Coatings of Polymers and Plastics Part 3 pptx

An importantguiding principal for all systems is percolation theory, which is used to predict theamount of filler required to make a single phase material conductive through filleradditi

Overview of the Automotive Plastics Market 39

F IG 27 Electrostatic paint transfer efficiency versus substrate conductivity forplaques painted in a test laboratory

Polar materials will have a faster inherent dissipation constant than nonpolarmaterials because of their higher dielectric constant Such charge dissipationprocesses that occur with highly resistive substrates produce characteristicallylow dissipation currents, in comparison to those of a conductive substrate.Figure 27 plots the transfer efficiency versus part conductivity for plaques

of polymers having various conductivities For these experiments, transfer ciencies were evaluated by paint thickness (theoretical yield is calculated based

effi-on percent solids in the paint and time of plaque exposure to the paint, which

is sprayed at a particular delivery rate) A plateau of transfer efficiency versussubstrate conductivity occurs in the region of approximately 10−5S/cm, whichshows that paint transfer efficiency equivalent to that of metal can be observed

at levels of polymer conductivity significantly below metallic (35)

Practical aspects to grounding a conductively modified part in a paintingshop are of equal importance as the theory Figure 28 illustrates that a chargewill typically travel a shorter distance if it moves toward the grounding clipthrough the bulk of a semiconductor instead of along the surface However, it

is not the physical distance, but instead the total resistance of the path that isthe determining feature for current distribution The resistance of any given path

is the average resistivity of the material traversed multiplied by the distancetraveled If substrate bulk conductivity is equal to or greater than that of the

40 Babinec and Cornell

F IG 28 Charge dissipation currents divide across all available paths using relativepower (IR drop) losses just as they would in a parallel electrical circuit

F IG 29 Theoretical Percolation Curve (From Ref 39.)

Overview of the Automotive Plastics Market 41

conductive primer, discharge currents will travel through the bulk (the shorterpath to ground) Practical details such as the shape of the part, the placement ofgrounding clips, and the number of grounding clips can clearly affect the rela-tive merits of the various charge dissipation routes, and thus the optimal conduc-tivity value The published literature consensus of a target conductivity for elec-trostatic painting appears to be a value of about 10−5to 10−6S/cm (35)

Typical fillers employed in the preparation of conductive polymers are tive carbon powders, fibers, and nanofibers The literature offers general guide-lines and many experimental examples of composite preparation An importantguiding principal for all systems is percolation theory, which is used to predict theamount of filler required to make a single phase material conductive through filleraddition This theory is based on the universal experimental finding that a criticalstate exists at which the fillers in an insulating matrix suddenly connect with eachother to create a continuous conductive network, as shown in Figure 29

conduc-The percolation threshold,ϕc, is the filler loading level at which the mer first becomes conductive, which is generally considered to be a value ofabout 10−8 S/cm Comprehensive experimental and theoretical treatments de-scribe and predict the shape of the percolation curve and the basic behaviors ofcomposites as a function of both conductive filler and the host polymer charac-teristics (36–38) A very important concept is that the porous nature of theconductive carbon powders significantly affect its volume filling behavior Thetypical inclusive structural measurement for conductive carbon powder porosity

poly-is dibutyl phthalate absorption (DBP) according to ASTM 2314 The higher theDBP, the greater the volume of internal pores, which vary in size and shape.The crystallinity of the polymer also reduces the percolation threshold, becauseconductive carbons do not reside in the crystallites but instead concentrate inthe amorphous phase Eq (2) describes the percolation curve (39)

ϕc= (1 − ζ)冉 1

where:

ϕc= volume at percolation onset

ρ = density of carbon (taken as 1.82)

ν = DBP absorption on crushed carbon in cm3

/g

ζ = crystalline volume fraction of the polymer

Table 14 compares the theoretical and experimental results for percolation

of two conductive carbon powders in a PP of two different melt flows, 4 and

44 g/10 min, when prepared by two melt-processing techniques, compression

42 Babinec and Cornell

T ABLE 14 Comparison of Theoretical and Experimental Electrical PercolationBehavior for PP

Predicted Experimental Experimentalpercolation percolation loading for

PP Melt flow Carbon threshold Sample threshold (σc) 10−5S/cm

of the carbon and formation of a preferred carbon network structure are ratelimiting in morphology development (39)

