Crosslinking of fluoroelastomers and the influence on final properties Matthias Lückmann, Wolfgang Steinhoff February, 13, 2014 Overview basics on curing of rubber and fluoroelastomers ionic cure with bisphenol requirements crosslinking mechanism effect of cross linker to accelerator ratio role of metal oxides and influences of level change activators radical cure with peroxides requirements crosslinking mechanism effect of cross linker and accelerator level on cure speed and mechanical properties different coagents Influence of peroxide level conclusions 2/11/2014 2 1
Some Basics on Curing of Rubber hydrocarbon rubbers are mainly cross-linked with systems based on sulfur or peroxides chemical crosslinking minimizes viscous flow and leads to material with high elasticity chemical crosslinks entanglements entanglements the occurring chemical bonds have following energy: C-S-C: 285 kj/mol C-S-S-C: 268 kj/mol with increasing bond energy the thermal stability is increasing special rubbers like FKM are used in demanding environments and the cross-linked materials have to withstand high temperatures and should have high chemical resistance crosslinking with bisphenol forms C-O-C bonds; bond energy C-O: 358 kj/mol crosslinking of FKM with peroxides forms C-C bonds; C-C: 352 kj/mol 2/11/2014 3 Ionic Cure vs. Peroxide Cure ionic cure: Bisphenol AF most common cross linker; phosphonium and ammonium salts most common accelerator curative Masterbatches support dispersion best scorch safety, low mould fouling and good mould release lowest compression set excellent heat resistance crosslinking of fluoroelastomers radical cure: cure sites are incorporated into the polymer G types) peroxide controls the rate of cure, DBPH most common peroxide coagent controls the number of crosslinks, TAIC most common coagent best resistance to hot water (or other aqueous fluids like coolants) improved chemical resistance (e.g. high and low ph) metal oxides not necessary but lead to higher heat resistance and more efficient cure 2/11/2014 4 2
Requirements for Curing with Bisphenol - CH 2 between two carbon (or longer) perfluorinated monomer units - easy dehydrofluorination (as for HFP:VF 2 :HFP or TFE:VF 2 :TFE) CF 3 group aids cross link formation later in the process Viton A; F-content 66% 2/11/2014 5 Requirements for Curing with Bisphenol B and F types also cured via C=C, formed at isolated VF 2 sequences but less efficiently because curatives are less soluble in the polymer - they cannot find the cure sites easily Viton B, F; F-content 66-70% HFP:VF 2 :HFP easy formation of C=C, crosslink very efficiently TFE:VF 2 :TFE 3 - H and F combine with acid acceptors to form water and metal fluorides 2/11/2014 6 3
Bisphenol Cure System Curing Mechanism 1. Formation of the soluble bisphenol monophosphoniumsalt, for nucleophilic reaction with the polymer [adapted from Schmiegel, Kautschuk Gummi Kunststoffe, 1978, 31, 137 ] 2/11/2014 7 Bisphenol Cure System Curing Mechanism 2. Creation of diene functionality in the polymer chain, through reaction of the soluble base (bisphenol monophosphoniumsalt) with the FKM (dehydrofluorination) 2/11/2014 8 4
Bisphenol Cure System Curing Mechanism 3. Crosslinking of two polymer chains 2/11/2014 9 Influence of Polymer Type on Cure Rate A types cure very quickly B types are slower F types need very high levels of accelerator Ratios for reasonable cure rates A type about 4 : 1 (BP-AF to Accelerator ratio) B type about 3 : 1 F type about 2 : 1 Test at 180C, 0.5, 3 mins Torque (dnm) 30 25 20 15 10 F type with 2.85 : 1 (BpAF : Accel) F type with 1.9 : 1 (BpAF : Accel) B type with 3.3 : 1 (BpAF : Accel) A type with 4.2 : 1 (BpAF : Accel) 5 0 0 0.5 1 2 3 Time (mins) 2/11/2014 10 5
