R. K. Y. Li

Rubber toughening in polypropylene: A review

The recent advances in the studies of the toughening methods and theories of polypropylene (PP)–elastomer blends are reviewed in the present article. Inclusions are key to toughening PP; they can play the role of agent-induced crazing, cause shear yielding of the matrix around them, and end the propagation of cracks. The major theories interpreting the toughening mechanisms of the blends are: multiple crazing, damage competition theory, shear-yielding theory, microvoids, and cavitation theories. The factors affecting the toughening effect are relatively complicated. Therefore, these theories have been verified only in some cases when they have been applied in relevant conditions. To achieve the objective of better toughening, it is important to improve the uniform distribution of dispersed-phase particle size and suitable filler size, as well as improving the dispersion of the inclusions formed in the matrix; in addition the matrix materials or fillers must be functional with suitable modifier in order to enhance the interfacial adhesion or to improve the interfacial morphological structure between the filler and matrix. However, the exact toughening mechanisms for PP–rubber blends have to be studied further because of complications resulting from the crystallinity of the matrix.

Morphological behaviour and instrumented dart impact properties of β-crystalline-phase polypropylene

Abstract Specimens consisting of high-purity β-phase polypropylene were prepared by adding a bicomponent β-nucleator consisting of equal amounts of pimelic acid and calcium stearate. Scanning electron microscopy (SEM), differential scanning calorimetry (d.s.c.), dynamic mechanical analysis and instrumented drop weight impact tests were used to characterize the morphology, thermal behaviour and the impact properties of the β-phase polypropylene. SEM examinations show that the β-spherulites exhibit a sheaf-like structure with no clear boundaries between them. This distinct spherulitic morphology results in a substantial improvement in the falling weight impact resistance. Fractographic analysis reveals that microfibrils and voids were formed in the fracture induction area of the β-form specimen. The greater impact strength observed in the β-form material is due to the larger energy dissipation which is associated with the formation of microfibrils. Furthermore, d.s.c. analysis showed that there is no β → α phase conversion during the impact test.

Brittle–tough transition in PP/EPDM blends: effects of interparticle distance and tensile deformation speed

Abstract The toughness of polypropylene (PP)/ethylene–propylene–diene monomer rubber (EPDM) blends containing various EPDM contents as a function of the tensile speed was studied. The toughness of the blends was determined from the tensile fracture energy of the side-edge notched samples. A sharp brittle–tough transition was observed in the fracture energy versus interparticle distance (ID) curves when the crosshead speed

An investigation on the processing of sisal fibre reinforced polypropylene composites

In this paper, a pre-impregnation technique has been introduced for the injection moulding of sisal fibre reinforced polypropylene (PP/SF) composites. The major advantasge of the pre-impregnation technique is that the PP/SF composites can be injection moulded with relatively lower barrel temperature, and therefore significant thermal degradation of the sisal fibres could be avoided. The resulting injection mouldings possessed lighter colour and no odor.

Flexible Transparent PES/Silver Nanowires/PET Sandwich-Structured Film for High-Efficiency Electromagnetic Interference Shielding

We have developed a kind of high-yield synthesis strategy for silver nanowires by a two-step injection polyol method. Silver nanowires and polyethylene oxide (PEO) (M(w) = 900,000) were prepared in a homogeneous-coating ink. Wet composite films with different thicknesses were fabricated on a PET substrate by drawn-down rod-coating technology. Silver nanowires on PET substrates present a homogeneous distribution under the assistance of PEO. Then PEO was thermally removed in situ at a relatively low temperature attributed to its special thermal behavior under atmospheric conditions. As-prepared metallic nanowire films on PET substrates show excellent stability and a good combination of conductivity and light transmission. A layer of transparent poly(ethersulfones) (PESs) was further coated on silver nanowire networks by the same coating method to prevent the shedding and corrosion of silver nanowires. Sandwich-structured flexible transparent films were obtained and displayed excellent electromagnetic interference (EMI) shielding effectiveness.

Mechanical behavior and fracture toughness evaluation of maleic anhydride compatibilized short glass fiber/SEBS/polypropylene hybrid composites

Short glass fiber (SGF) reinforced polypropylene composites toughened with styrene-ethylene-butylene-styrene (SEBS) triblock copolymer were injection molded. The effect of maleic anhydride (MA) functional group on the mechanical performance of the hybrid composites was investigated. In this study, MA was either grafted to PP (PP-g-MA) or SEBS copolymer (SEBS-g-MA). The mPP blend was prepared by compounding 95% PP with 5% PP-g-MA. The matrix of hybrid composites consists of either SEBS/mPP or SEBS-g-MA/mPP. Tensile tests showed that SGF exhibits a beneficial effect for restoring the stiffness of the SEBS/mPP blend. Impact measurements on SEBS/mPP exhibited superior impact strength. However, the incorporation of SGF into SEBS/mPP and SEBS-g-MA/mPP blends reduced the impact strength considerably. SEM observations revealed that the SGF surfaces of both SGF/SEBS/mPP and SGF/SEBS-g-MA/mPP hybrids are coated with a thin layer of matrix material. This implied that the MA functional group of mPP improves the adhesion between SGF and PP, and between SGF and SEBS. The essential work of fracture (EWF) method revealed that a strong interfacial bonding between SGF and PP is detrimental to the fracture toughness of ternary SGF/SEBS/mPP and SGF/SEBS-g-MA/mPP hybrid composites. Finally, dynamic mechanical analysis showed that SGF increases the storage modulus but decreases the intensity of damping factor for the hybrids.

