classes of polymeric materials

Copyright Joseph Greene 2001

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Classes of Polymeric MaterialsChapter 3: Specialty Polymeric

Products

Professor Joe Greene

CSU, CHICO


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Specialty Polymeric Products• Polymeric Fibers

– Many naturally occurring polymeric fibers; protein or cellulose based• Animal origin- wool, mohair, angora, fur, silk, cashmere, alpaca, llama, etc.• Vegetable origin- cotton, flax, linen, sisal, etc..

– Polymeric fibers are referred to as man-made, synthetic, or artificial fibers

• Used in a variety of applications, including filters, cords, cables, and fabrics• Fabrics require the fibers undergo a process which gives them a texture.• Figure 3.112- Fiber Texture for

– continuous filaments, staple filaments, or spun staple filaments

• Continuous monofilament tows or yarns are cut into staples which are subjected to a process often referred to as yarn spinning.

• Yarn spinning separates the monofilaments and tangles with a twist or spin into a spun yarn consisting of

– relatively short filaments whose mechanical interlocking give a reasonable strength,

– while loose ends afford a bulkier, less silky feel and appearance


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Specialty Polymeric Products• Polymeric Fibers

– Manufacture of fabrics consists of• Weaving, knitting and other mills

• Many types of fabrics– Woven fabrics or clothes are either

» Plain, patterned (dobby or jacquard), loop-type (terry or cut loop pile)

– Knitted fabrics are either circular (weft knit) or flat (warp knit)

– Tufted fabrics are produced as cut or uncut pile

– Stitch-bonded fabrics rely on a secondary fiber to hold the primary yarns in a given pattern.

– Non-woven fabrics are subdivided into

» Bonded webs that make use of a polymeric binder to hold continuous or staple yarn together

» Needle punch felts which involve a mechanical-type interaction.

• Fiber-forming polymers are normally crystallizing, uncross-linked type that have a high degree of orientation


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Specialty Polymeric Products• Characterization

– Long-established textile industry developed specific methods and an associated jargon to characterize fibers and associated units

– Fiber dimensions• Reported as titer in denier (mass in g of 9000 m of monofilament or

multifilament yarn)– Monofilaments range from 3 to 15 denier

» For a specific gravity of 1.3, a titer of 10 denier corresponds to a diameter of 0.030 mm (0.001 in)

» Hosiery usually involves filaments of 15-denier titer» Apparel may involve 12-filament twisted yarn with a global 50-denier titer» Tire cord may involve 2448-filament untwisted yarn with a global 15,000

denier titer– Fiber strength is often reported as tenacity in force (gf) per unit titer (denier)

» For a specific gravity of 1.3, a tenacity of 6 gf/denier corresponds to a tensile strength of 100 kpsi (700 MPa)

» Tenacity of common fibers is in the range of 3-9 gf/denier (30 to 150kpsi)» Fiber moduli range from 30-50 gf/denier, corresponding to .3-1 Mpsi


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Specialty Polymeric Products• Processing

– Formation of monofilaments involves two basic steps (Fig 3.113)• Step 1: Filament spinning- formation of the filament or “as-spun

monofilament”, which is semi-crystalline, but nonoriented• Step 2- “cold drawing” or “drawing”, confers most of the orientation

through a stretching, yielding, and drawing process that takes place in the solid state, but above the Tg of the crystallizing polymer

• Monofilament in final form is “drawn filament”

– Filament spinning, achieved in several ways• Chemical reaction during the fiber-forming stage, or• Transformation are only physical, involving heat and mass transfer

– Wet spinning (Fig 3.114) involves the extrusion of a liquid-like fluid through small holes (orifices) of a spinneret in a bath containing another fluid with which the extruded strand interacts, either chemically or through molecular exchange

• After sufficient interaction and residence time, the strand becomes solid and as-spun monofilament


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Specialty Polymeric Products• Processing

– Dry spinning (Fig 3.115) involves the extrusion of a concentrated polymer solution through small spinneret holes.

• Emerging strands are then dried (solvent is evaporated by cross-flow if air)

• Difficulty is in handling the solvents

– Melt spinning (Fig 3.116) involves extrusion of a molten polymer through relatively large spinneret holes and its cooling and solidification in a cross-flow of air.

• Difficulty is in the thermal stability of the melt and its high viscosity

– Strands are rapidly pulled (elongated) as they emerge from spinneret holes, primarily in order to reduce the diameter.

