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CRB 30204 RUBBER TECHNOLOGY – CHAPTER 2 Dr. SK Ong.

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1 CRB RUBBER TECHNOLOGY – CHAPTER 2 Dr. SK Ong

2 Chapter Outline  Rubber Additives  Compounding ingredient  Vulcanization systems  Rubber reinforcement theories Dr. Ong SK 2

3 Raw Rubber Processing Raw rubber Mastication Compounding Final product Shaping Vulcanization Rubber compound Testing: Viscosity/Plasticity Tool: Calendering, spreading, extruding, compression moulding, transfer moulding, injection moulding Tool: Two-roll mill or internal mixer Testing: Scorch & Cure Characteristic Testing: Tensile Test, Tear Strength, Compression Set, Abrasion, Resilience, Hardness, Ageing etc Dr. Ong SK 3

4 Viscosity Test – Mooney Viscometer Dr. Ong SK 4  Equipment: Mooney Viscometer  Properties measured: viscosity of raw rubber (incoming rubber bale)  Unit : Mooney units  Standards: IS0 289, ASTM D 1646, BS 903: Part A58, DIN

5 How Mooney Viscometer Works? Dr. Ong SK 5  A knurled knob (rotor) rotates (at 2 revolutions/min) in a closed heated cavity (like a mold), filled with uncured rubber.  Shearing action will develops btwn compound & rotor when the rotor rotate  This results in torque (resistance of the rubber to the turning rotor)  Mooney units is directly relate to torque  The higher the number, the higher the viscosity (higher resistance of the rubber against the rotor rotation)

6 Schematic Diagram of Mooney Viscometer Dr. Ong SK 6

7 Mooney Viscosity Results Dr. Ong SK 7

8 Wallace Plastimeter  Measures the plasticity or viscosity of cylindrically cut unvulcanized rubbers which is under a constant compressive force btwn 2 parallel plates  Very simple & crude methods for measuring flow of rubber  Disadvantages: operate at extremely low shear rate range ( to 1 s -1 )

9 Wallace Plastimeter (Cont’)  Determine the Plasticity Retention Index (PRI) of raw natural rubbers (ASTM D 3194)  PRI is a measure of the resistance of raw NR to oxidation.  Oxidation effect is assessed by measuring the plasticity before ageing (P 0 ) and after ageing for 30mins in the Wallace - MRPRA ageing oven at 140°C (P 30 )

10 Wallace Plastimeter (Cont’)

11 Tackiness (Rubber)  Tackiness: ability of an uncured rubber compound to stick to itself or another compound in short dwell time with moderate applied pressure  Important property when a rubber product was built by laying one calendered/extruded rubber ply on top of another  Uncured layers must hv good tackiness prior to curing  Tackiness can be deteriorate due to blooming of additives like sulfur, accelerators, waxes etc (tackifier can be added to improve this property)  Tool: Tel-Tak Tackmeter (no ATSM/ISO test std)

12 Mastication  Entangled rubber chains are ‘torn apart’ at weakest bond  Free radicals will recombine or add to a double bond in another rubber chain w/o O 2 present. (rubber chain not permanently broken)  With O 2 present, peroxy radical formed. (permanent breakdown of rubber chain)  Tool: Two-roll mill, internal mixer, injection molding etc Dr. Ong SK 12

13 Compounding/Mixing  Obj:  Obtain uniform blend with good dispersion of each ingredient  Produce consistent batches which are uniform in viscosity & degree of dispersion.  Main tools: Two-roll mill or internal mixer  Main constituents: Rubber, antioxidant, accelerator, vulcanizing agent(s), activator, reinforcing agents/filler, processing aids, extender/diluents & pigments. Dr. Ong SK 13

14 Main Compounding Ingredients  Rubber or blends of rubbers  Vulcanizing agent(s)  Accelerators of vulcanization  Activators of accelerators  Reinforcing agents or filler (carbon black, mineral filler)  Processing aids (peptisers, plasticizers, dispersing aids)  Extenders or diluents (extending oils, cheap inert mineral fillers)  Pigments or colouring materials  Antidegradants Dr. Ong SK 14

