Plasticizer loss in a complex system (polyamide 12): Kinetics, prediction and its effects on mechanical properties

Journal Pre-proof Plasticizer loss in a complex system (polyamide 12): Kinetics, prediction and its effects on mechanical properties Xin-Feng Wei, Kai J. Kallio, Stefan Bruder, Martin Bellander, Mikael S. Hedenqvist PII:

S0141-3910(19)30313-1

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.108985

Reference:

PDST 108985

To appear in:

Polymer Degradation and Stability

Received Date: 26 June 2019 Revised Date:

20 September 2019

Accepted Date: 23 September 2019

Please cite this article as: Wei X-F, Kallio KJ, Bruder S, Bellander M, Hedenqvist MS, Plasticizer loss in a complex system (polyamide 12): Kinetics, prediction and its effects on mechanical properties, Polymer Degradation and Stability (2019), doi: https://doi.org/10.1016/j.polymdegradstab.2019.108985. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Plasticizer loss in a complex system (polyamide 12): Kinetics, prediction and its effects on mechanical properties

Xin-Feng Weia,*, Kai J. Kalliob, Stefan Bruderc, Martin Bellanderc, Mikael S. Hedenqvista,*

a

Fibre and Polymer Technology, KTH Royal Institute of Technology, SE–100 44 Stockholm, Sweden

b

Polymer Centre, Volvo Car Corporation, SE–405 31 Göteborg, Sweden

c

Materials Technology, Scania CV AB, SE–151 87 Södertälje, Sweden

E-mail address: [email protected] (X-F. Wei) [email protected] (M.S. Hedenqvist) 1

Abstract: Plasticizer migration is a major concern for plasticized polymers because it leads to unwanted changes in mechanical properties and, in many cases, contamination of the environment. In cases of slow migration, it is of practical importance to be able to perform accelerated testing and estimate migration rates from high temperature experiments. Despite the importance, a critical evaluation of different ways of extrapolating mass loss data has hitherto not been reported. Therefore, in this article, three different methods (involving for the first time a master-curve approach to mass loss data) to estimate low temperature migration from high temperature data are presented and critically evaluated. The system chosen was a plasticized polyamide 12 pipe, an important component in vehicle fuel-line systems. This system was challenging since the lower part of the temperature range in which the material is used overlaps with the glass transition region. All three methods (using data at 80-145 °C) over-estimated, although to different extents, the low-temperature mass loss rate (60 °C). The main reason for the over-estimation was the partial overlap with the glass transition region. Hence, there is a built-in safety factor when predicting plasticizer loss over glass transition regions, and the predictions are conservative. It was observed that plasticizer loss and annealing effects were the main reasons for changes in mechanical properties (increase in flexural stiffness/strength) during ageing. Keywords: Plasticizer, Polyamide, Migration, Diffusion, Evaporation, Ageing, Prediction, Safety factor

2

1. Introduction Plasticizers, one of the most commonly used additives, are added to polymers to lower their glass transition temperature and improve their processability, mechanical flexibility, ductility and toughness [1-7]. However, the use of plasticized polymers has often been accompanied by the loss of plasticizer, leading to unwanted changes in mechanical properties and contamination of the surroundings (e.g. in food packaging) [8, 9]. Plasticizer loss decreases the mobility of polymer chains, and thus increases the glass transition temperature and stiffness/modulus but decreases the flexibility, extensibility and toughness of the polymer. For instance, the strain at break of a poly(vinyl chloride) (PVC) cable decreases from ca. 150 % to less than 10 % when the plasticizer content decreases from 20 wt.% to 5 wt.% [10]. To address these problems, efforts have been made to understand the underlying mechanisms and kinetics of plasticizer loss. The loss of plasticizer from polymers to a gas phase occurs through plasticizer diffusion within the polymer bulk to the surface and subsequent evaporation from the surface to the surrounding air/gas phase [2, 11]. The kinetics of the plasticizer loss is determined by the slowest of the diffusion and the evaporation processes, meaning that the system is either diffusion or evaporation controlled [12-14]. Different features are observed for these two cases. The initial plasticizer loss is linear with respect to the square root of ageing time in the diffusion controlled mode and linear with respect to the ageing time in the evaporation controlled mode [15, 16]. Diffusion is generally the slowest process at high temperatures whereas evaporation is the slowest process at low temperatures. For 3

