Hardening reaction

Fig. 20.3. The setting and hardening of Portland cement. At the start (a) cement grains ore mixed with water, H. After 15 minutes (b) the setting reaction gives a weak bond. Real strength comes with the hardening reaction ( ), which takes some days.

In what way would you expect the setting and hardening reactions in cement paste to change with temperature Indicate the practical significance of your result. 

Q = activation energy for hardening reaction R = universal gas constant T = absolute temperature. 

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

Between 10 and 15 parts of hexa are used in typical moulding compositions. The mechanism by which it cross-links novolak resins is not fully understood but it appears capable of supplying the requisite methylene bridges required for cross-linking. It also functions as a promoter for the hardening reaction. 

For technical purposes it is often convenient to both arrest the reaction prior to gelation by changing to a slightly alkaline pH and to remove some if not all of the water. The first hardening reaction may then be carried out when desired by changing once again to an acid pH. 

For example, a UF-resin for particleboard at the end of the 1970s would have had a F/U molar ratio of approx. 1.6-1.8. To day a UF-resin for the same application has a molar ratio of between 1.02 and 1.08, but the requirements for the boards, as given in the quality standards, are still the same. The degree of crosslinking of the cured resins as well as the reactivity of the hardening reaction depends on the availability of free formaldehyde in the system. 

Reactivity and hardening reactions of aminoplastic adhesive resins... 

The addition of urea to a phenolic resin causes several effects (1) decrease of the content of free formaldehyde (2) decrease of the viscosity of the glue resin (3) acceleration of the hardening reaction via the higher possible degree of condensation of the resin at the same viscosity (4) reduction of the costs of the resin. 

The acceleration of the hardening reaction [107,108]. This is unsuccessful with the hydrolyzable chestnut tannins used in ref [107] but successful with the use of polyflavonoid tannins such as in Ref. [108]. 

Usually no hardeners are added during the produetion of wood-based panels (PB, MDF, OSB, engineered wood produets). With speeial additives, an aecel-eration of the hardening reaction and hence shorter press times or lower press temperatures ean be aehieved. This is espeeially interesting for eold-setting systems as well as for the produetion of partieleboards. Possible catalysts are those traditionally used for the promotion of urethane formation [17,143],... 

The setting and hardening reactions are first outlined in general terms. They can be considered to take place in a number of overlapping stages. 

The wastes are not reusable as virgin matter because of the irreversibility of the hardening reaction. At best, they can be used like fillers after grinding. 

In order to obtain water resistant materials the curing temperature should be raised to 130 °C or more, because this increases the strength of the adhesion layer. But the presence of dressing formulations reduces the temperature of the hardening reaction, while retaining the degree of conversion, and this simplifies the technology... 

Processing aids are used to directly influence the synthesis process function as reaction controllers. Depending on their chemical state they can function as reaction accelerators (the actual catalysts and starters or initiator substances), crosslinkers and/or hardeners, reaction inhibitors or catalyst deactivators, molecular weight controllers, chain splitters or lengtheners. From a chemical standpoint (structure and method of function) the radical builders, mainly peroxides and azo compounds, are treated separately from the catalysts which are mainly metals, metal oxides, salts (redox systems) and organo-metal compounds. The carrier substances, promoters and deactivators are placed in the catalyst class of substances. 

As a preliminary step in the manufacture of unsaturated polyester thermoset plastic one uses low molecular weight linear polyester (Mr 10,000) obtained by a polycondensation of polyglycols with saturated and unsaturated dicarboxylic acids. The precondensate can then be dissolved and stored in the stabilized comonomer, e.g. styrene, with which it will be crosslinked later. The crosslinking polymerization reaction between the polyester chains and the styrene bridges is initiated with the help of organic peroxides which are added dispersed in plasticizers. The reaction begins at 60-90 °C and then proceeds exothermally. In addition to this a cold hardening reaction can also be carried out. For this reaction cold accelerators are necessary, e.g. tertiary amines or cobalt naphthenate. 

In the hardening of free tallow fatty acids in sc CO2 we measured iodine values (IV) below 1 g I2/IOO g product at a space velocity of 15 h1. In comparison to trickle bed hardening reactions with activated carbon and titania supported 2 wt. % palladium fixed bed catalysts, between 6 to 15 times higher space time yields were determined when the hardening was carried out in sc CO2. The hydrogen partial pressures in both processes were comparable (2.0 MPa H2). 

When the hardening reaction involves cationic polymerization induced by a Lewis acid, however, the functionality of each epoxy group is 2 and that of structure 1-20 is 4. The general hardening reaction is illustrated in Eq. (1-10) for initiation by BF3, which is normally used in this context as a complex with ethylamine, for easier handling. 

For this pmpose we may use an assessment criterion of the viscous-plastic state in any temperature-time regime of the polymer hardening reaction, as given by the following relationship [23] ... 

Typically, an unhardened cottonseed oil is composed of 2.5% stearic fatty acid and 22-23% palmitic fatty acid, which are the principal saturated fatty acids. The unsaturated fatty acids are composed of approximate 18% oleic, 54% linoleic, and less than 1 % each of linolenic and pahnitoleic fatty acids. If the hardening reaction was completely selective, before any hydrogen was absorbed by oleic fatty acid the linoleic fatty acid would have to be completely converted to oleic. On the disappearance of the linoleic fatty acid, the oleic would next absorb hydrogen to be converted to the fully saturated stearic fatty acid. This degree of selectivity is never attained in practice but how closely it is approximated depends on the catalyst type and dosage, temperature, and pressure. 

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