– Acrylic and acetate fibers are wet- or dry spun

– Polyamides, polyolefins, and polyesters are commonly melt spun


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Specialty Polymeric Products• Commercial Types

– Seven major types of polymeric fibers• Rayon- viscose rayon fibers are sold as regular, cross-linked, or high wet

modulus types

• Acetate- or triacetate fibers are based upon cellulose acetate

• Olefinics- include polyethylene and polypropylene

• Vinylinics- based on PVC, but also with copolymerization with vinyl acetate or vinylidenechloride

• Acrylics- based on PAN, but can involve copolymerization with PVC

• Polyamides- or nylons involve aliphatic polyamides

• Polyesters- involve PET

• Special Purpose or high performance fibers– Polyurethanes, aramids, extended chain, PBI, and PEEK


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Specialty Polymeric Products• Polymeric Films

– Widely used in the form of wide products of uniform thickness (gauge)

– Film is associated with a thickness of less than 0.25mm (thickness between 0.040 mm (0.001 in or 1 mil) and 0.4mm (10 mils)

• Dry cleaning garment bags are made from LDPE at a thickness of 0.013 mm or 0.5 mils thick.

– Sheet is thicker than 0.25mm.– Plastic film is manufactured in flat extrusion on chin rolls (film

casting or calendering) and tubular (bubble) extrusion blowing– Uni-axial or bi-axial molecular orientation can be obtained using a

flat stretching device or through the bubble process, improves properties

– Subjected to several standard tests• Burst resistance, tear resistance, puncture resistance, folding endurance, slip,

curl, and resealability


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Specialty Polymeric Products• Polymeric Film Materials

– Regenerated cellulose, CLE, (Cellophane)- used for many years. and can be coated with a thermoplastic for heat sealing.

– Cellulose Nitrate, CN, and Cellulose Acetate, CA- were the earliest films.

– LDPE and HDPE- most common film materials.– PP is generally used as oriented PP, OPP.– Ionomers (IO) or Surlyn are olefin related film materials.– PVC- is used in plasticized form.– PVDC, or Saran Wrap- is used in copolymer form with 10-15%

acrylonitrile, AN, or with ethylene-vinyl acetate, EVA, or ethylyne-vinyl alcohol, EVOH.

– PET is used in film form for mylar sheet.– PS is used as biaxially oriented film of for thermoforming sheet– PC, polysulphone (PSU), Polyimides (PI), polyetherimides (PEI)


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Specialty Polymeric Products• Polymeric Film Materials

– Composite films can be defined as parallel layers of different materials designed to offer a set of properties not possessed by either material

• Coating or lamination of materials, e.g., paper or foil.

• Multilayer films are made by coextrusion with each layer:– Mechanical strength: PET

– Sealing: PE

– Barrier : PVDC, EVOH

– Adhesives are needed to form bonds between layers– Complex coextrusion dies can handle over 10 layers are

are used for tubular or flat extrusion


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Specialty Polymeric Products• Polymeric Film Applications

– Mechanical packaging: applications that require mechanical resistance of the film

– Barrier packaging: displacing traditional packaging in glass or metal containers.

• Some are flexible, e.g., bags or pouches for food items• Some are rigid, e.g., yogurt containers, margarine tubs, etc.

– Packaging is essential for • Sanitary and conservation reasons and should retard the

deterioration (spoilage) of foods, e.g., decay, discoloration of meats, staleness of breads, etc.

– Barrier packaging involves control of • Oxygen, carbon dioxide, and water.• Food characteristics or aroma, odor, scent of food oils and fats


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Specialty Polymeric Products• Food containers

– Heat resistant packaging for boil-in, cook-in, and bake-in• Requires sterilization of 120°C (250°F) and control of oxygen,

nitrogen , and carbon dioxide.

– Liquid food stuffs includes soups, beverages, wine, soda, water, liquor can be packaged with plastics and are replacing glass containers.

• Carbonated beverages can develop up to 100 psi pressure and require resistance to carbon dioxide permeation.

– Semi-fluid food stuffs include dressing, mayonnaise, relishes, and tomato ketchup, sauces (BBQ, pasta), and jelly, jam, or preserves.


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Specialty Polymeric Products• Food containers (continued)– Most solid foods are candidates for all plastic packaging, but

there are large differences in requirements that are associated with the food and the intended use

• Animal products require odor control, water barrier, and controlled oxygen permeation (PVC)

• Frozen poultry is shrink wrapped

• Dairy products including milk (HDPE) and cheese wraps

• Bakery products (bread, cakes, pies) must be wrapped with suitable water barriers to prevent premature drying.

