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Corona treatment of PLA (polylactic acid)

PLA is an organic plastic material, totally compostable, increasingly used in the packaging industry. Ferrarini & Benelli performed a test to demonstrate good reactivity of this material to corona treatment.

Polylactic acid (PLA) is a polymer derived from plants rich in natural sugar like corn, wheat or beet. The dextrose is converted to lactic acid through a fermentation process and subsequently to versatile polymers that can be used to produce resins similar to plastic or fibre.

Wetting ability of PLA before and after corona treatment

Wetting abilityis an indicator that evaluates the possibility of implementing optimal printing and laminating processes on the material.

Ferrarini & Benelli created a test for corona treatment of PLA in flexible film format (EarthFirstTM EF BCP 25µ) supplied by SIDAPLAX in partnership with NatureWorks LLC which produces PLA under the Ingeo brandTM

For the measurement of results, the ASTM D25 78 method was employed with the use of Formamide and Cellosolve solutions and a brush applicator. The use of other applicators or different solutions may produce different results.

1 Corona treatment with ceramic electrodes and discharge roller coated with conductive ceramic

Test performed on laboratory equipment on material samples of PLA sheets. Initial surface tension: 38 dyne/cm.

Power used

Final surface tension

40 watt*min/m2

46 dyne/cm.

80 watt*min/m2

48 dyne/cm.

>100 watt*min/m2

48 dyne/cm.

2 Corona treatment with aluminium electrodes and discharge roller coated with silicone sleeve

Tests carried out on laboratory equipment on material samples of PLA sheets. Initial surface tension: 38 dyne/cm

Power used

Final surface tension

30 watt*min/m2

46 dyne/cm.

60 watt*min/m2

48 dyne/cm.

>100 watt*min/m2

48 dyne/cm

3 Corona treatment with ceramic electrodes and roller coated with conductive ceramic

Test carried out on winding-unwinding line on material samples of PLA reels. Initial surface tension: 38 dyne/cm.

Power used

Final surface tension

40 watt*min/m2

48 dyne/cm.

80 watt*min/m2

50 dyne/cm.

>100 watt*min/m2

50 dyne/cm

The use of plasma torch treatment has proved to be infeasible as the plasma-discharge has a very high temperature that deforms the material.

Conclusions

Tests show good reactivity of the PLA material to the corona effect. The levels of surface tension reached are quite high and this implies good wetting ability.  It can be assumed therefore that PLA is a material that lends itself well to being transformed in various stages of packaging (printing or laminating). The tests also show that the material:

  • after reaching a certain value presents a barrier and does not increase its wetting ability further even with the use of greater power
  • when treated with aluminium electrodes requires power coefficients lower than with the use of ceramic electrodes (25% less power)
  • when treated in line presents more satisfactory results compared to laboratory tests (surface tension greater than 2 dyne/cm under the same conditions).