Use of toughness improvers for increasing the maximum linear expansion of single-component heat-curing epoxy resin compositions

Methods in which toughness improvers based on terminally blocked polyurethane prepolymers are used to increase the maximum linear expansion of single component heat-curing epoxy resin compositions, in particular for joining substrates having different thermal expansion coefficients, in particular in the framework of transport agents or white goods.

TECHNICAL FIELD

The invention relates to the field of thermosetting epoxy resin compositions, especially for the bonding of substrates having different coefficients of thermal expansion, especially in the shell construction of modes of transport or white goods.

PRIOR ART

Thermosetting epoxy resin compositions have long been known. An important field of use of thermosetting epoxy resin compositions is in motor vehicle construction, especially in bonding in the shell construction of modes of transport or white goods. In both cases, after the application of the epoxy resin composition, the bonded article is heated in an oven, which also cures the thermosetting epoxy resin composition.

If two substrates having different coefficients of linear thermal expansion are bonded to one another by structural bonding, the result of the curing step in the oven at temperatures of 120-220° C. is that the two substrates expand to different lengths. The subsequent cooling thus gives rise to a high tension in the cured epoxy resin composition, which leads either to failure of the adhesive bond, to deformation of the substrates, or to “freezing” of the tension in the adhesive bond. As a result of such “freezing”, the adhesive bond during its lifetime is significantly more sensitive to static, dynamic and shock stresses, which can lead to weakening of the adhesive bond.

There is therefore a need for thermosetting epoxy resin compositions for structural bonding of substrates having different coefficients of thermal linear expansion, which firstly have adequate mechanical properties for structural bonding and secondly withstand the high stresses that occur in thermosetting without failure of the structural bond.

SUMMARY OF THE INVENTION

The object of the present invention is therefore that of providing thermosetting epoxy resin compositions suitable for structural bonding of substrates having different coefficients of linear thermal expansion, which firstly have adequate mechanical properties for structural bonding and secondly assure bonding in spite of the high stresses that occur in thermosetting without failure of the structural bond.

This object was surprisingly achieved by an inventive use as claimed in claim1.

Further aspects of the invention are the subject of further independent claims. Particularly preferred embodiments of the invention are the subject of the dependent claims.

EXAMPLES

Some examples which further illustrate the invention, but which are not intended to restrict the scope of the invention in any way, are cited below.