15 Example Dr. Ong SK 15

16 Accelerators  Majority contains nitrogen or sulphur atoms  Classified in term of cure rates or chemical structure Dr. Ong SK 16

17 Classification of Accelerators Dr. Ong SK 17

18 Classification of Accelerators (cont.) Dr. Ong SK 18

19 More Information on Selected Accelerators Dr. Ong SK 19 AcceleratorsCharacteristics DithiocarbamatesExample: Zinc dibutyl dithiocarbamate (ZDBC). Very scorchy & very fast curing. Useful in low temperature (down to 100 o C) vulcanization and in elastomers with low levels of unsaturation such as EPDM. Note that as temperature reduces scorch increases & cure rate decreases. ThiuramsExample: Tetramethylthiuram disulfide (TMTD). Somewhat less scorchy than dithiocarbamates & fast curing. TMTD is less scorchy in the absence of sulfur. In this case its function would be that of a cross-linking agent rather than an accelerator. Tetramethylthiuram monosulfide (TMTM) gives good compression set.

20 More Information on Selected Accelerators Dr. Ong SK AcceleratorsCharacteristics Thiazoleseg: Mercaptobenzothiazole (MBT). Moderate cure rate & scorch giving a low modulus vulcanizate. Guanidineseg: Diphenyl guanidine (DPG). Scorchy & slow curing. Most often used in combination with other accelerators. Sulfenamideseg: N-cyclohexyl-2-benzothiazolseu lfenamide (CBS). Long scorch with medium to fast cure. It would be a good choice when mixing compounds containing reinforcing furnace blacks which generate more heat. The sulfenamide N,N-dicyclohexyl-2-benzothiazyl sulfenamide (DCBS) gives longer scorch & slower curing. DCBS gives excellent adhesion when bonding brass coated steel to rubber, eg in tire production. 20

21 Examples of Accelerators & Its Structure Dr. Ong SK 21

22 Examples of Accelerators & Its Structure Dr. Ong SK 22

23 Vulcanization characteristics by various accelerators and combinations. Dr. Ong SK 23

24 Various Accelerators & Its Cure Time Dr. Ong SK 24

25 Activator  Increase vulcanization rate by activating the accelerator  Types of activators  Inorganic compounds, mainly metal oxides e.g. Zinc oxide 3-5 p.h.r.  Organic acids, high molecular weight monobasic fatty acids e.g. stearic acid 1-3 p.h.r.  Alkaline substances, mainly amines e.g. diethanolamine 2 p.h.r. used in silica-filled compounds, and compounds with acidic ingredients)  Normally ZnO & stearic acid are used together or zinc stearate to replace both  Zinc oxide reacts with stearic acid to form zinc stearate  Together w/accelerator they speed up the rate at which sulfur vulcanization occurs.  With sulfur alone, the curing process might take hours. With this curing system, it can be reduced to minutes. Dr. Ong SK 25

26 Vulcanizing Agents – Sulfur  It reacts chemically with the raw gum elastomer forming cross-links between the polymer chains, resulting in a more dimensionally stable and less heat-sensitive product.  Elastomers need to hv unsaturated bonds  Its cost is relatively low but its function is essential.  Grade for rubber application: cyclic sulfur in fine rhombic crystal shape (S 8 ) /amorphous sulfur  At high sulfur level in a compound, it can slowly bloom to the surface.  Sulfur donor like TMTD/TETD can be use also Dr. Ong SK 26

27 Chemistry of Sulfur Vulcanization Dr. Ong SK 27  Refer to Handouts

28 Sulfur Vulcanization System Cure systems (phr) Accelerator (phr) Conventional EV Semi-EV Dr. Ong SK 28

29 Sulphur Vulcanizing System  EV system:  Mainly monosulphide cross-link  Little modification except on pendant grps  CV system:  Mainly di-,poly- & cyclic sulphide cross-link  Sulphide cross-link easily decompose & causing main chain modification Note: C-C bond: strong; C-S bond: medium strong: S-S bond: weak; S-S x -S bond: weak Dr. Ong SK 29

