The flame melts the surface of the plastic making it smooth and shiny. This is called flame polishing.

The original color is red. When wear and tear occurs it leaves smalls scratches that cloud the surface making it look pale. When the scratches are melted away there is no more cloudiness and discoloration.

In addition to scratches, the plastic resin has become oxidized due to UV exposure (these seats probably get lots of sunshine!). Red pigment is really hard to protect, and with heavy UV exposure it turns cloudy (paint on red cars can also suffer the same problem).

This torch is melting away the oxidized top layer, allowing fresh red plastic to flow across the surface.

So, this opaque look in thermoplastic materials is caused by crystallization which may be caused by stress or UV- radiation. This is the same mechanism that colors your drink cap white when you scratch it with your finger. The dye is not lost but becomes less visible due to the crystals scattering the light instead of absorbing it (the parts of it that isn't red light). Heating the material up above its glass transition temperature breaks these crystals and the original structure is restored, letting the material once again reflect only the red light.

Video by @j.d.soplete (TikTok)

Wikipedia:

Fire polishing, also known as flame polishing, is a method of polishing a material, usually glass or thermoplastics, by exposing it to a flame or heat. When the surface of the material briefly melts, surface tension smooths the surface. Operator skill is critical with this method. When done properly, flame plastic polishing produces the clearest finish, especially when polishing acrylic. This method is most applicable to flat external surfaces. Flame polishing is frequently used in acrylic plastic fabrication because of its high speed compared to abrasive methods. In this application, an oxyhydrogen torch is typically used, one reason being that the flame chemistry is unlikely to contaminate the plastic.


Video by @j.d.soplete (TikTok)

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