CN111286070B - 一种超临界流体注塑发泡聚乳酸泡沫材料及其制备方法 - Google Patents

一种超临界流体注塑发泡聚乳酸泡沫材料及其制备方法 Download PDF

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CN111286070B
CN111286070B CN202010086835.9A CN202010086835A CN111286070B CN 111286070 B CN111286070 B CN 111286070B CN 202010086835 A CN202010086835 A CN 202010086835A CN 111286070 B CN111286070 B CN 111286070B
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polylactic acid
foaming
cellulose nanofiber
preparation
supercritical fluid
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CN111286070A (zh
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汪龙
郑文革
任倩
张利华
吴明辉
吴飞
罗海斌
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Ningbo Institute of Material Technology and Engineering of CAS
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Abstract

本发明公开了一种超临界流体注塑发泡聚乳酸泡沫材料及其制备方法。本发明利用生物填料纤维素纳米纤维来改性制备轻质高强聚乳酸发泡材料，通过利用纤维素纳米纤维作为成核剂来改善聚乳酸材料的结晶性能与熔体性能。将制备的纤维素纳米纤维改性聚乳酸复合材料与超临界流体发泡剂在微孔发泡注塑机中混合均匀，结合开合模发泡装置，进行微孔注塑发泡成型得到全生物基聚乳酸发泡材料。本发明制备了泡孔尺寸均匀、泡孔密度高、性能良好的微孔发泡纤维素纳米纤维/聚乳酸复合材料，且制备方法具有生产效率高、可操作性强、稳定可靠，易于工业化生产等突出优势。

Description

一种超临界流体注塑发泡聚乳酸泡沫材料及其制备方法
技术领域
本发明涉及高分子材料加工技术领域，具体涉及一种超临界流体注塑发泡聚乳酸泡沫材料及其制备方法。
背景技术
聚乳酸(Polylactide，PLA)是一种热塑性脂肪族聚酯，具有良好的力学性能、加工性能、透明性和独特的阻隔性能。PLA主要通过丙交酯开环聚合得到，来源于可再生资源如小麦、玉米、谷物、稻秆等中的淀粉和糖类。PLA使用废弃后完全可生物降解，最终分解产物为二氧化碳和水，不会对环境产生任何污染，是被广泛关注的绿色环保材料，有望替代传统石油基塑料被广泛应用于各个领域。近年来，聚乳酸发泡材料成为发泡材料领域研究的热点，有望广泛的用于日用品和包装等领域，取代传统的石油基泡沬塑料，解决困扰多年的“白色污染”问题。
然而，PLA的分子链呈现半刚性，导致其熔体强度低、结晶速率慢，可发泡性差，一般在发泡前需要先对PLA进行增强改性。目前提高PLA发泡能力的方法主要有如下三种：第一种方法是在PLA中加入另外一种聚合物形成聚合物共混物，第二组分的加入会在一定程度上改善聚乳酸的发泡性能；第二种方法是改变PLA分子链结构，如通过加入扩链剂和交联剂交联，比如公开号为CN1958668A的专利说明书公开了一种吸塑用聚乳酸泡沫塑料片材及制备方法，特点在于采用化学发泡剂，由挤出机挤出发泡、成型得到。第三种方法是加入填料，这是一种非常有效的改善PLA发泡能力的手段，同时填料的加入能增强PLA泡沫的机械性能。如公开号为CN103554858A的专利说明书公开了一种利用纳米黏土来改善PLA发泡能力的方法，得到了高发泡倍率的PLA发泡制品。但是，这些非生物基填料的加入会影响PLA材料的生物特性，降低材料的可降解性。
值得注意的是，目前PLA泡沫材料的开发还是传统化学发泡剂和物理发泡剂比如烷烃类作为发泡剂，所得泡沫泡孔尺寸大、力学性能差，此外大多数发泡工艺采用间歇式发泡工艺，难以大规模工业化应用。为此，有必要研究一种新的绿色轻质高强PLA泡沫材料的制备方法，以期解决上述问题。
发明内容
针对本领域存在的不足之处，本发明提供了一种超临界流体注塑发泡聚乳酸泡沫材料的制备方法，制备工艺简单，生产效率高，具有良好的工业化前景，且原料完全为生物可降解材料，所得PLA发泡材料泡孔尺寸小、泡孔密度高，泡孔均匀、发泡倍率高。
与纤维素纳米晶相比，纤维素纳米纤维具有更大长径比、更高表面积、以及更易相互交织成网状缠结结构等优点，从而更有利于对聚合物材料的增强以及熔体性能改善，目前还没有利用纤维素纳米纤维来改性制备聚乳酸发泡材料的研究。本发明中纤维素纳米纤维的直径为5～50nm，长径比为100～1000。
一种超临界流体注塑发泡聚乳酸泡沫材料的制备方法，包括步骤：
