Stress corrosion

cracking on acrylic-glass component parts

By stress corrosion cracking is meant chemical and/or electro-chemical corrosion of a material by simultaneous action of a corrosion agent and static tensile stress. In specialist jargon a media-supported stress crack formation is also referred to. The effects are cracks in the acrylic glass which are very fine at the beginning, but which in the final stage can lead to the destruction of the component part.

The molecular chains under stress are attacked by corrosion agents and destroyed. The first indications of the beginning of crack formation are crazed patterns which first become visible as silver shimmer points on the acrylic glass when viewed from a certain angle. Cracks
through chem. actions in the advanced stage are mostly branched off, and cannot be assigned to any exact direction. Mech. cracks, on the other hand, mostly run in one direction and indicate no branch-offs. The acrylic-glass surface seems to be completely in order, since
this crack formation beginning does not leave any detectable traces on the surface, yet it is contributing to preliminary damage.

The windows and Heki domes are manufactured of organic material (acrylic glass). They leave our automatically-controlled production facility with a very low stress levels. With installation of the windows and in use, additional stresses are set up which not do not result in crack formation in proper use. During the usage time, so-called skin stresses are additionally formed on the surfaces through temperature and wet/dry alternations. These are completely harmless and over many years usage time (presupposing material-orientated maintenance) also do not result in damage. However, if cleaning and care agents (corrosion agents) unsuitable for acrylic glass are employed, they will attack the window material.

Material which can be regarded as corrosion agents are e.g. alcohols, certain ten sides, organic solvents and plasticizers, to mention only a few. Such products can also be included in plastic care agents, such as polishing pastes and rubber maintenance products.

Crack formation initially occurs at more highly-stressed locations e.g. in case of windows in the area of the pane strip or near the retention elements. The crack formation is influenced by local action, duration of action, aggressively of the corrosion agent and the temperature. In individual cases, local crack formation can also be determined in the surface only, or at the edges. In case of corrosive rubber maintenance products, the crack formation occurs in the area of the seating face of the sealing rubber.

Caution: If crack formation is determined in the rubber contact zone and a window-pane replacement is implemented, the sealing rubber which has been handled must also be replaced. If the sealing rubber is not replaced, the danger exists that the new pane will also be destroyed.

Also a dilution of aggressive cleaning agents is sometimes helpful. A concentration increase occurs in most cases through the evaporation of the water content after application. Thus crack-causing chem. materials are present in concentrated form!

With proper maintenance, acrylic glass will keep its pore-free surface characteristic over years, and will not become yellow or embrittled. We recommend lukewarm water for cleaning and maintenance of acrylic-glass windows, to which can be added a little household cleaning agent. It is important that work is always carried out with a water surplus. Never clean the windows dry. For rubber maintenance, we recommend talcum powder or white Vaseline.