Method of devulcanization of sulfur-cured rubber

Abstract

This invention relates to a composition for devulcanization of sulfur-cured rubber and a method of devulcanization of sulfur-cured rubber, and can be used to recycle used rubber articles. The composition for devulcanization of sulfur-cured rubber, which contains a devulcanizing agent for the selective destruction of sulfide bonds according to this invention, has a mixture of triphenylphosphine and 1,8-diazabicyclo[5.4.0]undec-7-ene at a weight ratio from 5:1 to 1:5 as the devulcanizing agent. Further, it contains a compatibilizer compatible with both the devulcanizing agent and the sulfur-cured rubber, at a weight ratio of the devulcanizing agent to the compatibilizer from 1:15 to 1:70. The devulcanizing agent breaks sulfide bonds in sulfur-cured rubber very effectively and selectively, while the experimentally selected compatibilizer ensures an effective penetration of the devulcanizing agent into the rubber crumbs. As a consequence, the required concentration of the devulcanizing agent in the resulting compounded rubber does not exceed 1% (preferably, 0.03-0.3%).

Claims

1. A method of devulcanization of sulfur-cured rubber that includes mixing a composition for devulcanization of sulfur-cured rubber with rubber crumbs at a weight ratio of the composition to the rubber crumbs from 1:9 to 1:60, wherein said composition for devulcanization of sulfur-cured rubber contains a devulcanizing agent for selective destruction of sulfide bonds being a mixture which contains 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and triphenylphosphine (TPP) at a weight ratio of DBU to TPP from 5:1 to 1:5, and further contains a compatibilizer compatible with both the devulcanizing agent and the sulfur-cured rubber at a weight ratio of the devulcanizing agent to the compatibilizer from 1:15 to 1:70, followed by extrusion of this mixture at 40-120? C.

2. The method according to claim 1, wherein a product of petroleum distillation with a boiling temperature exceeding 250? C. is used as the compatibilizer in the composition for devulcanization of sulfur-cured rubber.

Description

EXAMPLE 1

(1) 0.3 kg of a devulcanizing agent consisting of mixture of DBU and TPP (ratio by weight 1:1) and 10 kg of the petroleum-based oil Viplex 530A (boiling range 320-500? C., liquid at mixing temperature 25? C.) were mixed in a 20 liter plastic pail by a blade mixer for 5 minutes at 150 rotations per minute (rpm). A weight ratio of the devulcanizing agent to the compatibilizer was 1:33.3. The resulting uniform composition was added to 120 kg of rubber crumbs (average particle size 0.5 mm; produced from used rubber tires and cleaned from metal particles) on the Ross mixer (capacity 500 liters; speed 30 rpm) and mixed for 20 minutes. This mixture was loaded into an NRM extruder (screw diameter=4.5 inches). The three heating zones of the extruder had the following temperatures: zone I45? C.; zone II55? C.; zone III65? C. At the end of the extruder, the mixture exited through flat holes 1.5 mm thick, which created the resistance necessary for effective shearing.

(2) To study the properties of the resulting devulcanized rubber, it was mixed with powdered sulfur and the activator of vulcanization dibenzothiazoledisulfide (DBTD) at a weight ratio of the devulcanized rubber to sulfur to DBTD 100:1:0.5 using mixing rubber rolls (rolls diameter 300 mm, rolls speed ratio 1:1.14). To make test samples, this secondary compounded rubber was vulcanized in an electrical press at 140? C. for 15 minutes. The test results are given in Table 1 (P-1).

EXAMPLE 2

(3) The composition for devulcanization of sulfur-cured rubber was made and used exactly as described in Example 1 except that 0.15 kg of mixture of DBU and TPP (ratio by weight 1:1) was used as a devulcanizing agent. A weight ratio of the devulcanizing agent to the compatibilizer was 1:66.7.

(4) The resulting devulcanized rubber was tested as described in Example 1. The test results are given in Table 1 (P-2).

EXAMPLE 3

(5) The composition for devulcanization of sulfur-cured rubber was made and used exactly as described in Example 1 except that 0.3 kg of mixture of DBU and TPP (ratio by weight 5:1) was used as a devulcanizing agent.

(6) The resulting devulcanized rubber was tested as described in Example 1. The test results are given in Table 1 (P-3).

EXAMPLE 4

(7) The composition for devulcanization of sulfur-cured rubber was made and used exactly as described in Example 1 except that 0.3 kg of mixture of DBU and TPP (ratio by weight 1:5) was used as a devulcanizing agent.

(8) The resulting devulcanized rubber was tested as described in Example 1. The test results are given in Table 1 (P-4).

EXAMPLE 5

(9) The composition for devulcanization of sulfur-cured rubber was made and used exactly as described in Example 1 except that 0.3 kg of TPP was used as a devulcanizing agent.

(10) The resulting devulcanized rubber was tested as described in Example 1. The test results are given in Table 1 (P-5).

EXAMPLE 6

(11) The composition for devulcanization of sulfur-cured rubber was made and used exactly as described in Example 1 except that 0.3 kg of DBU was used as a devulcanizing agent.

(12) The resulting devulcanized rubber was tested as described in Example 1. The test results are given in Table 1 (P-6).

EXAMPLE 7

(13) This example illustrates the real effect of a devulcanizing agent.

(14) 10 kg of Viplex 530A (without the devulcanizing agent) was added to 120 kg of rubber crumbs and extruded as described in Example 1.

(15) The extruded rubber was tested as described in Example 1. The test results are given in Table 1 (P-7).

(16) TABLE-US-00001 TABLE 1 The characteristics of the tested rubbers. Characteristics P-1 P-2 P-3 P-4 P-5 P-6 P-7 Scorching time* 120? C. t.sub.5, minutes 16 20 13 18 16 11 >25 Modulus at 100% elongation, MPa 2.53 2.56 2.44 2.54 1.95 2.42 0.86 Tensile strength at break, MPa 13.6 13.2 12.4 13.6 7.8 12.3 4.8 Elongation at break, % 390 380 350 380 250 330 225 Shore Hardness A, ASTM D2240 58 58 58 58 57 58 53 Rebound resilience, ASTM D2632, % 43 41 42 43 34 40 23 Tear resistance ASTM, D624, kN/m 107 97 100 111 38 62 18 *Scorching time was defined as the time necessary for a 5% increase of compounded rubber viscosity at 120? C.

(17) Examples 1-4 illustrate the current invention.

(18) Example 1 shows that the mixture of TPP and DBU as a devulcanizing agent produces together better properties of secondary rubber than any of them alone (examples 5 and 6), while increasing very important for technology scorching time.

(19) Example 2 shows that even twice lower amount of the mixture of TPP and DBU as a devulcanizing agent gives good mechanical properties of secondary rubber along with even longer scorching time.

(20) Examples 3 and 4 show that this synergistic effect of TPP and DBU appears even at a ratio different from 1:1.

(21) Example 5 shows that TPP is not very effective as a devulcanizing agent, giving relatively low modulus, tear resistance and tensile strength of secondary rubber.

(22) Example 6 proves high effectiveness of DBU as a devulcanizing agent, showing at the same time pretty short scorching time.

(23) Example 7 shows that without a devulcanizing agent a secondary rubber has dismal mechanical properties.