Adhesives from Recovered Polyurethane Waste

Introduction

The resuse of polymer waste is a goal of many sustainability initiatives. Much work has been done in this area but most of it has focused on commodity plastics such as those used in packaging. Relatively little effort has been devoted to applications for utilizing waste in adhesive formulations. However, there have been some developments in the area which could be forerunners of a new generation of "greener" adhesives.

Scrap polymeric materials, including those left over from consumption as well as those left over from production but not useful for a variety of reasons, comprise a vast range of reclaimable materials for potential use in other products. In the adhesive area, this type of activity has focused mainly on the reuse of the following polymer waste sources: polyurethane, polyester, and cellulosic.

This article will review the efforts on utilizing recovered polyurethane scrap in adhesive systems and attempt to provide insight into the opportunities and technical problems. The use of polymeric wastes other than polyurethane will be the subject of future SpecialChem articles.

The scope of the article will be limited to waste that is either internally generated within a formulators process or from larger companies producing products with similar resins. Much has previously been written on the use of agricultural waste as a renewable source for biopolymers, and this is outside of the scope of the current article.

Waste and Sustainability

Adhesives consume nonrenewable resources such as petroleum, and they contribute to environmental pollution and the waste stream. These waste streams can include residues of cured and uncured adhesives or other polymers. Waste is a well-known contributor to production costs with attached safety and health issues. More importantly, just the appearance of waste provides evidence that a feedstock (petroleum) with limited future availability is being lost to future generations.

In all approaches to sustainability there is a hierarchy of choices in decreasing order of desirability. This hierarchy is illustrated in Figure 1. The most efficient strategy is usually to eliminate or reduce the use of non-renewable materials and energy. This can be accomplished by optimizing design or by use of renewable resources. Reusing materials that would otherwise be introduced into the waste stream is the next most environmentally and economically beneficial strategy. Disposal is the least desirable strategy in that it takes up space, is costly, and could result in harmful emissions and contaminants.

Figure 1: Hierarchy of sustainable choices regarding the use of non-renewable materials (SpecialChem Fig. Ref.: Hierarchy of sustainable choices)

The possible use of polymer waste needs to be focused upon in several ways: INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (1) reuse of relatively pure scrap, INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (2) reuse of polymer mixtures, INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (3) burning of polymer scrap to recover their energy content, INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (4) chemical conversion of polymer waste to recover useful materials, and INSERT INTO [lzx].[dbo].[tb_new]([id],[type],[title],[source],[personal],[image],[contents],[time],[number]) VALUES (5) separation of polymers from mixed waste streams. These processes are not new and have been around for decades as evidenced by an excellent early review on the reuse of polymer waste.

Nearly 50 years ago, Ford Motor Company, with an eye toward the increasing number of junked automobiles at the time, developed a process to hydrolyze molded polyurethane and polyurethane foams, recovering chemicals useable in polyurethane production. Today, there is renewed interest due to dwindling petrochemical reserves, increasing and volatile oil prices, and concern about the environment and the ever increasing amount of waste being produced.

Early efforts were focused at reusing recovered resin for fillers in adhesives, construction materials, etc. These gained only small profit margins. However, more advanced chemical conversion processes have been developed and are being suggested aiming at the reuse of resin in a more profitable approach ? as an integral component in developing new polymers. Advances in this area have the potential to decrease costs and waste while protecting the environment.

The methods of recovery and recycling of polyurethane waste by the adhesive and sealant industry can be separated into three categories :

Energy recovery if often considered the only suitable disposal method for recovered material for which no markets exist or can be created. This strongly applies to scrap with polyurethane laminates to wood, leather or fabrics, or comingled materials. However, flame retardant materials make recovery and recycling very difficult. Incineration of a polyurethane foam results in a volume reduction of around 99% which has large implication to reducing the land filling of this material.