Plastic Product Material and Process Selection Handbook Part 9 ppsx

The process is performed at a temperature close to but less than the melt temperature, by stretching between rolls operating at a speed differential.. This action provides the different

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ratio and to reduce polymerization costs and emissions The final polymer/plastic may or may not be soluble in the solvent

Dry spinning involves a solution of plastics The solution is then heated above the boiling temperature of the solvent and the solution is extruded through a spinneret The solvent evaporates into the gas stream With wet spinning, the fiber is directly extruded into a coagulation bath where solvent diffuses into the bath liquid and the coagulant diffuses into the fiber The fiber is washed free of solvent by passing it through an additional bath Each process step generates emissions or wastewater Solvents used in production are recovered by distillation

Extruders have their part in producing fibers or filaments There are many variations and combinations of the basic processes that include:

be profiled to create fibers with improved wicking properties Common profiles include trilobal, quadrilobal, delta, and dogbone shapes The land length of the spinneret holes is typically in the range 5 to 15 times the hole diameter In turn they are immediately stretched or drawn (oriented), cooled, and collected at the end of the line During this process they may be subjected to other operations such as:

1 thermal setting and thermal relaxation processes to provide

dimensional stability;

2 twisting and interlacing to provide cohesion of the filaments with or without sizings;

3 texturing;

4 crimping and cutting to provide staple products

Speeds of certain lines using the melt and dry spinning processes can go from 2,000 m / m i n (6,600 ft/min) to at least 4,000 m / m i n (13,000 ft/min)

The spinneret passes the melt through a very large number of very small holes It is a rectangular plate die, typically provided with 200 or

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Figure 5~17 Example in using a gear pump to produce fibers (left) and example in using an

extruder and gear pump to produce fibers

more holes arranged in a grid formation This action gives the process a number of distinctive features The process demands a low melt viscosity High melt flow index grades are used, and at relatively high melt temperatures in the range of 230C to 260C Because the holes in the spinneret are very small, an exceptionally high degree of melt filtration is necessary to screen out foreign particles that would otherwise clog the spinneret or cause the filament to fracture in the stretching operation

This system of extrusion does not have the die m o u n t e d at the extruder outlet, the fiber-forming spinneret is m o u n t e d at some distance from the extruder and is connected to it by a heated melt manifold system From the extruder, the melt stream passes through a filter system As the melt emerges from the spinneret hole, it passes down a spin chimney where it is cooled or quenched in streamline airflow operating

at a controlled temperature and then hauled off by godet rolls The final step is yarn stretching or drawing to orient the molecular chains to

a high degree in the machine direction Orientation also reduces the diameter of the fibers The process is performed at a temperature close

to but less than the melt temperature, by stretching between rolls operating at a speed differential The orientation is set by an annealing step and the continuous filament yarn is w o u n d on spools or bobbins for subsequent use in textile operations

During fiber spinning they can be exposed to different conditions Different type finishes can be applied after cooling in-line to meet different requirements that include permitting processing improvement

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during weaving, handling of fibers, etc Texturing introduces crimp, whereby the straight filaments are given a twisted, coiled, or randomly kinked structure A yarn made up of these filaments is softer and more open in structure; it is more pleasing to the touch

Fibers are twist during fabrication The twist is the spiral turns about its axis per unit of length Twist may be expressed as turns per inch (tpi) The letters S and Z indicated the direction of the twist, in reference to whether the direction conforms to the middle-section slope of the particular letter 143 A fiber, yarn, or strand has what is known as an "S" twist if, when held in a vertical position, the spirals conform in slope to the central position of the letter S It has a " Z " twist if the spirals conform in slope to the central portion of the letter Z Fibers that are simply twisted (greater than 1 t u r n / i n , or 40 t u r n s / m ) will kink, corkscrew, a n d / o r unravel because their twist is only in one direction The plying operation normally eliminates this problem For example single yarns having a " Z " twist are plied with an "S" twist, thus resulting in a balanced yarn Depending on the twisting and plying operations, different yarn strengths, diameter, and flexibility can be obtained This action provides the different shaped and handling fabrics that are used to meet different performance requirements of plastic materials such as coated fabrics, reinforced plastics, pultrusions, etc Monofilament yarns consist of a single filament The filament size is much larger than those found in multifilament yarn Consequently, monofilament is relatively stiff and is used mainly for the production of rope and twine Fiber size range is typically 75 to 5000 denier Monofilament fiber is usually produced from polypropylene homo- polymer with a relatively low melt flow index in the range 3.5 to 5.0

