Plasticisers are a rather special type of additive. Without plasticisers for example, PVC would have been too brittle and fragile to be able to conquer the huge market it has taken over today; without plasticisers most injection molding compounds would be entirely unsuitable for that purpose, and without plasticisers, some blends of rubber simply could not be produced. Not only do plasticisers make plastics extensible, plastic, elastic and flexible at low temperatures, in many cases it is only possible to process polymer products on a commercial basis by incorporating a plasticiser. In fact, were it not for the invention of plasticisers, the plastics industry would very probably not have developed much beyond the stage it was at in the olden days, as the first plastics and modified natural polymers such as nitrocellulose or galalith were hard and brittle and therefore unusable for everyday uses. And of course it doesn’t exactly boost the marketability of a new material, if for example you have to look after your comb as if it were made of Meissen porcelain.
But plasticisers changed all that. The first one to be used in real plastics was camphor, a colourless substance, whose smell is reminiscent of many a cold remedy and that consists of small crystals extracted from the wood of a Chinese laurel bush known as Cinnamomum camphora. In 1869, an inventor named John Wesley Hyatt and his brother mixed camphor with nitrocellulose in order to make the nitrocellulose more malleable.
Hyatt was not the first person to attempt to make the brittle early products of an emerging plastics industry easier to handle through the use of additives, however. A no less ambitious inventor by the name of Alexander Parkes had already tried to achieve the same effect using wood tar and vegetable oils for example. He did, however, have problems in achieving the right recipe: his "Parkesin" was relatively easy to process, but after just a few weeks, items such as ladies’ earrings, combs and bracelets that had been made from this particular product would warp so badly that they could no longer be used - probably due to the fact that Parkes’ oils evaporated too quickly. Other inventors used banana oil or even fusel oil that was produced during the distillation of whisky - but without notable success, if the lack of records is anything to go by.
Soft as Chewing Gum Yet Hard as Horn
Hyatt’s new plastic was better equipped to meet the challenges posed by the newly emerging plastics business. Depending upon the quantity of camphor that he mixed with collodion - a solution of gun cotton, known chemically as nitrocellulose - he was able to produce a plastic that was transparent but that could be coloured and that was as hard as horn but as flexible as crude rubber. This plastic was celluloid. One of the very first applications of this malleable material, which at a moderate temperature of between 80 to 90°C could be produced in any desired shape thanks to Hyatt’s fragrant formulation, was false teeth. Even though they smelt more than a little of camphor and for this reason did not always meet with the full satisfaction of their owners, the fact that they could be produced in a suitable colour meant that they were a considerable improvement upon the hard rubber plates that had been used up until then. Later on, the same material made a name for itself as the ideal backing material for photographic films.
Origin of Modern Plastics
It is true that for some time, Hyatt’s celluloid had to overcome various problems that were due to its close relationship to gun cotton. Mini explosions were caused when billiard balls made of nitrocellulose collided with one another. Eyewitnesses reported that the explosions were enough to make cowboys standing around the pool table reach for their revolvers. A magazine article even related the story of a lady whose evening became even more uplifting when the celluloid buttons on her evening dress came too close to a fireplace and caught fire. On one occasion, an entire celluloid factory exploded. But all of this did not detract from the fact that Hyatt had invented the first thermoplastic, and that it was a plasticiser that helped bring us the plastics of today.
Even after Hyatt, the history of plastics remained closely linked to that of its plasticisers. In 1946 a use was being sought for several hundred tons of a brittle cellulose triacetate plastic that had been used, amongst other things, in the production of aircraft windows. It was lying unused in the grounds of a factory, when an inventive chemist had the idea of combining the material with a plasticiser. And this was how a new injection moulding material was born. In 1952, the newly plasticised Cellit was dubbed "Cellidor" and was the very epitome of versatility. In the 1950s, it was used to make casings for radios, dashboards, combs, hairslides, screwdriver handles, spectacle frames and so on and so forth.
Plasticisers in Rubber
Plasticisers were also used by the rubber industry to refine its products. Though prolonged kneading makes unvulcanised rubber as soft as chewing gum, this is only because the kneading process breaks down the long chain molecules of the polymer. Unfortunately, however, this means that other important properties of this valuable material are also then lost. For this reason, rubber scientists made an early start in mixing all manner of liquid components into their black formulations - coal tar, pitch, oils and terpenes (such as camphor!), paraffin and even Vaseline. This meant that even without reducing the size of the rubber molecules very much, the crude rubber mixture was sticky enough for the many solid ingredients such as carbon black to be easily kneaded together in the mixer.
From this we can see how important these inconspicuous plasticisers are when it comes to processing polymers - they can completely revolutionize the properties of what initially was a rather unprepossessing polymer material. Paradoxically, this remarkable power is reflected by the fact that even well known chemical historians find it hard nowadays to come by information concerning plasticiser technology. The "right" plasticisers are so important to the performance of a plastic that information gleaned about them over the decades has disappeared into the vaults of companies using polymers. As time went by, this meant that plasticisers became rather anonymous utilitarian substances.
