New TPE bonding technology and various overmolding processes for TPV applications. (Cover Story).
Novel bonding technology to various substrates
Olefinic based TPVs are typically co-processed via commonly used dual injection and extrusion processes along with polyolefin thermoplastic substrates such as polyethylene (PE) or polypropylene (PP). At the interface between TPV and the substrate, heat from the molten TPV melts the surface of the hard PE/PP substrate, fusing the compatible surfaces together to create high bond strength between the two materials. This practice has also been successfully applied to "filled" polyolefinic substrates which will be discussed later.
When applying this method to more polar engineering polymer substrates such as polycarbonate, polyamide or ABS, the low surface energy of the olefinic TPVs precludes fusion bonding. In many cases, the melt temperature of this TPV at standard processing conditions will not carry sufficient heat to melt the substrate. In many instances, adhesion promoters were used to induce a bond, but at a higher cost than the process if no adhesives were necessary.
With the recent availability of new alloys, many new TPVs are now available for bonding to many engineering thermoplastics, such as polyamide, polyester, ABS, PC, ASA, PMMA and PPO, without the need for adhesives.
The first of this series of materials is TPV alloys designed to bond to polyamide. These were introduced commercially in 1996 by the TPV supplier. The data in table 1 demonstrate the bond strength of two different hardnesses of this type of TPV to various substrates.
Optimum bond strength is achieved during two-shot injection molding, where the surface of the substrate is clean and warm for overmolding with polyamide bondable TPV. An insert molding process requires a preheat (up to 180 [degrees] C) of the substrate to achieve fusion of the surface and to develop good bond strength.
Many consumer hand tool and hardware handles successfully use polyamide bondable TPVs as the soft touch grip for comfort and for vibration damping.
The next progression was made in 1998, when another TPV was developed which bonds directly to polyester or polyamide fiber. These are commonly used as reinforcement for coated fabric, industrial hose and conveyor belting applications. These new resins do not require primer and adhesive. The elimination of primer and adhesive significantly increases the flexibility of the finished product while reducing finished product weight and production cost (ref. 2). Target peel strength for the interface between the elastomer and reinforcement is 21 N/cm. This new TPV generally exhibits values above 24 N/cm peel strength as shown in table 2.
In the last two years, many new TPVs have been developed to bond to various engineering plastics such as polycarbonate, ABS, ASA, etc. Table 3 illustrates the bond strengths from this new TPV to various substrates.
This brings the TPVs into many new application areas for medical equipment, electrical components, appliances, automotive, housewares and hardware.
Typical property comparison with many styrenic block copolymers (SBC) is shown in table 4. However, due to the non-vulcanized nature of these polymers, SBCs exhibit inferior performance to TPVs in the areas of high temperature performance, sealing properties (in terms of compression set) and fluid resistance. The new TPVs also offer cycle time advantage over many SBCs. Table 4 lists the data result of the comparison. Heat aging data are worth noting, as they simulate long term oxidation of the product in the air at room temperature.
Information in table 5, recording bond strength after a one-week exposure to commonly used fluids, provides additional guidance for design engineers when considering the environment to which the finished product will be exposed. Many SBCs exhibited good original bond strength which fell significantly under short exposure to these fluids. There are also differences in performance among SBC compounds.
Over the past few years, the multi-component injection molding of parts using combinations of thermoplastics and TPEs has taken on increasing importance. However, one of the main issues to overcome with these parts is developing adhesion between the substrate and the overmold. Adhesion is not only affected by the solubility parameters of the two materials, but also greatly influenced by the process parameters (e.g., temperature, pressure and time). The optimum conditions will vary, depending on the type of the substrate, and whether it is based on a crystalline or amorphous polymer. The effects of processing parameters on bond strength using both types of substrates, with focus on filled thermoplastic substrates, are discussed in the following section.
The effect of filler content on bond strength is shown in figure 1. Table 6 shows the substrates and the overmold studied. The high flow grades were selected for the study, since they can be more challenging for developing consistent adhesion in overmolding. The substrates selected are used in automotive interior and exterior applications such as flapper doors, cowl vent grill, etc. The grades selected for overmolding in this study have higher flow capabilities than general purpose grades of TPVs. Bond strength for high flow grades is more sensitive to process parameters, hence making them an ideal candidate for the study. TPV 121-50M100 has a flow ratio of 300-350, while TPV 111-45 has a flow ratio of 200-250. (Flow ratio is defined as the ratio of the longest flow length of the material per unit thickness).
