High pressure thermoplastic hose constructions utilizing TPVs.

Engineered thermoset and thermoplastic elastomers have been used successfully in hose constructions for the transfer of gas, liquid and solid materials, as well as for the transmission of energy. Elastomers have been used to provide flexibility to the hose construction while improving strength for burst resistance in both static and dynamic applications. This balance of performance in elastomeric properties must also accommodate hose motion, misalignment, vibration and portability. Flexibility of the hose is very important to enable easier routing and installation. Fittings and couplings are attached to the hose ends to facilitate connection of the hose to a pressure source.

The vast majority of hose constructions consists of the three elements shown in figure 1. The three components, tube, reinforcement and cover, are joined by some adhesive method to provide optimal performance. These composite constructions derive synergistic benefits from combining each material's inherent attributes. The materials used for cover and tube constructions are chosen based on their abrasion, fluid and temperature resistance.

[Figure 1 ILLUSTRATION OMITTED]

The requirements for the elastomeric components vary considerably, depending on the end use applications. Historically, the most demanding applications have been in the transfer of hydraulic fluids. The requirements for these hose constructions are typified by high working pressures, a wide use temperature range and fluid resistance.

Improper use of the hose or the use of a hose not designed for the intended service can result in personal injury and equipment damage. Factors which affect the life of a hose are flexing below the minimum bend radius, twisting, kinking, crushing and abrading the construction.

The present work covers the use of flexible TPVs (thermoplastic vulcanizates) as replacement materials for other engineered thermoset and thermoplastic elastomers in hydraulic hose constructions and specialty fluid transfer applications. The standard designations for thermoset hydraulic hose constructions are SAE 100RI and SAE 100R2, while the corresponding designations for the thermoplastic constructions are SAE 100R7 and SAE 100R8.

The SAE 100RI and SAE 100R2 constructions consist of a crosslinked nitrile rubber tube reinforced with one and two layers of wire braid reinforcement, respectively. These reinforced tube assemblies are covered with a crosslinked polychloroprene sheath to protect the braid from corrosive environments and abrasion. The SAE 100R7 and 100R8 thermoplastic analogues utilize a copolyetherester fluid resistant tube reinforced with a closed braid layer(s) of nylon or polyester fibers and covered by a thermoplastic urethane material for abrasion and splash fluid resistance. Both the thermoset and thermoplastic hoses are rated for use from 2,500 psi to 5,000 psi working pressures at temperatures ranging from -40 [degrees] F to 200 [degrees] F.

The function of the tube is to provide a pathway for different types of materials to be transferred. The tubing material must demonstrate satisfactory fluid resistance over the desired application temperature range of the hose. Failure to provide adequate fluid resistance results in weepage or permeation of the transfer medium through the tube assembly. The migration of transfer fluid through the tube often occurs during the life of the hose and represents the time at which degradation takes place and the assembly has to be replaced. Generally, the remainder of the hose assembly, the braided reinforcement and the cover material, offers very little resistance to migration of the transfer fluids and as a result rapid deterioration of the hose assembly occurs. In high pressure hose applications this can be a very hazardous situation and can result in an unsafe work environment and equipment failure. Rupture of the hose assembly primarily occurs at locations of severe degradation and/or at the couplings.

Coupling retention is an important property of any given hose construction. The coupling is the location on the hose assembly where the ends are joined. Coupling retention is a function of the tube, cover, braid and fitting design. Satisfactory coupling retention is also a reflection of how well the three components of the hose are adhered to one another.

Material properties which are considered to influence the coupling retention of a hose assembly are compression set, stress relaxation and the modulus versus temperature performance of the material. Good elastomeric candidates for hose tube and cover materials demonstrate low compression set, low stress relaxation and stable properties over the end use temperature range. Sometimes, materials that exhibit greater stiffness and higher compression set, which can be related to poor coupling retention, have been accommodated by using alternative fitting designs and crimp diameters.

The primary function of the cover is to provide a barrier layer for the braided tube assembly from potential corrosion, abrasion and other physical damage. Since the reinforcing braid in a closed braid construction is the major contributing component to the overall pressure rating of the hose, it is imperative to protect it from damaging elements.

