CA2267722A1 - Method and apparatus for determining moisture content - Google Patents
Method and apparatus for determining moisture content Download PDFInfo
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- CA2267722A1 CA2267722A1 CA 2267722 CA2267722A CA2267722A1 CA 2267722 A1 CA2267722 A1 CA 2267722A1 CA 2267722 CA2267722 CA 2267722 CA 2267722 A CA2267722 A CA 2267722A CA 2267722 A1 CA2267722 A1 CA 2267722A1
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- 238000000034 method Methods 0.000 title claims abstract description 51
- 238000005259 measurement Methods 0.000 claims abstract description 29
- 229920000139 polyethylene terephthalate Polymers 0.000 claims abstract description 20
- 239000005020 polyethylene terephthalate Substances 0.000 claims abstract description 20
- 239000004033 plastic Substances 0.000 claims abstract description 4
- 229920003023 plastic Polymers 0.000 claims abstract description 4
- -1 polyethylene terephthalate Polymers 0.000 claims abstract description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 31
- 238000012360 testing method Methods 0.000 claims description 24
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- 238000001816 cooling Methods 0.000 claims description 3
- 238000004891 communication Methods 0.000 claims description 2
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- 229920005989 resin Polymers 0.000 abstract description 11
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- 239000008188 pellet Substances 0.000 abstract description 5
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- 239000012080 ambient air Substances 0.000 description 15
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 6
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- 239000012159 carrier gas Substances 0.000 description 3
- 238000009826 distribution Methods 0.000 description 3
- 239000000383 hazardous chemical Substances 0.000 description 3
- 229910052757 nitrogen Inorganic materials 0.000 description 3
- 238000011084 recovery Methods 0.000 description 3
- 229920006395 saturated elastomer Polymers 0.000 description 3
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- Investigating Or Analyzing Materials Using Thermal Means (AREA)
Abstract
Method and apparatus for measuring levels of moisture in a sample, particularly plastic resins and articles such as polyethylene terephthalate (PET) pellets/preforms/containers. A
sample of the material/article is placed in an oven and heated while a flow of gas passing through the oven carries the moisture released by the sample and exits the oven as an output gas flow.
Three measurements are made of the output gas flow -- namely temperature, relative humidity, and flow rate. From these measurements, the mass flow rate of the output gas flow is calculated in order to determine the moisture content released by the sample.
sample of the material/article is placed in an oven and heated while a flow of gas passing through the oven carries the moisture released by the sample and exits the oven as an output gas flow.
Three measurements are made of the output gas flow -- namely temperature, relative humidity, and flow rate. From these measurements, the mass flow rate of the output gas flow is calculated in order to determine the moisture content released by the sample.
Description
METHOD AND APPARATUS FOR
DETERMINING MOISTURE CONTENT
Field of the Invention This invention relates to a simple and reproducible method and apparatus for determining levels of moisture in samples, such as polymer resins and articles, utilizing measurements of temperature, relative humidity and rate of gas flow.
to Background of the Invention Low moisture levels are required during various stages of the manufacture of injection blow-molded beverage containers made from polymers such as polyethylene terephthalate (PET), in order to achieve certain desired attributes such as clarity, strength, thermal resistance and impact resistance. For example, if the starting resin has excess moisture, heating the same 15 causes hydrolytic degradation which reduces the intrinsic viscosity and makes it difficult to mold a clear preform. A similar problem arises during reheating of the preform (prior to blow molding), i.e., moisture in the preform reduces the temperature at which haze (lack of clarity) occurs, particularly with preforms for hot-fill containers. Haze, a measure of the scattering of light as it is transmitted through an article, is defined as the ratio of light transmitted through the 2o article to_ the light transmitted through air. A value of 0% haze would indicate a perfectly clear (transparent) article, while a value of 100% haze would indicate a totally opaque article.
Another complication arising from excess moisture in the preform is a plasticizing effect which makes it difficult to maintain a uniform sidewall thickness during reheating and/or stretch blow molding. In effect, the plastic becomes too soft and there may be material flow into the 25 base. Still further, moisture in the final blown container causes problems because it is more likely to shrink either during cooling (e.g., after removal from the blow mold) or upon subsequent exposure to elevated temperatures. Again, the presence of moisture is a particular problem with hot-fill containers -- the containers will shrink when a hot beverage is introduced into the container.
3o There is thus a need to measure moisture levels during various stages of the PET
container manufacturing process, including the moisture content of each of the starting resin, preform, and container. Generally, the PET resin (flakes or pellets) will have from 20-70 ppm of moisture; it is desirable that the moisture content of the resin be no greater than 50 ppm. As for PET preforms, they may typically have from 100-1000 ppm of moisture and preferably no 35 greater than 100.
DETERMINING MOISTURE CONTENT
Field of the Invention This invention relates to a simple and reproducible method and apparatus for determining levels of moisture in samples, such as polymer resins and articles, utilizing measurements of temperature, relative humidity and rate of gas flow.
to Background of the Invention Low moisture levels are required during various stages of the manufacture of injection blow-molded beverage containers made from polymers such as polyethylene terephthalate (PET), in order to achieve certain desired attributes such as clarity, strength, thermal resistance and impact resistance. For example, if the starting resin has excess moisture, heating the same 15 causes hydrolytic degradation which reduces the intrinsic viscosity and makes it difficult to mold a clear preform. A similar problem arises during reheating of the preform (prior to blow molding), i.e., moisture in the preform reduces the temperature at which haze (lack of clarity) occurs, particularly with preforms for hot-fill containers. Haze, a measure of the scattering of light as it is transmitted through an article, is defined as the ratio of light transmitted through the 2o article to_ the light transmitted through air. A value of 0% haze would indicate a perfectly clear (transparent) article, while a value of 100% haze would indicate a totally opaque article.
Another complication arising from excess moisture in the preform is a plasticizing effect which makes it difficult to maintain a uniform sidewall thickness during reheating and/or stretch blow molding. In effect, the plastic becomes too soft and there may be material flow into the 25 base. Still further, moisture in the final blown container causes problems because it is more likely to shrink either during cooling (e.g., after removal from the blow mold) or upon subsequent exposure to elevated temperatures. Again, the presence of moisture is a particular problem with hot-fill containers -- the containers will shrink when a hot beverage is introduced into the container.
3o There is thus a need to measure moisture levels during various stages of the PET
container manufacturing process, including the moisture content of each of the starting resin, preform, and container. Generally, the PET resin (flakes or pellets) will have from 20-70 ppm of moisture; it is desirable that the moisture content of the resin be no greater than 50 ppm. As for PET preforms, they may typically have from 100-1000 ppm of moisture and preferably no 35 greater than 100.
One known method for determining moisture content in a polymer sample utilizes a "weight loss on drying" technique. U.S. Patent No. 4,165,633 to Raisaner describes such a system in which a sample is first weighed and then dried to remove any moisture. The sample is reweighed and the weight difference before and after drying corresponds to the moisture content released from the sample. The sensitivity of such devices is limited by the sensitivity of the balance in the weighing device. Moreover in a factory environment, vibrations and other external stimuli can cause a further decrease in sensitivity.
U.S. Patent No. 4,838,705 to Byers describes another "weight loss on drying"
apparatus for collecting and weighing a volatile fluid of interest from a sample under test. This apparatus 1 o uses an electromagnetic force restoration balance which restores the balance to its original position if the system is subjected to undesired forces. As with all "weight loss on drying"
systems, however, the main component is a balance device made of expensive, intricate parts.
