EBB 220/3ADAVANCED POLYMERIC MATERIALS DR AZURA A.RASHID Room 2.19 School of Materials And Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, P. Pinang Malaysia
Introduction • A number of new polymers having unique and desirable combinations of properties have been developed over the past several years. • This include: • A niches in new technologies and/or • A satisfactorily replaced other materials • Some of these include: • Ultrahigh molecular weight polyethylene (UHMWPE) • Liquid crystal polymers (LCPs) • Thermoplastic elastomers (TPe) • Nanotechnology
Ultrahigh Molecular Weight Polyethylene (UHMWPE) • Ultrahigh molecular weight polyethylene (UHMWPE) is a linear polyethylene that has an extremely high molecular weight. • Its typical Mw is approximately 4 x 106 g/molgreater than that of highdensity polyethylene • UHMWPE in fibre form has trade name ‘spectra’ • This material has a relatively low melting temperature its mechanical properties diminish rapidly with increasing temperature.
UHMWPE Characteristics • Some of the extraordinary characteristics of this material are as follows: • An extremely high impact resistance. • Outstanding resistance to wear and abrasion. • A very low coefficient of friction. • A self-lubricating and nonstick surface. • Very good chemical resistance. • Excellent low-temperature properties. • Outstanding sound damping and energy absorption characteristics. • Electrically insulating and excellent dielectric properties.
This unusual combination of properties leads to numerous and diverse applications for this material, including: • bullet-proof vests, • composite military helmets, • fishing line, • ski bottom surfaces, • golf ball cores, • bowling alley and ice skating rink surfaces, • biomedical blood filters, • marking pen nibs, • bulk material handling equipment (for coal, grain, cement, gravel, etc.), • bushings, pump impellers, and valve gaskets.
Liquid crystal polymers (LCPs) • The liquid crystal polymers (LCPs) are a group of chemically complex and structurally distinct materials that have unique properties and are utilized in diverse applications. • LCPs are composed of extended, rod-shaped, and rigid molecules. • In terms of molecular arrangement these materials do not fall within any of conventional liquid, amorphous, crystalline, semicrystalline classifications as a new state of matter-the liquid crystalline state, being neither crystalline nor liquid. • In the melt (or liquid) condition LCP molecules can become aligned in highly ordered configurations. • As solids this molecular alignment remains, and form in domain structures having characteristic intermolecular spacing.
Liquidcrystals Semi crystalline Amorphous liquid solid
High Heat Resistance Flame Retardant Chemical Resistance Dimensional Stability Moldability Heat Aging Resistance Adhesion Low Viscosity Weldable Low Cost weatherability. Form weak weld lines Highly anisotropic properties Drying required before processing High Z-axis thermal expansion coefficient Advantages of LCP Disadvantages of LCP
Processing and fabrication of LCP • The following may be said about their processing and fabrication characteristics: • All conventional processing techniques available for thermoplastic materials may be used. • Extremely low shrinkage and warpage during molding. • Exceptional dimensional repeatability from part to part. • Low melt viscosity, which permits molding of thin sections and/or complex shapes. • Low heats of fusion; this results in rapid melting and subsequent cooling, which shortens molding cycle times. • Anisotropic finished-part properties; molecular orientation effects are produced from melt flow during molding.
Typical LCP applications • Electrical/Electronic Applications • Automotive Applications • Parts, Engineering • Containers, Food • Appliances • Industrial Applications • Connectors • Optical Applications • Parts, Thin-walled • medical equipment industry (in components to be repeatedly sterilized), in photocopiers.
Thermoplastic Elastomers (TPe) • Thermoplastic elastomers elatosplastic are polymers that combine the processibility of thermoplastics and the functional performance of conventional elastomers. • Block copolymer that possess elastic properties within a certain range of temperature e.g from room temperature -70°C. • The elastic properties are due to physical crosslinks resulting from secondary inter molecules forces such as hydrogen bonding. • These crosslinks disappear when heated above certain temperature and reform immediately on cooling to develop elastic properties.
Of the several varieties of TPEs one of the best known and widely used is a block copolymer consisting of block segments of a hard and rigid thermoplastic elastomer. • These are dissimilar and incompatible with each other they act as individual phase. • The dominant soft segments are flexible, amorphous and have low Tg • The hard segments have high Tm, and tend to aggregate at ordinary temperatures into a rigid domains to form physically effective pseudo crosslinks. • When the block copolymer is heated to the processing temperature the forces that bind hard segments together will be destroyed. • It is then possible to process the polymer as a conventional thermoplastics the macromolecules are no longer bound together.
