Possible phase transformation toughening of thermoset polymers by poly(butylene terephthalate)

Mechanisms were explored by which particles of poly(butylene terephthalate) (PBT) are able to toughen a brittle epoxy. The epoxy studied was an aromatic amine-cured diglycidyl ether of bisphenol-A, which was toughened at about twice the rate with particles of poly(butylene terephthalate) as with particles of nylon 6, poly(vinylidene fluoride), or CTBN rubber. Many of the mechanisms of toughening are visible on the fracture surface of the PBT-epoxy blend, but a mechanism suggested to account for perhaps half of the increased toughness with PBT, phase transformation toughening, is not. The two types of experiment performed to detect phase transformation toughening were: (1) measurements of the rubber cavitation zone in PBT-CTBN rubber-epoxy ternary blends, which would detect an expansion of the PBT particles during fracture if it occurred, and (2) measurements of the fracture energy in PBT-epoxy blends in which the various mechanisms of toughening were selectively suppressed. Both types of experiment indicated the occurrence of phase transformation toughening in these PBT-epoxy blends.     

The effect of crystalline morphology of poly (butylene terephthalate) phases on toughening of poly(butylene terephthalate)/epoxy blends

In an effort to investigate the effect of the crystalline morphology of a poly(butylene terephthalate) (PBT) phase on the toughening of PBT/epoxy blends, the blends, having different degrees of perfectness of the PBT crystalline phase, were prepared by blending PBT and epoxy at various temperatures ranging from 200 to 240 °C. As the blending temperature decreases, the degree of perfectness of the PBT crystalline phase increases as a result of the increase of crystal growth rate. For PBT/epoxy blends, the change in crystalline morphology induced by processing may be the most important cause for the dependency of the fracture energy on blending temperatures. It has been found that PBT phases with a well-developed Maltese cross are most effective for epoxy toughening. This dependency reveals the occurrence of a phase transformation toughening mechanism. Also, the higher relative enhancement of fracture energy of a higher molecular weight epoxy system is further indirect evidence for a phase transformation toughening mechanism. Some other toughening mechanisms observed from the fracture surfaces, such as crack bifurcation, crack bridging, and ductile fracture of PBT phases, have been found to also be affected by the blending temperatures.     

Phase morphology of nanofibre interlayers: Critical factor for toughening carbon/epoxy composites

Electrospun thermoplastic nanofibres were employed to toughen carbon/epoxy composites by direct deposition on carbon fibre fabrics, prior to resin impregnation and curing. The toughening mechanism was investigated with respect to the critical role of phase morphology on the toughening effect in carbon/epoxy composites. The influences of solubility in epoxy and melting characteristics of thermoplastics were studied towards their effects on phase structure and delamination resistance. For the three different thermoplastic nanofibre interlayers used in this work, i.e. poly(ε-caprolactone) (PCL), poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) nanofibre interlayers, only PCL nanofibres produced toughening. Although cylinder-shaped fibrous macrophases existed in all three interlayer regions, only PCL nanofibres had polymerisation-induced phase separation with epoxy, forming ductile thermoplastic-rich particulate microphases on the delamination plane. These findings clearly show that the polymerisation-induced phase separation is critical to the interlayer toughening by thermoplastic nanofibres. An optimal concentration (15 wt.%) of PCL solution for electrospinning was found to produce composites with enhanced mode I interlaminar fracture toughness (G_IC), stable crack growth and maintained flexural strength and modulus.     

The phase transformation toughening and synergism in poly(butylene terephthalate)/poly(tetramethyleneglycol) copolymer-modified epoxies

The toughening of epoxy modified with poly(butylene terephthalate)/poly(tetra-methylene glycol) (PBT–PTMG) copolymers of various chemical composition was investigated. The fracture toughness of the brittle epoxy was highly enhanced by the inclusion of PBT–PTMG copolymer without loss of other intrinsic mechanical properties, such as modulus and yield stress. These modified epoxies also exhibited synergism in toughening. The remarkable enhancement and the synergism in fracture toughness of PBT/PTMG-modified epoxies is possibly due to the enhancement of the degree of phase transformation toughening, which is a result of the enhancement of the degree of perfectness of PBT spherulites in the presence of PTMG segments. The changes in micro-morphology of PBT/PTMG phases induced by the different chemical composition of copolymer is the most important cause of the dependency of the fracture energy on the processing variables, such as the relative PBT/PTMG composition and total amount of modifiers. Other toughening mechanisms, such as crack bifurcation, ductile fracture of PBT/PTMG phases, main crack-path alteration, and crack bridging, also contributed to toughness enhancement of the modified epoxies. © 1998 Chapman & Hall     

