Elsevier

Progress in Polymer Science

Volume 71, August 2017, Pages 144-189
Progress in Polymer Science

Review
Lifetime prediction of biodegradable polymers

https://doi.org/10.1016/j.progpolymsci.2017.02.004Get rights and content

Abstract

The determination of the safe working life of polymer materials is important for their successful use in engineering, medicine and consumer-goods applications. An understanding of the physical and chemical changes to the structure of widely-used polymers such as the polyolefins, when exposed to aggressive environments, has provided a framework for controlling their ultimate service lifetime by either stabilising the polymer or chemically accelerating the degradation reactions. The recent focus on biodegradable polymers as replacements for more bio-inert materials such as the polyolefins in areas as diverse as packaging and as scaffolds for tissue engineering has highlighted the need for a review of the approaches to being able to predict the lifetime of these materials. In many studies the focus has not been on the embrittlement and fracture of the material (as it would be for a polyolefin) but rather the products of degradation, their toxicity and ultimate fate when in the environment, which may be the human body. These differences are primarily due to time-scale. Different approaches to the problem have arisen in biomedicine, such as the kinetic control of drug delivery by the bio-erosion of polymers, but the similarities in mechanism provide real prospects for the prediction of the safe service lifetime of a biodegradable polymer as a structural material. Common mechanistic themes that emerge include the diffusion-controlled process of water sorption and conditions for surface versus bulk degradation, the role of hydrolysis versus oxidative degradation in controlling the rate of polymer chain scission and strength loss and the specificity of enzyme-mediated reactions.

Introduction

Plastics are ubiquitous in our modern culture, having excellent and tailorable material properties, with controllable flexibility and strength and the ability to be moulded into shape. They are also cheap, durable, relatively impermeable, sterilisable, and with a high strength to weight ratio. The application of plastic film as packaging and other disposable items is particularly important, with approximately 40 million tonnes of plastic film and sheet produced from polyethylene alone [1], [2], [3].

There has been considerable interest in the use and optimisation of biodegradable polymers as an alternative to polyolefins such as polyethylene for such applications. Much of this has been driven by increasing concerns about land, water and, in particular, marine pollution that arise from the inherent resistance of polyolefins to environmental degradation [4].

Biodegradable plastics can originate from renewable sources (e.g., starch and polyhydroxyalkanoates) or biodegradable synthetic polymers (e.g., petroleum derived polyesters). The most widely studied biodegradable polymers have been either polysaccharides (cellulose and its derivatives, particularly starch) or aliphatic and mixed aliphatic/aromatic polyesters. Fig. 1 summarises the stages in degradation for biodegradable polymers, where the primary mode of degradation is chain cleavage through hydrolysis (either through abiotic (non-enzymatic) hydrolysis or enzyme-promoted hydrolysis), unlike oxo-degradable systems which are very resistant to hydrolysis [5]. There are four key variables, and the relationships between them, which are critical to the mechanism of polymer erosion (covered in detail in Section 4.4):

  • The rate of water diffusion into the polymer (D) and the pseudo first order rate of hydrolysis (λ′)

  • The thickness of the specimen (L) and the critical thickness (Lcrit)

Under a surface erosion mechanism (λ′ > D; L > Lcrit), polymer is eroded from the surface and the core polymeric material remains intact (retaining average molecular weight M¯n and mechanical properties), as the load bearing capability decreases steadily until the thickness of the polymer is less than the critical thickness. At this point the mechanism of erosion shifts to bulk erosion (λ′ < D; L < Lcrit), where the time to failure becomes dominated by the rate of auto-acceleration of hydrolysis, where M¯n reaches a critical value M¯e. From this point, the polymer depolymerises into water-soluble products (oligomers and monomers), which are then assimilated by micro-organisms into biomass or mineralised to CO2, H2O, CH4 and other metabolic products.

However, the use of biodegradable plastics has been limited by their higher cost, moisture sensitivity, narrow processing windows, low heat deflection temperatures, and/or poor barrier and conductivity properties [5]. In addition, thorough life cycle assessments (cradle to grave) need to be carried out to assess the relative environmental impact of each polymer type.

While technological solutions are being developed for many of the property limitations described above, the core challenge remains: to understand the factors that will ultimately control the time over which biodegradable polymers will maintain their integrity and material properties when exposed to different environments. The environmental stresses usually considered in association with the deterioration of performance outdoors are elevated temperatures and solar radiation as well as mechanical stresses and rainfall/moisture. However, other factors such as chemical conditions and, particularly for soil burial, biological activities including enzymatic and other microbial and biological processes (such as impacts of roots and fungal hyphae) are also factors.

