Nylon moisture absorption

Nylon 6 and 6/6 possess the maximum stiffness, strength, and heat resistance of all the types of nylon. Type 6/6 has a higher melt temperature, whereas type 6 has a higher impact resistance and better processibility. At a sacrifice in stiffness and heat resistance, the higher analogs of nylon are useful primarily for improved chemical resistance in certain environments (acids, bases, and zinc chloride solutions) and for lower moisture absorption. 

Nylon-12,12. Nylon-12,12 [36497-34-4] [36348-71-7] was introduced into the marketplace by Du Pont in the late 1980s (174). This polymer possesses very low moisture absorption, high dimensional stabihty, and excellent chemical resistance, with a moderately high melt point (T = 185° C)... 

A number of high melting poiat semiaromatic nylons, iatroduced ia the 1990s, have lower moisture absorption and iacreased stiffness and strength. Apart from nylon-6 /6,T (copolymer of 6 and 6,T), the exact stmcture of these is usually proprietary and they are identified by trade names. Examples iaclude Zytel HTN (Du Pont) Amodel, referred to as polyphthalamide or PPA (Amoco) and Aden (Mitsui Petrochemical). Properties for polyphthalamide are given ia Table 2. A polyphthalamide has been defined by ASTM as "a polyamide ia which the residues of terephthaUc acid or isophthahc acid or a combination of the two comprise at least 60 molar percent of the dicarboxyhc acid portion of the repeating stmctural units ia the polymer chain" (18). 

The nylons are hygroscopic. Figure 18.16 shows how the equilibrium water absorption of different nylons varies with humidity at room temperature. Figure 18.17 shows how the rate of moisture absorption is affected by the environmental conditions. 

Figure 18.17. Effect of environmental conditions on rate of moisture absorption of nylon 66 ( 4 in...

Compared with nylon 66 fibres, the polyurethane fibres (known as Perlon U) have a tensile strength at the higher end of the range quoted for nylon 66, they are less prone to discolouration in air, are more resistant to acid conditions and they have a lower moisture absorption. On the debit side they are less easy to dye, are hard, wiry and harsh to handle and have too low a softening point for many applications. They are currently of little importance but have found some use in bristles, filler cloths, sieves and a few other miscellaneous applications. 

Other nylons are made by varying the molecular length of the diamines and the dibasic acids Nylon-fi. in u.ses sebacic acid (10 carbon atoms), nylon-11 uses an acid from castor oil, and nylon-12 uses butadiene. These variations decrease moisture absorption. Other variations use amines with a ring structure, e.g., the aromatic nylons to give polymers with softening points above 577 F,... 

Moisture absorption of nylons differs according to the distance between the amide groups. For example, nylon 4 has a higher moisture absorption than most other nylons, and it is approximately similar to that of cotton. This is a result of the higher hydrophilic character of nylon 4. 

There is another type of condition that results from exposure to high humidity. The alteration in electrical properties caused by moisture absorption in nylon and phenolics is reversible. When the moisture content is decreased, the properties of the materials recover to close to the original values. In some instances the exposure to moisture and electrical fields can cause irreversible damage that can lead to failure. 

Nylon-12,12. Nylon-12,12 [36497-34-4], [36348-71-7] was introduced into the marketplace by Du Pont in the late 1980s (174). This polymer possesses very low moisture absorption, high dimensional stability, and excellent chemical resistance, with a moderately high melt point (Tm = 185° C) (175). Its manufacture begins with the formation of dodecanedioic acid produced from the trimerization of butadiene in a process identical to that used in the manufacture of nylon-6,12. The other starting material, 1,12-dodecanediamine, is prepared in a two-step process that first converts the dodecanedioic acid to a diamide, and then continues to dehydrate the diamide to the dinitrile. In the second step, the dinitrile is then hydrogenated to the diamine with hydrogen in the presence of a suitable catalyst. 

Long-chain a,o)-dicarboxylic acids have been condensed with diamino-isohexides, forming linear polyamides suitable for producing fibers having a silklike texture and higher moisture-absorption properties than nylon-like polymers.206... 

All nylons absorb some moisture from environmental humidity. Moisture absorption characteristics must be considered in designing and joining these materials. They absorb from 0.5 to 2 percent by weight of moisture after 24-h water immersion. Freshly molded objects contain less than 0.3 percent moisture since only dry molding powder can be successfully molded. Once molded, these objects absorb moisture when they are exposed to humid air or water. The amount of absorbed moisture increases until an equilibrium condition is reached based on the relative humidity of the environment. Equilibrium moisture contents of two commercial nylon resins for two humidity levels are as follows ... 

Nylon 6/6 is the most widely used, followed by nylon 6, with similar properties except that it absorbs moisture more rapidly and its melting point is 21C (70F) lower. Also, its lower processing temperature and less crystalline structure result in lower mold shrinkage. Nylon 6/6 has the lowest permeability by gasoline and mineral oil of all the nylons. The 6/10 and 6/12 types are used where lower moisture absorption and better dimensional stability are needed. Nylons 11 and 12 have better dimensional stability and electrical properties than the others because they absorb less moisture. These more expensive types also are compounded with plasticizers to increase their flexibility and ductility. With nylon toughening and technology advancements supertough nylons became available. Their notched lzod impact values are over 10 J/m (20 ft-lb/in), and they fail in a ductile manner. 

The effect of moisture on nylon 6 resins is well-known and well-documented. (15) Because of this, the susceptibility of nylon block copolymers to moisture absorption was expected. Evaluation of requirements for exterior automotive body panels and other applications requiring dimensional stability indicated that early nylon block copolymer formulations would not meet specifications... 

The use of polyol rubber blocks In NBC gave unexpected Increase In moisture absorption over that of nylon 6. The effect of absorption on linear expansion Is shown In Figure 7 for three types of polyol blocks. Polyol level was 20% of copolymer. Polyols A and B not only Increased the total moisture absorption, but greatly Increased the rate of absorption as well. Reinforcement with glass fiber will give further improvement in dimensional stability, as illustrated below, but the degree of improvement is Insufficient to meet specifications for critical applications. 

Many different varieties of polyamides have been produced by varying the monomer composition. Variations include nylon 6,9 nylon 6,10 and nylon 6,12 (made from the 9-, 10-, and 12-carbon dicarboxylic acids, respectively) and nylon 11 and nylon 12 (via the self-condensation of 11-aminoundecanoic acid and lauryllactam, respectively). These specialty nylons exhibit lower moisture absorption—only one-third or one-fourth that of nylon 6 or nylon 6,6. 

Two of the most common concerns associated with nylon polymers are brittleness at low temperatures and moisture absorption. For example, a nylon material dried to a moisture content of 0.20% may flow 30-40% faster than the same material dried 10 times better, to 0.02% [21]. In some cases it could be taken as a sign of the polymer degradation. 

However, it is moisture absorption that gives nylon its toughness. Upon initial injection molding manufactured nylon profiles are typically very hard and brittle. After they left to cure, they absorb moisture from the air and become very impact resistant. The curing (or softening ) period for nylon profiles takes 2-3 weeks. Clearly, at outdoors conditions it usually takes much less time. 

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