new approaches for studying uhmw-pe thermal behavior · new approaches for studying uhmw-pe thermal...

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New approaches for studying UHMW-PE thermal behavior Daniel Istrate

a, Geert vd Poel

b, Marcel Meuwissen

c, Paul Smeets

d, Hans Schneiders

d, Patric

Meessend, Bert Tabor

d, Xavier Amils

e, Maarten Vierstraete

e,

a) DSM Resolve, Geleen, The Netherlands

b) DSM Egineering Plastics, Geleen, The Netherlands

c) DSM Ahead, Geleen, The Netherlands

d) DSM-Dyneema, Urmond, The Netherlands

e) NV Bekaert SA, Zwevegem, Belgium

Abstract

One of the determining advantages to use of synthetic high performance ropes is their lightweight

and high strength nature. In many applications it is critical however to assess the rope condition

1during use to assure its safe operation. Commonly this is achieved by visual inspection and by

tracking the history of usage of each rope. We hereby report less subjective thermal analysis

methods to evaluate the structural integrity of synthetic ropes or hybrid cables made of steel and

UHMW-PE.

Via a systematic study including isothermal, conventional and fast scanning calorimetry, it was

found that the material history influences the dependency of the melting peak temperatures on

the heating rate: generally the harsher the fatigue level, the more severe the displacement

towards higher temperatures of the melting peak will be. Nonisothermal kinetics methods reveals

the possibility to rank and monitor the structural integrity of UHMW-PE ropes or hybrid cables

made of steel and UHMW-PE. By testing a relevant number of samples collected up to the rope

failure, a health monitoring regression line can be obtained that will eventually allow further

extrapolating the time until retirement of a high performance synthetic or hybrid rope in practice.


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1. Introduction

In the last decades high performance fibers have entered the arena of engineered ropes because

of advantages like e.g. weight reduction, high strength per diameter, fatigue resistance, safety, life

time, chemical- and corrosion resistance.

These ropes are used in high-end application fields like e.g. marine and offshore, commercial

fishing, hoisting, transport and leisure/sports with high impact on man and machine in case

failures occur.

To date visual inspection is used to determine the quality and retirement of these ropes.

Several more scientific based, less subjective methods are being researched or under

development but not yet widely spread nor written down in guidelines or standards.

In this paper an analytic method is described which shows good potential to monitor the fatigue

life time of ropes based on high performance fibers, in this case with specific data for ropes based

on Dyneema®, an UHMW-PE fiber, under bending fatigue conditions.

2. Experimental

2.1. Conventional DSC

Differential scanning calorimetry (DSC) study has been performed using standard heat flux DSC

of Mettler Toledo. Samples of 1 mg mass are weighed with a precision balance and encapsulated

in (crimped) aluminium pans of known mass. An identical empty pan is used as a reference.

Nitrogen is purged at a rate of 50 ml min−1. Heating-Cooling-Heating cycles in the range -

10…200°C were applied for determining the parameters that numerically characterize the thermal

behavior of the investigated materials.

2.2. Fast Scanning Calorimetry

Measurements were performed with a DSC8500 from Perkin Elmer: its furnace has low mass and

small dimensions, ensuring2 faster heat transfer. Samples for Hyper-DSC experiments were

wrapped in aluminium foil of predetermined weight, and an empty aluminium foil wrap of equal

weight was placed in the reference cell of the DSC. Samples were then heated-cooled from

−85°C to 200°C at various scan rates, under a constant helium purge. Aluminium foil was used

instead of standard DSC pans, to facilitate effective heat transfer during such high scanning rates.

2.3. Deconvolution procedure

The DSC thermograms were analyzed using a deconvolution procedure provided by the

OriginLab 8.5 software package. In the deconvolution procedure, the only constraint applied was

the Gaussian type of the peaks; their position, height and width was allowed to freely move in

order to obtain the best fit.


