生物工程学报  2023, Vol. 39 Issue (5): 2053-2069
http://dx.doi.org/10.13345/j.cjb.230033
中国科学院微生物研究所、中国微生物学会主办
0

文章信息

张宗豪, 何宏韬, 张旭, 郑爽, 郑陶然, 刘絮, 陈国强
ZHANG Zonghao, HE Hongtao, ZHANG Xu, ZHENG Shuang, ZHENG Taoran, LIU Xu, CHEN Guoqiang
塑料的降解与可降解塑料——聚羟基脂肪酸酯的合成
The degradation of plastics and the production of polyhydroxyalkanoates (PHA)
生物工程学报, 2023, 39(5): 2053-2069
Chinese Journal of Biotechnology, 2023, 39(5): 2053-2069
10.13345/j.cjb.230033

文章历史

Received: January 14, 2023
Accepted: March 16, 2023
塑料的降解与可降解塑料——聚羟基脂肪酸酯的合成
张宗豪1,5 , 何宏韬1 , 张旭3,4 , 郑爽4 , 郑陶然6 , 刘絮6 , 陈国强1,2,3     
1. 清华大学生命科学学院, 北京 100084;
2. 清华大学合成与系统生物学中心, 北京 100084;
3. 清华大学化工系, 北京 100084;
4. 清华大学医学院, 北京 100084;
5. 青海大学畜牧兽医科学院, 青海 西宁 810016;
6. 北京微构工场生物技术有限公司, 北京 101309
摘要:近年来,塑料污染的问题始终困扰着人类社会。为了解决不可回收的塑料带来的环境问题,“降塑再造”的理念被提出。“降塑再造”主要包括塑料的降解和塑料的再生。而再生成为可降解的聚羟基脂肪酸酯(polyhydroxyalkanoates, PHA)则是实现塑料内循环的一种方式。PHA是一种可由多种微生物合成的生物聚酯,以其特有的生物相容性和可降解性以及热加工性能而被大家所关注。同时利用PHA的多样化的单体组成、加工技术和改性方法,可以进一步改善PHA的性能,产生类型多样、性能各异的PHA材料,也可以创造平衡耐久性和生物降解性的新产品,这些特性使PHA有望成为传统塑料的替代品之一。利用极端微生物进行生产的“下一代工业技术(next-generation industrial biotechnology, NGIB)”可以增加PHA的市场竞争力,为国家碳中和目标顺利实施提供参考。本文综述了各类塑料降解并生产PHA的可能性、PHA材料的基础材料属性、加工和改性方法及获得的新材料、新技术和独特的材料性质。
关键词降塑再造    合成生物学    生物材料    聚羟基脂肪酸酯    下一代工业生物技术    盐单胞菌    
The degradation of plastics and the production of polyhydroxyalkanoates (PHA)
ZHANG Zonghao1,5 , HE Hongtao1 , ZHANG Xu3,4 , ZHENG Shuang4 , ZHENG Taoran6 , LIU Xu6 , CHEN Guoqiang1,2,3     
1. School of Life Sciences, Tsinghua University, Beijing 100084, China;
2. Center for Synthetic and Systems Biology, Tsinghua University, Beijing 100084, China;
3. Department of Chemical Engineering, Tsinghua University, Beijing 100084, China;
4. School of Medicine, Tsinghua University, Beijing 100084, China;
5. Academy of Animal Husbandry and Veterinary Sciences, Qinghai University, Xining 810016, Qinghai, China;
6. PhaBuilder Biotech Co. Ltd., Beijing 101309, China
Abstract: In recent years, the petroleum-based plastic pollution problem has been causing global attention. The idea of "degradation and up-cycling of plastics" was proposed for solving the environmental pollution caused by non-degradable plastics. Following this idea, plastics would be firstly degraded and then reconstructed. Polyhydroxyalkanoates (PHA) can be produced from the degraded plastic monomers as a choice to recycle among various plastics. PHA, a family of biopolyesters synthesized by many microbes, have attracted great interest in industrial, agricultural and medical sectors due to its biodegradability, biocompatibility, thermoplasticity and carbon neutrality. Moreover, the regulations on PHA monomer compositions, processing technology, and modification methods may further improve the material properties, making PHA a promising alternative to traditional plastics. Furthermore, the application of the "next-generation industrial biotechnology (NGIB)" utilizing extremophiles for PHA production is expected to enhance the PHA market competitiveness, promoting this environmentally friendly bio-based material to partially replace petroleum-based products, and achieve sustainable development with carbon-neutrality. This review summarizes the basic material properties, plastic upcycling via PHA biosynthesis, processing and modification methods of PHA, and biosynthesis of novel PHA.
Keywords: degradation and up-cycling of plastics    synthetic biology    biomaterials    polyhydroxyalkanoates    next-generation industrial biotechnology    Halomonas    

