CA1070575A

CA1070575A - Polyparabanic acid laminates

Info

Publication number: CA1070575A
Application number: CA222,520A
Authority: CA
Inventors: Don J. Henderson
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1974-04-26
Filing date: 1975-03-19
Publication date: 1980-01-29
Also published as: SE7902400L; SE7504853L; DE2517671A1; NL7505012A; JPS5529821B2; BE828397A; JPS50146686A; SE420290B; GB1493029A; FR2268638A1; FR2268638B1

Abstract

ABSTRACT OF DISCLOSURE
This invention relates to processes and techniques for forming metal and/or plastic/polyparabanic acid (PPA) laminates particularly for use as a component of electrical and electronic systems, especially as flexible circuit materials. Laminate species not requiring intermediate adhesive layers, not fabricated from pre-formed PPA films and not requiring subsequent cross-linking or other curing reactions are preferred. Solvent casting of PPA is a preferred method of preparing such laminates.

Description

~o705~75 1 There is a strong existing need for flexible print-

2 ed circuit (FPC) materiAls which have good electrical insu-

3 lating properties, good mechanical properties, and high tem-

4 perature dimenslonal stability to enable them to withstand soldering operations above 400F-, preferably as high as 6 500F.
7 It is known in the prior art that various polyimide 8 films can be adhesively bonded to copper foil to form such 9 FPC articles. Nevertheless, the requirement of an adhesive l~yer adds an expensive and difficult process requirement.
11 Moreover, the adhesive layer is the weakest link in the over-l2 all laminate. Therefore, such laminates suffer from impor-13 tant temperature limitations, which adversely affect signifi-14 cant characteristics, such as bond strength, dimensional stsbility, environmentsl stability, and chemical stability.
l6 Another slternative taught by the art, see e.g., l7 U. S. 3,682,960, i~ to use solutions of polyamic acids and/or 18 amite-modified polyamic acids as intermediates which are ~o-19 luble in certain solvents such as dimethylformamide. These solutions are then coated on various metal ~ubstrates such 21 as copper sheets, copper foil, copper wire, etc. Subsequent-22 ly, it is required that the coated substrate be exposed to 23 the precisely proper temperature or free radical environment 24 conditions nece~sary to achieve full cyclization without cross-linking or without inadequate cyclization.
26 Improved cyclized polymers adhering to the surface 27 of a metal foil, which is to be used as a flexible printed 28 circuit suffer from the adverse characteristic of lacking 29 flexibility and foldability, which are ~he sine qua non of ~ flexible printed circuit.
31 The very existence of a flexible printed circuit 32 market is strongly indic~tive of the market place demsnd of ~ 2 - ~

~070575 a printed circuit which is flexible enough to be stuffed, jammed, wrapped, poked into what would ordinarily be inaccessible areas. FPC's which have been prepared from improperly cyclized films will not stand up under these conditionsnearly as well as the FPC articles according to the present invention.
Thus the present invention provides a process of forming laminate articles of polyparabanic acid and strongly adherent to a supportive substrate which comprises in combination the steps of forming a solution of polyparabanic acid having a viscosity of from 1,000 to 50,000 cps, ~Brookfield viscosity);
coating a thin layer of said solution directly on a substrate in the absence of intermediate adhesives; heating said thin layer of polyparabanic acid solution to evaporate said solvent from said film; and recovering a laminate having a relatively dry layer of polyparabanic acid strongly adhered directly to said substrate.
In accordance with the present invention there are provided essentially non-cross-lin~.ed polyparabanic acids (PPA)/laminates of two or more layers wlth the PPA coating adhered to the substrate without an inter-mediate adhesive layer and without the necessity or extensive cross-linking.
Preferably at least one layer of the laminate is metallic. Copper is the preferred metal; metal foil is the preferred shape of the metallic portion of the composite.
Novel and unobvious PPA/metal composite laminates are prepared according to the invention which are especially useful for preparing flexible printed circuits (FPC) which will withstand high soldering temperatures without distortion or delamination. The process of preparing such high performance laminate composites is considerably less expensive than comparable known processes as is the resulting composite, since if offers high performance coupled with relatively low polymer costs. Therefore, the composites of the invention are especially useful because of the combination of excellent A ~ 3 _ .

