Interpenetrating network of ring-containing allyl polymers and epoxy resin, and a laminate prepared therefrom

Abstract: Disclosed are polymeric materials comprising an interpenetrating network (IPN) prepared from cyclic moieties-containing allyl polymers and epoxy resins. The allyl polymers art crosslinked under the influence of a radical initiator, the epoxy resin is crosslinked by a curing agent. The curing agent is a polyhydric phenol or a cyclic anhydride not containing a polymerizable double bond. Preferably the curing agent is bisphenol-A or tetrabromo-bisphenol-A. Preferred IPN's are prepared from triallylcyanurate-based allyl polymers and phenol-type epoxy resin.

Description: The invention relates to a polymeric material comprising an interpenetrating network built up from a polymer that can be cross-linked under the influence of radicals and an epoxy resin comprising a cross-linking agent. The invention has for its object the manufacture of new materials especially suited to be used in the electronics industry (electronic materials for short). A polymeric material of the type mentioned in the opening paragraph is known from US-A-4 468 485. In this patent specification the polymer that can be cross-linked under the influence of radicals is an unsaturated polyester resin. The epoxy resin is of the DGEBA type (diglycidyl ether of Bisphenol-A) and is cross-linked using a polyfunctional amine. For the preparation of an interpenetrating network (IPN) without phase separation the above-mentioned patent specification describes a process in which use is made of microwave radiation of a frequency spectrum so chosen as to effect the cross-linking of the polyester, with the heat released in this process triggering the cross-linking reaction of the epoxy resin. Admittedly, in this manner there is obtained a polymeric material which has one single glass transition temperature (Tg) (in the range of 50°-100°C depending on the IPN's composition), but the process used is not universally applicable. Although said patent specification does teach the skilled man a process for the preparation of IPNs, it offers no teaching on how, by the proper selection of IPN components, suitable electronic materials can be manufactured. It is generally known that there is a need to replace the present electronic materials with materials meeting requirements such as low dielectric constant low electrical dissipation factor high Tg ready processability low price high dimensional stability high solvent resistance more satisfactorily. The invention employs IPN technology to obtain materials especially suited to be used in the electronics industry. The invention consists in that in a polymeric material of the above-mentioned known type the polymer that can be cross-linked under the influence of radicals is built up from at least one, cyclic moiety-containing polyallyl compound together or not with an aromatic difunctional methacrylate, and the cross-linking agent for the epoxy resin is a polyhydric phenol. Of course, the two IPN-forming polymers are mixed in such a ratio as to give a resin of which the properties are no longer determined by one of the IPN constituents individually. Although this ratio is to some extent dependent on the type of allyl compound and the type of epoxy resin used in general a weight ratio of 80/20 or 20/80 will be adhered to as a limit. Since both the allyl polymer and the epoxy resin provide a share of the favourable IPN properties, the preferred ratio is dependent on those properties which are especially envisaged. If it is desired to stress electrical or thermal properties, then a ratio of allyl polymer to epoxy resin of 70/30 - 60/40 will be selected; if the manufacture is desired of a laminate with a metal layer (such as for printed circuits), then a ratio of allyl polymer to epoxy resin of 30/70 - 40/60 will be chosen on account of the favourable peel strength. Preference is given to an IPM in which the properties of the two constituents come out optimally. This is the case for a ratio of allyl polymer to epoxy resin of 40/60 - 60/40, more preferably 50/50. It should be noted that the above-mentioned cross-linking agents for epoxy resin are known. However, what is novel and surprising is that