Curable composite manufacturing adhesive

Abstract

A curable adhesive that is modified to allow spray application and polymerization seamlessly during the process of epoxy resin vacuum infusion.

Claims

1. A cross-linking adhesive composition comprising: Melamine resin; and tackifier, wherein said adhesive composition is dissolved in an organic ketone; and one or more adducts; wherein said melamine resin has a carboxyl terminated butadiene nitrile adduct.

2. The adhesive composition of claim 1, wherein said melamine resin is hexa(methoxymethyl) melamine.

3. The adhesive composition of claim 1, wherein said organic ketone is acetone.

4. The adhesive composition of claim 1, wherein said tackifier is selected from a group of phenolic, novolac or resol resins or is isobutyl methacrylate.

5. The adhesive composition of claim 1, further comprising fumed silica filler.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present disclosure will be better understood by reading the Detailed Description of the Preferred and Alternate Embodiments with reference to the accompanying drawing figures, in which like reference numerals denote similar structure and refer to like elements throughout, and in which:

(2) FIG. 1A through 1D illustrates typical layers preform assembly and stages of resin infusion implemented in a typical embodiment of the process;

(3) FIG. 2 illustrates an embodiment of typical epoxy resin cross-linking reactions;

(4) FIG. 3 illustrates a first tabular presentation of initial adhesive formula performance testing with carbon fiber;

(5) FIG. 4 illustrates a first graphical presentation of initial adhesive formula performance testing with carbon fiber;

(6) FIG. 5 illustrates a second tabular presentation of further adhesive formula performance testing with fiberglass; and

(7) FIG. 6 illustrates a second graphical presentation of further adhesive formula performance testing with fiberglass.

DETAILED DESCRIPTION

(8) In describing the preferred and alternate embodiments of the present disclosure, as illustrated in the FIGS. 1-6 and/or described herein, specific terminology is employed for the sake of clarity. The disclosure, however, is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish similar functions.

(9) Manufacturers of epoxy-fiberglass or epoxy-carbon fiber structures using the vacuum infusion process need an adhesive product to hold fabrics together until infusion and curing is complete. Manufacturers of epoxy-fiberglass or epoxy-carbon fiber structures using the vacuum infusion process also need an adhesive product that does not act as a contaminant in the resin matrix. Manufacturers of epoxy-fiberglass or epoxy-carbon fiber structures using the vacuum infusion process also need an adhesive that is not composed of epoxy resins that cause skin sensitization, occupational allergic contact dermatitis and occupational asthma. Having previously successfully developed, an adhesive comprising of epoxy Diglycidyl ether of bisphenol A (DGEBA) epoxy resin, an adduct of carboxylic acid terminated butadiene nitrile rubber and Diglycidyl ether of bisphenol A epoxy resin and acetone, as described in U.S. Pat. No. 20120299216 A1, a new investigation was undertaken to develop an improved adhesive for use in multiple type liquid resin vacuum infusion and light resin transfer systems that would allow reduced exposure to the hazardous components of the adhesive described in U.S. Pat. No. 20120299216 A1. A complex series of trial and error experiments was conducted to conceive, analyze, identify, and create a new combination of materials that, when formulated together, would deliver heretofore unavailable results relative to vacuum infusion epoxy laminates, and according to an entirely original perspective relative to the previous epoxy resin adhesive. The result, after many modifications directed to improve particular characteristics, including stickiness, spray characteristics and product shelf-life, was a discovery of 2 final adhesive formulations composed of ingredients that are compatible with epoxy vacuum infusion resins and would allow for efficient spray application for preparation of vacuum infusion epoxy laminates, that would be able to hold many layers of reinforcing fabric in a vertical aspect, and that would integrate into the cured epoxy laminate structure rather than form a potentially weakening interface, that exposure to would not cause skin sensitization, occupational allergic contact dermatitis and occupational asthma, all with low VOC emissions.

