Natural rubber, pyrolysis products

From the time that isoprene was isolated from the pyrolysis products of natural mbber (1), scientific researchers have been attempting to reverse the process. In 1879, Bouchardat prepared a synthetic rubbery product by treating isoprene with hydrochloric acid (2). It was not until 1954—1955 that methods were found to prepare a high ar-polyisoprene which duplicates the structure of natural rubber. In one method (3,4) a Ziegler-type catalyst of trialkylaluminum and titanium tetrachloride was used to polymerize isoprene in an air-free, moisture-free hydrocarbon solvent to an all t /s- 1,4-polyisoprene. A polyisoprene with 90% 1,4-units was synthesized with lithium catalysts as early as 1949 (5). 

The usefulness of analytical pyrolysis in polymer characterization, identification, or quantitation has long been demonstrated. The first application of analytical pyrolysis can be considered the discovery in 1860 of the structure of natural rubber as being polyisoprene [10]. This was done by the identification of isoprene as the main pyrolysis product of rubber. Natural organic polymers and their composite materials such as wood, peat, soils, bacteria, animal cells, etc. are good candidates for analysis using a pyrolytic step. 

The dependence of the composition of the pyrolysis products on temperature can be exemplified by the study of the monomer and dimer formation during the flash pyrolysis of natural rubber at different temperatures [1]. Figure 4.1.2 shows the plot of monomer/dimer ratio for flash pyrolysis of rubber at discrete temperatures between 300° C and 500° C. The figure indicates the increase in monomer formation at higher temperatures. More examples will be given when pyrolysis for particular biopolymers is discussed. 

Other pyrolysis products besides isoprene and its dimer are also formed from rubber. The more volatile compounds with the maximum of five carbon atoms generated from the pyrolysis of natural rubber at 700 C are indicated in Table 6.1.1. 

Table 6.1.1. Volatile pyrolysis products (Cs or lower) from natural rubber.

The main oxidation products found during oxidative pyrolysis of natural rubber are given in Table 6.1.3. 

An important field of applications for analytical pyrolysis is the analysis of synthetic and natural rubber and of rubber blends. Some applications of analytical pyrolysis on this subject are discussed in Section 7.1 and 7.2. Many other applications are reported in literature, including some discussing the formation of various pyrolysis products in the processing of used tires [20, 21]. 

Polybutadiene, CAS 9003-17-2, is a common synthetic polymer with the formula (-CH2CH=CHCH2-)n- The cis form (CAS 40022-03-5) of the polymer can be obtained by coordination or anionic polymerization. It is used mainly in tires blended with natural rubber and synthetic copolymers. The trans form is less common. 1,4-Polyisoprene in cis form, CAS 9003-31-0, is commonly found in large quantities as natural rubber, but also can be obtained synthetically, for example, using the coordination or anionic polymerization of 2-methyl-1,3-butadiene. Stereoregular synthetic cis-polyisoprene has properties practically identical to natural rubber, but this material is not highly competitive in price with natural rubber, and its industrial production is lower than that of other unsaturated polyhydrocarbons. Synthetic frans-polyisoprene, CAS 104389-31-3, also is known. Pyrolysis and the thermal decomposition of these polymers has been studied frequently [1-18]. Some reports on thermal decomposition products of polybutadiene and polyisoprene reported in literature are summarized in Table 7.1.1 [19]. 

Table 7. 1.5. Oxidative pyrolysis products of natural rubber.

The GC results are computed automatically with the aid of a calculator. The natural rubber butadiene-styrene rubber butadiene rubber ratio in the sample material is printed out together with the initial data on pyrolysis product peak retention time, peak area, etc.). This system has been used successfully by Coulter and Thompson [69] for over 2 years in industrial analysis. As a result, the quality of the end product (tyres) was drastically improved. A similar device can be developed using a furnace-type pyrolyser. 

