Heat curing adhesives

Heat-cured adhesives. Heat curing by convection in box ovens is the most widely used and simplestmethod for curing adhesives. Heat is applied until a temperature is reached at which polymerization occurs and continues until the polymerization is complete. Time-temperature curing schedules are developed for each adhesive for which optimum properties are achieved. F or each of their adhesives, suppliers provide several cure schedules that are considered equivalent relative to final properties. The user may also develop... 

Fig. 7.10. Stress versus strain characteristics of three lap joints and an unbonded test coupon. Joint A, 25 mm X 12 5 mm overlap B, 25 mm X 190 mm overlap C, 25 mm X 25 0 mm overlap. Adherend mild steel, 16 gauge. Adhesive heat cured toughened epoxide (Permabond ESP105). Test coupon 25 X 75 mm, 16 gauge mild steel of the type used to fabricate the joints represented by continuous line. The classical form of the elastic/plastic deformation of the unbonded test coupon is clearly seen. (A) This specimen does not fail until after the test coupon has become plastic. (B) Although possessing 50% more bond area, the load required to fail specimen B is not very much greater than that needed to fail A however, the toughness of the adhesive and the spare capacity of the initially unloaded central area are clearly illustrated by the ability of the joint to sustain a load even though it has cracked. (C) This example emphasises the point made with specimen B. A performance such as this makes it difficult to say whether the adhesive has failed the steel or the steel has failed the adhesive.

Silicone Heat-Cured Rubber. Sihcone elastomers are made by vulcanising high molecular weight (>5 x 10 mol wt) linear polydimethylsiloxane polymer, often called gum. Fillers are used in these formulations to increase strength through reinforcement. Extending fillers and various additives, eg, antioxidants, adhesion promoters, and pigments, can be used to obtain certain properties (59,357,364). 

The adhesive-coated veneers are then stacked in the correct order (thickness and number of pHes) to make the desired product and sent to a cold press. The pressure appHed by the cold press assures uniform adhesive distribution across the pHes and fliU adhesion later. From the cold press the assembly is moved to a hot press. The hot presses have up to 50 openings between steam heated plates. Depending on the thickness of the plywood being manufactured, one or more (up to three) layers of uncured plywood assembHes are loaded into each opening. The press is closed and pressure of 1.2—1.38 MPa (175—200 psi) and temperatures of 110—166°C are appHed for the period of time needed to cure the adhesive. The cured plywood panels are removed... 

Parallam, or laminated strand lumber (LSL) is a beam made by a continuous manufacturing process composed of bigger-size wood needles (very elongated wood particles) reassembled with a structural exterior grade adhesive, the favorite adhesive being isocyanates (pMDI) when heat-curing and PRFs when cold-curing. 

The air in the autoclave is slowly heated to the cure temperature of the adhesive. Heating rate is monitored and controlled to within a given range. Increasing the temperature too slowly can cause partial cure of the adhesive at lower temperature... 

Pretreatment primers. In this method of use the silane may be applied from a solvent solution, by vapour phase deposition or by plasma deposition although solvent application is the more usual. The solution usually contains water and silane at a concentration of 1-2 wt%. The applied film may be water washed before subsequent coating/bonding and/or heat cured. The solvent(s) used may be important in both the stability of the solution and the performance, particularly in the wet adhesion. It has been shown that the presence of water either in the solution or as a final rinse is important, particularly in the case of AAMS and presumably other silanes [1]. Other factors which are important include the concentration of silane the pH of the solution the thickness of the silane film deposited. 

In the Tiller study, adhesive was applied to a glass bottle and cured. Highly purified water was placed over the adhesive, heated at 50 °C for 3 days, and analyzed with gradient reversed-phase HPLC. An LC/ITMS with ESI was used to profile the polyesters in the adhesive extracts with full-scan mass spectra and corresponding product ion spectra triggered by an ion abundance that surpassed a threshold. 

Medium-molecular-weight PMTFPS with vinyl or hydroxyl end blocks are used for adhesives and sealants. They are cured either at ambient temperature (RTV-room temperature vulcanization) or at elevated temperature. One-part moisture-activated RTV sealants have been available commercially for many years. Because of then-very high resistance to jet engine fuels, excellent flexibility at very low temperatures, and high thermal stability, they have been used in both military and civilian aerospace applications.78 Two-part, heat-cured fluorosilicone sealants have been used in military aircraft applications and for sealing automotive fuel systems.79 Special class of fluorosilicone sealants are channel sealants or groove injection sealants, sticky, puttylike compounds, which do not cure. They are used to seal fuel tanks of military aircraft and missiles.75... 

Many epoxy adhesives are capable of being B-staged. A B-staged resin is one in which a limited reaction between the resin and hardener has taken place so that the product is in a semicured but solid state. In the B-staged state, the polymeric adhesive is still fusible and soluble. On additional heat curing, the adhesive will progress from the B stage to a completely cured state. This will usually be accompanied by moderate flow. 