• Dry foods include cereals, biscuits, coffee, snack foods, chips require moisture and oxygen to be kept out and aroma to be kept in.

• Confectionery includes chocolate products whose high oil content requires oil resistant acrylonitrile based film


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Specialty Polymeric Products• Mechanical film packaging applications include

– LDPE and HDPE • Thick-gauged sacks for powdered or granular products

• Garbage and trash bags, general merchandise and tee-shirt bags

• Thin gauge dry-cleaning garment covers and florist wraps

• Heat shrinkable (biaxially stretched) can be made with LDPE, PVC, PP, etc.

– Shrinking is achieved with hot water, hot air convection or radiation

• Stretch wrapping by winding a thin plastic tape under some controlled tension

• Construction and public works uses for polymer film– Construction coverings, roof liners, industrial liners– Agricultural for silo covers, water reservoir liners


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Cellular Polymers• Polymers can be combined with a gas

– Forms voids or cells in the polymer causing the polymer to be very light

– Referred to as cellular, blown, expanded polymer, foam • Elastomeric foam- matrix (polymer) is an elastomer or rubber

• Flexible foam- soft plastic matrix, e.g., plasticized PVC (PPVC), LDPE, PU

• Rigid foams- PS, unsaturated polyesters, phenolics, urethanes (PU)

– Type of polymer matrix, thermoplastic or thermoset can form basis for classification

– Amount of gas added reflects the resulting density• Light foams: density = 0.01 to 0.10 g/cc (1 to 6 lb/ft3)

• Dense foams: density = 0.4 to 0.6 g/cc (25 to 40 lb/ft3)– Note: water = 1g/cc or 62.3 lb/ft3


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Cellular Polymers• Arrangement and distribution of gas in the cellular polymer

corresponds to the structure of the foam system.

• Two types (Figure 3.117)– Closed cell: spherical or roughly spherical voids (cells) are fully separated by

matrix material.

– Open cell: spherical or roughly spherical voids (cells) are interconnection occurs between the cells.

• Degree of interconnection can be assessed if a sample is subjected to a moderate vacuum and liquid is allowed to flow into the interconnections and causes the weight to increase.

• Cell size is important for heat and mass transfer– Cell density (number of cells per unit cross-section area or volume)

• Characterizes the courseness or fineness of a foam

– Structural foam: foamed core is sandwiched between solid skins• Structured foam between integral skins

– Foaming can give an inhomogeneous structure

Matrix

Closed cell

Gas Matrix

Open cell

Gas

Closed cell

Interconnection


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Cellular Polymers• Closed-cellular polymers

– Nature of entrapped gas may have an effect on certain properties or suitability for specific applications

• Air, nitrogen, water, pentane, methylene chloride, fluorohydrocarbon vapors can be used as blowing agent

• Amount of gas changes with time as the gas moves through the material and exits to the atmosphere leaving a cellular structure

• Mechanism for the formation of cellular structure– Aeration or frothing: mechanical agitation is used to incorporate

air into liquid resin system (latex, reactive urethane)– Physical blowing agent:

• Add N2 gas into solution or to liquid melt which comes out of solution when pressure is released and forms cells.

• Add liquids at room temperature and have low boiling point. The liquids vaporize upon heating or by chemical reaction heat.

– Aliphatic hydrocarbons (pentane), methylene chloride, trichloro-fluoromethane, or freon 11


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Cellular Polymers• Mechanism for the formation of cellular structure (continued)

– Chemical blowing agents are compounds that decompose under heat and liberate large amounts of and inert gas,

• N2, CO2, CO, water, ammonia, H2, etc.• Activators can sometimes be added to allow lower decomposition

temperature and release more gas at a lower temperature.• Early blowing agents were

– Sodium bicarbonate, which liberates CO2

– Other carbonates and nitrates liberate hydrogen or nitrogen.– Hydrogen can be generated in large quantities, but diffuses away quickly

• Organic compounds can be used for some high temperature thermoplastics– Toluene sulfonyl hydrazine– Oxybis benzene sulfonyl hydrazide– Toluene Sofonyl semicarbazide– Trihydrazinatrizine– Phenyl tetrazole

• Can be in finely divided solid form to create cellular structure• Nucleating agents and surfactants are used to control cellular structure


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Cellular Polymers• Examples– Polystyrene: PS or expanded polystyrene foam (EPS)

• Made from expandable polystyrene beads which are small spheres of polystyrene (diameter of 0.3 – 2.3 mm) containing 3-7% pentane as physical blowing agent

– Bulk density of beads (with air spaces) is 0.7 g/cc.

