Manufacturing Techniques for Polymer Composites Used in Construction
The two parts of this article concentrate upon the manufacturing techniques for civil engineering fibrereinforced thermosetting and thermoplastic polymer composites, respectively.
Manufacture of fibrereinforced thermosetting composites
There are three basic techniques used to manufacture advanced polymer composites for the civil engineering industry; each technique will have an influence on the mechanical properties of the final component. The techniques and their sub-divisions are:
- wet lay-up (either factory or site fabricated) and cold cured
- pressure bag methods, fabricated and cold cured in a factory or on site
- the vacuum assisted resin-transfer procedures (RTM, site fabricated and cold cured).
- resin injection (cold cured)
- low-temperature mould factory made impregnated fibre (prepreg) site fabricated and cured under pressure and elevated temperature.
- pultrusion (hot cured)
- filament winding (cold cured)
- the cold melt prepreg factory manufactured, cured under a vacuum assisted pressure of 1 bar and an elevated temperature
- injection moulding (cold cured).
The manual processes
The manual processes currently used in construction are variations of the general wet lay-up method. The commercial methods of manufacture of fibre– polymer composites by this process are:
- the REPLARK™ method
- the Dupont method
- the Tonen Forca method.
These techniques are basically the same with minor variations. The wet lay-up process consists of in-situ wetting of dry fibres in the form of sheets or fabrics impregnated in-situ with a polymer. These are wrapped around the structural member during rehabilitation or placed on a mould of the desired geometric shape.
Their size will depend upon the size of the member or mould; the fibre reinforcements are generally of widths varying between 150 and 1500 mm. The REPLARK process (REPLARK is a trade name used by Sumotomo Corporation, Europe PLC) is a commercially available method of rehabilitating (or retrofitting) a structural member to strengthen structures in flexure and shear by bonding the material to their tensile and/or shear faces; in these cases the surface of the structure forms the mould for the composite.
Furthermore, planar and non-planar composites can be manufactured independently and used as structural units. Pressure- and vacuum-bag moulding are similar lay-up systems, but pressure or vacuum is applied to the mould through a rubber membrane for compaction before curing commences (the voids in the composite material are considerably reduced, thus providing a glass fibre:polymer weight ratio of up to 55% for vacuum-bag mouldings and 65% for pressure-bag mouldings).
To protect fibres from exposure to the atmosphere and especially to moisture penetration of the interface of the fibre and matrix, a resin-rich coat, known as a gel coat, is sometimes applied to the surface of the composite. The thickness of the gel coat is generally about 0.35 mm. Sometimes a surface tissue mat is used to reinforce the gel coat.
A second method for rehabilitating/retrofitting procedures is known as the Dupont method, which uses Kevlar fibres; it is marketed as a repair system for concrete structures.
In addition to manufacturing structural components made from polymer composites by the wet lay-up or spray-up techniques, commercially available procedures to rehabilitate composite materials to existing structural members, to improve their tensile and shear strengths, do exist.
The autoclave is a modification of the pressurebag method; pressures of up to 6 bar are developed within the autoclave and the system produces a high-quality composite with a fibre:matrix weight ratio of up to 70%. The cost of production also increases. These methods have been described in Hollaway (1993) and Hollaway and Head (2001).
The semi-automated process for the rehabilitation of a structural material
The semi-automated processes used currently are:
- the resin infusion under flexible tooling (RIFT) process
- the resin transfer moulding process (RTM)
- the low-temperature factory-made pre-impregnated fibre (prepreg), cured under pressure and elevated temperature.
Resin-infusion moulding: The semi-automated resin infusion under flexible tooling (RIFT) process has been developed by DML, Devonport, Plymouth, to allow quality composites to be formed. In this process dry fibres are preformed in a mould in the fabrication shop and the required materials are attached to the preform before packaging. The preform is taken to site and is attached to the structure; a resin supply is then channelled to the prepreg. The prepreg and resin supply is then enveloped in a vacuum-bagging system. As the resin flows into the dry fibre preform it develops both the composite material and the adhesive bond between the carbon fiber-reinforced polymer (CFRP) and the structure.
The process provides composites with high fibre volume fractions on the order of 55%; these have high strength and stiffness values. XXsys Technologies, Inc., San Diego, California, has developed a wrapping system for seismic retrofitting to columns. The technology associated with the technique is based upon filament winding of prepreg carbon fibre tows around the structural unit, thus forming a carbon fibre jacket; currently, structural units to be upgraded would be columns. The polymer is then cured by a controlled temperature oven and can, if desired, be coated to match the existing structure.
Resin-transfer moulding is a low-pressure, closed mould semi-mechanised process. In the RTM process, several layers of dry continuous strand mat, woven roving or cloth are placed in the bottom half of a two-part closed mould and a low-viscosity catalysed resin is injected under pressure into the mould cavity, and cured. Flat reinforcing layers, such as a continuous strand mat, or a ‘preform’ that has already been shaped to the desired product, can be used as the starting material in this process.
The potential advantages of RTM are the rapid manufacture of large, complex, high-performance structures with good surface finish on both sides, design flexibility and the capability of integrating a large number of components into one part. This method can be employed to form large components for all composite bridge units but it is not often used.
