Thermal and visco-elastic properties of EVA-based hot-melt adhesives: relationship to peel behaviour

We have studied the visco-elastic and mechanical behaviour of hot-melt adhesive formulations based on mixtures of an ethylene vinyl acetate (EVA) copolymer, a terpenc phenolic tackifying resin, and increasing amounts of a highly crystalline synthetic Fischer-Tropsch wax. The visco-elastic behaviour has been characterized by oscillatory experiments at a fixed frequency (thermomechanical analysis), and by creep measurements at various temperatures. As a general feature, increasing the wax content leads, at room temperature (i.e. well below the melting point of the wax), to a progressive change from the behaviour of a visco-elastic liquid to the behaviour of a visco-elastic solid (the highly crystalline wax acts like a filler). This feature is clearly evidenced by the variations of the compliance function. The pcel behaviour on aluminium foils is dramatically affected by the wax content; this is closely related to the visco-elastic behaviour described above. One can establish, at a given peel rate, a general relationship between (1) liquid behaviour and cohesive failure at low wax contents, and (2) solid behaviour and interfacial failure at high wax contents. At a given wax content, increasing peel rate results in the generally observed transitions: cohesive→interfacial (rubberlike)→stickslip→interfacial (glasslike); these transitions are shifted towards the right side and the peel energy is decreased as the wax content is increased. The single glass transition temperature (Tg) of the formulations is a function of the polymer/resin ratio, and its variations as a function of resin content confirm the hypothesis of complete polymer/ resin compatibility. Furthermore, the Tg's determined by thermo-mechanical analysis (oscillatory experiments at fixed frequency) arc not affected by the wax content; this suggests that the mechanical transitions cannot be simply understood on the basis of time-temperature equivalence.     

Lower critical solution temperature behavior of ethylene propylene copolymers in multicomponent solvents

The lower critical solution temperature (LCST) locus for ethylene-propylene copolymers has been determined as a function of pressure in a variety of single and multicomponent solvents. The lower critical end-point temperature (LCEP), which is the intersection of the LCST locus with the vapor-pressure curve, was found to be predictable from the solvent density as previously established for single-component solvents by Charlet and Delmas.¹ Dissolving a low-molecular hydrocarbon gas such as propylene in an alkane has a dramatic effect on lowering the LCEP, and can reduce phase-separation temperatures to levels at which this technique becomes attractive as a practical method for polymer recovery from diluents such as those used in solution polymerizations. Temperatures considerably above the LCEP are needed to minimize the residual polymer concentration in the solvent in the two-liquid-phase region. The solvent critical temperature must be approached for essentially complete elimination of the polymer from the solvent phase. The LCST locus was found to be a linear function of pressure for all of the systems investigated, and the slope of the line, d(LCST)/dP, could be well correlated as a function of solvent density and critical temperature. From the relationship between the LCEP and solvent density and the correlation for d(LCST)/dP, the location of the LCST locus can be readily predicted from a knowledge of solvent properties.     

Melt‐Phase Nylon 612 Polycondensation Kinetics: Effects of Sodium Hypophosphite Catalyst

Effects of sodium hypophosphite (SHP) catalyst and water on nylon 612 melt‐phase polycondensation reactions were investigated using dynamic experiments with different catalyst levels (33, 113 and 249 ppm). The water concentration in the nylon melt was varied by altering the composition of a steam/nitrogen mixture bubbled through the molten polymer. Carboxyl and amine end‐group concentrations were determined for polymer samples collected during the experiments. A model was developed to describe the kinetics of nylon polycondensation using the SHP catalyst in the high‐temperature and low water operating region that corresponds to the final stages of industrial melt‐phase polyamidation processes.     

