Influence of Natural Fibers on the Phase Transitions in High-Density Polyethylene Composites Using Dynamic Mechanical Analysis
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The Seventh International Conference on Woodfiber-Plastic Composites ~ Influence of Natural Fibers on the Phase Transitions in High-Density Polyethylene Composites Using Dynamic Mechanical Analysis Mehdi Tajvidi, Robert H. Falk, John C. Hermanson, and Colin Felton Abstract Dynamic mechanical analysis was employed to tion was shifted to higher temperatures when fi- evaluate the performance of various natural fibers bers were present. The results also indicated that in high-density polyethylene composites. Kenaf, b transition is not a major transition in such com- newsprint, rice hulls, and wood flour were sources posites, whereas tan d curves of the composites of fiber. Composites were made at 25 percent and tended to deviate from the pure plastic curve at 50 percent by weight fiber contents. Maleic anhy- temperatures above a transition temperature. dride modified polyethylene was also added at 1:25 ratio to the fiber. Temperature scans in the range Introduction of -110° to +100°C were performed, and storage In recent years, the use of natural fibers as rein- modulus, loss modulus, and mechanical loss factor forcement or filler in the manufacture of fiber- (tan 6) were recorded over the selected tempera- thermoplastic composites has been of great inter- ture range. Different transitions were monitored, est to many researchers. These fibers have many and the effects of natural fibers on location and in- advantages, such as low density, high specific tensity of such transitions were investigated. The b strength and modulus, relative non-abrasiveness, transition was hard to detect, whereas the a transi- ease of fiber surface modification, and wide avail- ability. Natural fibers are much cheaper than syn- thetic fibers and could replace synthetics in many applications where cost outweighs high composite Tajvidi: performance requirements (1). The main disad- PhD Candidate, College of Natural Resources, Univer- vantages of natural fibers in composites are the sity of Tehran, Iran lower processing temperatures allowable, incom- Falk and Hermanson: patibility between hydrophilic natural fibers and Research Engineers, USDA Forest Service, Forest hydrophobic polymers, and potential moisture ab- Products Laboratory, Madison, Wisconsin, USA Felton: sorption of the fibers and the manufactured com- Technical Manager, Teel Global Resources, Baraboo, posite. To enhance the compatibility of the two Wisconsin, USA phases in such composites, a compatibilizer or Tajvidi, Falk, Hermanson, and Felton ~ 187 coupling agent is normally added to the mixture. sents the glass transition. The low-temperature g Many researchers have reported improvements in process is generally considered to originate in the mechanical properties when a compatibilizer was amorphous phase but may also have an important used or the fibers were chemically modified prior component associated with the crystalline phase to mixing (2-9). Composites of natural fibers and (15). In these polymers the dominant thermal thermoplastics are finding places in many indus- characteristic is the melting transition, which has tries, particularly the automotive industry (10). the primary characteristics of a first-order ther- Conventional static tests, including tensile, modynamic transition. In highly crystalline poly- bending, and impact tests, are usually performed to mers, an a or a, relaxation involving the crystal- characterize the mechanical properties of such line phase occurs below Tm and is the major composites. Because fiber-reinforced thermoplas- relaxation in these materials. tic composite materials can undergo various types Sirotkin et al. (16) summarize different theories of dynamic stressing during service, studies on the on the mechanisms of relaxations in high-density dynamic mechanical properties of these materials polyethylene. Three major transitions are ob- are of great importance. Furthermore, because of served over the temperature range. The g transi- the highly temperature-dependent mechanical tion is thought to be associated with short-range properties of such composites, the application of a method that monitors property changes over a motions in the amorphous phase. The b transition range of temperatures is critical. Similar to other is associated with inter-lamellar shear and is properties, dynamic mechanical properties depend thought to be dependent on the lamellar fold sur- on types of fiber, fiber length and orientation, fiber face morphology. The a transition is associated loading, fiber dispersion, and fiber-matrix adhesion with shear within the crystalline lamellae; it is de- (1 1-14). Dynamic mechanical analysis (DMA), or pendent only on the lamellar thickness and is un- dynamic mechanical thermal analysis (DMTA), is a affected by the lamellar fold surface morphology. sensitive technique that characterizes the mechan- The glass transition in highly crystalline poly- ical responses of materials by monitoring property mers is difficult to identify (15). This is true be- changes with