Thermoplastic Elastomers Based on Halogen-Containing Polyolefins

8 Thermoplastic Elastomers Based on Halogen-Containing Polyolefins 8.1 Introduction The presence of halogen atoms in elastomeric macromolecules lends them some very advantageous properties, such as chemical resistance, flame retardancy, and less swelling in hydrocarbon solvents and oils. Thermosetting elastomers containing halogens, such as polychloroprene, chlorinated polyethylene, fluoroelastomers, chlorobutyl rubber, and bromobutyl rubber, have been used commercially for decades. However, halogen-containing thermoplastic elastomers are relatively new, and some of them are currently still in development. The first halogen-containing polymer that exhibits rubber-like properties and still maintains thermoplasticity is plasticized poly(vinyl chloride) (PVC). Plasticizers used for this purpose are both low-molecular-weight liquids and high-molecularweight solids [1e5]. However, true thermoplastic elastomers are blends of PVC with cross-linked or elastomeric polymers. The first such material is a blend of PVC with acrylonitrile-butadiene rubber (ASTM designation NBR), prepared mainly as proprietary blends by flexible PVC fabricators. PVC is also compatible with other elastomers, such as copolyester elastomers (COPEs), and with TPU, and such blends are thermoplastic elastomers with unique properties. Another halogen-containing TPE is ALCRYNÒ, which was developed by the DuPont Company. It is a melt-processable rubber (MPR) introduced in the late 1980s as a fully compounded pelletized product for direct fabrication. It can be processed with equipment typically used to process thermoplastics, such as injection molding machines, extruders, calenders, and so on.

8.2 Blends of PVC with Nitrile Rubber Before 1970, PVC was added to copolymers of acrylonitrile and 1,3-butadiene (nitrile rubber (NBR))

to improve ozone and solvent resistance of its vulcanizates. Later, NBR grades, already available in the bale form, were developed and commercialized in powder form to be used specifically in the PVC processing industry. By the mid-1980s, more grades, including food grades and grades with improved processing, had been made available. Thermoplastic blends of PVC and NBR are produced when PVC is the predominant polymer. Typically, they also contain liquid PVC plasticizers for PVC, such as phthalates or adipates, fillers, stabilizers, etc. These blends bridge the gap between conventional liquid plasticized PVC (PVC pastes) and conventional NBR-cured rubber. Properly formulated PVC-nitrile rubber blends are rubber-like in appearance and feel. They are flexible at low temperatures; have a good tear strength, low compression set, and good abrasion resistance; and exhibit minimum swelling or extraction when immersed in oils or solvents [6]. The important compounding variables affecting the processing behavior and final properties are:  PVC/NBR ratio  Acrylonitrile (ACN) content in the NBR  Mooney viscosity of the NBR  Molecular weight of the PVC  Type and amount of liquid plasticizers  Type of stabilizer(s) used  Type and amount of filler(s) added In general, NBR elastomer used for such blends should have 30e40% of ACN to obtain single-phase homogeneous blends, with the optimal content being 40% [7]. The single-phase material exhibits a single value of glass transition temperature (Tg) as measured by differential thermal analyses [8] intermediate between the Tg values of the two polymers. The absence of two phases was established by electron photomicroscopy [9]. Some studies reported two

Handbook of Thermoplastic Elastomers. http://dx.doi.org/10.1016/B978-0-323-22136-8.00008-9 Copyright Ó 2014 Elsevier Inc. All rights reserved.

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Tg values [10] and micro-domains rich in one or the other of the components [11], which indicates that the morphological behavior of these blends is rather complex. The NBR used may be either without crosslinks or containing various amounts of cross-links. The blends with NBR, which does not contain crosslinks, have low viscosity, thus requiring relatively low energy to process. This provides a wider processing margin before thermal degradation of PVC occurs. The lower viscosity blends are suitable for injection molding. The blends with partially crosslinked NBR provide the lowest die swell, making them suitable particularly for extrusion or calendering. Products containing an increased cross-link density exhibit a lower compression set.

8.2.1 Melt Compounding and Processing As pointed out earlier, NBR used for blends with PVC is in the form of a free-flowing powder to be suitable for the handling and mixing common in the PVC technology. These powders consist of particles with an average size of about 0.5 mm (0.02 in) and contain about 10% of a partitioning agent, which may be PVC, calcium carbonate, or silica [12]. The NBR powder is usually added to the dry blending cycle after the PVC has absorbed all of the liquid plasticizers (“dry point”). The temperature of the PVC blend to which the NBR powder is added should not be higher than 40  C (104  F) to avoid rubber agglomeration [13]. The mixing can be done in conventional mixing equipment for thermoplastics, such as low-intensity or high-intensity mixers, singlescrew and twin-screw extruders, continuous mixers, and kneaders. The finished blends can be further melt compounded into pellets before processing or used directly to produce the finished products. In recent years, fully compounded commercial PVC-NBR blends have been marketed as alternatives to flexible vinyl, mid-performance elastomers and to “in-house” prepared PVC/NBR blends.

