CA1320779C

CA1320779C - Conductive primer composition

Info

Publication number: CA1320779C
Application number: CA000559228A
Authority: CA
Inventors: Henry Stever Tremper Iii
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1986-08-29
Filing date: 1988-02-18
Publication date: 1993-07-27
Anticipated expiration: 2010-07-27
Also published as: EP0328711A1

Abstract

TITLE
CONDUCTIVE PRIMER COMPOSITION
ABSTRACT

A primer composition containing solvents, a film forming binder and pigments of an electrically conductive carbon black pigment and the binder contains about (1) 40-70% by weight of a polyester copolymer having linear segment with hydroxyl groups and a hydroxyl number of about 15 - 300 and a number average molecular weight of about 300 - 3,000 and branched chains having hydroxyl groups and having a hydroxyl number of about 175 -300 and a number average molecular weight of about 500 - 2,000 and are attached to linear segment by esterification of the hydroxyl groups with a diacid or a anhydride; and (2) 30 - 60 % by weight of an alkylated melamine formaldehyde crosslinking agent;

wherein the composition forms a hard flexible coating on a substrate whereby the coated substrate can be bent 180° around a 1/2 inch mandrel with the coating facing outward and the coating exhibits substantially no breaks or cracks when tested at -18°C and the coating has a surface conductivity of at least 160 Randsburg units.

Description

~32~7i79 FP-0102Title CONDUCTIVE PRIMER COMPOSITION

BACKGROUND OF THE INVENTION
This invention is ~irected to a primer compostion and in particular to an electrically conductive and ~lexible primer composit.ion.
10 Primer compositions are well known in the art and are widely used in the manu~acture of automobiles and trucks as shown in Simon U.SO Patent 4,232,090 issued Nov. 4, 1989. High solids polyester coating compositions also are wPll known as ehown in Chang V.S. Patent 3,g94,851 issued N~v. 30, 197S.
Polyester copolymers having linear and branched chains useful ~or c~atings are shown in Sommerfeld et al U.S.
Patent 4,442,269 issued April 10, 1984. However, none of these composi~ions meet the current needs of the automotive and truck manufackuring plants.
In modern auto and truck manufacturing facilities, the top coat paint is applied by electro-static spraying which in g~neral has not been done previously. Electrostatic praying reduces paint loss and air pollution caused by paint over-spraying. To efficiently electrostatica~ly spray a paint~ the substrate m~st be ~onductive. Most autos and trucXs contain flexible plastic components and some auto bodies are eonstructed entirely from a polymer reinforced fiber glass and ~thers from plastic.
Obviously, these components ar~ not electrically conductive and electro-static spraying works poorly under the ~est conditions. Also, these components are flexible and require a primer and a topcoat that is flexible. There is a great need in auto and truck manufacturing for a high quality primer that is :~32~779 electrically conductive, flexible, has excellent adhesion to the substrate and provides a ~ur~ace to which conventional coatings will adhere.

SUMMARY OF THE INVENTION
A conductive, hard and flexible primer composition containing solvPnt6 and about 40~75% by weight of a film forming bindler and pigments in a pi~ment to binder ratio of about ~:100 50:100;
wherein the pigments comprise an electrically conductive carbon black pigment and the binder contains ~bout (1) 40-70% by weight, based on the weight of the binder, of a polyester copolymer having linear segment with hydro~yl groups that has a hydroxyl number of about 15 -300 and a number average molecular weiyht of about 300 - 3,000 and branched ~egments that have hydroxyl groups and have a hydroxyl number of about 175 - 300 and a number average molecular weight of about 500 - 2,000 and ~re ~ttached to linear ~egment by esterificatiQn of the hydroxyl groups with a diacid or an anhydride;

~2) 30-60 % by weight of an alkylated ~elamine formaldehyde crosslinking agent;

wherein the composition forms a flexible coating on a cubstrate whereby the cDated 6ubstrate can be bent lR0O around a 1/2 inch mandrel with the coating facing outward and the coating exhibits 6ubstantially no breaks or cracks when tested at -18C and the coating has a surface conductivity of at least 160 Randsburg units.

