超临界二氧化碳发泡过程中聚合物泡孔结构的控制
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摘要
聚合物泡沫材料通常具有高的冲击强度、韧性及低的介电常数和热导率,这些独特的性能使得它们广泛地应用于各个领域。聚合物泡沫材料的应用除了受基体本身性质的影响外,还由泡孔结构(泡孔大小、分布、泡孔密度及开/闭孔等)决定。从气泡成核、生长到固化的整个过程来看,泡孔密度由成核决定,其控制因素包括成核方式(均相、异相成核)、表面张力及压差。而气泡的生长和固化则强烈的依赖于基体的流变学性质。从气泡生长动力学来看,聚合物的粘弹性是一个主要的控制因素。在气泡成核生长初期需要小的粘度以利于气泡形成,在生长过程中需要粘度不断变大,具有应变硬化性能,使气泡壁生长均匀,最终达到固化成型的目的。因此,非常有必要对流变学性能和泡孔结构之间的进行深入了解,以达到更好的设计流变性能、控制泡孔结构的目的。
从发泡过程中的聚合物体系来看,总体可以分为两类:均相体系(指单一组分无定型聚合物)和非均相体系(包括结晶聚合物、共混物、添加成核剂及其他添加剂的复合体系等)。对均相体系而言,除了改变工艺条件来控制泡孔结构外,更重要的方法是对聚合物基体本身的结构改性,从而得到具有不同流变性质的基体。目前最常见的方法是扩链、长支化和交联改性,通过提高支化度或增加分子链长度降低泡孔直径。但是,这类研究工作目前均还停留在定性描述阶段,无法对泡孔结构进行设计与调控。为此,有必要对分子结构(特别是长支化结构)、流变性能、泡孔结构之间的关系进行深入研究,以求在发泡过程中对泡孔结构的形成实现严密调控。对非均相体系而言,聚合物体系的流变性质可以通过聚合物共混、添加成核剂、控制结晶度等方式进行调节。对结晶聚合物而言,球晶可以作为气泡的异相成核点,诱发气泡异相成核。由于晶区是不能发泡的,所以泡孔只能存在于无定型相中,这样容易使得形成的泡孔不均匀。但是目前只是定性知道结晶度大小对泡孔直径的影响,球晶的大小和数量对泡孔直径及密度的影响仍然有待进一步了解。为此,我们首先利用超临界二氧化碳作为物理发泡剂,进行了系统的发泡实验研究:探明超临界二氧化碳对聚合物玻璃化温度及粘度,晶型的影响,通过自由基反应制备聚有不同流变性能的长支化聚苯乙烯(PS)、聚丙烯(PP),通过拉伸流变仪、旋转流变仪对长支化结构进行了定量表征,利用扫描电镜及图片处理软件对泡孔结构进行了分析。另一方面,我们通过耦合聚合物发泡动力学模型、经典成核理论及pom-pom大分子动力学模型,计算研究了无定型聚合物在发泡过程中长支化分子链拓扑结构、流变参数、和泡孔结构之间的定量关系;对于结晶聚合物,我们通过异相成核理论计算了泡孔密度,探明了球晶数量及成核方式对泡孔结构的影响。本文的主要研究内容与结果如下:
1.超临界二氧化碳作为一种增塑剂,会增加聚合物的自由体积,降低玻璃化温度,以及体系的粘度。在发泡过程中,随着二氧化碳浓度的降低,体系的粘度会增加,所以有必要了解体系粘度随着二氧化碳浓度的变化关系。从实验结果来看,聚合物的玻璃化温度随着二氧化碳压力的升高而降低。由于受到实验条件的限制,二氧化碳的压力不可能无限增大,我们利用Chow model预测聚合物玻璃化温度随二氧化碳浓度的变化,得到的实验结果和模拟结果能很好吻合。我们进一步利用Chow model和WLF方程,预测了体系粘度随二氧化碳浓度的变化,并用于气泡生长过程的模拟计算。
2.超临界二氧化碳对结晶温度、结晶速度及晶型产生明显的影响。PP在一般条件下生成稳定的α晶(单斜晶系),只有在特殊条件下才能生成β(六方晶系)和γ晶(三斜晶系)。在实验中,我们研究了支化度、结晶温度和二氧化碳压力(或浓度)对对γ晶含量的影响。结果表明,γ晶的含量随支化度的增大而提高,随二氧化碳压力的升高接近线性增加,说明二氧化碳是形成γ晶的主导因素,并从γ晶的分子链排列方式进行了理论解释。由于α晶和γ晶片中分子链排列方式不同,造成从晶片中伸出的链段的密度的不同,即晶片表面的分子链段的密度不同。这一差异会对无定型相中分子的排列产生重大影响。对无定型相分子链的构象而言,只有当晶片中伸出的分子链重新折叠回到晶片中,其密度才可能小于晶区密度。而对γ晶而言,由于其晶片表面分子链密度小于α晶片表面分子链密度,所以从γ晶片中伸出的分子链并不需要重新折叠进入晶片。因此,任何有利于抑制链折叠的因素都会促进γ晶的形成。长支链结构增大了分子折叠链自由能,在结晶形成过程中,分子链更不容易折叠回到片晶中排列,因而长支化结构促进了γ晶的形成。而二氧化碳增大了聚合物的自由体积,从晶片中伸出的分子链有更多的空间进行排列,使得需要折叠返回晶片的分子链的数量相对减少,所以γ晶的含量大大增加。因此,在超临界二氧化碳和长支化结构的协同作用下,PP中γ晶的含量能达到所有结晶中的90%。
3.无定型聚合物的流变学性质,特别是熔体强度,对泡孔结构的影响是以PS为研究对象来实现的。首先通过自由基反应得到支化度在0.15-1.6个支化点/104个碳原子的长支化PS,通过小振幅振荡剪切试验数据得到了PS长支化结构的松弛时间和支化度。研究发现,剪切与拉伸流变性能依赖于长支化结构,支化改性样品在瞬态拉伸粘度中具有应变硬化性能,支化度越大应变硬化系数越高。通过研究这些具有不同支化度的样品在不同二氧化碳压力和温度下发泡结构表明:支化度增加,泡孔直径减小,泡孔尺寸分布变窄,泡孔密度也略有增加。我们进一步通过应变硬化系数,建立了流变性能和泡孔直径间的关系。由于在自由基反应中得到的聚合物为线性PS和三臂星形PS的混合物,支化链的长度变化不大,支化度的改变是由于支链的数目的改变造成的。在支链长度相近的情况下,支链数目越多,得到的泡孔直径越小,泡孔尺寸分布越窄。为了进一步了解不同支链长度和主链长度对泡孔直径的影响,我们利用pom-pom model描述聚合物大分子动力学行为,耦合泡孔生长动力学方程和经典均相成核方程,对气泡成核、生长过程进行了模拟。模拟结果和实验结果是一致的,首次得到了支链长度对泡孔直径的影响比主链长度对泡孔直径影响更大的结论。通过实验和理论模拟,建立了泡孔结构、流变性质和分子结构之间的关系,这一关系可以用来指导对聚合物结构进行设计和控制流变性质,以达到控制泡孔结构的目的。
