Removing Inorganic Impurities from Polypropylene by Swelling-Complexation Process

doi:10.19894/j.issn.1000-0518.250417

Chinese Journal of Applied Chemistry ›› 2026, Vol. 43 ›› Issue (3): 422-434.DOI: 10.19894/j.issn.1000-0518.250417

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Removing Inorganic Impurities from Polypropylene by Swelling-Complexation Process

Shuai ZHANG¹, Xuan LI², Zhao-Liang XING³, Ju-Yang LIU³, Huan GAO¹, Zhe MA¹, Li PAN¹, Chong ZHANG³(), Yue-Sheng LI¹()

^1.School of Materials Science and Engineering，Tianjin University，Tianjin 300350，China
^2.State Grid Corporation of China，Beijing 100031，China
^3.Institute of New Materials for Electrical Engineering，China Electric Power Research Institute Co. ，Ltd. ，Beijing 100192，China

Received:2025-10-31 Accepted:2025-12-26 Published:2026-03-01 Online:2026-03-26
Contact: Chong ZHANG,Yue-Sheng LI
About author:ysli@tju.edu.cn
zhangc_sgri@163.com；
Supported by:
the Headquarters Science and Technology Project of State Grid Corporation of China(5500-202358774A-3-6-ZX)

Abstract

Abstract:

To address the issue of residual inorganic impurities degrading the electrical properties of insulating materials， this study proposes a swelling-chelating deashing method for purifying polypropylene （PP）. Multiple complexing agents-including aliphatic alcohols， glycol diethers， tetramethylethylenediamine （TMEDA）， and acetylacetone etc. were employed in conjunction with polymer swelling to remove inorganic impurities （Mg， Al， Ti） from commercial PP. Among them， acetylacetone exhibited the highest removal rate， which can reduce the inorganic ash content of PP from 33.88 mg/L to 9.06 mg/L， with a removal rate of 73.3%. The ash content was further reduced to 3.63 mg/L by using a compound agent （acetic acid/acetylacetone）. For a high-impurity PP grade， ash content was reduced from 50.09 mg/L to 9.68 mg/L （80.7% removal）. Differential scanning calorimetry （DSC） and electrical tests confirmed that deashing enhanced crystallinity and insulation properties： breakdown strength increased by 29.1 and 50.5 kV/mm， while resistivity rose by 2.59×1013 and 4.34×1013 Ω·m for the two PP grades （PP1， PP2）， respectively. This work provides an effective strategy to advance PP′s application in high-performance cable insulation.

Key words: Polypropylene purification, Swelling-complexation deashing, Cable insulation materials

CLC Number:

O632

Shuai ZHANG, Xuan LI, Zhao-Liang XING, Ju-Yang LIU, Huan GAO, Zhe MA, Li PAN, Chong ZHANG, Yue-Sheng LI. Removing Inorganic Impurities from Polypropylene by Swelling-Complexation Process[J]. Chinese Journal of Applied Chemistry, 2026, 43(3): 422-434.

Figures/Tables 11

References 43

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Method	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Swelling-washing only	8.13	19.38	1.78	29.29
Methanol complexation	7.65	11.43	1.22	20.30
Ethanol complexation	2.62	11.02	1.20	14.84
Benzyl alcohol complexation	3.58	8.43	0.65	12.66
Isopropanol complexation	2.65	7.30	0.95	10.90
n-Butanol complexation	4.62	8.18	1.03	13.83
Isobutanol complexation	2.76	7.05	0.93	10.74
tert-Butanol complexation	3.70	8.68	0.81	13.19
Ethylene glycol complexation	8.02	14.89	1.41	24.32
1，2-Propanediol complexation	7.70	12.58	0.67	20.95

Method	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Swelling-washing only	8.13	19.38	1.78	29.29
Methanol complexation	7.65	11.43	1.22	20.30
Ethanol complexation	2.62	11.02	1.20	14.84
Benzyl alcohol complexation	3.58	8.43	0.65	12.66
Isopropanol complexation	2.65	7.30	0.95	10.90
n-Butanol complexation	4.62	8.18	1.03	13.83
Isobutanol complexation	2.76	7.05	0.93	10.74
tert-Butanol complexation	3.70	8.68	0.81	13.19
Ethylene glycol complexation	8.02	14.89	1.41	24.32
1，2-Propanediol complexation	7.70	12.58	0.67	20.95

