Thermoplastic elastomers (TPEs) have been traditionally compounded and manufactured from raw materials based on fossil fuels. Current trends in marketplace abounds sustainability programs. TPEs are no exception to this trend. In a recent editorial, the authors stated “Through research and application, sustainability can evolve from a catchphrase to a societal one”1. More than two decades ago the Brundtland Commission (formerly the World Commission on Environment and Development, WCED), deliberated sustainable development issue and gave a definition of sustainability:
“Sustainable development meets the needs of the present without compromising the ability of future generations to meet their own needs.1a
Keeping sustainability in mind, innovative developments are taking place in new TPE technologies based on bio-renewable resources. The challenge however, is to provide the TPE product designers and the TPE product manufacturers with flexible products meeting broadest range of useful properties those contain a bio-renewable content. Towards this objective, PolyOne has established “PolyOne Sustainable Solutions” certification to denote those products or services that meet defined standards for sustainability in areas such as renewability, recyclability, reusability, and eco-friendly composition or resource efficiency. Let’s take a quick look at each of these categories to better understand how new TPEs to be discussed in this article provide sustainable solutions.
Managing carbon in a sustainable manner is the central issue of sustainable development. Traditional polymers such as PE, PP or PS are based on fossil feed stocks whose rate of carbon fixation is in millions of years, whilst their rate of release of carbon into the environment is 1-10 years. Clearly this increases atmospheric CO2. On the other hand polymers which are produced from plant based, renewable feed stocks have their end-of-life CO2 release absorbed during the next planting season, thus balancing the CO2 release and trending towards a zero carbon footprint2. Polymers made from bio based materials may be derived from renewable resources by chemical methods, mechanical methods or produced directly by biological processes. TPE compounds based on renewable substances thus contribute to sustainable development by reducing the carbon footprint of the formulated TPE.
Recyclability considers solutions which incorporate post-consumer or post-industrial recycled content or which lend themselves to recycling such as PolyOne’s PlanetPak™ packaging system. Among the various other advantages that come from processing thermoplastic materials, recyclability is a key factor for many processors when specifying TPEs in place of thermoset rubbers. Not only can scrap produced during production (i.e. sprues) be ground and re-introduced with the virgin material, but finished articles can be reprocessed and recycled as well.
TPEs can also make use of recycled products as a component of their formulation, for example employing post-consumer waste such as recycled tires, or post-industrial waste such as rubber dust or leather fibres. Not only can these compound components provide sustainable solutions they can also reduce cost and provide functionality.
Reusability considers packaging and other logistics related systems which are easily returned or reused. Therefore, another important factor is to develop packaging & logistical solutions which help reduce waste and save costs. Furthermore, PolyOne has pioneered novel, proprietary dusting systems which allow bulk packaging of TPEs that might otherwise have required more wasteful and costly packaging.
Eco-friendly compositions are those solutions which respond to the ever-changing market needs by offering alternatives to traditional ingredients such as lead, bisphenol-A (BPA), phthalates and halogen-containing materials. In many of the markets served by TPEs, there are compounds available which address these concerns. For example in the healthcare market, TPEs are designed to be phthalate free, plasticizer free and with low extractable, meeting stringent regulations and addressing real or perceived patient safety concerns4. TPEs typically employ medical quality mineral oils which are much less scrutinized, but for the most sensitive applications plasticizer free TPEs offer the lowest level of leachable compounds. Combined with careful component selection and specialized compounding techniques, TPE compounds can offer a very high level of purity and safety.
Which types of Renewable TPEs are available in the current market?
A number of different reactor produced TPE polymers have been commercialized by various manufacturers over the last few years.
Arkema’s Pebax Rnew is a polyether-block-amide (PEBA) block copolymer consisting of segments of amino-11 and polyether. Since amino-11, is a derivative of castor oil, it is a biorenewable polymer with low environmental impact. Arkema’s biorenewable TPEs contain 26-100% renewable content and have hardnesses from 35 to 72 Shore D. They are promoted for various applications including electronics, shoes and automotive3,4.
