Resins are vicious, liquid polymers derived from organic or synthetic sources. Their benefit centres on their ability to transition from a liquid into a customisable solid, homogeneous structure.
In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers.
Plants secrete resins for their protective benefits in response to injury. The resin protects the plant from insects and pathogens. A clear example of the longevity and durability of resin is the encapsulation of insects in amber.
Many of these naturally occurring resins have practical uses today and equivalents have been manufactured from petrochemicals.
Resin usage as a single, homogeneous form is commonly used in the form of moulds as well as protective linings and coatings.
Polymers themselves are substances comprised of a built-up structure of a large number of similar units bonded together. Classic examples of synthetic polymers include resins and plastics.
There are two main types of resin: thermosetting and thermoplastic resin. See below.
Resins involved in composite production, as well as linings and coatings centre on two types: (1) thermosetting resins and (2) thermoplastic resins.
Resin Types
There are two main types of resin: thermoplastic and thermosetting resins.
Thermoplastic Resins
Thermoplastics account for a smaller part of the resins industry. They are sold as non-reactive solids, so no chemical reaction occurs during processing. Unlike thermosets, they require heat and pressure to create a finished material. Reheating and reforming is also possible.
Thermoplastic resins are able to soften and form viscous liquids when exposed to heat. They can then be transitioned to a solid under cooling. One of the key benefits of thermoplastic resins is their capacity for rapid moulding cycles since the curing process is independent of a chemical reaction. However, the primary focus of this article is on thermoset resins, a class of resins with applications in the construction, aerospace, marine and automotive sectors.
Thermoset Resins
Thermosetting resins are designed to react chemically after deposition to form a three-dimensional network. Thermoset resins need a curing agent to be added prior to the impregnation onto a reinforcement. Curing of this material results in a solidified structure that cannot be remelted or reformed.
For reinforced composites, the compounds usually include a resin system (curing agent, hardener, inhibitor,) and a filler/reinforcement.
Composites formed with thermoset resins have high dimensional stability, high-temperature resistance, and good resistance to solvents because of their three-dimensional cross-linked structure. The most frequently used thermosetting resins are polyesters, vinyl esters, epoxies, phenolics, polyamides (PA), and bismaleimides (BMI).
Examples of Thermoset Resins
Polyesters
There are two classes of polyester resin: saturated and unsaturated polyester resin.
Unsaturated polyester resin is a macromolecule – namely one with a molecular weight ranging from 1,000 to 3,000 kDa.
It is the result of a polycondensation reaction that converts dicarboxylic acids and dialcohols. A proportion of the dicarboxylic acids contain carbon-carbon double bonds – referred to as unsaturated bonds. Uncured and at room temperature, this resin is a highly viscous, largely transparent pink liquid.
The term ‘polyester resin’ is a shortened version of the term ‘unsaturated polyester resin’. Polyester resin is commonly used for fibreglass laminating applications. Glass fibre is added as a reinforcement material to create fibreglass, which can be used to build flat roofs, repair yacht hulls and more.
Epoxy
Epoxy resins contribute to the strength, durability and chemical resistance of a composite. They offer high performance at elevated temperatures, with hot/wet service temperatures up to 121°C.
Epoxies present themselves in liquid, solid and semisolid forms. They usually cure by reacting with amines or anhydrides.
Epoxies do not require a catalyst for curing, unlike polyester resins. Rather, a hardener (also called a curing agent). The hardener (part B) and the base resin (part A) co-react in an “addition reaction,” according to a specified ratio.
The correct mix ratio of resin to hardener is required to ensure the curing is effective. Incorrect mixing ratios will yield cures that are not fully cured with defective properties.
Epoxy resin can be reinforced and toughened using additives such as Kevlar and carbon fibre. This overcomes the brittleness of epoxy caused by a high degree of cross-linking.
When the resin is in a liquid state, the conditions and ingredients of the reaction (in combination with the laminating step and reinforcement material) will ultimately dictate the quality and performance properties of the composite. Tensile strength, Young’s modulus, flexibility in addition to water absorption capacity and chemical resistance are some of the measured metrics used to evaluate such performance.