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Extruded cast film can be slit into narrow tapes that are then uniaxially oriented in the machine direction becoming slit filament The chill roll may cast the film or water quenches processes The slit film fiber (plain tape) may be used as it is in weaving processes, or it may be given a subsequent fibrillation treatment to impart a fibrous character Slitting occurs with the film in tension upstream of the first set of godet rolls that anchor the tapes ahead of the orientation oven A second set of godet rolls applies a stretching tension to the tapes that are heated to a temperature just below the melting point Draw ratios typically range from 1:5 to 1:8 Heated rolls may be used as an alternative to the draw oven In the final step in this operation the tapes are annealed to set the orientation, and are wound on spools for subsequent use in textile operations

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Similar to slit film fiber, fibrillated tape fiber is produced However a draw ratio of up to 1:10 is used and a fibrillation step is added to the process The slit film tapes arc fibrillated by passing them over a rapidly rotating roll fitted with staggered rows of pins As the pins are traveling faster than the tapes, they make a series of short discrete cuts in the longitudinal direction of the tapes

Bulked continuous filament yarn is produced in exactly the same manner

as continuous filament but is given an additional treatment before final spooling The purpose of this treatment is to improve the handling, feel, and elasticity of a textile article manufactured from the yarn There are a number of different bulking treatments, the target always being to produce minor distortions to the filaments The principle is to heat the yarn close to the melting point by means that perturb the filaments, using steam jets, turbulent hot air, or a hot knife-edge

There is staple fiber that consists of intermingled short length multi- filament fibers similar to natural fibers such as wool or cotton They are produced in much the same manner as continuous filament but the filaments are finer and are collected together in large numbers in a loose rope or tow The spinneret contains a very large number of holes such

as 15,000 or more and sometimes ranging as high as 50,000 The tow

is passed through a crimper that imparts a small zigzag configuration to the filaments and is then chopped into short lengths from 5 to 60 mm The chopped fibers are baled in random orientation for use in any textile process conducted with staple fiber

Coextrusion

The different extrusion processes (film, profile, etc) using coextrusions that can range from two to seven or more layers are not u n c o m m o n (Chapters 3 and 17) 203, 204, 205 An example for just coextruding film or sheet is shown in Figure 5.18 Coextrusion is an economical competitor

to conventional laminating processes by virtue of its reduced materials handling costs, raw material costs, and machine-time cost Pinholing is also reduced with coextrusion, even when it uses one extruder and divides the melt into at least a two-layer structure Other gains include elimination or reduction of delamination and air entrapment It pro- vides an excellent way to integrate/entrap recycled contaminated plastic with one side or both sides using virgin plastic It provides combining the different properties of the different plastics that result in perfor- mance and cost advantages

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Figure 5 1 8 Schematic of a basic three layered coextrusion sheet or film system

The different materials used in the coextruded structure meets different performance requirements such as strength, stiffness, toughness, noise absorption, resist permeation (air, oxygen, moisture, gas, odor, etc.), adhesive qualities, and economics A single expensive plastic could be used to meet performance requirements, however it is possible to provide combinations of plastics to reduce the cost An example as used

in the marketplace; coextrusion has been adapted to the production of products like building profiles, pipes, and packaging films that incorporate

in a single extrusion structure several layers of different plastics, each offering varying degrees of different performance requirements

Compatible plastics can flow through a single manifold reducing any potential problem down-stream in the multimanifold Combinations can be made to provide different laminated designs However, the final exiting layer thickness distributions can be affected by the amount of die body deflection if the die is not properly designed to take the required pressure loads Any deflection causing distortion influences the melt flow channels (Chapter 17)

Many plastics can be bonded to each other There are those that require

a tie-layer Choosing an adhesive tie-layer is by no means a simple operation as in coinjection (Chapter 4) There are many different types, each with specific capabilities EVAs form the bulk being used Proper choice can improve performance, such as increasing melt strength and bubble stability in blown film High melt strength can also help in cast film used in thermoforming or coating Good melt draw is required to run higher take-up speeds and or thinner structures without causing flow distribution or edge-weave problems

