Solid vinyl ether terminated urethane curing agent

Details Solid vinyl ether terminated urethane curing agent Solid vinyl ether terminated urethane curing agent

1999-06-25

2001-09-18

Sergent, Rabon (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Compositions to be polymerized by wave energy wherein said...

C428S422800, C428S423100, C428S425100, C522S093000, C522S095000, C522S173000, C522S174000, C525S123000, C525S127000, C525S329500, C525S329900, C525S330500, C525S374000, C525S440030, C525S452000, C525S454000, C525S455000

Reexamination Certificate

active

06291541

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to vinyl ether terminated urethane curing agents. More particularly, this invention relates to vinyl ether terminated urethane curing agents that are derived from relatively safe, non-hazardous materials, and that are non-crystalline solids at room temperature to enable use in powder coatings.
BACKGROUND OF THE INVENTION
Vinyl ether terminated urethane resins are extremely reactive prepolymers which are known to undergo rapid polymerization when exposed to radiation. These compounds are particularly useful as curing agents in applications which require high speed curing of a resin formulation, such as in radiation curable coatings.
One disadvantage attendant to the use of such vinyl ether functionalized urethanes is that their commercial availability is relatively limited. In general, the available prepolymers constitute liquid or semi-solid (with extremely low Tg °C.) materials. U.S. Pat. No. 4,751,273 (Lapin, et al.) provides specific examples of such liquid and semi-solid vinyl ether terminated urethane resins.
These curing agents, though extremely useful in liquid radiation curable coatings, have only limited use in powder coatings. Typically, because of their liquid or semi-solid state, they cannot be used beyond a few percent (<5%) in powder coatings. Greater amounts typically cause the powder to block or sinter in storage, which renders the powder unsprayable during electrostatic coating operations.
Solid vinyl ether terminated urethane curing agents which are more conducive for use in radiation curable powder coatings have been proposed. For example, EP-A-0 636 669 (DSM, N.V.) provides one example of a crystalline vinyl ether functionalized urethane curing agent that remains a solid at room temperature (melt range of 90-108° C.). This curing agent arises from the reaction of hydroxybutyl vinyl ether (HBVE) with hexamethylene diisocyanate (HDI) monomer in a 1:1 (stoichiometric) mole ratio of hydroxy to isocyanate groups. The reaction product is a short chain crystalline urethane oligomer (HBVE-HDI-HBVE).
One disadvantage with the use of such a crystalline curing agent in powder coatings is that it makes manufacture of the powders extremely troublesome. Powders based on crystalline materials take longer to recrystallize after melt extrusion, making subsequent grinding and handling very messy and difficult.
Another disadvantage with the use of this curing agent is that monomeric HDI is known to be unsafe to handle because of its high toxicity. Thus, the presence of residual (unreacted) monomeric HDI in the curing agent will expose the end user to serious health hazards. For instance, HDI monomer has been known to cause skin sensitization, which can lead to serious respiratory disease in workers, including asthma and permanent decrease in lung functions. Furthermore, HDI monomer readily becomes airborne because of its high vapor pressure at room temperature which, in turn, increases the risk of inhalation of its vapors or mists.
It would be desirable to provide a vinyl ether terminated urethane curing agent that is solid at room temperature, is easier to melt process, is much safer to handle, and is effective in curing powder coatings.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a vinyl ether terminated urethane curing agent which does not suffer from the foregoing drawbacks.
It is another object of this invention to provide a vinyl ether terminated urethane curing agent that is derived from safer and less hazardous monomers.
Still another object of this invention is to provide a vinyl ether terminated urethane curing agent that remains a solid at room temperature.
And yet another object of this invention is to provide a method for preparing vinyl ether terminated urethane curing agents of the aforesaid character.
Another object of this invention is to provide a vinyl ether terminated urethane curing agent that can be effectively incorporated into powder coatings without degrading the shelf stability and electrostatic sprayability of the powder.
Still another object of this invention is to provide a vinyl ether terminated urethane curing agent that is a non-crystalline material, making powder coatings based on the same easier to melt process and handle during powder manufacture.
And still another object of this invention is to provide a vinyl ether terminated urethane curing agent that is extremely useful in curing powder coatings, particularly powder coatings that are curable by exposure to radiation, heat, or both, and especially those that can be used to coat heat sensitive substrates, such as wood and plastic, without causing permanent thermal damage to the substrate during curing.
The various objects, features and advantages of this invention will become more apparent from the following description and appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
This invention provides vinyl ether terminated urethane prepolymers that are prepared from safer and less hazardous materials, and that are non-crystalline solids at room temperature to enable them to be extremely useful as curing agents in powder coatings. This invention also provides a method for the preparation of the same.
Broadly stated, the desired vinyl ether terminated urethane curing agents of this invention can be prepared by reacting an aliphatic diisocyanate monomer with a polyol, and then reacting the product obtained with a hydroxy vinyl ether, or by reacting an aliphatic polyisocyanate with a hydroxy vinyl ether. In this invention, the reactants are chosen particularly from materials which are relatively safe and less hazardous to handle. Furthermore, the reaction product which is obtained by either of the above preparation methods will comprise vinyl ether terminated urethane prepolymers that are non-crystalline solids at room temperature or higher.
In the first embodiment of the invention, the desired product is prepared by the two-step reaction sequence wherein a non-crystallizing aliphatic diisocyanate monomer (with relatively low vapor pressure) is first reacted with a crystallizing or non-crystallizing polyol, the resulting material being an adduct of the diisocyanate with the polyol, and then the adduct so obtained is further reacted with a hydroxy vinyl ether to end-cap the adduct with a hydroxy vinyl ether, the resulting material being a non-crystalline solid vinyl ether terminated urethane prepolymer.
The first reaction between the aliphatic diisocyanate monomer and the polyol can be viewed as an addition reaction wherein an adduct of diisocyanate with a polyol is formed. The reaction conditions will be chosen so as to form an isocyanate terminated urethane oligomer to the virtual exclusion of alcohol terminated polymeric materials.
The aliphatic diisocyanate monomers which may be employed in the first reaction include those selected from materials that are non-crystallizing, possess a vapor pressure less than that of monomeric hexamethylene diisocyanate (HDI) at room temperature (i.e., less than 0.011 mm Hg at 25° C.), and that preferably contain isocyanates with different reactivities. The inventors have thus far identified only one material which meets the above criterion, which is isophorone diisocyanate (IPDI). Thus, in the preferred embodiment of the invention, isophorone diisocyanate (vapor pressure 0.00048 mm Hg at 25° C.) is employed in the first reaction.
The polyols which may be subjected to the first reaction include those selected from crystallizing or non-crystallizing polyols, although non-crystallizing polyols are preferred. Examples of suitable diols useful herein include ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,3-butylethyl propanediol, neopentyl glycol (2,2′-dimethyl-1,3-propanediol), 2-butyl-2-ethyl-1,3-propanediol (BEPD), 2-methyl-1,3-propanediol (MP diol), 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, 1,3-isobutanediol, 1,2-isobutanediol, 2,3-butanediol, 2-butenediol(1,4),

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