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Performance resins in tire compounding.

Regulatory pressures

In recent years, regulatory changes have affected how tire manufactures formulate their tire compounds. In 2010, the European Union (EU) banned (ref. 1) the use of high polycyclic aromatic hydrocarbon (PAH) containing processing oils, which dominated the market as plasticizers for rubber compounding. Several alternatives have been offered by oil suppliers, including treated distilled aromatic extracts (TDAE), which are produced by adding an extractive purification step to remove the polycyclic compounds, and naphthenic oils. Since the amount of aromatic character in the process oil affects the dynamic properties of the rubber (ref. 2), the oil options available require reformulation of the tire compound in order to compensate for the reduced aromatic content of these alternate oils.

An example of additional regulations (ref. 3) impacting tire compound formulations is the new EU labeling requirements that will allow consumers to easily evaluate tires based on fuel consumption performance, safety and noise. Also, regulations (ref. 4) requiring all tires in the EU to meet standard performance criteria for rolling resistance, wet grip (passenger car tires) and noise are being implemented stepwise between 2012 and 2020 for different types of tires for passenger cars, as well as for light and heavy duty vehicles (trucks and buses).

Competitive pressure and performance labeling regulations in various parts of the world are forcing tire manufacturers to simultaneously improve conflicting tire attributes, including safety, product life and fuel economy (ref. 5). In addition, the recent legislation banning use of the PAH-containing processing oils that dominated the market has further complicated this challenge. Hydrocarbon resins (HCR) provide a unique solution for tire formulators by simultaneously improving safety performance, while functioning as a processing aid with acceptable PAH content.

Resins used in industry

A variety of resins are used in the rubber industry as processing aids, tackifying agents, curing agents and reinforcing agents (ref. 6) (table 1). Phenol-formaldehyde based resins are the largest and probably most widely used group of resins. The reactive resol type resins react with unsaturation in the rubber to crosslink the compound, improving heat and moisture resistance, but reducing flexibility. Reactive novolak type phenol formaldehyde resins with branching will crosslink with methylene donors to reinforce the rubber compound.

A range of non-reactive resins can be used to modify the viscoelastic profile of polymers, resulting in rubber compounds with specific properties different from the polymer itself. The resins can be produced from feed streams that include linear novolak type phenol formaldehyde, natural products such as rosin or terpene, coal coke (coumarone, indene), styrenic monomers and petrochemical distillation column fractions loosely referred to as five-carbon (C5) or nine-carbon (C9) resins, although the composition of these petroleum fractions contains monomers with a range mixture of compounds having similar boiling points (ref. 7). Polymerization of C5-type monomers yields hydrocarbon resins that are aliphatic in character with varying amounts of unsaturation in the back bone. In contrast, polymerization of C9-type monomers yields an aromatic hydrocarbon resin. The two types of feed streams can be co-polymerized to produce a hydrocarbon with a mixed aliphatic-aromatic character. Additionally, resins may be hydrogenated to remove unsaturation and aromaticity. This changes the aliphatic-aromatic balance of the resin, reduces color and improves stability. Possible structures for these HC resins are given in figure 1.

Resin effects on compound properties

Historically, the compound viscosity, tack and strength (reinforced/cured) were the primary compound properties of interest. Today, the changes in compound viscoelastic properties and the resulting changes in traction, handling, tread-wear and rolling resistance are critical concerns. When formulating to adjust the performance properties of a tire compound, knowledge of how the different types of resins affect the viscoelastic properties of the final compound is important.

When a non-reactive resin is compatible with a rubber, it decreases the polymer entanglement density, which improves processability and decreases the compound modulus, as measured by dynamic mechanical analysis (DMA) (ref. 8) (figure 2). This is similar to the effect &adding hydrocarbon oil to the compound. The sorer rubber compound more readily deforms and provides better surface contact with the substrate. This improved contact provides opportunity for a greater amount of van der Waals interactions and increases tack.

