Abstract:
A treatment method for polyene resins used in thiol-ene formulations, especially norbornene resins, significantly stabilizes thiol-ene compositions made from batches of polyene resin which otherwise give compositions with poor dark stability. The treatment comprises contacting the polyene with an amphoteric treating agent selected from the group consisting of silicated magnesium oxide, basic aluminum oxide, silica gel, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide, and barium hydroxide, and then separating the resin from the treating agent prior to mixture with a polythiol to form a thiol-ene composition. The treatment method significantly reduces ionic content in polyene resins, particularly chloride ion but also sodium and potassium ions. Consequently, treatment is also beneficial with polyene resins which do not produce substantial dark stability problems. Reduction of the ionic content of the resin is desirable to produce cured thiolene polymers with low electrical conductivity and low corrosiveness.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application, Ser. No. 07/746,649, filed Aug. 16, 1991, U.S. Pat. No. 5,208,281, incorporated herein by reference, which is a continuation-in-part of Ser. No. 651,271, filed Feb. 5, 1991, U.S. Pat. No. 5,167,882, which is a continuation-in-part of Ser. No. 632,391, filed Dec. 21, 1990, abandoned. 
    
    
     DEFINITIONS 
     As used herein: 
     A polyene is a compound having a plurality of olefinically unsaturated groups. 
     A polythiol is a compound having plurality of organic thiol groups. 
     A thiol-ene composition is a mixture of a polyene and a polythiol formulated to cure at least predominately by addition of thiol groups across the olefinic double bond of the polyene. Generally the composition will include a free radical initiator such as a peroxy compound or a free radical photoinitiator to initiate the thiol-ene addition reactions. 
     Norbornenyl groups, as used herein, are broadly defined to encompass organic functional groups of the formula: ##STR1## where Q is CR 2   1 , O, S, NR 1  or SO 2 , R 1  is H or alkyl; and m is 0-10. Thus the term should be understood to encompass, in addition to true norbornenyl groups, various substituted analogs, nadic groups and various other cyclopentadiene Dieis-Alder polyadducts. 
     BACKGROUND ART 
     In thiol-olefin cooxidation reactions it has been reported that chloride ion and bromide ion shortened an initial slow oxygen-absorbing phase while iodide ion prolonged the initial phase and decreased its rate. Bredereck et al, Chemische Berichte, 93 2415-2423 (1960); D&#39;Souza et al, J. Org. Chem. 1987, 41, 1729-1740. Several hypotheses are offered in the latter reference as possible explanations of the difference between chloride/bromide and iodide in this reaction, but no conclusions are drawn as to which of the hypotheses are correct. 
     Organohalo compounds, such as tris(2,3-dibromopropyl) phosphate and tetrabromophthalic anhydride (0.1-10 % ) in combination with mercaptans (0.05-5% ) are said to improve melt spin properties of polyolefin compounds. Chem. Abst., 80:1222322 (abstracting JP 48004374). 
     The polymerization of methacrylate ester monomers during neutralization, washing and/or extraction with base is reported to be inhibited by adding 0.03-0,3 % NaNO 2  and/or KI to the solutions. Chem. Abst., 106; 156990r (abstracting CS 229529). 
     Heat stability of a nylon to polymer containing a copper stearate, KI, and 2-mercaptobenzimidazole is reported to be improved over comparable formulations omitting either the KI or the mercapto compound in Chem. Abst., 81:137494f (abstracting JP 49053944). 
     Halogen containing compounds such as 2,4-bis(trichloromethyl)-6-methyl-s-triazine are reported as components of an initiator system, which also uses an organic thiol component, for anaerobically curing formulations in Chem. Abst., 100: 69316h (abstracting U.S. Pat. No. 4,413,108). 
     In Bartlett et al, JACS, 72, 1051-1059 (1950), iodine was reported to have a dual effect on peroxide catalyzed polymerizations of vinyl acetate, rapidly accelerating polymerization at moderate concentrations but behaving as a rather strong inhibitor at low concentrations. I 3 .radical is postulated as a terminator of growing polymer chain to explain the inhibition affection. A polar mechanism is postulated to explain the acceleration at higher concentrations. 
     BACKGROUND OF THE INVENTION 
     Curable thiol-ene systems are described in U.S. Pat. No. 3,661,744; U.S. Pat. No. 4,119,617; U.S. Pat. No. 4,289,867; U.S. Pat. No. 3,445,419 and U.S. Pat. No. 4,808,638. Such formulations employ a polyene functional resin, a polythiol and a free radical initiator. 
