1
On the determination of thermal degradation effects and detection 1
techniques for thermoplastic composites obtained by automatic lamination 2
M.I. Martín
1
, F. Rodríguez-Lence
1
, A. Güemes
2
, A. Fernández-López
2
, L.A. Pérez-3
Maqueda
3
, A. Perejón
3,4
4
1
FIDAMC, Foundation for the Research, Development and Application of Composite 5
Materials, Avda. Rita Levi Montalcini, 29, Tecnogetafe 28906 Getafe (Madrid), Spain; 6
2
Departamento de Materiales y Producción Aeroespacial, E.T.S.I. Aeronáutica y del 7
Espacio, Universidad Politécnica de Madrid, Pza. de Cardenal Cisneros 3, 28040 8
Madrid, Spain; 9
3
Instituto de Ciencia de Materiales de Sevilla (C.S.I.C. – Universidad de Sevilla). C. 10
Américo Vespucio 49, Sevilla 41092. Spain; 11
4
Departamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla, 12
Sevilla 41071, Spain 13
14
15
Abstract 16
Automatic lay-up and in-situ consolidation with thermoplastic composite materials is a 17
technology under research for its expected use in the profitable manufacturing of structural 18
aeronautical parts. This study is devoted to analysing the possible effects of thermal degradation 19
produced by this manufacturing technique. 20
Rheological measurements showed that there is negligible degradation in PEEK for the 21
temperatures reached during the process. Thermogravimetric analysis under linear heating and 22
constant rate conditions show that thermal degradation is a complex process with a number of 23
overlapping steps. A general kinetic equation that describes the degradation of the material with 24
temperature has been proposed and validated. Attenuated total reflectance Fourier transform 25
infrared spectroscopy and X-ray photoelectron spectroscopy confirmed that there is no 26
remarkable degradation. The use of a combination of in-situ and ex-situ experimental techniques, 27
including kinetic modelling, not only provides reliable information about degradation but also 28
allows setting optimal processing conditions. 29
30
31
Keywords: Polymer-matrix composites (PMCs); Automated fibre placement (AFP); Process 32
Modeling 33
34
35
2
1. Introduction 36
With the goal of industrial application of thermoplastic composite materials in aeronautics, 37
similar techniques to those used with thermosets (automatic tape placement, fibre placement) 38
should be fine-tuned taking into account their demonstrated success in aircraft [1]. Current 39
automatic lamination with thermoplastic composite materials attempts to reach full consolidation 40
in only one-step, avoiding the secondary use of an autoclave. However, the lack of tackiness in 41
the material forces the use of heat sources that apply a punctual or surficial heating that lasts only 42
several seconds. 43
The operating principle of these machines is the sequential heating of individual layers before 44
being placed in contact with the pre-positioned layer for the production of laminates of different 45
shapes and ply orientations. Different heating sources have been used, with diode laser being a 46
preferred solution [2]. After heating to the melting temperature of the polymer, the layers are 47
compacted by a deformable roller. 48
Establishing the proper limits for the parameters interacting in the control loop of automatic lay-49
up and in-situ consolidation is a complicated task, owing to their coupling. Mathematical models 50
defining the responses of the material to these specific effects are needed. These models depend 51
on the way the specific effects are described, and the property used for their calculation. Diverse 52
effects should be estimated in advance, creating models that contemplate thresholds whilst 53
reaching the required power and speed, for acceptable levels of degradation, crystallization, 54
bonding, and so on [3]. Finally, the control system should implement all of them, giving proper 55
impact ranges and estimating the degree of deviation from the ideal result. 56
Owing to the elevated requirements imposed by aeronautics, high-performance thermoplastics 57
such as those in the poly-aryl-ether-ketone (PAEK) family are the focus of most of the lamination 58
endeavours to set up this manufacturing technology [4–9], with the exception of some works 59
devoted to the application of PPS (poly- phenylene-sulphide) [5,10,11], PEI ( polyetherimide) 60
[12], and current works with PA (polyamide) [13,14]. Poly-ether-ether-ketone (PEEK) is a linear, 61
aromatic, semi-crystalline thermoplastic polymer with excellent thermal stability, chemical 62
resistance, and mechanical properties. Its glass transition temperature appears at around 416 K 63
and its melting point is 616 K. Moreover, above 723 K, 4-phenoxyphenol and 1,4-64
diphenoxybenzene are detected, showing chain scission thermal degradation [15]. Other authors 65
suggest that the degradation produces cross-linking in the matr
ix structure, especially under an 66
oxidative atmosphere, which affects the viscosity and the ability to crystallize [16,17]. The 67
atmosphere has an important effect on the degradation, with differences in the time required for 68
degradation at the same temperature as high as 16% in either inert or oxidative atmospheres [18]. 69
70
For the automatic lamination processes of carbon fibre/PEEK composites (CF/PEEK), 71
