【Popular Science Tips on Polymers and Materials】--32 “Resin glasses”: Polymeric materials used in glasses worn on the eyes + People watching the sea
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Resin lenses? Optical resin lenses are organic materials whose interior features a polymeric chain structure that forms a three-dimensional network. The structure between molecules is relatively loose, allowing for relative movement between the molecular chains. The light transmittance of such lenses ranges from 84% to 90%, giving them good light transmission properties; moreover, optical resin lenses are highly resistant to impacts. Resin is a hydrocarbon secretion derived from various plants, especially conifers. It is valued for its unique chemical structure and its ability to be used as a latex paint and adhesive. It is a mixture of various polymer compounds, so it has different melting points. Resins can be divided into natural resins and synthetic resins. There are a vast number of types of resins, which are widely used in both light industry and heavy industry. They can also be seen frequently in everyday life, such as in plastics, resin glasses, paints, and so on. Resin lenses are lenses that are made by chemically processing and polishing resin as the raw material. Introduction to the characteristics of the type: CR-39 is an abbreviation for acryloyl diglycol carbonate; it is also known as Columbia resin. It belongs to the category of thermosetting materials. Its physical properties are as follows: 1.1 Refractive index (at 23°C): 1.498; 1.2 Abbe number: 53.6; 1.3 Transmittance: 92%; 1.4 Density: 1.32 g/cm2; 1.5 Heat distortion temperature: 140°C; 1.6 Rockwell hardness: M100; 1.7 Coefficient of thermal expansion: 9–10x10-5/°C. CR-39 material also has the advantages of being easy to dye, impact-resistant, and having good chemical stability. It was developed by the American company PPG in the 1970s, and its use has since expanded rapidly. China began producing this material in the 1980s. Lenses made from this material are most suitable for correcting the refractive errors of primary and secondary school students as well as children, as well as for use in safety glasses and reading glasses. However, for those with high degrees of myopia, resin lenses with medium to high refractive indices are used instead, as such lenses tend to be thicker. PM MA, commonly known as plexiglass, is polymethyl methacrylate and belongs to thermoplastic materials. Its physical properties are as follows: 2.1 Refractive index (at 23°C): 1.49; 2.2 Abbe number: 57.4; 2.3 Transmittance: 92%; 2.4 Density: 1.19 g/cm³; 2.5 Thermal deformation temperature: 65–100°C; 2.6 Rockwell hardness: M80–100; 2.7 Impact toughness: 2.2–2.8 KJ/m²; 2.8 Coefficient of thermal expansion: 7×10-5/°C; 2.9 Saturation water absorption rate: 2%. PMMA is soluble in chloroform, dichloroethane, propane, benzene, and formic acid, but it is insoluble in water, methanol, ethanol, and gasoline. PMMA has advantages such as light weight and low cost, and is easy to shape, especially by injection molding, allowing for mass production. Lenses were produced as early as the 1950s, but due to their tendency to deform when heated and their extremely poor wear resistance, they were never widely adopted in the eyewear industry. Today, it is mostly used in low-end sunglasses lenses. PC stands for polycarbonate, and it is a thermoplastic material. Physical properties include: 3.1 Refractive index (Nd at 23°C): 1.585; 3.2 Abbe number: 30.3; 3.3 Transmittance: 89%; 3.4 Density: 1.2 g/cm3; 3.5 Heat distortion temperature: 120–140°C; 3.6 Rockwell hardness: M70–118; 3.7 Impact toughness: 80–100 KJ/m2; 3.8 Coefficient of thermal expansion: 6.6x10-5/°C; 3.9 Saturation water absorption rate: 0.4%. This material is transparent with a slight yellow tint, is not easy to dye, is hard yet tough, and possesses extremely high impact strength—more than 10 times that of CR-39. It ranks among the best among thermoplastic materials. It can be designed to be quite thin, with a minimum core thickness of 1.0 mm, and it passes the drop ball test. It exhibits good stability to heat, thermal radiation, air, and ozone. It absorbs all ultraviolet rays below ~385nm, making it a truly safe lens. In addition, it has high heat resistance, resistance to oils, greases, and acids, low water absorption, and high dimensional stability; it is also an environmentally friendly material that can be reused countless times. In the United States, many people call it the dominant lens of the 21st century. The disadvantages are high stress, susceptibility to cracking, poor compatibility with other resins, a high friction coefficient, and no self-lubricating property. The processing is difficult; at present, only sunglasses and plain glasses can be produced in China, with optical lenses still relying on imports. Many industry professionals are optimistic about this material, believing that in a few years, PC lenses will dominate China’s lens market. Features? A. Low density: Due to the gaps between molecular chains, there are fewer molecules per unit volume, which gives resin lenses the advantage of having a low specific gravity and being lightweight – only 1/3 to 1/2 as heavy as glass lenses ； B. Moderate refractive index: The standard CR-39 acryl diglycol carbonate has a refractive index of 1.497–1.504. Among the resin lenses available in the Shenyang eyewear market, those with the highest refractive index are the aspherical, ultra-thin, hardened, and coated lenses from Japanese Nikon, whose refractive index is 1.67. Nikon of Japan has introduced resin lenses with a refractive index of 1.74. C. Its surface hardness is lower than that of glass, making it susceptible to scratches from hard