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固化技术外文文献翻译、中英文翻译,固化,技术,外文,文献,翻译,中英文
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8固化技术硫化原理首先，必须知道硫化胶或硫化制品所需的质量和职责，并获得这些性能，所涉及的问题是什么。作为一种实用的方法，硫化被认为是一种过程，在这种过程中，不同成分的材料和劣质的热导体受到加热过程的影响，目的是生产出均匀的产品。提供所需的物理属性。这样的结果是用天然橡胶或合成橡胶定期得到的，这是对改进的配方和技术的一种赞扬。使一种化合物，允许广泛的固化系统，没有任何有害的影响，以及工程设计，允许均匀加热。变量的可能性有很多。硫化的过程非常苛刻，因此必须对原料的种类和质量、胶料的开发和分散的均匀性进行严格的控制。还有其他原料。第二，精确控制固化时间、压力和操作温度是至关重要的。在许多应用中，配方设计的弹性体组合物已经证明对其整体性能和成功是非常关键的。然而，一个橡胶化学家的设想一次又一次的遭遇是，对这一关键材料的正确理解和发展常常被公司管理的幻想和幻想所忽视，或者留待t的最后阶段去完成。产品开发过程受交货期限的限制。这样做会对产品的耐久性和长期性能产生严重和不利的后果。与许多其他材料不同的是，橡胶的使用往往完全是由于其工程特性。此外，它几乎总是以高度复合的形式使用，以达到所需的物理效果。财产。化合物的理化性质是成功的关键因素。性能，但要获得正确的属性，用户必须首先协商“黑色艺术”的橡胶COM-冲击和固化。橡胶复合开发不是一项简单的任务.有超过35种不同的橡胶聚合物类型和无数的材料等级可供选择，而且通过复合和固化工艺可以大大改善所获得的物理性能。对于非专家来说，选择最合适的橡胶材料是非常困难的。即使在不适当的复合加工可能对其产生影响之前，也是如此。最后的产品及其功能可以考虑。因此，从橡胶技师那里获得不偏不倚的材料、建议和咨询意见，使之成为最重要的，是至关重要的。选择合适的橡胶和等级，而不是仅仅依靠单一的供应来源和误导的材料知识，以及对性能规格的错误评价。.将橡胶混合物适当混合并成型为共混物进行模压、压延和挤压，制成复合材料或手工成型，并衬在罐和容器等上，它应该在高压釜或模具中硫化。在液压机上制造模压零件，采用不同的成型工艺，如传递成型、注射成型等。成型和真空成型。在美国和日本的创新实践中，无模橡胶是采用专有技术制造的。不同硫化方法在橡胶内衬容器的硫化过程中，根据设备的尺寸和胶料配方，必须采用下列方法之一：高压釜固化，露天蒸汽养护，热水固化，自我硫化，冷粘合衬里使用容器本身作为高压釜进行固化。高压釜是一种压力容器，能够承受蒸汽产生的内部压力.图8.1显示了一个定制的大型高压釜，门是封闭的。高压釜处理是标准通用方法用于固化热固性产品。热固性复合材料的固化过程涉及到机械和化学两个方面。机械上，压力是用来清除被困的空气和体积。一块瓷砖，并巩固个别的胶合板和纤维.在化学上，必须先引发交联反应，然后完成交联反应，才能形成刚性基体。交联是最常见的初始化。通过应用热，虽然它也可能是由暴露在紫外线，微波，或高能电子(电子束固化)。在高压釜过程中，高压通过高压釜对橡胶内衬设备施加热.特定应用程序的治疗周期通常是通过经验确定的，因此，有几个治疗周期是可以确定的。开发一个单一的材料系统，以解释层压板厚度的差异，或优化固化部分的特殊合适的连接。典型的高压釜固化周期是一个两步的过程。首先，当温度上升到中间水平并保持不变时，就会施加真空和压力。在那里呆了很短时间。热量降低了聚合物的粘度，使其流动，使被困的空气和挥发分更容易逃逸。塑料聚合物也开始润湿这个阶段的纤维。在第二次上升阶段，将温度提高到最终的固化温度，并保持足够的时间来完成固化反应。在这个步骤中，橡胶复合物的粘度下降，但在温度下开始上升，保持时间不变，然后稳定在允许充分固结和纤维湿化的水平上。，同时避免过多的流动和随后的聚合物饥饿。这些控制因素也减慢了反应速度，防止了放热硫化过程产生的过热。一个高压釜固化的橡胶充气酸储罐如图8.2所示. 图8.1一个定制的大型高压釜用于硫化橡胶内衬罐。图8.2织物结构橡胶充气存放罐.对于内衬橡胶的容器，高压灭菌器中的工作压力通常为45kg/cm2。高压灭菌器的设计和施工必须考虑到这些工作压力要求。为此目的选择的高压釜必须容纳相当大比例的化工厂设备、管道等。工厂内高压釜的大小和布局是很重要的考虑因素。呃.内衬的设备被装上一辆手推车，然后在绞车电机的帮助下被推到高压釜里。锅炉压力保持在100 psig，蒸汽压力保持在内部。自动釜保持在60 psig，根据衬里所用化合物的类型，其恒温温度为130140C。固化周期通常在6至16小时之间。这在很大程度上取决于所使用的化合物和船只的大小。硫化后，高压釜打开，内衬设备和罐体卸下，进行整理和检验。如属露天蒸汽腌制及热水煮制-英制，橡胶内衬的容器要在外面绝缘。在粘合层上提供保温，从而保证橡胶与金属的正确结合。在使用容器本身作为高压釜进行固化的情况下，同样要设计T。同时，还要承受焊接过程中的温度和压力条件.这是由VES-SEL/坦克制造者来保证的。对船外绝缘是很好的做法。衬里用蒸汽固化。这种情况下的压力通常是1 atm。自硫化衬里不是很流行，除了在非关键设备.冷键合法采用预固化橡胶和室温固化粘接胶。这种方法主要用于大型坦克.在硫化过程中，发生了下列物理变化和化学变化：橡胶分子的长链通过与硫化剂反应而交联，形成三维分子结构。这种反应改变了软塑和弱塑力。橡胶材料成为弹性强的弹性产品。橡胶失去了原来的黏性，在溶剂中变得不溶于，并且对热、光和老化的变质和降解效果更有抵抗力。下面的硫化系统在COM-磅，以影响上述变化。硫磺无硫化工业用橡胶中最常见的是一般用途的合成橡胶和天然橡胶，它们分子结构中含有足够的不饱和性。用这些所谓的二烯橡胶，硫磺硫化是可行的.硫磺是目前使用最多的硫化剂.在硫的作用下，橡胶分子形成交联和环状结构。总硫数在交联和循环结构网络中结合的原子通常称为硫化系数1，定义为按重量计算的每100份橡胶中的硫含量。对于大多数橡胶来说，链中每200个单体单元就有一个交联链，就足以生产硫化产品。在一种有效的加硫固化体系中，大约有一两个硫原子交联，很少或没有形成循环基团。在不加速器的低效硫磺固化体系中，交联硫等于8个原子。自行车数量网络中的C和交联硫决定了橡胶制品的老化特性。硫氰酸二硫代硫脲可在不含元素硫的情况下进行硫化。我是硫化促进剂，或硒或碲产品，这是更耐高温老化。硫氰酸磺酸盐，有效的交联，含一到两个硫OMS也被发现，除此之外，硫柳拉姆加速器碎片起到抗氧化剂的作用。因此，用这种促进剂进行无硫或低硫治疗，可生产出老化性能较好的产品。过氧化物硫化饱和橡胶如丁基橡胶或乙丙橡胶单体不能用硫磺和促进剂交联.有机过氧化物是这些橡胶硫化所必需的.为什么过氧化物分解后，在聚合物链上形成自由基，然后这些链结合形成只有碳键的交联，与硫不同。硫化这些碳-碳键是相当稳定的。这种键也是通过使用复合橡胶的或X射线辐射进行硫化而形成的.一些橡胶可以通过使用来硫化。某些双官能化合物，形成桥式交联，例如，具有金属氧化物或丁基橡胶与二硝基苯的氯丁橡胶。硫化条件可以看出，每种类型的硫化体系与其他类型的硫化体系在种类和程度上的不同，共同产生了Vulca化状态。在硫化过程中因此，必须考虑到所涉及的产品的厚度、硫化温度和橡胶胶料的热稳定性等方面的差异。“固化”这个词指硫化被认为是由查尔斯古德伊尔发明的，这是橡胶工业界公认的术语2。治疗的条件会在很大范围内变化。五、要求硫化胶的种类及橡胶厂现有的设备。多因素必须预先确定产品的硬度，包括产品的整体尺寸，所需的生产周转量，以及硫化前橡胶原料的预处理。硬度通常会由股票的成分决定，但也会受到治疗状态的影响。厚度效应该产品的厚度是非常重要的，因为必须在橡胶的内部提供热量，并通过横截面保持一种均匀的固化状态。橡胶是较差的热传导体，因此需要考虑产品的热条件、热容量、产品的几何形状(如模塑和高压釜固化件)、热交换系统(I)。橡胶内衬设备所采用的开式蒸汽或热水固化工艺，以及某一颗粒化合物的固化特性，其中的制品厚度约为1/4。英奇正在被硫化。这是一个一般车间的做法，增加5分钟的固化时间，每四分之一英寸厚在成型的物品。在高压釜的情况下对于内衬罐和容器，当衬里厚度超过四分之一英寸时，缓慢升温至固化温度是合适的程序。对于大于q的厚度更小的英寸，比如说2或3英寸，就像使用硬铝石管和部件一样，最好采用热水固化技术。两种处理较厚物品的实用方法是：当使用一种以上的温度时，可采取升级疗法，(2)用缓慢的外部冷却代替缓慢的加热。在固化完成和橡胶保持在压力下之前，热就停止了。无论是在模具中还是在高压釜中进行大气冷却。尺寸的增加要求固化温度随时间的增加而降低，以在整个过程中获得均匀性。输卵管。但是，在开蒸汽中低温固化可能会导致橡胶的塑性流动，如果在该温度下不发生固化，则可能导致橡胶的塑性流动。本文采用了一种合适的复合技术解决这个问题。温度对固化时间的影响必须选择硫化温度，才能在最短的时间内生产出物理性能最好、均匀的产品。“硫化温度系数”一词可用来确定不同温度下不同硫化时间之间的关系。有了这些信息，最佳治愈时间在较高或较低的温度下，可以估计出许多已知硫化系数的橡胶化合物。对于大多数橡胶化合物，Vulca-n化系数为2.这表明当固化温度每升高10C时，固化时间必须减少2倍；若将温度降低到10C，则固化时间必须加倍。热稳定性的影响每种橡胶都有一定的回火电阻范围，这是必须考虑的。这些温度可能会有所变化，但重要的是不要超过每一种温度的最大值，因为会发生某种形式的恶化。这种效果可以通过成品的外观或其物理性质来表现。硫化技术橡胶制品的生产可以采用多种硫化方法。对于加工工业的模塑产品，其方法与其它产品的方法相似但前者可能有不同的化合物。在大多数行业中使用的方法都是基于普遍遵循的标准技术，下文将简要概述这些技术。压模法这种方法或根据方便性的要求对其进行修改，使用了橡胶工业中最常见的模具类型。这是全世界都遵循的斯坦达德方法，也是一种广泛的方法。从本质上讲，这种方法包括将预切毛坯或组合件放置或加载到两片模具中，然后关闭模具。液压机施加的压力迫使材料填充模具中的型腔，使所需的形状成形，毛坯中的少量多余橡胶从模具的边缘流出或通过通风口。这种多余的橡胶被称为飞边。普通压片尺寸为12“12”32“32”，由单液压压块操作。当涉及多个平台时，板的尺寸是无限的，传送带也不受限制。超过30英尺的长度是在更大的板子中产生的。这样的皮带被固定在末端，并在印刷机中固化，使它们无止境。为了更高的生产效率，印刷机将有多日光板。在冲压过程中常用的液压压力范围为1500至2000磅/英寸。蒸汽是最常用的加热介质，尽管电阻加热也适用于需要高温治疗的场合。对于制造橡胶到金属粘结的部件，优选的是压缩模具。硫化胶的收缩是设计模具时的一个重要因素。收缩取决于橡胶部件的类型模具的类型和固化的温度。对于一般橡胶模塑制品，选择1.5%-3%的任意数值作为收缩余量.