Which engineering plastic should you choose?

At a glance: density and glass transition. Ranges mix grades and states. PS prints its glass transition only in kelvin (360 to 370 K).

Which engineering plastic is the lightest?

PP is the lightest in our data at 0.9 g/cm³, followed by PE at 0.93 to 0.96 g/cm³; both are commodity plastics, as is PS at 1.04 to 1.05 g/cm³. The lightest engineering grades are ABS at 1.04 to 1.05 g/cm³ and the nylons: nylon 6 at 1.12 to 1.15 g/cm³ and nylon 66 at 1.13 to 1.14 g/cm³. PC and PMMA both read 1.19 g/cm³.

The heavier group is PLA at 1.22 to 1.24 g/cm³, PBT at 1.3 to 1.33 g/cm³, PET film at 1.35 to 1.39 g/cm³ and POM at 1.41 to 1.42 g/cm³. PVC has the widest spread, 1.18 to 1.62 g/cm³: rigid compounds read 1.32 to 1.54 g/cm³ and flexible compounds 1.18 to 1.62 g/cm³. Apart from PVC, no profile goes above 1.42 g/cm³.

Which engineering plastic is the strongest?

Dry nylon 66 has the highest value labelled yield among the materials compared here: 85 MPa for BASF Ultramid A3K dry, and 82 MPa for dry DuPont Zytel 101L NC010. The same grades give 54 and 55 MPa once conditioned. POM yields at 62 to 71 MPa: the two Hostaform copolymers at 62 and 64 MPa, the Delrin 500P homopolymer at 71 MPa. PC yields at 65 MPa (Makrolon 2207 MAS195, 23 °C, 50 mm/min).

PLA shows 51 to 65 MPa. PBT gives 51 to 55 MPa, ABS 45 to 47 MPa, rigid PVC 38.2 to 53.4 MPa, PP 30 to 34 MPa (30 MPa for a random copolymer, 34 MPa for a homopolymer) and PE 17 to 28.4 MPa. PS spans 20.3 to 55 MPa: general-purpose PS 158K is 55 MPa, high-impact grades about 20.3 MPa.

Three strength figures sit outside the yield comparison. PMMA cast sheet is listed at 80 MPa and nylon 6 at 70 to 76 MPa (70 MPa is the conditioned ECO 27 grade). Neither profile says whether the figure is yield or break. PET shows 190 to 210 MPa at break, but that is a 50 micron biaxially oriented film and cannot be set against moulded parts.

Which engineering plastic is the stiffest?

Among moulded grades, PMMA sheet gives 3,300 MPa (a short-term value), POM 2,600 to 3,100 MPa and dry nylon 66 up to 3 GPa. PC gives 2.35 to 2.4 GPa (the lower figure is flexural), ABS 2,400 MPa and PBT 2.4 to 2.6 GPa. Higher single values come from other forms. PLA spans 2.31 to 3.5 GPa, the top being an extrusion grade measured to ASTM D882. Rigid PVC reaches 3.72 GPa in a powder dry-blend compound, and PS runs 1.86 to 3.3 GPa from a high-impact grade to general-purpose grades. PET oriented film (3.8 GPa, 50 micron Mylar A, machine direction) is left out of the ranking because it cannot be set against moulded parts.

POM splits into 2,600 and 2,700 MPa for the copolymers and 3,100 MPa for the homopolymer, which matches dry nylon 66. At the soft end, PP random copolymer reads 1,150 MPa in tension and the PP homopolymer 1.48 GPa in bending. PE reads 0.8 to 1.27 GPa, a recycled HDPE tensile value and an HDPE flexural value.

Nylon 66 stiffness depends on water: it spans 1.2 to 3 GPa between the conditioned and dry states of the BASF grade, and DuPont Zytel gives 3,100 MPa dry and 1,400 MPa conditioned. Nylon 6 shows 2,215 to 3,100 MPa, but the two sheets are different grades in different states, so that gap is not purely a moisture effect.

Which engineering plastic handles heat best?

