1、 General classification of optical plastics
Plastic materials are generally divided into thermoplastics and thermosetting plastics. Most optical plastics are thermoplastics. Commonly used are polymethylmethacrylate PMMA, polystyrene PS, polycarbonate PC, etc, Thermosetting plastics means that the synthetic resin used softens at the beginning of heating and has plasticity. If it is heated continuously, it will harden with chemical reaction and the shape will not change
The commonly used materials are allyl diglycol carbonate CR-39 resin spectacle lens, epoxy optical plastics are thermosetting plastics
2、 Main optical plastics
1.Polymethyl methacrylate (PMMA), also known as acrylic
The molar weight is about 500000-1000000
Nd = 1.491, the dispersion coefficient VD = 57.2, is the "King Crown" material, the transmittance is about 92%, after accelerated aging, the transmittance can still reach 92% at 240h, after 10 years of outdoor use, it only drops to 88%, and it can pass through the ultraviolet light with wavelength above 270nm.
PMMA can transmit X-ray and y-ray, and its thin sheet can transmit α - ray and β - ray, but can absorb neutron line
The density of PMMA is 1.19kg/m3, and its average water absorption is 2% at 20 ℃ * 109pa. Among all optical plastics, PMMA has the highest water absorption, elastic modulus is 3.16 * 109pa, Poisson's ratio is 0.32, and tensile strength is (462-703) * 109 Pa.PMMA The linear expansion coefficient of K9 glass is 8.3 * 10-5k-1, which is 10 times larger than that of K9 glass, However, PMMA has better performance than glass in recovering its optical memory module from high temperature cooling. Its refractive index dn / DT is - 8.5 * 10-5, which is about 30 times larger than K9 glass, but it is negative. Its thermal conductivity is 0.192w / (m * k), specific heat capacity is 1465j / (kg * k), and its glass transition temperature is 105 ℃ and melting temperature is 180 ℃, PMMA has excellent weather resistance. It has been exposed in tropical climate for many years, and its transparency and color change are small. At present, PMMA is widely used in the manufacture of cameras, video recorders and projectors, The optical disk reading head and aspheric lens and reflector in military fire control and guidance system are also used to manufacture Fresnel lens, microlens array, contact lens, optical fiber, optical disk substrate and other parts
2. Polystyrene (PS, also known as styrene)
This is a kind of flint thermoplastic optical plastic. Although its anti ultraviolet radiation performance and anti scratch performance are not as good as PMMA, its refractive index is high, nd = 1.59-1.660, Abbe coefficient is small, VD = 30.8, so when it is combined with PMMA, it can be used as achromatic lens to correct f and C spectral lines. The correction of secondary spectrum is generally better than that of glass achromatic lens. Its transmittance is 88%, Its birefringence is large, and polystyrene is easy to turn yellow under the action of sunlight. PS can be freely colored, odorless, tasteless, nontoxic, and does not produce mold. Its hygroscopicity is small, and its hygroscopicity is only 0.02%. The thermal deformation temperature of PS is 70-98 ℃, which is related to the formula and post-treatment. Its maximum continuous use temperature is 60-80 ℃, and the molding shrinkage is 0.45%, The thermal conductivity of PS does not change with the temperature, so it can be used as a good refrigeration insulation material. The specific heat capacity of PS has obvious change, which is one of the plastics with low specific heat. In high vacuum and in 330-380 ℃, the heat will drop sharply. 43% of volatile matter is liberated, 41% of styrene is liberated, PS is resistant to some mineral oils, organic acids, bases, salts, low-energy alcohols and their aqueous solutions, softened by the attack of many ketones and higher fatty esters, and soluble in aromatic hydrocarbons such as benzene, toluene, ethylbenzene and styrene monomers, PS is one of the most easily processed resins. It has the characteristics of large difference between molding temperature and decomposition temperature, low melting viscosity and stable size. It can be molded by compression molding and also used in injection molding. However, its brittleness is greater than other optical plastics, so it is easy to crack. Attention should be paid to prevent cracking when cutting gate, It can also use general metal or wood processing tools for machining, such as drilling, sawing, cutting, etc
PS and PMMA are used to make achromatic lens and copy grating assembly
In order to improve the properties of PS, some improved varieties have been developed. For example, 70% polystyrene and 30% methyl acrylate are copolymerized to form a new optical plastic NAS. The other copolymer is acrylonitrile styrene copolymer, called San, which is mainly used in engineering plastic products. Optically, it is mainly used as a window, basically maintaining the transparency of PS, but it still has a tendency of yellowing, nd = 1.567, the maximum use temperature of 75-90 ℃, The thermal deformation temperature is 82-105 ℃, the linear expansion coefficient is (6.5-6.7) * 10-5k-1, and the density is 1.06kg/m3-1.08kg/m3
3. Polycarbonate (PC)
Thermoplastics with excellent comprehensive properties have good heat resistance, cold resistance, high mechanical strength in a wide temperature range (- 135 ℃ - + 120 ℃), good dimensional stability. When the temperature rises to 105 ℃, the linear size of the material increases by 0.07%, high impact strength, good ductility, uniform molding shrinkage, low water absorption, and only 0.13% weight gain after soaking in water for 24h, The density is 1.20kg/m3, the natural color is light yellow, add a little light blue to get colorless transparent products. Nd = 1.586, VD = 34.0, the transmittance is 88%. The mechanical properties of PC are tough and rigid, notchless impact strength is among the best in thermoplastic, molding shrinkage is stable in 0.5% - 0.7%, The corrosion of reducing agent, salt, oil, aliphatic hydrocarbon, alkali resistance, amine, ketone and aromatic hydrocarbon, swelling in many organic liquids as vapor and causing stress cracking, dissolving in dichloromethane, dichloroamphenicol, ethane, cresol and dioxane, causing hydrolysis and cracking in water for a long time, leading to embrittlement. The normal hygroscopicity of PC in water is 0.15%, that in warm water is 0.35%, and that in boiling water is 0.58%, The refractive index of the optical plastic is a function of wavelength λ (μ m), which can be calculated by the following formula:
n2(λ)=A0+A1λ2+A2λ-2+A3λ-4+A4λ-6+A5λ-8
The coefficients of the three plastics are as follows:
4. Copolymer of styrene and acrylic ester, abbreviated as NAS
It is a copolymer of 70% styrene and 30% acrylic ester. Its properties are better than those of polystyrene. Its transmittance can reach 90%, and its refractive index nd can vary from 1.533 to 1.567, VD = 35. It can be used as the second material of positive chromatic aberration, but it is generally only used as thin lens
5. Allgl diglycol carbonate (ADC or CR-39)
Nd = 1.498, VD = 53-57, white light transmittance 92%, wear resistance, impact resistance, strong chemical corrosion resistance, can withstand 100 ℃ high temperature, 150 ℃ in a short time, Because of its high shrinkage (up to 14% when cured), it is mainly used for spectacle lenses
6. New varieties
① Poly (tricyclic decane methacrylate) series oz-1000 and 101110121013 are new alicyclic resins developed by Hitachi chemical company of Japan
②ARTON
Developed by Japan Synthetic Rubber Company (JSR), it has the lightest specific gravity, low water absorption, better than PMMA, good transmittance, small color difference, lower birefringence than PC, better heat resistance than PMMA and PC, better tensile strength than PMMA, and better flexural modulus of elasticity than PC, so it is very suitable for aspheric lens
③ Cycloolefin copolymer (COC)
④ Cyclo olefin polymer (COP)
It is another kind of amorphous polyolefin (zeonex480, etc.) developed by Japanese ruiweng company
Performance comparison of zeonex480 with PC and PMMA
