Handbook of Plastics Technologies Part 6 docx

Particles with diameters on the order of the wavelength of light can be made with the emulsion process, whereas larger particles with occluded matrix resin is very typical of mass.. As a

FIGUR

duced: a styrene-acrylonitrile (SAN) continuous phase and a discrete grafted polybutadi-ene one The rubber particles are finely dispersed in the rigid phase Thus, a situation is created in which copolymerizing acrylonitrile and styrene in the presence of PBD results

in an amorphous molding polymer that is much better suited to automotive applications than the polystyrene homopolymer Although the addition of the more polar monomer brings about improvements in modulus, equally important is a step improvement in resis-tance to impact damage Figure 9.12 shows transmission electron microscopy images of two distinct morphologies present in commercial ABS resins The darker domains are droplets of grafted polybutadiene that have been stained by osmium tetraoxide One can see that the rubber particles are very different in terms of average diameter and appear-ance Particles with diameters on the order of the wavelength of light can be made with the emulsion process, whereas larger particles with occluded matrix resin is very typical of mass In mass ABS, control of the particle size and particle density allows for a broad range of gloss

The key function of the dispersed, grafted PBD phase is to dissipate energy in the case

of an impact event There are two types of energy absorption found in ABS: crazing and shear banding In the case of crazing, rubber particles can dissipate energy by initiating and terminating this type of microcracking The initial step in the dissipation process is the deformation of the rubber particles to the point of void formation This void formation, in turn, initiates additional crazes that are terminated at neighboring rubber particles Crazes characteristically have high levels of surface area in the form of fibrils spanning the craze direction This is the dominant mechanism for lower AN containing ABS resins In higher

AN containing ABS copolymers, the dominant mechanism becomes shear, yielding evi-denced by the appearance of banding Submicron-size particles are reported to facilitate this energy dissipation mode

The composition of the ABS has a very large bearing on the final performance of the resin Figure 9.13 illustrates the relationship between composition and structure in ABS Polymer scientists carefully balance the proportions of these monomers when designing ABS resins for particular applications Table 9.8 shows properties typical of some com-mercially available classes of ABS These material properties are suitable for injection molding automotive parts

ABS copolymers can be used in a wide variety of applications ranging from plated ex-terior grills to molded-in-color glove box doors However, advances in vehicle aerody-namics have resulted in much more sunlight exposure for interior parts and an overall increase in maximum temperatures As a rubber modified amorphous resin, the glass

tran-sition temperature (T g) of the rigid phase and the phase volume of the dispersed elastomer have a very large influence on its modulus response to temperature There are numerous approaches to extending the upper service temperatures of ABS These include alloying with higher-heat polymers, reinforcing with fibrous fillers, and terpolymerization This group of chemically modified resins is commonly referred to as high-heat ABS (HHABS)

Terpolymerization is a popular method of raising the heat resistance of ABS, and these resins account for a large portion of the volume used in automotive applications That is,

an additional monomer is added to the polymerization These monomers can act in two ways when incorporated in the rigid phase: chain stiffening and/or modification of the

co-hesive energy density Table 9.9 lists the T g of some common styrenic polymers You can see that the influence of comonomer AN is a modest 0.3 to 0.4ºC/percent The three most

commonly used monomers that increase the T g of ABS are substituted styrenes, maleim-ides, and maleic anhydrides (MAs) The majority of HHABS uses α-methylstyrene (αMS)

or N-phenylmaleimide (NPMI) for enhancing the T g The efficiency with which these

monomers raise the T g depend on the chemistry employed The most common monomer added to HHABS is αMS Reaction kinetics change significantly with the introduction of

TABLE 9.8 Typical Properties of ABS Resins for Automotive Applications

Property

Test method (ASTM)

High-flow mass

General-purpose mass

High-impact mass

General-purpose emulsion

Coefficient of linear thermal expansion

(cm/cm, °C)

D-696 7.6E-05 9.3E-05 9.3E-05 8.8E-05 Notched Izod impact strength (23°C, J/m) D-256 160 310 553 203

Heat deflection temperature (unannealed,

°C)

this substituted styrene, requiring that the emulsion process be used for higher levels of monomer incorporation Maleic anhydride (MAH) is an excellent monomer for modifying styrenics for heat resistance; however, it can’t be used in the polymerization of ABS be-cause of its tendency to induce cross-linking of the rigid phase This can be avoided by blending a styrene-MAH copolymer with ABS in a compounding step These two poly-mers are highly compatible Finally, NPMI has recently found its way into HHABS in both the emulsion and mass process This monomer is especially reactive with styrene and compatible with multiple processes

