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Recent advances in flamesprayed thermoplastic powder coatings
proSress in Organ& Coatings, 22 (1993)
195
196-199
Recent advances in flamesprayed thermoplastic powder coatings E.R. George Novon Products, 182 Tabor Road, Morris Plains, NJ 07950 (USA)
J. Reimer Plastic Flamecoat
Systems,
1613 Highway
3, League
City, TX 77573 (USA)
Abstract The types of thermoplastics suitable for the plastic flamespray process and the effect of the flamespray on the physical properties and degree of crystallinity in semicrystalline thermoplastics are investigated. Novel coating application techniques and the use of polymer blends to produce viable coatings are also reported. Ethylene-carbor@c acid copolymers, aliphatic polyketones, polyether block amides, and liquid crystal polymers as flamesprayable coating materials are reviewed. The flamespray process does not significantly affect the uysMinity in the polymers studied; however, polymers possessing functional hydrolyzable groups in the backbone such as the polyether block amide may experience some reduction in physical properties during the flamespray process.
wonis: Flamespray; Powder coatings; Thermoplastic; Liquid crystal polymer; Ethylene-carboxylic acid copolymer; Polyether block amides; Aliphatic polyketone
Key
Introduction The plastic flamespray process is the application of thermoplastic coatings via the transport of plastic powder through a combination air propane flame. It allows powder coatings to be applied in the field without some of the limitations of electrostatic powder coatings. The method and apparatus have been described [ 11. Many thermoplastic flamesprayed coatings exhibit resistance to impact, heat, solvents, and corrosion. In most cases only sandblasting without primer is sufhcient. There is no cure time involved, only the cooling of the thermoplastic. We have reported the effect of flamespraying on the degree of crystallinity in three representative thermoplastics [2]. The flamespray process does not significantly affect the crystallinity in the polymers studied; however, polymers possessing functional groups in the main chain backbone such as polyamides experience some reduction in physical properties during the flamespray coating process. We report here recent results on the application of aliphatic polyketones and liquid crystal polymers as flamesprayed thermoplastic powder coatings. Elsevier Sequoia
196
These polyketones and liquid crystal polymers are classified as engineering thermoplastic and high temperature polymer, respectively. Engineering thermoplastics include nylons, polyesters, and polyacetals which typically exhibit continuous use temperatures greater than 100 “C and tensile strengths greater than 6000 psi. High temperature plastics include aromatic polyketones, polyetherimides, and polysulfones. These polymers typically exhibit continuous use temperatures greater than 150 “C and tensile strengths greater than 10 000 psi. The aliphatic engineering polyketones studied in this investigation consist of a linear alternatingpolymer of carbon monoxide and at least one ethylenically unsaturated hydrocarbon, optionally blended with an ethylene-carboxylic acid copolymer [ 31. The liquid crystal polymers in this investigation are thermotropic polyesters synthesized via the condensation polymerization of terephthahc acid and various hydroquinones [4]. The application of engineering thermoplastics and thermotropic liquid crystal polymers as powder coatings are beset by difficulties. These higher melting point polymers require higher temperatures for melting and often exhibit a yield stress (Bingham fluidity) before the onset of melt flow. We have reported the novel use of polymer-polymer blends in order to achieve viable coatings from aliphatic engineering polyketones [ 3 1. The use of liquid crystal dihrents in liquid crystal polymers in order to lower the melting point and melt viscosity has also been reported [5, 61. The modification of the liquid crystal polymer backbone via the incorporation of asymmetric units, kinks, and flexible bonds has also been reported as a method to lower the melting point [7]. We report here recent results and novel coating techniques for flamespraying engineering polyketone blends with ethylene-carboxylic acid copolymers and the physical properties of a flamesprayed thermotropic liquid crystal polyester.
