As their peers have higher and higher requirements on the quality and stability of paper, they often purchase simple paper testing equipment for themselves and test the paper for internal reference, or compare it with the data provided by the supplier for revenue. Goods standard. However, the accuracy of test data often produces many unnecessary disputes or misunderstandings. In fact, deviations in the data are inevitable. In addition to the normal data fluctuations from the test sample itself, the sampling of the sample (Sampling), the processing of the sample before the test, the type of instrument used, the test techniques and methods Affect the test data. The only question is whether the deviation in the data is within our control. Therefore, the international standards for paper testing, such as TAPPI (Technical Association of the Pulp and Paper Industry) and ISO (International Standards Organization), will make the test procedures clear from how to sample, process, test methods, instrument requirements, and even report forms. It is stated that it is important to enable those who test according to the standard to obtain comparable test data. As for the processing procedure before the test of the paper sample, the residence is ignored by people. So take this opportunity to discuss its importance with you here.
Since paper is a hygroscopic material, it means that it will absorb moisture and release moisture with the change of relative humidity (RH). For example, when we move the paper from a place with low relative humidity to a place with high relative humidity, since the new environment has a higher water pressure (Vapour Pressure), there will be more water vapor entering the paper from the air Release, so the moisture content of the paper will increase until the relative humidity of the paper is raised to the same as the environment, then 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 high relative humidity to a place with low relative humidity, due to the lower water pressure in the new environment, the paper releases more water into the air than absorbed. 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 (Moisture Content). The paper moisture content also affects various paper characteristics, including:
(1) Mechanical properties of paper, such as Tensile Strength, Stiffness, etc.
(2) Structural properties of paper (Structural Properties), such as paper weight (Grammage), smoothness (Smoothness), etc.
(3) Other characteristics of the paper, such as the charging characteristics, dimensional stability, and absorption capacity of the paper.
Changes in various characteristics Difficulty Bursting strength (Burst) Tensile strength (Tensile) Tear strength (Tear) Folding strength (Fold) Stiffness (Stiffness) Relative humidity% Paper relative humidity and paper properties.
Suppose now that we are testing the moisture content of the same stack of paper in two different environments, 50% RH, 23 ° C, 10% RH, and 23 ° C. The former has a moisture content of about 6% of the paper weight, while the latter It is about 2%. If our stack of paper is 2000 lbs in an environment of 50% RH and 23 ° C, when calculating the weight of the stack of paper based on the percentage of moisture loss above, we will find that the paper weight will be as much as 80 lbs. Deviation. From this example, it is not difficult for the reader to find that, if accurate paper data is to be obtained, the stability of the water content of the paper sample is very important during each test. In other words, we must first pass the paper samples through the preconditioning and conditioning procedures to stabilize the water content before testing the paper.
In order to allow everyone to have a unified basis for environmental control during testing, international testing standards also provide us with the requirements for the testing environment (Table 1). Because the temperature changes, the relative humidity will change. 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, to control the relative humidity of the environment, we must also control the temperature of the environment. So from the requirements of the test standards, we will see that the temperature control is very strict. Furthermore, for certain types of paper such as synthetic paper or glue-treated paper, the influence of temperature is more important than the influence of specific humidity.
Relative Humidity RH RH Tolerance Difference Temperature Tolerance Difference Preconditioning atmosphere 10-35% None 22-40 ° C Conditioning atmosphere 50.0% ± 2% 23 ° C ± 1 ° C
Testing environment Testingat mosphere 50.0% ± 2% 23 ° C ± 1 ° C
Figure 1 Requirements of international standards for environmental control of paper test procedures
As I said earlier, it is important to test whether the moisture of the paper is stable during the test. The humidity adjustment procedure is designed to make the paper reach the equilibrium state with the environment before the test. However, the composition and weight of different papers are different. Of course, the required humidity adjustment time will be different. So how do we determine the humidity adjustment time required for different papers to confirm that the paper has reached equilibrium? In this regard, the easiest way is to refer to some recommendations provided by international test standards (Figure 2). As for how to set the humidity adjustment time in the standard? If we repeatedly test the same kind of paper, do we have a way to set a more accurate humidity adjustment time to shorten the test process time? As for the "pre-humidification" procedure that few people know about? What impact does he have on the test data? For these questions, we will continue to discuss with you in the next issue.
