JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009 - 108

KNOW-HOW Materials …/… Effect of tackifying binders on matrix properties particles of the binder fully dissolve in the resin in 40 minutes, and no further dissolution occurs from then on right up to the fully cured state. This is not entirely representative of what happens during the production of components. The binder is not in a powder state in the preform. It has been heated to initially bond to the cloth and then again in the lay-up and consolidation process where it is also placed under a vacuum. A section of consolidated preform was systematically delaminated to expose the state of the binder on the fabric surface, which can be seen in Figure 3. a microscopic layer from the surface of the specimen and, if repeatedly passed over a specific area, they can effectively machine away the surface of the specimen. This could be used to help visualize any areas of interest. Specimens were cut from a section of unidirectional composite and the surfaces were polished at 90º to the plane. Images were taken looking end-on to the fibres. Matrix/binder interface 0.5 mm 0.5 mm Time = 0 min Time = 40 min Fig. 2: Hot stage microscopy images of Neoxil binder in Huntsman LY564/HY2954 resin during the standard cure cycle. Fig. 4: Focused ion beam image of composite extracted from component showing undissolved binder. The FIB image in Figure 4 shows an interface in the matrix at the edge of a tow, indicating that this binder does not dissolve completely in the resin during the RTM process. The FIB imaging investigations identified the presence of undissolved Neoxil binder within the matrix of the composite. In order to determine what effect this has upon the matrix properties, bulk matrix samples were manufactured with different concentrations of two different binders: Neoxil 950 KS1 (polyester binder) and Hexcel E01 (epoxy binder, which by contrast to the Neoxil binder dissolves rapidly and completely in the LY564/HY2954 resin system). Determination of matrix properties incorporating polyester and epoxy binders Fig. 3: Neoxil binder on the surface of UD fabric extracted from consolidated preform. The binder appears to have softened and spread onto the surface of the tows, thus exposing a greater surface area to the resin during the RTM process. In principle, this would enable greater dissolution of the binder than in the hot stage microscopy tests. A focused ion beam (FIB) imaging apparatus was employed in an attempt to determine whether the binder had dissolved completely in the matrix. This works in a very similar way to a scanning electron microscope but utilizes ions rather than electrons to scan the surface of the specimen. The ions vaporize In order to gather as much data as possible on the shear modulus and strength of the resin, a torsion test was used on bulk resin specimens with testing continued up to failure. Machining of the specimens was critical. The epoxies were very sensitive to the cutting speed. Tools had to be very sharp and of the correct geometry (non-ferrous tooling geometry worked well). A precise setup of the machine tools was critical to produce dimensionally accurate specimens with a good surface finish. In order to gather good quality shear modulus results from the torsion test, an accurate method was used to measure the twist of the specimen gauge length during the test that would not itself affect the results. The method chosen for this was a laser reflection technique. Lightweight mirrors were held at either end of the gauge length on sprung metal mounts, as shown in Figure 5. 108 jec composites magazine / No47 March-April 2009

JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #47 - March/April 2009