This image was created with artificial intelligence – marked under Art. 50 of the AI Act, Regulation (EU) 2024/1689. It is a placeholder until the company's photos arrive.Polyester resins (UP resins)
What are polyester resins (UP resins)? Everything about properties, processing, areas of application and advantages for industry. Find out more now.
Polyester resins belong to the group of thermosets
Polyester resins are among the thermosetting plastics and are predominantly used as a matrix material for fibre-reinforced composites. They cure irreversibly through a chemical reaction and, after curing, form a dimensionally stable, temperature-resistant material.
Technically, polyester resins can be divided into unsaturated and saturated variants. For industrial applications, almost exclusively unsaturated polyester resins (UP resins) play a role. These can be processed in a targeted manner with hardeners and accelerators and offer a balanced ratio of mechanical strength, chemical resistance and cost-effectiveness.
Polyester resins are often used where large-area components, complex geometries or series production are required. Typical fields of application are mould making, vehicle construction, tank and apparatus construction as well as applications in the construction and structural engineering sector. A systematic classification in comparison with other resin systems is provided in the overview of resins at a glance.
Structure and chemical properties of polyester resins
Polyester resins are based on a reactive resin matrix of polyhydric alcohols and unsaturated dicarboxylic acids. The resulting polymer chains contain reactive double bonds that are chemically cross-linked during curing.
Curing takes place through the addition of a hardener, usually organic peroxides. This produces a three-dimensionally cross-linked polymer network. This process leads to high dimensional stability, temperature resistance and chemical resistance to many media.
Characteristic of polyester resins is their good wettability of glass fibres as well as the comparatively low viscosity in the uncross-linked state. As a result, they are particularly suitable for hand lay-up processes, fibre spraying processes and closed mould processes in series production.
In terms of material, polyester resins belong to the group of thermosetting resin systems. After curing, they can no longer be melted or reshaped, but reliably retain their properties even under mechanical and thermal load.
Mechanical and physical properties
Cured polyester resins are characterised by high dimensional stability and compressive strength. In fibre-reinforced applications, they handle the transmission of force between the fibres and ensure uniform load distribution in the composite.
Typical properties of polyester resins are good dimensional accuracy, a low tendency to creep as well as stable mechanical performance over a broad temperature range. The heat deflection temperature is significantly above that of thermoplastics of comparable density.
Chemically, polyester resins show high resistance to oils, greases, dilute acids and many aqueous media. Resistance to strong alkalis and concentrated acids must be assessed on a material-dependent basis.
The final characteristic values are determined to a decisive extent by fibre type, fibre content and manufacturing process. A practical classification of comparable resin systems can be found in the section on epoxy resins, which offer higher strengths at increased processing effort.
Processing and curing
Polyester resins can be processed at room temperature and are therefore suitable for manual, semi-automatic and industrial manufacturing processes. Processing takes place in the liquid state, with viscosity and reactivity specifically matched to the respective process.
Curing is initiated by adding hardeners and accelerators. The reaction rate can be controlled via the formulation as well as via temperature control. As a result, both short cycle times in series production and longer processing windows for complex components can be realised.
Typical processing methods are hand lay-up, fibre spraying, winding as well as closed processes such as RTM or vacuum infusion. The good flowability of the resins supports uniform impregnation of the reinforcing fibres.
After curing, a permanently cross-linked material with stable properties is formed. Information on selecting suitable resin types for specific manufacturing processes can be found in the overview of reactive resin systems.
Typical areas of application of polyester resins
Polyester resins are used in a wide variety of industrial applications, in particular where dimensionally stable, fibre-reinforced components are to be manufactured economically. Due to their balanced combination of mechanical properties, chemical resistance and processing properties, they are established in many industries.
A central area of application is mould and tool making. Polyester resins are used for master models, negative moulds and laminate tools where dimensional accuracy and reproducible surface quality are required.
In vehicle and commercial vehicle construction, polyester resins are used for body components, covers, trim elements and structural components. They are also used in tank and apparatus construction for tanks, pipe systems and housings that must be resistant to moisture and chemical media.
Further fields of application are the construction and structural engineering sector, the electrical industry as well as mechanical engineering. A classification of alternative resin systems for highly stressed applications is provided in the section on epoxy resins.
Distinction from other resin systems
Polyester resins are often used as an economical standard solution for fibre-reinforced plastics. Compared with other reactive resin systems, they differ in terms of strength, chemical resistance, processing effort and cost structure.
Compared with epoxy resins, polyester resins offer shorter curing times and simpler processing, but show lower mechanical characteristic values and lower adhesion to reinforcing fibres. For applications with very high structural requirements, epoxy resins are therefore often used.
Vinyl ester resins represent an intermediate solution. They combine higher chemical resistance and improved mechanical properties with a processing technology close to that of polyester resins. The material costs are accordingly above those of classic polyester resins.
A systematic comparison of the available resin types can be found in the superordinate section resin systems in comparison, which offers decision-making aids for specific applications.
Selection criteria and material decision
The selection of a suitable polyester resin depends on the technical requirements of the component, the intended manufacturing process as well as the ambient conditions in later use. A blanket material choice is rarely expedient in practice.
Central selection criteria are the desired mechanical load capacity, the chemical resistance, temperature exposure as well as requirements for surface quality and dimensional accuracy. Equally relevant are pot life, reactivity and viscosity of the resin in relation to the manufacturing process used.
For series production with short cycle times, fast-curing systems are often used, while for complex geometries longer processing windows are advantageous. The combination with glass or carbon fibres also has a decisive influence on resin selection.
An initial orientation for classifying suitable materials is provided by the overview of engineering plastics, which provides cross-material decision-making aids.
Advantages and limits of polyester resins
Polyester resins have become established as versatile materials for fibre-reinforced plastics. Their use is based on a balanced ratio of technical performance, simple processing and economical material availability.
The main advantages include good processability at room temperature, short curing times as well as suitability for manual and automated manufacturing processes. In addition, polyester resins offer solid chemical resistance to many media and enable the economical production of large-area components.
Limits become apparent under very high mechanical or thermal requirements. Compared with epoxy or high-performance resins, polyester resins exhibit lower strengths and limited long-term resistance under extreme conditions. For structurally highly stressed applications, alternative resin systems must therefore be considered.
A more in-depth examination of higher-performance alternatives is provided in the section on epoxy resins, which are used in particular in high-performance composite construction.
Frequently asked questions about polyester resins
What are polyester resins?
Polyester resins are thermosetting reactive resins that are predominantly used as a matrix material for fibre-reinforced plastics. After curing, a dimensionally stable, permanently cross-linked material is formed.
What does unsaturated polyester resin mean?
Unsaturated polyester resins contain reactive double bonds in the polymer structure. These enable chemical cross-linking during curing and form the technical basis for most industrially used polyester resins.
What are polyester resins typically used for?
Polyester resins are used above all in mould and tool making, in vehicle construction, in tank and apparatus construction as well as in the construction and structural engineering sector. Applications with fibre-reinforced composites are typical.
How do polyester resins differ from epoxy resins?
Polyester resins can be processed more easily and economically, but exhibit lower mechanical characteristic values. Epoxy resins offer higher strengths and better adhesion to fibres, but require greater processing effort. A classification can be found in the section on epoxy resins.
Are polyester resins chemically resistant?
Polyester resins show good resistance to oils, greases, water and many dilute chemicals. The specific resistance depends on the resin formulation and the medium used.
What must be observed during storage and processing?
Polyester resins should be stored cool, dry and protected from direct sunlight. During processing, the respective safety and processing instructions of the manufacturer must be observed.