Crosslinking of Polymer Beam - Indico [Home]

Crosslinking of Polymermaterials with ElectronBeamMichael BielmannDirector Access ebeam

2-Week Training School for the CI programme onAccelerators for Security, Health and Environment(ASHE)

28/05/18Daresbury ebeam Seminar : Polymer Crosslinking 1

List of Contents

§ The Journey of an Electron

§ Not all electrons are the same§ The key parameters of Electron Beam Processing§ Penetration, Dose, etc…§ Depth / Dose Profiles§ Low Energy EB and Medium/High Energy EB

§ Why crosslink polymers

§ Polymers and their reaction to EB

§ Practical Aspects of Polymer Crosslinking


Disclaimer

All Illustrations, Pictures, Products and Companies mentioned arefor the sole purpose of supporting the content and crediting theillustrations used

It is not intended to promote any specific company, brand, productor service


The Journey of an ElectronFundamentals

28 May 2018ebeam @ Borealis 4

• Mostly linear cathode designs• Some scanner based designs• Single or multi-filament• Pumped or sealed• Almost exclusively self-

shielded systems• Almost exclusively at

customers and not in servicecenters

• Ti Foil Thickness 9-~16um

• Exclusively scanner based• Always continuously pumped• Self shielded possible up to

~800kV• Some end customer

implementations (Cable, Tires)• Ti Foil Thickness ~40-50um

• Exclusively scanner based• Always continuously pumped• Bunker Shielding• Almost exclusively Service

Centers and Research

Low Energy EB(80-300kV)

Medium Energy(500kV -

High Energy<10 MeV)

Low Energyvs.Medium/HighEnergy EBThe main differences


Picture Sources: ebeam Technlogogies and IBA (Internet)

Materials Science with ebeam

The journey of one electron


1

4

1 create electron cloud

2 accelerate in high vacuum

sample

3 escape vacuum

4 fly through atmosphere

3

2

Polymer Crosslinking with ebeam



1

2

3

4

1 escape vacuum


3 create secondary electron (ionize)

4 create radical (ionize)

5

5 continue trajectory




1

2

4

35

1 escape vacuum








1

2

4

3

1 escape vacuum





5




1

2

1 escape vacuum








1

2

1 escape vacuum





6 *end*

6

44 4 4 4 4

4 4 44

44

Not all electrons are the sameThe key parameters of Electron Beam Processing


Dose (1 kGy = 1kJ / kg) mainlydepends on the total energy

delivered to the sample, whichdepends primarily on amount of

electrons hitting the surface

Depth mainly depends on kineticvelocity (keV) and density of the

sample

The two main parameters : Voltage and Dose

Depth Dose relationship


velocity

density

velocity

density

amount

amount

Depth (UAcceleration, Density) Dose (ISurface)




1

4

1 emission current [mA]

2 Acceleration voltage [kV]

sample

3 Foil Thickness and Support structure transmission

4 Air Gap distance

3

2

Sur

face

Dos

e an

d D

ose

Rat

e




1 escape vacuum





Dose distribution

Depth/dose curves of Low Energy EB


Valid for 10mm Air Gap, 9um Ti-equivalent window foil

• Air Gap has massive impact low voltages(80-120kV)

• Window Foil Thickness has great impacton Penetration Profile

• Window Foil is supported by CopperCooling structure

• Max. Dose at surface (< 200kV)

• Energy Loss in Air Gap is mostly negligeble

• Window Foil thickness does not vary muchand has reduced impact on losses

• Max. Dose is below surface

Low Energy EB(80-300kV)

Medium / High Energy(500kV -10MeV)

Low Energyvs.Medium/HighEnergy EBThe main differences


• Beam current is often used as a characteristicparameter for treatment intensity

• In this world, people talk «Surface DoseRate»

• Beam current [mA] has little meaning inapplication

• The Dose delivered to the surface isdependend on machine type andindividual configuration of theimplementation

• K-Value captures all machine dependentdifferences

Low Energy EB

The KIDSFormula

K I = D S

K K-value table for the chosen working point of the machine

I Emission Current

D Dose delivered to the SURFACE of Product

S Process Speed

The k-Value Tables are always machine and manufacturer specific and the k-Values change depending on:

Machine Type and Manufacturer, Window Foil Thickness, Support StructureTransmission, Voltage, Air Gap, ….


