Chopped glass and recycled newspaper as reinforcement fibers in injection molded poly(lactic acid) (PLA) composites: A comparative study

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Abstract

Natural/bio-fibers are replacing synthetic reinforcements traditionally used for the preparation of the environmentally friendly composites. Composite materials are also replacing conventional materials in various fields due to their ease of processability. Chopped glass fiber- and recycled newspaper cellulose fiber (RNCF)- reinforced poly(lactic acid) (PLA) composites were processed using a full size twin-screw extruder and an injection molder. Additionally, a glass-reinforced polypropylene (PP) composite was compounded and molded, and compared to PLA/RNCF and PLA/glass fiber composites. The tensile and flexural moduli of RNCF- reinforced composites were significantly higher when compared to the virgin resin. The morphology, evaluated by scanning electron microscopy, indicated uniform dispersion of both fibers in the PLA matrix. The mechanical and thermo-physical properties of PLA/RNCF, PLA/glass and PP/glass fiber composite were studied and compared using dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). DMA results confirmed that the storage and loss moduli of the PLA/RNCF composites increased with respect to the pure polymer, whereas the mechanical loss factor (tan delta) decreased. The results of the TGA experiments indicated that the addition of fibers increased the thermal stability of the biocomposites compared to neat PLA. The heat defection temperature of PLA/RNCF was found to be comparable to that of the glass fiber-reinforced PLA composites. Such studies are of great interest in the development of environmentally friendly composites from biodegradable polymers.

Introduction

There is growing interest in the use of natural/bio-fibers as reinforcements for biodegradable polymers because natural/bio-fibers have the functional capability to substitute glass fibers that are currently being used in the industry today and also have advantages such as wide availability, high specific strength and modulus, low density, ease of fiber surface modification, relative non-abrasiveness, and low cost [1], [2]. Furthermore, rising oil prices and increased activity with regards to environmental pollution prevention have also catalyzed the research and development of biodegradable polymers. Fiber reinforced composites have gained importance from automotive to geotextiles sectors, where high mechanical properties and dimensional stability must be coupled with low weight. Although glass fibers are widely used commercially in the composite industry, they have several disadvantages compared to natural/bio-fibers (Table 1) [3], [4], [5], [6], [7]. Vollenberg and Hinkens [8] reported that the specific moduli (the ratio of the composite modulus to the composite specific gravity) of high fiber volume fraction bast fibers/polypropylene (PP) composites are in the same range as glass fiber/PP composites. The specific tensile and flexural moduli of the 50% (w/w) kenaf coupled [2% (w/w) maleic anhydride grafted polypropylene] composites were marginally higher than typical values reported for the 40% (w/w) coupled glass/PP injection-molded composites (Table 2) [9]. In addition, natural fibers have high specific strength and stiffness, which is also comparable to that of glass fiber [2], [3]. Besides their low density, they reduce the abrasion of screw and barrel in the extruder and injection molds, as well as the energy input for distributing them in the polymer melt. Also, the biodegradability of this renewable raw material is important.

It is increasingly recognized that recycled newspapers and used paper products constitute a valuable source of fibers [7]. Newspapers are one of the most collected materials in most community recycling programs. Recycled newspaper materials consist of lignocellulosic material and other inorganic fillers, which invariably contain printing inks and other process aid materials. Recycled newspaper cellulose fiber (RNCF)-reinforced plastic composites may find applications as structural materials for the housing industry, such as load bearing roof systems, framing components, and non-structural products such as doors, furniture, and automotive interior parts that might be similar to wood-based composites [6], [7], [11]. Therefore, recycled newspapers are an ideal source of fibers and could be converted into high value composite materials using dry common techniques such as compression- or injection-molding.

