Method of monitoring solidification of a liquid composition

Abstract: Reference materials of differing indices of refraction are incorporated into liquid compositions to monitor the solidification process of these liquid compositions. Light waves are used to indirectly measure the difference between the index of refraction of the solidifying composition and the reference material. The method provides an accurate in situ indication of the completion of the solidification process as well as a monitor of the degree of solidification during the solidification process.

Description: Background of the Invention The present invention relates to a method for monitoring the solidification of a liquid composition as it passes from a liquid phase to a solid phase. The change of a liquid composition from a liquid phase to a solid phase occurs in many chemical reactions such as the polymerization of liquid monomers to a solid polymer, curing of thermoset resins such as epoxy resins or polyimides, and cross-linking of single chain polymers. Monitoring the degree of solidification, i.e., completion, of these exemplary types of reactions is advantageous to ensure that the solid product has the desired chemical and physical properties associated with the completion of the reaction, as well as to minimize the energy and time costs associated with maintaining the process conditions past the point of completion. For example, precision curing conditions are required in the production of cured thermosettable materials and composite materials containing these cured thermosettable materials that are finding increasing use in a variety of advance materials applications. The precision of the curing process is important because the ultimate mechanical properties of these materials depends primarily on completely and uniformly curing these materials. For instance, in a large molded structure of a thermoset resin, the degree of cure may vary from place to place within the structure due to the lack of homogeneity of the starting materials, the exothermic nature of the curing reaction, the thermodiffusivity of the material, and the thermal characteristics of the curing oven or the geometry of the mold. A common method for characterizing the degree of cure of thermosettable materials involves the measurement of the glass transition temperature (T g ) by one of a number of different techniques. Examples of these techniques include differential scanning calorimetry and thermoexpansion measurements. The equipment associated with these bulk methods are hard to adapt to local in situ measurements, that are necessary to accurately monitor the curing process throughout a large body. A technique known as dielectric spectroscopy has undergone development resulting in a method for the in situ measurement of the local dielectric properties of curing materials. A frequency range of 0.005 to 10,000 Hz is used to monitor changes in the dielectric loss factor that have been shown to correlate with different stages of the curing process. Another technique recognizes that the intensity and wavelength of fluorescence of certain molecules are dependent upon the viscosity of the environment surrounding the molecules. Because the viscosity of a composition changes with the degree of cure, it is possible to monitor the degree of cure by indirectly monitoring the change in the viscosity of the thermosettable composition and detecting the resultant effect on the fluorescence of the molecules. In still another technique, the attenuation and phase velocity of ultrasonic waves in thermosettable materials is known to vary during the curing process primarily due to the change in viscosity of the materials. An ultrasonic spectral analysis technique has been used to measure the frequency dependence of the ultrasonic velocity and attenuation during the curing process of a thermosettable composition at a fixed temperature. The prior techniques show the onset of the curing reaction in thermosettable materials. However, during the curing process, properties of the thermosettable material, such as viscosity, conductivity and glass transition temperature, change drastically at the onset of curing and then saturate at an upper bound as the curing process continues and nears completion. For example, as methylmethacrylate polymerizes and cures, the instantaneous glass transition temperature changes from -102.8° to 70°C, and the viscosity rises from about 0.1 to 1 x 10¹¹ centipoise. Using a fluorescent probe method, a fluorescent probe molecule added to the methylmethacrylate system exhibits an intensity change from 4 to 75 relative units. Unfortunately, by the time the glass transition temperature has reached 40°C, the viscosity has reached only 2 x 10⁴ centipoise and the fluorescent intensity has already reached 72.5. The final stages of the curing process must be measured by the remaining 2.5 relative units of the fluorescent intensity and therefore the final stages of the curing process are not observable with adequate resolution. The glass transition temperature and dielectric spectroscopy methods of monitoring the curing of thermosettable resin compositions also suffer from similar limitations, in addition, both these methods are not readily adaptable to in situ measurements of large structures, where the monitoring of the solidification process is most important. DD-238674 describes an optical fiber sensor to be used as a refractometer or for temperature measurement. An optical conductor is disposed in a curved tube. The optical transmission of the optical conductor depends on the refractive index of the substance surrounding it. This makes it possible either to measure this refractive index, or the temperature, when the curved tube is filled with a substance having a refractive index that is temperature dependent. In other respect, an optical non-destructive method for dynamic monitoring of chemical reactions is known (IBM Technical Disclosure Bulletin, vol. 25, N° 