COMPOSITES BASED ON ACRYLONITRILE BUTADIENE …Simona Nikolova, Mihail Mihaylov, Nikolay Dishovsky...

Simona Nikolova, Mihail Mihaylov, Nikolay Dishovsky

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COMPOSITES BASED ON ACRYLONITRILE BUTADIENE RUBBER DESIGNED FOR SEALANTS CONTACTING POTABLE WATER

Simona Nikolova1, 2, Mihail Mihaylov2, Nikolay Dishovsky2

ABSTRACT

The present work reports on the preparation and investigation of the properties of composites based on acry-lonitrile butadiene rubber (NBR) aimed for production of sealants in water supply networks carrying hot and cold potable water. The studies involve 12 NBR-based composites. A conventional, semi-efficient and efficient sulphur-vulcanization system is used for four of the composites. A vulcanizing agent, dicumyl peroxide, was used in the other eight composites. The results obtained show that the type of the vulcanization system used has a significant impact both on the vulcanization characteristics of the investigated rubber compounds and on the mechanical properties of their vulcanizates. The comparison of the results obtained and the requirements of standard EN 681-1:1996 refer-ring to the mechanical properties of the elastomer composites used for production of sealants for pipes carrying hot and cold potable water reveal that composition P-10 is the most suitable for the purpose. The microbiological characteristics of the water in contact with the composites indicate that no bacterial growth occurs in the samples tested as a result of this contact. The latter does not lead to a change of the organoleptic parameters as well. Higher values of heavy metals, nitrates or nitrites are not observed.

Keywords: acrylonitrile butadiene rubber, rubber composites, sealing elements, food contact, potable water.

Received 04 October 2018Accepted 03 December 2018

Journal of Chemical Technology and Metallurgy, 54, 2, 2019, 249-259

1Tehnoguma-Nikolov DOOEL, Stip, Macedonia2Department of Polymer Engineering University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected]

INTRODUCTION

The elastomers and the end products based thereon are widely used in various industrial fields. Besides the production of typical rubber industry products (tires, conveyor belts, technical rubber goods, etc.), their ap-plication in the pharmaceutical and cosmetic industries increases every year. This refers also to articles that have contact with food and drinking water. For more than 60 years the elastomer composites have been successfully used in the water supply networks where they are im-plemented as seals for different types of pipes, valves, hydrants, fittings, etc. Their performance is equally good regardless of the material of the water system elements (cast iron, steel, copper, PVC) [1, 2].

The development of a formulation for rubber com-

posites designed to produce sealing elements to contact drinking water is not an easy task. There are many and different reasons, but the most essential refer to: 1) the sanitary requirements for these products which are constantly increasing in view of the drinking water quality and human health issues; this is also valid for the elastomers and the ingredients used for their production; 2) the high requirements in respect to the mechanical properties of these products.

There is currently no uniform regulation in the European Union concerning the type of the elastomers and the ingredients used to manufacture rubber products in contact with food and drinking water. There is only one directive of the kind in force in Europe - 93/11/EEC, but it controls only the release of N-nitrosamines and N-nitrosatable substances from baby bottle rubber

Journal of Chemical Technology and Metallurgy, 54, 2, 2019

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teats and soothers [3]. However, all materials that come into contact with food and drinking water, including elastomeric materials, must comply with the general and specific requirements of Regulation (EC) 1935/2004 [4] and be produced in accordance with good manufactur-ing practice, including full traceability of the materials used, as referred to in Regulation (EC) 2023/2006 [5]. In addition to Regulation (EC) 1935/2004, all materials and articles that come into contact with food and potable water in Europe must also comply with Directive 89/109/EEC of the European Union [6].

