Rubber permeation

Because the membrane selectivity and pressure ratio achievable in a commercial membrane system are limited, a one-stage membrane system may not provide the separation desired. The problem is illustrated in Figure 8.16. The target of the process is 90% removal of a volatile organic compound (VOC), which is the permeable component, from the feed gas, which contains 1 vol% of this component. This calculation and those that immediately follow assume a feed gas mixture VOC and nitrogen. Rubbery membranes such as silicone rubber permeate the VOC preferentially because of its greater condensability and hence solubility in the membrane. In this calculation, the pressure ratio is fixed at 20... 

A. J. Woytek and. E. Gentilecore, "A New Blow Mol ding Process to Reduce Solvent Permeation of Polyolefin Containers," paper no. 13 presented at ddpances in Blow Molding Conference Rubber and Plastics Institute, London, Dec. 6,1977. 

Permeation rates are dependent on the ehemieal makeup of the eontamination. This ineludes the size of the eontaminant (how large or small the moleeule or partiele is) and on the pore size of the proteetive material (for instanee, impermeable rubber suits, tyveks, or eotton eoveralls). Chemieal eharaeteristies (i.e., polarity, vapor pressure, pH) of both the eontaminant and the proteetive material also determine permeability. Keep in mind that gases, vapors, and low-viseosity liquids tend to permeate more readily than high-viseosity liquids or solids [2],... 

In the past chemical cure linings have been employed on a wide scale. These linings, usually based on natural rubber or acrylonitrile-butadiene rubber consist of a standard lining compound with a chemical activator such as dibenzylamine incorporated in the formulation. Prior to the application of the lining to the substrate, the individual sheets of rubber are dipped or brush coated with carbon disulphide or a solution of a xanthogen disulphide in a solvent. The carbon disulphide or xanthogen disulphide permeates the rubber and combines with the dibenzylamine to form an ultra-fast dithiocar-bamate accelerator in situ, and thus the rubber rapidly vulcanises at ambient temperature. 

Preservative availability may be appreciably reduced by interaction with packaging materials. Examples include the permeation of phenolic preservatives into the rubber wads and teats of multi-dose injection or eye-drop containers and by their interaction with flexible nylon tubes for creams. Quaternary ammonium preservative levels in formulations have been significantly reduced by adsorption onto the surfaces of plastic and glass containers. Volatile preservatives such as chloroform are so readily lost by the routine opening and closing of containers that their usefulness is somewhat restricted to preservation of medicines in sealed, impervious containers during storage, with quite short use lives once opened. 

Glass-seal ampoules provide the most impervious barrier for gas transmission. A butyl rubber stock is used with rubber-stoppered products that are sensitive to oxygen because it provides better resistance to gas permeation than other rubber stocks. 

H. Vromans and J. A. Van Laarhoven. Study on water permeation through rubber closures of injection vials. Int. J. Pharm., 79, 301 (1992). 

Diffusion of a gas or liquid through a semi-permeable material. The permeability of elastomers to gases varies with the elastomer type and with the gas. Butyl rubber is much less permeable to air than is natural rubber hence its use in tyre inner tubes and similar apphcations. The rate of permeation is generally related to the size of gas molecule, i.e., the smaller the molecule the higher the rate. The exception is C02 which has a rate 10 to 100 times greater than that of nitrogen. 

Crude sulfur vesicants are relatively stable and stability increases with purity Distilled materials show very little decomposition on storage. Solvents such as carbon tetrachloride and chlorobenzene have been added to enhance stability of crude material. Agents can be stored in glass or steel containers, although pressure may develop in steel containers. Sulfur vesicants rapidly corrode brass and cast iron, and permeate into ordinary rubber. 

The heat of permeation of chlorine in a silicone rubber membrane is —3 to — 5 kcal mol-1 while that of nitrogen is 1.0-1.5 kcal mol-1. Separation of these gases therefore is helped by low temperature. The flux of chlorine actually increases as temperature is lowered, reflecting the increased sorption. The flux of nitrogen decreases, reflecting the lower diffusivity. 

Because of the slow permeation of water, a method is standardised in BS 903 Part 18 [18] for rubbers, in which the material is cut into small particles by cutting, rasping or grinding. This results in a large ratio of surface area to volume and a relatively short time to equilibrium. The same approach could be applied to plastics. 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

Figure 5 Gel Permeation Chromatograms of Chlorinated Rubber (a) Guayule CR, Cl - 60% (b) Commercial CR-20, Cl - 64-65%...

Uses. Comonomer for preparation of cross-linked polymers in production of ion exchange beads and gel permeation chromatography polystyrene beads polymerization monomer for synthetic rubber, drying oils, and casting resins... 

Permeation (naturally permeability) of gas through materials such as rubber hoses, elastomer seals, etc. (unless these parts have become brittle and thus leaky ). 

The copolymer of isobutylene with a few percent isoprenc (butyl rubber) can be cured to produce an ozone-resistant elastomer with low permeability to oxygen and nitrogen. Butyl rubber has a Tt of — 70 C a refractive index of 1.5081, and a coefficient of linear expansion of 5.7 X 10 cm/cxn C. Chloro and bromo butyl rubber are more resistant to the permeation of oxygen and nitrogen than butyl rubber. 

Because of their widespread use in the American workplace, butyl rubber, nitrile latex, neoprene latex, poly(vinyl alcohol), surgical rubber latex, and Viton elastomer were chosen for the present studies. The composite/bonded substances of this study were not in all cases presently available as commercial material for protective garments, but rather were chosen to determine their potential for resistance to solvent permeation. Likewise, Teflon gloves were included in these studies simply because they are commercially available. 

Of the remaining materials in Table IV, only Viton, neoprene rubber latex,poly(vinyl alcohol), butyl rubber, and butyl-coated nylon exhibited at least a 20-min breakthrough time forl,2 dichloro-ethane permeation to occur. The nitrile rubber latex, cement dipped nitrile rubber, polyethylene (medium density), and surgical rubber latex were all penetrated by 1,2-dichloroethane in less than 3 min and would be of little use in situations requiring the garment to be in constant contact with 1,2-dichloroethane. From the above, butyl rubber or Viton appear to be the best materials to protect the worker against 1,2-dichloroethane, but because of apparent lot-to-lot variations(20) in butyl properties, Viton appears to be the best suited material of these studies to protect the worker from this chemical. 

PIB exhibits a comparatively low gas permeation (56). In Table 6.5, gas permeation coefficients of some polyolefins are given. Oppanol B 200 is compared with natural rubber, high density polyethylene) and low density poly(ethylene). Certain other Oppanol types have roughly the same permeability to gases as Oppanol B 200. 

Polymerizations were conducted in 250 x 20 mm test tubes fitted with rubber septum caps. Typically, the covered tubes were cooled to the desired temperature, the solvent and monomer were charged, and the temperature allowed to equilibrate. At this time a suitable volume of a solution of Lewis add coinitiator was introduced. The polymerization was terminated by addition of 1-2 ml of methanol. The polymer was recovered by allowing the solvent and unreacted monomer to evaporate, and rinsing with methanol to remove coinitiator residues. To insure that no polymer was lost by rinsing with methanol the gel permeation chromatograms of a number of samples were obtained before and after rinsing. They were found to be identical. The polymer was dried for at least 48 hrs at room temperature under vacuum. 

---