10, March 1983, pages 4976, 4977). According to this method, a reactive vessel is provided with a reactive film upon its surface. A light beam is directed from outside the vessel to the reactive film. A detector measures the outcoupled light. The light interacts with the reactive film, so that the outcoupled light measured can be used to monitor the reaction. Summary of the Invention The present invention addresses the inadequacies of the prior methods used for the in situ monitoring of the solidification of a liquid composition, such as thermoset resins. The invention provides an accurate monitor of the solidification process that ensures the solid product will have the desired properties associated with the fully and uniformly solidified product. The method relies upon monitoring the difference between the index of refraction of the liquid composition and a reference material that are subjected to conditions that cause the liquid composition to solidify. In a preferred embodiment, the solidification process is monitored using an optical waveguide that passes through the liquid composition. The waveguide has an index of refraction that is greater than the index of refraction of the composition in a liquid state. When the composition is fully-solidified, it has an index of refraction that is substantially equal to the index of refraction of the waveguide. While the liquid composition containing the optical waveguide is subjected to solidifying conditions, light waves are introduced into the portion of the waveguide that passes through the liquid composition. Because several of the transmission characteristics of the waveguide (e.g., critical angle, mode structure and attenuation) are dependent on the difference between the index of refraction of the waveguide and the liquid composition, the degree of solidification is monitored by determining at least one transmission characteristic of the waveguide passing through the solidifying composition. For example, when the difference between the index of refraction of the waveguide and the liquid composition is at its greatest, that is, before the liquid begins to solidify, the intensity of the introduced light waves transmitted through the waveguide is at its maximum; however, as the liquid composition solidifies and the difference diminishes, the intensity of the transmitted light waves proportionally decreases because increasing light power is coupled out of the waveguide. Thus, the relative intensity of the light energy transmitted through the waveguide provides an indication of the degree of solidification of the composition. Other objects, features, and advantages of the present invention will be readily apparent from the following description of certain preferred embodiments thereof, taken in conjunction with the accompanying drawings. It is understood that variations and modifications may be affected without departing from the scope of the novel concepts according to the claims. Brief Description of the Drawings FIGURE 1 is a schematic diagram of an apparatus constructed in accordance with a preferred embodiment of the present invention. FIGURE 2 is a graph illustrating the relationship between the curing process and the intensity of transmitted light in accordance with a preferred embodiment illustrated in FIGURE 1. Detailed Description of the Invention The present invention is generally applicable to any process wherein it is desired to monitor a change of a liquid composition from a liquid phase to a solid phase. Examples of these types of processes include reactions wherein liquid monomers such as styrene, phenol-formaldehyde or phenolics are polymerized to solid polymers, liquid resins are cured to solid resins such as epoxy resins and polyimides and straight chain liquid polymers are cross-linked to solid polymers. Also included are other materials that undergo a physical change, particularly when passing from a liquid phase to a solid phase. These materials include metals, plastics, cement, concrete and the freezing of various liquids. Though not intended to be limited to a particular chemical reaction or family of liquid compositions, the present invention will be described with reference to the particular family of liquid compositions known as thermosettable resins. Thermosetting resins are well-known and are widely used as matrices for advanced composite materials and structural adhesives. The fabrication of strong composite materials or uniform adhesive joints requires curing the resin material under proper thermal and pressure conditions to an optimal degree. Thermoset resins are generally high polymers that solidify or set irreversibly when heated. This property is usually associated with a cross-linking reaction of the molecular constituents induced by heat or radiation. During the curing reaction, the thermosetting resin transforms from the viscous liquid state to a gel and then vitrifies to the solid glass state. In some instances, it may be necessary to add conventional curing agents such as organic peroxides and the like. Phenolics, alkyls, amino resins, polyesters, epoxides and silicones are usually considered to be thermosetting resins, but the term may also apply to materials where additive induced cross-linking is possible. An epoxy resin is a specific example of a thermoset resin. The most common epoxy resins are made from epichlorohydrin and bisphenol A, however, aliphatic polyols such as glycerol may be used instead of the aromatic bisphenol A. Epoxy resins of this type have glycidol ether structures in the terminal positions, many hydroxyl groups, and cure readily with amines. Another type of epoxy resin is made from polyolefins oxidized with acetic acid. These epoxy resins have more epoxide groups within the molecule as well as in terminal positions, and can be cured with anhydrides at high temperatures. The epoxy resins form a tight cross-linked polymer network, and are characterized by toughness, good adhesion, corrosion