4.结晶对泡孔结构的影响是通过改变PP的流变性质、成核方式、球晶大小及数量来实现的。通过对线性PP进行长支化改性,使得PP结晶速度大幅提高,球晶尺寸明显减小,球晶数量大量增加。对线性PP而言,泡孔直径随结晶时间的增加而减小,这一方面是由于基体粘度上升,阻碍了气泡的生长,另一方面是由于结晶的存在,气泡的成核方式由开始的均相成核变为异相成核方式。正是由于成核方式的改变,泡孔密度也随之增加,但是由于线性PP结晶速度慢,球晶大,晶体数目少,加上聚合物基体的粘度较低,泡孔间存在合并的情况,所以得到的泡孔直径相对较大,泡孔密度不高。经过长支化改性后,基体的粘度、熔体强度提高,泡孔直径明显减小,而且随着结晶时间的增加,泡孔直径越来越小,直到结晶完成时,气泡不能形成。而泡孔密度则随着结晶的发展,出现先增大后接近一个最大值的趋势。在结晶初期,由于LCB PP结晶速度较快,在很短的时间内便出现球晶,所以成核方式为异相成核,泡孔密度和球晶密度的增大趋势相近。结晶一段时间后,球晶密度改变不大,而泡孔密度仍然进一步增加,泡孔密度甚至大于球晶密度,这是由于出现了异相成核和均相成核共同成核模式造成的。而对线性PP和LCB PP异相成核模式而言,球晶的大小对于气泡成核活化能的降低效果接近,造成两者泡孔密度差异的主要原因在于球晶密度不同。通过实验与理论泡孔密度相比较,发现LCB PP中球晶的成核效率略高于线性PP。而对泡孔尺寸分布而言,它受流变性质和晶体结构的共同作用,线性PP和LCB PP泡孔尺寸分布均随结晶时间的延长而变窄,但是LCB PP的泡孔尺寸分布均比线性PP更窄。从实验结果可以看出,通过改变体系的流变学性质和晶体结构可以调节结晶聚合物的泡孔结构。
5.前面的实验都是在高压釜中进行间歇式发泡,而且用来发泡的基体需要先经过化学改性,整个实验周期较长,工艺比较复杂。为了提高效率,希望能够在挤出机上进行化学改性的同时直接挤出发泡。通过对螺杆排列组合设计、挤出机头的改善、调整工艺参数,得到了最佳的发泡工艺条件;通过前面间歇式发泡过程中对流变性质、分子结构和泡孔结构间关系的了解,进行了长支化改性配方设计,同时加入碳酸钙作为异相成核剂,实现了连续挤出发泡工艺过程,最终制得泡孔直径较小、分布均匀、消除泡孔结构缺陷(泡孔合并、贯通等)的泡沫材料。为低熔体强度聚合物的连续挤出发泡工艺过程的实现提供了一条简单有效的途径。
Polymeric foams are used in a wide range of commercial applications because of their properties of high impact resistance, toughness, low dielectric constant, and good thermal insulation. The applications of polymeric foams decide not only by martial properties but also by foam structure, such as cell size, cell size distribution, cell density and open/close etc. From the whole process of bubble nucleation, growth, and solidification, cell density decides by nucleation, whose control factors include nucleation mode (homogeneous or heterogeneous nucleation), surface tension and pressure. The growth and solidification process strongly depend on materials’rheological properties. From bubble growth dynamics, viscoelastic properties are the main factors controlling foam structure. It is desirable that the viscosity is low during nucleation and early stage of bubble growth, increases gradually as the bubbles grow further, and becomes very high in the late stage of foaming to stabilize the foam structures. So it is necessary to understand the relationship between rheological properties and foam structure to better control of rheological properties and foam structures.
Materials used to foam can be simply divided into two categories: single phase materials (amorphous polymer) and multi-component or multi-phase system (such as polymer blends, polymer with nucleation agents and crystalline polymer etc.). For single phase system, except for adjusting by foaming condition (such as saturation pressure, foaming temperature and pressure drop rate etc), an important method to tune foam structure is modification of polymers to get better rheological properties. The most popular modification methods include chain extending, long chain branching and crosslinking. It is just qualitatively described that increasing long chain branching level or molecular length will decrease bubble