Study of the curing process giving the rigid polyurethane foam by dynamic viscoelastic method

Dynamic viscoelastic properties of the rigid polyurethane foam, which is used in Japan as a thermal insulator for refrigerators were studied during the curing process using the RDS-7700 Dynamic Spectrometer. The curing process, in which large volume expansion and heat evolution occurred, was easily analyzed by this method. The apparent storage modulus (G′_app), the apparent loss modulus (G″_app), and the apparent loss tangent (tan δ_app) were measured as a function of cure time. The effects of the curing temperature and the amine catalyst on the curing process were investigated. It was found that the curing process proceeded through three stages: logarithm of G″_app (log G″_app) increased with increasing cure time in the first stage, log G″_app increased with increasing logarithm of cure time in the second, and the change of G″_app cannot be expressed simply in the final. Curing behavior can be estimated from changes of rheological parameters during the curing. The fluidity of reaction mixture can be also predicted from the peak time of tan δ_app.     

Comparison of thermal techniques for glass transition measurements of polystyrene and cross-linked acrylic polyurethane films

There are few quantitative comparisons in the literature between glass transitions (T_g) measured by differential scanning calorimetry (DSC) and by dynamic mechanical analysis (DMA). Also, in the case of DMA, two different operational definitions have been used to obtain the glass transition, namely, the loss modulus (E″) and damping (tan δ) peak temperatures. We propose a new DMA definition of T_g and demonstrate that it agrees with DSC T_g measurements within ±2°C for both thermoplastic polystyrene and thermoset cross-linked acrylic polyurethane films with measurable tan δ peaks. The glass transitions for a single polystyrene standard and several cross-linked acrylic polyurethane films were measured by DSC. Additionally, E″ and tan δ peak temperatures were measured by DMA as a function of frequency and temperature. Empirically, it was determined that the average of the E″ and tan δ peak temperatures measured at 1 rad/s oscillation frequency corresponds to the glass transition measured by the ASTM E1356 DSC test method. © 1994 John Wiley & Sons, Inc.     

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     

A dynamic mechanical analysis method for predicting the curing behavior of phenol-formaldehyde resin adhesive

In this work, filter paper was proven to be suitable as the substrate for the preparation of dynamic mechanical analysis (DMA) testing specimens to predict the curing behavior of phenol-formaldehyde (PF) resin adhesives for its stability during the curing temperature span. With this method, the curing behavior of PF resin was monitored by DMA in tensile-torsion mode. With the strain curves, the onset of curing temperature of PF resin could be determined clearly. The curing degree of PF resin could be calculated by the integral area in strain curves. The method to combine storage modulus (G′), tan δ, and strain curves together could explain the curing behavior of PF resin more comprehensively than the commonly used method using only G′ and tan δ curves. The DMA test results of PF resin with different viscosity and with accelerator implied the reliability of this novelty method.     

Viscoelastic properties of elastic polyurethaneureas in the curing process

Dynamic viscoelastic properties of a system, NCO-terminated polyurethane–aniline–cresol–formaldehyde liquid resin were studied during the curing process using an R-17 Weissenberg Rheogoniometer. For gel time determinations a new method was developed, the gel time being then difined as the cure time when the relaxation time τ = 3 s at a frequency of 0.1 Hz was achieved. The dynamic storage modulus G′, the dynamic viscosity η´ and the relaxation time τ have been determined as functions of curing time. The effects of cure temperature and of curing agent concentration on G′, η′, and τ were studied. The influence of cure temperature on the gel time and on the visco-elastic properties determined at 18ks (5 h) of curing was discussed. Based on the experimental results, it was concluded that curing at higher temperature led to the more crosslinked polymer. Activation energy values for the gel time were also determined and compared with the values for polyurethanes and polyurethaneureas reported elsewhere. A relationship between the relaxation time and the effect of frequency on modulus G′ was presented, and it appeared to be independent of cure temperature, curing agent concentration, and curing time, being then presumably a useful method for comparative studies of viscoelastic behavior of different materials in curing process. This relationship was compared with the approximate relationship between tan δ and d In G′/d In ω given by Staverman and Schwarzl. The agreement at shorter relaxation times and some discrepancy at longer times were obtained.     