Tailoring impact toughness of poly(L-lactide)/poly(ε-caprolactone) (PLLA/PCL) blends by controlling crystallization of PLLA matrix

Melt blending poly(L-lactide) (PLLA) with various biodegradable polymers has been thought to be the most economic and effective route to toughen PLLA without compromising its biodegradability. Unfortunately, only very limited improvement in notched impact toughness can be achieved, although most of these blends show significant enhancement in tensile toughness. In this work, biodegradable poly(ε-caprolactone) (PCL) was used as an impact modifier to toughen PLLA and a nucleating agent was utilized to tailor the crystallization of PLLA matrix. Depending on the nucleating agent concentrations in the matrix and mold temperatures in injection molding, PLLA/PCL blends with a wide range of matrix crystallinity (10-50%) were prepared by practical injection molding. The results show that there is a linear relationship between PLLA matrix crystallinity and impact toughness. With the increase in PLLA crystalline content, toughening becomes much easier to achieve. PLLA crystals are believed to provide a path for the propagation of shear yielding needed for effective impact energy absorption, and then, excellent toughening effect can be obtained when these crystals percolate through the whole matrix. This investigation provides not only a new route to prepare sustainable PLLA products with good impact toughness but also a fresh insight into the importance of matrix crystallization in the toughening of semicrystalline polymers with a flexible polymer.     

PBT树脂对核壳粒子增韧聚碳酸酯结构与性能的影响

合成了甲基丙烯酸甲酯-丁二烯-苯乙烯-甲基丙烯酸环氧丙酯(MBS-g-GMA)核壳粒子用于聚碳酸酯(PC)的增韧,并在PC/MBS-g-GMA共混物中引入聚对苯二甲酸丁二醇酯(PBT)以提高性能。PBT的加入使MBS-g-GMA在PC中分散更均匀;共混物缺口冲击强度进一步提高,PBT质量分数为20%时,冲击强度接近700J/m;PBT提高了PC/MBS-g-GMA共混物的熔体流动速率,使加工性能得到改善。断裂形态表明,PBT加入后共混物应力发白区尺寸增加,剪切屈服更明显,共混物韧性提高。     

Fracture toughness of the nano-particle reinforced epoxy composite

Although thermoset polymers have been widely used for engineering components, adhesives and matrix for fiber-reinforced composites due to their good mechanical properties compared to those of thermoplastic polymers, they are usually brittle and vulnerable to crack. Therefore, ductile materials such as micro-sized rubber or nylon particles are added to thermoset polymers are used to increase their fracture toughness, which might decrease their strength if micro-sized particles act like defects.In this work, in order to improve the fracture toughness of epoxy adhesive, nano-particle additives such as carbon black and nanoclay were mixed with epoxy resin. The fracture toughness was measured using the single edge notched bend specimen at the room (25 °C) and cryogenic temperature (−150 °C). From the experimental results, it was found that reinforcement with nano-particles improved the fracture toughness at the room temperature, but decreased the fracture toughness at the cryogenic temperature in spite of their toughening effect.     

核壳结构改性剂对PBT/PC共混物的增韧作用

采用丙烯腈-丁二烯-苯乙烯（ABS）核壳结构改性剂增韧聚对苯二甲酸丁二醇酯（PBT）/聚碳酸酯（PC）共混物。动态力学测试（DMTA）结果表明,PBT与PC为热力学不相容体系,ABS的引入导致PBT、PC玻璃化转变温度相互靠近,相容性提高;差示扫描量热（DSC）研究结果表明,随着ABS的加入,PBT/PC体系中PBT的...     