An investigation on the impact fracture characteristics of EPR toughened polypropylene

Abstract The effect of adding ethylene-propylene copolymer (EPR) into polypropylene (PP), with particular emphasis on the impact fracture characteristics, will be presented in this work. The impact fracture characterization techniques used include the notched Izod impact strength, drop-weight dart impact, and the impact fracture toughness G c . It was observed that different impact velocity needs to be selected for the different techniques in order to ensure a well defined failure situation and to obtain a well defined impact fracture history based on the force-time curves measured during impact. Whenever there was a blunt or sharp notch present, EPR particles dispersed inside the PP matrix can play an effective role as a toughening additive. Linear elastic fracture mechanics (LEFM) approach for the measurement of the impact fracture toughness G c can be applied not only to the PP homopolymer, which failed in a brittle manner under a high rate of loading, but was also found to be applicable to EPR modified PP, except that plastic zone correction was required.

Mechanical properties of injection moulded blends of polypropylene with thermotropic liquid crystalline polymer

Blends of a thermotropic liquid crystalline polymer (LCP) with polypropylene (PP) were injection moulded. The LCP exhibited a higher viscosity than that of PP. Static and dynamic mechanical measurements, Izod impact tests, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA) were performed on these blends. The static tensile tests show that the tensile modulus and strength of PP are improved with the addition of LCP. The improvement in mechanical properties is associated with the formation of LCP fibrils as evidenced by SEM observations. Dynamic studies on these blends show an increase in the storage modulus but a decrease in loss factor with the addition of LCP. Furthermore, TGA measurements show that the thermal stability of PP is improved substantially with the addition of LCP.

Impact behaviour of polypropylene/polyethylene blends

Abstract Due to the large volume consumption of polyolefins, the treatment of the resulting solid waste is becoming a major concern. One possible utilization of the polyolefin wastes is to form blends from recycled polypropylene (PP) and polyethylene (PE). This study is a preliminary investigation on the impact fracture behaviour for the PP/PE blends. The impact testing method employed in this study includes the conventional and instrumented Izod impact tests, the instrumented Charpy impact test, and the instrumented drop weight plate impact test (IDWPIT). In both conventional and instrumented Izod impact testing, PP homopolymer and PP/LDPE blend have similar impact strengths, while PP/HDPE blend exhibits slightly lower impact strength. However, the instrumented Charpy impact test indicates that both PP/LDPE and PP/HDPE have similar impact strengths, and both are slightly lower than PP homopolymer. This suggests that the type of impact test employed is playing an important role in the impact fracture behaviour of the PP/PE blends. The effect of temperature on the impact fracture behaviour for the PP/PE blends was evaluated by means of the Charpy and drop weight plate impact tests. During the tests, impact strengths reduced with decreasing testing temperature. In the Charpy impact test, the impact strength for PP homopolymer was higher than those of the two PP/PE blends at both 20 and 0°C. The E total measured from the IDWPIT indicated PP/LDPE has the highest impact strength at both 20 and 0°C.

Short glass fiber-reinforced polyamide 6,6 composites toughened with maleated SEBS

Abstract Polyamide 6,6 hybrid composites toughened with maleated styrene–ethylene butylene–styrene (SEBS-g-MA) and reinforced with 5, 10, 15, 20 and 30% short glass fiber (SGF) were prepared by melt blending and subsequent injection molding. The matrix blend of composites consisting of 80 wt.% PA6,6 and 20 wt.% SEBS-g-MA. The mechanical behavior and fracture resistance of the SGF/SEBS-g-MA/PA6,6 hybrid composites were investigated. Tensile test showed that the incorporation of SGF to the PA6,6/SEBS-g-MA 80/20 blend improves both the tensile strength and stiffness substantially at the expense of its tensile ductility. The essential work of fracture (EWF) methodology was used to evaluate the fracture toughness of the hybrids. EWF measurements indicated that the SGF additions are beneficial in enhancing the specific essential work of fracture (we) of the hybrids. The specific essential work of fracture appeared to increase with increasing fiber content up to 15%. It then decreased for the hybrids containing SGF content ⩾20%. Overall, the we value of hybrids was higher than that of the unreinforced PA6,6/SEBS-g-MA 80/20 blend. The correlation between the microstructure and the fracture resistance of the hybrids is discussed.