WHEATON Technical Data - page 12

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Technical Data, Glass
Sterilization of Glass Containers
Although most types of glass is sterilizable by either steam or dry heat certain techniques
are recommended for specific types of glass. Most Type I borosilicate glass is suitable,
when proper techniques are followed, for sterilization and de-pyrogenation. Type III is not
recommended for repeated steam sterilization, although this may be appropriate on a
single use basis. Recommended autoclave cycles are 121°C @ 15 psi for 20 minutes.
Closures should be left loose on the containers. Proper care must be given when venting
back to atmosphere or there may be damage to the containers.
Dry heat sterilization can be achieved at a temperature of 160°C for 2 to 3 hours, but
glass containers are capable of withstanding sterilization temperatures up to 500°C
without noticeable degradation of the glass. Repeated dry heat sterilization of containers
containing a fair amount of moisture may be susceptible to glass flaking. Inversion of
the container and good ventilation would prevent this from occurring. Inspect glass
containers for chips, cracks and scratches before each use and discard if damage is
evident, as breakage may occur during sterilization if used. Glass containers may also
be sterilized using gas or chemicals. Ethylene oxide (EtO), formaldehyde or peroxide gas
is generally used when heat and pressure cannot be used due to material limitations.
Chemical disinfectants normally used are quaternary ammonium compounds, iodophors,
formalin, benzalkonium chloride and ethanol.
Glass containers may also be sterilized using irradiation, however, the process changes
the color of the glass, which may not be acceptable for most applications. There is glass
tubing available that will not change color when irradiated. This would be available for
those interested in large quantity orders of tubing vials only.
Mold Lubricants and Residues
Modern high-speed mold production of glass containers requires the use of release
agents or coatings on the metal mold equipment to prevent sticking and malformation of
the bottles in the molding process. A variety of coatings or lubricants are used to provide
optimum viscosity and function according to the particular needs of the individual piece
of process equipment as well as service conditions.
The coatings are compounded from colloidal graphite and sulfur suspended in hydrocarbon
oils and waxes with small amounts of modifiers such as calcium soaps and greases. This
“mold dope” is replenished through periodic swabbing of the mold equipment. The hot
forming temperature and the subsequent lehr annealing (1000 – 1100°F) process will
burn off the volatile sulfur and organic oils and waxes. Portions of carbon can remain
since the major component (graphite) is very slowly decomposed and oxidized in the
process. Quality control in manufacturing employs a number of devises (both automatic
and manual) to eliminate the small percentage of product that has excess graphite spots.
Pressure and Vacuum in Glass Vessels
Because the conditions under which glassware is used vary widely, there is no guarantee
against breakage. Always exercise care to protect personnel and property when using
any vessel with vacuum or pressure. Never subject glassware showing visible signs of
damage (chipped, cracked or scratched) to pressure or vacuum.
Weathering of Glass Containers
When glass containers are formed, the surface of the glass is enriched in alkali.
The annealing process further enhances this effect. This phenomena is usually
of no practical consequence and goes unnoticed, but in certain circumstances,
it interferes with further processing of the container. As glass is exposed to the
atmosphere, a complex reaction occurs on the surface between the alkali on the
glass and gasses in the air. These reactions are commonly known as weathering.
The reaction produces salts, which can absorb water from the air. It is these salts
that are the source of surface related decorating problems. Weathering salts are
composed of a mixture of various hydrates of sodium carbonate and sulfate along with
minor amounts of similar calcium salts. Weathering is a normal condition and such salts
are always found on glass surfaces as they are exposed to the atmosphere. The quantity
and crystal appearance will vary depending upon time, humidity and temperatures of
storage. These salts are easily removed by water rinsing.
All glass weathers, but some are more resistant than others. Borosilicate glass is most
resistant, followed by durable soda-lime, and common soda lime. Since glass containers
can be decorated in a myriad of ways, the weathering of glass must be considered in
selecting a method that is effective and trouble free. Below is a weathering chart to be
used as is a rough guide to decorating and labeling of glass.
The surface treatments used to remove weather salts or remove the alkali that cause
weathering are somewhat limited. Since the salts are water soluble, a simple wipe with
a wet cloth or washing prior to decoration or pressure sensitive labeling is effective in
most cases.
Heat and humidity cycling or storing glass in a confined space promotes weathering.
Keeping the glass under constant low humidity is effective in slowing weathering as it
keeps the surface dry and reduces the salt build-up. Dry heating the container just prior
to decorating or pressure sensitive labeling is sometimes successful.
A layer of absorbed moisture on the glass prevents good adhesion of pressure sensitive
labels. Water based adhesives, however, would be no problem. Several possible solutions
to the problem can be suggested:
1. Simply storing the containers in an area of low humidity for several days may solve
the problem.
2. Washing or wiping the ware with warm water. This removes the weathering salts and
allows the achievement of a moisture-free surface.
3. Heating the ware will dry the surface and allow good adhesion of the labels. Heating
will not remove the salts so the heating must be accomplished shortly before labeling.
The presence of dry salts on the ware will not cause a problem, but the salts can again,
rapidly absorb moisture.
Weathering Chart
Borosilicate Soda-Lime
Rate of Weathering
Very slow (months)
Slow under low humidity
Rapid under high humidity
(typically weeks)
Pressure Sensitive
√ Success varies with
amount of weathering
Glue Labels
Ceramic Screen
Organic Screen
√ If not severely weathered
= O.K.)
1...,2,3,4,5,6,7,8,9,10,11 13,14,15,16,17,18,19,20,21,22,...27
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