Fracture resistance of polyblends and polyblend matrix composites Part III Role of rubber type and location in nylon 6,6/SAN composites

The role of rubber particle type, location and morphology on toughening in blends of nylon 6,6 with styrene acrylonitrile (SAN), with and without fibre reinforcements was examined in this study. The rubber used was ethylene propylene diene monomer (EPDM) rubber and the results were compared to a previous study that used butadiene rubber. The compositions of the blends ranged from pure nylon 6,6 to pure SAN. EPDM rubber was chemically compatibilized with one of the matrix phases rather than grafted, as in the ABS. In order to study the effect of rubber location on fracture behaviour, the approach was to compatibilize EPDM with either the minor phase or the major phase component of the blend. Attention was focused on fracture initiation toughness and fracture propagation toughness, measured through the parameters J _IC and J _SS, respectively. J _SS refers to the steady-state, or plateau value of the material R-curve and was therefore a measure of total toughness which included the additional component derived from crack extension. The results indicated that EPDM rubber was not as effective a toughening agent as was butadiene in the Acrylonitrile Butadiene Styrene (ABS) system, primarily due to the morphology of EPDM and its interface character with the nylon 6,6 or SAN matrix. It was demonstrated that the embrittlement effects of a second rigid polymer phase can be mitigated by selectively adding rubber to that phase in the alloy or blend. With regard to the role of fibre reinforcement, a strong fibre matrix interface was found to be essential for toughening. Further, the extent of rubber toughening was larger when fibres were present than when fibres were absent, provided the fibre matrix interface was strong. Fibres also, like rubber, enhanced local matrix plasticity as well as reduced the embrittlement effects associated with a second polymer phase.     

Fracture behaviour of liquid crystal epoxy resin systems based on diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene: Part II Effect due to blending with TACTIX 556 epoxy resin and phenolic monomers

The mechanical behaviours of unoriented, poured resin castings based on formulated blends containing the diglycidyl ether of 4,4′-dihydroxy-α-methylstilbene monomer are studied. It is found that the mechanical and fracture behaviours of these liquid crystalline epoxy (LCE) blends vary significantly. In general, the LCE blends possess much higher fracture toughness and fatigue crack resistance than conventional epoxy resins. At low temperatures (−40°C), the KIC values of the LCE blends are slightly higher than those measured at room temperature. The common fracture mechanisms observed in the ductile LCE blends are crack segmentation, crack branching, crack bridging and crack blunting. The fracture surfaces of the tougher LCE blends only exhibit limited ductile drawing (furrow pattern) at the slow crack growth region; no signs of shear lips on the edges of the starter crack region are observed. The optical microscopy and transmission electron microscopy work suggests that orientation and/or transformation toughening may be the source for such high fracture toughness of the LCE blends. The possible cause(s) of the unusual fracture behaviour of the LCEs is discussed. Approaches for making high performance LCE blends are also addressed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fracture resistance of polyblends and polyblend matrix composites: Part II Role of the rubber phase in Nylon 6,6/ABS alloys

A comprehensive study was undertaken on the specific role of rubber on toughening when other rigid polymer or non-polymer phases were present. Nylon 6,6/SAN blends of various SAN concentrations ranging from pure SAN to pure nylon 6,6 were investigated with and without fibre reinforcements. These results could be compared with the toughness values of unreinforced and fibre-reinforced nylon 6,6/ABS alloys from a previous study in order to elucidate the role of rubber. Fracture behaviour was investigated rigorously by characterizing the fracture initiation toughness, JIC, and the steady-state fracture toughness, Jss. These were then related to the microstructure and failure modes determined by microscopy and fractography methods. It was found that rubber increased both fracture initiation and propagation toughness in the presence of the rigid phase, while the rigid phase toughened the alloy only when the rigid phase/matrix interface was strong enough. The role played by glass fibres was found to be critically related to the fibre/matrix interfacial strength. Toughening was generally observed, both in the presence and absence of rubber, when the interface was strong. In all cases toughening could be related to the enhancement of plasticity in the crack tip by the presence of the rubber phase or the reinforcing glass phase. This revised version was published online in November 2006 with corrections to the Cover Date.     

