All-polypropylene Composites Research Articles

Thermosetting and thermoplastic impact attenuator under axial loading

High-performance composites are generally fabricated with continuous fibre and fabric reinforcements embedded in a thermosetting resin. Using thermoplastic matrices, there are substantial reductions in forming time and labour. More recently, the availability of all-polypropylene composites, achieved using the same thermoplastic polymer for both the fibre and the matrix phase, is also increasing because of their recyclability. In this perspective, the work aims to study the mechanical behaviour of a new fully thermoplastic composite, first showing the results of an experimental campaign for the mechanical characterization of the material properties, then examining the behaviour of structures made of such material under axial loading to evaluate their energy absorption capability. The second part of this work is divided into two steps. In the first step, crush tests on simple tubes were performed. In the second step, the behaviour of a specific impact attenuator for a Formula SAE racing car was analysed. Using the same geometry, different material solutions were tested. Beside traditional thermosetting composite structure, a new fully thermoplastic composite and a hybrid solution were used taking into account various feasibility problems in the manufacturing phases. Even if the thermoplastic attenuator does not exhibit the same absorption capability of the thermosetting solutions, an interesting crushing mechanism was noticed: no more brittle failure with formation of debris, but a ductile progression with a load distribution very close to an ideal absorber.

Non-destructive characterisation of all-polypropylene composites using small angle X-ray scattering and polarized Raman spectroscopy

Small angle X-ray scattering (SAXS) and polarized Raman spectroscopy were used to examine the structure of unidirectional all-polypropylene composites prepared at different consolidation temperatures. Analysis of the anisotropy of the X-ray scattering pattern provided a way to quantify the disorientation of the crystallites and a direct correlation has been found between a measure of overall orientation and the Young’s modulus of the composites. In the case of the Raman spectroscopic measurements, the molecular orientation state of the reinforcing PP fibres were evaluated by classical least squares (CLS) modelling with real reference spectra. Strong correlation was evinced between the estimated relative degree of orientation of the reinforcing fibres and the Young’s modulus of the multi-layered all-polypropylene composites. Based on these results, both SAXS and Raman spectroscopy are suitable methods to predict the mechanical performance of all-polymer composites, being especially sensitive to manufacturing and application conditions, in a non-destructive way.

Nonlinear creep response of oriented polypropylene tapes

The creep response of a highly oriented polypropylene tape used for the manufacture of self-reinforced polypropylene or all-polypropylene composites was studied over a range of stresses and temperatures. Similar to oriented polyethylene, the creep compliance is linear viscoelastic at short loading times, whereas pronounced stress dependence is observed at longer loading times. A mathematical model is proposed, where the total deformation of the fibre is regarded as being composed of a stress-linear delayed elastic component and a nonlinear plastic flow contribution. Model predictions are in good agreement with the experimental data.

Mechanical Properties of Woven Fabric Reinforced All-Polypropylene Composites

Random copolymer polypropylene (co-PP) and beta-nucleated PP (β-PP) produced with nucleant, which was not completely selective, were used as matrix materials. The effect of matrix type and consolidation temperature on the static and dynamic mechanical properties of woven tape-reinforced all-PP composites were investigated. The mechanical properties of all-PP composites strictly depended on the matrix type and the consolidation temperature. Co-PP-based composites had higher tensile strength and lower impact strength than β-PP-based composites. The maximum tensile strength and storage modulus was achieved at a consolidation temperature of 10°C above the melting point of both matrix materials.