diameter. But this result is not enough to tune foam structure accurately. So it is necessary to quantitatively know the relationship among molecular structure (especially long chain branching), rheological properties and foam structure to control foam structure accurately. For multi-phase system, the rheological properties of matrix can be tuned by blending, adding nucleation agent, and changing crystallinity. For crystalline polymer, spherulites can act as heterogeneous nucleation agent. But the foaming agent can’t enter crystal region, so the bubble can only exist in amorphous region and the bubbles can’t distribute uniformly in the matrix. It is qualitatively known that the crystallinity will influence cell diameter, while the influence of spherulites size and density on foam structure is still unknown. So in this paper, supercritical carbon dioxide is used as foam agent to carry out detailed foaming experiments. Firstly, the influence of supercritical carbon dioxide on glass transition temperature, viscosity and crystal form was studied. Then, by free radical reaction, long chain branching (LCB) polystyrene (PS) and polypropylene (PP) with different rheological properties were prepared and LCB level was quantitatively characterized by rheology method. The foam structure was observed by scanning electron microscopy (SEM) and analyzed by image software. On the other hand, bubble growth process of amorphous polymer was simulated by combined bubble growth dynamics, nucleation theory and pom-pom model, and the relationship among molecular structure, rheological properties foam structure was established. For crystalline polymer, the ideal cell density was calculated by heterogeneous nucleation theory and the influence of spherulites density and nucleation mode on foam structure was found. The main contents and results of the research list below:
1. As a plasticizer, supercritical carbon dioxide will increase system’s free volume, decrease glass transition temperature and viscosity. In the foaming process, the viscosity of the system will increase with decreasing of carbon dioxide concentration, so it needs to know the variation or viscosity with carbon dioxide concentration. From the experimental results, it is found that glass transition temperature decreases linearly with increasing pressure. But for the limitation of experiment setup, the pressure in testing can’t increase with freedom, Chow model can be used to predict the variation of glass transition temperature with carbon dioxide pressure. The experimental results are in accordance with the predictions. Combining Chow model with WLF equation, the changing of viscosity with carbon dioxide can be predicted further, and it is used in the simulation of bubble growth dynamics.