30 Dr. Ong SK 30

31 Vulcanizing Agents – Peroxide  Most common organic peroxide: dicumyl peroxide (DCP)  Elastomer do not need unsaturated bonds  Major curative for silicone rubber  Note: not recommended for some elastomers such as IIR or CIIR Dr. Ong SK 31

32 Chemistry of Peroxide Curing Dr. Ong SK 32  Peroxide decomposition:  Crosslinking of 2 polymeric free radicals addition of the peroxide-derived radical to a double bond of the polymer molecule abstraction of a hydrogen atom from an allylic position on the polymer

33 Chemistry of Peroxide Curing (Cont.) Dr. Ong SK 33  Or addition of polymeric free radicals to double bonds

34 Cares When Using Peroxide  Reason: to avoid unwanted interaction with peroxide  Example  Antioxidant selection  Contact with oxygen (air) should be avoided during vulcanization (such as in hot air ovens or autoclave curing).  Some ingredients, which are not part of the cure system, which are common in sulfur systems can interact with the peroxide in peroxide cure systems and thus interfere with cure. Dr. Ong SK 34

35 Peroxides Cure  Vulcanizate hv good resistance towards oxidative ageing & reversion due to stable C-C x/link  Compression set is also improved  Tensile strength, tear strength, and fatigue (dynamic deformation such as constant flexing) life are reduced. Dr. Ong SK 35

36 Peroxides Cure  Post cure (continued cure outside of the mold) is sometimes undertaken with peroxide cured vulcanizates, to complete the cure and remove unwanted byproducts.  x/link density of a peroxide cured compound can be increased by addition of chemicals called coagents, of which methacrylates are a good example.  This results in a higher state of cure with improvements in properties such as compression set Dr. Ong SK 36

37 Dynamic Vulcanization Dr. Ong SK 37  For thermoplastic elastomer (TPE)  Vulcanizing or x/linking of one polymer during its molten-state mixing with another polymer or with other polymers.  The polymers are first thoroughly mixed and then, during further mixing, one of the polymers will x/linked, whereas the remaining other polymeric material remains uncrosslinked.  The process produces a dispersion of x/linked polymer in a matrix or continuous phase of uncrosslinked polymer.

38 Filler  Function of fillers:  Reinforcement effect, i.e. to increase mechanical properties (eg, TS & resistance to tearing) in the vulcanizate & to increase stiffness  Compound cost reduction  Might be for coloring purpose  Types of filler  Reinforcing filler (Carbon black & silica): To improve the mechanical properties of the filled rubber vulcanizate  Non-reinforcing filler (Clay, Calcium carbonate etc) Stiffer & harder filled rubber vulcanizate will be obtained Dr. Ong SK 38

39 Particle Size vs. Reinforcement  Size>5000 nm: non-reinforcing  Size btwn 1000 – 5000 nm: weak reinforcing but with high loading, reinforcement can be obtain  Size< 1000 nm: Reinforcing  Size< 100 nm: True reinforcing Dr. Ong SK 39

40 Carbon Black  Hv various type with different physical & chemical properties  Multiple size, surface area, structure & surface activities  Its surface has functional grps like phenolic, ketones, carboxylic, lactones etc. these functional grps will be responsible in interacting with the rubber Dr. Ong SK 40

41 Dr. Ong SK 41

42 Structure, Aggregation & Agglomeration  Structure refers to degree of aggregation, a low structure means there are 30 particles/aggregate while high structure hv 200 particle/aggregate  Aggregate hv tendency to agglomerate during processing; these agglomerates are called secondary aggregate  Bonds btwn aggregates are weak vs. those bond of primary aggregate Dr. Ong SK 42

43 CB Aggregates Dr. Ong SK 43

44 Reinforcing  Filler-rubber interaction depends on:  External factor: total surface area of the filler which are in contact with rubber  Internal factor: surface activities & chemical properties  Geometry factor: filler’s structure & filler porosity (minor factor) Dr. Ong SK 44

45 Surface Area  The smaller the particle size the bigger the surface area  Bigger surface area results in higher interaction btwn filler & rubber  A s = surface area; d = diameter & e = filler density Dr. Ong SK 45

