CDB 4423 Polymer Process Engineering
May 2017
EXPERIMENT 6
: DETERMINE THE COATING ADHESION BETWEEN EPOXY AND STEEL BY PULL OFF ADHESION TESTER
GROUP
: 3
GROUP MEMBERS
: KOI ZI KANG
18868
NORANISAH BINTI JAMIAN
19339
SADADINE MAHAMAT YOUSSOUF
17839
LECTURER
: DR WAN ZAIREEN NISA YAHYA
DATE OF EXPERIMENT
: 6th JULY 2017
Table of Contents
Chapter 1: Introduction 1.1 Abstract .................................................................................................................... 1 1.2 Problem Statement ................................................................................................... 1 1.3 Objectives ................................................................................................................ 1
Chapter 2: Literature Review 2.1 Coatings ................................................................................................................... 2 2.2 Adhesion and Cohesion ........................................................................................ 2-3 2.3 Coating Deterioration............................................................................................... 3 2.4 Adhesion Testing Methods ...................................................................................... 3
Chapter 3: Methodology .........................................................................................................4-5
Chapter 4: Results and Discussions 4.1 Results ................................................................................................................... 6-7 4.2 Discussions .........................................................................................................7-10
Chapter 5: Conclusion ............................................................................................................. 11
References ........................................................................................................................... 11-12
CHAPTER 1: INTRODUCTION 1.1 Abstract
1.2 Problem Statement What are the effect of different curing time, curing temperature and effect of water on mechanical properties of coating material?
1.3 Objectives The experiment is designed to improve understanding on the concept of coating adhesion. The objectives of the experiment are: •
To understand the concept of coating adhesion
•
To relate the effect of different curing time, curing temperature and effect of water on mechanical properties of coating material
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CHAPTER 2: LITERATURE REVIEW 2.1 Coatings In general, coating is a protective layer that is applied to the surface of an object which may be for decorative or protective purposes (Tator, 2015). The coating itself may be an all-over coating, completely covering the substrate, or it may only cover parts of the substrate. Tator (2015) mentioned that coatings also provide light reflectivity, camouflage surfaces, reflect and absorb heat. Examples of coatings include product label on many drink bottles, non-stick PTFE-coated cooking pans and epoxy-coated pipelines. 2.2 Adhesion and Cohesion The bond performance of coating to substrate is related to adhesive strength and cohesive strength. Savage (2005) defined adhesive strength as the force required to pull substrates apart from each other that is held by adhesive while cohesive strength which is developed during curing process refers to the forces of attraction that hold the particles together in an adhesive structure (Adhesion/Cohesion Theory, n.d.). Adhesion usually determines the lifespan of the paint system. Low adhesion at the coating substrate interface can results in blisters forming at the interface or even lifting of the paint film (Tator, 2015).
2.3 Coating Deterioration According to Tator (2015), coating deterioration usually happens due to few possible reasons as follow: •
Poor surface preparation or insufficient coating thickness
•
Incorrect formulation of paint or coating
•
Coating used is not suitable for the environment
•
Improper or insufficient mixing of the coating at the time of application
•
The drying and/or curing condition of the coating after application is inadequate
•
Chemical, physical, and/or mechanical damage to the coating system during exposure
2.4 Adhesion Testing Methods There are several adhesion testing methods introduced to determine how well a coating is bonded to the substrate such as knife test, tape test and pull-off adhesion test. It is vital to record 2
if the bond failure was adhesive or cohesive after the test (Defelsko, n.d.). The respective standard procedure manual for each test is listed in Table 2.1 (Defelsko, n.d.). Table 2.1 Standard Procedure Manual for each Adhesion Testing Method Adhesion Testing Method
Standard Procedure Manual
Knife Test
ASTM D6677
Tape Test
ASTM D3359
Pull-off Adhesion Test
ASTM D4541 and ISO 4624
Scrap Test
ASTM D2197
In this experiment, pull-off adhesion test was used to determine the adhesive strength of the coating. Pull-off adhesion test is a more quantitative test in which a loading fixture, commonly called a dolly or stub, is bonded by an adhesive to a coating (Defelsko, n.d.). With the use of a portable pull-off adhesion tester, the applied load is increased until the dolly is pulled off. The force required to pull the dolly off is a measurement of tensile strength in pounds per square inch (psi) or mega Pascals (MPa). Failure will occur along the weakest area within the system that includes dolly, adhesive, coating system, and substrate. A pull-off adhesion tester model is shown in Figure 2.1.
Figure 2.1 Pull-off Adhesion Tester Model
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CHAPTER 3: METHODOLOGY Brief methodology of the experiment is described as follow: 1. The surface of the dollies and the test areas were cleaned to keep them free from oil, moisture and dust before starting the experiment to ensure a good bond between the dolly face and the coating. 2. Regular Araldite, a two-components epoxy paste were mixed in roughly equal portions. 3. Three conditions of curing were being examined: •
Three dollies were cured for 2 hours at room temperature. Then, the strength of adhesiveness of two dollies were measured.
•
Three dollies were cured in the oven at 120°C for 2 hours. Then, the strength of adhesiveness of two dollies were measured.
•
The third dollies from both curing conditions were immersed in water bath for 5 days. Then, the strength of adhesiveness was measured.