塑料制品自问世以来,以其轻便易得、易于加工、成本低廉等优势在人们生活的各个领域发挥着作用,随着经济的不断发展,世界各个国家对塑料制品的需求不断增加,对塑料制品的种类和质量要求也越来越高。目前主流合成塑料包括聚乙烯(polyethylene, PE)、聚丙烯(polypropylene, PP)、聚苯乙烯(polystyrenepolystyrene, PS)、聚氯乙烯(polyvinyl chloride, PVC)、聚对苯二甲酸二乙醇酯(polyethylene terephthalate, PET)、聚氨酯(polyurethanes, PU)等。此外,也有各类新型的塑料被不断挖掘生产,包括有聚对苯二甲酸丙二醇酯(polytrimethylene terephthalate, PTT)、聚丁二酸丁二醇酯[poly(butylene succinate), PBS]、聚己二酸对苯二甲酸丁二醇酯[poly (butyleneadipate-co-terephthalate), PBAT]。截至2020年,人类生产的塑料达到了80亿t,且每年以数亿t的规模在持续增长中,与此同时,在全球的塑料生产总额中,生物基塑料占比寥寥,据统计,在2020年塑料需求量达到了3.68亿t,其中生物基塑料只占241.7万t[1-2]。石油基塑料的过度使用,加剧了环境危机、资源危机和能源危机等负面影响,尤其是必须面对大量塑料被生产,却难以被降解而带来的不可逆转的全球环境问题[3]。对于已经生产出的塑料制品,如果能对其进行转化、分解,就能够较好地解决塑料污染物的问题。近年来,许多研究集中于“降塑再造”,特别是将塑料分解后再生产其他环境友好型的塑料类型,实现塑料自身碳资源的闭环自循环。这一理念也和我国提出的“碳达峰”“碳中和”的目标相一致。“降塑再造”主要致力于使用工业生物技术方式,实现塑料的生物解聚、生物降解和生物转化的一系列步骤(图 1)[4]

图 1 塑料的降解过程和PHA的合成 Fig. 1 The degradation of plastics and the production of PHA.

在这一过程中,将塑料逐步转化为聚羟基脂肪酸酯(polyhydroxyalkanoates, PHA)引起众多关注。具体来看,塑料转化为PHA要经历分类回收、清洗、解聚和降解至成为细菌能够利用的单体形式,再经过微生物的胞内转运,合成PHA的单体并将其聚合起来,之后还要对细菌胞内的PHA进行提取和再加工,使之成为可以日常使用的塑料制品。PHA作为可降解、可再生、生物相容性好的生物材料,在天然合成的生物聚合物中具有独特的材料性能和应用前景。PHA可在许多细菌、古菌中自然产生,这些微生物在生长成分(如氮、磷或钾)受限、碳源过剩的条件下,细胞内以颗粒的形式积累PHA,作为碳源和能量的储存材料[5]。PHA的产量还可以通过生物工程改造和培养基优化的方式得到继续提高[5]。此外,通过代谢工程动态或者静态调控PHA的合成路径,可以实现精确控制各类PHA单体以不同比例聚合形成多种PHA材料。根据单体的类型和组成,材料表现出或软或硬的弹性塑料性能,这也是PHA的材料性能多样性的体现。这一特点赋予了PHA相较于其他生物塑料更多的应用场景和发展潜力。在工业生产中表现出“从上到下”的设计理念,构建的工程菌在特定的环境下产生性能可调的PHA产品,可获得更好的材料热性能和机械性能。本文概述了塑料的降解手段、PHA的合成过程、PHA材料的组成和结构特性、PHA材料性能的加工和改进方法,最后对降解塑料生产PHA这一过程进行了展望。

