poly-oxepanone (pcl)
2-oxepanone homopolymer is commonly known as polycaprolactone, or PCL for short. It is closely related to the naturally-occurring cutin polymers made by plants, and historically got its name from caproic acid, a 6-carbon fatty acid found in goats' milk.
Along with several other polyesters, PCL was first reported by Du Pont (Frank van Natta, Julian Hill, and Wallace Carothers) in the mid 1930s. Unfortunately, it was not suitable for making "artificial silk", so they switched to developing polyamides instead.
PCL was then largely ignored until around 1960 when it was realised that similar polyesters could be broken down in vivo. This led to their use in medical application for resorbable sutures, implants and slow-release drug capsules.
In the same decade, these polymers were also found to biodegrade in nature, with PCL being particularly tractable. It is now known that PCL will biodegrade under both aerobic and anaerobic conditions, including in terrestrial, freshwater and marine environments.
Although poly-oxepanone (PCL) is one of the best biodegradable thermoplastics currently available, it is rarely used for consumer products. Compared to more durable polymers, it is quite expensive and requires careful design to get good results.
natural polyesters
The reason some polyesters are biodegradable in the environment is because nature is already using them. Microbes long ago evolved the enzymes needed to break down these naturally occurring plastics, allowing them to recycle the released fatty acids as food. Artificial polyesters with similar molecular structures are also attacked by these enzymes, hence their ability to biodegrade.
from insects & bacteria
Shellac is a complex polyester that melts around 80°C and is easily moulded into intricate shapes (e.g., gramophone records). Being soluble in alcohols and alkalis, today this natural thermoplastic is mostly used as a varnish or lacquer (such as French polish), and also sometimes as a food glaze (E904).
The PHAs are a class of (mostly) simple butane- & pentane-based polyesters that bacteria use to store energy. Known since the 1920s, mass-extraction was eventually pioneered by ICI in the 1970s, who sold it as "Biopol". Different PHAs exhibit a variety of material properties, but they can be difficult to melt-process.
from plants
Cutin forms the scaffolding for the cuticle, the waxy layer plants use to seal their leaves & fruit. Despite literally "growing on trees" it has never been successfully mass-harvested. Cutin is also quite variable in its composition, which makes it unsuitable for mass-produced items where consistency is important.
Suberin is a complex polyester containing both aliphatic (linear) and aromatic (ring-shaped) hydrocarbons. The aliphatic components are related to cutin, and the aromatic components are similar to the lignins found in wood. Like cutin, suberin is not currently extracted for use as a thermoplastic.
"synthetic cutin"
(WARNING! Skip this bit if you really, really don't like chemistry...)
When microbes encounter the polar C6 polymer poly-oxepanone (aka poly-caprolactone, or PCL for short), they treat it as another type of plant cutin. The enzymes they release break it down into natural fatty acids, just as they do with leaves.
Natural plant polyesters are quite diverse and can incorporate a wide range of fatty acids & alcohols. The exact composition varies significantly between different species and even different parts of the same plant. Acid, primary alcohol and diol components in the C8 to C28 range have been reported, with C16 and C18 being the most common (e.g., palmitic, stearic, oleic & linoleic).
The microbial enzymes responsible for degrading cutin and suberin (known as "cutinases") are consequently quite broad-spectrum since they have to deal with numerous polyester variations.
Although C6 (caproic) acid is uncommon in suberin or cutin, it is very closely related to those that are. It is therefore no surprise that when cutinase enzymes are applied to synthetic C6 polymer, it is also readily biodegraded.
This C6 polymer is poly-oxepanone, and its vulnerability to cutinase is further suggested by its structural correlations with the C16 to C20 polymers commonly used by plants. As a result, when microbes encounter poly-oxepanone (aka PCL), they treat it as just another form of cutin.
the magic of esters
An "ester" is formed when hydrocarbon molecules are interconnected by carbon dioxide.
They are very common, and are easily produced by reacting carboxylic acids with alcohols. If this is done with bifunctional molecules (i.e., acid at one end, alcohol at the other), it becomes possible to keep repeating the process, leading to the creation of poly-esters. This is exactly what nature does, and the production of many synthetic polyesters uses the same approach.
Polycaprolactone could also be made this way (from ω-hydroxy caproic acid), but on an industrial level, it is instead mostly produced by an alternative technique called "ring-opening polymerisation". This process takes the "cyclic ester" caprolactone (aka oxepanone), and with the help of suitable catalysts, opens these molecules up and links them together to form long macromolecules, i.e., poly-caprolactone (aka poly-oxepanone).
the reversible reaction
Compared to common everyday polymers, polyesters such as poly-oxepanone (PCL) are much easier to deconstruct. This is why polyesters have become dominate in the field of biodegradable applications. (Other polyesters include PGA, PLA, PHB, PBAT, PBS, et cetera.)
Depolymerisation of these materials is possible because CO2 reacts with water, even if it's incorporated into another molecule. Since polyesters contain a lot of CO2 links, they are inherently unstable in the presence of water.
This break-down mechanism is termed hydrolysis, and some polyesters are more susceptible to it than others. As a purely chemical reaction, hydrolysis is quite slow at ambient temperatures, but it can be greatly accelerated by suitable biochemical catalysts. These are collectively known as "esterases", and are exactly what many species of microorganism produce to help them decompose natural polyesters like shellac and cork suberin. (Cutinase is one example, lipase is another.)
