Novel high-performance all-aramid composites

9 March 2017
Jian Min Zhang, Beatriz Cortés-Ballesteros, and Ton Peijs
Self-reinforced polymer composites are prepared by applying an elaborately designed surface-dissolution method to fuse and consolidate poly(p-phenylene terephthalamide) fibers.

Advanced composite materials that are based on high-performance fibers (e.g., aramid or carbon fibers) exhibit excellent physical and mechanical properties. Such composites are therefore used in a wide range of applications, such as for transportation, sport, and shield materials. In most cases, composite materials are composed of three phases, i.e., the reinforcements, the matrix, and the interphase between the two. In recent years, however, a new class of ‘all-polymer’ or ‘self-reinforced polymer’ composites have emerged in which both the matrix and the reinforcements are made of similar or identical materials. These new composites have drawn much interest in both academia and industry because of their advantages—in terms of processing, interfacial properties, and recyclability—over traditional composites. Although many researchers are working on these all-polymer composites,1–13 they are mainly based on thermoplastic fibers that have moderate performance and temperature resistance, and their use in high-performance applications is therefore limited.

We have previously developed a new type of self-reinforced composite that is based on high-performance aramid fibers.14 To fabricate our ‘all-aramid’ composite, we applied a unique surface-dissolution method to fuse poly(p-phenyleneterephthalamide), PPTA, fibers together. Unfortunately, however, we have found that the dissolution behavior of the composite in highly concentrated sulfuric acid (H2SO4) was difficult to control (i.e., dissolution occurred in a matter of seconds). In addition, we observed undissolved PPTA fibers inside the composites, whereas the outer surface layer of the samples often exhibited heavily dissolved fibers. The resulting inhomogeneities in composite morphology thus gave rise to poor interface or matrix-dominated properties.

In this work,15 we have therefore proposed the use of milder acid solvents to achieve longer dissolution times and better process control. After we had conducted preliminary screening experiments, we selected a mixture of phosphoric acid (H3PO4) and H2SO4 (vp and vs, respectively) in 1:1 and 1:2 volume/volume (v/v) mixing ratios for further investigation. With these milder solvents, we require longer immersion times to partially dissolve the PPTA fiber surfaces, and we can realize a more controllable consolidation process to create the all-aramid composites (with more homogeneous microstructures and adhesion levels). Specifically, in our study, we immersed the PPTA fibers in the mixed solvents for periods of 5, 30, 60, and 90 minutes to dissolve the fiber surfaces. After subsequent acid extraction and drying (see Figure 1), we formed the consolidated all-aramid composite samples.


Schematic illustration of the aramid fiber surface-dissolution process for the preparation of the ‘all-aramid’ composites. The process involves (a) fiber alignment and constraint, (b) immersion in acid, (c) initial coagulation in water, (d) consolidation by pressure, (e) final coagulation, and (f) acid extraction.

In our study we also conducted tensile tests—in both the longitudinal and transverse directions—on our all-aramid composites. From these measurements we determined the optimum immersion time (i.e., interphase condition) as the point where we achieved a good balance between the longitudinal and transverse tensile strength (indicated by the gray bands in Figure 2). We also find (see Table 1) that the tensile properties of our all-aramid composites compare favorably with a conventional unidirectional aramid/epoxy composite.16 Furthermore, the results given in Table 1—for all-aramid composites prepared in 95wt% vs for 120 seconds14 and in a mixed solvent (1:1 vp/vs) for 60 minutes—show that the samples prepared in a more controlled manner (i.e., with a longer dissolution time) have significantly better transverse properties, albeit at the expense of the longitudinal properties.


Longitudinal and transverse tensile strength of the all-aramid composites, as a function of immersion time. Results are shown for the samples prepared with solvents of phosphoric acid (H3PO4) and sulfuric acid (H2SO4) in a (a) 1:1 volume/volume (v/v) and (b) 1:2v/v mixing ratio. The gray band in each diagram indicates the range of optimum immersion times, i.e., ten minutes either side of the immersion time at the crossing point of the two fitting curves.

