Use of a thermoplastic polyester as an additive for epoxy resins

Abstract: A curable composition is composed of (a) one or more epoxide group containing materials containing at least two epoxide groups per molecule; (b) a positive amount of an effectively thermoplastic polyester which does not exceed 40 percent by weight based on the total weight of the epoxide and polyester components of the composition, said polyester having a weight average molecular weight of less than 10,000 and being insoluble in the one or more epoxide group containing materials of (a); and (c) a curing agent adapted to cure the one or more epoxide group containing materials of (a).

Description: The present invention relates to the use of a thermoplastic polyester as a shrinkage reducing additive for epoxy resins. US-A-4,055,606 refers to copolyester-polyepoxide compositions composed of a copolyester and a polyepoxide mixture containing high, intermediate and low molecular weight components. The compositions are useful as adhesives. The copolyesters have a number average molecular weight from 6,000 to 50,000. The polyepoxides are present in an amount of at least 50 % but not more than 200 % by weight based on said copolyester. Adhesive compositions prepared from epoxy resins exhibit a number of advantages including good strength, adhesion, corrosion resistance and suitability for formulation at high solids levels. One marked disadvantage is that epoxy based adhesives deform the substrates to which they bond because of shrinkage during cure. In structural adhesive applications this deformation is of particular concern, especially in the automobile industry wherein the appearance of the substrate is of critical importance. This substrate deformation is not only unsightly, but it also necessitates the expenditure of additional costs in labor and materials in order to restore the intended appearance of the substrate. There is a need, therefore, for a curable epoxy based structural adhesive composition which exhibits excellent physical properties but has markedly reduced shrinkage for forming bonded structures without deforming the substrates. This object is achieved by the use of a thermoplastic polyester as a shrinkage reducing additive in a curable structural adhesive composition for forming a bonded structure between two surfaces of substrates without deforming said substrates, said adhesive composition comprising (a) one or more epoxide groups containing materials containing at least two epoxide groups per molecule, (b) a thermoplastic polyester which is not reactive with one or more epoxide groups containing materials and being insoluble in the one or more epoxide groups containing materials of (a) and having a weight average molecular weight of from 1000 to 8000 and being present in an amount ranging from 1 to 30 percent by weight based on the total weight of the epoxide and polyester components in the composition, (c) a curing agent for the one or more epoxide groups containing materials of (a), as structural adhesive to form a bonded structure between two surfaces of substrates. The curable structural adhesive composition to be used according to the present invention comprises three principal components. The first component is one or more epoxide group containing materials containing at least two epoxide groups per molecule. These materials are also referred to as polyepoxides. Hydroxyl groups may also be present and often are. In general, the epoxide equivalent weight can range from 70 to 4,000. These polyepoxides can be saturated or unsaturated, cyclic or acyclic, aliphatic, alicyclic, aromatic or heterocyclic. They can contain substituents such as halogen, hydroxyl and ether groups. One useful class of polyepoxides comprises the epoxy polyethers obtained by reacting an epihalohydrin (such as epichlorohydrin or epibromohydrin) with a polyphenol in the presence of an alkali. Suitable polyphenols include resorcinol, catechol, hydroquinone, bis(4-hydroxyphenyl)-2,2-propane, i.e., bisphenol A; bis(4-hydroxyphenyl)-1,1-isobutane; 4,4-dihydroxybenzophenone; bis(4-hydroxyphenol)-1,1-ethane; bis(2-hydroxyphenyl)-methane; and 1,5-hydroxynaphthalene. One very common polyepoxide is the diglycidyl ether of bisphenol A. Another class of polyepoxides are the polyglycidyl ethers of polyhydric alcohols. These compounds may be derived from such polyhydric alcohols as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,4-butylene glycol, 1,5-pentanediol, 1,2,6-hexanetriol, glycerol, trimethylolpropane, and bis (4-hydroxycyclohexyl)-2,2-propane. Another class of polyepoxides are the polyglycidyl esters of polycarboxylic acids. These compounds are produced by the reaction of epichlorohydrin or a similar epoxy compound with an aliphatic or aromatic polycarboxylic acid such as oxalic acid, succinic acid, glutaric acid, terephthalic acid, 2,6-naphthalene dicarboxylic acid and dimerized linoleic acid. Still another class of polyepoxides are derived from the epoxidation of an olefinically unsaturated alicyclic compound. These polyepoxides are non-phenolic and