DENSOLEN Tape Systems - compatibility with factory polypropylene coatings

DENSOLEN Tape Systems achieve a secure corrosion protection of the welded seam area of steel pipes not least because of the complete and durable seal of the transition to the adjoining factory coating. Adherence to the factory coating is provided by a primer (DENSOLEN Primer, a solution of rubber and unsaponifiable resins in bezine), which anchors mechanically in the rough base.

The bond to the DENSOLEN Tape System occurs through cold welding of the butyl adhesive layer of the tape with the primer. If the surface is cleaned and prepared in the same way, the adhesion of DENSOLEN systems to polyethylene and polypropylene coatings is identical. As there are also no chemical "incompatibilities," DENSOLEN Tape Systems are fully compatible with polypropylene factory coatings.

Can DENSO petrolatum products be used in contact with PE or PP?

DENSO petrolatum products are based on technical Vaseline, i.e. on a blend of inert, short-chain hydrocarbons. This means they are chemically related to polyethylene and polypropylene. 

Petrolatum products do not contain special reactive softeners. When using petrolatum products on  PE or PP, a distinction must be made between two kinds of use:

 

PE, PP coatings

There are no restrictions on the use of DENSO petrolatum materials on PE or PP anti-corrosion coatings. Experience shows that DENSO petrolatum products are fully compatible with such coatings. No negative effects are known. Petrolatum products are well suited to safe corrosion protection of PE or PP insulated pipes or installations. 

PE, PP pressure pipes (gas) 

The close chemical relationship between PE / PP and petrolatum makes diffusion of petrolatum components into the upper molecule layers of the pipe wall a possibility. This could influence the firmness of the pipe material and if there is pressure stress from the transported medium could theoretically lead to a stress crack in the synthetic material of the pipe wall. However, the stress crack behavior of PE pipes is very much dependent on the quality of the PE. As we have no information from the pipe manufacturers on the quality of the PE used in their pipes, we cannot for our part make a statement on compatibility. It is the case that all previous inspections and experience have shown no such stress crack behavior, and we also do not know of any occurrence of such damage. However, simply because of the theoretical possibility, we cannot recommend the use of petrolatum for the transition area of PE to steel on gas pressure pipes. As an alternative, the corresponding elements may be wrapped with tapes and filling compounds based on polyethylene and butyl rubber (DENSOLEN systems). As it is theoretically possible for the solvents in the primer to influence the PE material, no primer should be applied to the PE pipe. Despite the above, a secure seal in the transition area can be achieved by thoroughly cleaning the PE pipe surface and if necessary using a greater width of overlap for the PE pipes. 

Can DENSOLEN pipe protection mats be used instead of sand?

DENSOLEN DRM-PP pipe protection mats consist of rot-proof fleece material based on modified polypropylene, distinguished by high mechanical resilience. Their shock-absorbent and load-distributing effect means that they effectively protect the anti-corrosion coatings of steel and synthetic pipes from damage from rocks, stones and other constituent parts of the ground.

When using the DENSOLEN DRM-PP 1000 Plus pipe protection mat, material with the maximum size granulation of 63 mm (coarse gravel, classified according to DIN 4022 and DIN 18196 respectively) can be used instead of sand for embedding the pipes and infilling. The relevant provisions for embedding pipes, filling the trenches and sealing, in particular DIN 1610, DIN 18300 and also the DVGW (Deutscher Verein des Gas- und Wasserfachs e.V - German Association for the Gas and Water Industries) process sheets G 462, G 463 and G472, must be followed.

It will, given the great number of synthetic pipes on the market, be necessary to establish with the manufacturer in each individual case to what extent the durability of synthetic pipes is influenced by the use of the embedding and filling material described above.

C-30 or C-50 ?

Stress class C-30 and C-50 tape systems differ in two particular qualities

  • Peel/Shear resistance
  • Indentation resistance

The difference not only occurs at 50 °C and upwards, but also becomes apparent at 23 °C. A C-50 tape system, therefore, provides greater protection from mechanical damage or shearing of the coating even at room temperature. Considering that the PE factory coatings, for their part, have a markedly greater mechanical resistance than C-50 tape systems, a C-50 tape system is always to be preferred in post-factory wrapping if there are no aspects of the application process that indicate otherwise. All welded seams, pipes and pipe bends can, for instance, easily be wrapped with a C-50 tape system, without any disadvantages in the wrapping process. Only in the wrapping by hand of complex shaped components does a C-30 tape system consisting of a butyl rubber tape and a mechanical protection tape offer, on the whole, better corrosion protection. Here the lesser mechanical strength is compensated for by simpler application, more likely to avoid the development of folds and voids.

