Overview of Fluoropolymer
Foam Technology
Robert T. Young, DuPont Fluoroproducts
Wire New England ’07
WAI New England Chapter
October, 2007
2
Outline
• Resin Structure and Properties
• General aspects of Foaming
• Equipment requirements for Foaming
• Resin, Construction & Processing Considerations
• Summary/Conclusion
3
Structure & Properties of Fluoropolymers
Tefzel® ETFE
- CF2 - CF2 - CH2 - CH2 - n-
59% F
Teflon® PTFE
- CF2 - CF2 - n-
76% F
Teflon® FEP
- - CF2 - CF2 m CF2 - CF- n-
76% F
CF3
Teflon® PFA
- - CF2 - CF2 m CF2 - CF- nOC3F7
76% F
Property
PTFE
FEP
ETFE
PFA
Specific Gravity
2.15
2.15
1.70
2.15
Melting Point (2nd),°C
327
260
270
310
Tensile Strength, MPa
20
20
40
28
Elongation, %
300
300
300
300
Flexural Modulus, MPa
560
650
1,100
650
Temperature Rating,°C
260
200
150
260
Dielectric Constant
2.1
2.1
2.6
2.1
Coefficient of Friction
0.1
0.2
0.4
0.2
CutCut-Through, kg
4.5
4.5
18
4.5
Chemical Resistance
Excellent
Excellent
Very good Excellent
* Nominal values
• PTFE discovered in the 1930s but could not be melt extruded
• Modifications to polymer architecture led to the development of melt
processable fluoropolymers such as FEP, PFA, FEP, PVDF
• Ability to be melt extruded coupled with properties such as a low
dielectric constant and low flammability has made materials such as
FEP and PFA well suited for wire and cable applications
4
Foaming, Why do it?
- DIELECTRIC CONSTANT
- DISSIPATION FACTOR
- CAPACITANCE
- ATTENUATION
- WEIGHT
- WALL THICKNESS
- RELATIVE VELOCITY
5
DIELECTRIC CONSTANT
VERSUS VOID CONTENT
2.6
2.5
2.4
2.3
DIELECTRIC CONSTANT
2.2
2.1
FEP/PFA
2.0
1.9
POLYETHYLENE
1.8
ETFE
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0
10
20
30
40
50
60
70
VOID CONTENT, %
80
90
100
6
Technology Implemented For Foaming
1980
Low Pressure
Freon® Injection
Liquid Freon®
Injection
1985
1990
2000
High Pressure
Nitrogen Injection
Low Pressure
Nitrogen Injection
7
Mechanisms and Equipment for
Foaming Fluoropolymers
• Polymer is melted and conveyed (Extruder)
• Gas is introduced into melt, forms solution
(Injector and High Pressure Pump)
• Ratio of polymer to N2 controls final product void Content
• Polymer/N2 solution exits Crosshead under
pressure
• Gas comes out of solution, forming bubbles
• Nucleating agents assist bubble formation
• Polymer cools on wire, freezing in structure
8
Extruder Configuration
A traditional 3 to 4 stage screw is typically used
•
Feed, Compression,Metering, and Mixing
•
Typical compression ratio of around 3
A gentle mixing elements are recommended
•
Low shear, divide & recombine elements beneficial in distributing the N2
Corrosion resistant materials of construction are needed
Extruder and screw design needs to be sized properly
•
Too large (slow rpm) results in poor mixing and melting
•
Too small (high rpm) can result in excessive shear and heat
9
Gas Injection and Metering
Flow versus Pressure for Injector
with 0.118 mil Nozzle
Flow (cc/min)
140
Metering Orifice
Example of high pressure gas pump
(Fine International Corporation)
100
80
60
40
2000
Example of Injector (Maillefer Extrusion)
y = 0.0145x
R2 = 0.9989
120
3000
4000
5000
6000
7000
8000
9000
10000
Pressure (psig)
• Injector needs to be sized to deliver the
required gas flow at the desired pressure
• Flow to extruder controlled by orifice size &
gas pressure supplied to the injector
• For stability, desirable to run injector pressures
1.5X to 2X higher than the barrel pressure
10
Profile Extrusion and Drawing
Optimum to have material
foam once it is on the wire
Melt Draw Extrusion Foaming
for Fluoropolymers
Tooling calculations should
be based on the “pre-foamed”