轮胎通常在压缩模具的变形中被固化，其中气囊或充气气囊在硫化过程中迫使轮胎的绿色橡胶坯料保持在模具表面上。这种力再现了轮胎面的设计，并将蒸汽的热量引入到轮胎的囊内，以影响硫化过程。在管道系统中使用的小型橡胶膨胀接头通过压塑成型，而较大的尺寸是手工构建在模具上并在自动Clave中固化。厚厚的硬质塑料是通过逐步硫化的方法来硫化的.手工成型的模具也是由铸铁或铝制成的，用于管道膨胀节、烧碱工厂的柔性电池盖和大型产品。该产品是手工成型的橡胶库存，与织物或钢筋，需要在某些产品的情况下，然后固化在高压釜。传递模塑法传递成型涉及到未固化料从模具的一部分，称为锅，分配到实际的模具腔。这个过程允许成型复杂的形状或凹形。在许多复合产品中，插入物类似金属。这些程序在压缩模中是困难的。虽然模具比通过使用更高的温度和更好的传热，压缩模具的实际工艺允许更短的固化时间，这是由于施加更高的压力来强制压缩压力而获得的。进入模具。喷射造型法这种方法通常适用于塑料制品。然而，在小型橡胶部件的制造中，采用了具有设备改造的注射成型。通过仔细控制原料：橡胶制品可以在几分钟内硫化。这种方法可以通过适当的进料、注射和脱模循环来完全控制，从而导致低排斥反应。等压模造形法等静压成型是一种独特的成型工艺，成型压力均匀地施加在被制造零件周围的各个方向。这与压铸成型不同，后者的压力适用于只有一个方向。等静压成型很好地帮助自己制造一些奇怪的形状，如杯子和水桶。等静压成形件为网形。因为零件是近网成型的形状，显着减少材料将被使用，而不是更标准的使用块和钢瓶。成型后，该零件将适合于最终加工。锥形套筒和封闭端筒是常被模压等截面的常见形状。与挤压成型和模压成型不同的是，等静压成型的材料将具有高度的稳定性。材料特性该方法适用于聚四氟乙烯产品的制备。开放加工热风炉可用于硫化在挤压过程或压延过程中预压过的薄片和薄片，也可用于模具前兆的组合，然后再用后置热空气炉来硫化薄片和薄片。吴。后固化是为了去除过氧化物固化产品中的分解产物。由于热风传热能力差，该系统效率不高，较低的加工时间温度。必须防止未硫化或尚未硫化的橡胶坯形成孔隙或变形。热空气固化分为常温露天固化和常温空气固化。在密闭烤箱里治愈。通常情况下，空气治愈从80上升到120，在45-60分钟内。热风硫化胶的光泽度很高。开放蒸汽可用于封闭容器，如高压釜。这个过程包括在压力下使用饱和蒸汽。饱和蒸汽作为惰性气体，获得了较好的传热效果。高温可以使用，而且固化时间也可能更短，这使得这个过程比热风烘炉更容易处理。卧式加热器采用压力下干蒸汽。软管电缆橡胶内衬设备，橡胶辊，各种设备的挤压型材作为附件，和卡伦板是用这种方法固化的.在衬里用橡胶薄膜的情况下，一种布衬垫是用作衬垫，而它是缠绕在鼓上。这些衬里是湿的，并施加压力时，干燥，因为收缩。10-12毫米厚的薄片被缠绕在中空的桶上，厚度高达40-50毫米，用湿棉布衬里紧紧地包裹着。此方法在任何程度上都没有被任何其他更新的方法所取代。开式蒸汽在硫化橡胶中的应用金砖国家是最有效的方法。在固化过程中，充气织物保持正常的充气形态。维持充气压力，以平衡内部及外部压力。在加工接近完成时，允许空气进入蒸汽，使压力稳定而缓慢地下降。热水疗法可用于生产以下物品：不受浸泡在热水中的影响。该方法适用于厚壁制品和橡胶内衬设备，尤其适用于鄂博-奈特组合物。与水直接接触比水有更好的传热效果。热空气或蒸汽。因此，在固化过程中，该系统可以减少制品的变形。连续硫化体系该系统涉及使用某种形式的空气或蒸汽加热在一个室内，使硫化立即发生后，橡胶是在挤出机或日历形成。第四是一种适用于挤压型材和压延薄板和传送带的工艺。液体固化法也是。一个连续的过程，涉及到使用合适的热液浴，通过挤压型材可以通过和连续硫化。在200度的温度下，可以迅速治愈物品。要达到300C；无论如何，这些化合物都必须经过适当的设计才能排空孔隙，因为这是任何挤出物都会遇到的一个常见问题。适用于固化介质的材料包括铋锡合金，一种由硝酸钾和硝酸钠组成的共晶混合物(共晶是两种或两种以上成分的混合物，在最低冰点时从液体中同时凝固)。还有一些硅树脂。由悬浮在热空气中的小颗粒(玻璃珠)组成的流化床是一种有效的硫化体系。这通常用于挤压连续挤压。热传递比单独使用热空气高约50倍。冷硫化通过在溶液中浸渍或将其暴露于其蒸气中，可通过用硫单氯化物处理来硫化薄制品。这一过程已被使用超加速剂所取代，超加速剂是上限室温固化的可能性。高能辐射固化使用钴60的伽马辐射或电子束的系统已被用于硫化。电子束法用于硅橡胶的固化。确定最佳治愈率和治愈率是为某一特定成品选择库存的必要前提。这些通常是通过首先固化样品来获得的，比如说140C，测定不同固化时间的模量、抗拉强度和硬度。最佳固化通常是通过绘制模量、拉伸强度和硬度来确定的。最佳固化也可以固定在其他支柱的基础上，如撕裂强度、耐磨性或抗挠曲开裂能力。在美国，最佳的治疗方法主要是基于最佳模量在英国中，最佳抗拉强度为其次，确定最佳治疗方案。实际硫化的硫化速率图如图8.3所示，它包括三条基于三种物理测定的曲线，即极限器皿。硫化胶的强度、300%模量、硬度与化学测定法(即硫化胶中游离硫)相结合。从这三种物理性质出发，提出了最佳固化点。在图中。抗张强度曲线上以10磅斜率为基础的切线点，每分钟拉伸强度增加，表明最佳固化时间为46 min。系数，模数曲线上基于斜率为20磅的曲线上每分钟模量增加20磅的点表示最佳固化时间为43分钟。坚硬曲线上以0.2度(1/5度)的斜率为基础的曲线上每分钟硬度增加的点为48分钟。硬度比其他常量小，因为其中使用较少。这种物质的最佳加工方法是在138C时为45 MTS。 0游离硫比例 0.25 0.5 0.75 负荷每平方英寸极限抗张强度 1 2500 2000300% Modulus 1500硬度-海岸A度计 1000 500 0 15 30 45 60 75 90在138摄氏度下的加工最少时间图8.3最佳硫化实用硫化图上述有关最佳固化的细节仅与天然橡胶有关，而不适用于丁苯橡胶(SBR)，在此情况下，硬度和模量继续增加，超过Opti值。妈妈治疗点。然而，这给出了丁苯橡胶的固化状态。然而，抗撕裂性、切割生长和永久固化已被用来确定最佳的丁苯橡胶固化工艺。生产控制比重，可以很容易地判断，不是严格检查固化状态，而是检查可能影响固化的材料。对固化的最简单的检查是通过硬度的测定，这是在充分冷却到室温后进行的，最好是在24小时后进行。减少残渣可能是由未充分愈合或空气陷阱造成的。热电偶的固定可以在硫化设备的不同位置进行，也可以对固化温度进行准确的监测。橡胶与金属或橡胶与织物的粘合试验应在与固化设备中的产品一起保存的计数器样品中进行。对游离硫的化学分析提供了一种最普遍接受的方法-挖掘固化状态，这也是一种适用于该复合批次的方法。溶胀试验表明硫化程度。样品可从Prod-UCT本身切割和制备，并对其进行抗拉强度、模量和硬度测试。固化期硫化的总时间分为两个步骤：把橡胶加热到固化温度所需的时间，以及硫化温度达到后，固化橡胶所需的时间.后者是在给定的温度下预定和固定的，前者是加热库存的时间，可称为预热或预热时间。较短的固化时间不一定意味着蒸汽的使用，因为橡胶的单位重量需要一定量的热量。在含空气的开放疗法中，循环可大大缩短治愈时间。加热介质作为冷橡胶冷却空气直接接触，然后作为一个绝缘体。当压力下的饱和蒸汽用于固化内衬容器和罐体时，采用冷卢布面使蒸汽凝结，同时放弃大量的潜热。冷凝会导致压力降低，以吸引更多的蒸汽在该接触点。没有人的固化工艺具有所有的优点，那些可能是最理想的可能被拒绝的理由是最初的成本和设备的主要维修。例如，即使是a可固化的橡胶内衬容器可在常压下在开蒸汽中固化，在高压釜停机修理或维修时可采用这种方法。硫化胶中常见的缺陷成品中的大部分缺陷是由于化合物混合不当造成的。假设已进行适当的混合，产品缺陷可能是由偏差i引起的。规定的硫化条件，如压力、温度以及硫化本身的过程。一些最常见的缺陷，可能的原因，和接下来给出了留级拨号的措施。空气水泡空气起泡通常是由于在压延机或挤出机中的橡胶加工过程中或在产品的手工装配过程中被困空气造成的。应采取的补救措施关于:1修改工艺温度或压力，2给压延机或挤出机提供足够的橡胶料，3当化合物处于未硫化状态时，突出可见的水泡，4在橡胶衬里过程中，使用金属和摩擦片之间的放线。撕裂这是由于硫化过大造成的。其补救措施是减少固化时间和温度，或对复合配方进行修改，以降低固化速度。撕裂也可能是由清除方法引起的。当产品处于热状态时，从模具中取出。撕裂可以通过在足够冷却后从模具上取出产品，或者通过小心和缓慢的去除来消除。多孔性橡胶中的挥发性物质或复合成分或水分会导致气孔.模具中橡胶库存不足和固化不足也会导致气孔。以防止此缺陷避免使用含有挥发性物质的原料，测试所有原料的水分含量，让溶剂或粘合剂完全干涸，检查t的体积和形状。他的成品，并提高固化压力，如果可行的。金属脱脂不同成分、不同类型的橡胶与金属的结合，往往难以获得满意的效果。硫和促进剂的含量各种类型的影响粘着。必须选择这些，这样才不会出现硫化不足或硫化过高的情况。要获得适当的橡胶与织物之间的粘合，织物必须充分干燥，并均匀地粘合。用粘接剂。粘合不良是由于成分表面膨胀而造成的污染。在配制配方时，应避免表面开花的含蜡柔软剂。表面烧焦有时，由于加工温度高，储存条件不当，橡胶片的表面可能会被烧焦或预防。为了避免这种情况，修改COM-英镑合适。橡胶在加工过程中储存的最佳温度为20-24C。参考文献H.L.Stephens，美国俄亥俄州阿克伦聚合物科学大学副教授，美国橡胶的复合和硫化，单位：M。莫顿（ED.），橡胶技术，范诺兰德公司，美国纽约州，1973年。