摘要
聚丙烯(PP)是综合性能优良且发展最快的通用塑料之一,但由于PP分子链为线性结构,其熔体不能产生应变硬化行为,因此PP的熔体强度较低且耐熔垂性能较差,使其在一些领域中的应用受到了限制,如在挤出、压延或涂覆时出现边缘收缩和卷曲,在热成型时致使制品厚度不均,在发泡时泡孔易塌陷等缺陷,提高熔体强度可以有效地克服PP的这些缺点,因此对PP的熔体强度改性研究具有重要的应用前景。本文以提高PP熔体强度为目的,采用通用PP树脂通过一步反应挤出法,制备高熔体强度聚丙烯(HMSPP)。
首先,本文选用了乙烯基聚二甲基硅氧烷(VS)为第一接枝单体,苯乙烯(St)为第二接枝单体,在引发剂过氧化苯甲酰(BPO)存在的条件下,通过一步反应挤出法制备得到了无凝胶的HMSPP。FTIR、MFR、GPC、ICP-MS和流变学分析结果表明,St和VS都接枝到iPP分子上,St第二单体的加入不仅可以有效的控制降解,而且当VS和BPO的添加量固定,St的添加量逐渐增加时,HMSPP中St的接枝率逐渐上升,而VS的接枝量先逐渐上升后下降,说明HMSPP的接枝结构中形成了St和VS共聚的支链结构；在动态剪切条件下, HMSPP在低频下的弹性响应明显增强,HMSPP在低频区有较高的η*和G'值,在tanδ～ω图中出现平台,在G'～G"图中偏离了直线,说明接枝改性后HMSPP形成了长链支化结构；在拉伸时HMSPP在较高的拉伸速率1s-1下也出现显著的应变硬化行为,并且其断裂时应变值也提高到3以上。通过单因素优化确定了制备HMSPP的最佳工艺配方：当VS和St的添加量都为iPP的1wt%, BPO添加量为iPP的0.5wt%时,制得的PP-g-VS/St的熔体强度最大,为0.29N,相对于iPP的0.022N提高了12倍,此时其Mw,为384Kg/mol, MFR为0.55g/10min, VS的接枝量为0.027%。
其次,通过差示扫描量热仪(DSC)、偏光显微镜(POM)和广角X射线衍射(WAXD)的分析方法研究了HMSPP的结晶行为,分别利用Avrami方程、Hoffman结晶理论和Mo方程、Kissger方程对其等温和非等温动力学进行了描述。研究结果表明：随着熔体强度从0.031N增加到0.29N,HMSPP的结晶温度上升,同时HMSPP的熔融温度和结晶度也升高。相对于iPP,熔体强度最高为0.29N的PP-g-VS/St的结晶温度提高了14左右,熔融温度提高20C左右,结晶度提高6%左右。在等温结晶过程中,随着熔体强度的提高,HMSPP的Avrami指数逐渐减小,接近2,表明HMSPP中支化结构对HMSPP的结晶起到了异相成核剂的作用。在非等温结晶过程的早期,随着熔体强度的增加,HMSPP中的支化结构增多,在结晶的早期促进成核的作用逐渐增强,但在后期由于链段之间的相互作用增加,对球晶的生长的阻碍作用也逐渐增加；随着熔体强度增加,HMSPP中的支化结构增加,促使其折叠链表面自由能降低,而非等温结晶活化能增加；结晶形态方面,随着熔体强度的提高,HMSPP的晶粒逐渐变细,结晶数量逐渐增加,而且HMSPP中支化结构有利于HMSPP在无应力的情况下形成γ晶PP。
第三,采用超临界二氧化碳为发泡剂,通过釜式发泡法考察了HMSPP的发泡性能,重点考察了发泡温度和发泡压力对HMSPP泡孔结构的影响。结果表明：接枝单体对HMSPP的发泡性能有很大影响,由于St单体的存在使反应过程降解得到了控制,而VS单体具有高CO2溶解度和低表面能有利于气泡的成核,因此当采用VS/St双单体接枝体系时制备的HMSPP的发泡性能更优,得到的泡沫材料不仅具有较高的泡孔密度,并且具有更小、更均匀的泡孔尺寸；熔体强度对HMSPP的发泡性能具有很大影响,对比熔体强度分别为0.031N、0.23N和0.29N的PP-g-VS/St1、 PP-g-VS/St2和PP-g-VS/St的泡孔结构,发现泡沫样品的开孔泡随熔体强度的提高逐渐消失,泡孔尺寸逐渐变均匀,且只有达到一定的熔体强度的HMSPP,如PP-g-VS/St的熔体强度为0.29N,才能得到泡孔尺寸均匀的闭孔泡沫材料。对发泡性能最好的PP-g-VS/St来说,适宜的发泡温度区间为145～165℃,适宜的发泡压力区间为8～14MPa。在此温度和压力范围内,固定压力为12Mpa时,随着发泡温度的逐渐上升,材料的泡孔密度逐渐减小,平均泡孔直径逐渐增大,表观密度先减小后增大。当温度固定为160℃时，随着发泡压力的逐渐上升,材料的泡孔密度呈现逐渐增大,平均泡孔逐渐减小,表观密度先减小后增大的趋势。当发泡压力为14MPa,发泡温度为160℃时,PP-g-VS/St的发泡倍率最大为66倍,此时其泡沫的泡孔密度、平均泡孔直径和表观密度分别为5.8×107cell/cm3、139.8μm和0.0133g/cm3。
第四,本文还研究了注塑成型过程对HMSPP晶型的影响,结果表明在注塑条件下,HMSPP中形成α和p两种晶型,且注塑过程中的降温速率、保压压力、熔体温度和模具温度不是HMSPP形成p晶的关键因素,而注塑过程中剪切作用和快速冷却的共同作用是诱导HMSPP形成β晶的必要条件。以熔体强度最高为0.29N的PP-g-VS/St为例,用流变仪模拟注塑过程中剪切对HMSPP晶型影响时,发现随着剪切速率和剪切时间的增加,PP-g-VS/St中β晶含量呈现先增加后趋于平缓的趋势,且在170～230℃温度范围内,随剪切温度的增加而增加。当剪切速率为70s-1、剪切时间为60s、剪切温度为230℃和降温速率为280℃/min的情况下,PP-g-VS/St中β晶含量达到最大为35.6%,与注塑过程中p晶的含量相当。在力学性能方面,HMSPP的力学性能相对于iPP得到大幅度提高,且随着熔体强度从0.031N增加到0.29N,其HMSPP的拉伸强度、弯曲模量和冲击强度都呈现逐渐增加的趋势。PP-g-VS/St的提高幅度最大,其拉伸强度、弯曲模量和冲击强度相对于iPP分别提高了28.6%、40.0%和466.7%。进一步通过退火的方法将HMSPP注塑成型样条中β晶转化为α晶后,HMSPP的冲击性能有所下降,但是仍大幅度高于iPP提高,表明HMSPP注塑样品冲击性能的大幅度提高是由其本身的支化结构和p晶共同作用引起的。
