Flexibilisers

As a class the aliphatic polyalkenamers have low values due to a combination of low chain stiffness and low interchain attraction. The presence of double bonds has the effect of increasing the flexibility of adjacent single bonds (see Chapter 4) and overall this leads to a reduction in. Thus in the sequence from polydecenamer down to polypentenamer an increase in the double bond concentration leads to a lowering of Tg. On the other hand the Tg of polybutenamer, i.e. poly butadiene, is somewhat higher than that of polypentenamer, presumably because the proportion of stiff links, i.e. double bonds, becomes sufficiently high to override the flexibilising effect on adjacent chains. Consequently the polypentenamers have the lowest Tg values known for hydrocarbon polymers (cis- -114°C, trans- -97°C). 

They have found use as hardeners-eum-flexibilisers for epoxide resins (see Chapter 26) and are of interest in the production of thixotropic paints and adhesives. Related higher molecular weight materials are tough and flexible and find use as hot melt adhesives (Versalons). 

The polyglycol diepoxides, which are used as reactive flexibilisers, are considered in the next section. 

For a number of purposes the unmodified epoxide resins may be considered to have certain disadvantages. These disadvantages include high viscosity, high cost and too great a rigidity for specific applications. The resins are therefore often modified by incorporation of diluents, fillers, and flexibilisers and sometimes, particularly for surface coating applications, blended with other resins. 

In order to increase the flexibility, and usually, in consequence, the toughness of the resins, plasticisers and flexibilisers may be added. Non-reactive plasticisers such as the conventional phthalates and phosphates have proved unsuccessful. Monofunctional materials, which in some cases also act as reactive diluents, have been used but are not of great importance. 

More interest has been shown in polymeric flexibilisers, particularly the low molecular weight polyamides from dimer acid (see Chapter 18), the low molecular weight poly sulphides (Chapter 19), polyamines and the polyglycol diepoxides. 

Interesting amine flexibilisers have also been described. These materials are made by cyanoethylation of amine hardeners such as diethylenetriamine, to such an extent that only two reactive hydrogens remain and the material is only difunctional, e.g. Figure 26.21. 

Diluents, Flexibilisers and other Additives 771 The amine flexibilisers may be used in two ways ... 

Progressive replacement of amine hardener by a low-viscosity flexibiliser will reduce mix viscosity, increase pot life and reduce the heat distortion temperature of the cured system. Higher impaet strengths are achieved using approximately equivalent amounts of hardener and flexibiliser. 

Using flexibilisers in addition to the usual amount of hardener, very flexible produets may be obtained. 

Although in many respects they are similar to the liquid polysulphides, the amine flexibilisers differ in three important respects ... 

Table 26.10 compares the effect of the above classes of flexibiliser. 

Especially in recent years, there has been a tendency to develop stabilisers with higher-MW (>2000 Da) to prevent loss under severe conditions of application. Polymeric additives for polymers, including impact modifiers, flexibilisers, antistatic agents, and processing aids, have been reviewed [54]. 

Flame retardants - Fire retardants - Smoulder retardants -Smoke-suppressants Flexibilisers Flow improvers... 

Ethylene copolymers (high molecular weight flexibilisers) have been proposed to replace liquid plasticiser for low smoke FR PVC cable formulations (238). 

Examples of other plasticisers are an adduct of tetrahydrofurfural alcohol and ethylene oxide and the reaction products of ethylene glycol either with organic acids or with anhydrides. Polyols and natural and synthetic rubbers are examples of flexibilisers. 

Epoxy resins produced by the reaction of bisphenol A and epichloro-hydrin are versatile polymers with several useful properties (subsection 2.2.2.1). However, one significant weakness is their brittle nature. Incorporation of plasticisers is not very useful. Dibutyl phthalate is an exception, showing good compatibility but offering only limited ability to flexibilise the resin. Moreover, plasticisers affect the mechanical properties and chemical resistance of the cured system. With polyurethanes it is possible to complement the flexibility of the epoxy system. Numerous attempts have been made to combine the two types to achieve beneficial modifications (Lee and Nivelle, 1967). These modifications proved successful under high-temperature cure but inferior results were obtained for ambient cures. 

Another development has been reported by P.A. Lucas, W.E. Stamer and S.G. Musselman of Air Products and Chemicals Inc. (Lucas et al., 1994). An acrylate functional urethane flexibiliser has been used to modify epoxy resin, which optimises reactivity and is more compatible with epoxy. Urethane-acrylate flexibiliser offers very tough hybrid epoxy systems meeting the more demanding requirements of civil engineering applications. 

Flexibilisation of the scope of accreditation an important asset for food and water microbiology laboratories 155... 

In the context of multifaceted special demands of some sectors and companies, a series of special allocations and flexibilisation options was integrated into the regulations of the first German NAP. 

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