DuPont’s renewable Hytrel RS thermoplastic polyester elastomers are based on their Cerenol polyols derived from bio-1,3propanediol produced from corn sugar using a patented bacterial fermentation process. Hytrel RS thermoplastic polyester elastomers contain 20 to 60% renewable based material. They have hardnesses in the range of 30 to 83 Shore D and are promoted for use in applications such as hoses, tubing boots, energy dampers and airbag doors in automotive and industrial markets5.
Merquinsa’s Pearlthane ECO TPUs are produced from specially derived vegetable based polyols reacted with isocyanates in a novel patent pending process. Whilst polyurethane foams using plant based polyols have been around for some time, difficulty reacting these polyols has restricted their use in thermoplastic applications, until now. Merquinsa’s Pearlthane ECO TPUs contain from 30 to 90% renewable content according to ASTM D6866 and offer performance at least as good as traditional, high performance PCL based TPUs. Products are available in hardnesses from 85 to 95 Shore A6.
In 2008, GLS Thermoplastics Elastomers launched two ranges of OnFlex™ BIO renewable based thermoplastic elastomers. These product lines extended the functionality and performance of renewable thermoplastic elastomers available in the market opening up a wider range of applications and options for sustainable solutions. Based on Merquinsa’s patent-pending Pearlthane® ECO technology these products feature at least 20% renewable material as certified by ASTM-D6866, meeting global market needs for sustainable and environmentally-friendly products. The OnFlex™ BIO 5100 series are glass fiber reinforced compounds which offer high stiffness in combination with excellent abrasion and impact resistance, a low coefficient of thermal expansion and excellent mechanical properties. Typical applications for OnFlex™ BIO 5100 are industrial applications in demanding environments, sports equipment components such as ski bindings and caster wheels.
While OnFlex™ BIO 5300 series compounds are designed to offer the performance of traditional TPUs but with a wider range of hardness and easier processability. These compounds have been designed to process well in injection molding, extrusion and calendaring processes. Moreover these compounds offer exceptional abrasion, scratch and impact resistance, excellent tensile and tear performance, and adhesion to a wide range of substrates. OnFlex™ BIO compounds are available from hardnesses of 70 Shore A upwards. Typical applications for OnFlex™ BIO 5300 compounds include automotive instrument panel skins, door panel skins, interior trim, gear knobs, shoe soles and grips on sports equipment.
For the purpose of a meaningful discussion of bio-renewable TPEs, it is important to understand the significance of terms such as Bio-sourced, Bio-degradable or Compostable.
Bio-sourced, Biodegradable or Compostable
Bio-plastics may be produced from renewable resources, biodegradable, or both. Synthetic polymers either naturally or by virtue of chemical modification or the addition of certain additives can biodegrade making waste handling simpler. Polycaprolactone (PCL) is one such example7. Certain bio plastics may be sourced from renewable resources but are not biodegradable, such as OnFlex™ BIO and Versaflex™ BIO.
The term biodegradation is not consistently used or clearly defined. It may be used to mean fragmentation, loss of mechanical properties or degradation through the action of living organisms. Sometimes deterioration or loss of integrity is mistakenly considered to be biodegradation. Even the speed of degradation has to be taken into account. For example LDPE has been shown to biodegrade slowly to produce carbon dioxide (0.35% over 2.5 years)8.
Compostable means that materials will biodegrade only in a composting situation, that is, together with plant material in an aerobic environment. Compostable plastics will biodegrade completely in the industrial compositing facilities. Not all biodegradable plastics are suitable for composting, as degradation may take too long or require higher temperatures to biodegrade completely. The European standard EN13432 and the American standard ASTM D6400-04 helps to determine which plastics are compostable9.
Biodegradation of a material concerns only one aspect of its environmental impact and whilst it can be considered as part of an integrated waste management plan, it must be considered along with other issues such as the material source and carbon footprint. In durable applications such as industrial and automotive markets biodegradation is undesirable. In this case alternative methods of disposal must be investigated2.
The new TPE series, under the trade name VersaflexTM BIO those have been recently developed has opened up a new trend in this renewable TPE material segment.
