Development of a novel toughener for epoxy resins

BACKGROUND: Conventional tougheners for brittle epoxy resins form micrometre‐sized particles with ineffective interfaces, and meet toughening requirements with 10–20 wt% of toughener, which unfortunately causes deterioration of other desirable properties. RESULTS: We developed a novel polyurea toughener using a facile synthesis method from isophorone diisocyanate and polyoxyalkyleneamine. Polyurea forms thick‐interface particles, and creates an effective toughness improvement: 265% increase in fracture energy release rate. More importantly, polyurea causes no deterioration of other important properties. Different from conventional epoxy/rubber or epoxy/thermoplastic composites, the epoxy/polyurea composite shows Newtonian rheological behaviour, a convenient property for processing. CONCLUSION: This study describes a novel polyurea toughener. Its unique feature is that toughness can be significantly improved at low toughener content (5 wt%) without losing other desirable properties. The study illuminates the importance of the interface for toughening in polymer blends/composites. Copyright © 2009 Society of Chemical Industry     

Fracture properties of epoxy/poly(styrene‐co‐allylalcohol) blends

The epoxy/polystyrene system is characterized by a poor adhesion between the constituent phases, which determines its mechanical properties. The adhesion can be improved via blends based on epoxy resin and random copolymers, poly(styrene‐co‐allylalcohol) (PS‐co‐PA). In this work, the influence of PS‐co‐PA content and the good adhesion between the phases on the tensile properties and the fracture toughness achieved through instrumented Charpy tests have been investigated. The tensile strength and the deformation at break showed an increase in the PS‐co‐PA content while the Young's modulus remained the same. The tensile fracture surfaces revealed that the improvement of these magnitudes was mainly due to a crack deflection mechanism. Also, the fracture toughness of the blends was superior to that of the pure epoxy resin. The main operating toughening mechanism was crack deflection. The fractographic analysis showed that ～ 80% of the particles were broken, and the crack tended to divert from its original path through the broken PS‐co‐PA particles. The remaining particles were detached from the epoxy resin, and the holes left suffered plastic deformation. Analytical models were used to predict successfully the toughness due to these mechanisms. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Micro‐ and macrophase separation of thermosetting systems modified with epoxidized styrene‐block‐butadiene‐ block‐styrene linear triblock copolymers and their influence on final mechanical properties

BACKGROUND: The goal of this work was to establish the minimum degree of epoxidation needed to develop nanostructured epoxy systems by modification with poly(styrene‐block‐butadiene‐block‐styrene) (SBS) triblock copolymers epoxidized to several degrees, and also to investigate the effect of polystyrene (PS) content on the final morphologies. By using two SBS copolymers, the influence of the weight ratio of the two blocks on the generated morphologies and mechanical properties was also analysed. RESULTS: Nanostructured thermosets were effectively obtained through reaction‐induced microphase separation of PS blocks from the matrix. A minimum of 27 mol% of epoxidation, which corresponds to 4.8 wt% of epoxidized polybutadiene (PB) units in the overall mixture, was needed to ensure nanostructuring of final mixtures and thus their transparency. Hexagonally ordered nanostructures were achieved for PS contents of around 16–20 wt%, which agrees with our previous results for mixtures with other SBS copolymers with different ratios between blocks. The fracture toughness of the epoxy matrix was improved or at least retained with mixing. CONCLUSION: The degree of epoxidation of PB blocks needed to switch epoxy/SBS mixtures from a macrophase‐separated to a nanostructured state has been established. The generated morphologies in the epoxy systems are mainly dependent on the PS content in the mixture. Copyright © 2008 Society of Chemical Industry     

Toughness and strength improvement of diglycidyl ether of bisphenol‐A by low viscosity liquid hyperbranched epoxy resin

This article reports on the use of low viscosity liquid thermosetting hyperbranched poly(trimellitic anhydride‐diethylene glycol) ester epoxy resin (HTDE) as an additive to an epoxy amine resin system. Four kinds of variety molecular weight and epoxy equivalent weight HTDE as modifiers in the diglycidyl ether of bisphenol‐A (DGEBA) amine systems are discussed in detail. It has been shown that the content and molecular weight of HTDE have important effect on the performance of the cured system, and the performance of the HTDE/DGEBA blends has been maximum with the increase of content and molecular weight or generation of HTDE. The impact strength and fracture toughness of the cured systems with 9 wt % second generation of HTDE are 58.2 kJ/m² and 3.20 MPa m^1/2, which are almost three and two times, respectively, of DGEBA performance. Furthermore, the tensile and flexural strength can be enhanced about 20%. The glass transition temperature and Vicat temperature, however, are found to decrease to some extent. The fracture surfaces are evaluated by using scanning electron microscopy, which showed that the homogeneous phase structure of the HTDE blends facilitates an enhanced interaction with the polymer matrix to achieve excellent toughness and strength enhancement of the cured systems, and the “protonema” phenomenon in SEM has been explained by in situ reinforcing and toughening mechanism and molecular simulation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2504–2511, 2006     

