Page 5 - Infrared for plastics EN

Only the infrared radiation which matches the processed material is absorbed
Just like visible light, a part of the broad infrared radiation spectrum is relected from the surface of the material while the rest is either absorbed within the material or passes through the material. Every material consists of molecules and molecular structures that absorb speciic radiation wavelengths. The radiation wavelength that is absorbed by a material coincides with the wavelength of the molecular oscillation in that material. The absorbed radiation releases energy to the molecule generating heat in the material.
For instance, plastics generally absorb infrared radiation in the wavelength range above 2 μm. C-H-bonds are especially eficient because they absorb wavelengths between 3.2 μm and 3.5 μm.
Wavelength itting to the material
Thin materials such as foils are dificult to heat with short wave infrared because only a small component of the short wave radiation matches the absorption spectrum of the material. Thin materials are transparent to the infrared radiation and short wave IR is not eficient. Medium wave radiation, on the other hand, is absorbed more readily and the result is that the foil heats signiicantly faster at the same electrical power input.
To avoid the loss of radiation Heraeus offers a wavelength converter, consisting of a plate with mineral ibers. This absorbs the radiation which has passed through the material and radiates it back into the material at a different wave- length. The wavelength converter absorbs transmitted infra- red radiation, heats up to 500 – 600 °C and then radiates back medium- and long wave radiation.
For solid materials, because of low absorption rates, short wave infrared penetrates deep into the material and provides uniform volumetric heating. Medium wave radiation is absorbed in the material’s outer layer and generally heats only the surface. With the correct infrared emitters, heating plastics can be controlled according to very speciic require- ments. Pigments in coloured plastics increase the infrared absorption.
The power density determines the amount of
heat transferred
In turn, the amount of heat transferred to the material depends on the emitter’s power, its temperature and distance from the material. Once the product material has determined the spectrum, the spectrum ixes the tempera- ture and thus the electrical power of the emitter. Therefore, to increase the amount of heat transferred, the power density (radiation output per unit surface area) has to be increased. This is achieved by physically arranging the emitters, by using twin tube emitters and by additional relectors.
Radiation power (relative units)
250 200 150 100 50
UV
short wave
Halogen/NIR
2600 °C
medium w
short wave
2200 °C
fast response medium
The infrared spectrum of different Heraeus emitters.
The curves show the radiation intensity in the different wavelength ranges at the same electrical power ratings. While the halogen emitter in the short wavelength region provides the highest power output, carbon emitters and medium wave emitters have signiicantly higher outputs at wavelengths above 2 μm.
The absorption spectra for polyethylene and polyvinylchloride (PVC) show strong absorption for infrared radiation between 2.5 and 4 μm. For these materials medium wave emitters have a greater eficiency than short wave and halogen emitters.
This overlay of the absorption spectrum of water demonstrates that medium wave emitters also have a signiicantly higher eficiency than short wave emitters for drying applications.
ave
long wave
0 1 2 3
wavelength (μm)
1600 °C
carbon
1200 °C
wave
medium wave
900 °C
wavelength (μm)
Absorption (%)
100 50
short wave
2200 °C
short wave
medium wave
900 °C
medium wave
900 °C
Polyethylen 0,1 mm
PVC 0,02 mm
2200 °C
water
0 1 2 3 wavelength (μm)
5
Visible light