Abstract. Introduction

Transcription

1 An Approach to optimise Processing Additives for Silica Compounding Michael E. Wolers Performance Additives, Taman Perindustrian Sime UEP, 7600 Subang Jaya, Selangor Darul Ehsan, Malaysia Abstract With the introduction of the silica tyre concept to achieve an optimum between wet traction and rolling resistance, the mixing problems involved by incorporating large amounts of silica into rubber compounds created a demand for additives specially designed to solve these problems. The purpose of this paper is to show development approaches to optimize the efficiency of additives for this type of compounding. Introduction Since the introduction of the so-called Green Tyre concept in the early 90ies, the use of silica as reinforcing filler has spread and grown worldwide. The general advantages of silica over black filler are better rolling resistance by achieving at least equal wet traction while tread wear should not be adversely affected 1. The compound mixing procedure has a significant effect on the before mentioned performance criteria. Since the mixing and processing of silica compounds are considerably worse as compared to carbon black filled compounds, a number of processing promoters have been recommended to overcome these difficulties, such as zinc soaps and esters 2. Responding to requests to reduce the zinc content in rubber parts, we have looked into products based on non-zinc chemistry or drastically reduced concentrations. Other approaches were to replace the silica system with carbon black/mineral filler blends, such as aluminium trihydrate or clay without sacrificing too much in wear and wet traction 3. However, as it appears that the silica tyre will remain unchanged for the near future, the task of improving the processability of the compound still remains unchanged. Test parameter The test formulation is based on an academical tread compound (Table 1) that helps to differentiate the performances of processing additives.

2 SBR 1721 BR 01 Silica (VN 3) Silane X 50 S ZnO Stearic acid 6PPD Aromatic oil Sulphur TBBS Processing additives Table 1 The mixing was carried out in a 1-liter laboratory mixer at constant mixing time. The curing package was added on an open laboratory mill. All chemicals including the processing additives were added after the in-situ silanisation reaction was completed. Mixing Procedure 1 ltr. Lab mixer, 60 rpm rotor speed, starttemperatur 60 C 0.5 min. Polymer blending min Silica, silane and oil incorporation min Additives and small chemicals Curatives added at open mill. Table 2

3 Processing agents There are several commercially available chemicals, claimed to improve the overall processability of silica compounds. These materials are typically fatty acid derivatives such as metal soaps based on zinc or potassium or fatty acid esters, or blends of such chemicals. Taking the polarity of silica into account, an effective additive that helps dispersion should also be of similarly high polarity. The difficulty lies in the fact that high polarity materials are not very good lubricants for this compound type. However, good lubrication is vital to improve down-line processing! The use of higher molecular weight fatty acids can help to balance the dispersion effect and the lubrication effect. There are several ways to increase the polarity of materials based on fatty acids. Development Approaches Aryl groups : more polarity, chance of bloom, potential. Substitution of zinc by potassium : Higher melting point, less divalency and more polarity- good potential, particularly with high silica compounds! Table 3 One is the introduction of aryl groups, which showed some potential but created bloom problems at higher dosage levels. Another way is replacing zinc soaps in part or fully with potassium, which should reach a good compromise between high polarity and internal lubrication effects. The drawback for pure potassium soaps is a high melting point, which can lead to dispersion problems in the compound. Last but not least, polar fatty acid esters show good filler dispersion properties, however, have not as much effect on processability as metal soaps. Taking all of this into consideration, the following chemicals were chosen for evaluation:

4 Trial Products UF 700 S Znsoap 65% Ester 5 o C TP 3 K-soap 80% Ester 85 o C TP 35 K-soap 50% Ester 0 o C TP 36 K 35/Zn 15% Ester 95 o C TP 51 K 80/Zn 5% Fatty acid 0 o C TP 52 K 65/Zn 5% Ester 3 o C Table 3 ULTRA-FLOW 700 S is a commercially available product for silica compounding that we chose as a standard for comparison purposes. Even though, Potassium soap in its pure form shows a very high melting point, we could develop products containing up to 80% potassium soap at melting points of around 80 0 o C. (TP 3 TP 52) Results: Observations during compounding The control compounds showed poor mixing and silica incorporation at high energy consumption. The compounds were very dry and the accelerator package did not easily incorporate at the mill. All of the test compounds showed more or less dramatic improvements with regards to filler incorporation time, energy consumption and mill handling. It is interesting to notice in our first test series (Table,5), that the temperature pattern follows the energy pattern, except for the control, where the higher energy absorption and the lower dump temperature indicate a slower incorporation of the silica.

5 The test compounds incorporated the filler faster, but the additives as viscosity reducer caused the reduced energy consumption. Temperature/Energy Temperature Energy 15 C C 150 C 18 C 16 C C 700S C 700S C 750. Table Table 5 The second test series confirmed the high-energy/low-temperature for the control, however, the test compounds did show the low energy absorption but not the same similarity in the temperature development. The small differences of +/- one degree can be considered margin of error.( Table a/5a) Temperature/Energy (20016) Control 700 S Control 700 S T 860 Energy Table a Table 5a

6 Physical properties The tensile properties were improved with all additives, except for the TP 36, the product with the highest ester content. MPa Physical Properties Modulus TP 35 UF 700 S TP 3 Control TP % Table 6 Physical Properties Modulus (20016) MPa % TP 51 UF 700 S TP 52 TP 3 Control Table 6 a Modul differences are small at low elongation but become significant at higher elongation levels. This is confirmed by the Shore A Hardness which for all compounds is at equal level, +/- one point.

7 Physical Properties Tensile Strength MPa Control TP 3 TP 35 TP 36 UF 700 S Table 7 Physical Properties Tensile Strength (20016) MPa Control UF 700S TP 3 TP 51 TP Table 7a Similar behaviour pattern can be found for the tensile strength ( Table 7/7a) The vulcanization characteristics are shown in Tables 8/8a. Rheometer 150 C, t/t95 UF 700S TP 36 TP 35 TP t95 t Control Min. Table 8

8 Rheometer 150 C, t/t95 (20016) TP 52 TP 51 TP 3 UF 700S t95 t Control Min. Table 8a All additives provided a more or less pronounced retarding effect without significantly affecting t 95.In both test series, ULTRA-FLOW 700 S gave the highest scorch safety. This can be expected from a material with a fairly high zinc soap content since pure zinc soaps are known to have a delaying effect on cure. If this can not be tolerated, a reduction of stearic acid in the compound counteracts this effect. The compound viscosity is reduced in all cases, most effective being the products ULTRA-FLOW 700 S, TP 3 and TP 51. Silica compounds have a strong tendency to increase viscosity after prolonged storage periods caused by its ability to form a network with itself. This is explained by its poor compatibility to rubber, its polarity and the formation of hydrogen bonds. The surface hydrophobation with silane reduces this compatibility problem 5. However, as shown in Table 9 and 9a, a viscosity increase of the control compound after 72 hrs is still pronounced while all the additives reduced this effect to (almost) zero! Viscosity ML 1+, 2/72 hrs Control TP 3 TP 35 TP 36 UF 700S 2 72 Table 9

9 Viscosity ML 1+, 2/72 hrs (20016) Control UF 700S TP 3 TP 51 TP 52 Table 9a Dispersion The filler dispersion was determined by using an optical Dispergrader TM. The results are showing small differences in both classification methods. Big variations can be seen between the two test series, which leads to the conclusion, that the evaluation of silica dispersion with this type of equipment is not very reproducible. (Table / a) Phillips 6.6 Dispersion Rating Dispergrader 96 Cabot % Phillips Cabot [%] Cabot Scale : All C Control TP 3 TP 35 TP S Table