Preparation of high‐performance damping materials based on carboxylated nitrile rubber: Combination of organic hybridization and fiber reinforcement

The dynamic mechanical properties denoted by storage modulus (E′) and loss factor (tan δ) of binary and ternary systems consisting of carboxylated nitrile rubber (XNBR) filled with organic hindered phenol compound 2,2′‐methylenebis(6‐tert‐butyl‐4‐methylphenol) (AO‐2246) or/and short carbon fiber (SCF) were investigated. DMA results of binary XNBR/AO‐2246 system showed that by addition of AO‐2246, the tan δ peak maximum of XNBR was remarkably increased up to 3.5, and its peak position was also significantly shifted to room temperature, demonstrating that XNBR/AO‐2246 composite is a promising damping material. Nevertheless, application of such XNBR/AO‐2246 composite is limited due to its relatively low E′ value above glass transition temperature. Therefore, to develop a high‐performance damping material with high tan δ peak and high modulus as well as controllable tan δ peak position, the combination of organic hybridization and fiber reinforcement were adopted. DMA analysis of various ternary XNBR/AO‐2246/SCF systems revealed that by introduction of SCF, the E′ value of XNBR/AO‐2246 was increased remarkably while the tan δ peak maximum was still higher than 2.5. Thus, a new type of XNBR‐based high‐performance damping material was developed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

A new method to predict optimum cure time of rubber compound using dynamic mechanical analysis

The degree of vulcanization of a rubber compound has a big influence on the properties of the final product. Therefore, precisely defining the curing process including optimum cure time is important to ensure the production of final products having high performance. Typically, vulcanization is represented using vulcanization curves. The main types of equipment used for producing vulcanization curves are the oscillating disc rheometer (ODR) and the moving die rheometer (MDR). These can be used to plot graphs of torque versus time at a constant temperature to show how cure is proceeding. Based on the results obtained, optimum cure time (t₉₀) is calculated as the time required for the torque to reach 90% of the maximum achievable torque. In this study, the use of Dynamic Mechanical Analysis (DMA) for assessment of t₉₀ was assessed. DMA was carried out using shear mode isothermal tests to measure the changes in material properties caused by vulcanization. The results revealed that the shear storage modulus (G′), shear loss modulus (G′′) , and tan δ all reflect the vulcanization process, however, tan δ gave the best representation of level of vulcanization. Indeed, the curve of tan δ was able to be used to derive the t₉₀ for rubber compounds and showed good agreement with the results from an MDR. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 40008.     

Frequency and Temperature Dependence of Dielectric Properties of Au/Polyvinyl Alcohol (Co,Ni-Doped)/n-Si Schottky Diodes

The dielectric properties and AC conductivity of Au/polyvinyl alcohol (Co, Ni-doped)/n-Si Schottky diodes (SDs) were investigated in the frequency range 1 kHz–1 MHz and in the temperature range 80–400 K. The frequency and temperature dependence of dielectric constant (?′), dielectric loss (?″), loss tangent (tan δ), AC electrical conductivity (σ _ac ) and the real and imaginary parts of the electric modulus (M′ and M″) were found to be a strong function of frequency and temperature. The values of ?′, ?″ and tan δ decrease with increasing frequency, while they increase with increasing temperature, especially above 275 K. The values of σ _ac increase with both increasing frequency and temperature. Such temperature-related behavior of σ _ac can be attributed to the high mobility of free charges at high temperature. Electric modulus formalism was also analyzed to obtain experimental dielectric data. The values of M′ and M″ increase with increasing frequency, while they decrease with increasing temperature. The interfacial polarization, which more easily occurs at low frequencies and high temperatures, consequently contributes to the improvement of the dielectric properties of SDs.     

