REINFORCED PLASTICS SYMPOSIUM
THERMOSETTING LAMINATES ST. JOHN BAIN
JOHN C. PITZER
An indus&y, traditiondry regulated electrical industy and military stanhrds, turns at#mtiOn to the c h e m i w that determines the performance of its products he laminate manufacturer, until very recently, has T not been interested in the chemistry of the resin used and has relegated the formulation of the resin to the synthetic resin manufacturer or to staff polymer chemists within his own organization. Laminators have merely requested resins to produce products required for a given end use, and hence have shown little concern for the composition of the resin providing its processing characteristics. The laminate industry started almost simultaneously with the beginning of the synthetic phenolic resin industry. The first volume usage of thermosetting laminates was in World War I. ’ Immediately after the war the “build-it-yourself” radio hobby brought laminates to the attention of the general public. The National Electrical Manufacturers Association (NEMA) began collecting statistics on the sales of the industry in 1925. Because of the reporting method and the fact that the sales of decorative laminates were not separated from industrial laminates until 1949, it is only possible to show a truly reliable picture of the growth of industrial laminates since that date. Figure 1 shows the growth in industrial laminate salessince 1950. The properties of a laminate are influenced by the type resin used, the amount of the resin, and the filler to which the resin is applied. The mechanical properties by which laminates are most frequently evaluated are: flexural strength, Izod impact, and bonding strength. Water absorption is determined as a physical quality. Electrical properties consist of dielectric breakdown, dissipation factor, dielectric constant, and insulation resistance. Methods for testing these properties, as well as other less frequently used
tests, are fully given in the various American sodety for Testing and Materials Standards concerning electrical insulating materials. National Electrical Manufacturer’s Association (NEMA) has published at least six editions of standards which have been established by the industry for the properties of the more common grades of laminates (NEMA Standards Publication No. LI 1-1965 Industrial Laminated Thermosetting products, National Electrical Manufacturers Association, 155 East 44th Street, New York, N. Y. 10017). NEMA designates the variety of laminated products as “grades.” Military specifications for laminates refer to the various products as “types.” The NEMA standards give detailed minimum or maximum property values for all three available forms in which laminates are produced, namely, sheets, tubes, and rods. A convenient means for comparing the properties of grades is by use of a “comparator chart.” Here all the grades are compared for each property on a numerical scale (usually 1 through 10). No attempt is made to relate these comparative numbers to the actual property
AUTHORS St. John Bain is Director of Product Engineering f m fhe Formica Cmp., a who& owned subsidimy of Amnican
Cyanamid Co. HU coaufhm, John C. PitUr, is currenfIy M a n a p of its Indwfrial Appricafions D e p a r m f . Their extensivc combined experience wifh t h o s e t t i n g laminates strongly qual$es t k m to prepare this broad W Q C ~ of fhe chemistry and fechnology in f h u j e l d .
comhnf to fhe company, and was fmmnIy
Figwe 1. Growth of ihe ht@+we.ssure lamhie induaby (doto slaWical reports)
jTom N E M A
V O L 5 8 NO. 5 M A Y 1 9 6 6
37
values and if one grade is given a value of 2 and another is given 4, it does not mean that the second mentioned grade is twice as good as the first. Such a comparator chart is shown in Table I covering the grades discussed in this paper. I n general, the electrical, dimensional stability, and water absorption properties of a laminate are controlled by the type of resin and thickness on a given base. The mechanical properties such as flexural strength, Izod impact, and bonding (ply-adhesion) are controlled by the base material. Papers, cotton fabrics, and asbestos fabrics used in laminates have been discussed in many literature references heretofore and glass as a filler is discussed in the foregoing paper in this symposium; therefore, no further reference to filler requirements will be discussed here. Epoxy resins on glass fabric base constitute the most recent additions to the ever-increasing number of thermosetting laminates, but this type material is also discussed by another author in this symposium. The various laminates covered by this paper are classed as “high pressure” laminates since the majority of them are pressed at pressures of 700 p.s.i. and over. Phenolic resins used for making low pressure laminates are covered very thoroughly by a Military Specification MIL-R-9299B. This specification groups low pressure resins into two classes: 10-50 p.s.i. and 50-300 p.s.i. Phenolics
In the beginning of the industry the resins used for laminates were usually of the novolac-type prepared with acidic catalysts. These were converted into thermosetting resins by the addition of hexamethylenetetramine at the time the resin was applied to the filler. At present, however, all phenolic laminates are made
TABLE I .