instance, Ekelund et.al. [11] observed that the plasticizer (di-(2-ethylhenyl) phthalate) loss from PVC was controlled by diffusion at 120 and 155 ºC but by evaporation at ≤100 ºC. The actual limiting mode also dictates the plasticizer concentration profile in the material during ageing; in the diffusion controlled mode the concentration decrease towards the sample surface, whereas in the evaporation controlled mode the profiles are flatter [13, 17, 18]. Since the mechanical properties depend on the plasticizer content, the type of dominating mode also affects the mechanical properties during ageing. The plasticizer loss is often a slow process at lower temperatures, which means that the only practical way to estimate this loss is through accelerated testing at higher temperature. This is, however, for several reasons, not a straightforward task, and a clear critical evaluation of the approach to performing such extrapolation has hitherto not been undertaken/reported. In this work, an approach was used in which three methods were applied to estimate low temperature plasticizer loss from high temperature data. This approach was performed on a practically important system, plasticized polyamide 12 (PA12) used in vehicle fuel lines (where temperatures are commonly 60-80 °C during normal driving and even higher during long-term/high-speed driving). The loss of plasticizer (n-butyl benzenesulfonamide (BBSA), which is one of the most common plasticizers used in polyamides [19-21]), was investigated over a large temperature range (60-145 ºC). The effects of plasticizer loss on the mechanical properties were also determined. 2. Experimental 2.1. Materials 4

Monolayer PA12 pipes, with an outer diameter of 12 mm and a wall thickness of 1.5 mm, were supplied by Scania CV AB (Södertälje, Sweden). According to infrared spectroscopy and thermogravimetry coupled with gas chromatography/mass spectrometry analysis, the pipe contained 6.1 wt.% BBSA plasticizer (whose chemical structure is illustrated in Fig. 1) and 0.5 wt.% carbon black. The plasticizer is added to the PA12 pipe to improve its flexibility and low-temperature impact strength when used in vehicle fuel supply systems, and the carbon black is used as a UV stabilizer and pigment. 2.2. Ageing 70 mm long pieces of pipe, cut from the as-received pipes, were hung on aluminium stands and aged in dry air (<10 % RH) in several ULE-600 ventilated ovens (volume = 256 L; Memmert GmbH & Co., Germany) for different periods of time. The samples were positioned to avoid direct airflow from the gas inlet. The linear airflow rate was ca. 1 m s-1, and the air replacement rate was 10000 L h-1, corresponding to an oven volume replacement of 40 times each hour. The ageing was performed at 60, 80, 105, 125 and 145 (±1) ºC. The mass loss of the pipes during ageing was determined gravimetrically using a Mettler-Toledo balance (AG245, Mettler-Toledo, Switzerland). Prior to ageing, the samples were stored at an ambient temperature of 20 ºC and relative humidity of 20 %, which results in a 0.3 wt.% moisture content in the PA12 pipe. The sorbed moisture left the samples quickly during ageing at the elevated temperatures. The moisture content was excluded from the mass loss to obtain only the loss of plasticizer from the pipe. 5

