Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-02-12
2001-08-14
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S654000, C430S137170
Reexamination Certificate
active
06275049
ABSTRACT:
BACKGROUND OF THE INVENTION
The critical temperature required for a latex coating to form a strong, continuous film is important in the design and use of these coatings. This temperature is known as the minimum film formation temperature (MFFT).
Presently, an optical method is employed to measure the MFFT. The method involves the observation of the clarity of a cast film on a large metal table. A temperature gradient is applied to the table and the position on the table where the film is judged to be clear is noted. The temperature at that point on the table is determined to be the MFFT value. This technique may provide variable, operator dependent results. Another difficulty with this method is that only optically clear latexes can be tested. Pigments and other additives which are known to alter the MFFT cannot be used using this method. Accordingly a method to objectively measure the film formation temperature of a latex is desirable, especially a method which can also be used with pigmented latices.
SUMMARY OF THE INVENTION
One aspect of the present invention is a method for measuring the minimum film formation temperature comprising:
providing a latex;
measuring the initial conductivity (K) of the latex;
measuring the temperature of the latex (T);
heating the latex solution;
measuring the conductivity of the latex while it is being heated;
determining the point in which the conductivity of the latex changes sharply with temperature; and
determining the minimum film formation temperature by calculating the maximum point in the curve described by (+/−)d
2
K/dT
2
versus temperature.
A second embodiment of the present invention provides apparatus for the film formation temperature a latex. The apparatus comprises:
a container suitable for holding a latex solution;
means for removing water from the latex solution via osmotic suction;
means for measuring the conductivity of a latex;
means for measuring the temperature of a latex;
means for modifying the temperature of a latex provided in the container; and
means for determining the minimum film formation temperature by calculating the maximum point in the curve described by (+/−) d
2
K/dT
2
temperature.
The new method is based upon the discovery that the film formation properties of latex materials, such as paints and other coatings, can be dried in a controlled manner using osmotic suction to the point where latex becomes critically dependent on temperature. At the MFFT value the latex particles soften and deform, which in turn changes the solid drying curve. Surprisingly, we have also discovered that the conductivity of the latex is also sensitive to this same transition point and is an ideal parameter for monitoring this change.
As used throughout the specification, conductivity of the latex will be used to determine the MFFT. Those with skill in the art will realize that other electrical parameters could be used to detect the change in behavior such as dielectric constant, and these parameters are understood to be within the scope of the present invention. One reason the present invention employs conductivity in the description of the invention is the relative easy in which conductivity can be measured.
The conductivity of latex solutions is caused by the presence of free ions in the latex between the particles while under osmotic suction. At the MFFT point the deformation of the latex particles reduces the pore size and ion conductivity between the particles. Some latex samples demonstrate a sharp increase in conductivity with temperature. The conductivity change in the latex solution typically occurs at a solids level of from 60-85 weight percent, preferably from 65 to 80 weight percent and in a highly preferred embodiment at 75 weight percent by weight solids. This increase is probably due to the release of ionic materials from the interior of the latex particles at the softening temperature. The MFFT value is therefore determined as a sharp change in conductivity as the temperature of the latex sample is increased across the transition range when the film is formed. Therefore by plotting the conductivity of a latex solution at various temperatures it is possible to calculate the rate of change of the conductivity of the latex versus the rate of change of the temperature (dK/dT) versus temperature. This is the first also done by calculating the first derivative of the conductivity versus temperature curve.
By calculating the second derivative of the conductivity versus temperature (+/−d
2
K/dT
2
) versus temperature, the MFFT can be calculated. The maximum in this curve is at the minimum film formation temperature. This result is provided by fundamental principles of calculus. The ability to calculate the rate of change of the conductivity per the rate of change of temperature by taking a derivative is known by those with skill in the art.
The direction of the transition may be altered by the frequency at which the conductivity is measured. The present invention and the accompanying examples were conducted at 1000 HZ, although those with skill in the art could easily vary the frequency employed. The transition may be altered or enhanced by variation of the frequency without departing from the scope of the present invention.
REFERENCES:
patent: 3657640 (1972-04-01), Jelinek et al.
patent: 5140275 (1992-08-01), Schoerner et al.
patent: 5523959 (1996-06-01), Seegmiller
patent: 5644239 (1997-07-01), Huang et al.
patent: 5650255 (1997-07-01), Ng et al.
Karaman Marilyn Emily
Ninham Barry William
Pashley Richard Mark
Kerveros James
Metjahic Safet
The Australian National University
Welsh & Katz Ltd.
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