Vulcanisates, additives Antioxidants

Applications The broad industrial analytical applicability of microwave heating was mentioned before (see Section 3.4.4.2). The chemical industry requires extractions of additives (antioxidants, colorants, and slip agents) from plastic resins or vulcanised products. So far there have been relatively few publications on microwave-assisted solvent extraction from polymers (Table 3.5). As may be seen from Tables 3.27 and 3.28, most MAE work has concerned polyolefins. 

Lattimer and co-workers [46] have applied MS to the determination of organic additives (antioxidants and antiozonants) in rubber vulcanisates. Direct thermal desorption was used with three different ionisation methods (El, Cl, FI). The vulcanisates were also examined by direct FAB-MS as a means for surface desorption/ionisation. 

Rubber vulcanisation inhibitor, lubricant additive, antioxidant, fungicide, insecticide, stabiliser. Yellow plates. Mp 66.5°. 

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

As with c -polyisoprene, the gutta molecule may be hydrogenated, hydro-chlorinated and vulcanised with sulphur. Ozone will cause rapid degradation. It is also seriously affected by both air (oxygen) and light and is therefore stored under water. Antioxidants such as those used in natural rubber retard oxidative deterioration. If the material is subjected to heat and mechanical working when dry, there is additional deterioration so that it is important to maintain a minimum moisture content of 1%. (It is not usual to vulcanise the polymer.)... 

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

At Goodyear laser-desorption MS has been used for direct analysis of rubber additives (e.g. antioxidants, antiozonants, vulcanising agents, processing oils, silica fillers, etc.), in situ at the surface of an elastomeric vulcanisate [74,75]. 

Applications Shake-flask extraction nowadays finds only limited application in polymer/additive analysis. Carlson et al. [108] used this technique to extract antioxidants from rubber vulcanisates for identification purposes (NMR, IR, MS). Wrist-action shaking at room temperature was also used as the sample preparation step for the UV and IR determination of Ionol CP, Santonox R and oleamide extracted from pelletised polyethylene using different solvents [78]. BHT could be extracted in 98 % yield from powdered PP by shaking at room temperature for 30 min with carbon disulfide. 

Applications Conventional TLC was the most successful separation technique in the 1960s and early 1970s for identification of components in plastics. Amos [409] has published a comprehensive review on the use of TLC for various additive types (antioxidants, stabilisers, plasticisers, curing agents, antistatic agents, peroxides) in polymers and rubber vulcanisates (1973 status). More recently, Freitag [429] has reviewed TLC applications in additive analysis. TLC has been extensively applied to the determination of additives in polymer extracts [444,445]. 

FD-MS by itself provides only limited chemical information. Lattimer et al. [92] have also compared the analysis of extracted rubber vulcanisates by means of FD-MS and FAB-MS, using the aforementioned EI/FD/FT/FAB ion source. The systems investigated were neoprene/DOPPD, EPDM/(DOP, PBNA, paraffin wax), neoprene-SBR blend/(DOP, DOPPD, TDBHI). Certain compounds were observed by FD but not by FAB (wax, oil, isocyanurate antioxidant TDBHI). In FAB conditions some polymer additives suppress... 

The standard approach to reducing scorch (e.g. discoloration resulting from oxidation) involves addition of one or more antioxidants. Scorch retardants prevent premature decomposition of peroxides and cross-linking of polymers (pre-vulcanisation). 

The solubility of wax in vulcanised rubbers is low (of the order of 0.5% for NR) but enough wax has to be added to a rubber compound to ensure that once the compound has been vulcanised and the rubber cools, the rate of migrational movement of the wax from the rubber mass to the surface of the rubber is rapid. Dependant upon the application, the addition level of wax can be up to about 10 phr. Migration of the wax to the rubber surface will also carry other ingredients such as antioxidants, antiozonants and other materials (e.g., vulcanisation residuals), to enhance the surface protection. 

Vulcanised rubbers show viscoelasticity and the departure from perfect elasticity are evaluated by measurement of resilience, creep and stress relaxation. Compounding which contributes to a more tightly knit crosslinking system occupying the maximum possible volume proportion of the vulcanisate will enhance the elastic properties as displayed by resilience. Appropriate antioxidant protection of the polymer will give further improvement. At normal levels of addition softeners and plasticisers have little effect [7]. 

Rudewicz and Munson [45] used this technique for the direct determination of additives in PP. The technique has also been used to determine oligomers in polyacrylates, PEG, siloxanes and polycarbonates [87], polyglycols [88] and adhesion promoters, primers and additives in the surface of PET film [89], volatile antioxidants in styrene-butadiene rubbers [34, 50], mercaptobenzothiazole sulfenamide accelerator in rubber vulcanisates [90] and divinyl benzene in styrene-divinyl benzene copolymer [91]. 

Mass spectrometry techniques have been described for the analysis of rubber compounds. A GC/mass spectrometric procedure has been described [132] for the single injection separation and identification of allerogenic vulcanisation agents and antioxidants from isoprene rubber. Mass spectral fragmentation mechanisms were proposed for each of the additives studied. 

