WO2016058158A1

WO2016058158A1 - Process of preparing shaped composites

Info

Publication number: WO2016058158A1
Application number: PCT/CN2014/088732
Authority: WO
Inventors: Yonghua GONG; Yi Zhang; Jianle BAO; Jun Lian
Original assignee: Blue Cube Ip Llc
Priority date: 2014-10-16
Filing date: 2014-10-16
Publication date: 2016-04-21
Also published as: TW201615710A

Abstract

A process of preparing a shaped composite based on an epoxy resin matrix can be conducted at temperatures lower than that for composites based on a thermoplastic resin matrix; and the composite product prepared from the above process shows no cracks and shows satisfactory deformation resistance.

Description

Process of Preparing Shaped Composites

FIELD OF THE INVENTION

The present invention relates to a process of preparing a shaped composite.

INTRODUCTION

A thermoforming process (also known as a “thermal press molding process” , a “hot press forming process” , or a “high temperature compression molding process” ) is widely used to make composite parts with complex geometric shapes. This process involves simultaneously applying heat and pressure to a composite sheet in a two-part (male and female) mold to form a set shape. The composite usually comprises reinforcing fibers embedded in a polymer matrix. The thermoforming process is frequently employed in the composite industry for high volume production due to cost and time efficiency with a molding cycle time of usually several seconds to several minutes.

Due to the moldability (or shape-forming ability) of thermoplastic materials, thermoplastic materials are predominantly used as the polymer matrix for composites manufactured by the thermoforming process. During the thermoforming process, thermoplastic materials soften upon heating, and conform to the shape of the mold when the mold closes and pressure is applied to the mold. However, the temperature for thermoforming composites with a thermoplastic resin matrix is usually undesirably high. For example, commonly used thermoplastic materials usually have a melting point of 165 ℃ or higher. In industrial practice, the mold temperature for thermoforming usually needs to be at least 20 ℃ higher than the melting points of thermoplastic resin matrix in order for composites to form the shape. In addition, some types of thermoplastic materials may have insufficient wettability to a fiber reinforcement, which may cause interfacial defects and consequently unsatisfactory mechanical properties of the obtained composites.

Compared to thermoplastic materials, thermosetting materials (for example, radical cured acrylic resins and epoxy resins) may provide better wettability to a fiber reinforcement, due to their lower molecular weight and lower viscosity when dissolved in solvents. Thermosetting resins upon being fully curing are highly cross-linked and usually do not soften sufficiently to form a set shape even at a temperature higher than their glass transition temperature (T_g) . When composites comprising such matrix are subjected to the thermoforming process, cracks are usually generated within the resin matrix and/or at the interface between the resin matrix and reinforcing fibers.

To avoid cracking during thermoforming, partially cured thermosetting materials, also referred to as “B-staged” materials or “prepregs” , are usually used for thermoforming in a mold with a set shape, followed by post curing in the same mold. Prepregs usually have a T_g of 20-50 ℃ to give a non-sticky surface for handling convenience and to afford sufficient flexibility to resist cracking during thermoforming. However, the surface of prepregs tends to soften and deform at temperatures higher than room temperature (22±3 ℃) , which limits the type of surface modifications that can be made to the prepregs at elevated temperatures before shape forming. For example, a surface modification such as silk screen printing is usually operated at around 80 ℃, which is not suitable for the surface modification of a prepreg. In addition, the post curing step usually takes at least one hour, which causes prolonged molding cycle time and productivity inefficiency.

A resin injection molding (RIM) process is another approach for making shaped glass or carbon fiber reinforced composites using hot melt epoxy resin compositions (for example, liquid epoxy resins) . In the RIM process, fiber cloth is placed in a mold in advance and epoxy resin compositions injected into the mold is cured until the epoxy resin solidifies before the releasing of the obtained composites from the mold. This leads to both a long molding cycle time and low productivity.

Shaped composites obtained from the thermoforming process are also required to have sufficient deformation resistance so as to maintain their shape during storage or in use, particularly during the hot days of the summer season.

Therefore, it is desirable to provide a process for preparing a shaped composite that does not have the above described disadvantages associated with thermoplastic materials and associated with conventional processes for producing thermosetting resins such as thermoforming prepregs and the RIM process.

SUMMARY OF THE INVENTION

The present invention provides a novel process that thermoforms fiber reinforced composites comprising a fully cured, flexible epoxy thermoset matrix obtained by using a specific solvent-borne, difunctional solid epoxy resin cured with a curing agent. The composites can be thermally press-formed (i.e. , “thermoformed” ) at temperatures (typically 120-160 ℃) lower than for composites containing thermoplastic resin matrix (typically 185-310 ℃) . Thermoformed composites (that is, shaped composites) also have sufficient deformation resistance for storage and/or in use according to the test method described in the Examples section herein below. In addition, such composites have sufficient surface hardness at 10 to 80 ℃ to be surface modified at elevated temperatures up to 85 ℃ prior to thermoforming. The molding cycle time for the process of the present invention is also significantly shorter than a traditional RIM process.