1 塑料的回收与降解

塑料制品的诞生给人类带来了各种便利,但是不可降解的塑料也给世界带来了类似于“白色污染”“微塑料”等环境和健康问题。据统计,我国自1949–2018年之间,塑料累计消费22亿t,共产生废塑料14亿t,其中30.0%被回收再利用,14.0%被焚烧,36.0%进入填埋场,20.0%未经处理直接进入环境[6]。为了解决这一问题,各类塑料回收方法被提出,包括物理方法、化学方法和生物方法等。物理方法主要是将塑料进行重熔再造,对塑料的种类有一定的限制,且往往是“降级”再造,如将原本制造塑料瓶的PET材料用于制造长纤或短纤。但这一过程中还会伴随废水、废气的污染问题。使用化学方法将塑料进行裂解,可以生成塑料单体甚至是低分子量的燃油,但其技术难度较大,且中间耗费的人力物力同样是一个潜在的问题。如果能够使用生物方法对塑料进行降解,在未来其有可能应用于塑料废品处理的各个环节之中,包括填埋物的处理、新产生的塑料废品的回收等。目前生物降解的研究集中于挖掘可以降解不同塑料的微生物,进而探究其降解酶和降解机理,从而有望实现塑料的大规模降解。

1.1 聚酯型合成塑料的生物降解

在常用的塑料中,PET和PU为聚酯类塑料。PET的生物降解是目前最具有可行性的。从2005年起,有多种能够降解PET的微生物被鉴定,更进一步地发掘了一些PET的降解酶。如Yoshida等[7]在2016年报道了细菌大阪堺菌(Ideonella sakaiensis)可以利用PET为碳源生长,并通过2个酶的作用先将PET降解为单(2-羟乙基)对苯二甲酸[mono(2-hydroxyethyl) terephthalic acid, MHET],再进一步降解为对苯二甲酸,并被细菌利用。目前发掘的PET降解酶包括角质酶HiC[8]、Tfcut2[9]和LCC[10]等。改进的PET降解酶能够在10 h内降解90%的PET,并以每小时16.7 g/L的速度生产对苯二甲酸酯[11]。此外,也有一些公司,如法国的Carbios和瑞士的Gr3n开展了PET的回收和降解业务,未来几年有望实现商业化的PET降解。

PU是常用的六大塑料之一,2020年产量占所有塑料的7.8%[12],主要由不同的异氰酸酯和多元醇反应而成。近年来也有微生物降解PU的报道,包括细菌、真菌等。棒状杆菌(Corynebacterium sp.) B12铜绿假单胞菌(Pseudomonas aeruginosa) B16[13]、枯草芽孢杆菌(Bacillus subtilis) MZA-75、Pseudomonas aeruginosa MZA-85等微生物被报道有降解PU的能力[14],其中假单胞菌属和芽孢杆菌属的细菌是常见的能够降解PU的细菌[15]。PU的降解产物主要包括1, 4-丁二醇和己二酸。目前来看实现PU降解的可行性要高于烯烃类塑料材料,国际上有韩国公司SKC致力于PU的降解再造。

1.2 聚烯烃类合成塑料的生物降解

PE和PP是目前世界上用量最大的两种塑料材料。PE主要应用于各类包装材料,包括冷冻食品的包装、购物袋等。而PP由于能耐受更高的温度,可用于超过100 ℃的场景中,如使用微波炉加热的保鲜盒等。这两种塑料性质稳定,因而应用广泛,同时降解难度也很大。一些研究集中于从塑料的掩埋场等塑料丰富的场所分离能够降解对应塑料的细菌。目前报道泊库岛食烷菌(Alcanivorax borkumensis)、短芽孢杆菌(Brevibacilus argi)、短短芽孢杆菌(Brevibacilus brevis)、Brevibacilus sp.[16]、解硫胺素硫胺素芽孢杆菌(Aneurinibacillus aneurinilyticus)[17]、红球菌(Rhodococcus sp. strain) 36、Bacillus sp. strain 27[18]Bacillus sp. YP1[19]Bacillus subtilis H1584[20]、假单胞菌(Pseudomonas sp.) AKS2[21]、粘质沙雷氏菌(Serratia marcescens)[22]、狭食单胞菌(Stenotrophomonas panacihumi)[23]等微生物可以降解PE和PP。此外,还有一些昆虫具有消化PE的能力,如黄粉虫等,这种能力被认为是其消化酶和其肠道菌群共同作用的结果[24]

