Compatibilization of LDPE/PP Waste Blends Using Acrylamide-Functionalized Phenol- Formaldehyde

Every day, the world generates a vast stream of discarded plastic, much of it coming from packaging films, containers, and household goods. Two of the most common materials in this waste stream are low-density polyethylene and polypropylene. While these materials are chemically similar in that they are both non-polar plastics, they behave very differently when melted down together. Imagine trying to mix oil and water; no matter how hard you stir, they separate into distinct layers rather than forming a single, uniform liquid. In the world of recycling, this separation is a major obstacle. When these two plastics are melted together without help, they form a weak, brittle material that falls apart under stress, rendering the mixed waste useless for making new, strong products. This fundamental incompatibility means that mixed plastic waste often ends up in landfills or incinerators, even though the individual components could be valuable if they could be made to work together.

Researchers have long sought a way to bridge this gap, a method to force these two stubborn plastics to bond and function as a single, cohesive material. A team of scientists from Azerbaijan and France has now demonstrated a promising solution using a specially designed chemical additive. Their work focuses on taking unsorted waste containing both low-density polyethylene and polypropylene and treating it with a custom-made substance called an acrylamide-functionalized phenol-formaldehyde oligomer. This additive acts as a mediator, chemically reaching out to both types of plastic to hold them together. The result is a transformation of a problematic, mixed waste stream into a robust, high-performance material that can be processed and used again.

The journey began with raw materials sourced from a local polymer facility in Sumgait, Azerbaijan. The researchers started with waste granules of low-density polyethylene and polypropylene, cutting them into small, uniform pieces to ensure they would mix evenly. Before any blending could take place, these plastic particles were thoroughly dried in an oven to remove any moisture, a crucial step to prevent defects during the melting process. The key to the experiment was the creation of a new type of compatibilizer, a substance designed to improve the relationship between the two plastics. The team synthesized this additive by modifying a phenol-formaldehyde resin, a type of polymer often used in adhesives and coatings, by attaching acrylamide groups to its structure. This chemical modification was not random; it was carefully engineered to create a molecule that could interact effectively with the specific chemical nature of both polyethylene and polypropylene.

To ensure this new additive would survive the harsh conditions of industrial recycling, the team first tested its thermal stability. They heated the material to see at what point it would begin to break down. The results showed that the additive remained completely stable up to temperatures well above the 190 degrees Celsius used for melting the plastics. It did not evaporate or decompose prematurely. Instead, it held its structure intact, ready to perform its job of binding the two plastics together. The chemical analysis confirmed that the acrylamide groups were successfully attached to the resin backbone, creating a molecule with the right functional groups to act as a bridge between the two immiscible polymers.

With the additive ready and proven stable, the researchers moved to the blending phase. They mixed the waste plastics with the new additive in a small-scale machine that mimics industrial extrusion, the process used to melt and shape plastics. They used a fixed ratio of fifty percent low-density polyethylene and fifty percent polypropylene, adding the compatibilizer in amounts ranging from two to eight percent of the total weight. The mixture was heated to 190 degrees Celsius and forced through a screw mechanism that sheared and mixed the materials together. This reactive process allowed the additive to interact with the plastics while they were in a molten state, creating chemical links at the interface where the two materials met.

The most striking evidence of success came from looking at the microscopic structure of the resulting material. When the researchers examined the unmodified blend under a powerful electron microscope, the image revealed a rough, chaotic landscape. The two plastics had separated into large, distinct islands with jagged boundaries and visible gaps between them. These gaps were weak points where the material would easily crack. In contrast, the samples containing the new additive showed a completely different picture. The rough, separated domains disappeared, replaced by a smooth, continuous surface where the two plastics were intimately mixed. The boundaries between them became indistinct, and the material looked like a single, unified substance rather than a mixture of two. This visual change indicated that the additive had successfully reduced the tension between the layers, allowing them to adhere tightly and share stress effectively.

Further testing confirmed that this structural change translated into real physical improvements. When the team measured how the material flowed while molten, they found a significant shift. The unmodified waste plastic flowed easily, like thin honey, which is typical for a weak, unconnected mixture. The modified blend, however, flowed much more slowly and with greater resistance. This increase in viscosity suggested that the molecules were no longer sliding past each other freely but were instead held together by the new chemical connections formed by the additive. The material had become thicker and more cohesive, a sign of a stronger internal structure.

The thermal behavior of the blend also changed in a way that pointed to better integration. When the unmodified plastics were heated, they melted at two distinct temperatures, clearly showing that they were acting as two separate entities. The low-density polyethylene melted around 110 degrees Celsius, while the polypropylene melted near 165 degrees Celsius. However, the blend treated with the additive showed a more unified melting pattern. Instead of two sharp, separate peaks, the material exhibited a single, broader melting range. This shift suggested that the two polymers were interacting so closely that they began to organize themselves together, a phenomenon known as co-crystallization, which is a strong indicator of compatibility.

The durability of the new material was also tested by heating it until it burned away. The unmodified waste left behind a small amount of residue, approximately 1.0 wt% char, rather than turning completely into gas. The blends containing the additive, however, left behind a significantly larger amount of solid char. As the amount of additive increased, the amount of this protective residue grew, reaching 7.9 wt% in the samples with the highest concentration. This char formation is a sign that the additive created a more stable, crosslinked network within the plastic, one that could withstand higher temperatures and resist breaking down as quickly as the raw waste.

Finally, the researchers measured the density of the materials to see how tightly the molecules were packed. The unmodified waste had a lower density, indicating that there were tiny voids and gaps within the structure where the two plastics failed to touch. The modified blends were consistently denser, suggesting that the additive had pushed the materials closer together, eliminating those empty spaces and creating a more compact, solid structure. This densification, combined with the improved flow resistance and thermal stability, paints a clear picture of a material that has been fundamentally upgraded from a weak, separated mixture to a strong, integrated composite.

The study concludes that this approach offers a viable path forward for recycling mixed polyolefin waste. By using a tailor-made additive to force these incompatible plastics to bond, the researchers have demonstrated that it is possible to transform unsorted plastic waste into a structurally robust thermoplastic material. This method bypasses the need for expensive and time-consuming separation processes, turning a difficult waste stream into a resource for higher-performance applications. The findings suggest that with the right chemical intervention, the limitations of mixed plastic recycling can be overcome, paving the way for more sustainable and efficient use of plastic resources.