Impact performance of phenolic composites following thermal exposure

The notched and unnotched Izod impact properties of a series of phenolic-glass composites following thermal exposure at 180°C, 300°C, and 800°C have been investigated. Four phenolic resins; a resol, a novolac, a resol/novolac blend, and a furan-novolac/resol copolymer were used to prepare the composites. The notched and unnotched impact properties of all S-glass composites improved following thermal exposure at 180°C for times up to 28 days. The best results at 180°C were obtained for the copolymer-based composite. However, thermal exposure at 300°C for times greater than 1 day led to significant reduction in the performance of this composite. The best retention of impact properties folowing exposure at 300°C and 800°C was found for the composite made with the resol/novolac blend. The results indicate that the impact properties of phenolic composites made with modified resins, that is, a blended resol/novolac or a furan-novolac/resol copolymer resin, improve significantly. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 349–361, 1998     

Dynamic moduli for short fiber-CR composites

The dynamic moduli, E′ and E″, and tan δ for nylon–CR and PET–CR composites with unidirectional short fibers were studied as a function of temperature by using a Rheovibron. The temperature dependence of tan δ showed two dispersion peaks for nylon–CR composite. The peak at ?28°C corresponded to the main dispersion of CR and the peak at 100°C to the α-dispersion of nylon 6. For a PET-CR composite, in addition to the individual dispersion of CR and PET, a small and broad peak was observed at about 90°C. The angular dependence of E′ indicated that the short fibers assumed good orientation. The storage modulus for the composites was given by the parallel model as E′ = V_f′E_f + V_mE′_m., where E′_c, E′_f and E′_m were the storage modulus for the composite, fiber, and matrix and V_f and V_m were the volume fraction of fiber and matrix, respectively. In the transverse direction of fiber, the peak values of tan δ at ?28°C were given by the following equation; tan δ_c = tan δ_m ? δV_f, where tan δ_c and tan δ_m are the loss tangent for the composite and matrix, respectively, and α is coefficient depending on fiber type. The results indicated that a region with strong interaction was formed between fibers and CR matrix.     

Effect of hydrogen bond formation on dynamic mechanical properties of amorphous cellulose

The temperature dependence of the dynamic modulus (E′) and the mechanical loss tangent (tanδ) of amorphous cellulose prepared from cellulose triacetate by saponification was measured and compared with that of cellophane, recrystallized cellulose obtained by immersing amorphous cellulose in water, and cellulose triacetate. The E′ of amorphous cellulose decreased initially with increasing temperature and then began to increase at about 70°C with a maximum at 80°C, decreasing again at about 100°C. Another decrease in E′ was observed at 220°C accompanied by a discontinuity at 155°C. In the tan δ-versus-temperature curve, a medium peak at 60°C a shoulder peak at 146°C, and a broad peak at 200°C were observed. It was found that the transition at about 60°C was related to hydrogen bond formation by free OH groups. The transition at about 150°C was attributed to a recrystallization process by heating, and the relaxation at 200°C, to the glass transition of the polymer. The decrement in E′ observed at about 100°C was attributed to the cooperative motion of an individual pyranose ring in amorphous cellulose, juding from the E′ and tan δ assignment of other cellulose materials. The change in E′ was also measured isothermally as a function of time in the temperature range between 40°C and 80°C, where a maximum in tan δ and an increment in E′ were observed as the temperature dependence of the dynamic viscoelasticity. The change in E′ with elapsed time was analyzed kinetically, and an activation energy of 2.6 kcal/mole was calculated. This value is the expected activation energy of hydrogen bond formation.     

