WO2015102911A1

WO2015102911A1 - Toughening agents for epoxy systems

Info

Publication number: WO2015102911A1
Application number: PCT/US2014/070934
Authority: WO
Inventors: Bhawesh Kumar; Kandathil E. Verghese; George C. Jacob
Original assignee: Dow Global Technologies Llc
Priority date: 2013-12-30
Filing date: 2014-12-17
Publication date: 2015-07-09
Also published as: TW201529706A

Abstract

A curable composition comprising: a) 30 weight percent to 98 weight percent of an epoxy resin; b) a hardener; c) 1 weight percent to 10 weight percent of nanosilica; and d) 1 weight percent to 10 weight percent of a core shell rubber comprising a rubber particle core and a shell layer, is disclosed.

Description

TOUGHENING AGENTS FOR EPOXY SYSTEMS

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is related to epoxy systems useful in composite applications.

More particularly, the present invention is related to toughening agents for such epoxy systems.

Introduction

A variety of fillers have been used to toughen epoxy resin systems. In particular, hard and soft nanoparticle fillers have both been used. One example of hard nanoparticle fillers is nanosilica. Nanosilica can improve the fracture toughness, stiffness, and strength. However, nanosilica does not toughen epoxy resins as much as soft nanoparticle fillers. Epoxies toughened with hard nanoparticle fillers do not meet the high toughness and stiffness requirements of aerospace composite applications.

Combinations of hard and soft nanoparticle fillers have also been used in epoxy applications. For example, nanosilica has been used in combination with micro rubber particles. However, these systems have exhibited losses in other desirable properties, such as glass transition temperature (Tg), modulus, and strength. Additionally, the resulting compositions do not have a balance of fracture and fatigue (cycles to failure) performance.

Therefore, epoxy resin systems having a balance of fracture toughness and fatigue performance without compromising other properties such as viscosity, glass transition temperature, and modulus, would be desirable.

SUMMARY OF THE INVENTION

In one broad embodiment of the present invention, there is disclosed a curable composition comprising, consisting of, or consisting essentially of: a) 30 weight percent to 98 weight percent of an epoxy resin; b) a hardener c) 1 weight percent to 10 weight percent of nanosilica; and d) 1 weight percent to 10 weight percent of a core shell rubber comprising a rubber particle core and a shell layer. DETAILED DESCRIPTION OF THE INVENTION

The curable composition of the present invention includes at least one epoxy resin. The epoxy resin may be saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic and may be substituted. The epoxy resin may also be monomeric or polymeric.

The epoxy resins, used in embodiments disclosed herein for component (a) of the present invention, may vary and include conventional and commercially available epoxy resins, which may be used alone or in combinations of two or more. In choosing epoxy resins for compositions disclosed herein, consideration should not only be given to properties of the final product, but also to viscosity and other properties that may influence the processing of the resin composition.

Particularly suitable epoxy resins known to the skilled worker are based on reaction products of polyfunctional alcohols, phenols, cycloaliphatic carboxylic acids, aromatic amines, or aminophenols with epichlorohydrin. A few non-limiting embodiments include, for example, bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, resorcinol diglycidyl ether, and triglycidyl ethers of para- aminophenols. Other suitable epoxy resins known to the skilled worker include reaction products of epichlorohydrin with o-cresol and, respectively, phenol novolacs. Further epoxy resins include epoxides of divinylbenzene or

divinylnaphthalene. It is also possible to use a mixture of two or more epoxy resins.

The epoxy resins useful in the present invention may be selected from commercially available products; for example, D.E.R®. 331, D.E.R. 332, D.E.R. 383, D.E.R. 334, D.E.R. 580, D.E.N. 431, D.E.N. 438, D.E.R. 736, or D.E.R. 732 epoxy resins available from The Dow Chemical Company or Syna 21 cycloaliphatic epoxy resin from Synasia. As an illustration of the present invention, the epoxy resin component (a) may be a mixture of a liquid epoxy resin, such as D.E.R. 383, an epoxy novolac DEN 438, a cycloaliphatic epoxide Syna 21, and a divinylarene dioxide, divinylbenzene dioxide (DVBDO) and mixtures thereof.

Generally, the epoxy resin is present in the composition in an amount in the range of from 30 weight percent to 98 weight percent, based on the total weight of the composition. The epoxy resin can be present in an amount in the range of 70 to 85 weight percentage in various other embodiments.

The composition also includes an amine hardener. Examples of amine hardeners include but are not limited to primary and secondary polyamines and adducts thereof, anhydrides, and polyamides. Examples include aromatic amines, aliphatic amines, cycloaliphatic amines, and amidoamines. For example, polyfunctional amines may include aliphatic amine compounds such as diethylene triamine (D.E.H. 20, available from The Dow Chemical Company (TDCC)), triethylene tetramine (D.E.H.™ 24, available from TDCC), tetraethylene pentamine (D.E.H.™ 26, available from TDCC), as well as adducts of the above amines with epoxy resins, diluents, or other amine-reactive compounds. Aromatic amines, such as metaphenylene diamine and diamine diphenyl sulfone, aliphatic polyamines, such as amino ethyl piperazine and polyethylene polyamine, and aromatic polyamines, such as metaphenylene diamine, diamino diphenyl sulfone, and diethyltoluene diamine, can also be used.

