Plastic Surface Treating Terminology
Listed below are some of the key terms we use when discussing our clients plastic surface treating challenges.
Chemical surface treatment, usually involving chlorinated solvents or strong acids or bases, attack the polymer surface breaking bonds, creating free radicals and roughening the surface. Chlorinated priming of a surface prior to bonding or painting is a good example of chemical surface treatment. Still being used extensively in the automotive painting of plastics, chemical surface treatment and priming are slowly giving way to other types of surface treatment as EPA and OSHA rules become more stringent.
Coating is a process whereby a coating is applied to a substrate. The coating can be solvent based, water based, UV curable, or electron beam curable. Sometimes coating can mean extrusion coating whereby a substrate is coated with a molten polyolefin.
Contact Angle Measuring Device
This method of surface energy uses a microscope to look a measured droplet of liquid on the surface of the plastic. The tangential angle between the liquid droplet and the surface is a very accurate measurement of the surface energy. Unfortunately, this method can only be used on flat plastic substrates of a very small size.
Corona Discharge Treatment
Corona treatment is the method by which a high frequency corona discharge is directed through a small air gap at the surface of the part to be treated. Historically corona discharge has been used for the surface treatment of film and other flat substrates. The advantage of corona discharge is that there is no shrinkage, warpage, scrap, or fire hazard. The disadvantage of corona treatment is that the electrode which emits the discharge or treatment has to be in close proximity of the part to be treated, so if there are several different parts to be surface treated, several electrodes and additional set up time is required.
Dyne solutions have been used to measure wetting and predict adhesive potential of plastics and coated paperboard. This method is widely used to test the treatment level of plastic films intended for printing, laminating, and coating. Test results are based on how varying surface tension solutions react when applied to a non-absorbing surface: If a given solution wets the surface, its dyne level is lower than the substrate’s; if, instead, it rapidly forms beads, its dyne level exceeds that of the substrate. This method offers adequate precision and reliability for most capability studies, materials research, and process control.
Dynes/cm or Newtons/meter
Dynes/cm or Newtons/meter is the unit of force in the centimeter-gram-second absolute system, the force which imparts an acceleration of 1 centimeter per second to a body having a mass of 1 gram. Symbol, dyne. In SI units, one dyne = 10 -5 newton. To convert from to multiply by dyne (dyn) newton (N) 1.0 E-05.
Spraying of molds with mold releases will make surface treatment difficult if not impossible. When the surface of a plastic part is coated with a mold release the mold release will be surface treated and not the plastic part surface. This will render the surface treatment ineffective.
Contaminants in the plant atmosphere such as oil and water vapor, and sticky particulate which may be transferred to the plastic part can also make surface treatment difficult or impossible.
This form of surface treatment uses a hot oxidizing flame; usually natural gas or propane as fuel in the range of 2000 F (1093 C) to 5000 F (2760 C). It is brought into direct contact with the surface of the plastic to be treated. The general rule of thumb is the hotter the flame the faster and more effective the treatment. The major advantages of flame are the ability to vaporize some contaminates on the plastic surface and a very fast treating rate. It is important to minimize flame exposure to the plastic surface to avoid heat distortion, warpage, wrinkling and the dulling of high gloss finishes. One drawback of flame treating is the inability to surface treat irregular shaped plastic parts. Other drawbacks include the safety hazard associated with open flames in plants, and high-energy consumption.
Surface functionalization introduces chemical functional groups to a surface. This way, functional materials can be designed from substrates with standard bulk material properties. Prominent examples can be found in semiconductor industry and biomaterial research. In both cases, plasma processing technologies were successfully employed.
Gas Plasma Treatment
Gas plasma treatment is a batch treatment method within a chamber at very high vacuums. A plasma is formed in a gas, typically helium, argon, oxygen, or nitrogen at a high vacuum with either radio or microwave frequency. Advantages of gas plasma treatment are high surface treatment levels, the ability to treat irregularly shaped parts, and the ability to create specific functional groups on a plastic surface. To date, due to the high cost of the equipment and the batch method of surface treatment, gas plasma has been limited to high value products, such as those found in the specialty medical industry.
There are many types of internal additives in all plastics. The amount of each additive and the total concentration can effect surface treatment. All internal additives are lighter weight molecular groups than the plastic itself. These internal additives will, over time, migrate or rotate to the surface. If this occurs before surface treatment, they may make it difficult to treat the surface. If the migration occurs after surface treatment, they will mask over the treatment and the surface will react with the adhesive as if it were never treated. Some of these internal additives are: mold release agents, lubricants, antistats, etc.
