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Volume 2 Paper 28


Influence of Ageing on Mechanical Properties of Epoxide Coating

D. Kotnarowska

Radom Technical University, Al. Chrobrego 45, 26-600 Radom, Poland

Abstract

The paper describes the influence of environmental factors such as: ultraviolet radiation, thermal shocks, aggressive media (5% water solutions of: sulphuric acid, potassium hydroxide and sodium chloride) as well as weathering on the course of mechanical characteristics of an epoxide coating. It was stated that ageing processes that occur in the epoxide coating cause deterioration of its mechanical properties, such as: hardness, tensile strength as well as unit elongation. They also influence changes in mechanical characteristics obtained by means of dynamic mechanical thermal analyse, i.e. elongation and dynamic storage modulus E’.

Keywords: epoxide coating, ageing, mechanical properties

Introduction

Optimal choice of organic coatings that protect technical equipment – during the design process – requires a knowledge of wear processes of these coatings. Therefore, it is necessary to carry on model (accelerated) tests in order to predict the coating service life because multi-layer metal-organic coatings perform their protective role for a long time, even for fifty years. Unfortunately, there is not yet developed an universal accelerated test and it is not found the correlation between results of accelerated tests and weathering. Such situation results from a non-sufficient knowledge of destruction processes of an organic coating under the influence of ambient factors. The multicriterion investigations of physico-chemical properties of organic coatings aged under various conditions that are carried out by the author complete the knowledge from this field. The paper describes the effects of destruction of epoxide coatings aged with ultraviolet radiation, thermal shocks, aggressive media and weathering. The effect of these factors on the course of mechanical characteristics was estimated as well. The author uses the investigation results for prediction the organic coating service life.

Investigation Methods

Materials and Sample Preparation

The model investigated coating was made of chemically resistant epoxide paint. The main components of this paint were: epoxy resin Epidian 119, pigments (iron oxide red), fillers (barium sulphate, microtalk) and diluent (ethyloglicol). Polyaminoamide was used as the curing agent. Its mass fraction (calculated stoichiometrically) amounted 30% of the whole composition. After addition of the curing agent the paint was stirred for half an hour and then, after two hours, the coating application started. Three-layer coatings were obtained by means of air operated spraying on the polyester sheeting as an apparent base. The layers were cured in two stages: at 20oC for 24 hours and at 120oC for 0.5 hour. The samples were acclimatised before testing (according to the Polish Standard PN-86/C-81510) for 10 days at ambient temperature 20oC and at relative humidity (65�5)%.

Ageing Conditions of the Epoxide Coating

Destruction kinetics of the epoxide coating aged with 5% water solutions of sulphuric acid, potassium hydroxide and sodium chloride as well as ultraviolet radiation, thermal shocks and weathering was investigated. The ageing conditions are presented in Table 1. After ageing samples were washed with distilled water and then – after wiping with absorbent paper – dried for 24 hours at the temperature 20oC.

Table 1.  Ageing conditions of the epoxide coating

No.

Ageing kind

Ageing conditions

1

2

3

1.

Ultraviolet radiation (UV)

Two lamps LRF 250E40 without glass bulbs were used as the source of ultraviolet radiation (wavelength of 300-460 nm). Power of each of them was 250 W. Epoxide coating samples were placed in the distance of 300 mm from the UV source. Ageing time - 1008 h.

2.

Thermal shocks (ST)

Ageing in the climatic chamber under the influence of thermal shocks with the following cycle:

for 16 h at the temperature (34�2)oC,

for 8 h at the temperature (-23�2)oC.

Ageing time - 1008 h.

3.

5% water solutions of:

- sulphuric acid (H2SO4),

- potassium hydroxide (KOH),

- sodium chloride (NaCl)

Immersion in the 5% water solutions of these substances for 1008 hours.

4.

Weathering (WN)

Influence of climatic factors on the epoxide coating samples attached to the stands in the climatic station in Radom.

Ageing period: 19.09.97 to 31.10.97 (1008 h)

Methods of Investigation of Physico-Chemical Properties of Epoxide Coatings

Estimation of wear processes of the epoxide coatings aged according to Table 1 was done with the help of methods presented in Table 2.

Table 2. Methods of epoxide coating investigations

No.

Investigation method

1

2

1.

Infrared spectroscopic investigations (FTIR) of the degree of ageing with the use of the spectrophotometer I-seriesTM

2.

Thermogravimetric investigations with the use of the derivatograph Q 1500D

3.

Microscopic investigations with the use of an electron microscope S246N, optical microscope (Neophot) and scanning electron microscope LEO 435 Pi

4.

