DE4225684A1 - Prepn. of stable general purpose rubber vulcanisate with ageing resistance - Google Patents

Abstract

Diene rubber vulcanisates are prepd. by vulcanising in the presence of (A) 1-4.5 parts crosslinking substance of general formula X(CH2)nX (I); (B) 0.05-0.3 parts sulphur; and (C) (i) 1-2.5 parts mercapto-accelerator or (2) 0.2-0.8 parts sulphenamide accelerator or (3) 0.3-2.5 parts mercapto-accelerator and 0.1-0.8 parts sulphenamide accelerator; to 100 parts general purpose rubber at 140-200 deg. C. X is -S-S-C(=S)N(Bz)2; Bz = benzyl; and n = 2-6. The rubber is pref. oil treated and pref. consists of natural rubber, cis-polyisoprene, cis-polybutadiene, emulsion SBR, soln. SBR, vinyl-polybutadiene, vinyl SBR, 3,4-polyisoprene, nitrile rubber or polyoctenamer or mixts.

Description

Diene rubbers are among the most commonly used all-purpose chews chuken. This is understood to mean polymers and copolymers from butadiene, Styrene and also isoprene. Natural rubber as well as synthetic Polyisoprene are to be regarded as diene rubbers. These are rubbers inexpensive, available in large quantities and have good general properties on. A disadvantage of this rubber group is a structure-related only limited to insufficient aerobic and anaerobic aging resistance and heat stability of their sulfur vulcanizates.

Anaerobic aging is the behavior of the vulcanizate air exclusion, including z. B. in the heating form, where longer reaction times leads to so-called reversion in many diene rubbers. Below one understands the waste of important material properties like tensile strength speed, elongation at break, modulus and dynamic properties when the mixture is heated longer than the optimum. The phenomenon of reversion leaves The easiest way is to use a so-called vulcanometer curve according to DIN Watch 53 529.

On the other hand, aerobic aging encompasses all natural processes Aging in the presence of atmospheric oxygen, even at elevated temperatures, such as B. Vehicle tires under operating conditions.

The described shortcomings in aerobic and anaerobic aging are in Natural rubber and polyisoprene particularly pronounced.

In order to at least partially compensate for this disadvantage, one has in addition to the Use of antioxidants a number of alternative vulcanization recipes ren applied. These include so-called sulfur donors such as tetrame thylthiuram disulfide (TMTD), dimorpholine disulfide (DTDM) and similar ver bindings or so-called EV systems that use the sulfur used use more by adding a large amount of accelerator at low higher sulfur dosage. With these vulcanization systems however, due to the shorter scorch time, the processing is safe  frequently impaired. Also the Vulka manufactured with EV systems nisates have various disadvantages, especially their increased ones Susceptibility to fatigue under dynamic continuous load. Furthermore exists when using TMTD and generally with low molecular weight dithio carbamates the risk of forming carcinogenic nitrosamines in considerable amount. But above all, the achievable improvements are Aging stability is not yet convincing, but mostly leaves still to be desired.

Another way to improve the thermal stability of Dienkau Tschuk Vulkanisaten was proposed in EP-OS 0 385 072 and 0 432 417 gen: Here, as the sole crosslinking agent, compounds with the Usual sulfur vulcanization accelerating residues of the general structure

• with R = ethyl and n = 2 (EP-OS 0 385 072), hereinafter referred to as BDTE, and
• with R = benzyl, n = 2 (EP-OS 0 432 417), hereinafter referred to as BDBzTE,
• - and with n = 6, hereinafter referred to as BDBzTH (EP-OS 0 432 417).

The examples show that e.g. B. with 4.5 parts BDTE or 7 parts BDBzTE per 100 parts of rubber vulcanizates of very good aging stability are preserved. BDBzTH with a longer network bridge is also used as Ver same example used (EP-OS 0 432 417, Table 1a, Examples 1a or Da). However, the disadvantages of the Systems:

• a) BDTE is susceptible to the formation of dangerous nitrosamines.
• b) With the crosslinking agents BDBzTE and BDBzTH, the vulcanization reaction proceeds much more slowly (t ₉₀ values of 29 min. and 34.3 min., respectively, so that uneconomically long vulcanization times of 40 min. and longer are required at 150 ° C.
• c) Due to the high molecular weight of BDBzTE and BDBzTH, in order to to achieve useful vulcanizate properties, at least 7 parts per 100 parts of rubber are used, which is an economical Bela represents the process according to EP-OS 0 385 072 and 0 432 472.