The present invention includes a process of preparing a shaped composite. In one embodiment, the process comprises:

(i) forming a curable epoxy resin composition comprising (a) an epoxy resin, wherein the epoxy resin is a difunctional epoxy resin having an epoxide equivalent weight of from 1,000 to 2,400 grams per equivalent of epoxy group (g/eq) , (b) a solvent, and (c) a curing agent；

(ii) impregnating a reinforcing substrate with the epoxy resin composition；

(iii) heating the impregnated substrate with the epoxy resin composition at a temperature sufficient to fully cure the epoxy resin composition to form a cured composite； and

(iv) thermoforming the cured composite to a set shape in a mold.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of a series of stages (A) – (D) of the thermoforming step in a process of preparing a shaped composite described herein.

DETAILED DESCRIPTION OF THE INVENTION

“Fully cured” herein, with reference to a curable epoxy resin composition, refers to more than 90％of epoxide groups in a curable epoxy resin composition having reacted with a curing agent. The degree of reaction (that is, degree of crosslinking) may be assessed by measuring residue heat release of a composite using differential scanning calorimetry (DSC) .

“Aset shape” herein refers to any shape the same as the shape of a mold used for thermoforming.

The curable epoxy resin composition may comprise one or more difunctional epoxy resins. The difunctional epoxy resins herein refer to epoxy resins having two epoxide ending groups per molecule. The difunctional epoxy resins may have an epoxide equivalent weight (EEW) of 1,000 to 2,400 g/eq, according to the ASTM D-1652 method. Some embodiments of the difunctional epoxy resins may include for example diglycidyl ethers of bisphenol A or their derivatives such as diglycidyl ethers of bisphenol A, diglycidyl ethers of tetrabromobisphenol A, or oligomeric and polymeric diglycidyl ethers of bisphenol A and tetrabromobisphenol A； diglycidyl ethers of bisphenol F or their derivatives； epoxy novolac resins； epoxy cresol novolac resins； or mixtures thereof. A preferred embodiment of the difunctional epoxy resin useful in the present invention may include for example a diglycidyl ether of bisphenol A. Suitable commercially available epoxy resins may include, for example, D. E. R. ^TM 667 epoxy resin available from The Dow Chemical Company (D. E. R. is a trademark of The Dow Chemical Company) .

The EEW of the difunctional epoxy resin may be 1,000 g/eq or more, 1,200 g/eq or more, 1,300g/eq or more, 1,400g/eq or more, 1,500g/eq or more, or even 1,600 g/eq or more, and at the same time, 2,400 g/eq or less, 2,300 g/eq or less, 2,200 g/eq or less, 2,100 g/eq or less, or even 2,000 g/eq or less, according to the ASTM D-1652 method.

The concentration of the difunctional epoxy resin may be 90％by weight or more, 95％by weight or more, 98％by weight or more, or even 100％by weight, based on the total weight of epoxy resins in the curable epoxy resin composition.

In addition to the difunctional epoxy resin described above, the curable epoxy resin composition may also comprise one or more additional epoxy resins (or “second epoxy” ) that are different from the difunctional epoxy resin described above. The additional epoxy resins preferably have an epoxy functionality of two. When the additional epoxy resin is used, the average functionality of total epoxy resins in the curable epoxy resin composition is preferably two. Examples of suitable additional epoxy resins include D. E. R. 858 oxazolidone ring-containing epoxy resin available from The Dow Chemical Company, D. E. R. 383, D. E. R. 354, D. E. R. 331, D. E. R. 330, and D. E. R. 671 bisphenol A epoxy resins； D. E. N. ^TM 438 and D. E. N. 439 epoxy novolac resins all available from The Dow Chemical Company (D. E. N. is a trademark of The Dow Chemical Company) ； or mixtures thereof. The additional epoxy resin should be used in an amount that does not compromise the thermoforming property and the deformation resistance of the shaped composites made from the curable epoxy resin composition. For example, when the additional epoxy resin is used, the concentration of the additional epoxy resin may be less than 10％by weight, less than 5％by weight, or even less than 2％by weight, based on the weight of total epoxy resins in the curable epoxy resin composition. In some preferred embodiments, the curable epoxy resin composition is substantially free of the additional epoxy resin.

