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Substrates and printability criteria
Different substrates

Paper and cardboard
In the case of paper and cardboard non coated, gloss coated, matt coated), the surface condition is characterised by the smoothness.

The smoothness enables the degree of polish or evenness of a paper to be evaluated. Paper that is very smooth has a uniform surface. On the otherhand, a low degree of smoothness is characterised by a surface with numerous asperities and a rough appearance. The smoother the paper, the better its printability.

Papers undergo different types of treatment that allow a greater or lesser amount of surface smoothness to be obtained:
Mechanical finishes: smoothing, calendaring, etc.
Top coating finishes: surfacing, spread coating, etc.

Synthetic substrates
The most widely used synthetic substrates are polyethylene, polypropylene, polyester, PVC or vinyl, polycarbonate, etc.
Synthetic substrates are principally characterised by their surface energy: the interactions between the solid surface of a substrate and a liquid element (an ink or overprint varnish) is defined by the evaluation of the surface energy of the substrate and the surface tension of the liquid. These two energies are measured in dynes /cm and are physical characteristics that make it possible to quantify the affinity of the liquid for the solid substrate. Simple means are available to measure the surface energy of a substrate, such as Sherman type felt tip pens.
To ensure good printability, it is necessary necessary that the ink and the substrate to
be printed have compatible levels of surface tension and surface energy.
> The surface tension of the ink must be lower than the surface energy of the substrate.

Determining the surface energy is therefore very important in guaranteeing the quality of the print work. 
Did you know?
Good printability requires a surface energy of between 38 and 45 dynes/cm, depending on the type of substrate and the type of ink used.
Tracing papers, chromo coated papers and parchment papers are considered as non-absorbent substrates.
Metallised and metallic substrates also need to be considered separately. 
Printability criteria

Two main criteria influence printability: the nature of the substrate – characterised by the porosity – and its surface condition.

The porosity
Porosity induces the notion of permeability and therefore printability. It determines the capacity of the substrate to absorb inks and varnishes. The porosity of a substrate results from both the size and the number of pores present on its surface.

It allows us to distinguish 5 categories of substrate:
Non coated papers
Gloss coated papers
Matt coated papers
Cardboard
Synthetics and non-absorbent substrates

Non coated papers are macro-porous, while coated papers and cardboards are micro-porous.
Synthetic and non-absorbent substrates are non-porous or very slightly micro-porous.

The surface condition
The surface condition of a substrate may be characterised by different parameters such as the smoothness, the roughness, the surface energy and the cleanliness (absence of impurities such as grease, waxes, etc.). A large amount of treatments are available that allow the surface condition of a substrate to be modified or prepared in order to make it printable (in particular for synthetic substrates).
Did you know?
Different treatments allow the surface energy or surface condition of a substrate to be modified:
“Corona” electric treatment:
An electrode, linked to a high alternating and high frequency voltage generator (13 to 15,000 volts) emits an electrical discharge in the form of blue sparks, accompanied by the emission of ozone.

The substrate to be treated passes under the electrode, at a distance of several millimetres, and the surface is both physically and chemically modified. This treatment enables the surface energy of the substrate to be increased (by oxidation) and generates a degree of micro-porosity, which improves adhesion and therefore printability. Some presses now have inline CORONA treatment.

It should however be noted that excessive treatment can embrittle the substrate or even destroy the superficial coating. This causes poor adhesion of the ink film and problems of blocking in the spool.

The effectiveness of CORONA treatment is limited over time: the surface energy inevitably drops and returns to its initial level.

Adhesion primer:
The surface energy becomes that of the film of primer deposited. It is composed of low levels of polymer dissolved in a solvent. Polypropylene, polyester and some metallic substrates can be treated using an adhesion primer. This solution offers the advantage of not causing embrittlement of the substrate.

Top coating / Pre-lacquering:
Top coating consists in applying a coat of varnish on one or both sides of a substrate, in order to modify its surface condition. Top coating is generally carried out by the substrate manufacturer by means of emulsion based polymers (acrylic, PVDC, etc.). In certain cases, very large amounts can be deposited (up to 6 - 7 g/m2).

Pre-lacquering is carried out using a varnish (water or solvent based) and can be done by either the substrate manufacturer or the printer. The amount deposited varies from 1 to 2 g/m2. Since the surface to be printed is coated, it is important to know the chemical nature of the top coat or pre-lacquer (cellulosic, acrylic, etc.) in order to optimise printability.
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