No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Infiluence of Polymer Structure on High Resiliency Urethane Foams
Abstract
Several relationships have been studied between polymer structure and the physical properties of high resiliency flexible urethane foams. This study revealed that ambient temperature polymer properties are affected primarily by the amount and nature of disubstituted urea sequences. The amount of disubstituted urea sequences can be controlled by the molar amount of water and the molar amount of isocyanate in the urethane formulation. The nature of the disubstituted urea sequences is controlled by the type of isocyanate and by the choice of amine or organometallic catalyst. Polymer properties were also shown to be dependent on crosslink density. Further, it was demonstrated that the modulus of urethane foam polymers can be increased substantially without affecting the glass transition temperature through the addition of the vinyl polymer dispersions contained in polymer-polyols. Extensive experimental data are presented in curves.
The behaviour of open cell flexible polyurethane foams in energy management applications, such as automotive seating, where static and dynamic comfort are the main functional attributes, is governed by two material properties: the effective stiffness; and energy lost through hysteresis under the conditions that prevail in use. In most applications the foam is subject to combined stresses (tension, shear and compression) and in some characterization procedures, such as indentation-force-deflection and ball rebound, these conditions prevail. However to elucidate the mechanisms responsible for behaviour a simple deformation regime is normally employed, in particular, simple compression. The imposed static strains may be large (40–70%) which means in practical terms that we are dealing with a highly non-linear static and dynamic mechanical situation. This is manifest by the fact that under high intensity harmonic excitation a flexible foam-based vibration isolating system exhibits behaviour similar to that of classical chaos [1].
Polyurethanes are chemically complex polymers, usually formed by the reactions of liquid isocyanate components with liquid polyol resin components.
A study was made of the variation of the degree of self-association of urethane groups and mechanical properties of polyurethanes, according to the length of flexible and rigid segments, the type and density of chemical crosslinking networks. In polyurethanes obtained from trifunctional polyethers with M⩾5000 chemical crosslinking does not change the degree of self-association, compared with linear polyurethanes. When introducing low-molecular weight crosslinking agents crosslinking reduces the degree of self-association to a greater extent than a network of the same density formed with polyester. Results confirm the structural micro-heterogeneity of polyurethanes with high density of crosslinking.
It is likely that the development of urea technology for flexible seating foam applications has been hindered simply by a perceived incompatibility between the rapid isocyanate-amine reaction and the traditional 'long' gel time foam processing requirements. We will show that the amine-isocyanate reaction rate can be controlled, thereby making urea technology useful in flexible foam applications. Conventional processability evaluations on low pressure metering equipment have shown that amine terminated polyether resins can be used in place of conventional polyols in typical foam formulations without jeopardizing desirable processing parameters. In addition, nonpolymer polyol reinforced polyurea foam matrixes have hardness properties comparable to or better than conventional polymer polyol filled urethane foams. Other foam properties such as humid aged compression set and resiliency are unchanged. Improvements provided by the incorporation of the polyurea technology in regard to overall durability and fatigue performance will be discussed.
Several formulating principles have been studied for the production of high-density, high resilience slabstock foams. The results show that the performance of dibutyltindilaurate, as compared with stannous octoate, offers significant improvements in controlling density, density gradients and ILD values for low water/high density foams. In addition, the adverse effect of higher concentrations of amine catalysts on foam load-bearing properties also is demonstrated. A broad range of high-density, high resilience slabstock foam formulations halve been developed using the tin catalyst, dibutyltindilaurate, and polymer polymer- polyols. These formulations provide various grades of foam in the density range of 2-3 pcf and firmness values at 25% ILD of about 20-40 lbs/50 in2. The outstanding foam physical properties which contribute to cushioning comfort and support are maintained for each of these high-density, high resilience slabstock foams. These formulating principles have been proven in large-scale, commercial production of high-density, high resilience slabstock foams. Such foams are being produced successfully on various conventional and Maxfoam slabstock machines in the United States.
This study compares the mechanical and physical properties of foams derived from typical automotive seating formulations. The ability of polymer-polyols to increase the load bearing characteristics of flexible urethane foams without general acceptance in high resiliency molding applications. In addition, various high resiliency foam formulations are discussed which show the broad processing latitude and physical property changes that may be achieved with polymer-polyols.
Humidity resistance of HR-foam is not so poor when placed without stress, but, when placed with - stress or in compressed state, Wet Compression Set becomes much higher. Wet Compression Set could be improved by the increase of crosslink density; lower molecular weight polyol, reduction of ratio of polymeric isocyanate and lower level of amine catalysts. High molecular weight polyol is preferred as a material of high resilient foam. but it is not suitable for improving wet compression set of foam. And cell structure of foam gives much influence, too. Such silicone surfactant that gives coarser and regular cell should be required.