2.2. Thermogravimetry (TG) The evaporation rate of the pristine plasticizer (BBSA, 99 %, Sigma-Aldrich, Sweden) at different temperatures was determined with a TG/DSC 1 analyser (Mettler-Toledo, Switzerland). 70 µL alumina crucibles with ca. 40 mg of liquid BBSA were heated from 20 °C to 80, 105, 125 and 145 °C, at a rate of 10 °C/min and held isothermally at each final temperature for up to 6 h to record the mass loss. All the tests were carried out in a nitrogen atmosphere with a gas flow rate of 50 mL/min. To reveal the carbon black content of the pipe, the TG curve of the as-received pipe was obtained at 20-800 °C using a heating rate of 10 °C min–1, the testing gas being switched from nitrogen to oxygen when temperature is higher than 600 °C. 2.3. Dynamic mechanical analysis (DMA) The changes in the glass transition temperature of the pipe aged at 125 ºC were investigated with a dynamic mechanical analyzer (Q800, TA Instruments, USA) in tensile mode at a strain amplitude of 0.08 % and a frequency of 1 Hz. Samples representing the whole pipe wall, with a length of 30 mm and a width of 3.5 mm, were cut out from the pipes and used for the DMA tests. The measurements were run from -40 to 140 °C at a heating rate of 3 °C/min, and the glass transition temperature (Tg) was reported as the temperature of the peak in the loss modulus. 2.4. Three-point bending tests Three-point bending tests were carried out on the aged pipe specimens at 23 ± 1 °C and 50 ± 2.5 % RH in an Instron 5566 Universal Tensile Testing Machine, in accordance with ASTM D790. 70 mm lengths of pipe were bent at a flexural strain 6

rate of 0.1 mm/mm/min with a support span of 60 mm. The flexural stress was calculated by using the equation [22]: =

(

(1)

)

where F is the flexural force; D and d are the outer and inner diameters of the pipe, respectively; L is the support span. The flexural strength was reported as the maximum value of the flexural stress. The flexural modulus was determined from the slope in the linear region of flexural stress-strain curve with a strain range of 0.5-3%. 2.5. Differential scanning calorimetry (DSC) The crystallinity of the unaged pipes and pipes aged at 125 ºC was assessed by a Mettler-Toledo DSC 1 analyser. Samples, weighing ca. 5 mg and cut evenly so as to represent the whole pipe wall, were placed in 40 µL aluminium cups and heated from 20 to 270 °C at a rate of 10 °C/min in nitrogen with a gas flow rate of 50 mL/min. The degree of crystallinity of the PA12 was calculated as: =∆

∆

×(

(2)

)

where ∆ and ∆

are the melting enthalpies of the sample and of

100 % crystalline PA12 (209 J/g) [23, 24], respectively, and w is the actual plasticizer mass fraction. 2.6. Modelling plasticizer loss The plasticizer loss was modelled with Fick’s second law [25, 26] for a cylinder geometry [11]: =

1

( )

(3) 7

where r, C and

( ) are the radius, plasticizer concentration and plasticizer

diffusivity, respectively. The concentration dependence can be expressed as [11, 26]: ( )=

!" #



%$(4)

where Dco and α are the zero-concentration diffusivity and plasticization power, respectively. Dco is the plasticizer diffusivity in a plasticizer-free material, and the plasticization power describes the degree of plasticization caused by the plasticizer. Dcmax is also defined here as the diffusivity at the initial/maximum plasticizer concentration (Cmax, 6.1 wt.%). The diffusivity averaged over the whole concentration-range (Dav) is obtained with eq.5. &'

=%

()*

+

%()*

!" #



%$,-

(5)

The choice of using the eq. 4 is based on that it has been used successfully in both non-polar and polar solute-polymer systems to describe the exponential plasticisation

effects

on

the

solute

diffusivity,

as

observed

with

both

sorption/desorption integral and isotherm (stepwise changes in solute activity) data [27-31]. Although there exist more elaborate/detailed free-volume relationships [7, 30-32], the present semi-empirical equation is useful for engineering purposes because of its simplicity. The concentration profiles were generated with Matlab® software, using an implicit multi-step backwards differentiation formula described earlier [33]. The profiles were integrated to yield plasticizer loss-versus-time curves, which were fitted to the corresponding experimental loss data, using . and Dco as adjustable parameters. The

