SUSTAINABLE POLYMERS FOR HEALTHCARE HELP ADVANCE THE CIRCULAR ECONOMY

Citation: Fix A, de Brouwer H, “Sustainable Polymers for Healthcare Help Advance the Circular Economy”. ONdrugDelivery, Issue 112 (Sep/Oct 2020), pp 38-42.

Alexander Fix and Hans de Brouwer provide an update on activities undertaken by SABIC’s Specialties business to address the need for sustainable materials in the pharma world.

Sustainable materials are receiving increased attention by customers across industries. While efforts are primarily focused on food packaging, the healthcare industry is also looking for more environmentally responsible solutions. This article looks at the activities SABIC’s Specialties business is undertaking in order to address the use of sustainable materials in the pharmaceutical industry.

THE PLASTIC DILEMMA

“A study from McKinsey
and the Ocean Conservancy shows that land-based plastic waste ending up in the oceans will reach up to 20 million metric tons by 2030 if necessary corrective actions are not taken.”

As they have for many decades, plastics support innovation in the healthcare industry and can help improve medical treatment and outcomes for patients. At the same time, plastics have received criticism due to plastic waste being discarded in the environment. A 2018 article from McKinsey and Company reported that around 260 million tonnes of plastic waste are being created every year.1 This quantity of plastic waste is equal to the mass of around 26,000 Eiffel Towers.2

Only 16% of plastic waste is collected for recycling.1 Eight million metric tons of plastic escape into the oceans every year, as reported in National Geographic.3 A study from McKinsey and the Ocean Conservancy shows that land-based plastic waste ending up in the oceans will reach up to 20 million metric tons by 2030 if necessary corrective actions are not taken.4 Beverage bottles, caps and lids are among the top single-use items found on ocean shores.5

Next to these waste problems, production, processing and incineration of plastics generate carbon dioxide (CO2) contributing to increased climate change.

CHEMICAL UPCYCLING OF PLASTIC WASTE

How can polymer technology continue to drive innovation in the medical industry while helping to create meaningful uses for plastic waste and reduce CO2 emissions?

One answer is chemically upcycled polybutylene terephthalate (PBT) resin that is synthesised from single-use polyethylene terephthalate (PET) bottles. This solution is created by reusing post-consumer plastic waste to produce a drop-in replacement for virgin PBT, which avoids the need to consume fossil-based feedstocks to produce more virgin resin. This approach can help support the reduction of CO2 emissions, plastic waste, energy demand and water consumption, according to a Lifecycle Assessment study completed by SABIC in 2020.6 The study featured a comparison with virgin PBT produced with the terephthalic acid process.

MAKING PBT MORE SUSTAINABLE

“SABIC is using its chemical upcycling technology to produce PBT from post-consumer PET. Rather than mixing molten recycled PET into a new plastic, SABIC uses PET as a feedstock in a chemical process that deconstructs the material down to its basic building blocks.”

PBT is an engineering plastic that is often used in the healthcare industry. It processes more predictably than PET and, being a semi-crystalline resin, offers excellent chemical resistance. PBT is made from 1,4-butanediol and either terephthalic acid or dimethyl terephthalate building blocks. These monomers are derived from crude oil – a fossil-based, non-renewable resource – through several intermediate steps. At each step, purification takes place, resulting in a well-defined polymer product.

Efforts to replace both the diol monomer and the diacid monomer with alternatives that are more sustainable, such as bio-sourced components, typically lead to polymers with different processing behaviour and material performance. Even small structural changes to either monomer can have large effects on processing, crystallisation behaviour and material performance. The resulting polymer may be interesting on its own, but it is never a drop-in solution for an existing application.

Performance changes can also occur when mechanically recycled plastics are mixed with new material. First, the composition of most mechanically recycled plastics from post-consumer sources shows greater variation compared with non-recycled plastics. Mechanically recycled plastic may also be a mixture of different types of plastic and contain traces of its first life: colourants, stabilisers, flame retardants and the like. Some of these components are unacceptable in healthcare-grade plastics. They can restrict colour options and affect food contact compliance and biocompatibility.

UPCYCLING PET TO HIGHER-VALUE PBT

Figure 1: Approximately 67 0.5 L PET bottles go into each kilogram of virgin-quality LNP™ ELCRIN™ iQ engineering resin.

To help address this challenge, SABIC is using its chemical upcycling technology to produce PBT from post-consumer PET. Rather than mixing molten recycled PET into a new plastic, SABIC uses PET as a feedstock in a chemical process that deconstructs the material down to its basic building blocks. These building blocks are then purified with similar rigor as chemicals coming from a fossil source, before being fed into the polymerisation process to prepare PBT resin. SABIC’s process uses dissolved monomers and oligomers from PET and adds fossil-based 1,4-butanediol to produce LNP™ ELCRIN™ iQ resin (PBT).