46 Surface Activities & Chemical Properties  Chemical properties of a filler depends on the presence of functional grps which hv oxygen  Polar rubber hv better affinity to polar filler Reinforcement factor = surface area x surface activity Dr. Ong SK 46

47 Chemical Functions on CB Surface Dr. Ong SK 47

48 Structure  Primary structure refers to degree of aggregation  Secondary structure refers to agglomeration of aggregates; agglomeration is a result of Van der Waals forces Dr. Ong SK 48

49 Structure Dr. Ong SK 49

50 Dr. Ong SK 50

51 CB loading & Selected Properties Dr. Ong SK 51

52 Carbon Black Classification  Following ASTM D 1765  1 st alphabet refer top rate of vulcanization; N refer to normal rate & S is slow rate  1 st digit after the alphabet refers to particle size; smaller the number, smaller size.  2 nd & 3 rd digit after the 1 st digit don’t hv special meaning  5 major reinforcing CB are N110, N220, N330, N341 & N550 Dr. Ong SK 52

53 ASTM Grades ofCB Dr. Ong SK 53

54 ASTM Grades of CB Dr. Ong SK 54

55 Silica  More expensive vs. CB but yet performance similar as CB  Special performance: improvement in tear strength, reduction in heat buildup, & increase in compound adhesion in multicomponent products such as tires  Preparation method of silica: fume silica & precipitate silica  Fume Silica: smallest filler size (7–15 nm); obtained from high temp rxtn btwn silicon tetrachloride and water vapor  Precipitate silica: better reinforcing performance & reinforcing factor determine via particle size. Dr. Ong SK 55

56 Silica (cont.)  Important properties of silica in reinforcement:  Ultimate particle size  Extent of hydration  Others important physical properties: pH, chemical composition & oil absorption Dr. Ong SK 56

57 Characteristic of Silica Filled Vulcanizates vs CB Filled Vulcanizates  No change in modulus as much as CB  Abrasion resistance is good  TS is good  Much better tear strength  Used when non-black reinforcement required; eg shoe sole, non black side wall tire or good tear resistance needed Dr. Ong SK 57

58 Physical Properties of Silica  Silica is an amorphous, consists of silicon and oxygen arranged in a tetrahedral structure of a 3- D lattice  Particle size ranges from 1–30 nm & surface area from 20–300m 2 /g.  There is no long-range crystal order, only short- range ordered domains in a random arrangement with neighbouring domains Dr. Ong SK 58

59 Chemical Properties of Silica  Surface silanol concentration (silanol groups —Si— O—H) influence the degree of surface hydration  Surface acidity is controlled by hydroxyl grps on the surface & is intermediate btwn those of P—OH & B—OH.  This intrinsic acidity can influence peroxide vulcanization, although in sulphur curing, there is no significant effect  Rubber–filler interaction is affected by these sites Dr. Ong SK 59

60 Chemical Properties of Silica (cont.)  Surface hydration caused by water vapour absorption is affected by [surface silanol]  High levels of hydration can adversely affect final compound physical properties  Silicas are hydroscopic & thus require dry storage conditions Dr. Ong SK 60

61 Dr. Ong SK 61 Silica loading in SBR: 50phr

62 Types of Silanols  Isolated  Geminal (2 —OH grps on the same silicon atom)  Vicinal (on adjacent silicon atoms)  Siloxane bridge Dr. Ong SK 62

63 Silica Grps Dr. Ong SK 63

64 Problems with Silica Filler  Increase in viscosity during compounding; especially high surface area one  Deactivation of accelerator system  longer cure time Dr. Ong SK 64

65 Solutions  Using combination of 2 or more accelerators eg one from thiazole or sulfenamide & the other one from guaidine, tiurams or ditiocarbamate  Use glycol activator (eg polyethylene glycol or diethylene glycol)  to prevent rxtn btwn moisture  Using EV system Dr. Ong SK 65

66 Silane Coupling Agent  Function: to improve the affinity btwn rubber and silica  Silane coupling agent: bifunctional compounds which react with silica surface & sulphur containing grps in vulcanized rubber  Results: increased in modulus, resilience, abrasion resistance but decrement in tear strength Dr. Ong SK 66