with resoles (the term applied to phenol-aldehyde condensation products) which are alkaline catalyzed. Phenolic and cresylic laminates, the latter being a substituted phenol type, comprise eight major groups with respect to end usage and the resole used in each group is differentiated by some variation in formulation. Table I1 gives some of the different end uses of the laminate together with a broad classification of the resole type. These types can vary according to whether phenol or a cresol is used. Other variables are theformaldehyde ratio, the type catalyst, the use of plasticizers, and the degree of advancement. All of these resoles are applied to the filler either by dipping or coating, then the solvent is dried in a coating machine before the material is cut into sheets for sheet stock. The dried material is taken to the tube roller in roll form if it is to be used for tubing. Punching stock (grades X P or XPC) accounts for one of the greater volume items of paper base products. This type material may be made to punch either hot or cold depending on the plasticizer used and the method for compounding it into the resole. The distinction between ‘(hot” and “cold” punching qualities is also dependent on the thickness of the material beingpunched, the contour of the part, and the “sharpness)’of the punching die. Hot punching stock is more rigid and has higher flexural strength and less deformation under load than cold punching laminates do. Grade XX is a paper-base grade for general insulating used especially in the electrical power distribution and usage industries. I t is possible to produce sheets of this grade up to 2 inches thick whereas punchable materials are made up only to inch thick. Grade XX has higher physical properties than X P or X P C and also has lower water absorption. The use of grade XXXPC, or X X X P which is a hot
COMPARATIVE PROPERTIES OF INDUSTRIAL LAMINATES
(Properties are rated numerically, in ascending order from 1 through 10)
Base
N E M A Grade Phenolic Grades XP and XPC
XX XXXPC
-I I
C
L G-3
Mechanicol Flexural Izod Strength Impact
Paper Paper Paper Cotton fabric Fine-weave cotton fabric Glass cloth
Physical Properties, Water Absorption
Elecfrical Dielectric Breakdown
Dissipation Factor
1 3
1
’3
2
4
4
4
2 5 4
10 2 3
9 1 2
10 1 2
7
4
2
2
Melamine Gradesa
MC” G-5 G-9 Allylic Grades Z-80b FF-33b a
38
Deleted as a N E M A grade.
Cotton fabric Glass cloth Glass cloth
3 9 10
3 9 10
3 6 8
2 3 9
2 2 6
Cotton fabric Glass cloth
4 7
4
7
8
8
6 9
10
A n individual manufacturer’s grade.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
No N E M A designation for these grades.
4
punching laminate having similar properties, has increased extensively with the increased use of printed circuits by the radio and television industry. This grade has the highest insulation resistance and the lowest dielectric losses of any of the phenolic paper base grades. Most of the production of these grades is supplied with copper foil on one or two surfaces. After cutting the sheet into panels, a wiring diagram of a circuit is masked on the copper surface, and the remainder of the copper is etched away. After punching and/or drilling the necessary holes, components are mounted on the panel and connection between the component leads and the etched circuit is accomplished by a standard solder dip operation. Grade X X X P C should have good fabricating qualities and remain dimensionally stable under conditions of variable temperature and humidity in the ultimate application. Materials for printed circuits must have several properties not required in other laminates: (1) The surface must not be attacked by the etchants (usually ferric chloride, ammonium persulfate, or chromic acid) used to etch off the unwanted copper; (2) the insulation resistance should be retained after the panel has been processed; (3) there must be good internal bond between the laminations and adhesion of the copper to the laminate so that there will not be any penetration of plating solutions into the material in punched holes if the circuit is required to have “plated-through holes” ; (4) the adhesion of the copper to the laminate must not show great deterioration of peel strength when the panel is subjected to heat for a long period of time as will occur in a television set. The requirements for copper-clad laminates have been standardized in the NEMA standards. I n recent years there has been an increasing concern
TABLE II.
