Fillers effect

Class and Chu demonstrated that if a tackifier is chosen that is largely incompatible with the elastomer, a modulus increase due to the filler effect is observed and little change in Ta results, and once again a PSA would not be obtained. This was observed for mixtures of low molecular weight polystyrene resin and natural rubber. The same polystyrene resin did tackify SBR, a more polar elastomer that is compatible with the resin. Hydrogenating the polystyrene to the cycloaliphatic polyvinylcyclohexane changed the resin to one now compatible with the less polar natural rubber and no longer compatible with SBR. These authors also provide... 

Block copolymer chemistry and architecture is well described in polymer textbooks and monographs [40]. The block copolymers of PSA interest consist of anionically polymerized styrene-isoprene or styrene-butadiene diblocks usually terminating with a second styrene block to form an SIS or SBS triblock, or terminating at a central nucleus to form a radial or star polymer (SI) . Representative structures are shown in Fig. 5. For most PSA formulations the softer SIS is preferred over SBS. In many respects, SIS may be treated as a thermoplastic, thermoprocessible natural rubber with a somewhat higher modulus due to filler effect of the polystyrene fraction. Two longer reviews [41,42] of styrenic block copolymer PSAs have been published. 

Methods for Estimating the Filler Effect on Polymer Matrices... 

Note that, apart from the filler particle shape and size, the molecular mass of the base polymer may also have a marked effect on the viscosity of molten composites [182,183]. The higher the MM of the matrix the less apparent are the variations of relative viscosity with varying filler content. In Fig. 2, borrowed from [183], one can see that the effect of the matrix MM on the viscosity of filled systems decreases with the increasing filler activity. In the quoted reference it has also been shown that the lg r 0 — lg (MM)W relationships for filled and unfilled systems may intersect. The more branches the polymer has, the stronger is the filler effect on its viscosity. The data for filled high- (HDPE) and low-density polyethylene (LDPE) [164,182] may serve as an example the decrease of the molecular mass of LDPE causes a more rapid increase of the relative viscosity of filled systems than in case of HDPE. When the values (MM)W and (MM)W (MM) 1 are close, the increased degree of branching results in increase of the relative viscosity of filled system [184]. 

The authors of a very recent work [259] simulated the filler effect by applying an effective tensile stress crz in the direction perpendicular to the direction of the tensile stress aT. To determine erz they relied on the condition of equality of the work of external forces which cause deformation of a unit volume of the composite and matrix subjected to the above-mentioned stresses (asymmetric tri-axial stretching). They came up with a formula of the form ... 

When plastics act as a physical cross-link and strength properties are indirectly related to the modulus of hard phase and morphology of the blend, the filler effect is analyzed by the following equation ... 

The above equations gave reasonably reliable M value of SBS. Another approach to modeling the elastic behavior of SBS triblock copolymer has been developed [202]. The first one, the simple model, is obtained by a modification of classical rubber elasticity theory to account for the filler effect of the domain. The major objection was the simple application of mbber elasticity theory to block copolymers without considering the effect of the domain on the distribution function of the mbber matrix chain. In the derivation of classical equation of rabber elasticity, it is assumed that the chain has Gaussian distribution function. The use of this distribution function considers that aU spaces are accessible to a given chain. However, that is not the case of TPEs because the domain also takes up space in block copolymers. 

Figure 1.13—occur because of simultaneous crystallization and polymerization at 150°C. This temperature is near the maximum crystallization rate temperature ( 145°C) of nylon 6 homopolymer [66]. The presence of solid crystallites increases the complex viscosity of the polymerizing system because of a filler effect. 

The gluability of the lignin-epoxy resin adhesives was found to be improved by the addition of calcium carbonate (50% by weight) to the liquid resin. This must be attributed to the nature of the weak alkali in calcium carbonate as a cure accelerator, and to the reinforcement effect of fillers. Since wood surfaces are acidic, the addition of alkaline fillers effectively alters the pH of the glue line. 

The character of the filler effect depends on the affinity of the silica surface for the copolymer, and the rigidity of macromolecules. For MEDDE-DVB and DMGE-DYB copolymers, the introduction of both methyl-, and methyl,hydride-containing silicas results in the formation of amorphous structures. A greater degree of disorder was detected in the presence of silicon hydride groups on the filler surface. 

Many studies have been devoted to determining the exact role of these organic species. Stucture directing effects, void-filler effects, and stabilization of specific secondary building units in the solution have been considered ( 4-6). ... 

Gaylord and Lohse (10) have calculated the stress-strain relation for cilia and tie molecules in a spherical domain morphology using the same type of three-chain model as Meier. It is assumed that the overall sample deformation is affine while the domains are undeformable. It is predicted that the stress increases rapidly with increasing strain for both types of chains. The rate of stress rise is greatly accelerated as the ratio of the domain thickness to the initial interdomain separation increases. The results indicate that it is not correct to use the stress-strain equation obtained by Gaussian elasticity theory, even if it is multiplied by a filler effect correction term. No connection is made between the initial dimensions and the volume fractions of the domain and interdomain material in this theory. 

The rubbery shear modulus is higher when hard segment crystallinity is present because of the filler effect. 

A generalized kinetic model of cure is developed from the aspect of relaxation phenomena. The model not only can predict modulus and viscosity during the cure cycle under isothermal and non-isothermal cure conditions, but also takes into account filler effects on cure behavior. The increase of carbon black filler loading tends to accelerate the cure reaction and also broadens the relaxation spectrum. The presence of filler reduces the activation energy of viscous flow, but has little effect on the activation energy of the cure reaction. 

The model described in Eq.(l) not only can predict the cure behavior measured by standard curometers, but also can explain filler effects on the cure reaction. The model enables one to predict scorch time and cure time of elastomers at various filler loadings and cure temperatures. In the following discussion, this kinetic model of cure will be extended to explain and predict the modulus or viscosity of elastomers/thermosets during... 

The filler effects on the chemoviscosity of thermosetting resins have not been studied extensively, but are vital to understanding the rheology of filled thermosets. For example, the effects of filler concentration on viscosity can be used in process control to monitor batch-to-batch variations or to provide essential information for research into alternative filler/resin batches. Ng and Manas-Zloczower (1993) examined an epoxy-resin system with silica filler and established that the elastic modulus of the resin can be expressed in terms of... 

We are now in a position to examine the chemorheology and modelling of filled reactive systems, by combing our knowledge of cure, shear-rate and filler effects, as described above, with an examination of the current literature on filled reactive systems. 

Many of the same basic raw materials shown in Table II for RIM fascia systems are also used in high modulus systems. Additionally, however, polyether polyols "filled" with dispersions of polyureas are used( 2) These are the so-called PHD polyols developed by Bayer AG, the PHD being an abbreviation for Polyharnstoff-Dispersion. These polyols provide the same "filler" effect as the graft polyols (Table II) for increasing the modulus of the polymer without increasing the amount of extender. 

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