Cure and mechanical behavior of elastomeric compounds containing devulcanized materials.

Elastomer usage is dominated by the automotive industry[ref. 1]. Ford Motor Co. is committed to utilizing elastomer compounds with recycled content wherever they meet performance requirements and make economic sense. Ford has already used post-consumer recycled tire rubber in applications such as parking brake pedal pads and Aerostar step plates. Other applications including tires are being vigorously explored.

The infrastructure to obtain steady, reliable sources of post-consumer recycled materials (PCR) of the various types of elastomers used in the vehicle is developing. The automobile companies are creating a demand for PCR in order to help stimulate this growth of the infrastructure as it makes economic sense in a free market place environment. However, PCR needs to be recognized as a compounding tool. That is, the compound should be developed initially using PCR and not simply adulterated with PCR after the fact.

Most rubber compounds contain only 30-50% of base elastomers mixed with a variety of other materials (fillers, plasticizers, extenders, stabilizers and curatives) in order to achieve the specific performance required. Because elastomer compounds are designed to maximize the performance, life and stability of components, these materials do not deteriorate rapidly and accordingly accumulate in disposal areas. However, the materials being disposed of may have value. The recycled rubber examined in this report is the result of a new proprietary technology for devulcanizing rubber[refs. 2 and 3].

Most elastomer compounds form permanent chemical bonds and cannot be easily remolded. Thermoset elastomers are either ground into particles, devulcanized or reclaimed through a severe chemical process which significantly alters the elastomer properties[refs. 1 and 4]. Ground cured elastomers have been used in modest amounts as a component of rubber compounds in the past. Reclaimed rubber can be suitable for use in some rubber compounds[refs. 5 and 6]. In the past, use of reclaimed rubber and ground rubber was not always held to stringent performance requirements (e.g. in some low cost after market tires). Advances in the surface treatment of ground rubber have significantly improved the physical properties of the compounds in which they were included[refs. 7-10]. Devulcanization technology is the newest in the arsenal of methods which present opportunities for reutilization of cured rubber[refs. 1, 2 and 11].

Tires account for about half of the rubber used in automobiles. Because of the large quantity of existing scrap tires (over 2 billion) and the fact that they are often concentrated in disposal areas, tires will remain the major source of postconsumer rubber for the near term[refs. 1 and 12]. Technologies such as devulcanization which further the economic reutilization of tire rubber have the potential for a major impact on the recycling of elastomers.

Experimental

Materials

Elastomer compounds prepared for this study contained natural rubber SMR-L. The engine mounts and bushing compounds also contained smaller amounts of bromobutyl [X.sub.2]. (bromine 2.20%, Mooney viscosity ML 1+8 at 125 [degrees] C = 41-52.), polyisobutylene (Vistanex L-140, viscosity average molecular weight 2.11 x [10.sup.5] and terminal unsaturation) and ethylene-propylene rubber (Vistalon 805, Mooney viscosity ML 1+4 at 125 [degrees] C = 35, ethylene 75% by weight and medium MWD). The above were supplied by Exxon Chemical Co.

Accelerators, activators and vulcanizing agents were provided by R.T. Vanderbilt Co. and used in compounding as received. The De-Vulc (black material) was supplied in the slab form by STI-K Polymers of America (density [20 [degrees] C] = 1.14 g/[cm.sup.3]). It was prepared by combining on a two-roll laboratory mill (229 x 500 mm) tire crumb rubber (passenger car, 100 phr), proprietary De-Link reactant (7 phr), Struktol 60NS/40MSF (2 phr) and raw natural rubber (6 phr).

Compound preparation

Virgin natural rubber compounds for tire tread and related applications. engine mounts and high damping bushings were compounded (table 3). Mixing was done in an internal mixer (BR laboratory size). After masticating the elastomer or elastomers for two minutes, the activators, antioxidants and antiozonants were added and mixed for one minute. The incorporation of carbon black for two minutes was followed by the addition of plasticizer for one minute. The mix was dumped after the curatives were mixed for one minute. The compound temperature just before dumping was about 55 [degrees] C. Mixing was finished on a 229 x 500 mm two-roll laboratory mill. The incorporation of the virgin compounds with the De-Vulc materials was also carried out in the BR mixer. Table 4 provides the mixing conditions for the De-Vulc blends. Again, the final mixes were finished on the two-roll mill as with the virgin materials.

