Curing at Room Temperature

In order to obtain /227s,norm(tcure=0) as the initial state of the adhesive prior to curing, 12275,norm(tcure) IS extrapolated. 

The results substantiate increased isocyanate conversion rates in the vicinity of any of the three metals, even in ultrathin films on Au, to a greater extent on Al, and to the most pronounced extent on Cu (Fig. 6.4b). As the film thickness 

Except for ultrathin PU films on Cu, isocyanate conversion (2275 cm , e.g., in Fig. 6.3) is advanced but stiU incomplete after 72 h of cure at room temperature. Hence, post-curing is imposed on all samples in order to obtain Jully cured polyurethane. 

---

Resorcinol is to phenol as melamine is to urea. Resorcinol—formaldehyde (RF) is very expensive, produces dark and waterproof gluelines, but will cure at room temperature. As with melamine and urea, resorcinol is often combined with phenol to produce phenol—resorcinol—formaldehyde (PRF) adhesives, thus producing an exceUent adhesive with some of the economy of phenol. These adhesives are the mainstay of the laminated timber industry which generally requites a room-temperature cure with durable, waterproof gluelines. 

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

The soft segments made from asymmetrical (amorphous) polyols are important for two-component structural adhesives and one-component moisture-curing adhesives. These materials are applied and usually cured at room temperature. 

Two-component systems consist of (1) polyol or polyamine, and (2) isocyanate. The hardening starts with the mixing of the two components. Due to the low viscosities of the two components, they can be used without addition of solvents. The mass ratio between the two components determines the properties of the bond line. Linear polyols and a lower surplus of isocyanates give flexible bond lines, whereas branched polyols and higher amounts of isocyanates lead to hard and brittle bond lines. The pot life of the two-component systems is determined by the reactivity of the two components, the temperature and the addition of catalysts. The pot life can vary between 0.5 and 24 h. The cure at room temperature is completed within 3 to 20 h. 

Liquid resins are usually reinforced with fibers (glass, asbestos), because of their brittleness. They are almost always used for process plant construction. As liquid resins they can be catalyzed to cure at room temperature and low pressures. Relatively cheap wooden molds are required to build quite large items such as tanks and ducting on a one-off basis. The principal materials in this group of plastics are described below. 

Unsaturated polyester finishes of this type do not need to be stoved to effect crosslinking, but will cure at room temperature once a suitable peroxide initiator cobalt salt activator are added. The system then has a finite pot life and needs to be applied soon after mixing. Such a system is an example of a two-pack system. That is the finish is supplied in two packages to be mixed shortly before use, with obvious limitations. However, polymerisation can also be induced by ultra violet radiation or electron beam exposure when polymerisation occurs almost instantaneously. These techniques are used widely in packaging, particularly cans, for which many other unsaturated polymers, such as unsaturated acrylic resins have been devised. 

These materials are reviewed elsewhere in this book except RTV. The RTV (room temperature vulcanization) silicone plastic is a very popular type. It solidifies by vulcanization or curing at room temperature by chemical reaction, made up of two-part components of silicones and other elas-tomers/rubbers. RTV are used to withstand temperatures as high as 290° C (550°F) and as low as —160° C (—250°F) without losing their strength. Their rapid curing makes them... 

For a number of applications curing at room temperature is desirable. This so-called cold cure is brought about by using a peroxy initiator in conjunction with some kind of activator substance. The peroxy compounds in these cases are substances such as methyl ethyl ketone peroxide and cyclohexanone peroxide, which as used in commercial systems tend not to be particularly pure, but instead are usually mixtures of peroxides and hydroperoxides corresponding in composition approximately to that of the respective nominal compounds. Activators are generally salts of metals capable of undergoing oxidation/reduction reactions very readily. A typical salt for this purpose is cobalt naphthenate, which undergoes the kind of reactions illustrated in Reactions 4.6 and 4.7. 

The electrocatalysts for oxygen reduction were prepared as follows. These complex compounds were inoculated onto the carbon (AG-3, BET area near 800 m2/g) by means of adsorption from dimethylformamide solutions. The portion of complex compound weighed so as to achieve 3% of Co content was mixed with the carbon, then 5 ml of dimethylformamide per 1 g of the carbon were added and the mixture was cured at room temperature for 24 hours. Series of samples obtained were thermally treated (pyrolyzed), and the resulting grafted carbons were tested as electrode materials in the reaction of molecular oxygen reduction. 

