Why is plastic transparent

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In the construction industry, acrylic glass and polycarbonate are used as load-bearing building materials. In addition, the following materials are mainly used as transparent plastics:

  • Acrylic glass (polymethyl methacrylate, PMMA)
  • Polycarbonate (PC)
  • Polyvinyl chloride (PVC)
  • Polystyrene (PS)
  • Polyphenylene ether (PPO)
  • Polyethylene (PE)
The formation of macromolecules is crucial for the manufacture of plastics. A distinction is made between polymerization, polyaddition and polycondensation. During polymerization, unsaturated monomers react in a chain reaction that is triggered by initiators such as radicals or ions to form thermoplastics. In polyaddition, monomers react under pressure and heat in the presence of a catalyst to form a long-chain polymer. In addition to a macromolecule, polycondensation creates another substance by splitting off (e.g. water). Polycondensates are usually more rigid due to a better link between the individual chains.

Thermoplastics can be produced by polyaddition, polycondensation or polymerisation, thermosets only by polycondensation and polyaddition. In contrast to thermosets, thermoplastics become soft when exposed to heat and can generally be processed more easily.

The advantages of transparent plastics over glass are in particular their lower specific weight, which is only around half that of glass. Other material properties can also be set in a targeted manner. Plastics are also much easier to work with than glass. The low modulus of elasticity and the low surface hardness as well as the massive drop in strength at temperatures above approx. 70 ° C and the fire behavior have a disadvantageous effect. The strong time dependency of the strength and the embrittlement due to aging of most of the plastics used must also be taken into account.

Acrylic glass
The best-known transparent thermoplastic material is acrylic glass, which was developed in the laboratories of the Röhm company in the late 1920s and is now sold under the brand names Plexiglas or Perspex, for example. Acrylic glass is a high-polymer plastic (polymethyl methacrylate, PMMA) that is produced by polyaddition of methyl methacrylate monomers using a peroxide as a catalyst. Usually, the monomers are heated until a viscous mass is formed and placed in a mold that consists of two glass plates and a spacer that determines the thickness. The complete polymerization takes place in an oven. The glass surfaces give the polymer its smooth surface, which is excellent in the cast form of acrylic glass.

The material is available in the form of solid sheets, blocks, tubes or rods in thicknesses from 1.5 to 150 mm. With the extruded form, a good surface quality is achieved and solid sheets as well as tubes, multi-skin sheets and corrugated sheets are produced. Solid sheets from 1.5 to 25 mm thick and multi-skin sheets from 16 to 32 mm thick are available here. The standard formats are 3.00 x 2.00 m for cast acrylic glass and 4.00 x 2.00 m for extruded. It can be colored with different colors in the mass.

Polycarbonates
Polycarbonates (PC) are polyesters of carboxylic acid. In the case of transparent polycarbonates, a polymer made of bi-phenol acetones and the carbonate is produced by polycondensation using carbonyl chloride. This creates hydrochloric acid as a by-product. Polycarbonates are thermoplastics with a very strong network structure. They are therefore much more impact-resistant, thermally and chemically resistant, but also more brittle than acrylic glass. When they are burned, no poisonous gases are produced, only CO₂ and CO. In addition to lower UV resistance, another disadvantage compared to acrylic glass is that this material is 1.5 times more expensive. Polycarbonate, which is known, for example, under the brand name Makrolon, is extruded and processed into solid sheets or multi-skin sheets. It is available in thicknesses of 1 to 12 mm for solid sheets and 6 to 25 mm for multi-skin sheets in standard formats up to 4.00 x 2.00 m. Glass fiber reinforced polycarbonates are also produced. These are used in vehicle and container construction, but are only translucent.

Use of acrylic or polycarbonate sheets
As so-called light panels, acrylic and polycarbonate panels are intended for spatial situations that should be provided with daylight, but only have low demands on thermal insulation and a view. Their areas of application are, for example, hall light strips in the roofs or facades of industrial buildings, ramp roofs or carports as well as greenhouses. Light panels can be constructed with one or more layers. The multi-shell structure is achieved by an intermediate layer with a honeycomb or prism-shaped hollow chamber structure made of the same material. Thanks to the air inclusions, it ensures improved insulation properties and less glare in direct sunlight. However, the possibility of seeing through is reduced by the layer structure. Depending on the nature and number of plastic layers, light panels are transparent to translucent and have different intensities of light scattering.

Both acrylic glass and polycarbonate can be glued together very well with suitable, transparent adhesives. Both building materials can be both cold-formed and hot-formed. During hot forming, it must be ensured that the panels have to be tempered before they are removed from the form in order to reduce the residual stresses.

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