WO1993007464A1

WO1993007464A1 - Surface energy and surface charge meter

Info

Publication number: WO1993007464A1
Application number: PCT/US1992/004999
Authority: WO
Inventors: Charles H. Seiter; Roger P. Woodward
Original assignee: Wda Contracts Corporation
Priority date: 1991-10-08
Filing date: 1992-06-15
Publication date: 1993-04-15

Abstract

A surface energy/surface charge meter (10) includes one probe (18) or two probes (18a-b). Each probe (18, 18a-b) has two spaced apart electrodes (38 and 40) with a centrally located fluid dispensing head (36) which dispenses a volume of electrically conductive fluid onto a surface of a material (22) having an unknown surface energy or surface charge. When a fluid of known surface tension is dispensed onto a material (22) having unknown surface energy, the surface energy is determined by the volume of a fluid drop (42) which bridges between the electrodes (38 and 40). When both a positively charged fluid and a negatively charged fluid are dispensed onto a material (22) using probes (18a-b), the surface charge of the material (22) can be determined from the electrical conductivity or resistance measurements made after the electrodes (38 and 40) have been bridged. After bridging the electrodes (38 and 40), the rate at which the positively and negatively charged fluids spread across the surface is a function of surface charge.

Description

SURFACEENERGYAND SURFACECHARGE

METER

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation in part (OP) of the 5 patent application having Serial Number 07/774,533, filed October 8,

1991, entitled "SURFACE ENERGY METER", by the applicants of this patent application, Charles H. Seiter and Roger P. Woodward, now U.S. Patent 5,121,636, and that patent and patent application is herein incorporated by reference.

10 DESCRIPTION

BACKGROUND OF THE INVENTION

Field of the Invention

The invention is generally related to instrumentation used for detecting the surface characteristics of a material in an automated 15 fashion. More particularly, the invention is concerned with instrumentation useful for determining the surface energy of a material, the surface tension of a liquid, the surface charge of a material, and the identification of surface groups on a material.

Description of the Prior Art

Λ 20 Knowledge of the surface energy of a material is particularly important in the plastics field. For example, the ability * to print or coat a material is highly dependent on the surface energy of the material. In addition, the surface energy of the material will determine its suitability for medical or biotechnology applications. The surface energy of a material is a measure of the thermodynamic energy needed to increase surface area and is measured in dynes/cm. For historical reasons, the same parameter in liquids is called surface tension, rather than surface energy. Surface energy measurements have been routinely made for several years by a variety of different techniques.

One method of making surface energy measurements involves optically determining the contact angle of a drop of pure water on the surface of the material of interest. Since the surface tension of pure water is 72 dynes/cm at 25°C, knowledge of the contact angle can yield the surface energy of the material from the Girifalco-Good-Fowkes- Young equation which is set forth in Equation 1 below:

Eq.l

CosQ =-1+2 >1__) U2-__^S∑ ^ γ L γ L

The Girifalco-Good-Fowkes- Young equation is a thermodynamic expression which relates solid surface energy γ_s to liquid tension γ_L and contact angle θ, with a small correction term. for vapor pressure ^ and is discussed in /. Phys. Chem. 61: 904 (1957), which is herein incorporated by reference.

Another method of making surface energy measurements involves the use of a plurality of fluids, each with a known surface tension, where the technician sequentially smears different fluids on the surface of the material of interest. The surface energy of the material is determined when one of the fluids is found to just form a continuous film. Still another method of making surface energy measurements involves dipping the material into a fluid of known surface tension and then weighing the material. The weight of fluid which adheres to the surface of the material is then used to provide a measure of the 5 surface energy of the material.

A problem with the prior art methods of making surface energy measurements is that they rely heavily on the judgement and ability of skilled technicians. Providing a surface energy meter which requires minimal training for use, but which can provide accurate and 10 reliable data, would be very beneficial in the plastics field.

In addition to providing a surface energy meter for detecting the surface energy of a material, it would be particularly advantageous to have a device suitable for determining the surface charge of a material as well as the types of chemical groups present at the surface 15 of a material. In this way, inks, coatings, as well other types of materials which may be deposited on a plastic substrate may be chosen to be compatible with the surface of the plastic substrate. Presently, the surface charge and the charge contributing surface groups of a plastic material is assumed from the knowledge of the 20 plastic material and its treatment process. For example, polystyrene treated with 0₂ plasma is known to have a negative charge and a high density of carboxyl groups and polypropylene treated with ammonia plasma is known to have a positive charge and a high density of amine groups. If one is unaware of the nature of a plastic material to 25 be used or its treatment history, knowledge of the surface charge and active surface groups is difficult to ascertain.

SUMMARY OF THE INVENTION

^■f

It is therefore an object of the invention to provide an automated surface energy meter which is easy to use and which provides reliable measurements.

It is another object of the invention to provide a surface energy meter which can be used to detect the surface energy of a material with a fluid having a known surface tension or which can be used to detect the surface tension of a fluid with a material having a known surface energy.

It is yet another object of the invention to provide a surface energy meter which utilizes electrical sensing to determine surface energy of a material.

It is still another object of the invention to provide a surface energy meter which is compact and portable.

It is still another object of the invention to provide an automated surface charge meter which is easy to use and which provides reliable measurements.