8.2.2 Physical and Mechanical Properties Physical properties of the PVC-NBR blends are similar to those of mid-performance thermoset rubber materials. The stressestrain curves for three different compounds [14] are shown in Fig. 8.1. It is obvious that with increasing cross-linking of NBR and viscosity, the stiffness of the compound increases

Figure 8.1 Stressestrain curves for PVC/NBR rubber compounds (see Table 8.1).

slightly and the elongation decreases. Other physical and mechanical properties are in Table 8.1. It can be seen that the compression set at low and high temperatures is slightly improved with increasing cross-linking. Other properties are not affected significantly. Physical properties of a typical PVC/NBR blend and two typical thermoset rubber materials were compared at temperatures up to 121  C (250  F). At room temperature, the tensile strength of the blend was comparable to that of thermoset rubber but was considerably lower at higher temperatures [9]. The relatively high tensile strength of the blend, containing slightly cross-linked NBR, can be attributed to the cross-links in the elastomer combined with the strong hydrogen bonding between PVC and the NBR. As the temperature increases, the hydrogen bonding becomes gradually weaker and eventually the tensile strength depends only on the partially cross-linked NBR. Thus, the values cannot match those of a fully cross-linked elastomer. Liquid low-molecular-weight plasticizers are used to improve the flexibility of PVC/NBR blends at low temperatures. Adipate and triglycol ester-type plasticizers are more effective than phthalate ester types in maintaining both their low-temperature flexibility and resistance to swelling in certain liquids, such as ASTM Reference Fuel B.

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Table 8.1 Flexible PVC Compounds Based on Powdered Nitrile Rubber Grade

5123P

5125P

5128P

Acrylonitrile content, %

33

33

33

Mooney viscosity, ML-4

30

55

80

Precross-linking

None

Medium

High

Instantaneous

68

68

69

15-s delay

64

64

63

H-18 wheel, 1000 g load, 2000 rev, g loss

0.657

0.644

0.713

Tear strength, ASTM D 624, die C, kN/m

38.5

42

40.3

NBR rubber properties

Shore A hardness

Taber abrasion resistance

Brittle point, ASTM D746 ( C)

36

37

36

Compression set, ASTM D 395 B (22 h, 25% compression) 23  C (%)

23

20

19



71

70

65

100 C (%) Ref. [14].

8.2.3 Other Properties PVC/NBR blends exhibit a fair resistance to swelling in ASTM Reference Fuel B. The degree of swelling is reduced slightly as the degree of crosslinking of the NBR component increases. The compounds containing dioctyl phthalate (DOP) harden excessively during swelling in ASTM No. 1 Oil and exhibit low volume swelling at the same time. This is attributed to the extraction of the DOP during the immersion [14]. In such a case, the use of a polymeric plasticizer that resists extraction by the swelling media alleviates the problem, although at a somewhat higher formulation cost. Because the nitrile rubber contains double bonds, it is susceptible to oxidation. To prevent that, antioxidants must be added to the compound if a longterm resistance to aging is required. A compound without an antioxidant becomes brittle and shows a considerable loss of elongation and considerable hardening when aged in an oven at 113  C (235  F) for 1 week [15]. The addition of an antioxidant brings about less reduction in elongation, but the compound will still harden when it contains a liquid plasticizer(s). The hardening is the result of the loss of the plasticizer due to its volatility. The use of

a nonvolatile polymeric plasticizer will minimize the hardening as pointed out in the previous paragraph. The PVC/NBR compounds are not sufficiently resistant to UV radiation, so if they are intended for a long-term outdoor exposure, an addition of an adequate UV-protection package is required. This will contain pigmentation (titanium dioxide, zinc oxide, or carbon black) or UV absorbers or a combination of both.

8.3 Blends of PVC with Other Elastomers 8.3.1 Blends of PVC with COPEs COPEs, random block copolymers consisting of crystallizable tetramethylene terephthalate (4 GT) hard segments and amorphous elastomeric polytetramethylene ether glycol (PTMEG-T) soft segments, as described in Chapter 12, are compatible with plasticized PVC. Their blends yield materials that combine the elastomeric properties of the COPEs with the excellent processing characteristics of plasticized polyvinyl chloride [16]. The COPEs used for these blends are usually low-melting grades,

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which typically contain about 33% 4GT and melt below 180  C (356  F). The addition of COPEs to plasticized PVC formulation improves its low-temperature flexibility and impact resistance, abrasion and tear resistance, and resistance to oils and fuels [17]. As little as 25% of COPE improves these properties but also increases hardness and torsion modulus at room temperature [18]. A compound containing equal amounts of PVC and COPE exhibits properties that are significantly better than those typical for conventionally plasticized PVC. The blend has a superior low-temperature flexibility, heat aging resistance, abrasion resistance, and greater elongation at break both before and after oil immersion. Additionally, it exhibits good electrical properties (dielectric strength, volume resistivity), good resistance to water, chemicals, and cut growth. Because of that, it is used in wire and cable applications, such as cable jacket compound [17]. Compounds for outdoor applications have to be protected against effects of ultraviolet (UV) radiation. A typical protective package contains a UV absorber (e.g., a benzotriazole), a light stabilizer (e.g., a hindered amine), a hindered phenol antioxidant at 0.1e0.2 phr (parts per hundred parts of rubber), and a small amount of rutile TiO2. If color is not an issue, a small amount of carbon black (typically 2.5 phr) provides an effective UV screen [17]. The PVC component of the blend is prepared by mixing suspension grade PVC with plasticizer(s), stabilizer(s), etc. in a standard fashion using a highintensity mixer or a heated ribbon blender. The COPE can be mixed into the plasticized PVC powder blend before the melt compounding, or the PVC and COPE can be metered independent of the melt compounding equipment, such as a Banbury mixer, kneader, or single-screw or twin-screw mixing extruder. Maximum melt temperature should be less than 190  C (374  F) to prevent degradation of PVC. The product is dried to less than 0.10% moisture and packaged in moisture-barrier packaging [19]. PVC/COPE blends are processed with standard equipment used for thermoplastics, such as injection molding presses and extruders. Because COPEs tend to degrade at processing temperatures, it is absolutely necessary that the compound be dry. The melt temperature must not exceed 190  C (374  F) and ideally should be kept in the 160e170  C (320e338  F) range, and the heat history has to be

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kept at the minimum to prevent degradation of the PVC component [19].