Thermally resistant carbazole‐modified phenol–formaldehyde resins

Phenol–formaldehyde resins were modified with carbazole in order to improve their thermal resistance. Attempts to incorporate carbazole rings into novolac and resol resins were made using three methods: (1) the addition of N‐(hydroxymethyl)carbazole (HMC) into a phenol–formaldehyde mixture, (2) the addition of carbazole into a phenol–hydroxymethyl derivative of acetone mixture, where the hydroxymethyl derivative of acetone was used as formaldehyde donor, and (3) by prolonging the time of high‐temperature reaction between phenol, carbazole and formaldehyde. The temperature and time of reaction were critical for incorporation of carbazole, which successfully led to highly temperature‐resistant carbazole‐modified novolacs for the latter procedure. When carbazole was incorporated into novolac structure at a level of 8 mol%, the thermal resistance increased by 118 °C measured as 5% mass loss temperature. Other procedures led to solids containing carbazole or HMC as physical admixtures. The obtained composites revealed variable thermal resistance effects; the carbazole‐modified resol containing 9 mol% of carbazole showed 47 °C increase of thermal resistance in comparison with non‐modified resol, measured as 5% mass loss temperature. © 2015 Society of Chemical Industry     

Dynamic mechanical analysis of fiber-reinforced phenolics

Dynamic mechanical analysis (DMA) was used to investigate the thermomechanical behavior and the effects of postcuring on a range of glass-reinforced phenolics. The materials examined were a pure resol (reinforced with S- and E-glass), a pure novolac (reinforced with S-glass), and three derivatives of the resol and/or novolac: a resol/novolac blend, a phenolic–furan graft copolymer, and a rubber-modified resol (all reinforced with S-glass). The blend and copolymer were prepared to obtain phenolic resins with improved impact strength, without degeneration of their high-temperature performance. They have a more loosely crosslinked structure compared to the pure resol or novolac. The rubber-modified resol was prepared with the intention of reducing the brittleness of the resin structure by incorporating an elastomeric phase within the resol resin matrix. It was found that the stiffness and glass transition temperature (T_g) of the materials could be increased by postcuring, which also produced a decrease in their damping capacity. Knowing that the postcure process is a function of time and temperature, a master curve was constructed that allowed prediction of the T_g of the resol/novolac blend over a broad range of postcure times and temperatures. The effect of frequency on the storage modulus of the pure resol (S-glass), copolymer, and blend was also studied from 0.01 to 100 Hz. Master curves were constructed by time–temperature superpositioning that allowed prediction of the storage modulus at times and temperatures that are not experimentally accessible. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 649–658, 1999     

New epoxy-imide resins cured with bis(hydroxyphthalimide)s

New epoxy-imide resins were synthesized using bis(hydroxyphthalimide)s (BHPIs). Among these resins, that cured with BHPI(DDS), synthesized from 4,4′-diaminodiphenylsulfone, exhibits the best thermal resistance, reaching a tan δ maximum temperature of 230°C. This resin also features a tensile lap shear adhesive strength of 320 kgf/cm² when applied to steel test pieces. The cure reaction was followed by infrared spectroscopy and dynamic mechanical analysis. The ring-opening reaction between the phenolic hydroxyl group of BHPI and the epoxy group is observed, and accelerated by a tertiary amine catalyst, triethylamine.     

Activation energy and curing behavior of resol‐ and novolac‐type phenolic resins by differential scanning calorimetry and thermogravimetric analysis

The thermal behavior, thermal degradation kinetics, and pyrolysis of resol and novolac phenolic resins with different curing conditions, as a function of the formaldehyde/phenol (F/P) molar ratio (1.3, 1.9, and 2.5 for the resol resins and 0.5, 0.7, and 0.9 for the novolac resins) were investigated. The activation energy of the thermal reaction was studied with differential scanning calorimetry at five different heating rates (2, 5, 10, 20, and 40°C/min) between 50 and 300°C. The activation energy of the thermal decomposition was investigated with thermogravimetric analysis at five different heating rates (2, 5, 10, 20, and 40°C/min) from 30 to 800°C. The low molar ratio resins exhibited a higher activation energy than the high molar ratio resins in the curing process. This meant that less heat was needed to cure the high molar ratio resins. Therefore, the higher the molar ratio was, the lower the activation energy was of the reaction. As the thermal decomposition of the resol resins proceeded, the activation energy sharply decreased at first and then remained almost constant. The activation energy of the thermal decomposition for novolac resins with F/P = 0.5 or F/P = 0.7 was almost identical in all regions, whereas that for novolac resins with F/P = 0.9 gradually decreased as the reaction proceeded. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2589–2596, 2003     