It is yet another object of the invention to provide a surface charge meter which can be used to determine surface charge of a material using two charged fluids.

It is yet another object of the invention to provide a surface charge meter which utilizes electrical sensing to determine the surface charge on a material.

It is yet another object of the invention to provide a surface charge meter which can be used to determine the nature of the constituents at the surface of a material under test. It is still another object of the invention to provide a surface charge meter which is compact and portable.

It is still another object of the invention to provide a combined surface energy meter and surface charge meter.

According to the invention, a digital surface energy/surface charge meter includes a microprocessor, a control panel, a syringe pump assembly and probes for contacting a sample under test. The probes have two separated electrodes and a fluid outlet therebetween for dispensing an electrically conductive fluid onto the surface of a material under test. The electrodes may be pins, circular contacts, a grid arrangement, or other suitable construction.

In the surface energy meter mode, a stepper motor or other suitable device is connected to control the rate at which the electrically conductive fluid is dispensed from a fluid chamber onto the surface of a material under test. When fluid is dispensed onto the surface of the material, a drop forms between the electrodes and expands as the volume of the drop increases. When the drop bridges the two electrodes, a current is conducted through the electrically conductive fluid and the presence of the current acts as a switch which halts the operation of the stepper motor or other device. At the time the drop bridges the two electrodes, the volume of fluid dispensed can be accurately determined (e.g., from the number of steps of the stepper motor, etc.). The microprocessor uses the volume of fluid dispensed to determine surface energy of an unknown material or surface tension of an unknown fluid. To speed operation, the microprocessor can include a look-up table which relates volume to surface energy.

When surface energy of an unknown material is being measured, fluids of known surface tension are dispensed from the probe. Because plastic materials can have a wide range of surface energies, it is preferable to use a number of different fluids, each of which is optimum for a specific range of surface energies. Identification of the fluid being used and its surface tension characteristics can be performed using a built-in bar code reader on the surface energy meter. When surface tension of an unknown fluid is being measured, the unknown fluid is dispensed from the probe onto materials of known surface energy. Similar to the case of measuring the surface energy of an unknown material, it is preferable when measuring for the surface tension of an unknown fluid to have a number of different known materials, each with surface energies that are optimum for a specific range of fluid surface tensions.

In the surface charge mode, preferably two probes are used, one of which has a connected fluid chamber filled with a positively charged fluid and the other of which as a connected fluid chamber filled with a negatively charged fluid. The probes are operated as described above where a fluid is controllably dispensed onto the material surface to a point where a droplet bridges between two spaced apart electrodes, afterwhich fluid dispensing ceases. Immediately following the point at which the droplets bridge between the spaced apart electrodes and for periodic (e.g., one minute) intervals thereafter (e.g., up to five minutes or more), the electrical conductivity or resistivity of the fluid is measured. Since the droplet will spread with time, the electrical conductivity of the fluid will decrease and its resistivity will increase during the period of conductivity/resistivity measurements following the fluid bridging the electrodes.

By using a positively charged fluid in one probe and a negatively charged fluid in the other probe, the surface charge of the material can be readily determined as either positive or negative under the principle that fluids which are oppositely charged relative to the material under test will spread faster and have a correspondingly faster decrease in conductivity or increase in resistance than fluids which have the same charge as the material under test. For example, a positively charged fluid such as dodecyltrimethylammonium bromide (DTAB) will spread faster on a negatively charged wetted surface such as polystyrene treated with oxygen plasma than a negatively charged fluid such as sodium dodecyl sulfate (SDS). Hence, the rate at which conductivity decreases or resistivity increases for the positively charged fluid relative to the negatively charged fluid can be used to determine whether the surface of a material is positively charged, negatively charged, or neutral.

In a preferred mode of operation, after a suitable time period, such as five minutes, a ratio of the measured conductivity or resistivity of the positively and negatively charged fluids is determined. The ratio provides an index of the surface charge of the material under test. It has been found by experimentation that the index generally ranges between 0.3 and 2. For example, with a polystyrene surface treated with 0₂ plasma which is known by ESCA analysis of derivatized samples to have a high density of carboxyl groups and a correspondingly negative charge, it has been found that the index after five minutes when using DTAB has a positively charged solution and SDS as a negatively charged solution is approximately 2. Conversely, with a polypropylene material treated with a ammonia plasma which is known to have a high amine group density and corresponding positive charge, it has been found that the index after five minutes when using DTAB and SDS is 0.3. Other surfaces have been found to give values inside the 0.3 to 2 range and neutral surfaces give index measurements of close to 1.

Making periodic measurements of the conductivity or resistivity subsequent to the fluid bridging the electrodes may be used to speed up the determination of the index value. Specifically, a computer controller may mathematically form an asymptotic value from a shorter time series of measurements to predict the final conductivity or resistivity value for the positively and negatively charged fluids at longer times. In addition to using surfactants such as DTAB and SDS in the surface charge testing scheme, solutions containing charged polymers or other suitable compounds may also be used. Moreover, other alkane based detergents of longer and shorter chain length can be used. In this way, the nonpolar components of spreading can be taken into consideration.

