natural rubber ⋅ rubber poison ⋅ copper the role of copper ... · the ratio of the wallace...

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS

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Natural rubber ⋅ Rubber poison ⋅ Copper compounds ⋅ Thermooxidation ⋅ PRI ⋅ MRI

Oxygen in the air is responsible for the thermooxidation of rubber and is cata-lysed by “rubber poisons”, mainly by copper and manganese compounds. In this study, a variety of copper com-pounds were used such as copper stea-rate, copper oxide and copper sulphate. They were mixed with rubber. The ther-mooxidation of the rubber was evalua-ted using the traditional plasticity re-tention index (PRI) and the alternative Mooney retention index (MRI). The vul-canization behaviour of the mixtures contaminated by copper compounds was studied using a rubber process analyzer (RPA). Mechanical properties like tear strength and elongation at break were also tested.

Rolle von Kupferverbindungen als Katalysator bei der Thermo oxidation Naturkautschuk ⋅ Kautschukgift ⋅ Kupferverbindungen ⋅ Thermooxidation ⋅ PRI ⋅ MRI

Der Luftsauerstoff verursacht die Ther-mooxidation des Kautschuks und wird durch „Kautschukgifte“, vor allem durch Kupfer- und Manganverbindungen, ka-talysiert. In dieser Arbeit wurden diver-se Kupferverbindungen, wie Kupfer-stearat, Kupferoxid, Kupfersulfat, usw. verwendet. Diese Verbindungen wur-den mit Kautschuk gemischt. Die Ther-mooxidation des Kautschuks wurde mit dem traditionellen Plastizitätsretenti-onsindex (PRI) und mit der alternativen Mooney-Methode (MRI) bewertet. Das Vulkanisationsverhalten der durch Kup-ferverbindungen kontaminierten Gemi-sche wurde mit Rubber Process Analy-zer (RPA) untersucht. Behandelt wurden auch mechanische Eigenschaften wie Weiterreißwiderstand oder Bruchdeh-nung.

Figures and Tables: By a kind approval of the authors.

IntroductionThe degradation of natural rubber, like other polymers, is accelerated by sun-light (mainly by ultraviolet radiation) and by increased temperatures when oxygen is present. Degradation can lead to changes in the processing properties of natural rubber and also to changes in some mechanical properties of the final product. Some metal impurities can be present in rubber as well as in fillers and other chemicals used to process rubber. These metal impurities – rubber poisons - can accelerate the oxidation process.

Oxidative degradation is a process con-sisting of two competing reactions – scis-sion and crosslinking. In the case of natu-ral rubber, scission is the dominant phe-nomenon due to a methyl group in the alpha position of the double bond in the polyisoprene chain. The oxidation is also called autoxidation because it is an auto-catalytic process in which hydroperoxides, formed as the primary product of the re-action, decompose to produce free radi-cals – alkoxy, peroxy and hydroxyl - which initiate the free radical chain mechanism. This reaction begins slowly and increases its rate as hydroperoxides are produced. The scission of polyisoprene chains leads to a decrease in the molecular weight and viscosity of natural rubber. [1, 2]

Vulcanized rubber evolves differently during oxidative degradation due to sul-phur linkages created by the vulcaniza-tion. Many theories regarding mecha-nisms of oxidative degradation have been established. Norling et al. [3] notes that many types of linkages with differ-ent levels of stability can be created dur-ing vulcanization. If monosulfide or car-bon-carbon linkages are created, then cleavage of the polyisoprene chain takes place. Polysulfide bonds are more sensi-tive to oxidative degradation than the main chain. This was confirmed by Col-clough [4] and Mathew [5]. Oxidative degradation also depends on the type of vulcanization system, so conventional vulcanization systems have a different course of degradation than efficient vul-canization systems. Although the degra-dation of vulcanized rubber differs from that of raw rubber, the main degradation process takes place mainly on the poly-isoprene chain in both cases.

Some metal compounds, especially salts dissolved in NR, can play the role of a catalyst of rubber degradation. The most catalytic effects are associated with copper, manganese, iron and cobalt compounds. The catalytic effects are based on a change in the metal’s oxida-tion state. Scheme shows the oxidation and reduction of copper ions by hydrop-eroxides. As a result of these reactions, free radicals are created and the activa-tion energy of the initial autoxidation step is decreased. [3]

ROOH + Cu2+ → ROO∙ + Cu+ + H+ROOH + Cu+ → RO∙ + Cu2+ + OH-

The amount of rubber poisons in rubber should not exceed 5·10-3 wt. % according to analytical chemistry standards. [6] This value is not general for all rubber poisons because the catalytic effects of different metal compounds are not same.

