Examination of selected synthesis and room‐temperature storage parameters for wood adhesive‐type urea–formaldehyde resins by 13C‐NMR spectroscopy. V

The effects of posttreatments of particleboard adhesive‐type urea–formaldehyde resins were studied. The resins were synthesized with formaldehyde/first urea (F/U₁) mol ratios of 1.40, 1.60, 1.80, 2.10, and 2.40 and then the second urea was added to give a final formaldehyde/urea ratio of 1.15 in alkaline pH. The resins were posttreated at 60°C for up to 13.5 h and the 2.5‐h heat‐treated resin samples were stored at room temperature for up to 27 days. Resins sampled during the posttreatments were examined by ¹³C‐NMR and evaluated by bonding particleboards. In the posttreatments, hydroxymethyl groups on the polymeric resin components dissociated to formaldehyde and reacted with the second urea, and methylene and methylene–ether groups were formed from reactions involving the second urea. Methylene–diurea and urea groups bonded to UF polymers were identified. As a result, the viscosity of the resins initially decreased but later increased along with the cloudiness of the resins. Bond‐strength and formaldehyde‐emission values of particleboard varied with posttreatment variables as well as with the F/U₁ mol ratios used in the resin syntheses. The results would be useful in optimizing resin synthesis and handling parameters. Various reaction mechanisms were considered. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1896–1917, 2003     

Thin film initiation of cracks for fracture toughness measurements in epoxy resins

The first step in accurately measuring the fracture toughness of a polymeric material is to generate a sharp crack in the sample. Often this is accomplished by tapping a razor blade into the sample, allowing a natural crack to grow; however, it can be difficult to control the crack propagation. Alternate methods for initial crack generation are investigated, including scoring the sample with a razor blade and inserting thin films or foils into the samples during cure. Fluoropolymer films and aluminum and stainless steel foils of various thicknesses are examined in a number of epoxy‐amine resins with a range of toughness and glass transition values. None of the alternative methods replicate the results of starting with a natural crack. Furthermore, it is difficult to form satisfactory test samples using fluoropolymer films. For relative toughness comparisons, either the scoring method or thin, ≤25.4 μm, foils can be used to initiate cracks with similar results. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44364.     

Thermoplastic‐modified epoxy resins cured with different functionalities amine mixtures: Morphology,thermal behavior,and mechanical properties

Epoxy matrices inherent brittleness and poor crack resistance make necessary some form of toughening. In this work, to improve their fracture toughness and ductility epoxy matrices were modified by changing its architecture and by the addition of a third component. The matrices architecture were modified by stoichiometrically reacting a bifunctional epoxy resin with different functionalities amine mixtures, one of which being a monoamine that plays the role of chain extender. In the modification by the addition of a third component, poly(methyl methacrylate) (PMMA) was selected as modifier. PMMA is initially miscible with epoxy/amine systems but can phase separate during curing. The kinetics and miscibility of these systems were studied previously. At constant curing conditions, materials from completely opaque (phase separated) to transparent (miscible) can be obtained with the increase in monoamine content. In this work, the effects of the modifier content and of the monoamine : diamine ratio in stoichiometric epoxy/amine mixtures on the resultant morphologies as well as on their thermal and mechanical properties was studied.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Environmentally friendly mixed tannin/lignin wood resins

We obtained lignin‐based wood adhesives satisfying the requirements of relevant international standards for the manufacture of wood particleboard. These were based on two different low‐molecular‐mass lignins. These lignin‐based wood adhesives did not use any formaldehyde in their formulation; formaldehyde was substituted with a nonvolatile nontoxic aldehyde, namely, glyoxal. The last formaldehyde present, contributed by a fortifying synthetic phenol–formaldehyde resin, was also eliminated by the substitution of the phenol–formaldehyde resin with a natural, vegetable polyflavonoid tannin extract to which no aldehyde was added. This substitution brought the total content of natural material up to 80 wt % of the total adhesive. The adhesives yielded good internal bond strength results of the panels, enough to pass relevant international standard specifications for interior‐grade panels. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characterization and modeling of diglycidyl ether of bisphenol‐a epoxy cured with aliphatic liquid amines

