Laminate of Fiber-Reinforced, Crosslinked Polypropylene

This invention relates to a laminate for the manufacture of printed circuits and corresponding flat structures from one or more conductor layers and one or more insulating layers, consisting of one or more impregnated cover and core layers containing impregnants that are novel for this purpose.

Laminates serve predominantly as substrates of printed circuits or the like and are made up of identical or different, resin-impregnated sheets which are cut into leaves or panels, superimposed in layers, and, under the effect of heat and pressure, joined to form a unit and solidified. Numerous resin binders for the panel materials are known for layered pressmolded materials, also called laminates and, respectively, multilayer materials, in order to satisfy, in particular, maximum requirements with respect to physical electrical and chemical properties. Laminates constitute an indispensable basic material of modern electronics, as described, for example, by K. Borchert, "Elektroanzeiger Essen" [Electrician's News Essen], No. 18: 27-30 (1966). The reinforcing substrate layers used for the binder resins are predominantly glass fabric sheets, also paper sheets, where glass-fiber-reinforced epoxy resin laminates result in the highest quality laminates. The laminates can be covered on one or both sides by laminating with a metallic foil or copper or aluminum, or can be provided with metal layers by electrochemical methods. Laminates can carry for the inner plies of the up to 10 core layers, as well as for the outer plies of the cover layers a uniformly identical substrate material and the same resin, or different substrate materials and/or reinforcing layers of glass fabric, glass mats, glass paper, paper or cellulose materials, polyester knits, asbestos fibers, or the like, and various resin coats which as a rule are condensation resins. The manufacture requires according to the state of the art a wet impregnation with resin solutions, drying and preliminary condensation, final condensation, and lastly press-molding in heated presses to which several layers are subjected, with the resins being condensed.

Laminates of the conventional type, in spite of their absolute indispensability, exhibit a number of drawbacks and entail a cumbersome manufacturing procedure due to the necessity of condensation of the resins and a cumbersome handling during press-molding of the numerous layers. Laminates with glass fiber reinforcement are furthermore difficult to machine mechanically, for example by drilling, cutting or punching, especially since high precision is required in the production of printed circuits. Attempts have been made to improve machining by combining glass-fiber-reinforced layers with core layers of paper-bound sheets. Also, the same laminate plies have been utilized with a different resin binding, wherein also polyesters can replace the epoxy resins and wherein synthetic fiber materials have been used. Further, core layers have been suggested made of hardened synthetic resin foams with closed cells and cover layers of resin-impregnated fiber plies.

However, there is always the disadvantage that the condensation resins and the numerous layers bring about a high rigidity of the laminate, and a yieldability and flexibility, desired in many cases, are either difficult to achieve or unattainable with the known materials.

Therefore, an object of the present invention resides in producing, in a simple way a laminate, or corresponding flat structure provided with metal layers and containing a fiber reinforcement, exhibiting especially favorable electrical properties, such as low relative permittivity (dielectric constant) and a low dissipation factor, as well as improved mechanical machinability.

Optionally, synthetic resin materials are to be employed which heretofore have not served for the production of printed circuit laminates. In particular, a good and improved solder bath stability according to DIN-IEC 52 is to be achieved wherein, at 260.degree. C., the specimen is stored floating on liquid solder and tested for changes. It has been found surprisingly that crosslinked polypropylene in conjunction with fibers that can be melted as well as fiber that cannot be melted withstands the required high temperatures for an adequate length of time and thus meets the requirement of good soldering ability.

Furthermore, numerous improvements in quality have been found, in particular a substantially enhanced mechanical workability by punching, cutting, drilling, or the like. The manufacturing method is different and substantially simplified.

Consequently, the invention relates to a laminate containing a uniquely crosslinked polypropylene as a binder resin.

Another aspect of the invention resides in a process for the manufacture of these laminates.

It has been found surprisingly that polypropylene, which per se cannot be crosslinked, or can be crosslinked only with great difficulties and with a low crosslinking density, exhibits in accordance with this invention a degree of crosslinking of at least 30% and, with certain measures, a degree of crosslinking in the range of 70-90%, especially if certain measures according to the invention work together. In particular, when producing the laminates in the hot press, temperatures should be maintained of at least 200.degree.-230.degree. C. for an adequate time period, on the order 0.2 to 5.0 minutes, but these temperatures can also be exceeded somewhat, depending on the temperature stability of the laminates. Furthermore, a prerequisite for adequate crosslinking is the choice of crosslinking materials, especially the cooperation of specific silanes, specific peroxides, hereinafter described, and the preferred use of silanol condensation catalysts based on tin compounds. Moreover, the type of polypropylene employed also exerts a substantial influence.

