Polyamide nanocomposites with improved toughness

Polymer nanocomposites containing several percent of exfoliated layered silicates are materials with a unique weight/performance ratio. The only parameter that is not enhanced, but even decreased, is toughness. This work focused on the toughness enhancement of these advanced systems with polyamide matrix prepared via melt‐mixing (i.e., by a conventional method of polymer processing having an advantage of easy simultaneous addition of other components). Analogously to ternary polyamide blends with improved mechanical behavior, containing finely and separately dispersed elastomer and rigid polymer, elastomer particles with an average size of 60 nm were incorporated in the nanocomposite. The very low particle size was achieved by in situ reactive compatibilization by using suitably functionalized elastomers. The simultaneously increasing viscosity of the system enhanced exfoliation of the silicate. Melt exfoliated nanocomposites containing 3 wt % of clay and 5 wt % of elastomer particles exhibit increased toughness without significant loss of other properties. Elastomer particles increase toughness by both acting as stress concentrators (by initiating energy absorbing microdeformations) and influencing the clay‐induced matrix crystalline structure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 288–293, 2005     

Properties of bulk-polymerized thermoplastic polyurethane nanocomposites

The thermal, rheological, and mechanical properties of bulk-polymerized thermoplastic polyurethane nanocomposites of reactive and non-reactive layered silicate clay were characterized as a function of the state of dispersion of particles. True exfoliated nanocomposites were produced by mixing reactive clay particles with polymer chains carrying residual isocyanate groups. On the other hand, non-reactive clay particles yielded only intercalated composites. Most significant improvement in mechanical properties were obtained when clay particles were fully exfoliated, e.g. 110% increase in tensile modulus, 170% increase in tensile strength, 110% increase in tear strength, 120% increase in fracture toughness, and 40% increase in abrasion resistance over pristine polyurethane with 5 wt% clay. In addition, the terminal dynamic rheological data showed strong dependence on the clay content, indicating substantial hindrance to chain relaxation by tethering clay particles. The peak location and the area under the peak of hydrogen-bonded carbonyl showed two distinct zones of temperature dependence, which indicate additional hydrogen bonding between polymer chains and organic modifier of reactive clays.     

Microstructural,mechanical, and thermal characteristics of recycled cellulose fiber‐halloysite‐epoxy hybrid nanocomposites

Epoxy hybrid‐nanocomposites reinforced with recycled cellulose fibers (RCF) and halloysite nanotubes (HNTs) have been fabricated and investigated. The dispersion of HNTs was studied by synchrotron radiation diffraction (SRD) and transmission electron microscopy (TEM). The influences of RCF/HNTs dispersion on the mechanical properties and thermal properties of these composites have been characterized in terms of flexural strength, flexural modulus, fracture toughness, impact toughness, impact strength, and thermogravimetric analysis. The fracture surface morphology and toughness mechanisms were investigated by SEM. Results indicated that mechanical properties increased because of the addition of HNTs into the epoxy matrix. Flexural strength, flexural modulus, fracture toughness, and impact toughness increased by 20.8, 72.8, 56.5, and 25.0%, respectively, at 1 wt% HNTs load. The presence of RCF dramatically enhanced flexural strength, fracture toughness, impact strength, and impact toughness of the composites by 160%, 350%, 444%, and 263%, respectively. However, adding HNTs to RCF/epoxy showed only slight enhancements in flexural strength and fracture toughness. The inclusion of 5 wt% HNTs into RCF/epoxy ecocomposites increased the impact toughness by 27.6%. The presence of either HNTs or RCF accelerated the thermal degradation of neat epoxy. However, at high temperature, samples reinforced with RCF and HNTs displayed better thermal stability with increased char residue than neat resin. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

Polyurea/vinylester hybrid thermoset resins with in situ produced silicate filler: Preparation and static mechanical properties

