Effects of combined use of additives

Oxidation induction time

The OIT of B3S and C2000 modified with both P-EPQ as well as Licowax C is compared with neat and thermally stabilized polymers in Figure 6-8. The use of

Neat P-EPQ P-EPQ + Licowax C

0 10 20 30 40 50 60

Oxidation induction time (OIT) [min]

OIT of C2000 at 340°C OIT of B3S at 320°C

Figure 6-8 Complex viscosity of neat and modified C2000 subjected to temperature profile P2 under oxidative and inert atmosphere.

both additives yields a further increase of the thermo-oxidative stability. The OIT

of multi-functionalized B3S is enhanced by 23 % to 57 min in comparison to B3S modified with P-EPQ only. For C2000, an additional increase of the OIT by 19 % is yielded when Licowax C is added to the single-modified C2000. The synergistic effect of P-EPQ and Licowax C is attributed to increased mobility of the molecular chains due to less friction induced by the lubricant. This facilitates diffusion of antioxidants as well as radicals and enables a more effective stabilization.

Mass loss

Table 6-4 summarizes the mass loss recorded for neat and multi-functionalized B3S and C2000 when exposed to temperature profile P1 in an oxidative surrounding.

Table 6-4 Mass loss of neat and multi-functionalized (MF) B3S and C2000 recorded during subjection to temperature profile P1 under an oxidative atmosphere.

Process Step B3S_P1 MF-B3S_P1 C2000_P1 MF-C2000_P1

Powder-coating 0.49 0.80 0.74 1.54

Tape consolidation 0.88 1.49 2.99 4.60

Laminate production 1.08 2.05 1.71 6.23

Thermoforming 1.79 2.70 1.35 7.90

The results from TGA reveal a different trend than the measured OIT. The mass loss detected for multi-functionalized polymers are ever larger than for non-modified B3S and C2000. Substantial mass losses are recorded for the multi-functionalized polymers during the laminate production and thermoforming. These mass losses are attributed to the added lubricant as less decomposition reactions were present in polymers modified with P-EPQ only. The melting range of Licowax C lies between 139 and 144 °C but the temperatures used for temperature profile P1 are much higher and can lead to early decomposition of Licowax C. Parts of the detected mass loss can be attributed to decomposed lubricant. Additionally, degradation of the lubricant may result in an enhanced amount of radicals. The reduced friction between molecular chains may promote the diffusibility of the formed radicals and accelerate degradation reactions. At the same time, the efficiency of antioxidants is improved due to increased diffusibility but decreases with the amount of newly formed radicals.

Complex viscosity

As presented for single-modified polymers, the complex viscosity of neat and multi-functionalized B3S and C2000 is plotted against the single process steps of temper-ature profile P2 under an oxidative and inert atmosphere in Figure 6-9. In contrast

0

Figure 6-9 Complex viscosity of neat and multi-functionalized a) B3S and b) C2000 subjected to temperature profile P2 under oxidative and inert atmosphere.

to the thermogravimetric results, a synergistic effect on melt viscosity by combin-ing antioxidant with lubricant is found for both B3S and C2000. Compared to neat polymers, the combination of P-EPQ and Licowax C yielded an overall decrease in complex viscosity of 13 % for B3S and 50 % for C2000 in air atmosphere. Under exclusion of oxygen, the viscosity was increased by 2 % for MF-B3S and reduced by 68 % for MF-C2000.

Four-point bend test

After the production of intermediates with neat and multi-functionalized polymers reinforced by CF-TP fibers, test panels were pressed with half and full dwell times.

This serves to identify potential effects on the viscosity and mechanical properties.

It is expected that adding P-EPQ and Licowax C positively influences the impreg-nation progress by reducing the viscosity. The results obtained by four-point bend testing longitudinal (0°) and transverse (90°) to the fiber direction are presented in Figure 6-10. The results for longitudinal flexural strength σ_f1 were normalized to the measured fiber volume content that is stated in the appendix in section A.2.