Surface profile investigations with the use of the profile measurement gauge Hommel T2000

5.

Investigations of mechanical properties using the tensile testing machine Tiratest-2160

6.

Dynamical strength investigations (DMTA) during cyclic tension with the use of PL-Dynamical Thermal Analyser MK II

7.

Hardness investigations with the use of Erichsen hardness tester (according to the Polish Standard

PN-79/C81530)

Effect of Ultraviolet Radiation on Chemical Ageing of the Epoxide Coating

The changes occuring with time in the chemical structure of the epoxide coating superficial layer were studied in infrared spectroscopy investigations. It was found an increase of absorption of infrared radiation in the range 1800-1392 cm-1 what indicates ageing processes of the epoxide. The band 1800-1552 cm-1 included in this range represents oxidation products [1�3]. It contains the band that represents carbonyl group vibration. In case of the epoxide coatings aged with thermal shocks, weathering and with 5% water solutions of sodium chloride or potassium hydroxide it was stated an increase in carbonyl group intensity in the band 1807-1700 cm-1 – with the maximum at 1740 cm-1. Coatings aged with ultraviolet radiation were characterised by the highest carbonyl group intensity (in the whole measured range) and the band representing these groups was 1807-1640 cm-1 – with the maximum at 1717 cm-1 [1]. The band of carbonyl groups was not found in the range of 1807-1640 cm-1 in case of unaged coatings and coatings aged with 5% water solution of sulphuric acid [1]. The increase of infrared absorption observed in the bands 1472-1424 cm-1 and 1391-1344 cm-1 corresponds to the increase of amount of CH3 groups that are formed as a result of bond cleavage in epoxide. Increase of infrared absorption in the band 1422-1392 cm-1 corresponds to the increase of amount of CH2 groups (adjacent to carbonyl groups) what gives an evidence of epoxide oxidation progress [2]. The increase of intensity of the band having the maximum at 3400 cm-1 with ageing time was also found. This band represents valence vibration of hydroxyl group [1, 3].

It results from the total absorbance characteristics obtained with the use of infrared spectroscopic investigations that the ageing changes in the chemical structure of the superficial layers of the epoxide coating are non-homogenous. The most intensive ageing is due to ultraviolet radiation. The profilometric and microscopic investigations proves, however, a non-homogenous physical wear (cracking, pigment and filler spalling) of the epoxide coating surface under the influence of ambient factors. The results of microscopic investigations allow to state that number and size of the cracks depend on the ageing kind. For instance, cracks generated under the influence of 5% water solution of sulphuric acid were, in general, in the shape of rings with the diameter of 40 to 90 μm m (for 1008 h ageing) situated around blisters. These cracks showed tendency to become larger with ageing time. However, cracks generated under the influence of other media were in the shape of bands 150 to 190 μm long and 2.5 to 10 μm wide. In the case of ultraviolet radiation action, number and size of the cracks increased with ageing time. Moreover, these cracks were characterised by silver shines in the reflected light of an optical microscope what was observed in case of weathering too. Shape and shine of these cracks were similar to the "silver cracks" described by I. Narisava [4] but they were ten times wider.

Substantial chemical and physical ageing effects of investigated epoxide coatings was caused by the action of 5% water solution of sulphuric acid as well as UV radiation what, in particular, can be seen analysing the courses of thermogravimetric characteristics. The greatest decreases of the decomposition start temperature as well as the most intensive thermal decompositions (Fig. 1) were found for these ageing kinds in the whole ageing range (the measurements were done after 240, 360, 744 and 1008 hours).

 

Fig. 1. Influence of ageing for 1008 h on the temperature of thermal decomposition (Tu) of the 5, 10, 40% of the epoxide coating mass (0 – unaged coating)

Essential ageing changes, in case of action of 5% sulphuric acid water solution, were confirmed in the investigations by the means of dynamic mechanical thermal analyse (DMTA). A substantial increase (by 30%) of the glass transition temperature was found for 1008 hour ageing (Fig. 2). A decrease of the glass transition temperature – on the average by 10-15% - was found for the other ageing kinds (Fig. 2).

1)

2) 

Fig. 2. Influence of temperature on loss factor (tan δ) during dynamical tension of the epoxide coating aged for 1008 hours

Action of 5% sulphuric acid water solution caused an increase of waviness depth from 1.9 μm (for the unaged coating) to 63 μm (for the coating aged for 1008 hours) [1]. The greatest changes of the coating roughness, estimated with arithmetic mean roughness (Ra) and mean peak-to-valley height (Rz) were caused by the action of ultraviolet radiation (Fig. 3, 4).