Versaflex™ BIO series in Table 1 consists of a range of unique, patent pending renewable TPE compounds with some very special aspects. These are based on novel and innovative compatible blends of a reactor produced TPE polymer, cross-linked elastomer and additives. These are the newest and most advanced renewable TPE solutions available globally. They were launched in 2009 at the NPE exhibition in Chicago. Versaflex™ BIO compounds are unique for their highest renewable content (up to 80%) for the hardness range of 20 Shore A to 90 Shore A with lowest specific gravity. They are the softest renewable products on the market today, achieving performance currently unattainable with reactor polymer technologies.
Table 1 Physical Properties of Versaflex™ BIO series
Not only do Versaflex™ BIO compounds offer very high levels of renewable content, they also help to reduce environmental impact through their low specific gravity, recyclability and ease of processability.
Dynamic Mechanical Analysis (DMA): Viscoelastic and Phase behaviour
DMA is particularly useful for determining viscoelastic behavior and phase transitions of a material, such as glass transition temperature (Tg) which can usually be easily identified by the peak of tan ? However, the peak location depends on the heating-rate and the frequency which may lead to different results than those obtained by differential scanning calorimetry (DSC)10. DMA was conducted on samples of Versaflex™ BIO in order to understand the phase behavior of these novel complex blends and to predict the performance of these compounds at different temperatures.
The storage modulus G/ and loss modulus G// curves for two Versaflex™ BIO compounds across a range of temperatures are shown in Figure 1. These curves indicates that these Versaflex™ BIO compounds have good flexibility between -30°C and above 100°C. TPEs show a single transition as seen by the data.
Figure 1. Storage Modulus, G' and Loss Modulus. G'' of Versaflex™ BIO at 1 Hz
Tan ? is another measure of the mobility which is the ratio of the storage modulus and loss modulus as shown below:
Tan ? = G// / G/
Figure 2, shows Tan ? curves for two Versaflex™ BIO compounds across a range of temperatures and indicates good flexibility and toughness of the TPEs in functional performance.
The unique ingredients employed in Versaflex™ BIO provide a wide range of hardnesses, flexibility across a wide temperature range and a high renewable content. A single Tg at low temperature clearly indicates the miscibility of the softer segment of the reactor based polymer TPE with the cross-linked elastomer. There is a little shoulder around 75oC which is the Tg of amorphous phase of the reactor produced TPE elastomer in the compounded TPE.
A number of microscopic techniques are available to understand the morphology of multiphase polymer blends. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM) are the most commonly used methods. AFM technique is the most user friendly technique requiring minimum sample preparation.
For TPEs, AFM is usually conducted in the tapping mode, meaning the cantilever is oscillated by peizo motion and brought into a light contact with the surface. As the oscillating cantilever taps the surface, energy is lost due to the contact of the tip with the surface. By measuring the cantilever amplitude information about surface morphology can be obtained. AFM characterizes hard and soft phases, based on the effect of the modulus of these regions on the cantilever11. This is particularly interesting for TPE blends which are typically composed of multiple ingredients of different hardness, crystallinity and other features.
Information collected in an AFM scan is quantitative in three dimensions. A micrograph can be presented as a topographical image with the elevation of each point encoded with a false color scale. A 3D image can be manipulated with software, viewed from different angles, and measurements may be made on horizontal and vertical distances. Surface roughness can be calculated along the lines or over planes.
Figure 3. Atomic Force Micrographs (AFM) of Versaflex BIOTM 70A and 80A Morphology
AFM micrographs of two different Versaflex™BIO compounds are shown in Figure 3. These micrographs clearly show phase separation on the µm scale. Interestingly, there is a phase change between 70 and 80 Shore A compounds; the co-continuous cross-linked elastomer (light) phase becomes dispersed into particles in a continuous reactor produced TPE matrix shown by the dark phase. These micrographs explain the improved elastic behavior and compression set for the 70 Shore A TPE compared to 80 Shore A with a penalty in tensile strength and 100% modulus.