Cost savings in changing to a C-30 would in any case only be achieved if a two-ply tape system was used. However, because of the insufficient seal in the overlapping tape in such systems, their use is not recommended, as long-term corrosion protection is not guaranteed. High-performance C-30 tape systems offer no advantages compared to C-50 versions.

Are DENSOLEN tapes UV resistant?

DENSOLEN AS- and S-tapes are three-ply anti-corrosion tapes with a polyethylene carrier film and butyl rubber adhesive layers on both sides.

Both the polyethylene and the butyl rubber used are stabilized by means of appropriate additives to prevent decay due to UV radiation.

Unfortunately, even stabilized synthetics suffer long-term damage if constantly exposed to UV radiation, which can make them lose some of the qualities of the material. The durability of stabilized polyethylene can be estimated at 3 - 5 years, that of the corresponding butyl rubber is even higher.

The multi-layered construction of DENSOLEN tapes and of the DENSOLEN systems means that the coating as a whole has long-term UV stability. The butyl rubber outer tape layer protects the tape layers and the layers of wrapping beneath. Even if this outer layer were to have its material qualities destroyed by UV radiation, it would still continue to keep the UV radiation off the underlying layers of tape and wrapping.

The formation of a certain surface roughness and weathering of the outer butyl rubber layer ("elephant hide") is not a fault of the tape system and is caused by weathering.

What is the importance of a coating's capacity for self-healing?

The ability of anti-corrosion materials to close up a small spot of damage in the coating by the cold flow of the coating material is known as the self-healing effect.

Picture 1: Principle of  "Self-healing" in a laboratory test

Self-healing effects only occur in materials with very soft  adhesive coatings, which can flow independently without pressure or under very light pressure. Such materials naturally have  a low peel and shear resistance. To put it another way, the capacity to "heal" small spots of damage increases the danger of the coating being damaged by peel and shear stress.

Self-healing, then, in no way means complete restoration of the coating quality, but merely the electrically insulating covering of the steel surface with a soft adhesive compound. The firmer carrier materials, which make the major contribution to a coating's mechanical resistance, are not involved in the self-healing. The result is that the mechanical strength of the coating is clearly less at the "healed" spot than in its undamaged original condition. Self-healing, therefore, cannot be an alternative to genuine repair of flaws that occur before pipe laying and trench filling.

Also, where defects that do not arise until the pipelines are in operation are involved, one cannot be very confident that a "healed" flaw with lower mechanical strength will be able to withstand long-term the mechanical stress that has already caused the damage to the far more resistant original coating.

A further ideal condition in Picture 1 and in the customary "laboratory conditions" for demonstrating self-healing effects is the fact that the adhesive can flow freely, i.e. that the cause of the damage has been removed from the surface of the pipe. In practice, of course, this scenario is unrealistic. It is more likely that the cause of the damage, generally a rock or stone, will continue to press into the coating. The consequence is that the adhesive - in contrast to the laboratory trial - cannot flow freely or indeed that the ground or parts of the ground are preventing the flow of the adhesive. This means that there is no difference in practice between self-healing and non-self-healing coatings. It is therefore not surprising that the effectiveness of self-healing coatings under industry practice conditions has not been demonstrated even today.

Conclusion 

  • Self-healing does not mean the complete restoration of the original coating quality.
  • Self-healing only functions if
    • the flaws are small in area
    • the adhesive flows freely
  • Under laboratory conditions, self-healing anti-corrosion coatings can close up a flaw of a defined size. The resistance of the coating, its mechanical strength and the thickness of the layers are however less than is the case with the original undamaged material.
  • Self-healing is a laboratory effect; its efficiency in practice has not as yet been demonstrated. 
  • The capacity for self-healing brings with it an increase in the risk of coating damage due to peel and shear stress.