Inside the Glass: Exploring the Multifaceted Role of Borates in Innovation

In our March article, we explored how boric acid enhances glass performance — acting as a flux, improving tensile strength, and boosting thermal stability and refractive index. You can revisit that post here:👉 Boric Acid and Its Applications in the Glass Industry

Today, we dive deeper — uncovering how borates play distinct yet critical roles in different glass types, from insulation fiber glass to borosilicate and optical glass.

I. Core Functions and Application Categories of Borates in Glass

In glass manufacturing, boron oxide—added in the form of borates—plays an irreplaceable or high-cost-substitute role. Its core functions include:

• Acting as a flux to lower processing temperatures (especially when alkali addition is restricted)

• Providing low thermal expansion for improved thermal shock resistance

• Enhancing chemical stability

• Reducing electrical conductivity

Borate applications in glass can be broadly classified into three main categories—insulation fiber glass, textile glass fiber, and borosilicate glass—along with other specialty glass types. Each category achieves specific performance characteristics by precisely adjusting borate content and form.

II. Three Core Glass Applications and Technical Details

(1) Insulation Fiber Glass (IFG, also known as Glass Wool)

Application & Structure:Widely used as a primary insulation material in the construction industry, with limited use in industrial, automotive, and household applications. It consists of ultra-fine fibers (~5 µm diameter) spun from melted glass in electric or gas furnaces. Insulation and soundproofing are achieved by trapping air and reducing infrared radiation transfer. Performance depends on:

• Fiber diameter (smaller is better)

• Bulk density (higher is better)

• Material thickness (thicker is better)

Acoustic performance can be optimized by increasing density, thickness, and reducing fiber diameter.

Borate Role & Parameters:Typical borate content: 4–7%, mainly added as borax pentahydrate or boron-containing minerals (e.g., ulexite).Functions include:

• Assisting in glass melting and preventing devitrification

• Enhancing water resistance (avoiding fiber breakage or binder degradation)

• Improving recovery after compression (reducing transport volume)

• Increasing infrared absorption (enhancing thermal performance and production efficiency)

• Improving fiber solubility in lung fluid (enhancing inhalation safety)

Production Requirements:Produced with Cr/Ni superalloy spinnerets at up to 1050 °C; fiberization viscosity must be suitable for 1050 °C, with a liquidus temperature ≤ 100 °C to prevent crystallization at spinnerets. Formula adjustments are made based on cost and capacity needs.

(2) Textile Fiber Glass (Continuous Filament Fiber Glass)

Product Type & Features:Mainly E-glass (alumino-borosilicate calcium glass), characterized by low or zero alkali content (≤ 1.0–1.5%) and fiber diameters of 7–25 µm. It combines high tensile strength and durability. Despite technical advances, borates remain key fluxing agents that help lower melting and fiberizing temperatures while maintaining viscosity and controlling fiber breakage.

Borate Addition Forms:Added as colemanite, boric acid, or borax pentahydrate (within alkali limits). These lower melting points and raw material costs.

Main Applications:Used in:

• Automotive composites (engine covers, body panels)

• Durable consumer goods (appliances, sports equipment)

• Construction (showers, flooring)

• Industrial equipment (tubes, tanks)

• Electrical and electronic components (circuit boards, connectors)

• Shipbuilding, aerospace, roofing, reinforced PVC flooring, gypsum boards, filters, and concrete reinforcement.

(3) Borosilicate Glass (B₂O₃ content 5–20%)

Characterized by high boron content, borosilicate glass combines thermal shock resistance, chemical durability, and low thermal expansion. It can be subdivided into several major applications:

1. “PYREX® Type” Glass:

   Contains ~12.5% B₂O₃; expansion coefficient 33 × 10⁻⁶ /K (4.5% B₂O₃ equivalent). A blend of boric acid and borax pentahydrate.

   Used in cookware, microwave ware, laboratory glassware, and lighting tubes.

   Features excellent thermal shock resistance, chemical durability, and physical strength.

