What is Prepreg?
Composite components now account for up to 85% of today’s Formula 1 cars, but only roughly 20% of the weight. An achievement only made possible by continuously exploiting and developing the capabilities of carbon fiber to manufacture intricate parts. These are built by laying up individual plies of carbon fiber prepreg on top of one another at different angles to ensure mechanical performance in all directions. This process is often discussed, however, what is frequently forgotten is the manufacturing process required to make these carbon fiber prepregs in the first place.
Advanced composite materials, such as prepregs, have been utilized by the Formula 1 and motorsport markets for almost 40 years. Prepreg is essentially a material format using carbon, glass or aramid fibers and fabrics that have been impregnated with a polymer matrix. Functionally, fibers give the composite strength and stiffness along their fiber direction. Careful selection of fabric prepregs allow component stresses to be considered, and directed along critical load paths. The combination of woven fabrics usually oriented at 0° and 90° and unidirectional formats allow optimal performance to be designed whether quasi-isotropic or anisotropic properties are needed. Typical applications of unidirectional materials include suspension components, whereas woven reinforcements are mostly used for bodywork.
The primary role of the thermoset resin matrix is to act as an adhesive, bonding and fixing the fibers mass together. However, resins can also be optimized with fillers and particles to increase their functionality, improving toughness, impact and flammability. Fundamentally each fiber is immersed in resin, not only does this allow them to absorb higher compressive loads, but the loads are transferred between the fibers resulting in an overall better distribution of external loads. The most commonly used resin in Motorsport is Epoxy which is essentially made from an epoxy molecule and an amine. Mixing these two together initiates a chemical reaction that results in these molecules forming a closed chain that is more regular in structure than other polymers such as vinylesters.
‘In the simplest terms, manufacturing Prepreg is effectively combining a resin matrix and an appropriate reinforcement in a strictly controlled process. In general resin components are blended to initiate the reaction of polymerization,’ explains Jed Illsley, Product Manager at TenCate, who manufacture prepregs for Formula 1. ‘Working with prepreg is a sensitive operation so great care is taken throughout our processes. Once that chemical reaction starts, you have a finite time to complete the manufacturing operation, guaranteeing the customer quality and performance. Some of the resins we produce have an outlife of only 50 hours, so time management is critical and any time lost would mean outlife time lost for our customers.’
The resin matrices used in prepreg manufacture are Thermosets which do not melt under heat, they instead disintegrate. Uncured, the short molecular chains that make up this group of polymers have low viscosity, which is ideal for the impregnation of fibers. The resin constituent raw materials are blended together in large mixers and the chemical reaction induced by the hardeners form bonds between these short chains, creating a 3D ‘cross linked’ network which is incredibly strong.
‘The mixing process can be quite complicated. Every resin system in our portfolio has a documented and controlled mixing process using sophisticated dispensing methods,’ highlights Illsley.
Once the resin is mixed there are 2 principle conversion processes. Firstly, direct coating, this process evenly distributes the resin immediately onto the fiber reinforcement and is favored for heavyweight fabrics such as those used in wind energy. The second option is film coating, this method coats a thin film of resin onto release paper and is favored in lightweight fabric/fiber constructions such as Motorsport. ‘Film coating allows you to test and calibrate resin prior to impregnating the fiber. Fundamentally it is all about process control, we know exactly the weight of resin before committing to fiber impregnation,’ adds Illsley.
Fiber selection starts as a price versus performance consideration. Each fiber format, Glass, Aramid and Carbon have a distinct value proposition when looking at the end use application. In motorsport applications where weight optimization is critical, carbon fiber provides the best solution, but not all carbons are equal. Each type of carbon fiber exhibits different strength and stiffness per unit weight. A 3K fiber for example, means there are 3,000 filaments in every tow, where a tow is a bundle of filaments.
UD and Woven
Carbon fiber reinforcement for the Formula 1 market generally comes in two formats: woven or uni-directional (UD). Woven carbon fiber often use a twill weave that visually have a diagonal pattern. This format achieves an open ‘looser’ weave pattern, allowing it to drape more easily and is particularly useful when laminating mould surfaces with complex curves and contours. For this reason, woven fabrics are widely used in Motorsport, however in applications where strength is only required in a load direction, UD can be used, surrounded by woven fabric to hold it together. The principle for Uni-directional material is to get all the fibers aligned in the same direction and thereby maximizing the benefit of the high strength achieved in that direction.
To manufacture UD prepregs, multiple bobbins of fiber are aligned on a creel, each supplying one tow of carbon fiber with filaments that are spread out and lined up. The resin film is then applied to the fiber through the manufacturing process. The amount of resin applied is carefully controlled to match the tolerances demanded by Formula 1 teams. ‘Formula 1 teams demand an optimized balance of reinforcement and matrix resin. For example, some of our prepregs that have been homologated for the chassis now utilize lower resin contents combined with very lightweight fabrics,’ highlights Graham Roberts, Business Development Manager at TenCate. ‘This ability to customize elements of our materials, whilst maintaining high tolerances, at this stage of the process allows teams to benefit from incremental gains which they can use to their competitive advantage.’
Producing woven prepreg follows a similar method, with the reinforcement being delivered to the manufacturing line as woven fabric, typically in widths of 1.25m. Once laminated together with the resin, the process controls the level of impregnation.
The vast array of directional properties that can be achieved with carbon fiber lead to endless end-use possibilities. It is easy therefore to see why composites are used so extensively on many areas of modern racecars. ‘Not only can we change the type of fiber and therefore the strength, modulus and number of filaments, but we can also change the type of resin, the way we apply as well as the ratio of resin to fiber,’ highlights Illsley. ‘This is why manufacturing composites to achieve optimum performance is so complicated because there are so many parameters that you can modify.’