Structural effect of phenalkamines on adhesive viscoelastic and thermal properties of epoxy networks

Phenalkamine, the Mannich reaction products from cardanol, formaldehyde, and polyamines were prepared using ethylene diamine, diethylene triamine and triethlene tetraamine. These products were characterized by high‐pressure liquid chromatography (HPLC), infrared spectroscopy, and nuclear magnetic resonance spectroscopy (1H NMR). Clearly resolved peaks due to presence of triene, diene, monoene, and saturated side chain containing species of cardanol were observed in HPLC. The presence of characteristic methylene linkages of Mannich bases at δ 3.5–4.0 ppm was observed by 1H NMR. These curing agents were reacted with diglycidyl ether of bisphenol‐A at room temperature and the curing times were optimized. The cured resins showed good adhesion with different metal surfaces particularly higher values were observed with copper due to its high surface energy. The viscoelastic properties of the cured samples were determined by dynamic mechanical thermal analysis. The storage modulus (E′) was found to be in the order of 10⁹ Pa and tan δ values are around 90°C. A reduction in storage modulus (E′) and an increase in tan δ values on postcuring were observed. Thermogravimetry analysis showed two‐stage degradation above 250°C for the cured samples. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4741–4748, 2006     

Thermal and electrical properties of glass fiber reinforced o‐cresol novolac epoxy composites cured with allylated phenol novolac hardener

An allyl group was introduced into a phenol novolac hardener to improve the thermal and electrical properties of glass fiber reinforced o‐cresol novolac epoxy composites. Based on ¹H NMR analysis, it was found that the degree of allylation was measured as 12.5% and 15.3% of allylated moieties underwent Claisen rearrangement. Thermal and electrical properties of the prepared composites were investigated by thermal mechanical analysis, dynamic mechanical analysis and dielectric measurements. Their thermal expansion coefficient and glass transition temperature measured by thermal mechanical analysis varied from 20.1 to 18.0 ppm K^?1 and 132.5 to 170.9 °C, respectively, due to allylation of the phenol novolac hardener. From the storage modulus at the rubbery plateau, it was confirmed that these improvements were mainly caused by an increase in the crosslink density of the matrix resin owing to allyl groups. The electrical properties of the composites, however, showed no noticeable changes by allylation of the phenol novolac hardener. Copyright © 2012 Society of Chemical Industry     

Transition magnitudes and impact improvement in rubber-modified plastics

Impact strength at room temperature and dynamic mechanical properties over a temperature range have been studied for a number of rubber reinforced glassy state plastics. The rubber phases in every case were butadiene copolymers of known composition and particle size and selected for their good dispersion after blending into the various matrices. This dispersion was checked by electron microscopy and the in situ particle size evaluated. The matrices were based on homo- and co-polymers of styrene, methyl methacrylate and acrylonitrile. A vibrating reed apparatus was employed to measure the storage component of Young's modulus (E′) and loss factor (tan δ) at essentially constant frequency (～300 Hz) through the rubber relaxation region. The Izod impact strength was measured in accordance with the standard method ASTM D-256. A gross parallel was found between impact strength and transition magnitude as measured by the change in modulus between ?100°C and 20°C (ΔE′) or the tan δ peak area with, for example, increasing volume fraction of rubber phase. However, when the same rubber was dispersed in different matrices a more subtle effect was an inverse proportionality of tan δ area with E′ measured at the peak temperature. Conversely ΔE′ after correction for matrix modulus change was shown both theoretically and experimentally to be directly proportional to E′ of the matrix at room temperature. The impact strength actually increases with ΔE′ and not with tan δ area in these cases. However, a more important requirement for good impact is compatibility between the rubber and matrix, but neither ΔE′ nor tan δ reflect this. After correction of tan δ areas to constant matrix modulus there remains an increase of area with particle size. Impact strength also increases strongly with particle size for compatible systems. The applicability of Hashin's central equation and Mackenzie's equation in describing the systems is discussed.     

