Thermosets – Definition

Thermosets are non-meltable, hard plastics with a highly cross-linked, three-dimensional molecular structure. Once cured, they can no longer be reshaped or melted down. This clearly distinguishes them from other types of plastics such as thermoplastics and elastomers.

Thermosetting plastics are indispensable when high strength, heat resistance, and chemical resistance are required.

They are used in a wide range of applications in electrical engineering, automotive engineering, the construction industry, and many consumer goods sectors. Thermosets are particularly suitable for safety-critical and function-critical applications.

The most important properties of thermosetting plastics are:

• high temperature and dimensional stability – thermally stable up to approx. 300 °C
• very high mechanical strength and hardness – permanently dimensionally stable
• chemical resistance – resistant to oils, solvents, and chemicals
• one-time curing – no subsequent deformation possible

These properties make thermosets ideal materials for demanding industrial scenarios.

Thermoplastics can be molded and remelted as often as desired when heated, whereas thermosets cannot. Elastomers, on the other hand, are soft and rubber-like, while thermosets remain hard and brittle.

Thermosets are used in many industries, including:

• Electrical engineering and electronics: Insulators, switches, measuring transformers, bushings for medium-voltage technology
• Automotive engineering: heat-resistant engine covers, housings, structural parts, trim strips, cockpit paneling
• Construction industry: durable, weatherproof components
• Household appliances and furniture: heat-resistant handles, molded parts, kitchen sinks, kitchen countertops, sanitary parts

• high temperature resistance – ideal for applications with high heat loads
• excellent strength and dimensional stability – even under continuous mechanical stress
• chemical resistance – resistant to solvents, oils, and many chemicals
• durability – high resistance to aging and weathering, even with long-term use in demanding environments

Thermosets are processed by curing, e.g., in a compression or injection molding process. Curing times vary depending on the type of resin. The tool quality and process parameters such as temperature, pressure, and curing time are decisive for the subsequent component properties. Since the process is irreversible, thermosets cannot be remelted like thermoplastics.

The best-known thermosetting plastics include:

• Bakelite – the first plastic used in industry, excellent electrical insulation
• Epoxy resins – the basis for electrical casting compounds, high-performance adhesives, and composite materials
• Melamine resins – scratch- and heat-resistant surfaces, e.g., kitchen countertops
• Phenolic resins – resistant to heat, used in machine and vehicle construction
• PMMA kitchen sinks

Optional recycling options are

• mechanical recycling (shredding and reuse as filler) and
• energy recovery (combustion for energy production).

Intensive research into the development of bio-based and more easily recyclable thermosets is continuing.

Our high-performance fillers play an important role in the manufacture of thermosetting plastics. The high demands placed on the wide range of thermosets, such as high strength and high heat resistance, can only be met by functional fillers. In addition, their use in finished parts results in visually appealing, durable surfaces.

The following overview shows which properties of thermosets are influenced by the use of our high-performance fillers:

Due to their good adhesion, excellent heat and chemical resistance, and outstanding electrical properties, epoxy resins have been an important raw material for electrical engineering and electronics for decades.

In electronics, epoxy resin molding compounds are used as encapsulation systems, as laminating resins for circuits, and as auxiliary materials for printed circuit board manufacturing. In electrical engineering, epoxy resins are used in transformer construction and for the manufacture of insulators and dry-type transformers.

The required mechanical, thermal, and electrical characteristics of epoxy resin are largely determined by the functional filler selected. Fillers based on silica,  wollastonite, mica, aluminum hydroxide, and white fused alumina are used.

For decades, the high-performance fillers SILBOND^® silanized fine quartz powder and MILLISIL^® quartz powder have proven themselves in EP resin systems. On the one hand, they are used because of their outstanding mechanical properties, and on the other hand, they make an important economic contribution.

High-performance quartz-based fillers are characterized by the following properties:

• high resistance to weathering and chemicals
• high mechanical strength
• good filling levels
• excellent workability
• low thermal expansion

Due to the excellent integration of surface-modified quartz powder (SILBOND^® W 12 EST) into the EP polymer system, this has long been the standard for weather-resistant outdoor applications. SILBOND^® quartz powder is now also increasingly being used for indoor cast resin parts, including for optically glossy, high-quality product surfaces.

SILBOND^® 126 EST is a further development of the established silanized quartz powder SILBOND^® W 12 EST, offering an increase in the filling degree at constant viscosity. This allows for more cost-effective formulations, as less resin is required.

EP casting compounds and metallic materials are exposed to severe thermal cycling in electrical engineering. To prevent damage to the component, the temperature-related dimensional changes of the different materials must be as similar as possible.

Epoxy resin has a thermal expansion coefficient of 65 x 10^-6/K, whereas metals have a thermal expansion coefficient of between 12-24 x 10^-6/K.

The use of selected fused silica fillers with a low thermal expansion coefficient in EP casting compounds minimizes the different temperature-related dimensional changes between the casting compound and the metal. This is the only way to manufacture crack-sensitive parts (large metal castings or complex geometries).

Block-shaped and needle-shaped TREMIN^® wollastonite are characterized by good electrical properties, low thermal expansion, and very good reinforcing properties.

SILATHERM^® Plus was developed specifically for applications requiring a combination of electrical insulation and thermal conductivity. This top-quality product allows high filler contents to be achieved without compromising its excellent flow properties.

Many of our fillers are available with surface treatment. Products with optimized grain size distribution for higher filling levels are also available. Please contact us for more information!