Rubber crosslinking systems

The versatile properties of mbbers result from their low glass transition temperatures (Tg) and the ability to manipulate (increase) this by various types of crosslinking or vulcanization (also known as curing).Sulphur and peroxide, respectively, are the most widely-used crosslinking agents. 

Vulcanization is a very complex reaction and involves activators for the breakage of die sulphur ring (Sg) and accelerators for the formation of sulphur intermediates, which facilitate sulphur-to-double bond crosslinking. Elastomer vulcanization by sulphur wifliout any accelerators takes several hours and is of no commercial importance. By using accelerators in the sulphur curing system, die optimum curing time can be decreased to as little as 2-5 min. 

The production of free radicals is the driving force for peroxide crosslinking. Radicals are atoms or molecular fragments with unpaired electrons. These radicals cause an unstable situation and react to allow the electron to pair with another. Rubber peroxide crosslinking reaction consists of three basic steps as follows.  

When peroxide is heated to a sufficient temperature, the oxygen-oxygen bond ruptures. The resultant molecular fragments from these ruptures are called radicals, which are highly energetic, reactive species.  

Radicals that have been formed from the peroxide decomposing are reactive toward hydrogen atoms in chains. Hydrogen abstraction is a process where the radical removes a hydrogen atom from another nearby atom, and is a very important step in the peroxide curing reaction, as it is the mechanism by which radicals are transferred from peroxide molecular fragments to the rubber backbone.  

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Kraton Polymers has developed a multiarm SIS (Kraton 1320X [37,46,47,50]) and SBS (Kraton KX-222C, [48,49]) for rapid UV/e-beam cure. Besides heat resistance improvements, plasticizer resistance is also improved in cured rubber-based systems. The dioctyl phthlate plasticizer common in PVC backing films is soluble in the styrenic domains of SBCs. Crosslinking of the mid-block provides cohesion even after plasticizer attack [51]. 

A term apphed to vulcanisation systems in which sulphur or a sulphur donor is used very efficiently for crosslinking the rubber. EV systems produce vulcanisates with crosslinks that are mainly monosulphidic, which are thermally and mechanically stable. 

In another system, miscible blends of PE and lauryl methacrylates (LMA) were in situ polymerized/crosslinked to yield submicrometer rubber particle sizes ranging from 70 to 400 nm [55]. Divinyl benzene (DVB) was used as a crosslinking system for LMA (rubber precursor). Typical TPV morphologies consisting of a crosslinked PLMA rubber dispersion (gel content >90%) in a PE matrix and, consequently, typical TPV solid-state properties are obtained. 

The importance of crosslinked polymers, since the discovery of cured phenolic formaldehyde resins and vulcanized rubber, has significantly grown. Simultaneously, the understanding of the mechanism of network formation, the chemical structure of crosslinked systems and the motional properties at the molecular level, which are responsible for the macroscopic physical and mechanical properties, did not accompany the rapid growth of their commercial production. The insolubility of polymer networks made impossible the structural analysis by NMR techniques, although some studies had been made on the swollen crosslinked polymers. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenolic structure renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxylic acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubility is controlled by chemical and polarity differences rather than molecular size. 

Thus, the level of sophistication which one may consider for the application of rubber-like elasticity theory to epoxy networks may depend on the application. For highly crosslinked systems (M < 1,000), a quantitative dependence of the rubbery modulus on network chain length has recently been demonstrated , but the relevance of higher order refinements in elasticity theory is questionable. Less densely crosslinked epoxies, however, are potentially suitable for testing modern elasticity theories because they form via near quantitative stepwise reactions. Detailed investigations of such networks have been reported by Dusek and coworkers in recent studies ... 

With room temperature vulcanizable two-component silicone rubbers, the polymer and the crosslinking component are mixed immediately before application. Two different crosslinking systems are used, one based on polycondensation and the other on polymer addition. 

Several performance characteristics of rubber such as abrasion resistance, pendulum rebound, Mooney viscosity, modulus, Taber die swell, and rheological properties can be modeled by Eq 7.34. " A complex mathematical model, called links-nodes-blobs was also developed and experimentally tested to express the properties of a filled rubber network system. Blobs are the filler aggregates, nodes are crosslinks and links are interconnecting chains. The model not only allows for... 