Furthermore, when using such large quantities of chemicals, there is a risk that that the resulting products in the course of vulcanization in Ela are no longer sufficiently soluble, but to undesirable ones Efflorescence.

Both patents explicitly refer to the negative Effect of sulfur additives in general on the stability of the Vulcanized against reversion.

The aforementioned European patent applications therefore do not provide anything Clue on how to make diene rubber vulcanizates of excellent aging Stability with economically and technically justifiable amounts of ver can be obtained with acceptable vulcanization times.

The two DE-PS 22 65 382 and 22 56 511 deal very generally with the compounds of the general formula

A-S-S-R-S-S-A ′ (b)

where the residues A and A 'a large number of accelerator residues, u. a. also N-substituted thiocarbamoyl residues, and R any one represent divalent organic radical. This results in an extreme wide range of substances, some of which are included in the examples be set. There is also an analog substance according to the general formula (a) with R = methyl and n = 2, hereinafter referred to as BDMTE. This is also used together with sulfur in DE-PS 22 65 382 (Column 31/32, Table VII), however, the experiments show that ge low amounts of sulfur (0.3 to 1%) to undesirable reversion  lead. It is expressly conceded that approaches without sulfur have a higher resistance to reversion (column 32, Lines 1 to 11).

In DE-PS 22 56 511 BDMTE and BDTE are used as crosslinking substances set. Here is the unfavorable crosslink density of the vulcanizates pointed out as a disadvantage (column 21, lines 12 to 27) by additions of 0.5-1.5 parts of sulfur can be compensated, with which one however, the rubber mixtures are again prone to reversion negotiates.

In general, the latest state of the art must be complained about apply that the absence of reversion phenomena in the rheometer test fung is used as the only criterion for thermal stability. However, as we said at the beginning, the rheometer test tells everyone if anything about anaerobic aging - and this is also incomplete constantly, since the rheometer curve is only a statement about the torque of the vulcanizate. No information is given in DE-PS 22 65 382 and 22 56 511 about aerobic aging resistance, which made a lot more relevant clues to the resistance of the vulcanizate in practice supplies. These statements are preferably obtained through La storage of vulcanizate samples in a recirculation cabinet and at one the temperature load of 100 ° C over a period of 3, 7 or 14 Days (according to DIN 53 508). After this time, that for natural chewing Tschuk, cis-polyisoprene, cis-polybutadiene and their blends one extremely relevant load, all relevant Elasto properties such as tensile strength, elongation at break, stress value, hardness, Rebound resilience, possibly also the dynamic properties and the abrasion resistance measured. So you get a comprehensive one Insight into the thermal aging resistance of the elastomer.

It should therefore be noted that the known prior art does not provides usable process to get to ther under reasonable conditions Mo- and aging-stable vulcanizates of diene rubbers under practi conditions of use.

The object of the present invention is therefore a thermal resistant d. H. both a reversion-resistant and insbe special aging-stable crosslinking system for natural rubber and syn to find synthetic diene rubbers, which have the disadvantages of the known Sy steme avoids. In particular, this means:

• 1. Vulcanization speed as with conventional sulfur / misting niger systems.
• 2. Crosslinking yield, measured by vulcanizate properties such as tension strength, stress value and compression set, as with konventio nelle sulfur / accelerator systems.
• 3. Reversion resistance, d. 76 H. Anaerobic aging resistance.
• 4. Excellent aerobic aging resistance of the vulcanizates.
• 5. Reduction in the amount of crosslinking agent used for technical reasons (Risk of blooming) as well as for economic reasons.

A solution to the problem while fulfilling all requirements has now been found in Surprisingly achieved by crosslinking agents with the formula

With
Bz = benzyl,
n = 2 (BDBzTE) or
n = 6 (BDBzTH),

while using very small, almost catalytic amounts Sulfur as well as a small amount of mercapto- or sulfenamide accelerator or mixtures thereof as a vulcanization system puts. The accelerator is used for fine-tuning the control the vulcanization kinetics.

