visualising tread wear - guildford tyre... · • bulk of tyre treads and akron test pieces retain...
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Tun Abdul Razak Research Centre (TARRC) A RESEARCH & PROMOTION CENTRE OF THE MALAYSIAN RUBBER BOARD
Visualising Tread Wear Dr Pamela Martin, Materials Scientist
4th International Tyre Colloquium, April 2015
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• Consumers want fuel economy, safety, low cost and long service life.
• Legislators are introducing steps to reduce the impact of transport.
• Growing concerns about climate change and oil price/availability
demands for renewable materials with low carbon footprints.
Tyre Performance
• Resistance to wear is a critical
attribute for tyre performance
EU Tyre label 2012
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Passenger tread compounds: • Carbon black substituted with silica filler fuel efficient tires • sSBR/BR silica/silane technology low rolling resistance and improved wet grip • Free of aromatic oil (PAHs) Health and environment
Last 25 Years of Tire Tread Technology
Truck/Bus tread compounds: • NR/BR or NR/SBR blends • Black-filled (Non-rubbers in NR consume or block coupling silane during
silica mixing)
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• An ecologically sustainable rubber from renewable resource
• Is more polar than any other tire rubber inherently good interaction with silica
• Has a large negative carbon footprint Rubber trees sequester carbon dioxide
• Suitable for both passenger and commercial vehicles
• Ekoprena-25: Commercial grade 25% epoxidised natural rubber
• Produced by epoxidation of natural rubber latex
Current Developments Tire Tread Technology Industry aims:
• Continual improvements in rolling resistance and wet grip
• Environmental sustainability
Solution:
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Truck/Bus Tyre Tread Formulations
Truck/Bus tread compounds: • Ekoprena-25 with 55 phr silica filler, low level of coupling silane • NR/BR (70:30) with 53 phr N234 black
Physical properties matched: • Hardness • Modulus • Tensile strength
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• Used in silica-filled tyre treads Ekoprena provides
Silica-filled Ekoprena Tyre Treads
Significantly improved ice grip possibly extending the range of
useable temperatures of low rolling resistance tyres
Very low rolling resistance good fuel economy
Exceptionally high wet grip good safety
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Service data
Coach trials – rear drive axle Ca. 18,000 km
52 km/day urban
Akron Abrasion Test
BS 903 A9 1988 Method B 5500 cycles per test at 15° slip angle
250 rpm ± 5 rpm Carborundum & fuller’s earth (2:1)
Techniques Used for Assessing Wear
Correlation of laboratory results to in-service road test results is notoriously poor!
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Coach Wear Trials
Tyres fitted on rear drive axle
Tread samples removed after ca. 18,000 km urban usage (52 km/day)
Tyres retreaded at TARRC
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Circumferential
direction NR/BR black Ekoprena silica
Worn Truck/Bus Tyre Tread Samples
ca. 0.80 nm depth loss per cycle ca. 1.26 nm depth loss per cycle
LM Macro-texture
Ra = 3.3 μm Ra = 7.3 μm
Periodic abrasion pattern observed No abrasion pattern
Dust and debris on surface
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Rubber-silica Interaction by ‘Network Visualization’–TEM: methodology
• Swell vulcanisate in polymerisable monomer, e.g. styrene • Polymerize monomer • Cut ultrathin sections of embedded ‘swollen’ vulcanizate • Stain section to identify rubber unsaturation, or unstained
for estimation of microdispersion • Image via TEM
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Sections not stained to see only filler particles Example: Network Visualization TEM
Ekoprena silica
0
20
40
60
80
100
100 1000 10000 100000
Cu
mu
lati
ve f
req
ue
ncy
, %
Aggregate area, nm2
Ekoprena
NR
Particle size distribution
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Sections stained to see rubber network
Example: Network Visualization TEM
NR Rubber
Network
Polystyrene
Mesh size relates to physical cross-link density
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Sections stained to see rubber network and rubber-filler Interactions Example: Network Visualization TEM
Polystyrene Vacuoles
NR Rubber
Network
Zinc oxide
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Vacuole
SBR Rubber Network
Silica
No silane coupling agent Silane coupled
Example: Network Visualization TEM
Ladouce, Bomal, Flandin and Labarre
Paper No. 33, ACS Rubber Division Meeting, April 4-6 2000
Rubber Chem. Technol., Vol 76, 145, (2003)
Sections stained to see rubber network and rubber-filler Interactions
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Example: Network Visualization TEM
SBR
Silica
Relative surface occupied by vacuoles decreased with increasing silane content.
