DOVER, Del. – Dow Chemical and DuPont are merging to form a company valued at about $130 billion as they try to counter falling commodities prices and weakness in some key markets that have pressured their giant agriculture and chemicals businesses.

The two companies, whose research has brought the world products ranging from Ziploc bags and Saran wrap developed by Dow to DuPont’s Teflon coatings and Nylon and Kevlar fibers, will first form DowDuPont, then separate into three independent publicly traded companies focused on agriculture, material science and specialty products.

The proposed merger, announced Friday, would temporarily create the world’s second-largest chemical company, behind BASF. It comes as both Dow and DuPont Co. have seen recent declines in agricultural performance and been pressured by activist shareholders to control spending and shift away from commodities to faster-growing parts of their businesses.

“Overall, this transaction represents a tectonic shift in an industry that has been evolving over the last many years,” said Dow Chairman and CEO Andrew Liveris, calling the merger a seminal event for employees and customers of the two companies, which have a combined workforce of more than 110,000.

DuPont Chairman and CEO Edward Breen said the “industrial logic” behind the deal was compelling.

“When I look at DuPont and Dow, I see businesses that fit together like hand and glove,” Breen said.

DuPont shares slid 5.5 percent to $70.44. Dow Chemical closed down 2.8 percent at $53.37.

Breen took over as DuPont CEO following the abrupt resignation in October of Ellen Kullman, who just a few months earlier fended off a proxy challenge by Trian Fund Management, a hedge fund led by activist investor Nelson Peltz. Peltz has called for DuPont’s agriculture, nutrition and health and industrial biosciences units to be combined into a single growth company, separate from the more cyclical businesses of performance materials, safety and protection, and electronics and communication.

Similarly, Dow has been pressured by hedge fund Third Point LLC, led by activist investor Dan Loeb, to split its specialty chemical and petrochemical businesses.