Results
and discussion
After much trial and error, we have learned that there are three key requirements for producing viable flamesprayed coatings from ahphatic engineering polyketones: (1) use low molecular weight polyketones (LVN < 1.2); (2) the polyketone must contain more than 10% NucrelTMor PrimacorTM; (3) the particle size must be less than 70 mesh. The LVN refers to logarithmic viscosity number determined from solution viscosity in m-cresol. An LVN of 1.2 is a number average molecular weight of about 20 000. Lowering the molecular weight gives rise to a concomitant decrease in melt viscosity and subsequently better flow and wet-out of the coating. Blends were prepared with Nucrelm or PrimacorTMby melt mixing in a rotating twin screw extruder and subsequently chopping the strand into
197
pellets. NucrelTMand Primacorru are ethylen+carboxylic acid copolymers available from DuPont and Dow, respectively. The pellets were cryogenically ground to a size that would pass through a 70 mesh screen. Table 1 lists the physical properties for an aliphatic engineering polyketone containing 15 wt.% NucrelTM 535. The coating was flamesprayed onto a substrate that was only sandblasted. These coatings exhibited good impact resistance as evidenced by the Gardner impact test. The lower molecular weight polymer did not produce a polymer with high elongation. The pencil hardness corresponded to a medium hard coating. The hardness was found to be inversely proportional to Nucrelm content, which is expected since the ethylene-carboxylic acid copolymer is a softer material than the polyketone. The blending of the aliphatic engineering polyketone with an ethylene-carboxylic copolymer produced a powder coating with good flow and physical properties, and heat and solvent resistance, but the coatings often exhibited inadequate adhesion to the substrate, particularly for coatings thicker than 20 mils. A process was invented that improved adhesion to the metal substrate [8]. A primer layer was developed in situ by purpose fully degrading a layer of polyketone coating onto the sandblasted metal surface. Subsequently, a smooth nondegraded topcoat of polyketone was applied. This application technique has yielded viable high performance coatings which have demonstrated corrosion resistance to methyl tertbutyl ether, (MTBE) a high octane enhancer in gasoline, for over one year. We were the first group to successfully apply thermotropic liquid crystal polyesters (TLCPs) as flamesprayed powder coatings. These coatings exhibited outstanding properties compared to other high temperature thermoplastics (Table 2) applied by electrostatic powder coating, provided that the coating was heat treated for 20 ruin at 250 “C after application. The addition of flow enhancers [6] might eliminated the need for treatment. Table 2 compares the properties of poly(phenylene sulfide), fluorinated ethylene-propylene copolymers, ethylene-tetrafluoroethylene copolymers [ 9 ] and TLCP. The TLCPs are less dense than typical high performance coatings, giving rise to greater coverage and exhibiting a very high melting point (= 650 “F’) and continuous use temperatures comparable with those of other high performance coatings. The reverse Gardner impact resistance was lower for the TLCPs than for the other representative high performance coatings, an indication that TLCPs are relatively brittle. The impact resistance of these TLCPs is surprisingly TABLE 1 Physical properties of polyketone-ethylene carboxylic acid powder coating Property
Method
Value
Gardner impact (in. lb) Conical-Mandrel bend (%) Pencil Hardness
ASTM 2794 ASTM D622-60 ASTM 3363
180 8 H
198
TABLE 2 Comparison of high performance powder coatings with TLCPs
Pmerty
PPS
FEP
ETFE
TLCP
Physical proper& Density Coverage (ft? lb-’ mil-I)
1.35 143
2.16 89
1.70 112
1.26 152
525-530 400
600-504 400
520-524 350
650-680 53410
475
=500
Thennolp?WpWties Melt range (“F) Continuous use temperature (“F) Short term use temperature (“F) Wear proper&s Taber weight loss (mg), revolving disc (CS 17) 100 cycles 1000 cycles
1.7 19.0
2.2 14.8
2.8 13.4
0.8 12.5
Mechanical j3ropertf.s~ Gardner impact Face (in. lb) Reverse (in. lb) Tensile strength (psi)
160 160 10800
160 160 2200
160 160 6500
160 80 = 10000
3500 10”
1400 > 10’8
Electrical properties Dielectric strength (V/n@ Volume resistivity (a cm)
2300 10’6
‘Heat treated. PPS = polyphenylene sulfide. FEP = fluorinated ethylene-propylene (polymers). ETFE = ethylene-tetrafluoroethylene (copolymers). TLCP = thermotropic liquid crystal copolymer.