Ladouce, Bomal, Flandin and Labarre
Paper No. 33, ACS Rubber Division Meeting, April 4-6 2000
Rubber Chem. Technol., Vol 76, 145, (2003)
Vacuole
No silane coupling agent
Sections stained to see rubber network and rubber-filler Interactions
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Ekoprena silica NR/BR black
Worn Truck/Bus Tyre Tread Samples
• Detached and partially detached particles observed • Larger particle in CB NR/BR sample
TEM (unstained)
Samples styrene swollen and polymerised
Polystyrene
Road dirt
ca. 1.26 nm depth loss per cycle ca. 0.80 nm depth loss per cycle
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Worn Truck/Bus Tyre Tread Samples
• Voids observed around filler particles – weakened rubber-filler interactions
TEM (stained)
ca. 1.26 nm depth loss per cycle ca. 0.80 nm depth loss per cycle
Vacuoles
NR/BR black Ekoprena silica
BR phase
CB
NR phase
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Worn Truck/Bus Tyre Tread Samples
• Voids around filler – weakened rubber-filler interactions
Vacuoles
TEM (stained)
NR/BR black Ekoprena silica
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Worn Truck/Bus Tyre Tread Samples TEM (stained)
• Voids around filler – weakened rubber-filler interactions
Vacuoles
NR/BR black Ekoprena silica
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Worn Truck/Bus Tyre Tread Samples
• Bulk does not show weakened rubber-filler interactions
NR/BR black Ekoprena silica TEM (stained)
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Circumferential
direction
Laboratory Wear Performance – Akron
Abrasion Resistance Index 119% ca. 1.3 nm depth loss per cycle
Abrasion Resistance Index 220% ca. 0.7 nm depth loss per cycle
Akron Abrasion Resistance Index relative to Standard 50 phr black-filled NR
LM macro-texture
• Coarser/rougher surface corresponds to higher wear rate • Pitch of ridges changes across test piece
NR/BR black Ekoprena silica
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NR/BR black Ekoprena silica
Carborundum & fuller’s earth
Laboratory Wear Performance – Akron
Polystyrene
Filler
Abrasion Resistance Index 119% Abrasion Resistance Index 220%
TEM (stained)
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Modeling Tyre Wear Processes
Model: • Movement of tread block surface during revolution of tyre • Deformations of tread surface by road asperities
• Heat generation at surface during tyre use
• Contact and slippage between tyre and road
What might we model for the surface layer?
Image taken from www.thermal-cameras.com
Load
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Modeling a Tyre Going Over an Asperity
1 mm radius rigid spherical asperity
Results: • Asperity penetrates rubber by 0.5 mm
vertical displacement (mm)
Model parameters: • Rubber dimensions much larger than contact region • Frictionless contact • Neo-Hookean rubber model
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Conclusions
• Wear pattern seen for worn CB-filled NR/BR tyre tread samples
significantly different to that observed for Akron test pieces
• Reduced rubber-filler interaction observed at or near surface of on-
the-road worn tyres not observed for Akron test pieces
• Initiated weaknesses in rubber-filler interaction close to the surface
of on-the-road worn tyres lead to particle detachment
• Bulk of tyre treads and Akron test pieces retain good rubber-filler
interaction
• Wear mechanism for on-the-road tyres not replicated by Akron test
• Modelling the surface layer is of interest
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Dr Pamela Martin, Dr Stuart Cook and Mr Paul Brown Thanks to Dr Robin Davies, Kathy Lawrence & Jolanta Bonfante for TEM images
Thanks to Dr Julia Gough for modelling
Visualising Tread Wear
@pmartinTARRC
Tun Abdul Razak Research Centre (TARRC) A RESEARCH & PROMOTION CENTRE OF THE MALAYSIAN RUBBER BOARD