respect to temperature and/or fre- cause in such cases Tg is a minor event, masked by quency of oscillation. The technique separates the crystallinity, and because crystalline polymers fre- dynamic response of materials into two distinct quently have multiple transitions arising from re- parts: an elastic part (E') and a viscous or damping laxations associated with the amorphous phase, component (E"). The elastic process describes the the crystalline phase, or both. The controversy energy stored in the system, whereas the viscous concerning Tg of polyethylene centers around as- component describes the energy dissipated during signing it to one of the three temperature regions, the process. In a natural fiber thermoplastic com- -33°C, -83°C, and -123°C. Turi (15) suggests that posite, both phases exhibit viscoelastic behavior. evidence favoring the -33°C temperature range DMA provides rapid assessment on the viscoelastic has gained considerable credibility in recent properties of these materials. years. On the other hand, it is suggested that b re- Incorporation of natural fibers into polymers laxation is associated with the relaxation of branch can cause significant changes in the dynamic re- points. This seems to be true because in low-den- sponse of the composites. Phase transitions are sity polyethylene, which is a branched polymer, a major phenomena in many amorphous polymers clear b transition peak can be detected (17). Siro- where mechanical properties may change on the tkin et al. (16) report that for high-density polyeth- order of decades when the material goes through a ylene, the b relaxation is usually absent. This re- glass-rubber transition known as Tg. For semi- crystalline polymers in the temperature range be- laxation is therefore generally attributed to tween the crystalline melting point and liquid ni- segmental motions in the non-crystalline phase. trogen temperature (-196° C), at least three The objectives of this study were to develop ba- relaxation processes are often found. The high- sic information on the influence of various natural temperature a process is often related to the crys- fiber types and contents on the engineering and talline fraction. The b process in these polymers is viscoelastic properties and phase transitions of related to the amorphous phase and usually repre- such composites through DMA. 188 ~ Tajvidi, Falk, Hermanson, and Felton Table 1. ~ Composition of evaluated formulations (% wt.). Compatibilizer Formulation Specimen codea Fiber content Resin content Compatibilizer type content - - - - - - - - - - - - (%) - - - - - - - - - - - - - - (%) - - - 1 PE 0 100 -- 0 2 PE-WF-25 25 74 MAPE 1 3 PE-WF-50 50 48 2 4 PE-WF-25-0 25 75 -- 0 5 PE-WF-50-0 50 50 -- 0 6 PE-KF-25 25 47 MAPE 1 7 PE-KF-50 50 48 WE 2 8 PE-RH-25 25 74 WE 1 9 PE-RH-50 50 48 MAPE 2 10 PE-NP-25 25 74 MAPE 1 11 PE-NP-50 50 48 MAPE 2 a PP = polypropylene; WF = wood flour; KF = kenaf fiber; RH = rice hulls; NP = newsprint; PE = polyethylene; MAPE = maleated polyethelene. Materials and Methods pounded materials were then ground using a pi- lot-scale grinder to prepare the granules. Materials High-density polyethylene, Chevron HiD ® Preparation of DMA specimens 9035, with a melt flow index of 40 g/10 min. (190°C, The granules of the various composite formu- 2.16 kg) and a density of 0.952 g/cm 3 , was used as lations were injection molded to produce stan- the polymer matrix. Wood flour, kenaf fibers, dard ASTM impact specimens. Injection molding newsprint, and rice hulls were used as the discon- was performed using a 33-ton Cincinnati Mila- tinuous phase (filler or reinforcement) in the com- cron 32-mm reciprocating screw injection molder posites. Wood flour (supplied by American Wood with an L/D ratio of 20:1. Mold temperature was Fibers, Inc., Schofield, WI) was 40-mesh maple 93°C, and barrel and nozzle temperature were set flour. Kenaf fibers (supplied by Kengro Corpora- to 188°C. Prior to injection, all materials were tion, Charleston, MI) contained approximately 97 dried for at least 4 hours at 105°C to ensure that percent bast fiber. Rice hulls (supplied by Riceland moisture contents were below 0.5 percent. Speci- Foods, Stuttgart, AK) were 20-80 mesh ground rice mens for DMA testing were cut from the impact hulls. Newsprint fibers were obtained by grinding specimens using a table saw. They were further old newspapers at Teel Global Resources. MAPE machined to a nominal thickness of 2 mm using a (maleic anhydride modified polyethylene) was knee-type Bridgeport vertical milling machine. A Fusabond ® C modified polyethylene, product fly cutter with a carbide insert tool was used. The MB-100D, and was supplied by DuPont Industrial specimens were held in place using a vacuum Corporation. chuck specifically manufactured for this project. Care was taken to obtain the specimens from the Methods same area of the impact specimens. Each side of Composites preparation the specimen was machined to produce a bal- Polymer, fibers, and compatibilizer were ini- anced DMA specimen at the desired thickness. tially weighed and bagged according to the various The final specimen dimensions were 52 by 8 by 2 fiber contents indicated in Table 1.