Melt flow instabilities, such as interracial instability, melt fracture, surging, a n d / o r layer nonuniformity, can become problems that could cause quality problems with the coextruded product 2~176 There are several options to correct these problems The key to success is to select

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the best option or combination of options to eliminate the problem and minimize other possible adverse effects Flow instabilities can cause limited production rates Appropriate operating conditions of the coextrusion process combined with the proper plastic selections can successfully be used to solve the problems

Orientation

Orientation consists of a controlled system of stretching TP molecules

in unioriented [unidirectional (UD)] or bioriented [biaxial direction (BD)] UD orientation can be in the machine direction (MD) or transverse direction (TD) 2~ Orientation improves strength, stiffness, optics, electrical, a n d / o r other properties with the usual result that improved product performance-to-cost occur This technique is used during the processing of many different products such as films, sheets, pipes, fibers, tapes, etc

Depending on the properties of a specific plastic product the stretch ratio may vary from 21/2:1 to as high as 10:1 Some specialty films may have an even higher stretch ratio Used for almost a century, it became prominent during the 1930s for stretching fibers up to 10 times Later

it was adapted principally to films and other products such as stretched blow molded bottles Practically all TPs can undergo orientation, although certain types find it particularly advantageous (PET, PP, PVC,

PE, PS, PVDC, PVA, PC, etc.) Many different markets use oriented plastic products The largest market for plastics worldwide, consuming about 20wt% of total, is oriented plastic film

Orienting by stretching is influenced by factors such as:

1 the lowest temperature will give the greatest orientation (tensile

strength, modulus, etc.)

2 the highest rate of stretching will give the greatest orientation at a

given temperature and percent stretch

3 the highest percent stretch will give the greatest orientation at a

given temperature and rate of stretching,

4 the greatest quench rate will preserve the most orientation under

any stretching condition

With orientation, the thickness is reduced and the surface area enlarged

If film is longitudinally stretched, its thiclmess and width are reduced in the same ratio If lateral/transverse contraction is prevented, stretching reduces the thickness only Orientation temperature is normally 60 to

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75% of the range between the plastic's glass transition temperature (Tg) and melting point (Tm) Equipment can be used in-line or off-line to gain output yield and properties

Examples of many oriented products include the heat-shrinkable material found in flat or tubular films or sheets and fibers The orientation in these cases is terminated downstream of an extrusion-stretching operation when

a cold-enough temperature is achieved Reversing the operation, shrinkage occurs when a sufficiently high temperature is introduced The reheating and subsequent shrinking of these oriented plastics can result in a useful property It is used, for example, in heat-shrinkable flamc-rctardant PP tubular or flat communication cable wraps, heat-shrinkable furniture webbings, pipe fittings, medical devices, and many other products

Mechanical properties depend directly upon the relationship between the axis of orientation of the plastic molecules and the axis of mechanical stress upon the molecules Modulus, strength, etc increases in the direction of stretch and decreases in the perpendicular direction This is the mechanism

of pscudoplastic and thixotropic rhcology typical of a non-Newtonian plastic flow behavior After processing, some loss in properties may occur (insignificant) when subjected to heat during further processing, such as thermoforming, heat sealing, and solvent sealing

Biaxial orientation of crystalline plastics generally improves clarity of films This occurs because stretching breaks up large crystalline structures into smaller than the wavelength of visible light With uniaxial orientation, the result is an anisotropic refractive index and thus birefringence, especially

in crystalline plastics

Orientation decreases electrical dissipation factors in the direction of the orientation, and increase occurs perpendicular to orientation Since modulus changes in the opposite way, this indicates that polar vibrations along a stretched plastic molecule are decreased, while transverse vibrations between the stretched molecules are increased Different methods are used An online ordinary common blown or cast film line uses a machine direction oricntcr ( M D O ) on the front end of biaxially oriented film heated chamber extrusion line If only the machine direction is to be stretched, a series of precision controlled heated rolls can be used Film is fed through a series of rolls where it is sequentially hcated, drawn around rolls that increase in rotational speed providing the stretching action, annealed around larger diameter roll(s), and cooled on a final roll(s)

