Plastics Processing.pdf

PLASTICS PROCESSING

Introduction

Plastics are classiﬁed as thermoplastic or thermosetting resins, depending on the

effect of heat. Thermoplastic resins, when heated during processing, soften and

ﬂow as viscous liquids; when cooled, they solidify. The heating/cooling cycle can

be repeated many times with little loss in properties. Thermosetting resins liq-

uefy when heated and solidify with continued heating; the polymer undergoes

permanent cross-linking and retains its shape during subsequent cooling/heating

cycles (see T HERMOSETS ). Thus, a thermoset cannot be reheated and molded again.

However, thermoplastics can be melt-reprocessed, and hence readily recycled (see

R ECYCLING ,P LASTICS ).

Thermoplastic Resins. Almost 85% of the resins produced are thermo-

plastics (1,2). Although a number of chemically different types of thermoplastics

are available in the market, they can be divided into two broad classes: amorphous

and crystalline. Amorphous thermoplastics shown in Table 1 (3) are characterized

by their glass-transition temperature T g a temperature above which the modu-

lus decreases rapidly and the polymer exhibits liquid-like properties; amorphous

thermoplastics are normally processed at temperatures well above their T g (see

A MORPHOUS P OLYMERS ). Semicrystalline resins shown in Table 2 (3) can have dif-

ferent degrees of crystallinity ranging from 50 to 95%; they are normally processed

above the melting point T m of the crystalline phase (see S EMICRYSTALLINE P OLY -

MERS ). Upon cooling, crystallization must occur quickly, ie, in a few seconds. Addi-

tion of nucleating agents increases the crystallization rate (see C RYSTALLIZATION

K INETICS ). Additional crystallization often takes place after cooling and during the

ﬁrst few hours following melt processing.

Over 70% of the total volume of thermoplastics is accounted for by the

commodity resins: polyethylene, polypropylene, polystyrene, and poly(vinyl

2 PLASTICS PROCESSING

Vol. 11

Table 1. Glass-Transition Temperature of Amorphous

Thermoplastics a

Polymer

T g , ◦ C

Polyamideimide (PAI)

295

Polyethersulfone (PES)

230

Polyarylsulfone (PAS)

220

Polyethermide (PEI)

218

Polyarylate (PAR)

198

Polysulfone (PSU)

190

Polyamide, amorphous (PA)

155

Polycarbonate (PC)

145

Styrene–maleic anhydride (SMA)

122

Chlorinated PVC (CPVC)

107

Poly(methyl methacrylate) (PMMA)

105

Styrene–acrylonitrile (SAN)

104

Polystyrene (PS)

100

Acrylonitrile–butadiene–styrene (ABS)

100

Poly(ethylene terephthalate) (PET)

Poly(vinyl chloride) (PVC)

a Ref. 3.

chloride) (PVC) (1) (see E THYLENE P OLYMERS , HDPE; E THYLENE P OLYMERS , LDPE;

E THYLENE P OLYMERS , LLDPE; P ROPYLENE P OLYMERS (PP); S TYRENE P OLYMERS ;

V INYL C HLORIDE P OLYMERS ). They are made in a variety of grades and because of

their low cost are the ﬁrst choice for a variety of applications. Next in performance

and in cost are acrylics, cellulosics, and acrylonitrile–butadiene–styrene (ABS)

Table 2. Melting Temperature of Semicrystalline

Thermoplastics a

Polymer

T m , ◦ C

Polyetherketone (PEK)

365

Polyetheretherketone (PEEK)

334

Polytetraﬂuoroethylene (PTFE)

327

Poly(phenylene sulﬁde) (PPS)

285

Liquid crystal polymer (LCP)

280

Nylon-6,6 b

265

Poly(ethylene terephthalate) (PET)

260

Nylon-6 b

220

Nylon-6,12 b

212

Nylon-11 b

185

Nylon-12 b

178

Acetal resin c

175

Polypropylene (PP)

170

High density polyethylene (HDPE)

135

Low density polyethylene (LDPE)

112

a Ref. 3.

b Nylons are polyamides (qv).

c Acetal resin in polyoxymethylene (POM).

Vol. 11

PLASTICS PROCESSING 3

terpolymers (see A CRYLIC E STER P OLYMERS ;A CRYLONITRILE AND A CRYLONITRILE

P OLYMERS ;C ELLULOSE E STERS ,I NORGANIC ). Engineering thermoplastics (qv) such

as acetal resins, polyamides (qv), polycarbonates (qv), thermoplastic polyesters

(qv), and poly(phenylene sulﬁde), and advanced materials such as liquid crystal

polymers, polysulfones (qv), and polyetheretherketones are used in high per-

formance applications; they are processed at higher temperatures than their

commodity counterparts.

With few exceptions, thermoplastics are marketed in the form of pellets.

They are shipped in containers of various sizes, from 25-kg bags to railroad hop-

per cars. Resins are conveyed to silos for storage and from there to the process-

ing equipment. Colored resins are available, but frequently it is more convenient

and economical to buy uncolored resins and blend them with color concentrates.

Using concentrates avoids handling dusty pigments and ensures uniform color

distribution.

The packaging requirements for shipping and storage of thermoplastic resins

depend on the moisture that can be absorbed by the resin and its effect when the

material is heated to processing temperatures. Excess moisture may result in

undesirable degradation during melt processing and inferior properties. Conden-

sation polymers such as nylons and polyesters need to be specially predried to very

low moisture levels (4,5), ie, less than 0.2% for nylon-6,6 and as low as 0.005% for

poly(ethylene terephthalate) (PET), which hydrolyzes faster.