8.3.2 Blends of PVC with Thermoplastic Polyurethane Elastomers Thermoplastic polyurethane elastomers (TPUs) (see Chapter 9 for a detailed description) are multiblock copolymers consisting of sequences of amorphous or low melting soft segments and rigid, hard segments, which have a crystalline melting point above room temperature. Many TPUs are compatible with PVC, and their blends exhibit only one major glass transition whose position on the temperature scale is raised with increasing levels of PVC [20]. The melt compounding and subsequent processing of the PVC/TPU blends are very similar to those of the PVC/COPE blends discussed in the previous section. Because of the heat sensitivity of PVC, only the softest TPU grades (i.e., those with hardness values of Shore A of 80) can be melt mixed with it safely. The PVC/TPU blends are also sensitive to moisture during processing, so it is necessary to dry them to less than 0.03% moisture content to maintain optimum properties [21]. For protection of the PVC/TPU compounds against UV radiation during outdoor exposure, either benzotriazole UV absorber for neutral or color compounds at amounts up to 2% loading or carbon black up to 5% loading for black formulations is recommended. Antioxidants, such as hindered phenols or organosulfur types, will extend the life of the products in outdoor exposures [21]. Blends of PVC with TPUs combine the toughness of the TPU with the stiffness and high modulus of the PVC. It is possible to obtain a wide range of hardness values by blending PVC with different hardness grades of TPU and added varied amounts of plasticizers to the PVC resin. The blend of PVC/TPU in the ratio of 70:30 by weight is equivalent to a commercial plasticized PVC compound in all respects yet displays a higher abrasion resistance and low-temperature flexibility. The oil resistance of PVC/TPU blends is also improved over the plasticized PVC compounds. The immersion of such material in ASTM No. 3 Oil for 7 days at ambient temperature has a negligible effect on the volume swell and causes no decrease in tear strength. The flexural performance improves with the increasing content of TPU: a compound containing

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30% of TPU is markedly better than a commercial plasticized PVC material of the same hardness in both flex life and the cut growth resistance after oil immersion, and the trend continues with increasing proportion of TPU [21]. There appears to be an optimum TPU content of 40% for oil resistance and an optimum of 50% for the other properties [14].

8.4 Melt-Processable Rubber This family of materials is described as “alloys of proprietary ethylene interpolymers and chlorinated polymers, in which the ethylene component has been partially cross-linked in-situ” [22]. They are composed of a blend of molecularly miscible polymers having a single Tg (see Fig. 8.2). These polymers can be used for compounds containing usually different additives, such as carbon black, clay, plasticizers, and stabilizers, which give them desired processing characteristics and end-use properties.

8.4.1 Physical and Mechanical Properties Unlike two-phase materials, such as most other TPEs (e.g., SeBeS and dynamically vulcanized TPVs), these are single-phase, primarily amorphous polymeric systems, soft and flexible with a good

Figure 8.2 Glass transition of a 70 Shore A MPR from Rheovibron data.

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recovery. Their stressestrain curves are essentially identical to those of typical cross-linked thermoset elastomeric systems (see Fig. 8.3). Because of these inherent factors of rubbery character, they are defined as melt-processable rubber (MPR), in contrast to the two-phase systems, which are referred to as thermoplastic elastomers (TPUs) and thermoplastic vulcanizates (TPVs). When comparing MPR to TPV, vulcanized NBR, and vulcanized poly(chloroprene) (ASTM designation CR) at hardness approximately 70A durometer, it can be clearly seen that MPR has the initial slope of the stressestrain curve almost identical to those of NBR and CR. Because the initial slope is the measure of stiffness of the material, the much higher slope of the TPV indicates that the material is much stiffer than the others at the same durometer hardness. The tensile stress of TPV at 25% elongation is about three times higher than that of the other materials at the same point. In addition to being stiffer, TPV displays a plastic behavior by yielding at about 35% elongation, while MPR, NBR, and CR remain elastic well beyond 100% elongation. Hysteresis curves comparing MPR with TPV with equal hardness (Fig. 8.4) also indicate that MPR is more resilient, having much lower hysteresis than TPV. Thus, based on above observations and field experience, MPR is considered to be a true rubber [23].

Figure 8.3 Stressestrain curves of different 70 Shore A hardness materials.

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thermoset rubber materials (ranging between 0.4 and over 1.0) (see Table 8.4). This is almost twice that of similar TPVs and styrenic TPEs [25].

8.4.2 Chemical Resistance MPR has broad fluid resistance for a thermoplastic elastomer. In general, the harder grades swell slightly less than the softer ones, but the differences are small. MPR exhibits a high resistance to oil, higher than vulcanized polychloroprene (CR) or chlorosulfonated polyethylene (CSM) and comparable to NBR with medium content (33%) of acrylonitrile. It resists petroleum-based oils, greases, fuels, and solvents more than TPV; it exhibits less than half of the volume swelling after a 7-day immersion in hydrocarbon oil at 100  C. Under these conditions, SBC would practically dissolve. MPR is also resistant to most mineral acids, bases, ethylene glycol, and other automotive fluids (except proprietary hydraulic fluid SKYDROL 500), to silicone grease (unlike TPV), to lithium grease, to insecticides and agricultural sprays, to mineral and vegetable oils, and to fuels (gasoline, diesel, and kerosene). It holds up in ASTM Reference Fuels A and B, has fair resistance to unleaded gasoline and gasohol, but is moderately to severely attacked by ASTM Reference Fuels C and D. MPR resists alcohols, amines, and paraffinic hydrocarbons, but it has poor resistance to aromatic hydrocarbons, and is severely attacked by ketones, esters, and chlorinated solvents [26].