PHENOLIC RESOLESUSED FOR LAMINATES Typical
Laminate End Use
NEMA Grade
Ty#e Resole Used
Punching stock High insulation resistance Flame retardant
X P and XPC XXXPC
Plasticized phenolic Oil modified cresylic
FR-2
Mechanical application Postforming
C or L
Oil modified cresylic with flame-resistant additives Cresylic acids
Dimensionally stable Chemical resistant (to acids and mild alkali) Heat resistant
CHWa CN-7”
CF
Cresylics or phenolics not highly cross-linked Water soluble phenolic Phenolic modified with xylenol Highly cross-linked cresylics or phenolics
-___ a An individual manufacturer’s grade. these grades.
No NEMA designation f o r
over laminates propagating flame in case a fire occurs in a television set or other electronic equipment. A grade of laminate has been developed to reduce this hazard (NEMA grade FR-2). This grade is formulated with a resole which imparts flame-resistant properties to the laminate. The other properties are similar to grade XXXPC. Among the first uses of phenolic resins in laminates was the application to cotton fabrics to produce the grades now known as C and L. These materials quickly found use as gears and insulation for railroad track signal systems. Strictly speaking, cresylic acid resoles are used for these mechanical application grades rather than phenolics because cresylics give greater resiliency and impact strength. Cotton fabrics varying in weight between 2 ounces and 32 ounces per square yard are used depending on the thickness of the sheet to be produced and the end use of the product. NEMA standards designate laminates using fabrics weighing over 4 ounces per square yard as grade C and 4 ounces and less per square yard as grade L. Izod impact strength of these grades ranges from 1.2 to 2.5 ft. lb. per 1-inch notch tested in the edgewise direction; whereas paper base grades are less than one half these values. Postforming laminates, especially fabric base grades, are formulated with a resole containing cresol fractions which are not highly cross-linked in the resin preparation. These give flexibility to the sheet at postforming temperatures after it has been subjected to the normal heat and pressure cure cycle. The amount of crimp in the cotton fabric being used has an important influence on the postforming qualities of a sheet. The usual physical properties of NEMA grade CF are essentially the same as for grade C, but certain other properties are also necessary to provide a sheet which will fabricate by the postforming technique. Among these are minimum hot bending radius and a specific depth of draw for every thickness. Postforming is done in matched dies after the material is heated to a temperature somewhat higher than the initial curing temperature for a short period of time. Postforming grades are heated to approximately 325’ F. for forming, and material should not blister at these temperatures if heated in accordance with the manufacturer’s recommendations. Some special applications of phenolic laminates, such as pump impeller blades, require that the part have extreme dimensional stability since these blades are used in applications such as air tools and compressors where water from compressed air is encountered. Such laminates are usually made using fine-weave cotton fabric. These fabrics are given a first impregnation with a water soluble phenolic resole to give better penetration into the fiber. This impregnation resists the capillary wicking of water when the blade is fabricated and in service. This wicking of water is the cause of swelling of the laminate, which causes “freezing” of the blade in the pump rotor. Xylenol base resoles are used in making phenolic laminates which are to be used in mild alkaline or dilute acid chemical applications. Guides for wire feed into VOL. 5 8
NO.
