Special Rubber - Fluoro Rubber (FPM) and Silicone Rubber

1. Fluoro Rubber (FPM) Structure

Between 1957 and 1959, DuPont developed Fluoro Rubber (FPM) to meet the needs of aerospace sealing materials. FPM is a polar rubber with a fully saturated molecular chain side group containing fluorine.

Structural Features:

- High fluorine electronegativity, with a high C-F bond energy.

- Fluorine atoms, with their large radius, are closely arranged on both sides of the main chain, shielding the C-C main chain, providing FPM with high-temperature resistance and corrosion resistance.

2. Performance of FPM

1. Excellent High-Temperature Resistance (Class H):

  - Can operate for extended periods at 250°C.

  - Outstanding weather and ozone aging resistance.

  - Examples of usage duration at different temperatures:

     - 232°C: 3000 hours

     - 260°C: 1000 hours

     - 288°C: 240 hours

     - 316°C: 48 hours

2. Outstanding Oil Resistance (Class K):

  - Fluorine content reaches up to 70%, making it the best among all rubbers in terms of oil resistance.

3. Resistance to Chemicals and Corrosive Media:

  - Resistant to "aqua regia" corrosion, unlike EPDM.

  - Not resistant to water, low-molecular-weight esters, ethers, ketones, and amine compounds.

4. Flame Retardant and Self-Extinguishing:

  - Exhibits flame retardancy higher than Chloroprene Rubber (CR) and is considered a self-extinguishing rubber.

Drawback:

  - Poor elasticity and low-temperature resistance.

The failure of seals used in aviation is often attributed to low temperatures leading to the loss of elasticity and subsequent oil leakage.

FPM has strong intermolecular forces, resulting in poor processability, low flowability, corrosion of molds, and the need for secondary vulcanization.

3. Vulcanization of Fluoro Rubber

Commonly used vulcanizing agents include diamines or organic peroxides. Under the action of an amine, the FPM molecular chain loses fluorine hydrogen, forming a double bond. The double bond then reacts with the diamine to create crosslinks. Common diamines include bis(aminocyclohexyl)methane and bis(aminophenyl)thiourea. Common peroxides include tert-butyl perbenzoate and benzoyl peroxide.

Silicone Rubber

1. Types of Silicone Rubber

Silicone rubber has a main chain of Si-O, with side groups being saturated, non-polar, and organic link structures. Due to its inorganic main chain and organic side groups, it has a hybrid chain structure—semi-organic and semi-inorganic.

Examples of Silicone Rubber Types:**

1). Dimethyl Silicone Rubber (MQ):

  - Side group: (Methyl + Methyl)

2). Methyl Vinyl Silicone Rubber (VMQ):

  - Side group: (Methyl + Vinyl)

  - Characterized by easy vulcanization during the processing.

3). Methyl Vinyl Phenyl Silicone Rubber (PVMQ):

  - Side group: (Methyl + Vinyl) + (Methyl + Phenyl)

  - Good low-temperature resistance and less prone to crystallization.

4). Fluorosilicone Rubber (FMQ):

  - Side group: (Methyl + Methyl) + (Methyl + 2,2-difluoropropyl)

  - Excellent oil resistance.

2. Structure and Properties of Silicone Rubber

Structural and Property Characteristics:

- Long Si-O bonds, large bond angles, low rotational hindrance within the main chain.

- Weak intermolecular forces, resulting in good chain flexibility and low surface energy.

- Si-O bond energy higher than C-C bond energy, contributing to excellent chemical stability, high-temperature resistance, and aging resistance.

Performance:

1). Wide Operating Temperature Range:

  - From -100°C to 350°C, making it the most versatile in terms of temperature.

2). Excellent Resistance to Ozone, Heat, and Natural Aging:

  - Resistant to deterioration caused by ozone, thermal oxidation, and natural aging.

3). Excellent Electrical Insulation:

  - Silicone rubber exhibits excellent electrical insulation properties.

4. Low Surface Energy:

  - Low surface energy, good physiological inertia, and aging resistance, suitable for implantation into the human body.

5). High Breathability:

  - Silicone rubber has high permeability, making it suitable for use in fresh-keeping/separation materials.

Drawbacks:

- Low strength; can be reinforced with white carbon black.

- Poor acid and alkali resistance; also poor oil resistance.

3. Applications

- Household: 20%

- Medical: 25%

- Automotive industry: 40%

- Machinery, construction, instruments: 15%