     In U.S. Pat. No. 4,808,638 there are described photocurable thiol-ene compositions comprising a norbornene functional resin, a polythiol and a free radical photoinitiator. Further description of this system may be found in Jacobine et al, Proceedings of ACS Division of Polymeric Materials: Science and Engineering, Vol. 60, pp. 211-216 (1989). 
     Norbornenyl functional resins and their uses are described in U.S. Pat. No. 4,808,638 and references cited therein. The disclosed use of such resins in this reference is as the polyene resin in radically cured thiol-ene formulations. The aforementioned U.S. patent is incorporated herein by reference. 
     Further norbornenyl functional compounds useful in thiol-ene compositions as described in U.S. Pat. No. 4,808,638, U.S. Pat. No. 5,034,490, U.S. Pat. No. 5,171,816 and U.S. Pat. No. 5,182,360 and copending applications Ser. No. 07/315,737 filed Feb. 24, 1989 and Ser. No. 07/619,068 filed Nov. 28, 1990. 
     Dark stability of thiol-ene curable formulations has been a problem for a number of these systems, particularly those where the polyene is a plural norbornene compound. The norbornene-thiol system, however, is an especially desirable system because of its very high cure speed, good cured polymer properties and the ready availability of norbornene resins from acrylate ester precursors. 
     Various attempts have been made to improve upon the dark stability of thiol-ene systems by use of improved stabilizers. References pertaining to these efforts include U.S. Pat. No. 3,619,393; U.S. Pat. No. 5,208,281; and EP 428,342. Such improvements, however, are not always satisfactory in achieving consistent, commercially acceptable shelf lives. In particular, it has been noticed that certain batches of ene resins are extremely difficult to formulate into shelf-stable thiol-ene compositions. 
     SUMMARY OF THE INVENTION 
     It has now been discovered that treatment of a norbornene resin, prior to mixture with a polythiol to form a thiol-ene composition, significantly stabilizes compositions made from batches of ene resin which have previously given the greatest stability problems. The treatment comprises contacting the polyene with an amphoteric treating agent selected from the group consisting of silicated magnesium oxide, basic aluminum oxide, silica gel, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide, and barium hydroxide, and then separating the resin from the treating agent. 
     It has been found that treatment with such amphoteric substances significantly reduces ionic content in polyene resins, particularly chloride ion but also sodium and potassium ions. Consequently, treatment is considered beneficial even with polyene resins which do not produce substantial dark stability problems. Reduction of the ionic content of the resin is desirable to produce cured thiol-ene polymers with low electrical conductivity and low corrosiveness. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The amphoteric substance used to treat the polyene resins is selected from the group consisting of silicated magnesium oxide, basic aluminum oxide, silica gel, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, barium oxide and barium hydroxide. The average particle size of the amphoteric substance is preferably in the range of about 2 microns to about 200 microns. A particularly preferred amphoteric agent is Magnesol® Polysorb 30/40 hydrated silicated magnesium oxide, which has a particle size range of from about 2 to about 200 microns and an average particle size of 50 microns. 
     Contact time with the polyene resin will vary depending upon the temperature employed and the moisture content and viscosity of the resin. The time should be for sufficient time to reach a Cl -  content of less than 1.0 ppm, preferably to 0.5 ppm or less, or for sufficient time to improve the shelf life of a thiolene composition prepared by mixing the polyene with a polythiol after the amphoteric treating agent has been separated from the polyene. The treatment can also be used to reduce sodium and potassium levels below 1 ppm, suitably 0.5 or less. Typical contact times will be between 1 hour and 3 days. Treatment efficiency can be improved, lessening necessary contact time, if the polyene has a small moisture content, suitably, 0.01%-1.0%. When a dry resin is used and moisture can readily be removed subsequent to treatment, e.g. by vacuum stripping, it may be desirable to add moisture to the treatment mixture. 
     Contact temperatures will also vary depending on viscosity of the polyene. Effective ion removal can be obtained at temperatures between 0 ° C. and 100 ° C. provided that the polyene is sufficiently non-viscous at the temperature employed to allow for good agitation of the mixture. 