5
MAY 1966
39
pickling baths in a wire mill are a typical application for this type of laminate. Considerable amount of tubing for use in electroplating applications is also made from this type product. Heat-resistant phenolic laminates comprise the most recent addition to the variety of phenolic resole applications. This group of products is made with glass or some other inorganic fillers. The phenolic resoles are compounded so that when they are fully cured they are highly cross-linked. T o maximize heat-resistant properties, long postcuring at temperatures up to 600’ F. are employed. The choice of resole formulation and filler used as well as the postcuring procedure depends to a great extent on the nature of the ultimate application. If the application is to be subjected to ablation temperatures for a period of only seconds duration, then one type resin with a carbon fabric may be used to give a char layer when in service. If the application requires a product to withstand temperatures of 300’. to 500’ F. for long periods of time such as 1000 hours, then another combination of resole and filler will be chosen. Military Specification MIL-P-46040, for “High Temperature Resistant Laminates,” requires that the laminate have a flexural strength of 30,000 p s i . minimum when tested at 260° C. (500’ F.) after being subjected to 260’ C. for 96 hours. Melamines
The use of melamine resins for industrial laminates had its greatest impetus during World War 11. It was found that melamine resin on glass fabric base produced a laminate which had high impact strength, was flame resistant, and did not carbonize across the surface when subjected to an electric arc. Such a product was needed for circuit breaker bases and panel boards on naval ships where slate was being broken by shock from shell fire. By melamines is actually meant “melamine-formaldehyde resins’); these are classified as amino resins. Melamine is commonly prepared from dicyandiamide, or sometimes more recently, urea. The melamine is dissolved in a neutral formalin at 85’ to 90’ C. and further heated under reflux through several stages of condensation. The condensation is arrested at the desired stage of water miscibility by cooling. This resinous solution may be used as is or it may be spray dried to give better stability and more economic shipping conditions. The spray dried resin is dissolved in water-alcohol mixtures for use in impregnating fillers for industrial laminates. The earlier melamine resins were inorganic caustic buffered and because of the presence of free caustic the laminates had very low dielectric breakdown properties after being subjected to water or high humidity for several days. The production of the nuclear powered submarine brought a demand for a panelboard having much higher initial dielectric breakdown values and which would not degrade so rapidly when subjected to moisture. An improved melamine was developed using organic buffers plus the addition of coreactants. This 40
INDUSTRIAL A N D ENGINEERING CHEMISTRY
type resin in conjunction with improved finishes on glass fabric permits the production of glass base laminates (Military type GME) having much higher initial breakdown values (60 kv. per inch as compared to 23 kv.) but even more important was the improvement after being subjected to water. Type GME is required to maintain a breakdown value of 45 kv. for ‘/2 inch thick material after being immersed in water at 50’ C. for 336 hours whereas the earlier GMG required only 5 kv. after a 48hour immersion a t 50’ C. Another application of melamine resins for industrial applications is a cotton fabric base grade designated MC. This product has good caustic resistance. The primary applications are in the electroplating industry. While the use of melamine laminates is small compared to the volume of phenolics, melamines have a very important application where good arc resistance and nonburning properties are required. Allylics
Allyl resins are a class of polyesters. They are colorless monomeric liquids of low viscosity and low volatility. When a catalyst such as benzoyl peroxide is dissolved in the monomer and heat is applied, the liquid gradually thickens to form a gel which hardens into an infusible solid on further heating. Hence, this class of resins is included in the thermosetting group Allyl resins, like melamines, are used to a very limited extent in laminates, but two desirable properties of allylics are outstanding when compared to other thermosetting resins used for laminates. These properties are uniform dielectric loss factor over a wide frequency range (10’ to 1O1O cycles per second) and high flexural strength at elevated temperatures. These particular properties make glass base allylic laminates useful in applications such as radomes, antenna housings, and airborne computer potentiometer cards. I t should be noted also that the chemical-resistant characteristics of allyl resins, such as diallylphthalate (DAP), are better than either phenolics or melamines. Future Developments
The possibilities for further new developments in the three aforesaid resins for use in laminates would appear to be limited. Currently among the demands for improvement in laminates are such requirements as higher temperature resistance together with maintenance of good insulating properties at these temperatures; less drift of dielectric loss with variations in service temperatures; and very much lower dissipation factor values. Epoxy resin laminates are finding a very useful place in meeting some of these requirements. New resin systems such as polyphenylene oxides, polyimides, and poly-p-xylylenes are now commanding the laminate engineer’s time as these new developments in the resin field give promise of meeting some of the current needs of laminate consumers, especially in the high temperature applications.