PS和PVC也属于烯烃类的塑料材料。PS可应用于各类日用品中,如漱口杯、尺子等,由于PS透明度高,也可用于一次性餐盒等场景中。关于PS的降解有一些报道,如微小杆菌(Exiguobacterium sp.) YT2[25]、微杆菌(Microbacterium sp.) NA23、类芽孢杆菌(Paenibacillus urinalis) NA26[26]、赤红球菌(Rhodococcus ruber) C208[27]等微生物可以降解PS的薄膜,但存在和PE塑料类似的降解速率慢等问题。PVC主要应用于管材、线材和框架等工业领域,由于其良好的绝缘性也被用于电线的包裹材料等方面。针对PVC的降解也有一些报道,Zhang等[28]发现了草地贪夜蛾幼虫取食PVC薄膜并能利用其能量短暂存活,并对该机制进行了探究,确认了在该幼虫肠道中的菌群参与了这一过程,鉴定出一株克雷伯氏菌(Klebsiella sp.) EMBL-1,并发掘其有降解PVC的能力。

整体而言,这些降解烯烃类塑料的研究还停留在实验室阶段,降解速率较慢,距离真正的实际应用任重道远[29]

2 PHA简介与合成 2.1 PHA简介

PHA作为一种储备的碳源物质[30],在菌体中以不溶于水的包涵体形式存在(图 2)。从合成过程上来看,PHA是通过生物过程直接得到的生物基可降解材料[31]。PHA的化学本质是由同一种或不同种的羟基脂肪酸单体相互缩合,以酯键连接而成的高分子聚酯。

图 2 细菌胞内PHA包涵体透射电镜图及其结构式 Fig. 2 Microbial intracellular PHA granules under transmission electron microscopy (TEM) and its molecular structure.
2.1.1 PHA的结构和分类

放眼生物界,微生物的种类最为繁多、形态多样,其中有不少都具备产生和存储PHA的能力。一般情况下,天然微生物只能积累特定类型的PHA。目前,已发现组成PHA的羟基脂肪酸单体有150多种[32],单体的种类、碳链长度以及侧链基团,影响和决定了PHA的结构和性能[33],同时它们也是PHA的分类依据。

就单体种类而言,组成特定PHA的单体为同一单体时,此PHA为均聚物,如聚3-羟基丁酸酯[poly(3-hydroxybutyrate), P3HB]、聚3-羟基丙酸酯[poly(3-hydroxypropionate), P3HP]、聚4-羟基丁酸酯[poly(4-hydroxybutyrate), P4HB]、聚3-羟基戊酸酯[poly(3-hydroxyvalerate), P3HV];组成该PHA的单体为2种及以上时,此PHA为共聚物,如聚3-羟基丁酸-3-羟基戊酸共聚酯[P(3HB-co-3HV)]、聚3-羟基丁酸-3-羟基丙酸共聚酯[P(3HB-co-3HP)]、聚3-羟基丁酸-4-羟基丁酸共聚酯[P(3HB-co-4HB)]、聚3-羟基丁酸-3-羟基己酸共聚酯[(poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), P(3HB-co-3HHx)]等共聚物。又根据不同单体连接的规律,分为嵌段共聚物和随机共聚物[33]。就单体碳链长度而言(图 2),由碳链长度为3–5的单体组成的PHA为短链PHA (short-chain-length PHA, SCL PHA)[34];由碳链长度为6–14的单体组成的PHA为中长链PHA (medium-chain-length PHA, MCL PHA)[35]。此外,一些单体的侧链上还带有功能基团(如氨基、苯环、双键、羧基),这样的PHA具有一般的PHA所不具有的一些性质,如可以发生交联反应等,有助于研究更多种可以满足不同需求的材料[36]

