EP 342: Materials Science: Synthetic Polymers

PVA Polyvinyl acetate

Structure

Properties

Applications

As an emulsion in water, PVAc emulsions are used as adhesives for porous materials, particularly for wood, paper, and cloth, and as a consolidant for porous building stone, in particular sandstone. Uses:

• as wood glue PVAc is known as "white glue" and the yellow as "carpenter's glue" or PVA glue.
• as paper adhesive during paper packaging converting
• in bookbinding and book arts, due to its flexible strong bond and non-acidic nature (unlike many other polymers). The use of PVAc on the Archimedes Palimpsest during the 20th century greatly hindered the task of disbinding the book and preserving and imaging the pages in the early 21st century, in part because the glue was stronger than the parchment it held together.
• in handicrafts
• as envelope adhesive
• as wallpaper adhesive
• as a primer for drywall and other substrates

The stiff homopolymer PVAc, but mostly the more soft copolymer, a combination of vinyl acetate and ethylene, vinyl acetate ethylene (VAE), is used also in paper coatings, paint and other industrial coatings, as binder in nonwovens inglass fibers. sanitary napkins, filter paper and in textile finishing.

Polyvinyl acetate is also the raw material to make other polymers like:

• Polyvinyl alcohol -[HOCHCH₂]-: Polyvinyl acetate is partially or completely hydrolysed to give polyvinyl alcohol. This reversible saponification and esterification reaction was a strong hint for Hermann Staudinger in the formulation of his theory of macromolecules.
• Polyvinyl acetate phthalate (PVAP): Polyvinyl acetate is partially hydrolyzed and then esterified with phthalic acid.

BoPET

Biaxially-oriented polyethylene terephthalate

Also known as: Mylar, Melinex, Hostaphan

Structure

Properties

The manufacturing process begins with a film of moltenpolyethylene terephthalate (PET) being extruded onto a chill roll, which quenches it into the amorphous state. It is then biaxially oriented by drawing. The most common way of doing this is the sequential process, in which the film is first drawn in the machine direction using heated rollers and subsequently drawn in the transverse direction, i.e. orthogonally to the direction of travel, in a heated oven. It is also possible to draw the film in both directions simultaneously, although the equipment required for this is somewhat more elaborate. Draw ratios are typically around 3 to 4 in each direction.

Once the drawing is completed, the film is "heat set" or crystallized under tension in the oven at temperatures typically above 200 °C (392 °F). The heat setting step prevents the film from shrinking back to its original unstretched shape and locks in the molecular orientation in the film plane. The orientation of the polymer chains is responsible for the high strength and stiffness of biaxially oriented PET film, which has a typical Young's modulus of about 4 GPa. Another important consequence of the molecular orientation is that it induces the formation of many crystal nuclei. The crystallites that grow rapidly reach the boundary of the neighboring crystallite and remain smaller than the wavelength of visible light. As a result, biaxially oriented PET film has excellent clarity, despite its semicrystalline structure.

If it were produced without any additives, the surface of the film would be so smooth that layers would adhere strongly to one another when the film is wound up, similar to the sticking of clean glass plates when stacked. To make handling possible, microscopic inert inorganic particles are usually embedded in the PET to roughen the surface of the film.

Biaxially oriented PET film can be metallized by vapor deposition of a thin film of evaporated aluminium, gold, or other metal onto it. The result is much less permeable to gases (important in food packaging) and reflects up to 99% of light, including much of the infrared spectrum. For some applications like food packaging, the aluminized boPET film can be laminated with a layer of polyethylene, which provides sealability and improves puncture resistance. The polyethylene side of such a laminate appears dull and the PET side shiny.

Other coatings, such as conductive indium tin oxide (ITO), can be applied to boPET film by sputter deposition.

Applications

Flexible packaging and food contact applications, covering over paper, insulating material, solar marine and aviation applications, science, electronic and acoustic applications, graphic arts, balloons

PVP: Polyvinylpyrrolidone

Commonly called:
-polyvidone
-povidone

Structure

Properties

Applications

PVP is used in many technical applications:

• as an adhesive in glue stick and hot-melt adhesives
• as a special additive for batteries, ceramics, fiberglass, inks, andinkjet paper, and in the chemical-mechanical planarization process
• as an emulsifier and disintegrant for solution polymerization
• to increase resolution in photoresists for cathode ray tubes (CRT)
• in aqueous metal quenching
• for production of membranes, such as dialysis and water purification filters
• as a binder and complexation agent in agro applications such as crop protection, seed treatment and coating
• as a thickening agent in tooth whitening gels
• as an aid for increasing the solubility of drugs in liquid and semi-liquid dosage forms (syrups, soft gelatine capsules) and as an inhibitor of recrystallisation
• as an additive to Doro's RNA extraction buffer
• as a liquid-phase dispersion enhancing agent in DOSY NMR

Kevlar

Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids, such as Nomex and Technora.

