Development of a shatter protection coating for laboratory glass articles
28.08.2008
Initial situation
Glass is the preferred material for the packaging and handling of liquid chemicals as well as medical, biological and pharmaceutical samples. It is basically inert, transparent and has a surface which is easy to clean. Its disadvantage is its tendency to breakage.
Why protect bottles and flasks?
When a bottle containing dangerous chemicals or infectious substances (mineral acids, strong alkali, bactericidal substances etc.) is dropped, the consequences range from widespread contamination of glass and chemical contents in sensitive areas, to a worst case scenario of serious injury of laboratory personnel.
A protective coating on the bottle contains all the glass and hazardous chemicals in case of an accident, minimizes risk and contamination, and makes clearing up the hazard much easier.
Why should a protective coating be steam-sterilizable?
In order to prevent biological, medical and pharmaceutical samples from pathogenic germs, the bottles and flasks have to be decontaminated by steam sterilization. Since the BSE crisis it has become generally accepted that the steam sterilization process has to be carried out at 134 °C in order to kill the BSE prions. In case of dangerous contents the glass containers need a plastic coating. The plastic envelope should be transparent and must have a good temperature resistance in order to endure the steam sterilization process.
Until 1994, a fluidized bed coating powder on the basis of ethylene vinyl alcohol copolymer resins produced by Bayer AG was available, which was the only shatter guard system on the market that was steam-sterilizable at 134 °C. For economic reasons Bayer AG stopped the production of this coating system in 1994. At present there is no steam-sterilizable coating available. But the market (biotechnology, pharmaceutical and medical technology) still demands glass bottles and flasks with a steam-sterilizable envelope.
Development of a steam-sterilizable transparent plastic coating
Our aim was the development of a transparent plastic coating with good temperature resistance as a shatter guard system for laboratory glass articles. The first task was to look for an appropriate coating material with high temperature resistance. The second task was to develop a primer based on hybrid polymer materials which increases or adjusts the adhesion of the coatings on glass.
The plastic coating material must have a melting point above 134 °C, a good water vapor stability, toughness and chemical resistance. The coating system should be able to build up a layer thickness of 500 µm or more for high stability. At present a thick covering of DuPont Surlyn® is used as standard shatter-proof system. But Surlyn®, an ethylene and methacrylacid copolymer, has a melting point below 100 °C and is, therefore, not steam-sterilizable.
Thermo-setting coating materials like some electrostatic coating powders have a high shrinkage while baking. This means that layers can be only produced between 50 and 150 µm thickness without cracks in the coating. To achieve a higher layer thickness, thermoplastic coating materials are advantageous because of their lower shrinkage and tendency to cracking.
A thermoplastic powder coating material based on Polyamide 12 is available on the market with a high melting point (176 °C), low water absorption, high chemical resistance and low water vapor permeability. This material can be used together with the fluidized bed coating technology to make a shatter resistant plastic envelope.
The fluidized bed coating process
Since 1952, the fluidized bed coating technology has been used for powder coatings. This technology combines fluidized bed and dipping processes. Preheated objects (surface temperature above the melting temperature of the coating powder) are dipped into a powder bed that is fluidized by compressed air. Impinging powder particles melt and adhere to the surface. Depending on the object to be coated and its specific heat and temperature, the coating material becomes a persistent film after some time or after renewed heating.
Development of a primer based on hybrid polymer materials
Some time, typically a week after the coating process with PA 12, the adhesion of the PA 12 coating on the glass surface decreases or is lost completely. This depends on the humidity of the environment. Therefore, it is necessary to improve the adhesion by a primer.
During the past years ISC scientists have developed hybrid polymers with good adhesion to glass. These materials are based on a sol-gel process using silanes as precursors. Si- OH-groups are responsible for the direct bonding to the Si- OH-groups of the glass surface.
These hybrid polymers contain organic moieties to which reactive groups of the PA 12 can bond.
A major goal of the project has been the adjustment of the chemical composition of the hybrid polymers in order to optimize the bonds to both the inorganic glass surface and the polyamid coating.
A formulation was found which as a primer increases the long-term adhesion of the PA 12 coating to the glass surfaces under ordinary climatic conditions but particularly also in humid atmospheres (14 days condensation constant climate, DIN 50017). This formulation also withstands the harsh process of steam sterilization (134 °C, 3 bar of water vapor atmosphere) without loss of adhesion.
Conclusion
A powder coating material was found that endures the steam sterilization process. A primer based on hybrid polymers was developed to realize good bonding between the plastic coating and the glass surface. The final investigations in this project concentrate on the increase of the layer thickness of the plastic coating in order to enhance the protective effect.
Your contact
Karl J. Deichmann
Phone +49 (0)931 4100-624
Fax +49 (0)931 4100-698
E-Mail: deichmann@isc.fraunhofer.de
Dr. Johanna Kron
Phone +49 (0)931 4100-631
Fax +49 (0)931 4100-698
E-Mail: krpon@isc.fraunhofer.de
Dr. Karl-Heinz Lochner
Phone +49 (0)9342 9221-713
Fax +49 (0)9342 9221-799
E-Mail: lochner@isc.fraunhofer.de