【作者】 王必勤；

【导师】 彭宗林； 张隐西；

【作者基本信息】 上海交通大学 ， 材料学， 2006， 博士

【摘要】 橡胶发泡材料是一种传统的材料,已广泛应用于农业、工业和国防等领域。然而有关橡胶发泡材料的研究甚少,且仅仅停留在对硫化和发泡匹配问题探讨,加工条件和配方因素对泡孔结构以及其力学行为的影响等方面。目前EPDM橡胶发泡材料采用低补强填料,导致其力学性能不高,而减轻材料重量、提高材料力学性能一直都是工业界追求的目标。高补强填料能提高基材的力学性能,因而发泡材料的力学性能也随之提高。本论文采用高耐磨炭黒 N330为橡胶的补强填料,研究了促进剂、发泡温度、填料、发泡剂以及石蜡油用量对发泡材料结构的影响,深入探讨橡胶硫化和发泡剂分解速率匹配问题,指出硫化和发泡速率匹配并不是成功发泡的充分条件,在分析基础上提出了橡胶预硫化发泡新工艺,研究了动态预硫化对混炼胶的流变性能、凝胶含量及发泡材料结构的影响。采用自催化模型分析了硫化和发泡动力学,研究了泡沫材料的压缩和应力松弛行为。本论文的主要研究内容和结果如下:研究了N330填充补强EPDM混炼胶的发泡工艺条件和配方因素对泡孔结构的影响。结果表明,高耐磨炭黒 N330用于EPDM橡胶的补强,改变不同促进剂种类,未发泡EPDM硫化胶的力学性能基本不变。将混炼胶中添加发泡剂于195oC下发泡,采用促进剂M/TMTD/TRA为1/1或TMTD/CZ为1/0.5/0.5时,橡胶热硫化前期速度快,发泡剂开始分解时橡胶的交联程度高达50%以上,导致橡胶发泡材料整体破裂。而在175到195oC范围内,在促D作用下橡胶硫化与发泡剂AC分解相匹配,发泡剂分解进程包含在橡胶硫化进程之中,制备出表面光滑、平整的橡胶发泡材料,SEM显示气泡呈现为变形的圆形或椭圆形,以闭孔结构为主,较大的泡孔镶嵌在众多的小泡丛中,成为一种具有两个尺寸水平孔洞的层次性泡沫结构材料。随着发泡剂用量的增加,发泡材料泡径明显增加,材料密度从1.04 g/cm3降低到0.12 g/cm3。随着N330用量增加,能显著提高EPDM混炼胶的粘度和模量,较大的泡径逐渐变小,密度开始缓慢升高,当填料用量达到60份时,泡孔的镶嵌结构消失,且泡壁破裂的开孔结构明显增多,其密度显著升高到0.73 g/cm3,难以制备低密度的发泡材料。橡胶中加入石蜡油能降低橡胶分子间作用力,提高混炼胶的流动性能,但随石蜡油用量的增加,发泡材料的密度仅从0.73 g/cm3降低到0.53 g/cm3,即使增加发泡剂用量,该配方也难于制备低密度的发泡材料。尽管石蜡油用量增加和炭黒用量减少所导致混炼胶的硫化和发泡特征的变化趋势基本相似,但两者对发泡材料结构的影响呈现较大差别,表明硫化和发泡速率的匹配问题不是实现成功发泡的充分条件。为了解决发泡剂利用率低的难题,首次提出一种新的发泡加工工艺:先将混炼胶在哈克中动态预硫化,通过控制硫黄用量来控制橡胶的预交联程度,经预硫化的橡胶再混入发泡剂和硫化剂,然后在热空气中发泡。考察橡胶预硫化过程,预硫化橡胶的流变特性,及其对发泡的影响,研究结果表明,混炼胶在HAAKE中动态预硫化,随着硫黄用量的增加,其平衡扭矩升高,凝胶含量增加,表观粘度、储能模量和损耗模量则呈线性上升。随频率增加,预硫化混炼胶储能模量和损耗模量上升,而表观粘度下降,EPDM混炼胶是一种假塑性流体,具有剪切变稀行为。硫化仪测试结果表明,EPDM混炼胶经动态预硫化后,并未影响其再次硫化速度。考察不同预硫化程度的混炼胶在190oC下发泡,结果表明,随预硫化程度增加,较大的泡孔直径逐渐减小,而小气泡的尺寸基本没有变化,发泡材料的密度先下降后上升。考察发泡剂用量对适宜预硫化程度的混炼胶发泡能力的影响,结果表明随发泡剂用量增加,发泡材料的密度先缓慢降低,随后急剧下降,最后又缓慢降低。随发泡材料的密度降低,其压缩模量、邵尔C硬度降低,而压缩永久变形增加。在相同应力作用下,随发泡材料密度增加,其应力松弛速度明显加快,且应力松弛程度提高。将动态硫化技术应用于难发泡的EP4703混炼胶的油变量体系,实验结果表明能显著提高混炼橡胶的可发性,进一步验证动态硫化技术的普适性。采用振荡圆盘式硫化仪研究了发泡剂AC等温分解和EPDM等温硫化特性。结果表明,AC分解和橡胶硫化反应并不遵循n次反应速率方程,而是遵循自催化反应动力学模型,在不同温度下,其反应级数并不相同,AC的分解活化能比橡胶硫化的活化能高,表明随着温度的升高,发泡剂AC的分解速度要比橡胶的硫化速度上升的更加迅速。由于表面张力作用,气泡内的气体通过泡壁相互扩散,形成了较大的气泡镶嵌在众多的小气泡中的多层次性的泡孔结构。在橡胶配方基本相同的条件下,用两种加工工艺,制备出不同结构的发泡材料。对于相对密度较低的发泡材料,其压缩应力应变曲线在较低的应变下为线弹性,接着出现较宽的泡孔塌陷平台,最后为密实化区域,是典型的弹性体发泡材料的压缩应力应变曲线。而对于相对密度高的发泡材料,其压缩曲线仅显示两个区,相对较长的线弹性区和应力徒然上升的密实化区域,表现出与密实的固态基材相似的力学性能。随发泡材料相对密度的增加,发泡材料的相对模量也随着增加,在低相对密度时,Gibson-Ashby模型能预测发泡材料的压缩模量。随相对密度的升高,气体在压缩过程产生的应力对材料受到的总应力的贡献降低。密实化应变与材料的相对密度有关,显示不同密度的发泡材料具有不同的压缩变形机制。在较低的允许应力下,相对密度较低的发泡材料其较高的吸收能量的能力源于发泡材料本身较大的变形。在较高的允许应力下,较高相对密度的发泡材料具有较高的能量吸收能力主要是因为发泡材料具有较高的模量以及在大应力作用下较高的变形性。能量吸收的效益曲线、理想参数以及能量吸收图提供了判断和优选合适材料的方法。通过并联Maxwell模型模拟了不同相对密度发泡材料的应力松弛行为,拟合出来的比例系数与实测材料的相对密度基本吻合,其平衡应力随着发泡材料的相对密度的下降而上升,橡胶材料的应力松弛时间明显呈现多分散性,不同相对密度下EPDM橡胶基材的应力松弛时间和表观松弛强度差别较小。而气体通过橡胶基材的扩散松弛时间明显较长。本论文的主要创新之处:(1)将动态预硫化技术应用于橡胶发泡过程,提出了一种发泡加工的新工艺,即混炼胶在哈克中先动态预硫化,通过控制硫黄用量来控制橡胶预交联程度,预硫化混炼胶再混入发泡剂和硫化剂体系,然后在热空气中硫化并发泡,该工艺不突出强调橡胶硫化和发泡剂分解速率的严格匹配,采用常用原材料、简单配方,容易控制的工艺条件,制备出外观良好、结构可调的发泡材料。具有容易控制、生产周期短、节省发泡剂用量等特点。(2)橡胶基体材料是构成和影响发泡材料力学性能最关键的因素,基体材料的力学性能提高必然导致发泡材料的力学性能的上升。采用高耐磨炭黒 N330作为EPDM的补强填料,系统的研究了促进剂种类,加工温度,发泡剂用量,炭黒及油的用量对发泡行为的影响,制备出表面光滑、平整的橡胶发泡材料。(3)研究了发泡材料的应力-应变和应力松弛行为,特别是依据发泡材料的应力松弛特点,提出发泡材料的应力松弛模型来研究发泡材料的应力松弛行为。更多还原

【Abstract】 Rubber foams are a conventional material, which have been widely used in the field of agriculture and industry etc. However, the study of the rubber foams is still in its infancy with a few correlative published papers, in which the studies on the rubber foams mainly involved preparation methods and mechanical properties of testing, especially focus on the delicate balance of two simultaneously occurring reaction---the curing of the rubber and the decomposition of the blowing agent. The relationships among the preparation, morphology and mechanical properties of the rubber foams are still unclear.In general, the conventional formulation contains non-reinforcing filler