Under a 1.8 MPa load, PC deflects at the highest temperature among the materials compared here, once an unverified nylon 6 value is excluded: 123 °C for Makrolon 2207 MAS195, with a glass transition of 142 to 144 °C and a Vicat softening point of 143 °C. The excluded value is 200 °C for nylon 6 (Vinmar sheet, listed at 1.8 MPa). It is far above nylon 66's 70 to 75 °C under the same load and the profile gives no reason, so confirm it with the supplier.

Other 1.8 MPa values: POM 95 to 101 °C, ABS 80 to 98 °C (two grades: 80 °C for SABIC MG94F and 98 °C annealed for Chimei PA-757F, so not like for like with PC) and rigid PVC 57 to 72.2 °C (unannealed bars). PS shows 82.2 to 98 °C, a range that includes a 0.45 MPa value. Load changes the answer more than the family does: PBT reads 54 to 55 °C at 1.8 MPa and 150 to 165 °C at 0.45 MPa, PP reads 80 to 97 °C at about 0.45 MPa, and PLA spans 54 to 149 °C, from amorphous moulded Ingeo 3100HP to nucleated crystalline grades.

Glass transition is stored for only four of the thirteen: PC, PET at 76 °C (a bottle-grade copolymer from a paper, not film data), PBT at about 60 °C and PLA at 55 to 60 °C. It is not stored for ABS, PP, PE, PVC, PMMA, the nylons or POM, and PS has only a value printed in kelvin, 360 to 370 K.

Service limits differ in meaning. PMMA is rated 80 °C permanent in air, PVC 60 °C continuous for two flexible cable grades (no rigid value), and PET film 150 °C in air uncoated. PBT lists 150 to 200 °C, a short-cycle rating, not continuous. For long heat ageing the temperature index is 135 °C for PBT at 20,000 hours, and 118 °C at 5,000 hours and 101 °C at 20,000 hours for nylon 66.

How much water do engineering plastics absorb?

Nylons absorb the most among the materials with a value. Nylon 6 holds 2.2 to 10 %: 2.2 % at 23 °C and 50 % relative humidity and 8.5 % at saturation for Ultramid B27 EX. Durethan B 30 S prints 3 % at 50 % RH and 10 % on immersion, and the Aquafil sheet 10 %, all as maximum limits. Nylon 66 holds 1.8 to 8.5 %: 2.6 to 3.1 % at 50 % RH and 8.5 % at saturation in the Zytel 101L NC010 and Ultramid A3K sheets, and 1.8 % for a 33 % glass-filled grade. The 50 % RH values compare best, and PBT takes 0.25 % there. PC shows 0.58 % at equilibrium (LEXAN FXE154, humidity not stated) and PMMA 2.1 % mass maximum on immersion (ISO 62, a 2 mm cast sheet); these are different tests.

In the BASF nylon 66 sheets, conditioning lifts elongation at yield from 5 to 25 % as strength and stiffness fall. LANXESS gives a length growth of 0.2 to 0.3 % per 1 % water absorbed for unreinforced nylon 6, and a larger change in thickness.

No water absorption value is stored for ABS, PP, PE, PS, POM, PET, PVC or PLA.

What chemicals and sunlight damage each plastic?

The profiles state most of these weaknesses qualitatively, often with no test time.

ABS: yellows and turns brittle with UV or excessive heat. Poor in chlorinated solvents and alcohols.

PC: yellows from the surface, with most of the photodamage in the top 25 µm; the Makrolon 2207 grade is UV-stabilised. No usable chemical table is stored.

PMMA: the makers differ. One UV-transmitting, detector-grade acrylic without a UV absorber lost 47.5 % of its transmittance at 360 nm after 296 J/cm2 of sunlight, while another sample changed by +1.8 %. Commercial PMMA normally carries UV absorbers, so that loss does not describe ordinary sheet (general engineering knowledge, not in our data). Good with dilute acids and alkalis and limited with organic solvents, per its maker.