Aqueous polynitrile oxide curing compositions, with good storage stability, have been patented (525). The compositions comprise aqueous dispersions containing nitrile oxides and are useful for coating systems that are cured at room temperature without the release of byproducts. Latexes are cured by mixing a polymer latex with a stable polynitrile oxide, for example, 2,4,6-triethylbenzene -1,3-dicarbonitrile oxide, and removing water from the mixture. 

Cold cure silicone rubbers and available as pastes. These pastes are mixed with an organometallic catalyst and silicate and cured at room temperature. These are used as adhesives, and as encapsulating materials for electronic components. They are also used for textile coating and in moulds. 

The polyester product is first dissolved in styrene at a level of 60 % polyester and 40 % styrene. The initiators, either benzoyl peroxides (BPOs) or methyl ethyl ketone peroxide (MEKP), with appropriate co-catalysts, are added. The mixture is then poured into a glass mold and cured at room temperature for the MEKP system or at 100 °C for the BPO systems. A post-cure at 100 °C or 130 °C, respectively, for 5 h is then carried out. Blending experiments were carried out by using a dicyclo pentadiene (DCPD) resin, 61-AA-364, from GLS Fiberglass (Woodstock, IL, USA). This sample is pre-promoted , so we could only use the MEKP system. 

PEER polymers can be cured with traditional radical initiators such as methyl ethyl ketone (MEK) peroxides and benzoyl peroxide (BPO). Curing can be carried out either at room temperature or at elevated temperature. A PEER polymer containing 30 % maleic anhydride can be cured at room temperature with MEK peroxides in 10 to 60 min, depending on the type of peroxide used (Table 22.2). To cure a PEER resin with MEK peroxides at room temperature, a co-catalyst is needed. The commonly used cobalt naphthenate works very well in this case, while another co-catalyst, dimethyl aniline, is very efficient for the BPO system. 

The formulated RTV silicone is usually cured at room temperature for 16 hours and then at 120°C for 4 hours to ensure the complete removal of organic solvent. A rubbery and non-tacky elastomer is usually obtained after the curing cycle. 

EP-4 developed by ERDL is a very flexible polyester based on polyethylene glycol with molecular weight-200 (PEG-200), isophthalic acid (IPA) and maleic anhydride (MAn). Before its use, it is blended with styrene monomer (1 1) and cured at room temperature using cobalt naphthenate (as an accelerator) and methyl ethyl ketone (MEK) peroxide (as a catalyst). This meets the requirements of the main inhibitor and is used for inhibition of DB and CMDB propellants after the application of a barrier coat (generally a rigid polyester such as PR-3). However, it is observed during manufacture of EP-4 that there is a lot of batch-to-batch variation in properties in spite of the strict quality control measures adopted during its manufacture. 

Melamine molding compounds tend to cure at room temperature, and consequently have a relatively short storage life. Molded parts... 

Thin articles may be vulcanized by treatment with sulphur monochloride by dipping in a solution or exposure to its vapours. This process has been replaced by using ultra accelerators which are capable of curing at room temperature. 

Dynamic mechanical testing of the epoxy matrix/epoxy sizing blends was done on a DuPont model 983 DMA interfaced to a 9900 model controller. Stoichiometric mixtures of DER 383 and DACH were first prepared, and then sufficient size was added to produce the desired concentration on a weight percent basis. The mixture was degassed and poured into silicone RTV-664 [10] molds that contained four cavities, 3.2 x 12.5 x 60 mm. The specimens were cured at room temperature for 16 h in a desiccator, placed in a forced convection oven, and ramped to 80°C at 5°C/min and held at that temperature for 2 h. The samples were allowed to cool to room temperature and then removed from the mold. The specimens were replaced in the oven on a metal sheet and postcured at 175°C for 2 h. The free surface of the specimens was ground on a Struers Abramin polisher using 320 grit SiC paper and water to produce parallel faces on the specimens. 

SIMS Data From Two Part Epoxy Cured at Room Temperature for 24 Hours and at 250°F for One Hour. 

---