[FIGURE 1 OMITTED]
Specimen preparation and testing
T bar specimens are molded using a two-shot process in an 80 ton Engel machine. Figure 2 shows the configuration of the T bar specimen. The bond strength was measured by pulling the tabs of the T bar specimen using a tensile tester. The rate of peel was 50 mm/sec. (2 in./sec.). The force to peel the materials apart was recorded as a measure of the bond strength. The peel strength results reported here are average values of five peel tests.
[FIGURE 2 OMITTED]
It has been shown by Skourlis et al. (ref. 3) that process parameters (pressure, temperature, speeds) play a role in determining the final skin/core structure for glass reinforced materials. In fiber reinforced injection molded parts, morphological characteristics such as the fiber length distribution and fiber orientation distribution, can vary markedly from position to position and through the thickness of the part. Specifically, fiber orientation is a result of the manner in which the cavity fills and is usually present in this form through thickness layers. The region close to the mid-plane of the part, where the flow is mainly extensional, is called the core region, and the region adjacent to the mold wall, where the flow induced orientation of the fibers takes place, is called the shell/skin region. The degree of molecular orientation (thickness of the skin and the core region) depends on processing variables like injection speed, melt temperature, mold temperature, cooling time and other variables (ref. 4). Control of such processing parameters while molding the substrate can result in significant changes in bond strengths.
In multicomponent molding, whether it is insert molding or two-shot molding, it is very important to achieve a resin rich substrate surface for optimum bond strength, since a thicker skin can hinder adhesion. The processing conditions for the substrate and the overmold are listed in tables 2 and 3, respectively. The processing conditions are selected as per (refs. 3-5).
Bond strength was measured by injection molding the substrate samples at the low end of their processing conditions. The processing conditions of the overmold and substrate combination are shown in tables 7 and 8. Hold pressure was changed when molding the substrates at the low end of the processing conditions. The overall cycle time for molding the T bar samples was 30 seconds. Figure 3 shows the difference in the bond strength at high level and at the low level of processing conditions. It can be seen that the bond strength significantly decreases when the substrate was processed at a low level of their processing conditions. This can be attributed to the rich fiber surface in the skin region. Mechanical properties of the samples are not tested.
[FIGURE 3 OMITTED]
Similar studies were performed on amorphous substrates such as ABS and ASA to study the effect of process parameters on bond strength. The single most important variable that influenced bond strength was the melt temperature.
Recent advances in TPVs that can heat fuse with several semicrystalline and amorphous substrates are commercially available. Depending on the type of application, the material and the process can be selected. The bond strength is influenced by processing parameters and will vary from amorphous and crystalline substrates. The influence is significantly noticed in fiber reinforced thermoplastics. A high injection speed, mold and melt temperature while processing fiber filled reinforced PP substrate will result in improved bond strength.
(This article is based on a paper given at the October, 2000 meeting of the Rubber Division.)
[FIGURE 4 OMITTED]
Appendix Property ASTM test method Tensile strength ASTM D 412 Elongation, ultimate ASTM D 412 100% modulus ASTM D 412 Specific gravity ASTM D 792 Hot air aging ASTM D 573 Compression set ASTM D 395, Method B Fluid resistance, % weight change ASTM D 471 * 1 Adhesion to fabric to rubber ASTM D 413 * 2 Peel resistance of adhesives (T-peel test) ASTM D 1876-95 Table 1 -- TPV bond strength to polyamide (refer to appendix for test procedures) Substrates TPV 55A, bond strength N/cm (pli) Polyamide 6 32 (18) Polyamide 6, 33% glass filled 30 (17) Polyamide 6, 6/6 and 33% glass filled 35 (20) Substrates TPV 85A, bond strength N/cm (pli) Polyamide 6 58 (33) Polyamide 6, 33% glass filled 56 (32) Polyamide 6, 6/6 and 33% glass filled 88 (50) Table 2 -- textiles bondable TPVs properties and performance (refer to