JEC COMPOSITES MAGAZINE - Issue #52 - October 2009 - 64

KNOW-HOW LFI Development of Class-A-surface polyurethane LFI composites Recent advancements in the polyurethane Long Fibre Composite Injection (LFI) process have resulted in a dramatic increase in its commercial interest for producing a wide range of products, including PWCs, entry door skins, and truck-body and spa panels. One of the main reasons for this success is a development in the LFI process’s polyurethane chemistry that allows long gel times on an open hot mould, while maintaining a relatively short demould time. This paper discusses the chemistry and processing of LFI material, with emphasis on the structure/property relationship, density reduction through dissolved CO2, and the development of high-surface-quality (Class A) composites. USAMA YOUNES, PRINCIPAL SCIENTIST, BAYER MATERIALSCIENCE ne of the major challenges to PU Long Fibre Injection technology has been the formation of defect-free surfaces, which can lead to Class A surfaces. The nature of the LFI process makes it difficult to produce such surfaces, because the process itself deposits a mixture of glass, PU and air onto the mould surface, which results in entrapment of air bubbles on or near the surface of the part. It also deposits glass fibres onto the surface, resulting in glass read-through. One way to overcome the surface defects due to both bubbles and glass is to deposit a PU barrier coat between the in-mould paint and the LFI composite. While this eliminates surface defects and the glass read-through, it creates a new problem with increased orange peel. This is attributed to two factors: barrier coat deformation due to the heat generated from the LFI reaction and the different coefficients of linear expansion of the various layers. O then accurately and consistently positions the yet unreacted components into the open cavity of a heated mould. After the pour is complete, the mould is closed for a specified time. Later, the mould is opened and the part is removed. One of the early challenges to the advancement of LFI technology was its inability to make large parts, because current mixing equipment has a maximum throughput of 0.45 to 0.68 kg/sec. of polyurethane. Therefore, it was up to the PU chemical design to create a longer gel time. Recent developments in polyurethane chemistry for LFI have extended the gel time on the heated mould to about 120 seconds, as discussed below, allowing for the production of glass reinforced parts in excess of 90 kg. Tab. 1: Gel time of developed systems and their expected demold time. Gel time, sec. at 70°C 30 45 70 120 Demould, min. 3 4 6 20 Demould, min. with barrier coat 1.5 3 5.5 7 LFI technology LFI is a fast-growing composite moulding technology. It involves dispensing chopped fibres and polyurethane liquid components simultaneously into an open heated mould. The polyurethane components are metered through a urethane metering machine and delivered into a specialized mixing head which chops the glass fibre to a specified and variable length (13 to 100 mm) and coats it with the mixed urethane liquid components. A robot Gel time Controlling the chemistry makes it possible to extend the gel time on a hot mould while maintaining a relatively short demould time. Table 1 shows various chemical systems developed and compares the gel time at 70°C versus its demould time. It is important to note here that the gel time determines material flow within the mould, as well as the quality of the part jec composites magazine / No53 November - December 2009

JEC COMPOSITES MAGAZINE - Issue #52 - October 2009

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