Two different systems can be used to create metallized paper:
This process involves a gluing process on the paper, normally a non-coated one, to a sheet of aluminiun with a thickness between 9 and 12 micron. This system is used increasingly less owing to the high consumption of aluminium.
This system, which we are going to look at in depth, is the most used and it consists of heating aluminium or another material in a vacuum chamber, vaporizing or sublimating it, and depositing a thin layer on the surface of the a coated substrate. In this case, the main advantage is that 300 times less aluminium is used than in the previous case.
In both cases this type of substrate offers a solution to achieve the distinctive, metallic finish on packaging with only a fraction of the aluminium content of traditional foils. Let's see some more details about the vacuum production process.
As mentioned before the best way to produced a metallized paper it's through a vacuum metallization.
The process of producing the metallized paper by the direct high vacuum method consists of three independent processes, which are the following:
A thin layer of varnish is applied to the coated substrate in order to prepare the surface for subsequent metallization. The paper is pre-coated with lacquers that are applied to smooth out the surface of the paper to act as a barrier between the paper and the metallized layer and to improve the adhesion of the vaporized metal. The varnish is applied in the varnisher through a process similar to gravure printing. Once the varnish has been applied, the paper passes through the hot air drying unitin order to remove the solvents of the varnish and therefore dry the paper until it has a moisture level of between 2 and 3%. Next, and within the same varnisher, the paper passes through a set of refrigerated rollers which prevent the paper from sticking in the winder.
This phase is probably the most critical point during the production process. Once the reeks of paper have undergone the varnishing process they are sent to the metallization area, where they are introduced one by one in order for the layer of aluminium to be applied to the paper. The pre-coated paper is then metallized directly. The varnished paper is placed in a "metallization chamber" where the layer of aluminium is applied through a vaporizing or sublimating process.
All metallized papers are then top coated and re-moisturized as a final process. Once the reel is placed in the lacquering machine, the metallized paper passes through corona treating that facilitates the anchorage of the lacquer on the aluminium. This corona treating acts by increasing the surface tension of the metallized surface. Once the corona process has taken place the lacquer is applied using a system of gravure, as in the varnisher, and is subsequently dried using hot air tunnels.
When the product is coloured, it is the lacquer itself that is coloured with the final colour of the product. The classic colour is gold, but from a technical point of view other colorants could be used if this were necessary. Once dry, the metallized paper passes through cooling rollers, and the water and the dorsal treatment are subsequently applied. This is to ensure that the paper has the final moisture necessary for the finished product and to maintain flatness when it is transformed into sheets or labels.
The coating protects the aluminium from damage and serves as a print receptive primer for multiple printing applications. A thin layer of lacquer is applied to give the final characteristics of the paper such as printing features, colour such as for gold susbtrates or matte / gloss finishings.
The product obtained is recyclable in a similar way to other special papers.
The metallized surface of the product is plastic in nature, or in other words, it is waterproof and non-absorbent. It offers a series of barrier properties to the coated paper, forming a barrier against light, water vapour, oxygen and odours.