from an epoxy resin to be cross-linked with the aforesaid materials and ring-containing allyl polymers an IPN is obtained with one glass transition temperature (Tg), since it is known from US-A-4 468 485 already referred to above that, if the method described in that specification is not followed, in general IPN synthesis will give rise to structures with phase separation. It should further be noted that in WO 85/03515 and WO 86/02085 an epoxy resin is described which is used for the manufacture of electronic materials. The advantage to the IPNs according to the invention is that, compared with these electronic materials which are made up exclusively of epoxy resin, they are less expensive, have more favourable electrical properties, display a higher solvent resistance, and are fire retardant at a lower bromine content, which is beneficial to the environment. The polymer that can be cross-linked under the influence of radicals is built up from ring-containing polyallyl compounds. To prepare the polymer the allyl compounds may be employed either in the monomeric or the oligomeric (prepolymeric) form. As suitable polyallyl compounds may be mentioned triallyl cyanurate (TAC), triallyl isocyanurate (TAIC), and aromatic polyallyl esters such as diallyl phthalate, diallyl isophthalate, diallyl terephthalate, triallyl trimellitate, tetrallyl pyromellitate, and diallyl tetrabromophthalate. TAC and TAIC prepolymers can be prepared by the partial polymerisation of TAC monomer or TAIC monomer in, say, methylethyl ketone (MEK) in the presence of a peroxide of a relatively low decomposition temperature, e.g. dibenzoyl peroxide or di(orthomethylbenzoyl) peroxide. The monomer conversion, the viscosity, and the molecular weight distribution of the TAC or TAIC oligomers can be controlled by means of the amount of peroxide employed and the reaction time. Optionally, use may be made in the polymerisation process of a chain regulator such as carbon tetrabromide to prevent the prepolymerisation resulting in gelling. As is known to the skilled man, it is possible to remove monomers from TAC or TAIC prepolymers by selective precipitation, using a suitable organic solvent. Aromatic polyallyl esters can be illustrated using the following general structural formula: wherein n = 2, 3 or 4 and each X individually represents hydrogen or halogen (especially bromine). Of course, suitable ring-containing polyallyl compounds are not restricted to the above-indicated structural formulae. Preferably, the polyallyl compound used is TAC (in the monomeric or prepolymeric form), since this will give an IPN with optimal thermal properties. Moreover, TAC has the advantage that it may also serve as a solvent in the IPN preparation; consequently, additional solvent may largely be omitted. Aromatic difunctional methacrylates may be used to partially replace the polyallyl compounds in the IPN. Suitable methacrylates may be of the following structural formula: wherein R and R′ may be the same or different and represent H or CH₃, n and m may be the same or different and 0, 1, 2, 3 or 4, with n + m being 4 maximum, and wherein A represents a hydrocarbon group having 1-6 carbon atoms, or else stands for Preferably, use is made of 2,2-di(4-methacryloxy-ethoxyphenyl)propane (BMEPP). The polymerisation of ring-containing polyallyl compounds is carried out under the influence of an initiator that will generally be employed in a ratio of 0,1-5 wt.%, calculated on the allyl compound. Examples of suitable initiators include peroxides, such as t-butylperoxy benzoate, t-butylperoxy-3,5,5-trimethyl hexanoate, and benzoyl peroxide. By the term "epoxy resin" is meant a curable composition of oxirane ring-containing compounds. Such compounds have been described in C.A. May's "Epoxy Resins", 2nd. Edition, Marcel Dekker Inc., New York & Basle, 1988. As examples of epoxy resins may be mentioned phenol types, such as those based on the diglycidyl ether of Bisphenol-A, on polyglycidyl ethers of phenol-formaldehyde Novolac or cresol-formaldehyde Novolac, on the triglycidyl ether of tris(p-hydroxyphenol)methane, or on the tetraglycidyl ether of tetraphenyl ethane; amine types, such as those based on tetraglycidyl methylene dianiline or on the triglycidyl ether of