CN102683631A - 双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法 - Google Patents

双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法 Download PDF

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CN102683631A
CN102683631A CN2012101309511A CN201210130951A CN102683631A CN 102683631 A CN102683631 A CN 102683631A CN 2012101309511 A CN2012101309511 A CN 2012101309511A CN 201210130951 A CN201210130951 A CN 201210130951A CN 102683631 A CN102683631 A CN 102683631A
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范建国
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Abstract

本发明公开了一种双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,是通过以下的步骤实现的:(1)将纳米氧化铝在四异丙基二钛酸酯中进行表面预处理;(2)将预处理后的纳米氧化铝均匀混入聚合物中,加入量为聚合物质量的0.5%~2.5%,对原料进行熔融,采用双向拉伸法制成基膜;(3)将基膜在5~155℃、拉伸速度为0.1~30m/min,拉伸比为1~5倍的条件下进行拉伸形成微孔结构;(4)微孔薄膜进行再拉伸,拉伸比为0.5~1.5,100~165℃定型,形成聚烯烃微孔膜产品。本发明聚烯烃微孔隔膜产品与现有产品相比,提高隔膜的厚度均匀性、隔膜的机械强度,减小弓形效应。

Description

双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法
技术领域
本发明涉及一种双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,属于聚烯烃隔膜技术领域。
背景技术
聚丙烯隔膜是一种拉伸强度高、耐高温性能良好的隔膜制品,广泛用于工业生产,例如锂电池隔膜。这种隔膜制品需要具有阻碍电子传导的能力,现有技术通常是在聚丙烯材料中添加无机颗粒材料,如二氧化钛和二氧化硅材料,在电池正常状态下可减少电池内部漏电的能力一般,则在电池遇到高温或外界压力的紧急状况下容易发生正负极短路。原因是上述无机粒子一般分布于隔膜表面,并没有混入隔膜内部,在使用时容易脱落。
现有的聚丙烯隔膜最大的缺点是厚度不均匀,一般为了简化生产流程,不进行横向拉伸,即横向机械强度不高。在后续制备流程中,就出现了隔膜厚度不均的问题。
发明内容
本发明提供一种双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,解决了普通双向拉伸中纵向拉伸结晶取向后横向拉伸结晶难以取向的问题,提高隔膜的厚度均匀性、隔膜的机械强度,减小弓形效应。
本发明是通过以下的技术方案实现的:
一种双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,是通过以下的步骤实现的:
(1)将纳米氧化铝在四异丙基二钛酸酯中进行表面预处理,具体为将所述纳米氧化铝质量的1~4%的四异丙基二钛酸酯和聚丙烯与纳米氧化铝进行混合,在120~145℃条件下硫化20min,再在150℃条件下二次硫化3h;
(2)将预处理后的纳米氧化铝均匀混入聚丙烯中,加入量为聚丙烯质量的0.5%~2.5%,对原料进行熔融,采用双向拉伸法制成基膜,制成的基膜经70~155℃,10min~30h退火;
(3)将基膜在5~155℃、拉伸速度为0.1~30m/min,拉伸比为1~5倍的条件下进行拉伸形成微孔结构;