Despite the absence of a specific directive, many countries have their own list of materials and ingredients that can be used for making rubber products in contact with food and potable water. Many countries (France, Germany, Italy, the Netherlands, etc.) have also their own requirements regarding the amount of substances migrat-ing from the rubber products into the food or potable water. In Germany, the quality of the rubber products in contact with food and potable water is controlled by Recommendation XXI of the German Federal Institute for Health Protection of Consumers and Veterinary Medicine (BgVV). It is worth noting that this document is only of a recommendation nature. However, due to the absence of a formal EU directive, the document pointed above is widely used not only in Germany but also in most European countries [2, 7].

All those directives and requirements provide for that the substances contained in the various articles (packaging, sealants, conveyor belts, etc.) under normal or foreseeable utilization conditions do not migrate to the food stuff or potable water in quantities above which they can threaten human health, cause unacceptable changes in the composition of food products or cause deterioration of their organoleptic characteristics.

In Europe, the properties of rubber products in con-tact with potable water are controlled by standard EN 681-1:1996 [8]. The document specifies both the proper-ties of the materials used and those of the end products. That includes requirements concerning the mechanical properties (tensile strength, elongation at break, residual elongation, IRHD hardness) prior to and after thermal aging, compression set, stress relaxation, ozone resist-ance, volume change in water, microbiological growth, migration of toxic substances, etc. The requirements for these products are extremely strict and depend on the temperature of the potable water they will be into contact

with. Therefore, the standard includes requirements for the products contacting both cold and hot potable water.

The purpose of the present work is to develop and test the properties of acrylonitrile butadiene rubber (NBR) based composites aimed for production of seal-ing elements in water supply networks carrying hot and cold potable water. The choice of NBR as an elastomeric matrix is not accidental. Typically, natural rubber (NR), ethylene-propylene terpolymer (EPDM) and some other synthetic elastomers are also used besides NBR in such a type of production. However, it is known that various substances are added to potable water, most often acting as disinfectants, for instance, free chlorine (most frequently), and recently various chloramines [9, 10]. A serious dis-advantage of the disinfectants is that both free chlorine and chloramines have a harmful effect on the elastomeric composites used as sealants in plumbing installations. Moreover, according to the literature, the chloramines, in particular, which are significantly less harmful to human health, lead to quite faster destructive processes in the elastomeric composites [11, 12]. NBR and EPDM are the two most resistant elastomers to the destruction processes caused by these substances. This in turn determines nowa-days their widest application in this area [1].

EXPERIMENTALMaterials

Acrylonitrile butadiene rubber SKN 3340 made by Krasnoyarsk Synthetic Rubber Plant JSC was used as a polymer matrix. The bound acrylonitrile content in the elastomer was 35%. Two types of carbon black were used as fillers, namely CORAX N550 and CORAX N772, both produced by Orion Engineered Carbons. The characteristics of the fillers were as follows:

l CORAX N550 – Iodine Adsorption of 43 mg/g, Surface Area of 39 m2/g, Oil Absorption Number of 121 ml/100g;

l CORAX N772 - Iodine Adsorption of 30 mg/g, Surface Area of 30 m2/g, Oil Absorption Number of 65 ml/100g.

All other ingredients were also commercially availa-ble and used without further processing. They referred to:

l Shell Ondina 941 - a perfectly purified paraffin oil containing no stabilizers (it finds application as a plas-ticizer in rubber mixtures intended for the manufacture of articles having contact with pharmaceutical and food products, potable water, etc.);


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l Vulkanox BKF (6,6’-di-tert-butyl-4,4’-dimethyl-2,2’-methanediyl-di-phenol) – a bisphenol type non-coloring antioxidant (Vulkanox BKF complies with Recommendation XXI of the German Federal Institute for Health Protection of Consumers and Veterinary Medicine (BgVV). Its use in the production of rubber articles contacting food products and potable water is admissible). Vulkanox BKF was manufactured by Lanxess).

l Pilflex 13 (N-(1,3-dimethylbutyl)-N’-phenyl-p-phenylenediamine (6PPD)) – an antioxidant and an-tidegradant manufactured by Nocil Limited (according to the technical documentation, Pilflex 13 complies with Recommendation XXI of BgVV and its use in manufac-turing rubber products in contact with food and potable water is admissible).