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2.4. General description of the kinetic method

When an effect is recorded during a DSC experiment, while the sample is subjected to a

controlled temperature ramp, it is reasonable to assume that it reflects a transformation which can

be represented for the simplest case as3:

(1)

where B0 is the material before transformation and B1 the material after transformation. Any

conversion is accompanied by an absorption or release of heat. In a quantitative way it is

expressed by means of the enthalpy of the process4:

(2)

where ΔH is the enthalpy, t is the time, T is the temperature, cp is the heat capacity and β is the

heating rate:

(3)

The degree of conversion, α, is then calculated from the DSC curve assuming that the area under

the peak up to a given time is proportional to the degree of conversion:

(4)

where PAt is the integral of the peak up to time t and PAtotal is the overall peak area.

With this definition of conversion the reaction rate can be written as:

(5)

where dα/dt is the reaction rate, k(T) is rate constant as a function of the absolute temperature, T,

and f(α) is an unknown function of conversion.

Assuming that the rate constant obeys Arrhenius equation:

(6)

where Ea is the activation energy of the reaction, T the absolute temperature, R the universal gas

constant, and A is the pre-exponential factor, one may re-write eqn. 5 in a expanded form as:

(7) dα/dt = A f(α) exp(-Ea/RT)

The function of conversion, f(α), in eqn. 7 is chosen according to experimental data and is

assumed to describe the reaction mechanism. There are many different proposed functions for

the function f(α). A quite general one is Sesták- Berggren equation5,6

:

(8)

where the values of n, m, p allows retrieving the particular form of kinetic models of

1Bk

B 0

2

1

2

1

t

t

p

T

T

p dtcdtcH

dt

dT

total

t

PA

PA

fk(T)dt

d

RT

EaexpAk(T)

p1lnm1nf


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heterogeneous reactions.

A convenient change of the variable time (t) into temperature (T), with definition of β from

eqn. 3 and the general form of eqn. 8 turns eqn. 7 into7:

(9)

This equation allows obtaining the group (Ea, A, f(α)), called the kinetic triplet which is

considered to characterise kinetically the investigated process. Any approach of solving eqn. 9 for

calculating the kinetic parameters of thermally initiated chemical or physical processes has to

take into account the particular form of the signal, for instance whether it displays single or

multiple peaks. In the case of a multiple peak pattern, eqn. 9 stands only for separate processes.

For complex competitive or reversible reactions sequences or those complicated by diffusion Ea

varies with α. Under such circumstances eqn. 9 remains valid only locally and the kinetic

parameters have to be calculated for each experimental point or rather, on small intervals for

which they are considered as constants8. The alternative is that a complicated pattern is

separated into elementary processes (deconvoluted) to enable further analysis.

There are several methods for calculating the kinetic triplet, Ea, A and f(α), from thermal

analysis data. In some cases, a certain reaction model is assumed, implying a specific analytical

form of the kinetic function, f(α). There are also the model-free methods which allow calculating

the value of the activation energy, Ea, without any knowledge of the reaction pathway. The

advantage of analysing kinetic data using model-free methods is that these methods do not

assume any model or mechanism beforehand, and thus they are able to describe the most

complicated reaction behaviour at different temperatures. The solely assumption involved by the

use of model-free methods is that the reaction mechanism does not change with temperature and

heating rate9.

2.4.1. The activation energy, Ea

The forced fitting of experimental data to simple reaction-order kinetic models can produce

significant errors when predicting rates outside the experimental range of temperatures. For this

reason, model-free methods are the suitable approach. These are also known as the

isoconversional methods10

, meaning that the data for calculation are acquired for the same

degree of conversion from a series of experiments conducted at different heating rates. Literature

considers them as the best approach for taking into account reaction mechanisms outside the

experimental range of temperatures.

A simple approach in this sense is due to Kissinger11

. The method is derived from the condition

that the maximum of the signal is formed for the same degree of conversion for any heating rate,

β. The apparent activation energy can be determined by Kissinger method without a precise

knowledge of the reaction mechanism, using the following equation:

(10)

where A: pre-exponential factor, Ea: Energy barrier and f(α): unknown reaction mechanism, Tp:

peak temperature (K), R: the gas constant

RT

EaexpAln

β

1

dT

dα pmn 11


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A plot of ln(β/Tp2 ) against 1/Tp gives rise to a straight line whose slope yields the activation

energy Ea. For a first order kinetic model, f(α)= -1, the preexponential factor A may be extracted

from the intercept.