2. Supercritical carbon dioxide will influence crystallization temperature, dynamics and crystal form. PP can simultaneously crystallize into three crystalline forms, namely, the most commonly observed monoclinic orαform, the hexagonal orβform and the orthorhombic orγform. In this part, the influence of LCB level, crystallization temperature and carbon dioxied pressure (or concentration) on the content ofγform crystal was studied. It is found that the content ofγform crystal increases with LCB level and carbon dioxide pressure. Carbon dioxide is the main factor to formγcrystal. The mechanism of formingγcrystal is explained from the arrangement of molecular chain. Because the arrangement of lelics inαandγcrystal lamellae is different, the number of chains that emerge from the lamellae surface (which means that the density of molecular chain on lamellae surface) will be different. This difference has a remarkable effect on the amorphous phase. Forαcrystal, only when the chains emerged from the lamellae surface fold back, the density of amorphous phase will be lower than that of crystalline phase. But forγcrystal, its density of molecular chain on lamellae surface is lower than that ofαcrystal, so only a few chains emerged fromγcrystal lamellae need to fold back. It can be said that any factor that suppressed chainfolding made the formation ofαcrystal difficult and promoted the formation of theγcrystal. LCB structure increases the chain folding energy and makes molecular chain fold back more hardly, so it will facilitate the formation ofγcrystal. Carbon dioxide increases the free volume of polymer, the chains emerged from lamellae have more space to arrange, which makes less chains fold back to lamellae. So the content ofγcrystal will increase with carbon dioxide pressure. With the synergistic effect of carbon dioxide and LCB structure, the content ofγcrystal in PP can be as high as 90% in total crystal.
3. The influence of rheological properties, especially the melt strength, on the foam structure was carried out by the foaming experiments of PS. A series of LCB PS with LCB level range from 0.15 to 1.6 branching point/104 carbon atom was prepared by free radical reaction and their corresponding relaxation time were gotten from frequency sweep data. It is found that shear and elongational rheological properties are dependent on LCB structure, LCB samples show obvious strain hardening behavior and strain hardening coefficient increases with LCB level. From the foaming experiments, it is found that with increasing LCB level, cell diameter decreases, cell size distribution becomes narrower and cell density increases lightly. By strain hardening coefficient, the relationship between rheological properties and cell diameter can be established. The LCB PSs are the blends of linear PS and three-armed star PS and the length of arm is almost the same, so the changing of LCB level is because of the number of arm. When the arm length is close, the higher content of arms, the small the cell diameter is and the narrower the cell size distribution is. In order to know the influence of the length of arm and backbone on the foam structure, pom-pom used to describe the molecular dynamics of polymer, coupled with bubble growth dynamics and classical nucleation theory, the simulation on bubble nucleation and growth process is performed. The simulation results are in accordance with experimental results. It is found that the arm length had greater influence on the cell radius than the backbone length. Based on the experiment and simulation results, the relationship among rheological properties, molecular structure and foam structure can be established. The relationship can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.
4. The influence of crystal on foam structure was achieved by changing the rheological properties, nucleation mode, spherulits size and density of PP. By LCB modification of linear PP, its crystallization rate improves, spherulites size decreases and spherulits density increases greatly. For linear PP, cell diameter decreases with crystallization time. There are two reasons: on the one hand, the increasing of viscosity (because of crystallization) hinders bubble growth; on the other hand, the nucleation mode changes from homogeneous one to heterogeneous one. At the same time, heterogeneous nucleation increases the cell density. But because of the slow crystallization speed, big spherulites and small number of spherulites of linear PP, its cell size is bigger and cell density is not very high compared to that of LCB PP. After LCB modification, the viscosity and melt strength increase, which leads to smaller cell diameter. Cell diameter decreases with crystallization time and the bubbles will disappear when crystallization finishes. Cell density will increase first and then reach a plateau. At the earlier stage of crystallization, the crystal forms quickly, and it acts as the heterogeneous agent. The increasing trend of cell density is similar to that of spherulites density. After a period of crystallization, the spherulites density almost keeps constant while cell density still increases and even higher than spherulites density at last. This is because there is homogeneous and heterogeneous nucleation at the same time. For the heterogeneous nucleation of linear PP and LCB PP, the decreasing of nucleation barrier by different shperulites size is almost the same. The difference of spherulites density is the main reason for the difference of cell density of linear and LCB PP. By comparison of experimental and ideal cell density, it is found that the nucleation efficiency of spherulites in LCB PP is a little higher than that of linear PP. For cell size distribution, it is decided by rheological properties and crystal structure. It becomes narrower with increasing crystallization time, but LCB PP has narrower cell size distribution than linear PP. From the experiments, it can be concluded that the foam structure of crystalline polymer can be tuned by rheological properties and crystal structure.