Normally additives are not a concern when limited to the resin manufactures guidelines. Our past experience in the plastic industry has been that as long as adhesion takes place shortly after surface treatment, migratory additives are not a serious problem.
kVA vs. kW
kVA is kilovolt-ampere; kilovolt-amperes and kW is a unit of power equal to 1000 watts.
LectroTreat Plasma Treatment
LectroTreat method is designed to electrically treat the surface of three dimensional plastic objects. The LectroTreat works well on all types of molding processes, injection molding, blow molding, extruded, thermoformed, and vacuum formed plastic parts. Unlike typical corona devices, the suppressed spark system does not use high frequency, so it is far safer for employees in a plant operation, and close proximity to an electrode is not required. The equipment does not have the disadvantages that are experienced with flame treating such as shrinkage, warpage, and dulling of high gloss finishes, and/or fire hazards due to open flame in the plant. More importantly, the LectroTreat process yields an even, consistent surface treatment to the plastic part. The LectroTreat reaches a higher level of energy than all typical corona discharge devices by supplying what is referred to as directional plasma in air. The equipment normally operates on an input electrical requirement of 240 volts, 60 cycle current, transformed into high voltage and supplied it to specially design high voltage capacitor type plates.
The LectroTreat equipment consists of a dielectric “tunnel” that the parts to be treated pass through. The plates are situated parallel and covered with a perforated polyethylene shield that will distribute directional plasma in air evenly throughout the air gap. The surface of a plastic object passing between the plates is bombarded by charged particles creating a treated surface. The equipment normally has two sets of plates; one set mounted 90 degrees to the other for vertical and horizontal electron flow. This arrangement allows 360 degree treatment of the part passing through the treating area.
Electrical operating parameters are factory set for maximum operating efficiency. The only variable for treatment of the part is conveyor speed. Conveyor speed is variable so the surface treatment may be optimized for each particular situation. As the conveyor system is an integral part of the equipment, automation is easily achievable. With the LectroTreat there is no vacuum, or batch processing. Different sizes of parts are normally not a concern and can be treated simultaneously. As long as the parts fit within the treating tunnel the parts will be consistently and evenly treated.
Mechanical abrasion of a plastic surface can aid in the bonding of an adhesive. This is normally done in conjunction with another type of surface treatment to give good adhesion. The drawbacks of mechanical abrasion are numerous and obvious.
A colorless gaseous substance obtained (as by the silent discharge of electricity in oxygen) as an allotropic form of oxygen, containing three atoms in the molecule. It is a strong oxidizer, and probably exists in the air, though by ordinary tests it is liable to be confused with certain other substances, as hydrogen dioxide, or certain oxides of nitrogen. It derives its name from its peculiar odor, which resembles that of weak chlorine. Ozone generators deliver high ozone concentration at minimal power levels Ozone generators are used to advance adhesion in extrusion coating and laminating.
Ozone Destruct Systems
Reduces ozone emissions to less than 0.1 ppm. Used in areas where ozone emissions to the atmosphere are controlled. The system uses metal oxide catalyst pellets to convert ozone to carbon dioxide and water. Catalyst will last indefinitely unless contaminated.
Increase of energy and decrease of contact angle, usually correlates directly with improved bonding.
Some plastic molders measure surface tension, or surface treatment, by the ” water test” where the treated part is usually dipped into a pail of water; if the water holds as a continuous sheet on the plastic part for two seconds, the treatment is considered good; if the water does not hold on the plastic part and breaks into droplets, the treatment is considered bad, and the part has to be retreated. The “water test ” is the most stringent test for surface tension or treatment, and passing of the test indicates surface tension or treatment levels of at least 72 dynes, which is far more than sufficient for any adhesion requirement.
Watt Density is a measurement of the amount of energy being applied to the web. It is measured in Watts/ft2/minute. Watt density takes into account the amount of power being applied (watts), the time it is being applied (minute) and the amount of material it is being applied to (ft2). Once the watt density is known to get a particular material to a certain dyne level, it can be used to predict the results if any of the parameters change such as line speed.
Is the ability of a liquid to spread on or “wet “ a surface. Wettability is a measure of work overcoming resistance and is referred to as surface tension. The measurement of surface energy or wettability is defined in the terms of dynes/cm2.
The easiest method of determining if a plastic surface has been treated and to what level is to see how wettable the surface is and compare this wettability with the surface before treatment. In most cases, wettability correlates directly with surface energy and adhesion, so in most practical applications determining the wettability of a surface will tell you whether you will get good adhesion.
The object of surface treating is to improve the wettability of the surface, thereby improving the ability to bond to solvents, adhesives, coatings, and extrusion coating. In order for a surface to be properly wet by a liquid, the surface energy of the plastic must be higher than the surface tension of the liquid. Surface energy is measured in dynes per centimeter. This surface energy is the wetting tension.