In particular, it has been discovered, surprisingly, that the molecular weight of the polypropylene homopolymer or of the copolymers should be higher than 5.times.10.sup.5 ; thus surprisingly a very high molecular weight material, is a prerequisite for an adequate crosslinking density. With very great preference, the so-called partially crystalline, isotactic homopolymer of propylene, or corresponding copolymers, are utilized, which have crystalline proportions. Further influence is exerted by the melt flow index of the polypropylene, in such a way that polypropylene having a melt flow index MFI (230/5) of 3 can be crosslinked only up to a degree of crosslinking of about 30%; whereas, material having an MFI of about 1 exhibits a degree of crosslinking of up to 70%, and polypropylene with an MFI (230/5) of 0.1-0.3 g/10 min permits attainment of practically complete crosslinking, with a crosslinking degree in the range of 70-95%. The degree of crosslinking is here measured by determining the insoluble proportion in * according to the standard method.

It is not necessary to achieve complete crosslinking for the purposes of the present invention.

The laminates of this invention can be of a multilayer structure or the laminates are preferably one-ply, which entails considerable simplifications of the manufacturing process as compared with conventional laminates. The laminates can be covered by laminating on one side or on both sides with a metal foil, in particular a copper foil or optionally an aluminum foil, if desired with the presence of an adhesion promoter as an intermediary. Also, intermediate layers of metallic foils are possible. In case of multilayer materials, the various layers can contain various polypropylenes, can exhibit differing crosslinking densities, or can contain different reinforcing layers. Furthermore, the concomitant use of conventional layers made of resin-impregnated laminates, so-called prepregs, is possible.

The supporting layer of the laminates or the layers thereof can consist of glass fibers or sufficiently temperature-resistant organic fibers in the form of mats, woven fabrics, knit fabrics, random layers, or the like; in case of continous production, this supporting layer can consist of lengths of these materials available in rolls.

Synthetic fiber reinforcing materials, especially those of thermoplastic synthetic resins, are to exhibit Vicat softening temperatures, measured according to DIN 53 460, procedure B, with a weight load of 5 kp, of higher than 1,200.degree. C., and are to be meltable or, as polyacrylonitrile fibers, are not to have any melting point, but do have dimensional stability under heating in accordance with Iso/R 75 which is high and is, for example, higher than thermoplastic linear polyester.

Suitable thermoplastic synthetic resins for the reinforcing sheets are, in particular, polyesters and/or mixed polyesters, such as polybutylene terephthalate, poly-1,6-cyclohexylene dimethyl terephthalate, polyethylene terephthlate, polyethylene terephthalate adipate and/or poly-butylene terephthalate adipate. Furthermore, suitable for this purpose are polyimides and polyamides, among these polyarylamides. Further, suitable fibers can be made of polyacrylonitrile and/or styrene-acrylonitrile. Also, suited are fibers of polysulfone, polyether sulfone, polyacrylic ethers, polyphenylene sulfide, modified polyphenylene oxide and/or polyparahydroxybenzoate.

The carrier layers or reinforcing layers can also be built up of two different fibers; moreover, so-called bicomponent fibers can be employed. In many cases, glass fibers or mineral fibers are preferred. An advantageous embodiment of the reinforcing layers can be made up on the basis of polyethylene terephthalate or polybutylene terephthalate or poly-1,6hexylene terephthalate, or copolyesters of terephthalic acid with a second dicarboxylic acid, e.g. isophthalic acid and/or aliphatic saturated dicarboxylic acids of 2-6carbon atoms. The substrate can also contain, for example, bicomponent fibers, i.e. one of the aforementioned sheet-like structures, provided with an additional bond or reinforcement by molten fibers, strips, powder, or the like of thermoplastic synthetic resins, for example polyethylene terephthalate or copolyesters; amounts of up to 30% by weight, based on the sheet-like structure, can be present. It is also possible to use, as binders in the form of a powder spread onto sheets, or as additionally applied fibers, polyacrylates, polyvinyl acetates, polyvinyl propionate, polyurethane, melamine resins, unreaformaldehyde resins and/or phenolic resins, whereby heat-setting of even thin mats can be attained.