A TP's molecular orientation can be accidental or deliberate Accident can occur during the processing of TPs where excessive frozen-in stresses develop, however with the usual proper process control, there is no

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accidental orientation (Chapter 3) The frozen-in stresses with certain TPs can bc extremely damaging if products are subjected to stress cracldng in certain environments, crazing in the presence of heat or chemicals, etc Initially the molecules are relaxed Molecules in the amorphous regions arc

in random coils; those in crystalline regions are relatively straight and folded

During processing (extrusion, injection, blow molding, etc.) the molecules tend to be more oriented than relaxed, particularly when the melt is subjected to excessive shearing action After temperature-time- pressure is applied and the melt goes through restrictions (mold, die, etc.), the molecules tend to be stretched and aligned in a parallel form The result can be undesirable changes in the directional properties and dimensions immediately when processed a n d / o r thereafter when in use

if stress relaxation occurs

By deliberate stretching, the molecular chains of a plastic are drawn in the direction of the stretching, and inherent strengths of the chains are more nearly realized than they arc in their naturally relaxed con- figurations Stretching can take place with heat during or after processing

by extruding film, blow molding, injection molding, thermoforming, etc Products can be drawn in one direction or in two opposite directions, in which case many properties significantly increase uniaxially or biaxially

The amount of change depends on the type of TP, the amount of restriction, and, most important, its rate of cooling The faster the rate, the more retention there is of the frozen orientation After processing, products could be subject to stress relaxation, with changes in perfor- mance and dimensions With certain plastics and processes there is an insignificant change If changes arc significant and undesirable, one must take action to change the processing conditions during a n d / o r after processing As an example during processing increase the cooling rate Annealing of the product is an after processing condition approach

The processing hardware of orientation is fairly expensive It increases the cost per unit weight of the product However, the yield increases considerably, its quality improves greatly, and the product cost reduction occurs Many products arc made much stronger, flexible, tougher, etc resulting in significant cost advantages

Blown Film

The blown products, such as upward blown film, are basically natural for providing orientation (Figure 5.19) The blow-up ratio determines the degree of circumferential orientation, and the pull rate of the bubble determines the longitudinal orientation

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Figure 5.19 Example of upward extruded blown film process for biaxially orienting film

The optimum stretching heat for amorphous plastics (PVC, etc.) is usually just above its glass transition temperature (Tg;; Chapter 1) Generally the orientation temperature is 60 to 75% between the Tg and

T m (melt temperature) For crystalline plastics (PE, PET, etc.) generally

it is below the Tg Stretching can take place in-line or off-line with or without tenter flames using the appropriate temperature-pull rates as the plastic travels first through a series of heated rolls For unidirect- ional orientation just the rolls are used

In addition to the upward blown film there is also the downward extrusion It allows the tube to be quenched rapidly in a water bath after which it is collapsed as a layflat for passage over nip and idler rollers Film passes through a reheating tunncl where it is raised to a temperature above the softening point but below the melting point The heated tube is then inflated by internal air pressure that forms a bubble in which thc film is stretched in all directions Some machine-

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direction stretch may take place in the ovens upstream of the bubble; the haul-off rate can be adjusted if necessary to secure an orientation balance An air ring similar to that used in other blown film processes provides bubble cooling Subsequent calibration and bubble collapsing operations are also similar

Flat Film

Using some type of tenter flame can biaxially orient fiat products Orienting film is accomplished by mechanically stretching the film in the tenter machine This takes its name from the tenter flame originally used for stretching cloth between grips known as tentcrhooks Simultaneous tcntcring is possible, involving complex movements of the film edge grips so that the film is stretched in the machine and transverse directions at the same time However, the process can be mechanically complicated and can be difficult to adjust the balance between the stretch directions for certain plastics To eliminate potential problems the two-step tenter process is used (Figure 5.20) The process starts with the production of a cast film This is then stretched in the machine direction by passing it around heated rollers rotating at controlled and increasing speeds in excess of the extrusion output speed Varying the roll speeds controls the degree of stretch Typically, stretch in the machine direction is about 4.5:1