The chemical nature or concentration of the charged groups on a test surface may also be identified by using a series of positively and negatively charged fluids, where the pH of the fluids in the series is varied. For example, if the index (e.g., ratio of the conductivity or resistivity detected with the positively charged fluid relative to the conductivity or resistivity detected with the negatively charged fluid) is closer to 1 at pH 3 rather than pH 7, it suggests that the charged groups of the material surface have been titrated to neutrality at this pH. Some surfaces may have the an index closer to 1 at pH 11 rather than pH 7 and, as above, the change in index is explained by titration of the surface charges to neutrality. By using a series of. positively and negatively charged solutions at a series of pH steps, titration curves are prepared from the observed change in the index measurement and these titration curves can be used to identify the chemical environment of the charged groups from standard handbook data.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

Figure 1 is an isometric view of the surface energy meter/surface charge meter according to the present invention;

Figure 2 is a side view of a probe used in the surface energy meter/surface charge meter;

Figure 3 is a bottom view of a probe used in the surface energy meter/surface charge meter;

Figures 4a-c are sequential side views of a drop expanding on a surface to electrically connect the electrodes of the probe of the surface energy meter/surface charge meter;

Figures 5a-c correspond with Figures 4a-c and are sequential top views of the drop expanding on the surface; and

Figure 6 is an isometric view of the surface energy meter/surface charge meter set up with two probes for conducting surface charge measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to the drawings, and more particularly to Figures 1 and 6, there is shown a digital surface energy/surface charge meter 10 which includes a syringe 12 (Figure 1) or syringes 12a and 12b (Figure 6), a pump 14, a control panel 16, and a probe 18 (Figure 1) or probes (Figure 6). Except where indicated, identical numerals in Figures 1 and 6 indicate identical elements. The surface energy/surface charge meter 10 is shown in Figure 1 to be ideally set up for surface energy measurements and in Figure 6 to be ideally set up for surface charge measurements. It will be apparent, however, that the single syringe 12, single probe 18 arrangement of Figure 1 could be used for surface charge measurements as discussed in conjunction with Figure 6 simply by changing the fluid within the syringe 12. With respect to surface energy measurements and with particular reference to Figure 1, the syringe 12 contains an electrically conductive fluid which is pumped to the probe 18 via conduit 20 and onto the surface of a material 22. The pump 14 is preferably driven by a stepper motor 24 which sequentially moves plate 26 downward on stem 28 of the syringe 12. Precise volumes of fluid are delivered with each step of the motor 24 where the volumes depend on the size syringe 12 which is used and degree of movement of plate 26 per step. Other pumps capable of metering out precise volumes of fluid could also be used within the practice of the invention including electronically controlled pumps, and clock-drive-type pumps. A clear plastic cover 29 can be positioned over the pump 14 and syringe 12 to provide e vironmental protection.

Probe 18 includes two electrodes, discussed in detail below, which are electrically connected to the surface energy meter 10 via conductive wires 30. While the probe 18 can be fitted on a stand 32, it is preferably of a size and has the flexibility in terms of conduit 20 length and wire length which allows it to be positioned on any flat surface. Those skilled in the art will recognize that the probe 18 and surface energy/surface charge meter 10 unit can be readily adapted for hand-held use in the field where a battery powers the surface energy/surface charge meter 10 and the technician holds the probe 18 against the surface of interest. In a preferred embodiment, the probe 18 is one centimeter in diameter; however, the probe 18 could be made smaller or larger for accommodating a variety of different surfaces.

The surface energy/surface charge meter 10 is preferably microprocessor controlled. Control panel 16 allows entry of fluid, material, and other information into the microprocessor inside the surface energy/surface charge meter 10, as well as displays surface energy measurement results in display area 34. The double arrow keys 110 on control panel 16 allow the operator to drive the stepper motor rapidly backwards (< <) and forwards (> >). The microprocessor should also be programmable to drive the stepper motor at a prescribed rate. The arrow keys 112 under the display area 34 allow the operator to change the values presented in program menus used for programming the surface energy/surface charge meter 10.

Figures 2 and 3 show the probe 18 can have a centrally located fluid dispensing head 36 connected to conduit 20. Two electrodes 38 and 40 can be located on opposite sides of the probe 18. However, different electrode arrangements (e.g., circular contacts or electrode grids) might also be used in the probe 18 wherein, for example, conductance between various points on the grid would be used in a similar manner to electrodes 38 and 40. The electrodes 38 and 40 may extend beyond the probe 18 bottom or be even therewith. The function of the electrodes 38 and 40 is to provide a means of sensing the presence of a fluid droplet. Fluid dispensed by the fluid dispensing head 36 is made electrically conductive so that current passes between electrodes 38 and 40 when the fluid droplet provides an electrically conductive pathway between them.