high since injection molded parts of these polymers are quite brittle and exhibit very low elongations. The wear resistance was excellent for the TLPCs. The tensile strength and volume resistivity are outstanding for the TLCPs. The dielectric strength was relatively low, attributed to the fact that craters and pinholes were sometimes present in these TLCP coatings. More development is needed to produce viable reproducible coatings from TLCPs.
Conclusions
We report here the first application of aliphatic engineering polyketones and thermotropic liquid crystal polymers (TLCPs) and flamesprayed thermoplastic powder coatings. Novel blends and coating techniques are described in order to produce viable coatings from polyketones, The TLCPs produced coatings with a performance characteristic similar to other high
199
temperature thermoplastics. More development is required to produce viable coatings from TLCPs.
References J. Reimer, US Patent No. 4 632 309 (1986). E.R. George and J. Reimer, Polyp. Eng. Sci., 31 (1991) 789. E.R. George, US Patent No. 4861675 (1989). DA. Hutchings, US Patent No. 4 600 765 (1986). E.R. George and R.S. Porter, iVcxcmmok&s, 19 (1986) 97. E.R. George and R.S. Porter, US Patent No. 4690836 (1987). G.W. Calundann and M. J&e, Robert A. Welch Coqf Chemical Research, Houston, TX, USA, Nov. 15-17, 1982. 8 E.R. George, US Pate& No. 4 985 278 (1991). 9 J.P. Blackwell, D.G. Brady and H.W. Hill, J. Coat. Technd., 50 (1978) 643. 1 2 3 4 5 6 7
195
196-199
Recent advances in flamesprayed thermoplastic powder coatings E.R. George Novon Products, 182 Tabor Road, Morris Plains, NJ 07950 (USA)
J. Reimer Plastic Flamecoat
Systems,
1613 Highway
3, League
City, TX 77573 (USA)
Abstract The types of thermoplastics suitable for the plastic flamespray process and the effect of the flamespray on the physical properties and degree of crystallinity in semicrystalline thermoplastics are investigated. Novel coating application techniques and the use of polymer blends to produce viable coatings are also reported. Ethylene-carbor@c acid copolymers, aliphatic polyketones, polyether block amides, and liquid crystal polymers as flamesprayable coating materials are reviewed. The flamespray process does not significantly affect the uysMinity in the polymers studied; however, polymers possessing functional hydrolyzable groups in the backbone such as the polyether block amide may experience some reduction in physical properties during the flamespray process.
