Moisture-proof packaging technology design (2)

The moisture permeability under any temperature and humidity conditions can be obtained as follows:
Qθ = R·K·Δh (4-9)
Where: Qθ--measurement of moisture permeability at any temperature θ°C (g/m2·24h)
R-- measured moisture permeability at 40°C (g/m2·24h)
K-- The coefficient of different packaging materials when the ambient temperature of the packaging is stored at θ°C, see Table 4-5.
Δh--At any temperature of θ°C, the relative humidity inside and outside the package is poor.
In order to express the moisture permeability at a specified temperature of 40°C, the following equation can be used:
W·(C2-C1)·10-2
R = ------------- (4-10)
S·t·(h1-h2)·K
The R value calculated by Equation 4-10 is the moisture permeability of the packaging material when the moisture content of the product inside the package is just at the limit value, and this moisture permeability is characterized by the standard test, but the value and the related chart are generally The values ​​are not equal, so the factors that affect moisture permeability should also be considered in the design, and the thickness of the packaging material should be properly determined, and the moisture permeability of the actual moisture-proof packaging material should be slightly less than that of Formula 4-10. Calculated R value.
When examining whether a known packaging container meets the moisture protection requirements of the product, the expiration date of the moisture-proof packaging can be calculated based on known conditions (calculation method), and the effective moisture-proof period (experimental method) can also be measured through practical tests. Regardless of the method described above, the value obtained or measured must be slightly greater than the actual moisture-proof period.
If you use the calculation method, you must know the exact data of the package container, the environmental conditions, and the product's limit moisture content. Equation 4-10 can be used to calculate the allowable moisture-proof validity period, that is:
W·(C2-C1)·10-2
t = ------------- (4-11)
R·S·(h1-h2)·K
Second, to prevent the moisture content of the packaged goods to increase moisture packaging method and design to prevent moisture increase packaged articles moisture packaging method is to store the amount of desiccant in the package. The solid desiccant and the product are simultaneously placed in a closed packaging container. The desiccant can absorb moisture from the wet material and store it in itself, thereby reducing the humidity in the packaging container or reducing the moisture of the packaged product. Such a moisture-proof packaging method is strong in moisture resistance and reliable, and has a low moisture-proof requirement for packaging containers, but is only applicable to short-term moisture-proof packaging of foods, medicines or other Small electromechanical products.
This kind of moisture-proof packaging requires the use of moisture-proof packaging materials with low moisture permeability because the moisture content inside and outside the packaging is increased after the desiccant is stored in the packaging. If the moisture permeability of the packaging materials used is large, moisture outside the packaging will be faster. Into the package, the limited desiccant in the package quickly loses its effect, resulting in wet deterioration of the packaged product. The desiccant used for moisture proof packaging must have certain characteristics:
(1) Strong moisture absorption capacity, and the unit volume of moisture absorption should be as large as possible;
(2) It has physical stability, tasteless, non-toxic and non-volatile;
(3) It has chemical stability and no chemical change after moisture absorption;
(4) At room temperature, temperature has no significant effect on moisture absorption capacity;
(5) It can be regenerated by repeated drying such as baking and reused. Commonly used desiccants are silica gel, molecular sieves (zeolites and synthetic zeolites), anhydrous sodium chloride, bauxite and aluminum paste, diatomaceous earth and active algal lands.
When designing a moisture-proof packaging using a desiccant, the amount of desiccant should be calculated based on the purpose of the packaging and the packaging conditions. The calculation parameters must be given at design time:
1 Determine the moisture permeability R (g/m2·24h) of the moisture-proof material used
2 According to the relationship between the packaged items and humidity, determine the humidity inside the package h2 (%)
3 According to the hygroscopicity isothermal curve of the desiccant used, determine the maximum moisture content C2 (%) of the desiccant corresponding to the humidity value h2 inside the package, and select the original moisture content of the desiccant C1 (%)
4 Design the surface area S(m2) of the packaging container, and the packaging is valid for t(d). If the external environment temperature is θ°C and the relative humidity is h1(%), the coefficient K is determined according to θ°C and h1(%) conditions. value.
By substituting the above parameters into 4-10, the amount of desiccant W (g) can be obtained.
R·S·t·(h1-h2)·K
W = --------------- (4-12)
(C2-C1)×10-2
If we know the weight of the desiccant in the packaging container, we can use the above parameters and 4-11 formula to calculate the validity of the packaging.