~320779 DETAILED DESCRIPTION OF THE INVENTION
The primer composil:ion forms finishes that are hard, ~lexible and electrically conductive and have excellent adhesion to a variety of ~ubstrates such as cold roll ~teel., phosphatized ~teel, polyester r~inforced fiber glass, reac1:ion injection molded urethanes, partially crystal;Line polyamides and other plastic substrates and provides a sur~ace to which conventional topcoats will adhere. The primer is particularly useful on the aforementioned plastic substrates since the resulting finish i5 conductive and topcoats can be applied by electrostatic spraying which is currently being used by ~any auto and truck manufacturing facilities.
Flexibility of the finish is determined by the mandrel bena test in which a panel coated with about l.0 mil thick fully cured film of the coating decomposition is held at -18C and then bent around a l/2 inch diameter steel mandrel with the coated ~ide of the panel Pacing outward. The coating should not break or crack.
Surfac~ conductivity of the coating compo~ition is measured by a Sprayability Meter manufactured by Randsburg Electrocoating Corporation, Indianapolis, Indiana. The composition has a surface conductivity vf at least l60 Randsburg units and preferably, 165-350 Randsburg units.
The primer compositiDn has a film forming binder content of about 40-75~ ~y weight and correspondingly about 25-60% by weight of a liquid carrier which llsually is a solvent for the binder. The composition contains pigments in a pigment to binder weight rat.io of about 5:lO0 - 50:lO0. The pigments must conta.in an electrically conductive pigment such 13~0779 .

as an electrically conductive carbon blac~ pigment.
Other pigments also can be used in the compo~ition.
Preferably, the film orming binder of the composition contains about 50-80% by weight of ~
polyester copolymer and about 20-50% by weight of an alkylated melamine for~aldehyde crosslinking agent.
~ne highly preferred composition contains ~bout ~0-70~
by weight of the polyester co,pol~mer and 30-40% of the cro~slinking age~t are used and forms a high quality composition.
The polyester copolymer used in khe co~position has ~ linear polyester 6egment with hydroxyl groups that has a hydroxyl number 3f about 15-300 and a number averaqe molecular weight of about 300-3,000 and has branched polyest~r 6egments. These branched segments contain hydroxyl groups and have a hydroxyl number of about 175-300 and a number average molecular weight of about 500-2,000 and are attached 2 to the linear segment by esterification of the hydroxyl groups of the the two segments with a diacid or an anhydride. The polyester copolymer has a number average ~olecular weight of about 800 ~ 3,500 and a hydroxyl number of about 50 - 170.
~olecular weight is determined by gel permeation chromatography u~ing polymethyl methacrylate a~ the standard.
The aforementioned Somm~rfeld et al 3 u.S. Patent 4,442,269 shows a method for preparing the polyester copolymer used herein. Also, preferred polyester compositions are disclosedO

The linear segment is formed ~rom a straight chain diol and a dicarboxylic acid. Typical diols that are useful have from 2-10 car~on atoms and ~re for example ethylene ylycol, propylene glycol, butane ~3207~

diol, pentane diol, neopentyl glycol, hexane diol, octane diol, Esterdiol*204 (a commercial diol produced by Union ~arbide) and the like. Preferred diols are neopentyl glycol and 1,6 hexane diol ~ince these diols form high quality polyesters.
Typical dicarboxylic acids that can be used are aromatic acids ~uch as phthalic acid, orthophthalic acid, isophthalic acid, and aliphatic ~cids such ~s adipic acid, azelaic acid and the like.
The branched ~egment is formed rom a ~traight chain dio~, a branched chain polyol and a dicarboxylic acid. The above straight chain diols and dicarboxylic acids are used. Typical branched chain polyols that can be used are trimethylol propane, triethylol propane, pentaerythritol and the like~
The linear and branched ~egments are attached by esterification of the hydroxyl groups of each ~egment with a diacid or an anhydride. The aforementi~ned acids can be used but preferably an anhydride is used. Ty~ical anhydrides are orthophthalic anhydride, ~uccinic ~nhydride, glutaric anhydride and the like.
one preferred polye~ter copolymer has a linear chain ~egment of a polyester of neopentyl glycol, hexane diol, isophthalic acid and azelaic acid and branched chain ~egment~ of a polyester of neop~ntyl glycbl, trimethylol propane, isophthalic acid and azelaic acid and are esterified with orth~phthalic anhydride. The ~olar ratio of linear chain se~ment/anhydride/branched chain ~egment is lJl/l and the copolymer has a nu~ber average Dolecular weight of about 800 - 1,500 and a hydroxyl number o~
125-155.
The polyester ~egments and polyester copolymer are prepared by conventional esterification * denotes trade mark 132~17~

