Non-destructive Determination of Curing State of Rubbers during Vulcanization Process

Ceren Ülger1, Erdem Aydın1, H. İlker Yelbay2, Ali Erkin Kutlu1, C. Hakan Gür2,3 1 Standart Profil, TR, [email protected] 2 Middle East Technical University Welding Tech. & Nondestructive Testing Research/Application Center, Ankara-TR, [email protected] 3 Middle East Technical University Metallurgical & Materials Eng. Dep., Ankara-TR

http://www.ndt.net/?id=22825 Abstract

EPDM (ethylene-propylene-diene monomer) elastomer is one of the most widely used synthetic rubbers in several industries ranging from automotive to heating, ventilation, and air conditioning. Gaskets, bumpers, auto parts, auto brake systems, electrical installation, dust covers, weather stripping and conveyor belts are typical application

More info about this article: examples. Quality control is important to ensure the rubber compound products meet the requested mechanical properties. Satisfactory properties can be obtained only by proper compounding and by appropriate vulcanization. Vulcanization is a chemical process that converts natural rubber and other polydiene elastomers into cross-linked polymers which have much improved mechanical properties. The aim of this study is to develop a non- destructive quality control method to determine the curing state, and thus the mechanical properties of elastomers by measuring ultrasonic wave velocity. A series of samples were prepared by curing the EPDM compound at 125oC for different periods ranging from 5 to 90 minutes. The results of the mechanical tests and ultrasonic measurements showed that ultrasonic wave velocity, hardness, and the maximum force increase with increasing curing time, in contrast to the tangent delta values.

1. Introduction

Rubbers are one of the mostly used polymer that can be classified as elastomers. Due to advantageous properties, such as high elasticity and high damping, the elastomers are widely used in tyres, dampers, gaskets, seals, conveyor belts, seismic isolators, sea fenders, harpoon bands, etc. In order to obtain the desired properties, the rubber compound, i.e., a mixture of rubber, fillers, oils, activators, cross linkers, accelerators and several ingredients, must be cross linked to form an elastic and shape persistent material [1]. Ethylene-propylene-diene monomer (EPDM) is a synthetic rubber that is formed by the combination of ethylene, propylene and unsaturated diene molecules. It has some advantages like good mechanical properties, saturated structure and resistance to aging, oxidation, low temperature and ozone [2-4], thus it is typically used in weather strips, belts, hoses, wipers and mounts in automotive industry [5]. The typical service temperature range is between –50°C and +150°C. The mechanical properties of the EPDM compound depend on the mixing parameters and the curing process (vulcanization). During curing, a three-dimensional network of polymer chains forms by cross-bonds among linear rubber chains, and therefore, a significant increase in the

Creative Commons CC-BY-NC licence https://creativecommons.org/licenses/by-nc/4.0/ resistance against external forces are observed [6,7]. During the curing process, EPDM compounds are heated to a temperature of about 200°C and then cooled. Curing reaction can be defined with 3 stages (Figure 1). First stage (the scorch time) is the safe process period because the curing does not start. Second stage is the curing period in which the cross links form between the polymer chains. Third stage is the over-curing period in which reversion, equilibrium, or marching mechanisms can be observed depending upon the rubber type and the accelerator system [8].

Figure 1. A typical vulcanisation curve [8]

Non-destructive techniques can be used for determining the mechanical properties of the EPDM compounds during or after the vulcanisation process. For instance, the curing procedure can be analysed by measuring the sound velocity differences, i.e., it is possible to avoid over-curing. Jaunich and Stark [9] monitored the vulcanization of natural rubber at different curing temperatures, and reported that the ultrasound wave velocity decreases with the increase in temperature. Sun et al. investigated the reaction kinetics of EPDM by using a non-invasive and non-destructive ultrasonic technique. Several compounds varying amounts of sulphur and accelerator are analysed [10]. The aim of this study is to develop a non-destructive quality control method to determine the curing state, and thus the mechanical properties of elastomers by measuring ultrasonic wave velocity.

2. Material and Method

The EPDM compound (Table 1) was prepared by the Standard Profile company. Disc shaped samples (12 mm thickness, 30 mm diameter) were cured at 125oC using a laboratory type vulcanisation press for different curing times (5, 7, 10, 12, 15, 20, 25, 35, 45, 60, 90 minutes). A low temperature was intentionally selected to obtain a lower curing rate and thus to monitor the vulcanisation process. The maximum force (Fmax) values were measured at 50% deformation in the compression test. Shore A and IR HD N hardness tests were also performed. In each test, at least three samples were used. To obtain the vulcanization curve, the isothermal curing test at 125°C was carried out using a Rubber Process Analyser (RPA) which is an advanced rotorless rotational shear rheometer.

2 Table 1. The EPDM compound Compounding materials Phr (parts per hundred parts of rubber) EPDM 100 Carbon Black 150 White Filler 20 Oil 30 Activators 5 Sulfur and Curing Agents 3

Dynamic Mechanical Analysis (DMA) was done at the ambient temperature using the compression plates. The DMA instrument deforms the 0.05 mm thick samples, and measures the phase shift as a response in the dynamic mode. Static strain and frequency values were 0.15 and 3 Hz, respectively. The modulus and the tangent delta (tan) values were calculated using the equations 1 to 4.

