JEC COMPOSITES MAGAZINE - Issue #52 - October 2009 - 65

surface. If the material starts to gel prior to mould closing, incomplete mould fill and poor surface will result. Therefore, proper selection of gel time to part volume is critical. However, lower-density LFI systems present a special challenge, namely the addition of water as an environmentally friendly blowing agent, which also decreases the gel time of the systems. It was important to develop a blowing agent which would not impact the gel time of polyurethanes. Dissolved carbon dioxide was used. Careful control of the amount of carbon dioxide in either the polyol or the isocyanate is essential, as one must stay below the solubility limits of CO2 in the components. If not, large bubbles will form in the parts. Figure 1 shows the froth density of the polyol at or below its solubility limits. Using this technology, one can vary the density of the composite at will, just by adjusting the amount of CO2 dissolved in the polyol. By doing so, none of the other process parameters need to be changed. Table 2 shows the properties of LFI composites made with 30 wt% glass fibres at various densities. Barrier coat technology An attractive feature of polyurethane LFI technology is the elimination of the need to post-paint parts, because the technology allows in-mould painting of parts as is commonly done with non-smooth Class A surfaces, such a parts with a grained surface. However, the demand for Class A surfaces has grown over the years. Until recently, LFI was capable of delivering such a high-quality surface only through the use of a painted thermoplastic film. Bayer MaterialScience’s development work concentrated on a less costly alternative to painted films. It involved spraying a polyurethane thermosetting film as a barrier coat between the in-mould paint and the LFI. The PU thermosetting film protects the thin in-mould paint from glass read-through caused by the LFI process. The development of a proper barrier coat material is critical, in that it can cause surface defects due to deformation by the LFI reaction’s heat and the difference in coefficient of linear expansions between the barrier coat and the LFI composite. This new class of barrier coats was developed to offer a high glass transition temperature and a low roughness value as determined by atomic force microscopy (AFM), as described below. Effect of crosslink density on Tg There are several ways to increase the glass transition temperature of polyurethanes. A typical PU will have a low soft segment transition, which is responsible for the elastomeric properties of polyurethanes – such as elongation and impact – and a hard segment glass transition which is responsible for properties such as stiffness and heat distortion. The hard segment transition can be increased by increasing the crosslink density of the system, as shown in Figure 3. 1.2 0.64 0.45 3.7 8.456 11,066 382,836 Fig. 1: Polyol froth density as a function of CO2 concentration Tab. 2: Properties of LFI composite at various densities achieved with dissolved CO2 [CO2] in polyol, g/l Composite density, g/cc PU density, g/cc Inst. dart impact, ft-lbs Un. Izod impact, ft-lbs/in Flexural strength, psi Flexural modulus, psi 0 1.33 0.93 8.66 19.278 38,390 0.6 1.12 0.78 7.18 14.678 35,773 0.8 0.96 0.67 7.2 13.876 33,354 1 0.8 0.56 5.6 10.58 15,196 576,637 1,302,088 1,207,804 1,044,759 Additionally, Figure 2 shows unnotched Izod impact properties achieved with the same system but with different densities, by simply controlling the amount of CO2 dissolved in the polyol. Fig. 3: Tg vs. Crosslink density. Atomic force microscopy The ability of the barrier layer to provide a good Class A finish was determined using AFM, which measures the deflection of a micro-scale probe brought about by forces on the surface of the Fig. 2: Unnotched Izod of LFI at various densities achieved with dissolved CO2. No53 November - December 2009 / jec composites magazine

JEC COMPOSITES MAGAZINE - Issue #52 - October 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #52 - October 2009