Pigment dispersions Binder systems

The binder film in a pigment print is a three-dimensional structure, the third dimension of which is of rather less importance than the other two. The binder is a film-forming substance made up of long-chain macromolecules which, when applied to the textile together with the pigment, produce a three-dimensionally linked network. The links are formed during some suitable ‘fixing’ process, which usually consists of dry heat and a change in pH value, bringing about either self-crosslinking or reaction with suitable crosslinking agents. The degree of crosslinking should be limited, to prevent the macromolecules becoming too rigidly bonded, thus preserving some extensibility.

Binder systems

The important criteria, which ensure that the pigment within the crosslinked binder film is fast to wear and cleaning, are elasticity, cohesion and adhesion to the substrate, resistance to hydrolysis, as little thermoplasticity as possible and absence of swelling in the presence of dry-cleaning solvents. The binders used are all addition polymers, preferably copolymers such as structure The technique used is that of emulsion copolymerisation, which leads to a product containing 40–45% binder dispersed in water. These ‘dispersion binders’ look like milk, are comparatively readily produced, and can be easily transported for use. Moreover, they have the advantage of high concentrations of active binding substances, together with low flammability because they contain no organic solvents.


Depending upon the properties required in the binding film (softness, elasticity, plasticity, solvent stability, light and weather fastness), binders can be ‘tailor-made’ by choosing suitable base products [3,4]. Typically, unsaturated monomers are used, such as vinyl chloride, dichloroethene, acrylic acid, methacrylic acid, acrylamide, acrylonitrile, acrylic acid esters, vinyl ethers and vinyl esters, styrene and diolefins like butadiene. The monomers are dispersed by using sufficient amounts of suitable surfactants, and polymerisation is initiated by means of free radicals originating from redox reactions such as that between potassium persulphate and sodium bisulphite. These add to the monomers, producing more radicals that also have the capacity to accumulate monomers. As a result of this reaction, macromolecules in chain form are produced. Their growth is limited either by the combination or disproportionation of two radicals, or by chain transfer to a monomer or to another macromolecule to give branching. The addition of ‘regulating’ substances can influence, if necessary, the length of the polymer chain, in order to give the required properties to the end-product or to prevent premature crosslinking.


The size of dispersed polymer particles is determined for the most part by the type and amount of surfactant present during [CH2 CH CO OC4H9 CH2 CH CO OC4H9 CH2 CH CO OC4H9 CH2 CH]n CN 5.1 05.p65 143 4/12/02, 4:59 pm 144 DIRECT PRINT COLORATION polymerisation, and in the case of mechanically stable dispersion binders is in the region of 120–300 nm. While the prints are being dried, a film is formed from the dispersed binder. Its formation takes place in two stages: flocculation (or coagulation) and coalescence.

During the first stage of film formation, water and surfactants are removed from the binder by absorption and evaporation. The dispersed solids coagulate to form a gel-like layer of very tightly packed ‘balls’, which have only poor solidity and adhesive properties. If the mechanically more stable, more redispersible, dispersion binders are used, these coagulated particles can be brought back to their original form by rubbing them with water. During the second phase, the gel particles flow together to form a continuous film. The lowest temperature at which a film can be formed depends upon chemical constitution, but for pigment printing it is usually around 5°C. The speed at which the film is formed depends upon the range of particle size.


 Poly(butyl acrylate), for instance, can form a film at 0 °C, whereas the more polar polyacrylonitrile is a very poor film-former even at high temperatures. For pigment printing, such a film would require to be softened by copolymerisation with, for example, butyl acrylate, in a ratio of butyl acrylate:acrylonitrile in the range 3:1–5:1. The higher the ratio the softer the film becomes, but at the same time it becomes more thermoplastic and develops poorer fastness to dry cleaning. Binder systems for w/o pigment printing have been based on the reaction products of polyols with saturated and unsaturated mono- and di-carboxylic acids, combined with the hydrophobic butyl ethers of urea– or melamine–formaldehyde condensates. More recently, emulsion copolymers based on butadiene have been added to improve the dry-rubbing fastness of the prints. Binders can also be made from high-r.m.m. polyols (mainly based on polyethers) and di- or tri-isocyanates.


 The polyurethanes thus obtained produce a soft, elastic binder film with excellent binding powers and fastness properties, but the printing pastes have to be water-free. These have a very short pot-life and are of interest to the textile printer only for special articles, since in general there is a preference for printing pastes that present no problems when applied and which can be stored for a long period of time. Although it has been possible to produce polyurethanes in the form of aqueous dispersion binders, by incorporating hydrophilic groups, these have not been able to compete with the dispersed binders obtained from emulsion copolymerisation because of their relatively high cost. Crosslinking Elasticity and improved adhesion of the film to the substrate is achieved by 05.p65 144 4/12/02, 4:59 pm PIGMENT PRINTING 145 crosslinking.



The crosslinking reaction must produce covalent bonds which are insensitive to hydrolysing agents (washing liquors, body sweat, industrial atmospheres). The reaction should be activated only during fixation and not while the binder and the printing pastes are in storage. The simplest crosslinking reaction would be the condensation of carboxyl groups with hydroxyl groups of film-forming macromolecules. The disadvantages of this process are that it needs very high temperatures and an acid medium, and thus entails the risk of the textile yellowing, and that an ester bond is formed which is relatively sensitive to hydrolysis.

Crosslinking through N-methylolamide groups takes place under milder conditions, also in acid media (Scheme 5.1, where B is the principal part of the binder molecule). The compounds formed are relatively fast to hydrolysis as the consequence either of a reaction of methylol groups with each other, or of a reaction of methylol groups with hydroxyl groups which are also present in the binder copolymer.


The reaction equilibrium requires that water is removed from the reaction system. Hot air above 120 °C is therefore suitable for this type of reaction. Steam can have adverse effects on crosslinking and consequently on the fastness of the print. High-temperature steam is of course able to remove water, but it is nevertheless itself water vapour and the reaction cannot go completely to the right. The effect of dry hot air is better.