Moisture absorbed from atmosphere

Irrespective of pretreatment procedure, optimum bond strengths can be attained if the laminates are dried before bonding to remove any moisture absorbed from the atmosphere. This should be carried out, immediately before joint assembly, in an air-circulating oven at a temperature at which no thermal damage will be imparted to the laminate. 

Taking up Moisture from Atmosphere. Adhesives that react by taking up atmospheric moisture remain fluid while in storage but absorb water after being applied and this initiates cross-linking, gelation, and hardening. 

Tendency of a substance to absorb moisture from its surroundings specifically, absorption of water vapor from atmosphere. 

ANHYDRIDE ARSENIQUE (French) (1303-28-2) Incompatible with acids, aluminum, halogens, rubidium carbide, strong alkalis, zinc. Gradually deliquesces (absorbs moisture from atmospheric air, becoming liquid) on exposure to air, forming arsenic acid. Contact with moisture, water, steam forms arsenic acid. Incompatible with sulfuric acid, caustics, ammonia, aliphatic amines, alkanolamines, amides, organic anhydrides, isocyanates, vinyl acetate, alkylene oxides, epichlorohydrin. Contact with acids or acid mists releases deadly arsine gas. Corrosive to metals in the presence of moisture. 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

A 300-ml. three-necked flask is equipped with a dropping funnel and Avith a short condenser, both isolated from atmospheric moisture by means of calcium chloride drying tubes. If the reaction is not to be carried out in a hood, a tube should lead from the drying tube at the top of the condenser to a trap for absorbing gaseous hydrogen chloride. 

This water absorption is caused by hydration of the sodium cation (12) Fig 7 shows the moisture absorbed by a silicate formulation when exposed to different relative humidities In this case the silicate ratio is 3 32 1 The slopes of the curves on Fig 7 correspond to the rate of moisture absorption from humid atmospheres It is important to notice that the absorption is linear with time, at least until the HpO/Na ratio equals one ... 

Although samples are vacuum-dried during the process of cellulose extraction (see above), it has been demonstrated that moisture is readily absorbed from the air (Edwards et ah, 1994). Furthermore, due to the small sample size, any moisture can have a significant effect on the final analysis. It is therefore important to adhere to the following procedure designed to remove all traces of atmospheric moisture that may cause sample contamination. 

In practice, it is necessary to avoid using amorphous resins in structural applications above, or even near to, the T. (This restriction applies to all thermosets, because they are invariably amorphous, but it does not apply to crystalline thermoplastics, otherwise polypropylene chairs, with a of around -15°C, could only be used safely in Alaska and in Siberian winters). A margin of safety of 20°C is required for reasonable durability, and a much larger one in humid conditions. Traces of water, or any other solvent, lower the Tg to below its usual value, so the maximum safe working temperature of a load-bearing amorphous resin is lower in humid conditions than in a dry environment. There is a widely quoted rule of thumb that each 1% moisture absorbed by the matrix lowers the Tg by 20°C. This comes from epoxy resin data in wet atmospheres and different figures apply to other resins and liquids. 

Storage Hygroscopic readily absorbs moisture and CO2 from atmosphere store under nitrogen Uses Phosphor activator infrared-absorbing glass... 

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