I-
Spray cans are ubiquitous in the day-
A spray can comprises: a metallic can, a pickup tube, a valve in the centre of the cup which is set on top of the can, an actuator, the end product, the propellent and sometimes a small marble to help for blending (homogenizing) the product while one shakes the can.
The end product is in fact the product to be reclaimed: penetrant, developer, degreaser, white contrast paint, supplied in bulk quantities to the filling plant.
2-
Propellents come in two types:
-
-
3-
-
-
-
For decades till the '80s the CFCs have been the most widely used propellent family. They generally came as a blend of CFC 11 (trichlorofluoromethane) and CFC 12 (dichlorodifluoromethane).
Their very specific characteristics made them very successful:
-
-
-
-
-
-
-
-
-
-
On Jan 1, 1989 the Montreal Protocol went in effect which phased out production and use of several substances believed to be the cause for the stratospheric ozone layer depletion. CFCs were among the targeted chemicals.
The largest ozone hole recorded
in September 2006 above Antarctica
(Wikipedia picture)
If everything goes well it is anticipated that the ozone layer would come back to
its 1980 condition in 2055-
4-
No other propellent matches CFCs qualities.
In the '70s, due to the stringent requirements of the French nuclear industry for fluorine and chlorine content of any material, it became obvious that the CFCs were no longer the propellents of choice! Two of the main PT materials suppliers chose a blend of butane and propane (please refer to the Penetrant Testing History Table, by Pierre CHEMIN and Patrick DUBOSC).
At the same time in France aircraft manufacturers as well as nuclear industry primes
required chlorine-
The other industries which had no concern about chlorine and fluorine content were not happy with these new spray cans.
Indeed some users were against, even alarmed by, any use of these extremely flammable propellents.
Among them DME was used mainly in Netherland, but without success. Its vapour pressure is 510 kPa (5.1 atm; 75 psi) at 20 °C (68 °F).
Some tests were carried out using non flammable propellents: nitrogen, nitrogen protoxide, carbon dioxide...which were more or less successful!
Nitrogen protoxide is classified as pollutant by the KYOTO protocol. It is the 4th in line of the greenhouse effect gases when considering its action on the Earth warming, water vapour (H2O) , carbon dioxide (CO2) and methane (CH4) being respectively number 1, 2 and 3.
5-
Once again there is a fundamental difference between compressed gases and liquefied gases.
5.1-
A compressed gas is almost not soluble in the end-
The compressed gas acts as a piston on the product and spraying is said as "airless".
The end-
-
-
Compressed gases lead to a dramatic drop of the pressure, down to close the atmospheric pressure when using the spray can. When almost no gas is left the spray can "splutters".
Indeed the spray can internal pressure for compressed gases roughly follows the MARIOTTE's law established for perfect gases.
P x V = Constant.
At a given temperature.
Equation in which P is the pressure and V the volume.
Only a very small quantity of gas is put in the can: some 7 to 11 grams (0.25 to 0.39 oz).
One of the problems with compressed gases is that the cans shall be used vertically, or close to. Tilting them too much, or worse, having them upside down, makes the gas going out at a fast rate. Very often there is then not enough gas left in the can to empty it!
5.2-
The expansion ratio of the gases is the most important data for understanding the difference between compressed gases and liquefied gases used in spray cans.
The expansion ratio is always measured at a given temperature, for instance at 20 °C (68 °F). As you know, liquids and gases expand with temperature, gases far more than liquids.
This ratio is given when measuring the volume of a given mass of gas at the standard pressure (101,325 Pa, 1013.25 mbar (*), 14.7 psi, 1 atm.) divided by the volume of this same mass of the same gas when compressed or when liquefied.
In other words:
-
Expansion ratio = Volume occupied by a given mass of gas at the standard atmospheric pressure / Volume occupied by the same mass of the compressed gas
-
Expansion ratio = Volume occupied by a given mass of gas at the standard atmospheric pressure / Volume occupied by the same mass of the liquefied gas
We may assume that gases used as propellents are "perfect gases" as per the MARIOTTE's law, due to the "low" pressures used. If a gas is compressed at 7 atm. (102.9 psi), when coming back to the standard pressure it will show a ratio of 7.