Microcrazing in the struts of flexible polyurethane foams was discovered during compressive deformation and observed directly in the scanning electron microscope. Attributed to this phenomena was the decrease in stress at maximum compression and the intensity of acoustic emission during compressive cycling. The higher content of styrene–acrylonitrile (SAN) copolymer in these foams resulted in higher modulus, more severe microcrazing, an increase in acoustic emission activity, and a decrease in the stress at maximum compression as cycling progressed.
This article is concerned with the general theory of expansion, manufacturing processes, properties, and applications of foamed (cellular) plastics. The high strength-to-weight ratio, excellent insulating properties, and cushioning properties of cellular plastics have contributed to the development and growth of the broad range of cellular plastics in use. The total usage of foamed plastics in the United States has risen from 1.4 million metric tons in 1982 to 2.2 million metric tons in 1992 and has been projected to rise to about 3.1 million metric tons in 2002.
A novel class of polyols for polyurethane production was described in 1966¹. Their unique feature was the presence of an in situ polymerized vinyl polymer which markedly enchanes the modulus of the resultant polyurethane. The final product is a conventional polyol that contains a dispersion of the vinyl polymer. From among the numerous products investigated, a poly-(acrylonitrile) polyol was selected because of the excellent stability of the dispersion. Poly (acrylonitrile) polyols are opaque, cream-colored liquids with viscosities ranging from 1500 to 4500 cps, depending upon the nature of the polyol precursor. Particle size of the dispersed phase was reported to be between 0.25 and 0.5 micron as determined by electron microscopy. Poly (acrylonitrile) polyols have been used in slab-stock flexible foams as additives to conventional polyether polyols to improve load-bearing properties. More extensive utility has been identified in molding applications. Whitman described their use in producing integral skin urethane foams for uses such as automotive arm rests, horn buttons, mirror covers, etc.. Dunleavy discussed the use of polymer polyols in molded, microcellular urethane elastomers for automotive trim parts, particularly for "cosmetic" bumpers. More recently, Patten and Priest showed their utility in highly resilient automotive seating foam. In all of these applications the modulus enhancing characteristics of these polyols are particularly important.
The deformation of a foamed elastic material, both in tension and compression, and its resistance to tearing and to tensile rupture, have recently been derived on the basis of a model consisting of a large number of thin threads joined at their ends to form a three-dimensional network. A general account of this theoretical treatment, and the evidence for it, is presented. The theory is extended to deal with small deformations of closed-cell foams; relations for Young's modulus and Poisson's ratio are derived. The viscous damping of open-cell foams due to air flow through the network of threads is also discussed.
Geometrical shapes of interstices of two types of closest packing of uniform spheres, 1) hexagonal closest packing, 2) face centered cubic closest packing are studied, and the structures of interstices of these two types of packing are used to represent those of the actual foamed elastomers.
Equivalent elastic constants for these two structures are calculated in terms of the slenderness of a thread (which is a function of voids content) l'/A where l is the length of a thread and A its cross-sectional area, and of the elastic constants of the interstices. The calculated value of Poisson's ratio of a model containing 67 percent of the interstices of hexagonal closest packing and 33 percent those of face centered cubic packing corre lates fairly well with existing experimental data.
The load–compression behavior of a foam reflects its geometric structure and the physical properties of the matrix polymer. Quantitative relations between these parameters have been established in the present study. Based on both theoretical analyses and experimental data obtained on a flexible polyurethane foam, it is shown that the compressive stress can be factored into the product of two terms: (1) a dimensionless function of the compressive strain, ψ(ε), calculated from experimental load–compression data and reflecting the buckling of the foam matrix; and (2) a factor, εEf, where Ef is the apparent Young's modulus of the foam (which is a function primarily of the modulus of the base polymer E0 and of the volume fraction of polymer, φ). Thus the compressive stress behavior of a foamed polymer is determined by E0, φ, and the matrix geometry, the latter described by the function ψ(ε). Using these established relations, it now is possible to delineate precisely the structural features a foam must possess—density, cell shape and size distribution, and modulus of the base polymer—to meet a given load–compression specification.
A recording instrument for measuring the dynamic shear modulus and mechanical damping of plastic and rubber‐like materials using the principle of the torsion pendulum has been constructed. The mechanical oscillations are converted into electrical potentials for recording by a torque measuring device which is actuated by a differential transformer. The apparatus is capable of measuring the modulus and damping of materials over an extremely wide range.