POLYURETHANE PREPOLYMERS HAVING IMPROVED THERMAL STABILITY
Publication Date: 2003-Aug-20
The IP.com Prior Art Database
A prepolymer composition of a curing hot melt resin made from a polyester polyol copolymer (butanediol adipate:ethylene glycol adipate / 85:15 (w/w)), 0.2% by weight 2,2'-dimorpholinodiethylether (an endcapping and cure promoting catalyst), and 4,4'-methylene diisocyanate (NCO:OH / 2.0:1.0) was evaluated for green strength time after 2 and 16 hours aging at 250°F. Compositions containing no and 0.1% by weight UNILIN 550 wax were tested. Without the wax the green strength time was found to change from 7 minutes after 2 hours of aging to 8.5 minutes after 16 hours of aging. When wax was added the results were 4 minutes at both aging times. Similar results were obtained when the butanediol adipate:ethylene glycol adipate ratio was 90:10 (w/w). In this case, green strength times without the wax were 9.5 and 10.5 minutes at 2 and 16 hours heat aging respectively. With the wax they were 7.5 minutes at both aging times. This indicates the use of a crystalline, reactive wax provides both shortened green strength times and improved thermal stability.
Polyurethane Prepolymers Having Improved Thermal Stability
The use of crystalline, reactive waxes for nucleating crystalline polyester-based, moisture curing hot melts is known. Another feature of such reactive, crystalline, reactive waxes pertains to assembly operations where rapid and consistent crystallization is desired to speed up the production line. In a typical adhesive bonding operation, the adhesive is applied to the substrate and a second member is placed on the substrate to be bonded thereto by the hot melt composition. For a certain period of time after the hot melt composition has been applied, the second member may be moved, adjusted or repositioned. Once that period of time (i.e., the set time) elapses, the second member can no longer be repositioned. Another important parameter in assessing the performance of hot melt compositions is the initial strength or "green strength" of a bond made therewith. Green strength is often measured by the ability of the bond to support a given weight for at least a certain time. Therefore, green strength and set time are frequently related since set time can be regarded as the time that must pass before the bond can support a given weight for a certain time. In certain instances it is particularly desirable to employ a hot melt composition having a short set time; that is, a hot melt composition that rapidly achieves a certain green strength. One example is an automated assembly line.
A current problem is that, if the hot melt resin composition is kept molten for extended periods (eg., in a heated dispenser), the time to reach a desired green strength tends to drift higher, in some cases by as much as 50% to 100%. If a crystalline, reactive wax, such as UNILIN 550, is added to the curing hot melt composition this time remains much more consistent, that is, it exhibits little or no change. In other words, the use of a crystalline, reactive wax provides improved thermal stability, with respect to green strength times, when the resin is exposed to heat aging.
By “crystalline” it is meant the wax displays a crystalline melting point greater than room temperature (about 20°C to 22°C) when measured by differential scanning calorimetry at a heating rate of 5°C/minute. The peak temperature of the observed endotherm is regarded as the crystalline melting point. In one embodiment the wax does not have a crystalline melting point above 150°C so as to facilitate processing of the hot melt composition. By “reactive” it is meant the wax includes a moiety that is reactive with a principal component of the hot melt composition, including a polyurethane prepolymer, a polyisocyanate, and selected crystalline polyester polyols, or that is reactive with another constituent of the hot melt composition that itself is reactive with a princ...