The invention especially resides in a process, wherein polypropylene homopolymer or propylene-ethylene copolymer of up to 12% by weight of ethylene, containing 5 to 60 wt.-% of textile or fiber reinforcement, the balance of 40 to 95 wt.-% being polypropylene material, the polypropylene material having a molecular weight of higher than 5.times.10.sup.5 and the MFI melt flow index (230/5) being then 0.1 to 3.0 g/10 min. is worked for 0.2 to 10 min. at 200.degree. to 230.degree. in the presence of 0.1 to 5.0 wt. %, based on the amount of polypropylene material, of said alkoxy-silane of said formula and 0.1 to 0.4 wt.-% , based on polypropylene material, of one or more peroxides having a one-minute half-life temperature of about 160.degree. to 240.degree. C., this will lead to laminates of sufficient degrees of crosslinking of 30 to 95%, measured by determining the insoluble portion in decahydronaphthaline, said laminates thus being no longer thermoplastic but becoming duroplastic thus now being stable in a solder bath of 260.degree. C. for more than 20 seconds.

The invention resides also in the so produced laminates, wherein the polypropylene material is silane grafted and cross-linked.

The quantity of textile or fiber reinforcements for the binder containing layers of the laminates is to be 5-60% by weight, preferably 7-45% by weight, with the balance being polypropylene binder thereby achieving the desired flexibility or rigidity of the laminate, as well as one the necessary electrical properties.

Suitable crosslinking is possible, especially by the addition of silanes, preferably together with organic peroxides of a suitable kind and furthermore preferably together with silanol condensation catalysts based on tin compounds. Amounts of the silanes added range from 1.0-5.0% by weight, based on the polypropylene employed. Suitable silanes are, in particular, alkoxy silane compounds having the formula: ##STR1## wherein R.sub.1 is hydrogen or an alkyl radical of 1-4 carbon atoms; R.sub.2 is a straight-chain alkylene radical of 1-10 carbon atoms; R.sub.3 is an alkoxy radical of 1-5 carbon atoms which can optionally be interrupted by an oxygen atom; and m and n are equal to 0 or 1. Among these, preferred are .gamma.-methacryloxypropyl-trimethoxysilane, .gamma.-acryloxypropyltrimethoxysilane, as well as the corresponding triethoxysilanes. Further, suitable silanes are identified in the examples. A suitable selection of the silanes is possible, those having boiling points higher than 150.degree. C. being preferred, and furthermore, silanes which contain acrylic groups, methacrylic groups, and vinyl groups, but also alkoxy groups are split off in a suitable way and bring about crosslinking.

Preferably, silanes are used together with peroxides. The amounts of peroxide are to amount preferably to 0.1 -0.8% by weight, based on the polypropylene employed. A suitable choice of the peroxides is possible based on their half-life temperature of about 160.degree.-240.degree. C. Dicumyl peroxide is highly preferred, also because of easy availability. Further suitable peroxides are set forth in the examples.

An especially suitable silanol condensation catalyst is dibutyltin dilaurate. *

It has been found surprisingly that the laminates of this invention exhibit low numerical values for the dissipation factor and thus highest insulating effect, as well as very low relative permittivities. The solder bath stability, depending on amount and type of the fiber material, is high. There is the great advantage of being able to set the stiffness of the material or the flexibility at intentional values, and also to be able to select thin layer thicknesses. The laminates according to the invention can be provided, especially when serving as substrate material for printed circuits, with metal or wires on the surface or inside during the production process, layers, or the laminates can be provided subsequently with metallic layers by adding metal foils or wires or by means of electrochemical methods. Furthermore, it is especially possible in this way to advantageously manufacture ribbon cables, i.e. metallic conductor paths arranged in parallel and preferably of a flat structure, with a joint covering of layers of the laminate according to this invention and terminals at each end of a conductor path.

The manufacturing process deviates from the customary production methods for laminates as follows:

First of all, a premix is to be prepared from the aforementioned polypropylene and the crosslinking agents, in finely divided form by mixing or by compounding and subsequent granulating; during this step, suitable, finely divided fillers, flame retarders, pigments, stabilizers, antioxidants, etc., can also be added.

For a production of a small number of pieces, the process resides in strewing granules or powder made up of the aforementioned components onto the mold surfaces of the heated presses usually employed in laminate production, at temperatures of 170.degree.-240.degree. C. and under pressure of 10-200 bar, whereupon the reinforcing layer is inserted and again granules or powder is or are spread on. One or several metal layers can be inserted; also, several layers of granulated material or powder and several layers of reinforcement are possible. It is likewise possible to manufacture several laminate plates simultaneously, if corresponding separating layers are utilized.

