Oxygen permeability plastics

Oxygen-sensitive membrane electrodes are commercially available. The electrode in such a system is covered with an oxygen-permeable plastic membrane, thus protecting it from impurities. The current is proportional to the activity of dissolved molecular oxygen, and at low concentration, to the amount of DO. Before the measurement of DO in the sample, calibrate the electrodes using standards of known DO concentrations (determined from iodometric titrations). 

Fig. 4. Oxygen permeability in a vinylidene chloride copolymer film at selected levels of plasticizer (Citroflex A-4). Plasticizer level in parts pet hundred resin...

Hard lenses can be defined as plastic lenses that contain no water, have moduli in excess of 5 MPa (500 g/mm ), and have T well above the temperature of the ocular environment. Poly(methyl methacrylate) (PMMA) has excellent optical and mechanical properties and scratch resistance and was the first and only plastic used as a hard lens material before higher oxygen-permeable materials were developed. PMMA lenses also show excellent wetting in the ocular environment even though they are hydrophobic, eg, the contact angle is 66°. 

A large membrane pack of this type will act like an artificial gill, permitting a swimmer to breathe like a fish and remain submerged for much longer periods of time than are possible with scuba equipment. Speculative fiction has man returning to live in the seas, and this type of application may make it possible. Their application in spacecraft is obvious as a part of a continuously recycled air support system. The oxygen permeability of silicone materials is just one example of the selective permeability of plastics. 

Polystyrene (PS), 7 610t 23 326, 348, 358. See also Polystyrenes Styrene Styrene plastics biodegradation of, 23 376 brominated, 11 470-474 chain transfer to, 23 383 colloidal suspensions, 7 275 crystalline syndiotactic, 23 388 decomposition of, 14 109 effect of orientation on oxygen permeability, 3 393t... 

The traditional definition of a barrier polymer required ao oxygen permeability less than 2 nmol/(m s GPa) (originally, less dian (i cc-mil)/(100 mX d atm)) at room temperature. This definition was based partly on function and partly on conforming to the old commercial unit of permeability. The old commercial unit of permeability was created so that the oxygen permeability of Saran Wrap brand plastic film, a trademark of The Dow Chemical Company, would have a numerical value of 1. 

Finally, PSPs based on the OEP metal complexes in various cellulose derivative polymers also have been introduced. Cellulose derivative polymers such as ethyl cellulose, cellulose acetate etc., as shown in Fig. 9, also provide a mechanical strength to thin film. The oxygen permeability, diffusion constant, and solubility for oxygen for typical cellulose derivative polymers are listed in Table 1. Cellulose derivative polymer films with plasticizer tributyl phosphate (TBP) have large oxygen permeability. Cellulose acetate with TBP... 

The effect of plasticizers and temperature on the permeability of small molecules in a typical vinylidene chloride copolymer has been studied thoroughly. The oxygen permeability doubles with the addition of about 1.7 parts per hundred resin (phr) of common plasticizers, or a temperature increase of 8°C (91). The effects of temperature and plasticizer on the permeability are shown in Figure 4. The moisture (water) vapor transmission rate (MVTR or WVTR) doubles with the addition of about 3.5 phr of common plasticizers (92). The dependence of the WVTR on temperature is a litde more complicated. WVTR is commonly reported at a constant difference in relative humidity and not at a constant partial pressure difference. WVTR is a mixed term that increases with increasing temperature because both the fundamental permeability and the fundamental partial pressure at constant relative humidity increase. Carbon dioxide permeability doubles with the addition of about 1.8 phr of common plasticizers, or a temperature increase of 7°C (93). 

Figure 2 shows the oxygen permeability coefficient for EvOH-32 as a function of water content at 20C. The oxygen barrier effectiveness of EvOH-32 decreases markedly with the absorption of water because of plasticization of the EvOH-32 matrix. 

Polymer Molecular Structure. We may expect polymer molecular structure to have an enormous effect on the rate of transport of small molecules through them. This fact may be most easily appreciated by examining gas transport because solubility of these molecules is quite low and, thus, not affected by plasticization effects as mentioned earlier. Table I gives the oxygen permeability in a wide range of polymers at 25 C as collected from a variety of published (22) and unpublished sources. Among the olefins listed. 

The addition of plasticizer used for the improvement of mechanical properties leads generally to an increase in the oxygen permeability coefficient due to the higher mobility of the polymer chain and higher free volume [102]. On the contrary, the dispersion of nanoclays in PLA makes it possible to divide the permeability coefficient by 2 or 3 depending upon the type of the nanoclays (e.g. organomodified montmorillonite, cloisite 25A or 30B, organomodified synthetic fluorine mica) and exfoliation [129-131]. 

Clay nanocomposites are also being developed as barrier coatings for film and for containers. The nanocomposite is deposited on the film from a solution of PVOH/ EVOH copolymer in a mix of water and isopropyl alcohol which has been used in a supersonic dispersion system to nano-disperse 7 nm diameter silica and titanium dioxide particles. The ratio of polymer to silica depends on the barrier properties required. Typical microgravure equipment can be used to coat the solution onto a plastic substrate. The result reportedly is a transparent barrier coating which is superior to silica- and alumina-coated films, and is comparable to aluminum-coated materials. Oxygen permeability at a coating thickness of 2 pm is less than 1 cc/m d atm, and moisture permeation less than f g/m d. Costs are reported to be competitive with ceramic coatings [4]. 

You are designing a multilayer plastic structure with 4 layers. The total oxygen permeability is required be 30 cm mil/m day atm. Calculate the required thickness of the barrier layer (layer 2) in mil. (Answer x = 2.55 mil)... 

---