With the improvement of the living standards of our people, environmental protection, safety, and health have increasingly become the goals pursued by everyone. Food safety is particularly important in the fields of food, clothing, housing, and transportation. The safety of external packaging as a component of food has become the focus of everyone's attention.
Non-fried foods, as well as some less-tasting foods such as milk powder, biscuits, cereal, chocolate, etc., are very sensitive to the taste of residual solvents. Therefore, reducing the amount of solvent residues in food packaging bags is a pressing problem that many packaging material printing plants urgently need to solve.
There are several factors that affect the solvent residue:
Solvent residue generated in the standard printing process for determining whether or not the solvent residue is acceptable Residual solvent in the compounding process Sample and detection accuracy and accuracy Solvent residue during storage and transportation
First, the standard to determine whether the residual solvent is qualified
1, the national standard
There are two standards for solvent residues in China: one is that the standard for solvent-resistant cooking film bags of GB/T 10004-1998 stipulates that the residual solvent amount is less than or equal to 10 mg/m2, but it is not specified which solvent is used. The other is GB/T 10005-1998 “BOPP/LDPE composite film, pouch†standard stipulates that the solvent residue is less than or equal to 10mg/m2, in which benzene is less than or equal to 3.0mg/m2 . Both of these standards are recommended standards and are not national mandatory standards.
2. Internal standard of the company The instant noodles production factory generally requires that the solvent residue of the packaging bag is less than or equal to 10mg/m2, among which the benzene content is less than or equal to 3.0mg/m2; the requirement of a certain milk powder manufacturer for the residue of the packaging bag is less than or equal to 10mg/m2, among which benzene Less than or equal to 2.0mg/m2; chocolate factory generally requires solvent residue is less than or equal to 5mg/m2, does not contain benzene solvent.
Why do you need to control the amount of solvent residue?
According to medical tests, organic solvents invade the human body can cause cumulative poisoning (chronic poisoning), and are classified according to the toxicity of the solvent:
A. Non-toxic: Water B. Weak toxicity: isopropanol, ethyl acetate, butyl acetate C. Toxicity: benzene, xylene, methanol, butanone D. Strong toxicity: dichloroethane, tetrachloroethane
Several common solvents harm the human body:
Toluene: The stimulation of the central nervous system is likely to cause chronic poisoning in the human body, causing loss of appetite, weight loss, leukopenia, anemia, and certain damage to the liver and kidneys.
Xylene: strong on skin irritation, inhibition of the central nervous system.
Butanone: It is toxic, has anesthetic effect, can cause a decline in central nervous function, stimulate the eyes and trachea.
Isopropanol: It has irritation and anesthetic effects on the upper respiratory mucosa of the human body.
Ethyl acetate: Anesthetic, irritating eyes and mucous membranes.
Second, the printing process control of residual solvent
1. The influence of the printed material The solvent residue is very different due to the different types of printed substrates.
According to the ability to absorb toluene from the largest to the smallest: glassine, K-coated BOPP, BOPP, PA, PET.
According to the ability to absorb ethyl acetate from big to small: glassine, K-coated BOPP, BOPP, PET, PA.
According to the ability to absorb butanone from big to small: glassine, K-coated BOPP, BOPP, PET, PA.
We must fully consider the above factors when designing our product structure.
2. Printing screen design product color sequence; how thick the ink layer; solid area size; whether full version of white ink, etc. are related to the size of the residual solvent.
3. The type of ink used in the ink type has a great influence on the size of the solvent residue. The alcohol-soluble ink and the water-soluble ink do not substantially contain toluene, ethyl ester, and xylene. The residual solvent content of benzene-containing ink (mainly chlorinated polypropylene resin and aromatic hydrocarbon polyurethane resin) mainly depends on the release properties of the ink resin to the solvent, the better the solubility of the solvent to the resin, and the greater the affinity of the resin and the solvent. The poorer the release, the higher the solvent residue. Chlorinated polypropylene resin inks have poorer releasability to toluene and xylene than aromatic hydrocarbon polyurethane resin inks.
