Almost all Hot Melt Pressure Sensitive Adhesives (HMPSA) are composed of various Styrenic Block Copolymers (SBC), tackifiers, and mineral oils. The mixture is a thermoplastic material which does not crosslink or cure during manufacturing and processing.
When a HMPSA is used to bond onto plasticized PVC objects, the pressure sensitive adhesion, particularly the holding power, will deteriorate gradually. PVC alone is a polar and rigid material without any low molecular weight ingredients. It will not affect the performance of any HMPSAs. However, when PVC is plasticized by certain plasticizer, e.g. phthalates, it is softened and flexible. The plasticized PVC becomes a diverse and very useful material for use in our daily life. They are widely used in the following major application markets: furniture, electronics, automobile, construction, clothing, medical device and films, tapes and labels, etc. Those incorporated phthalates however do not crosslink with PVC polymer; they are simply dispersed in the PVC polymer matrix. Those phthalates will slowly separate from PVC and gradually evaporate to the air even though at a regular storage condition. The evaporation rate may be accelerated by increasing surrounded temperature or any forms of energy.
When a material, such as a HMPSA, contacts the surface of plasticized PVC, the dispersed phthalate will gradually migrate to the interface of PVC and HMPSA; and penetrate into HMPSA. This phenomenon will continue to proceed until it reaches a thermodynamic equilibrium state. Because of this feature, the formulation of the used HMPSA is gradually changed with service time. As phthalates are very polar low molecular weight plasticizers, they can easily associate with the styrene domain of SBC, due to their excellent compatibility, and gradually destroy the cohesive strength of those physically cross-linked domain. In practice, most HMPSAs may exhibit an optimum, balanced, adhesion performances initially, but they will creep or separate cohesively from plasticized PVC objects after a certain period of time depending upon the percentage of plasticizers included in PVC.
To resolve the above troublesome feature, many HMPSA formulators will try to formulate a radiation curable HMPSA. Though the molecular weigh of polymer becomes infinite after curing, the included tackifiers, particularly for those polar ones, are still soluble with those plasticizers used in PVC. As a result, the holding power may be less affected by plasticized PVC, the entire pressure sensitive adhesion performances will still be altered anyway.
Disregarding the pressure sensitive adhesion performances, when a conventional hot melt adhesive (HMA) is used to bond onto a plasticized PVC, interfacial failure is observed typically. Even though plasticizers are not compatible with most EVA and APO polymers, they will associate with and dissolve some polar tackifiers. Additionally, plasticizers often migrate to the interface and form a thin layer of plasticizer (oil) between PVC and adhesive, which will eventually cause an adhesive failure with a rather low force.
In summary, no HMAs or HMPSAs are suitable for bonding plasticized PVC, unless the incorporated plasticizers are high molecular weight and do not migrate to the surface of PVC. Generally, when a HMPSA is bonded onto plasticized PVC, a fracture mode of cohesive failure is observed. On the other hand, a HMA will present an adhesive failure with very low separation force.
For more information call or email Pierce Covert,
Glue Machinery Corporation