8

two parameters yielding the best fit were obtained with the Matlab built-in non-linear Nelder-Mead Simplex Method “fminsearch”. 3. Results and Discussion 3.1. Mass loss

Fig. 1. Experimental and fitted mass loss versus the square root of time of the PA12 pipe aged at different temperatures. The inset shows the chemical structure of BBSA. The Dco and α values corresponding to the fitted curves are given in Fig. 4. All the mass loss curves of the PA12 pipe samples showed a linear relationship (not s-shape/sigmoidal shape) with the square root of ageing time in the first part of the curves (Fig. 1), indicating that the plasticizer loss was diffusion controlled in the temperature region investigated [13]. The mass loss could be described well with the present diffusion model using a concentration-dependent diffusivity (Eq. 4). Fig. 2 shows the evolution of the plasticizer concentration profiles (generated with the diffusion model) during ageing at 105 ºC. The plasticization of the PA12 by BBSA led to steep concentration gradients close to the surface in the early stage of the mass loss (Fig. 2), in contrast to the case of a concentration-independent diffusivity where 9

the decrease in concentration would be more even through the pipe thickness (not shown). Fig. 3 shows that the concentration gradients became steeper with decreasing ageing temperature due to the increase in the plasticization power (α) (see below).

Fig. 2. Plasticizer profiles (generated with the diffusion model (eqs. 3 and 4, using Dco and α from Fig. 4) of the PA12 pipe aged at 105 ºC. The diffusion coefficients obtained from the modelling showed linear Arrhenius behaviour in the 145-105 ºC region with fitted activation energies of 108, 60 and 33 kJ/mol for Dco, Dav and Dcmax, respectively (Fig. 4a, Table 1). However, the diffusion coefficients at 80 ºC deviated from the linear behaviour and the data at all four temperatures were fitted better with a quadratic function (Fig. 4a, Table 1). Average activation energies over the four temperatures (122, 68 and 44 kJ/mol, Table 1), obtained by approximating the non-linear behaviour with a linear Arrhenius curve, became obviously higher than those obtained by excluding the 80 °C data. The decrease in diffusion activation energy when moving from the unplastisized system to 10

that containing the maximum amount of plasticizer is expected, due to the plasticizing effect. The increase in α with decreasing temperature in Fig. 4b reflects the fact that plasticizer-induced plasticization is more effective at lower temperature where the molecular mobility of the starting unplasticized material is lower. The data could be fitted to a quadratic function (Table 1). Due to this effect, the difference between Dco and Dcmax increased significantly with decreasing temperature (Fig. 4a). For instance, Dcmax was less than twice that of Dco at 145 ºC (8.6× 10-8 cm2/s, compared to 4.5× 10-8 cm2/s) but ca. 100 times that of Dco at 80 ºC (7.9× 10-9 cm2/s, compared to 7.1× 10-11 cm2/s). It should be noted here that D(C) calculated by Eq. 4 with Dco and α obtained by the modelling is strictly valid only in the plasticizer concentration interval investigated. If Eq. 4 is used to “extrapolate” diffusivity data to larger plasticizer concentrations (>6.1 wt.%) the diffusivities at the different temperatures actually cross at some concentration point and yield unphysical trends (a higher diffusivity at a lower temperature).

11

Fig. 3. Plasticizer profiles of the samples with the same “central” normalized concentrations of 0.8 and 0.5, respectively, at different ageing temperatures. Arrows point towards lower temperature. The curves were generated with eqs. 3 and 4, using Dco and α from Fig. 4.

