Self-reinforced Polypropylene Composites Research Articles

Stretch Forming Simulation of Woven Composites Based on an Orthotropic Non-Linear Material Model

Characterisation experiments have been conducted on a woven self-reinforced polypropylene composite (SRPP) including uniaxial and bias extension tests. Outcomes of these experiments were employed to develop a non-linear orthotropic material model within an incremental deformation framework. The material model of the woven composite was implemented into a finite element simulation to predict stretch forming behaviour of SRPP specimens. The predicted strain paths at the pole of specimens were verified against experimental outcomes. It was shown that specimens possessing different aspect ratios deform under a wide range of deformation modes from uniaxial extension to biaxial stretch modes. Finally, the effect of different forming parameters on the strain path evolution of the woven composite was elucidated through numerical simulations. It was shown that the aspect ratio of the samples plays an important role in forming behaviour of woven composites. Development of a reliable and accurate numerical model for predicting forming behaviour of woven composites and understanding their main forming mechanisms promote and encourage the extensive application of these materials systems in a wide range of mass producing industries. Adopting woven composites in manufacturing industrial components facilitates addressing environmental concerns such as recyclability and sustainability issues.

The influence of process parameters on the properties of hot compacted self-reinforced polypropylene composites

Hot compacted self-reinforced polypropylene composites have good tensile properties and excellent impact resistance, but they have a limited processing window. Therefore, the influence of compaction temperature, dwell time and the application of interleaved films on the tensile and impact properties was assessed. Increased compaction temperature allows more molecular relaxation, thereby melting more matrix and creating a stronger interlayer bonding. This results in reduced 0° tensile properties and penetration impact resistance, while the 45° tensile properties and non-penetration impact resistance are maintained or improved. The dwell time only has minor influences on tensile and impact properties, while interleaved films have a similar influence as increased compaction temperature. These films increase the interlayer bonding, which increases the tensile strength and non-penetration impact resistance, but reduces penetration impact resistance. This paper demonstrates a wide property range depending on the processing parameters, helping in future tailoring of self-reinforced composites to specific applications.

Tensile behaviour of intralayer hybrid composites of carbon fibre and self-reinforced polypropylene

To improve the toughness of carbon fibre composites, unidirectional carbon fibre prepregs were hybridised with highly oriented polypropylene (PP) tapes. The latter tapes are used in self-reinforced PP composites (SRPPs) with high toughness, but relatively low stiffness. The tensile behaviour of intralayer hybrids of oriented PP tapes and CFPP prepreg tapes was investigated by changing the layup and weave pattern, and by adding interleaved PP films. While stiffness and strength of these hybrids was decreased compared to CFPP, their ductility significantly increased by adding oriented PP tapes. The parallel behaviour of the two constituent materials was caused by delaminations, which developed after the CFPP failure and spreads over the entire sample. This behaviour was modelled and proven to be independent of layup and weave pattern. Interleaved films, which are often necessary for thermoformability, limited the delamination development by increasing the interlayer bonding and yielded a lower ultimate failure strain.

Flexural and impact studies on stitched self-reinforced polypropylene composites

Self-reinforced composite (SRC) has gained significant interest due to its lightweight and good recyclability. In the present work, SRCs were prepared from woven polypropylene tapes. The woven tapes were further stitched by lock stitch and its effects on flexural and impact properties of the composites were studied. Flexural and impact results show that stitching parameters such as stitches per inch, sewing yarn count and the direction of stitching have a significant effect on the improvement of mechanical properties compared with the control sample (unstitched sample) without stitching. The morphological studies were carried out using optical microscopy and failure mechanism was analysed.

Self-extinguishing polypropylene with a mass fraction of 9% intumescent additive - A new physical way for enhancing the fire retardant efficiency

Multilayer self-reinforced polypropylene composites, fire retarded with APP based intumescent systems, were found to have surprisingly good fire retardancy. The combination of V-0 classification with excellent mechanical properties could be realized at significantly lower additive levels than expected. Modifying the structure of the composites allowed the mechanism behind the observed synergistic effect to be understood. The outstanding efficiency recorded can be explained by the beneficial physical cooperation between the expanding process caused by intumescent fire retardants and the shrinking of the reinforcing tapes. Polymer systems with these components, when exposed to heat, form a special, compact charred surface layer, which hinders the release of pyrolysis gases and where the flame retarding action of P- and N- containing compounds occurs at smaller volume, and thus at higher concentration, resulting in effective fire extinction. The observed novel synergism is promising to be efficiently utilized in the formation of cost-effective, fire retardant, self-reinforced composites.