2.1.2 PHA的性质

由于PHA的碳链长度和侧链R基团的变化多样,使得PHA单体种类极其丰富,加上单体之间的排列方式不同,使PHA具备了截然不同的材料学性质[37]表 1列举了传统塑料和几种典型PHA的材料常规性能对比[38-39]

表 1 传统塑料与几种典型的PHA的材料常规性能对比 Table 1 Comparison of the properties of traditional plastics versus several PHA materials
Material Melting point (Tm, ℃) Glass-transition temperature (Tg, ℃) Tensile strength (MPa) Elongation at break (%)
PP 175 –20 31 200
PET 262 69 56 8 300
HDPE 135 –80 29 ND
PHB 178 4 43 5
PHBV (20 mol% HV) 145 –1 20 50
PHBHHx (10 mol% HHx) 127 –1 21 400
P34HB (23 mol% 4HB)
MCL-PHA
152
53
–7
–44
13
9
626
189
PP: Polypropylene; PET: Polyethylene glycol terephthalate; HDPE: High-density polyethylene; PHB: Poly-3-hydroxybutyrate; PHBV: Copolymers of 3-hydroxybutyrate and 3-hydroxyvalerate; PHBHHx: Copolymers of 3-hydroxybutyrate and 3-hydroxyhexanoate; P34HB: Copolymers of 3-hydroxybutyrate and 4-hydroxybutyrate; MCL-PHA: Poly(3HHx-co-3HO-co-3HD-co-3HDD). ND: Not determined.

按照不同单体组成的PHA展现出完全不同的材料性质。短链PHA,结构简单规整,结晶度达60%–80%,高硬度和强度,但柔韧性差;中长链PHA材料结构较复杂,柔韧性好,具有弹性塑料的特点,但是其结晶度、熔融温度和玻璃态转化温度过低。短链和中长链PHA各自的材料特点限制了它们的应用。随着PHA生产技术的发展,将短链单体和中长链单体共聚后,这种共聚物具有比短链PHA更好的韧性和延展性,比中长链PHA更优异的硬度和结晶速率以及更高的熔融温度[40-42]。将PHA进行各种修饰改性后,会呈现出多种多样的材料性能,而且随着不断发展的新技术、各种修饰和改性方法的应用,不仅可以改善原有PHA材料的不足之处,还能按照需求制造出新特性的材料。因此,PHA材料较传统材料在应用范围上具有更广泛前景。

PHA极具优势的生物特性是可降解性和生物相容性[43]。多年来,石油基一次性塑料的不可降解性造成了严重的环境污染,其他生物基可降解材料往往需要特定的环境条件才能降解,比如PLA需要进行集中堆肥处理[44]。PHA在自然条件下,可以被多种微生物降解[45-47]。其在土壤、海水、河流和湖泊等自然环境中的生物降解性已经被研究了多年,PHA的生物降解速率受许多因素的影响,例如环境中的微生物数量、营养物质以及温度、水质和pH值等[48],通过调控这些因素,可以有效地控制PHA的降解速率,以达到不同的应用目的[49]。生物相容性是评判材料是否属于生物材料的一个重要因素,高生物相容性的聚合物材料评估为生物安全性好,能够与人体组织接触而不会引起负面反应,不会损伤组织。生物相容性是医学领域应用材料,尤其是长期植入体内材料的强制性要求之一[50]。PHA一般被认为有良好的生物相容性,在多种植入物的研究中取得了较好的结果,如骨组织支架、皮肤再生修复和神经导管等[51-53]。另一方面,很多PHA在体内降解后会产生3-羟基丁酸(3HB),3HB作为最为常见的PHA组成单体,也是人体代谢中酮体的组成成分之一,并能在极端条件(如饥饿、大量运动后)下为机体供能[54]。同时也有多种报道阐述了3HB有治疗各类疾病的潜力,如肠炎、阿尔兹海默症、癫痫等疾病[55-57]。在2007年,以P4HB为原料的可吸收缝合线被美国食品药品监督管理局(Food and Drug Administration, FDA)批准上市,更加确认了PHA材料具有较好的生物相容性[58]

