what does water absorption factor mean in plastics? 5

[msbharaj]

What does water absorption mean? Suppose with 1% factor will the volume increase by 1% ot the weaight? And where does this water absorbed, in the molecules or with in the gap ? Any good links for the explanation will be helpfull.

 

[CoryPad]

According to ASM Engineered Materials Handbook Volume 2: Engineering Plastics, water absorption is defined as: "[t]he ratio of the weight of water absorbed by a material to the weight of the dry material. From the same source, "[t]he primary value of this data is to assess qualitatively the sensitivity of a plastic to water. The higher the absorption level, the more concern the designer should have about potential changes in mechanical and electrical propeties, as well as part dimensions."

This condition can be measured using ASTM D570-98, Standard Test Method for Water Absorption of Plastics.

The water moves into the free volume between the polymer chains, sometimes causing noticeable volume expansion (swelling).

The ASM Handbook is available at:

http://www.asminternational.org

The ASTM standard is available at:

http://www.astm.org

 

[patprimmer]

Water absorbtion applies to some extent to many plastics, but is mostly negliable.
The plastic most often affected to a noticable degree by water is nylon. The swelling varies with the type of nylon, fillers used, environmental factors and time. Various graphs are avaliable from the manufactures such as DuPont, BASF, Allied Signal (now HoneywellI believe), Bayer, and a long and volatile list of others.

 

[Soupcat]

I agree with Patprimmer, water absorbtion is a particular problem with Nylons. Du Pont (

http://www.dupont.com)

should be able to help you out with more information. They should be able to correlate % factor with dimensional stability because ultimately this is the culminating problem resulting from a taking on of water.
Regards

Kieran

http://www.moulddesigner.com

 

[ve7brz]

I'll throw a curve in here. To my understanding the absorbed water in nylon also influences its strength and/or flexibility. Comments?

 

[Soupcat]

Nylon will absorb water to as high a level as 10% of its weight. Water absorption affects the rigidity of a Nylon and it's linear strength. Moisture absorption by nylon has been a source of great study for many years. Although all polymers absorb some amount of moisture, on none does it have such a significant effect as on nylons.
Dimensional changes of 0.7% can result in nylon parts from the "as-molded" state to equilibrium at 50% R.H. environments. This change occurs in approximately 150 days for a 0.060 inch (1.5 mm) thick part. Molecular mobility is increased through the absorption of water. The increase in spacing between nylon molecules lowers the secondary forces allowing easier translational motion. This is the major reasons for the change in physical properties discussed above. There is less resistence to applied stress from the decrease in intermolecular friction. The change in molecular mobility is significant enough that molded nylon parts can relieve molded in stresses as they absorb moisture.
The absorption of moisture by nylon is a completely reversible physical reaction. Drying in an oven will drive off all but a small percentage of the water molecules which can only be removed through dissolution of the nylon molecular matrix. The rate of absorption/desorption varies with type of nylon as well as temperature and relative humidity. Addition of fillers reduces the effect of moisture both due to volume reduction of the ammount of nylon polymer in the mixture, and by sharing the attraction of the molecules somewhat reducing polarity and the available space for moisture molecules. Reinforcements reduce the effects more than fillers due to nylons strong affinity for reinforcement. In addition to the mechanisms which take place with fillers, the adhesion of the nylon molecular matrix to dimensionally stable reinforcements is stronger than than polar bonding of the water molecules and it dominates.
Another as important area where moisture has significant effects on nylons is in processing. Heated to molding temperatures while wet (ie., >0.2 % water) will result in hydrolytic degradation and a significant loss of physical properties. (Hydrolytic degradation is a chemical reaction which occurs at high temperature with some polymers in the presence of water. It causes primary bonds in the molecular chains to be severed thus reducing molecular weight.) Over drying (ie., <0.08% water) will remove the plasticizing effect of the water molecules and make the resin very viscous and hard to flow. The plasticizing effect in processing has to do with mobility and relative spacing of the nylon molecules, the same influence as on physical properties. This low level of moisture does not cause significant degradation during processing. The absorption of moisture by nylon must be considered in mold making. The shrinkage factor used in designing the mold must take the the potential for change in post molded dimensional into account. Although moisture causes problems in working with nylons, it does contribute to: better dyeability, toughness, softness and greater flexibility in nylon parts.

Hope this helps

Kieran

http://www.moulddesigner.com

 

[msbharaj]

Thanks a lot, Kieran. Your indepth explanation of the subject has answered all my question and cleared lot of doubts.

Maninder

 

[ve7brz]

WOW! That deserves a star!

 

[PrentP]

See the following link at

http://162.128.70.45/comm/GeneralPoly/9_96gptip.html

Regards,

Peter P