Influence of thermal and mechanical histories on the viscoelastic behavior of drawn polyethylene

Storage and loss elasticity complex moduli E′ and E″ and temperatures at which the α relaxation takes place are studied with respect to thermal history, deformation speed, and molecular weight distribution of drawn linear polyethylene. Maximum values of E′ and E″ increase with draw ratio of the hot-drawn samples, and the α relaxation temperatures increase by around 10°C when the polyethylene filaments are annealed at 110°C. The activation energy of the process, considered as a single one because the symmetrical shape of the maxima, increases with draw ratio, and this increase is less pronounced when the filaments are annealed. Annealing of the filaments produces a decrease in their E′ values, but this decrease is almost negligible for filaments obtained from polyethylene with a broad molecular weight distribution. The final crystallinity of the filaments drawn at room temperature and subsequently annealed is higher for the filaments obtained at lower drawing speed.     

Effects of hygrothermal aging on the thermal–mechanical properties of vinylester resin and its pultruded carbon fiber composites

Hygrothermal aging was carried out on vinyl ester (VE) resin cast and its pultruded carbon fiber reinforced composite (CF/VE) by immersing them in distilled water at 65 and 95°C. Hygrothermal aging effects on the samples were studied in terms of thermal–mechanical properties, as well as moisture absorption behavior, interfacial adhesion, and transverse mechanical properties. Moisture absorption behaviors of the VE casts and the CF/VE composites were characterized as Fickian behavior. Dynamic mechanical thermal analysis (DMTA) tests showed that the tan δ peak temperatures of the VE casts and CF/VE composites decreased with immersion time at 65 and 95°C. Moreover, there existed a splitting in the tan δ peaks at 95°C, which was reversible and could be recovered by dehydration. Three‐point flexural test indicated that flexural strengths of both the VE casts and the composites decreased by hygrothermal aging with a trend related to their moisture absorption behaviors, while flexural modulus of the composites was less affected. The ILSS of the CF/VE composites was also depressed by deterioration in interfacial adhesion, which was proved by the interfacial adhesion parameters, A and α. POLYM. COMPOS., 2009. © 2009 Society of Plastics Engineers     

Phenolic‐epoxy matrix curable by click chemistry—synthesis,curing, and syntactic foam composite properties

Alkyne functional phenolic resin was cured by azide functional epoxy resins making use of alkyne‐azide click reaction. For this, propargylated novolac (PN) was reacted with bisphenol A bisazide (BABA) and azido hydroxy propyloxy novolac (AHPN) leading to triazole‐linked phenolic‐epoxy networks. The click cure reaction was initiated at 40–65°C in presence of Cu₂I₂. Glass transition temperature (Tg) of the cured networks varied from 70°C to 75°C in the case of BABA‐PN and 75°C to 80°C in the case of AHPN‐PN. DSC and rheological studies revealed a single stage curing pattern for both the systems. The cured BABA‐PN and AHPN‐PN blends showed mass loss above 300°C because of decomposition of the triazole rings and the novolac backbone. Silica fiber‐reinforced syntactic foam composites derived from these resins possessed comparable mechanical properties and superior impact resistance vis‐a‐vis their phenolic resin analogues. The mechanical properties could be tuned by regulating the reactant stoichiometry. These low temperature addition curable resins are suited for light weight polymer composite for related applications. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41254.     

Some structure-property relationships in polymer flammability: Studies of phenolic-derived polymers

The investigation of the thermal degradation of the char-formaing phenol–formaldehyde resins is conducted to provide information for the systematic design of high temperature flame-resistant phenolics. Three different processes of curing are used: (1) Formaldehyde or s-trioxane is reacted with m-substituted phenol–formaldehyde oligomers under acidic conditions to give the methylene bridged-novolac resins. (2) Phenol and m-substituted phenols are reacted with CH₂O under basic conditions and then heated to give the methylene bridged resol resins. (3) p-Terephthaloyl chloride and m-and p-substituted novolac oligomers are reacted to give cured resins with ester linkages. The evaluation of the effect of various substituents indicates that the oxygen index (OI) increases from about 33 for unsubstituted phenolics to about 75 for meta-halogen substituted phenolics. The evaluation of the effect of various crosslinking agents shows that the OI for CH₂O-cured phenolics is 75 as compared to 50 for the trioxane cured phenolics and to 40 for the terephthaloyl chloride cured phenolics. A set of phenolic copolymers with different weight percentage content of halogen substituted phenols are synthesized as novolacs and resols. The results surprisingly indicate no increase of OI for the cured novolac copolymers, whereas the increase is observed for the cured resol copolymers. The activation energy for the thermooxidative degradation of the cured novolacs is about 12–15 kJ/mol lower as compared to that of the curd resols.     