Figure 8.4 Hysteresis curves: comparison of MPR and TPV.

MPR is also compared with other polymeric materials in Table 8.2. Based on stiffness (initial slope of the stressestrain curve), yield strain (the percent elongation at which the material exhibits plastic yielding), and ultimate tensile strength, MPR is clearly the most rubber-like material of this group. Like other thermoplastic elastomers, MPR is inferior to thermoset rubber materials in high temperature compression set and creep (see Table 8.3). As for the estimated service temperature in terms of retention of mechanical properties and heat aging without embrittlement, MPR has a somewhat lower performance, compared with TPV and copolyester [24], with 120  C versus 135  C and 150  C for TPV and copolyester, respectively. The coefficient of friction of MPR is another attractive property of MPR. For example, a compound with a hardness of 70 Shore A has a friction coefficient well over 1.0, comparable to that of most

8.4.3 Weather and Flame Resistance MPR has a saturated polymeric backbone and therefore is not attacked by ozone and resists the

Table 8.2 Tensile Properties of MPR Compared with Other Materials

a

Material

Stiffness,a MPa

Yield Strain,b %

Tensile Strength,c MPa

CR vulcanizate, 70 Shore A

10

>100

10

MPR, 70 Shore A

10

>100

10

TPV, 70 Shore A

40

35

12

Copolyester, 55 Shore D

200

10

35

Nylon

1200

3

80

Initial slope of the stressestrain curve. b Elongation (%) at which the material exhibits plastic yielding. c Stress at break. From: Holden G, Legge NR, Quirk RP, Schroeder HE, editors. Thermoplastic elastomers, 2nd ed. Munich: Hanser Publishers; 1996.

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Table 8.3 Compression Set and Creep of MPR Compared with Other Elastomers Elastomer

Compression Set,a %

Creep in Compression,b %

Creep in Tension,c %

Stress Relaxation,d %

MPR 1

78

31

e

21

MPR2

e

e

40

e

EPDM rubber

79

18

e

25

COPE

91

9

e

19

S-EB-S

e

e

90

e

NBR

46

18

30

15

TPV

68

50

170

30

TPU

100

26

e

30



a

25% compression, constant strain, 1000 h at 212 F. 25% compression, constant load, 1000 h at RT. c 10% elongation, constant load, 1000 h at RT. d 15% compression, 1000 h at RT. Ref. [23]. b

Table 8.4 Coefficient of Friction of Different Elastomeric Materialsa Dry

Wet

Material

Steel

Glass

LuciteÒ

Steel

Glass

LuciteÒ

EPDM rubber

3.1

3.5

3.9

2.2

1.0

2.2

CR (polychloroprene)

2.2

1.4

3.8

1.3

1.3

1.0

MPR, 60 Shore A

2.6

2.6

1.8

1.4

0.4

1.5

MPR 70 Shore A

2.3

2.8

2.7

1.2

0.5

1.5

TPV 70 Shore A

0.9

0.8

1.2

0.7

0.6

0.8

0.9

1.3

2.2

0.8

0.9

1.0

Ò

S-EB-S (Kraton G) a

ASTM D 1894. AlcrynÒ, Tech notes, COF (2/98), Advanced Polymer Alloys, Wilmington (DE).

effects of sunlight, UV radiation, and weather much better than other TPEs (SBC, TPV, TPU, and COPE), as well as many thermoset rubber materials. Black grades of MPR remain unchanged over long-term outdoor exposure without any antidegradants. However, light-color grades need the addition of light stabilizers, such as hindered amine light stabilizers (HALS) and benzotriazole, if an enhanced protection against the effects of weather is required. Because most grades contain 9e20% by weight of chlorine, they are flame resistant even as sold, several of them even having the UL-94 HB (horizontal burning) rating [27]. For the higher flame resistance rating, UL-94 V-0 (vertical burning), a flame retardant such as antimony trioxide must be added.

8.4.4 Electrical Properties MPR grades are suitable for low voltage applications (600 V) only [28], but their major benefit is the ability to dissipate static charge as sold. Some comparable data are provided in Table 8.5 [29]. This feature can be further enhanced by the addition of conductive fillers to obtain semiconductive materials.

8.4.5 Grades of MPR Because of its many true properties of rubber, MPR is unique in that it can be processed with both plastic and certain modified rubber equipment. Since its market introduction in 1986, a multitude of products with specific processing behavior, physical and mechanical properties (mainly hardness), and

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Table 8.5 Antistatic Properties of Different Materialsa

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adhesion properties are designed for overmolding applications. They will bond to rigid plastics such as polycarbonate, ABS, SAN and ASA, polycarbonate/ ABS alloys, COPE, TPU, and PVC.

Material

Static Level, kV

Rigid PVC

10.0

Typical commercial carpet

3.5

Flexible PVC (Shore A 70)

2.8

Carpet for computer rooms

2.0

MPR (AlcrynÒ 3065NC)

1.0

AlcrynÒ 2070NC/ flexible PVC (50/50)

1.0

 COPE improves compression set, tear resistance, and low temperature properties.