Determination of Isocyanate Content

The isocyanate content was determined in % by weight by means of a back-titration with di-n-butylamine used in excess and 0.1 M hydrochloric acid. All determinations were conducted in a semi-manual manner on a Mettler-Toledo DL50 Graphix titrator with automatic potentiometric endpoint determination. For this purpose, 600-800 mg in each case of the sample to be determined was dissolved while heating in a mixture of 10 ml of isopropanol and 40 ml of xylene, and then reacted with a solution of dibutylamine in xylene. Excess di-n-butylamine was titrated with 0.1 M hydrochloric acid, and the isocyanate content was calculated therefrom.
Maximum Linear Expansion (Max. Elongation) and Maximum Force (Max. Force)
Maximum linear expansion (Max. elongation) and maximum force (Max. force) were determined as described above under “Description of test method for maximum linear expansion (Max. elongation) and maximum force”. A triple determination was conducted for each epoxy resin composition. The maximum linear expansion Max. elongation was determined from the distance traversed. The measurement was evaluated by taking, from the measurement protocol, the averages of the maximum linear expansion Max. elongation (εM2) at maximum force (σM2).
Tensile Strength, Elongation at Break and Modulus of Elasticity (DIN EN ISO 527)
An adhesive sample was pressed between two Teflon papers to a layer thickness of 2 mm. After curing at 175° C. for 35 min, the Teflon papers were removed and the specimens were die-cut to the DIN standard state. The test specimens were examined under standard climatic conditions at a strain rate of 2 mm/min. Tensile strength, elongation at break and the 0.05-0.25% modulus of elasticity were determined to DIN EN ISO 527.
Lap Shear Strength (LSS) (DIN EN 1465)
Cleaned test specimens of H420+Z steel (thickness 1.2 mm) that had been reoiled with Anticorit PL 3802-39S were bonded with the adhesive over a bonding area of 25×10 mm with glass beads as spacer in a layer thickness of 1.5 mm, and cured under the curing conditions specified.
Curing conditions: a) 35 min at oven temperature 175° C.
Lap shear strength was determined on a tensile tester at a strain rate of 10 mm/min in a triple determination to DIN EN 1465.
T-Peel Strength (DIN 53281)
130×25 mm test sheets of DC-04+ZE steel (thickness 0.8 mm) were prepared. Test sheets were processed at a height of 30 mm with a suitable die-cutting machine (90°). The cleaned 100×25 mm surfaces that had been reoiled with Anticorit PL 3802-39S were bonded with the adhesive with glass beads as spacer in a layer thickness of 0.3 mm, and cured for a dwell time of 35 min from attainment of an oven temperature of 175° C. T-peel strength was determined on a tensile tester at a strain rate of 100 mm/min in a duplicate determination as peel force in N/mm in the traversed distance range from ⅙ to ⅚ of the distance covered.
Impact Peel Strength (to ISO 11343)
The specimens were produced with the adhesive and DC04+ZE steel with dimensions of 90×20×0.8 mm. The bonding area here was 20×30 mm at a layer thickness of 0.3 mm with glass beads as spacer. Impact peel strength was measured in each case at the temperatures specified (RT=23° C., −30° C.) as a triple determination on a Zwick 450 impact pendulum at 2 m/s. The impact peel strength reported is the average force in N/mm under the measurement curve from 25% to 90% to ISO11343.
The adhesives were cured at oven temperature 175° C. for 35 min.
Viscosity
Viscosity measurements of the adhesives were effected 1 d after production on an Anton Paar MCR 101 rheometer by oscillation using a plate-plate geometry at a temperature of 25° C. or 50° C. with the following parameters: 5 Hz, 1 mm gap, plate-plate distance 25 mm, 1% deformation.
The following commercial products were used for the production of impact modifiers 1 to 9:

225 g of PolyTHF 2000, 225 g of Poly bd R45V and 2.25 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 90.83 g of isophorone diisocyanate (IPDI) and 0.059 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 3.05%.
To the resultant NCO-terminated polymer were added 0.117 g of dibutyltin dilaurate (DBTDL) and 44.35 g of phenol, and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 5 h. Measured free NCO content: (directly after preparation) 0.53%, (1 day after preparation) 0.24%.

200 g of PolyTHF 2000, 200 g of Poly bd R45V and 2.00 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 80.64 g of isophorone diisocyanate (IPDI) and 0.053 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 2.81%.
To the resultant NCO-terminated polymer were added 0.106 g of dibutyltin dilaurate (DBTDL) and 47.93 g of 4-methoxyphenol (HQMME), and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 5 h. Measured free NCO content: (directly after preparation) 2.82%, (1 day after preparation) 0.09%.

350 g of PolyTHF 2000 and 3.50 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 77.82 g of isophorone diisocyanate (IPDI) and 0.048 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 3.35%.
To the resultant NCO-terminated polymer were added 0.096 g of dibutyltin dilaurate (DBTDL) and 50.76 g of 4-methoxyphenol (HQMME), and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 5 h. Measured free NCO content: (directly after preparation) 0.48%, (1 day after preparation) 0.23%.
Viscosity (1 day after preparation): 422 Pa*s at 25° C., 82 Pa*s at 50° C.

400 g of PolyTHF 2000 and 4.50 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 88.62 g of isophorone diisocyanate (IPDI) and 0.055 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 3.61%.
To the resultant NCO-terminated polymer were added 0.110 g of dibutyltin dilaurate (DBTDL) and 59.64 g of benzoxazolinone, and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 3 h. Measured free NCO content: 0.24%.

225 g of PolyTHF 2000, 225 g of Poly bd R45V and 2.25 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 77.82 g of isophorone diisocyanate (IPDI) and 0.058 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 3.08%.
To the resultant NCO-terminated polymer were added 0.116 g of dibutyltin dilaurate (DBTDL) and 38.11 g of 3,5-dimethylpyrazole, and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 2 h. Measured free NCO content: 0.0%.

