The causes of some of the differences in properties between compounded natural rubber and compounded synthetic poly (isoprene) have been traced to the insoluble non-rubber material in natural rubber. This material is mostly denatured proteins and is responsible for the higher modulus, faster scorch time, higher heat buildup, and higher hot tear strength of natural rubber. These properties may be related to the pigment effect of the denatured protein to act as a reinforcing filler at low concentrations (3–4 per cent by wt) as well as a curing activator. The greater green strength of compounded natural rubber has been related to its more perfect configurational regularity which contributes to faster crystallization. The crystallite concentration increases with increasing stress and the crystallites act like a reversible reinforcing pigment which disappears when the stress is released. The faster plastication rate has been related to the synthetic stabilizers used. Natural rubber hydrocarbon has been shown to be a high molecular lactone arranged in a six membered ring. We speculate natural rubber forms as a prosthetic group connected through a lactone linkage (or the δ-hydroxy acid precursor to the lactone) to a protein molecule in the cell of hevea brasiliensis. It is this structure of a high molecular weight hydrocarbon (natural rubber) attached to a (denatured) protein molecule that accounts for the remarkable dispersability of the insoluble fraction of natural rubber in rubber solvents : the rubber end of the structure tends to dissolve in the rubber solvent while the highly polar, insoluble protein end prevents solution. This structure is the reverse of a micelle in water in principle.
High resolution NMR has undergone a revolution in the last ten years or so. The ability to manipulate spin systems to a high degree in the pulse FT NMR experiment, coupled with advances in NMR computing systems, has led to the design of many multipulse and two-dimensional (2-D) NMR experiments which can provide considerably more information than a standard spectrum. In addition, experiments to obtain high resolution NMR spectra of solid materials have opened a whole area of chemistry to NMR. The new interpretive techniques for spectra obtained in solution have been applied to synthetic polymers or polymer chemicals only recently. However, the solid-state methods already have seen wide application in the polymer area. In this report, we describe some work from our laboratory employing some of these advanced methods in both solution and the solid state. This is not meant to serve as a detailed discussion of the techniques employed, but rather as an introduction to potential applications in the rubber and polymer industries.
Only 1,5-polyenes with three or more double bonds crosslinked in a curing recipe with 2-benzothiazolyl-N-morpholyl disulfide (BMD) or bis(dimethylthiocarbamyl) disulfide (TMTD) as the crosslinking agents. A suitable chemical model for 1,4-polybutadiene (PB) is cycIohexadeca-1,5,9,13-tetraene (CHT). The preparation of CHT is described. The crosslink density of a CHT vulcanizate agreed well with the chemical crosslink density of cis- 1,4-polybutadiene rubber, similarly cured. The crosslink density of the CHT vulcanizate was measured by isolating, identifying, and weighing the various crosslink structures. The non-crosslink structures of the network and the extranetwork structures were also identified and quantified. Various amounts of curative fragments attach to the PB network structure, as exemplified by the amount of curative fragments attached to CHT. About 16% of the lauric acid attached to the network structure. The curing agent, BMD, divided into three fragments: 2-thiobenzothiazole, morpholyl, and thio groups. About 67% of the thio groups, 25% of the 2-thiobenzothiazole groups, and about 6% of the morpholyl groups attached to the network structure. About 57% of the thio groups formed crosslinks. The main crosslink structure was 85% bis(allylic) monosulfide and 15% bis(allylic) disulfide. The length of the disulfide crosslink was only 60 pm greater than the length of the monosulfide crosslink. This compares with the 0.2 nm length of a sulfur atom. Very little cis-trans isomerization occurred in the unreacted CHT as a result of vulcanization. This indicated almost no cis-trans isomerization of unreacted segments of PB in PB vulcanizates. However, extensive cis-trans isomerization occurred in the CHT that crosslinked, which indicates that cis-trans isomerization in PB vulcanizates is confined to the reaction sites of the PB molecules. The configurational isomerism was essentially exclusively cis-to-trans. No trans-to-cis isomerism was observed. The insoluble solid in the vulcanizate was a mixture of cadmium bis(2-thiobenzothiazole), cadmium sulfide, and morpholinium sulfate. Material balances of the reaction products with the curatives showed that 90% by weight of the reaction products of the vulcanization of CHT were identified. The hydrogen transfer balance showed that CHT is the main hydrogen donor during crosslink formation. The morpholyl group from BMD was the main hydrogen acceptor and formed morpholine and morpholinium sulfate. Escape of volatile substances during vulcanization of PB resulted in much reduced crosslinking. For this reason, a hydraulic reactor for vulcanization of liquids, such as CHT, without loss of volatile substances was described.
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