Proprietary polyurea‐based thermosets were produced from polyisocyanate and water glass (WG) using a phosphate‐type phase transfer catalyst. WG was dispersed in the polyisocyanate resulting in water‐in‐oil (W/O) type emulsion. The polyurea matrix, formed after crosslinking, contained the WG derived silicate in coarse particles showing a broad particle size distribution. The mean particle size of the silicate was markedly reduced and its distribution narrowed when the polyisocyanate was hybridized with a peroxide crosslinkable vinylester resin (VE) when the amount of the latter was <75%. This resin hybridization strongly improved the mechanical (flexural) properties of the related thermosets, however, at cost of the fracture mechanical characteristics (fracture toughness and energy under mode I condition). This was mostly attributed to the formation of a conetwork or interpenetrated network between the polyurea and VE. The static flexural and fracture mechanical properties were determined as function of the resin hybridization ratio. It was found that the mechanical properties change according to the additivity rule as a function of the resin hybridization in the first approximation. The silicate dispersion and the failure behavior in the polyurea/VE hybrids were studied by scanning electron microscopy (SEM) and discussed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 853–859, 2007     

Modification of phenolic resin composites by hyperbranched polyborate and polybenzoxazine

Hyperbranched polyborate (HBPB) is a novel polymer with highly branched structures, plenty of functional end groups, and excellent thermal resistance, which could be used as an effective modifier for thermoset resin to greatly improve the thermal resistance and toughness of the resin, simultaneously. With the assistance of benzoxazine (BOZ) resin which has low viscosity, high thermal resistance, and no by‐product release upon curing, modification effects of HBPB on the thermal resistance and toughness of phenolic resin (PR) can be adequately realized. The optimal combination of HBPB and BOZ modified PR (PR‐BOZ‐HBPB) is obtained by the orthogonal test. The PR‐BOZ‐HBPB blend prepared with the optimal combination (90 portion of PR, 10 portion of BOZ, and 5 portion of HBPB) has a char yield up to 67.4%; meanwhile, flexural strength, interlaminar shear strength, flexural modulus of this glass/PR‐BOZ‐HBPB composite are higher than those of glass/PR composite by 17.6%, 23.3%, and 9.6%, respectively. The novel thermoset resin, PR‐BOZ‐HBPB blend, which consists of good processability, great thermal and mechanical properties, is obtained. POLYM. COMPOS., 33:1960–1968, 2012. © 2012 Society of Plastics Engineers     

Preparation of high‐performance flame‐retardant hybrid material by hyperbranched polyphosphate ester

A novel hyperbranched polyphosphate ester (HPPE) and a high‐performance hybrid flame‐retarding material HPPE/diglycidyl ether of bisphenol‐A (DGEBA) epoxy resin are prepared. The effect of HPPE content and molecular weight on the mechanical performance, thermal performance, and flame‐retarding property of the hybrid material is studied. The results show that the overall performance of the hybrid material first increases and then decreases with the increase of HPPE content as well as the molecular weight and reaches maximum with 8–12 wt% of HPPE. When compared with the performance of pure DGEBA, the tensile strength, flexural strength, impact strength, fracture toughness, and limited oxygen index of the HPPE‐2/DGEBA hybrid material are enhanced by about 114%, 50%, 48%, 95%, and 20%, respectively. The toughening and reinforcing mechanism of the hybrid system is a novel in situ effect as revealed by the micrographs of the fracture surfaces using scanning electron microscopy. POLYM. COMPOS., 32:36–43, 2011. © 2010 Society of Plastics Engineers     

Mode I and Mode II delamination resistance and mechanical properties of woven glass/epoxy–PU IPN composites