PET: an amorphous film's elongation fell from 16.84 % to 5.61 % after one hour of UV in a chamber (a bottle-type film, not Mylar). It embrittles when more than 0.1 % water is present at heat.

PBT: hydrolyses in warm humid air and in hot water above 60 °C; Celanex 2003HR is a hydrolysis-resistant grade.

POM: affected by sunlight at 290 to 400 nm, and UV-stabilised grades exist. Attacked by chlorine at 3 to 5 ppm and above, depolymerised by zinc chloride, and rated unsatisfactory in 10 % hydrochloric and 30 % sulphuric acids by DuPont. Copolymer POM resists hot water and strong caustic better than homopolymer.

PP: needs antioxidants and UV absorbers, and its profile holds no retained-strength figure. It becomes brittle below 0 °C, per the profile. No usable chemical table is stored.

PE: yellows with age, though UHMWPE is described as very resistant to UV. HDPE grades are not attacked by strong acids or bases, but polyethylene can crack under stress in stress-cracking agents.

PVC, PLA, PS and the nylons: a producer note calls one PVC compound excellent in weathering, with no test. PLA loses viscosity above 0.025 % moisture when melted. No usable chemical table is stored for PS, and no UV result for PS, the nylons or PLA.

General engineering knowledge, not in our data: PC can crack under stress in some solvents and hydrolyse in hot water, and nylon and POM creep under steady load.

Terms used

Yield strength

the stress where a plastic starts to stretch for good. Design loads are set below it. Some sheets print only a tensile strength, which may be the break value.

Tensile and flexural modulus

stiffness measured in pulling or in bending. The two differ for the same grade, so a chart that mixes them is not like for like.

Heat deflection temperature (HDT)

the temperature where a test bar bends a set amount under a set bending load. It changes with the load: these profiles use 0.45 MPa and 1.8 MPa.

Glass transition temperature (Tg)

the range where the amorphous part of a plastic turns from hard and glassy to soft and rubbery.

Vicat softening temperature

the temperature where a loaded needle sinks a set depth into the plastic. It is a different test from HDT, so the numbers are not interchangeable.

Temperature index

a heat-ageing rating (IEC 60216): the temperature at which tensile strength falls to half after a stated number of hours.

Conditioned and relative humidity (RH)

conditioned means stored in humid air until the part holds water in balance with it; 50 % RH is the usual test air. Dry means as moulded, before it takes up water.

Hydrolysis

breakdown of polymer chains by water, faster when hot. Polyesters such as PBT and PET are sensitive to it.

What to check before you order

1. Ask for the grade code, not the polymer name, and the datasheet state: dry or conditioned, annealed or not.
2. For nylons, ask for the modulus and yield at 50 % relative humidity as well as dry, and the allowed dimensional change in service.
3. For heat, ask for the HDT load (0.45 or 1.8 MPa), the Vicat load and rate, and a stress-against-temperature or creep curve at your service temperature. Do not treat HDT or Tg as a use limit.
4. For outdoor or window use, ask for a UV-stabilised grade and exposure results at your dose.
5. For POM, check for chlorine, bleach and strong acids in cleaning fluids. For PBT and PET, check for hot water, steam or humid heat.

What do we not know yet?

• Glass transition is missing for ABS, PP, PE, PVC, PMMA, the nylons and POM, and is weak for PS, PET and PBT (see the heat section).
• Maximum service temperature is stored for only four plastics, on four bases, and water absorption for only the nylons, PBT, PC and PMMA.
• Yield and modulus sit under different property names across profiles, so no single chart covers all thirteen. Impact values use several names and test types, so no toughness comparison is made.
• The nylon 6 heat deflection value of 200 °C is unverified. PET is film only, PVC mixes rigid and flexible compounds, and PE mixes HDPE and UHMWPE, so none of them speaks for its family.
• No creep, fatigue, wear, friction or cost property is stored in these profiles, and filled grades are mostly absent. The profiles hold no matched UV exposure results across plastics.
• Polyurethane and cellulose acetate are not covered, and the high-temperature plastics have their own article.

Questions people ask