Synthetic polyisoprene rubbers activators

We will now apply to natural rubber, i.e., NR, biosynthesis what we have learned so far. As mentioned before, in 2005 approximately 10 million tons of NR was produced worldwide for commercial use, from which about 15% was consumed in the United States. While the United States is self-sufficient in S5mthetic mbber production, with substantial export activities, no NR is produced domestically. The development of a U.S.-based supply of NR is recognized in the Critical Agricultural Materials Act of 1984 (Laws 95-592 and 98-284). The Act recognizes that NR latex is a commodity of vital importance to the economy and the defense of the nation. It is important to emphasize that synthetic polyisoprene does not match the performance of imported Hevea NR in several applications, so NR remains irreplaceable. 

Natural rubber invariably contains fatty acids which have an activating effect in accelerated sulphur vulcanisation systems. This type of activator is absent in synthetic polyisoprenes and must be added in sufficient quantity (1 -5-2 0 phr) to ensure full crosslink development. Alternative activators of a similar chemical nature, such as zinc stearate or zinc 2-ethylhexanoate, have been used at lower levels (see Section 7). 

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. 

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). 

For rubbers such as NR (or synthetic 1,4-polyisoprene) (Santangelo and Roland, 1998) and polybutadiene (both 1,2- and 1,4-isomers) (CareUa et al., 1986), LCB increases the temperature sensitivity of the viscosity and terminal relaxation time. Thus, by comparing apparent activation energies, or, in the more usual case where the behavior is non-Arrhenius, the temperature dependence of the ratio of relaxation times for an unknown and a linear sample of the same polymer, inferences can be drawn concerning LCB (Figure 3.9). For polymers such as 1,4-polyisoprene, which have a dipole moment parallel to the chain, dielectric measurements of the normal mode can be used to measure the temperature dependence and thus assess the presence of LCB. 

For rubbers such as NR (or synthetic 1,4-polyisoprene) [175] and polybutadiene (both 1,2- and 1,4-isomers) [176], LCB increases the temperature-sensitivity of the viscosity and terminal relaxation time. Thus, by comparing apparent activation energies, or, in the more usual case where the behavior is... 

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