Polimeros

Polymers 2010, 2, 554-574; doi:10.3390/polym2040554

polymers

ISSN 2073-4360

www.mdpi.com/journal/polymers

Article

Functionalization, Compatibilization and Properties of Polyolefin Composites with Natural Fibers

Mariano Pracella 1,*, Md. Minhaz-Ul Haque 2 and Vera Alvarez 3

1 Institute of Composite and Biomedical Materials, IMCB- CNR, Via Diotisalvi 2, Pisa 56122, Italy

2 Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, Pisa 56122, Italy; E-Mail: minhaz1978@yahoo.com

3 Polymer Division, INTEMA-CONICET, University of Mar del Plata, Mar del Plata, Argentina; E-Mail: alvarezvera@fi.mdp.edu.ar

* Author to whom correspondence should be addressed; E-Mail: mariano.pracella@diccism.unipi.it; Tel.: +39-0502217829; Fax: +39-0502217866.

Received: 14 October 2010; in revised form: 2 November 2010 / Accepted: 10 November 2010 / Published: 15 November 2010

Abstract: The article is focused on analyzing the effect of functionalization and reactive processing on the morphological, thermal, rheological and mechanical properties of composites of isotactic polypropylene (PP), polystyrene (PS), poly(ethylene-vinyl acetate) (EVA), with cellulose fibers, hemp or oat as natural fillers. Both polymers and fibers were modified with bi-functional monomers (glycidyl methacrylate, GMA; maleic anhydride, MA) capable of facilitating chemical reactions between the components during melt mixing. Polyolefin copolymers containing reactive groups (PP-g-GMA, SEBS-g-MA, PS-co-MA, etc.) were used as compatibilizers. Optical and SEM microscopy, FTIR, RX, DSC, TGA, DMTA, rheological and mechanical tests were employed for the composites characterization. The properties of binary and ternary systems have been analyzed as a function of both fiber and compatibilizer content. All compatibilized systems showed enhanced fiber dispersion and interfacial adhesion. The phase behavior and the thermal stability of the composites were affected by the chemical modification of the fibers. Marked changes in the overall crystallization processes and crystal morphology of PP composites were observed owing to the nucleating effect of the fibers. The tensile mechanical behavior of the compatibilized composites generally resulted in a higher stiffness, depending on the fiber amount and the structure and concentration of compatibilizer.

OPEN ACCESS

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Keywords: polymer composites; natural fibers; functionalization; compatibilization;

morphology; physical-mechanical properties

1. Introduction

Polymer composites based on natural fillers are currently receiving great attention as innovative

materials for industrial applications in several sectors, such as automotive, building, appliance,

packaging and biomaterials. The main advantage of employing natural fibers is that these are

biodegradable and renewable, and exhibit low cost, low density and high toughness. However, the

weak compatibility between fibers and polymer matrix, the low dispersion degree of the fibers, as well

as its poor moisture resistance, generally leads to low performance materials, limiting their use. Other

factors, which can largely affect the composite properties, are concerned with the size, geometry and

dispersion of filler particles in the matrix. In order to improve the interfacial interactions between

polymer and fibers, surface modification of the fibers and/or polymer functionalization, as well as

addition of compatibilizers is required (Figure 1) [1].

Figure 1. Structure-processing-property relationships of thermoplastic composites with natural fibers.

POLYMER/NAT. FIBER COMPOSITES

PROCESSING FIBER MODIFICATION

MORPHOLOGY

PROPERTIES

CRYSTALLIZATION

PROCESSES

POLYMER

FUNCTIONALIZATION

INTERFACIAL

INTERACTIONS

Enhanced interfacial adhesion for composites containing natural fibers can be achieved by

chemical/physical treatments of the fibers or by use of specific interfacial additives [2-3]. Traditional

chemical treatments of the fibers include extraction with alcohol, benzene or NaOH (delignification,

bleaching, etc.). The other important possibility is the insertion of functional molecules which can be

exploited in secondary reactions (i.e., radical grafting) with the polymer matrix, providing a stable

network of bonding between the components. Effective methods of chemical modification of fibers

have been developed by means of reaction with various reactive monomers, such as acetic anhydride,

stearic acid [4], maleic anhydride [5,6], glycidyl methacrylate [7], silane and isocyanate [8]. Grafting

of methyl methacrylate (MMA) onto sisal fibers was found to improve the surface adhesion and

dispersion of the fibers in composites with PP matrix, giving rise to enhanced thermal stability and

mechanical properties [9]. Similar findings were reported for PVC based composites reinforced

henequen cellulose fibers grafted with MMA [10]. Rozman et al. [11] showed that compounding MA

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treated EFB (palm empty fruit bunch) with PP matrix in the melt, in the presence of peroxide, resulted in a marked change of mechanical properties due to the grafting reaction of MA double bond with PP chains.

