Relation between mechanical properties and crystallinity ratio 53

To eliminate the plasticizing effect of moisture on crystallinity and more impor-tant on mechanical properties, all test panels were dried at 80 °C for more than 60 h under vacuum. This temperature lies above T_g of B3S and belowT_g of C2000.

The temperature for drying was chosen to ensure that moisture is removed most efficiently and the mechanical properties remain comparable to Chapter 2. The diffusion rate of water increases with increasing temperature due to increased mo-bility of molecular chains [103]. Considering the opposite of moisture removal, the water absorption rate increases from 1.05 x 10⁻¹²m²/s at 40 °C to 14.19 x 10⁻¹²m²/s at 80 °C [104] of reinforced PA66. PA66 and PA6 resemble each other in their water absorption due to the same ratio of CH₂/CONH groups. Drying at 40 °C, below T_g of B3S, would therefore require a long time. However, secondary crystallization cannot be excluded by drying at temperatures above T_g.

Pressure-induced changes in crystallinity during test panel manufacture can be excluded since significant crystal modifications are usually obtained at pressures above 500 MPa [105]. All test panels were manufactured with a pressure of 1 MPa.

Table 3-6 gives an overview of the produced test panels along with matrix mass fraction determined by acid digestion according to DIN EN 2564 method B [106]

at the central laboratory of SGL Carbon GmbH. In case of test panels, the

ther-Table 3-6 Overview of produced test panels along with matrix mass fraction determined by acid digestion.

Cooling rate

[ °C/min] Designation Manufacturing

process

Matrix mass fraction [wt%]

2 CF-TP/B3S_02 Static press 36.50±0.69

20 CF-TP/B3S_20 Static press 35.30±0.62

≈380* CF-TP/B3S_TF Thermoforming 37.20±0.20

2 CF-TP/C2000_02 Static press 27.20±2.16

20 CF-TP/C2000_20 Static press 31.43±1.10

≈320* CF-TP/C2000_TF Thermoforming 35.17±1.19

*measured by thermocouple

mal history and the process-induced crystallinity is to be investigated. Thus, ΔHf

from the first heating (including ΔHcc) is divided by the maximum possible ΔHc

as described by CR in Equation 3-6. Maximum ΔHc was yielded by cooling the samples with 5 °C/min for B3S and 20 °C/min for C2000. As investigated on neat polymers, ΔHc reached maximum values for these cooling rates (see Figure 3-10).

The relation of meanCRto the mean transverse flexural strengthσ_f2and the mean

transverse flexural modulus E_f2 is presented in Figure 3-12 for both CF-TP/B3S

In case of CF-TP/B3S, the CR exceeds 1 at a cooling rate of 2 °C/min. This is attributed to the fact that all samples were cooled at 5 °C/min. This cooling rate was selected to reach maximum crystallinity as the results from neat polymers sug-gested. The values for ΔHc at a cooling rate of 2 °C/min and 5 °C/min however are very close to each other. Thus, the maximum crystallinity may be achieved at 2 °C/min instead of 5 °C/min leading to an CR>1.

As expected from non-isothermal measurements on neat B3S, CR decreases with increasing cooling rates. In addition to DSC measurements on dried samples, the CR of undried specimen was determined to revise possible secondary crystalliza-tion induced by pre-drying. Undried samples show slightly lower CR than dried specimen. However, the overlapping error bars indicate the deviation is not sig-nificant. The CR of CF-TP/C2000 develops also accordingly to measurements on neat polymer. At 20 °C, CR reaches its maximum. In general, fast cooling rates that occur during thermoforming have a larger impact on the crystallinity of C2000

than on B3S as noticed for measurements on neat polymers.

Usually, increasing crystallinity leads to improved mechanical properties such as stiffness and strength. However, the opposite behavior is observed regarding strength σ_f2 for both polymers. Increases inCR involves decreased strength and vice versa.

On the other hand, stiffness E_f2 declines with decreasingCR as expected.