Thermal and mechanical properties of PPO filled epoxy resins compatibilized by triallylisocyanurate

A series of blends has been prepared by adding a poly(phenylene oxide) (PPO), in varying proportions, to an epoxy resin cured with dicyandiamide. All the materials show two‐phase morphology when characterized by SEM and DMA. SEM and DMA indicate that partial mixing exists in all the blends especially with high PPO content. This implies that the epoxy oligomer or low crosslinking density epoxy exists in the PPO phase after curing. The tensile strength and modulus of these blends are nearly independent of the PPO content, while the fracture toughness (G_IC) is improved by PPO. However, the two‐phase particulate morphology is not uniform. In order to improve the uniformity and miscibility, triallylisocyanurate (TAIC) has been used as an in situ compatibilizer for the polymer blends of epoxy and PPO. SEM and DMA reveal improvement of miscibility and solvent resistance for this system. The fracture toughness of these TAIC‐modified systems are also improved by adding TAIC (0–20 phr). © 2000 Society of Chemical Industry     

Polylactide toughening with epoxy‐functionalized grafted acrylonitrile–butadiene–styrene particles

Glycidyl methacrylate functionalized acrylonitrile–butadiene–styrene (ABS‐g‐GMA) particles were prepared and used to toughen polylactide (PLA). The characteristic absorption at 1728 cm^?1 of the Fourier transform infrared spectra indicated that glycidyl methacrylate (GMA) was grafted onto the polybutadiene phase of acrylonitrile–butadiene–styrene (ABS). Chemical reactions analysis indicated that compatibilization and crosslinking reactions took place simultaneously between the epoxy groups of ABS‐g‐GMA and the end carboxyl or hydroxyl groups of PLA and that the increase of GMA content improved the reaction degree. Scanning electron microscopy results showed that 1 wt % GMA was sufficient to satisfy the compatibilization and that ABS‐g‐GMA particles with 1 wt % GMA dispersed in PLA uniformly. A further increase of GMA content induced the agglomeration of ABS‐g‐GMA particles because of crosslinking reactions. Dynamic mechanical analysis testing showed that the miscibility between PLA and ABS improved with the introduction of GMA onto ABS particles because of compatibilization reactions. The storage modulus decreased for the PLA blends with increasing GMA content. The decrease in the storage modulus was due to the chemical reactions in the PLA/ABS‐g‐GMA blends, which improved the viscosity and decreased the crystallization of PLA. A notched impact strength of 540 J/m was achieved for the PLA/ABS‐g‐GMA blend with 1 wt % GMA, which was 27 times than the impact strength of pure PLA, and a further increase in the GMA content in the ABS‐g‐GMA particles was not beneficial to the toughness improvement. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synergetic effects on the mechanical and fracture properties of epoxy composites with multiscale reinforcements: Carbon nanotubes and short carbon fibers

The ball‐milling/liquid‐phase oxidation (BMLPO) method was used to fabricate surface‐modified short carbon fibers (SCFs). Multiscale epoxy composites reinforced with a combination of SCFs and multiwalled carbon nanotubes (MWNTs) were prepared. Atomic force microscopy observations and contact angle measurement were used to investigate the modification effect of the BMLPO method. Mechanical tests and scanning electron microscopy observations were used to study the effects of the SCFs, MWNTs, and their combination on tensile properties, impact strength, and fracture toughness of the epoxy composites. The results show that the surface roughness of the SCFs after BMLPO treatment increased, and the wettability of the SCFs was improved as well. The combined use of the SCFs and MWNTs had a synergetic effect on the tensile strength, fracture toughness, and impact strength of the epoxy composites. The addition of MWNTs promoted the plastic deformation of the epoxy matrix and decreased the stress‐concentration level near the SCF/matrix interface; these were considered the main causes of the synergetic effect. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43500.     