Fig. 4. Diffusion coefficient (left) and plasticization power (right) of the PA12 pipe versus reciprocal of temperature. Table 1. Parameters obtained from fittings. Polynomial fitting (Y=Ax2+Bx+C) and linear fitting (Y=Bx+C) where x is reciprocal temperature (K);

Y Log Dco

A a

Log Dco b Log Dco c Log Dav a Log Dav b Log Dav c Log Dcmax a Log Dcmax b

-3.93×106

-2.15×106

-2.97×106

B

C

R2

∆E (kJ/mol)

14175.6

-18.78

0.999

-5632.3 -6371.2 7664.4

6.1 8 -13.3

0.997 0.990 0.997

108 122

-3118.6 -3565.4 13191.4

0.25 1.36 -21.7

0.999 0.989 0.989

60 68

-1708.6

-2.99

0.999

33

12

Log Dcmax c Log α

a

Log αT a

-3.59×106 -1.3×10-4

Log αT b Log αT c Ln kevap

-2320.5 20662

-1.46 -27.9

0.953 0.996

10.1

-12.8

0.999

-4229.9 -4628.5 -10508.2

10.6 11.7 27.2

0.998 0.994 0.982

a

polynomial fitting.

b

linear fitting based on the three data points at 105, 125 and 145 ºC.

c

linear fitting based on all four data points including the data at 80 ºC.

44

81 89 87

Fig. 5. Time taken for the half (50%) and 95% loss of the plasticizer plotted as a function of temperature and reciprocal of temperature. Fig. 5 shows that the time taken for 50 % and 95 % of plasticizer loss increased exponentially with decreasing temperature. 95 % plasticizer loss from the pipe took only 36 and 130 h at 145 and 125 ºC, respectively, showing the high loss rate of BBSA from the PA12 at elevated temperature. At 80 ºC, it took 24 days and 1.4 years to lose 50 % and 95 % of plasticizer, respectively. Hence, the loss of the first 50 % of plasticizer was much faster than that of the second 50 %, especially at lower temperature, which was caused by the strong decrease in both the diffusivity (Fig. 4a 13

and Eq. 4) and plasticizer gradient (Figs. 2 and 3) with decreasing plasticizer concentration.

Fig. 6. (a) Master curve of mass loss with 125 ºC as the reference temperature, and (b) the Arrhenius plot for the shift factor. 3.2. Mass loss master curve Fig. 6a shows the master curve of the mass loss, having a high degree of overlap at the reference temperature chosen (125 ºC), which was obtained by horizontally shifting the entire mass loss data at the different temperatures along the log time axis. Fig. 6b shows that the shift factor followed linear Arrhenius behaviour with a fitted activation energy of 81 kJ/mol in the 145-105 ºC temperature region, but deviated from the linear relationship at lower temperature, displaying the same trend as the diffusivity data (Fig. 4a). As in the case of the diffusivity data, if the trend over all four temperatures with a linear Arrhenius curve is approximated, the estimated activation energy is higher (89 kJ/mol). The non-linear Arrhenius behaviour could also be fitted with a quadratic function (Fig. 6b and Table 1). The activation energies

14

from the shifting to a mass loss master curve were well within the range of those obtained from the diffusivity data. It should be mentioned that the use of the master-curve concept involving horizontal shifting of mass loss data has not been reported previously.

Fig. 7. (a) Mass loss curves of liquid BBSA at different temperatures as a function of time and (b) Arrhenius plot of the evaporation rate of pure BBSA. 3.3. Evaporation rate of BBSA The evaporation characteristics of BBSA at different temperatures were investigated with TGA. Fig. 7 shows that the mass of the liquid BBSA decreased linearly with time after an initial regime of non-linear mass loss. The initial non-linear mass loss has also been observed for liquid DEHP, which was, as in this case, due to the evaporation of absorbed water [14]. The evaporation rate (kv) of pure BBSA, obtained by dividing the slope of the linear region of the mass loss with the cross-sectional area of the TG crucible, followed the Arrhenius law with an activation energy of 87 kJ/mol, similar to the values obtained from the mass loss master curve.

