Sofia Martinez
Flame treatment is a process that improves the adhesion of plastics to other materials, such as epoxy. It involves controlled combustion to generate a flame plasma that oxidizes the plastic surface, increasing its surface energy and enhancing its bond with other substances. This technique is particularly useful for plastics like ABS and HDPE, where it can significantly improve adhesion. Various methods for testing the flammability of plastics are available, including horizontal and vertical burning tests, glow wire testing, and needle flame tests. These tests assess the material's tendency to extinguish or spread a flame, its ignition resistance, and its burning behavior, assigning flammability ratings accordingly.
| Characteristics | Values |
|---|---|
| Flame treatment | Done with a flame (propane torch, butane, M.A.A.P, or natural gas) |
| A heat gun will not work | |
| Optimal results are achieved through proper control of flame chemistry, treatment distance, and dwell time | |
| Brief contact time is controlled by how fast the part is passed through the flame and is dependent on burner width, plasma output, and type of resin | |
| Over-treatment can result in artificially high surface energy wetting test results | |
| Removes most surface debris and some hydrocarbon contaminants | |
| Generates flame plasma by controlled combustion of a hydrocarbon fuel | |
| Used to improve wettability and adhesion of plastics | |
| Testing | Water break test |
| Horizontal burning test | |
| Vertical burning test | |
| Glow wire testing | |
| Needle flame test |
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What You'll Learn
Flame treatment improves bonding and adhesion
Flame treatment is a well-known and cost-effective method of improving the bonding and adhesion of plastics. It is a process that involves exposing the surface of a plastic material to an oxidizing flame for a short duration, typically between 0.2 and 3 seconds. This treatment is particularly effective for polyethylene (HDPE, LDPE, UHMW) and polyolefin-based plastics before polymer bonding or printing.
The key to successful bonding with plastics is surface preparation. Flame treatment oxidizes the surface of the plastic, transforming it from a low-energy surface to a high-energy surface. This oxidation process introduces polar reactive groups, such as hydroxyl, carbonyl, carboxyl, and amide groups, which increase the surface free energy. As a result, the wettability and adhesion of the plastic surface are significantly enhanced. This treatment method is especially beneficial for plastics like ABS and HDPE, where adhesion can be challenging without it.
The effectiveness of the flame treatment can be easily tested using a water break test. When water is applied to a treated surface, it should sheet out and wet the surface, indicating a high-energy surface. On the other hand, if water beads up and rolls off, it suggests that the surface has lower energy. This simple test provides a clear visualization of the surface energy changes brought about by flame treatment.
Flame treatment offers several advantages over other adhesion-promoting methods. It is a safe and environmentally friendly process that does not produce ozone or require the use of solvents, resulting in reduced solvent emissions. Additionally, flame treatment is versatile and can be applied to non-uniform shapes with robotic burners, ensuring better stability and longevity of the treatment.
Overall, flame treatment is a reliable and cost-effective solution for improving the bonding and adhesion of plastics. By altering the surface energy and introducing reactive groups, flame treatment enhances the wettability and adhesion properties, making it a valuable technique in various industrial applications.
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Water break test to check treatment effectiveness
Flame treatment is a process that involves applying the exhaust from a gas flame to the surface of a plastic material to improve adhesion. The flame treatment process changes the surface of the plastic from a low-energy surface to a high-energy surface. An example of a low-energy surface is a waxed car hood, where water beads up and rolls off. Conversely, an unwaxed car hood would be a high-energy surface, where water sheets out and wets the surface.
The water break test is a simple and effective method to determine if the flame treatment was successful. To perform this test, water with a dye added for contrast is applied to the treated surface. If the flame treatment was effective, the water will sheet out and wet the surface. On the other hand, if the treatment was ineffective, the water will bead up and roll off the surface.
It is important to note that the water break test should be conducted immediately after the flame treatment for the most accurate results. Over time, the surface oxidation from the flame treatment will dissipate, and the water may bead up even on a successfully treated surface. In addition, the plastic surface should be cleaned before the test to remove any contaminants that may interfere with the test results.
For the flame treatment process itself, it is recommended to use a propane torch, with the flame just touching the surface and moved at a rate of 12 to 16 inches per second. Two or three passes over the bonding area are typically sufficient, with a total exposure time of one second. It is crucial to keep the torch moving and slightly overlap each pass to ensure even treatment. Additionally, a flame spreader can be used to cover a larger area, creating a "W" shape with the flame.
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Horizontal burning test for flammability
The Horizontal Burning Test is one of the preselection test programs conducted on plastic materials to measure their flammability characteristics. The flammability rating of the UL94 Horizontal burning test is HB, which is the lowest flame retardant rating in the UL94 standard.
