Viscosity and polymerization

Chemical modification of hydroxyethylcellulose or hydroxypropylcellulose with long-chain hydrocarbon alkylating reagents, such as C8-C24 epoxides or halides, has been reported to yield novel water-soluble compositions exhibiting enhanced low-shear-rate solution viscosities and polymeric surfactant properties [ 104,105]. Patents have also been issued for water-soluble phosphonomethylcellulose and phosphonomethylhydroxyethylcellulose [106,107]. 

According to O Driscoll, / (rei) should increase with pressure in the cases discussed, the counteracting effects of viscosity changes in monomer viscosity and polymeric coil dimensions are very pronounced. 

The anisotropy x l of the magnetic susceptibility must be determined in a separate experiment. The relaxation time after a rapid rotation by a small angle obeys Fq. (43). For angles above 45°, and especially for angles around 90°, the director relaxation will be strongly inhomogeneous due to backflow [36]. This effect can be utilized to determine shear viscosities besides the rotational viscosity for high viscosity and polymeric materials. 

Table 2 Inherent Viscosity and Polymerization Data of PPBIs...

As the polymer molecular weight increases, so does the melt viscosity, and the power to the stirrer drive is monitored so that an end point can be determined for each batch. When the desired melt viscosity is reached, the molten polymer is discharged through a bottom valve, often under positive pressure of the blanketing gas, and extmded as a ribbon or as thick strands which are water-quenched and chopped continuously by a set of mechanical knives. Large amounts of PET are also made by continuous polymerization processes. PBT is made both by batch and continuous polymerization processes (79—81). 

Low viscosity urethane polymers have been prepared from castor od and polymeric isocyanates (82). These low mix viscosity systems are extremely usehd for potting electrical components where fast penetration without air voids, and fast dispensing cycles are desirable. Very low viscosity urethane systems containing castor polyols have been prepared for use in reclaiming water-logged buried telephone cable and for encapsulating telephone cable sphces (83—86). 

Random copolymers of vinyl chloride and other monomers are important commercially. Most of these materials are produced by suspension or emulsion polymerization using free-radical initiators. Important producers for vinyl chloride—vinyUdene chloride copolymers include Borden, Inc. and Dow. These copolymers are used in specialized coatings appHcations because of their enhanced solubiUty and as extender resins in plastisols where rapid fusion is required (72). Another important class of materials are the vinyl chloride—vinyl acetate copolymers. Principal producers include Borden Chemicals Plastics, B. F. Goodrich Chemical, and Union Carbide. The copolymerization of vinyl chloride with vinyl acetate yields a material with improved processabihty compared with vinyl chloride homopolymer. However, the physical and chemical properties of the copolymers are different from those of the homopolymer PVC. Generally, as the vinyl acetate content increases, the resin solubiUty in ketone and ester solvents and its susceptibiUty to chemical attack increase, the resin viscosity and heat distortion temperature decrease, and the tensile strength and flexibiUty increase slightly. 

The choice of initiator system depends on the polymerization temperature, which is an important factor in determining final product properties. Cold polymers are generally easier to process than hot polymers and in conventional cured mbber parts have superior properties. The hot polymers are more highly branched and have some advantages in solution appHcations such as adhesives, where the branching results in lower solution viscosity and better cohesion in the final adhesive bond. 

Commercial chloroprene polymerization is most often carried out in aqueous emulsion using an anionic soap system. This technique provides a relatively concentrated polymerization mass having low viscosity and good transfer of the heat of polymerization. A water-soluble redox catalyst is normally used to provide high reaction rate at relatively low polymerization temperatures. 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

For rosins and rosin esters, the produets having high aeid numbers are the most susceptible to oxidation and have inferior viscosity stability and colour stability in adhesive formulations. Thus, when stability properties are essential in adhesives, rosin esters rather than high aeid number rosins are used. However, the high acid number resins are polar and display better adhesion to polar elastomers and polymeric surfaces. 

As another consequence of the properties of the siloxane bond, the value of n in the common linear trimethylsiloxy-endblocked-PDMS, (M-D -M) can vary from zero to tens of thousands giving a range of viscosity from 0.65 to 2,500,000 centipoise to the polymeric material. This relationship between viscosity and polymer chain length allows PDMS polymers to vary in form from water-like fluids to a flowable gum, while retaining the same chemical character. 

The influence of temperature on the copolymerization was investigated at constant absorbed dose of 0.12 and 0.16 KGy for copolymerization of AM-AANa [17,54] and AM-DAEA-HCl [22], respectively. The results are shown in Figs. 9 and 10, which show that the Rp values increase while the intrinsic viscosity and the degree of polymerization decrease with increasing the polymerization temperature. However, the increase in the temperature of the polymerization medium increases the swell-... 

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