Лако-красочные материалы — производство

Waterborne resins for surface coatings are not only prepared by an emulsion copolymerisation technique. They can be prepared using a solution technique with a water soluble organic solvent, or in water. This is the least common.

The solution technique uses water soluble solvents, such as alcohols, (e. g. butanol, propanol) glycol ethers (e. g. butyl glycol), and acetates of glycol ethers (e. g. Dowanol range), following the same principles and techniques used for conventional solvent based systems. The two major differences are the inclusion of a functional comonomer, to induce water dispersibility, and neutralisation and dispersion after polymerising the resin in solution.

The most common functionality is acid with methacrylic or acrylic acid normally being used to give the acrylic resin an acid value of about 50-80 mg KOH/g. The resin manufacturer may offer this acid functional resin in solution to formulators of coatings for them to neutralise and disperse in water. Alternatively, the resin manufacturer may partially or fully neutralise the acrylic resin, with an amine or ammonia before dispersing it in water.

This gives a much lower viscosity product than the un-neutralised resin solutions. With the need for lower and lower VOCs, less water soluble solvent must be used for the solution stage resulting in higher and higher solids. Unlike some emulsions, water reducible systems will always contain some solvent, However, solvents are also frequently added to emulsions. Many people refer to the dispersing of an organic solvent based resin into water as inversion. The neutralised particles are ‘solubilised’ by the amine. Because this approach can be done by both resin manufacturers and coating formulators, the process of neutralisation, suitable anions and other details are considered in the waterborne application section (Chapter VII).

Before considering some formulations in detail, it should be remembered that all of the comments which applied to solvent based polymerisation also apply to solution polymerisation in water soluble solvents, with the added problem of limited choice of suitable solvents. These solution polymerisations may or may not be run under reflux conditions, depending upon the solvent present.

The alternative route of preparing water reducible resins using emulsion polymerisation of an acid functional acrylic resin has already been considered under emulsion example 7 — preparation of a water soluble acrylic copolymer.

The majority of water reducible resins prepared by the solution route are for thermoset applications if they are to be used as the principle film forming resin. As such hydroxyl functionality is normally incorporated if the resin contains acid functionality for neutralisation, which is normal. Typically, the hydroxyl value would be of a similar value to the acid value, i. e. 50 — 100 mg KOH/g. Note that there are two methods of quoting acid and hydroxyl values for vinyl and acrylic solution resins. Values can be quoted

either as determined in the solution or corrected for the measured non-volatile content. Unlike resins formed by acid or hydroxyl reactions (polyester, alkyds, etc.) the acid and hydroxyl values should not change during processing.

It does not matter in practice which value is quoted, provided it is clear whether it relates to solution or solids. However, it is normally preferred that the values on 100% solids are used because this automatically removes any variation from variation in non-volatile content.

Electro deposition systems (which are considered later) are the only main exception to acid functional acrylic resins, where cathodic deposition which is increasingly favoured requires amine functional acrylics.

Some of the water based solvents, such as butyl glycol, higher homologues and acetates, have relatively low volatility compared to the traditional solvents used for solution polymerisation such as methyl ethyl ketone, xylene or butyl acetate. Thus, the measurement of solids must be conducted at a temperature high enough and for a time long enough for all of these ’slow’ solvents to evaporate. A fan assisted oven at 150°C for ‘/2 — 1 hour is a good starting point provided no polymer degradation occurs.

Butyl glycol is a unique solvent in that it is not truly water soluble. At about 45-50°C a butyl glycol, water mixture will go cloudy indicating immiscibility/insolubility. Below these temperatures the solution is clear. However, when a third component is present this is not a problem because it normally helps solubilise the butyl glycol. An explanation for the partial water solubility is that butyl glycol consists of both hydrophobic and hydrophillic constituents and is of a sufficiently high molecular weight for them both to have an effect.

Some straightforward examples of preparing water reducible resins follows, due to the similarity with solution polymerisation, comments will only be made where appropriate.

