How To Measure PET Pellets Using CTHT

PET(E)-extruded pellets are created from Polyethylene Terephthalate, a form of thermoplastic polyester resin derived from the combination of ethylene glycol and either purified terephthalic acid or dimethyl terephthalate. These resins possess qualities which lend them well for use in a variety of applications; they have high strength and clarity, are resistant to heat, and have good moisture and gas barrier properties. Typically, these pellets are clear or translucent and are used for the production of plastic bottles and containers. Other applications include use in textiles as spinning fiber for the carpet industry, geosynthetic materials, and fiberfill. PET pellets are also widely used in the injection molding of many plastic products. The material from which they are made can be either virgin or recycled; strict requirements for quality assessment and control often exist for PET processing due to the inclusion of recycled materials in their manufacturing.

In the compounding of PET pellets, molten polymer is drawn through dies in order to form resin strands. These strands are then either advanced through a water bath for solidification and subsequently cut into specified lengths, or they can, through use of underwater pelletizers, be cut immediately upon exiting the dies. At this stage in the process, PET pellets are transported to vibrating shaker tables containing screening, such that material can be segregated by size. There are optimal size requirements for individual pellets in this application, and those that are dimensionally unsuitable are separated from acceptable product at this area. At this point in the process, the ColorTrend HT online color measurement instrument can be utilized in order to determine product color. Of particular importance are typically the L and b values, which signify lightness to darkness and yellowness to blueness. While specifications differ, L values must usually be greater than 73 units and b values less than 3.0. Some companies also require a values to be approximately –1.0 to –1.5 and have Yellowness Index (YI) requirements in the range of 4-16 units. The Whiteness Index can also be measured. Of concern to the manufacturer are also additive quantities used to adjust strength, clarity, and UV properties of the pellets. Changes in these often result in colorimetric changes to the product, which are detectable by the ColorTrend HT. For clear or translucent pellets, a bed depth of 0.5" is typically necessary. This can be achieved by either controlling the flow rate of pellets; placing a small sheet of plating under the sensor, so that the pellets cannot fall through at this location; or installing a small dam after the sensor, such that the pellets accumulate under the sensor and the requisite bed depth can be maintained. Ideally, the sensor should be mounted such that the viewing window is 3-4" from the product surface. The ColorTrend HT has the capability of compensating for height, so small differences in the presentation of the pellets to the sensor is not of concern, so long as the minimum bed depth of 0.5" is met.