15

Fig. 8. Experimental, predicted, and fitted mass loss from the PA12 pipe at 60 oC plotted as a function of the square root of time. 3.4. Prediction of plasticizer mass loss at 60 ºC Since the plasticizer loss occurs over a very long time (several years) at temperatures lower than 80 ºC (as indicated in Fig. 5), it is of interest to find out whether it was possible to predict the mass loss at lower temperature based on the experimental data at higher temperatures (80-145 °C). Three different approaches were used to predict the mass loss at 60 °C. In the first approach, the mass loss was assumed evaporation controlled at 60 °C and the evaporation rate at this temperature was obtained by an Arrhenius extrapolation using the evaporation rates and activation energy obtained at 80-145 °C (Fig. 7, Table 1). The mass loss using the extrapolated evaporation rate (0.014 mg/(h cm2)) at 60 ºC is shown in Fig. 8 together with experimentally obtained plasticizer mass loss data. This approach clearly

16

over-estimated the mass loss rate, which is understandable since the initial linear experimental mass loss (as a function of square root of time) indicates that the desorption of BBSA is actually diffusion controlled at 60 °C. Hence, the switch from diffusion-controlled mass loss to evaporation-controlled mass loss, as the temperature is lowered, occurs below 60 °C. In the second approach, Eqs. 3 and 4 were used with the parameters obtained at 80-145 °C, extrapolated by the fitted quadratic functions (Fig. 4, Table 1), to model the mass loss at 60 °C (Dco = 2.3×10-12 cm2/s and α = 108 g polymer/g plasticizer). This approach also over-estimated the mass loss rate, but the prediction was better than in the first approach. In the third approach, the master curve and the shift factors (extrapolated by the fitted quadratic function, Fig. 6, Table 1) were used to obtain the mass loss curve at 60 °C. As observed in Fig. 8, this prediction was similar to that in the second approach in the low plasticizer loss region (<2.5 wt.%) but was poorer in the high plasticizer loss region. To conclude, all-three approaches over-estimated the plasticizer mass loss, with the second approach using Eqs. 3 and 4 yielding the least over-estimation. The second approach predicted 50 % and 95 % mass loss after 7000 h (0.8 years) and 309 000 h (35.3 years), respectively. The curve showing the full plasticizer loss at 60 °C, using the available experimental short-term data, was estimated by using the α-value from the extrapolation (108 g polymer/g plasticizer) and an adjustable Dco (determined to be 7.5×10-13 cm2/s). It predicted 50 % and 95 % mass loss after 22500 h (2.5 years) and 921600 h (105 years), respectively.

17

Fig. 9. (a) DMA loss modulus curves of the pipes aged at 125 ºC for different time and (b) their Tg plotted as a function of plasticizer concentration.

Fig.10. (a) Flexural stress-strain curves of the unaged pipe and the pipes aged at 125 ºC, and their flexural (b) modulus and (c) strength plotted as a function of plasticizer concentration. 3.4. Glass transition temperature region

18

Fig. 11. (a) DSC heating curves and (b) crystallinity of the PA 12 pipe aged at 125 ºC for different times, and its flexural (c) strength and (d) modulus plotted as a function of crystallinity. Fig. 9a shows the shift of the glass transition temperature/region, recorded by the loss modulus, of the PA12 pipe towards higher temperature with increasing ageing time due to the plasticizer loss. Fig. 9b shows that Tg increased from ca. 7 ºC (unaged) to 37 ºC (almost plasticizer-free sample). It is clear from Fig. 9a that, at 80 °C, and even more so at 60 °C, the plasticizer loss occurs from a material that is partly in the glass transition region. This is the main reason for the non-linear Arrhenius behaviour

19

in the 80-145 °C temperature region and the predicted over-estimation of the mass loss at 60 °C. The fact that, for several polymers, extrapolation of higher temperature mass loss data to lower temperatures of interest occurs within a glass transition region makes the prediction more complicated. However, since the mass loss rate will always be lower in the glass transition region than predicted by higher temperature data, the predictions can be considered as having a “built-in” safety factor for plasticizer loss/migration. It should be pointed out that a similar type of non-linear Arrhenius behaviour is also expected over larger temperature regions because of a temperature-dependent activation energy. Nevertheless, as mentioned above, the main effect is considered to be the presence of the glass transition region.