The characterization by DMA and compressive stress‐strain behavior of an epoxy resin cured with a number of liquid amines is studied in this work along with predictions of the associated properties using Group Interaction Modeling (GIM). A number of different methods are used to assign two of the input parameters for GIM, and the effect on the predictions is investigated. Excellent predictions are made for the glass transition temperature, along with good predictions for the beta transition temperature and modulus for the majority of resins tested. Predictions for the compressive yield stress and strain are less accurate, due to a number of factors, but still show reasonable correlation with the experimental data. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3130–3141, 2013     

New investigation of 1‐substituted imidazole derivatives as thermal latent catalysts for epoxy‐phenolic resins

Novel 1‐substituted imidazole derivatives ( 4 – 10 ) were synthesized by imidazole and the corresponding substituted reagents (chloromethylpivalate, diphenylphosphinicchloride, di‐tert‐butyldicarbonate, 1,1′‐oxalylchloride, pyrazine, phneylisocyanat, and p‐toluensulfonylchloride). Polymerization of diglycidyl ether of bisphenol A (DGEBA) with 1‐substituted imidazole derivatives, two commercial available catalysts (imidazole and 1‐cyanoethyl‐2‐ethyl‐4‐methylimidazole) and N‐benzylpyrazinium hexafluoroantimonate were investigated as model reactions of epoxy resin systems with respect to the thermal latency and storage stability of the catalysts. The catalytic activity of 1‐substituted imidazole derivatives 4 – 10 depended on the steric and withdrawing electronic effect of the substitution groups. To characterize the cure activation energy and the viscosity‐storage time, the order of thermally latent activity is 1‐tosylimidazole ( 6 ) > 1,1′‐oxalyldiimidazole ( 8 ) > N‐benzylpyrazinium hexafluoroantimonate (BPH, 3 ) > 1‐tritylimidazole ( 9 ) > N‐phenyl‐imidazole‐1‐carboxamide ( 5 ) > 3‐(diphenylphosphinoyl)imidazole ( 7 ) > tert‐butyl‐1H‐imidazole‐1‐carboxylate ( 4 ) > 1‐cyanoethyl‐2‐ethyl‐4‐methylimidazole (2E4MZ, 2 ) > 1‐[(pivalyloxy)methyl]imidazol ( 10 ) > imidazole ( 1 ). In comparison with commercially available catalysts imidazole ( 1 ) and 1‐cyanoethyl‐2‐ethyl‐4‐methylimidazole ( 2 ) and a cationic latent catalyst N‐benzylpyrazinium hexafluoroantimonate (BPH, 3 ) as the standard compounds, in addition to 1‐[(pivalyloxy)methyl]imidazole ( 10 ), the 1‐substituted imidazole derivatives ( 4 – 9 ) revealed better thermal latency. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Curing and thermal behaviors of inorganic–organic hybrid polyarylacetylene resins with polyhedral oligomeric octa(propargylaminophenyl)silsesquioxane

Polyhedral oligomeric octa(propargylaminophenyl)silsesquioxane (OPAPS) was used to prepare composite resins with prepolyarylacetylene (prePAA). The curing and thermal behaviors of the PAA/OPAPS composites were studied through Fourier‐transform infrared (FTIR), X‐ray diffraction (XRD), differential scanning calorimetric (DSC), thermogravimetric (TGA), and scanning electron microscopic (SEM) analysis and by direct observation. The morphologies of the PAA/OPAPS resins proved that there was good compatibility between PAA and OPAPS. FTIR analysis indicated formation of a conjugated diene and aromatic ring groups in the thermal curing process of the resins. DSC analysis implied that the addition of OPAPS to prePAA could decrease the exothermic heat and widen the temperature range in the curing process of prePAA. According to TGA analysis, a 10 wt % addition of OPAPS to PAA can maintain the thermal stability of PAA in N₂ atmosphere and somewhat enhance the thermal‐oxidative stability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and properties of methyl hexahydrophthalic anhydride‐cured fluorinated epoxy resin 2,2‐bisphenol hexafluoropropane diglycidyl ether

