sieved to mesh size 60. The wood fibers ␤-cellulose and total cellulose contents are respectively 63.6% and 92.8%. The silane coupling agent (A172-vinyltris(2-methoxyethoxy) silane was supplied by Union Carbide and donated by Osi Specialities Ltd. Dicumylperoxide (DCUP) was supplied by Atochem-Pennwalt S/A and used without further purification. The solvents employed (research grade) were donated: methanol (PROSINT), toluene (CSN), o-dichlorobenzene (Hoechst), iso-octane (Polibrasil S/A) and n-hexane (CENPES/Petrobras). 2.2. Wood fiber treatment CTMP aspen pulp oven dried (60°C) for 24 h was refluxed for 3 h in a 4% w/w A172 methanol solution containing 2% w/w dicumylperoxide in a glass reactor. At the end of the reaction, methanol was eliminated by vacuum and the pulp was oven dried (60°C) for 24 h. The treated pulp was ground and screened to 60-mesh size (average aspect ratio L/D ⫽ 9).

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2.3. Polypropylene coating 10 g (2% w/v coating) or 50 g (10% w/v coating) of MAPP (maleated polypropylene) was mixed with 500 ml of o-dichlorobenzene in a 1 l glass reactor and, after 24 h of swelling, the mixture was heated to 180°C until complete dissolution. When the temperature of the viscous solution reached 130°C, the A172 treated wood fibers were added. The mixture was then cooled to room temperature, thoroughly washed with toluene and hexane and finally dried at 60°C for 24 h. The resulting solid mixture was then ground. 2.4. Composites preparation The mixtures of polypropylene (PP) or maleated polypropylene (MAPP) and coated A172 treated wood fibers were compounded in a Haake Rheomix 600 equipped with roller blades rotor at 180°C for 10 min at 60 rpm. After the addition of polypropylene, the filler was added as soon as the registered torque indicated that the polymer melt had reached a steady state (approx. 2 min). Tensile and flexural specimens were obtained by compression molding. Rectangular specimens were cut from the pressed sheets to size (100 ⫻ 8 ⫻ 0.9 mm). They were measured with the aid of a micrometer for tensile (ASTM D882-83) and flexural tests (ASTM D790M-84 three bending, adapted to the available support span). The mechanical property measurements were carried out in an Instron Tester (model 4204) with a load cell of 1 kN, crosshead speed of 5 mm/min in tensile tests and 6.5 mm/min in flexural tests. A minimum of four samples were tested. Impact (Charpy-ASTM D256a) and heat distortion temperature (HDT-ASTM D648) tests were performed with injection molded samples. 2.5. Apparatus and operating conditions For thermoanalytical studies, a Perkin–Elmer Thermal Analysis System 7 with DSC-7 and TGA-7 were used. DSC measurements were carried out with 5 mg of sample at a heating rate of 20°C/min in a N2 atmosphere. Thermogravimetric measurements were done with the same amount of sample and heating rate. The loss in material weight was registered in the range from 30 to 600°C at heating rate of 10°C/min and nitrogen flow of the oven and sample of 60 ml/min and 33 ml/min, respectively. 3. Results and discussion Table 1 shows twelve composites and their composition while Table 2 present their performance in tensile and flexural tests. These composites were prepared at the best mixing temperature [6]. It can be observed that compared to pure PP or MAPP, all composites are reinforced although the performance of composites of MAPP matrix is higher. The increase in the tensile strength compared to pure polymer suggests that satisfactory interfacial adhesion was developed with both types of matrix [5,7]. The best results of the composites prepared with MAPP as matrix are due to the polar characteristics of the polymer. Fig. 1(a) presents a SEM micrograph of a fracture of composite 1 and it shows adhesion between fiber and matrix. Although Fig. 1(b) is of a fracture

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Table 1 Composition of wood-fiber/MAPP or PP composites Composite

Fiber content in the composite (%)

1 2 3 4 5 6 7 8 9 10 11 12

MAPP coating on the fiber Type of matrix (%)

10 10 10 10 20 20 20 20 30 30 30 30

2 2 10 10 2 2 10 10 2 2 10 10

PP MAPP PP MAPP PP MAPP PP MAPP PP MAPP PP MAPP

Table 2 Tensile and flexural performance of composites of Table 1 Composite

1 2 3 4 5 6 7 8 9 10 11 12

Stress at maximum loada (MPa) 35.0 40.8 36.9 41.3 34.7 44.2 39.4 41.6 38.6 48.5 40.6 46.6

Stress at maximum loadb (MPa) 43.2 47.2 42.7 38.9 50.6 56.2 45.0 44.0 59.5 57.4 46.3 48.5

Modulus1(MPa)

1085 1245 1113 990 1266 1442 1248 1086 1472 1441 1487 1461

Modulus2(MPa)

5184 4333 4319 4231 5759 5156 4791 3962 6356 5158 4916 5269

Elongation at yielda (%)

8.3 12.3 11.7 13.8 5.8 8.1 8.5 10.1 5.7 9.6 9.1 10.6

a

Tensile test (ASTM D 882-83), crosshead speed 5 mm/min. Flexural test (ASTM D-790M-84), crosshead speed 6.5 mm/min. Mechanical properties of pure polymers (PP/MAPP): stress at maximum load1: 33.1/36.7 MPa; stress at maximum load2: 39.0/42.1 MPa; Modulus:a 1096/1030 MPa; modulusb: 3996/3759 MPa; elongation at yield: 14.8/12.6%; experimental error: tensile stress: ⫾ 1.4 MPa; flexural stress: ⫾ 3.2 MPa; tensile modulus ⫾ 110 MPa; flexural modulus: ⫾ 615 MPa; strain at yield: ⫾ 0.9%. b

of a composite of same composition as composite 1, before mixing of fibers and matrix the treated and coated wood fibers were dried at 150°C instead of 60°C. It can be seen that no adhesion was developed between fiber and matrix. This composite presented poor mechanical properties and fiber degradation was also suggested by a brown color of the fibers, usually yellowish. In relation

