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Relaxation properties of particle filled coatings: Experimental study and modelling of a screw joint
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0003-3201-5138
2006 (English)In: Progress in organic coatings, ISSN 0300-9440, E-ISSN 1873-331X, Vol. 55, no 2, 112-118 p.Article in journal (Refereed) Published
Abstract [en]

The present study describes the mechanical behaviour of powder coatings used under very high compressive loads in clamping force joints. Carboxyl functional polyester powder coatings cured with hydroxyl functional P-hydroxyalkylamides with variations in amount of filler have been studied. The coatings were subjected to relaxation tests in tension and in compression. The tests in compression were performed in specially designed tests developed to study the behaviour of powder coatings under compressive loads in clamping force joints. The relaxation results for the matrix were used in a unit cell in micromechanical finite element (FE) model to predict the homogenised viscoelastic properties of the particle composite. These constitutive properties were subsequently used to evaluate the behaviour on a macromechanical scale in a screw joint. The model corresponds well with experimental data at ambient temperature. When increasing the temperature above the glass transition of the coating, however, the model predictions and experimental data differ. Experiments in compression show a much lower relaxation its compared to the FE model. The relaxation simulations of the coating under compressive loads from screw joints showed a significant sensitivity to the Poisson's ratio of the polymer matrix. As the Poisson's ratio approaches 0.5, the matrix becomes hydrostatically incompressible, which resulted in a negligible relaxation of the coating at the screw joint.

Place, publisher, year, edition, pages
2006. Vol. 55, no 2, 112-118 p.
Keyword [en]
powder coating, mechanical properties, compression, screw joint, finite element modelling, relaxation
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:kth:diva-6079DOI: 10.1016/j.porgcoat.2005.09.008ISI: 000235627300006Scopus ID: 2-s2.0-32144432885OAI: oai:DiVA.org:kth-6079DiVA: diva2:10686
Note
QC 20100921. Conference: Coatings Science International Conference. Noordwijk, NETHERLANDS. JUN 26-JUL 01, 2005Available from: 2006-09-05 Created: 2006-09-05 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Thermoset polymers and coatings subjected to high compressive loads
Open this publication in new window or tab >>Thermoset polymers and coatings subjected to high compressive loads
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

This study describes the mechanical response of thermoset polymers under high compressive loads. The study is divided into two parts. The first part focuses on the behaviour of a powder coating when used in a clamping force joint and how the properties vary when the chemical and physical structure of the coating is changed. The second part discusses the fundamental understanding of the behaviour of thermoset polymers with small thickness-to-width ratio subjected to compressive stresses, the aim being to develop mathematical material models for viscoelastic materials under high compressive loads.

In the first part polyester powder coatings were used with variations in molecular weight, number of functional groups of the resin, amount and type of filler and thickness of the coating. The coatings were subjected to conventional tests for coatings and polymers and also to specially designed tests developed to study the behaviour of powder coatings in clamping force joints.

The high compressive loads in a clamping force joint put high demands on the relaxation and creep resistance of the coating and the study shows the importance of crosslink density, filler content, and also coating thickness in order to achieve the desired mechanical properties of a coating.

A high reactivity of the resin, facilitating a high crosslink density and hence a high Tg, is the most important property of the coating. A film with high crosslink density shows increase in relaxation time and in apparent yield strength under compression, and also an increase in relaxation modulus and storage modulus in tension at temperatures above Tg.

Addition of fillers reduces the deformation during compression and tension, but also induces a lower strain at break and hence a more brittle coating. The reinforcing effect of the fillers is pronounced when increasing the crosslink density of the coating, especially in the compression tests. The effect is evident in compression even at low amounts of fillers, where the relaxation time and resistance to deformation are strongly increased. The combination of high crosslink density and addition of fillers is therefore desirable since fillers then can be used moderately in order to achieve a reinforcing effect in compression while minimising embrittlement.

The study also showed that increased coating thickness will give rise to defects in the coating, especially voids and blisters due to evaporation of water formed during the curing of the polyester powder coating. These defects will give rise to stress concentrations and increased plastic deformations in the coating, impairing the properties of the clamping force joint.

The results from relaxation tests in tension were used to create a micromechanical model. This model was used in finite element modelling to estimate the loss of clamping force in a screw joint and to correlate with the experimental results of the powder coatings.

In the second part of the study a well-defined free radically cured vinyl ester resin was used and studied in six different geometries in order to determine the dependence of apparent mechanical properties on the particular size and shape of a sample when it is subjected to high compressive loads. Variation of the specimen thickness, boundary conditions and loading conditions reveals that the geometry of the sample has a significant effect on the mechanical performance of the polymer. The apparent modulus and the yield strength increase dramatically when the thickness-to-width ratio of the sample is reduced, whereas they decrease when the friction between the sample and the compression plate is reduced. The creep strain rate decreases when the thickness of the material is reduced and it decreases even more when the amount of material surrounding the compressed part of the sample is increased.

Creep and strain recovery tests on large specimens were used to develop a mathematical model including non-linear viscoelastic and viscoplastic response of a thermoset vinyl ester. The model was used in FEM calculations where the experimental results were compared with the calculated results in order to model the trends of the material response when varying the sample geometry.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 59 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:22
Keyword
polymers, organic coating, thermoset, mechanical properties, compression
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:kth:diva-4091 (URN)91-7178-425-X (ISBN)
Public defence
2006-09-22, F3, KTH, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20100921Available from: 2006-09-05 Created: 2006-09-05 Last updated: 2010-09-21Bibliographically approved

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Publisher's full textScopushttp://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6THD-4HNSPN0-1&_user=4478132&_coverDate=02%2F01%2F2006&_alid=444168613&_rdoc=1&_fmt=summary&_orig=search&_cdi=5280&_sort=d&_docanchor=&view=c&_acct=C000034958&_version=1&_urlVersion=0&_userid=4478132&md5=581a53620a75d8e0d19babd48dc1982f

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Johansson, Mats

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