Thermal insulation principle of thermos cup
The thermos cup usually has a double-layer structure, and the inner liner of the cup and two layers of stainless steel are welded together, so that vacuum is pumped without heat transfer; The cock of the thermos cup has good sealing performance and less convection heat dissipation. Copper or silver plating between the liner and two layers of stainless steel can effectively reduce radiation heat loss. Therefore, it can keep hot water at a certain temperature. In addition, this structure can also "keep out the cold" because its structure makes it difficult for internal heat to dissipate and external heat to enter.
There are three ways of heat transfer: heat conduction, heat convection and heat radiation. The vacuum cup with cover cuts off the convection channel; Cut off its conductive path with vacuum; The only thing left is radiation, and the glass liner of the thermos bottle solves the radiation problem.
Why is the thermos cup poisoned?
CCTV did an experiment about thermos cups. They bought a batch of children's thermos cups from the market, then used them to make tea and sealed them for three days to see what would happen. As a result, I didn't expect the inner wall of the thermos cup to be corroded in just three days, and the tea also gave off a strange smell. After subsequent professional testing, it was found that the materials used to make these thermos cups contained a lot of heavy metals, such as nickel, manganese and chromium. For example, nickel poisoning can cause respiratory tract irritation and skin damage, and manganese poisoning can cause acute corrosive gastroenteritis or irritating bronchitis and pneumonia.
Children's physique is inferior to that of adults. If they drink this water containing heavy metals for a long time, it is easy to be chronically poisoned and even life-threatening. The media exposed many poisoning incidents of thermos cups.
The reason is that the liner material of the thermos cup is generally stainless steel. In order to maintain the inherent corrosion resistance of stainless steel, steel must contain more than 18% chromium and more than 8% nickel. Cr 18 and Ni8 marked on many thermos cups are 18% chromium and 8% nickel. In addition, some manufacturers will add chromium and molybdenum, which can easily passivate the surface of stainless steel and form an oxide film.
Because stainless steel contains alloying elements such as chromium, nickel, manganese and copper, trace impurities such as lead, arsenic and cadmium may be brought in the smelting process, and metal ions will be precipitated. The alloy composition, residual elements, surface state and application environment of stainless steel have important effects on the precipitation of metal ions. The stainless steel material of the thermos cup shall be in strict accordance with the national food safety standards "Determination of As, Cd, Cr and Pb in Food Contact Materials and Products" and "Determination of Migration Amount of As, Cd, Cr, Ni, Pb, Sb and Zn". Under normal use, the content of chromium in stainless steel tableware that meets national standards is very small, so there is no need to worry about affecting health.
At present, there are three kinds of stainless steel materials on the market: 20 1, 304, 3 16. 304 stainless steel is internationally recognized as the first choice of food-grade stainless steel, which is rust-proof, corrosion-resistant and environmentally friendly. 3 16 stainless steel has strong corrosion resistance, but its relative cost is high. These two kinds of stainless steel metals have less ions, which meet the national standards.
Many manufacturers will choose to use 20 1 stainless steel and CCTV exposure. 20 1 stainless steel is a high-quality material for making various watchcases and watchbands. Mainly used for decorative pipes, industrial pipes and some shallow stretching products. Can only be used in industry. In order to save costs, many manufacturers will choose to use 20 1 stainless steel as 304 stainless steel.
These conditions will also lead to excessive metal precipitation in ordinary vacuum cups.
Even improper use of thermos cups that meet international standards will produce excessive metal precipitation. Scientists bought eight different brands of vacuum cups for testing. After adding acidic solution, all the heavy metals in the eight vacuum cups were precipitated, and all the manganese in the solution exceeded the standard, which was 6-34 times higher than the standard. Experts say that acidic solution will accelerate the precipitation of metal ions in stainless steel, and it is very likely that more heavy metals will migrate in a short time, thus causing heavy metal poisoning.
In daily life, fruit juice, carbonated drinks, etc. This is an acidic solution and can't be put in a thermos.
As for hot milk and soybean milk, you can use a thermos cup, but you must drink it in a short time. Microbes in milk and soybean milk will multiply in large quantities at a proper temperature, which will lead to the deterioration of drinks and easily cause stomach discomfort symptoms such as abdominal pain and diarrhea after drinking. Moreover, in the high temperature environment, the acidic substances in milk will react with the inner wall of the thermos, which will affect human health. So drink it as soon as possible.
Traditional Chinese medicine can't be put in a thermos cup, and many acidic substances are also dissolved in cooked traditional Chinese medicine.
As for the tea scale after making tea in a thermos cup, it will corrode the cup, and then precipitate heavy metals, leading to chronic poisoning. This is a complete rumor. Researchers in China have done experiments. The inspector took nine tea cups made of different materials and three filter tea sets made of different materials as samples. Tea cups are made of stainless steel, plastic, glass, ceramics and disposable paper cups, among which 7 kinds are thermos cups, and filter tea sets are made of stainless steel, ceramics and silica gel. Step 1: Weigh 3 grams of tea leaves, put them into each cup, filter the tea set, and soak them in the same amount of boiling water.
Step 2: After brewing for 60 minutes, filter out the tea leaves, pour them directly into test tubes, and determine the contents of lead, arsenic, cadmium and chromium in the tea leaves by ICP-MS analysis.
After testing and analysis by testing personnel, it can be seen from the test results that the detection amounts of lead, total arsenic, chromium and cadmium in tea leaves of three kinds of filtered tea sets and nine kinds of tea cups with different materials are relatively low, which will not affect the human body.
How to identify
Some parents will continue to buy unqualified "death-killing thermos cups" even if they know that they will endanger their children's health and that heavy metals may even be fatal. We must point out that this behavior is incorrect.
So how to choose a qualified thermos cup?
1. When consumers buy these stainless steel thermos cups, electric kettles and filter tea sets, they should try to buy them from regular manufacturers, and then see if there are the words SUS304, SUS3 16 or 18/8 on the inner container, because SUS304 or 18/8 means 304 or. If there is no thermos cup with these words on the inner container, it is not selected, because it may be made of 20 1 stainless steel.
2. There will be some detailed information on the outer packaging of most thermos cups, including the manufacturer's information. Parents can check whether some relevant information of the manufacturer is correct through the description on the package. If there is no relevant information on the outer packaging or packaging box, parents should buy it carefully.
3. When most qualified new thermos cups are just opened, the inner container should have no odor, so be careful if the price is too cheap.
Matters needing attention in using thermos cup
Before the new product is used, it must be scalded with boiling water or detergent several times and sterilized at high temperature to achieve the sterilization effect.
Please preheat it with boiling water for 5- 10 minutes before use, for better insulation effect.
Don't add too much water to avoid boiling water overflowing and scalding when tightening the lid.
After drinking, please tighten the lid to ensure hygiene and cleanliness.
To prevent odor or stains, please clean it after use and let it dry completely.
Finally, as long as it is qualified, it can be used and cleaned normally, so don't worry too much.