procedures in which the components are charged into a polymerization vessel with solvents and u~ually a esterification cataly6t and heated to about 100-200C.
~or about 30 minutes to 5 hours with removal of water that is formed during the process.
The crosslinking agent u~ed in the composition is an alkylat2d ~elamine formaldehyde crosslinking resin. Typically, these resins have 1-8 1 carbon atoms in the alkyl group. Methylated melamine ~ormaldehyde resins ~uch as Cymel*3~0 and methylated melamine formaldehyde resins containing methoxymethyl imino groups 6uch as "~ymel"325 are preferred to form a high quality primer. One preferred combination is a blend o~ 40-60% by wei~ht of "Cymeln380 and correspondingly about 60~40% by weight of "Cymel"325.
The composition contains carbon black pigments that are electrically conductive ~uch as a conductive furnace black pigment. One preferred pigment is a Vulcan*XC-72-R furnace black pigment that does not have a ~urface treatment. Other pigments can be us~d in the composition provided that the conductvity remains at least 160 Ranshurg units.
Typical pigments that can be used re filler pigments such as talc, china clay, barytes, carbonates, ~ilicates, metallic oxid~s such as ~itanium dioxide, zinc oxide and iron oxide and organic colored pigments and dyes.
The pigments can be introduced into the primer composition by first ~orming a mill base with the polyester copolymer or with another compatible polymer or dispersant by conventional techniques such as sand grinding, ball milling, attritor grinding or two roll milling. The mill ~ase is blended with other constituents used in the composition.
* denotes trade mark :~320779 Any of the cDnventional ~olvents or ~lends of solvents can be used to form the primer composition provided that the ~election of Golvents i~ ~uch that the polymeric binder constituents are compatible and give a high quality primer. The following are examples of solvents that can be used to prepare the composition: methyl ethyl ketone, methyl amyl ketone, methyl isobutyl ketone, toluelle, xylene, acetone, ethylene glycol monobutyl ether acetate and other esters, ethers, ketones and aliphatic and aromatic hydrocarbon solvents that are conventionally used.
The composition can contain about 0.01-2% by weight, based on the weight of the binder, of ultraviolet light ~tabilizers which term includes ultraviolet light absorbers, ~creen~rs and quenchers.
Typical ultraviolet light ~tablizers include b~nzophenones, triazines, triazols, benzoates, hindered amines and blends of thereo~.
~ he primer composition can be applied to a flexible or rigid substrate by conventional techniquec such as spraying, electrostatic epraying, dipping, ~rushing, flowcoating and th~ like. As mentioned above the preferred method is electrostatic ~praying. ~fter application, the composition i8 baked at about 100-200C for about 5-45 ~inutes tor ~or~ a primer coating layer ~bout 0.1-2.0 mils thick. Generally the primer layer is about 0.5-1.5 mils thick. Conventional solvent borne or water borne acrylic enamels or lacquers, acrylic polyurethane coatings, polyesterurethane coatings, alkyd enamels and the like can bP applied and then baked to form a durable automotive ~r truck fini~h on the substrate.
The following example illustrates the invention. All parts and percentages are on a weight basis unl~ss otherwise indicated. ~olecular weights ~2~77~