The curable epoxy resin composition useful in the present invention may comprise one or more solvents. The solvents may be added to lower the viscosity of the above composition. Examples of suitable solvents include ketones such as acetone and methyl ethyl ketone, alcohols such as methanol and ethanol, glycol ethers such as ethylene glycol methyl ether and propylene glycol monomethyl ether, acetate of glycol ethers such as propylene glycol monomethyl ether acetate, amides such as N, N-dimethylformamide, aromatic organic solvents such as toluene and xylene； aliphatic hydrocarbons, cyclic ethers, halogenated hydrocarbons, or mixtures thereof. The amount of the solvent employed may generally range from 20 to 60％by weight, from 30 to 50％by weight, or from 35 to 45％by weight, based on the total weight of the curable epoxy resin composition.

The curable epoxy resin composition useful in the present invention may comprise one or more curing agents. The curing agents may be selected from an amine compound, a phenolic novolac resin, an anhydride compound, or mixtures thereof.

In some embodiments, the curing agent used in the curable epoxy resin composition may comprise one or more amine compounds. Examples of suitable amine compounds include cycloaliphatic polyamines such as isophorone diamine (IPDA) ； 1, 3-cyclohexanebis (methylamine) (1, 3-BAC) ； 4, 4’-methylenebis (cyclohexylamine) (PACM) ； 1, 2-diaminocyclohexane (DACH) and 4, 4’-dianinodicyclohexylmethane (DMDC) ； dicyandiamide, guanamine, guanidine, amino guanidine, diaminodiphenylmethane, diaminodiphenylsulfone, and derivatives thereof； polyoxyalkylene polyamines such as polyoxypropylenediamine； or mixtures thereof. In some preferred embodiments, dicyandiamide is used as the curing agent in the curable epoxy resin composition.

In some embodiments, the curing agent in the curable epoxy resin composition may comprise one or more phenolic novolacs. Examples of suitable phenolic novolacs include bisphenol A novolac, bisphenol F novolac, or mixtures thereof. The phenolic novolacs are usually prepared by reacting phenols, cresols, bisphenol A, bisphenol F, or other alkyl phenols with formaldehyde. In some preferred embodiments, the phenolic novolac resin used as the curing agent in the curable epoxy resin composition is a phosphorus-containing phenol resin.

The phosphorus-containing phenol resin useful in the present invention may be formed by condensing an etherified resole with a phosphorous compound having the following structure,

H-P (＝X) R¹R²,

wherein X is selected from the group consisting of sulfur, oxygen, and a lone electron pair； each R¹ and R² is independently an aliphatic moiety having 1 to 20 carbon atoms, an aromatic hydrocarbon moiety having 6 to 20 carbon atoms, where the aliphatic moiety and the aromatic hydrocarbon moiety can be joined to form a cyclic structure, RX-, wherein R is an aliphatic moiety having 1 to 20 carbon atoms, or wherein R¹ and R² together are Ar¹X-, wherein Ar¹ is benzene, naphthalene, or biphenyl. An aliphatic moiety herein includes saturated or unsaturated linear or branched hydrocarbon groups, which include, for example, alkyl, alkenyl, and alkynyl groups. An aromatic hydrocarbon moiety herein includes mono-or polynuclear aromatic hydrocarbon groups.

In some preferred embodiments, R¹ and R² together are Ar¹X-, wherein X is oxygen and Ar¹ is biphenyl to give 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (H-DOP) . The structure of H-DOP is shown as follows,

Examp les of suitable etherified resoles used to react with the phosphorous compound include butyl ether bisphenol-Aresoles, which are made with bisphenol A, formaldehyde and n-butanol. The etherified resole is typically a mixture of monomeric, dimeric and oligomeric structures. The condensation reaction of the phosphorous compound and the etherified resole may be carried out under conditions known in the epoxy art including, for example, as described in US2012/0289663A1. For example, the H-DOP and the etherified resole are typically combined at a H-DOP to etherified resole weight ratio in the range from 10: 1 to 1: 10, from 5: 1 to 1: 5, from 2: 1 to 1: 2, or from 1.1: 1 to 1: 1.1 based on total solids content of the reaction mixture of the H-DOP and the etherified resole. If desired, other materials such as catalysts or solvents may be added to the reaction mixture. In some preferred embodiments, the curing agent comprises the reaction product of H-DOP with butyl ether bisphenol A resole (hereinafter as “DOP-BN” ) . Suitable commercially available phosphorous-containing phenolic novolac curing agents include, for example, XZ-92741 solution available from The Dow Chemical Company.

