Mutual attraction: mating hard & soft materials takes the right chemistry.

Mating Hard & Soft Materials Takes the Right Chemistry

TPEs are pairing off with rigid plastics in everything from soft-touch consumer goods to flexible automotive air ducts. New resins improve adhesion and broaden the range of combinations.

Hard-soft material combinations are popular in products ranging from personal care to lawn care to auto parts. This trend is keeping suppliers of thermoplastic elastomers busy stretching the range of rigid substrates to which their materials can bond. Hard-soft combinations have improved the aesthetics and ergonomics of housewares, toothbrushes, hair dryers, hand tools, lawn-care equipment, and sporting goods. Although much of the attention has been focused on consumer products, hardsoft applications are also making inroads in automotive interiors, lens gaskets, and air ducts. Most of the development activity has centered on injection molding ("overmolding") by two-shot or insert methods, but some has involved coextrusion and sequential multi-material blow molding.

New developments in elastomers help solve adhesion issues that have faced molders of "soft-touch" products in the past, such as difficulty in attaching nonpolar PP-based TPEs to rigid polar substrates. Better adhesion between materials eliminates costly or labor-intensive steps involving mechanical interlocks or adhesives.

Odd couple: TPV and nylon

Two new series of PP/EPDM thermoplastic vulcanizates (TPVs) are said to be the first such materials to bond well to nylon 6. One is the Santoprene PA line from Advanced Elastomer Systems. The new grades, developed in cooperation with nylon producer AlliedSignal, include a small amount of nylon block copolymer as a compatibilizer. Santoprene PA comes in Shore A durometers of 55, 70, and 85. The new grades are priced 15-20% higher than g-p Santoprene. Potential applications include appliance knobs, automotive hoses and lever grips, fluid-delivery systems, power tools, furniture, electronics, and sporting goods.

AES has demonstrated that its nylon bondable grades are suitable for two-shot and insert injection molding and coextrusion. In two-shot molding tests, T-bar samples of nylon 6 and 85 A Santoprene PA showed bond strength of 33 lb/in. Bonds with 55 A Santoprene measured 18 lb/in. However, when Santoprene PA was tested in two-shot injection molding with a nylon 66 substrate, acceptable adhesion (20 lb/in.) was achieved only with the 85 A grade.

New nylon-bondable grades of Sarlink TPV come from DSM Thermoplastic Elastomers. These adhesive grades are based on the Sarlink 3000 series. Two grades are available: Sarlink X-3565B (65 Shore A) and X-3575B (75 A), as well as an experimental 55 A grade.

Good bonding to both nylon 6 and 66 has been achieved with two-shot and insert injection molding as well as coextrusion, DSM reports. Peel tests of the 65A grade insert molded with various substrates showed a bond strength of 25 lb/in. with neat nylon 6; 24 lb/in. with impact-modified, 33%-glass nylon 6; and 37 lb/in. with 33%-glass nylon 66/PP alloy.

Another line of TPVs that bond to engineering plastics is being developed by HiTech Polymers for two-shot molding and coextrusion. M.A. Hanna Engineered Materials is also working on TPVs for overmolding onto rigid substrates.

More soft-touch olefinics

DuPont Dow Elastomers is working to broaden the range of bondable substrates for its Engage polyolefin elastomer (POE). Although it naturally bonds well to PP (a toothbrush is one commercial example), market potential for POE/PP combinations is limited mainly to personal-care products, observes Engage development manager Mike Garratt. "If you want to expand beyond that market area, you really have to develop something that sticks to engineering thermoplastics," he says. "We are trying to deliver adhesion to PC,ABS, PC/ABS, and nylon - and also materials like cellulose propionates and butyrates." He stresses that new grades are not yet close to commercialization.

"The big push for anyone in this field is to achieve softer grades and great adhesion," Garratt notes. Durometers for typical soft-touch applications are said to be 40-50 A. DuPont Dow's primary focus is on two-shot molding, although Garratt is also interested in exploring possibilities in blow molding. In coextrusion, development efforts are focused on profiles where Engage could replace thermoset rubber, TPVs, and even flexible PVC.

Garratt notes that POEs offer cost advantages over other TPEs.

Engage base resins are priced from the low 90[cents]lb range to $1.20/1b. TPVs cost around $1.60-1.70/lb; SEBS ranges from about $1.70 to $2.25/lb.

The newest player in the TPE business,Advanced Polymer Alloys, is developing grades of Alcryn chlorinated PE alloy that will bond to a wider range of substrates. Alcryn naturally bonds well to polycarbonate, PC/ABS, and rigid PVC. Current work is focused on expanding that range to include ABS and nylon by the end of this year. The company is working closely with DuPont (which sold the Alcryn line to Advanced Polymer Alloys last year) to develop compatibilizers for Alcryn and nylon. Primary targets are household and industrial appliances, according to v.p of sales and marketing Dave Santoreli. Alcryn ranges in hardness from 50 to 80 Shore A. He expects the new grades to cost as much as standard Alcryn.