The fluorinated epoxy resin, 2,2‐bisphenol hexafluoropropane diglycidyl ether (DGEBHF) was synthesized through a two‐step procedure, and the chemical structure was confirmed by ¹H n uclear magnetic resonance (NMR), ¹³C NMR, and Fourier transform infrared (FTIR) spectra. Moreover, DGEBHF was thermally cured with methyl hexahydrophthalic anhydride (MHHPA). The results clearly indicated that the cured DGEBHF/MHHPA exhibited higher glass transition temperature (T_g 147°C) and thermal decomposition temperature at 5% weight loss (T₅ 372°C) than those (T_g 131.2°C; T₅ 362°C) of diglycidyl ether of bisphenol A (DGEBA)/MHHPA. In addition, the incorporation of bis‐trifluoromethyl groups led to enhanced dielectric properties with lower dielectric constant (D_k 2.93) of DGEBHF/MHHPA compared with cured DGEBA resins (D_k 3.25). The cured fluorinated epoxy resin also gave lower water absorption measured in two methods relative to its nonfluorinated counterparts. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2801–2808, 2013     

Synthesis,characterization, and properties of low viscosity tetra‐functional epoxy resin N,N,N′,N′‐ tetraglycidyl‐3,3′‐diethyl‐4,4′‐diaminodiphenylmethane

Tetra‐functional epoxy resin N,N,N′,N′‐tetraglycidyl‐3,3′‐diethyl‐4,4′‐diaminodiphenylmethane (TGDEDDM) was synthesized and characterized. The viscosity of TGDEDDM at 25°C was 7.2 Pa·s, much lower than that of N,N,N′,N′‐tetraglycidyl‐4,4′‐diaminodiphenylmethane (TGDDM). DSC analysis revealed that the reactivity of TGDEDDM with curing agent 4,4′‐diamino diphenylsulfone (DDS) was significantly lower than that of TGDDM. Owing to its lower viscosity and reactivity, TGDEDDM/DDS exhibited a much wider processing temperature window compared to TGDDM/DDS. Trifluoroborane ethylamine complex (BF₃‐MEA) was used to promote the curing of TGDEDDM/DDS to achieve a full cure, and the thermal and mechanical properties of the cured TGDEDDM were investigated and compared with those of the cured TGDDM. It transpired that, due to the introduction of ethyl groups, the heat resistance and flexural strength were reduced, while the modulus was enhanced. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40009.     

Melamine formaldehyde/polyvinyl alcohol composite fiber: Structure and properties manipulated by hydroxymethylation degree

Composite fibers made of different hydroxymethylation degree (HMD) melamine formaldehyde (MF) resins and polyvinyl alcohol were prepared by wet spinning. The structures of MF resins and composite fibers were studied using Fourier transform infrared spectrometer, optical microscope, scanning electron microscope, differential scanning calorimetry, X‐ray diffraction, and Kjeldahl. The fibers' mechanical properties, fire retardant properties, softening point in hot water, and thermal stability were also tested. Results show that with the increase of HMD, the compatibility between MF and PVA gets better; the breaking strength and thermal stability of the fibers increase, while the nitrogen‐loss rate, the elongation at break, and SP decrease. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40678.     

Gallate derivatives as antioxidant additives for polypropylene

Octyl gallate (OG), dodecyl gallate (DG), and hexadecyl gallate (HG) were synthesized and characterized by Fourier transform infrared (FTIR) and NMR, and their thermal stability was measured by thermogravimetric analysis (TGA). The antioxidant effect of these derivatives in polypropylene (PP) was measured by oxidation induction temperature measurement on DSC, and by measurement of carbonyl groups in the polymer on attenuated total reflectance FTIR spectroscopy after they were aged at 120°C. The results indicated that the gallate derivatives could provide long‐term stabilization to PP under conditions of oxidative degradation. The antioxidant performance of HG with longer alkyl chain was superior to the OG and DG, making the former a good candidate to be used as antioxidant additive in PP. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39850.     

Synthesis of amine‐cured,epoxy‐layered silicate nanocomposites: The influence of the silicate surface modification on the properties

Fluorohectorites were rendered organophilic through the cation exchange of sodium intergallery cations for protonated monoamine, diamine, and triamine oligopropyleneoxides and octadecylamine, benzylamine, and adducts of octadecylamine and benzylamine with diglycidyl ether of bisphenol A (DGEBA). The influence of the silicate surface modification and compatibility on the morphology and thermal and mechanical properties was examined. Surface modification with protonated octadecylamine and its adduct with DGEBA promoted the formation of microscale domains of silicate layers separated by more than 50 Å, as evidenced by transmission electron microscopy and wide‐angle X‐ray scattering. Young's modulus of these two nanocomposites increased parabolically with the true silicate content, whereas conventionally filled composites exhibited a linear relationship. The highest fracture toughness was observed for conventionally filled composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2643–2652, 2002     

