Plasticizer diffusion

Because of a number of problems—e.g., weakness of the perforated plates, too low an ozone concentration, too large a volume of air, and costly maintenance of the compressor and other equipment because of corrosive action of ozone— this method has been modified in later installations. The perforated plates have been eliminated, and the air is diffused through plastic diffusers. Stainless steel rotary, liquid ring compressors are used. In some cases, even the Otto method of emulseurs and selfcontact columns is employed. 

Two types ofDV actions occur (1) volatile components diffuse to the plastic-vapor interface (called diffusional mass transport), and (2) volatile components evaporate at the interface and are carried away (called conveaive mass transport). If (1) is less than (2), the process is diffusion-controlled. This condition represents most of the plastic devolatilization processes because plastics diffusion contents are usually low. 

Figure 5 Problem geometry. The surface at = 0 separates the plastics film from the ambient gas. The metal layer extends for y > 0. The region underneath the metal film (gray area) is assumed to be characterized by a diffusivity Z>(0) different from the iDulk plastics diffusivity (i3 -oo)).

Plasticizers diffusion is important in many practical applications. " These include ... 

Several analytical methods are used in studies of plasticizer diffusion weight uptake glass transition temperature infrared imaging sorption kinetics ... 

In this and previous sections we have discussed the first step - the plasticizer diffusion towards the surface. At the interface the migration will depend on the properties of other eomponent (surrounding medium). In this respect we may distinguish four cases ... 

Plasticizer diffusion, deposition, and accumulation on a material surface are controlled by the degree of the compatibihty between the plasticizer and the matrix, the surface energy of the liquid/solid interface, and the volatility of plasticizer. Volatihty and migration are controlled by different properties of the plasticizers therefore they are not related (Figure 7.8). Plasticizers have high boiling points so evaporation is typically slow. The rate can be calculated from the Hertz equation ... 

Rates of plasticizer evaporation are 10 to 100 times slower than typical diffusion rates. If plasticizer has good compatibility and good wetting characteristics for particular solid surface its surface spreading will hamper diffusion because diffusion is gradient controlled. Theories to explain surface deposition and its effect on plasticizer diffusion are yet to be developed. 

Below are several general rules related to plasticizer diffusion and mobiUty ... 

From thermal degradation studies it was shown that temperatures available in environmental conditiorrs ( 100°C) accelerate plasticizer diffusion. Thirs plasticizer loss may... 

Figure 11.61 shows principles of diffusivity of plasticizers. Diffusion coefficient, k, comes from the following equation ... 

The retention volumes on poly(vinyl chloride) powders were correlated with their capacity for incorporating the plasticizer [153]. The diffusion of plasticizer in the polymer powder being controlled by the external smface, by the plasticizer diffusion coefficient and by a shape factor, some connection with gas chromatographic measurements is to be expected. The plasticizer adsorption was found to take place only at a temperature slightly above T,. 

One can see that, in the sample bulk, the polymer remains in a rubbery state. On the contrary, in the superficial layer of thickness the polymer vitrifies and thus becomes brittle. Since the coefficient of plasticizer diffusion varies by at least one order of magnitude on both sides of the glass transition, the real gradient will display rather the shape of Fig. 12.10. In such cases, the diffusion front is very abrupt and the sample weight decreases proportionally with time. 

In flexible vinyl, the plasticizer system maintains equilibrium throughout the plastic matrix and facilitates the diffusion of the biocide. As the plasticizer diffuses to the surface, the biocide is always available to inhibit microbial growth. 

There are two leading theories concerning the mechanism of activity for plasticizer molecules. The Tubricating theory suggests that as the polymer is heated, the plasticizer diffuses into the polymer and disrupts the van der Waal interactions... 

Plastic Diffusion coefficient Solubility Temperature Source... 

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