Moisture resistance

Moisture is the substance that often causes the greatest difficulties in terms of environmental stability for bonded or sealed joints. Water can be an exceptional problem because it is very polar and permeates most polymers. Other common fluids, such as lubricants and fuels, are of low or zero polarity and are not as likely to permeate and weaken adhesive or sealant joints. 

Moisture can degrade a cured adhesive joint in three distinctive ways. 

The hostility of certain moisture environments can be seen in Fig. 15.11 Aluminum joints were bonded with room temperature curing epoxy-polyamide adhesive and aged in a hot, wet (tropical) environment and in a hot, dry (desert) environment. Excellent durability is achieved under dry conditions while significant degradation is caused by the wet conditions. 

Ambient moisture can also affect certain types of uncured adhesive, either as it is being mixed and applied to a substrate or as it is stored in a container waiting to be applied. The degradation mechanism before cure of the adhesive is discussed in Chap. 3. 

Plastic foams have advantages over other thermal insulation when exposed to moisture, particularly where subjected to a combination of thermal and moisture gradients. In some cases the foams are also exposed to freeze-thaw cycles. The behavior of plastic foams has been studied under laboratory and field conditions. 

In plastic roof insulations under controlled thermal gradients, the moisture gains found in polyurethane are greater than those of bead polystyrene, and much greater than those of extruded polystyrene [9]. 

We conclude from findings on moisture absorption and freeze-thaw resistance of various insulations, and the effect of moisture on thermal performance, that in protected-membrane roofing applications the order of resisting moisture pickup is extruded polystyrene polyurethane molded polystyrene [57]. Water absorption values for insulation in use for 5 years were 0.2vol% for extruded polystyrene, 5vol% for polyurethane without skins, and 8-30 vol% 

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Tetrahydrofurfuryl alcohol is used in elastomer production. As a solvent for the polymerization initiator, it finds appHcation in the manufacture of chlorohydrin mbber. Additionally, tetrahydrofurfuryl alcohol is used as a catalyst solvent-activator and reactive diluent in epoxy formulations for a variety of apphcations. Where exceptional moisture resistance is needed, as for outdoor appHcations, furfuryl alcohol is used jointly with tetrahydrofurfuryl alcohol in epoxy adhesive formulations. 

Specifications, Standards, Quality Control, and Health and Safety Factors. Formerly, there was an Insulation Board Institute representing the insulation board industry, but the decline in the market and number of producers has led to its demise. Currently (ca 1997), the industry is represented by the American Hardboard Association (AHA). Specifications and standards are found in American National Standards Institute (ANSI) standard for CellulosicFiberboard (7). The standard includes descriptions of the various types and classes of ftberboard, as well as requirements for physical and dimensional stabiUty properties. QuaUty control tests are limited to a few basic strength and stabiUty tests, including bending strength, bond strength, and moisture resistance. 

Polyacrylamide powders are typically shipped in moisture-resistant bags or fiber packs. Emulsion and solution polymers are sold in dmms, tote bins, tank tmcks, and tank cars. The transportation of dry and solution products is not regulated in the United States by the Department of Transportation, but emulsions require a DOT NA 1693 label. 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Residential Construction. Owing to rising energy costs, the cost and low thermal conductivity are of prime importance in wall and ceiling insulation of residential buildings. The combination of insulation efficiency, desirable stmctural properties, ease of appHcation, abiHty to reduce air infiltration, and moisture resistance has led to use of extmded polymeric foam in residential constmction as sheathing, as perimeter and floor insulation under concrete, and as a combined plaster base and insulation for walls. 

Buoyancy. The low density, closed-ceUed nature of many ceUular polymers coupled with their moisture resistance and low cost resulted in their immediate acceptance for buoyancy in boats and floating stmctures such as docks and buoys. Since each ceU in the foam is a separate flotation member, these materials caimot be destroyed by a single puncture. 

Expanded polystyrene bead mol ding products account for the largest portion of the drinking cup market and are used in fabricating a variety of other products including packaging materials, iasulation board, and ice chests. The iasulation value, the moisture resistance, and physical properties are inferior to extmded boardstock, but the material cost is much less. 

Polyimides of 6FDA and aUphatic diamines with good low temperature processkig and low moisture swelling are known to be useful as hot-melt adhesives (109). Aluminum strips bonded by this polymer (177°C/172 kPa (25 psi) for 15 min) exhibited a lap-shear strength of 53 MPa (7690 psi) at room temperature and 35 MPa (5090 psi) at 100°C. The heat- and moisture-resistant 6F-containing Pis useful ki electronic devices are prepared from... 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Bitumen Ionomers. Moisture-resistant asphalts (qv) have been prepared by reaction of metal oxides with acid-functionalized bitumens (75). Maleic anhydride or sulfur trioxide/trimethylamine complexes have been used successfully for introduction of acid groups into asphaltic bitumens. 