Synthesis of isocyanurate-based imidazole carboxylate as thermal latent curing accelerator for thermosetting epoxy resins

A novel thermal latent curing accelerator, 1-(2-cyanoethyl)-2-methylimidazole/tris (2-carboxyethyl) isocyanurate adduct (2MICN-T), was successfully synthesized through an acid–base neutralization of tris(2-carboxyethyl)isocyanurate (TCEIC) and 1-(2-cyanoethyl)-2-methylimidazole (2MICN). It was further added into diglycidylether of bisphenol A based epoxy resin/methylhexahydrophthalic anhydride mixture to form one-component curing systems. With the addition of 2 wt% of 2MICN-T, the one-component system could be steadily stored for more than 1 month at room temperature, while the shelf life of 2MICN curing system was only 2 days. Nonisothermal differential scanning calorimeter also demonstrated the excellent thermal latency of 2MICN-T in low-temperature region and rapid initiation of the curing reaction when raising temperature. Compared to the cured resins with original 2MICN as accelerator, the resulted thermosets exhibited enhanced glassy storage modulus, glass transition temperature, and thermal stability when 2 wt% of 2MICN-T was applied. It was attributed to the chemical incorporation of the isocyanurate moieties with multi carboxyl groups and nitrogen-contained heterocyclic ring, effectively increasing the crosslinking density, chain rigidity, and heat resistance of the cured resin. Thus, it is suggested that 2MICN-T can play both roles as latent curing accelerator and modifier for one-component epoxy compounds, and is particularly recommended for application in electronic packaging fields.     

Properties and origins of high‐performance poly(phenylene oxide)/cyanate ester resins for high‐frequency copper‐clad laminates

A high‐performance matrix is the key base for the fabrication of high‐frequency copper‐clad laminates. A high‐performance resin system based on commercial poly(phenylene oxide) (PPO) and 2,2′‐bis(4‐cyanatophenyl) isopropylidene (BADCy), coded as PPO‐n/BADCy (where n is the weight parts of PPO per 100 weight parts of BADCy), was developed. The effect of PPO on the key properties, including the dielectric and thermal properties, water resistance, and toughness, of the cured resins was investigated extensively. The results show that PPO not only catalyzed the curing reaction of BADCy but also reacted with BADCy to form a single‐phase structure. Furthermore, compared with the cured BADCy resin with 1 phr epoxy resin as a catalyst, the cured PPO‐n/BADCy resins had significantly increased impact strengths and decreased dielectric constants, loss, and water resistance. The reasons behind these desirable improvements are discussed from the view of structure–property relationships. These results suggest that the PPO‐n/BADCy system has great potential to be used as a matrix for high‐frequency copper‐clad laminates or other advanced composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Characterization of epoxies cured with bimodal blends of polyetheramines

DGEBA was cured with bimodal blends of polyetheramines as well as with single molecular weight amines while maintaining stoichiometry. Glass transition temperatures (T_gs) and moduli were measured using dynamic mechanical analysis (DMA). Fracture properties were measured using the compact tension geometry and testing was performed at both ambient and non‐ambient temperatures, investigating toughness changes as a function of temperature. For constant amine average molecular weights, the addition of high molecular weight amines caused increased glassy moduli at a constant T ? T_g and decreased densities while broadening the glass transition without changing the fracture toughness. The fracture behavior, specifically the slip‐stick to brittle transition, was affected by the broadened transitions. T_g, breadth of T_g, and total damping were found to be proportional to the volume fraction of amine in the system. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1621–1631, 2013     

Epoxide‐terminated hyperbranched polyether sulphone as triple enhancement modifier for DGEBA

Epoxide‐terminated hyperbranched polyether sulphones (EHBPESs) with different backbone structures were synthesized and used as tougheners for diglycidyl ether of bisphenol‐A (DGEBA) curing system, which result in nonphase‐separated cured networks. Effects of backbone structure (at comparable degree of polymerization) and loading contents on the mechanical and thermal properties of cured hybrids were investigated. The hybrid containing EHBPES3, which has the most flexible backbone, shows the best mechanical performance and highest glass transition temperature (T_g). Compared with unmodified system, the impact strength, tensile strength, elongation at break of the hybrid containing 5% EHBPES3 increased by 69.8%, 9.4%, and 60.2%, respectively. The balanced improvements were attributed to the increased crosslink density and fractional free volume as well as the unique inhomogeneous network structure because of incorporation of hyperbranched modifiers with proper structure and loading contents. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41910.     