Alternatively, there are commercially available "Karl-Fischer" instruments which perform a chemical analysis to determine low moisture levels in PET samples. A
cut-up PET
sample is heated in an oven where moisture is driven off. High-purity, medical-grade nitrogen is used as a carrier gas to pass through the oven and carry off the moisture released from the sample. The carrier gas exiting the instrument passes through chemicals which absorb the moisture and any other impurities in the carrier gas. The moisture in the chemicals is then analyzed to determine an amount of water released by the sample.
2o Although Karl-Fischer instruments are quite precise at measuring low moisture levels, they present a number of problems. First, they are only accurate and reproducible with a well-trained operator. Secondly, they utilize hazardous chemicals, which have to be changed often, and thus present a hazard to the operator. In addition, the chemicals pass through small-diameter tubing which may clog or become dislodged, and many of the key components are made of fragile glass, and thus are subject to breakage. Cleaning and replacement requires additional operator exposure to the chemicals. Still further, introducing a preform sample into the Karl-Fischer instrument is labor-intensive -- the preform must be cut into pieces and then the pieces accurately weighed.
For these reasons it would be desirable to provide an improved method and apparatus that 3o is capable of determining moisture levels in a variety of materials under a variety of processing conditions, which method and apparatus are relatively simple and easy to use and the results of which are reproducible.
U.S. Patent No. 4,838,705 to Byers describes another "weight loss on drying"
apparatus for collecting and weighing a volatile fluid of interest from a sample under test. This apparatus 1 o uses an electromagnetic force restoration balance which restores the balance to its original position if the system is subjected to undesired forces. As with all "weight loss on drying"
systems, however, the main component is a balance device made of expensive, intricate parts.
Alternatively, there are commercially available "Karl-Fischer" instruments which perform a chemical analysis to determine low moisture levels in PET samples. A
cut-up PET
sample is heated in an oven where moisture is driven off. High-purity, medical-grade nitrogen is used as a carrier gas to pass through the oven and carry off the moisture released from the sample. The carrier gas exiting the instrument passes through chemicals which absorb the moisture and any other impurities in the carrier gas. The moisture in the chemicals is then analyzed to determine an amount of water released by the sample.
2o Although Karl-Fischer instruments are quite precise at measuring low moisture levels, they present a number of problems. First, they are only accurate and reproducible with a well-trained operator. Secondly, they utilize hazardous chemicals, which have to be changed often, and thus present a hazard to the operator. In addition, the chemicals pass through small-diameter tubing which may clog or become dislodged, and many of the key components are made of fragile glass, and thus are subject to breakage. Cleaning and replacement requires additional operator exposure to the chemicals. Still further, introducing a preform sample into the Karl-Fischer instrument is labor-intensive -- the preform must be cut into pieces and then the pieces accurately weighed.
For these reasons it would be desirable to provide an improved method and apparatus that 3o is capable of determining moisture levels in a variety of materials under a variety of processing conditions, which method and apparatus are relatively simple and easy to use and the results of which are reproducible.
Summary of the Invention A method is provided for determining moisture content in a sample from temperature, relative humidity and rate of gas flow measurements. In one embodiment, a sample having a moisture content is heated in an oven to release moisture. A gas flow is introduced into and passes through the oven, carrying the moisture out of the oven as an output gas flow to a number of measurement devices. At each of a plurality of successive time intervals over a test period, the devices measure the temperature, relative humidity and flow rate of the output gas. From these measurements, the moisture content in the sample is determined. The method further comprises a step of determining a termination point for data collection.
i o According to the present invention, one can reproducibly measure moisture levels of at or below 50 parts-per-million (ppm) in PET resin (flakes or pellets), and moisture levels of at or below 100 ppm in PET preforms. By varying the gas flow rate, one can easily adjust the moisture range to be measured from below 50 ppm to 1000 ppm. The method/apparatus avoids the use of fragile glassware, small diameter tubes, and hazardous chemicals.
The measurements, from which the moisture content is determined, are relatively easy to make. It is possible to mount an entire preform in the oven (sample chamber) without having to cut up the sample. This reduces preparation time and expense, as well as reducing variation in the results.
The invention will be further described with respect to the following detailed description and drawings, which are given by way of example only and are not restrictive.
Brief Description of the Drawings Fig. 1 is a block diagram of a moisture analyzer according to one embodiment of the present invention;
Fig. 2 is a flow diagram illustrating the process performed by the moisture analyzer of Fig. l;
Fig. 3 is a graph of mass flow rate, M", versus time for a test period of less than 20 minutes, according to the process described in Fig. 2; and Fig. 4 is a graph similar to Fig. 3 but demonstrating the extrapolation of data beyond the test period of 20 minutes, according to the process described in Fig. 2.
Detailed Description The invention provides an apparatus and method for determining moisture content in a sample from temperature, relative humidity and flow rate measurements. It can be used for measuring levels of moisture in PET and other polymeric samples during preform/container manufacture or any other process requiring a moisture determination.
Fig. 1 shows one embodiment of a moisture analyzer 10 according to the present invention. The analyzer 10 includes an oven 12 having an interior sample chamber 15 coupled to receive a pressurized input gas flow (e.g., 10 psi of substantially dry air or nitrogen) from a gas flow controller 14, via input tube 16. The gas flow controller 14 may be, for example, a Sierra 810C unit having a variable flow rate of 0-2000 cubic centimeters per minute (cc/min), sold by Sierra Instruments, Monterey CA. The oven 12 is in communication with a number of output l0 measurement devices 22, 24. In this embodiment, a stainless steel coil 18 functions as a pathway for the output gas flow exiting the oven, and coil 18 couples the oven 12 with the measurement devices 22 and 24. A first measurement device is a temperature and relative humidity (T/RH) monitor 22 which may be, for example, a Vaisala HMM 30C, sold by Vaisala, Inc., Woburn, MA. Coupled to the T/RH monitor 22 is a gas flow monitor 24 for measuring the rate of the output gas flow, e.g., a Sierra 820 C unit also available from Vaisala, Inc.
The sample 30, in this case an entire PET preform, is mounted in the sample chamber 15 on a vertical support 17 extending from input tube 16. The vertical tube support 17 allows mounting of large preform samples -- e.g., preforms weighing up to 150 grams.
This arrangement does not require the prior art sample preparation step -- i.e., reducing the sample _ 2o size to fit into a chamber or tray of a prescribed area. Disadvantages of the prior art include the extra time needed to reduce the sample size and the effect of surface area on the moisture content (i.e., a cut-up sample having a larger surface area per unit weight may absorb/release more moisture from the environment thus effecting the measurement). Alternatively, the present moisture analyzer 10 is adaptable to analyze the water content of smaller samples by removing vertical support 17 and replacing it with a sample holder of another design, e.g., a tray to hold pieces of resin.
The tube support 17 has ports 19 through which the input gas flow from controller 14 enters the oven chamber 15. Sample 30 in chamber 15 is heated to release moisture as a water vapor gas. The chamber may operate in a range of 400-470°F (204 -243°C). As moisture is 3o released from the sample, it is mixed with the input gas flow passing through the chamber to output tube 18, where it exits as an output gas flow.
i o According to the present invention, one can reproducibly measure moisture levels of at or below 50 parts-per-million (ppm) in PET resin (flakes or pellets), and moisture levels of at or below 100 ppm in PET preforms. By varying the gas flow rate, one can easily adjust the moisture range to be measured from below 50 ppm to 1000 ppm. The method/apparatus avoids the use of fragile glassware, small diameter tubes, and hazardous chemicals.
The measurements, from which the moisture content is determined, are relatively easy to make. It is possible to mount an entire preform in the oven (sample chamber) without having to cut up the sample. This reduces preparation time and expense, as well as reducing variation in the results.
The invention will be further described with respect to the following detailed description and drawings, which are given by way of example only and are not restrictive.
Brief Description of the Drawings Fig. 1 is a block diagram of a moisture analyzer according to one embodiment of the present invention;
Fig. 2 is a flow diagram illustrating the process performed by the moisture analyzer of Fig. l;
Fig. 3 is a graph of mass flow rate, M", versus time for a test period of less than 20 minutes, according to the process described in Fig. 2; and Fig. 4 is a graph similar to Fig. 3 but demonstrating the extrapolation of data beyond the test period of 20 minutes, according to the process described in Fig. 2.
Detailed Description The invention provides an apparatus and method for determining moisture content in a sample from temperature, relative humidity and flow rate measurements. It can be used for measuring levels of moisture in PET and other polymeric samples during preform/container manufacture or any other process requiring a moisture determination.
Fig. 1 shows one embodiment of a moisture analyzer 10 according to the present invention. The analyzer 10 includes an oven 12 having an interior sample chamber 15 coupled to receive a pressurized input gas flow (e.g., 10 psi of substantially dry air or nitrogen) from a gas flow controller 14, via input tube 16. The gas flow controller 14 may be, for example, a Sierra 810C unit having a variable flow rate of 0-2000 cubic centimeters per minute (cc/min), sold by Sierra Instruments, Monterey CA. The oven 12 is in communication with a number of output l0 measurement devices 22, 24. In this embodiment, a stainless steel coil 18 functions as a pathway for the output gas flow exiting the oven, and coil 18 couples the oven 12 with the measurement devices 22 and 24. A first measurement device is a temperature and relative humidity (T/RH) monitor 22 which may be, for example, a Vaisala HMM 30C, sold by Vaisala, Inc., Woburn, MA. Coupled to the T/RH monitor 22 is a gas flow monitor 24 for measuring the rate of the output gas flow, e.g., a Sierra 820 C unit also available from Vaisala, Inc.
The sample 30, in this case an entire PET preform, is mounted in the sample chamber 15 on a vertical support 17 extending from input tube 16. The vertical tube support 17 allows mounting of large preform samples -- e.g., preforms weighing up to 150 grams.
This arrangement does not require the prior art sample preparation step -- i.e., reducing the sample _ 2o size to fit into a chamber or tray of a prescribed area. Disadvantages of the prior art include the extra time needed to reduce the sample size and the effect of surface area on the moisture content (i.e., a cut-up sample having a larger surface area per unit weight may absorb/release more moisture from the environment thus effecting the measurement). Alternatively, the present moisture analyzer 10 is adaptable to analyze the water content of smaller samples by removing vertical support 17 and replacing it with a sample holder of another design, e.g., a tray to hold pieces of resin.
The tube support 17 has ports 19 through which the input gas flow from controller 14 enters the oven chamber 15. Sample 30 in chamber 15 is heated to release moisture as a water vapor gas. The chamber may operate in a range of 400-470°F (204 -243°C). As moisture is 3o released from the sample, it is mixed with the input gas flow passing through the chamber to output tube 18, where it exits as an output gas flow.
In this embodiment, the output gas flow is cooled to a desired temperature prior to being measured. Preferably, the desired temperature is 25 °C. A fan 20 is directed to cool the coils of the stainless steel output tubing 18. Here the tubing 18 is 1/4 inch thick, the coils are 6 inches in diameter, and five coils are used to provide adequate cooling.
In the T/RH monitor 22 the temperature of the cooled output gas flow is measured by a resistive temperature device (RTD), and the relative humidity is measured by a capacitive thin-film, solid-state humidity sensor. The term "relative humidity" herein refers to a ratio of the mositure content in a gas under measured conditions, compared to the moisture content in the gas under saturated conditions at the same temperature. For example, air at 100%
relative humidity (RH) is saturated with moisture. A processing device, here computer 26, receives a temperature signal on line 22a and a relative humidity signal on line 22b from the T/RH
monitor 22, and an output gas flow rate signal on line 24a from gas flow monitor 24. These measurements are analyzed by software 28 in a processing device, e.g., computer 26, to provide the moisture content released by the sample per sample weight in units of parts per million (ppm). The analysis software allows a user to control the instrumentation and perform the data acquisition, and then displays the control parameters and output data on a graphical user interface. The computer 26 also provides an input gas flow rate signal to the input gas flow controller unit 14 on line 14a.
The operation of the moisture analyzer 10 will now be described. Referring to the flow 2o chart in Fig. 2, an initial step is determining a baseline mass flow rate 31 without a sample 30 in the chamber 15. This baseline mass flow rate corresponds to the moisture content already present in the input gas, and thus must be subtracted from the calculated total sample value (described below). In step 32, the sample is inserted and its weight is input in step 34. Steps 36 and 38 comprise measuring the relative humidity 36 and temperature 38 of the output gas flow respectively with the T/RH monitor 22, and step 40 comprises measuring the rate of output gas flow with the monitor 24. A plurality of such temperature, humidity and output gas flow rate measurements are taken over a test period at predetermined time intervals. As an example, in one embodiment the test period is 20 minutes, and each of the measurements is taken at a predetermined time interval of one second. Thus, refernng back to Fig. 1, every second for a 3o period of 20 minutes, the T/RH monitor 22 sends an updated temperature signal on line 22a and relative humidity signal on line 22b to the computer 26, the monitor 24 sends an output gas flow signal on line 24a to computer 26, and the computer 26 sends an input gas flow signal on line 14a to controller 14. Using this series of temperature, relative humidity and gas flow measurements, the step of calculating a mass flow rate (step 42) is described below.
In the present invention, heating of the sample causes it to emit a moisture content (water vapor) that mixes with the input gas flow and exits the oven as an output gas flow. The input gas must be inert with respect to water vapor, e.g., medical grade nitrogen. Due to the unreactive nature of the input gas flow / water vapor mixture, the output gas flow can be modeled as an ideal gas. An "ideal gas" refers to a gas that exists as an isolated entity, i.e. free of any intermolecular interactions. Moreover, the pressure of a gas in a mixture of gases can be individually determined from the law of partial pressures. According to this law, the total 1 o pressure of a gas mixture is the sum of the partial pressures of each gas in the mixture. In the present invention, the total pressure of the output gas flow, P, is equal to the partial pressures of the input gas, Pa, and the water vapor, P~, as expressed in equation 1:
P = Pa+P~ (1) The partial pressure of the water vapor (moisture) in the output gas flow can be obtained from the relative humidity. As explained previously, air saturated with water vapor has 100 relative humidity. The saturated water vapor pressure at various temperatures can be obtained from water vapor pressure tables, commonly known by those of ordinary skill in the art. A
2o simple relationship exists between the partial pressure of water vapor in the output gas flow, P,"
its relative humidity, ~, and the pressure of saturated water vapor at 25°C, Plat@zs >a as expressed in equation (2):
P,, _ ~ x Psa~@zs °c (2) By way of specific example, assume that the pressure of the output gas flow is atmospheric pressure, 101 kPa (kiloPascals), the relative humidity is 75%, and the temperature is 25 °C. The saturation pressure of water vapor at 25°C is 3.169 kPa, Solving equation 2 provides a P~ value of 2.38 kPa.
3o The "ideal gas equation", which relates the pressure, volume and temperature of an ideal gas with an amount of the ideal gas, can be used in determining the amount of water vapor in the output gas flow. Applying this equation to the method of the present invention results in equation 3:
_7_ n-RAT (3) where n is the number of moles of water in the output gas flow, P~ is the partial pressure of water in the output gas flow, V is the volume of the output gas flow, R" is an ideal gas constant for water vapor, and T~ is the temperature of the output gas flow. The value R" is 0.4615 kPa~m'~kg~
' ~K-' and T~ is obtained by measurement. Because the volume V of the output gas flow is not easily determinable, V can be substituted with a volumetric output gas flow rate variable, Q~.
Therefore, instead of determining the number of moles of water in the output gas flow, the mass flow rate of water in the output gas flow, M," can be calculated from equation 4:
to M - P~Q~ (4) RAT
For example, given an output gas flow rate of 2,000 cubic centimeters per minute (cm3/min), an output gas flow temperature of 25 °C (298 K), an output gas flow relative humidity at 25 °C of 75%, and an output gas flow pressure at atmospheric pressure (101 kPa), M~ is:
2.3kPa.2000 ~m M - min 1m 10003Ng. 1min =576.97 Ng V 0.4615 kpa.m 3.298K. 1003cm 3 . 1 kg 60s s kg.K
where "min" is minutes, "s" is seconds, "m" is meters, "cm" is centimeters, "kg" is kilograms, "K" is degrees Kelvin, "kPa" is kiloPascals, and "~.g" is micro grams. From this example, in one second at 75% relative humidity, 576.97 gg of water passes by the relative humidity sensor 22.
As described with reference to Fig. 2, successive measurements of the relative humidity, air temperature and the gas flow are taken at every predetermined time interval ( 1 second) for the test period (20 minutes), in order to obtain successive calculated values of the mass flow rate, 2o M~. A graph of mass flow rate, M~, versus time, for a PET preform sample, is shown in Fig. 3.
This graph displays calculated M~ values on the y-axis versus time (in seconds) on the x-axis and extends from a period of 0 to approximately 12.5 minutes. Each point along the curve 50 is an _g_ M" value (~g/s) for that time (s). Curve 50 has a first peak 53 (adjacent the origin) where the sample is loaded into the sample chamber 15, which results in the introduction of ambient air.
The following steep dip is caused by an equilibrium between the ambient air moisture being driven off and heating of the sample to emit moisture. The second shallow peak 54 and subsequent gradual dip is due to moisture emitted from the sample.
To determine the total water emitted from the sample, the area under the curve 50 is calculated using mathematical integration techniques (step 46 in Fig. 2). The water content of the output gas flow is determined in parts per million (ppm) of moisture by dividing the weight of the emitted moisture by the weight of the sample (where the sample is weighed before 1 o heating/insertion into the oven). Thus, the moisture content of the output gas flow can be calculated by equation 5 below:
t = test period Moisture content in - J M~(t)dt / sample weight 15 output gas flow (ppm) t = °
where t is time and M,, and sample weight are both in units of grams.
Integration of curve 50 is started at the origin (t = 0 seconds) so that the moisture introduced with the ambient air is included with the moisture released by the sample. The size 2o and shape of the curve will depend on the material and ambient conditions.
It has been found that the amount of moisture introduced with the ambient air is generally sufficiently small that it need not be subtracted from the moisture in the output gas flow in order to obtain a useful measure of the sample moisture. For example, when measuring flakes/pellets, a smaller amount of ambient air is generally introduced, which produces a smaller error, e.g., 2 ppm (compared to 2s 20-70 ppm of moisture released by the flakes/pellets). When measuring preforms, where more air is introduced with the sample, a larger error of 10 ppm is typical, but again this is small compared to a typical preform moisture content of 50-100 ppm or greater.
Alternatively, one can determine and subtract the moisture introduced with the ambient air (an ambient air offset) as described below.
3o As mentioned previously, the flowchart of Fig. 2 includes the step of determining a baseline mass flow rate (step 31 ). Assuming the moisture content of the input gas is constant throughout the test period, a constant baseline mass flow rate, M~(t)base~i~e, is simply subtracted from the mass flow rate, M~. For illustrative purposes, an area 52 is indicated in Fig. 3, providing an estimate of the baseline mass flow rate. The resulting adjusted mass flow rate (total - baseline) is accumulated to determine the moisture content in the sample during the time period of the test, according to equation 6.
t = test period Moisture content in - f [M~(t) - M,,(t)base,i~e]dt / sample weight (6) sample (ppm) t-°
Referring again to the flowchart of Fig. 2, steps 36-42 may comprise a loop.
In one 1o embodiment, steps 36-42 are repeated every second for the entire test period.
Another embodiment of the present invention is a method for determining a termination point for collecting the plurality of M~ data points. If a sample has a very low moisture content, a subroutine at step 44 of Fig. 2 will allow the test to terminate at a time less than the test period.
For example, if the moisture content in the sample can be determined within five minutes and the test period is set at 20 minutes, the moisture analyzer would operate needlessly for 15 minutes.
An advantage of subroutine step 44 is that it allows the measurement to terminate after five minutes. Determining the termination point involves assessing whether a most recent portion of the plurality of M~ data points meets a set of criteria. Preferably, the set of criteria is based on a regression analysis performed on the portion of the plurality of M~ data points to assess the linearity, stability and steady state nature of the portion of M~ data points.
More preferably, the set of criteria comprises determining: ( 1 ) the steady state nature - whether a slope of regression is within about ~ 0.01, i.e., the mass flow rate is changing by less than 0.01 grams during each two-minute time interval; (2) the y-intercept of the regression is less than the sum of about 0.5 and the baseline mass flow rate, i.e. there is less than about 0.5 grams of moisture remaining in the sample; and (3) the linearity and stability - the mean square error is less than about 0.1.
Determination step 44 preferably occurs at each predetermined time interval.
At subroutine step 44 of Fig. 2, when it is determined that all three criteria are satisfied, data acquisition is terminated. At step 46 the mass flow rate, M~, values are integrated over the test period to provide a total moisture content of the sample in ppm, step 47.
3o As an example of subroutine 44, the moisture content of a sample is determined where measurements are taken at every predetermined time interval of 1 s, and the portion of the plurality of M~ data points comprise 120 M~ data points taken over 120 s.
Referring to Fig. 3, the portion of M~ data points is shown as range 60 (four such ranges 60a, 60b, 60c, 60d are marked below the horizontal time axis). Point 62 corresponds to the 120th M~ data point. The first 120 s of the measurements (range 60a) are ignored to allow sufficient time for moisture in the ambient air which entered the chamber along with the sample, to pass through the system. At point 64, which corresponds to data point 240, the subroutine 44 becomes operable and performs a linear regression over range 60b, i.e. data points 121 to 240. The portion of data points between points 62 and 64 will not satisfy any of the criteria because curve 50 at this point is non-linear and shows a relatively steep slope. After point 64, subroutine 44 is performed every second. As exemplified in Fig. 3, the data shows various rises and decreases within the first ten minutes (600 1 o seconds) of data collection. At point 66, subroutine 44 determines that the portion of M~ data points encompassed by range 60c does not meet the criteria. At point 68 (about t = 750 seconds), however, subroutine 44 determines that the portion of M,, data points encompassed by range 60d shows stability, linearity, steady state nature and the y-intercept of the regression falls below the sum of 0.5 and the baseline mass flow rate. Data acquisition thus stops at point 68.
