Johanne Denault

Evaluation of bias-extension and picture-frame test methods for the measurement of intraply shear properties of PP/glass commingled fabrics

Abstract Efforts are currently devoted to the development of numerical tools for predicting the forming process of continuous fiber-reinforced thermoplastic composites. To ensure good predictions, reliable experimental measurements of the laminate properties must be performed at the processing temperature. This paper presents experimental results of the intraply shear properties measured on a polypropylene/glass fabric using two different methods known as the bias-extension and the picture-frame testing methods. Based on kinematic equations, both methods of measurement are discussed in regards to their respective weaknesses, mainly related to the difficulty to measure precise shear angles in the bias-extension method and the undesired fiber tension invalidating the load–displacement curve in the picture-frame test. To remove these difficulties, a modified version of the picture-frame test is proposed. This new method avoids fiber tensioning during testing and is considered more appropriate to capture the influence of the fabric architecture on the measured properties. Good correlations between the load–displacement curves and viscosities measured with the modified picture-frame and bias-extension methods were obtained compared to the standard picture-frame method.

The effects of injection molding on the mechanical behavior of long-fiber reinforced PBT/PET blends

Abstract Long-fiber reinforced thermoplastics have received much attention because of their processability by conventional technologies for thermoplastics. However, fiber degradation represents a major problem. This work has been undertaken with the objective of investigating the effects of injection molding parameters on fiber degradation and fracture performance in PBT-PET blend/glass fiber composites. The influences of six parameters and their interactions are analyzed, i.e. peak cavity pressure, holding pressure, back pressure, screw speed, melt temperature and barrel profile. The results show that most of the molding variables, as well as their interactions, affect the properties of the composite. Fiber-length retention is not the sole parameter to be optimized; the matrix system is also affected by molding conditions and has significant effects on composite properties. The influence of the composite microstructure, which is controlled by the molding process, on the rigidity, strength and toughness of the composite is also discussed.

Parameters regulating interfacial and mechanical properties of short glass fiber reinforced polypropylene

The performance of thermoplastic composites is known to depend on the intrinsic properties of the two composite components, the quality of the fiber-matrix interface, and the crystalline properties of their matrix. The objective of this work is to characterize the effect of the addition of modified polypropylene (PP) and silane coupling agent on the mechanical and interfacial properties of short fiber reinforced PP composites. Differential scanning calorimetry (DSC), single fiber composite fragmentation tests (SFC), and mechanical testing are used to understand the different parameters regulating the interfacial properties of composites. No influence of the modified PP on the level of crystallinity is observed. Some differences in the size of the spherulites are observed for acrylic acid grafted PP (PP-g-AA). Those samples also show lower mechanical properties in spite of good interfacial interactions. Maleic anhydride grafted PP (PP-g-MAh) leads to better mechanical performances than PP-g-AA. A high MAh content PP-g-MAh grade with low viscosity is the best polymeric additive used in the present work.

Mode I interlaminar crack propagation in continuous glass fiber/polypropylene composites: temperature and molding condition dependence

Abstract The mode I interlaminar fracture toughness of unidirectional continuous glass fiber/polypropylene composites above (23°C) and below (−40°C) the glass transition temperature of the PP matrix was investigated. Three molding conditions, leading to different levels of fiber dispersion and matrix microstructure, were studied. Fracture toughness testing performed employing double-cantilever beam (DCB) specimens showed that the molding conditions strongly influenced the fracture toughness of the composites studied. Similar values of fracture toughness were obtained at the two test temperatures investigated (23 and −40°C). Fractographic observations revealed that crack propagation differed significantly for each of the molding conditions and test temperatures studied. Depending on the fiber dispersion and matrix microstructure resulting from the different molding conditions, crack propagation occurred either at the fiber-matrix interface or in the matrix interspherulitic regions. A transition from stable crack propagation behavior at 23°C to unstable crack propagation behavior at −40°C was noted only when the crack propagated at the fiber–matrix interface. This effect was attributed to the ductile–brittle transition of the amorphous PP phase, more concentrated at the fiber-matrix interface, at the glass transition temperature near 0°C.

Thermoforming-Stamping of Continuous Glass Fiber/Polypropylene Composites: Interlaminar and Tool–Laminate Shear Properties:

The results of interlaminar and tool–laminate shear tests performed on a twill 2 2 PP/glass fabric are described in this paper. The influence of the laminate temperature, pullout velocity and normal pressure on the interlaminar shear stress and friction coefficient are evaluated, as well as the effect of cooling the specimen from the melt to simulate real forming conditions. Opposite trends were observed for the variation of the shear stress and friction coefficient whether the tests were performed above the melt temperature of the matrix or above the crystallization temperature (135, 140, and 155 after cooling from the melt temperature. For the interlaminar shear tests, this was caused by the shift from an interlaminar to an intralaminar shear deformation mode occurring. For the tool–laminate shear tests, this was caused by the shift from matrix shear at the interface tool–laminate to direct Coulomb friction of the fibers with the tool with an increase of the normal pressure and/or an increase of the matrix viscosity with decreasing temperatures. Above the melt temperature of the matrix, the friction coefficient and shear stress were higher at the tool–laminate interface than in the interlaminar region while at temperatures close to the crystallization temperature they became lower at the tool–laminate interface. A summary of these observations is made and a discussion of their possible impact on the forming of parts is enlightened.