The reinforcement braid angle is very important to the overall hose performance. The purpose for the selection and designation of the braid angle is to balance the hoop stresses with those of the end stresses for a given hose construction. This angle is called the "neutral" angle and is equivalent to 54 degrees and 44 minutes. When the braid angle is greater than "neutral," the hose will tend to increase in length and the diameter will decrease upon pressurization. If the braid angle is less than "neutral," the hose will decrease in length and the diameter will increase when pressurized. Coupling and fitting failures can result from increases in hose diameter during hose pressurization.

It is through the use of the reinforcing braid that the hose is capable of performance at moderate to high pressures. The key to high pressure hose constructions is the linkage and adhesion of the tubing and cover materials with that of the reinforcing braid. This reinforced composite construction enables the transfer of material at high pressures over a wide temperature range.

Experimental

The scope of this study focused on constructing a thermoplastic hydraulic hose utilizing TPVs as both tube and cover materials and evaluated various methods to promote adhesion.

The greatest barrier for widespread acceptance of TPVs in closed braid hose constructions has been due to the lack of adhesion to the reinforcing braid. TPVs, produced by the process of dynamic vulcanization, consist of crosslinked EPDM rubber particles in a continuous phase of polypropylene. The olefinic or non-polar nature of the TPV makes it difficult to bond to different surface chemistries using conventional adhesive systems. Polypropylene, for example, exhibits low surface energy levels of 30-33 dyne/cm. The low surface energy of olefins and the lack of polar, functional groups make it difficult for satisfactory wetting using epoxy, urethane and acrylic structural adhesives. These adhesive types exhibit higher surface energy than polypropylene or polyethylene and, as a result, tend to bead up on the surface of the TPV rather than form a continuous adhesive film.

The low surface energies of these TPVs can be improved by using surface treatments and primers. Mechanical pretreatments such as plasma discharge and flame treatment, and chemical solvent treatments are used throughout the marketplace to improve adhesion to thermoplastic olefins. The overall impact of these pretreatments on the adhesive characteristics of TPVs varies, depending on many process and compositional factors. The level of rubber and its composition within the TPV formulation have a significant impact on the adhesive performance.

The effect of mechanical pretreatments is related to the degree of oxidation on the adhering surface. Oxidation of the polyolefin results in increases of the surface energy, thereby making it easier to adhere and coat. The ability to oxidize a surface is also strongly dependent on the composition of the TPV. Natural rubber based TPVs have been shown to be receptive to halogenation treatments. Oils and fillers, which are prevalent in the crosslinked rubber phase of a TPV, will deter oxidation and surface modification.

Primers such as chlorinated polyolefins (CPO) have been used successfully as "tie-coats" to improve the paintability and adhesion of thermoplastic olefin surfaces. These primers, which often are carded in solvent, increase the surface energy of the TPV through the polar functionality. Equally important, these materials impart decreased crystallinity on the surface of the TPV, resulting in improved adhesion performance using conventional coatings and structural adhesive systems.

Results

Table 1 shows the effect of pretreatments on the surface energy of TPV materials. Untreated TPVs all exhibit very low surface energy. The corona pre-treatment was conducted on flat material stock at a line speed of 17 feet per minute. There was a marked improvement in the surface characteristics of the TPV after corona treatment. The response of the TPV to the corona treatment improved as the hardness of the material increased. This improvement can be correlated to the level of thermoplastic in the TPV. The chlorinated polypropylene primer evaluated in this study did not show the same improvement in surface energy. Based on these results, corona treatment would be the choice surface pretreatment used in combination with conventional adhesive systems.

Table 1 - the effect of surface pre-treatments on TPV
surfaces

                                Corona             CPO
TPV           Untreated        treatment        treatment

55 Shore A    30-32 dyne/cm   40-41 dyne/cm   34-36 dyne/cm
75 Shore A    30-32 dyne/cm   41-42 dyne/cm   36-38 dyne/cm
50 Shore D    30-32 dyne/cm      64 dyne/cm   36-38 dyne/cm

The prevalent adhesive systems used within the high pressure thermoplastic hose industry are single component urethane adhesives. These moisture cure urethanes have been used for over 20 years to bond copolyetherester and TPU materials to textile reinforcement. These liquid systems are available in solvents, as well as in 100% solids form. Generally, it takes 72 hours for the adhesive system to fully cure with moisture to develop final adhesive properties. Once crosslinked, these urethane systems exhibit excellent fatigue and temperature resistance.

