Processing window

Considering the laminate production step of temperature profile P2, the effect of the significant increase of the melt viscosity on the impregnation time is calculated by using the model developed in Chapter 4. The initial fiber bundle height was set to 500 μm to account for woven fabrics or NCF. The initial DOI was set to 0 and is representative of processing powder-coated tows in a static press for 60 min as presented in Chapter 2.

Figure 5-14 shows the development of the DOI of CF-TP/B3S over time for a con-stant viscosity of 185 Pa s, which is the minimum viscosity measured for B3S at the beginning of the laminate production step, compared to the viscosity increase as a function of time.

a) Constant η= 185 Pa s (B3S) b) Viscosity increase over time (B3S)

Figure 5-14 Development of the DOI of CF-TP/B3S as a function of a) constant viscosity and b) viscosity development as measured for temperature profile P2 during the laminate production step.

The increase in viscosity directly affects the impregnation time. For a constant viscosity, 100 % impregnation is yielded after 1526 s. The increase in viscosity as

measured for the laminate production step of temperature profile on neat B3S re-sults in an increase of the impregnation time by 13 %.

In Figure 5-15a, the DOI of CF-TP/B3S is presented as a function of time for a constant viscosity of 195 Pa s that was measured for C2000 at the beginning of the laminate production step compared to the viscosity increase as a function of time (Figure 5-15b).

a) Constant η= 195 Pa s (C2000) b) Viscosity increase over time (C2000)

Figure 5-15 Development of the DOI of CF-TP/C2000 as a function of a) constant viscosity and b) viscosity development as measured for temperature profile P2 during the laminate production step.

The effect of the viscosity increase on the impregnation time is more pronounced for C2000. Applying the constant viscosity of non-degraded C2000, complete im-pregnation is achieved after 1609 s. When the viscosity profile measured during the laminate production step is used, the DOI reaches 42 % after 1700 s.

To ensure processing at a constant viscosity and a time-efficient production of thermoplastics with B3S and C2000 it is recommended to restrict each process step to a dwell time of less than 5 min. This can be achieved by consolidation of powder-coated tows to tapes in a double-belt press prior to laminate produc-tion. The processing window applies in particular to woven fabrics or NCF with large flow paths. In addition, this processing window describes the time until the viscosity starts to increase and hence is a conservative value still allowing some impregnation progress.

5.4.6 Conclusion and implications

To enable gradual impregnation during the production of thermoplastic compos-ites, B3S and C2000 were investigated with regard to thermal cycling as present during processing. Two temperature profiles were derived from the process steps powder-coating, tape consolidation, laminate production and thermoforming re-quired for CFRTP production. The profiles revealed two different dwell times and were applied to both polymer types.

The methods DSC, TGA, and rheometry served to detect changes in thermal prop-erties and complex viscosity as a result from thermal cycling. The results obtained from all methods led to a more comprehensive understanding of degradation reac-tions that occur during thermal cycling. In the DSC with oxidative atmosphere, de-creasing melting temperatures T_m indicated thermo-oxidative degradation of both B3S and C2000. The results from TGA that show increasing mass loss of B3S and C2000 with extending dwell times in the presence of oxygen support the indica-tions from DSC. In rheological studies, significant increases in complex viscosity η^∗ for both B3S and C2000 were detected when subjected to long dwell times in presence of oxygen. The results obtained from all methods confirm the thermo-oxidative degradation of both polyamide types during thermal cycling throughout the production of CFRTP. No correlation was found between the development of the complex viscosity η^∗ and changes in number-average molecular weightM_n and weight-average molecular weight M_w as measured by GPC.

In inert atmosphere, the complex viscosityη^∗ of B3S remained unaffected, indicat-ing a low susceptibility to thermal degradation when additionally exposed to shear.

In contrast, C2000 showed considerably enhanced complex viscosity also under ex-clusion of oxygen.

Altogether, the results from this study set the basis for instructions during process-ing but also durprocess-ing recyclprocess-ing of both investigated polyamide types. The viscosity of both B3S and C2000 was increased by secondary reactions of thermo-oxidative degradation. Using the model developed in Chapter 4, the effect of increasing vis-cosity during processing was simulated for both B3S and C2000. The increase in viscosity is more moderate for B3S than for C2000 but results in a rise in im-pregnation time by 13 % compared to a viscosity that remains at a constant level.

The effect of increasing viscosity during processing was much more pronounced for C2000. While a constant viscosity would yield complete impregnation after 1700 s, increasing viscosity as measured on C2000 during the laminate production step hinders complete impregnation. The significant increase in viscosity led to insuffi-cient impregnation indicated by a DOI of 42 %.

The number of thermal cycles was found to have a negligible effect on thermo-oxidative degradation of B3S and C2000 when dwell times are limited to 5 min.

Considering the concept of gradual impregnation during the production of ther-moplastic composites, complete impregnation is hindered by additionally increased viscosities. Therefore, the impregnation process is recommended to be completed before thermo-oxidative degradation is initiated which leads to a processing win-dow of less than 5 min per process step for the investigated polyamide types. As this processing window describes the time until the viscosity starts to increase, it is a conservative value that still allows some impregnation progress.

promotion of polyamides

The effect of thermal cycling on neat polymers was studied in Chapter 5 and re-vealed significant increases in viscosity when B3S and C2000 were subjected to dwell times of more than 5 min in an oxidative atmosphere. Considering gradual impregnation of partially impregnated intermediates throughout CFRTP produc-tion, the substantial viscosity increase can slow down the impregnation progress and even hinder complete impregnation in case of C2000.

By adding various antioxidants to both B3S and C2000, the polymers are thermally stabilized to prevent drastic increases in viscosity. In addition, several lubricants are selected and compounded to both polymers to further decrease the viscosity facili-tating gradual impregnation. After investigations on single-modified polymers with only one additive, the most effective antioxidant and lubricant is further combined and potential interactions are studied on multi-functionalized B3S and C2000. By coating CF-TP fibers with the multi-functionalized powder, consisting of antiox-idant, lubricant and polymer, intermediates are produced. These are stacked and pressed to multi-ply test panels. Flexural properties are determined from these test panels to investigate the effect of these additives on the mechanical behavior on the composite level. Parts of the following section have previously been published in [144].

6.1 Thermal stabilization

The addition of stabilizers cannot prevent the initiation of thermo-oxidative degra-dation [104] but has proven to limit it during reprocessing or recycling of polyamides [165, 168]. Antioxidants are commonly classified into chain breaking (CB) and pre-ventive antioxidants due to their functional principle [175].