p-aminoglycol; cycloaliphatic types, such as those based on 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. By the term "epoxy resin" are also meant reaction products of compounds (e.g. of the foregoing types) containing an excess of epoxy with aromatic dihydro compounds. These dihydro compounds may be halogen substituted. The same epoxy resins were described in WO 86/02085 referred to above. Preference is given to epoxy resins of the phenol type, especially because of their low price. It should be noted that, as a rule, one unequivocal structural formula is used to illustrate epoxy resins. Differing products resulting from side reactions which occur in the preparation of epoxy resins should, as the skilled man knows, be considered to be included therein. As these side products form a standard constituent of cured epoxy resins, so they make up a standard constituent of the IPNs according to the invention. It is of importance that the epoxy resin be so cured as to give a non-segregated IPN. Moreover, the curing method has its effect on the IPN's final material properties. To obtain a non-segregated IPN of favourable properties the epoxy resin is cured using the type of cross-linking agent referred to above. As examples of polyhydric phenols may be mentioned aromatic dihydroxy compounds of such formulae as wherein A represents an aliphatic or alicyclic hydrocarbon group having 1-12 carbon atoms, Y stands for halogen, notably bromine or chlorine, R or OR, with R being a hydrocarbon group having 1-10 carbon atoms; and wherein n = 0 or 1 and m = 0, 1, 2, 3 or 4. Alternatively, Novolac resins such as phenol/formaldehyde, cresol/formaldehyde or phenol/p-hydroxybenzaldehyde may serve as polyhydric phenol cross-linking agent. A preferred crosslinking agent for the epoxy resin is Bisphenol-A or tetrabromo Bisphenol-A. A further object of the invention is a laminate comprising a layer of synthetic material which consists of an IPN as described above, reinforced with fibre or not, and a layer of metal. Another object of the invention is a printed circuit board manufactured from said laminate. The invention will be further illustrated with reference to the following unlimitative examples. Further explanation: Epoxy resin A is a polyglycidyl ether of a phenol-hydroxy-benzaldehyde condensate having an epoxy equivalent weight (EEW) of 220 and an average epoxy functionality of 3,5. Epoxy resin B is a phenol-formaldehyde Novolac epoxy resin having an average epoxy functionality of 3,5 and an EEW of 178. Epoxy resin C is a Bisphenol A bisepoxide resin having an EEW of 174. Epoxy resin D is a 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate having an EEW of 137. Determination of properties The glass transition temperature (Tg) was determined with a Differential Scanning Calorimeter (DSC) manufactured by Dupont, basic module 990 with DSC module 910 at a heating rate of 10°C/min in an atmosphere of nitrogen. The coefficient of thermal expansion (TEC) in the z-direction and also Tg were determined using a Dupont Thermal Mechanical Analyser (TMA), basic module 990 with TMA module 942 at 100 ml/min in an atmosphere of nitrogen. Values were determined both during heating (5°C/min) and cooling (2°C/min). The decomposition behaviour of neat resins was studied with a Dupont Thermo-Gravimetric Analyser (TGA), basic module 990 with TGA module 951 at a heating rate of 10°C/min in an atmosphere of nitrogen. The study of laminates was conducted in air. Cured resins were tested for fire retardancy by means of a manual test in which the sample to be tested was held in a flame for 30 seconds. The sample was considered to be fire retardant, if it stopped burning immediately upon being removed from the flame. Laminates were tested for fire retardancy in accordance with the UL-94 test, which is known to the skilled man. Example 0 Preparation of TAC prepolymer (used in Examples 1, 8, 9, 11, 12 and 13) To 1 kg of molten TAC monomer in a reactor (fitted with a cooling apparatus and a stirrer) were added 223 g of MEK. Next, the contents of the reactor were heated to 82°C, with stirring, and over a period of 1 hour there was added dropwise a solution of 4,4 g of di(ortho-methylbenzoyl)peroxide in 100 g of MEK phlegmatized with 1,2 