Synthetic polyisoprene rubbers compounding

Jenke [73] studied the extractabihty of aniline, diphenylguanidine, dedenzyl-amine, and triisopropanolamine from a synthetic polyisoprene rubber similar to the material used in pharmaceutical applications. Rubber samples were autoclaved (121 °C) in contact with water or NaCl 0.9% solution for lh.Table 33 presents the concentration of each compound in solution after the extraction procedure using 2g rubber material. Extraction profiles ranged between 1.64 and 3.73 mg/L, with the exception of diphenylguanidine, whose extraction yield reached 11.76 mg/L. 

Between the 1920s when the initial commercial development of mbbery elastomers based on 1,3-dienes began (5—7), and 1955 when transition metal catalysts were fkst used to prepare synthetic polyisoprene, researchers in the U.S. and Europe developed emulsion polybutadiene and styrene—butadiene copolymers as substitutes for natural mbber. However, the tire properties of these polymers were inferior to natural mbber compounds. In seeking to improve the synthetic material properties, research was conducted in many laboratories worldwide, especially in the U.S. under the Rubber Reserve Program. 

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. 

Natural rubber (NR) and guttapercha consist essentially of polyisoprene in cis-l, 4 and trans-1,4 isomers, respectively. Commercially produced synthetic polyisoprenes have more or less identical structure but reduced chain regularity, although some may contain certain proportions of 1,2- and 3,4-isomers. Microstructure differences not only cause the polymers to have different physical properties but also affect their response to radiation. The most apparent change in microstructure on irradiation is the decrease in unsaturation. It is further promoted by the addition of thiols and other compounds.130 On the other hand, antioxidants and sulfur were found to reduce the rate of decay of unsaturation.131 A significant loss in unsaturation was found, particularly in polyisoprenes composed primarily of 1,2- and 3,4-isomers.132,133... 

Rubber tyres are by far the most visible of rubber products. Identification is trivial and collection is well organized. Recycling and disposal, however, are less evident. A major route for tyres is their use as a supplemental fuel in cement kilns. Major compounds in tyres are styrene-butadiene rubber (SBR), synthetic and natural polyisoprene rubber, steel cord, carbon black, zinc oxide, sulphur and vulcanization-controlling chemicals. Tyres can be retreaded, which is economic for large sizes (truck tyres), or ground to crumb or powder (cryogenic grinding). Such materials have some limited market potential as an additive in asphalt, and in surfaces for tennis courts or athletics. 

Before reviewing in detail the fundamental aspects of elastomer blends, it would be appropriate to first review the basic principles of polymer science. Polymers fall into three basic classes plastics, fibers, and elastomers. Elastomers are generally unsaturated (though can be saturated as in the case of ethylene-propylene copolymers or polyisobutylene) and operate above their glass transition temperature (Tg). The International Institute of Synthetic Rubber Producers has prepared a list of abbreviations for all elastomers [3], For example, BR denotes polybutadiene, IRis synthetic polyisoprene, and NBR is acrylonitrile-butadiene rubber (Table 4.1). There are also several definitions that merit discussion. The glass transition temperature (Tg) defines the temperature at which an elastomer undergoes a transition from a rubbery to a glassy state at the molecular level. This transition is due to a cessation of molecular motion as temperature drops. An increase in the Tg, also known as the second-order transition temperature, leads to an increase in compound hysteretic properties, and in tires to an improvement in tire traction... 

From the point of view of the physical characteristics of rubber compounds, their processing and vulcanisate properties, the significant differences between natural rubber and c/5-polyisoprene and among different sources of the synthetic polymers relate to ... 

In general, synthetic polyisoprenes may be compounded and processed using the same or very similar formulations and techniques to those used for natural rubber. 

For these processing operations rubber compounds undergo high shear deformation and the flow characteristics of synthetic polyisoprenes can be used to advantage. 

Following the mixing operation polyisoprenes should have higher Mooney viscosities than equivalent natural rubber compounds but, nevertheless, will still exhibit faster flow under high shear. Compounds based on the synthetic polymer may be cold fed to extruders in which the necessary prewarming prior to extrusion is easily achieved or, alternatively, for hot feed extruders premilling should be kept to a minimum to avoid too... 

The generally accepted upper limit of temperature for moulding natural rubber compounds is around 180 °C above which there is a tendency to foul moulds. Due to their lower content of non-rubbers synthetic polyisoprenes may tolerate mould temperatures of 200 °C. The advantage of lower fouling tendency is evident in reduced down time of machines while moulds are changed for cleaning. 

In gum or mineral filled compounds, synthetic polyisoprenes exhibit longer times to optimum cure than natural rubber and significant differences between the various synthetic polymers are observed with some vulcanisation systems. The lack of naturally occurring amine activators present in natural rubber becomes evident in a simple thiazole-accelerated sulphur cure system. This is very well illustrated by observing the incremental addition of diphenylguanidine (DPG) to a MBT-accelerated sulphur system (Table 5) where the lack of added amine accelerator shows a very slow cure for the synthetic polymer but, if sufficient is added, the differences between the rubbers become indiscernible. 

From the practical standpoint, two minor alterations in cure system should be considered if the cure characteristics of a synthetic polyisoprene need to be altered to match exactly that of a natural rubber compound. A secondary accelerator (e.g. 0T-0-2phr of TMTD) may be added to a sulphur/sulphenamide system to reduce time to optimum cure. An increase of between 5 and 15 %, pro rata, in both sulphur and accelerator will improve the degree of cure, as indicated by maximum torque value on a Monsanto rheometer. 

This also shows that the lower compound green strength associated with the synthetic polyisoprenes relative to natural rubber is not an insurmountable problem. However, the incipient problem has been generally avoided by not making a total replacement of the natural rubber in tyre compounds which are subject to substantial deformation during building operations. 

Catalysts. Iodine and its compounds ate very active catalysts for many reactions (133). The principal use is in the production of synthetic mbber via Ziegler-Natta catalysts systems. Also, iodine and certain iodides, eg, titanium tetraiodide [7720-83-4], are employed for producing stereospecific polymers, such as polybutadiene mbber (134) about 75% of the iodine consumed in catalysts is assumed to be used for polybutadiene and polyisoprene polymeri2a tion (66) (see RUBBER CHEMICALS). Hydrogen iodide is used as a catalyst in the manufacture of acetic acid from methanol (66). A 99% yield as acetic acid has been reported. In the heat stabiH2ation of nylon suitable for tire cordage, iodine is used in a system involving copper acetate or borate, and potassium iodide (66) (see Tire cords). 

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