Diffusion of Dithiophosphate Accelerators in Tire Compounds

Transcription

1 ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS Tire s diffusion of accelerators dithiophosphates equilibrium distribution Considering the performance of multilayer elastomer products like tires the distribution of crosslinking chemicals caused by diffusion following the assembly step during storage and vulcanization is an interesting aspect for covulcanization, crosslink density and influences on the adhesion systems. In this manner the diffusion behavior and the equilibrium distribution of the dithiophosphate accelerator, di-(2-ethyl)hexylphosphorylpolysulfide (SDT) between different rubber s used in tires was investigated by means of Fourier Transform infrared spectroscopy (FT-IR spectroscopy) and swelling. The equilibrium distribution of SDT differs as a function of the rubber matrices involved, with the consequence that the concentration decreases in the direction tread undertread steel cord mixture. Diffusion von Dithiophosphatbeschleunigern in Reifenmischungen Reifenmischungen Beschleunigerdiffusion Dithiophosphate Gleichgewichtsverteilung Für die Leistungsfähigkeit von mehrschichtig aufgebauten Elastomerprodukten wie Reifen ist die diffusionsbedingte Verteilung von Vernetzungschemikalien nach der Konfektionierung, während der Zwischenlagerung und Vulkanisation ein für die Covulkanisaton, Vernetzungsdichten und Beeinflussung von Haftsystemen interessanter Aspekt. So wurden die Diffusionseigenschaften und die Gleichgewichtsverteilung des Dithiophosphatbeschleunigers, Di-(2-ethyl)- hexylphosphorylpolysulfid (SDT) zwischen verschiedenen Reifenmischungen mittels FT-IR-spektroskopischer Methoden und Quellung untersucht. Die mit verschiedenen Methoden ermittelten Diffusionskoeffizienten zeigen untereinander eine gute Korrelation und können zur Vorhersage der Diffusionsgeschwindigkeit des Beschleunigers genutzt werden. Die Gleichgewichtsverteilung von SDT führt zu verschiedenen Konzentrationen in Abhängigkeit der Mischungsmatrices, wobei in der Richtung Laufflächenmischung, Laufflächenhaftmischung, Stahlcordhaftmischung ein abnehmender Konzentrationgradient gemessen wurde. Diffusion of Dithiophosphate Accelerators in Tire Compounds For the performance of elastomer products with a multilayer structure tires, for example insights into the inter diffusion and distribution of vulcanization chemicals during post-assembly intermediate storage and vulcanization are of particular interest, with attention focusing on the aspects of a covulcanization or of the positive or negative synergisms as regards resulting crosslink densities and crosslink structures in the case of sulfur crosslink. It is especially evident in observing the behavior of thiazoles and sulfenamides, also taking into account the synergistic effects of secondary accelerators and different sulfur accelerator ratios on application properties like scorch times, attainment of maximum crosslink density and the crosslinking kinetics in rubbers with different reactivity and microstructure (1). In much the same way as for multiphase blends, the different polarity ratios among the partners involved and different crosslinking chemical solubilities in conjunction with the reactivity of the polymers concerned has a resulting impact on crosslink density in the discrete polymer phase, which as, for example, in the case of the combination of NR and BR employing the same vulcanization system results in a step-up in NR crosslink density (2, 3). A. J. Tinker has exhaustively investigated the influence of alkyl thiurame substitutes on the distribution of crosslink densities and on the mechanical properties of NR/NBR blends as well as the impact of the double-bonding concentration in polymer, taking the example of NR/EPDM blends (4). The literature basically describes numerous measuring methods for characterizing the diffusion of low-molar components and, in specific, of crosslinking chemicals in polymer matrices. Included are methods like microinterferometry, light-optical microscopy, ATR-FT-IR spectroscopy (ATR ¼ attenuated total reflectance) and swelling (2, 4 8). Specifically for determination of cases of equilibrium concentration