Applied Research Centre for Polymers/Centre de recherche appliquee sur les polymeres

The Applied Research Centre for Polymers (Centre de recherche appliquee sur les polymeres, CRASP), established at Ecole Polytechnique in the summer of 1988, groups the research activities of 11 professors, four research associates and six research engineers from the chemical engineering, mechanical engineering and applied mathematics departments. This is the most important research centre at Ecole Polytechnique with over 45 graduate students preparing a master's or PhD thesis in one of the following areas.

Structure analysis of plastic composites: - fracture mechanisms in composites; - multilaminate analysis; - damage and aging phenomena; - fatigue and creep in composites; - performance characteristics of parts.

Characterization of polymer systems: - polymer structure and morphology; - rheology of polymers, constitutive

equations; - surface properties and permeability; - mechanical and viscoelastic

properties; - component interactions in blends

and composites; - friction and abrasion responses of

polymers.

Materials development: - thermoplastic blends and

composites, engineering plastics

and liquid-crystalline polymers; - composite materials with cellulose

fibre reinforcement; - emulsion polymerisation of high-molecular

weight polymers; - additives and compounds for high

frequency heating.

Polymer processing: - modelling of single-screw extruders; - modelling of extrusion dies,

blown-film; - profile extrusion; - melt fracture of polyolefines; - injection moulding of composites; - new simulation methods.

Gears performance and tribology: - development of software for parts

analysis; - contact temperature; - development of new geometries; - plastic-plastic and plastic-metal

wear; - real surface contact.

CRASP is a major Canadian research facility on polymers. It has equipment for rheological and thermal characterization and for polymer processing. The following is a summary of some of our most recent achievements.

High Frequency Heating

of Plastics

Some thermoplastics like Teflon and ultra-high-molecular-weight polyethylene (UHMWPE) cannot be processed by regular screw plastification techniques - injection moulding and extrusion. Instead they are processed by compression moulding in a hot mould or by ram extrusion in an electrically-heated extruder. As thermoplastics are good thermal insulators these two processes, based on conductive heating, are extremely slow. To speed up the process, we are using high-frequency heating. Research is being done on UHMWPE and other hard-to-process polymers such as polyimide (PI) and polyphenylenesulfide (PPS). Since the UHMW polyethylene is transparent to high frequencies, a sensitizer must be added to the polymer. Major project components are: sensitizers screening and characterization, determination of extrusion conditions for sensitizers of interest, development of mixing and moulding processes, and optimization of finished product properties.

Analysis of Blown-Film

A series of projects has been undertaken to study the film blowing process. The first project analyzes the flow of polymer melt through the extruder and the helical diea and a computer-aided design software has been developed. The second project is concerned with the optimization of operating conditions of the system for different resins. Data on the temperature of the melt outside the die, the shape of the bubble, the pressure inside the bubble, the cooling rate are analyzed and compared to model predictions. Recent developments include the fabrication of a barrier film using coextrusion techniques and blends.

Modelling of Resin Transfer

Moulding (RTM)

In industry today, glass-fibre reinforced polyester parts such as boat hulls, truck cabins, bathtubs, etc., are moulded by contact moulding. Unfortunately with open-mould processing, styrene (used as solvent with polyester resin), evaporates causing an environmental problem. Resin transfer moulding (RTM) is the best alternative for contact moulding. In RTM, the woven and non-woven continuous fibre reinforcements are first placed into the mould. The mould is then closed and the liquid resin injected. This closed mould processing avoids styrene evaporation. However, the lack of knowledge and design tools for parts and moulds have restrained considerably the use of the RTM process. The purpose of this project is to develop design tools to predict the pressure distribution in the cavity as well as the position of the flow front during the filling cycle. A computer program, based on finite elements, is being developed for that purpose. Modelling of the process is complicated by the fact that several types of reinforcement can be used. Cores could also be added for sandwich construction, complicating even further the filling and the flow pattern. The results of our research can also be applied to other processes such as structural-reaction injection moulding (SRIM) or vacuum bag and autoclave moulding for prepregs.

Extrusion Die Simulation

Recent software developed at CRASP includes two computer programs to analyze the flow of molten polymer in coat hanger and wire coating dies. The models are user-friendly and can be used on personal computers. In the analyses of the flow in coat hanger dies, most numerical methods deal with a 2-D geometry, but only a few of them have considered non-isothermal flows. A new model was developed using a modified FAN method (Flow Analysis Network introduced by Tadmor) for 2-D non-isothermal flow. The FAN technique is combined with a finite-difference scheme for the calculation of temperature. To simulate the wire coating, the flow in the axisymetrical section of the die is analyzed with the lubrication approximations combined with a non-isothermal and non-Newtonian fluid behaviour. The system of equations is solved using a broken-section technique.

Interaction in Polymer

Systems

Most polymers are used in complex combinations with additives including fillers, pigments, plasticizers, stabilisers, etc. There is much qualitative evidence to suggest that these additives do not act independently of each other, but that they interact and therefore create combinations of effects which are not easily predicted from the individual properties of the materials.

Analytic techniques, based on principles of gas chromatography, have been developed to provide quantitative characterization of interactions among the components of complex polymer systems. Several projects are underway to show the applicability of the new capability. In one program, it has been shown that the effectiveness of thermal stabilizers in polyolefins varies with the surface properties of pigments and fillers added to the system. Results were rationalized by concepts of acid/base interaction using chromatographic data. The effectiveness of fluoropolymers as suppressants of shark-skin and slip-stick failure in the processing of polyolefins and other polymers has been established through the principles of component interaction. The adhesion of polyurethanes to substrate uses in the formulation of high-performance composites also is following the dictates of specific interactions at adhesive/substrate interfaces. The stability of pigment dispersions in automotive polyester vehicles has been related to the adsorption of the polymer on the pigment surface; in turn shown to be a strong function of acid/base forces acting at the interface.

One unusual feature of the centre CRASP is its multidisciplinary staff composition: chemical and mechanical engineers, chemists, physicists and one applied mathematician. The centre can count on the experience of old-timers (Pierre Bataille, MCIC, Pierre Carreau, FCIC, B. Fisa, R. Gauvin, M. Grmela, H.P. Schreiber, FCIC, T. Vu-Khanh, H. Yelle) and on the enthusiasm of younger staff (R. Boukhili, Basil Favis, MCIC, A. Fortin, Pierre Lafleur, MCIC, T.M. Malik, Q.X. Nguyen, B. Sanchagrin). The centre is presently directed by Carreau and Gauvin is the co-director.

PHOTO : CRASP has a 45mm Killion extrusion and blown-film unit among the equipment available for use.