Maro Publications

Hot Melt Adhesives

Notes

*11/21/2013  
from 8/30/2012

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Adhesives

Polyolefins

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Notes

A hot melt adhesive (HMA) is a thermoplastic polymer system applied in a molten state. The molten adhesives are applied to a substrate and then placed in contact with another substrate. The adhesive cools and solidifies to form a bond between the substrates. Hot melt adhesives are widely used in the packaging industry, for example, to seal and close cartons or to laminate multilayer papers.

The HMA should have good adhesion over a wide temperature range, e.g. at low temperatures for packaging applications in the frozen-goods sector, and a low viscosity melt to facilitate application to the substrate, especially for automatic processing. Where applied from a heated reservoir in which it is maintained as a melt, for example, typically at 177.degree. C. (350.degree. F.) in the prior art, for an extended period of time, the adhesive should have oxidative resistance and good thermal stability, which is also called "pot life." The HMA should have a moderate to long open time, defined as the time span between adhesive application to a first substrate and assembly of the parts to be joined. On the other hand, a fast set time is required to quickly build up bond strength on fast-running packaging machines. The characteristic set time is defined as the time needed for the hot melt adhesive to solidify to the point where it possesses enough bond strength to form bonds to give substrate fiber tear when pulled apart, e.g., the bond is sufficiently strong such that sealed substrates will not pop open upon exit from the compression section on a packaging line. The bond may continue to build additional strength upon further cooling; however, compression is no longer required to maintain adhesion between the substrate surfaces. In reference to automatic packaging lines, the set time is referred to as the duration of the application of adhering pressure, which is generally at least equal to or greater than the characteristic set time of the HMA.

A wide variety of thermoplastic polymers, particularly ethylene-based polymers such as ethylene-vinyl acetate copolymers (EVA) have traditionally been used in HMA, but often the adhesive formulation requires substantial use of low molecular weight ingredients such as wax and tackifier to adjust the viscosity and glass transition temperature to useful ranges. Thus, EVA formulations are typically exemplified by a wax with either a low molecular weight or a low crystallization temperature. An HMA for packaging applications such as case and carton sealing is typically composed of a polymer, a tackifier or diluent, and a wax. The polymer has largely influenced the flow and mechanical properties: (a) viscosity and rheology characteristics, (b) cohesive strength, (c) flexibility, and (d) adhesive strength. The viscosity of the polymer has typically been a few orders of magnitude higher than the viscosity of the wax. Low viscosity wax has been used to reduce the high viscosity of the polymer and resin to ensure efficient mixing. This viscosity reduction is particularly important during the application stage where a low viscosity HMA is required to pump the molten adhesive from the storage tank to the application area and to ensure proper surface wetting when applied.

Currently used packaging adhesives comprise primarily petroleum-derived waxes such as paraffin and microcrystalline wax, and synthesis wax such as Fischer-Tropsch waxes. The lower molecular weight of paraffin wax is the primary choice when formulating low application temperature adhesives. The waxes used also generally have a crystallization temperature of less than 100.degree. C. Due to high molecular weight of the base polymer, low molecular weight wax is necessary to reduce the viscosity of the HMA to an applicable range. Large amounts of wax are also required for set time control since wax helps control the set time. However, in certain polymers, large amounts of wax can form large segregated domains or migrate onto the bonding surface. The large difference in the molecular weight between the polymer and the wax has contributed to the wax migration.

[Jiang et al, US Patent 8,242,198 (8/14/2012)]

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Interested!!
Bookmark this page to follow future developments!.
(RDC 6/5/2012)

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Roger D. Corneliussen
Editor
www.maropolymeronline.com

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Tel: 610 363 9920
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E-Mail: cornelrd@bee.net  

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Copyright 2012 by Roger D. Corneliussen.
No part of this transmission is to be duplicated in any manner or forwarded by electronic mail without the express written permission of Roger D. Corneliussen
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* Date of latest addition; date of first entry is 8/30/2012.