Effect of the filler size and content on the thermomechanical properties of particulate aluminum nitride filled epoxy composites

Polymer matrix composites based on brominated epoxy as the matrix and aluminum nitride (AlN) particles as the filler were prepared. The influences of the size, content, and size distribution of AlN on the thermomechanical properties, including the glass‐transition temperature (T_g), coefficient of thermal expansion (CTE), dynamic storage modulus (E′), dynamic loss modulus (E″), and loss factor (tan δ), of the composites were investigated by thermomechanical analysis and dynamic mechanical analysis. There was a total change trend for T_g; that is, T_g of the composites containing nano‐aluminum nitride (nano‐AlN; 50 nm) was lower than that of the micro‐aluminum nitride (micro‐AlN; 2.3 μm) filled composites, especially at high nano‐AlN contents. The T_g depression of the composites containing nano‐AlN was related to the aggregation of nano‐AlN and voids in the composites. On the other hand, the crosslink density of the epoxy matrix decreased for nano‐AlN‐filled composites, which also resulted in a T_g depression. The results also show that E′ and E″ increased, whereas tan δ and CTE of the composites decreased, with increasing the AlN content or increasing nano‐AlN fraction at the same AlN content. These results indicate that increasing the interfacial areas between AlN and the epoxy matrix effectively enhanced the dynamic modulus and decreased CTE. In addition, at a fixed AlN content of 10 wt %, a low E′ of pre‐T_g (before T_g temperature) and high T_g were observed at the smaller weight ratio of nano‐AlN when combinations of nano‐AlN plus micro‐AlN were used as the filler. This may have been related to the best packing efficiency at that weight ratio when the bimodal filler was used. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Characterization of modulus and glass transition phenomena in poly(L‐lactide)/hydroxyapatite composites

In order to study the dynamic‐mechanical properties of Poly(L‐lactide)/Hydroxyapatite (PLLA/HA) composites, two different molecular weight (inherent viscosity (η_inh): 4.0 (dL/g), and 7.8 (dL/g)) poly(L‐lactide) (PLLA) were synthesized by bulk polymerization and filled with 10%, 30%, and 50% (w/w) with medical grade HA (size range: 25–45 μm and Ca/P = 1.69). The plain PLLA polymers and PLLA/HA composites were compression molded and machined to yield 50 × 3 × 2 mm³ specimens. The composites were investigated by dynamic mechanical thermal analyzer (DMTA) of imposed bending load on rectangular specimens over a temperature range from 30 to 120°C using multiple frequencies (0.3–50 Hz). The results showed that the bending storage modulus (E′) of the composites increased linearly with the percentage of the filler, reaching at 37°C and 0.1 Hz about 2.5, 3.7 and 5.0 GPa with 10, 30 and 50% of HA respectively. The glass transition temperature, evaluated at the tan δ peaks, were in the range 70–80°C and 50–70°C for PLLA matrix and PLLA composites respectively. The activation energies at the glass transition temperature were calculated from the Arrhenius plot in the range of 102–111 Kcal/mol for the composites, whereas 132 and 148 Kcal/mol were found for low and high molecular weight of PLLA respectively. The content of amorphous phase was evaluated from the intensity of tan δ peak. Results showed that HA causes an amorphous phase with a greater mobility with respect to the pure PLLA.     