Why Crosslink Polymers


MaterialsSciencewithebeam


ebeam is a physical wayof doing chemistry

(ebeam creates radicalsin any organic material, similar to other

types of ionizing radiation)

Some(personal)Definition


Crosslinking is the process where bonds areformed between polymer chains to form a 3dimensional network




How ebeam works

The 3 effectsGet radicalBreak a bondand then …


ebeam creates radicals

in any organic material

Cut

Link

Paste

… leave it broken

Example: Sterilization

… let it bond to itself

Example: cross-linking ofpackaging material andcables

… let it bond tosomething else

Example: Reactivecompounding, grafting ofbiocompatible materialson membranes

usedto..

cut(scission)

link(crosslinking/cure)

paste(grafting)

a brand of the COMET Group

ebeam

createsradicals

physical

non-thermal

proportional to Dose(not Arrenius)

..serve afunction

mechanical

surface properties

adhesive

curing

Industrialhistory ofElectronBeam


(Raychem - 1950ies)

Raychem Corp.now Tyco (cable and wire)

Ethiconnow J&J (Sterilization)

WL Grace / CryovacNow Sealed Air (Heat shrink)

3M (Solventless PSA)

Sekisui (PE Foams)

Firestone (Tires)

Others (XL- PVC)

Tetra Pak (Printing)

Ford (Dashboard Coatings)

Microporous (Battery Separator)

1957 70ies 80ies 90ies 2000 2010

Tetra Pak(Sterilization)Manhattan

Project

40ies

The firstcomprehensivestudy on Polymersand Radiation


Crosslinked PE polymerNon-crosslinked PE polymer

What isCrosslinking ?


Irradiationprocess by

ebeam

crosslink

Materials Science withebeam

EBXL –explainedvisually

28/05/18Daresbury ebeam Seminar : Polymer Crosslinking 28Source: Herotron Services GmbH

Source: Herotron Services GmbH

PolymerCrosslinkingalternatives


Crosslinking Method Radiation (EB) Peroxide Silane

Major Products treated Wire & cable, tube, pipe,film, foam

Wire & cable, tube, pipe,foam Wire & cable, pipe

Polymer PE m m mPP m o oPVC m o oEngineering Plastics m u uPTFE m u uFluoropolymer m m o

Product thickness restriction < 10 cm > 0,3 mm > 0,3 mm

Rate of crosslinking High Low Low

Degree of crosslinking Adjustable High Low

m In practical use

o Technically possible but no practical use

u Hard to apply


Thermoplasts to Thermosets - the upgrade approach


amorphousthermoplasts

thermoplasticElastomers

partially crystallinethermoplasts

PVC

TPU

TPA

TPC

TPV

TPO

TPS

PMP

PP

PE

PA

PBT

PET

PEEKLCP

FP

Elas

tom

ers

PES

PEIPI

PC

PMMA

ABS

COC

PS

SAN

economically crosslinkablePolymers

Source: BetaGammaServices

Polymers can be classified as :- standard commodity materials (i.e.

PVC, PP, PE),- engineered materials

(i.e. PA, POM, PMMA)- performance polymers

(i.e. PPS, PEEK)

EB crosslinking of certain standardlower cost polymers (i.e. PE) liftsperformance into higher areas ofcommercial value

Crosslinking of Polymer materials

Main Purpose of EBXL in the industry


PE Properties Improvement

§ Service Temperature§ Impact strength§ Tensile strength§ Creep and fatigue resistance§ Stress-crack resistance§ Abrasion resistance§ Heat resistanceChemical resistance§ Higher Barrier properties§ ...

EBXL-Materials

§ Thermoplastics§ HDPE,LDPE,LLDPE,UHMWPE§ PVF, ECTFE, PVDF, ETFE§ EVA§ PA6, PA6-6, PA12§ PBT§ Polyacrylates§ PVC§ ...

§ Elastomers§ natural, synthetic, silicone, nitrile rubber§ PU§ PB§ EPDM§ EPR§ SBR§ ...

Why Crosslink Polymers

Polymers and their reaction to EB


The challengewith Polymersin EBXL


not all polymers are created equal

ebeam crosslinking of polymermaterials

Factorsaffecting thebalance


▪ Inertatmosphere

▪ Pressure▪ Temperature▪ Amorphous▪ Promoters

▪ Oxygen▪ Cristallinity▪ Antioxidants,

fillers

Both effects happen at the same time !

G(S)chain scissioning

G(X)crosslinking

▪ Molecular weight (distribution)▪ Catalyst system▪ Chemical composition▪ Dose and Dose rate

Crystalinity andPolymerMorphology


crystalline• low radical mobility• will cause scission with O2

exposure• NO CROSSLINKING HAPPENS

HERE

amorphous• high radical mobility• will potentially crosslink and

get consumed• CROSSLINKING HAPPENS

HERE


How dodifferentpolymersreact?


Keizo Makuuchi, Song Cheng (Ed.) (2012): Radiation Processing of Polymer Materials and its Industrial Applications.