Figures 4a-c and corresponding Figures 5a-c provide a description of the operation of the probe 18 of the surface energy/surface charge meter 10 and are best understood with reference to Figures 1-3. Figures 4a and 5a show that at the beginning of a measurement run, the fluid dispensing head 36 deposits a controlled volume of fluid 42 on a surface 22 at a central location between the probes 38 and 40. Figures 4b and 5b show that the diameter of the fluid drop 42 increases as more fluid is pumped onto the material surface 22. Figures 4c and 5c show that the diameter of the fluid drop 42 will ultimately become large enough to bridge electrodes 38 and 40 as more fluid is pumped onto surface 22. When the fluid drop 42 bridges electrodes 38 and 40, the stepper motor 24 is shut off. The volume of the fluid drop 42 required to bridge the electrodes 38 and 40 can be precisely determined by knowing the number of steps required by the stepper motor 24 to bridge the electrodes 38 and 40 and the volume dispensed per step. In the case where the surface energy/surface charge meter 10 is to be used to determine the surface energy of an unknown solid material, a volume of a fluid having a known surface tension is pumped onto the unknown solid until the electrodes 38 and 40 are bridged. Stored in the microprocessor prior to testing are values which relate stepper motor counts (a volume measurement) for the particular fluid having the known surface tension being used to the surface energy of at least two known solids which preferably have correspondingly expected higher and lower surface energies than the solid under investigation (although having higher and lower surface energies is not required). The volume of fluid having the known surface tension which was dispensed on the solid having the unknown surface energy is then used to calculate the surface energy of the unknown solid by first determining a variable X according to Equation

2 as follows:

Eq.2

_j. _ Low ^vMeas ^VLow~^VHigh

where V,^, is the volume in stepper motor counts of the fluid having the known surface tension which is required for bridging the electrodes when a solid having a low surface energy is measured, V_High is the volume in stepper motor counts of the fluid having the known surface tension which is required for bridging the electrodes when a solid having a high surface energy is measured, and V_Meas is the volume in stepper motor counts of the fluid having the known surface tension which was required for bridging the electrodes for the solid under investigation. After the variable X is calculated according to Equation 2 above, the surface energy of the solid under investigation is computed according to Equation 3: Eq.3

S. E. ( dynes/ cm) = E_Low+ (E_Hlgh-E_Low) -x

where E-^ is the known surface energy of the low surface energy solid used for V^, and E_High is the known surface energy of the high surface energy solid used for V_High. EXAMPLE 1, discussed below, presents test data where the above operation and equations were successfully used for determining the surface energy of test surfaces. Likewise, in the case where the surface energy/surface charge meter 10 is to be used to determine the surface tension of a fluid, the fluid with unknown surface tension is pumped onto a surface of a material having a known surface energy until the electrodes are bridged. The surface tension can then be calculated from the volume of the fluid having unknown surface tension required to bridge the electrodes 38 and 40. Specifically, volume values in terms of stepper motor counts for two fluids having known surface tensions which are preferably higher and lower than the expected surface tension of the unknown fluid (although having higher and lower surface tensions is not required) and which relate the two fluids to a surface having a known surface energy are used in a calculation like equation 2 after the volume value for the fluid having unknown surface tension is determined on the surface having a known surface energy. Subsequently, the surface tension is determined in a calculation like that of equation 3 where the surface tensions of the known fluids are combined with a variable which considers the volume required for the fluid having unknown surface tension to bridge the electrodes when a solid of known surface energy is used.

In a preferred embodiment of the invention, a microprocessor in the surface energy/surface charge meter 10 can include a look-up table which relates volume information to surface energy or tension. In this manner, the speed of determining a surface energy or surface tension measurement will be increased since calibration runs with fluids having known surface tensions and solids having known surface energies would not need to be performed each time the surface energy of an unknown solid or the surface tension of an unknown fluid are to be determined. The calibration runs would be storable in memory and would only need to be performed periodically.

The principle of operation of the surface energy/surface charge meter 10 for surface energy measurements can be understood by_. contrasting a test where a drop of water is used to determine the surface energy of an untreated polyethylene material with a test where a drop of water is used to determine the surface energy of a polyethylene material that has been subjected to a plasma or corona discharge. In each test, the water would include a small amount of salt for conductance purposes. If the water is pumped onto the untreated polyethylene surface, the water will form a very tall drop with a contact angle of 90° or more, a condition sometimes referred to as "beading" up. In this situation, a large amount of water would need to be dispensed before a fluid drop 42 having sufficient diameter to bridge electrodes 38 and 40 is formed. The large volume of water required to bridge the electrodes 38 and 40 would correspond to the untreated polyethylene having a low surface energy. By contrast, when the water is pumped onto the plasma or corona treated polyethylene surface, which are two common ways to make a polyethylene surface "printable" with water-based inks, the water will form a very flat drop with a contact angle of 10° or less. In this situation, a relatively small volume of water is required to obtain a fluid drop 42 of sufficient diameter to bridge electrodes of 38 and 40. The small volume of water required to bridge the electrodes 38 and 40 would correspond to a high surface energy.

Ideally, fluids having higher contact angles on the surface of interest are better for detecting the surface energy. This is because very low contact angles are not easily repeatable. Since no single fluid is known which will be capable of having a suitable contact angle on all surfaces, it is preferred to have a large number of fluids with different known surface tensions when determining the surface energy of a material of interest. In a preferred embodiment, five or more fluids will be used in the surface energy meter 10 where the surface tension of each of the fluids is selected to be suitable for accurately testing the surface energy of materials within a specific range of surface energies. The surface tensions of each of the fluids can be indicated on the syringe 12 using a bar-code so that the technician is not required to input the surface tension each time a test is performed. A bar-code reader could be attached to or built-in the surface energy meter 10 so that each time a different fluid of known surface tension is placed in the surface energy meter 10, the microprocessor automatically considers the surface tension when determining the surface energy of the material under investigation. If a plurality of fluids having known surface tensions are to be used, a means for assuring that the pump 14 empties all remaining fluid of a prior syringe 12 stored in conduit 20 before the new fluid is used in a test. Likewise, if the surface tension of an unknown fluid is to be determined, it would be advantageous to have a plurality of materials, each of which has a known surface energy that is suitable for determining surface tensions within different ranges. In addition, if an unknown fluid is to be tested, it must first be made electrically conductive by adding a small amount of salt.