The Role of Copper Compounds as Thermooxidation Catalysts

AuthorsDrahomír Čadek, Kateřina Zvolská, Alena Kadeřábková, Zdeněk Hrdlička, Jakub Havlín, Prague, Czech Republic Corresponding Author:Drahomir CadekDepartment of PolymersFaculty of Chemical TechnologyThe University of Chemistry and Technology, Prague Technicka 5166 28 Praha 6 - DejviceCzech RepublicE-Mail: [email protected]


32 KGK · 01-2 2020 www.kgk-rubberpoint.de

This study is focused on copper com-pounds and their influence on the oxi-dative degradation of NR. Not many studies have been published on this topic, and those that have been are old-er. Tuampoemsab et al. [7] studied the influence of some copper compounds on the molecular weight (M

_n and M

_w) of

purified NR. It is well known that NR contains natural antioxidants such as tocotrienol, proteins and amino acids. For the study of rubber poisons, the nat-ural antioxidants were removed from the NR. The most significant decrease in molecular weight after ageing was ob-served in the NR with copper complex – [Cu(NH3)4]

2+ – and the NR with CuSO4. The study confirmed that the degrada-

tive effect of copper depends on the form of compound present.

Villain [8] studied the influence of vari-ous copper compounds on the tensile strength of vulcanized natural rubber. He mentioned two theories that deal with oxidative degradation. In the first theory, the copper oxide is responsible for the deg-radation, because it can arise from the re-action of a copper compound with zinc oxide. In the second theory, the degrada-tion is caused by a copper ion. The most considerable decrease in the tensile strength of vulcanized NR after ageing was caused by compounds of easily dissociative copper acids such as chloride, sulphate and acetate. A similar decrease was observed with stearate and resinate. Their signifi-cant influence on degradation was attrib-uted to their dissolution in natural rubber.

The plasticity retention index (PRI) is a standardized method (ISO 2930) for de-termining the resistance of rubber to thermooxidation and for predicting rub-ber processing. It is measured with a Wallace plastimeter. PRI is expressed as the ratio of the Wallace plasticity of a rubber sample aged in an air oven with air circulating at 140°C for 30 minutes (P30) and the Wallace plasticity of an un-aged rubber sample (P0). The resulting value was expressed as a percent:

𝑃𝑃𝑃𝑃𝑃𝑃 = 𝑃𝑃30𝑃𝑃0

∙ 100

Bateman and Sekhar [9] studied the influ-ence of some NR metal impurities on the PRI. Increasing the amount of copper compounds negatively influenced the PRI.

The thermooxidation of rubber can also be studied via thermogravimetric analysis (TGA). Galiani et al. [10] showed in their work that the thermooxidation of natural rubber is not a one step process, but a pro-cess consisting of two or three steps.

The oxidative degradation of NR and its acceleration by three different copper compounds were studied in this work.

ExperimentalNatural rubber GH 10 (Ghana, Ghana Rubber Estates Limited) was used in this study. Three copper compounds – copper stearate, copper oxide and copper sul-phate – were mixed into the NR on a two-roll mill. At the beginning, a master batch of each copper compound was prepared – the total amount of the cop-per compound was mixed into 50 g of NR. This master batch was divided into four parts, and then each part was added to 200 g of NR. The mixing of the master batch with the NR was done on a two-roll mill at room temperature. The final concentrations of copper in the NR were approximately 5·10-4, 5·10-3, 5·10-2 and 5·10-1 wt. %. These values were verified by atomic absorption spectroscopy (AAS, ASTM D 4004-93).