4. The effects of printing presses mainly include the following aspects:
A, length B of the oven, whether the air volume is sufficient C, the angle D of the mouthpiece of the oven, the air pressure E, the distance F between the outlet of the mouthpiece and the film, the presence or absence of the terminal drying device G, and the direction of the air duct.
Focus here on the terminal drying equipment. My company's printing press is of an earlier model, there is no terminal drying equipment, and the solvent residue is always large. Later, we contacted the printing press manufacturer to install a set of terminal drying equipment on the top of the eight-color printing press in 12 colors. Two sets of terminal drying equipment were installed on the top of the machine and the effect was very obvious after use.
5. The influence of the solvent The ink itself contains a large amount of solvent, and at the same time, a large amount of mixed solvent such as toluene, xylene, methyl ethyl ketone, and ethyl acetate is added during printing. The residual solvent amount has a large relationship with the boiling point and vapor pressure of the solvent. The isopropanol boiling point is 82.5 degrees, the ethyl acetate boiling point is 77.1 degrees, the butanone boiling point is 79.6 degrees, the toluene boiling point is 110.6 degrees, and the xylene boiling point is 140 degrees. The higher the boiling point of xylene, the harder it is to discharge, and should be used as little or as little as possible. Butanone has a low volatilization residue, but it has a large odor and should be used as little as possible. Coking toluene is strictly prohibited.
6. Printing process Because most of the packaging structure is not changeable by packaging manufacturers, non-benzene inks are much more expensive than benzene-containing inks. Therefore, how to reduce the residual solvent of benzene-containing inks is something we must resolve in production. of.
A. Since the amount of methyl ethyl ketone and xylene used in printing is relatively small, it is easy to control if it can be used less or not.
B, the oven temperature to increase step by step, the latter oven than the previous oven 3-5 degrees higher, the general field ink, white ink temperature should be set at 65-75 degrees.
C. The amount of air intake and the amount of exhaust air should be basically balanced. The amount of exhaust air should be slightly larger than the amount of air intake to avoid the escape of hot air.
D. The printing speed should be properly controlled so that there is sufficient drying time.
E. The printed film is to be worn by 1-2 printing units so that the solvent can evaporate further.
F. The humidity in the workshop is controlled between 50% and 70%. It should not be too high. Humidity is not conducive to the volatilization of the solvent. When the humidity is doubled, the drying speed of the ink will be delayed by nearly twice.
G. It can increase the layout of the hair under the premise of not affecting the printing quality, speed up solvent evaporation.
H. Control the purity of the solvent. If the solvent is not high in purity, there are many high-boiling components. The solvent with strong volatility will be emitted first, and the volatile will remain weak. The volatility of the solvent will gradually decrease, and the residual amount of the solvent will be relatively high. .
I. The ventilation of the workshop is better, especially in the workshop with better sealing performance (such as the GMP workshop), the intake air volume and the exhaust air volume must be balanced, otherwise it is easy to cause many residual solvents in the workshop air and the odor is very large.
J. The winding tension shouldn't be too large. It is easy to press with a slight deformation. The diameter of the winding should not be too large, generally less than 500mm.
K. The air blow of the oven should be aligned or slightly upwards with the printing film. The baffle should be added at the bottom of the oven to block the escape of hot air.
L, can not be produced in excess production capacity, such as the 1000mm width of the press, although some can print 1050mm or even 1070mm printed film, but the solvent residue is much higher than the use of 1000mm film.
M, control the solvent ratio. Because the ethyl ester is easier to discharge, ethyl ester should be used as much as possible without using toluene. The ink in the field can be basically free of toluene, and the toluene, ethyl ester=1:1 ratio can be used in the hierarchical edition. We have found through comparative tests that the solvent ratios vary greatly depending on the ratio of the solvents under the same conditions.
N. Plate roller depth. The shallower the plate roller, the lower the residual solvent.
O. The secondary air return flow should be controlled well and not too large.
P, with the test report to enter the next process, if it is a single layer of film solvent residue exceeds the standard, after drying and rewinding it is easy to come down, but it is difficult to decline after compounding.