Coupling agents increase the performance of composite materials

In recent decades, composite materials have become an essential part of everyday life. In many areas, they enable technological change , for example as an essential component of wind turbines and electric vehicles. In many cases, composites are a combination of cross-linked organic polymers, the matrix, and inorganic components such as fillers and fibers. Even if these inorganic materials take on the function of a cost-effective filler in some areas, they still lead to an improved property profile of the overall component. This manifests itself, for example, in improved surface quality, flame resistance or abrasion resistance. Fiber materials such as glass and carbon fibers make a significant contribution to improving the mechanical properties of the composite material while at the same time reducing its weight. Without this inorganic reinforcement, the lightweight materials required to reduce emissions, increase fuel efficiency, and use renewable energies would not be available.

Despite the beneficial influence of the inorganic components on the composite material, the interface between the matrix and the inorganic component can prove to be the weakest link in the overall system and limit its maximum performance. Improved bonding between the resin matrix and the dispersed phase can therefore lead to even further improved mechanical properties. A simple way to optimize the desired bonding is to use additives that can crosslink with the resin and at the same time form a stable bond with the filler or fiber.

Coupling agents create a bridge between the organic resin matrix and the particulate fillers or fiber materials by forming strong bonds to the surface of the filler or fiber and crosslinking with the matrix during the thermoset curing process (Figure 1). This strong bond between the organic and inorganic components of the formulation leads to increased mechanical properties of the composite material produced. In general, a coupling agent contains at least two types of functional groups: firstly, a reactive group that copolymerizes with the matrix during the curing process, and secondly, a surface affinic group capable of adhering to the surface of the particle or fiber. In a radically curing system, e.g. an unsaturated polyester or acrylate resin, the coupling agent has polymerizable double bonds, for example. If the thermoset is cured via a two-component mechanism - so-called 2pack systems, such as most epoxy or polyurethane resins - the coupling agent can be reactive towards the resin or the hardener.

As the coupling agent acts as a bridging agent between the matrix and the dispersed phase, the additive must necessarily be designed for a specific combination of matrix and inorganic phase. There is therefore no universal coupling agent that is suitable for all constellations. However, an increasing number of these additive agents are now available for the various combinations of matrix and reinforcement. Some examples of applications where the use is worthwhile are presented below.

Typical applications for quartz-filled unsaturated polyester systems are polymer concrete, artificial marble, and artificial stone. In these application fields, it is desirable to increase the mechanical performance for a particular article or to reduce the thickness of the article while maintaining the same mechanical properties, thereby reducing costs and saving material. Both can be achieved by using a coupling agent. For example, the flexural strength increases significantly at a dosage of only 0.2% by weight of the additive (relative to the filler) (Fig. 2).

Under the electron microscope, it can be seen that, in the presence of coupling agents, cavities are minimized and resin and filler are tightly bonded together (Fig. 3). Once the coupling agent has formed the largest possible number of connection sites, however, a further increase in its dosage cannot lead to further enhancement of mechanical properties.

In addition to improving the mechanical properties, the systems optimized with the coupling agent can offer additional benefits. Examples of this include increased resistance to hot water and chemicals. This extends the service life of a component and minimizes bubble formation when exposed to moisture and temperature.

Carbon fiber reinforced plastics (CFRP) are used when high material strength and low weight of composite parts need to be reconciled. This is the case in the aerospace industry, the automotive industry, as well as in wind energy applications, among others. [1] In these industries, composite materials are often manufactured using the RTM (resin transfer molding) process, prepregs, pultrusion or SMC (sheet molding compound) technology, in each case in combination with various resin matrix systems. Suitable coupling agents can provide advantages for the bending and tensile strength of chopped fibers as well as fabrics and scrims (Fig. 4).

Similar to particulate fillers, the mechanical improvement can also be observed in fiber-reinforced systems under the electron microscope (Fig. 5). In the absence of a coupling agent, only a few resin residues are detectable on the fiber surface after material failure. This underlines the insufficient adhesion between fiber and matrix. In contrast, in the presence of a suitable coupling agent, considerable amounts of resin residue can be seen on the fiber surface. In some cases, the adhesion between carbon fiber and matrix is even so strong that fiber fracture occurs.

Glass fibers are the most commonly used fiber material for reinforcing composites. Examples of such glass fiber-reinforced systems can be found in the wind energy, shipbuilding, sports, sanitary and construction sectors. The transverse tensile strength of these materials can be significantly increased by adding a suitable coupling agent to the formulations used.

It is well known that the quality of fiber sizing is reduced under certain storage conditions, which is why aged fibers can lead to downgraded performance of the final system. Coupling agents can compensate for this aging process, enabling composites with properties similar to those of virgin fiber material. This effect helps to improve the manufacturing process by increasing reliability and reducing batch-to-batch variation.

Coupling agents can also provide higher mechanical properties for radically curing systems. This allows the system to be adjusted according to the performance requirements, even if these go beyond the typical properties of the material itself (Fig. 6).

The optimization of mechanical properties plays a key role in the design and manufacture of composites. Even if it is possible to further improve individual components such as resins, fillers or fibers, there are limits to the influence of these components on the properties of the overall material. An alternative approach is to optimize the bond between the resin matrix and the reinforcing phase dispersed in it. A simple way to do this is to use additives that crosslink with the resin and at the same time form a stable bond to the filler or fiber. The structure of these coupling agents has to be adapted to the chemistry of the matrix and dispersed phase. 

One example of this are the coupling agents of the BYK-C 8000 product family available from BYK. They form a strong bond between the resin and the reinforcing material, enabling more robust and durable materials. This also provides greater freedom for material design. The improved properties of composites can be crucial in bringing the cost/performance ratio of systems into the desired range. The use of coupling agents therefore offers both access to new technical solutions and cost savings.

Coupling agents can be used for a wide range of systems, including epoxy, unsaturated polyester, vinyl ester and acrylate resins. They can be combined with various inorganic reinforcing materials such as quartz, aluminum hydroxide, glass fibers, and carbon fibers. They can also be used at various stages of processing, for example in the production of reinforcing materials (surface treatment of fillers and fibers), as part of the resin formulation (use as an additive in the resin) and in the composite production itself (e.g. as second sizing or directly before or during application). Coupling agents can therefore be used at various steps along the value chain. This includes market players as diverse as manufacturers of fillers and fibers, resin houses, and composite manufacturers. Companies can use the additives to gain competitive advantages over conventional systems on the market.