Research progress of silver-plated glass mirror back coating

Research progress of silver-plated glass mirror back coating

Research Progress on Mirror Back Coatings of Silver−plated Glass

Glass mirrors are mainly divided into two types: silver-plated glass mirrors and aluminized glass mirrors, of which silver-plated glass mirrors are further divided into copper-plated silver-plated glass mirrors and copperless silver-plated glass mirrors
.
Copper-plated silver-plated glass mirror is plated with a layer of metal silver reflection layer of about 700mg/m2 on the glass surface through the reaction of silver mirror on flat glass, and then evenly coated with a layer of copper of about 200mg/m2 on the silver plating, and then the curtain mirror back coating; Copperless silver-plated glass mirror is directly plated on the flat glass with a silver reflection layer of about 900mg/m2, in order to improve the adhesion between the silver plating layer and the mirror back coating, it is necessary to use tin fluoride and other compounds to passivate the silver plating layer, and then curtain mirror back coating
.
Due to the serious copper ion pollution in the production process of copper-plated silver-plated glass mirrors and the inability of visible light reflectivity to keep up with the market demand, copperless silver-plated glass mirrors are gradually replacing the online coatingol.
com
of copper-plated silver-plated glass mirror coatings.
The aluminized glass mirror is a sputtering of metallic aluminum through vacuum magnetron, and a layer of metallic aluminum is deposited on the flat glass as a reflective layer, and then the curtain mirror is backed by a coating
.

At present, China's silver-plated glass mirror production line up to more than 200, of which more than 100 silver mirror production lines are introduced from German Knuber, Italy Bowen, the United States Somaca and other foreign companies, the rest of the silver mirror production line for domestic technology, silver-plated glass mirror annual production capacity of 1.
05×108m2, export silver-plated glass mirror annual more than 1 million t, and formed Zhejiang Dangshan, Shandong Tengzhou, Hebei Shahe and other mirror-making industry clusters
.
The raw materials for the production of silver-plated glass mirrors mainly include: flat glass, plating solution and silver-plated glass mirror mirror back coatings, of which the annual demand for silver-plated glass mirror mirror back coatings exceeds 20,000 tons
.
With the increasingly stringent environmental protection requirements, the solvent-based silver-plated glass mirror back coating used now can no longer meet the environmental protection requirements, and the research and development of high-performance waterborne silver-plated glass mirror back coating cannot be delayed
.

The particularity of silver-plated glass mirror back coating mainly depends on 5 aspects: the adhesion of silver-plated glass mirror back coating to the silver-plated layer; Anti-corrosion and scratch protection of silver plating; It is required to bake at low temperature for a short time to avoid the change of glass crystal phase after high temperature and long-term baking; In order to save production time, the curtain coating silver-plated glass mirror mirror back coating process is adopted; As well as the cutting and edging
of the mirror during the use of the silver-plated glass mirror.
These peculiarities make it difficult
for silver-plated glass mirror back coatings to select resins, corrosion inhibitors, and formulations.
Starting from the development process of silver-plated glass mirror back coatings, this paper discusses the basic requirements
for the selection of waterborne resin and waterborne corrosion inhibitors for mirror-backed coatings of waterborne silver-plated glass mirrors.

1 Solvent-based copper-plated silver-plated glass mirror back coating

The development of solvent-based copper-plated silver-plated glass mirror back coatings has gone through three stages: (1) the primer and topcoat contain lead pigments; (2) Primer containing lead pigment, topcoat without lead pigment; (3) The primer and topcoat do not contain lead pigment
.

1.
1 Copper-plated silver-plated glass mirror mirror back coating containing lead in the primer and topcoat

Lead-containing corrosion inhibitors such as red dan dissociate Pb2+ and phenolic groups or conjugated double-bond hydrolysates in the resin in water to form insoluble compounds, which are adsorbed on the surface of the copper plating layer, which can prevent the penetration
of water molecules in the coating film.
In order to improve the corrosion resistance of the copper-plated silver-plated glass mirror back coating and topcoat, as well as the dry and wet adhesion, and adapt to the curtain coating process, the film-forming resin of the copper-plated silver-plated glass mirror mirror back coating mainly adopts rosin phenolic modified medium oil degree tong linoleic acid alkyd resin, adds a small amount of butyl etherated melamine amino resin, and reacts with a small amount of hydroxyl in the film-forming resin to improve the crosslinking density
of the coating.
The conjugated double bonds in the film-forming resin are crosslinked
by composite drying agents such as naphthenic acid (isooctanoic acid) cobalt, naphthenic acid (isooctanoic acid) lead, naphthenic acid (isooctanoic acid) calcium, naphthenic acid (isooctanoic acid) zinc, and naphthenic acid (isooctanoic acid) rare earth.
There are also acrylic modified medium oil dry alkyd resin followed by epoxy modified resin, acrylic modified medium oil dry acid alkyd resin and then toluene diisocyanate modified resin, cashew phenol, formaldehyde and hexamethyltetramine condensation modified petroleum resin
。 Lead bottom, top coating, most of the modified medium oil degree self-drying alkyd resin and amino resin, solid mass ratio is generally (7 ~ 8): 1, amino resin content is too high, mirror back paint in the assembly line is difficult to bake cross-linking completely, and this kind of copper-plated silver-plated glass mirror back coating contains red dan, yellow pill and other toxic anti-corrosion pigments, the content is 7% ~ 10%
of the quality of the entire formula.