l Pilnox TDQ (2, 2, 4-trimehyl-1, 2-dihydroquino-line) – an antioxidant manufactured by Nocil Limited (according to recommendation XXI of the BgVV, the use of this antioxidant for production of rubber items con-tacting food and potable water is restricted. Normally, its quantity in such articles should not exceed 1 phr).

l Antilux 654 L - a mixture of selected paraffins mi-crocrystal wax protecting the rubber articles from ozone aging. Antilux 654 L was produced by Rhein Chemie and allowed to be used for articles contacting food.

The vulcanization agents used were: triallylisocya-nurate – TAIC 70 (by Ketlitz), tetramethylthiuram disul-phide - Vulkacit Thiuram/C (by Lanxess), N-Cyclohexyl-2-benzothiazole sulphenamide – Pilcure CBS (by Nocil Limited), dicumyl peroxide - Peroxan DC40-PK (by Per-gan), N-phenyl-N-(trichloromethylsulphenyl)-benzene sulphonamide - Vulkalent E/C (by Lanxess) and sulphur.

MeasurementsThe vulcanization characteristics of the investigated

rubber compounds were determined using a Monsanto MDR 2000 rheometer according to ISO 6502-3:2018 [13]. The cure curves were taken at 165°C.

Tensile stress-strain properties of the investigated vulcanizates were determined according to ISO 37:2017 [14]. The thermal aging of the vulcanizates was deter-mined according to ISO 188:2011 [15].

The compression set test of the studied composites was performed according to ISO 815-1:2014 [16] and ISO 815-2:2014 [17] referring to experiments conducted at room and elevated or low temperatures, correspond-

ingly. The hardness (IRHD) of the investigated vulcani-zates was determined according to ISO 48-2:2018 [18].

The stress relaxation of the composites studied was determined according to ISO 3384-1:2011 [19].

The change in the volume of the investigated vul-canizates in water was monitored according to ISO 1817:2015 [20].

The ozone resistance test of the investigated vul-canizates was conducted by an accredited laboratory according to ISO 1431-1:2012 [21].

The microbiological analysis of the water that had been in contact with the studied composites for two days was carried out in the laboratory of Sofiyska Voda Ltd. accredited according to EN ISO/IEC 17025:2006 [22]. The samples of the tested water were taken and transported according to the laboratory requirements - in sterile containers.

Preparation of the rubber compositesThe rubber compounds were prepared on a labora-

tory rubber mixing mill (rolls L/D 320x160) according to the formulations presented in Tables 1 and 2.

Twelve mixtures were produced. The first four, presented in Table 1, comprised a conventional (NBR and C1), semi-efficient (SE2) and efficient (E3) sulphur vulcanization system. The compounds, whose composi-tion is presented in Table 2, contained a peroxide vul-canization system. The peroxide amount ranged from 2 phr to 8 phr. That was necessary to determine the optimal amount of dicumyl peroxide (containing 40% of the active substance) required to produce vulcanizates of desired properties and aging resistance. The investigated rubber compounds were vulcanized on an electrically heated hydraulic press at 165°C and a pressure of 10 MPa at the optimal vulcanization time of each compound determined by its vulcanization characteristics.

RESULTS AND DISCUSSION

Properties of the composites comprising convention-al, semi-efficient and efficient vulcanization systems

Our study began with a composition containing carbon black at 65 phr (NBR, Table 1) in the presence of paraffin oil Shell Ondina 941 at 12 phr and a convention-al sulphur vulcanization system. However, as expected, although acceptable for manufacturing articles having contact with food products and potable water, this non-


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polar plasticizer turned out to be incompatible with polar NBR. To avoid the use of a plasticizer, we reduced the fillers amount to 55 phr in the next formulation (C1). In addition, we developed two more compositions contain-ing a semi-efficient and efficient sulphur-vulcanization

system (SE2 and E3). The poor compatibility between the elastomer used and the plasticizer yielding poor dispersion of the powdered ingredients in the rubber composite, referred to as NBR, was the reason for its exclusion from further research.