Table 2 shows the adhesion results for a 64 Shore A hardness TPV bonded to E-coated steel using surface pretreatments and moisture cure urethane adhesives. The peel strength results show that the chlorinated polyolefin based primer treated TPV surface adhered better than the corona treated TPV. The mode of failure for all of the samples was adhesive to the TPV pretreated surface. These peel results in combination with the data in table 1 indicate that adhesion between the urethane and the TPV is not solely dependent on the wettability of the surface. There may be an influence on the surface morphology of the TPV which facilitates greater adhesive peel strengths using chlorinated polyolefin primers in combination with conventional adhesive systems. While the primer treated TPV gives better overall peel strengths, the level of adhesion is still too low compared to that required for typical hose applications. The industry as a whole is looking for 42 N/2 cm adhesion.

Table 2 - adhesive peel strengths of a 64 Shore A TPV
on E-coated steel

Adhesive           Pretreatment           Type

Solvent based      CPO primer         Moisture cure
  urethane
100% solids        CPO primer         Moisture cure
  urethane
Solvent based      Corona             Moisture cure
  urethane
100% solids        Corona             Moisture cure
  urethane

Adhesive           Peek peel str.       Average peel
                   N/2 cm (pli)       str. N/2 cm (pli)

Solvent based       28.0 (8)             21.0 (6)
  urethane
100% solids         26.3 (7.5)           21.0 (6)
  urethane
Solvent based        3.5 (1)              1.8 (0.5)
  urethane
100% solids          3.0 (1)              1.8 (0.5)
  urethane

The adhesion results show that in order to use a structural adhesive system to bond the TPV to reinforcing fibers or wire, the TPV surface must be pretreated with a primer. Many of these primers are solvent based. While these pretreatments improve adhesion strength, the failure modes are typically adhesive to the TPV surface. Another disadvantage of these systems is that assembly times for the moisture cure urethanes vary with environmental conditions.

The goal of the project was to develop an all thermoplastic hose construction using TPV materials that demonstrates high pressure and fluid resistant performance. The choice of reinforcement for the hose construction was important in establishing the high pressure rating of the hose assembly. Steel wire was chosen based on its ability to mechanically lock onto the thermoplastic tubing material during the braiding operation. The fabrication issues were then focused on developing adhesion between the TPV cover and the wire reinforcement.

The study evaluated an alternative approach to using liquid adhesive systems for a bonded TPV assembly for use in specialty fluid transfer applications. This work was conducted to replace solvent-based adhesives and other necessary pretreatments for the fabrication of TPV hose assemblies. Due to the low hydraulic fluid resistance exhibited by tubing made with olefinic TPV systems, a fluid resistant barrier layer was coextruded with the TPV to replace nitrile thermoset elastomers and copolyetherester materials as tubing materials. Nylon 6 was chosen as the inner portion or sleeve of the tube construction to provide the required fluid resistance. A 0.2 mm wall of the polyamide was coextruded with 0.6 mm of an 85 shore A nylon bondable grade of TPV to provide flexibility and fatigue resistance to the tube construction. The nylon bondable TPV grade was developed to adhere directly to polyamide 6 materials during insert molding and co-extrusion. The functionality present in this TPV allows for adhesion to metal surfaces.

The coextruded tube was reinforced with brass plated steel braid commonly used in thermoset hose and belting applications. An elastomeric cover/jacket made of the same nylon bondable TPV was then crosshead extruded over the reinforced hose carcass. An alternative design utilized a coextruded tube consisting of a maleic anhydride modified polyolefin tie layer in combination with a standard olefinic TPV crosshead extruded over the reinforced tube to function as the cover material. Anhydride and/or acid functionality provides adhesion to the metal surface, while the olefinic backbone provides adhesion to the continuous thermoplastic phase of the TPV system.

