10 things food technologists might not know about emulsifiers in polymers
Is it a new application?
Emulsifiers such as GMS (Glycerol Monostearate) have been used in the polymer industry for more than thirty years, primarily as anti-static additives, but as the understanding of emulsifiers increased in the food industry, more and more emulsifiers are used in plastics in a large variety of applications.
Are they the same emulsifiers as in food?
Many of the very same emulsifiers used in food have proven effective as polymer additives. As an example, we can take an emulsifier from a margarine recipe and incorporate it into plastic to achieve an anti-static effect. However, the diverse emulsifier chemistry can also be used to make completely new and tailor-made molecules which are optimized for each specific application in plastics.
Are the emulsifiers used as polymer additives food grade?
One of the great strengths of using emulsifiers as polymer additives is the inherent food grade status. They represent a group of safe and non-hazardous additives in a world where increasing attention on additives in plastic packaging has resulted in more regulation and control with what comes in direct contact with food. Thereby, emulsifiers present themselves as effective alternatives to current additives of concern. It is, however, possible to make non-food grade derivatives of emulsifiers which are then used outside of food packaging applications.
How can emulsifiers be polymer additives - how does it work?
Most of the emulsifiers are mixed into the polymer, much like margarine in a dough, after which the polymers are shaped, not into cookies or muffins, but into long sheets or objects through extrusion or injection molding. After production and depending on the emulsifier, it will migrate to the surface instantly or within days where it will position itself in the interface between plastic and air. Here it will make the plastic surface more water loving, thus attracting minuscule amounts of water which helps dissipate static electricity, making the surface anti-static or it can spread the droplets on a foggy surface into a clear water film, giving an anti-fogging effect.
Are they any good/ Are they better than current additive alternatives?
In some cases, with specifically engineered emulsifiers or blends, superior performance can be achieved, but in others they simply meet the current standards in the industry. In addition to their performance, one of the greatest strengths of emulsifiers is the combination of food-grade and their bio-based nature which frees them from much of the concerns currently surrounding polymer additives used by the polymer industry today. And with increasing customer demands for safe packaging, this trend will only get stronger as legislation continues to tighten in years to come.
What makes an emulsifier a good polymer additive?
The key is the amphiphilic nature, that it is both hydrophilic and lipophilic, which is also utilised in food applications. However, instead of acting in the oil/water interface they migrate to the surface of the polymer to the polymer/air interface. This migration is very much controlled by the polymer type and its crystallinity, but the chemical buildup of the emulsifier also plays a big role. Shorter-chained glycerides migrate to the surface almost instantly during processing which makes them good for mold-release in injection molding or when an instant anti-static effect is needed in further processing. Medium and longer chained glycerides can take days to reach the surface and thereby give a delayed effect, which in turn often lasts longer. Polyglycerols of the same types of fatty acids have also shown very good performance in challenging applications such as anti-fogging in green-house film where a long performance is needed.
Are there any drawbacks to using emulsifiers as additives in plastic?
One of the challenges for the polymer industries with these products is their physical appearance. Some are pellets which can be handled by many producers, but some are paste-like or liquid. This means that the producers must heat them and dose them as a liquid which can be hard to handle on an extrusion line, if they don't have the right equipment. This is also why one of the biggest customer segments for emulsifiers used as polymer additives is masterbatch producers. They can blend the additives with the polymer on their extrusion lines creating a concentrated mix, a masterbatch, in pellets which is easy to handle for converters who shape the plastic into usable pieces such as chairs or buckets.
Another drawback is the need to hit the right concentration. This is most often not a problem, but unfortunately it isn't so that the effect gets better the more you put in, just as too much emulsifier can create an undesirable crumb in a cake or make the chocolate to liquid. In the case of polymer additives, too much emulsifier will cause excessive migration to the surface creating a visible, greasy layer on the plastic surface – much like fat blooming in chocolates. In the case of, for example a black and shiny coffee machine, you wouldn't want a milky layer like this on the surface and at that point, when the product starts to bloom, it no longer gives the anti-static or anti-fogging effect. Therefore, it is important not to overdose when using emulsifiers as polymer additives.
Emulsifiers show excellent performance within several applications in plastics.
- Anti-static. Here, it prevents static buildup during production, handling, and usage of plastic products. Static buildup can be a hazard during production as it can result in a sudden electrostatic discharge which can hit personnel or short circuit the machinery. During handling and use, static electricity can make the plastic products virtually impossible to separate or cause dust attraction, which makes it hard to seal the packaging properly or causes the product to look dirty and uninviting
- Anti-fogging, where they improve the visual presentation of the product by spreading the water in a thin and transparent film instead of having small droplets obscuring the view. Thus, it does not remove the water droplets, but retains the water on the plastic surface in a thin film which also helps prevent the forming of a water pool in the bottom of for example a bag of salad
- Mold release, where it prevents adhesion of the plastic to the metal dies/tools used in e.g. injection molding, making the processing faster, more efficient and with less flawed products. Here, the emulsifiers lubricate the plastic surface in a thin layer without affecting subsequent sealing or printing on the product
How is the performance evaluated?
Emulsifier performance in the various applications are evaluated quite differently from the food industry as texture, emulsion stability and water content are not used in the evaluation of polymer additives.
After being processed into a film or an injection molded piece, the anti-static performance of the final product is evaluated through static decay time or surface resistivity. Static decay time is an expression of how long it takes the surface to discharge from 5000 volts to 500 volts i.e. 10% of the charge. A pure polymer will not even be able to charge to the 5000 volts required to begin the experiment and a good anti-static additive will show a static decay time in less than 2 seconds. In surface resistivity, a current is run through two electrodes with a given distance on the surface of the plastic piece and the resistance of the surface is determined over the measured area. An anti-static surface is in the range of 1010-1011 ohm/square, while the pure polymer is insulative with a surface resistivity at 1012 ohm/square and up.
The anti-fogging performance of plastic films is evaluated visually on a scale from A to E, where A is completely fogged with very small droplets and E is a transparent film with no droplets. The film is fixed to a container with water and the evaluation is then performed at either 5 °C in a refrigerated environment, to simulate cold storage of for example salads and meat, or in a water bath at 60 °C to simulate hot applications such as newly cooked meals.
Can they be used in bio-based or biodegradable plastics?
Bio-based emulsifiers are especially well-suited for both bio-based and biodegradable plastics. Here, it is important to distinguish between the two types of “bio” plastics.
Bio-based means originating from bio-based materials high in carbon, such as sugar canes. This type can be transformed into an exact copy of their fossil-based counterparts, such as PE (polyethylene), which are not biodegradable. Thereby, emulsifiers can be an effective and safe additive in bio-based plastics just like in their fossil-based counterparts.
PLA (polylactic acid) is an example of a polymer which is both bio-based and biodegradable. The biodegradability in PLA is inherent, due to its ester backbone, which, like the emulsifiers, will break down to non-harmful compounds under hydrolysis. This makes emulsifiers an obvious candidate for additives, as it is fully biodegradable and safe for the environment.
Therefore, emulsifiers are an excellent choice for both bio-based and biodegradable plastics, which is a growing segment in the plastics industry as consumers are demanding greener alternatives, free of fossil fuels. Recently, scientists are even working on polymers based on CO2 from the air, to help clean the atmosphere of excess CO2, which is the perfect raw material for the carbon-based polymers.