10 Phillips.6 Dispersion Rating Dispergrader (20016) 83.5 Cabot % Phillips Cabot [%] Cabot Scale : All E Control UF 700 S TP 3 TP 51 TP Table a Dynamic Testing As shown in the past, metal soaps of higher molecular fatty acids, in particular based on zinc, can influence dynamic compound properties 6. It is therefore necessary to evaluate the effect on Heat-Built-Up and hysteresis when adding this type of chemicals to a compound. Tests for heat built-up and blow-out time were carried out on a Goodrich Flexometer, according to ISO 666/3. The zinc soap/ester combination (ULTRA-FLOW 700 S ) gave the lowest HBU, the longest blow-out time and the lowest set. The Potassium based additives (TP 3/35, TP 51/52 ) were inferior but still showed some improvement over the control. Worst results, even against the control, gave the ester/zinc/potassium combination (TP 36). Results are shown in table 11/11a and 12/12a. Dynamic Properties HBU,Delta T/ Set, % o C % Delta T [C] Set ( % ) kg/.5 mm stroke,55 o C/25 min. Control 700 S TP 3 TP 35 TP 36 5 Table 11

11 Dynamic Properties HBU,Delta T/ Set, % (20016) o C % Delta T [C] Set ( % ) kg/.5 mm stroke,55 o C/25 min. Control UF 700S TP 3 TP 51 TP 52 5 Table 11a Dynamic Properties Blow Out Temp./Time o C 19 3 Min. BO Temp.[C] BO Time[min] kg/6.35 mm stroke,0 o C Control 700 S TP 3 TP 35 TP Table 12 Dynamic Properties Blow Out Temp./Time (20016) o C M in. BO Temp.[C] BO Time[min] kg/6.35 mm stroke,0 o C Control UF 700 S TP 3 TP 51 TP Table 12a

12 Flow, surface appearance Bloom has always been a major concern for the use of processing additives in tyre compounding. The complete loss of green tack after a few days of storage is an only too well known phenomenon. To evaluate the bloom, the uncured compounds were stored for 21 days at room temperature, thus well exceeding the usual storage times, while the cured sheets were kept for 1 days under the same conditions. None of them showed bloom! The flow test was carried out using a transfer mould, measuring the differences in sample weight and size compared to the control compound. Table 13 shows the results as a percentage improvement over the control. The zinc soap/ester combination (ULTRA-FLOW 700 S ) and the potassium soap/ester blends with low to medium potassium levels performed best. Against expectations that increased potassium content combined with zinc would lead to increased flow, that was not the case. Both products, TP 51 and 52 performed worse then the control. Flow Improvement % 5 % over Control 0-5 TP 3 TP 35 TP 36 TP 51 TP S Table 13 Conclusion It appears that the original assumption that higher polarity leads to improved silica compounding is still valid. That is true for the potassium/ester combinations as in TP 3. Looking at the results of the potassium/zinc combinations (TP 51/52), it seems in both cases that the zinc soap part creates a detrimental effect on the flow properties; a totally unexpected effect! Heat built-up and Blow-out time are best with the zinc soap/ester combination (ULTRA-FLOW 700 S), followed by the other test products.

13 The control shows wide differences between the two test series, which says something about the reproducibility of a silica compound! Very positive are the stabilizing effects on viscosity after storage; all test compounds show a more or less pronounced effect, the zinc soap/ester combination being the most stable. All products improved the mixing and mill handling and reduced the energy input. It is obvious that further development work will be necessary on the subject of silica compounding. The available standard additives work, but according to feedback from the industry, still more effective products are desirable. We have decided to pursue the direction of potassium/ester combinations for further development. Reference : 1. S.Wolff, Rubber Chem. Technology 69, Steger et al., Paper presented at ITEC Mouri et al., Paper presented at ITEC 96. Mouri et al., Paper presented at ITEC Schwaiger et al., Tyretech 2000, Warsaw, Brian G.Crowther, paper presented at Rubbercon 89, Prague Compounding and testing carried out at Sime Darby Technical Center, Kuala Lumpur, Malaysia.