Лако-красочные материалы — производство

When used as a surface coating, an emulsion must form a coherent film. The evaporation of water during the final stages of drying causes stress in the film and a brittle film will crack under this stress. The temperature at which no stress cracking occurs is termed the minimum film forming temperature, MFFT or MFT. This is measured by a MFFT bridge which consists of a stainless steel slab with a slight hollow running along its length which can be filled with emulsion. Each end of the slab is turned down to provide a contact with a thermostatically controlled water bath; three or four small cavities along the length of the slab are used to house thermometers.

If the two bath temperatures have been chosen correctly, the emulsion will dry to a continuous film at the hot end of the bridge and will be cracked and crazed at the cold end. The boundary between the cracked and intact films occurs at the minimum film forming temperature. The MFFT is generally slightly lower than the Tg because of the plasticising effect of water on the polymer film.

It is critical that the MFFT is below the temperature prevailing during the drying of the film, otherwise a discontinuous, hazy film is formed because the polymer did not film form at that temperature. The formation of a polymer film form or latex film is different to all other types of film formation. Not only has the water to disappear by evaporation or absorption into substrate, but during this process the polymer particles must fuse together.

A polymer emulsion consists of polymer particles suspended in a relatively mobile liquid. The particles are free to move but electrostatic charges prevent particles approaching each other too closely. As the water evaporates, the volume of water separating the particles decreases and the particles are forced closer together. Electrostatic repulsion is reduced by the counter ions, generally sodium, associating with the particle charges. At some stage during the drying, the particles become so tightly packed that they cannot move and a gel structure results. This is the flocculation stage of drying and the gel is dispersible in water. Further drying causes coalescence of the particles which is an irreversible process. The rate of water loss is relatively fast up to the flocculation stage and varies little with particle size. Subsequent drying is much slower with smaller particle sized latices drying more slowly than larger ones.

The latex may be 50% non volatile content which means that 50% of the initial coating is water. As this is removed, the particles are physically drawn together on a simple shrinkage basis. However, a point is reached where the capillary forces of attraction take over and these forces are strong enough to overcome the latex stabilisation forces. The polymer particles touch and for coherent film formation to occur the polymer particles should flow into one another. This is called coalescence. The remaining water should be removed during this stage. Lack of the particles cold flowing gives rise to poor films.

It is often stated that coalescence is caused by surface tension forces, but it is probable that these forces only operate in the final stages of particle coalescence. The pressure needed to cause particle deformation prior to coalescence may be caused by film contraction caused by loss of water. Soft films deform more readily than harder ones and this means that soft polymers require less energy to start coalescing. When coalescence starts, soft polymers flow more easily and therefore integrate more readily into the film.

In an emulsion the interfacial tension between the polymer and water is reduced by surfactact at the interface. Once the water has evaporated the surfactant will tend to form inverted micelles with the hydrophilic groups forming the core. Sulphate end groups and any grafted surfactant or colloid on the particle surface will provide a nucleus for such inverted micelles. Most colloids and some ionic surfactants are not compatible with polymers and will form a dispersed phase giving a cloudy film. Most nonionic surfactants and polyvinyl pyrolidone are soluble in polymers and would not reduce film clarity. It is often claimed that large particle size emulsions give films with a lower tensile strength than that for a smaller particle emulsion of the same polymer. The argument runs that large particles do not fully integrate during drying of the water and that the discontinuity in the film reduces the film strength. The gloss of an unpigmented film from an emulsion polymer increases as the particle size decreases. This finding has led to a range of small particle size polymer emulsions being suggested for gloss or semi-gloss applications.

Like the theory of emulsion polymerisation, there are many differing theories about the mechanism of film formation. The reader is advised to consult specialist works for more details.

The common themes from many of the theories relate to the following ‘driving forces’ for coalescence.

a) Capillary forces

b) Surface tension of both polymer/water and water/air interfaces

c) Inter-diffusion of both polymer and surfactant molecules between contiguous particles

d) The resistance of any particle to physical deformation (i. e. ‘cold flow’).

The improvement of film properties with respect to time is believed to be evidence for autohesion (inter-diffusion) where residual levels of water and solvent do not vary.

Water can be considered to be removed in three stages. Van der Hoff et al<6) defined the three stages as;

i) an initial constant evaporation stage (or absorption) where the particles still retain some mobility

ii) an intermediate stage, when particles start to come into irreversible contact and the rate of water evaporation is much less than that in stage i) (some 5-10% of the rate)

iii) loss of water from the film by diffusion which is very slow and can occur over days. It is believed that the water diffuses along hydrophilic networks.

Loss of water by evaporation alone means that water at the surface of the latex is lost first creating a concentration gradient between the top and bottom of the film. Coalescence occurs first on the surface and further loss of water is controlled by the rate of diffusion (evaporation rate is higher than diffusion rate).

