CHAPTER. Polymer II. Characteristics, Applications and Processing

Transcription

1 CHAPTER 15 Polymer II Characteristics, Applications and Processing

2 ISSUES TO ADDRESS... What are the tensile properties of polymers and how are they affected by basic microstructural features? Hardening, anisotropy, and annealing in polymers. How does the elevated temperature mechanical response of polymers compare to ceramics and metals? What are the primary polymer processing methods?

3 Content I 15.1 Introduction 15.2 Stress Strain Behavior 15.3 Macroscopic Deformation 15.4 Viscoelastic Deformation 15.5 Fracture of Polymers 15.6 Miscellaneous Mechanical Characteristics 15.7 Deformation of Semicrystalline Polymers 15.8 Factors That Influence the Mechanical Properties of Semicrystalline Polymers 15.9 Deformation of Elastomers Crystallization Melting The Glass Transition

4 Content II Melting and Glass Transition Temperatures (Tg) Factors That Influence Melting and Tg Plastics Elastomers Fibers Miscellaneous Applications Advanced Polymeric Materials Polymerization Polymer Additives Forming Techniques for Plastics Fabrication of Elastomers Fabrication of Fibers and Films

5 1. Introduction An engineer should know something about the characteristics, applications, and processing of polymeric materials. Polymers are used in a wide variety of applications, such as construction materials and microelectronics processing. Thus, most engineers will be required to work with polymers at some point in their careers. Understanding the mechanisms by which polymers elastically and plastically deform allows one to alter and control their moduli of elasticity and strengths Also, additives may be incorporated into polymeric materials to modify a host of properties, including strength, abrasion resistance, toughness, thermal stability, stiffness, deteriorability, color, and flammability

6 2. Stress-Strain Behavior On the basis of stress strain behavior, polymers fall within three general classifications (Figure 15.1): brittle (curve A), plastic (curve B), and highly elastic (curve C). Polymers are neither as strong nor as stiff as metals. high flexibilities, low densities, and resistance to corrosion make them the materials of choice for many applications. The mechanical properties of polymers are sensitive to changes in temperature and strain rate. With either rising temperature or decreasing strain rate, modulus of elasticity diminishes, tensile strength decreases, and ductility increases.

7 Polymers Stress-Strain Behavior brittle polymer plastic elastic moduli less than for metals elastomer Fracture strengths of polymers ~ 10% of those for metals Deformation strains for polymers > 1000% for most metals, deformation strains < 10% 7

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9 Influence of T and Strain Rate on Thermoplastics Decreasing T increases E -- increases TS -- decreases %EL Increasing strain rate same effects as decreasing T. s (MPa) ºC 20ºC 40ºC Plots for semicrystalline PMMA (Plexiglas) 20 60ºC to e 9

10 Initial Mechanisms of Deformation Brittle Crosslinked and Network Polymers Near Failure s (MPa) x brittle failure Initial Near Failure x plastic failure aligned, crosslinked polymer e network polymer 10

11 3. Macroscopic deformation Tensile strain-stress curve for a semicrystalline polymer.

12 4. Viscoelastic Deformation Viscoelastic mechanical behavior, being intermediate between totally elastic and totally viscous, is displayed by a number of polymeric materials. This behavior is characterized by the relaxation modulus, a timedependent modulus of elasticity. Magnitude of the relaxation modulus is very sensitive to temp. Glassy, leathery, rubbery, and viscous flow regions may be identified on a plot of logarithm of relaxation modulus versus temperature (Figure 15.7). The logarithm of relaxation modulus versus temperature behavior will depend on molecular configuration degree of crystallinity, presence of crosslinking, and so on. (Figure 15.8).

13 (a). load-time; (b)- (d) strain-time Totally elastic Viscoelastic Viscous

14 Time-Dependent Deformation Stress relaxation test: -- strain in tension to e o and hold. -- observe decrease in stress with time. e o tensile test Relaxation modulus: E r ( t) s( t) e o strain s(t) time There is a large decrease in E r for T > T g E r (10 s) in MPa rigid solid (small relax) T(ºC) Representative T g values ( C): PE (low density) PE (high density) PVC PS PC transition region viscous liquid (large relax) T g (amorphous polystyrene) Leathery Rubbery Rubbery flow 14

15 5. Fracture of polymers Fracture strengths are low relative to metals and ceramics. Both brittle and ductile fracture modes are possible. Some thermoplastic materials experience a ductile-to-brittle transition with a lowering of temperature, an increase in strain rate, and/or an alteration of specimen thickness or geometry. In some thermoplastics, the crack-formation process may be preceded by crazing; crazes are regions of localized deformation and the presence of microvoids (Figure 15.9). Crazing can lead to an increase in ductility and toughness of the material.