The wire reinforcement was mechanically fixed to the nylon bondable TPV tube surface under tension during the braiding operation. This mechanical lock was sufficient to provide adequate resistance to burst and kinking.

Table 3 shows the adhesion performance of several combinations of TPVs to wire reinforcement using various tie layer adhesive systems. The data shows a marked improvement in adhesion using both tie layer adhesives in combination with a conventional olefinic TPV and the nylon bondable TPV. The "T" test data show excellent adhesion to the wire reinforcement using the modified polyolefins and the nylon bondable grade.

Table 3-TPV adhesion results to wire reinforcement
(ASTM D 2229-73)

TPV                 Tire layer adhesive    Wire type and size
                                           (mm)

80 Shore A          None                   Brass plated (0.36)
80 Shore A          Adhesive A             Brass plated (0.36)
80 Shore A          Adhesive B             Brass plated (0.36)
80 Shore A          Adhesive C             Brass plated (0.36)
85 Shore A          None                   Brass plated (0.36)
 nylon bondable

TPV                 Adhesion
                    (N/4 cm)

80 Shore A           50
80 Shore A          520
80 Shore A          450
80 Shore A          410
85 Shore A          390

This level of adhesion between the TPV and the reinforcing wire enabled these hose designs to pass stringent impulse and burst testing. A primary discovery was the excellent fluid resistance demonstrated by the hose assembly. The multilayer hose was pressurized with hydraulic fluid at 100 [degrees] C for one month without weepage or leakage of the fluid through the hose layers or at the fittings.

The use of a tie layer adhesive in combination with a newly developed series of TPVs (TPV B) made for a promising hose cover construction. Figure 2 shows the comparison of abrasion resistance for several thermoplastic materials using a Taber abrader. The 90 Shore A hardness material, TPV B, compares very favorably in its abrasion resistance with a competitive polyether based TPU of identical hardness. TPV B also shows an improvement in abrasion resistance over more conventional TPV (TPV A) grades. This new series provides a potential alternative to thermoplastic urethane materials for hose cover and jacket applications.

[Figure 2 ILLUSTRATION OMITTED]

Conclusion

A thermoplastic hydraulic hose construction has been successfully fabricated utilizing TPV materials for the cover and as intermediate layers of the tubing assembly.

The preferred tube assembly consists of a thin layer of an impact modified polyamide 6 or a pure polyamide 6 resin to provide resistance to hydraulic oil migration and weepage. This fluid resistant layer was coextruded with a nylon bondable TPV which demonstrated excellent adhesion to polyamide materials in the melt phase. The coextrusion of a 0.2 mm thick inner layer of polyamide with the nylon bondable TPV as the outer tube layer provided a flexible, fluid resistant tube construction.

The reinforcing layer between the tube and cover was comprised of metallic wires which were braided to provide satisfactory resistance to burst during exposure in moderate to high pressure environments. Adhesion between the elastomeric TPV and the wire reinforcement was achieved through use of specific tie layer adhesives and modified TPV materials. Rubber tearing bonds were achieved between the TPV and the wire through the use of select adhesive tie layers and understanding the surface characteristics of the reinforcement.

The hose assembly fabricated utilizing the polyamide and TPV tube demonstrated satisfactory performance during extended testing. The hose assembly did not exhibit any weepage of the hydraulic oil after one month testing at 100 [degrees] C.

References

[1.] I. Skeist, Handbook of adhesives, New York, Chapman and Hall, 1990.

[2.] R.A. Ryntz, "Coating adhesion of low surface free energy substrates in the automotive industry," Polymer News, Vol. 18, pp. 101-106, 1993.

[3.] D. Briggs, "Surface treatments for polyolefins" in Surface Analysis and Pretreatment of Plastics and Metals, New York, MacMillian Publishing Co., 1982.

[4.] R.A. Ryntz, "Recent advances in the understanding of the paint adhesion mechanism to automotive plastics," Proc. 25th Anniv. Symp. Polym. Inst., pp. 308-30, 1994.