最后,利用HMSPP在注塑过程中可以形成β晶的特点,以PP-g-VS/St为例,考察了HMSPP作为高分子p晶成核剂时对iPP结晶温度、结晶结构、结晶形态及力学性能影响。结果表明：PP-g-VS/St作为高分子β晶成核剂能有效诱导β-iPP的形成,加快iPP的结晶速度,提高iPP的结晶温度,并可细化球晶尺寸,使晶粒分布更均匀,并可以同时改善iPP的刚性和韧性；当PP-g-VS/St的添加量为50wt%时,此时成核iPP注塑样条中p晶的相对含量达到32.8%,结晶温度提高约10℃。当PP-g-VS/St的添加量为70wt%时,成核iPP的冲击强度、弯曲模量和拉伸强度相对于iPP分别提高了260.0%、54.7%和17.2%。
Polypropylene (PP) is one of the conventional thermoplastics with a number of desirable basic properties. However, the commercially available linear iPP is ill-suited for melt-state processing operations such as foaming, thermoforming and blow molding, because of its lower melt strength. Currently, it has great demand for high melt strength PP (HMSPP) on the market, but there are no domestic industrial products supplied. Therefore, the studies on the PP melt strength modification have an important application prospect. The purpose of this research is to improve the melt strength of commercially linear PP, and develope a novel and feasible method for preparing HMSPP by one-step reactive extrusion.
Firstly, HMSPP was successfully prepared by melt grafting reaction in the presence of macromonomer vinyl polydimethylsiloxane (VS), co-monomer styrene (St) and initiator benzoyl peroxide (BPO) via one step reactive extrusion, and had no gel. The results of FTIR, MFR, GPC, ICP-MS and rheology analysis showed that St and VS were grafted onto the PP backbones successfully, and St not only can effectively control the degradation of PP, but also can react with VS to form branched structure, because when the concentration of VS and BPO was constant, the grafting degree of St increased with the increase of the concentration of St, and the grafting degree of VS increased, too. Dynamic shear rheological tests showed that the elastic response of HMSPP at low frequencies was significantly enhanced in comparison with that of iPP, such as higher G'and η*at lower frequency, appearing a platform in tanδ～ω polt and deviating from linear iPP in G'～G" plot. Meanwhile, HMSPP also appeared strain harding behaviour when the tensile rate was ls-1, and the fracture strain value increased to more than3. The optimum conditions for preparing HMSPP was as follow:when the concentration of VS, St and initiator BPO was1wt%,1wt%and0.5wt%of isotactic polypropylene (iPP), reapectively, the melt strength of PP-g-VS/St was the highest, up to0.29N, which was12times higher than that of iPP, and the Mw, MFR and grafting degree of VS was up to384Kg/mol,0.55g/10min and0.027%, respectively.