5. The above discussed experiments are all batch foaming process, and the polymers need modification before foaming, so the experimental period is long and the process is complicated. In order to improve efficiency, it is desirable to modification and foaming at the same time. By design of screw, die, and foaming condition, the continuous reactive foaming process is carried out successfully. By design of formulation and adding nucleation agent, the foam with small cell size and uniform distribution can be fabricated. This is a simple but effective method to extrusion foaming of low melt strength polymer.
从发泡过程中的聚合物体系来看,总体可以分为两类:均相体系(指单一组分无定型聚合物)和非均相体系(包括结晶聚合物、共混物、添加成核剂及其他添加剂的复合体系等)。对均相体系而言,除了改变工艺条件来控制泡孔结构外,更重要的方法是对聚合物基体本身的结构改性,从而得到具有不同流变性质的基体。目前最常见的方法是扩链、长支化和交联改性,通过提高支化度或增加分子链长度降低泡孔直径。但是,这类研究工作目前均还停留在定性描述阶段,无法对泡孔结构进行设计与调控。为此,有必要对分子结构(特别是长支化结构)、流变性能、泡孔结构之间的关系进行深入研究,以求在发泡过程中对泡孔结构的形成实现严密调控。对非均相体系而言,聚合物体系的流变性质可以通过聚合物共混、添加成核剂、控制结晶度等方式进行调节。对结晶聚合物而言,球晶可以作为气泡的异相成核点,诱发气泡异相成核。由于晶区是不能发泡的,所以泡孔只能存在于无定型相中,这样容易使得形成的泡孔不均匀。但是目前只是定性知道结晶度大小对泡孔直径的影响,球晶的大小和数量对泡孔直径及密度的影响仍然有待进一步了解。为此,我们首先利用超临界二氧化碳作为物理发泡剂,进行了系统的发泡实验研究:探明超临界二氧化碳对聚合物玻璃化温度及粘度,晶型的影响,通过自由基反应制备聚有不同流变性能的长支化聚苯乙烯(PS)、聚丙烯(PP),通过拉伸流变仪、旋转流变仪对长支化结构进行了定量表征,利用扫描电镜及图片处理软件对泡孔结构进行了分析。另一方面,我们通过耦合聚合物发泡动力学模型、经典成核理论及pom-pom大分子动力学模型,计算研究了无定型聚合物在发泡过程中长支化分子链拓扑结构、流变参数、和泡孔结构之间的定量关系;对于结晶聚合物,我们通过异相成核理论计算了泡孔密度,探明了球晶数量及成核方式对泡孔结构的影响。本文的主要研究内容与结果如下:
1.超临界二氧化碳作为一种增塑剂,会增加聚合物的自由体积,降低玻璃化温度,以及体系的粘度。在发泡过程中,随着二氧化碳浓度的降低,体系的粘度会增加,所以有必要了解体系粘度随着二氧化碳浓度的变化关系。从实验结果来看,聚合物的玻璃化温度随着二氧化碳压力的升高而降低。由于受到实验条件的限制,二氧化碳的压力不可能无限增大,我们利用Chow model预测聚合物玻璃化温度随二氧化碳浓度的变化,得到的实验结果和模拟结果能很好吻合。我们进一步利用Chow model和WLF方程,预测了体系粘度随二氧化碳浓度的变化,并用于气泡生长过程的模拟计算。
2.超临界二氧化碳对结晶温度、结晶速度及晶型产生明显的影响。PP在一般条件下生成稳定的α晶(单斜晶系),只有在特殊条件下才能生成β(六方晶系)和γ晶(三斜晶系)。在实验中,我们研究了支化度、结晶温度和二氧化碳压力(或浓度)对对γ晶含量的影响。结果表明,γ晶的含量随支化度的增大而提高,随二氧化碳压力的升高接近线性增加,说明二氧化碳是形成γ晶的主导因素,并从γ晶的分子链排列方式进行了理论解释。由于α晶和γ晶片中分子链排列方式不同,造成从晶片中伸出的链段的密度的不同,即晶片表面的分子链段的密度不同。这一差异会对无定型相中分子的排列产生重大影响。对无定型相分子链的构象而言,只有当晶片中伸出的分子链重新折叠回到晶片中,其密度才可能小于晶区密度。而对γ晶而言,由于其晶片表面分子链密度小于α晶片表面分子链密度,所以从γ晶片中伸出的分子链并不需要重新折叠进入晶片。因此,任何有利于抑制链折叠的因素都会促进γ晶的形成。长支链结构增大了分子折叠链自由能,在结晶形成过程中,分子链更不容易折叠回到片晶中排列,因而长支化结构促进了γ晶的形成。而二氧化碳增大了聚合物的自由体积,从晶片中伸出的分子链有更多的空间进行排列,使得需要折叠返回晶片的分子链的数量相对减少,所以γ晶的含量大大增加。因此,在超临界二氧化碳和长支化结构的协同作用下,PP中γ晶的含量能达到所有结晶中的90%。