In this example butyl glycol is mixed with butanol. Depending upon the ratios, the reflux temperature will be modified. Chain transfer agent tertiary dodecyl mercaptan is present to control molecular weight. The initiator is added separately being slurried with solvent.

The hard monomers styrene and methyl methacrylate are used. Styrene is lower cost than methyl methacrylate and as much as possible is used to reduce costs. Methyl methacrylate confers certain advantages to the film, depending upon end application, thus it cannot be totally replaced.

Procedure

1. Charge the reactor with butyl glycot^ and butanot^ and heat to 130°C.

2. Commence the addition of the premixed monomers at a constant rate over 3 hrs. at 130°C.

3. Add 90% of the initiator charge simultaneously over the same period of time. Note that if required butanol can be replaced by butyl glycol.

4. Hold at 130°Cfor 1 hour and then add the remainder of the initiator charge.

5. Hold at 130°C until the non volatile content is 49%. If the solids do not increase above 49%, add further booster shots of peroxide. When the non volatile content is > 49%, cool and discharge or cool to 95°C, neutralise and disperse in water in the reactor.

Properties

50+1%

66 mg KOH/g (on 100% solids) 65 mg KOH/g (on 100% solids)

When neutralised with amine and blended with crosslinking agents such as hexamethoxymethyl melamine the following formulation is suitable for clear or pigmented stoving lacquers.

Procedure

1. Charge reactor with n-butanol and heat to reflux.

2. Pre-mix monomers and catalyse and add to the refluxing solvent at a controlled rate of addition over three hours.

3. Check degree of conversion and, if necessary, add additional booster shots of peroxide 0.02% at one hour intervals.

4.

Cool, adjust to required solid content, if necessary.

Another approach to preparing aqueous solutions, is to prepare the resin as an addition solution polymer and then remove or partially replace the organic solvent. Two methods can be used, either distillation of solvent prior to neutralisation of the resin and subsequent addition of water or neutralisation and addition of water prior to distillation of organic solvent possibly as an azeotrope. The major disadvantage of the former approach is the high viscosities of the remaining resin solution prior to neutralisation. As a generalisation, some organic solvent always remains in the final resin solution and normally water miscible solvents are used.

FORMULATION 2-12:

LABORATORY SCALE PREPARATION OF AN ACRYLIC COPOLYMER
CONTAINING FUNCTIONAL GROUPS

30g acrylamide and 60g itaconic acid are dissolved in 300g isopropanol; after filtering

500g ethyl acrylate are added and 6g benzoyl peroxide are dissolved in the mixture.

Procedure

1. 300g isopropanol are added to a two litre flask fitted with a stirrer, reflux condenser and tap funnel.

2. The flask is heated until the isopropanol starts to reflex when the monomer mixture is added at a constant rate over two hours; reflux is continued for a further 30 minutes giving 97% conversion.

3. The reflux condenser is moved and set for distillation.

4. 450g isopropanol are distilled off when 200g deionised water are added. Distillation is continued to remove the balance of the isopropanol as an azeotrope containing 92% isopropanol.

5. 80g of a 27% solution of ammonia and 300g water are added with stirring giving an aqueous solution of49-50% non-volatile content.

The larger proportion of initiator as compared with some of the examples, tends to give more rapid reaction and a lower molecular weight. Note that the method of monomer addition allows a large proportion of the copolymer to have a random sequence of monomer units with the overall composition of the added mixture. Isopropanol is often used to produce copolymers of low molecular weight owing to its high chain transfer activity, although quantitative values are not available. Another interesting point in this example is the distillation of the (expensive) solvent and the eventual production of a solution in water which is very cheap. The distillation would not be possible for a higher molecular weight polymer and it does require a high energy input.

It should be noted that this is an expensive production route even if the solvent is reclaimed and re-used. Only in extreme cases is this approach used industrially.

The neutralisation of this resin and the blending of a crosslinking agent would be as follows. This material is suitable as an anti-corrosive water based spray applied coating, the formulation of which is given in the Waterborne Chapter (Chapter 7).

FORMULATION 2-13:

PREPARATION OF AN AQUEOUS POLYMER