Secondly, the crystallization behavior and crystal morphology HMSPP was studied by differential scanning calorimetry thermal (DSC), the polarization microscopy (POM), and wide angle X-ray diffraction (WAXD) analysis methods, and isothermal and non-isothermal crystallization kinetics was described using Avramie quation, Hoffman theory, Mo equation and Kissger equation, respectively. The results showed that the crystallization temperature, melting temperature and crystallinity of HMSPP increased with the increase of melt strength from0.03IN to0.29N. For the highest melt strength sample PP-g-VS/St, the crystallization temperature increased by14℃, melting temperature increased by2℃, and the crystallinity increased by about6%than that of iPP. In the isothermal crystallization process, the Avrami index of HMSPP decreased to2with the increase of melt strength, which indicated that branching structures of HMSPP acted as a heterogeneous nucleating agent. In the non-isothermal crystallization process, the presence of branching structure accelerated nucleation of HMSPP at the early stage, and then hindered the growth of crystal at the later stage. Meanwhile, the fold surface free energy of HMSPP decreased, and the non-isothermal crystallization activation energy of HMSPP increased gradually with the increase of melt strength. In the crystal morphology, the presence of branching structures made the crystal size of HMSPP decreased, and the number of crystal increased.
Thirdly, The foaming ability of HMSPP was investigated by using supercritical carbon dioxide as the blowing agent, and the effects of foaming temperature and foaming pressure on the cell structure of PP-g-VS/St foam were studied to find the optimal foaming process. The results showed that grafting monomer has a significant effect on foaming ability of HMSPP. Since the monomer St can control the degration during the reactive extrusion and the monomer VS can increse the solubility of CO2, the HMSPP prepared by grafting VS and St had the best foaming ability, such as higher cell density, smaller and more uniform cell size. In the other hand, the melt stength will affect the foaming ability of HMSPP when the grafting monomer was the same. Compared with the cell structure of PP-g-VS/Stl, PP-g-VS/St2and PP-g-VS/St, whose melt strength was0.031,0.23and0.29N respectively, it can be found that the open cell disappeared gradually. PP-g-VS/St had an excellent foaming properties, and the suitable foaming temperature range was from145to165℃, and the suitable foaming pressure interval was from18to14MPa. When the foaming pressure was14MPa and the foaming temperature was160℃, the maximum expansion ratio of PP-g-VS/St was up to66times, and the cell density, the cell diameter and the foam apparent density was5.8×107cell/cm3,139.8μm and0.0133g/cm3, respectively.
Fourthly, the effect of the injection molding process on the crystal structure of HMSPP was investigated. The results showed that HMSPP forms α and β crystal in the injection molding conditions, and the cooling rate, packing pressure, melt temperature and mold temperature were not the key factors for the formation of the β crystal of HMSPP, while the shearing and high cooling rate in the injection molding process was the necessary condition for the generation of β crystal of HMSPP. When a rheometer was used to simulate the effect of shearing on the crystal of PP-g-VS/St, it can be found that the β crystal content of HMSPP increased at first and then leveled off with increasing shearing rate and shearing time, and increased with the increase of shearing temperature from170to230℃. The β crystal content of PP-g-VS/St was up to35.6%when the shearing rate, shearing time, shearing temperature and cooling rate was70s-1,60s,230℃and280℃/min, respectively, which was almost equal to that of injection molded sample. The mechanical properties of HMSPP increased with the melt strength from0.031N to0.29N, the increased range of PP-g-VS/St was the highest, whose tensile strength, flexural modulus and impact strength increased by28.6%,40.0%and466.7%respectively compared to iPP. After annealing, the β crystal of injection molding sample of PP-g-VS/St was converted into β crystal, and the impact strength of PP-g-VS/St decreased, but it was still higher than that of iPP, which indicated that the excellent impact strength of HMSPP was caused by branching structures and the β crystal together.
Finally, due to PP-g-VS/St can form β crystal during the injection process, the effects of PP-g-VS/St used as polymeric β crystal nucleating agent on the crystallization temperature, crystal structure, crystal morphology and mechanical performance of iPP were investigated. The results showed that PP-g-VS/St can effectively induce β-iPP, and increase the crystallization rate, crystallization temperature, rigidity and toughness of iPP. When the addition content of PP-g-VS/St was50wt%, the relative content of β crystal of nucleated iPP reached32.8%, the crystallization temperature increase by about10℃. When the addition content of PP-g-VS/St was70wt%, the impact strength, flexural modulus and tensile strength increased by260.0%,54.7%and17.2%at room temperature, respectively.
引文
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