For any industry, it would be a spectacular windfall: a near 50 per cent fall in the cost of its major input.

That is what has happened to European petrochemical companies over the past year, as the price of crude oil plunged, delivering a once-in-a-generation boon to a sector that in recent years has struggled to survive.

But the effect has not been uniform. European industries further down the supply chain, such as plastic packagers which buy their raw material from the petrochemical companies, are seeing little or no benefit from the oil price decline.

This is partly because years of chronic underinvestment has left petrochemical companies’ plants facing a flurry of unscheduled shutdowns — leading to a shortage of plastic that has pushed up prices to new highs despite it costing less to make.

For chemicals companies, however, the oil price plunge has provided a significant profitability boost.

“Up until the fall in the oil price we had a situation where the European chemical industry was in a pretty parlous state generally because of the cost of energy and feedstock,” says Tom Crotty, group director at Ineos, the Swiss chemicals group. Now it is enjoying a much-needed respite, he adds.

European chemicals companies have long been seen as the poor relations of their US counterparts. The US shale boom had given American petrochemical producers a huge cost advantage over European and Asian rivals in an industry where feedstock and energy consumption account for as much as 85 per cent of total operating costs.

Meanwhile, the main feedstock for Europe’s petrochemical industry is naphtha — a raw material derived from crude oil. The price of naphtha has historically been high, reflecting crude’s extraordinary bull-run throughout the 2000s.

“The differential between [petrochemical plants known as] crackers in the US running on gas and those in Europe running on oil was absolutely massive. It was a huge competitiveness issue,” says Crotty, whose company also runs crackers in America.

European petrochemical companies operate crackers that break down naphtha into ethylene, a chemical used as a building block for many plastics.

As the value of Brent crude has fallen since the summer of last year by about 45 per cent, so has the price of naphtha — reducing the US-Europe competitiveness gap. Naphtha has plunged from a high of $975 per tonne in June 2014 to a low of $373 in January. It has since risen to $525.

But the competitiveness gap still exists. Ben van Beurden, chief executive of Royal Dutch Shell, says the cost of feedstock in the US is still lower than the European equivalent. “In terms of fundamental competitiveness, Europe is still behind the US and the Middle East because the feedstock is still more expensive,” he adds.

While the good times are back for European petrochemical groups, plastic converters — companies that turn the material into products ranging from food packaging to pipes for the construction industry — have warned that many are struggling to survive in the face of rising prices.

The price of polyethylene, a common plastic made by petrochemical companies, fell to a low of 1,085 euro per tonne in early February, but it has subsequently jumped to about 1,635 euro — past its previous high of 1,452 euro in July last year.

“The assumption is that everyone in the plastics industry is making money because the cost of the derived material has come down, but that’s not the case,” says Ron Marsh, who used to head up RPC Group, a British packaging company.

Instead, plastic prices have rocketed as the supply of the material has dwindled, following a series of so-called “forces majeures” which saw chemical companies shut down their plants.

“What was initially a windfall from lower oil prices has actually turned into a moment of intense supply constraint, which is making the prices plastic converters are exposed to really not much better than they were before the oil price fall,” says Paul Ray of ICIS, a consultancy.

Several of the big petrochemical companies have been hit by stoppages this year, including Shell, BP and Ineos.

Shell says its Netherlands manufacturing site in Moerdijk, which comprises four main production units, had two incidents last year that impacted production.

BP says it only declares a force majeure when there “are reasons beyond our usual control for not being able to produce as per contracts”. Ineos says it is in its interests to have all plants operating.

There have been 41 shutdowns of European petrochemical plants in the past six months, compared with 26 for the whole of last year, according to EuPC, the trade body which represents 50,000 companies involved in plastics conversion.

Industry insiders say the reason for the petrochemical plant stoppages is a chronic lack of investment by companies over the past decade.

Michael Kundel, chief executive at Renolit, a German packaging manufacturer, describes the current situation as “precarious”. “As far as I can remember in my 25 years of history we’ve never had such a high number [of petrochemical plant shutdowns] in such a short period of time,” he says.

The shortage of plastic in Europe has been made worse by a rise in import duties since January, which made it more expensive for companies to source the material from outside the region.

Paul Hodges of International eChem, a chemicals and commodities consultancy, says the current situation is not simply an issue of producers pushing up prices. He notes that on top of the force majeures, the second quarter of the year is typically the strongest time for demand and that the limited rally in the value of oil at the beginning of 2015 caused buyers to rush to build plastics inventory in advance of expected price increases.

Alexandre Dangis, director-general of EuPC, says the most important thing for manufacturers using plastics in their products is to understand is how well-placed the European petrochemical companies really are to supply the market over the coming years.

“The confidence of the plastic converters towards their suppliers is at a very low level,” he says. “We really need to understand what is going on. Is there a need to import more polymers from outside Europe? If so then we need to open the ways to be able to import more.”

Financial Times