Table 3 - rubber formulations without recyclate material

Ingredients            R-272   R-273   R-276
NR, SMR-L               100      80      60
Bromobutyl X2           ---      20      40
Vistanex L-140          ---     ---      20
Vistalon 805            ---      10     ---
Stearic acid              2       2       2
Zinc oxide                5       5       5
Carbon black (N-330)     50     ---     ---
Carbon black (N-550)    ---      35     ---
Carbon black (N-660)    ---     ---      73
Sundex 790               10       5     ---
Sunpar 130              ---     ---       5
Vulcanox DDA (HS)       ---     ---     1.5
Vulcanox MB7/MGC        ---     ---     1.5
Sulfur (#104)          2.00    0.60    0.40
Amax                    ---    0.70    0.55
Cure Rite 18            ---    1.40    1.80
Durax                  1.30     ---     ---
Agerite Superflex       ---    2.00     ---
Agerite resin D         ---    2.00     ---
Antozite 67P            ---    1.00     ---
Table 4 - cure and aging data of De-Vulc and blend materials

Compound   Cure time(a)     Scorch   Cure state
              t95, min.  time ts2,     (MH-ML)
                              min.         dNm

127                  18      1.58        35.90
272                  12      2.17        81.90
272D                 25      1.30        75-80
273                  13      2.85        42.95
273D                  8      2.06        44-30
276                  10      2.12        50.90
276D                  8      2.38        47.60

Compound           Compression
                       set(b)
            60 [degrees] C  100 [degrees] C

127              15.5             31.0
272              15.8             47.0
272D             16.0             31.0
273              16.0             20.0
273D             15.6             29.8
276              28.0             27.0
276D             32.8             34.0

(a) All samples cured at 160 [degrees] C

(b) Samples under compression for 22 hours

Molding method

In general the determination of the molding procedure was done by curing the materials to 95% of optimum cure at 160 [degrees] C. Molding of compression set buttons (28 mm diameter by 13 mm thick) and ASTM test sheets (150 x 150 x 2 mm) were carried out according to ASTM D 395 (method B) and D 3182 procedures, respectively. Tensile (die C) and tear (die B) specimens were cut from cured sheets with a punch press.

Property measurements

The tensile (ASTM D412) and tear (ASTM D624) of unaged and heat aged samples were measured with an Instron electromechanical tester (model 5565). In doing this testing, the procedure is set up on the screen display of a Gateway 2000 personal computer (PC) run by the Windows-based Merlin software. In addition to the testing unit attached to the PC, an Instron (series 2663) non-contacting video extensometer is interfaced for accurately determining the strain applied to the specimen undergoing deformation during mechanical testing. The control panel on the load frame is used to run the actual test. All data analysis is handled automatically by the software and reported numerically and graphically on the screen and an HP laser jet III printer.

Both of these tests are run at ambient temperature at a crosshead speed of 500 mm/minute. Sample heat aging is conducted in an air circulating oven at 100 [degrees] C for 72 hours. Hardness of the vulcanizates was measured according to ASTM D2240 using a Shore A durometer. A ten second relaxation was allowed before the final reading was taken. Compression set measurements were run according to ASTM D395 method by compressing the buttons for 22 hours at 60 [degrees] C and 100 [degrees] C in an air-circulating oven. Cure data were obtained with a Monsanto ODR 2000 rheometer. The uncured samples were freshened on a small two-roll mill before cure to assure a homogeneous mix. During testing the rheometer rotor oscillates through a 3 [degrees] arc at 1.66 Hz and exerts a shear strain on the sample as that sample cures. Cure profiles of rubber compounds are usually displayed as torque in dNm against time in minutes. The thermal behavior of the materials was determined with a Perkin-Elmer TGA7 thermogravimetric analyzer controlled by a DEC personal work station. The analyzer permits the measurement of weight changes in a sample material as a function of temperature or time. Although the weight changes can be due to a wide variety of phenomena, volatility and differences in breakdown patterns are of particular interest in this study. The dynamic tests were conducted by programming the sample temperature from 30 [degrees] C to 80 [degrees] C at 10 [degrees] C per minute.

Results and discussion

In order to examine a range of potential applications, compounds were prepared typical of three types of applications: tire tread (R-272), engine mount (R-273) and high damping bushings (R-276). Further, these applications also represented a range of cure systems. The tire tread compound (R-272) was cured with a system which approaches a conventional cure system and which gives a range of disulfidic and polysulfidic crosslinks with very few monosulfidic crosslinks. The engine mount compound (R-273) used is a semi-EV (semi-efficient vulcanization) cure system which is directionally closer to a conventional cure system. It thereby produced more monosulfidic crosslinks than the conventional cure but less than an average semi-EV system. The high damping bushing compound (R-276) used a semi-EV cure system which produced more monosulfidic crosslinks than the usual semi-EV cure system but less than expected from an EV cure system. A summary of the sulfur/sulfenamide weight ratios is shown in table 1 with the actual recipes shown in table 3.

Table 1 - cure systems used with De-Vulc blends

Cure     Ingredients    Sulfur/sulfenamide   Type of cure
system                     weight ratio

A        Sulfur                3.00          Approaching
         Durax                 1.95          conventional

B        Sulfur                0.86          Semi EV
         Amax                  3.00          (closer to
         Cure Rite 18                        conventional)

C        Sulfur                0.48          Semi EV
         Amax                  3.00          (closer to EV
         Cure Rite 18                        system)

The characterization of the cure information is shown in table 4. The effect of the De-Vulc material varied depending on the cure system and compound. There was a reduction in scorch safety with R-272 and R-273, but an increase in scorch safety with R-276. The cure time was almost double for R-272 with the De-Vulc material, about half as long for the R-273 with De-Vulc and slightly shorter for the R-276 compound. When the final cure state was reached, it was essentially unchanged by the presence of the DE-Vulc material.