The Modern-Day Descendants of Camphor
What we do know, however, is that in addition to Hyatt, others soon started to use camphor. In fact, even today two thirds of all camphor produced worldwide is used in the production of celluloid. In a chapter entitled "Plasticisers," a chemistry encyclopaedia published in 1931 also lists terpene, as well as phthalates and glycerol esters and organic phosphates such as tricresyl phosphate. Not only do these make plastics flexible, but they also enhance their fire retardant ability, so much so that cellulose acetate, a successor to celluloid, managed to remove one of the major disadvantages of the original plastic, namely its flammability, after it had been elastified with a mixture of camphor and phosphates. State-of-the-art technology knows approximately 400 substances - "worldbeaters" and exotic ones - that are used in one form or another as a plasticisers. About one hundred of these are of significant commercial value.
Halfway through the 1990s more than 4.2 million metric tons of plasticisers were being used. Around 90 percent of all plasticisers are nowadays used in PVC, a plastic that in its basic form is almost as brittle as glass and would be entirely unusable for most applications if plasticisers did not account for up to 55 percent of its content. Even rigid PVC can contain up to 12 percent plasticisers that improve its processability. Depending upon the application, other polymers make use of plasticisers in a range of different quantities. Paper contains around five percent, thermoplastic materials up to 10, elastomers sometimes as much as 60 percent; some plastics even have a plasticiser content of 95 percent!
How Plasticisers Work
All plasticisers are actually based upon the same principle and one that is almost self-explanatory, if we understand how plastics are made up on the inside. "Plastic" always consists of very long chain molecules that under extremely high magnification would look like long threads. A plastic in which these threads are loosely tangled together is flexible. In the case of many plastics, however, these threads have a tendency to lie on top of one another like packed spaghetti. In fact, anyone who casually throws spaghetti into a pan, does not stir it while it cooks and sieves it afterwards, will then find that in addition to strands of pasta that are loosely tangled together, there will also be areas in which the strands of pasta are still stuck together as they were in the bag. These lumps appear somewhat harder than the rest, although the pasta itself is fully cooked and soft.
A similar thing happens in the case of the chain molecules of plastics. A rigid structure that is similar to the strictly regular composition of crystals allows the plastic to appear rigid on the outside. In the pan as well as in the test tube, the rule is: loosely tangled is flexible, rigid structure is hard.
This is where the plasticisers come in. What it comes down to in the majority of cases, irrespective of whether we are talking about camphor or mineral oil, is molecules that are very much smaller than the chain molecules of the polymer material and that are interwoven into their spaghetti-like structure as the plastic is processed. They then push their way between the adjacent threads of plastic molecules, setting them apart from one another, and act rather in the same way as oil does on a plate of spaghetti, in that it allows the strands of pasta to slide past one another. This means that a loose, freely mobile structure can be created - the plastic becomes flexible and that the more plasticiser is added, the more flexible it becomes.
This simple analogy explains a whole range of items that form the bread and butter of a plastics chemist’s job. A large part of the expertise of a materials developer lies in being able to find substances that are the most ideally suited to the plastic being used. Hydrophilic substances cannot be interwoven into water-repellent molecules such as those of unvulcanised rubber for example, as the two substances would separate just like water and oil. Furthermore it is important to select a plasticiser that provides an optimum fit in respect of its own molecular configuration and the plastic chain molecules in question. Chain molecules are not as similar to one another as the strands of spaghetti - some polymers do remind us of flat pasta, while others have a zig-zag appearance or look like a chain of thick neon tubes, connected together with thin wires; others look like necklaces made of very fat pearls. After all, Hyatt’s celluloid only achieved the success it did because the camphor molecules were able to fit so well between the molecules of the gun cotton, which were shaped like a pearl necklace.
Not all plasticisers are suitable for every polymer. An additional factor is that each one has a different effect upon its "host molecule." While one provides a greater degree of flexibility at lower temperatures, another is specially designed to prevent plastics from melting at high temperatures. Others, meanwhile, not only make plastics more flexible, but also act as a kind of built-in fire extinguisher that can snuff out flames in their very early stages. They achieve this by decomposing in the presence of heat to produce flame-retardant substances.
One Problem - Many Solutions
Over the decades, the plastics industry’s order book for plasticisers has grown into a veritable menagerie of chemicals. It is however dominated by a number of "major families" of products. Phthalates are used in PVC cables and films, coatings and cellulose adhesives. Dicarbonates make flexible PVC elastic at low temperatures, phosphates are used as a flame-retardant and also as hydraulic fluid. Fatty acid esters - which are distant relatives of margarine - are used to plasticise rubber and vinyl resin floor coverings. For certain applications, plastics technologists also turn to esters of citric acid and tartaric acid.
