Injection molding of long sisal fiber-reinforced polypropylene: effects of compatibilizer concentration and viscosity on fiber adhesion and thermal degradation
The incorporation of natural fibers into thermoplastic polymer matrices reduces cost and weight, as these fibers are cheaper than synthetic polymers and have lower density. The incorporation of cellulosic natural fibers in thermoplastic composites has increased in the past years. This trend is driven mainly by government regulations that promote the use of renewable and recyclable materials, aimed at reducing the environmental impact of much agricultural fiber waste.
Natural cellulosic fibers can be used to improve some mechanical properties of unfilled plastics [1]. Synthetic fibers usually display higher modulus and strength values, but for massive applications of medium mechanical properties requirements, natural fibers may be a cost-efficient alternative. Another desirable feature is recyclability; thermoplastic composites are expected to be recyclable without much loss in mechanical properties and appearance.
The preparation of composites made out of thermoplastics reinforced with natural fibers is usually a two-stage process. The first stage consists of compounding the composite by blending the fibers and the polymer together, and a twin-screw extruder is used to achieve good fiber impregnation and distribution. The second stage is the molding of the parts, usually by an injection molding process.
Polyolefins–particularly polypropylenes (PP)–are often used as matrices for natural fiber-reinforced composites. They have low prices, low density, processing temperatures that can be kept low to avoid or reduce cellulose and can be recycled.
Sisal fiber (SF) is one of the natural fibers used most in the automotive industry. It is renewable, nonabrasive, and biodegradable. SF shows a quite high specific strength and stiffness, and can be obtained at a low price (0.36 US$/Kg.) [2]. SF has already been used as an effective reinforcing material in polymeric resin matrices to make useful structural composite materials [2].
While being a very interesting pair with many potential applications, SF and PP share important problems derived from two facts: the weak interfacial bonding between the polar fiber surface and the hydrophobic matrix, and the high viscosity of molten PP [3]. The polymer adhesion to the fiber surface controls the stress transfer between the matrix and the reinforcing fibers. Fiber impregnation depends mainly on processing variables, and the polymer-fiber adhesion depends on chemical identities [4]. For these cases of polar fibers and hydrophobic matrices, poor mechanical properties can be linked to weak interfacial bonding. Also, the PP high viscosity is assumed to be responsible for poor penetration of the molten PP into the fiber bundles, thus diminishing the SF/PP contact area. The viscosity of regular injection-grade polypropylenes at the temperature range at which the natural cellulosic fibers can be processed is high enough to reduce fiber bundle impregnation. For the injection molding process, high shear rates at the mold filling stage–combined with a high PP viscosity–cause bulk temperature rises that can easily degrade the cellulosic fibers by thermal depolymerization of hemicellulose and the glycoside linkages of cellulose [5].
These problems of interfacial bonding and high melt viscosity need to be solved, for a cost-efficient use of SF reinforcing. For improved SF/PP interface bonding, coupling agents can be used. Maleic anhydride grafted polypropylene (MA-g-PP) is an excellent choice, because the anhydride functionality of MA-g-PP reacts with cellulosic fiber hydroxyl groups and esterification gives stronger links between the fiber surface and the PP matrix [6]. Reducing the PP molecular weight lowers the viscosity of the molten polymer, reduces the shear stress between individual fibers and between fibers and PP melt [7], and reduces the viscous heat generation; the composite may then be processed by injection molding at lower temperatures.
Other aspects of the problem must also be considered. Commercial grades of MA-g-PP with high level of functionality (1.0 wt% MA) are very expensive (about 5 US$/Kg), and therefore these must be used in small quantities to keep the SF/PP composite economically competitive.
Some details of the preparation of composites made out of thermoplastics reinforced with natural fibers must also be analyzed. The first stage usually consists of compounding the composite by blending the fibers, the polymer matrix, and a coupling agent together in a twin-screw extruder. With this practice, the maleic anhydride bonding efficiency is reduced by this dilution of the MA-g-PP into the PP matrix, because many anhydride functional groups may not reach the hydroxyl groups at the SF surface. The second stage is the molding of the parts, by an injection molding process. For good mechanical properties, a high molecular weigh PP is used for the continuous thermoplastic matrix. Therefore, the injection molding process can generate bulk temperature and shear stress higher than allowed for low fiber thermal degradation and SF attrition. This is a very important feature of this process, since most of the fiber breakage found for these processes occurs inside the twin-screw extruder [8]. The combined operations of compounding and injection molding thermoplastics/natural fiber composites generally reduce the initial fiber length because of the severe temperature condition and shear stress imposed. The efficiency of the composite depends on the amount of stress transferred from the matrix to the fibers. This can be maximized by improving the interaction and adhesion between the two phases and also by maximizing the length of the fibers retained in the final composite [9].