Under the now found conditions one can with the vulcanization of diene rubbers such as B. Natural rubber (NR), cis-polyisoprene (cis- IR), cis-polybutadiene (cis-BR), styrene-butadiene rubber (SBR) or de  Ren cut the amount of crosslinkers BDBzTE or BDBzTH used to 1 to 4.5 parts, preferably 2 to 4 parts, per 100 parts of rubber redu grace if you simultaneously 0.05 to 0.3 parts, preferably 0.1 to 0.2 parts, sulfur and 1 to 2.5 parts, preferably 1.3 to 1.8 parts, Mercapto accelerator or 0.2 to 0.8 parts, preferably 0.3 to 0.5 Parts, Sulfenamid accelerator used without the mechanical Properties after aging have the least drawbacks compared to Vulkanisa ten from slow crosslinking with 6 to 7 parts of these crosslinkers al have to be made.

However, mixtures with 0.3 to 2.5 parts are preferred, especially before prefers 0.6 to 1.5 parts, mercapto and 0.1 to 0.8 parts, especially before add 0.2 to 0.6 parts, sulfenamide accelerator, in addition to the above amounts of crosslinker and sulfur.

Zinc-2-mercaptobenzothiazole (ZMBT), Dibenzothiazyl disulfide (MBTS) and N-tert-butyl-2-benzothiazylsulfenamide (TBBS) use other mercapto or sulfenamide accelerators however, can be used within the scope of the invention.

The values found show that compared to the exclusive use of the crosslinkers even significant improvements can be achieved. This result was due to the known unfavorable influence of Sulfur on the reversion and aging behavior of elastomers not to be expected.

Natural rubber, cis-polyisoprene, cis-poly are suitable as diene rubbers butadiene as well as for conventional SBR, which is based on both solution and can also be produced by emulsion processes. In addition come in glei vinyl-polybutadiene and vinyl-SBR. The application too of nitrile rubber and polyoctenamers (TOR) is possible. Same good Results are also blended with each other of these polymers receive. In addition, the rubbers can be oil-free or oil-stretched for Come into play.  

The rubber types listed are usually used in compounds tongue vulcanized, d. H. in mixtures containing fillers such as carbon black or Si silicon dioxide, oils, rubber auxiliaries, zinc oxide, stearic acid, antioxidant dantien and, if necessary, antiozonants in the usual amounts hold. The use of the above-mentioned antioxidants (ASM) is for us However, the systems according to the invention are not required. That invented The system according to the invention also supplies vul Kanisate as ASM in a conventional system. The manufacture of the chew Chuk mixes are carried out in the usual way in internal mixers (kneaders), if necessary also on rolling mills.

The preparation of the crosslinking substances is described in EP-OS 0 385 072 and 0 432 417 described in detail, so that ver can be waived.

The vulcanization of the finished mixtures can take place at the usual tempera doors, d. H. at 140-200 ° C. The advantage of the invention System is that you can vulcanize at higher temperatures without reversion symptoms with the known negative effects noticeable effects on the vulcanizate properties. You get gum Articles of excellent aging resistance that are suitable for ver different areas of application are of great importance, such as. B. for driving grab, technical elastomer items such as engine mounts and many others Applications at elevated temperatures.

In addition to the above especially preferred accelerators also come prinzi triazine-based accelerators in question.

The advantages of the crosslinking systems according to the invention are shown in the following examples can be seen. The ones listed in the tables Properties are determined according to the test regulations of the relevant DIN standard men determined.

The tensile strength and elongation at break were determined in accordance with DIN 53 504. The tension value, also called module, was at 100 or 300% elongation determined according to DIN 53 504.  

The structural strength was determined according to Pohle (see S. Boström, Kautschuk-Hand book, volume 5, page 123).

The permanent elongation (tension set) was determined according to DIN 53 518 Right.

The hardness (Shore A) was determined in accordance with DIN 53 505.

The rebound resilience (elasticity) was determined in accordance with DIN 53 512 The abrasion was determined in accordance with DIN 53 516.

The compression set was determined according to DIN 53 517 Right.

