Saturday, September 18, 2010

The Role Of waxes in coatings and inks

The Role Of waxes in coatings and inks

What is wax?

For centuries, the word wax meant ‘beeswax’ only. It was mainly used for ‘burning’ in candles and also, occasionally, for waterproofing ships. In the 17th imported into Europe from brazil. At that time, the question ‘what is wax?’ could easily be answered. Later, fossil materials such as montan wax, solid paraffin wax and microcrystalline wax here added to the family of waxes.

Today there is a wide variety of waxes available on the market. Many are synthetic and the question ‘what is wax?’ becomes more difficult to answer. Figure 1 gives a survey of the most popular natural and synthetic waxes. Figure 2 gives some insight into the location of ‘wax’ (in this case polyethylene wax) as being a particular molecular weight in the long chain from oil to ‘plastic’.

How waxes work in coatings

Basically, there are two main theories : the ball bearing effect and the floating effect. Schematically, this is illustrated in Figure 3. Both theories are supported by strong arguments.

The ball bearing theory explains the beneficial effect of waxes on mar and scratch resistance in coatings by saying that they only work as long as their particle size is similar to the film thickness of the system, or even a little larger. This theory is backed up by the following observations:

a). Extremely fine waxes are less effective than coarser ones.

b). In very fast drying films, the wax would have no time to float to the surface.

c). In high viscosity coatings, floating of the wax will be impossible because of the very restricted mobility in the film.

These statements are based mainly upon observations in printing inks where thin films, in the range of 3 to 5 microns, are normal. Also, drying times of gravure and flexographic inks are very short indeed. The last argument against floating is the fact that waxes also work in highly viscous coatings, such as offset inks, where it is very unlikely that they could float to the surface.

The Floating Theory assumes that waxes act by virtue of their capacity to float to the surface of the film. This theory is also supported by some strong arguments:

a) Fine particle size waxes (3 to 5 microns) are effective in thick films (well in excess of 100 microns in certain solventless system).

b) ‘Heavy’ components eg PTFE (Specific gravity 2.15), are not always effective in thick films because only a minor part reaches the surface.

c) The flatting effect of waxes is stronger in thick films compared to thin (see Figure 12). It seems that a sort of ‘reservoir effect’ is present: the more wax that is available in the film, the more that can float to the surface.

one may conclude that both ‘religions’ (as is often the case) have some merit, depending upon one’s standpoint, be it inks (thin and fast drying films), or coatings (thicker films with usually longer ‘open’ times)

How To Get The Wax Into The System

The performance of waxes is coatings and inks depends to a great extent on the method of incorporation. Here again – there is no ‘one and only’ correct method. Figure 4 shows the four techniques most frequently used.

Figure 4: Techniques of incorporation

1). Molten Wax: although most waxes are very resistant to solvents at room temperature, many of them can be dissolved in hot solvents. Melting the wax was one of the hot wax and solvent method is that only solvents which have boiling points higher than the melting point of the wax can be used. One can get around this problem by using closed, high pressure kettles but this calls for special equipment not normally found in coatings or (ink) plants.

In addition it is very difficult to achieve ‘batch to batch’ uniformity with this method because it depends largely on the rate of cooling of the molten wax. (….), more and more companies have abandoned this troublesome technique and changed to another incorporation method or else buy ready made Wax melts from specialist producers.

A further limitation of the melting method is that PTFE cannot be used as it has no melting point at all.

2). Emulsified waxes: it is also possible to emulsify wax into water. Even more than the ‘molten wax in solvent’ technique, this calls for specialized know-how and equipment. Once again PTFE cannot be used. Besides the limitation that they can only be used in water borne systems, wax emulsions also contain a certain percentage of surfactants, which can have a negative effect on the water resistance of the coating or ink.

3). Dispersed Waxes: Grinding the wax into solvent and/or vehicle in a ball mill – or triple roll mill – is a frequently used method. It has the advantage that it is suitable for practically all wax/solvent/resin combinations – including PTFE. Though the equipment needed is conventional, it still requires special knowhow to achieve uniform results. Besides this, batch sizes are often too small to justify such an operation within a paint or ink factory. Here again specialized wax (………..) houses can often do a better job.

Wax dispersions have another advantage: because the wax particles are rounded off they have even less influence on gloss than the same wax in micronized form.

4). Micronised Waxes: the most popular incorporation method is certainly the use of micronized waxes – if a micronized version of the wax is available. Not all waxes can be micronized: they must be ‘brittle’ enough.

Because 100% active material is bought, there is no limitation at all as far as compatibility is concerned: and the same wax can be used for all kinds of organic solvent, or solvent free systems – as well as aqueous coatings.

Incorporation can be effected by means of a high speed impeller in a few minutes. A further advantage of micronized waxes - …….- is their matting ability, provided that the right kind of wax is selected.

Why Are Waxes Used?

The following properties are obtained by small additions of waxes to coatings and inks:

• Improved mar resistance and resistance to metal marking.

• Silky and soft feel of the surface

• improved anti-blocking effect.

• enhanced slip

• prevention of hard settlement when used in combination with silicas

• improved water repellency and less dirt pick-up.

• good matting ( with proper wax selection )

Numerous coatings and ink systems will therefore show improved surface properties with small wax additions. Some examples are: can and coil coatings, furniture acquers, stoving and air drying enamels, leather finishes, wood stains, gloss and semi-gloss latex paints. UV curing coatings and inks, textile finishes, overprint varnishes. Flexo, gravure, and offset inks, and last but not least, powder coatings.

Although all waxes will, to a greater or lesser extent, give these surface improvements, there are very important differences between the various waxes and, as previously mentioned, the method of addition.

What Is The Best Wax?

This question is as easy to answer as the question ‘what is the best resin?’ or ‘what is the best pigment?’ the answer is the same: it all depends on the specific properties you require and the system in which the wax is to be used. Of course there are some basics involved when selecting waxes. One of the main functions of a wax in a coating system is lowering the ‘friction’ or, in other words, the surface energy. Figure 5 demonstrates the surface energy of various materials.

Figure 5: surface energy

It is obvious that PTFE has by far the lowest surface energy. It is even lower than silicone-oils and compounds. This, together with its high heat resistance, explains the widespread use of PTFE in frying pans and cake tins (anti blocking!). its low coefficient of friction makes it an ideal coating for highly sophisticated skis. Both are used in PTFE tapes for sealing purposes.

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