Polymers and plastics

POLYMERSOR

PLASTICS

GLASS TRANSITON TEMP. OF SOME THERMOPLASTICS

POLYETHYLENE--PE -110 °C

POLYPROPYLENE---PP

-18 °C

POLYVINYL ACETATE--PVA

29 °C

POLYVINYL CHLORIDE--PVC

82 °C

POLYSTYRENE---PS 75-100 °C

POLYVINYL CHLORIDE

PVC

Polyvinyl chloride (PVC)

• Manufactured since 1927• Vinyl chloride (CH2=CHCl) is a carcinogenic gas,

but PVC does not have “free” monomer

The IAPD Plastics Primer, Module 3

Polyvinyl Chloride (PVC)CHARACTERISTICS

• Low cost• Good chemical resistance• Versatile• Naturally UV resistant• Good strength


Polyvinyl Chloride (PVC)Limitations

• Heat sensitive• Low impact strength• Poor wear resistance


Polyvinyl Chloride (PVC)Applications

• Pipe, valves and fittings • Thermoformed parts for transportation, exhibits

and machinery parts• Pollution control equipment parts• Packaging

POLYETHYLENEPE


Polyethylenes (PE)Key Characteristics

• Classified by density and molecular weight

• Excellent chemical resistance• Zero moisture absorption• Light weight (1/8 weight of steel)• Excellent insulator• High impact strength


Polyethylenes (PE)Applications

• Packaging• Wire and cable • Industrial


Low Density Polyethylene (LDPE)Key Characteristics

• Good chemical resistance• Very flexible• High impact strength• Low heat resistance (60-79°C/140-

175°F)• Used extensively in packaging industry


High Density Polyethylene (HDPE)Key Characteristics

• Higher rigidity and tensile strength than low and medium density polyethylenes

• Good formability• Slightly lower impact strength than LDPE


Low Density Polyethylene (LDPE)High Density Polyethylene (HDPE)

Applications

• Films• Industrial trash bags• Liners• Shipping bags• Marine industry• Playgrounds• Bathrooms• Pipe• Automotive

Table 1 Typical Property Values for Polyethylenes• Property HDPE LLDPE/LDPE----------------------------------------------------------------------------------------• Density (mg/m3) 0.96–0.97 0.90–0.93• Tensile modulus (GPa) 0.76–1.0 —• Tensile strength (MPa) 25–32 4–20• Elongation at break (%) 500–700 275–600• Flexural modulus (GPa) 0.8–1.0 0.2–0.4• Vicat soft point (C) 120–129 80–98• Brittle temperature (C) 100 to 70 85 to 35• Hardness (Shore) D60–D69 D45–D55• Dielectric constant (106 Hz) ------ 2.3• Dielectric strength (MV/m) — 9–21• Dissipation factor (106 Hz) — 0.0002• Linear mold shrinkage (in. / in.) 0.007–0.009 0.015–0.035

POLYPROPYLENEPP

Polypropylene (PP)

• First marketed in 1957• Propylene monomer: CH2=CHCH3


Polypropylene PP comes from Propylene!


Polypropylenes (PP)Key Characteristics

• Excellent chemical resistance• Excellent fatigue strength — referred to as

“living hinge”• Lower impact strength than polyethylenes• Available in homopolymer and co-polymer• Co-polymer offers better impact, clarity

and cold temperature properties


Polypropylenes (PP)Applications

• Packaging• Automotive• Consumer/durable goods• Vacuum formed parts• Fiber/carpet

POLYSTYRENE PS

Polystyrene (PS)

• Since 1937• Styrene monomer: CH2=CH-C6H5

• Mix with petrol ‘napalm’ bomb, because PS burns very well

The polystyrene PS monomer is made of carbon-carbon

double bonds with a benzene ring

Polystyrene (PS) Strengths

• Inexpensive• Easy to bond• Quick to bond • Easy to decorate

Polystyrene PS• ITS inexpensive. • IT can be hard or soft.• It can be shaped into just about anything.• Toys• Hairdryers• Drinking cups• Computers• Packaging• Car parts• Kitchen appliances• StyrofoamTM is one brand of polystyrene foam

Polystyrene (PS) Limitations

• Low impact resistance• Brittle after UV exposure• Cannot be used at elevated temperatures• Mechanical stress

Properties of Polystyrene PS

1. The Glass transition temperature of Polystyrene is 100º C.

2. It melts at 240ºC . 3. It softens in acetone (nail polish

remover). 4. It is recyclable.