Structure

Properties

When Kevlar is spun, the resulting fiber has a tensile strength of about 3,620 MPa, and a relative density of 1.44. The polymer owes its high strength to the many inter-chain bonds. These inter-molecular hydrogen bonds form between the carbonyl groups and NH centers. Additional strength is derived from aromatic stacking interactions between adjacent strands. These interactions have a greater influence on Kevlar than the van der Waals interactions and chain length that typically influence the properties of other synthetic polymers and fibers such as Dyneema. The presence of salts and certain other impurities, especially calcium, could interfere with the strand interactions and care is taken to avoid inclusion in its production. Kevlar's structure consists of relatively rigid molecules which tend to form mostly planar sheet-like structures rather like silk protein.

Kevlar maintains its strength and resilience down to cryogenic temperatures (−196 °C); in fact, it is slightly stronger at low temperatures. At higher temperatures the tensile strength is immediately reduced by about 10–20%, and after some hours the strength progressively reduces further. For example, at 160 °C (320 °F) about 10% reduction in strength occurs after 500 hours. At 260 °C (500 °F) 50% strength reduction occurs after 70 hours.

Applications

Protection - cryogenics, armor, personal protection

Sports - personal protection, equipment, shoes

Music - audio equipment, bowed-string instruments, drumheads, woodwind reeds

Other - fire dancing, frying pans, rope, cable, sheath, electricity generation, building construction, brakes, expansion joints and hoses, particle physics, smartphones, marine current turbine and wind turbine 

PVC

Polyvinyl chloride

Structure

Properties (Rigid PVC vs. Flexible PVC)

Applications

-Pipes

-electric cables

-unplasticized poly(vinyl chloride) (uPVC) for construction, signs, clothing and furniture, healthcare, plasticizers

Polyethelene

Structure

Properties

Applications

Used as insulating material for medium and high voltage cable insulation, for hot water pipes and molded parts in electrical engineering, plant engineering and in automotive industry

Polytetrafluoroethylene (PTFE)

Also known as Teflon

Structure

Properties

Applications

PTFE (Teflon) is best known for its use in coating non-stick frying pans and other cookware, as it is hydrophobic and possesses fairly high heat resistance.

PTFE is a versatile material that is found in many niche applications:

• It can be stretched to contain small pores of varying sizes and is then placed between fabric layers to make a waterproof, breathable fabric in outdoor apparel.
• It is used widely as a fabric protector to repel stains on formal school-wear, like uniform blazers.
• It is used as a film interface patch for sports and medical applications, featuring a pressure-sensitive adhesive backing, which is installed in strategic high friction areas of footwear, insoles, ankle-foot orthosis, and other medical devices to prevent and relieve friction-induced blisters, calluses and foot ulceration.
• Powdered PTFE is used in pyrotechnic compositions as an oxidizer with powdered metals such as aluminium andmagnesium. Upon ignition, these mixtures form carbonaceous soot and the corresponding metal fluoride, and release large amounts of heat. They are used in infrared decoy flares and as igniters for solid-fuel rocket propellants. Aluminium and PTFE is also used in some thermobaric fuel compositions.
• In optical radiometry, sheets of PTFE are used as measuring heads in spectroradiometers and broadband radiometers (e.g., illuminance meters and UV radiometers) due to PTFE's capability to diffuse a transmitting light nearly perfectly. Moreover, optical properties of PTFE stay constant over a wide range of wavelengths, from UV down to near infrared. In this region, the relation of its regular transmittance to diffuse transmittance is negligibly small, so light transmitted through a diffuser (PTFE sheet) radiates like Lambert's cosine law. Thus PTFE enables cosinusoidal angular response for a detector measuring the power of optical radiation at a surface, e.g. in solarirradiance measurements.
• Certain types of hardened, armor-piercing bullets are coated with PTFE to reduce wear on firearms's rifling that harder projectiles would cause. PTFE itself does not give a projectile an armor-piercing property.
• Its high corrosion resistance makes PTFE useful in laboratory environments, where it is used for lining containers, as a coating for magnetic stirrers, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which will dissolve glass containers. It is used in containers for storing fluoroantimonic acid, a superacid.
• PTFE tubes are used in gas-gas heat exchangers in gas cleaning of waste incinerators. Unit power capacity is typically several megawatts.
• PTFE is widely used as a thread seal tape in plumbing applications, largely replacing paste thread dope.
• PTFE membrane filters are among the most efficient industrial air filters. PTFE-coated filters are often used indust collection systems to collect particulate matter from air streams in applications involving high temperatures and high particulate loads such as coal-fired power plants, cement production and steel foundries.
• PTFE grafts can be used to bypass stenotic arteries in peripheral vascular disease if a suitable autologous veingraft is not available.
• Many bicycle lubricants contain PTFE and are used on chains and other moving parts.
• PTFE can also be used for dental fillings, to isolate the contacts of the anterior tooth so the filling materials will not stick to the adjacent tooth.
• PTFE sheets are used in the production of butane hash oil due to its non-stick properties and resistance to non-polar solvents.