The effect of polyurethane on the mechanical properties and Mode I and Mode II interlaminar fracture toughness of glass/epoxy composites were studied. Polyurethanes (PU) synthesized using polyols and toluene diisocyanate were employed as modifier for epoxy resin by forming interpenetrating polymer network. The PU/Epoxy IPN was used as matrix material for GFRP. PU modified epoxy composite laminates having varying PU contents were prepared. The effect of PU content on the mechanical properties like interlaminar fracture toughness (Mode I, G_1c and Mode II, G_IIc), tensile strength, flexural strength, and Izod impact strength were studied. The morphological studies were conducted on the fractured surface of the composite specimen by scanning electron microscopy (SEM). Tensile strength, flexural strength, and impact strength of PU‐modified epoxy composite laminates were found to increase inline with interlaminar fracture toughness (G_1c and G_IIc) with increasing PU content to a certain limit and then it was found to decrease with increase in PU content. It was observed that toughening of epoxy with PU increases the Mode I and Mode II delamination toughness up to 17 and 120% higher than that of untoughened composite specimen, respectively. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Simultaneous effects of silanized coal fly ash and nano/micro glass fiber on fracture toughness and mechanical properties of carbon fiber‐reinforced vinyl ester resin composites

In this study, the simultaneous effects of both silanized coal fly ash (S‐CFA) and nano/micro glass fiber (nGF) on fracture toughness and mechanical properties of vinyl ester (VE) resin filled with carbon fiber‐based composite materials were investigated. The CFA was treated with (3‐trimethoxysilyl) propyl methacrylate to introduce the methacryloxy groups into the surface of CFA, and was confirmed by using FTIR technique. The nGF and S‐CFA with different weight ratios were well mixed with VE resin by using of high‐speed mechanical stirrer, and ultrasonic technique before using as matrices for fabrication of carbon fiber‐based composite materials via sheet molding compound (SMC) method and hot curing processing. Many characteristics of both cured VE resin composites and carbon fiber‐based composite were examined such as mechanical properties, fracture toughness, and morphology. The results showed that by adding of both 0.1 wt% nGF and 1 wt% S‐CFA into VE resin the tensile strength, tensile modulus, flexural strength, K_IC, impact strength as well as the Mode I interlaminar fracture toughness (G_IC) of VE composites and carbon fiber based composites get optimum values and increased about 61.39%; 39.83%; 36.21%; 103.1%; 81.79%; 48.61%, respectively when compared with pristine materials. POLYM. ENG. SCI., 59:584–591, 2019. © 2018 Society of Plastics Engineers     

High performance graphene oxide‐modified polybenzoxazine resin

High performance polybenzoxazine resin can be obtained by introducing partially reduced graphene oxide (PRGO) into bis‐benzoxazine (B‐BOZ) resin to overcome the defects such as low char yield, weak mechanical properties, and poor toughness of B‐BOZ resin. By virtue of the good thermal resistance and graphitization acceleration of PRGO, thermal resistance especially char yield of B‐BOZ resin can be greatly improved to 62.1% at 800°C in nitrogen. Moreover, with the folds deformation and the crack deflection of PRGO in B‐BOZ resin, the mechanical properties such as flexural strength and interlaminar shear strength of the carbon fiber reinforced B‐BOZ composite, could be greatly increased by PRGO without a decrease on modulus of B‐BOZ composite. POLYM. COMPOS., 37:1507–1514, 2016. © 2014 Society of Plastics Engineers     

Differences in physico-mechanical behaviors of resol(e) and novolac type phenolic resin based composite bipolar plate for proton exchange membrane (PEM) fuel cell

Composite bipolar plates for Proton Exchange Membrane Fuel Cell (PEMFC) are prepared by compression molding technique using polymer as binder and graphite as electric filler material with some other reinforcements. Study on the effect of resole and novolac type phenolic resin on the properties of composite bipolar plate, such as bulk density, porosity, bulk conductivity, hardness, flexural strength, etc. shows that both of the resin shows different physico-mechanical properties. Moreover, single cell performance analysis also shows variation for resole and novolac based composites. A novel concept of triple continuous structure to provide graphite polymer blends with high electrical conductivity, high shore hardness, high flexural strength, less porosity and low density has been proposed and study on the effect of different types of phenolic resin on the properties and performance of bipolar plate reveals that novolac type powdered phenolic resin gives better mechanical properties than resole type phenolic resin. However, resole type phenolic resin compound has slightly higher electrical conductivity due to more number of polar -OH group presents on its cured form. But due to the less porosity and higher mechanical strength, bipolar plates with novolac type phenolic resin gives better performance in I-V analysis than bipolar plates with resole type phenolic resin.     