     The amphoteric treating agent must be removed from the polyene resin prior to formulation into a curable composition, such as a thiol-ene composition. Separation can be accomplished by filtration but in some cases where the amphoteric treating agent has a clay-like consistency addition of a filter aid such as Celite® to the treatment mixture prior to filtration is necessary for effective filtration. Addition of the filter aid does not appear to influence the effectiveness of the amphoteric treating agent in removing ionic species or in improving shelf-life stability of thiolene compositions produced from the treated resin. 
     The plural norbornene functional compounds useful in the invention are known from U.S. Pat. No. 4,808,638, incorporated herein by reference, and have recently also been described in Jacobine et al, &#34;Photoinitiated Cross-linking of Norbornene Resins with Multifunctional Thiols&#34;, Chapter 13 of Radiation Curing of Polymeric Materials, ACS Symposium Series #417, American Chemical Society, 1990, and U.S. Pat. No. 5,167,882, also incorporated herein by reference. Particularly preferred norbornene compounds are norbornenemethyl norbomenecarboxylate and norbornene carboxylate esters of polyols such as 1,6-hexanediol, trimethylolpropane, ethoxylated bisphenol A, poly(tetramethyleneoxide) and mixtures thereof. 
     The polythiol component of the inventive compositions may be any compound having two or more thiol groups per molecule. Suitable polythiols are described in U.S. Pat. No. 3,661,744 at col.8, In 76-col.9, In 46; in U.S. Pat. No. 4,119,617, col.7, Ins 40-57; U.S. Pat. No. 3,445,419; and U.S. Pat. No. 4,289,867. Especially preferred are polythiols obtained by esterification of a polyol with an α or β-mercaptocarboxylic acid such as thioglycolic acid, or β-mercaptopropionie acid. Particularly preferred polythiols are pentaerythritol tetramercaptoacetate and pentaerythritol tetrakis-β-mercaptopropionate (PETMP). 
     The ratio of the polyene to the polythiol component can be varied widely. Generally it is preferred that the ratio of thiol to ene groups be between 0.7:1 and 1.3:1, but ratios outside this range may occasionally be usefully employed without departing from the invention hereof. 
     While a curable composition using norbornene functional compounds of the invention may include both difunctional norbornenyl compounds and difunctional thiol compounds, it will be understood that at least a portion of at least one of these components should contain more than two functional groups per molecule to produce a crosslinked product when cured. That is, the total of the average number of norbornene groups per molecule of norbornene functional compound and the average number of coreactive thiol groups per molecule of the thiol functional compound should be greater than 4 when a crosslinked cured product is desired. This total is referred to as the &#34;total reactive functionality&#34; of the composition. 
     The initiator used in the curable thiol-ene formulations is suitably a free radical photoinitiator. Examples of free radical photoinitiators include benzoin and substituted benzoin compounds, benzophenone, Michler&#39;s ketone, dialkoxybenzophenones, dialkoxyacetophenones, peroxyesters described in U.S. Pat. Nos. 4,616,826 and 4,604,295, etc. The photoinitiator is employed in an amount effective for initiating cure of the formulation upon irradiation with UV light, suitably 0.1-10%, typically 0.5-5%. 
     The formulations also preferably include a stabilizer. Preferred stabilizers are described in EP 428,342. Such stabilizers are non-acidic nitroso compounds, particularly N-nitrosoarylhydroxylamines and salts thereof. Particularity suitable stabilizer compounds are the ammonium and aluminum salts of N-nitrosophenylhydroxylamine which may be usefully employed at levels between about 10 ppm and 2%, preferably 10-5,000 ppm. 
     As described in U.S. Pat. No. 5,208,281, triiodide and other polyiodides are useful shelf-life stabilizers for thiol-ene formulations. 
     The invention is illustrated by reference to the following non-limiting examples. 
     EXAMPLE 1 
     Dinorbornene resins were produced by Dieis-Alder cycloaddition of cyclopentadiene to ethoxylated bisphenol A. The same lot of acrylate starting material was used in both the control and the treated samples. Resins were filtered at 50 ° C. through a course frit Celite® C bed after synthesis. The treated sample was mixed with 2 wt % Magnesol® Polysorb 30/40 silicated magnesium oxide, stirred three hours at 45° C. and allowed to sit overnight. Celite® filter aid, 3 wt % was added, mixed for one hour at 45° C. and then the mixture filtered at 70° C. using a 1μ filter pad. The dinorbornene resins were then formulated into thiol-ene formulations using equivalent weights of pentaerythritol tetramercaptopropionate, 2 wt % Darocure® 1173 photoinitiator and 1000 ppm aluminum N-nitrosophenylhydroxylamine. Samples of the formulations were then stored at room temperature and at 5° C. under nitrogen. Viscosities of the formulations were taken at periodic intervals to ascertain their relative storage stability. Results are given in Table I. 
     