Ratio of Bisphenol to Accelerator Impact on Properties Higher curative level results in : - higher modulus - higher hardness - lower elongation - better compression set - poorer TR-10 - better flow - better mould release Lower BP-AF : Accelerator ratio results in : - faster cure - increases scorch - poorer compression set - more mould fouling Higher BP-AF : Accelerator ratio results in : - slower cure - less scorch - better compression set 2/11/2014 11 BPAF / BTPPC Ratio Impact on Curing 2/11/2014 12 6
BPAF / BTPPC Ratio Impact on Properties BTPPC 0,6 0,5 0,4 Contour Plot of CS 70 200 vs BTPPC;; BPAF CS 70 200 < 16,5 16,5 18,0 18,0 19,5 19,5 21,0 21,0 22,5 22,5 24,0 24,0 25,5 25,5 27,0 27,0 28,5 28,5 30,0 > 30,0 0,3 0,2 1,2 1,4 1,6 BPAF 1,8 2,0 2,2 2/11/2014 13 Effect of Metal Oxides During Bisphenol Cure cure site creation and acid acceptors usual metal oxides are Ca(OH)2 and MgO : 6 phr Ca(OH)2 and 3 phr MgO effects of metal oxide levels higher Ca(OH)2 results in faster curing but poorer compression set and properties higher MgO results in better heat resistance and better bonding high metal oxide levels adversely affect for flow (injection) metal oxides promote mould sticking and fouling types of MgO usual levels are 3 phr high activity or 15 of low activity MgO (processing!!!) Metal oxides are hygroscopic and are often the cause of scorch problems 2/11/2014 14 7
Effect of Metal Oxides During Bisphenol Cure using Calcium Hydroxide to control cure rate MDR, 180C, 0.5 deg, 12 mins 25 20 Torque (dnm) 15 10 5 6 CaOH2 4 CaOH2 6 CaOH2, 0.5 VC-30 6 CaOH2, 1.0 VC-30 0 0 2 4 6 8 10 12 14 Time (mins) General Trends: reducing calcium hydroxide level from 6 to 4 phr reduces cure rate similar reduction in cure rate by adding 0.5 phr VC-30 but increasing the modulus adding 1.0 phr VC 30 significantly reduces cure rate but increases modulus adding VC-30 gives a reduced cure rate because of a higher curative to accelerator ratio 2/11/2014 15 New Bisphenol Curable Grade standard bisphenol compounds provide poor resistance to organic acids due to MO development with new cure activator and good acid resistance Viton VTR 9307 new bisphenol curable precompound test conditions: acetic acid, 504h @ 100C VTR 9307 vs Viton A331C Viton VTR 9307 FKM Copolymer FKM Terpolymer X X X X 2/11/2014 16 8
Requirements for Curing with Peroxides Viton GAL-S, GBL-S, GF-S 65.5-70%F peroxide cured fluoroelastomers for best hot water resistance, improved acid and base resistance and low post cure capability bromine or iodine containing cure site monomers have to be incorporated no need of metal oxides but could be incorporated for improved cure efficiency and heat resistance Viton GLT-S, GBLT-S, GFLT-S 6-67%F 2/11/2014 17 Peroxide Cure System Curing Mechanism 1. t-butoxy radical generation and beta scission to the methyl radical and acetone 2/11/2014 18 9
Peroxide Cure System Curing Mechanism 2. Adding methyl radical to TAIC and abstracting bromine from polymer chain more likely the TAIC radical polymerizes 2/11/2014 19 Peroxide Cure System Curing Mechanism 1. polymer radical is formed by reaction of methyl radical with CSM 2. polymer radical reacts with the (poly)coagent Reality: TAIC Oligomer with some FKM crosslinks 2/11/2014 20 10