resistance, chemical resistance, and good dielectric properties. Many modifications of both types of epoxy resins are available commercially. Another example of thermoset resins include the polyimides. Polyimides are derived from a pyromellitic dianhydride and an aromatic diamine. The polyimides exhibit excellent frictional characteristics, good wear resistance at high temperatures, and resistance to radiation and combustion. The thermosettable polyimides have found widespread application in high-temperature coatings, laminates and composites for aerospace vehicles, and as adhesives. In the present discussion, the term "liquid composition" generally includes the thermoset resin in an uncured state. The term "solidifying composition" generally includes the thermosetting resin that is subjected to curing conditions and is passing from the liquid phase to the solid phase. The term "solid composition" generally includes the thermoset resin in a cured solid form. When referring to other liquid compositions, it should be understood that the term "uncured composition" can be used interchangeably in the present specification and claims with the term "liquid composition." Likewise, the term "curing composition" can be used interchangeably with the term "solidifying composition" and the term "cured composition" can be used interchangeably with the term "solid composition." The term "solid product" or "cured product" refers to the product of the solidification process that is being monitored. The method of the present invention, as it applies to thermosetting resins, relies upon monitoring the difference between the index of refraction of the solidifying composition and the index of refraction of a reference material during the curing, i.e., solidifying process in the resins. The method incorporates a reference material having a known index of refraction into the uncured composition and monitors the difference between the indices of refraction of the reference material and the curing composition by detecting the waveguiding, reflecting, or scattering effects on light waves passing through the curing composition or the incorporated reference material. The reference material is preferably chosen so that its index of refraction is substantially equal to the index of refraction of the cured product. The term "substantially equal to" as used herein means that the difference between the index of refraction of the reference material and the cured product is such that light waves passing from the cured product, having a first index of refraction to the reference material, having a second index of refraction are virtually unaffected by the difference in the indices of refraction. The index of refraction of the reference material provides the baseline or reference standard to which the index of refraction of the curing composition or cured product is indirectly compared. A direct measurement of the indices of refraction is not made, but rather the difference between the index of refraction of the curing composition or cured product and the reference material is measured by monitoring the reflection or scattering of light resulting from the differing indices of refraction or the optical waveguiding properties of certain structures fabricated preferably from the reference material and incorporated in the curing composition. In accordance with a preferred embodiment of the present invention, an optical waveguide is passed through the uncured composition and used to monitor the degree of cure in the curing composition. The optical waveguides useful in the present invention are generally fibers of circular cross section having a waveguiding composition at least on the periphery of the waveguide. These fibers range in diameter from about 10 to about 1000 microns and may be made from materials such as glass, plastic and the like. The optical waveguide must have a lightguiding region when surrounded by the uncured or curing composition. In addition, the transmission characteristics of this lightguiding region must be sensitive to changes in the index of refraction of the curing composition. Therefore, one may use a fiber of uniform composition having an index of refraction n₁ greater than the index of refraction n₂ of the uncured composition. The optical waveguide is preferably an optical fiber made of the uncured composition in a cured form. This ensures that the optical waveguide has an index of refraction n₁, that is substantially equal to the index of refraction n₃, of the cured product and greater than the index of refraction n₂, of the uncured composition. The use of optical waveguides of the cured composition is also desirable because the index of refraction of the waveguide is affected by the process conditions, for example, pressure and temperature, in the same manner as the index of refraction of the cured product. Therefore, any error in monitoring the curing process attributable to the differing effect of the process conditions on the index of refraction of the waveguide and the cured product is eliminated by using waveguides prepared from the cured composition. Light is supplied to the optical waveguide at a first end from a conventional light source. Examples of such conventional light sources include a laser, a light emitting diode and the like. The light source supplies the light to the optical waveguide that will be preferably detected at the second end of the waveguide and used to determine the transmission characteristics of the waveguide such as the critical angle, mode structure or the attenuation of the waveguide. Any wavelength of light waves that the waveguide will transmit may be used, although the wavelength of the light should be such that its attenuation in the waveguide surrounded by the curing composition is not excessive, preferably, the wavelength is about 400 to about 1500 nanometers. In order for an optical waveguide to effectively transmit light, the index of refraction n₂, of the medium