Evaluation of Filler Effects on the Dynamic Mechanical Behavior of HTPB-Elastomer Used as Binder in Exemplary Composite Formulations

ABSTRACT: This study aims to evaluate the dynamic mechanical properties of a binder, used in composite propellants, with increasing amount of different solid fillers. Dynamic Mechanical Analysis (DMA) is performed to determine the region of glass-rubber transition temperature (Tg). Further parameters were obtained from the description of the loss factor curves (tanδ) with models based on exponentially modified Gauss (EMG) distribution function. Polyurethane binders employing the polyol HTPB and diisocyanate were filled with 20, 40, and 60 mass-% of ammonium perchlorate (AP), aluminum (Al) or cyclotrimethylene trinitramine (RDX) particles, using fine and coarse mean sizes. A special turning device manufactured by Fraunhofer ICT was installed inside the curing oven in order to avoid sedimentation during curing. The cured composites were characterized by DMA in torsion mode from -100 °C to 70 °C. The good homogeneous distribution of fillers in the samples was evaluated by computational X-ray micro-tomography. The data on tanδ showed a complex structure describable at first with 2 overlapping peaks. The peak at lower temperatures is due to mobility of unrestricted binder parts (Tgunr) and the second broader peak at high temperature is due to mobility of restricted binder parts (Tgres). Results of tanδ, calculated areas of EMG curves and apparent activation energies of the distinct relaxation processes indicate that Al particles have higher interaction with HTPB-based polyurethane than AP and RDX. Probably Al is bonded to the binder network via isocyanate coupling with OH groups present in the surface. Ammonium perchlorate and cyclotrimethylene trinitramine particle sizes and its morphology influenced the viscoelastic properties. AP and RDX cause more changes in intensity of first peak of loss factor than Al. Increasing amount of these both particles enhances storage (G’) and loss (G”) shear moduli. As a whole tanδ intensity is lowered in the main peak and Al showed an intensity increase in the second apparent peak.

摘要：本研究旨在评估复合固体推进剂用黏合剂在添加不同含量固态填料时的动态力学性能。采用动态力学分析（Dynamic Mechanical Analysis, DMA）测定玻璃化转变温度（glass-rubber transition temperature, Tg）的区间；进一步基于指数修正高斯（exponentially modified Gauss, EMG）分布函数构建模型，通过对损耗因子曲线（tanδ）的拟合得到相关参数。本研究以端羟基聚丁二烯（hydroxyl-terminated polybutadiene, HTPB）多元醇与二异氰酸酯制备聚氨酯黏合剂，分别添加20%、40%、60%质量分数的高氯酸铵（ammonium perchlorate, AP）、铝（aluminum, Al）或环三亚甲基三硝胺（cyclotrimethylene trinitramine, RDX）颗粒，填料采用细、粗两种平均粒径。为避免固化过程中填料发生沉降，将弗劳恩霍夫 ICT（Fraunhofer ICT）制造的专用旋转装置安装于固化炉内。固化后的复合材料采用DMA扭转模式在-100 ℃至70 ℃区间内进行表征；通过X射线显微计算机断层扫描（computational X-ray micro-tomography）评估样品内填料的均匀分散性。tanδ曲线数据呈现出复杂的结构，最初可通过两个重叠峰进行描述：低温处的峰源于未受限黏合剂链段的运动（Tgunr），高温处更宽的峰则源于受限黏合剂链段的运动（Tgres）。tanδ数据、EMG曲线拟合面积以及不同松弛过程的表观活化能结果均表明，铝颗粒与端羟基聚丁二烯基聚氨酯的相互作用强于高氯酸铵与环三亚甲基三硝胺。推测铝颗粒可通过表面羟基与异氰酸酯基团的偶联作用结合至黏合剂网络中。高氯酸铵与环三亚甲基三硝胺的粒径及形貌会影响材料的黏弹性能；相较于铝颗粒，二者对损耗因子第一峰的强度影响更为显著。提高这两种填料的添加量会提升储能剪切模量（G'）与损耗剪切模量（G''）。整体而言，主损耗峰的tanδ强度有所降低，而铝颗粒则使第二表观峰的强度出现提升。