除此之外,PHA还具有材料多变性、非线性光学性能、电压性能、气体阻隔性能、热塑性和可食用性等特性。PHA种类多样性、材料性质的多样性,新材料的开发及新技术的应用,将更加开阔PHA产品的应用范围[32, 59]

2.2 PHA的合成 2.2.1 可被利用的塑料单体

在塑料的组成单体之中,有一些组分可以被微生物重新利用,这主要包括一些二元醇、二元羧酸等。如PU的组成成分,己二酸(adipic acid, AA)和1, 4-丁二醇(1, 4-butanediol, BDO),乙二醇(ethylene glycol, EG)都可以被假单胞菌利用[60]。此外,PET的降解物对苯二甲酸(terephthalate, TPA)等也有被生物利用的潜力[60]。而对于烯烃类塑料,其降解效率较低且降解机制和产物的研究还不够清楚,所以往往通过一些物理、化学手段(如热裂解等)对其进行预处理,再将处理后的产物用于培养细菌。如果使用热裂解方法,一般产生有8–32碳的饱和或不饱和的烃类[61];而如果使用臭氧氧化并加热的方法对其进行预处理,会产生一些有羟基、羧基和醛基等基团的降解产物[62]。以上这两种处理烯烃类塑料的方式产生的小分子都有被细菌利用的潜力。

2.2.2 塑料单体转化为PHA的研究

来源于塑料降解物的单体,如何转变为PHA是目前的研究方向之一。有研究开发了短链二元醇转化为PHA的平台,可以将1, 3-丙二醇、1, 4丁二醇和1, 5-戊二醇等二醇转化为PHA,扩展了这些塑料降解物的可利用性[63]。该方法首先通过醇脱氢酶和醛脱氢酶,将二醇逐步转化为羟基酸;进一步地,通过辅酶A连接酶,将羟基酸活化,再通过PHA合酶(PhaC)的聚合作用,将此单体整合进PHA之中。也有研究表明可以将对苯二甲酸转化为PHA产品[64]。长链二元酸如己二酸等,也可以用于合成PHA[65]。这些研究表明,将塑料单体转化为PHA是可行的,而PHA本身是一种从生产到降解完全实现了“碳中和”的可降解塑料。此外,也有报道使用经过处理的烯烃类塑料的裂解产物来培养细菌生产PHA。其中,铜绿假单胞菌(Pseudomonas aeruginosa) PAO-1使用热裂解后的烃类作为碳源发酵,可以生产出占细胞干重25%的PHA[62]。而使用臭氧氧化裂解的方式产生的底物作为碳源据报道培养钩虫贪铜菌(Cupriavidus necator) H16,可以获得占细胞干重48%的PHA[66]

2.2.3 PHA的代谢合成途径

目前已达数十种自然合成PHA的细菌代谢途径被报道。其中,主要的合成途径有3种:第一种是目前分布最广泛、研究最多的途径,由乙酰辅酶A通过3步酶反应并消耗NADPH,最终直接合成PHA的代谢途径,该途径主要合成短链PHA;第二种是脂肪酸进入细胞作为碳源供能时,经过β-氧化途径转变成PHA的合成前体3-羟基酯酰辅酶A,最终由合成酶PhaC催化,合成中长链PHA的途径;第三种是独立地利用脂肪酸合成途径,以乙酰辅酶A作为起始物质,从头合成PHA的途径(图 3)。

图 3 细菌形成PHA的3种主要代谢途径 Fig. 3 Three major metabolic pathways through which PHA are synthesized.

