MAKING THE MOST OF THE VNC - PDF Free Download

MAKING THE MOST OF THE VNC Eric Sheard (Lescon Inc) and Bryan Willoughby Presented to the PMA Annual Meeting April 17-19, 2005

The VNC

Rapra s Vibrating Needle Curemeter For cure profiling liquid elastomers especially polyurethane In the lab or on the shop floor For recording and archiving processing behaviour Provides quality checks on feedstocks In terms of fitness for purpose i.e. curing performance Assists manufacture and speeds up develoment Puts in-house knowledge on a formal basis Helps maintain a competitive edge

Vibrator and needle of the VNC Needle (probe) is actually a parallel-sided rod (CFRP)

VNC Operating Principles Amplitude of vibration decreases with increasing viscosity plot A versus t for continuous cure trace Back EMF provides a measure of amplitude no need for displacement transducers makes for simple and robust device Instrument operates close to resonance Can be made to scan & locate the resonance maxm. And can do this as fast as every second So it can track the resonance peak during cure this is the Scanning VNC follows both amplitude and frequency changes

SVNC provides two continuous cure traces A/t and F/t

Making the Most of the VNC - scope of the paper Calibration Operational parameters starting amplitude, probe diameter, probe depth temperature scanning or fixed frequency System variables stoichiometry, replication, etc. Outputs work life, gel time, etc. phase separation effects Conclusions

Calibrating a VNC - a challenge? No such thing as a standard cure Uncured mixes don t keep even stability in a freezer cannot be guaranteed and reheating from cold a potential variable Even ingredients go off (oxidation, hydrolysis, etc) And fresh batches of ingredients may be different Also sources of variability in weighing, mixing, etc. We use the VNC to check on variability How do we check on the VNC?

Calibrating a VNC- check the instrument, not the cure m 1 slope = k 1/F r 2 m 0 Measure the effective spring constant of the vibrator For a system at resonance, frequency F r is given by: F r 2 = (spring constant, k) x (vibrating mass m) Vibrator itself has a mass m 0, Measure F r for a range of added masses m 1 and plot m 1 vs 1/F r 2

Initial amplitude effects A/t Amplitude is a digital value derived from back EMF Here one cure starts at 14,000 and the other at 10,000 Amplitude decreases faster from a higher initial amplitude

Initial amplitude effects F/t A has higher initial amplitude than B Frequency increases faster from a higher initial amplitude Cure profiles are different for different starting amplitudes

Probe diameter effects A/t C & D are with 4 mm probe, E is with 1.5 mm probe Greater sensitivity to initial viscosity with wider probe

Probe diameter effects F/t Anomalous features when trying to monitor elastic changes with wider probe in the standard sample well

Probe depth effects A/t Amplitude decreases faster with a greater probe depth (greater depth = lower height above base of sample well) Curve G is for probe only 0.5 mm above base

Probe depth effects F/t Frequency increases faster with a greater probe depth (reduced height above base) Cure profiles are different for different probe depths

Influence of geometry critical Initial amplitude, probe diameter and probe depth influence geometry of system Ratio A/h where A is the amplitude and h is the height above the base of the sample well Defines the (max.) strain in the sample So, increasing A or reducing h increases strain viscoelastic systems have non-linear responses hence initial amplitude or probe depth critical Need consistent geometry for consistent performance

Fixed frequency monitoring A/t Curves H & I are fixed frequency, J is scanning All similar formulations, but different mixing histories Fixed frequency gives a greater decrease in amplitude

Software provides numerical comparison of cure profiles

Effect of temperature A/t K & M same stoichiometry (95.5) but K hotter (97 C vs 82 C) L is 96.4 stoichiometry and 110 C

Effect of temperature F/t Marked differences in cure rate here (K & M are only 15 apart) PU cure very sensitive to temperature

Operational Parameters Consistent performance with the VNC/SVNC requires: Same starting amplitude Same diameter probe Same probe depth in the sample and same sample size and geometry Same mix and mix history Same temperature consistent temperature for mixing consistent temperature for cure

Effect of stoichiometry A/t N = 92.6 (index 1.08) O = 93.9 (index 1.065) P = 96.4 (index 1.04) Stoichiometry has a critical influence on cure rate

Effect of stoichiometry F/t Cure rate is P > O > N Fastest cure with optimum stoichiometry

Effect of stoichiometry A/F A/F plot is route map of cure. Lowest stoichiometry (highest index) formulation is following a different path from the other two. Expect quite different properties.

Low TDI & standard prepolymers A/t Q is low TDI & R is standard material. Low TDI is slower curing Spike suggests transient debonding effect

Low TDI & standard prepolymers F/ t Spike in A/t trace corresponds with a step-change in F/t trace more on this later

Low TDI & standard prepolymers A/F Low TDI (Q) and standard (R) start on different courses But come together as the cure progresses In time, they may develop similar products

Replication of cure profiles A/t Keeping instrumental parameters constant and taking care with formulation & mixing allows replicate profiles to be generated. With this level of reproducibility, the VNC is a sensitive indicator of variance.

Outputs 1- processing The development of a molecular network is a continuous process of chain extension and crosslinking No step changes on the A/t or F/t expected So where do we find pot life, work life, gel time etc.? Best to calibrate empirically for diagnostic points And use VNC software to tabulate and archive Gel time always a topic of interest near point of max slope in fixed frequency A/t trace between max slope and minimum in scanning A/t trace where F just starts to increase in F/t trace

Outputs 2- property development Property development in PU can rely on more than just network structure later stages of cure may be characterized by phase separation effects Phase separation leads to self-reinforced structure Is this a sudden or gradual effect? A sudden effect i.e. nucleation could give rise to a step change in the A/t or F/t traces We are seeing discontinuities in the A/t and F/t traces in the later stages of the cure

Step-change features in SVNC cure profiles F/t All cures based on PTMEG and Curene 107 (different stoichiometries and temps) note features at ca. 200 Hz.

More step-change features in SVNC cure profiles F/t Same range of systems as before step features all occurring in region 180-200 Hz.

Step-change features in F/t traces for thermoplastic crystallisation F/t during cooling for a molten polyolefin. Spike & step in trace occurs at point of crystallisation.

Phase separation in PU observable with the SVNC? The features on the PU cure traces look like those for phase separation in a cooling thermoplastic melt Step change in F/t indicates a step change in modulus Shrinkage may cause spikes in either A/t or F/t traces i.e. transient debonding Points to domain formation in PU as a spontaneous effect in the progress of cure nucleated as in crystallisation A deeper insight into PU cure emerging with the SVNC expect further exploration here

Conclusions Calibration Check the spring constant of the vibrator Operational parameters Need consistent settings for consistent performance i.e. for: initial amplitude, probe diameter, probe depth, temp. System variables Stoichiometry, mixing history, etc., important variables Replication shows variables under control. Outputs Continuous profiles pass smoothly through gel point Can calibrate empirically for any diagnostic point of interest Pot life, work life, gel time, setting time, etc. Discontinuities recognisable later in the cure Looks like phase separation