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
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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
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The Journey of an ElectronFundamentals
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• 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
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Picture Sources: ebeam Technlogogies and IBA (Internet)
Materials Science with ebeam
The journey of one electron
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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
The journey of one electron
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1
2
3
4
1 escape vacuum
2 fly through atmosphere
3 create secondary electron (ionize)
4 create radical (ionize)
5
5 continue trajectory
Polymer Crosslinking with ebeam
The journey of one electron
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1
2
4
35
1 escape vacuum
2 fly through atmosphere
3 create secondary electron (ionize)
4 create radical (ionize)
5 continue trajectory
Polymer Crosslinking with ebeam
The journey of one electron
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1
2
4
3
1 escape vacuum
2 fly through atmosphere
3 create secondary electron (ionize)
4 create radical (ionize)
5 continue trajectory
5
Polymer Crosslinking with ebeam
The journey of one electron
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1
2
1 escape vacuum
2 fly through atmosphere
3 create secondary electron (ionize)
4 create radical (ionize)
5 continue trajectory
Polymer Crosslinking with ebeam
The journey of one electron
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1
2
1 escape vacuum
2 fly through atmosphere
3 create secondary electron (ionize)
4 create radical (ionize)
5 continue trajectory
6 *end*
6
44 4 4 4 4
4 4 44
44
Not all electrons are the sameThe key parameters of Electron Beam Processing
28 May 2018ebeam @ Borealis 12
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
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velocity
density
velocity
density
amount
amount
Depth (UAcceleration, Density) Dose (ISurface)
Materials Science with ebeam
The journey of one electron
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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
Polymer Crosslinking with ebeam
The journey of one electron
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1 escape vacuum
2 fly through atmosphere
3 create secondary electron (ionize)
4 create radical (ionize)
5 continue trajectory
Dose distribution
Depth/dose curves of Low Energy EB
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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
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• 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, ….
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Why Crosslink Polymers
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MaterialsSciencewithebeam
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ebeam is a physical wayof doing chemistry
(ebeam creates radicalsin any organic material, similar to other
types of ionizing radiation)
Some(personal)Definition
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Crosslinking is the process where bonds areformed between polymer chains to form a 3dimensional network
Polymer Crosslinking with ebeam
The journey of one electron
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How ebeam works
The 3 effectsGet radicalBreak a bondand then …
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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
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(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
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Crosslinked PE polymerNon-crosslinked PE polymer
What isCrosslinking ?
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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
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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
Polymer Crosslinking with ebeam
Thermoplasts to Thermosets - the upgrade approach
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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
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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
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The challengewith Polymersin EBXL
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not all polymers are created equal
ebeam crosslinking of polymermaterials
Factorsaffecting thebalance
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▪ 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
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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
Materials Science with ebeam
How dodifferentpolymersreact?
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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
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cros
slink
ing
degr
adin
gre
sista
nt
...but there are exceptions to the “rules”!
Keizo Makuuchi, Song Cheng (Ed.) (2012): Radiation Processing of Polymer Materials and its Industrial Applications.
ebeam Crosslinking
Effect ofEBXL ondifferentpolymers(Trends)
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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
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Different Doses applied(25-200kGy)
Oven90°C
EVA Film
Crosslinking of film material
How to reduce the necessary dose with ProRads / Promoters
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• 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
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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?
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Example: TheEffect ofTemperatureand GlassTransitionTemperature
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Keizo Makuuchi, Song Cheng (Ed.) (2012): Radiation Processing of Polymer Materials and its Industrial Applications.
EB exposure below the Tg, crosslinking will not happen due toinsufficient mobility of radicals
Examples ofImprovementswithCrosslinking
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Keizo Makuuchi, Song Cheng (Ed.) (2012): Radiation Processing of Polymer Materials and its Industrial Applications.
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
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Example: VSP(Vacuum SkinPack)
00
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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
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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
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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
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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
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Picture Sources: IBA, PTS Marketing, Ebeam Services, BGS (Internet)
Crosslinking of shrink film material
Single-side or double-side treatment for thick films
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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
Crosslinking of film material
Dealing with Thermal Input / Heat-up of Material
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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.
Crosslinking of film material
Dealing with Outgassing and Process Gas
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• 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
Crosslinking of film material
Dealing with Electrostatic Chargeup
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• 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!!)
Crosslinking of film material
Trapped Radicals and Coloration
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• 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
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▪ 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
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Recommended further reading for the curious
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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…
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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
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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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