PVC floor tile is constructed with many layers of materials. Typically, from top to bottom, there is a transparent PVC film with various thickness (abrasion resistance), a printing film (aesthetic purpose), a white PVC film (background for printing film), and a composite PVC body (form a rigid shape).
The composite body is compounded with several components: recycled PVC, virgin PVC, plasticizer, calcium carbonate, stearic acid derivatives, PE wax, stabilizer, pigment, and etc. Since those recycled PVCs are received from various resources, the total content of plasticizer in the composite is always uncertain.
For self stick PVC floor tile, hot melt pressure sensitive adhesive (HMPSA) is coated on the top of the composite body piece by piece with a top feed roll coater. The coating speed is up to 160 pieces per minute for 1 square foot tiles. The adhesive surface is automatically covered with a piece of release paper immediately after coating.
Two major difficulties are commonly observed on commercially available self stick PVC floor tiles.
- Most PVC floor tiles contain some amount of plasticizers, such as DOP and DINP. These plasticizers will associate or soften the coated HMPSA after adhesive is coated on the composite body. Many HMPSAs will eventually lose their cohesion and become very sticky or gummy. As a result, adhesive side stick will appear after the tile is placed on the floor.
- HMPSA is applied on intermitted, piece by piece, surface at elevated temperatures. The coated adhesive is instantaneously cooled by the cold tile and will string between roller and the edge of those coated leaving tiles. These adhesive strings left on the machine and tile surfaces must be removed and cleaned by solvent after coating. To minimize this undesired phenomenon, many manufacturers suggest raising the melting and coating temperatures up to 450F. At this high temperature, the viscosity of adhesive becomes very thin and will not cause any string after coating. However, most HMPSAs are completely thermal aged at this high temperature. The coated adhesive on PVC floor tile will be further attacked by plasticizers migrating from the composite body. Very soon, the adhesive becomes very gummy and sometimes loses the tackiness when the consumers obtain those tiles.
How to formulate an appropriate HMPSA to overcome the above two problems is always a challenge to most formulators. Following are two suggested formulation tips.
- The adhesive formulation should reserve some high molecular weight polymer to resist the softening effect from plasticizers.
- The formulated adhesive should impart high elasticity. As a result, those adhesive strings can be easily broken and snap back to both roller and composite surface without leaving any undesired string on either machine or the printed side of PVC floor tiles.