Q&A About Polyurethanes
April 1, 2011
A particular application will require me to bond a variety of optically clear polymers to each other and to glass. What adhesive options are available?A particular application will require me to bond a variety of optically clear polymers to each other and to glass. What adhesive options are available?
A variety of polymeric film adhesives are on the market that may work for your application. The main types are optical aliphatic polyurethanes (OATPUs), plasticized polyvinyl butyral (PVB), and reactive adhesives that cure by heat or light. In protective glazing structures, the adhesive layer is the key factor in creating a robust, successful design.
OATPU films or sheets are used as adhesive inter-layers between layers of glass, polycarbonate or acrylics. They are currently one of the materials of choice for adhesive films because of their ability to bond to a variety of substrates and their optimal performance over an extended time period.
OATPU adhesive inter-layers are inherently soft due to their molecular block structure. They are synthesized using an aliphatic diisocyanate, macrodiol (polyol) and short chain diol (extender). These urethanes maintain their physical properties over a wide temperature range and therefore exhibit excellent impact resistance and anti-spalling properties of the laminated structure. The inherent soft elastic properties of OATPU prevent delamination by allowing rigid glass and polycarbonate substrates to expand and contract over a range of temperatures. Since they are based on aliphatic isocyanates, these TPU films exhibit long-term color stability, which eliminates yellowing problems associated with exposure to ultraviolet light. They also show long-term optical clarity and low light distortion.
Typical applications include bonding glass/glass and glass/polycarbonate for a variety of security and military applications, such as forced entry protection and ballistic and explosive attacks.
PVB provides good bonding ability at low cost and offers some level of protection against spalling, but it has a major disadvantage in that its plasticizer content is not compatible with polycarbonate. Also, the PVB laminating temperature is in excess of 260°F; this high temperature level causes stress cracks in a polycarbonate coating. Liquid resin adhesives are cast or poured into voids between the glass, polycarbonate or acrylic substrates and cured by ultraviolet light or heat. These resin systems are used in special design applications such as curved laminates and laminates containing inserts.
I am familiar with the use of waterborne polymers for textile bonding applications, but I am interested in a 100% solids system. What options exist for the use of polyurethane polymers?Thermoplastic polyurethane films are used in textile applications to serve as an adhesive or barrier layer. A typical adhesive application is in the manufacture and application of emblems. The emblems are a multi-layer construction typically made from polyester and non-woven or woven fabrics. TPU films are used to bond these layers of fabric together and serve as the adhesive backing on the emblem, which is then hot-pressed onto articles of clothing.
The TPU adhesive films are based on polyester polyols and aromatic isocyanates. These ester-based films are preferred over ether-based films because they are inherently tackier when molten and generate better bond strength at the film/fabric interface. A temperature of 300°F is used to laminate fabric layers together or adhere the emblem to clothing. Emblems constructed with ester-based TPU adhesive films are able to withstand laundering and dry cleaning cycles.
TPU films function both as an adhesive and a barrier layer in automotive applications, where foam-in-place manufacturing techniques are used for the production of arm and head rests and seats. For these applications, either an aromatic ether-based or an ester-based TPU film can be used. Typically, a tri-laminate of fabric or vinyl/soft porous PU foam/TPU film is made by a flame lamination process, in which the soft porous foam is sandwiched between the fabric and the TPU film. This three-layer structure is then cut and sewn to form the cover of the part.
The sewn assembly is placed into a mold with the outer fabric layer contacting the mold surface. The reactive components used to make the structural PU foam are then poured into the mold, and the foam expands to form the part. The TPU film acts as a barrier that prevents the reacting foam from bleeding through the layer of soft porous foam. An unwanted bleed-through could cause the part to be rejected for a dimpled appearance or a rigid feel in the outer surface of the assembly.
For additional information on the topics addressed or to ask another question, e-mail email@example.com with the subject line “Polyurethane Q&A.”
Any views or opinions expressed in this column are those of the author and do not represent those of Adhesives & Sealants Industry, its staff, Editorial Advisory Board or BNP Media.