1.
2 Primer lead-containing and lead-free copper-plated silver-plated glass mirror mirror back coating

With the deepening of human understanding of the hazards of lead, the lead content of paint, especially the lead content in contact with the human body, has been strictly controlled
.
The coating resin adopts a hyperbranched acrylic resin obtained by the copolymerization reaction of monofunctional acrylic monomer, multifunctional acrylic monomer and titanate coupling agent containing double bonds, or a modified fluoroacrylic resin, but the data from the literature show that these coating systems without amino resin crosslinking and curing have not reached the standard
due to insufficient crosslinking density.

1.
3 Copper-plated silver-plated glass mirror mirror back coating with lead-free primer and topcoat

Copper-plated, silver-plated, glass-backed coating resins are polymerized into hyperbranched acrylic-modified polyester resins using unsaturated polyester resins
and acrylic monomers.
There are also solvent-based copper-plated silver-plated mirror-backed coating resins
modified with acrylic acid and then modified with epoxy resin to synthesize ultra-short oiliness.
Because the primer and topcoat of copper-plated silver-plated glass mirror back coating are lead-free, in order to improve the performance of mirror back coating, the solid mass ratio of film-forming resin and amino resin is (4~5): 1, which increases the proportion of amino resin in the formula and increases the crosslinking density of the coating, but with the increase of the proportion of amino resin in the formula, it is necessary to use closed p-toluenesulfonic acid at the same time to reduce the baking temperature and baking time of the resin system, and only ordinary coloring pigments and talcum powder, barium sulfate, calcium carbonate and other fillers are used in the literature.
Without the use of suitable preservatives, the development of mirror back coatings did not meet the performance requirements
of copper-plated silver-plated glass mirrors.

The use of epoxy modified medium oil alkyd resin and a small amount of butyl etherized melamine amino resin crosslinking, the use of lead-free corrosion inhibitor cyanamide zinc to meet the anti-corrosion requirements of copper-plated silver-plated glass mirror, the zinc cyanamide is prepared by zinc oxide, urea, dicyandiamide and monocyanamide reaction, in order to prevent the hydrolysis of zinc cyanamide too quickly, the use of silane coupling agent coating
.
Due to the strong polarity of CN- produced by the hydrolysis of zinc cyanamide in the coating film, it is adsorbed on the copper plating layer or silver plating layer to form a corrosion-resistant precipitation film
.

2 Solvent-based copper-free silver-plated glass mirror back coating

The production process of copper-plated silver-plated glass mirrors will produce a large amount of wastewater containing copper ions, polluting the environment
.
Moreover, in the production process, over-baking will cause the copper and silver metal atoms in the copper and silver plating layer to interpenetrate, resulting in the visible light reflectivity of the copper-plated silver-plated glass mirror is difficult to exceed 85%.

The ion radius of silver and copper is very different, although the copper plating layer and the silver plating layer are congeners of elements, but the coating resin with good adhesion to the copper plating layer is not necessarily good for the adhesion of the silver plating layer, and the choice of preservatives is also different from the copper plating layer
.