Functional Performance: Oil Resistance
Sebum oil and Skin So SoftTM exposure simulates functional performance in consumer applications, where as IRM- 903 Oil reflects the functional performance in demanding industrial and automotive applications. We have compared test data from Versaflex™ BIO compounds with a typical TPE-S and TPE-V to benchmark their performance in Figure 5. We have looked at the % tensile change and % weight change after immersing samples in oil at specified temperatures for two weeks. We have chosen a 40 Shore A and 60 Shore A versions of the Versaflex™ BIO compounds for these evaluations and 60 Shore A for the TPE-S (TPE-styrenic block copolymer) and TPE-V (TPE-vulcanizate of EPDM in PP). The sebum resistance of VersaflexTM BIO is compared against typical TPE-S and TPE-V compounds. VersaflexTMBIO 5500-60 shows outstanding sebum resistance and is comparable to TPE-V.
Figure 4. Sebum Resistance of Versaflex BIOTM, TPE-S and TPE-V (2 weeks @ 40°C)
The soft, grippy nature of TPEs makes them attractive for use in grip applications. Among all renewable TPEs available Versaflex™ BIO is particularly appropriate for soft-touch grips due to its wide hardness range. In grip applications contact with various moisturizers and hand oils can lead to deterioration of performance, so Versaflex™ BIO was tested in contact with well known body oil, Skin So SoftTM lotion, a product of Avon Products Inc, and compared to traditional TPEs as a reference (Figure 5).
Figure 5. Skin So SoftTM Lotion Resistance of VersaflexTM BIO, TPE-S and TPE-V (2 weeks @ 23°C)
In Figure 6 performance of Versaflex™ BIO compounds in IRM 903 oil are compared to a traditional TPE-S and a TPE-V compound. Despite the highly aggressive nature of IRM 903 oil, Versaflex™ BIO performs very well, with almost no weight gain and very little change in tensile performance over the testing period. Not only does this corroborate the performance in contact with sebum and hand oils to which it might be exposed in consumer applications, but it also demonstrates performance that might be useful in industrial and automotive applications.
Figure 6. IRM 903 oil resistance of of VersaflexTM BIO, TPE-S and TPE-V (2 weeks @ 23°C)
Rheology, Processing and Applications
The hardness range and novel combination of properties make Versaflex™ BIO suitable for a very wide range of applications. The capillary rheologies of Versaflex™ BIO TPEs are shown in Figure 7. The viscosity shows shear thinning behavior and is very similar to a typical robust TPE grade which has fabrication versatility. The capillary viscosities are nearly invariant for different hardness compounds at the measured range of shear rates. .
Figure 7. Capillary rheology @200°C of VersaflexTM BIO
VersaflexTM BIO exhibit very robust processing using multiple fabrication methods. Figure 8 shows three different fabricated forms produced from Versa flex™ BIO compounds. Figure 8 a) shows stepped plaques injection molded from Versaflex™ BIO. Differing thicknesses demonstrate the translucency of the compound. Figure 8 b) shows a cast film extruded from Versaflex™ BIO, and Figure 8 c) shows a tube or hose extruded from Versaflex™ BIO.
Figure 8. 70 Shore A Versaflex™ BIO a) Molded Plaques, b) Extruded Sheet , c) Extruded Hose
Figure 9 shows a blow-molded bottle produced from Versaflex™ BIO. This can provide durability, toughness and flexibility for demanding applications with limited environmental impact. Furthermore, Versaflex™ BIO can be processed by multiple fabrication methods and can be colored to suit desired economics and aesthetics.
Figure 9. 80 Shore A Versaflex™ BIO Blow-Molded Bottle
A limited over-molding evaluation performed on these compounds indicates that they do not bond to polyolefins and have poor bonding to engineering plastics such as ABS and polycarbonate. Further evaluations for other substrates are underway. At this time, Versaflex™ BIO is not recommended for over-molding applications.
Biorenewable TPE for Overmolding Applications
There have been tremendous interests and commercial applications for soft TPE over-molded to rigid plastics. These over-molded TPEs can provide enhanced ergonomic feel and touch, gripability, aesthetics, cushion against impact, vibration isolation, etc. Versaflex BIO materials described above can be used as a stand alone TPE, however, they are not designed for over-molding applications. Recently a new line of renewable TPE with very high renewable content was developed for over-molding applications.