67 Silane Coupling Agent (cont.) Dr. Ong SK 67 :

68 Dr. Ong SK 68

69 Dr. Ong SK 69

70 Dr. Ong SK 70

71 Dr. Ong SK 71

72 Other Fillers – Clay  Commonly used white filler, due to:  Low cost  Reinforcing efficiency is btwn low to medium  Ease of processing especially for extrusion & calendering  Grades of China clays:  Soft clays  Hard clays  Calcined clays  Treated clays Dr. Ong SK 72

73 China Clays  Soft clays:  Particle size > 2 nm  Often used at high loading for low-cost compounds  Hard clays  Particle sizes < 2 nm  Higher TS, tear resistance & abrasion resistance are obtainable by using the hard clays vs. soft clays. Dr. Ong SK 73

74 China Clays (cont.)  Calcined clays:  Obtained from hard clays in which the combined water has been removed  Give higher hardness, TS & electrical resistivity vs. hard clays  Treated clays:  Hard clays which have been chemically treated offer better filler reinforcement than the untreated clays Dr. Ong SK 74

75 Other Fillers  Calcium carbonate is used as a low-cost filler in rubber products for static applications such as carpet underlay.  Calcium carbonates grades:  Ground limestone  Whiting  Precipitated whiting  Treated whiting  Titanium dioxide finds extensive use in white products such as white tire sidewalls where appearance is important Dr. Ong SK 75

76 Effect of Particle Size on Hardness of Vulcanizates Carbon black type Average particle size (nm) Parts of filler per point increase in hardness ASTMCodeCRIIRNRSBR N 220ISAF N 330HAF N 660GPF N 990MT Non black Hydrated silica Calcium silicate Hard clay2 X Soft clay10 X Whiting12 X Dr. Ong SK 76

77 Factors Contributes to Modulus of CB Filled Vulcanizates  Pure gum: directly proportional to x/link density;  x/link denstiy,  modulus  Filler-rubber bonds: physical and/or chemical interaction btwn filler & rubber increase apparent x/link density of the system  Structure: at moderate filler loading, high structure fillers formed 3-D network through the rubber matrix; filler network can increase modulus up to 2% (due to Van der Waals-London attraction. At abt 10% strain, 90% of the network broke down Dr. Ong SK 77

78 Factors Contributes to Modulus of CB Filled Vulcanizates (cont.)  Strain amplification (hydrodynamic effect(  When rubber filled with CB/silica, an equivalent volume fraction of rubber is replaced with rigid non deforming filler.  When deformed, filler does not undergo deformation  So the microscopic strains experienced by rubber chains is greater than the macroscopic strains  This is called strain amplifications  it depends on filler loading & filler type Dr. Ong SK 78

79 X/link Density  x/lonk density=degree of x/link= how many x/link formed  Method to determine x/link density:  Swelling – Flory Rehners  Physical method – Mooney Rivlin  Rheocurve (indirect)  Both method hv pros & cons, selection depends…  For research purpose, both method hv to be included for consistency of results Dr. Ong SK 79

80 Swelling Method – Flory Rehners  Need to hv  value (solubility parameters)  Takes long time as need to wait until equilibrium of swelling achieve  Method: Vulcanized rubber immersed in a solvent until equilibrium weight reached  For a given solvent, higher x/link density  lower swelling  For a given degree of x/link, more powerful solvent give higher degree of swelling Dr. Ong SK 80

81 Flory-Rehner Eq. Dr. Ong SK 81 V s =molar volume of the solvent v r =volume fraction of rubber in the swollen gel  = interaction constant, for NR usually is 0.4 in good solvent (determine the cohesive energy density of solvent & polymer & the swollen gel) Note: Effectiveness depends on obtaining good estimation of  At high cure state, further increase in cure only results in small changes in volume fraction of rubber (poor accuracy)

82 Mooney Rivlin Method  Carried out based on stress-strain measurement of the samples  Must hv ‘straight line’  Intercept at y-axis must be +ve  Takes long measurement time since low x/head speed is used Dr. Ong SK 82

