Лако-красочные материалы — производство

Some peroxides can be attacked and directly decomposed by the action of solvents. In general for an initiator is increased in the presence of solvents in order of chemical type, as shown below.

Alcohols < ethers < aliphatic hydrocarbons < aromatic hydrocarbons < halogenated solvents.

Factors of two or three between t^ in the different solvents are not uncommon.

Increasing pressure tends to increase t^. Under normal polymerisation conditions, the effect of pressure is insignificant when compared with the effect of temperature on free radical generation. However, where gaseous monomers are involved, the polymerisation is often carried out under conditions of high pressure, and the effect of pressure on Ц becomes more relevant.

In practice, an initiator with a t^ in the range 30 minutes — 2 hours, is preferred for commercial processes.

Azo initiators such as AZDN do not cause the oxidation effects which can occur with peroxy compounds, particularly where labile hydrogen atoms are present (e. g. acrylate monomers).

As a general rule, a greater degree of graft polymerisation takes place, when peroxy compounds are used as initiators, compared to circumstances involving the use of azo compounds. This is due to the greater potential for hydrogen abstraction when peroxides are used as initiators, than when azo compounds are employed as polymerisation initiators.

The use of AZDN is accompanied by the release of nitrogen into the reaction mixture. This would not normally present a problem, but where the polymer is produced by a suspension or bead polymerisation process, the presence of nitrogen can lead to voids being formed in the polymer bead. These alter the specific gravity of the particle causing it to float. This leads to practical difficulties, and yield loss when filtering and collecting the final polymer.

1. Solvent Selection

This section deals with solvent selection in general terms with respect to the basic principles on which the choice is based. Solvents and their effects are dealt with in more detail in the chapters dealing with the applications of solvent based coatings.

The principle factors affecting solvent selection for a specific polymer and end use are cost, solvent power, evaporation rate, flash point, toxicity and customer requirements.

The popular, cheap solvents are hydrocarbons such as xylene, alcohols such as the isomers of butanol and propanol, and ketones such as methyl ethyl ketone and methyl isobutyl ketone. The more expensive solvents used tend to be esters such as acetates, ethers and ethoxylated ethers such as butyl glycol (butoxy ethanol). Other speciality solvents are also occasionally encountered, but generally the quantities used are small due to the cost factor.

The viscosity of a polymer solution is lowest in a “good” solvent and highest in a “poor” solvent for a given molecular weight. There is often a synergistic effect when more than one solvent is employed, particularly where aromatic hydrocarbons or chlorinated solvents are involved, enhancing the solvating power of the solvents involved and resulting in a solvent combination which performs as a better solvent than their solubility parameters would predict. Similarly, mixtures of solvents often perform better than a single solvent.

The evaporation rate of the solvent is an important factor in some film forming mechanisms. In general, the evaporation rate depends upon the vapour pressure of the solvent which will decrease with increasing boiling point. Solvents generally evaporate more slowly than their vapour pressure would suggest.

The flammability of a solvent is normally measured by its flash point, which is defined as the temperature at which the mixture of air and solvent vapour above the solvent will just ignite. There are two distinct types of apparatus used to measure this parameter. One is a closed cup, which contains the vapour as it evaporates, and the other is an open cup where the vapour is free to escape into the atmosphere. Both methods are frequently employed and it is normal to quote the method used when recording the results of flash point determinations because different values result from the different methods.

A table comparing boiling point with flash point for some common solvents is shown below:

The film forming of coatings based on polymer solutions is designed to give a suitable application time. When a mixture of solvents is used, a change in solvent composition may occur during film formation. Should this change result in partial precipitation of the polymer, film defects can result.

When formulating with solvent blends, care must be taken to ensure that the polymer will remain in solution throughout the drying process. This can be achieved by ensuring the best solvent of the mixture has the highest boiling point (and hence, lowest evaporation rate) of the solvents used. The degree of humidity of the atmosphere will affect solvent evaporation rates. Solvent evaporation will cause a temperature drop at the film surface and at high humidity, this may result in the surface temperature falling to below the dew point of the atmosphere. This will cause water to condense on the surface of the film, causing localised precipitation of the polymer from solution.

Water is the ideal solvent from the cost and pollution viewpoints, but it is a non-solvent for many surface coating polymers. It will dissolve a small number of homopolymers, notably those derived from acrylamide, acrylic acid, itaconic acid, vinyl methyl ether, vinyl pyrrolidone and vinyl sulphonic acid, but none of these homopolymers forms flexible films of use in the coatings industry. While copolymers of acrylic or methacrylic acids with acrylate esters are generally insoluble in water, their salts are soluble when the acid content is over 5% (for hydrophilic monomers) and 12% (for hydrophobic monomers). Such polymers can be prepared in solution, or in emulsion, but not in aqueous solution. This is because the acrylate esters are insoluble in water. The acid is copolymerised in the un-ionised form because the ion is unreactive to free radicals. In emulsion polymerisation, care has to be taken to avoid homopolymerisation of the acrylic or methacrylic acid in the water phase. Suppression of homopolymerisation requires a low concentration of acid throughout the polymerisation process. This can be achieved by using a long reaction period and slow addition of monomer mixture, or by careful pH buffer selection.

Sodium, potassium and ammonium salts will all solubilise acidic copolymers but, in practice, ammonium salts are used because the ammonia evaporates slowly from the polymer film. After ageing, the polymer film reverts to the un-ionised acid form and is insoluble in water with a lower water sensitivity than the salt form. Another method of reducing water sensitivity is to add certain heavy metal salts of which zinc and zirconium salts are the most important. This technology will be discussed more fully in the section on waterborne coatings.

