Comparison of high-performance matrix materials as candidates for additive processes
Topic(s) : Material science
Co-authors :
Melany MCBEAN (UNITED KINGDOM), Nan YI (UNITED KINGDOM), Adam CHAPLIN (UNITED KINGDOM), Oana GHITAAbstract :
High performance thermoplastics are gaining interest as potential substitutes for thermosetting polymers in various composite manufacturing processes. Their ability to be reshaped, remoulded, or recycled makes them environmentally attractive in aerospace, space, motor, and defence applications. Other issues such as short shelf life, long curing times, through-thickness curing and chemical resistance cease to be a concern when using thermoplastics. However, despite their above benefits and excellent mechanical performance, semi-crystalline thermoplastic parts can suffer from high shrinkage rates and warpage when melt processed. The fast crystallisation kinetics combined with increasingly demanding and complex geometries makes the use of semi-crystalline polymers challenging.
There are two methods that can be used to address the shrinkage and warpage of thermoplastic parts: 1) the use of fillers and fibres such as carbon fibre, or 2) alterations of the matrix polymer chemistry. Driven by the need for more materials with low shrinkage and warpage rates, 3D printing has made significant progress in understanding the requirements for processing and has led to the development of new semi-crystalline grades with tailored processing behaviour. This study presents the differences in rheological and crystallization kinetics between three high performance semi-crystalline grades: Victrex 450 PEEK, Arkema Kepstan PEKK and Victrex LMPAEK. Victrex 450 PEEK is a fast-crystallising grade used extensively in conventional manufacturing processes such as injection moulding, whilst Arkema Kepstan PEKK and Victrex LMPAEK are known as slow crystallising polymers, each with unique molecular architectures, designed specifically for additive processes.
The shrinkage is dominated by the volume change taking place during crystallization under different thermal profiles, which equally links to the flow properties, polymer architecture and orientation under flow. Rotational rheometry was used to characterize the viscosity of the samples across a range of shear rates, as well as to measure the linear viscoelastic region (LVR), whilst differential and flash scanning calorimetry were used to determine the crystallization rates as a function of isothermal temperatures. Results obtained demonstrated that whilst Victrex 450 PEEK has high shear thinning properties, it has a fast crystallization rate and low critical strain limit making it less suitable for additive processes such as 3D printing, large format direct extrusion or automated tape layup. The molecular structure is also considered when discussing and comparing the results.
There are two methods that can be used to address the shrinkage and warpage of thermoplastic parts: 1) the use of fillers and fibres such as carbon fibre, or 2) alterations of the matrix polymer chemistry. Driven by the need for more materials with low shrinkage and warpage rates, 3D printing has made significant progress in understanding the requirements for processing and has led to the development of new semi-crystalline grades with tailored processing behaviour. This study presents the differences in rheological and crystallization kinetics between three high performance semi-crystalline grades: Victrex 450 PEEK, Arkema Kepstan PEKK and Victrex LMPAEK. Victrex 450 PEEK is a fast-crystallising grade used extensively in conventional manufacturing processes such as injection moulding, whilst Arkema Kepstan PEKK and Victrex LMPAEK are known as slow crystallising polymers, each with unique molecular architectures, designed specifically for additive processes.
The shrinkage is dominated by the volume change taking place during crystallization under different thermal profiles, which equally links to the flow properties, polymer architecture and orientation under flow. Rotational rheometry was used to characterize the viscosity of the samples across a range of shear rates, as well as to measure the linear viscoelastic region (LVR), whilst differential and flash scanning calorimetry were used to determine the crystallization rates as a function of isothermal temperatures. Results obtained demonstrated that whilst Victrex 450 PEEK has high shear thinning properties, it has a fast crystallization rate and low critical strain limit making it less suitable for additive processes such as 3D printing, large format direct extrusion or automated tape layup. The molecular structure is also considered when discussing and comparing the results.