What Standard Do I Use to Verify Yellowness Index Performance?

The Yellowness Index from ASTM International provides a measurable standard for assessing the yellowness in white or light-colored materials, such as plastics. This standard determines the degree of yellowness present, allowing companies to perform quality control processes to maintain product consistency. The test uses a spectrophotometer to quantify the color measurement.

The APHA color system is the most common measurement scale for measuring yellowness in liquids. It serves industries ranging from chemical production to pharmaceutical development.

Materials That Require Yellowness Observations

Various plastics require yellowness observations due to the changes these materials undergo in processing. These plastics include:

• Polyvinyl chloride (PVC)
• Polycarbonate (PC)
• Purified terephthalic acid (PTA)
• Polyethylene terephthalate (PET)

Yellowness appears in PET due to oxidation. With the increasing use of recycled PET (rPET), manufacturers must measure color to counter yellowness and determine the correct ratio of optical brighteners required to mitigate it.

What Is the Yellowness Index?

The Yellowness Index (YI) measures yellowness due to product changes. The ASTM E313 Yellowness Index is the most common method for determining yellowness in materials like plastics. The standard gives defined indices for measuring the shift from ideal white. It is calculated according to the following formula:

YI = (100CxX - CzZ)/Y

The X, Y, and Z variables are CIE Tristimulus values, which express color and how it is perceived. Cx and Cz depend on the illuminant and observer. During calculations, users choose the values for these coefficients from a table to correspond with spectrophotometer settings. The ASTM E313 combines the calculations into a single value, defining the yellowness amount in a sample. The lower the number, the less yellow the material is.

The YI value is commonly reported as the difference in yellowness between a product sample and the original or reference, designated as delta YI (∆YI).

How to Test for Yellowness in Plastics

You can find the Yellowness Index value of a material by using a spectrophotometer. This solution collects CIE Tristimulus values and has set numbers for the illuminant and observer portions of the Yellowness Index formula to simplify calculations.

Benefits of Using the Yellowness Index

Using this ASTM standard means measuring yellowness in plastic according to a set guideline. It's essential because yellowness in unpigmented polymers often signals degradation due to temperature, UV exposure, or chemicals. Off-color plastics can also affect how consumers perceive quality and influence their purchasing decisions. When yellowness is identified using a spectrophotometer, companies can adapt their processes or raw materials, adjusting optical brightener addition to achieve the ideal color.

What Is the APHA Color System?

While the Yellowness Index serves solids, the American Public Health Association (APHA) color scale or Hazen scale measures transparent liquid purity on a scale from 0 to 500, where 0 is the clearest. The measurement is based on dilutions of platinum-cobalt solution (PtCo). ASTM standard D1209 establishes a test for measuring clear liquids in terms of APHA units.

The APHA color scale has applications in numerous industries, including petroleum, chemicals, and pharmaceuticals. 

How to Test for Yellowness in Liquids

There are no physical liquid standards specifically designed to verify the Yellowness Index of liquid samples over time. The Yellowness Index is defined as a mathematical function such that a perfect clear of 100% transmission, typically represented by the transmission cell filled with DI water, will have a YI value of 0.

However, a good workaround solution is to use a very stable APHA/Pt-Co liquid standard in the yellowness range of your product as an application diagnostic to monitor your liquid measurement over time.

1. Purchase an APHA/Pt-Co 500 color standard that conforms to ASTM D1209. Prepare an intermediate APHA/Pt-Co liquid standard close in yellowness to your sample by diluting an APHA/Pt-Co 500 color standard with DI water following ASTM D1209.
2. Standardize the HunterLab sphere solution in TTRAN LAV mode using your cell filled with DI water.
3. Read the cell filled with DI water as a PQ (Performance Qualification) step after standardization. You should obtain CIE L* = 100, a* = 0, b* = 0, and YI = 0.0  if the solution is set up and standardized correctly.
4. Read the cell with the intermediate APHA/Pt-Co liquid color standard and baseline the Yellowness Index value of this stable yellow liquid on the first day. From that point on, read this APHA-Pt-Co liquid color standard after standardization as a further Application Diagnostic. The YI value should match the baseline value closely over time.
5. Create a statistical process control (SPC) chart with the YI values of the DI water and the dilute APHA/Pt-Co liquid color standard over time as a record of solution performance in the same region of color space as your sample.

Measure Yellowness With a Spectrophotometer

HunterLab has over 70 years of experience in spectrophotometry and market-leading solutions for measuring color, including yellowness. We offer innovative options for the plastics industry and various liquids, including chemicals and pharmaceuticals. Contact us to learn more about our products or for pricing and ordering advice.