NBR C1 SE2 E3 NBR 3340 100.0 100.0 100.0 100.0 Zinc Oxide 5.0 5.0 5.0 5.0 Stearic Acid 1.0 1.0 1.0 1.0 TAIC-70 - - - - Sulphur 2.0 2.0 1.0 0.4 Vulkanox BKF 1.0 1.0 1.0 1.0 Antilux L 2.0 2.0 2.0 2.0 Pilflex 13 (6PPD) 0.8 0.8 0.8 0.8 Pilnox TDQ 0.5 0.5 0.5 0.5 Carbon Black SRF N772 45.0 35.0 35.0 35.0 Carbon Black N 550 20.0 20.0 20.0 20.0 Shell Ondina 941 12.0 - - - Vulkacit Thiuram/C - TMTD 1.2 1.2 2.4 3.0 Pilcure CBS 0.15 0.15 0.15 0.15 Vulkalent E/C 0.2 0.2 0.2 0.2 Peroxan DC-40 PK - - - -

Table 1. Compositions of acrylonitrile butadiene rubber based compounds com-prising conventional, semi-efficient and efficient vulcanization systems (phr).

P4 P5 P6 P7 P8 P9 P10 P11 NBR 3340 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Zinc Oxide 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 Stearic Acid - - - - - - - - TAIC-70 1.0 1.25 1.5 2.0 2.5 3.0 3.5 4.0 Sulphur - - - - - - - - Vulkanox BKF 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 Antilux L 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Pilflex 13 (6PPD) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Pilnox TDQ 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Carbon Black SRF N772 35.0 35.0 35.0 35.0 35.0 35.0 35.0 35.0

Carbon Black N 550 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0

Vulkacit Thiuram/C - TMTD - - - - - - - -

Pilcure CBS 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Vulkalent E/C - - - - - - - - Peroxan DC-40 PK 2.0 2.5 3.0 4.0 5.0 6.0 7.0 8.0

Table 2. Compositions of acrylonitrile butadiene rubber based compounds comprising a peroxide vulcanization system (phr).


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Fig. 1 shows the cure curves of the investigated rubber compounds. As evident there are significant dif-ferences in the course of the cure curves of the rubber mixtures subjected to sulphur vulcanization depend-ing on their being conventional, semi-efficient and efficient. Although the type of sulphur vulcanization does not affect the minimum torque or the viscosity of the rubber compounds, it is seen that the maximum torque and hence the difference between the maximum and minimum torque decreases in the following order: conventional > semi-efficient > efficient. So, the density of the formed crosslink network is lower in the semi-efficient and efficient vulcanization system. This is expected because of the lower amount of vulcanizing agent (sulphur) in the semi-efficient and efficient vul-canization system. However, in this case, it is believed that the sulphidity of the crosslinks obtained is lower, which provides better heat resistance of the vulcanizates. That is due not only to the lower amount of sulphur but also to the increased amount of accelerator, in this case tetramethylthiuram disulphide, which acts as a sulphur donor. By increasing the accelerator amount a greater amount of sulphur is introduced, which contributes to the vulcanization process. Though this sulfur is not 8 atomic and mainly monosulphide bonds are thus formed. As Fig. 1 shows, the type of sulphur vulcanization system used does not affect the scorch time. On the other hand, the cure time decreases in the following order: efficient > semi-efficient > conventional.