FLUORINATED THERMOPLASTICS

• Fluoropolymers, or fluoroplastics, are a family of fluorine-containing thermoplastics that exhibit some unusual properties. These properties include inertness to most chemicals, resistance to high temperatures, extremely low coefficient of friction, weather resistance, and excellent dielectric properties.

• Mechanical properties are normally low but can be enhanced with glass or carbon fiber or molybdenum disulfide fillers.

FLUORINATED THERMOPLASTICS

FluoropolymersMaterials

• Fluorinated ethylene propylene (FEP)• Ethylene chlorotrifluoroethylene (ECTFE)• Ethylene tetrafluoroethylene (ETFE)• Polychlorotrifluoroethylene (PCTFE)• Polytetrafluoroethylene (PTFE)• Polyvinylidene fluoride (PVDF)• Perfluoroalkoxy (PFA)

FluoropolymersKey Characteristics

• Superior flammability properties• Excellent heat resistance properties• Chemical resistance• Very low COF• Good dielectric properties

FluoropolymersApplications

• Chemical processing• Pulp and paper• Oil and gas• Cable and wire• Outdoor signs• Coatings• Decorative film

Polytetrafluoroethylene (PTFE)Key Characteristics

• Lowest COF• Excellent low loss characteristics• Exceptional resistance to most

chemicals• High limiting oxygen index• High impact strength

Polychlorotrifluoroethylene (PCTFE)Key Characteristics

• Outstanding barrier properties• High optical transparency• High stress crack resistance

PROCESSING OF PLASTIC MATERIALS

PROCESSING OF PLASTIC MATERIALS

There are different methods to process the plastic materials and forming several products

Polymers are formed by many low temperature processes.

8Extrusion 8Film Blowing8Injection Molding 8Blow Molding8Compression Molding8Reaction Injection Molding

Polymer Processing Methods

PROCESSING OF PLASTIC MATERIALSInjection Molding PROCESS

• Injection molding is a manufacturing technique for making parts from thermoplastic material.

• Molten plastic is injected at high pressure into a mold, which makes the product's shape.

http://en.wikipedia.org/wiki/Manufacturing

http://en.wikipedia.org/wiki/Plastics

http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/Molding_(process)

INJECTION MOLDING PROCESS

• The mold is made usually either steel or aluminum.

• Injection Molding is the most common method of production, with some commonly made items including bottle caps and outdoor furniture .

http://en.wikipedia.org/wiki/Steel

The polymer is heated to the liquid state, but it is prepared in metered amounts, and the melt is forced into a mold to create the part. It is not a continuous process. Many toys are made by injection molding.


Injection Molding Process


• There is two types of injection molding machines 1- Plunger Type

• 2- Screw Type

• In injection molding process plastic granules from a hopper are inserted into the injection cylinder onto the surface of a rotating screw which carries them forward and moves to the dies and during the whole path constant pressure is maintained and constant heating to melt the plastic grains and after giving them desired shape they made cool in open air or in water.

Injection Molding Machine

Injection Molded Parts

The polymer is heated to the liquid state and forced through a die under pressure resulting in an endless product of constant cross section. 60% of polymers are prepared in this way. Examples: tubing, pipes, window frames, sheet, insulated wire.

Extrusion Process

.• Using the same method as extrusion the

material coming out of the die is blown into a film.

• An example is plastic wrap.

Film Blowing Process

Film blowing Process

Blow Molding (Vacuum molding) Process

• The melted polymer is put into a mold, then compressed air is used to spread the polymer into the mold. It is used to make many containers such as plastic soda containers and milk jugs.

BLOW MOLDING PROCESS

• Blow Molding is accomplished by vertically extruding a hollow tube of molten plastic. This tube is called a "parison" in blow molding terms. The parison is then clamped between two mold halves and expanded into the desired shape by inflating it with compressed air. After cooling, a hollow part emerges.

