Natural Rubber Layer

The proven compounding practice for the natural rubber layer is to load it with 30 phr of graphite, 30 phr of semi-reinforcing carbon black filler and 50 phr of inert filler, china clay. A low sulfur curing system is adopted for heat resistance with a suitable antioxidant to prevent flex-cracking. A typical chlorine resistant soft natural rubber compound formula is given in Table 3.2. 

S. Varghese, J. Karger-Kocsis, K. G. Gatos (2003) Melt compounded epoxidized natural rubber/layered silicate nanocomposites structure-properties relationships. Polymer 44, 3977. 

A thin layer of a mix of natural rubber, sulfur, precipitated silica, water, and some additives, such as carbon black and vulcanizing agents, is extruded on a paper support belt, calendered, and vulcanized as a roll in an autoclave under elevated pressure and temperature ( 180 °C). A modi-... 

Elastomers Solutions of NR have been used for laminating textiles for over a century. The Macintosh raincoat, invented in 1825, consisted of two sheets of cotton adhered by an inner layer of natural rubber. SBR is used as an adhesive in carpet backing and packaging. Neoprene (polychloroprene) may be blended with a terpene or phenolic resin and used as a contact adhesive for shoes and furniture. 

Solutions of NR have been used for laminating textiles for over a century. The mackintosh raincoat consists of two sheets of cotton adhered by an inner layer of natural rubber. 

The effect of polymer-filler interaction on solvent swelling and dynamic mechanical properties of the sol-gel-derived acrylic rubber (ACM)/silica, epoxi-dized natural rubber (ENR)/silica, and polyvinyl alcohol (PVA)/silica hybrid nanocomposites was described by Bandyopadhyay et al. [27]. Theoretical delineation of the reinforcing mechanism of polymer-layered silicate nanocomposites has been attempted by some authors while studying the micromechanics of the intercalated or exfoliated PNCs [28-31]. Wu et al. [32] verified the modulus reinforcement of rubber/clay nanocomposites using composite theories based on Guth, Halpin-Tsai, and the modified Halpin-Tsai equations. On introduction of a modulus reduction factor (MRF) for the platelet-like fillers, the predicted moduli were found to be closer to the experimental measurements. 

These same types of compounds are also more resistant to many acids at high temperatures than natural rubber can handle. Neoprene should not be used in parts which are bonded to metal for hydrochloric acid service because acid migration can cause failures. For hydrochloric acid service ebonite lined mild steel equipment is the correct selection. Ebonites form rubber hydrochloride film in contact with natural rubber and this film is the protective layer against corrosion. 

Antiozonents protect the rubber surface by the formation of a protection layer, the ozonides on the surface of rubber by reaction of the antiozonents with ozone. Certain polymers also provide good ozone protection. The use of 10-20 parts of EPDM, a low diene rubber, in natural rubber compound significantly increases ozone resistance. 

For wet grinding of copper ore in the ball mills at ambient temperature a 6 mm thick natural rubber compound of 45°-50°A was used successfully as an abrasion resistant layer. 

Several NMR relaxation studies using carbon-black-filled natural rubber (NR), EPDM and butadiene (BR) rubbers have shown that a layer of immobilised, tightly bound rubber is formed on the carbon black surface [20, 62, 79, 87, 89] (Figure 10.9). 

Another example of a commercially available three layer tile is available from Burke Rubber Co. as part number BR-8899-117. This tile is a foamed natural rubber with added fillers, and was designed to have an echo reduction of 7.5 dB at frequencies above 7.5 kHz. Its performance as measured in our laboratory is shown in Figure 3. 

The stability of latex is due to a thin layer of proteins on particles, which acts as a colloid stabilizer. Natural rubber is practically obtained by the precipitation and drying of the latex. The precipitation is done with acids (acetic acid is commonly used for this purpose) when the isoelectric point of the protecting protein is reached (pH 4.6). The macromolecules have a MW between 5 10 to 3 10 Dalton and contain between 600 to 50,000 units of isopentene. Due to the double bond, both cis and trans isomers are possible for the monomer units. It was determined that natural rubber is an isotactic polymer formed exclusively from cis units and has the following (idealized) structure (in reality the polymer is not perfectly planar) ... 

Thermal aging of styrene-butadiene rubber (SBR) and natural rubber (NR) exposed to air leads to a hard surface layer which grows in thickness up to... 

When there is a relative motion between the corroding liquid and the metal or rubber surface the rate of attack of the damage to the surface is increased. The process is called sweating off with the corrosion product thus exposing the base surface again to corrosion. Otherwise the corrosion product (as a newly formed protective layer) would have prevented or slowed down further corrosion, just as in the case of hypochlorous acid solution on natural rubber lining, where the protective corrosion products exhibit very low cohesion and as such prone to be wiped off by the liquid unlike wet chlorine or hydrochloric acid which form a strong layer of corrosion product well adhered to the rubber surface. 

There is yet another type of lining called the Triflex lining. Triflex is a three ply, soft-hard-soft natural rubber construction. The soft rubber is compounded for maximum rubber to metal adhesion. The middle hard rubber layer, a flexible ebonite composition,... 

This is red lead monoxide, used as in inorganic accelerator for the curing of soft natural rubber lining in an autoclave. High temperature curing leads to blooming of lead film, which is the chemical resistance layer, but impairs bonding with metal. 

While sheeting in a calender, the stock must be soft so that they penetrate the interstices of the layers easily, and tacky so that ready adhesion between plies is ensured. Inevitably, such stocks can give rise to handling problems and call for compromise compounding . Natural rubber and Neoprene rubbers compounded with resins such as coumarone indene or petroleum resin produce good sheeting stocks. 

Consider the disposable diaper. The outer, waterproof layer is composed of polyethylene. The polymeri2ation reaction that produces polyethylene is shown in Section 12.4. The diapers have elastic to prevent leaking. The elastic is made of a natural polymer, rubber. The monomer from which natural rubber is formed is 2-methyl-l,3-butadiene. The common name of this monomer is isoprene. As we will see in coming chapters, iso-prene is an important monomer in the synthesis of many natural polymers. 

Rubber consumption is dominated by tyre production. In these, conveyor belts, and pressure hoses, thin layers of either steel wire or polymeric fibre reinforcement take the main mechanical loads. These layers, with rubber interlayers, allow flexibility in bending, whereas the reinforcement limits the in-plane stretching of the product. The applications are dominated by natural rubber and styrene butadiene copolymer rubber (SBR). Other rubbers have specialised properties butyl rubbers have low air permeability, nitrile rubbers have good oil resistance, while silicone rubbers have high and low temperature resistance. Rubbers play a relatively small role in this book, but the rubbery behaviour of the amorphous phase in semi-crystalline thermoplastics is important. 

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