are determined by gel permeation chromatography us.ing polymethyl methacrylat~ a~ the ~tandard.
EX~MPLE
A polyester 601ution is prepared by oharging the folowing constituent~ into a polymerization ve~sel equipped with a mechanical stirrer, an electric heatin~ mantle, a nitrogen inlet tub~, a water separator, ~ thermometer, an addition funnel and a water cooled reflux condenser:
Parts by weight Portion 1 Branched polyester ~olution526.22 (75% solids in xylene of a polyester of neopentyl glycol, trimethylol propane, iso~
phthalic acid, and azelaic acid in a molar ratio of 0.7/0.6/
0.25/0.75 and having a hydroxyl 20 number of about 200 230 and a number average molecular wei~ht of about 1000) Orthophthalic anhydride 62.53 Portion 2 25 Linear polyester ~olu~ion 247.22 (90% ~olids in xylene of a polyester of neopentyl glycol, 1,6 hexane diol~
iso-phthalic acid and azelaic acid in a molar ratio of 1.~8/0.32/0.25/0.75 and having a hydroxyl number of about 200-225 and a number average molecular weight of about 500) Xylene 7.20 ~320779 Por~ion 3 Xylene 20.00 Por~ion 4 Me~hyl ethyl ketone 75.08 Total 938.25 Portion 1 is charged into the polymerization v~ssel and with ~tirring heatled to 125-150C for about 1 hour. Portion 2 is then added and waker is distilled off at a temperature of about 220-225C. Portion 3 is added and then Portion 4 is added and the composition is cooled to room temperature. The resulting polyester solution has a ~olids content of about 80% and the polyester has a hydroxyl number of about 120-150 and a number average ~olecular weight of about 1,200 and has a ~olar ratio of branched polyester/orthophthalic ~hydride/linear polyester of 1~
A ~ill base is prepared by charging the following constituents into a sand mill and grinding to a 0.5 ~il fineness:
Parts by weiqht polyester solution (prepared above) 40.01 "~ymel" 380 (B0% ~olids of a polymeric 15.63 ~e~hylated melamine fo~maldehyde resin in isobutanol) Conductive carbon black pigment 6.20 tVulcan XC-72-R conducti~e furnace black without a ~urface treatment) Dii~obutyl ketone 26.73 ~ethyl ethy:L ketone 11.~6 Total 100.00 ~207~

A coatin~ composition was prepared by blending together the following constituents:

Parts b~ Weight ~ill base (prspared above)56008 Resiflow* (50% solution of an acrylic 0.26 t2rpclymer flow control agent in Solvesso*100) Syloid* 37B (~latting agent of 0.97 synthetic silica having a partical size of 3-5 microns) Polyester æolution (prepared above) 13.89 "Cymel"325 (B0% solids in isobutanol 6.38 of a methylated melamine formaldehyde resin having methoxymethyl imino functionality and a degr e of . polymerization of 2.3~
W screener (10% solution of TinuYin* 1089 900 2-r2-hydroxy-3,5-di(l,l-dimethyl (benzyl)phenyl~-2H-benzotriazole in xylene~
Ethylene glycol ~onobu~yl ether acetate 14.38 Butanol - 8.04 Total 101.89 The resulting primer composition has a solids content of 46.5%, a pigment/binder ratio of 13.5/100 and is black in color.
The primer was reduced to a spray viscosity of 351- measured with a No.2 Fisher cup ~t 25~C with methyl amyl ketone and sprayed onto a cold roll ~teel panel, a phosphatized æteel ~anel, a fiber glass reinforced polyester panel, a reaction injection * den~tes trade mark :~3~779 molded urethane panel and a partially crystalline polyamide panel. The ~teel pan~ls were baked for 30 minutes at about 125C and the remaining panels were baked at 120C for about 30 minutes. The panels had had a black color,a dry film thickness of about 0. a mils and a glo~s measured at 60C of about 30.
The coatings on the panels had the ollowing properties:
mar resistance - good adhesion - good ~andability - good knife scrape r~sistance - good solvent resistance - 75 double rubs (1:1 ratio p-naphtha/isopropanol) conductivity - >165 Randsburg Units humidity resistance 96 hour at 38C/100% rel. humidity _ 101 dry crosshatch and tape adhesion _ 101 mandrel bend lB0~ (-18C) - 5~
~-29C) 42 Rating system ~ 0 - 10, 10 - best, 0 - worst 2 0 - 5, 5 - best, 0 - worst

