Scientists researching the way in which polymers crystallise have made a breakthrough which has far-reaching implications for the plastics industry’s ability to manufacture polymers consistently. The research could be relevant to 75% of polymers. But so far the research team has been unable to raise funds to apply its findings in process plants.
The problem first came to light with filter coffee sachets made by Four Square for Flavia vending machines. The sachet containing the coffee consists of a filter paper within a polypropylene laminate. The bottom of the sachet is sealed by a different method from the sides, so that when boiling water is introduced through the top the sachet bursts at the bottom, allowing the coffee to run into the cup.
Occasionally a batch of sachets would burst in the wrong place, despite being made under ostensibly the same conditions.
Working with Four Square and Flavia, the University of Manchester Institute of Science and Technology’s materials science centre and the Central Laboratory of the Research Council’s Daresbury Laboratory, won grant funding under the Realising Our Potential awards scheme to investigate the science behind the industrial process.
They set up a 1/40th scale model of the process used to extrude polypropylene beads into a film from which sachets are made. The film changes from an amorphous to a crystalline state during extrusion. The scientists used a synchotron X-ray beam (a very intense X-ray source) at Daresbury to find the point of cystallisation and observe it.
Wide angle X-ray scattering allowed the detailed molecular level to be seen while less sensitive narrow angle scattering showed the formation of large molecules.
The results challenged conventional wisdom about the process. The theory states that crystallisation begins in one place first and then this propagates to form the polymer – a long chain of linked molecules. `We didn’t see that at all,’ said UMIST researcher Dr Patrick Fairclough.
Instead, narrow angle scattering results showed that groups of molecules first get into the right orientation to form the polymer chain and these separate out from the rest of the material before any chemical bonds are formed. The density of this separate phase gradually increases until crystallisation occurs.
`This is the reverse of the theory,’ said Fairclough.
The team now want to apply this in the plant, but though industry funding is available, the team has been unable to get a matching research council grant.
It would not be possible to use synchotron X-rays in a plant, but `now we know what we’re looking for we can use, for example, light or infra-red scattering to say it crystallises here,’ he said.
Defining the point of crystallisation would enable the conditions at the critical stage to be controlled precisely, so that the product was always the same.
By David Fowler