The Plasticization Effect of Hydroxylated Palm Stearin Plasticizer (HPS) Towards High-Density Polyethylene (HDPE) Via Compression Moulding

The world is awash in plastic, a material celebrated for its durability yet criticized for its persistence. Much of this plastic waste comes from single-use items that are used briefly and then discarded, overwhelming landfills and polluting ecosystems. In response, scientists are constantly searching for ways to make these materials more manageable, either by making them break down faster or by finding new ways to process them using renewable resources. One promising avenue involves tweaking the chemical makeup of common plastics to make them easier to work with or to introduce bio-based ingredients that could eventually help the material degrade. At the heart of this effort is the concept of a plasticizer, a substance added to a hard plastic to make it softer and more flexible, much like adding oil to dough to make it easier to knead. While synthetic plasticizers have long been the standard, researchers are increasingly turning to plant-based alternatives to reduce reliance on fossil fuels and improve the environmental profile of these materials.

In a recent study, researchers from Universiti Teknologi MARA in Malaysia explored a specific plant-based solution for one of the most common plastics in use today: high-density polyethylene, or HDPE. This is the tough, rigid plastic used to make everything from milk jugs to detergent bottles. The team investigated a new, chemically modified ingredient derived from palm oil, known as hydroxylated palm stearin, or HPS. Their goal was to see if this natural substance could act as an effective plasticizer for HDPE, improving how the plastic flows when it is melted and molded, without destroying its essential strength. By blending different amounts of this palm oil derivative into the plastic, the researchers aimed to create a bio-hybrid material that is easier to manufacture and potentially more environmentally friendly, while still retaining the useful properties that make HDPE so widely used.

The researchers began by taking solid pellets of HDPE and mixing them with the HPS powder in a machine that heats and stirs the mixture until it becomes a uniform, molten blend. They tested three different recipes, adding 10, 20, and 30 percent of the palm oil derivative by weight. As soon as the HPS was introduced, the machine's resistance dropped dramatically. The more palm oil derivative they added, the easier it became to mix the material. In the blend with the highest amount of HPS, the effort required to mix the plastic fell by nearly 80 percent compared to the pure plastic. This indicated that the plant-based ingredient was successfully slipping between the long chains of the plastic molecules, allowing them to slide past one another more freely. This reduction in resistance meant the material would be much easier and less energy-intensive to process in a factory setting.

However, this increased ease of processing came with a trade-off in strength. When the researchers pulled the molded plastic strips apart to test their durability, they found that the material became weaker as more HPS was added. The pure HDPE could withstand a significant amount of force before breaking, but the blend with 30 percent HPS lost nearly half of that strength. The material also became less stretchy; while the pure plastic could stretch to nearly five times its original length before snapping, the blend with the most HPS stretched to less than one-third of that distance. Looking closely at the broken surfaces under a powerful microscope, the team saw that the pure plastic had a tight, compact structure. In contrast, the blends with HPS showed small dark holes or voids, suggesting that the two materials did not fully merge into a single, seamless substance. These gaps, along with the increased space between the plastic chains, explained why the material became softer and less able to bear heavy loads.

The study also examined how the plastic behaved under heat. When heated, the pure HDPE melted at a specific temperature, but the addition of HPS lowered this melting point slightly, making the material ready to be molded at a lower temperature. Interestingly, the plant-based ingredient seemed to encourage the plastic to start forming its solid crystal structure at a slightly higher temperature as it cooled down. Despite these shifts, the overall amount of crystal structure within the plastic remained largely the same, meaning the fundamental nature of the HDPE was preserved even as it was softened. However, the heat resistance of the material did take a hit. The pure plastic could withstand temperatures up to over 400 degrees Celsius before it began to break down, but the blend with the most HPS started to degrade at a much lower temperature, dropping by more than 100 degrees. This suggests that while the new ingredient makes the plastic easier to melt and shape, it also makes the final product more sensitive to high heat.

Finally, the researchers used a technique that analyzes the light absorbed by the material to check for any chemical reactions between the HDPE and the palm oil derivative. The results showed that no new chemical bonds were formed; the two substances simply sat side by side, interacting physically rather than chemically. This confirmed that the HPS acted as a true plasticizer, working by physically separating the plastic chains rather than by chemically altering them. The study concludes that hydroxylated palm stearin is a viable, bio-based option for making HDPE easier to process, significantly reducing the energy needed to melt and mix the plastic. While it does weaken the material and reduce its heat resistance, the findings offer a clear path toward creating more sustainable plastic blends, provided that future work can find ways to improve how well the plant-based ingredient mixes with the synthetic plastic to maintain strength.