Regardless of the source material, in all cases the end results are the simple hydroxy fatty acids that were used to construct the polyester in the first place. These small hydrocarbons are easy to consume as food, and are thereby ultimately returned to nature.
material properties
Physically, poly-oxepanone (aka poly-caprolactone, PCL) is a white, semi-crystalline thermoplastic that resembles polyethylene. It is flexible and reasonably strong (but with a relatively low yield point), and retains its flexibility even at sub-zero temperatures. It has a very low thermal conductivity, which means it can stay in a molten state for many minutes; It melts at just +64°C.
These thermal characteristics make it a somewhat difficult material to work with, but the low Tg and Tm do also help make it highly biodegradable. PCL's low melting point and low heat capacity also mean it requires far less energy to process than other common thermoplastics.
On the downside, the low melting point means poly-oxepanone is not suitable for high temperature applications, although short-term excursions above 40°C are usually safe. Prolonged exposure, however, can result in material creepage and deformation - especially under load.
Poly-oxepanone (PCL) is also a relatively expensive polymer, and consequently it is not commonly used for injection-moulded products. The main exception being some high value medical applications:
Compared to PGA or PLA, it hydrolyses more slowly in the body and is better suited for slow-release drug delivery capsules; Due to its low melting point, PCL is used for making external medical & dental casts.
More common industrial uses include coatings, adhesives, as a masterbatch carrier (typically for colourants), and for making biodegradable bags & films.
The fact that poly-oxepanone is easy to melt in hot water, yet does not actually feel hot to touch, means it has found some use as a convenient modelling resin. In this form it typically sells for just(!) £20-30 per kg. However, the exact source and history of these online resins is rarely disclosed, hence they lack the provenance and formal independent evaluation needed for reliable use in biodegradable applications.
Some glow-in-the-dark PCL modelling resin containing Nemoto's high-persistence aluminate phosphors.
compared to nylon
Despite some online assertions, poly-oxepanone (aka poly-caprolactone, PCL) is nothing like nylon; A closer comparison would be with poly-ethylene.
Versus nylon 66 (usually the material of choice for cable/zip ties), poly-oxepanone is much softer and more flexible, especially at low temperatures. However, it provides only about ⅓ the strength of nylon, and is not suitable for even moderately high temperature applications. Replacing a nylon product with a PCL alternative is therefore not as simple as just swapping the material; A complete redesign is usually required.
The two materials have the following physical properties:
| Property | PCL | PA66 |
|---|---|---|
| Formal name | (1,7)-poly-oxepan-2-one | "polyamide 6-6" |
| Common name | poly-caprolactone | nylon (66) |
| Molecular weight | 114 | 226 |
| Crystallinity (post-processing) | 50-60% | 35-45% |
| Water at saturation | <1% | 8% |
| Glass transition | -64°C | 40°C |
| Melting point | 64°C | 260-280°C |
| Specific heat capacity | 2.0 J/g/K | 2.5 J/g/K |
| Enthalpy of fusion | 70 J/g | >180 J/g |
| Energy to melt 1g (from 25°C) | 220 J | 870 J |
| Thermal conductivity (at 25°C) | 0.05 W/m/K | 0.25 W/m/K |
| Yield stress | 17.5MPa | >50MPa |
| Yield strain | 25% | 20% |
| Elongation at break | >700% | >100% |
| Flexural modulus | 410MPa | ca. 1000MPa |
| Shore hardness | 95A/50D | 80D |
| Density (apx) | 1.15 g/cm3 | 1.10 g/cm3 |
| CAS number | 24980-41-4 | 32131-17-2 |
recycling & compatibility
Biodegradable plastics are not generally suitable for post-consumer recycling as they can contaminate the waste stream. However, at concentrations of a few percent, poly-oxepanone is potentially safe to recycle because it is compatible with a wide range of other common polymers, including the following:
| PE (Polyethylene, LD & HD) |
| PP (Polypropylene) |
| PVA (poly vinyl acetate) |
| EVA (ethylene vinyl acetate) |
| PVC (poly vinyl chloride) |
| ABS / PS (Polystyrene) |
| Polybutadiene |
| PMMA (Acrylic) |
| PA (Polyamide / Nylon) |
| PU (Polyurethane) |
| Natural rubber |
| Butyl rubber |
| ... |
Poly-oxepanone (PCL) is also compatible with most other biodegradable polyesters, such as PLA (poly lactic acid), PBS (poly butylene succinate) and PBAT (poly butylene adipate terephthalate). In many cases, the addition of a few percent PCL will significantly improve the flexibility and impact resistance of the resulting compound. It has been used to this effect in some proprietary biopolymer blends.
biodegradable vs sustainable
Consumers are increasingly demanding more sustainable products: renewably-resourced, recyclable, and ultimately biodegradable in the environment. Unfortunately, there is currently no material that readily fulfills all of these requirements.
No bioplastics are currently circularly recycled, and trade-offs must be made between polymers that are renewable, degradable and practically useful.
The renewably-resourced polymers that are currently available (TPS, PHA, PLA) tend to have poor physical properties, and they often end up in proprietary compounds that try to mitigate this. Conversely, the polymers that work best as thermoplastics (PBAT, PBS, PVOH) are either non-renewable or only modestly biodegradable in the environment.
Despite the cost, we consider poly-oxepanone (PCL) to be the best compromise polymer available at present. Although it is not currently produced from renewable resources, it is technically possible to do so (and may become economically possible in the future). Also, because PCL is a very low melting-point polymer, it requires far less energy to process than the alternatives.
This table summarises the current state-of-the-art; Hover over the table elements for additional details:
| Polymer | RR? | Recycle? | Degrades? | Utility? |
|---|---|---|---|---|
| TPS |  |
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| PHA | |
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| PLA | |
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| PCL | |
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| PBS | |
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| PBAT | |
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| PVOH | |
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