Polymer modification with unsaturated polar molecules (such as anhydrides, epoxides, amines, etc.) is another way that has been explored to enhance interfacial adhesion, especially for polyolefin based composites [3]. Polymer modification appears to be a quick, effective method to provide good interfacial adhesion, in contrast to fiber modification, which generally involves solvent based processes. Maleated PP (PP-g-MA) have been extensively used as compatibilizers in various polyolefin composites with natural fibers [12-14]; glycidyl methacrylate grafted PP (PP-g-GMA) has been employed in composites of PP with hemp fibers [15].

We analyzed the effect of reactive compatibilization processes on the morphological, thermal and mechanical properties of thermoplastic composites, both with crystalline and amorphous matrix, based on isotactic polypropylene (PP), polystyrene (PS), poly(ethylene-vinyl acetate) (EVA) and polyesters, containing hemp, cellulose or oat as natural fillers, respectively. In the case of PS based systems, the properties of composites reinforced with CaCO3 particles were also investigated. In particular, the study was aimed at evaluating the role of various compatibilization methods, processing conditions and compatibilizer structures, as well as the type and concentration of filler, on the interfacial interaction phenomena, the crystallization processes of the polymer matrix and, finally, the degradation behavior of these materials.

2. Experimental Section

2.1. Materials

Isotactic polypropylene (PP) with MFR = 12 g/10 min was purchased from Targor (Italy). Polystyrene (PS) was a (1:1) mixture of crystal type and high impact Polystyrene. Ethylene-vinyl acetate (EVA) with 28% vinyl acetate, trade name Elvax 220W (density = 0.951 g/cm3, MFR(190 °C/2.16 Kg) = 150 g/10 min), was supplied by DuPont (Italy).

Hemp fibers (cannabis sativa) were supplied by Technical University of Poznan (Poland); the fibers were ground in a laboratory miller (Frish-Pulverisette 14, GmbH) up to a length less than 250 m and then treated with caustic soda to remove surface impurities (Hemp-OH). Oat particles (avena sativa) derived from ground oat husks (size < 75 m) were supplied by Chemical Net (Poland). According to the supplier, the particles contained 50 wt.% cellulose, 25 wt.% hemicellulose and 3–5 wt.% lignin. Cellulose fibers (Technocel 75, 165 and 500—Here referred to as Cell, Cell-1 and Cell-5) having an average length of 75, 190 and 340 m, respectively, and cellulose content of 99.6%, were provided by Neuchem (Italy). Maleic anhydride (MA), purity min. 99.8%, was purchased from RPE, Milan, Italy. Glycidyl methacrylate (GMA), purity 97%, was purchased from ALDRICH, Italy.

2.2. Chemical Modification and Compatibilizers

Hemp and cellulose fibers were modified with GMA following the procedure reported by Rozman et al. [7]. The fibers were reacted at 90 °C with glycidyl methacrylate/triethylamine

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(3:7 vol/vol) solution. Hydroquinone was added in order to minimize the effect of free radical reaction at the unsaturated end of GMA molecules. After reaction, the modified fibers were separated by filtration and rinsed with acetone to remove the unreacted reagent and possible GMA homopolymer formed during the reaction. Modification of cellulose with MA (0.5 mol/mol of anhydro glucose unit) was carried out into acetone suspension by refluxing the solvent at 55 °C. The modified cellulose was then cooled to room temperature, separated by filtration, washed with water and acetone, then dried at 50 °C under vacuum.

PP was modified with glycidyl methacrylate (PP-g-GMA, 5 wt.% GMA) according to a melt radical grafting procedure reported elsewhere [16] and employed as compatibilizer for PP/Hemp composites. Commercial samples of a styrene-(ethylene-butene)-styrene three-block copolymer grafted with maleic anhydride (SEBS-g-MA) by Shell (Kraton FG-1901X, MFR= 20–25 g/10 min) containing 1 wt.% MA, and a poly(styrene-co-maleic anhydride) copolymer (PS-co-MA) by Sigma-Aldrich (Mw= 224.000, MFR= 1.7 g/10 min) containing 7 wt.% MA, were used as compatibilizers for PS/Cell samples. Low molecular weight poly(ethylene glycol) (PEG) with Mw = 600

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