Thermo-Stamping Process of Glass and Carbon-Fibre Reinforced Polymer Composites

In this work, manufacturing tools for thermoplastic (TP) composites have been developed. The chosen process involves the stacking alternately of oriented dry fabrics and TP films and does not use semi-products in order to reduce material costs. This study was specifically directed towards optimizing the impregnation of continuous glass and carbon fibres reinforcing two TP amorphous matrices, the polyphenylsulfone (PPSU) and polyetherimide (PEI), to obtain semi-finished products employed for aeronautical structures. The impregnation quality of inter and intra-yarns is analyzed and validated by optical and scanning micrographic observations conducted with an optical and a Scanning Electron Microscopies (SEM), respectively. The study showed that besides the process parameters and porosity distribution in the core of warp yarns, the impregnation quality depends on the surface properties of constituents. Desizing treatment has been carried out to improve the wettability of fibres by the TP matrices.


Introduction
Despite the widespread use of thermosetting resins (TS) in composites for aeronautic applications over the past four decades, their processing shows some drawbacks such as the low storage temperature, long curing cycles and irreversi-ble process [1]. Thermoplastic (TP) composites with high-performance matrices (Polyphenylene Sulfide (PPS), polyphenylsulfone (PPSU), polyetherimide (PEI), polyetheretherketone (PEEK), etc.) can be a promising alternative to these issues. The advantages of TP composites, especially carbon-fibre reinforced polymer (CFRP) and glass-fibre reinforced polymer (GFRP), include a healthier manufacturing environment, good impact properties, reduced production cycle time as well as their easier recyclability when compared to TS composites [2] [3] [4] [5]. Crystallization of TP polymers is an important factor due to its strong influence on the mechanical and chemical properties: the crystal phase increases the stiffness and tensile strength, while the amorphous phase absorbs the impact energy [1] [6] [7]. The crystallinity degree is determined by many factors, including the polymer type and the processing conditions. Investigations of a wide range of semi-crystalline polymers show that the crystalline morphology of the thermoplastic matrix in the composite depends on the molding temperature, the residence time during melting and the cooling rate. For this reason, the consolidation process of a continuous fibrous reinforcement by an amorphous TP polymer matrix is complicated [8]. However, TP composites become attractive in the aeronautical field due to some technical properties such as increased toughness and inherent flame retardancy, as well as reduced cost processing by employing manufacturing processes like thermoforming, stamping, welding and co-consolidation [6]. As the curing phase required for TS matrices is replaced by a controlled and fast cooling phase, large-scale productions are then possible even if the manufacturing temperatures are generally higher than those of TS [8]- [13]. Several aircraft parts can be manufactured from stamped TP composites reinforced with woven continuous-carbon fibres such as ribs and spars of engine nacelles [6]. From one manufacturer to another, the mechanical and physical characteristics may change in large proportions affected by the manufacturing method. Furthermore, the high viscosity of TP polymers makes the impregnation of reinforcement more difficult compared to TS matrices [8]- [13]. Consequently, some semi-products have emerged to overcome this drawback but with a restricted number of suppliers. The forming process of continuous fiber-reinforced thermoplastic is generally conducted at the melting temperature of the thermoplastic resin. In this case, the resin can be considered as liquid, and the processing parameters including the maximum applied pressure, compaction rate, and consolidation time must be evaluated [8]- [13]. Polymer melt viscosity is a crucial property used for plastics processing. Its dependency on strain rate is primary knowledge for successful adjustment of fabrication conditions. Besides temperature and pressure conditions are the other manufacturing variables affecting polymer viscosity. For pressure, it is complicated to evaluate its influence; for this, it is usually omitted. This can be accepted for "low-pressure" technologies, as for example, casting, but not for injection molding or extrusion, where pressures up to 100 MPa or 40 MPa, respectively [14].
Internal stresses generated during the cooling phase and residual porosities after molding are important issues for TP laminate composites. They are asso-ciated with the difference in thermal expansion of their components, the organization of those ingredients, and the processing history of the material. In a unidirectional (UD) composite, the thermal expansion of the fibres is nearly zero while that of the matrix phase is large. When the composite is cooled from its processing temperature, the matrix tries to shrink but is prevented by the stiff inextensible fibres, which leads the resin and the fibres under tensile and compressive stress, respectively [15] [16]. In addition to these phenomena, the lay-up sequence must be stacked symmetrically to the neutral plane so as to not introduce residual stresses in the composites. Although there are large volume changes between the melting point and the glass transition temperature (Tg), three-quarters of the total stress is generated between Tg and ambient temperature during the cooling phase [17]. Internal stresses contribute directly to the performance of the composite structure by defects introduction such as microcracking initiation and delamination under mechanical loading [18]- [23].
The development of autonomous means for the design and manufacture of TP composite structures would be a vital strategic asset, especially for the transport industries. For this, this study focuses on the implementation of composites with continuous woven fibres and TP matrices. It is mainly directed towards impregnation optimization of the reinforcements (glass and carbon fibres) by two high-performance amorphous TP matrices (PPSU and PEI), to obtain semi-finished products that can be used directly for the forming of aircraft parts. The choice of two amorphous matrices is related to the end application of the composite materials developed in this study that will be used for an aeronautical structure subjected to the impact and hydraulic crash loadings. GFRP and CFRP plates with an average thickness of 3 mm are implemented using a thermo-stamping process proposed under vacuum with an optimized porosity level.
The originality of this study lies in the proposal of a new innovative experimental protocol for the rapid manufacture of TP composite plates. The porosity rate of the developed materials will be evaluated by a simple determination of the material density, while the impregnation quality of the fibrous reinforcement by the two TP matrices (PPSU and PEI) will be checked by micrographic inspections carried out in the thickness of TP specimens.
In the next sections of this paper, the viscosity investigation of the two TP matrices will be briefly presented followed by a discussion of the proposed method that will be used to evaluate the porosity rate. Thermo-stamping parameters process will be experimentally investigated and the residual porosities will be evaluated. The study is completed with optical and scanning micrographics obtained from the through-thickness analysis of several specimens at different locations of the manufactured TP plates.