(1)将干燥的纤维素纸浆放入有机溶剂中然后在室温下搅拌得到改性纤维素纳米纤维(Cellulosic Nanofiber，CNF)浆液，依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液；
(2)将聚乳酸通过双螺杆挤出机的主加料口加入，再将所述纤维素纳米纤维水溶液由所述双螺杆挤出机的辅助进料口加入，熔融共混挤出，同时通过所述双螺杆挤出机料筒上的排气口排除水蒸气，所得挤出料再次经所述双螺杆挤出机熔融共混，最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料；
(3)将所述聚乳酸/纤维素纳米纤维复合材料置于微孔发泡注塑机中塑化熔融，利用超临界流体设备注入超临界流体发泡剂，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)将所述聚合物熔体注射到模具的型腔里，经保压、开合模、发泡，冷却成型，得到轻质高强聚乳酸泡沫材料。
由于PLA熔体强度低，在发泡过程中容易形成大泡孔和不均匀泡孔，因此，本申请加入了纤维素纳米纤维(CNF)作为纳米填料来改善PLA的熔体强度和结晶性能，CNF的加入能提供异相成核点，提高泡孔数量和泡孔密度，改善泡孔结构。此外，本申请中PLA/CNF复合材料采用双螺杆挤出机熔融加工共混制备，制备简单，可大批量生产，具有良好的工业化应用前景。
本发明除了在原料上进行改进，还在工艺设备上使用了微孔发泡注塑技术。传统超临界流体发泡耗时长，为非连续性生产(间歇性生产)，生产周期长，而本发明上述步骤(3)、(4)均可在微孔发泡注塑机中连续性完成，耗时短，只需几分钟甚至几十秒即可得到高性能的泡沫材料产品。
作为优选，步骤(1)中，所述有机溶剂为甲苯、乙酸、高氯酸与乙酸酐的混合液，其中甲苯、乙酸、高氯酸与乙酸酐的体积比为100:80:0.2～1.2:1～10，所述改性纤维素纳米纤维浆液中纤维素纳米纤维的质量分数为5％～40％，所述室温下搅拌的时间为1～5小时。
作为优选，步骤(1)中，所述纤维素选自苎麻纤维、亚麻纤维、剑麻纤维、黄麻纤维、大麻纤维、竹纤维中的一种。
作为优选，步骤(2)中，所述聚乳酸为左旋聚乳酸和/或右旋聚乳酸，重均分子量为10～40万。
作为优选，步骤(2)中，所述聚乳酸/纤维素纳米纤维复合材料中纤维素纳米纤维的质量分数为1％～20％。
作为优选，步骤(2)中，第一次熔融共混时料筒温度为93～99℃，螺杆转速为100～200rpm；第二次熔融共混时料筒温度为140～180℃，螺杆转速为100～200rpm。
作为优选，步骤(3)中，所述超临界流体发泡剂选自超临界二氧化碳、超临界氮气中的至少一种。
进一步优选，所述超临界流体发泡剂为超临界二氧化碳，所述超临界二氧化碳占所述聚合物熔体的质量分数为3％～8％；或者，
所述超临界流体发泡剂为超临界氮气，所述超临界氮气占所述聚合物熔体的质量分数为0.2％～0.8％。
步骤(3)中，所述超临界流体发泡剂的压力为18～30MPa，优选为20～28MPa，所述微孔发泡注塑机的螺杆温度为160～240℃，优选为180～220℃。
步骤(4)中，所述注射的速度为1～300mm/s，优选为70～150mm/s，所述模具的温度为40～90℃，优选为40～70℃，所述保压的压力为5～100MPa，优选为20～60MPa，所述保压的时间为0.2～10s，优选为3～7s，所述冷却的时间为5～300s，优选为40～110s。所述开合模的速度优选为10～30mm/s，所述发泡倍率优选为2～20倍。
本发明还提供了所述的制备方法制备得到的轻质高强聚乳酸泡沫材料。所述轻质高强聚乳酸泡沫材料是一种全生物基泡沫材料，为三明治结构，两个外表面层分别为完全不含泡沫的实心层，夹芯层为具有泡孔结构的泡沫层；
所述轻质高强聚乳酸泡沫材料的密度为0.05～0.7g/cm3，比拉伸强度最大可达60MPa，比杨氏模量最大可达2.2GPa。
所述轻质高强聚乳酸泡沫材料可应用于汽车、轨道交通、通讯电子、建筑保温、包装运输、航空航天等领域。
本发明利用生物填料纤维素纳米纤维来改性制备轻质高强聚乳酸发泡材料，通过利用纤维素纳米纤维作为成核剂来改善聚乳酸材料的结晶性能与熔体性能。将制备的纤维素纳米纤维改性聚乳酸复合材料与超临界流体发泡剂在注塑机中混合均匀，结合开合模发泡装置，进行微孔注塑发泡成型得到全生物基聚乳酸发泡材料。本发明制备了泡孔尺寸均匀、泡孔密度高、性能良好的微孔发泡纤维素纳米纤维/聚乳酸复合材料，且制备方法具有生产效率高、可操作性强、稳定可靠，易于工业化生产等突出优势。
本发明与现有技术相比，主要优点包括：
(1)本发明利用乙酸酐来表面改性CNF，该复合材料具有高强度和高模量。同时，利用双螺杆挤出共混的方法制备PLA/CNF复合材料，制备简单，适合大批量工业化生产。
(2)利用CNF一方面可以在PLA中形成填料网络，改善PLA的熔体强度；另一方面，CNF还可以作为PLA的结晶成核剂，促进PLA的结晶性能。为此，CNF的加入能显著提高PLA的发泡能力。
(3)本发明利用微孔注塑成型技术制备PLA泡沫，其泡孔尺寸、泡孔密度、发泡倍率和力学性能都可精确调控，生产效率高，能满足不同的应用需求。
(4)本发明操作简单、发泡效率高、生产稳定可靠、实用性广，易于工业化生产。
附图说明
图1为实施例2中原料聚乳酸(PLA)和制备的聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料的流变曲线图；
