PPDS

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

In one experiment the effect of ppd assay was correlated to scorch safety. As the ppd degrades Hberate free amine, scorch time decreases and cure rate is faster. The degradation products apparentiy serve to activate the cure, since both the induction time, and cure time, decrease with decreasing ppd assay. However, the effect on unaged properties is minimal. 

In contrast, antioxidants can have an opposite effect when peroxide curing. Because peroxide cross-linking involves a free-radical mechanism, and antioxidants are designed to scavenge free radicals, it is obvious that peroxide efficiency can be compromised by the addition of antioxidants. Thus the decomposition products of the ppds were acting as accelerators (29). 

Parallel Pore and Solid Diffusion Control With a linear isotherm, assuming equilibrium between the pore fluid and the solid adsorbent, batch adsorption can be represented in terms of an equivalent solid diffusivity = ( pD i + ppD, )/( p + pp Q). Thus, Eqs. (16-96) and (16-99) can be used for this case with D, replaced by D. ... 

PPD Positive pressure powered dust hood and blouses... 

ISO EN 7730 Ciomfort evaluation in moderate environ-nienrs Mode.tate thermal environments Deterrnina tion of the PMV and PPD index and specification of the conditioirs for thermal comfoi r... 

ISO EN 7730 standardizes the PMV-PPD index as the method for evaluation of moderate thermal environments. To quantify the degree of comfort, the PMV (predicted mean vote) index gives a value on a 7-point thermal sensation scale -t-3 hot, +2 warm, +1 slightly warm, 0 neutral, -I slightly cool, -2 cool, -3 cold. An equation in the standard calculates the PMV index based on the six factors (clothing, activity, air and mean radiant temperatures, air speed, and humidity). 

The PMV index predicts the mean value of the thermal preferences of a large group of people exposed to the same environment. But individual votes are scattered around this mean value, and it is useful to predict the number of people likely to feel uncomfortably warm or cool. The PPD (predicted percentage of dissatisfied) index establishes a quantitative prediction of the number of thermally dissatisfied people. The PPD predicts the percentage of a large group of people likely to feel too warm or cool, i.e., voting hot (-t-. , warm (4-2), cool (-2), or cold (-3) on the 7-point thermal sensation scale. 

Once the PMV value has been determined, the PPD can be found from Fig. 6.2 or from the equation... 

FIGURE 6.2 Predicted percentage of dissatisfied (PPD) as a function of predicted mean vote (PMV). 

Thermal comfort is defined as the condition of mind that expresses satisfaction with the thermal environment. Dissatisfaction may be caused by thermal discomfort of the body as a whole as expressed with the PMV and PPD indices, or it may be caused by unwanted cooling (or heating) of a particular part of the body. Due to individual differences, it is impossible to specify a thermal environment that will satisfy everybody. There will always be a percentage of dissatisfied occupants, but it is possible to specify an environment predicted to be acceptable by a certain percentage of the occupants. 

Due to local or national priorities, technical developments, and climatic regions, in some cases a higher thermal quality (fewer dissatisfied) or a lower quality (more dissatisfied) may be sufficient. In both cases the PMV and PPD indices, the model of draft, and the relation between local thermal discomfort parameters and the expected percentage of dissatisfied people may be used to determine different ranges of parameters for the evaluation and design of the thermal environment. 

Predicted percentage of dissatisfied Category (PPD) Predicted mean vote (PMV) Percentage of dissatisfied due to draft (DR) Percentage of dissatisfied due to vertical air temperature difference Percentage of dissatisfied due to warm or cooi floor Percentage of dissatisfied due to radiant asymmetry... 

The numbers of dissatisfied persons in Table 6.3 are not additive. Some of the people experiencing general thermal comfort (PMV-PPD) may be the same as the people experiencing local thermal discomfort. In practice, a higher or lower number of dissatisfied persons may be found using subjective questionnaires in field investigations (ISO 10551). 

Radiant asymmetry <10 °C for cold wall < 23°C for warm wall < 14 °C for cold ceiling < 5 °C for warm ceiling PPD < 5% with respect to cold wall, warm wall, cold ceiling, and warm ceiling... 

ISO 7730 1984, Modern thermal environments - determination of the PMV and PPD indices and specification of the conditions for thermal comfort. 

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