The Viscosity of TP Polymers
TP polymers exhibit great potential for recycling due to their reversible process.
However, their significant viscosity makes the impregnation of the dry rein-  [25].
Viscosity varies as a function of shear rate γ , commonly represented by the power-law proposed for the first time by Ostwald in 1925. An extension of the power-law is the flow relationship of Carreau which is written as a function of shear rate γ , temperature T and pressure p [24]. To evaluate the viscosity of TP at high temperatures by simultaneously varying shear rate γ , temperature T and pressure p, Sedlacek et al. have developed a rheometer with a backpressure device (a capillary rheometer Göttfert) to generate backpressure and thereby to raise the pressure in the polymer melting process. Temperature and pressure dependencies of the shear viscosity were recorded via measurements in temperature and pressure regions covering the regular processing conditions of PPSU.
The acquired flow curves were fitted according to the Carreau-Yasuda model, and therefore, temperature and pressure effects on viscosity behavior were evaluated [14]. Figure 1 illustrates the results obtained at different pressures and strain rates. As indicated at 345˚C and ambient pressure, the viscosity of PPSU is 1116 Pa.s. The same value is also found for the PEI at 370˚C [25]. It confirms the complexity of the fibrous reinforcement impregnation by these TP matrices during the consolidation process. Nevertheless, under a low strain rate, two parameters (T, p) have presented essential contributions for an optimum consolidation of TP plates.

