What is HCR Silicone – The Complete Guide

Heat-cured rubber (HCR) silicone is a versatile material with many uses. This article explores what HCR silicone is and how its properties make it suitable for various applications.

Let’s explore important fundamental aspects about HCR:

What is Heat Cured Rubber Silicone?

Heat-cured rubber silicone, also known as HCR silicone, is a type of silicone elastomer. An elastomer is a polymer with rubber-like properties and flexibility.

You can think of HCR silicone as silicone rubber that cures, or hardens, with the application of heat. Unlike other silicone types that use chemicals or radiation to cure, HCR silicone uses heat as the curing agent.

The primary component in HCR silicone is polydimethylsiloxane, a silicone polymer. This polymer gives HCR silicone its rubber-like flexibility as well as heat and cold resistance.

For you to initiate the curing process, HCR silicone formulations contain chemical additives like platinum complexes.

These additives promote cross-linking reactions between polymer chains when heated. Cross-linking transforms the liquid silicone into a solid elastomer with resistant properties.

Benefits of HCR Silicone

HCR silicone offers several advantages compared to other materials. Some of the main benefits you can get from HCR include:

Temperature Resistance

Resistance to extreme temperatures from -55°C to 200°C. Therefore, you can use it in applications with fluctuating temperatures.

Emits Less Volatile Organic Compounds

Being heat-cured also means HCR silicone emits less volatile organic compounds during curing. Additionally, exposure to heat helps ensure deeper cures within thick HCR parts for maximum strength and stability.

Resistant to Chemicals

HCR silicone is also resistant to chemicals, weathering, and aging. It maintains its flexibility over a wide range of conditions. Thanks to its inert nature, HCR silicone does not deteriorate when in contact with oils, solvents, and chemicals found in industrial and lab settings.

Its durability reduces the need for replacement and maintenance.

Properties of Heat-cured Rubber Silicone

As a silicone elastomer, heat-cured rubber has many distinguishing properties that result from its molecular structure and composition.

Non-toxic and Hypoallergenic

This makes your HCR safe for applications involving direct contact with skin, such as in the medical and food industries.

High Tensile Strength

If you are looking for silicone rubber with high strength then choose HCR. HCR silicone also exhibits high tensile strength (the ability to withstand pulling forces).

High Flexibility

Combined with flexibility, this lets HCR components like gaskets and hoses efficiently cope with compression and stretching.

Low Compressibility

Additionally, HCR has very low compressibility, so it can withstand high pressures without breaking or bursting.

Heat Cured Rubber Silicone Chemical Structure

On a molecular level, HCR silicone consists of silicone polymers with methyl groups attached to the silicon atoms instead of hydrogen atoms.

When you add methyl groups, it gives your silicone its flexibility as their size prevents the chains from packing tightly together and allows free movement.

Between polymer chains are cross-links formed from the curing process. These cross-links serve as connection points that bind polymer chains to create a cohesive, three-dimensional network.

The cross-linking gives HCR its elasticity as well as heat, chemical, and compression resistance by restricting chain movement. Without cross-links, the material would remain a liquid forever.

HCR Silicone Chemical Structure
HCR Silicone Chemical Structure

Difference between HCR and LSR Silicone

You may have heard of liquid silicone rubber or LSR silicone. While both HCR and LSR are types of silicone elastomer, they differ in how curing occurs.

Normally, you will find that LSR silicone uses platinum-catalyzed addition reactions that initiate at room temperature. This allows LSR to cure inside injection molds for complex fabrications.

On the other hand, HCR silicone utilizes heat to generate condensation reactions between polymer chains. As such, it requires baking in an oven after shaping, often in compression or transfer molds.

Compared to LSR, HCR offers higher hardness and temperature resistance. It also exhibits less mold fouling so molds last longer. However, LSR molding enables more intricate parts.

Curing Behavior of LSR and HCR
Curing Behavior of LSR and HCR

How to Manufacture HCR Rubber

If you are planning to convert liquid HCR silicone into a solid component – compression molding offers perfect option. In this process, the silicone is first mixed, degassed to remove air bubbles, and then poured into a pre-heated mold cavity.

The mold and filled cavity are then compressed together using a press.

Compression Molding
Compression Molding

Excess material flows out while compressing helps the silicone fill intricate mold features. The mold is cooled before opening to recover the finished HCR component.

Alternatively, injection molding can be used for high-volume production. It involves injecting HCR silicone into a mold at high pressures to fill it. After shaping, the mold undergoes a post-cure bake cycle to complete cross-linking.

Liquid Injection Molding Process
Liquid Injection Molding Process

Curing Mechanism for HCR Silicone

As mentioned, the key factor initiating curing for HCR silicone is heat.

When you apply heat, chemical groups within the prepolymer break down and generate active sites on the polymer chains. Nearby active sites can then join together (a process called condensation).

This results in the formation of crosslinks between polymer chains. As crosslinking proceeds throughout the HCR material, individual polymer chains become chemically bonded together into an intertwined rubber network.

The completion of this heat-induced condensation reaction transforms the liquid HCR compound into a solid elastomer with durability and flexibility.

Curing temperatures often fall between 140°C – 205°C depending on the specific formulation. Newer equipment allows for accelerated curing at even higher temperatures to reduce cycle times.

Precise temperature control ensures uniform cures across large industrial batches for consistent final properties.

HCR Silicone Applications

  • With its advantages, HCR silicone has found widespread usage. Some applications include gaskets, o-rings, hoses, seals, encapsulation of electronics, footwear, prosthetics, and medical devices.
Silicon Rubber Gaskets
Silicon Rubber Gaskets
  • HCR pads and strips serve as insulation or damping materials. It also acts as a binder for high-temperature coatings and casting compounds.
  • In the automotive industry, HCR helps seal components like door trims from water, dust, and vibration.
  • For Kitchenware, you can use to HCR for its heat resistance in pan and pot handles.
HCR Silicone Kitchenware
HCR Silicone Kitchenware
  • Home appliances also employ HCR’s flexibility in items like washing machine gaskets.
  • Although not as formable as LSR, thin HCR sheets offer optical clarity for lighting components.

Conclusion

Heat-cured rubber silicone is a valuable material thanks to its flexibility, chemical inertness, and wide service temperature range. It’s curing mechanism and properties make HCR suitable for numerous demanding applications across various sectors.

 

More resources:

Where does Silicone Rubber Come from?  – Source: KDM

What is RTV-1 Silicone – Source: Legenday

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