Processing of Advanced Thermoplastic Composites 99 the combining process,impregnation may be accomplished during this step of combining fibres and matrix,or later during "in-situ"fabrication of laminates and parts.Based on APC-2 samples fabricated from pre-impregnated products and from products relying on a post impregnation taking place during laminate consolidation.Cogswell [197]showed that pre- impregnated strategies available today lead to better mechanical performance (see Table 29). He attributed this difference to the "fibre attrition during the post moulding impregnation stage where the forces necessary to squeeze the resin into spaces between the fbres also force the fibres together;by contrast in preimpregnated products each fibre is lubricated with a protective coating of viscous polymer".Techniques for combining fibres and matrix are described below. 5.4.1 Hot Melt Coating Hot melt coating [1.55,195,198]is probably the process that is the most commonly employed to combine fibres and matrix.Figure 41 shows a melt extrusion process used by Chung [16]but there are many possible variations in this process (1981.Initially.fibre tows are unwound from a spool or a creel of spools without twisting and they may go through a comb to achieve a better collimation.Fibres are then spread by a roller or air jet to expose as many fibres as possible to the polymer and reduce the gaps in the prepreg material.The fibres are fed into a die where the molten polymer is either added or furnished as a resin coating on release paper which is later removed.At this stage.pressure is applied on the melt in order to coat the individual fibres and not only the fibre bundles.Considerable pressure may be needed to acheive a total impregnation of the fibres (195].At the die exit,the hot tape is cooled and rolled up. No solvents are needed in this process.In the case of semi-crystalline polymers such as PEEK and PPS for which there are no known solvents in which to prepreg.hot melt coating is currently the impregnation method of choice [1.8].The wet out is generally excellent with a low void content but the prepreg obtained is stilf.boardy and tackless.The hot melt coating method is not appropriate for thermoplastic polymers possessing a very high melt viscosity. There is some danger of thermally degrading the polymer when heating it to lower its viscosity. 5.4.2 Solution Processing Solution processing [1.55.77.1951 is very well established for the thermoset polymers. The technique consists of dissolving the resin in a suitable solvent and wetting the fibres with the solution.As it reduces the viscosity of the thermoplastic polymers,full impregnation of fibres becomes much easier.The complete devolatilization of the prepreg will result in a tackless and boardy thermoplastic tape or fabric.If some of the solvent is left,a certain degree of tack and drape can be obtained.The drawbacks associated with this technique are two fold [8,55,77].First,if the prepreg is not devolatlized,the prepreg must be handled by conventionalProcessing of Advanced Thermoplastic Composites 99 the combining process, Impregnation may be accomplished during this step of combining fibres and matrix, or later during “in-situ” fabrication of laminates and parts. Based on APC-2 samples fabricated from pre-impregnated products and from products relying on a post impregnation taking place during laminate consolidation, Cogswell [ 1971 showed that pre￾impregnated strategies available today lead to belter mechanical performance (see Table 29). He attributed this difference to the “Mbre attrition during the post moulding impregnation stage where the forces necessary to squeeze the resin into spaces between the fibres also force the fibres together; by contrast in preimpregnated products each fibre is lubricated with a protective coating of viscous polymer”. Techniques for combining fibres and matrix are described below. 5.4.1 Hot Melt Coating Hot melt coating [ 1, 55, 195. 1981 is probably the process that is the most commonly employed to combine fibres and matrix. Figure 4 1 shows a melt extrusion process used by Chung [16] but there are many possible variations in this process [198]. Initially. fibre tows are unwound from a spool or a creel of spools without twisting and they may go through a comb to achieve a better collimation. Fibres are then spread by a roller or air jet to expose as many fibres as possible to the polymer and reduce the gaps in the prepreg material. The fibres are fed into a die where the molten polymer is eilher added or furnished as a resin coating on release paper which is later removed. At this stage, pressure is applied on the melt in order to coat the individual fibres and not only the fibre bundles. Considerable pressure may be needed to acheive a total impregnation of the fibres [ 1951. At the die exit, the hot tape is cooled and rolled up. No solvents are needed in lhis process. In the case of semi-crystalline polymers such as PEEK and PPS for which there are no known solvents in which to prepreg. hot melt coating is currently the impregnation method of choice [ 1,8]. The wet out is generally excellent with a low void content but the prepreg obtained is stiff, boardy and tackless. The hot melt coating method is not appropriate for thermoplastic polymers possessing a very high melt viscosity. There is some danger of thermally degrading the polymer when heating it to lower its viscosity. 5.4.2 Solution Processing Solution processing [ 1.55.77.1951 is very well established for the thermoset polymers. The technique consists of dissolving the resin in a suitable solvent and wetting the fibres with the solution. As it reduces the viscosity of the thermoplastic polymers, full impregnation of fibres becomes much easier. The complete devolatilization of the prepreg will result in a tackless and boardy thermoplastic tape or fabric. If some of the solvent is left, a certain degree of tack and drape can be obtained. The drawbacks associated with this technique are two fold I8.55.771. First, if the prepreg is not devolatilized, the prepreg must be handled by conventional