Application of genetic algorithm in calculation of the phase behavior of binary and ternary polymer solutions

A genetic algorithm as an optimization procedure has been developed to predict the phase behavior of polymer solutions. The phase equilibrium diagrams of binary and ternary polymer solutions have been determined using the appropriate form of Flory–Huggins free-energy function for polymer solutions. A concentration and temperature dependent form of the interaction parameter has been used to reflect the effect of temperature and polymer properties in the free-energy form. The proposed genetic algorithm is applied to compare the phase behavior results of some typical polymer solutions with the results of the classical determination methods and then applied to some conventional ternary polymer solutions as polymer–solvent–nonsolvent systems. The proposed algorithm use a set of individual states as the initial chromosomes and uses the general rules as crossover, mutation, and with use of a fractional objective function determines the binodal points or the phase diagram boundaries of polymer solutions. The properties of an industrially relevant polymer solution, a polystyrene–cyclohexane solution, have been used to emphasize on the industrial application of the proposed algorithm. The algorithm has been used to predict the phase behavior of the two polymer–solvent–nonsolvent systems as polystyrene-butanone-methanol and polystyrene-butanone-propanol at three different temperatures and results show good agreement with the experimental observations. The algorithm also has the capability to predict both the concentration-independent and concentration-dependent interaction parameters among the different components. The genetic algorithm is an easy-to-use, state-of-art, and very fast optimization tool, and has very strong capability to solve nonlinear systems in chemical and polymer engineering topics.     

A method for determining the permeability and solubility of sulfur in poly(dimethylsiloxane) (RTV)

A method has been devised whereby the S₈ permeability and solubility in silicon resin are determined by observing the reaction between sulfur vapors and silver particles within the polymer. The particles (～10 μm diameter), which may vary between 0.1 and 100 μm in diameter, are dispersed in the polymer at a concentration up to 20% within a glass cylinder (also an aluminum container). The polymer is cured according to procedure and exposed to S₈ saturated vapors at various temperatures (55°–125°C), leaving one end of the cylinder open. The Ag particles are a perfect sink for sulfur, which is consumed as soon as it reaches the reaction boundary that separates the reacted and unreacted Ag particles. Consequently, a distinct black region containing Ag₂S product is left behind as the boundary advances. The line displacement, measured at time intervals for the several temperatures, is used to calculate the gas permeability in the polymer as a function of temperature on the basis of a mass transport model developed from diffusion theory. The S₈ solubility in the polymer is calculated from the permeability and diffusivity. The latter is determined independently by measuring the time that the Ag₂S reaction is delayed when a layer of plain polymer separates a silver surface and the sulfur environment.     

Phase structure,morphology and phase boundary diagram in an aromatic polyimide (BPDA–PFMB)/m-cresol system

An organo-soluble aromatic polyimide has been synthesized from 3,3′, 4,4′-bis(phenyltetracarboxylic) dianhydride (BPDA) and 2,2′-bis (trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) via a one-step polymerization in m-cresol. The phase boundary diagram for this system has been established by differential scanning calorimetry, polarized light microscopy (PLM) and wide angle X-ray diffraction (WAXD) experiments. A crystallosolvate form I has been found over the entire concentration region at low temperatures. When the temperature is increased, an isotropic phase has been observed below concentrations of about 40%. In the relatively high concentration region between 45 and 95%, a transition from the crystallosolvate from I to a crystallosolvate form II has been observed. Form II exhibits a different WAXD pattern. In a narrow concentration region between the isotropic and the crystallosolvate form II (40–45%), a biphase behavior has been found. The birefringence of this region may be an indication of a liquid crystalline phase. The BPDA–PFMB polymer crystal has been found in the very high concentration region (>95%) for temperatures >250°C. The morphologies of these phases have been investigated via PLM and transmission electron microscopy. Above the gel/sol transition temperature, the form I shows negative birefringent spherulites consisting of thin lamellae. The form II exhibits a tendency of positive birefringent spherulites. Possible mechanisms of the formation of the metastable phase morphology and their associations with the mechanical gel/sol transition are also discussed.     

