Liquid silicone rubber (LSR) is primarily made by processing raw materials derived from silica into silicone polymers, which are then formulated into a two-part liquid compound that cures into a durable elastomer.
The fundamental process begins with extracting silicon from its natural source, silica, commonly found as quartz sand.
Initial Steps: From Silica to Silicon
- Silicon Isolation: To obtain the silicon needed for silicone, silicon atoms must be isolated from the silicon dioxide compound, silica (SiO₂). This critical step involves heating large volumes of quartz sand to extremely high temperatures, often up to 1800 °C. This pyrometallurgical process, typically performed in an electric arc furnace, reduces the silica to elemental silicon.
- Silane Synthesis: From here, there are several processes where silicon is combined with methyl chloride and heated. This reaction, known as the Rochow process (or direct process), yields a mixture of methylchlorosilanes. The most important product is dimethyldichlorosilane (Me₂SiCl₂).
Creating Silicone Polymers
The methylchlorosilanes are then purified and hydrolyzed. The hydrolysis step involves reacting the silanes with water, which removes the chlorine atoms and forms silanol groups (-Si-OH). These silanol groups then condense (react with each other, releasing water) to form siloxane bonds (-Si-O-Si-), creating long polymer chains called polysiloxanes.
- Linear Polysiloxanes: Using primarily dimethyldichlorosilane results in linear polysiloxane chains, the backbone of silicone polymers.
- Functionalization: For LSR, the ends of these linear chains are typically terminated with reactive groups, such as vinyl groups (-CH=CH₂), to enable the curing process.
Formulating Liquid Silicone Rubber (LSR)
LSR is not just a single polymer; it's a carefully formulated compound designed for liquid processing and rapid curing. It typically consists of a two-part system (Part A and Part B) that are mixed just before use.
Here are the key components found in a typical LSR formulation:
- Part A (Base Polymer & Catalyst): Contains vinyl-terminated polysiloxane polymers (the main silicone base) and a platinum catalyst (the curing agent).
- Part B (Crosslinker & Inhibitor): Contains more vinyl-terminated polysiloxane polymers, a hydride-functional siloxane (the crosslinker, which reacts with the vinyl groups), and an inhibitor (to prevent premature curing at room temperature).
- Fillers: Often includes reinforcing fillers like fumed silica or precipitated silica to improve mechanical properties such as strength, tear resistance, and hardness. Non-reinforcing fillers might also be used.
- Additives: Pigments for color, heat stabilizers, or other processing aids may be included.
The Curing Process
LSR cures via an addition reaction (also called hydrosilylation or platinum-catalyzed curing). When Part A and Part B are mixed, the inhibitor's effect is overcome, and the platinum catalyst facilitates the reaction between the vinyl groups on the base polymer and the hydride groups on the crosslinker. This reaction forms strong, stable crosslinks (-Si-CH₂-CH₂-Si-) between the polymer chains, transforming the liquid mixture into a solid, elastic network. This curing process is typically accelerated by heat, allowing for rapid molding cycles, often through injection molding.
| Key Stages in LSR Production | Description |
|---|---|
| 1. Raw Material (Silica) | Sourcing quartz sand (silica). |
| 2. Silicon Isolation | Heating silica to high temperatures (up to 1800 °C) to produce elemental silicon. |
| 3. Methylchlorosilane Synthesis | Reacting silicon with methyl chloride (combined with methyl chloride and heated) to form silanes. |
| 4. Polysiloxane Synthesis | Hydrolyzing and condensing silanes to form functionalized silicone polymer chains. |
| 5. LSR Formulation | Compounding functionalized polysiloxanes with catalyst, crosslinker, fillers, and additives into two liquid parts. |
| 6. Processing & Curing | Mixing the two parts and heating (e.g., via injection molding) to initiate rapid addition curing. |
In summary, making liquid silicone rubber involves isolating silicon from silica through high-temperature processes, synthesizing specific silicone polymers, and formulating these polymers into a reactive, two-part liquid system ready for molding and curing.
