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Silicones, Silicates, and Glass

Table of Contents

Overview

This chapter focuses on three closely related classes of inorganic materials that are all based on silicon and oxygen: silicones, silicates, and glass. Although they share common building blocks, differences in chemical bonding and structure lead to very different properties and applications.

We will:

Silicones

Chemical Nature and Basic Structure

Silicones (more precisely: polysiloxanes) are synthetic polymers with an inorganic backbone and organic substituents.

The characteristic structural motif is the siloxane group:

$$
\mathrm{Si{-}O{-}Si}
$$

A typical linear silicone polymer can be represented as:

$$
\mathrm{[-Si(R_2)-O-]_n}
$$

where:

Common example: polydimethylsiloxane (PDMS)

$$
\mathrm{[-Si(CH_3)_2-O-]_n}
$$

Key points:

Structural Variants

Silicones can have different architectures:

The degree of branching and crosslinking strongly influences mechanical properties:

Important Properties and Their Structural Origins

  1. Thermal stability
    • Si–O bonds are strong and have high bond energies.
    • Result: Silicones are stable over a wide temperature range and resist thermal degradation better than many organic polymers.
  2. Flexibility and low glass transition temperature
    • Si–O–Si bond angles can vary and rotate relatively freely.
    • The backbone is highly flexible at the molecular level.
    • Result: Many silicones remain flexible at low temperatures and behave as fluids or soft elastomers.
  3. Hydrophobic and water-repellent
    • Organic substituents (e.g., $\mathrm{CH_3}$ groups) form a nonpolar surface.
    • Result: Water does not wet silicone surfaces easily; silicones often act as water repellents and release agents.
  4. Chemical resistance
    • Resist many chemicals and weathering (UV, oxygen).
    • However, strong acids and bases can cleave Si–O bonds.
  5. Electrical insulation
    • Silicones are good electrical insulators.
    • Combined with thermal stability, this makes them valuable as insulating materials under demanding conditions.
  6. Low surface tension and low viscosity change with temperature
    • Leads to good spreading and lubricating behavior.
    • Viscosity remains relatively constant over a wide temperature range.

Production (Outline Only)

Silicone production typically involves:

  1. Forming chlorosilanes (e.g., $\mathrm{SiCl_4}$, $\mathrm{(CH_3)_2SiCl_2}$) from elemental silicon and organic halides.
  2. Hydrolysis of chlorosilanes to silanols ($\mathrm{Si{-}OH}$) and subsequent condensation to siloxanes ($\mathrm{Si{-}O{-}Si}$).
  3. Polymerization and crosslinking to the desired molecular weight and network structure.

Details of industrial process engineering are treated elsewhere; here the focus is on the resulting material classes.

Applications of Silicones

Silicones are used wherever combinations of flexibility, thermal resistance, chemical resistance, hydrophobicity, and electrical insulation are needed.

Typical examples:

Structure–property link:

Silicates

General Structural Principles

Silicates are inorganic compounds built from silicon–oxygen tetrahedra:

$$
\mathrm{[SiO_4]^{4-}}
$$

The way in which these tetrahedra connect (by sharing oxygen atoms) leads to a variety of silicate structures. Each shared oxygen reduces the negative charge per tetrahedron, and the resulting anionic framework is charge-balanced by metal cations (e.g., $\mathrm{Na^+}$, $\mathrm{K^+}$, $\mathrm{Ca^{2+}}$, $\mathrm{Mg^{2+}}$, $\mathrm{Al^{3+}}$).

Types of Silicate Structures

The classification of silicates is based on how many corners (oxygen atoms) of the tetrahedra are shared:

  1. Nesosilicates (orthosilicates) – isolated tetrahedra
    • Tetrahedra do not share corners with each other.
    • Basic unit: $\mathrm{[SiO_4]^{4-}}$.
    • Cations link and charge-balance isolated tetrahedra.
    • Typically form dense, hard minerals.
  2. Sorosilicates – double tetrahedra
    • Two tetrahedra share one oxygen atom:
      $$
      \mathrm{[Si_2O_7]^{6-}}
      $$
    • Less common subgroup.
  3. Cyclosilicates – ring silicates
    • Tetrahedra share two oxygens each, forming ring structures.
    • Example unit: $\mathrm{[Si_6O_{18}]^{12-}}$ (six-membered ring).
  4. Inosilicates – chain silicates
    • Tetrahedra share two oxygens in a repeating pattern to form chains.
    • Single chains:
      $$
      \mathrm{[SiO_3]^{2-}}_n \quad \text{(often written as } \mathrm{[Si_2O_6]^{4-}_n})
      $$
    • Double chains: more complex sharing patterns.
  5. Phyllosilicates – sheet silicates
    • Each tetrahedron shares three oxygens.
    • Form 2D sheets with composition approximating:
      $$
      \mathrm{[Si_2O_5]^{2-}}_n
      $$
    • Interlayers can contain cations or water molecules.
  6. Tectosilicates – framework silicates
    • Each tetrahedron shares all four oxygens with neighbors.
    • 3D framework with overall composition:
      $$
      \mathrm{[SiO_2]_n}
      $$
      or frameworks with partial substitution (e.g., $\mathrm{Al^{3+}}$ for $\mathrm{Si^{4+}}$) and compensating cations (e.g., feldspars, zeolites).

