Self-healing and repair in elastic materials and components

8th December 2017

Dr Ian German

Advanced Materials Manager, Gnosys Global

Overview

• Gnosys sector activity and motivation for self-healing materials

• Scope of Gnosys engagement with Self-healing materials

• Gnosys self-healing solids

• Self-healing in thermoplastic elastomers

• Non-molecular routes to self repair materials and assets

Gnosys Activity in Advanced Materials

Gnosys Global11 years

14 staff

Collaborative research and innovation

Self-healing

and repair materials

Application-specific

polymers and composites

Utilities: Power and gas

distribution industry

Plasma processes

and treatment

Spectroscopic hardware and

software

Demanding environment components

Power cable repair challenge and previous solutions

• Damage events occur at several points over a cable lifetime that can lead to catastrophic failure – what is realistically repairable?

?

Power cable repair challenge and previous solutions• Commercially available solutions:

– Prysmian Superseal: cellular semisolid repair system – poor uptake due to incompatibility with cost-efficient manufacturing and subsequent high unit cost

– Water-blocking tapes: Rapid failures in testing and high cost due to incompatibility with extrusion processes have led to very poor uptake

Power cable applications for self-healing and repair materials• Damage restoration: healing vs repair

– Fully healing materials guarantee repair, however service and function constraints may make that difficult to achieve – effective repair action must be autonomous and tailored

– For power cables, Gnosys has addressed repair by three routes, each activated upon damage to close defects prevent contaminant entry and cable failure:

OCF

Oxygen-curing insulation fluids deliver self-repair function for fluid-

filled cables. In cable trials under trademark ‘Anagen’

Power cable applications for self-healing and repair materials• Damage restoration: healing vs repair

– Fully healing materials guarantee repair, however service and function constraints may make that difficult to achieve – effective repair action must be autonomous and tailored

– For power cables, Gnosys has addressed repair by three routes, each activated upon damage to close defects prevent contaminant entry and cable failure:

H-TPE

Hydro-swelling

thermoplastic elastomer

blends as sub-sheath for

cables in marine

environments

SHT

Self-healing thermoplastic

elastomer blends as sub-

sheath for underground

cables

Example MVAC cable design

Factors affecting cable material selection

• Cable material standard requirements

– Thermal stability

– Electrical insulator (dependent on position within cable assembly)

– Mechanical property parameters (in particular hardness, tensile strength (10MPa), elongation-at-break (300%))

• Processability: compatible with cable production methods

– Intrinsic healing mechanisms are prioritised, as extrinsic (particularly those with irreversible reaction or liquid components) require substantial adaptation

• Material cost and availability

– Typical material cost tolerance of £8/kg, with bulk production and supply accessible.

SHT: Self-healing material selection• Intrinsic polymer self-healing often involves a property balance, due to the

necessity for chain mobility but also strength requirement

• Linear self-healing polymer prepared for chain mobility, with dynamic covalent bond

• Elastic behaviour observed attributed to node formation through weak secondary supramolecular interactions

Dynamic covalent bond

Secondary supramolecular interaction

Dynamic covalent bond activation mode

SHT: Self-healing material selection• Self-healing polymer prepared with rapid room-temperature self-healing

• 80% tensile strength healing efficiency – but only 0.25MPa strength ca. cable material standard requirement of 10MPa

• Some poly(ethylene-co-vinyl acetate) TPEs have shown the capacity for supporting self-healing while maintaining high tensile strength – so used as matrix due to homogeneous miscibility

0

50

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150

200

250

300

0 100 200 300 400 500 600St

and

ard

fo

rce

[kP

a]Strain [%]

Self-healing polymer tensometry

Intact

Healed 15mins

SHT: Self-healing material performance

Cross-section of SHT fused to HDPE Sheath

• Mechanical analysis (300% elongation minimum requirement) and healing efficiency (70°C, representative of temperature experience in service)

• Water barrier recovery test following puncture – at 25°C, to test impermeability following simulated installation damage

• Compatibility with cable sheath material extrusion processes

SHT: Self-healing material performance

Cross-section of SHT fused to HDPE Sheath

• Water barrier recovery test

MaterialIntact(1bar)

Punctured (0.25 bar)

Water barrier pressure rating following healing at 25°C for 24h (bar)

0.25 0.50 1.0

EVA

Optimal SHT blend

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0 100 200 300 400 500 600 700 800 900

Stan

dar

d f

orc

e [

MP

a]

Elongation [%]

SHT Self-Healing

Intact 50% cut Healed 24h 70°C

40% Tensile strength recovery

60% Elongation-at-break

recovery, exceeding cable

material standard

H-TPE: Self-repair material selection

• H-TPEs include a hydrophilic and hydrophobic component.

• Hydrophilic component must rapidly absorb water and swell within hydrophobic matrix.

– Polyanionic substance used to maximise water binding and allow free cation to aid absorption by osmotic effect

• Hydrophobic component is required to expand elastically, retaining mechanical properties and preventing water transport.

– TPE used by Gnosys was SEBS, to facilitate extrusion processability –however crosslinked elastomers are envisioned as alternatives for service conditions that include high temperatures

StyreneS

Ethylene/butyleneEB

StyreneS

Potential functional site

H-TPE: Self-repair material performance target

Swell response of h-TPE

closes breach in cable

sheath. Water ingress limited

to these areas.

H-TPE: Self-repair material performance

• Within elastic SEBS matrix, a water-swelling hydrophilic (ionomeric) microphase is blended

• Water contact triggers expansion of hydrophilic phase and overall swelling of blends

Water

SEBS

Hydrophilic phase Ionic site

H-TPE: Self-repair material performance• Pressure chamber quantifies swell-response and tests water blocking

under pressurised seawater to 7bar – typical North sea cable depth

• Swell response within 15 minutes showing responsive force and water containment

Future challenges for self-healing and repair materials for cables • Improving the efficiency and temperature range of effective healing

– Chemistry selection and network manipulation for stronger materials pre- and post-healing

– Ongoing development partnership with Hayes group at University of Reading

Future challenges for self-healing and repair materials for cables • Improving the efficiency and temperature range of effective healing

– Chemistry selection and network manipulation for stronger materials pre- and post-healing

– Ongoing development partnership with Hayes group at University of Reading

Future challenges for self-healing and repair materials for cables • Improving the efficiency and temperature range of effective healing

– Chemistry selection and network manipulation for stronger materials pre- and post-healing

– Ongoing development partnership with Hayes group at University of Reading

Future challenges for self-healing and repair materials for cables

• Secondary property development for TPE and H-TPE in progress at Gnosys

– Modification of material composition for improved damage resistance, thermal transport or repair support

– Focus on polymer grafting and addition of fillers

Self-healing grafted SEBS

Wet-adhesion enhanced SEBS

Perspective• Self-healing and repair materials an area of interest for expansion of

offshore and underground power networks – multiple developments are at demonstrator cable development and trial stage

• Projected £40m pa. saving on UK underground network maintenance alone is realisable

• Avoiding subsea burial can save 80% of installation cost (ca. £250m per 100km)

• Improvement in self-healing and repair materials is ongoing with our partners: