Explore the humidity control program before paper testing
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As the industry's requirements for the quality and stability of paper are getting higher and higher, they often add simple paper testing equipment for their own testing, for internal reference, or compared with the data provided by suppliers. Goods standard. However, the accuracy of test data often leads to many unnecessary disputes or misunderstandings. In fact, deviations in the data are inevitable, except for the normal data fluctuations from the test sample itself, the sample sampling (Sampling), the sample processing before the test, the type of instrument used, the test techniques and methods. Data that affects the test. The question is only whether the deviations in the data are within our control. Therefore, international standards for paper testing such as TAPPI (Technical Association of the Pulp and Paper Industry) and ISO (International Standards Organization) will clearly test the procedures, how to sample, process, test methods, instrument requirements, and even report forms. Make it clear that the people who are tested according to the standard can get comparable test data. For the processing procedures before the paper sample test, the residence is ignored. Therefore, I would like to take this opportunity to discuss the importance of this with everyone here.
Since paper is a hygroscopic material, it means that it absorbs moisture and releases moisture as the relative humidity (Relatively Humidity - R.H.) changes. For example, when we move the paper from a place with a relatively low relative humidity to a place with a relatively high humidity, the new environment has a higher water pressure (Vapour Pressure), the moisture entering the paper from the air will be released more, so the water content of the paper will increase until the relative humidity of the paper rises to the same level as the environment. At this time, we will say that the paper and the environment have reached equilibrium ( Equilibrium). On the contrary, when we move the paper from a place with a relatively high relative humidity to a place with a relatively low relative humidity, the water released into the air will absorb more water due to the lower water pressure in the new environment. The water content of the paper will decrease until the relative humidity of the paper is the same as the environment. From the above example, we can understand that the relative humidity of the environment is closely related to the moisture content of the paper (MoistureContent). The moisture content of the paper also affects the various characteristics of the paper, including:
(1) Mechanical properties of paper, such as Tensile Strength, Stiffness, etc.
(2) Structural properties of paper, such as Grammage, Smoothness, etc.
(3) Other characteristics of the paper, such as the charging characteristics of the paper, the stability of the size, and the absorption capacity.
Various characteristics change difficulty Burst tensile strength Tensile tear strength (Tear) folding resistance (Fold) stiffness (Stiffness) relative humidity % paper relative humidity and paper characteristics
Suppose now that we test the water content of the same stack of paper in 50% RH, 23 ° C and 10% RH, 23 ° C, respectively. The water content of the former is about 6% of the paper weight, while the latter Then it is about 2%. If our stack of paper is 2000 lbs in a 50% R.H., 23 ° C environment, we will find that the weight of the paper will vary by as much as 80 pounds based on the above water loss percentage. From this example, it is not difficult for the reader to find out that the stability of the water content of the paper sample is very important for each test, in order to obtain accurate paper data. In other words, we must stabilize the water content by pre-conditioning and conditioning before the test to test the paper.
In order to allow everyone to have a unified basis for environmental control during testing, the international testing standards also provide us with a test environment requirements (Table 1). Because the temperature changes, the relative humidity changes. For example, if the temperature of the test environment suddenly changes by 1 °C, the relative humidity of the test environment will change by about 3%. Therefore, we must control the relative humidity of the environment, we must also control the temperature of the environment. So from the requirements of the test standard, we will see that the temperature control is very strict. Furthermore, for certain types of paper, such as synthetic paper or glued paper, the effect of temperature on the effect of humidity is important.
As I said before, it is very important that the moisture of the paper is stable during the test. The humidity control program is designed to balance the paper with the environment before testing. However, the composition and weight of different papers are different. Of course, the required humidity control time will be different. So how do we determine the conditioning time required for different papers to confirm that the paper has reached equilibrium? In this regard, the simplest approach is to refer to some of the recommendations provided by international testing standards (Figure 2). As for the humidity control time in the standard is how to order it? If we repeatedly test the same paper, do we have a way to set a more accurate humidity control time to shorten the test process time? What is the "pre-wetting" procedure that few people know? What effect does he have on the data tested? For these questions, we will continue to discuss with you in the next issue.
We have already discussed the main purpose of Conditioning, which is to make the water content of each paper sample reach a stable state before the test, so that the test data has higher repeatability. Due to the different humidity control time of different papers, we also quote some suggestions on the time required for the paper to adjust the humidity in the test standard for your reference.