L.A.Mernagh，“实际硫化”，载于：W.J.S.Naunton(编辑)，“橡胶应用科学”，爱德华阿诺德，英国伦敦，1961年，p.1053.57Anticorrosive Rubber Lining. /10.1016/B978-0-323-44371-5.00008-6Copyright 2017 Elsevier Inc. All rights reserved.8 Curing TechnologyPrinciples of VulcanizationIt is essential to know first what qualities and duties are desired in a vulcanizate or a cured prod-uct, and to attain these properties what would be the problems involved. As a practical approach, vulcani-zation is considered a process whereby a material of different compositions and a poor conductor of heat is subjected to a heating process with the intention of producing a uniform product having the desired physical properties. That such a result is obtained regularly with either natural rubber or synthetic rub-ber is a tribute to improved compounding ingredients and techniques of formulating a compound, permit-ting a wide range of curing systems without any deleterious effect, and also engineering design that allows uniform heating. The possibilities of variables are many in the process of vulcanization and they are so serious that it is important that utmost control is exercised over the type and quality of raw materials, compound development, and the uniform dispersion of sulfur and other ingredients. Second, a precise control of curing time, pressures, and temperatures adopted in the operation is essential.In so many applications the elastomeric compo-nent of the formulation design has proven to be abso-lutely critical to its overall performance and success. However, one scenario that a rubber chemist encounters time and time again is that proper understanding and development of this key material is so often overlooked by the whims and fancies of company management, or left to the last stages of the product development pro-cess forced by constraints of delivery deadlines. Doing so can have serious and adverse consequences for the durability and long-term performance of the product.Unlike many other materials, rubber is often solely used as a result of its engineering properties. Additionally, it is almost always used in a highly compounded form to achieve the required physical properties. It is the physical and chemical properties of the compound that are the key factors to successful performance, but to get the right properties the user must first negotiate the “black art” of rubber com-pounding and curing. Rubber compound develop-ment is no simple task.With over 35 different rubber polymer types and countless material grades to choose from, plus the fact that the physical properties obtained can be modified greatly by compounding and curing technology, it can be very difficult for nonspecialists to select the most suitable rubber compound for their application. This is true even before the effects that improper compound processing could have on the final products and its functionalities can be considered. Therefore it is abso-lutely crucial to obtain impartial materials advice and consultations from a rubber technologist to enable the most appropriate rubber and grade to be chosen, rather than simply rely on a single source of supply and a misguided knowledge of materials as well as a misun-derstanding of the performance specifications.After the rubber compound has been properly mixed and shaped into blends for molding, calen-dering, and extruding, fabricated into composites or hand formed, and lined on the tanks and vessels, etc., it should be vulcanized either in the autoclave or in molds. Molded components are made in hydraulic presses and different types of molding processes are adopted, such as transfer molding, injection molding, and vacuum molding. In innovative practice in the United States and Japan, moldless rubber compo-nents are made with proprietary technology.Different Methods of VulcanizationIn the vulcanization of rubber-lined vessels, one of the following methods would be necessary depend-ing upon the size of the equipment and the compound formulation: Autoclave curing, Open steam curing,Anticorrosive Rubber Lining58 Hot water curing, Self-vulcanizing, Cold bond lining, and Curing by using the vessel itself as an autoclave. An autoclave is a pressure vessel capable of with-standing an internal pressure generated by steam. Fig. 8.1 shows a custom built large size autoclave with door closed. Autoclave processing is the most common method used for curing thermoset products. The curing of thermoset composites involves both mechanical and chemical processes. Mechanically, pressure is applied to remove trapped air and vola-tiles, and to consolidate the individual plies and fibers. Chemically, a crosslinking reaction must be initiated and taken to completion to form a rigid matrix. Crosslinking is most commonly initiated through the application of heat, though it also may be initiated by exposure to ultraviolet light, micro-waves, or high-energy electrons (e-beam curing). In the autoclave process, high pressure and heat are applied to the rubber-lined equipment through the autoclave atmosphere. The cure cycle for a specific application is usually determined empirically and, as a result, several cure cycles may be developed for a single material system to account for differences in