15 It has been established that resin/preform/container moisture content has an important effect on the process of manufacturing a PET bottle. Another variable which may have an effect on the blow molding process is the distribution of that moisture throughout the thickness of the preform. For example, Fig. 3 shows a plot for a preform which has little embedded moisture relative to the total amount and thus most of the moisture is extracted from the sample in the first 20 400 seconds (62/a minutes) of the test period -- i.e., the data acquisition ends after about 750 seconds ( 12'/z minutes), with little moisture remaining in the sample. The distribution of the moisture throughout the preform was such that most of the moisture was contained near the surface and had only a short distance to permeate through the sidewall and be carned off by the gas flow.
25 Fig. 4 shows a curve 70 for a PET preform which contains a high amount of embedded moisture. This preform, which has a nearly equal distribution of moisture throughout the thickness of the sample, releases moisture quickly at first, and as the process of permeation through the sidewall reaches a steady state at about 400 s, the rate of moisture release gradually slows over a long period of time. To release all of the moisture from this sample would require 30 over forty-five minutes. Because the test period was set at 20 minutes, the mass flow rate is extrapolated (beyond 1200 seconds as shown) back to the baseline (72) in order to obtain an estimate of the total area underneath the curve 70.
More accurate values of moisture content can be obtained by several methods.
As mentioned previously, added moisture from ambient air (introduced when the sample is loaded into the sample chamber) is included in the above-calculated moisture content.
One calibration technique for approximately determining this ambient air moisture involves opening the sample chamber to introduce ambient air, without inserting a sample, then determining the amount of moisture in the output gas flow, subtracting the known amount of moisture in the input gas flow, and using the result as an ambient air offset value. This offset value can then be subtracted from the values obtained for the preform/resin samples. Because moisture in ambient air may vary from day to day, a daily offset value may be obtained. _ 1 o Another factor that may affect the determined moisture content is moisture lost in traveling from the oven to the measuring devices or some other ''system"
error. A "recovery"
value can be obtained from a system calibration based on known amounts of moisture. For example, five capillary tubes of different sizes are each filled with a different known amount of water, and placed one at a time in the oven and a moisture content determined from the output 15 gas flow. The known amounts are plotted versus the determined moisture content values and a slope of the plot is obtained. A perfect recovery value of 100% would provide a slope of 1. If the slope resulting from the calibration is 912.7 ~.g of water per ~.L
(microliter), a multiplication factor of 1.09 would be required to adjust the slope of the calibration line with the theoretically perfect slope of 1. Thus, a determined sample moisture content would be multiplied by a factor 20 of 1.09 to compensate for the system error ("recovery") The plot described in the preceding paragraph can also provide an alternative method to calculate the offset value. If no moisture from ambient air was introduced into the system during the calibration, the calibration curve would extend to the origin regardless of the value of the slope. If ambient air is introduced into the system, the calibration curve would have a y-25 intercept. The y-axis is measured in units of micrograms; the value of the Y intercept equals the offset value that may be substracted from the amount of moisture determined for the preform/resin samples.
One advantage of the present invention is that the gas flow rate can be adjusted (via an adjustable gas flow input device), depending on the moisture content of the sample. For 3o example, resins generally have a moisture content ranging from 20 - 70 ppm, for which a gas flow rate of 150 cm3/min may be optimal in a given system. Preforms generally have higher moisture contents ranging from 100 - 1000 ppm, for which a higher flow rate, e.g., 2000 cm3/min, may be optimal. The preferred gas flow rate may be determined by factors such as the optimum (most accurate) measurement range of the relative humidity device. For example, some relative humidity devices do not function at relative humidity levels greater than 40%. Other relative humidity devices are accurate at relative humidity levels below 75%.
Thus, the gas flow rates can be adjusted depending on the moisture content of the various samples to obtain the most accurate and reproducible results.
The oven can be constructed to heat the sample in a number of ways. The oven can be an electric heating device. Other heating methods can include convection heating, microwave heating, infrared heating and the like. Alternatively, the sample can be heated by utilizing a hot 1o input gas flow, wherein the hot input gas flow which carries the released moisture from the oven chamber also heats the sample. The sample may be heated by any of the methods described previously above or in combination with a hot input gas flow to expedite the release of the moisture from the sample.
Those skilled in the art will appreciate that all parameters listed herein are meant to be 15 exemplary and that actual parameters will depend upon the specific application for which the methods and apparatus of the present invention are used.
The invention thus provides a method and apparatus for determining the moisture content of a PET sample using low-cost components by measuring relative humidity, temperature, and output gas flow rate at predetermined time intervals over a desired test period, and calculating, 2o for each time interval, a mass flow rate value indicating an amount of water vapor released from the sample during the respective time interval. By integrating the results received over the time period, a total water content released from the sample can be determined. The present invention provides a method and apparatus for performing moisture analysis using commercially-available measurement devices, without the use of hazardous chemicals and expensive equipment of the 25 prior art.
Although the above description has provided specific examples of moisture analysis for a PET sample, it should be understood that the method and apparatus may be used to analyze the moisture content of any material that dissipates water vapor when heated. In addition, although certain commercially-available components have been described, it should be understood that 30 other components performing similar functions may be substituted without affecting the scope of the invention.
Having thus described various embodiments of the invention, numerous modifications within the scope of the invention will occur to those skilled in the art.
Thus, this description and accompanying drawings are provided by way of example only and are not intended to be limiting.
In the T/RH monitor 22 the temperature of the cooled output gas flow is measured by a resistive temperature device (RTD), and the relative humidity is measured by a capacitive thin-film, solid-state humidity sensor. The term "relative humidity" herein refers to a ratio of the mositure content in a gas under measured conditions, compared to the moisture content in the gas under saturated conditions at the same temperature. For example, air at 100%
relative humidity (RH) is saturated with moisture. A processing device, here computer 26, receives a temperature signal on line 22a and a relative humidity signal on line 22b from the T/RH
monitor 22, and an output gas flow rate signal on line 24a from gas flow monitor 24. These measurements are analyzed by software 28 in a processing device, e.g., computer 26, to provide the moisture content released by the sample per sample weight in units of parts per million (ppm). The analysis software allows a user to control the instrumentation and perform the data acquisition, and then displays the control parameters and output data on a graphical user interface. The computer 26 also provides an input gas flow rate signal to the input gas flow controller unit 14 on line 14a.
The operation of the moisture analyzer 10 will now be described. Referring to the flow 2o chart in Fig. 2, an initial step is determining a baseline mass flow rate 31 without a sample 30 in the chamber 15. This baseline mass flow rate corresponds to the moisture content already present in the input gas, and thus must be subtracted from the calculated total sample value (described below). In step 32, the sample is inserted and its weight is input in step 34. Steps 36 and 38 comprise measuring the relative humidity 36 and temperature 38 of the output gas flow respectively with the T/RH monitor 22, and step 40 comprises measuring the rate of output gas flow with the monitor 24. A plurality of such temperature, humidity and output gas flow rate measurements are taken over a test period at predetermined time intervals. As an example, in one embodiment the test period is 20 minutes, and each of the measurements is taken at a predetermined time interval of one second. Thus, refernng back to Fig. 1, every second for a 3o period of 20 minutes, the T/RH monitor 22 sends an updated temperature signal on line 22a and relative humidity signal on line 22b to the computer 26, the monitor 24 sends an output gas flow signal on line 24a to computer 26, and the computer 26 sends an input gas flow signal on line 14a to controller 14. Using this series of temperature, relative humidity and gas flow measurements, the step of calculating a mass flow rate (step 42) is described below.
