Measuring High- and Low-Density Content in Polyethylene Blends
How to Determine the Amount of PE Grade in a Given Product
06 November 2018
Polyethylene (PE) is a polymer used to make many everyday items such as milk jugs, grocery bags, films and more. There are primarily three grades of PE: high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE). Each grade has its own set of mechanical properties; because of that, various grades of PE are often blended together to produce an article with the desired properties.
There are many manufacturers of PE and each produces many resins for a particular grade, with each resin finding utility for a given application. For example, HDPE resins may differ in their viscosity in the melt and require different molding conditions or be used in different applications. Many manufacturers require help to determine how much of each PE grade is in a given article. While this request is seemingly simple, the answer can be quite complex, in part because of the diversity of PEs that are available in the marketplace. Below is a look at how Intertek can address the question, based on previous experience.
The obvious answer to the question is to measure the density of the article and then calculate the relative amounts of each grade – after all, we are combining a high-density material with one of low density. The density of the article should be somewhere in between and dictated by the blend ratio. The problem is that the difference in density between HDPE and LDPE is relatively small – on the order of 0.05 g/cm3. Additionally, given the number of different resins for a given PE grade, there is a variation in density. When you combine that fact with the myriad of resin choices, the idea of using density alone to determine the composition of a PE blend is no longer feasible.
Even though measuring density is not a viable option for determining the composition of PE blends, there is a series of thermal, mechanical, and spectroscopic methods that can aid in answering the blend composition question. While these methods do not produce an exact composition, they certainly can provide a good estimate for the blend composition. A series of PE blends of known composition can be prepared and evaluated using analytical methods in order to validate this approach. Over a broad composition range, the predicted composition is typically within 10 percent of the actual composition.
Learn more about Intertek's polymer testing services and how PE grade can be determined by checking out our polymer testing webpage.
Menas Vratsanos is Chief Scientist and Research Fellow for Intertek at their laboratory in Allentown, Pennsylvania. He is responsible for the development, execution, and management of non-standard analytical test plans for clients, especially as they relate to polymers. He is a member of the American Chemical Society, the Society for Plastics Engineers, the Society for Plastics Institute and the Society for the Advancement of Material and Process Engineering. He holds a Bachelor's degree in Chemical Engineering from Columbia University and a Master's degree and Ph.D in Polymer Science and Engineering from the University of Massachusetts.