Naturally, the more complex the composition of the substances to be pyrolysed, the more characteristics are needed for identification. For example, in identifying isoprene rubbers (NK, SKN-3, SKIL, Natsyn, Coral, Cariflex IR), the characteristic pyrolysis products are isoprene and dipentene, whereas with butadiene rubbers (SKB, SKD, Budene, Diene NF, Buna CB, Asadene NF, Cariflex BR, Ameripol CB) they are butadiene and vinylcyclohexane. With copolymer rubbers, the number of characteristic products necessary for identification increases to three, viz., butadiene, vinylcyclohexene and styrene are used for butadiene -styrene rubbers (SKS-10, SKS-30, Buna S. Europrene-1500, Solprene) and butadiene, vinylcyclohexene and methylstyrene are used for butadiene-methylstyrene rubbers (SKMS-10, SKMS-30) [139, 140]. Fig. 3.12 [139, 140] shows as an example pyrograms of individual general-purpose rubbers and a four-component mixture of rubbers. The shaded peaks correspond to those components in the pyrolysis products which are used for identification. The ratio of the pyrolysis products changes depending on the composition of the copolymer and the structure of the polymer. 

Mixtures, formulated blends, or copolymers usually provide distinctive pyrolysis fragments that enable qualitative and quantitative analysis of the components to be undertaken, e.g., natural rubber (isoprene, dipentene), butadiene rubber (butadiene, vinylcyclo-hexene), styrene-butadiene rubber (butadiene, vinyl-cyclohexene, styrene). Pyrolyses are performed at a temperature that maximizes the production of a characteristic fragment, perhaps following stepped pyrolysis for unknown samples, and components are quantified by comparison with a calibration graph from pure standards. Different yields of products from mixed homopolymers and from copolymers of similar constitution may be found owing to different thermal stabilities. Appropriate copolymers should thus be used as standards and mass balance should be assessed to allow for nonvolatile additives. The amount of polymer within a matrix (e.g., 0.5%... 

Pyrolysis in an inert atmosphere under precisely controlled conditions (Figure 4) generates duplicable amounts of products, which are separated by capillary GC and provide an estimate of the ratio of polymeric constituents. Natural rubber produces iso-prene and limonene as two of the characteristic products, which distinguish it from polybutadiene (BR), styrene-butadiene co-polymer (SBR), butyl rubber (HR), and some of the other polymers. Quantification involving a mixture of polymers requires calibration curves derived from similar combinations of polymers (Figure 5). Cured and uncured formulations require separate calibrations and the differences in the microstructure of a polymer affect the products obtained on pyrolysis. 

MS has been used as a means of obtaining accnrate information regarding breakdown products produced upon pyrolysis of polymers. This includes applications to PS [152, 153], PVC [154], polyethers [155], PVC-polycarboxy piperadine polyurethanes [156], phenolics [157], PTFE [158], polybenzimidazole epoxies [159], ethylene-vinyl acetate copolymers [160], ethylene-vinyl alcohol copolymers [161], polybenzoxazines [162], polyxylyene sulfides [163], trimethoxysiloxy-snbstitnted polyoxadisilpentanylenes [164], chlorinated natural rubber [165], and polyacrylonitrile [166]. 

Krishen [42] obtained the products listed in Table 4.10 by pyrolysis of ethylene-butadiene rubber and ethylene-propylene-diene terpolymer. He showed that the 2-methyl-2-butene peak was linear with the natural rubber content of the sample. Styrene-butadiene rubber was determined from the peak area of the 1,3-butadiene peak. The ethylene-propylene-terpolymer content was deducted from the 1-pentane peak area of the pyrolysis products. 

The major degradation product of natural rubber is l-methyl-4-(l-methylethenyl)cyclo hexene. The presence of this compound as the major degradation product along with 2-methyl-1,3-butadiene (monomer) and groups of compounds containing 15 and 20 carbon atoms (three and four monomer units) in the pyrolysate of a rubber is sufficient to identify it as natural rubber. Similarly, the presence of l-chloro-4-(l-chloroethenyl)cyclohexene and 2-chloro-l, 3-butadiene, the cyclic dimer and monomer of poly(chloroprene) rubber, in the pyrolysate of a rubber identify it as poly(chloroprene) rubber. A correlation between the crosslink density and the product ratio of isoprene dimer species to isoprene formed from pyrolysis of natural rubber vulcanisates has been reported 697436 [a.232]. The major products of the isoprene dimer species were l,4-dimethyl-4-vinylcyclohexene and... 