Thermoplastic Processing. A variety of processing equipment and

shaping methods are available to fabricate the desired thermoplastic product

(6–10). Extrusion (qv) is the most popular. Approximately 50% of all commod-

ity thermoplastics are used in extrusion process equipment to produce proﬁles,

pipe and tubing, ﬁlm, sheet, wire, and cable (1). Injection molding (qv) follows as

a preferred processing method, accounting for about 15% of all commodity ther-

moplastics. Other common methods include blow molding (qv), rotational molding

(qv), thermoforming (qv), calendering, and, to some extent, compression molding.

Details on the amounts of resins converted annually in the United States in terms

of processes and products can be found in the following year’s January issue of

Modern Plastics . Computer-aided design software for molds and extruder screws

is commercially available. These programs assist in the selection and fabrication of

processing equipment, thereby saving research and development time. Modeling

and simulation of polymer processing is described in specialized textbooks (11–17).

The range of processes that may be used for fabricating a plastics product is

determined by the scale of production, the cost of the machine and the mold, and

the capabilities and limitations of the individual processes. For example, complex

and precise shapes can be achieved by injection molding, hollow objects via blow

molding and rotational molding, and continuous lengths by extrusion.

Thermoplastics processing operations produce emissions into the air,

wastewater, and solid waste resulting from both polymers and additives. Most im-

portant are volatile organic compounds emitted from heated cylinders and molds.

The identiﬁcation of such volatiles and the development of analytical techniques

for measuring their concentration in the workplace are of paramount importance

to establish or revise threshold limit values that would minimize exposure to

hazardous chemical substances. Environmental issues in polymer processing are

reviewed in References 18 and 19.

4 PLASTICS PROCESSING

Vol. 11

Extrusion

Extrusion is deﬁned as continuously forcing a molten material through a shaping

device. Because the viscosity of most plastic melts is high, extrusion requires the

development of pressure in order to force the melt through a die. Manufacturers of

plastic resins generally incorporate stabilizers and modiﬁers and sell the product

in the form of cylindrical, spherical, or cubic pellets of about 2–3 mm in diameter.

The end-product manufacturers remelt these pellets and extrude speciﬁc proﬁles,

such as ﬁlm, sheet, tubing, wire coating, or as a molten tube of resin (parison) for

blow molding or into molds, as in injection molding.

To provide a homogeneous product, incorporation of any additives, such as

antioxidants (qv), colorants (qv), and ﬁllers (qv), requires mixing them into the

plastic when it is in a molten state. This is done primarily in an extruder. The

extruder, as shown in Figure 1, accepts dry solid feed (F, E, J) and melts the plastic

by a combination of heat transfer through the barrel (B, C) and dissipation of work

energy from the extruder drive motor (I). In the act of melting, and in subsequent

sections along the barrel, the required amount of mixing is usually achieved.

Venting may also be accomplished to remove undesirable volatile components,

usually under vacuum through an additional deep-channel section and side vent

port. The ﬁnal portion of the extruder (L) is used to develop the pressure [

≤

Fig. 1. Parts of an extruder: A, screw; B, barrel; C, heater; D, thermocouple; E, feed

throat; F, hopper; G, thrust bearing; H, gear reducer; I, motor; J, deep-channel feed section;

K, tapered channel transition section; and L, shallow channel metering section (20).

50 MPa

(7500 psi)] for pumping the homogenized melt through a ﬁltering screen (optional)

and then through a shaping die attached to the end of the extruder.

Extruders are deﬁned by their screw diameter and length, with the length

expressed in terms of the length-to-diameter ratio (L/D). Single-screw extruders

Vol. 11

PLASTICS PROCESSING 5

Fig. 2. Intermeshing corotating twin-screw extruder: A, motor; B, gear box; C, feed port;

D, clam shell barrel; E, vent port; F, screw shafts; G, conveying screws; H, kneading paddles;

I, barrel valve; and J, blister rings. Courtesy of APV Chemical Machinery Inc.

range from small laboratory size (6 mm diameter) to large commercial units

(450 mm diameter) capable of processing up to 20 t/h. Melt-fed extruders run

at a L/D of about 8; solids-fed extruders run at a L/D of about 20–40 depending

on whether intermediate venting is provided.

Varieties of twin-screw extruders are also utilized, particularly when the in-

gredient mixing requirements are difﬁcult to fulﬁll or require multiple staging, as

in reactive extrusion. Twin-screw extruders are classiﬁed as being tangential or

intermeshing, and the latter as being counter- or corotating. These extruders are

generally supplied with slip-on conveying and kneading screw elements and seg-

mented barrels. These elements, shown in Figure 2, give the processor improved

mixing and pumping versatility. Single-screw extruders are usually ﬂood-fed, with

the feed rate determined by screw speed. Twin-screw extruders are generally

starve-fed. Starve feeding permits greater ﬂexibility in operation as screw speed

now becomes a process control variable, independent of feed rate.

Reactive extrusion is the term used to describe the use of an extruder as a con-

tinuous reactor for polymerization or polymer modiﬁcation by chemical reaction

(21,22). Extruders are uniquely suitable for carrying out such reactions because of

their ability to pump and mix highly viscous materials. Extruders readily permit

multiple process steps in a single machine, including melting, metering, mixing,

reacting, side-stream addition, and venting.

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