A compatible, non-reactive hydrocarbon resin also raises the glass transition temperature (Tg) of the compound compared to the Tg of the neat rubber (ref. 9). This higher compound Tg shifts the DMA tan delta curve and changes the indicated compound wet traction and rolling resistance (figure 3). Typically, the tan delta value at 0[degrees]C is increased, indicating improved wet traction. If the resin is not fully compatible or if the resin loading exceeds its miscibility in the rubber compound, then a second peak from the resin is seen in the tan delta curve, and a smaller shift in the tan delta curve is observed.

Several models have been proposed regarding adhesion and dynamic rubber friction (ref. 10). Nordseik correlated elastomeric properties to mechanical loss factors to develop the temperature performance spectrum of tire tread compounds (ref. 11). These and other changes in the DMA curves can be correlated to various tire performance properties. Thus, ice and snow traction were shown to correlate to tan delta measured below 0[degrees]C, and rolling resistance correlates to tan delta measured between 35-75[degrees]C, while wear correlates to tan delta measured below 50[degrees]C. It is very difficult to manipulate the performance balance of traction, treadwear and rolling resistance. These three properties are commonly referred to as the magic triangle or the golden triangle of compounding. Almost invariably, improvements in one property are met with comprised performance in one or both of the other two properties. This article will explore how the use of hydrocarbon resins helps to overcome common compromises often observed between grip and rolling resistance.

Experimental

Materials

All rubbers, additives and performance hydrocarbon and traditional phenolic resins used were commercially available and used as received without further purification. Hydrocarbon (HC) resins based on non-aromatic feedstock and without aromatic rings were classified as "aliphatic." HC resins based on aromatic feedstock were classified as "aromatic," and HC resins with both aromatic rings and aliphatic character were classified as "mixed."

Formulation and mixing

A general passenger car light truck (PCLT) tread formulation using a 70/30 by weight blend of solution-styrene-butadiene rubber (sSBR) and butadiene rubber (BR) was prepared (table 2). The reference formulation contained 20 parts per hundred rubber (phr) treated distillate aromatic extract (TDAE) oil. The effect of adding resins or additional oil to this formulation was evaluated by adding 10 phr of TDAE oil or of resin to the reference formulation. The formulation of the model compounds containing only rubber, resin and curing agents is given in table 3.

Mixing was done in three stages using an internal mixer and a two-roll mill (table 4).

Test methods

The Mooney ML (1+4) viscosities were measured according to ASTM D 1646 at 2 rpm, 100[degrees]C, with serrated grooved rotors.

Moving die rheometry (MDR) was performed according to ASTM D 5289 at 1.67 Hz and 0.5[degrees] amplitude.

Tear strength was measured according to ASTM D 624 Die T.

Abrasion resistance was measured according to D1N 53 516/ASTM D5963-04 (2010).

Dynamic mechanical analysis (DMA) was performed on cured samples in simple shear configuration unless otherwise specified. Temperature sweep was conducted from-100[degrees]C to +150[degrees]C at 1 Hz and 0.5% strain. Strain sweeps were conducted at 30[degrees]C, 10 Hz, from -0.1 to 20% strain. For tread compounds, the values for tan delta at 60[degrees]C, G' storage modulus at 30[degrees]C and J" loss compliance at 30[degrees]C are reported at 5% strain. Model compounds without filler were tested in compression at 1% strain at 1 Hz.

Results

Comparison of phenolic resins and hydrocarbon (HC) performance resins in a PCLT tread formulation (figure 4) shows similar improvement in compound viscosity for the two types of resins, but HC resins have reduced modulus (dry handling) compared to traditional tackifying phenolic resins. Similarly, the HC and phenolic resins affect the balance between tan delta at 0[degrees]C and at 60[degrees]C differently (figure 5). HC resins provide significantly greater improvement in tan delta at 0[degrees]C (wet traction) without the significant increase in tan delta at 60[degrees]C (rolling resistance) that is seen with traditional tackifying phenolic resins. Additionally, the balance between tan delta at 0[degrees]C and 60[degrees]C can vary, depending on the HC resin used, providing additional formulation options to the compounder.