而塑料单体进入PHA的途径则更为多样,以二元羧酸己二酸为例,其进入细胞后首先可以被活化成带有辅酶A的形式,再通过酰基辅酶A脱氢酶DcaA、烯酰辅酶A水合酶DcaE、酮脂酰辅酶A还原酶DcaF、羟基辅酶A脱氢酶DcaC和硫解酶DcaH生成琥珀酰辅酶A和乙酰辅酶A[65]。乙酰辅酶A即是PHA代谢的核心中间代谢产物之一,琥珀酰辅酶A可以通过三羧酸循环变为琥珀酸,再通过琥珀酸半醛的方式成为4-羟基丁酰辅酶A,进而成为PHA中的4-羟基丁酸(4HB)组分[67]。乙二醇和对苯二甲酸是PET的单体成分,乙二醇可以通过逐步氧化生成乙醇酸,2个乙醇酸可以合成一个乙醛酸、进一步,乙醛酸可以生成甘油酸,进而进入中心碳代谢;而对苯二甲酸,可以通过多步反应生成β-酮基己二酰辅酶A。与己二酸的代谢类似,β-酮基己二酰辅酶A通过硫解酶生成琥珀酰辅酶A和乙酰辅酶A,从而进入PHA的合成之中[68]

3 PHA的加工和改性 3.1 PHA的加工方法

PHA的加工方法能进一步改良材料性能。PHA作为一种生物塑料,可以使用常规的塑料加工手段进行加工,如注塑、挤压等。对于生产PHA纤维,PHA纺丝加工方法主要有溶液纺丝和熔融纺丝加工两种。优化PHA材料的加工方法,可以提升产品的品质,如在溶液加工过程中加入不同的溶剂[69]、在熔融加工中加入成核剂和增塑剂[70]。利用不同PHA的共混溶液以及采用不同纺丝工艺[71],可以得到性能更加优良的PHA产品,尤其是静电纺丝纳米纤维在医学组织工程中得到了广泛应用[72]

3.2 PHA修饰和改性方法

PHA材料虽然具有一些良好的加工特性,例如回收率、体积表面积比高;孔径小、不溶于水和加工温度较低等特点。但是天然PHA材料性能较差,非天然PHA产量又过低,这些不利条件限制了PHA的商业化应用。因此,对PHA进行不同改性可以增强PHA的物理、化学以及机械性质,提高其材料性能。PHA的改性研究,改性方式可分为物理、化学及生物改性[73],其中化学改性可分为官能化反应和接枝反应(表 2)。

表 2 PHA的改性和修饰方法 Table 2 PHA material processing and chemical modifications
Methods Modification methods Substances Properties and functions
Physical modification Blending with other materials Lignin Enhanced thermal stability
Increased crystallization[74-75]
Cellulose Enhanced gas barrier, mechanical and rheology[76]
Starch Enhanced tensile strength, elongation at break, thermal stability et al[77]
PLA Enhanced tensile and impact strengths[78-79]
PHA of different structures and thus different properties Enhanced tensile and impact strengths, fracture elongation, material flexibility, biodegradability and/or biodegradability[80-81]
Hydrophilic or hydrophobic Enhanced water or oil absorption[82]
Chemical grafting Halogen addition Chlorine Higher melting and glass transition temperatures[83-84]
Bromine The catalytic site can be provided[85]
Fluorine New contrast agents[86]
Introduction of hydroxyl groups Hydroxylation allows for the synthesis of customized PHA with other functional groups[87-89]
Carboxylation Enhance hydrophilicity[90-91]
Introduction of epoxy groups High elasticity[92-93]
Other grafting Graphene Enhanced thermal stability and electrical conductivity[94]
Multiple steps synthesis Higher thermal stability and controllable hydrophilicity, protein adsorption capacity, thermal responsiveness and hydrophilicity[95-96]
Photo cross-linking Controlled softness, flexibility, and porosity[97]
Rare earth metals Rare-earth-modified fluorescent PHA with biocompatibility[98]
Joint physical and chemical modification Chemical and then physical processes Antimicrobial, adhesive and/or hydrophilicity with biocompatibility[99]
Biological modification Functional monomers Formation of the newly functionalized PHA[100-101]