distribution, conventional methods of analysis are applied such as extraction and chromatography, or element analyses in conjunction with special test configurations like sandwich structures consisting of a diffusant reservoir and the diffusion matrix (9, 10). In the realm of nitrosamine-free accelerator systems, dithiophosphate accelerators are increasingly used because of their comparatively high solubility and their good dispersibility (11, 12). Opacity tests have shown that in EPDM matrices, for example, dithiophosphate accelerators, which are fluid at room temperature, exhibit a solubility limit exceeding that of TMTD or TBzTD by a factor of five (12, 13(, for which reason dithiophosphates are also suitable for reducing or avoiding blooming effects. As far as the crosslinking of diene rubber is concerned, dithiophosphates, acting as synergistic accelerators, exhibit outstanding crosslinking efficiency in combinations with sulphenamides or mercaptobenzthiazole accelerators (12, 14 16). Alongside these advantages, there is concern that when used in tires, that in the tread, for instance, or in the tread adhesion dithiophosphates as corrosion protection can negatively influence rubber-metal adhesion. This is of major importance in the area of the steel cord adhesion due to a certain metal reactivity of this material class (17). The paper s next interest is in characterizing the diffusion and the equilibrium distribution of dithiophosphates in tires for example, between tread, tread adhesion and a steel-cord that does not, in principle, contain any dithiophosphate relates not only to a synergistic effect on crosslinking but also C. Rosca, U. Giese, R. H. Schuster, Hannover, T. Früh, Mannheim Corresponding author: Ulrich Giese Deutsches Institut für Kautschuktechnologie e.v. Eupener Straße 33, Hannover Tel.: 05 11/ Fax: 05 11/ KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 11/

2 to the contamination of rubber-to-metal bonding. Fundamentals of diffusion As a rule, diffusion in polymers is understood to refer to the transfer processes involving foreign molecules such as dissolved, low-molar substances. For instance, from one volume unit of polymer into another, with the reaching a state of equilibrium i.e. a concentration balance constituting its driving force (18, 19). The theory of free volume and free chain mobility generally explains the diffusion mechanism in polymers at a temperature above the glass transition temperature (19 21). The basis for the mathematical description of diffusion processes is Fick s law that there is an analogy between heat flux and mass flux in a polymer based on free chain mobility (22). The transfer of a substance in the polymer matrix is thus proportional to the concentration gradient within a volume unit (19). F ¼ F ¼ Transfer rate (mass flux) per unit of volume and cross-sectional area c ¼ Concentration of the substance being diffused (diffusate) x ¼ Distance of diffusion perpendicular to the cross-section D ¼ Diffusion coefficient (unit: cm 2 s 1 ) The diffusion coefficient D is a substancespecific variable that can be regarded as constant at low concentrations, while a concentration dependency prevails at higher concentrations. With respect to equation (1), Fick s second law generally applies for one-dimensional diffusion in isotropic substances, e.g. in the direction of the x axis (18, ¼ 2 ð2þ t ¼ time of diffusion If the diffusion coefficient and the concentration of diffusant in dependency of the distance x is known, the concentration profiles across polymer-polymer interfaces of diffusants after any time can be described using the following solution of equation (2) (9): cðx; tþ ¼ 1 2 c 0 erf h x p 2 ffiffiffiffiffi h þ x þ erf p Dt 2 ffiffiffiffiffi Dt ð3þ c(x,t) ¼ concentration of diffusant at increment x after time t ¼ initial concentration c 0 D ¼ average diffusion coefficient t ¼ time of diffusion erf ¼ mathematical error function: erfðyþ ¼ 2 y3 y p 3 þ y5 y7 5*21 7*31 þ ::: y ¼ h p x 2 ffiffiffiffiffi Dt Experimental aspects Materials The recipes for the investigations of the diffusion behaviour of the dithiophosphate accelerator in combination with typical s for tread, undertread and steelcord adhesion