Flexural and Charpy impact behaviour of epoxy/glass fabric treated by nano-SiO2 and silane blend

A sizing formulation, containing compatible and incompatible silane coupling agents with epoxy resin in conjunction with nanoscale colloidal silica, was used to modify the surface of glass fabric. The modified glass fabric/epoxy resin composite panels were fabricated and characterised by flexural test, Charpy impact test and scanning electron microscope (SEM). By combining nano silica with silane blend in the fabric sizing, more energy was consumed under bending and impacting, which resulted in an improvement of the toughness in composites. The flexural strength, bending stain and Charpy impact strength of the epoxy composite/glass fabric treated with 1?wt-% nano silica and silane blend were ～42, ～22 and 35%, respectively, higher than those of silane blend coated glass fabric-reinforced composites (without nano silica). Furthermore, the change of the brittle fracture of the composite into ductile fracture was investigated by SEM micrographs. A possible toughening mechanism was also proposed.     

Synthesis and characterization of fluorinated poly(etherimide)s toughening for carbon fiber‐reinforced epoxy composites

Novel‐fluorinated poly(etherimide)s (FPEIs) with controlled molecular weights were synthesized and characterized, which were used to toughen epoxy resins (EP/FPEI) and carbon fiber‐reinforced epoxy composites (CF/EP/FPEI). Experimental results indicated that the FPEIs possessed outstanding solubility, thermal, and mechanical properties. The thermally cured EP/FPEI resin showed obviously improved toughness with impact strength of 21.1 kJ/m² and elongation at break of 4.6%, respectively. The EP/FPEI resin also showed outstanding mechanical strength with tensile strength of 91.5 MPa and flexural strength of 141.5 MPa, respectively. The mechanical moduli and thermal property of epoxy resins were not affected by blending with FPEIs. Furthermore, CF/EP/FPEI composite exhibited significantly improved toughness with Mode I interlaminar fracture toughness (G_IC) of 899.4 J/m² and Mode II interlaminar fracture toughness (G_IIC) of 1017.8 J/m², respectively. Flexural properties and interlaminar shear strength of the composite were slightly increased after toughening. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Preparation and Mechanical Properties of Polypropylene/Maleic Anhydride Compatibilized Polypropylene/Organo-Vermiculite Nanocomposites

Polypropylene/Maleic anhydride compatibilized polypropylene (MPP)/Organo-Vermiculite nanocomposites with different clay loadings were prepared via melt mixing using a twin screw extruder. The vermiculite (VMT) was pre-modified with Maleic anhydride (MA) by ball milling. The mechanism of VMT modification by MA was discussed. The resultant PP/MPP/OVMT nanocomposites possess an intercalated or exfoliated structure as confirmed by both wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). The mechanical property tests show that the tensile strength, flexural strength of these nanocomposites increase dramatically with the OVMT loading; the fracture toughness remain almost unchanged, and the Charpy impact strength decrease slightly.     