Impact fracture behaviour of nylon 6-based ternary nanocomposites

This study focuses on achieving high stiffness/strength and high fracture toughness in nylon 6/organoclay nanocomposites prepared via melt compounding by incorporating a maleic anhydride grafted polyethylene–octene elastomer (POE-g-MA) as a toughening agent. Mechanical test results indicated that the ternary nanocomposites exhibited higher stiffness than nylon 6/POE-g-MA binary blends at any given POE-g-MA content. More importantly, the brittle–ductile transition of nylon 6/POE-g-MA blends was not impaired in the presence of organoclay for the compositions prepared in this study. TEM analysis shows that organoclay layers and elastomer particles were dispersed separately in nylon 6 matrix. In the binary nanocomposite, no noticeable plastic deformation was observed around the crack tip. In the ternary nanocomposites, the presence of organoclay in the matrix provided maximum reinforcement to the polymer, while their absence in the elastomer particles allowed the latter to promote high fracture toughness via particle cavitation and subsequent matrix shear yielding. The partially exfoliated clay layers also delaminated and hence, adding to the total toughness of the nanocomposites.     

Toughening of epoxies through thermoplastic crack bridging

The fracture toughness and toughening mechanism of two epoxy matrices containing varying concentrations of pre-formed polyamide-12 particles was investigated. The pre-formed thermoplastic modifier was used to keep the physical and morphological characteristics of the second phase constant while varying the matrix intrinsic toughness to simplify the interpretation of toughening results. We observed that these particles toughened the epoxies through a crack bridging mechanism involving large plastic deformation of the second phase.This mechanism was found to be effective independent of the potential of the matrix for plastic deformation since the increasing fracture toughness was accomplished without significant amounts of plastic deformation in the epoxy matrix. A quantitative model was adapted to account for the increase in toughness due to the crack bridging mechanism. From this model, it was possible to determine the factors which are most important when attempting to toughen a material through thermoplastic crack bridging. A better understanding of the specific factors which influence the efficiency of the crack bridging mechanism enables the fracture properties of brittle materials to be further improved with thermoplastic addition. This was shown to be very important when attempting to enhance the toughness of materials which are believed to be un-toughenable by conventional rubber modification, or materials whose other mechanical properties suffer from the addition of elastomeric materials.     

AIM 核壳增韧剂/ PVC 树脂共混体系性能

采用自制的丙烯酸丁酯接枝甲基丙烯酸甲酯核壳结构增韧剂( PBA-g-PMMA, AIM) 与聚氯乙烯( PVC) 树脂熔融共混, 制备了AIM/ PVC 共混物。研究了增韧剂粒子在PVC 中的分散和空洞化、AIM/ PVC 共混物的断裂与脆韧转变。结果表明, 球形的增韧剂粒子能够在PVC 树脂中均匀分散并对PVC 有很好的增韧作用, 在PVC 树脂中加入质量分数为615 %AIM 时, 冲击样条以韧性方式断裂; 样条冲击断面周围应力发白区域内产生了空洞。提出AIM 增韧PVC 的机理是橡胶粒子的空洞化与塑料基体剪切屈服协同作用。      

PPS／PBT共混体系的研究

在Ｂｒａｂｅｎｄｅｒ混合仪中，用熔融混合法制备结晶／结晶共混体系ＰＢＴ／ＰＰＳ。采用ＤＳＣ、ＷＡＸＤ和ＳＥＭ对共混物的结晶，熔融，相容性和形态进行了研究。结果表明，ＰＢＴ／ＰＰＳ共混物是不相容的，各自自己的微区内进行结晶。ＰＢＴ加入可使窝本粘度明显上升。     

超韧尼龙6体系的流变与力学行为EI

马来酸酐接枝聚烯烃类热塑弹性体 (TPEg)对尼龙 6有显著的增韧效果。研究了基体粘度和界面改性剂 (CE-96 )的使用对尼龙 6 / TPEg共混体系缺口冲击强度的影响。在 TPEg分散相粘度大于尼龙 6基体粘度情况下 ,TPEg对高粘度尼龙 6的增韧效果明显好于低粘度尼龙体系。CE- 96的加入通过增大尼龙 6基体粘度和增强界面偶联显著地改善了 TPEg分散质量。在给定的分散相含量下 ,分散相颗粒尺寸的减小更有利于引发基体剪切屈服 。     