175 g of PolyTHF 2000, 175 g of Poly bd R45V and 1.75 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 70.56 g of isophorone diisocyanate (IPDI) and 0.046 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 2.90%.
To the resultant NCO-terminated polymer were added 0.92 g of dibutyltin dilaurate (DBTDL) and 37.52 g of dibutylamine, and the isocyanate groups were depleted by reaction at 70° C. under reduced pressure for 3 h. Measured free NCO content: 0.0%.

200 g of PolyTHF 2000, 200 g of Poly BD R45V and 2.00 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 80.74 g of isophorone diisocyanate (IPDI) and 0.052 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 2.91%.
To the resultant NCO-terminated polymer were added 0.104 g of dibutyltin dilaurate (DBTDL) and 34.82 g of 2-butane oxime (MEKO), and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 1 h. Measured free NCO content: 0.00%.

290 g of PolyTHF 2000 and 2.90 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 64.25 g of isophorone diisocyanate (IPDI) and 0.046 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 3.36%.
To the resultant NCO-terminated polymer were added 0.092 g of dibutyltin dilaurate (DBTDL) and 104.76 g of 2,2-diallylbisphenol, and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 5 h. Measured free NCO content after 5 h: 0.74%.
Measured free NCO content the next day at RT: 0.36%.

200 g of PolyTHF 2000, 200 g of Poly bd R45V and 2.00 g of BHT as stabilizer were dewatered at 90° C. under reduced pressure with minimal stirring for 1 h. Subsequently, 80.74 g of isophorone diisocyanate (IPDI) and 0.053 g of dibutyltin dilaurate (DBTDL) were added. The reaction was conducted under moderate stirring at 90° C. under reduced pressure for 2 h in order to obtain an isocyanate-terminated polymer: Measured free NCO content: 3.026%.
To the resultant NCO-terminated polymer were added 0.106 g of dibutyltin dilaurate (DBTDL) and 47.01 g of caprolactam, and the isocyanate groups were depleted by reaction at 110° C. under reduced pressure for 3 h. Measured free NCO content: 0.00%.

Example Adhesives 1 to 9

The impact modifiers SM1 to SM9 prepared in examples 1 to 9 were each used for production of epoxy resin compositions according to table 2.

Compositions and proportions for epoxy resin compositions containing one of impact modifiers 1 to 9 are summarized in table 2. SM-X relates to the impact modifiers prepared above SM1, SM2, etc.

TABLE 2Parts byChemicalweightcompositionFunction23.0Epoxy resin based on bisphenolEpoxy resinA, liquidmatrix12.0Epoxy resin based on bisphenolEpoxy resinA, solidmatrix0.5p-tert-Butylphenyl glycidyl etherReactive diluent50.0Blocked polyurethane, SM-XImpact modifier2.43DicyandiamideCuring agent0.13UroneCuring agentaccelerator5.0CaCO3Filler6.0Calcium oxideMoisturescavenger8.0Fumed silicaThixotropicagentTotal:107.06Dicy index:5.50 mol EP/dicy
The respective epoxy resin compositions were mixed in a planetary mixer in a batch size of 350 g. For this purpose, the mixing vessel was filled with the liquid components, followed by the solid components, and they were mixed at 70° C. under reduced pressure. During the mixing operation (about 45 min), the vacuum was broken several times and the mixing tool wiped clean. After a homogeneous mixture had been obtained, the epoxy resin composition was dispensed into cartridges and stored at room temperature.
Tables 3 and 4 show the results of the evaluation of the resultant epoxy resin compositions with the impact modifiers SM1-9.

Impact modifiers SM2, SM3 and SM4 were used in different concentrations to produce adhesives according to table 5. The epoxy resin “bisphenol A epoxy resin” used is a mixture of 2 parts liquid epoxy resin based on bisphenol A and one part solid epoxy resin based on bisphenol A.
Tables 6-8 show the results of the evaluation of the resultant adhesives 10-25 with the impact modifiers SM2, SM3 and SM4.
The maximum linear expansion as a function of the proportion of the impact modifier is shown inFIGS.2to4.