83 Mooney-Rivlin Eq. Dr. Ong SK 83 = extension ratio C 1, C 2 =elastic constants f=force A=cross sectional area of the specimen f/A=nominal stress

84 Kinetic Theory of Elasticity & x/link Density Dr. Ong SK 84 = extension ratio =number of physical crosslinks per cm 3 R=gas constant T= absolute temp Physical x/link density can be calculated from elastic constant C 1 (intercept of f/A ( -  -2 ) vs. -1 where ph = 2C 1 /RT

85 Dr. Ong SK 85 Plot of f/A ( -  -2 ) vs. -1 for a range of NR vulcanizates. Sulfur content increases from 3 to 4%, with time of vulcanization & other quantities as variables

86 Smallwood-Guth-Gold Eq Dr. Ong SK 86 E f = modulus of filled rubber E o = modulus of rubber matrix  = volume fraction of the filler

87 Rheocurve  T max – T min : measure moduli (M100, M300)  T max – T min lower, M100 lower Dr. Ong SK 87

88 Reinforcing Efficiency  Measurement:  Rheometer (T max – T min )  Tensile modulus  Modified Guth-Smallwood eq. Dr. Ong SK 88

89 Processing aids (peptisers, plasticizers, dispersing aids)  Function: to aid (help) processing  Types:  Processing aids  Plasticizer  Factice  Low molecular weight polyethylene/Wax  Fatty acids  Superior processing rubber (SP rubber)  Reground crumb  Reclaimed rubber Dr. Ong SK 89

90 Plasticizer  Types of plasticizers:  The chemical plasticizers (peptiser)  The physical plasticizers (softeners & extenders)  Peptiser was used to ease NR mol. chain break down (Added to the rubber at the start of mastication)  Physical plasticizer don’t react chemically with the rubbers but function by modifying the physical characteristics of either compounded rubber/ finished vulcanizate Dr. Ong SK 90

91 Eg of Peptizer ProducerTrade nameChemical name Min. operative temp. ( o C) Anchor Chemical Pepton 22Di (benzamido- phenyl)disulphide 115 Pepton 65Zinc-2-benzamido- thiophenate 65 BayerRenacit IVZinc salt of pentachlorothiophe nol 70 Renacit VIIPentachlorothiophen ol with activating dispersing additives. 70 Dr. Ong SK 91

92 Plasticizer –Softeners/Extenders  Petroleum oils are the most commonly used physical plasticisers for processing  Types of petroleum oils used as processing aids and extenders are classified under three headings:  Paraffinic  Napthenic  Aromatic Dr. Ong SK 92

93 Factice  Types of factice: white factice & brown factice  Function: to facilitate processing by improving the incorporation & dispersion of powders & reduce power consumption  Presence of 5-30 phr of factice may be used to control die swell, improve surface quality & prevent distortion of shape during open steam vulcanization  Large loadings of factice e.g phr are used in very soft compounds such as pencil erasers Dr. Ong SK 93

94 Low MW PE/Wax  Functions:  As processing aid  As release agent to prevent sticking together of rubber compounds  As lubricant to help could flow & extrusion & prevent blocking in dies Dr. Ong SK 94

95 Fatty Acid  Function:  As a plasticizer  Aids dispersion of CB and other fillers  Minimizes any tendency for sticking to the mill rolls  Eg: stearic acid & is commonly used btwn 1-2 phr for sulfur vulcanization system Dr. Ong SK 95

96 Superior Processing Rubber  Function:  Controlling "nerve" during mixing processes  Improving shaping operations  Ensuring dimensional stability of rubber compounds  E.g. PA 80 which consists of 80% vulcanised and 20% unvulcanised NR & PA57 which consists essentially of 70 parts of PA80 with 30 parts of a non-staining processing oil. Dr. Ong SK 96

97 Crumb  Function: to remove air  reduce blisters in rubber products  Can be obtained either by cryogenic grinding using liquid nitrogen or buffing process such as tyre buffings  Crumb can be used at a level of 5-20 phr Dr. Ong SK 97

98 Reclaimed Rubber  Function:  To reduce compound cost  To improve processing such as reducing shrinkage, & swell & increasing calendaring & extrusion rates  Obtained from treatment of vulcanized scraps by the application of heat & chemical agents, i.e. from devulcanization or depolymerization Dr. Ong SK 98