Repeated low energy impact behaviour of self-reinforced polypropylene composites

The repeated impact behaviour of self-reinforced polypropylene composite has been characterised by tensile impact and instrumented falling weight tests. The nature of the tapes is highly anisotropic with strain hardening failure. Plastic deformation of the tape is the dominant mechanism, and the resulting penetration mode is a highly localised “star”-shaped hole. Damage and perforation thresholds are 5 J and 31.4 J respectively. Impact fatigue life exceeds 500 impact events up to 13 J, but drops sharply for 14 J. Strain-hardening is the origin of the trend of peak load increase and plastic deformation decrease with impact events. As a consequence, the amount of energy absorbed by each impact is reduced. However, when tape breaking takes place the absorbed energy increases up to perforation.

Investigation of the Weldability of the Self-Reinforced Polypropylene Composites

In the present work the weldability of self-reinforced composite was investigated. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials of two kinds of random polypropylene copolymer (with ethylene) were used. The composite sheets were produced by film-stacking method and compression molded with different thickness (1 mm, 2 mm) with different contents at different processing temperatures keeping the holding time and pressure constant. The SRPPC sheets were welded by ultrasonic welding machine with different welding parameters. The welds were qualified by mechanical and microscopic tests. The results showed that the thermoplastic reinforcement has not got melted; therefore the reinforcement was kept the strength-increasing effect.

Effect of the consolidation degree on the fracture and failure behavior of self‐reinforced polypropylene composites as assessed by acoustic emission

AbstractIn this work, the fracture and failure behavior of self‐reinforced polypropylene composites (SRPPC) was studied. As reinforcement woven fabric, whereas as matrix materials α and β crystal forms of isotactic polypropylene (PP) homopolymer and random PP copolymer (with ethylene) were used. Composite sheets were produced by a film‐stacking method and compression molded for constant holding time and at constant pressure but at different processing temperatures to obtain SRPPC sheets with different consolidation quality. The failure behavior of tensile specimens was assessed by the acoustic emission (AE) technique and the typical failure behavior was deduced for the differently consolidated composites. Both the number of AE events and the shape of the cumulative AE events versus deformation curve depend on the adhesion between phases. Correlations between the dominant failure mechanisms and AE events amplitude for model specimens were established which can be used to monitor the damage growth process in SRPPCs. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Effect of consolidation on the flexural creep behaviour of all-polypropylene composite

The long-term viscoelastic behaviour of self-reinforced polypropylene composites (SRPPC) was studied by short-term flexural creep tests at different temperatures. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethyl- ene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures (5, 20, 35°C above the melting temperatures of the matrices) keeping the holding time and pressure constant. The manufactured specimens were subjected to isothermal creep tests at different temperatures rang- ing from -20 to 80°C under an applied load. The time-temperature superposition principle was verified for the creep data. An Arrhenius type relationship described the shift data obtained from the creep tests. It was found, that with improving con- solidation (increasing processing temperature) the creep compliance decreased and good correlation was found between creep compliance and density/peel strength.

Development of woven fabric reinforced all-polypropylene composites with beta nucleated homo- and copolymer matrices

In the present work self-reinforced polypropylene composites (SRPPC) were developed and investigated. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethylene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures keeping the holding time and pressure constant. The quality of the composite sheets was assessed by optical microscopy, density and peel-strength measurements. The SRPPC specimens were subjected to static tensile and flexural, and dynamic falling weight impact tests and the related results were analyzed as a function of processing temperature and polymorphic composition. Based on the results the optimum processing temperature was determined and found by 20–25 °C above the related matrix melting temperature. It was established that the β-modified PP homopolymer based one-component SRPPCs possessed similar attractive mechanical properties as the intensively studied α-random PP copolymer based two-component ones.

Impact resistance of all-polypropylene composites composed of alpha and beta modifications

The impact behaviour of self-reinforced polypropylene (PP) composites was studied. α and β polymorphs of isotactic PP homopolymer and random copolymer (with ethylene) were used for matrix materials, whereas the reinforcement was a fabric woven from highly stretched split PP yarns. The composite sheets were produced by the film-stacking method and consolidated by hot pressing at 5 and 15 °C above the melting temperature ( T m) of the matrix-giving PP grade. The composite sheets were subjected to static tensile, dynamic falling weight impact and impact tensile tests at room temperature. Dynamic mechanical thermal analysis (DMTA) was also performed on the related composites and their constituents. The results indicated that the β-modification of the PP homopolymer is more straightforward than that of the PP copolymer. Stiffness and strength usually increased while the toughness (tensile impact strength, perforation impact energy) decreased with increasing temperature of consolidation. This was assigned to differences in the failure mode based on fractographic results.

