advanced building skins 14 | 15 June 2012 - lamp.tugraz.at - Graz ...

2 Material PMMA
Advanced Building Skins
Polymethyl methacrylate, PMMA for short, is the chemical name of a highly transparent plastic.
Brand names for it are Plexiglas® or Acrylite® for example. PMMA belongs to the group of
transparent thermoplastics and stands out due to good weathering resistance and a relative hard surface
as well as its transparency. For sheet material there are mainly two manufacturing processes, extrusion
and cast.
The material characteristics are dependent on the type of manufacturing and the environmental
influences such as temperature and exposure time. The most important material characteristics under
standard climate conditions according to DIN 7823 are summarized in Table 1.
Table 1: Material characteristics according to DIN 7823
Extruded PMMA Cast PMMA Unit
Young´s Modulus ≥ 2900 ≥ 3000 MPa
Tensile Strength ≥ 60 ≥ 70 MPa
Tensile Strain ≥ 2 ≥ 4 %
Flexural Strength 100-115 100-115 MPa
Density 1,19 1,19 g/cm³
Thermal Expansion
Coefficient
7 x 10 -5 7 x 10 -5 1/K
In Figure 1 the dependence that Young’s Modulus has with the temperature is shown. At 23°C you can
see a young´s modulus of 3300 MPa. At 60°C it drops down to 2500 MPa.
Figure 1: Temperature dependency of young´s
modulus [Evonik Industries]
- 2 -
Figure 2: Relaxation [Evonik Industries]
The strain can be divided into three different parts. An energy elastic strain occurs immediately when
a load is applied. The second part is an entropic elastic strain, with which the position of the molecule
chains relative to one another is changed. This part of the strain is slowly reduced after the load is
relieved. The third part is a viscous flow that represents an irreversible deformation. In Figure 2, the
stress relaxation over the duration of the load is plotted. With a steady deformation, the stress is
relaxed significantly over time. The relaxation is dependent on the level of stress that is applied.
PMMA has a very high transmission, which results in the material heating up only a very small
amount in the sun.