For CF-TP/B3S, drastic changes in mechanical properties effected by undercooling are not anticipated since CR remains nearly constant for a wide range of different cooling rates. C2000 reacts much more sensitive to different cooling rates. CR of CF-TP/C2000 reduces from 0.98 at 20 °C/min to 0.25 at 320 °C/min. Despite the major effect on CR, the mechanical properties do not change within the same mag-nitude. However, a significant increase in strength is observed for the lowest CR.

The gain in strength is believed to arise rather from the lowest measured fiber vol-ume content than from a change in crystallinity. The transverse strength typically increases with decreasing fiber volume content as less defects are present.

3.4 Conclusion and implications

The isothermal measurements on neat polymers by using DSC revealed hetero-geneous nucleation behavior for both B3S and C2000. For C2000, the nucleation is even independent of the temperature revealing that only foreign particles act as crystal nuclei. These foreign particles are assumed to be the monomers X that are added to PA10T monomers by the manufacturer during synthesis (PA10T/X).

The analysis of the isothermal crystallization kinetics according to Avrami yielded fibrils for B3S and lamellae for C2000 as crystal forms. B3S revealed a crystalliza-tion rate slower than C2000 by two magnitudes. If it is desired to reach maximum crystallinity after manufacturing, B3S is required to be cooled to 202 °C and held constant for approximately 10 min. C2000 may be cooled from melt to 228 °C and held at this temperature for approximately 5 min to achieve maximum crystallinity.

Although analysis of the non-isothermal crystallization kinetics on neat polymers failed by using the method according to Ozawa [81] and Liu et al. [100], important results for processing were obtained. B3S revealed the maximum crystallization at a cooling rate of 5 °C/min with decreasing crystallization ability for increas-ing coolincreas-ing rates from 2 to 50 °C/min. In contrast, C2000 showed the maximum crystallization at a cooling rate of 20 °C/min and a rapid decline of crystallinity towards larger cooling rates up to 50 °C/min.

The analysis of epoxy- and polyamide-sized carbon fibers on the crystallization of B3S and C2000 showed a strongly nucleating behavior of the epoxy-sized fibers on both polymers. There was no influence of the polyamide-sized carbon fibers on the crystallization of C2000. B3S showed a slightly enhanced enthalpy of crystallization

with increasing the cooling rate from 2 to 20 °C/min for B3S. Carbon fibers can act as nucleating agents. The low nucleating effect of the polyamide-sized fibers is attributed to the chemical similarity of the sizing to B3S and C2000.

Test panels produced from CF-TP/B3S in a static press cooled at 2 and 20 °C/min as well as in a thermoforming unit cooled with approx. 380 °C/min revealed only slight differences in the CR and hence negligible differences in transverse flexu-ral strength. The transverse flexuflexu-ral modulus slightly decreased with decreasing CR. CR remained constant for CF-TP/C2000 when pressed and cooled at 2 and 20 °C/min resulting in comparable strength but slightly decreased stiffness. A dras-tic drop inCRfor CF-TP/C2000 thermoformed and cooled at 320 °C/min revealed a strong sensitivity of C2000 to large cooling rates. However, the transverse mod-ulus was not affected in the same manner and was found to be slightly decreased.

The strength was even increased despite of the low CR and attributed to lower fiber volume content of the thermoformed test panel compared to the statically pressed panels.

The introduced CR proved to be a suitable method to describe and compare the amount of crystallinity in fiber reinforced B3S and C2000.

In this chapter, a model is developed to observe the impregnation progress during consolidation of powder-coated tows to tapes and laminates. The governing equa-tions for the transport phenomena that occur during transverse flow are presented.

Additionally, analytical models to represent the processing phenomena of the indi-vidual constituents, fiber and matrix, are summarized. Based on existing analytical approaches, a one-dimensional (1D) model is derived to compute the impregnation progress through the thickness during processing of powder-coated tows and con-solidated tapes.

The experimental work comprises the development of a method to determine the degree of impregnation (DOI). This technique is further used to assess the results of an impregnation study which is set up by using design of experiments (DOE). By varying the most important process parameters that govern impregnation - time, temperature, and pressure - the study serves to verify the derived model experimen-tally for the material combinations CF-TP/B3S and CF-TP/C2000. In addition, the interlaminar shear strength (ILSS) is determined for both intermediates and related to the DOI.

