About STP CPRX Compound

STP CPRX Compound is a rubberised bituminous adhesive built for one job that Indian sites face constantly: fixing insulation and bituminous membranes to a substrate reliably, without dragging a torch or a bitumen boiler onto the deck. It arrives ready to use as a brushable, brown liquid of roughly 1.02 specific gravity. Because it is cold-applied, a two-man team can coat both faces, wait for the film to break, and press the boards home, all without an open flame. That single fact removes a lot of fire risk on occupied buildings, inside cold stores, and around refrigeration pipework where hot work is simply not allowed.

The chemistry is what makes it more than a plain cutback bitumen. STP blends rubber into the bituminous base, so the cured film is elastic rather than brittle. It holds thermocol, extruded and expanded polystyrene, insulex soft boards and fibreglass slabs against walls and ceilings, and it tolerates the small movements and thermal cycling those assemblies see in service. The same rubberised film is inherently waterproof and weather-resistant, which is why applicators also reach for it to bond bituminous felt and to seal lap joints within membrane systems. Space Arc Engineering supplies CPRX Compound across Delhi NCR and Pan-India, with the sizing and application guidance to match it to the deck it is going onto.

The technical insight

CPRX works as a solvent-release, contact-type adhesive rather than a chemically curing one. When brushed on, the carrier begins to flash off and the film “breaks” over 15-20 minutes, darkening from brown toward black as the rubberised bitumen concentrates at the surface. Pressing the two coated faces together at that point brings the tacky films into intimate contact, so bonding is by fusion of the two adhesive layers plus mechanical keying into the substrate texture. The rubber fraction is the reason the bond is not brittle: it lets the cured line flex with the insulation board and the deck instead of shearing when they expand and contract at different rates. Because the mechanism is evaporative, the adhesive needs open, ventilated conditions to break properly and needs the full 24 hours to build ultimate strength as the last of the carrier leaves the joint, which is why no further trade should load the completed surface within that day. The 120-micron dry film per coat is a useful check: it tells you the coverage figures assume a controlled, even brush film, not a flood coat, and that starving the surface will cost you grab.

Where it is used

• Bonding expanded and extruded polystyrene (thermocol) insulation boards to concrete, plaster and metal decks
• Fixing insulex soft boards and fibreglass insulation slabs to walls and ceilings
• Lining cold-storage walls and ceilings where good adhesion to insulation is critical
• Sticking thermocol for the insulation of refrigeration and chilled-water pipework
• Bonding bituminous felt and membranes to concrete, masonry and metal substrates
• Sealing lap joints within bituminous waterproofing membrane systems
• Damp-proofing and membrane bonding on masonry and concrete surfaces
• Adhering insulation sheet to insulation sheet in built-up insulated assemblies

Key advantages

• Cold-applied, so no torch, gas or bitumen boiler is needed on site
• Eliminates hot work, making it safe for occupied buildings, cold stores and pipework
• Brushable ready-to-use consistency that a small crew can apply quickly
• Rubber modification gives an elastic, flexible bond that tolerates movement
• Cured film is inherently waterproof and weather-resistant
• High bond strength once fully cured over 24 hours
• Bonds across mixed substrates, concrete, timber, metal and other insulation sheets
• Non-toxic and not harmful if inhaled, easing use in enclosed insulated spaces

Limitations & considerations

• It is an adhesive for bonding insulation and membranes, not a standalone liquid-applied waterproofing coating
• Solvent-release curing needs ventilation and open time; it will not break correctly in sealed or badly ventilated spaces
• Full bond strength takes 24 hours, during which the surface must not be loaded or worked
• Only 6 months shelf life in the unopened sealed drum, so stock must be rotated
• Standard grade is optimised for insulation and membrane bonding; tile and linoleum fixing needs the special grades
• Performance depends on clean, dust-free surfaces and an even film, so surface prep and workmanship are decisive

Selection & best practice

Specifications

Industry use cases

How we position STP CPRX Compound

STP CPRX Compound sits in Space Arc’s insulation and membrane-bonding line-up as the cold, brush-applied way to fix thermocol, insulation slabs and bituminous felt without hot work. Within the STP family, pair it with STP’s bituminous membranes such as the Superthermolay APP range where CPRX bonds the felt and seals the laps, and step up to STP’s special CPRX grades when the substrate is tile or linoleum rather than insulation. Where the specifier actually needs a continuous liquid-applied waterproofing film on the deck rather than an adhesive, our Fosroc line answers that: Nitoproof 600 for a bitumen-polymer brush-applied membrane, or Brushbond for a cementitious tanking membrane on tanks, basements and wet areas. In short, choose CPRX to bond and adhere; choose a Fosroc or STP membrane to waterproof.

Related Waterproofing products

View all Waterproofing products →