laminate thickness or to optimize particular proper-ties in the cured part.The typical autoclave cure cycle for a fabric-rein-forced inflatable is a two-step process. First, vacuum and pressure are applied while the temperature is ramped up to an intermediate level and held there for a short period of time. The heat reduces the polymer viscosity, allowing it to flow and making it easier for trapped air and volatiles to escape. The plastic polymer also begins wetting the fibers at this stage. In the second ramp-up, the temperature is raised to the final cure temperature and held for a sufficient length of time to complete the cure reaction. During this step, the viscosity of the rubber compound con-tinues to drop, but starts raising at temperature ramp rates and hold times, and then is stabilized at a level that permits adequate consolidation and fiber wet-ting, while avoiding excessive flow and subsequent polymer starvation. These control factors also slow the reaction rate, which prevents excessive heat gen-eration from the exothermic vulcanization process. An autoclave-cured fabricated rubber inflatable acid storage tank is shown in Fig. 8.2.For rubber-lined vessels, the working pressure in an autoclave is normally 45 kg/cm2. The design and construction of the autoclave have to take into consideration these working pressure requirements. The autoclave selected for this purpose has to accommodate a fairly large proportion of chemical plant equipment, piping, etc. The size and layout of the autoclave in the factory are important consider-ations. The lined equipment is loaded onto a trolley and then pushed into the autoclave with the help of a winch motor. The boiler pressure is maintained at 100 psig and the steam pressure inside the auto-clave is kept at 60 psig resulting in a constant tem-perature of 130140C, depending upon the type of compounds used for lining. The curing cycle time normally varies between 6 and 16 hours, depending largely on the compound used and the size of the vessel. After vulcanization, the autoclave is opened and the lined equipment and tanks are unloaded for finishing and inspection.In the case of open steam curing and hot water cur-ing, the rubber-lined vessel is to be insulated outside Figure 8.2 Fabric-constructed rubber inflatable stor-age tank.Figure 8.1 A custom built large size autoclave for vulcanizing rubber lined tanks.598: Curing Technologyto provide heat retention at the adhesive layer so that proper bonding of the rubber to metal is ensured. In the case of curing by using the vessel itself as an autoclave, the same is to be designed to withstand the temperature and pressure conditions of the cur-ing process as well. This is to be ensured by the ves-sel/tank fabricator. It is good practice to insulate the outside of the vessel while the lining is cured with steam. The pressure in this case is usually 1 atm. Self-vulcanizing linings are not very popular except in the case of noncritical equipment. The cold bonding method involves the use of precured rubber and room temperature curable bonding adhesive. This method is mostly followed for large-sized tanks.During vulcanization the following physical and chemical changes occur: The long chains of the rubber molecules become crosslinked by reaction with the vulcanizing agent to form a three-dimensional molecular structure. This reaction transforms the soft and weak plastic-like rubber material into a strong elastic resilient product. The rubber loses its original tackiness, becomes insoluble in solvents, and is more resistant to deterioration and degradation effects of heat, light, and aging. The following vulcanization systems in the com-pounds are followed to effect the foregoing changes.Sulfur and Sulfurless VulcanizationThe most common rubbers used in industry are the general-purpose synthetic types and natural rubber, which contain adequate unsaturation in the molecular structure. With these so-called diene rubbers, vulcani-zation with sulfur is possible. Sulfur is the most com-monly used vulcanizing agent. With sulfur, crosslinks and cyclic structures of rubber molecules are formed. The total number of sulfur atoms combined in the crosslink and cyclic structure network is usually called the coefficient of vulcanization 1, and is defined as the parts of sulfur combined per 100 parts of rubber by weight. For most rubbers, one crosslink for about each 200 monomer units in the chain is sufficient to produce a vulcanized product. In an efficient accel-erated curing system about one or two sulfur atoms crosslink with little or no cyclic group being formed. In an inefficient