Contents:
4. Secondary Chemical Strengthening
As demand for high-performance, reliable glass grows across industries, advanced strengthening techniques have become essential for meeting modern safety and durability standards.Today, we examine four primary glass strengthening processes widely employed in construction, consumer electronics, automotive, and specialised applications: physical tempering, semi-tempering, chemical strengthening, and secondary chemical strengthening. Each method offers distinct advantages in strength, stability, and application suitability, reflecting both the rapid advancement of glass processing technology and the industry's demand for safer, more robust materials.
Principle:
Physical tempering is achieved by heating glass to near its softening point (approximately 700°C) followed by rapid cooling (air-cooling). This rapid cooling causes the glass surface to contract quickly while the interior cools more slowly, creating a compressive stress layer on the surface and a tensile stress layer within. This stress distribution significantly enhances the glass's strength and impact resistance.
Process:
Cutting and Edging: The glass is first cut to the required dimensions, and the edges are ground.
Cleaning: Impurities and dust are removed from the glass surface.
Heating: The glass is conveyed into the tempering furnace and heated to its softening point.
Rapid Cooling: The glass surface is uniformly cooled by high-pressure air blowing.
Inspection and Packaging: Quality and strength are verified; compliant glass is packaged.
Strength is 4-5 times that of ordinary glass.
Breaks into small particles upon shattering, offering high safety.
Cannot be reprocessed (cutting, drilling, etc.).
Principle:
The principle of semi-tempering resembles physical tempering, but with slower cooling rates, resulting in lower surface compressive stresses. Internal tensile stresses are also relatively reduced, yielding less pronounced strength enhancement than full tempering.
Process:
Cutting and edging: Identical to fully toughened glass.
Cleaning: Removal of surface impurities.
Heating: Heated to softening point, though at a slightly lower temperature than full tempering.
Slow cooling: Slower cooling rate with lower air pressure.
Inspection and Packaging: Quality checked prior to packaging.
Characteristics:
Strength is 2-3 times that of ordinary glass.
Breaks into larger fragments, yet remains safer than ordinary glass.
Limited secondary processing (e.g. cutting and drilling) is possible.
Distinctions
Strength: Fully toughened > Semi-toughened > Ordinary glass.
Safety: Fully toughened glass breaks into harmless small particles; semi-toughened glass produces larger fragments with blunted edges.
Processability: Fully toughened cannot be reprocessed; semi-toughened permits limited processing.
Applications
Fully Tempered: High-rise building curtain walls, doors and windows, shower enclosures, automotive glass, and other scenarios demanding high safety standards.
Semi-toughened: Interior partitions, furniture, decorative glass, and other applications requiring moderate strength but lower safety standards.
Should you have specific application requirements or wish to explore further details, we would be pleased to discuss this further!
The core principle of chemical glass strengthening is deep ion exchange.Classified by material, single-treatments include primary and secondary tempered glass. Sodium-calcium glass and aluminosilicate glass achieve service requirements with primary treatment, whereas lithium-aluminosilicate glass requires secondary treatment. Post-tempering stress penetration depth for sodium-calcium glass typically ranges from 8μm to 14μm. Chemically tempered aluminosilicate glass achieves 35μm to 45μm, while lithium-aluminosilicate glass reaches approximately 85μm to 200μm after secondary treatment.
Single chemical strengthening is also termed standard strengthening. This process enhances glass strength by altering its surface chemical composition, primarily serving applications demanding exceptional strength, flatness, and thinness—such as smartphone displays, smart wearables, tablet cover glass, aerospace glass, and military-grade glass.
Fundamental Principle
Achieved through ion exchange. Glass is immersed in a molten alkaline salt bath at high temperatures (potassium nitrate KNO₃ for primary strengthening, KNO₃, NaNO₃, LiNO₃, or Na₃PO₄ for secondary strengthening). Smaller-radius alkali metal ions (e.g., Na⁺) in the glass surface exchange with larger-radius alkali metal ions (e.g., K⁺) in the molten salt.Upon entering the glass surface, these larger ions exert greater volume pressure, effectively "crowding" the surface layer. This creates a dense layer of compressive stress within the glass surface.
Process flow:
1. Pre-treatment: Clean the glass surface to ensure it is free from contamination.
2. Heating and Immersion: Immerse the glass in a molten potassium nitrate salt bath at approximately 400°C for several hours to over ten hours.
