Anomalous particle size shift during post-milling storage.
OBJECTIVE
To investigate the anomalous phenomenon of particle size shift during post-milling storage.


METHODS
Crystallised and ball-milled adipic acid were stored under different humidity conditions. Analyses were carried out to characterise changes in particle size distribution (laser diffraction), morphology (SEM), bulk flow properties (annular shear tester), surface adhesion forces (AFM) and crystallinity (PXRD and DVS).


RESULTS
It was observed that the particle size distribution of milled adipic acid can shift to finer fractions, remain unchanged, or even shift to coarser fractions depending on storage conditions. SEM analysis showed that milled adipic acid is composed of agglomerates, which can undergo de-aggregation or further agglomeration via re-crystallisation. Empirical analysis ruled out the effects of electrostatic charges on the particle size shift. In addition, an improvement in powder flow in terms of bulk tensile strength was seen for milled adipic acid stored under high relative humidity but not under low humidity.


CONCLUSIONS
Storage of milled adipic acid below the critical relative humidity led to localised disintegration from the agglomerate surface and particle size reduction, which was not influenced by moisture sorption or loss. This evidence supports that "stress relaxation" mechanism behind particle breakage of post-milled particles. Appropriate storage conditions are important in maintaining the stability of milled powders.
Crystallisation and Particle Sciences, Institute of Chemical and Engineering Sciences, Jurong Island, Singapore. ng_wai_kiong@ices.a-star.edu.sg
Wai Kiong
WK
Ng
Jin Wang
JW
Kwek
Reginald B H
RB
Tan
D000268:Adhesiveness; D000272:Adipates / Q000737:chemistry; D000327:Adsorption; D000465:Algorithms; D002152:Calorimetry, Differential Scanning; D002626:Chemistry, Pharmaceutical; D003460:Crystallization; D004339:Drug Compounding; D004356:Drug Storage; D005740:Gases; D018625:Microscopy, Atomic Force; D008855:Microscopy, Electron, Scanning; D046529:Microscopy, Electron, Transmission; D010316:Particle Size; D055672:Static Electricity; D014961:X-Ray Diffraction
D000272:Adipates; D005740:Gases; C029900:adipic acid