Toughness of nanoscale and multiscale polyamide‐6,6 composites

Fracture initiation and fracture propagation toughening (R‐curve behavior) of polyamide 6,6 (PA‐66) polymers with different types of layered silicate clay having nanoscale (fully dispersed) or multiscale (mixed nanoscale/microscale) structure were studied. These results were compared to fracture data for conventional kaolin clay particulate reinforcements and a PA‐66 polyblend containing rubber and rigid poly(styrene‐co‐acrylonitrile) particulates. The stiffness increase due to the intercalated clay was the same as would be predicted by classical models for conventional elongated reinforcements at the same volume fraction level. The special benefit of the nanoscale reinforcement derived from their high surface area of contact with the matrix. Toughness in layered silicate clay composites was enhanced by better dispersion of the clay, by exfoliation of the clay layers, and by a stronger clay/matrix interface. A multiscale microstructure was found to be the more desirable microstructure, combining toughness from the nanodispersed clay with resistance curve behavior from the micrometer‐sized particulates. Fracture toughness was proportional to the crack‐tip plastic zone size at fracture, indicating that the clay reinforcements, by influencing shear deformation in the crack tip region, played an important role in promoting toughness. There was indirect evidence for the formation of a zone of damage within the crack‐tip plastic zone that could explain why toughness was not optimal.     

Performance enhancement of nylon/kevlar fiber composites through viscoelastically generated pre‐stress

Kevlar‐29 fibers have high strength and stiffness but nylon 6,6 fibers have greater ductility. Thus by commingling these fibers prior to molding in a resin, the resulting hybrid composite may be mechanically superior to the corresponding single fiber‐type composites. The contribution made by viscoelastically generated pre‐stress, via the commingled nylon fibers, should add further performance enhancement. This paper reports on an initial study into the Charpy impact toughness and flexural stiffness of hybrid (commingled) nylon/Kevlar fiber viscoelastically pre‐stressed composites at low fiber volume fractions. The main findings show that (i) hybrid composites (with no pre‐stress) absorb more impact energy than Kevlar fiber‐only composites; (ii) pre‐stress further increases impact energy absorption in the hybrid case by up to 33%; (iii) pre‐stress increases flexural modulus by ∼40% in the hybrid composites. These findings are discussed in relation to practical composite applications. POLYM. COMPOS., 35:931–938, 2014. © 2013 Society of Plastics Engineers     

Mechanical properties,thermal stability,and flame retardance of novolac‐type phenolic resin blended with poly(dimethylsiloxaneadipamide)

Mechanical properties (flexural strength, flexural modulus, and notched Izod impact strength), thermal stability, and flame retardance of poly(dimethylsiloxane adipamide) (PDMSA)‐toughened novolac type phenolic resin were investigated. Mechanical properties of modified novolac‐type phenolic resin increase with PDMSA contents, because the soft segment of PDMSA absorbs the loads in the network of brittle novolac‐type phenolic resins. TGA results show that the thermal degradation temperatures are higher than 400°C, and the temperature of 10% weight loss increases with increasing the PDMSA content. The char yield increases with novolac‐type phenolic resin content. The morphologies of the fracture surface of the modified novolac‐type phenolic resin were investigated by scanning electron microscopy (SEM). Morphological results agree with those from mechanical properties of the modified novolac‐type phenolic resin. The modified novolac‐type phenolic resin also shows excellent flame retardance that is UL‐94, V‐1, and the limited oxygen index is higher than 35. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 631–637, 2001     

Toughness and reinforcement of diglycidyl ether of bisphenol-A by hyperbranched poly(trimellitic anhydride-butanediol glycol) ester epoxy resin

Hyperbranched epoxy resin shows best comprehensive performance in epoxy resin system and is considered as a kind of toughness and reinforcement additive. This article reports on the use of novel hyperbranched poly(trimellitic anhydride-butanediol glycol) ester epoxy resin (HTBE) prepared by us as a functional additive to an epoxy amine resin system. The effect of molecular weight and content of the HTBE on the performance of the diglycidyl ether of bisphenol-A (DGEBA)/HTBE hybrid resin are discussed in detail, and their performance has maximum with the increase of content and molecular weight of HTBE. The impact strength and fracture toughness of the hybrid resin containing 9 wt% second generation of HTBE are 48.2 kJ/m² and 2.71 MPa m^1/2, and which almost are 2.77 and 1.5 times of DGEBA performance respectively. Furthermore, the tensile and flexural strength can also be enhanced about 17%. The fracture surface micrograph of hybrid resin shows no microphase separation of the HTBE/DGEBA blends that facilitates an enhanced interaction to achieve excellent toughness and strength enhancement of the cured systems by scanning electron microscope (SEM). A novel situ reinforcing and toughening mechanism and model are discovered and confirmed by SEM, molecular simulation, and dynamic mechanical thermal analysis technology. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Nanocomposite adhesives: Mechanical behavior with nanoclay

