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Glass VS Acrylic: A Comparison

Acrylic glass is often chosen as an alternative for laminated glass. Both Acrylic and laminated glass are transparent building materials which withstand high pressures and can be used for applications like aquariums in marine museums and yacht windows.

Acrylic is a monolithic material made of organic substance, hence its nickname "organic glass". Plexiglas© is a popular brand name for acrylic. Another one is Perspex©.

Laminated glass is made of several layers of mineral glass (also called flat glass, Borosilicate glass or "Echtglas"- real glass, in German). These sheets of glass are separated by transparent liners which are fused together. This makes them particularly strong and prevents inclusions. The materials used for the liner are PVB & SGP. Nowadays SGP is chosen for most applications, because of its hydrophobic properties.

Looking at different physical aspects we want to compare the performance of the two materials.

1. Thermal Conductivity

The thermal conductivity of acrylic is lower than that of laminated glass. In other words the thermal insulation of acrylic is better than of laminated glass.

Material Insulation
Air 0.03 W/mK
Acrylic 0.19 W/mK
Water 0.57 W/mK
Glass, Laminated 0.79 W/mK

Implications on Pool Underwater Windows :

  • No condensation
  • less heat loss of the pool water
  • better insulation of heated buildings
Temperature

Hydrosight Underwater Window in a control room for a ROV. The water is just above 0°C, still there is no condensation on the screen. Click to see more examples

2. Optical Transmission

Acrylic transmits more light than glass. Up to 92% of visible light is transmitted through acrylic. Mineral glass transmits 80-90%, depending on the type of glass and manufacturer. This is especially relevant, when comparing load bearing transparent materials with the same depth. These windows are usually several centimetres thick, which makes differences in the optical transmittance particularly noticeable. Bullet proof glass is a well known type of load-bearing glass and transmits, similarly to other types of mineral glass, a white- to green-type of light. This phenomenon is not observable with Acrylic.

(Update September 08, 2014: Optiwhite is a laminated glass with improved optical transmittance. This is achieved through the addition of iron oxide compounds, which improve the quality of the glass. Optiwhite glazing can withstand pressures of up to 0.1 bar (1000mm water). This means that Optiwhite is only suitable for very small, shallow underwater windows.

Glass needs to be very thick and laminated in order to bear water pressure. It has a greenish tint. Acrylic is optically superior and has no visible tint.

3. Scratch Resistance

The surface of acrylic is softer and more easily scratched than mineral glass. It is advised to check the acrylic cleaning guidelines in order to prevent abrasive or chemical damage. It is easy to polish acrylic in order to remove blemishes. Scuffed mineral glass would on the other hand need to be completely replaced.

The Turkish new build "Dream On" with acrylic ship windows provided by Hydrosight. Click for more examples.

4. Shattering and Cracking

Acrylic has a higher impact strength than glass and does not shatter when exposed to high strains. Shattering into small, blunt chunks is desirable for applications in which tempered glass is used, e.g. in automobiles.
However, shattering is not desired for the use of glazing underwater. The crack resistance is especially valuable, when large underwater windows are craned into their location. The installation of large and heavy glazing bears a certain risk for the installation company when using laminated safety glass. Point loads while hanging on the crane or due to uneven seating might lead to material failure. Acrylic is more tolerant to these forces and rather flexes then breaks. Acrylic's high impact strength, durability and applicability make it the ideal material even when human lives are at stake. Therefore acrylic is used in deep sea submarine windows.

Polycarbonate is also a great, shatter-proof material.

Impact test for an acrylic spheric window, with impact mass of 560kg. Courtesy of J.Stachiw.

5. UV Resistance

Using acrylic outdoors exposes the material to potentially high amounts of ultraviolet radiation (UV). It is a common misconception that UV rays lead to the yellowing of acrylic glazing. This might be true for low quality acrylic. Plexiglas and other well known brands are immune even against high UV radiation. Up to 30 years are guaranteed by the manufacturer.

Safety glass consists of multiple layers of mineral glazing, separated by plastic laminate. Normal plastic laminates are not resistant against UV and are especially sensitive to humidity.

Acrylic and Ultraviolet Light

Acrylic Pool Windows don't Mind UV - this one has been installed by Hydrosight in Italy. Click for More Impressions

6. Weight

Acrylic glass has a lower density, which can range from 1150-1190 kg/m³. This is less than half the density of glass which ranges from 2400 to 2800 kg/m³. Transportation and assembly of acrylic building materials is consequently easier and cheaper.

Installation of a Hydrosight Control Window for a Glacier Water Tank in Island. Click for more examples.

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