Gel times

High ortho novolaks have faster cure rates with hexa. Typical properties of a 2inc acetate-cataly2ed high ortho novolak are also shown in Table 4. The gel time with hexa is one-third of that with a strong acid-cataly2ed novolak. 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

Another unique redox system used for extending gel times consists of cumene hydroperoxide and manganese naphthenate, which provides consistent gel times of between two and four hours over a temperature range of 25—50°C. 

The recovery of sand from foundry molds and cores is much easier when binders made water soluble by use of sodium alumiaate are used ia place of iasoluble resin binders (35,36). Sodium alumiaate acts as a setting accelerator for Portiand cement (qv) (37). In similar appHcation, addition to concrete provides a longer gel time before fully curing (38). 

When the hydrosol ceases to flow like a Hquid (the gel time), it is termed a hydrogel (Fig. 11a). As formed, the pores are filled with the medium (usually water) in which the gel is prepared. The hydrogel may be washed to remove the by-product salt and sold in that form, in which case it may consist of up to 70% water. Because the water is trapped in the pores, the final product can stiU be a relatively free-flowing powder. 

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

The relative reactivity of thermosetting powders can be easily deterrnined by the gel time or stroke-cure test. A small amount of powder is placed on a hot plate, usually at 200°C, and the time until the coating composition gels, or no longer forms fibers, is deterrnined. Powders are characterized by relative gel times (cure rate) as shown in Figure 1. 

Fig. 1. Gel time as a function of temperature for A, residual heat curing B, fast curing C, medium-fast curing and D, slow curing powders.

Figure 23.15. Effect of pH on the gel time of a P F cast resin. (After Apley )...

Quality control tests on the resins most commonly employed are for specific gravity, viscosity, colour, clarity and gel time under standard conditions,... 

Helmut Orth first reported the use of laetones to accelerate phenolic resole cure in 1957 [161]. A year later, Orth discovered that this effect could be extended to aliphatic esters as well [162], Despite the dramatic nature of the acceleration seen, Orth s observations were not applied in industry for a decade. In 1967, Sumitomo and BASF applied esters to soil grouting and wood uses [133,163, 164]. Neither of these applications were commercially successful, however, and commercial success would not occur until 1980 when Borden introduced ester-cured sand binders for foundry [165]. This technology was highly successful in UK and spread to the US, where it was applied immediately to foundry in 1981 and eventually to wood products in 1990 [119,166-173]. Esters are capable of reducing the gel times of resoles from several weeks to less than 30 s at room temperature. Both gaseous and liquid esters are applicable [119,166]. 

As with resoles, the central issue in design of novolacs is molecular weight. The effects of formaldehyde-to-phenol molar ratio and formaldehyde conversion on molecular weight of novolacs has been well studied and reported [192,193]. The effects of molecular weight on most of the important properties are also available [193]. These include Tg, melt viscosity, gel time, hot-plate flow, glass-plate flow. 

Gel times scaler proportionality constant Scale of segegation (m)... 

Polybutadienes, polycaprolactones, polycarbonates, and amine-terminated polyethers (ATPEs) are shown in Scheme 4.4 as examples of other commercially available polyols. They are all specialty materials, used in situations where specific property profiles are required. For example, ATPEs are utilized in spray-applied elastomers where fast-reacting, high-molecular-weight polyamines give quick gel times and rapid viscosity buildup. Polycarbonates are used for implantation devices because polyuredtanes based on them perform best in this very demanding environment. Polycaprolactones and polybutadienes may be chosen for applications which require exceptional light stability, hydrolysis resistance, and/or low-temperature flexibility. 

Physical properties (ASIM methods in parentheses) Gel time = 2-5 min hardness (D2240) = 49 Shore D (>90 Shore A) tensile strength (D412) = 2393 pounds per square inch (psi) ultimate elongation (D412) = 103% die C tear (D624 — 91) = 341 foot-pounds per inch (Ibf/in.) split tear (D3489 — 90) = 97 lbf/in. I g(DMA) = -50°C. 

The material should have these properties gel time = 2—5 min hardness (D2240 ) = 95 Shore A Bashore resilience (D2632) = 60 % tensile strength (D412) = 5400 psi ultimate elongation (D412) = 500% die C tear (D624) = 610 Ibf/in. 

Gel time values of the three systems measured as abrupt change in the slope of G (t) under isothermal curing conditions show that gelation occurs earlier in PWE system at all temperatures considered as shown in Table 11.27. ETPI behaves like a catalyst for the primary epoxy-amino reaction which dominates the cure until vitrihcation occurs. Dynamic mechanical analysis and dielectric spectroscopic analysis carried out by the authors also confirm the above conclusions. 

The rheological behaviour in the range of LM pectin was analyzed and the sol-gel diagram established [59] for different stoichiometric ratios. In their paper, these authors determined the gel times for sodium pectate during calcium-induced gelation and the variation of the gel time with polymer concentration, stoichrometric ratio and temperature. 

Solution Si(0C2H5)4 (8) h2o r c2h5oh (ml) Cone, of Si02(wt%) Spinna- bility Gel. time(h)... 

---