The Kissinger method allows a simplification of the kinetic analysis and obviously produces a

single value of the activation energy for any process regardless of its actual kinetic complexity. As

a result, the activation energy determined can adequately represent only single-step kinetics. An

adequate representation of the commonly encountered multi-step kinetics would normally require

more than a single value of the activation energy. Therefore, it is necessary to use an

isoconversional (integral and/or differential) method to back up the veracity of the Kissinger

estimates.

Ozawa, Flynn and Wall12

tried to rewrite Eq. (5) in an integral form, followed by a replacement of

the integrant by an approximation function. This treatment has led to establishment of the

following equation.

(11) ln(β) = ln[AEa/R] - G(α) - 5.3305 - 1.052 Ea/(RT)

Under the isoconversion assumption, the function G(α) reaches a given value thus a constant.

Therefore the plot of ln(β) against 1/T results in a straight line with the slope being -1.052 Ea/(RT)

which allows calculating the activation energy for the corresponding conversion degree.

Five heating rates, namely 1,3,5,10,15 °C/min were employed in the kinetic study.

2.4.2. Life-time and stability predictions13 Integration of equation (7) leads to:

(12) dtRT

EaA

f

dt

00

)exp()(

g

where g(α) is the integral form of the reaction model.

The integral in equation (12) does not have an analytical solution for an arbitrary temperature

program. However, an analytical solution can be obtained for an isothermal temperature program:

(13) tRT

EaA

expg

In the case of a single-step process taking place under isothermal conditions (constant

temperature, T0), the time to reach a given extent of conversion can be determined by rearranging

equation (13) as:

(14) )/exp(

)(t

0RTEA

g

Equation (14) can be used to predict the lifetime of a material under isothermal conditions at the

temperature T0. The prediction requires knowledge of the kinetic triplet for the process that

causes decay in the property of interest. The triplet can be determined from either isothermal or

non-isothermal experiments.


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2.5. Sample information

Cyclic-bending-over-sheave (CBOS) tests have been carried out on small diameter braids (5 mm nominal

diameter 12 strand braid based on Dyneema® SK78 and ICO-DYN-10 coating) using a dedicated

laboratory scale tester (see Figure 1). The tensile load on the braid was set to 4 kN which

corresponds to 20% of its average breaking load. The machine stroke was 0.5 m. The

experiments were carried out under standard laboratory conditions (23°C, 50% RH). No additional

provisions were taken to cool the braid during the test. Due to heat dissipation, temperature

increase in the braid may occur. The measured temperature during the test remained below 45°C

(measured by a thermocouple in the core of the braid). The time for one full cycle (forward and

reverse rotation of driving sheave over ±90°) is 6 seconds. The average number of cycles to braid

failure under these conditions is 16000 ± 2000.

Figure 1 Equipment used for performing bending fatigue experiments on Dyneema® braids

CBOS tests were conducted up to a fixed number of cycles: 10%, 25%, 50%, and 90% of the

lifetime of the braid. After reaching these cycle numbers, the CBOS test was stopped and

samples for the DSC analysis were taken at the center of the bending zone of the braid.

3. Results and Discussion

The advantage of the synthetic ropes is their performance and economic advantages induced by

amongst others their lightweight nature. It is critical however to assess the rope condition to

assure its safe operation. Commonly this is achieved by visual inspection and by tracking the

history of usage of each rope as there are no other generally accepted guidelines or standards for

measuring the integrity of the synthetic ropes yet. We hereby report less subjective thermal

analysis methods to evaluate the structural integrity of synthetic ropes or hybrid cables made of

steel and UHMW-PE.

