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New Process for Producing Optical Quality, Low Cost Plastic Films
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The market for devices requiring precision plastic optical components has exploded over the past ten years, with a corresponding demand for liquid crystal displays used in these devices -- computers, cellular phones, personal digital assistants, global positioning systems (GPS), head-up avionics instrumentation, and a host of other applications. In fact, the use of LCDs has become so widespread because low-cost, high-quality LCDs are readily available. The evolution of the LCD from early displays requiring expensive glass substrates to those being manufactured today using plastic substrates has enabled these devices to be used in many applications calling for rugged, high-quality, low-cost displays. Rohm and Haas Company of Philadelphia, Pennsylvania has developed an alternative manufacturing process that delivers both high-quality and low-cost plastic films suitable for use as plastic substrates and in other optical applications.
Optical Quality Plastic Films
Flatness, surface smoothness, and low optical retardation (related to birefringence -- the rotation of polarized light as it passes through a substrate) are the main properties for an optical-quality plastic film. Relevant properties needed for a plastic LCD substrate are as follows:
| Property | Value |
|---|---|
| Thickness Variation | < 5% |
| Surface Waviness | < 0.2 microns over 20mm |
| Surface Roughness | < 0.2 microns |
| Optical Retardation | < 10nm |
Each of these properties is affected by the manufacturing process chosen to make the plastic film. A wide variety of processes could be used, including:
- Compression molding
- Injection molding
- Extrusion
- Casting
Each process has distinct advantages and disadvantages: Compression molding yields a high quality plastic film, but is typically slow and costly. Injection molding produces high stress films which have high optical retardation; however, itīs a relatively low-cost, automated process. Extrusion is a low-cost, continuous manufacturing process, but it, too, produces high stresses and high optical retardation. Casting creates a film by evaporating a solvent from a plastic solution that has been put onto a metal or glass plate. It produces a low-stress plastic film but is limited to thin films and involves handling a solvent. All of these processes have a common deficiency: They use molds or rollers that impart surface features to the plastic -- any defect in the mold or roller surface leaves a defect in the film surface. The net result is that none of these processes is capable of producing optical quality plastic films at low cost.
The Science of High Quality, Low Cost Plastic Films
The key to Rohm and Haasī new technology is a stress-free optical (SFO) manufacturing process that utilizes a new process paradigm for producing optical quality plastic films. This novel, continuous process can form a variety of thermoplastic resins (acrylics, polycarbonates, and so forth) into optical quality films.
SFO Process Benefits
As a result of its research, Rohm and Haas has been able to overcome virtually all of the disadvantages of traditional plastics processing. The SFO process can continuously produce film over a range of sizes and thicknesses, enabling robust, low cost manufacturing. The process requires no solvents and provides films with ultra-smooth surfaces and low internal stresses, resulting in ultra-low optical retardation and low thermal shrinkage.
SFO Process Applications
The SFO process was originally invented as a means to manufacture film for plastic substrates in LCDs. With the advent of low cost, high quality plastic substrates, substantial growth in new display applications is possible. In addition, films made using the SFO process may find applications in other areas that require their unique properties, including optical data storage media.