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Fig. 1. SEM micrographs (a) fracture of composite 1; (b) fracture of composite of same composition as composite 1 prepared with wood fibers dried at 150°C.

to impact tests, composite 1 and 8 presented impact strengths of 22.2 and 18.5 J/m respectively, while pure MAPP and PP were 13.9 and 14.8 J/m respectively. The addition of treated wood fiber seems to have improved energy absorption, therefore, and increased the impact strength [8]. Two extreme conditions were chosen for evaluation of composites: immersion in water at ambient temperature for 90 days (condition 1) and immersion in boiling water for 1 h (condition 2). As shown in Table 3, that presents tensile properties, composites 1, 2, 4, 8, 10, and 12 were submitted to condition 1 and composites 2, 3, 4, 5, 6 and 7 to condition 2. It was verified that composites submitted to condition 1 presented high dimensional stability and resistance to water as the samples maintained their dimensions (cross-section areas, width ⫻ thickness) [9] and tensile properties did not change significantly. A slight increase in stress at maximum load and decrease in elongation at yield may be attributed to a higher ordering of the pressed samples stored for 90 days before the test. However, samples submitted to condition 2 presented a large increase in

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Table 3 Tensile performance of composites submitted to extreme conditions Composite

Stress at maximum load (MPa)

1a 2a 4a 8a 10a 12a 2b 3b 4b 5b 6b 7b a

Tensile modulus (MPa)

35.1 40.9 42.7 39.0 44.3 40.9 46.4 39.9 45.5 34.7 45.7 43.6

Strain at yield (%)

1196 1414 1060 1191 1553 1494 1359 1106 1168 1495 1232 1394

8.4 9.5 10.6 8.6 9.4 9.2 11.1 12.5 11.7 6.9 9.1 9.1

Composite submitted to condition 1. Composite submitted to condition 2.

b

tensile modulus due to water absorption and had their dimensions altered. The enhancement in tensile stress of composites submitted to condition 2 should not be considered an improvement in tensile property since eventually the water absorbed will tend to leave. This result was expected since the heat distortion temperature (HDT-ASTM D848)) of composite 2 and 6 was 74.5 and 82.5°C respectively, lower than the boiling point of water. Compared to the pure MAPP (HDT ⫽ 65.5°C), these composites present higher thermal stability. Table 4 presents thermal analytical data of polymers, composites and treated wood fiber used in this work. The onset temperature is defined as the temperature where the curve first begins to deviate from the baseline. Coated wood Table 4 Thermal analysis data of wood fiber, polymers and composites Material Composite 6 Composite 5 Composite 8 Treated and coated wood fibera Treated and coated wood fiberb Treated wood fiberc MAPP PP a

Onset (°C)

Peak (°C)

Tm (°C)

Tc (°C)

320/450 303/445 307/451 306/453

349/469 348/465 349/469 350/474

165 162 163 nd

116 116 115 nd

nd

nd

164

120

303 447 441

351 475 469

– 164 159

– 114 110

A172 wood fiber submitted to MAPP coating in o-dichlorobenzene solution of 2% w/v. A172 wood fiber submitted to MAPP coating in o-dichlorobenzene solution of 10% w/v. c A172 treated wood fiber: nd—not determined. b

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fiber present a two stage degradation process, the first onset and peak values are related to the wood fiber presence and the second onset and peak values are due to the polymer contribution. Compared to the onset of MAPP, the coated wood fiber present a higher onset which suggests that some interaction between fiber and MAPP occurred causing an enhancement of thermal stability. The crystallization temperature (Tc) of composites and coated fibers are higher than that of pure polymers. The presence of wood fibers may be viewed as a nucleation agent for polypropylene. Wang and Harrison [10] suggest that if crystallization occurs, preferably in the wood fiber surface, as well as improvement in mechanical properties wood fibers gain a barrier against moisture. 4. Conclusion It can be concluded that polypropylene–wood fiber composites may present good mechanical performance, is water resistant and presents higher thermal stability than the pure matrix. Acknowledgements The authors thank Professor B.V. Kokta from Centre de Recherche en Paˆtes et Papiers/Universite´ du Quebec a` Trois-Rivie`res/Canada for sending the aspen pulp and for some suggestions made during the experimental part of this work and PADCT/FINEP, CNPq, PADCT/CNPq, OPP Petroquı´mica S.A. and Polibrasil Polı´meros S.A for financial or material support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Sheldon RA. Chemtech 1994;24(3):38–47. Zaikov GE, Artsis MI, Polishchuk AYA. Polymer News 1996;21:323–4. Simionescu CRI. Cellulose Chem Technol 1990;24:293–307. Guillet J. Macromol Symp 1997;123:209–24. Costa THS. Doctorate thesis, IMA/UFRJ, Rio de Janeiro, RJ, Brazil, 1997. Coutinho FMB, Costa THS, Carvalho DL. J Appl Polym Sci 1997;65:1227–35. Felix JM, Gatenholm P. J Appl Polym Sci 1991;42:609–20. Raj RG, Kokta BV. ACS Symp SER (Emerging Technology Mater Chem Biomass) 1992;476:76–87. Maldas D, Kokta BV. Polym Comp 1990;11(2):77–89. Ge Wang, Harrison I. In: Conference Proceedings Plastics Engineering, Society of Plastic Engineers, May 1–4, ANTEC, San Francisco, 1994;II.