AlcrynÒ 2070NC

0.7

 TPU improves toughness, abrasion, and oil resistance.

a Test Method 134-1986 (Association of Textile Chemists and Colorists). AlcrynÒ, Tech notes, Static (10/97), Advanced Polymer Alloys, Wilmington (DE).

colors have been available. A complete listing is in Ref. [23] and Appendix 5. Extrusion/calendering grades produce relatively viscous melts as shown in Fig. 8.5 and are well suited for extrusion, extrusion blow molding, and calendering. Because all grades of MPR are compatible with chlorinated plastics, they form a completely fused extrudate when coextruded with rigid or semirigid PVC. In these hard/soft composite profiles, MPR provides the flexible rubbery seal on a rigid PVC support. Injection molding grades have enhanced melt flow compared with the extrusion/calendering grades, low mold shrinkage, and significantly superior dimensional stability. Certain grades with enhanced

8.4.6 Blends with Other Polymers MPR is compatible in all proportions with PVC and certain types of COPEs and TPUs. Each of these polymers was found to add a specific benefit [30]:  PVC lowers cost and improves tear resistance.

Mixtures of these components yield property combinations not available in any single component of the mixture, such as the property enhancement of flexible PVC by the addition of MPR and COPE. One improvement is a greater resistance of plasticizer extraction by dry cleaning solvent (perchloroethylene); another one is an improvement of the resistance to stiffening animal and vegetable fats, which often occurs in food processing applications. These improvements are achieved without sacrificing the good resistance to detergents, which is also required in such environments. Flexural resistance, the key mechanical property, is improved by a factor of 5. Various acrylic processing aids, used alone or in combination with each other, increase the melt flow of high-viscosity MPRs, in some cases up to 100-fold. Some of them do not significantly change the key physical properties, while others increase hardness and stiffness along with the melt flow [31].

8.4.7 Processing

Figure 8.5 Viscosity at 190  C as a function of shear rate for three extrusion/calendering grades of MPR.

The rheology of MPR differs from that of conventional thermoplastic materials because of its partially cross-linked structure. Because MPR is essentially amorphous, there is no significant drop in viscosity to the hard-segment glass transition temperature or crystalline melt point. The melt flow can be only induced by application of a shearing force in combination with elevated temperature. The viscosity is very high and is more sensitive to shear rate than to temperature. Pseudoplastic flow (shear thinning) (see Fig. 8.6) is the main mechanism by which different shapes are produced from MPR. The shear thinning effect is clearly evident on the

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Figure 8.6 Flow curves for Newtonian and pseudoplastic materials.

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short overall molding cycles and a very high productivity. MPR can be processed on reciprocating screw machines. General purpose, gradual transition screws with compression ratios 2.5:1 to 3.5:1 and with L/D ratios >20:1 are usually suitable for molding MPR [32]. Flow passages through the machine have to be carefully streamlined to eliminate melt stagnation and subsequent degradation. MPR degrades if overheated to temperatures 400  F (204  C) or higher or if held at processing temperatures for longer than 30 min. The onset of degradation is around 385  F (196  C), and during the degradation, gaseous products, including HCl, evolve. Therefore, the use of corrosion-resistant materials for the screw, the barrel liners, and nickel-plated steel for molds is recommended to maximize equipment life. The rheology of MPR makes it suited to the high shear of injection molding. A combination of barrel heat and shear is necessary to attain a properly fluxed, uniform melt. The melt temperature measured at the nozzle should be maintained between 340 and 375  F (171 and 191  C). Typical temperature settings are given in Table 8.6, and operating conditions are given in Table 8.7 [32]. If the machine is shut down for longer than 1 h and the temperature is above 350  F (177  C), purging with low-viscosity, low-density polyethylene is recommended.

8.4.7.2 Extrusion Figure 8.7 Viscosity vs shear rate at 171  C (340  F).

viscosities of four different grades of MPR at 340  F (171  C) (see Fig. 8.7).

8.4.7.1 Injection Molding During injection, high shear must be maintained to keep the melt viscosity low through the use of rapid first stage injection rates (1e3 in3/s) and smalldiameter nozzles, runner system, and gates. Such rapid fills (0.5e2 s) require generous mold venting to avoid burning and facilitate complete mold fill. MPR does not exhibit discontinuous volume change due to crystallization upon cooling. Molded parts rapidly develop strength in the mold and therefore can be demolded while relatively hot. The combination of rapid injection and demolding hot means

As pointed out in the previous section, the polymer is brought to the melt stage more by shear than by heat. Heat generated by the shearing of the melt has to be removed to avoid degradation of the material. Therefore, it is necessary to provide an efficient cooling to avoid overheating the melt. On the other hand, the temperature profile has to be designed so as to give maximum pressure at the die and ensure an equal flow rate across all sections. Table 8.6 Temperature Settings for Injection Molding of MPR (Reciprocating Screw Injection Molding Machine) Barrel,  F ( C) 



Nozzle, F ( C) 



Mold, F ( C) Ref. [32].

Rear

340e350 (171e177)

Front

340e350 (171e177) 340e350 (171e177) 70e120 (21e49)

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Table 8.7 Operating Conditions for Injection Molding of MPR (Molding Stages), Reciprocating Screw Machine Injection speed (fill rate)

1e3 in3/s (16e49 ml/s)

Injection pressure

700e1200 psi (4.83e8.27 MPa)

Injection time (first-stage boost)

0.5e2 s

Second-stage pressure

300e800 psi (2.07e5.52 MPa)

Second-stage time

3e10 s

Cooling time

2e20 s

Screw speed

50e100 rpm

Back pressure

30e80 psi (0.2e0.6 MPa)

Shot size

Control to fill mold

Ref. [32].