                       TABLE I______________________________________         FORMULATION VISCOSITIESSTORAGE INTERVAL           (Cumulative Change)(Temperature)   Example 1    Control______________________________________ 0               7178         6864 7 days (RT*)    7904 (10%)   8024 (17%) 28 days (RT)    8595 (20%)  10441 (52%) 90 days (RT)    8985 (25%)  14728 (115%)180 days (RT)   10218 (42%)  21627 (215%)180 days (5° C.)            7520 (5%)    9809 (43%)______________________________________ *RT = room temperature (approximately 21° C.). 
    
     EXAMPLE 2 
     The procedure of Example 1 was repeated using the same batch of norbornene resin for both control and treated samples. Results are shown in Table II. 
     
                       TABLE II______________________________________         FORMULATION VISCOSITIESSTORAGE INTERVAL           (Cumulative Change)(Temperature)   Example 2    Control______________________________________ 0               7187         7204 7 days (RT)     8418 (17%)   53520 (643%) 28 days (RT)    9714 (36%)   69865 (870%) 90 days (RT)   14431 (101%)  92660 (1186%)180 days (RT)   15302 (113%) 104348 (1348%)180 days (5° C.)            8808 (23%)   33213 (361%)______________________________________ 
    
     Analysis for selected ions in the norbornene resins used in this Example gave the following results: 
     Untreated control: 11.8 ppm Cl -  ; &lt;0.5 ppm Br -  ; 6.4 ppm Na +  ; 1.5 ppm K + . 
     Treated resin: &lt;0.5 ppm Cl -  ; &lt;0.5 ppm Br -  ; &lt;1 ppm Na +  &lt;1 ppm K + . 
     EXAMPLE 3 (COMPARATIVE EXAMPLE) 
     The treated norbornene resin used in Example 2 was spiked with 7.2 ppm Cl - , as NaCl in a methanol solution using an equivalent weight of 18-Crown-6 to assure transfer of the salt into the resin. The spiked resin was then formulated as per Example II. After 7 days, the viscosity of the formulation had increased 44%, more than double the viscosity increase of the formulation made from the treated, unspiked resin. 
     EXAMPLE 4 
     The procedure of Example 2 was repeated except that the norbornene monomer used was hexanediol dinorbornene carboxylate and the viscosity was monitored for only seven days at room temperature. Results are given in Table III. 
     
                       TABLE III______________________________________           FORMULATION           VISCOSITIESSTORAGE INTERVAL  (Cumulative Change)(Temperature)     Example 4   Control______________________________________0                 215         2107 days (RT)       330 (53%)   917 (337%)______________________________________ 
    
     EXAMPLE 5 
     Treatment of triallyl-1,3,5-triazine-2,4,6(1H,3H,3H)-trione in the manner of Example I gave a reduction of chloride content from 5.2 ppm to 0.2 ppm. The treatment did not affect storage stability of a thiol-ene formulation prepared from this resin but the reduction in chloride content was considered desirable for electrical insulating and corrosion resistance properties of the formulation. 
     EXAMPLE 6 
     Samples of the untreated dinorbornene resin used in Example 2 were treated in the same manner as in that example except that the 2 wt % Magnesol® Polysorb 30/40 and 3 wt % Celite® filter aid, respectively, were replaced with, 2 wt % basic aluminum oxide and 1 wt % Celite®; 2 wt % silica gel and 1 wt % Celite®; and 3 wt % Magnesol® Polysorb 30/40 and no Celite®. Hydrolyzable ion content was determined by ion chromatography on two trials for each sample and results are shown in Table IV. 
     
                                           TABLE IV__________________________________________________________________________                                PotassiumSample Treatment        Chloride (ppm)                Bromide (ppm)                        Sodium (ppm)                                (ppm)__________________________________________________________________________None         9.8     None Detected                        7.5     1.7        11.7    None Detected                        9.3     2.02 wt % basic aluminum        0.1     None Detected                        &lt;0.2    None Detectedoxide and 1 wt % Celite ®        &lt;0.1    None Detected                        &lt;0.2    None Detected2 wt % silica gel and 1        0.1     None Detected                        &lt;0.2    None Detectedwt % Celite ®        0.1     None Detected                        &lt;0.2    None Detected3 wt % Magnesol ®        &lt;0.1    None Detected                        &lt;0.2    None DetectedPolysorb 30/40 and no        &lt;0.1    None Detected                        &lt;0.2    None DetectedCelite ®__________________________________________________________________________ 
    
     Estimated detection limits for bromide were 0.5 ppm and for potassium were 0.2 ppm.