Groves and co-workers [a.237] analysed the oil derived from the pyrolysis of natural rubber in a Py-GC at 500 °G. These researchers showed that the major products were the monomer, isoprene, and the dimer dipentene, with other oligomers up to hexamer also being formed in significant concentrations. It was suggested that the isoprene monomer was formed via a depropagating mechanism in the polymer chain, and that dipentene dimer was formed either by intramolecular cyclisation followed by scission, or by monomer recombination via a Diels-Alder reaction. 

Products obtained by pyrolysis of other polymers is reviewed in Table 4.5. Some specific applications of the chromatography-MS technique to various types of polymers include the following PE [34,35], poly(l-octene) [29], poly(l-decene) [29], poly(l-dodecene) [29], CPE [36], polyolefins [37, 38], acrylic acid-methacrylic acid copolymers [39, 40], polyacrylate [41], nitrile rubber [42], natural rubbers [43, 44], chlorinated natural rubber [45, 46], polychloroprene [47], PVC [48-50], polysilicones [51, 52, 53], polycarbonates [54], styrene-isoprene copolymers [55], substituted olystyrene [56], PP carbonate [57], ethylene-vinyl acetate [58], Nylon 66 [59], polyisopropenyl cyclohexane-a-methyl styrene copolymers [60], cresol-novolac epoxy resins [61], polymeric flame retardants [62], poly(4-N-alkyl styrenes) [63], polyvinyl pyrrolidone [64], polybutyl-cyanoacrylate [65], polysulfides [66], poly(diethyl-2-methacryl-oxy) ethyl phosphate [67, 68], polyetherimide [69], bisphenol-A [70], polybutadiene [71], polyacenaphthalene [72], poly(l-lactide) [73], polyesterimide [74], polyphenylene triazine [75], poly-4-N-vinyl pyridine [76], diglycidylether-bisphenol-A epoxy resins [77], polyvinylidene chloride [78] and poly-p-chloromethyl styrene [79]. 

J.A. Hiltz and T. Foster, Direct Exposure Probe/Mass Spectrometry and Pyrolysis-Gas Chromatography/Mass Spectrometry Study of the Effect of Gamma Radiation Exposure Thermal Degradation Products of Natural Rubber Polycaprolactone Mixtures, Report No. DREA-TM-95/214, Defence R D, Canada, 1995. 

The thermal black process, which was developed in the 1930s, is still used for the production of coarse carbon blacks (nonreinforcing carbon blacks) for special applications in the rubber industry. Contrary to the above-described processes, energy generation and the pyrolysis reaction are not carried out simultaneously. Natural gas eventually blended with vaporized oil is used as both a feedstock and a fuel. 

This is why Py-GC seems to be the best analytical method [7, 70]. Impulse pyrolysis of rubbers yields characteristic products (volatile monomers or dimers) of the sample polymer materials, the nature of the resulting products being only slightly dependent on the presence of non-polymeric ingredients and the degree of polymerization. 

The reaction of the pyrolysis vapors with mercury(II) oxide will differentiate between these materials. To do this, heat a dry sample of the plastic in the pyrolysis tube closed with a piece of prepared filter paper. To prepare the paper, drench it with a solution of 0.5 g yellow mercury(ll) oxide in sulfuric acid (1.5 ml concentrated sulfuric acid added to 8 ml water, carefully). If the vapor gives a golden yellow spot, this indicates polyisobutylene, butyl rubber, and polypropylene (the latter only after a few minutes). Polyethylene does not react. Natural and nitrile rubber as well as polybutadiene yield a brown spot Waxlike greases are the products in the pyrolysis of polyethylene and polypropylene. Polyethylene smells like paraffin, and polypropylene is slightly aromatic. 

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