3.无定型聚合物的流变学性质,特别是熔体强度,对泡孔结构的影响是以PS为研究对象来实现的。首先通过自由基反应得到支化度在0.15-1.6个支化点/104个碳原子的长支化PS,通过小振幅振荡剪切试验数据得到了PS长支化结构的松弛时间和支化度。研究发现,剪切与拉伸流变性能依赖于长支化结构,支化改性样品在瞬态拉伸粘度中具有应变硬化性能,支化度越大应变硬化系数越高。通过研究这些具有不同支化度的样品在不同二氧化碳压力和温度下发泡结构表明:支化度增加,泡孔直径减小,泡孔尺寸分布变窄,泡孔密度也略有增加。我们进一步通过应变硬化系数,建立了流变性能和泡孔直径间的关系。由于在自由基反应中得到的聚合物为线性PS和三臂星形PS的混合物,支化链的长度变化不大,支化度的改变是由于支链的数目的改变造成的。在支链长度相近的情况下,支链数目越多,得到的泡孔直径越小,泡孔尺寸分布越窄。为了进一步了解不同支链长度和主链长度对泡孔直径的影响,我们利用pom-pom model描述聚合物大分子动力学行为,耦合泡孔生长动力学方程和经典均相成核方程,对气泡成核、生长过程进行了模拟。模拟结果和实验结果是一致的,首次得到了支链长度对泡孔直径的影响比主链长度对泡孔直径影响更大的结论。通过实验和理论模拟,建立了泡孔结构、流变性质和分子结构之间的关系,这一关系可以用来指导对聚合物结构进行设计和控制流变性质,以达到控制泡孔结构的目的。
4.结晶对泡孔结构的影响是通过改变PP的流变性质、成核方式、球晶大小及数量来实现的。通过对线性PP进行长支化改性,使得PP结晶速度大幅提高,球晶尺寸明显减小,球晶数量大量增加。对线性PP而言,泡孔直径随结晶时间的增加而减小,这一方面是由于基体粘度上升,阻碍了气泡的生长,另一方面是由于结晶的存在,气泡的成核方式由开始的均相成核变为异相成核方式。正是由于成核方式的改变,泡孔密度也随之增加,但是由于线性PP结晶速度慢,球晶大,晶体数目少,加上聚合物基体的粘度较低,泡孔间存在合并的情况,所以得到的泡孔直径相对较大,泡孔密度不高。经过长支化改性后,基体的粘度、熔体强度提高,泡孔直径明显减小,而且随着结晶时间的增加,泡孔直径越来越小,直到结晶完成时,气泡不能形成。而泡孔密度则随着结晶的发展,出现先增大后接近一个最大值的趋势。在结晶初期,由于LCB PP结晶速度较快,在很短的时间内便出现球晶,所以成核方式为异相成核,泡孔密度和球晶密度的增大趋势相近。结晶一段时间后,球晶密度改变不大,而泡孔密度仍然进一步增加,泡孔密度甚至大于球晶密度,这是由于出现了异相成核和均相成核共同成核模式造成的。而对线性PP和LCB PP异相成核模式而言,球晶的大小对于气泡成核活化能的降低效果接近,造成两者泡孔密度差异的主要原因在于球晶密度不同。通过实验与理论泡孔密度相比较,发现LCB PP中球晶的成核效率略高于线性PP。而对泡孔尺寸分布而言,它受流变性质和晶体结构的共同作用,线性PP和LCB PP泡孔尺寸分布均随结晶时间的延长而变窄,但是LCB PP的泡孔尺寸分布均比线性PP更窄。从实验结果可以看出,通过改变体系的流变学性质和晶体结构可以调节结晶聚合物的泡孔结构。
5.前面的实验都是在高压釜中进行间歇式发泡,而且用来发泡的基体需要先经过化学改性,整个实验周期较长,工艺比较复杂。为了提高效率,希望能够在挤出机上进行化学改性的同时直接挤出发泡。通过对螺杆排列组合设计、挤出机头的改善、调整工艺参数,得到了最佳的发泡工艺条件;通过前面间歇式发泡过程中对流变性质、分子结构和泡孔结构间关系的了解,进行了长支化改性配方设计,同时加入碳酸钙作为异相成核剂,实现了连续挤出发泡工艺过程,最终制得泡孔直径较小、分布均匀、消除泡孔结构缺陷(泡孔合并、贯通等)的泡沫材料。为低熔体强度聚合物的连续挤出发泡工艺过程的实现提供了一条简单有效的途径。
Polymeric foams are used in a wide range of commercial applications because of their properties of high impact resistance, toughness, low dielectric constant, and good thermal insulation. The applications of polymeric foams decide not only by martial properties but also by foam structure, such as cell size, cell size distribution, cell density and open/close etc. From the whole process of bubble nucleation, growth, and solidification, cell density decides by nucleation, whose control factors include nucleation mode (homogeneous or heterogeneous nucleation), surface tension and pressure. The growth and solidification process strongly depend on materials’rheological properties. From bubble growth dynamics, viscoelastic properties are the main factors controlling foam structure. It is desirable that the viscosity is low during nucleation and early stage of bubble growth, increases gradually as the bubbles grow further, and becomes very high in the late stage of foaming to stabilize the foam structures. So it is necessary to understand the relationship between rheological properties and foam structure to better control of rheological properties and foam structures.