TPOs play hard & soft roles

Flexible grades of TPO are also finding their way into soft-touch applications. They are suitable for many jobs that don't justify the higher cost of TPVs, in which the rubber phase is crosslinked to provide superior properties. That's the view of John Cowperthwait, new business manager of Montell USA, which makes TPOs and TPVs. "If someone wants to make a pen with a soft barrel, I wouldn't try to sell them a TPV as much as a soft TPO," he says.

TPO is overmolded on PE in packaging for personal-care products. The TPO has a durometer of 40 D - relatively hard, but still soft compared with PP or PE, Cowperthwait notes. TPO can also be a cost-effective replacement for soft-vinyl bicycle grips and skins in auto interiors. He says Montell is working to improve scuff and mar resistance, uv stability, and soft feel of TPOs for such applications. Most reactor TPO grades have durometers on the Shore D scale, but Montell is developing grades of 60-80 A.

As hard TPOs make inroads in auto interiors, they may open up opportunities for all-TPO hard-soft combinations, according to Andre Ferland, OEM marketing manager of Solvay Engineered Products. For example, the company's 68 Shore D Sequel 1733 TPO replaced PC last year in an IP upper on the Saturn. Ferland imagines TPOs performing both hard and soft roles in IPs, knee bolsters, glove-box doors, and consolebox lids. The company has a flexible Sequel 1600 series (65 A to 70 D) in development for soft overlays.

Lots of activity in styrenics

A new family of styrenic-based TPEs developed for co-molding and coextrusion has just been introduced in North America by AlphaGary Corp. These COGEE grades are part of a broad line of Evoprene TPEs that has been available in Europe for several [TABULAR DATA OMITTED] years. based on Shell Chemical's Kraton G SEBS, COGEE grades range in hardness from 49 to 70 A. The company is farthest along in developing grades to bond to PC, according to global business director Derek Fraser. He says AlphaGary just came up with a formulation that bonds well to ABS and is continuing to develop materials that adhere to nylon 6, 66, and 11. The company has also started to look at bonding to cellulose acetate and to GE Plastics' Noryl PPO alloys.

New SEBS/SEPS-based elastomers of 35-70 Shore A from M.A. Hanna reportedly bond well to PC, ABS, PC/ABS, and nylon 6. Other potential substrates are PBT, acetal, and cellulose acetate propionate. A general-purpose SEPS grade of 45 Shore A reportedly shows adhesive strength of 10.3 lb/in. when co-molded with ABS, 11.6 lb/in. with PC, and 12.7 lb/in. with nylon 6.

Four new styrenic-olefinic alloys for co-molding with polycarbonate, ABS, and acrylic are available from Multibase. A grade for co-molding with nylon 6 is also being developed. These new TEA ("thermo-elastic alloy") grades range in hardness from 55 A to 45 D and are priced around $2/lb. Current applications include automotive sealing profiles that are overmolded on ABS and acrylic lenses. TEA has also been overmolded on tools, household appliances, and SAN and cellulose propionate toothbrushes.

New grades of Tekron styrene block-copolymer elastomers are being developed by Teknor Apex for bonding to engineering thermoplastics. They will probably be introduced within the next two to three months, says Phil Morin, industry manager for general-purpose compounds. Hardnesses will range from 20 to 60 A.

R&D at Synergistics is aimed at improving the bondability of its Synprene SEBS elastomer to polar substrates like nylon, PC, and ABS, as well as to polyolefins. The company is working with a customer on an insert-molding application and expects to have commercially available products by the end of the year.

Specialty TPEs for bonding

DSM Engineering Plastics is working on soft-touch surfaces of its Arnitel polyester TPE bonded to engineering thermoplastics. Arnitel naturally adheres well to PBT, PC, and ABS, according to Arnitel research and technology manager Rain Borggrava. Hard-soft applications for Arnitel include tools, appliances, and hand-held electronics.

One interesting hard-soft application is sequential blow molding of polyester TPE with PBT in automotive air ducts. Both DSM's Arnitel and DuPont's Hytrel have been used in this manner.

Pebax, a nylon 12-based polyether block polyamide from Elf Atochem, is another specialty TPE that is used in overmolding with TPU, plasticized PVC, and nylons 11 and 12. Widely used in athletic shoes, Pebax comes in hardnesses from 25 to 70 A. Grades with 40 A hardness and higher bond more successfully, because the melt temperatures are higher, says Gary Dennis, CPI marketing manager.