   
   

2. Neutral / Pharmaceutical Glass:

   Contains 8–10% B₂O₃; structure stabilized with Al₂O₃ (to prevent phase separation and crystallization), BaO, CaO, and K₂O.

   Ideal ratio: 1:3 ([BO₄] network units), minimizing non-bridging oxygen for chemical stability.

   Sterilizable above 120 °C, with minimal alkali release.

   Applications: ampoules, vials, cosmetic containers, thermos bottles.

   Typically produced via tubing draw processes (clear/white/brown).

   
   

3. Lighting Glass:

   Around 8% B₂O₃ for sealed-beam headlights (high resistance, chemical durability, and heat shock tolerance).

   High-pressure mercury lamps use hard borosilicate glass for molybdenum sealing.

   Sodium-resistant glasses (for sodium vapor lamps) contain up to 48% B₂O₃—the highest in commercial glasses.

   Also used in fluorescent lamp tubes and sodium glass bulbs to improve melting and strength.

   
   

4. TFT-LCD Glass:

   Alkali-free boroaluminosilicate with ~10% B₂O₃ (as boric acid or B₂O₃).

   Maintains optimal viscosity and liquidus temperature for float or overflow fusion production of 0.7 mm sheets.

   Used for display panels in TVs and monitors.

   
   

5. Sealing Glass:

   Borosilicate glass for glass–glass or glass–metal seals (like low-temperature soldering).

   Must be engineered for specific softening temperature, viscosity, and expansion.

   Hard borosilicate glass is used for stress-free sealing with tungsten/molybdenum in lamps and electronics.

   Glass frits (PbO-based, low-softening) serve as solder coatings.

   
   

6. Heat-Resistant Glass (Cookware / Laboratory):

   Expansion coefficient 33–45 × 10⁻⁷ /°C (lower than soda-lime glass).

   Flat borosilicate glass made by float process for fire-resistant architectural use.

   
   

III. Other Types of Borate Glass

1. Optical Glass:

   Contains 0–40% B₂O₃ (rare-earth borate glass). Refractive index and light dispersion can be customized.

   Often produced in platinum-lined tanks to avoid contamination. Used for lenses, prisms, photomasks, and IC lithography.

   
   

2. Glass Microspheres:

   • Solid microspheres: low refractive index (~1.5, soda-lime) for road reflectors; high index (1.9–2.5) from B₂O₃/TiO₂/BaO compositions (3–5% B₂O₃).

   • Hollow microspheres: lightweight, high compressive strength, excellent thermal/acoustic insulation; used as fillers in composites.

     
     

3. Glass Ceramics:

   Produced via controlled crystallization of glass; nonporous and customizable.

   Some contain 8.5% B₂O₃ for machinability; used in precision electrical insulators and telescope frames.

   New B–Al–Ca/Sr/Ba glass ceramics (< 1000 °C sintering) offer low expansion, high strength, and electrical performance—ideal for microelectronic substrates and seals.

   
   

4. Art Glass:

   Modified soda-lime-silicate formulations with added borates to improve workability for decorative applications.

   
   

5. Soda-Lime-Silicate Glass:

   Accounts for over 80% of global glass production (containers, flat glass).

   Modern formulations rarely include borates—only 0.3–1.5% in premium containers (e.g., cosmetic bottles) to enhance moisture resistance and furnace efficiency.

   
   

6. Vycor Glass:

90% SiO₂, ultra-low expansion (0.8 × 10⁻⁶ /°C), high thermal shock and chemical resistance.Produced by forming 30% borosilicate glass, leaching out non-silica components, and heat-consolidating into a transparent product.Applications: quartz–Pyrex joints, biotechnological porous materials, aerospace heat shields, and lamp covers.

7. Space Protection Glass:

   Cesium borosilicate glass used for satellite solar cells to resist space radiation.

   
   

8. New Specialty Glasses:

   Includes photochromic, photosensitive, laser, and conductive coating glasses, as well as graded-index types.

   Automotive (large, heat-insulating windows) and architectural (low-emission coatings) markets drive innovation.

   Optical glass dominates in TV, ophthalmic, and fiber-optic industries supported by optoelectronic R&D.