物理改性是非常简单有效的改性方法,主要通过PHA加工工艺调整或与其他材料共混后再加工。获得的新型共混聚合物材料通常可克服原PHA材料的缺点。由于采用了低成本的常规技术,因此具有很高的应用潜力。如PHA与木质素共混,可以提高材料的热稳定性[74]和结晶度[75];与纤维素共混,可以改善材料的阻隔性、机械性和流变性等[76];与淀粉共混后,可以改善材料的拉伸强度、延伸率和热稳定性[77];与其他生物多聚物共混,比如与聚乳酸(polylactic acid, PLA)共混,可以改善材料的拉伸和冲击强度[78]。不同PHA之间的共混,也能够出色地拓展材料性能。如PHB与PHBHHx[聚(3-羟基丁酸酯-co-3-羟基己酸酯)]共混后,提高了断裂伸长率[79];PHB与中长链PHA共混后,提高了材料柔韧性,加工时容易变形[80]。与纯PHA相比,PHA与其他PHA共混物通常也显示出更好的生物降解性[81]。此外,用相分离方法制备的疏水聚PHBHHx,具有吸收疏水溶剂和油的能力,可以清洁环境中的油或溶剂污染[82]

化学改性是针对不饱和PHA材料的改性方法,不饱和PHA中含有较活泼的碳-碳双键,能够引入各种化学基团。例如将卤族元素通过加成和取代反应加入到PHA形成功能性材料[73]:引入氯原子形成氯化PHB (PHB-Cl)和氯化PHO (PHO-Cl),具有更高熔化和玻璃化转变温度[83-84]。添加溴化和炔基重复官能团后,为下一步“化学点击”提供催化位点[85];氟化PHA (PFDT)具有新型造影剂的潜力[86](图 4)。

图 4 PHA的各类化学修饰 Fig. 4 Halogenated PHA modification (A) and hydroxylation, carboxylation and epoxidation (B).

还有一些化学改性方法是对PHA的双键进行羟基化、羧基化和环氧化。羟基化可以改善亲水性[87];羧基化用于合成特定功能的PHA[88-89],其材料的水渗透能力和亲水性都得到了加强[90-91];环氧化的PHA材料具有高弹性[92-93] (图 4B)。

化学接枝是一种化学改性方法。聚合物单元以共价形式,从侧链连接构成共聚物,这种共聚物在保留原始性能的基础上,还能增加额外的特性。例如,不饱和聚(3-羟基十二酸酯-co-3-羟基-9-癸酸酯),即P(3HDD-co-3H9D),接枝石墨烯[含量为(0.2–1.5) wt%]后形成的石墨烯纳米复合材料,在其生物相容性和降解性不变的情况下,提高了热降解温度和导电性,与物理共混材料相比,导电性能更加优异[94]

生物改性方法主要包括通过功能蛋白、多肽或细胞因子结合到PHA材料完成修饰,如将抗菌肽与PHA颗粒结合蛋白(PhaP)融合表达,而后利用PhaP与PHA的疏水作用力结合到PHA表面,形成的新材料具有良好的抑菌能力和亲水性,有利于成纤维细胞增殖和伤口愈合[100]。另外,将人表皮生长因子利用相同策略结合到PHA纳米颗粒后,新材料具有了靶向投送药物的能力[101]

4 挑战与展望

传统不可降解塑料带来的环境污染、“微塑料”等问题,与每个人息息相关。整体来看,为了解决这一问题,可以从两个方面入手:逐步从生产不可天然降解塑料向生产可天然降解塑料转变;对已经存在的塑料制品进行回收再利用。如果能够回收降解已经存在的石化塑料,并利用它为原料生产更加环保的可降解塑料,这样的工艺流程与上文提及的两个方面都具有一定的契合度。通过对现有塑料中碳资源的回收,生产可在天然环境下降解的PHA产品,是解决塑料污染问题的一个有发展前景的思路。本文从3个方面综述了塑料降解后合成PHA的过程,包括塑料的回收与降解、PHA的合成和PHA的改性方法。在整个过程中,还有诸多的问题需要解决,如石化塑料的降解速率较低、降解不够充分、PHA的生产成本较高、PHA的性能还不够优越等。随着对塑料降解细菌和降解酶的不断挖掘、进化,会逐步改善目前降解效率不足的问题。目前,基于人工智能等手段逐步应用于塑料的降解领域,这将大大加快降解酶的筛选速度[102]。而对于PHA的生产成本问题,通过菌种、发酵和分离等各个阶段的优化,基于“下一代工业生物技术”的手段,有利于实现PHA的产业化生产[103]。当PHA产业化具有可行性后,各类改性手段随之发展,逐步优化PHA性能,使之能够适应更多的应用场景中。

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