The vulcanization was carried out according to DIN 53 529.

The ball wear was carried out according to the method from S. Boström, rubber Manual, 5th volume, Berliner Union, Stuttgart, 1962, pp. 149-150.

Examples 1-23 according to the invention listed in Tables 2 and 4 prove the progress in the aging stability of diene rubber Vulcanizates compared to the prior art and also not Compositions according to the invention of comparative examples I to XII Tables 3 and 4.

Examples 1 and 2 represent a preferred vulcanization recipe with the basic mixtures A (natural rubber) and B (cis-polyisoprene). The values after 14 days of aging show that the residual elongation values (elongation at break of the vulcanizates after 14 days of aging compared to the value of the ungain aged vulcanizates were still 73 and 78%, in contrast, comparative example V (7 parts BDBzTE according to EP-0 432 417) had only 62% residual elongation. Above all, however, the slow vulcanization (t ₉₀ value 26.9 min) of the comparative example in a completely unacceptable value for the compression set at both 70 and 100 ° C.

Even an increase in the vulcanization temperature to 180 ° C did not lead to any improvement: the t ₉₀ value was still 9.6 minutes, so that the 10 minutes vulcanization time customary at this temperature would also not be sufficient. The economic disadvantages and the risk of efflorescence at such high doses of crosslinking agent in comparative example V have already been dealt with.

In Comparative Example IV, a natural rubber mixture with a conventional vulcanized thiuram system. After 14 days of aging in the forced air cabinet at 100 ° C counter the disadvantages of thiuram vulcanization over the system according to the invention clearly: Not only is the Residual elongation dropped to 57% of the initial value; but also that Structural strength according to Pohle (tear resistance) is compared to that examples according to the invention dropped sharply. Damage to the vulka nisates also comes from the significantly falling resilience 75 ° C.

An EV system has similar disadvantages (comparative example II). Also here the value of the Wei has already decreased sharply after 7 days tensile strength and the sharp drop in rebound after 14 days elasticity at 75 ° C very clearly a strong damage to the Vulkanisa tes and therefore insufficient aging stability compared to the systems of the invention.

A conventional sulfur accelerator system with 0.6 parts sulfenamide accelerator and 2.5 parts sulfur in a natural rubber mixture was only included in the table as a comparison for the sake of completeness game I added: The values after aging drop so sharply that not at all for practical use under thermal stress is to be thought.

But also the use of BDBzTE with a sulfenamide accelerator alone does not lead to the goal, as Comparative Example III shows. First one times the vulcanization is even slower than with 7 parts BDBzTE alone, which manifests itself in an unacceptable compression set value. After 14 Days of aging, however, is reflected in the declining values for hardness and rebound resilience as well as the increasing compression set the values after 7 days of aging that a clear degradation of the Vulkanisates is progressing.

Comparative Example VII shows the effect of too high a dosage Accelerator, using the example of a basic mixture based on a cis Polyisoprens with the accelerator zinc-2-mercaptobenzothiazole (ZMBT)  will be shown. After aging for 14 days at 100 ° C in a hot air cabinet against the sharply dropped values for tensile strength and tear propagation resistance speed (structure according to Pohle) that the vulcanizate of this comparative example is significantly less resistant to aging than the example according to the invention 3, which has a significantly better retention of physical values such as train has strength, elongation at break and compression set.

In addition to the preferred embodiments of the invention according to the Examples 1 and 2 explained also lead to modified formulations men of the specification according to the invention to results that the state of the art nik are clearly superior.

In Example 3, the vulcanization of cis-polyisoprene with 3 parts of BDBzTE as well as with 0.3 parts sulfur and 0.5 parts zinc-2-mercaptobe zothiazole (ZMBT). The improved aging stability compared to comparative example VII, reference was made above sen.

In Examples 4 and 5, BDBzTH is used as the crosslinker. The bei games show that with this crosslinker the same excellent old woman Achieving stability is like with the networker BDBzTE. With expansion When the hot air ages from 7 to 14 days, the tear resistance drops not significantly (structure according to Pohle), while that in the same Period of increasing resilience at 75 ° C no damage supply of the elastomer network. This result is at cis- To achieve polyisoprene neither with sulfur donors nor with EV systems.