The impact response of environmental-friendly sandwich structures

The low-velocity impact response of sandwich structures based on fully-recyclable skin and core materials has been investigated. Particular attention has been focused on structures based on self-reinforced polypropylene skins combined with a polypropylene honeycomb core. Two types of skin designs were considered, the first being based on a single ‘as-supplied’ monolithic self-reinforced polypropylene laminate and the second being manufactured from thin self-reinforced polypropylene sheets bonded together using a hot-melt polypropylene film. For comparative purposes, a limited number of tests have also been carried out on a more conventional GFRP/aluminium honeycomb sandwich structure. Drop-weight impact test have shown that all-polypropylene honeycomb sandwich structures absorb significant energy through plastic deformation in the composite skins as well as plastic buckling in the honeycomb core. It has also been shown that the design of the self-reinforced polypropylene skin has a significant effect on the energy-absorbing characteristics of the sandwich structure, with the performance of systems based on multiple layer skins greatly exceeding that observed following tests on a monolithic design. Tests on plain laminates also yielded similar conclusions, with multilayer systems offering much higher perforation resistances than their plain counterparts. Finally, it has been demonstrated that when the impact data are normalised by their respective areal densities, the all-polypropylene composites significantly out-perform GFRP/aluminium honeycomb sandwich structures.

Development and characterisation of injection moulded, all-polypropylene composites

In this work, all-polypropylene composites (all-PP composites) were manufactured by injection moulding. Prior to injection moulding, pre-impregnated pellets were prepared by a three-step process (filament winding, compression moulding and pelletizing). A highly oriented polypropylene multifilament was used as the reinforcement material, and a random polypropylene copolymer (with ethylene) was used as the matrix material. Plaque specimens were injection moulded from the pellets with either a film gate or a fan gate. The compression moulded sheets and injection moulding plaques were characterised by shrinkage tests, static tensile tests, dynamic mechanical analysis and falling weight impact tests; the fibre distribution and fibre/matrix adhesion were analysed with light microscopy and scanning electron microscopy. The results showed that with increasing fibre content, both the yield stress and the perforation energy signifi- cantly increased. Of the two types of gates used, the fan gate caused the mechanical properties of the plaque specimens to become more homogeneous (i.e., the differences in behaviour parallel and perpendicular to the flow direction became neg- ligible).

Injection moulded all-polypropylene composites composed of polypropylene fibre and polypropylene based thermoplastic elastomer

Abstract All-polypropylene composites comprising of polypropylene based thermoplastic elastomer (TPE) matrix and polypropylene homopolymer (hPP) fibre reinforcement were prepared and processed by injection moulding. For the injection moulding of plaque specimens pre-impregnated pellets, prepared by combined filament winding and film stacking were used. The processing–structure–property relationships of the all-PP composites were studied on specimens cut from the plaques produced at different melt temperatures (120, 140, 160 °C). The properties determined covered the moulding-induced shrinkage, flexural stiffness via dynamic mechanical analysis, static tensile and high-speed perforation impact behaviours. It was demonstrated that by using TPE as matrix the processing window of this novel all-PP composite could be significantly increased. The static tests demonstrated that the yield stress and tensile modulus of the all-PP composites were prominently increased. Optical micrographs taken from polished sections of the composites confirmed good interfacial adhesion between the matrix and the fibres. It was established that the shrinkage behaviour of the TPE based all-polypropylene composites is similar to that of conventional polypropylene.

Fracture Behavior of Recyclable All-Polypropylene Composites Composed of α- and β-Modifications

The fracture behavior of all-PP composites was studied under quasi-static loading conditions. Fracture toughness was evaluated by means of different fracture mechanics approaches depending on materials’ behavior. Composites consolidated at low temperature exhibited pop-in features and the failure occurs typically by delamination and tape stretching and fracture. With increasing consolidation quality – i.e., with increasing processing temperature – the delamination became less pronounced, and so the tape stretching occurred, before the specimens break. In composites consolidated at the highest temperature investigated (190°C), the laminate-like structure typical of self-reinforced composites produced according to film-stacking method was lost. Accordingly, composites behave as if they were only α-PP and β-PP matrices: α-rPP exhibited typical brittle fracture of α-PP, while β-rPP exhibited the stable behavior with fully yielded ligament before crack propagation commonly observed for β-PP. In general, stress–strain behavior changed from stable to unstable and fracture toughness strongly decreased as consolidation quality increased. Based on these results and previous findings, it can be concluded that the properties of self-reinforced PP composites can be tailored for a given application through the quality of consolidation.