temperatures in the range from 623 K to 673 K, or even higher, should be applied to achieve 72
melting and bonding among layers. Thus, the processing temperature is very close to the onset of 73
degradation [18]. Moreover, considering that the temperature control is manual, instances of 74
overheating could discretely appear, thereby changing the material elementary structure. 75
Considering the normal trends in automatic lamination with carbon fibre-reinforced 76
thermoplastics, the relevant speeds of lamination differ from consulted groups of study. As a 77
parameter still under analysis, it is possible to find values as low as 10 mm/s [19] and as high as 78
400 mm/s [20], which imply times of maximum temperature stabilization in the range of seconds, 79
3
depending on the heating spot. Additionally, there is an important effect of polymer molecular 80
weight, which affects the capacity of chains to move and, therefore, the maintenance time needed 81
to reach full healing. Thus, for the sake of establishing comparisons, all these parameters should 82
be carefully considered. Owing to the fact that thermal degradation is not only affected by 83
temperature but, also by the dwell time at this temperature, both parameters should be considered 84
in any degradation model. For a proper kinetic model, a specific monitoring parameter (mass loss, 85
linkage % content, molecular weight, or viscosity) should be considered. 86
Degradation kinetics permit one to obtain a mathematical formula that relates the extension of 87
conversion (degradation by means of different parameters) to time–temperature profiles. Kinetic 88
thermal analysis is a powerful tool, which is widely employed to estimate in advance the 89
behaviour of a material subjected to a specific thermal process condition. It is a valuable tool in 90
many scientific branches and has already been used with PEEK [15,21,22]. 91
In particular, for the PAEK family, several studies have tried to obtain information about the 92
degradation mechanisms. Most of them focused on the detection and explanation of the 93
degradation mechanism from a physical-chemical point of view [16,23–26] and a low number of 94
works elucidate the kinetic model. Some of them, related to kinetics, are focused on pyrolytic 95
tests [27], oxidative atmosphere tests [28], or both. In the particular case of oxidative degradation, 96
the authors consider that there are only two mechanisms interacting from the beginning (0% mass 97
loss) to the end (100% mass loss). Nevertheless, the great dispersion observed in activation 98
energies, even for the first 30% of mass loss, suggests the presence of more than one mechanism 99
in this small range. 100
Furthermore, there are many works reporting that, before the appearance of any kind of 101
measurable degradation by mass loss, PEEK already experiences modifications that affect its 102
structure. This effect is related to the unstable behaviour of ether and carbonyl groups, which is 103
mainly detectable by rheological testing and by appreciable changes in the crystallization 104
behaviour [17,29]. Other authors consider that mass loss fails when one tries to associate its 105
appearance with detrimental mechanical properties [30]. 106
This study aims to analyse several effects in PEEK degradation and CF/PEEK composites. It 107
includes different experimental measurements for the determination of possible degradation 108
effects after diode-laser-irradiation processing of the composite material. Moreover, a simplified 109
kinetic equation is proposed that predicts the behaviour of neat resin under any heating profile by 110
considering two monitoring parameters independently: mass loss
and viscosity changes. 111
2. Theoretical background of thermal degradation kinetics 112
Equation 1 is commonly used for studying the kinetics of solid-state reactions [31]: 113
𝑑𝛼
𝑑𝑡
𝐴
𝑒𝑥𝑝
𝐸
𝑅𝑇
𝑓
𝛼
(1)
where A is the pre-exponential Arrhenius factor, E
a
the activation energy, α the reacted fraction 114
or conversion, dα/dt the reaction rate, and f(α) the kinetic model. 115
This equation can be rewritten considering the evolution of conversion with temperature instead 116
of its time dependency, by means of the reaction rate, 𝛽. 117
4
𝑑𝛼
𝑑𝑇
𝐴
𝛽
𝑒𝑥𝑝
𝐸
𝑅𝑇
𝑓
𝛼
(2)
From Equation 2, it follows that it is necessary to obtain the values of the kinetic triplet (A, E
a,
, 118
f(α)) in order to complete the information that properly describes the kinetics of a reaction. 119
The use of the so-called ‘isoconversional’ methods facilitates this task, because the reaction rate 120
at a constant extent of conversion is considered to be only a function of temperature. Thus, the 121
activation energy values can be extracted without any consideration of the reaction model. One 122
of the most used differential isocoversional methods is that of Friedman [31], which for 123
nonisothermal heating programs, has the form (3): 124
𝑙𝑛 𝛽
𝑑𝛼
𝑑𝑇
,
𝑙𝑛
𝑓
𝛼
𝐴
𝐸
𝑅𝑇
,
(3)
The index ‘i’ represents each of the set of dynamic heating programs used for the analysis. The 125
left-hand side of the equation is plotted against
; the plot consists of straight lines linked to 126
each conversion value and with a number of points that depends on the number of heating 127
programs used. The slope of the lines gives the value of the activation energy 𝐸