(4)微孔薄膜进行再拉伸,拉伸比为0.5~1.5,100~165℃定型,形成聚丙烯微孔膜产品。
采用四异丙基二钛酸酯对纳米氧化铝进行预处理,纳米氧化铝颗粒与聚丙烯之间有良好的相容性,能够在其中形成良好的分散状态。
所述步骤(2)中熔融温度为190~270℃。
所述步骤(3)中拉伸具体为:首先在10~100℃,拉伸比为1~5倍的条件下进行冷拉伸,使串晶结构破裂,形成微小银纹缺陷为止;然后在90~155℃,拉伸比为1~4倍的条件下进一步拉伸形成微孔结构。
所述步骤(3)中的微孔结构的微孔为10nm~30nm。
本发明的有益效果为:本发明隔膜平面相互垂直的两个拉伸方向的拉伸强度都比普通隔膜高,如果在隔膜的不同方向上具有相同的拉伸度,隔膜中长链分子沿平面上各方向的取向是平衡的,隔膜的性能就比较均衡。与单向拉伸相比隔膜的厚度均匀性、隔膜的机械强度都较好,可以减小弓形效应,对电池制造十分有利。
具体实施方式
以下结合实施例,对本发明做进一步说明。
实施例1
采用100Kg聚丙烯作为原料。
(1)取纳米氧化铝1Kg,取25g四异丙基二钛酸酯加入上述纳米氧化铝中,搅拌混合,放入加热釜,控制温度在120~145℃之间,硫化20min,再在控制温度为150℃条件下二次硫化3h;
(2)取上述预处理后的纳米氧化铝0.8Kg均匀混入完全100Kg聚丙烯中,对上述混合原料进行熔融,熔融温度为190~270℃,采用双向拉伸法制成基膜,制成的基膜经控制温度在70~155℃,10min~30h进行退火;
(3)将上述基膜控制拉伸比为1倍条件不变,控制20℃条件下冷拉伸,使串晶结构逐渐破裂,形成微小银纹缺陷为止;然后在拉伸比为3倍,95℃条件下进一步拉伸形成微孔结构,保证微孔在10~15nm停止;
(4)上述微孔薄膜进行再拉伸,拉伸比为0.5,控制温度在100~125℃定型10min,形成聚烯烃微孔膜产品。
实施例2
采用100Kg聚丙烯作为原料。
(1)取纳米氧化铝3Kg,取40g四异丙基二钛酸酯加入上述纳米氧化铝中,搅拌混合,放入加热釜,控制温度在120~145℃之间,硫化20min,再在控制温度为150℃条件下二次硫化3h;
(2)取上述预处理后的纳米氧化铝2.3Kg均匀混入完全100Kg聚丙烯中,对上述混合原料进行熔融,熔融温度为190~270℃,采用双向拉伸法制成基膜,制成的基膜经控制温度在70~155℃,10min~30h进行退火;
(3)将上述基膜控制拉伸比为3倍条件不变,控制55~75℃条件下冷拉伸,使串晶结构逐渐破裂,形成微小银纹缺陷为止;然后在拉伸比为4倍,125~130℃条件下进一步拉伸形成微孔结构,保证微孔在20~25nm停止;
(4)上述微孔薄膜进行再拉伸,拉伸比为1,控制温度在110~130℃定型2~4h,形成聚烯烃微孔膜产品。
实施例3
采用100Kg聚丙烯作为原料。
(1)取纳米氧化铝2Kg,取20g四异丙基二钛酸酯加入上述纳米氧化铝中,搅拌混合,放入加热釜,控制温度在120~145℃之间,硫化20min,再在控制温度为150℃条件下二次硫化3h;
(2)取上述预处理后的纳米氧化铝1.8Kg均匀混入完全100Kg聚丙烯中,对上述混合原料进行熔融,熔融温度为190~270℃,采用双向拉伸法制成基膜,制成的基膜经控制温度在70~155℃,10min~30h进行退火;
(3)将上述基膜控制拉伸比为5倍条件不变,控制80~100℃条件下冷拉伸,使串晶结构逐渐破裂,形成微小银纹缺陷为止;然后在拉伸比为3倍,140~155℃条件下进一步拉伸形成微孔结构,保证微孔在25~30nm停止;
(4)上述微孔薄膜进行再拉伸,拉伸比为1.5,控制温度在120~165℃定型6~8h,形成聚烯烃微孔膜产品。
本发明生产的微孔隔膜制品的微孔在10~30nm之间,当添加少量纳米氧化铝制成的聚丙烯隔膜,可以在较低温度下制成微孔直径较小的隔膜制品;当添加纳米氧化铝量较大时,聚丙烯隔膜可以提高制成温度,隔膜制品的微孔也可以达到30nm,其力学强度更高。本发明与单向拉伸相比隔膜的厚度均匀性、隔膜的机械强度都较好,可以减小弓形效应,对电池制造十分有利。

Claims (4)