and lower-surface-area carbon black, which would lead to lower mechanical properties of resultant EPDM foams, so that the EPDM foams cannot satisfy the requirement of high mechanical properties. In this paper the EPDM foam was prepared in a circular hot air, which contains higher-surface-area carbon black N330, used as reinforce filler. The effects of accelerator, temperature, filler content and blowing agent content etc. on the structure of EPDM foam were investigated. On the basis of the analysis on the match between curing and blowing, a new processing route of EPDM precured technology was put. The effects of precured compound on the rheological properties and gel content were investigated, the autocatalytic model was used to analyze the reaction kinetics of curing and blowing, the compressive stress-strain and relaxation behaviors were studied. The main contents and conclusions are listed as follows.The effect of accelerator on the macroscopical appearance was investigated. The results show that when TMTD/CZ (1/1) or M/TMTD/TRA (1/0.5/0.5) used as accelerator, the cure rate of EPDM is very rapid so that the resultant foam articles broken and becomes a useless product. However, when D used as accelerator, the initiating time of vulcanization is earlier than that of blowing and the end time of vulcanization is later than that of blowing, which leads to a successful blowing process and indicates that vulcanization and blowing are well matched each other. SEM shows that the foams have a closed-cell structure and the larger cells inlaying among the smaller cells. With AC content increasing, the density of foam article decrease and the cell size increase. With N330 content increasing, the viscosity and modulus of uncured EPDM compound increase obviously, the density of foam article increase and the cell size decrease. The sunpar 2280 could make the modulus and viscosity of uncured EPDM compound decrease, but it cannot obviously decrease the density of foam article, which indicate that the match between curing and blowing is not sufficient condition for the blowing process.In order to resolve the difficulty about the lower efficiency of blowing agent, a new processing route of EPDM precured technology was put, that is, EPDM foam was prepared by dynamically vulcanizing EPDM compound in a HAAKE rheometer firstly, then mixing the partially precured EPDM compound with a blowing agent and a sulphur vulcanizing system on a two roll mill, finally, the compound was foamed in a circulating hot air oven. The effects of pre-crosslink degree of EPDM compound on the rheological properties and gel content were investigated. The results show that pre-crosslink degree of EPDM compound could be effectively controlled by adding sulphur content in dynamic vulcanizing process. With sulphur content increasing, the gel content, the dynamic storage modulus (G′), the loss modulus (G″) and the complex viscosity (η*) of precured EPDM compound increase. With increasing the frequency, the dynamic storage modulus (G′) and the loss modulus (G″) increase, while the complex viscosity (η*) decreases, which indicates the pseudoplastic behavior of the compounds. The cure rate of partially precured EPDM compound is not affected by the precure in the HAAKE. With increasing the crosslink density of precured EPDM compound, the foam density of EPDM compound begins to decrease then to increase. For optimum partially precured EPDM compound, the density of EPDM foam decrease with AC content increase, when AC content is 12 phr, the density of foam could decrease to 0.09 g/cm3. The dynamic vulcanization technology could be used for other blowing