Additives to increase the service life of synthetic rubber hydronic heating hose.

Radiant heating by circulation of a fluid through hose buried in, or attached beneath, flooring has become popular because of the cost efficiency of the method and ease of installation, particularly in new construction. The use of synthetic rubber hose for transmission of the heating fluid is attractive because of the flexible yet rugged character of the product. Such hose consists of several layers. The outside layer makes use of one of several types of synthetic rubber having good resistance to oil or other spills, to high ozone levels in the air and to incidental cuts or damage by abrasion. Unlike many plastics, synthetic rubber compositions contain numerous added ingredients. The outer layer of the hose, the jacket, is formulated for strength and long term resistance to the elements. It may also be colored distinctively. A further object of formulation is to provide relatively easy processing into the final article, in this case by means of extrusion.

The inner layer of the hose, the hose tube, is formulated from synthetic rubber having, in addition to strength and resistance to damage, inertness to the fluid that will be circulated. Between these layers, a woven braid contributes to the burst strength of the hose.

Many of the hose tubes in the field are based on nitrile rubber (NBR), known for resistance both to water and various fluids. The recommended fluid for circulation in hydronic systems comprises half water, half a stabilized propylene glycol coolant. The proper coolant scavenges trace metals from the fluid and inhibits the growth of microorganisms, as well as providing protection from freezing. The resistance of commercial NBR hose tubes to such a circulating fluid is very good. Heating systems employing such hoses are best run in the range of 100 to 160 [degrees] F. Suitability for such service is determined by subjecting the hose tube to an unrealistically high temperature, immersed in the fluid. The results are demonstrated in figure 1, where the elastic modulus, or stiffness, is compared for samples of an unused hose tube with others that were subjected to accelerated aging of 100 hours at 250 [degrees] F in the above fluid. The vertical coordinate represents stiffness, the horizontal, the load placed on the sample. The data were obtained by dynamic mechanical analyses (DMA) of a number of samples.

[Figure 1 ILLUSTRATION OMITTED]

This severe exposure had very little effect on the rubbery response of the NBR hose tube. One would expect a very long lifetime at 100-160 [degrees] F. NBR is, however, less resistant to air at high temperatures. Its oxidation by air is catalyzed by copper and iron, available from metal components in the heating system. Therefore, a hydronic system that had been installed without adequate safeguard to prevent air entry (proper placement of correct expansion equipment) or had been operated at excessive temperatures, or had not used proper coolant containing metal-scavenging additives, or, more likely, some combination of these conditions, would show evidence of hose tube degradation. This proceeds as hardening, eventually leading to failure of the hose.

A number of such hardened samples was obtained from residential installations in the field. These were samples that had not cracked but could be bent only with some effort, as compared with the very flexible nature of unused hose. In figure 2, stiffness versus load is compared for partly hardened samples before and after accelerated aging at 250 [degrees] F.

[Figure 2 ILLUSTRATION OMITTED]

Several conclusions can be drawn. In addition to being stiffer than unused hose, the partly hardened samples also showed a much greater increase in stiffening with accelerated aging, and were much more sensitive to increases in applied load. In fact, after accelerated aging, these samples broke part way through the test.

In the unused hose tube, flexibility is gained by a balance in the recipe between the NBR rubber itself, solid ingredients (reinforcing fillers) that add strength and damage resistance, and liquid ingredients (plasticizers) that contribute to rubbery behavior and to ease of processing prior to vulcanization. The latter step, carried out after the hose is formed to the correct shape, irreversibly converts the rubber composition from one that can be processed into an article of permanent shape and set of properties. Vulcanization involves the reaction of several ingredients in the recipe to generate chemical bonds between the rubber molecules, forming a network. The correct extent of network formation is important in obtaining a useful set of physical properties.

It is a characteristic of NBR (and numerous other synthetic rubbers) that air oxidation at temperatures above the recommended service range has the effect of increasing the extent of vulcanization. The article becomes harder, stiffer and less flexible from having the network of bonds between the rubber molecules increased beyond what is optimum. Countering this tendency to become harder by oxidative degradation are additives, antioxidants, that interfere with the action of oxygen. Eventually, however, they are consumed. Service under conditions of hardening may also involve loss of plasticizers from the hose tube into the fluid medium.

It was, therefore, thought to investigate conditions under which additional antioxidant and plasticizer could be added to partly hardened hose tubes in the field in an effort to repair the effects of degradative hardening. To that end, a test stand was set up for circulation of fluid under recommended conditions. This is shown in figure 3.

[Figure 3 ILLUSTRATION OMITTED]

Using this apparatus, fluid based on 50% water, 50% propylene glycol coolant was circulated at 160 [degrees] F, considered the maximum normal service temperature for such systems. The plasticizer chosen for the study was oleyl nitrile, available from Harwick Chemical under the trade name Plasticizer OLN. This plasticizer is characterized by very high compatibility with NBR, particularly in comparison to aqueous media, maximizing the drive for its absorption into the hose tube. It is also resistant to hydrolysis by, or other reaction with, the circulating fluid. The antioxidant chosen was Vanox 1320, available from R.T. Vanderbilt & Co. It is a liquid analog of the BHT used in food packaging and other applications, a fast acting antioxidant completely soluble in OLN. Vanox 1320 is also resistant to reaction with the circulating medium.

The two ingredients were mixed in the proportion of five parts of OLN to one part of Vanox 1320. Although more than one level of addition was investigated, optimum results were obtained with the use 0.6 gm of the above combination per running foot of 3/8 in. hose. The corresponding levels are 0.8 gm for 1/2 in. hose and 1.4 gm for 3/4 in. hose, both again per running foot in the system. Fluid containing these levels of additives was allowed to circulate through the hose at 160 [degrees] F for 48 hours prior to determining the effect on stiffness. The results are shown in figure 4.

[Figure 4 ILLUSTRATION OMITTED]

Two conclusions are evident. The unused hose tube samples show very little effect and retain the characteristic resistance to accelerated aging in the recommended medium. The samples that had been partly hardened by service in the field were largely restored to the rubbery characteristics of the original product. None of the treated samples broke under load.

It must be emphasized that the evidence to date indicates that such repair can only be effected in an actual system in the field if the installation can be brought to ideal operating conditions, that is, operation within recommended temperature limits, use of a fluid containing metal stabilizers, such as 50/50 water/propylene glycol, and system design so as to prevent inadvertent entrance of air into the fluid.

If a particular system is properly designed but has suffered hose hardening through operation at unduly high temperatures or use of a poor choice of fluid, the following steps are suggested:

* Determine by contacting the supplier whether the hose tube is based on NBR. There are no data regarding the above treatment on other types of synthetic rubber.

* If the hose tube is based on synthetic rubber other than on NBR, the above additive package is of little value. It should be determined that the system is of reasonable design and is operating at specified temperatures with a recommended fluid.

* If the hose tube is NBR-based, the above treatment should be considered as a restorative measure, using proper circulating fluid, and avoiding excess temperatures. Sections of hose that show actual cracking should be replaced. In most cases, it will require an educated guess as to the exact footage involved. This should be done with some care; it must not be assumed that excess will be beneficial.

Richard Grossman is technical director with Halstab, a supplier of lead based and other heat stabilizers. He began his career in the rubber industry in 1957 with Anaconda Wire & Cable. He has also worked for Polysar, Synthetic Products, Cooke Color and Polymer Services.