METHOD FOR MEASURING THE
QUANTITY OF A POLYMERIC OR PRE-
BACKGROUND OF THE INVENTION 5
The invention relates to measuring the quantity of a polymeric or pre-polymeric composition within a given volume.
Processes for manufacturing polymeric compositions 10 (e.g., adhesives such as structural adhesives) often require adding or combining precise amounts of pre-polymeric components forming these compositions, particularly where these components react together to form the composition. Devices dispensing these components can malfunction peri- ^ odically and/or systematically, resulting in the deposition of an incorrect mix of the components. These malfunctions can significantly affect the quality of the resulting products.
It is also desirable to be able to measure the quantity of polymeric or pre-polymeric material in any given volume of 20 an article incorporating the material. For example, in the case of a structural adhesive joining two substrates together, it is desirable to measure the thickness of the adhesive throughout the adhesive joint to determine whether the thickness is uniform. Non-uniformities can affect the per- 25 formance of the joint, causing it to under perform in some circumstances.
SUMMARY OF THE INVENTION
In general, the invention features a method for measuring the quantity of a polymeric or pre-polymeric composition within a given volume that includes combining the polymeric or pre-polymeric composition with a plurality of microparticles having a non-ferromagnetic or nonferrimagnetic core provided with a coating that is ferromagnetic, ferrimagnetic, or conductive, to form an admixture in which the microparticles are substantially uniformly dispersed throughout the composition. The microparticles have a detectable electromagnetic characteristic which correlates with the amount of the composition within a given volume. The electromagnetic characteristic of the microparticles is then measured to determine the quantity of the composition within a given volume.
As used herein, a "pre-polymeric composition" refers 45 both to compositions whose molecular weight has not been sufficiently advanced to qualify as a polymeric composition (e.g., partially polymerized pre-polymeric syrups), as well as individual reactants in the form of monomers or oligomers that react with themselves or with other reactants to form a 5Q polymeric composition.
In preferred embodiments, the core of the microparticles is selected from the group consisting of glass bubbles, glass beads, glass fibers, fumed silica particles, fused silica particles, mica flakes, polymeric particles, and combinations 55 thereof, with glass bubbles being particularly preferred. The coating (which may be provided over substantially all or a portion of the surface of the core) is preferably a ferromagnetic or ferrimagnetic material. Examples of suitable ferromagnetic or ferrimagnetic materials include nickel, cobalt, go iron, alloys thereof and oxides thereof. Stainless steel coatings are particularly preferred. Other preferred coatings include electrically conductive coatings.
The dimensions of the microparticles preferably have a major dimension between about 10 micrometers and about 65 1 millimeter. The average thickness of the coating preferably ranges from about 0.1 nanometers to about 5 micrometers,
more preferably from about 1 nanometer to about 200 nanometers. The amount of microparticles provided in the admixture preferably ranges between about 0.01 and 50% by volume.
In one preferred embodiment, the method is used to measure the quantity of a polymeric or pre-polymeric composition being dispensed, e.g., into a reaction mixture. According to this embodiment, the admixture containing the microparticles and pre-polymeric or polymeric composition is dispensed while measuring an electromagnetic characteristic of the microparticles to determine the quantity of the polymeric or pre-polymeric composition being dispensed.
In yet another preferred embodiment, a first polymeric or pre-polymeric composition and a second polymeric or prepolymeric composition are combined to form a reaction mixture. At least one of the polymeric or pre-polymeric compositions is combined with the microparticles prior to combining the first and second polymeric or pre-polymeric compositions together. In a preferred embodiment, the electromagnetic characteristic of the microparticles in the reaction mixture is measured. The microparticles can be placed in both the first and second polymeric or pre-polymeric compositions. The microparticles in the first and second polymeric or pre-polymeric compositions can be different from each other. Another embodiment includes combining the microparticles with one of the polymeric or prepolymeric compositions and measuring the electromagnetic characteristic of the reaction mixture to determine the ratio of the first and second polymeric or pre-polymeric compositions to each other.
In another preferred embodiment (useful, e.g., for quality control measurements), the admixture is deposited on or between a substrate and the electromagnetic characteristic of the microparticles is measured to determine the amount of the admixture deposited on the substrate. In this way, for example, variations in the thickness of the deposited material can be detected.
One example of a useful polymeric composition is an adhesive composition. Specific examples of preferred polymeric compositions include epoxy resins (e.g., base cured epoxies, acid cured epoxies, and addition cured epoxies), polyurethanes, acrylates, silicones, and phenolics.
The invention provides a low-cost, reliable method for measuring the quantity of a polymeric or pre-polymeric composition within a given volume using microparticle "tags" having a detectable electromagnetic characteristic. The microparticles are easily fabricated and are generally chemically inert and stable over reasonable periods of time.
Moreover, certain properties of the microparticles are very similar to their uncoated counterparts. For example, metal-coated glass microbubbles impart substantially the same Theological behavior and mechanical properties as their uncoated counterparts. Thus, the microparticles can be substituted virtually one-for-one for their uncoated counterparts on a volume basis without adversely affecting the properties of the final composition.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plot of inductive reactance versus loading of coated microparticles in percent volume fraction.
FIG. 2 is a plot of inductive reactance versus coating thickness.