Under cure

Materials and additives that are chemically basic in nature have a detrimental effect on the curing of cationic-initiated epoxy systems. These substances can either stop the curing mechanism completely or produce under-cured polymers. Therefore such additives as amines or imides that are known to be adhesion promoters cannot be used in the EB-curable epoxy adhesive formulations. 

In dissimilar rubber/rubber blends, vulcanization ingredients also preferentially end up in only one of the phases, resulting in overcuring of this phase and under-curing of the other one. An example of this effect is the solubility of insoluble sulfur, which is much lower for NBR than for SBR and EPDM. 

Volatile substances in the rubber or compounding ingredients or moisture can cause porosity. Insufficient stock of rubber in the mould and under cure also cause porosity. To prevent this defect from occurring, avoid use of raw materials containing volatile materials, test all raw materials for moisture content, allow the solvents or adhesives to dry up completely, check up the volume and shape of the finished product and increase curing pressure if feasible. 

Porosity-generally inside the centre Under cure and reduced pressure. Increase time, temperature and pressure. 

Under cure - A low state of cross linking. Often an under cured lining will be low in durometer hardness and fluid and acid resistance will be poor. 

It is quite probable that the resin is insufficiently cured at the junction between latewood surfaces because the water has not been efficiently removed from the film. One observation which substantiates this is that when the specimens are heated, the sheer strength and wood failure values both increase, and the distinction between earlywood and latewood disappears. This definitely indicates that the problem is due to under cure. 

At one time formability was obtained by under-curing the laminate at the pressing stage, but this had several disadvantages ... 

However, improvements in plasticized resin systems have obviated to a large extent the need for under-curing and overcome such deficiencies. 

From previous accelerated and real time ageing studies, it is known that rubber compounds exhibit complex property changes over time (1,2). Oftentimes, this can lead to less than desirable characteristics in the rubber, such as increased stiffness, embrittlement, poor abrasion characteristics, and others. In thin latex rubber products, it is common practice to under cure the rubber. As the product ages, certain properties will tend to improve for a while before starting a decline. For example, latex rubber condoms are stored in hermetically sealed packages, which limit exposure to oxygen, ozone and UV therefore, their ageing characteristics are more predictable and less pronounced than tires or footwear outsoles, since they can establish a three to five year shelf-life (3). 

It is hypothesized that improved ageing characteristics of rubber compounds could be facilitated by reducing the amount of sulfur and/or under curing the... 

The cure times are determined via an Eekon Rheometer. An uncured sample is placed between two heated discs (150°C). As the sample cures, the viscosity increases. The cure condition, e.g. T90, is determined by the time required for the viscosity to increase to 90% of the difference between the minimum and maximum values. Samples cured at T60 and T75 represent various degrees of under curing. Samples cured at T90 and T90+1 (T90 plus one minute) represent optimal curing. To make an adequate matrix of conditions, each of the four portions was cured at two conditions, either T60 and T90, or T75 and T90+1. Current production would be represented by 100% T90+1 , as a bold X in Table I. 

The most surprising result for the baseline conditions was that severe under curing (T60) and wide variations in sulfur content did not produce any out-of-spec material. This result was not anticipated, and showed that the specification may not identify poorly formulated and/or processed materials. 

It is immediately apparent from the previous figures that the lower sulfur content (70% of standard) provides a high level of consistency and superior tensile strength when compared to the compounds with higher sulfur content. This was apparent at a cure of T90+1 as well as the under cured, T75, sample. 

Dual Cure. Films were prepared for Dynamic Mechanical Analysis (DMA). All films were cast on release paper with a 4.5 mil draw down bar, and partially cured with two 200 watt/inch lamps at half power and a belt speed of 200 ft/min. The films were intentionally under cured to facilitate cutting with minimal flaws. After the films were cut into ii-inch test pieces, they were cured with two 200 watt/inch lamps at 100 ft/min, equal to 260 millijoules/cm dose. The instrument used for the DMA work was a Rheometrics RSA II Solids Analyzer. All tests were made at a frequency of 11 hz with a nominal strain of 0.05%, under nitrogen. Both temperature scans, at 2°C/minute, and isothermal runs were made. 

