Glass surface reactions

Introduction – chemistry of the glass surface

Glass for pharmaceutical packaging can be present in different compositions. For parenteral packaging usually Type I Borosilicate glasses are used. The composition is shown in table and figure 1.

These glasses contain a significant amount of boron oxide and aluminum oxide. Additionally in order to make the glass more feasible to convert they can contain Alkali (Na and K) and Alkaline-earth (Mg, Ca, Ba) ions; the amount is dependent on the tubing manufacturer. By adding more Alkali and Alkaline-earth ions to the composition a Type III (Soda-lime glass ) is generated which is usually used for solid, oral or non-Pharma applications.
Fig. 1 Schematic composition of Borosilicate glass
Chemical composition of
Borosilicate glasses with CTE of ~ 5.0
SiO2 72 - 75
B2O3 9 - 11
Al2O3 5 - 7
Na2O/K2O 6 - 9
MgO/CaO/BaO 1 - 3

Tab. 1 Chemical composition of Borosilicate glass
The glass surface can be described as more or less hydrophobic or hydrophilic. In figure 2 the relation between wettability and the water contact angle is shown. Usual Type I Borosilicate glasses are known to have a water contact angle of below 90° being rather hydrophilic (1, 2, 5).
Fig. 2 Schematic comparison of a hydrophilic and hydrophobic surface

Building up a protection layer

As the glass surface is in constant contact with its surrounding a variety of different reactions are possible. As shown in figure 3 the surface area is not a smooth and even surface but rather a surface with several reaction zones. On one hand there are positively charged Hydrogen ions and negatively charged non-binding oxygen ions. Also internal binding flaws like a 3-coordinated silicon atom change the local reactiveness. One significant influencing factor is the water from the air humidity. In the first instance it is adsorbed physically onto the surface. However, it does further react with the silicon dioxide network and by that diffusing into the glass network.
Fig. 3 Different reaction possibilties on the glass surface
The „water film“ that is slowly built on the surface additionally collects particles from the surrounding like organic molecules and other adsorbates (3, 4). This layer acts like a lubricant protecting the glass surface from scratches and making it smoother.
Fig. 4 Different layers that build up on the glass surface

Building up a protection layer

When the glass is freshly heated or cleaned the surface possesses a quite high free energy making it most hydrophilic with water contact angles down to a few degrees only. By the time elapsing the surface reacts with adsorbates from the surrounding like e.g. water from humidity and saturates with the same. In figure 5 the reaction of water with the surface is shown schematically. First the water is only adsorbed physically. However, after a few weeks the water diffuses into the surface as described in figure 4. This adsorption layer acts like a cover on the glass protecting the glass from scratches.
Fig. 5 Reaction of water with a clean glass surface
But this layer does not only protect the glass from scratches it also plays a significant role as a lubricant decreasing the friction between two glasses or glass and metal. Particularly in the Pharmaceutical filling lines this property is quite important as it can limit the speed of the filling lines and the amount of scratches the containers receive in the process. The negative results can e.g. be seen when two vials are sticking together.

This development can also be followed by measuring the roughness which shows that in a clean state the roughness is higher than in a state were the surface is saturated with a lubricant layer (see figure 6).

The protection layer influences the friction of the glass

In figure 6 a measurement from SCHOTT Pharma Services is shown comparing the friction (basically the roughness of the surface) of three vial pairs after production, after storage for several months, after cleaning and after sterilization at 300°C for 30 min. It is clearly visible that after storage for several months the friction is significantly decreased. However, after cleaning or depyrogenation resp. this layer is basically evaporated causing a higher friction again.

This phenomenon of the building and evaporation of the protection layer is a phenomenon equally valid for all Borosilicate glasses. There is no difference correlated to the composition of the glasses. This also means that scratches and sticking behavior is rather a matter of glass handling than of the glass composition itself.
Fig. 6 Friction measurement on glass vials after different treatments
Dr. Bettine Boltres
Product Manager Pharmaceutical Tubing
Business Segment Tubing
SCHOTT AG, site Mitterteich

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Reference list:

  1. Palzer, S. et al. Einfluss der Rauhigkeit einer Feststoffoberfläche auf den Kontaktwinkel. Chemie Ingenieur Technik 2001 (73): 1032 – 1038
  2. Tessmer I., et al. Comparison of adhesive properties of different surface coatings using atomic force microscopy (AFM) – Report
  3. Birch, W. et al. Influence of cleaning on the surface of model glasses and their sensitivity to organic contamination. Surface Contamination and Cleaning, Vol. 1 (2003): 85–107
  4. Trier F., Ranke U.. The Glass Surface and Ways of its Modification.
  5. Sumner A., L., et al. The nature of water on surfaces of laboratory systems and implications for heterogeneous chemistry in the troposphere. Phys. Chem. Chem. Phys. 2004 (6): 604 – 613
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