Thermoset polymers and coatings subjected to high compressive loads

• Main Author: Ståhlberg, Daniel
• Format: Others
• Language: English
• Published: KTH, Fiber- och polymerteknologi 2004
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1699; http://nbn-resolving.de/urn:isbn:91-7283-685-7

This study describes the mechanical response of thermosetpolymers under high compressive loads. The study is dividedinto two parts. One part is focusing on the behaviour of apowder coating when used in a clamping force joint and how theproperties varies when varying the physical structure of thecoating. The other part is regarding the fundamentalunderstanding of the behaviour of thermoset polymers with smallthickness-to-width ratio subjected to compressive stresses inorder to develop mathematical material models for theviscoelastic materials. The first part describes the mechanical behaviour of powdercoatings used under very high compressive loads in clampingforce joints. Carboxyl functional polyester powder coatingscured with hydroxyl functional â-hydroxyalkylamides wereused with variations in coating thickness and amount and typeof filler. The coatings were subjected to conventional testsfor coatings and polymers and also to specially designed testsdeveloped to study the behaviour of powder coatings in clampingforce joints.The results show the importance of correct coatingthickness and filler content in order to achieve the desiredmechanical properties of a coating when used under highcompressive loads. Increased thickness will give rise todefects in the coating, especially voids and blisters due tothe evaporation of water formed during the curing of thepolyester powder coating. The surface roughness of the coatingis also affected by the coating thickness, but the maininfluence originates from the type and amount of filler used.The high compressive loads in a clamping force joint put highdemands on the stability of the coating and the defects must bekept to a minimum. A rough surface and defects such as voidswill give rise to stress concentrations and increased plasticdeformations in the coating, impairing the properties of theclamping force joint. In the second part of the study a well-defined freeradically cured vinyl ester resin has been used and studied insix different geometries in order to determine the dependenceof apparent mechanical properties on the particular size andshape of a sample when a sample is subjected to highcompressive loads. Variation of the specimen thickness,boundary conditions and loading conditions reveal that thegeometry of the sample has a significant effect on themechanical performance of the polymer. The apparent modulus andthe yield stress increases dramatically when thethickness-to-width ratio of the sample is reduced, whereas theydecrease when the friction between the sample and thecompression plate is reduced. The creep strain rate decreaseswhen the thickness of the material is reduced and it decreasesstill more when the amount of material surrounding thecompressed part of the sample is increased. This effect isimportant when designing parts to be used under compressiveloads. Properties measured macroscopically may not correlatewith the behaviour of the designed part since the geometry mayhave either a reinforcing or destabilising effect on thematerial. Creep and strain recovery tests on large specimensare used to develop a mathematical model including non-linearviscoelastic and viscoplastic response of a thermoset vinylester. The model is used in FEM calculations where theexperimental results are compared with the calculated resultsin order to model the trends of the material response whenvarying the sample geometry.