BLOW MOLDING PROCESS

Blow/Vacuum Moulding Process

Solid polymer is placed in a mold, the mold is heated and puts pressure on the polymer to form the part.

COMPRESSION MOLDING PROCESS

COMPRESSION/TRANSFER MOLDING PROCESS

• These type of molding is used for the materials like Phenol-formaldehyde, Urea-formaldehyde and melamine-formaldehyde

CHARACTERISTICSTooling Cost - high

Production Rate -high Tolerances are held tightly using injection molding

No trimming is required Wide variety of rubber compounds give varying costs

and material properties.


• Compression and Transfer Molding is accomplished by placing a pre-weighed amount of rubber in a matched metal mold and closing the mold. The heat and pressure cause the rubber to liquefy and flow into the voids in the tool where it chemically reacts and hardens into the final shape. Very large shapes can be molded in compression presses.


TRANSFER MOLDING PROCESS

• In the Resin Transfer Molding, or RTM process, dry ( i.e., unimpregnated ) reinforcement is pre-shaped and oriented into a skeleton of the actual part known as the preform, which is inserted into a matched die mold. The mold is then closed, and a low-viscosity thermoset resin is injected into the tool. During this time, the resin "wets out" the fibers and the air is displaced and escapes from vent ports placed at the high points. Heat is applied to the mold to activate the polymerization that solidifies the resin. The resin cure begins during filling and continues after the filling process. Once the part develops sufficient green strength, it is moved or demolded.

REACTION INJECTION MOLDING PROCESS

Liquid monomers are placed in the mold avoiding the need to use temperature to melt the polymer or pressure to inject it. The monomers polymerize in the mold forming the part.

ENGINEERING THERMOSETS

THERMOSETS

• Thermosetting resins are used in molded and laminated plastics. They are first polymerized into a low-molecular-weight linear or slightly branched polymer or oligomer, which is still soluble, fusible, and highly reactive during final processing. Thermoset resins are generally highly filled with mineral fillers and glass fibers. Thermosets are generally catalyzed and/or heated to finish the polymerization reaction, cross-linking them to almost infinite molecular weight. This step is often referred to as cure. Such cured polymers cannot be reprocessed or reshaped. The high filler loading and the high cross-link density of thermoset resins results in very high densities and very low ductility, but very high rigidity and good chemical resistance.

Phenolic ResinsPhenolic resins combine the high reactivity of phenol and formaldehyde to form prepolymers and oligomers called resoles and novolacs. These materials are combined with fibrous fillers to give a phenolic resin, which when heated provides rapid, complete cross linking into highly cured structures. The high crosslinked aromatic structure has high hardness, rigidity, strength, heat resistance, chemical resistance, and good electrical properties.

Uses. Phenolic applications include automotive uses (distributor caps, rotors, brake linings), appliance parts (pot handles, knobs, bases, electrical/electronic components (connectors, circuit breakers, switches), and as an adhesive in laminated materials (e.g., plywood).

Epoxy Resins

The most common epoxy resins are prepared from the reaction of bisphenol A and epichlorohydrin to yield low-molecular-weight resins that are liquid either at room temperature or on warming. Each polymer chain usually contains two or more epoxide groups. The high reactivity of the epoxide groups with amines, anhydrides, and other curing agents provides facile conversion into highly crosslinked materials. Cured epoxy resins exhibit hardness, strength, heat resistance, electrical resistance, and broad chemical resistance.

Epoxy Resins

• Uses. Epoxy resins are used in glass reinforced, high-strength composites in aerospace, pipes, tanks, pressure vessels; encapsulation or casting of various electrical and electronic components (printed wiring boards, etc.); adhesives; protective coatings in appliances, flooring, and industrial equipment; and sealants.

Unsaturated Polyesters

Unsaturated polyesters are prepared by the condensation polymerization of various diols and maleic anhydride to give a very viscous liquid that is dissolved in styrene monomer. The addition of styrene lowers the viscosity to a level suitable for impregnation and lamination of glass fibers. The low-molecularweight polyester has numerous fumarate ester units that provide easy reactivity with styrene monomer. Properly formulated glass-reinforced unsaturated polyesters are commonly referred to as sheet molding compound (SMC), or reinforced plastics. In combination with reinforcing materials such as glass fiber, cured resins offer outstanding strength, high rigidity, impact resistance, high strength-to-weight ratio, and chemical resistance. SMC typically is formulated with 50% calcium carbonate filler, 25% long glass fiber, and 25% unsaturated polyester.