properties and low cost which is not hitherto known.
The novel composition of the invention is achieved through processing by preferably:
(1) direct solution casting of PPA polymer from a volatile solvent onto a substrate, or (2) hot compression molding of PPA powder onto a substrate, or - 3a -, 107057s (3) direct heat lamination of thermoplastic PPA
film to a substrate.
In ~he preferred embodiment, it is significant that satis-factory adhesion of PPA to copper can be achieved by using commer-cially available clean, high purity copper foils (as are common for FPC) without additional surface pret~eatments. Known coating or film casting techniques and equipment can be employed. It is of course recognized that copper surface cleanliness and preparation will have a major influence on the resulting PPA-copper bond strength and temperature characteristics. Likewise, inorganic and organic primers applied to the copper surface before coating will affect bond proper-ties.
It has also been found that the exact drying conditions of the cast PPA layer affects adhesion between the PPA and copper. Dry-ing oven~ blanketed with nitrogen are useful to prevent oxidation of the copper. In general, processing conditions found to maximize PPA
cast film properties also maximize the properties of PPA when coated directly on copper. It is important to note that the PPA polymers used in these coating~ are fully formed and further chemical modifi-cation, i.e., cross-linking, does not need to be performed during the coating process as is necessary in the art of many "high temperature"
polymer films and coa~ings. Cyclization is also not necessary.
Thé PPA layer can be modified by any suitable technique for flame retardation, stabilization, reinforcement, blending, foaming and plasticization.
In certain situations adhesion can be improved by mixing into the PPA formulation or coating on the metal foil various adhe-sion promoters or coupling agents such as silanes as is known in the art. In such instances, preferably at least 25~ of the PPA resin must be of an aromatic type; however, it need not be a homopolymer but may consist of copolymers with PPA or blends of PPA polymers.
The thickness of the layers will be determined by the particular end-use contemplated. Typically the PPA layer is in the range of 0.1 to 8 mils (preferably 1 to 5 mils), and typical~y the copper layer is from 1/2 to 10 mils. Thicker coatings may be built up by applying several thinner layers and drying between applications.
Likewisé, structures coated on more than one side or of alternating PPA and copper layers can be prepared.
Polyparabanic acids have been described in several issuea patents. Illustrative are U.S. Patents 3,547,897 and 3,661,859.
For the purpose of thi~ invention the molecular weight of the utilizable polyparabanic acids can vary over a wide range. The lower limit i9 determined by that molecular weight which will form an integral supported coating on the surface of a metallic substrate.
Normally, this coating will be hard and relatively non-scratchable to the ~ouch and the fingernail.
Nevertheless, upon occasion it might be desired to use such a coating as an intermediate layer upon which other films can be la-minated. Thus a rela~ively low molecular weight tacky PPA coating -on a metallic laminate can be subsequently covered with a continuous film of Kapton*, which is the duPont polyimide film; this can be then laminated to the copper by direct heat lamination. Since the inter-mediate PPA material has excellent high temperature properties, this intermediate adhesive offers the advantage of being considerably heat * Trade Mark _ 5 _ 107~3575 stable at higher temperatures than the ordinarily employed intermedi-ate adhesives. Another material useful in this manner is fluorinated ethylene propylene polymer film.
Alternatively, the tacky PPA coating may be applied to a film of Kapton* or a film of fluorinated ethylene propylene polymer and the composite, utilizing the PPA film adjacent the metal, is ap-plied to a metallic layer by direct heat lamination.
The general solution viscosities are 1,000 to 50,000 centi-poise~ (cps, Brookfield viscosity), preferred 5,000 to 40,000, most preferred 10,000 to 20,000.
The inherent viscosity of the PPA polymer should be in the range of 0.4 to 2.5, more preferably 0.6 to 2.0, most preferably 0.8 to 1.2.
Viscosities of less than 0.4 tend to give brittle substrates But the intermediate adhesive layer, if utilized, can employ PPA polymers having inherent viscosities of less than 0.4 preferably at least 0.05.
Polymers having an inherent viscosity of less than 0.4 form brittle layers, which are unsuitable except as intermediate adhesives.
Brittleness is evidenced by inability to resist finger creasing with-out cracking.
The polyimide or polyimide-amide films of the prior art which require a heat reaction actually undergo a cyclization reaation which is accompanied by random cross-linking. This requires a criti-cal control of temperature and other conditions to make sure that the total reaction is not overdone or underdone. If either one of these extremes is encountered, the resulting coatings will be unsatisfac-* Trade Mark . ~ .

