In automotive lighting system, almost all current car models use polyester materials with good impact resistance and light transmittance, make full use of its characteristics of easy molding and processing. For example, headlight, reflector, lamp body, etc. are all injection molded from polycarbonate, which has great design flexibility and is easy to process, which cannot be replaced by inorganic glass.
1. Application scope of polycarbonate in car lights
In the 1980s, affected by energy crisis, Japan improved automobile functions and designed new models that saved energy consumption and raw materials. Automobile lamps are made of injection molding materials, and a manufacturing process suitable for large-scale injection molded parts of lamps is proposed, which greatly reduces cost of manual production and further improves efficiency of automated production of automobile lamps.
Car lights can be divided into headlights, rear lights, turn signals, fog lights, license plate lighting, etc. PC’s impact strength is 250 times higher than that of inorganic glass and 30 times higher than that of polymethylmethacrylate sheets. It was the first to replace glass for headlight covers (also known as light distribution lenses). Since outer cover is made of PC, lamp body is made of modified polypropylene, lampshade and lamp body are assembled using adhesive bonding.
In addition, decorative function is an important part of car light shape. PC has good optical properties and coloring properties, can be used to make decorative strips (circles) in car lights to embellish decorative effect of car lights.
Decorative strips are generally made of transparent and colored PC. An auxiliary spray primer can be used to set off color, or aluminization can be used to turn it into a metallic color for decoration; decorative ring is usually aluminum-plated into a metallic color and then embedded outside lighting lamp; reflector in lamp is an important component. In the past, it was made of die-cast aluminum-plated parts. Now it is all made of PC injection molding and aluminum-plated parts, which reduces quality and simplifies process.
Luminous color of bulb in lamp generally cannot be changed, and indicator light emits colored light. For this reason, color of light is adjusted through color of internal light distribution mirror. Generally, colors of colored transparent PC are red, yellow, green, blue, etc.
2. Characteristics of polycarbonate applications
Because most PC injection molded parts of automotive lamps need to be vacuum aluminum plated before they can meet requirements for use in lamps. In particular, actual environment in which lamps are used is affected by sunlight, wind, rain and seasonal climate changes. In order to extend service life of PC products, ensure anti-corrosion performance and decorative capabilities of lamps, materials are modified, or surfaces of parts are vacuum-coated and UV-coated.
3. High heat-resistant polycarbonate
Experiments show that after continuous lighting for 6 hours in a lighting box with an ambient temperature of 50℃, surface temperature of some models of car light mirrors reaches 120℃ (or even higher), which requires parts such as light mirrors to have high heat resistance. Otherwise, these parts may soften and become atomized. Water vapor, material volatiles, etc. may condense on inner surface of light distribution lens, forming bead-like condensation, which may endanger safety of lights.
Based on thermal light source used in existing car lights, heat generated under working conditions can produce a high temperature of about 200℃ in a narrow space. Heat spreads by radiation and conduction through thermal convection, which requires PC plastic parts to withstand long-term high temperature test of 160℃. Heat resistance of general PC meets long-term use temperature of 130℃.
Therefore, for safety reasons, high-temperature PC materials must be used in lamps, and heat resistance can be as high as 185℃ or more. Bayer Materials Technology Co., Ltd.’s copolymer polycarbonate Apec (referred to as PC-HT) is a material with high thermal resistance and stability. It is a polycarbonate produced based on bisphenol A (which forms continuous phase of polycarbonate) and trimethylcyclohexane bisphenol. Its heat resistance temperature is 165~185℃, and some models have a heat resistance temperature of nearly 200℃.
Apec has high surface gloss, good dimensional stability, good processing performance, mechanical properties and UV resistance. It is widely used in automobile lamp housings (light distribution mirrors), deep-bottom lamp internal reflectors, high-light reflector hoods and other products, that is, used in places close to light source, such as light distribution mirrors or reflectors.
4. Transparent colored polycarbonate
Due to need to adjust color of light source and decoration, taking advantage of colorable characteristics of polycarbonate, material manufacturers directly add pigments (masterbatch) and some additives during production process according to user requirements to make transparent and colored PC materials. At present, main companies that produce this type of PC materials are Bayer Company and SABIC Company. The former produces transparent colored PC materials based on PCHT, and the latter produces transparent colored PC materials based on general PC.