Generally, the amine hardener is present in the composition in an amount in the range of from 0.1 to 100 parts hardener per one hundred parts epoxy resin by weight. The amine hardener can be present in an amount in the range of from 20 to30 parts hardener per one hundred parts epoxy resin in various other embodiments.

The toughening agent of the composition includes nanosilica particles.

In various embodiments, the nanosilica used is a colloidal silica sol in a resin matrix with surface modified, spherically shaped silica nanoparticles having a diameter of 50 nm. The particles are synthesized from aqueous sodium silicate solution and then undergo a process of surface modification with organosilane and matrix exchange, to produce a masterbatch of 40 wt (26 vol. %) silica in the epoxy resin. In various embodiments, the nanosilica particles have a mean particle size of about 20 nm, with an extremely narrow range of particle-size distribution and most particles are generally between 5 and 35 nm in diameter. In various embodiments, Nanopox^® F400 is used.

Generally, the nanosilica is present in the composition in an amount in the range of from 1 weight percent to 10 weight percent. In another embodiment, the nanosilica is present in the composition in an amount in the range of from 2.5 weight percent to 7.5 weight percent, based on the total weight of the composition.

The toughening agent of the composition also includes a core-shell rubber. A core shell rubber is a polymer comprising a rubber particle core formed by a polymer comprising an elastomeric or rubbery polymer as a main ingredient and a shell layer formed by a polymer graft polymerized on the core. The shell layer partially or entirely covers the surface of the rubber particle core by graft polymerizing a monomer to the core. Generally the rubber particle core is constituted from acrylic or methacrylic acid ester monomers or diene

(conjugated diene) monomers or vinyl monomers or siloxane type monomers and

combinations thereof. The core-shell rubber may be selected from commercially available products; for example, Paraloid EXL 2650A, EXL 2655, EXL2691 A, each available from The Dow Chemical Company, or Kane Ace® MX series from Kaneka Corporation, such as MX 120, MX 125, MX 130, MX 136, MX 551, or METABLEN SX-006 available from Mitsubishi Rayon.

Generally, the core-shell rubber is present in the composition in an amount in the range of from 1 weight percent to 10 weight percent. In another embodiment, the core-shell rubber is present in the composition in an amount in the range of from 2.5 weight percent to 7.5 weight percent, based on the total weight of the composition.

Optionally, catalysts can be added to the curable composition. Examples of catalysts that can be used include, but are not limited to 2-methyl imidazole (2MI), 2-phenyl imidazole (2PI), 2-ethyl-4-methyl imidazole (2E4MI), l-benzyl-2-phenylimidazole (1B2PZ), boric acid, triphenylphosphine (TPP), tetraphenylphosphonium-tetraphenylborate (TPP-k) and mixtures thereof.

The catalyst is generally present in the curable composition in an amount in the range of from 0.01 weight percent to 20 weight percent, based on the total weight of the curable composition. In another embodiment, the catalyst is present in an amount in the range of from 0.05 weight percent to 10 weight percent, and is present in an amount in the range of from 0.02 weight percent to 3 weight percent in yet another embodiment.

In one or more embodiments, the curable composition can also include fillers.

Examples of fillers include but are not limited to silica, zinc oxide, alumina, titanium dioxide, aluminum trihydrate (ATH), magnesium hydroxide, and combinations thereof.

In one or more embodiments, the curable composition can contain a solvent.

Examples of solvents that can be used include, but are not limited to methyl ethyl ketone (MEK), dimethylformamide (DMF), ethyl alcohol (EtOH), propylene glycol methyl ether (PM), propylene glycol methyl ether acetate (DOWANOL™ PMA), 1,4-butanediol diglycidyl ether and mixtures thereof.

The composition can be produced by any suitable process known to those skilled in the art. In an embodiment, core-shell rubber particles are dispersed in an epoxy matrix. In various embodiments, the dispersion is performed at room temperature. The fillers can then be mixed into the epoxy resin using high speed shear mixing, or any other mixing method known to those skilled in the art. The hardener and any other desired components, such as the optional components described above, are then added to the mixture. In various embodiments, the mixture is then poured into a mold and is left at room temperature for one hour and is then cured at about 70°C for about 7 hours.

The cured products are generally used in composite applications such as windmill blades, automotive parts, pressure vessels, sporting goods etc.

EXAMPLES

D.E.R. 383 (DER 383) - DGEBA type liquid epoxy resin from The Dow Chemical Company.

