Focus on solvents in polymer production

The use of solvents is always critical as far as environmental protection and occupational health & safety are concerned. Their sustainability is another major issue, particularly when a solvent can be manufactured on the basis of renewable raw materials, comparable to a biofuel.

Whatever else is a matter of controversial debate, everyone agrees about one thing: the world as we know it would be inconceivable without solvents. Staff at analytical, food or paint laboratories use them, as do the manufacturers of chemicals, cosmetics, food products, plastics and adhesives, not to mention pharmacists, artists, chefs and house painters. Put in simple terms, solvents are more or less low- or high-boiling inorganic and organic liquids that are able to carry out the dissolution, dilution and fine distribution of gases, liquids or solids, but without changing them chemically. This means that water is probably the most important solvent of all. In addition to this, solvents that can be characterised by their carbon and hydrogen compound content and that have been obtained primarily from the fossil energy sources crude oil and natural gas up to now are required for a practically unlimited number of applications. According to a Study, global demand for solvents amounted to 17.9 million tonnes in 2005, which the writers of the study calculated to be worth € 8 billion.

 
 

Reduction, substitution and sustainability

In the course of time, it has been determined that a large number of chemicals and solvents not only have properties that make them attractive for different technical applications but also are potentially harmful to human beings and the environment. A finding that has prompted either the users to dispense with these hazardous substances voluntarily and to look for suitable alternatives or the government to carry out its duty to consumers by introducing complete bans on the use of dangerous solvents or by issuing laws and regulations to control the use of them to differing extents. These activities also relate incidentally to the proper handling of left-over solvents and solvent residues, which in some cases have to be disposed of by the user in laborious and expensive processes. The increasingly strict conditions have led to greater environmental awareness, which can be summarised effectively by a new name that is now firmly established: “green chemistry”. What is meant by this is the biggest possible reduction in the use of poisonous or hazardous substances and the substitution of hazardous substances by more environmentally sound alternatives. A closer examination reveals that sustainability is a factor that is also taken into account here, which by definition means the use of renewable resources.

 
 

Experts are convinced that the sub-standard glycerine (SSG) which is produced in biodiesel manufacturing has an important role to play in the creation of natural solvents. This SSG can be used to obtain extremely pure biogenic glycerine that is already being used on a large scale by industry, e.g. to produce medical drugs, cosmetics, lubricants, inks, paints and lacquers. Biogenic glycerine in turn forms the basis for a number of interesting “natural” solvents that have the properties of a quasi-homologous group. What we are talking about here are:

• Isopropylidene glycerine
(4-hydroxymethyl-2,2-dimethyl-1,3-dioxolane) CAS 100-79-8
• Glycerine formal
(4-hydroxymethyl-1,3-dioxolane) CAS 5464-28-8
• Glycerine carbonate
(4-hydroxymethyl-1,3-dioxolane-2-on) CAS 931-40-8

The homology of these three substances is based on the underlying ring system of the 1,3-dioxolane and the hydroxymethyl group in the fourth position. As a result of this, the compounds are first and foremost primary alcohols, while they are also cyclical ethers at the same time. In contrast to a genuine homologous chemical series, which differs due to changes in chemical substitutes of similar structure – e.g. alkyl groups of increasing size – the substitutes in this group differ in position two in type and bonding: isopropylidene glycerine, a cyclical ketal, has two methyl groups; glycerine formal is a cyclical acetal with two hydrogen atoms, while glycerine carbonate is in turn a cyclical carbonate with its double oxygen bond on the ring C atom. All three substances have a similar property profile in spite of their differences:

 
 

Physical properties

Isopropylidene glycerine, glycerine formal and glycerine carbonate are liquid at room temperature; their solidification point is far below zero. The three substances can be included in the group of high boilers, even though the vapour pressure levels of isopropylidene glycerine and glycerine formal at 20°C can be 0.1 mbar or just over. Glycerine carbonate has a (calculated) boiling point of 354°C and a vapour pressure level at 20°C of less than 0.01 mbar. With 11 and 14 cP (25°C), isopropylidene glycerine and glycerine formal tend to be liquids with low viscosity, whereas the viscosity level of glycerine carbonate (85 cP) can already be described as medium. By way of comparison: the viscosity level of glycerine is 945 cP (25°C).

