Лако-красочные материалы — производство

It should be noted that for internal applications compliance with FDA 175.300 and local regulations are required. This limits the types of materials which can be used for internal coatings. External coatings will only be considered here, because solvent based acrylic resins have little usage in internal lacquers other than PVC organosols. In the following sections, systems based on resins other than acrylics have been included to illustrate the types of formulation acrylic resins have replaced and reasons why, to compare alternative formulations for the same end use and to give an idea of the types of systems acrylic systems may be used in conjunction with.

It should be noted that in some of the examples given the actual ratios of crosslinker to resin on a solids/solids basis are different to those recommended by the suppliers of the resin. The recommended levels have been quoted and reasons why different ratios have been used are given.

If the same raw material is used more than once, a description of it along with its reason for inclusion is given the first or second time it is used.

A. General Line Systems

This section of the metal packaging industry covers the manufacture of articles such as oil cans, paint cans, fancy boxes for chocolates or biscuits (i. e. usually no retorting or pasteurisation requirement and frequently no internal lacquer). The metal substrate is normally tinplate with TFS used to a lesser degree. A three coat external system is often used. This consists of a vinyl size coat, alkyd white coating and alkyd overprint varnish.

A typical formulation might be:

In this formulation, a relatively low solid content is chosen to aid the consistent application of film weights between 0.75-1.0 gm/m2 with good substrate wetting. The slow evaporating solvents ensure a stable viscosity during application and the combination of solvents uses hydrocarbon for economy and the minimum amount of ketone for good solubility of the vinyl and epoxy resins. BE 659 (Ex BIP) is a n-butylated benzoguanamine containing a non-silicone flow agent to give improved flow and levelling. The combination of resins gives the cured form properties of good adhesion and flexibility after baking at 205°C for 10 minutes peak metal temperature (pint).

Disperse items 1-4 using a Cowles mixer or bead mill. Many excellent grades of titanium dioxide are available with good dispersibility by either Cowles or bead mill techniques. Typical examples might be Tiona 472 from SCM Chemicals, or Ті-Pure from Du Pont. The acrylated alkyd resin (Synolac 93S (Ex CV)) also provides an excellent medium for pigment dispersion and coating opacity is optimised at a pigment to binder ratio between 1.2:1.3. Small levels of timers are often incorporated as a further aid to appearance. These are usually blue or violet pigments; a blue toning gives a cleaner yet less opaque appearance and a violet toning a redder but more opaque appearance.

Coating weights of 15-20 gsm would normally be applied and cured by an oxidation process assisted by the metal driers when baked at about 170°C/10 minutes at peak metal temperature. Under these conditions, coated sheets can be stacked several stillages high without problems of blocking. Many companies do not favour the use of metal driers due to potential yellowing problems, particularly if the coating will be subjected to subsequent stovings.

The use of a very slow evaporating aromatic hydrocarbon solvent gives excellent flow out and a good surface for the acceptance of printing inks.

The choice of overprint varnish will be dictated by the flexibility requirements of the system. For example, a welded paint can body will need very little flexibility and hardness and mar-resistance are more important to avoid damage during filling and transportation.

Uralac AN621 S2-60 (Ex DSM) is a short oil alkyd resin based on Versatic acid (Ex Shell). It is normally used with medium reactivity melamine resins in the ratio 80/20-70/30 (solid/solid). In this example this ratio is drastically altered. The inherent hardness of the system is derived from the high level of crosslinking at an alkyd/melamine ratio approaching 1:1 [56/44] (solid/solid). The combination of lanolin and BYK301 provides slip and mar-resistance to the varnish, while the solvent balance of butanol and aromatic hydrocarbon provides good wet inking properties and viscosity stability. Application is normally at 4-6 gsm and it is baked at 150°C for 10 minutes at peak temperature. This resin may yellow if stoving temperatures exceed 180°C.

If, however, a greater degree of flexibility is required in the system for tooling lids for biscuit tins or drawn polish tin bodies, then a softer varnish would be chosen.