(10) In reference now to FIGS. 3 and 4, an adhesive formula was discovered with strength recovery of over 105% in preliminary testing. This strength recovery estimate was estimated during short beam shear testing by dividing the average measured strength of ten samples of an epoxy cured carbon fiber laminate structure with adhesive 10 applied by the average measured strength of ten samples of an epoxy cured carbon fiber laminate structure without adhesive 10 (as shown, 52.02 mPA/49.53 mPA=1.05). Those skilled in the art recognize results above 105% are very good for such short beam shear testing comparisons, and further testing, discussed herein below, further confirmed the unexpectedly zero impact of adhesive 10 on the epoxy cured laminate structure.

(11) In reference now to FIGS. 1A through 1D, vacuum infusion laminate adhesive 10 holds laminate layers together as epoxy resin is driven into a laminate structure. Adhesive 10 comprises properties that cross-link with epoxy resin as it cures. Generally, the laminate layers include the assembly of epoxy resin reinforced with fiberglass and/or carbon fiber. Present infusion molding used to fabricate epoxy resin structures is improved with the use of adhesive 10 and the methods related thereto described herein.

(12) The presently described technique encapsulates carbon fiber and/or fiberglass with multiple type resins while the resin cures, resulting in superior structural strength while allowing for low VOC emissions. The presently described process enables the use of adhesive 10 to hold components in place in a vertical aspect while the laminate is bagged and subsequently infused with epoxy resin under vacuum. Cross linkable adhesive 10 enables the creation of strong connections between laminate layers, wherein adhesive 10 preferably cures with epoxy resin and becomes an integral part of the cured structure, as discussed further herein. During curing, low shrinkage is observed. In addition, maximum tensile shear strength may be obtained.

(13) In a typical embodiment, adhesive 10 is enclosed within a spray can and is applied to hold dry materials together and onto structural surfaces, ultimately curing with the epoxy resin to result in a single, uninterrupted structural formation. The polymeric mixture, Melamine CTBN adduct resin and phenolic resin spray of adhesive 10 does not interfere with or contaminate the curing process of epoxy resins, wherein adhesive 10 instead cross links and/or otherwise structurally integrates and hardens along with the epoxy resin to form an integrated chemical structure.

(14) It should be understood that adhesive 10 may be enclosed in a canister or other suitable container, or otherwise applied in a manner desirable relative to the work-piece.

(15) Adhesive 10 is preferably comprised of a formulated hexamethoxymethyl melamine CTBN adduct resin base, preferably modified with phenolic resin tackifiers. The unique compatibility of the base of adhesive 10 with the epoxy resin of the target vacuum infusion procedure facilitates delivery of superior infusion results. However, it is the further modifications to that base that provide for the preferred tacky nature of adhesive 10 after the carrier solvent, preferably acetone, has evaporated. That is, in a typical implementation, adhesive 10 is prepared by dissolving hexamethoxymethyl melamine CTBN adduct resin and one or more tackifiers in a solvent, preferably acetone. Acetone is quick to evaporate, is exempt from VOC regulation, and is therefore preferred as a carrier solvent. However, it should be recognized by one skilled in the art that other carrier solvents could be utilized.

(16) According to the preferred embodiment, adhesive 10 is a mixture of a phenolic and a melamine resin, which has a carboxyl terminated butadiene nitrile (CTBN) adduct. Although a different combination or a single resin may alternately be utilized, the preferred mixture delivers improved toughness, elasticity, and tack of the hexamethoxymethyl melamine CTBN adduct portion of adhesive 10. Additionally, tackifier selection preferably optimizes stickiness or tack of adhesive 10, wherein tackifiers in the form of phenolic, novolac or resol or ployamide resins are preferred, but other commonly known tackifiers may perform suitably.

