Process for producing reclaimed rubber or unvulcanized reclaimed rubber

A process for producing reclaimed rubber has the steps of: mixing waste vulcanized rubber with unvulcanized new rubber and a devulcanizing agent; and kneading the resulting mixture in a heating atmosphere for mastication of the unvulcanized new rubber and simultaneously reclamation of the waste vulcanized rubber. Furthermore, the obtained unvulcanized reclaimed rubber can be vulcanized with a vulcanizing agent.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for producing reclaimed rubber 
or unvulcanized reclaimed rubber from waste vulcanized rubber. 
2. Description of the Related Art 
Rubber articles such as automotive tires are usually molded from vulcanized 
rubber, which has sulfur crosslinks (such as --S-- and --S--S--) made by 
vulcanization between molecule chains of unvulcanized rubber. Waste rubber 
articles are conventionally reclaimed by the process shown in FIG. 2 (flow 
chart). First, waste vulcanized rubber is crushed, incorporated with a 
devulcanizing agent, and heated. This step severs the sulfur crosslinks 
entirely or partly, thereby giving rise to fluid decomposed rubber. 
Meanwhile, unvulcanized new rubber which comprises an uncrosslinked 
polymer is separately masticated by milling. Second, the decomposed rubber 
is thoroughly mixed and kneaded with the unvulcanized new rubber. Third, 
the resulting mixture is incorporated with a vulcanizing agent and finally 
vulcanized to obtain vulcanized reclaimed rubber. 
The conventional process mentioned above has a disadvantage of requiring 
entirely separate two steps for converting waste vulcanized rubber into 
decomposed rubber and masticating unvulcanized new rubber. Therefore, it 
complicates procedures, consumes much time and work and comes to the 
increase of production cost of reclaimed rubber. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an economical and 
simple process for producing reclaimed rubber or unvulcanized reclaimed 
rubber without deteriorating the quality of the reclaimed rubber. 
To achieve this object, the present inventors performed various 
experiments. Through these efforts, they discovered that sulfur crosslinks 
can be effectively severed by applying shearing force to waste vulcanized 
rubber in the presence of a devulcanizing agent, and eventually conceived 
that it would be possible to accomplish reclamation (devulcanization) of 
waste vulcanized rubber and mastication of unvulcanized new rubber 
simultaneously by mixing and kneading waste vulcanized rubber with 
unvulcanized new rubber in the presence of a devulcanizing agent, contrary 
to the common belief. Conventionally, waste vulcanized rubber has to be 
devulcanized by heating in the presence of a devulcanizing agent to sever 
sulfur crosslinks. Therefore, it has never been believed that 
devulcanization is achieved by shearing force that occurs during kneading 
even in the presence of a devulcanizing agent. 
The first aspect of the present invention resides in a process for 
producing unvulcanized reclaimed rubber, comprising the steps of: 
mixing waste vulcanized rubber with unvulcanized new rubber and a 
devulcanizing agent; and 
kneading the resulting mixture in a heating atmosphere, thereby performing 
mastication of the unvulcanized new rubber and reclamation of the waste 
vulcanized rubber simultaneously to obtain unvulcanized reclaimed rubber. 
The second aspect of the present invention resides in a process for 
producing reclaimed rubber, comprising the steps of: 
mixing waste vulcanized rubber with unvulcanized new rubber and a 
devulcanizing agent; 
kneading the resulting mixture in a heating atmosphere to obtain 
unvulcanized reclaimed rubber; and 
subsequently vulcanizing the unvulcanized reclaimed rubber with a 
vulcanizing agent to obtain reclaimed rubber. 
According to the first and the second aspects of the present invention, the 
process for producing reclaimed rubber or unvulcanized reclaimed rubber 
involves severing sulfur crosslinks in waste vulcanized rubber and 
simultaneously masticating unvulcanized new rubber. Therefore, the process 
of the present invention needs a less number of steps and takes a shorter 
time than the conventional process. This contributes to increased 
productivity and reduced production cost. Moreover, it can be carried out 
in a single apparatus, thereby reducing equipment cost as well.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the present invention may be applied to waste vulcanized 
rubber which includes used rubber products and those in crushed form and 
vulcanized rubber scraps that occur during rubber molding. 