A liquefied gas on the other hand does not follow the MARIOTTE's law as there is
a change of phase (from gaseous to liquid). When a liquid evaporates the vapour takes
up a volume which is FAR LARGER than that of the liquid. For instance butane ratio
is 239, and 311 for propane. Though we have not found the data for DME we guess it
close to 500! That means that the liquefied gas, dissolved in the end-
This effect is ABSOLUTELY needed to spray end-
Using a liquefied gas allows for a constant pressure from the first use of the spray can to the very last one. This is due to the equilibrium between the liquid and the gaseous phases at a given temperature. A small part of the liquefied gas evaporates as the propellent is used. Outside temperature is an important data: if it rises, pressure rises. If it lowers pressure in the can lowers.
Liquefied gas pressure may be adjusted to fit TECHNICAL specific needs (certain products can require a higher pressure) by the respective relative butane and propane contents: the more propane, the higher the pressure.
Product atomization takes place as the product is ejected at the tip level. The gas
dissolved in the product at the contact of the outside environment (at atmospheric
pressure) is going to brutally expanse. The propelled end-
Liquefied gases being soluble in products, it is very easy to have a large propellent quantity in the can.
With a liquefied gas propellent, there is a constant pressure which results from the equilibrium between pressures of the liquid and gaseous phases. For that a part of the liquefied propellent evaporates in the can and becomes gas. The can's temperature is an important data: when temperature increases internal pressure increases, and on the other hand when temperature lowers so does the internal pressure.
When using a liquefied gas -
In a low temperature condition pressure in a liquefied gas-
(*) bar having for symbol bar, it remains invariable.
PROPELLENTS FOR PT/MT SPRAY CANS
6-
The propellent mass in every spray can depends on the propellent: some grams (fractions of an ounce) for compressed gases, several tens, or even far more for liquefied gases.
The net volume is the total of the liquid phase comprising the end-
Cans sizes are standardised, for example: 510 ml, 650 ml. This is their capacity, i.e. the inside volume of the cans. For every size there is a filling margin to meet. A 650 ml can cannot contain more than 500ml. A 510 ml can cannot be filled at more than 400 ml, roughly 75%.
Some unscrupulous suppliers gave in their Price Lists the size of the spray can instead of the net volume, giving the false feeling that they were cheaper than competitors'! Don't be tricked!
Let us have a look to the end-
Using an LPG there are ca 250 ml of the end-
Using CO2 there are 400 ml of end-
The main problem with CO2 cans, as mentioned above, is that the can shall be upside
up when spraying to prevent any propellent loss. If not within very few seconds all
the gas goes out-
The writers got complaints from some customers because there was no gas left while
some 50 ml of end-
Net weight is the total weight of the end-
When CFCs were replaced by LPG, users got in hand far less weighty cans: all they
imagined that the suppliers put less end-
It has been even more blatant when trichloroethane (T-
Once again an unscrupulous supplier of PT/MT products played the game of: "my competitors give you less for the same price"!
7-
We think Swiss suppliers were the first in Europe to offer this new kind of cans.
Beginning '90s nuclear industry in France was looking for spray cans which may be used in any position, including the upside down one so as to ease inspection in very remote areas.
The pouched-
These cans comprise a rigid outside can, very similar to the familiar ones and an inside flexible pouch, made generally of aluminium or some plastic material.
A small hole at the bottom of the outside can allows for injection of a compressed
gas, which may be as simple as compressed air: this propellent is NEVER in contact
with the end-
The propellent leans on the pouch and, as soon as the actuator is pressed down, the
end-
Products high in solids such as NAWDs for PT, white contrast paints for MT, cannot be applied this way. Only the liquefied gases may be used and these spray cans cannot be used upside down!
For home applications by far the most widely used product in pouched-
Improvements are seen on this type of spraying system.
8-
VOC content is a constant worry for many suppliers. Concerned with environment needs they modify their products.
Pouch-
LPG and DME are VOC. One way to lower VOC content is to use CO2 or nitrogen as replacement.
More end-
Just by looking to our niche market of NDT, and in this niche, by looking to the
"sub-
Furthermore a non flammable compressed gas used with Flammable, or easily Flammable,
or Highly Flammable end-
Using compressed gas, as already written several times in this paper, implies the
can be used in a quasi-
Modifying formulas of end-
Liquefied gas-
This type of spray can will be the reference for a long time ahead.
THE END
