Analysis on the cause of tunnel phenomenon in bag

2022-07-30
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Cause analysis of tunnel phenomenon in bag making with polyurethane solvent composite film (Part 1)

after using two-component polyurethane adhesive, there is a "tunnel" phenomenon near the hot sealing edge, which is easier to peel off after heating, and the adhesive on the film is still sticky after peeling off. This accident is called "incomplete curing"

I. causes of incomplete curing

I. insufficient curing agent

II. The curing time is less than, and the bag making operation is carried out in advance. Different isocyanates have different reaction rates, which should be paid special attention to

III. the solvent contains water, free alcohol, acid and other impurities, which consumes the NCO group in the curing agent

IV. when operating in high humidity environment, the moisture in the air enters the glue. The first two causes can be controlled through the data obtained from the test. As for the moisture and impurities in the solvent, at present, the general printing plants are unable to detect them quantitatively, resulting in frequent quality accidents, and the cause of the accident is difficult to define. What quality of ethyl ester can be used for polyurethane adhesive

the national standard GB requires that the purity of ethyl ester should not be less than 99% and the moisture content should not be less than 0.2%. Obviously, this standard is too wide. The so-called "ammonia fat grade" solvent of Henkel requires purity greater than 99.9%, moisture less than 300ppm and free alcohol less than 200ppm, which seems too strict. We believe that the evaluation standard should be established according to the high purity of ethyl ester in China. The actual measurement of the consumption of water and impurities in ethyl ester on NCO base is more focused on the local development of ALT company. It is feasible to adjust the proportion of curing agent to eradicate incomplete curing accidents

determination of diaminolipid solvent

the solvent that can be used for polyurethane adhesive is called ammonia lipid solvent. Compared with general industrial products, in addition to water, the content of free alcohol and acid must be reduced as much as possible to avoid reaction with NCO group. There is an index to consider whether the solvent belongs to ammonia resin grade, namely "isocyanate equivalent". Isocyanate equivalent refers to the weight of the solvent required to consume 1 mole of equivalent NCO group, while the substances that consume NCO group include the sum of impurities and moisture contained in the solvent. The isocyanate equivalent of domestic "urethane grade" ethyl ester can reach 5600g, and the greater the value, the better the stability. Printing compound enterprises are different from chemical enterprises. In the measurement method we recommend, we use "isocyanate value" to measure, which is intuitive and convenient

the test is divided into three steps. First, tmp-td1 adduct (such as yh10 curing agent of GAOMENG company) is used to react with ethyl ester, specifically with water, alcohol and acid in ethyl ester. Then, di-n-butylamine reacts with excess isocyanate to form urea:

r-nco+hn (C4H9) 2 - rnhcon (C4H9) 2

the remaining di-n-butylamine in the third step is titrated with hydrochloric acid:

hcl+hn (C4H9) 2- (C4H9) NH · hcl

reagent:

① yh10 curing agent

② 2mol/l di-n-butylamine toluene solution. 258g of anhydrous di-n-butylamine is re evaporated, diluted to 1L with anhydrous toluene, and stored in a brown reagent bottle

③ bromocresol green indicator. Dissolve 1g bromocresol green in ethanol to prepare a 100 ml solution

④ titrant. 0.5mol/l hydrochloric acid solution

⑤ absolute toluene, absolute ethanol

operation:

at the same time, accurately weigh two parts of yh10 curing agent, with the same weight, each gram. Put them into two conical flasks respectively. One of them is used for blank test, and the other uses a pipette to add 20 ml of ethyl acetate to be tested, so that the curing agent is dissolved and placed at room temperature for 5 hours. Then add 10 ml of dibutylamine toluene solution with another pipette, shake it violently to mix it, and leave it at room temperature for 50 minutes. After full reaction, add 50 ml of absolute ethanol and 5 drops of bromocresol green indicator into the flask, titrate with 0.5mol/l hydrochloric acid solution, and the end point is when the blue color of the sample solution disappears and the sample solution turns yellow and remains unchanged for 15 seconds

after the blank test flask is put into the curing agent, add 20 ml of anhydrous toluene, shake well and add dibutylamine solution. After 20 minutes at room temperature, add ethanol and indicator agent, and then titrate

the isocyanate value consumed is calculated by the following formula:

nco% = (a-b) × zero point five × 4.2/weight of curing agent (g)

a and B are the volume consumed by hydrochloric acid titrant (ML), where a is the consumption of ethyl acetate sample and B is the consumption of blank sample

the above-mentioned isocyanate equivalent 5600g ethyl ester consumes about 4.5% NCO, and the isocyanate equivalent 2500 g ethyl ester consumes 10% NCO. Of course, it can not be called "ammonia fat grade" solvent

discussion on analysis results

taking 5600g ethyl ester equivalent of isocyanate as an example, 20ml ethyl ester (18G) consumes 4.5% NCO group in 3G curing agent, and the weight ratio of ethyl ester to curing agent is 6:1. The greater the dilution of ethyl ester, the greater the consumption of isocyanate value in the curing agent. Take some varieties of GAOMENG company as an example:

name common working concentration Main: solid: ethyl ester, ethyl ester and curing agent ratio

yh200 brings confidence to extruder enterprises 0d 35% 5:1:6.9 6.9:1

yh501s aluminum coated adhesive 35% 10:1.5: to solve employment problems and improve product competitiveness 13.2 8.8:1

yh2000s aluminum foil adhesive% 10:1.5: 3 8.. 8: 1

yh501sl 25% 10:1.5:22.5 18:1

yh502 cooking gum 25% 10:1:123 The accuracy of the force sensor is 12:1

in the table, the ratio of common working concentration ethyl ester of 501s to curing agent is about 9:1, and the consumption of nco%:

9 × 4.5/6 =6.75%

isocyanic acid value of curing agent is 14%, and NCO base consumed by solvent accounts for about half of the total, so it should be safe. The reason is:

firstly, the ratio of NCO group to OH group in the adhesive ratio is about 2:1, and the excess NCO group is used for the consumption of water impurities in the solvent

secondly, the reaction rates of NCO groups are different. If the relative reaction rate of primary alcohol group is 1, the rate of water is 5 times slower, and that of carboxylic acid is 18 times slower than that of primary alcohol. Even if the relative reaction rate of free alcohol and OH group in the main agent is the same, their reaction probability is also related to the content in the gel

what percentage of isocyanate consumed by ethyl ester is the safety critical point? How much curing agent should be added if it exceeds the limit? This will be decided by experiment. What is important is that we have the means to monitor the "ammonia fat grade" ethyl ester and the quantitative analysis method, which can be used to study more complex dynamic changes, such as the impact of moisture in the gas during production under high humidity on the glue in the ink tray. Here, I hope colleagues in the industry can make wonderful statements

(to be continued)

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