Mechanical properties of borate crosslinked poly(vinyl alcohol) gels

Boric acid does not introduce crosslinks in poly(vinyl alcohol) solutions, but gelation does occur in the presence of cations. In this experimental study, the dynamic mechanical properties of these gels were determined using test-tube torsion pendulums and an air-bearing torsion pendulum. The modulus at a fixed concentration of polymer and boric acid increases with increasing sodium ion concentration up to the point where the atom ratio of sodium to boron reaches 1. Higher sodium concentrations do not increase the modulus. The log decrement, on the other hand, decreases with increasing sodium concentration continuously without reaching a plateau at the equal atom ratio. Log decrements as low as 0.02 can be measured. The storage modulus depends on the logarithm of borate concentration and on the 4.7 power of poly(vinyl alcohol) concentration. Only a very small portion of the borates in solution take part in effective crosslinks. The activation energy for breaking individual bonds in a function of temperature and the cation to boron ratios. At a fixed cation concentration, this activation energy is more negative with increasing amount of boron ions due to a transformation of monomeric crosslinks into polymeric crosslinks, so that the storage modulus which measures crosslink density decreases as a temperature rises.     

1H n.m.r. measurements on aqueous solutions of poly(methacrylic acid)

¹H n.m.r. spectra of aqueous solutions of poly(methacrylic acid) have been measured at a Larmor frequency of 60 MHz. It is shown that the methyl linewidth can be analysed in terms of triad placements of methyl groups, having individual linewidths that are relaxation determined. The contribution of the magnetic dipolar interaction between water protons and methyl protons to the methyl proton linewidth is shown to be negligible. The influence of polymer tacticity, concentration and charge was studied as a function of temperature. The linewidth is found to be strongly dependent on the amount of polymer charge at charge densities up to 50%; the actual shape of this dependence varies with polymer tacticity and concentration. The variable temperature experiments yield energies of activation (3–4 kcal/mole) that are in agreement with results of pulsed n.m.r. experiments on the same system. Values of the self-diffusion constant of the polymer, obtained by n.m.r. pulsed gradient experiments, are given; no simple relationship between this constant and the methyl linewidth is found, while the polymer charge dependence of the self-diffusion constant is opposite to the charge dependence of the diffusion constant in a concentration gradient.     

Kinetics of Adhesion Interaction of Polyolefins with Metals under Conditions of Contact Thermooxidation. II. Dissolution and Diffusion of Iron Compounds into the Bulk of Polymer

The kinetics of dissolution and penetration of iron compounds (IC) into the bulk of a polyethylene layer during oxidation in contact with steel as a function of contact temperature and thickness of a polymer layer has been studied by X-ray spectral fluorescence analysis (XSFA). The measured intensity of dissolving species correlates with the rate of contact oxidation. Small amounts of IC dissolve in the absence of oxygen. The depth profiles of IC content were determined by electron probe micro-analysis (EPMA). It was observed that the depth of penetration of IC does not exceed 15 μm up to a contact time of 3.6 ks at a temperature of 473 K. Judging from the value of the effective diffusion coefficient of IC, the process of transfer of IC is diffusion controlled. The results of some model experiments show that the most likely reaction for IC generation is the formation of iron carboxylates by the interaction of iron oxide with the products of contact oxidation of the polymer (carboxyl groups).     

E.p.r. study of radicals produced mechanically in PGMA and their interaction with oxygen

The vibrational grinding of poly(ethyleneglycol methacrylate) (PGMA) in vacuo at the liquid nitrogen temperature gives rise to polymer radicals in high concentrations. Changes in the radical concentration as a function of temperature in the presence and absence of oxygen were followed by means of electron paramagnetic resonance. It was found that polymer radicals reacted at very low temperatures with oxygen with simultaneous formation of polymer peroxy radicals and of a non-paramagnetic polymer tetroxide. This polymer tetroxide, which has been proved indirectly, can decompose to yield polymer peroxy radicals and non-paramagnetic products; the observed anomalies on the curve of the thermal decomposition of radicals may be thus elucidated. The relative participation of polymer tetroxide depends on the oxygen concentration, on the temperature of the sample in contact with oxygen and on the concentration of polymer radicals arising by grinding predominantly on the surface of polymer particles.     

