Rubbers, diffusion

The reciprocal of the permeability is called the impedance, so that the above expression means that the impedance is additive. The same property has been observed for gas-rubber diffusion systems (3). 

The rubber biosynthesis was found to take place on the rubber surface of rubber particles by the incorporation of IPP into a terminal allylic diphosphate group of rubber molecules. It was assumed that the growing hydrocarbon chain of rubber diffuses into the rubber particles, while the hydrophilic diphosphate end group remains in the serum phase where it can react with IPP bound to the active site of the rubber transferase, as shown in Figure 2.2. It has been confirmed that the rubber transferase is tightly bound to the Hevea rubber particles even after purification by washing. 

Crystalline structures have a much greater degree of molecular packing and the individual lamellae can be considered as almost impermeable so that diffusion can occur only in amorphous zones or through zones of imperfection. Hence crystalline polymers will tend to resist diffusion more than either rubbers or glassy polymers. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

Adhesion development depends on diffusion of the CPO component of the primer through the crystalline boundary layers followed by swelling and entanglement with the rubber rich layer [75]. 

Some rubber base adhesives need vulcanization to produce adequate ultimate strength. The adhesion is mainly due to chemical interactions at the interface. Other rubber base adhesives (contact adhesives) do not necessarily need vulcanization but rather adequate formulation to produce adhesive joints, mainly with porous substrates. In this case, the mechanism of diffusion dominates their adhesion properties. Consequently, the properties of the elastomeric adhesives depend on both the variety of intrinsic properties in natural and synthetic elastomers, and the modifying additives which may be incorporated into the adhesive formulation (tackifiers, reinforcing resins, fillers, plasticizers, curing agents, etc.). 

One of the most common rubber adhesives are the contact adhesives. These adhesives are bonded by a diffusion process in which the adhesive is applied to both surfaces to be joined. To achieve optimum diffusion of polymer chains, two requirements are necessary (1) a high wettability of the adhesive by the smooth or rough substrate surfaces (2) adequate viscosity (in general rheological properties) of the adhesive to penetrate into the voids and roughness of the substrate surfaces. Both requirements can be easily achieved in liquid adhesives. Once the adhesive solution is applied on the surface of the substrate, spontaneous or forced evaporation of the solvent or water must be produced to obtain a dry adhesive film. In most cases, the dry-contact adhesive film contains residual solvent (about 5-10 wt%), which usually acts as a plasticizer. The time necessary... 

The dry adhesive films on the two substrates to be joined must be placed in contact to develop adequate autoadhesion, i.e. diffusion of polymer rubber chains must be achieved across the interface between the two films to produce intimate adhesion at molecular level. The application of pressure and/or temperature for a given time allows the desired level of intimate contact (coalescence) between the two adhesive film surfaces. Obviously, the rheological and mechanical properties of the rubber adhesives will determine the degree of intimacy at the interface. These properties can be optimized by selecting the adequate rubber grade, the nature and amount of tackifier and the amount of filler, among other factors. 

The diffusion process in natural and polychloroprene rubber adhesives can be explained by Campion s approach [1] which considers the concept of molecular free volume. This free volume is mainly affected by the solvent mixture of the adhesive (which will determine the degree of uncoiling of rubber chains) and by the ingredients in the formulation (mainly the amount and type of tackifier). 

Plasticizers reduce hardness, enhance tack and reduce cost in rubber base adhesive formulations. A plasticizer must be easily miscible and highly compatible with other ingredients in the formulations and with the surfaces to which the adhesive is applied. The compatibility and miscibility of plasticizers can be estimated from the solubility parameter values. Most of plasticizers have solubility parameters ranging between 8.5 and 10.5 hildebrands. However, the high miscibility and compatibility also lead to easier diffusion of the plasticizer to the surface, decreasing the adhesion properties. Therefore, plasticizers should be carefully selected and generally combinations of two or more of them are used. 

An antiozonant should have adequate solubility and diffusivity characteristics. Since ozone attack is a surface phenomenon, the antiozonant must migrate to the surface of the rubber to provide protection. Poor solubility in rubber may result in excessive bloom. 