Compression set is a property related to cure state, and it was also essentially unchanged from the virgin compound when the testing was done at 60 [degrees] C. At 100 [degrees] C, the situation changed. R-272 was drastically improved by the presence of the De-Vulc material. R-273 was substantially worse, and R-276 about unchanged. Clearly, the effects of De-Vulc on compounding are not universal and must be carefully characterized for each individual compound.

As we look at the physical properties of the compounds with and without De-Vulc (table 5), it should he pointed out that compound optimization is probably necessary to achieve optimum physical properties. Unfortunately, that was not possible within the scope of the current work. Tensile strength, elongation at break and tear strength all showed significant reduction by the presence of De-Vulc in all three compounds. However, the values of these properties would probably be suitable for a number of applications. Modulus and hardness were essentially unaffected by the presence of De-Vulc. The effect of heat aging on the physical properties (table 6) yielded results which essentially mirrored those of the unaged materials except for the tear strength on the R-273 compound with De-Vulc which showed anomalously high tear strength.

Table 5 - physical properties of De-Vulc and blends with virgin compounds

Sample #    Tensile   Elongation       Tear       50%      100%
           strength            %   strength   Modulus   Modulus
                MPa         kN/m        MPa       MPa
R-127         10.60          302      24.90      1.15      4.50
R-272         29,80          529     143.00      1.50      7.15
R-272D        21.30          408      91.10      1.40      6.50
R-273         24.00          642      85.90      0.90      3.20
R-273D        14,40          481      72.40      1.03      3.75
R-276         15.20          420      74.80      1.70      7.10
R-276D        11.30          343      48.70      1.40      5.40

Sample #    Hardness
             Shore A

R-127             61
R-272             60
R-272D            63
R-273             50
R-273D            51
R-276             64
R-276D            62
Table 6 - effect of aging on properties of De-Vulc and blends

Sample #      Tensile   Elongation       Tear       50%      100%
             strength            %   strength   modulus   modulus
                  MPa                    kN/m       MPa       MPa
R-127(1)         9.85          290      21.80      1.06      4.10
R-272A(2)       21.04          350      94.40      1.85      9.25
R-272DA(3)      14.05          268      56.10      1.75      8.50
R-273A          17.50          537      51.30      0.83      3.70
R-273DA         11.07          375      65.75      1.02      4.10
R-276A          14.50          355      58.20      1.90      8.00
R-276DA         10.30          265      44.10      1.70      7.10

(1) R-127A is the De-Vulc material aged at 100 [degrees] C for 72 hours

(2) R-272A, R-273A and R-276A are the aged virgin compounds

(3) R-272DA, R-273DA and R-276DA are the aged blends of De-Vulc with the virgin materials

Conclusion

The effect of blending De-Vulc material into a rubber compound on the cure properties depends on the type of cure system and possibly the other components in the compound. Thus, no generalizations can be made and each compound will have to be examined and optimized separately.

The physical properties of tensile strength, elongation at break and tear, strength were generally reduced in the presence of De-Vulc. However, compound optimization may provide relief in these important properties. Work communicated from suppliers already indicates that this is correct. It should be pointed out that the physical properties actually obtained with De-Vulc present still represent a relatively high quality rubber compound. The modulus and heat aging characteristics are essentially unaffected by the presence of De-Vulc.

While there are many questions raised by this initial work, use of devulcanized rubber (De-Vulc) as a compounding tool appears very promising.

Table 2 - mixing cycles for De-Vulc blends

Mixing       Processing        Mixing time,   Mix temp.,
equipment    action              minutes      [degree] C

BR lab       Mastication of        0.50           --
internal     virgin rubber
mixer        compound

             Addition of           2.50          32-38
             De-Vulc (black)
             material

Two-roll     Completion of         4.00          27-32
laboratory   mixing of blend
mill         from mixer

References

[1.] J.R. Dunn, ACS National Meeting, Washington, D.C. (1994).

[2.] M. Moore, Rubber and Plastic's News, February 27 (1995).

[3.] K. Hon, F. Siesseger, S. Sin and B. Sekhar, ACS Rubber Division Meeting, Cleveland, Ohio (1995).

[4.] J.J. Leyden, Rubber World, 203, 28 (1991).

[5.] W.B. Klitzgensmith, Rubber World, 203, 16 (1991).

[6.] A.A. Phadke, A.K. Bhattacharya. S.K. Chakraborty and S. K. De, Rubber Chemistry and Technology, 56, 727 (1983).

[7.] B.D. Bauman, ACS Rubber Division Meeting, Chicago, IL (1994).

[8.] I. W. Dierkes, ACS Rubber Division Meeting, Philadelphia, PA (1995).

[9.] F.J. Stark, Jr., U.S. Patent 4,481,335 (1984).

[10.] E.M. Fesus, ACS Riebber Division Meeting, Chicago, IL (1994).

[11.] W.C. Warner, ACS Rubber Division Meeting, Orlando, FL (1993).

[12.] R.H. Snyder, Tire Technology Conference, Clemson University, Greenville, SC (1986).