Dynamic mechanical analysis of polypropylene composites based on surface-treated sepiolite

The dynamic mechanical properties in terms of the storage modulus G′, loss modulus G″, and loss tangent δ have been studied for polypropylene-sepiolite composites. The effect of surface treatment on the relaxation spectra has been elucidated, and quantitative values indicating the extent of polymer-filler interaction have been given. Analysis of the results show that the addition of sepiolite leads to an increase in the values of the modulus, a value of the glass transition temperature lower than that of unfilled polypropylene, and displacement towards higher temperatures of the α_c relaxation associated with the crystalline regions. Surface treatment of the sepiolite, by esterification with organic acids of different chain length, does not have a marked effect on the relaxation spectra of the composites, even though the polymer-filler interaction parameter decreases with respect to untreated sepiolite.     

Short jute fiber‐reinforced polypropylene composites: Dynamic mechanical study

Short jute fiber‐reinforced polypropylene (PP) composites were prepared using a high‐speed thermokinetic mixer. A compatibilizer was used to improve the molecular interaction between jute and PP. Both the percent weight fraction of the jute fiber and compatibilizer were varied to study the dynamic mechanical thermal (DMT) properties. Dynamic parameters such as storage flexural modulus (E′), loss flexural modulus (E″), storage shear modulus (G′), loss shear modulus (G″), and loss factor or damping efficiency (tan δ) were determined in a resonant frequency mode. The transition peak nature, amplitude, and temperature of E′, E″, G′, G″, and tan δ of different compositions were shown to indicate possible improvements of molecular interaction in the presence of a compatibilizer. The modulus retention term, a plot of the reduced modulus with the weight fraction of the jute fiber, also indicate its improvement. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 531–539, 1999     

Rheological behavior of partially hydrolyzed poly(vinyl acetate-co-ethylene)

The rheological behavior of partially hydrolyzed poly(vinyl acetate-co-ethylene) (VAE) was investigated, using a Rheometrics Dynamic Mechanical Spectrometer. For the investigation, measurements of storage modulus (G′), loss modulus (G″) and loss tangent (tan δ) of the materials were recorded as functions of temperature at a fixed frequency (i.e., using temperature scans) and, also, as functions of frequency at a fixed temperature (i.e., using frequency scans). For the hydrolysis, three different grades of copolymer (containing 34, 47, 62 mol% of vinyl acetate) were used to yield poly(vinyl acetate-co-ethylene-co-vinyl alcohol) (VAEOH) with varying amounts of hydroxyl group. For comparison purposes, the viscoelastic properties of blends of VAE copolymer with poly(vinyl alcohol) (PVOH), having varying blend compositions, were also measured. The blends were prepared by first mixing a solution of VAE copolymer, which was dissolved in a toluene/dichloroethane mixed solvent, and an aqueous solution of PVOH, and then freeze drying the mixture to a constant weight. Also measured were the glass transition temperature (T_g) of the VAE copolymers and the T_g and melting point (T_m) of the VAEOH terpolymers, using a DuPont Thermal Analyzer equipped with a 910 DSC Module. It has been found that (1) introduction of hydroxyl group into the backbone of the amorphous VAE copolymer has made the resulting VAEOH terpolymer semicrystalline; (2) the T_m, T_g, G′, and the complex viscosity (η^*) of the VAEOH terpolymers increase with increasing amounts of hydroxyl group; (3) the physical blending of PVOH with VAE copolymer has not affected the T_g of the VAE copolymer; (4) the G′ and η^* of the VAE/PVOH blends are found to increase with the amount of PVOH in the blend. The use of logarithmic plots of G′ versus G″ has been found to be very useful for discerning the differences in the structure of the materials tested.     

DMA analysis of the damping of ethylene–vinyl acetate/acrylonitrile butadiene rubber blends

