Mechanisms of antioxidant action: Auto-synergistic antioxidants based on thiol rubber adducts

The mechanism of by-product formation in the reaction of a thiol antioxidant, 3,5-di-tertbutyl-4-hydroxybenzyl mercaptan (BHBM), with rubber latices is shown to involve two processes, viz dimerization of the intermediate thiyl radical to give disulphide (BHBDS) and the reaction of the intermediate quinonemethide with water to give the aldehyde and with BHBM to give the monosulphide (BHBMS). The two sulphides themselves react with rubber in a slower reaction.The bound antioxidants obtained with natural rubber and nitrile-butadiene rubber are very much more effective than the present generation of commercial heat ageing antioxidants under conditions where the antioxidants can be lost from the rubbers by volatilization or solvent or detergent leaching. This is partly due to their resistance to removal from the polymers but there is also evidence that the auto-synergistic antioxidant bound to the rubber is more effective than single function antioxidants, due to the presence of the sulphide linkage.     

Mechanisms of antioxidant action: The synthesis of bound antioxidant concentrates in nitrile rubber

A phenolic thiol antioxidant and two thiol amides have been reacted with nitrile-butadiene rubber both in the latex and during melt processing to give masterbatch concentrates. BHBM (I) reacts to give higher bound antioxidant levels at 20% masterbatch concentration than the amides in the latex but MADA (II) gives the highest level of binding by the mechanochemical procedure (80% masterbatch concentration). The primary initiation process is mechanochemical but secondary peroxide initiated adduct formation occurs during the later stages.     

Vibration damping materials based on interpenetrating polymer networks of carboxylated nitrile rubber and poly(methyl methacrylate)

Interpenetrating polymer networks (IPNs) based on carboxylated nitrile rubber (XNBR) and poly(methyl methacrylate)s were synthesized. Crosslinked XNBR was swollen in methyl methacrylate containing benzoyl peroxide as initiator and tetraethylene glycol dimethacrylate as crosslinking agent. The compositions of the IPNs were varied by changing the swelling time of the rubber in the methacrylate monomer. The dynamic mechanical properties of the IPNs were studied. The dynamic mechanical properties in the range 1–10⁵ Hz were obtained by the time‐temperature superposition of the data under multifrequency mode, which indicated high tan δ with good storage modulus in the entire frequency range. This indicates the suitability of these IPNs as vibration and acoustic dampers. Copyright © 2002 John Wiley & Sons, Ltd.     

Mechanisms of antioxidant action: Stabilisation of polypropylene during processing and on exposure to light

The most important initiators for the photo-oxidation of processed polypropylene are thermally formed hydroperoxides. Peroxide decomposing antioxidants (ZnDEC and NiDEC) effectively control hydroperoxide formation, and hence oxidation, during processing. A chain breaking antioxidant (1076) is effective and a typical uv absorber (HOBP) completely ineffective. The uv stabilising activity of these compounds is NiDEC > HOBP > 1076 > ZnDEC; the first two show a strong concentration dependence whereas the last two do not. The two dithiocarbamates are relatively more effective at high temperatures and they synergise effectively with the uv absorber. The synergism obtained with ZnDEC is higher than that with NiDEC.     

Mechanisms of antioxidant action: Mechanochemical addition of thiol antioxidants to ABS

It is shown that thiol-containing antioxidants and stabilisers can be chemically reacted with ABS in high yields and in substantial concentrations under conditions of high temperature and high shear during processing. The resulting antioxidant concentrates are shown to be very effective thermal and photo-oxidative stabilisers for ABS when used as masterbatch additives for unstabilised ABS. Synergistic combinations of a bound antioxidant (BHBM (I(a))) and a bound uv stabiliser (EBHPT (II)) confer a high level of uv stability on ABS when incorporated into ABS. All the antioxidants show a high resistance to removal from the polymer by solvent extraction. The effect of the mechano-chemical process on the efficiency of adduct formation and on the effectiveness of the stabilisers so formed is discussed.     

Mechanisms of antioxidant action: Formation of antioxidant adducts with rubbers through sulphur by a mechanochemical procedure

It is shown that antioxidants containing a thiol group can be mechanochemically reacted with both natural rubber (NR) and styrene-butadiene-rubber (SBR). Oxygen inhibits the reaction and the optimum temperature is in the region of 70°C. Increase in the antioxidant concentration in the rubber leads to increasing levels of adduct formation. Evidence is presented to show that although a substantial part of the adduct is formed during mechanochemical treatment, about 20% of the total binding occurs during the vulcanisation reaction.     