UHMW-PE crystallizes during polymerization into small crystals that thicken on annealing below

the melting temperature to a maximum value of 25 nm14

. They melt above 140 ºC, a high melting

Driving sheave (D=300mm)

Test sheave (D=50mm)


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temperature normally found for “chain-extended” polyethylene crystals, which are extremely thick

(>1 μm).

The melting temperature of nascent UHMW-PE is independent of the polymerization conditions,

but the high melting temperature above 140 ºC is lost on a second heating DSC ramp and the

normal melting temperature of 135 ºC is recovered. The high melting temperature is reported as

representing not the true melting temperature of nascent UHMW-PE and that kinetic factors play

an important role in the melting process14

.

The kinetic interference in the melting of the nascent crystals is easily proven by annealing

experiments at specific isothermal temperatures after which the sample is cooled down to room

temperature and reheated (at 10 °C min−1) to 200 °C.

Figure 2 Temperature profile (left) respectively the typical DSC thermogram recorded during the

final heating ramp (right). The general assignment of the endotherms is mentioned on the figure

On re-heating, two melting peaks are generally observed (see Figure 2). The first peak

(temperatures around 135 °C) is associated with the melting of the material fraction which was

molten and re-crystallized during cooling from the annealing temperature and the second peak

(temperatures around 140 °C) to the crystal domains in the initial nascent state. The ratio

between the areas of the two peaks changes with the annealing time at the given annealing

temperature Ta.

With the final goal of monitoring the status of the synthetic or hybrid ropes for establishing and

maintaining an accurate retirement criterion15

, the usefulness of nonisothermal kinetic methods in

capturing the kinetic factors interfering in the nascent UHMW-PE melting process is herein

investigated.

Thermodynamic melting is generally recognized in a calorimetric experiment when the onset

temperature of the endothermic peak levels off to a constant value independent of heating rate; of

course this implies a very good control of the sample size submitted to analysis for avoiding

differences caused by a thermal lag.

Figure 3 exhibits the thermal behaviour of the laboratory scale fatigued materials when using a

low heating rate (i.e. 1°C/min). Generally sharply melting events characterize each material.

Repetitive bending and stretching cycles occurring when a rope is bended over e.g. sheaves are

shown to be decently well reflected by a decrease in the peak height.


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Slightly increasing the heating rate (see Figure 4) drastically changes the melting behavior.

Repetitive bending and stretching cycles are shown to be reflected not only by the decrease in

the peak height but also by a different displacement towards higher temperatures: almost in a

linear manner, the more fatigued material tends to melt at higher temperatures.

Further altering the scanning rate (see Figure 5) magnifies the differences in samples thermal

behavior: interesting features are not only the different displacement of the onset melting

temperatures but also the presence of several fractions exhibiting different melting dependencies

on heating rate.

Figure 3 Conventional DSC @ 1°C/min for materials sampled up to the rope failure

Figure 4 Conventional DSC @ 10°C/min for materials sampled up to the rope failure


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Figure 5 Fast Scanning Calorimetry measurements on materials sampled after 50 respectively

90% of the bending cycles up to the rope failure. The non-fatigued material is added as reference

while the heating rates are mentioned on the plot

Generally, the 3 fractions obtained via deconvolution of the thermograms from Figure 5 were

found to have different heating rate dependencies (see Figure 6). The endothermic peak of the

first fraction levels off to a constant value independent of heating rate and of previous bending

history: behavior characteristic to thermodynamic melting. The other two fractions show a linear

dependency on the heating rate, however at different extents.

Figure 6 Deconvolution procedure (left) respectively the heating rate dependency of the peak

temperatures (right) of the 3 fractions characterizing the non-fatigued material


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Figure 7 Slope of the heating rate dependency of the peak temperatures of the 3 fractions

obtained via deconvolution: higher the slope, greater the displacement of the melting endotherm

towards higher temperatures by increasing the heating rate

The material history seems to influence the dependency of the peak temperatures on the heating

rate: in general the harsher the mechanical treatment, the more severe the displacement towards

higher temperatures (see Figure 7).