Investigation of Strain-rate Effects in Self-reinforced Polypropylene Composites

The strain-rate sensitivity of a hot-compacted formed self-reinforced thermoplastic composite, based on polypropylene fibers in a polypropylene matrix, was investigated through a series of quasi-static and dynamic tensile tests. Characterization of the mechanical property dependence on strain-rate for a self-reinforced thermoplastic is an important issue when the material forms one part of a hybrid system, as in the case of a thermoplastic fiber—metal laminate that is prone to localized impact loading events. Strain-rates in the range from quasi-static (10—4 s—1) up to 10 s—1 were achieved in the gauge region of rectangular specimens loaded in a servo-hydraulic test machine. A measurable rate effect was observed in key mechanical properties of the self-reinforced composite, and constitutive equations were successfully applied to characterize the strain-rate behavior of the yield stress. Failure of the longitudinal ply fibers was the dominant failure mechanism, whilst the degree of inter-ply delamination varied over the dynamic loading range.

High Performance Self-Reinforced Polypropylene Composites

Highly oriented polypropylene fiber reinforced random PP copolymer composites were produced by film-stacking method. The reinforcing fibers were carded and needle punched and the film-stacked packages were compression molded at different processing temperatures (T=150…170°C) and holding times (t=90…600 s). For characterization of the consolidation of the composite sheets interlaminar strength was determined and further the polished sections were studied by light microscopy. Static tensile and dynamic impact (instrumented falling weight impact - IFWI) tests were performed on the specimens cut from the sheets. It was established that the best properties can be achieved when the processing conditions are 165°C and 90s. Increasing temperature and improving consolidation reduced perforation impact energy owning to better fiber/matrix adhesion and the smaller extent of delamination between the film-stacked layers.

Hot consolidated all-PP composites from textile fabrics composed of isotactic PP filaments with different degrees of orientation

Self-reinforced polypropylene composites (SRPPC) were hot pressed from textile layers (carded mat, knitted fabrics) at three different temperatures (160, 165 and 170°C) by setting a constant pressure (6 MPa) and constant holding time (2 min). Both textiles consist of two kinds of isotactic polypropylene (iPP) fibers differing in their orientation. In these compositions the highly oriented iPP works as reinforcement and the less oriented one fulfills the role of the matrix after hot consolidation. Two textile assemblies, viz. carded (also needle punched) mats and knitted fabrics were hot pressed; and the properties of the resultant SRPPC plates were investigated under tensile and falling weight impact conditions. The microstructure of the SRPPC was analyzed by light microscopy using polished sections and by SEM using cut surfaces.

On consolidation of self-reinforced polypropylene composites

Self-reinforced polypropylene composites (SRPPC) were manufactured with film stacking method at different processing temperatures (T=150–175°C) at a constant pressure of 5.5 MPa and holding time of 90 s with a nominal reinforcement content of 50 wt-%. For reinforcement carded PP, for matrix random PP copolymer films with two different melt flow rate (MFR) values were used. Static tensile and dynamic falling weight impact tests were performed on the manufactured SRPPC sheets. To characterise the consolidation light microscopic pictures were taken on the polished cross-section, and the density and the interlaminar strength were determined. Based on the results, the optimal processing temperature was deduced.

Transfer of load from matrix to fiber in self-reinforced polymer composites

The tensile properties of self-reinforced polypropylene composites, obtained by rapid extension of an isotactic polypropylene/atactic polypropylene melt, have been measured and correlated to morphological parameters derived from x-ray experiments. The longitudinal morphology of the core-fibrils is found to be independent of sample composition, while the lateral thickness of the fibers varies between 220 A for iPP and 110 A for the blend containing 50 wt.% aPP. Critical fiber lengths, as a function of sample composition and the elastic modulus and the yield stress of the fibers, could be determined. While the number of corefibrils increases with mass fraction of atactic polypropylene, the length of the lateral interface between fiber and matrix and the interface volume decreases with increasing aPP mass fraction. It is shown that this interface is responsible for the mechanical behavior of the composite by effecting the transfer of load from matrix to fiber.