Lattimer and co-workers [25] have applied mass spectrometry (MS) to the determination of antioxidants and antiozonants in rubber vulcanisates. Direct thermal desorption was used with three different ionisation methods [electron impact (El), chemical ionisation (Cl), field ionisation (FI)]. The vulcanisates were also examined by direct fast atom bombardment mass spectrometry (FAB-MS) as a means for surface desorption/ionisation. Rubber extracts were examined directly by these four ionisation methods. Of the various vaporisation/ionisation methods, it appears that field ionisation is the most efficient for identifying organic additives in the rubber vulcanisates. Other ionisation methods may be required, however, for detection of specific types of additives. There was no clear advantage for direct analysis as compared to extract analysis. Antiozonants examined include aromatic amines and a hindered bisphenol. These compounds could be identified quite readily by either extraction or direct analysis and by use of any vaporisation/ionisation method. 

GC has been used extensively for the determination of more volatile components of polymers, e.g., monomers, residual solvents, and antioxidants, and when coupled with complementary techniques such as pyrolysis, photolysis, and MS for the elucidation of the structure of additives (Table 7.1) [31]. Halmo and co-workers [32] have used capillary GC for the determination of antioxidants in vulcanised rubbers. 

This method is potentially the most versatile, since it can be carried out in a variety of ways and adds relatively little to the cost of the final polymer. The nitroso-ene reaction in natural rubber already referred to is one example of such a modification. However, in order to avoid the necessity to gear the modification process to the vulcanisation reaction, we have concentrated in our own work on antioxidant adduct formation to the rubber double bonds either by vinyl grafting or by the Kharasch thiol addition reaction... 

Although it would be interesting to study s-NMR for rubber vulcanisates, this nucleus has such low abundance and sensitivity that it is now not possible. On the other hand, P s-NMR is of more interest because of the sensitivity of the nucleus and lack of polymeric matrix interference the spectra can usually be acquired in a relatively short time. The main applications in polymer/additive deformulation are found in the analysis of phosphorous containing additives such as secondary antioxidants (e.g. Irgafos 168 and Sandostab P-EPQ), flame retardants and transesterification suppressants, as well as in quantitative determinations. P s-NMR is an efficient tool for the stmctural analysis of insoluble polyphosphates and melamine phosphates. 

Some more specific polymer chemistry applications for TG-FTIR are solvent and water retention, curing and vulcanisation reactions, isothermal ageing, product stability, identification of base polymer type and additives (plasticisers, mould lubricants, blowing agents, antioxidants, flame retardants, processing aids, etc.) and safety concerns (processing, product safety, product liability, fire hazards) [357]. A wide variety of polymers and elastomers has been studied by TG-FTIR [353,358,359]. The potential applications of an integrated TG-FTIR system were discussed by various authors [346,357]. 

Lykke et al. [218] have carried out LD, LI and two-step LDI of pure poly-TMDQ. The lack of signal from the ionisation-only experiment is indicative of the low volatility of this oligomeric HALS additive. The same authors have reported the 308 nm ionisation-only spectrum from a vulcanisate (direct analysis) showing the presence of the antioxidant di-r-octyldiphenylamine (DODPA, MW 393). 

Lykke et al. [177,262] have used L MS (ToF-MS, FTMS) in resonant and non-resonant mode for the molecular analysis of complex materials, including polymer/additive systems. Different wavelengths for the post-ionisation step (near-UV, far-UV, VUV) permit selectivity that provides important additional information on the chemical constitution of these complex materials. LDI techniques render more accessible analysis of complex materials such as polymers and rubbers containing a wide variety of additives and pigments. Lykke et al [218] also compared laser desorption, laser desorption/post-ionisation and laser ionisation in both direct and extract analysis of three vulcanised rubbers (natural rubber, SBR and poly(c/5 -butadiene)). Desorption (532, 308, 266 nm)/post-ionisation (355, 308, 266, 248, 213, 118 nm) was carried out with various lasers. Desorption (308 nm)/post-ionisation (355 nm) with REMPI detection allows preferential detection of various additives (antiozonant HPPD, m/z 268, 211, 183, 169 antioxidant poly-TMDQ, m/z 346, 311) over the ubiquitous hydrocarbons in a rubber (Fig. 3.13). 

As Pfisterer and Dunn have confirmed, this result is equal to that obtained with NBR grades containing antioxidants which are bound to the polymer chain. Nevertheless, difficulties are still posed by the ageing of NBR parts in certain technical lubricating oils" ° whose additives render the vulcanisates useless even at only moderately elevated temperatures. Apart from the use of hydrogenated NBR no generally effective remedy is known. 

The same study revealed that antioxidants in general provide little if any additional stability to bromobutyl vulcanisates at temperatures below 150°C, that many antioxidants contribute significantly to heat stability at higher temperatures, and that exceptional heat resistance is achieved with MBI/MgO, with diphenylamine/acetone reaction products plus MgO or, particularly, with combinations of these two antioxidant systems. The author states that magnesium oxide was included with each system because, although its role in bromobutyl vulcanisation is not clear, there is no doubt that it contributes to processing safety, shelf life and heat resistance when used in conjunction with curing systems that do not contain elemental sulphur. 

An examination is made of the antioxidant and antiozonant effects of Q-Flex QDI, a quinone diimine produced by Flexsys, in sulphur vulcanised unsaturated rubbers. It is also shown that this additive acts as a scorch retarder and viscosity modifier in NR compounds, and that it modifies the viscoelastic properties of NR compounds resulting in reduced rolling resistance of tyre treads. 5 refs. 

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