When the substrate absorbs water, water is lost from the base of the film until the substrate reaches saturation point. Obviously the absorbency of the substrate is important to the mechanism of the film formation. If the substrate is too absorbent then all of the water is removed before coalescence can commence. An everyday example to overcome this is the excessive dilution of a latex paint for the first coat on a very porous substrate.

Thus, the initial removal of water from the film depends upon the balance of loss by evaporation and absorption. It is important to realise that at some point during film formation an irreversible change must occur for the coating to be of any practical use.

Coalescing solvents are believed to work in two ways. Sullivan<7) considered two types of coalescing solvents and their different mechanisms. The film is considered to have a hydrophilic network where polar coalescing solvents like ethylene glycol partition, and a polymer phase where solvents like ethylene glycol monobutyl ether acetate partition.

Ethylene glycol facilitates coalescent evaporation because it swells the hydrophilic network, thereby creating a larger pathway for diffusion. The rate of initial water evaporation is unaffected by coalescing solvents, but they may retard final evaporation. The more polar the solvent, the faster it will evaporate. The use of a water soluble solvent enables water evaporation to be faster than the rate of diffusion during the final stages. In the absence of solvents, or in the presence of water insoluble solvents, water evaporation is diffusion controlled.

Thus, most practical latex systems require small additions of coalescing solvents to increase the drying rate. These solvents are generally not retained in the film despite having low evaporation rates. It is possible to mix the solvent so that one partitions to the hydrophilic network and the other the polymer phase. Surprisingly, they do not interfere with the action of each other.

Obviously, the particle size is important because it affects the capillary and other film formation forces, However, varying particle size has minimal effect on MFFT. The Tg of the comonomers and the presence of external plasticisers are most important for film formation.

Sullivan(7) gives a detailed treatment of the effects of water and solvent evaporation from latex and pigmented latex films.

Colloids have an important effect upon film formation. Achievement of physical properties of a film occurs more rapidly in the presence of colloids, with colloidal particles bridging gaps between polymer particles. Indeed, colloidal particles can fill residual voids in the resin matrix, thereby increasing the strength of the film. Colloid free systems can take days before optimum strength is attained.

When porous substrates are to be coated, it is important that the particle size of the latex is larger than the size of the pores, to stop the particles being absorbed into the pores. It is in this area that colloids can have an important effect on film formation. Colloid containing latices tend to have a larger particle size than a colloid free system. In addition, the colloids can be large enough to stop absorption into the pores by bridging the pores.

The pigment volume concentration influences solvent evaporation. Below the critical pigment volume concentration (CPVC) non-porous pigments act as barriers to solvent passage, whilst above the CPVC voids and polymer discontinuity affect solvent loss. At concentrations around the CPVC solvent evaporation rates are minimal.

Each emulsion polymer or co-polymer is characterised by its minimum film forming temperature (MFFT), which, if found to be too high for a given application, can be lowered by the addition of plasticiser or high boiling point coalescing solvent.

The MFFT of the emulsion as formulated must be lower than any temperature at which the paint is likely to be applied. For an indoor paint in the UK it is unlikely that temperatures below 10°C will be encountered, whilst the outdoor limit might be 5°C. Damage to paint films can occur should the ambient temperature fall below the Tg of the film. The Tg of a hard polymer, such as vinyl acetate, styrene or methyl methacrylate, can be reduced either by copolymerisation with a soft monomer or by adding an external plasticiser, such as dibutyl phthalate. On a cost effective basis external plasticisation is better for reducing the Tg. The snags are, firstly, that plasticisers are slowly lost through evaporation and, secondly, they can also diffuse into the substrate. The use of an internally plasticised polymer overcomes both problems and also saves the processing time necessary to incorporate an external plasticiser.

Ethylene, VeoVa and 2-ethylhexyl acrylate are most often used to soften vinyl acetate. Ethylene is the cheapest of the three monomers and is also the most efficient softener (see Table 7-7). It requires a very large capital investment for a high pressure reactor and the relatively long reaction times are expensive. 2-ethylhexyl acrylate is difficult to incorporate because of adverse reactivity ratios and it also gives the weakest films of the three copolymers. Veova is the easiest of the three monomers to incorporate and it gives films with surprisingly good water and alkali resistance. It is not an efficient softening monomer on a weight basis and is very inefficient on a cost basis. The use of alpha-olefins such as 1-hexene or 1-dodecene is popular in patents, but the olefins are very difficult to polymerise by a free radical mechanism and they are not used commercially.

Acrylic ester polymers and their copolymers with styrene or methyl methacrylate can be readily prepared with a wide range of Tg’s, but vinylidine chloride presents problems. Its Tg is normally given as 255°K, which is the glass transition temperature of the amorphous polymer. Polyvinylidine chloride behaves as a crystalline polymer for which the glass transition occurs at about 370 — 410°K and calculations of glass transition temperatures are pointless for this monomer.