(17) When the composition is to be delivered by a spray can, as preferred, adhesive 10 is formulated with a lower viscosity to enable pressurized placement with gas for satisfactory adhesive spray, wherein viscosity is preferably influenced and balanced in the formula of adhesive 10 with the addition of more acetone carrier. In the preferred embodiment, especially for spray delivery, the fumed silica filler AEROSIL is added, resulting in maintenance of a uniform spray and promotion of improved short beam shear strength.

(18) In another embodiment, when the composition is packaged in a canister, a small amount of propane-isobutane, or other gas and/or hydrocarbon is used and pressurized with nitrogen or other suitable gas to a higher pressure. In such an embodiment, a higher viscosity may be utilized, thereby accommodating a higher solids level in the basic composition. That is, the higher the concentration in terms of weight percent solids to the total weight of the mix, the higher the viscosity, wherein canisters can generally withstand higher pressure than cans.

(19) In use, laminates, or composites, are preferably prepared from layers of carbon fiber material held together with adhesive 10. These composites are vacuum infused with epoxy resin. Samples prepared according to such process and with adhesive 10, after curing, were subjected to testing using ASTM D 2334, Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates, to determine the short-beam strength of the high-modulus fiber-reinforced composite materials, wherein no weak spots were detected in the compositions formed using adhesive 10. That is, the interlaminar shear strength was determined by comparative flexing of composite specimens by delivery of controlled forces thereto until breakage occurred, and confirmation of the structural integration of adhesive 10 into the cured laminate structure was realized.

(20) Exemplary Test Data

(21) In order to test the efficacy of hexamethoxymethyl melamineCTBN adduct adhesive 10, laminate samples were prepared and analyzed following a procedure similar to ASTM D 2334. Fiberglass laminate layers were prepared: first, with no adhesive, second, with representative multi-purpose aerosol adhesive, 3M SUPER 77, and third, with melamine adhesive 10 with phenolic resin tackifier, and fourth, melamine adhesive 10 with acrylic resin tackifier. Ten samples were tested for each variation. Maximum shear stress (MPa) repeatedly confirmed the unexpected benefits of epoxy adhesive 10, as compared to the representative traditional, multi-purpose adhesive. Sample data and measured results are presented in FIG. 5, with graphical representation in FIG. 6. Both the phenolic and the acrylic tackified versions of adhesive 10 demonstrated strength recoveries double that of traditional adhesive. The acrylic tackified version of adhesive 10 demonstrated 97.9% strength recovery, while the performance of phenolic tackified adhesive 10 demonstrated 108% strength recovery. An 8% higher than no-adhesive, is an unexpectedly synergistic improvement for use in epoxy laminate applications. The traditional adhesive demonstrated a strength recovery of only 48.6%.

(22) In the procedure, laminates and fiberglass were thus either sprayed with adhesive 10 with phenolic resin tackifier, or sprayed with adhesive 10 with acrylic resin tackifier, or sprayed with representative traditional adhesive, or placed together with no adhesive. The assembled laminates were placed into a vacuum bag, and epoxy resin and hardeners were appropriately introduced. Vacuum remained until resin curing was complete. The completed samples, of dimensional specifications as noted in FIG. 5, were subjected to short beam shear testing, with failure load recorded for each sample, also as displayed in FIG. 5. The performance of phenolic tackified adhesive 10 relative to the control epoxy laminate structure without adhesive was remarkable, and the magnitude of improvement of shear strength with both phenolic tackified and acrylic resin tackified adhesive 10 as compared to traditional adhesive was unexpected. The testing results indicate that adhesive 10 with phenolic tackifier may be utilized in epoxy laminate applications to improve the resulting laminate structure.

(23) Having thus described exemplary embodiments of the present apparatus and method, it should be noted by those skilled in the art that the within disclosures are exemplary only, and that various other alternatives, adaptations, and modifications may be made within the scope of the present disclosure. Accordingly, the present disclosure is not limited to the specific embodiments illustrated herein, but is limited only by the following claims.