Screen printing 1000 questions (200-216)

200. Why is the volatilization rate of solvent in ink lower than pure solvent?
A: The volatilization process of the solvent in the ink is more complicated than the volatilization of the pure solvent. Because the organic solvent in the ink dissolves the resin, the volatilization rate of the solvent in the binder decreases, and the solvent evaporation rate in the ink is lower than the solvent evaporation rate in the binder. This is due to the fact that the resin molecules and pigment particles are present on the surface of the ink, and there is a large traction between the solvent molecules and the resin molecules and the pigment molecules, making it difficult for the solvent molecules to escape.

201. What effect does the pigment have on the volatilization rate of the solvent?
A: The greater the percentage of pigment in the ink, the lower the volatilization rate of the solvent; the smaller the radius of the pigment particles, the larger the specific surface area and the greater the effect on the solvent volatilization rate. And different types of pigments have different effects on the release of solvents.

202. What is the relationship between solvent toxicity and allowable concentration?
A: The permissible concentration of solvent refers to the highest concentration of solvent that can be used in air. The greater the allowable concentration, the lower the toxicity of the solvent. The maximum allowable concentration is usually expressed in parts per million (ppm) of the solvent vapor capacity in air. For example, the allowable concentration of ethanol is 1000ppm. At this time, the concentration of toxic substances in the air is 1914mg/m2·20°C; the allowable concentration of isophorone is 10ppm, and the concentration of toxic substances in the air is 57mg/m2·20°C. .

203. How is the mixture concentration of solvent vapor and air expressed?
A: The mixed concentration of solvent vapor and air is mainly represented by the volume percentage of solvent vapor in the mixture, ie, % (VOL).
204. What is the range of combustion of the solvent (explosion range)?
A: Flammable solvents Under certain temperatures and pressures, the vapors will form a flammable mixture (explosive mixture) with the oxygen in the air. If the flammable solvent in the mixture does not reach a certain concentration value, then the supply of energy will be large and no combustion will occur; once the concentration reaches a certain range, combustion will occur. The range of possible compositions (concentrations) that initiate combustion is called the range of combustion or the range of explosion.

205. What is the flash point of flammable solvents?
A: The flash point indicates the temperature at which the flammable solvent is heated to the surface of the liquid and the mixture of vapor and air is in contact with the flame and the flash fire occurs. The lower the flash point, the more flammable the solvent.

206. What are the methods to determine the flash point?
Answer: There are two methods to determine the flash point: open cup method and closed cup method. The open cup method is used to determine high flash point liquids; the closed cup method is used to determine low flash point liquids.

What is the flash point of a dangerous goods?
A: The solvent with a flash point below 28°C is defined as a primary hazardous product.

208. What is the ignition point of the solvent?
A: The ignition point, also known as the ignition point, refers to the lowest temperature at which the flammable liquid is heated to the surface of the mixture of the vapor and air in contact with the flame, and it immediately ignites and can continue to burn.

209. How does the concentration limit of a flammable solvent burn or explode?
A: Flammable solvents have a certain combustion range. The wider the combustion range, the greater the danger. The range is generally expressed as the upper limit and the lower limit. The lowest concentration that will burn or explode when exposed to an ignition source is the lower limit; the highest concentration is the upper limit. For example, the lower explosion limit of acetylene is 2.5% (VOL) and the upper limit is 80% (VOL); the lower explosion limit of ethane is 3.22% (VOL) and the upper limit is 12.45% (VOL). It can be seen that the explosion limit of acetylene is 8.4 times larger than that of ethane, which means that the explosion risk of acetylene is more than 8 times larger than that of ethane.

210. What is the burning or explosion limit of commonly used flammable solvents?
Answer: The combustion or explosion limits of commonly used flammable solvents can be found in Table 1.
211. Which solvents are liable to catch fire and explode?
A: The reaction of ethers with oxygen can generate explosive peroxides; non-flammable solvents such as carbon tetrachloride can also react explosively with metals such as sodium, potassium, calcium, magnesium, and barium; trichloroethylene and sodium hydroxide Contact with potassium hydroxide can generate dichloroacetylene; dichloroacetylene can oxidize and explode.

212. What should I pay attention to when using flammable solvents?
A: Frequent use of flammable solvents in screen printing production. Please note the following 3 points when using it:
(1) The solvent must be stored in a closed container; the container should be protected from light and should not be stored high when stored.
(2) The solvent should not be near the fire source, because the solvent vapor is heavier than air, and it is easy to reach the explosion limit at the low point;
(3) Pay attention to ventilation in the workplace. Since the flow of solvent vapor is prone to electrostatic accumulation, the printing equipment should be grounded.

213. Why should the surface of the plastic printing material before printing?
A: In order to improve the adhesion of the plastic surface to the ink, to get the ideal text and pattern, often need to clean or modify the plastic surface before printing.

214. What are the common surface treatment methods for plastic substrates?
Answer: The commonly used surface treatment methods for plastic substrates include water washing, solvent washing, solvent etching, mechanical abrasion, chemical etching, flame treatment, corona discharge treatment, plasma treatment, ultraviolet rays, and other ray irradiation or adding additives to the material. Commonly used surface treatment methods for plastic products include corona discharge treatment and flame treatment.

215. What kinds of polymers are mainly used for corona discharge treatment?
A: It is mainly used for the treatment of polyolefin films and other non-polar films. The effect of continuous processing of the web film is also very good. Also used in the treatment of fluorocarbon polymers, polyesters, polyvinyl chloride, silicones, nylons, polycarbonates, etc.

216. What is the principle of corona discharge treatment of polyethylene surfaces?
A: The corona discharge method uses electrons, protons, excited atoms, and ions emitted from corona discharge to break the carbon-carbon bond, hydrogen-hydrogen bond on the plastic surface, generate free radicals, and can exist for a long time. It reacts with oxygen and nitrogen in the air, introducing polar groups into the plastic surface, making the plastic surface easier to wet. After polyethylene film was treated by corona discharge, the polyethylene structure changed, and if subjected to deep corona discharge treatment, the crystal area could be completely destroyed. The critical surface tension of discharge treated polyethylene can reach 39.5×10-5N/cm, while the critical surface tension of low density polyethylene without corona discharge treatment is only 31×10-5N/cm.

Source: Ke Yin Media