In the present invention, heating of the sample causes it to emit a moisture content (water vapor) that mixes with the input gas flow and exits the oven as an output gas flow. The input gas must be inert with respect to water vapor, e.g., medical grade nitrogen. Due to the unreactive nature of the input gas flow / water vapor mixture, the output gas flow can be modeled as an ideal gas. An "ideal gas" refers to a gas that exists as an isolated entity, i.e. free of any intermolecular interactions. Moreover, the pressure of a gas in a mixture of gases can be individually determined from the law of partial pressures. According to this law, the total 1 o pressure of a gas mixture is the sum of the partial pressures of each gas in the mixture. In the present invention, the total pressure of the output gas flow, P, is equal to the partial pressures of the input gas, Pa, and the water vapor, P~, as expressed in equation 1:
P = Pa+P~ (1) The partial pressure of the water vapor (moisture) in the output gas flow can be obtained from the relative humidity. As explained previously, air saturated with water vapor has 100 relative humidity. The saturated water vapor pressure at various temperatures can be obtained from water vapor pressure tables, commonly known by those of ordinary skill in the art. A
2o simple relationship exists between the partial pressure of water vapor in the output gas flow, P,"
its relative humidity, ~, and the pressure of saturated water vapor at 25°C, Plat@zs >a as expressed in equation (2):
P,, _ ~ x Psa~@zs °c (2) By way of specific example, assume that the pressure of the output gas flow is atmospheric pressure, 101 kPa (kiloPascals), the relative humidity is 75%, and the temperature is 25 °C. The saturation pressure of water vapor at 25°C is 3.169 kPa, Solving equation 2 provides a P~ value of 2.38 kPa.
3o The "ideal gas equation", which relates the pressure, volume and temperature of an ideal gas with an amount of the ideal gas, can be used in determining the amount of water vapor in the output gas flow. Applying this equation to the method of the present invention results in equation 3:
_7_ n-RAT (3) where n is the number of moles of water in the output gas flow, P~ is the partial pressure of water in the output gas flow, V is the volume of the output gas flow, R" is an ideal gas constant for water vapor, and T~ is the temperature of the output gas flow. The value R" is 0.4615 kPa~m'~kg~
' ~K-' and T~ is obtained by measurement. Because the volume V of the output gas flow is not easily determinable, V can be substituted with a volumetric output gas flow rate variable, Q~.
Therefore, instead of determining the number of moles of water in the output gas flow, the mass flow rate of water in the output gas flow, M," can be calculated from equation 4:
to M - P~Q~ (4) RAT
For example, given an output gas flow rate of 2,000 cubic centimeters per minute (cm3/min), an output gas flow temperature of 25 °C (298 K), an output gas flow relative humidity at 25 °C of 75%, and an output gas flow pressure at atmospheric pressure (101 kPa), M~ is:
2.3kPa.2000 ~m M - min 1m 10003Ng. 1min =576.97 Ng V 0.4615 kpa.m 3.298K. 1003cm 3 . 1 kg 60s s kg.K
where "min" is minutes, "s" is seconds, "m" is meters, "cm" is centimeters, "kg" is kilograms, "K" is degrees Kelvin, "kPa" is kiloPascals, and "~.g" is micro grams. From this example, in one second at 75% relative humidity, 576.97 gg of water passes by the relative humidity sensor 22.
As described with reference to Fig. 2, successive measurements of the relative humidity, air temperature and the gas flow are taken at every predetermined time interval ( 1 second) for the test period (20 minutes), in order to obtain successive calculated values of the mass flow rate, 2o M~. A graph of mass flow rate, M~, versus time, for a PET preform sample, is shown in Fig. 3.
This graph displays calculated M~ values on the y-axis versus time (in seconds) on the x-axis and extends from a period of 0 to approximately 12.5 minutes. Each point along the curve 50 is an _g_ M" value (~g/s) for that time (s). Curve 50 has a first peak 53 (adjacent the origin) where the sample is loaded into the sample chamber 15, which results in the introduction of ambient air.
The following steep dip is caused by an equilibrium between the ambient air moisture being driven off and heating of the sample to emit moisture. The second shallow peak 54 and subsequent gradual dip is due to moisture emitted from the sample.
To determine the total water emitted from the sample, the area under the curve 50 is calculated using mathematical integration techniques (step 46 in Fig. 2). The water content of the output gas flow is determined in parts per million (ppm) of moisture by dividing the weight of the emitted moisture by the weight of the sample (where the sample is weighed before 1 o heating/insertion into the oven). Thus, the moisture content of the output gas flow can be calculated by equation 5 below:
t = test period Moisture content in - J M~(t)dt / sample weight 15 output gas flow (ppm) t = °
where t is time and M,, and sample weight are both in units of grams.
Integration of curve 50 is started at the origin (t = 0 seconds) so that the moisture introduced with the ambient air is included with the moisture released by the sample. The size 2o and shape of the curve will depend on the material and ambient conditions.
It has been found that the amount of moisture introduced with the ambient air is generally sufficiently small that it need not be subtracted from the moisture in the output gas flow in order to obtain a useful measure of the sample moisture. For example, when measuring flakes/pellets, a smaller amount of ambient air is generally introduced, which produces a smaller error, e.g., 2 ppm (compared to 2s 20-70 ppm of moisture released by the flakes/pellets). When measuring preforms, where more air is introduced with the sample, a larger error of 10 ppm is typical, but again this is small compared to a typical preform moisture content of 50-100 ppm or greater.
Alternatively, one can determine and subtract the moisture introduced with the ambient air (an ambient air offset) as described below.
3o As mentioned previously, the flowchart of Fig. 2 includes the step of determining a baseline mass flow rate (step 31 ). Assuming the moisture content of the input gas is constant throughout the test period, a constant baseline mass flow rate, M~(t)base~i~e, is simply subtracted from the mass flow rate, M~. For illustrative purposes, an area 52 is indicated in Fig. 3, providing an estimate of the baseline mass flow rate. The resulting adjusted mass flow rate (total - baseline) is accumulated to determine the moisture content in the sample during the time period of the test, according to equation 6.
t = test period Moisture content in - f [M~(t) - M,,(t)base,i~e]dt / sample weight (6) sample (ppm) t-°
Referring again to the flowchart of Fig. 2, steps 36-42 may comprise a loop.
In one 1o embodiment, steps 36-42 are repeated every second for the entire test period.
Another embodiment of the present invention is a method for determining a termination point for collecting the plurality of M~ data points. If a sample has a very low moisture content, a subroutine at step 44 of Fig. 2 will allow the test to terminate at a time less than the test period.
For example, if the moisture content in the sample can be determined within five minutes and the test period is set at 20 minutes, the moisture analyzer would operate needlessly for 15 minutes.
An advantage of subroutine step 44 is that it allows the measurement to terminate after five minutes. Determining the termination point involves assessing whether a most recent portion of the plurality of M~ data points meets a set of criteria. Preferably, the set of criteria is based on a regression analysis performed on the portion of the plurality of M~ data points to assess the linearity, stability and steady state nature of the portion of M~ data points.
