Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 124

The conveyor belt transports the container to the annealing lehr for stress‐relaxation. Once the container has left the IS‐machine, a first inspection often takes place with cameras that record the infrared images of the hot containers. From these images, defects can be identified and immediate corrections of the process can be initiated. This is very beneficial as the feedback between defects and applied corrections is direct and without the time delay that would occur if the container were first annealed and then inspected.

Before entering the annealing lehr, the outside body (not the finish) of the containers receives a hot‐end coating of a 5–15 nm thickness. Even so thin, the hot‐end coating serves different important functions. It first saturates the highly reactive surface bonds that are present at the surface of the new glass container. It also provides a surface suitable for good adhesion of the cold‐end coating, which is applied later. Furthermore, it may slightly increase the strength of the container by disabling surface flaws that have been introduced during the forming process.

As precursor for hot‐end coating most frequently used is monobutyltin‐trichloride, C₄H₉SnCl₃ (MBTC) or tin‐tetrachloride, which both gives rise to a SnO₂ coating on the container. The process is chemical vapor deposition under air at atmospheric pressure (atmospheric CVD) and is supported by the moisture of the air. The reactions that take place are:

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(7)

Other precursors based on titanium or titanium‐silicium are also in development to yield a TiO₂ or TiO₂‐SiO₂ coating. As explained, after forming and hot‐end transport, the containers from different sections experience different cooling whereas the surfaces of a given container cool faster than its bulk, the rate being higher for the outer than for the inner surface. These differences create tensile stresses in the container, which can lead to spontaneous breakage. Containers are thus reheated in the glass transition range in a continuous‐annealing lehr long enough to ensure complete stress relaxation. The annealing times depend on the size of the containers, but are typically between 45 and 60 minutes. The containers can then cool down to room temperature homogeneously.