Fig. 2 shows the hardness of the investigated vul-canizates prior to and after heat aging. Obviously, prior to the aging, the hardness of the composites studied is not affected significantly by the type of the sulphur vulcanization system used. The trend after aging is similar. In all cases, the hardness of the composites studied increases with aging as well as with increas-ing the aging temperature. The phenomenon is most pronounced in vulcanizates obtained by conventional sulphur vulcanization (C-1). The result is expected as part of the added sulphur may not have reacted during the vulcanization process, i.e. it remains as free sulphur. During the aging, this free sulphur can further crosslink the rubber macromolecules, whereby the hardness of the resulting composites increases. On the other hand, the conventional sulphur vulcanization forms predominantly di- and poly-sulphide crosslinks. During aging, these bonds are restructured to form a crosslink network of higher density, but of lower sul-phidity. That increases also the hardness of the rubber composites after aging.

According to standard EN 681-1:1996, the IRHD hardness of rubber vulcanizates intended for sealant for pipes carrying hot and cold potable water should not increase by more than 8 units after aging. As seen from Fig. 2, after 7 days of aging at 70°C, the hardness of the composites studied increases within the limits allowed by the standard. However, after 7 days of ag-ing at 125°C, the hardness of the vulcanizates exceeds significantly the admissible values, making them unsuit-able for this purpose.

Fig. 2. IRHD hardness of the studied composites comprising conventional, semi-efficient and efficient vulcanization systems prior to and after aging.

Fig. 1. Cure curves of the investigated acrylonitrile buta-diene rubber based compounds comprising conventional, semi-efficient and efficient vulcanization systems.


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Fig. 3 shows the tensile strength of the investigated vulcanizates. It is obvious that the type of sulphur vul-canization system used does not have a significant impact on this parameter. The tensile strength values for all three vulcanizates are quite above the minimum requirements (> 9 MPa) of EN 681-1:1996.

Naturally, the tensile strength of the investigated vulcanizates decreases after aging. As Fig. 4 shows the effect of aging is most pronounced in the vulcanizates obtained by conventional sulphur vulcanization (C-1). Those vulcanizates particularly undergo tensile strength deterioration by more than 20% after aging which is inadmissible by the standard. The changes of the tensile strengths of vulcanizates obtained by semi-efficient and

efficient vulcanization system (SE-2 and E-3) are in the range of 10 % - 15 %.

The results referring to elongation at break are analo-gous to those of the tensile strength. They are illustrated in Figs. 5 and 6. Prior to aging (Fig. 5), all vulcanizates exceed the minimum requirements (min 300%) of EN 861-1:1996 regarding the elongation at break. However, after heat aging, there is a significant deterioration in this parameter. The elongation at break of the vulcanizates investigated decreases by more than the admissible 30% regardless of the aging temperature (70°C or 125°C).

Fig. 7 presents the compression set of the vul-canizates studied depending on the type of the sulphur vulcanization used. In this case, the test is conducted at the highest temperature (125°C) specified in EN 681-

Fig. 3. Tensile strength of the studied composites comprising conventional, semi-efficient and efficient vulcanization systems.

Fig. 4. Aging coefficients in terms of tensile strength of the studied composites comprising conventional, semi-efficient and efficient vulcanization systems.

Fig. 5. Elongation at break of the studied composites comprising conventional, semi-efficient and efficient vulcanization systems.

Fig. 6. Aging coefficients in terms of elongation at break of the studied composites comprising a conventional, semi-efficient and efficient vulcanization systems.


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1:1996. The maximum admissible value amounts to 20% according to the latter. As seen from Fig. 7, the compres-sion set of all vulcanizates is below 20% and therefore it can be said that they meet the standard pointed above.

