Expert in Plastic & Optical Materials & Applications Engineering/Light Management for Photonics

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Lubricants are used to reduce static and kinetic friction in mechanical parts that slide past each other. Lubricants that are fluid at the operating temperatures can migrate to the edges of the respective components. This particularly true when the lubricants are formulated from a mixture of mobile semi-liquid phases (e.g., hydrocarbon greases admixed with dry lubricants such as molybdenum sulfide, graphite, or Teflon). This condition is aggravated if one or both contacting surfaces are textured. This condition tends to support partitioning of the lubricant formulation.

His experience is specific to the lubrication rotating lens barrel elements in camera lens systems. The selection of chemically deposited surface blackening agents to minimize lens flare from the surfaces of machined barrel elements led to formation of crystalline copper oxides. This surface exhibited a partitioning surface, which separated the mobile component of commercial grease formulated with solid dry lubricant. After ambient environmental aging for one week, the increase in interbarrel friction was evident, resulting in “freezing” of the desired rotational motion. The mobile component was now distributed only at the edges of each barrel element.

He elected to use a fatty acid amide (e.g. “Kenamide”, a derivative of erucic acid), which exhibits good room temperature stability, and maintains its solid state condition in the camera use environment. This material was applied to the lens barrels by dipping these elements in a dilute solution of the amide dissolved in isopropyl alcohol. Following evaporation of the solvent, the lens barrel elements could be assembled readily.

Sliding friction was significantly reduced, such that when the barrel elements were assembled and placed in a vertical geometry, gravity tended to cause the topmost element to self-rotate spontaneously! This condition was corrected by inserting an additional metal device next to the sliding element, to restore sufficient friction to prevent this gravity action from shifting lens focus during customer use.

This writer is concerned primarily with applications of polycarbonate in extrusion and injection molding as related to fabricating optical components.

This plastic offers many properties that form the basis for its popularity as a commodity resin. Among the physical and optical properties of interest to this writer include its high refractive index, toughness and resistance to impact forces (e.g., ballistic resistance), its availability in different melt flow grades*, moldability in stress-free configuration despite its high stress optic coefficient, its ability to dissolve organic dyes, its solvatochromic shift to longer wavelengths for dyes compared with PMMA or polystyrene (i.e., it is a better “solvent matrix”), and its capability to incorporate solid fillers without loss of mechanical properties.

He has used polycarbonate as a preferred matrix for incorporating heat-sensitive dyes used for near-infrared light management. Specifically, laser light absorbing goggles for the military for wearer eye protection was an important personnel safety item in the Gulf War period (1989-1991) and is still a critical item included in lenses for gas masks, binoculars, and sighting devices where the user of such optical equipment must be protected from both enemy and friendly laser ranging and weapon sources.

Its ability to dissolve organic dyes as mentioned above makes it a favored matrix for color filters intended for light management applications.

Its mechanical properties do not degrade as much as PMMA with change in molecular weight. This forms the basis for selecting easier flow properties without sacrificing the impact resistance needed for ejection from mold cavities. Also, its refractive index is only slightly affected when shifting between flow grades. Nevertheless, the optical surface design may need to be adjusted for these small changes that become apparent only after initial molding trials reveal focus changes.

Dyes may also be incorporated in sheet extrusion applications. One caveat is that this operation may induce machine direction (MD) orientation, resulting in orientation of (usually) aromatic dyes. This may result in shifting of the absorbance and emission (fluorescence) spectrum of molecular structures having a “rigid” geometry.

Both extrusion and injection molding can induce stress birefringence in the polycarbonate part. This effect may be partially relieved by annealing, and in molding, by designing the tooling to employ injection-compression molding technology. Primary chemical manufacturers can advise on thermal annealing schedules, but this may mostly relieve surface rather than bulk stress birefringence.

Optical plastic selection is based not only on their specific light refracting properties, but on the convenience of their fabrication by commercial machinery. Thus extrusion in continuous film format is useful if material thicknesses less than ~0.030” (30 mils) will suffice to the application. Thicknesses greater than 30 mils (up to ~60+mils) are classified as sheet products, but are usually cut into finite lengths immediately following the extrusion step. Options in film format suitable for optical applications include polycycloölefins, normal and impact-modified PMMA, polycarbonate, polyester (PET and PEN), polystyrene, certain (clarified) polyolefins, and other extrudable polymers whose applications may include packaging materials, barrier films (oxygen, moisture, etc. permeability control), etc.

Optical plastic molding is similarly limited to those plastic materials available primarily as a side benefit of commercial volumes of the general purpose grades, wherein their specific optical uses are considered as secondary to their inherent transparency in the visible spectrum. Thus while polyethylene has unique transparency in the infrared spectrum, its more common use is in food product packaging film and as blister packaging material for consumer products. PMMA and polycarbonate are the dominant polymers used for optical applications.

Mechanical, electrical, thermal, and other physical properties specific to an application for polymers used to substitute for metal must be explored individually, according to the most critical function property. Thus applications for exposure to solvents and heat, as in the case of under-the-hood automotive uses, must also consider their ease of fabrication, tooling and polymer cost, mechanical strength, and the volume of product to be produced. In some instances, polymers modified with property-enhancing additives can economically compete with traditional metal-formed components. This activity is also driven by need to reduce the weight of molded automotive components.

Raw material suppliers are the best source of material property information, as they compete on the basis of material cost and ease of fabrication. Specialty converters have raised the art of application-specific material modification to competitive levels, such that there is virtually no limit to the variety of options available to product designers.

For example, plastics welding using laser-induced heating can be supported by incorporating infrared dyes that absorb specific laser wavelengths in order to heat local area requiring fusion. In other applications, outdoor resistance to thermal warping of PVC siding can be improved by incorporating infrared-reflecting pigments in the extrusion of that plastic product.

While this discussion is about PMMA as an injection molding plastic, its optical properties are held in common with (non-injection moldable) cast grades, which are much higher in molecular weight and often crosslinked. These changes in chemistry provide higher temperature resistance than injection grades, and afford machinability and fabrication in large physical formats.

Addition of light-controlling dyes to acrylic makes it a useful component of optical systems (e.g., color filters) in which the rearward elements and image detectors require elimination of spectral components that could affect their function or environmental (e.g.,UV) stability. It can dissolve many dyes required in optical systems with specific performance requirements. Except for poly(cycloölefins), it offers the best transmittance and stability of optical plastics, requiring UV protective additives only if used below ~350nm.

While it is inherently brittle, modified grades are available that improve its impact strength, without undue sacrifice of its transparency. PMMA's poor impact strength made it vulnerable to breakage when first used in covers for camera strobe lights (strobe shields). By blending impact modifiers with PMMA, the resistance to potential fracture from exploding flash tubes was eliminated, as well as from accidental contact with sharp corners of household objects. Thus blending of standard PMMA with impact-modified grades (ratio varied from 40:60 to 20:80) prevented shattering in severe (steel) ball-drop tests. While cracks developed at stressed locations in such tests, the strobe shields never lost their physical integrity. Color balancing dyes incorporated in the acrylic plastic provided concurrent spectral control of the blue-rich output from the strobe tube behind the shields.