(X) 20/ (S) 10

(X) 1420/ (S) 480

(X) 100 / (S) 3000

Example:One 100keV electron yields

G-Value Number of reacted molecules per 100eV adsorbed (Chemical Yield)

G(X) Crosslinking yieldG(S) Chain scissioning yield

ebeam effect on polymers

Polymers andtheir reactionto ebeam


cros

slink

ing

degr

adin

gre

sista

nt

...but there are exceptions to the “rules”!


ebeam Crosslinking

Effect ofEBXL ondifferentpolymers(Trends)


Polymer Type EBXLEffect Comment

Polyethylene

LDPE, LLDPE,HDPE

and copolymersEAA, EVA, EVOH

C Crosslinking used for shrink films / physical propertyenhancement

Polypropylene BOPP F No crosslinking without Additives. Acceptable for sterilizationand as a printing substrate

ElastomersNatural rubber,polybutadiene,

SBR C Tire industry, battery separators, etc..

Polysiloxanes Silicones C Silicone release liners

FluoropolymersPTFE (Teflon)

PVDF

D-CF

PTFE (Teflon) crosslinkable @ 340° C in inert atmosphere.

Little effect on PVDF at normal cure doses.

PVC PolyVinylChloride DDegraded by EB. Properties may still be acceptable at normalcure doses. Films will yellow. Color is easily visible in a roll butmay be usable when viewing a single layer of film.

Polyester PET F Very resistant to EB effects. No crosslinking

Polystyrene PS F Very resistant to EB effects. No crosslinking

Polyamides Nylon F- C Little effect on mechanical properties at typical cure dose levels.Color may be formed which fades and disappears in a few hoursWidely used in Automotive under-the-hood

EB crosslinking– SimpleProcess Control


Different Doses applied(25-200kGy)

Oven90°C

EVA Film

Crosslinking of film material

How to reduce the necessary dose with ProRads / Promoters


• Prorads “sensitize” the polymer and shift the balance strongly towards crosslinking• ..to achieve XL at much reduced Dose• ..to achieve XL in highly AO loaded Polymers with Fillers and Flameretardants• ..to make polymers crosslinking that are normally chain scissioning

• Classical Pro-Rads• Mono-/di- or Trifunctional Acrylates• Examples: TAIC, TAC, TMPTMA, .....• Specific Prorads for specific polymers

• gaseous Prorads are reported, but little practical relevance• Acetylene• Nitrous Oxide

Drobny, Jiri George (2013): 4 - Electron Beam Processes. In Jiri George Drobny (Ed.): Ionizing Radiation and Polymers : Plastics Design Library: William Andrew Publishing, pp. 83–99. Available onlineat http://www.sciencedirect.com/science/article/pii/B9781455778812000043.

EB Crosslinking

Hall of FameMost relevant

Cable and Wire Hot melt adhesive Tires Heat shrinkables

Foams Food PackagingInjection Mouldedcomponents for

Electrical

Injection mouldedparts for

Automotive

28/05/18Munich ebeam Seminar 41

Crosslinking as a process enabler Crosslinking for final productperformance improvement

Tires ManufacturingSiC Fibre Production

Vacuum Skin PackagingFoams

Cable and WireSrink Film

Injection Moulded PartsPressure Sensitive Adhesives

What is therole of EBcrosslinkingin a specificapplication?


Example: TheEffect ofTemperatureand GlassTransitionTemperature



EB exposure below the Tg, crosslinking will not happen due toinsufficient mobility of radicals

Examples ofImprovementswithCrosslinking



General Trends

• Propertry improvements normally are connected to higher servicetemperature

• Tensile strength of thermoplastics is usually reduced at RT• Elongation at Break is reduced at RT

• Testing materials at Room Temperature after crosslinking can actuallyshow decrease in many mechnical properties

• The true benefits are often only apparent at higher temperatures

• Most applications of crosslinking are therefore driven by temperaturerelated effects


Example: VSP(Vacuum SkinPack)

00


https://www.youtube.com/watch?v=czqFKikWYI0

The Why The What

The Process

The Requirement

• Increase transitory temperature resistance of outer skin• Keep sealing temperature of inside layer unchanged

00:20 – 00:50

Shrink film application

Use of Depth /Dose curves


120 kV energy level100 kGy dose level

Outer Skin

Sealing Layer

7-11 Layers

Example of a multi layerpackaging film of 150

microns thickness and 0,95kg/dm3 density

The How

Practical Aspects in Polymer Crosslinking


Voltage usually between 125-300kV

Beam Current usually between 30mA - 1000mA

Narrow (0.25m) to Wide (up to 2m)

Product Thicknesses up to 1mm (Density 1, 2 sided)

