Microwave-Safe Thread: Can You Microwave Polyester Thread?

As we get deeper into winter, the idea of microwavable heating bags gets ever more attractive. Cozy, warming, and almost as quick to make as they are to heat, they’re a great way of staving off the cold.

Even though they’re a breeze to construct, crafters looking to make their own heating bags have a few important questions to consider…. namely, what kind of threads and fabrics can safely be used in the microwave? Here, we take a look at some of the keys things you need to know.

Can You Put Polyester Thread in The Microwave?


You’ve probably read numerous accounts of crafters using any thread they find for their microwavable bags without ever experiencing any problems. Well, all power to them. The problem with following anecdotal advice is that at some point or another, it’s almost guaranteed to come face to face with cold, hard scientific facts. And the cold, hard fact in this case is that polyester shouldn’t come within striking distance of a microwave.

First up, what exactly is polyester thread? If you’re anything like the rest of us, it’s probably the one you’re using right now. Since achieving popularity in the 1970s, polyester thread has become the most common type of thread for sewing, and second only to Rayon for embroidery. Made, as the name suggests, from polyester (in itself a petroleum-based synthetic material), polyester thread is the kind of all-singing, all-dancing, all-purpose thread that forms the basis of any good crafter’s kit.

Polyester thread has a number of advantages: it’s strong, it’s durable, it’s got great elasticity, and it’s as resistant to mold, sunlight, and abrasion as any fabric you’re likely to find. What it also has in abundance is plastic. While some plastics are microwave safe, others are most certainly not. Polyester thread falls most decidedly into the latter category.

Unless you want the thread to melt or even start a fire in the microwave, keep it well away.

Can Mercerized Cotton Thread be Microwaved?


For those new to sewing, the question of whether mercerized cotton can be microwaved is more than likely preceded by another, far more pressing question. What, exactly, is mercerized cotton?

For the uninitiated, mercerized cotton is cotton that’s gone through a special, textile finishing treatment. Mercerization involves treating cotton with a solution of 20-30% sodium hydroxide, followed by a thorough washing. As well as giving the thread a beautiful luster, the treatment imparts a number of other great benefits, including improving dye uptake, reducing shrinkage, and increasing tear strength. Most crafters will also find it a breeze to work with, even those who normally find sewing more a chore than a pleasure.

So, now we know what it is, onto the question of whether it can be microwaved. In a nutshell, yes. It may be a pimped-up version of cotton, but when all’s said and done, it’s still cotton- and cotton is one fabric that can be treated to a few blasts in the microwave with no repercussions.

Microwave-Safe Thread Options

When it comes to microwave-safe thread options, the best choice is cotton. Synthetic and acrylic threads are liable to melt or even catch fire when exposed to high temperatures. Cotton, on the other hand, can be used without concern, as can other threads made from 100% natural materials (although beware of any blends containing synthetic materials).

Can fabric Go in The Microwave?

If you’re interested in making heating pads, you’re probably wondering whether it’s ever a wise idea to put fabric in a microwave. After all, microwaves are for food, not fabrics, right? In fact, the answer is a bit more complicated than it seems. While some fabrics are a no-no when it comes to any kind of heating (via the microwave or otherwise), others can be safely zapped with no cause for concern.

When it comes to choosing suitable fabrics, the main thing you need to consider is whether it’s made from natural materials or synthetics. As a general rule, synthetic materials made of plastics should always be avoided. Natural fabrics like cotton are generally safe (usually the worst that can happen is that they get quite hot, but unless you’re planning on cooking them for hours at an end, there’s very little chance they’ll burn, and no chance at all they’ll melt (two things synthetic materials are more than capable of). Along with cotton, wool and muslin can also be used without worry, providing they’re free of any artificial dyes. Read more about fabrics in the microwave in our post Can you microware fabric.

Can Minky Fabric be Microwaved?


For those who haven’t worked with Minky fabric before, a brief introduction might be needed before we address the question of whether it can be microwaved. Although it comes in a variety of types (with smooth Minky, dimple dot Minky, ribbon Minky and print Minky being the most popular), all are made of the same basic material: polyester. As warm as fleece, as luxurious as fur, and as slinky as silk, it’s as popular for making clothes as it is for making crafts… even if it is a pain to handle.

Introductions over, on to the point in hand- can it be microwaved? In a word, no. Minky is made from 100% polyester, making it unsuitable for exposure to high temperatures. Make a microwavable bag from Minky and you’re as likely to end up with a melted mess (or at worse, a fire) as you are a cozy hand warmer.

It’s worth bearing in mind that Minky should never be exposed to high temperatures of any kind, whether they’re emanating from the microwave or not. Regardless of whether you want to straighten a seam or flatten a kink, resist the temptation to run an iron over this kind of fabric: not only can it melt, it’s also likely to become distorted before hardening into the kind of hard, rough fabric that’s the very opposite of what you want.

Can 100% Cotton Batting be Used in the Microwave?

Batting (sometimes known as wadding) is used in a variety of sewing and quilting projects, providing a warm, heavy layer of insulation between fabrics. The range of battings available is almost as wide as the number of ways it can be used, with some of the most widely known including:

Cotton batting -made from all-natural fibers, cotton batting is the preferred choice for those looking for a soft texture and maximum comfort. Typically speaking, batting made from 100% cotton is around 1/8" thick.