The thermooxidation of the raw NR was evaluated using PRI measurements done with a Wallace rapid plastimeter, Mk V-P14 (Wallace Instruments), accor-ding to ISO 2930. The thermooxidation of the raw NR was also evaluated using a Mooney viscometer, V-MV 3000 (Mon-Tech). This new measurement was an al-ternative to the PRI and is called the Mooney retention index (MRI). Approxi-mately 25 g of NR was closed in a round cell with a rotor heated to 100°C. The test was Mooney viscosity ML (1+4)/100°C (ML – Mooney Large, 1 minute of prehe-ating without rotating and 4 minutes of measuring with the rotor turning) at 2 rpm. The MRI is expressed as the ratio of the viscosity of the aged rubber (M30, 30 minutes at 140°C) and the viscosity of the rubber that was not aged (M0):

𝑀𝑀𝑃𝑃𝑃𝑃 = 𝑀𝑀30𝑀𝑀0

∙ 100

The rubber mixtures were prepared ac-cording to the ACS I recipe (table 2). 125 g of NR with a copper compound was used in each mixture on the two-roll mill.

The vulcanization behaviour of the NR mixtures was measured with the rubber process analyzer, RPA 2000 (Alpha Tech-nologies), at 160°C (amplitude 0.5°, fre-quency 100 cpm). The dimensions of the

Fig. 1: PRI of raw NR and NR with varying amounts of copper compounds (in logarithmic coordinates)

1

1 Influence of copper compounds on PRI [9]Metal impurity phr PRI [%]– – 94

manganese(II) sulfate0.1 940.2 72

ferric sulfate0.05 830.1 580.2 44

2 ACS rubber mixture recipe (MBT – 2-mercaptobenzothiazole)Ingredient phrZinc oxide 6Stearic acid 0.5Sulphur 3.5MBT 0.5



vulcanized rubber sheets prepared in the hydraulic press were 160 x 160 x 1 mm. From each mixture, 4 sheets were pre-pared to be aged in an air oven with air circulating at 100°C for 2, 4 and 24 hours. One sheet was not aged, for reference.

The samples of vulcanized rubber (which weighed precisely 0.15g) were swollen in toluene for 10 days at room temperature. The toluene was changed twice (after 3 days and 6 days). After 10 days, the samples were removed from the toluene, their surfaces were quickly dried and then they were weighed. After 10 days of free air drying, the samples were weighed once more. The crosslink density of the vulcanized rubber was calculated according to the Flory-Rehner equation [11], where ν is the crosslink density, VS is the molar volume of the toluene, νr is the volume fraction of the crosslinked rubber at the swelling equi-librium with toluene, and χ is the Hug-gins interaction parameter:

𝜈𝜈 = − 1𝑉𝑉𝑆𝑆

∙ 𝑙𝑙𝑙𝑙(1 − 𝑣𝑣𝑟𝑟) + 𝑣𝑣𝑟𝑟 + 𝜒𝜒𝑣𝑣𝑟𝑟2

𝑣𝑣𝑟𝑟13 − 0,5 𝑣𝑣𝑟𝑟

The tensile properties of the rubber vul-canizates were measured using an In-stron 3365 according to ISO 37.

Results and discussionThe thermooxidation of raw NR and NR with copper compounds was evaluated using classical PRI measurements (figure 1). The PRI of NR with copper stearate decreased and then increased with more copper stearate in the mixture. The de-crease could be connected to an acceler-ation in degradative reactions when there is a higher amount of stearate. The increase in the PRI was probably caused by the hardening effect of the stearate. The highest concentration of copper was approximately 0.5 wt. %, but the concen-tration of stearate was ten times higher. Such a high amount of stearate can play a role as a hardening component. Helaly et al. [12] used zinc stearate in their re-search. They prepared rubber mixtures with higher amounts of zinc stearate and observed its influence on mechanical properties. The mechanical properties of the rubber vulcanizates increased with the amount of zinc stearate up to 9 phr. Then the mechanical properties rapidly decreased. It is possible that the copper stearate had a hardening effect on the non-vulcanized rubber as well. Copper (II) oxide contributed to the thermooxidation of the NR, but figure 1

makes it evident that its catalytic effect was not so strong. There was no harden-ing effect, unlike with copper stearate. A decrease in the PRI for NR with copper sulphate is also displayed in figure 1. Its trend is similar to that of copper stea-rate, but there was no explanation for hardening in the NR with the highest amount of copper sulphate. For this rea-son, MRI measurements were taken.

The MRI was measured in a way simi-lar to the PRI (same ageing time and temperature) with a Mooney viscometer (figure 2). The NR with copper stearate and copper (II) oxide had very similar trends, as was the case with the PRI

measurements. The MRI measurements for the two sheets of NR with the highest amounts of copper sulphate were differ-ent from the PRI measurements. Their Mooney viscosity values were very low, at the limit of what was measurable. Mooney viscosity, in contrast to Wallace plasticity, evaluated the viscosity of rub-ber samples not so complex. The MRI is more sensitive to the flow of material than the PRI. These measurements are comparable, but they are not suitable for evaluating the rubber poison content. The PRI and more so the MRI are appro-priate measurements for predicting the behaviour of rubber during processing.

