Effect of Degree of Cure on Viscoelastic Behavior of Polymers

Sammanfattning: Reinforced polymer composites consist of continuous fibers embedded in a polymer matrix. The matrix is usually a thermoplastic or thermosetting resin. When thermosetting matrices are cured during the manufacture of composite parts, residual stresses develop within the part during the manufacture due primarily the thermally and chemically induced volumetric strains imposed on them. This can lead to shape distortions and sometimes weakening of the structure itself.Curing is the manufacturing process in which the thermoset resin is transformed from a liquid to a solid material. The molecular mechanisms involved in this process are quite complex and not well understood. In the macro-level, in addition to volumetric strains, heat is also generated since most thermoset polymerization reactions are exothermic. The mechanical properties of the thermoset also undergo dramatic changes. The material changes from its initial liquid state to a rubbery gel and finally to its vitrified glassy state.In modern day composite manufacturing, to accommodate for the shape distortions caused due to residual stress formation, the mold geometry is compensated. To do this, accurate predictions of the distortion behavior is required via computer simulations. This in turn requires simple mathematical models that can replicate the complex processes that take place during manufacture. One such process that requires attention is the curing of the thermoset. While models exist that assume elastic behavior during cure, they are not accurate throughout the entire cure process. Models based on viscoelastic material during cure offer better prospects in this perspective. However, currently models that are based on full viscoelasticity are either not well defined or are computationally tasking. Viscoelastic materials can be classified further in to thermorheologically simple and complex materials depending on their molecular weights. In layman’s terms, thermorheologically simple materials re those that obey the principles of time-temperature superposition (TTS). TTS requires that all response times (i.e., all relaxation or retardation time), depend equally on temperature. This is expressed by the temperature shift function. Master curves can be then generated extending the time scale beyond the range that could normally be covered in a single experiment. However to fully understand the development of viscoelasticity during cure it is also necessary that the effects of the degree of cure of the thermoset on these times be included in the model definition. This requires defining a cure shift function along with the temperature shift function. In the presented work, an attempt is made to develop a simplified methodology to characterize the viscoelastic material properties during curing. In the first paper, two different methods are investigated in a DMTA instrument to determine the effects of curing on the glassy state of the resin system LY5052/HY5052. A cure shift function was identified in the process. Based on observations it was concluded that the total shift function could be possibly defined as a product of the temperature and cure shift functions. Unique super-master curves were generated as a result. However, these curves showed a dependency of the rubbery modulus on the degree of cure. Hence, in the second paper, the effect of the degree of cure on the rubbery modulus was investigated. Following this a model was reformulated from an existing one and this was used to further simplify the super-master curves.