In conductive polymer blends, for example, TPO, another phenomenon

must be taken into account—the localization of the conductive filler in only one

of the available phases Such composites characteristically acquire conductivity

at lower filler loading levels than would be achieved by either of the two vidual polymer phases This advantaged percolation using localization of filler

indi-in a sindi-ingle phase of a polymer blend is called “double percolation.” Filler ization has been reported in a large number of conductive blends (40–54).The driving force for localization is believed to be the thermodynamics ofpolymer/filler interaction, as described by Young’s equation Sumita et al havecalculated a carbon black wetting coefficient,ωp1−p2, Eq (3), from Young’s equa-

local-Overview of the Automotive Plastics Market 43

F IG 30 Transmission electron micrograph (TEM) of the morphology of a tive TPO

conduc-tion in order to predict the thermodynamically controlled locaconduc-tion of the filler

γc −p1= interfacial tension between conductive carbon and polymer 1

γc −p2= interfacial tension between conductive carbon and polymer 2

γp1−p2= interfacial tension between polymer 1 and polymer 2

θ = contact angle of the polymer on the carbon

Prediction:

ωp1−p2> 1 = carbon in the P1phase

ωp1−p2< −1 = carbon in the P2phase

−1 < ωp1−p2< + 1 = carbon at the P1/P2interface

In blends of polar and nonpolar polymers, the carbon typically resides

in the more polar phase For blends of low-surface-energy polymers, such aspolyolefins, there are conflicting accounts of positioning of the carbon(21,39,55,56) It has been reported that conductive fillers are least likely toreside in a PP phase, which is related to its exceptionally low surface energy

44 Babinec and Cornell

When the conductive filler localizes in a minor phase of a blend, thatphase must be at least partially continuous for the composite to be globallyconductive Morphology is often adjusted to keep a conductive minor phasevolume to a minimum, while maximizing continuity in an attempt to minimizethe additional cost incurred for the conductive filler For example, in a rubber-modified polypropylene, the carbon resides in the minor rubber phase Figure

30 shows that the minor phase rheology of a conductive TPO For this, theconductive carbon resides fully in the elastomer phase, which is the dark region.The minor elastomer phase morphology has been adjusted to be somewhat la-mellar so that the conductive domains can be continuous within the composite

at low-volume fractions

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1 Chem Eng News 58(40):29, 1980

2 BRGTownsend Inc., P.O Box F, Suite 130, 500 International Drive North, MountOlive, NJ 07828

3 Automotive Plastics Report—2000, Market Search Inc

4 RW Carpenter Electroplating: back to the basics The Society of Plastics EngineersRegional Educational Technical Conference March 1992

5 Standards and guidelines for electroplated plastics American Society of plated Plastics Englewood Cliffs, NJ: Prentice-Hall, Inc, 1984

Electro-6 B Factor, T Russell, M Toney Physical Review Letters, 1991, p 1181

7 Merriam-Webster’s Collegiate Dictionary, Tenth Edition Merriam-Webster, Inc

2001, p 14

8 S Granick MRS Bulletin 33–36, 1996

9 D Brewis, D Briggs Polymer 22:7–16, 1981

10 RA Ryntz The Influence of Surface Morphology on the Adhesion of Coatings toThermoplastic Polyolefins Under Stress In: The Annual Meeting of the AdhesionSociety, 18th 1995

11 E Tomasetti, R Legras, B Henri-Mazeaud, B Nysten Polymer 41:6597–6602, 2000

12 D Bergbreiter, B Walchuk, B Holtzman, H Gray Macromolecules 31:3417–3423,1993

13 P Schmitz, J Holubka Journal of Adhesion 48:137–148, 1995

14 S Rzad, DC, M Burrell, J Chera In: SAE International Congress and Exposition.Detroit SAE, 1990

15 R Bongiovanni, BG, G Malucelli, A Priola, A Pollicino J Materials Science 33:1461–1464, 1998

16 M Hailat, HX, K Frisch J Elastomers and Plastics 32:195–210, 2000

17 T Ouhadi, JH, U.S Patent 5,843,577, 1998

18 T Ouhadi, JH, PCT WO 95/26380, 1995

19 B Miller Plastics World 15, 1996

20 J Helms Electrostatic painting of conductively modified injection molded plastics In: Coating Applications of Specialty Substrates Society of ManufacturingEngineers, 1995

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21 J Helms Conductive TPOs for improved painting efficiency of bumper fascia In:TPOs in Automotive Conference 1996

22 T Derengowski, EB, J Helms In SAE 1998

23 D Edge, DG, C Doan, S Kozeny, P Kim The benefits of conductive TPO forimproving the painting of automotive part In: TPOs in Automotive 1998