The results obtained show that there is no compo-sition, among those obtained by conventional, semi-efficient and efficient sulphur vulcanization of acryloni-trile butadiene rubber, which meets completely all the requirements of standard (EN 681-1:1996) and hence cannot be used for the production of sealants for pipes carrying hot and cold potable water. That is due, on one hand, to the heat resistance of nitrile rubber which is worse than that of some other synthetic elastomers and, while on the other hand, to the sulphur vulcanization itself. The conventional sulphur vulcanization of diene elastomers leads to the predominant formation of di- and polysulphide crosslinks. These bonds are thermally unstable and cause a more pronounced thermal aging of the rubber products [23 - 26]. When one fails to achieve the necessary results using also a semi-efficient or ef-ficient vulcanization system, he refers to the so-called peroxide vulcanization, which yields only C-C bonds [27 - 29] in the rubber vulcanizates. Therefore, some extra NBR-based compositions have to be developed, whose sulphur alloy system is replaced by a peroxide one. The peroxide amounts in those compositions range from 2 phr to 8 phr. That is necessary to determine the optimal amount of dicumyl peroxide (containing 40 % of the active substance) required to produce vulcanizates of the desired properties and resistance to aging.

Properties of the composites comprising a peroxide vulcanization system

Fig. 8 shows the cure curves of the rubber com-pounds containing the peroxide vulcanization system. As seen, almost no thermoplasticity is observed in this case. However, that is quite expected and is one of the major drawbacks of this kind of vulcanization. Although the scorch time is significantly shorter, the process is slower in presence of peroxide than in case of sulphur vul-canization. The figure shows that the maximum torque values and the difference between the maximum and the minimum torque increases, respectively, with peroxide amount increase. This indicates also that the formed

Fig. 7. Compression set of the studied composites comprising a conventional, semi-efficient and efficient vulcanization systems.

Fig. 8. Cure curves of the investigated acrylonitrile buta-diene rubber based compounds comprising a peroxide vulcanization system.

Fig. 9. IRHD hardness of the studied composites com-prising a peroxide vulcanization system prior to and after aging.


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crosslink network gets denser with peroxide amount increase. The data in the figure show that the elastomeric macromolecules are not completely crosslinked, i.e. the resulting composites are of poor vulcanization at low organic peroxide concentrations (2 phr - 4 phr).

The fact that the composites containing small amounts of peroxide (2 phr - 4 phr) are poorly vulcan-ized is also confirmed by the hardness shown in Fig. 9. Obviously, the hardness of these composites is much lower (62 IRHD - 64 IRHD) prior to aging than that of the composites whose dicumyl peroxide content is higher than 4 phr. As the amount of the vulcanizing agent increases, the hardness of the composites studied increases and reaches about 70 IRHD. The vulcanizates hardness increases quite regularly after thermal aging due to the byproducts of organic peroxide decomposi-tion. These byproducts lead to further crosslinking of the rubber macromolecules during the aging. According to EN 681-1:1996 the hardness change after aging of the composites intended for sealants for pipes carrying hot and cold water should not exceed their hardness prior to aging by more than 8 units. According to Fig. 9 this requirement is met when the curing agent concentration is higher than 6 phr.

Figs. 10 and 11 illustrate the tensile strength and the elongation at break of the studied composites obtained by peroxide vulcanization. The figures show that as the amount of peroxide increases, the tensile strength increases while the elongation at break decreases. The vulcanizates of the mixture containing 2 phr of dicumyl peroxide (P-4) have the lowest tensile strength - about 9 MPa. The elongation at break of the same vulcanizates (P-4) is extremely high – of ca 800%. That is not typical of the elastomeric composites vulcanized by peroxides. That is another verification of our assumption that they are poorly vulcanized in presence of small amounts of a vulcanizing agent. The tensile strength increases to about 18 MPa, while the elongation at break decreases to ca 300 % with peroxide amount increase of 4 phr to 8 phr. Despite the serious differences in the tensile strength and the elongation at break of the composites studied depending on the peroxide concentration, the parameters achieved in all cases exceed the minimum values required by EN 681-1:1996.

Figs. 12 and 13 illustrate the dependence of the aging coefficients on the tensile strength and the elon-gation at break. The figures show that both in terms of

tensile strength and elongation at break, the vulcanizates containing more than 6 phr of peroxide are much more resistant to thermal aging than those produced by sulphur vulcanization. The data presented in the figures reveal that the changes concerning both parameters after aging

Fig. 10. Tensile strength of the studied composites com-prising a peroxide vulcanization system.