16 Crazing During Fracture of Thermoplastic Polymers Craze formation prior to cracking during crazing, plastic deformation of spherulites and formation of microvoids and fibrillar bridges aligned chains fibrillar bridges microvoids crack 16

17 6. Miscellaneous Mechanical Characteristics Impact strength Fatigue Tear strength Hardness

18 7. Deformation of semicrystalline polymers During the elastic deformation of a semicrystalline polymer having a spherulitic structure that is stressed in tension, the molecules in amorphous regions elongate in the stress direction (Figure 15.12). The tensile plastic deformation of spherulitic polymers occurs in several stages as both amorphous tie chains and chainfolded block segments (which separate from the ribbonlike lamellae) become oriented with the tensile axis (Figure 15.13). Also, during deformation the shapes of spherulites are altered (for moderate deformations); relatively large degrees of deformation lead to a complete destruction of the spherulites to form highly aligned structures

19 Mechanisms of Deformation Semicrystalline (Plastic) Polymers s (MPa) x brittle failure fibrillar structure onset of necking plastic failure near failure x unload/reload undeformed structure amorphous regions elongate e crystalline regions align crystalline block segments separate 19

20 Elastic Deformation of a semicrystalline polymer

21 Plastic deformation

22 8. Factors that influence mech. Prop. Of semicryst. polymers Molecular weight TS = TS - (A/M n ) Degree of crystallinity TS increases with crystallinity Predeformation by drawing Analog to strain hardening Heat-treating Improve crystallinity

23 Predeformation by Drawing Drawing (ex: monofilament fishline) -- stretches the polymer prior to use -- aligns chains in the stretching direction Results of drawing: -- increases the elastic modulus (E) in the stretching direction -- increases the tensile strength (TS) in the stretching direction -- decreases ductility (%EL) Annealing after drawing decreases chain alignment -- reverses effects of drawing (reduces E and TS, enhances %EL) Contrast to effects of cold working in metals!

24 9. Mechanisms of Deformation Elastomers s (MPa) x brittle failure plastic failure x elastomer x initial: amorphous chains are kinked, cross-linked. e deformation is reversible (elastic)! final: chains are straighter, still cross-linked Compare elastic behavior of elastomers with the: -- brittle behavior (of aligned, crosslinked & network polymers), and -- plastic behavior (of semicrystalline polymers) 24

25 Elastomers (Rubbers) Natural rubber: Produced from latex of Havea Brasiliensis tree. H CH3 H H C C C C H H n Polyisoprene 聚異戊二烯 Vulcanization: Heating rubber with sulfur and lead carbonate. Increases tensile strength. Restricts molecular movement by crosslinking of molecules. Figure 7.41

26 10. Crystallization During the crystallization of a polymer, randomly oriented molecules in the liquid phase transform into chain-folded crystallites that have ordered and aligned molecular structures. Nucleation and growth process Avrami equation Fraction of crystallization y = 1 exp(-kt n ) 100% crystallinty is impossible

27 Melting and glass transition temp. Melting The melting of crystalline regions of a polymer corresponds to the transformation of a solid material, having an ordered structure of aligned molecular chains, to a viscous liquid in which the structure is highly random. The Glass Transition The glass transition occurs in amorphous regions of polymers. Upon cooling, this phenomenon corresponds to the gradual transformation from a liquid to a rubbery material, and finally to a rigid solid. With decreasing temperature there is a reduction in the motion of large segments of molecular chains.

28 13. Melting & Glass Transition Temp. 28

29 14. Factors influence melting and glass transition temp. T mobile liquid viscous liquid Callister, rubber Fig tough plastic T m T g crystalline solid partially crystalline solid Molecular weight The magnitudes of T m and T g increase with increasing chain stiffness; stiffness is enhanced by the presence of chain double bonds and side groups that are either bulky or polar. At low molecular weights T m and T g increase with increasing M.

30 15. Plastics: Thermoplastics vs. Thermosets Thermoplastics: -- little crosslinking -- ductile -- soften w/heating -- polyethylene polypropylene polycarbonate polystyrene Thermosets: -- significant crosslinking (10 to 50% of repeat units) -- hard and brittle -- do NOT soften w/heating -- vulcanized rubber, epoxies, polyester resin, phenolic resin 30

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33 16. Elastomers Table 15.4 lists properties and applications of common elastomers; these properties are typical and, of course, depend on the degree of vulcanization and on whether any reinforcement is used. Natural rubber is still used to a large degree because it has an outstanding combination of desirable properties. However, the most important synthetic elastomer is SBR, which is used predominantly in automobile tires, reinforced with carbon black. NBR, which is highly resistant to degradation and swelling, is another common synthetic elastomer.