MPR can be extruded on extruders commonly used for the processing of PVC or polyolefins, with typical L/D ratios between 20:1 and 24:1 (with 24:1 preferred) and compression ratios between 2.5:1 and 3.5:1. For most extrusions, a simple three-zone screw, having a transition (compression) zone at least one third of the screw length, is recommended [33]. As in extrusion, machine parts coming in contact with the melt should be corrosion resistant. Efficient cooling to remove the shear heat is essential for high productivity and to prevent degradation. Some general extruder configurations with examples of typical temperature guidelines are given in Table 8.8 [33]. Overhead and tangential-type feed throats usually used with single-screw extruders can be used with MPR. Water cooling of the throat is recommended to

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prevent excessive heating of the resin as it is entering the screw and to serve as protection of the drive bearings. Hopper drying is not usually necessary for MPR [33]. Melt temperature depends on the grade used and should be 180  10  C (356  18  F) for general-purpose grades and 185  5  C (365  9  F) for injection molding grades [33].

8.4.7.3 Extrusion Blow Molding Having sufficiently high melt strength, MPR can be fabricated into hollow articles by extrusion blow molding. Continuous extrusion, accumulator head, and reciprocating screw systems can be used with proper residence times and temperature control [34]. For the extrusion of MPR, continuous extrusion systems are preferred; second choice is the accumulator head system; and third choice is the ram-accumulator system. Essentially, the same guidelines, such as basic screw design, avoiding polymer degradation, and the use of corrosion resistant machine parts that were discussed earlier, apply here. The melt temperature measured at the parison die or at the exit to the accumulator head should be in the range 160e185  C (320e365  F). Barrel temperatures are set so that the feed zone is set to a higher temperature than the extruder head. Moderate pressures (i.e., between 200 and 700 kPa gauge (30e100 psig)) are typical and depend on the part size and parison wall thickness [34].

8.4.7.4 Calendering Unsupported sheets and coated substrates such as fabrics can be produced on three- or four-roll calenders commonly used for calendering plastics. The calender roll temperatures for calendering MPR

Table 8.8 Temperature Profile for the Extrusion of General-Purpose MPR Grades Length/ Diameter Long

Compression Ratio High

Type of Profile Increasing Flat

Short

Low

Reverse

Feed 

Transition/ Meter 

Adapter/Die

300 F

320e340 F

325  F

(150  C)

(160e171  C)

(163  C)

325  F

325  F

325  F

(163  C)

(163  C)

(163  C)

350  F

340e320  F

325  F

(177  C)

(171e160  C)

(163  C)

Note: Temperature should be increased 18e27  F (10e15  C) for extrusion injection molding grades. Ref. [33].

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need to be controlled in the range between 160 and 185  C (320e365  F). The stock is usually fluxed in batch or continuous mixers or a hot roll mill and then fed into the calender as a metered strip or a “pig” from a roll mill. It must be well fluxed and maintained at temperatures between 165 and 185  C (320e365  F). The top rolls of the calender should be set to the same temperature in the range 160e185  C, and the temperature of the lower rolls should be high enough to maintain tracking but low enough to prevent sticking to the rolls (as low as 140  C or 285  F). Embossing is easily accomplished by using an embossing roll while the stock is still warm. Uneven speed between adjacent rolls prevents the formation of blisters in the calendered sheet. A small uniform bank must be maintained at the roll nip. A small, pencil-thin bank between the second and third rolls will prevent blisters or blemishes in the sheet. The usual speeds between adjacent rolls are frequently uneven with a speed ratio of 1.05:1 to prevent blisters. In most cases, the MPR stock has enough internal lubrication to be released from the calender rolls. Only under extreme conditions, such as very thin gauges, high calendering speeds, and high roll temperatures, may it be necessary to add internal lubricant (such as oxidized PE waxes) to avoid sticking of the stock to the rolls. The internal lubricants have to be incorporated during the pellet fluxing process [35].

8.4.7.5 Compression Molding Compression molding is a less efficient way to produce molding parts but is useful for preparing test plaques, prototypes, and unusual parts. The singlepress method is rather cumbersome because it requires that the mold with the part requires cooling down to about 120  F (50  C) before the part can be removed from the mold. Two-press method (one heated, one cold) is more efficient, because the hot mold is transferred into the second press for cooling down to the demolding temperature [36]. The compression mold procedure consists of the following steps: 1. Heating the press and the empty mold to 350  F (177  C). 2. Loading the mold with a sufficient amount of milled preform to ensure complete fill and some flash (excess stock).

229

3. Dropping platen until it touches the preform and heating the filled mold for 1e2 min or 5 min for thicker parts. 4. Closing the press and bringing it the full pressure and holding for at least 1 min; then cooling at full pressure to about 120  F (50  C) before releasing pressure. 5. Removing from press and demolding parts. Note: With two presses, the mold is moved from the hot to the cold press, and the pressure is raised quickly to maximum and then held until cooled to 120  F (50  C). Then the part is demolded (see Step 5).

