Failure Formaldehyde resins

In this study, up to about 50% of the phenol-formaldehyde was replaced with carbohydrates and the modified resins used to bond wood veneer panels. The carbohydrate modified resins were formulated and cured under neutral conditions. The resins bond wood with acceptable dry- and wet-shear strengths, and wood failures. Reducing as well as nonreducing carbohydrates can be used as modifiers. The carbohydrate modifiers are being incorporated into the resin via ether linkages between the hydroxyls of the carbohydrate and methylol groups in the phenol-formaldehyde resin. The resins formulated under neutral conditions are very light in color. 

Figure 4. Variation of the dry- and wet- (hatched) wood failures of modified phenol-formaldehyde resins. The moles of modifier were varied as indicated. The mole ratio of phenol to formaldehyde was 1 2.3. The wood failures of xylose- and prehydrolysate-modified resins were similar and were averaged.

WeatherBest solid 28% HDPE, 60% wood flour, 12% talc + phenol-formaldehyde resin solid, density 1.20 g/cm3 No degradation effect No failure in 40 min... 

In the manufacture of a phenol-formaldehyde resin, the temperature was allowed to rise to 341 K. Attempts to control it failed, despite addition of water. Insufficient thought had been given in scaling-up. The lack of emergency relief venting resulted in failure of the vessel. 

The primary causes of accidents in the chemical industry are technical failures, human failures and the chemical reaction itself (due to lack of knowledge of the thermochemistry and the reaction kinetics) [156]. As discussed previously, polymerization reactions are subject to thermal runaway, so that it is not surprising to learn that polymerization reactions (64 from 134 cases) are more prone than other processes to serious accidents [157]. Among the polymerization processes, the phenol-formaldehyde resin production seems to be the worst case, although incidents have been reported for vinyl chloride, vinyl acetate and polyester resins polymerization processes. 

Overall, therefore, the available literature supports the generally held view that the durability of UF-bonded wood products is governed by the susceptibility of cured UF resin bonds to scission by both hydrolysis and swell/shrink stresses. Note, moreover, that in either case, the most likely product of scission will ultimately be formaldehyde and further that mechanical stress enhances the rates of many chemical reactions (37). In fact, simplistic calculations based on formaldehyde liberated from bond ruptures at least indicate the possibility that formaldehyde from swell/shrink stress rupture could contribute significantly to total emission. Assume, for example, that board failure occurs due to rupture of one chemical bond type which liberates one molecule of formaldehyde and consider two cases (a) a conservative one in which only 5 percent of those bonds rupture in 50 years, i.e., probable board durability greater that 50 years, and (b) a much less conservative case in which 30 percent of those bonds rupture in 20 years, i.e., probably failure in 20 years or less. Case (a) leads to a first order scission rate constant of 3.3 x 10 s and a hypothetical board emission rate (see Appendix 3a) that is below the maximum liberation rate permitted by the German E-1 standard (7). However, Case (b) leads to a first order scission rate constant of 5.7 x 10 s and a hypothetical board emission rate above that allowed by the HUD standard (8). (FormaIdehyde-wood interactions and diffusion effects would... 

Comparable wood failure data (hard maple) for resorcinol-formaldehyde and epoxy resins are 92 2 and 30%, respectively (from same sources as given under footnote 5). 

Woo (1982, personal communication) has described the exterior plywood adhesive formulation based on Tannaphen and used commercially by New Zealand Forest Products Ltd. since 1981 as Tannaphen - 56 parts, a phenol-formaldehyde fortifying resin - 21 parts, olivestone flour - 18 parts, and paraformaldehyde - 5 parts on weight of solids. These adhesives are tolerant of open assembly times of 20 minutes, closed assembly times of up to 4 hours, and veneer moisture contents of up to 15%. Both shear strengths and wood failure were comparatively high even after 5 weeks in water at 25 °C. 

Although the commercial phenol-formaldehyde control resin did not perform well with a short press time of 2 minutes, giving average wood failure of 35%, it provided satisfactory bond quality, with an average of 84% wood failure, at the manufacturer s recommended press time of 3 minutes. It did very well (average of 92% wood failure) at the longer press time of 4 minutes. 

The theories of gelation presented here can only be applied when it is clear that the assumption of equal reactivity of functional groups is satisfactory. In terms of the polymerizations described earlier, the theories generally are applicable to the formation of polyester and polyurethane networks, but not to the formation of formaldehyde-based resins and epoxy resins. Failure of the principle of equal reactivity for the latter systems results from modification of the reactivity of a particular functional group by reaction of another functional group in the same molecule of monomer. 

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