Optical Material: Plastics is the third in a series of blog posts that aims to highlight the various optical materials used to design and manufacture optical components for the photonics industry.
Although glass has been the traditional choice for optical components, the advent of plastics transformed and expanded their application potential in modern optics. Plastics have several advantages over glasses and ceramics, and in many recent applications have served as a formidable replacement for traditional optical materials. From a chemical perspective, plastics are essentially polymers of hydrocarbons. They are extensively used as packaging materials, and functional or protective components in medical equipment, construction tubing, contact lenses, fabrics etc. Within that wide range of plastic materials, there is a special category of plastics that exhibits excellent optical properties such as high-transmission and refractive index comparable to float glass. Such plastics are well suited as substrates in the manufacture of a range of optical components like lenses, optical fibers, gratings, etc.
Plastics in Optical Applications
In this class of plastics, by far the most widely used plastic is PMMA or Poly Methyl Methacrylate. It is also known as plexiglass or acrylic glass. PMMA has a refractive index of 1.49 at 589.3 nm and a 3 mm thick sheet transmits 92% of light in the visible spectrum. PMMA absorbs UV light with wavelengths shorter than 400 nm, and IR wavelengths higher than 2800 nm. These properties make it an excellent replacement for glass as an optical material. Some notable applications of PMMA are:
As compared to glass, PMMA does not shatter, which makes it a safer choice in many applications. PMMA is even used in infrastructure applications such as windows in aircraft, roof panels, aquariums, and automobile headlights. Due to its light weight, PMMA headlights, taillights, indicators, and other parts in automobiles are extremely desirable and contribute towards the overall efficiency of the system.
Due to its affinity to human tissue and excellent stability, PMMA was the first material to be used in intraocular lens implants. Although, silicones are more popular today due to the added advantage of flexibility.
PMMA is also used as a core material in optical fibers or polymer/plastic optical fibers (POF). Although these fibers have higher attenuation as compared to silica fibers, they are less brittle and can operate with smaller bend radius as compared to silica counterparts. They are cheaper than silica fibers and better suited for short distance and low-speed communication links (<150 m). Short PMMA fibers also can be seen in artistic and decorative indoor applications involving fiber bundles.
Polycarbonates (PC) are a class of thermoplastics that have excellent optical transmission in the visible range along with superior toughness as compared to other optical plastics. PCs absorb UV wavelengths shorter than 400 nm and have about 90% transmission in the visible range. They have a refractive index of 1.59. Due to their toughness and optical clarity they are used for the following applications:
PCs are an excellent material choice for eyeglass lenses, goggles, and protective eyewear. They are impact resistant, filter out UV and do not shatter or crack like glass. As a result, they are suited for users who engage in sports, or work in high-activity environments and for children.
Substrates for Filters, Reflectors, CDs/DVDs
Optical components with PCs can be produced with high dimensional accuracy and low defects. This makes them suitable as substrate materials for optical filters, mirrors and reflectors and CDs/DVDs where high transparency is required with track spacings in the range of 0.75-1.6 microns. CDs and DVDs generally have reflective metal coatings of gold or aluminum over the PC substrates. Additionally, multilayer coatings can be deposited on PC substrates for optical filter applications.
Polyethylene Terephthalate (PET)
Commonly known as PET, this plastic is one of the most versatile and widely used materials. PET is used to make a range of products from the ubiquitous water bottles, polyester fabric to thin film solar cells. PET substrates are highly transparent with a transmission of greater than 90% across the visible spectrum and a refractive index of 1.57. Common applications of PET.
Touch Panels and Solar Cells
PET substrates have low haze, high transparency, and high optically smoothness, that makes them ideal for applications in touch panels and solar cells. Conductors such as Indium Tin Oxide (ITO) and silver nanoparticles can be coated on PET substrates. The ability to realize transparent conducting surfaces is essential for touch screens in smartphones, tablets and, more recently, flexible displays.
Polyether Ether Ketone (PEEK)
PEEK plastics are commonly used in engineering applications that require high-temperature and mechanical stability. They melt at around 340oC and have a high tensile strength and creep resistance. Additionally, PEEK is a flame retardant and does not easily dissolve in water or common organic solvents. These makes them suited for highly demanding industrial applications. PEEK plastics are used as buffer materials in optical fibers, automobile parts, and aluminum replacements in aerospace applications.
CR-39 (Allyl Diglycol Carbonate or ADC)
CR-39 is most commonly used in eyeglass lenses due to its high optical clarity and scratch resistance. It is lighter than glass and naturally absorbs UV radiation (<350 nm), making it ideal for eyeglass applications. As compared to PC, CR-39 shatters under impact. Hence, it is not recommended for high-risk applications.
Polystyrene, particularly General Purpose Polystyrene (GPPS) is another optical plastic material which is clear, hard, and brittle. It has a refractive index of 1.59, which is similar to PCs but higher than PMMA. PS has a high transmission of 90% in the visible spectrum. Some applications of GPPS are:
Diffusers for Lighting Applications
GPPS is quite inexpensive and a good choice for diffuser sheets and plates in lighting applications. Surface texturing can be easily incorporated to create patterned sheets for light guides, and architectural glass-like plates.
Optical silicones are polymers and have the advantage of being optically transparent and flexible. PMMA, Polystyrene, and Polycarbonates are rigid plastics, whereas silicones have found uses in new applications, particularly where flexibility is essential. Among silicones, poly dimethyl siloxane (PDMS) is a widely used optical silicone. PDMS has around 90% optical transmission from 250-1000 nm, which makes it suitable for optical components such as lenses and flexible substrates. Additionally, PDMS is a viscoelastic polymer, whose viscosity and elasticity can be tuned by controlling the cross-linking between polymer chains. This allows one to impart different levels of flexibility depending on the application. Silicones are used for the following applications:
Silicones have replaced PMMA as the material of choice for intraocular lenses due to their flexibility and tissue compatibility. They can be molded into biconvex, meniscus and other lens shapes, and are typically used after cataract surgeries as lens replacements.
EMF has successfully coated 1.7 million plastic optics on substrates like Polycarbonate, PMMA, PET, Acrylic, PEEK and CR-39.
Advantages of Optical Plastics
In contrast to glasses and ceramics that are typically processed and formed at very high temperatures (1,000 oC or even higher), plastics can be reliably molded at much lower temperatures at typically 150-200oC. A considerably lower amount of energy is needed during the plastic manufacturing process, which makes it cheaper to produce. Additionally, plastics can be synthesized from petrochemicals in the lab, thereby offering ease of processing advantage.
Scalability in Manufacturing
Plastic optical components can be produced using injection molding, which is extremely scalable and yields a high-volume of components. Complex surfaces like aspherics can be generated using standard molding approaches. Additionally, casting, texturing and embossing techniques can be applied to plastics for applications such as diffusers.
Optical plastics have a lower density as compared to glasses or ceramics. So, for a given volume, plastics weigh 2-5 times less than their glass counterpart while achieving the same function. In applications where the weight of the optical system is a critical constraint, plastics definitely take preference over glass.
Safer Alternative to Glass
Conventional glasses shatter under impact causing splinters that can cause serious harm to humans. Plastics are a safer choice in applications that have the risk of breakage and frequent use. For instance, helmets and face shields are made from plastics, which provide optical clarity and a safer alternative to glass as they do not pose a threat in case of breakage.