Syntactic foam plugs

The present invention provides an improved syntactic foam plug for plug assist thermoforming wherein such plug is composed of a thermoplastic material having a relatively high melting and/or glass transition temperature. Accordingly, the subject invention encompasses diminishing or removing the undesirable characteristics of thermoset syntactic plugs while increasing the ease by which these articles may be constructed for a given use, by forming such plugs from syntactic foams containing thermoplastics.

EXAMPLES 
 Examples 1-4 and Comparative Experiment A A series of nylon syntactic foam samples were made using hollow glass micro-spheres (commercially available from 3M under the trade name Scotchlite Glass Bubbles) and caprolactam, along with a catalyst (sodium lactamate) and an initiator (hexamethylene diisocyanate). The process for forming the samples composed of the formulations set forth in Table 1 is as follows. The caprolactam monomer was melted and added to a closed mixer maintained under nitrogen. The remaining ingredients (except the catalyst) were added under nitrogen in such a manner that excludes moisture and air. The mixture was than heated to 160-170° C. and the catalyst was added to and mixed with the monomer mixture for polymerization. Upon polymerization, the solid object was then removed and annealed for stress relief. The termal conductivity values for each sample was tested and reported also in Table 1. The melting point of the nylons was 210° C. 1 TABLE 1 Volume % Thermal glass micro-spheres Conductivity * Examples Resin and type (W/m ° K) Comparative A ** Nylon 6 Unfilled 0.25 1 Nylon 6 30 Vol. % of K20 0.23 microspheres (specific gravity 0.20, average diameter 65 &mgr;m) 2 Nylon 6 55 Vol. % of K20 0.18 microspheres (specific gravity 0.20, average diameter 65 &mgr;m) 3 Nylon 6 55 Vol. % of K1 0.17 microspheres (specific gravity 0.125, average diameter 65 &mgr;m) 4 Nylon 6 45 Vol. % of K20 0.19 microspheres (specific gravity 0.22, average diameter 40 &mgr;m) Notes: * Measurements performed in accordance with ASTM E-1530 ** Unfilled thermoplastic polyurethane resins were also tested and had a Thermal Conductivity of 0.35 W/m ° K 
 Example 5 Syntactic foam plugs were prepared on an industrial scale from the formulation set forth in Table 2-A. The industrial scale process for the polymerization included adding melted caprolactam (monomer) to a closed mixer maintained under nitrogen. The microspheres filler, initiator, stabilizer and pigment were added under nitrogen in such a manner that excludes moisture and air. The temperature of the mixture was then raised to 145° C. and the vessel evacuated to less than 25 mm Hg to remove entrained air and other volatiles. The vacuum was broken with nitrogen and the evacuation cycle was repeated 2-3 times. The mixture was then transferred to a mold preheated to 160-170° C. for polymerization. The catalyst was added to and mixed with the monomer mixture as the material was being transferred into the mold. After polymerization, the solid object was removed from the mold and annealed for stress relief. 2 TABLE 2-A Weight, lbs. (Kg) Weight (%) Caprolactam 56.65 (25.72) 79.78 K20 Glass 9.3 (4.22) 13.10 Initiator solution 1.13 (.513) 1.60 Catalyst solution 3.09 (1.40) 4.35 Heat stabilizer 0.17 (.077) 0.24 901 (blue) pigment 0.66 (.299) 0.93 100% The properties of the plugs were tested, in accordance with ASTM test methods, and are reported in the following Table 2-B. 3 TABLE 2-B Property Example 1 ASTM Density (&rgr;) 720 (kg/m 3 ) D-792 Coefficient of 26 × 10 −6 in/in/° F. E-831 Thermal Expansion (47 × 10 −6 m/m/° C.) (CTE) (21-150° C.) Compressive 6,512 psi D-645 Strength &lsqb;44.9 Mpa&rsqb; Compressive 231 Kpsi D-645 Modulus &lsqb;1.59 Gpa&rsqb; Service 351° F. N/A Temperature &lsqb;180° C.&rsqb; When these plugs were used in a thermoforming process for forming polypropylene cups, the resulting cups were extremely clear (transparent without haze) with more uniform wall thickness as compared with cups resulting from a process using unfilled polyurethane plugs. 
 Example 6 Industrial scale plugs were prepared in a thermoforming process from a formulation corresponding to Example 4 (45 Vol. % of S22 microspheres) having properties as shown in Table 3. 4 TABLE 3 Property Example 5 ASTM Density (&rgr;) 43-47 lb/ft 3 D-792 &lsqb;740 kg/m 3 &rsqb; Specific Heat (C p ) 0.43 BTU/lb · ° F. E-1530 per mass &lsqb;1.80 kJ/(kg · ° C.)&rsqb; Coefficient of 28 × 10 −6 in/in/° F. E-831 Thermal Expansion &lsqb;50 × 10 −6 m/m/° C.&rsqb; (CTE) (21-150° C.) Compressive 6,300 psi D-645 Strength &lsqb;43.4 Mpa&rsqb; Compressive 180 Kpsi D-645 Modulus &lsqb;1.24 Gpa&rsqb; Service 350° F. N/A Temperature &lsqb;180° C.&rsqb; Although particular embodiments of the invention have been described in detail for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.