A primary HC resin attribute that affects the compound viscoelastic properties is aromatic content of the resin, with a given resin being predominantly aliphatic, predominantly aromatic or mixed aliphatic-aromatic in character. The particular balance of aliphatic-aromatic character affects the final tread compound properties, as illustrated in figure 6 for a mixed HC resin and an aromatic HC resin. The plot is indexed such that values greater than 100 indicate an improvement in performance relative to the reference compound in the study. In general, HC resins reduce compound viscosity and lengthen scorch time, but cure time may also be lengthened. Additionally, the compound tan delta at -10[degrees] and 0[degrees]C and loss compliance at 30[degrees]C are all increased by both HC resins, indicating improved ice traction, wet traction and dry traction, respectively, without a large negative effect on tan delta at 60[degrees]C.

The mixed aliphatic-aromatic character HC resin gave improved tear performance compared to the reference compound and compared to the compound containing aromatic HC resin. The mixed resin increased the compound storage modulus at -20[degrees]C, indicating reduction in traction to cold road surfaces in winter (lower indexed value in the figure), while the aromatic HC resin did not significantly alter the modulus at -20[degrees]C relative to the reference compound.

The variation with different HC resins in the balance between tan delta at 0[degrees] and 60[degrees]C, depending on which HC resin is used in the compound, is shown in figure 7. The resin attributes such as aliphatic-aromatic balance, glass transition temperature and molecular weight distribution all affect the final balance of compound performance.

To assist in understanding the impact of resin attributes, a model system containing only rubber (sSBR or BR), 30 phr resin and curatives was studied. Resins were chosen with similar aliphatic-aromatic balance and varying molecular weight. The resin number average molecular weight, Mn, is strongly correlated with the resin Tg for these resins. As the resin molecular weight increases, the model compound tan delta curve shifts to higher temperature and the peak height is reduced (curves A-C in figure 8). This trend continues until the resin molecular weight is sufficiently high to reduce resin miscibility in the rubber, and the tan delta peak broadens and the peak temperature shifts to lower temperature compared to lower molecular weight resins (curve D). The reduced miscibility of the highest molecular weight resin (D) is also evidenced by a second peak in the tan delta curve.

A similar trend is seen with these resins in a tread compound containing 10 phr resin, 80 phr Si and a 70/30 blend of sSBR and BR (figure 9). The tan delta peak temperature shifts with the addition of resin, and the height of the tan delta peak is reduced (figure 10). The magnitude of shift in the tan delta curve is less than observed in the model compounds as a result of the lower resin content (10 phr) in the tread compounds. Additionally, at this loading, the miscibility of the highest molecular weight resin in the rubbers is not exceeded in the tread formulation, and a second tan delta peak is not observed. This is in agreement with work done previously by J. Class (ref. 12) showing that the aromatic-aliphatic balance, molecular weight and other resin attributes of the HC resin will determine the miscibility limit of a given resin in a particular rubber or rubber blend.

Conclusions

Although resins have been used in tire compounding for many years, only recently have they been utilized to affect more than processing properties and tack. Hydrocarbon (HC) resins were shown to provide a better balance between tan delta at 0[degrees]C and 60[degrees]C (wet grip and rolling resistance) with a trade-off in dry handling when compared with traditional phenolic resins. In addition to the processing benefits of lower viscosity, better scorch safety and increased tack compared to a reference formulation, HC resins generally provide improved wet, dry and ice traction, and improved tear resistance, with trade-offs including cure time and dry handling.

The degree of change in compound properties depends on the resin type and molecular attributes, resin loading and the rubber(s) used. For a model compound of resin and rubber, a direct relationship was shown between resin molecular weight, resin compatibility in rubber and the compound tan delta values. For more complicated systems, the relationships are also more complicated, although similar trends were seen. An understanding and knowledge of the resin attributes is important in selecting the correct resin to adjust compound properties.