Of course it must also not be forgotten that despite the beneficial effect they have had upon plastics technology, plasticisers also have their detractors. In recent times, phthalates have been suspected of being harmful to health. Though no conclusive evidence has yet been forthcoming, investigations are currently taking place. Fortunately, whatever the outcome of this debate, it does not mean that all plasticisers are to be condemned: after all, one plasticiser is not the same as the next, as shown by camphor, which is a natural product.
In the meantime, some very useful products have been developed that provide an alternative to the phthalates. These products take the form of a family of substances whose members are known as "alkyl sulfonates". Alkyl sulfonates have for a long time been recognized as having no harmful effects and in many countries have even been authorized as being safe for food use. They are already used to replace the controversial phthalates in dolls and toy figurines, gloves, membranes for water beds, are found in sealants for the construction industry and for use in swimming aids and wellington boots. The material also possesses a whole host of further advantages such as, for example, the fact that unlike many other plasticisers, it is not attacked by the elements or water and gives rise to products that lend themselves well to printing. And this is not an unimportant factor when it comes to producing brightly coloured children’s paddling pools made from PVC film, amongst other things.
In its raw form, PVC is a brittle, almost glass-like plastic that would be virtually unusable if it were not for plasticisers. Alkyl sulfonates make it elastic, resist saponification and weather resistant.
Drifting Plasticiser Molecules
Industry researchers have recently developed a solution to the problem of drifting plasticiser molecules. The tiny particles of plasticiser contained within plastics are astonishingly mobile. In certain conditions they move around like honey in a sponge. The journey that some plasticiser molecules make through the plastic itself sooner or later comes to an end when they arrive on the surface, forming an unsightly greasy film. And housewives are not the only ones who are aware of this. Special plasticisers that have been trained to remain in one place within the polymer - for example by giving them long chain molecules - can prevent plastic and rubber from developing a dulled or greasy surface. Scientists have also developed customized halogen-free plasticisers for electronic circuit boards, so as to prevent the brittle plastic that is used to produce them from shattering when the boards are punched, drilled and soldered.
Growth and Development of Industry
The plasticiser industry is mostly involved with traditional products, and though devoting so much effort to product development is rather unusual, it certainly pays off. In the last two years, sales of some specialist plasticisers have grown by around 15 percent, even though the plastics market only managed to achieve growth of around 4 percent in the same period. This alone is enough to show that the final chapter in the long history of plasticisers has yet to be written: new plastics and the new demands placed upon the products made from them continually require new solutions, and only the pooled brainpower of plastics experts is sufficient to do justice to these.
But back to plasticisers. The plasticising chemicals that emerge from the flasks and crucibles of the chemical industry are of course not the only ones that exist in the world at large. As nature also partly consists of polymers, it also requires substances to ensure that these polymers remain flexible. Proteins, DNA, starches and wood and even stones basically consist of long and sometimes spatially interlinked chain molecules. Water is the main plasticiser that is used by nature. Without their water content, natural fibres such as silk, wool or cotton would be brittle.
Surplus water also keeps muscular proteins flexible. In advanced age, water content decreases and fats more or less successfully take on part of the function of a plasticiser. Not only the plastics industry depends upon plasticisers - Mother Nature herself could not manage without them. Made up of chain molecules like those found in plastics, muscle fibres, just like modern polymers, contain their own plasticisers in the form of water and fat molecules. Even quartz, which is an extremely hard material, can be softened using water: hard, natural quartz consists of only 0.01 percent water, whilst artificial quartz for technical reasons contains around ten times that amount. Artificial quartz can be shaped in a similar way to plaster at a temperature of 400 degrees Celsius, a temperature that minerals can usually withstand with comfort, whilst "dry" natural quartz literally remains "rock solid" up to a temperature of 1000 degrees.
This is a prime example of the fact that the application of plasticisers in technology is not necessarily confined to plastics. Anyone who thinks hard about the curved handle of his umbrella must have an idea that some sort of process must exist to make wood flexible. And of course hot water vapour can be used to soften wood, but liquid ammonia, combined with organic solvents such as polyethylene glycol, dimethyl sulfoxide or tetrahydrofuran, does the job even better. This particular formulation even allows you to tie knots in walking sticks. Once the ammonia has evaporated, the wood returns to the state it was in when its freshly cut state.
Emerging From The Plastics Middle Ages
More than a century ago, it was Hyatt’s invention that brought us plastics. And so it comes as no surprise that professions that until now have been involved with the preservation of antique pigments are now having to get to grips with plasticiser problems. For example, restorers are currently fighting to preserve the space suits that the Apollo astronauts wore on the moon and returned to Earth undamaged. They contained PVC tubes that had been plasticised using a phthalate; after spending more than 30 years in a museum, this liquid substance, in a way that is typical of all drifting plasticisers, has diffused out of the polymer, causing the supply tubes to become fragile. What was once the ultimate in space suit technology has turned out to be less long-lasting than knights’ harnesses that are hundreds of years old. Looking at things from this point of view, we are therefore still in what amounts to the Middle Ages of plastics technology.