Comparing reinforcement strategies for epoxy networks using reactive and non‐reactive fortifiers

Improvements in physical, mechanical, and thermal properties of an epoxy network are made with the use of a unique class of molecules that reinforce the network at the molecular scale. These molecules are commonly referred to as antiplasticizers or fortifiers. In this contribution, two types of fortifiers are incorporated into the model epoxy network. One, dimethyl methylphosphonate, is a simple additive while the other, diethyl phosphoramidate, contains a reactive amine and is cured as part of the network. The two approaches provide some unique differences in the physical and mechanical properties of the networks. Several mechanisms of fortification are discussed and correlated to the observed properties. In addition, it is shown that the fortifiers improve the rheological characteristics of the epoxy resin and act as fire‐retardants in the cured network. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4606–4615, 2006     

Electrical ac and dc behavior of epoxy nanocomposites containing graphene oxide

This article deals with the investigation of electrical properties of epoxy‐based nanocomposites containing graphene oxide nanofillers dispersed in the polymer matrix through two‐phase extraction. Broadband dielectric spectroscopy and dc electrical conductivity as a function of electric field have been evaluated in specimens containing up to 0.5 wt % of nanofiller. Nanocomposites containing pristine graphene oxide do not show significant changes of electrical properties. On the contrary, the same materials after a proper thermal treatment at 135°C, able to provoke the in situ reduction of graphene oxide, exhibit higher permittivity and electrical conductivity, without showing large decrease of breakdown voltage. Moreover, a nonlinear behavior of the electrical conductivity is observed in the range of electric fields investigated, i.e. 2–30 kV mm^?1. A new relaxation phenomenon with a very low temperature dependence is also evidenced at high frequency in reduced graphene oxide composites, likely associated to induced polarization of electrically conductive nanoparticles. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41923.     

Selection of thinner for epoxy–amine system used as binder for polymeric mortars

Epoxy resin mortars are mainly used in the implementation of precast items used in the civil engineering field. The workability of these mortars represents a major obstacle to the development of this field. In order to improve this property, different types of thinners are selected to study their effect on the rheological properties of the epoxy–amine system. Rheological tests are performed in order to estimate the resin gel point and to measure its viscosity during the crosslinking reaction. It appears that viscosity is the lowest for mixtures prepared with methyl octanoate, o‐xylene, and butyl glycidyl ether. Methyl octanoate is chosen as an ecofriendly thinner. The curing process is evaluated by varying the amount of the selected thinner using isothermal and dynamic DSC to determine the end of crosslinking. The mechanical properties using dynamic mechanical analysis (DMA) and tensile resistance testing of the cured epoxy resin with different amounts of methyl octanoate are also investigated. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43970.     

A facile way to prepare two novel DOPO‐containing liquid benzoxazines and their graphene oxide composites

The design, preparation, and properties of two DOPO‐containing compounds based on a cardanol‐allylamine‐paraformaldehyde benzoxazine (abbreviated as BZc‐a) were described in this article. By controlling the amount of DOPO, it could react with BZc‐a and generated different products. When DOPO: BZc‐a <1 : 1, DOPO reacted only with allyl group, and generated a containing single DOPO group benzoxazine (P₁B); When DOPO: BZc‐a ≧ 1 : 1, DOPO reacted not only with allyl group but also nucleophilic addition reaction, got a N‐substituted derivative P₂B, which contain double DOPO groups. The chemical structure of P₁B and P₂B was characterized by ¹H‐NMR and ¹³C‐NMR. Effect of DOPO on characteristics of BZc‐a was investigated. Fire test demonstrated that P₁B and P₂B showed better flame retardance than BZc‐a, and prepared two kinds of flame‐retardance resin based on BZc‐a. However, Field emission electron microscope observations showed the surface of P₁B and P₂B was very fragility, graphene oxide (GO) was chosen as to improve the surface performance. P₁B/GO‐3 wt % and P₂B/GO‐3 wt % composites were prepared by solution blending, and the thermal stability was studied by thermogravimetric analysis. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41634.