In recent years, we have seen great interest in the use of composites in many industries, in transport and aviation, in order to reduce weight, provide better aesthetics, achieve increased resistance to environmental conditions or wider design options. There is a growing consumer demand for products with increased performance parameters, which are demonstrated by composites. But what are composites really? What are they made of, what is their structure and properties? And finally, what is the best technique to combine them?

Composite bonding - why is it worth choosing this bonding method?

Composites, apart from the already traditional mechanical and thermal joining techniques, require new techniques to further optimize the strength and aesthetics of the structures made. As of today, it is possible to use various types of structural adhesives (epoxy, acrylic, polyurethane) to connect composites without the need to use reinforcements in the form of rivets, clamps, welding or mechanical fasteners. Therefore, bonding may prove to be the best option for joining composites and plastics.

Advantages of bonding composites

Advantages of using an adhesive joint for composites:

• structural adhesives work well in the process of gluing composite materials with other plastics and with many other substrates, e.g. wood, glass, metals, without losing their functional properties;

• gluing allows you to maintain the best ratio of weight to stiffness, desired structures are becoming lighter while maintaining their required stiffness.;

• glued joints in many aspects also meet the requirements of tightness, resistance to water, rain or drafts;

• gluing sets the bar high, especially when it comes to aesthetics - very often adhesive joints are invisible to the eye and the elements made using them are aesthetic and ready for immediate use;

• strengthening laminates with rivets or may weaken them, while gluing allows to distribute the load evenly and helps to increase the overall force transmitted by the structure.

Disadvantages of composite bonding

The bonding of composites is not only advantages but also difficulties that are worth mentioning. Bonding is primarily the cost of the adhesive, which is often higher than the price of a specific composite per kilogram. Gluing is an element of the production process, which also requires technological time. It happens that glued elements require the use of additional tools to stabilize and immobilize them after joining and to store them in specific positions until they can be subjected to further processes. Some adhesives require proper surface preparation of glued elements, degreasing and application of primers, which additionally increases the cost of the entire technological process. An important aspect that should be taken into account when choosing a composite bonding technique is the difference in coefficients of thermal expansion. Aesthetic considerations also turn out to be important, e.g. sometimes it is necessary to remove glue flakes, which is associated with an additional cost of the glue.

Construction adhesives - selection of the right adhesive

Methacrylic adhesives are the best for high-strength bonding of composites. Thanks to their versatility and high strength, they allow for resistant and durable adhesive joints. An example of this type of adhesive is Melkib MMA Power 10. When choosing an adhesive, it is worth paying attention to a few important issues:

- how fast glue do we need? What is the required open time and curing time? This is very important when implementing the adhesive into production and adjusting it to the current process.

- what will be the optimal viscosity - consistency of the glue and how will it be dosed?

If you have a problem with choosing a product, please contact our specialists.

Fiber-reinforced plastics - what are composites?

A composite is a material that is created as a result of combining two or more materials, one of which is a binding material and the other plays a reinforcing role and is introduced in a fibrous, granular or layered form. As a result of this combination, a structure with much greater mechanical, aesthetic and strength properties than those of individual materials is created. Fibre-reinforced plastics (a composite of two materials) are the most commonly used and we will take a closer look at them. Fiber composites consist of a matrix matrix and reinforcing fibers. The matrix surrounds the reinforcing fibers.

Both the matrix and the reinforcing fibers can be individually selected and adapted to the requirements. Combining these two ingredients together makes the resulting matterł shows much greater strength than that which each of them has individually. The matrix may be e.g. synthetic resin or thermoplastic and the fibers may be e.g. glass or carbon. A composite of several layers of fibers and a matrix is called a laminate. The main task of the fibers is to strengthen the material and absorb the force that acts on it. The matrix is designed to arrange the fibers in the expected geometrical arrangement, transfer forces between the fibers and protect the fibers from the effects of weather conditions. Fiber composites have properties depending on the direction of the fibers.

Photo #2 Example of fiber arrangement

Duroplastics and thermoplastics are used as the matrix material, and less often ceramics and metals.

