Novel TPVs exhibit excellent adhesion to textile fibers during melt processing.

Engineered thermoset and thermoplastic elastomers are used in combination with reinforcing materials such as textile cords and fabrics for engineered composites and mechanical rubber goods. Some examples of these applications are industrial hose, lightweight conveyor belting and fabric reinforced sheet. The level of adhesion between the elastomer and the reinforcement plays a major role in establishing the ultimate strength and overall performance of the finished composite structure. In general, elastomers require the use of a yarn treatment or adhesive system to facilitate adhesion to the reinforcing substrate.

Treated yarns are chemically adhered to the thermoset rubber materials during the vulcanization step. These treated yarns chemically bond through the adhesive polymer's reactive end groups. These end groups typically have hydroxyl, carboxyl and amine functionality. The principle treatments used for adhesion of the yarn to thermoset elastomers are based on resorcinol, formaldehyde and latex (RFL), or isocyanate systems (ref. 1). The application of these liquid yarn treatments takes place during a dip operation. The adhesion of the dip treatment to the yarn surface then occurs during drying and curing by either direct or indirect fired ovens.

The resulting adhesion between the fiber and the dip, and the dip to the thermoset rubber is a strong function of the curing time and temperature experienced following the dip operation. If the combination of time and temperature is insufficient, then the level of adhesion will be unsatisfactory between the treatment and the yarn. If the dipped yarn is overexposed during curing and drying then the adhesion between the treatment and the fiber may be satisfactory but the bond between the treatment and the thermoset rubber will suffer because of fewer reactive groups in the dip layer.

The adhesion of thermoplastics to textile reinforcement in flexible composite structures takes place through the use of reactive and/or hot melt adhesives usually applied after melt processing. Melt processing is a generic term used for describing thermoplastic forming processes. These processes include injection molding, extrusion, blow molding and thermoforming. The cycle times associated with these processes are very short in comparison to the analogous thermoset rubber operations due to the vulcanization step. The lack of time available at elevated temperatures associated with thermoplastic processing prevents the widespread use of dip treatments for the adhesion of thermoplastic elastomers to yarns.

As a result, fibers are typically adhered to the formed thermoplastic surface using some type of structural adhesive. For instance, in the manufacture of high pressure thermoplastic hose, fibers are bonded to the thermoplastic tube and cover materials using moisture cure urethane adhesives. These adhesives generally require 72 hours to achieve a full cure. In addition, these adhesive systems tend to add both weight and stiffness to the finished construction. The decrease in flexibility is due to the adhesive's tendency to wick into the braided reinforcement. Once cured, the adhesive impregnated fibers cause increased stiffness and reduced flexibility of the hose assemblies.

The present work covers the development of novel TPVs which have been formulated to exhibit excellent adhesion to polyamide and polyester reinforcing materials during melt processing. These novel TPV formulations have shown peel strengths greater than 50 N/2cm (15 pli) to either polyester and polyamide fibers during crosshead extrusion. This excellent level of adhesion to textiles combined with the lightweight and flexible characteristics of TPVs contributes to improved performance of engineered composite structures.

Experimental

Evaluating existing adhesive techniques

This technical effort initially focused on developing a method for adhering TPVs to fiber reinforcement to enable greater penetration into flexible composite applications and constructions.

The greatest barrier for widespread acceptance of olefinic TPVs in fiber reinforced constructions has been due to its poor adhesion characteristics. Olefinic TPVs, which are produced by the process of dynamic vulcanization, typically 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 types of adhesives exhibit higher surface energy than polypropylene or polyethylene and, as a result, tend to bead up on the surface of the TPV, exhibiting poor wetting, rather than form a continuous adhesive film.

The low surface energies of these olefinic TPVs can be improved by using surface treatments and primers. Mechanical pretreatment such as plasma discharge, flame treatment and chemical/solvent treatments are used throughout the marketplace to improve adhesion to thermoplastic olefins (refs. 2-4). 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 pretreatment is related to the degree of oxidation imparted to the adhering surface. Oxidation of the polyolefin results in an increase 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 (ref. 5). 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 their 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.

Table 1 shows the effect of pretreatments on the surface energy of olefinic TPV materials. Untreated TPVs all exhibit very low surface energy. The corona and flame pretreatments were carded out 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 undergoing either type of treatment. The response of the TPV to the surface pretreatment improved as the hardness of the material increased. This improvement can be correlated to the level of polypropylene 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 would be the choice surface pretreatment used in combination with conventional adhesive systems.