Figure 5,20 Example of two-step tenter process

When machine direction stretching is complete, the tenter machine applies transverse stretching This consists essentially of a temperature- regulated tunnel in which the film edges arc gripped by chain-driven tension clips running on divergent paths As the film passes through the tunnel it is progressively stretched in the transverse direction as the clips diverge The edge grip mechanism must withstand high cross loads of

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up to 3.5 kN per clip, and be capable of operating at line speeds of up

to 450 m / m i n Transverse stretch is controlled by varying the divergence of the edge clip paths A ratio of 8:1 is typical Feeding the orientation system can be from an extruder or an extruded roll of film

Fiber

It is accomplished usually by just stretching when the fibers are produced They can also be stretched by a set of heated rolls where each of their rotating speeds (rpm) are increased A fiber or thread of nylon 6 / 6 , that is an unoriented glassy polymer, has a modulus of elasticity of about 2,000 MPa (300,000 psi) Above the Tg (glass transition temperature) its elastic modulus drops even lower, because small stresses will readily straighten the kinked molecular chains (Chapter 1) However, once it is extended and has its molecules oriented in the direction of the stress, larger stresses are required to produce added strain The elastic modulus significantly increases

The next stop is to cool the nylon below its Tg without removing the stress, retaining its molecular orientation The nylon becomes rigid with

a much higher elastic modulus in the tension direction [15,000 to 20,000 MPa (2 to 3 x 106 psi)] This is nearly ten times the elastic modulus of the unoricnted nylon-66 plastic The stress for any elastic extension must work against the rigid backbone of the nylon molecule and not simply unkink molecules

Other process

Different processes take advantage in applying orientation to gain certain properties in certain products Major product lines include stretched blow molded bottles/containers (Chapter 6), thermoformed oriented containers (used for such products as fruits, vegetables, and baked goods) (Chapter 7), tapes, etc

Postforming

There is off-line (for small quantity of products) or in-line post- forming The in-line refers to forming/shaping the extrudate (tube sheet, etc.) just after it emerges from the extruder die but before the plastic has a chance to cool It provides specialty products with performance and cost advantages Upon leaving the extruder's die and

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while it is retaining its heat, the plastic is continually post-formed producing products such as embossing patterns on flat, twisted, curved, etc shapes 2~ Also coiled form of tube, rod, profile, etc.; twisting different extrudates; and small or large (up to at least 6 ft) corrugated tubing or piping Examples of these type actions are shown in Figure 5.21

Compounding

Compounding deals with mixing and dispersing additives, fillers, pig- ments, stabilizers, reinforcing agents, antioxidants, inhibitors, foaming agents, processing aids, etc into a melted polymer by different equipment (Figures 5.22 and 5.23) Of major importance are single and multiscrew extruders with the multiscrews most important Each type extruder provides different capabilities As an example the twin and multiscrews produce excellent mixing by forcing the melt back and forth from one screw to another This breaks up the flow patterns Venting of the extruder also is efficient with the dual screw giving good exposure of the plastic in the vent zone Often twin-screw extruders have self-wiping screws, which can be purged rapidly and efficiently Some of these extruders are designed to be extra gentle with respect to mechanical working of the plastic They are used to give a low melt temperature and very little decomposition of the plastic 2~

Single-screw extruders for compounding have relatively long L / D ratios (from 32:1 to 40:1), usually with several barrier sections Venting

is almost always employed to remove the molten polymer of unwanted vapors (Chapter 3)

Corotating intermeshing twin-screw extruders are frequently used in compounding Nonintermeshing screws are used in devolatilizing extruders where very large surface area renewal is required to allow for diffusion of unwanted vapors from the melt Intermeshing conical counter-rotating screw machines are most often used for compounding

of PVC Ko-Kneaders are multiple screw machines featuring rotating shafts that reciprocate in an axial direction This type of machine is widely used for plastics such as PP and PVC

Reclamation/recycling

Recycled plastic is used to fabricate products (Chapter 2) Reclaiming plastics can require special equipment because the feedstock for the

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Figure 5,21 Few examples of the many different postformed shapes and cuts with some showing dies continued

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Figure 5,22 Examples and performances of compounding equipment

Figure 5.23 Schematic of compounding PVC