Fig. 2: MRI of raw NR and NR with varying amounts of copper compounds (in logarithmic coordinates).

2

Fig. 3: Vulcanization curves of the reference NR mixture and NR mixtures with copper compounds (grey symbols approx. 5∙10-4 wt. %, black symbols approx. 5∙10-1 wt. %).

3


34 KGK · 01-2 2020 www.kgk-rubberpoint.de

The influence of copper compounds on vulcanization was not suspected. Lower amounts of copper compounds (approx. 5·10-4 wt. %) had less influence on vulcanization behaviour. The vulcan-ization curve of the rubber mixture with copper stearate has a lower degree of vulcanization. The highest amounts of copper compounds (approx. 5·10-1 wt. %) had a more significant influence on vulcanization. The vulcanization of the rubber mixture with copper stearate was slower than the reference rubber mixture. The same trend also exists in

the case of the mixture with copper sul-phate. It is evident that the degree of vulcanization is significantly lower. Maybe the reason for the deceleration of the vulcanization could be found in the acidity of the system, but that was not confirmed. The degradation of the arising net might be the reason for the decrease in the degree of vulcanization. Copper (II) oxide in the rubber mixtures had no influence on vulcanization (fig-ure 3).

The changes in the extent of crosslink-ing during the ageing of rubber vulcaniza-

tes (100°C) were observed using crosslink density. The crosslink density of the rub-ber vulcanizates with higher amounts of copper compounds (approx. 5·10-1 wt. %) was lower than the crosslink density of the vulcanizate without copper at all times during ageing. The crosslink density increased with ageing time for vulcaniza-tes with all types of copper compounds. Similar trends in crosslink density were observed by Mathew and De [5] in aged natural rubber vulcanizates. Lower amounts of copper oxide (approx. 5·10-4 wt. %) in the rubber mixture led to an in-crease in crosslink density at all times during ageing. It was probably caused by some aftercuring reactions that formed more bonds (figure 4).

The mechanical properties of the NR vulcanizates with copper compounds were evaluated by tensile strength and elongation at break. The ageing of the vulcanizates was the same as in the case of crosslink density measurements. The most considerable decrease in the tensile strength of rubber vulcanizates occurred between 4 and 24 hours of ageing (figure 5). According Mathew and De [5], the highest rate of degrada-tion is just between 4 and 24 hours of ageing at 100°C. Vulcanizates with low-er amounts of copper compounds (ap-prox. 5·10-4 wt. %) had higher tensile strength than the reference vulcanizate at all times during ageing. It is probably for the same reason as for the crosslink density. There could be some aftercur-ing reactions during ageing. They formed stronger bonds.

The tensile strength of the reference vulcanizate and vulcanizates with higher amounts of copper stearate and copper (II) oxide (approx. 5·10-1 wt. %) are rela-tively similar during ageing. Vulcanizates with copper sulphate experienced the most significant change in tensile strength. The reason could be found in the same way as in the case of crosslink density measurements. Copper sulphate was a more efficient catalyst of vulcani-zate degradation than copper stearate or copper (II) oxide.

Measurements of elongation at break show trends similar to those of tensile strength (figure 6).

Conclusions This study confirmed the harmful effects of copper compounds on the properties of raw natural rubber – the PRI/MRI and its vulcanizates – crosslink density, ten-sile properties.

Fig. 4: Relationship between the crosslink density of the reference NR vulcanizate / NR vulcanizates containing copper compounds (grey symbols approx. 5∙10-4 wt. %, black symbols approx. 5∙10-1 wt. %) and the time of ageing (in logarithmic coordinates).

4

Fig. 5: Relationship between the tensile strength of the reference NR vulcanizate / NR vulcanizates with copper compounds (grey symbols approx. 5∙10-4 wt. %, black symbols approx. 5∙10-1 wt. %) and the time of ageing (in logarithmic coordinates).

5



Fig. 6: Relationship between the elongation at break of the reference NR vulcanizate / NR vulcanizates with copper compounds (grey symbols approx. 5∙10-4 wt. %, black symbols approx. 5∙10-4 wt. %) and the time of ageing (in logarithmic coordinates).