Table 1 - the effect of surface pretreatments on
TPV surfaces (x[10.sup.-5]N/cm)

TPV          Untreated     Flame      Corona        CPO
                         treatment   treatment   treatment

55 Shore A     30-32       40-42       40-41       34-36
75 Shore A     30-32       44-45       41-42       36-38
50 Shore D     30-32       55-56        64         36-38

The prevalent adhesive systems used in industry to bond elastomers to yarns for high pressure thermoplastic hose are single component urethane adhesives. These moisture cure urethanes have been used for over 20 years to bond copolyetherester, polyamide 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 olefinic 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 performed better than the corona and flame surface treatments. 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 of 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 was still too low for consideration in flexible composite applications. The flexible composite industry as a whole is looking for levels of greater than or equal to 42 N/2 cm adhesion.

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

Adhesive       Pretreatment    Type    Peak peel   Average
                                          str.     peel str.
                                        N/2 cm      N/2 cm
                                         (pli)       (pli)

Solvent based   CPO primer   Moisture   28.0 (8)   21.0 (6)
 urethane                     cure
100% solids     CPO primer   Moisture  26.3 (7.5)  21.0 (6)
 urethane                     cure
Solvent based     Corona     Moisture   3.5 (1)    1.8 (0.5)
 urethane                     cure
100% solids       Corona     Moisture   3.0 (1)    1.8 (0.5)
 urethane                     cure
Solvent based     Flame      Moisture   3.0 (1)    1.6 (0.5)
 urethane                     cure
100% solids       Flame      Moisture   3.0 (1)    1.8 (0.5)
 urethane                     cure

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

Development of a textile bondable TPV

Based on the poor results obtained using conventional adhesive techniques, an effort was initiated to develop a TPV material which would adhere directly to the fiber surface during melt processing without the use of pretreatments, adhesives or yarn dip treatments.

Standard olefinic TPVs generally exhibit poor adhesion to polyester and polyamide fibers during melt processing. Typical values for adhesive peel strength to textile fibers range from 1.0 to 3.0 N/2cm. This low level of adhesion is due to olefinic TPVs' inherent lack of reactive functionality and poor physical and chemical bonding to the textile fiber. This low level of adhesion was observed even when the single yams were fully embedded and mechanically locked within the TPV during high pressure compression molding of peel samples. In closed braided composite constructions, this inability to adhere to the textile fibers would be detrimental to the reinforced product's overall performance.

Statistically designed experiments were used for the development of a new TPV which would have a minimum of 50 N/2cm (15 pli) adhesion to both polyester and polyamide fibers during melt processing. Screening experiments were used to assign and quantify the factor and interaction effects of the TPV ingredients during the initial stages of the development. Analysis of the screening studies indicated which factors were significant in affecting the adhesive performance. The screening experiments used during the development of this TPV were: Plackett-Burman designs; fractional factorial designs; and factorial designs

Plackett-Burman designs are very good at determining the factor effects, while the factorial designs predict the significance of the interaction terms in addition to the factor effects. Interaction effects are attributed to the synergistic behavior of two or more variables. An interaction term may have significant impact on properties even though a factor contained in the interaction term may not be significant as a primary effect.

Response surface designs were used for optimization of the TPV formulation once the significant factors were identified using the screening type of experiments. Examples of response surface designs are: Central composite designs; Box-Behnken designs; and face-center cube designs.

A series of central composite designs were used during the optimization and scale-up of the textile bondable TPV.

This technical overview focuses on the identification of key ingredients which improve the adhesion of TPVs to various yams in the melt as well as the final optimization studies used in the development of this grade of TPV.

Discussion of the results

A five factor fractional design was one of the screening studies designed to analyze the effect of various polymers and modifiers on the TPV's ability to adhere to polyester and polyamide yams. In this design, the fifth factor was deliberately confounded with the interaction of the four remaining factors, thereby enabling the estimation of the primary factor and first order interaction effects. The advantage for choosing this design is the reduction in compounding and testing time. This design calls for 16 formulations in comparison to the full factorial design which requires 32 formulations.

Polymer modifiers A, B and C were included to identify the significance of their contribution to adhesion, physical properties and processing characteristics. The term "polymer modifiers" describes both thermoset and thermoplastic components of the TPV which potentially enhance the adhesive characteristics. Levels of oil and filler were also evaluated to determine their effect on adhesive performance, as well as possible improvement in the formulation economics.

