Rifeng® PMI structural foam cores can be used with nearly all commercially available adhesive systems. It is resistant to solvents and is distinguished by a closed cell structure. Bonding of the adhesive system is improved by mechanical anchoring in the cut cells.
Owing to the large number of available adhesives and the multitude of substances which may be bonded to Rifeng® PMI foam cores, it is difficult to provide complete information on the methods and amounts, and application of adhesives, as well as the drying and curing times. However, in case of special problems, we, together with adhesives manufacturers, shall be glad to look for a practical solution. Still, for most bonding problems, Rifeng® PMI foam cores offers the advantages of solvent resistance and heat distortion resistance for hot curing (epoxy resins up to 180 °C) advantages which should not be underestimated. Practically all commercial adhesives can therefore be used.
It is indispensable that before bonding, the Rifeng® PMI foam cores surfaces shall be freed from dust by suction or by blowing the dust off with oil-free compressed air.
The adhesives are usually classified in three groups:
1. Solvent-based adhesives
2. Emulsion adhesives
3. Solvent less adhesive systems
Since Rifeng® PMI foam cores is very impervious to solvent diffusion, and great care must be taken when large areas of Rifeng® PMI foam cores are to be bonded to Rifeng® PMI foam cores or other diffusion-impervious materials by adhesives of groups 1 and 2, that the adherents dry off well after the adhesive has been applied to both sides before they are joined together under pressure. Joints made with these adhesive systems (generally rubber based) normally remain a little elastic and have good peel-off strength. If it is possible to heat-seal the joint, the quality of the bond can be greatly improved.
After applying the adhesive to both sides, the adhesive is dried off, then placed on top of each other and bonded under pressure at a temperature of 80 - 120 °C. The pressure may be up to 40 % of the maximum compression strength at 20 °C of the particular foam plastic and it should be maintained for 1 - 5 minutes. The bonded materials should be removed at a temperature below 80 °C.
The solvent less system includes hot-melt adhesives, two-component adhesives like epoxy, polyester or methacrylic resins should be allowed to cure under sufficient pressure(0.05-0.3N/mm2) or be very fluid during the application so that the cells of the foam are well filled. The cure can be accelerated by heat (up to 160°C). The joints become very hard and rigid.
Adhesive films and hot-melt adhesives need heat for bonding and can therefore normally be applied by heat-sealing. Adhesive films must be sufficiently thick (100 -200 g/m²) in order to anchor them firmly in the cut Rifeng® PMI foam cores cells. Two-component adhesive films which give off volatile constituents like solvents or, in the case of phenolic resin films, water during the cure, should be warmed-through with gentle pressure. Before bonding, the press should be briefly bumped to allow the volatile constituents to escape.
When hot-melt films are used, it has frequently proved useful to perforate them before bonding in order to avoid air bubbles. For the purpose of better deaeration in difficult cases, prior grooving of the Rifeng® PMI foam cores sheet surfaces will help. Grooves about 1 - 1.5 mm deep and 2 mm wide have proved useful.
When Rifeng® PMI foam cores is to be bonded to other materials the adhesive may generally be selected according to its suitability for these materials. For instance, for metals and laminated plastics, epoxy resins, rubber or PU adhesives, for acrylic glass/methacrylic resins, for wood and paper or for glass fiber reinforced polyester sheets, polyester resins, epoxy resins and PU adhesives systems.
To obtain perfectly straight sandwich sheets, it is important for both sides of the Rifeng® PMI foam cores sheet to be simultaneously bonded to the skin. Both skins must be of the same material and have the same thickness. Uniform heating and cooling on both sides are other basic conditions for the manufacture of flat sandwich sheets.
If the Rifeng® PMI foam cores surface is treated with a needle roller before the application of the adhesive, the obtainable bond strength will meet very high requirements.
We recommend, in all cases, the use of oversize Rifeng® PMI foam cores sheets (0.5 – 1 mm) for bonding skins, and to run the press to firm stop, lest the Rifeng® PMI foam cores sheet be overstressed by the molding pressure at the high curing temperatures, or the dimensions be below the required tolerance through thermo elastics creep.