sulfur curing system without the addi-tion of an accelerator, the crosslinked sulfur is equal to 8 atoms. The amount of cyclic and crosslinked sulfur in the network governs the aging characteristics of the rubber products. Vulcanization can be effected without elemental sulfur by the use of thiuram disulfide com-pounds, which are accelerators of vulcanization, or with selenium or tellurium products, which are more resistant to heat aging. With the thiuram sulfides, effi-cient crosslinks containing one or two sulfur atoms are found and in addition to this, the thiuram accelerator fragments act as antioxidants. Therefore sulfurless or low-sulfur cures with such accelerators produce prod-ucts with better aging characteristics.Vulcanization With PeroxidesThe saturated rubbers such as butyl or ethylene-propylene-diene-monomer cannot be crosslinked by sulfur and accelerators. Organic peroxides are neces-sary for the vulcanization of these rubbers. When the peroxides decompose, free radicals are formed on the polymer chains and these chains can then combine to form crosslinks of the type where only carboncarbon bonds are formed, unlike in sulfur vulcani-zation. These carboncarbon bonds are quite stable. Such bonds are also formed by vulcanization using gamma or X-ray radiation of compounded rubbers. Some rubbers can be vulcanized by the use of certain bifunctional compounds, which form bridge-type crosslinks, for example, neoprene with metal oxides or butyl rubber with dinitrosobenzene.Vulcanization ConditionsIt can be seen that every type of vulcanization sys-tem differs from every other type in kind and extent of the various changes that together produce the vulca-nized state. In the vulcanization processes, consider-ation must be made for the difference in the thickness of the products involved, the vulcanization tempera-ture, and thermal stability of the rubber compound. The word “cure” used to denote vulcanization is believed to have been coined by Charles Goodyear and this has been a recognized term in rubber industry circles 2. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors Anticorrosive Rubber Lining60must be predetermined including the desired hardness of the product, its overall dimensions, the production turnover required, and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock, but it can also be influenced by the state of cure.Effect of ThicknessThe thickness of the product is highly significant because of the necessity of providing heat in the inte-rior of the rubber and preserving a uniform state of cure through the cross-section. Rubbers are poor con-ductors of heat and thus it is necessary to consider the heat conditions, heat capacity, geometry of the product in the case of moldings and autoclave-cured items, heat exchange system in the case of open steam or hot water-cured processes adopted for rubber-lined equipment, and the curing characteristics of a particu-lar compound where articles thicker than about one-quarter of an inch are being vulcanized. It is a general shop floor practice to add an additional 5 min to the curing time for every one-quarter inch thickness in the molded articles. In the case of autoclave curing of lined tanks and vessels, a slow rise in temperature up to the curing temperature is the proper procedure when the thickness of lining is more than a quarter inch. For thicknesses larger than a quarter inch, say 2 or 3 inches as in the case of ebonite pipes and components, it is desirable to adopt a hot water curing technique. Two practical methods of dealing with thicker articles are (1) step-up cures where more than one temperature is employed and (2) slow external cooling in place of slow heating. Heat is discontinued before the cure is complete and the rubber is kept under pressure either in the mold or in the autoclave for atmospheric cool-ing. An increase in dimensions necessitates a reduc-tion in curing temperature with increased time to obtain uniformity throughout the product. However, low-temperature cure in open steam may lead to plas-tic flow of the rubber, if onset of cure does not com-mence at that temperature. A suitable compounding technique is followed to offset this problem.Effect of Temperature on Curing TimeThe vulcanization temperature must be chosen to produce a well-cured product having uniform and optimum physical properties in the shortest possible time. The term “temperature coefficient of vulcaniza-tion” can be used to identify the relationship between different cure times at different temperatures. With this information, optimum cure times at higher or lower temperature can be