. 128
Polymer thermal degradation is usually described by complex mechanisms depending on the 129
number of reactions implied [32]. The presence of multiple reactions complicates the application 130
of isoconversional methods in order to obtain the activation energy, whose evolution is highly 131
dependent on conversion. For this reason, it is not realistic to determine the corresponding 132
activation energy values for each stage using the isoconversional methodology. It is important to 133
clarify that the interaction of simple individual mechanisms is not certified by the fact that a 134
constant activation energy with respect to conversion was obtained, because in any case, a deeper 135
analysis is needed [33]. 136
Attempting to overcome the difficulties when more than one stage is involved in the overall 137
process, several solutions have been suggested. One of these solutions considers the 138
deconvolution of the experimental curves of the reaction rate (
) into different subcurves that 139
represent each stage individually [34,35]. This methodology is applied in the present work in an 140
attempt to obtain the equation that describes the thermal degradation of PEEK. 141
Thus, after deconvoluting the experimental curves for each heating rate, they are analysed 142
independently for each degradation stage [36]. The calculus of the kinetic triplet is based on the 143
combined kinetic analysis method [37]. For the combined kinetic analysis methodology, the 144
general kinetic equation is converted into the linear form shown in Equation 4: 145
ln
ln
𝑐𝐴
(4)
However, the plot of the left-hand side of this equation versus the inverse of the temperature yields 146
a straight line only if the correct kinetic function, 𝑓
𝛼
is considered. Moreover, the proposed 147
kinetic functions are idealized physical models that may not be useful for all the solid-state 148
reactions, in which factors such as the particle size, the particle shape, etc. have an important 149
influence on the reaction mechanism. It has been demonstrated that this limitations can be 150
overcome if the modified Sestak-Berggren equation is considered as 𝑓
𝛼
: 151
5
𝑓
𝛼
𝛼
1𝛼
(5)
Thus, it has been shown that this expression fits all f(α) corresponding to the ideal kinetic models 152
proposed in the literature and even their deviations from the ideal conditions [38]., by adjusting
153
the parameters c, n and m. Equivalent reduced Sestak-Berggren equations for each ideal kinetic 154
model have been proposed [38]. Substituting equation (5) into equation (4) the general kinetic 155
expression for the combined kinetic analysis method is obtained: 156
ln
ln
𝑐𝐴
(6)
157
The kinetic triplet is obtained by plotting the left-hand side of Equation 6 against reciprocal 158
temperature, independently for all the experimental data corresponding to each degradation stage. 159
From the values of n and m that give the best linearity fit, the kinetic model followed by each 160
decomposition stage is obtained. The linearity is evaluated by the coefficient of linear correlation. 161
Afterwards, the values of the activation energy and pre-exponential factor are obtained from the 162
slope and the intercept of the straight line. Normal ranges of α considered for optimisation are 163
0.1–0.9 or 0.05–0.95 in order to avoid experimental errors that are more relevant for low and high 164
values of α [37,39]. 165
166
3. Experimental 167
168
3.1 Materials 169
To study the thermal degradation process experienced by the carbon fibre/thermoplastic material 170
used in automatic lamination and in-situ consolidation manufacturing processes, tests have been 171
conducted on a neat PEEK resin (without treatment) and the CF/PEEK composite material. Thus, 172
two different materials have been used in this study, neat polymer PEEK 450G Victrex in pellet 173
form and APC2/AS4 Solvay pre-impregnated PEEK/long carbon fibre material, with fibre content 174
close to 60% in volume. 175
Both materials were tested as received by the supplier (without any treatment) and after being 176
processed by a specific manufacturing technique. The studied samples were classified as shown 177
in Table 1 for the sake of clarity. The neat PEEK resin was studied as received (sample C1) and 178
after reaching 2% degradation (sample C2). In the latter case, the sample was heated in the 179
thermogravimetric analysis (TGA) apparatus up to the point when the degradation, measured as 180
the mass loss, reached 2%; it was then quenched to room temperature (~298𝐾). For the pre-181
impregnated material, it was studied as received (sample C3) and after laser irradiation treatment 182
(samples C4 and C5). Diode-laser-irradiated samples were prepared using a gantry style machine 183
developed by MTorres (Pamplona, Spain) using a 500 N compaction force and a speed of 1 mm 184
min
−1
. An operator, based on noncontact indications given by thermographic camera, controlled 185
the applied power supplied by a 500 W laser with a wave length of 980 nm. Sample C4 was 186
irradiated with low energy (normal operation conditions) that induces a sample temperature in the 187
range of 623–673 K, whereas sample C5 was irradiated with higher energy (over-irradiation 188
conditions) inducing a higher sample temperature, in the range of 673–723 K. 189