are obtained by epoxidation of alicyclic olefins, for example, by oxygen and selected metal catalysts, by perbenzoic acid, by acid-aldehyde monoperacetate or by peracetic acid. Among such polyepoxides are the epoxy alicyclic ethers and esters well known in the art. Useful polyepoxides also include those containing oxyalkylene groups in the epoxy molecule. Such oxyalkylene groups have the general formula: where R is hydrogen or alkyl, preferably a lower alkyl having from 1 to 6 carbon atoms, m is 1 to 4 and n is 2 to 50. Such groups are pendant to the main molecular chain of the polyepoxide or are part of the main chain itself. The proportion of oxyalkylene groups in the polyepoxide depends upon many factors, including the chain length of the oxyalkylene group, the nature of the epoxy and the degree of water solubility desired. Another class of polyepoxides consists of the epoxy novolac resins. These resins are obtained by reacting an epihalohydrin with the condensation product of aldehyde and monohydric or polyhydric phenols. A typical example is the reaction product of epichlorohydrin with a phenol- formaldehyde condensate. The polyepoxides can be partially defunctionalized by carboxylic acids, alcohol, water, phenols, mercaptans or other active hydrogen-containing compounds to give hydroxyl-containing polymers if desired. As has been stated above, the used curable composition can comprise more than one epoxide group containing material. Any number of epoxide group containing materials can be present in the mixture in any desired proportion. Usually the curable composition to be used according to the present invention contains from 70 percent by weight to 99 percent by weight of epoxide group containing component, (a). Preferably there is present from 70 percent by weight to 90 percent by weight of the epoxide group containing component, the percentages being based upon the total weight of the epoxide and polyester components of the composition. The second principal constituent of the curable composition to be used according to the present invention is an effectively thermoplastic polyester having a weight average molecular weight from 1000 to 8000. One very important feature of the aforesaid polyester is that it is insoluble in the one or more epoxide group containing materials. By "insoluble" is meant that the mixture of epoxide component and effectively thermoplastic polyester results in a hazy, heterogeneous phase. By "effectively thermoplastic" is meant that the polyester is not reactive with the one or more epoxide group containing materials. Preferably the effectively thermoplastic polyester is a saturated polyester although it should be understood that these saturated polyesters can contain some percentage of unsaturated units. The amount of unsaturation can vary widely so long as the thermoplastic nature of the polyester is maintained, i.e., it does not react into the curable composition but rather is present as a blended ingredient. In one preferred embodiment the saturated polyester contains no more than 10 percent by weight of unsaturated units, the percentage being based on the total weight of all the ingredients of the polyester. In a further preferred embodiment, the effectively thermoplastic polyester is substantially free of aromatic units. By "substantially free" is meant that it contains no more than 10 percent by weight of aromatic units, the percentage based upon the total weight of all of the ingredients of the polyester. As used herein, an aromatic unit is intended to mean which has a molecular weight of 76. The aromatic units are generally derived from aromatic acids or anhydrides such as phthalic acid, isophthalic acid and terephthalic acid. Polyesters with weight average molecular weights of greater than 10,000 are not useful, because the viscosity of the curable compositions becomes too high for satisfactory application. Molecular weight is determined by gel permeation chromatography (GPC) using a differential refractometer as the detector. The instrument is first calibrated using a polystyrene standard. Polystyrene standards used were purchased from pressure Chemicals Company, Pittsburgh, Pennsylvania. The polystyrene standards have dispersities (dispersity = weight average molecular weight/number average molecular weight) ranging from 1.05 to 1.10. The viscosity average molecular weights of the polystyrene standards used were 900,000; 233,000; 50,000; 17,500; 4,000. To obtain a calibration curve, a set of 0.1 percent (10 milligram polystyrene/1.0 ml tetrahydrofuran) polystyrene solutions in tetrahydrofuran were prepared and a 0.5 ml sample size was injected into the columns and a GPC chromatogram was obtained. The elution volume of each peak corresponding to a given molecular weight of the polystyrene standard was measured and the data was plotted on a semilogarithmic paper (logarithm scale in the ordinate and linear scale in the abscissa). A linear least squares plot of log₁₀ (molecular weight) versus elution volume in milliliters is used as a calibration curve. The lowest molecular weight