Porosity Evaluation
Generally, many methods can be employed for porosity evaluation. However, only a few such as microtomography [ [36] can be used to quantify the voids or closed gaps which are inaccessible by external agents (for e.g., mercury, nitrogen, argon). The microtomography is a recent 3D nondestructive technique, providing a volume image from the distribution of the linear absorption coefficient (μ) of the X-ray. Since the materials do not have the same X-ray absorption coefficients, it is possible to obtain a 3D mapping of the TP fibrous composite [26]. It is then easy to calculate the volume fraction of these gaps inside the material, but also to analyze their morphology, distribution, etc. However, this technique is relatively expensive and applicable to a minimal volume, not necessarily representative of the part, particularly due to the very large amount of the generated data per sample. Chemical degradation (or burn-out test) involves heating the sample at high temperature to cause combustion of the polymer matrix. The major issue of this procedure lies mainly in the evaluation of the density of composite material by hydrostatic weighing (a Mohr weighing scale according to standard NF EN ISO 1183). It may alter the density measurements due to the nature of composite materials that generally present porosities and, therefore, allow the liquid diffusion into their voids.
For reasons as mentioned above, the present study proposes evaluating the porosity using the following method:  The total volume percentage (100%) is the sum of the volume fraction of the fibres V f and matrix V m while adding their volume porosity ratio V p (Equation (1)).
( ) Knowing  (1) as seen in Equation (2): The volume fraction of porosity V p can be determined by Equation (3) with the mass ratio of the fibers where ρ, ρ f , and ρ m are the densities of composite, fibres and matrix, respectively. Fibres volume fraction, V f , is given by Equation (4) where N is the number of plies, A s is the fabric area weight (g/m 2 ) and h is the sample thickness.
The CFRP and GFRP specimens are cut by water jet with their dimensions (length*width*thickness) as an average of three measurements carried out at different locations for each edge by a digital micrometer (accuracy ± 2 µm), and their weights are measured by an electronic balance at high resolution (10 −4 g).
The ratio of mass to volume results in the density ρ.

Experimental Study and Results
The materials employed for the manufacturing of the composite plates are presented in this section. They consist of two types of TP matrices, PPSU and PEI, and two 2/2 twill weave fabrics, made from glass and carbon fibres. Also, the experimental protocol to obtain the composite plates is described.

Amorphous TP Matrices
Ultem 1000-1000 natural PEI (amber) is a high-performance engineering thermoplastic polymer, displaying high strength and modulus combined with good processability. It is presented in film form with a thickness of 125 µm. Its characteristics are similar to those of PEEK. However, it has a lower cost. PEI has a tensile modulus E of 3.1 GPa and a density ρ PEI = 1.27 g/cm 3 . It is reported by the manufacturing company (Ajedium Films, Solvay) to have high heat resistance, being able to withstand long-term exposure at elevated temperatures with excellent thermal stability. Otherwise, as an amorphous polymer, it is usually processed at temperatures sufficiently higher than its glass transition temperature (215˚C). The reported processing temperatures of PEI are between 340˚C and 400˚C. Radel R5000 PPSU natural film (clear) is an amorphous, high-performance thermoplastic polymer offering high heat distortion temperature, outstanding mechanical properties over a wide temperature range (better impact resistance than polysulfone or PEI), also at very low temperatures, extraordinary hydrolysis, and chemical resistance when compared to other amorphous thermoplastics. It is supplied by "Ajedium films" society in the form of sheets having a thickness of 125 µm. PPSU has a modulus E = 2.35 GPa, a glass transition temperature Tg = 220˚C and a density ρ PPSU = 1.29 g/cm 3 . PPSU needs high manufacturing temperatures (stiff macromolecules of PPSU need to be machined above 330˚C), e.g., preferred melt temperature during injection moulding is usually in the range of 335˚C -400˚C. Thus, PPSU is often used for medical, automotive and aerospace applications.

Fabrics
In this study, the two types of 2/2 twill fabrics are supplied by "Porcher Composites-France", made with E-glass fibres and 3K carbon fibers, respectively. Table 1 summarizes their references and characteristics. The density of E-glass fibre is ρ E-Glass = 2.54 g/cm 3 with a diameter ϕ E = 9 µm. The total number of fila-  column 5 of Table 1, warp yarns are shown by a black color, whereas white color is attributed to weft yarns. During the manufacturing fibres, they are surface-treated with an aqueous solution. This treatment of the fibre surfaces is called sizing. Sizing influences the properties of the interface between fibres and their matrix, and subsequently affects the mechanical properties of composites. It is known that sizing is effective for TS matrices, but far from being mastered for TP matrices. Although several studies were conducted investigations on the evaluation of sizing chemical formulation applied to glass and carbon fibres, it remains highly industrial and confidential (noted "-" in Table 1). The yarns are equilibrated in warp and weft directions for the 3K carbon and E-glass 2/2 twill fabrics (Table 1).

