Moisture is the water content in paper and cardboard, that is, the ratio of the mass reduced by the sample after drying in accordance with the specified method to the mass at the time of sampling, generally expressed as a percentage. The water content of paper and board plays an important role in printing, post-press processing, product storage, and product use. The moisture content of paper has a great impact on printing, the moisture is too low, the paper is brittle, and the printing is easy to produce static electricity; If the content is too high, it will make the ink difficult to dry. Water changes have a great impact on the various characteristics of paper, with the change of the fraction, its quantitative strength, tensile strength, flexibility, folding resistance, etc., will change, the size of the paper will be stretched, and even curling, warping, wrinkling, flanging and other phenomena. The measurement of moisture is of great significance to the performance and quality of paper and board. At present, there are mainly three commonly used test methods for the moisture of paper and cardboard:

The first: high frequency principle, suitable for field detection, portable.

Testing instrument: PN-FDG1 paper moisture meter

The second: international general oven method, suitable for laboratory use, economic, long test time.

Testing instrument: DHG-9003BS electric thermostatic air drying oven

The third: halogen moisture meter method, suitable for laboratory use, a rapid detection of moisture method, integrated drying box and balance function pre-integration, high efficiency, high precision, short test time.

Testing instrument: XY105MW halogen moisture tester

The moisture content of paper and paperboard is determined in accordance with GB/T 462-2008 "Determination of moisture content of Paper, paperboard and pulp analysis Samples", which is modified by ISO 287:1985.

1. Test instruments and method standards

GB/T 462, ISO 287, TAPPI T412

Ii. Test purpose

Water changes on the various characteristics of the paper is also a great impact, with the change of water, its quantitative, tensile strength, flexibility, folding resistance will change, the size of the paper will also be telescoped, and even curling, warping, wrinkling, flanging phenomenon. Control the performance and quality of paper and board through inspection.

Third, the test principle

Weigh the sample at the time of sampling, then dry to constant weight, and weigh the mass again. The ratio of the mass difference before and after drying to the mass before drying is the moisture content of the sample.

Iv. Sample taking, processing and container preparation

Sample according to GB/T 740, GB/T 450.

Note: If the sampling place is warm and humid, the sample should be avoided from contamination or water loss, and it is best to wear rubber gloves during operation. In order to avoid changes in the moisture of the sample due to exposure to the atmosphere, all samples should be packed into the container immediately after sampling.

Preparation of containers: Before sampling, a sufficient number of clean and dry containers should be numbered and balanced in the atmosphere, and then each container will be weighed and covered until the sample is loaded.

5. Test procedures and methods

(1) First drying and load bearing

The container containing the sample is dried in an oven which can keep the temperature at (105±2) ℃. For paper samples, the first drying time should not be less than 2h. When drying, the lid of the container with the sample can be opened, and the sample can be taken out of the container and spread out, but the sample and the container should be dried in the same oven at the same time. During this period, it should be ensured that no new samples are placed in the oven.

When the sample has been completely dried, the sample should be quickly placed in the container and covered, and then the container in the dryer to cool (cooling time can be estimated according to the different containers). Open the lid of the container and close it immediately, balance the air pressure inside and outside the container, and then weigh the container containing the sample, and calculate the sample mass after drying.

Figure 1 Moisture test equipment

(2) Dry and weigh until constant weight is reached

Put the container containing the sample in the oven again for a period of time, repeat the drying and load-bearing steps in 5.1 until the constant weight is reached, and the drying time is at least half of the first drying time.

When the difference in weighing is not greater than 0.1% of the mass of the sample before drying for two consecutive times at the specified time interval, the sample reaches constant weight.

6. Calculation and analysis of results

The moisture X (%) of paper and cardboard is calculated according to formula (1) :

Formula:

m1 - sample mass before drying, g;

m2- Sample mass after drying, g.

Two measurements were made at the same time, and the arithmetic mean value was taken as the measurement result. The measurement results should be revised to the first decimal place, and the absolute error between the two measurements should not exceed 0.4.

7. Possible wrong operation and error sources

(1) The sample is not fully mixed in time

Because no matter how the container is sealed, over time, the sample will eventually start to inhale water from the surface, resulting in uneven moisture content at each point of the sample.

(2) Placing too many samples

Placing too many samples in the oven will result in insufficient heat absorption, incomplete evaporation, and low moisture results.

(3) The oven temperature uniformity is poor

The use of ordinary oven will cause uneven heating, should try to choose the standard requirements of the test oven.

(4) Drying time is not enough

The drying time specified in the standard is 2h, which is suitable for most areas. However, the exact time required for the sample to reach the constant weight is affected by the local humidity and altitude. For example, Zhengzhou dry climate is shortened drying time, the southern climate is humid, drying 2h can not fully reach the constant weight.

(5) Electronic balance accuracy is not enough

According to the standard requirements, the quantity value should be accurate to 0.001g, and in theory, it is enough to use a balance with a sensitivity of 0.001g. However, in the actual work, it is very likely to encounter the end of the balance drift, can not guarantee the accuracy. In order to ensure accuracy, it is best to choose an electronic balance with a sensitivity of 0.00001g. The weighing results are accurate to three decimal places.

(6) After each weighing, the indicating value does not return to zero

The balance is not levelled, not preheated, not preloaded, poor linearity and not calibrated after changing position may result in this situation where the indicator value does not return to zero. The electronic balance is an electronic instrument, which should be preheated for more than 30min before measurement, so that the internal electronic devices can achieve a thermal stable balance. Preloading is a short pressure and pressure relief operation on the electronic balance, because after the balance is not used for a long time, the internal sensor is in a dormant state, and the metal shrapnel also stops working, and the performance of the electronic balance is in an unstable state, and the indicating stability and return to zero are not good. Before using the electronic balance for the first time, adjust the level first, the hotter the boot, pre-load before measurement, and calibrate regularly.

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This paper was published in the magazine Carton World in February 2023