Cr 39Edit

CR-39, short for Colu[m]bia Resin #39, is a clear polymeric plastic of wide significance in both optics and radiation detection. Originating as an ophthalmic lens material and later finding extensive use as a solid-state detector in nuclear and space physics, CR-39 is the diethylene glycol bis(allyl carbonate) polymer that earned its name from the Columbia Resin Company during mid-20th century materials development. Today, CR-39 remains a benchmark material for optical clarity and processability in some applications, while its etchable properties make it a standard in passive radiation detectors as well.

CR-39 and related materials have shaped practical choices in several industries. In eyewear, CR-39 lenses have been prized for their optical quality, light weight, and ease of fabrication. In radiation detection, CR-39 detectors are valued for their simple, passive operation and their capability to reveal charged-particle tracks after chemical etching. The dual life of CR-39—as both an optical medium and a detector—is a notable example of how a single material can serve distinct technical communities.

History CR-39 traces its origins to the mid-20th century, when the Columbia Resin Company developed a family of diethylene glycol bis(allyl carbonate) polymers for high-quality optics. The designation “CR-39” refers to a particular resin in that line. Early adoption occurred in the eyewear industry, where the material offered an appealing combination of high optical clarity, relatively low density, and good environmental stability. The same chemical system later attracted the attention of researchers in radiation physics, who discovered that CR-39 could record charged-particle tracks after etching, turning a lens polymer into a practical detector material. The historical transition from a consumer optics material to a laboratory detector reflects the versatility of polymer chemistry and manufacturing processes.

Composition and properties Chemical composition: CR-39 is a polymer formed by the polymerization of diethylene glycol bis(allyl carbonate). This backbone yields a clear, glassy solid with favorable optical transmission in the visible spectrum and a relatively low index of refraction that makes it suitable for thin, lightweight lenses.

Optical properties: CR-39 is renowned for optical clarity and transmission across the visible range. Its refractive index is typically around 1.49–1.50, with an Abbe number in the mid- to high-50s range, contributing to low chromatic aberration in well-made lenses. The material accepts coatings for scratch resistance, UV protection, and anti-reflective performance, expanding its applicability in modern eyewear.

Mechanical and chemical properties: CR-39 offers good dimensional stability, weather resistance, and chemical resistance for a polymer. However, standalone CR-39 is relatively softer than polycarbonate and can be more susceptible to scratches. This motivates surface coatings to enhance durability in everyday wear. In nuclear-detector contexts, its chemical etching behavior is a central feature, not a limitation, because the same reactivity that reveals latent tracks also enables quantitative analysis of radiation exposure.

Manufacturing and processing CR-39 sheets and blocks are produced through controlled polymerization and curing processes that yield optically clear, flat substrates suitable for grinding, shaping, and polishing into finished lenses. The material can be molded or machined, allowing an array of thicknesses and tolerances to meet prescription needs in ophthalmic lenses. When used as a detector, CR-39 is formed into sheets and exposed to a radiation field; after exposure, the material is chemically etched to reveal conical pits that record particle tracks. This dual-use manufacturing capability—precision optics and controlled etchability—stems from the polymer’s chemistry and the established processing infrastructure around it.

Uses Ophthalmic lenses: The primary historical use of CR-39 has been in eyeglass lenses. Its combination of excellent optical quality, light weight, and cost-effectiveness made it a standard option alongside glass and, later, polycarbonate. Lens-makers often apply coatings to improve scratch resistance and UV protection. In consumer markets, CR-39 lenses have served as a reliable, high-clarity alternative where extreme impact resistance is not the sole priority, and where thinner lenses with good optical performance are valued.

Nuclear track detectors: CR-39 is also a widely used solid-state detector for ionizing radiation. When exposed to charged particles, latent tracks form within the polymer’s structure. After chemical etching, these tracks become visible pits that can be counted and characterized to infer the type, energy, and fluence of the incident radiation. CR-39 detectors are favored in some cosmic-ray studies, as well as in dosimetry and space physics, because they are passive, lightweight, and capable of high sensitivity to heavy ions. The detectors often require calibration against known sources and careful etching protocols to extract quantitative information.

Other applications: Beyond lenses and detectors, CR-39’s optical quality and processing traits have kept it of interest in niche optical components, art conservation contexts, and experimental setups where a stable, clear polymer is advantageous.

Performance, comparisons, and debates CR-39 versus polycarbonate: In ophthalmic use, a central trade-off is optical quality versus impact resistance. CR-39 offers superior intrinsic optical clarity and lower chromatic dispersion than many polycarbonate formulations, which has made it preferred for high-precision vision requirements in some markets. Polycarbonate, by contrast, provides higher impact resistance and inherently better shatter resistance, a critical factor for children’s eyewear, sports goggles, and safety spectacles. This has led practitioners and consumers to weigh the benefits of CR-39’s optical excellence against polycarbonate’s resilience, and to consider coatings and multi-layer designs to optimize performance.

Scratch resistance and coatings: Because CR-39 is relatively softer than some plastics, scratch resistance depends heavily on surface coatings. Modern ophthalmic practice often pairs CR-39 with anti-scratch and anti-reflective coatings to preserve optical quality over time. In detector contexts, the chemical processing step is a deliberately designed feature that reveals tracks; the term “scratch resistance” is less relevant there, but surface quality can still affect track visibility in some setups.

UV protection and coloration: CR-39 can transmit UV wavelengths unless blocked by coatings or additives. Many lenses incorporate UV-absorbing coatings or dyes to protect wearers from ultraviolet exposure, aligning with broader health and safety considerations about UV radiation and eye health.

Environmental and regulatory considerations: Like other resin-based materials, CR-39 production and disposal intersect with environmental considerations around manufacturing solvents, monomers, and waste streams. In detector contexts, the lifetime and storage of exposed CR-39 sheets require careful handling, calibration, and disposal according to relevant lab and safety regulations. Debates in procurement and industry practice often center on lifecycle cost, durability, and the balance between upfront material costs and long-term maintenance or replacement needs.

Controversies and debates In the sphere of eyewear, practitioners occasionally debate the long-term value proposition of CR-39 versus alternative materials like polycarbonate or high-index plastics. Proponents of CR-39 emphasize optical fidelity, low weight, and cost-effectiveness for a broad user base, while opponents stress that higher-impact requirements in sports or safety-critical situations may favor polycarbonate or other engineered materials. In detector physics, the choice of CR-39 hinges on a balance between sensitivity, track resolution, and processing practicality; researchers must calibrate etching procedures to ensure reliable track extraction across different experimental conditions. Across these contexts, the discussion centers on optimizing performance, durability, and total cost of ownership rather than on any political or ideological position.

See also - ophthalmic lenses - polycarbonate (plastic) - nuclear track detector - diethylene glycol bis(allyl carbonate) - etching (chemistry) - spectacle lens - Columbia Resin