Large structural thermoplastic composite panels
(Published on October 2008 – JEC Magazine #44)
GOLNAZ SHOKOUHI, ENGINEERING MANAGER, AS COMPOSITE
CFTP materials can be formed into any shape using only a combination of heat and pressure. Hence, they are much easier and safer to process because they do not produce any harmful emissions, and are therefore environmentally friendly. Moreover, unlike traditional thermoset composites, thermoplastics can be reheated and reused a number of times, and they are easy to recycle.
However, their industrial development is hampered by a number of manufacturing difficulties. Only press machines can simultaneously apply high temperature to melt the matrix and high pressure to consolidate the material. These low-rate machines produce limited volumes of small- to medium-sized parts, which is not cost-effective for thermoplastic products. The system developed by AS Composite consists in roll forming the raw material using both heat and pressure in a continuous manner. The main product is the ASC panel, a sandwich panel made of thermoplastic composite skins covering a core panel. The core is a lightweight material such as honeycomb or foam, which is mainly used to provide the required panel thickness, strength and stiffness.
The AS sandwich panel is manufactured by a machine that was developed by AS Composite’s engineering department in close cooperation with the National Research Council of Canada (NRC). They decided to create a new machine to overcome the restrictions inherent to the machines and presses currently available on the market, namely manufactured panel dimensions, fabrication time, mould heating and significant energy and raw material loss.
The process offers several advantages:
- Easy production of large parts − panel length is no longer limited by the press length.
- Production of parts with different types of surface finish − the machine easily tolerates additional thin films on the surface of the finished product. These films can be used to improve properties such as colouring, joining, paint adhesion, fire resistance, and more.
- Manufacturing rate of 1.8 m/minute for a panel up to 2.4 m wide.
- Panel thickness ranging from 5 to 150 mm with 0.5 to 3 mm-thick skins.
- The profiles can be roll-formed after consolidation using forming rollers. A pre-heated reinforced TP laminate passes through a series of compaction rollers and is transformed into a partially consolidated flat strip. Instead of being laminated with a core material to make a sandwich panel, the flat strip then continues passing through pairs of forming rollers and, finally, the desired profiles (V, L, U, etc. shape) of continuous fibre thermoplastic composites exit at the end of the process.
The most critical aspects of the process are:
- Defining optimum parameters (window Process) including the roller speed, the temperature and pressure applied at each nanufacturing stage.
- Thermoplastic laminate heating speed. The centre of the laminate should reach the required temperature without surface degradation.
- Surface temperature of the skin and core, that must fuse together without crushing the honeycomb core.
- Temperature of the consolidation rollers to ensure gradual cooling and avoid significant temperature variations in the laminate thickness.
The main business development opportunities for ASC panels are found in the construction industry.
- Concrete formwork
The annual market for concrete formwork in Canada and the USA is estimated at 400 million dollars. The use of conventional plywood or steel panels is limited due to the inconvenience of these materials in concrete formwork. Thermoplastic composite panels with PP honeycomb core can favourably replace the current formwork with no changes required, offering the following advantages:
- Lighter panels (17 kg vs. 25 kg for a 1.2 m x 2.4 m plywood panel), which results in 20% higher productivity (1.1 sq.m/man-hour on average compared to 0.9 sq.m/man-hour for plywood),
- No oil required for dismantling,
- No water or humidity absorption,
- Corrosion resistance,
- 100% recyclable − used panels can be shredded to produce new reinforced plastic parts using thermoplastic processes like injection, extrusion, etc.,
- Foldable and suitable to create 3-D parts,
- Higher impact resistance.
- Bridge decks
Designing and manufacturing innovative all-composite bridge decks is the objective of a project launched in 2003 in cooperation with Sherbrooke University (Sherbrooke, Qc. Canada) and NRC (Boucherville, Qc. Canada). The new deck slab design was intended to replace the traditional timber decks used on short-span bridges.
Two prototypes were fabricated in 2005, 2006 and tested under static and dynamic loading at the laboratory of civil engineering of the University of Sherbrooke. The test results were very encouraging as the slabs were able to resist a complex combined loading mode representing a 70-ton truck. The prototypes were 1.2 m wide and 3.2 m long strips representing two continuous spans of 1.45 m each.
The roll-formed reinforced thermoplastic webs were assembled between two flat skins of a deck slab to form the bridge deck prototype, as shown in Figure 1. Expanding structural foam was used to fill the inner space of the webs. The innovative deck slab shows several advantages over current timber decks. It is much more durable, weighs less per surface area, shows excellent fatigue resistance, and is much easier to install and replace. It is believed that the biggest advantage of thermoplastic composite bridge decks over their thermosetting counterparts is their potential for rapid and cost-effective processing in a continuous process as developed by AS Composite.
- Panellized housing system
The 2005 hurricane season brought unprecedented destruction to the US gulf coast. Thousands of homes, businesses and lives were destroyed or disrupted. In addition, the South Asian tsunami in December 2004 killed hundreds of thousands of people and displaced millions. The earthquake in Pakistan left thousands exposed to the ravages of winter. War and famine in Africa have filled refugee camps with displaced broken families. Soaring energy and construction costs have stretched the world’s working class to the breaking point.
AS Composite has developed a 10 cmthick panel made of a lightweight foam core adhesive bonded to two strong thermoplastic composite surfaces. The panel was used to develop a new building system to meet the current housing needs worldwide. The system is a panellized home or structure that will change the dynamics of emergency and sustainable permanent housing. The floor plans of these homes can be easily altered to accommodate whatever provisions are necessary for privacy, family size, family composition, or people with disabilities.
Extensive research and loading, deflection and suction tests have been carried out with collaboration of NRC. Architectural Testing, Inc. (ATI) in York, PA, tested the panel according to the Florida Building Code Test Protocols for High Velocity Hurricane Zone, TAS 201- 94 Impact Test.
These modular homes are designed to meet several needs. They can be erected on temporary foundations (block piers) and be outfitted with individual waste disposal systems, generators and bottled gas. They can be erected in isolated lots or grouped in urban parking lots or parks. They can be retrofitted with permanent foundations and hook-ups, or be disassembled and re-erected at a permanent location or stored for future deployment. They can also be built on stationary piers, basements or crawl spaces for permanent use.
These homes can be manufactured, shipped, and erected rapidly for temporary housing or remain as permanent housing. Local, state, and housing authorities will start to pursue the same methods for community development. Emergency management agencies can strategically stock pile kits for rapid deployment following a disaster. A prototype was built on blocks in four days by a team of four people at the 2005 ThinkTec conference in Charleston, South Carolina (Figure 2).
The prototype home surpassed the Southern Dade County Standards of 225 km/h wind forces. Due to the composite integrity of these panels, they are mould-, water-, vermin- and termiteproof, as well as seismic-, flood- and hurricane-resistant.