图2为实施例2所得聚乳酸泡沫材料的扫描电子显微镜(SEM)照片。
具体实施方式
下面结合附图及具体实施例，进一步阐述本发明。应理解，这些实施例仅用于说明本发明而不用于限制本发明的范围。下列实施例中未注明具体条件的操作方法，通常按照常规条件，或按照制造厂商所建议的条件。
本发明中流变曲线、泡孔密度、泡孔直径的测试方法如下：
流变曲线：所用仪器为AR2000EX应力控制型流变仪，测试所用夹具为25mm的平板夹具。测试所采用的间距值为1.0mm，并采用了应变控制模式。为保证所有材料试样的流变性质测定都是在线性粘弹区内进行的，所选应变量为1.0％，并在180℃温度下进行动态频率扫描测试。
泡沫密度：按照GB/T6343-2009标准采用排水法测量泡沫材料的密度。
泡孔直径：将微孔发泡材料经液氮淬断，断面喷金后，采用扫描电子显微镜(SEM)观察发泡材料内部的泡孔结构。采用Image J软件测量泡孔尺寸。
本发明采用日本制钢所(JST)180吨锁模力的注塑机(微孔发泡注塑机)以及美国Trexel公司开发的MuCell增压设备(SII-TR-35A)来制备微孔发泡注塑制品。
实施例1
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:0.2:1的混合液中，其中纤维素纳米纤维的质量分数为5％，在室温下搅拌1小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为40万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维水溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为99℃，螺杆转速为100rpm；第二次挤出共混料筒温度为140℃，螺杆转速为140rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为1％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为180℃，利用超临界流体设备注入超临界氮气，氮气压力为20MPa，含量为聚合物熔体的0.2wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为70mm/s，模具温度为40℃，保压压力为20MPa，待3s保压时间之后，动模具一侧以开合模速度为10mm/s快速打开，发泡倍率设为2倍，开始发泡过程，发泡结束后冷却40s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
实施例2
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:0.4:3的混合液，其中纤维素纳米纤维的质量分数为10％，在室温下搅拌2小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为25万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为97℃，螺杆转速为130rpm；第二次挤出共混料筒温度为150℃，螺杆转速为150rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为5％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为190℃，利用超临界流体设备注入超临界氮气，氮气压力为22MPa，含量为聚合物熔体的0.5wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为100mm/s，模具温度为50℃，保压压力为30MPa，待4s保压时间之后，动模具一侧以开合模速度为20mm/s快速打开，发泡倍率设为5倍，开始发泡过程，发泡结束后冷却60s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
实施例3
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:0.6:5的混合液，其中纤维素纳米纤维的质量分数为20％，在室温下搅拌3小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为15万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为95℃，螺杆转速为160rpm；第二次挤出共混料筒温度为160℃，螺杆转速为200rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为10％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为210℃，利用超临界流体设备注入超临界氮气，氮气压力为24MPa，含量为聚合物熔体的0.8wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为120mm/s，模具温度为60℃，保压压力为40MPa，待5.5s保压时间之后，动模具一侧以开合模速度为30mm/s快速打开，发泡倍率设为10倍，开始发泡过程，发泡结束后冷却80s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
