With the continuous improvement of plastic processing and modification technology, the application field has expanded rapidly. Different application fields have increasing requirements for plastic surface decoration, material protection, improving bonding and other properties, but various plastic structures and components are different, and the corresponding surface properties are also significantly different. A variety of surface treatment technologies and products for different applications have emerged.
To meet the different needs of plastic surface treatment, a variety of treatment technologies have been developed. Commonly used technologies are solvent cleaning (degreasing), corona treatment, short-wave ultraviolet radiation treatment, sandpaper treatment, sandblasting treatment, plasma etching, chemical etching, heating treatment, etc. It is often necessary to choose different treatment methods for different materials.
Selection of surface treatment methods
Due to the low surface energy of most plastics, many treatment methods such as decoration, printing, spraying, etc., cannot be directly applied, and surface treatment needs to be carried out first. The adhesion of plastics to a variety of different materials is a key problem to be solved in surface treatment. Generally speaking, the bonding properties of plastics are related to the structure and composition of the material.
Structural influence
Polyolefin materials such as polypropylene (PP) and polyethylene (PE) have very low surface energy, usually only 30 to 34 dynes. To achieve good bonding, it is generally required that the surface energy is not less than 40 dyne. The bonding test shows that the bonding strength of PE can be increased by 10 times after plasma treatment. After chromic acid treatment, the bonding performance can be increased by about 5 times. After the same treatment, the bond strength of PP increased by about 200 times after ionization treatment, and 600 times after chromic acid treatment.
Why is the treatment effect of chromic acid on PP so significant, but not on PE? This is because each carbon atom in the PP segment has a methyl group (-CH3). Methyl groups are easily oxidized by carboxyl groups after oxygen ionization or chromic acid treatment. Moreover, even if only a few methyl groups are oxidized, the bonding properties and polarity of PP are significantly improved due to the presence of carboxyl groups. PE does not have this group.
It can be seen that the chemical structure of the polymer is an important factor that must be considered when performing surface treatment.
Compositional influence
For various blends or copolymers, the material composition will also affect the choice of surface treatment method. Fluoropolymers and their copolymers, for example, have lower surface energies than polyolefins, typically ranging from 18 to 26 dynes. For high fluorine content resins such as polytetrafluoroethylene (
PTFE), after sodium naphthenate etching, the bonding performance is improved by 10 times, but after oxygen or argon plasma treatment, it is only improved by 3 times. The trend for PE is the opposite.
However, the bonding properties of the copolymer of fluorine resin and PE were increased by 10 times after plasma treatment or sodium naphthenate treatment. It can be seen that plasma treatment has more interaction with PE, while sodium naphthenate treatment has more interaction with fluorine resin. It can be seen that the processing properties of different materials can be improved by copolymerization. For copolymers of different components, it is also necessary to choose the corresponding treatment method according to the characteristics of the material.
Selection technique
Different treatment methods have different effects on different polymer structures and components, so the choice of surface treatment methods should also be based on the structure and composition of the material.
For low surface energy plastics (<35 dynes), the selection is mainly based on experience. The high surface energy plastic, because of its good adhesion, almost every treatment method is applicable, and can be selected according to the convenience of use.
In general, the lower the surface energy of the plastic, the more treatment is required. However, some polymers have lower surface energy and can also be bonded directly with solvents, such as acrylonitrile-butadiene-styrene plastics (ABS), polycarbonate (PC), polystyrene (PS), zinc stearate (AC) and polyvinyl chloride (PVC).
In fact, AC can be bonded because many acrylic adhesives have their own solvent action. For those anti-solvent materials, such as polyformaldehyde (POM), polyphenylene ether (PPE),
Polyphenylene Sulfide (PPS) and other polymers containing benzene rings, surface oxidation or hair treatment is usually required. Materials that are more difficult to bond such as polyamides and polyimides often require surface etching treatment to bond.