The adhesion between the TPE and rigid thermoplastics substrates was measured by a “90 degree peel test” which is a modified ASTM D903 method. This test is done on over-molded plaques with TPE on top of rigid thermoplastic substrates. A 25 mm wide strip of TPE is cut and pulled at a 90o angle toward the substrate using an Instron tensile tester. The substrate is locked in its place on wheels in order to maintain the 90o angle as the elastomer is pulled. The adhesion strength, i.e., peel strength, is measured by the force required to pull the elastomer from the substrate and is reported as an average or a maximum strength over 50 mm of pulling. The adhesion is also categorized based on a visual observation of the failure mode, i.e., an adhesive failure if no TPE residue is left on the substrate or a cohesive failure if the failure is in TPE.
Table 4 below listed the mechanical properties of these new renewable TPEs. These TPEs are developed from compatible blends of commercially available reactor grade TPE with crosslinked elastomers and plasticizers. The materials have a renewable content of 60-70%, and are available from 40 to 70 Shore A hardness, however, broader range of hardness can be developed based on similar chemistry. The bonding strength to polar substrates is considered good (>10 pli) to excellent (>15 pli). These are the softest and highest renewable content TPE with good overmolding properties on polar substrates.
Table 2. Physical Properties of New Bio-renewable TPE for over-molding Applications
1. J.C. Crittenden and H.S. White; Harnessing Energy for a Sustainable World, J. Am. Chem. Soc., 132 (13), pp. 4503-4505 (2010)
1a. Our Common Future, Report of the World Commission on Environment and Development, World Commission on Environment and Development. 1987
2. Narayan, R., Bioplastics or Biobased (Renewable) Materials 101. Michigan State University. The authors here use “free” as an indication that there is no intention to include the particular chemical in the mixture.
3. Eustache, R., Lé G., Silagy, D. PEBA made from Renewable Resources or How to Offer Simultaneously Sustainability and High Performance. The Eleventh International Conference on Thermoplastic Elastomers. Rapra 2008
4. Arkema, www.pebax.com
5. DuPont, www.dupont.com
6. Julia, J., Santamaria, J. New Thermoplastic Polyurethane (TPU) from Renewable Sources. The Eleventh International Conference on Thermoplastic Elastomers. Rapra 2008
7. Kulshreshtha, A. K., Vasile, C. Handbook of Polymer Blends and Composites (Vol 4, pp: 632). Smithers Rapra Technology, 2002
8. Bastioli, C. Handbook of Biodegradable Polymers. (pp: 2, 219, 257, 272). Smithers Rapra Technology, 2005
9. Renewable Polymers. www.nnfcc.co.uk, The National Non-Food Crops Centre, 2009
10. Ramachandran, V.S., Paroli, R. N., Beaudoin, J.J., Delgado, A. H., Handbook of Thermal Analysis of Construction Materials. William Andrew, 2002
11. Mark, J. E., Erman, B., Eirich, F. R., Rubber Science and Technology. (pp: 606). Academic Press, 2005
Dr. Krishna Venkataswamy is the Senior Director, Global Research and Development for GLS Thermoplastic Elastomers, PolyOne Corporation. He has been active in the TPE industry for the past 22 years and at GLS since 2001. He has held management and technology leadership positions at GAF Materials Corporation, Advanced Elastomer Systems (ExxonMobil) and Monsanto. He has developed many commercially successful products in the consumer, medical, automotive and industrial applications. Krishna has authored more than 50 technical papers and patents. He has received “SPE Fellow” award in 2006 for his technical and leadership contributions in Thermoplastic Elastomers. In addition, he has received SPEs “Outstanding Achievement Award for Leadership” in 1999. He has been the recipient of “Sherwin-Williams” award in Applied Polymer Science from the American Chemical Society in 1986 for a paper from his Ph.D work. Krishna has been very active in SPE; Current Chairman of New Technology Forum, Past Chair of EPSDIV and Board member Akron Section, Past Chair of SPE TPE Topical conferences including the first two-day International Symposium on TPEs during ANTEC during mid-90s. He has a BS in Chemical Engineering from the Indian Institute of Technology, Madras, an MS in Materials Science & Engineering from University of Florida and a PhD in Polymer Science and Engineering from Case Western Reserve University.