surrounding the optical waveguide must be less than the index of refraction n₁, of the optical waveguide so that a light ray traveling within the optical waveguide and striking the surface of the waveguide at an angle of incidence, measured with respect to the direction perpendicular to the surface of the waveguide, greater than the critical angle, ϑ C , given by sin ϑ C = n₂/n₁, will be guided by total internal reflection. As the indices of refraction approach each other, the critical angle ϑ C increases, therefore less light can be guided. Any light ray traveling in the waveguide and striking the surface of the waveguide at an angle of incidence less than ϑ C escapes the optical waveguide. Therefore, in the context of the present invention, because of the differences in the indices of refraction between the optical waveguide and the curing composition at the beginning of the curing process, a certain fraction of the light introduced into the optical waveguide is internally reflected and transmitted by the optical waveguide through the length of the waveguide which is surrounded by the curing composition. As the degree of cure increases, and the index of refraction n₂, of the curing composition increases and approaches the index of refraction n₁, of the optical waveguide, the transmission characteristics of the waveguide will change, e.g., the intensity of light internally reflected and transmitted by the optical waveguide through the curing composition diminishes. The intensity of light transmitted by the optical waveguide and other transmission characteristics of the waveguide such as the critical angle and mode structure can be determined by a photodetector positioned at the second end of the optical waveguide. Conventional photodetectors commercially available can be employed. Optionally, it is possible to reflect the lightwave back into the second end of the waveguide by placing an optically mismatched material on the second end of the waveguide. In this instance, the light source and the photodetector will be coupled to the first end of the optical waveguide in a manner that allows the light source to provide the light to the waveguide and allows the detector to detect the reflected light. The transmission characteristics of the optical waveguide surrounded by the curing composition can be compared to predetermined reference standards for a given light source to monitor the degree of cure during the curing process. These reference standards for a given light source provide a relationship between the waveguiding characteristics of the optical waveguide and the degree of cure during the curing process. The relationships can be established by simultaneously monitoring the degree of cure by an independent means such as Differential Scanning Calorimetry or Differential Thermal Analysis and relating this information to the waveguide characteristics of the optical waveguide during the curing process. When the optical waveguide is employed, it is possible to determine the completion of the curing of the resin without referring to reference standards. Because the optical waveguide has an index of refraction n₁, substantially equal to the index of refraction n₃, of the cured product, and is preferably of the same composition as the cured product, when the resin is completely cured, the indices of refraction are substantially equal, and substantially all of the light introduced into the optical waveguide will escape from the optical waveguide into the cured product. In other words, the critical angle will be substantially 90 degrees when the curing of the resin is complete and the attenuation of the waveguide as detected by the photodetector will be at a maximum as evidenced by a substantial absence of any detectable intensity of light being transmitted by the optical waveguide. When the optical waveguide includes the preferred cured composition, the waveguides can be prepared by forming a puddle of the uncured composition and allowing it to cure to a point where the viscosity of the composition allows it to adhere to a tool and be drawn into a fiber. Alternatively, the waveguide can be prepared by extruding the uncured or partially cured composition through a die that ensures a waveguide with smooth walls. The waveguide can also be made by molding the uncured or partially cured composition in a form that provides a waveguide with a smooth surface. In each method, after the waveguide is formed, it is subjected to curing conditions to provide a waveguide of the cured composition. The degree of cure can be monitored in any portion of the uncured composition by passing the optical waveguide through that particular region of the uncured composition. It is within the scope of the present invention to use more than one optical waveguide to monitor the degree of cure in different regions of the curing composition. Because the optical waveguides will remain in the cured product when the curing process is completed, the optical waveguides are preferably positioned so as not to hinder the strength or function of the cured product. Referring to FIGURE 1, an optical waveguide 10 passes through the curing composition 16. The first end of the optical waveguide 10 is optically coupled to a light source 2 by means of an input fiber 6 that is clad to prevent the escape of light from the input fiber 6. Light that is introduced into the optical waveguide 10 by the input fiber 6 is transmitted by internal reflection through the unclad optical waveguide 10 to its second end where it enters an output fiber 8 that is optically coupled to a photodetector 4. The output fiber 8 is clad in a similar manner as the input fiber 6 to prevent any escape of the transmitted light 12 that enters the output fiber 8 from the optical waveguide 10. The transmission characteristics of the optical waveguide 10 are determined from the transmitted light 12 that is detected by the photodetector 4 and provides information that may be related to reference