TPUs are a good candidate for overmolding applications because of their resistance to abrasion and chemicals and their natural adhesion to nylon, PBT, PC, and acrylic, according to Roger Huarng, product manager of Elastollan TPU at BASF. The company is developing insert-molding and coextrusion applications such as abrasion-resistant tool handles.

BASF offers a 60 A TPU for bonding. Although it has resulted in stress cracking with substrates of PC and ABS, this problem does not occur with crystalline polymers such as nylon 6. BASF is also developing a new soft grade that reportedly eliminates the risk of stress cracking.

BFGoodrich is working on softer grades of its Estane TPU for bonding, according to Kevin Gase, engineering manager of custom molding. BFG has also been developing co-molding applications combining soft Estane and hard Estaloc glass-filled TPU.

Co-molded soft-touch auto-interior applications are being pursued by Bayer, according to Nick Nardo, technical marketing manager for the Texin TPU line. TPU is also overmolded on acrylic and PC taillights.

More bondable TPEs

Co-moldable TPEs from two more major suppliers are difficult to categorize because the firms decline to identify their polymer composition. For example, overmolding and coextrusion grades of what are described as TPE alloys were introduced last year by GLS Corp. They reportedly adhere to PC and ABS. Development continues on bonding to nylon and PC/ABS. These new Versaflex OM1040 and OM1060 alloys are available in hardnesses of 40 and 60 A. Applications include soft-touch handles for consumer products and tools. Prices are $2.50-$3.00/lb in small quantities, and the specific gravity of both products is below 1.0.

Bondable elastomers have been supplied for years by J-Von. V.P. Edwin Tam reveals only that the Hercuprene bondable grades are terpolymers. Hercuprene S-2954 is available in hardnesses of 50, 60, and 75 A and is bondable to ABS, PC, PC/ABS, cellulose acetate propionate, and polyesters. It can be processed by two-shot and insert molding or coextrusion. J-Von is also developing materials to bond to nylon 6, 66, and 6/66 copolymers.

In peel tests, 50-60 Shore A grades of Hercuprene have a bond strength of 24-28 lb/in. when molded onto PC/ABS. Tam notes that drying the resin is critical to forming a strong bond. Hercuprene is priced in the $5/lb range, but J-Von is working to bring the price down to the $4/lb level.

TPE-friendly rigid resins

Approaching adhesion from the other side of the bond, some suppliers of rigid materials are developing grades that stick well to TPEs. For example, EMS-American Grilon has developed four Grilon HM glass-filled nylon 6 grades that are said to bond very well to Santoprene 191PA. Besides good adhesion, the materials do not require preheating in order to achieve a good bond when insert molded, and they can be processed at temperatures below 500 F, according to Bill Burnham, product manager for injection molding. Grilon HM reportedly also adheres to g-p Santoprene.

EMS-American Grilon also offers a line of Grilamid TR transparent nylons that bond well to TPU. Applications have involved TPU gaskets and seals with Shore A hardnesses of 70 and above. Grivory GTR45, a less expensive transparent nylon from the same supplier, offers the same bonding performance with TPU.

Several years ago, Creanova (formerly Huls America) developed a technology for bonding rigid thermoplastics to rubber without adhesives, adhesion promoters, or clamps. The technology is established in Europe and is now being re-introduced to the North American market. Targets are automotive parts like suspension bushings, spring seats, door locks, and window seals.

Creanova offers special bondable versions of three rigid substrate resins in neat and glass-filled grades - Vestoran PPE, Vestamid nylon 612, and Vestodur PBT. They adhere to rubbers such as EPDM and SBR when coextruded or co-molded by injection or transfer processes. Recently, the technology has been expanded to bond PBT to silicones and PPE to fluoroelastomers.

RELATED ARTICLE: How Processing Affects Adhesion

Makers of TPEs agree that processing conditions affect the quality of the bond in hard-soft combinations. Most sources say that achieving a high enough melt temperature at the interface is essential. For example, Advanced Elastomer Systems notes that its standard Santoprene TPV molds at 420 F, but its nylonbondable PA grades require 480-520 F for good adhesion. Several sources advise that substrates be preheated before insert molding with TPE. On the other hand, too high a temperature may inhibit bonding.

Length of contact in the molten state may matter also. AES notes that in TPV/nylon overmolding, the bond tends to be stronger at the gate, where the elastomer first comes into contact with the nylon. In a similar vein, a Teknor Apex source reports that aging or annealing co-molded parts can double or triple the bond strength between styrenic TPEs and rigid substrates.

As for some other general tips: J-Von cautions that presence of mold release or lubricants can affect adhesion, so check with the supplier of the rigid substrate resin. You must also prevent dirt or oil from contaminating substrate surfaces before overmolding with TPE. And AES advises that overmolding glass-filled substrates requires a resin-rich surface, rather than a glass-rich surface.