6

In praxis, the very often used PRI can predict the behaviour of rubber during processing, but it cannot determine the rubber poison content or its influence on processing. Real concentrations of cop-per compounds in rubber are significant-ly lower than those used in this study. The MRI could be used as an alternative to the PRI. It proved to be more sensitive to the flow of rubber.

In praxis, the total amount of copper in the rubber and in other chemicals, like fillers, is measured. In some cases, these measurements can´t provide the infor-mation necessary to forecast the aging of rubber vulcanizates. This study showed how much the type of copper compound matters regarding rubber degradation. Copper sulphate had the most considera-ble effect on the thermooxidation of the

NR. This was probably caused by its disso-ciation. Copper stearate also had a signif-icant effect due to its solubility in NR, but it was not useful for study because of its hardening effect. Copper (II) oxide could be used as a model compound.

References[1] Shelton, J. R., Rubber Chem. Technol. 45

(1972) 359.[2] Kumar, A.; Commereuc, S.; Verney, V., Polym.

Degrad. Stabil. 85 (2004) 751.[3] Norling, P. M.; Lee, T. C. P.; Tobolsky, A. V., Rub-

ber Chem. Technol. 38 (1965) 1198.[4] Colclough, T.; Cunneen, J.; Higgins, G. M. C., J.

Appl. Polym. Sci. 12 (1968) 295.[5] Mathew, N. M.; De, S. K., Polymer 24 (1983)

1042.[6] Research Association of British Rubber Man-

ufacturers, Rubber Chem. Technol. 20 (1947) 821.

[7] Tuampoemsab, S.; Sakdapipanich, J.; Tanaka, Y., Rubber Chem. Technol. 80 (2007) 159.

[8] Villain, H., Rubber Chem. Technol. 23 (1950) 352.

[9] Bateman, L.; Sekhar, B. C., Rubber Chem. Technol. 39 (1966) 1608.

[10] Galiani, P. D.; Malmonge, J. A.; Soares, B. G.; Mattoso, L. H. C., Plast. Rubber 42 (2013) 334.

[11] Flory, P. J., Rehner, J., J. Chem. Physics. 11 (1943) 521.

[12] Helaly, F. M.; El Sabbagh, S. H.; El Kinawy, O. S.; El Sawy, S. M., Materials & Design 32 (2011) 2835.

14th Fall Rubber Colloquium – Call for papers

EVENTS

KHK 2020 Since 1994 the DIK e. V. has hos-ted the internationally acknowledged „Fall Rubber Colloquium“ every two years in Han-nover. This conference has developed into a discussion forum that has proven to be highly effective for the interactive transfer of innovative ideas and knowledge between universities, institutes and industry. A poster session will offer opportunities for more at-depth discussions with young scientists in the field. Additionally, several posters will be awarded. The 14th Fall Rubber Colloquium takes place from 10th to 12th of November, 2020 in Hannover (Germany). The organi-zers invite prospective presenters to upload on this homepage an abstract on any confe-rence themes. The abstract written in Eng-lish will be - once accepted by the Scientific Committee - the final document published in the Conference Proceedings.

Conference topics: ■ Materials ■ Reinforcement ■ Vulcanization ■ Processing ■ Aging and Lifetime ■ 3D-Printing ■ Analysis ■ Environment ■ Tires ■ Physics ■ Simulation ■ Sustainability

Accepted abstracts will be published in the Conference Procee-dings. The organiser recommends to follow the guidelines which could be find in temp-late files at https://www.dikautschuk.de/khk/call-for-papers/call-for-papers/

Those who use different software than Microsoft Word should use the style definitions defined in the guidelines. The ab-stract should be uploa-ded on https://www.di-kautschuk.de/khk/call-for-papers/abstract-up-load/ as a pdf-file.Submission deadlines:

Abstract Lectures: March 2nd, 2020 Notifi-cation of acceptance: June 15th, 2020; Abs-tract Posters: September 9th, 2020 Notifica-tion of acceptance: October 9th, 2020. n www.dikautschuk.de/khk

Prof. Ulrich Giese, Director DIK e.V. Bildquelle: Dr. Etwina Gandert/Redaktion KGK

natural rubber ⋅ rubber poison ⋅ copper the role of copper ... · the ratio of the wallace...

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