Volumen II - SAM

In this paper, the influence of average particle size of micaceous iron oxide (dispersion time) on water film
permeability is studied. In the first stage in a ball mill of 1 liter of total capacity, under optimized operating
conditions in a previous study [7], six dispersion times were employed for each one of the considered PVC
values (Pigment Volume Concentration). Then, starting from these results, three dispersion times were
selected for each formulation for attaining the pre-established average particle sizes, Table 3. To evaluate the
degree of dispersion of the micaceous iron oxide (mean particle size), optical microscopy was applied [8].
Mean diameter Dp was calculated for each dispersion time using the expression Dp= Ni Di/ Ni, where Ni
is the number of particles of Di diameter present in the system. The film permeability evaluation was carried
out on free dry films (100-110 m thickness) by using the Gardner method [9-11]. The water quantity that
permeated through the film under steady state condition at 25°C was determined by gravimetry; the results
were expressed in weight per unit area and time. Permeability is the average calculated starting from those
values of coatings based on the several flame retardant pigments (no significant scattering was observed).
- Zinc hydroxyphosphite. This pigment (aspect, white powder; density, 4.0 g.cm -3 ; oil absorption, 19 g/100 g;
mean particle diameter, 3 m) is chemically active as phosphates, chromates and molibdates. It acts as
anodic inhibitor and displays low toxicity: for example, its LD 50 for rats is higher than 5000 mg.kg -1 (oral
acute); as reference, the aspirin and the caffeine have values about 1750 and 200 mg.Kg -1 , respectively.
Other characteristics of this pigment are: water solubility, 0.04 mg.l –1 ; specific resistance, 9700 ohm.cm -1 ;
acute oral toxicity by inhalation, more than 5.1 mg.l -1 and no primary irritation of skin and eyes.
- Flame retardant pigments (FRP). Antimony trioxide (aspect, white fine powder; density, 5.4 g.cm -3 ; oil
absorption, 12 g/100 g; mean particle diameter, 0.90 m) is a pigment of verified flame retardant action. By
itself it is not effective; nevertheless, in combination with organohalogen compounds it shows in gas phase
an adequate flame retardant behavior [12, 13]. Its mechanism of action is attributable to interference on
flame propagation rate produced by the antimony trichloride and/or oxychloride generated during the
combustion. Many studies were made during the last years for attaining the partial or total substitution of
antimony trioxide because of the following reasons: smoke generation [14], metallic substrate corrosion and
possible toxicity [15-17]; improvement of the relation cost/efficiency, etc. In the present work, the
substitution of antimony trioxide by zinc borate (ZnO.3B2O3.7.5H2O; aspect, white powder; density, 3.9
g.cm -3 ; oil absorption, 28 g/100 g; mean particle diameter, 5.20 m) and trihydrated alumina (Al2O3.3H2O,
aspect, granulated fine powder; density, 2.4 g.cm -3 ; oil absorption, 53 g/100 g; mean particle diameter, 0.65
m) as flame retardant pigments was studied, Table 2.
Zinc borate was prepared by reaction between zinc oxide and boric acid avoiding the presence of free zinc
oxide in the final product due to its negative effect on thermal stability of the chlorinated alkyd resin.
Thermo-gravimetric Analysis (TGA) and Differential Thermal Analysis (DTA) of flame retardant pigments
selected as partial or complete substitutes for antimony trioxide were performed.
-Opaque pigment. Rutile titanium dioxide (aspect, very fine white powder; density, 4.1 g.cm -3 ; oil
absorption, 39 g/100 g; mean particle diameter, 0.25 m) improves the hiding power; besides, it shows an
important physical activity since it requires a high quantity of heat to reach the pyrolysis temperature.
Binder. In all cases, a chlorinated alkyd resin was employed as flame retardant film forming material
prepared in laboratory by means of soya oil, chlorendic anhydride and pentaerythritol. The activity of the
mentioned resin is based on a free radicals mechanism. At high temperatures, it releases hydrides that react
with the hydroxyl radicals present at the flame and also with the activated hydrogen atoms bringing
incombustible gases that interfere the access of air oxygen. Generally, the activity of halogenated products is
not reduced only for acting in gaseous phase since they also increase the carbonaceous remainder during
polymer pyrolysis (solid phase activity). Considering that the level of chlorine acts on flame retardant
capacity, a level of 24.9% in weight was selected.
Solvent mixture. Mineral white spirit was used as solvent because of its high flash point (39°C) and boiling
point (160-200°C) related to the other solvents usually selected in coatings. Besides, white spirit shows also
a reduced pollution power (maximum permissible in the air, 500 ppm or 200 mg.m -3 ).
Additives. Several additives were used: an electro-neutral compound as dispersant, a product of inorganic
nature as rheological agent and zirconium and cobalt naphthenates as dryers.
Pigment/binder ratio. To attain a satisfactory anticorrosive capacity [18], 45, 50 and 55% values were
defined taking into account that they are lower than the corresponding CPVC, Table 3.
To estimate the mentioned CPVC values, several experimental coatings with PVC from 40 to 70% were
formulated and prepared at laboratory scale; starting from the permeability values in function of PVC, for
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