Proposed Thermo-Stamping Process
The manufacturing processes of TP plates with continuous fibers are still homemade, and everyone has its expertise depending on the targeted applications.
The process used in this study was optimized to obtain flat plates with parallel faces, uniform thickness, and good impregnation quality. To meet these requirements, the chosen manufacturing method was the compression molding process by thermo-stamping. It is performed using a rapid stamping press of "COGIT Composites-France" society, which exerts a maximum closing pressure P max between two heating trays. The CFRP and GFRP plates should have a thickness of 3 mm and a fibre volume fraction of 40%. According to Equation

Compaction Cycle Parameters
Three different cycles were applied to the two TP polymers PPSU and PEI reinforced with glass and carbon fibres. In the first (Figure 3(a)), p is maintained at 17% of P max for 14 minutes and reaches the maximum pressure during the last minute of cycle 1. The processing temperature is T 0 = 355˚C. During the second and third cycles (Figure 3 (Figure 4(a)) shows horizontal black lines and some transparent areas between these lines, which are less apparent on plate 2 ( Figure   4(b)). Plate 3 presents a degradation of the TP resin (Figure 4(c)). The various areas on plate 1 reflect poor and heterogeneous impregnation of reinforcement by its TP resin. This defect is mainly due to the abrupt transition of pressure from 17% to 100% of P max . A stepwise increase in the pressure prevents this defect (Figure 4(b)). On the other side, a temperature increase of 10˚C has an adverse effect on plate 3. Thus the transformation temperature of 355˚C is considered the most suitable to obtain a melt PPSU and PEI.
Optical micrographs are conducted on the thickness of samples cut from the three plates. Figure 5 Figure 6 at the fiber scale (20 µm). In close confirmation with micrographic observations already discussed, plate 2 presents the lowest porosity fraction with a mean value V p = 3% ± 0.2% evaluated in Table 2    So, the real challenge in thermo-stamping is the optimization of the time-pressure window at the adequate consolidation temperature: at a low strain rate and constant transformation temperature, the key parameter of the thermo-stamping process is the compaction pressure. According to the three proposed cycles, it is essential to adjust the pressure by steps so as not to suddenly close the spaces between the fibers of the same strand and therefore prevent impregnation by the capillarity phenomenon. On the other hand, the temporal pressure adjustment must not be over a very long duration (the pressure should be maintained just during a few minutes) in order to promote a reasonable industrial production.
Industrially, only warp filaments undergo sizing and porosities located essentially in warp strands let us conclude that impregnation defects are due to sizing. For this reason, we decide to continue the thermo-stamping process of TP composites by applying the same experimental protocol (cycle 2, Figure 3(b)) but while removing the sizing (desizing) of the two 2/2 twill glass and carbon fibres.

Effects of Desizing Treatment a) Procedure
The heat treatment consists of putting the dry glass and carbon fabrics before consolidation in a calcination furnace at 400˚C for 4 hours. Unlike glass fibre, carbon can react with oxygen in the air to form CO or CO 2 and can volatilize. During the manufacturing process, the permeability of glass fabric is thus higher (more spaces between filaments), which allows easier yarn impregnation than with carbon fibres. Materials Sciences and Applications

Conclusion
In this study, a manufacturing process of TP (PPSU and PEI) composite plates reinforced with 2/2 twill glass and carbon fibres was proposed using a thermo-stamping molding approach. Three stamping cycles were tested, including a working temperature, a pressure range and holding times. It was shown that the transformation temperature of 355˚C is considered the most suitable to obtain a melt PPSU and PEI resins. Also, a stepwise increase of compacting pressure allows a better consolidation with a low porosity fraction. Similarly, an increase in the cycle duration promotes the capillarity of TP resins inside the strand. Inter/intra-ply impregnation quality was evaluated following optical and SEM micrographic observations conducted on the thickness of TP composite plates. Porosity evaluation was performed using a new simple approach based on the density evaluation of composite material. Porosity concentration was focused especially on warp yarns. For this reason, desizing of fabrics was established to decrease the surface tensions of filaments and facilitate the impregnation of glass and carbon fibres. As the structure of a fabric, yarn dimensions and shape, number of fibers per yarn and fibre diameters affect the rate at which matrix impregnation and fiber wetting occur and therefore influence the time to consolidate, the impregnation difficulty of carbon yarn is mainly due to the small diameter of filaments and their high number that participates in the constitution of the yarn. The viscous resin cannot coat 3000 filaments with a very small diameter (7 µm). As the current sizing solutions are very compatible with TS matrices, the definition of the appropriate new solutions with TP matrices constitutes a very promising research challenge.