wonis: Flamespray; Powder coatings; Thermoplastic; Liquid crystal polymer; Ethylene-carboxylic acid copolymer; Polyether block amides; Aliphatic polyketone
Key
Introduction The plastic flamespray process is the application of thermoplastic coatings via the transport of plastic powder through a combination air propane flame. It allows powder coatings to be applied in the field without some of the limitations of electrostatic powder coatings. The method and apparatus have been described [ 11. Many thermoplastic flamesprayed coatings exhibit resistance to impact, heat, solvents, and corrosion. In most cases only sandblasting without primer is sufhcient. There is no cure time involved, only the cooling of the thermoplastic. We have reported the effect of flamespraying on the degree of crystallinity in three representative thermoplastics [2]. The flamespray process does not significantly affect the crystallinity in the polymers studied; however, polymers possessing functional groups in the main chain backbone such as polyamides experience some reduction in physical properties during the flamespray coating process. We report here recent results on the application of aliphatic polyketones and liquid crystal polymers as flamesprayed thermoplastic powder coatings. Elsevier Sequoia
196
These polyketones and liquid crystal polymers are classified as engineering thermoplastic and high temperature polymer, respectively. Engineering thermoplastics include nylons, polyesters, and polyacetals which typically exhibit continuous use temperatures greater than 100 “C and tensile strengths greater than 6000 psi. High temperature plastics include aromatic polyketones, polyetherimides, and polysulfones. These polymers typically exhibit continuous use temperatures greater than 150 “C and tensile strengths greater than 10 000 psi. The aliphatic engineering polyketones studied in this investigation consist of a linear alternatingpolymer of carbon monoxide and at least one ethylenically unsaturated hydrocarbon, optionally blended with an ethylene-carboxylic acid copolymer [ 31. The liquid crystal polymers in this investigation are thermotropic polyesters synthesized via the condensation polymerization of terephthahc acid and various hydroquinones [4]. The application of engineering thermoplastics and thermotropic liquid crystal polymers as powder coatings are beset by difficulties. These higher melting point polymers require higher temperatures for melting and often exhibit a yield stress (Bingham fluidity) before the onset of melt flow. We have reported the novel use of polymer-polymer blends in order to achieve viable coatings from aliphatic engineering polyketones [ 3 1. The use of liquid crystal dihrents in liquid crystal polymers in order to lower the melting point and melt viscosity has also been reported [5, 61. The modification of the liquid crystal polymer backbone via the incorporation of asymmetric units, kinks, and flexible bonds has also been reported as a method to lower the melting point [7]. We report here recent results and novel coating techniques for flamespraying engineering polyketone blends with ethylene-carboxylic acid copolymers and the physical properties of a flamesprayed thermotropic liquid crystal polyester.
Results
and discussion
After much trial and error, we have learned that there are three key requirements for producing viable flamesprayed coatings from ahphatic engineering polyketones: (1) use low molecular weight polyketones (LVN < 1.2); (2) the polyketone must contain more than 10% NucrelTMor PrimacorTM; (3) the particle size must be less than 70 mesh. The LVN refers to logarithmic viscosity number determined from solution viscosity in m-cresol. An LVN of 1.2 is a number average molecular weight of about 20 000. Lowering the molecular weight gives rise to a concomitant decrease in melt viscosity and subsequently better flow and wet-out of the coating. Blends were prepared with Nucrelm or PrimacorTMby melt mixing in a rotating twin screw extruder and subsequently chopping the strand into
197