Unsaturated Polyesters• The highly filled nature of SMC results in high density

and brittle easily pitted surface. Bulk molding compound (BMC) is formulated similar to SMC except -in. chopped glass is used. The shorter glass length gives easier process but lower strength and impact.

Uses. The applications include large body parts for automobiles, trucks, trailers, buses, and aircraft, small- to medium-sized boat hulls and associated marine equipment, building panels, housing bathtub and shower stalls, appliances, and electrical/electronic components.

Alkyd Resins• Alkyd resins are based on branched prepolymers from

glycerol, phthalic anhydride, and glyceryl esters of fatty acids. Alkyds have excellent heat resistance, are dimensionally stable at high temperatures, and have excellent dielectric strength (14 MV/m), high resistance to electrical leakage, and excellent arc resistance.

Uses. Alkyd resin applications include drying oils in enamel paints, lacquers for automobiles and appliances; and molding compounds when formulated with

reinforcing fillers for electrical applications (circuit breaker insulation, encapsulation of capacitors and resistors, and coil forms).

Diallyl Phthalate

Diallyl phthalate (DAP) is the most widely used compound in the allylic family. The neat resin is a medium-viscosity liquid. These low-molecular-weight prepolymers can be reinforced and compression molded into highly cross-linked, completely cured products. The most outstanding properties of DAP are excellent dimensional stability and high insulation resistance. In addition, DAP has high dielectric strength, excellent arc resistance, and chemical resistance.

Uses. DAP applications include electronic parts, electrical connectors, bases and housings. DAP is also used as a coating and impregnating material.

Amino ResinsThe two main members of the amino family of thermosets are:The melamine Urea resinsThey are prepared from the reaction of melamine and urea with formaldehyde. These materials exhibit extreme hardness, scratch resistance, electrical resistance, and chemical resistance. Uses. The melamine Resins are used for dinnerware, decorative laminates, countertops, tabletops, furniture surfacing, switchboard panels, circuit breaker, arc barriers, armature, slot wedges, adhesives, coatings.

The urea resins are used in particleboard binders, decorative housings, closures, elecrical parts, coatings, and paper and textile treatment.

THERMOSETTING PLASTICSPROCESSING

Processing of thermosetting plastics

Thermosets are usually processed by using the following molding techniques.

• Compression molding. • Transfer molding. • Injection molding.

TRANSFER MOLDING PROCESS

COMPRESSION MOLDING PROCESS OF THERMOSETTING PLASTICS

• Step 1 The mold consists of two halves into which cavities are machined material is placed on the lower mold.The mold driven by hydraulic cylinder is heated by electric heater.

COMPRESSION MOLDING PROCESS Step 2

• Heated moldscompress the materialallowing it to fill the wholecavity chemical reaction takeplace and the part is cured.

COMPRESSION MOLDING PROCESS Step 3

• After curing the partthe molds are open and the part is ejected fromthe molds. The excess materialcalled as flash is then trimmed.

Advantages of compression molding process

• Lowest cost.• Better for large parts.• Lower labor cost.• Minimum wastage of material &

improved efficiency.• Internal stresses are minimum.• High dimensional stability and shrinkage

is minimum.

Transfer molding process of thermosetting plastics

• Step 1 The molds heated byelectric heater are open and the material is placed In the transfer pot .

Transfer molding process Step 2

• The molds are closed the plunger compresses the material which passes through gate into cavities.

Transfer molding process Step 3

• After punching the molds are open the cured part is

ejected . The flash are then trimmed.

Advantages of transfer molding process

• 1- It is quick method because loading material in “pot” takes less time than loading in each mold cavity.

• 2- The flash produced in transfer molding are comparatively less.

Injection molding process of thermosetting plastics

Injection molding is done with the help of two mechanisms.

• 1- with plunger• 2- with a reciprocating rotating

screw

Injection molding process with a plunger

Injection molding process with a reciprocating rotating screw

Injection molding process

END

Engineering

Polymers and plastics