The waste vulcanized rubber is a used elastomer or a used rubbery substance 
having sulfur bonds (such as --S--, --S--S--, and --S--S--S--) between 
carbon main chains of an organic compound or between polymers of silicone 
rubber. Examples of the organic compound include butadiene rubber, 
isoprene rubber, butyl rubber, ethylene-propylene rubber, 
styrene-butadiene rubber, chloroprene rubber, nitrile rubber, acrylic 
rubber, epichlorohydrin rubber, chlorosulfonated polyethylene, chlorinated 
polyethylene, fluoro rubber, and EPDM (ethylene-propylene-diene 
terpolymer) which are in unvulcanized form. 
The unvulcanized new rubber denotes any polymer which is a collective of 
organic compounds composed of main carbon chains or a collective of 
silicone rubber molecules, and not yet crosslinked (or vulcanized) with 
sulfur or sulfur compounds. Examples of such compounds include those 
listed above. 
The waste vulcanized rubber and the unvulcanized new rubber should 
preferably be of the same kind, so that they are readily miscible with 
each other, thereby reducing kneading time, as well as yielding reclaimed 
rubber with uniform and good dynamic properties. 
The devulcanizing agent is not specifically restricted so long as it is 
capable of severing sulfur crosslinks in the waste vulcanized rubber. Any 
commonly used one will suffice. Typical examples include reclaiming agents 
such as diaryldisulfide, dixylyldisulfide, phenylhydrazine-ferrous 
chloride, and thiophenol-butylamine salt. It is not always necessary for 
the devulcanizing agent to sever all of the sulfur crosslinks in the waste 
vulcanized rubber. All that is required of the devulcanizing agent is an 
ability to sever the sulfur crosslinks partly to such an extent that no 
problems are involved in mixing with the unvulcanized new rubber. 
Kneading in the step of masticating and reclaiming may be accomplished by 
using any means which is capable of applying a certain amount of shearing 
force to the waste vulcanized rubber and the unvulcanized new rubber so 
that the former undergoes decomposition and the latter undergoes 
mastication. Such kneading may be accomplished by a mixing mill or an 
extruder. A preferable amount of shearing force is 1 to 150 kg/cm.sup.3. 
Shearing force less than 1 kg/cm.sup.3 is not enough for mixing required, 
because decomposition takes a long time. Shearing force exceeding 150 
kg/cm.sup.3 will sever even the main chains of rubber molecules, 
deteriorating properties of obtained reclaimed rubber. 
The heating atmosphere for masticating and reclaiming should preferably be 
at 50.degree.-400.degree. C. Heating temperatures less than 50.degree. C. 
will slow down the rate of decomposition of the waste vulcanized rubber, 
causing the production of the unvulcanized reclaimed rubber to take a long 
time. Heating temperatures exceeding 400.degree. C. will sever the main 
chains of rubber molecules, deteriorating properties of obtained reclaimed 
rubber. 
According to the second aspect of the present invention, the step of 
masticating and reclaiming is followed by the step of adding a vulcanizing 
agent and vulcanizing masticated and reclaimed rubber. This vulcanizing 
step may be accomplished in the same manner as in the conventional 
technology. 
The reclaimed rubber obtained according to the first and the second aspects 
may be incorporated with optional additives which are commonly used for 
ordinary rubber in any step of the process. 
EXAMPLE 1 
The process pertaining to the first and the second aspects of the present 
invention will be explained with reference to FIG. 1. 
As shown in FIG. 1, the process in this example consists of the steps of: 
mixing waste vulcanized rubber with unvulcanized new rubber and a 
devulcanizing agent; masticating the resulting mixture in a heating 
atmosphere, thereby performing mastication of the unvulcanized new rubber 
and reclamation of the waste vulcanized rubber simultaneously to obtain 
unvulcanized reclaimed rubber; and subsequently incorporating the 
unvulcanized reclaimed rubber with a vulcanizing agent to obtain reclaimed 
rubber. The process will be described below in more detail. 