9.2.2.7 PC/ABS Blends. Polymer blends have an important role in bringing property options for automotive applications that can’t be reached with a single material Unfortu-nately, due to unfavorable thermodynamics, only a few examples of miscible blends have found their way into commercial use in this industry In contrast, many compatible (or par-tially miscible) blends have reached commercial importance and can be found in key ap-plications Numerous examples are impact modified resins: ABS, TPO, PVC, IM-PC, and

so forth However, as a binary mixture of resins approaches equal volume fraction in the blend, the requirements for compatibility at the phase interfaces increases dramatically and excludes the majority of combinations One such blend having properties that make it unique and useful in automotive applications is PC/ABS The concept of blending PC and ABS dates back to 1964 with U.S patent no 3130177, Borg Warner’s grandfather patent

in the area Additional inventions have ensued: a patent was granted to Teijin in 1974 that covers impact strength and rubber location in PC/ABS/MBS blends (U.S patent no 3582394) U.S patent no 3880783 was issued in 1975 to Bayer, which demonstrates that polycarbonate and ABS (emulsion polymer) blends give high gloss and good impact U.S patent no 4098734 was issued in 1978 to Monsanto, showing that improved properties are obtained for PC/ABS alloys when bimodal rubber particles are employed Based on its versatility, this blend is considered a mainstay engineering material for applications such

as automotive instrument panels, body panels, and wheel covers

Although ABS and PC are extremely useful amorphous resins in their own right (see the ABS and PC sections), they have some limitations that can be solved by their blends Although PC has exceptional clarity, toughness, and heat resistance, it is notch sensitive and more difficult to process than ABS resins Similarly, ABS is a tough material, very processable, and adheres to paint and foams well Due to exceptional compatibility be-tween the phases, alloys of these two resins result in a resin that has a unique combination

of their properties PC/ABS blends are noted for having high heat properties, stiffness, and toughness with less notch sensitivity, improved processability, and versatile surface char-acteristics

TABLE 9.9 Properties of Some HHABS Resins, Medium and High Heat Range

MFR (g/10 min, 220°C,

10 kg)

Charpy impact (kJ/m2)

Vicat (°C) Medium

heat

MAGNUM™

3325MT

Dow Chemical Co Mass, low residuals 10 18 101

High heat MAGNUM™

3416SC

Ronfolin

HX-10

Since there are at least two major, immiscible phases in the PC/ABS blends, it is not surprising that careful attention has to be paid to developing an optimum morphology dur-ing processdur-ing Figure 9.14 shows the morphology developed in a commercial grade of PC/ABS Note that the darkened phase is ABS surrounded by PC Thus, scales as diverse

as molecules through to phase morphology, and part scale millimeters to meters, contrib-ute to useful parts Numerous studies have been done to understand the role that composi-tion of the neat polymers, phase volumes, compatibilizacomposi-tion, and processing play in the ultimate performance of this blend

The thermodynamics of polymer blends, although semiquantitative, play a key role in understanding phase morphology For the ABS composition, Callaghan et al.8 determined that an optimum AN level exists at about 25 percent This gives the highest level of phase adhesion and compatibility Morphology development at the micron scale must also be op-timized so as to get the most desirable toughness Figure 9.15 shows the response of im-pact resistance to the weight percent of PC in the blend Clearly, toughening is optimum near the composition at which the ABS phase first becomes co-continuous (or 65/35 per-cent by weight PC/ABS)

Stability of both the morphology and the components must be considered in this sys-tem Although it can undergo oxidative chain scission under very extreme temperatures, mass ABS is known to be robust during molding processes Polycarbonate, with its active carbonyls, is known to undergo hydrolysis and can be attacked by basic impurities result-ing in molecular weight losses Figure 9.16 shows the effect of time and temperature on the molecular weight of the PC portion of the blend Note that, if PC molecular weight falls below 20,000 amu, then embrittlement of that phase occurs The rate of degradation

is a function of some the common impurities and even moisture Depending on the level of these agents, degradation can be changed dramatically Plastics producers understand the

PC/ABS blend (phases light to dark: SAN, PC, polyb-utadiene)

need to start with PC and ABS with a minimum of impurities to have molecular weight control through compounding and processing into parts Impurities in ABS are known to

be strongly dictated by the polymerization process employed The process to manufacture emulsion ABS involves substances that are detrimental at trace levels (surfactants, flow aids, heat stabilizers, and so on) compared to mass ABS In addition, the mass process ex-poses the polymer to less heat history during its manufacture than emulsion Likewise, im-purities in polycarbonate or PC blends, particularly those capable of an alkaline reaction, can reduce its resistance to moisture Stability of the melt is especially significant as resins suppliers push to minimize viscosities during molding via lowering the feedstock molecu-lar weights High-flow PC/ABS resins must start with high-purity feedstocks to assure that molecular weight attrition doesn’t lead to brittle parts

Table 9.10 shows the properties typical of commercial PC/ABS resins Four perfor-mance types are listed: general-purpose, high-flow, blow molding and low-gloss grades Some important characteristics of PC/ABS can be learned from this table For example, these PC/ABS resins are all engineered with low-temperature ductility and a robust modu-lus The rheology is modified to meet the forming applications