The water absorption behaviour of all-polypropylene composites and its effect on mechanical properties

Woven fabric based self-reinforced polypropylene (SRPP) or all-polypropylene (all-PP) composite laminates based on co-extruded tapes were analysed for water uptake and the effect that this had on mechanical properties. Comparisons were made to a commercial glass mat reinforced PP (GMT) and natural fibre mat reinforced PP (NMT). Although the all-PP composites described in this paper can show significant water uptake through the presence of voids between tapes in the crimp region, the amount of which depends on the fabric architecture and level of consolidation, and their mechanical properties are seen to be largely unaffected by water absorption, even after freezing and thawing. Conversely, the NMT used in this investigation demonstrated significant mechanical degradation with increasing water uptake.

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.

Direct Forming of All-Polypropylene Composites Products from Fabrics made of Co-Extruded Tapes

Many technologies presented in literature for the forming of self-reinforced or all-polymer composites are based on manufacturing processes involving thermoforming of pre-consolidated sheets. This paper describes novel direct forming routes to manufacture simple geometries of self-reinforced, all-polypropylene (all-PP) composites, by moulding fabrics of woven co-extruded polypropylene tapes directly into composite products, without the need for pre-consolidated sheet. High strength co-extruded PP tapes have potential processing advantages over mono-extruded fibres or tapes as they allow for a larger temperature processing window for consolidation. This enlarged temperature processing window makes direct forming routes feasible, without the need for an intermediate pre-consolidated sheet product. Thermoforming studies show that direct forming is an interesting alternative to stamping of pre-consolidated sheets, as it eliminates an expensive belt-pressing step which is normally needed for the manufacturing of semi-finished sheets products. Moreover, results from forming studies shows that only half the energy was required to directly form a simple dome geometry from a stack of fabrics compared to stamping the same shape from a pre-consolidated sheet.

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.

Fatigue Properties of Highly Oriented Polypropylene Tapes and All-Polypropylene Composites

This paper describes the fatigue behaviour of newly developed all-polypropylene (all-PP) tapes and composites, with reference to the composite processing conditions, testing temperature and making a comparison with commercial alternatives. All-PP tapes are highly oriented and their failure behaviour follows that of other highly oriented polymers. All-PP woven composites fail ultimately due to PP tape failure. However, this failure mode is accompanied by delamination of fabrics in the woven structure. Consolidation pressure plays a decisive role in controlling the interlaminar properties and hence the delamination resistance and furthermore the fatigue limit of the composite. Comparison of all-PP woven composites with commercial alternatives based on glass and natural fibres reveals the excellent relative performance of all-PP composites under fatigue loads. Fatigue properties of all-PP composites are however sensitive to the testing temperature, and elevated temperatures can lead to a rapid reduction of the fatigue resistance of these all-polymer systems.

The effect of temperature and strain rate on the impact performance of recyclable all-polypropylene composites

Highly oriented polypropylene (PP) tapes, with high tensile strength and stiffness achieved by molecular orientation during solid state drawing, are consolidated to create fully recyclable, high performance “all-polypropylene” (all-PP) composites. These composites possess a large processing temperature window (>30 °C) and a high volume fraction of highly oriented PP reinforcement phase (>90%). This large processing window is achieved by using co-extruded, highly drawn PP tapes. This paper investigates the relationship between the impact resistance of all-PP composite laminates based on these highly oriented co-extruded PP tapes, and the temperature and velocity of impact. Unlike isotropic PP, the highly oriented nature of all-PP composites means that a significant influence of glass transition temperature is not observed and so all-PP composites retain high impact energy absorption even at low temperatures. Finally, the ballistic impact resistance of all-PP composites is investigated and compared with current commercial anti-ballistic materials.