In some embodiments, the curing agent in the curable epoxy resin composition may comprise one or more anhydride compounds. The anhydride compounds may include cycloaliphatic anhydrides, aromatic anhydrides, or mixtures thereof. Examples of suitable anhydride compounds include phthalic acid anhydride and derivatives thereof, nadic acid anhydride and derivatives thereof, trimellitic acid anhydride and derivatives thereof, pyromellitic acid anhydride and derivatives thereof, benzophenonetetracarboxylic acid anhydride and derivatives thereof, dodecenyl succinic acid anhydride and derivatives thereof, poly (ethyloctadecanedioic acid) anhydride and derivatives thereof, or mixtures thereof. Particularly suitable anhydride compounds useful in the present invention may include hexahydrophthalic anhydride； methyl hexahydrophathalic anhydride； tetrahydrophthalic anhydride； methyl tetrahydrophthalic anhydride； methyl nadic acid anhydride； methyl- (endo) -5-norbornene-2, 3-dicarboxylic anhydride； pyromellitic dianhydride； hemimellitic anhydride； ciscyclopentanetetracarboxylic acid dianhydride； trimellitic anhydride； naphthalene-1, 8-dicarboxylic acid anhydride； phthalic anhydride； dichloromaleic anhydride； glutaric anhydride； maleic anhydride； succinic anhydride； copolymers of styrene and maleic acid anhydrides； or mixtures thereof.

The curing agent useful in the present invention is generally used in an amount to fully cure the curable epoxy resin composition. For example, the molar ratio of total epoxy functionality of the epoxy resin (s) to total active hydrogen functionality of the curing agent may be from 0.4: 1 to 10: 1, from 0.5: 1 to 5: 1, from 0.6: 1 to 1.3: 1, or from 0.7: 1 to 1: 1.

The curable epoxy resin composition useful in the present invention may further comprise one or more curing catalysts. The catalyst may be used to promote the reaction between the epoxy resins and the curing agent. Examples of suitable curing catalysts include Lewis acids such as boron trifluoride, and a derivative of boron trifluoride with an amine such as piperidine or methyl ethylamine； tertiary amines such as triethylamine, tripropylamine, tributylamine, benzyldimethylamine, diethyl aminopropylamine, and tris (dimethylaminomethyl) phenol； imidazole derivatives such as 1-methylimidazole, 2-methylimidazole, 2-ethyl-4-methyl imidazole, 2-isopropylimidazole, 1-propylimidazole, 1-cyanoethyl-2-ethyl-4-methylimidazole, 2-phenyl imidazole, and 2-heptadecylimidazole； onium compounds such as ethyltriphenylphosphonium tetrahaloborate, tetrabutylphosphonium chloride, tetrabutylphosphonium acetate, tetrabutylphosphonium diacetate, ethyltriphenylphosphonium acetate, tetrabutylphosphonium tetrahaloborate, butyltriphenylphosphonium tetrabromobisphenate, butyltriphenylphosphonium bisphenate, butyltriphenylphosphonium bicarbonate, benzyltrimethylammonium chloride, benzyltrimethylammonium hydroxide, benzyltrimethylammonium tetrahaloborate, tetramethylammonium hydroxide, tetrabutylammonium hydroxide, and tetrabutylammonium tetrahaloborate； ureas compounds such as 3- (3-chloro-4-methylphenyl) -1, 1-dimethylurea； phenyldimethylurea； p-chlorophenyl-N, N-dimethylurea； 3-phenyl-1, 1-dimethylurea； 3,4-dichlorophenyl-N, N-dimethylurea； N, N-dimethylurea； N-isobutyl-N’, N’-dimethylurea； or mixtures thereof. In some embodiments, the curing catalyst is selected from triethylamine, tripropylamine； 2-methyl imidazole； 2-ethyl-4-methylimidazole； 1-cyanoethyl-2-ethyl-4-methylimidazole； ethyltriphenylphosphonium acetate； benzyldimethylamine； 1, 1-dimethyl-3-phenylurea； or mixtures thereof.

The curing catalyst useful in the present invention may be present in an amount of from 0.001 to 2％by weight, from 0.01 to 1.5％by weight, or from 0.1 to 1％by weight, based on the total weight of the curable epoxy resin composition.

The curable epoxy resin composition useful in the present invention may further comprise one or more of the following additives: stabilizers, surfactants, flow modifiers, matting agents, degassing agents, fillers, flame retardants such as aluminum trihydroxide and magnesium hydroxide, curing initiators, curing inhibitors, wetting agents, colorants or pigments, thermoplastics, processing aids, ultraviolet (UV) blocking compounds, fluorescing compounds, UV stabilizers, antioxidants, mold release agents, and mixtures thereof . The combined concentration of the additives may be, based on the total weight of the curable epoxy resin composition, from 0 to 30％by weight, from 0.01 to 20％by weight, or from 0.1 to 10％by weight.