estimated for many rubber compounds with known coefficient of vulcanization. For most rubber compounds the coefficient of vulca-nization is 2. This indicates that the cure time must be reduced by a factor of 2 for each 10C increase in cure temperature or if the temperature is reduced to 10C, the cure time must be doubled.Effects of Thermal StabilityEach type of rubber has a definite range of temper-ature resistance, which has to be considered for vul-canization. These temperatures may vary somewhat but it is quite important not to exceed the maximum for each since some form of deterioration will occur. This effect is shown either by the appearance of the finished product or by its physical properties.Techniques of VulcanizationMany methods of vulcanization are available for manufacturing a rubber product. In the case of molded goods for process industries, the methods fol-lowed are similar to those for other products except that the former may have a different compound for-mulation. The methods used in most industries are based on universally followed standard techniques, which are briefly outlined next.Compression MoldingThis method or a modification of it as required by convenience utilizes the most common type of mold used in the rubber industry. This is the stan-dard method followed throughout the world and a vast assortment of molded products known as gen-eral rubber-molded goods are cured in this method. Essentially, this method consists of placing or loading into a two-piece mold a precut blank or a composite material and then closing the mold. The pressure applied by the hydraulic press forces the material to fill up the cavity in the mold bringing the required shape into formation and the slight excess rubber in the blank flows out of the rims of the mold or through vents. This excess rubber is known as 618: Curing Technologyflash. The common platen sizes range from 12 12 to 32 32 when the presses are operated by single hydraulic rams. When multirams are involved the platen size is unlimited and conveyor belts of more than 30 feet in length are produced in larger platens. Such belts are fastened at the ends and cured in the presses to make them endless. For higher produc-tivity the presses will have multi-daylight platens. The usual hydraulic pressures applied during press cures range from 1500 to 2000 lb/inch. Steam is the most often used heating medium although electri-cal resistance heating also is used whenever high-temperature cures are required. For manufacturing rubber to metal-bonded components compression molds are preferred. Shrinkage of cured rubber is an important factor when designing molds. Shrinkage depends on the type of rubber compound, type of mold, and the temperature of the cure. An arbitrary figure of 1.5%3% is chosen for general rubber-molded goods as shrinkage allowance.Tires are normally cured in a modification of the compression mold where a bladder or an inflated airbag forces and holds the green rubber stock of the tire against the mold surface during vulcaniza-tion. This force reproduces the design of the tire tread and the heat from the steam is introduced into the bladder to effect the vulcanization. Small size rubber expansion joints used in piping systems are molded by compression molding, whereas larger sizes are hand built on molds and cured in an auto-clave. Thick ebonite moldings are vulcanized by a step-up cure process.Hand-formed moldings are also produced by molds made of cast iron or aluminum for products such as expansion joints for pipelines, flexible cell covers for caustic soda factories, and large-sized valve diaphragms. The products are hand formed by building up the stock of rubber manually along with fabric or steel reinforcement required in the case of some products and then cured in the autoclave.Transfer MoldingTransfer molding involves the distribution of the uncured stock from one part of the mold, called the pot, into the actual mold cavity. This process permits the molding of intricate shapes or the introduction of inserts like metals in many composite products. These procedures are difficult in compression molds. Although molds are relatively more expensive than compression molds the actual process permits shorter cure times through the use of higher temperatures and better heat transfer, which is obtained because of higher pressure applied to force the compound into the mold.Injection MoldingThis method is normally followed for plastic prod-ucts. However, injection molding with modifications of equipment is adopted for the manufacture of small rubber components. By careful control of the