Pot-life

Since the chemical reaction of the two components A and B begins in a pot immediately after mixing, this ready-made adhesive mixture requires speedy application. Otherwise the reaction for the formation of the AB polymer (the adhesive layer) will have developed to such an extent already prior to the application to the adherends that the expected strength of the bonded joint is impaired. Between the mixing of the adhesive mix and its application to the adherends and their fixing only a certain time span is allowed - which may vary for the individual reactive adhesives. This time is called the pot life. Depending on the reactivity of the A and B monomers, pot life can lie in the range of minutes or even hours. 

The time- and temperature-dependent gel point of a reactive adhesive represents the state at which the adhesive transfers from the state of increasingly higher viscosity to the solid state until it reaches its final strength. 

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Foam Products. Latices are made into foams for use in cushioning appHcations. The latices are frothed with air and then chemically coagulated for thick appHcations, or heated to induce coagulation for thinner appHcations. The latter method allows for infinite pot life during production (see Foamed plastics). 

Inhibitors are often iacluded ia formulations to iacrease the pot life and cute temperature so that coatings or mol dings can be convenientiy prepared. An ideal sUicone addition cure may combine iastant cure at elevated temperature with infinite pot life at ambient conditions. Unfortunately, real systems always deviate from this ideal situation. A proposed mechanism for inhibitor (I) function is an equUibtium involving the inhibitor, catalyst ligands (L), the sUicone—hydride groups, and the sUicone vinyl groups (177). 

Epoxy cross-linking is cataly2ed by TYZOR TPT and TYZOR TBT, alone or with piperidine, and by TYZOR TE. The soHd condensation product from 3 TPT 4 TEA (triethanolamine) has also been appHed to epoxy curing (490). Titanate curing is accelerated by selected phenoHc ethers and esters at 150°C the mixtures have along pot life at 50°C (491) (see Epoxyresins). 

Diluent CAS Registry Flexi-bil Effi-cien Solvent Acid resistance Pot-life... 

Ambient-cure systems are often based on lower molecular-weight soHd epoxy resins cured with aUphatic polyamines or polyamides. Curing normally occurs at ambient temperatures with a working life (pot life) of 8—24 h, depending on the formulation. Epoxy—poly amine systems are typically used for maintenance coatings in oil refineries, petrochemical plants, and in many marine appHcations. Such coverings are appHed by spray or bmsh. These are used widely where water immersion is encountered, particularly in marine appHcations (see COATINGS, MARINE). 

Attempts to overcome the short pot life have focused on development of special equipment in which the components can be mixed in the proper proportion. Another approach has been to develop curing agents that give long pot life compared to conventional systems. 

Similar properties are exhibited by dimethylaminopropylamine and diethyl-aminopropylamine, which are sometimes preferred because they are slightly less reactive and allow a pot life (for a 500 g batch) of about 140 minutes. 

These hardeners are extremely aetive. The pot life for a 500 g bateh may be as little as 10 minutes. 

A hardener consisting of a blend of the two reaction products shown in the above equation is a low-viscosity liquid giving a 16-18 minute pot life for a 500 g batch at room temperature. 

The greater the degree of cyanoethylation the higher the viscosity of the adduct, the larger the pot life and the lower the peak exotherm. The products are skin sensitive. 

Meta-phenylenediamine, a crystalline solid with a melting point of about 60°C, gives cured resins with a heat distortion temperature of 150°C and very good chemical resistance. It has a pot life of six hours for a 200 g batch at room temperature whilst complete cures require cure times of four to six hours at 150°C. About 14 pts phr are used with the liquid epoxies. The main disadvantages are the need to heat the components in order to mix them, the irritating nature of the amine and persistent yellow staining that can occur on skin and clothing. The hardener finds use in the manufacture of chemical-resistant laminates. 

Many other amines are catalytic in their action. One of these, piperidine, has been in use since the early patents of Castan. 5-7 pts phr of piperidine are used to give a system with a pot life of about eight hours. A typical cure schedule is three hours at 100°C. Although it is a skin irritant it is still used for casting of larger masses than are possible with diethylenetriamine and diethylaminopropylamine. 

Tertiary amines form a further important class of catalytic hardeners. For example, triethylamine has found use in adhesive formulations. Also of value are the aromatic substituted tertiary amines such as benzyldimethylamine and dimethyldiaminophenol. They have found uses in adhesive and coating applications. A long pot life may be achieved by the use of salts of the aromatic substituted amines. 

Hardener Parrs used per 100 pts resin Pot life (500 g batch) Typical cure schedule Skin irritant Max HOT cured resin rcj Features Applications... 

Ethyl hexoate salt of above 10-14 3-6 h — yes — long pot life encapsulation... 

The number of hardening agents used commercially is very large and the final choice will depend on the relative importance of economics, ease of handling, pot life, cure rates, dermatitic effects and the mechanical, chemical, thermal and electrical properties of the cured products. Since these will differ from application to application it is understandable that such a wide range of material is employed. 

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. 

A typical condensation system involves the reaction of a silanol-terminated polydimethylsiloxane with a multi-functional organosilicon cross-linking agent such as Si(RO)4 Figure 29.8). Pot life will vary from a few minutes to several hours, depending on the catalysts used and the ambient conditions. Typical catalysts include tin octoate and dibutyl tin dilaurate. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Neoprene AD-G. It is a Neoprene AD grafted with methyl methacrylate. It provides longer pot life (i.c. the time after which a mixture of two components in an adhesive maintains its properties) in two-part adhesives systems. 

Inhibitor Various organie or organosilieone types Delays eure at room temperature and inereases pot life... 

When formulating a silicone adhesive, sealant, or coating, based on hydrosilylation addition cure, one must consider the following properties of the uncured product pot life, dispensing technique, rheology, extrusion rate, cure performance. These characteristics directly affect the processing properties of the polymer base or crosslinker parts. The degree of cure conversion at the temperature of interest is determined by properties such as tack free time, cure profile and cure time. Once... 

Pot life is several hours versus several days for conventional non-reactive hot melts. A good reactive urethane is one which exhibits a viscosity rise of less than 10%/h. The slow increase in viscosity with urethane adhesives is due to chain extension via the slow reaction of the active hydrogen of the urethane groups with... 

Note that, in each case, water is split off to form each derivative. These blocked amines and blocked amino-alcohols can be stirred into isocyanate-terminated urethane prepolymers. Each system has a certain pot life. The pot life can vary from... 

The ketimine of isophorone diamine is formed by reacting it with methyl isobutylketone, splitting off water in the process. When said ketimine is added to an isocyanate-terminated prepolymer based on IPDI, a semi-stable system is established with a pot life of several hours. The ketimine is a Schiff base and thus can react even in the absence of water. The complexities and advantages of this system are reviewed by Bock and Halpaap [75] ... 

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