Cure time and temperatures

Unsaturated polyester resins predominate among fiber-reinforced composite matrices for several reasons. A wide variety of polyesters is available and the composites fabricator must choose the best for a particular appHcation. The choice involves evaluation of fabrication techniques, temperatures at which the resin is to be handled, cure time and temperature desked, and requked cured properties (see Polyesters, unsaturated). 

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

Thermocouples were placed in the curing part so that the model could be compared to the actual process. The model accurately predicted the cure time and temperature curing profiles for several parts with different geometries and curing conditions. 

Observed monomer concentrations are presented by Figure 2 as a function of cure time and temperature (see Equation 20). At high monomer conversions, the data appear to approach an asymptote. As the extent of network development within the resin advances, the rate of reaction diminishes. Molecular diffusion of macromolecules, initially, and of monomeric molecules, ultimately, becomes severely restricted, resulting in diffusion-controlled reactions (20). The material ultimately becomes a glass. Monomer concentration dynamics are no longer exponential decays. The rate constants become time dependent. For the cure at 60°C, monomer concentration can be described by an exponential function. 

Crosslinking time is directly dependent on the rate of decomposition of the peroxide. The effectiveness of the overall crosslinking reaction is thus dependent on the type of peroxide and polymer radicals produced during the process. Cure time and temperature can, in a peroxide cure system, be determined solely from knowledge of the rate of peroxide thermal decomposition. 

Additional drawbacks to the use of polyimide insulators for the fabrication of multilevel structures include self- or auto-adhesion. It has been demonstrated that the interfacial strength of polyimide layers sequentially cast and cured depends on the interdiffusion between layers, which in turn depends on the cure time and temperature for both the first layer (Tj) and the combined first and second layers (T2) [3]. In this work, it was shown that unusually high diffusion distances ( 200 nm) were required to achieve bulk strength [3]. For T2 > Tj, the adhesion decreased with increasing T. However, for T2 < Tj and Tj 400 °C, the adhesion between the layers was poor irrespective of T2. Consequently, it is of interest to combine the desirable characteristics of polyimide with other materials in such a way as to produce a low stress, low dielectric constant, self-adhering material with the desirable processabiHty and mechanical properties of polyimide. 

It is well known that curing times and temperatures influence the final properties of thermoset epoxy resins.(2 )... 

One possible explanation for different results between isothermal and dynamic analysis was first given by Prime [89]. It is related to the fact that the degree of conversion is a function of the curing time and temperature therefore, we have Equation 2.26 ... 

A very wide range of curing times and temperatures exists for epoxy resin systems. Therefore, they offer a great deal of design flexibility for use both as structural bonding adhesives and protective surface coatings. 

This is due to over-vulcanization. The remedy is to reduce cure times and temperatures or modify the compound formulation to reduce the speed of cure. Tearing can also be due to method of removal of the product from the mould while it is in hot condition. Tearing can be eliminated by removing the products from the mould after sufficient cooling or by careful and slow removing. 

The epoxy/siloxane/PACM-20 mixture was poured into a hot (120 °C) RTV-silicone mold of the precise shapes to be used for solid-state testing. The mixture was cured at 160 °C for 2.5 hours. The curing time and temperature chosen were considered to provide enough mobility for network formation. This conclusion was partially based on earlier studies which found a glass transition temperature of 150 °C for Epon 828/PACM-20 3S). 

Some polysiloxanes are curable with lead monoxide, with a consequent reduction in both curing time and temperature. High-frequency electrical energy vulcanizes in one case at least. Zirconium naphthenate imparts improved resistance to high temperatures. Barium salts are said to prevent blooming. Sulfur dichloride is also used. Some resins are solidified by pressure vulcanization, using di-f-butyl peroxide. Improvements are to be found in lower condensation temperatures and shorter times of treatment... 

FIGURE 1.1 Characteristics of a particular room temperature curing epoxy adhesive under different cure time and temperature conditions.1... 

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. 

Cure time and temperature are as required and measured at the adhesive bond. 

The epoxy resin data and the post-cure data, taken together, show that the dipolar relaxation is associated with the temperature dependence of the polymer chain mobility in the vicinity of the glass transition. The WLF analysis of the dipolar relaxation during cure has not been carried out. In order to complete the analysis, correlated measurements of Tg, extent of cure, and dielectric properties must be made as functions of cure time and temperature. In the absence of such definitive studies, various indirect methods have been employed to analyze dielectric relaxations in curing systems, as described below. 

Hutchings, D. Kuzmak, B. Sakr, A. Processing considerations for an EC latex coating system influence of curing time and temperature. Pharm. Res. 1994, 11 (10), 1474-1478. 

Because the fracture toughness depends both on cure time and temperature, the arbitrary selection of time and temperature for accelerated tests may not be appropriate for reliable prediction of longterm service life of joints (J7). In order to reduce test variability and improve the durability prediction of adhesive joints, it would be necessary first to control the cure temperature and time required to produce a level of fracture toughness that does not change further (14). The study is thus an excellent example of a multidisciplinary approach combining chemistry, fracture mechanics, and wood science in the investigation of the adhesive bonding of wood. 

Effect of Curing Time and Temperature on Tg and Degree of Cure. Samples having different H/R compositions were also cured with schedule B. The dependence of Tg on composition is shown in Figure 3 (open squares). The same samples were subsequently heated a second time in the calorimeter to determine the Tg s (closed squares) of the now fully cured samples (no measurable reaction enthalpy). The Tg s of these samples are the same as under schedule A (solid line). 

Cohesive forces Act between molecules in the adhesive layer. Sufficient cohesive strength depends on the observance of the determined curing time and temperature as well as on a homogeneous mixture of the adhesive components. 

Key properties required of the polymer system before application are acceptable pot life compatibility with other components and viscosity, cure time, and temperature suitable for particular applications. After application and curing, the coatings should have low moisture absorption, low moisture permeability, and good resistance to other outside influences that may tend to deteriorate the performance of the device. 

The other factors which affect the properties of crosslinked polymers such as the molar ratio of the functional groups, the curing time and temperature etc. were kept constant for the individual classes of the curing agents. 

Automatic Bonding Evaluation System (ABES) The ABES consists of a small press and a tiny testing machine in a single unit. It enables bonds to be formed under highly controlled conditions the joints that contain the bonds which are to be measured are pressed against heated blocks for a certain time, cooled within a few seconds, and pulled immediately thereafter in shear mode. Repetition of this procedure at different curing times and temperatures yields the points (a point for each specimen) of a near-isothermal strength development curve. 96,97... 

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