Claims

1. A primer composition comprising solvents and about 40-75% by weight of a film forming binder and pigments in a pigment to binder ratio of about 5:100 - 50:100; wherein the pigments comprise an electrically conductive carbon black pigment and the binder consists essentially of about (1) 40-70% by weight, based on the weight of the binder, of a polyester copolymer having linear segment with hydroxyl groups and having a hydroxyl number of about 15 -300 and a number average molecular weight of about 300 - 3,000 and branched segments having hydroxyl groups and having a hydroxyl number of about 175 - 300 and a number average molecular weight of about 500 - 2000 and being attached to linear segment by esterification of the hydroxyl groups with a diacid or an anhydride;

(2) 30-60 % by weight of an alkylated melamine formaldehyde crosslinking agent;

wherein the composition forms a hard flexible coating on a substrate whereby the coated substrate can be bent 180° around a 1/2 inch mandrel with the coating facing outward and the coating exhibits substantially no breaks or cracks when tested at -18°C and the coating has a surface conductivity of at least 160 Randsburg units.

2. The coating composition of claim 1 in which the coating has a surface conductivity of 165 -350 Randsburg units.

3. The coating composition of claim 2 in which the linear chain of the polyester comprises neopentyl glycol, hexane diol, isophthalic acid and azelaic acid.

4. The coating composition of claim 3 in which the branched chain segment of the polyester comprises neopently glycol,trimethylol propane, isophthalic acid and azelaic acid.

5. The coating composition of claim 4 in which the anhydride is orthophthalic anhydride.

6. The coating composition of claim 1 in which the alkylated melamine formaldehyde crosslinking agent is a blend of methylated melamine formaldehyde resin containing alkoxy imino groups and a methylated melamine formaldehyde resin.

7. The coating composition of claim 1 containing about 0.01-2% by weight, based on the weight of the binder, of an ultraviolet light stabilizer.

8. The coating composition of claim 1 containing about 50-80% by weight, based on the weight of the binder, of the polyester copolymer and about 20-50% by weight, based on the weight of the binder of the alkylated melamine formaldehyde crosslinking agent.

9. The coating composition of claim 8 in which the alkylated melamine formaldehyde crosslinking agent consists essentially of a blend of a methylated melamine formaldehyde resin having methoxymethyl imino groups and a methylated melamine formaldehyde resin.

10. The coating composition of claim 1 containing a binder consisting essentially of about (1) 60-70% by weight, based on the weight of the binder, of a polyester copolymer having a linear segment of neopentyl glycol, hexane diol, isophthalic acid and azelaic acid and branched chain segments of neopentyl glycol, trimethlylol propane, isophthalic acid and azelaic acid, and the polyester copolymer has a number average molecular weight of about 800 - 3,500 and a hydroxyl number of about 50-170.

(2) 30-40% by weight, based on the weight of the binder, of a blend of alkylated melamine formaldehyde crosslinking agents comprising about 40-60% by weight, based on the weight of the crosslinking agents, of a methylated melamine formaldehyde resin having methoxymethyl imino groups and 60-40%
by weight, based on the weight of the crosslinking agents of a methylated melamine formaldehyde resin;
and the coating has surface conductivity of 165-350 Randsburg units.

11. A flexible substrate coated with a dried and cured layer of the composition of claim 1.

12. The flexible substrate of claim 11 in which the substrate is a plastic.

13. The flexible substrate of claim 11 in which the substrate is a polyester reinforced with fiberglass.

14. The flexible substrate of claim 11 in which the substrate is a partially crystalline polyamide resin.