City Plastics, Glazing

City Plastics Glazing

Acrylic sheet is commonly used as a clear glazing material. Benefits over glass are light weight, clarity and high light transmission.

Light Transmission
Clear acrylic sheet has better than glass clarity and 92% visible light transmission.

Sound Transmission
A barrier’s sound reduction depends on its weight, surface area, rigidity, and air tightness. Air tightness is important, acrylic sheet with its good sound reduction may not improve the performance of a non-airtight window. In airtight surroundings, sound reduction increases with increased sheet thickness and decreases with increased area. Doubling the area increases transmission several decibels. Use the following values of sound pressure and loudness reduction for acrylic in airtight, resilient settings.

Sound Reduction

	Average Sound Transmission (dB) Loss 100-3200 Hz	STC Rating
3.0mm (1/8”)	22	23
4.5mm (3/16”)	25	26
6.0mm (1/4”)	28	28
9.5mm (3/8”)	30	30

Thermal Conductivity
A material’s thermal conductivity, or ability to conduct heat, is called the k-factor. This is an inherent property independent of thickness and surroundings.

The k-factor of Acrylic sheet =	1.3 BTU
	(hr.)(sq. ft.)(°F/in. thick)

The k-factor is a physical property of a material.
The U-factor, or overall heat transmission coefficient, is the amount of heat which will pass through one square foot of a specific thickness of material per hour. This factor, expressed in BTUs, measures heat loss or gain through a window.
U-factors depend on specific conditions (e.g., single or double-glazing) and differ for summer and winter. Listed below are U-factors for several thicknesses of acrylic sheet for single-glazed, vertical installations.

U-Factors—BTU/(hour)(sq.ft.)(°F)
Acrylic Sheet Thickness	Summer Conditions	Winter Conditions
3.0mm (1/8”)	0.98	1.06
4.5mm (3/16”)	0.94	1.02
6.0mm (1/4”)	0.90	0.97
9.5mm (3/8”)	0.83	0.89

Total heat loss or gain through a window due to temperature differences, expressed in BTUs/hr., = window area, sq. ft. x (indoor temp-outdoor temp)°F x U-factor (from above table). Include solar-radiated heat gain within a structure to find total heat gain.

Acrylic sheet insulates better than glass. When single-glazed, its U-factor is about 10% lower than that of equally thick glass. This material is also more resistant than glass to thermal shock and stresses caused by temperature differences between the opposite surfaces of a window.

Hardness and Rigidity
Handle and clean acrylic sheet carefully. The surfaces are not as hard as glass. Acrylic sheet is less rigid than glass or metals. Under wind load, deflection causes acrylic to bow and foreshorten.
Consider maximum wind load and window size in selecting glazing panel thickness. Forming acrylic sheet into ribbed or domed shapes improves rigidity and reduces deflection. Large, flat acrylic sheets, if insufficiently supported, may deform permanently due to their weight or continuous loads. Increasing rigidity by forming minimizes cold flow.

Strength and Stresses
Though acrylic sheet has a tensile strength that is high at 74 MPA, continuous loads can cause crazing (numerous tiny cracks in the material). Avoid localized, concentrated stresses. For this reason, and due to thermal expansion and contraction, install panels 1200 mm x 1200 mm or larger in frames, do not use screws or bolts.

Expansion and Contraction
Like most plastics, acrylic sheet expands and contracts 3 to 8 times as much as glass or metals. Allow for size variation in frame systems and other fixtures. A 1200 mm long panel expands or contracts about 6mm over a temperature change of 40 °C. Calculate expansion and contraction allowances before installing. Acrylic sheet absorbs water in high relative humidity's, leading to expansion. At relative humidity's of 100%, 80%, and 60%, respective dimensional changes are 0.5%, 0.3%, and 0.2%. A temperature and/or humidity difference on opposite sides of a sheet may cause slight bowing toward the higher value. As the differential lessens, the sheet resumes its original position.
Use acrylic sheet at temperatures from -34°C to +88°C(–30°F to +190°F) Do not exceed 60°C (160°F) in continuous service or 90°C (190°F) intermittently.

Glazing materials and Gaskets
Acrylic sheet is manufactured from polymethyl methacrylate, based on oil and natural gas. Some gasket materials used with glass contain plasticizers (solvents). These can dissolve acrylic sheet, as can nail polish remover, paint thinner, or alcohol.