Rubber Fractionation

Fig. 2.2 Osmotic molecular weights as a function of limiting viscosity number for unfractionated rubber after mastication in air (a) unfractionated (b) fractionated rubber (c) theoretical curve for a random distribution of chain lengths based on curve b [7].

When a more delicate fractional vacuum-distillation is required, the flask and column shown in Fig. ii(b), p. 26, may be used, the side-arm of the column being fitted directly into receiver C (Fig. 14). A rubber stopper must then be used to fit the flask on to the fractionating column, and it should also carry a capillary tube leading to the bottom of the flask, to provide the usual fine stream of bubbles to prevent bumping. 

Now filter the ether through a fluted filter-paper directly into a 100 ml. distilling-flask, and then equip the latter with a 100° thermometer and a double-surface condenser to the end of the latter attach a receiver with a rubber delivery-tube precisely as before. Place the flask cautiously in a water-bath, the contents of which have previously been heated to about 60° at some distance from the apparatus arrange the depth of the flask in the water-bath so that the ether distils slowly over. Collect the fraction boiling between 34-39°. Yield, 25 g. (35 ml.). Not more than a verv small residue of etlianol should remain in the flask. 

Solvents with boiling points below 90-95°. A steam bath or water bath should be employed. Alternatively, the apparatus of Fig. 77,13, 3, but with a Alter flask as receiver, may be used the end of the rubber tubing attached to the tubulure is either placed in the sink or allowed to hang over the bench. If a distillation is ultimately to be conducted in the flask from which the solvent is removed, the apparatus depicted in Fig. 77,13, 4 is recommended the distilling flask may be replaced by a Claisen flask or a Claisen flask with fractionating side arm, particularly if the subsequent distillation is to be conducted under diminished pressure. 

If a bolt-head fiask is used and a rubber stopper is permissible, there is ample room for the insertion of a capillary tube and the fractionating column in the wide neck. 

Higher alcohols. These may be purified by drying with anhydrous potassium carbonate or with anhydrous calcium sulphate, and fractionated after filtration from the desiccant. Bark corks (or ground glass joints) should be used rubber stoppers are slightly attacked. The boiUng points of the fractions to be collected are as follows —... 

An important application of Eq. (3.39) is the evaluation of M, . Flory et al.t measured the tensile force required for 100% elongation of synthetic rubber with variable crosslinking at 25°C. The molecular weight of the un-cross-linked polymer was 225,000, its density was 0.92 g cm , and the average molecular weight of a repeat unit was 68. Use Eq. (3.39) to estimate M. for each of the following samples and compare the calculated value with that obtained from the known fraction of repeat units cross-linked ... 

Foamed polymers Foamed sheet Foamed silicone rubber Foam fractionation... 

Antioxidants may be assessed in a variety of ways. For screening and for fundamental studies the induction period and rate of oxidation of petroleum fractions with and without antioxidants present provide useful model systems. Since the effect of oxidation differs from polymer to polymer it is important to evaluate the efficacy of the antioxidant with respect to some property seriously affected by oxidation. Thus for polyethylene it is common to study changes in flow properties and in power factor in polypropylene, flow properties and tendency to embrittlement in natural rubber vulcanisates, changes in tensile strength and tear strength. 

Whilst the Tg of poly(dimethylsiloxane) rubbers is reported to be as low as -123°C they do become stiff at about -60 to -80°C due to some crystallisation. Copolymerisation of the dimethyl intermediate with a small amount of a dichlorodiphenylsilane or, preferably, phenylmethyldichlorosilane, leads to an irregular structure and hence amorphous polymer which thus remains a rubber down to its Tg. Although this is higher than the Tg of the dimethylsiloxane it is lower than the so that the polymer remains rubbery down to a lower temperature (in some cases down to -100°C). The Tg does, however, increase steadily with the fraction of phenylsiloxane and eventually rises above that of the of the dimethylsilicone rubber. In practice the use of about 10% of phenyldichlorosilane is sufficient to inhibit crystallisation without causing an excess rise in the glass transition temperature. As with the polydimethylsilox-anes, most methylphenyl silicone rubbers also contain a small amount of vinyl groups. 

Solutions of the two recipes were blended in varying proportions to provide tie coats of continuously varying composition. The patent shows an example of eight plies or layers of graded composition between the rubber and the metal substrate. Because of the high fraction of reactive filler, the material closest to the metal substrate would be the most rigid and polar. The stiffness and polarity... 

Wi is the weight fraction of the elastomer, W2 the tackifier, W3 a further compatible additive, such as an oil, and so forth, for the remaining components in the formulated PSA. Application of the Fox equation to the poly (/-butylstyrene) tackified natural rubber adhesive (cited above) gives a value of —11°C, in good agreement with the interpolated value of — 13°C. 

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. 

The demand for isoprene for Butyl rubber led to the development of a recovery process for this Cj diolefin. Extractive distillation with acetone was the first process used but it has been replaced with acetonitrile (ACN ). The first step in the process is the fractionation of steam cracker debutanizer bottoms in a conventional two tower system to produce a C5 cut containing 30% isoprene. The first tower rejects C and heavier while the second rejects C4 and lighter materials. 

Figure 12.12 Coupled SEC-RPLC separation of Plioflex rubber stock (a) SEC (b) RPLC ti ace of fraction 1, Wingstay 100 (Eive-peak pattern is representative of diarylphenylenedi-amine isomers) (c) RPLC ti ace of fraction 2, mixed disulfide and MBTS (2,2 -thiobis (ben-zothiazole)). Obtained under the same conditions as given for Eigure 12.11. Reprinted from Journal of Chromatography, 149, E. L. Johnson et al, Coupled column chromatography employing exclusion and a reversed phase. A potential general approach to sequential analysis , pp. 571-585, copyright 1978, with permission from Elsevier Science.

Rubber particle size (/i.m) Large particles in blends (%) Rubber phase volume fraction Notched Izod impact strength ft. Ibs/in. Gloss... 

Isoprene is the second important conjugated diene for synthetic rubber production. The main source for isoprene is the dehydrogenation of C5 olefins (tertiary amylenes) obtained by the extraction of a C5 fraction from catalytic cracking units. It can also be produced through several synthetic routes using reactive chemicals such as isobutene, formaldehyde, and propene (Chapter 3). 

Methyl Acetone. Commercial product .a water-white, anhydr liq, consisting of various mixts of acet (45 to 65%), methyl acetate (20 to 30%) and methanol (20 to 40%). Density about 0.83g/cc, boiling range 50 to 70°, flash p near 0°F. Miscible with hydrocarbons, oils and w. Obtained as a by-product in the manuf of acet and methanol from wood distn, representing fractions which cannot be economically sepd. The mixt can be used as such, being an excellent solv and plasticizer for NC and other cellulose esters, such as the acetate. It also dissolves rubber, gums, resins, lacquers, paint and varnish Refs 1) T.H, Durrans, Solvents , Van-Nostrand, NY (1938), 122 2) CondChemDict... 

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