are listed in Table 1. Additional NR, lightly crosslinked with peroxide, is used as reference and model in comparison to the mixtures from practise. Futhermore it is important, that SDT is stable during time and temperature used for the experiments. The chemical composition of the used dithiophosphate, di- (2-ethyl) hexylphosphorylpolysulfide (SDT), is shown in Fig. 1. The statistical mean value for S x is x ¼ 3.5. Sample preparation - Mixing The s were mixed in two stages. First the polymers, zinc oxide and stearic acid as well as fillers and other components are mixed using a temperature of 60 8C and a rotor speed of 50 rpm for seventeen minutes by means of a laboratory kneader (type: Haake Rheocord 90 with a Rheomix 600E mixing chamber). In the second step the curatives were added on a laboratory Tab. 1. Recipes of tread, undertread and steelcord adhesion Component Tread [phr] Undertread [phr] Steelcord adhesion [phr] SBR (Buna VSL ) BR (Buna CB 24) NR (SMR CV 50) 10.0 NR (RSS 1) Carbon black (N 234) 10,0 Carbon black (N 330) Silica (Vulkasil N) 7,5 Silica (Ultrasil GR 7000) 70.0 Oil (TDAE-oil/ Thyrex 20) Antioxidant (IPPD) 1.0 Antioxidant (6PPD) Antioxidant (TMQ) Light stability agent (Antilux 654) 1.0 Processing aid (Aktiplast 20) 0,2 Processing aid (Aktiplast ST) Adhesive cement (Rhenogran Resorin-80) 1.6 Adhesive cement (Cohedur A 150) 4.0 Co-stearate 1.3 ZnO Stearic acid Activator (Rhenofit GE 1715) 10.0 DCBS TBBS 2.0 CBS DPG (Rhenogran DPG-80) 2.5 SDT (Rhenofit SASD) 5.0 1) 5.0 1) Sulphur, unsoluble (Rhenogran IS 90-65) 6.0 Sulphur ) if it is used as reservoir in a sandwich arrangement. 594 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 11/2004

3 Fig. 1. Chemical composition of SDT mill for four minutes period. Plates of 2 mm thickness were vulcanized in an electric heated press using t 90 at 160 8C. Methods Determination of diffusion coefficients by swelling Cured samples were immersed in the liquid accelerator (SDT) at room temperature of 24 8C until a swelling equilibrium was reached. The specimen s dimensions were 2.15 mm thickness and 16.2 mm diameter. The weight of initial sample (M 0 ) of 0.8 g was measured before immersing in accelerator. The weight of swollen samples at different times (M t ) was determined after removing the SDT by blotting with filter paper from the surface of the sample. The swelling degree (SD) in dependency on time is calculated using the following equation: SD ¼ M t M 0 M 0 *100% ð4þ in relationship to a constant signal of e.g. the rubber matrix is used. This is estimated with the time lag method using the following equation (19) D ¼ l2 ð6þ 6t l ¼ thickness of the film t ¼ average passing time of the diffusant through the polymer film (time lag). The passing time taken for the diffusant to pass into the polymer film can be determined with the help of an ATR investigation on an arrangement consisting of a rubber film (100 lm in thickness) and the accelerator (liquid), which is placed on the horizontal surface of the Ge-crystal at room temperature of 24 8C (Fig. 2) (7, 24). The time of contact is defined as t ¼ 0. The diffusion process happens perpendicularly to the contact surface of the Ge-crystal. The chronological change of concentration of the diffusant at the surface of the Ge-crystal is determined by measuring the increase in signal intensity of the typical IR-signal of C P group at 1003 cm 1 in the case of dithiophosphates. The slope of the linear behavior of the signal intensity vs. time gives the diffusion coefficient. Determination of equilibration concentrations For information about equilibrium concentrations of SDT in the sandwich arrangement (Fig. 3) in the case of dithiophosphates (25), the determination of phosphor as a significant element in SDT by photometric method is carried out in the different layers or by FT-IR spectroscopy on the extracts. The sandwich arrangement was kept in oven at 60 8C for various times. After the layers have been separated they were investigated using element analysis after extraction. The absorbed mass of SDT (M t ) at a defined time of swelling is calculated from the difference M t M 0. In the equilibrium state of swelling the absorbed mass of SDT corresponds