and control standards to determine the degree of cure in the curing composition 16. As the curing process approaches completion, the amount of light introduced into the optical waveguide 10 from input fiber 6 that escapes within the curing composition (as represented by arrows 14) increases until substantially all of the light that is introduced into the optical waveguide 10 passing through the curing composition 16 escapes within the curing composition 16. This results in a substantial absence of transmitted light 12 being detected by the photodetector 4. It is to be understood that modifications and changes to the preferred embodiment of the invention herein described and shown can be made without departing from the scope of the invention as claimed. The following examples are set forth to illustrate the present invention. They are not, however, to be considered as limitations thereof. EXAMPLE An optical waveguide one (1) cm in length and tapered from 1 mm to about 0.5 mm in diameter is fabricated from DEVCON ® 5-Minute ® Epoxy, available from Devcon Corp., Danvers, MA, by drawing a fiber from a pool of the partially cured epoxy resin and permitting the fiber to cure. The fiber has a gentle S-bend shape, similar to an integral sign. Each end of the fiber is epoxied to a 600 µcore step index plastic fiber available from Poly-Optical Products, Inc., Santa Ana, CA. The input plastic fiber attached to the 1 mm diameter end of the epoxy fiber is illuminated with a GaAs laser, available from General Optronics, Edison, NJ, and the output plastic fiber attached to the 0.5 mm diameter end of the epoxy fiber directs the transmitted light to a silicon photodetector, available from United Detector Technology, Hawthorne, CA. Reference is made to FIGURE 1 for a schematic illustration of the apparatus. The GaAs laser was modulated at 1000 Hz, and the output of the silicon cell is detected using a lock-in amplifier to remove any effect from ambient lighting. In air, the epoxy resin fiber transmits light with a detectable signal of 470 relative units. The epoxy resin fiber is then immersed in a freshly mixed pool of the 5-Minute ® Epoxy. The detected signal level immediately falls to 90 relative units and as the pool of 5-Minute ® Epoxy is cured under ambient conditions, the detected signal decreases and falls to six relative units after about 5 minutes. The relationship between the degree of cure as a function of time (X-axis) and the intensity of transmitted light (Y-axis) is illustrated in FIGURE 2 by line 44. Referring to FIGURE 2, as the degree of cure increases with time, the intensity of the light decreases from 90 relative units to about 6 relative units, this decrease in transmitted light provides adequate resolution, to accurately monitor the final stages of the curing process using the method in accordance with the present invention. Particular embodiments of the present invention described above are to be considered in all respects as illustrative and not restrictive. The scope of Letters Patent granted thereon is to be limited only by the definition set forth in the appended claims and equivalents thereof.

Claims: A method of monitoring in situ a process, wherein a composition passes from a liquid phase, which has a refractive index n₂, to a solid phase, which has a refractive index n₃, by using a variation of the optical properties of the composition as a function of the process, characterized in that it comprises the steps of : a) immersing an optical waveguide made of the composition in its solid state and having an index of refraction n₁ in the composition when in its liquid phase, n₁ being greater than n₂ and substantially equal to n₃ ; b) subjecting the liquid composition containing the optical waveguide to conditions that cause the composition to solidify ; c) coupling the optical waveguide to an optical source ; d) coupling the optical waveguide to a photodetector ; e) measuring the intensity of the light transmitted by the optical waveguide during the process. The method of claim 1, further comprising the step : f) comparing the transmission characteristic determined in step e) with reference standards to monitor the degree of solidification of the liquid composition. The method of any of claims 1 and 2, wherein the completion of the solidification of the composition is indicated by a substantial absence of a detectable intensity of the light transmitted by the optical waveguide. The method of any of claims 1 to 3, wherein the composition is a thermosettable resin composition. Apparatus for monitoring a process in situ, wherein a composition passes from a liquid phase, which has a refractive index n₂, to a solid phase, which has a refractive index n₃, by using a variation of the optical properties of the composition as a function of the process comprising a light source and a photodetector, characterized in that it comprises : an optical waveguide made of the composition in its solid phase and coupled to the light source and to the photodetector, said waveguide having an index of refraction n₁ greater than n₂ and substantially equal to n₃ and intended to be immersed in the composition, means for measuring the intensity of the light from the light source transmitted by the waveguide. Apparatus of claim 5 comprising means for comparing the intensity of the light transmitted with reference standards to monitor the degree of solidification of the liquid composition. Apparatus of any of claims 5 to 6, wherein the completion of the solidification of the composition is indicated by a substantial absence of a detectable intensity of the light transmitted by the optical waveguide. Apparatus of any of claims 5 to 7, wherein the composition is a thermosettable resin composition.

Inventors: AFROMOWITZ MARTIN A, AFROMOWITZ, MARTIN A.
Applicants: UNIV WASHINGTON, THE BOARD OF REGENTS OF THE UNIVERSITY OF WASHINGTON
Application year: 1989
Main classification: G01N21
IPC codes: G01N33, G01N21, G01N25, G01N3