Stress environmental

Although a considerable amount of work has been reported on the effect of environmental conditions on bodily function, particularly with reference to thermoregulation (G5) and although there is a considerable literature on the body s response to injury (e.g., see bibliography of this review), little work has been done on the interaction of environmental conditions and injury. However, studies on the effects of environmental stress are of relevance to the problem of the metabolic response to injury. 

Heroux and Gridgeman (HIO) compared tissue weights in rats adapted to 30°C or to 6 0, and found that cold adaptation had no effect on brain, genitalia, and lung weights, but reduced growth of muscle, pelt, fat, skeleton, spleen, and thymus. Liver, gut, kidney, heart, and adrenals were hypertrophied at 6°C. 

At high environmental temperatures, food intake and oxygen consumption will fall, as will voluntary activity. Thus a reversal of the effect secondary to food intake noted for cold exposure may be expected. Whether or not adrenal activation occurs is of interest, since one tends to consider environmental temperatures other than those normally found in the individual s natural habitat to constitute a stress, but all environmental temperatures below body surface temperature will impose an obligatory heat loss on the animal, and may thus be considered stressful. 

Body surface temperature in the rat is about 34°C, depending on skin blood flow, environmental temperature, and the interpretation of surface. The results of Hale et al. (H2) that adrenal weight is lowest at 34°-35°C would support the above, insofar as adrenal weight may or may not reflect adrenal activity. 

Exposure to both cold (K3) and heat (34°C) (K4, K5) will lead to stimulation of corticosteroid secretion within 24 hours, but we have shown that exposure to 30° C results in plasma corticosterone levels below those of controls at 20°C in the rat (T4). 

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Propellants cast into rockets are commonly case-bonded to the motors to achieve maximum volumetric loading density. The interior of the motor is thoroughly cleaned, coated using an insulating material, and then lined with a composition to which the propellant binder adheres under the environmental stresses of the system. The insulation material is generally a mbber-type composition, filled with siUca, titanium dioxide, or potassium titanate. SiUca-filled nitrate mbber and vulcanizable ethylene—propylene mbber have been used. The liner generally consists of the same base polymer as is used in the propellant. It is usually appHed in a thin layer, and may be partially or fully cured before the propellant is poured into the rocket. 

PVDF is used in many diverse industrial applications for products that require high mechanical strength and resistance to severe environmental stresses. 

A case provides mechanical support and protection for the devices, interconnects, and substrate mounted in it it also helps to dissipate heat during component operation and offers protection to the contents of the package from environmental stresses, contaminants, and, in the case of hermetic packages, moisture. 

Mechanical properties of plastics can be determined by short, single-point quaUty control tests and longer, generally multipoint or multiple condition procedures that relate to fundamental polymer properties. Single-point tests iaclude tensile, compressive, flexural, shear, and impact properties of plastics creep, heat aging, creep mpture, and environmental stress-crackiag tests usually result ia multipoint curves or tables for comparison of the original response to post-exposure response. 

MSTM D1693, ed. Test Methodfor Environmental Stress-Cracking of Ethylene Plastics, Vol. 8.01, ASTM, Philadelphia, Pa., 1988. 

ISO 4599, Plastics Determination of Resistance to Environmental Stress-Cracking, Pent Strip Method, ISO, Geneva, Swit2edand, 1986. 

Resistance to Chemical Environments and Solubility. As a rule, amorphous plastics are susceptible, to various degrees, to cracking by certain chemical environments when the plastic material is placed under stress. The phenomenon is referred to as environmental stress cracking (ESC) and the resistance of the polymer to failure by this mode is known as environmental stress cracking resistance (ESCR). The tendency of a polymer to undergo ESC depends on several factors, the most important of which are appHed stress, temperature, and the concentration of the aggressive species. 

For reasons that are not fiiUy understood, PPSF exhibits generally improved compatibiUty characteristics over either PSF or PES in a number of systems. An example of this is blends of PPSF with polyaryletherketones (39,40). These blends form extremely finely dispersed systems with synergistic strength, impact, and environmental stress cracking resistance properties. Blends of PPSF with either PSF or PES are synergistic in the sense that they exhibit the super-toughness characteristic of PPSF at PSF or PES contents of up to 35 wt % (33,34). The miscibility of PPSF with a special class of polyimides has been discovered and documented (41). The miscibility profile of PPSF with high temperature (T > 230° C) polysulfones has been reported (42). 

Polyesters are known to be produced by many bacteria as intracellular reserve materials for use as a food source during periods of environmental stress. They have received a great deal of attention since the 1970s because they are biodegradable, can be processed as plastic materials, are produced from renewable resources, and can be produced by many bacteria in a range of compositions. The thermoplastic polymers have properties that vary from soft elastomers to rigid brittie plastics in accordance with the stmcture of the pendent side-chain of the polyester. The general stmcture of this class of compounds is shown by (3), where R = CH3, n = >100, and m = 0-8. 

Pacemaker Interfaces and Leads. Problems of existing pacemaker interfaces and pacemaker lead materials made from siUcones and standard polyurethanes are environmental stress cracking, rigidity, insulation properties, and size. 

Under mechanical and environmental stresses, composites are dimensionally stable. They maintain their shape and functionaHty, a critical requirement in such appHcations as dish antennas, constmction girders, and in appHance and business machines. Color and surface texture can often be molded into an FRP product for long lasting, low maintenance permanent surface appearance. Boats are a good example. The surface color is molded in and requires minimum maintenance, an advantage in saltwater environments. 

Levitt, "Responses of Plants to Environmental Stresses." Academic Press, New York, 1972,... 

The very low density materials (VLDPEs) introduced in the mid-1980s are generally considered as alternatives to plasticised PVC (Chapter 12) and ethylene-vinyl acetate (EVA) plastics (see Chapter 11). They have no volatile or extractable plasticisers as in plasticised PVC nor do they have the odour or moulding problems associated with EVA. Whilst VLDPE materials can match the flexibility of EVA they also have better environmental stress cracking resistance, improved toughness and a higher softening point. 

The susceptibility of low molecular weight grades to environmental stress cracking. 

Polypropylene appears to be free from environmental stress cracking problems. The only exception seems to be with concentrated sulphuric and chromic acids and with aqua regia. 

It is less resistant to aliphatic hydrocarbons than polyethylene and polypropylene and in fact pipes may be solvent welded. At the same time the resistance to environmental stress cracking is excellent. 

The homopolymer finds a variety of uses, as an adhesive component, as a base for chewing gum, in caulking compounds, as a tackifier for greases, in tank linings, as a motor oil additive to provide suitable viscosity characteristics and to improve the environmental stress-cracking resistance of polyethylene. It has been incorporated in quantities of up to 30% in high-density polyethylene to improve the impact strength of heavy duty sacks. 

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