A number of fluids, each of which has a different known surface tension, can be made simply by combining different ratios of known fluids. For example, a mixture of isopropanol and water can be easily adjusted to produce a test fluid ranging between 30 dynes/cm and 72 dynes/cm, which are the surface tensions of pure isopropanol and pure water, respectively. In a particular embodiment, by mixing 90 gms of water with 10 gms of isopropanol, a fluid with a surface tension of 55 is produced. Solutions are best prepared using ingredient weights rather than volumes because weight can be specified with greater accuracy than volumes, especially for large quantities. In a preferred embodiment, the solutions are adjusted to approximately 0.0005 M NaCl for detecting a sharp conductivity change and may be provided with trace amounts of a blue dye for the technician to observe the spreading of the fluid drop under the probe. Other salts could be used instead of NaCl with the only requirement being that the amount of salt be large enough to promote electrical conductivity between the electrodes 38 and 40, but small enough not to interfere with the surface energy measurement. Isopropanol and water mixes are inexpensive and present minimal corrosion and plastics compatibility problems; however, it should be understood that other organic compounds that are soluble in water can be used in like manner. For example, water could be mixed with formamide, dimethyl formamide, and acetone to make a series of solutions with surface tensions of different values. In addition, formamide might be combined with water/isopropanol formulations to fine tune the mixtures to achieve particular surface tension numbers. Moreover, the range of surface tensions for the test fluids can be expanded beyond isopropanol/water mixtures. For example, hexane, with a surface tension of 18 dynes/cm can be mixed with n-butanol, at 25 dynes/cm and toluene at 29 dynes/cm to obtain a range of mixtures below 30 dynes/cm. Potassium carbonate solutions, with a surface tension of 85 dynes/cm for a 2M concentration, may be used to obtain test fluids with surface tension above 72 dynes/cm.

With respect to surface charge measurements and with particular reference to Figure 6, the surface energy/surface charge meter 10 includes two syringes 12a-b connected to two probes 18a-b by two conduits 20a-b. The probes 18a-b are shown contacting adjacent portions on the surface of material 22; however, they need not contact adjacent portions of the material. As explained above, the probes 18a-b and surface energy/surface charge meter 10 are readily adaptable to field use and can include battery power. The probes 18a-b are identical in configuration to those described in . conjunction with probe 18 in Figures 1-5 and include spaced apart electrodes 38 and 40 which are bridged by electrically conductive fluids dispensed from syringes 12a-b. In the preferred configuration, the probes 18a-b are each one centimeter in diameter; however, the could be smaller or large for accommodating a variety of different surfaces. As noted above, the electrodes 38 and 40 (shown best in Figures 2-5) could be replaced with spaced circular contacts or by a grid arrangement wherein the conductance between various points on the grid can be measured. Figure 6 shows the use of two stepper motors 24a-b or like devices to move plates 26a-b downward on stems 28a-b of syringes 12a-b to deliver precise volumes of fluid; however, it should be understood that a single stepper motor could operate a single plate that operates against both stems 28a-b. When the surface energy/surface charge meter 10 is operated in the surface charge detecting mode, one of the syringes 12a-b preferably contains a positively charged fluid while the other preferably contains a negatively charged fluid. A suitable positively charged fluid is the surfactant dodecyltrimethylammonium bromide

(DTAB) and a suitable negatively charged fluid is sodium dodecyl sulfate (SDS). Each of these surfactants (detergents) contains an alkane constituent of identical length so that nonpolar spreading properties for the two surfactants are the same. To evaluate the nonpolar spreading properties on a material surface, one might use a series of longer and shorter chain length alkane based detergents. The positively and negatively charged fluids need not be surfactants. Other suitable solutions may include positively charged and negatively charged polymers dissolved or dispersed in a carrier fluid. The chief requirements for the choice of fluids surface charge measurements with the surface energy/surface charge meter would be that they have similar nonpolar spreading properties, as would be the case if the nonpolar constituents of the two fluids are of approximately the same size, and that they have approximately equal and opposite charge characteristics.

When detecting the surface charge of a material under test, both the positively charged fluid and the negatively charged fluid should be balanced to pH 7. For purposes of illustration, assume a 4*10^"^ DTAB solution is present in syringe 12a and a 4*10^'4M SDS solution is present in syringe 12b (it being understood that the relative position of the fluids and the choice of fluids can be altered significantly within the practice of this invention). Each of these solutions give an initial resistance across the electrodes 38 and 40 of the respective probes 18a and 18b of approximately 200K ohms, depending on the surface and drop size on which they are deposited, and these resistance values change by a factor ranging between two and three on a time scale of five minutes.