Super-tough poly(butylene terephthalate) based blends by modification with core-shell structured polyacrylic nanoparticles

Core-shell structured polyacrylic nanoparticles (named CSPN) impact modifiers consisting of a rubbery poly(n-butyl acrylate) core and a rigid poly(methyl methacrylate) shell with a size of about 352 nm were synthesized by seed emulsion polymerization. The CSPN modifier with core-shell weight ratio 80/20 was used to toughen poly(butylene terephthalate) (PBT) by melt blending. With an increase in CSPN content, the impact strength and the elongation at break of PBT/CSPN blends increased significantly compared with those of PBT; however, the tensile strength decreased. It was found that the polymerization had a very high instantaneous conversion (> 93%) and overall conversion (99%). The core-shell structure of CSPN was examined by means of transmission electron microscope. Scanning electron microscope was used to observe the morphology of CSPN particle and fractured surfaces of the blends. The dynamic mechanical analyses of PBT/CSPN blends showed two merged transition peaks of PBT matrix, with the presence of CSPN modifier, which was responsible for the improvement of PBT toughness. The results indicated that the notched impact strength of PBT/CSPN blend with a weight ratio of 80/20 was 8.61 times greater than that of pure PBT where the brittle-ductile transition point appeared.     

Interactions of a liquid crystalline polymer with polycarbonate and poly(ethylene terephthalate)

Thermal behaviour of blends of a liquid crystalline copoly(ester amide) (Vectra B950) with two isotropic polymers has been studied by differential scanning calorimetry. One of the isotropic polymers is an amorphous polymer – polycarbonate, the other is a semi-crystalline polymer – poly(ethylene terephthalate). It was found that the glass transition temperature of polycarbonate decreases with increasing Vectra concentration in the blend, suggesting a partial miscibility between the Vectra liquid crystalline polymer (LCP) and polycarbonate. The miscibility is enhanced through heat treatment at elevated temperatures presumably due to a transesterification reaction. Moreover, the presence of the amorphous poly- carbonate hinders the crystallization of the liquid crystalline polymer in the blends. It was also observed that heat treatment of the Vectra LCP and poly(ethylene terephthalate) blends causes a loss in crystallinity and shifts in transition temperatures of poly(ethylene terephthalate), indicating that exchange reactions occur between Vectra B950 and poly(ethylene terephthalate). Based on these results, a new strategy, in situ compatibilization, is proposed to improve the interfacial adhesion between an LCP and an isotropic polymer. This revised version was published online in November 2006 with corrections to the Cover Date.     

Toughening of polyamide 6 with β-nucleated thermoplastic vulcanizates based on polypropylene/ethylene–propylene–diene rubber grafted with maleic anhydride blends

The toughening of polyamide 6 (PA 6) with β-nucleated thermoplastic vulcanizates (TPVs) based on polypropylene (PP)/ethylene–propylene–diene rubber grafted with maleic anhydride (EPDM-g-MAH) blends was studied. A series of TPVs without and with different dosage of β-nucleating agent (β-NA) were prepared and used to toughen PA 6 at the same proportion. Differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) measurements showed that β crystals of PP were effectively induced in the TPVs. The PA 6 blends toughened with β-nucleated TPVs (β-TPVs) exhibit significantly enhanced toughness, balanced mechanical properties and thermal properties compared with PA 6 toughened by TPV without β-NA or only by EPDM-g-MAH. Phase morphologies of the blends characterized by scanning electron microscopy (SEM) showed that better interfacial adhesion caused by the migration of β-NA from PP to PA 6/PP interface and PP/EPDM-g-MAH interface gives rise to more uniform dispersion and smaller size of the dispersed phase; moreover, the core–shell structure comprised of rubber particles enveloped by PP on the surface, brings about easier and stronger interference of the stress field of EPDM phase.     

共混改性尼龙11合金的研究进展

论述了用聚烯烃、橡胶、液晶高分子、树形大分子及无机刚性粒子等对尼龙11进行增韧增强改性的研究发展,其中以聚烯烃类改性最为广泛,但需要一定的相容剂;而无机刚性粒子增韧增强是近年发展起来的新方法,它可以在提高材料韧性的同时,提高材料的拉伸强度。