VENTAJAS E INCONVENIENTES DEL USO DE CFD

Polymers based on acrylic acid have been known since the 1930s but it was not until the late 1970s that the use of thickening agents based on them came into prominence in textile printing [369]. Typical repeat units are shown in 10.118 and 10.119. Commercial linear products represented by 10.118 can have n = 50–750 but crosslinked grades of higher relative molecular mass are also available. The products represented by 10.119 cover a range of n values from 3200 to 30 000. Only the longer-chain grades are of significant interest for textile printing in the form of their sodium or ammonium salts. However, the scope for

CH₂ CH CH CH₂ C

C

O OH

O OH

n 10.118

CH₂ CH CH₂ C

O OH

CH C OH O

n 10.119

CH₂ CH CH₂ C

O OH

CH CH₂ CH C

O OH

CH CH₂ CH CH₂ CH CH₂ CH CH₂

C C

O OH O OH

CH CH₂ CH CH₂

C

O OH

100 100

100

100 100

10.120

forming copolymers and crosslinked variants is virtually limitless. A schematic representation of a crosslinked copolymer, using acrylic acid and divinylbenzene at a molar ratio of 100:1, is shown in 10.120 [370].

It is important to make a clear distinction between acrylic binders used in pigment printing and acrylic thickeners. Binders are generally copolymers and usually contain an integral crosslinking agent [371]. The thickeners also find their greatest use in pigment printing. Their biggest drawback is their sensitivity to electrolytes, although this is less of a problem in pigment printing than in printing with dyes. The sensitivity of poly(acrylic acid) to electrolytes can be reduced by copolymerising with acrylamide [371], although only relatively small proportions can be incorporated before a deterioration in thickening efficiency occurs. Two important and interrelated parameters for acrylic thickeners are relative molecular mass and degree of crosslinking. Simply increasing the molecular mass of linear poly(acrylic acid) yields thickeners that give stringy pastes unsuitable for use in printing. Hence a degree of crosslinking is necessary to minimise stringiness by decreasing the water solubility and promoting dispersibility. Figure 10.50 illustrates the effect of crosslinking for three acrylic acid polymers of the same molecular mass [371]. The balance of molecular mass and degree of crosslinking influences other properties, such as degree of penetration, levelness of ground colours and sharpness of the print.

These products are usually supplied to the printer as partially neutralised polyacids.

Further neutralisation is carried out by the printer when making up the print pastes. This neutralisation is often a critical process. For certain applications, as with resin-bonded pigments, neutralisation is carried out with ammonia. This has the advantage that during subsequent baking the ammonia is driven off to liberate the free polyacid, which then catalyses activation of the resin binder. In other cases neutralisation is carried out with non-volatile alkalis such as sodium hydroxide. It is particularly important to use the latter in reactive printing, since ammonia would be evaporated off during fixation leading to a lowering of pH and consequently poor fixation. Moreover, reactive dyes can be deactivated by reaction with ammonia to form their non-reactive amino derivatives.

The commercial success of acrylic thickeners in pigment printing is attributable to the

Viscosity

A B

C

A B C

Highly crosslinked Moderately crosslinked Slightly crosslinked

Thickener concentration

fact that they can be designed to give properties very similar to those of emulsion thickenings. These were previously the only systems used in pigment printing and they are dealt with in section 10.8.3. It is important to realise that an acrylic thickener intended for use with pigment systems may be unsuitable for use with dyes. This is because commercial thickeners, available as solutions, emulsions, liquid dispersions or powders, often contain additional chemicals to improve their stability and performance in particular systems. For pigment systems, for example, the thickener may also contain additives (surfactants or polyelectrolyte dispersing assistants), such as the ammonium or sodium salt of linear poly(acrylic acid) (10.121), which not only modify the behaviour of the acrylic thickener but also assist dispersion of the pigment [371]. Surfactant additions are undesirable with reactive dyes because they promote colour bleeding, whilst the ammonia is undesirable because of deactivation of reactive groups, the lowering of pH that occurs by its evolution during the fixation process and the subsequent difficulty in washing-off of the residual thickener, now bereft of its solubilising ammonium ions.

Figure 10.50 Effect of crosslinking on thickener efficiency [371]

CH₂ CH C

O O M

n 10.121

M = Na or NH₄

_ +

A major drawback of synthetic thickeners when used with dyes is their sensitivity to electrolytes. Most soluble dyes behave as highly ionised electrolytes and disperse dyes contain anionic polyelectrolyte dispersing agents unless they have been formulated with nonionic systems specifically for use with acrylic thickeners. Consequently there is a loss of viscosity; this can be quite pronounced although it depends on circumstances, particularly on the dye concentration. As already mentioned, this can be alleviated to some extent by copolymerisation with acrylamide during manufacture. Otherwise it is necessary to try to eliminate all electrolytes from the system or to increase the concentration of thickener. Such measures have their limitations in practice, however. Alternative synthetic thickening

CH₂ CH₂ CH CH

C C

O O O

n 10.122

agents include poly(vinyl alcohol) and copolymers of maleic anhydride with alkenes (10.122).

A detailed comparative study of the rheological properties of four acrylic thickeners varying in relative molecular mass from 1.25 × 10⁶ to 4 × 10⁶ and of two crosslinked ethylene-maleic anhydride copolymers has been published [345]. In respect of some properties, comparisons were also made with a starch ether and an alginate. Amongst other factors, the influence of molecular mass was demonstrated (Figure 10.51), showing that the higher the molecular mass of the acrylic polymer, the less the amount of thickener required to achieve a given viscosity. Nevertheless, earlier comments in relation to the stringiness of linear acrylic polymers should be borne in mind, i.e. factors other than viscosity need to be considered. Viscosity develops as water is absorbed, causing swelling and rearrangement of the polymer chains, a process that is assisted by, indeed is critically dependent on, neutralisation. Figure 10.52 gives a schematic representation of this swelling and also illustrates the similarity in behaviour with oil-in-water emulsions [371]. It is important that the degree of crosslinking is the optimum required to maintain the polymer in this swollen state and prevent it from dissolving, which would result in loss of desirable properties.

0.2 0.6 1.0 1.4

Thickness concentration/%

10 20 30 40 50

Viscosity/Pa s

M_r 4 × 10⁶ M_r 3 × 10⁶ M_r 1.75 × 10⁶ M_r 1.25 × 10⁶ Acrylic thickeners

Crosslinked ethylene–

maleic anhydride copolymers

No 1 No 2

Figure 10.51 Comparison of rheological behaviour of acrylic and copolymeric thickeners [345]

Dispersed polyacid/pH 4

Neutralised, swollen polyacid/pH 9

Kerosene emulsion in water Base

Figure 10.52 Schematic diagram illustrating swelling of polyacid thickener and comparison with oil-in-water emulsion [371]

A novel concept [371] was evaluated to seek a replacement for alginates in printing with reactive dyes. This approach utilised copolymers of poly(acrylic acid) onto which starch has been grafted by free radical polymerisation, the free radical initiator being a potassium persulphate redox system. Both native starch and starch pre-oxidised with sodium hypochlorite were included in the study. It was found that an acrylic–starch graft copolymer effectively replaced 25% sodium alginate, whilst a copolymer of poly(acrylic acid) and oxidised starch replaced 50% sodium alginate. A detailed rheological study was presented but the evaluation lacked economic analysis.