Morphology and toughness characterization of epoxy resins modified with amine and carboxyl terminated rubbers

The morphology (dispersed phase composition, size distribution, and particle/matrix interface shapes) of epoxy resins modified with 5–15 parts by weight (pbw) of carboxyl (CTBN) and amine (ATBN) terminated butadiene acrylonitrile rubber have been characterized. The characterization techniques were transmission electron microscopy coupled with energy dispersive X-ray analysis, differential scanning calorimetry and proton and ¹³C nuclear magnetic resonance. ATBN modified epoxies have a diffuse-appearing interface between the dispersed rubber phase and the epoxy matrix, in contrast to the sharp boundaries of CTBN particle interfaces. Interface mixing of epoxy and rubber, hypothesized initially to explain the interface diffuseness in ATBN modified epoxy, was not found in either CTBN or ATBN modified epoxy. The difference in interface appearance is attributed to ATBN particles having highly irregular shapes compared to the nearly spherical CTBN particles. Bimodal particle size distributions are observed with both modifiers. Both rubber modifiers also produce essentially identical toughness values which do not increase with rubber content in the range 5–15 pbw despite a commensurate increase in the population of large particles.     

Toughening of an epoxy resin with hydroxy‐terminated poly(arylene ether nitrile) with pendent tertiary butyl groups

Hydroxy‐terminated poly(arylene ether nitrile) oligomers with pendent tert‐butyl groups (PENTOH) were synthesized by the nucleophilic aromatic substitution reaction of 2,6‐dichlorobenzonitrile with tert‐butyl hydroquinone in N‐methyl‐2‐pyrrolidone medium with anhydrous potassium carbonate as a catalyst at 200°C in a nitrogen atmosphere. The PENTOH oligomers were blended with diglycidyl ether of bisphenol A epoxy resin and cured with 4,4′‐diaminodiphenyl sulfone. The curing reaction was monitored with infrared spectroscopy and differential scanning calorimetry. The morphology, fracture toughness, and thermomechanical properties of the blends were investigated. The scanning electron micrographs revealed a two‐phase morphology with a particulate structure of the PENTOH phase dispersed in the epoxy matrix, except for the epoxy resin modified with PENTOH with a number‐average molecular weight of approximately 4000. The storage modulus of the blends was higher than that of the neat epoxy resin. The crosslink density calculated from the storage modulus in the rubbery plateau region decreased with an increase in PENTOH in the blends. The fracture toughness increased more than twofold with the addition of PENTOH oligomers. The tensile strength of the blends increased marginally, whereas the flexural strength decreased marginally. The dispersed PENTOH initiated several toughening mechanisms, which improved the fracture toughness of the blends. The thermal stability of the epoxy resin was not affected by the addition of PENTOH to the epoxy resin. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Mechanical properties and fracture behaviors of epoxy composites with phase‐separation formed liquid rubber and preformed powdered rubber nanoparticles: A comparative study

Epoxy composites filled with phase‐separation formed submicron liquid rubber (LR) and preformed nanoscale powdered rubber (PR) particles were prepared at different filler loading levels. The effect of filler loading and type on the rheological properties of liquid epoxy resin suspensions and the thermal and mechanical properties of the cured composites as well as the relative fracture behaviors are systematically investigated. Almost unchanged tensile yield strength of the cured epoxy/PR composites is observed in the tensile test compared with that of the neat epoxy; while the strength of the cured epoxy/LR composites shows a maximum value at ∼4.5 wt% and significantly decreases with increasing LR content. The glass transition temperature (T_g) of the cured PR/epoxy has shifted to the higher temperature in the dynamic mechanical thermal analysis compared with that of the cured pure epoxy and epoxy/LR composites. Furthermore, the presence of LR results in highly improved critical stress intensity factor (K_IC) of epoxy resin compared with the corresponding PR nanoparticles. In particular, the PR and LR particles at 9.2 wt% loading produce about 69 and 118% improvement in K_IC of the epoxy composites, respectively. The fracture surface and damage zone analysis demonstrate that these two types of rubber particles induce different degrees of local plastic deformation of matrix initiated by their debonding/cavitation, which was also quantified and correlated with the fracture toughness of the two epoxy/rubber systems. POLYM. COMPOS., 36:785–799, 2015. © 2014 Society of Plastics Engineers     

Epoxy/clay nanocomposites: further exfoliation of newly modified clay induced by shearing force of ball milling