There are literature using phenolic modified short oily alkyd resin, butanol etherized melamine resin, ultrafine lead oxide and zinc oxide, 2500 mesh filler, of which the solid mass ratio of film-forming resin and amino resin is (7~8): 1, the use of inorganic crosslinking agent lead oxide and zinc oxide to improve the adhesion of copper-free silver plating layer, reduce the baking temperature and shorten the baking time of the mirror back coating, the use of single-layer curtain coating copper-free silver-plated glass mirror various performance can reach the performance indicators
of copper-plated silver-plated glass mirror 。 There is also the use of phenolic and epoxy modified acrylic acid modified short oily alkyd resin, and amino resin solid mass ratio of (4 ~ 5): 1, after the crosslinking reaction, and then with silane coupling agent and titanate coupling agent instead of lead-free preservative, because silane coupling agent and titanate coupling agent is not a good preservative for silver plating, the modification did not achieve good results
.
The oil absorption of fine flake filler is large, and the high addition amount will increase the rheological index of the silver-plated glass mirror back coating, so as to achieve the stability of the curtain, and the small addition amount cannot achieve the anti-corrosion effect
of the silver-plated glass mirror.
The method of coating talc with coupling agent can reduce the oil absorption of talc, and a large amount of talcum powder can be used in silver-plated glass mirror back coating without excessive rheological index and affect curtain stability, the literature uses hydroxyacrylic resin and closed polyurethane resin crosslinking reaction, which has good performance under single coating conditions, of which the copper accelerated acetate mist test (CASS) performance reaches 480h
.

Due to the curtain coating process used in the production line, two-component polyurethane coatings and two-component epoxy coatings have a limited activation period and are not suitable for the assembly line production process
of silver-plated glass mirrors.
There are also UV curing resins, trifunctional aliphatic urethane acrylates and perfluoropolyether-containing urethane acrylates, and 2-hydroxyethyl methacrylate phosphate as a corrosion resistance additive grafted on the resin, but can not meet the performance requirements
of copper-free silver-plated glass mirrors.

3 Water-based copper-free silver-plated glass mirror back coating

Due to the large surface tension of water and the high latent heat of evaporation, water-based coatings are easy to burst during the baking process, and the drying interval
between layers after baking must be considered.
Considering the particularity of silver-plated glass mirror back coatings, the selection of waterborne mirror back coating resins and preservatives is particularly important
.

At present, there are literature using water-based cobalt drying agent, water-based manganese drying agent to dry conjugated double bond water-based acrylic modified medium oil dry alkyd resin, and then modified with epoxy, which can be self-drying or baked dry
.
However, the prepared paint did not show good corrosion
resistance.

Zhang Chongming et al.
use water-based hydroxyacrylic resin, water-based epoxy ester resin after cold setting and fully methyl etherized melamine resin, a small amount of water-based blocking TDI polyurethane curing agent, preservatives using aluminum tripolyphosphate, zinc phosphate and methylbenzotriazole mixture, and pyridine blocking p-toluenesulfonic acid to reduce baking temperature and time
.
Phosphate ions in aluminum tripolyphosphate and zinc phosphate cannot precipitate on the copper plating layer and silver plating layer to form chelates, and they are not good preservatives
for silver plating.

Guo Shihua et al.
mixed with waterborne resin after copolymerization of epoxy resin, acrylic monomer and silane monomer and phenolic epoxy and polyether modified flexible epoxy resin, using 2-ethyl−4-methylimidazole, dicyandiamide and 2-methylimidazole as curing agents, and the CASS performance of the copper-free silver-plated glass mirror prepared by spraying process reached more than 600h, but the two-component was not suitable for the existing curtain production process
.

Our research group used N-hydroxymethylmethacrylamide and 2-propylamide-2-methylpropanesulfonic acid copolymerization and then diluted methyl etherized N-hydroxymethacrylamide resin with methanol etherification instead of alkali-neutralized aqueous resin and aqueous emulsion, and then combined with high iminoamino resin to prepare waterborne single-coated copper-free silver-plated glass mirror mirror back coating
.
The waterborne mirror back coating can fully meet the performance indicators of copper-free silver-plated glass mirrors with a single coating, and it is inferred that a large number of imino (NH) groups and sulfonate groups in the coating are the key to
improving the corrosion resistance of waterborne copper-free silver-plated glass mirror back coatings.

4 Conclusion

Water-based copper-free silver-plated glass mirror back coating resin can not use alkali-neutralized water-based resin, should use high NH group and sulfonate group resin, on the one hand, high NH group and sulfonate group can make mirror back coating resin water-based, on the other hand, high NH group and sulfonate group can improve the dry and wet adhesion of water-based copper-free silver-plated glass mirror back coating in the silver-plated layer, thereby improving the corrosion
resistance, acid resistance, alkali resistance, humidity and heat resistance and wear-resistant edge cutting properties of copper-free silver-plated glass mirror.

Waterborne copper-free silver-plated glass mirror back coatings require sulfonate-containing corrosion resistants
.

Single-coating waterborne copper-free silver-plated glass mirror mirror back coating is the development direction
of waterborne copper-free silver-plated glass mirror mirror back coating.