8.4.7.6 Bonding and Welding Although MPR can be joined to itself or other materials by friction or snap fit or mechanical fasteners, other methods of assembly may give better joints often at a lower cost. Various adhesive systems provide satisfactory bonds to thermoset rubber materials, metals, plastics, textiles, leather, wood, etc. The disadvantages of adhesive bonding include handling hazards and difficulties, material costs, environmental considerations (the use of solvents or toxic substances), and processing speed. Direct overmolding is used where possible, because it is rather simple and fast and provides very good bonds to a very large number of substrates. It is feasible only if the part is made in large series because a mold, often very costly, is required. In many other instances, welding is used as it is simple, fast, and reliable and in most cases gives excellent bonds. The welding methods applicable to MPR include ultrasonic welding, external heating by hot plate or gas, radiofrequency, and electromagnetic induction [37].

8.4.7.6.1 Bonding MPR to Itself Bonding at room temperature requires an adhesive. There are two classes of adhesives; one includes adhesives commonly used for rigid and flexible PVC (e.g., Hercules Plastic Pipe Cement or Waxman PLUMBCRAFT Cement). The second class includes urethane adhesives, such as Lord 7540 A/B (Lord Corporation, Erie, PA). This system is a two-part adhesive and requires that the joints be first coated by a primer (e.g., ChemlockÒ 480). Adhesive bonding at elevated temperatures is used mainly for laminations. For that purpose, specialty adhesives such as a combination of ChemlockÒ 480

230 with ChemlockÒ Curative 44 (Lord Corporation, Erie, PA) are used. Best results are obtained at laminating temperatures above 200  F (93  C) and pressures at least 100 psi (690 kPa) with dwell time minimum 10 min. Melt bonding techniques are very common in bonding MPR to itself because of its thermoplastic nature. Butt welding on hot knife is done mainly for extruded shapes at temperatures in the range from 500 to 600  F (260e315  C). Heating the surfaces to be joined with a hot gun is another method to obtain good bonds but requires precise technique. Ultrasonic welding has been successful in sections up to 40 mils (1 mm) thick. Dielectric (RF) welding is accomplished at a frequency of 27.17 MHz [36].

8.4.7.6.2 Bonding MPR to Other Materials Bonding to thermoset rubber is accomplished in a similar way as bonding to itself, namely by using, for example, the combination of ChemlockÒ 480 primer and LordÒ 7540 adhesive for room temperature bonding and ChemlockÒ 480 and ChemlockÒ Curative 44, for example, for bonding at temperatures above 200  F (93  C) as described in the previous section. Bonding to metals requires that the metal surface be prepared by making it rough (blasting with sand, grit, etc. or by chemical etching). In the next step, primer and bonding adhesive are applied to the surface to the metal. The bonding can be accomplished at room temperature by a proper adhesive system (see previous section) or by using the pretreated metal as an insert, placing it into a mold and injecting MPR (melt temperature 340e360  F, or 170e180  C) to fabricate the part. Bonding of MPR to other materials, such as different plastics, textile, wood and leather, requires specific adhesive systems and the applicable procedures are covered in detail in Ref. [36].

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Figure 8.8 Schematic drawing of the structure of a thermoplastic fluorocarbon elastomer.

grades on the market that claimed good transparency, low extractables, and an excellent chemical resistance [38]. Typical properties are listed in Table 8.9. This product was produced and marketed by Daikin Industries under the trade name DAI-EL Thermoplastic until 2013, at which time it was discontinued. Table 8.9 Typical Properties of Thermoplastic Fluorocarbon Elastomer Property Specific gravity

Value 1.88

Hardness, JIS A 

67e73 

Melting point, C ( F)

220 (428)

Pyrolysis initiation temperature,  C ( F)

380 (716)

Tensile strength, MPa (psi)

17 (2470)

Elongation at break, %

600

Tear strength, kN/m (pli)

29 (154)

Rebound resilience, %

10

Coefficient of friction

0.6

Taber abrasion (Wheel CS-17, 1000 g), mg/1000 cycles

2

Low temperature torsion test, Gehman T50,  C

8.5 Thermoplastic Fluorocarbon Elastomer Originally, there was one commercial product, which was essentially a thermoplastic fluorocarbon elastomer. Its structure was that of a block copolymer with hard segment consisting of a fluororesin and a soft segment formed by fluoroelastomers (see Fig. 8.8). At that time, there were two

9

Compression set (24 h at 50  C (122  F)), %

11

Volume resistivity, ohm-cm

5  1013

Dielectric breakdown strength (kV/mm)

14

Dielectric constant (23  C, 103 Hz)

6.6

Ref. [38].

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Table 8.10 Comparison of Hardness and Tensile Strength of FluoroXpreneÒ and a Standard FKM Cured Compound Property Hardness Shore A

Cured FKM 70e95

FluoroXprene 70e100

Tensile strength, MPa

6.0e12.0

2.0e25.0

Elongation at break, %

100e300

10e350

Compression set, %

15e50

27e55

231

solents and can be processed by conventional melt processing techniques, including injection molding, extrusion, and blow molding. The potential applications are seals, O-rings, tubes, and linings of tanks and containers. The physical and mechanical properties can be varied by compounding, mainly by the ratio of fluoroelastomer and fluoroplastic used. The properties of a typical FluoroXpreneÒ formulation are compared with a standard FKM cross-linked FKM in Table 8.10. Tables 8.11e8.13 illustrate swelling behavior of three different FluoroXpreneÒ formulations and that of a standard cured FKM in several liquids. (All the data were obtained from E.H. Park.)