Peroxide Cure System Idealized Mechanism 3. Reaction of the polymer radical with TAIC 4. The coagent provides three potential network points 2/11/2014 21 Peroxide Cure System Peroxide / Coagent Ratio Impact on Curing Basic recipe = 100 phr Viton GLT-600S, 30 phr N-990, 3 phr ZnO, 0.5 phr VPA #2, Peroxide and Coagent are variables ts1 Ts1 mins vs Diak 7, Luperox 101XL45 0.414 0.390 0.366 MH MH dnm vs Diak 7, Luperox 101XL45 28.01 27.38 0.342 26.75 0.402 26.12 25.49 24.86 24.23 1.0 1.2 0.378 0.354 1.4 1.6 1.8 2.2 Luperox 101XL45 phr 2.4 1.0 1.2 23.60 1.4 1.6 1.8 2.2 Luperox 101XL45 phr 2.4 tc50 Tc-50 mins vs Diak 7, Luperox 101XL45 0.64 0.72 0.56 tc90 Tc-90 mins vs Diak 7, Luperox 101XL45 1.3 1.1 0.9 0.76 1.4 0.68 0.60 0.52 1.2 1.0 1.2 1.4 1.6 1.8 2.2 2.4 1.0 1.2 1.4 1.6 1.8 2.2 2.4 2/11/2014 Luperox 101XL45 phr Luperox 101XL45 phr PRESENTATION TITLE 22 1.0 0.8 11
Peroxide Cure System Peroxide / Coagent Ratio Impact on Properties Basic recipe = 100 phr Viton GLT-600S, 30 phr N-990, 3 phr ZnO, 0.5 phr VPA #2, Peroxide and Coagent are variables Tensile Tensile Strength vs Diak 7, Luperox 101XL45 22.1 22.9 Elongation Elongation @ Break vs Diak 7, Luperox 101XL45 296 280 20.5 21.3 312 288 18.9 19.7 1.0 1.2 1.4 1.6 1.8 2.2 Luperox 101XL45 phr 2.4 304 328 320 1.0 1.2 1.4 1.6 1.8 2.2 2.4 Luperox 101XL45 phr Duro Shore A pts vs Diak 7, Luperox 101XL45 70.8 69.6 70.2 Comp Set Comp Set % vs Diak 7, Luperox 101XL45 (70hrs @ 200C) 20.0 21.2 20.6 21.8 68.4 69.0 22.4 2 67.2 67.8 23.6 24.2 1.0 1.2 1.4 1.6 1.8 2.2 2.4 2/11/2014 1.0 1.2 1.4 1.6 1.8 2.2 2.4 23 Luperox 101XL45 phr Luperox 101XL45 phr Influence of Different Coagents on Cure Speed - Coagent controls the number of cross links - the most common coagents are - Triallylisocyanurate (TAIC) should be used with Viton APA polymers - Trimethallylisocyanurate (TMAIC) not recommended for Viton APA polymers - Triallylcyanurate (TAC) sometimes used - TMAIC and TAC - will give slow and inefficient curing with Viton APA polymers 30 25 TAIC - 3 Torque (dnm) 20 15 10 TMAIC - 1 TMAIC - TMAIC - 2 5 0 0 1 2 3 4 TAC 3 Time (mins) 2/11/2014 24 12
Conclusions two relevant curing mechanisms ionic and radical Ionic Mechanism Bisphenol AF cure for high efficiency,vf 2 units surrounded by HFP units BTPPC and BPAF commonly used for ionic cure ratio of BPAF and BTPPC influences cure rate, cure state and mechanical properties Novel BPAF cure system overcomes deficiencies in dilute acids radical cure requires cure site monomer that contains bromine or iodine most common peroxide and coagent are DBPH and TAIC cure rate depends mainly on peroxide level cure state depends mainly on coagent level properties influenced by ratio of peroxide and coagent 2/11/2014 25 The information set forth herein is furnished free of charge and is based on technical data that DuPont believes to be reliable and falls within the normal range of properties. It is intended for use by persons having technical skill, at their own discretion and risk. This data should not be used to establish specification limits nor used alone as the basis of design. Handling precaution information is given with the understanding that those using it will satisfy themselves that their particular conditions of use present no health or safety hazards. Since conditions of product use and disposal are outside our control, we make no warranties, express or implied, and assume no liability in connection with any use of this information. As with any product, evaluation under end-use conditions prior to specification is essential. Nothing herein is to be taken as a license to operate or a recommendation to infringe on patents. Caution: Do not use in medical applications involving permanent implantation in the human body. For other medical applications, discuss with your DuPont customer service representative and read Medical Caution Statement H-50103-3. Copyright 2014 DuPont. All rights reserved. DuPont DuPont de Nemours or its affiliates. 13