Methods of producing FRP laminates with polyesters have been described in Chapter 2. The major process today is the hand layup technique in which the resin is brushed or rolled into the glass mat (or cloth) by hand (see Figure 2.44). Since unsaturated polyesters are susceptible to polymerization inhibition by air, surfaces of the hand layup laminates may remain under-cured, soft, and, in some cases, tacky if freely exposed to air during the curing. A common way of avoiding this difiiculty is to blend a small amount of paraffin wax (or other incompatible material) in with the resin. This blooms out on the surface and forms a protective layer over the resin during cure. 

Curing agents are intentional additives and are used to crosslink resinous systems to improve bulk properties, and ultimately to reach the thermoset state. For under-cure of the system or excess application of curing agents, so that some of the curing agents remain, migration of these chemicals (which are toxic to different extents) may occur. 

Health hazards (mostly allergens) observed with UP composites are usually either due to non-crosslinked UP, mainly due to remnants as mentioned previously (such as, styrene, cobalt naphthanate, phthalates or tricresyl phosphate, benzoyl peroxide and other catalysts) or to the under-cured resin (i.e., UP automobile repair putty). Benzoyl peroxide is known to be a strong skin irritant [70]. 

Under-cured Cure timer setting Check... 

The migration ofcuratives was observed to happen very quickly (e.g., 3 satl50°C). A diffusion gradient may be produced between the dissimilar elastomers well before any vulcanization occurs. The practical consequences of the solubility and diffusion differences in elastomer blends are curative imbalance between the component phases and associated over- and under-cure. 

Lewis and Bronsted acids, activate the epoxy ring toward ring opening by various nucleophilic species, most often hydroxyl or other epoxy groups (Eqs. 2-4). Both types of curatives can take the form of catalytic species, such as tertiary amines and Lewis acids, or coreactants, such as primary amines, mercaptans, and dicarboxylic acids. When the curatives are catalytic species, the properties of the cured adhesive are due primarily to the epoxy resin and the stability/activity of the catalyst in the resin under cure conditions. Coreactant curatives offer much greater latitude in choosing the final cured adhesive properties because the physical characteristics of the... 

Since rehabilitation is primarily conducted under ambient conditions, there is also potential for under-cure or slow progression of cure of the adhesives. Thus, in situations where the adhesive cured in ambient conditions does not produce a fully cured system which consequently has an inadequate glass transition temperature, a postcure procedure at a temperature above the glass transition temperature of the cured adhesive will be necessary to achieve the desired result (Custodio et al., 2011 Custodio, 2009). 

Figure 3.13 Pyrogram of (a) under cured rubber 30 gg (b) optimally cured rubber 20 pg (c) over cured rubber 20 pg - Continued overleaf. (Source Author s own files)...

Under-cure degree of cure that is less than optimum. May be evidenced by tackiness, loginess or inferior physical properties. 

However, at higher thickness the strip remains under-cured as the time of bonding is kept constant resulting in a reduction in bond strength. 

Under Cure. The reqnirement of full cure is relatively easy to obtain if proper cure time and temperatnre are nsed. One measure of the cure level is Tg. Periodic measurements of Tg are an excellent check for material and process consistency. Another way to check cure is to make two consecutive measurements of Tg If the epoxy is only partially cured, it will continue to cure during the first measurement and a higher Tg will be detected on the second measurement. A shift inTg of more than 5°C is an indication of under cure. This measurement is typically performed on a TMA. Figure 27.39 shows an example of an epoxy TMA run note that the second TMA run has a delta of 3°C. 

From an analytical perspective, the erosslinking materials and process should be well understood. A reactive peroxide, for example, will generally not be evident in the analysis of a polymer crosslinked with this material. This will be evident when not fully decomposed during a cure process, for example, and the presence of non-reaeted peroxide may be a valuable indieation of an under-cure condition. Typically, the deeomposition products of dicumyl peroxide are what will be found analytically. These include aeetie aeid, eumyl aleohol, acetophenone, and others. Heating of most thermoplastics to a level that initiates deeomposition of the curing agent would also oxidize the base polymer. It is therefore neeessary to proteet the polymer with an antioxidant. GC/MS analysis of a typical peroxide-crosslinked polyethylene will reveal the peroxide decomposition products, an antioxidant, and no trace of the original peroxide. 

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