24 J Pryweller Plastics News 4, 1997

25 Plastics Engineer 32, 1998

26 VTA News 2, 1993

27 B Miller Plastics World 73–77, 1996

28 JD Gaspari Plastics Technology 14–15, 1997

29 B Miller Plastics World 15, 1996

30 H Scobbo, DG, T Lemmen, V Umamaheswaran In: SAE Conference 1998

31 D Garner, AE J Coatings Technology 63(803):33–37, 1991

32 D Garner, AE J Coatings Technology 64(805):39–44, 1992

33 C Speck, AE Transactions on Industry Applications 27(2):311–315, 1991

34 A Elmoursi In: IEEE Industry Applications Society Annual Meeting 1991

35 K Sichel Carbon Black-Polymer Composites—The Physics of Electrically ducting Composites New York: Marcel Dekker, 1982

Con-36 N Probst Carbon Black—Science and Technology, Second Edition—ConductingCarbon Black ch 8, 1993

37 A Medalia Rubber Chem and Technology 59:432, 1985

38 S Babinec, RM, R Lundgard, R Cieslinski Advanced Materials 12(23):1823–1834,2000

39 K Miyasaka, KW, E Jojima, H Aida, M Sumita, K Ishikawa J Materials Science17:1610, 1982

40 S Asai, KS, M Sumita, K Miyasaka Polymer Journal 24(5):415, 1992

41 M Sumita, KS, S Asai, K Miyasake, H Nakagawa Polymer Bulletin 25:265, 1991

42 G Geuskens, E DK, S Blacher, F Brouers European Polymer Journal 27:1261,1991

43 F Gubbels, RJ, Ph Teyssie, E Vanlathem, R Deltour, A Calderone, V Parente, JBredas Macromolecules 27:1972, 1994

44 F Gubbels, SB, E Vanlathem, R Jerome, R Deltour, F Brouers, Ph Teyssie molecules 28:1559, 1995

Macro-45 B Soares, FG, R Jerome, Ph Teyssie, E Vanlathem, R Deltour Polymer Bulletin35:223, 1995

46 M Sumita, KS, S Asai, K Miyasaka In: Sixth Annual Meeting—PPS Nice, France,1990

47 F Gubbels, RJ, E Vanlathem, R Deltour, S Blacher, F Brouers Chem Materials 10:

1227, 1998

48 R Tchoudakov, OB, M Narkis Polymer Networks Blends 6:1, 1996

49 M Narkis, RT, O Breuer In ANTEC 95 1995

50 R Tchoudakov, OB, M Narkis, A Siegmann Polymer Engineering and Science36(10):1336, 1996

51 C Zhang, HH, X Yi, S Asai, M Sumita Compos Interfaces 6:227, 1999

52 J Feng, CC Polymer 41:4559, 2000

53 A Ponomarenko, VS, N Enikolopyan Advances in Polymer Science 126, 1996

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55 J Helms, EB, M Cheung U.S Patent 5,484,838 1996

56 No 38 was given in the original document

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63 Teltech Resources Network Corp Adhesives Age 38–44, 1996

64 BN McBane Automotive Coatings Monograph, Federation of Cosieties for ings Technology Blue Bell, PA: SAE, 1987, p 39

Coat-65 IA Hamley Introduction to Soft Matter, Polymers, Colloids, Amphiphiles, and uid Crystals New York: Wiley, 2000

Liq-66 RJ Young, PA Lovell Introduction to Polymers, 2nd ed London: Chapman andHall, 1991, ch 4

67 MF Ashby Materials Selection in Mechanical Design, 2nd ed Oxford: worth/Heinemann Publishing, 1999

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Processing flexibility plays an important role in the overall plastics nario in two ways First, flexibility makes it possible for plastics to be used inthe design of complex shapes that often cannot be produced with metals or othertypes of materials Second, the precision and rapid cycling that can be realized

sce-in everyday manufactursce-ing produces a compellsce-ing economic scenario that isdifficult or impossible for other material-process combinations to match.This chapter will provide a broad overview of the variety of plastics pro-cesses that are used to produce component parts To gain a perspective, therelationship between raw materials and processes will be explored Next, thefundamental physical mechanics of conversion will be outlined to develop anappreciation for how specific techniques are used to create processes that can

be applied to achieve specific goals

With this foundation in place, the factors used to make the underlyingprocess selection decision will be discussed Because the selection of processhas important implications to coating of raw molded components, a method ofincorporating coating considerations into the decision will be introduced

47

48 Stretch

Finally, an organized summary of key plastics conversion processes will bepresented as an aid to making the best coating decisions for specific applicationscenarios