Considering the effect of the dwell time on σ_f1 overlapping error bars indicate no significant influence. For all dwell times, the multi-functionalized laminates yielded slightly lowerσ_f1. Laminates produced from non-modified and multi-functionalized C2000 show increased values for σ_f1. A better alignment of the fibers along 0°-direction may be the reason for increased σ_f1.

The effect of the dwell time on σ_f2 is more pronounced for B3S and reveals in-creased values for both non-modified and multi-functionalized test panels. The op-posite behavior is observed for test panels produced from non-modified and multi-functionalized C2000 showing decreasedσ_f2 with increased dwell time. The overall lowerσ_f2 for C2000-based test panels can be explained by the lower tensile strength

CF-TP/

Figure 6-10 Results from four-point bend testing a) in fiber direction and b) perpendicular to fiber direction of test panels produced from non-modified and multi-functionalized polymers at different dwell times in a press.

of 62 MPa compared to 90 MPa of B3S. In general, a lower σ_f2 is found for test panels made from multi-functionalized polymers. As presented, the suppression of thermo-oxidative degradation reactions with the help of P-EPQ can avoid the substantial increase in viscosity when the polymers are subjected to high tempera-tures for long dwell times. The increase in viscosity is explained by crosslinking and post-condensation reactions (see Chapter 5 for a more detailed discussion). If these reactions are suppressed by adding antioxidants a decrease in mechanical perfor-mance is expected. In addition, the modification with P-EPQ and Licowax C may alter the compatibility to the polyamide-based sizing of CF-TP fibers. The molec-ular composition of P-EPQ and Licowax C is different from the host matrices and from the polyamide sizing. The intercalation of the additives in the host matrices can act as disturbing factors in the interface region and reduce the establishment of covalent bonds.

Effect on molecular composition

The effect of adding P-EPQ and Licowax C to B3S and C2000 on the molecular composition was tested on extracted specimens from laminates that were tested in the four-point bend setup. For means of comparison, the MWD of as-received (non-processed), single-modified and multi-functionalized polymer samples is pre-sented in Figure 6-11.

Referring to Figure 6-11 a, the MWD of B3S modified with P-EPQ after subjection to temperature profile P2 in air results in a similar, slightly broadened MWD of non-processed B3S. However, M_w is reduced significantly and shifted to lower M_n when multi-functionalized B3S was investigated. The long subjection to high tem-peratures (P2) is expected to result in a shift to lower masses as observed for multi-functionalized polymers due to stabilization reactions. Potential post-condensation

1x10² 1x10³ 1x10⁴ 1x10⁵ 1x10⁶

Figure 6-11 MWD of as-received without processing, single-modified and multi-functionalized polymers after subjection to temperature profile P2 in air of a) B3S and b) C2000;

MWD of non-modified and multi-functionalized samples extracted from four-point bend test panels of c) B3S and d) C2000.

reactions promoted by P-EPQ but suppressed by Licowax C are assumed to cause a similar MWD as found for neat B3S. A different behavior is observed for samples based on C2000 (Figure 6-11 b): modification with P-EPQ only and with both ad-ditives yield an almost superposable MWD. In comparison to as-received C2000, the MWD reveals lower M_w and M_n. This confirms the assumptions of stabilizing reactions without crosslinking reactions.

Comparing the MWD of samples extracted from the composite samples (Figure 6-11 c, d), M_w is reduced and shifted to lower M_n when adding P-EPQ and Li-cowax C to both B3S and C2000 in comparison to non-modified polymers. M_w of non-modified samples is higher than for multi-functionalized B3S and C2000 whereas more pronounced for B3S. The largerM_w with higher M_nof non-modified samples is attributed to the stabilization reaction from P-EPQ. The antioxidant suppresses crosslinking and post-condensation reactions by reacting with emerging radicals. The given MWDs also support the outcome from four-point bend testing.

By suppressing crosslinking reactions, smallerM_n and lower M_w lead to a decrease in strength.