Rotational molding

TU Berlin UniBw München

The Use of Silica-Fume and PEG to The Use of Silica-Fume and PEG to Enhance Sintering of LLDPE Powders inEnhance Sintering of LLDPE Powders in

Rotomolding ProcessRotomolding Process

What is ROTATIONAL MOLDING

?

a technology for manufacturing hollow seamless articles

a technology for manufacturing hollow seamless articles

by heating, melting,

sintering

by heating, melting,

sintering

of powderof powder

1 mm1 mm

raw reactor powder of LLDPEraw reactor powder of LLDPE

ProblemsProblems

many bubbles

many bubbles

crack resistance

is lowis low

overheating overheating

to

ExperimentalExperimental

RESULTSRESULTS

Set-up forSet-up for

blendingblending

Set-up forSet-up for

sinteringsintering

Uniaxial Uniaxial

RotomoldingRotomolding

PE powderPE powder

1 mm1 mm

after 10 min of heatingafter 10 min of heating

1 mm1 mm

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.

PiPi

ΔP = Pi - Patm = 2T/rΔP = Pi - Patm = 2T/r

PatmPatm

Dissolving the bubblesDissolving the bubbles

PE powder+

SF (800 ppm)

PE powder+

SF (800 ppm)

1 mm1 mm

after 10 min of heatingafter 10 min of heating

1 mm1 mm

PE powder + SF (800 ppm): PE powder + SF (800 ppm): very good free-flowvery good free-flow

PE powder + SF (800 ppm): PE powder + SF (800 ppm): very good free-flowvery good free-flow

42.5o42.5o

PE powder: good free-flowPE powder: good free-flowPE powder: good free-flowPE powder: good free-flow

45o45o

Example of Gas Permeable Polymers:

Example of Gas Permeable Polymers:

Silicone Rubber: high flexibility of the silicon-oxygen chain in siloxane provides “openings” which are freevolume and permit gas diffusion, see Zhang and Cloud (2006).

Silicone Rubber: high flexibility of the silicon-oxygen chain in siloxane provides “openings” which are freevolume and permit gas diffusion, see Zhang and Cloud (2006).

Gas Permeation Gas Permeation

Siloxane – high flexible chainSiloxane – high flexible chain (Si-O-Si bond)

PEG – high flexible chainPEG – high flexible chain (C-O-C bond)

Gas Permeation of Polymers, BarrerGas Permeation of Polymers, Barrer

1

10

100

1000

10000

100000

1000000

LDPE Silicone PEG est.

N2

O2

CO2

H2O

PE powder+

PEG (0.4%)

PE powder+

PEG (0.4%)

1 mm1 mm

PEG beadsafter 10 min of heating

PEG beadsafter 10 min of heating

1 mm1 mm

PE powder + PEG (0.4%) PE powder + PEG (0.4%) at 72at 72ooC: no free-flowC: no free-flow

PE powder + PEG (0.4%) PE powder + PEG (0.4%) at 72at 72ooC: no free-flowC: no free-flow

70o70o

PEG + Silica FumePEG + Silica FumePEG + Silica FumePEG + Silica Fume

PE powder+

PEG (0.4 %)+

SF(800 ppm)

PE powder+

PEG (0.4 %)+

SF(800 ppm)

1 mm1 mm

after 10 min of heating

after 10 min of heating

1 mm1 mm

PE + PEG(0.4%) + SF(800ppm)PE + PEG(0.4%) + SF(800ppm)at 72at 72ooC: free-flow properties C: free-flow properties

PE + PEG(0.4%) + SF(800ppm)PE + PEG(0.4%) + SF(800ppm)at 72at 72ooC: free-flow properties C: free-flow properties

60o60o

PE powder+

PEG+CA (0.4%)

PE powder+

PEG+CA (0.4%)

1 mm1 mm

after 10 min of heating

after 10 min of heating

1 mm1 mm

PE powder+

PEG+CA (0.4%)+

SF (800ppm)

PE powder+

PEG+CA (0.4%)+

SF (800ppm)

1 mm1 mm

after 10 min of heating

after 10 min of heating

1 mm1 mm

Silica Fume

Novel PPA: PEG+

Silica Fume

Silica Fume

PEG +

Carboxylic Acids+

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.

Process comparison

Customer

Dust Collector

Blower

Bag

PE pellets Cyclone

Sieves

GrindingHead

OversizeParticles

Powder

85-105oC

Source: Practical Guide to Rotational Mouldingby R Crawford & M Kearns

POWDERPELLETS and

MICRO-PELLETS

Source: Gala Industries

Bag

Centrifugal Dryer

Extruder

PE Powder

Die Plate

PDVCutter

Process Water

Process WaterPump

WaterBath

190-210oC

Particle size distribution comparison.

0

10

20

30

40

50

60

70

0 50 106 150 212 250 425 500 600 710 850 1000

Particle Size, Micron

% B

y W

eig

ht

Powder MicroPellets

Ave = 300-350 µm

Ave = 550 µm

IMPROVED ECONOMICS AND MORE OPTIONS FOR THE MOULDER

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

Micro Pellet Tank Trial

20

25

30

35

40

45

50C

ycle

Tim

e (m

ins)

Cook Time (min) Cool Time (min)

Cool Time (min) 17 16 16 17

Cook Time (min) 28 24 25 24

Powder Tank - 1 Tank - 2 Tank - 3

Meet requirements of tank standard impact testing with no ductile or brittlefailures using a 9.06kg drop dart @ 1.4m.

Oven machine, Tank Size = 2,250L, Shot Weight = 55kgs, Wall thickness = ~5mm, Colour = Merino.

> 12%

Rotomolding PPAs: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.

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.

Extrusion PPAs: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.

Silica Fume (SF): Thickening Agent.

PEG+SF+Anti-Oxydants: Processing Aid (PA).

PEG + Carboxylic Acids (CA): Advanced PA.

Novel Processing Additives: PEG + CA + SF.

Process comparison

MICRO-PELLETSWATER COOLING

Source: Gala Industries

Bag

Centrifugal Dryer

Extruder

Powder

Die Plate

PDVCutter

Process Water

Process WaterPump

WaterBath

190-210oC

Bag

Extruder

Powder

Die Plate

PDVCutter

125-135oC

Air Cooler

MICRO-PELLETSAIR COOLING

Little things can make BIG CHANGES

Cost Effectivepatent pending

Acknowledgements:

Financial support: the German Science Foundation (Deutsche Forschungsgemeinschaft).

Technical support (PE powder samples): ExxonMobil Chemical SCG Chemicals