The process of preparing a shaped composite of the present invention may comprise:

(i) forming the curable epoxy resin composition described above；

(ii) impregnating a reinforcing substrate with the curable epoxy resin composition；

(iv) thermoforming the cured composite to a set shape in a mold.

In step (ii) of the process of the present invention, the curable epoxy resin composition is applied to or impregnated into the substrate by rolling, dipping, spraying, or other known techniques or combinations thereof. The reinforcing substrate useful in the present invention may be a woven fabric. Examples of suitable woven fabric include a woven glass fabric, a woven carbon fabric or combinations thereof.

In step (iii) of the process of the present invention, the impregnated substrate with the epoxy resin composition may first be partially cured to form a B-staged product and then subsequently the B-staged product can be completely cured at a later time to form the fully cured composite. For example, the impregnated substrate may be “B-staged” by heating at a temperature sufficient to draw off solvent in the epoxy resin composition and to partially cure the epoxy resin composition, so that the impregnated substrate can be handled easily. The “B-staging” step is usually carried out at a temperature of from 90 ℃ to 210 ℃ and for a time of from 1 minute to 15 minutes. The B-staged product is usually fully cured at a temperature of from 100 ℃ to 230 ℃ and for a time of from 10 minute to 200 minutes.

The impregnated substrate that results from B-staging is called a “prepreg” . As one illustrative embodiment, the prepreg can be used to form a laminate using a lamination process. For example, one or more sheets of prepregs may be stacked or laid up. The laid-up sheets may be pressed at high temperature and pressure for a time sufficient to cure the epoxy resin and form a composite. The temperature of this lamination step is usually between 100 ℃and 230 ℃, or between 165 ℃ and 190 ℃. The lamination step may also be carried out in two or more stages, such as a first stage between 100 ℃ and 150 ℃ and a second stage between 165 ℃ and 190 ℃. The pressure is usually between 0.5 MPa and 20 MPa. The lamination step is usually carried out for a time of from 10 minutes to 200 minutes, from 20 minutes to 120 minutes, or from 45 minutes to 90 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes) . The obtained cured composite may have a glass transition temperature from 70 ℃to 150 ℃, or from 80 ℃ to 120 ℃, according to the test method described in the Examples section herein below.

The cured composite may be subject to a surface treatment process prior to the thermoforming step (iv) in the process of the present invention. The cured composites have better surface hardness than prepregs at 10 ℃ to 85 ℃, thus they are capable of being surface treated at temperatures even higher than room temperature, for examples, up to 85 ℃. The surface treatment process can be any known surface treatment techniques in the epoxy field, in particular, those techniques that require the surface to be treated having sufficient surface hardness to sustain heat and/or pressure. For example, the surface treatment may be a surface printing process such as silk screen printing process, which is usually carried out at about 80 ℃.

The process of the present invention further comprises step (iv) , the step of thermoforming the cured composite to form a desired or set shape. “Thermoforming” is the pressing or squeezing of a heated pliable material into a final shape. The pliable material can be heated by infrared, natural gas, or other heaters or ovens to the pliable material’s forming temperature. Then, the pliable material can be stretched over or into a temperature-controlled mold. The mold can be etched with patterns ranging from fine to coarse to simulate a natural, an artificial, or a grain-like textured look. The pliable material is held against the mold surface unit until cooled. The formed part is then trimmed from the pliable material. There are several categories of thermoforming including, for example, vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending. Various thermoforming processes are disclosed in books such as J. L. Throne, “Understanding Thermoforming, ” Hanser Gardner Publications, Inc. , Cincinnati, Ohio, 1999； and J. L. Throne, “Technology of Thermoforming, ” Hanser Verlag, Munich, 1996, both of which are incorporated herein by reference.

The thermoforming step in the process of the present invention may be conducted in a mold that is conventionally used for thermoforming thermoplastic materials. The thermoforming step may be carried out at temperatures at least 40 ℃ higher than the glass transition temperature of the cured composite. In some embodiments, the thermoforming temperature is from 100 ℃ to 200 ℃ or from 120 ℃ to 160 ℃, which is lower than that for composites containing thermoplastic resin matrix (typically 185-310 ℃) . The thermoforming step may be carried out for a time period of from 1 second to 10 minutes, or from 1 minute to 5 minutes. In some embodiments, the thermoforming step includes pre-softening the composite sheet to temperatures above its glass transition temperature, cutting flat regions (i.e., blanks) from the sheet, and transferring blanks by gravity or mechanical means into matched molds whereby the blanks are shaped into articles by heat and pressure. In some preferred embodiments, the process of the present invention allows the preparation of a shaped composite sheet with an angle of 90 degrees (°) and no cracks, according to the test method described in the Examples section herein below.