Mechanisms of antioxidant action: Chemical binding of a thiol antioxidant into polypropylene by means of uv irradiation

3,5-Di-t-butyl-4-hydroxybenzyl mercaptan (BHBM) became chemically combined to polypropylene to the extent of 68% of the antioxidant present when films containing BHBM were exposed to uv irradiation. In addition to the radical coupling reaction of the thiyl radical derived from BHBM with the polymer, a second technologically less useful process also occurs, leading to the formation of a derived stilbenequinone. The latter absorbs strongly both in the visible and in the near uv and consequently slows down the subsequent loss of the bound antioxidant from the polymer.     

Mechanisms of antioxidant action: rubber bound antioxidants based on nitrones—I. Non-sulphur vulcanizates

The antioxidant activities of a number of N-phenyl nitrones containing phenolic functions were compared with N-methyl nitrones and conventional antioxidants in peroxide and TMTD sulphur-less vulcanizates. The N-phenyl nitrones were found to be effectively bound during vulcanization whereas the N-methyl nitrones were not. Unhindered phenols reacted to a higher degree and were relatively more effective than the typical hindered phenol antioxidant structures. A conventional bisphenol was effective in the sulphurless vulcanizate after solvent extraction but this is considered to be due to the formation of an insoluble zinc salt. Synergism was observed in the TMTD sulphurless vulcanizate both before and after extraction.     

Mechanisms of antioxidant action: Effects of rubberbound antioxidants in rubber modified polypropylene

The effect of thermal and photo-oxidation on rubber (ethylene-propylenediene terpolymer) modified polypropylene is examined and it is concluded that, although the rate of photo-oxidation increases with the amount of modifier, the rate of loss of impact resistance decreases. Antioxidants and uv stabilisers improve the oxidation resistance of the impact modified polypropylene but conventional stabilisers are removed by solvent leaching. Antioxidants bound to the EPDM through sulphur show a similar stabilising performance to commercial stabilisers before extraction but are much more effective after extraction.     

Mechanisms of antioxidant action: Activity of sulphur-linked bound antioxidants in natural rubber

A number of phenolic antioxidants containing a sulphide linkage have been found to react with natural rubber latex in the presence of a redox radical generator. They are highly effective under conditions of solvent extraction but the extent of adduct formation is only about 50%.     

Mechanisms of antioxidant action: Sulphur compounds as inhibitors of mechanochemical degradation of rubber

It is shown that the stability of sulphur vulcanised natural rubber to mechanodegradation (fatigue) is associated with the formation of labile oxidation products. The addition of sulphides and their oxidation products to a non-sulphur vulcanisate has a similar effect. Stress relaxation shows that the initial rate of chain scission under stress is directly related to the fatigue resistance of the vulcanisate. However, the oxidation process also produces peroxidolytic antioxidants (PD-C). It is concluded that the fatigue resistance of sulphur vulcanisates is due to a combination of facile restructurisation and an oxidation chain-breaking mechanism.     

Mechanisms of antioxidant action: identification of the antioxidant principle produced in an “efficient vulcanization” of rubber

The soluble zinc mercaptobenzothiazolate (ZMBT) and the derived insoluble basic zinc salt are both present in an efficient CBS vulcanization (EV). These substances appear to be primarily responsible for the very good thermal-oxidative stability of this vulcanizate and the basic salt is as effective as the soluble complex as an antioxidant in spite of its incompatability. MBT itself and rubber-bound MBT residues are very much less effective as antioxidants. Maximum oxidative stability is developed at optimum cure time.     

Mechanisms of antioxidant action: polymer grafted antioxidants

It is shown that the antioxidant 3,5-di-tert-butyl-4-hydroxybenzyl acrylate, may be grafted to the surface of polymer artefacts. The antioxidant activity of the resulting “efficiently dispersed” antioxidant is very much higher than that of low molecular weight antioxidants containing the same functional group. Even after extraction with an effective solvent for the antioxidant-monomer and derived polymer, the oxidative stability of the polymer films is very much greater than that of polymer containing the antioxidant-monomer or antioxidant-polymer incorporated by a conventional melt technique without extraction. Optimum activity is believed to be associated with the formation of frequent short chain grafts in the surface of the polymer and is achieved by the use of a polymerization photo-activator such as benzophenone which also terminates kinetic chains.Surface grafted antioxidants are also found to be effective u.v. stabilizers for polypropylene in contrast to the normal ineffectiveness of chain terminating antioxidants incorporated into the polymer by conventional techniques.     

