There is no such thing as the universal material to meet every single requirement in the laboratory. The decision to use glass or plastics will depend on the application and on the design of the instrument, taking into account the specific properties of the materials, and cost aspects.
Glass excels by its very good chemical resistance against water, saline solutions, acids, alkalis and organic solvents, and exceeds most plastics in this regard. It is only attacked by hydrofluoric acid and at elevated temperatures by strong alkalis and concentrated phosphoric acid. Further advantages of glass are its dimensional stability even at elevated temperatures, and its high transparency.
Specific properties of individual glasses: For the laboratory, various technical glasses with different properties are available.
Soda-lime glass (e.g. AR-Glas®): Soda-lime glass (e.g. AR-Glas®) has good chemical and physical properties. It is suitable for products which are usually subjected to short-term chemical exposure, and to limited thermal stress (e.g. pipettes, culture tubes)
Borosilicate glass (BORO 3.3, BORO 5.0): Borosilicate glass has very good chemical and physical properties. DURAN® (BORO 3.3) is considered the technical all-round glass for applications requiring very good chemical and thermal resistance (including resistance to thermal shock), and high mechanical stability. Typical applications are e.g. components for chemical apparatus, round-bottom flasks, and beakers.
Working with glass
When working with glass, it is essential to consider its limitations regarding resistance to thermal shock and to mechanical stress. Strict safety measures must be observed: Exothermic reactions, such as the diluting of sulphuric acid or dissolving solid alkaline hydroxides must always be carried out under stirring and cooling, and in suitable vessels such as Erlenmeyer flasks – never in graduated cylinders or volumetric flasks!
Do not heat up volumetric instruments, e.g. measuring cylinders and flasks, on heating plates.
Glass instruments must never be exposed to sudden temperature changes. Therefore, never take them out of a drying cabinet while hot, and place them on a cold or even wet lab bench. This applies especially to thick-walled glass instruments such as filtering flasks or desiccators.
When assembling apparatus, support the components in a way to ensure stability and to avoid mechanical stress. To compensate stresses or vibrations, use e.g. PTFE bellows.
Never subject glass instruments to sudden pressure changes; e.g. never admit air abruptly into evacuated glass apparatus. Glass vessels with flat bottom (e.g. Erlenmeyer flasks or flat-bottom flasks) must never be evacuated. The only exceptions are instruments specifically designed for vacuum use (e.g. desiccators, filtering flasks).
Never exert force on stiff stopcocks, ground joints, or glass/tubing connections. Apply effort only in a steady and controlled way upon empty glass instruments; never when they are under pressure or evacuated. Use appropriate safety devices, e.g. protective gloves, goggles, screens, etc.
During the production and processing of glass, hazardous thermal stresses may be introduced. During the cooling of the melted glass, in the range between the upper and lower annealing point, the transition from the plastic state to the brittle state takes place. At this stage, existing thermal stresses can be eliminated through a carefully controlled annealing process.
Once the lower annealing point is reached, the glass may be cooled more rapidly, without introducing any major new stresses. Glass responds in a similar way when heated, e.g. through direct exposure to a Bunsen flame, to a temperature higher than the lower annealing point. Uncontrolled cooling may result in the "freezing in" of thermal stresses which would considerably reduce resistance to breakage and mechanical stability.
To eliminate inherent stresses, glass must be heated up to a temperature between the upper and lower annealing point, be kept at this temperature for approx. 30 minutes and be cooled by observing the prescribed cooling rates.
From a technical viewpoint, glasses behave in an ideally elastic way. This means that, after exceeding the limits of elasticity, tensile and compressive stresses do not result in plastic deformation, but breakage occurs. The tensile strength is relatively low and may be greatly diminished even further by scratches or cracks. For safety reasons, therefore, the tensile strength of DURAN® in apparatus and plant design is calculated at 6 N/mm². The compressive strength, however, is about ten times as high
Resistance to temperature changes
When glass is heated to a temperature below the lower annealing point, the thermal expansion and the poor thermal conductivity result in tensile and compressive stresses within the glass surface. If, due to improper heating-up or cooling rates, the permissible mechanical loads are exceeded, breakage occurs. Apart from the expansion coefficient a, which varies with each kind of glass, the wall thickness, the geometry of the glass body, and any existing scratches must be taken into account. Therefore, it is difficult to state specific numerical values for thermal shock resistance. However, a comparison of the a values shows that DURAN® is much more resistant to thermal changes than e.g. AR-Glas®.
Chemical interaction with water and acids
The interaction of water and acids with the glass surface is negligibly small; only very small amounts, primarily of monovalent ions, are dissolved from the glass. Through this, a very thin, almost non-porous layer of silica gel forms on the glass surface, inhibiting further attack. Exeptions are hydrofluoric acid and hot phosphoric acid which prevent the formation of the inert layer.
Chemical interaction with alkalis
Alkalis attack the glass surface as concentration and temperature increase. In the case of borosilicate glass 3.3 the surface erosion is limited to the µm range; however, after a certain time of exposure, this may result in change of volume or destruction of graduation of volumetric instruments.
Properties of Borosilicate glass 3.3 (DURAN®):
Water resistance: DURAN® meets the requirements of hydrolytic resistance class 1 of glass grains to ISO 719-HGB 1. In the determination by the glass grain titration method to DIN 12 111/ISO 719, after heating of 1g of glass grains in water at 98°C for 1 hour, only 0.026ml of HCl (0.01mol/l) are consumed. This corresponds to an alkali release of 0.008mg Na2O per 1g of glass grains.
Acid resistance: DURAN® meets the requirements of acid resistance class 1. In the determination of acid resistance to DIN 12 116, an exposure of fire-polished DURAN® surfaces to boiling 18% hydrochloric acid for 3 hours results in a weight loss of only 0.3mg/dm2.
Alkali resistance: DURAN® meets the requirements of alkali resistance class 2 to ISO 695-A2. In the determination of alkali resistance to DIN 52 322/ISO 695, an exposure of fire-polished DURAN® surfaces to boiling in a mixture of equal volumes of sodium hydroxide solution (1mol/l) and sodium carbonate solution(0.5mol/l) for 3 hours results in a weight loss of only 134mg/dm2.
Source: BRAND GMBH + CO KG. All rights reserved