Additives for thermosets

Blog Archive | 7 minutes  | Author: Koen Nickmans , Ph.D.

Ever since the discovery of Bakelite in the early 20th century, thermosets have established themselves as an exceptional class of materials that provide an unmatched combination of robust mechanical properties with high temperature resistance. These days thermoset plastics can be found across a diverse range of industries including automotive, construction, electronics, and advanced composites.

Broadly speaking, the desirable properties of thermoset plastics stem from their high degree of chemical crosslinking. The crosslinking is irreversible which provides the high-temperature resistance, a property that is commonly lacking in thermoplastic materials. Moreover, thermoset plastics are typically cured from flowable resins which are easy to apply and mould before they are cured into place. A downside of thermoset plastics is that they are more difficult to recycle.

In this article, we introduce the basic components of thermoset plastics and take a closer look at the additives that play a role in finetuning their properties.

 

In this technical article:

 

Thermosets typically consist of the following components:

  • Polymer matrix - a thermosetting resin that cures to bind together the other components of the formulation. The matrix protects the other components from environmental influences. The main classes of industrially relevant thermosetting polymers are epoxies, vinyl esters, unsaturated polyesters, acrylics, and polyurethanes. Other thermosetting resin types are available for more demanding applications, for example the unique Nouryon Thioplast™ EPS for increased toughness, chemical resistance, and adhesion properties.

  • Fillers & pigments - used to embed desirable properties to the thermoset. This can be as simple as providing an attractive colour, using TiO2 or coloured pigments. The functions of fillers can also be more complex, for example providing fire-retardant properties using ATH or lightweighting using Glass Bubbles.  Fillers can also be used to reduce cost by partially replacing the resin, for example using calcium carbonate, or by partially replacing pigments, for example using calcined kaolin.

  • Reinforcements - these are added to improve the mechanical and physical properties of the thermoset and result in what is classed as a thermoset composite material. Reinforcements are usually in the form of fibres which can include glass fibre, carbon fibre, or natural flax fibre or wollastonites. Fibres predominantly add stiffness to the material and vary broadly in length depending on the application and the chosen manufacturing technology.

  • Curatives - used to initiate and/or control the rate of the curing reaction. The choice of curing agent depends largely on the chemistry of the thermosetting polymer as well as the desired properties (see for example our blog article on catalysts for PUR).

  • Additives - the subject of this article. Additives are typically used in small amounts (< 1 wt%), but they are essential parts of any thermoset formulation. Additives are necessary to prevent defects, enhance longevity of the products, and generally improve the cost performance ratio of the thermoset plastic. They can also enhance properties such as rheology, wetting behaviour, and surface finish.

 

Infographic of thermoset components: polymer matrix, fillers and pigments, reinforcements, curatives and additives

Figure 1. Commonly used components of thermosets.

 

Additives for thermosetting polymers

Foam controlling additives

Entrapped air in the form of air bubbles can lead to significant issues in thermoset applications. Air bubbles manifest themselves as micro- and macro-foam and remain present after curing. This will lead to considerably porosity and mechanical weak points in the bulk of the material, as well as various defects at the surface leading to vulnerable points and an unattractive finish.

Due to the viscous nature of thermosetting resins, air can become easily entrapped. Air bubbles can find their way into the resin either during processing steps (e.g. mixing) or during application (e.g. brush rolling) of the resin. To further complicate matters, most resins contain small amounts of surface-active substances in the form of soluble impurities, which cause the entrapped air to be stabilised, making it difficult to get rid of the air bubbles once they are formed.

Luckily, this can be counteracted by adding small amounts of foam controlling additives that use clever chemistry to destabilise the entrapped air, and to prevent it from being formed in the first place. Essentially these materials result in the coalescence of the air bubbles, increasing their size, and allowing them to rise easily to the surface. The additives are also known as ‘defoamers’, ‘antifoaming agents’, ‘air release agents’, and ‘deaerating agents’. There is no distinction between the various names since modern foam controlling additives control all of these aspects.

The chemical nature of foam controlling agents depends on the thermoset resin as well as application, but can typically be divided into polysiloxanes, acrylic copolymers, and low molecular weight nonionic surfactants.

 

Test methods for defoaming

Figure 2. Foam controlling agents are vital performance additives for achieving optimal bulk and surface properties in thermoset resins.

 

Dispersing agents

Dispersing agents are another common additive in thermoset resins. They are particularly important when high filler and pigment loadings are required, or when the formulator is faced with difficult to disperse pigments (e.g. carbon black).

The inclusion of particles into any resin invariably increases its viscosity, which can lead to processing difficulties. Dispersing agents help to lower the viscosity of the resin by wetting the surface of the filler particles and thereby acting as the interface between the particles and the resin (Figure 3). In practise, the selection of an appropriate dispersing agent will usually mean that the filler loading can be further increased, resulting in either a cost-saving or an increase in performance. The opposite can also be true - for example in the case of pigments where the colour strength is determined by the quality of the dispersion. In this case the selection of a suitable dispersing agent will increase the colour strength thereby allowing the formulator to reduce the pigment concentration.

Dispersing agents also have a stabilising effect on the filled resin, which means that they can prevent flocculation and settling of the filler particles over time. A well-dispersed resin is paramount to maintaining properties such as processability, gloss, and colour strength.

 

Mechanism of pigment stabilisation via electrostatics and sterics

Figure 3. Dispersing agents are active at the interface between filler particles and the thermosetting resin, thereby reducing viscosity and preventing agglomeration.

 

Surface modifiers

This class of additives includes wetting agents, levelling agents, and slip agents. There are subtle differences between these materials which are further explained in a previous blog post. Basically, these materials are used to balance surface tension differences at the various interfaces, to improve flow and to prevent surface defects. Like foam control agents, surface modifiers are also surface-active agents and therefore an interaction between these classes of materials can be expected in a given formulation. Since surface modifiers are active at the interface, these materials can also impart additional functionalities such as slip and scratch resistance.

 

Diagram showing surface tension of two different liquids

Figure 4. The surface tension of a thermosetting resin can be optimised by an appropriate surface modifier to prevent surface defects. 

 

Conclusion

In summary, additives are a critical component of thermoset formulations; ensuring reliable production processes as well as providing thermoset products with flawless surfaces and outstanding mechanical properties.  If you think that a foam controlling additive, dispersing agent, or surface modifier might be beneficial to your project then please contact us to speak with one of our technical sales team. They will be able to advise on which are the most suitable grades for your application. More information is also available on our product page and the MÜNZING CHEMIE website.

Author: Koen Nickmans , Ph.D.

Koen studied chemistry at the Catholic University of Leuven in Belgium with a specialisation in polymers. He subsequently obtained a PhD in the field of responsive polymeric coatings from the Eindhoven University of Technology in the Netherlands. He has been with Lawrence Industries since 2019 as a technical sales manager, covering all areas involving polymers: thermoplastics, thermosets, and elastomers.