Figure 1 is a schematic perspective view of a series of stages or operations of the thermoforming step in one embodiment of the present invention. With reference to Figure 1, there is shown a composite sheet 11 comprising two opposing primary surfaces 11 (a) and 11(b) , respectively； and two

iron cube members

12 and 13. Each iron cube comprises at least two adjacent primary surfaces. For example, the two primary surfaces for cube 12 are 12 (a) and 12 (b) ； and the two primary surfaces for cube 13 are 13 (a) and 13 (b) . The composite sheet 11 is firstly pre-softened at the thermoforming temperature described above and then placed on top of a first iron cube 12 as shown in the first and second stages (A) and (B) . At least a portion of sheet 11 contacts the first iron cube 12 so that at least a portion of one primary surface 12 (a) of the first iron cube 12 contacts at least a part of one primary surface 11 (a) of the composite sheet 11 as shown in stage (B) . A second iron cube 13 is placed on top of the other part of the composite sheet 11, so that at least a portion of one primary surface 13 (a) of the second cube 13 contacts at least a portion of the opposing primary surface 11 (b) of the composite sheet 11 as shown in stage (C) . The second cube 13 is then vertically pressed in a downwardly direction as indicated by arrow 13 (c) to bend the other part of the composite sheet 11 along at least a portion of the adjacent primary surface 12 (b) of the first iron cube 12 and along at least a portion of the adjacent primary surface 13 (b) of the second iron cube 13 to transform the composite sheet 11 to a shaped composite 14 such as for example a shaped composite with a desired angle as shown in stage (D) .

The shaped composites prepared by the process of the present invention may be used as housings, covers, or cases for a variety of end-uses including home appliances, frames, automotive panel parts, and electronics equipment, in particular, smart phone cases, tablet computer cases and regular computer cases. The shaped composites may have various thicknesses depending on the requirements dictated by the application in which the shaped composite is used. For example, shaped composites may have a thickness of from 0.1 to 5 millimeters (mm) , from 0.5 to 1.5 mm, or from 0.8 to 1.2 mm.

EXAMPLES

The following examples illustrate embodiments of the present invention. All parts and percentages are by weight unless otherwise indicated.

D. E. R. 383 (EEW＝180) , D. E. R. 671 (EEW＝513) , D. E. R. 664 (EEW＝895) , D. E. R. 667 (EEW＝1800) , and D. E. R. 669 (EEW＝3250) resins are diglycidyl ethers of bisphenol A and all available from The Dow Chemical Company.

D. E. R. 858 resin (EEW＝400) , available from The Dow Chemical Company, is an oxazolidone ring-containing epoxy resin obtained by reacting a diglycidyl ether of 2, 2-bis (4-hydroxyphenyl) propane with a polyisocyanate to form an epoxy-terminated polyoxazolidone.

XZ-92530 epoxy resin solution, available from The Dow Chemical Company, is a phosphorus-containing high functional epoxy resin having a functionality between 2 and 3.6, and has a solids content of 75 ％by weight in a mixture of methyl ethyl ketone and propylene glycol monomethyl ether.

Dicyandiamide ( “DICY” ) used as a hardener, 2-methylimidazole ( “2-MI” ) used as a catalyst, and 2-phenylimidazole ( “2-PI” ) used as a catalyst are all available from Sinopharm Chemical Reagent Co., Ltd.

XZ-92741 solution, available from The Dow Chemical Company, is a modified, phosphorus-containing phenolic novolac solution with a solids content of 57 ％by weight in a mixture of propylene glycol monomethyl ether and butanol, and is used as a hardener. The phosphorous-containing phenolic novolac is an reaction product of 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide with butyl ether bisphenol A resole ( “DOP-BN” ) .

DOWANOL^TM PM propylene glycol monomethyl ether is available from The Dow Chemical Company (DOWANOL is a trademark of The Dow Chemical Company) .

The following standard analytical equipment and methods are used in the Examples.

Epoxide Equivalent Weight (EEW) Analysis

EEW of an epoxy resin is measured according to the ASTM D1652 method.

Glass Transition Temperature (T_g)

T_g of composites is measured by DSC. A 5-10 milligram (mg) sample is analyzed in an open aluminum pan on a TA Instrument DSC Q2000 fitted with an auto-sampler under nitrogen atmosphere with a heating ramp of 20 ℃/min. Two consecutive cycles are conducted to obtain two T_gs, T_g1 (1^st cycle) and T_g2 (2^nd cycle) , as determined by the method of half height.