Among these transparent colored PC materials, heat resistance of color masterbatch is very important. Melt temperature of PC-HT materials is 310~340℃, and melt temperature of general PC materials is 280~310℃. Currently, basic colors such as transparent red, transparent yellow, transparent green, transparent blue, transparent white, and transparent black are available. On this basis, other transparent colors can also be adjusted.
5. Polycarbonate surface treatment
Polycarbonate materials are prone to aging during use, including physical aging and aging under environmental effects such as thermal degradation, thermo-oxidative aging, photo-oxidative aging, and contact with water aging.
Bisphenol A is produced when polycarbonate ages and degrades, causing material’s appearance, physical properties, mechanical properties, and electrical properties to deteriorate or fail. Polycarbonate absorbs ultraviolet rays, long-term ultraviolet irradiation will cause material to turn yellow and reduce its impact strength, indicating that polycarbonate lamps have poor weather resistance under environmental influence.
In order to control product aging and improve product weather resistance, ultraviolet stabilizers (anti-UV agents) and a small amount of release agents are added to PC material synthesis, or final product is subjected to ultraviolet shielding coating.
After adding ultraviolet absorber, PC particles will appear blue and translucent, injection molded parts obtained by injection molding with PC containing anti-UV agent will also appear blue and translucent. Simple method of identification is to stack 4 to 5 layers of injection molded parts. If transparent color is blue and clear by visual inspection, it is OK. However, if it turns yellow, it means that anti-UV agent has been destroyed and meaning of anti-UV is lost. For PC covers that do not have anti-UV agents added, UV shielding treatment (UV film coating) can also play a role in weather resistance.
Surface of PC plastic parts is relatively brittle, especially transparent parts. When it is stained with dust, even if it is wiped with an electrostatic cloth, surface of light distribution mirror can be easily scratched, thus seriously affecting appearance of the entire lamp. Generally, when PC is used as a surface part outside a lamp, surface of product must be processed before use.
Currently, surface silicone wear-resistant coating is mainly used, which includes following steps:
1) Application of base coating. Apply base paint on the surface of polycarbonate optical material in a clean environment and dry;
2) Application of intermediate coating. Apply midcoat paint in a clean environment and dry;
3) Application of wear-resistant coating. Apply wear-resistant coating in a clean environment and dry to obtain a polycarbonate optical material treated with a silicone wear-resistant coating.
Hard transparent wear-resistant coating produced by this method has excellent wear resistance, weather resistance, and optical properties. It can significantly improve surface properties of polycarbonate optical materials, extend their service life, and can be used in surface modification of optical lenses, etc.
PC light distribution mirror after surface hardening treatment is not easy to scratch, and hardness is enhanced – surface hardness of product can be maintained between 2H ~ 3H, that is: use a 2H ~ 3H Chinese advanced drawing pencil to evenly draw 3 times on different parts of specimen (loading 750~1000g, angle between pencil and horizontal is 45°), once for each position, and clean it with an eraser. When all pencil lead debris on the surface of coating is wiped off, damage is easier to assess. You can use a magnifying glass (6 to 10 times) to evaluate: there are no indentations on coating surface, no visible scratches and no coating damage, all are qualified.
In the past, parts such as reflectors or viewing circles in lamps were electroplated or aluminum-plated. Metal parts were heavy, complex to process, and costly. Nowadays, PC injection molded parts are used instead. Molding process is simple and complex shapes can be processed. Surface obtained by aluminum plating process has a metallic luster and is light in weight.
Among engineering plastics, adhesion between PC substrate surface and aluminum layer is still good.
Basic process of PC aluminum plating is to evaporate (or sputter) metallic pure aluminum in a vacuum to deposit it on coated object (called substrate), which is also called vacuum aluminum plating. Aluminum-plated parts of automobile lamps are basically lamp body and reflective viewing circle, which are installed inside lamp to play the role of reflection, refraction and decoration. Thickness of aluminum plating is 0.8~1.2μm, surface is smooth and has high gloss. There are generally two methods of aluminum plating.
One way is to directly plate aluminum on substrate and then coat it with a protective film. Advantage of aluminum-plated protective film is that it has bright metallic gloss, excellent gas and light barrier properties, good moisture resistance, heat resistance, and puncture-resistant properties, so that items are not contaminated and corroded during transportation, storage, and use, original smoothness and shine are protected.