Polyester batting – if you want a batting that holds its shape well, is thick but light, and adds warmth without weight, polyester batting may be the ideal choice. Like many synthetic fabrics, it lacks breathability, but what it lacks on that score it makes up for in mold and mildew resistance. When it comes to thickness, most polyester battings come in measurements of 10 oz (equating to 1" thickness), 6 oz (1/2" thickness), 8 oz (3/4" thickness), and 4 oz ( 3/8" thickness).

Wool batting- lightweight, warm, springy, crease-resistant, and remarkably easy to work with, wool batting makes a great choice for crafters looking to keep things all-natural. In terms of thickness, we’re looking at ½" for most types.

Cotton/ Poly blends - cotton/poly batting is typically available in blends of 80% cotton and 20% polyester and is a great choice for those looking to combine the benefits of cotton with the strengths of polyester.

Bamboo batting - with a combination of 50% bamboo and 50% cotton, bamboo batting is a breathable, machine washable material that’s often used for machine quilting.

Bonded batting - unlike other types of batting, bonded batting (as the name suggests) is bonded together with adhesive on both sides to make sure the batting fibers don’t move around or “beard” (the technical name for when fibers push through the fabric).

Fusible batting - fusible batting is easy to use, practical, and contains a type of fusible web that allows you to baste layers together.

Needle punch batting - if you’re looking for a supremely durable backing for quilts, attire, and blankets, needle punch batting may be the ideal choice. The batting is felted together mechanically using needles to create a dense, firm material that offers excellent insulating qualities.

So, now we know the types, which is best for items intended for the microwave? If you guessed cotton, give yourself a pat on the back. Avoid blends containing synthetic fabrics and keep to 100% cotton: natural materials, unlike synthetics, contain zero plastic, and while it’s likely the material will get quite hot in the microwave (tip to the wise: let it cool slightly before removing), it’s guaranteed not to leave a puddle or catch alight.

Will Velcro Melt in The Microwave?


If you’ve ever made a heating bag, you’ll know that Velcro makes an easy way to close the outer cover. The question, however, remains… is it safe?

Some crafters argue that Velcro can be safely microwaved for a few minutes providing the heat from any liquids doesn’t come too close, or the material itself doesn’t get too hot.

However, accidents can and do happen– add a few extra minutes onto the timer by accident, and you could have a disaster (or at the least, a melted mess) on your hands.

If you’re too bowled over by the convenience of Velcro to give it up entirely, there are a few things you can do to lessen the potential for catastrophe. Number 1 is to never use ordinary glue to attach the Velcro to the material: when heated, glue (especially superglue) can become unstable, releasing the kind of fumes you really don’t want to be breathing in. If you can’t face the hassle of sewing the Velcro on by hand, be sure to opt for a non-toxic, microwave-safe adhesive instead.

Can Fleece be Microwaved?

Fleece might be cozy, comfy, and all kinds of other good things, but the one thing it’s not is microwavable. While its name may suggest it’s made from the wool or “fleece” of sheep, it’s in fact 100% synthetic.

Fleece, like Minky, is made from polyester, which in turn is made of plastic. Occasionally, you may find a fleece material made from recycled materials such as plastic water bottles: while this may up its eco-credentials, it doesn’t negate the fact that plastic is still plastic, and as we’ve learned, plastic doesn’t belong in the microwave.

If you intend on making a heating pad from fleece, opt for a fleece cover that can be removed before placing the pad in the microwave. This will allow you to benefit from the coziness of the material without risking a catastrophe in the process.

For the ultimate heating pad with detachable fleece cover, try out this step by step guide.

Rice Heating Bag With Detachable Fleece Cover

What you’ll need

  • 1-yard fleece.
  • 1-yard plain cotton.
  • ​A large bag of rice.
  • ​Cotton thread.
  • ​Sewing materials (a needle, scissors, pins, etc.)
  • ​Snaps.
  • A sheet of paper.

The Method

Step 1- The Rice Bags

  • Cut 2 rectangles out of the cotton. The rectangles should measure 16 inches by 32 inches.
  • Fold each rectangle in half to make two squares.
  • ​With right sides together, sew around the square leaving a 2-inch gap for tuning and a half-inch seam allowance.
  • ​Turn right side out, fashion the sheet of paper into a funnel, and pour around 3- 4 cups of rice into each bag.
  • Sew the gap shut to seal.

Step 2- The Bag

  • Cut a rectangle from the fleece measuring 16 by 32 inches.
  • Cut two further pieces of fabric measuring 16 inches by 17.5 inches, and a further piece measuring 16 inches by 12 inches.
  • Take the piece measuring 16 x 12 inches and fold over each length (these will be the sides measuring 16 inches) under 1 inch. Sew along the edge using a zig-zag stitch. Repeat the process with the two pieces measuring 16 by 17.5 inches, folding under one side measuring 16 inches under 1 inch.

Step 3 – Construct

  • Take the piece of fabric measuring 16 by 32 inches, fold and mark halfway along with a pin.
  • Repeat the process with the rectangle.
  • ​Line up the pins with the wrong sides of the fabric facing.
  • ​Using small, straight stitches, sew along the center of the rectangle.
  • Pin the two squares to the large rectangle on either side and sew to attach.
  • ​Turn right side out, attach the snaps to the inside of the bag’s corners and at the halfway point.
  • To use, simply heat the rice bag inserts in the microwave before popping into the pockets.

I hope you’ve found the information in today’s post useful. If you know any other crafters who might feel the same, please feel free to share.

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