Materials used to foam can be simply divided into two categories: single phase materials (amorphous polymer) and multi-component or multi-phase system (such as polymer blends, polymer with nucleation agents and crystalline polymer etc.). For single phase system, except for adjusting by foaming condition (such as saturation pressure, foaming temperature and pressure drop rate etc), an important method to tune foam structure is modification of polymers to get better rheological properties. The most popular modification methods include chain extending, long chain branching and crosslinking. It is just qualitatively described that increasing long chain branching level or molecular length will decrease bubble diameter. But this result is not enough to tune foam structure accurately. So it is necessary to quantitatively know the relationship among molecular structure (especially long chain branching), rheological properties and foam structure to control foam structure accurately. For multi-phase system, the rheological properties of matrix can be tuned by blending, adding nucleation agent, and changing crystallinity. For crystalline polymer, spherulites can act as heterogeneous nucleation agent. But the foaming agent can’t enter crystal region, so the bubble can only exist in amorphous region and the bubbles can’t distribute uniformly in the matrix. It is qualitatively known that the crystallinity will influence cell diameter, while the influence of spherulites size and density on foam structure is still unknown. So in this paper, supercritical carbon dioxide is used as foam agent to carry out detailed foaming experiments. Firstly, the influence of supercritical carbon dioxide on glass transition temperature, viscosity and crystal form was studied. Then, by free radical reaction, long chain branching (LCB) polystyrene (PS) and polypropylene (PP) with different rheological properties were prepared and LCB level was quantitatively characterized by rheology method. The foam structure was observed by scanning electron microscopy (SEM) and analyzed by image software. On the other hand, bubble growth process of amorphous polymer was simulated by combined bubble growth dynamics, nucleation theory and pom-pom model, and the relationship among molecular structure, rheological properties foam structure was established. For crystalline polymer, the ideal cell density was calculated by heterogeneous nucleation theory and the influence of spherulites density and nucleation mode on foam structure was found. The main contents and results of the research list below:
1. As a plasticizer, supercritical carbon dioxide will increase system’s free volume, decrease glass transition temperature and viscosity. In the foaming process, the viscosity of the system will increase with decreasing of carbon dioxide concentration, so it needs to know the variation or viscosity with carbon dioxide concentration. From the experimental results, it is found that glass transition temperature decreases linearly with increasing pressure. But for the limitation of experiment setup, the pressure in testing can’t increase with freedom, Chow model can be used to predict the variation of glass transition temperature with carbon dioxide pressure. The experimental results are in accordance with the predictions. Combining Chow model with WLF equation, the changing of viscosity with carbon dioxide can be predicted further, and it is used in the simulation of bubble growth dynamics.