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35 17. Polymer Types Fibers Fibers - length/diameter >100 Primary use is in textiles. Fiber characteristics: high tensile strengths high degrees of crystallinity structures containing polar groups Formed by spinning extrude polymer through a spinneret (a die containing many small orifices) the spun fibers are drawn under tension leads to highly aligned chains - fibrillar structure 35

36 18. Polymer Types Miscellaneous Coatings thin polymer films applied to surfaces i.e., paints, varnishes protects from corrosion/degradation decorative improves appearance can provide electrical insulation Adhesives bonds two solid materials (adherands) bonding types: 1. Secondary van der Waals forces 2. Mechanical penetration into pores/crevices Films produced by blown film extrusion Foams gas bubbles incorporated into plastic 36

37 19. Advanced Polymers Ultrahigh Molecular Weight Polyethylene (UHMWPE) Molecular weight ca. 4 x 10 6 g/mol Outstanding properties high impact strength resistance to wear/abrasion low coefficient of friction self-lubricating surface Important applications bullet-proof vests golf ball covers hip implants (acetabular cup) UHMWPE 37

38 Advanced Polymers Thermoplastic Elastomers Styrene-butadiene block copolymer styrene hard component domain butadiene soft component domain 38

39 20. Polymer Formation There are two types of polymerization Addition (or chain) polymerization Monomer units are attached one at a time in chainlike fashion to form a linear molecule. Condensation (step) polymerization Involve stepwise intermolecular chemical reactions that may include more than a single molecular species.

40 Addition (Chain) Polymerization Initiation Propagation Termination 40

41 Condensation (Step) Polymerization 41

42 21. Polymer Additives Improve mechanical properties, processability, durability, etc. Fillers Added to improve tensile strength & abrasion resistance, toughness & decrease cost ex: carbon black, silica gel, wood flour, glass, limestone, talc, etc. Plasticizers Added to reduce the glass transition temperature T g below room temperature Presence of plasticizer transforms brittle polymer to a ductile one Commonly added to PVC - otherwise it is brittle 42

43 Polymer Additives (cont.) Stabilizers Antioxidants UV protectants Lubricants Added to allow easier processing polymer slides through dies easier ex: sodium stearate Colorants Dyes and pigments Flame Retardants Substances containing chlorine, fluorine, and boron 43

44 22. Processing of Plastics Thermoplastic can be reversibly cooled & reheated, i.e. recycled heat until soft, shape as desired, then cool ex: polyethylene, polypropylene, polystyrene. Thermoset when heated forms a molecular network (chemical reaction) degrades (doesn t melt) when heated a prepolymer molded into desired shape, then chemical reaction occurs ex: urethane, epoxy 44

45 23. Forming techniques for plastics, fibers and films Fabrication of plastic polymers is usually accomplished by shaping the material in molten form at an elevated temperature, using at least one of several different molding techniques compression (Figure 15.23), transfer, injection (Figure 15.24), and blow. Extrusion (Figure 15.25) and casting are also possible. Some fibers are spun from a viscous melt or solution, after which they are plastically elongated during a drawing operation, which improves the mechanical strength. Films are formed by extrusion and blowing (Figure 15.26) or by calendering.

46 Processing Plastics Compression Molding Thermoplastics and thermosets polymer and additives placed in mold cavity mold heated and pressure applied fluid polymer assumes shape of mold 46

47 Processing Plastics Injection Molding Thermoplastics and some thermosets when ram retracts, plastic pellets drop from hopper into barrel ram forces plastic into the heating chamber (around the spreader) where the plastic melts as it moves forward molten plastic is forced under pressure (injected) into the mold cavity where it assumes the shape of the mold Barrel 47

48 Processing Plastics Extrusion thermoplastics plastic pellets drop from hopper onto the turning screw plastic pellets melt as the turning screw pushes them forward by the heaters molten polymer is forced under pressure through the shaping die to form the final product (extrudate) 48

49 24. Processing Plastics Blown-Film Extrusion 49

50 Limitations of polymers: -- E, sy, Kc, Tapplication are generally small. -- Deformation is often time and temperature dependent. Thermoplastics (PE, PS, PP, PC): -- Smaller E, sy, Tapplication -- Larger Kc -- Easier to form and recycle Elastomers (rubber): -- Large reversible strains! Thermosets (epoxies, polyesters): -- Larger E, sy, Tapplication -- Smaller Kc Summary I Table 15.3 Callister & Rethwisch 8e: Good overview of applications and trade names of polymers.