Superheating and/or kinetic arrest are seen to count for this behaviour. Superheating (heat

supplied faster than the crystal can melt and reorganize to a random coil) of extended‐chain

crystals was demonstrated by qualitative DTA and isothermal quantitative calorimetry16

for

polyethylene extended-chain crystals. Smaller crystals and folded‐chain crystals showed

decreasingly less superheating17

.

The first fraction showing thermodynamic melting (less superheating and/or kinetic arrest) is

therefore hypothesised to reflect a certain amount of folded-chain type material. This supports a

melting mechanism which calls for melting starting from chain ends or folds.

Figure 8 suggest that the fatigue-induced structural changes of synthetic ropes involves at least

partial recrystallization/ reorganization of the extended-chain crystals into folded chain lamellae.

Figure 8 Averaged fractions composition determined via deconvoluting the endotherms given in

Figure 5 and using the procedure described in Figure 6

If the heating rate is fast enough to suppress the kinetic process, thermodynamic melting may be

achieved. Figure 9 indicates therefore a kinetic control rather than the superheating phenomenon


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to count for the endotherm displacement towards higher temperatures with increasing the heating

rate: at ~ 500°C/min the onset temperature of the endothermic melting peak levels off to a

constant value independent of heating rate and previous bending history. The peak shape

suggests as well single transitions, no additional shoulder being noticeable.

Figure 9 Fast Scanning Calorimetry measurements on materials sampled after 50 respectively

90% of the bending cycles up to the rope failure. The non-fatiqued material is added as reference

while the heating rates are mentioned on the plot

Stimulated by this observation we attempted to extract the kinetic parameters governing the

melting behaviour of the extended-chain crystals of UHMW-PE and their usefulness in properly

reflecting the structural integrity of the investigated materials.

Figure 10 exhibits the Kissinger apparent activation energies determined as described in

paragraph 2.4.1. from conventional DSC data (i.e. heating rates of maximum 15°C /min) and reveals the possibility to rank UHMW-PE ropes according their fatigue level. By testing a relevant

number of samples collected up to the rope failure, a health monitoring regression line can be

obtained that will eventually allow further extrapolating the time till retirement of a synthetic or

hybrid rope in practice.


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Figure 10 Kissinger activation energy dependency on the previous bending history. Ref stays for

the non-fatigued material while the percentages for the applied bending cycles up to the rope

failure

Assuming a first order kinetic model, the pre-exponential factor was further inferred from the

intercept. The results of the kinetic analysis were used for predicting the half-life of the nascent

crystals at a given temperature that is, in other words, the time needed for half of the crystals to

undergo melting on annealing.

Figure 11 shows that the Kissinger estimates allows predicting the thermal stability of UHMW-PE

nascent crystals with reasonable accuracy. The calculated half-life time of 6 minutes at 142°C

matches the experimental findings: only 50% of the original enthalpy characterizing the remaining

nascent crystals is recorded after an isothermal exposure to the above specified conditions.

In spite of this encouraging result, one has to be aware about the limitations of such a simple

approach: the Kissinger method assumes a constant activation energy with the conversion

degree and that for a given DSC curve with the heating rate, β, one observes the maximum

reaction rate at the peak temperature.


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Figure 11 Reference material first heating, respectively heating scan after an initial exposure of 6

minutes at 142°C. Hatching highlight the melting of the nascent crystals

Figure 12 exhibits the apparent activation energies values determined from conventional DSC

data via the isoconversional method of Flynn-Wall-Ozawa that do not assume any model or

mechanism beforehand, thus being able to describe the most complicated reaction behaviour at

different temperatures. One can notice that at a conversion degree of 50% (that is approximately

at the peak temperature), the apparent activation energies are similar to the Kissinger estimates,

therefore validating their accuracy. Similar discrimination of the fatigue level is obvious over the

whole conversion.

Figure 12 Flynn Wall Ozawa activation energy variation during conversion (i.e. melting) However, one can easily notice that the magnitude of the activation energy is overall, but

especially in the beginning of the melting transition, quite high. Such values are usually


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characterizing thermodynamic processes, therefore validating our hypothesis on the nature of the

first fraction.