Examples 6 to 8 show the use of BDBzTE with less Amounts of sulfur and 0.4 to 0.6 parts of the TBBS sulfenamide accelerator. The vulcanization is slower, but the pressure is higher deformation values both before and after aging of the vulcanizates significantly better than when using the crosslinker alone, or only with Additional accelerator (Comparative Examples III and V). The values of the tear elongation after 14 days of aging at 100 ° C in relation to the initial values before aging, are particularly in Examples 6 and 7 80 or 74% exceptionally high. These examples show that an increase  The dosage of BDBzTE does not necessarily mean the best in all criteria results in our values.

Examples 9 and 10 show the vulcanization of styrene-butadiene chew tschuk (SBR) by 3 to 4 parts of the crosslinker BDBzTE and addition of 0.1 up to 0.2 parts of sulfur and 0.4 part of the TBBS additional accelerator. At Both vulcanizates are excellent abrasion and compression sets achieved after 14 days of aging, in the formulation according to Example 10, also less hardening and higher residual elongation.

These two examples show that SBR is known to be hardening aerobic aging tends to lower doses of sweat and a quantity of crosslinking agent BDBzTE or BDBzTH deliver cheaper results if you have the lowest possible Aiming for module increase.

Comparative Examples VIII to X illustrate the effects ner compared to Examples 9 and 10 (0.2 and 0.1 parts of sulfur) 0.3 to 0.6 parts of increased sulfur dosage in an SBR mixture, both with the additional accelerator TBBS (X: 0.2 part) and without this (VIII and IX). After 14 days at 100 ° C, the residual dehyde falls Mixtures without additional accelerators to only 18 to 27% of the Baseline. The comparative example VIII also contains the residual values for Tensile strength and elongation at break after 14 days of aging in a circulating air cabinet at 100 ° C at a significantly lower level than the invention according to Examples 9 and 10, so that the effectiveness of the invention Procedure clearly emerges.

In comparative example VI, the effect of an excessively high sulfur dose is demonstration of a polyisoprene mixture (the amount of additive accelerator TBBS was reduced from 0.4 to 0.2 parts to ver to achieve comparable stress values for the vulcanizates). If you compare with Example 2 according to the invention with 0.2 parts of sulfur, so falls that after 7 days of aging at 100 ° C, the elongation to  <300% (64% of the initial value compared to 77% in Example 2) is. The tear resistance is also higher at higher sulfur doses lower in front.

While in Example 2 the property level after 14 days at 100 ° C compared to 7 days has hardly dropped further, can be seen in Ver same example VI a significant further decrease in tensile strength, Elongation and elasticity, which indicates damage to the network. The system according to the invention thus has the same effect on cis Polyisoprene as well on SBR.

In Example 11 the vulcanization of a blend from natural chewing tschuk, cis-polybutadiene and SBR, as used in a tread compound Winter tires are used with the crosslinking system according to the invention stem shown. Here too there is excellent retention of the printing process residual formation and the resilience to observe.

In a further preferred embodiment, di benzothiazyl disulfide (MBTS) is used very advantageously either in combination with TBBS or else alone. Here it is possible to further shorten the vulcanization times. A polyisoprene mixture according to Examples 14 and 15 has a t ₉₀ value of only 14-15 minutes. At the same time, the dosage of BDBzTH can be reduced from 3.3 to 2.5 parts without the tension value of the vulcanizate dropping. If one looks at the aging properties, it becomes clear that, despite the reduced amount of BDBzTH, similarly excellent values of structural strength, hardness and elasticity are achieved as in Examples 2 and 4.

However, the amount of this additional accelerator should not be allowed alone keep it low. With only 0.8 parts MBTS (comparative example XI) obtained insufficient vulcanizate and unfavorable aging values after 14 Days at 100 ° C, which is particularly low in structural strength and Rebound resilience as well as higher heating in the ball attrition test never beat.  

Examples 17-19 demonstrate the advantages of the invention Procedure for using MBTS as an additional accelerator from SBR. After aging at 100 ° C for 14 days, the tensile strength of the Vulkanisa lies tes still at 20 MPa and the elongation at break of the same samples still at 380-500%, while in the comparative examples VIII-X only more 6-11, 8 MPa tensile strength and 69-196% elongation at break can be achieved.