of the polystyrene standard used was 4,000, and the calibration curve beyond that was extrapolated down to 100. The upper and lower exclusion limits of this set of columns are 5,000,000 and 100, respectively, in terms of polystyrene molecular weight. The sample whose molecular weights are to be determined was prepared as a 1.0 percent tetrahydrofuran solution. After filtration through a 0.5 micron filter, available from Millapore Corporation, a 0.5 ml sample size was injected into the columns and a GPC chromatogram obtained under the same experimental conditions as the calibration. From the resulting calibration curve of molecular weight versus retention time, a molecular weight relative to the standard can be assigned to the retention times of the sample. The height (H) of the curve at the corresponding retention times is recorded by the computer. From these height-molecular weight (M) combinations the following averages are calculated: This is the number reported. Suitable polyesters are those which satisfy the aforedescribed requirements and are prepared from polyfunctional acids and polyhydric alcohols. Examples of saturated acids for preparing the preferred saturated polyesters include adipic acid, azelaic acid, sebacic acid and the anhydrides thereof where they exist. When some proportion of unsaturation is present, it is commonly introduced by the use of unsaturated polyfunctional acids such as maleic acid and fumaric acid. Commonly utilized polyhydric alcohols are ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, butylene glycol, glycerol, trimethylolpropane, pentaerythritol and sorbitol. Preferably the polyester is prepared from a diol and diacid. The polyesters useful in the present invention are prepared by standard procedures well known in the art. One very useful procedure involves charging the diols and diacids to a reaction vessel, heating to a temperature ranging from 200°C to 300°C and removing the water of condensation by distillation. Care must be taken to avoid the loss of any of the reactants while removing the water since this of course will have an effect on the composition and molecular weight. Solvents such as xylene or toluene may be employed in the reaction mixture to help remove water by azeotropic distillation and an esterification catalyst such as a tin compound, for example, dibutyltin oxide or butyl stannoic acid can also be employed to increase the rate of reaction. Of course, functional derivatives of the diacids such as esters anhydrides or acid chlorides may be employed. It should be understood that the term polyester as used herein also applies to resins which contain oil or fatty acid modification, i.e. alkyd resins. As was mentioned above, the effectively thermoplastic polyester is present in the claimed curable compositions to be used according to the invention in a positive amount which does not exceed 30 percent by weight based on the total weight of the epoxide and polyester components of the composition. Preferably the polyester is present in an amount ranging from 10 to 30 percent by weight. The third principal component, the curing agent to cure the one or more epoxide group containing materials, can be selected from a variety of materials. Suitable curing agents for the epoxide(s) include aliphatic, cycloaliphatic and aromatic polyfunctional amines; and polyamides. Examples of suitable amines include ethylene diamine, diethylene triamine, triethylene tetraamine, tetraethylene pentamine, 1,4-diaminobutane; 1,3-diaminobutane, hexamethylene diamine, 3-(N-isopropylamino) propylamine, diaminocyclohexane, and polyoxypropyleneamines commercially available under the trademark designation JEFFAMINE®. Aromatic amines, although not preferred, can also be utilized herein, for example metaphenylene diamine, P,P'-methylene dianiline, and 1,4-aminonaphthalene. Also very useful herein are latent curing agents such as boron trifluoride monoethylamine complex, boron trifluoride diethylamine complex, boron trifluoride triethylamine complex, boron trifluoride pyridine complex, boron trifluoride benzyldimethylamine complex, boron trifluoride benzylamine, boron trifluoride etherate, and dicyandiamide. By latent is meant that these materials are inactive until activated by the application of heat. Polyamides which are useful in the present compositions are those derived from fatty acids or dimerized fatty acids or polymeric fatty acids and aliphatic polyamines. For example, the materials commercially available from Henkel under the trademark designations VERSAMIDE® 220 or 125 are quite useful herein. The aforedescribed curing agent is present in the claimed curable compositions in an amount ranging from 1 percent by weight to 50 percent by weight, preferably from 3 to 15 percent by weight, the percentages being based on the total weight of the curable composition. The curable compositions to be used according to the present invention can contain a variety of additives including pigments, reinforcements, thixotropes, plasticizers, extenders, stabilizers and antioxidants. The compositions