PLASTIC TECHNOLOGY

KMÜ 407October 20, 2015

ADDITIVES and COMPOUNDING

The term “compounding” means incorporationof selected additives with the polymer to give ahomogenous mixture.

The traditional compounding device in theplastic industry is the two-roll mill. Otherdevices in common use are internal mixers,paddle blenders and blenders.

ADDITIVES Fillers and reinforcementsPlasticizersAntioxidantsColorantsFlame retardantsStabilizersBiocidesProcessing additivesCompatibilizers

AdditivesFillers: These materials enhance dimensional stability, impact

resistance, tensile and compressive strength, abrasionresistance and thermal stability.

Two types of fillers are available:i) Particulate: sand, quartz, clay, mica, talc, glass, metal

powders, limestone and etc.ii) Fibrous: cellulosic fibers, synthetic fibers (nylon, acrlyic,

polyester), carbon fibers.

Recently, the use of mineral fillers has increased markedly.

Plasticizers: They improve flow properties, processability and reduce the brittlenesss of the product bylowering the Tg of polymer to room temperature.

A plasticizer must be compatible with the polymerand permanent. It must be miscible with the polymer.This implies a similarity in the intermolecular forcesactive in the two components.

Phtalate esters (such as DOP), phosphate esters, adipates,epoxy plasticizers, fatty acid esters, sulfonamids,glycerol derivatives are commonly used plasticizers.

Antioxidants: They prevent or inhibit theoxidation of polymers. Phenols, aromaticamines and thio compounds are importantantioxidants.

• Colorants: They may be oil-soluble dyes or organic pigments and they lead to transparent colored products when incorporated into transparent plastics. Others, including inorganic pigments, impart opacity to the plastic.

Flame Retardants: The flammability of materialscan be reduced:

i. elimination of volatile fuel (by cooling)ii. production of a thermal barriers ( by charring)iii. quenching the chain reactions in the flame (by

adding suitable radical scavengers).

The mostly used flame retardant is antimonytrioxide.

Stabilizers: Carbon black is used to preventphotochemical degradation as in PE and UV-light absorbers are used to improve the lightstability of both plastics and their colorants.

Biocides: It is a chemical that control or destroys bacterialgrowths.

Alternative terminology:Fungicides; fungistats; algicides; germicides; bactericides.

It must be toxic for microorganisms but safe to humansand animal life.

Polyolefins and vinyl polymers are RESISTANT tobacterial attack.

Natural rubber, cellulose and cellulose derivatives andsome polyesters are susceptible to microbial attack.

Biocides are generally used in; Latex paints Textiles Food packaging (must be FDA approved)

Widely used biocides consist Tributyltin oxide

Lubricants: 2 types are available;1. Internal lubricants: They improve flow during

processing by reducing melt viscosity.

2. External lubricants: By reducing adhesionbetween metallic surface of the processingequipment and the polymer melt.

The major market for processing lubricants is PVC------- stearates are often used.

Other lubricants include; Mineral oil Low MW polyolefins

Most polymers are poor electrical conductors

Therefore

Static electrical charges can form on the surfaceof plastics.

Causes

Dust collection and sparking

Antistatic agents:

Hygroscopic additives are called as antistats. They areeffective for dissipating static electrical charges.

External or topical antistats are applied by spraying,wipping or dipping the plastic surface.

Internal antistats: They diffuses to the plasticsurface where it absorbs moisture and provides aconductive layer of water.

Examples: Phosphate and fatty acid esters sulfatedwaxes quaternary ammonium compounds andamines.

Curing Agents: “Curing” means the process of applying heat

(and pressure) to change the properties ofrubber or thermosetting resin.

Curing agents including sulfur-containingcompounds are used to promote thevulcanization (crosslinking of rubber).

Compatibilizers :Many polymers are immiscible and therefore

phase separation occurs during processing.That is why

Their mechanical properties are often poor(immiscible blends) due to inadequateinterfacial strength between the dispersedphase and matrix.Additives can be used to promote miscibility

by reducing interfacial tension.

PROCESSING METHODS

Compression moldingInjection moldingBlow moldingReaction-injection molding (RIM)Thermoset moldingTransfer molding

1) Molding

2) Extrusion CoextrusionFilm extrusion

3) Other Processing Methods

Foaming FormingLaminating

CalenderingCastingCoating

MOLDING

Plastic is forced by the aplication of heat andpressure to flow into, fill and conform to theshape of cavity (mold).

