Fabrication of Mg/CF/PEEK laminated composites

for the outflow of PEEK resin and carbon fibers.

A rough surface could result in better bonding between the Mg alloy and the PEEK resin.

Accordingly, the AZ31 alloy was roughened with a #100 SiC abrasive paper to make a rough bonding surface and to remove the oxides present at the Mg surface prior to laminating with APC-2 prepreg. Consequently, the resulting laminated composite revealed a slight improvement in the interface bonding, as compared with the laminated composite without any surface pretreatment, but the problem of delamination still occurred.

In order to overcome the delamination problem, a polyolefin-based adhesive was firstly tried. Three layers of polyolefin adhesive films with a layer thickness of ~0.1 mm were placed between the AZ31 sheet and APC-2 prepreg to act as a binder. The AZ31 sheets were also roughened with SiC abrasive paper prior to lamination. The forming temperatures were kept low at 200^oC in preventing from adhesive decomposition, with the same forming pressure of 1.4 MPa and time duration of 15 min. It was shown that the Mg/APC-2 laminate bonded with a 3-layer polyolefin adhesive at 200^oC showed no delamination at the interface, but the adhesive would be squeezed by the forming pressure and flow out of the sheets. The viscous flow of the adhesive could result in sliding of the AZ31 sheet and the APC-2 prepreg with each other under a pressure of 1.4 MPa at 200^oC. A lower holding temperature coupled with a lower forming pressure would reduce the sliding problem, but the bonding quality became degraded. The adhesive seemed to offer a bonding with minimum delamination;

however, the bonding strength was found to be low. It appears easy to tear off the lamina from the interface. Meanwhile, the service temperature of the bonded laminated composite with adhesive would be again lower than 100^oC.

In addition to the use of polyolefin-based adhesive, chemical etching on Mg alloy seems

to be important to produce a surface with affinity to PEEK resin. With this concern, the picric acid ((NO2)3C6H2OH) based etchant was applied to enhance the interface affinity. The fabrication of the laminated composite was, again, carried out without any application of polyolefin-based adhesive, except that the Mg sheets were etched by the picric acid. As a result, this etchant could yield better bonding, but partial delamination still occurred from the composite edge. Except for the use of picric acid and many acid chemicals tried in follow, it was later found that the CrO3/HNO3 based coupling agent could result in satisfactory bonding. The laminate pretreated by this coupling agent reveals no delamination throughout the processing and subsequent storage. As shown in Figs. 3.1 to 3.7, the poor bonding aspect was improved by such proper pretreatments, including surface roughening and chemical etching. But the main problem was still present, that is the outflow of PEEK resin and carbon fibers during lamination. In Figs. 3.1 to 3.7, the bonding temperature was kept at 400^oC, and the pressures were varied from 0.7 to 1.4 MPa. The other changes in the fabrication conditions were the number of plies in the APC-2 layer and the stacking sequence. It is shown that no matter how the changes in the conditions of fabrication, there is still a problem of outflow in PEEK resin carbon fibers.

It was later advised by Professor Kao [146] that the outflow of PEEK resin and carbon fibers could be solved completely by proper and uniform pressuring on the laminated composite during fabrication. Thus, the follow-up was conducted in Professor Jen’s lab, Department of Mechanical and Electro-Mechanical Engineering, NSYSU, using another kind of vacuum hot press suitable for laminating polymer based composites. And the etchant was changed to be the combination of CrO3 and Na2SO4. The stacking sequence was finalized to Mg/APC-2/Mg/APC-2/Mg. At first trial on this FRP-suitable vacuum hot press, the Mg/CF/PEEK laminated composite was successfully fabricated. It reveals no delamination and no resin or fiber outflow. The bonding characteristics between the Mg alloy and the

APC-2 appear satisfactory.

The laminate pretreated by this promising coupling agent, i.e. CrO3/Na2SO4, revealed no delamination throughout the processing, subsequent machining and tensile loading. The possible postulated reactions are below:

MgCrO₄ + C O 400^oC

CrO O Mg

O C

+ H₂O

PEEK Coordination

CrO₃ H₂O

H₂CrO₄

H₂CrO₄ + Mg²⁺ MgCrO₄ + 2 H⁺

(3.1)

(3.2)

(3.3)

The electron configuration of chromium might account for the unique characteristics of the CrO3 based coupling agent; the unoccupied d orbital in the electron configuration of the chromium atom could make its ion characteristically forming coordination compounds [153].

In this case, the oxygen atom in the backbone of PEEK has lone-paired electrons and, consequently, this oxygen atom may be able to form a ligand bonding to the chromium ion, resulting in coordinate covalent bond. Instead of the week van der Waals bonding formed between Mg and PEEK when using other surface echants, the laminate pretreated with chromium oxide base coupling agent could impart a characteristic coordinate covalent bond at the interface between the APC-2 prepreg and Mg phase. As a consequence, this laminate exhibited a good bonding and no delamination along the interface during slow or rapid cooling, as well as following aging and machining.

Figure 2.3 shows the enlarged schematic drawing of the resulting Mg based laminated composite, containing five layers. The thickness of the resulting laminated composites depends on the layer number. For example, a five layer composite containing three Mg foils (each ~0.55 mm thick) and two APC-2 plies (each ~0.55 mm thick with 4 APC-2 layers) will measure ~2.7-2.8 mm in thickness. In that case, the resulting Mg bases composite after hot pressing would contain 61 vol% AZ31 Mg and 39 vol% APC-2 prepreg (i.e., ~24 vol%

carbon fibers and ~15 vol% PEEK polymer).