Network characteristics of hydrogenated nitrile butadiene rubber networks obtained by radiation crosslinking by electron beam

The effects of high-energy radiation on hydrogenated nitrile butadiene rubber (HNBR) copolymer structure and properties were studied. Characterization by FTIR spectroscopy, swelling and mechanical measurements of irradiated and un-irradiated sample permit us to correlate the change in structure with properties. The modifications obtained are dependent on the radiation dose of the incident electron beam. FTIR spectroscopy in absorption mode shows that irradiation of HNBR first induces trans-vinylene bond formation and secondly small amounts of carbonyl (CO) groups. Moreover, more significant changes were observed with swelling method and mechanical behaviour showing the effect of crosslinking on the elastomer.     

Viscoelastic properties of nanostructured natural rubber/polystyrene interpenetrating polymer networks

The effects of the blend ratio and initiating system on the viscoelastic properties of nanostructured natural rubber/polystyrene‐based interpenetrating polymer networks (IPNs) were investigated in the temperature range of ?80 to 150 °C. The studies were carried out at different frequencies (100, 50, 10, 1, and 0.1 Hz), and their effects on the damping and storage and loss moduli were analyzed. In all cases, tan δ and the storage and loss moduli showed two distinct transitions corresponding to natural rubber and polystyrene phases, which indicated that the system was not miscible on the molecular level. However, a slight inward shift was observed in the IPNs, with respect to the glass‐transition temperatures (T_g's) of the virgin polymers, showing a certain degree of miscibility or intermixing between the two phases. When the frequency increased from 0.1 to 100 Hz, the T_g values showed a positive shift in all cases. In a comparison of the three initiating systems (dicumyl peroxide, benzoyl peroxide, and azobisisobutyronitrile), the dicumyl peroxide system showed the highest modulus. The morphology of the IPNs was analyzed with transmission electron microscopy. The micrographs indicated that the system was nanostructured. An attempt was made to relate the viscoelastic behavior to the morphology of the IPNs. Various models, such as the series, parallel, Halpin–Tsai, Kerner, Coran, Takayanagi, and Davies models, were used to model the viscoelastic data. The area under the linear loss modulus curve was larger than that obtained by group contribution analysis; this showed that the damping was influenced by the phase morphology, dual‐phase continuity, and crosslinking of the phases. Finally, the homogeneity of the system was further evaluated with Cole–Cole analysis. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1680–1696, 2003     

Mechanisms of antioxidant action: The reaction of hindered phenols with rubber in the presence of free radicals

Simple hindered phenols, containing ortho or para methyl groups, react with natural rubber latex in the presence of oxidizing free radicals (alkoxy or acyloxy), giving up to 20% yields of bound antioxidant. This reaction is most efficient when the concentration of the phenolic antioxidant in the rubber is less than 1%. As the concentration is increased, side reactions involving both the phenol (stilbenequinone formation) and the rubber (cross-linking) supervenes. The efficiency of the bound antioxidants is much higher than conventional antioxidants under aggressive conditions of air oven ageing and solvent or detergent extraction. This effect is due primarily to the non-removal of the bound antioxidants under these conditions whereas the conventional antioxidants are removed completely.     

Mechanisms of antioxidant action: Auto-synergistic behaviour of nitrones

The antioxidant activities of a number of nitrones have been studied in model systems initiated by both alkoxy radical generators (hydroperoxides) and an alkylperoxy radical generator (azo-bis-isobutyronitrile in the presence of oxygen). Simple nitrones are ineffective in the presence of the latter but are highly effective in the presence of the former. An auto-synergistic mechanism has been shown to be involved due to the ability of nitrones to destroy hydroperoxides in a stoichiometric reaction and at the same time to scavenge alkoxy radicals formed by homolysis of hydroperoxides. Nitrones containing additional conventional phenolic antioxidant functions are effective in an alkylperoxy radical initiated system but do not show the usual structure-activity relationships.     

Mechanisms of antioxidant action: Radical addition reactions of the dialkyl dithiophosphoric acids and their derivatives to natural rubber

It is shown that the dialkyldithiophosphoric acids and their ammonium salts can be readily reacted with natural rubber (NR) latex to give adducts containing up to 50% of the compound added. The free acids tend to coagulate the latex at relatively low concentrations but the use of the neutral ammonium salt permits relatively high loadings of adduct to be obtained in NR.The same additives and a related disulphide were also reacted mechanochemically with NR at temperatures between 110°C and 140°C but the adduct yields were somewhat lower than in the latex procedure.     

Mechanisms of antioxidant action: Time-controlled stabilisation of polypropylene by transition metal dithiocarbamates

Combinations of iron and nickel dithiocarbamates approach the ‘ideal’ delayed action photo-activators for polypropylene. They give an induction period to photo-oxidation which can be controlled by the amount of the nickel complex and a rapid rate of photo-oxidation at the end of the induction period which is primarily determined by the iron concentration. Combinations of other photo-activators and uv stabilisers show a less ideal behaviour and do not permit such accurate time control.