For polar plastics, such as polyester, epoxy, polyurethane, polyamide, etc., the surface treatment method also has different requirements. In general, the less polarity, the less processing is required. Of these materials, polyester and epoxy resins are the most polar and need to be bonded on the surface after brushing. Rigid polyurethanes are not highly polar and can usually be bonded with polyurethane adhesives, but require surface treatment with epoxy resins. Polyamide is one of the least polar and can be bonded without treatment.
For the actual processing process, it is usually necessary to consider the economy of the processing, so that it can better meet the actual processing needs. The various process parameters usually involved, such as processing time, temperature, exposure, drying conditions, etc., need to be carefully considered.
When selecting the treatment method, it is necessary to take into account the chemical characteristics of the corresponding material, the structure of the polymer segment, and the special requirements of the application field. High reliability bonding usually requires more surface treatment.
(1) thermomolecular bonding processing technology
FTS has developed a thermal-molecular bonding processing technology (AtmaP) that can effectively improve the bonding properties of materials, improve product quality, and have good environmental friendliness.
The AtmaP technology is realized by using Cirqual burners. AtmaP processing is mainly to graft a layer of chemical coupling agent on the surface of olefin based plastic parts to improve bonding properties. The combustion flame provided by the Cirqual burner is the only driving force for the coupling agent to spread on the surface of the plastic part. The lightweight aluminum structure of the burner allows for quick maintenance and operation, especially for automatic handling.
The product is mainly suitable for surface treatment that requires spraying, bonding, decoration, lamination, printing or adhesive tape bonding. According to reports, other similar processes used today are unable to achieve the results achieved by AtmaP.
(2) Light curing coatings are applied to automotive plastic parts
Many automotive components already use engineering plastics or polymer-based composites, which require not only coatings to improve their surface properties, but sometimes also to improve the properties of the material. The use of plastic instead of glass in automotive lamp shades and mirrors benefits from the treatment technology of light curing coatings.
PC has the advantages of easy processing and forming, light weight and strong flexibility not easy to break, but its surface strength is not enough, not resistant to scratching and scratching and poor weather resistance, easy to yellow. The use of light curing coatings to improve the surface properties can not only greatly save the coating time, but also have good optical rub resistance and meet the requirements of long-term weather resistance. It is precisely because of the advancement of new technologies that PC lampshades have now completely replaced glass lampshades.
Automotive mirrors are also made of plastic, but they must have high reflective performance. In order to achieve this, the plastic surface must be treated with ultraviolet irradiation three times.
First of all, the plastic must be irradiated by ultraviolet to produce photochemical reaction on the surface to increase the surface tension, so as to facilitate the leveling and adhesion of the photocurable coating; The surface of the plastic becomes flat and easy to be metallized after curing with coated light curing varnish. Metal deposition is then completed in a vacuum deposition chamber. After the plastic surface is metallized, it is also necessary to apply a layer of light curing coating to protect the metal reflective layer.
(3) Improve surface properties through modification
Due to the brittleness of PP (especially low temperature brittleness), high crystallinity, small molecular polarity, and other polymers (such as plastics, rubber) and inorganic fillers, the blending and adhesion of PP is poor, limiting its application in some fields.
Through solid phase grafting modification, related products have been developed, such as the chlorinated modified PP (MCPP) resin produced by Eastman. The solid phase grafting method was used to modify isotactic PP to obtain MPP, and the solid powder resin MCPP was obtained by chlorination of MPP. Modified PP (MPP) and MCPP, as special PP materials, greatly expand the application range of PP. The chlorination modified resin has strong adhesion, improved bonding performance, and is easy to blend or bond with other resins.
(4) Surface treatment of plastic film
Plastic film is one of the largest varieties of plastic, accounting for about 35% of the total amount of plastic. Plastic film is difficult to print, difficult to bond, difficult to compound, easy to produce droplets, easy to generate static electricity and other problems are more prominent. In China, corona technology is used in the surface treatment of plastic film, but it is not suitable for many large-scale uses.
Plasma surface treatment technology has not made a fundamental breakthrough so far, and it is difficult to meet the surface modification needs of large-scale industrial products. The development of new surface modification technology is of great significance for expanding the application market of plastic film.
