Reactive rubbers

The ebonite compound before cure is a rather soft plastic mass which may be extruded, calendered and moulded on the simple equipment of the type that has been in use in the rubber industry for the last century. In the case of extruded and calendered products vulcanisation is carried out in an air or steam pan. There has been a progressive reduction in the cure times for ebonite mixes over the years from 4-5 hours down to 7-8 minutes. This has been brought about by considerable dilution of the reactive rubber and sulphur by inert fillers, by use of accelerators and an increase in cure temperatures up to 170-180°C. The valuable effect of ebonite dust in reducing the exotherm is shown graphically in Figure 30.3. 

In formulating adhesives, it is desirable to use materials with low cost. For specialty adhesives such as the acrylics, it is preferred to use commodity chemicals with a range of other uses. Minor components such as reactive rubbers, functional monomers and some additives are specially synthesized for acrylics, but these are expensive due to low volume. 

These authors found that to achieve supertoughness in PET by shear yielding, a reactive modifier is superior to a non-reactive rubber modifier and that a dispersed particle size and interparticle distance of 200 and 50 nm, respectively, are... 

For epoxy networks modified by liquid reactive rubbers, it is not so easy to discuss these parameters separately, because they are interdependent. For example, an increase in the acrylonitrile content of the carboxy-termi-nated butadiene acrylonitrile rubber (CTBN) induces a size reduction of the rubbery domains but also a higher miscibility with the epoxy-rich phase, leading to a higher amount remaining dissolved in the matrix at the end of cure (Chapter 8). It is not possible to separate the influence of these two effects on toughness. 

Interfacial adhesion between particles and matrix is necessary and can be achieved using reactive rubbers. 

Liquid reactive rubbers were also used for UP and vinyl ester formulations (Suspene et al., 1993 Siebert et al., 1996). Increases in fracture energy and fatigue- crack resistance were reported for some systems, although no significant improvements were observed for some other systems. These different behaviors are probably related to the heterogeneous structure of the matrix (Chapter 7). Toughening mechanisms in three-phase systems are not yet well established. 

The use of liquid reactive rubbers to toughen thermosets leads to a decrease of both T — g and the Young s modulus (see Fig. 13.7). Thermoplastics can be added to these formulations to improve thermal and mechanical properties. 

Woo et al. (1994) studied a DGEBA/DDS system with both polysul-fone and CTBN. The thermoplastic/rubber-modified epoxy showed a complex phase-in-phase morphology, with a continuous epoxy phase surrounding a discrete thermoplastic/epoxy phase domain. These discrete domains exhibited a phase-inverted morphology, consisting of a continuous thermoplastic and dispersed epoxy particles. The reactive rubber seemed to enhance the interfacial adhesive bonding between the thermoplastic and thermosetting domains. With 5 phr CTBN in addition to 20 phr polysul-fone, Glc of the ternary system showed a 300% improvement (700 Jm-2 compared with 230 J m 2 for the neat matrix). 

Fig. 13 is a TTT cure diagram of three systems a neat epoxy resin and the same epoxy modified with two reactive rubbers at the same concentration level. The times to the cloud point, gelation and vitrification are shown for each system. The cloud point is the point of incipient phase separation, as detected by light transmission. The modified system with the longer times to the cloud point and gelation, and the greater depression of Tg, contains the more compatible of the two rubbers. The difference in compatibility could then be used to account for differences in the volume fractions of the phase separated rubber-rich domains and in the mechanical properties of the neat and the two rubber-modified systems. 

Table 19.1 Comparisons of increases in epoxy resin fracture toughness achieved using liquid reactive rubbers and core-shell paitides at equivalent volume fractions of particles...

Among the most effective modifiers allowing an increase in the impact viscosity and cracking resistance of epoxy polymers are the oligobutadiene acrylonitrile rubbers with reactive end groups. The structures of the reactive rubbers that are commonly used to increase the impact viscosity of the epoxy resins are as follows. CTBN is a... 

PPG-24 butyl ether PPG-33 butyl ether PPG-40 butyl ether PPG-53 butyl ether intermediate, reactive rubber Methyl butynol Methyl pentynol intermediate, reactive specialty chemicals Methyl pentynol Polybutene intermediate, reactive surfactants PPG-5 butyl ether PPG-9 butyl ether PPG-15 butyl ether PPG-18 butyl ether PPG-22 butyl ether... 

Table 11,2 Some common reactive rubbers and tougheners for polyamides (From Akkapeddi 2001) ...

Reactive rubber/toughener Fimctirarality Reactivity Other features... 

Fig. 11.28 (a) Notched Izod impact strength versus rubber particle diameter in PA 6,6/reactive rubber blends (curve A, 10 wt.% rubber curve B, 15 wt.% curve C, 20 wt.% rubber), (b) The same Izod impact strength data plotted versus interparticle distance (From Wu (1985) reproduced with permission of Elsevier)... 

Fig. 19.10 Schematic illustration of interfacial graft copolymers in polyamide/reactive rubber blends (Note Balanced end group PA (1-amine/chain) forms a mono-graft copolymer. A diamine terminated PA forms di-graft copolymers. The latter can lead to entanglement at interface. Both graft copolymers stabilize and strengthen the blend interface)...

Table 19 1 Some common reactive rubber and other types of impact modifiers used for meltblending and impact modification of polyamides...

One of the first approaches was to use a blend of liquid reactive rubbers and the much higher molecular weight elastomer (carboxylated nitrile rubber). This led to a bimodal distribution of the precipitated secondary phase, which was seen as a distinct improvement over either the elastomer or the RLP on its own. 

Matrix-rubber particle adhesion is an important parameter for rubber toughening. For effective rubber toughening, rubber particles must be well bonded to the thermoset matrix. The poor intrinsic adhesion across the particle-matrix interface causes premature debonding of particles, leading to catastrophic failure of the materials. Nearly all the studies [9, 193, 2-10] have been concerned with reactive rubbers as toughening agents, and showed that dispersed particles have interfacial chemical bonds as a consequence of chemical reactivity. 

Some authors have refused to accept the role of interfacial adhesion on the toughening of thermoset resins. Lavita and co-workers [190] reported that non-reactive rubber can toughen BPA-modified epoxy, but the mechanism was not fully discussed. Huang and co-workers [194] showed that when the second phase consists of micron-size rubber particles, the interfacial bonding has only a modest effect on the fracture properties of blends. 

One of the most important methods for controlling the yield behaviour of polymers is rubber modification, which is widely used to increase fracture resistance. The technique was first used in 1948 to modify the properties of polystyrene, and has since been extended to most of the major plastics, including polypropylene, polycarbonate, and rigid PVC, and to a number of the less highly crosslinked thermosets, notably epoxy resins. Between S and 20 % of a suitable rubber is added in the form of small particles, which are typically between 0.1 and S /im in diameter. Chemically reactive rubbers are preferred, because they form bonds with molecules of the surrounding matrix which can withstand tensile stress. The rubber particles have low moduli, and therefore act as stress concentrators. Accelerated deformation in the matrix adjacent to the rubber particles results in a lowering of the yield stress. 

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