Table 3 shows the levels of the variables used for this screening analysis. Table 4 summarizes the analysis of these five factors and their first order interaction effects on the resulting peel strength of the developmental TPV formulations bonded to woven "adhesive activated" polyester yarn. The adhesive activated spin finish typically used for polyester fibers is a dilute aqueous-based epoxy solution applied by the fiber manufacturer (ref. 1).

Table 3 - specification of levels for the five factor
fractional experimental design

Variables - factors   Description    -1       1

X1                    Polymer A     5 phr   30 phr
X2                    Polymer B     5 phr   30 phr
X3                    Polymer C     5 phr   30 phr
X4                    Oil A         5 phr   40 phr
X5                    Filler A      5 phr   40 phr
Table 4 - 90 [degrees] peel strength analysis for the experimental
TPVs bonded to adhesive activated polyester yarn

Trial   X1   X2   X3   X4   Response   Effect     Factor
                             Peel      estimate
                            strength    Peel
                             (pli)     strength
                                        (pli)

  1     -1   -1   -1   -1      24       32.0       mean
  2      1   -1   -1   -1      42       15.6         X1
  3     -1    1   -1   -1       7        6.6         X2
  4      1    1   -1   -1      39        6.4       X1X2
  5     -1   -1    1   -1      33        3.2         X3
  6      1   -1    1   -1      32       -5.0       X1X3
  7     -1    1    1   -1      33        5.6       X2X3
  8      1    1    1   -1      42       -3.3       X4X5
  9     -1   -1   -1    1      16        0.9         X4
 10      1   -1   -1    1      40        1.4       X1X4
 11     -1    1   -1    1      34        8.0       X2X4
 12      1    1   -1    1      42       -9.8       X3X5
 13     -1   -1    1    1       5       -3.8       X3X4
 14      1   -1    1    1      42        5.7       X2X5
 15     -1    1    1    1      42       -1.6       X1X5
 16      1    1    1    1      40       -2.0         X5

The response column of table 4 shows the actual mean value for the peel strength of the TPVs to the polyester yarn for each trial (formulation) evaluated. These trials were randomized during compounding and testing. Significant effects were determined by quantitatively comparing both the magnitude of the assigned estimated effects and by comparison with the overall mean value for the study which is listed in the estimated effects column.

The analysis in table 4 shows that the magnitude of the factor effect of the polymer modifier A was 15.6. This value was large compared to the magnitude of the other factors, interactions and the average value for peel strength for all trials which was 32 pli. The addition of polymer modifier A over the range of levels chosen for the design revealed a significant positive increase in peel strength between the TPV and the woven yam as the level increased. It is important to note that the sign of the estimated effect signifies whether the variable has a positive or negative effect on the response measured. A factor which has a significantly large negative effect will show a decrease in peel strength as the factor level increases.

Another significant positive contribution to peel strength was observed after the level of polymer modifier B was increased. This factor was judged to be significant, even though the magnitude of this value was less than that observed for polymer modifier A. The increase in polymer modifier C, oil and filler level did not show a significant effect on the TPV's overall ability to adhere to the polyester yarn. Some of the interaction terms were significant in magnitude relative to the overall mean. These combinations of factors were to be treated as either significant or at least controlled during subsequent design of experiments to minimize variability on the compounded properties.

Physical properties of the TPVs were also evaluated as responses during these screening studies to ensure that the engineered characteristics of the various compounded materials were maintained.

Response surface designs were then constructed for the optimization of key variables such as polymer modifiers A and B which were identified during the screening studies like those shown in tables 3 and 4.

Central composite designs were used for generating response equations for the determination of the optimum formulation. Multiple response equations for two or three variables can be quickly optimized by layering of contour plots for these equations. Response equations are determined through regression analyses of the data. Equation 1 is an example of a regression equation for two independent variables.

(1) Response = [A.sub.0] + [A.sub.1]X + [A.sub.2]Y + [A.sub.11][X.sup.2] + [A.sub.22][Y.sup.2] + [A.sub.12]XY

Figures 1-3 are contour plots constructed from the data analysis of a central composite design used for optimization of polymer modifiers A and B. The axis of these plots represent the actual weight percent of these two ingredients used in the TPV formulations. One approach used for determining the optimal formulation window for the textile bondable TPV was through inspection and overlay of contour plots.

[GRAPHS OMITTED]

Figures 1-3 show the trends in adhesive peel and ultimate tensile strength as the levels of polymer modifier A and B change. Other responses were measured and were included in the optimization analyses. Visualization through the use of contour plots was very useful in developing intuition and knowledge of compounding for these engineered properties.

