Producing Long Chain Branched Polymers from Linear Polyolefins

摘要

Polyolefins, (polyethylene and polypropylene) are arguably the most commonly used thermoplastics used today in a broad range of processes and applications. There are hundreds of types of polyolefins that can be produced directly from the polymerization reactor considering co-monomer type and concentration, molecular weight and molecular weight distribution etc. However, even with today's state of the art process technology, not all of the polymer attributes required by many applications are achievable or commercially viable to produce at the polymerization reactor level. One of those attributes is good melt processability or high melt strength. Of the polyolefin family, only low density polyethylene inherently exhibits excellent melt strength resulting from its highly long chain branched structure. While the physical properties of linear low density polyethylene (higher tear and puncture resistance), high density polyethylene (higher stiffness, toughness) and polypropylene (higher stiffness, clarity, high heat resistance) may be more desirable for a particular application, they all exhibit poor melt strength due to their linear structure. This paper reviews the technique and practical application of modifying readily available "reactor grade" linear polyolefins to produce long chain branching using high energy electron beam modification. Some of these radiation modified materials exhibit rheological properties comparable to or exceeding the melt strength of low density polyethylene while maintaining 100% of their original physical properties. Examples of their commercial practicality will be discussed in terms of value in application.