Method or complexing agent	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Swelling-washing only	8.13	19.38	1.78	29.29
Ethylene glycol monomethyl ether	3.76	8.27	1.10	13.13
Ethylene glycol dimethyl ether	2.32	7.98	0.90	11.20
Diethylene glycol monomethyl ether	3.35	8.07	1.08	12.50
Diethylene glycol dimethyl ether	1.74	7.67	0.88	10.29
Tetramethylethylenediamine	2.91	7.51	0.81	11.23
Pentamethyldiethylenetriamine	2.65	6.88	1.25	10.78

Method or complexing agent	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Swelling-washing only	8.13	19.38	1.78	29.29
Ethylene glycol monomethyl ether	3.76	8.27	1.10	13.13
Ethylene glycol dimethyl ether	2.32	7.98	0.90	11.20
Diethylene glycol monomethyl ether	3.35	8.07	1.08	12.50
Diethylene glycol dimethyl ether	1.74	7.67	0.88	10.29
Tetramethylethylenediamine	2.91	7.51	0.81	11.23
Pentamethyldiethylenetriamine	2.65	6.88	1.25	10.78

Method or complexing agent	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Swelling-washing only	8.13	19.38	1.78	29.29
Acetylacetone	1.46	6.85	0.75	9.06
2，3-Butanedione	8.03	8.43	1.53	17.99
Benzoylacetone	2.43	6.73	1.28	10.44
Ethyl acetoacetate	1.59	7.26	0.82	9.67
Diethyl malonate	2.36	7.38	0.97	10.71

Removing Inorganic Impurities from Polypropylene by Swelling-Complexation Process

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Sample	Melting peak temperature/℃	Melting enthalpy/（J·g^-1）	Crystallinity/%
Untreated sample PP1	157.9/164.6	95.32	45.6
PP1 swelling only	161.8	111.83	53.5
PP1 swelling-complexation	163.1	99.81	47.8
Untreated sample PP2	160.7	101.02	48.3
PP2 swelling-complexation	162.0	111.71	53.4

Sample	10^-4M_n	10^-4M_w	PDI
Untreated sample PP1	6.14	31.98	5.21
PP1 swelling only	7.54	30.30	4.02
PP1 swelling-complexation	8.13	28.63	3.52
Untreated sample PP2	8.56	32.16	3.76
PP2 swelling-complexation	9.12	32.78	3.60

Sample	Ash content/（mg·L^-1）	10^-13Volume resistivity/（Ω·m）	Breakdown strength/（kV·mm^-1）
Untreated sample PP1	33.88	8.75	421.5
PP1 swelling-complexation	3.63	11.34	450.6
Untreated sample PP2	50.09	5.88	355.6
PP2 swelling-complexation	9.68	10.22	406.1

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Method	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Acac 1 mL	1.46	6.85	0.75	9.06
Acac 5 mL	1.43	4.60	0.41	6.44
Acac 2×1 mL	1.47	4.87	0.66	6.94
Acac 4×1 mL	1.46	4.47	0.45	6.38
Acac 1 mL/Acetic acid 1 mL	1.47	2.00	0.16	3.63
Untreated sample PP2	10.52	38.45	1.12	50.09
Acac 1 mL/Acetic acid 1 mL	4.23	5.42	0.03	9.68

Method	ρ（Mg）/（mg·L^-1）	ρ（Al）/（mg·L^-1）	ρ（Ti）/（mg·L^-1）	ρ（total metal residue）/（mg·L^-1）
Untreated sample PP1	9.12	22.91	1.85	33.88
Acac 1 mL	1.46	6.85	0.75	9.06
Acac 5 mL	1.43	4.60	0.41	6.44
Acac 2×1 mL	1.47	4.87	0.66	6.94
Acac 4×1 mL	1.46	4.47	0.45	6.38
Acac 1 mL/Acetic acid 1 mL	1.47	2.00	0.16	3.63
Untreated sample PP2	10.52	38.45	1.12	50.09
Acac 1 mL/Acetic acid 1 mL	4.23	5.42	0.03	9.68