A 3‐D finite element model for gas‐assisted injection molding: Simulations and experiments

To gain a better understanding of the gas‐assisted injection molding process, we have developed a computational model for the gas assisted injection molding (GAIM) process. This model has been set up to deal with (non‐isothermal) three‐dimensional flow, in order to correctly predict the gas distribution in GAIM products. It employs a pseudo‐concentration method, in which the governing equations are solved on a fixed grid that covers the entire mold. Both the air downstream of the polymer front and the gas are represented by a fictitious fluid that does not contributeto the pressure drop in the mold. The model has been validated against both isothermal and non‐isothermal gas injected experiments. In contrast to other models that have been reported in the literature, our model yields the gas penetration from the actual process physics (not from a presupposed gas distribution). Consequently, it is able to deal with the 3‐D character of the process, as well as with primary (end of gas filling) and secondary (end of packing) gas penetration, including temperature effects and generalized Newtonian viscosity behavior.     

Viscosity and retention of sulfonated polyacrylamide polymers at high temperature

The viscosity and retention of several copolymers of acrylamide (AM) with sodium salt of 2‐acrylamido‐2‐methylpropane sulfonic acid (PAMS), and also hydrolyzed polyacrylamide (HPAM) have been studied under aerobic condition with and without the sacrificial agent, isobutyl alcohol (IBA) added at a temperature of 80°C. Parallel experiments have been performed in synthetic seawater (SSW) and 5 wt % NaCl. The viscosity at high temperature has been studied as a function of aging time, shear rate, sulfonation degree, molecular weight, and concentration of IBA. The retention in porous medium for sulfonated polyacrylamide polymers was measured in core floods using outcrop Berea sandstone. For the studied polymer sacrificial agent may protect polymer structure at high temperature. Higher sacrificial agent concentration gives better thermal stability in both 5 wt % NaCl and SSW solvents. Sulfonation degree also has a direct effect on thermal stability, i.e., higher sulfonation degree lead to better thermal stability in terms of viscosity. By increasing temperature, less relative reduction in polymer solution viscosity was observed for the polymer with lower molecular weight. The presence of divalent ions at high temperature leads to strong reduction of HPAM polymer solution viscosity, but the viscosity is better maintained for PAMS copolymer solution at high temperature. The precipitation of HPAM first occurred after 3 months at 80°C and for PAMS copolymer with lowest sulfonation degree precipitation started after 7 months. For the studied polymers the retention was found to be relatively independent of temperature and compared to HPAM a much lower retention is observed for the sulfonated copolymers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Correlations between water uptake and effective fixed charge concentration at high univalent electrolyte concentrations in sulfonated polymer cation-exchange membranes with different morphology

Water uptakes properties and effective fixed charge concentration have been determined in aqueous electrolyte solutions (LiCl, NaCl and KCl) for different commercial sulfonated polymer cation-exchange membranes with different morphology. Differences in the water uptake properties and in the membrane effective fixed charge concentration has been found, which have been analyzed on the basis of the different membrane structures. The experimental results show correlations between the water uptake and the loss of the membrane selectivity at high electrolyte concentrations which are dependent on the membrane morphology. Relationships are found which permit to estimate the membrane effective fixed charge concentration from equilibrium and morphological properties with the advantage of avoiding the need for membrane potential measurements.     

A fragment of the phase diagram of polysulfone in dimethyl acetamide

Conclusions -- The temperature—concentration limits of the solubility of PS in DMAc have been determined, and it has been shown that the solubility increases with rise in temperature.-- The phase breakdown of solutions of PS in DMAc has been identified as amorphous without the intervention of crystallization.-- A fragment of the phase diagram for the ternary system PS-DMAc-H₂O shows that the region of solutions is strongly narrowed on increasing the concentration of the polymer.Translated from Khimicheskie Volokna, No. 4, pp. 20–21, July–August, 1990.     