g of water. After a total reaction period of 8 hours at 82°C the 75% solution of TAC prepolymer in MEK was cooled down to room temperature. Next, MEK was stripped off under reduced pressure. The result was a TAC prepolymer containing less than 2% of MEK residue with a (Brookfield) viscosity of 1250 mPa.s at 23°C, a monomer conversion (mc) of 37%, a number average molecular weight (Mn) of 8000, and a degree of dispersion (D) of 13 (by HPLC relative to Styrene standards). Example 1 66,7 g of a solution prepared from 52,76 g (0,194 equivalents) of tetrabromo Bisphenol-A (TBBPA), 47,24 g of epoxy resin A (0,215 equivalents), and 33,3 g of MEK were mixed, with stirring, with 50 g of TAC prepolymer (mc 37%, Mn 8000, D 13, viscosity 1250 mPa.s). The part by weight of TAC prepolymer thus was equal to that of epoxy resin A plus TBBPA. To the mixture, which contained 12,5 wt.% of methylethyl ketone (MEK), was added 0,5 g of a 10 wt.% solution of 2-ethyl-4-methyl imidazole (2E4MI) in MEK and, subsequently, 0,5 g of tert. butylperoxy perbenzoate. The resin solution was then poured into aluminium dishes in such a way as to give a layer thickness for the resin solution of about 1 mm. Next, the samples were heated to 60°C in a forced-circulation air oven and kept at this temperature for 1 hour. The temperature of the oven was then raised to and kept for 30 minutes at 100°C, followed by 3 hours at 150°C and, finally, 30 minutes at 180°C. After the orangish-brown, homogeneously transparent plates had been cooled slowly and released from the mould, the samples were post-cured for a further 2 hours at 200°C and then cooled to room temperature. The following properties were measured on the flat, cured plates of a thickness of Example 2 The prepreg and laminates were prepared on the basis of a resin solution consisting of: 921 g of a 76%-solution of 330,68 g of epoxy resin A and 369,32 g of TBBPA in MEK; 933 g of a 75%-solution of TAC prepolymer (having a monomer conversion of 36%, Mn = 9035, D = 15,2, prepared with the use of benzoyl peroxide as initiator) in MEK; 4,7 g of a 15%-solution of 2-ethyl-4-methyl imidazole in acetone; 7,0 g of t-butylperoxy benzoate; and 50 g of acetone. The resin solution in which the TAC prepolymer part by weight was equal to that of epoxy A plus TBBPA had a viscosity of 50 mPa.s determined in accordance with the instructions by Brookfield at 23°C. The E-glass fabric type 7628 (finishing agent Z6040) much used in the electronics industry was manually impregnated with the resin solution. Next, the impregnated fabrics were kept at a temperature of 160°C in a forced-circulation air oven for 286 seconds, resulting in tack-free prepregs of excellent appearance being obtained in the B stage. The percentage by volume of resin in the prepreg was 65%. Eight prepregs stacked one on top of the other were moulded in an autoclave at a pressure of 15 atm. and a temperature of, successively, 150°C for 30 minutes, 180°C for 30 minutes, and, finally, 200°C for 30 minutes. Heating and cooling was at a rate of 3,5°C/min. In this manner both a laminate coated on one side with copper (1 ounce, electrodeposited type) and an uncoated laminate of an overall thickness of 1,60 mm were made. The properties of the laminate and two reference laminates are given in Table 1. Reference laminate 1 is a purchased standard FR4 laminate that is used on a very wide scale in the printed board industry and contains, in addition to E-glass fabric, a resin made up of brominated Bisphenol A bisepoxy, with dicyanodiamide as curing agent. The preparation of reference material 2 was analogous to that of the laminate described in Example 2, except that the 65%-solution of resin in MEK contained 490 g (2,227 equivalents) of epoxy A, 544 g (2,000 equivalents) of TBBPA, and 0,72 g of 2-methyl imidazole. Prepregs of this type of resin were prepared in an oven at 175°C for 60 seconds and moulded to form a laminate over a period of 60 minutes at a pressure of 15 atm. and a temperature of 175°C. The properties of the two reference laminates, each 1,6 mm thick, are given in Table 1. Besides the aforementioned techniques the following methods/processes were employed to determine a number of additional properties. Tg was determined with a type