实施例4
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:1.2:10的混合液，其中纤维素纳米纤维的质量分数为40％，在室温下搅拌5小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为10万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为93℃，螺杆转速为200rpm；第二次挤出共混料筒温度为180℃，螺杆转速为200rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为20％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为220℃，利用超临界流体设备注入超临界二氧化碳，二氧化碳压力为28MPa，含量为聚合物熔体的8wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为150mm/s，模具温度为70℃，保压压力为60MPa，待7s保压时间之后，动模具一侧以开合模速度为20mm/s快速打开，发泡倍率设为10倍，开始发泡过程，发泡结束后冷却110s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
实施例5
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:0.4:3的混合液，其中纤维素纳米纤维的质量分数为10％，在室温下搅拌2小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为25万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为97℃，螺杆转速为130rpm；第二次挤出共混料筒温度为150℃，螺杆转速为150rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为5％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为190℃，利用超临界流体设备注入超临界二氧化碳，二氧化碳压力为24MPa，含量为聚合物熔体的3wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为100mm/s，模具温度为50℃，保压压力为30MPa，待4.5s保压时间之后，动模具一侧以开合模速度为20mm/s快速打开，发泡倍率设为2倍，开始发泡过程，发泡结束后冷却60s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
实施例6
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:0.4:3的混合液，其中纤维素纳米纤维的质量分数为10％，在室温下搅拌2小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为25万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为97℃，螺杆转速为130rpm；第二次挤出共混料筒温度为150℃，螺杆转速为150rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为5％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为190℃，利用超临界流体设备注入超临界二氧化碳，二氧化碳压力为26MPa，含量为聚合物熔体的5wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为100mm/s，模具温度为50℃，保压压力为30MPa，待4s保压时间之后，动模具一侧以开合模速度为20mm/s快速打开，发泡倍率设为12倍，开始发泡过程，发泡结束后冷却60s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
实施例7
(1)将干燥的纤维素纸浆放入甲苯、乙酸、高氯酸与乙酸酐体积比为100:80:0.4:3的混合液，其中纤维素纳米纤维的质量分数为10％，在室温下搅拌2小时得到改性纤维素纳米纤维浆液，然后依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液。
(2)将重均分子量为25万的PLA通过双螺杆挤出机的主加料口加入，再将上述改性纤维素纳米纤维溶液由挤出机的辅助进料口加入，利用螺杆的高剪切力熔融共混，同时通过挤出机料筒上的排气口排除水蒸气，重复挤出2次，第一次挤出共混料筒温度为97℃，螺杆转速为130rpm；第二次挤出共混料筒温度为150℃，螺杆转速为150rpm；最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维(PLA/CNF)复合材料，其中纤维素纳米纤维的质量分数为5％；