This upcycling process converts commodity plastic waste into virgin-quality engineering resin (approximately 67 0.5 L PET bottles – see Figure 1 – go into each kilogram) at reduced CO2 emission levels compared with the production of virgin resin. Not only does 60% of the material originate from the waste stream, but this resin’s performance is equivalent to that of traditional PBT resin. In addition, PBT may be used for more-durable applications than PET, such as medical device housings. In this way, upcycling can extend the lifespan of PET beyond its original application in disposable water bottles.

As shown in Figure 2, the upcycled PBT reduces global warming impact by 29%, has 43% less cumulative energy demand and consumes 15% less water compared with virgin PBT. All these values contribute to an improved environmental footprint.6

Figure 2: Comparison of resin cradle-to-gate footprint data.6

MATERIAL PROPERTIES FOR HEALTHCARE APPLICATIONS

The medical industry typically requires healthcare-grade materials to meet specific criteria, such as biocompatibility pre-assessment; management of change; plant and formulation lock; long-term supply guarantee; and production according to good manufacturing practices (GMPs). Further, each end application has its own set of material requirements.

Technical datasheets provide many of the important properties of a material: mechanical performance, electrical properties, rheological parameters and heat resistance, for instance. The properties of the upcycled PBT material are similar to those of a standard, unfilled PBT of comparable viscosity. Table 1 compares key properties of a representative grade for the upcycled PBT, LNP ELCRIN W1000JiQ resin, with those of a reference PBT material, SABIC’s VALOX™ HX260HPR resin. This comparison reveals the strong similarities between the two materials. Even a parameter such as mould shrinkage, which is sensitive to small material differences, was identical (1.2%) in a side-by-side comparison on a tensile part specimen.

LNP™ ELCRIN™ W1000JiQ resinVALOX™ HX260HPR resinTest Method
Tensile modulusMPa24502400ISO 527
Stress at YieldMPa5454ISO 527
Stress at BreakMPa2528ISO 527
Strain at Yield%3.33.5ISO 527
Strain at Break%9384ISO 527
Impact Strength (23C)kJ/m255ISO 180/1A
Impact Strength (-30C)kJ/m255ISO 180/1A
Vicat softening temp.°C165170ISO 306, 50N
Heat deflection temp.°C122123ISO 75 0.45MPa
Heat deflection temp.°C5254ISO 75 1.8MPa
Mould shrinkage parallel%1.21.2SABIC method

Table 1: Mechanical performance comparison.

Environmental stress cracking resistance (ESCR) was tested using PDI’s (NJ, US) Sani-Cloth® AF3 Germicidal Disposable Wipes, which include one of the strongest chemicals used on medical devices, as well as Banana Boat® Sunscreen lotion (Edgewell Personal Care, CT, US), which represents an aggressive skin contact situation. The tests featured tensile bars in continuous contact with these chemicals under 1% strain at room temperature for seven days. Table 2 shows that both materials performed very well against these harsh chemicals. A positive rating (+) in this table indicates over 90% retention of yield strength and 80–140% retention of elongation after exposure, indicating no significant change.

Retention of:LNP™ ELCRIN™ W1000JiQ resinVALOX™ HX260HPR resin
Yield StrengthElongation at BreakYield StrengthElongation at Break
PDI Sani-Cloth® AF3++++
Banana Boat® Sunscreen++++

Table 2: ESCR testing results.

ANALYSING TWO PBT MATERIALS

However, there is much more to a material than what is shown on a datasheet. Analytical techniques allow us to collect additional information, down to a material’s molecular level. Infrared spectroscopy detects molecular vibrations and is often used for fingerprinting purposes: it can determine if materials are the same without interpreting and labelling every individual peak in the spectrum.

The infrared spectra overlay graph (Figure 3) shows matching fingerprints for the standard PBT and the upcycled PBT materials, indicating no significant difference between the two.

Figure 3: Infrared spectra of the two types of PBT indicate no structural differences. Peaks are the same in both samples.

Other analytical techniques allow us to study extractables. These components can potentially migrate out of a material under extreme conditions. Extractables depend, not only on the type of polymer, but also on the quality of the raw materials used to build the molecules and the specifics of the chemical process. We studied both volatile and non-volatile organic components and ran an elemental analysis with a focus on metals.