Grade XXP is better than Grade XX in electrical and moisture-resisting properties and more suitable for hot punching. It is intermediate between Grades XP and XXXP in punching and cold flow characteristics. 

Grade G-9, glass fabric with moisture-resistant melamine resin binder, is similar to Grade G-5 but with better electric strength properties under wet conditions. Electrical appHcations should be limited to operating temperatures of 50°C (122°F) or less. 

The polymeric encapsulating resin, modified by additives, must possess adequate mechanical strength, adhesion to package components, manufacturing and environmental chemical resistance, electrical resistance, GTE matching, as weU as thermal and moisture resistance in the use-temperature range. 

Acryhc resins have excellent moisture resistance, dielectric properties, and reworkabiUty, but poor abrasion resistance. Their dielectric constant, which decreases with increasing frequency, makes them attractive candidates for high frequency appHcations. 

Laminates. Laminate manufacture involves the impregnation of a web with a Hquid phenoHc resin in a dip-coating operation. Solvent type, resin concentration, and viscosity determine the degree of fiber penetration. The treated web is dried in an oven and the resin cures, sometimes to the B-stage (semicured). Final resin content is between 30 and 70%. The dry sheet is cut and stacked, ready for lamination. In the curing step, multilayers of laminate are stacked or laid up in a press and cured at 150—175°C for several hours. The resins are generally low molecular weight resoles, which have been neutralized with the salt removed. Common carrier solvents for the varnish include acetone, alcohol, and toluene. Alkylated phenols such as cresols improve flexibiUty and moisture resistance in the fused products. 

The pelargonic acid by-product is already a useful item of commerce, making the overall process a commercial possibiUty. The 13-carbon polyamides appear to have many of the properties of nylon-11, nylon-12, or nylon-12,12 toughness, moisture resistance, dimensional stabiUty, increased resistance to hydrolysis, moderate melt point, and melt processibiUty. Thus, these nylons could be useful in similar markets, eg, automotive parts, coatings, fibers, or films. Properties for nylon-13,13 are = 56 (7 and = 183 (7 (179). 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

I Jnqiiaternized DMAEMA copolymers afford resins that are mildly cationic and less hydroscopic. They provide more moisture-resistant fixatives (146). further refinements have been accompHshed by adding a third comonomer such as A/-vinylcaprolactam (V Cl). In this case, replacement of VP with VCl results in a terpolymer (VP/VCl/DMAEMA) with even greater high humidity moisture resistance and cud retention. 

Urea—formaldehyde (UF) resias commonly were used ia the past. However, because of the lack of moisture resistance and the potential for the resias to hydroly2e ia the presence of moisture and decompose iato urea and formaldehyde, they are not used as much now. Governmental regulations are under development that eliminate the use of UF resia ia wood products. This would limit the exposure of the pubHc to formaldehyde, a Hsted carciaogen, formed by the decomposition of UF resia. Today most wood products use pheaol—formaldehyde (pheaoHc) resias, but urethane-based resias are becoming more common. 

The standard sized sheets are four ft (1.22 m) wide and from 8 to 16 ft long. Sheets are available in thicknesses from j up to 1 in. (0.63 to 2.5 cm), but the most common thicknesses are S and 5/8 in. (1.3 and 1.6 cm). There are special products for bathrooms, such as moisture-resistant board (3% of market) and sheathing appHcations (2% of market), but the vast majority of the product sold is either standard board (60% of market) or the fire-rated Type X board (29% of market). Type X (1.6 cm thick) is the basis of most of the one-hour fire walls in buildings built since the 1960s. 

Amino resins are lighter in color and have better tensile strength and hardness than phenoHc resins their impact strength and heat and water resistance are less than those of phenoHcs. The melamine—formaldehyde resins are harder and have better heat and moisture resistance than the urea resins, but they are also more expensive. The physical properties of the melamine—formaldehyde laminates are Hsted in Table 1. 

Plastic laminated sheets produced in 1913 led to the formation of the Formica Products Company and the commercial introduction, in 1931, of decorative laminates consisting of a urea—formaldehyde surface on an unrefined (kraft) paper core impregnated with phenoHc resin and compressed and heated between poHshed steel platens (8,10). The decorative surface laminates are usually about 1.6 mm thick and bonded to wood (a natural composite), plywood (another laminate), or particle board (a particulate composite). Since 1937, the surface layer of most decorative laminates has been fabricated with melamine—formaldehyde, which can be prepared with mineral fiUers, thus offering improved heat and moisture resistance and allowing a wide range of decorative effects (10,11). 

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