Elevated temperature cure system

The resins are hardened in situ by mixing with an acidic substance just before application. A typical curing system would be four parts of toluene-p-sulphonic acid per 100 parts resin. The curing may take place at room temperature if the resin is in a bulk form but elevated temperature cures will often be necessary when the material is being used in thin films or coatings. 

Both room temperature and heat-curable epoxy adhesives are used in medical applications. The room temperature curing systems require metering and mixing, and the cure time is generally slow (several hours). Elevated-temperature cures could affect temperature-sensitive substrates. 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

The chemical structures of important amines for curing epoxy resins in adhesive systems are identified in Fig. 5.1. Diethylenetriamine (DETA), triethylenetetramine (TETA), ra-aminoethylpiperazine (AEP), diethylaminopropylamine (DEAPA), ra-phenylenediamine (MPDA), and diaminodiphenyl sulfone (DDS) are the most commonly used members of this class. They are all primary amines. They give room or elevated temperature cure at near stoichiometric ratios. Ethylenediamine is too reactive to be used in most practical adhesive formulations. Polyoxypropyleneamines (amine-terminated polypropylene glycols) impart superior flexibility and adhesion. 

The high elevated-temperature cures are damaging to adhesive systems due to a mismatch in thermal expansion coefficient that can occur between the epoxy and the substrate. The difference in rate of expansion when returning to room temperature from the cure temperature can lead to significant internal stress within the adhesive joint, which results in poor adhesion. 

Suitable curatives for the polysulfide-epoxy reaction include liquid aliphatic amines, liquid aliphatic amine adducts, solid amine adducts, liquid cycloaliphatic amines, liquid amide-amines, liquid aromatic amines, polyamides, and tertiary amines. Primary and secondary amines are preferred for thermal stability and low-temperature performance. Not all amines are completely compatible with polysulfide resins. The incompatible amines may require a three-part adhesive system. The liquid polysulfides are generally added to the liquid epoxy resin component because of possible compatibility problems. Optimum elevated-temperature performance is obtained with either an elevated-temperature cure or a postcure. 

Carboxy-terminated curative, such as CTBN, provides excellent toughening in part due to its miscibility in many epoxy resins. Phase separation during cure is required to obtain toughening, and generally the phase separation requires an elevated-temperature cure. However, by prereacting the CTBN with a portion of the epoxy to obtain an adduct, a room temperature curing toughened epoxy is possible. Adduction reduces the likelihood of early phase separation and maintains the solubility of the elastomer in the uncured resin system. 

Whereas most room temperature curing epoxy adhesives are cured with aliphatic amines, polyamides, or amidoamines, most elevated-temperature curing epoxy adhesives are cured with aromatic amines, modified aliphatic amines, alcoholic and phenolic hydroxyls, acid anhydrides, Lewis acids, and a host of other curatives. Latent curing agents, such as dicyan-diamide and imidazoles, are typically used in one-component epoxy adhesives systems. 

Many structural adhesives require heat as well as pressure to cure. Even with conventional room temperature curing systems, most often the strongest bonds are achieved by an elevated-temperature cure. With many adhesives, tradeoffs between cure times and temperature are permissible. Generally, the manufacturer will recommend a certain curing schedule for optimum properties. 

Formulation details are then presented in Chapters 11 through 14 for the various possible forms of epoxy adhesive systems room temperature and elevated-temperature curing liquids, pastes, and solids. The more or less unconventional forms of epoxy adhesives are also identified and discussed, since these are now achieving prominence in industry. These include uv and electron beam radiation curable, waterborne systems, and epoxy adhesives capable of curing via the indirect application of heat or energy. 

The rheological equipment requires certain capabilities in order to effectively measure composite adhesive resin systems. The transducer must have a dynamic range from 1 to 10 poise. This is a result of achieving a very low viscosity during an elevated temperature cure... 

The purpose of the study was a) to determine the effect of relative resin/hardener composition and ambient and elevated temperature curing schedules on the glass transition temperature, Tg, and on the degree of cure of the epoxy system, and b) to evaluate the agreement between the calculated degree of cure using the nth-order kinetic equation (1.1) (1, 2, 3) and the measured degree of cure. 

To improve high temperature stability over amine cured systems and to give better physical and electrical properties above their heat distortion temperatures, it has been general practice in epoxy resin systems to use anhydride curing agents with DGEBA epoxy resins (8 ). Most anhydride formulations require elevated-temperature cures with the ultimate properties dependent on postcures at temperatures of 150 C or higher. 

In room-temperature curing it is obviously necessary to add the resin to the reinforcement as soon as possible after the curing system has been blended and before gelation can occur. Benzoyl peroxide is most commonly used for elevated-temperature curing. It is generally supplied as a paste ( 50%) in a liquid such as dimethyl phthalate to reduce explosion hazards and to facilitate mixing. 

Thermosetting adhesives are provided as one- and two-part systems. The one-part systems usually require elevated temperature cure and have a limited shelf life. The two-part systems have longer shelf lives and can usually be cured slowly at room temperature, or somewhat faster at moderately higher temperatures. A disadvantage is their need for careful metering and mixing to make sure that the prescribed proportions are blended and that the resultant mixture is homogeneous. Once the adhesive is mixed, the useful life is limited. ... 

When the bonding is performed in a factory, the environmental parameters, such as temperature and humidity, may be controlled. If the bonding is performed on the construction site, environmental conditions can typically only be monitored, not controlled. Two component adhesive systems should be used in on-site bonding as, typically, they do not require elevated temperature cure. On-site bonding becomes impractical if the areas to be bonded are large. Elevated temperature cure requires an external heat source. In a factory this is typically an oven and on-site a radiator, a hot air blower or a heating blanket. The last three may also be used in a factory environment but maintaining the same level of temperature control as in an oven may not be possible (see also 6.2.6). 

Compared with aliphatic amine cures, the pot life of anhydride cures is usually long and exotherm is low. Elevated-temperature cures at up to 200°C are necessary and long post-cures are required to develop ultimate properties. Electrical and physical strength properties are good over a wide temperature range. Compared with amine-cured systems, anhydride cures offer better chemical resistance to aqueous acids, and less chemical resistance to some reagents. 

However, chemical resistance is inferior to that of aromatic amines. Becanse cycloaliphatic amines are less reactive than acyclic aliphatic amines, their nse results in a longer pot life and in the ability to cast larger masses. Unmodified cycloaliphatic amines require elevated temperature cure, but modified systems are RT-curable. Properly formulated, they can give an excellent balance of properties fast cure, low viscosity, low toxicity, good adhesion to damp concrete, and excellent color stability. They are, however, more expensive than other types of curing agents. 

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