The effect of temperature and strain rate on the mechanical properties of highly oriented polypropylene tapes and all-polypropylene composites

The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of recyclable “all-polypropylene” (all-PP) composites, with a large temperature processing window and a high volume fraction of highly oriented PP (>90%). The wholly thermoplastic nature of these ‘self-reinforced’ composites implies that the mechanical performance may vary with temperature. This paper describes the mechanical performance of all-PP composites by measuring the mechanical properties of highly oriented PP tapes and subsequent all-PP composites at a range of temperatures by static and dynamic testing methods. The time–temperature equivalence of all-PP composites is investigated by creating mastercurves of dynamic modulus and tensile strength. A comparison of the performance of these composites with commercial glass fibre reinforced polypropylene composites is included.

Mechanical properties of textile-inserted PP/PP knitted composites using injection–compression molding

Abstract This paper concentrates on the experimental investigation of the self-reinforced all-polypropylene composites. There exists an optimum processing condition to produce high quality specimens by injection–compression molding. Tensile and 3-point bending properties of the virgin PP materials were nearly unaffected by the introduction of reinforcing knit layer(s) due to very low fibre content of the knitted fabrics used. 3-point bending properties were also unaffected by the surface of indentation-flexure. The applied impact energy was maintained at 5 J for the homo-PP and 27 J for the block-PP materials, respectively, to cause penetration during drop-weight impact tests. It is interestingly noteworthy that the self-reinforced homo-PP composites exhibited superior energy absorption capability when compared with the virgin matrix materials. The corresponding plate bending performances of the self-reinforced homo-PP composites also revealed consistent improvement as compared to their virgin counterparts. On the other hand, although virgin block-PP material exhibited better impact performances than its composite reinforced by the homo-PP knitted fabric, a notably small increase in the reinforcement fibre content revealed considerable improvement in the impact properties comparable to those of the virgin block-PP matrix materials. These self-reinforced homo-PP/block-PP materials have clearly indicated that they have the potential to out-perform the block-PP materials via modification and/or manipulation of the reinforcement knit structural/geometrical parameters and the content of reinforcement fibres. Both static and dynamic impact properties are likely to be affected by the local area properties of the tested face under indentation, and thereby contributing to the improved performances of the composite specimens with the knit face under the impact.

The mechanical properties of woven tape all-polypropylene composites

The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of recyclable “all-polypropylene” (all-PP) composites, with a large temperature processing window (>30 °C) and a high volume fraction of highly oriented PP molecules (>90%). This paper describes all-PP composites made from woven tape fabrics and reports the tensile and compressive properties of these, with reference to composite processing conditions and compares these mechanical properties to those of commercial alternatives.

Low velocity impact performance of recyclable all-polypropylene composites

Highly oriented polypropylene (PP) tapes, with high tensile strength and stiffness achieved by molecular orientation during solid state drawing are consolidated to create high performance recyclable “all-polypropylene” (all-PP) composites. These composites possess a large temperature processing window (>30 °C) and a high volume fraction of highly oriented PP (>90%). This large processing window is achieved by using co-extruded, highly drawn PP tapes. This paper investigates the impact resistance of these all-PP composites, and the relationship between penetrative and non-penetrative impact behaviour, and composite consolidation conditions. The response of all-PP composites to falling weight impact is reported together with a comparison to conventional commercial glass reinforced polypropylene composites. A model for energy absorption is proposed by comparison with previous studies based on interfacial and tensile failure of tapes and composites.

The mechanical properties of unidirectional all-polypropylene composites

The creation of highly oriented, co-extruded polypropylene (PP) tapes allows the production of recyclable ‘all-polypropylene’ composites, with a large temperature processing window (>30 °C) and a high volume fraction of highly oriented PP (>90%). These composites show little deviation of mechanical properties with compaction temperature. This paper introduces all-polypropylene composites and reports the tensile and compressive properties of unidirectional composites. These composites show good retention of tape properties despite the relatively high temperatures used in composite manufacture.

Processing of all-polypropylene composites for ultimate recyclability

Processing of all-polypropylene composites for ultimate recyclability