Modulus G = Stress / Strain (1) Elastic (storage) modulus G’ = G cos       Viscous (loss) modulus G’’ = G sin (3) Tangent delta   tanG’’/G’ (4)

Ultrasonic wave velocities were measured just after the curing process by using ultrasonic test equipment and a straight beam probe with a central frequency of 5 MHz).

3. Results and Discussion

The vulcanisation curve of the EPDM compound is given in Figure 2. It is known that mechanical properties of EPDM compounds significantly improve by vulcanization reaction. During vulcanization, three-dimensional networks form and they lead to increase the mechanical properties such as stiffness, hardness, and modulus.

Figure 2. The results of the isothermal curing test at 125°C

Figure 3 shows that IRHD N and ShA hardness values significantly increase with the curing time. However, after 25 minutes hardness stays almost constant due to the saturation of sulphur bonds, which indicates the termination of the cross-linking reaction.

The results of the DMA showing the change in the tan value and the maximum force (Fmax) with the curing time is given in Figure 4. Fmax increases with the curing time

3 however, it stays almost constant after 45 minutes. The tan value is related to the viscoelastic behaviour of the elastomer, and it shows a tendency that is opposite to that of the vulcanization curve. Figure 5 gives the vulcanization curves for the storage modulus (S’) and the tan value. S’ was determined for the curing time of 45 minutes at 125°C. It is seen that S’ increases whereas tan decreases with curing time. The lower the tan value the higher the elastic modulus.

70 65

60 60

55

50 50

45

Hardness - ShoreA Hardness Hardness - IR N HD Hardness 40

40

30 35 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Curing Time (min) Curing Time (min) Figure 3. Variation of hardness with the curing time

5000 0.25

4000 0.20

3000 0.15

tand

Fmax,N

2000 0.10

1000 0.05 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 Curing Time (min) Curing Time (min) Figure 4. Variations of Fmax and tan with the curing time

Figure 5. Vulcanization curves for the storage modulus and tan

Figure 6 shows that the ultrasonic velocity increases with the curing time. However, over- curing starts after 20 minutes and the velocity becomes almost constant. The modulus of

4 rubber increases during vulcanization due to the rising of the cure degree or cross-link density, thus, the ultrasonic wave velocity increases. Similar results had been previously reported [11].

1475

1450

1425

1400

1375

ultrasound wave velocity, m/s velocity, wave ultrasound

1350

0 10 20 30 40 50 60 70 80 90 Curing Time (min)

Figure 6. Variation of longitudinal wave velocity with curing time

4. Summary and Conclusion

A series of samples were prepared by curing the EPDM elastomer at 125oC for various time periods. IRHD N and ShA hardnesses, maximum force, ultrasonic wave velocity were measured and the tan values were calculated. With increasing curing time the elastic modulus increases correspondingly hardness, maximum force and longitudinal wave velocity increase while tandecreases. The measurement of ultrasonic wave velocity seems to be a practical method to monitor the curing state as an alternative to the conventional cure-meters.

5. References

1. M Jaunich, W Stark and B Hoster, “Monitoring the vulcanisation of elastomers: Comparison of curemeter and ultrasonic online control”, Polymer testing, 28(1), pp 84-88, 2009. 2. A Ciesielski, “An introduction to rubber technology”. Smithers Rapra Publishing, 1999. 3. DL Tillier, J Mesldijk, G Hohne, PM Frederik, O Regev and CE Koning, “About morphology in ethylene-propylene(-diene) copolymers-based latexes”, Polymer, 46(18):7094-108, 2005. 4. M Alagar, SM Abdul Majeed, A Selvaganapathi and P Gnanasundaram, “Studies on thermal, thermal aging and morphological characteristics of EPDM-g- VTES/LLDPE”, Eur. Polym. J., 42(2), pp 336-347, 2006.

5 5. O Faruk, J Tjong and M Sain, “Lightweight and Sustainable Materials for Automotive Applications”., CRC Press, 2017. 6. D Zaimova, E Bayraktar and N Dishovsky, “State of cure evaluation by different experimental methods in thick rubber parts”, J. Achiev. Mater. Manuf. Eng., 44(2), pp 161-167, 2011. 7. SR Khimi, KL Pickering, “A new method to predict optimum cure time of rubber compound using dynamic mechanical analysis”, Journal of Applied Polymer Science, 131(6), 2014. 8. B Karaagac, B Inal and V Deniz, “Predicting optimum cure time of rubber compounds by means of ANFIS”, Materials and design 35, pp 833-838, 2012. 9. M Jaunich, W Stark, “Monitoring the vulcanization of rubber with ultrasound: Influence of material thickness and temperature”. Polymer testing, 28(8), pp 901- 906, 2009. 10. H Sun, K Liang, J Li and S Guo, “Real-time Monitoring the Vulcanization of EPDM Rubber with Ultrasound: A Study on the Reaction Kinetics”, Journal of Macromolecular Science, Part B, 52(10), pp 1341-1354, 2013. 11. L Congmei, S Huimin, W Shan, H Jian, L Jiang and G Shaoyun, “In-process ultrasonic measurements of orientation and disorientation relaxation of high-density polyethylene/polyamide-6 composites with compatibilizer”, Journal of Applied Acience 116, 320, 2010.

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