formulation, which is difficulty to complete the successful foam in conventional methods.The vulcanization and blowing characteristics of the rubber compounds were measured on a moving die rheometer. The results show that kinetic parameters determined from the autocatalytic model equation have good agreement with the experimental results, and the sum of the values of n and m related to reaction order changes with temperature. The calculated activation energy of AC decomposition is higher than that of rubber cure, which indicates the rate of blowing agent AC decomposition accelerates more quickly than that of rubber vulcanization with increasing temperature. In the course of bubble growth, there exists cell-to-cell diffusion for small cell having higher gas pressure, which leads to the structure that the larger cells inlay among the smaller cells.The compressive and relaxation behaviors were measured on an Instron 4465 electromechanical tester. The results show that the compressive stress-strain curve of foams with lower relative density includes three regimes: first is a linear elasticity at small deformations, followed by a wide collapse plateau, which leads to densification where stress rises steeply, while for the foams with higher relative density, the compressive stress-strain curve shows two regimes: the comparatively longer linear elastic region, then leads to densification. The energy absorption capabilities are closely relationship with the relative density of foam article and the permitted stress. The efficiency of energy absorption (E), the ideality parameter (I) and energy-absorption diagram approach could offer methods to evaluate and compare the performance and suitability of the energy absorption of different foam, which optimizing the choice of foam for appropriate utilization.The innovation points of this paper are listed as follows.(1) The dynamic cure processing has been first applied to rubber blowing procedure, which is a new thought to prepare the rubber foams. That is, EPDM foam was prepared by dynamically vulcanizing EPDM compound in a HAAKE rheometer firstly, then mixing the partially precured EPDM compound with a blowing agent and a sulphur vulcanizing system on a two roll mill, finally, the compound was foamed in a circulating hot air oven, and the foam articles have good appearance and adjustable cell structure. The craft emphasizes the pre-crosslinking degree of EPDM compound, not the strict match between the curing rate of rubber and decomposing rate of blowing agent, which use the commonly used raw materials, the simple formula and easy to control the technological conditions. Thus, the craft has many advantages, such as the production cycle short, easy to operate and decreasing blowing agent content etc.(2) The key factor that impacts the mechanical properties of foam articles is the composition of EPDM compound. The filler with lower surface area carbon black could give a lower reinforcing effect, which lead to lower mechanical properties of resultant EPDM foam. Thus, it is very important to adopt the higher-surface-area carbon black N330 as reinforcing filler to achieve the higher mechanical properties of EPDM foam. The effect of accelerator, temperature, filler content and blowing agent content etc. on the structure of EPDM foam were investigated systematically, and the foam articles with smooth surface are prepared.(3) The compressive stress-strain and relaxation behavior are studied. Especially based on the stress relaxation characteristics of foam articles, the stress relaxation model is proposed, which could explain the relaxation behavior of foam article very well.更多还原