Lab-scale Thermoforming Test

A lab-scale thermoforming test, as illustrated in Figure 1, is used to simulate a thermoforming process on industrial scale equipment. A composite sheet with a thickness of 0.8-1.2 mm and a size of 3 centimeters (cm) by 4.5 cm is placed into an oven at a pre-set thermoforming temperature described in Table 1 and each example below for 1 minute (min) . The composite sheet is then taken out and quickly placed on a 3 cm*3 cm*3 cm first iron cube preheated on a hot plate at 50 ℃, so that the first iron cube contacts a part of the composite sheet. A second iron cube preheated to 50 ℃ is then placed at the edge and on top of the other part of the composite sheet and slowly pressed down to bend the portion of the composite along the edge of the first iron cube to form a shaped composite with an angle of about 90°. The shaped composite is then cooled down to room temperature. If the composite sheet after thermoforming (that is, the shaped composite) shows no cracks visible to the naked eye, the composite passes inspection. If the composite sheet after thermoforming breaks at the composite’s bending position or shows cracks visible to the naked eye, the composite fails inspection.

Deformation Resistance Test

A deformation resistance test is conducted to evaluate the ability of a shaped composite to resist deformation under environmental conditions. The shaped composite with an angle formed by the thermoforming process described above is placed in an environmental test oven at 60℃ and 85％relative humidity (RH) for 72 hours. The shaped composite is then taken out to measure its deformation angle. The deformation angle is the angle difference between the angles before and after the deformation resistance test. If the deformation angle is less than 28.5°, the shaped composite passes the deformation resistance test. If the deformation angle >＝ 28.5°, the shaped composite fails the deformation resistance test.

Lab Punch Cutting Test

A punch cutting test is conducted in lab to evaluate the ability of a composite to resist cracking at the edge during cutting. A composite sheet fixed by a clamp is placed under the blade of a cutter (Di-Acro, Model No. 24 HAND SIEAR) . The blade is moved down toward the composite at a rate of 30 mm/second to cut the composite sheet into two sample pieces. Newly formed edges of the sample pieces after cutting are visually checked by the naked eye. If the sample pieces show cracks as evidenced by several non-continuous whitening zones along the newly formed edges, the composite has poor anti-crack property. If the sample pieces show no cracks on the newly formed edges, the composite has satisfactory anti-crack property.

Examples (Exs) 1 and 2

Part A. Preparation of epoxy resin solution, hardener solution and catalyst solution

Fifty five (55) grams of D. E. R. 669 epoxy resin and 45 grams of methyl ethyl ketone (MEK) were added into a round flask. The obtained mixture was heated at 80℃ to reflux for 0.5 hour to dissolve all resin solids. A clear solution of D. E. R. 669 resin with 55％solids was obtained. The solutions of other epoxy resins used in Examples were prepared substantially the same as the preparation of D. E. R. 669 resin solution.

Catalysts 2-MI and 2-PI were dissolved in DOWANOL PM to prepare 10％solutions of the catalysts, respectively.

DICY was dissolved in dimethyl formaldehyde (DMF) to prepare a 10％solution of DICY.

Part B. Preparation of varnish composition

Varnish compositions were prepared based on formulations shown in Table 1. The epoxy resin solution, the hardener solution and the catalyst solution prepared in Part A above were added into a glass bottle. The glass bottle was tightly capped and stirred for at least 0.5 hour until all components were homogeneously mixed to form the varnish compositions (that is, curable epoxy compositions) .

Part C. Preparation of prepregs by hand painting/dipping

Prepregs were prepared by pouring the varnish composition prepared in Part B above into an open steel pan in a fume hood and then dipping a glass cloth or a carbon fabric cloth (30 cm*30 cm) into the varnish composition.

The resulting impregnated fabric was then transferred to a heated and ventilated oven at a pre-set temperature of 171 ℃ for 3 minutes to remove the solvents and to partially cure the varnish composition to form prepregs. Glass fiber reinforcement used is #7628 glass cloth available from Grace Fabric Corporation. Carbon fiber reinforcement used is 0°/90° plain weave 12K 400GSM carbon fiber cloth available from Hantrong Investment Co. Ltd. , wherein 12K indicates that the number of carbon fibers is 12, 000 within one tow, and 400GSM means that the unit weight by area of the carbon fabric is 400 grams per square meter.

Part D. Preparation of composite

Several plies of prepregs prepared in Part C above were stacked together and placed in a vacuum press machine (Lauffer LC 40/1 press) to make cured composites based on different types of reinforcing substrates and lamination conditions described in Table 1.

Part E. Preparation of shaped composite by thermoforming

The cured composites obtained from Part D above were thermal press formed into L-shape composites under the conditions described in Table 1, according to the lab-scale thermoforming test described above.

