Report by Liz White, editor
As the push for wider use of renewable energy sources continues so associated industries start to reap the benefits. For the adhesives industry, the growing market for wind farms means the sector needs adhesives for joining turbine blade sections together, for example.
Adhesives companies generally have a big advantage with composites: in order to assemble the parts, "there is only one technology that works," and that is adhesives, commented Eugenio Toccalino, marketing manager with Dow Automotive.
So the wind energy market throws up new opportunities and challenges for suppliers of adhesives. And polyurethane also has a share of this bonding market, according to experts UTI spoke to at the JEC Composites show in Paris, 29-30 March.
How much adhesive is used in one wind blade depends on blade size, but it can vary from 300 kg up to 1 tonne in very large blades, said Hartmut Pilz of Henkel AG's focus segment wind in EMEA (Europe, the Middle-East and Africa). Pilz said currently the maximum blade length is about 62 m.
Although it is difficult to estimate market size because different technologies are being used, Pilz said in a 29 March interview at the JEC Composites show that he has heard a figure of 60 000 or 70 000 blades made globally in 2009.
Even at only 300 kg of adhesive per blade, this would give a minimum of 18 kilotonnes of adhesive needed for wind blades globally each year.
However, Daniel Vogt, market engineer for renewable energies with Sika, noted that for the longer blades which are now being proposed, the design may change. And some of these changes are because manufacturers want to minimise the use of adhesives, he said.
Epoxy adhesives are widely used as blade adhesives, as are polyesters and some MMA (methyl methacrylate), said Pilz. Polyurethane adhesives are already in use in this area and have been for some time, but their application depends on the blade structure.
Blades with a box beam inside, such as those made by Vestas or Gamesa, "use PU adhesives so far. But it is a different class of adhesives to ours," said Pilz.
"Some people consider this type of adhesive as non-structural," he added.
Polyurethanes for spar web blades
With blades that have a spar web or shear web type of construction, where the loading goes through the spars, there is higher stress on the bond, and the adhesive is considered to be load bearing, Pilz noted.
For this type of construction, polyester, MMA and epoxy adhesives are all used currently, as far as Pilz is aware. Henkel is now promoting its PU types for such use (see box).
Pilz said one issue with epoxy adhesives is that they are exothermic, which causes problems and could result in blade distortion.
"Heat may create damage via expansion and shrinkage, it may bring in tensions. You have some parts made of composite foams and they may start to deteriorate when exposed to high temperature over a too long time," Pilz noted.
Vogt agreed, commenting that as the polyurethane curing process is less exothermic, PU adhesives offer less risk of cracks being formed in the blade due to internal stresses.
Pilz also suggested that polyurethane allows a faster process than epoxy as it is faster-setting. This means "you can speed up the process with less risk of damage to the blade parts by too high a temperature."
Temperatures needed with PU depend on the quantity of adhesive, but in its literature all of Henkel's data show that PU operates around 75°C.
Toccalino claims that Dow 1s one of the leaders in adhesive bonding. The group supplies structural adhesives. It also offers solutions for modular bonding, "where you put together dissimilar materials with different coefficients of linear expansion," Toccalino said.
Combining materials such as plastics and aluminium needs a very elastic assembly, he pointed out.
Toccalino said Dow has historically been very strong in resins for wind energy. "Now we bring the adhesive as well."
Meanwhile in automotive: "We have been historically very strong in the adhesives and now we enter with the resins," he added.
And here, Toccalino feels that long term there is going to be a very synergistic effect between the adhesive used in assembly and the resin used in these high-end composites, whether polyurethane or epoxy.
PU CAN COMPETE IN WIND BLADE STRUCTURE
Bayer MaterialScience’s Harry George, who is US manager for new applications is highly optimistic about the PU composites business. Asked is the mainstream composites market one PU can compete in, his reply was emphatic: "Definitely. That's why BMS has so many people working here."