The major objective for this research was to examine the role of epoxy-clay nanocomposites in the area of epoxy bonding to porous stone (granite) substrates. Two bisphenol A epoxy systems were selected based on the prior work that determined optimal adhesive properties from a larger set of epoxy systems to determine the role of viscosity on the intercalation and exfoliation of the clay tactiods in the epoxy resin. The systems were characterized and mechanically tested at varying levels of intercalated and exfoliated organic clay tactiods. In the first stage of the work, epoxy-clay systems were characterized by wide-angle X-ray diffraction (WAXD) to detect inter-laminar distances of clay layers and to determine if the mixing procedures had indeed dispersed and exfoliated the clay layers sufficiently. The second stage of the work involved examining mechanical properties of the epoxy-nanoclay systems. Fracture behavior was studied using granite stone substrates in notched double lap configuration. Compressing a wedge between the cover plates induced the fracture. Fracture toughness was approximated by the load at fracture. Tensile properties were measured using cast dog bone tensile samples. The better layered silicate nanocomposite performance was seen with the lower viscosity resin. The most noticeable improvements in mechanical properties for the lower viscosity resin system were found to be maximum stress, elastic modulus, and yield stress. Increased toughness and stress whitening at 1% by weight nanoclay loading revealed that the clay can act as a shear-yielding toughening agent in this epoxy system.     

Effect of quasi‐carbonization processing parameters on the mechanical properties of quasi‐carbon/phenolic composites

In this work, quasi‐carbon fabrics were produced by quasi‐carbonization processes conducted at and below 1200°C. Stabilized polyacrylonitrile (PAN) fabrics and quasi‐carbon fabrics were used as reinforcements of phenolic composites with a 50 wt %/50 wt % ratio of the fabric to the phenolic resin. The effect of the quasi‐carbonization process on the flexural properties, interfacial strength, and dynamic mechanical properties of quasi‐carbon/phenolic composites was investigated in terms of the flexural strength and modulus, interlaminar shear strength, and storage modulus. The results were also compared with those of a stabilized PAN fabric/phenolic composite. The flexural, interlaminar, and dynamic mechanical results were quite consistent with one another. On the basis of all the results, the quasi‐static and dynamic mechanical properties of quasi‐carbon/phenolic composites increased with the applied external tension and heat‐treatment temperature increasing and with the heating rate decreasing for the quasi‐carbonization process. This study shows that control of the processing parameters strongly influences not only the mechanical properties of quasi‐carbon/phenolic composites but also the interlaminar shear strength between the fibers and the matrix resin. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

PET interleaving veils for improved fracture toughness of glass fibre/low‐styrene‐emission unsaturated polyester resin composites

The use of interleaved polyethylene terephthalate (PET) veils to increase the interlaminar fracture toughness of glass fiber‐reinforced, low‐styrene emission, unsaturated polyester resin composites, was investigated. PET, being chemically similar to the unsaturated polyester resin, was expected to exhibit good wetting and strong interaction with the matrix. Composite laminates were manufactured by hand lay‐up, with the veil content varying up to 7%. The effects of PET veils on the interlaminar shear strength, flexural strength, flexural modulus, glass transition temperature, damping parameters, and Mode‐I interlaminar fracture toughness of the composite were studied. The veils were found to enhance most of these properties, with only minor negative effects on flexural stiffness and T_g. The PET/resin bonding did indeed prove to be strong, but the enhancement of fracture toughness was not as much as expected, because of the weaker glass/resin interface providing an alternative crack propagation path. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42877.