4.1 Transverse resin flow

Under application of pressure and at elevated temperatures, intermediates are consolidated that are initially pre-impregnated or pre-formed. Consolidation pro-cesses aim to complete impregnation, remove the entrapped air and surplus matrix.

Double-belt press forming or processing of film-stacked prepregs, pre-impregnated tows or powder-coated tows belong to the class of consolidation processes [107].

This process is commonly described by resin flow or resin impregnation.

Models to describe composites manufacturing processes generally combine funda-mental laws such as conservation of mass, energy and momentum with specific empirical models for e.g. viscosity and permeability [108]. The governing equations of mass conservation consider a representative volume element (ΔV) to average the material properties of all constituents forming a composite. Having two con-stituents in the composites, the equation for mass conservation is applied to both fibers and matrix [109, 110]:

∂V_f

∂t +∇(Vfu¯_f) = 0 (4-1)

57

∂(1−V_f)

∂t +∇((1−V_f)¯u_m) = 0 (4-2) V_f denotes the fiber volume fraction, ¯u_f describes the average velocity of the fiber bed whereas ¯u_m indicates the average velocity of the matrix.

The conservation of momentum is described by Darcy’s Law [111] assuming satu-rated laminar flow and neglecting gravity:

(1−V_f)(¯u_m−u¯_f) = −K¯¯

η∇P (4-3)

¯¯

K represents the permeability tensor of the fiber bed,η describes the dynamic ma-trix viscosity, andP the applied pressure. Darcy’s Law has been used by many re-searchers [25, 109, 110, 112–120] to describe thermoplastic matrix flow through fiber beds. Originally, Henry Darcy formed the specialized momentum balance equation for flow of water through a granular bed of sand in 1856. As a consequence, several assumptions have to be made and fulfilled for thermoplastic matrix flow through a unidirectional carbon fiber bed, the focus of the present work. The boundary conditions of Darcy’s Law and during processing of intermediates into parts made of thermoplastic composites are compared in Table 4-1.

Table 4-1 Comparison of boundary conditions as present in Darcy’s Law and thermoplastic matrix flow through carbon fiber bed [121].

Darcy’s Law CFRTP processing

Fluid Newtonian (water) non-Newtonian, visco-elastic Porous media

isotropic anisotropic homogeneous inhomogeneous Flow

characteristics 1D

1D (unidirectional)

quasi-1D, multidimensional (woven fabrics)

Darcy’s Law is based on ideally rigid, porous medium with spherical elements similar in size (sand). In case of spread unidirectional fiber tows, the distribution is assumed to be homogeneously as well in contrast to e.g. woven fabrics. The fibers are also more or less spherical elements with regard to their cross-sectional area but with extension in length. Thermoplastics are generally characterized by non-Newtonian behavior considering the dependance of the viscosity on the ap-plied shear rate. Under sufficiently low shear rates, thermoplastics behave quasi-Newtonian and fulfill therefore the requirements of Darcy’s law. In addition, the validity of Darcy’s Law is restricted to laminar flow with a Reynolds number < 1.

Generally, the high melt viscosity of most thermoplastics causes low Reynolds num-bers and laminar flow [122].

In principle, spontaneous impregnation can occur when matrix and carbon fiber sizing increase the capillary forces. For thermoplastics known for high melt viscosi-ties, spontaneous impregnation is less likely to be achieved in a reasonable time.

Hence, external pressure is generally applied [118].

The pressure is assumed to be carried by fibers and matrix during the impregnation and consolidation process. In comparison to the applied pressure with a range of 5 to 40 bar, the capillary pressure is low and hence disregarded [118]. Neglecting inertia-related forces, the stress equilibrium yields [109]

∇P +∇σ¯¯= 0, (4-4)

where ¯σ¯ denotes the average effective stress tensor that acts on the fiber bed.∇P describes the applied pressure [123].

Im Dokument Gradual impregnation during the production of thermoplastic composites  (Seite 85-91)