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PUR hotmelts reduce clamp times.

PUR hotmelt adhesives adapt to many processes and applications due to their very favorable properties.

The evolution of manufacturing processes includes the adaptation to change. Many of today's modern production facilities are using the latest in CNC equipment as well as other enhanced production machinery to maximize their output. This increased productivity has created serious bottlenecks in the area of gluing.

Many basic assembly operations use adhesives with set times and bond characteristics using technology developed during the World War II era which has since remained relatively unchanged. These adhesives, most specifically PVAs, while suitable in some applications, have clamp times (30 to 60 minutes) and recommended stand time to machinability (24 hours) that is unsuitable for today's advanced plant productivity rates.

Polyurethane reactive hotmelts (PUR) represent a quantum leap in adhesive technology that provides greater production rates and exceptional bond quality to cabinet and furniture manufacturers as well as other areas of assembly bonding.

Polyurethane moisture reactive hotmelt technology was developed approximately 13 years ago and has been used industrially since 1986. Originally designed to replace solvent-based adhesives, PURs have found use in many industries including automotive for dashboards, cockpit and door panel assembly; windows for profile wrapping of vinyls and veneers; flooring for the in-line production of 2-ply parquet; and the RV and mobile home industry.

The PUR Process

Polyurethane hotmelt adhesives adapt to many processes and applications due to their favorable properties. PURs are 100 percent solids adhesives, containing no volatile organic compounds. The chemical composition of PUR hotmelt adhesives include easily melting polyurethanes along with free isocyanate groups.

The curing process begins with a physical hardening of the glue line followed by a chemical cross-linking reaction. These two stages begin in parallel but complete at different speeds.

The physical setting stage completes upon cooling, while the chemical curing stage continues. The physical hardening (stage 1) consists of two closely related processes. Initially, the hotmelt goes from a liquid to a solid state driven by loss of temperature, i.e. it cools. This happens quickly while at the same time a second process occurs more slowly whereby polyruer molecules form. These two processes in conjunction form the quick initial bond typical of all hotmelts.

The chemical curing (stage 2) is a humidity-driven reaction between the free isocyanate groups to cross-link with the now formed polymer molecules. This pro cess results in the improved mechanical and chemical properties of the PUR hotmelt glue line.

This chemical reaction increases the bond strength and produces a bond that has temperature resistance to as low as 40F and higher than 300F. This bond is also resistant to solvents and moisture and may resist saturated steam for four hours.

Quick-Setting Bonds

Due to the accelerated bonding, furniture manufacturers are able to see clamp times reduced from minutes, and sometimes hours, down to seconds. Cleanup is minimal since squeeze-out is minimal, and in many cases eliminated. Assemblies can move through subsequent processes in a fraction of the time, which translates to more product out the door with fewer man-hours.

Since current processes are devised to compensate for long clamp time adhesives, additional steps can often be eliminated. The quick bond strength allows for the elimination of pin nails and staples. This eliminates the need for subsequent patching, puttying and hand sanding. This also eliminates the possibility of splitting and staple "blowout" which requires the replacement of costly parts. Also, the removal of pin nails sometimes eliminates damage to costly sandbelts.

PUR hotmelts will also bond to a wide variety of substrates - both porous and non-porous. Today, many shops and manufacturers are faced with gluing or fastening a wider range of materials. These may include various plastics (PVC and ABS), metals, melamine board, glass, ceramics, and an ever-increasing variety of woods. PUR hotmelts will bond these materials in addition to the surface of most finishes.

To realize the full scope of benefits that PUR hotmelts provide, potential users must closely evaluate how best to implement them. The manufacturer should partner with an adhesive technician who is knowledgeable in PUR technology, is experienced with the application equipment, as well as having a working knowledge about a range of manufacturing methods and fully understanding the conditions and circumstances that can cause processes to slow down and productivity to decrease. These technicians should be able to observe the current assembly process and make workable suggestions to streamline and improve performance.

One case in point has been the ability with some types of sub-assembly applications to combine multiple clamp stations to one assembly and gluing station increasing parts per man-hour 200 to 300 percent or more. Rather than approach this adhesive as simply a plug-in alternative to the current adhesives being used, users should look carefully at how desired advantages can be gained with the system and design their manufacturing processes around it.

While it may be cost prohibitive, PURs could simply replace another adhesive in an existing process, but the full array of advantages come from a well-thought-out process. In many cases, that process may be radically different from the existing one. This most often entails some planning, changing and experimentation, but the effect will be dramatic.

PUR hotmelts are tools that can and are changing furniture manufacturing processes for the better. This new technology allows the manufacturer to realize lower labor costs, higher production rates, improved bond strengths and reduced cull rates in many assembly applications. The use of moisture reactive polyurethane hotmelts in assembly is a fast-growing and high-tech application. The customer must be educated to the material. He needs to understand the capabilities as well as the nuances inherent in the use of this type of adhesive. With the proper knowledge and training coupled with the willingness to adapt to change, the manufacturer can be a part of a radical development which could change the way furniture is assembled.

PUR Advantages and Benefits:

Polyurethane reactive hotmelts may provide many advantages and benefits over contemporary adhesives and fastening methods.

Potential advantages include:

* Increased bond strengths

* Extreme cold (-40F) and high heat (300F) resistance

* High moisture and steam resistance

* Solvent resistance

* Diversity in methods of application

* Fast set speeds

* Variety of open times to suit specific applications

* Suitable for exterior use (weather resistant)

* Bonds to wide variety of substrates

* Will bond to many painted, lacquered and finished surfaces

Potential benefits include:

* Reduced labor costs

* Increased productivity

* Reduced cull rates

* Elimination of pin nails and staples

* Parts can be handled almost immediately, eliminating need for additional fastening screws, brackets and other hardware

* Eliminating lengthy clamp or set times

Archie Thompson is the marketing director for Southeastern Adhesives Co., a manufacturer of quality adhesives for the woodworking industry. Thompson can be reached at (800) 359-1880.
COPYRIGHT 1998 Vance Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1998, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:polyurethane reactive
Author:Thompson, Archie
Publication:Wood & Wood Products
Date:Jun 1, 1998
Words:1117
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