Лако-красочные материалы — производство

By contrast to the now extensive use of acrylics in waterborne coatings for DWI beer/beverage can coatings, this is the only other sector where acrylics find significant use (Table 4-1). This coatings sector still uses mainly solvent borne coatings with some use of UV cured coatings. The main function of the coating on the outside of the cap, closure or general line can is to decorate and promote the sale of the package. Therefore, very high quality appearance is very important, high gloss finishes are popular. Once again, this can only be achieved if the coating has good adhesion to the substrate and good scratch and scuff resistance to withstand damage resulting from handling in the manufacturing, filling and transport of the finished article.

Thermoset acrylics are suitable for these coating uses (as well as for other end uses (already discussed in this volume)), in particular:

• good resistance to yellowing and discoloration

• ability to form hard coatings by increasing Tg

• good pigment wetting (enabling pigmented coatings with good hiding power to be formulated)

• ability to get good plate coverage at low film thickness (typically coatings are applied a 5-10 microns)

The chemistry of the acrylic polymer is exactly as described elsewhere in this volume. Acrylic monomers are polymerised by addition polymerisation in solvent using free radical chain initiators. Batch polymerisation is typically used, the chosen monomer mixture being added at a predetermined rate to the solvent with stirring and close temperature control. The temperature and choice of chain initiator determines the rate of decomposition of the chain initiator into free radicals. This determines the concentration of free radicals, which is one factor in controlling the molecular weight of the polymer formation. The choice of solvents is also important as the solvent can interact with free radicals to a greater or lesser extent, and so change the course of the polymerisation independently of other factors, by, for example, acting as chain transfer agents and terminating the polymerisation of one molecule (hence limiting molecular weight growth), and starting a new molecule.

By careful choice of:

• acrylic monomer types and weight ratio

• chain initiators), type and amount

• batch temperature

• solvent type

• monomer addition rates acrylic polymers can be tailored to optimise:

• pigment wetting

• hardness

• gloss

• flexibility

• reactivity with amino crosslinkers

• adhesion

• other specific properties (e. g., resistance to staining on contact with product to be packed)

Thermoset acrylics are, therefore, extremely versatile polymers and can be formulated into coatings and varnishes which are applied by the typical rollercoating/mandrel coating techniques used in the industry. A typical coating sequence may be:

a) Size coat (low film weight/thickness 1-2 microns) to provide a key coat for subsequent coatings. The use of size coats is decreasing all the time to reduce costs ( not only materials, but also another coating and stoving operation ) and size coats are now only mainly used where there are exceptional tooling requirements.

b) White basecoat (film thickness 5-10 microns) to provide a solid background for decoration.

c) Inks

d) Varnish (may be wet-on-wet with inks).

Each coat is applied and stoved before the next coat is applied, except in the case of inks and varnish which are often applied wet-on-wet and given one stoving. This technique places an additional requirement on the varnish, i. e., it has to be formulated to wet out perfectly over wet inks.

It is in inking and varnishing that the use of ultraviolet (UV) curing technology is most advanced. This is a technology that has grown around the increased availability and use of acrylics. UV coatings for free radical cure are formulated from polymers which contain residual unsaturation (double bonds) that are still capable of being polymerised by free radicals. The polymers are dissolved in ‘monomers’ which themselves retain residual unsaturation (double bonds). The mixture will contain a photo initiator which will release free radicals when exposed to the right frequency of UV light. The UV ink or varnish will be applied by a conventional method and then the coated substrate passed under a bank of UV lights. This will decompose the photo initiator and generate free radicals that will initiate the polymerisation of the polymer (and monomers in which they are dissolved) to form a densely crosslinked coating film. The use of UV coatings has been limited by:

• problems with irritancy of the monomers used

• shrinkage of the coating on cure (i. e., the cured coating occupies a smaller volume than the uncured coating). The shrinkage of the coating during the curing process causes movement relative to the substrate and gives rise to poor adhesion.

• the inherent high crosslink density and poor adhesion limit flexibility.

• the inability to obtain complete cure by UV can mean that a thermal bake is also required.

UV coatings do have the potential advantage of being 100% solids, i. e., containing no volatile organic solvents and are, therefore, an alternative route to complying with the increasing amount of environmental regulations restricting the emission of solvents to the atmosphere.

UV curable systems are not restricted to acrylate technology. Cationically cured UV systems based on cycloaliphatic epoxy resins, vinyl ethers and iodonium salt photoinitiators are also used for metal decorating.

There are advantages and disadvantages between acrylates and cationic systems; for more information see Oldring et al(8)(9).