To induce montmorillonite (MMT) to be further exfoliated and homogeneously dispersed in epoxy matrix (diglycidyl ether of bisphenol A) curing in the presence of diaminodiphenyl sulfone and obtain improved mechanical properties, a promising new method has been developed to prepare highly reinforced epoxy/MMT nanocomposites through exerting shearing force on epoxy/MMT solution by ball milling. Modifying agents, being combined with dodecylbenzyldimethylammonium chloride and meta‐xylylenediamine, were used to organically modify the clay (MMTII). Different resultant products were characterized by Fourier‐transform infrared spectroscopy, X‐ray diffraction, transmission electron microscopy, etc, and the fracture surface of the mechanically tested specimens was also observed by scanning electronic microscopy. Results show that the novel MMTII possess a reactive interaction surface with the epoxy matrix. Exfoliation of the retained sandwich structure (being intercalated) or large agglomerates (undispersed MMTII) can be promoted by external shearing force of ball milling, and homogeneously dispersed MMTII nano‐sheets in epoxy matrix nanocomposites are typically observed. Mechanical properties, especially impact toughness, can be increasingly enhanced by the newly structured MMTII and ball milling. Impact strength is increased up to 48.1 kJ m^?2 from 32.1 kJ m^?2 at 3 wt% MMTII content, which is about 50 % higher than that of pristine matrix, and the flexural strength can also be enhanced by about 8 % higher. Copyright © 2004 Society of Chemical Industry     

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     

Structure and properties of epoxy resins modified with acrylic particles

The morphology and material properties of dicyandiamide (DICY)‐cured epoxy resins modified with acrylic particles consisting of a PBA (polybutyl acrylate) core and a PMMA (polymethyl methacrylate) shell and epoxy resins modified with acrylic rubber (PBA) particles alone were studied. It was found that the epoxy system modified with core/shell acrylic particles showed higher fracture toughness, indicating that the modification had a larger effect on improving the material properties of the epoxy resin. A characteristic shown by the core/shell acrylic particles is that they swell along with the epoxy resin under exposure to heat and gel before the latter cures. In this process, the epoxy resin penetrates the surface of the shell layer and a bond is formed between the epoxy matrix and the core/shell acrylic particles. This suggests that the epoxy matrix around the core/shell acrylic particles has the effect of increasing the level of energy absorption due to plastic deformation of the matrix. This is thought to explain why the epoxy resin modified with core/shell acrylic particles showed higher fracture toughness. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2955–2962, 1999     

The effect of chemical modification on the fracture toughness of montmorillonite clay/epoxy nanocomposites

The change in fracture toughness and its dependence on the content of clay nanoplatelets and adhesion at the interface between clay nanoplatelets and anhydride-cured epoxy matrix are discussed. Three clay nanoplatelets with different chemical modifications were used in this investigation. To fabricate nanocomposites, the clay nanoplatelets were sonicated in acetone for 2 h. The role of the clay nanoplatelets in the mechanical/fracture properties was investigated by transmission electron microscopy (TEM). Bright-field TEM micrographs showed excellent dispersion of clay nanoplatelets in epoxy matrix. Both intercalation and exfoliation of clay nanoplatelets were observed depending on clay modification. Compact tension specimens were used for fracture testing. The fracture toughness increased with increasing clay content. The fracture toughness of clay/epoxy nanocomposites varied with the clay morphology in the epoxy matrix. Different morphologies of the fracture surfaces, highly dependent on the morphology of dispersed clay nanoplatelets, were observed using environmental scanning electron microscopy (ESEM). The fracture toughness was found to be correlated with the fracture surface roughness measured by confocal laser scanning microscopy (CLSM).     

Structure-property relationships in rubber-toughened epoxies

Epoxies toughened with two reactive liquid rubbers, an epoxy-terminated butadiene acrylonitrile rubber (ETBN) and an amino-terminated butadiene acrylonitrile rubber (ATBN), were prepared and studied in terms of their structure property relationships. A two-phase structure was formed, consisting of spherical rubber particles dispersed in an epoxy matrix. A broad distribution of rubber particles was observed in all the materials with most of the particles ranging in size from 1 to 4 μm, but some particles exceeding 20 μm were also found. Impact strength, plane strain fracture toughness (K_IC), and fracture energy (G_IC) were increased, while Young's modulus and yield strength decreased slightly with increasing rubber content and volume fraction of the dispersed phase. Both G_IC and K_IC were found to increase with increasing apparent molecular weight between crosslinks and decreasing yield strength. The increased size of the plastic zone at the crack tip associated with decreasing yield strength could be the cause of the increased toughness. An ATBN-toughened system containing the greatest amount of epoxy sub-inclusion in the rubbery phase demonstrated the best fracture toughness in this series. In the present systems, rubber-enhanced shear deformation of the matrix is considered to be the major toughening mechanism. Curing conditions and the miscibility between the liquid rubber and the epoxy resin determine the phase morphology of the resulting two-phase systems. Kerner's equation successfully describes the modulus dependence on volume fraction for the two-phase epoxy materials.     