1.一种双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,其特征在于是通过以下的步骤实现的:
(1)将纳米氧化铝在四异丙基二钛酸酯中进行表面预处理,具体为将所述纳米氧化铝质量的1~4%的四异丙基二钛酸酯和聚丙烯与纳米氧化铝进行混合,在120~145℃条件下硫化20min,再在150℃条件下二次硫化3h;
(2)将预处理后的纳米氧化铝均匀混入聚丙烯中,加入量为聚丙烯质量的0.5%~2.5%,对原料进行熔融,采用双向拉伸法制成基膜,制成的基膜经70~155℃,10min~30h退火;
(3)将基膜在5~155℃、拉伸速度为0.1~30m/min,拉伸比为1~5倍的条件下进行拉伸形成微孔结构;
(4)微孔薄膜进行再拉伸,拉伸比为0.5~1.5,100~165℃定型,形成聚丙烯微孔膜产品。
2.如权利要求1所述的双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,其特征在于所述步骤(2)中熔融温度190~270℃。
3.如权利要求1所述的双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,其特征在于所述步骤(3)中拉伸具体为:首先在10~100℃,拉伸比为1~5倍的条件下进行冷拉伸,使串晶结构破裂,形成微小银纹缺陷为止;然后在90~155℃,拉伸比为1~4倍的条件下进一步拉伸形成微孔结构。
4.如权利要求1所述的双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法,其特征在于所述步骤(3)中的微孔结构的微孔为10nm~30nm。
CN201210130951.1A 2012-04-27 2012-04-27 双向同步拉伸均匀可调微孔结构耐高温隔膜的制备方法 Expired - Fee Related CN102683631B (zh)

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103762327A (zh) * 2014-01-17 2014-04-30 苏州鼎机新能源材料科技有限公司 一种锂离子电池隔膜及其生产工艺
CN106564205A (zh) * 2016-09-30 2017-04-19 中材科技股份有限公司 一种高透湿聚四氟乙烯薄膜的制备方法

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004017367A (ja) * 2002-06-13 2004-01-22 Sumitomo Chem Co Ltd 二軸延伸機構及び該二軸延伸機構を備えた二軸延伸試験機並びに二軸延伸機
CN101695869A (zh) * 2009-10-30 2010-04-21 沧州明珠塑料股份有限公司 聚烯烃微孔膜制备方法
CN102064299A (zh) * 2010-12-25 2011-05-18 佛山塑料集团股份有限公司 一种锂离子电池用聚烯烃多层多孔隔膜及其制备方法
CN102271791A (zh) * 2009-04-17 2011-12-07 Cs特科有限公司 制造微孔聚合物膜的方法和通过其制造的微孔聚合物膜

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101710614B (zh) * 2009-12-16 2012-06-20 南京大学 介孔纳米粒子改性锂电池隔膜
CN101798183A (zh) * 2010-04-09 2010-08-11 上海交通大学 掺铝氧化锌薄膜的制备方法
JP5793332B2 (ja) * 2010-04-09 2015-10-14 川研ファインケミカル株式会社 非水電解液電池用セパレータおよびリチウムイオン二次電池
CN102412378A (zh) * 2011-09-28 2012-04-11 哈尔滨工业大学 一种聚合物电解质隔离膜、其制备方法及其应用
CN102386357B (zh) * 2011-10-28 2013-11-20 浙江南都电源动力股份有限公司 一种高性能锂离子电池聚合物复合隔膜的制备方法

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2004017367A (ja) * 2002-06-13 2004-01-22 Sumitomo Chem Co Ltd 二軸延伸機構及び該二軸延伸機構を備えた二軸延伸試験機並びに二軸延伸機
CN102271791A (zh) * 2009-04-17 2011-12-07 Cs特科有限公司 制造微孔聚合物膜的方法和通过其制造的微孔聚合物膜
CN101695869A (zh) * 2009-10-30 2010-04-21 沧州明珠塑料股份有限公司 聚烯烃微孔膜制备方法
CN102064299A (zh) * 2010-12-25 2011-05-18 佛山塑料集团股份有限公司 一种锂离子电池用聚烯烃多层多孔隔膜及其制备方法

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103762327A (zh) * 2014-01-17 2014-04-30 苏州鼎机新能源材料科技有限公司 一种锂离子电池隔膜及其生产工艺
CN103762327B (zh) * 2014-01-17 2016-07-27 苏州鼎机新能源材料科技有限公司 一种锂离子电池隔膜及其生产工艺
CN106564205A (zh) * 2016-09-30 2017-04-19 中材科技股份有限公司 一种高透湿聚四氟乙烯薄膜的制备方法
CN106564205B (zh) * 2016-09-30 2019-04-05 中材科技股份有限公司 一种高透湿聚四氟乙烯薄膜的制备方法

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