Careful readers may find that there is no detailed definition of all paper types on the market in the recommendations for humidity control time. So when we actually perform the humidity control program, we may have questions about the classification of the paper and the required humidity control time. Moreover, it is recommended that the humidity adjustment time of different papers tends to increase by a multiple. If we want to test different types of paper regularly as a pre-production quality certification, it is not very time consuming. If we can set an accurate humidity control time for the paper we use, the time required for the entire test process can be greatly reduced.
In fact, there are some guidelines for different test standards to define the actual humidity control time for different paper samples. In a simpler way, we can measure the weight of the sample at least every hour during the humidity control process (the exact time period can be adjusted according to the characteristics, weight and size of the paper sample), and the change in equivalent weight is less than before. When measuring 0.25% of the weight at a time, it means that the paper sample in the humidity control has basically reached a stable equilibrium state, and various other tests can be performed. The other method is conceptually the same as previously mentioned, in the form of a chart record to find a more accurate humidity control time (Figure 1). In the graph, the X axis is the logarithm of the humidity control time (Logarithm), and the Y axis is the weight of the paper sample. When we record the required time and sample weight during the humidity control process and plot the data into a curve, when the curve is averaged with the X axis (the sample weight has not changed), the time required is the humidity control time of the sample.
Sample weight A B C log (humidification time)
Record the weight of the sample before conditioning
If the paper sample is below 50% RH before conditioning, the humidity adjustment process will increase the weight of the sample until it reaches the equilibrium with the environment RH. At this time, the weight of the sample will not change and change with the humidity adjustment time. (point B) Determine the humidity control time of the sample according to the humidity curve and the average of the X axis (point C)
When the data we test is for the physical properties of the paper (such as tensile strength, stiffness, etc.); or when more accurate data is required, when compared with the data tested elsewhere, we will find that we only rely on humidity control. The procedure is not enough. In fact, the relative humidity of the storage environment before the paper is conditioned, will directly affect the water content of the sample after conditioning, and the accuracy of the test results. For example: If we have two identical test samples, each from a relative humidity of more than 50% and less than 50%, when the humidity is adjusted, the paper water content can vary by as much as 1.6% to 6%. The deviation from the physical property test data is as high as 5 to 25%. This phenomenon of error caused by the history of relative humidity stored on paper is called "Hysteresis" (Figure 2). Since we can't completely rule out this phenomenon, we can't accurately predict the range of water content fluctuations caused by it, and the error of the test results, so we must take some steps to minimize his impact.
Paper water content%15QP 0050100B C Decrease R.H. Increase R.H. Relative humidity (R.H.)% (Fig. 2) Paper "Hysteresis" phenomenon When the paper is stored in a place below 50% R.H., move to 50% R.H. to adjust humidity. The water content of the paper will change according to the curve C, and the final water content is P. When the paper is moved from a place higher than 50% R.H. to 50% R.H., the humidity is adjusted. The water content of the paper will change according to the curve B, and the final water content is Q. The history of the water content before the paper is conditioned (stored relative humidity) will cause a certain uncertainty in the water content of the humidity-changing paper - Uncertainty ( The distance between Q and P in the figure), causing errors in the test data
The experiment found that if the paper is stored in a place with a relative humidity of less than 50% for a period of time before the paper is conditioned, it is helpful to control the fluctuation of the sample water content within 0.15%, so we call this process. For preconditioning. As for the temperature and humidity requirements for pre-conditioning, you can refer to the previous issue, and the time required is generally 24 hours. If we want more accurate pre-conditioning time, we can also refer to some suggestions for pre-conditioning in the test standard.
In the actual paper testing process, you may find it difficult to control the environment in three stages. But in fact, in addition to the humidity control process may encounter a little trouble, basically the general household equipment can cope with the following, the following are some environmental control recommendations, you may wish to use as a reference:
1. Pre-humidification - Since the requirements for ambient temperature and relative humidity are relatively loose, we only need to use air conditioning devices to carefully control the paper storage environment.
2. Humidity control - Due to strict requirements and long storage time, it is recommended to use a constant temperature, constant humidity storage device.
3. Test environment - Due to the short test time, we only need to properly use the air conditioning device and the dehumidification equipment to control the temperature and humidity of the environment during the test time.