4. As the dollies cured for 2 hours at room temperature were not sufficiently cured as one of the dollies failed the adhesion test, the curing time for the remaining two dollies was extended to 24 hours. 5. The results were recorded in accordance with validity of the test result as follow: •
coating was fully adhered to the dolly (valid)
•
coating was adhered to part of the area of the dolly (partial adhesion failure)
•
no coating present on the dolly (failure of the adhesive)
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CHAPTER 4: RESULTS AND DISCUSSIONS 4.1 Results Table 4.1: Results for Pull-off Adhesion Test of Epoxy Coating 1st Sample Condition
Strength
Validity of Test
Strength
Validity of
(MPa)
Result*
(MPa)
Test Result
No
Invalid due to
strength
partial adhesion
N/A
N/A
recorded
failure
5.1
N/A
N/A
7.5
7.0
Valid
N/A
N/A
N/A
N/A
Cured at room temperature for 2 h
2nd Sample
Cured at room temperature for 24 h Cured at 80 °C for 2 h Cured at room temperature for 24 h and immersed in
Valid
3.9
water bath for 5 days Cured at 80 °C for 2 h and immersed in water bath for 5
5.1
days *Test result is considered valid if the coating fully adheres to the dolly. **There are only one sample cured at room temperature for 2 h and 24 h respectively due to incomplete curing when left to cure for 2 h.
4.2 Discussions 1. Record the strength of your sample. The results were recorded in Table 4.1. 2. Discuss the results and explain the difference between the cases. Case 1: One sample cured at Room Temperature for 2 h The adhesion test failed as the epoxy coating was not cured sufficiently. Although the curing time of the epoxy stated in the manual is 2 h at room temperature, the slow curing process is
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probably due to the broken cap of the curing agent which causes oxidation of the curing agent. Therefore, another sample was left to cure for 24 h to allow the sample to be cured sufficiently.
Case 2 and Case 3: One sample cured at Room Temperature for 24 h and another sample cured at 80 °C for 2 h As curing temperature increases, the molecules are highly mobilized due to increased kinetic energy, thus crosslinking density increases. The increase in crosslinking density will reduce the free volume of the cured resin (Abdelkader & White, 2005). Besides, the increased degree of crosslinks will strengthen the bonding between coating and substrate which enhances the adhesion strength of the coating. Therefore, the sample cured at higher temperature will require to larger force required to remove the coating away from the substrate.
Case 3 and Case 4: One sample cured cured at 80 °C for 2 h and another sample cured at 80 °C for 2 h and immersed in water bath for 5 days As epoxy coating suffers inherent porosity due to hydrolytic degradation, water could penetrate through the holes in epoxy coating and reach the coating substrate interface and corrodes the metal (Ramezanzadeh & Attar, 2011). Presence of water could speed up the formation of rust on the metal surface and that will lift the coating and greatly reduce the bonding between coating and substrate (Tator, 2015). That is why the sample immersed in water bath has a lower adhesive strength as compared to the sample without immersed in water bath.
Case 4 and Case 5: One sample cured at room temperature for 24 h and another sample cured at 80 °C for 2 h and both samples were immersed in water bath for 5 days Sample cured at lower temperature (room temperature) has lower degree of crosslinking as compared to sample cured at higher temperature (80 °C). Therefore, water absorption of the former sample will be much higher as water has more access in the former sample due to less hindrance or more free volume in the epoxy matrix. Therefore, there will be more formation of rust in the former sample that will cause more reduction in adhesive strength of the coating as compared to the sample cured at higher temperature. The experimental result is in accordance with this theory.
3. Give TWO (2) examples of application and discuss it. One of the coating application is for corrosion protection. Corrosion process or electrochemical reaction occurs at the metal-coating interface when corrosive media could reach the metal 6
substrate surface in the presence of water (Wan et al., 2017). Therefore, superior adhesive strength between coating and metal substrate provides longer period of protection as well as better protection to the substrate surface by resisting penetration of water or other corrosive media to the metal-coating interface. Another application of coating is in paint industry. Paint coatings are widely used to decorate and protect surfaces of buildings, cars or consumer products. Excellent adhesive strength of paint coating is highly desired to increase durability and to maintain performance of the coating especially when the coating is exposed to harsh environment.
CHAPTER 5: CONCLUSION
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REFERENCES Abdelkader, A., & White, J. (2005). Water absorption in epoxy resins: The effects of the crosslinking agent and curing temperature. Journal of Applied Polymer Science, 98(2005), 2544-2549. Adhesion/Cohesion Theory. (n.d.). Retrieved July 11, 2017 from http://www.adhesives.org/adhesives-sealants/adhesives-sealants-overview/structuraldesign/adhesion-cohesion-theory Defelsko. (n.d.). Adhesion Testing Methods. Retrieved July 11, 2017 from http://www.defelsko.com/technotes/adhesion_methods.htm Ramezanzadeh, B., & Attar, M. M. (2011). Studying the corrosion resistance and hydrolytic degradation of an epoxy coating containing ZnO nanoparticles. Materials Chemistry and Physics, 130(3), 1208-1219. doi:http://dx.doi.org/10.1016/j.matchemphys.2011.08.065 Savage, G. (2005). Practical Aspects of Failure Prevention in Bonded Joints on Primary Load Bearing Structures. Anales de Mecánica de la Fractura, 22, 273-282. Tator, K. B. (2015). Coating Deterioration. ASM Handbook, 5B, 462-473. Wan, H., Song, D., Li, X., Zhang, D., Gao, J., & Du, C. (2017). Failure Mechanisms of the Coating/Metal Interface in Waterborne Coatings: The Effect of Bonding. Materials, 10(4). doi:10.3390/ma10040397
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