Interlaminar fatigue crack propagation in continuous glass fiber/polypropylene composites

Abstract The mode II interlaminar fatigue crack propagation behavior of unidirectional continuous glass fiber (GF) composites with a polypropylene (PP) matrix obtained under three different molding conditions has been studied with the use of the end-notch flexure (ENF) geometry. The microstructure and mechanical performance, especially the interlaminar fatigue crack propagation, are strongly affected by the molding conditions. Comparative results reveal a major influence of the fiber–matrix interface and the matrix morphology on the crack propagation resistance. The distribution of the ductile amorphous PP phase in the semi-crystalline PP matrix appears to be the controlling parameter determining the fatigue crack propagation resistance of the PP/GF composite. Fractographic observations clearly showed the role of this phase.

Processing-structure-property relations in PEEK/carbon composites made from comingled fabric and prepreg

Although the processability of PEEK/carbon prepreg has received much attention, the effects of processing on the microstructure and performance of the co mingled fabric system is not well known. In this paper, the effect of molding temperature, molding time and cooling rate on the mechanical performance of PEEK/carbon compos ites made from both prepreg and comingled forms are discussed. To obtain good mechanical properties of PEEK/carbon fiber composites, processing must be carried out at a temperature sufficiently high to destroy the previous thermal history of the PEEK matrix. While the fiber/matrix interaction in the prepreg system does not seem to be af fected by the thermal processing conditions, this interaction in the comingled system must be created during the molding process. Fiber/matrix adhesion in the comingled system depends on the molding temperature, residence time at melt temperature as well as cooling rate. This is probably a result of several complex mechanisms such as matrix adsorption on the fiber surface, matrix degradation leading to chemical bonding and interfacial crystallization.

Non-isothermal crystallization behavior of clay-reinforced polypropylene nanocomposites

Abstract In this study, the non-isothermal crystallization kinetics of polypropylene (PP) in the presence of nanoclay particles were investigated using differential scanning calorimetry (DSC) with various cooling rates varying from 0.5°C/min to 80°C/min. Such kinetics were compared with those obtained for the pure PP matrix. The modified Avrami analysis was used to describe the non-isothermal crystallization process. The results obtained indicate that the presence of nanoclay significantly affected the crystallization rate of the PP resin, since an increase of the crystallization temperature as the nanoclay content increased was observed. This was attributed to the nucleating ability of these particles. Moreover, it was seen that the increase of both the cooling rate and the nanoclay content decreased the Avrami exponent n, suggesting a change in the obtained crystalline shape. For the nanocomposite materials, as well as for neat PP, the mechanism of crystallization was found to undergo two transitions, at about 5°C/min and 40°C/min. This suggests that the surface-induced nucleation at the clay particles follows the same mechanisms as those of the complete spherulitic structure. However, a lower value of activation energy for crystallization was obtained as the clay content increased, confirming the nucleating effect of clay particles.

Effect of the Molding Conditions on Mode II Interlaminar Crack Propagation in Continuous Glass Fiber/Polypropylene Composites

The effect of molding conditions on the resistance to Mode II interlaminar crack propagation under monotonic and cyclic loading of unidirectional continuous glass-fiber composites with a polypropylene (PP) matrix was studied. The distribution of the fibers and of the crystalline and amorphous components of the matrix phase, the melting temperature and the amount of crystallinity are related to the molding conditions employed and to the resulting flexural strength and modulus, apparent interlaminar shear strength and Young’s modulus. Mode II crack propagation, either cyclic or monotonic, is strongly affected by the fiber-matrix interface and matrix morphology. The distribution of the soft amorphous PP phase in the semi-crystalline PP matrix appears to be the controlling parameter determining the fracture and fatigue resistance of the composite. The fractographic features clearly show the role that this phase plays during crack propagation. A relationship between the shear cusp size measured on the fatigue surfaces and the cyclic strain energy release rate is proposed.

Chemicals from agricultural biomass: chemoenzymatic approach for production of vinylphenols and polyvinylphenols from phenolic acids

Abstract A two-step chemoenzymatic process for the preparation of polyvinylphenols from phenolic acids (PAs), being abundant aromatic constituents found in agricultural biomass, was developed. In the first step, conversion of 4-hydroxycinnamic acid derivatives to the corresponding vinylphenols, mediated by a recombinant phenolic acid decarboxylase, was evaluated using a variety of bioprocessing technologies that include biphasic whole cell and cell free extract biotransformations, a combination of biocatalyst with adsorbent resins for in situ product recovery, and fixed bed reactors using immobilized whole cells. The best yield (90%) with a high space time yield of 4.83 g/l/h was the result of a combination of crude enzyme extracts of the recombinant Escherichia coli (E. coli) with water immiscible organic solvents such as toluene. In the second step, cationic and radical polymerizations were tested to produce polyvinylguaiacol (PVG) from vinyl phenols. Characterization of PVG by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and nanoindentation test are reported here for the first time. The feasibility of the chemoenzymatic process for the production of aromatic polymers from biomass was demonstrated by the production of polymers from a mixture of ferulic acid (FA) and p-coumaric acid (pCA), obtained from alkaline hydrolysis of corn bran. Interestingly, nanoindentation tests showed that both PVG and “mixed” PVG polymers showed significantly higher performances than a commercial polystyrene polymer.