~070575 tory for use as FPC materials.
The invention will be further illustrated by the following examples:
Example I
PPA prepared from the hydrolyzed reaction product of methy-lene diisocyanate with hydrogen cyanide (designated PPA-M) was dis-solved in dimethylformamide to give a 17.0%, 20,000 Vi8 . sol. Two-ounce (0.0028 inch thick) copper foil (Hi-Bond*, Anaconda, American Brass Company) was coated, as received, on the matt side with a 13-mil wet coating, and then heated at 400 for eight minutes in a ni-trogen atmosphere. This laminate was subsequently dried in a circu-lating air oven for one hour at 250C. to give a 3-mil PPA coating on the copper.
The product laminate was tough, flexible and creasable with-out delamination or cracking. The structure was unaffected when im-mersed for 15 seconds in a 500F. solder bath.
Example II
~ sing the coating procedure of Example I, 2 oz. copper was coated with PPR-M ~inherent viscosity 0.83) from a 15% solution in N-methyl-2-pyrrolidone. The coating was dried at 455F. for eight minutes.
The resulting product had a smooth surface and excellent physical properties. The PPA layer could not be peeled from the o~er.
Example III
A continuous roll of PPA-M~copper laminate was prepared in commercial type equipment using a reverse roll coater as solution ap-plicator and in-line drying ovens. The resulting laminate was batch *Trade Mark L~
~ - 7 -dried at temperatures up to 300C. to remove last traces of DMF.
This dry laminate was used to prepare sample flexible printed circuits by chemical etching techniques.
Example IV
PPA-M powder was hot compression molded on copper - 7a -1 foil to give 3/16 PPA layer. The resulting structure had 2 good properties with the copper layer adequately adhered.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process of forming laminate articles of poly-parabanic acid and strongly adherent to a supportive substrate which comprises in combination the steps of:

a) forming a solution of polyparabanic acid having a viscosity of from 1,000 to 50,000 cps, (Brookfield viscosity);

b) coating a thin layer of said solution directly on a substrate in the absence of intermediate adhesives;

c) heating said thin layer of polyparabanic acid solution to evaporate said solvent from said film; and d) recovering a laminate having a relatively dry layer of polyparabanic acid having an inherent viscosity of 0.4 to 2.5 strongly adhered directly to said substrate.

2. The process of claim 1 wherein said substrate is copper foil.

3. The process of claim 2, wherein said polyparabanic acid layer has a thickness of 0.1 to 8 mils and said copper layer has a thickness of 1/2 to 10 mils.

4. A coated precursor for a laminate article comprising one component of the ultimate laminate coated with a layer of polyparabanic acid having an inherent viscosity of 0.4 to 2.5.

5. The coated precursor for a laminate article as claimed in claim 4 comprising one component of the ultimate laminate coated with a layer of polyparabanic acid having an inherent viscosity of at least 0.05 but less than 0.4 wherein at least 25 percent of the polyparabanic acid is of an aromatic type.

6. The coated precursor of claim 4 or 5 wherein said coated component is metallic copper.

7. The coated precursor of claim 4 or 5 wherein said coated component is copper foil.

8. Coated articles comprising copper coated with a polyparabanic acid layer having an inherent viscosity of 0.4 to 2.5.

9. Coated articles as claimed in claim 8 comprising copper coated with a polyparabanic acid wherein at least 25 percent of the polyparabanic acid is of an aromatic type and the inherent viscosity of the polyparabanic acid layer is at least 0.05 but less than 0.4.

10. The article of claim 8 or 9 wherein said copper is copper foil.