Almost exclusively Foil / Film Products treated

• Shrink Film for Food Packaging

• Vacuum Skin Pack for Food Packaging

• Polymer Foils for highly demanding applications

Low EnergyEB forCrosslinkingMost relevant applications


Voltage usually between 500kV and 2MeV

Beam Current usually between 25mA - 100mA

Rarely very wide scan horns

Product Thicknesses < 5cm (Density 1)

Cable and WireTire and RubberPipes and Thick wall tubingInjection Moulded Parts for ElectricalInjectino Moulded Parts for Automotive

Medium/HighEnergy EB forCrosslinkingMost relevant applications


Picture Sources: IBA, PTS Marketing, Ebeam Services, BGS (Internet)

Crosslinking of shrink film material

Single-side or double-side treatment for thick films


Single-sided treatment

§ Applies a uniform dose throughfull thickness in one pass byusing higher voltage

§ Sometimes, apply gradientDose to retain sealingcapability on the backside

Double-sided treatment

§ Uses lower voltage so dosedecreases along the cross-section ofthe film

§ The EB treatment is applied to bothsides

§ The two doses combined make a fulltreatment through the tube

Film Thickness

Electron Beam Dose

2.4x


Dealing with Thermal Input / Heat-up of Material


Dose is an dissipated energy in the target material

D [kGy] = 1 kJ / 1kg

The (adiabatic) Temperature increase can becalculated with the specific heat capacity c [J/kg K]

Example: PE Crosslinking with D=200kGy leads totheoretical DT of 86°C

Consequence:

• Chill Rolls mostly needed (film)• Web tension control can become challenging• multi-pass often needed (Cable)• physical limitations on processing speed despite

higher machine capability

Makuuchi K, Cheng S. Radiation processing of polymer materials and its industrial applications. Hoboken, N.J: Wiley; 2011.


Dealing with Outgassing and Process Gas


• EB produces Ozone (when in Air) which must be properly treated• Can have a mayor influence on Organo-leptic performance of Polymers. Treatment in Nitrogen

Atmosphere therefore is sometimes necessary

• Crosslinking induces the evolution of volatile species• Example: H2 in PE, EVA

• The gas(es) need to diffuse out of the polymer• Effect: At high dose (rate), bubble formation can occur. Puts practical limits to processing speed

• When the gases have escaped the polymer, they need to be safely handeled• Example: Prevention of accumulation of H2 in EB on Double Bubble Process below the

Flamability Limits


Dealing with Electrostatic Chargeup


• EB injects charge into the Polymer Film

• Proper Grounding of equipment, idler rolls, unwinder and rewinder is a must

• Especially, proper grounding is cruical with metalized film (capacitor!!)


Trapped Radicals and Coloration


• EB can induce long living trapped radicals in Polymers• Example: Radicals in UHMWPE can survive for months due to insufficient mobility. Annealing can

give them the mobility needed for

• EB can induce color centers in some materials• Example: PVC and Nylon can show mild to substantial color. The color effect can be temporary

(minutes) to permanent

ebeam Crosslinking of polymer materials

General Advantages and Challenges


▪ Reaction takes place at room temperature

▪ Reaction completed in a fraction of second (only one step), hence high output is obtained.

▪ Reaction can take place without any additives, additives can boost reactivity

▪ Highly suitable for relatively thin insulating layers

General Challenges with EB Crosslinking:

▪ Diversity of Polymers is enourmous. Real world test and development always needed to optimize

product

▪ Most converters have limited Polymer understanding to understand and optimize bottom-up

▪ Difficult to cross-link irregular shapes (but this is changing with the Development of 3D-EB

curing systems)

Recommended Reading


Recommended further reading for the curious


Drobny JG. Ionizing radiation and polymers:Principles, technology and applications.Norwich, N.Y, London: William Andrew;Elsevier Health Sciences [distributor]; 2013.ISBN: 978-1-4557-7881-2

Makuuchi K, Cheng S. Radiationprocessing of polymer materials andits industrial applications. Hoboken,N.J: Wiley; 2011. ISBN: 978-0470-5876-90

Charlesby A. Atomic Radiationand Polymers: Elsevier; 1960.ISBN: 978-1-4831-9776-0

A word of warning…


Books and Articles seem often conflicting and contradictory

They never report with sufficient detail the starting point, just usually polymer class (p.ex. LDPE)

The randomly report on treatment conditions (In Nitrogen, in Air, in Vacuum, etc)

They should therefore serve as «starting point» rather than be taken at face value

In practice, don’t be surprised if you’re getting surprised (sometimes things don’t work the wayyou think!)

Polymer Crosslinking with EB retains a strong empirical component


Get in touch

ContactMichael Bielmann

[email protected]+41 78 801 16 01

Comet AGebeam TechnologiesHerrengasse 103175 Flamatt Switzerland

www.ebeamtechnologies.com

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