To conduct the test, a specimen of 50mm by 150mm is placed horizontally on wire gauze. One end of the specimen is clamped, and the other is ignited by a specified burner for 60 seconds. The burner is inclined by 45 degrees towards the specimen, and the flame is applied to the free end at the lower edge for 30 seconds, ensuring the flame reaches a depth of 6mm.
The time it takes for the flame to pass over a specified span is recorded, as well as the afterflame time, afterglow time, and the damaged specimen length. If the flame continues to spread, timing starts when the flame passes the first reference line and stops when the flame goes out, provided it does not reach the second reference line. If the flame reaches the second line, timing is stopped when it reaches that line. The travel distance of the flame between the two reference lines divided by the time is the linear burning rate.
Based on the test results, the materials can be classified into different flammability ratings, including HBF, HF-1, or HF-2. These ratings are used to distinguish a material's burning characteristics and relate to low-density foam materials commonly used in fabricating speaker grills and sound-damping materials.
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Glow wire testing for flammability
Glow wire testing is a method used to determine the flammability characteristics of plastic materials. It is a pre-selection test program that measures the material's tendency to either extinguish or spread a flame once ignited. This test is particularly relevant for assessing the burning behaviour of compounded thermoplastics, as it takes into account not just the material's properties but also the shape and wall thickness of the application.
In Europe, the Glow Wire Test is conducted according to IEC 695-2-1 to assess the flammability of plastic materials. This test simulates thermal stresses produced by sources of heat or ignition, such as overloaded resistors, to evaluate the potential fire hazard. The sample undergoes a pre-treatment of 24 hours at 23°C and 50% relative humidity before being subjected to the glow wire at temperatures ranging from 550°C to 960°C, depending on the relevant specification.
During the test, the specimen is considered to have passed if one of the following criteria is met: no flame or glowing is observed on the sample; any flames or glowing extinguish within 30 seconds of removing the glow wire; and the cotton or paper underlay remains unignited and does not burn. This test is crucial for evaluating the safety of plastic materials and ensuring they meet the required flammability standards.
Additionally, there are other test programs to assess the flammability of plastics. UL 94, the Standard for Tests for Flammability of Plastic Materials, outlines two test programs. The first determines the material's tendency to extinguish or spread a flame, while the second measures ignition resistance to electrical ignition sources. These tests are designed to assess the fire behaviour of plastics used in devices and appliances, ensuring they meet safety standards and minimise fire risks.
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Flame treatment removes surface debris and contaminants
Flame treatment is a surface treatment process used to chemically modify a plastic container's surface for better adhesion. This process is typically used on low-energy surfaces that can be difficult to adhere to, such as plastics and composites. The treatment is also very gentle, posing little risk to the material.
The process uses specially designed equipment to create a controlled blend of natural gas and air that produces a flame. This oxidizing flame is then exposed to the plastic surface for 0.2 to 0.3 seconds. As the plastic is exposed to this flame, dirt, dust, fibres, oils, and other contaminants that can affect adhesion are removed. The plastic's surface is activated by breaking the molecular chains and adding polar functional groups. On a microscopic level, this process creates a "'rougher'" finish that provides a clean, high-energy surface ideal for accepting inks, paints, coatings, or bonds.
The best way to test if the flame treatment was effective is with a water break test. Oxidizing the plastic surface with a flame changes the surface from a low-energy surface to a high-energy surface. An example of a low-energy surface is a waxed car hood, where water beads up and rolls off. An unwaxed hood would be a high-energy surface, where water sheets out, wetting the surface.
Flame treatment is an important step in the manufacturing process of plastic containers intended for applications with labels. Polyolefin plastics, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), and polypropylene (PP), are commonly used in plastic packaging applications. However, these plastics have low surface energy when produced, making it difficult for inks, paints, glues, and other labelling mediums to adhere to. Flame treatment helps form a long-lasting, durable bond between the label and the container surface.
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Frequently asked questions
The best way to test if the flame treatment was effective is with a water break test. An untreated plastic surface is a low-energy surface, like a waxed car hood, where water beads up and rolls off. When the plastic surface is oxidized with a flame, it becomes a high-energy surface, like an unwaxed car hood, where water sheets out, wetting the surface.
The flame treatment needs to be done with an actual flame; a heat gun will not work. You can use a propane torch, butane, M.A.A.P., or natural gas.
Flame treatment involves the controlled combustion of a hydrocarbon fuel to generate flame plasma. A laminar flame is a mixture of fuel and an oxidizer that is thoroughly mixed before combustion.
Flame treatment improves wettability and adhesion of plastics. It is a safe and established pretreatment process that has been used for decades to enhance the surface attributes of thin packaging web films.
Underwriters Laboratories (UL) is an independent organization that provides product safety testing and certification. Their UL94 testing procedures and rating system for thermoplastics flammability are the generally accepted standard throughout most of the world. Different tests include the Horizontal Burn test and the Vertical Burn test.