PARALOID™ EXL 2619A Impact Modifier (EXL 2619A) - core-shell butadiene rubber, from The Dow Chemical Company

Jeffamine^® D230 - polyetheramine hardener from Huntsman

Vestamin^® IPD - cycloaliphatic hardener from Evonik

D.E.H.™ 52 (DEH 52) - polyamine/polyamide hardener from The Dow Chemical Company Nanopox^® F400: silica reinforced bisphenolA-based epoxy resin with surface modified, spherically shaped silica nanoparticles having diameter of 50 nm, available from Evonik

Method of making the plaques

The required amounts of core-shell rubber EXL 2691 A was added to DER 383 and homogenized using an IKA UltraTurrax T25 homogenizer for 30 minutes. This initial mixture was kept for 3-5 days for soaking which helps in dispersion of rubber particles in epoxy matrix. After the soaking step, the required amount of Nanopox^® F400 was added and mixed with an overhead stirrer at 500-700 rpm for 10 minutes. Following this step, the resin with fillers was homogenized for 2 more hours. The mixture was then degassed at 2-5mm Hg for 15-20 minutes. Subsequently, the required amount of a hardener component comprising Jeffamine D-230 (64 weight percent), Vestamin IPD (33 weight percent) and DEH 52 (3 weight percent) was added and hand mixed. The mixture was then degassed for 5 minutes and poured into the mold. The mixture was kept in the mold at room temperature for one hour and was then placed in an oven for 7 hours at 70°C. The plaque was then slowly cooled to room temperature.

Various toughened systems with Nanopox^® F400 (silica nanoparticles masterbatch) as a hard filler and EXL-2691A were produced as shown in Table 1 below. Three different loading levels of the filler loading (5 weight percent, 7.5 weight percent and 10 weight percent) were also considered. The weight fraction of hard fillers and soft fillers varied for each filler loading. For example, for 5% filler loading, 5 different compositions of Nanopox' F400 and EXL-2691A were considered as mentioned in Table 1. A total of 16 plaques were fabricated as mentioned above.

Table 1

The properties of various examples are shown in Table 2, below.

Table 2

Test Methods

Glass Transition Temperature (Tg)

Glass transition temperature was determined by Differential Scanning Calorimetry (DSC) using a Q2000 machine from TA Instruments. Typically, a thermal scan ranges from room temperature to 250 °C and heating rate of 10 °C/min was used. Two heating cycles were performed, with the curve from the second cycle used for Tg determination by "middle of inflection" method.

Fracture Toughness (KT_C)

Fracture toughness was determined by ASTM D5045 performed on a typical Instron or MTS equipment. Samples were cut to dimension using a water jet cutter. A starter crack was introduced using a cold razor blade by gently tapping into the chevron notch in the specimen at room temperature. The crack tip should be sharp to achieve the singularity of stress field. Specimens were checked to make sure notch was not too long in which case the specimen was not tested. Razor blades were placed in a container of dry ice for 30 minutes before use. The test was conducted on an Instron 5566 mechanical test frame equipped with a 200 lb load cell with a constant crosshead speed of 0.02 in/min under room temperature conditions. Samples were loaded using a clamp and dowel pin. Data was recorded using Instron Bluehill software. In most cases, five specimens were analyzed.

Modulus

Modulus was determined by dynamic mechanical analysis (DMA). DMA of the cured samples was done using a TA instrument RSA3 DMA. Tests were carried out using a three point bend configuration. Sample size was typically 10-11 mm wide and 2.6-2.8 mm thick. The length of the sample was 40 mm which was controlled by the simply-supported points in the three point bend fixture. Three runs were done for each sample. Temperature was varied from room temperature to 110°C using compressed gas and internal heater as temperature modulators. Frequency was kept constant at 1 Hz. The strain value was determined by a strain sweep experiment and the standard value was chosen where the modulus variation is linear. Typically the strain value was around 0.0015 to 0.002. TA Orchestrator software controlled the instrument and data was analyzed by taking ASCII raw data and plotting in EXCEL™. Modulus was measured at 35°C.

Viscosity

Viscosity was determined by a Brookfield viscometer model number DVII+ Pro. Two types of cylindrical spindles were utilized SC-4-15 and SC-4-21 from Brookfield on different occasions. RPM was adjusted until torque value reached 90-98% for each sample. For the viscosity measurements, only part A was considered with various loadings of the toughening agents.

Claims

WHAT IS CLAIMED IS:

1. A curable composition comprising:

a) 30 weight percent to 98 weight percent of an epoxy resin; b) a hardener;

c) 1 weight percent to 10 weight percent of nanosilica; and

d) 1 weight percent to 10 weight percent of a core shell rubber comprising a rubber particle core and a shell layer.

2. A curable composition in accordance with claim 1 wherein the core shell rubber comprises a functional group selected from the group consisting of carboxyl groups, hydroxyl groups, carbon-carbon double bonds, anhydride groups, amino groups, amide groups, and combinations thereof.

3. A curable composition in accordance with any one of the preceding claims wherein the epoxy resin is selected from the group consisting of a diglycidyl ether of bisphenol-A, a diglycidyl ether of bisphenol-F, and combinations thereof.

4. A composition in accordance with any one of the preceding claims wherein the hardener is selected from the group consisting of aromatic amines, aliphatic amines, cycloaliphatic amines, anhydrides, amido amines, polyamides and combinations thereof.

5. A process for preparing the composition of any one of the preceding claims comprising the steps of:

a) dispersing the core shell rubber into the epoxy resin to form a dispersion; and b) incorporating nanosilica and a core-shell rubber into the dispersion.

6. A composite made from the composition of any one of the preceding claims.