51 Summary II Polymer Processing -- compression and injection molding, extrusion, blown film extrusion Polymer melting and glass transition temperatures Polymer applications -- elastomers -- fibers -- coatings -- adhesives -- films -- foams -- advanced polymeric materials

52 Plastics Recycling PET Polyethylene Terephthalate 聚對苯二甲酸乙二酯 Two-liter beverage bottles, mouthwash bottles, boil-in-bag pouches. HDPE High Density Polyethylene Milk jugs, trash bags, detergent bottles. PVC Polyvinyl Chloride Cooking oil bottles, packaging around meat. LDPE Low Density Polyethylene Grocery bags, produce bags, food wrap, bread bags. PP Polypropylene Yogurt containers, shampoo bottles, straws, margarine tubs, diapers. PS Polystyrene Hot beverage cups, take-home boxes, egg cartons, meat trays, cd cases. OTHER All other types of plastics or packaging made from more than one type of plastic.

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54 Action to save the earth 3R 5R Reduce Reuse Recycle Refuse Respect

55 成大教授 : 台灣過度塑化自我覺醒吧 聯合報 記者蔡永彬 / 台 北報導 塑化劑風波延 燒, 成功大學 昨天發表研究 指出, 台灣室 內灰塵的 DEHP( 鄰苯 二甲酸二酯 最常用的塑化 劑之一 ) 含量 全世界最 高 ; 木質地 板或打蠟也會 使室內塑化劑 濃度變高 ; 過 度使用塑膠容 器還會提高孩 童氣喘機率, 或讓女童性早 熟 成大環境醫學 研究所教授李 俊璋呼籲政府, 在化妝品 保 養品內應該全 面禁用塑化劑, 器皿要訂溶出 標準, 民眾在 生活習慣上也 要 自我覺 醒 少用塑 膠 這項研究由成 大副校長蘇慧 貞 環醫所教 授李俊璋 高 雄第一科技大 學環境與安全 衛生工程系副 教授李家偉等 人共同合作, 在國科會 永 續發展整合研 究成果發表會 上發表 他們 從大台南地區 挑選一百九十 三戶, 每戶挑 選幼兒最常活 動的五個地方 採樣 ; 李俊璋 指出, 從中位 數看來, 每克 的灰塵含了 1.23 微克 DEHP, 比起 十五年內歐洲 美國的研究都 高出許多 圖 / 聯合報 提供 研究團隊另 採七十一位 性早熟女童 的尿液分析, 發現其中鄰 苯二甲酸酯 類濃度是正 常女童尿液 中的一點三 至兩倍, 李 俊璋認為, DEHP 會刺激 女童大腦神 經中樞, 讓 她們第二性 徵提早發育 李俊璋指出, 不少室內裝 潢採用木質 地板, 上面 的亮光漆和 防水層就可 能有塑化劑, 打蠟時蠟油 中也可能有 李家偉則說, 電器的電線 外皮有塑化 劑, 如果疏 於打掃, 通 電時電線溫 度提高, 塑 化劑可能揮 發附著在灰 塵上, 或逸 散到空氣中 建議大家電 器不用時就 拔掉插頭, 不只節能 減碳, 也保 護身體健 康 台灣已經 過度 塑 化 李 俊璋認為, 某些攤販不 想洗盤子, 就在盤子外 面套一層塑 膠袋, 塑 膠袋耐熱是 保證 不變 形, 不代 表 不溶出 塑化劑! 他也不建議 用塑膠奶瓶 幫小孩泡牛 奶, 萬一洗 瓶子刮傷內 壁, 下次泡 就溶出更多 要吃便利商 店的微波食 品, 最好買 回家倒在盤 子裡, 用 那種容器我 不敢吃! 李俊璋強調, 他不是希望 政府 全部 禁掉 ( 塑 膠 ) 他 舉例, 化妝 品 保養品 的定香劑可 用天然產物 取代, 就應 該禁用塑化 劑 ; 器皿方 面, 政府應 對所有常用 塑化劑訂定 溶出標準 作 正面表 列, 告訴 廠商什麼能 用 延伸閱讀 0531 更新 / 含有受塑 化劑汙染起 雲劑之產品 資料表 衛生署食 品藥物管理 局網站 / 起 雲劑 DEHP 汙 染專區 2011/06/02 / :25 am