9.2.2.8 LGF PP and ABS. Estimates are that about 30 percent of the 2 million Mton of e-glass fiber consumed globally for polymer reinforcement is used in thermoplastic com-posites Glass-filled thermoplastic composites have been growing at a very healthy pace of

15 to 20 percent per year, largely fueled by automotive applications Two key reasons that glass-reinforced thermoplastics are becoming so important are their recyclability and com-patibility with the injection-molding process One of the most revolutionary technologies

to come to the forefront is the long glass fiber-reinforced (LGF) resins Specifically, the use of long glass fiber reinforcement in polypropylenes has allowed the use of a lower-cost

composition

TABLE 9.10 Properties of Some Typical Automotive Grades of PC/ABS

Properties ISO test method Unit

General-purpose

Easy-processing

Blow molding

Low- gloss

Notched Izod impact

strength

(23°) C

(–30°) C

(–40°) C

180-4A; 1993 180-4A; 1993 180-4A; 1993 180-4A; 1993

kJ/m2 kJ/m2 kJ/m2 kJ/m2

— 50 36 24

— 46 34 25

— 53 25 20

— 52 32 24

DTUL (0.45 MPa),

unannealed

DTUL (1.8 MPa),

unannealed

Melt flow rate (MFR),

260°C 3.8 kg

polymer to be used in structural, engineered applications LGF-PP is especially interest-ing, since the glass fiber’s enhanced reinforcing abilities and the cost/performance of PP make it a very economical engineering material This material is quickly gaining in impor-tance, as evidenced by annual growth rates of >35 percent over the last four years

Long glass fiber PP derives its unique properties by artfully combining a low-density, semicrystalline PP resin with compatibilized e-glass fibers in such a way as to preserve the filler aspect ratio All references to fiber length will be for the molded part, not the starting fiber length It has long been understood that this is a key to maximizing the potential of glass fiber reinforcement of PP; however, the combination of material and transformation science has just now gelled to make this a popular commercial option Composites theory can be used to describe the effect of fillers on modulus as a function of aspect ratio.* (Note that this predicts modulus in the direction of fiber orientation.) These relationships can be seen in Fig 9.17 Another important property improvement seen in LGF-PP is impact re-sistance Figure 9.18 compares the falling dart impact (FDI) strength and tensile properties

of long and short glass-reinforced PP Clearly, an impressive level of toughening is achieved with long glass fiber reinforcement Material scientists attribute this drastic im-provement† in impact energy management to a mechanism whereby energy is dissipated due to slippage at the long glass/PP interface

FIGUR

Another important element to attaining optimum properties via LGF-PP is the addition

of a compatibilizing, grafted additive Because of the chemical mismatch at the interface of

PP and e-glass fibers, without additional treatments, there is the potential for very poor en-ergy transfer between fiber and matrix Numerous microscopic studies have verified poor adhesion of glass fibers to PP and motivated the development of grafted polypropylenes to improve the adhesion The addition of maleic anhydride grafted PP (MA-g-PP) has been shown to improve the adhesion between the two major phases Figure 9.19 compares the relative adhesion of a fracture surface with and without modification by MA-g-PP Smooth fibers indicate very poor adhesion and stress transfer, whereas a roughened glass surface is indicative of cohesive failure at the glass-PP interface Better interfacial bonding has been shown to be a strong contributor to modulus in these types of composites

LGF-PP properties are very much dependent on the conversion process used to incor-porate the fibers and form the parts Thus, representative properties should be reported for each of the major processes and glass levels Table 9.11 gives a summary of properties from both direct compression and pellet injection Three glass fiber levels were chosen:

20, 30, and 40 percent

PP can be modified with long glass fibers and formed into articles in a number of ways However, two very distinct methods are used to incorporate PP and long glass fibers: di-rect and pellet processes In the didi-rect process, glass roving is fed to a portion of the form-ing process, and the fiber filled melt is transferred to either an injection or compression molding process Alternatively, if existing injection molding capital must be used, then molders have the option of purchasing a material from the pellet process

In the direct process, glass roving is added to a stage of the process where shear can be carefully controlled and fiber lengths are maximized Two major direct processes are com-mercially practiced: injection and compression The direct injection process uses a cou-pled process whereby a compounding extruder is coucou-pled to an injection molding press Typically, the extruder is operated continuously and fills an accumulating tank that, in turn, feeds the molding machine In this case, the extruder can clearly be seen mounted on the top of the press Direct compression is also linked to the forming process, and glass modified, molten “buns” are transferred to a compression press

Pellet processes are practiced by suppliers who may also do more conventional mold-ing The original process, begun by Fiberfill Co in the 1950s, consisted of simply pulling glass rovings through wire coating dies This incomplete wetting of the glass fibrils

left had no compatibilizer, sample on right had MA-g-PP added