to M 1. The diffusion coefficient (D) was calculated by means of the following equation (7, 23): M t M 1 ¼ 4 l M t 1 D*t 2 ð5þ p ¼ absorption of SDT at swelling time t M 1 ¼ absorption of SDT in equilibrium state of swelling l ¼ initial thickness of the dried sample t ¼ time. ATR-FT-IR spectroscopy time lag method The time lag method that is based on Fick s laws offers the possibility of measuring the diffusion coefficient by means of ATR-FT-IR spectroscopy. The time taken for the diffusant to penetrate a film with a constant thickness is a measure of the mobility and allows calculating the diffusion coefficient. For the extrapolation of the time t in equation (6) the area of a characteristic signal of the diffusant that increases in time Fig. 2. Schematic description of the ATR diffusion experiments (time lag method) Fig. 3. Sandwich arrangement used for element analysis and extracts investigations in the determination of equilibrium concentrations KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 11/

4 Tab. 2. Diffusion coefficients from swelling investigation calculated from the slope of the first step of the curve in Fig. 5 System D [10 8 * cm 2 /sec] Steelcord adhesion 0,16 Undertread 0,9 NR-model 0,9 The element analyses for phosphor were carried out as follows: to an orthophosphate solution ammonium molybdate and ammonium vanadate are added in order to form an orange-yellow water-soluble complex of molybdo-vanadate phosphor acid. The reaction is made in diluted nitric acid at a ph ¼ 1,5. The color has a maximum absorption typically at nm and is stable for 24 h. The results of the above mentioned analyses are used to calculate a distribution coefficient K for SDT by means of the following equation: K ¼ c A ð7þ c B c A ¼ concentration of phosphor in film c B ¼ concentration of phosphor in reservoir Results and discussion Swelling investigations for determining the diffusion coefficients Fig. 4 shows the swelling degree (SD) as a function of time for SDT/steel cord adhesion, SDT/undertread and SDT/ NR at 24 8C. The swelling degrees are calculated using equation (3). The results presented in Fig. 4 indicate that the equilibrium swelling degrees of the three matrices are different caused by the solubility of SDT, different filler content and crosslink density of the s. The equilibrium uptake by NR vulcanisate is much higher than for the steel cord adhesion and undertread. The equilibrium state is reached after 277 hours (¼ 1*10 6 sec) for steelcord adhesion, after 550 hours (¼ 2*10 6 sec) for undertread and 2000 hours (¼ 7*10 6 sec) for NR-model. Fig. 4. Swelling degree of steel cord adhesion, undertread and NRmodel in SDT at 24 8C The diffusion coefficient for SDT in Steelcord adhesion, undertread and NR-model by evaluation of the swelling experiments is determined considering pffiffi the SDT uptake rate M t /M 1 against t presented in Fig. 5: The curves in Fig. 5 show two steps considering the slope of the linear parts. The final value is reached by an asymptotically approach. The different three parts of the reception of the SDT accelerator can be interpreted in the following way: the first molecules diffuse into the polymer matrix, which is absolutely SDT free (1st step), the diffusion coefficient is independent of SDT concentration in the. the weight of the sample is increasing, because the chains of polymer matrix are expanded, based on reception of SDT, and the surface of the sample is increasing (2nd step); Fig. 5. SDT uptake rate from swelling experiments Tab. 3. Extrapolated diffusion coefficients System D [10 8 * cm 2 /sec] Steelcord adhesion 2,5 Undertread 1,8 NR-model 2,1 The asymptotically approach, in which the swelling equilibrium is reached, is defined by chemical and physical crosslink density and solubility. The diffusion coefficient is determined by calculating the slope (linear regression) and using equation (5). The results are summarized in Table 2: Because the chemical composition of the rubber material is changed during swelling, the diffusion coefficient depends on swelling degree or swelling time. So it is necessary to extrapolate the diffusion coefficient against a concentration of zero SDT in the rubber material as is presented in Fig. 6. The dependency of the diffusion coefficient on the absorbed amounts of SDT (Dm) for all three systems, steelcord adhesion, undertread and NR-mo- 596 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 11/2004