Mechanical properties of the all-aramid composites prepared in 95wt% H2SO4for 120 seconds14 and in the 1:1v/v mixed H3PO4/H2SO4solvent for 60 minutes, and of a conventional unidirectional aramid/epoxy composite (Twaron1000).16

All-aramidAll-aramid Twaron1000/
95% H2SO4,H3PO4/H2SO4epoxy
120s1:1, 60min
Tensile
strength [GPa]1.401.100.94
Tensile
modulus [GPa]6548.315.6
Strain
at failure [%]4.042.15
Transverse
strength [MPa]2.66.0

In the next part of our work we investigated the failure modes and morphology of our all-aramid composites. Although Figure 2 shows that an immersion time of 30 minutes is close to the theoretical optimal dissolving time for the 1:1 vp/vs solvent, our morphological measurements (see Figures 3 and 4) indicate that an immersion time of 60 minutes resulted in a better interphase structure. Indeed, the immersion time of 60 minutes was sufficient to fuse the remaining fiber cores together. This provides a good level of adhesion between the fibers, but leaves a suitable fraction of the fiber core as the reinforcing phase.


Images of the all-aramid composites showing the failure mode of the samples prepared with the (a) 1:1v/v H3PO4/H2SO4(vp/vs) and (b) 1:2v/v (vp/vs) solvents for immersion times of 5, 30, 60, and 90 minutes.


Scanning electron microscope images of cross sections through the all-aramid composites that were prepared in the (a) 1:1v/v (vp/vs) and (b) 1:2v/v (vp/vs) solvents for immersion times of 5, 30, 60, and 90 minutes.

We also used a dynamical mechanical analysis to examine the thermomechanical properties of our all-aramid composites. Our results indicate, for instance, that the composites still retain a reasonably high modulus (35GPa) for the 1:1 vp/vs solvent, 60 minute immersion time sample. These high modulus characteristics at elevated temperatures arise because the fibers and matrix of the composites are made from the same high-temperature-resistant PPTA material, which makes them particularly suitable for high-temperature applications. In addition, x-ray diffraction profiles that we obtained show the crystal size and apparent crystallinity of our samples decrease with increasing solvent immersion time. This is a result of the increasing dissolution of the highly crystalline fiber skins and the formation of the non-crystalline, or partially crystalline, matrix phase. We note, however, that the decrease in crystal size is not as dramatic as in our previous work,14 which suggests there was an increase in the remaining crystal modification as a result of the weaker dissolution effect of the mixed solvent.

In summary, we have successfully prepared all-aramid composites by using a process of selective dissolution and fusion of aramid fiber surfaces in a mixed solvent of H3PO4 and H2SO4. Our resulting all-aramid composites have oriented aramid fiber cores that are fused together by an aramid interphase or matrix. Our tensile test results and morphological observations indicate that the composites prepared with a 1:1 vp/vs solvent and an immersion time of 60 minutes exhibited good overall mechanical properties. We also find that our all-aramid composites have much higher mechanical and thermomechanical properties than other all-polymer composites, which means they are good candidates for high-performance (especially high-temperature) applications. In our future work, we hope to ultimately optimize the vp/vs mixing ratio and to further refine the immersion time in our process. Meanwhile, we are continuing to look for new ways to prepare high-performance all-polymer composites.


Authors

Jian Min Zhang
Qingdao University

Jian Min Zhang is a lecturer in the School of Mechanical and Electrical Engineering. Her research includes the fields of all-polymer composites, nanocomposites, and the application of multiscale carbon materials.

Beatriz Cortés-Ballesteros
Queen Mary University of London

Beatriz Cortés-Ballesteros was a research assistant in the School of Engineering and Materials Science.

Ton Peijs
Queen Mary University of London

Ton Peijs is a professor in the School of Engineering and Materials Science. His research interests cover all aspects of processing–structure–property relationships of polymers and their composites. He has made notable contributions to several fields, including hybrid composites, interfaces, failure mechanisms, biobased materials, natural-fiber-reinforced plastics, polymer fibers, thermoplastic composites, smart materials, and nanocomposites.


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DOI:  10.2417/spepro.006900