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     

Effect of blending sequence on the morphology and impact toughness of poly(ethylene terephthalate)/polycarbonate blends

The morphology of PET/PC/E‐GMA‐MA blends made by different mixing sequences was studied by transmission electron microscopy (TEM). The results suggest that migration of the E‐GMA‐MA copolymer from the PET phase to the PC phase occurred during the mixing of the (PET/E‐GMA‐MA) pre‐blend with the PC at 10% copolymer content. As a result of the migration, the E‐GMA‐MA particles are located in the PC phase rather than in the PET phase. This finding is not in agreement with the prediction made previously by others based on the possible reaction between the epoxy group of GMA and carboxyl group of PET. Core‐shell (PC/E‐GMA‐MA) particles formed in situ during blending and the size of the core‐shell particles was controlled by the blending sequence used. Mechanical properties of the ternary blends were tested at various temperatures. Although the blending sequence does not have a noticeable effect on the yield strength and modulus of the blends, it has a strong influence on the morphology formed, which determines the impact toughness. For blends made under optimum processing conditions, the brittle‐ductile transition occurred at a lower temperature and lower elastomer content. A study of the toughening mechanism suggested that the major toughening events were cavitation plus matrix shear yielding. It is postulated that the very high impact toughness found with the (PC/E‐GMA‐MA)/PET blend (at 10% E‐GMA‐MA) originated from the bimodal particle size distribution of the core‐shell particles formed in situ.     

Sequences of fracture toughness micromechanisms in PP/CaCO3 nanocomposites

Mechanical properties and fracture toughness micromechanisms of copolypropylene filled with different amount of nanometric CaCO₃ (5–15 wt %) were studied. J‐integral fracture toughness was incorporated to measure the effect of incorporation of nanoparticle into PP matrix. Crack‐tip damage zones and fracture surfaces were studied to investigate the effect of nanofiller content on fracture toughness micromechanisms. It was found that nanofiller acted as a nucleating agent and decreased the spherulite size of polypropylene significantly. J‐integral fracture toughness (J_c) of nanocomposites was improved dramatically. The J_c value increased up to approximately two times that of pure PP at 5 wt % of nano‐CaCO₃. The fracture micromechanisms varied from rubber particles cavitation and shear yielding in pure PP to simultaneous existence of rubber particles cavitation, shear yielding, filler particles debonding, and crazing in PP/CaCO₃ nanocomposites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Experimental assessment on potential for processiblity of carbon nanotubes/epoxy system used as nanocomposites

In this study, carboxylic acid functionalized carbon nanotubes (CNTs) were used to modify epoxy with intent to develop a nanocomposite matrix for hybrid multiscale composites combining benefits of nanoscale reinforcement with well‐established fibrous composites. CNTs were dispersed in epoxy by using high energy sonication, followed by the fabrication of epoxy/CNTs composites. The processibility of CNTs/epoxy systems was explored with respect to their dispersion state and viscosity. The dependences of viscosity, mechanical and thermomechanical properties of nanocomposite system on CNTs content were investigated. The dispersion quality and reagglomeration behavior of CNTs in epoxy and the capillary infiltration of continuous fiber with the epoxy/CNTs dispersion were characterized using optical microscope and capillary experiment. As compared with neat epoxy sample, the CNTs nanocomposites exhibit flexural strength of 126.5 MPa for 1 wt% CNTs content and impact strength of 28.9 kJ m^?2 for 0.1 wt% CNTs content, respectively. A CNTs loading of 0.1 wt% significantly improved the glass transition temperatures, T_g, of the nanocomposites. Scanning electron microscopy (SEM) was used to examine the fracture surface of the failed specimens. It is demonstrated that the properties of CNTs/epoxy system are dispersion‐dominated and interface sensitive. POLYM. ENG. SCI., 2013. © 2012 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