Formulating principles remain basically the same, but the major differences lie in the choice of a more flexible resin (Synolac 25S (Ex CV)) and much reduced crosslinking density. BE 615 (Ex BIP) is a n-butylated melamine resin of medium reactivity. Application conditions will again be much the same at 4-6 gsm and bake conditions of 150°C for 10 minutes at pmt.

Under certain circumstances, even greater flexibility might be required from the decorated sheet. Under such conditions vinyl systems may be used.

Disperse items 1-3 in a Cowles mixer or bead mill. Then disperse items 4-7 in a similar manner. The vinyl white is formulated at a much reduced pigment to binder ratio of approximately 0.8:1 for optimum opacity. The appearance and applied cost of this type of coating is significantly inferior to the alkyd system previously discussed, and it is possible to compromise the properties of both systems by blending vinyl and alkyd resins.

One of the major disadvantages of both vinyl and alkyd systems is the potential loss of colour on overbake. This is particularly significant in general line systems, where multiple ink stovings are commonplace. Acrylic coatings were the first to gain favour,
showing good flexibility, adhesion and colour retention. These systems also offered excellent performance when used in conjunction with UV cured printing inks.

FORMULATION 4-16: ACRYLIC COATING

130-140" F4 @ 25°С

61.0 + 2.0%

15-17 gm/m2

140°C at 10 minutes peak metal temperature

Disperse items 1-3 by a Cowles mixer or bead mill. The above formulation offers a very good standard of flow and opacity. The RD-IS grade of titanium dioxide is readily dispersed in the acrylic resin and is well suited to the Cowles process. Uracron CR 203M1 (Ex DSM) is a hydroxy functional acrylic resin which gives optimum properties with benzoguanamine resins in the ratios 90/10 — 85/15 (solid/solid). The acrylic is crosslinked with a benzoguanamine resin [Uramex BF892 BF (Ex DSM)] for improved flexibility, whilst the high molecular weight LTH polyester resin (Ex Huls) gives improved adhesion. The ratio of acrylic/benzoguanamine is slightly different to that recommended (80/20), because the polyester resin will also be crosslinked, thereby removing some of the available functional groups on the benzoguanamine for the acrylic resin. Additional flexibility can be obtained by incorporating a small percentage of a low molecular weight, liquid grade epoxy resin. Nacure 155 is chosen as the acid catalyst for the system rather than the more commonly used PTSA. This is thought to offer more complete crosslinking between the acrylic and benzoguanoamine resins and discourage self-condensation of the amino resin.

As further economies were sought in the industry, the elimination of size coat was investigated. Acrylic coatings were found to be well suited to this process, showing properties of good adhesion and wetting over plain tinplate.

For operatives who had traditionally worked with alkyd systems, the acrylics received some criticism on account of the odour. For this reason, polyesters were found to offer flexibility advantages over acrylics, but suffered from pinholing tendencies and proved to be more critical for adhesion in the case of UV printing inks.

130-140" F4 @ 25°C

60.0 ± 2.0%

15-17 gm/m2

180°C for 10’ peak metal temperature

Disperse items 1&2 using a Cowles mixer or bead mill. The polyester resin SN800 S2G3-60 (Ex DSM) can be cured with HMMM at a 85/15 (solid/solid) ratio or by a butylated benzoguanamine (Uramex BF829 В [Ex DSM]). An acid catalyst such as PTSA (2-2.5% on solids ) would be incorporated for curing with HMMM. In this polyester formulation, a butylated guanamine resin is used at the middle of the recommended range for Uramex BF829 В of 80/20 ( 70/30-90/10 recommended). The high boiling benzyl alcohol is used for a combination of flow and viscosity stability. The hydrocarbon solvent is split between Solvesso 150 and Solvesso 200, thus giving both good flow and solid content in the coating, as Solvesso 150 gives faster viscosity reduction, CAB 551-0.2 is used to stop pinholing and to give assistance to UV ink adhesion.

With both the acrylic and polyester coatings, the tendency has been to retain the use of the traditional alkyd over print varnishes. This is mainly for reasons of economy and good application over wet ink.