Important Natural Silicates

Silicates dominate the Earth's crust and mantle. Examples:

These natural silicates are key components of rocks, soils, and sediments, and they influence many environmental and geological processes.

Industrially Used Silicates

Besides naturally occurring minerals, silicates are processed and modified for technological purposes.

Examples:

Structure–Property Relationships in Silicates

Glass

What Is Glass?

Glass is an amorphous (non-crystalline) solid that forms when certain melts are cooled so quickly or under such conditions that a regular crystal lattice cannot develop. The atoms are frozen into a disordered arrangement, somewhat like a "solidified liquid."

Although various types of glass exist (including non-silicate glasses), the most important and widespread are silicate glasses, particularly soda–lime–silica glass.

Structure of Silicate Glass

Silicate glass is based on $\mathrm{SiO_4}$ tetrahedra, as in silicates, but:

Network Formers and Network Modifiers

Silicate glass can be understood as a network modified by additional cations:

  1. Network formers
    • Primarily $\mathrm{SiO_2}$ (silica).
    • Other possible network formers in specialized glasses: $\mathrm{B_2O_3}$ (boron oxide), $\mathrm{P_2O_5}$ (phosphorus pentoxide).
  2. Network modifiers
    • Cations such as $\mathrm{Na^+}$, $\mathrm{K^+}$, $\mathrm{Ca^{2+}}$, $\mathrm{Mg^{2+}}$, introduced via oxides (e.g., $\mathrm{Na_2O}$, $\mathrm{CaO}$).
    • They break some Si–O–Si linkages, creating non-bridging oxygens and modifying the network properties.

As a result:

Common Types of Silicate Glass

Soda–Lime–Silica Glass

The most common commercial glass type.

Typical composition (approximate):

Properties:

Uses:

Borosilicate Glass

Contains significant amounts of $\mathrm{B_2O_3}$ in addition to $\mathrm{SiO_2}$.

Typical features:

Uses:

Lead Glass and Other Specialty Glasses

Glass Formation and Processing (Conceptual)

Glass Formation

A glassy structure can form if:

Silicate glass is particularly suitable for vitrification because the $\mathrm{SiO_4}$ network can easily adopt a disordered structure.

Melting and Forming

Typical industrial steps for soda–lime–silica glass:

  1. Batch materials
    • Silica sand ($\mathrm{SiO_2}$).
    • Sodium carbonate ($\mathrm{Na_2CO_3}$) → provides $\mathrm{Na_2O}$.
    • Calcium carbonate ($\mathrm{CaCO_3}$) → provides $\mathrm{CaO}$.
    • Minor additives (colorants, refining agents).
  2. Melting
    • Materials are melted in furnaces at high temperatures (over 1400 °C).
    • Carbonates decompose and oxides mix to form a homogeneous melt.
  3. Forming into shapes
    • Float glass process for flat glass: the melt is floated on a bath of molten tin to make uniform sheets.
    • Blow-and-blow or press-and-blow processes for containers.
    • Drawing, pressing, or fiber-forming methods for other shapes.
  4. Controlled cooling (annealing)
    • The glass is cooled carefully to relieve internal stresses.

Properties of Glass and Their Origins

  1. Transparency
    • The disordered structure and electronic band structure allow visible light to pass through with minimal absorption (for colorless compositions).
    • Lack of grain boundaries (as in polycrystalline materials) reduces scattering.
  2. Hardness and brittleness
    • The rigid, continuous network yields high hardness and scratch resistance.
    • However, limited plastic deformation leads to brittle fracture: cracks can propagate suddenly.
  3. Chemical resistance
    • Silicate networks resist many chemicals.
    • Sensitive to strong bases, which can attack the silicate network.
    • Ion exchange at the surface (e.g., leaching of alkali ions) can influence durability.
  4. Thermal properties
    • Above the glass transition temperature, glass softens continuously rather than melting sharply.
    • Thermal expansion can be tuned via composition (soda–lime vs. borosilicate glass).
    • Resistance to thermal shock depends on both expansion coefficient and thickness.
  5. Electrical insulation
    • Silicate glasses are good electrical insulators at room temperature.
    • Used in insulators and dielectric applications.

Comparison: Silicones, Silicates, and Glass

Although all are based on silicon and oxygen, their bonding and structures differ fundamentally:

Understanding how the same elements, silicon and oxygen, can form materials as different as flexible silicone rubber, hard crystalline silicate minerals, and transparent glass illustrates the central role of atomic structure and bonding in determining material properties.

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