pellets. NucrelTMand Primacorru are ethylen+carboxylic acid copolymers available from DuPont and Dow, respectively. The pellets were cryogenically ground to a size that would pass through a 70 mesh screen. Table 1 lists the physical properties for an aliphatic engineering polyketone containing 15 wt.% NucrelTM 535. The coating was flamesprayed onto a substrate that was only sandblasted. These coatings exhibited good impact resistance as evidenced by the Gardner impact test. The lower molecular weight polymer did not produce a polymer with high elongation. The pencil hardness corresponded to a medium hard coating. The hardness was found to be inversely proportional to Nucrelm content, which is expected since the ethylene-carboxylic acid copolymer is a softer material than the polyketone. The blending of the aliphatic engineering polyketone with an ethylene-carboxylic copolymer produced a powder coating with good flow and physical properties, and heat and solvent resistance, but the coatings often exhibited inadequate adhesion to the substrate, particularly for coatings thicker than 20 mils. A process was invented that improved adhesion to the metal substrate [8]. A primer layer was developed in situ by purpose fully degrading a layer of polyketone coating onto the sandblasted metal surface. Subsequently, a smooth nondegraded topcoat of polyketone was applied. This application technique has yielded viable high performance coatings which have demonstrated corrosion resistance to methyl tertbutyl ether, (MTBE) a high octane enhancer in gasoline, for over one year. We were the first group to successfully apply thermotropic liquid crystal polyesters (TLCPs) as flamesprayed powder coatings. These coatings exhibited outstanding properties compared to other high temperature thermoplastics (Table 2) applied by electrostatic powder coating, provided that the coating was heat treated for 20 ruin at 250 “C after application. The addition of flow enhancers [6] might eliminated the need for treatment. Table 2 compares the properties of poly(phenylene sulfide), fluorinated ethylene-propylene copolymers, ethylene-tetrafluoroethylene copolymers [ 9 ] and TLCP. The TLCPs are less dense than typical high performance coatings, giving rise to greater coverage and exhibiting a very high melting point (= 650 “F’) and continuous use temperatures comparable with those of other high performance coatings. The reverse Gardner impact resistance was lower for the TLCPs than for the other representative high performance coatings, an indication that TLCPs are relatively brittle. The impact resistance of these TLCPs is surprisingly TABLE 1 Physical properties of polyketone-ethylene carboxylic acid powder coating Property
Method
Value
Gardner impact (in. lb) Conical-Mandrel bend (%) Pencil Hardness
ASTM 2794 ASTM D622-60 ASTM 3363
180 8 H
198
TABLE 2 Comparison of high performance powder coatings with TLCPs
Pmerty
PPS
FEP
ETFE
TLCP
Physical proper& Density Coverage (ft? lb-’ mil-I)
1.35 143
2.16 89
1.70 112
1.26 152
525-530 400
600-504 400
520-524 350
650-680 53410
475
=500
Thennolp?WpWties Melt range (“F) Continuous use temperature (“F) Short term use temperature (“F) Wear proper&s Taber weight loss (mg), revolving disc (CS 17) 100 cycles 1000 cycles
1.7 19.0
2.2 14.8
2.8 13.4
0.8 12.5
Mechanical j3ropertf.s~ Gardner impact Face (in. lb) Reverse (in. lb) Tensile strength (psi)
160 160 10800
160 160 2200
160 160 6500
160 80 = 10000
3500 10”
1400 > 10’8
Electrical properties Dielectric strength (V/n@ Volume resistivity (a cm)
2300 10’6
‘Heat treated. PPS = polyphenylene sulfide. FEP = fluorinated ethylene-propylene (polymers). ETFE = ethylene-tetrafluoroethylene (copolymers). TLCP = thermotropic liquid crystal copolymer.
high since injection molded parts of these polymers are quite brittle and exhibit very low elongations. The wear resistance was excellent for the TLPCs. The tensile strength and volume resistivity are outstanding for the TLCPs. The dielectric strength was relatively low, attributed to the fact that craters and pinholes were sometimes present in these TLCP coatings. More development is needed to produce viable reproducible coatings from TLCPs.
Conclusions
We report here the first application of aliphatic engineering polyketones and thermotropic liquid crystal polymers (TLCPs) and flamesprayed thermoplastic powder coatings. Novel blends and coating techniques are described in order to produce viable coatings from polyketones, The TLCPs produced coatings with a performance characteristic similar to other high
199
temperature thermoplastics. More development is required to produce viable coatings from TLCPs.
References J. Reimer, US Patent No. 4 632 309 (1986). E.R. George and J. Reimer, Polyp. Eng. Sci., 31 (1991) 789. E.R. George, US Patent No. 4861675 (1989). DA. Hutchings, US Patent No. 4 600 765 (1986). E.R. George and R.S. Porter, iVcxcmmok&s, 19 (1986) 97. E.R. George and R.S. Porter, US Patent No. 4690836 (1987). G.W. Calundann and M. J&e, Robert A. Welch Coqf Chemical Research, Houston, TX, USA, Nov. 15-17, 1982. 8 E.R. George, US Pate& No. 4 985 278 (1991). 9 J.P. Blackwell, D.G. Brady and H.W. Hill, J. Coat. Technd., 50 (1978) 643. 1 2 3 4 5 6 7