PROCESS DECRIPTION

Chemical strengthening is the name given to glass products that have been strengthened by means of an ion-exchange process. It is a surface treatment which occurs at a temperature lower than glass melting temperature. This process increases the thermal and mechanical strengths of common annealed soda lime silicate glass without affecting its optical properties. The process is particularly useful for thin glass, tiny glass and shape glass which cannot be tempered by ordinary physical tempering.

The glass to be treated is dipped into a bath of dissolved potassium salts at a temperature about 380oC for duration from 4 to 30 hours, producing an ionic exchange between the superficial sodium ions in the glass and potassium ions inside the bath. The cycle time would be greatly reduced if the glass is made of certain elements such as lithium or magnesium because ion mobility between potassium and these elements is a lot faster. The process parameters such as ion exchanging time and temperature would be modified according to the type of glass to be treated and the required strengthen specification.

The introduction of potassium ions which are larger in size than the sodium ions results in the establishment of a system of residual stress characterized by compression stretches on the surface counterbalanced by traction stretches within the glass

Such a configuration involves different behavior under the action of external stress. In the case of breakage, thermal temper glass breaks up into small blunt pieces whereas chemical strengthen glass breaks into bigger pieces which are not as sharp as a non-toughened glass.

The surface compression condition which is higher in the case of a chemical strengthen glass also involves an increase of flexion resistance, which is one of the main characteristics of chemical strengthen glass.

For a comparison, the following table outlines the flexion resistance values of float glass, thermal temper glass and chemical strengthen glass:

WHY CHEMICAL STRENGTHEN

When glass is submitted to traction stress, the presence of microscopic cracks (fissures) on the glass surface, leads to the apex becoming over stretched which increases the size of the cracks until the glass breaks.

This is the main reason for low mechanical resistance of glass when it is stressed by traction. Industrial methods to improve the mechanical characteristics of glass are based on the introduction of a system of residual stress characterized by a superficial pre-compression condition the aim of which is to contrast the effect due to the presence of the cracks.

This condition of superficial compression is obtained by a toughening process which can be achieved by thermal tempering or chemical strengthening.

In thermal tempering, the residual stress system is obtained through transitory thermal gradients which occur over the melting temperature.

The chemical strengthen process allows some of the limitations of thermal tempering to be overcome. Any shape and thickness of glass can be chemically heat strengthened. It retains the flatness of the glass as the temperature of the process is lower than the glass melting temperature. In addition, high levels of toughening can be achieved.

Chemical strengthening is necessary in the following situations:

1. When the glass thickness is less than 2.5mm, thermal tempering is almost impossible.

2. Certain complex curvatures or dimensional characteristics make thermal toughening impossible.

3. In some industrial or architectural applications where mechanical resistance needs to be higher than usually obtained by thermal tempering.

4. Where impact resistance needs to be higher than the level obtained by traditional thermal tempering.

5. For application in the industrial or automotive sectors where there are high optical requirements or where no superficial distortions on the glass are required.

MECHANICAL PROPERTIES

Heat strengthen glass has excellent impact and flexural (bending) strength. Strength is specified by a Modulus of Rupture (MOR) test, surface compression test, and/or by Depth of Layer (DOL) test.

Chemical strengthened glass is classified by two strength components: surface compression and depth of layer (DOL). Surface compression values relate to flexural (bending) strength (MOR) (see Figure 1), impact strength, hardness penetration (scratching) (see Figure 2) and thermal shock resistance.

For a thermal heat strengthen glass to break it requires a stress 5 times higher than the stress imparted to float glass, to break a chemically toughened glass 10-15 times more stress in required.

In addition to higher flexion resistance, the other important feature of chemically heat strengthen glass is impact resistance.

Figure 3 shows the impact resistance with expression of energy (joules) versus glass thickness. If a thermal heat strengthen glass has an impact resistance of approximately 2.5 times than ordinary float glass (using a steel ball), the resistance of a chemical strengthen glass is 5 times higher than the resistance of float glass.

Processing times and temperatures as well as pre-heat/cool-down procedures affect both values and are determined based upon the application requirements. Because the chemical strengthening process treats the entire surface and all edges, fabrication processes should be completed prior to treatment.

THERMAL PROPERTIES

Heat strengthened glass has a resistance to thermal shock cycling that will vary based upon the actual surface compression created. In applications requiring capability to withstand thermal shock, heat tempered or chemically strengthened glass should be specified.

OPTICAL PROPERTIES

During chemical processing, the glass temperature is lower than glass melting temperature and meanwhile, the glass does not move. So the optical quality is much better than those glass done by thermal tempering which the glass oscillates and travel on ceramic roller at softening temperature.

There will be a visible stress pattern observed when the part is placed between polarizing sheets that are oriented to each other at 90 degrees. This stress pattern is a result of the residual surface and edge compression introduced by heat strengthening.

OTHERS

The values of the international system adopted to indicate the data relevant to the measurement of strength and pressure/stress are as follows:

NEWTON (N) is the force required to impact to a mass of one kilo the acceleration of a meter in a second per second.

Strength: mass x acceleration
F=ma
1 kgf = 1 kg x 9.81 m/s2 = 9.81kg x m/s2 = 9.81N

To simplify the calculation:

1 kgf = 10N

1 KN (Kilonewton) = 103 N
1 MN (Meganewton) = 106 N

PASCAL (Pa) is the relationship between a strength and a surface

1 Pa= 1 N/m2

To define the material’s resistance characteristics, the mega pascal (Mpa) is used.
1 Mpa = 1 N/mm2

The relationship between the international system of measuring and the technical one is decoded by the following equivalence factors:

1 Kg/cm2 =0.1 N/mm2 (MPa) = 10 N/m2 (Pa)

1 N/mm2 (MPa) = 10 Kg/cm2 = 10 N/m2 (Pa)

PSI (Pound per square inch) is used to indicate the surface compression level of the glass after the toughened cycle.

1 Mpa = 145 PSI
1 PSI = 0.07031 kg/cm2

1. Custom design is on request.
2. All data and information listed in this document is subject to final confirmation.