And will this extend to uses such as wind turbine blades? Again George said yes. His view is that polyurethane has, "much better properties than vinyl ester and epoxies. Yes everybody knows epoxy and is fam1har with it, but our static properties are very similar." And "PU's dynamic properties are so much better," he continued.
BMS has done studies comparing vinyl ester, epoxy and PU samples, each with a deli berate flaw. The samples each had the same glass fibre construction, with a hole drilled to simulate an air bubble. These were sent to outside laboratory for tensile fatigue tests with a 10 000 lb (4356 kg) load.
Vinyl ester lasted 50k cycles, epoxy broke at 95 k cycles, but the urethane lasted 850k cycles. A factor almost of ten better, said George. "For a wind blade where all that flexing is involved," this very good dynamic behaviour i s a compelling reason to use polyurethanes, George indicated.
Repair of wind blades is a challenging task, the Bayer expert pointed out, with windfarms in the English Channel, for example, repaired by a technician rappelling down the blade to mend cracks.
And this can only get more difficult in future as wind blade length rises from 40m to 60m to perhaps 100 m i n future, George said.
BMS is in contact with people making wind turbine who are interested in assessing use of polyurethane, and doing tests, George said, noting t he extensive certification such material s must undergo.
In terms of costs. PU is probably equivalent to vinyl ester, and say 10 percent less than epoxy. But George said, "Cost is not a key driver: it's how fast can you make the blade." With PU, users should be able to halve blade production time. Using much the same process as at present, resin infusion and layup.
Asked could the process be at all automated with PU, George said i t is "such a complicated part design, with many layers and crossovers i n the fabrics - and that takes most of the time."
Nevertheless, even using conventional processing, "We think PU can offer 50 percent improvement in cycle times," he reiterated.
DESIGN DICTATES ADHESIVE
Discussing adhesives for wind blades. Sika's Daniel Vogt, market engineer for renewable energies, also noted that the technology used is "a little bit dependent on the blade design."
Most of the market is covered by epoxy, but in the case of pre-preg technology, mostly urethane adhesives are chosen, Vogt said.
Processibility can also be an advantage with PU as it is easier to pump, he added.
Users need fast but controllable adhesion, with an open time of 50-60 min, Vogt said. Also the adhesive is often applied under "perhaps quite harsh environmental conditions." for example in India or China, where humidity and also temperature might be high. For this reason, "you need an adhesive with high moisture resistance," Vogt said. Then, "You need instant adhesion once you put the two parts together."
The latest challenge is that customers are looking to cut cycle times from 24 to 12 hours, Vogt said.
Adhesive application and curing are quite time offer fewer bottlenecks during production.
Pre-preg production is a lengthy part of the process in conventional technology for blade manufacture, Vogt said, adding that some manufacturers are moving towards combining prepreg and a RIM (reaction injection moulding) approach.
Some new blade designs involve bonding cured parts together and in this case a fast reacting adhesive is needed, hence the bottleneck is cut, Vogt added.
Asked if customers are interested in PU as a blade material, Vogt said yes. Most resins now are based on epoxy which has limits in times and with PU, "you could significantly cut cycle times," he added. But, he noted, "It is not so easy to develop something that works with PU."
Sika currently supplies adhesives for wind blades from its SikaForce portfolio of structural 2- component PU adhesives for harsh environments. These are designed to be strong and crack resistant, with fast curing but long open times.
Sika also produces tailored products to meet customers' specific requirements in terms of glass transition temperature, open time or resistance to moisture.
The adhesives are used not only for bonding blade elements together, but also to finish and repair surfaces. They are designed to flow and fill gaps without leaving air bubbles on the repaired area.
Vogt commented that most of Sika's materials, say 80 percent, are used in construction, with the rest going into industrial uses, which includes composites.
Sika also supplies epoxy resins, but PU materials dominate the Swiss group's construction/industrial materials range of sealants and adhesives.