On cooling the hard segments re-associate into rigid domains and material shows elastomeric properties once again • Suitable solvents also able to destroy the pseudo crosslinks when solvent is evaporated the hard segments re-associate into rigid domains. • The properties of block copolymers can be adjusted by • Varying the ratio of the monomers • Varying the lengths of hard and soft segments. • The polymers become harder and stiffer as the ratio of the hard to soft phase is increased. • The upper service temperature of the block copolymers depend on the softening point of the hard phase. • The low temperature properties and fluid resistance controlled largely by the soft segments.
Morphology of thermoplastic elastomer Rigid domain (physical crosslinking) Soft amorphous domain
Advantages of TPes • The practical advantages of TPes include: • Little or no compounding is required. Most TPes are “ready to use”materials eliminating batch to batch variations compared to conventional elastomers compounding. • Easy processing can be processed on conventional thermoplastic equipments (e.g blow molding, injection molding etc) very fast processing with lowered costs. • TPE parts may be reformed into other shapes The scrap can be easily recycled results in lower production cost. • Product consistency is better than comparable vulcanized elastomer higher productivity • Very easy to colour with many types of pigments or dyes and less skilled labour is needed
Disadvantages of TPe • The disadvantages of TPes including: • They melt at elevated temperatures with results not suitable for applications requiring brief exposures beyond the upper service • They may require drying before processing not a common step with conventional elastomer but in fabrication of thermoplastic products. • There is a limited number of low modulus compound • They have higher compression set and less thermal stability do not allow these materials to be used in areas where compression set is important and the working conditions are critical. • E.g: at temperatures above normal and at high strain
Nanotechnology • The design, characterization, production and application of structures, devices and systems by controlling shape and size at the nanoscale. • The understanding and control of matter at dimensions of roughly 1 to 100 nanometers where unique phenomena enable novel applications. • Eight to ten atoms span one nanometer (nm) The human hair is approximately 70,000 to 80,000 nm thick. • It becomes dominant when the nanometer size range is reached Materials reduced to the nanoscale can suddenly show very different properties compared to what they show on a macroscale.
Applications & potential benefits • With nanotechnology, a large set of materials with distinct properties (optical, electrical, or magnetic) can be fabricated. • Nanotechnologically improved products rely on a change in the physical properties when the feature sizes are shrunk. • Nanoparticles for example take advantage of theirdramatically increased surface area to volume ratio. • When brought into a bulk material, nanoparticles can strongly influence the mechanical properties, such as the stiffness or elasticity. • E.g., traditional polymers can be reinforced by nanoparticles resulting in novel materials e.g. as lightweight replacements for metals. • Such nanotechnologically • enhanced materials will enable a weight reduction • accompanied by an increase in stability and • an improved functionality.
Example of applications • Medicine • The biological and medical research communities have exploited the unique properties of nanomaterials for various applications (e.g., contrast agents for cell imaging and therapeutics for treating cancer)’ • Functionalities can be added to nanomaterials by interfacing them with biological molecules or structures. • The size of nanomaterials is similar to that of most biological molecules and structures; therefore, nanomaterials can be useful for both in vivo and in vitro biomedical research and applications. • Tissue engineering • Nanotechnology can help to reproduce or to repair damaged tissue called “tissue engineering” makes use of artificially stimulated cell proliferation by using suitable nanomaterial-based scaffolds and growth factors. • Tissue engineering might replace today’s conventional treatments, e.g. transplantation of organs or artificial implants. .
Energy • The most advanced nanotechnology projects related to energy are: storage, conversion, manufacturing improvements by reducing materials and process rates, • energy saving e.g. by better thermal insulation, and enhance renewable energies sources • Household • The most prominent application of nanotechnology in the household is self-cleaning or “easy-to-clean” surfaces on ceramics or glasses • Common household equipment like flat irons has improved smoothness and heat-resistance due to nanoceramic particles. • Optics • The first sunglasses using protective and antireflective ultrathin polymer coatings are on the market. • For optics, nanotechnology also offers scratch resistant coatings based on nanocomposites.
Textiles • The use of engineered nanofibers already makes clothes water- and stain-repellent or wrinkle-free. • Textiles with a nanotechnological finish can be washed less frequently and at lower temperatures. • Nanotechnology has been used to integrate tiny carbon particles membrane and guarantee full-surface protection from electrostatic charges for the wearer. • Sports • Tennis rackets with carbon nanotubes have an increased torsion and flex resistance. • The rackets are more rigid than current carbon rackets and pack more power. • Long-lasting tennis-balls are made by coating the inner core with clay polymer nanocomposites. • These tennis-balls have twice the lifetime of conventional balls.
Example of the exams question • What is liquid crystal polymers? • Discuss the advantages of thermoplastic elastomer. • What are the benefits of nanotechnologhy?