Duromer (duroplastic) matrix systems:

• unsaturated polyesters (UP)
• vinyl ester resin (VE)
• epoxy resin
• phenol-formaldehyde (PF) resin
• polyurethane resin (PUR)

Thermoplastic matrix systems:

• polypropylene (PP)
• pyliamide (PA)
• polyetherimide (PEI)
• Phenyl polysulfide (PPS)
• polyetheretherketone (PEEK)

For a matrix to be suitable for a given material, the following selection criteria should be considered:

✔ manufacturing process (thermoplastics or recyclable thermoplastics);

✔ price;

✔ properties of the matrix material, e.g. mechanical properties, limiting application temperatures, resistance to media and radiation, moisture absorption;

✔ properties during production, e.g. viscosity, processing time, conditions and curing time;

✔ labor protection, toxicity.

Materials with a duromer matrix in the processing process are characterized by low viscosity before curing, long curing time, usually good susceptibility to gluing, and after curing they are infusible. Materials with a thermoplastic matrix in the processing process are characterized by high viscosity, the need to use short processing cycles, relatively low adhesion of fibers and the matrix, they stick poorly, and after hardening they are fusible and may deform.

The following types of fibers are most often used in fiber composites:

1. Glass fibers

2. Carbon Fibers

3. Aramid fibers

4. Natural fibers

5. Boron fibers

6. Ceramic or mineral fibers

Glass fibers are the most commonly used types of fiber - 90% of all fibers. They are characterized by a shapeless structure without a specific direction. The properties of the materials produced from them are not dependent on the direction of the fibers. Glass fibers are relatively cheap and the spinning process is done with a melt. The starting material for the production of glass fibers is silicon dioxide SiO2 mixed with soda and/or potassium (technical potassium carbonate). Due to their properties, glass fibers are used, for example, in boatbuilding or in the production of rotor blades.

Carbon fibers are also quite popular, characterized by a strong dependence of properties on the direction of their arrangement. They show a negative coefficient of thermal expansion in the longitudinal direction. When in contact with metals, carbon fibers corrode them. Glass fibers are expensive and are usually produced by carbonizing polyacrylonitrile (PAN) fibres. They are used, for example, in aviation, astronautics and in the production of sports equipment. Aramid fibers, also known as Kevlar fibers, are characterized by a low specific weight, high tensile strength and excellent thermal resistance. Aramid fibers do not corrode, are non-flammable and wear-resistant. Composites with aramid fibers are used in the production of bulletproof vests, helmets and protective helmets.

Natural fibers are an ecological alternative to glass and synthetic fibers used in the production of fiber-reinforced composites. Due to their mechanical properties, natural fibers such as flax, sisal, jute or hemp can easily compete with glass fibers in terms of strength. Natural fibers are characterized by high availability, low price, renewability, good mechanical properties and low density contributes to reducing the weight of the finished product. Natural fibers degrade under the influence of UV radiation and as a result of the action of acids and alkalis.

Ceramic (mineral) fibers due to their high thermal and chemical resistance and good insulating properties, they are mainly used as asbestos substitutes for the production of thermal insulation, insulation, sealing and filtration materials.

Boron fibers are much less frequently used due to their price and used inspecial structures where high rigidity is required.

Pros and cons of composites

Fiber composites are generally more stable than one-component materials made of plastics of the same weight. The low mass of fiber composites is used, for example, in mechanical engineering, where the aim is to reduce mass inertia. Another advantage is the high strength of the fiber composites and stiffness with a very low density at the same time. Fiber composites are characterized by low thermal conductivity and, depending on the direction of the fibers, they have a high energy absorption capacity.

Unfortunately, plastics also have disadvantages. Compared to, for example, metals, their price is high and high demands are placed on the design of components throughout the entire process chain. They exhibit thermal expansion and due to the structure it is difficult to recognize damage and repair is expensive. One of the significant problems faced by the production and processing of composites is the collection and segregation of waste generated as a result of these processes. Most of them do not degrade in the short term. Protection of the environment against plastic waste consists primarily in improving the process of its disposal. Ideally, the composites should be recyclable or self-degradable. At worst, there should be ways to remove them at relatively low cost.

The Melkib company offers various types of flexible, methacrylic and structural adhesives, which are perfect for bonding composites, both with other plastics as well as with metals, wood, glass, etc. It also has specialized people who professionally advise adhesives suitable for a given technological process, meeting the highest requirements.