More preferably, the set of criteria comprises determining: ( 1 ) the steady state nature - whether a slope of regression is within about ~ 0.01, i.e., the mass flow rate is changing by less than 0.01 grams during each two-minute time interval; (2) the y-intercept of the regression is less than the sum of about 0.5 and the baseline mass flow rate, i.e. there is less than about 0.5 grams of moisture remaining in the sample; and (3) the linearity and stability - the mean square error is less than about 0.1.
Determination step 44 preferably occurs at each predetermined time interval.
At subroutine step 44 of Fig. 2, when it is determined that all three criteria are satisfied, data acquisition is terminated. At step 46 the mass flow rate, M~, values are integrated over the test period to provide a total moisture content of the sample in ppm, step 47.
3o As an example of subroutine 44, the moisture content of a sample is determined where measurements are taken at every predetermined time interval of 1 s, and the portion of the plurality of M~ data points comprise 120 M~ data points taken over 120 s.
Referring to Fig. 3, the portion of M~ data points is shown as range 60 (four such ranges 60a, 60b, 60c, 60d are marked below the horizontal time axis). Point 62 corresponds to the 120th M~ data point. The first 120 s of the measurements (range 60a) are ignored to allow sufficient time for moisture in the ambient air which entered the chamber along with the sample, to pass through the system. At point 64, which corresponds to data point 240, the subroutine 44 becomes operable and performs a linear regression over range 60b, i.e. data points 121 to 240. The portion of data points between points 62 and 64 will not satisfy any of the criteria because curve 50 at this point is non-linear and shows a relatively steep slope. After point 64, subroutine 44 is performed every second. As exemplified in Fig. 3, the data shows various rises and decreases within the first ten minutes (600 1 o seconds) of data collection. At point 66, subroutine 44 determines that the portion of M~ data points encompassed by range 60c does not meet the criteria. At point 68 (about t = 750 seconds), however, subroutine 44 determines that the portion of M,, data points encompassed by range 60d shows stability, linearity, steady state nature and the y-intercept of the regression falls below the sum of 0.5 and the baseline mass flow rate. Data acquisition thus stops at point 68.
15 It has been established that resin/preform/container moisture content has an important effect on the process of manufacturing a PET bottle. Another variable which may have an effect on the blow molding process is the distribution of that moisture throughout the thickness of the preform. For example, Fig. 3 shows a plot for a preform which has little embedded moisture relative to the total amount and thus most of the moisture is extracted from the sample in the first 20 400 seconds (62/a minutes) of the test period -- i.e., the data acquisition ends after about 750 seconds ( 12'/z minutes), with little moisture remaining in the sample. The distribution of the moisture throughout the preform was such that most of the moisture was contained near the surface and had only a short distance to permeate through the sidewall and be carned off by the gas flow.
25 Fig. 4 shows a curve 70 for a PET preform which contains a high amount of embedded moisture. This preform, which has a nearly equal distribution of moisture throughout the thickness of the sample, releases moisture quickly at first, and as the process of permeation through the sidewall reaches a steady state at about 400 s, the rate of moisture release gradually slows over a long period of time. To release all of the moisture from this sample would require 30 over forty-five minutes. Because the test period was set at 20 minutes, the mass flow rate is extrapolated (beyond 1200 seconds as shown) back to the baseline (72) in order to obtain an estimate of the total area underneath the curve 70.
More accurate values of moisture content can be obtained by several methods.
As mentioned previously, added moisture from ambient air (introduced when the sample is loaded into the sample chamber) is included in the above-calculated moisture content.
One calibration technique for approximately determining this ambient air moisture involves opening the sample chamber to introduce ambient air, without inserting a sample, then determining the amount of moisture in the output gas flow, subtracting the known amount of moisture in the input gas flow, and using the result as an ambient air offset value. This offset value can then be subtracted from the values obtained for the preform/resin samples. Because moisture in ambient air may vary from day to day, a daily offset value may be obtained. _ 1 o Another factor that may affect the determined moisture content is moisture lost in traveling from the oven to the measuring devices or some other ''system"
error. A "recovery"
value can be obtained from a system calibration based on known amounts of moisture. For example, five capillary tubes of different sizes are each filled with a different known amount of water, and placed one at a time in the oven and a moisture content determined from the output 15 gas flow. The known amounts are plotted versus the determined moisture content values and a slope of the plot is obtained. A perfect recovery value of 100% would provide a slope of 1. If the slope resulting from the calibration is 912.7 ~.g of water per ~.L
(microliter), a multiplication factor of 1.09 would be required to adjust the slope of the calibration line with the theoretically perfect slope of 1. Thus, a determined sample moisture content would be multiplied by a factor 20 of 1.09 to compensate for the system error ("recovery") The plot described in the preceding paragraph can also provide an alternative method to calculate the offset value. If no moisture from ambient air was introduced into the system during the calibration, the calibration curve would extend to the origin regardless of the value of the slope. If ambient air is introduced into the system, the calibration curve would have a y-25 intercept. The y-axis is measured in units of micrograms; the value of the Y intercept equals the offset value that may be substracted from the amount of moisture determined for the preform/resin samples.
One advantage of the present invention is that the gas flow rate can be adjusted (via an adjustable gas flow input device), depending on the moisture content of the sample. For 3o example, resins generally have a moisture content ranging from 20 - 70 ppm, for which a gas flow rate of 150 cm3/min may be optimal in a given system. Preforms generally have higher moisture contents ranging from 100 - 1000 ppm, for which a higher flow rate, e.g., 2000 cm3/min, may be optimal. The preferred gas flow rate may be determined by factors such as the optimum (most accurate) measurement range of the relative humidity device. For example, some relative humidity devices do not function at relative humidity levels greater than 40%. Other relative humidity devices are accurate at relative humidity levels below 75%.
Thus, the gas flow rates can be adjusted depending on the moisture content of the various samples to obtain the most accurate and reproducible results.
The oven can be constructed to heat the sample in a number of ways. The oven can be an electric heating device. Other heating methods can include convection heating, microwave heating, infrared heating and the like. Alternatively, the sample can be heated by utilizing a hot 1o input gas flow, wherein the hot input gas flow which carries the released moisture from the oven chamber also heats the sample. The sample may be heated by any of the methods described previously above or in combination with a hot input gas flow to expedite the release of the moisture from the sample.
Those skilled in the art will appreciate that all parameters listed herein are meant to be 15 exemplary and that actual parameters will depend upon the specific application for which the methods and apparatus of the present invention are used.
The invention thus provides a method and apparatus for determining the moisture content of a PET sample using low-cost components by measuring relative humidity, temperature, and output gas flow rate at predetermined time intervals over a desired test period, and calculating, 2o for each time interval, a mass flow rate value indicating an amount of water vapor released from the sample during the respective time interval. By integrating the results received over the time period, a total water content released from the sample can be determined. The present invention provides a method and apparatus for performing moisture analysis using commercially-available measurement devices, without the use of hazardous chemicals and expensive equipment of the 25 prior art.
Although the above description has provided specific examples of moisture analysis for a PET sample, it should be understood that the method and apparatus may be used to analyze the moisture content of any material that dissipates water vapor when heated. In addition, although certain commercially-available components have been described, it should be understood that 30 other components performing similar functions may be substituted without affecting the scope of the invention.
Having thus described various embodiments of the invention, numerous modifications within the scope of the invention will occur to those skilled in the art.
Thus, this description and accompanying drawings are provided by way of example only and are not intended to be limiting.
