HomePET Knowledge BaseColorimetric Analysis of Bottle-Grade PET Chips: Understanding L and B Values in the PET Industries

Colorimetric Analysis of Bottle-Grade PET Chips: Understanding L and B Values in the PET Industries

PET(Polyethylene Terephthalate), a polymer derived from the reaction of terephthalic acid (PTA) and ethylene glycol (MEG), is a highly crystalline material that can appear colorless, transparent, or slightly milky white. When selecting PET chips for bottle production, visual inspection of color, luster, and uniformity aids in determining the quality of the material.

In the production of PET bottles, the true transparency of bottle-grade PET chips is revealed in the molten state. The color tone of these PET chips is quantified using parameters such as L value (brightness), a value (red-green tone), and b value (yellow-blue tone), with L value and b value being particularly critical for quality assurance.


1. Understanding L value and b value
L value (Brightness): It represents the luminosity or whiteness of the color. The scale ranges from 0 (black) to 100 (white). Higher L values indicate greater brightness and transparency in PET chips, while lower values suggest a darker or grayer appearance.
b value (Yellow-Blue Tone): Indicates the yellow-blue color bias. A positive b value suggests a yellow tint, while a negative b value indicates a blue tint. In PET chips, an ideal b value is close to 0 to prevent color deviations.


2. Case Analysis: L value and b value of WK-801 bottle-grade PET chips
L value ≥ 83: This threshold signifies that WK-801 possess significant brightness and whiteness, which is essential for producing PET bottles with high transparency. A high L value enhances product visual appeal and market competitiveness.
b value ≤ -0.5: This suggests that the color tone of the WK-801 leans slightly towards blue. With an ideal b value typically ranging from -3 to 0, a b value of -0.5 indicates a very low yellow index, approaching colorless, thus helping to maintain color neutrality in the final product and prevent color deviations.

These metrics collectively ensure that WK-801 exhibit high transparency and color stability, making them suitable for manufacturing high-quality PET bottles, particularly in applications where transparency and color consistency are paramount, such as in food, beverage, and personal care product packaging.


3. Innovation: Wankai's Color Value Control Method in Bottle-grade PET Production
Recognizing the need for color value stability and customer satisfaction, the industry employs measures to control the color value of bottle-grade PET resins. However, conventional methods are often inefficient and result in delayed color value readings, impacting production efficiency.

Wankai New Materials Co., Ltd. (    Wankai) has pioneered an innovative approach to color value control in the production of bottle-grade PET chips, which streamlines the process, improving production efficiency while maintaining color value consistency. This method involves:


a. Establishing color value control system targets for bottle-grade PET resins, setting predetermined color value reference points.

b. Utilizing a color value analyzer to measure the a and b values of the polyester melt at the outlet of the final polymerization reactor.

c. The color value control system then transmits these readings to the pneumatic control valves of the red and blue agents via the DCS console's calculation module.

d. By adjusting the flow of the red and blue agents through these valves, the color value is regulated to the desired levels.


Conclusion: In selecting PET resins for PET bottle production, the color tone, particularly L value and b value, is a vital indicator for quantifying color characteristics. Ideal PET resins should exhibit high L values and near-zero b values to ensure high transparency and prevent color deviations. Wankai's commitment to quality control innovation is reflected in the introduction of this advanced color value control method, which enhances product quality and customer satisfaction.

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