Different ways to express the molecular weight of polyamides

1. Viscosity of polyamides
Ted Brink

2. Page  2
Effects of molecular weight
The molecular weight of a polymer influences:
• melt viscosity (flow behaviour)
• processing behaviour
• mechanical properties
• crystallization behaviour
• optical properties

3. Page  3
Methods to measure molecular weight
• Melt viscosity as a function of shear rate
• MFI (Melt Flow Index)
• MVR (Melt Volume Rate)
• RSV (Relative Solution Viscosity)
• VN (Viscosity Number)
• GPC (Gel Permeation Chromatography)
• SEC (Size Exclusion Chromatography)

4. Page  4
MFI and MVR
Disadvantages
for polyamide
characterization:
• moisture sensitivity
• polymer reactivity
Units:
• MFI in g/10 min
• MVR in cm3/10 min
Capillary
( D = 2.0955 mm, L = 8.000 mm )
Barrel
( D = 9.5504 mm )
Piston
( D = 9.4742 mm )
Temperature
Control
( ± 0.2oC )
Thermometer
or PRT
( ± 0.1oC )
Load
(Wt + Piston = ± 0.5% )
 Standardized test (ASTM D1238, ISO 1133)
 Constant load test
 Fixed die diameter
 Fixed die L/D ratio
 Mass / time measured
 MFR  1/Viscosity
 May be related to, but is not a
strict function of Mw

5. Page  5
Limitations of MFI measurement
100100010000
0.01 0.1 1 10 100 1000
γ
η
Two types of materials
with the same MFI value
but completely different
rheological behavior
Hygroscopic and chemical
character of PA6 influence
accuracy of measurement
melt index
conditions
A
B

6. Page  6
RSV and VN according to ISO 307
• solvents used:
- 90 % formic acid
- 96 % sulphuric acid
- 100 % m-cresol
- others
• concentration 1 g polymer/100 ml
• measured at 25 °C
• Ubbelohde capillary
RSV = Tsolution/Tsolvent
VN = (Tsolution/Tsolvent)/c

7. Page  7
GPC/SEC
0
5
10
15
20
25
15 20 25 30 35
Ve (ml)
RawRIsignal(mV)
0
0.2
0.4
0.6
0.8
1
1.2
3 3.5 4 4.5 5 5.5 6
LogM
dw/dLogM

8. Page  8
Methods to express molecular weight
η0 at
260 °C
MFI at
250 °C
Mn Mw VN
RSV
H2SO4
RSV
HCOOH
Pa.s g/10 ‘ g/mol g/mol cm3/g
1070 7.7 18000 35000 197 3.3 3.0
1280 7.1 20000 40000 210 3.6 3.2
2600 3.3 22000 47000 245 4.0 3.6

9. Page  9
Relation MFI and RSV (in formic acid)
0
10
20
30
2 2.5 3 3.5 4
Meltflowindex[g/10min]
Relative solution viscosity in formic acid
MFI measured at 235 °C and 2.16 kg

10. Page  10
Contact
Ted Brink
tel.: +31 (0) 651109899
email: ted.brink@extrusionist.com
internet: www.extrusionist.com