The catalyst does not make up part of the final epoxy network structure or have a significant effect on the final properties of the cured resin. Thus, the final cured properties of the epoxy system are primarily due to the nature of the epoxy resin alone. Homopolymerization normally provides better heat and environmental resistance than polyaddition reactions. However, it also provides a more rigidly cured system, so that toughening agents or flexibilizers must often be used. In adhesive systems, homopolymerization reactions are generally utilized for heat cured, one-component formulations. 

Another characteristic difference between a two-component, room temperature curing epoxy adhesive system and a one-component, heat curing system is the shelf life. The shelf life of a one-component epoxy can be 3 to 6 months when stored at room temperature and may even require refrigeration or freezing for a practical shelf life. The shelf life of each component in a two-component system is generally 6 months to 1 year. 

There are several ways by which the formulator can moderately improve the heat or chemical resistance of room temperature curing epoxy adhesives. Using an elevated-temperature cure or a postcure will, of course, improve the temperature resistance by virtue of improved crosslink density. However, this section describes formulations that have been developed for moderately improved heat resistance after only a cure at room temperature. Optimal (heat-curing) high-temperature and chemically resistant epoxy adhesives are discussed in Chap. 15. 

Resolution Performance Products, Starting Formulation 4018, High Strength Room Temperature or Heat Cure Adhesive, Houston, Tx, August 2003. 

Heat curing epoxy systems may consist of one or two components. The nomenclature is similar to that employed with the room temperature curing adhesives (e.g., resin component, curing agent component, 2Ksystem, etc.). Many heat curing epoxy adhesives systems are liquids or pastes. However, heat curing systems also can be processed into solid adhesive forms (e.g., powders and films). This chapter describes the paste and liquid formulations solid adhesive forms are considered separately in Chap. 13. 

Several important factors must be considered relative to the elevated temperatures that are required to cure these adhesives. The curing temperature and time specified represent the temperature and time that the adhesive film actually experiences. For example, if a cure of 60 min at 150°C is recommended, this does not mean that the assembly should be simply placed in a 150°C oven for 60 min. The temperature is to be measured at the adhesive within the joint. A large assembly will act as a heat sink and may require substantial time for an adhesive within the joint to reach the necessary temperature. In this example,... 

Although room temperature curing epoxy adhesive can also be cured at elevated temperatures, there are important differences in morphology and internal stresses between a room temperature cured adhesive and a heat cured epoxy adhesive (see Chap. 3). One should not assume that the properties would be equivalent or better if the adhesive were cured at elevated temperature rather than room temperature. A number of factors need to be considered in such a deliberation. 

Other anhydrides such as dodecyl succinic anhydride (DDS A) or adducts of DDS A with polyglycols, can also be used for formulating heat cured epoxy adhesives. These have excellent electrical properties and good thermal shock resistance. Anhydride cured epoxies are also useful for bonding plastics, notably polyester such as Mylar.8... 

Epoxy adhesives are most commonly used as liquids or pastes. However, certain types of epoxies can be employed in the form of a solid. The components in these adhesives are mixed and processed to a stage where the resulting adhesive product is in a solid but still fusible (uncrosslinked) state. When the applied solid adhesive is heated, it melts, flows, and wets the substrate. Additional heating time then causes the adhesive to cure completely into a strong, thermosetting structure. 

Preformed sticks or tubes of solid epoxy may also be applied much as a solder is.6 To apply, the area to be bonded is heated until sufficiently hot to cause the stick solder to flow when drawn across the surface. After application, the adhesive is cured by conventional thermal techniques. If two substrates are to be bonded, both must be preheated prior to... 

EPOXY ADHESIVES THAT CURE BY INDIRECT HEATING... 

FIGURE 15.16 Time to failure versus stress for two adhesives in a warm, high-humidity environment, (a) One part, heat cured modified epoxy adhesive. (b) Flexibilized amine cured epoxy adhesive.43... 

Heat cured adhesive systems are generally more resistant than room temperature cured systems. 

Room temperature curing, two-part epoxy adhesives are usually considered to have a lower level of outdoor performance than the heat cured adhesives. Their performance, however, can be improved by heat treatment following the room temperature cure. 

Figure 15.22 shows the long-term effect of a heat cured one-part epoxy adhesive to various chemical environments. As can be seen, the temperature of the immersion medium is a significant factor in the aging properties of the adhesive. As the temperature increases, the adhesive generally adsorbs more fluid, and the degradation rate increases. 

TABLE 16.3 Effect of Surface Treatment on Aluminum Alloy Joints Bonded with a Heat Cured Epoxy Adhesive (EC-3443, 3M Company)6... 

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