• Manufacturing (Figure 3.118)– Beads are pre-expanded with the use of a steam chamber to a bulk density of

0.02-0.05 g/cc.– Beads are cooled and reached equilibrium with air penetrating the cells.– Placed back in steam chamber and molded into final foamed shape.

» Forms basic cellular structure is closed cell type– Large blocks are molded which are cut into insulating boards or molded into

custom products» Cups, insulating containers, protective elements

– Extrusion process can be used with blowing agent» Meat trays, egg cartons

Expandable bead

Initial Stage

Void

Cellular Polymer

Preexpansion Mold Filling Final ExpansionPre-Expanded bead


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Cellular Polymers• Examples

– Polyurethane can be made in cellular form• Stiffness can vary widely from that of a soft elastomer to a rigid plastic.

• Density can vary widely from 0.03 g/cc (rigid foam) to 0.08 g/cc (flexible)

• Cell structure varies from open cell structure for flexible and closed cell structure for rigid foam which traps the blowing agent (Freon 11)

• Produced with a water-blown Carbon dioxide blowing agent

– Manufacturing• Continuous formation of rigid or flexible foam of large block (log, bun,

loaf)

• Uses a suitable mold using a mixing head on a boom that is placed on top of a carrousel with several molds. The resin is injected in one mold while others are curing.

• Typical cross section is 2m x 1m and a typical linear speed of production is 4m/min

• Called foamstock– Subsequent products are cut from foamstock using hot wires


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Cellular Polymers• Manufacturing (continued)

– Another method involves permanently placing the foam in a cavity of a product, called in-situ (In-place) foaming

• For insulation, buoyancy, structural or combined purposes.

• Requires good adhesion to the cavity walls and may require treatment (degreasing, carona discharge, etc..)

– Spray-on method• Liquid or frothed resin is projected against a surface (substrate) but rises

freely on the opposite side.

• External insulation of tanks, vessels, roofs, truck boxes

– Molding method with molds • Parts are to a specific complex shape. (Steering wheel covers, foam seats)

• Demolding will require the use of a external spray and internal release agent– Usually soap based zinc stearate

• Pressure generated during molding requires adequate control, otherwise– Dimensions may vary significantly and poor formation of integral skin and cells


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Cellular Polymers• Manufacturing (continued)

– Frothing method corresponds to 2-stage expansion.

• Suitable low boiling point blowing agent is incorporated to the resin under pressure (4-5 atmospheres) (1 atmosphere = 14.69 psi) to prevent expansion

• Pressure release at the exit of the dispensing nozzle causes the immediate formation of a froth (foamed cream) corresponding to a pre-expansion ratio of 10X. Subsequent expansion is associated with the curing reaction which causes the vaporization of the other blowing agent with expansion of 3X.

• Pressure developed in a cavity and temperature variations are lower than in the case of direct liquid feeding and mush larger than by successive layer build-ups.


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Cellular Polymers• Structural Foam– Feature cellular core and solid skins– Based upon thermoplastic or thermosets– Produced in a variety of methods

• Low pressure (Union Carbide) process Fig 3.119– Forms the foam in an accumulator from which it is transferred into

mold cavity under moderate pressure (35 atm or 500 psi)

– Tooling is inexpensive, Surface finish is not very good

• High pressure process (United Shoe Machinery). Fig 3.120– Conventional injection of the melt containing a blowing agent

– High pressures (15kpsi to 20 kpsi) prevents foaming and allows for better surface finish

– Tooling is expensive, Surface finish is very good

– Mold cavity is enlarged (expansion mold) to allow molten core to foam

– Reaction Injection Molding Process can produce urethane structural foam parts


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Cellular Polymers• Applications

– Mechanical properties are very good on per weight basis• Core materials in conjunction with composites

– Composite floor pans

• Thermal insulation properties are outstanding– Closed cell are used as insulation board and for packaging of frozen or

perishable foods, e.g., ice cream, fish, poultry.

• Floatation devices for closed cell

• Shock absorption and vibration resistant applications– Automotive occupant protection

– Automotive bumper impact, urethane foam and expanded PP beam foam

• Acoustic insulation or dampening materials

• Open cellular structures used in filtering and humidifying applications

classes of polymeric materials

Business

fabrics fabrics

polymeric fibersprotein

polymeric binder

yarn spinning

triacetate fibers

artificial fibers

types of fabrics

spun yarn