 
 

Compatibility

What is probably the most important and outstanding characteristic of all three dioxolanes is their complete miscibility with water, isopropylidene glycerine proving to be a practically universal substance: it is not fundamentally incompatible with any of the substance groups tested. There is miscibility to a sufficient extent even with such apolar substances as aliphates in some cases. Isopropylidene glycerine is completely miscible with cycloaliphates and aromates. It dissolves to a large extent in vegetable oils too, although the situation is somewhat different in the other direction: vegetable oils do not dissolve as much in isopropylidene glycerine.

The polarity of the three compounds increases from isopropylidene glycerine to glycerine formal to glycerine carbonate, which is expressed in decreasing compatibility with apolar or low-polar compounds: whereas it is still possible to a small extent to mix hydrocarbons (aliphates, cycloaliphates, aromates) with glycerine formal too, miscibility with glycerine carbonate is for all practical purposes zero. The same is true of vegetable oils.

With minor restrictions, the compatibility of ethers and chlorinated hydrocarbons with glycerine formal is similar to isopropylidene glycerine, whereas there are larger gaps with glycerine carbonate here too. With this diversified compatibility pattern, the three solvents form a selection system that is suitable for many different substance combinations.

 
 

Solvent capacity for polymers

In view of the tremendous diversity of the different polymers, which are manufactured by almost every supplier on the basis of proprietary formulations and therefore have highly specific properties, it is not possible to make any definite statements about their solubility in the above-mentioned dioxolanes. Appropriate tests need to be carried out in the context of the specific applications to make this possible. The excellent solvent capacity for the many, very different macromolecules, such as polyesters, polyacetates, polyacrylates, epoxides, polyurethanes, urea-formaldehyde resins and melamine resins, has been emphasised in the literature for decades now and is documented in particular in extensive patent literature (indicate the source). More complex macromolecules such as polyesterimides can be dissolved with these substances too, however. The cresols that have been popular in this area up to now even though there are objections to them can as a result be substituted to a large extent.

 
 

The way a chemical or a solvent is used and the extent to which it is used depends not least of all on whether it is harmless or not. Isopropylidene glycerine, glycerine formal and glycerine carbonate are non-toxic, cause no or hardly any irritation, have only a slight but pleasant odour and are therefore highly compatible in contact with human beings. They are neither dangerous nor are they required to be identified: the flash points are close to 100°C or even far higher (glycerine carbonate). Their environmental performance is very good too.

From a historical point of view, isopropylidene glycerine, glycerine formal and glycerine carbonate are not new substances. Production of them was described as long ago as the end of the 19th century. Their excellent characteristics as solvents and raw materials for chemical reactions have been well known for several decades too and have been described on many occasions. What have stopped them being used on a large scale up to now have, however, been the small amounts of them that have been produced in the past and the high procurements costs for the user associated with this. This is now a thing of the past, however: isopropylidene glycerine, glycerine formal and glycerine carbonate can now be produced and bought in large quantities and at affordable prices.

The arguments in favour of an increase in use of them apart from the high quality and wide range of possible applications for these solvents include in particular their biogenic background. Isopropylidene glycerine, glycerine formal and glycerine carbonate satisfy the requirements of “green chemistry”; they can be manufactured sustainably, while they are harmless and safe to use. In addition to this, the use of a by-product of a different process saves primary raw materials and reduces the use of non-renewable fossil energy sources. When biogenic glycerine is used as the basis for producing them, isopropylidene glycerine, glycerine formal and glycerine carbonate are harmless to human beings and the environment, while they are safer to manufacture and use. GDeußing