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]. 

Proper selection of crosslinking system, for example a low sulfur and thiuram accelerated formulation and choice of mineral fillers like clay, activated calcium carbonates, etc., can increase appreciably the maximum service temperature of the rubber. Combination with good heat resistant antioxidant system is capable of increasing the typical service temperature of natural rubber from around 70 °C to 100 °C and sometimes above this for intermittent exposure. 

In order to deal with the four non-crystalline forms in a unified way, we define a network chain in a crosslinked system, as the section of network between neighbouring crosslinks (Fig. 3.6). The shape of both a network chain in a rubber, and a molecule in a polymer melt, can be changed dramatically by stress, and both can respond elastically. However, when the polymer is cooled below Tg, the elastic strains are limited to a few per cent (unless a glassy polymer yields), so the molecular shape is effectively fixed. If the melt or rubber was under stress when cooled, the molecular shape in the glass is non-equilibrium. This molecular orientation may be deliberate, as in biaxially stretched polymethylmethacrylate used in aircraft windows, or a by-product of processing, as the oriented skin on a polystyrene injection moulding. Details are discussed in Chapter 5. 

From the reaction of such polyiners with silicone crosslinkers, which need to have at least three reactive groups, three-dimensional networks with rubber-like elasticity are obtained. The chemical nature of the silicone crosslinker to be used depends on the crosslinking system, further details of which are given in the next Chapter. 

There are two different crosslinking systems available to vulcanize RTV-2 silicone rubbers ... 

For RTV-2 silicone rubbers used for mold making, there are further distinguishing features, besides the crosslinking system, that are very important ... 

Figure 27.5 Structure of the networks formed by the four vulcanization techniques (a) DCP, (b) EV, (c) mixed, (d) CV (left) and mole percent -hexane uptake of natural rubber with different crosslinking systems (right). (Reprinted from ref. 26 with the permission of Copyright 1997 John Wiley and Sons, Inc.)...

If the rubber has a carboxylic acid as a cure site, then one can use compounds with two or more functional groups that will react with the —COOH group. Examples of such compounds are polyhydroxy compounds such as ethylene glycol and pentaer-ythritol, diamines such as hexamethylenediamine and methylenebis(aniline), diisocyanates such as toluene diisocyanate, and so on. The crosslinking systems used for other rubber compounds will depend upon the cure site and are readily available from the rubber supplier. 

Preparation of the Blends of Plastics and Crosslinked Rubbers 137 Table 3.3 Attributes of different crosslinking systems for unsaturated polymers. 

Scagliusi, R.S., Cardoso, E.C.L., Lugao, A.B. Radiation-induced degradation of butyl rubber vulcanized by three different crosslinking systems. Radiat. Phys. Chem. 81, 991-994 (2012)... 

Table 4.1 Examples of crosslinkers used in different rubber-based systems with diverse applications... 

Aromatic polyesters were investigated to examine their suitability as a toughening agent for epoxy, and found to be effective for a highly crosslinked system. Liquid rubber is less efficient in toughening a highly crosslinked system. lijima and co-workers [132, 133] synthesised a series of polyesters by the reaction of 1,2 ethane diol and aromatic dicarboxylic acid. The latter and the derivatives contained pthalic anhydride. 

Crosslinking is a widely used method for modification of pol5nner properties. The process consists in a formation of a tii-dimensional network structure, a gel, causing substantial changes in the material properties. Vulcanized rubbers and thermosetting resins can be mentioned as the most common examples of crosslinked systems. However, crosslinked polyolefins are of significant interest as well. 

Several rubbers may be crosslinked using divalent metal oxides, usually zinc oxide. There are a limited number of polymers that utilise this method, which is used with halogenated polymers such as polychloroprene [8], chloro- and bromobutyl, and chlorosulfonated polyethylene and carboxylated nitrile rubbers. The system may utilise the metal oxide alone or in combination with the organic accelerators used with sulfur-curing systems. In the case of halogenated polymers, magnesium oxide may be added to act as an acid scavenger. 

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