5 Tab. 4. Diffusion coefficients D calculated by means of time lag method System Steelcord adhesion D [10 8 * cm 2 /sec] 2,2 0,153 1) Undertread 2,8 0,173 1) NR-model 1,3 0,115 1) 1) mean values of three measurements (n ¼ 3) Fig. 7. ATR-FT-IR spectra of SDT and NR/SDT in dependency on time del, represented in Fig. 6 are used for extrapolation. The values of the diffusion coefficients obtained are summarized in Table 3. The results from swelling experiments for diffusion coefficients of SDT in the three s show small differences only. Considering the results in Table 3 higher values for D are indicated. This is to be expected, because by extrapolation an ideal concentration gradient results and the values for D are independent of concentration of SDT in the rubber matrix. ATR-FT-IR spectroscopy for determining the diffusion coefficients Fig. 7 presents the FT-IR spectra of SDT accelerator (a) and the ATR-FT-IR spectra of Fig. 6. Dependency of diffusion coefficients on absorbed amounts of SDT (Dm) NR with SDTat different times for diffusion ((b), (c)) from investigations on arrangements as it is illustrated in Fig. 1. The spectrum of pure SDT (standard) shows the characteristic signal of the P-O-group in the region of 1003 cm 1, CH 2 - and CH 3 - groups cause the signals at wavelength of 2950, 2930, 2850 and 1462 cm 1, which are not specific for dithiophosphates but typical for organic substances. Curves (b) and (c) in Fig. 7 present in the region of 1003 cm 1 one broad peak with two maxims, the second smaller maxim is related to natural rubber. As the diffusion process progresses, there is an increase in SDT content in film, resulting in an increase in the signal at 1003 cm 1. For all the arrangements studied, the behavior was similar. These findings indicated that an increasing of the signal in the region of a wavelength of 1003 cm 1 in dependency on diffusion time occurs. For the calculation of diffusion coefficient two peak areas have been measured: one peak characteristic to SDT that changes with time due to its diffusion and one peak specific to the rubber matrix that remains unchanged during the diffusion experiment. After that, a ratio between the areas of these peaks was used. The plot in Fig. 8 presents the ratio between the areas of the two representative peaks chosen as a function of time for NR. As it can be seen in Fig. 8, there is an initial built-up period, but eventually a linear relationship develops. This plot has an intercept on the time axis known as the time lag. This is related to the diffusion coefficient that can be calculated by equation (6). The diffusion coefficients determined by means of the time lag methods for steelcord adhesion, undertread and NR-model are enumerated in Table 4. The results indicate that the diffusion coefficients determined by time lag method are slightly different to the results obtained in swelling measurements. The mobility of SDT in undertread and steelcord adhesion is very similar. Determination of equilibrium concentrations and distribution of SDT The results of the determination of phosphorus amounts in the single layers of the sandwich reservoir and film the distribution coefficient K is calculated by means of equation (7). For these calculations, data are used from swelling experiments with diffusion times higher than seven days that means the equilibrium state has been reached. The equilibrium distribution of SDT is different between the systems undertread/ tread (K ¼ 0.9) or steelcord adhesion / undertread (K ¼ 0.7). Overall in direction from tread-undertread- steelcord KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 11/