This article is based on a paper presented at the 184th Technical Meeting of the Rubber Division, ACS, October 2013.

References

(1.) 1906/2007/EC (REACH) Annex XVII--Entry 50.

(2.) Regulation (EC) No. 1222/2009 of the European Parliament and of the Council of November 25, 2009, on the labeling of tires with respect to fuel efficiency and other essential parameters, Commission Regulation (EU) No. 228/2011 of March 7, 2011, amending Regulation (EC) No. 1222/2009 of the European Parliament and of the Council with regard to the wet grip testing method for C1 tires; text with EEA relevance.

(3.) A. Ahagon, T. Kobayashi and M. Misawa, Rubber Chem. and Technol., 61, 14 (1988).

(4.) Commission Regulation (EU) No. 1235/2011 of November 29, 2011, amending Regulation (EC) No. 1222/2009 of the European Parliament and of the council with regard to the wet grip grading of tires, the measurement of rolling resistance and the verification procedure; text with EEA relevance.

(5.) "Tyre Labeling 101," August 20, 2012, http://www.tyrepress, com/news/legislation/labelling/26070.html.

(6.) B. Stuck, "Tackifying, curing and reinforcing resins." In Rubber Technology, Second Edition, J.S. Dick, Ed.; Hanser Publications: Cincinnati, 2009; pp. 438-447.

(7.) P O. Powers, Hydrocarbon Resins. Kirk-Othmer Encyclopedia of Chemical Technology, Second Completely Revised Edition, Interscience: New York, 1966; Vol. 11, pp. 242-262.

(8.) H.P. Weyman, Naval Stores Review, 6 (1965).

(9.) J.B. Class and S.G. Chu, J. Appl. Polym. Sci., 30, 805 (1985).

(10.) A. Schallamach, Wear 6, 375 (1963), Yu B. Chernyak and A.I. Leonow, Wear 108, 105 (1986), G. Heinrich, Rubber Chem. and Technol. 70, 1 (1997), J.T. Byers, Rubber Chem. and Technol., 75, 527 (2002).

(11.) K.H. Nordsiek, Kautschuk Gummi Kunstoffe 38, 178 (1985).

(12.) J.B. Class and S.G. Chu, J. Appl. Polym. Sci. 30, 815 (1985).

by Terri R. Carvagno, Antoinette van Bennekom, Soumendra K. Basu and Gert-Jan van Ruler, Eastman Chemical

Table 1--resin types used in the rubber industry

              Reactive resins                     Non-reactive resins

Curing resins         Reinforcing resins          Tackifying resins
Phenol-formaldehyde   Phenol-formaldehyde         Phenol-formaldehyde
Resol type            Novolak type                Novolak type
                      High styrene resins         Hydrocarbon resins
                      Methylene donors            Terpene derivatives
                      Resorcinol (formaldehyde)   Coumarone indene
                      Polybutadiene               Rosin derivatives
                      Styrene-acrylonitrile       Tall oil derivatives
                      Polyvinyl chloride resins   Phenol-acetylene
                                                  condensation

Table 2--tread formulation used for resin evaluation

                                 Reference    Reference    Resin
Components                          (phr)    + oil (phr)   (phr)

sSBR: 25% styrene, 42% vinyl,       6570.0          70.0    70.0
  Mooney
BR: 44 Mooney, high-cis               30.0          30.0    30.0
Silica: [N.sub.2] surface area        80.0          80.0    80.0
  175[m.sup.2]/g, 6.5 pH
Carbon black N 234                    10.0          10.0    10.0
Si 69 (silane)                         8.0           8.0     8.0
TDAE oil                              20.0          30.0    20.0
Santoflex (6PPD)                       2.0           2.0     2.0
IPPD                                   1.0           1.0     1.0
TMQ                                    0.5           0.5     0.5
Wax                                    1.0           1.0     1.0
Zinc oxide                             3.0           3.0     3.0
Stearic acid                           1.0           1.0     1.0
Resin                                  0.0           0.0    10.0
Sulfur                                 1.5           1.5     1.5
CBS                                    1.5           1.5     1.5
DPG                                    2.0           2.0     2.0