Polyurethane
Polyurethane resin is a more viscous resin which upon curing forms a softer and more flexible structure than epoxy or polyester.
It has a broad range of applications and is used in construction and civil engineering as well as marine and medical industry.
In construction, it is used for resin floors and resin-bound driveways, which combine the flexibility of polyurethane with the high mechanical strength of aggregate.
Phenolic Resins
Phenolic resins are thermosetting polymers derived from phenol and formaldehyde. They are known for their excellent heat resistance, flame retardancy, and dimensional stability. Phenolic resins are commonly used in applications requiring high-temperature performance, such as in the production of brake linings, electrical components, and moulded parts for the automotive and aerospace industries. These resins are defined by their high cross-linking density, which contributes to their exceptional mechanical and thermal properties, particularly in combination with carbon fibre reinforcement.
Vinyl Ester Resins
Vinyl ester resins are a hybrid between polyester and epoxy resins, combining some of the advantages of both. They are corrosion-resistant and offer excellent chemical resistance, making them well-suited for use in aggressive environments such as chemical processing plants and marine structures. Vinyl esters exhibit superior fatigue resistance and toughness compared to traditional polyester resins. They are commonly used in applications requiring both structural integrity and resistance to harsh chemicals, such as tanks, pipes, and storage vessels.
Resin Curing
Resin curing is a process that transforms the properties of resins through the cross-linking of polymer chains. This transformation involves converting the resin from a liquid to a solid state, playing a fundamental role in thermoset polymer production.
The driving forces behind the curing process are diverse. Essentially, individual monomers and oligomers are combined, sometimes with a curing agent (often a catalyst), to initiate a reaction leading to the creation of a three-dimensional polymeric network. Initially, the reaction generates molecules with varying structures.
As the reaction progresses, the molecular weight of the structure increases in proportion to the reaction rate. In addition to chemical changes, physical alterations occur, such as reduced solubility, heightened viscosity, and increased density. This progression continues until the reaction’s completion, resulting in a uniform and homogenous network.
Curing is triggered by several factors
Heat Curing
Heat curing, also known as thermal curing, is a widely used method in composite manufacturing. It involves subjecting the composite material to elevated temperatures for a specific duration. This process activates the chemical reactions within the thermosetting resin, causing it to harden and solidify. Heat curing is effective for resins like epoxy and polyester and is often performed in ovens or autoclaves to ensure uniform temperature distribution.
Radiation Curing
Radiation curing uses high-energy radiation sources, such as electron beams or ultraviolet (UV) light, to initiate polymerization reactions in certain resin systems. Photoinitiation, using UV light, is common in composites where rapid curing is essential. This method offers precise control over the curing process and can be ideal for applications like dental composites or coatings.
Moisture Curing
Moisture curing involves the use of ambient humidity to trigger the curing process in specific resin formulations, such as certain polyurethane adhesives and sealants. The presence of moisture initiates cross-linking reactions within the resin, resulting in a hardened material. It’s a convenient method for applications where temperature control is challenging.
Activators or Catalysts
Many thermosetting resins require the presence of activators or catalysts to facilitate curing. These additives accelerate the curing process by lowering the activation energy required for the chemical reactions to occur. Common catalysts include amines for epoxy resins and peroxides for polyester resins. They are often mixed into the resin system just before application.
Depending on the resin type, there is a bulk of research committed to investigating novel methods and optimising existing ones.
The choice of curing depends on factors such as resin type, application, and desired shrinkage level. For the most part, curing is driven by two main categories:
Ultimately, the type of curing depends on several factors including, but not limited to the type of resin, application and the accepted level of shrinkage.
Chemically-induced curing
Resins like epoxy and polyester are cured via additive addition. Such additives are referred to as hardeners and induce resin curing.
Chemical-free hardening
Certain resins are cured without additives and instead rely upon heat, as is the case with thermoplastics.