Fig. 11. Elongation at break of the studied composites comprising a peroxide vulcanization system.

Fig.12. Aging coefficients in terms of the tensile strength of the studied composites comprising a peroxide vul-canization system.


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are rather lower than the 20 % maximum tolerance for tensile strength and 30 % for the elongation at break specified in the standard.

Fig. 14 plots the compression set of the vulcanizates tested versus the peroxide amount used. It is seen that the compression set of all vulcanizates is below 20 %, indicating that they meet the requirements specified in the standard. Obviously, the amount of the organic peroxide used as a vulcanizing agent has also a signifi-cant impact on the parameter. The compression set of the studied composites decreases with increase of the peroxide amount, i.e. with increase of the density of the crosslink network formed.

In our opinion, considering the results obtained

and the requirements of standard EN 681-1:1996, the composition designated as P-10 is the most suitable for the manufacture of sealants for pipes carrying hot and cold potable water. Table 3 lists results referring to some other more specific and labor-intensive but important parameters of that composition, namely: compression set at -10°C, stress relaxation, a volume change in water and ozone resistance. The data presented in the table shows that the values of these parameters meet completely the requirements of the standard.

As mentioned in the introduction, besides the high requirements in respect to the mechanical properties of elastomeric composites used to produce sealants for pipes carrying hot and cold potable water, it is also of great importance that they do not release different sub-stances therein, especially in quantities above which the human health may be endangered. The same is valid for the changes in water composition causing deterioration in its organoleptic characteristics. Therefore a micro-biological analysis of the water that had been in contact with the studied composites for two days is run in the laboratory of Sofiyska Voda Ltd. accredited according to EN ISO/IEC 17025:2006. The samples of the tested water are taken and transported in sterile containers ac-cording to the laboratory requirements.

The data from the analyses are summarized in Table 4. It presents also the reference values of the parameters according to Ordinance 9/16.03.2001 issued by the Min-ister of Health, the Minister of Regional Development and Public Works and the Minister of Environment and Water of the Republic of Bulgaria referring to the qual-ity of potable water for household purposes. Obviously,

Fig. 13. Aging coefficients in terms of the elongation at break of the studied composites comprising a peroxide vulcanization system.

Fig. 14. Compression set of the studied composites comprising a peroxide vulcanization system.

Table 3. Mechanical properties of the composites pre-pared by the rubber compound designated as P-10.

Property Unit Result Requirements

(EN 681-1:1996)

Compression set, 72 hours at 23°С 24 hours at 70°С 72 hours at -10°С

% % %

4.6 6.2 3.9

max. 12 max. 20 max. 50

Stress relaxation, 7 days at 23°С 100 days at 23°С

% %

7.5 11.2

max. 16 max. 23

Volume change in water, 7 days at 70°С 7 days at 95°С

% %

+1.85 +2.11

+8/-1 +8/-1

Ozone resistance - No cracking

No cracking when viewed

without magnification


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the microbiological parameters of the samples tested are in full compliance with the requirements of the above-mentioned regulation, i.e. no bacterial growth, as well as changes in the organoleptic parameters of the water studied are observed.

CONCLUSIONS

The work presents studies on 12 compositions based on acrylonitrile butadiene rubber designed for the production of sealants used in the water carrying networks. A conventional, semi-efficient and efficient sulphur-vulcanization system is implemented to produce four of the compositions. Dicumyl peroxide is used as a vulcanizing agent for the other eight compositions. The vulcanization characteristics of the studied rubber compounds show that the type of sulphur vulcaniza-tion does not affect the minimum torque or viscosity of the mixtures tested. In this case, the maximum torque and hence the difference between the maximum and the minimum torque decreases in the following order: conventional > semi-efficient > efficient sulphur vulcani-zation. The rate of the vulcanization process decreases in the order of efficient > semi-efficient > conventional sulphur vulcanization. No thermoplasticity is observed in case of rubber compounds vulcanized by peroxide. That vulcanization occurs at a slower rate and the den-sity of the formed crosslink network increases with the