2. Supercritical carbon dioxide will influence crystallization temperature, dynamics and crystal form. PP can simultaneously crystallize into three crystalline forms, namely, the most commonly observed monoclinic orαform, the hexagonal orβform and the orthorhombic orγform. In this part, the influence of LCB level, crystallization temperature and carbon dioxied pressure (or concentration) on the content ofγform crystal was studied. It is found that the content ofγform crystal increases with LCB level and carbon dioxide pressure. Carbon dioxide is the main factor to formγcrystal. The mechanism of formingγcrystal is explained from the arrangement of molecular chain. Because the arrangement of lelics inαandγcrystal lamellae is different, the number of chains that emerge from the lamellae surface (which means that the density of molecular chain on lamellae surface) will be different. This difference has a remarkable effect on the amorphous phase. Forαcrystal, only when the chains emerged from the lamellae surface fold back, the density of amorphous phase will be lower than that of crystalline phase. But forγcrystal, its density of molecular chain on lamellae surface is lower than that ofαcrystal, so only a few chains emerged fromγcrystal lamellae need to fold back. It can be said that any factor that suppressed chainfolding made the formation ofαcrystal difficult and promoted the formation of theγcrystal. LCB structure increases the chain folding energy and makes molecular chain fold back more hardly, so it will facilitate the formation ofγcrystal. Carbon dioxide increases the free volume of polymer, the chains emerged from lamellae have more space to arrange, which makes less chains fold back to lamellae. So the content ofγcrystal will increase with carbon dioxide pressure. With the synergistic effect of carbon dioxide and LCB structure, the content ofγcrystal in PP can be as high as 90% in total crystal.
3. The influence of rheological properties, especially the melt strength, on the foam structure was carried out by the foaming experiments of PS. A series of LCB PS with LCB level range from 0.15 to 1.6 branching point/104 carbon atom was prepared by free radical reaction and their corresponding relaxation time were gotten from frequency sweep data. It is found that shear and elongational rheological properties are dependent on LCB structure, LCB samples show obvious strain hardening behavior and strain hardening coefficient increases with LCB level. From the foaming experiments, it is found that with increasing LCB level, cell diameter decreases, cell size distribution becomes narrower and cell density increases lightly. By strain hardening coefficient, the relationship between rheological properties and cell diameter can be established. The LCB PSs are the blends of linear PS and three-armed star PS and the length of arm is almost the same, so the changing of LCB level is because of the number of arm. When the arm length is close, the higher content of arms, the small the cell diameter is and the narrower the cell size distribution is. In order to know the influence of the length of arm and backbone on the foam structure, pom-pom used to describe the molecular dynamics of polymer, coupled with bubble growth dynamics and classical nucleation theory, the simulation on bubble nucleation and growth process is performed. The simulation results are in accordance with experimental results. It is found that the arm length had greater influence on the cell radius than the backbone length. Based on the experiment and simulation results, the relationship among rheological properties, molecular structure and foam structure can be established. The relationship can be used as a guidance to control the foam structure by designing and controlling the molecular structures and the corresponding rheological properties.
4. The influence of crystal on foam structure was achieved by changing the rheological properties, nucleation mode, spherulits size and density of PP. By LCB modification of linear PP, its crystallization rate improves, spherulites size decreases and spherulits density increases greatly. For linear PP, cell diameter decreases with crystallization time. There are two reasons: on the one hand, the increasing of viscosity (because of crystallization) hinders bubble growth; on the other hand, the nucleation mode changes from homogeneous one to heterogeneous one. At the same time, heterogeneous nucleation increases the cell density. But because of the slow crystallization speed, big spherulites and small number of spherulites of linear PP, its cell size is bigger and cell density is not very high compared to that of LCB PP. After LCB modification, the viscosity and melt strength increase, which leads to smaller cell diameter. Cell diameter decreases with crystallization time and the bubbles will disappear when crystallization finishes. Cell density will increase first and then reach a plateau. At the earlier stage of crystallization, the crystal forms quickly, and it acts as the heterogeneous agent. The increasing trend of cell density is similar to that of spherulites density. After a period of crystallization, the spherulites density almost keeps constant while cell density still increases and even higher than spherulites density at last. This is because there is homogeneous and heterogeneous nucleation at the same time. For the heterogeneous nucleation of linear PP and LCB PP, the decreasing of nucleation barrier by different shperulites size is almost the same. The difference of spherulites density is the main reason for the difference of cell density of linear and LCB PP. By comparison of experimental and ideal cell density, it is found that the nucleation efficiency of spherulites in LCB PP is a little higher than that of linear PP. For cell size distribution, it is decided by rheological properties and crystal structure. It becomes narrower with increasing crystallization time, but LCB PP has narrower cell size distribution than linear PP. From the experiments, it can be concluded that the foam structure of crystalline polymer can be tuned by rheological properties and crystal structure.
5. The above discussed experiments are all batch foaming process, and the polymers need modification before foaming, so the experimental period is long and the process is complicated. In order to improve efficiency, it is desirable to modification and foaming at the same time. By design of screw, die, and foaming condition, the continuous reactive foaming process is carried out successfully. By design of formulation and adding nucleation agent, the foam with small cell size and uniform distribution can be fabricated. This is a simple but effective method to extrusion foaming of low melt strength polymer.
引文
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