insulation diameter
Draw Ratio Balance (DRB)
• Balance between the rates the outside and
inside of the cone draw down
• R1 is die diameter divided by coated wire
diameter
• R2 is tip diameter divided by bare wire
diameter
• DRB = R1/R2
• DRB of 1.02 to 1.10 is typical
Die
Foaming
Slight
Melt
Draw
Tip
Wire
Drawdown Ratio (DDR)
• Ratio of the cross sectional areas of the
annular tooling gap to that of the coating
on the wire
• Lower DDR gives a more stable cone
• Higher DDR facilitates faster production
rates
• DDR of 5:1 to 30:1 is typical
11
EXAMPLE OF FEP RHEOLOGICAL BEHAVIOR
Typical
operating
region
for foam
resins
Shear Stress (MPa)
Typical operating
region for solid resins
Melt Fracture
Die wall
Adhesion
Screw RPM
Stick
Slip
Slipping
“Super Shear”
Draw Down
Temperature
Resin MFR
Smooth
Increasingly
rough
Smooth &
Clear
Increasing Shear Rate
Distorted
extrudate
12
Construction Considerations
Twisted Pair
Cat 5e
10 Gig
Spacer
Cat 6
Jacket
Jacket Spacer
• Resin and processing requirements highly dependent on construction
• Thicker wall (> 20 mil), higher void content (> 45%) constructions
typically utilize lower MFR (< 15) resins and lower draw down tooling
(RG cable, etc.)
• Thinner wall (< 20 mil), lower void content (< 45%) often made with higher
MFR resins (> 15) and higher draw down tooling (Cat 6, 10 Gig, etc.)
13
Successful Foam = Good Return Loss/Adhesion
Return Loss measures the amount of signal that is lost due to
reflections along the length of the cable
14
What is required for good Return Loss?
Consistency, Consistency, Consistency !!!!!
- Small and uniform cell structure
- Consistent and good bond between the
conductor & insulation (maintains spacing)
- Uniform material along the conductor
(voids along conductor are detrimental)
15
Processing & Formulation = Performance
• Good return loss can be achieved by combining material and processing
parameters
• Polymer and nucleant formulation are contributing factors to producing
uniform cells and better adhesion to the conductor
16
Performance via Formulation
Variability in Diameter & Capacitance vs Nucleant Package
Nucleant A
COV % (Standard Deviation/Average)
0.60%
0.55%
Nucleant A
Nucleant B
0.50%
0.45%
0.40%
0.35%
Nucleant B
0.30%
0.25%
0.20%
0.15%
0.10%
Diameter
Capacitance
• Nucleant type and loading can have an effect on processing
and cell structure which impacts electrical performance
17
Processing Effects on Product Attributes
Return Loss Adhesion Capacitance
Cone Length
Wire Preheat Temperature
Quench Point
• Process variables typically used to control items such as
diameter & capacitance can have large impact on performance
of the cable
• Have to keep a good balance between finding a robust, stable
process while maintaining desired electrical performance
18
Alternative Methods for Processing
Foam / Skin
Film / Foam / Skin
Film (Solid layer next to conductor)
• Provides void free uniform material next to the conductor
• Serves to help improve return loss and adhesion
Skin (Solid layer on external Surface)
• Provide layer to help minimize blow through
• Smoother surface for better contact for processes such
as shielding/braiding
• Better processing control for items such as the cone
19
Summary and Conclusions
• Foaming of fluoropolymer resins such as FEP/PFA
with gas injection provides a safe & economical way
to produce plenum rated cables of reduced size with
desirable electrical performance
• Implementation of foamed products being facilitated
with improvements in material formulations and
processing technology
Makes for exciting times in the Wire and Cable Industry