In operation, the DTAB or other positively charged fluid is pumped out of syringe 12a under the precise control of stepper motor 24a until a drop is created on the surface of material 22 which just bridges the electrodes 38 and 40 in probe 18a. At this point, the stepper motor 24a is stopped and a series of electrical measurements including either or both the conductivity through the drop or the resistance through the drop are taken at timed intervals (e.g., one minute) for a preset period of time (e.g., five minutes). Either subsequently or at the same time, the SDS or other negatively charged fluid is pumped out of syringe 12b under the precise control of stepper motor 26a until a drop is created on a different portion of the surface of material 22 which just bridges the electrodes 38 and 40 in probe 18b. In like manner, conductivity and/or resistivity measurements are made on the drop at probe 18b at timed intervals for a preset amount of time. The measurements for both probes 18a and 18b show a decrease in conductivity or increase in resistivity as the drops spread with time. Spreading is enhanced with DTAB and SDS because of their surfactant character.

The essence of the measurement is that fluids which have a charge which is opposite to that of the material 22 surface spread faster on the wetted surface than do fluids which have the same charge as the wetted surface. For example, DTAB, a positively charged fluid, spreads faster on an 0₂ plasma treated polystyrene surface, which is known to be negatively charged and have a high density of carboxyl groups, than does SDS, a negatively charged fluid. Likewise, SDS spreads faster on an ammonia plasma treated polypropylene surface which is known to have a high amine group density and corresponding positive charge. It is proposed that the phenomena of quicker spreading of the fluids having an opposite charge on the surface of material 22 is related to adsorption of the oppositely charged fluid into the Stem interfacial layer. This quicker spreading is reflected by a faster decline in conductivity of the drop or faster increase in resistivity. Hence, a qualitative charge determination for the surface of material 22 can be made by observing whether or not the positively charged fluid drop at probe 18a has a conductivity that declines at a different rate from the negatively charged fluid drop at probe 18b. For example, when the measured conductivity for the positively charged fluid at probe 18a declines more rapidly than the negatively charged fluid at probe 18b, the surface of material 22 has a negative charge. Conversely, when the measured conductivity declines more slowly, the surface of the material 22 has a positive charge, and when the measured conductivity declines at the same rate, the surface of the material 22 is neutral.

A more quantitative measure of the surface charge of the material 22 can be obtained by determining a ratio of the conductivity of the positively charged fluid drop relative to the conductivity of the negatively charged fluid drop after a preset period of time. This ratio is then used as an index from which the surface charge of the material can be determined. In trial experiments with 0₂ plasma treated polystyrene and ammonia plasma treated polypropylene, the index ranges from 0.3 to 2 after a five minute interval. Specifically, after five minutes the ratio of conductivity (and resistivity) for a DTAB drop on 0₂ plasma treated polystyrene material relative to an SDS drop on the same material is 2, whereas after five minutes the ratio of conductivity (and resistivity) for a DTAB drop on ammonia plasma treated polypropylene relative to an SDS drop on the same material is 0.3. Other surfaces have been found to give index values inside the 0.3 to 2 range, and neutral surfaces give readings close to 1. Hence, an index number can be used to determine if a surface has a positive or negative charge or is neutral. To speed the measurement process along, a computer controller in the surface energy/surface charge meter 10 may be used to mathematically form an asymptotic value from a shorter time series of measurements to predict the final value at longer times (e.g., that which would occur at five minutes).

While two probes 18a-b are discussed in conjunction with Figure 6 when surface charge measurements are being made, it should be understood that a single probe (e.g., Figure 1) can be used for making surface charge measurements where the syringe would e alternately filled with positively and negatively charged fluids and the conductivity and/or resistivity measurements would be made in sequential order. The nature and concentration of the surface charge contributing components of a material may also be evaluated with the surface energy/surface charge meter 10. For surface charge measurements, the pH of the positive and negative charge fluids was balanced at pH 7; however, when evaluating the nature and concentration of the surface charge contributing components of a material, a series of positively and negatively charged solutions having the pH adjusted to various levels is required. In particular, having a set of solutions with the pH stepped from pH 3 to pH 11 would be beneficial. For example, a highly negatively charged surface may yield an index of 2 for positively and negatively charged fluids pH balanced at pH 7; however, the same surface may yield an index closer to 1 when the pH of the positively and negatively charged fluids is adjusted to pH 3. This would occur when the negatively charged groups of the underlying polymer surface are titrated to neutrality by the positively charged solution. This change in index would occur when the carboxyl derivatives of the underlying polymer are the main charge components. Likewise, a highly positively charged surface may yield an index of 0.3 for positively and negatively charged fluids pH balanced at pH 7; however,^' the same surface may yield an index closer to 1 when the pH of the positively and negatively charged fluids is adjusted to pH 11. This would occur when the positively charged groups of the underlying polymer surface are titrated to neutrality by the negatively charged solution. This change in index would occur when the amine derivatives of the underlying polymer are the main charge components. If solutions of positively and negatively charged fluids are formulated at a series of pH steps, it is possible to construct titration curves of the charged species on the polymer surface. The titration curves can be compared to standard handbook data to identify the chemical environment of the charged groups.

