Fabrication And Tribology Of Fluoroelastomer-based Blends Enhanced With Multiwall Carbon Nanotubes For Stator Of Oil Extraction Progressive Cavity Pump

The use of oil extraction progressive cavity pump(PCP)has increased gradually over last two decades due to its ability to handle high viscosity oil with sand,gases and wax.PCP has low capital and operating cost,and it is considered the only oil lifting means for heavy oil and directional well applications.The pumping elements of PCP consist of one start helix metallic rotor turns inside two start helix elastomeric stator,with an interference fit exist between them.The stator is a quick-wear part in the oil extraction PCP due to the wear by steel rotor,action of chemicals and solids in the crude oil and high temperature at downhole.The common oil-field elastomers nitrile rubber(NBR),hydrogenated nitrile rubber(HNBR)and fluoroelastomer(FKM)are used for stator of oil extraction PCP due to their good mechanical and chemical resistance properties.FKM has superior chemical resistance and thermal stability than most oil-field elastomers.However,FKM is very expensive compared to NBR and HNBR.It is also quite difficult to be reinforced with filler due to its high viscosity and spatial shielding effect.Blending of elastomer is a feasible technique to obtain commercially valuable products by improving properties and/or reducing the cost.In addition,reinforcement of elastomer with carbon nanotube(CNT)is an effective method to improve their performance.The main problem in the fabrication of elastomer/CNT nanocomposite is how to disperse the CNTs into the matrix,because CNTs tend to form agglomerates due to their strong van der waals force.The aim of the research is to develop FKM-based blend nanocomposites with best tribological performance that meets requirement of oil extraction PCP.Firstly,FKM were blended with NBR and HNBR to obtain optimum ratio and blending method for best tribological properties.Then,effect of pristine,oxidized and fluorinated multiwall carbon nanotubes(MWCNTs)on mechanical,thermal and tribological properties of FKM/CB/MWCNT hybrid nanocomposites were investigated.Finally,MWCNT with best functionalized method were incorporated to optimum ratio of FKM/NBR and FKM/HNBR blends,and the mechanical and tribological parameters of blend nanocomposites under dry,un-sandy crude oil and sandy crude oil were determined.The results of FKM/HNBR blend(100/0,75/25,50/50,25/75 and 0/100)prepared by dry and melt methods showed that melt mixed methods showed more compatibility and finer phase morphology than dry-mixed methods.This resulted in better mechanical,thermal,crude oil swelling and dry sliding wear properties for melt-mixed than dry-mixed ones.The performance of both dry-mixed and melt-mixed blends was found to decline gradually with increased HNBR ratio.The optimum blending method and ratio was noticed for melt-mixed FKM/HNBR(75/25).It exhibited mechanical properties superior to both blend component,and specific wear rate very close to FKM.Based on previous results of FKM/HNBR blends,melt-mixed FKM/NBR blends with ratios of(90/10,80/20,70/30,60/40 and 50/50)and same hardness were prepared.All blends showed co-continuous morphology of 200-400 nm phase size,except FKM/NBR(90/10)which exhibited partially continuous phase morphology of 100-250 nm phase size.The results of swelling and sliding wear under crude oil revealed that FKM/NBR(90/10 and 80/20)have crude oil swelling percentage and specific wear rate very close to FKM.The main mechanochemical effect of unswollen and swollen worn surfaces was fracture of macromolecular chains,in addition to oxygenated degradation detected with increasing NBR ratio in the blends.The fracture of macromolecular chains resulted in slight fatigue wear mechanism,which was also observed in the morphology of the worn surfaces.Pristine,oxidized and fluorinated MWCNTs were used with carbon black(CB)as hybrid fillers to reinforce FKM.The dispersion of pristine MWCNT within FKM matrix was very poor,resulted in decline of mechanical and tribological properties of nanoomposites with increasing pristine MWCNT.The reason for this is that the pristine MWCNT agglomerates in nanocomposites generated stress concentrations points inside matrix.The dispersion of oxidized and fluorinated MWCNTs were significantly enhanced compared to pristine MWCNTs,especially fluorinated MWCNTs.there was also a remarkable increase in the cross-linking density of the nanocomposites associated with increase of the fluorinated-MWCNTs.Therefore,FKM/CB/fluorinated-MWCNT nanocomposites exhibited better mechanical,thermal and sliding wear properties than FKM/CB/oxidized-MWCNT ones.The mechanical and tribological parameters were found to enhance with increasing oxidized and fluorinated MWCNTs to a certain ratio,then began to decline gradually.The optimum mechanical and tribological properties of FKM/CB/fluorinated-MWCNT were at CB/fluorinated-MWCNT loading of 20/10 and 17.5/12.5,respectively.In contrast,the optimum properties of FKM/CB/oxidized-MWCNT were at CB/oxidized-MWCNT loading of 25/5.The main wear mechanism for the FKM/treated-MWCNTs nanocomposites was fatigue,compared to adhesive wear marks appeared in the nanocomposites with a high ratio of pristine-MWCNT.Flourinated-MWCNTs which afford significant enhanced properties to FKM were incorporated with CB into melt-mixed FKM/NBR(80/20)blend and melt-mixed FKM/HNBR(80/20)blend.The well dispersion of Flourinated-MWCNTs into both phases of the blends was achieved by mixing F-MWCNTs firstly with FKM then mixing with NBR or HNBR.All blends exhibited compatible blend systems.The co-cont:inuous phase morphology of blend nanocomposites was found to become finer with increasing Flourinated-MWCNTs.The contact angle of crude oil on the surfaces of blends nanocomposites show that increasing of F-MWCNTs enhanced wettability of these surfaces to crude oil.The optimum tribological properties of blend nanocomposites were at CB/fluorinated-MWCNT loading of 15/15,compared to 17.5/12.5 for FKM/CB/F-MWCNT nanocomposites.This means that blend FKM with NBR or HNBR enhanced the compatibility of nanocomposite blends to F-MWCNTs.The existence of the sand in crude oil transformed the two-body fatigue wear mechanism of nanocomposites to three-body abrasion and increased the COF and specific wear rate of the nanocomposites.F-MWCNTs exhibited better resistance to sand effect on tribological behaviors of blend nanocomposites.