99 Degradation of Unsaturated Rubber Dr. Ong SK 99

100 Antidegradants  Function: to extend the service life of vulcanized elastomer by protecting them from oxygen, ozone, light, heat, and flex fatigue  Why?...due to the unsaturated backbones  Types:  Staining antioxidant & antiozonant  Non-staining antioxidant & antiozonant  Antiozonant  Waxes Dr. Ong SK 100

101 Antioxidant  A/O react with  Oxygen to prevent oxidation of vulcanized rubber  Free radicals that degrade vulcanized rubber  2 main classes of staining: polymerized dihydroquinolines & diphenylamines  Non staining hv 4 grps: phosphites, hindered phenols, hindered bisphenols & hydroquinones Dr. Ong SK 101

102 Eg of Non Staining Antioxidant Dr. Ong SK 102

103 Eg of Staining Antioxidant Dr. Ong SK 103

104 Antiozonant  2 classes: staining & non-staining antiozonants  Non-staining types are less powerful & less versatile vs to the staining types  Most common antiozonant: para-Phenylenediamines (PPDs)  General structure:  Also improve resistance to fatigue, oxygen, heat, and metal ions Dr. Ong SK 104

105 Waxes  Used to improve rubber ozone protection primarily under static conditions.  Hv 2 categories: Microcrystalline wax & paraffin wax  Microcrystalline wax:  T m : in the region of 55 to 100°C  Extracted from residual heavy lube stock of refined petroleum  Paraffin wax:  T m : in the region 35 to 75°C  Obtained from the light lube distillate of crude oil Dr. Ong SK 105

106 Dr. Ong SK 106

107 Selection Criteria of Antidegradant – 1. Discoloration & Staining  In general, phenolic antioxidants tend to be nondiscoloring and amines are discoloring.  Thus for elastomers containing CB, more active amine antioxidants are preferred as discoloration is not important. Dr. Ong SK 107

108 Selection Criteria of Antidegradant – 2. Volatility  As a rule, the higher the MW of the antioxidant, the less volatile it will be  Hindered phenols tend to be highly volatile compared with amines of equivalent molecular weight  Correct addition of antioxidants in the compound mix cycle is critical if loss of material is to be avoided Dr. Ong SK 108

109 Selection Criteria of Antidegradant – 3. Solubility  Low solubility of an antidegradant  material to bloom to the surface  loss of protection  Therefore, solubility of antidegradants, particularly antiozonants, controls their effectiveness.  The materials must be soluble up to 2.0 phr, must be able to migrate to the surface  Must not be soluble in water or other solvents such as hydraulic fluid so as to prevent extraction of the protectant from the rubber. Dr. Ong SK 109

110 Selection Criteria of Antidegradant – 4. Chemical Stability  Antidegradant stability against heat, light, oxygen, & solvents is required for durability Dr. Ong SK 110

111 Selection Criteria of Antidegradant – 5. Concentration  Most antidegradants have an optimum concentration for max effectiveness after which the material solubility becomes a limiting factor.  para-Phenylenediamines: good oxidation resistance at a loading of 0.5 – 1.0 phr & antiozonant at the range 2.0 – 5.0phr. Above 5.0 phr para- phenylenediamines tend to bloom Dr. Ong SK 111

112 Selection Criteria of Antidegradant – 6. Environment, Health, & Safety  For ease of handling & avoidance of dust & inhalation, antidegradants should be dust free while free flowing. Dr. Ong SK 112

113 Principal Theories of Antiozonant Mechanisms  Scavenger theory  Postulates that the antiozonant competes with the rubber for ozone.  Ozonized antiozonant forms a protective film on the surface of the vulcanized rubber, preventing further attack  3 rd mechanism postulated is that the antiozonants react with elastomer ozonide fragments, relinking them and essentially restoring the polymer chain  4 th theorized mechanism suggests that Criege zwitterions are formed from the ozonide produced. Dr. Ong SK 113

114 Some Common Antidegradant Dr. Ong SK 114


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