The document discusses using silica fume and polyethylene glycol (PEG) to enhance the sintering of linear low-density polyethylene (LLDPE) powders in the rotomolding process. It presents experimental results showing that adding silica fume and PEG improves the free-flow properties of LLDPE powders and reduces bubbles in molded parts. The additions help dissolve bubbles by increasing the gas permeability of the polymer melt. The document concludes that silica fume acts as a free-flow agent while PEG combined with silica fume acts as a densification and processing aid for rotomolding LLDPE powders.

1. The Use of Silica-Fume and PEG to
Enhance Sintering of LLDPE Powders in
Rotomolding Process
TU Berlin
UniBw München

2. What is?
ROTATIONAL
MOLDING

3. a technology for manufacturing
hollow seamless articles

4. by
heating,
melting,
sintering

5. of powder

6. 1 mm
raw reactor powder of LLDPE

7. Problems

8. many
bubbles

9. cr c e
ac ta n
s is is low
k re

10. overheating

13. to

14. Experimental
RESULTS

15. Set-up for
blending

16. Set-up for
sintering

17. Uniaxial
Rotomolding

18. 1 mm
PE powder

19. after
10 min of heating
1 mm

20. Dissolving the bubbles
Patm
Pi
ΔP = Pi - Patm = 2T/r
If melt is in diffusion equilibrium with atmosphere
gas in the bubble tend to flow outwards because
of pressure difference ΔP until radius r of the
bubble diminishes to zero.

21. 1 mm
PE powder
+
SF (800 ppm)

22. after
10 min of heating
1 mm

23. o
42.5
PE powder + SF (800 ppm):
very good free-flow

24. PE powder: good free-flow
o
45

25. Example of
Gas Permeable Polymers:
Silicone Rubber: high flexibility of
the silicon-oxygen chain in siloxane
provides “openings” which are free
volume and permit gas diffusion, see
Zhang and Cloud (2006).

26. Gas Permeation

27. Siloxane – high flexible chain (Si-O-Si bond)
PEG – high flexible chain (C-O-C bond)

28. Gas Permeation of Polymers, Barrer
1000000
100000
10000
N2
1000 O2
CO2
100
H2O
10
1
LDPE Silicone PEG est.

29. 1 mm
PE powder
+
PEG (0.4%)

30. PEG beads
after
10 min of heating
1 mm

31. o
70
PE powder + PEG (0.4%)
o
at 72 C: no free-flow

32. PEG + Silica Fume

33. 1 mm PE powder
+
PEG (0.4 %)
+
SF(800 ppm)

34. after
10 min of heating
1 mm

35. o
60
PE + PEG(0.4%) + SF(800ppm)
o
at 72 C: free-flow properties

36. 1 mm
PE powder
+
PEG+CA (0.4%)

37. after
10 min of heating
1 mm

38. 1 mm
PE powder
+
PEG+CA (0.4%)
+
SF (800ppm)

39. after
10 min of heating
1 mm

41. Silica
Fume

42. Novel PPA: PEG
Silica
+
Fume

43. PEG
+
Carboxylic Acids
Silica +
Fume

44. What are micro-pellets?
• Small regular oval shaped
plastic granules with a
diameter between 500-600µm.
• Produced from the same base
material as powder, i.e. Qenos
711UV etc.
• Differ from powder by way of
the manufacturing process and
particle size distribution.

45. Process comparison
PELLETS POWDER
and
MICRO-PELLETS
Cyclone
Process Water PE pellets
Oversize
Centrifugal Dryer Particles
Sieves
Cutter Dust Collector
PDV
PE Powder Powder
190-210oC Grinding Blower
Water Head
Bath Bag 85-105oC
Extruder Bag Customer
Source: Practical Guide to Rotational Moulding
Die Plate Process Water
Source: Gala Industries by R Crawford & M Kearns
Pump

46. Particle size distribution comparison.
70
Ave = 550 µm
60
% By Weight
50
Ave = 300-350 µm
40
30
20
10
0
0 50 106 150 212 250 425 500 600 710 850 1000
Particle Size, Micron
Powder MicroPellets

47. What are the benefits of micro pellets?
Manufacturing Process
• Consistent quality and size
• Less contamination
• Dust free during mould filling
Shape and Particle Size Distribution
• Potential for faster cook times
• Potential for lower peak internal air temperature
• Greater bulk density (0.51kg/m3 cf. 0.34 kg/m3)
• Improved flow (10 sec cf. 32 sec)
• Potential for improved mechanical properties
• Improved filling and definition of difficult to fill mould areas
• Moulded parts with more uniform wall thickness
• Maintained / improved surface finish
Other
• Cheaper raw material
IMPROVED ECONOMICS AND MORE OPTIONS FOR THE MOULDER

48. Micro Pellet Tank Trial
Oven machine, Tank Size = 2,250L, Shot Weight = 55kgs, Wall thickness = ~5mm, Colour = Merino.
50
Cycle Time (mins)
45
40
35
30
25 > 12%
20
Powder Tank - 1 Tank - 2 Tank - 3
Cool Time (min) 17 16 16 17
Cook Time (min) 28 24 25 24
Cook Time (min) Cool Time (min)
Meet requirements of tank standard impact testing with no ductile or brittle
failures using a 9.06kg drop dart @ 1.4m.

49. Rotomolding PPAs:
Silica Fume (SF): "Free-Flow"
Agent.
PEG+SF: Densification &
Processing Aid (D&PA).
PEG + Carboxylic Acids (CA):
Advanced D&PA.
Novel Processing Additives:
PEG + CA + SF.

50. Extrusion PPAs:
Silica Fume (SF): Thickening
Agent.
PEG+SF+Anti-Oxydants:
Processing Aid (PA).
PEG + Carboxylic Acids (CA):
Advanced PA.
Novel Processing Additives:
PEG + CA + SF.

53. Process comparison
MICRO-PELLETS MICRO-PELLETS
WATER COOLING AIR COOLING
Process Water
Centrifugal Dryer
Cutter Cutter
PDV PDV
Powder Powder
190-210oC 125-135oC
Water
Bath Bag Extruder
Extruder
Die Plate Die Plate
Source: Gala Industries Process Water
Pump Bag
Air Cooler

54. Little things can make
BIG CHANGES

55. Cost Effective
patent pending

56. Acknowledgements:
Financial support:
the German Science Foundation
(Deutsche
Forschungsgemeinschaft).
Technical support
(PE powder samples):
ExxonMobil Chemical
SCG Chemicals