Comparative (Comp) Exs A and B

Composites of Comp Exs A and B were prepared according to the procedure substantially the same as Exs 1 and 2, based on formulations and conditions described in Table 1.

Table 1

Comp Exs C-L

Composites of Comp Exs C-L were prepared according to the procedure substantially the same as Exs 1 and 2 based on curable epoxy resin compositions described in Table 2, except that the thermoforming step for Comp Exs C, F, G and H was carried out at 200 ℃ for 1 minute. Blends of D. E. R. 664 and D. E. R. 858 epoxy resins were used in Comp Exs G and H at a weight ratio of 80/20 and 60/40 by solids, respectively. A blend of D. E. R. 669 and D. E. R. 383 epoxy resins at a weight ratio of 95/5 by solids was used in Comp Ex L.

Table 2

Weight ratio is the solids weight ratio of total epoxy resin (s) /curing agent/catalyst

Part F. Results

The properties of the obtained composites in the above Exs and Comp Exs are provided in Table 3. Composites obtained from curable epoxy compositions comprising a difunctional epoxy resin having an EEW of 1800 (Exs 1 and 2) were first cut into pieces of a defined size according to the punch cutting test. The obtained pieces were pre-heated at around 140-150 ℃ and thermoformed into L-shape composites with 90° angle according to the thermoforming test described above. Both composites showed no cracks after thermoforming. The obtained shaped composites of Exs 1 and 2 were also evaluated for deformation resistance property according to the test method described above. As shown in Table 3, the shaped composites of Exs 1 and 2 both passed the deformation resistance test. In addition, the composite from Ex 1 showed no evidence of cracks at the newly formed edges when punch cut.

XZ-92530 epoxy resin was cured by DICY in the presence of imidazole to give a cured composite with T_g of 120-125 ℃ (Comp Ex A) . Comp Ex B used only imidazole as a catalyst (no hardener) to promote homopolymerization of the XZ-92530 epoxy resin to form a composite with T_g of about 96-100 ℃ (Comp Ex B) . When cured composites prepared from Comp Exs A-C were bended to form 90° angle, according to the thermoforming test described above, all composites were broken at the bending position even when preheated at much higher temperature (180 ℃) . Although composites prepared from Comp Exs D-L were able to pass the thermoforming test, the shaped composites obtained all failed the deformation resistance test with a deformation angle bigger than 28.5°.

Table 3

Claims

A process of preparing a shaped composite, comprising:

(i) forming a curable epoxy resin composition comprising:

(a) an epoxy resin, wherein the epoxy resin is a difunctional epoxy resin having an epoxide equivalent weight of from 1000 to 2400 g/eq as measured by the ASTM D1652 method,

(b) a solvent, and

(c) a curing agent；

(ii) impregnating a reinforcing substrate with the epoxy resin composition of step (i) ；

(iii) heating the impregnated substrate with the epoxy resin composition at a temperature sufficient to fully cure the epoxy resin composition to form a cured composite； and

(iv) thermoforming the cured composite to a set shape in a mold.
The process of claim 1, wherein the cured composite is formed by partially curing the epoxy resin composition to form a B-staged product and subsequently curing the B-staged product completely at a later time to form the cured composite.
The process of claim 1, wherein the difunctional epoxy resin has an epoxide equivalent weight of from 1600 to 2000 g/eq.
The process of claim 1, wherein the curing agent is selected from an amine compound, a phenolic novolac resin, an acid anhydride, or mixtures thereof.
The process of claim 4, wherein the curing agent is dicyandiamide.
The process of claim 4, wherein the curing agent is a condensation product of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide with an etherified resole.
The process of any one of claims 1-6, wherein the difunctional epoxy resin is a diglycidyl ether of bisphenol A, a diglycidyl ether of bisphenol F, or a mixture thereof.
The process of any one of claims 1-6, wherein the difunctional epoxy resin is present in an amount of from 90％to 100％by weight, based on the weight of total epoxy resins in the curable epoxy resin composition.
The process of claim 8, wherein the average functionality of total epoxy resins in the curable epoxy resin composition is 2.
The process of any one of claims 1-6, the cured composite has a glass transition temperature of from 70 ℃ to 150 ℃.
The process of any one of claims 1-6, wherein the thermoforming step is conducted at temperatures at least 40 ℃ higher than the glass transition temperature of the cured composite.
The process of any one of claims 1-6, wherein the reinforcing substrate is a woven fabric.
The process of claim 12, wherein the reinforcing substrate is selected from a woven glass fabric, a woven carbon fabric, or combinations thereof.
The process of any one of claims 1-6, wherein prior to the step of thermoforming the cured composite, the process further comprises the step of treating the cured composite with a surface printing process.