EXAMPLE 1

Mylar with no calendaring agents and glass treated with a hydrocarbon plasma were used as solids with known surface energies. The surface energies of the materials were determined to be 44 dynes/cm for the Mylar and 60 dynes/cm for the plasma treated glass by repeated tests with wetting solutions according to the established prior art technique of smearing a series of wetting solutions on the surface of the materials. A plasma was then used to produce two treated glass surfaces with unknown surface energies which were below 60 dynes/cm (the plasma treatment conditions used to create the known surface were modified to create a glass with lower surface energy). The fluid used in the experiments for determining the surface energies was water with 0.005 M NaCl and a trace amount of blue dye, which had a known surface tension of 70 dynes/cm. The measured volume in steps for the 60 dynes/cm plasma treated glass surface was 219, while the measured volume in steps for the 44 dynes/cm Mylar surface was 420. One of the plasma treated glass test surfaces had a fluid volume value of the 316 steps which corresponds to a surface energy value of 52 dynes/cm, and the other plasma treated glass test surface had a fluid volume value of 443 steps which corresponds to a surface energy value of 42 dynes/cm. These surface energy values were determined to be correct using the prior art technique of smearing a series of fluids having known surface tensions on the test surfaces.

EXAMPLE 2

A 4*10^_ M DTAB solution and a 4*10^**M SDS solution are prepared and pH balanced at pH 7. The solutions are separately applied to the surfaces of both a polystyrene material treated with 0₂ plasma and a polypropylene material treated with ammonia plasma using a 1 cm diameter probe with spaced apart electrodes. The DTAB and SDS are deposited to a point where a droplet just bridges the electrodes of the probe. The conductivity and resistivity ratio for the DTAB droplet relative to the SDS droplet after five minutes was 2 for the 0₂ plasma treated polystyrene and 0.3 for the ammonia plasma treated polypropylene.

While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Claims

CLAIMSHaving thus described our invention, what we claim as new and desire to secure by Letters Patent is as follows:

1. A surface charge meter, comprising: a means for holding a first electrically conductive fluid which is positively charged; a means for holding a second electrically conductive fluid which is negatively charged; a means for dispensing quantities of said first and second electrically conductive fluids onto surfaces of a material; a means for determining at least one first electrical conductivity or resistance value for said quantity of said first electrically conductive fluid on said material; a means for determining at least one second electrical conductivity or resistance value for said quantity of said second electrically conductive fluid on said material; and a means for determining a surface charge characteristic of said material from a comparison of said first and second electrical conductivity or resistance values.

2. A surface charge meter as recited in claim 1 wherein said means for holding said first electrically conductive fluid which is positively charged and said means for holding said second electrically conductive fluid which is negatively charged are the same holding vessel.

3. A surface charge meter as recited in claim 1 wherein said means for holding said first electrically conductive fluid which is positively charged and said means for holding said second electrically conductive fluid which is negatively charged are each syringes.

4. A surface charge meter as recited in claim 1 wherein said first electrically conductive fluid which is positively charged is dodecyltrimethylammomum bromide and said second electrically conductive fluid which is negatively charged is sodium dodecyl sulfate.

5. A surface charge meter as recited in claim 1 wherein said first electrically conductive fluid which is positively charged and said second electrically conductive fluid which is negatively charged are both surfactants.

6. A surface charge meter as recited in claim 1 wherein said first electrically conductive fluid which is positively charged and said second electrically conductive fluid which is negatively charged are fluids containing positively and negatively charged polymers respectively.

7. A surface charge meter as recited in claim 1 wherein said means for dispensing quantities of said first and second electrically conductive fluids includes at least one stepper motor.

8. A surface charge meter as recited in claim 1 wherein said means for determining a first electrical conductivity or resistance value of said first electrically conductive fluid on said material and said means for determining a second electrical conductivity or resistance value of said second electrically conductive fluid on said material each include a pair of spaced apart electrodes which are bridged by said first and second electrically conductive fluids.

9. A surface charge meter as recited in claim 1 wherein said means for determining a first electrical conductivity or resistance value of said first electrically conductive fluid on said material and said means for determining a second electrical conductivity or resistance value of said second electrically conductive fluid on said material each include an electrically conductive grid which is bridged by said first and second electrically conductive fluids.

10. A surface charge meter as recited in claim 1 wherein said means for determining a surface charge characteristic of said material from a comparison of said first and second electrical conductivity or resistance values comprises a means for evaluating the rate of decline of said first electrical conductivity values or rate of increase of said first electrical resistance values with the rate of decline of said second electrical conductivity values or rate of increase of said second electrical resistance values.

11. A surface charge meter as recited in claim 1 wherein said means for determining a surface charge characteristic of said material from a comparison of said first and second electrical conductivity or resistance values comprises a means for determining an index value which relates a first electrical conductivity or resistance value for said first electrically conductive fluid with a second electrical conductivity or resistance value for said second electrically conductive fluid.

12. A surface charge meter as recited in claim 11 wherein said index ranges from 0.3 to 2.0.

13. A method for determining an index of surface charge of a material, comprising the steps of: dispensing a first volume of a first electrically conductive, charged fluid onto a surface at a point between a first pair of spaced apart electrodes, said first volume providing a pathway between said first pair of spaced apart electrodes; waiting a specified time for said first volume to spread out across said surface; measuring a first electrical resistance associated with said first charged fluid with said first pair of electrodes; dispensing a second volume of a second electrically conductive, charged fluid onto said surface at a point between a second pair of spaced apart electrodes, said second volume providing a pathway between said second pair of spaced apart electrodes; waiting a specified time for said second volume to spread out across said surface; measuring a second electrical resistance associated with said second charged fluid with said second pair of electrodes; and relating the ratio of said first and said second electrical resistances to surface charge of said surface.