The elemental analysis was conducted using inductively coupled plasma mass spectrometry (ICP-MS) on an acidic liquid extract that had been in contact with the polymer material for three hours. This mimics a worst-case scenario as metal ions are typically extracted more actively under such conditions than in a pH-neutral aqueous environment. Of the 39 elements analysed, 34 were not detected above their individual quantification limits or the 0.010 ppm reporting limit.* Quantified elements, which are listed in Table 3, are low levels of common elements of no particular concern. In this particular data set, the levels of these elements in the upcycled PBT are lower than those in the virgin PBT.**

Volatile organic components (VOCs) were measured by analysing the gas emitted from a sample kept at 150°C for 45 minutes using gas chromatography with time-of-flight mass spectroscopy (GCMS). Under these extreme conditions, tetrahydrofuran (THF) was detected in both samples (Table 3). THF is a known by-product of PBT synthesis. In a polymerisation with terephthalic acid (virgin PBT), it is present at higher levels than in the chemically upcycled PBT, which is formed from transesterification, consistent with the literature.7

Figure 4: Structural formula of cyclic PBT dimer.

Further, ethanol was used to extract organic components from the samples at 70°C for 24 hours. The extracts were analysed for semi-volatiles with liquid chromatography coupled to a quadruple mass spectrometer (LCMS), and for volatiles using GCMS. Quantification was executed using an internal standard. Both techniques revealed the presence of short cyclic polymer chains of PBT (Figure 4, Table 3).

ElementLNP™ ELCRIN™ W1000JiQ resin
(ppm)
VALOX™ HX260HPR resin
(ppm)
Method
Sodium (Na)0.190.50Elemental analysis, ICP-MS
Magnesium (Mg)0.070.14Elemental analysis, ICP-MS
Sulphur (S)< 0.020.05Elemental analysis, ICP-MS
Potassium (K)0.070.25Elemental analysis, ICP-MS
Potassium (K)0.400.60Elemental analysis, ICP-MS
THF4.936.8headspace, GCMS
cyclic PBT dimer<0.112.0ethanol extract, GCMS
cyclic PBT dimer3.212.0ethanol extract, GCMS
cyclic PBT dimer0.72.2ethanol extract, GCMS
cyclic PBT tetramer<0.10.1ethanol extract, GCMS

Table 3: Results from elemental analysis.

These molecules are an integral part of the PBT molecular weight distribution. During the polymerisation process, all molecules pass through the stage of short chain oligomers. Molecules that happen to react with themselves form cyclic structures that stop growing due to the absence of a reactive chain end (either a carboxylic group or a hydroxyl group). As such, their presence is to be expected. The process to form LNP ELCRIN iQ resin results in fewer of these structures than the virgin PBT process in the current comparison. Neither sample showed any unidentified components. In a similar extract using hexane, lower levels of the same components were found.

DROP-IN SOLUTION FOR TRADITIONAL PBT

All snowflakes appear the same but none are identical. The same is true of plastics. It all depends on how deeply they are evaluated. This study was not restricted to a high-level comparison of macroscopic properties for upcycled and traditional PBT. It looked closely at the subtle differences between SABIC’s sustainability driven PBT material and the oil-derived commodity product. Importantly, there were no significant differences between the two products: the same components were observed, and a quantitative comparison confirmed the cleanliness of the upcycled PBT product.

This analysis indicates that LNP ELCRIN W1000JiQ resin can be considered a more-sustainable, drop-in replacement for traditional PBT in many healthcare applications. This material also features formulation lock, US FDA food contact compliance and a stringent management of change process – and is produced according to GMPs.

* Not detected above quantification or reporting limit: Li, Be, B, Al, P, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, As, Se, Sr, Zr, Nb, Mo, Ag , Cd, Sn, Sb, Cs, Ba, La, Ce, Hf, W, Hg, Tl, Pb

** Please note that even though the data in this document have been generated with the utmost care and most analyses have been executed in duplicate, the sample set is restricted to a single representative sample of LNP ELCRIN W1000JiQ PBT resin and VALOX HX260HPR PBT resin. It does not include statistical analyses of multiple lots, nor does it provide a complete picture of all available virgin PBT materials. Hence, a customer should always independently assess suitability of the material for the intended application.

REFERENCES

  1. Hundertmark T, McNally C, Simons T J and Vanthournout H, “No time to waste: What plastics recycling could offer”. McKinsey & Company, September 21, 2018.
  2. https://www.wonders-of-the-world.net/Eiffel-Tower/Weight-of-the-Eiffel-tower.php
  3. Parker L, “The world’s plastic pollution crisis explained”. National Geographic, June 7, 2019.
  4. “Saving the ocean from plastic waste”. McKinsey & Company, November 1, 2015.
  5. “Plastic in the ocean: the facts, effects and new EU rules”. News European Parliament, October 24, 2018.
  6. SABIC Lifecycle Assessment Study, 2020, data available upon request. (ReCiPe 2016 Midpoint (H) V1.03 / World 2010 H, Cumulative Energy Demand V1.11 and IPCC 2013 GWP100a V1.03 used to measure CED and GW Impacts.)
  7. Devroede J. “Study of the THF formation during the TPA-based synthesis of PBT”. Eindhoven: Technische Universiteit Eindhoven, 2007.

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