ISO 489 Plastics - Determination of Refractive Index

The ISO 489 Plastics - Determination of Refractive Index standard, published by the International Organization for Standardization (ISO), describes two test methods used to determine the refractive index of plastics:

• Method A: This is a refractometric method used to measure the refractive index of molded parts, cast or extruded sheets or films using a refractometer. It is applicable not only to isotropic transparent, translucent, colored or opaque materials, but also to anisotropic materials.
• Method B: This is an immersion method used to determine the refractive index of transparent materials in powder or granular form using a microscope (utilizing the Becke line phenomenon). Monochromatic light is usually used to avoid dispersion effects.

Refractive index is a fundamental property that can be used to control purity and composition, identify materials, and design optical components. The variation of refractive index with temperature can provide an indication of the transition points of materials.

Preparation of test samples:

• Method A: Samples are cut from the specimen to a size that will fit on the surface of the fixed half of the refractometer prism. For plate samples, the thickness should be between 3 mm and 5 mm.

For maximum accuracy, the surface of the test specimen in contact with the prism (measuring surface) must be optically flat and well polished. Burrs from cutting or any contaminants adhering to the specimen must be removed. Satisfactory contact between the specimen and the prism is indicated by the dividing line between the light and dark halves of the eyepiece field appearing sharp and straight. The edge of the specimen (perpendicular to the first) must be optically flat and fairly well polished. The two polished surfaces must intersect along a sharp line without any beveled or rounded edges. For film specimens, the thickness must be the actual film thickness.

• Method B: The test specimen consists of particles of the material to be examined, e.g., powder, granules, or chips. The particles should be small in size and distributed sufficiently to allow simultaneous observation of approximately equal areas of the specimen and its surroundings within the field of view. The thickness of the test specimen should be significantly less than the working distance of the microscope objective.

For sheets or films, five samples are required. For powders, pellets, and granules, a sufficient quantity of samples is needed to perform five measurements.

Unless otherwise specified in the relevant material specification, samples shall be conditioned for at least 88 hours prior to testing at a temperature of 23 degrees ± 2 degrees Celsius and a relative humidity of 50 percent ± 5 percent, in accordance with ISO 291. The testing apparatus shall be set up in an environment maintained at a temperature of 23 degrees ± 2 degrees Celsius and a relative humidity of 50 percent ± 5 percent.

In short, the ISO 489 standard falls under the category of plastic testing (ICS 83.080.01) and is widely used in materials science, quality control, and the manufacturing of plastic products.

The refractive index measures how much light bends when passing through a material from a vacuum (or air). It is defined as the ratio of the speed of light in a vacuum to the speed of light in the material. This property is very important for plastics because it helps with:

• Identifying materials: Different polymers have characteristic refractive indices (e.g., polycarbonate 1,58-1,59, PMMA approximately 1,49).
• Evaluating optical quality: This is important for clear plastics used in lenses, plates, films, or screens.
• Detecting variations: Identifying variations in composition, additives, or processing conditions that affect transparency or performance.
• Applications supported: In optics, packaging, automotive parts, and medical devices.

A higher refractive index generally means a greater ability to bend light, which can indirectly affect transparency, color, and even mechanical properties.

This standard specifies two basic test methods for determining the refractive index:

• Method A: Refractometric method (preferred and most versatile): Uses a refractometer to directly measure the critical angle of total internal reflection. Applicable materials include:
• Molded parts, cast or extruded sheets or films.
• Isotropic (homogeneous in all directions) and anisotropic (direction-dependent) materials.
• Transparent, translucent, colored, or even opaque plastics.

The sample must have a flat, polished surface for contact with the refractometer prism. Monochromatic light (e.g., sodium D-line at 589 nm) is generally used for sensitivity.

• Method B: Immersion method (alternative for some cases): The sample is immersed in liquids with known refractive indices until matching (Becke line test under a microscope) minimizes the visibility of the sample edges. This method is suitable for isotropic translucent or colored materials. However, it is not applicable to opaque or anisotropic materials. Accuracy is comparable to Method A but requires more effort.

Both methods recommend controlled conditions (e.g., temperature according to ISO 291 standard) and report results to four decimal places for accuracy.

In industry, compliance with the ISO 489 standard ensures consistent material characterization. It is used as a reference in data sheets from suppliers (and laboratories providing testing services). For example, it is crucial in verifying optical-grade plastics for eyeglasses, windows, or electronic displays. The ISO 489 standard plays a quiet but critical role in developing reliable plastic materials for everyday products.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services to businesses in accordance with the "ISO 489 Plastics - Determination of Refractive Index" standard.