Claims (28)
1. An apparatus for determining a moisture content in a sample, comprising:
an oven to heat the sample;
an input gas flow which passes through the oven and accepts moisture released by the sample to generate an output gas flow which exits the oven;
measuring devices in communication with the output gas flow for measuring values of temperature, relative humidity and flow rate of the output gas flow;
and a processing device, coupled to receive and process the values of temperature, relative humidity and flow rate to determine the moisture content in the sample.
an oven to heat the sample;
an input gas flow which passes through the oven and accepts moisture released by the sample to generate an output gas flow which exits the oven;
measuring devices in communication with the output gas flow for measuring values of temperature, relative humidity and flow rate of the output gas flow;
and a processing device, coupled to receive and process the values of temperature, relative humidity and flow rate to determine the moisture content in the sample.
2. The apparatus of claim 1, wherein the processing device comprises:
means for accumulating a plurality of the temperature, humidity and flow rate measurements over a test period, each of the plurality of measurements being taken at predetermined time intervals; and means for integrating the accumulated plurality of temperature, humidity, and flow rate measurements to determine the moisture content in the sample.
means for accumulating a plurality of the temperature, humidity and flow rate measurements over a test period, each of the plurality of measurements being taken at predetermined time intervals; and means for integrating the accumulated plurality of temperature, humidity, and flow rate measurements to determine the moisture content in the sample.
3. The apparatus of claim 1, including an adjustable gas flow input device coupled to the oven, for providing the input gas flow at a predetermined rate based on the moisture content of the sample.
4. The apparatus of claim 1, including a tube extending into the oven and providing a support on which to mount the sample and apertures through which the input gas flow enters the oven.
5. The apparatus of claim 1, including an output cooling duct coupled to the oven and the measuring devices to receive the output gas flow from the oven.
6. The apparatus of claim 5, wherein the output duct comprises a metal coil in which the output gas flow is cooled.
7. The apparatus of claim 1, wherein the processing device provides the moisture content in the sample per sample weight in units of parts per million (ppm).
8. The apparatus of claim 7, wherein the apparatus determines moisture levels below 50 ppm.
9. A method comprising:
providing a sample having a moisture content;
heating the sample to release the moisture content;
providing an input gas flow, allowing the input gas flow to combine with the released moisture content to produce an output gas flow;
measuring values of temperature, relative humidity and flow rate of the output gas flow; and determining the released moisture content from the measured values.
providing a sample having a moisture content;
heating the sample to release the moisture content;
providing an input gas flow, allowing the input gas flow to combine with the released moisture content to produce an output gas flow;
measuring values of temperature, relative humidity and flow rate of the output gas flow; and determining the released moisture content from the measured values.
10. The method of claim 9, wherein the sample is heated in an oven, the oven having an input port for receiving the input gas flow and an output port through which the output gas flow exits the oven.
11. The method of claim 10, wherein a predetermined rate of the input gas flow into the oven is selected based on the moisture content of the sample.
12. The method of claim 10, wherein the output gas flow exits the oven and enters a number of measuring devices for determining the measured values.
13. The method of claim 9, wherein the measuring step includes accumulating a plurality of the temperature, humidity and flow rate measurements over a test period, each of the plurality of measurements being taken at predetermined time intervals, and the determining step includes integrating the accumulated plurality of temperature, humidity, and flow rate measurements to determine the released moisture content.
14. The method of claim 13, wherein the determining step includes determining a partial pressure of water vapor in the output gas flow, P v, from the measured value of the relative humidity of the output gas flow, .PHI., according to the equation:
P v ~ .PHI. x P sat@25 °C
wherein P sat@25°C is a saturation pressure of water vapor at 25 °C.
P v ~ .PHI. x P sat@25 °C
wherein P sat@25°C is a saturation pressure of water vapor at 25 °C.
15. The method of claim 14, wherein the determining step includes determining a mass flow rate of the output gas flow, M v, according to the equation:
wherein Q v is a measured value of the volumetric output gas flow rate, R v is an ideal gas constant for water vapor, and T v is the measured value for the temperature of the output gas flow.
wherein Q v is a measured value of the volumetric output gas flow rate, R v is an ideal gas constant for water vapor, and T v is the measured value for the temperature of the output gas flow.
16. The method of claim 15, wherein the released moisture content is determined according to the equation:
wherein M v(t)baseline is a baseline mass flow rate for the moisture content of the input gas flow.
wherein M v(t)baseline is a baseline mass flow rate for the moisture content of the input gas flow.
17. The method of claim 9, wherein the measured values are collected at a predetermined time interval over a test period.
18. The method of claim 17, wherein the predetermined time interval is one second and the test period is 20 minutes.
19. The method of claim 17, further comprising determining a termination point less than the test period for terminating further collection of the measured values.
20. The method of claim 19, wherein the step of determining the termination point includes assessing the linearity, stability and steady state nature of a portion of the measured values.
21. The method of claim 20, wherein the portion of the measured values is the most recent portion of the measured values.
22. The method of claim 20, wherein the step of determining the termination point includes assessing the portion of the measured values at successive predetermined time intervals.
23. The method of claim 19, wherein the step of determining the termination point includes a regression performed on a portion of the measured values.
24. The method of claim 23, wherein a slope of the regression is no more than about +/- 0.01 µg/s, a y-intercept of the regression is no more than about 0.5 µg plus a baseline mass flow rate for the moisture content of the input gas flow, and a mean square error of the regression is no more than about 0.1.
25. The method of claim 9, wherein the released moisture content is determined to values of less than about 50 ppm.
26. The method of claim 9, wherein the sample is a plastic sample.
27. The method of claim 26, wherein the plastic is polyethylene terephthalate (PET).
28. A method comprising:
providing a sample having a moisture content;
heating the sample to release the moisture content;
providing an input gas flow which combines with the moisture content to produce an output gas flow; and determining a mass flow rate of the output gas flow from measured values of temperature, relative humidity and gas flow rate of the output gas flow in order to determine the moisture content.
providing a sample having a moisture content;
heating the sample to release the moisture content;
providing an input gas flow which combines with the moisture content to produce an output gas flow; and determining a mass flow rate of the output gas flow from measured values of temperature, relative humidity and gas flow rate of the output gas flow in order to determine the moisture content.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US4872498A | 1998-03-26 | 1998-03-26 | |
| US09/048,724 | 1998-03-26 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2267722A1 true CA2267722A1 (en) | 1999-09-26 |
Family
ID=29709176
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2267722 Abandoned CA2267722A1 (en) | 1998-03-26 | 1999-03-24 | Method and apparatus for determining moisture content |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2267722A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102015219054A1 (en) * | 2015-10-01 | 2017-04-06 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method and device for determining the amount of liquid on an object |
| WO2024042156A1 (en) * | 2022-08-26 | 2024-02-29 | Wageningen University | Method for determining the amount of volatile surface liquid on a solid extrudate |
-
1999
- 1999-03-24 CA CA 2267722 patent/CA2267722A1/en not_active Abandoned
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102015219054A1 (en) * | 2015-10-01 | 2017-04-06 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method and device for determining the amount of liquid on an object |
| DE102015219054B4 (en) * | 2015-10-01 | 2020-06-10 | Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. | Method for determining the amount of a liquid substance on an object |
| WO2024042156A1 (en) * | 2022-08-26 | 2024-02-29 | Wageningen University | Method for determining the amount of volatile surface liquid on a solid extrudate |
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