appendix for test procedures) Properties TBTPV 65A TBTPV 80A TBTPV 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 90 [degrees] Peel strength N/cm Polyester -- Woven 26 35 44 Polyamide 6/6 -- Woven -- 25 50 Table 3 -- bond strength of 55A new TPV to various engineering plastics (refer to appendix for test procedures) Substrates TPV 55A bond strength, N/cm (pli) Polycarbonate (PC) 51 (29) PC with 30% glass filled 40 (23) PC/PBT 30 (17) PC/ABS 39 (22) ABS 33 (19) Polystyrene (PS) 51 (29) ASA 40 (23) PMMA 35 (20) Table 4 -- property comparison between TPV and SBC ** Properties * TPV 55A SBC SEBS #1 Bond to ABS N/cm (pli) 33 (19) 79 (45) Bond to PC N/cm (pli) 51 (29) 86 (50) Bond to PS N/cm (pli) 51 (29) 70 (40) IRM 903 oil (168 hrs. @ 23 [degrees] C) Survived Dissolved Cooling time, sec *** 25 85 Hot air Survived Product (672 hrs. @ 100 [degrees] C) (24% change in failed **** at properties) one week Comp. set (22 hrs. @ 70 [degrees] C) 65% 81% Properties * SBC SBC SEBS #2 SEBS #3 Bond to ABS N/cm (pli) 20 (11) 21 (12) Bond to PC N/cm (pli) 23 (13) 12 (7) Bond to PS N/cm (pli) 21 (12) 18 (10) IRM 903 oil (168 hrs. @ 23 [degrees] C) Dissolved Dissolved Cooling time, sec *** 45 35 Hot air Product Survived (672 hrs. @ 100 [degrees] C) failed **** (29% change at one in properties) Comp. set week (22 hrs. @ 70 [degrees] C) 97% 64% * Refer to appendix for test procedure ** Commercial SBCs (SEBS type) are chosen from both U.S. and German sources *** ISO plaque cycle time, thickness 2 mm **** Product cannot be tested Table 5 -- bond strength durability comparison * to ABS, unit: N/cm (% change) TPV 55A SBC SBC SBC SEBS #1 SEBS #2 SEBS #3 Original bond strength 19 45 11 12 Water 18 25 10.5 3.7 (168 hrs. @ 23 [degrees] C) (-5%) (-44%) (-5%) (-69%) IRM 903 Oil 5 (168 hrs. @ 23 [degrees] C) (-74%) Dissolved Dissolved Dissolved Ethanol 14 14 7.4 2 (168 hrs. @ 23 [degrees] C) (-26%) (-69%) (-33%) (-83%) 10% HCL 18 27 8.8 3.5 (168 hrs. @ 23 [degrees] C) (-5%) (-40%) (-20%) (-71%) 20% NaOH 19 25 11 7.5 (168 hrs. @ 23 [degrees] C) (No change) (-44%) (0%) (-38%) * Refer to appendix for test procedure Table 6 -- materials evaluated for bond strength No. Substrate Substrate Overmold Overmold characteristics characteristics 1 PRC25MG3 30% mineral and 121-50M100 Santoprene, glass reinforced PP high flow grade 2 PRC25MG3 30% mineral and 111-45 Santoprene, glass reinforced PP high flow grade 3 TPP30AJ41 30% talc filled PP 121-50M100 Santoprene, high flow grade 4 TPP30AJ41 30% talc filled PP 111-45 Santoprene, molding grade Table 7 -- processing conditions for different substrates at high and low level of processing conditions Substrate Melt temperature Injection speed Substrate Substrate Substrate Substrate Low High Low High [degrees] C [degrees] C mm/sec. mm/sec. ([degrees] F) ([degrees] F) (in./s) (in./s) PRC25MG3 193 (380) 237 (460) 12.7 (0.5) 101.6 (4.0) TPP30AJ41 187 (370) 232 (450) 12.7 (0.5) 101.6 (4.0) Substrate Mold temperature Substrate Substrate Low High [degrees] C [degrees] C ([degrees] F) ([degrees] F) PRC25MG3 21 (70) 43 (110) TPP30AJ41 21 (70) 43 (110) Table 8 -- processing conditions for different TPVs studied for bond strength Specimen Melt Injection Mold temperature speed temperature TPV overmold mm/s (in./s) overmold [dgrees] C [dgrees] C ([degrees] F) ([degrees] F) 121-50M100 221 (430) 89 (3.5) 27 (80) 111-45 221 (430) 89 (3.5) 27 (80)
(1.) Peter Bemis and Susan Braun, Bemis Manufacturing Co. Structure Plastics `99 Conference, April 1999, "Co-injection molding: Not just for recycling option, "pp. 91-93.
(2.) Marvin Hill, Advanced Elastomer Systems, 156th Rubber Division Meeting, September 1999, "Novel thermoplastic vulcanizates which exhibit excellent adhesion to textile fibers during melt processing."
(3.) T. Skourlis, S. Mehta, C. Chassapis and S. Manoocheri, "Impact fracture behavior of injection molded long fiber reinforced polypropylene," Polymer Engineering and Science, Vol. 38, No. 1, pp. 78-89 (1998).
(4.) R. Bailey and B. Rzepka, "Fiber orientation mechanisms for injection molding of long fiber composites," Intern. Polymer Processing, VI, pp. 35-41 (1991).
(5.) P.F. Bright and M.W. Darlington, "Factors influencing fiber orientation and mechanical properties in fiber reinforced thermoplastics injection moldings," Plastics and Rubber Processing and Applications, Vol. 1, No. 2, pp. 139-147 (1981).
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|Date:||Oct 1, 2001|
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