Fig. 12. Predicted flexural moduli (lines) using 125 °C modulus data (Fig. 10) and mass loss data (Fig. 1), and experimental data as a function of ageing of the PA12 pipe. 3.5. Mechanical properties Fig. 10a shows the increase in pipe flexural stiffness (modulus) and strength

20

during ageing at 125 °C. Both the flexural strength and modulus increased linearly with decreasing plasticizer concentration (average plasticizer content over the pipe wall, Figs. 10b and c). After complete loss of plasticizer, the flexural strength nearly doubled from 23 to 44 MPa while the flexural modulus increased 2.3 times (from 350 to 817 MPa). During ageing, the annealing of PA12 led to the appearance of a new melting peak at ca. 140 ºC for the aged samples (Fig. 11a) [10, 34]. As a consequence, the crystallinity increased by 4 % (from 27.3 (unaged) to 31.5 % after 100 h of exposure at 125 ºC, Fig. 11b). The increase in crystallinity should also contribute to the stiffening of the pipe, but the relation between the flexural strength/modulus and the crystallinity was weaker (Figs. 11c and d) than with the plasticizer concentration (Figs. 10b and c). Hence, the plasticizer loss, increasing the Tg of the sample, was the main reason for the loss of flexibility of the pipe during ageing in the mass loss phase. It was interesting to find out if it is possible to predict the size and time dependence of the flexural modulus at different temperatures from the relationship between flexural modulus and plasticizer concentration at one temperature, combined with known mass loss rate data at different temperatures. Fig. 12 shows the calculated flexural modulus during ageing at 80-125 °C using the flexural modulus data at 125 °C (Fig. 10) combined with the mass loss data at 80-125 °C (Fig. 1). For the 80 °C data, the prediction is relatively good, but the prediction is poorer at the other temperatures. It is evident that, when the plasticizer is gone, or little remains, the annealing effects become more important, and the increase in crystallinity/change in morphology yields a higher than predicted stiffness, the latter based on solely the 21

plasticizer effects. The crystallinity was 33.5 % for the sample aged for the longest time at 125 °C (Fig. 12). It should be noted that the annealing effect becomes smaller when the ageing temperature approaches Tg due to a reduced molecular mobility. 4. Conclusions It was shown that the plasticizer (BBSA) loss-rate from the PA12 pipe was controlled by diffusion over the whole temperature interval investigated. This was also the reason that the evaporation-based model yielded the poorest prediction of plasticizer loss. The best prediction, although conservative, was also obtained with a proper diffusion model. The plasticizer diffusivity displayed a linear Arrhenius behaviour in the high temperature region but deviated from linearity at 80 ºC. The time-temperature shift factor of the mass loss master curve showed the same type of temperature dependence as the diffusivity. The non-linearity and over-estimation of the loss-rate was caused mainly by the fact that the lower temperatures investigated (60 and 80 °C) overlapped with the onset of the glass transition region. As expected, the plasticizer loss increased the Tg and flexural modulus/strength of the sample. The increase in stiffness and strength during the mass loss phase was due mainly to the loss of plasticizer, whereas at longer times the main effect was due to annealing-induced changes in crystallinity. It was also shown that it is possible to use the master-curve-shifting concept for mass loss data, similar to its frequent use for mechanical data.

Acknowledgments 22

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1. The kinetics and activation energy of plasticizer migration from a polyamide 12 pipe were determined. 2. The master-curve concept was introduced for plasticizer loss data and prediction purposes. 3. Three methods were employed to predict the plasticizer loss at a low temperature by using data obtained at higher temperatures. 4. The effects of plasticizer loss on the glass transition temperature and mechanical properties of the pipe were revealed.