3. Ion Exchange: During immersion, sodium ions on the glass surface exchange with potassium ions in the molten salt.
4. Cooling and Cleaning: Remove the glass, cool it slowly to room temperature, then wash off surface salt residues.
Key Characteristics:
Exceptional Strength: Features a profoundly deep and uniformly distributed surface compressive stress layer, yielding strength 6–8 times greater than standard glass, or even higher.
Exceptional Flatness: The entire process occurs below the glass softening point, preventing deformation and yielding a perfectly flat surface.
Suitable for thin, small components: Applicable to ultra-thin glass ≤3mm thick.
No risk of spontaneous breakage: Unlike physically toughened glass, it does not exhibit spontaneous breakage issues.
Disadvantages:
High cost: The process is time-consuming, energy-intensive, and requires expensive salt materials.
Unstable stress layer: At elevated temperatures (exceeding tempering temperatures), ions may undergo reverse exchange, potentially reducing strength.
Unsafe fragmentation pattern: When broken, fragments resemble conventional glass—large, sharp shards—and do not qualify as safety glass.
4. Secondary Strengthening (Deep Strengthening)
"Secondary toughened glass" is not a standard materials science term but rather an industry colloquialism. It refers to glass that, after undergoing an initial chemical toughening process, is subjected to a second chemical toughening treatment involving longer duration, higher temperatures, and more specialised salt bath composition. Typically applied to lithium-aluminosilicate glass, the stress depth of chemically toughened aluminosilicate glass can reach 35–45 μm, whereas secondary toughening of lithium-aluminosilicate glass yields a stress depth of approximately 85–200 μm.
The process is as follows: The chemically toughened glass is immersed in a salt bath containing different components (such as caesium nitrate, CsNO₃). A second ion exchange occurs at a higher temperature (e.g., above 450°C) and for a longer duration. The effects are as follows:
Deepening the stress layer: The second exchange promotes ion migration deeper into the glass, significantly increasing the depth of the compressive stress layer (DOL).
Optimising stress distribution: A smoother, deeper stress gradient forms within the glass. This enables the impact force to be effectively absorbed and dispersed by a thicker toughening layer upon impact, preventing inward crack propagation caused by abrupt stress changes.
Key Characteristics and Advantages of Secondary Tempered Glass
1. Exceptional drop resistance: This constitutes its core advantage. The deeper compressive stress layer enables it to withstand greater bending deformation without fracturing, delivering significantly superior performance to standard tempered glass during accidental drops.
2. Superior impact resistance: Enhanced resilience against sharp object impacts, such as corner-first drops or stone strikes.
3. Enhanced surface durability: Whilst the increase in Mohs hardness is not pronounced, the deeper tempering layer ensures that even if minor surface scratches occur, the underlying glass retains high strength support, making it less prone to cracking from the scratch point.
4. Maintains high light transmission: The chemical strengthening process involves ion exchange, which does not alter the glass's inherent optical properties, ensuring extremely high light transmittance remains intact.
However, drawbacks and challenges of secondary-toughened glass
1. Significant cost implications: The process involves greater complexity, extended duration, increased energy consumption, and requires specialised salt bath formulations, resulting in substantially higher costs compared to single-stage chemically toughened glass.
2. Potential issues: Improper process control may result in excessive secondary strengthening, causing slight warping (bending) of the glass itself or introducing invisible micro-defects on the surface, which can adversely affect yield rates.
Typical applications of secondary-toughened glass
Premium flagship smartphones: Certain brands opt for double-toughened glass as screen covers or rear panels to achieve ultimate drop resistance.
Specialised and military equipment: Sectors demanding exceptionally high screen reliability.
Premium automotive sector: Environments such as in-car infotainment screens and instrument panels that must withstand vibration and potential impacts.
Through standard chemical strengthening and secondary strengthening processes, glass exhibits significantly enhanced strength, impact resistance, and service life, delivering more reliable safety assurance for diverse high-end applications. As technology advances, chemical strengthening is increasingly becoming the preferred solution across industries. From consumer electronics to architectural decoration, and from precision optics to transportation equipment, advanced strengthening techniques are driving glass products towards higher performance and greater stability. We remain committed to monitoring industry trends, bringing you the latest and most practical insights into glass strengthening technologies.
Ready to take your glass project to the next level? Contact us today to discuss your custom glass needs and get a quote!
contact us