A mixture was prepared from 100 parts per hundred parts of rubber (phr) of 
unvulcanized EPDM (as unvulcanized new rubber), 30 phr of vulcanized EPDM 
(as waste vulcanized rubber), 10 phr of reclaiming oil, and 1 phr of 
diaryldisulfide (as a devulcanizing agent). 
The mixture was kneaded by milling with 0.1 mm opening at a temperature of 
100.degree. C. for 20 minutes for masticating and reclaiming. In this way 
unvulcanized reclaimed rubber in which the waste vulcanized rubber and the 
unvulcanized new rubber are uniformly mixed was obtained. 
The unvulcanized reclaimed rubber was incorporated with 2 phr of sulfur (as 
a vulcanizing agent), 1 phr of Nocceler-NS-P (as a vulcanization 
accelerator), 3 phr of zinc oxide, and 2 phr of stearic acid. The 
resulting rubber compound was kneaded for 10 minutes and then vulcanized 
to obtain reclaimed rubber of this example. 
The process in this example permits one to perform mastication of 
unvulcanized new rubber and reclamation of waste vulcanized rubber (or 
severing of sulfur crosslinks) simultaneously. The result is improved 
productivity and reduced production cost due to reduced manufacturing 
steps, labor, and time. 
EXAMPLE 2 
In this example, three samples (1 to 3) of reclaimed rubber were prepared 
according to the process of the present invention, and they were compared 
with three comparative samples (C1 to C3) of reclaimed rubber prepared 
according to the conventional process. 
Sample 1 was prepared by the process shown in Example 1. (EPDM was used as 
waste vulcanized rubber and unvulcanized new rubber.) Samples 2 and 3 were 
prepared from isoprene rubber and butyl rubber, respectively, as waste 
vulcanized rubber and unvulcanized new rubber. The formulation and process 
were the same as in Example 1, except that reclaiming oil was not added to 
samples 2 and 3. 
Comparative samples C1 to C3 were prepared according to the process shown 
in FIG. 2. First, a mixture was prepared from 100 phr of waste vulcanized 
rubber, 10 phr of reclaiming oil (not added to C2 and C3), and 1 phr of 
diaryldisulfide (as a devulcanizing agent). The resulting mixture was 
heated under steam in an autoclave at 200.degree. C. for 5 hours to obtain 
decomposed rubber. Meanwhile, unvulcanized new rubber was kneaded for 
mastication for 10 minutes in a mixing mill. 
Second, 100 phr of the unvulcanized new rubber (which had undergone 
mastication) was mixed and kneaded with 30 phr of the decomposed rubber 
for 10 minutes. Finally, 100 phr of the resulting unvulcanized new rubber 
was incorporated with 2 phr of sulfur (as a vulcanizing agent), 1 phr of 
Nocceler-NS-P (as a vulcanization accelerator), 3 phr of zinc oxide, and 2 
phr of stearic acid. The resulting mixture was kneaded for 10 minutes and 
then vulcanized to obtain a sample of reclaimed rubber by the conventional 
process. 
The aforementioned comparative samples C1 to C3 were prepared from EPDM, 
isoprene rubber, and butyl rubber, respectively, as waste vulcanized 
rubber and unvulcanized new rubber. 
Each sample of the reclaimed rubber was formed into sheet measuring 30 
cm.times.30 cm.times.5 mm, and dumbbell test specimens (conforming to JIS 
K6301) were cut out of the sheet. The specimens were tested for tensile 
strength according to JIS K6301. The results are shown in Table 1. 
TABLE 1 
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Tensile strength 
Breaking elongation 
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Sample 1 200 kg/cm.sup.2 
650% 
Comparative Sample C1 
190 kg/cm.sup.2 
630% 
Sample 2 180 kg/cm.sup.2 
500% 
Comparative Sample C2 
180 kg/cm.sup.2 
480% 
Sample 3 150 kg/cm.sup.2 
600% 
Comparative Sample C3 
140 kg/cm.sup.2 
590% 
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It is noted that the process of the present invention yields reclaimed 
rubber comparable to that obtained by the conventional process although it 
is simpler than the conventional process.