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Compatibilisation of blends of TPUs and a fluoroelastomer
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Miraglia, Azzurra Ramona (2021) Compatibilisation of blends of TPUs and a fluoroelastomer. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3764580
Abstract
The rapid expansion of different industrial sectors (e.g. automotive, footwear, cable insulation) over the recent past, has precipitated the demand for high-performance materials. The blending of polymers that are already available in the market is among the low-cost alternatives for developing new polymeric materials that allow improvements in properties not necessarily exhibited by the components alone. An unusual combination of mechanical, thermal and chemical properties can be provided by blending. Several different types of polymer blends have been developed whose properties are tailored for specific applications. Thermoplastic vulcanisates (TPVs) are a particular class of blends of thermoplastic elastomers (TPEs), where rubber-like properties are achieved by dispersing and crosslinking a rubber phase in a thermoplastic matrix, upon dynamic vulcanisation. The selection of proper vulcanizing agents (e.g. sulphur, peroxides) is therefore vital for the preparation of TPVs so as to achieve desired properties. However, preparing high-quality blends is a non-trivial process as the vast majority of polymers are incompatible resulting in the formation of immiscible blends showing inferior properties relative to those of the individual components (such as poor tensile strength, elongation at break and compression set). Nonetheless, immiscible polymer blends are often preferred over the miscible types, as they can combine some of the important characteristics of each component providing enhanced performance, and offering low-cost alternatives, rather than having to synthesize new monomers and polymers yielding similar properties. For this reason, several compatibilization routes (e.g. addition of a third component such as a block copolymer or reactive compatibilization) have been developed to address miscibility between blend components, in an attempt to achieve a more stable morphology and better mechanical properties than immiscible polymer blends. Another important challenge is the selection of the appropriate crosslinking system, e.g. peroxide type when producing TPVs. Indeed, blending of a thermoplastic with rubber necessitates the mixing to be done above the melting point of the thermoplastic, and very few crosslinking systems have a long enough half-life to prevent the degradation of the peroxide initiator and the premature crosslinking of the rubber. These challenges limit the development of such composites on an industrial scale.
The work described in this thesis focuses on the preparation and characterization of different blends of various thermoplastic polyurethanes (TPUs) with a fluoroelastomer (FKM) in the attempt to obtain novel TPVs with enhanced properties (e.g. mechanical and solvent resistance), addressing the requirement for compatibility between blend components, as well as optimizing the processing conditions for the preparation of TPVs.
In particular, different blend compositions of TPUs with FKM were prepared by both physical blending and dynamic vulcanization techniques, and different processing strategies explored, particularly for the compounding of TPVs (different types of equipment, processing condition, feeding protocol). Indeed, this investigation revealed the relative ease at which TPU: FKM blends can be prepared using conventional polymer processing techniques, such as extrusion and batch mixing.
The use of novel compatibilisers (C1, C2, C3, and C4) in tuning blend properties was also investigated. Moreover, two different peroxide systems were selected in an attempt to dynamically crosslink the rubber phase when mixed in the thermoplastic phase. The morphology and the thermal, rheological, dynamic-mechanical and quasi-static mechanical properties of the pristine materials and their compounds were investigated through a wide range of characterization techniques. The thermal stability of the pristine components in the processing temperature window of interest was confirmed by thermogravimetric analysis (TGA). Moreover, this analysis confirmed the scorch-protection nature of Luperox. Indeed, it was seen that the decomposition of Luperox starts at about 20oC higher than that of Varox, which is vital to improving scorch protection during compounding. Scanning electron microscopy (SEM) showed an ‘island-sea’ structure for unvulcanised TPU: FKM blends at a ratio of 80:20vol%, with the morphology approaching a co-continuous phase at intermediate compositions (i.e. 60:40vol% and 50:50vol%), and reaching a phase inversion when the FKM content was further increased (i.e. 40:60, and 20:80 vol%). Moreover, the addition of C1 to the blends led to finer and more uniform structure, due to increased adhesion between the two polymeric phases.
Transmission electron microscopy (TEM) imaging of the vulcanised TPU2: FKM blends at a ratio of 80:20 (wt%) revealed the presence of non-monodisperse dispersed FKM particles, ranging in diameter from ca. 500 nm to 5 μm, within the TPU2 matrix. Moreover, the presence of the compatibiliser surrounding the FKM particles was seen when C1 and C2 were added to extruded blends (compounded with Varox), indicating that the migration/diffusion of the compatibilisers from the bulk to the interface of the blend components had occurred. These observations may explain the improvements in Young’s modulus, strength, and ductility obtained with the addition of C1 with respect to the corresponding incompatible blend, indicating the contribution of C1 in improving the compatibility between TPU2 and FKM. Similar improvements in Young’s modulus, strength, and elongation at break were achieved for the blends compounded with C3. An important outcome of this work, obvious from the stress-strain behaviour of both vulcanised and unvulcanised blends, showed that blending TPUs with FKM yielded in materials with interesting mechanical properties intermediate between those of the individual components, making the blends very interesting for exploitation in various sectors, some of possible interest to the industry partner. Even though the evidence for FKM vulcanization in the TPU matrices is not fully confirmed, the blends prepared both with and without compatibiliser showed useful mechanical properties intermediate between the individual components. The industrial partner (Lubrizol Corporation) believes these blends, i.e. where the crosslinking of the rubber is not critical, may have application, such as adhesives, in stain-resistant wearable electronics and cable insulation.
Item Type: | Thesis (PhD) | ||||
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Subjects: | T Technology > TA Engineering (General). Civil engineering (General) T Technology > TP Chemical technology |
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Library of Congress Subject Headings (LCSH): | Polymers, Thermoresponsive polymers, Elastomers, Polyethylene, Vulcanization, Thermoplastic composites | ||||
Official Date: | June 2021 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Warwick Manufacturing Group | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | McNally, Tony | ||||
Sponsors: | Warwick Manufacturing Group ; University of Warwick ; Lubrizol Corporation | ||||
Format of File: | |||||
Extent: | xxi, 243 leaves : illustrations | ||||
Language: | eng |
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