feed-stock the rubber products can be vulcanized in less than several minutes. This method can be completely controlled by proper feed, injection, and demolding cycles resulting in low rejection rates and lower fin-ishing costs.Isostatic MoldingIsostatic molding is a unique process where mold-ing pressures are applied evenly in all directions around the part being made. This differs from com-pression molding, which has pressure applied in only one direction. Isostatic molding lends itself nicely to making odd shapes such as cups and buckets. An isostatically molded part is made to near net shape. Because the part is molded to near net shape, sig-nificantly less material would be used as opposed to the more standard use of blocks and cylinders. After molding, the part would be suitable for final machining.Tapered sleeves and closed end cylinders are common shapes that are often molded isostatically. Unlike extruded and compression-molded parts, materials isostatically molded will have highly con-sistent material properties. This method is followed for making polytetrafluoroethylene products.Open CuresHot air ovens can be used to vulcanize thin articles and sheets that have been preshaped in the extru-sion process or by calendering, or by a combina-tion of precuring in a mold followed by postcuring. Postcuring is done to remove decomposition prod-ucts from products cured with peroxides. This system is not very efficient because of the poor heat transfer of hot air. Longer cure times at lower temperatures Anticorrosive Rubber Lining62are necessary to prevent the formation of porosity or deformation of the unvulcanized or still-to-be-vulcanized rubber stock. Hot air cure is divided into open air cure at atmospheric temperature and cure at closed ovens. The usual conditions are an air cure rising from 80 to 120C within 4560 min. Hot air-cured vulcanizates give a glossy finish.Open steam can be used in closed vessels such as autoclaves. The process involves using saturated steam under pressure. The saturated steam acts as an inert gas and better heat transfer is obtained, thus high temperatures can be employed and shorter cure times are possible making the process more desir-able than hot air ovens. Dry steam under pressure is used in horizontal heaters. Hoses, cables, rubber-lined equipment, rubberized rolls, extruded profiles for various equipment as accessories, and calen-dered sheets are cured by this method. In the case of rubber sheeting for lining, a cloth liner is used as a backing while it is wound on drums. These liners are wet and exert pressure when dried up because of shrinkage. Sheets of 1012 mm thickness are wound over hollow drums to a thickness as high as 4050 mm and tightly wrapped with wet cotton cloth liner. This method has not been replaced by any other newer methods to any extent. The use of open steam for vulcanizing rubberized fabrics is the most efficient method. During cure the inflated fabrics are kept in their normal inflated shape. The inflated pressure is maintained to balance the inter-nal and external pressure of the steam. Near the completion of the cure, air is allowed into the steam to give a steady and slow drop in pressure.Hot water cures can be used for articles that are not affected by immersion in hot water. This method is useful for thick-walled articles and rubber-lined equipment and is especially most suitable for ebo-nite compositions. Direct contact with water pro-duces better heat transfer than with hot air or steam. Consequently, this system gives less deformation of products during cure.Continuous Vulcanization SystemThis system involves the use of some form of heat-ing by air or steam in a chamber in a manner such that the vulcanization occurs immediately after the rubber is formed in an extruder or calender. This is a suitable process for extruded profiles and calendered sheets and conveyor belts. The liquid curing method is also a continuous process that involves the use of suitable hot liquid baths through which extruded profiles can be passed and vulcanized continuously. Items can be cured rapidly at temperatures from 200 to 300C; how-ever, the compounds must be suitably designed to pre-vent porosity because this is a common problem with any extrudate. Suitable materials for curing medium include bismuth-tin alloys, a eutectic mixture of potas-sium nitrate and sodium nitrate (a eutectic is a mixture of two or more constituents that solidify simultane-ously out of the liquid at a minimum freezing point), polyglycols, and certain silicone fluids. Fluidized beds consisting of small particles (glass beads) suspended