Photosensitive adhesive and photosensitive film for screen printing (2)

The sensitization materials used in the early platemaking process include chromic salts as sensitizers, and water-soluble natural polymer materials such as egg folds, gelatin, and chrome barium as a film-forming material. However, since the development of the water-soluble synthetic polymeric substance polyvinyl alcohol, it has been specifically used as a film-forming material.

1. Dichromate photosensitive principle. The hexavalent chromium ion in dichromate absorbs ultraviolet light and reduces to trivalent chromium, which is well known from a long time ago. After the water-soluble polymer polyvinyl alcohol was widely used in plate-making sensation materials, long-term use of dichromate ammonia as a sensitizer was added to PVA in silk screen making, but it has been discontinued due to chromic contamination issues. .

With regard to the photochemical reaction of dichromate, the current general theory is that when a hexavalent chromium ion undergoes photoreduction to become a trivalent chromium ion, a part of its hydroxyl group is oxidized if it coexists with PVA, and the water solubility of its PVA is also So reduce. At the same time, due to the non-consensus electrons of the generated trivalent chromium ions, the hydroxyl groups of polyvinyl alcohol are formed and coordinated, and the insoluble water of PVA is further deepened through the cross-linking reaction, resulting in photocuring of the colloid, as shown in Figure 2-56. .

2. The sensitizing principle of diazo resin. (Figure 2-57, 2-58). Dichromate has become more and more difficult as a sensitizer because of the cause of the pollution. Diazo resins have now been developed. Diazo resin has been widely used as a photosensitizer for offset printing PS plates, and for screen making, since the hydrophilic PVA is mixed, the diazo radical of the photosensitive layer is hydrolyzed, so the PS plate is made of diazo resin. More difficult.

The photohardenability of polyvinyl alcohol emulsions of diazo resins is insoluble in water, and the crosslinking reaction of polyvinyl alcohol molecules is clear.

The diazo resin has the smallest molecule, n is in the range of 2 to 3, and if n is 2, the diazo dimer resin has less hydroxyl groups of the lost polyvinyl alcohol due to the condensation of the polyvinyl alcohol in the photocrosslinking. To make it lose its hydrophilicity, although there is no obvious effect, the polyvinyl alcohol molecule of the chain structure due to cross-linking will turn into a three-dimensional network structure, which has a great influence on the insolubility of the photohardened water. .

3. Photosensitivity of allyl alcohol esters (Figure 2-59, 2-60). Acrylic acrylonitrile group is very important as a photosensitive resin photosensitive base, its high sensitivity, good storage, can be used as a screen PS photosensitive material. Acrylic monomers or low polymers are photosensitive resins obtained by condensation with various polyacrylates. When used as a stencil-type photosensitive material, they need to be combined with polyvinyl alcohol that can be developed by water.

Pentaerythritol tripropenate, which has high sensitivity, has three photofunctional groups bonded by double bonds and exhibits excellent photohardenability, which is insoluble in water after hardening. Therefore, mixing with water-soluble polyvinyl alcohol becomes difficult. The soluble organic solvent polyethylene with low saponification value and acrylate are mixed and dissolved together to make a photosensitive material for a screen PS plate.

The double bond of the sensitizer acrylic resin absorbs ultraviolet rays, and an initiator having a radical is used to initiate free radical formation of the active molecule. If such an active molecule is polymerized with polyvinyl alcohol, a number-domain molecule of a three-dimensional network structure is formed, making the polyvinyl alcohol water-insoluble. At this time, the hydroxyl group of polyvinyl alcohol is insoluble in water due to the active molecule. Of course, there is still not much clarity about this part of the photohardening theory.

As a photosensitive material for screen printing, the most used one is emulsifying vinyl acetate in a polyvinyl alcohol solution and then adding a diazo resin as a photosensitive agent. The photosensitive film formation mechanism is shown in Fig. 2-61. The photosensitive film hardens due to exposure and becomes a water-insoluble plate film. The role of polyvinyl acetate is to strengthen the adhesion of the screen fibers, and the unexposed image portions are flushed with water by the uncured polyvinyl alcohol during development.

Fifth, the introduction of sensitive plastic classification

Photosensitive adhesives are generally classified into two types, solvent-resistant and water-resistant. Solvent-resistant photosensitive adhesive, resistant to various organic solvents, suitable for the printing of oily inks; water-resistant photosensitive adhesives, suitable for the printing of water-based inks (water-soluble printing coatings, such as textile printing pastes), the former can be removed from the screen Recycling; the latter generally cannot be removed by film. Dedicated water resistance photosensitive adhesives include nylon and diazo photosensitive adhesives. The user must select the appropriate photosensitive adhesive according to the printing object and referring to the relevant manual.

Sensitive adhesive is a silk screen-printing photosensitive material used for direct legal printing. It can also be divided into single liquid type and double liquid type. Single-liquid type photosensitive adhesive has been added to the emulsion in the production of the photosensitive agent, the use of coating without the need to be coated; two-liquid type photosensitive adhesive before use, the photosensitive agent should be formulated according to release instructions, and then dispersed in the mixed Latex can be applied after defoaming.

Sensitive plastic model, a wide variety, the manufacturer's formula is not uniform, when selected according to the substrate requirements, refer to the product manual appropriate choice.

1. Dichromate emulsion. The sensitizer of such photosensitive adhesive is ammonium dichromate, and the film forming agent is a water-soluble high molecular material such as gelatin, PVA and PVAC+PVA. This kind of emulsion can be self-formulated, the typical formula is as follows:

1 gelatin emulsion formula

Gelatin 16g

Citric acid 0.5g

Ammonium dichromate 4.5g

Ammonia 3ml

Water 70ml

The source of the material for this glue is convenient, inexpensive and easy to develop water. However, the glue is apt to be frozen, the coating is not strong, the wet strength is poor, the clarity is poor, and it will become brittle for a long time. Therefore gradually replaced by synthetic polymer materials.