increase of the amount of peroxide used. The type of the vulcanization system used has a significant impact on the mechanical properties of vulcanizates investigated. The results obtained and the requirements of EN 681-1:1996 referring to the mechanical properties of elastomeric composites used to produce sealants for pipes carrying hot and cold potable water reveal that composite P-10 is the most suitable for this purpose. It contains dicumyl peroxide at 7 phr. The microbiological characteristics of the water in contact with the composites of the formu-lations are also studied. The results show no bacterial growth in the samples tested. The contact of the water with the developed composites does not lead to a change in its organoleptic parameters. No increase in the values of heavy metals, nitrates or nitrites contents in the water tested is observed.

REFERENCES

1. T.D. Rockaway, G.A. Willing, R.M. Schreck, K.R. Davis, Performance of Elastomeric Components in Contact with Potable Water, Denver, CO, USA, IWA Publishing, 2007.

2. N. De Coster, H. Magg, NBR in contact with food, potable water, pharmaceutical and cosmetic applica-tions, KGK - Kautschuk Gummi Kunststoffe, 56, 7-8, 2003, 405-411.

3. Commission Directive 93/11/EEC of 15 March 1993

Table 4. Microbiologic characteristics of the water being in contact with the composites studied.

№ Type Unit Standard/Test method Result Reference values

1 Escherichia coli KOE/100ml EN ISO 9308-1:2014 0 0

2 Number of Coliform bacteria KOE/100ml EN ISO 9308-1:2014 0 -

3 Number of Clostri-dium perfringens (incl. spores)

KOE/100ml EN 26461-2:2004 0 0

4 pH pH units BSS 3424-81 7.56±0.04 6.50-9.50 5 Turbidity FNU EN ISO 7027-1:2016 <0.50 <2 6 Color mg/l Pt EN ISO 7887:2012 2.4±0.1 <15 7 Conductivity µS/cm EN 27888:2002 116.6±4.0 2000.0 8 Ammonium ions mg/l EN ISO 14911:2002 <0.010 0.500 9 Nitrites mg/l EN ISO 10304-1:2009 <0.010 0.500 10 Nitrates mg/l EN ISO 10304-1:2009 1.076±0.055 50.000 11 Aluminum µg/l EN ISO 11885:2009 77±3 200 12 Iron µg/l EN ISO 11885:2009 32±2 200 13 Mangan µg/l EN ISO 11885:2009 <10 50


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Concerning the release of the N-nitrosamines and N- nitrosatable substances from elastomer or rubber teats and soothers, Official Journal of the European Communities L93, 36, 1993, 37-38.

4. Regulation (EC) No 1935/2004 of the European Parliament and of the Council of 27 October 2004 On materials and articles intended to come into con-tact with food and repealing Directives 80/590/EEC and 89/109/EEC, Official Journal of the European Communities L338, 47, 2004, 4-17.

5. Commission Regulation (EC) No 2023/2006 of 22 December 2006 On good manufacturing practice for materials and articles intended to come into contact with food, Official Journal of the European Com-munities L384, 49, 2006, 75-78.

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27. P.R. Dluzneski, Peroxide vulcanization of elasto-mers, Rubber Chemistry and Technology, 74, 3, 2001, 451-492.

28. J. Class, A review of the fundamentals of crosslink-ing with peroxide, Rubber World, 220, 5, 1999, 35.

29. M. Alvarez, Novel Co-Agents for Improved Prop-erties in Peroxide Cure of Saturated Elastomers, PhD Thesis, University of Twente, Enschede, the Netherlands, 2007.

COMPOSITES BASED ON ACRYLONITRILE BUTADIENE …Simona Nikolova, Mihail Mihaylov, Nikolay Dishovsky...

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