MK1 dynamic, mechanical, thermal analysis (DMTA) apparatus of Polymer Laboratories at 10 Hz under an atmosphere of nitrogen and at a heating rate of 5°C/min. The water absorption was determined in accordance with I.P.C. (Institute for Interconnecting and Packaging Electronic Circuits) TM 650 method 2.6.2.1. The dichloromethane absorption was determined in conformity with I.P.C. TM 650 method 2.3.4.3., i.e. over a period of 30′. A second measurement was carried out after 1 week. The dielectric constant and the dissipation factor were measured at 1 MHz on samples which, after having been dried for 30′ at 50°C and cooled in a dessicator, were stored for at least 40 hours at 23°C in an atmosphere of 50% relative humidity. The copper peel strength was measured at 23°C at an angle of 90°C on a laminate sample of 14 x 2 on which there was a strip of copper of 3 mm wide. The measuring process was effected by removing excess copper from a copper laminate in a known manner by etching. The peel strength was also measured on samples of which the copper side had been place on hot solder for 10″ (Solder float test). Examples 3 through 9 Analogous to the description in Example 1 resins of the composition indicated in Table 2 were prepared in Examples 3 through 9. The properties of the resulting cured resins are to be found in the same table. Example 10 Analogous to the Examples 1 and 3 through 9, except that a portion of the allyl compound was replaced with 2,2-bis(4-methacryloxy-ethoxyphenyl)propane (BMEPP). The composition of the resin formulation and the properties measured on the cured resin are listed below. TAC monomer 16 g BMEPP 24 g Epoxy resin A 28,34 g (0,130 equiv.) TBBPA 31,66 g (0,116 equiv.) fraction by weight TAC plus BMEPP calculated on overall weight 0,40 10% of 2MI in 2-methoxy-isopropanol 3,0 g t-butylperoxy perbenzoate 1,2 g MEK 20,4 g calculated bromine % 18,6 Tg (°C) by TMA 150 TEC z ( p pm/°C) 30 TGA loss at 300°C (%) 2 decomposition maximum (°C) 320 residual at 650°C (%) 25 flame test, fire retardant? yes In Example 12 a non-flame retardant interpenetrating network resin composition was prepared analogous to Example 1. In Example 12 Bisphenol A (BPA) was employed as curing agent for the epoxy. The composition and properties are compiled in Table 3. Example 13 Analogous to Example 1 an interpenetrating network resin composition was prepared of which composition and properties are outlined below:

Claims: A polymeric material comprising an interpenetrating network (IPN) built up from a polymer that can be cross-linked under the influence of radicals and an epoxy resin comprising a cross-linking agent, characterized in that the polymer that can be cross-linked under the influence of radicals is built up from at least one, cyclic moiety-containing polyallyl compound together or not with an aromatic difunctional methacrylate, and the cross-linking agent for the epoxy resin is a polyhydric phenol A polymeric material according to claim 1, characterized in that the ring-containing polyallyl compound is triallyl cyanurate or a prepolymer of triallyl cyanurate. A polymeric material according to claim 1 or claim 2, characterized in that 40-60 % by weight of the interpenetrating network is made up of epoxy resin. A polymeric material according to any one of the preceding claims, characterized in that the epoxy resin is a phenol type epoxy resin. A polymeric material according to any one of the preceding claims, characterized in that the cross-linking agent for the epoxy resin is Bisphenol-A or tetrabromo Bisphenol-A. A laminate at least comprising a layer of synthetic material and a layer of metal, characterized in that the layer of synthetic material consists of an IPN, reinforced with fibre or not, according to any one of the preceding claims. A printed circuit board manufactured from a laminate according to claim 6.

Inventors: BUSER ANTONIUS JOHANNES WILHEL, SCHUTYSER JAN ANDRE JOZEF, SLOTS HENDRIK JOHAN, ZUURING PIETER HENDRIK, BUSER, ANTONIUS JOHANNES WILHELMUS, SCHUTYSER, JAN ANDRE JOZEF, SLOTS, HENDRIK JOHAN, ZUURING, PIETER HENDRIK, Schutyser, Jan André Jozef
Applicants: AKZO NV, AKZO N.V.
Application year: 1990
Main classification: C08F283
IPC codes: C08G59, B32B15, C08L63, H05K1, C08F283