STYRENE–BUTADIENE RUBBER/EPOXIDIZED NATURAL RUBBER BLENDS: THE EFFECT OF VULCANIZATION SYSTEM ON BLENDS PROPERTIES

The dynamic properties, curing characteristics, and swelling behavior of the blends cured using two types of vulcanization systems [i.e., conventional vulcanization (CV) and semiefficient vulcanization (semi-EV)] were investigated. Results indicate that the maximum elastic torque (S′ @MH) and the torque difference, S′ @MH–S′ @ML (maximum elastic torque minus minimum elastic torque) increased with the increasing epoxidized natural rubber (ENR) composition in the blend. However, a reverse trend was observed for tan δ @MH and viscous torque (S″ @MH). At a similar blend ratio, the CV system gives a higher S′ @MH and torque difference but a lower tan δ @MH and S″ @MH than the semi-EV system. The scorch time, t ₂ and cure time, t ₉₀, decreased with increasing ENR composition in the blends. Semi-EV system blends exhibit shorter t ₂ and t ₉₀ than CV system blends. The swelling degree decreased with increasing ENR composition in the blends and the CV system blends show better oil-swelling resistance.     

Dynamic mechanical analysis study of the curing of phenol‐formaldehyde novolac resins

The curing reaction of typical commercial phenol‐formaldehyde novolac resins with hexamethylentetraamine (HMTA) was followed by dynamic mechanical analysis. The evolution of the rheological parameters, such as storage modulus G′, loss modulus G″, and tanδ (G″/G′), as a function of time, for samples of the phenolic resins on cloth, was recorded. The curing reaction, leading to the formation of a crosslinked structure, is described by a third‐order phenomenological equation. This equation takes into account a self‐acceleration effect, as a consequence not only of the chemical reaction of crosslinking after the gel point but of phase segregation as well. This rheokinetic model of the curing of phenolic novolac resins permits the determination of the numerical values of the kinetic equation constants. The influence of the composition, structure, and physical treatment on the curing kinetics of the novolac resins is evaluated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 1902–1913, 2001     

Preparation and properties of halogen-free flame retardant epoxy resins with phosphorus-containing siloxanes

Novel epoxy resin modifiers, DOPO–TMDS and DOPO–DMDP were synthesized by addition reaction of divinylsiloxane with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Halogen-free flame retardant epoxy resins were obtained through modification of o-cresol novolac epoxy resin cured by phenol novolac resin using DOPO–TMDS and DOPO–DMDP which were characterized by ¹H NMR, ¹³C NMR, ³¹P NMR and FT-IR measurements. Effects of the phosphorus-containing siloxanes on thermal stabilities, mechanical properties and flame retardant properties of the epoxy resins were investigated. The cured epoxy resins exhibited better mechanical properties and greatly improved flame retardant properties due to the presence of phosphorus-containing siloxanes. The cured epoxy resins with phosphorus loading of 2.0 wt% showed LOI values of 32–33 and achieved UL94V-0 ratings.     

Effect of crosslink structures on dynamic mechanical properties of natural rubber vulcanizates under different aging conditions

The effects of crosslink structures on the dynamic mechanical properties (DMPs) of unfilled and carbon black N330‐filled natural rubber (NR) vulcanizates cured with conventional (CV), semiefficient (SEV), and efficient (EV) cure systems and having about the same total crosslink densities were investigated before and after aerobic and anaerobic aging at 100°C. The three unfilled NR vulcanizates cured with the CV, SEV, and EV systems had about the same mechanical loss factor (tan δ) values at about 0°C but showed some apparent differences in the tan δ values in the order EV > SEV > CV at relatively high temperatures of 40–80°C before aging. However, N330‐filled NR vulcanizates gave higher tan δ values than the unfilled vulcanizates and showed little effect of the crosslink types on the tan δ at different temperatures over the glass‐transition temperature (T_g) before aging. Aerobic heat aging increased the T_g and tan δ values of the vulcanizates over a wide range of temperatures from ?80 to 90°C that was mainly due to the changes in the total density and types of crosslinks. The unfilled vulcanizates cured with the CV system showed the greatest change in DMP because of their poor resistance to heat aging. Aerobic heat aging of NR vulcanizates caused a more significant change in the DMP than anaerobic heat aging because of the dominant effect of the oxidative degradation during aerobic heat aging on the main‐chain structure, crosslink structures, and DMPs of the vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 710–718, 2001