The fibrous materials that make up pulp, paper and board are hydrophilic. Therefore, changes in ambient temperature and humidity will inevitably cause changes in the moisture content of the paper, and changes in the moisture content of the paper will change the bonding state of the fibers, thereby affecting the technical performance of the paper. Therefore, the paper sample is before the test. Pre-testing requires conditioning at a certain relative humidity and temperature to allow the moisture to reach equilibrium so that accurate and comparable results can be obtained.
1 definition
(1) Relative humidity is the percentage of the actual water vapor content of the atmosphere and the saturated water vapor content of the atmosphere at the same temperature and pressure.
(2) Temperature and humidity treatment is a process . When the weight of the two samples separated by more than 1h before and after is not greater than the specified value, it is considered to be in equilibrium.
Note: The weighing interval depends on the timing of the sample. The requirements for the degree of agreement between the front and back weighing should take into account the cycle characteristics of the designated laboratory, and the paper can be considered to be in a stable physical state if the moisture is indeed balanced. But in special circumstances. The temperature and humidity treatment time of the sample may be extended until the required equilibrium conditions are reached.
2 principle
When the sample is exposed to the specified constant temperature and humidity atmosphere, when its moisture content enters a repeatable state, the sample and the atmosphere will reach equilibrium.
3 standard atmosphere
The standard atmosphere for testing pulp, paper and cardboard should be (23±1)°C, 50% 2%RH, and tropical countries can use (27±1)°C, 65 %±2%RH. The allowable deviation of relative humidity is ±4%, and the deviation time cannot be more than 1 min in every 15 min.
4 Control of the test atmosphere
4.1 Temperature and humidity control system
Control equipment should be in good working order to ensure that the air in the room circulates evenly. Air should flow through the room at an adequate rate, with complete air changes approximately every 5 minutes. All cooling, heating, humidification, and dehumidification should be done outdoors. Temperature and humidity are controlled by sensing elements in the room or at the air duct opening. The airflow needs to enter evenly from the micro-holes on the top and be withdrawn from the floor, so that the indoor temperature can be accurately controlled at a level with little fluctuation.
(1) Temperature fluctuation
When the air conditioner is in normal operation, the difference between the maximum and minimum temperature at any point in the room shall not exceed 1.0°C in any 30 minutes within 24 hours, and the difference between the average temperature within any two separate 30 minutes within 24 hours shall not exceed 1.0°C. more than 0.5°C. The temperature difference between any two points in the room shall not exceed 0.5°C at any time.
(2) Relative humidity fluctuations
When the air conditioner is running normally, the difference between the maximum value and the minimum value of relative humidity at any point in the room is not greater than 2%RH in any 30-minute period within 24 hours, and any two 30-minute periods within 24 hours are relatively The difference between the humidity averages cannot exceed 1%RH. The difference in relative humidity between any two points in the room cannot exceed 2% at any time.
(3) Ventilated wet and dry bulb hygrometer
The graduation value of the thermometer is 0.2°C, the reading is accurate to 0.1°C, and the ventilation speed is (4±1) m/s . The mercury bulb of the wet-bulb thermometer in the hygrometer should be wrapped with degreasing gauze. Pay attention to keep the gauze clean and replace it regularly, and fill it with water to keep it saturated. Hang the hygrometer anywhere in the room, first check whether the gauze wrapped by the wet bulb is wet, then start the hygrometer fan, and after 2 minutes, read the temperature values on the dry and wet bulbs on the hygrometer, and record them in detail. Then find out the relative humidity of the atmosphere at the time of measurement through the relative humidity meter.
4.2 Steps of temperature and humidity treatment
(1) Sample pretreatment
When the sample is subjected to temperature and humidity treatment. Its original humidity has a certain influence on the performance index of paper. Because the sample is in a certain humidity state to balance to the standard humidity state. Transitioning from a high-humidity state to a standard humidity state, the moisture content is always higher than when transitioning from a low-humidity state to a standard humidity state. The change of moisture content caused by this "hysteresis phenomenon" will inevitably have a certain impact on the technical performance of the paper sheet. In order to eliminate the impact of "hysteresis" on paper performance, it is generally required to process the paper sample from a low-humidity state to a standard humidity state. Therefore, the samples should be pretreated for 24 h in an atmosphere with a relative humidity of 10%–35%RH and no higher than 40 °C before treatment. If it is predicted that the equilibrium moisture content after the temperature and humidity treatment is equivalent to the moisture content when the moisture absorption process reaches equilibrium. Then this preprocessing can be omitted.
(2) temperature and humidity treatment
Hang up the cut sample, let the temperature and humidity air flow freely touch all sides of the sample, until the moisture content reaches the equilibrium state with the water vapor in the atmosphere. When the weighing is no more than 0.25% of the total mass twice before and after an interval of 1h, it is considered to have reached equilibrium. For high-weight paper, the time between two weighings should be extended appropriately, and the degree of coincidence should take into account the influence of laboratory cycle characteristics.
Note: For laboratories with good circulation conditions, the temperature and humidity treatment of paper is usually sufficient for 4 hours. For paper with a higher basis weight, it usually takes 5~8 hours. For high-value cardboard and specially treated materials, the temperature and humidity treatment time should be 48 hours. or longer.
