Solid freeform fabrication of epoxidized soybean oil/epoxy composite with bis or polyalkyleneamine curing agents

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Abstract

Extrusion freeform fabrication has been used to make bars of fiber-reinforced epoxidized soybean oil (ESO)/epoxy resin. Freeform fabrication methods build materials by the repetitive addition of thin layers. The mixture of epoxidized soybean oil (ESO) and epoxy resin are modified with a gelling agent to solidify the materials until curing occurs. The high strength and stiffness composites are formed through fiber reinforcement. Glass, carbon and mineral fibers are used in the formulations. It is shown that the fiber orientation follows the direction of motion of the write head that deposits the resins and has a large influence on the properties of the composite. In addition, the effects of curing agents, curing temperature, epoxy/ESO ratio, and fiber loading on mechanical properties of composites are studied and reported.

Introduction

Fiber-reinforced composites offer great potential for use in aircraft and automotive primary structure. They are generally manufactured by using fibers as reinforcement and polymeric resin as a matrix. During the last few years, there has been a growing interest in the use of polymers obtained from renewable resources because advantages of these polymers include their biodegradable properties and, in many cases, lower cost [1]. The importance of natural products for industrial applications also becomes very clear with increasing social emphasis on issues of the environment, waste disposal, and the depletion of non-renewable resources. United States agriculture produces over 20 billion pounds of soybean oil annually, only 590 million pounds used in industrial application, and frequently carry-over exceeds one billion pounds. The major use of soybean oil is directed to food products such as salad and cooking oil, shortening, and margarine. Less than 3% of soybean oil is utilized in non-food applications since a number of former vegetable oil markets were lost to petroleum products. Development of economically feasible new industrial products from soybean oil or commercial processes is highly desirable. In our previous paper [2], we reported the preparation of epoxidized soybean oil (ESO) based composites. These composites reinforced with glass, short carbon, Franklin Fiber® H-45, and Fillex® 17-AF1 fibers demonstrated thermophysical and mechanical properties comparable to petroleum-based soft rubbery polymeric materials. In order to obtain a variety of viable polymeric materials ranging from elastomers to rigid plastics meeting a wide variety of market demand, a combination of ESO and epoxy resin is considered.

Epoxy resins are widely used as polymeric matrix in composites. However, epoxy resins are similar to other engineering resins in that they are either brittle, notch-sensitive, or both. For load-bearing purposes, this means that the product may be subject to catastrophic failure. A major effort over the years focused on improving the toughness of epoxy structural systems. As a result, epoxy resins have been used for structural applications such as adhesives, encapsulation of electronic devices, and composites such as electrical laminates, aerospace parts and automotive parts.

Sue et al. [3] have reviewed rubber-toughening of epoxy resins. This method has been most effective and can improve toughness substantially. Qureshi et al. [4] reported that the use of 25% epoxidized crambe oil (ECO) as reactive diluents in bisphenol A and cycloaliphatic epoxy compounds gave improvement in resistance to fatigue crack propagation without significant sacrifice in tensile or impact strength and Young’s modulus. Raghavachar et al. [5] recently reported rubber-toughening epoxy thermosets with epoxidized crambe oil. They reported the phase separation in thermoset matrices of diglycidyl ether bisphenol (DGEBA)-4,4′-diaminodiphenylmethane (DDM)-ECO and improvement in toughness of the epoxy matrices. Fracture toughness values of the epoxy thermosets were increased approximately 100% by both 5% and 10% epoxidized crambe oil. It is clear that a combination of epoxidized vegetable oil, epoxy resin and curing agents can be formulated to meet a wide variety of market demands. Soy-based polymers will support global sustainability and provide an alternative to synthetic polymers for many manufacturing applications [6].

Solid freeform fabrication (SFF) is a method of making shapes without molds. It is best known in its stereolithography forms as a method of rapid prototyping. In stereolithography a laser photopolymerizes successive thin layers of monomer to build up a solid object. Extrusion solid freeform fabrication was developed by the University of Arizona in collaboration with Advanced Ceramic Research (Tucson, AZ) [7]. It functions essentially as a three-dimensional (3D) pen plotter. A fine stream of liquid resin or slurry is pumped from a syringe as it moves over the surface of a hot-plate to trace out a layer of materials. The syringe then moves up one step and writes the next lager as the first continues to cure (Fig. 1). The shape to be produced may be derived from a 3D CAD program or from standard drawing packages. This method has the potential to produce new materials and complex composites that could not be made in any other way.

In this work, we report the preparation of ESO/epoxy based composites by extrusion solid freeform fabrication method. High strength and stiffness parts can be obtained through fiber reinforcement. These high performance composites will be tested in agriculture equipment, the automotive industry, civil engineering, marine infrastructure, rail infrastructure, and the construction industry. Recent advances in genetic engineering, natural fiber development, and composite science offer significant opportunities for improved materials from renewable resources with enhanced support for global sustainability.

Section snippets

Materials

The resin used as a co-matrix is EPON® 828, provided by the Shell Chemical Company (Houston, Texas). EPON® 828 is a bisphenol A/epichlorohydrin based epoxy resin, which is the most widely used epoxy. ESO (7.0% oxirane oxygen) was purchased from Alf Atochem Inc. (Philadelphia, PA). Calcium sulfate microfiber, Franklin Fiber® H-45 used in the experiments was provided by the United States Gypsum Company, (Chicago, IL). Wollastonite mineral fiber, Fillex® 17-AF1 fiber is surface-modified

Composite morphology

Scanning electron microscopy (SEM) was performed to characterize the morphology of soybean oil-based filled composite materials. In our previous paper [2], we reported SEM micrographs of four different kinds of fibers: (1) the commercial milled glass fibers with a nominal length of 1/32 in. and fiber diameter of 10 μm, the aspect ratio distribution reported according to Peng [9] shows two main peaks at values of 3 and 7; (2) calcium sulfate microfiber, Franklin Fiber® H-45 with average length of

Conclusions

The application of solid freeform fabrication to reinforced composites has been explored. Fiber reinforced epoxidized soybean oil/epoxy composites can be formed with high strength and stiffness. Different fiber types, E-glass fiber, carbon fiber, and mineral fibers are used in reinforced composites. It was found that glass fiber and carbon fiber show better reinforcing effects than mineral fibers. By writing a series of test bars with write axes at different angles to the long axis, modulus can

Acknowledgement

The authors gratefully acknowledge Dr. Arthur Thompson for help in SEM, and Ms. Jiong Peng for help in three point bending test.

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