Thermoset matrices are very common in motorsports application. Actually, they are also well known in the industry due to its reasonable trade-off between lightweight and good mechanical properties. Usually, the most common in this field is the polyester and epoxy resin, which are used for components made in processes like vacuum fusion and autoclave pre-preg. This article provides a summary of the main characteristics of the thermoset polymers regarding their manufacturing, types and applications.
Manufacturing process
The thermoset polymers are usually liquid or semi-solid states before curing. In the point of view of chemistry, these materials are designed in order to be moulded in several ways. It can be manufactured by different kind of technologies, the simplest one is the vacuum fusion. This is based in the reinforcement of the dry fabric which is put into the structure to be built. Then, a vacuum bag and some valves are responsible for inclusion of the liquid resin in order to fill and impregnate the reinforcement. Another process is the pre-preg, which is based in the process of putting together fiber and reinforcement. This keeps the fabric that have both ingredients stabilized in a room. Hence, it is just necessary to cut the different plies and to prepare the lay-up for the components or structures under construction.
Curing process
Regarding the curing process, the epoxies and thermosets are characterized by chains that are strongly interconnected one to the other. Hence, at the end of curing it is developed a three dimensional pattern, which is the so-called cross-links. These bonds are extremely strong and characterized by chemical bonds. In this way, it is obtained final parts or components that, differently from thermoplastics, it can not be melted or re-manufactured. The 3D pattern of bonds is realized by two different parts. The objective is to have a progressive reaction. Hence, the polymer chains are connected together with a monomer, thus it is possible to realize small interconnected structures with linear chains.
In this case, the material is characterized by lower mechanical properties and shrinkage. This makes the material exposed to damage due to heat. On the other hand, when the method is complex, the different chains are interconnected, it is obtained a structure which is characterized by higher mechanical properties. There are different ways to start the reaction. For standard materials, the reaction is activated by temperature and, in some cases, by humidity absorption. Another way is the radiation, that is considered for resins used in detail applications, which requires an UV lamp to cure the part. A mixed reaction is combination of two factors, time and humidity, or time and temperature. This can be the case of epoxies, which there is both ingredients of time and temperature, which polyurethane has the humidity as the main factor for its curing.
The effect of having 3D branched structures or a more linear one are noted on the mechanical properties as strength and elastic modulus. If the cross-links are very far one from the other, it is possible to observe that, the strength and the elastic modulus decrease. This can be also observed in thermoplastics, which the chains can slide one over the other when heat is applied. Hence, it is increased the distance between the bonds, thus reducing the mechanical properties. In the case of thermoplastics, the bonds are broken, while for thermosets it is considered the number of the cross-links and how they are realized on the structure. If the structure is strongly interconnected, the elastic modulus and the strength of the polymer itself can reach the maximum of it.
Types of thermoset matrix
Basically, the thermosets for motorsport applications can be divided into:
- Epoxies;
- Polyesters.
The most used thermoset matrices can be divided into different chemical structures. The most used is the epoxy resin, which comes from very simple chemical bond structures. This means that, along their bonds the molecule can rotate, thus the molecule has some mobility. The polyester is a very common thermoset. Its structure is composed by a double bond between two carbons, thus the structure is much more rigid, in the point of view of the rotation. Generally, epoxies are polymers characterized by good mechanical properties. In addition, their adhesion with fibers is good and it is helped by sizing. This is a surface treatment addressed to fibers, it enhances the chemical bonds between the matrix and fibers. The epoxies can be coupled with a lot of different types of fibers. The polyester resins are less expensive than epoxies. They are widely used together with glass fibers. Polyesters, or vinilesters in some case, are used in compression moulding process.
Polyesters
The vinilester is widely used in transports, for interiors and reinforcements of trucks, buses and tractors. In the racing field, this matrix is also used with carbon fibers also for interiors and reinforcements. The reason is that, regarding the mechanical properties, when vinilester is used with fibers, they are characterized by low adhesion with the other materials. Hence, reduced mechanical properties are obtained. Therefore, polyesters are more used for lower budget cost, which has higher batch. An example of this kind of matrix in body panels, is the new generation of NASCAR cars and some Lotus ones. These are made from glass fibers and vinilester resins. They are lighter with respect to other materials and can be changed easily after a crash or shank. Then, in the costing point of view, it is much less expensive than carbon fibers. Another common application of these resins is the marine market. In some cases they are used for radomes, because it works well with glass fibers, that is rather transparent. From this point of view, it is a good application.
However, regarding the mechanical properties, polyesters have a lot of problems with shrinkage, because in the cross-linking process it makes the resin to shrink more than 7%. This occurs due to the chemistry of the polymer. In the point of view of surface finishing, polyester matrices are characterized by a quite low surface quality. Polyester resins are not characterized by good mechanical properties, in particular this is due to the chemistry of the material and, depending on the number of the cross-links, it is possible to obtain higher or lower properties. In addition, due to the fact that, it is necessary a reacting diluent, a sort of part re-accelerator to start the process, the part concentration defines the mechanical properties. This is important in the case of buckling fusion, which the process starts with a liquid state resin, when adding diluent, the reaction is accelerated. This can be effective for both small and large parts. For this one, the diluent is important since it is required to polymerize all the fusion lines at the same time.
Epoxy resins
Epoxy resins are characterized by high mechanical properties and good thermal ones. Their chemistry is different from polyester, the structure is a simple bond along different carbons. This means that, the epoxy resins can be easily modified also due to external functional groups, because there is a lot of open bonds in which the different parts can be bonded together. Comparing it with polyester resins, the cost is higher and the shrinkage effects are low. However, the moulds should be designed considering the effects of temperature and shrinkage. The phenomena is originated from the cross-links since the polymer chains arrange in a three dimensional structures that could be also very complex in the point of view of the different chains arrangements. These are the origins of the thermal treatments, thus when heat is applied, the structure is polymerized and the chains, that started from a linear structure, become a big and entangled structure. Hence, this tends to shrink with respect to the critical dimensions. This is a physical effect due to the presence of the cross-links. The shrinkage epoxy resins is lower with respect to polyester, because the chemistry of this polymer is different and the monomer repetition generate a more stable structure with respect to the polyester. Hence, the shrinkage depends on the chemistry and the distribution of electrons inside the molecule.
References
- This article is based on lecture notes written by the author during the Design for Composite Structures of Racing Cars lectures attended at Unimore.
Automotive Papers 