can be prepared in a number of ways either as a one-package type of composition or as a two-package composition. One package compositions can be prepared with the use of a latent curing agent. The curable compositions are typically prepared by combining the ingredients at the time immediately before use. If a one-package composition is prepared, this can be prepared in advance of use and stored. The compositions can be applied in a number of ways including spraying, extrusion, or by hand with a blade. They can be cured by allowing them to stand at ambient temperature, or a combination of ambient temperature cure and baking, or by baking alone. The compositions can be cured at ambient temperature typically in a period ranging from 1 hour to 48 hours, preferably from 3 hours to 5 hours. If ambient temperature and baking are utilized in combination, the composition is typically allowed to stand for a period of from 5 hours to 24 hours followed by baking at a temperature of from 75°C to 200°C, preferably from 100°C to 150°C, for a period of time ranging from 20 minutes to 1 hour. The number average molecular weight is determined in the same fashion as is detailed above for weight average molecular weight determinations. Each of the components has been discussed in detail above therefore further elaboration is not deemed to be necessary. The curable compositions to be used according to the present invention exhibit excellent physical properties and in addition have markedly reduced shrinkage. This is quite important in applications where the appearance of the substrate is critical, such as automotive applications, because the reduced shrinkage translates into substantially reduced deformation of the substrate. As a result, there is a substantial labor and cost reduction because the need for additional labor to restore the deformed substrate is markedly reduced. The use for preparing an adhesive bond between two surfaces of substrate to form a bonded structure comprises applying to at least one of the surfaces the curable composition detailed above and then at least partially curing the curable composition to form the bonded structure. Both the polymerizable composition and the mode of effecting cure have been discussed in detail above. As has been mentioned, the use of the curable composition is particularly applicable to structural building components of vehicles, e.g., doors, frames, hoods and the like of cars, vans, buses, and other vehicles. The following Examples are illustrative of the invention and are not intended to limit it to their details. Example In this example several adhesive compositions were prepared and evaluated as is detailed below. Each of the adhesive compositions was prepared by combining the ingredients together with mild agitation. Each composition was tested for physical properties as follows. Lap Shear Strength : Lap shear bonds for testing were prepared using two strips of cold rolled steel 2.5 x 10.16 x 0.16 cm (1 inch x 4 inches x 0.062 inch). A 152.4 µm (6 mil) thick film of a composition was applied onto one of the metal strips and then a second strip was placed over top of the first strip so that only 6.5 cm² (one square inch) strip overlapped. The composition was cured at 177°C. for 20 minutes. The lap shear strength of the bond in N/m² (pounds per square inch (psi)) was determined according to ASTM D-1002-65. (The ends of the strips were pulled with an INSTRON TESTER device and the lap shear strength of the bond measured.) Flexural Strength : A panel for testing was prepared by coating a 2.54 x 15.2 x 0.08 cm (1 inch x 6 inches x 0.031) inch metal panel with a 2.8 mm (0.110 inch) thick layer of a composition. The composition was cured by baking the panel for one hour at 177°C and then the panel was cooled to room temperature. Flexural strength was measured in pounds by using a three point loading system utilizing center loading on a simply supported beam. That is, the panel was supported at two edges and the load was placed in the center. A load was applied at a rate of 5.1 mm (0.2 inch) per minute and the variable measured was the amount of force in pounds required to deflect the specimen a distance of 3.2 mm (0.125 inch). Each value is an average of two separate determinations. Tensile Strength and Tensile Modulus : A 2.54 mm (0.100 inch) thick layer of a composition was applied onto a glass panel measuring 30.5 x 30.5 x 0.254 cm (12 inches X 12 inches X 0.100 inch). A second glass panel of the same dimensions was placed over top of this first panel and the panels were clamped together. The composition was cured by baking at 177°C for 40 minutes and then the panels were cooled to room temperature. The clamps were removed and the free adhesive film was removed from between the panels. The free films were prepared and evaluated for tensile strength and tensile modulus according to ASTM D 638. Each value in N/m² (psi) is an average of two separate determinations. Metal Deformation : A panel was prepared by applying a 2.54 x 15.2 x 0.28 cm (1 inch x 6 inches x 0.11 inch) adhesive bead onto a