09-Chemical and Physical Changes in Lead–Acid Battery Separators Resulting from Oxidation

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The polymer matrix within the separator is oxidized, allowing PbSO4 to eventually grow to the positive plate, resulting in short-circuit.
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ELECTROCHEMICAL OXIDATION RESISTANCE
Mineral Oil Naphthenic Oil #2 Naphthenic Oil #1 Naphthenic Oil #3
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C-13 NMR ANALYSIS (AROMATICS)
Naphthenic Oil #1
Naphthenic Oil #3 Naphthenic Oil #2 Mineral Oil
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EVIDENCE OF SHEAR DEGRADATION
Soxhlet extraction
Para-xylene 138 °C
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CHAIN SCISSION VS. CROSSLINKING IN BATTERY ENVIRONMENT
More crosslinking → less extractables
UHMWPE chains are susceptible to chain scission during twin screw extrusion
Reactive end groups
Hydrogen abstraction Oxidation Crosslinking
R● + O2 → ROO●
ox time (hrs)
Polymer content : XLR > LR > STD
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PEROX 80 --- PUNCTURE STRENGTH VS EXPOSURE TIME
0.25 mm backweb
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formula 0.25 LR 0.25 std 0.25 XLR
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Puncture (N)
800 700 600
formula 0.15 LR 0.15 XLR 0.25 std
XMD % strain
500 400 300 200 100 0 0 10 20 30 40 50 60 70
ox time (hrs)
Polymer content : XLR > LR > STD
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CAN A 0.15 BW SEPARATOR BE DESIGNED TO HAVE AS GOOD OF OXIDATION RESISTANCE AS 0.25 BW ?
AGM Failure ~ 35 hrs Fully extracted 0 % oil
Polymer content : XLR > LR > STD
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ELECTROCHEMICAL OXIDATION RESISTANCE (CONT’D)
Even after aggressive oxidation test, 0.15 XLR has comparable puncture strength to pristine 0.25 STD
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PEROX 80 --- % XMD ELONGATION VS EXPOSURE TIME
0.25 mm backweb
800 700 600
formula 0.25 LR 0.25 std 0.25 XLR
XMD % strain
500 400 300 200 100 0 0 10 20 30 40 50 60 70
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PROCESS OILS ARE COMPLEX CHEMICAL MIXTURES
Naphthenic Oil #1
Naphthenic Oil #2
X X
C-13 NMR Analysis provides a chemical fingerprint for process oils
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C-13 NMR ANALYSIS (ALIPHATICS)
After 20 hrs in H2SO4 at 80 °C
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After 20 hrs in H2SO4/H2O2 at 80 °C
PEROX 80 --- % XMD ELONGATION VS EXPOSURE TIME
பைடு நூலகம்
Polymer content : XLR > LR > STD
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PEROX 80 --- PUNCTURE STRENGTH VS EXPOSURE TIME
─ ┼
5 kg Lead Weight H2SO4, 1.30 S.G. at 24°C Glass Plate Negative Electrode Lead Plate Lead Plate Positive Electrode Glass Plate Polypropylene Support
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SEPARATOR OXIDATION RESISTANCE Test methods
Perox 80 Potassium dichromate Oxidation induction time Electrochemical oxidation test High temperature battery life test
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SURFACE SEM --- BEFORE AND AFTER PEROX 80 TEST
STD Separator
BEFORE
AFTER
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SURFACE SEM --- BEFORE AND AFTER PEROX 80 TEST
XLR Separator
BEFORE
AFTER
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ELECTROCHEMICAL OXIDATION TEST
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SILICA AGGREGATES DISPERSED THROUGHOUT THE POLYMER MATRIX
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OIL EXTRACTION + SOLVENT DRYING → POROUS SEPARATOR
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UHMWPE fibrils
Silica Aggregates
DEGRADATION MECHANISM --- SHEAR
Scatterplot of Puncture (N) vs ox time (hrs)
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formula 0.15 LR 0.15 XLR 0.25 std
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Puncture (N)
10
5
0 0 10 20 30 40 50 60 70
ox time (hrs)
Polymer content : XLR > LR > STD
ENTEK International LTD ENTEK International LLC
September 9, 2015
UHMWPE GEL PROCESSING
Silica
Twin Screw Extrusion
UHMWPE
(-CH2-CH2-) x
Oil
2
UHMWPE + OIL + SHEAR → CHAIN DISENTANGLEMENT
0.15 mm backweb
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formula 0.15 LR 0.15 std 0.15 XLR 0.15 XLR no oil
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Puncture (N)
10
5
0 0 10 20 30 40 50 60 70
ox time (hrs)
Polymer content : XLR > LR > STD
Material considerations
Polymer matrix Residual oil Oil / PE ratio
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CHEMICAL RESISTANCE VS. OXIDATION RESISTANCE
Phenolic/polyester
PE/SiO2
Phenolic/glass
Separator Test Sample
5cm x 5cm specimens were overcharged with 200mA/cm² current between 2 pure lead plates at 75°C:
Three specimens per separator type Samples “fail” when ΔV/Δt ≥ 0.2 V/min
CHEMICAL AND PHYSICAL CHANGES THROUGHOUT THE LIFE OF A BATTERY SEPARATOR
R. Waterhouse, D. Walker, A. Brown, D. Merritt, J. Emanuel, E. Hostetler, C. La, C. Rogers, and R.W. Pekala
Fully Extracted 0% oil
Polymer content : XLR > LR > STD
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SUMMARY
PE/SiO2 separators are susceptible to many chemical and physical changes both during their manufacture and within a Pb-acid battery environment The polymer content and oil/PE ratio are two critical parameters in the design and engineering of battery separators ENTEK has introduced a series of LR and XLR separators that have low electrical resistance, good mechanical properties, and excellent oxidation resistance Although some OEMs are reluctant to use thinner backweb separators in their battery designs, 0.15 XLR outperforms many 0.25 STD separators.