Molecular Origin of the Influence of the Temperature on the Loss Factor of a Solid Propellant

This study focuses on the viscoelastic behavior of an industrial hydroxyl‐terminated polybutadiene (HTPB) based solid propellant. The analysis of the loss factor as function of temperature enables the investigation of the molecular mechanisms participating in the nonlinear viscoelastic behavior. A design of experiments determines the influences of the filler fraction, of the NCO/OH ratio, of the plasticizer content, and of the presence or absence of filler‐binder bonding agents. For all the tested materials, the loss factor as function of temperature exhibits two distinct peaks when measured by Dynamic Mechanical Analysis. Exponentially modified Gaussian distributions are applied on each peak to characterize the behavior. While the first peak is commonly associated with the rubber‐glass transition of the material, the second peak has not been clearly associated with a molecular mechanism. This study shows that the second peak of the loss factor in HTPB‐based solid propellants originates from the flow of free polymer chains in the polymer network with a reptation mechanism. The sol polymer fraction controls the area of the second peak, whereas its temperature at the maximum corresponds to an activation temperature determined by the molar masses of the sol polymer. Finally, when the propellant is stretched, a decrease in area and an increase in the temperature of the peak show that the reptation of the sol polymer chains is constrained by the network.     

Heat sealing of semicrystalline polymer films. I. Calculation and measurement of interfacial temperatures: Effect of process variables on seal properties

A finite element analysis (FEA) modeling technique was used to predict the interfacial temperature as a function of time during the sealing of semicrystalline polymer films. An experimental technique using micro-thermocouples to measure rapidly changing interfacial temperatures during sealing was also developed. Agreement between predicted interfacial temperature profiles and measured values for polyethylene films was good except at temperatures substantially above the final melting point of the polymer. This deviation is caused by film-thickness changes occurring during sealing that are not taken into account in the calculations. The effect of heat-sealing process variables (seal bar temperature, dwell time, and pressure) on seal properties (seal strength, seal elongation, and seal energy) of polyethylene films has also been quantitatively determined. Seal properties are determined primarily by the maximum temperature achieved at the interface during heat sealing. Dwell time must be sufficiently long to bring the interfacial temperature to a desired level, but longer times at a given interfacial temperature do not improve seal properties at the conditions of our experiments. A slight pressure is helpful in bringing two microscopically uneven film surfaces into intimate contact, but higher pressure has no beneficial influence on seal properties. However, increased pressures and dwell times at temperatures above the final melting point of the polymer are detrimental to seal appearance due to material deformation in the sealing area. © 1994 John Wiley & Sons, Inc.     

Application of the kinetic composite methodology to autocatalytic-type thermoset prepreg cures

Using a commercial epoxy/carbon fiber prepreg as a model system, cure kinetics of an autocatalytic-type reaction were analyzed by a general form of conversion-dependent function first proposed for degradation kinetics of polymers and composites. The characteristic feature of conversion-dependent function was determined using a reduced-plot method where the temperature-dependent reaction rate constant was analytically separated from the isothermal data. Assuming two elementary reaction mechanisms that were expressed by the nth order and autocatalytic kinetic models, they were combined with a composite methodology capable of predicting overall kinetic behavior. The activation energies were determined and favorably compared for both isothermal and dynamic-heating differential scanning calorimetry experiments in the temperature region for standard epoxy cures at 177°C (350°F). Finally, the proposed model equation demonstrated excellent predictive capability and broad applicability in describing various types of thermoset polymer cure for both isothermal and dynamic heating conditions. © 1993 John Wiley & Sons, Inc.     