The application of heat causes a reduction in viscosity prior to the onset of crosslinking. This leads to an increase in the oligomer chains present – a process referred to as gelation. At this point, the resin becomes immobile and rigid. Mass transport of molecules within the structure is limited until eventual curing. To achieve vitrification, an increase in temperature is required post-gelation.
Certain resins may see the inclusion of a catalyst activated by UV light. The curing process is a well-studied area, since curing dictates the qualities of the resultant resin structure as a means to improve the physical and chemical outcome.
What makes a good resin?
The properties of a resin are highly specific to the project at hand; there is no one-size-fits-all all, particularly as the project becomes more specialised. This has given rise to various categories and sub-categories. Some of the more general qualities deemed beneficial are detailed in the table below.
- Adhesion: Strong bonding to various materials.
- Flame Retardancy: Enhances safety in case of fire
- Mechanical Strength: High strength to withstand loads.
- Flexibility: Absorb energy and resist cracking.
- Chemical Resistance: Resistant to chemicals and environmental factors.
- Thermal Stability: Maintains properties over a wide temperature range upon curing.
- Low Shrinkage: Minimizes internal stresses and warping.
- Viscosity: Ensures easy fibre impregnation and minimizes voids.
- Curing Time: Efficient processing and production.
- UV Resistance: Protects against UV radiation.
Resin Composites
Whilst resin paints alone offer effective surface protection, their performance is limited in more demanding situations. Polyester resin without glass fibre reinforcement, for instance, is significantly more brittle and fragile than with reinforcement and has limited applications.
The introduction of wood as a support structure has been around for millennia. Limitations in wood, namely strength and durability saw steel become a popular option. However, steel is heavy and has a propensity to rust, limiting the number of applications available. Furthermore, steel, like aluminium, requires heat treatment (welding) to ensure the correct size and shape is obtained.
Resins provide a useful solution in the form of surface protection and stand-alone structures themselves. The combination of a resin with a reinforcement material (e.g. glass fibre or carbon fibre) has given rise to composites.
Resin-based composites are a combination of two or more materials with different chemical and physical characteristics. When combined, they form a material that is superior to the individual characteristics of these materials. This often results in superior performance in terms of strength, flexibility, chemical resistance and durability.
In the context of resins, composites combine both resin and reinforcement ingredients, initially in a malleable state whereby the resin is a liquid prior to curing.
Upon curing, the transition of the resin from a liquid to a solid state leads to the formation of a strong, rigid and impervious structure. This structure overcomes the limitations of these individual ingredients alone (resin is brittle, reinforcement is permeable and soft) to ultimately create a superior structure such as fibreglass or carbon fibre.
Classic examples include fibreglass and carbon fibre, which present seamless, joint-free features resistant to rust, a broad range of chemicals whilst also being durable and demonstrating a high strength-to-weight ratio.
Frequently Asked Questions (FAQs)
Q. Which resin is right for my project?
A. This depends on the project requirements and reinforcement material used. Polyester resin should not be used alone unless it is in the form of a more flexible topcoat. Epoxy can be used as a surface coating to protect an existing composite via a repair layer, for instance. At Resin Library, we supply a range of fibreglass and carbon fibre kits. These have been paired up with polyester and epoxy resins respectively.
Q. How long does it take for laminating resin to cure?
A. Cure times vary depending on the resin type. They typically range from a few hours to 24 hours, but can be further enhanced in properties or accelerated in curing times by using an autoclave.
Q. What’s the shelf life of laminating resin?
A. Shelf life varies by brand but is typically several months to a year when stored in a cool, dark and dry place. Polyester resin is typically 6 months whilst epoxy resin is 12 months.
Q. What are 3 cure stages of thermoset resins?
A. Thermoset Resin Curing Process: (1) the resin and curing agent are mixed, but the chemical reaction has not yet started, (2) the chemical reaction has started, and the viscosity and tackiness of the mixture have begun to increase and (3) the resin is completely cured.