For waterborne systems, derived from solution polymerised monomers, the solvent system used for the solution polymerisation stage must be fully soluble or miscible with water, otherwise there may be problems during the neutralisation and dispersion of the resin in water.

In the case of aqueous solutions, the rate of evaporation is proportional to the dryness of the air and can be generalised as being :-

rate of evaporation ~ (100 — RH) where: RH is the relative humidity.

At very high humidity water based coatings will not easily dry.

Organic liquids which are not ‘true’ solvents for resins are frequently used in solvent based surface coatings. These are termed diluents and are used in combination with a true solvent for the resin. Alone they will not dissolve the resin, but when added to an existing resin solution they can decrease the viscosity of the system. Diluents are frequently incorporated to improve some aspects of the application characteristics of the coating, such as substrate wetting, particularly on metal. The addition of too much or the wrong type of diluent to a resin solution may result in precipitation of the resin, causing the solution to become hazy and/or separate out. Diluent can be used to reduce the cost of the solvent system and alter the solvent balance.

Solvent power is generally viewed in terms of a solubility parameter approach.

(i) Solubility and Solubility Parameters

Huggins and co-workers related the free energy of mixing between a solvent and a linear homogeneous polymer to:

a) the molar volume of the solvent

b) the molar volume of the repeat unit of the polymer

c) the volume fractions of both polymer and solvent

d) the gas constant (RT)

e) an empirical constant dependent upon the co-ordination number of the polymer lattice

f) a constant representing the heat of mixing for the particular mixture in question

AH — «(^(pp

where: AH is the heat of mixing

К is a constant

cps and фр are the volume fractions of solvent and polymer respectively

Scatchard showed that:

AH = Ф8ФР[ф1-ф^

are the molar cohesive energies of solvent and polymer respectively

are the molar volumes of solvent and polymer respectively.

Combining the above equations:

К = [(~)1/2-(-^)1/2] = (5S-5P)2

5 = (^V)1/2

8s and 5p are Hildebrand’s solubility parameters for solvent and

polymer respectively

E/V is the Cohesive Energy Density

Gibbs Free Energy of Mixing AGm is related to AH„ the heat change that occurs on mixing by the equation

AGm — AHm — TASm

where: T is the temperature in °K

ASm is the change in entropy due to the mixing process

For mixing to occur AGm must be negative.

As a general rule ASm is positive and much smaller than AHm. Thus the magnitude of AHm dictates whether solution occurs or not. As a general rule as AHm approaches zero, then solution will occur because ATSm becomes the dominant factor. Thus, for effective solution (8S — 6P)2 should approach zero. Obviously, this can be achieved by selecting a solvent with a similar solubility parameter to the polymer to be dissolved.

For simple molecules, such as the organic solvents normally encountered in surface coating applications, the solubility parameter 8 may be readily determined from measurements of the heat of vaporisation.

However, most acrylic polymers degrade well before vaporisation temperature is reached and hence the experimental determination of 8P by measurement of the heat of vaporisation is not practical. 8P is normally measured practically by reference to the solubility of the polymer in a range of solvents of known solubility parameters. Other practical methods include calculation from determination of the swelling characteristics or vapour pressure.

The reliability of the solubility parameter approach to solvent selection is adversely affected by the fact that volume changes can occur as a result of mixing. This is particularly so with the largely polar aciylic polymers and the polar solvent normally employed in surface coating apphcations.

Hydrogen bonding between these polar species also plays a part in distorting the relationship between the solubility parameter and the actual solubility of polymer in solvent. The presence of crystallinity in the polymer also affects tms relationship, leading to anomalous results. Many film forming polymers have solubility parameters in the range 8.5-10.0. Solvents in this range are preferred.

The following table lists solubility parameters for some typical acrybc homopolymers and solvents.

It can be seen from the above that aromatic hydrocarbons have solubility parameters nearer to those of acrylic polymers than those of aliphatic hydrocarbons. Thus, it is not surprising that aromatic hydrocarbons are generally “better” solvents for acrylic resins than aliphatic hydrocarbons.

This can be illustrated by white spirit, where high aromatic content grades are solvents for certain low molecular weight acrylic polymers, whilst white spirits with low aromatic content are non-solvents.

Inspection of solubility parameters provides a good guide on which to base solvent selection. For example, acrylic polymers with long alkyl chains are more soluble in aliphatic solvents, since their solubility parameters are similar. Conversely, they will have good resistance to alcohols, water and glycol, due to the large differences in the relative solubility parameters.

Polymers of acrylonitrile are predicted to have good resistance to attack by aliphatic hydrocarbons (e. g. white spirit). These polymers will also be most soluble in solvents with higher solubility parameters (e. g. butyl glycol).

An improvement to the original solubility parameter approach is provided by separation of the solubility parameter into three components, each contributing to the overall solubility parameter. The components are related to dispersion, polarity and hydrogen bonding effects on the cohesive energy of the molecule of polymer in solution.

The table below shows the component values for solvents commonly used in surface coatings:

For most acrylic polymers the three components of the solubility parameter are in the range 7.5-8.0; 2.5-3.5 and 0.5-1.2 respectively.

It is possible to relate solubility parameters to the solvent and polymer interaction parameters %.

The theory was originally derived from the lattice theory.

The original Flory-Huggins Theory assumed random distribution of contiguous segments of polymer chains. Modem polymer solution theories have diverged significantly from this original concept and contain many correction terms for non-ideal behaviour.

Despite the multiplicity of theories, factors and adjustments the solubility parameter still offers a guide to solvent selection. The concept of interaction parameters is useful as a guideline, but in practice the solubility parameter theory is at best a rule of thumb and at worse unreliable.