PLA is a well-behaved thermoplastic, made from 100% renewable resources like corn, sugar beets, wheat and other starch-rich products [1], [12], [13], [14]. It belongs to the family of aliphatic polyesters made from α-hydroxy acids. These polymers are biodegradable and compostable [1], [4], [12], [13], [14] and several studies on the physical and mechanical properties of the PLA and other biocomposites have been reported [12], [13], [14], [15], [16]. Fang and Hanna [17] reported that the shear viscosity of PLA decreased as the temperature increased, which made flow easier. Moreover, as the shear rates increased, the viscosities of the PLA melts decreased significantly. This was caused mainly by the breaking of the chains of the PLA molecules due to the strong shear forces. Teramoto et al. [18] reported that although no weight loss was observed for neat PLA and PLA/acetic anhydride-treated-abaca composite, the PLA/untreated abaca composite showed an approximate 10% weight loss at 60 days, which was caused by the preferential degradation of the fiber. Generally, PLA polymers are made into useful items using thermal processes, such as extrusion and injection molding [15], [19]. PLA can be melt-processed with standard processing equipment at temperatures below those at which natural fibers start to degrade and at a relatively low cost. Hence, PLA is a versatile material with applications in the medical, textile and packaging industries [14], [19], [20]. PLA exhibits many properties that are equal to or better than many petroleum-based plastics, which makes it suitable for a variety of applications. Despite its high modulus and tensile strength, the low deformation at break and quite elevated price of PLA in comparison to the traditional thermoplastics may limit its applications. Since it is possible to overcome brittleness and poor processability of stiff and hard polymers by a combination of materials, composites are a necessity in the evolution of engineering materials. Most research on PLA composite ultimately seeks to improve the mechanical properties to a level that satisfies a particular application. Some authors consider the enhanced toughness, which is the main advantage of biofibers in composites. The strength of a material, like PLA for instance, may be improved by processing with biofibers. These composites may be easily processed by common techniques such as compression- and injection-molding [19]. The properties of composite materials can be determined by the characteristics of the polymer matrices, together with reinforcements, and the adhesion of matrix/fiber interface and the bonding strength at the interface [21]. As a consequence of these characteristics, sensitive techniques must be used for this aim, such as DMA which monitors changes in the mechanical properties, and serves as an important thermal analysis technique for characterizing the fiber–matrix interface [22], [23], [24]. It is essential to understand the thermal behavior and phase morphology of the cellulose fiber-reinforced composites, which may affect the mechanical properties and biodegradation behaviors.

Though interest in biofiber composites for industrial applications in advanced countries has increased significantly, the lack of availability of extensive property data is an important contributing factor limiting the wide spread application of biofibers in composites. Considerable research effort is needed to develop and exploit fully the potential of these biofiber materials. The objective of this work is to evaluate the mechanical and thermo-physical properties of the RNCF-reinforced PLA as well as glass fiber reinforced PLA composite material that were processed using a full size twin-screw extruder and injection molder. It was possible to prepare cellulose fiber reinforced PLA composites by extrusion in nearly the same way as polypropylene (PP). The glass-reinforced PP composite was compounded and molded with a fiber content of 30 wt% and compared to PLA/RNCF composite. PLA/glass fiber (70 wt%/30 wt%) composites were compared to the PLA/RNCF (70 wt%/30 wt%) composite as well.

Section snippets

Materials

Poly(lactic acid) (PLA; Mw: 20 kDa; Mn: 10.1 kDa) (product name – Biomer L 9000) was obtained from Biomer, Krailling, Germany. Polypropylene (Pro-Fax 6523) was supplied by Basell Polyolefins, Elkton, MD. Glass fibers (Chopped Stand 735: fiber glass chopped stands for polypropylene) (currently priced at $0.95/lb) were provided by Johns Manville, Toledo, OH. Average glass fiber lengths were 413 and 387 μm after injection for PLA/glass (70 wt%/30 wt%) and PP/glass (70 wt%/30 wt%), respectively, from an

Tensile properties of the composites

The stress–strain curves of the PLA- and PP-based composites are shown in Fig. 1, which shows the effect of chopped glass fibers and RNCF on the tensile strengths of virgin and reinforced PLA. The tensile properties of PLA/fiber composites were compared to PP/fiber composites. The results of the tensile tests performed with the composites and the pure PLA and PP are shown in Table 3. PLA has better mechanical properties than PP. The pure PLA has a tensile strength of 62 MPa and a modulus of 2.7 

Conclusions

The mechanical and thermo-physical properties of glass fiber- and RNCF-reinforced PLA composites as well as of glass fiber reinforced PP composites have been investigated. The mechanical properties of the fiber-reinforced PLA composites were found to compare favorably with the corresponding properties of PP composites. Compared to the neat resin, the tensile and flexural moduli of PLA composites were significantly higher as a result of reinforcement by the cellulose fiber. The stiffness of PLA

Acknowledgments

The financial support from USDA-MBI Award Number 2002-34189-12748-S4057 for the project “Bioprocessing for Utilization of Agricultural Resources” as well as NSF-DMI 2004 award #0400296 for the Project “PREMISE II: Design and Engineering of Green Composites from Biofibers and Bioplastics” is gratefully acknowledged. The authors also express their appreciation to CreaFill Fibers Corp., Chestertown, MD, to Johns Manville, Toledo, OH, to Basell Polyolefins, Elkton, MD, USA, and to Biomer, Germany

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