Some analogies in the mechanical behavior of filled polymers

The temperature and frequency dependences of complex shear modulus and mechanical losses were studied for expoxy resin composition in the presence of different amounts of quartz and polystyrene fillers. The data obtained were analyzed by the use of the Williams-Landell-Ferry method. It was shown that in the mechanical behavior of filled polymers, except for the well-known temperature-time analogy there exist some lows connected with the presence of filler. The change in filler concentration leads to the same change in the real part of complex modulus as change in frequency (concentration–time analogy), and change in temperature is equivalent to concentration change (temperature–concentration analogy). The existence of these analogies is explained by a change in deformation conditions for polymeric matrix in the presence of different amount of filler, by the existence of surface layers of polymer at the interface with solid filler, and by peculiarities of the mechanical behavior of filler. It is also established that the thickness of surface layer which was determined from experimental data depends on temperature and has an extremum in the temperature region of the α-transition.     

Control of the permeability of a porous media using a thermally sensitive polymer

Experiments explore the reduction in permeability of a porous bead pack when a suspension of thermally responsive polymer is injected and the temperature then increased above the thermal activation temperature. The change in permeability is greater with higher polymer concentration, provided that the ionic concentration of the solution is sufficient for floc formation. The time for activation of the blocking effect is within tens of seconds to minutes of when the polymer solution is heated. This is consistent with the timescale for diffusion‐limited aggregation, although the detailed value depends on the geometry and polymer concentration. Dynamical experiments demonstrate that once the porous media is blocked, adding additional polymer has no effect. The mechanism for permeability reduction may be modeled in the context of a pore‐network model, and we build a simple model to illustrate the permeability reduction as a function of the fraction of pores links which are blocked. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1193–1201, 2014     

Rheological properties of concentrated polymer solution: Polybutadiene in good and θ solvents

The zero shear viscosity, η° of three polybutadiene samples having different molecular weights over a wide range of concentration (1.0–35.0% polymer) in good and θ solvents has been studied. Superposition of viscosity data has been made to give a single composite curve for each solvent by shifting them vertically by a factor (M°/M)^3.4, where M° represents the molecular weight of the reference sample. The shift factor is found to be proportional to M^3.4 in the region of higher concentration, which indicates that the 3.4-power law is valid for the data of polybutadiene. The double-logarithmic plots of relative viscosity η°_r as a function of c⁵M^3.4 yielded a single composite curve approximating a straight line with slope of unity at the higher values of the variables. The results indicate that over a considerable range of the variables (molecular weight and concentration) at a constant temperature, the relative viscosity is a single function of c⁵M^3.4. The results for double-logarithmic plots of zero shear specific viscosity η°_csp as a function of concentration confirmed those observed in polycholoroprene samples studied earlier that the η0_sp values in θ solvents at higher concentration region are found to be higher than those found in good solvents, whereas in the moderately concentrated region the values are just opposite in θ and good solvents. The viscosity crossover in θ solvents is not as sharp as is found in case of polychloroprene samples and that crossover, too, has taken place in the range of concentration of 11.7–31.6% polymer, which is comparatively higher than that of polychloroprene samples (6.06–21.0% polymer). The results indicate some relation between viscosity crossover and polymer polarity, supporting the idea of enhanced intermolecular association in poor solvents. To correlatethe viscosity data obtained in good and poor solvents, two methods, one given by Graessley and the other given by Dreval and coworkers involving the correlating variable c[η], were considered. The plots of relative viscosity η°, versus the correlating variable c[η] in benzene (good solvent) yielded one curve, but in the case of θ solvents (dioxane and isobutyl acetate), the same plots yielded three separate curves instead of a single curve, which is rather unusual. The appropriate correction on the correlating variable for chain contraction in the concentrated region in a good solvent moved the data to a common curve, especially in lower concentration region, but at the higher concentration region a slight overestimation of data seems to have been effected. On the other hand, the plots of log η as a function of correlating variable c[η] yielded a single curve for three samples in the good solvent benzene, but in poor solvents (diozane and isobutyl acetate) the same plots yielded three separate curves for three samples instead of a single curve, the reason for which is not known at present. However, the normalization of the correlating variable c[η] with Martin constant K_M reduced all experimental data of the polymer samples to a common curve. The correlation of the viscosity data by either of the two methods seems to be possible in the case of the nonpolar flexible polymer, polybutadiene.