The quality, integrity and ability of a rope to carry loads safely is a function of the tensile strength.

Residual break strength of ropes are commonly undertaken after service to asses condition.

Figure 13 reports the observed relation between the Kissinger apparent activation energy and

the residual break strength of ropes after service.

Figure 13 Kissinger apparent activation energy plotted versus residual break strength after

service of ropes

Given the correlation and the observations above, one can consider the described thermal

analysis approach for performing either in-service or post-service analysis of ropes to achieve

performance feedback or understand any deterioration levels. Systematic testing of used rope by

users and rope manufacturer to develop patterns can lead to development of retirement practices,

avoiding unplanned downtime and improve service life of ropes. Of course this also imply

increased attention and uniformization of the sampling procedure for subsequent DSC analysis;

obviously, inhomogeneous ageing across the rope diameter or its length can induce severe

limitations for an accurate retirement assessment. This must be therefore topic of further

research. Additionally, only by testing a relevant number of samples will allow firmly concluding

on the practical applicability of the described thermal analysis approach and on the relevance of

the correlations and observations reported herein.

4. Conclusions

Material fatigue history seems to influence the dependency of the melting peak temperatures on

the heating rate: generally the harsher the mechanical treatment, the more severe the


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displacement towards higher temperatures. Fast Scanning Calorimetry allows separating the

thermodynamic from the kinetic arrested fractions.

Structural changes of synthetic ropes as a result of repetitive bending cycles involve

recrystallization/ reorganization of the extended-chain crystals into folded chain lamellae. A

melting mechanism which calls for melting starting from chain ends or folds is found to be

UHMW-PE characteristic.

Nonisothermal kinetics reveals the possibility to monitor the structural integrity of UHMW-PE

ropes. Further research is however needed to develop this technology as an inspection and

retirement tool for ropes in use, in several application areas. By testing a relevant number of

samples collected up to the rope failure, including sampling areas, a health monitoring regression

line can be obtained that will allow further extrapolating the time until retirement of a synthetic or

hybrid rope in practice.

5. Literature

1

2 G.Vanden Poel, V.Mathot, Thermochim. Acta 446 (2006) 41-54

3 D. Istrate, C. Popescu, M. Möller, Macromolecular Bioscience, Volume 9, Issue 8, p 805-812,

2009

4 Wortmann, F. J.; Popescu, C.; Sendelbach, G. Biopolymers 2006, 83, 630-635

5 Pielichowski, K.; Czub, P.; Pielichowski, J. Polymer 2000, 41, 4381-4388

6 Sestak, J.; Berggren, G. Thermochim. Acta 1971, 3, 1-12

7 Popescu, C.; Segal, E. Int. J. Chem. Kinet. 1998, 30, 313-327.

8 Vyazovkin, S. Int. J. Chem. Kinet. 1996, 28, 95-101

9 Fernandez d‘Arlas, B.; Rueda, L.; Stefani, P. M.; de la Caba, K.; Mondragon, I.; Eceiza, A.

Thermochim. Acta 2007, 459, 94-103

10 Vyazovkin, S.; Wight, C. A. Annu. Rev. Phys. Chem. 1997, 48, 125-149

11 Kissinger, H.E., Anal. Chem., 29:11, 1702-1706 (1957)

12 Flynn, J. H.; Wall, L. A. J. Res. Natl. Bur. Stand. 70A 1966, 487–523

13 Vyazovkin, S. (2011). Thermochimica Acta 520, 1–19

14 Rastogi, S.; Lippits, D.R.; et all, Nature Materials 2005, doi: 10.1038/nmat1437

15 M. Vierstraete, X. Amils, Internal Bekaert Report, 4 November 2008

16 E. Hellmuth, B. Wunderlich, J. Appl. Phys. 36, 3039 (1965);

17 B. Wunderlich, T. Davidson, J. Polymer Sci. Part A-2: Polymer Physics, Volume 7, Issue 12,

pages 2043–2050, December 1969

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