Paper type standard recommended time Generally used pounds of paper Generally, 4 hours of humidity is enough for light-weight cardboard. At least 4-8 hours of heavy-weight cardboard or special water-resistant paper. At least 16 hours of paper treated with wax. Minimum humidity adjustment 72 hours
Figure 2 Recommendations of international standards on the time required for paper humidity control
Earlier we have 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 is more repeatable. Due to the difference in the humidity adjustment time of different papers, we also quoted some suggestions in the test standards for the time required for the humidity adjustment of papers for your reference.
Attentive readers may find that all the types of paper on the market are not defined in detail in the recommendations for adjusting the humidity time. So when we actually carry out the humidity adjustment procedure, we may have questions about the classification of the paper and even the required humidity adjustment time. In addition, the humidity adjustment time of different papers in the proposal is often increased by multiples. If we want to regularly test different types of papers as pre-production quality verification, it is not very time-consuming. If we can set an accurate humidity adjustment time for our commonly used paper, the time required for the entire test process can be greatly reduced.
In fact, different test standards also provide some guidelines for us to define the actual humidity adjustment time of different paper samples. A relatively simple method, we can measure the weight of the sample at least every hour during the humidity adjustment process (the exact period can be adjusted according to the characteristics, weight and size of the paper sample), when the measured weight changes 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, and uses a chart record to find a more accurate humidity adjustment time (Figure 1). In the graph, the X axis is the logarithm of the humidity adjustment 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 adjustment process, and draw the data as a curve, when the curve is averaged with the X axis (the sample weight has not changed), the time required is the humidity adjustment time of the sample.
Sample weight ABC log (humidification time)
Figure Humidity control curve of paper sample
Record the weight of the sample before adjusting the humidity
If the paper sample is below 50% RH before humidity adjustment, the humidity adjustment process will increase the sample weight until it reaches a state of equilibrium with the environmental RH. At this time, the sample weight will no longer fluctuate and change significantly with the humidity adjustment time (Point B)
Determine the humidity adjustment time (point C) of the sample according to the average of the humidity adjustment curve and the X axis
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 for comparison with data from other places, we will find that humidity alone The procedure is not enough. In fact, the relative humidity of the paper storage environment before humidity adjustment will also directly affect the water content of the sample after humidity adjustment, and even the accuracy of the test results. For example: if we have two identical test samples from a place where the relative humidity is higher than 50% and lower than 50%, when the humidity adjustment is completed, the deviation of the paper water content can be as high as 1.6% to 6%, so that The deviation to the physical characteristic test data is as much as 5 to 25%. This phenomenon caused by the history of relative humidity in paper storage is called "Hysteresis" (Figure). Since we cannot completely rule out the occurrence of this phenomenon, nor can we accurately predict the range of water content fluctuations caused by it and the error of the test results, we must take some steps to minimize his impact.
Paper water content% 15Q P 0050100B C Reduce RH and increase RH relative humidity (RH)% (Figure 2) Paper "Hysteresis" phenomenon.
When the paper is stored from a place below 50% RH, move it to a place with 50% RH to adjust the 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 stored from a place above 50% RH, move it to a place with 50% RH to adjust the humidity. The paper water content will change according to curve B, and the final water content is Q.
The history of the water content of the paper before humidity adjustment (storage relative humidity) will make the water content of the humidity adjustment paper have an uncertainty-Uncertainty (the distance between Q and P in the figure), causing errors in the test data.
Experiments have found that if the paper is stored in a place with a relative humidity below 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 processing procedure Preconditioning. As for the temperature and humidity requirements of pre-conditioning, you can refer to the previous period, and the time required is generally 24 hours. If we want more accurate pre-humidification time, we can also refer to some recommendations for pre-humidification in the test standard (table).
Paper Type Standard Recommended Time Paper (Single Sheet) Less than 1 hour Lightweight Cardboard 1 to 2 hours Heavy Paperboard or Watton Cardboard 5 to 10 hours Paperboard or Watton Cardboard Finished Products 12 to 16 hours Water-resistant treated paper Or cardboard for not less than 24 hours
Table 3 Standard recommended time required for pre-conditioning the paper
In the actual paper testing process, you may find it very difficult to control the environment in three stages. But in fact, except that the humidity control process may encounter a little trouble, basically the general household equipment can handle it. The following are some suggestions for environmental control, you may wish to use it as a reference:
1. Humidity adjustment in advance-As the requirements for ambient temperature and relative humidity are relatively loose, we only need to use an air conditioning device to carefully control the paper storage environment.
2. Humidity adjustment-due to strict requirements and long storage time, it is recommended to use storage equipment with constant temperature and constant humidity.
3. Test environment-due to the short test time, we only need to properly use air conditioning equipment and dehumidification equipment to control the temperature and humidity of the environment within the test time.
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