objects; therefore, it needs to be hardened. The material used for hardening is silica, but its hardness still cannot compare to that of glass. Therefore, wearers need to pay attention to the maintenance of the lenses ； D. It has good elasticity; due to the space available for displacement between the molecular chains of organic materials, its relative strength is 23-28 times that of glass sheets. This determines another key characteristic of resin sheets: their good impact resistance. Europe, the United States, and Japan **prohibit children under 16 from wearing glass lenses** ； E. Auxiliary functions: Additional features can be added to provide protection against harmful rays, color change, and other functions. F. Workability: Excellent; allows for diverse processing in full-frame, semi-frame, and frameless versions. It is also possible to perform penetrant staining to color the lenses in one’s preferred color. G. Chemical properties: As it is an organic compound, its use is strictly prohibited in acidic or alkaline environments. H. Thermal performance: It must not be used in environments with temperatures above 60 degrees, otherwise the anti-reflective coating on the surface will crack, and in severe cases, the coating may even peel off. I. Price: Varies depending on quality, features, brand, and grade. J. Dimensional design of lenses: According to the standards set by the U.S. Food and Drug Administration, the minimum central thickness can be 0.6 millimeters, but the strength requirements decrease accordingly. For this reason, manufacturers in Europe, the United States, Japan, and other countries generally design lenses for myopia or hyperopia of 100 degrees or more with a central thickness of 1.0 millimeter or more, in order to ensure the overall strength of the lenses.
Optical properties: 1. The surface gloss and smoothness of resin lenses are in no way inferior to those of conventional glass lenses. 2. The refractive index of resin lenses is lower than that of ordinary glass lenses, so resin lenses with the same degree are thicker. 3. The dispersion of resin lenses is extremely similar to that of ordinary glass lenses. 4. The light transmittance of resin lenses reaches 92%, which is more than 2% higher than that of ordinary glass lenses. 5. The surface reflection of resin lenses is lower than that of ordinary glass lenses, and they are also less glaring, owing to their higher light transmittance and lower refractive index. 6. The bifocal lenses made of resin are formed as a single piece, rather than being created by fusing together like ordinary glass bifocal lenses; therefore, resin bifocal lenses do not have chromatic aberration. 7. The optical properties of resin lenses are extremely stable, and they do not change under either high or low temperatures. Mechanical properties: 1. Resin lenses can be cast into lenses of various shapes with high transparency and meeting optical requirements. 2. Resin lenses are easier to frame on the side than ordinary glass lenses. 3. Resin lenses are highly susceptible to staining, and can be colored into lenses with various levels of light transmittance as desired. Physical properties: 1. Resin lenses are lightweight, weighing only half as much as ordinary glass lenses. 2. Resin lenses have extremely strong impact resistance. 3. When a resin lens is shattered due to impact, there are few fragments, and the broken pieces are large in size with blunt edges ； Minimize injuries to the eyes and face. 4. Even with long-term use, resin lenses are not prone to cracking on their surface. 5. Resin lenses have strong impact resistance against objects that are small in size and operate in high temperatures; when such objects collide with resin lenses, they are immediately repelled ； It won’t, like ordinary glass lenses, easily develop dents and spots. Therefore, when welding or using a grinding wheel, resin lenses can be used as protection to prevent the eyes from being damaged by flying sparks. 6. Since resin lenses have low thermal conductivity, their anti-fogging performance is better than that of ordinary lenses. Chemical properties: 1. Resin lenses can resist a wide range of chemicals and chemical solvents. Household chemicals and chemical solvents almost never cause damage to resin lenses. 2. Resin lenses have low thermal conductivity; they exhibit excellent heat resistance when heated, and are less likely to undergo warping or deformation at certain high temperatures.
Maintenance and Use? 1. When not in use, glasses should be stored in their case, and the outer surface of the lenses should not come into contact with hard objects. 2. Rinse the lens with tap water before wiping it. If there is oil stain, drop a little dishwashing detergent, rinse it off with tap water, and then use a soft paper towel to absorb the water. 3. Use a dedicated fiber cloth to wipe the lenses. If the fiber cloth gets dirty, it can be washed and reused. 4. When using coated resin sheets or space sheets, be careful to avoid high temperatures; do not take hot baths while wearing glasses, and certainly not use a sauna while wearing them ； Do not leave glasses in an unoccupied car in the summer sun ； Do not direct the hot air at the lenses while blowing. 5. Although the surface of resin lenses is treated with special hardening processes, it is still slightly inferior to glass; therefore, contact with hard objects should be avoided. Try not to wear it when swimming in the sea.
This type of lens isn’t afraid of drops, but it is afraid of scratches, right? The temperature resistance is also low
This improvement is commendable – the glasses have become lighter, and the discomfort caused by pressure on the nose bridge has been greatly reduced. There’s no risk of them breaking