Compression Molding is the simplest andoldest one. The polymer is put betweenstationary and movable members of a mold.The mold is closed and T and P are applied.

INJECTION MOLDING

• PURPOSE:To liquify the plastic material and then toinject the liquid into the mold.

STEPS OF INJECTION MOLDING PROCESS1) The resin is usually introduced into the injection molding

machine through a hopper. • The hopper can be fed manually or pneumatically.• Several hoppers can be clustered on one machine so that

fillers, colorants, or other additives can be fed simultaneously.• However, because of the limited size (barrel length) of most

injection molding machines, mixing capability is poor and so most fillers and other resins are blended in a separate extrusion operation.

• Drying is very common on injection molding hoppers because of the hygroscopic nature of several resins, such as nylon, PC, PET and ABS.

2) The resin enters the barrel of the injection molding machine by gravity from the hopper through a hole in the top of the barrel called the feed throat.

The barrel is a heavy steel cylinder.

3) Two systems have been used in injection molding machines to melt and inject the resin.

• Reciprocoating screw (most commonly used): It is similar to extruder screw.

• Ram injector

4) The screw rotates to pick up theparticulate polymer, compact and melt it,mix the melt and deliver it to the entranceto the mold.

5) The polymer melt has solidified in the cool mold, mold is opened and the product is removed.

Injection Molding

Reciprocoating Screw

• It has 3 sections:--Feed section:to advance the resin--Compression section:to melt the resin--Metering section:to homogenize the resin and pump it forward

INJECTION MOLDING

• Ram injector machine was developed before the reciprocoating screw machine and is generally less expensive to built.

• Today, the reciprocoating screw dominates the market.• Advantages of reciprocoating screw machines:

-better melting characteristics-easier to control-require less thermal heat because they also induce mechanical heating.-more uniform melt.

INJECTION MOLDING

• Advantages:-Wide variety of shapes-high degree of repeatability-high level of automation are possible.-All thermoplastics and many thermoset resins can be injection molded.

• Disadvantages:-High cost of the equipment and high cost of the

molds.

Blow Molding: This operation can be carried out either on anextruder or an injection machine. A section of molten polymertubing (parison) is extruded into an open mold. By means ofcompressed air or steam the plastic is blown into theconfiguration of mold.

This technique is used for the manufacture of bottles and similararticles.

Reaction-Injection Molding (RIM): This is a relatively new process. Polymerization takes place in the mold.

To date, RIM has been used for the molding of PU’s.

Thermoset Molding

• Compression and transfer molding are most common.

• Some elastomers can be extruded and injection molded.

• When large sheets are desired, the elastomers can be calendered.

• Molding is usually done at elevated temperatures and sufficient time is required.

• Thermoset Molding: The injection molding of thermosets differs slightly from that of thermoplastics. The ingredients are heated just enough to make them fluid but not to initiate cure.

EXTRUSION

In this process, polymer is propelledcontinuously along a screw through regions ofhigh T and P where it is melted andcompacted.

Finally, it is forced through a die shaped to givethe final object.

EXTRUDER

Equipments:-Hopper (to feed)-Barrel and screw (to melt)-Die

EXTRUSION

• Advantages:-Continuous-High production volumes-Low cost per pound-Efficient melting-Many types of raw materials-Good mixing

• Disadvantages:-Limited complexity of parts-Uniform cross-sectional shapes only

SCREW

• The differences in the feed and compression sections in screws depends upon differences in melting characteristics of resins.

SCREW• L/D (length/diameter) of the screw.• L/D is a measure of the capability of the screw

to mix materials.• High L/D ratios indicate good mixing and good

melting capabilities but also high energy requirements.

• Typical L/D ratios are 16:1 to 32:1.• Elastomeric materials such as thermoset

rubbers would have smaller L/D ratios.

EXTRUSION

• Thermoplastics are the most common plastics that are extruded.

• However, some thermosets (especially rubber) can also be extruded, provided the temperature of the extruder is kept below the temperature that initiates the cure.

Extrusion

OTHER PROCESSING METHODS

Calendering: Granular resin is passed betweenpairs of highly polished heated rolls under highpressure. This process is used for thecontinious manufacture of sheet or film.

Casting: A liquid material is poured into amold and solidified by physical (e.g., cooling)or chemical (e.g., polymerization) means andthe solid product is removed from the mold.

Foaming: The production of plasticfoams is accomplished by generating agas in a fluid polymer, usually atelevated temperature.