Jiae Kim introduces isosorbide, a plant starch-derived diol made from renewable feedstock that has wide ranging applications across polymer materials for a variety of industries.
When discussing climate change, plastic packaging is often considered to be “the problem”. However, it has the potential to become the solution, with estimates suggesting that switching plastic out for alternatives, such as aluminium and glass, could result in a 2.7 fold increase in greenhouse gas emissions.1 Establishing a circular economy, in which plastics are efficiently recycled and reused, is essential, but so too is the development of new, sustainable feedstocks and high performance materials. Enter isosorbide (Figure 1); perfectly safe and produced from an annually renewable feedstock, isosorbide has exciting potential for a range of packaging applications.
WHAT IS ISOSORBIDE AND HOW IS IT MADE?
Isosorbide is a plant starch-derived bicyclic diol with rich functionality for a range of applications in the packaging, coating, adhesives, sealants and elastomers (CASE) and automotive sectors.
Roquette, the world’s leading manufacturer of isosorbide, has refined and optimised this manufacturing route to produce stable, high purity isosorbide in industrial quantities.
Figure 2 illustrates how isosorbide is made. Starch from annually renewable feedstocks is hydrolysed to produce glucose, which is then converted to sorbitol and subsequently isosorbide by hydrogenation. Over the last two decades, Roquette, the world’s leading manufacturer of isosorbide, has refined and optimised this manufacturing route to produce stable, high purity isosorbide in industrial quantities. The company’s flagship production facility produces isosorbide of three different grades, each tailored to specific industrial applications.
As a sustainable plant-based feedstock with a carbon footprint of just 0.09 kg CO2/kg of product, isosorbide is an attractive substitute for monomers such as bisphenol A, which has a carbon footprint around 60 times higher (according to an internal comparative study by Roquette based on lifecycle analysis methodology and peer-reviewed by an external auditor).
However, isosorbide’s impressive environmental credentials are just the start (Figure 3). The material is:
- Non-endocrine disrupting
- EU Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) compliant
- Suitable for food contact and for the manufacture of cosmetics and pharmaceuticals, such as blisters and connectors, thanks to the chemical resistance.
These attributes make the performance benefits of isosorbide of interest and value for a diverse range of applications.
WHAT ARE THE PROPERTIES OF ISOSORBIDE AND HOW CAN IT BE USED?
Isosorbide can be incorporated alongside other monomers to modify the properties of, for example, polyethylene terephthalate (PET), polycarbonates (PC), polyurethanes (PU) and epoxy resins. When used in this way it can enhance:
- Thermal properties, such as increasing glass transition temperature (Tg)
- Chemical resistance to both water and organic solvents (such as acetones and esters)
- Optical performance, such as transparency and birefringence
- UV resistance, reducing yellowing upon exposure to the sun
- Mechanical properties, such as scratch, impact and bending resistance
- Adhesion, a crucial property for the CASE sector
- Appearance, such as imparting high gloss.
Isosorbide can reduce the environmental burden associated with the use of existing materials, while maintaining their performance, or enabling the realisation of new performance profiles, to meet the exacting requirements of different markets.
INDUSTRIAL APPLICATIONS FOR ISOSORBIDE: PET
PET is a copolyester of ethylene glycol and terephthalic acid, both of which are essentially petrochemical feedstocks. However, by replacing some of the ethylene glycol with isosorbide you can create polyethylene co-isosorbide terephthalate (PEIT). A defining feature of this material is an elevated Tg, with Tg increasing relative to PET by over 1°C for every one mole percent of diols switched to isosorbide.
Depending on the extent of isosorbide replacement, PEIT is either semi-crystalline or amorphous; both types of polymer have compelling properties for innovative packaging and both are recyclable and reusable (Figure 3).
Semi-crystalline polymers have an ordered structure and sharp melting point. Prized for their ability to produce tough finished components with excellent strength and stiffness, they can be difficult to thermoform.
The higher Tg of PEIT, relative to PET, is particularly advantageous for aerosol packaging. Plastic aerosols are lightweight, chemically resistant, low cost and safer to store than their metal counterparts. Unlike PET, semi-crystalline PEIT (PEITsc) has the thermal resistance required to meet the criteria laid out in European Aerosols Federation (FEA) Standard 647.2 It therefore answers directly to the trend away from metal towards plastic aerosol packaging.
The enhanced thermal properties of PEITsc are equally advantageous when it comes to beverage packaging, for hot filling and pasteurisation. With hot filling, vegetable and fruit juices, water, teas and other soft drinks are heated and and filled at high temperature to ensure sterilisation of the bottle and closure system. Pasteurisation is an alternative preservation technique for drinks such as beer. PEITsc eliminates any requirement for a vacuum panel, increasing design flexibility. PEITsc beer bottles have proven resistant to deformation during pasteurisation at 60°C for 15 minutes.
All these performance benefits are accessible with no loss of recyclability relative to PET
Beyond thermal properties, PEITsc has excellent chemical resistance making it a strong contender for personal care and cosmetic packaging, particularly for products containing polar molecules, such as nail polish remover, shaving cream and make-up remover. Incorporating isosorbide produces chemically inert, high gloss packaging with good optical properties and excellent impact resistance. This is a compelling packaging profile for high value products.
All these performance benefits are accessible with no loss of recyclability relative to PET. PEITsc can be recycled directly into existing PET waste streams with recycled materials containing up to 50% modified PET fulfilling all the European PET Bottle Platform (EPBP) Testing Protocols for assessing PET recycling compatibility.3 PEITsc can also carry the Resin Identification Code (RIC) #1.4
Amorphous polymers are dominated by randomly ordered molecular structure and tend to soften with the application of heat, rather than exhibiting a sharp melting point. This makes them easy to thermoform, to manufacture products with high dimensional stability and good impact resistance.
Amorphous PEIT (PEITam) shares the chemical resistance of PEITsc, making it similarly attractive for personal care and cosmetic packaging depending on the product and packaging design (Figure 4). It can be processed in just the same way as PET, using existing equipment.
When it comes to reusable plastic packaging, PEITam is more environmentally friendly than PET, but with no loss of performance. For example, PEITam reusable sports bottles have excellent scratch and fingerprint resistance, while PEITam reusable food containers are dishwasher-proof and offer better staining resistance relative to PET. Safety and the assurance of a non-endocrine disrupting monomer are highly advantageous for these applications.
PEITam carries the RIC #74 and is recycled along with other polymers, such as PC and other bioplastics.
Isosorbide is a plant-based, sustainable monomer with a low carbon footprint and significant potential for a wide range of applications such as blisters and connectors. Substituting isosorbide for existing petrochemical ingredients not only lightens the environmental impact of industrial polymers, it also drives up performance. Isosorbide plastics are set to enable innovations such as new types of aerosol packaging, touch screens with enhanced clarity and scratch resistance, and better adhesives with improved UV and impact resistance. These are just some of the products that isosorbide is already beginning to deliver as polymer chemists and formulators learn exactly what it can do.
- “Plastic packaging: the unloved that would benefit from being better known”. PlasticsleMag, Mar 2019.
- “FEA 647 – Plastic aerosol dispensers – Technical requirements”. European Aerosol Federation, Mar 2020.
- “How we do a Recycling Evaluation!”. European PET Bottle Platform web page.
- “Standard Practice for Coding Plastic Manufactured Articles for Resin Identification.” ASTM International, 2020.
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