The method according to the invention is also particularly suitable for Vulka nization of oil-extended rubbers. This is illustrated by Examples 20 to 23 occupied. As can be seen from Table 4, with a Combination of TBBS and MBTS as additional accelerator (example 20) or good vulcanization conditions even with MBTS alone (Examples 21 to 22) reached. The table are also the excellent residual values for Tensile strength, elongation at break, structural strength (according to Pohle) and Compres sion set, as well as for the dynamic values with only moderate at the same time increasing hardness after 14 days of aging at 100 ° C.

In contrast, in the comparative examples VIII-X only 6-11.8 MPa Tensile strength and 69-196% elongation at break achieved.

Examples 1 to 23 listed demonstrate in comparison to the Comparative Examples I to XI not according to the invention the advantages of Method according to the invention:

Use of only 2 to 4 parts of the crosslinking substances according to the invention BDBzTE or BDBzTH per 100 parts of rubber with simultaneous use tion of 0.05 to 0.3, but preferably 0.1 to 0.2, parts of sulfur and 0.3 to 2.5, but especially 0.5 to 1.8 parts of mercapto acceleration such as zinc 2-mercaptobenzothiazole (ZMBT), 2-mercaptobenzothiazole (MBT) Dibenzothiazyl disulfide (MBTS), or 0.1 to 0.8, preferably 0.3 to 0.5, parts of sulfenamide accelerators such as N-tert-butyl-2-benzothiazylsulfe namid (TBBS), but preferably TBBS or ZMBT among them; of the far ren especially mixtures of 0.7 to 1.8 parts Merkapto and 0.2 to 0.6 parts of sulfenamide accelerator, with MBTS and TBBS preferred here Find use.  

When using economical quantities and avoiding Efflorescence in the vulcanizate after a relatively short vulcanization period ten elastomers of outstanding aging properties not previously achieved durability and reversion resistance.  

Table 4

Basic mix E

Basic mix F

Basic mix G

Basic mix H

Claims (8)

1. A process for the preparation of diene rubber vulcanizates with very high aging stability and resistance to reversion, characterized in that the vulcanization in the presence of

• a) 1 to 4.5 parts of crosslinking substances with the formula where n is 2 or 6 and Bz is benzyl, and
• b) 0.05 to 0.3 parts sulfur, and c1) 1 to 2.5 parts Merkaptob accelerator
  or
  c2) 0.2 to 0.8 parts of sulfenamide accelerator
  or
  c3) 0.3 to 2.5 parts of mercapto accelerator and 0.1 to 0.8 part of sulfenamide accelerator,

based on 100 parts of diene rubber, at temperatures from 140 to 200 ° C is carried out. 2. The method according to claim 1, characterized in that per 100 parts of diene rubber

• a) 2 to 4 parts of crosslinking agent, and
• b) 0.1 to 0.2 parts sulfur, and c1) 1.3 to 1.8 parts mercapto accelerator
  or
  c2) 0.3 to 0.5 parts of sulfenamide accelerator
  or
  c3) 0.6 to 1.5 parts mercapto accelerator and
  0.2 to 0.6 parts of sulfenamide accelerator

be used. 3. The method according to claims 1 to 2, characterized, that the diene rubber made of natural rubber, cis-polyisoprene, cis-polybu tadiene, emulsion SBR, solution SBR, vinyl polybutadiene, vinyl SBR, 3,4-polyisoprene, nitrile rubber or polyoctenamer and from a mixture against the same. 4. The method according to claim 3, characterized, that the diene rubber is oil-extended. 5. Use of

• a) 1 to 4.5 parts of crosslinking substances with the formula where n is 2 or 6 and Bz is benzyl, and
• b) 0.05 to 0.3 parts sulfur, and c1) 1 to 2.5 parts Merkaptob accelerator
  or
  c2) 0.2 to 0.8 parts of sulfenamide accelerator
  or
  c3) 0.3 to 2.5 parts of Mercury accelerator and
  0.1 to 0.8 parts of sulfenamide accelerator,

based on 100 parts of diene rubber for vulcanization of diene rubber chuken.