Fig. 3. Arithmetic mean roughness (Ra) of the epoxide coating in dependence of the kind of ageing for 1008 hours

Fig. 4. Mean peak-to-valley height (Rz) of the epoxide coating in dependence of the kind of ageing for 1008 hours

This was connected with pigment and filler spalling from the coating structure as a result of the loss of coherence with the epoxide. This kind of ageing contributes also to the greatest decrease of the coating hardness (Fig. 5). After 1008 hour ageing the coating hardness decreased by more than 25%. In case of other ageing kinds the observed hardness decrease was from 5 to 13% - except for the ageing with thermal shocks, when the hardness had almost the constant value.

Fig. 5. Influence of ageing on the coating hardness (according to the Polish Standard PN-79/C-81530)

Ageing of the coating contributes to the decrease of its tensile strength (Fig. 6) [1].

Fig. 6. Influence of ageing on the tensile strength of the epoxide coating.

Kind of the ageing factor:

The highest tensile strength was obtained after ageing with ultraviolet radiation lasting from 0 to about 700 hours. This should be explained by additional after-bake curing of the coating. After 800 hours of ageing, the coating showed high embrittlement what made carrying on the investigation impossible because the coating ruptured when being fixed in the tensile testing machine. The lowest tensile strength – in almost the whole range of investigation – was obtained in case of weathering. It was also observed in this case the highest unit elongation at maximal tensile stress and at rupture (Fig.7) [1].

 

Fig. 7. Influence of ageing on the unit elongation at rupture of the epoxide coating

Ruptureability of epoxide coatings, estimated by means of dynamic mechanical thermal analyse (DMTA), has changed with ageing time. The unaged coating and the coatings aged for 240 and 360 hours ruptured in the temperature range from 120 to 150oC – except for the coating aged with 5% sulphuric acid water solution (Fig. 8) [1]. On the contrary, the coatings aged for 744 and 1008 hours did not rupture in the temperature range from 0 to 200oC (Fig. 9) [1].

1) 

2)

Fig. 8. Influence of kind of ageing for 360 hours on the epoxide coating elongation

1) 

2) 

Fig. 9. Influence of kind of ageing for 1008 hours on the epoxide coating elongation

In case of the coatings aged with the most destructive factors, such as ultraviolet radiation or 5% sulphuric acid water solution, it was stated in DMTA measurements that elongation increased with ageing time and with temperature. In case of other ageing kinds, the coating elongation decreased in the temperature range of 0�200oC. The coating aged with 5% water solution of sulphuric acid showed an increase in dynamic storage modulus E’ – in the high-elastic state of the epoxide (Fig. 10) and an increase of the glass transition temperature (Fig. 2) what gives the evidence of increased crosslinking of such aged coatings [5,6].

1) 

2) 

Fig. 10.  Influence of temperature on the value of dynamic storage modulus E’ during dynamic tension of the coating aged for 1008 hours

Conclusions

Run of mechanical characteristics of the epoxide coating substantially depends on the ageing kind.

Ageing of the epoxide coating causes deterioration of its mechanical properties, such as: hardness, static tensile strength, unit elongation. It also influences changes in mechanical characteristics obtained using dynamic mechanical thermal analyse, i.e. elongation and dynamic storage modulus E’.

Embrittlement of epoxide coatings increases with ageing time. It is also affected by the kind of ageing. The coating aged with ultraviolet radiation showed the highest embrittlement. It resulted from significant oxidation degree of the epoxide.

As a result of ageing of the epoxide, pigment and filler particles lose coherence with it and are spalled from the coating structure. This results in an increase of the coating roughness. The coating aged with ultraviolet radiation showed the highest roughness.

References

  1. D. Kotnarowska, Influence of ambient factors on operational parameters of the coatings protecting technical devices, Monograph No. 40/1999, Radom Technical University (in Polish)
  2. D. Kotnarowska, Influence of ageing process on epoxide coating durability, Monograph No. 12/1994, Radom Technical University (in Polish)
  3. L.A. Kazicyna, N.B. Kupletska: Methods of spectroscopic determination of the organic compound structure, Warszawa , PWN 1976.
  4. I. Narisava, Resistance of polymeric materials, Moscow, Chemistry 1987.
  5. Z. Brojer, Z. Hertz, P. Penczek, Epoxide resins, Warszawa, WNT 1982.
  6. W. Schlesing: Dynamische thermomechanische Analyse an freien Lackfilmen. Farbe und Lack 1993 N. 11 S. 918-923.