Isoprene shortage changes the way adhesives are formulated

Isoprene is used as a major raw material in SIS, or styrene isoprene styrene. It is one of the most commonly used rubber polymers in the production of pressure-sensitive adhesives. Leading suppliers say that isoprene is in short supply primarily because it is very expensive to produce so over the last decade global isoprene capacity has not increased.

There is a shortage "because the price has been low for a number of years, " says Yuri Sebreqts, global business manager C4/C5, Shell Chemicals, "and nobody had an incentive to invest.

In addition, a couple of major U.S. production sites have had plant outages for the last six months. Meanwhile, demand has gone up dramatically in the last couple of years and more isoprene is being used to produce tires.

Fixed capacity, increased demand and especially due to higher demand in tires, there is a resulting shortage for the adhesives industry.

This shortage is having a serious impact on tape and label manufacturers around the world. Manufacturers have traditionally used styrene-isoprene-styrene (SIS) block copolymers in their formulations because SIS-based hot-melt pressure-sensitive adhesive (HMPSA) formulations can be used over a range of temperatures. They also provide an aggressive tack, enabling tailored peel adhesion properties with good cohesive strength. SIS is formulated into adhesives with good color, clarity and low odor.

How is isoprene made?

Isoprene can be made intentionally or as a byproduct from the gas oil production of ethylene and propylene. Ethylene and propylene are two of the most important starting raw materials for organic chemicals. Most polyol plastics start with either ethylene or propylene, so they're very economically important raw materials.

"Commercially viable quantities of isoprene are extracted under tightly controlled conditions from a byproduct stream of ethylene manufacture," said Sebreqts, "and are therefore essentially derived from crude oil via a number of extraction steps.

"Unlike many other isoprene producers, Shell chemicals companies are also ethylene producers," Sebreqts said, "affording us better and more consistent access to crude C5s, the feedstock for isoprene production. In Europe, our production facilities are in the same location as our customers' consuming facilities, minimizing both the handling and transport of isoprene."

Has isoprene production been affected by fuel prices?

"Gas price has a direct effect on the polymers and it has a direct effect on the price of the resins," said Marc Jackson, adhesives marketing manager, Arizona Chemical Co. "Some formulas, like the formulas that have to rely on the hydrocarbon-based products, are going to remain more expensive so long as gas prices remain more expensive, so SIS systems because they only use hydrocarbon resins and because they use these styrene-isoprene-styrene ... are going to be much more expensive for the medium-range outlook than with the SBS alternatives.

Jackson said Arizona "is the biggest producer of pine-based tackifiers; we compete with the guys who make hydrocarbon-based tackifiers like Exxon and Eastman. So historically, Arizona Chemical's products have been used predominantly in EVA-based hot-melt adhesives," such as packaging and book-binding adhesives.

"For now, because of the diminished use of SIS and the increased use of SBS and SIBS, both of which are much more compatible with pine chemicals, we're seeing a very nice increase in demand and interest in the pressure-sensitive segment."

Who produces SIS?

Companies like Kraton, Dexco [Dexco Polymers LP, a limited partnership of affiliates of the Dow Chemical Company and ExxonMobil Chemical Company, produces both SIS and SBS products from its dedicated facilities in Plaquemine, Louisiana.] produce SIS but can't get the isoprene to convert it to the polymer so what they're doing is they're putting people on sales allocation. They're saying, we'll sell you 90% or 80% of what we sold you last year.

And of course the price has gone up. I've heard some people say that SIS prices have gone up by as much as 30% so that average market pricing is around a dollar, maybe a dollar twenty five.

Who Makes Isoprene?

Because it is an expensive process, few North American companies produce isoprene. Shell's Sebreqts said that extraction capacity is operated by Nippon Zeon, Goodyear, Equistar and Petrobras. Goodyear also toll processes Crude C5 for other majors. There is also isoprene production capacity in Russia from pentane-dehydrogenation, Sebreqts said.

What can be done about the shortage?

The good news is that there are a number of options for adhesive formulators to manage the isoprene shortage.

"Several formulation strategies can be used to help mitigate the current isoprene shortage," said Mark A. Peters, Group Leader, Adhesive Applications Technology, Eastman Chemical Co. These strategies can allow PSA manufacturers to continue producing PSA tapes while reducing isoprene content by as much as 50%. In addition, it results in a quality end product, as well as significant production efficiencies, reductions in labor requirements, and reduced cost.

First, styrene-isoprene-butadiene-styrene, or SIBS, can be used in place of SIS in a traditional SIS formula. In this solution, PSAs will require changes to the tackifier resins in the formulation due to the addition of butadiene. Performance characteristics of resins depend on the compatibility of the polymer used with them. Peters recommends aromatically modified C5 hydrocarbon resins "because they are more compatible with SIBS elastomers and offer better tack and peel strength properties than their aliphatic cousins. In fact, aliphatic resins are not compatible with the SIBS polymer, giving very low tack and low adhesion."

The isoprene in SIBS-based PSAs can be further extended by raising the styrene content of the SIBS blend, Peters said. As with high-styrene SIS formulations, only highly aromatic modified hydrocarbon tackifiers with a low softening point are recommended. Lab tests conducted using Piccotac 7050 hydrocarbon resin showed an overall reduction of isoprene of 30-50% depending on the PSA tape formulations under consideration.

Secondly, to further extend their existing inventories of isoprene-based polymer, manufacturers can also blend SBS with SIS. Peters said that "Eastman studies showed that SIS block copolymers diluted with SBS in conjunction with aromatic-modified, aliphatic hydrocarbon tackifiers permit a reduction of isoprene content of 23% while maintaining peel and tack properties."

Marc Jackson said, "Historically, an SIS adhesive was formulated with a polymer, an aliphatic or a non- or low-polar tackifying resin, and an oil. When people go to SBS they're able to get similar or better performance by using SBS and more polar tacifiers like rosin esters.

"The use of SBS creates a new way of formulating adhesives for the different type of tackfier; you still use the same type of oils." There are many types of tackifiers available that will work with SIBS to give equivalent performance, and he says the resultant adhesive will work just as well.

Jackson said SBS has one major deficiency to SIS - it tends to be more difficult to apply. "It's more difficult to coat it because SBS can get gummy, sticky, on the process and SIS does not. SIS can be used for very fast line speeds, is a very fast roll coater; if you're trying to make tape as fast as you possibly can, nothing works quite as well as SIS."

Thirdly, manufacturers can extend their supply of SIS by adding amorphous polyolefins (APOs), by as much as 30% of the SIS in PSA formulations. APOs are offered in a range of forms and offer good compatibility with a variety of polymers and tackifiers. They have little impact to clarity, odor, or tack when properly formulated, Peters said. These blends require the use of hydrogenated hydrocarbon tackifiers to produce reasonable adhesive properties.

Peters suggests that manufacturers check with their APO suppliers for specific recommendations in their PSA formulations.

Future outlook for isoprene

Some people have said that toward the end of this year they think isoprene will loosen up, the situation will improve, because they think that, well US producers have got their plants back online, they're running again, and also purpose-made isoprene is coming in from Russia. So people are hoping that things will improve toward the end of the year. But many people are very skeptical now about formulating with an SIS so they hedge their bets, they're changing their formulas to use more SBS and more SIBS - they don't want to be sole-sourced on SIS.

Shell's Sebreqts said it will remain tight for the rest of 2005.