Thermal Shock - An Explanation of Why Glass Breakage Occurs and How to Prevent It

Alper Karairmak, Aegg's Technical Engineering Manager for glass packaging gives the lowdown in this technical article...

What is Thermal Shock

When a glass container is heated from the outside or is filled with a hot liquid, there is an immediate difference between the outside and the inside temperatures. This difference is denoted as ΔT, and due to this difference, stresses develop in the glass through the mechanism outlined below.

The length the glass would like to be if not constrained:

When Thermal Shock Happens

If the top surface was physically constrained by something when it was trying to expand, compressive stresses would build up within the glass.
Similarly, if the lower surface was prevented from shrinking when it was trying to contract, tensile stresses would build up in the glass.
So, if we pour hot liquid into a jar, the inside surface will have compressive stresses, and the outside will go into tension. The higher the temperature differential, the greater the stresses and this can result in breakage.
Normally, thermal shock breakage has a very simple pattern, with a crack that circles the base and rises up the body of the container, as illustrated below.

Explaining Stress Levels

The level of stress developed by this temperature difference is dependant on how much the glass wants to expand and is therefore dependant on the coefficient of expansion. For Pyrex, a borosilicate glass, there is very little expansion over the first 300°C, so stresses are minimal. Hence why you can pour boiling water into it or put it in the oven without it breaking.

For normal soda–lime–silicate container glass, which tends to expand significantly, the standard test allows for a temperature differential of 42°C. This is tested in the factory by immersing containers in hot water for 5 minutes and then plunging them into cold water. The tensile stresses generated in the outside surface can then open up any micro-cracks or manufacturing imperfections, if they are present, to cause failure. This simulates the same mechanism as hot filling, with hot liquid on the inside and a colder outer surface.

Bottles tend to be more resilient if heated from the outside, as the inner surface is blown and free from some of the manufacturing imperfections caused by moulds or general handling damage that may weaken the glass.

Ways to Mitigate Thermal Shock

• Add ingredients to the glass batch that reduce the coefficient of expansion.
  
  
• Reduce surface imperfections, namely surface fractures, deep baffle marks or deep mould joints.
  
  
• Test more rigorously to ensure actions taken have improved the glass strength. This is done by increasing the temperature differential (ΔT) to 60°C - 65°C to further stress the glass, which helps identify any issues in production.
  
  
• Pre-heat (condition) bottles before hot filling. This is done by storing pallets of glass inside during cold weather, and possibly pre-heating containers on the filling line before filling.
  

For more information on this article, please get in touch

This article is part of Aegg's Technical Articles series, explaining the technical aspects of glass and plastic packaging for the food and drinks industry.  Click the link to read more articles from the series.

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