Chlorinated rubber is also used to promote the adhesion of solvent-borne CR adhesives to metals and plasticized PVC. Addition of a low molecular weight chlorinated rubber (containing about 65 wt% chlorine) improves the shear strength and creep resistance of polychloroprene adhesives [75] but a reduction in open time is also produced. A heat reactivation (process in which the surface of the adhesive film is raised to 90-100°C to destroy the crystallinity of the film and allowing diffusion to produce polymer chain interlocking more rapidly) restores tack to the polychloroprene adhesives. 

Surface evaporation can be a limiting factor in the manufacture of many types of products. In the drying of paper, chrome leather, certain types of synthetic rubbers and similar materials, the sheets possess a finely fibrous structure which distributes the moisture through them by capillary action, thus securing very rapid diffusion of moisture from one point of the sheet to another. This means that it is almost impossible to remove moisture from the surface of the sheet without having it immediately replaced by capillary diffusion from the interior. The drying of sheetlike materials is essentially a process of surface evaporation. Note that with porous materials, evaporation may occur within the solid. In a porous material that is characterized by pores of diverse sizes, the movement of water may be controlled by capillarity, and not by concentration gradients. 

Finally, a property of practical importance which may be noted is the ability of noble gases, especially He, to diffuse through many materials commonly used in laboratories. Rubber and PVC... 

Explds at 60° after 13 sec in a sealed glass tube (Ref 4). Explds spontaneously when frozen and then thawed. Compd is a violent expl, extremely sensitive to impact or friction. It Jalso explds on exposure to strong light (sunlight or diffused), or when in contact with P, As, ozone, fused alkalies, and organic matter such as turpentine rubber, but not with sugar or resins. Metals strong acids do not cause it to expld,... 

Blends of carboxylated nitrile rubber (XNBR) with EPDM are likely to provide an attractive combination of properties including oil resistance, heat and ozone resistance, high tensile strength, modulus, and hardness. However, the polar curing ingredients often diffuse from the nonpolar to polar component, thereby producing cure rate mismatch and inferior properties. Three different measures have been used to overcome the cure rate mismatch [29] ... 

FIGURE 11.1S Diffused particles in natural mbber/ethylene-propylene monomer/rranj-polyoctylene rubber (NR-EPM-TOR) blend (a) are much smaller than in blends without TOR (b). (From Chang, Y.-W., Shin, Y.-S., Chun, H and Nab, C., J. Appl. Polym. Set, Ti, 749, 1999.)... 

Unlike a plastic blend where the properties largely depend on the properties of the individual component and the compatibUizer used, those of a rubber blend depend on the solubility and diffusivity of the curatives, reaction rates, scorch time, etc. Figure 11.16 gives relative cure rate and scorch time for a number of accelerators. Hence, in designing a rubber blend, aU these parameters have to be taken into consideration in order to obtain good properties along with good processability. 

The most effective antiozonants are the substituted PPDs. Their mechanism of protection against ozone is based on the scavenger-protective film mechanism [68-70]. The reaction of ozone with the antiozonant is much faster than the reaction with the C=C bond of the rubber on the rubber surface [56]. The rubber is protected from the ozone attack tUl the surface antiozonant is depleted. As the antiozonant is continuously consumed through its reaction with ozone at the mbber surface, diffusion of the antiozonant from the inner parts to the surface replenishes the surface concentration to provide the continuous protection against ozone. A thin flexible film developed from the antiozonant/ozone reaction products on the mbber surface also offers protection. 

Diffusion Coefficients for iV-lsopropyl-A/ -Phenyl-p-Phenylenediamine (IPPD), A/-(1,3-Dimethylbutyl)-iV -Phenyl-p-Phenylenediamine (6PPD), and A/-(1 -Phenylethyl)-A/ -Phenyl-p-Phenylenediamine (SPPD) (See Figure 15.14), in Different Rubbers and at Different Temperatures... 

The lower diffusion coefficient observed for A -(l-phenylethyl)-A -phenyl-p-phenylenediamine (SPPD) compared to that of IPPD and 6PPD was explained by an increased MW and/or increased compatibility with the rubbers. 

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