Metal Painting and Coating Operations

Table of Contents  Background  Regulatory Overview  Planning P2 Programs  Overview of P2  Surface Preparation
Alternatives to Solvent-Borne Coatings  Application Techniques  Curing Methods  Equipment Cleaning

Curing Methods

Once a coating is applied to the workpiece it then undergoes a curing or drying process. While the terms drying or baking are commonly used in the painting industry to refer to curing, there is a distinction between drying (baking) and curing. In curing, the resin must be converted into a new resin, while drying refers to the loss of the solvent so that the resin remains the same. Curing and drying both use the two same methods to harden a coating: air/force dry and baking (newer curing methods such as radiation curing are discussed as they apply to specific coatings in the preceding chapters). Table 44 compares air/force dry and bake methods.

  • Air Drying. In air drying, a coating film is formed by the evaporation of solvent, which leaves behind a solid film. The rate of drying is governed by how quickly the solvent evaporates. Moderate heat (below 194°F) can be applied to accelerate evaporation (called force drying), however, the process still basically remains one of air drying.

  • Elevated Temperature Curing/Baking. Elevated temperature curing uses one of three means: conduction, convection or radiation to apply heat to the coated part (SME, p. 28-7).

Selecting air/force dry or bake coating (baked at elevated temperatures above 250°F) is an important consideration in choosing a P2 alternative. Baked coatings usually have better physical and chemical-resistant properties, but they also have some limitations. Air/force dried coatings (defined by EPA as those that cure below 194°F) have special VOC limits that are usually higher than baked coatings (EPAq, p. 92). Table 45 lists the typical RACT VOC limits for metal part coating.

Table 44. Air/Force Dry vs. Bake (EPAq, p.91)

 

Air/Force Dry

Bake
Curing Time
  • Takes longer to achieve thorough hardness, which can affect production schedules
  • After baking and cool-down, the coated parts are usually ready for assembly or shipping
Clean-Up Requirements
  • Overspray dries on spray booth filters, floors and walls; therefore, maintenance is not a significant problem
  • Uncured overspray remains sticky, making it awkward to walk on spray booth floors
  • Maintenance is more costly because of difficulty handling the sticky material
Substrate Versatility
  • Can be applied to all substrates (e.g., metal, plastics, wood, rubber and masonry)
  • Can be applied over porous materials such as sand castings, wood, and paper
  • Can only be applied on metals and substrates that can withstand high baking temperatures. Generally not suitable for heat-sensitive products such as plastics, wood and rubber.
  • Should not be applied over machined or other surfaces that are sensitive to warpage, unless taking adequate precautions
  • Can cause outgassing on sand castings and other porous substrates. Preheating workpiece can often overcome problem, but adds an additional step to the process
RACT Regulations
  • Some regulations have higher VOC limits for air/force-dry than for bake coatings
  • Same
Heating Requirements
  • Can dry and cure at temperatures from ambient up to 194oF by EPA definition
  • Solvent-borne coatings do not require an oven although a low temperature oven will speed up the drying process
  • Water-Borne coatings would benefit from a low temperature oven that will speed up the drying process
  • Offers lower energy use
  • Generally must cure at a minimum of 250oF. A typical curing schedule is 10 minutes at 350oF. Curing times are inversely proportional to temperature. A cool-down staging area is required.
  • Requires high temperature oven, and therefore greater energy use
Physical/Chemical Requirements
  • Most single-component coatings, such as alkyds and modified alkyds, do not exhibit superior physical and chemical properties
  • Single-component moisture-cured polyurethanes, however, do perform comparably to two-component polyurethanes and baked coatings
  • Often have excellent physical and chemical-resistant properties, sometimes similar to two-component polyurethanes
Appearance Defects
  • Surface defects, such as orange peel, often do not flow out during the drying and curing process. Force drying at elevated temperatures below 194oF can partially alleviate this.
  • Films tend to flow out better when in the oven, providing smooth finishes and eliminating surface defects such as orange peel.

Table 45. Typical RACT Limits for Miscellaneous Metal Parts Coating (EPAq, p. 93)

 

Air/Force Dry

Bake

lb/gal

g/L

lb/gal

g/L
California 2.8 340 2.3 275
Most other states 3.5 420 3.0 360