The regression analyses were validated by using various statistical tests to ensure the accuracy of the findings. The plots were then generated.

The difficulty in utilizing this approach lies in visualizing the formulation window for more than two measured responses or observations. The desirability function approach was utilized to simultaneously optimize the TPV formulation for the multiple response equations. A comprehensive treatment of this approach is referenced in Deringer and Suich (ref. 6) and Harrington (ref. 7). In brief, the observed physical property and peel strength response equations were transformed into individual desirability functions which were optimized using univariant techniques. These multiple responses were then imbedded into an overall response function. This overall desirability function was then analyzed to determine the optimal formulating region used for the textile bondable TPV grades.

Tables 5 and 6 show the physical property characteristics of three optimized TPVs which adhere to both adhesive activated polyester and polyamide yarns during melt processing. In addition to their unique adhesion characteristics, these TPVs exhibit excellent abrasion resistance and are very low in specific gravity. The abrasion resistance of these materials is very beneficial to engineered elastomeric applications such as hose and belting, while the low specific gravity enables both lighter weight and lower cost products.

Table 5 - physical property summary for textile
bondable TPVs

Material                       TPV A      TPV B      TPV C

Hardness                        65A        80A        85A
Tensile Strength (Mpa)          8.3        10.3        11
Elongation (%)                  650        700        800
100% modulus (Mpa)              2.4        3.1         4
Specific gravity                0.9        0.9        0.9
Abrasion-1,000 cycles          40 mg      25 mg      25 mg
90 [degrees] peel strength
Polyester - woven            105 N/2cm   55 N/2cm   50 N/2cm
Nylon 6/6 - woven               -        25 N/2cm   50 N/2cm
Table 6 - peel strength of the textile bondable TPV C to various yarns
(N/2cm)

                  Supplier A        Supplier A        Supplier B

Fibers             Polyester         Polyester         Polyester
(Greige)         1,100 denier      1,100 denier      1,100 denier
Spin finish           Oil          Adhesive act.     Adhesive act.
Peel str.             21               105+              105+
 (N/2cm)

                  Supplier C        Supplier D

Fibers             Polyester         Polyamide
(Greige)         1,100 denier      1,260 denier
Spin finish      Adhesive act.         None
Peel str.            105+              105+
 (N/2cm)

Table 6 shows a comparison of the adhesion characteristics of the textile bondable TPV on several different yarns. The textile bondable TPV demonstrates excellent adhesion to the adhesive activated polyester fabrics in comparison to the polyester yams which utilize a standard oil spin finish. As mentioned earlier, the adhesive activated spin finish typically used for polyester fibers is a dilute aqueous-based epoxy solution applied by the fiber manufacturer (ref. 1). This type of spin finish adds minimal cost to the overall construction while increasing the adhesive peel strength to the textile bondable TPV. The type of spin finish on the polyester fiber was very important for achieving high levels of adhesion to the developmental TPVs, whereas adhesion to the polyamide 6/6 fiber was independent of the surface finish.

Conclusions

Using design of experiments, a series of novel, high performance TPVs were successfully developed to adhere to textile yarns during melt processing. Unlike traditional olefinic TPVs, these adhere very well to common yarns and fabrics without the use of adhesive systems. The level of adhesion to textiles during melt processing far exceeded the minimum requirement of 42 N/2cm peel strength. This unique performance attribute opens up new market and application opportunities to olefinic TPVs. These textile bondable TPVs offer the following properties in comparison to other engineered thermoplastic and thermoset elastomers: Lighter weight; lower finished goods cost/lower specific gravity; excellent abrasion resistance; improved flexibility; matte finish; and exceptional flex fatigue

The combination of these properties and characteristics allows for the replacement of competitive thermoset and thermoplastic elastomers in reinforced elastomeric constructions. Examples of reinforced TPV products include applications like industrial hose, lightweight conveyor belting and fabric reinforced sheet products. These novel TPVs are a good fit for applications where cost savings is an important factor in a product's development.

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.

(5.) M.D. Ellul and D.R. Hazelton, "Chemical surface treatments of natural rubber and EPDM thermoplastic elastomers: Effects on friction and adhesion," Rubber Chemistry and Technology, Vol. 67, No. 4, Sept. - Oct. 1994.

(6.) Deringer and Suich, "Simultaneous optimization of several response variables,", Journal of Quality Technology, Vol. 12, No. 4, October 1980.

(7.) Harrington, E.G. Jr., "The desirability function," Industrial Quality Control, Vol. 21, No. 10, 1965, pp. 494-498.