In parallel with the use of polymers in the external environment, there is the increasing use of controlled-lifetime polymers in biomedical applications such as drug delivery, tissue engineering, scaffolds and prosthetics. In this case the environment of concern is a particular part of a human or animal body. In these applications, “lifetime” has a different meaning depending on the function the polymer is performing in the body. The medical applications of a biodegradable polymer are the most challenging of all due to:

  • The need for compatibility with body tissue of both the original polymer and its degradation products;

  • The requirement for properties to continually change as the medical function is progressively met, e.g., a scaffold for tissue regeneration must progressively weaken so the new tissue can assume the biological function and replace the implant;

  • In the case of polymer-controlled drug delivery, the kinetics of release will depend on whether the degradation of the carrier polymer is controlling release or whether this occurs through migration following water uptake and swelling. The rate of biodegradation may be less important if the polymer is orally administered compared to subcutaneous or pulmonary delivery.

If one is able to focus on the physical and chemical property changes in the polymer when exposed to different environments, then results in one application may be translatable to others. The key principle is the extent of degradation of the polymer that constitutes end-of-life when in that particular application. If the rate of change of the property is known for this environment then the lifetime can, in principle, be predicted.

Lifetime prediction therefore requires the measurement of the kinetics of the chemical, physical and/or biological reactions that result in bond scission and subsequent chemical transformations that constitute the degradation process under the combined environmental stresses (shown in Fig. 1), together with knowledge of the extent of degradation that constitutes the end of the safe service life [7], [8], [9].

In this paper, the fundamental principles that underlie the biodegradation of biodegradable polymers are summarised and then recent literature on the environmental performance and prediction of the lifetime of these polymers is reviewed.

Section snippets

Definitions

The literature associated with biodegradable polymer degradation and biodegradation is inconsistent with respect to the terms used to describe different stages and aspects of degradation. In this review, we have adopted the definitions as listed in the Standards, PD CEN/TR 15351:2006 and ASTM D883 [10], [11]:

Aerobic biodegradationBiodegradation under aerobic conditions (oxygen present)
Anaerobic biodegradationBiodegradation under anaerobic conditions (oxygen absent)
BioassimilationConversion of a

Polymer degradation – an overview

The short overview of the principles of polymer degradation as a whole that is covered in this section is not a comprehensive review of the field but rather summarises the core concepts and formulae that need to be understood in order to undertake lifetime prediction in biodegradable polymers.

Polymer degradation can be defined as “a deleterious change in the chemical structure, physical properties, or appearance of a polymer, which may result from chemical cleavage of the macromolecules forming

Hydrolytic biodegradation

The processes involved in hydrolytic biodegradation are complex, in that the interactions of living organisms with susceptible (biodegradable) polymers such as polysaccharides, polyesters and their aliphatic and aromatic copolymers, and polyamides play a large role. Such polymers can be degraded through a variety of mechanisms (via photo, thermal, mechanical and chemical degradation), which can act alone or in combination, often synergistically [28]. It has, for example, been observed that the

Enzyme promoted degradation – effect on kinetics and mechanism

So far, this review has focussed on abiotic processes and their modelling. However, biodegradation is in large part driven by enzymatic processes. Enzymes work through lowering of the activation energy of a reaction such that the reaction rate can be increased under conditions that are otherwise unfavourable, e.g., at room temperature in water at neutral pH. In the presence of enzymes, an increase in reaction rates by 108 to 1020 can commonly be observed [192]. Enzymes are proteins that have a

Environmental biodegradation

Enzymatic degradation of polymers in a controlled aqueous environment, although complex, is still relatively predictable. In contrast, real world (environmental) biodegradation becomes much more complex to understand and predict. To our knowledge, to date, there is no model for predicting lifetimes in these circumstances for any class of biodegradable polymer. The prediction of polymer lifetime requires all elements of degradation to be accounted for [272] and this can be difficult to achieve

Summary and conclusions

The ultimate goal of lifetime modelling for all classes of polymers is to predict the degradation rate, taking all controlling variables as input. However, at this point, both existing models and the fundamental understanding of degradation mechanisms and interactions, particularly in a natural environment, are not sufficiently advanced as to be able to achieve this with a single unified theory. From this review, common approaches have emerged that are able to be translated from the disparate

Acknowledgements

The authors would like to thank and acknowledge the Cooperative Research Centre for Polymers and Integrated Packaging for financial support of this work. Dr Paul Luckman is acknowledged with thanks for his assistance with graphic design and artistry.

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