However, it is more expedient to encase an endless substrate sheet, covered with the crosslinking polymer, n suitable coating units by press-molding to both sides the plasticized polymers which exit, for example, from an extruder slot die.

Especially suitable is a cluster roll stand with 3 or 4 rolls including a subsequently connected smoothing device. In this way, lengths of sheet containing the substrate can be prefabricated in a suitable way, these lengths of sheets having been partially crosslinked already.

In the aforementioned heated presses, sections of these sheet lengths or packs of thin sheet sections are then press molded together with an arbitrary number of metal foils and are crosslinked in the desired way during the press molding step. Presses with automatic feeding and discharging are preferred.

A mixture of 100 parts by weight of polypropylene with an MFI (230/5) of<0.3 g/10 min and 1.2 parts by weight of .gamma.-methacryloxypropyltrimethoxysilane and 0.4 part by weight of dicumyl peroxide and 0.05 part by weight of dibutyltin dilaurate and 0.3 part by weight of an antioxidant * is homogeneously blended and, in a mold, uniformly spread on the bottom and, after placing thereon a glass mat having a m.sup.2 weight of 80 g (Example 1) and, respectively, 227 g (Example 2), on this glass mat, and the resulting composite is heated up in a press under 100 bar to 230.degree. C. within 5 minutes. In a second working step, the single-layer laminates are bonded by laminating to a 0.035 mm copper foil at 200.degree. C. and under 60 bar. The properties of these copper-laminate layered materials are listed in Table 1.

The crosslinking polypropylene mixture mentioned in Examples 1 and 2 is fed to an extrusion device with 2 extruders and a 4-roll smoothing stand. Extrusion takes place with a mass temperature of 220.degree.-230.degree. C. A nonwoven glass mat of endless glass fibers having a m.sup.2 weight of 80 g (Example 3) and, respectively, a glass mat of 660 g (Example 4) is fed into the first slot of the 4-roll smoothing unit. Penetration of the nonwoven glass mat with the polypropylene is effected from fishtail dies on both sides of the fiber web. Subsequently, the resultant sheet is cut to size and, as described in Examples 1 and 2, laminated to a copper foil.

The properties obtained are also shown in Table 1.

It is correspondingly possible to layer several plies of sections of thin sheets in the press, one on top of the other, and provide these plies with a copper foil or with several layers of copper foil. Furthermore, additional layers of conventional, resin-bound prepregs with reinforcements of glass fiber, synthetic fibers, paper, or the like, can be inserted in the press.

The laminating process is conducted according to the same procedure as described in Examples 1 and 2 with polyester nonwoven mats based on polyethylene terephthalate having the following m.sup.2 weights:

Ex. 5: 250 g/m.sup.2 (polyethylene terephthalate)

Ex. 6: 140 g/m.sup.2 (polyethylene terephthalate)

Ex. 7: 135 g/m.sup.2 (polyethylene terephthalate)

Ex. 8 200 g/m.sup.2 (polyethylene terephthalate)

Ex. 9: 120 g/m.sup.2 (polyethylene terephthalate)

The properties are illustrated in Table 2.

Laminates are produced as set forth in Examples 1 and 2, but using, in place of the nonwoven glass mats employed, fabrics of aramid, e.g. Kelvar (registered trademark of Dupont) having varying m.sup.2 weights and differing fabric weaves:

Ex. 10: linen weave 220 g/m.sup.2

Ex. 11: twill weave 220 g/m.sup.2

Ex. 12: linen 161 g/m.sup.2

Ex. 13: linen 80 g/m.sup.2

The material proportions and properties are indicated in Table 3.

Polypropylene (PP-H) having an MFI at 230/5 of 0.3 g/10 min is processed together with various peroxides and silanes according to Example 4 on a 3-roll stand with subsequently connected smoothing rollers. In Example 17, a propylene copolymer (PP-CO) containing 8% by weight of ethylene is processed together with vinyltrimethoxysilane and .gamma.-methacryloxpropyltrimethoxysilane, and peroxide A. Depending on the setting of the roll nips, sheet material having thicknesses of 0.2 to 1.5 mm can be obtained; this material is cut and then laminated in a press with copper or Al foil. The electrical values correspond to those of Table 4.