Low Green Strength

MDI-based materials go through an initial cheesy state with very poor tear and tensile strengths. Once they have had their full cure, they are also very tough. 

RC failure at the polyurethane cement interface CP failure at the cover cement primer interface CM failure at the metal primer interface R failure in the polyurethane 

The polyurethane failure can be further broken down into a number of subgroups. These classifications are based on normal rubber, and that is why the R appears frequently. These include  

SR spotty polyurethane. This appears on the metal surface looking like splattered-on polyurethane. It is caused by either surface contamination with dust or the fact that the sprayed-on adhesive has dried as it leaves the spray nozzles. 

TR thin polyurethane. There is an even but very thin residue of polyurethane on the metal surface. It is normally found with either butyl or polyurethane stocks that are very highly oil (or plasticizer) 

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Cheesy appearance Often MDI systems Low green strength Insufficient cure Off ratio Poor mixing Incorrect temperatures... 

Following the extrusion, several alternative flow paths are possible. In most cases, because the pellets from expander-extruders are relatively wet and have low green strength, the discharge is carefully positioned onto a belt dryer, which dries-off excess moisture at minimum mechanical stress while the final binding mechanism is developed (e.g., by recrystallization of dissolved substances, increased viscosity of molasses. 

China clay - This is a ceramics industry term synonymous widi what is today ealled simply kaolin. China clay is substantially pme white or near-white kaolinite characterized by low plasticity, low green strength, and good fired whiteness. 

There will be promising applications for cement-free refractory castables in the future, too. The low green strength of these castables will be compensated for by superb properties in the hot state. 

A more recent application involves inner tubes which currently are made of compounds based on butyl rubber but which have the following disadvantages high cold flow and low green strength in the uncured state, large losses by hysteresis, low resilience and high permanent set in service. 

Polyester diols are often combined with polyether diols to provide green strength through crystallization or elevated r . Most prevalent and least expensive is hexamethylene diol adipate (HDA) with a Tm of about 60°C. A variety of polyesters are available with various levels of crystallinity — from wax-like to amorphous — and crystallization rate, and with values ranging well below 0°C to above room temperature. Polybutadiene diols are the most expensive and most hydrophobic. They provide low surface tension and thus good wet out of non-polar surfaces. 

Fig. 4.1.17 Graphic illustration of Forster-type resonance energy transfer from aequorin to Aequorea GFP. In the vessel at left, a solution contains the molecules of aequorin and GFP randomly distributed in a low ionic strength buffer. The vessel at right contains a solution identical with the left, except that it contains some particles of DEAE cellulose. In the solution at right, the molecules of aequorin and GFP are coadsorbed on the surface of DEAE particles. Upon an addition of Ca2+, the solution at left emits blue light from aequorin (Xmax 465 nm), and the solution at right emits green light from GFP (Xmax 509 nm).

In printing inks P.Y.173 it is a very green shade opaque yellow with excellent light, weather and solvent fastness properties, but low color strength. It is recommended for paste and liquid inks where high fastness properties are required as well as for water based decorative inks. 

Apart from the above three types there are custom built rubber products such as expansion joints, flexible cell covers and large size rubber foils for the caustic soda industry, and many inflatables, fabric reinforced products and thick moulded sheets for specialty applications in certain process plants. These are all hand formed in aluminium or cast iron moulds or forms by laying up process and then cured in autoclave. Here the flow of the un-vulcanized rubber during cure is not very important as the shape is already formed rather the green strength and the stiffness of rubber stock with a low scorch time are the important requisites. A rubber expansion joint made by a hand layup method and cured in autoclave is shown in the following figure 14.1. 

Another way of implementing the rules of green chemistry is the miniaturization of solid electrodes this offers many practical advantages, including a reduction in sample consumption. Furthermore, the significant decrease in resistance makes for easier voltammetric measurements in low-ionic-strength water samples.112... 

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