The mechanical and damping properties of blends of ethylene–vinyl acetate rubber (VA content > 40% wt) (EVM)/acrylonitrile butadiene rubber (NBR), with 1.4 phr BIPB [bis (tert‐butyl peroxy isopropyl) benzene] as curing agent, were investigated by DMA and DSC. The effect of chlorinated polyvinyl chloride (CPVC), silica, carbon black, and phenolic resin (PF) as a substitute curing agent, on the damping and mechanical properties of EVM/NBR blends were studied. The results showed that 10 phr CPVC did not contribute to the damping of EVM700/NBR blends; Silica could dramatically improve the damping of EVM700/NBR blends because of the formation of bound rubber between EVM700/NBR and silica, which appeared as a shoulder tan δ peak between 20 and 70°C proved by DMA and DSC. This shoulder tan δ peak increased as the increase of the content of EVM in EVM/NBR blends. The tensile strength, modulus at 100% and tear strength of the blend with SiO₂ increased while the elongation at break and hardness decreased comparing with the blend with CB. PF, partly replacing BIPB as the curing agent, could significantly improve the damping of EVM700/NBR to have an effective damping temperature range of over 100°C and reasonable mechanical properties. Among EVM600, EVM700, and EVM800/NBR/silica blend system, EVM800/NBR/silica blend had the best damping properties. The EVM700/NBR = 80/10 blend had a better damping property than EVM700/NBR = 70/20. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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     

Dielectric constant and loss investigations of pristine,iodine‐doped and annealed polyetherimide film

Temperature‐ and frequency‐dependent dielectric behaviour has been investigated for pristine, iodine‐doped and annealed polyetherimide (PEI) in the temperature range 303–523 K at various frequencies (120 Hz, 1 kHz, 10 kHz and 100 kHz). In pristine PEI, the rapid decrease in the dielectric constant (ε′) in the temperature range 303–373 K is governed by the Kirkwood model whereas the gradual decrease in ε′ in the temperature range 373–473 K is governed by β‐relaxation (dipolar) and αβ‐relaxation (hybrid) processes. The occurrence of these relaxations is confirmed by the appearance of tan δ loss peaks at ca 403 and 443 K, respectively, in ε″–T curves. The temperature‐independence of ε′ in the temperature range 473–523 K is mainly governed by the α‐relaxation process associated with large segmental groups. The tan δ loss peak appearing at 503 K confirms this relaxation process. In iodine‐doped samples, an overall increase in ε′ is attributed to the formation of charge‐transfer complexes in the polymer structure. The dominance of a new relaxation process due to interaction of iodine with ether linkages neutralizes the 1/T rule in the low‐temperature region. A significant decrease in ε′ in annealed samples below 393 K is due to the suppression of the dipolar relaxation process. The enhancement in ε′ above 393 K is due to the dominance of the α‐relaxation process. The shift in the high‐temperature tan δ loss peak towards higher temperature with increasing frequency shows the distributive nature of relaxation time for this relaxation process. Copyright © 2011 Society of Chemical Industry     

Synthesis and characterization of acrylamide‐based aluminum flocculant for turbidity reduction in wastewater

Polymeric flocculants of aluminum hydroxide‐poly[acrylamide‐co‐(acrylic acid)], AHAMAA, were prepared by solution polymerization using aluminum hydroxide as a coagulant in the presence of acrylamide (AM) and acrylic acid (AA) as a comonomer pair with N,N′‐methylenebisacrylamide as a crosslinking agent. The crosslinking was initiated by ammonium persulfate with N,N,N′,N′‐tetramethylethylenediamine as an initiator. The water absorbency of crosslinked poly[AM‐co‐AA] was always higher than that of AHAMAA and was found to be correlated to the storage modulus of the polymers, which was higher for AHAMAA than that of crosslinked poly[AM‐co‐AA]. The residual aluminum concentration of AHAMAA (0.09–0.2 mg L^?1) indicated the stability of the polymer flocculant which was in good agreement with the observed tan δ and the higher G′′ and G′ values. Both the crosslinked poly[AM‐co‐AA] and AHAMAA satisfactorily reduced the turbidity of kaolin suspensions, but the latter gave a better reduction performance. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Supramolecular composites of photo‐cured acrylated castor oil and fibers formed by the self‐assembly of low molecular weight organic gelators