(3)将上述PLA/CNF复合材料置于微孔发泡注塑机中塑化熔融，熔融温度为190℃，利用超临界流体设备注入超临界二氧化碳，二氧化碳压力为28MPa，含量为聚合物熔体的8wt％，在螺杆中溶解混合，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)利用注塑机螺杆将上述熔体经由喷嘴注入模具的型腔里，注射速度为100mm/s，模具温度为50℃，保压压力为30MPa，待3.5s保压时间之后，动模具一侧以开合模速度为20mm/s快速打开，发泡倍率设为20倍，开始发泡过程，发泡结束后冷却60s后，模具顶出制件，得到纤维素纳米纤维增强聚乳酸泡沫材料。
本发明实施例1～3和实施例7的聚乳酸泡沫材料的密度、平均泡孔直径的测试结果如表1所示。
表1
实施例1 实施例2 实施例3 实施例7
密度(g/cm<sup>3</sup>) 0.607 0.248 0.128 0.062
平均泡孔直径(μm) 8.2±1.4 28.9±4.3 83.5±8.5 115.6±11
从表1可知，本发明制备的PLA泡沫材料的密度可在大范围(0.06～0.6g/cm3)内的精确调控，且泡沫具有均匀细小的泡孔结构。图1为实施例2制备的PLA/CNF复合材料的流变曲线，可以看到CNF的加入极大增强了PLA的熔体性能，所得PLA/CNF复合材料具有更佳的粘度和强度。图2为实施例2制备的聚乳酸泡沫材料内部泡孔结构的SEM照片。
此外应理解，在阅读了本发明的上述描述内容之后，本领域技术人员可以对本发明作各种改动或修改，这些等价形式同样落于本申请所附权利要求书所限定的范围。

Claims (10)

1.一种超临界流体注塑发泡聚乳酸泡沫材料的制备方法，其特征在于，包括步骤：
(1)将干燥的纤维素纸浆放入有机溶剂中然后在室温下搅拌得到改性纤维素纳米纤维浆液，依次用乙醇、去离子水清洗，得到表面改性的纤维素纳米纤维水溶液；纤维素纳米纤维的直径为5～50nm，长径比为100～1000；
(2)将聚乳酸通过双螺杆挤出机的主加料口加入，再将所述纤维素纳米纤维水溶液由所述双螺杆挤出机的辅助进料口加入，熔融共混挤出，同时通过所述双螺杆挤出机料筒上的排气口排除水蒸气，所得挤出料再次经所述双螺杆挤出机熔融共混，最后经挤出、水下冷却、切粒得到聚乳酸/纤维素纳米纤维复合材料；第一次熔融共混时料筒温度为93～99℃，第二次熔融共混时料筒温度为140～180℃；
(3)将所述聚乳酸/纤维素纳米纤维复合材料置于微孔发泡注塑机中塑化熔融，利用超临界流体设备注入超临界流体发泡剂，得到单一均相的聚合物/气体混合的聚合物熔体；
(4)将所述聚合物熔体注射到模具的型腔里，经保压、开合模、发泡，冷却成型，得到轻质高强聚乳酸泡沫材料；所述发泡倍率为2～20倍；所述轻质高强聚乳酸泡沫材料的密度为0.05～0.7g/cm3。
2.根据权利要求1所述的制备方法，其特征在于，步骤(1)中，所述有机溶剂为甲苯、乙酸、高氯酸与乙酸酐的混合液，其中甲苯、乙酸、高氯酸与乙酸酐的体积比为100:80:0.2～1.2:1～10，所述改性纤维素纳米纤维浆液中纤维素纳米纤维的质量分数为5％～40％，所述纤维素纳米纤维的直径为5～50nm，长径比为100～1000，所述室温下搅拌的时间为1～5小时。
3.根据权利要求1所述的制备方法，其特征在于，步骤(1)中，所述纤维素选自苎麻纤维、亚麻纤维、剑麻纤维、黄麻纤维、大麻纤维、竹纤维中的一种；
步骤(2)中，所述聚乳酸为左旋聚乳酸和/或右旋聚乳酸，重均分子量为10～40万。
4.根据权利要求1所述的制备方法，其特征在于，步骤(2)中，所述聚乳酸/纤维素纳米纤维复合材料中纤维素纳米纤维的质量分数为1％～20％。
5.根据权利要求1所述的制备方法，其特征在于，步骤(2)中，第一次熔融共混时螺杆转速为100～200rpm；第二次熔融共混时螺杆转速为100～200rpm。
6.根据权利要求1所述的制备方法，其特征在于，步骤(3)中，所述超临界流体发泡剂选自超临界二氧化碳、超临界氮气中的至少一种。
7.根据权利要求6所述的制备方法，其特征在于，所述超临界流体发泡剂为超临界二氧化碳，所述超临界二氧化碳占所述聚合物熔体的质量分数为3％～8％；或者，
所述超临界流体发泡剂为超临界氮气，所述超临界氮气占所述聚合物熔体的质量分数为0.2％～0.8％。
8.根据权利要求1所述的制备方法，其特征在于，步骤(3)中，所述超临界流体发泡剂的压力为18～30MPa，所述微孔发泡注塑机的螺杆温度为160～240℃。
9.根据权利要求1所述的制备方法，其特征在于，步骤(4)中，所述注射的速度为1～300mm/s，所述模具的温度为40～90℃，所述保压的压力为5～100MPa，时间为0.2～10s，所述开合模的速度为10～30mm/s，所述冷却的时间为5～300s。
10.根据权利要求1～9任一权利要求所述的制备方法制备得到的轻质高强聚乳酸泡沫材料，其特征在于，为三明治结构，两个外表面层分别为完全不含泡沫的实心层，夹芯层为具有泡孔结构的泡沫层；
所述轻质高强聚乳酸泡沫材料的最大比拉伸强度为60MPa，最大比杨氏模量为2.2GPa。
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