in a stream of heated air is an efficient vulcanization system. This is normally used for continuous vulca-nization of extrusions. The heat transfer is approxi-mately 50 times greater than with hot air alone.Cold VulcanizationThin articles may be vulcanized by treatment with sulfur monochloride by dipping in a solution or exposing them to its vapors. This process has been replaced by using ultra accelerators, which are capa-ble of curing at room temperature.Cure With High-Energy RadiationSystems using either gamma radiation from cobalt 60 or electron beams have been used for vulcaniza-tion. The electron beam method has been used for curing silicone rubbers.Optimum CureDetermination of optimum cure and rate of cure are essential prerequisites in the selection of stocks for a particular finished product. These are generally obtained by first curing the samples at say 140C and determining the modulus, tensile strength, and hardness at various cure times. The optimum cure is normally fixed by plotting modulus, tensile strength, and hardness against various cure times. Optimum cure may also be fixed on the basis of other prop-erties such as tear strength, abrasion resistance, or resistance to flex cracking. In the United States the optimum cure is mainly based on optimum modulus. In the United Kingdom optimum tensile strength is 638: Curing Technologythe one followed for determining optimum cure. A cure rate chart of practical vulcanization is shown in Fig. 8.3 comprising three curves based on three physical determinations, namely, ultimate tensile strength, 300% modulus, and hardness together with one based on chemical determination, namely, free sulfur in the vulcanizate. The point of optimum cure is taken from the three physical properties in the graph.Tensile StrengthThe point on the curve that has a tangent based on a slope of 10 lb increase in tensile strength per minute of cure denotes an optimum cure time of 46 min.ModulusThe point on the curve that has a tangent based on a slope of 20 lb increase in modulus per minute of cure denotes an optimum cure of 43 min.HardnessThe point on the curve that has a tangent based on a slope of 0.2 degree (1/5 degree) increase in hardness per minute of cure denotes 48 min. Hardness is less specific than other constants and because of this is less frequently used.The optimum cure of such a stock is quoted as 45 mts at 138C.15050010001500200025001.251.000.750.500.2503045Time of cure in mins. at 138 C.607590656055Shore A DurometerLoad in Ib. per sq. in.% Free SulphurUltimate Tensile Strength300% ModulusHardness Shore A DurometerFigure 8.3 Practical vulcanization chart for optimum cure.Anticorrosive Rubber Lining64The aforementioned details on optimum cure are related only to natural rubber and do not apply to styrene-butadiene rubber (SBR) in which case hardness and modulus continue to increase beyond the optimum cure point. However, this gives an indication of the cure state of the SBR compounds. Tear resistance, cut growth, and permanent set, however, have been used to determine the optimum cure of SBR.Control of Production Cures Specific gravity, which can be easily deter-mined, is not strictly a check on cure state but on materials that may affect cure. The simplest check on cure is by a hardness determination, which is to be done after the vul-canizate is sufficiently cooled to room tempera-ture; the test is best done after 24 hours of cure. Reducing scraps can be caused by undercure or air trapping. Fixing of thermocouple can be done at vari-ous locations in the vulcanization equipment as well as monitoring the temperature of cure accurately. Adhesion tests of rubber to metal or rubber to fabric are to be done in counter samples kept along with the products in the curing equipment. Chemical analysis of free sulfur provides one of the most generally accepted methods of deter-mining the state of cure, which is also appropri-ate for the compound batch. A swelling test indicates the degree of vulcanization. Samples can be cut and prepared from the prod-uct itself and tests should be conducted on them for tensile strength, modulus, and hardness.Curing TimeThe total time of vulcanization is split into two steps: 1. The time taken to heat the rubber to curing temperature, and 2. The time taken to cure the rubber after the cur-ing temperature has been attained. The latter is predetermined and fixed for a given temperature, and the former is the time for warm-ing up the stock and can be referred as the warm-ing or preheating time. A shorter curing time does not necess
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