Assessing Flowability of Cohesive Powders from a Small Sample Quantity

细小的粘结性粉末的加工非常困难，经常受到粉末流动行为不一致的影响，这对工厂的可靠性和生产率产生了不利影响。制药、精细化工和核工业就是很好的例子，在这些行业，少量粘性粉末的投加和分散在技术上非常具有挑战性。例如，对于通过肺部的药物输送，干粉吸入器的功能在很大程度上依赖于弱致密散装粉末的流动性。目前评估粉末流动性能的测试方法(无侧限压缩和剪切小室测试)需要相对大量的粉末(至少100克)。对于核能和制药等行业来说，这是不可取的，因为前者的电离辐射和后者的毒性、药物成本以及在开发早期阶段缺乏供应。此外，传统的测试方法不适合测试非常弱的压实度，而且填充方式可能不同于无侧限压缩和剪切单元等常用方法。因此，样品的流动性必须在其自己的容器中进行评估。本研究项目旨在评估压痕探针法在小尺度(通常为几个立方毫米)和极低载荷下对细小粘结粉末的变形和流动行为的分析。为了实现这一目标，提出了一种综合的方法，包括分析单个颗粒的力学性质和大块流动行为的实验工作，以及将大块行为与一次粒子性质联系起来的模拟工作。所描述的程序将在受压缩的颗粒组件的微观和宏观行为之间建立联系，并将阐明颗粒形成强致密的情况。具体而言，交付成果包括：-基于无约束屈服应力的粉末流动函数与几立方毫米样品的压痕特征之间的关联；-用于测量少量粘性粉末整体屈服应力的经过验证的测试方法；-对低负荷下粉末变形的理解。

Processing of fine and cohesive powders is very difficult and often marred by inconsistencies in powder flow behaviour which adversely affects plant reliability and productivity. Good examples are in pharmaceutical, fine chemicals and nuclear industries, where dosing and dispersion of small quantities of cohesive powders is technologically very challenging. For instance for drug delivery through lungs the functionality of dry powder inhalers is strongly dependant on flowability of weakly compacted bulk powders. Current test methods for assessing the flow behaviour of powders (unconfined compression and shear cell testing) require a relatively large amount (at least 100 g) of powders. This is undesirable for industries such as nuclear and pharmaceutical due to ionising radiation for the former and toxicity, cost of drugs and lack of availability at the early stages of development for the latter. Furthermore the traditional test methods are not suitable for testing very weak compacts and the packing may be different from that used in the common methods such as unconfined compression and shear cell. Therefore the flowability of the sample has to be assessed in its own container. This research project is formulated to evaluate the analysis of the deformation and flow behaviour of fine cohesive powders at small scales (typically a few cubic mm) and at very low loads by the indentation probe method. An integrated approach is proposed to achieve the goal, which includes experimental work to analyse the mechanical properties of the single particles and bulk flow behaviour and simulation work to relate the bulk behaviour to the properties of primary particles. The described procedure will establish a link between the microscopic and macroscopic behaviour of particle assemblies subjected to compression and will elucidate the circumstances under which the particles form a strong compact. In particular the deliverables are:-a correlation between powder flow function, based on the unconfined yield stress, and indentation characteristics for samples of a few cubic mm;-a validated test methodology for measurement of the bulk yield stress of small quantities of cohesive powders;-an understanding of powder deformation at low loads.