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Influence of Keratin on Polylactic acid/Chitosan Composite Properties. Behavior upon Accelerated Weathering Iuliana Spiridon, Oana Maria Paduraru, Mirela Fernanda Zaltariov, and Raluca Nicoleta Darie Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie400848t • Publication Date (Web): 25 Jun 2013 Downloaded from http://pubs.acs.org on July 6, 2013
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Industrial & Engineering Chemistry Research
Influence of Keratin on Polylactic acid/Chitosan Composite Properties. Behavior upon Accelerated Weathering
Iuliana Spiridon, Oana Maria Paduraru, Mirela Fernanda Zaltariov, Raluca Nicoleta Darie∗
Romanian Academy, “Petru Poni” Institute of Macromolecular Chemistry, 41A Grigore Ghica Alley, 700487, Iasi, Romania
Keywords: composites; polylactic acid; chitosan; keratin; accelerated weathering
Summary In view of producing environmentally friendly materials without compromising the properties, new composites containing polylactic acid as matrix and chitosan, with or without keratin fibers have been obtained. The morphological, mechanical, rheological and thermal characterizations of the composites were performed before and after accelerating weathering. In normal conditions, the presence of keratin improved the toughness and thermal stability of PLA/chitosan material. Upon accelerated weathering, thermal degradation of PLA matrix was faster in the presence of chitosan, as compared to the composite containing chitosan and keratin. Fractured surfaces of the exposed composites are rougher related to the unexposed samples, as revealed by SEM. XRD analysis recorded selective degradation of amorphous part of the materials. The decrease of the complex viscosity values after UV exposure of the blends indicated that chain scission was the most prominent phenomenon in accelerated weathering tests.
∗
Corresponding author:
[email protected] Tel: + 40 232 217454; Fax: + 40 232 211299
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1. Introduction The applications of biopolymers for economic and environmentally sustainable development are growing in last years in order to replace existing materials, as industry attempts to lessen the dependence on petroleum-based fuels and products. Their combination with natural fibers represents an opportunity to improve the composite materials market. Polylactic acid (PLA) has been used in composites mainly as packaging material or in agricultural applications.1 It is synthesized from renewable resources through lactic acid fermentation and represents a highly transparent and rigid material with thermal and mechanical properties comparable to poly(ethylene terephthalate) (PET).2 Pure PLA has the advantage that can degrade to carbon dioxide, water, and methane in the environment over a period between several months to two years, compared to other petroleum plastics needing very longer periods.3 PLA is fully compatible with existing polymer processing equipment to make conventional thermoplastics and maintains the necessary flexibility for desired thermal and mechanical properties. Molecular level packing of pure PLA includes gas permeability.4,5 Blends of PLA with several synthetic and biopolymers have been prepared in an effort to enhance the properties of PLA. PLA blends with collagen, poly (butylenes succinate adipate), poly(hydroxybutyrate valerate), poly(ethylene glycol), poly(methyl methacrylate), polyethylene, poly(ethylene oxide), and poly(butylenes adipate-co-terephthalate) have been reported to improve the properties, such as toughness, modulus, impact strength, and thermal stability, compared to the neat polymer.6.7 PLA composites with different natural fibers have strength properties lower as compared to unfilled PLA due to the poor interfacial adhesion between filler and matrix.8 PLA brittleness and low toughness restricts its applications.9 Chitosan is the deacetylated product of chitin, a natural polymer found in the cell wall of fungi and microorganisms. The chemical structure of chitosan is nearly similar to that of ACS Paragon Plus Environment
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cellulose, except for the replacement of one NH2 group instead of OH on the glucose molecule. Recent studies revealed that it is widely used for medicine, edible packaging or coating, food additives, cosmetic, water treatment and antifungal agents.10-12 Keratin fibers are wastes from poultry farms and therefore are available and inexpensive. Fiber-forming keratins are distinguished from other structural proteins by their amino acid composition, especially by the number of cystine units in the keratin molecule. The most important grafting sites are located at thiol groups on cystine amino acid, but NH and OH groups through H-bond have also been found to act as reactive sites.13, 14 They have limitations because of fragility and the poor mechanical properties, which restrict processing and applications. It was found that blending with other synthetic polymers resulted in blends with improved mechanical properties as compared to components of blend or composites.15-19 In the same time, some studies revealed that PLA composites with different non-wood cellulosic fibers presented decreased strength properties as compared to pure PLA.20,21 These components have different surface chemistry: PLA is hydrophobic, chitosan is hydrophilic and keratin fibers are both hydrophilic and hydrophobic. In many instances, incompatibility between the components of composite material results in an inferior interface that does not adequately transfer stress to the load-bearing filer. Composites may thus be formulated which have properties superior to those of the components alone. Based on interest in formulations of ‘green materials’ from natural sources, composites of PLA and chitosan with keratin as an additive were developed and tested for performance. Accelerated weathering was the stress applied to composite formulations, and TGA, FT-IR, SEM, X-ray diffraction, mechanical tests and dynamic rheology were used to measure and evaluate the results from composite formulations.
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Their characterization and behavior to accelerated weathering was studied in order to identify possible application of these composites.
2. Experimental Section 2.1. Materials The PLA used in this study was NatureWorks 2002D. This PLA had a density of 1.24 g/cm3 and melt flow index (210ºC/2.16kg) of 5-7 g/10 min (ASTM D1238). Chitosan produced by Vanson Inc. (Redmond, WA, USA) with an average molecular weight of 1200 KDa and acetylation degree of 34% was added in a ration of 30% in composite materials. The feathers were washed with ethanol and dried to clean white, sanitized and odor-free. Fibers were ground in a Retsch PM 200 planetary ball mill, uniform lengths being obtained through grinding and sieving. Fractions of sieved fibers greater than 30 µm and smaller than 70 µm were used in this analysis. The resulting fibers had average diameter/length ratios of 50 µm/0.1−0.2 cm, as determined by laser diffraction (Mastersizer 2000, Malvern Instruments). 2.2. Composite Preparation Before blend preparation, PLA pallets, chitosan and feather fibers were dried in a vacuum oven for 6 h at 80ºC. Components compounding was performed at 175 ºC for 10 minutes at a rotor speed of 60 rpm using a fully automated laboratory Brabender station. Specimens for the mechanical characterization were prepared by compression molding using a Carver press at 175 ºC with a pre-pressing step of 3 min at 50 atm and a pressing step of 2 min at 150 atm. A neat PLA sheet was prepared in the same conditions and acted as a reference. Samples composition and preparation are summarized in Table 1. Table 1 ACS Paragon Plus Environment
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2.3. Weathering Procedure PLA and composite samples were placed in a laboratory chamber to accelerate weathering, being exposed to artificial light of a mercury lamp (200