Due to presence of ester groups in PC main chain, especially in normal temperature environments with high relative humidity, water absorption rate of base material is high (0.15% ~ 0.19%). Injection molded products must be aluminum-plated within 2 hours. Once stored for a long time, fogging may occur after aluminum plating, which is caused by base material absorbing water;
Another way is to apply a corresponding primer on base material. After aluminum plating, adhesion is good, product has a strong metallic feel and a good gloss. After applying primer, it can cover some defects on base material. No matter which of above methods is used, adhesion of aluminum-plated parts must be checked and used only if they pass test.
Stress relief of polycarbonate parts. Polycarbonate is an engineering plastic with excellent comprehensive properties. It has high impact strength, but its parts often have large residual internal stress and are prone to stress cracking.
There are many factors that affect internal stress of PC parts, such as product structure, injection process, heat treatment, etc. Melting temperature of PC is high, melt viscosity is high, fluidity is poor, and cooling rate is fast, resulting in greater internal stress in product.
PC molecular chain has high rigidity and is not easy to relax after molecules are oriented, making it difficult for residual internal stress in product to eliminate itself. It must be post-processed to eliminate internal stress. By raising temperature to extent that molecular chains in plastic parts are activated, frozen molecular chains are relaxed and disordered after heating, thereby eliminating residual stress.
Heat treatment of PC products is the simplest way to eliminate or reduce internal stress. Method is to put product into oven at room temperature, raise temperature to 105℃ in oven and keep it for 10 to 20 minutes, continue to raise temperature to 125℃ and keep it for 30 to 40 minutes, then cool product in oven to below 60℃ and take it out. Heat treatment medium is Air.
For larger parts such as lamp housings (light distribution mirrors), it is suitable to be tempered in hot air circulation according to purpose and surface requirements of light distribution mirror. Generally, a tunnel-type far-infrared tempering furnace is used, which is placed next to injection molding machine. Molded plastic parts are placed on tracks of tempering furnace, products are trimmed at the end of the tempering furnace while waiting for them to be released. Add a closed channel at a certain distance at the end of tempering furnace to reduce exit temperature of product and facilitate flow of light distribution mirror into next process (as shown in Figure 2).
Tempering can greatly reduce residual stress in product. Tempering temperature of optical lens is (120±2)℃, and tempering time is (15±1)min. Temperature of tempering furnace is adjustable, tempering time can also be achieved by adjusting crawler transmission speed so that product stays in a certain tunnel length for 15 minutes.
For PC parts placed inside lamps, tempering is generally not used. Instead, a mold temperature machine is used during injection molding to adjust impact of mold temperature on internal stress of product.
Formation of internal stress is basically caused by different thermal shrinkage rates during cooling. When product is formed, its cooling gradually extends from surface to inside. Surface shrinks and hardens first, then gradually reaches inside. During this process, internal stress is generated due to difference in shrinkage speed.
When residual internal stress in plastic part is higher than elastic limit of resin, or when it is corroded by a certain chemical environment, cracks will occur on the surface of plastic part. Mold temperature is the most basic condition for controlling internal stress. A slight change in mold temperature will greatly change residual internal stress.
Generally speaking, each product and resin has its minimum mold temperature limit for acceptable internal stress. Mold temperature controlled by mold temperature machine should be as high as possible. PC products can be successfully demoulded at a mold temperature of 140℃, so mold temperature is generally maintained at 85 to 140℃. When mold temperature is too high, plastic parts cool slowly, molding cycle is extended, and mold is easy to stick, causing parts to deform during demoulding process.
The easiest way to check internal stress of PC products is to immerse products in carbon tetrachloride. Untreated products will crack at stress concentration location in less than 1 minute, while treated products will show no change even if soaked for 10 minutes.
Other methods can be used with a polarizing light transmittance instrument. Put a transparent plastic part between two polarizers with perpendicular transmission directions, and observe color pattern through white light. When pressure changes, color pattern also changes, indicating that birefringence properties change with stress, and size of internal stress can be determined. As shown in Figure 3, shaded areas on stress diagram are areas where stress is concentrated.
6. Conclusion
With progress of automobile safety and lightweight, polycarbonate, as a very important raw material, is used more and more widely in automobiles. Application of polycarbonate in automotive lamps is also becoming more and more important. Because polycarbonate has some inherent shortcomings, it requires internal changes in material, product appearance treatment, etc., and more processing steps are required to extend service life of product.
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