Table 8.11 Swelling of FluoroXpreneÒ and of Standard Cured FKM Compound in Different Organic Liquids (Swelling, Volume %)

Skydrol 150  C 41

Toluene 24  C 12

Hexane 70  C 7

TrichloroEthylene 70  C 15

Methanol 24  C 2

33

11

7

16

1

5

2

15

2

e

e

85e95

Sample No. 1

Fuel C 100  C 19

No. 2

8

No. 3

14

0

DI

FKM

35e40

e

e

Diesel 65  C 4 4

Table 8.12 Swelling of FluoroXpreneÒ and of Standard Cured FKM Compound in Sodium Hydroxide Solution and Diluted Sulfuric Acid (Swelling, Volume %)

Sample No. 1

Sodium Hydroxide (1 M), 65  C 19

No. 2

8

No. 3

14

FKM

35e40

Table 8.13 Fuel Permeation Resistancea of FluoroXpreneÒ and Standard Cured FKM

Sulfuric Acid, 50% 70  C 4

Sample FKM 1 (Ter-) FKM 2 (Co-)

4

FluoroXprene

0 e

Ò

Permeation Rate, g/m2/day 15

Permeation Constant, g-mm/m2/day 28

29

55

1e4

2e8 

a

ASTM D814, CE10 Fuel, 30 mdays, 40 C.

References 8.6 New Commercial Development In the meantime, Freudenberg-NOK developed and commercialized a TPV-type fluorinated elastomer based on a cross-linked fluorocarbon elastomer (FKM) dispersed in a fluoroplastic, such as PVDF, ECTFE, or similar fluorinated polymer or copolymer [39]. The material with trade name FluoroXpreneÒ is resistant to heat, bases, fuels, and

[1] Robeson LM, McGrath JE. Polym Eng Sci 1977;17(5):300. [2] Hammer CF. In: Paul DR, Newman S, editors. Polymer blends, vol. 2. New York: Academic Press; 1978. p. 219. [3] Hofmann GH. In: Walsh DJ, Higgins JS, Maconnacie A, editors. Polymer blends and mixtures, NATO ASI Series, No. 89. Dordrecht: Nijhoff; 1985. p. 117.

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[4] Hofmann GH, Statz RJ, Case RB. In: Proceedings of 51st SPE-ANTEC, vol. XXXIX; 1993. p. 2938. [5] Asay RE, Hein MD, Wharry DL. J Vinyl Tech 1993;15(2):76. [6] Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 143. [7] Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 144. [8] Landi VR. Appl Polym Symp 1974;25:223. [9] Stockdale MK. J Vinyl Tech 1990;12(4):235. [10] Oganesove YG, et al. Polym Sci USSR (Engl Transl) 1969;11:1012. [11] Matsuo M, Nozaki C, Jyo Y. Polym Eng Sci 1969;9:197. [12] Milner PW, Duval GR. Thermoplastic elastomers 3. Sudsbury (UK): RAPRA Technology, Ltd; 1991. p. 7. [13] Duval GR, Milner PW. PVC 87. Brighton (UK); April 28e30, 1987. [14] Kliever B, DeMarco RD. Rubber Plast News; February 15, 1993:25. [15] Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 146. [16] Crawford RW, Witsiepe WK. U.S. Patent 3,718,715; 1973. [17] Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 148. [18] Brown M. Rubber Ind June 1975;102. [19] Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 149. [20] Hourston DJ, Hughes ID. J Appl Polym Sci 1981;26(10):3467. [21] Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 151. [22] Wallace JG. In: Walker BM, Rader CP, editors. Handbook of thermoplastic elastomers. 2nd ed.

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[23]

[24]

[25]

[26] [27] [28]

[29]

[30] [31] [32]

[33] [34]

[35] [36]

[37] [38]

[39]

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New York: Van Nostrand Reinhold Company; 1988. p. 143. AlcrynÒ MPRÔ product and properties guide (3/10/05), Advanced Polymer Alloys, Wilmington (DE). Hofmann GH. In: Holden G, Legge NR, Quirk R, Schroeder HE, editors. Thermoplastic elastomers. 2nd ed. Munich: Hanser Publishers; 1986. p. 133. AlcrynÒ tech notes, coefficient of friction (ASTM D 1894), COF (2/98), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ fluid resistance guide, fluid (2/98), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ tech notes, flammability (horizontal burn), HB flame (10/97). Hofmann GH. In: Holden G, Kricheldorf HR, Quirk R, editors. Thermoplastic elastomers. 3rd ed. Munich: Hanser Publishers; 2004. p. 126. AlcrynÒ tech notes, antistatic properties, static (10/97), Advanced Polymeric Alloys, Wilmington (DE). Myrick RE. In: Proceedings of the 52nd SPE ANTEC, vol. LX; 1994. Hoffmann GH. In: Proceedings of the 47th SPE ANTEC, vol. XXXV; 1989. p. 1752. AlcrynÒ injection molding guide, INJGUIDE (02/01), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ extrusion guide, extrusion (6/99), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ extrusion blow molding guide, BlowMolding (3/98), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ calendering guide, calendering (2/98), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ tech notes, compression molding procedure, CompMold (7/01), Advanced Polymer Alloys, Wilmington (DE). AlcrynÒ bonding guide, bonding AlcrynÒ to various substrates, bonding guide (1/02). DAI-EL Themoplastic, http://www.daikinchem. com.cn/en/pro/daiel/sam.html, Daikin Industries Ltd, Chemical Division. Park EH, Walker FJ. Base resistant FKM-TPV elastomer. U.S. Patent 7,718,736; May 18, 2010 to Freudenberg-NOK, General Partnership.