The sheer number and variety of processes used for converting plastic raw rials into components can be daunting Thankfully, it is possible to make sense

mate-of it all by understanding the nature mate-of the raw materials that can be used and

by viewing processes in terms of the basic physical mechanics that are involvedfor each This approach will make it relatively easy to understand which materi-als can be used with what processes and what a given process is able to accom-plish

Plastic materials are based on hydrocarbons, a class of organic compounds thatcontain hydrogen and carbon The primary source of hydrocarbons today iscrude oil, although it is possible to produce them from coal, shale, or otherforms of fossil fuel It is also possible to produce hydrocarbons from otherorganic matter, such as cereal grains

Hydrocarbons are interesting compounds because some of them lend

themselves to reaction by polymerization This type of reaction produces plastic

materials from simple molecular building blocks The building blocks combineinto chains that result in polymer molecules that are very large (in atomic terms)

The term polymers is from the Latin poly (meaning many) and mers (meaning

units) So plastics are described as hydrocarbons that are composed of “manyunits.”

2.1.1 Raw Material Form

The first source of processing variety comes into play when the question ofwhen and how this polymerization reaction takes place Raw materials may beliquid components or they may be solids in the form of powders, granules, orpellets The raw materials may be presented for processing in a prepolymerizedform (polyethylene or acrylonitrile/butadiene/styrene [ABS]), they may be in apartially reacted form (urethanes = polyols + isocyanates), or they may be inthe state of their precursor raw materials (phenolics and alkyds)

The general path from hydrocarbons to molding materials is shown ingraphical form in Figure 1 There are several considerations that determine theform that raw materials take

Physical Form of the Polymer Building Blocks. The raw materials orunits used to produce plastics are normally compounds that are in the form of

Number and Type of Building Blocks Used in the Polymerization Process.

In some cases, the polymerization may involve the use of a single buildingblock that is caused to repeatedly link to itself and form long chains, as is thecase of ε-caprolactam to form nylon 6 This type of reaction is referred to as

addition polymerization In other situations, two building blocks are combined

to form a polymer, such as the polymerization of hexamethylenediame (HMDA)

and adipic acid to form nylon 6/6 This type of reaction is called condensation

polymerization When liquid raw material constituents are used, it may be ble to directly convert them to parts, but this may be prevented if other factorsare involved

possi-Environment in Which the Reaction Must Take Place. In some tions, raw materials may react and polymerize readily when simply mixed to-gether, such as epoxies In others, unusual conditions of heat and pressure may

situa-be required to accomplish the polymerization For example, high pressures andtemperatures are required to produce polycarbonate If unusual conditions areneeded, then raw materials are converted in a separate, dedicated process thatproduces polymers in a basic form

Presence of Other Physical Agents to Achieve the Desired Results. Most

polymerization reactions require the presence of one or more catalysts A lyst is a compound or agent that promotes a reaction but that is not consumed

cata-by the reaction and is not normally an important constituent of the finishedpolymer Because catalysts are often expensive, it often makes sense to usethem within the confines of a specific manufacturing process so that they can

be controlled and used again

50 Stretch

Once the base polymer is prepared, it may be ready for commercial use

In many cases, however, further work is required to enhance the physical erties offered by the polymer and to make it more forgiving during processing.Some commercial products are produced by blending polymers together and inmany situations the base polmer is compounded with inorganic additives.Inorganic additives such as glass fibers, pigments, heat stabilizers, UVstabilizers, and flame retardants are commonly incorporated into basic polymerformulations to impart special behavior In most cases, these additives are com-pounded into the base polymer after the reaction and presented for processingafter a separate operation is performed

prop-2.1.2 Direct vs Indirect Conversion

The starting point for any process is thus defined by the nature and form of theraw materials available The scope of the process can be described to explainhow the transition from raw materials to finished product is made The term

direct conversion is applied to processes that start with raw materials and

pro-duce parts in one step Extrusion and injection molding are examples of thistype of process

Other processes require that intermediate steps be performed to the raw

materials on the path to finished parts Examples of indirect conversion are

thermoforming, which first requires the production of plastic sheet and injectionblow molding, which requires the production of a preform before finished partscan be produced Intermediate steps may be accomplished in-line as a definedportion of the process, or the raw materials may be the result of a separateprocess that was performed at a different location The mechanisms of howthese intermediates are produced will, of course, have an impact on processflexibility and economics

pow-way are often referred to as being melt processable One of the chief advantages

of thermoplastics is that they can be remelted and used more than once (withinlimitations) by the processor

Other materials are delivered in a form in which the polymerization tion has not taken place or is only partially completed In the case of polyure-thanes, the isocyanate component is essentially unreacted and the raw material