14. A method for determining an index of surface charge of a material as recited in claim 13 wherein said first and said second fluids are oppositely charged detergents.

15. A method for determining an index of surface charge of a material as recited in claim 13 wherein said first and said second fluids include oppositely charged polymers.

16. A method for determining an index of surface charge of a material as recited in claim 13 wherein said first fluid is dodecyltrimethylammomum bromide and said second fluid is sodium dodecyl sulfate.

17. A method for determining an index of surface charge of a material as recited in claim 13, further including the steps of: periodically measuring an electrical resistance associated with said first charged fluid to obtain a first set of preliminary resistance values; periodically measuring said electrical resistance associated with said second charged fluid to obtain a second set of resistance values; and using said first and said second preliminary resistance values to predict final resistance values.

18. A method for determining an index of surface charge of a material as recited in claim 13, further including the steps of: varying the pH levels of said first and said second fluids from basic to acidic; and constructing titration curves for said surface used to identify charged species on said surface.

19. A method for determining the surface charge of a material, comprising the steps of dispensing quantities of a first electrically conductive fluid which is positively charged and a second electrically conductive fluid which is negatively charged onto surfaces of a material; determining at least one first electrical conductivity or resistance value for said quantity of said first electrically conductive fluid on said material; determining at least one second electrical conductivity or resistance value for said quantity of said second electrically conductive fluid on said material; and determining a surface charge characteristic of said material from a comparison of said first and second electrical conductivity or resistance values.

20. A surface energy meter, comprising: a vessel for holding an electrically conductive fluid; at least two spaced apart electrodes which can be bridged by said electrically conductive fluid; a means for dispensing said electrically conductive fluid from said vessel onto a surface of a material at a point centrally located between said two spaced apart electrodes; means for determining a volume of said electrically conductive fluid required to be dispensed onto said surface of said material to bridge said two spaced apart electrodes and provide an electrical pathway therebetween; and means for relating said volume of said electrically conductive .fluid to a surface energy of said material and a surface tension of said electrically conductive fluid.

21. A surface energy meter as recited in claim 20 wherein said means for dispensing includes a pump.

22. A surface energy meter as recited in claim 21 wherein said pump is electronically controlled.

23. A surface energy meter as recited in claim 22 wherein said pump is driven by a stepper motor.

24. A surface energy meter as recited in claim 23 wherein said means for determining said volume includes a means for counting a number of steps of said stepper motor.

25. A surface energy meter as recited in claim 20 wherein said means for relating includes a stored value for a surface energy of said material.

26. A surface energy meter as recited in claim 20 wherein said means for relating includes a stored value for surface tension of said electrically conductive fluid.

27. A surface energy meter as recited in claim 26 wherein said stored value for said electrically conductive fluid ranges from 18 to 85 dynes per centimeter.

28. A surface energy meter as recited in claim 20 wherein said means for dispensing and said two spaced apart electrodes are both contained in a probe, said probe being positionable on said surface of said material.

29. A method for determining the surface energy of a material, comprising the steps of: dispensing a first volume of an electrically conductive fluid having a known surface tension onto a surface of a first material at a point centrally located between two spaced apart electrodes, said first volume providing an electrical pathway between said two spaced apart electrodes; and calculating a first surface energy of said first material from said first volume of said electrically conductive fluid.

30. A method as recited in claim 29 wherein said step of calculating includes the steps of: determining a second volume of said electrically conductive fluid having said known surface tension which is required to provide an electrical pathway between said two spaced apart electrodes when said electrically conductive fluid is dispensed onto a surface of a second material having a known second surface energy; determining a third volume of said electrically conductive fluid having said known surface tension which is required to provide an electrical pathway between said two spaced apart electrodes when said electrically conductive fluid is dispensed onto a surface of a third material having a known third surface energy; and computing said first surface energy from said first, second and third volumes of said electrically conductive fluid and said second and third surface enerqies.

32. A method as recited in claim 30 wherein said steps of determining said second and third volumes of said electrically conductive fluid having said known surface tension are performed in advance of said step of determining said first volume of said electrically conductive fluid having said known surface tension and are stored in a memory for use during said computing step.

32. A method for determining the surface tension of a fluid, comprising the steps of: assuring that a first fluid having a surface tension to be measured is electrically conductive; dispensing a first volume of said first fluid onto a surface of a material having a known surface energy at a point centrally located between two spaced apart electrodes, said first volume providing an ..electrical pathway between said two spaced apart electrodes; and calculating a first surface tension of said first fluid from said first volume.

33. A method as recited in claim 32 wherein said step of calculating includes the steps of: determining a second volume of a second electrically conductive fluid having a known second surface tension which is required to provide an electrical pathway between said two spaced apart electrodes when said second electrically conductive fluid is dispensed onto said surface of said material having said known surface energy; determining a third volume of a third electrically conductive fluid having a known third surface tension which is required to provide an electrical pathway between said two spaced apart electrodes when said third electrically conductive fluid is dispensed onto said surface of said material having said known surface energy; and computing said first surface tension from said first, second and third volumes of said first, second and third electrically conductive fluids and said known surface energy of said material.

34. A method as recited in claim 33 wherein said steps of determining said second and third volumes of said second and third electrically conductive fluids having said known surface tensions are performed in advance of said step of determining said first volume of said first fluid, and are stored in a memory for use during said computing step.