6 Fig. 8. Intensity of relativ SDT signal in dependency on time (time lag method) for NR-model Fig. 9. SDT-distribution between tread/undertread and steelcord adhesion after seven days at 22 8C Tab. 5. Equilibrium concentrations of SDT presented by the phosphorus amounts and distribution coefficients K Sandwich %P K ¼ c A Tread Undertread Steel cord Undertread/Tread 1) :127 0:144 ¼ 0:9 Steelcord adhesion / :126 Undertread 1) 0:90 ¼ 0:7 1) reservoir with 5 phr SDT (t ¼ 0) adhesion the equilibrium concentration of SDT is decreasing. On the base of the equilibrium concentrations of SDT between undertread and tread and the distribution coefficients in Table 5, the SDT distribution in Fig. 9 results. Considering that in the experimental arrangement with 5 phr much higher SDTconcentrations for initial load (reservoir) are used than in recipe from practice and the observed time for diffusion is much longer than the normal storage time of rubber mixtures in the process, we can ignore the SDT-concentration in steelcord adhesion after a diffusion time of e. g. one day and room temperature (22 8C). Conclusions As it is to be expected the accelerator di-(2- ethyl)-hexylphosphorylpolysulfide (SDT) diffuses between different tire mixtures like tread, undertread and steelcord adhesion. The diffusion coefficients were determined by means of swelling investigations and time lag method using ATR-FT-IR spectroscopy. Different determination methods lead to values for the diffusion coefficients with small differences. The values can be used for the calculation of the diffusion rate of SDT in practice. Considering the used s the mobility of SDT is nearly the same in undertread and steelcord adhesion, but the equilibrium concentrations of SDT is decreasing in the direction tread undertread steelcord adhesion, so that for short storage times of unvulcanized tires and for the vulcanization process in practice no interferences of SDTon the vulcanization of steelcord adhesion or on the adhesion to the steelcord are to be expected. The authors Cristina Rosca, Ulrich Giese, Robert Hans Schuster, Deutsches Institut für Kautschuktechnologie e. V., Hannover Thomas Früh, Rhein Chemie Rheinau GmbH, Mannheim Germany References [1] T. Kleiner, H.-J. Weidenhaupt, Kautsch. Gummi Kunstst. 56 (2003) 510. [2] A. V. Chapman, A. J. Tinker, Kautsch. Gummi Kunstst. 56 (2003) 533. [3] F. X. Guillaumond, Rubber Chem. Technol. 49 (1976) 105. [4] A. J. Tinker, Rubber Chemistry Technol. 68 (1995) 461. [5] J. Brooke Gardiner, Rubber Chem. and Technol. 42 (1969) [6] J. Brooke Gardiner, Rubber Chem. and Technol. 41 (1968) cb [7] J. Deiters, Dissertation Universität Hannover [8] H. Ishida, Rubber Chemistry Technology 60 (1980) 497. [9] D. A. Lederer, K. E. Kear and G. H. Kuhls, Rubber Chem. and Technol. 55 (1982) [10] A. B. Ahmad, A. J. Tinker and D. W. Aubrey, Journal of Natural Rubber Research 9 (1) (1994) 1. [11] TRGS 552-Technische Regel für Gefahrstoffe N- Nitrosamine März 1996/Fassung November 1997, Carl Heymanns Verlag, Köln. [12] H. J. Graf and H-M. Issel, Kautschuk Gummi Kunstst. 9 (1995) 600. [13] T. Früh, G. Müller: Vortrag Curing Time Reduction in Dithioate Accelarated Sulfur Cure for NR and Improved Co-Vulcanization of Rubber Blends with NR by Using Dithioates, Worldwide Amazon Rubber Conference Manaus-Brazil, November [14] T. Früh, L. Heiliger: Vortrag State of the Art Vulcanization by Dithiophosphates South Brazilian Rubber Conference, Bento Consalves-Brazil, September [15] J. Kiesekamp, H. R. Schmidt, R. H. Schuster, M. L. Hallensleben, Efficiency of Sulfenamides and Dithiophosphates in Crosslinking Polydien Rubbers, Poster at the IRC Paris, (1998). [16] T. Früh, G. Müller: Rubber World, (8), (2000) based on paper 22 presented at the 156th Meeting of the Rubber Division, ACS, Orlando, [17] Römps Chemie Lexikon, Band 6, Frankh sche Verlagsbuchhandlung, Stuttgart [18] G. J. Van Amerongen Rubber Chem. and Technol. 37 (1964) [19] J. Crank, The Mathematics of Diffusion, 2nd ed., Clarendon Press, Oxford (1975). [20] P. Meares, J. Polym. Sci. 27 (1958) 391. [21] H. Fujita, A. Kishimoto, K. Matsumoto, Trans. Faraday Soc. 56 (1960) 424. [22] A. Fick, Ann. Phys. Lpz. 170 (1855) 59. [23] J. Crank, G. S. Park, Diffusion in Polymers, Academic Press, London, New York (1968). [24] C. Rosca, U. Giese, T. Früh, Poster presented at South Brazilian Rubber Conference, Bento GonÓalves RS-Brazil, October, [25] U. Giese, V. Peterseim, R. H. Schuster, Gummi Fasern Kunstst.(GAK) 51 (1988) KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 11/2004