Table 3--model compound formulation

                                Reference   Resin
                                   (phr)    (phr)

Rubber                                100     100
Resin                                  -       30
Stearic acid   Cure activator         1.5     1.5
Zinc oxide     Cure activator         1.9     1.9
Sulfur         Crosslinker            1.5     1.5
CBTS           Accelerator            1.3     1.3
DPG            Accelerator            1.5     1.5

Table 4--mixing and curing protocol for tread preparation

Components                       Reference   Reference   Resin
                                     + oil
                                               Stage 1
sSBR: 25% styrene, 42% vinyl,           70          70      70
  Mooney 65
BR: 44 Mooney, high-cis                 30          30      30
Silica: N2 surface area                 80          80      80
  175 [m.sup.2]/g, 6.5 pH
Carbon black N 234                      10          10      10
Si 69 silane coupling agent              8           8       8
TDAE oil                                30          20      20
Resin                                    -           -      10
Stearic acid                           1.0         1.0     1.0
Zinc oxide                             3.0         3.0     3.0
Compounding conditions           BR Banbury, FF 75%, Start
for stage 1 Record: 1)           60[degrees]C, rpm 60. Increase rpm
rpm of rotor vs. time            as needed to reach 150-160[degrees]C
2) temperature and energy        by 4 min. 0'00" to 1'0" polymer
   input vs. time                1'00" to 3'00" 50 phr Si, 5 phr Si69,
3) Dump temperature              ZnO, stearic acid and carbon black
                                 and resin or 10 phr oil (Ref. +)
                                 3'00" to 4'00" 30 phr Si, 3 phr Si69,
                                 20 phr oil ram raise at 4' 00"
                                 for for 30"
                                 4'30" to 6'30" hold temperature
                                 at 150-160[degrees]C
                                 Dump at 6'30"
                                 RT milling for 2'00" after dump.
                                 60[degrees]C, 2 mm gap, 16-20 rpm,
                                 friction 1 to 1.14

                                               Stage 2
Santoflex (6PPD)       Antioxidant     2.0         2.0     2.0
IPPD                   Antiozonant     1.0         1.0     1.0
TMQ                    Antioxidant     0.5         0.5     0.5
Wax                                    1.0         1.0     1.0

Compounding conditions for       BR Banbury, FF 75%, start
stage 2 Record same as           60[degrees]C, rpm 35 rpm
for stage 1                      At 0'00" to 1'00" stage 1 batch
                                 At 1' 00" to 1" 30" antioxidants, wax
                                 At V30" to 4'00" increase rpm as
                                 needed to reach 150-160[degrees]C
                                 by 4'00"
                                 4'00" to 6'00" hold temperature
                                 at 150-160[degrees]C
                                 Dump at 6'00"
                                 RT milling for 2'00" after dump,
                                 as above

                                              Stage 3
Rubbermakers, sulfur                   1.5        1.5      1.5
CBTS                   Accelerators    1.5        1.5      1.5
DPG                    Accelerators    2.0        2.0      2.0

Compounding conditions for       Open two-roll mill, start
stage 3 Record dump              60[degrees]C, 16-20 rpm
temperature                      0'00" to 1'00" cooling down the
                                 compound 1'00" to 5'00" sulfur,
                                 accelerators (actual addition
                                 1'00" to 1'30")
                                 5'00" to T00" homogenization
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Author:Carvagno, Terri R.; van Bennekom, Antoinette; Basu, Soumendra K.; van Ruler, Gert-Jan
Publication:Rubber World
Date:Feb 1, 2014
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