The low-temperature mould factory-made preimpregnated fibre (prepreg) is cured either in the factory, for the production of pre-cast plates, or on site if the prepreg composite is to be fabricated onto a structural member. In the latter case a compatible film adhesive is used and the film adhesive and the prepreg components are cured in one operation under an elevated temperature of 65°C applied for 16 hours or 80°C applied for 4 hours; a vacuumassisted pressure of 1 bar is applied for simultaneous compaction of the composite and the film adhesive.
This method is new but it is estimated that it will be used increasingly for rehabilitating degraded structural members (see Chapter 43, section 43.5). In the UK the manufacturing specialist in the production of hot-melt factory made pre-impregnated fibre for the construction industry is ACG, of Derbyshire.
The automated processes that are available to the construction industry are:
- filament winding
- the pultrusion technique.
The filament winding technique is a highly mechanised and sophisticated technique for the manufacture of pressure vessels, pipes and rocket casings when exceptionally high strengths are required. In the construction industry filament winding has been used to form high-pressure pipes and pressure vessels. Sewerage pipes have also been manufactured by filament winding.
Continuous reinforcement, usually rovings, is fed through a traversing bath of activated resin and is then wound on to a rotating mandrel. If resin preimpregnated reinforcement is used, it is passed over a hot roller until tacky and is then wound on to the rotating mandrel. Figure 1 illustrates the process and it is evident that the angle of the helix is determined by the relative speeds of the traversing bath and the mandrel. After completion of the initial polymerisation, the composite is removed from the mandrel and cured, for which process the composite unit is placed in an enclosure at 60°C for 8 hours.
The pultrusion technique and the pull-winding technique are used within a closed-mould system, utilising heat to produce high-quality units. Owing to the high capital equipment outlay, particularly for the manufacture of the metal moulds and the initial set-up of runs, it is essential that large production runs are performed.
Only a small skilled workforce is required owing to the mechanisation of the system. The technique consists of impregnating continuous strands of a reinforcing material with a polymer and drawing them through a die, as shown in Fig. 2.
Thermosetting polymers are used in this process, although research is currently being undertaken to pultrude thermoplastic materials. Curing of the thermosetting composite component is undertaken when the die is heated to about 135°C. A glass content of between 60 and 80% by weight can be achieved.
Composites manufactured by this method tend to be reinforced mainly in the longitudinal direction with only a relatively small percentage of fibres in the transverse direction. A technique was developed (Shaw-Stewart, 1988) to ‘wind’ fibres in the transverse direction simultaneously with the pultrusion operation. The process is known as pull-winding and gives the designer greater flexibility in the production of composites, particularly those of circular cross-section. The pultrusion technique is the process used extensively in the civil engineering industry and is an important technique to form flexural/shear structural units and also to manufacture high-pressure water and sewerage system pipes (using the pull-winding procedure).
The finished pultrusion sections are generally straight and dies can be manufactured to give most geometrical shapes; the most common of these are I, L, Tee, and circular sections. Curved-in-plan pultrusion sections can also be manufactured.
Manufacture of fibre-reinforced thermoplastic composites
Reinforced thermoplastic composites can be manufactured by most of the thermoplastic processing techniques such as extrusion, blow-moulding and thermoforming of short-fibre reinforced thermoplastics. However, the most important technique for civil engineering industry use is injection moulding.
It is a similar technique to the manufacture of un-reinforced thermoplastics but the melt viscosity is higher in the reinforced polymer process, consequently injection pressures are higher. With all the techniques, production difficulties can occur because the reinforced composite is stiffer than the unreinforced one. The cycle time is less but the increased stiffness can affect ejection from the mould, so the mould design has to be modified from that of the un-reinforced polymer mould.
One of the problems of thermoplastic composites is that they use short fibres (typically 0.2–0.4 mm long) and consequently their full strength is not developed. Continuous fibre tapes and mats in the form of prepregs can help to overcome this problem. The best known examples of these systems are the aromatic polymer composites (APC) and the glassmat-reinforced thermoplastic composites (GMT).
The systems use unidirectional carbon fibre in a matrix of polyethersulphone (PES) or polyetheretherketone (PEEK). The material for APCs comes in a prepreg form of unidirectional or 0°/90° fibre, and for GMT in a tape prepreg form. The composite is manufactured by the film-stacking process, with the prepregs arranged in the desired directions.
Film-stacking products can be made from prepreg reinforcement, and one system uses a polyethersulphone polymer content of about 15% by weight. The final volume fractions of fibre and resin are obtained by adding matrix in the form of a polymer film.
The film-stacking process, therefore, consists of alternating layers of fibre impregnated with insufficient matrix, with polymer films of complementary mass to bring the overall laminate to the correct fibre volume ratio. The required stacked sequence is rolled around a central mandrel (PTFE material) and is placed into one part of a split steel mould; the two half moulds are joined and placed in an oven for a specific time.
The difference in thermal expansion of the PTFE and steel causes pressure to be applied to the curing polymer. This technique is used mainly for high-technology composites in the aerospace and space industries.