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Headlight structure and materials

Headlights are made from either glass or resin. The most common type today uses polycarbonate resins. This is one of the strongest and most heat resistant types of plastic, but it also has weaknesses. We will begin by reviewing the material and structure.

Material Acrylic resin Polycarbonate Glass
Features Glass-like transparency
Excellent durability
Hard to break
Does not easily burn
Scratch-resistant
High durability
Nonflammable
Weaknesses Easily deforms at high temperatures
Flammable
Scratches easily
Poor weather resistance
Breaks easily
Difficult to manufacture
Heavy
Transparency 93% 86% 92%
Outdoor weather resistance Good Easily deteriorates due to ultraviolet rays Very good
Strength High Very high Very low
Surface hardness About 2H–3H (pencil hardness) About 1H–HB About 1H–HB
Flame resistance Flammable Self‐extinguishing Nonflammable
Relative density 1.2 1.2 2.5
UV protection Good Good (absorbs UV rays) Poor (none)

Plastic materials and properties

PP (polypropylene)

PP has excellent moldability, allowing for a variety of molding methods. Among commodity plastics, PP is the lightest (relative density: ~0.9) and most heat-resistant type. It is resistant to chemicals such as acids, alkalis and organic solvents. Its physical properties can be reinforced with fillers, glass fibers, etc. Furthermore, like other polyolefin resins, it can be recycled. These strengths have made PP the most commonly used plastic for automobiles. PP has a very wide range of applications, including exterior parts like the bumper and door trim, interior parts like the instrument panel, electrical components and around the engine.

PE (polyethylene)

PE is mainly used for air ducts, fenders, fuel tanks and cushioning materials in vehicles, taking advantage of its lightness, low cost and resistance to low temperatures.

ABS resin

ABS resin is mainly used for interior parts like the instrument panel and exterior parts like the front grille, taking advantage of its excellent balance of physical properties, high-quality coloring, dimensional stability and moldability.

PVC (polyvinyl chloride)

While usage in the automotive industry has decreased in recent years owing to environmental concerns, PVC remains an indispensable material for automobiles due to its flexibility, texture and cost performance.

PA (polyamide, nylon)

PA is an engineering plastic most commonly used for interior parts due to its excellent heat resistance, chemical resistance, mechanical properties and moldability.

POM (polyacetal)

POM is an engineering plastic that exhibits excellent mechanical properties over the short and long term. It also has excellent moldability and a good balance of physical properties, including resistance to friction and wear and chemical resistance. POM has been in high demand in the automotive industry lately. It is used for many parts of the vehicle, including interior parts like steering wheels, exterior parts like windshield wipers and parts around the chassis like gas pedals.

PBT (polybutylene terephthalate)

PBT is becoming increasingly popular for automotive parts of every kind due to its outstanding balance of electrical properties, heat resistance, heat aging resistance, chemical resistance and resistance to friction and wear. It is frequently used for interior parts such as air outlets, exterior parts such as outer door handles and various electrical components.

PC (polycarbonate)

PC is an engineering plastic that excels in dimensional stability, resistance to low and high temperatures and mechanical properties such as transparency and impact resistance. It is often used for headlight lenses, meter covers and door handles.

There are various kinds of plastic resins, but acrylic resin and polycarbonate stand out as materials with glass-like transparency.

Weaknesses of polycarbonate

Low surface hardness

Ordinary automobile paint has a hardness of 1H–2H, while polycarbonate is only about 1H–HB. This softness means that it scratches and clouds easily.

Poor solvent resistance

Polycarbonate is very weak against alkaline and organic solvents, which makes it prone to dissolution and cracking.

Poor UV resistance

Polycarbonate is highly susceptible to deterioration from ultraviolet rays, including yellowing and cracking. Prolonged exposure to ultraviolet rays causes the surface of polycarbonate resins to undergo a chemical change involving a rearrangement of its components. The product of this rearrangement appears yellow or reddish-brown, with a surface that absorbs ultraviolet rays. For that reason, after a certain point, polycarbonate stops changing in reaction to ultraviolet rays. This loss of transparency and yellow discoloration is called "yellowing."

Susceptibility to heat

Polycarbonate is superior to other plastic resins in terms of heat resistance, but when polishing a headlight with a polisher, the build up of heat may exceed its limits and lead to cracking. As such, caution is required.

As you can see, polycarbonate itself has low surface hardness, scratches easily and is weak to solvents. In the next section, we will explain how polycarbonate headlights are designed and structured to compensate for these weaknesses.

Related products

Zen-Xero Vario
●Size: 100ml
●Main ingredient: Organic Polysilazane
●Application method: By hand
Ease of Use:☆☆☆☆☆
Durability:☆☆☆☆☆
Luster:☆☆☆☆☆
Now polycarbonate resin, rather than glass, is used for a material of headlights, and a various shapes of headlights exist. Headlights are important parts to determine the design of a vehicle. Deterioration and white turbidity of the hard coat layer of the surface of headlights are considered as a weak point of polycarbonate. It is common to polish and remove the hard coat with a water-resistant paper, buff the surface with compound to recover the transparency, and then apply a coating or paint on the surface to fix this problem. However, a product specially designed for headlights is needed to give the maximum effect to the material and conditions which are different from the painted surfaces. Zen-Xero Vario is formulated specially for headlights, and it prevents the deterioration of the coat itself and protects polycarbonate.
Ease of use: Average
Finish:☆☆☆☆
Durability :☆☆☆
This product uses a formula that is as close to inorganic as possible and free of unnecessary ingredients to maximize the thickness and hardness of the coating. While its ease of use is inferior to the other two products, the main ingredient, organosiloxane, provides a very strong coating. This high-level headlight coating is applied by hand.
Lens Reformer2
●Set contents:Main agent ・hardener
1. Adheres to any kind of plastic without primer.
2. Improves the appearance of the vehicle by restoring the transparency and gloss of the headlights.
3. Maintains the appearance of the vehicle for a long period with excellent weather resistance.
4. These environmentally friendly products contain no substances subject to PRTR laws or the Ordinance on Prevention of Hazards Due to Specified Chemical Substances.

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