Heat soaking could be about to change

Phil Brown, European regulatory marketing manager at Pilkington UK, discusses the benefits of heat soaking toughened glass and outlines potential changes for the heat soaking process.

When glass breaks, it can cause surprise, shock, fear or confusion for those in close proximity to it. Toughened glass in particular can cause alarm when it fractures unexpectedly.  

Toughened glass can break for several reasons, including impact damage (deliberate or accidental), edge damage (during handling), poor installation (tight glazing, missing setting blocks, proud fixings/hard surface contact with glass), poor design (insufficient clearances, structural movement) and inclusions in the glass (foreign particles/impurities). Breakage due to inclusions, more specifically nickel sulphide (NiS) inclusions, are what tend to create most concern. 

Flat glass manufacturers have taken huge steps in recent years to remove nickel from the manufacturing process, however, NiS inclusions can still be present in glass. During the toughening process, any NiS in the glass undergoes a structural change at high temperature, but the rapid cooling during the quenching process does not allow time for it to convert back to its stable form. NiS is effectively ‘frozen’ into toughened glass in its unstable form. 

Over a period of time, the conversion back to its stable form is accompanied by an increase in volume. If the NiS inclusion is sufficiently large and present in the tensile zone of the toughened glass, then the expansion may be sufficient to cause the glass to fracture.

Heat soaking is an additional process that can be carried out on thermally toughened glass to reduce the risk of breakage during service from inclusions in the glass. The process puts the glass through a heat cycle to encourage the glass to break under test if it is at risk of inclusions. The result is a pane with a much lower risk of fracture than ‘ordinary’ thermally toughened glass.

There are a number of applications where toughened glass should be heat soaked. These include safety critical areas such as sloping or horizontal overhead applications or free standing glass protective barriers. In the Pilkington Planar structural glazing system, all toughened glass is heat soaked as standard. Locations where the cost of re-glazing is relatively high may also make heat soaking an economic consideration. 

Heat soaked thermally toughened soda lime silica glass is defined in a European standard, BS EN 14179-1 (Part 1). First published in 2005, this standard specifies the heat soak process, along with requirements for tolerances, flatness, edgework and fragmentation. It sets out a holding phase of 290oC for two hours, with the aim of achieving a residual risk of no more than one breakage per 400 tonnes of heat soaked thermally toughened glass.

A revised version of Part 1 was approved by CEN, the European Committee for Standardisation, in May 2016 and subsequently published as a national standard by BSI, the UK standards body, on August 31, 2016. Several changes have been made in this revision, including edge lift, roller wave distortion and the introduction of the air cushion manufacturing process.

Perhaps the most significant change is the reduction of the glass temperature during the holding stage of the heat soak process cycle. This is based on studies undertaken on NiS re-transformation, examining NiS containing different proportions of sulphur and with different levels of iron contamination. For some critical compositions, this research showed that the conversion rate decreases when the temperature is above 280oC. On this basis, the revised standard introduces a new temperature of 260oC (+/- 10oC), down from 290oC (+/- 10oC). 

In the foreword to the European standard, it’s recommended that all conflicting national standards should be withdrawn by no later than January 2017. So does this mean that processors should start working to the new version of BS EN 14179-1? Not yet. 

This is because Part 2 of EN 14179 – the mandatory aspect of the standard – still refers to the old version of Part 1. Formally, it will not be possible for manufacturers to CE Mark their heat soaked toughened glass based on the new regime until a revised Part 2 is cited in the Official Journal of the European Union. For processors with heat soaking ovens, it may be prudent to prepare for these changes in advance, including commissioning new calibrations. 

It’s certainly worth keeping an eye on developments. The volume of toughened glass requiring heat soaking treatment looks set to increase over the next few years. Our report Building Confidence found that architects and specifiers expect more glass to be used in new build facades, which often require safety glass such as toughened glass, which is increasingly specified as heat soaked, and laminated glass (including laminated toughened glass). Processors need to be on standby to react to the changes of standards such as BS EN 14179 in order to meet growing demand.