ADDITIVES | COMPATIBILISERS
Figure 3: The repolymerisation effect of phosphato titanate catalysis on a 50/50 LDPE/PP regrind blend after six heat cycles demonstrated by melt index values. Control contains no catalyst Source: Salvatore Monte/Kenrich Petrochemicals
mechanically tough product out of PE/PP blends. Their latest work, related to compatibilising isotactic PP and HDPE, is described in the Ameri- can Chemical Society’s journal Macromolecules, 2018 51 (21), 8585-8596. “Tensile tests revealed a dramatic enhancement in toughness based on the strain at break, which increased from 10% for the unmodified blend to more than 300% with just 0.2 wt% BCP and over 500% with the addition of 0.5 wt% BCP or greater,” the researchers say. “Incorporation of BCPs in blends also improved the impact toughness, doubling the Izod impact strength to a level comparable to the neat HDPE with just 1 wt% additive.” The researchers believe these large improve-
ments are based on molecular entanglements and co-crystallisation of tetrablocks and diblocks, respectively. “These results demonstrate exciting opportunities to recycle the world’s top two polymers through simple melt blending, obviating the need to separate these plastics in mixed waste streams,” they say. Addition of the compatibiliser does not require
any reactive compounding, a process that the Cornel team say presents many drawbacks. Firstly, they point out that reactions must be completed faster than the residence time in blending, which greatly limits the range of viable chemical reac- tions. In addition, they say the formed chemical bonds need to be stable under processing conditions involving relatively high temperatures and high shear, hazardous catalysts with heavy metal components are often needed to promote the reactions, and cross-linking often complicates control over morphology during processing. On the other hand, nonreactive compatibilisa- tion involves just the simple addition of premade compatibilisers along with the PP and PE during the blending procedure. “Ideally, the compatibilis- ers migrate to and remain at the interfaces defining the separate phases, ‘emulsifying’ the blend and facilitating the formation of finer dispersions of the minor phase,” say the researchers. Asked about the commercial prospects for the development, Coates told Compounding World magazine: “We have a partner and are gaining momentum. With that said, resin prices being so low are making it a tough business case.” He said he hoped to have more information to share on future developments by the end of this year.
Reactive options Salvatore Monte, President of Kenrich Petrochemi- cals, does see potential in the use of reactive compounding techniques and has long promoted the use of titanium and zirconium catalysts to regenerate various filled and unfilled plastics through repolymerisation. At the Society of Plastics Engineers’ PO2019 conference in Houston in Texas, US, in February, he presented plenty of data examples showing how they can work with mixes of polyethylene and polypropylene. “For example, where delamination occurs in the injection moulding of HDPE parts containing more
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