A mixture of acrylated castor oil (ACO) with N‐carbobenzyloxy‐L ‐isoleucylaminooctadecane (CIA) or (R)‐12‐hydroxystrearic acid (HSA) containing photo‐initiators was heated to 130°C and gradually cooled to room temperature to give bio‐based gelatinous material. The photo‐curing of the gel generated a crosslinked ACO composite containing CIA or HSA. The optical micrographic and differential scanning calorimetry analyses revealed that a supramolecular fibrous network is formed for photo‐cured ACO (cACO)/CIA and that spherulitic crystals are formed for cACO/HSA. The tan δ peak temperatures measured by dynamic mechanical analysis for cACO/CIA and cACO/HSA composites were higher than that of cACO. The flexural strength and modulus of cACO/CIA composites increased with an increase of CIA content, and those values were higher than those of cACO/HSA composites. POLYM. COMPOS., 33:2001–2008, 2012. © 2012 Society of Plastics Engineers     

Compatibilization and characterization of blends of styrene–maleic anhydride copolymer with modified polyethylenes

Potentially reactive blends of styrene–maleic anhydride (SMAH) with ethylene/methyl acrylate/glycidyl methacrylate (E‐MA‐GMA) and nonreactive blends of SMAH with ethylene/methyl acrylate (E‐MA) were produced in a Brabender batch mixer and in a corotating twin‐screw extruder. The products were characterized in terms of rheology, morphology, and mechanical properties to understand the reaction characteristics between anhydride/epoxy functional groups. Storage modulus, G′, loss modulus, G″ and complex viscosity, η* of the reactive blends were higher than those of nonreactive ones. At 25% E‐MA‐GMA content, maximum in η* was obtained for the reactive blends. The reactive blends showed finer morphology than the nonreactive ones at all concentrations studied. Mechanical characterization showed that reactive SMAH/E‐MA‐GMA blends had higher tensile strength, % strain at break, and tensile modulus than the nonreactive blends for all corresponding modified polyethylene contents. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 790–797, 2001     

Synthesis,characterization, and properties of novel fluorine containing aromatic polyamides

A series of novel fluorine containing aromatic polyamides were synthesized by the direct polycondensation of various fluorine containing aromatic diamines and commercially available 5‐t‐butyl isophthalic acid. These polyamides have good solubility in several organic solvents such as dimethylformamide, N,N‐dimethylacetamide, 1‐Methyl‐2‐pyrrolidone, dimethyl sulfoxide, and tetrahydrofuran. The synthesized polymers exhibited inherent viscosities up to 0.93 dL/g and M_w up to 1,52,000 with PDI of 2.49. The polyamides exhibited good thermal stability up to 489°C for 10% weight loss in nitrogen and high glass transition temperature up to 273°C. Dynamic mechanical analysis showed a very good retention of storage modulus up to the glass transition temperature. The tan δ peak value at 1 Hz was used to calculate the T_g and these values are in good agreement with differential scanning calorimetry data. The polyamide films were flexible with tensile strength up to 72 MPa, elongations at break up to 14%, and modulus of elasticity up to 1.39 GPa depending on the exact repeating unit structure. X‐ray diffraction measurements indicate that these polyamides are semicrystalline. Rheology study showed same trend of melt viscosity behavior with different shear rate for all polymers. Water absorption study indicates the hydrophobic nature of the polymer. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

微螺旋炭纤维的微波电磁参数及其环氧复合材料的吸波性能研究

采用化学气相沉积法制备了微螺旋炭纤维(CMCs),研究了其在频率为8.2￣12.4GHz范围内的微波介电特性;以环氧树脂为胶粘剂制备了CMCs/环氧复合吸波涂层,并对其吸波性能进行了研究。结果表明,随着CMCs含量的增加,复介电参数实部ε′、虚部ε″和介电损耗tanδ均有所增加,当CMCs含量为1%￣5%时,ε″和tanδ增幅很小;当CMCs含量为10%时,两者增幅显著增大。ε′表现出高频减小的趋势,同时tanδ表现出高频增大的趋势,这些均有利于实现高频吸波。磁损耗tanξ较小且CMCs含量对其影响不明显。CMCs/环氧复合吸波涂层的反射率随着CMCs含量的增加而明显减小,表现出高频衰减略有增加的趋势,同时出现多吸收峰,这有利于实现宽频吸波。