metal panel measuring 10.16 x 30.5 x 0.08 cm (4 inches x 12 inches x 0.031 inch) and baking at 177°C. for 20 minutes. The panel was visually observed for the amount of metal deformation which was an indication of the shrinkage of the composition. Solubility of each Polyester in the Epoxide Component : The solubility of each of the polyesters in the epoxide component of each the compositions was determined by mixing the polyester with the epoxide component at room temperature. After the air bubbles had disappeared the clarity or haziness of the mixture was noted. A hazy mixture indicated that the polyester was insoluble in the epoxide component while a clear mixture indicated that the polyester was soluble in the epoxide component. The results of each of the tests is recorded in Table I. As the data shows, the polyesters which were insoluble in the epoxide component gave compositions which resulted in the least amount of metal deformation. The control composition, A, which contained no polyester; and the composition F, which contained a polyester soluble in the epoxide component resulted in severe metal deformation. Compositions Solubility of Polyester in Epoxy Component Flexural Strength (pounds) Kilograms Lap Shear Strength x10⁴N/m²(psi) Tensile Strength (psi) x10⁴N/m² Tensile Modulus (psi) x10⁸N/m² Metal Deformation A (control) contained no polyester 19.5 (32) 2639 (3828) 4013 (5821) 45(650504) severe B not soluble 10.9 (24) 2220 (3220) 3560 (5164) 33(483484) none C not soluble 14.1 (31) 2243 (3543) 2671 (3874) 38(554546) very slight D not soluble 11.4 (25) 2007 (2911) 3112 (4513) 29(422029) none E not soluble 15 (33) 1789 (2593) 3789 (5496) 41(588123) slight F soluble 3.3 (7.2) 227 (329) 552 (800) 11(163191) severe G not soluble 12.7 (28) 2021 (2931) 2119 (3073) 35(503869) slight H not soluble 11.1 (24.5) 2539 (3682) 3794 (5502) 39(564099) slight

Claims: Use of a thermoplastic polyester as a shrinkage-reducing additive in a curable structural adhesive composition for forming a bonded structure between two surfaces of substrates without deformation of the substrates, said adhesive composition comprising (a) one or more epoxide groups containing materials containing at least two epoxide groups per molecule, and (c) a curing agent for the one or more epoxide groups containing materials of (a); and wherein said thermoplastic polyester (b) is not reactive with one or more epoxide groups containing materials and being insoluble in the one or more epoxide groups containing materials of (a) and having a weight average molecular weight of from 1000-8000 and being present in an amount ranging from 1-30 w% based on the total weight of the epoxide and polyester components in the composition. Use of a thermoplastic polyester as a shrinkage-reducing additive in a curable structural adhesive composition for forming a bonded structure between two surfaces of substrates without deformation of the substrates, said adhesive composition comprising (a) one or more epoxide groups containing materials containing at least two epoxide groups per molecule, and (c) a curing agent for the one or more epoxide groups containing materials of (a), wherein said thermoplastic polyester (b) is not reactive with one or more epoxide groups containing materials and being insoluble in the one or more epoxide groups containing materials of (a) and having a number average molecular weight of from 400-5000 and being present in an amount ranging from 1-30 w% based on the total weight of the epoxide and polyester components in the composition. The use of claims 1 or 2, characterized in that the polyester is a saturated polyester containing not more than 10 percent by weight of unsaturated units, the percentage based on the total weight of all the ingredients of the polyester. The use of claim 3, characterized in that the saturated polyester is substantially free of aromatic units. The use of claims 1 or 2, characterized in that (a) is comprised of a single epoxide group containing material which is a diglycidyl ether of bisphenol A. The use of claims 1 or 2, characterized in that (a) is comprised of a mixture of two or more epoxide groups containing materials. The use of claims 1 or 2, characterized in that the curing agent is an amine curing agent. The use of claim 7, characterized in that the amine curing agent is dicyandiamide. The use of claims 1 or 2, characterized in that the curing agent is present in the curable composition in an amount ranging from 1 percent by weight to 50 percent by weight, the percentages based on the total weight of the composition. The use of any of the preceding claims by applying to at least one of the surfaces of substrates the curable composition according to any of the preceding claims and at least partially curing the curable composition to form the bonded structure.

Inventors: PICCIRILLI ROBERT MICHAEL, SCHAPPERT RAYMOND FRANCIS, PICCIRILLI, ROBERT MICHAEL, SCHAPPERT, RAYMOND FRANCIS
Applicants: PPG INDUSTRIES INC, PPG INDUSTRIES, INC.
Application year: 1987
Main classification: C08L63
IPC codes: C08L63, C08G59, C08L67